Heat Exchangers. Volume II: Mechanical Design, Materials Selection, Nondestructive Testing, and Manufacturing Methods [3 ed.] 9781032399348, 9781032399355, 9781003352051

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Heat Exchangers Volume II Heat Exchangers Volume II: Mechanical Design, Materials Selection, Nondestructive Testing, and Manufacturing Methods covers mechanical design of pressure vessels and shell and tube heat exchangers, including bolted flange joint design, as well as selection of a wide spectrum of materials for heat exchanger construction, their physical properties, corrosion behavior, and fabrication methods like welding. Discussing the basics of quality control, the book includes ISO Standards for QMS, EMS, EnMS, and OSHAS and references modern quality concepts such as Kaizen, TPM, and TQM. It presents Six Sigma, including Lean tools, for heat exchangers manufacturing industries. The book explores heat exchanger manufacturing methods such as fabrication of shell and tube heat exchangers and brazing and soldering of compact heat exchangers. The book serves as a useful reference for researchers, graduate students, and engineers in the field of heat exchanger design, including pressure vessel manufacturers.

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Heat Exchangers Volume II Mechanical Design, Materials Selection, Nondestructive Testing, and Manufacturing Methods Third Edition

Kuppan Thulukkanam

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Designed cover image: courtesy of Villa Scambiatori Srl Italy Third edition published 2024 by CRC Press 2385 Executive Center Drive, Suite 320, Boca Raton, FL 33431 and by CRC Press 4 Park Square, Milton Park, Abingdon, Oxon, OX14 4RN CRC Press is an imprint of Taylor & Francis Group, LLC © 2024 Kuppan Thulukkanam First edition published by Marcel-​Dekker 2000 Second edition published by CRC Press 2013 Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, access www.copyri​ght.com or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-​750-​8400. For works that are not available on CCC please contact [email protected] Trademark notice: Product or corporate names may be trademarks or registered trademarks and are used only for identification and explanation without intent to infringe. Library of Congress Cataloging‑in‑Publication Data Names: Kuppan, T., 1957– author. Title: Heat Exchangers. (v.2) Mechanical design, materials selection, nondestructive testing, and manufacturing methods / Kuppan Thulukkanam. Other titles: Heat exchanger design handbook. Selections (Mechanical design) Description: Third edition. | Boca Raton, FL : CRC Press, 2024. | Series: Heat Exchangers | Includes revised material previously published in the author’s Heat exchanger design handbook. | Includes bibliographical references and index. Identifiers: LCCN 2023021011 (print) | LCCN 2023021012 (ebook) | ISBN 9781032399348 (hardback) | ISBN 9781032399355 (paperback) | ISBN 9781003352051 (ebook) Subjects: LCSH: Heat exchangers – Design and construction – Handbooks, manuals, etc. Classification: LCC TJ263 .K872 2024 (print) | LCC TJ263 (ebook) | DDC 621.402/5 – dc23/eng/20230506 LC record available at https://lccn.loc.gov/2023021011 LC ebook record available at https://lccn.loc.gov/2023021012 ISBN: 978-​1-​032-​39934-​8 (hbk) ISBN: 978-​1-​032-​39935-​5 (pbk) ISBN: 978-​1-​003-​35205-​1 (ebk) DOI: 10.1201/​9781003352051 Typeset in Times by Newgen Publishing UK

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Dedicated to my parents, S. Thulukkanam and T. Senthamarai, my wife, Tamizselvi Kuppan, and my mentor, Dr. Ramesh K. Shah

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Contents Preface................................................................................................................................................xi Acknowledgments............................................................................................................................. xv About the Author.............................................................................................................................xvii

Chapter 1 Mechanical Design of Shell and Tube Heat Exchangers.............................................. 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 1.11 1.12 1.13 1.14 1.15 1.16 1.17 1.18 1.19 1.20 1.21 1.22 1.23 1.24

Pressure Vessels.................................................................................................. 1 Mechanical Design of Pressure Vessels and Heat Exchangers........................... 5 Stress Analysis.................................................................................................. 15 Stress Categories............................................................................................... 16 Design Methods and Design Criteria................................................................ 19 Key Terms in Pressure Vessel and Heat Exchanger Design.............................. 24 Pressure Vessels Design.................................................................................... 25 Mechanical Design of STHE............................................................................ 30 Fundamentals of Tubesheet Design.................................................................. 35 Tubesheet Design Procedure: Historical Background...................................... 38 Tubesheet Design as per ASME Code Section VIII Div. 1............................... 46 Tubesheet Design as per TEMA Standards (Appendix A-​Non-​mandatory Section)............................................................................................................. 49 Flanged Tubesheets: TEMA Design Procedure A.1.3.3................................... 58 Rectangular Tubesheet Design.......................................................................... 58 Curved Tubesheets............................................................................................ 60 Conventional Double Tubesheet Design........................................................... 60 Cylindrical Shell, End Closures, and Formed Heads Under Internal Pressure............................................................................................................. 61 Bolted Flange Joint (BFJ)................................................................................. 70 Taper-​Lok® Heat Exchanger Closure............................................................. 100 Expansion Joints............................................................................................. 102 Openings and Nozzles..................................................................................... 119 Nozzles............................................................................................................ 120 Supports.......................................................................................................... 124 Lifting Devices and Attachments.................................................................... 128

Chapter 2 Material Selection and Fabrication............................................................................ 135 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11

Material Selection Principles.......................................................................... 135 Equipment Design Features............................................................................ 151 Raw Material Forms Used in the Construction of Heat Exchangers.............. 152 Materials for Heat Exchanger and Pressure Vessel Construction................... 155 Plate Steels...................................................................................................... 156 Pipes and Tubes............................................................................................... 160 Plate Steels Weldability Problems.................................................................. 163 Hot Cracking................................................................................................... 172 Laboratory Tests to Determine Susceptibility to Cracking............................. 176 Service-​Oriented Cracking.............................................................................. 178 Welding-​Related Failures................................................................................ 178 vii

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2.12 2.13 2.14 2.15 2.16 2.17 2.18 2.19 2.20 2.21 2.22 2.23 2.24 2.25 2.26 2.27 2.28 2.29 2.30 2.31 2.32 2.33 2.34 2.35 2.36

Selection of Cast Iron...................................................................................... 178 Selection of Carbon Steels.............................................................................. 179 Low-​Alloy Steels............................................................................................ 183 Quenched and Tempered Steels...................................................................... 185 Chromium-​Molybdenum Steels...................................................................... 187 Stainless Steels................................................................................................ 192 Ferritic Stainless Steels................................................................................... 220 Duplex Stainless Steels................................................................................... 225 Super Duplex Stainless Steel.......................................................................... 234 Superaustenitic Stainless Steels...................................................................... 235 Aluminum Alloys: Metallurgy........................................................................ 237 Copper............................................................................................................. 249 Nickel and Nickel-​Base Alloys Metallurgy and Properties............................ 260 Titanium: Properties and Metallurgy.............................................................. 269 Zirconium........................................................................................................ 281 Tantalum.........................................................................................................284 Materials for High Temperature Heat Exchangers......................................... 285 Graphite........................................................................................................... 285 Glass................................................................................................................ 290 Teflon.............................................................................................................. 291 Ceramics......................................................................................................... 292 Composite....................................................................................................... 294 Alloys for Subzero/​Cryogenic Temperatures.................................................. 294 Cladding.......................................................................................................... 303 Post-​Weld Heat Treatment of Welded Joints in Steel Pressure Vessels and Heat Exchangers.......................................................................... 320

Chapter 3 Quality Control, Inspection, and Nondestructive Testing......................................... 337 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.12 3.13 3.14 3.15 3.16 3.17 3.18 3.19 3.20 3.21 3.22

Quality Control and Quality Assurance.......................................................... 337 Quality Control System (QC)......................................................................... 339 Quality Manual............................................................................................... 341 Elements of Quality Costs.............................................................................. 343 Quality Review and Evaluation Procedures.................................................... 344 Documentation................................................................................................ 344 Quality Management....................................................................................... 345 Quality Tools and Quality Improvements Methods........................................ 347 Inspection........................................................................................................ 362 Welding Design............................................................................................... 364 Nondestructive Testing Methods..................................................................... 374 Visual Examination......................................................................................... 385 Liquid Penetrant Inspection............................................................................ 391 Magnetic Particle Inspection........................................................................... 396 Magnetic Rubber Technique (MRT)............................................................... 403 Radiographic Testing...................................................................................... 404 Ultrasonic Testing........................................................................................... 418 Advanced UT Methods................................................................................... 439 Acoustic Emission Testing.............................................................................. 444 Eddy Current Testing...................................................................................... 449 Tube Inspection with Magnetic Flux Leakage................................................ 459 Remote Field Eddy Current Testing................................................................ 460

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3.23 3.24 3.25 3.26 3.27 3.28 3.29 3.30 3.31 3.32 3.33 3.34 3.35 3.36 3.37 3.38 3.39 3.40 3.41 3.42 3.43 3.44

Tube Inspection with Near Field Testing........................................................ 461 Pulsed Eddy Current (PEC)............................................................................ 462 Heat Exchanger Tube Inspection Methods..................................................... 464 Tubesheet Diagram for Windows.................................................................... 465 Alternating Current Field Measurement (ACFM).......................................... 465 Acoustic Pulse Reflectometry (APR).............................................................. 466 Barkhausen Noise Analysis............................................................................. 467 Automated Corrosion Mapping...................................................................... 468 Drones Use in Nondestructive Testing............................................................ 468 Dynamic NDT Methods.................................................................................. 468 Electromagnetic Sorting of Ferrous Metals.................................................... 469 Electromagnetic Acoustic Transducers........................................................... 469 Optical Holography NDT............................................................................... 470 Magnetic Flux Leakage................................................................................... 470 Microwave Nondestructive Testing................................................................. 472 Smart Pig......................................................................................................... 472 Replication Metallography.............................................................................. 472 Shearography.................................................................................................. 473 Thermography................................................................................................. 474 PAIRT.............................................................................................................. 474 Leak Testing.................................................................................................... 474 In-​Service Examination of Heat Exchangers for Detection of Leaks............. 482

Chapter 4 Fabrication, Brazing, and Soldering of Heat Exchangers......................................... 494 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 4.12 4.13 4.14 4.15 4.16 4.17 4.18 4.19 4.20 4.21 4.22 4.23

Introduction..................................................................................................... 494 Fabrication of the Shell and Tube Heat Exchanger........................................494 Vendor’s Responsibilities................................................................................ 497 Details of Manufacturing Drawing................................................................. 497 Details of Manufacture of STHE.................................................................... 501 Plate Bending.................................................................................................. 508 Welding of Shells, Checking the Dimensions, and Subjecting Pieces to Radiography..................................................................................... 509 Tubesheet and Baffle Drilling......................................................................... 514 Tube Bundle Assembly................................................................................... 519 Tubesheet to Shell Welding............................................................................. 526 Tube-​to-​Tubesheet Joint Fabrication..............................................................527 Tube-​to-​Tubesheet Joint Welding................................................................... 553 Assembly of Channels/​End Closures with Shell Assembly............................ 577 Preparation of Heat Exchangers for Shipment................................................ 581 Making Up Certificates................................................................................... 582 Foundation Loading Diagrams/Drawings....................................................... 582 Heads and Closures......................................................................................... 583 Heads and Closures Forming Methods........................................................... 585 Brazing............................................................................................................ 593 Elements of Brazing........................................................................................ 595 Quality Control and Quality Assurance System for Brazing of Heat Exchangers......................................................................................... 605 Brazing Methods............................................................................................. 605 Brazing of Aluminum..................................................................................... 617

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4.24 Microchannel Heat Exchangers...................................................................... 627 4.25 Cuprobraze Heat Exchanger........................................................................... 629 4.26 Brazing of Heat-​Resistant Alloys, Stainless Steel, and Reactive Metals............................................................................................... 632 4.27 Post-​Braze Cleaning after Lucasmilhaupt...................................................... 634 4.28 Inspection and Testing of Brazed Joint........................................................... 634 4.29 Nondestructive Testing Methods..................................................................... 635 4.30 Destructive Testing Methods........................................................................... 636 4.31 Soldering of Heat Exchangers........................................................................ 637 4.32 Nondestructive Testing of Soldered Heat Exchanger..................................... 644 4.33 Properties of Brazed Joints............................................................................. 645 4.34 Corrosion of Brazed Joints and Corrosion Control Methods......................... 645 4.35 Corrosion of Soldered Joints........................................................................... 649 4.36 Evaluation of Design and Materials of Automotive Radiators....................... 650 Annexures.................................................................................................................. 651

Index............................................................................................................................................... 661

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Preface INTRODUCTION This edition showcases recent advances in heat exchanger and pressure vessels mechanical design, construction, operation, and their maintenance and pressure vessels Codes and Standards; materials selection principle for a wide spectrum of materials and fabrication issues, conventional NDT methods for heat exchangers inspection and advances in many NDT methods, lean manufacturing and shop floor quality improvement methods, advances in brazing methods and newer types of heat exchangers manufactured by brazing, etc. are discussed. The book will be a centerpiece of information for practicing engineers, research engineers, academicians, designers, and manufacturers involved in heat exchange between two or more fluids.

COVERAGE CHAPTER 1 MECHANICAL DESIGN OF PRESSURE VESSELS AND SHELL AND TUBE HEAT EXCHANGERS Safe design and construction of pressure vessels is critical to ensure safety in operation and prevent catastrophic failure if any. There are country specific pressure vessel codes that govern how pressure vessels should be designed and built in order to meet various safety guidelines and requirements. This chapter discusses mechanical design of pressure vessels and shell and tube heat exchangers, pressure equipment devices, design parameters, stress analysis, classification of stresses and stress category concept, weld joint categories and joint efficiency, design by rule and design by analysis, key terms in pressure vessel and heat exchanger design. Detailed discussions on various pressure vessel codes including ASME, CODAP, BS EN 13445, PD 5500, 2014/​68/​EU, AD 2000 and heat exchanger standards such as TEMA, HEI, API, etc. Calculation of minimum thickness of pressure retaining components like shell and end closures, tubesheet design principles, modeling of shell and tubesheet connection, tubesheet design procedure as per ASME Code and TEMA (non-​mandatory). Bolted flange joint (BFJ) design including Flange and Casket Standards, gasket selection, flange design including identification and marking, material selection and bolting design, etc. heat exchanger flanged joints and gaskets, design of expansion joints like flanged and flued type and membrane type, EJMA standard, openings, nozzles, and vessel supports.

CHAPTER 2 MATERIAL SELECTION AND FABRICATION Proper material selection is important for desired thermal performance, strength considerations, safe operation, and achieving the expected life and economy. Thus it is necessary to have a thorough knowledge of various heat exchanger materials, their fabricability, and performance during service. This chapter discusses the material selection guidelines for heat exchangers and pressure vessels. A wide spectrum of metals both ferrous and non ferrous metals and non-​metals, and for every material, its composition, ASTM standards, physical properties, fabrication, forms of corrosion and control, fabrication and welding are discussed. The list of metals selection discussed are carbon steels, low alloys steels, chromium-​molybdenum steels, martensitic stainless steel, austenitic-​, ferritic-​, superferritic-​, duplex-​superaustenitic stainless steels, aluminum and aluminum alloys, copper and copper alloys, nickel and nickel alloys, nickel-​iron-​chromium alloys, titanium, zirconium and tantalum, alloys for high-​temperature applications, alloys for subzero/​cryogenic temperatures, requirements of materials for low-​temperature applications and their fabrication and welding; non

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metals such as graphite, glass, Teflon, ceramics and composites and their engineering properties for heat exchangers and pressure vessels are discussed. Cladding purpose and principle, cladding metals, ASTM standards for clad plates, clad plate manufacture by weld overlay, roll cladding and explosive cladding, processing of clad plates and their forming, welding, post-​weld heat treatment of welded joints are discussed in details.

CHAPTER 3 QUALITY CONTROL AND QUALITY ASSURANCE, INSPECTION, AND NONDESTRUCTIVE TESTING Quality of goods and equipment manufactured in the world market has become a matter of concern in recent years. For heat exchangers and pressure vessels, the overriding goal is to avoid the consequences of failure, which can be catastrophic in human, monetary, and environmental terms. This chapter discusses quality control and quality assurance program, quality system, elements of quality costs, quality gurus and their contribution for quality management, seven quality tools and quality improvements methods like ISO 9001, PDCA Cycle, Total Quality Management, 5S, Lean tools, Kaizen, TPM, Six Sigma, Lean-​Six Sigma, etc. inspection, scope of inspection of heat exchangers, welding quality design, standards for welding, welding procedure qualification, and welding defects. It also discusses NDT principles, standards for NDT, NDT methods and procedures, evaluation of indications and reporting, acceptance criteria, third-​party inspection, etc. It also discusses in detail commonly used NDT methods like visual, dye penetrant, magnetic particle, radiography, ultrasonic, acoustic emission, eddy current testing and other NDT methods, like automated and remote ultrasonic testing, advanced ultrasonic backscatter technique, alternating current field measurement, electro-​magnetic acoustic transducer, guided wave ultrasonics, internal rotary inspection system, phased array UT, time of flight diffraction, magnetic flux leakage, long range ultrasonic testing, phased array ultrasonics inspection, remote field electromagnetic testing, near filed testing, pulsed eddy current, radioscopy, thermal imaging, computer tomography, acoustic pulse reflectometry, replication, heat exchanger tubes inspection methods, leak testing methods for tubes and heat exchangers, including helium mass spectrometer test.

CHAPTER 4 FABRICATION OF THE SHELL AND TUBE HEAT EXCHANGER, BRAZING AND SOLDERING OF COMPACT HEAT EXCHANGER Thermal design followed by heat exchanger unit fabrication is carried out by shop floor operations. Beyond the theoretical background, a knowledge of shop floor practices is required for a manufacturer and to achieve the desired quality and performance. This chapter discusses fabrication requirements, basics of quality assurance program, inspection and test plan, elements of quality control system, stage inspection, and nondestructive examination for manufacture of pressure vessels and heat exchangers. It also discusses detailed methods of fabrication of shell and tube heat exchangers (STHE), brazing of compact heat exchangers, automobile radiators, micro channel heat exchanger and coils, cuprobraze heat exchanger and soldering of radiators. Shop floor practices for STHE fabrication such as identification of materials, fabrication of shell, tube bundle assembly, tube-​to-​tubesheet joints, tube rolling/​expansion methods, tube-​to-​tubesheet joint welding, assembly of channels/​end closures, hydrostatic testing, stamping, preparation of heat exchangers for shipment, Principles and elements of brazing and soldering, quality control and quality assurance system for brazing, brazing methods like torch or flame brazing, dip brazing, furnace brazing, vacuum brazing, post-​braze cleaning, aluminum brazing methods, controlled atmosphere brazing, NOCOLOK® flux brazing, cuprobraze heat exchanger, brazing of heat-​resistant alloys, stainless steel, nickel-​ based alloys, cobalt-​based alloys and reactive metals, post-​braze cleaning, inspection and NDT of brazed and soldered joints, leak testing of brazed joints, etc. corrosion of brazed and soldered joints, corrosion protection methods are discussed.

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The preparation of this book was facilitated by the great volume of existing literature contributed by many scholars in this field and sources in website. I have tried to acknowledge all the sources and have sought the necessary permissions. If omissions have been made, I offer my sincere apologies. Most materials manufacturers, heat exchanger fabricators and research organizations responded to my inquiries and supplied substantial useful data, figures and informative material. They are all acknowledged either directly or through references. This edition is abundantly illustrated with over 400 drawings, diagrams, photos, and tables. Heat Exchanger Design, Volume 2 is an excellent resource for mechanical, chemical, and petrochemical engineers; heat exchanger, process equipment and pressure vessel designers and manufacturers, consultants, industry professionals and upper-​level undergraduate and graduate students in these disciplines.

DISCLAIMER The text of this book is based upon open literature resources like standards, codes, authentic books on heat exchangers and pressure vessels, technical literature from leading heat exchanger and pressure vessels manufacturers, and technical information from many websites, etc. No Indian railway related technical information is adopted in this book.

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Acknowledgments A large number of my colleagues from Indian Railways, well-wishers, and family members have contributed immensely toward the preparation of the book. I mention a few of them here, as follows: Jothimani Gunasekaran, V. R. Ventakaraman, Amitab Chakraborty (ADG), O. P. Agarwal (ED) and M. Vijayakumar (Director), RDSO Lucknow; Member Mechanical and senior Officials of Railway Board, New Delhi, and T. Adikesavan of Southern Railway; K. Narayanan for CAD drawings, Satheeh Kumar S., Sundar Raj A., V. Baskaran, and Er. K. Praveen for their assistance. I have immensely benefited from the contributions of scholars such as Dr. Ramesh K. Shah, Dr. K. P. Singh, and Dr. J. P. Gupta, Ministry of Railways, and the library facilities of IIT- M, IIT- K, IIT- D, and RDSO, Lucknow. A large number of heat exchanger manufacturers and research organizations have spared photos and figures, and their names are acknowledged in the respective figure captions.

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About the Author Kuppan Thulukkanam, Indian Railway Service of Mechanical Engineers (IRSME), Ministry of Railways, retired as Principal Executive Director, CAMTECH, Gwalior, RDSO. He has authored an article in the ASME Journal of Pressure Vessel Technology. His various roles have included being an experienced administrator, staff recruitment board chairman for a zonal railway, and joint director, Engine Development Directorate of RDSO, Lucknow (Min. of Railways). He was also involved in design and performance evaluation of various types of heat exchangers used in diesel electric locomotives and has served as chief workshop engineer for the production of rolling stocks like coaches, diesel and electric multiple units, wagons, electric locomotive, etc. and as Director, Public Grievances (DPG) to the Minister of State for Railways, Railway Board, Government of India. Kuppan received his BE (Hons) in 1980 from the PSG College of Technology, Coimbatore, Madras University, and his MTech in production engineering in 1982 from the Indian Institute of Technology, Madras, India.

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1.1 PRESSURE VESSELS A pressure vessel is a container designed to withstand internal pressure of a stored fluid or external pressure. The term covers a wide variety of vessels including separation vessels, columns, storage vessels, reactors, and heat exchangers. Because of the risk associated with the accidental failure of pressure vessels, many countries have come up with regulations known as pressure vessel code to govern pressure vessel design and production. Pressure vessels are produced and used in a wide variety of geometrical shapes, capacities, and sizes for use in a large number of applications. Pressure vessels for internal containment are the most common and are designed to store liquid, gas, or vapor at pressures greater than 15 psi. Pressure vessels are either fired, like boilers, or unfired, such as storage tanks, processing vessels and heat exchangers. The pressure vessels are typically constructed in accordance with ASME BPVC Section VIII or other recognized international pressure vessel codes, or as approved by the jurisdiction. These codes typically limit design basis to an external or internal operating pressure no less than 15 psig (103 kPa). However, vessels can also operate at lower pressures. External pressure on a vessel can be caused by an internal vacuum or by fluid pressure between an outer jacket and the vessel wall. Boiler, steam generator, columns, towers, drums, reactors, heat exchangers, condensers, air-​cooled heat exchanger, feedwater heater, bullets, and accumulators are common types of industry pressure vessels. A few types of PV are shown in Figure 1.1. Pressure vessels are either fired, like boilers, or unfired such as storage tanks, processing vessels, and heat exchangers.

1.1.1 Types of Pressure Vessels There are many kinds of pressure vessels, with the three most common being storage vessels, heat exchangers, and process vessels. The shape and size of a pressure vessel are determined by the design requirements, product being stored, the amount of space at the job site, and a company’s budget. The two most common pressure vessel shapes are cylindrical and spherical. Based on installed orientation, they are either horizontal or vertical and based on heat content as fired and unfired pressure vessels [1a, 1b].

1.1.2 Fired and Unfired Pressure Vessel There are two forms of pressure vessels: fired and unfired. A fired pressure vessel is partially or totally open to burners and combustion gases. On the other hand, unfired pressure vessels can act like heat exchangers, used to cool and heat fluid when combined with another fluid. A fired pressure vessel is used to hold gases or liquids usually at a high pressure of 15 psig or more. They can be DOI: 10.1201/9781003352051-1

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FIGURE 1.1  Examples of pressure vessels –​ (a) cylindrical PV, (b) leg support PV, (c) lug/ring support PV, (d) spherical PV, (e) skirt supported PV, (f) cryogenic PV, (g) industrial boiler, (h) schematic of natural circulation drum boiler, and (i) shell and tube heat exchanger.

used as a direct or indirect heat source in order to maintain a gas or liquid at a high pressure. A fired pressure vessel is subjected to a direct or indirect heat source (often coal, oil or gas-​fired boilers). Due to this, they are at a higher risk of overheating than unfired pressure vessels. 1.1.2.1 Unfired Pressure Vessels These are vessels designed to contain fluids under internal pressure or vacuum and not heated directly through the combustion of fuels or other external heat sources. They are found in commercial and industrial facilities. Heat can be generated from chemical reactions within vessel or by applying a heating medium within the vessel or circulating it around the vessel (jacket). Examples include

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FIGURE 1.1  (Continued)

heat exchangers, compressed air tanks, propane tanks, LPG tank, liquid oxygen storage tanks, de-​ aerators and condensate tanks, steam-​jacketed kettles, etc. 1.1.2.2 Heat Exchangers A heat exchanger is an unfired pressure vessel. It consists of heat transfer elements such as a core consisting tubes or plates or solid matrix containing the heat transfer surface, and fluid distribution

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FIGURE 1.1  (Continued)

elements such as headers or tanks, inlet and outlet nozzles or pipes, etc. Usually, there are no moving parts in the heat exchanger; however, there are exceptions, such as a rotary regenerator in which the matrix is driven to rotate at some design speed or fixed matrix with moving hoods which convey the fluids into the heat transfer matrix and a scraped surface heat exchanger in which a rotary element

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FIGURE 1.2  Cross sectional view of a STHE.

with scraper blades continuously rotates inside the heat transfer tube. This chapter discusses design of shell and tube heat exchanger (STHE). Figure 1.2 shows cross sectional view of a STHE. 1.1.2.3 Hazards Due to Failures of Pressure Vessels Both fired and unfired pressure vessels can present hazards to employee safety and facility operability, yet fired pressure vessels in particular are at greater risk of overheating. Due to this, the safe design, installation, and maintenance of pressure vessels is critical, and compliance with codes and standards is necessary to ensure employee safety and prevent damage to your facility [2].

1.2 MECHANICAL DESIGN OF PRESSURE VESSELS AND HEAT EXCHANGERS Mechanical design involves the design of pressure-​retaining and non-​pressure-​retaining components and equipments to withstand the design loads and the deterioration in service so that the equipment will function satisfactorily and reliably throughout its codal life. Mechanical design is done as per the procedure given in the construction codes and standards. Where no guidance is provided by the codes and standards, the procedure may be arrived at by mutual agreement between the purchaser and the fabricator.

1.2.1 Standards and Codes Standards and codes were established primarily to ensure safety against failure. The need for safety standards is obvious in a world growing increasingly aware of the hazards posed to people, property, and the environment due to failures of pressure vessels and heat exchangers in any industrial plant [3]. Failures may occur due to design inadequacies, use of inferior materials for construction, poor workmanship in fabrication and welding, and inadequate quality control checks. Hence, it is essential that due consideration is given at all stages of design, manufacturing, and installation. The codes and standards give guidance and in some cases govern the design, manufacture, construction, operation, and maintenance of heat exchangers and pressure vessels. The codes and standards are published periodically by issuing organizations or associations. 1.2.1.1 Standards A standard can be defined as a set of technical definitions and guidelines, or how-​to instructions for designers and manufacturers [4]. A standard is developed by the consensus process, involves

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technical experts from the producers and the users. Standards are mostly voluntary in nature. They serve as guidelines but do not themselves have a force of law. The standards can be of the following major four types: 1. company standards 2. trade or manufacturer’s association standards 3. national standards 4. international standards. 1.2.1.2 Trade or Manufacturer’s Association Standards Trade or manufacturer’s association standards are the rules and the recommendations of various manufacturers of common interest, developed based on experience in design, manufacture, installation, and operation. While making the standards, feedback from users is normally included. Manufacturer’s association standards that are most prominent among heat exchanger manufacturers are Tubular Exchanger Manufacturers Association (TEMA) [5], Heat Exchange Institute (HEI) [6–​8], and API Standards [9]. There are also Expansion Joint Manufacturers Association (EJMA) Standards for membrane type expansion joint [10] for the design of membrane-​type expansion joints and American National Standards Institute (ANSI) standards for design of fittings, flanges, valves, piping, and piping components. 1.2.1.3 National Standards National standards are followed in the country where the standard has been issued by subcontractors or license holders in other countries or complied with when the purchasers have so specified. A few national standards are shown in Table 1.1. 1.2.1.4 Benefits of Standardization For producers, standards rationalize the products and manufacturing process, reduce inventories of both raw material and finished products, and reduce the cost of manufacture. For customers, standards assure the quality of goods purchased and services received, provide better value for money, and are convenient for settling disputes, if any, with suppliers.

TABLE 1.1 National Standards on Pressure Vessels

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Abbreviation

Organization

Country

ANSI SCC AFNOR BSI CEN DIN JISC BIS ISO

American National Standards Institute Standards Council of Canada Association Francaise British Standards Institute Committee of Euripean Normalization Deutsches Institute for Normung Japanese Industrial Standards Committee Bureau of Indian Standards International Organizations for Standards

United States Canada France United Kingdom Europe Germany Japan India Worldwide

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1.2.2 Design Standards Used for the Mechanical Design of Heat Exchangers Some design standards used for the mechanical design of heat exchangers include the following: TEMA, HEI, and API. TEMA standards founded in 1939, the Tubular Exchanger Manufacturers Association, Inc. is a group of leading manufacturers of shell and tube heat exchangers who have pioneered the research and development of heat exchangers for over 60 years. TEMA standards are followed in most countries of the world for the design of shell and tube heat exchangers. 1.2.2.1 TEMA Standards Scope and General Requirements (Section 5, RCB-​1.1.1) TEMA standards [5] are followed in most countries of the world for the design of shell and tube heat exchangers. TEMA recognizes three classes of mechanical standards, R, C and B, reflecting the acceptable designs for various service applications and their definitions are given below: Definition of TEMA class “R” exchangers The TEMA Mechanical Standards for Class “R” heat exchangers specify design and fabrication of unfired shell and tube heat exchangers for the generally severe requirements of petroleum and related processing applications. Definition of TEMA class “C” exchangers The TEMA Mechanical Standards for Class “C” heat exchangers specify design and fabrication of unfired shell and tube heat exchangers for the generally moderate requirements of commercial and general process applications. Definition of TEMA class “B” exchangers The TEMA Mechanical Standards for Class “B” heat exchangers specify design and fabrication of unfired shell and tube heat exchangers for chemical process service. Each section is identified by an uppercase letter symbol, which precedes the paragraph numbers of the section and identifies the subject matter. Also, TEMA classes R, C, and B have been combined into one section titled class RCB. The scope of TEMA Standards is shown in Table 1.2. 1.2.2.2 Shell and Tube Heat Exchanger A shell and tube heat exchanger (STHE) is a class of heat exchanger designs. It is the most common type of heat exchanger in oil refineries and other large chemical processes, and is suited for higher-​ pressure applications. The shell and tube is very adaptable and flexible, thus it can be used for nearly all applications. As its name implies, this type of heat exchanger consists of a shell (a large pressure vessel) with a bundle of tubes inside it. One fluid runs through the tubes, and another fluid flows over the tubes (through the shell) to transfer heat between the two fluids. The set of tubes is called a tube bundle. Typically, the ends of each tube are terminated through holes in a tubesheet. The tubes are mechanically rolled or welded into tube sheet face. Tubes may be straight or bent in the shape of a U, called U-​tubes. In process industries, shell and tube heat exchangers are used in great numbers, far more than any other type of exchanger. STHEs are the “workhorses” of industrial process heat transfer. They are the first choice because of well-​established procedures for design and manufacture from a wide variety of materials, many years of satisfactory service, and availability of codes and standards for design and fabrication. They are produced in the widest variety of sizes and styles. 1.2.2.3 Scope of TEMA Standards The TEMA mechanical standards are applicable to unfired shell and tube heat exchangers with inside diameters not exceeding 100 in. (2540 mm), a maximum product of nominal diameter (in.) and design pressure (psi) of 100,000 psi (20,684 kPa), or a maximum design pressure of 3,000 psi.

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TABLE 1.2 Scope of TEMA Standards Parameter

Limit

Inside diameter Nominal diameter × pressure Design Pressure Shell wall thickness Stud diameters (approx.) Construction code Pressure source

100 in. (2540 mm) 100, 000 psi (17.5×106 kPA) 3,000 psi(20,684 kPa) 3 in. (76 mm) 4 in. (102 mm) ASME Sec VIII, Div. 1 Indirect (unfired units only)

TEMA Standards Contents Section

Symbol

Paragraph

1 2 3 4 5

N F G E RCB

6 7 8 9 10 Appendix (Non-​Mandatory)

V T P D RGP A

Nomenclature Fabrication tolerances General fabrication and performance information Installation, operation, and maintenance Mechanical standards TEMA class RCB heat exchangers Flow-​induced vibration Thermal relations Physical properties of fluids General information Recommended good practice Tubesheets

The intent of these parameters is to limit the maximum shell wall thickness to approximately 3 in. (76 mm) and the maximum stud diameter to approximately 4 in. (102 mm). Section 5 has mechanical standards that apply to three classes of heat exchangers R, C, and B. The contents of TEMA Standards are given in Table 1.2. 1.2.2.4 Differences Among TEMA Classes R, C, and B Differences among TEMA classes R, C, and B have been summarized and are listed in Chapter 4 of “Heat Exchangers: Classification, Selection, and Thermal Design”. 1.2.2.5 Other Standards for STHE 1. Heat Exchange Institute Standards The HEI, Cleveland, Ohio, is an association of manufacturers of heat transfer equipment used in power generation. The association promotes improved designs by developing equipment design standards. It publishes standards for tubular heat exchangers used in power generation. Such exchangers include surface condensers, feedwater heaters, and other power plant heat exchangers [6–​8]. Among these standards are: 1. Standards for Direct Contact Barometric and Low Level Condensers, 9th Edition, 2014. 2. Standards for Shell and Tube Heat Exchangers, 5th Edition, 2013. 3. Standards for Steam Surface Condensers, 12th Edition, 2017.

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4. Standards for Steam Jet Vacuum Systems, 7th Edition, 2017. 5. Standards and Typical Specifications for Tray Type De-​aerators, 10th Edition, 2016. 6. Standards for Closed Feedwater Heaters, 9th Edition, 2015. 7. Standards for Air-​Cooled Condensers, 2nd Edition, 2016. 8. ASME Standards: a. Steam Surface Condenser, ASME PTC 12.2-​2010(2020). This performance test code establishes equipment performance metrics with the philosophy of promoting testing. b. Closed Feedwater Heaters, ASME PTC 12.1-​2020. This code applies to all horizontal and vertical heaters except those with partial pass full-​length drain cooling zones. The heater design is based on a specific operating condition that includes flow, temperature, and pressure. This specific condition constitutes the design point that is found on the manufacturer’s feedwater heater specification sheet. 3. API 660-​2015, Shell and Tube Heat Exchangers This standard specifies requirements and gives recommendations for the mechanical design, material selection, fabrication, inspection, testing, and preparation for shipment of shell and tube heat exchangers for the petroleum, petrochemical, and natural gas industries. This standard is applicable to the following types of shell and tube heat exchangers: heaters, condensers, coolers, and reboilers. This standard is not applicable to vacuum operated steam surface condensers and feed-​water heaters [9]. 4. ISO 16812-​2019 Petroleum, petrochemical and Natural Gas Industries –​Shell and Tube Heat Exchangers 5. PIP VESSM001-​2017 Specification for small pressure vessels and heat exchangers with limited design conditions 6. Codes Many pressure vessel codes including ASME Boiler and Pressure Vessel Code Sec VIII Div. 1 discuss STHE design.

1.2.3 Codes A code is a system of regulations or a systematic book of law often given statutory force by state or legislative bodies [11]. Codes provide rules/​specific design criteria: (1) in respect of permissible materials of construction, allowable working stresses, and load sets that must be considered in design; (2) to determine the minimum wall thickness and structural behavior due to the effects of internal pressure, thermal expansion, dead weight, live loads, or other imposed internal or external loads; (3) for design requirement for components such as valves, flanges, standard fittings, and non-​standard fittings; and (4) for reinforcement of the openings in a pressure vessel. Among the codes, the ASME Code [12–​17] for the construction of boilers and pressure vessels including heat exchangers is the most widely used and is referred to code in the world today. Apart from ASME Code, many other codes are issued by various countries such as CODAP, PED European Pressure Equipment Directive, BS EN 13445, PD 5500. AD Merkblatter, etc. Codes followed by few other countries are shown in Table 1.3. 1.2.3.1 Structure of the Codes The general structures of all codes are similar: they all include chapters respectively devoted to general provisions, materials and procurement, design, construction examination and inspection, etc.

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TABLE 1.3 International Design Codes for the Mechanical Design of Pressure Vessels Code Name

Country

ASME BPVC BS EN 13445:2021 PED 2014/​68/​EU GB 150 EN 13445 CODAP AD2000 PD 5500 IS 2825-​1969(2022) JIS B 8265-​2017 GOST R 52857 AS1210-​2010 CSA B51-​2019

USA Europe European Union China Europe France Germany Great Britain India Japan Russia Australia Canada

1.2.3.2 Introduction to some International Codes for Unfired Pressure Vessels 1. BS EN 13445 The BS EN 13445 series comprises all parts of the European pressure vessels standards. 2. PD 5500:2021. Unfired Fusion Welded Pressure Vessels 3. 2014/​68/​EU–​Pressure Equipment Directive The Pressure Equipment Directive (PED) applies to the design, manufacture and conformity assessment of stationary pressure equipment with a maximum allowable pressure greater than 0.5 bar. The directive entered into force on 20 July 2016. The Pressure Equipment Directive aims to guarantee free movement of the products in its scope while ensuring a high level of safety. 4. AD 2000 The German AD 2000 (AD stands for “Arbeitsgemeinschaft Druckbehalter”) is a code of practice for pressure vessels and other pressure equipment. It was drawn up by the German Pressure Vessel Association which includes a large number of German associations and institutions specialized in boilers and pressure vessels. 5. CODAP CODAP is the French Code for Construction of Unfired Pressure Vessels. The Division 1 is essentially intended for the construction of the most common vessels to be manufactured from common materials. The Division 2 intended for the construction of more complex vessels: 1. CODAP Division 1: 2015(Rev. 2018) –​Code for Construction of Unfired Pressure Vessels 2. CODAP® Division 2: 2020 Division 2 specifies the criteria for fabrication of more complex unfired pressure vessels (including many innovations)

1.2.4 ASME Codes 1.2.4.1 What is the ASME Boiler and Pressure Vessel Code (BPVC)? ASME Code establishes minimum rules of safety governing the design, fabrication, inspection, and testing of boilers, pressure vessels, and nuclear power plant components. It covers new construction and rerating the existing equipment. The existence of the code stamp on a pressure vessel, with

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the indicated pressure and temperature, establishes the design conditions, new and old. The service conditions such as corrosion, erosion, change in operating pressure, and/​or temperature may be the reasons to rerate the unit, but the original stamping remains valid. Supplemental stamping is a requirement to address rerating [18]. The main objective of ASME Code rules is to establish the minimum requirements that are necessary for safe construction and operation. The ASME Code protects the public by defining the material, design, fabrication, inspection, and testing requirements that are needed to achieve a safe design. 1.2.4.2 ASME Code: Historical Background The steady increase in boiler explosions in the 40 years from 1870 to 1910 excited public feeling to make rules and regulations for safe operation of steam boilers. With the publication of the ASME Code for Construction of Boiler and Pressure Vessels in 1914, serious boiler explosions steadily decreased despite the fact that the number of boilers in use has increased enormously. Primarily as a result of the ASME Boiler Code, boiler explosions and the consequent loss of life and damage to property are a rarity today [19]. To become familiar with the important aspects of ASME Codes, refer to Refs. [3, 20], Nichols [21] and Refs. [22–​26]. Readers are advised to refer to the latest codes and standards to know the state of the art. Unless otherwise mentioned, the mention of ASME Code throughout this book refers to ASME Code Section VIII, Div. 1, only. 1.2.4.3 ASME Codes The various sections are as follows: ASME BPVC Sections 1. Rules for construction of power boilers. 2. Material specifications: Part A –​ferrous materials Part B –​nonferrous materials Part C –​welding rods, electrodes, and filler metals Part D –​material properties(customary/​metric). 3. Rules for construction of nuclear facility components –​Division 1-​appendices. Subsection NCA –​general requirements for Division 1 and Division 2. • Appendices • Division 1 – Subsection NB –​Class 1components – Subsection NCD –​Class 2 and Class 3 components – Subsection NE –​Class MC components – Subsection NF –​Supports – Subsection NG –​Core support structures. • Division 2 –​Code for concrete containments • Division 3 –​Containment systems for transportation and storage of spent nuclear fuel and high-​level radioactive material • Division 5 –​High temperature reactors. 4. Rules for construction of heating boilers. 5. Nondestructive examination. 6. Recommended rules for care and operation of heating boilers. 7. Recommended rules for care of power boilers. 8. Pressure vessels, Division 1. Pressure vessels, Division 2 alternative rules. Pressure vessels, Division 3 alternative rules for construction of high pressure vessels.

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9. Welding, brazing, and fusing qualifications. 10. Fiberglass-​reinforced plastic pressure vessels. 11. Rules for in-​service inspection of nuclear power plant components: Division 1, rules for inspection and testing of components of light-​water-​cooled plants Division 2, requirements for reliability and integrity management (RIM) programs for nuclear power plants. 12. Rules for construction and continued service of transport tanks. 13. Rules for overpressure protection. Addenda Interpretations Code cases Boilers and pressure vessels Nuclear components Address trade inquiries to the following address: ANSI/​ASME –​Boiler and pressure vessel codes American National Standards Institute 11 West 42nd Street New York, NY 10036 Sections relevant for the fabrication of heat exchangers other than nuclear power plant units 3 are Section II, Section V, Section VIII, and Section IX. The ASME Code does not dictate what section of the code to use. The law or regulatory body at the point of installation determines what section to use. Various sections of ASME Codes are shown in Figure 1.3. 1.2.4.4 Scope of the ASME Code Section VIII Pressure Vessels Pressure vessels are typically designed in accordance with the ASME Code Section VIII. Section VIII is divided into three divisions: Division 1, Division 2, and Division 3. Division 1 is used most often since it contains sufficient requirements for the majority of pressure vessel applications.

FIGURE 1.3  ASME BPV Code Sections.

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13

Division 1 provides requirements applicable to the design, fabrication, inspection, testing, and certification of pressure vessels operating at either internal or external pressures exceeding 15 psig. Such vessels may be fired or unfired. This pressure may be obtained from an external source or by the application of heat from a direct or indirect source, or any combination thereof. Specific requirements apply to several classes of material used in pressure vessel construction, and also to fabrication methods such as welding, forging, and brazing. 1.2.4.5 Structure of Section VIII This Division is divided into three Subsections, Mandatory Appendices, and Non-​ mandatory Appendices. The contents of the above three subsections of Sec VIII Div. 1 of the code are shown in Figure 1.4. Division 1 also contains the following appendices: Mandatory Appendices address subjects that are not covered elsewhere in the code. The requirements that are contained in these appendices are mandatory when the subject that is covered is included in the pressure vessel under consideration. Non-​mandatory Appendices provide information and suggested good practices. The use of these non-​mandatory appendices is not required unless their use is specified in the vessel purchase order. Division 2 (ASME BPVC Sec VIII Div. 2-​2021) This Division contains mandatory requirements, specific prohibitions, and non-​mandatory guidance for the design, materials, fabrication, examination, inspection, testing, overpressure protection, and certification of pressure vessels. Division 3 The rules of this Division constitute requirements for the design, construction, inspection, and overpressure protection of metallic pressure vessels with design pressures generally above 10 ksi (70 MPa). However, it is not the intent of this Division to establish maximum pressure limits for either Section VIII, Division 1 or 2, nor minimum pressure limits for this Division. Division 3 requirements are applicable to pressure vessels operating at either internal or external pressures generally above 10,000 psi. It does not establish maximum pressure limits for either Section VIII,

FIGURE 1.4  Various sub sections of ASME BPV Code Section VIII Div. 1.

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TABLE 1.4 Comparison of Design Criteria of ASME Code Section VIII Div. 1 vs Div. 2 Parameters

Div. 1

Div. 2

Code pressure limit Design criteria Failure theory Effects of combined stress

Up to 3000psi “design-​by-​rules” normal stress theory Does not explicitly consider the effects of combined stress and does not give detailed methods on how stresses are combined considers a biaxial state of stress combined in accordance with the maximum principal stress theory 3.5 Not mandatory

No limits; usually more than 600 psig “design-​by-​analysis” maximum distortion energy (Von Mises criteria) Provides specific guidelines on classes of stresses, stresses categories, and how they are combined

Stress analysis

Safety factor Fatigue Evaluation Professional Engineer Certification Hydrostatic Test Pressure

Normally not required

1.3 times design pressure

considers all stresses in a triaxial state combined in accordance with the maximum shear stress theory 3 Criteria for determining when a vessel must be analyzed for fatigue are specified Professional Engineers’ Certification of User’s Design Specifications as well as Manufacturer’s Design Report 1.25 times design pressure

Note:  Refer to ASME Code Sec VIII Div. 1 and Div. 2.

Divisions 1 or 2, nor minimum pressure limits for this Division. Rules pertaining to the use of the ASME Certification Mark with the U3, UV3 and UD3 designator are also included. This fatigue analysis is mandatory for Division 3 vessels. Division is divided into eight parts. 1.2.4.6 Comparison of ASME Code Section VIII Div. 1 versus Div. 2 Design pressure: when the design pressure exceeds 3000 psi (210 Kg/​cm2); design according to Div.2 is required. Additionally, Div. 1 cannot be used for pressures below 15 psi (1.054 kg/​ cm2) and the main differences are: allowable stresses, stress calculations, cyclic service design, general design criteria, inspection and quality control, inspection, NDT and fabrication. Salient features and differences among code rules between Div. 1 and Div. 2 are discussed in Ref. [27–​29]. Main differences between Div. 1 and Div. 2 are hereunder [29] and also shown in Table1.4 1.2.4.7 Section X Fiber-​Reinforced Plastic Pressure Vessels Provides requirements for construction of a fiber-​reinforced plastic pressure vessel (FRP) in conformance with a manufacturer’s design report. It includes production, processing, fabrication, inspection and testing methods required for the vessel. 1.2.4.8 Section XIII, Rules for Overpressure Protection This Section provides rules for the overpressure protection of pressurized equipment such as boilers, pressure vessels, and piping systems. This standard provides requirements for topics such as design, material, inspection, assembly, testing, and marking for pressure relief valves, rupture disk devices, pin devices, spring-​actuated non-​reclosing devices, and temperature and pressure relief valves.

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1.2.4.9 Code Interpretations 1. Requests for code interpretations should be accompanied by the following information: a. Inquiry. The Inquirer should propose a condensed and precise Inquiry, omitting superfluous background information and, when possible, composing the Inquiry in such a way that a “yes” or a “no” Reply, with brief limitations or conditions, if needed, can be provided by the Committee. b. Reply. The Inquirer should propose a Reply that clearly and concisely answers the proposed Inquiry question. c. Background Information. 2. Requests for code interpretations should be limited to an interpretation of a particular requirement in the code or in a code case. 1.2.4.10 Submittals Submittal. Requests for code interpretation should preferably be submitted through the online Interpretation Submittal Form. Interpretation Request. Upon submittal of the form, the Inquirer will receive an automatic e-​mail confirming receipt. If the Inquirer is unable to use the online form, the Inquirer may mail the request to the following address: Secretary ASME Boiler and Pressure Vessel Committee Two Park Avenue New York, NY.

1.3 STRESS ANALYSIS Stress analysis is the determination of the relationship between external forces applied to a vessel and the corresponding stress. The stress analysis of heat exchangers and pressure vessels is similar to other structural members in that it involves mathematical operations with unknown forces and displacements. In the evaluation of the stress field in heat exchangers and pressure vessels, the problem is considerably simplified due to these reasons [30]: (1) The pressure-​retaining components such as shell, heads, and cones are surfaces of revolution; (2) pressure loading –​the primary mechanical loading is spatially uniform; (3) the thickness of a pressure vessel is small compared to its characteristic dimensions; and (4) with little accuracy loss, we can assume that the meridian, tangential, and through-​thickness directions are principal directions.

1.3.1 Classes and Categories of Stresses Classes of stress, categories of stress, and allowable stresses as permitted by codes are based on the type of loading that produced them and on the hazard they cause to the structure.

1.3.2 Stress Categories The combined stresses due to a combination of loads acting simultaneously are called stress categories.

1.3.3 Stress Classification The stresses that are present in pressure vessels are separated into various classes in accordance with the types of loads that produced them and the hazard they pose to the vessel. The reason for

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classifying stresses into various groups is that not all types of stresses require the same safety factors in protection against failure. Limit analysis theory indicates that some stresses may be permitted to a higher level than other stresses. Before discussing stress classification, membrane stress and primary stress are defined.

1.3.4 Membrane Stress When the thickness is small in comparison with other dimensions (Rm/​t > 10), vessels are referred to as membranes, and the resulting stresses due to contained pressure are called membrane stresses [31]. The membrane is assumed to offer no resistance to bending. When the wall offers resistance to bending, bending stresses occur in addition to membrane stresses.

1.3.5 Primary Stress Primary stress is a normal stress or a shear stress developed by the imposed loading that is necessary to satisfy the laws of equilibrium. The basic characteristic of a primary stress is that it is not self-​limiting. Primary stresses that exceed the yield strength will result in plastic deformation, gross distortion, or failure. Thermal stress is not classified as a primary stress. It is classified as a secondary stress only. Classes of stress and categories of stress are dealt in detail by Refs. [30–​32], among others.

1.4 STRESS CATEGORIES The object of the elastic analysis is to ensure that the vessel has adequate margins of safety against three failure modes: gross plastic deformation, ratcheting, and fatigue. This is done by defining three classes or categories of stress, which have different significance when the failure modes are considered. These three stress categories are assigned different maximum allowable stress values in the code: the designer is required to decompose the elastic stress field into these three categories and apply the appropriate stress limits. The total elastic stress which occurs in the vessel shell is considered to be composed of three different types of stress primary, secondary, and peak. In addition, primary stress has three specific sub-​categories. The ASME stress categories and the symbols used to denote them in the code are given below: 1. Primary Stress General Primary Membrane Stress, Pm Local Primary Membrane Stress, PL Primary Bending Stress, Pb 2. Secondary Stress, Q 3. Peak Stress, F.

1.4.1 Primary Membrane Stress, Pm The component of primary stress that is obtained by averaging the stress distribution across the thickness of the pressure vessel is referred to as the primary membrane stress. It is the most significant stress class. An important characteristic of the primary membrane stress is that beyond the yield point, redistribution of stresses in the structure does not take place. It is remote from discontinuities such as head –​shell intersections, nozzles, and supports. Design codes limit its value to the allowable stress for the component material. Examples for primary membrane stresses are as follows:

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1. circumferential (hoop) and longitudinal (meridian) stresses due to internal or external pressures 2. stress due to vessel weight 3. longitudinal stress due to the bending of the horizontal vessel over the supports 4. membrane stress in the nozzle wall within the area of reinforcement due to pressure or external loads 5. stress caused by wind and seismic forces.

1.4.2 Primary Bending Stress, Pb In contrast to a cylindrical shell, certain structural shapes cannot resist external loadings without bending, and the resulting stress is known as primary bending stress. Primary bending stress is capable of causing permanent distortion or collapse of the vessel. Some examples of primary bending stress are the following: 1. bending stress due to pressure in a flat cover 2. bending stress in the crown of a torispherical head due to internal pressure 3. bending stress in the ligaments of closely spaced openings, such as bending stress in the tubesheet averaged across the ligament. Primary general stresses are divided into primary membrane and primary bending stresses, and the reason for such a division is that the calculated value of a primary bending stress may be allowed to go higher than that of a primary membrane stress.

1.4.3 Local Membrane Stress, PL Local (primary) membrane stress is produced either by pressure load alone or by other mechanical loads. It has some self-​limiting characteristics. Since the loads are localized, once the yield strength of the material is reached, the load is redistributed to stiffer portions of the vessel. Typical examples for local primary membrane stress are stresses at supports and stresses due to internal pressure at structural discontinuities [32].

1.4.4 Secondary Stress Secondary stress (Q) is a normal or shear stress arising because of the constraint of adjacent material or by self-​constraint of the structure. These stresses arise solely to satisfy compatibility conditions and are not required to satisfy laws of equilibrium. They are self-​limiting in nature. Local yielding can relieve the conditions that lead to the development of these stresses and limit their maximum value. Failure from secondary stress is not to be expected. The concept of primary and secondary stresses is not relevant for brittle materials. Two sources of secondary stresses are (1) temperature and (2) gross structural discontinuity. Secondary stresses can be subdivided into two major categories: (1) load-​actuated secondary stresses and (2) temperature-​actuated secondary stresses. Examples for these classes are given next. Some examples of load-​actuated secondary stresses are the following: 1. bending stress in a shell where it is connected to a head or to a flange 2. bending stress in a shell or a head due to nozzle loads 3. bending stress in the knuckle at a head-​to-​shell joint.

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Some examples of temperature-​actuated secondary stresses are the following: 1. stresses caused by axial temperature variation in a shell 2. both membrane and bending stresses due to differential thermal expansion between two adjoining parts of a structure such as nozzle to shell or shell to head. Examples of secondary bending stress, Qb, are the following: 1. bending stress at a gross structural discontinuity due to relenting loads only, such as nozzles and lugs 2. the non-​uniform portion of the stress distribution in a thick-​walled vessel due to internal pressure.

1.4.5 Thermal Stresses Thermal stress is induced in a body when the temperature of the body is raised or lowered and the body is not allowed to expand or contract freely. Temperature gradients, thermal expansion or contraction and thermal shocks are things that can lead to thermal stress.

1.4.6 Peak Stress, F Peak stresses are the additional stresses due to stress concentration in highly localized areas. They are caused by mechanical and thermal loads, and they apply to both limiting and self-​limiting loads. Peak stresses are added to the primary and secondary stresses to give the total stress at a point. A peak stress does not cause any noticeable distortion. The determination of peak stress is necessary only for fatigue analysis or a source of stress corrosion cracking, or it can be a possible source of brittle fracture [32]. Peak stress applies to membrane, bending, and shear stresses. Examples for peak stresses due to thermal and mechanical loads are given next. Some examples of peak stresses due to thermal loads are as follows: 1. thermal stress in the cladding or weld overlay of a tubesheet, shell, or vessel head 2. thermal stresses in a wall caused by a sudden change in the surface temperature (thermal shock). Some examples of load-​actuated peak stresses for specific situations are as follows: 1. peak stress in a ligament (uniform ligament pattern) 2. stress at a local structural discontinuity 3. stress at corner of a discontinuity 4. stress due to notch effect or stress concentration or small radius fillet, hole, or incomplete penetration [32] 5. additional stresses developed at the fillet at a nozzle-​to-​shell junction due to internal pressure or external loads.

1.4.7 Failure Modes, Stress Limits, and Stress Categories One of the most important aspects of a stress analysis is to make sure that various stresses are assigned to the proper stress categories. Limits on primary stress are set to prevent plastic deformation and burst. Secondary stress limits are set to prevent excessive plastic deformation which may lead to incremental collapse and to ensure the validity of the use of an elastic analysis for making a fatigue analysis. Peak stress limits are set to prevent fatigue failure due to excessive cyclic loadings.

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1.4.8 Discontinuity Stresses Pressure vessel components and sections usually contain regions of different thickness, material, diameter, and abrupt changes in geometry. The juncture at these locations is known as “discontinuity” areas. Examples include the skirt junction with the shell/​vessel and head and shell. The stresses induced in the respective parts at or near discontinuity areas are called discontinuity stresses. Discontinuity stresses are necessary to satisfy compatibility conditions at discontinuity regions. They are not serious under static loads such as internal pressure with ductile materials if they are kept within limits by the design, but they are important under cyclic loads [32].

1.4.9 Stress Intensity Pressure vessels are subject to multiaxial stress states, such that yield is not governed by the individual components of stress but by some combination of all stress components. Most Design by Rules make use of the Tresca criterion but in the DBA approach a more accurate representation of multiaxial yield is required. The theories most commonly used to relate multiaxial stress to uniaxial yield data are the von Mises criterion and the Tresca criterion. For simplicity consider general three dimensional stress described by its principal stress compoents, σ1 , σ 2 , and σ3 and define the principal shear stresses by [33]:



τ1 =

1 σ − σ3 2 2

(

)

or τ1 =

1 1 (σ3 − σ1 ) or τ1 = σ1 − σ 2 2 2

(

)

According to the Tresca criterion, yielding occurs when

τ = max ( τ1 , τ 2 , τ 3 ) =

1 σ 2 y

where σ y is the uniaxial yield stress obtianed from tensile tests.

1.4.10 Fatigue Analysis When a vessel is subjected to repeated loading that could cause failure by the development of a progressive fracture, the vessel is said to be in cyclic service.

1.5 DESIGN METHODS AND DESIGN CRITERIA There are two basic design methods used by the codes for the design of heat exchangers and pressure vessels; these are termed, “design-​by-​rule” and “design-​by-​analysis”. [33, 34]

1.5.1 Design Loads The forces that influence pressure vessel design are internal/​external pressure; dead loads due to the weight of the vessel and contents; external loads from piping and attachments, wind, and earthquakes; operating-​type loads such as vibration and sloshing of the contents; and startup and shutdown loads. The code considers design pressure, design temperature, and, to some extent, the influence of other loads that impact the circumferential (or hoop) and longitudinal stresses in shells. It is left to the designer to account for the effect of the remaining loads on the vessel. Various national and local building codes must be consulted for handling wind and earthquake loading [34].

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1.5.2 Design Criteria The code design criteria consist of basic rules specifying the design method, design load, allowable stress, acceptable material, and fabrication inspection certification requirements for vessel construction.

1.5.3 Design by Rule (DBR) The design formulas used in the “design by rule” method are based on the principal stress theory and calculate the average hoop stress. The principal stress theory of failure states that failure occurs when one of the three principal stresses reaches the yield strength of the material. Assuming that the radial stress is negligible, the other two principal stresses can be determined by simple formulas based on engineering mechanics. DBR uses design pressure, allowable stress, and a design formula compatible with the geometry of the part to calculate the minimum required thickness of the part. This procedure minimizes the amount of analysis required to ensure that the vessel will not rupture or undergo excessive distortion.

1.5.4 Design by Analysis The design by analysis (DBA) procedure is intended to guard against eight possible pressure vessel failure modes by performing a detailed stress analysis of the vessel. The failure modes considered are [33]: 1. excessive elastic deformation including elastic instability 2. excessive plastic deformation 3. brittle fracture 4. stress rupture/​creep deformation (inelastic) 5. plastic instability –​incremental collapse 6. high strain –​low cycle fatigue 7. stress corrosion 8. corrosion fatigue.

1.5.5 Stress Categorization An important part of the DBA methodology is the categorization (also termed classification) of stresses in a pressure vessel, so that they have proper relevance to the various failure modes that are considered. Figure 1.5 shows the DBA stress categories [33]. Pressure vessel design has been historically based on Design by Formula. Standard vessel configurations are sized using a series of simple formulae and charts. In addition to the Design by Formula method, many national codes and standards for pressure vessel and boiler design do provide for a Design by Analysis (DBA) method, where the admissibility of a design is checked, or proven, via a detailed investigation of the structure’s behavior under the external loads (or ‘actions’) to be considered. Nevertheless ‘Design by Formula’ remains the dominant approach.

1.5.6 ASME Code Section VIII Design Criteria The ASME boiler and pressure vessel code (BPVC) establishes rules of safety governing the design, fabrication and inspection during construction of boilers, pressure vessels, and nuclear power plant components. The objectives of the rules are to assure reasonably certain protection of life and property and to provide a margin for deterioration in service. These rules do not provide criteria for thermal performance, but rather set minimum necessary guidelines for structural integrity to ensure safe operation during the expected component life.

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21

FIGURE 1.5  DBA stress categories.

The BPVC provides a systematic approach to evaluating the stresses and applying material properties in a way which provides a safe pressure vessel design. The rules are established through a structured voluntary consensus code writing system and are implemented through specific contract terms and/​or adoption by various jurisdictions

1.5.7 Strength Theories The three commonly used theories to predict failure are: 1. the maximum principal stress theory 2. the maximum shear stress theory 3. the distortion energy theory.

1.5.8 Allowable Stress Allowable stresses are used in the design of pressure vessels, heat exchangers, structures, machine elements, etc. The code gives tables for allowable stresses in tension for most structural materials at discrete temperatures. The allowable stresses in compression depend on the slenderness of the pressure components and are therefore presented in terms of slenderness ratio. The basis of the ASME Code allowable stress values is discussed in Ref. [35].

1.5.9 Combined-​Thickness Approach for Clad Plates In general, the code does not permit using the clad thickness as additional thickness to resist pressure but rather treats it only as a corrosion allowance. As an exception, for clad material conforming to SA-​263, SA-​264, and SA-​265, the cladding thickness after deducting the corrosion allowance can be used for the thickness calculation purpose. As per paragraph UCL-​23(c), if the nominal thickness of base plate is tb, then the allowable combined thickness, tt, that can be used for pressure calculations is given by



S  t t = t b +  c  tc (1.1)  Sb 

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Mechanical Design of Shell and Tube Heat Exchangers

where Sc is the maximum allowable stress of cladding at design temperature, Sb is the maximum allowable stress of the base material at design temperature, tc is the nominal thickness of cladding less corrosion allowance, where Sc is greater than Sb, the multiplier Sc/​Sb shall be taken equal to unity.

1.5.10 Welded Joints The most common way to fabricate a pressure vessel is by welding. For vessel members with weld joints, all thickness formulas shall contain the weld joint efficiency term E inserted into the equation. The multiplication of the efficiency term E with the code allowable stress, S, gives the effective allowable stress, SE, for the weld seam. If no E term is contained in the formula, the allowable stress may have to be modified by a quality factor of 80% [25]. Special restrictions prevail at the weld joint for the following cases: 1. the vessel contains a lethal substance 2. the vessel will operate at a temperature lower than −20°F 3. the vessel is an unfired steam boiler with design pressure exceeding 50 psi 4. the vessel is subjected to direct firing. In these cases, all joints are restricted to butt joints and full penetration welds. 1.5.10.1 Welded Joint Efficiencies In industry, radiographic examination (RT) is the most common technique to establish soundness of the weld joints. Depending on the type of weld joint (single or double butt, double full fillet lap, single-​welded butt joint without backing strip, etc.), and also on the extent of RT used to check the soundness of the joint, most of the pressure vessel codes prescribe a “joint efficiency” E to be used in the thickness formulas. The code recognizes full radiography, spot radiography, and none. As per ASME Code Table UW-​12, for a double-​welding butt joint, the corresponding efficiencies would then be fully radiographed 100%, spot radiographed 85%, and none 70%. The decrease in joint efficiency from 100% to 70% when no spot radiographic examinations are made on the welded joints means that a fabricator must provide more thickness. 1.5.10.2 Joint Categories As per paragraph UW-​3 of ASME Code Section VIII, Div. 1, the term “category” is used to define the location of a joint in a vessel, but not the type of the joint. Categories are established for the purpose of specifying special requirements regarding joint type and degree of inspections of certain welded pressure vessels. ASME Code categorizes various joint locations into the following four types: Category A locations, Category B locations, Category C locations, and Category D locations. These locations are schematically shown in Figure 1.6. Some examples for Category A, B, C, and D are given next. For complete details, refer to ASME Code Section VIII, Div. 1. Category A Locations Category A locations are longitudinal welded joints within a main shell, and welded joints within a sphere, within a formed or flat head, or within the side plates of a flat-​sided vessel; circumferential welded joints connect hemispherical heads to main shells and several other locations.

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FIGURE 1.6  ASME Code joint category designation.

FIGURE 1.7  Types of Weld Joint –​(a) Type 1, (b) Type 2, (c) Type 3, (d) Type 4, (e) Type 5, and (f) Type 6.

Category B Locations Category B locations are circumferential welded joints within the main shell, and circumferential welded joints connecting formed heads other than hemispherical to main shells, to transitions in diameter, to nozzles, or to communicating chambers and several other locations. Category C Locations Category C locations are welded joints connecting flanges, tubesheets, or flat heads to the main shell, to formed heads, to transitions in diameter, and to nozzles; any welded joints connect one side plate to another side plate of a flat-​sided vessel and several other locations. Category D Locations Category D locations are welded joints connecting communicating chambers or nozzles to main shells, to spheres, to heads, or to flat-​sided vessels, those joints connecting nozzles to communicating chambers, and several other locations. 1.5.10.3 Weld Joint Types The category of the weld joint determines permissible joint types, weld examination requirements, and associated weld joint efficiencies used in pressure part thickness calculation. The code defines six weld joint types (UW-​2); weld joint types are schematically shown in Figure 1.7 and their definitions are given in Table 1.5.

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TABLE 1.5 Weld Joint Types Type

Description

Joint type 1

Double-​welded butt joint, or by other means that produce the same quality of weld on the inside and outside. Welds using metal backing strips that remain in place are excluded Single-​welded butt joints with backing strip which remains in place after welding Single-​welded butt joints without backing strip Double full fillet lap joint Single full fillet lap joints with plug welds Single full fillet lap joint without plug welds

Joint type 2 Joint type 3 Joint type 4 Joint type 5 Joint type 6

1.6 KEY TERMS IN PRESSURE VESSEL AND HEAT EXCHANGER DESIGN 1.6.1 Design Pressure Design pressure for a pressure vessel or a heat exchanger is the gauge pressure at the top of the vessel and, together with the coincident design metal temperature, is used in the design calculations of a pressure vessel for the purpose of determining the minimum thickness of the various pressure-​ retaining components of the vessel. Since a heat exchanger is made of two different pressure zones –​ tubeside and shellside –​at least two design pressures shall be defined. ASME Code encourages (UG-​21) that the design pressure be higher than the normal operating pressure with a suitable safe margin to allow for probable pressure surges in the vessel up to the setting of pressure relief valves (UG-​134). When vessels are subjected to inside vacuum and external positive pressure on the outside, then the maximum difference between the inside and outside of the vessel shall be taken into account.

1.6.2 Design Temperature This is the temperature stamped on the nameplate along with the design pressure. This temperature shall not be less than the mean metal temperature expected across the thickness, under the operating conditions for the parts under consideration (UG-​20). Design temperature can be different for the different pressure parts if the operating conditions ensure a defined temperature variation [35]. For example, in a multipass shell and tube heat exchanger in which there is an appreciable temperature drop or rise on the tubeside, the inlet headers and outlet headers can have different design temperatures. In no case shall the design temperature exceed the temperature corresponding to the code allowable stress for the material used in the thickness calculations or the allowable working temperature for the material specified in the code.

1.6.3 Maximum Allowable Working Pressure The maximum allowable working pressure (MAWP) is the gauge pressure for a specified operating temperature that is permitted for the vessel in operation, such that, together with any other likely loadings other than pressure, the stresses computed using code formulas do not exceed the code allowable stress values. Metal thickness specified as corrosion allowance is not considered for the calculation of thickness. It is the basis for the pressure setting of the pressure-​relieving devices that protect the vessel. The MAWP is normally specified for two conditions –​new (uncorroded) and old (corroded).

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1.6.4 Operating Temperature or Working Temperature As per ASME Code, this is defined as the temperature that will be maintained in the metal of the part of the vessel being considered for the specified operation of the vessel.

1.6.5 Operating Pressure or Working Pressure As per ASME Code, this is defined as the pressure at the top of the vessel at which it normally operates. It shall not exceed the MAWP and it is kept at a suitable level below the setting of the pressure-​relieving devices to prevent their frequent opening.

1.6.6 Corrosion Allowance The corrosion allowance is the additional thickness of metal added to allow for material lost by corrosion and erosion, or scaling. Corrosion is a complex phenomenon and it is not possible to give specific rules for the estimation of the corrosion allowance required for all circumstances.

1.7 PRESSURE VESSELS DESIGN The structural integrity of pressure vessels and heat exchangers depends on proper mechanical design arrived at after detailed stress analysis keeping in view all the static, dynamic, steady, and transient loads. Design involves parameters such as maximum safe operating pressure and temperature, safety factor, corrosion allowance and minimum design temperature, materials selection, and choice of fabrication methods. This chapter discusses pressure vessel design and mechanical design of heat exchangers. Also discussed in this chapter are the fundamentals of mechanical design, stress analysis, classification of stresses and stress category concept, allowable stress, weld joint efficiency and joint category, and various design terms. References [36–​44] give basic guidelines for pressure vessel design and fabrication.

1.7.1 Pressure Vessel Shapes The shape and size of a pressure vessel are determined by the design requirements, product being stored, the amount of space at the job site, and a company’s budget. The most common pressure vessel shapes are cylindrical pressure vessels and spherical pressure vessels. based on orientation they may be horizontal or vertical pressure vessels. As for material, pressure vessels can be manufactured from a variety of materials but they are most commonly fabricated from carbon or stainless steel.

1.7.2 Types of Pressure Vessel Heads Cylindrical, horizontal, and vertical pressure vessels are the most common kinds of vessels and they all require specialized ASME caps on each end. These caps are called “heads” and there are three primary kinds –​hemispherical head, ellipsoidal head, and flanged & dished heads. Their design is discussed later.

1.7.3 Construction Details of Pressure Vessels Pressure vessels are composed of a shell, head, and supports, with additional attachments as needed for the vessel’s particular application. The main pressure vessel components are shell, head, support, nozzle, openings, filling and drain pipes, pressure relief valves, etc. Common external attachments

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which are connected to pressure vessels are platforms, ladders, piping, insulation, gauges, drains, etc. and vessel internals include trays, beds, linings, etc. Each component of a pressure vessel will have a different design and material composition based on its intended use. Pressure vessel manways. The manway of a pressure vessel is the means by which workers can access the vessel. There are a number of design aspects that must be taken into consideration when fabricating a pressure vessel manway.

1.7.4 Venting and Relief Devices In order to prevent collapse of the vessel, ASME Codes require that a certified pressure relief device be installed on all pressure vessels. Appropriate relief devices include [45]: 1. safety valves 2. relief valves 3. safety relief valves 4. pressure safety relief valves 5. rupture disks. Depending on the application for which the pressure vessel is being used, a combination of devices may be necessary in order to meet ASME Code.

1.7.5 Outlets and Drains Pressure vessels typically contain a drain or outlet at or near the lowest point of the tank. The outlet and joint must be able to withstand the pressure held in the tank to avoid leaks or blowouts. Drains for containers in sanitary services must be placed carefully and at minimal lengths to ensure that the vessel will drain completely.

1.7.6 Pressure Equipment Devices Some common technical categories of pressure equipment used in the mainstream engineering industries. From the engineering viewpoint, pressure equipment types, and component [46]: 1. simple pressure vessels 2. simple receivers, air receivers 3. oxygen, liquid nitrogen, LPG, LNG tanks, etc. 4. O2, CO2, acetylene, etc. gas cylinders 5. LPG cylinders (transportable) 6. unfired pressure vessels –​condensers, air-​cooled heat exchangers, superheaters, reheater, economizers, high pressure feed water heaters, shell and tube exchangers, gasketed plate heat exchangers, etc. 7. boilers and steam generator 8. valves 9. pipelines and fittings –​power piping, service piping, flanges, pipework fittings, pressurized accessories, etc. 10. miscellaneous pressure equipments.

1.7.7 Pressure Vessel Design Codes Pressure vessels have to be designed and manufactured following specific design codes. The commonly used pressure vessel codes are ASME VIII Division 1, ASME VIII Division 2, PD5500,

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BS EN13445, etc. The purpose of using design codes is to avoid disasters due to poor design that can affect humans and environment. The codes provide guidance on design, selection of materials of construction, fabrication methods, quality control, inspection and testing, shipment, etc. In the U.S. and many other countries, pressure vessels are typically constructed in accordance with ASME BPVC Section VIII which is divided into three parts, Division 1, Division 2, and Division 3.

1.7.8 Design Considerations Factors that should be taken into account in the design process for pressure vessels include: 1. internal and external static and dynamic pressures 2. ambient and operational temperatures 3. weight of vessel and contents, snow load if any 4. wind loading, seismic load if any 5. stress analysis –​residual stress, localized stress, thermal stress, etc. 6. reaction forces and moments from attachments, piping, etc. 7. fatigue including thermal fatigue 8. corrosion 9. creep.

1.7.9 Data Required for a Pressure Vessel Design Basic data required for a pressure vessel design [47–​49]: 1. vessel function 2. fired/​unfired pressure vessel 3. process fluids and service conditions 4. operating conditions (temperature and pressure): • design/​operating pressure and temperature • Maximum Allowable Working Pressure (MAWP) • Minimum Design Metal Temperature (MDMT). 5. design code 6. design loads 7. wind and seismic load data 8. materials of construction 9. shape, dimensions and orientation 10. type of vessel heads to be used 11. openings and connections required 12. heating/​cooling requirements 13. vessels supports 14. specification of internal fittings.

1.7.10 Geometry Definition To define the geometry of a pressure vessel, the inner diameter of the equipment and the distance between tangent lines is used. The inner diameter should be used, since this is a process requirement. Figure 1.8 shows general configuration and dimensional data for pressure vessel shells and heads [31].

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FIGURE 1.8  General configuration and dimensional data for pressure vessel shell and head, after Moss, D.R. [31].

1.7.11 Minimum Wall Thickness There will be a minimum wall thickness required to ensure that any vessel is sufficiently rigid to withstand internal and external pressures, self weight and any incidental loads.

1.7.12 Materials of Construction The materials to be used in pressure vessels must be selected from code approved material specifications. This requirement is normally not a problem since a large catalogue of tables listing acceptable materials is available. Factors that need to be considered in picking a suitable materials are: cost and availability in market fabricability service condition (wear and corrosion resistance, operating temperature) strength requirements, creep and fatigue strength. The range of materials used for pressure vessels and heat exchanger is wide and includes, but is not limited to, the following: carbon steel (with less than 0.25% carbon), carbon manganese steel (giving higher strength than carbon steel), low alloy steels, high alloy steels, austenitic stainless steels, non-​ferrous metals including aluminum, copper, nickel and alloys, titanium, zirconium, tantalum, etc. and nonmetals such as glass, graphite, ceramics, teflon, composites like FRP, etc.

1.7.13 Design Parameters Consider the design parameters as discussed in ASME Code Section VIII Div. 1 and Ref. [50]: 1. Minimum thickness of pressure-​retaining components. 2. Plate undertolerance. 3. Pipe undertolerance.

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FIGURE 1.9  Pressure vessel design parameters.

4. Corrosion allowance in design formulas. 5. Methods of fabrication in combination. A vessel may be designed and constructed by a combination of the methods of fabrication 6. Materials in combination. 7. Special constructions. Combination Units. A combination unit is a pressure vessel that consists of more than one independent or dependent pressure chamber, operating at the same or different pressures and temperatures. 8. Design temperature –​maximum and minimum. 9. Design pressure. 10. Loadings. 11. Maximum allowable stress values. 12. Corrosion allowances. PV design parameters are shown in Figure 1.9.

1.7.14 Methods of Construction of Pressure Vessels Several different methods are used to construct pressure vessels. Most pressure vessels are constructed with welded joints. Cylindrical shell are usually made by rolling plate at either elevated or ambient temperature. The cylinder is formed by welding the ends of the rolled plate together. This yields a cylinder with a longitudinal weld. Hot forging is another method of making cylindrical vessels.

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1.7.15 Compliance with ASME Code Important elements for compliance with ASME Code include [45]: 1. design 2. fabrication 3. fabrication, inspection and testing 4. installation. Compliance with ASME Code is authorized by inspectors commissioned by the National Board of Boiler and Pressure Vessel Inspectors –​licensed by state or provincial governmental authority charged with enforcement. 1. installation 2. operation 3. inspection 4. repairs and alterations 5. routine safety checks.

1.7.16  Common Causes of Failures and Explosions in Pressure Vessels Pressure vessel failures and hazards are caused due to [50]: 1. errors in design, construction, and installation 2. improper installation, human failure, and inadequate training of operators 3. corrosion/​erosion of construction materials 4. failure or intentional defeat of safety devices; failure or override of automatic control devices 5. failure to inspect and test thoroughly, properly, and frequently 6. improper application of equipment; overfiring 7. lack of planned preventive maintenance.

1.8 MECHANICAL DESIGN OF STHE This type of heat exchanger consists of a shell with an internal tube bundle that is typically supported by tubesheets and intermittent tube support plates known as baffles. Two fluids, of different inlet temperatures, flow through the heat exchanger. One flows through the tubes (the tubeside) and the other flows outside the tubes but inside the shell (the shellside). The tube bundle may be composed of several types of tubes supported by baffles. The tube pitch (center to center distance of adjoining tubes) is typically a minimum of 1.25 times the tube outer diameter or larger. Typically, the ends of each tube are terminated through holes in a tubesheet. The tubes are mechanically rolled or welded into tube sheet face. Tubes may be straight or bent in the shape of a U, called U-​tubes.

1.8.1 STHE Types 1. Fixed Tubesheet Heat Exchanger. Heat exchanger with two stationary tubesheets, each attached to the shell and channel. The heat exchanger contains a bundle of straight tubes connecting both tubesheets. 2. U-​tube Heat Exchanger. Heat exchanger with one stationary tubesheet attached to the shell and channel. The heat exchanger contains a bundle of U-tubes attached to the tubesheet.

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31

3. Floating Tubesheet Heat Exchanger. Heat exchanger with one stationary tubesheet attached to the shell and channel, and one floating tubesheet that can move axially. The heat exchanger contains a bundle of straight tubes connecting both tubesheets. The following components perform a function mainly related to pressure and fluid containment. Their design is carried out in accordance with the relevant pressure vessel code: 1. Shell. 2. Dished heads and flat heads. 3. Tubesheets. Tubesheets design is as per ASME Code whereas TEMA guidelines are non-​mandatory. 4. Bolted flanged joint. 5. Expansion joint. 6. Nozzle. 7. Vessel support. 8. Other pressure and non-​pressure parts.

1.8.2 Pressure Vessel Codes and Standards Mechanical design involves the design of pressure-​retaining and non-​pressure-​retaining components and equipments to withstand the design loads and the deterioration in service so that the equipment will function satisfactorily and reliably throughout its codal life. Mechanical design is done as per the procedure given in the construction codes and standards. The pressure parts of a shell and tube heat exchanger are designed in accordance with a pressure vessel design code such as ASME, PD 5500, EN 13445, A. D. Merkblatter 2000, and so on, but a pressure vessel design code alone cannot be expected to deal with all the special features of shell and tube heat exchangers. To give guidance and protection to designers, fabricators, and purchasers alike, a supplementary code, viz. a Manufacturers Standard is desirable which provides minimum standards for design, materials, thicknesses, corrosion allowances, fabrication tolerances, testing, inspection, shipment details, installation, operation, maintenance, and guarantees for shell and tube heat exchangers. One such universally accepted standard for STHE is the Standards of the Tubular Exchanger Manufacturers Association, known as TEMA. For power plant heat exchangers like condenser and feedwater heater, HEI Standards are followed.

1.8.3 ASME Code Section VIII Div. 1 Part UHX Rules for Shell and Tube Heat Exchangers 1. The rules in Part UHX cover the minimum requirements for design, fabrication, and inspection of shell and tube heat exchangers. 2. The rules in Part UHX cover the common types of shell and tube heat exchangers and their elements but are not intended to limit the configurations or details to those illustrated or otherwise described herein. Designs that differ from those covered in this Part shall be in accordance with U-​2(g).

1.8.4 Required Information for Mechanical Design For mechanical design of shell and tube heat exchangers, certain minimum information is required [58]. The following listing summarizes the minimum information required: 1. Thermohydraulic design details in the form TEMA or an equivalent specification sheet. 2. TEMA class, type of TEMA shell, channels/​heads.

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3. Shellside and tubeside passes. 4. Number, type, size, and layout of tubes. 5. Diameter and length of shell, channel/​head, and its configuration. 6. Design temperatures and pressures. 7. Joint efficiency for longitudinal joint and circumferential joint. 8. External pressure if the equipment is under external pressure or is under internal vacuum. 9. Worst-​case coincident conditions of temperature and pressure. 10. Nozzle, wind, and seismic loads, impact loads (including water hammer, if any). 11. Superimposed loads due to insulation, piping, stacked units, etc. 12. Corrosion properties of the fluids and the environment in which the unit will be installed and the expected service life. This will help to specify corrosion allowances or better material selection to reduce the material loss due to corrosion. 13. Materials of construction except tube material, which is arrived at the thermal design stage. 14. Fouling characteristics of the streams to be handled by the exchanger. This will determine if closures are required for frequent cleaning of internal parts of the exchanger. Many fixed tubesheet heat exchangers, if not specified otherwise, may be of welded head and shell construction. 15. Flow rate to size the nozzles and to determine whether impingement protection is required. 16. Special restrictions imposed by the purchaser on available space, piping layout, location of supports, type of material, servicing conditions, etc. 17. Construction code and standard to be followed. 18. Installation –​vertical or horizontal. 19. Installation and operation considerations like startup, transients, shutdown, and upset conditions that decide tubesheet thickness [40]. 20. Handling of lethal or toxic fluids, which demand more stringent welding and NDT requirements. When high-​pressure fluid is routed through the tubeside, the effect of tube failure on the low pressure shellside should be considered. It is essential to provide excess pressure protection on the shellside.

1.8.5 Sequence of Decisions to be made During Mechanical Design In addition to the information required at the mechanical design stage as mentioned already, certain decisions are also to be made at the mechanical design stage. Soler [39] summarizes a typical sequence of decisions that must be made at the mechanical design stage of a heat exchanger design. Some of the points are as follows: 1. What kind of connections (welded, flanged, or packed) should be provided at the front head, tubesheet, and rear head? 2. What style of flanged joint should be used –​e.g. ring-​type gasket or full-​face gasket? 3. What kind of closures (hemispherical, ellipsoidal, torispherical, conical, etc.) should be used? 4. What combination of load will govern the pressure part design? (Typical loads are shellside pressure, tubeside pressure, differential thermal expansion, self-​weight, mechanically transmitted vibration, seismic vibration, etc.) 5. Type and style of openings. 6. Type of nozzle connections, such as self-​reinforcing forging stock versus pipe schedule. 7. Details of vent and drain design. 8. Minimum bend radii for U-​tubes.

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9. Whether an expansion joint is required. If so, what is the best type and style of the expansion joint? 10. Whether installation is horizontal or vertical, to decide the type/​style of heat exchanger supports. 11. Evaluation of the ability of the exchanger to withstand operational transients, startup, and pressure testing. Each of these decisions and evaluation steps requires a proper adjudication among various possibilities; many of these considerations require and/​or are amenable to mathematical analysis, while others are derived from past experience or experimental data [59].

1.8.6 Content of Mechanical Design of Shell and Tube Heat Exchangers Mechanical design of shell and tube heat exchangers involves at a minimum the following components design and the determination of stresses induced in that component: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

Shell thickness. Shell flange and channel flange design. Dished end calculation. Design of openings and nozzles. Tubesheet thickness. If the differential expansion between shell and tubes is excessive, then an expansion joint is to be designed and thus final tubesheet thickness is arrived at. Shell longitudinal stress and bending stress. Tube longitudinal stress, both at the tube bundle inside and at the periphery. Channel longitudinal stress and bending stress. Tube-​to-​tubesheet joint load. Flat cover thickness. Design of supports. Additionally consider the cost of material, fabrication and labor.

1.8.7 Software for Mechanical Design of Heat Exchanger Nowadays most of the mechanical design of heat exchangers and pressure vessels are done by software. Typical software for mechanical design include Advanced Pressure Vessel of Computer Engineering, Inc. MO, Intergraph® PV Elite™, Houston, TX, and COMPRESS of CODEWARE, Houston, TX (www.codew​are.com). Typical software program structure and output of mechanical design of shell and tube heat exchanger is shown in Table 1.6.

1.8.8 Mechanical Design Procedure A typical mechanical design procedure is discussed by Singh [60]. It is the following: 1. identify applied loadings 2. determine applicable codes and standards 3. select materials of construction (except for tube material, which is selected during the thermal design stage) 4. compute pressure part thickness and reinforcements if any 5. select appropriate welding details 6. establish that no thermohydraulic conditions are violated 7. design non-​pressure parts

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TABLE 1.6 Typical Software Program Structure and Output of Mechanical Design of Shell and Tube Heat Exchanger Design conditions Cylinder

Heads/​covers Nozzles

Flanges

Tubesheets

Expansion joints Supports Maximum allowable working pressures (MAWP) Minimum design metal temperature (MDMT) External loadings on nozzles Lifting lug design Rigging analysis Vessel natural frequency Vortex shedding loads due to wind Wind deflection Calculation and documentation Tubesheet layout details Drawings Others Bill of materials

Concise summary of design pressures, temperatures, corrosion allowances, weights (empty, full, bundle) Diameters, Code calculated thickness, TEMA minimum thickness, external pressure minimum thickness, actual thickness, maximum external pressure, maximum length for external pressure, materials of construction. Vessel cross sectional areas and moment of inertia checks. Vessel longitudinal stress check-​Stress analysis due to the combined effect of pressure, live loads, dead weights, etc. Calculations are performed using the ID or OD for Flanged and Dished, Torispherical, Ellipsoidal, Hemispherical, Conical, Flat, Toriconical heads, etc. Diameters, Code calculated thickness, actual thickness, reinforcement pad diameter and thickness, materials of construction. Nozzle weld load, stress, strength of connection, shear and path of failure ANSI B 16.5 flange: Computes pressure rating based on Class, Grade and design temperature. Custom flange calculations as per Appendix 2 and Taylor Forge method for min. thickness and MAWP. Flange outer diameter, bolt circle, bolt diameter, bolt number, gasket outer diameter, gasket width, gasket thickness, Code calculated flange thickness, actual flange thickness, lap joint ring dimensions, hub dimensions, materials of construction Diameters (front and rear), bending thickness, shear thickness, flange extension thickness, effective thickness, recesses, actual thickness, clad thickness, tubing layout details, outer tube limit, materials of construction Number of joints, diameter, flexible element thickness, dimensions, spring rates, cycle life, materials of construction Support dimensions, gussets, hole dimensions, wear plate thickness, Zick stress analysis; For vertical vessel base ring or legs and/​or lugs analyzed. materials of construction. MAWP for all code components at design and ambient conditions, with controlling components flagged Calculations performed per UCS-​66 for all pressure components composed of UCS-​23 materials

Local stresses in cylinders with applied loads, design conditions, maximum loads and moments, interaction diagram Calculation for both horizontal and vertical lifting lugs as per standard engineering methods Bending and shear stresses generated in a vessel while it is being lifted are computed. These stresses are compared to their allowable values Computes natural frequency of the vessel in filled, empty and operating conditions Computes fatigue stresses and number of hours of safe operation, based on loads generated by dynamic wind vibration (mostly for vertical unit) Computes the elemental deflection, angular rotation and critical speed Formulas used and intermediate results for verification of Code and TEMA calculations Number of tubes per row, distances offset from horizontal and vertical center lines for each row, tie rod locations, pass partition locations, balance of tubes per pass, baffle cut dimensions Setting plan, sectional drawing, bundle layout, tubesheet layout, etc. Fatigue analysis, Seismic loads, graphics, thermal and mechanical design interface, material database, supporting Standards and Codes Quantity of all components, their dimensions and specifications, costing

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35

8. design supports 9. select appropriate inspection procedure.

1.8.9 Design Loadings A list of loadings to be considered in designing a heat exchanger or a pressure vessel part is given in UG-​22 of ASME Code Section VIII, Div. 1. They include the following: 1. internal and/​or external design pressure (as defined in UG-​21) 2. weight of vessel and normal contents including the static head of liquid 3. local loadings on the shell, such as those due to internals, vessel supports, lugs, etc. 4. cyclic and dynamic reactions due to pressure or thermal variations, mechanical loadings, etc. 5. wind, seismic, and snow loadings where required 6. impact loadings, such as those due to fluid shock.

1.8.10 Design of STHE Components Calculation of minimum thickness and design of the components such as: (a) shell, (b) dished end and flat cover, (c) tubesheet, (d) determination of stresses such as shell longitudinal stress and tube longitudinal stress, (e) tube-​to-​tubesheet joint loads at the periphery of the tube bundle, (f) expansion joint, (g) flange, (h) nozzle openings and reinforcement of nozzle openings, and (i) supports.

1.9 FUNDAMENTALS OF TUBESHEET DESIGN A tubesheet is an important component of a heat exchanger. It is the principal barrier between the shellside and tubeside fluids pressures. The cost of drilling and reaming the tube holes as well as the overall cost of the tubesheet of a given dimension will have direct bearing on the heat exchanger cost. Additionally, proper design of a tubesheet is important for safe and reliable operation of the heat exchanger. In this section, fundamentals of tubesheet design such as classification of tubesheets and constructional features are discussed.

1.9.1 Tubesheet Connection with the Shell and Channel Tubesheets are mostly flat circular plates with uniform pattern of drilled holes. Tubesheets of surface condensers are rectangular in shape. The tubesheet is connected to the shell and the channel either by welding (integral) or bolts (gasketed joints) or a combination thereof. As defined by ASME Code Section VIII, Div. 1, there are four categories for fixed tubesheet heat exchanger and six categories for U-​tube and floating head heat exchanger and they are discussed below.

1.9.2 Fixed Tubesheet Heat Exchanger A fixed tubesheet heat exchanger is shown in Figure 1.2. The tubesheets may have one of the four configurations shown in Figure 1.10. 1. Configuration a: tubesheet integral with shell and channel. 2. Configuration b: tubesheet integral with shell and gasketed with channel, extended as a flange. 3. Configuration c: tubesheet integral with shell and gasketed with channel, not extended as a flange. 4. Configuration d: tubesheet gasketed with shell and channel.

36

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Mechanical Design of Shell and Tube Heat Exchangers

FIGURE 1.10  Tubesheet connection with the shell and channel of a fixed tubesheet heat exchanger.

1.9.3 U-​tube Heat Exchanger Tubesheet The tubesheet may have one of the six configurations shown in Figure 1.11 1. Configuration a: tubesheet integral with shell and channel. 2. Configuration b: tubesheet integral with shell and gasketed with channel, extended as a flange. 3. Configuration c: tubesheet integral with shell and gasketed with channel, not extended as a flange. 4. Configuration d: tubesheet gasketed with shell and channel. 5. Configuration e: tubesheet gasketed with shell and integral with channel, extended as a flange. 6. Configuration f: tubesheet gasketed with shell and integral with channel, not extended as a flange.

37

Mechanical Design of Shell and Tube Heat Exchangers

37

FIGURE 1.11  Tubesheet connection with the shell and channel of a U-tube heat exchanger and stationary tubesheet of a floating head heat exchanger.

38

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Mechanical Design of Shell and Tube Heat Exchangers

1.9.4 Floating Head Heat Exchanger Various possibilities of the connection of tubesheet with shell and channel of stationary tubesheet are same as U-​tube heat exchanger as given in Figure 1.11.

1.10 TUBESHEET DESIGN PROCEDURE: HISTORICAL BACKGROUND TEMA set up rules for the design of U-​tube and floating head heat exchangers in 1941. These rules were simple but do not ensure an overall safety for all heat exchangers. Hence, many researchers [41–​88] published papers on tubesheet design and interpreted code rules from 1948 onward. References [89–​93] discusses design of curved and rectangular tubesheets. The codes and standards periodically updated the procedure for tubesheet design as and when better methods were published. Design procedure for tubesheets varies among the code rules and standards, and hence designers get different thickness for the same design condition since different assumptions were made in the model analysis. With this background knowledge, principles of tubesheet design is explained next. Subsequently, the tubesheet design procedure for fixed, floating head, and U-​tubesheet procedure as per ASME Code Section VIII Div. 1 and TEMA procedure which is included in the non-​ mandatory section is dealt with. Design aspects of double tubesheets, rectangular tubesheets, and curved tubesheets are also covered later.

1.10.1 Assumptions in Tubesheet Analysis While analyzing the tubesheets, certain assumptions are made in their models by many researchers. The tubesheets are treated as thin plates compared to their radial dimension, both circumferential and radial stresses vary linearly through the thickness of the tubesheets, and shear stresses vary parabolically from zero at one face to zero at the other face with a maximum at the center. Other assumptions include the following: 1. The tubesheet is uniformly perforated over its whole area; the unperforated annular rim is not considered by some standards. For example, TEMA Standards do not consider the unperforated tubesheet portion for all classifications of tubesheets. 2. The membrane loads in the tubesheets are negligible as compared to the bending loads [64]. 3. No slip occurs at the junction between the tubes and the tubesheet. 4. The tubes are adequately stayed by baffle plates to enable them to stand up to the calculated loads without sagging. 5. The bending moments in the tubes at their attachment with the tubesheet are neglected. 6. The exchanger is axis symmetrical and symmetric about the plane midway between the tubesheets. 7. Modeling of the tube bundle: The tubes are assumed uniformly distributed over the whole tubesheet and in sufficient number (Nt) so as to act as a uniform elastic foundation of modulus Kw. The expression for Kw is

Kw =

Nt Kt

πR2

(1.2)

where Kt represents the axial rigidity of one tube as given by

Kt =

π Et t ( d − t ) L

(1.3)

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Mechanical Design of Shell and Tube Heat Exchangers

39

Note: The elastic modulus for a half bundle, kw, is equal to 2Kw, and the axial rigidity of one tube is kt, half tube is equal to 2Kt. 8. Modeling of the tubesheet: The perforated tubesheet is replaced by an equivalent solid plate of effective elastic constants E* and v* (the determination of effective elastic constants is discussed separately). The flexural rigidity of the perforated plate Kt*, in terms of the flexural rigidity of unperforated plate Kt and deflection efficiency η, is given by

η=



K t* Kt

(1.4)

where Kt and Kt* are given by

Kt =

E *T 3 ET 3 * and K = (1.5) t 12 (1 − ν*2 ) 12 (1 − ν2 )

One of the drawbacks of the work of Gardner [62, 63] and Miller [64] is the assumption that the Poisson ratio of the perforated tubesheet is the same as that for the unperforated tubesheet; accordingly, a constant value of v* =​0.3 was assumed in their treatment. 9. The maximum stress in the perforated plate will be the maximum stress in the homogeneous plate divided by the ligament efficiency, μ. 10. The analysis is based on the optimum design of tubesheets within their elastic behavior of all components attached to the tubesheet. If the temperatures are high enough, creep becomes of primary importance [67]. 11. The deflection of the tubesheet is small, and hence the angular distortion of the tube ends due to the bending of the tubesheet can be neglected. 12. The effect of rotational resistance of the tubes is negligible since it is minor in nature. 13. Boundary restraint parameters X and Z. Tubesheets are weakened due to drilling holes, whereas they are stiffened by the tube bundle and tubesheet edge restraint offered by the shell and the channel connected with the tubesheet by welding. Based on the tubesheet connection with the shell and the channel, the edge-​restraint condition is treated as simply supported, clamped, and an intermediate case. The stresses induced in the shell, channel, and tubesheet depend on a dimensionless parameter, X (which is equal to the ratio of the axial tube bundle rigidity to the bending rigidity of the tubesheet) that accounts for the support afforded by the tube bundle to the tubesheet and the perforations that weaken it. It may vary from almost zero as in U-​tube heat exchanger to about 50 (very stiff tube bundle as compared to the tubesheet). Common values generally lie between two and eight. A second parameter Z, which represents the degree of rotational restraint of the tubesheet by the shell and channel, is also important. 14. Effective elastic constants of perforated plates: While designing the perforated tubesheets, the weakening effect due to the tube hole perforations has been taken into account by replacing the plate by an equivalent solid plate with new elastic constants known as the effective Young’s modulus, E*, and effective Poisson’s ratio, v*. The values of E* and v* are such that the equivalent plate has the same deflection as that of the original unperforated plate. This is known as the equivalent solid plate concept. The equivalent solid plate concept has been found to be quite useful in the design and analysis of perforated plates by equating strains in the equivalent solid material to the average strains in the perforated material [87].

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Mechanical Design of Shell and Tube Heat Exchangers

These effective elastic constants must be evaluated correctly, especially in fixed tubesheet heat exchangers [88]. If they are too low, the stresses at the junction with the shell and the head will be lower than that in real units. If they are too high, the stress at the center of the plate, which may be a maximum, will be too low. 15. Determination of effective elastic constants: Pressure vessel codes such as ASME, CODAP, and UPV present charts to determine effective elastic constants. TEMA does not determine the effective elastic constants. It assumes a constant value of 0.178 for deflection efficiency. An excellent review of about 60 papers on the elastic constants was done by Osweiller [88]. Osweiller proposed curves for the determination of effective elastic constants that have been adopted in CODAP. 16. Ligament efficiency: The ligament efficiency is a very useful dimensionless parameter for the analysis of perforated plates. The ligament efficiency, defined in terms of the tube layout pattern and pitch ratio in TEMA, is known as mean ligament efficiency, η and in terms of pitch ratio is known as minimum ligament efficiency, μ in codes such as CODAP and ligament efficiency in ASME Code Section VIII, Div. 1. The general expression for ligament efficiency is

µ=



p−d (1.6) p

where d is the tube outer diameter p is the tube pitch p − d is the minimum ligament width. The effect of ligament efficiency in the calculations of tubesheet thickness and tube-​to tubesheet joint strength is discussed in the later section. The ligament of a perforated tubesheet is given in Figure 1.12.

1.10.2 Basis of Tubesheet Design The basis of tubesheet design procedure is discussed here. This discussion closely follows the method of Galletly [67] and Ref. [78], which was further expanded by Osweiller [81, 84] for inclusion in CODAP.

FIGURE 1.12  Minimum ligament width definition of a perforated plate.

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Mechanical Design of Shell and Tube Heat Exchangers

41

1.10.2.1 Analytical Treatment of Tubesheets The analytical treatment has the same basis in UPV, CODAP, and ASME rules and has been widely presented in papers, Soler [78] and Osweiller [81, 84]: 1. Thin circular plate on elastic foundation. Most tubesheet design analysis treats the tubesheet as a thin circular plate on an elastic foundation. The elastic foundation is provided by the tube bundle. (See Figure 1.13) 2. The tubesheet is disconnected from the shell and channel. A shear force VE and a moment ME are applied at the tubesheet edge as shown in Figure 1.13b. 3. The perforated tubesheet is treated as a solid equivalent circular plate of effective elastic constants E* (effective modulus of elasticity) and v* (effective Poisson’s ratio) depending on the ligament efficiency μ* of the tubesheet. 4. The tubes are replaced by an equivalent elastic foundation of modulus kw. In U-​tube heat exchangers, the tubes do not act as an elastic foundation and hence kw =​0. 5. Classical thin plate theory is applied to this equivalent tubesheet to determine the maximum stresses in the tubesheet, tubes, shell, and channel. The analytical aspect is based on the following terms: 1. ligament efficiency, μ* ASME ligament efficiency μ*. It accounts for an untubed diametral lane of width UL (through the effective tube pitch p*) and for the degree of tube expansion ρ (through the effective tube diameter d*). 2. effective elastic constants E* and v* Effective elastic constants E* and v* given by curves as a function of μ* in ASME Code. 1.10.2.2 Design Analysis The heat exchanger is assumed to be surface of revolution and symmetrical about a plane midway between the tubesheets, so as to analyze a half exchanger as shown in Figure 1.13a. Figure 1.13b shows a circular plate of thickness T resting on an elastic foundation. To minimize the complexity, the untubed tubesheet portion is neglected, and the tubesheet is integral with both the shell and the channel. The tubesheet is disconnected from the remainder of the exchanger, i.e. from shell and channel. The plate is elastically restrained against deflection and rotation around its periphery, θc by (1) an axial reaction VE due to the end load acting on the head and to the axial displacement Δs of the half shell, and (2) a reactive bending moment ME

FIGURE 1.13  Basis of tubesheet analysis –​(a) half heat exchanger, (b) circular plate on an elastic foundation, and (c) force and moment at the tubesheet edge. (From [81] Osweiller.)

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Mechanical Design of Shell and Tube Heat Exchangers

(−Kθθc) as shown schematically in Figure 1.13c. The plate is subjected to a uniform net effective pressure q(r) given by [66, 67, 80]



(

)

 N p − p ( d − t )2  γ t s  + 2 νs psQ − kw  w (r ) − + ∆ s  (1.7) q (r ) = ps fs − pt ft − νt  2t 2 2 R     

In the expression for q(r), the first term takes into account the differential pressure acting on the equivalent plate, which is corrected for the tube hole areas by the shellside drilling coefficient, fs, and tubeside drilling coefficient, ft, respectively; the second and third terms take into account the loads resulting from the axial displacements of tubes and shell by the Poisson effect of shellside pressure, ps, and tubeside pressure, pt, respectively (Q is the ratio of rigidity of tube bundle to the shell); and the fourth term traduces the reactive effect of the elastic foundation. In this, the term w(r) is the deflection of the plate at a distance r from the center axis, and γ/​2 is the differential thermal expansion between the tubes and the shell, which is given for the half exchanger by



γ L = α t ( θt − θamb ) − α s (θs − θamb ) (1.8) 2 2

The expressions for fs, ft, and Q are

fs = 1 −

Nt d 2 4 R2

(1.9)

N t ( d − 2t )



ft = 1 −



Q=



=

4 R2 Nt Kt Ks

(1.10)

(1.11)

1 (1.12) K

where K is the ratio of axial rigidity of the shell (Ks) to the axial rigidity of the tube bundle (NtKt). The parameter K signifies the ratio of the force required to produce a given strain in the shell to the force necessary to produce the same strain in the tube bundle. It is thus the measure of the ability of the shell to resist movement of the two tubesheets relative to the tube bundle [61]. However, when there is a considerable thermal expansion between the shell and the tube bundle, the high rigidity of the shell may induce very high thermal stresses in the tube bundle and the shell. This ability is reduced by the introduction of an expansion joint into the shell. An externally packed floating head exchanger for purposes of tubesheet design is considered as a perfect expansion joint, and for exchangers so constructed, the value of K is zero. The expression for the axial rigidity of the shell Ks is given by

Ks =

πts ( Do − ts ) Es L

(1.13)

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Mechanical Design of Shell and Tube Heat Exchangers

From the expression for Kt (Equation 1.3) and Ks (Equation 1.13), the resulting expression for K is given by

K=



(

Es ts Do − ts

) (1.14)

Et N t t ( d − t )

where Do is the shell outside diameter. The expression for axial rigidity of the half shell, ks, is equal to 2Ks, and axial rigidity of the half tube, kt, is equal to 2Kt. 1.10.2.3 Deflection, Slope, and Bending Moment From classical thin-​plate theory, the deflection of a solid circular plate of elastic constants E*, v*, and flexural rigidity D* resting on elastic foundation kw, subjected to net effective pressure q(r) and elastically restrained at its periphery, is given by



d 4 w 2 d 3 w 1 d 2 w 1 dw q (r ) + − + = * (1.15) dr 4 r dr 3 r 2 dr 2 r 3 dr D

The solution of this equation is of the form



w (r ) = ABer ( x ) + BBer ( x ) +

γ P* − ∆ s + (1.16) 2 kw

Deflection, slope, and bending moment



(

)

 N p − p ( d − t )2  t s  + 2 νs psQ (1.17) p* = ps fs − pt ft − νt  2t 2 R    x = kr =



4

kw D*

r (1.18)

In Equation 1.16, Ber(x) and Bei(x) are the modified Bessel functions of the first kind, and A and B are unknown constants. At the periphery of the tubesheet (i.e. r =​R), x becomes X. It represents the relative rigidity of the tube bundle with respect to the tubesheet. It may vary from 0 (no tube in the bundle) to above 50 for very stiff tube bundle. The expression for X is as follows:





X = kR =

=

4

4

kw D*

R (1.19)

π N t Et t ( d − t ) 12 (1 − ν*2 ) R 4 π R2 L /2

E *T 3

(1.20)

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Mechanical Design of Shell and Tube Heat Exchangers

Substituting for NtEt (d − t) by Ests (Do − ts)/​K and for E*/​(1 − v*2) by ηE/​(1 − v2) and Do − ts ≈ 2R, X becomes

X=



4

)

KL

=



(

24 Es ts Do − ts R 2 (1 − ν2 )

4

ηE

(1.21a)

6 (1 − ν2 ) Es ts  2 R  3   (1.21b) η KLE  T 

From w(r), one may determine the shear forces, the bending moment, and the slope at any point in the tubesheet. By employing the Kirchoffs-​Kelvin equation, the expressions for slope, dw/​dr, and bending moment, M, are given by dw (1.22) dr

θ=



 d 2 w ν* dw  M = D*  2 + (1.23)  dr r dr 



The two constants of integration A and B and the axial displacement of the half shell, Δs, are determined by the following three boundary conditions: 1. At r =​R, the deflection w(r) at the edge of the plate due to bending is zero. 2. At r =​R, the radial bending moment at the edge of the plate equals the moment exerted by the rotational spring, i.e.  d 2 w ν* dw  D*  2 +  dr r dr 



= − Kθ r=R

dw (1.24) dr r = R

where Kθ is the spring constant for half shell. Its value is the sum of the bending rigidities of the shell and the channel. 3. The vertical force at the edge, VE, is given by the net effective force acting on the plate in terms of net effective pressure q(r): R

2 π ∫q (r ) dr = 2 π RVE (1.25)



o

where



VE =

π R 2 pt − ks ∆ s 2π R

(1.26)

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Mechanical Design of Shell and Tube Heat Exchangers

1.10.2.4 Parameter Z Though the parameter Z is not figuring in the earlier relations, it is required to find equivalent pressure in CODAP and other rules. The expression for Z [67, 80] is given by

Z=

Kθ kD*

=

Kθ 4

kw D*3

(1.27)

where Kθ is the edge moment coefficient. It depends on the bending rigidities of the shell (δs) and the channel (δc). It represents the degree of elastic restraint of the tubesheet offered to it by the shell and the channel connected to it. Its value varies between the two extreme values: 0, which corresponds to the simply supported case, and ∞, which corresponds to the clamped case. K θ = 2  δs + δ c 

 Es ts2.5 Kθ = 2   12 (1 − ν2 ) 0.75 D + t s s  

(

)

0.5

+

Ec tc2.5 12 (1 − ν2 )

0.75

(

  0.5 Dc + tc 

)

(1.28)

1.10.2.5 Supported Tubesheet and Unsupported Tubesheet Heat exchanger tubes other than a U-​tube heat exchanger may be considered to act as stays that support or contribute to the strength of the tubesheets in which they are attached. In the case of fixed and floating head heat exchangers, the tube bundle behaves like an elastic foundation. However, the floating nature of one of the tubesheets makes the staying action partial. This is especially true for an outside packed floating-​type heat exchanger. In the case of U-​tube heat exchangers, tubes provide only a reactive bending moment to the tubesheet bending. According to the level of support provided by the tubes, TEMA classifies the tubesheets as (1) supported tubesheet and (2) unsupported tubesheet; examples are as follows: 1. unsupported tubesheets, e.g. U-​tube tubesheets 2. supported tubesheets, e.g. fixed tubesheets and floating tubesheets. 1.10.2.6 Factors that Control Tubesheet Thickness Being a plate-​type structure, the tubesheet resists the lateral pressure by bending, and the membrane loads are negligible. Hence, the limiting stress is the primary bending stress only. The factors that control the tubesheet thickness are the following: 1. Tube pitch and layout pattern, which define the ligament efficiency of perforated tubesheets. 2. The manner in which the deformation of tubesheet is influenced by the support being provided by the tube bundle to the tubesheets. For the same process conditions and tubesheet diameter, tubesheet thickness decreases in the order of the following exchanger types: a. U-​tube heat exchanger b. floating head exchanger c. fixed tubesheet exchanger. 3. Mean metal temperatures of tubesheet, tube, and shell. 4. Number of tubes. 5. Limits of the tube field and the extent of untubed portion.

46

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Mechanical Design of Shell and Tube Heat Exchangers

6. Method of joining the tubes to the tubesheets, e.g. rolled, seal welded, or strength welded. 7. Shell and the channel connection with the tubesheets, e.g. gasketed and integral. 8. Shell and the channel thickness.

1.11 TUBESHEET DESIGN AS PER ASME CODE SECTION VIII DIV. 1 Part UHX Rules for Shell and Tube Heat Exchangers UHX-​1 1. Rules for U-tube heat exchangers are covered in UHX-​12. 2. Rules for fixed tubesheet heat exchangers are covered in UHX-​13. 3. Rules for floating tubesheet heat exchangers are covered in UHX-​14.

1.11.1 General Conditions of Applicability for Tubesheets 1. The tubesheet shall be flat and circular. 2. The tubesheet shall be of uniform thickness, except that the thickness of a tubesheet extension may differ from the center thickness. 3. The tubesheet shall be uniformly perforated over a nominally circular area, in either equilateral triangular or square patterns. However, untubed lanes for pass partitions are permitted. 4. The channel component integral with the tubesheet shall be either a cylinder or a hemispherical head. The hemispherical head rules shall be used when the head is attached directly to the tubesheet and there are no cylindrical sections between the head and the tubesheet. 5. The tubeside and shellside pressures are assumed to be uniform. These rules do not cover weight loadings or pressure drop. 6. The design pressure or operating pressure defined in the nomenclature is the applicable pressure in the shellside or tubeside chamber, including any static head, not the coincident pressure  defined. For the design-​pressure-​only conditions (design loading cases), the design pressure shall be used. For the operating thermal-​pressure conditions (operating loading cases), the operating pressure shall be used. If the operating pressure is not available, the design pressure shall be used for all loading cases. 7. The design rules are based on a fully assembled heat exchanger.

1.11.2 Tubesheet Characteristics These rules cover the determination of the ligament efficiencies, effective depth of the tubeside pass partition groove, and effective elastic constants to be used in the calculation of U-tube, fixed, and floating tubesheets.

1.11.3 Design Considerations 1. Elastic moduli and allowable stresses shall be taken at the design temperatures. However, for cases involving thermal loading, it is permitted to use the operating temperatures instead of the design temperatures. 2. When the values calculated in this section are to be used for fixed tubesheets, they shall be determined in both the corroded and uncorroded conditions. 3. Determination of Effective Dimensions and Ligament Efficiencies. 4. Determination of Effective Elastic Properties. 5. Determine the values for E*/​E and ν* relative to h/​p. 6. Materials and Methods of Fabrication.

47

Mechanical Design of Shell and Tube Heat Exchangers

47

Materials and methods of fabrication of heat exchangers shall be in accordance with ASME Code Section VIII Div. 1 Subsections A, B, and C.

1.11.4 Rules for The Design of Fixed Tubesheets of Fixed Tubesheet HEx Rules for fixed tubesheet heat exchangers are covered in UHX-​13. These rules cover the design of tubesheets for fixed tubesheet heat exchangers. The tubesheets may have one of the four configurations as shown in Figure 1.10. 1.11.4.1 Conditions of Applicability The two tubesheets shall have the same thickness, material and edge conditions. 1.11.4.2 Design Considerations 1. It is generally not possible to determine, by observation, the most severe condition of coincident pressure, temperature, and differential thermal expansion. Thus, it is necessary to evaluate all the anticipated loading conditions to ensure that the worst load combination has been considered in the design. Specify all the design and operating conditions that govern the design of the main components of the heat exchanger (i.e. tubesheets, tubes, shell, channel, tube-​to-​tubesheet joint). These shall include, but not be limited to, normal operating, start-​up, shutdown, cleaning, and upset conditions. For each of these conditions, the following loading cases shall be considered to determine the effective pressure, Pe to be used in the design formulas: a. Design Loading Cases. Table UHX-​13.4-​1 provides the load combinations required to evaluate the heat exchanger for the design condition. b. Operating Loading Cases. Table UHX-​13.4-​2 provides the load combinations required to evaluate the heat exchanger for each operating condition. c. Differential pressure design. d. The designer should take appropriate consideration of the stresses resulting from the pressure test required. 2. The elastic moduli, yield strengths, and allowable stresses shall be taken at the design temperatures for the design loading cases and may be taken at the operating metal temperature of the component under consideration for operating condition . 3. As the calculation procedure is iterative, a tubesheet value T shall be assumed for the tubesheet thickness to calculate and check that the maximum stresses in tubesheet, tubes, shell, and channel are within the maximum permissible stress limits, and that the resulting tube-​to-​tubesheet joint load is acceptable. Because any increase of tubesheet thickness may lead to overstresses in the tubes, shell, channel, or tube-​to-​tubesheet joint, a final check shall be performed, using in the equations the nominal thickness of tubesheet, tubes, shell, and channel, in both corroded and uncorroded conditions. 4. The designer shall consider the effect of radial differential thermal expansion between the tubesheet and integral shell or channel. 5. The designer may consider the tubesheet as simply supported.

1.11.5 Rules for the Design of U-​tube Heat Exchanger Tubesheets Rules for U-tube heat exchangers are covered in UHX-​12. These rules cover the design of tubesheets for U-tube heat exchangers. The tubesheet may have one of the six configurations shown in Figure 1.11.

48

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Mechanical Design of Shell and Tube Heat Exchangers

1.11.5.1 Design Considerations The various loading conditions to be considered shall include, but not be limited to, normal operating, startup, shutdown, cleaning, and upset conditions, which may govern the design of the tubesheet. 1.11.5.2 Calculation Procedure for Simply Supported U-​tube Tubesheets This procedure describes how to use the rules of UHX-​12.5 when the effect of the stiffness of the integral channel and/​or shell is not considered. 1.11.5.3 Calculation Procedure 1. As the calculation procedure is iterative, a value T shall be assumed for the tubesheet thickness to calculate and check that the maximum stresses in tubesheet, shell, and channel are within the maximum permissible stress limits. 2. The designer may consider the tubesheet as simply supported in accordance with UHX-​12.6. 3. The elastic moduli, yield strengths, and allowable stresses shall be taken at the design temperatures for the design loading cases.

1.11.6 Rules for the Design of Stationary Tubesheet of Floating Head Heat Exchanger 1. These rules cover the design of tubesheets for floating tubesheet heat exchangers that have one stationary tubesheet and one floating tubesheet. Three types of floating tubesheet heat exchangers are covered as shown in Figure UHX-​14.1. a. immersed floating head b. externally sealed floating head c. internally sealed floating tubesheet. 2. Stationary tubesheets may have one of the six configurations shown in Figure 1.11 3. Floating tubesheet may have one of the four configurations as shown in Figure 1.13: a. Configuration A: tubesheet integral. b. Configuration B: tubesheet gasketed, extended as a flange. c. Configuration C: tubesheet gasketed, not extended as a flange. d. Configuration D: tubesheet internally sealed. 1.11.6.1 Conditions of Applicability The two tubesheets shall have the same thickness and material. 1.11.6.2 Design Considerations 1. The calculation shall be performed for the stationary end and for the floating end of the exchanger. Since the edge configurations of the stationary and floating tubesheets are different, the data may be different for each set of calculations. 2. It is generally not possible to determine, by observation, the most severe condition of coincident pressure, temperature, and radial differential thermal expansion. Thus, it is necessary to evaluate all the anticipated loading conditions to ensure that the worst load combination has been considered in the design. Specify all the design and operating conditions that govern the design of the main components of the heat exchanger (i.e. tubesheets, tubes, shell, channel, tube-​to-​tubesheet joint). These shall include, but not be limited to, normal operating, start-​ up, shutdown, cleaning, and upset conditions. For each of these conditions, the following loading cases shall be considered to determine the effective pressure Pe to be used in the design equations: a design loading cases

49

Mechanical Design of Shell and Tube Heat Exchangers

49

b operating loading cases c differential pressure design d The designer should take appropriate consideration of the stresses resulting from the pressure test. 3. The elastic moduli, yield strengths, and allowable stresses shall be taken at the design temperatures for the design loading cases and may be taken at the operating metal temperature of the component under consideration for operating condition x. 4. As the calculation procedure is iterative, a value h shall be assumed for the tubesheet thickness to calculate and check that the maximum stresses in tubesheet, tubes, shell, and channel are within the maximum permissible stress limits and that the resulting tube-​to-​tubesheet joint load is acceptable. 5. The designer shall consider the effect of radial differential thermal expansion adjacent to the tubesheet.

1.11.7 Tubesheet Extension 1. Tubesheet extensions, if present, may be extended as a flange (flanged) or not extended as a flange (unflanged). 2. These rules cover the design of tubesheet extensions that have loads applied to them. 3. The required thickness of the tubesheet extension may differ from that required for the interior of the tubesheet. 1.11.7.1 Design Considerations 1. The designer shall take appropriate consideration of the stresses resulting from the pressure test required. Special consideration shall be required for tubesheets that are gasketed on both sides when the pressure test in each chamber is conducted independently and the bolt loading is only applied to the flanged extension during the pressure test. 2. If the tubesheet is grooved for a peripheral gasket, the net thickness under the groove or between the groove and the outer edge of the tubesheet shall not be less than Tr. Tubesheet Calculation Procedure shall be as per ASME Code.

1.12 TUBESHEET DESIGN AS PER TEMA STANDARDS (APPENDIX A-​NON-​MANDATORY SECTION) ASME Code, Section VIII, Division 1, Part UHX, provides specific rules for calculating the thickness of tubesheets for shell and tube heat exchangers for both bending and shear. Basics of tubesheet design as per ASME Code discussed above, refer to the appropriate section of ASME Code, as the calculations are too extensive to be included in this chapter. When ASME may not be the design code or the specific design may not be addressed by ASME, in these cases, the minimum tubesheet thickness calculations may be based on the following TEMA rules in Appendix A (Non-​mandatory).

1.12.1 Tubesheet Formula for Bending The formula for minimum tubesheet thickness to resist bending is given by



T=

FG 3

P (1.29) ηS

50

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Mechanical Design of Shell and Tube Heat Exchangers

where F is the parameter used to account for the elastic restraint at the edge of the tubesheet due to shell and channel connections G is the diameter over which the pressure is acting P is the effective design pressure S is the ASME Code allowable stress η is the mean ligament efficiency (it depends on the mean width of the ligament), given in terms of tube layout pattern angle θ and pitch ratio p/​d whose expression is given by

η = 1− = 1−



= 1−

π

4 (sin θ )( p /d )

2

0.785

( p /d )2 0.907

( p /d )2

for θ = 90° and 45° (1.30) for θ = 60° and 30°

The minimum values of η are 0.42 (triangular pitch) and 0.50 (square pitch); therefore, for a given value of ligament efficiency, the tubesheet thickness is lower for square pitch than for triangular pitch. But in real cases, η will generally range between 0.45 and 0.60, which leads to a decrease of T by about 10%–​15%. TEMA ligament efficiency η is significantly higher than ASME ligament efficiency μ* (generally 0.25 ≤ μ* ≤ 0.35). ASME ligament efficiency μ* is based on the minimum width of the ligament, which leads to lower values than TEMA. For these reasons, tubesheet thickness obtained by ASME is generally thicker than TEMA, Osweiller [81]. The values of F, P, and G differ for supported and unsupported tubesheets. For a fixed tubesheet exchanger, G shall be the shell inside diameter. For other types of exchangers, refer to TEMA for the definition of G. For a fixed tubesheet exchanger, the effective design pressure, P, shall be calculated as per A.1.3.1. The definition of F for supported and unsupported tubesheets is discussed next. For all types of tubesheets, the thickness shall be calculated both for uncorroded and for corroded conditions.

1.12.2 Parameter F 1. Supported tubesheet Gasketed both sides, e.g. stationary tubesheet and floating tubesheet and floating head exchanger:

F = 1.0 2. Clamped or integral tubesheet Integral on both sides or a single side, e.g. stationary tubesheet of fixed tubesheet exchanger and floating head exchanger: When the tubesheet is integral with both sides or a single side, F is determined by curve H in Fig A.1.3.1. The curve is presented in terms of the ratio of wall thickness to internal diameter (ID) of the shell or channel, i.e. (t/​ID), whichever yields the smaller value of F. For the shellside integral condition, use the shell ID to find F. The H curve can be represented by

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Mechanical Design of Shell and Tube Heat Exchangers

F = 1.0 for =



t ≤ 0.02 ID

17 − 100 ( t /ID ) 12

= 0.8 for

for 0.5 ≥

t > 0.02 (1.31) ID

t > 0.05 ID

As per this condition, the minimum value of F is 0.8 and the maximum value is 1.0. Both the tubesheets of the fixed tubesheet exchanger are satisfied by this condition, and both the tubesheets shall have the same thickness, unless the provisions of RCB-​7.166 are satisfied. Note: The F value for the tubesheet at the floating head for all configurations is 1.0. Unsupported tubesheet, for example, U-​tubesheet: 1. When gasketed at both sides, F =​1.25. 2. For tubesheets integral on both sides or a single side, F shall be determined by curve U in Figure A.1.3.1 of TEMA. The curve is presented in terms of the ratio of wall thickness to ID of the integral side, i.e. (t/​ID). The U curve can be represented by F = 1.25 for =



t ≤ 0.02 ID

17 − 100 ( t /ID )

= 1.0

15 for

for

0.05 ≥

T > 0.02 (1.32) ID

t > 0.05 ID

As per this condition, the maximum value of F is 1.25 and the minimum value is 1.0. Effective Design Pressure: The term P is the effective design pressure where P =​ps +​ pb or pt +​ pb; pb is defined as equivalent bolting pressure when the tubesheet is extended as a flange. The expression for pb is given by

pb =

−6.2 M * (1.33) F 2G3

where M* is defined in Paragraph a.1.5.2.

1.12.3 Shear Formula The effective tubesheet thickness T to resist shear is given by

T=

0.31De P (1 − d /p) S (1.34)

where De is the equivalent diameter of the perforated tubesheet (=​4Ap/​C) C is the perimeter of the tube layout measured stepwise in increments of one tube pitch from center to center of the outermost tubes Ap is the total area enclosed by the perimeter C.

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FIGURE 11.14  Tubesheet tube holes layout pattern –​(a) triangular 30˚ pattern, (b) rotated triangular 60˚ pattern, (c) square 90˚ pattern, and (d) rotated square 45˚ pattern.

There are four types of tube layout patterns, viz. (a) triangular layout, (b) square layout, (c) rotated triangular layout, and (d) rotated square layout as shown in Figure 1.14. Figure 1.15 shows the perimeter for a typical triangular and square tubesheet layout patterns. The shear stress formula was derived by limiting the maximum allowable shear stress to 0.8 times the code allowable stress S. The shear formula controls the tubesheet thickness only in high-​ pressure and small-​diameter cases. Since the quantities C and Ap are available after the tube layout is finalized, TEMA provides a formula to check whether shear stress will be controlling the tubesheet thickness or not. Shear formula will not control the tubesheet thickness if 2



 d P < 1.6  1 −  (1.35) S p 

1.12.4 Minimum Tubesheet Thickness as per TEMA TEMA RCB-​7 Tubesheets TEMA RCB-​7.1 Tubesheet Thickness. The tubesheet thickness shall be per code rules. When the heat exchanger specification does not include rules for tubesheets as per ASME Code Part UHX, Appendix A of this TEMA Standard, or the manufacturer’s method may be used.

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FIGURE 1.15  Perimeter of tubesheet layout for shear stress calculation as per TEMA method –​(a) triangular layout and (b) square layout.

1.12.5 Stress Category Concept in TEMA Formula The primary stress in the tubesheet is limited to ASME Code Section VIII, Div. 1, allowable stress S, and the primary stress plus the thermal stress (secondary stress) is limited to 2S. Accordingly, the expression for effective pressure is halved before being used in the thickness formula.

1.12.6 Determination of Effective Design Pressure, P The determination of (1) effective shellside design pressure involves the terms ps′ , equivalent differential expansion pressure pd, and equivalent bolting pressure (pbs and/​or pbt) and (2) effective tubeside design pressure involves pt′, pd, and pbs, and/​or Pbt. The expressions for pd equivalent bolting pressure (pbs, pbt), ps′, and pt′ are given next.

1.12.7 Equivalent Differential Expansion Pressure, pd The equivalent differential expansion pressure, pd, is given by [63]



(

)

(

)(

)(

4 JTs Es α s θs − θamb − α t θ t − θamb  Do − ts Do − 2ts pd = 1 + JKFq

)

2

(1.36)

where Do is the shell outer diameter, Fq a parameter that is a function of X (G4 of Miller [44] and H4 of Galletly [46, 47]),



θ1 = θ1,m − θamb θs = θs,m − θamb



and where J is the expansion joint flexibility parameter (=​1.0 for shells without expansion joint).

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The expressions for J and Fq and the final expression for pd follow. 1. The expansion joint flexibility parameter J is given by J=



(

1

1 + K s /S j

)

(1.37)

)

(1.38)

Substituting the expression for Ks, J is given by

J=

(

SjL

S j L + π Do − ts Es ts

J can be assumed equal to zero for a shell with expansion joints if the following condition is satisfied: Sj
(1.54) 1 − FS  rG 2Λ 

where Λ=



2 π 2 Et Sy

(1.55)

rG is the radius of gyration of the tubes,

rG = 0.25 d 2 + ( d − 2t ) (1.56) 2

where kl is the equivalent unsupported buckling length of the tube k is a factor that takes into account the tube span end conditions l is the unsupported tube span between two baffles Fs is the factor of safety.

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1.12.13 Tube-​to-​Tubesheet Joint Loads (A.2.5) The maximum effective tube-​to-​tubesheet joint load, Fj, acting on the tubes located at the periphery of the tube bundle is given by



Fj =

π Fq pt*G 2 4Nt

(1.57)

where pt* is determined as per TEMA definition. This joint load is to be less than the maximum allowable joint load calculated as per ASME Code Section VIII, Div. 1.

1.12.14 Maximum Allowable Joint Loads In the design of shell and tube heat exchangers other than U-​tube construction, the maximum allowable axial load on tube-​to-​tubesheet joints shall be determined in accordance with the code formula. The basis for establishing allowable loads for tube-​to-​tubesheet joints loads is given in ASME Code Appendix AA.

1.13 FLANGED TUBESHEETS: TEMA DESIGN PROCEDURE A.1.3.3 Formulas are included in the TEMA for the calculation of the minimum thickness required on the flanged portion of the tubesheet of fixed, floating, and U-​tube exchangers. The calculation procedure is based on the work of Singh et al. [85]. The purpose of this new method is to insure that there is sufficient tubesheet thickness to withstand bending moments transmitted by the adjoining flange.

1.13.1 Tubesheet Extended as Flange The thickness of the portion of the tubesheet extended as a flange, Tf, is given by [85]



{

( )}  ) 

 M r 2 − 1 + 3.72 ln r f f Tf = 0.98   S DT − G 1.0 + 1.86rf2 

(

)(

0.5

(1.58)

where M is the bolting moment due either to gasket seating condition or to operating condition, whichever is greater; if a joint is integral (welded connection), then the corresponding edge moment is zero; DT and G are the tubesheet outer diameter and effective gasket diameter over which the pressure under consideration is action respectively; and S is the code allowable stress at the design temperature. The quantity rf is the ratio of DT to G:

rf =

DT G

(1.59)

1.14 RECTANGULAR TUBESHEET DESIGN The rectangular tubesheet of a surface condenser and idealized representation of tubesheet loading are shown in Figure 1.16 and Figure 1.17 respectively. For Rectangular tubesheet design refer to

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Mechanical Design of Shell and Tube Heat Exchangers

59

FIGURE 1.16  Rectangular tubesheet of a surface condenser (partially drilled), after Bernstein, M. D. and Soler, A. [89].

FIGURE 1.17  Idealized representation of rectangular tubesheet loading, after Bernstein, M. D. and Soler, A. [89].

HEI Standards [6]. The basis of the rectangular tubesheet design is discussed by Bernstein et al. [89] and in Singh [93] and Soler [94].

1.14.1 Methods of Tubesheet Analysis A condenser tubesheet is assumed to be a partially perforated plate on an elastic foundation, with the tubes comprising the foundation. The hydrostatic pull from the water box and surface pressure on the tubesheets are the dominant loads. Irregular tube patterns and variations in edge

60

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Mechanical Design of Shell and Tube Heat Exchangers

boundary conditions make solutions of the plate problem difficult. The HEI Standards mention the following four methods by which the structural integrity of the tubesheet and tubes may be demonstrated [89]: 1. interaction analysis using plate and shell formulas 2. experimental modeling techniques or prior services 3. FEA (elastic or elastic-​plastic) 4. beam strip on elastic foundation (single or multiple strips).

1.15 CURVED TUBESHEETS Traditionally, heat exchangers use thick flat tubesheets. The problem of replacing thick flat tubesheets by thinner curved tubesheets was suggested first by Rachkov and Morozov [90]. They designed a much thinner semi-​ellipsoidal curved tubesheet based on membrane theory of shells. The design of shallow spherical curved tubesheets for heat exchangers is discussed by Paliwal et al. [91].

1.16 CONVENTIONAL DOUBLE TUBESHEET DESIGN In many heat exchange applications, intermingling of shellside and tubeside fluids may cause undesirable results, loss of quality of product, safety hazard. Therefore, prevention of any fluid leakage between shellside and tubeside of shell and tube heat exchangers becomes a prime design consideration. One method to inhibit mixing of component fluids is to employ double tubesheet construction. Double tubesheet construction has also been found in large power plant condensers. In the condenser application to large rectangular tubesheets, the primary concern has been prevention of contamination of treated and demineralized water due to the leakage of circulating water (raw water) into the condenser steam space. The double tubesheet configuration consists basically of two tubesheets, which may be closely spaced, at either or both ends of the exchanger, connected to each other by the tubing. The two tubesheets can have different edge conditions, and may be close enough to act together as a single “sandwich” plate under mechanical and thermal loading. This configuration may be employed in U-​tube exchangers, floating head units, or in fixed tube construction. A conventional double tubesheet construction is shown in Figure 1.18. Design of double tubesheets is discussed by Singh and Soler [30].

1.16.1 Conventional Double Tubesheet Design TEMA Guidelines RCB-​7.1.2 Double Tubesheets 1. Paragraphs RCB-​7.1.2.4, RCB-​7.1.2.5, and RCB-​7.1.2.6 provide the design rules for determining the thickness of double tubesheets for some of the most commonly used construction types. 2. When double tubesheets are used, special attention shall be given to the ability of the tubes to withstand, without damage, the mechanical and thermal loads imposed on them by the construction. 3. The tubesheets are connected in a manner which distributes axial load and radial thermal expansion loads between tubesheets by means of an interconnecting element capable of preventing individual radial growth of tubesheets.

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Mechanical Design of Shell and Tube Heat Exchangers

61

FIGURE 1.18  Conventional double tubesheet (schematic).

1.17 CYLINDRICAL SHELL, END CLOSURES, AND FORMED HEADS UNDER INTERNAL PRESSURE The following symbols are used in the formulas required to calculate the minimum thickness of a cylindrical shell and various end closures (some symbols are listed in ASME Code Section VIII Div. 1): • t is the minimum required thickness of shell or heads or end closures • P is the internal design pressure, psi (see UG-​21) (or MAWP, see UG-​98) • C is a factor depending upon the method of attachment of head, shell dimensions, and other items as listed in ASME Code • D is the inside length of the major axis of an ellipsoidal head, or inside diameter of a torispherical head, or inside diameter of a conical head at the point under consideration measured perpendicular to the longitudinal axis • Do is the outside length of the major axis of an ellipsoidal head, or outside diameter of a conical head at the point under consideration measured perpendicular to the longitudinal axis • d is the dimension of the short span for flat heads • G is the mean gasket diameter (not the effective gasket diameter as defined in the ASME Code) • h is the maximum inside depth of the ellipsoidal head, exclusive of the flange • hG is the gasket moment arm (i.e. radial offset between the circle and the bolt circle) • IDD is the inside depth of torispherical head • K is the factor in the formulas for ellipsoidal heads depending on the head proportion D/​2h • L is the inside spherical radius or crown radius

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Mechanical Design of Shell and Tube Heat Exchangers

• • • • • • • • • •

Lo is the outside spherical radius or crown radius M is the factor in the formulas for torispherical heads depending on the head proportion L/​r R is the inside radius of the shell course under consideration Ro is the outside radius of the shell course under consideration r is the inside knuckle radius S is the maximum allowable stress value, psi (see UG-​23 and the stress limitations specified in UG-​24) t is minimum required thickness of shell or flat cover E is the joint efficiency, or the efficiency of appropriate joint in cylindrical or spherical shells, or the efficiency of ligaments between openings, whichever is less (in decimal form) W is the total bolt load α is the one-​half of the included (apex) angle of the cone at the centerline of the head.

1.17.1 Cylindrical/Spherical Shell Under Internal Pressure Design of cylindrical shell is carried out as per UG-​27 of the ASME Code. The design formulas in the code are based on equating the maximum membrane stress to the allowable stress corrected for weld joint efficiency. As per ASME Code procedure, the thickness of shells under internal pressure shall not be less than that computed by the following formulas. In addition, provision shall be made for any of the other loadings listed in UG-​22, when such loadings are expected (see UG-​16).

1.17.2 Thin Cylindrical/Spherical Shells 1. Circumferential Stress (Longitudinal Joints) When the thickness does not exceed one-​half of the inside radius, or P does not exceed 0.385SE, the following formulas shall apply:

t=

PR SE − 0.6 P

or P =

SEt (1.60) R + 0.6t

2. Longitudinal Stress (Circumferential Joints) When the thickness does not exceed one-​half of the inside radius, or P does not exceed 1.25SE, the following formulas shall apply:

t=

PR 2 SE + 0.4 P

or P =

2 SEt (1.61) R − 0.4t

The minimum thickness or MAWP of cylindrical shell shall be the greater thickness or lesser pressure as given by Equations 1.60 and 1.61. These formulas are presented in Table 1.7 along with the formulas based on shell outside dimension; Table 1.8 gives formulas for thick cylindrical shells. 3. Spherical Shells. When the thickness of the shell of a wholly spherical vessel does not exceed 0.356R, or P does not exceed 0.665SE, the following formulas shall apply:



t=

PR 2 SE − 0.2 P

or P =

2 SEt (1.62) R + 0.2t

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Mechanical Design of Shell and Tube Heat Exchangers

TABLE 1.7 ASME Code Formulas for Minimum Thickness of Thin Cylindrical Shell to Withstand Internal Pressure, P Thickness, t

Member Longitudinal joints

Circumferential jointsa In terms of outside radius Ro

Maximum Internal Pressure, P

Limitation

t=

PR SE − 0.6 P

P=

SEt R + 0.6t

P ≤ 0.385SE t ≤ 0.5R

t=

PR 2 SE + 0.4 P

P=

2 SEt R − 0.4t

P ≤ 1.25SE t ≤ 0.5R

t=

PRo SE + 0.4 P

P=

SEt Ro − 0.4t

P ≤ 0.385SE t ≤ 0.5R

Source: ASME Boiler and Pressure Vessel Code, Section VIII, Division 1 − Pressure, American Society of Mechanical Engineers, New York, 2021 Edition. Formulas based on stress across circumferential joint will govern only if circumferential joint (Category B) efficiency is less than one-​half of the longitudinal joint efficiency.

a 

1.17.3 Thick Spherical Shells When the thickness of the shell of a wholly spherical vessel or of a hemispherical head under internal design pressure exceeds 0.356R, or when P exceeds 0.665SE, the following equations shall apply. The following equations may be used in lieu of those given in UG-​27(d). When P is known and t is desired,

   −0.50.P    0.50.P   t = R exp   − 1 = Ro 1 − exp    (1.63)  SE    SE    

where t is known and P is desired,



 R   R + t P = 2 SE loge  = 2 SE loge  O  (1.64)   R   Ro − t 

1.17.4 Design for External Pressure and/​or Internal Vacuum There is no straightforward formula as in the case for the design under internal pressure, because buckling has to be taken into account. The procedure to be followed is given in the ASME Code Section VIII, Div. 1, Paragraph UG-​28, and requires the use of various charts given in Appendix 5.

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Mechanical Design of Shell and Tube Heat Exchangers

TABLE 1.8 ASME Code Formulas for Minimum Thickness of Thick Cylindrical Shells to Withstand Internal Pressure, P Member

Thickness, t

Maximum Internal Pressure, P

Limitation

Circumferential stress (Longitudinal joint) Longitudinal stress (Circumferential joint)

When P is known and t is desired

Where t is known and P is desired

P > 0.385SE

   − P  P  t = R exp   − 1 = ​Ro 1 − exp     SE    SE    

 R   R + t P = SE loge  = S​ E loge  O   R   Ro − t 

t > 0.5R

P =​ SE(Z − 1) where Z is as defined earlier

P > 1.25SE

( ( =R

t=R

o

) Z − 1) where

Z −1

Z

  P Z= +1  SE 

 R + t Z=  R  R  =  o  R

t > 0.5R

2

2

(R

Ro2

o

−t

)

2

Source: ASME Boiler and Pressure Vessel Code, Section VIII, Division 1 − Pressure Vessels, American Society of Mechanical Engineers, New York, 2021 Edition.   Formulas based on stress across circumferential joint will govern only if circumferential joint (Category B) efficiency is less than one-​half of the longitudinal joint efficiency.

a

1.17.5 End Closures and Formed Heads End closures for heat exchangers and pressure vessels are in the form of either flat covers or formed heads. ASME Code Section VIII, Div. 1, recognizes the following end closures: 1. flat cover 2. hemispherical cover 3. ellipsoidal cover 4. torispherical cover (flanged and dished) 5. conical/​toriconical cover. These end closures are shown schematically in Figure 1.19, and some formed heads are shown in Figure 1.20. TEMA designates the front and rear covers by B and M, respectively. Closures other than flat heads are normally formed type; sometimes for low-​pressure application, cast heads are also used. Closures are designed as per UG-​32 or UA-​4 of the ASME Code. First, a brief description of various end closures is presented and then the determination of minimum thickness to retain internal pressure is presented. 1.17.5.1 Flat Cover Flat covers are easy to fabricate in any thickness from plates or forgings. They are widely used from low –​to high-​pressure applications. Since a flat cover resists pressure load only by bending, its thickness is significantly greater than that of the cylindrical section to which these are attached as a closure. 1.17.5.2 Hemispherical Hemispherical heads are used for high-​pressure service since their thickness is about half that of a cylindrical shell. The degree of forming and accompanying costs are greater than any other heads, and the available sizes from single plates are limited [34].

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65

FIGURE 1.19  End closures –​(a) flat head, (b) ellipsoidal head, (c) torispherical head, (d) hemispherical head, (e) toriconical head, and (f) conical head.

1.17.5.3 Ellipsoidal These are widely used for low –​to intermediate-​pressure services. When the minor-​to-​major axis ratio is 0.5 (most common), the head thickness is almost the same as that of the cylindrical shell. This simplifies the joining of these two and minimizes the discontinuity stresses at the joint. Another popular ellipsoidal head is with minor axis 25% of D. 1.17.5.4 Torispherical Among the various types of formed heads, the torispherical head is the most widely used in the industries, particularly for low-​pressure service, i.e. up to 200 psi [40]. For pressures over 200 psi gauge, ellipsoidal heads are used. The torispherical head is characterized by four geometric parameters: inside head diameter D, crown radius L, knuckle radius r, and head thickness t. Figure 1.21 shows the details of the torispherical head geometry. In Figure 1.21, the depth of dish b is a geometric function of crown radius L and knuckle radius r, and the straight cylindrical flange is integral with the dished end. By varying the ratios of L/​D and L/​r, heads of different shapes can be manufactured. Heads wherein L ≈ D, L ≈ 16⅔r, and r =​0.06D are referred to as ASME flanged and dished heads in the pressure vessel industry. Another popular variation is the 80:10 head where L ≈ 0.8D and r =​0.1L.

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FIGURE 1.20  Some types of formed heads –​(a) cone made of titanium Gr. 7, (b) elliptical head, (c) semielliptical head made of SA-​516 Gr. 70, and (d) hemispherical head made of SA-​533. (Courtesy of König +​Co. GmbH, Netphen, Germany.)

D FIGURE 1.21  Dimensional details of flanged and dished torispherical head. a = ; b = L − 2 2 2 D AB = − r ; BC = L − r ; AC = ( BC ) − ( AB ) ; and OA = t + b + Sf 2

( BC )2 − ( AB)2 ;

1.17.5.5 Conical These are used for low –​and intermediate-​pressure service with the half apex angle generally not more than 30°. A knuckle portion is provided to minimize the discontinuity stresses where it joins the shell.

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1.17.5.6 Torispherical Head (or Flanged and Dished Head) These heads have a dish with a fixed radius (r1), the size of which depends on the type of torispherical head. The transition between the cylinder and the dish is called the knuckle. The knuckle has a toroidal shape. The most common types of torispherical heads are: 1.17.5.7 ASME F&D Head Commonly used for ASME pressure vessels, these torispherical heads have a crown radius equal to the outside diameter of the head (r1 =​Do), and a knuckle radius equal to 6% of the outside diameter (r2 =​0.6Do). The ASME design code does not allow the knuckle radius to be any less than 6% of the outside diameter. 1.17.5.8 Klöpper Head This is a torispherical head. The dish has a radius that equals the diameter of the cylinder it is attached to. The knuckle has a radius that equals a tenth of the diameter of the cylinder, in compliance with DIN28011 standard. 1.17.5.9 Korbbogen Head This is a torispherical head also named semi ellipsoidal head (DIN 28013). A Korbbogenboden according to DIN 28013 has a crown radius of 80% of the diameter of the cylinder (CR =​0.8 x D0) and the radius of the knuckle is (KR =​0.154 x D0). 1.17.5.10 Elliptical head 2:1 Elliptical heads 2:1 are very similar to the Korbbogen heads, i.e. the Korbbogen heads are the European Elliptical heads and the Elliptical heads 2:1 are the American Korbbogen heads. 1.17.5.11 Minimum Thickness of Heads and Closures The required thickness at the thinnest point after forming of ellipsoidal, torispherical, hemispherical, conical, and toriconical heads under an internal pressure shall be computed by the appropriate formulas in UG-​16 of the ASME Code. In addition, provision shall be made for any of the other loadings given in UG-​22. The head design formulas in the code are based on equating the maximum membrane stresses to the allowable stresses corrected for weld joint efficiency. 1. Flat Cover a. As per ASME UG-​34, the minimum required thickness of flat unstayed circular heads, covers, and blind flanges shall be calculated by the following formula (refer to Figure 1.19a):



t=d

CP (1.65) SE

d =​diameter, or short span, measured as indicated in Figure 1.19a. b. The minimum required thickness of flat head, cover, and blind flange attached by bolts causing an edge moment is given by the formula: 0.5



 CP 1.9WhG  + t = d (1.66) SEd 3   SE

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where, hG =​gasket moment arm, equal to the radial distance from the centerline of the bolts to the line of the gasket reaction W =​total bolt load given for circular heads. 2. Ellipsoidal Heads The thickness or the MAWP of a dished head of semi-​ellipsoidal form, in which half the minor axis equals one-​fourth of the inside diameter of the head skirt, shall be determined by (refer to Figure 1.19b and Table 1.9): t=



PD 2 SE − 0.2 P

or P =

2 SEt (1.67) D + 0.2t

3. Torispherical Heads The required thickness or the MAWP of a torispherical head for the case in which the knuckle radius is 6% of the inside radius and the inside crown radius equals the inside diameter of the skirt (i.e. L ≈ D, Li ≈ 16⅔r, and r =​0.06D) shall be determined by (refer to Figure 1.19c and Table 1.9): t=



0.885PL SE − 0.1P

or P =

SEt (1.68) 0.885L + 0.1t

4. Hemispherical Heads When the thickness or the MAWP of a hemispherical head does not exceed 0.356L, or the internal pressure P does not exceed 0.665SE, the required thickness or the MAWP of a hemispherical head is given by (refer to Figure 1.19d and Table 1.9): t=



PL 2 SE − 0.2 P

or P =

2 SEt (1.69) L + 0.2t

5. Conical Heads and Sections (without Transition Knuckle) The required thickness of conical heads or conical shell sections that have a half apex angle α not greater than 30° shall be determined by (refer to Figure 1.19e and Table 1.9):



t=

2 SEt ( cos α ) PD or P = (1.70) 2 ( cos α ) ( SE − 0.6 P ) D + 1.2t ( cos α )

These formulas and formulas for minimum head thickness referred to outside head dimension are given in Table 1.9. Salient features of fabrication of various heads are discussed in Chapter 4.

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TABLE 1.9 ASME Code Formulas for Minimum Thickness of Heads/​End Closures of Pressure Vessels to Withstand Internal Pressure Head Ellipsoidal

Minimum Thickness, t

Maximum Pressure, p

PD 2 SE − 0.2 P PDK t= 2 SE − 0.2 P PDo K t= 2 SE − 2 P ( K − 0.1)

2 SEt D + 0.2t 2 SEt P= KD + 0.2t 2SEt P= KDo − 2t ( K − 0.1)

t=

P=

where K= Torispherical

0.885PL SE − 0.1P PLM t= 2 SE − 0.2 P PLo M t= 2 SE + P ( M − 0.2 ) t=

1  D 2 +   6   2h 

2

  

SEt 0.885L + 0.1t 2 SEt P= LM + 0.2t 2 SEt P= MLo − t ( M − 0.2 ) P=

where

Hemispherical

Conical

M=

1   L 3 +   4   r 

P=

2 SEt L + 0.2t

t=

PL 2 SE − 0.2 P

t=

PD 2 ( cos α ) ( SE − 0.6 P )

p=

t=

PDo 2 ( cos α ) ( SE + 0.4 P )

P=

0.5

  

2 SEt ( cos α ) D + 1.2t ( cos α )

2SEt ( cos α ) Do + 0.8t (cos α )

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1.18 BOLTED FLANGE JOINT (BFJ) Pressure vessels require flanged joints to permit their assembly/​disassembly, inspection and cleaning. A flange joint consists of three subcomponents: the flange ring, the gasket, and the bolting. The successful operation of the flange depends on the correct choice, design and assembly of these subcomponents. From a conceptual standpoint, flanged joints may be subdivided into two major categories: 1. bolted joints 2. pressure actuated joints. The “bolted joint” is by far the most common type. “Pressure actuated joints” find application in the higher pressure range, typically over 2000 psi. Pressure-​actuated joints exploit the header pressure force to compress and to seal the gasket. Pressure-​actuated joints find application in the higher pressure range, typically over 2000 psi [30]. The main difference between these two joint types lies in the manner by which the pressure load is resisted and leak-​tightness is achieved. A typical flanged connection is comprised of three parts: (1) the flanges, (2) the gaskets, and (3) the bolting. A gasket is normally inserted between the two mating flanges to provide a tighter seal. Using flanges adds flexibility by allowing for easier disassembly and improved access to system components. They provide easy access for cleaning, inspection or modification. Flange joint for connecting two pipes is shown in Figure 1.22 [95] Proper controls must be exercised in the selection and application for all these elements to attain a joint that has acceptable leak tightness. Special techniques, such as controlled bolt tightening, are described in ASME PCC-​1-​2022 [96]. Components and materials in a BFJ include flange joints, threaded fasteners (bolts/​studs, nuts, and washers), and gaskets. The goal of a BFJ is to create a tight-​leak sealing load on the gasket material. Typically BFJs are used for above ground service for water, wastewater, air, oil, and other liquids where rigid, restrained joints are needed in pressure containing equipment like pressure vessels and pipes. Important sources for information on bolted flanged joints include these references [95–​100].

1.18.1 Forces Acting on the Bolted Flange Connection A bolted flanged joint typically consists of a flange pair, bolts, nuts, a gasket and, where applicable, washers. The elements of bolted flange connection as shown in Figure 1.23 (97) is a mechanical

FIGURE 1.22  Basic elements of a bolted flange connection [95].

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FIGURE 1.23  The elements of bolted flange connection.

FIGURE 1.24  Forces acting in a bolted flange connection.

system whose components must be selected and assembled properly to provide reliable sealing over a wide range of operating conditions. The components consist of the piping, or vessels, the flange(s), the gasket(s) and bolts [95, 96].

1.18.2 Maintaining the Seal When the bolted flange connection is assembled, the gasket is subject to compressive load between the faces of the flanges as shown in Figure 1.24 [97]. Under operating conditions the compressive forces on the gasket is reduced by the hydrostatic end load and influenced by other factors, such as thermal expansion behavior of flanges and bolts, lever arms, etc. Misalignment of piping and surface waveness of flanges should be minimized. Flange misalignment creates additional loads that the bolts have to overcome before the bolt loads can be applied to the gasket [95, 96].

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FIGURE 1.25  Parameters of flange, gasket and bolts which affect the integrity of leak free joint.

1.18.3 BFJ Technical Requirements A BFJ constitutes a system serving the following functions: 1. The successful operation of the flanged joint depends on the correct choice, design and assembly of the three subcomponents, viz., flange, gasket and fasteners. The parameters of these three subcomponents which control the integrity of leak free flanged joint is shown in Figure 1.25 [97]. 2. Quality assurance measures must be in place to ensure the leak tightness of the bolted flanged joint. Next, the design aspects of components of BFJ are discussed.

1.18.4 Flange A flange is a forged or cast ring of steel or other metals designed to connect sections of pipe or join pipe to a pressure vessel, pump or any other integral flanges assembly. Flanges are joined to each other by bolting and joined to the piping system by welding or threading. Flanges play a crucial role in piping systems, connecting valves with other equipment. Pipe flanges are the second-​most commonly used joining mechanism after welding. Using flanges provides added flexibility, allowing easier assembly and disassembly of pipe systems. For details on flanges, refer to references[101–​106]. 1.18.4.1 Flange Types The most common flanges used in industrial applications follow. When applying gasket and sealing components to these flanges, the user must take into consideration sizing limitations, available clamp load, optimum surface finish, and gasket placement to minimize flange rotation. Pressure ratings for ASME standard flanges are classified by pressure class of 150, 300, 400, 600, 900, 1500 and 2500.

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1.18.4.2 Flange Classes and Types Flanges may be classified by shape (weld neck, blind, socket weld, threaded, etc.), by facing type (RF, FF, RTJ), by material grade (cast and forged), and by specification (the ANSI/​ASME B16.5 and B16.47 specifications cover US standard flanges for pipe flanges). The basic types of flanges are welding neck flange, long weld neck flanges, lap joint flange, ring flange, socket welding flange, slip-​on-​flange, blind flanges, orifice flanges, threaded flange, and reducing flange. As a general rule, welding neck, slip-​on, and socket weld flanges are used for high-​pressure applications that require long-​lasting flanged joints. Threaded flanges can be used with a lower pressure piping system, and if vibrations are not present. Lap joint flanges are used in connection with stub ends either to facilitate the alignment of the bolts of the two mating flanges or to reduce the cost of noble materials in high-​grade flanged joints (for example, in an Inconel piping system, the stub end connected to the pipe may be in Inconel, whereas the lap joint flange can be of a lower grade, thus saving the overall weight of the expensive Inconel material). Figure 1.26 and Figure 1.27 show the above types of flanges. 1.18.4.3 Flanged Joint Construction Based on the width of gasket, flange joints are classified as (1) ring-​type gasket joint and (2) full-​face gasket joint. Constructional details of flanged joints is shown in Figure 1.28. 1.18.4.4 ASME Code Classification of Circular Flanges for Design Purposes For computation purposes, ASME Code Section VIII, Div. 1, classifies circular flanges with ring type gaskets as: 1. loose-​type flanges 2. integral-​type flanges 3. optional-​type flanges 4. flanges with nut stop 5. reverse flanges. 1.18.4.5 Flange Facings The geometric details of the mating flange surfaces on which the gasket seats are known as flange facings. Flange facings are prepared to suit the gasket type, the kind of application, and the service conditions. Some of the classifications of the flange facings are as follows: 1. unconfined and prestressed: flat face and raised face (RF) 2. semiconfined and prestressed (male and female joint) 3. confined and prestressed: tongue and groove, double step joint, ring joint 4. self-​energizing: O-​rings, metallic, elastomer, etc. There are different types of flange faces, viz. flat face (FF), raised flange face (RF), ring joint face (RTJ), tongue and groove facing, and male and female face flange. The above types of flange faces are shown in Figure 1.29. 1.18.4.6 Flange Finish on Gasket Performance The ability of a flanged joint to maintain a leak-​proof joint depends on a number of parameters, of which the gasket and flange facing details are the most important [30]. A critical and fundamental aspect of sealing is the level of friction between the flange and gasket surfaces. Alignment, parallelism, and flange finish must be within specified limits in order to achieve an optimal result. Lack

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FIGURE 1.26  Types of flanges.

of alignment and surface waviness of a flanged joint is shown in Figure 1.30 [97]. A typical surface finish recommendation for these types of metal contact seals is 64 AARH/​RMS or smoother. 1.18.4.7 Flange Face Finish Definition and Common Terminology The type and texture of surface finish are important for leak tightness of a flanged joint. The flange face finish is determined by the standard used and measured as an arithmetical average roughness height (AARH). There are five distinct styles of surface finish that are commonly used in the industry: rounded nose spiral finish, spiral serrated finish, concentric serrated finish, smooth finish, and cold water finish as shown in Figure 1.31 [98]. 1.18.4.8 Flange Standards There are many different flange standards to be found worldwide. To allow easy functionality and interchangeability, these are designed to have standardized dimensions. Further many of the

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FIGURE 1.27  Types of flanges (contd)..

FIGURE 1.28  Flanged joint construction –​types of bolted flanges –​(a) loose ring flange, (b) fusion lap welded ring flange, (c) fusion lap welded hubbed flange, (d) fusion through welded ring flange, and (e) fusion butt welded hubbed flange.

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FIGURE 1.29  Flange facings –​(a) flat face, (b) raised face, (c) recessed face, (d) confined, (e) tongue and groove, (f) ring joint, and (g) O-​ring.

FIGURE 1.30  Flanged joint (a) Flanges with surface waviness and (b) Misaligned flanges.

flanges in each standard are divided into “pressure classes”, allowing flanges to be capable of taking different pressure ratings. Again these are not generally interchangeable. These pressure classes also have differing pressure and temperature ratings for different materials. For details on flange standards, refer to reference[107–​110] . 1.18.4.9 Flange Standards –​ASME Specifications The following ASME standards apply to main and companion flanges in pipe works:

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FIGURE 1.31  Type and texture of flange surface finish.

1. ASME B16.1-​2020: Gray Iron Pipe Flanges and Flanged Fittings Classes 25, 125, and 250. It includes: a. pressure-​temperature ratings b. sizes and method of designating openings of reducing fittings c. marking d. materials e. dimensions and tolerances f. bolting and gaskets g. pressure testing. 2. ASME B16.5-​2020 Pipe Flanges and Flanged Fittings: NPS ½ through NPS 24 Metric/​Inch Standard. 3. ASME B16.47-​2020 –​Large Diameter Steel Flanges NPS 26 through NPS 60 Metric/​Inch Standard. 4. ASME B16.48-​2020 Line blanks. 5. MSS SP-​44 shall be used for steel pipeline flanges for sizes smaller than ASME B16.47 where the material grade is not listed in ASME B16.5. 6. Flanges of unlisted materials and flanges not covered by the above standards shall be designed in accordance with ASME Section VIII Div. 1, Appendix 2, and for blind flanges, in accordance with ASME Section VIII Div 1, Section UG-​34. 7. Standards for Flange marking. ANSI/​MSS SP-​25-​2018 –​Standard Marking System for valves, fittings, flanges, and unions. (Note: MSS is Manufacturers Standardization Society of the Valve and Fittings Industry, Inc. USA). Flanges shall be marked as required in MSS SP-​25. 8. Identification Markings. Identification markings include: a. Name. The manufacturer’s name or trademark shall be applied. b. Materials. c. Rating Designation. d. Conformance. The designation B16 or B16.47 shall be applied. e. Temperature.

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f. Size. The NPS identification number. g. Ring-​Joint Flange. h. Multiple Material Marking. 1.18.4.10 Flange Material Selection Flanges can be manufactured from many different materials. The construction material chosen depends upon the service conditions the flange is likely to encounter. Some important service condition factors are temperature, pressure, and corrosion rate. For details on flange materials selection refer [111–​117]. Forged carbon steel pipe flanges are the most common type present within the market. Additionally, one can find flanges manufactured from cast iron, stainless steel, aluminum, copper, bronze, specialty metals such as chrome-​moly, nickel, monel, Inconel, super alloys, titanium, etc. The type of material used depends on the industrial application and piping systems. 1.18.4.11 ASME Code Approved Flange Materials 1. Materials used in the construction of bolted flange connections shall comply with the requirements given in ASME Code UG-​4 through UG-​14. 2. Flanges made from ferritic steel shall be full-annealed, normalized, normalized and tempered, or quenched and tempered, etc. as per requirements. 1.18.4.12 ASTM Standards for Flange Material Material range is varied from carbon steel to alloy steel, all grades of stainless steel, non-​ferrous metal (brass, Al-​brass, copper-​nickel), and a wide range of clad steel consisting of corrosion resistant materials. The key carbon steel and alloy steel material grades for flanges are hereunder: 1. Carbon steel is the primary material of carbon steel flange and end flange connectors, examples include ASTM 105, ASTM A350, and ASTM A694. 2. ASTM A36 is a low-​carbon steel favored for its welding properties and is suitable for machining, making it a common material for steel flanges. 3. Flange material based on ASTM Standards: a. ASTM A105 for forged carbon steel flange b. ASTM A350 for forged carbon steel, low alloy steel flange c. ASTM A182 for alloy steel flange and stainless steel flange d. ASTM A216 for casting carbon steel flanges e. ASTM A352, ASTM A217 f. ASTM A515 Gr 70, 65 for plates flange g. ASTM A516 Gr 70, 65 for carbon and low alloy steel plates flange h. ASTM A203, A204 i. ASTM A387 Gr 11, 22, 91 CL 2 for alloy steel plates flange j. ASTM A240 Grade 304/​L, 316/​L for stainless steel plate flange k. ASTM A536 covers ductile iron, which is used for backing flanges l. ASTM A694/​A694M-​16 –​Standard Specification for carbon and alloy steel forgings for pipe flanges, fittings, valves, and parts for high-​pressure transmission service. 1.18.4.13 Selection of Flange 1. First, identify by determining whether a flange has a flat face, threaded bore, lap joint, weld neck, socket weld, or tongue and groove. It is required that each flange be stamped with the manufacturer’s name, nominal pipe size, pressure classification, flange facing, bore, material

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designation, ring gasket number (when using a ring type joint flange facing) and heat number or code. 2. Face Type A flange face is the shape of the area where the gaskets are placed and the flanges are tightened to enable a seal. The strength and robustness of the seal is a factor of the flange face making it an important characteristic of the flange design that plays a key role in both the performance and service life of the flange. Different face types exist today and each with its own advantage; the most common ones are flat face, raised face, ring joint face, tongue and groove, and male and female [117]. 3. Flange Dimensions/​Sizes Flange dimensions are determined by the pipe size and the pressure class required for the application. Common considerations include outside diameter, flange thickness, bolt circle diameter, pipe size, nominal bore size and bolt holes. Some of the flange dimensions are shown in Figure 1.32. 4. Manufacturing Methods There are typically three methods used: a. Plate flanges are manufactured when metal slabs are sent through rollers until they reach the desired thickness. b. Cast flanges are manufactured by machining casting to the appropriate specifications. c. Forged flanges are made through a process of heating and forming the material, and then machining the part to the proper specifications. 5. Pressure-​Temperature rating for Flanges Flanges are often classified based on their ability to withstand temperatures and pressures. This is designated using a number and either the “#”, “lb”, or “class” suffix. These suffixes are interchangeable but will differ based on the region or vendor. Common classifications include 150#, 300#, 600#, 900#, 1500# and 2500#. Exact pressure and temperature tolerances will vary by materials used, flange design, and flange size. The only constant is that in all cases, pressure ratings decrease as temperatures rise. Pressure –​temperature

FIGURE 1.32  Some of the flange dimensions.

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Mechanical Design of Shell and Tube Heat Exchangers

ratings are the maximum allowable working gauge pressures at the temperatures for the applicable material and class designation. For intermediate temperatures, linear interpolation can be done.

1.18.5 Bolting Material The term “bolting material, as used in this specification, covers rolled, forged or strain hardened bars, bolts, screws, studs, and stud bolts. These alloy steel fasteners are used for bolting pressure vessels, valves, flanges, and fittings for low-​temperature service. Typical bolting materials list includes ASTM A193/​A193M, ASTM A307, ASTM A320/​A320M, ASTM A437/​A437M, ASTM A453/​ A453M, ASTM A489, ASTM A540/​A540M, ASTM A1014/​A1014M, ASTM A1082/​A1082M, ASTM F738M, ASTM F432, ASTM F468/​F468 M, ASTM F541, ASTM F593, ASTM F2281, ASTM F2882/​F2882M, ASTM F3042, ASTM F3042, etc. [118].

1.18.6 Gaskets The gasket is a key component in a flange joint assembly. Commonly used flange designs have a soft gasket squeezed between harder flange surfaces to form a leak-​free seal. The complex interaction between these sub-​components under bolting up and operating conditions determines the successful operation of the flange. A gasket fills the microscopic spaces and irregularities of the flange faces, and then it forms a seal that is designed to keep in liquids and gases. However, it is through the gasket that any leakage occurs and for this reason it receives most attention. A wide range of gasket materials and types are used, most of them to national standards which specify quality and dimensions. The various gasket materials are rubbers, elastomers (springy polymers), soft polymers covering a springy metal (e.g. PTFE covered stainless steel), and soft metal (copper or aluminum). For details on gasket selection refer to references[119–​131]. 1.18.6.1 Gasket Categories Gaskets can be segregated into three main categories: 1. non-​metallic (soft) 2. semi-​metallic 3. metallic. 1.18.6.2 Gasket Material Considerations A most important factor in selecting the proper gasket is selecting the suitable material that will be compatible with the application service. Mechanical factors are important in the design of the joint but the primary selection of a gasket material is influenced by different factors: 1. the temperature of the fluid or gas in the service 2. the pressure of the fluid or gas in the service 3. the corrosive characteristics of the fluid or gas to be contained 4. flange compatibility. 1.18.6.3 Effecting or Creating a Seal To achieve a successful seal, the gasket must be resilient enough to conform to any irregularities in the mating surfaces. The gasket must also be sufficiently tough (rugged) enough to resist extrusion, creep and blowout under the operating conditions and unexpected pressure/​temperature excursions. To avoid leaky joint, ensure:

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1. proper design of flanged joint 2. selection of the optimum gasket material 3. proper installation procedures. Correct installation of damage-​free gaskets and damage-​free flange faces is a requirement for a leak-​free flange connection. Keep the following questions in mind when specifying gaskets [124]: 1. S: Size of the gasket 2. T: Temperature range in which the gasket will operate 3. A: Application –​is it a heat exchanger? a valve? a pump? etc. 4. M: Media –​what will the gasket prevent from leaking 5. P: Pressure range in which the gasket will operate. 1.18.6.4 Gasket Seating There are two major factors to be considered with regard to gasket seating: 1. first, minimum design seating stress 2. second, the other major factor to take into consideration must be the surface finish of the gasket seating surface. 1.18.6.5 Gasket Types Based on Material Types of gaskets based on materials of construction are shown in Figure 1.33. 1. Metallic Gaskets 2. Semi-​Metallic Gaskets Commonly used semimetallic gaskets are spiral wound, metal jacketed, camprofile, and a variety of metal-​reinforced graphite gaskets. Semimetallic gaskets are designed for the widest range of operating conditions of temperature and pressure. Semimetallic gaskets are used on raised face, male-​ and-​female, and tongue-​and-​groove flanges. Examples include: a. spiral wound gaskets b. kammprofile or camprofile gaskets c. metal jacketed gaskets d. double jacketed metal gaskets. Flanged joints with a spiral wound gasket and Kammprofile gasket are shown in Figures 1.34 [97] and 1.35 [97]. 3. Non-​Metallic gaskets Non-​Metallic gaskets are usually composite sheet materials which are used with flat face and raised face flanges in low pressure class applications. Non-​metallic gaskets are manufactured from arimid fiber, glass fiber, elastomer, Teflon® (PTFE), graphite, etc. 1.18.6.6 Gasket Standards Gasket dimensions are covered in ASME B16.5 up to NPS 24 “and in ASME B16.47 Series A and B for NPS 26” and above. The following are three of the most commonly used international standards

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FIGURE 1.33  Types of gaskets based on materials of construction.

for gaskets specified in process plants designed to ASME B31 codes. They include dimensions, tolerances, fabrication, and marking of gaskets: 1. ASME B16.20-​2017, Metallic Gaskets for Pipe Flanges, Ring Joint, RJ, Spiral Wound, SW, Jacketed Gaskets, JA and Grooved Metal Gaskets with Covering Layers, GM 2. ASME B16.21-​2021, Nonmetallic Flat Gaskets for Pipe Flanges 3. API 6A-​2019, Specification for Wellhead and Christmas Tree Equipment.

1.18.7 Guidelines for Bolted Flanged Joint Assembly Procedure 1. Visual inspection prior to assembly Due care must be exercised to ensure that the flange sealing faces are clean, flat and free from damage. Bolts, nuts and washers must be clean and free from damage. 2. Lubrication and lubricants To minimize friction forces, the sliding faces of the bolts, nuts and washers must be coated with suitable lubricants prior to bolt-​up.

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FIGURE 1.34  Metallic and semimetallic gaskets.

3. Gasket installation issues Misalignment of piping and flanges should be minimized. Flange misalignment creates additional loads that the bolts have to overcome before the bolt loads can be applied to the gasket. 4. Gasket Installation Procedures Successfully sealing a flanged connection is dependent upon all elements of a well-​designed flange system working well together. Special tools are required for cleaning and tensioning the fasteners.

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FIGURE 1.35  Flanged join with a semimetallic gasket and Kammprofile gaskets.

5. Install and tighten bolts The sequence in which the bolts and nuts are tightened has a major influence on the distribution of the loads acting on the gasket (gasket seating stress). Tighten the nuts in multiple steps: Step 1. Tighten all nuts initially by hand (larger bolts may require a small hand wrench). Step 2. Torque each nut to approximately 30% of full torque. Step 3. Torque the nuts to approximately 60% of full torque. Step 4. Torque each nut to full torque, again using the cross bolt tightening pattern (large-​ diameter flanges may require additional tightening passes). Step 5. Apply at least one final full torque to all nuts in a clock-​wise direction until all torque is uniform (large-​diameter flanges may require additional tightening passes). 6. Retightening Do not retorque elastomer-​based, asbestos-​free gaskets after they have been exposed to elevated temperatures unless otherwise specified. Retorque fasteners exposed to aggressive thermal cycling. All retorquing should be performed at ambient temperature and atmospheric pressure.

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7. Storage of gaskets Gaskets must be stored such as to rule out damage due to external forces. Generally, gaskets or sheet gaskets must be stored flat to avoid tensions and warping. Positive identification of the gaskets must be ensured. The following ideal storage conditions are recommended [123, 132 and 132.1]: a. room temperature < 25 °C b. relative humidity 50 to 60 % c. darkened room (protection from direct solar radiation) or as specified by the manufacturer.

1.18.8 Flanged Joints in Shell and Tube Heat Exchanger Flanges are often employed to connect two sections by bolting them together so that the sections can be assembled and disassembled easily. In heat exchangers, the flange joints are used to connect together the following components: 1. channel and channel cover 2. heads or channels with the shell/​tubesheets 3. inlet and outlet nozzles with the pipes carrying the fluids 4. to close various openings such as manholes, peep holes, and their cover plates. The flanged joints play an important role from the standpoint of integrity and reliability of heat exchangers. Improper design of flanges causes leakage of heat exchanger fluids. Therefore, preventing the liquid or gas leaks is one of the most important considerations while designing flanged joints. Important sources for heat exchanger gasket selection for bolted flanged joints include these references [133–​135]. 1.18.8.1 Heat Exchanger Gaskets Heat exchanger gaskets come in several forms based on the type of heat exchanger or application. Most common heat exchanger gaskets are: 1. Double-​jacketed gaskets are gaskets, in which the gasket material is enclosed by an outer metal cover. 2. Solid gaskets from metal, stamped into a gasket shape. 3. Corrugated metal gaskets. These are metal gaskets, usually incorporating a filler material in the well of the corrugations, in which the seal is formed between the peaks of the corrugations and the mating flanges. 4. Kammprofile gaskets –​a metal gasket with grooved –​serrated faces, with or without resilient sealing layer on surfaces. These gaskets are used in areas where extreme temperatures and excessive movement due to thermal expansion exist. Various profiles of heat exchanger gaskets are shown in Figure 1.36 [98]. 1.18.8.2 Factors to Consider for Heat Exchanger Gaskets Choosing the correct gasket material depends on the operating conditions; such as, temperature, pressure, and what materials the gasket is in contact with. Heat exchangers can be difficult to seal since they require special gasketing attention for proper safety and maximum efficiency. Gasket material can also affect the performance of heat exchangers.

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FIGURE 1.36  Heat exchanger gaskets profiles [98].

1.18.8.3 Flange Types Found in Shell and Tube Exchangers Three types of flanges are found in shell and tube exchangers, namely: 1. girth flanges for the shell and channel barrels 2. internal flanges in the floating head exchanger to allow disassembly of the internals and removal of the tube bundle 3. nozzle flanges where the flange and gasket standards, the size and pressure rating will be set by the line specification. Figure 1.37 shows types of heat exchanger flanged joints and Figure 1.38 shows gasketed flanged joints. Details of gasket types and gasket materials are discussed in RCB-​6 and gasketed flanged joints refer E-​3.2.4 of TEMA [5]. 1.18.8.4 Girth Flange Girth flanges are used for connecting two segments of a pressure vessel. Girth flanges are found in shell and tube exchangers for the shell and channel barrels. Girth flanges can be made in carbon steel, low temperature steel, alloy steel, stainless steel, etc. A girth flange is shown in Figure 1.39 (also shown in Figure 1.40). 1.18.8.5 Collar Bolts in Shell and Tube Heat Exchanger Collar bolts are used for removable bundle heat exchangers to hold the bundle in place and remove the channel without interrupting or breaking the seal between the tubesheet and shell. TEMA standards recommend that the use of the collar bolts in the removable bundles with B-​Type bonnet

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FIGURE 1.37  Types of STHE flanged joints.

are added under part RCB-​11.8 as summarized below and as per the recommended configuration shown in Figure 1.40. This configuration shows shellside and tubeside girth flanges. When specified by the purchaser, collar stud shall be used on units with removable tube bundles. Collar stud are recommended for B-​type bonnets. The OD of the stationary tubesheet shall match the mating flange OD, and shall be through-​bolted. Every fourth stud in the bolt circle (with minimum of four) shall be a collar type I or type II as shown in Figure 1.40. Collar bolts are only used to maintain the gasket integrity and position when the channel is removed and torqued prior to pressurizing [136].

1.18.9 Design of Bolted Flange Joints The objectives in flange design are to ensure that the residual gasket stress levels and the pressure induced in the flange during bolt preload, as well as under operating conditions, do not exceed allowable stress values in the structural members. The earliest treatment of the problem of flange design to receive widespread recognition was that of Waters et al. [137], which gave the general basis for the design rules in the ASME Code. Design of flanged joints with ring-​type gaskets is carried out as per Appendix 2 of ASME Code Section VIII, Div. 1. Appendix S of the code gives general guidelines for bolting requirements of flanges. The code method for the design of integral-​type flange and ring flange is briefly described here. 1.18.9.1 Design Procedure The integrity and reliability of a bolted flanged joint depend to a large extent upon the correct choice of materials, dimensions, and loads on the gasket. The flange design procedure can be summarized as three separate elements: 1. gasket design 2. bolting design 3. flange design.

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FIGURE 1.38  Examples of STHE Gasketed flanged joints.

To start with, materials of construction of flange, bolting, and gasket, and gasket properties are chosen. For design calculation, the following information is essential [138]: 1. type of flange 2. shell or pipe dimensions 3. design pressure and temperature 4. flange material.

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FIGURE 1.39  Girth flange.

And additionally this information is very helpful –​governing code, media contained, shell or pipe material, bolt and gasket material, facing style, and corrosion allowance. 1.18.9.2 Gasket Parameters A leak-​proof joint with metal-​to-​metal surfaces without a gasket is difficult to achieve even with the use of accurately machined fine-​finish surfaces. Surface irregularities of only a few millionths of an inch will permit the escape of a fluid under pressure [52]. Being a semi-​plastic material, the gasket deforms under load, which in turn seals the minute surface irregularities and prevents leakage of the fluid. 1.18.9.3 Selection of Gasket Material A gasket is essentially an elastoplastic material that is softer than the flange faces. In the gasket seating condition, the entire bolt load is borne by the gasket. Hence, the gasket must be strong enough to withstand load due to bolting and operating conditions without crushing or extruding out. Therefore, soft materials like asbestos and organic fibers are precluded for high-​pressure applications. Also the gasket material shall withstand the operating temperature and exhibit corrosion resistance to the fluid contained in the pressure vessel [139]. 1.18.9.4 Gasket Materials Gaskets are made out of a myriad variety of materials. Good references are available from many gasket manufacturers for the selection of proper gaskets for the intended applications. Table 2-​5.1 of the ASME Code gives a list of many commonly used gasket materials.

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FIGURE 1.40  Collar bolts used in shell and tube heat exchanger.

1.18.9.5 Gasket Profile Commonly used gasket profiles include (1) single jacketed, (2) double jacketed, (3) double-​jacketed corrugated, (4) double shell jacket, (5) solid metal, (6) solid profile, (7) double-​corrugated jacket with non-​asbestos fill, and (8) two-​piece French type. Figure 1.41 shows various gasket crosssectional profiles and gasket with welded bars are shown in Figure 1.42 and Figure 1.43. The welded bar design eliminated one of the major problems of conventional gaskets which are cracks in the radius [140].

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FIGURE 1.41  Types of gasket profiles –​(a) double jacketed, (b) solid metal with flexible graphite covering, (c) solid metal profiled, (d) single jacketed overlap, selected metal with felt, (e) selected metal and Cerafelt®, (f) single jacketed, (g) double jacketed profile, (h) single jacketed profile, (i) solid metal, (j) double jacketed covered, (k) solid metallic corrugated, and (l) double shell jacket.

FIGURE 1.42  Some of multipass heat exchanger gasket profile.

1.18.9.6 Gasket Size The size restrictions for heat exchanger gaskets depend only on the available sizes of the materials. Heat exchanger gaskets are commonly made in diameters up to 120 in., with rib widths up to 31 mm (1.25 in.) and thicknesses up to 6.4 mm (1/​4 in.). When ordering gaskets for heat exchangers, specify these information, (1) model/​style number, shape, thickness, and material (metal or metal and filler) and (2) outside diameter, inside diameter, rib width, radius of rib, bolt circle radius, distance from centerline of gasket to centerline of ribs, radius around bolt, and size and number of bolt holes. The choice of the gasket material is often based upon the required gasket width. If the gasket is made too narrow, the unit pressure on it may be excessive, whereas if the gasket is made too wide, the bolt load will be unnecessarily high [52]. Standard metals include 304 Stainless, 316L Stainless, Inconel® 600, Inconel® 625, Incoloy® 800, Incoloy® 825, Hastelloy® C276, and Monel® 400, and sealing elements include flexible graphite, ePTFE, and combination graphite and ePTFE.

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FIGURE 1.43  Heat exchanger metallic gasket profile.

1.18.9.7  Gasket Factors The basic behavior of the gasket is defined by the gasket factor m and gasket or joint contact surface unit seating load y, which are tabulated in the ASME Code, Section VIII, Div. 1. 1.18.9.8  Gasket Factor, m This is the ratio of the residual stress on the gasket under operating pressure to that pressure. In other words, m =​(bolt load − hydrostatic end load)/​(gasket area × internal pressure). 1.18.9.9  Gasket or Joint Contact Surface Unit Seating Load, y This is the stress required to make the gasket surface take up the shape of the flange faces, or the gasket stress required to contain zero internal pressure. The factor y is usually expressed as a unit stress in pounds per square inch and is independent of the pressure in the vessel. Table 2-​5.1 of the ASME Code gives suggested design values of gasket factor m and minimum design seating stress y. 1.18.9.10  Gasket Dimensions A relationship for making a preliminary estimate of the proportions of the gasket may be derived as follows [30]:

Residual gasket force =​gasket seating force –​hydrostatic pressure force

(1.71)

The residual gasket force cannot be less than that required to prevent leakage of the internal fluid under operating pressure. This condition results in the following expression:

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do



di

=

y − pm (1.72) y − p ( m + 1)

where do is the gasket outside diameter m is the gasket factor y the minimum design seating stress. To determine m and y, refer to ASME Code [15]. The inside diameter of the gasket (in inches) is normally as follows: di = B + 0.01 (1.73)



where B equals the shell inside diameter for weld neck flange and shell outside diameter for ring flange. 1.18.9.11  Gasket Width and Diametral Location of Gasket Load Reaction The steps involved in arriving at the gasket width and diametral location of gasket load reaction are as follows: 1. Calculate the gasket width, N, given by N=



(d

o

− di 2

) (1.74)

2. Select the gasket width such that it is not less than the minimum specified width of the gasket as specified in Table 2-​5.2 of the ASME Code, and a representative value for one case is shown in Table 1.10. 3. Calculate the basic gasket width, bo, given by Table 2-​5.2 of the ASME Code. Location of gasket load reaction is shown in Figure 1.44

TABLE 1.10 Basic Gasket Seating Width Facing sketch (exaggerated)

(1a)

(1b)

Basic gasket seating width, bo Column–​I

Column–​II

N/​2

N/​2

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FIGURE 1.44  (a–​c) Location of gasket load reaction.

1.18.10 Bolting Design With the size and shape of the gasket established, next determine the bolting required. The bolting should be designed to maintain the required compression on the gasket with the internal pressure acting. Various design aspects of bolting are discussed next. Design rules limit the analysis to consideration of the flange moment that results from: 1. bolt load 2. gasket load 3. face pressure load 4. hydrostatic end load. 1.18.10.1 Determination of Bolt Loads The bolt loads, W, required under the following conditions could be considered: 1. gasket seating condition in the absence of internal pressure 2. operating conditions. 1.18.10.2 Gasket Seating Conditions The gasket seating conditions are the conditions existing when the gasket or joint contact surface is seated by applying an initial load on the bolts when assembling the joint, at atmospheric pressure and temperature. The minimum initial load considered to be adequate for proper gasket seating is a function of the gasket material and the effective or contact area to be seated. The minimum initial bolt load required for this purpose, Wm2, shall be determined using the following formula:

Wm2 = πbGy (1.75)

The need for providing sufficient bolt load to seat the gasket or joint contact surfaces in accordance with this formula will prevail on many low-​pressure designs and with facings and materials that require a high seating load, and the bolt load calculated for operating conditions is not sufficient to seat the joint. When formula (1.75) governs, flange dimensions will be a function of the bolting instead of internal pressure. As per code formulas, for flange pairs used to contain a tubesheet (both sides gasketed) for a floating head or a U-​tube type of heat exchanger, or for any other similar design, and where the

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95

flanges and/​or gaskets are not the same, Wm2 shall be the larger of the values obtained from formula (1.75) as individually calculated for each flange and gasket, and that value shall be used for both flanges. 1.18.10.3 Operating Conditions The operating conditions are the conditions required to resist the hydrostatic end force (H) of the design pressure, which tends to part the joint and to maintain on the gasket or joint contact surface sufficient compression force (Hp) to assure a tight joint at all operating conditions. The minimum load is a function of the design pressure, the gasket material, and the effective gasket area or the effective contact area to be kept tight under pressure. The required bolt load Wm1 for the operating condition is given by



Wm1 = H + H p =

π 2 G P + 2 πbGmP (1.76) 4

Various flange forces are shown schematically in Figure 1.45 and for more details on flanger design refer to [138]. 1.18.10.4 Bolt Area at the Root of the Threads The necessary bolt area at the root of the threads, Am, required for both the gasket seating and the operating conditions is the greater of the values Wm1/​Sh and Wm2/​Sa as given by the following expression:



Am = max

Wm1 Wm 2 , (1.77) Sb Sa

FIGURE 1.45  Dimensional data and flange forces –​weld neck flange.

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From the required bolt area, determine the minimum number of bolts required (generally in multiples of four) and observe the minimum sizes as recommended by TEMA. From the number of bolts chosen, find out the actual bolt area, Ab. In no case shall Ab be less than Am. At this point, the designer should sketch a tentative layout showing the desired location and size of gasket or contact surface, hub thickness, and diameter of bolts and bolt circle, and from these set the outside diameter of the flange. 1.18.10.5 Flange Bolt Load, W The bolt loads used in the flange design shall be the values obtained from these formulas: 1. For the gasket seating condition,

W=



(A

m

)

+ Ab Sa 2

(1.78)

2. For the operating condition,

W = Wm1 (1.79)

1.18.10.6 Pitch Circle Diameter In general, the pitch circle diameter for each particular size of bolt considered should be kept small, to keep the flange bending moment and flange outside diameter small [141]. 1.18.10.7 Minimum Bolt Size Small bolts should be avoided wherever possible, owing to the case with which they may be overstressed by torsion applied with a wrench. Bolts, studs, nuts, and washers must meet the code requirements. Appendix 2 of the code recommends not using bolts and studs smaller than 0.5 in. (12.7 mm). If bolts or studs are smaller than 0.5 in. (12.7 mm), alloy steel bolting material must be used. Precautions must be taken to avoid overstressing small-​diameter bolts. TEMA Standards give guidelines for minimum bolt size in RCB-​1. The minimum bolt size is 0.75 in. for R, 0.5 in. for C, and 0.625 in. for B class exchangers. 1.18.10.8 Minimum Recommended Bolt Spacing Bolts should be spaced far enough apart to permit the clearance necessary for socket wrenches and to insure a uniform compression on the gasket. Likewise, the bolt circle on hubbed or integral flanges should have sufficient diameter to permit a generous fillet between the back of the flange and hub. Waters et al. [137] recommends a bolt spacing of at least 2.25 times bolt diameters between centers to avoid high stress concentration. In the TEMA guidelines, the minimum chordal pitch between adjacent bolts and minimum recommended wrench and nut clearances may be read from TEMA Table D-​5. 1.18.10.9 Maximum Recommended Bolt Spacing The bolt spacing should not be so great as to result in an appreciable reduction in gasket pressure between bolts. Waters et al. [137] recommends a spacing of 3.5d (d is the nominal diameter of bolt) between bolt hole centers as a reasonable maximum. An empirical expression given by Taylor Forge and Pipe Works [138] expresses the maximum bolt spacing in the form

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Bmax = 2 d +

6Tf m + 0.5

(1.80)

where Bmax is the maximum bolt spacing for a tight joint (in.) d is the nominal bolt diameter (in.) Tf is the flange thickness (in.) m is the gasket factor, which is obtained from ASME Code Table 2-​5.1. This is included in TEMA, RCB-​11.2,2. 1.18.10.10 Load Concentration Factor As per TEMA RCB-​11.2.3, when the distance between bolt centerlines exceeds the recommended Bmax, the total flange moment determined shall be multiplied by a load concentration factor equal to [3]

B (1.81) Bmax

where B is the centerline-​to-​centerline bolt spacing. Note: To prevent overstressing of bolted flanged connections, the designer should, wherever possible, set the lengths of wrench to be used. 1.18.10.11 Relaxation of Bolt Stress at Elevated Temperature Under high temperatures, the leak tightness of bolted joints is compromised due to the loss of the bolt load as a result of creep of not only the gasket and bolt materials but also the flange material.

1.18.11 Flange Design After the gasket and bolting design, next determine the flange dimensions required to withstand the bolt load without exceeding the allowable stress for the flange material. The outside diameter of the flange must be large enough to seat the bolt with manufacturing tolerance. In addition to bolting data, some more details on flange dimensions can be read from TEMA Table D-​5. Since the flange design procedure is iterative in the case of integral weld neck flange and slip-​on flange, initially assume a flange thickness. In the case of the ring flange, a closed-​form solution for flange thickness is possible. The next step is to determine the moment arm of the various forces and reactions. 1.18.11.1 Flange Moments In the calculation of flange stress, the moment of a load acting on the flange is the product of the load and the moment arm. Various forces acting during the operating condition are the hydrostatic end force on area inside of the flange, HD, the pressure force on the flange face, HT, and the gasket load under operating conditions, HG (forces acting on the flanged joint and their moments are shown in Figure 1.45):

π B2 P (1.82) 4



HD =



H T = H − H D (1.83)



H G = W − H (1.84)

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TABLE 1.11 Moment Arms for Flange Loads Under Operating Conditions Type of Flange

hD

hT

hG

Integral flange Loose or ring flange Lap flange

R +​0.5g1 0.5(C − B) 0.5(C − B)

0.5(R +​g1 +​ hG) 0.5(hD +​ hG) 0.5(C − G)

0.5(C − G) 0.5(C − G) 0.5(C − G)

where W is bolt load, Wm1 or Wm2, whichever is greater. For the operating condition, the flange moment Mo is the sum of the three individual moments MD, MT, and MG. Determine the moment arms hD, hT, and hG for flange loads under operating conditions from Table 1.11 for the three types of flanges. Calculate MD (moment due to HD), MT (moment due to HT), and MG (moment due to HG) as given by

M D = H D hD (1.85)



M T = H T hT (1.86)



M G = H G hG (1.87)

and

M o = M D + M T + M G (1.88)

For the gasket seating condition, the total flange moment M o′ (ASME Code uses the term Mo for moment due to gasket seating condition also), which is opposed only by the gasket load W, is given by

M o′ =

W (C − G ) 2

(1.89)

1.18.11.2 Flange Thickness For a ring flange, the flange thickness tf required is the greater of the gasket seating condition or operating condition, given by



 M Y M ′Y  o o Tf = max  ,  (1.90) S B S B  tb ta

where Y is the shape constant, defined in Section 1.5.2, Step 5. For the weld neck integral flange, as mentioned earlier, flange thickness is calculated by iterative process until such time as the flange stress falls within allowable stress for the flange material. If not within the limit, increase the flange thickness and continue the steps mentioned earlier. The stresses induced in the flange shall be determined for both the operating condition and the gasket seating condition, whichever controls. The procedure to determine flange stresses is listed in the step-​by-​ step procedure given in the Appendix.

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1.18.11.3 Bolting Procedures In a flanged connection, all components must be correct to achieve a seal. The most common cause of leaky gasketed joints is improper installation procedures. To ensure leak-​free joint, the bolting procedures as recommended by Garlock Sealing Technologies, Palmyra, New York, are given as follows: 1. Place the gasket on the flange surface to be sealed. Bring the opposing flange into contact with the gasket. 2. Clean the bolts and lubricate them with a quality lubricant, such as an oil and graphite mixture. 3. Place the bolts into the bolt holes. 4. Finger-​tighten the nuts. 5. Follow the bolting sequence as shown in Figure 1.46. During the initial tightening sequence, do not tighten any bolts more than 30% of the recommended bolt stress. Doing so will cause cocking of the flange, and the gasket will be crushed. Upon reaching the recommended torque requirements, do a clockwise bolt-​to-​bolt torque check to make certain that the bolts have been stressed evenly. 6. Due to creep and stress relaxation, it is essential to prestress the bolts to ensure adequate stress load during operation.

1.18.12 Step-​by-​Step Procedure for Integral/​Loose/​Optional Flanges Design The ASME Code procedure for bolted flange joints for integral/​loose/​optional design is given here. The design procedure for reverse flange design is not covered. Certain steps may not be relevant for any one or two of these varieties. The procedure given here is similar to the ASME Code procedure and Taylor and Forge Company Bulletin on flange design [138]. The essential steps on bolted flange design are as follows: 1. selection of material for flange, gasket, bolts 2. calculation of load for gasket seating condition 3. calculation of load to withstand hydrostatic pressure known as operating condition 4. bolting design and number of bolts decided 5. thickness of flange estimation to withstand governing moment 6. calculation of stress in the flange and to verify that the calculated stresses are within code allowable stress.

FIGURE 1.46  Flange bolts tightening sequence.

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FIGURE 1.47  Pipe fittings.

1.18.13 Pipe Fittings Pipe fittings are a piping components that help in changing the direction of the flow, such as elbows and tees, changes the size of the pipe, such as reducers, and reducing tees, connect different components such as couplings and stop the flows such as caps, etc. There are different types of pipe fitting used in piping. There are several types of pipe fittings. Piping systems designed and fabricated to ASME B31.3 utilize forged, wrought and cast fittings. Fitting components commonly used include elbows, couplings, unions, reducers, o-​lets, tees, crosses, caps, blanks (blinds), and plugs. Some of the pipe fittings used in piping work are shown in Figure 1.47.

1.19 TAPER-​LOK® HEAT EXCHANGER CLOSURE It is common practice to design and build large-​diameter, high-​pressure heat exchangers with a welded diaphragm plate to seal the channel opening. While this solution does provide a seal for the closure, there are a few inherent problems. Generally, there are several thermal cycles encountered during the service of the heat exchanger. These cycles induce stress cracks due to thermal expansion

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at weld of the diaphragm plate, resulting in a leak. Leaks may also occur from less than perfect welds. Leaking welded diaphragms of high-​pressure heat exchangers in hydro treating and hydro processing service units are a serious problem for the petrochemical industry. Repairing the weld to stop leaks is very costly. The reoccurring maintenance process to cut and re-​weld is cumbersome as it usually requires a nitrogen purge, stress relief at the weld, and nondestructive weld testing. The patented Taper-​Lok seal (of Taper-​Lok Corporation, Houston, Texas) offers the most simplified and reliable seal for high pressure exchanger designs. Taper-​Lok seals are a proven technology for durability in sealing heat exchangers, hard to hold or lethal fluids, especially in high-​pressure or high-​temperature environments. Being “self energized” under operating conditions imparts an even tighter seal, as the internal pressure acts on the wedge-​shape geometry of the seal ring creating greater tension on the wide area sealing surfaces, effectively creating a leak-​free, zero fugitive emissions. Taper-​Lok closure is typically used for high pressure up to 20,000 psi rating applications. Like a Separated-​Head, the Taper-​ Lok connection uses independent tubeside and shellside flanges, bolting, and gasketing. The unique feature this closure offers is a reusable Taper-​Lok ring that acts as a self-​energizing seal connection. Construction: The Taper-​Lok seal is a metal-​to-​metal seal ring with dual converging tapered contact surfaces (Figure 1.48). The assembly consists of male flange, female flange, seal ring, studs, and nuts. The pressure-​energized seal ring seats into a pocket in the female flange and is wedged and seated by a male nose located on the male flange. Utilizing this concept, the exchanger channel cylinder would contain the female pocket, while the channel cover would have the male nose geometry (Figure 1.49). In the pre-​bolted condition, the Taper-​Lok seal ring lip stands off of the face of the channel. The converging seal surfaces are brought together like a wedge during bolt up. This wedging motion forces the seal ring onto the male nose and into the female pocket forcing a compressive hoop stress. Minimal bolt load is required to achieve the required contact stress on the seal surfaces. The converging angles of the seal ring create a wedge or “doorstop” effect. As the equipment internal pressure increases, the seal seats tighter into this sealing wedge. The seals can be made from the same material as the process equipment (exchanger channel and cover) to ensure that thermal expansions are consistent across all components. The effects of bimetallic (galvanic) corrosion are eliminated. A baked-​on molybdenum disulfide coating is applied to the seal to prevent galling [142]. For more details on Taper-​Lok applications, see Ref. [142]

1.19.1 Zero-​gap Flange In polymer processing systems, the gap between conventional flanges can cause turbulence and/​or stagnation in the flow of polymer materials that are transported through the pipelines. This, in turn, can cause the materials to solidify and build up in the pipeline at the connector. As the solidified

FIGURE 1.48  Taper-​Lok closure metal-​to-​metal seal ring with dual converging tapered contact surfaces. (Courtesy Taper-​Lok Corporation, Houston, TX.)

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FIGURE 1.49  Taper-​Lok closure for a high-​pressure heat exchanger. (Courtesy Taper-​Lok Corporation, Houston, TX.)

material builds up, the flow through the pipeline is restricted. The flanges must be periodically dismantled and the solidified polymer removed. This is an extremely expensive process, due to both the cost of dismantling/​cleaning the connectors and the cost associated with downtime of the system. Taper-​Lok zero-​gap connection consists of a through bore, as the male nose is landed onto the female flange to eliminate the gap between the two flanges (Figure 1.50). The resulting through bore significantly reduces problems with crevice corrosion and polymer solidification. The connector offers an advantage in critical services with crevice corrosion, such as alkylation units with hydrofluoric acid and sulfuric acid. Flanges in these highly corrosive environments suffer from significant corrosion specifically in the crevice between the two flanges.

1.20 EXPANSION JOINTS Differential longitudinal expansion between the shell and the tube bundle is a well known problem in fixed tubesheet heat exchanger design. The differential expansion occurs from two sources: (a) temperature and (b) pressure. Expansion joints are promising for accommodating differential longitudinal expansion between the shell and the tube bundle and pipelines carrying high-​temperature

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103

FIGURE 1.50  Taper-​Lok’s zero-​gap connection. (Courtesy Taper-​Lok Corporation, Houston, TX.)

fluids. Differences in the axial expansion of the shell and the tube bundle due to high mean metal temperature differentials warrant incorporation of expansion joints in heat exchangers. This is particularly true for fixed tubesheet exchangers. For fixed tubesheet exchangers, when the difference between shell and tube mean metal temperatures becomes large (greater than approximately 50°C for carbon steel), the tubesheet thickness and tube end loads become excessive [23].

1.20.1 Flexibility of Expansion Joints Expansion joints used as an integral part of heat exchangers or other pressure vessels shall be designed to provide flexibility for thermal expansion and also to function as a pressure-​retaining structural element. Hence, an expansion joint must compromise between two contradictory loading conditions [143]: (1) pressure-​retaining capacity and (2) flexibility to accommodate the differential thermal expansion. In many cases, the design for a particular application will involve a compromise of normally conflicting requirements. For example, to retain a high pressure, usually a thick-​walled bellows is required, whereas high flexibility and high fatigue life require a thin-​walled bellows.

1.20.2 Classification of Expansion Joints Expansion joints are broadly classified into two types: 1. formed head or flanged-​and-​flued head 2. bellows or formed membrane.

1.20.3 Formed Head or Flanged-​and-​Flued Head Formed head expansion joints, also called thick-​walled expansion joints, are characterized by higher spring rates (i.e. force required for unit deflection of a bellow) and usually a lower cycle life than thin-​walled bellows. The convolutions on the bellows may be U-​shaped or toroidal depending on the design conditions. Because of the higher wall thickness, this type of expansion joint is rugged and the most durable from the standpoint of abuse, but it has the disadvantage of very limited flexibility. Construction details of formed head expansion joints are discussed in Refs. [30, 35] and by Singh [144]. The location of a thick-​walled expansion joint in a shell and tube heat exchanger as per TEMA RCB 8.5 is shown in Figure 1.51 and a thick walled expansion joint is shown in Figure 1.52. Thick Wall Expansion Joint designs are identified by a thick ply and high convolution height. They are typically used in heat exchanger applications. Thick wall expansion bellows are usually are manufactured in single convolution with one or more thick ply’s usually starting from 0.125” (~3.2mm) thickness and up. Design of such bellows is covered by ASME Code Section VIII Div. 1,

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FIGURE 1.51  Criteria for the location of a flanged-flued or thick-walled expansion joint in a shell and tube heat exchanger.

FIGURE 1.52  Thick-walled expansion joint with a single convolution. (Courtesy of U.S. Bellows, Inc, Houston, TX, www.usbell​ows.com.)

Appendix CC. Heavy wall bellows are rugged, generally having a wall thickness equal or near to the shell wall. Because of material thickness, no cover or shroud is necessary. The disadvantage is that a lot of fluid can be trapped in these corrugations, and a drain is sometimes required. Those bellows are formed by welding flanged-​and-​flued plates together, thus creating one, two, or three U-​Shaped corrugations. ASME Code inspection and U-​2 stamp are required.

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FIGURE 1.53  Constructional details of thick-​walled expansion joint. Refer to ASME Code [12] for more details.

1.20.3.1 Construction Formed head expansion joints are made in two halves from flat annular plates. The outside edges of the plates are formed in one direction (flanged), and the inside edges are formed in the other direction (flued). The two halves are welded together and then welded into the heat exchanger shell Figure 1.53. Figure 1.53 shows two “flanged and flued heads” welded together. To know more design details refer to ASME Code [15]. A “flanged and flued head” consists of these elements: (a) an outer shell, (b) two outer tori (flange), (c) two annular plates, (d) two inner tori (flue), and (e) two

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inner shells with butt weld to the two portions of the heat exchanger shell. The inner and outer tori serve to mitigate the severity of geometric discontinuities between plate and shell type members. The annular plate contributes significantly to lowering the spring rate. The radii of the tori are seldom made less than three times the expansion joint thickness. The inner and outer tori serve to mitigate the stress concentration due to geometric discontinuities between the shell and the annular plates. The radii of the tori are seldom less than three times the expansion joint thickness [30]. The annular plate contributes to lower the spring rate of the joint. Where the flexibility requirement is rather feeble, annular plate and the outer shell are eliminated. Some of the configurations of the formed head expansion joints, i.e. the flexible element discussed in ASME Code includes the flanged-​only head, the flanged-​and-​flued head, the annular plate, or the flued-​only head, as appropriate to the expansion joint configuration. The flexible element may be fabricated from a single plate (without welds) or from multiple plates or shapes welded together. 1.20.3.2 Design of Formed Head Expansion Joints Applicable design codes include ASME B&PV Code Section VIII Div-​1, Appendix 5 ASME B&PV Code Section VIII Div-​1 Appendix 26 [15]. The most common application for thicker wall/​flanged and flued expansion joints is heat exchangers and large diameter piping systems. 1.20.3.3 Kopp and Sayre Model Kopp and Sayre [145] are generally credited for the first comprehensive work to determine analytically the axial stiffness of “flanged-​only” expansion joints. The method of analysis is based upon replacing the geometric configuration by an equivalent geometry. The outer torus (total length πr/​2) is replaced by an equivalent corner end. One-​half of the meridian of the outer torus is assigned to the annular plate and the other half to the outer shell. Figure 1.54 shows their idealized model. 1.20.3.4 Singh and Soler Model Singh and Soler [30] upgraded the Kopp and Sayre [145] solution by using classical plate and shell solutions in place of “beam” solutions. This model suffers from the limitation of considering one standard expansion joint geometry only. In practical applications, myriad variations of the standard flanged-​and-​flued configuration are employed. Hence, Singh [80] present generalized treatment of various forms of FSE geometry, while retaining its modeling assumptions, which were directly borrowed from Kopp and Sayre.

FIGURE 1.54  Kopp and Sayre model for flanged-​and-​flued expansion joint. (a) actual model and (b) idealized model. Note: Dimensions a and b are radii. (From [145] Kopp, S. and Sayre, M.F.)

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FIGURE 1.55  Singh and Soler model for flanged-​and-​flued expansion joint. (a) general model, (b) equivalent Kopp and Sayre model, and (c) final idealized model. (From [73] Singh, K.P.)

The basics of the Singh [144] model are discussed next without the details of stress analysis and determination of spring constant. The most general form of the FSE is shown in Figure 1.55. The FSE is one half of a standard expansion joint. Two flexible elements together make a standard expansion joint, such as the one shown in Figure 1.55. The FSE is reduced to two concentric shells of radii a and b connected by an annular plate. The Young’s moduli of the three elements can be different. The three elements of the FSE can be characterized as follows: 1. inner shell of thickness t1, equivalent Young’s modulus E1, equivalent length l1, and radius a 2. outer shell of thickness t2, equivalent Young’s modulus E2, length l1, and radius b 3. annular plate of thickness te, with inner and outer radii a and b, respectively. The equivalent lengths l1 and l2 warrant further comment. For the inner shell, l1 should be taken sufficiently long such that the edge effects (at the annular plate and shell junction) die out. Taking l1 =​2.5 (at)0.5 will suffice, unless the shell is shorter, in which case the actual length should be used. Similarly, the length l2 is actually the half-​length of the top shell in the expansion joint. Analysis for Axial Load and Internal Pressure The resultant loading and internal stress acting on the elements along their inner junction A (the interface between the main shell element and annular plate) and outer junction B (the interface between the annular plate element and the outer shell) are shown in Figure 1.56. The equilibrium of one-​half of the joint, in the axial direction, gives F2 in terms Fax [31]:

2 π F2 b = 2 π Fax a + π (b2 − a 2 ) ps (1.91)

Equation (1.91) can be written as

F2 = Fax

a b2 − a 2 + ps (1.92) 2b b

The load deflection relations for short shell and annular plate elements to assemble the stiffness equations are derived in their work. They are not repeated here. 1.20.3.5 Finite Element Analysis Cross section of a flanged-​and-​flued expansion joint for FEA is shown in Figure 1.57. Elements of a flanged-​and-​flued expansion joint for FEA is shown in Figure 1.58. Wolf and Mains[143] applied FEA and elements as per their model is shown in Figure 1.59. As per their model, the flanged and flued expansion joint is broken into its basic geometric components –​a short cylinder, a toroidal

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FIGURE 1.56  Loading and internal stress resultant acting on the flanged-​and-​flued expansion joint –​ (a) loading pattern and (b) forces and moments acting at the joints. (Adapted and modified from [30] Singh, K.P. and Soler, A.I.)

segment, a flat annular plate, another toroidal segment, and a semi-​infinite cylinder as shown in Figure 1.58. FEA of Flanged-​and-​Flued Expansion Joint –​the elements to be considered for FEA analysis are shown in Figure 1.59, and various possibilities to be considered to arrive at an appropriate/​optimum design solution include bellow without straight crown and cuffs, bellow with straight crown but without straight cuffs, and bellow with straight crown and straight cuffs as shown in Figure 1.60. 1.20.3.6 ASME Code and TEMA Procedure Rules for designing the formed head expansion joint exist in TEMA, ASME Code Section VIII, Div. 1, ANCC VSR1P, and AD Merkblatter, among others. HEDH [23] summarizes the salient features of flanged-​and-​flued type expansion joint design. ASME VIII-​1 mandatory Appendix 5 provides guidelines for the design of flanged-​and-​flued expansion joints. 1.20.3.7 TEMA Procedure TEMA Paragraph RCB-​8 FSE. This section shall apply to fixed tubesheet exchangers that require flexible elements to reduce shell and tube longitudinal stresses and/​or tube-​to-​tubesheet joint loads. Light gauge bellows type

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FIGURE 1.57  Cross section with elements and nodes of a flanged-and-flued expansion joint for FEA. (Courtesy of Pressure Vessel Engineering Ltd. Waterloo, Ontario, Canada.)

FIGURE 1.58  Elements of flanged-and-flued expansion joint for finite element analysis. (From [143] Wolf, L.J. and Mains, R.M.)

expansion joints within the scope of the Standards of the Expansion Joint Manufacturers Association (EJMA) or the code are not included within the scope of this section. Flanged-​only, flanged-​and-​ flued, flued-​only, and corner-​corner types of expansion joints, as shown in TEMA Figure RCB-​8.2, are examples of flexible shell element (FSE) combinations. The location FSE from one end of tubesheet shall be as per TEMA Figure RCB 8.5 and the same is shown in Figure 1.51. The designer shall consider the most adverse operating conditions specified by the purchaser. This section provides rules and guidelines for determining the spring rate and stresses using axisymmetric finite element model (FEA) methods for the FSEs or combinations of FSEs.

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FIGURE 1.59  Various combinations of bellow elements considered for a typical design of case of a flanged-​ and-​flued expansion joint by FEA –​(a) bellow profile, (b) bellow without straight crown and cuffs, (c) bellow with straight crown but without straight cuffs, and (d) bellow with straight crown and straight cuffs. (Courtesy of Pressure Vessel Engineering Ltd. Waterloo, Ontario, Canada.)

1.20.3.8 TEMA RCB –​8.1.1 Analysis Sequence The sequence of the analysis shall be as follows: 1. 2. 3. 4. 5.

select a geometry for the flexible element construct the FEA model apply the axial load for pring rate analysis perform FEA for displacement and determine spring rate using a tubesheet analysis method, determine the induced axial displacement as required for loading conditions 6. apply appropriate loads and displacements to the model 7. perform FEA to determine stresses 8. compute the membrane and bending tresses according to stres classifications 9. if necessary, perform a fatigue analysis 10. compare the flexible element stresses to the appropriate allowable stresses per the code for all applicable load conditions 11. repeat steps 1 through 10 as necessary. More than one analysis may be needed to evaluate the hydrotest and uncorroded conditions. 1.20.3.9 Design Method as per ASME Code Section VIII Div. 1 Mandatory Appendix 5 Flexible shell element expansion joints used as an integral part of heat exchangers or other pressure vessels shall be designed to provide flexibility for thermal expansions and also function as pressure-​ containing elements. The rules in this Appendix are intended to apply to typical single-​layer flexible shell element expansion joints and are limited to applications involving only axial deflections. Materials Materials for pressure-​retaining components shall conform to the requirements of UG-​4. For carbon and low alloy steels, minimum thickness exclusive of corrosion allowance shall be 0.125 in. (3 mm) for all pressure containing parts. The minimum thickness for high alloy steel shall conform to requirements of UG-​16.

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Fabrication 1. The flexible element is the flanged-​only head, the flanged-​and-​flued head, the annular plate, or the flued-​only head, as appropriate to the expansion joint configuration per Figure 5-​1 of ASME Code. The flexible element may be fabricated from a single plate (without welds) or from multiple plates or shapes welded together. 2. Nozzles, backing strips, clips, or other attachments shall not be located in highly stressed areas of the expansion joint, i.e. inner torus, annular plate, and outer torus. 1.20.3.10 Lack of Baffle Support (to the Tube-​Bundle) Due to a Flanged and Flued Expansion Joint For horizontal exchangers with a flanged and flued joint, an internal sleeve is purposely required for mechanical reasons. Internal liners or sleeves are a recommended accessory to extend the life of many metallic pipe expansion joints because of their ability to protect the convolutions from direct flow impingement, which can cause erosion and flow-​induced vibration. Where it is necessary to hold friction losses to a minimum and smooth flow is desired. Where flow velocities are high and could produce resonant vibration of the bellows. The tube bundle structure for internal sleeve fitted expansion joint is shown in Figure 1.60 [146].

1.20.4 Bellows or Formed Membrane A bellows-​type expansion joint containing one or more bellows is used to absorb dimensional changes, such as those caused by thermal expansion or contraction of a pipeline, duct, or vessel. A bellows is defined as a flexible element of an expansion joint, consisting of one or more convolutions and the end tangents, if any. The bellows-​type expansion joint is also known as a “thin-​walled expansion joint”. The name “thin-​walled expansion joint” is used to mean any form of expansion joint whose thickness is less than the thickness of the heat exchanger shell. Bellows-​type expansion joints are made primarily for piping applications. Figure 1.61 shows bellows-​type expansion joints for piping applications and is shown schematically in Figure 1.62 and Figure 1.63. Expansion joints consist of end fittings (on both sides of bellows element) such as flanges to allow connection to the adjacent piping or equipment. Flexibility of the bellows is achieved through bending of the convolution sidewalls, as well as flexing within their crest and root radii. In most cases, multiple convolutions are required to provide sufficient flexibility to accommodate the expected expansion and contraction of the piping system. 1.20.4.1 Construction Bellows are manufactured from relatively thin-​walled tubing to form a corrugated cylinder. The corrugations, commonly referred to as convolutions, add the structural reinforcement necessary for the thin-​wall material to contain system pressure. The bellows designer selects the thickness and convolution geometry to produce a bellows design that approaches, and often exceeds the capacity of the adjoining pipe to contain system pressure at the specified design temperature. Generally, the thin-​walled bellows formed from a thin plate whose thickness does not exceed 1/​ 8 in. (3.2 mm) of corrosion-​resistant material such as austenitic stainless steel or nickel-​base alloys or high-​alloy material using manufacturing processes like (1) disk or diaphragm forming (2) elastomeric forming (3) expansion forming (4) hydraulic forming and (5) pneumatic tube forming.

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FIGURE 1.60  Guidelines for construction of expansion joint of a STHE (a) Joint with flow sleeve – acceptable construction and (b and c) Unacceptable construction. (A) and unacceptable construction (B and C). Adopted from Ref. [146].

FIGURE 1.61  Bellows-​type expansion joint for piping applications. (Courtesy of U.S. Bellows, Inc, Houston, TX, www.usbell​ows.com.)

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FIGURE 1.62  Bellows-​type expansion joint attachment with a pipe (schematic).

FIGURE 1.63  Bellows-​type expansion joint attachment with a pipe, different configuration (schematic).

1.20.4.2 Applications Heat exchanger shell bellows can also be thin-​walled multi-​convolution bellows, ring reinforced for higher pressures. When used in heat exchangers, they are costly and delicate [143]. Design of thin-​ walled bellows is covered by ASME Code Section VIII Div. 1 mandatory Appendix 26. Thin-​walled bellows can be formed by expanding mandrel, roll forming, or hydro forming. An external cover or shroud is required for these bellows to protect against mechanical damage. Code inspection and U-​2 stamp are required. 1.20.4.3 Movement Capabilities Movement Capabilities of bellows type expansion joints are here under after EJMA [147]: 1. Axial Compression. Reduction of the bellows length due to piping expansion. 2. Angular Rotation. Bending about the longitudinal center line of the expansion joint. 3. Torsion. Twisting about the longitudinal axis of the expansion joint can reduce bellows life or cause expansion joint failure and should be avoided. Expansion joints should not be located at any point in a piping system that would impose torque to the expansion joint as a result of thermal change or settlement. 4. Axial Extension. Increase of the bellows length due to pipe contraction. 5. Lateral Offset. Transverse motion which is perpendicular to the plane of the pipe with the expansion joint ends remaining parallel.

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FIGURE 1.64  Movement capabilities of bellows, after EJMA, Ref. [147].

6. Squirm or buckling. An internally pressurized bellows behaves in a manner similar to that of a slender column under compressive load. The above movement capabilities are shown in Figure 1.64. 1.20.4.4 End Fittings Expansion joints will include appropriate end fittings such as flanges or butt-​weld ends that should match the dimensional requirements and materials of the adjoining pipe, or equipment. Small diameter compensators are available with threaded male ends, butt weld ends, or copper sweat ends. Threaded flanges may be added to the threaded end compensators if a flanged connection is preferred. Accessories. Flow liners are installed in the inlet bore of the expansion joint to protect the bellows from erosion damage due to an abrasive media or resonant vibration due to turbulent flow or velocities which exceed. 1.20.4.5 Flow Turbulence Expansion joints that are installed within ten pipe diameters downstream of elbows, tees, valves, or cyclonic devices should be considered to be subject to flow turbulence. The actual flow velocity should be multiplied by four to determine if a liner is required per the above guidelines. Actual or factored flow velocities should always be included with design data, particularly flow that exceeds 100 ft./​sec. which require heavy gauge liners.

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115

External covers are mounted at one end of the expansion joint, providing a protective shield that spans the length of the bellows. Covers prevent direct contact with the bellows, offering personnel protection, as well as protection to the bellows from physical damage such as falling objects, weld splatter, or arc strikes. Covers also provide a suitable base for external insulation to be added over an expansion joint. Some insulating materials, if wet, can leach chlorides or other substances that could damage a bellows. Tie rods eliminate pressure thrust and the need for main anchors required in an unrestrained piping system. Axial movement is prevented with the use of tie rods. Designs that have only two tie rods have the additional ability to accommodate angular rotation. Limit rods are similar, however they accommodate a specified axial capability. 1.20.4.6 Design of Bellows or Formed Membranes Bellows-​type expansion joint design shall conform to the requirements of EJMA Standards, the ANSI Piping Codes, and the ASME Codes as applicable. The design of structural attachment shall be in accordance with accepted methods, based on elastic theory. In addition to EJMA Standards, design analysis and rules are also included in Appendix BB, ASME Code Section VIII, Div. 1, for circular-​type bellows with single-​ply reinforced and non-​reinforced bellows with thickness less than 3.2 mm (0.125 in.). 1.20.4.7 EJMA Standards The Expansion Joint Manufacturers Association Inc. is a group of leading manufacturers of bellows-​ type expansion joints. This association issues standards on the design of bellows-​type expansion joints known as EJMA Standards [10]. The bellows-​type expansion joints are employed primarily in piping systems to absorb differential thermal expansion while containing the system under pressure. Design of expansion joints is discussed later. 1.20.4.8 Shapes and Cross Section The bellows are available both for circular shells and for rectangular shells. Rectangular shapes are used for surface condensers. 1.20.4.9 Bellows Materials The bellows material shall be specified and must be compatible with the fluid handled, the external environment, and the operating temperature. Particular consideration shall be given to possible corrosion attack. 1.20.4.10 Bellows Design: Circular Expansion Joints The design of bellows-​type expansion joints involves an evaluation of pressure-​retaining capacity, stress due to deflection, spring rate, fatigue life, and instability (squirm). The spring rate is a function of the dimensions of the bellows and the bellows material. The determination of an acceptable design further involves the bellows parameters such as material, diameter, thickness, number of convolutions, pitch, height, number of plies, method of reinforcement, manufacturing technique, and heat treatment. Specification sheet for circular bellows is shown in Figure 1.65. 1.20.4.11 Cycle Life In most applications, design movements cause the individual convolutions to deflect beyond their elastic limits, producing fatigue due to plastic deformation, or yielding. One movement cycle occurs each time the expansion joint deflects from the installed length, to the operating temperature length, and then back again to the original installation length.

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FIGURE 1.65  Specification sheet for circular-​type bellows type expansion joint. (Courtesy of U.S. Bellows, Inc. Houston, TX, www.usbell​ows.com.)

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In the majority of applications, total shutdowns are infrequent, therefore a bellows with a predicted cycle life of one or two thousand cycles is usually sufficient to provide reliable fatigue life for decades of normal service. High cycle life designs may be desirable for service applications that include frequent start up/​shut down cycles. The bellows designer considers such design variables as material type, wall thickness, the number of convolutions and their geometry to produce a reliable design for the intended service with a suitable cycle life expectancy. 1.20.4.12 ASME Code Section VIII Div. 1 Bellows Expansion Joints Article 26 The rules in this Appendix cover the minimum requirements for the design of bellows expansion joints used as an integral part of heat exchangers or other pressure vessels. These rules apply to single or multiple layer bellows expansion joints, unreinforced, reinforced, or toroidal subject to internal or external pressure and cyclic displacement. The bellows shall consist of single or multiple identically formed convolutions. They may be as formed (not heat treated), or annealed (heat treated). The suitability of an expansion joint for the specified design pressure, temperature, and axial displacement shall be determined by the methods described herein. 1.20.4.13 Limitations and Means to Improve the Operational Capability of Bellows Single-​ply bellows are used for low-​pressure applications. They are fragile and hence they are easily damaged; external covers to protect personnel against the hazards of bellows blowout due to failure are necessary. Drainable varieties are expensive, and external supports may be required to maintain alignment of the shell sections welded to the expansion joint. Additionally, single-​ply bellows are susceptible to instability. Since a bellows is a thin shell of revolution with repeated U-​shaped convolutions, there exist a large number of natural vibration modes. Basically, these vibration modes are classified into three types: axial accordion modes, lateral bending modes, and shell modes, among which the former two are easily excited [148]. Methods and improved designs to overcome various shortcomings are discussed in the EJMA Standards. The following measures are normally adopted by designers to improve the single-​ply expansion joint: 1. Use of external reinforcement. 2. Use of multi-​ply construction and thicker convolutions. 3. Pressure-​balanced expansion joints. 4. Flow sleeve inside the convolutions. 5. Universal expansion joint assemblies. a. External Reinforcement A combination of high internal pressure-​retaining capacity and large deflection can be achieved by external reinforcement of the U-​shaped bellows. The external reinforcement offers circumferential restraint and supports the root radius against collapse from internal pressure loading. Reinforcing rings are also added where instability or squirm of the bellows is a concern. Equalizing and reinforcing ring devices used on some expansion joints fitting snugly in the roots of the convolutions are shown in Figure 1.66. Equalizing rings are made of cast iron, carbon steel, stainless steel, or other suitable alloys and are approximately “T” shaped in cross section. Reinforcing rings are fabricated from tubing or solid round bars of carbon steel, stainless steel, or other suitable alloys. b. Multi-​Ply Construction and Thicker Convolutions The pressure-​retaining capacity of a bellows can be increased by the use of multi-​ply construction and by increasing the thickness of the convolutions; however, the latter significantly reduces the bellows flexibility.

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FIGURE 1.66  External reinforcement for bellows.

FIGURE 1.67  Inline pressure-​balanced expansion joint. (Courtesy of U.S. Bellows, Inc., Houston, TX, www. usbell​ows.com.)

c. Pressure-​Balanced Expansion Joints The pressure-​balanced expansion joints (Figure 1.67) are used for applications where pressure loading upon piping or equipment is considered excessive. The major advantage of the pressure-​ balanced expansion joint design is its ability to absorb externally imposed axial movement and/​or lateral deflection while restraining the pressure thrust by means of tie devices interconnecting the flow bellows with an opposed bellows also subjected to line pressure. Their design should be as per EJMA Standards.

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FIGURE 1.68  Flow sleeve to overcome FIV of bellows-​type expansion joint (schematic).

d. Flow Sleeve inside the Convolutions To overcome flow-​induced vibration, install a sleeve inside the convolutions as shown in Figure 1.68. Internal liners or baffles are a recommended accessory to extend the life of many metallic pipe expansion joints because of their ability to protect the convolutions from direct flow impingement, which can cause erosion and flow-​induced vibration. Internal liners should be used when internal flow conditions exceed the following criteria [149]: Internal sleeves shall be specified for all pipe expansion joints, regardless of the metal of the bellows in the following cases: i. where it is necessary to hold friction losses to a minimum and smooth flow is desired ii. where flow velocities are high and could produce resonant vibration of the bellows. For recommended flow velocities for installation sleeves, refer to [149]. When sleeve is installed, the bellow is thought to be two coaxial cylinders consisting of the convolutions and the sleeve, and the coupled vibrations through the fluid in the annular region may significantly affect the lateral vibration of the convolutions. e. Universal Expansion Joint Assemblies A universal expansion joint is one containing two bellows connected by a common connector for the purpose of absorbing any combination of the three basic movements, i.e. axial movements, lateral deflection, and angular rotation. Universal expansion joints are usually furnished with control rods to distribute the movement between the two bellows of the expansion joint and stabilize the common connector. Figure 1.69 shows expansion joints with flow sleeve inside the convolutions and universal expansion joint assembly.

1.21 OPENINGS AND NOZZLES 1.21.1 Openings Openings in pressure vessels and heat exchangers refer to the cuts made in shells, flat covers, channels, and heads for accommodating the nozzles and to provide manholes, peepholes, drains and vents, instrument connections, etc. Openings can be circular, elliptical, or oblong. Whenever an opening is made in the wall of the shell or in the head, the wall is weakened due to the discontinuity in the wall and decrease in cross-​sectional area perpendicular to the hoop stress direction. To keep the local stresses within the permissible limits, reinforcements to the openings are made.

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FIGURE 1.69  Expansion joints –​(a) with an internal sleeve and (b) universal expansion joint. (Courtesy of KE-​Burgmann Expansion Joint Systems, Santee, CA.)

1.21.2 Reinforcement Pad The design of reinforcement is covered in UG-​36 to UG-​42 of the ASME Code by an area-​to-​area method. Reinforced pads whenever required as per drawings/​codes shall be of the same material or equivalent to the component to which they are welded. Even though a reinforcement pad can be applied on either the outside or the inside of the shell, it is the common practice to provide it at the outside due to easiness, and no need to meet the requirement of compatibility of the pad material with the process fluids, except that the pad should be resistant to general corrosion and of weldable equality. The factors to be kept in mind while considering the reinforcement pad are the following: 1. the pad should match the contour of the component to which it should be attached 2. provide a telltale hole to release the entrapped gases during welding and to check the soundness of the welding.

1.21.3 Reinforced Pad and Air-​Soap Solution Testing As per ASME Code UW-​15, reinforcing plates and saddles of nozzles attached to the outside vessel shall be provided with at least one telltale tapped hole (maximum size NPS 1/​4 tap) for compressed air-​soap solution test for tightness of welds that seal off the inside of the vessel. Air pressure of 1.25 kg/​cm2 is suggested for these tests or as per applicable code. Higher test pressures are not recommended because the soap bubbles have a chance to blow off. Telltale holes in the reinforcing pads may be left open or plugged when the vessel is in service.

1.22 NOZZLES Nozzles are incorporated to convey process fluids into the heat exchanger and out of it. Their sizes are arrived after calculating permissible fluid velocity limited by erosion-​corrosion, impingement attack, pressure drop, etc. Minimum wall thickness is arrived at using the cylindrical shell formula. Good nozzle design involves better distribution of process fluids, ability to withstand operating load and the other loads, and should provide easy accessibility to connect or disconnect the pipes. A well-​ designed nozzle should have a very low-​pressure drop. Shell nozzles shall not protrude beyond the

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FIGURE 1.70  Some pressure vessel nozzle types.

shell inside contour if they interfere with tube bundle insertion or removal. A pressure vessel nozzle consists of three parts: 1. a flange connection (for flanged connection with pipe) 2. nozzle neck part and 3. reinforcement (in case required). Figure 1.70 shows some pressure vessel nozzle types.

1.22.1 Types of Nozzles Set-​in Nozzle. Nozzle is projected inside the vessel surface. The pressure vessel opening diameter in the shell/​head coincides with the outer diameter of the neck. Set on Nozzle. Nozzle is seated on the vessel. The diameter of the pressure vessel opening in the shell or head coincides with the ID (inner diameter) of the nozzle neck. A set-​in nozzle is shown in Figure 1.71 Nozzles with added reinforcement. Additional reinforcing plate is added to withstand external nozzle loading. Self-​reinforced nozzles. Nozzle thickness itself is sufficient to withstand external nozzle loads and so an additional RF pad is not provided. Preferred for fatigue or cyclic loading. They are of two types –​nozzles with straight hub and nozzles with variable hub. Also, the nozzles in a pressure vessel can be placed perpendicular or in an angular position with respect to the shell axis. It can be intersecting the vessel axis or offset. Nozzles with reinforcement is shown in Figure 1.72. Nozzle

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FIGURE 1.71  Some pressure vessel nozzle types- (a and b) Set on and (c). Set in.

openings can be circular, elliptical, and oblong. Nozzles are connected by weldment to the shell by butt welding, through type and reinforcing pads. In addition to the welded-​type connections, brazed, threaded, studded, and expanded connections are also employed.

1.22.2 Design of Pressure Vessel Nozzles Nozzle loading. 1. With an increase in flange rating, the nozzle load-​carrying capability increases. 2. With an increase in nozzle size the load-​carrying capability increases. The capability of a vessel to withstand the external nozzle loading is decided based on WRC or FEA calculations. WRC calculation can be done using Caesar II, PV-​elite, Start-​Prof, or Code-​cal software. FEA calculation can be done using Nozzle-​pro, Nozzle-​FEM, or Ansys software. Most nozzles are sized to match the next schedule piping. The openings in the barrels require reinforcement in accordance with the relevant pressure vessel code. For the design of pressure vessel nozzles, various codes and standards are available including ASME Code Section VIII. Nozzle design basically means three parts: 1. deciding the nozzle size or nozzle opening 2. designing and Selecting Nozzle thickness and 3. calculating the reinforcement requirement based on pressure and external loads. The size of the nozzle opening is normally decided by the process team depending on the volume of fluid input and output in the pressure vessel. Once the nozzle opening size is fixed the nozzle thickness requirement is calculated based on the design pressure of the contained/​flowing medium.

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FIGURE 1.72  Nozzle opening reinforcement –​(a) thick forged-​ blank nozzle, (b) compensation by reinforcement pad, and (c) thick-​walled nozzle.

The calculated thickness is normally converted into standard nozzle thickness as per standard pipe thickness available following ASME B36.10 or ASME B36.19 standards. In the next step, the requirement of nozzle reinforcement is which is normally performed following the area compensation method. The design of pressure vessel nozzles is done as per ASME Section VIII, Div. 1 UG 36 to UG 45.

1.22.3 Nozzle Openings Reinforcement Nozzle openings are reinforced by: 1. using thick forged-​blank nozzle (Figure 1.72a) 2. opening compensated by reinforcement pad (Figure 1.72b) 3. welding of thick-​walled nozzle pipe (Figure 1.72c). Nozzles design shall be as code. Design aspects of various nozzle reinforcements are discussed by Schoessow et al. [150]. Requirements of reinforcement for openings in shells and formed heads are covered in UG-​37 to UG-​42, UG-​82, and attachment welds in UW-​15, and exemption from reinforcement in UG-​36. As far as possible, nozzle design should avoid the separate reinforcement plate being welded to the shell, because the weld metal cracks at the interface between the reinforcement pad plate and the shell plate pose additional problems. Nozzles are forged from hot-​rolled bar, hot-​rolled billet, or forged billet. All raw materials used are electric furnace, vacuum degassed and fully killed steel with stringent quality specifications. Heat treatment, machining, drilling, and contouring are standard controlled processes.

1.22.4 Standards for Nozzle Design Nozzle design is carried out as per code and /​or WRC guidelines. Considerations in nozzle design should include the inspectablity of the nozzle-​ to-​ pipe and nozzle-​ to-​ vessel weld inspection . WRC Guidelines for Design of Pressure Vessel Nozzles Stresses on pressure vessel nozzles and attachments are checked using the Welding Research Council Bulletin 107 and WRC 537.

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FIGURE 1.73  Force/​moment convention used for a cylindrical vessel and for a spherical vessel as per WRC 107/​537.

1. WRC 107–​1965 Local Stresses in spherical and cylindrical shells due to external loadings. It provides an analytical tool to evaluate the vessel stresses in the immediate vicinity of a nozzle. 2. WRC 537-​2022: Precision Equations and Enhanced Diagrams for Local Stresses in Spherical and Cylindrical Shells Due to External Loadings for implementation of WRC Bulletin 107. Note: Welding Research Council, Inc. will no longer deliver WRC Bulletin 107 when requested for purchase. WRC Bulletin 537 provides the same content in a more useful and clearer format that also facilitates computer implementation. 1.22.4.1 WRC 107/​537 Analysis The following load sets are applicable: 1. sustained-​primary loads, typically weight +​pressure +​forces 2. expansion-​secondary thermal expansion loads 3. occasional-​irregularly occurring loads such as wind loads, seismic loads, and water hammer. WRC 107/​537 force/​moment convention used for a cylindrical vessel and for a spherical vessel is shown in Figure 1.73.

1.23 SUPPORTS Pressure vessels are normally supported by one of the following methods such as skirts, support legs, support lugs, ring girders and saddles and the supporting members shall be attached to the

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vessel wall. Skirts are typically used for vertical vessels because they are the most economical. Leg-​ supported vessels are normally lightweight, and the legs provide easy access to the bottom of the vessel. A lug support system depends on the stiffness of the shell and its ability to adequately resist the bending moments. Horizontal vessels are normally supported by saddles. Stiffening rings may be required if the shell is too thin to transfer the loads to the saddles. Thermal expansion is typically accommodated by having one end of the vessel on a sliding support.

1.23.1 Design Basics The design of supports shall normally conform to good engineering practice. The supports should be designed to resist internal and external pressures and accommodate the self-​weight of the unit and contents, including the flooded weight during hydrostatic test. Based on their installation, the supports differ for horizontal and vertical installation. The selection of the type of support for a pressure vessel is dependent on parameters such as the elevation of the vessel from the ground level, the materials of construction, and the operating temperature [52].

1.23.2 Design Loads While designing the supports of a vessel, care should be taken to include all the external loads likely to be imposed on it. Such external loads include (1) wind loads, (2) loads due to connected piping, (3) superimposed loads, (4) shock loads due to surging or hydraulic hammer, and (5) seismic vibration. As per TEMA Standards, supports for a removable tube bundle heat exchanger should be designed to withstand a pulling force equal to 1.5 times the weight of the tube bundle, and when additional loads and forces from external nozzle loadings, wind loads, and seismic forces are assumed for the purposes of supports design, the combinations need not be assumed to occur simultaneously. Care should be taken that the thermal stresses in external supports do not exceed those permitted by the code.

1.23.3 Horizontal Vessel Supports Horizontal vessels are subject to longitudinal bending moments and local shear forces due to the weight of their contents. They are generally supported by three types of supports: (1) saddle supports, (2) ring supports, and (3) leg supports. Saddle support is used most commonly for heat exchangers. It is shown in Figure 1.74. Whenever possible, horizontal vessels shall be supported by two supports only, with holes for anchor bolts. If more than two supports are used, the distribution of the reaction is affected by difference in support level, the straightness and local roundness of the vessel, and the relative stiffness of different parts of the vessel against local deflection [151]. 1.23.3.1 Saddle Supports Saddle supports may be used for vessels whose wall is not too thin. Horizontal vessels when supported on saddle supports such as in Figure 1.75 behave as beams, and with these kinds of supports, the maximum longitudinal bending stresses occur at the supports and at the mid span of the vessel. Hence, the location of supports from the mid span of the vessel or head tangent is critical to minimize the bending stresses at the supports. Consideration shall be given to ensure that the saddles should be preferably extended over at least 120° of the circumference of the vessel. The limitation, which is imposed by most codes of practice, is an empirical one based on experience with large vessels [151]. While designing, ensure that saddle supports can withstand a tube bundle pulling force equal to the weight of the removable bundle.

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FIGURE 1.74  Examples of horizontal supports of pressure vessels –​(a) saddle support, (b) ring support, and (c) lug support.

FIGURE 1.75  A vessel on horizontal saddle support –​(a) schematic and (b) details of the saddle support.

1.23.3.2 Zick Stress Zick [152] developed a method for analyzing supports for the horizontal cylindrical shells. The analysis gives a detailed derivation of the equations for longitudinal bending stresses at the supports and at the mid span. These stresses are named as Zick stress. Zick’s method is discussed in detail in Refs. [30, 32, 53], among others.

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1.23.3.3 Ring Supports Ring supports as shown in Figure 1.74b are preferred to saddle supports for large thin-​walled vessels, vacuum vessels, and in the case of saddles located away from the head. Ring supports are also preferred when supporting a vessel at more than two cross sections becomes inevitable. The welds attaching ring supports should have a minimum leg length equal to the thickness of the thinner of the two parts being joined together. 1.23.3.4 Leg Supports Leg supports as shown in Figure 1.1 are usually permitted for small vessels by the usual code practice because of the severe local stresses that can be set up at the connection of the support to the vessel wall.

1.23.4 Vertical Vessels Supports for the vertical units may be skirt supports, ring supports, and lugs (columns). Some of these vertical supports are shown in Figure 1.76. 1.23.4.1 Skirt Supports Skirt supports (Figure 1.76a) are recommended for large/​tall vertical vessels. Skirt supports are preferred because they do not lead to concentrated local loads on the shell, offer less restraint against differential thermal expansion, and reduce the effect of discontinuity stresses at the junction of the cylindrical shell and the bottom [151]. The skirt supports shall be provided with at least one opening for inspection unless there is a provision to examine the bottom of the vessel accessible from below.

FIGURE 1.76  Examples of vertical supports of pressure vessels –​(a) leg support/​skirt supports, (b) lug support, and (c) ring support.

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1.23.4.2 Lug Supports Vertical vessels may be supported by a number of posts or lugs as shown in Figure 1.76b. Lug supports are ideal for thick-​walled vessels. For thin-​walled vessels, it is not convenient unless proper reinforcements are used or many lugs are welded. Brackets or lugs offer many advantages over other types of vessels [30]: They are inexpensive, can absorb diametrical expansion by sliding over greased or bronze plates, and requirements of welding are minimal.

1.23.5 Procedure for Support Design 1.23.5.1 TEMA Rules for Supports Design (G-​7.1) TEMA rules for supports for horizontal units are listed in G-​7.11 and for vertical units in G-​7.12. For calculating resulting stresses due to the saddle supports, references are suggested under TEMA G-​7.13. The “Recommended Good Practice” section of TEMA Standards provides additional information on support design. 1.23.5.2 ASME Code ASME Code requirements for supports design are covered in UG-​54. Appendix G contains suggested good practices for support design.

1.24 LIFTING DEVICES AND ATTACHMENTS TEMA rules for the design of lifting devices are given in G-​7.2. ASME Code rules for the construction of lifting devices and fitting attachments are covered in UG-​82. Some of the TEMA Standards for design of lifting devices are as follows: 1. Channels, bonnets, and covers that weigh more than 60 lb are to be provided with lifting lugs. 2. Lifting devices are designed to lift the component to which they are directly attached. When lifting lugs are required by the purchaser to lift the complete unit, the device must be adequately designed. 3. The design load shall incorporate an appropriate impact factor. 4. Lifting devices and attachments shall be formed and fitted to conform to the curvature of the component surface to which they are attached. 5. The lifting lugs on shell shall be designed to take the weight of complete unit with full of content.

REFERENCES 1a 1b 2 3

www.palagr​oup.com/​com​mon-​types-​of-​press​ure-​vess​els/​ www.iqsdi​rect​ory.com/​artic​les/​press​ure-​ves​sel.html www.tuv​sud.com/​en-​us/​servi​ces/​ins​pect​ion/​arise-​inc/​fired-​press​ure-​vess​els Hirschfeld, F., Codes, standards and certificate of authorization program: Part 1 –​Establishing safety standards, Mech. Eng., January, 33–​39 (1979); (b) Hirschfeld, F., Codes, standards and certificate of authorization program: Part 2 –​Policies, programs and organization, Mech. Eng., February, 31–​37 (1979). 4 Statement of the Council on Codes and Standards of the American Society of Mechanical Engineers on the Federal Role in International Standards, Trans. ASME J. Pressure Vessel Technol., 112, 425–​426 (1990). 5 Standards of the Tubular Exchanger Manufacturers Association (TEMA), 10th edn., 2019, Tubular Exchanger Manufacturers Association, Inc., Tarrytown, NY. 6 Standards for Steam Surface Condensers, 12th Edition, 2017, HEI, Cleveland, Ohio 7 Standards for Closed Feedwater Heaters, 9th Edition, 2015, HEI, Cleveland, Ohio

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8 Standards for Shell and Tube Exchangers, 5th edn., 2013, Heat Exchange Institute, Cleveland, OH. 9 API 660, Shell and Tube Heat Exchangers, 9th Edition, 2015, American Petroleum Institute, Washington, DC. 10 EJMA, Standards of the Expansion Joint Manufacturers Association, 10th edn., Expansion Joint Manufacturers Association, Tarrytown, NY, 2017. 11 Codes, standards and the ASME: Part 3 –​Standards, safety standards, Mech. Eng., August, 24–​25 (1972). 12 ASME Boiler and Pressure Vessel Code, Section VIII, Division 1 –​Pressure Vessels, American Society of Mechanical Engineers, New York, 2021. 13 ASME Boiler and Pressure Vessel Code, Section VIII, Division 2, Pressure Vessels –​Alternative Rules, American Society of Mechanical Engineers, New York, 2021. 14 ASME Boiler and Pressure Vessel Code, Section VIII, Division 3, Pressure Vessels –​Alternative Rules for High Pressure Vessels, American Society of Mechanical Engineers, New York, 2021. 15 ASME Boiler and Pressure Vessel Code, Section II Parts A, B, C and D for Materials specification, 2021 16 ASME Boiler and Pressure Vessel Code, Section V Nondestructive Examination, 2021. 17 ASME Boiler and Pressure Vessel Code, Section IX Welding, Brazing, and Fusing Qualifications, 2021. 18 Cepluch, R. J., The ASME Boiler and Pressure Vessel Code Committee –​Challenges: Past and future, Trans. ASME J. Pressure Vessel Technol., 112, 319–​322 (1990). 19 Blackall, F. S., Jr., ASME standards save lives and dollars, Mech. Eng., December, 979–​981 (1953). 20 Yokell, S., A Working Guide to Shell and Tube Heat Exchangers, McGraw-​Hill, New York, 1990. 21 Nichols, R. W. ed., Pressure Vessel Codes and Standards: Developments in Pressure Vessel Technology—​5, Elsevier Applied Science, London, U.K., 1987. 22 (a) Codes, standards and the ASME: Part 1 –​Codes, Mech. Eng., June, 24–​25 (1972); (b) Codes, standards and the ASME: Part 2 –​ASME boiler and pressure vessel codes, Mech. Eng., July, 16–​18 (1972); (c) Codes, standards and the ASME: Part 4—​International Standardization Committee, Mech. Eng., September, 20–​24 (1972). 23 Schlunder, E. U. (editor-​in-​chief), Mechanical design codes, in Heat Exchanger Design Handbook, Vol. 4, Hemisphere, Washington, DC, 1983, Section 4.1.6. 24 Mase, J. R. and Smolen, A. M., ASME pressure vessel code: Which division to choose, Chem. Eng., January, 133–​136 (1982). 25 Farr, J. R., The ASME boiler and pressure vessel code: Section VIII—​Pressure vessels, in Pressure Vessel Codes and Standards, Developments in Pressure Vessel Technology –​5 (R. W. Nichols, ed.), Elsevier Applied Science, London, U.K., 1987, pp. 35–​58. 26 Farr, J. R., The ASME boiler and pressure vessel code: Overview, in Pressure Vessel Codes and Standards, Developments in Pressure Vessel Technology –​5 (R. W. Nichols, ed.), Elsevier Applied Science, London, U.K., 1987, pp. 1–​34. 27 https://​lets​fab.in/​dif​fere​nce-​betw​een-​asme-​sect​ion-​viii-​div1-​div2-​and-​div3/​ 28 https://​whati​spip​ing.com/​dif​fere​nce-​betw​een-​asme-​sec-​viii-​div-​1-​and-​div-​2/​ 29 Javier Tirenti, Pressure vessels online course, Part I: Pressure Vessel Design, Shell, Head, Nozzle and Basic Flange, study notes, pp. 1–​19. 30 Singh, K. P. and Soler, A. I., Mechanical Design of Heat Exchangers and Pressure Vessel Components, Arcturus, Cherry Hill, NJ, 1984. 31 Moss, D. R., Pressure Vessel Design Manual, Gulf Publishing Company, Book Division, Houston, TX. 32 Bednar, H. H., Pressure Vessel Design Handbook, Von Nostrand Reinhold, New York, 1981. 33 The Design-​by-​Analysis Manual, European Commission Joint Research Centre, EPERC and European Pressure Equipment Research Council, 1999, EUR 19020 EN, European Commission, Directorate-​ General, Joint Research Centre, Petten, The Netherlands. 34 Livingston, E., Scavuzzo, R. J. “Pressure Vessels” The Engineering Handbook. Ed. Richard C. Dorf Boca Raton: CRC Press LLC, 2000 35 Singh, K. P., Mechanical design of tubular heat exchangers –​An appraisal of the state-​of-​the-​art, in Heat Transfer Equipment Design (R. K. Shah, C. Subbarao, and R. M. Mashelekar, eds.), Hemisphere, Washington, DC, 1988, pp. 71–​87. 36 www.wer​mac.org/​equipm​ent/​pre​ssur​eves​sel.html#gsc.tab=​0 37 www.pipro​cess​inst​rume​ntat​ion.com/​inst​rume​ntat​ion/​press​ure-​meas​urem​ent/​arti​cle/​15563​207/​info​ grap​hic-​compar​ing-​des​ign-​by-​analy​sis-​ver​sus-​des​ign-​by-​rule-​for-​press​ure-​vess​els

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130 38 39 40 41 42 43 44 45 46

47 48 49 50 51 52 53

54

55 56 57 58 59

60

61 62 63 64 65 66 67 68

Mechanical Design of Shell and Tube Heat Exchangers Pressure Vessels from American Alloy, Norristown, PA. americanalloyfab.com, pp. 1–​12 www.allw​eld.ca/​blog/​import​ant-​aspe​cts-​of-​press​ure-​ves​sel-​ins​pect​ion-​and-​test​ing/​ www.Pveng.Com/​Home/​Pipe-​Str​ess-​Analy​sis/​Types-​Of-​Stres​ses-​In-​Pip​ing-​Syst​ems/​ Arturs Kalnins, Pressure Vessels and Piping Systems–​Stress Classification, Encyclopedia of Life Support Systems (EOLSS). Peter Smith, Volume One, The Fundamentals of Piping Design Drafting and Design Methods for Process Applications 2007, Process Piping Design Handbook, Gulf Publishing Company, Houston, Texas. sales@Pressure Vessels & their Components americanalloyfab.com americanalloyfab.com www.resea​rchg​ate.net/​publ​icat​ion/​329636857_​Design_​of_​Vertical_​Press​ure_​Vess​ll_​U​sing​_​ASM​ E_​Co​des https://​ameri​cana​lloy​fab.com/​press​ure-​vess​els/​ Engineers’ Guide to Pressure Equipment –​The Pocket Reference Clifford Matthews BSc, CEng, MBA, FIMechE Professional Engineering Publishing Limited, London and Bury St Edmunds, UK First published 2001. The Publishing Editor, Professional Engineering Publishing Limited, Northgate Avenue, Bury St Edmunds, Suffolk, IP32 6BW, UK. ISBN 1 86058 298 2 © 2001 Clifford Matthews www.suns​trea​mglo​bal.com/​fact​ors-​to-​consi​der-​while-​design​ing-​a-​press​ure-​ves​sel/​ Abdolreza Toudehdehghan and Tan Wai Hong 2019 IOP Conf. Ser.: Mater. Sci. Eng. 469 012009 www.ther​mope​dia.com/​cont​ent/​1058/​ www.total​mate​ria.com/​page.aspx?ID=​Check​Arti​cle&site=​kts&NM=​373 Dr. S.D. Allen Iske, Accident Prevention Manual for Business & Industry, Engineering & Technology, 13th edition, National Safety Council. Brownell, L. E. and Young, E. H., Process Equipment Design, John Wiley & Sons, New York, 1968. (a) Escoe, K. A., Mechanical Design of Process Systems, Vol. 2 –​Shell and Tube Heat Exchangers, Rotating Equipment, Bins, Silos, Stacks, Gulf Publishing Company, Book Division, Houston, TX, 1995; (b) Escoe, K. A., Mechanical Design of Process Systems, Vol. 1, The Engineering Mechanics of Pressure Vessels, Gulf Publishing Company, Book Division, Houston, TX, p. 19xx, Chapter 4 Pressure Vessel Design Guides & Procedures, Authors/​Compilers Committee, 2011, G. Ghanbari M.A. Liaghat A. Sadeghian A. Mahootchi I. Sokouti R. Heidary M.H. Mohammadi A. Ansarifard M. Seraj, pp. 1–​429. Bednar, H. H., Pressure Vessel Design Handbook, Von Nostrand Reinhold, New York, 1981. Harvey, J. F., Pressure Component Construction, Von Nostrand Reinhold, New York, 1980. Chuse, R., Pressure Vessels –​The ASME Code Simplified, 6th edn., McGraw-​Hill, New York, 1984. Gupta, J. P., Fundamentals of Heat Exchanger and Pressure Vessel Technology, Hemisphere, Washington, DC, 1986. Soler, A. I., Expert system for design integration –​Application to the total design of shell and tube heat exchangers, in Proceedings of the ASME, Thermal/​Mechanical Heat Exchanger Design, Karl Gardner Memorial Session, Vol. 118 (K. P. Singh and S. M. Shenkman, eds.), ASME PVP, 1985, pp. 135–​138. Singh, K. P., Mechanical design of tubular heat exchangers –​An appraisal of the state-​of-​the-​art, in Heat Transfer Equipment Design (R. K. Shah, C. Subbarao, and R. M. Mashelekar, eds.), Hemisphere, Washington, DC, 1988, pp. 71–​87. Gardner, K. A., Heat exchanger tubesheet design, Trans. ASME J. Appl. Mech., 70A, 377–​385 (1948). Gardner, K. A., Heat exchanger tubesheet design –​2, Fixed tubesheets, Trans. ASME J. Appl. Mech., 74, 159–​166 (1952). Gardner, K. A., Heat exchanger tubesheet design –​3, U-​tube and bayonet tubesheets, Trans. ASME J. Appl. Mech. Ser. E., 82, 25–​33 (1960). Miller, K. A. G., The design of tube plates in heat exchangers, in Proceedings of Institution of Mechanical Engineers, Sect. B, Vol. 1, London, U.K., 1952–​1953, pp. 672–​688. Yu, Y. Y., Rational analysis of heat exchanger tubesheet stresses, Trans. ASME J. Appl. Mech., 78, 468–​473 (1956). Galletly, G. D. and Garbett, C. R., Pressure vessels –​Let the tubes support the tubesheet, Ind. Eng. Chem., 50, 1227–​1230 (1958). Galletly, G. D., Optimum design of thin circular plates on an elastic foundation, Proceedings of the Institution of the Mechanical Engineers, IMechE, Vol. 173, London, U.K., 1959, pp. 689–​698. Boon, G. B. and Walsh, R. A., Fixed tubesheet heat exchangers, Trans. ASME J. Appl. Mech., June, 175–​180 (1964).

131

Mechanical Design of Shell and Tube Heat Exchangers

131

69 Gardner, K. A., Tubesheet design: A basis for standardization, in Proceedings of the First International Conference on Pressure Vessel Technology: Part I, Design and Analysis, Delft, the Netherlands, 1969, pp. 621–​668. 70 Chiang, C. C., Closed form design solutions for box type heat exchangers, ASME publication 75-​WA/​ DE, New York, 1975. 71 Hayashi, K., An analysis procedure for fixed tubesheet exchangers, in Proceedings of the Third International Conference on Pressure Vessel Technology: Part I, Analysis, Design and Inspection, Tokyo, Japan, 1977, pp. 363–​373. 72 Malek, R. G., A new approach to exchanger tubesheet design, Hydrocarb. Process., 165–​169 (1977). 73 Singh, K. P., Analysis of vertically mounted through tube heat exchangers, Trans. ASME J. Eng. Power, 100, 380–​390 (1978). 74 Soler, A. I. and Soehrens, J. E., Design curves for stress analysis of U-​tube heat exchanger tubesheet with integral channel and head, Trans. ASME J. Pressure Vessel Technol., 100, 221–​232 (1978). 75 Soehrens, J. E., Tubesheet thicknesses and tube loads for floating head and fixed-​tubesheet heat exchangers, ASME J. Pressure Vessel Technol., 106, 289–​299 (1984). 76 Cascales, D. H. and Militello, C., Tubesheet thicknesses and tube loads for fixed tubesheet heat exchangers [Letter to the editor], Trans. ASME J. Pressure Vessel Technol., 107, 318–​323 (1985). 77 Singh, K. P. and Soler, A. I., An elastic-​plastic analysis of the integral tubesheet in U-​tube heat exchangers –​ Towards an ASME code oriented approach, in ASME Proceedings of the 1985 PVP Conference, Vol. 98, New Orleans, LA, 1985, pp. 39–​51. 78 Soler, A. I., Caldwell, S. M., and Singh, K. P., Tubesheet analysis –​A proposed ASME design procedure, in Proceedings of the ASME, Thermal/​Mechanical Heat Exchanger Design –​ Karl Gardner Memorial Session, Vol. 118 (K. P. Singh and S. M. Shenkman, eds.), ASME PVP, ASME, New York, 1985, pp. 93–​101. 79 Soehrens, J. E., Stress analysis of heat exchangers, in Proceedings of the ASME, PVP Vol. 118, Thermal/​Mechanical Heat Exchanger Design –​Karl Gardner Memorial Session, Vol. 118 (K. P. Singh and S. M. Shenkman, eds.), ASME PVP, New York, 1985, pp. 79–​91. 80 Osweiller, F., Basis of the tubesheet heat exchanger design rules used in the French pressure vessel code, Trans. ASME J. Press. Vessel Technol., 114, 124–​131 (1992). 81 Osweiller, F., Tubesheet heat exchangers: New common design rules in UPV, CODAP and ASME, ASME J. Press. Vessel Technol., 12(3), 317–​324 (August 2000). 82 ASME Section VIII –​Div. 1 –​Appendix AA: July 2001 Edition (Addenda July 2002) –​Section UHX, 2002. 83 Osweiller, F., Analysis of TEMA tubesheet design rules –​Comparison with up to date code methods, in Proceedings of the 1986 Pressure Vessel and Piping Conference, Vol. 107, Chicago, IL, 1986, pp. 1–​9. 84 Osweiller, F., New common design rules for U-​tube heat exchangers in ASME, CODAP and UPV Codes, in Proceedings of the ASME PVP Conference, Vol. 439 (H01237), Vancouver, British Columbia, Canada, August 4–​8, 2002. 85 Singh, K. P. and Marks, P., Proposed extension of the TEMA tubesheet design method to determine tubesheet rim thickness, in Proceedings of the ASME, Thermal/​Mechanical Heat Exchanger Design –​ Karl Gardner Memorial Session, Vol. 118 (K. P. Singh and S. M. Shenkman, eds.), ASME PVP., ASME, New York, 1985, pp. 111–​119. 86 Kuppan, T., Alternate design charts for fixed tubesheet design procedure included in ASME boiler and pressure vessel code, Section VIII, Div. 1, Trans. ASME J. Pressure Vessel Technol., 117, 189–​194 (1995). 87 O’Donnell, W. J. and Langer, B. F., Design of perforated plates, ASME J. Eng. Ind., 84, 307–​320 (1962). 88 Osweiller, F., Evolution and synthesis of the effective elastic constants concept for the design of tubesheets, Trans. ASME J. Pressure Vessel Technol., 111, 209–​217 (1989). 89 Bernstein, M. D. and Soler, A. I., The tubesheet analysis method in the new HEI condenser standards, Trans. ASME J. Eng. Power, 100, 363–​368 (1978). 90 Rachkov, V. I. and Morozov, V. H., Designing curved tube plates, Khim-​i-​reft. Masb, 7, 14 (1968). 91 Paliwal, D. N. and Sinha, S. N., Design of shallow spherical curved tubesheet for heat exchangers, Int. J. Press. Vessels Pip., 17, 185–​192 (1984).

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92 Sang, Z.-​F. and Widera, G. E. O., Stress analysis of elliptical tube plates in heat exchangers, Trans. ASME J. Press. Vessel Technol., 109, 310–​314 (1987). 93 Singh, K.P., Soler, A.I. (1984). Rectangular Tubesheets –​Application to Power Plant Condensers. In: Mechanical Design of Heat Exchangers. Springer, Berlin, Heidelberg. 94 Structural Design Concepts for Increased Reliability and Safety in Power Plant Condensing Systems FP-​507, Volume 2 Research Project 372-​1 Final Report, September, Principal Investigator A. I. Soler Research Associate C. Shahravan Prepared for Electric Power Research Institute, Palo Alto, California. 95 https://savree.com/en/encyclopedia/flange-types 96 ASME PCC-1-2022: Guidelines for Pressure Boundary Bolted Flange Joint Assembly 97 GASKET HANDBOOK, 1st Edition, FLUID SEALING ASSOCIATION, Wayne, PA and the EUROPEAN SEALING ASSOCIATION, Morzine, France, 2017, pp. 1–138. 98 Gasket & Fastener Handbook, A Technical Guide to Gasketing& Bolted Flanges, Lamons Corporate Headquarters 7300 Airport Blvd. Houston, Texas, pp. 1–214. 99 Gasket Handbook, A Technical Guide to Gasketing& Bolted Flanges, Revision 02.2012, LAMONS, Houston, Texas, pp. 1–167. 100 Guideline for Bolted Flanged Joint Assembly in Process Plants, 2016, pp. 1–21 101 Flange sealing guide, gaskets and bolted connections, Form No. 073837, 08/12, Chesterton, Griveland, MA. 102 What is a Spiral Wound Gasket? Applications, Types, Construction, and Specifications of Spiral Wound Gaskets (PDF) – What Is Piping 103 https://www.texasflange.com/weld-neck-flanges/ 104 https://www.texasflange.com/blog/what-are-various-types-of-flanges-used-in-piping-applications/ 105 https://www.wermac.org/flanges/flanges_welding-neck_socket-weld_lap-joint_screwed_blind.html 106 6 Ways To Identify A Flange. Posted on December 9, 2018 December 14, 2018 by Hydrastar 107 ASME B16.1-​2020: Gray Iron Pipe Flanges and Flanged Fittings Classes 25, 125, and 250. 108 ASME B16.5-​2020 Pipe Flanges and Flanged Fittings: NPS ½ Through NPS 24 Metric/​Inch Standard 109 ASME B16.47-​2020 –​Large Diameter Steel Flanges NPS 26 Through NPS 60 Metric/​Inch Standard 110 ANSI/​MSS SP-​25-​2018 111 www.neoim​pex.com/​fla​nge-​out​let-​manuf​actu​rer.html 112 www.dines​hind​ustr​ies.com/​titan​ium-​flan​ges-​manufa​ctur​ers/​ 113 www.linke​din.com/​pulse/​what-​girth-​flan​ges-​vicky-​zhang/​ 114 www.linke​din.com/​pulse/​what-​girth-​fla​nge-​mary-​fitt​ing-​fla​nge-​kong/​ 115 www.api​int.com/​resou​rce-​cen​ter/​how-​to-​cho​ose-​pipe-​fla​nge-​materi​als/​ 116 www.unifie​dall​oys.com/​blog/​flan​ges-​101 117 www.cway​expo​rts.com/​blog/​5-​fact​ors-​to-​consi​der-​bef​ore-​choos​ing-​right-​fla​nge-​type-​for-​your-​pip​ ing-​proj​ect/​ 118 https://​torqb​olt.com/​bolt​ing-​spe​cifi​cati​ons-​standa​rds-​codes-​materi​als-​manufa​ctur​ers-​suppli​ers 119 www.boar​dman​inc.com/​news.html/​2022/​09/​07/​vol​ume-​xlv​iii.i-​gask​ets-​how-​do-​they-​work-​and​why-​do-​we-​need-​them-​part-​1/​ 120 www.thepr​oces​spip​ing.com/​intro​duct​ion-​to-​gas​ket/​ 121 Gasket dimensions according to DIN EN and ASME normative, GARLOCK GMBH and EnPro Industries family of companies, Neuss, Germany, pp. 1–​23. 122 www.wer​mac.org/​gask​ets/​gask​ets.html 123 www.vci.de/​langfa​ssun​gen/​langfa​ssun​gen-​pdf/​vci-​guidel​ine-​for-​bol​ted-​flan​ged-​joint-​assem​bly-​in-​ proc​ess-​pla​nts.pdf 124 www.delmar​comp​any.com/​dou​ble-​jacke​ted-​metal-​gask​ets 125 Peter Smith, Process Piping Design Handbook, Volume One, The Fundamentals of Piping Design Drafting and Design Methods for Process Applications 2007, Gulf Publishing Company, Houston, Texas. 126 www.valve​sonl​ine.com.au/​ref​eren​ces/​fla​nge-​tab​les/​ 127 Gasket Dimensions According to DIN EN and ASME normative, Garlock, pp. 1–​23. 128 Garlock Gasketing Products, pp. 1–​63 and www.garl​ock.com 129 www.delmar​comp​any.com/​heat-​exchan​ger 130 www.canada​rubb​ergr​oup.com/​en-​ca/​blog/​heat-​exchan​ger-​gask​ets 131 www.klin​ger-​kempc​hen.de/​wp-​cont​ent/​uplo​ads/​2020/​08/​103-​104-​METAL-​PROF​ILE-​GASK​ETS.pdf

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132 Guideline for Bolted Flanged Joint Assembly in Process Plants, 17, March 2016, Verband der Chemischen Industrie e. V, Frankfurt a. M., pp. 1–​21 132.1 www.vci.de/​langfa​ssun​gen/​langfa​ssun​gen-​pdf/​vci-​guidel​ine-​for-​bol​ted-​flan​ged-​joint-​assem​bly-​in-​ proc​ess-​pla​nts.pdf 133 https://​gasket​ech.com.au/​what-​are-​heat-​exchan​ger-​gask​ets/​ 134 www.delmar​comp​any.com/​heat-​exchan​ger 135 https://​dobso​ngas​ket.com/​produ​cts/​heat-​exchan​ger-​gask​ets 136 www.linke​din.com/​pulse/​col​lar-​bolts-​shell-​tube-​heat-​exc​hang​ers-​baher-​elshe​ikh/​ 137 Waters, E. O., Wesstrom, D. B., and Williams, F. S. G., Design of bolted flanged connections, in Pressure Vessel and Piping Design, Collected Papers 1927–​959, American Society of Mechanical Engineers, New York, 1960, pp. 58–​61. 138 Modern Flange Design, 7th edn., Taylor Forge International, Chicago, IL, Bull., 502, 1979. 139 Bickford, J. H., Gasketed joints and leaks, in An Introduction to the Design and Behavior of Bolted Joints, 2nd edn., Marcel Dekker, New York, 1990, pp. 495–​548. 140 www.tektr​ade.ee/​ass​ets/​Uplo​ads/​Pdf/​Heat-​Exchan​ger-​gask​ets.pdf 141 Lake, G. F. and Boyd, G., Design of bolted, flanged joints of pressure vessels, Proc. Inst. Mech. Eng., 171, 843–​858 (1957). 142 https://​afgh​oldi​ngs.com/​wp-​conte​nts/​uplo​ads/​2020/​05/​AFG-​107​5_​Ta​per-​LokCo​nnec​tors​_​Cat​alog​_​ Low​Res.pdf 143 Wolf, L. J. and Mains, R. M., The stress analysis of heat exchanger expansion joints in the elastic range, Trans. ASME J. Eng. Ind., 145–​150 (1973). 144 Singh, K. P., A rational procedure for analyzing flanged and flued expansion joints, Trans. ASME J. Pressure Vessel Technol., 113, 64–​70 (1991). 145 Kopp, S. and Sayre, M. F., Expansion joints for heat exchangers, in ASME Winter Annual Meeting, New York, 1952. 146 www.hydroc​arbo​npro​cess​ing.com/​magaz​ine/​2022/​april-​2022/​heat-​trans​fer/​uncom​mon-​less​ons- ​ shell-​and-​tube-​heat-​exc​hang​ers-​part-​1 147 www.ejma.org/​bell​ows/​ 148 Morishita, M., Ikahata, N., and Kitamura, S., Simplified dynamic analysis methods for metallic bellows expansion joints, Trans. ASME J. Press. Vessel Technol., 113, 504–​510 (1991). 149 https://​usbell​ows.com/​resour​ces/​expans​ion-​joint-​acce​ssor​ies/​expans​ion-​joint-​inter​nal-​liner/​ 150 Schoessow, G. J. and Brooks, E., Analysis of experimental data regarding certain design features of pressure, in Pressure Vessel and Piping Design, Collected Papers 1927–​1959, American Society of Mechanical Engineers, New York, 1960, pp. 24–​34. 151 Spencer, T. C., Mechanical design and fabrication of exchangers in the United States, Heat Transfer Eng., 8, 58–​61 (1987). 152 Zick, L. P., Stresses in large horizontal cylindrical pressure vessels on two saddle supports, in Pressure Vessel and Piping; Design and Analysis, ASME, New York, 1972.

Suggested Readings Baylac G. and Koplewicz D., EN 13445 Unfired pressure vessels, background to the rules in part 3 design, Issue 2, August 20, 2004, pp. 1–​143.www.bell​ows-​syst​ems.com/​thick-​wall-​expans​ion-​joint/​ Osweiller, F., Methode de calcul des exchangers a deux ictes fixés le CODAP, Etude CETIM, No. 14B031, 1986. Cascales, D. H. and Militello, C., A model for fixed tubesheet heat exchanger, Trans. ASME J. Press. Vessel Technol., 109, 289–​296 (1987). Derenne, M., Marchand, L., Payne, J. R., and Bazergui, A., Elevated temperature testing of gaskets for bolted flanged connections, WRC Bull., 391, 2003. Paliwal, D. N., Design of fixed tubesheet for heat exchangers, Trans. ASME J. Press. Vessel Technol., 111, 79–​85 (1989). Roach, G. H. and Wood, R. M., Shell and tube exchangers having improved design features, Heat Transfer Eng., 7, 19–​23 (1986).

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Soler, A. I., Expert system for design integration—​Application to the total design of shell and tube heat exchangers, in Proceedings of the ASME, Thermal/​Mechanical Heat Exchanger Design, Karl Gardner Memorial Session, Vol. 118 (K. P. Singh and S. M. Shenkman, eds.), ASME PVP, 1985, pp. 135–​138. Singh, K. P., Study of bolted joint integrity and inter-​tube pass leakage in U-​tube heat exchangers –​Part II: Analysis, Trans. ASME J. Eng. Power, 101, 16–​22 (1979). https://​whati​spip​ing.com/​press​ure-​ves​sel-​noz​zle-​types/​ www.unifie​dall​oys.com/​blog/​flan​ges-​101 http://​ceme​ntec​hnol​ogy.ir/​Libr​ary/​Engine​ers.Guide.to.Press​ure.Equipm​ent.pdf https://​blog.tbai​ley.com/​types-​of-​press​ure-​vess​els

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2.1 MATERIAL SELECTION PRINCIPLES In engineering practice, the selection of materials of construction depends on the equipment being designed and the service requirements imposed on them. Selection of materials involves the thorough understanding of their availability, source, lead time, product forms, and size. In simple terms, the following factors are to be considered while selecting the materials for heat exchanger and pressure vessel components: 1. compatibility of the materials with the process fluids 2. compatibility of the materials with the other component materials 3. ease of manufacture and fabrication by using standard methods like machining, rolling, forging, forming, and metal-​joining methods such as welding, brazing, and soldering 4. material strength and ability to withstand operating temperature and pressure 5. cost 6. availability.

2.1.1 Desired Material Requirements Features While selecting materials, one has to balance many requirements; these requirements include the following [1]: • • • • • • • •

expected total life of plant or process expected service life of the material reliability (safety, hazard, and economic consequences of failure) material costs fabrication costs maintenance and inspection costs availability in required size, shape, thickness, and so on, and delivery time return on investment.

Materials are selected based on past experience, corrosion tests, the literature, and the recommendations of material suppliers. While selecting materials for the construction of heat

DOI: 10.1201/9781003352051-2

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exchangers and pressure vessels, the following points may be considered for the desired performance and the life of the equipment [2]: 1. Review of operating process. 2. Review of design. 3. Selection of material. 4. Evaluation of material. 5. Specification. 6. Finally, the material should be cost-​effective. Do cost-​benefit analysis for an optimum material selection. With this background information, the logical sequence of material selection for the construction of pressure vessels and heat exchangers is discussed next.

2.1.2 Review of Operating Process The foremost step in the material selection is the thorough review of the process environment and equipment operating conditions like operating temperature, pressure, and phases of the fluids. The following operating data are required by the design engineer [3]: 1. Environment –​nature and composition of fluid handled, water chemistry, steam quality, constituents/​concentration of a solution, conductivity, pH, aeration, impurities, and so on. 2. Pressure –​average and range, constant or cyclic, internal and external loadings. 3. Temperature –​average and range, constant or variable, thermal gradients, and thermal shock. 4. Velocity –​flow rate, linear velocity, nominal and range, degree of agitation, turbulence, etc. In addition to operating temperature and pressure, other factors such as the start-​ups and shutdowns, intermittent operation, transients and pressure surges, and momentary failure of the system must be considered for the satisfactory performance of a material.

2.1.3 Material Selection Factors The selection of proper materials for wetted and non-​wetted parts, pressure and non-​pressure parts, and equipment support is an important design step. Material Standards like American Society for Testing and Materials (ASTM), DIN, BS, ISO, and JIS give data on a large number of ferrous and nonferrous materials. Many of these materials have been adopted in the Pressure Vessel Codes by the country of origin in the Material Standards. The materials to be used in pressure vessels must be selected from code-​approved material specifications. Factors that need to be considered in picking a suitable material are: • • • • •

cost fabricability service condition (wear, corrosion, operating temperature) availability strength requirements.

2.1.4 ASME Code Material Requirements All materials used for pressure-​retaining parts shall be as per ASME Codes.

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2.1.4.1 Section II Materials All materials used for pressure-​retaining parts must meet the ASME Code (ASME Boiler and Pressure Vessel Code, Section II –​Material Specifications and Section VIII Div. 1. 1. Part A covers Ferrous Material 2. Part B covers Nonferrous Material 3. Part C covers Welding Rods, Electrodes, and Filler Metals 4. Part D covers Material Properties in both customary and metric units of measure. The manufacturer should deliver the material after thorough inspection, with the chemical composition and metallurgy and properties as stipulated in the relevant specification. Only code-​specified material shall be used in the vessel construction or repair. These specifications contain requirements for chemical and mechanical properties, heat treatment, manufacture, heat and product analyses, and methods of testing. 2.1.4.2 Section VIII Div.1 Subsection C –​Requirements Pertaining to Classes of Materials. 1. Part UCS –​Requirements for Pressure Vessels Constructed of Carbon and Low Alloy Steels, Non-​mandatory Appendix UCS-​A. 2. Part UNF –​Requirements for Pressure Vessels Constructed of Nonferrous Materials. Non-​mandatory Appendix –​UNF-​A Characteristics of the Nonferrous Materials. 3. Part UHA –​Requirements for Pressure Vessels Constructed of High Alloy Steel Non-​ mandatory Appendix. 4. UHA-​A Suggestions on the Selection and Treatment of Austenitic Chromium-​Nickel and Ferritic and Martensitic High Chromium Steels. 5. Part UCI –​Requirements for Pressure Vessels Constructed of Cast Iron. 6. Part UCL –​Requirements for Welded Pressure Vessels Constructed of Material with Corrosion Resistant Integral Cladding, Weld Metal Overlay Cladding, or Applied Linings. 7. Part UCD –​Requirements for Pressure Vessels Constructed of Cast Ductile Iron. 8. Part UHT –​Requirements for Pressure Vessels Constructed of Ferritic Steels with Tensile Properties Enhanced by Heat Treatment. 9. Part ULW –​Requirements for Pressure Vessels Fabricated by Layered Construction. 10. Part ULT –​Alternative Rules for Pressure Vessels Constructed of Materials Having Higher Allowable Stresses at Low Temperature. 2.1.4.3 Section VIII Div. 1 Requirements for Pressure Vessels Constructed of Nonferrous Materials The rules in Part UNF are applicable to pressure vessels and vessel parts that are constructed of nonferrous materials and shall be used in conjunction with the general requirements in Subsection A, and with the specific requirements in Subsection B that pertain to the method of fabrication used. Some of the uses of nonferrous materials are to resist corrosion, to facilitate cleaning of vessels for processing foods, to provide strength or scaling-resistance at high temperatures, and to provide toughness at low temperatures.

2.1.5 The Unified Numbering System The traditional designation systems for metals and alloys in the United States have been developed by a large group comprising trade associations (Aluminum Association (AA), the American Iron

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and Steel Institute (AISI), and the Copper Development Association (CDA)), metal producers, professional societies (ASTM International, the American Welding Society (AWS), and the Society of Automotive Engineers (SAE)), and the U.S. government. The unified numbering system (UNS) is an alloy designation system widely accepted in North America. Each UNS number relates to a specific metal or alloy and defines its specific chemical composition, or in some cases a specific mechanical or physical property. A UNS number alone does not constitute a full material specification because it establishes no requirements for material properties, heat treatment, form, or quality. For example, the UNS numbering categories for aluminum and aluminium alloys is A00001 to A99999 and for copper and copper alloys is C00001 to C99999.

2.1.6 International Material Specifications TEMA Standard includes ASME material specifications and a cross reference of materials produced to equivalent material specifications like UK-​BS, GERMANY-​DIN, JAPAN-​JLS, CHINA-​GB, EUROPE-​EN, FRANCE-​AFNOR and ITALY-​UNI. Material groupings are presented based on similarity of chemistry and alloying elements.

2.1.7 Functional Requirements of Materials Function of equipment has frequently been equated with long, trouble-​free service [4]. Selected material has certain functional requirements, which ultimately reflect on the function of the equipment. Functional requirements of materials given in Ref. [5] have been enlarged to include additional information and presented in Table 2.1. 2.1.7.1 Strength While the term “strength” applies to many mechanical properties, it most commonly refers to tensile strength and yield strength. These strengths are best understood by examining a stress-​strain curve. Stress-​strain diagram (schematic) for some of the heat exchanger materials is given in Figure 2.1. Other related strengths are compression, bending (flexural), shear, and torsional strength. It is better to use high-​strength materials to keep the thickness of parts less.

TABLE 2.1 Functional Requirements of Materials Characteristics

Functional Requirements

Strength Physical properties

Tensile, torsion, bending, compression, shear, buckling, fatigue, creep, rupture strength, etc. Design may require certain properties peculiar to the materials (thermal, electrical, acoustic, magnetic, etc.), coefficient of thermal expansion, thermal conductivity, modulus of elasticity, specific gravity (strength to weight ratio). Low temperature: subzero and cryogenic; intermediate temperature; elevated temperature. Stiffness of the structure, effect of sustained loading, etc. Ability to cope with a sudden load, impact, low-​temperature operation, etc. Ability to be formed, forged, machined, joined, ease of heat treatment, etc. Compatibility of the materials with the process fluids, with the other component materials in contact; resistance to ambient environment; passivity; hydrogen attack. Loss of surface oxide film (passive films), which is helpful in corrosion resistance.

Temperature resistance Geometric stability Toughness Fabricability Corrosion resistance Wear and abrasion

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FIGURE 2.1  Stress-​strain diagram of some common ferrous materials for heat exchanger.

FIGURE 2.2  S-​N curve (stress-​number of cycles of stress reversals).

2.1.7.2 Fatigue Strength Fatigue is the failure of a metal by fracture when it is subjected to cyclic stress. The usual case involves rapidly fluctuating stresses that may be well below the tensile strength. As stress is increased, the number of cycles required to cause failure decreases. A stress-​number of cycles (S-​N) curve is conventionally plotted for a metal as shown in Figure 2.2. Fatigue failures are most likely to occur at welded joints, which, by virtue of their design, give rise to significant stress concentration effects. Some of the material and design characteristics favorable to fatigue resistance are as follows [6]: increased tensile strength via alloying or heat treatment, minimum work piece thickness, polished surfaces, low design stress, smaller grain size, absence of metallurgical discontinuities and residual stresses, presence of surface compressive stress, minimizing environments conducive to corrosion and fatigue, and avoidance of metallic coating, such as Cr, Ni, or Cd plating, which can reduce fatigue strength due to microcracks in the plating. For pressure vessels, hydrostatic pressures that exceed the yield strength of the metal can relieve residual stresses and can improve fatigue strength [7]. 2.1.7.3 Brittle Fracture Brittle fracture is the term given to describe failure by rapid crack propagation at nominal stress below the yield strength. Brittle fracture has been mostly observed in body-​centered cubic (bcc) crystal structure metals, notably ferritic steels, in which complete rupture takes place under certain

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conditions at stress levels not only below the ultimate tensile strength but also even lower than yield strength. The conditions associated with brittle fracture of steels include the following [8, 9]: 1. a preexisting defect 2. the presence of residual and/​or applied tensile stress across the defect, which is more serious if triaxial 3. a sufficiently low temperature 4. several metallurgical factors − deoxidation practice, composition, rolling practice, and subsequent heat treatment [8]. The subject of brittle fracture is of interest to the designers of long-​life pressure vessels for the following reasons [9]: 1. Even if the vessel is to operate at an elevated temperature, where the brittle fracture is not to be expected, there is a danger of failure of the vessel during the preservice cold hydrostatic test; another case of practical importance is the temper embrittlement of Cr-​Mo steels, a condition caused by elevated temperature service after a long time in refinery service. 2. Cyclic variations of load and temperature during long service may enlarge small initial defects to the critical size and result in brittle fracture during start-​up, shutdown, or cold periodic tests. 3. In the special case of nuclear vessels, neutron bombardment can produce large increase in the transition temperature (TT). In welded structures, brittle fractures invariably initiate from weld defects. Weldment failure due to brittle fracture is dependent on through-​thickness toughness, the critical defect/​notch size, residual welding strains, mechanical constraints, grain size, inclusion content, and temperature [10]. It must not be overlooked that a normally ductile material can fail in a brittle manner at locations where complex and triaxial stress fields exist [11]. Measures to minimize brittle fracture. Brittle fracture may be prevented by selecting a notch-​tough material and avoiding notches. The following design practices will help to minimize brittle fracture [12]: 1. Eliminate all defects, which is impracticable. 2. Employ design stress less than required for propagation, but this may not be justified economically. 3. Select materials whose TT is below operating temperature. 4. If the structures are not stress relieved, residual stresses may be superimposed on the applied stress and will cause local yielding in the vicinity of a defect and thus initiate a brittle crack. Some simple practices to minimize the chance of non-​service failure have been suggested by Bravenec [13]. These recommendations include [13] the following: 1. Sheared and torched edges should be conditioned by grinding prior to forming. 2. Warm forming should be used whenever the ambient temperature is less than 75°F. 3. Material selection should meet end use requirements and code requirements; however, it must be recognized that code stipulates only a minimum standard. 2.1.7.4 Toughness A material’s toughness is its ability to absorb energy and deform plastically before fracture, and the amount of energy absorbed during both the deformation and the fracture is the measure of toughness.

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Toughness rather than strength is the most important low-​temperature mechanical property. Notch toughness is influenced by chemical composition, microstructure, grain size, grain flow pattern, section size, hot and cold working temperature, method of fabrication, and surface conditions like carburization and decarburization [6]. Basic tests. The most common toughness tests use notched specimens and measure either notch toughness, usually by an impact test, or fracture toughness, which is measured at a relatively low strain rate. Impact tests reveal low-​temperature strength, lack of ductility, and brittleness, and these tests are particularly relevant to ferritic materials, which may show a transition from ductile-​to-​ brittle (DBT) behavior. Two commonly used methods are the Charpy and Izod tests. Charpy V-​notch, CVN (ASTM E23). The Charpy V-​notch test is considered the most appropriate because a part or structure will generally fail due to a notch or other stress concentration. This test is used to obtain absorbed energy, lateral expansion, and fracture appearance from a 10 mm × 10 mm × 55 mm piece of a material subjected to impact loading. The specimen has a 0.254 mm (0.010 in.) radius notch. Test results given in foot-​pounds measure the capacity to absorb energy and thus signify the material ability to resist failure at points of local stress concentration. Precracked Charpy V-​notch. This is a test often used as a correlative measure of fracture toughness. Nil-​ductility transition temperature, NDTT (ASTM E208). This test defines the temperature at which a small flaw will initiate when subjected to yield loads. It is used with a Charpy V-​notch test to establish the reference fracture toughness in ASME Code. Fracture toughness. The notch toughness approach does not work well with high-​strength steels and nonferrous metals because of the gradual transition from ductile to brittle behavior [6]. Also it is felt that CVN test results alone are not reliable enough to predict whether an existing crack of a certain size will grow slowly, causing leakage before failure, or fail catastrophically under the very slow strain rates likely to be encountered in liquefied natural gas (LNG) containment vessels [14]. In these situations, fracture toughness value is used to assess metal’s toughness, and the approach is known as fracture mechanics approach. Plane-​strain fracture toughness, or KIc, values are used to calculate the critical crack size in a stressed component that can result in an abrupt failure. KIc values, unlike notch toughness values, are independent of test specimen shape [6]. Fracture toughness (ASTM E399 and E813). These are the only two tests that yield “ASTM valid” fracture toughness values. E399 is for linear-​elastic fracture toughness KIc, and E813 is for the “J-​ Integral”, which can be converted to KIc. 2.1.7.5 Creep Creep is defined as slow deformation of material with time, with the deformation taking place at elevated temperatures without increase in stress. It is usually associated with the tertiary stage of creep, and brings about the onset of creep failure. It can, however, initiate at the relatively early stages of creep, and develop gradually throughout creep life. Creep damage is manifested by the formation and growth of creep voids or cavities within the microstructure of the material. Creep curve is shown in Figure 2.3. Creep failure is the time-​dependent and permanent deformation of a material when subjected to a constant load or stress. The study of creep is essential because it helps to assess the lifespan of materials or equipment in a given environment. Creep failures are expected for superheaters and reheaters operating at design conditions and deviations from these parameters will promote early

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FIGURE 2.3  Creep curve.

failures. The phenomenon of creep is well understood, the basis of design is clear, and no special problems need be encountered. It is worth, however, sounding two notes of warning [15]: 1. If the design is poor so that unforeseen high local stresses occur, then the local creep deformation may exceed the creep ductility and lead to premature failure. 2. If the structures are not stress relieved, residual stresses may be superimposed on the applied stress and again lead to premature creep failure. Creep test. Tests for resistance of a material to elevated temperature generally fall into three classes: short-​time tests (tension, compression, bending, shear, torsion, and impact) run at elevated temperature, short-​time tests run at room temperature after elevated-​temperature exposure for various lengths of time, and longtime tests run at room temperature. Longtime strength tests, in turn, can be classified as constant load (creep, stress-​rupture, and many fatigue tests) or constant deflection (stress relaxation and some fatigue tests) tests [6]. The standard test followed to determine creep is ASTM El39-​11(2018) –​Standard practice for conducting creep, creep-​rupture, and stress-​rupture tests of metallic materials. The Larsen-​Miller parameter is considered reliable for using data from high-​temperature tests to predict creep-​rupture time. The Larson-​Miller parameter P, used for predicting stress-​rupture data for low-​alloy steels is given by

P = 1.8T ( k + log t ) × 10 −3

where T is temperature (K) t is time in hours k is a constant (=​20) for low-​alloy steels. For temperature in °R, eliminate the constant 1.8 in the equation. 2.1.7.6 Temperature Resistance Temperature resistance of construction materials is discussed with reference to various temperatures of operation, since basic material behavior is intimately related to operating temperature. Accordingly, the temperature of operation is classified into the following four ranges:

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1. subzero and cryogenic operation 2. low-​temperature operation (up to 200°C) 3. intermediate-​temperature operation (200°C–​650°C) 4. high-​temperature operation (>650°C). Subzero and cryogenic operation. A major requirement of materials for subzero applications is toughness at the handling temperature. Steels and nonferrous materials are used in cryogenic services for containment, handling, and transportation of liquefied gas and liquefaction of gases. Some metals display a marked loss of ductility in a narrow temperature range below room temperature or nil-​ductility temperature (NDT). This is called the ductile-​brittle transition temperature (DBTT). The application of metals below their NDT is avoided because of the danger created by brittle crack propagation. Below the NDT, very little energy is required for crack propagation [16]. Intermediate-​temperature operation. The intermediate temperature category includes most of the vessels used in petroleum and petrochemical services. Here the most important consideration is that materials be used to best advantage. The principal criteria in this respect are design stress values, adequacy of design formulas, and ductility of material. High-​temperature operation. In high-​temperature service, the materials operate in a pseudo-​plastic or fully plastic state, and designs must be based on stresses that will keep the inelastic deformations below limits that would permit failure [12]. High-​temperature design stress limits are established on the basis of longtime creep strength, as well as by the metal’s scaling resistance [17]. For the use of high-​temperature applications, the selected materials should exhibit certain properties that are given in Table 2.2 [18]. At temperatures above 800°F (425°C), materials should be heat-​resistant and must maintain corrosion resistance, mechanical strength, metallurgical stability, creep resistance, oxidation resistance, stress-​rupture strength, toughness, and surface stability against combustion gas and chemicals handled [19]. Alloying elements for heat-​resistant material. Heat-​resisting materials must contain chromium for oxidation resistance. Cobalt, aluminum, silicon, and rare earths enhance the formation, stability, and tenacity of the oxide surface layer. Nickel provides strength, stability, toughness, and carburization resistance. Tungsten and molybdenum increase high-​temperature strength [19]. Material groups and their service temperature. Approximate operating boundaries for various heat exchanger materials along with ceramic are shown in Chapter 6 of “Heat Exchangers: Classification,

TABLE 2.2 Mechanical Properties for High-​Temperature Operation Tensile strength or yield strength Creep and creep rupture strength Thermal fatigue characteristics Thermal shock characteristics Temper embrittlement Corrosion due to sulfides, high-​temperature sulfidation for carbon steels above 260°C when sulfur content is above 0.2% Metallurgical stability, grain growth, recrystallization, phase change, etc. Resistance against surface oxidation and carburization Precipitation of intermetallic phases at grain boundary, weld decay

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Selection, and Thermal Design”. For temperatures above 600°C, the following material groups are usually considered: 1. ferritic steels with 17%–​27% Cr and Mo addition 2. austenitic high-​temperature, high-​strength steels (Cr-​Ni-​Mo steels having more than 8% Ni) like Type 321, 347, 316, and 310 3. cast stainless steel (SS) heat-​resistant alloys: HC, HK, HT, HP, HX 4. solution-​hardened nickel-​base alloys with the addition of Cr and Fe, such as 800H, Incoloy 800HT, and alloy 330. 5. precipitation-​hardened nickel-​base alloys with the addition of Cr, such as X-​718 and X-​750 6. cobalt-​base alloys: N-​155, 188, L605, 25 7. advanced ceramics and other highly refractory material (>1200°C). Items 4–​6 are often called superalloys. Steels for high-​temperature applications. 1. Low-​alloy steels for high-​temperature service. C-​Mo alloy steels, chromium-​molybdenum alloy steels, and manganese-​molybdenum-​(nickel) alloy steels are regarded as steels for high-​ temperature service. 2. SSs for elevated-​ temperature service. While carbon steels can be conveniently used at temperatures up to about 752°F (400°C) and low-​alloy steels up to 1112°F–​1200°F (600°C–​ 650°C), their reliability ceases beyond these temperatures due to creep. While nickel-​base and cobalt-​base superalloys can withstand elevated temperatures without much loss of mechanical properties, their economics and availability may not permit their use for general applications such as in fossil fuel power plants operating in the middle temperature range of 1112°F–​ 1247°F (600°C–​675°C). For this temperature range, austenitic SS is preferred [20]. While selecting SSs for high-​temperature applications, in addition to creep strength, consider high-​ temperature corrosion, oxidation, scaling and carburization, and precipitation of secondary phases. Among the ferritics, E-​Brite 26-​1 (Allegheny Ludlum) appears to fill many gaps associated with the drawbacks of type 316 SS. 3. Developments in ferrous alloy technology for high-​temperature service. During the past 25 years, new alloys with improved strength and corrosion resistance were developed for use in nuclear, fossil, and petrochemical industries. Specific groups of alloys include vanadium-​ modified low-​alloy steels, 9Cr-​1Mo-​V steel, niobium-​modified lean SSs, and high-​chromium-​ nickel-​iron alloys [21]. Alloying additions of vanadium, niobium, titanium, and nitrogen have been effective in strengthening both ferritic and austenitic alloys. 4. Nickel-​base heat-​resistant material. Inconel (nickel-​ chromium alloys) and Inco nickel-​ chromium alloys resist oxidation, carburization, and other forms of high-​temperature deterioration. Typical heat-​resistant alloys include (1) Inconel alloy 600, Inconel alloy 601, Inconel alloy 617, Inconel alloy 625, Inconel alloy 718, Inconel alloy X-​750, and Inco alloy HX; (2)  nickel-​ iron-​ chromium alloys and Inco nickel-​ iron-​ chromium alloys such as Incoloy alloy 800, and Incoloy alloy 800HT, and Incoloy alloy 825; and (3) other newer alloys such as 253MA, 556, HR-​120, HR-​160, 45TM 230, and 214. Table 2.3 shows material groups and their service temperatures. Materials for high temperature heat exchangers are further discussed in Section 2.28. 2.1.7.7 Heat and Corrosion Alloys that have both heat and corrosion resistance are necessary for successful long-​term operation of high-​temperature processing equipment, particularly where the environment may be contaminated by corrosive constituents [22].

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TABLE 2.3 Material for High-​Temperature Applications Temperature Range

Alloys

Up to 1202°F (650°C) 1202°F–​1472°F (650°C–​800°C) 1472°F–​1832°F (800°C–​1000°C) 1832°F–​2192°F (1000°C–​1200°C) >2192°F (1200°C)

Carbon steels having 2 in. Thick 0.46 0.48 —​

Note:  For applicability of various conditions, refer ASTM Specification A20.

Maximum carbon equivalent as per ASTM A20.As per ASTM Specification A20, for weldability considerations, the plates shall be specified with a specific maximum CE value based on heat analysis. The CE shall be calculated using the following formula:

CE = C +

Mn Cr + Mo + V Ni + Cu + + 6 5 15

The maximum values of the CE for C-​steels, including C-​Mn, C-​Mn-​Si, and C-​Mn-​Si-​Al steels, are given in Table 2.9. Caution about using the carbon equivalent formulas to predict preheating temperature. Because the CE is calculated from the base metal composition and includes no other variables related to filler metal and welding procedures, it is only an approximate measure of weldability or predicting susceptibility to hydrogen-​induced cracking. 2.7.1.2 Underbead Cracking Underbead cracks are cold cracks that are most frequently encountered when welding a hardenable base metal. This is found at the toes of weld deposits and is promoted by these factors [56]: (1) increased base metal hardenability contributed by high carbon and alloying element content; (2) depositing small welds so that the cooling rates after welding favor the formation of hard, brittle HAZ; (3) weld designs that impose a high degree of mechanical restraint on the weld metal; and (4) the presence of hydrogen. Underbead cracking can be minimized or prevented by using low hydrogen electrodes or by preheating joints to the range of 250°F–​400°F and avoiding the already mentioned factors that cause underbead cracking. 2.7.1.3 Lamellar Tearing Lamellar tearing is a cold-​cracking phenomenon that occurs beneath welds and is principally found in rolled steel plate fabrications (Figure 2.9). The tearing always lies within the parent plate, often outside the visible HAZ. It is characterized by step-​like cracking parallel to the rolling plane [15, 56, 61, 62]. Lamellar tearing is due to the presence of planar inclusions lying parallel to the plate surface, and it is shown schematically in Figure 2.9a. For a given type of welded joint –​T (Figure 2.9b), L, or cruciform configuration –​the occurrence of lamellar tearing will be governed by the drop in strength and ductility of a plate in the through-​ thickness direction (z direction) as compared with the rolling direction.

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FIGURE 2.9  Lamellar tearing principle and susceptible welded joints: (a) Lamellar tearing due to planar inclusion lying parallel to the surface, (b) Lamellar tearing susceptible welded T-joint, (c and e) Shell to fixed tubesheet joint and (d and f) T-joint such as nozzle set through a rigid plane like channel.

2.7.1.3.1  Conditions That Promote Lamellar Tearing For lamellar tearing to occur, these conditions must be satisfied [56, 62]: 1. Strains must develop in the short transverse direction of the plate. These strains arise from weld metal shrinkage in the joint, but can be greatly increased by strains developed from reaction with other joints in restrained structures. 2. The fusion boundary is roughly parallel to the plate surface. 3. Susceptible base material with a local concentration of inclusions, particularly those extended in planar directions. 4. Section thickness. Lamellar tearing does not normally occur in welds of lighter gauge plates because of insufficient constraint. Most reported occurrences of tearing are in plates greater than 25 mm in thickness [62].

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Factors affecting weldment cracking due to lamellar tear (subcritical mode) include [10] through thickness ductility, size, shape, and distribution of inclusions, matrix-​inclusion cohesion, residual welding strains, mechanical constraints, and temperature. 2.7.1.3.2  Structures/​Locations Prone to Lamellar Tearing Any joint may be subject to lamellar tearing under certain conditions where a restrained weld is laid against a plate surface rather than the edge. From an analysis of fabrication failures, it appears that three major categories of structure type are commonly associated with the problem [62]: 1. shell to tubesheet connection on fixed-​tubesheet heat exchangers (Figure 2.9c) 2. stiffeners or end closure plates in cylindrical structures 3. nozzle or penetrator set through a rigid plate (T joint) (Figure 2.9d). 2.7.1.3.3  Prevention of Lamellar Tearing The risk of lamellar tearing can be solved by directing attention toward steel quality, appropriate design, and welding and fabrication techniques (Figure 2.10). These approaches are discussed next. Melting practice. The most reliable method of avoiding lamellar tearing involves special melting and solidification technique. Any steel-​making technique that reduces the inclusion content of the steel will improve the steel properties in the through-​thickness direction (z direction) and reduce the risk of tearing. A list of techniques are described in detail in Refs. [56, 61, 62, 63] and by Gross-​ Wordemann and Dittrich [64]. Design improvements. Many instances of lamellar tearing can be avoided in practice by improvements in design: • • • •

replacement of cruciform joints by offset T configurations replacement of T or L joints by butt joints location of T joints in regions of lower restraint replacement of plates by forgings, castings, or extrusions in critical T and L joints, or to avoid fillet welds.

Design improvements to overcome lamellar tearing are shown in Figure 2.10. Welding procedural factor. The following can be helpful measures: • • • • • • •

buttering or grooving and buttering, and in situ buttering of the plate surface [62] selection of sequence of turns to reduce strains in plates susceptible to lamellar tearing balanced welding control of preheating and interpass temperature to minimize tensile strains around the joint following low-​hydrogen welding practice shot-​peening the weld beads.

Welding procedures such as buttering and balanced welding are shown in Figure 2.10b. Complementary information test for lamellar tearing. Conventional pulse echo techniques, although useful for detecting laminations in plate, cannot reliably detect small inclusions, which can give rise to lamellar tears [62]. Hence for further assurance, additional testing is conducted at room temperature, known as the through-​thickness tensile test. ASTM Specification A770 covers procedures and

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FIGURE 2.10  Lamellar tearing –​(a) principle, (b) methods to overcome lamellar tearing-​welding techniques, and (c) methods to overcome lamellar tearing-​T-​joint design: original design and improved design. (Courtesy of TWI Ltd, UK, www.twi.co.uk.)

acceptance standards for through-​thickness tension tests. The room-​temperature through-​thickness tensile tests specimens of any one of the following can be used [65]: 1. Type 1. These tests are recommended for all thicknesses and fitted with welded extensions. They may be machined from friction-​welded assemblies or stud-​welded or fusion-​welded extensions. Friction-​or stud-​welded extensions are recommended in order to test the plates as close to the surface as possible. 2. Type 2. Test pieces of this type are recommended for plates having a thickness over 25 mm.

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2.7.1.3.5  Detection of Lamellar Tearing after Welding For surface tears, visual, dye penetrant and magnetic particle testing will be satisfactory. For subsurface tears, ultrasonics is probably the most widely used technique, but there may be problems in distinguishing true lamellar tears from inclusion bands and other forms of cracking. Therefore, pay particular attention to the position of the cracking in relation to the plate thickness and weld fusion boundary, to avoid confusion with lack of penetration defects, entrapped slags, etc. [62]. 2.7.1.4 Fish-​Eye Cracking Fish-​eye cracking is a form of hydrogen-​induced cracking that occurs in weld metal and appears as small bright spots on the fractured faces of broken specimens of weld metal. These small bright spots are similar to fish eyes and hence the name fish-​eye cracking. The fish eye usually surrounds some discontinuity in the metal, such as a hydrogen gas pocket or a nonmetallic inclusion, which gives the appearance of a “pupil in an eye”. The conditions that lead to fish-​eye cracking can be minimized by using dry low-​hydrogen electrodes or by heating the weldments for some period in the temperature range of 200°F–​300°F. Longer times are required with lower temperatures.

2.8 HOT CRACKING Hot cracking occurs during solidification and cooling of a weld, in the weld metal or in the HAZ. It occurs above solidus temperature of the lowest melting phase present. During the final stages of solidification, narrow solid bridges separating areas of low melting liquid are subject to the maximum proportion of the shrinkage-​induced strains. An increase in the amount of low-​melting phase or the inherent strain resulting from solidification shrinkage may cause fracture of these solid bridges, thus resulting in hot cracks [66]. Hot cracking is the subject of most fabrication weldability testing, so much so that many fabricators equate the term “weldability” with “hot cracking” and use the terms interchangeably [48].

2.8.1 Factors Responsible for Hot Cracking Most mechanisms proposed deal with the metallurgical factors that can lead to hot cracking. Hot cracking will occur due to (1) the segregation of low-​melting-​point elements, (2) the sufficient stress applied to a susceptible microstructure, and (3) the mode of solidification.

2.8.2 Susceptible Alloys These include (1) copper-​base alloys like silicon bronzes, aluminum bronzes, and copper-​nickels, (2) aluminum alloys, (3) fully austenitic and superaustenitic SSs, and (4) nickel-​base alloys.

2.8.3 Types of Hot Cracking Various types of hot cracking are the following: 1. solidification cracking 2. HAZ liquation cracking, microfissuring 3. reheat cracking (RC) or stress-​relief cracking 4. ductility dip cracking 5. chevron cracks 6. crater cracks.

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FIGURE 2.11  Solidification cracking (schematic).

2.8.3.1 Solidification Cracking Solidification cracking of the weld metal takes place within a few hundred degrees of the nominal liquidus temperature of the weld metal. It is caused by welding stresses and the presence of low melting-​point constituents that form as a result of segregation during solidification. Solidification cracking is shown schematically in Figure 2.11. 2.8.3.1.1  Elements Contributing to Solidification Cracking Increased amounts of carbon, phosphorus, and sulfur enhance the solidification cracking tendency. Resistance to solidification cracking may be obtained if the concentrations of sulfur and phosphorus are restricted. Cracking is less troublesome in carbon steel than in the high-​alloy steel. Susceptibility to solidification cracking is given by the following empirical formula [48]:

UCS = 230C + 190S + 75P + 45Cb − 12.3Si − 5.4 Mn − 1

As a rating of 20, a T fillet weld is likely to crack. 2.8.3.1.2  Welding Procedure-​Related Factors Responsible for Solidification Cracking These include the following: 1. Excessive travel speed, high thermal stress, small weld, and low weld current. 2. No or low preheat, high restraint, poor joint preparation, poor weld profile, poor fit up, inadequate jigging or tacking, unbalanced heat input, and poor electrode manipulation. 3. Weld metal used is not chosen correctly to accommodate the impurity level in the plate. 2.8.3.2 Heat-​Affected Zone Liquation Cracking HAZ liquation cracking is a type of high-​temperature weld cracking that occurs in the HAZ adjacent to the fusion boundary during welding. In the region adjacent to the fusion boundary, where high peak temperatures are attained during welding, local melting may occur at the grain boundaries

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FIGURE 2.12  Liquation cracking (schematic).

in ferritic steels due to the formation of a sulfide eutectic. The phenomenon has been known for many years to occur on steels heated at too high a temperature during processing and is known as “burning” [15]. The metallurgical basis for cracking involves the presence and persistence of liquid films at grain boundaries and the inability of these films to accommodate the thermal and/​or mechanical strains experienced during weld cooling [68]. Impurity elements like sulfur and phosphorus promote hot cracking in ferrous alloys. The degree of HAZ liquation cracking will be affected by the welding process used. HAZ liquated cracking is illustrated schematically in Figure 2.12. This problem can be controlled by restricting the sulfur level in the parent material and increasing the manganese, in sufficient concentrations [66, 67]; usually a 20:1 ratio (manganese to sulfur) ties up most of the available sulfur as globular manganese sulfide and leaves no sulfur for the formation of low-​melting intergranular sulfide films. Welding processes characterized by a relatively high heat input, such as submerged arc and electroslag welding, are likely to produce a greater degree of HAZ liquation cracking than processes such as the manual metal arc (MMA) and metal inert gas (MIG) welding processes [15]. Cleanliness and purity of the base material and welding consumables also lessen the tendency to hot cracking. 2.8.3.2.1 HAZ Liquation Cracking Susceptible Alloys HAZ liquation cracking is often encountered during welding of austenitic SSs, nickel-​and aluminum-​ base alloys [68], and superaustenitic SS and duplex SSs. HAZ liquation cracking in austenitic SS is known as microfissuring. 2.8.3.3 Reheat Cracking or Stress-​Relief Cracking RC is defined as intergranular cracking in the HAZ, and occasionally in the weld metal of a welded joint, being initiated during heat treatment in the temperature range of 950°F–​1200°F or during service at a sufficiently high temperature [69]. The process is driven by the relaxation of residual welding stresses. The extent of cracking during heat treatment depends on composition, the microstructure of the HAZ, heat treatment temperature, and time at temperature; for some steels, it is greater with slow cooling, as in stress relieving. RC, stress-​relief cracking (SRC), and stress-​rupture cracking are equivalent forms of cracking. Causes include poor creep ductility in HAZ coupled with thermal stress, accentuated by severe notches such as preexisting cracks or tear at weld toes, or in the fused root of a partial penetration weld.

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2.8.3.3.1  Susceptible Alloys RC is reported in low-​alloy structural steels, Q&T steels, ferritic creep-​resisting steels (Cr-​Mo and Cr-​Mo-​V steels), nickel-​base alloys, and austenitic SSs. Chromium, molybdenum, and vanadium contribute to this crack susceptibility. The appearance of the cracking and its position will depend on the local factors, such as [15] geometry of the joint, the relative properties of the weld metal and HAZ, and presence or absence of long-​range stresses. Phosphorus was found to enhance the RC susceptibility of Cr-​Mo steels, as it segregates to some extent to the austenite grain boundaries during the weld thermal cycle. For a particular alloy, there exists a critical level of P above which embrittlement is apparent. The addition of a small amount of Ti (0.07%) decreases the RC susceptibility [69]. 2.8.3.3.2 Empirical Check for Reheat Cracking Nakamua et al. [70] observed that susceptibility to stress-​rupture cracking (ΔG) in the HAZ of welds is related to composition as follows:

∆G = %Cr + 3.3 (%Mo ) + 8.1(%V ) − 2

According to this equation, cracking is possible if ΔG is greater than zero. However, experience has shown that this relationship does not always give a reliable estimate of crack susceptibility. Furthermore, factors other than chemical composition are known to affect crack susceptibility. 2.8.3.3.3 Avoidance of Reheat Cracking It is possible to weld any steel without the risk of RC if proper design and welding procedures are followed. Some procedures that may be used to minimize SRC in the steels include [69] the following: 1. Welding process changes. Proper selection of consumables, joint cleanliness, and proper preheat. Use of lower strength weld metal than that of the HAZ. Use of temper beads, that is small stringer beads placed over the last pass to refine the grain. Structure of the HAZ. 2. Complete normalization after welding. 3. Use low PWHT temperatures and high heating rates (provided stresses due to temperature gradients can be avoided). 4. Dress weld to remove discontinuities at weld toes and minimize stress concentrations in the weld joint. 5. Reduce weld stresses by weld sequencing, back stepping techniques, and interpass PWHT; shot-​ peen each layer of the weld metal to reduce residual tensile stresses at the surface of the weld. 6. Lower the overall level of restraint. 7. Control of sulfur level to avoid liquation cracking will also be beneficial since it has been established that liquation cracks can be potent initiation points for RC [15]. 2.8.3.3.4 Underclad Cracking RC taking place under clad surfaces is known as underclad cracking (UCC). UCC of low-​alloy steels is a potential problem associated with the fabrication of internally clad pressure vessels, in particular nuclear reactor vessels [69]. The incidence of UCC has been reduced considerably by measures such as [71]: 1. Avoid liquation and hydrogen-​induced cracks. 2. Use low-​susceptibility alloys, such as SA508 C13.

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3. Use clean steels: restrict impurity levels and minimize residual solidification segregation remaining in the surfaces to be clad. 4. Reduction of carbide and carbide-​ forming elements consistent with the compositional requirements to achieve adequate strength. 5. Adopt cladding procedures that can ensure a fine-​grained structure in the entire HAZ. 2.8.3.4 Ductility Dip Cracking Ductility dip cracking may occur in either weld metal or the HAZ of austenitic steels, cupronickels, aluminum bronzes, and some nickel-​base alloys while the weld cools through a range of temperatures where the ductility of the particular metal is inherently low. Cracking occurs if sufficient restraint is present as the metal cools through the ductility dip temperature range. The ductility dip temperature for austenitic steels is 100°C–​300°C (212°F–​600°F) below the equilibrium solidus, and the cracks formed are generally much less extensive than solidification cracks [66]. 2.8.3.5 Chevron Cracking Transverse cracking in submerged arc welds has been observed when welding 2.25Cr-​1Mo steel for pressure vessels, carbon-​manganese-​niobium steel, and 1Ni-​0.5Cr-​0.5Mo steel. This form of cracking lies at 45° to the weld surface in multipass welds in thick-​walled plate. Due to the inclination of these cracks, they are called chevron cracks. Chevron cracking is associated with large weld beads resulting from high heat inputs [59], the moisture content in the submerged arc fluxes and when high basicity (low-​oxygen-​potential) submerged arc fluxes are used. They are not easily detected by ultrasonic inspection method and usually found only with detailed metallographic examination. 2.8.3.6 Crater Cracks These are cracks formed from a circular surface with a depression either in the weld or at the end of a weld. They are caused by a volume contraction of molten metal during solidification, usually the result of abrupt interruption of the welding arc in the root run. Crater cracks are avoided by taking back the electrode and keeping stationary for a moment at the weld pool while stopping the weld.

2.9 LABORATORY TESTS TO DETERMINE SUSCEPTIBILITY TO CRACKING 2.9.1 Weldability Tests Tests for lamellar tearing have been already discussed. Some tests used to determine hot cracking, cold cracking (delayed cracking), or underbead cracking are as follows: • • • • • • • •

spot Varestraint test, for hot cracking longitudinal Varestraint test, for hot cracking sigmajig, for hot cracking hot ductility test, for hot cracking lehigh restraint test, for cold cracking controlled thermal severity test, for cold cracking cruciform cracking test, for cold cracking battelle underbead cracking test, for cold cracking.

The HAZ liquation cracking susceptibilities of several commercial austenitic and duplex SSs were evaluated using the spot Varestraint test by Lippold et al. [72]. According to them, low ferrite

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FIGURE 2.13  Configuration for testing weld cracking –​(a) spot varestraint test, (b) longitudinal varestraint test, and (c) varestraint test sample.

potential (FP) heats (FP 0–​1) of Type 304L were found to be more susceptible to cracking than a Type 304 alloy with FP 8, whereas duplex SSs, ferralium 255 and alloy 2205, were roughly equivalent to that of the low-​FP austenitic SSs. Tigamajig is the original name for the spot Varestraint test. Additional tests are discussed and detailed in Ref. [73]. The drawings in Figure 2.13 show some common tests for weld cracking [74, 75]. Only the Varestraint test is discussed next.

2.9.2 Varestraint (Variable Restraint) Test This test measures the hot-​cracking tendency of a weld as it is being laid down on a specimen plate. The Varestraint is one of the few hot-​cracking tests that tests for cracking by imposing an external stress. The strain can be calculated from the geometry of the test setup, and it can be reproduced time and time again, independent of the welding procedure. The applied augmented strain, ε, can be varied by adjusting the radius of the die block, R, using the equation ε =​ t/​(2R +​t), where t is the specimen thickness. In this method, the Varestraint test plate (Figure 2.13a) is bent to a preset radius while an autogenous weld bead is being arc-​melted past point X.

2.9.3 Multitask Varestraint Weldability Testing System The Multitask Varestraint (MTV) Weldability Testing System (Figure 2.14) is used to evaluate the solidification, liquidation, and HAZ cracking susceptibility of materials. This system can perform both conventional (i.e. consecutive) and simultaneous Varestraint tests for three types of welds: spot, transverse, and longitudinal.

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FIGURE 2.14  Multitask varestraint weldability testing system. (Courtesy of D. L. Wright, Inc., Zanesville, Ohio.)

2.10 SERVICE-​ORIENTED CRACKING 2.10.1 Temper Embrittlement or Creep Embrittlement Cr-​Mo low-​alloy steels are susceptible to temper embrittlement, a condition caused by long-​term, elevated-​ temperature exposure in the 370°C–​ 560°C (700°F–​ 1050°F) range, coupled with the presence of impurity elements such as phosphorus, tin, antimony, and arsenic. The phenomenon results in the progressive reduction of the notch toughness of the material as embrittlement develops. As the material toughness decreases, its resistance to brittle fracture decreases. This phenomenon is discussed further in the Section 2.15.5 on Cr-​Mo steels.

2.11 WELDING-​RELATED FAILURES Welding-​related failures take place (1) during welding, e.g. lamellar tearing, cold cracking, and hot cracking; (2) during stress-​relief annealing, e.g. temperature-​induced embrittlement, elongation-​ induced embrittlement, and SRC; and (3) during operation, e.g. stress-​ induced corrosion, intergranular corrosion, SCC, crack growth, and creep embrittlement [76]. Inadequate welding techniques often degrade the inherent corrosion resistance of parent metals.

2.12 SELECTION OF CAST IRON 2.12.1 Cast Iron For low-​pressure applications, cast-​iron bonnets and covers are used. According to ASME Code Section VIII, Div. 1, cast-​iron vessels shall not be used for the following services:

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• to contain lethal or flammable substances, either liquid or gases • for unfired steam boilers • for direct firing. The design pressure and temperature limitations for the vessels and the vessel parts constructed of cast iron are listed in ASME Code Section VIII, Div. 1, UCI-​3.

2.13 SELECTION OF CARBON STEELS 2.13.1 Steels Grades of steel may vary in chemical composition from almost pure iron to a material of complex composition made up of several elements. Widely dissimilar properties and qualities may be obtained from a carbon steel within specified chemical limits of a given grade by changes in steel making and mill processing. In the selection of the proper quality within a given grade, the end use and method of fabrication should be considered. Various practices are employed in all phases of steel production that determine the type and quality of the finished product. Carbon steel is the steel for which no minimum content is specified or required for alloying elements such as, chromium, cobalt, columbium, molybdenum, nickel, titanium, tungsten, vanadium, or zirconium, or any other element added to obtain a desired effect; when the specified minimum for copper does not exceed 0.40%; or when the maximum content specified for any of the following elements does not exceed the percentages noted: manganese 1.65, silicon 0.60, and copper 0.60. Carbon steel depends on carbon and manganese in conjunction with proper processing to improve mechanical properties. High-​strength low-​alloy (HSLA) steel comprises a group of steels with chemical composition specially developed to impart higher values of mechanical properties –​in certain of these steels, greater resistance to atmospheric corrosion than that is obtainable from conventional carbon structural steels. It is not considered to be alloy steel, even though an intentionally added alloy would qualify it as such. The minimum yield point and tensile strength requirements most often specified are 42–​80 ksi (290–​550 MPa) yield point and 60–​90 ksi (415–​620 MPa) tensile strength. These steels achieve high strength, high toughness, good formability, and good weldability. Low-​alloy steels are divided into groups according to their chemical composition, that is molybdenum alloy steel, manganese-​molybdenum-​nickel alloy steel, and chromium-​molybdenum alloy steel, and each group is further divided according to strength level. Steels are commercially classified as low-​alloy steels when they have: 1. manganese content exceeding 1.65% 2. silicon content exceeding 0.60% 3. copper content in excess of 0.60% 4. a range or a minimum amount of any other alloying element such as nickel, molybdenum, vanadium, chromium, etc. specified or added to obtain any alloying effect. Alloy steels can utilize a wide variety of alloying elements and heat treatments to develop the most desirable combination of properties.

2.13.2 Steel Making Process Improvements Improved refining techniques, vacuum induction melting (VIM) technique, low-​sulfur steel-​making methods, refining processes such as argon-​oxygen decarburization (AOD, process primarily used in

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stainless steel making and other high grade alloys with oxidizable elements such as chromium and aluminum), and vacuum oxygen decarburization (VOD), electroslag remelting, ladle metallurgy, vacuum degassing, pressure casting, continuous casting, controlled rolling, etc. resulted in clean, low-​carbon-​equivalent steels of high design strengths and improved weldability, increased corrosion resistance, and increased toughness.

2.13.3 Carbon Steels Carbon steels are alloys of iron and carbon in which carbon does not usually exceed 1%, manganese does not exceed 1.6%, and copper and silicon do not exceed 0.60%. Other alloying elements are normally not present in more than residual amounts. The properties and weldability of carbon steels mainly depend on carbon content. Carbon steel can be classified according to various deoxidation practices, as rimmed, capped, semikilled, or killed steel. Variations in carbon content will influence the mechanical properties. Carbon steels are normally used in the as-​rolled condition, although some may be in the annealed or normalized condition. 2.13.3.1 Types of Steel In steel making, the principal reaction is the combination of carbon and oxygen to form a gas. If the oxygen is not removed, the gaseous product continues to evolve as the steel changes from a liquid to a solid. Silicon and/​or aluminum are added to remove the oxygen. Control of the amount of gas evolved during solidification determines the type of steel. If practically no gas is evolved, the steel is completely deoxidized and is termed “killed” because it lies quietly in the molds. Increasing degrees of gas evolution result in semikilled, capped, or rimmed steels [77]. Killed steels are fully deoxidized and are characterized by more uniform chemical composition and mechanical properties as compared with other types. Semikilled steels are characterized by variable degrees of uniformity in composition that are intermediate between those of killed and those of rimmed steel. They are deoxidized less than killed steels. Rimmed and capped steels are characterized by marked difference in chemical composition across the section and from top to bottom of the ingot. Steels containing more than 0.30% carbon cannot be made to rim at all. Steels that are completely deoxidized cannot be rimmed [77]. 2.13.3.2 Product Forms • Steels are available in plates, tubes, pipes, forgings, etc. • Carbons steels for pressure vessels. • These are specified as A285, Grade A, B, C; A229; A442, Grade 55, 60; A515, Grade 55, 60, 65, 70; A516, Grade 55, 60, 65, 70; A537, Grade 1 (Q&T condition) and Grade 2 normalized and tempered condition. 2.13.3.3 Use of Carbon Steels Due to low cost, carbon steels are most widely used, despite their low corrosion resistance, in large quantities as heat exchanger material in the chemical industries, fossil fuel power plants, petroleum refining operations, etc. Apart from cost, the other properties that favor low-​carbon steels are (1) easy availability and (2) ease of fabrication by conventional methods. Features that favor carbon steels for feedwater applications and refinery applications are discussed next.

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2.13.3.4 Carbon Steel Tubes for Feedwater Applications In recent years, manufacturers were forced to abandon copper alloy tubes for high-​ pressure feedwater heater tubing. At the elevated feedwater temperatures used in modern power-​plant cycles, the corrosion of copper tubes by the feedwater, although very slight, gives rise to the problem of carryover of copper into the steam generator and possible blockage in turbine units [47, 78]. Sometimes low-​molybdenum alloy steel is also used. The factors that favor plain carbon steel or low-​molybdenum alloy steel are better weldability, high strength, and low cost. 2.13.3.5 Refinery Operations In refinery operations, carbon steel is an acceptable material for crudes up to 260°C to resist hydrogen attack and sulfide corrosion. At higher temperatures, corrosion rates increase and alloy steels or ferritic or SS may be used. 2.13.3.6 Corrosion Resistance The corrosion resistance of carbon steel is somewhat limited and is dependent on the formation of an oxide film on its surface. Carbon steel should not be used in contact with dilute acids. Carbon steels are susceptible to SCC by several corrosives including aqueous solutions of amines, carbonates, acidified cyanides, hydroxides, ammonium nitrate, anhydrous ammonia, coal gas liquors [2, 79], and sulfide cracking of steels used in sour oil fields containing hydrogen sulfide. The temperature and concentration limits for the susceptibility of carbon steel to SCC in caustic soda are fairly well defined and given in chart form [24]. Of all the metallic materials, carbon steel is the most economical for handling solutions of up to 50% concentration at temperatures up to 190°F (88°C). If iron contamination is permissible, steel can be used to handle caustic soda up to 75% concentration and up to 100°C (212°F). Stress relieving should be employed to reduce caustic embrittlement. Brine and seawater corrode steel at a slow rate. Steel is little affected by neutral water and most organic chemicals. In general, the presence of oxygen and/​or acidic conditions will promote the corrosion of carbon steel, while alkaline conditions will inhibit the corrosion of carbon steel. General aqueous corrosion. Principal factors governing the overall corrosion of carbon steel include solution pH, dissolved oxygen in the solution, temperature, dissolved salts, and solution. The effects of these factors are well discussed by Kirby [80]. Salient features of carbon steel in aqueous solutions are discussed next [80]: Effects of pH. Acidic solutions (pH less than 5) are highly corrosive to carbon steel. The protective oxide is soluble at pH values below 5.0 and above 2.0. Corrosion is low over a wide band of pH values [81]. Effects of dissolved oxygen. At ambient temperatures in near-​neutral solutions, the overall corrosion rate is proportional to the oxygen concentration, up to air saturation. Temperature effects. High temperatures increase the corrosion rate by accelerating the diffusion of oxygen through cathodic layers of hydrated iron oxide. Effects of dissolved salts. The presence of acid or neutral salts may increase the corrosion rate, whereas the presence of alkaline salts may lower them. Effects of velocity. In general, increasing the solution velocity steps up corrosion rates, especially if NaCl is present. To prevent failures due to the impingement of natural waters on equipment walls, velocities should be limited to a maximum of 8 ft/​s.

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Carbon steels exhibit passivity in alkaline environment and have minimal corrosion rates. This property is exploited in phosphating. Corrosion prevention measures for carbon steels include metallic coatings, painting, cathodic protection, addition of inhibitors, alloying additions, removal of oxygen from environment, elimination of galvanic couples, and organic coatings.

2.13.4 Fabrication 2.13.4.1 Plate Cutting Carbon steel plates are to cut to size by flame cutting and/​or machining. All plate edges after cutting must be examined for laminations and to ensure that the sheared edges are free from cracks. Dye penetrant and ultrasonic check of edges forming main weld seams is recommended, especially for higher thickness plates when these plates are used for low-​temperature or hydrogen service. 2.13.4.2 Rating of Weldability The weldability of a steel in terms of its susceptibility to cracking can be roughly estimated by CE. The influence of CE on weldability has already been covered in Section 2.7. 2.13.4.3 Weldability Considerations While welding carbon steel, consider the following factors: 1. hydrogen-​induced cold cracking 2. solidification cracking 3. lamellar tear 4. hardness limitation for refinery service and sour gas service. These problems are also discussed by Campbell [82] and in Ref. [83]. Factors 1–​3 were discussed in Section 2.7. 2.13.4.4 Hardness Limitation for Refinery Service The prevention of in-​service weld cracking is of special concern to the refining industry because carbon steel is widely used for refinery pressure vessels, heat exchangers, tanks, and piping. Three primary factors are believed to be involved in producing in-​service cracking of welds: corrosive environment, total stress, and hardness. NACE Standard RP-​04-​72 establishes a hardness limitation of 200 HB on completed P-​l welds. 2.13.4.5 Welding Processes Most carbon steels can be welded using covered electrodes and appropriate welding procedures including preheating when required. Welding processes used for carbon steel are gas metal arc welding (GMAW), flux-​cored arc welding (FCAW), submerged arc welding (SAW), electroslag and electrogas welding, and oxyacetylene welding (OAW). 2.13.4.6 Arc Welding of Carbon Steels Selection of welding electrodes is based on compatibility between base metals to be joined and the service requirements of the weldment. The essential factors in the selection of electrodes for the carbon steels are (1) mechanical properties, (2) material composition, (3) welding current, (4) position of welding, and (5) cost. Precautions should be taken to store electrodes in dry places. The minimum thickness of low-​carbon steel that can be welded by shielded metal arc welding (SMAW) depends on welder’s skill, welding position, characteristic of the current, type of joint, fit

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TABLE 2.10 Carbon Steel Welding Defects Associated with Various Arc Welding Methods SMAW

FCAW

SAW

GMAW and GTAW

Slag inclusion

Due to excessive arc voltage:

Weld porosity

GMAW

Contaminants in the flux

Weld-​metal cracks Porosity Inclusions Incomplete fusion Lack of penetration Undercutting Excessive melt-​through

Porosity Wormhole porosity Undercuts Hot cracking Cold cracking Centerline cracking Underbead cracks Base metal cracks Microfissuring Weld craters Arc strikes Oxidation Gaps from incomplete fusion Sink or concavity Weld reinforcement overlapping Excessive weld spatter

Porosity Heavy spatter Undercutting Due to excessive current: Convex weld beads Large droplet Due to travel speed: Slag interference Convex weld bead

Insufficient flux coverage Excessive viscous fluxes Arc blow Cracking Solidification cracking Hydrogen cracking Chevron cracking Underbead cracking Slag inclusions

Wire feed stoppage Inadequate shielding GTAW Contamination of tungsten electrodes Tungsten inclusion Electrode extension

Source:  Compiled from [83] Nippes, E.F.

up, class and size of electrode, amperage, arc length, and welding speed. Steel as thin as 0.036 in. (0.91 mm) has been successfully welded with flux-​covered electrodes. In terms of thick sections, all commercial thicknesses can be welded successfully by SMAW provided that the root of the joint can be reached with the electrode. 2.13.4.7 Welding Defects Since carbon steels are used extensively, the welding defects associated with various arc welding methods are given in Table 2.10.

2.14 LOW-​ALLOY STEELS 2.14.1 Selection of Steels for Pressure Vessel Construction The most important factors to be considered in the selection of steels for pressure vessels and heat exchangers are tensile strength, creep strength, notch toughness, weldability, and heat treatment. In general, thinner steel plates are preferred for these reasons [29]: low weight, ease of fabrication, and low foundation cost; ease of welding, heat treatment and stress relief, NDT, erection, higher fatigue strength, and resistance to brittle fracture. The inherently poorer properties of thicker sections of carbon steels are well known. For example, increased thickness offers greater internal restraint at a defect and thus increases the likelihood of defect growth by brittle fracture. Therefore, the trend is to use high-​strength steels in place of thicker plates.

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Material Selection and Fabrication

2.14.2 Low-​Alloy Steels for Pressure Vessel Constructions Low-​alloy steels used in pressure vessel and heat exchanger construction are the following: 1. carbon-​molybdenum steels 2. carbon-​manganese steels 3. carbon-​manganese-​molybdenum steels 4. manganese-​molybdenum-​nickel alloy steels 5. chromium-​molybdenum steels or creep-​resistant steels such as Cr-​Mo, Cr-​Mo-​V, and modified Cr-​Mo steels. Items 1–​4 are covered here, and chromium-​molybdenum steels are covered separately.

2.14.3 Applications of Low-​Alloy Steel Plates Principal applications for low-​alloy steel plates include [18] the following: 1. boiler drums for thermal power plants 2. reactor pressure vessels, steam generators, and pressurizers for nuclear power plants 3. various types of reactors, converters, steam drums, separators, pressurizers, heat exchangers, and hydrocrackers for chemical plants. Of all these types of equipment, large nuclear reactors and heavy oil desulfurizers require particularly heavy steel plates, ranging from 150 to 300 mm in thickness and 30 to 50 tons in unit weight.

2.14.4 Carbon-​Molybdenum Steels Carbon-​molybdenum steels are similar to carbon steels except they have an addition of approximately 0.5% Mo. The addition of 0.5% Mo and a little boron to carbon steels has a considerable effect on the transformation to lower bainite at normal rates of cooling [84]. This has a relatively high strength, which is retained to a moderately high temperature. These steels include ASTM A204 Grades A, B, and C; A302 Grades A and B; and ASTM A182 Grade Fl forging. Steels with 0.5% Mo find considerable use in the petroleum industry, where they have replaced carbon steel because of their resistance to hydrogen at higher temperatures. The possibility of graphitization limits their use to a maximum temperature of 875°F (470°C) [29]. These steels are produced in the as-​rolled or normalized condition. The weldments should be given PWHT.

2.14.5 Carbon-​Manganese Steels These steels find use in pressure vessels, ships, and other large fabrications. Manganese is added to steel for a number of reasons such as manganese increases the strength of the steel by forming a solid solution with the iron, manganese retards the transformation of austenite to ferrite and pearlite and manganese increases the strength of the steel by forming a solid solution with the iron [15, 85]. The properties of carbon-​manganese steels can be further improved in a number of ways [15, 85]: 1. by grain refining: the addition of aluminum to a killed steel refines its grain size to produce greater toughness 2. by normalizing 3. by controlled rolling –​this leads to relatively high strength and toughness 4. quenching and tempering: if carbon-​manganese steels are quenched, products with a good combination of strength and toughness can be obtained.

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185

To avoid the formation of hard constituents in weld metal susceptible to hydrogen cracking, slow the cooling rate by increasing heat input to the weld; in multipass welding, proper sequencing can reaustenize previous passes to form softer microconstituents and temper hard ones [58].

2.14.6 Carbon-​Manganese-​Molybdenum Steels The addition of molybdenum to carbon-​manganese steel increases the strength at ambient temperature slightly but has a marked effect on the strength at higher temperature. Carbon-​manganese steels containing about 0.5% Mo have found extensive use in pressure vessels. However, their creep ductility is rather limited, and for that reason and for their improved resistance to high-​ temperature corrosion and oxidation, the chromium-​molybdenum steels may be preferable in some cases.

2.15 QUENCHED AND TEMPERED STEELS An important group of steels is those in which the hardenability is increased by alloy additions so as to produce martensite throughout the plate thickness when the steel is quenched or even air-​cooled from the austenite range. The steels are then tempered to give the required properties. The total alloy content of such steels may be relatively high, but they have a very attractive combination of strength and toughness. These steels are known as Q&T steels. They are also relatively easy to weld since the carbon content to achieve a given strength can be kept low.

2.15.1 ASTM Specifications 1. ASTM A542/​A542M-​19 –​Standard Specification for Pressure Vessel Plates, Alloy Steel, Quenched-​and-​Tempered, Chromium-​Molybdenum, and Chromium-​Molybdenum-​ Vanadium. 2. ASTM A553/​A553M-​17e1 –​Standard Specification for Pressure Vessel Plates, Alloy Steel, Quenched and Tempered 7, 8, and 9 % Nickel.

2.15.2 Compositions and Properties These steels contain not more than 0.25% carbon and a total content of alloying elements (not including Mn and Si) ranging from 0.85% to about 16%. Q&T steels are furnished in the heat-​ treated condition with yield strengths ranging from 50 to 150 ksi depending on chemical composition, thickness, and heat treatment. They have high strength in combination with good ductility. Various combinations of toughness, fatigue strength, and corrosion resistance can be developed to meet the requirements of pressure vessels for use in atmospheric conditions, at elevated temperature, or at cryogenic temperatures. Some Q&T steels fall within the ASTM carbon steel classification, and others within the alloy steel classification. Typical Q&T plate steels are given in Table 2.11. A brief description of some of these Q&T steels is given next, and important sources on Q&T steels include Refs. [18, 43, 86]. 1. A517 Alloy steel. Fifteen grades, quenched and tempered to yield strengths (minimum) of 90 ksi (more than 2.5–​6 in.) and 100–​125 ksi (less than 2.5). Amounts of C, Mn, Si, and other minor alloying elements such as Ni, Cr, Mo, B, V, Ti, Zr, and Cu vary with grades. 2. A533 Alloy steel. Four types, quenched and tempered to three tensile strength ranges, 80–​100 ksi, 90–​115 ksi, and 100–​125 ksi. Composition: 0.50Mo (Type A), 0.50Mo +​0.55Ni (Type B), 0.85Ni (Type C), and 0.30Ni (Type D). This steel, with a somewhat higher carbon content, has the least susceptibility to hot cracking because it has a high manganese-​to-​sulfur ratio, usually

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Material Selection and Fabrication

TABLE 2.11 Q&T Steel Plates, ASTM Specification A203 Grade F A517 (15 Grades) A533 Type A Cl. 1 A533 Type A Cl. 2 A533 Type A Cl. 3 A533 Type B Cl. 1 A533 Type B Cl. 2 A533 Type B Cl. 3 A533 Type C Cl. 1 A533 Type C Cl. 2 A533 Type C Cl. 3 A533 Type D Cl. 1 A533 Type D Cl. 2 A533 Type D Cl. 3 A537 Cl. 2

A542 Type A Cl. 1 A542 Type A Cl. 2 A542 Type A Cl. 3 A542 Type A Cl. 4 A542 Type A Cl. 4a A542 Type B Cl. 1 A542 Type B Cl. 2 A542 Type B Cl. 3 A542 Type B Cl. 4 A542 Type B Cl. 4a A542 Type C Cl. 1 A542 Type C Cl. 2 A542 Type C Cl. 3 A542 Type C Cl. 4 A542 Type C Cl 4a A543 Type B Cl. 1 A543 Type B Cl. 2 A543 Type B Cl. 3 A543 Type C Cl. 1 A543 Type C Cl. 2 A543 Type C Cl. 3

A553 Type 1 (8% and 9% Ni steel)

A645 5% Ni steel A724 Grade A A724 Grade B A724 Grade C A734 HSLA Type A A734 HSLA Type B A738 Grade A A738 Grade B A782 Class 1 A782 Class 2 A782 Class 3

Note: A543 Grade is: Q&T up to 4 in. A738 Grade A is N or Q&T up to 2.5 in.; Q&T over 2–​5 in.

about 50:1. ASTM A533 Grade B steel is used in thick section for nuclear pressure vessels, and at its highest strength level (Class 3), it may be used for thin-​walled or layered pressure vessels. 3. ASTM A537 (Class 2) carbon steel is used in pressure vessels where high notch toughness is required. 4. A542 Alloy steel. Five classes, quenched and tempered to tensile strengths (minimum) of 85, 95, 105, and 115 ksi. Two types of 2.25Cr-​1Mo steels, one type of 3Cr-​1Mo-​0.25V-​Ti-​ B steel. 5. A543 Alloy steel. Three classes, quenched and tempered to tensile strengths (minimum) of 90, 105, and 115 ksi. Two types of Ni-​Cr-​Mo steels. 6. A553 Alloy steel. Two types, quenched and tempered to 85 ksi yield strength (minimum). 7. ASTM A592 used as forgings where good notch toughness is needed in a steel having a yield strength of 100 ksi.

2.15.3 Weldability The carbon content of Q&T steels generally does not exceed 0.22% for good weldability. The alloying elements are carefully selected to ensure the most economically heat-​treated steel with the desired properties and acceptable weldability. The development of good notch toughness in the HAZ of Q&T steels depends on the rapid dissipation of welding heat to permit the formation of martensite and bainite on cooling. This requirement may increase the susceptibility of the Q&T steel to hydrogen-​induced cold cracking.

Material Selection and Fabrication

187

2.15.4 Joint Design Appropriate joint design, good workmanship, and adequate inspection are needed to take advantage of the high strength of Q&T steels and to optimize the serviceability of weldments made from these steels.

2.15.5 Preheat Preheat for welding Q&T steels must be used with caution because it reduces the cooling rate of the weld HAZ. If the cooling rate is too slow, the reaustenized zone adjacent to weld metal can transform to ferrite with regions of high-​carbon martensite or coarse bainite, with loss of strength and toughness.

2.15.6 Welding Processes Welding processes such as SMAW, SAW, GMAW, FCAW, and GTAW can be used to join Q&T steels having minimum yield strength up to 150 ksi and carbon content up to 0.25%. Filler metal should be selected with care. Adopt low-​hydrogen welding practices: use properly dried low hydrogen electrodes, clean and dry flux, moisture-​free shielding gas, clean low-​hydrogen electrode wire, etc.

2.15.7 Post-​weld Heat Treatment The heat treatment for most of these steels consists of austenitizing, quenching, and tempering. A few are given a precipitation hardening (PH; aging) treatment following hot rolling or a hardening treatment. Welded structures fabricated from these steels generally do not need further heat treatment except for a stress relief in special situations. Stress relief is necessary for these circumstances: (1) the steel has inadequate notch toughness after cold forming or welding, (2) to maintain dimensional stability after fabrication, and (3) the weldment with high-​residual stress is susceptible to SCC.

2.15.8 Stress-​relief Cracking The weldments of many quenched and low-​alloy steels are susceptible to SRC, also known as RC. Chromium, molybdenum, and vanadium contribute to this type of cracking, but other carbide forming elements also contribute to SRC. Measures to overcome SRC have been discussed in the earlier Section 2.8.3.3 weldability consideration.

2.16 CHROMIUM-​MOLYBDENUM STEELS Chromium-​molybdenum steels, also referred as creep-​resistant low-​alloy steels, are used for fabricating pressure vessels to be operated under high-​temperature and high-​pressure conditions. These steels contain varying amounts of chromium up to a nominal composition of 9%, and 0.5% or l% molybdenum. The carbon content is normally less than 0.20% to achieve good weldability, but the alloys have high hardenability. The chromium provides improved oxidation and corrosion resistance, and the molybdenum increases strength at elevated temperatures. These steels usually come from the steel manufacturer in the annealed, normalized and tempered, or Q&T condition. The most popular grades of creep-​resistant Cr-​Mo steels are 0.5Cr-​0.5Mo-​0.25V, 1Cr-​0.5Mo, and 1.25Cr-​ 0.5Mo and 2.25Cr-​1Mo (with or without vanadium). 1Cr-​0.5Mo (Grade 11) and 2.25Cr-​1Mo steel (Grade 22, Class 2) have long been used to fabricate heavy sections of pressure vessel and piping for

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Material Selection and Fabrication

TABLE 2.12 ASTM A387/​A387M Cr-​Mo Steel Nominal Composition Grade

Cr%

Mo%

2 12 11 22.22L 21.21L 5 7 9 91

0.50 1.00 1.25 2.25 3.00 5.00 7.00 9.00 9.00

0.50 0.50 0.50 1.00 1.00 0.50 0.50 1.00 1.00

TABLE 2.13 Nominal Composition of ASTM A387/​A387M Cr-​Mo Steel Plate Composition (%) Grade 12 11 22 5 91

Generic Name

C

Mn

Si

Cr

Mo

1.00Cr 1.25Cr 2.25Cr 5.00Cr 9.00Cr

0.05–​0.17 0.05–​0.17 0.05–​0.15 0.15 max 0.06–​0.15

0.40–​0.65 0.40–​0.65 0.30–​0.60 0.30–​0.60 0.30–​0.60

0.15–​0.40 0.50–​0.80 0.50 max 0.50 max 0.50–​1.00

0.80–​1.15 1.00–​1.50 2.00–​2.50 4.00–​6.00 8.00–​10.00

0.45–​0.60 0.45–​0.65 0.90–​1.10 0.45–​0.65 0.90–​1.10

fossil power, petroleum, and petrochemical applications. 2.25Cr-​1Mo steel is frequently used for its superior hot strength; plates are covered by specification SA-​387, Grade 22, Class 2, and forgings by specification SA-​336, Grade F22. 5Cr-​1Mo and 9Cr-​lMo are widely used in the petrochemical industry for their superior corrosion and oxidation resistance [87].

2.16.1 Composition and Properties ASTM A387/​ A387M-​ 17a –​ Standard Specification for Pressure Vessel Plates, Alloy Steel, Chromium-​Molybdenum. This specification covers chromium-​molybdenum alloy steel plates for welded broilers and pressure vessels designed for elevated temperature service. Materials considered under this specification are available in grades 2, 12, 11, 22, 22L, 21, 21L, 5, 9, and 91. The steel materials shall be killed and shall be thermally treated. ASTM specification A387, Grades 2, 5, 7, 9, 11, 12, 21, 22, and 91, cover plates, and ASTM A182 Grades F2, F5, F7, F9, F11, F12, F21, and F22 deal with forging. The nominal chemical compositions of certain grades of Cr-​Mo steels are given in Tables 2.12 and 2.13. ASTM A387 steel plate is applied in temperature range(316°C–​593°C). The elevated temperature properties of it is still of particular interest, even with the current trend towards applications with high cvn toughness requirements. ASTM A387 steel tend to be fine grain, low silicon, low sulfur, calcium treated and occasionally produced Q&T to give the highest levels of toughness. ASTM specifications for various product forms are shown in Table 2.14.

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TABLE 2.14 ASTM Specification for Chromium-​Molybdenum Steel Product Formsa Steel Type

Plate: ASTM Spec.

Tube: ASTM Spec.

0.5Cr-​0.5Mo 1Cr-​0.5Mo l.25Cr-​0.5Mo 2.25Cr-​1Mo

A387-​Gr2, Cl. 1,2 A387-​Gr12, Cl. 1,2 A387-​Gr11, Cl. 1,2 A387-​Gr22, Cl. 1,2, A542

3Cr-​1Mo

A387-​Gr2l

5Cr-​0.5Mo

A387-​Gr5, Cl. 1,2

7Cr-​0.5Mo

A387-​Gr7

9Cr-​1Mo

A387-​Gr9, Cl. 1,2

A213-​T2 A213-​T11, T12 A199-​T11, A200-​T11, A213-​T11 A199-​T22, A200-​T22 A213-​T22 A199-​T21, A200-​T21 A213-​T21 A199-​T5, A200-​T5 A213-​T5 A199-​T7, A200-​T7 A213-​T7 A199-​T9, A200-​T9 A213-​T9

Max. Service Temperature 450°C–​475°C 450°C–​500°C 500°C–​550°C 500°C–​550°C 550°C 550°C 550°C–​600°C

  Forgings and bar-​ASTM A182, Pipe A 335, Welded fittings A234.

a

2.16.2 Applications Chromium-​molybdenum steels are primarily used for service at elevated temperatures up to about 1300°F (704°C) in applications such as power plants and petroleum refineries, and in chemical industries for pressure vessels and piping systems. However, the normal application temperature range is 400°C–​550°C. These alloys exhibit excellent resistance to refinery corrosives like sulfur, elevated temperature, and hydrogen attack. For improved corrosion resistance, these alloys are often overlaid with SS.

2.16.3 Creep Strength The optimum creep strength is developed in chromium-​molybdenum steels by tempering following normalizing or quenching. In the more highly alloyed steels, the tempering treatment results in the precipitation of fine particles of alloy carbide, which are very effective in producing good creep strength. It should be noted, however, that there is usually a corresponding decrease in the fracture toughness. A further increase in creep strength is achieved by the addition of vanadium to give normal compositions such as 1Cr-​0.5Mo-​0.25V and 0.5Cr-​0.5Mo-​0.25V. The presence of a fine dispersion of V carbides makes the steels more stable than other chromium and molybdenum carbides. Like other grades, vanadium is added to 2.25Cr-​1Mo steel to improve creep strength.

2.16.4 Welding Metallurgy Cr-​Mo steels are weldable, but they require a higher degree of welding design and control than low-​ carbon steel. The primary difference is the air hardenability of alloy steels. These steels are susceptible to cracking from inadequate ductility. Such types of cracking include lamellar tearing, hot cracking, and reheat or SRC. The welding procedures must incorporate low-​hydrogen welding practices to prevent hydrogen-​induced cracking in the weld metal and in the HAZ. They are welded in various heat-​ treated conditions: annealed, normalized and tempered, or quenched and tempered. Welded joints are often heat-​treated prior to use to improve ductility and toughness and reduce welding stresses.

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Material Selection and Fabrication

2.16.4.1 Preheating Preheating is extremely important when welding air-​hardening Cr-​Mo steels. Preheating is required to prevent hardening and cracking. Preheating reduces stresses, limits or tempers martensitic areas, and reduces the amount of hydrogen retained in the weld. 2.16.4.2 Welding Processes Most fusion-​ welding processes can be applied for joining Cr-​ Mo steels. Typical welding processes include SMAW, GTAW, plasma arc welding (PAW), SAW, electroslag welding, GMAW, and FCAW. 2.16.4.3 Filler Metal The filler metal composition should be nearly the same as that of the base metal, except for carbon content to obtain uniform strength and also resistance to heat and corrosion. The carbon content of the filler metal is usually lower than that of the base metal. A matching carbon content is required when the weldment is to be quenched and tempered or normalized and tempered. 2.16.4.4 Temper Embrittlement Susceptibility Cr-​Mo steels, especially 2.25Cr-​1Mo steels, are susceptible to temper embrittlement, a condition caused by long-​term, elevated-​temperature exposure in the 370°C–​560°C (700°F–​1050°F) range, associated with the presence of impurity elements, such as phosphorous, tin, antimony, and arsenic [88–​90]. The phenomenon results in the progressive reduction of the notch toughness of the material as embrittlement develops. In other words, due to embrittlement, steels ductile at room temperature tend to become brittle during service. Temper embrittlement susceptibility of 2.25Cr-​1Mo steel is very high among Cr-​Mo steels and ranks next to (but almost equal to) that of 3Cr-​1Mo steel [91]. It has been established that higher silicon and manganese levels will have an even greater effect on toughness degradation. Requirements for resistance against temper embrittlement and low-​temperature toughness are becoming severer recently, especially for the Cr-​Mo steel to be used for fabricating the pressure vessels, particularly in the petroleum industry, since it indicates that the potential for brittle fracture increases with service time in the critical temperature range. The resulting implication is that certain units may become susceptible to brittle failure during start-​up or shut down without any warning [92]. Temper embrittlement of all 2.25Cr-​1Mo plates, forgings, and weld metal can be minimized by ordering all material to chemical specifications that limit the elements that cause temper embrittlement. Temper embrittlement resistance is often specified by various factors that combine the effect of these and other elements. Two of these factors, Watanabe number or J factor and X factor, for 2.25Cr-​1Mo can be specified [93]: J = (%Si + %Mn ) (%P + %Sn ) × 10 4

X=

(10P + 5SP + 4Sn + As) 100

( weight %)

(all elements in ppm )



The other known contributors to temper embrittlement, arsenic and antimony, are less effective in 2.5Cr-​1Mo steel when they are controlled under 0.020% and 0.004%, respectively [91]. Good resistance to temper embrittlement is generally obtained with a J factor below 200 shut in 40-​ft-​lb transition temp. [94] –​most recent specifications require a J factor less than 150 [88] –​but there is appreciable scatter in the correlation between the J factor and the shift in the 54-​J (40 ft-​lb) CVN TT for individual heats [95].

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Material Selection and Fabrication

Temper embrittlement is measured by testing the notch toughness of the material before and after a slow cooling cycle from 595°C to 315°C. The shift in an energy TT, usually the 54-​J transition, is the measure of embrittlement. Modern ASTM A387 steels are most often fine grain, low silicon, low sulfur, calcium treated for inclusion shape control and occasionally quenched and tempered to provide the highest possible toughness levels. It can be stated with reasonable confidence that Q&T 2.25Cr-​1Mo plate should not experience rapid embrittlement in vessels operating at or below 400°C (750°F) [92]. 2.16.4.5 Step-​cooling Heat Treatment Some specifications additionally require a step-​cooled simulation treatment, which is an accelerated embrittling treatment used to predict temper embrittlement susceptibility in a relatively short test period. Manufacturers will perform this laboratory step-​cooling treatment and meet commonly specified shifts in the 40 ft-​lb CVN TT. 2.16.4.6 CVN Impact Properties Improved Charpy V-​notch properties can be met for A387 steels with Fineline Processing (0.010% or 0.005% maximum sulfur, vacuum degassed, with calcium treatment for inclusion shape control). When thick plates are specified with high CVN toughness requirements, a quenching and tempering heat treatment may be required. Multiple austenizations may also be utilized for increased toughness. 2.16.4.7 Temper Embrittlement of Weld Metal Correlation of the J factor with the temper embrittlement of weld metal is poor. This is probably due to significantly different chemical composition and microstructure of weld metal compared to the wrought product. Therefore, the susceptibility of the weld metal to temper embrittlement is frequently determined by a direct measurement of CVN impact toughness using the following equation [95]:

vTr 40 + 1.5∆vTr 40 < 100F

where vTr40 is the TT for embrittlement weld metal ΔvTr40 is the change in TT for weld metal that has been step-​cooled to cause temper embrittlement. 2.16.4.8 Control of Temper Embrittlement of Weld Metal Control of temper embrittlement in weld metal of 2.25Cr-​1Mo is considerably more difficult than that with plate and forgings. Higher silicon and manganese contents are necessary to sound weld metal. Basic fluxes generally provide the minimum susceptibility to temper embrittlement consistent with high-​temperature strength requirements [88, 96]. 2.16.4.9 Post-​weld Heat Treatment (Stress Relief) ASME boiler and pressure vessel code requires PWHT, or stress relieving, primarily to soften the HAZ, minimizing the presence of hard zones and stabilizing its microstructure. Otherwise, hydrogen attack or creep embrittlement could occur in service. 2.16.4.10 Larson-​Miller Tempering Parameter The Larson-​Miller parameter is usually employed to obtain some idea of the change in a given property of a material during a heat treatment process performed at different temperatures and with different holding times [8, 96]. It is widely used because of its usefulness in summarizing the

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Material Selection and Fabrication

heat treating characteristics of low-​alloy steels and in estimating their longtime strengths at given temperatures [18]. Cr-​Mo steel has a hardened structure in the as-​welded condition. This is given PWHT for changing it to a softened structure (tempered bainite) before use. By softening the structure, the toughness recovers gradually, but the heat treatment parameter commonly known as the Larson-​Miller parameter, P =​ T(20 +​log t), where T is the absolute temperature in Kelvin (K) and t is heat treatment time in hours, becomes large. Excessive increase in this parameter causes reduction in the toughness. Therefore, it is necessary to carefully examine the heat treatment conditions. 2.16.4.11 Reheat Cracking in Cr-​Mo and Cr-​Mo-​V Steels Phosphorus and sulfur were found to enhance the RC susceptibility of Cr-​Mo steels. For a particular alloy, there exists a critical phosphorus content below which embrittlement will not take place. Measures such as (1) reduction of P and S, (2) addition of a small quantity of titanium (0.07%), which decreases the RC susceptibility due to phosphorus effects, and (3) the addition of calcium or rare earth metals, in accordance with sulfur, improve resistance to RC. 2.16.4.12 Modified 9Cr-​1Mo Steel Small additions of niobium (0.06%–​0.10%) and vanadium (0.18%–​0.25%) to the standard 9Cr-​1Mo elevated-​temperature steel result in a steel that is stronger and more ductile. This new steel exhibits improved long-​term creep properties, lower thermal expansion, higher thermal conductivity, and better resistance to SCC [97]. Proposed applications for the modified 9Cr-​1Mo steel include boilers, reaction vessels, breeder reactor systems, pressure vessels for coal liquefaction and gasification, oil refining hydrotreating equipment, and geothermal energy systems. 2.16.4.13 Advanced 3Cr-​Mo-​Ni Steels Advanced 3Cr-​Mo-​Ni steels have been developed for use in thick-​section pressure vessels, specifically for coal liquefaction and gasification, by minor alloy modifications to commercial 2.25Cr-​ 1Mo (ASTM A387, Grade 22, Class 2 steel) [98]. Specifically, they show significantly improved hardenability (i.e. fully bainitic microstructures following normalizing of 400 mm (16 inch) plates), enhanced strength (i.e. yield strengths exceeding 600 MPa), far superior hydrogen attack resistance and better Charpy V-​notch impact toughness, with comparable tensile ductility, creep rupture resistance and temper embrittlement resistance.

2.17 STAINLESS STEELS Stainless steels (SSs) are those alloy steels that have a normal chromium content of not less than 12%, with or without other alloy additions. The thin but dense chromium oxide film which forms on the surface of a stainless steel provides corrosion resistance and prevents further oxidation. SSs are more resistant to rusting and staining than plain carbon steel and low-​alloy steels. They have superior corrosion resistance because of relatively high contents of chromium. These metals are available in both wrought and cast forms.

2.17.1 Classification and Designation of Stainless Steels SS may be classified into five families, according to alloying elements and metallurgical structure: 1. Martensitic SS 2. Austenitic SS 3. Ferritic SS

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4. Duplex SS 5. PH SS. Austenitic stainless steels include the 200 and 300 series of which type 304 is the most common. The primary alloying additions are chromium and nickel. Ferritic stainless steels are non-​hardenable Fe-​Cr alloys. Types 405, 409, 430, 422 and 446 are representative of this group. Martensitic stainless steels are similar in composition to the ferritic group but contain higher carbon and lower chromium to permit hardening by heat treatment. Types 403, 410, 416 and 420 are representative of this group. Duplex stainless steels are supplied with a microstructure of approximately equal amounts of ferrite and austenite. They contain roughly 24% chromium and 5% nickel. Their numbering system is not included in the 200, 300 or 400 groups. Precipitation hardening stainless steels contain alloying additions such as aluminum which allow them to be hardened by a solution and aging heat treatment. They are further classified into sub groups as martensitic, semiaustenitic and austenitic precipitation hardening stainless steels. They are identified as the 600-​series of stainless steels (e.g. 630, 631, 660). 600 series stainless steels are also called as precipitation hardening stainless steels. They have advantages of both austenitic stainless steels and martensitic stainless steels –​that is, corrosion resistance, hardness, and strength. Originally, 600 series stainless steels were designed out for aerospace and aircraft industries. Nowadays, they are applied into making precision turned components. Before machining, they are usually supplied in annealed conditions and are hereafter doing heat treatment. 17-​4PH is now the most extensively applied with industries.

2.17.2 Designations Wrought SSs are assigned designations by the American Iron and Steel Institute (AISI) according to composition: the Cr-​Ni-​Mn austenitic SSs as 2xx series, and the Cr-​Ni austenitic SSs as 3xx series and 4xx series. The PH grades are assigned designations based on their Cr and Ni contents.

2.17.3 ASTM Specification for Stainless Steels Most of the AISI types –​austenitic, duplex, ferritic, and martensitic SSs, plus some special stainless like superferritic and superaustenitic steels –​are included in A240.

2.17.4 Guidance for Stainless Steel Selection Guidelines for selection of the types of available SSs are discussed by Brown [99] and Debold [100]. According to Brown, the following guidelines will serve to select a proper grade for the service under consideration: Select the level of corrosion resistance required for the application. Select the level of strength required. For special fabrication problems, select one of the basic alloy modifications that provides the best fabrication characteristics. Do a cost-​benefit analysis to include the initial raw material price, cost of installation, and the effective life expectancy of the finished product. Determine the availability of the raw material at the most economical and practical choice.

2.17.5 Martensitic Stainless Steel The martensitic SSs are on the lower scale of corrosion resistance, because they contain only 11%–​ 18% Cr, with carbon content usually less than 0.4%. The lower limit on chromium is governed by

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corrosion resistance, and the upper limit by the requirement for the alloy to convert fully to austenite on heat treatment [101]. The key feature of this family is that it can be hardened by heat treatment. Its utility as heat exchanger material in aqueous environments is limited. But these steels do exhibit a useful combination of strength, ductility, toughness, and corrosion resistance in mild environments. Resistance to corrosion is obtained only when the material is fully hardened and tempered [102]. AISI 410 is the most widely used of the martensitic grades. It is occasionally used in heat exchangers.

2.17.6 Austenitic Stainless Steel Properties and Metallurgy Austenitic SSs make up approximately 80%–​90% of the SSs in use today [103]. This class of SS includes both the 200 and 300 series alloys that are hardenable by cold working. The 200 series alloys are originally developed to conserve nickel by replacing it with manganese at a ratio of 2% manganese for each 1% of nickel replaced. The common austenitic SS Type 300 series are low carbon, iron, and chromium alloys sufficiently alloyed with nickel and sometimes manganese or nitrogen, or combinations of these elements, to have an austenitic structure, most or all of it when the steel is cooled rapidly to room temperature. The chromium content is between 15% and 32%, nickel between 8% and 37%, and carbon is restricted to a maximum of 0.03%. Chromium provides oxidation resistance and resistance to corrosion in certain media. An important source book on SSs is Llewellyn [104]. 2.17.6.1 Types of Austenitic Stainless Steel The conventional types of 3xx SS include types such as 304, 304L, 309, 310, 316, 316L, 321, 347, and 348. The basic alloy Type 304 SS contains 18% chromium and 8% nickel. It has moderate strength, excellent toughness, and moderate corrosion resistance. Additional resistance to chloride pitting was achieved by the addition of Mo, creating Types 316 and 317. Types 316 (18% Cr, 12% Ni, 2.5% Mo) and 317 (18% Cr, 15% Ni, 3.5% Mo) have greater resistance to corrosion in chloride environments than Type 304. The characteristics of austenitic SSs are as follows: • • • • • • •

nonmagnetic, ductile, work hardenable nonhardenable by heat treatment single phase from 0 K to melting point crystallographic form –​face-​centered cubic easy to weld not subject to 885°F (475°C) embrittlement and hydrogen embrittlement not subject to DBT temperature transition.

2.17.6.2 Alloy Development The 18Cr-​8Ni austenitic SSs have been successfully used in freshwater and mildly corrosive industrial conditions for more than 50 years. The corrosion resistance, weldability, and strength of the austenitic family of alloys have been constantly improved for more demanding industrial applications by changing the basic chemical composition [105]. Such features include the following: 1. Molybdenum is added to enhance corrosion resistance in chloride environments, such as AISI Types 316 and 317. These steels possess a greatly increased resistance to chemical attack as compared to that of the basic Cr-​Ni Type 304. 2. Low-​carbon steels (Types 304L, 316L, and 317L) are resistant to carbide precipitation in the 800°F–​1600°F range and can thus undergo normal welding without reduction in corrosion resistance. These steels are generally recommended for use below 800°F.

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3. Nitrogen is added to compensate for the reduced strength of the low-​carbon-​grade steels (L grades); it increases strength at all temperatures –​cryogenic through elevated –​improves localized corrosion resistance in acid chloride solutions, and improves pitting resistance and phase stability. Nitrogen addition also improves passivation characteristics and enhances the effects of other alloy element additions, particularly, Cr and Mo, which add corrosion resistance [106]. Nitrogen addition may be of the order of 0.1%–​0.25%. Maximum nitrogen content in austenitic SSs is typically restricted to 0.25% by weight to avoid problems such as ingot porosity, hot workability, and nitride precipitation that are associated with excess nitrogen content. Nitrogen addition is denoted by the suffix N. 4. Chromium is increased to enhance pitting and crevice corrosion resistance. 5. Nickel is added to stabilize the austenitic microstructure and to improve resistance to SCC as well as general corrosion in reducing environments. The effect of nickel on SCC of SSs is explained by the Copson curve. 6. Stabilized grades. The addition of titanium and niobium forms stable carbides, which prevents chromium depletion by the formation of complex chromium carbides, thereby avoiding sensitization of weldments or heat-​treated parts; examples are Type 321 (Ti stabilized) and Type 347 (Nb stabilized). 7. LR stands for low residuals, and in this case the restrictions are on carbon for corrosion resistance. Reducing the carbon also allows the Nb to be reduced and so minimizes the risk of Nb-​rich interdendritic films. There is also restriction on Si, S, and P for liquation cracking resistance. Manganese is usually raised to improve resistance to solidification cracking.

2.17.7 Stainless Steel for Heat Exchanger Applications Stainless steels are used extensively for heat exchangers because their ability to remain clean and hence enhances heat transfer efficiency. Austenitic SSs are used primarily because of their low cost, corrosion resistance, and good mechanical properties over a broad temperature range from cryogenics to high temperature. They have been applied successfully in a large variety of environments including acids, freshwater, and seawater. On the other hand, the martensitic and ferritic SSs have acquired a more restricted field of application due to low toughness at room temperature [107, 108]. 2.17.7.1 Newer Stainless Steels for Heat Exchanger Service New steel-​making technologies like AOD and VIM in the last two decades have introduced a variety of new grades of ferritic, austenitic, and duplex SSs with low-​impurity elements and a wide range of alloying elements tailored to the requirements of specific applications. Against these good properties, the following are the demerits of SSs [109]: 1. sensitive to crevice corrosion under deposits 2. sensitive to pitting corrosion and SCC in the presence of chloride ions if temperature exceeds 50°C 3. sensitization leads to intergranular corrosion 4. sensitive to fouling.

2.17.8 Properties of Austenitic Stainless Steels SSs are known for excellent fabricability, weldability, good mechanical properties (strength, toughness, and ductility) over a broad temperature range, and corrosion resistance in many environments. Other relevant properties are lower melting points, higher electrical resistance, lower thermal conductivity, and higher coefficients of thermal expansion than carbon steels.

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2.17.9 Mechanical Properties at Cryogenic Temperature and Elevated Temperature Although austenitic SSs are used primarily because of their high corrosion resistance, they also possess excellent mechanical properties over a wide range of temperature from cryogenic to elevated temperature. Cryogenic applications. Unlike ferritic materials, austenitic SSs do not exhibit DBT transition. They maintain a high level of toughness at cryogenic temperatures. Austenitic SSs types such as 304, 304L, 316, 316L, and 347 are used in cryogenic applications for liquid gas storage and transportation vessels. Elevated temperature strength. Austenitic SSs exhibit good creep-​rupture strength at temperatures up to 600°C. If still higher creep strength and elevated temperature strength are required, addition of V, Nb, and Ti is necessary. Addition of these elements can lead to an increase in strength, but also a reduction in the low-​temperature toughness. SSs also exhibit high-​temperature oxidation resistance due to the oxide layer formed on the surface.

2.17.10 Alloying Elements and Microstructure The microstructures most important in weldable SSs are ferrite and austenite. Although chromium and nickel are the principal alloying elements in austenitic SSs, other elements are added to meet specific requirements and therefore consideration must be given to their effects on microstructure. Molybdenum, columbium, and titanium promote the formation of delta ferrite in the austenitic matrix and also form carbides similar to that of chromium. These elements are known as ferrite-​forming elements. On the other hand, copper, manganese, cobalt, carbon, and nitrogen have a similar effect to nickel and promote the formation of austenite. These elements are known as austenite-​forming elements. 2.17.10.1 Composition of Wrought Alloys The compositions of typical wrought austenitic SSs are given in Table 2.15. There are several variations for some of those listed. ASTM Spec for bars, sheets and strips are given below: 1. ASTM A484/​A484M-​22 –​Standard Specification for General Requirements for Stainless Steel Bars, Billets, and Forgings. TABLE 2.15 Nominal Composition of Typical Wrought Austenitic SS Grade C (max) Mn (max) P (max) S (max) Si (max) Cr

Ni

304 304L 316 316L 317 317L 321 347

0.08 0.03 0.08 0.03 0.08 0.03 0.08 0.08

2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0

0.045 0.045 0.045 0.045 0.045 0.045 0.045 0.045

0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

18.0–​20.0 18.0–​20.0 16.0–​18.0 16.0–​18.0 18.0–​20.0 18.0–​20.0 17.0–​19.0 17.0–​19.0

8.0–​12.0 8.0–​12.0 10.0–​14.0 10.0–​14.0 11.0–​15.0 11.0–​15.0 9.0–​12.0 9.0–​2.0

348

0.08

2.0

0.045

0.03

1.0

17.0–​19.0 9.0–​2.0

Mo

Other Elements

—​ 2.0–​3.0 2.0–​3.0 3.0–​4.0 3.0–​4.0 Ti =​5C min (0.70 max) Nb +​Ta =​10C min (1.10 max) Nb +​Ta =​10C min (1.10 max)

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2. ASTM A666-​15 –​Standard Specification for Annealed or Cold-​Worked Austenitic Stainless Steel Sheet, Strip, Plate, and Flat Bar.

2.17.11 Alloy Types and their Applications The workhorse materials for the process industries are Types 304, 304L, 316, and 347. SS is used as a heat exchanger material for condensers, feedwater heaters, and other heat exchangers and has wide use in refineries, chemical process industries, fertilizer industries, pulp and paper industries, food processing industries, etc. The properties and their usage of AISI Types 304, 310, 316, 321, and 347 are discussed next. 2.17.11.1 Type 304 Type 304 (18Cr-​8Ni) is the most popular grade in the series and is used in a wide variety of applications that require a good combination of corrosion resistance and formability. Its homogeneous structure, high ductility, and excellent strength ensure excellent performance in cold forming, deep drawing, and spinning. It is nonmagnetic in the annealed condition. Its excellent toughness at low temperature is utilized for the construction of cryogenic vessels. Type 304 is highly resistant to ordinary rusting and immune to foods, most of the organic chemicals, dyes, and a wide range of inorganic chemicals. It resists scaling up to 1600°F. For intermittent heating and cooling applications, temperatures should not exceed 1500°F. The maximum temperature for continuous service is 1650°F [110]. 2.17.11.2 Type 310 Type 310 (25Cr-​20Ni) represents the most highly alloyed composition in the popular range of austenitic SSs and exhibits the greatest resistance to corrosion and oxidation. 2.17.11.3 Type 316 The addition of molybdenum gives this grade the highest resistance to pitting corrosion of any of the chromium-​nickel grades, which makes this grade particularly suitable for applications involving severe chloride corrosion. Therefore, alloys 316 and 316L are “workhorse” materials in both the chemical industries and the pulp and paper industries [111]. Type 316 has excellent resistance to sulfates, chlorides, phosphates, and other salts. Nevertheless, the resistance of Types 316 and 316L to pitting and crevice corrosion is often not high enough, particularly in stagnant or slow-​moving seawater ( GMAW > FCAW > SMAW > SAW Highly basic fluxes have been reported to be beneficial to the as-​welded impact toughness of the duplex stainless steels. When welding a duplex grade to carbon steel a Type 309L filler is usually a good choice for achieving a sound weld. 2.19.10.8  Gas Shielding Gas shielding is generally pure argon although argon/​helium mixtures have given some improvements by permitting faster travel speeds. Nitrogen, a strong austenite former, is an important alloying

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element, particularly in the super/​hyper duplex steels and around 1 to 2% nitrogen is sometimes added to the shield gas to compensate for any loss of nitrogen from the weld pool. Nitrogen additions will, however, increase the speed of erosion of the tungsten electrode. 2.19.10.9 Welding Practices Welding practices should ensure cleanliness, provide inert gas shielding, and avoid carbon contamination. Since ferrite is susceptible for hydrogen embrittlement, low-​hydrogen welding practices should be followed. It is recommended to achieve a ferrite content of approximately 22%–​70%, which is equivalent to 30–​100 FN. A corrosion test in ferric chloride is also carried out as per ASTM G48 to assess the corrosion resistance of the weldment. As per this test, the CPT is specified at 22°C for duplex and 35°C for super duplex steel. 2.19.10.10 Ferrite in Duplex Stainless Steels HAZs in duplex SSs have a tendency to form a high level of ferrite, and this provides a special problem for ferrite measurement. The ferrite content can be kept within acceptance limits by controlling the composition of the base metal and by controlling the heat input. In duplex SSs, HAZ ferrite content determination using a precisely defined metallographic point counting procedure is recommended. Ferrite content by magnetic means can be determined as per ANSI/​ AWS A4.2. 2.19.10.11 Welding Practices to Retain Corrosion Resistance Welding practices to retain corrosion resistance include the following [162]: 1. Special attention is needed to control weld spatter, slag residues, and oxide formation, since these have adverse effects on resistance to pitting and crevice corrosion. 2. As far as possible, post-​weld cleaning is to be done; when cleaning is not possible, restrict oxygen to values of 10 and 25 ppm maximum inert backing gases. To promote arc stability and penetration, small additions of CO2 and O2 may be made to argon shields when employing GMAW technique. 2.19.10.12 Post-​weld Stress Relief Post-​weld stress relief for duplex is not necessary as duplex steels are sensitive to short exposure temperature of 300°C–​1000°C (572°F–​1832°F). In this temperature range, precipitation of alpha prime phase can take place, which in turn reduces toughness and corrosion resistance. Any heat treatment applied to duplex steel should be a PWHT of full solution anneal typically at temperature above 1000°C followed by water quenching.

2.19.11 Nondestructive Testing of Duplex SS Radiographic and dye penetrant inspection procedures are readily applicable. NDT by conventional ultrasonic techniques with angle beam shear waves can be difficult, owing to the anisotropy of the parent materials and weld deposits, especially the tendency to epitaxial columnar structures in the latter [162]. Improved techniques using angle beam compression wave probes and creep wave probes at diminished frequencies are reported. 2.19.12 NORSOK Standard The NORSOK standards are developed by the Norwegian petroleum industry to ensure adequate safety, value adding and cost effectiveness for petroleum industry developments and operations. This standard is applicable to the following material grades and product forms:

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Material Selection and Fabrication

1. duplex SS: all grades, product forms and dimensions 2. high alloyed austenitic SS: all grades, product forms and dimensions 3. nickel base alloys: castings 4. titanium and its alloys: castings. 2.19.12.1 NORSOK M650 Mill Certification NORSOK M650 is the standard that sets down a qualification procedure for mills that want to cast and forge materials specified under NORSOK M630, specifically Duplex Stainless Steels such as A182 F51, High Alloy Austenitic Stainless Steel like 254 SMO, Nickel Base Alloys like Monel and Inconel, as well as Titanium and its alloys. If focuses on the calibration and consistency of the product issued by a mill’s furnaces and the quenching and heat treatment procedures.

2.19.13 Welding Methods for Modern Duplex Stainless Steels Welding Modern duplex stainless steels have generally good weldability. The most common arc welding methods for stainless steels can be used with good results: • • • • •

gas tungsten-​arc welding (GTAW or TIG) gas metal-​arc welding (GMAW or MIG/​MAG) shielded metal arc welding (SMAW or MMA) submerged arc welding (SAW) flux-​cored arc welding (FCAW).

Higher nitrogen contents are extremely helpful in avoiding excessive ferrite content, especially with lower-​alloyed grades. Weld metal toughness is strongly related to the welding process. Non-​flux processes providing greater toughness. Super Duplex stainless steels. Super Duplex stainless steels possesses good weldability and can be joined to itself or other materials by shielded metal arc welding (SMAW), gas tungsten arc welding (GTAW), plasma arc welding (PAW), flux cored wire (FCW), or submerged arc welding (SAW).

2.19.14 Expansion of Tube to Tubesheet Joints Duplex stainless steels have high proof and tensile strength. Test results and practical experience have shown that expansion of tubes can be successfully carried out into tubesheets of considerably lower strength. Normal expanding methods can be used, but the expansion requires higher initial force and should be undertaken in a one-​step operation. As a general rule, tube to tubesheet joints should be welded if the service conditions include a high chloride concentration, thus limiting the risk for crevice corrosion.

2.20 SUPER DUPLEX STAINLESS STEEL 2.20.1 Properties and Characteristics of Super Duplex Stainless Steel • Like other duplex grades, the super duplex grades are not suitable for high or low temperature service. • High strength. • High resistance to pitting, crevice corrosion resistance. • High resistance to chloride stress corrosion cracking, corrosion fatigue and erosion. • Low coefficient of thermal expansion. • Good sulfide stress corrosion resistance. • Good workability and weldability.

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Material Selection and Fabrication

2.20.2 Difference between Duplex and Super Duplex Stainless Steels Super Duplex stainless steel has all of the same benefits as Duplex Stainless Steel but the main difference being that this metal has a higher chromium, nitrogen and molybdenum content, which provides it with increased corrosion resistance.

2.21 SUPERAUSTENITIC STAINLESS STEELS Superaustenitic SSs are alloys with an fcc structure, highly alloyed, usually containing substantial amounts of chromium, molybdenum, and nitrogen, along with sufficient nickel to stabilize a fully austenitic microstructure. They are classified by their molybdenum content, which is in the range of 4.5%–​7%. The addition of 0.3%–​0.5% nitrogen provides yield strength typically twice that of conventional SSs [116]. Relatively high nickel content (18%–​31%) and high chromium content and Mo content give the alloys excellent resistance to SCC. Copper is intentionally added to some of the alloys and improves resistance to reducing media such as hot phosphoric acid, acetic acid, and dilute sulfuric acid. In this section, superaustenitics are covered under two headings: 4.5% Mo superaustenitic steels and 6% MO superaustenitic steels.

2.21.1 4.5% Mo Superaustenitic Steels The 4.5% Mo austenitic alloys like AL 904L, Ciramet, 254SLX, and JS700 have demonstrated sufficient resistance to corrosion in seawater to perform reasonably well as tubing. They are readily weldable and workable, available in a wide range of product forms such as tubing, sheet, plate, and forgings. Nitrogen content in the range of 0.4%–​0.5% gives good resistance to pitting and crevice corrosion [115]. The high nickel (25%) and molybdenum (4.5%) contents provide good resistance to chloride SCC. They also exhibit resistance to intergranular corrosion. Chlorination to control microbiological fouling is necessary to minimize under-​deposit corrosion; flange faces and gasketed surfaces are subject to crevice corrosion in brackish water and seawater [164].

2.21.2 6% Mo Superaustenitic Stainless Steel The 6% Mo superaustenitics are now well established in the chemical process industries. They contain about 20% chromium, nickel contents range from 18% to 25%, and they are enhanced with more than 0.10% nitrogen. Nitrogen addition serves to improve strength, stabilize the austenitic structure, and improve pitting corrosion resistance. The 6% Mo superaustenitics exhibit excellent toughness and ductility characteristic of the 300 series austenitics. The compositions of several 6% Mo superaustenitic SSs (along with Types 304 and 316), are shown in Table 2.27 and their product forms and ASTM/​ASME Code references are listed in Table 2.28.

TABLE 2.27 Nominal Composition of Selected Superaustenitic SS along with Types 304 and 316 UNS No.

Alloy

S30400 S31600 S31254 N08367 N08926 N08366

304 316 254SMO AL-​6XN 1925hMo 25-​6 MO AL-​6X

C

Cr

Ni

Mo

N

Others

0.03 0.03 0.02 0.02 0.01 0.018

18 17 20.0 21 20 21

 8 12 18 24.5 25 24.5

—​ 2.5 6.2 6.5 6.5 6.5

—​ —​ 0.2 0.2 0.2

—​ —​ 0.75Cu 0.75Cu, max 1.1Cu 1.39Mn, 0.41Si

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Material Selection and Fabrication

TABLE 2.28 ASTM/​ASME Spec. for Various Product Forms of AL-​6XN N 08367 Alloy Name

UNS Designation

Sheet, Plate, and Strip

Heat Exchanger Tube Pipe

AL-​6XN

N08367

A 240, B/​SB688

A/​SA 249

A312; B/​SB675, 690, 829

Forgings

Fittings

Rod, Bar, and Wire

B/​SB564, A 182 F62

B462

B/​SB691

TABLE 2.29 Pitting Resistance Number for Selected Superaustenitic Stainless Steels Composition Required to Calculate PREN Alloy

UNS Designation

Cr

Mo

N2

PRENa

S31254 N08367 N08926 N08366 S31277

254SMO™ AL-​6XN ™/​1925hMo™ 25-​6 MO™ AL-​6X 27-​7Mo™

20.0 21 20.5 21 22

6.2 6.5 6.5 6.5 7.2

0.2 0.2 0.2 —​ 0.35

46.46 45.65 45.15 42.45 57.26

 PREN =​%Cr +​3.3 (%Mo) +​30 (%N).

a

2.21.3 Corrosion Resistance In general, the high-​alloy superaustenitic steels show superior resistance to uniform corrosion, pitting and crevice corrosion, and SCC [165]. The 6% Mo superaustenitics resist both localized corrosion and SCC in oxidizing chloride and sulfide/​chloride-​containing solutions, as well as a broad range of process chemicals. They are widely used in severe seawater applications. The corrosion performance of 6% Mo superaustenitics falls between Types 316 and 317 SSs and the nickel-​based alloys 625 and C-​276 [115]. In order to be resistant to some corrosive environments, some steels also contain additions of copper, which improves its resistance to acids in general. The fully austenitic 6% Mo steels are primarily characterized by a high (Cr +​Mo) content, with a PRE given as [166] follows:

PRE N = %Cr + 3.3 (%Mo ) + 30 (%N )

exceeding 35 in all of the steels. Pitting resistance number for selected superaustenitic stainless steels is given in Table 2.29.

2.21.4 Applications In process industries, the 6% Mo superaustenitic SSs have replaced common austenitic SSs that have failed by pitting, crevice corrosion, and chloride SCC [167]. They have been used extensively in the offshore and desalination industries, in seawater handling, in chlorine and chlorine dioxide stages and bleach plants in the pulp and paper industries, and in flue gas desulfurization plants. Equipment fabricated of 6% Mo austenitics includes pressure vessels, columns, seawater-​cooled condensers, evaporators, heat exchangers, crystallizers, pumps, piping, and components.

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237

2.21.5 Welding Alloy 6Mo has a good weldability and the methods used for welding conventional austenitic steels are used. Because of Alloy 6Mo is more sensitive to hot cracking it shall be welded with low heat input. Recommended solution annealing temperature interval is 1130°C–​1190°C with rapid quenching in water which guarantees a stable microstructure without the presence of secondary intermetallic chi and sigma phases. In general, fully austenitic 6% Mo superaustenitic SSs exhibit satisfactory weldability. The main concern when using the superaustenitic SSs is adequate corrosion resistance in welds. During welding, particular concern has to be paid to the following three phenomena: 1. hot cracking 2. molybdenum microsegregation 3. precipitation of intermetallic phases. Since the carbon content of the 6% Mo steels is low (99% Cu >96% Cu Cu-​Zn Cu-​Zn-​Pb Cu-​Zn-​Sn-​Pb Cu-​Al-​Ni-​Fe-​Si-​Sn Cu-​Si-​Sn Cu-​Ni-​Fe

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possible blockage in turbine units. With the advent of the high-​pressure supercritical units, the use of carbon steel tubes in feedwater heaters became common practice in order to eliminate the main source of copper carryover into the steam generator [47, 78]. 2.23.3.2 Copper-​Nickels (Cu-​Ni-​Fe), C70000 –​ C79900 Copper-​nickels or cupronickels, C70400, C70600, C71000, C71500, and C72200, are single-​phase solid solution alloys, with nickel as the principal alloying element. Alloys containing 10% and 30% nickel are important from the heat exchanger construction point of view. They are resistant to fresh-​, brackish, and seawater. Copper-​nickel alloys with the addition of iron and sometimes manganese are resistant to erosion-​corrosion and SCC. Iron enhances the resistance to impingement attack of these alloys, if it is in solid solution. The presence of iron in small microprecipitates can be detrimental to corrosion resistance. Copper-​nickels are an alternative to inhibited admiralty metal tubes where cooling-​water velocity is high. General corrosion rates for C70600 and C71500 in seawater are about 1 mpy. Maximum design velocities for condenser tubes are 3.6 m/​s (12 ft/​s) for C70600 and 4.6 m/​s (15 ft/​s) for C71500. At elevated temperatures, the creep strength of cupronickels is greater than that of copper and brasses. Therefore, they find use in high-​temperature and high-​pressure feedwater heaters and heat exchangers [187]. The cupronickels are highly resistant to SCC. Of all the copper alloys, they are the most resistant to SCC in ammonia environments. Therefore, they are sometimes installed in the air removal sections of large-​surface condensers. They are prone to non-​uniform and pitting corrosion, if sulfide is present, for instance, during intermittent service [109]. Dealloying has rarely been seen in the cupronickels. The 10% copper-​ nickel (90Cu-​ 10Ni), C70600, finds applications in heat exchangers and condensers in seawater applications, feedwater heaters, and refinery heat exchangers. The 30% copper-​nickel (70Cu-​30Ni), C71500, is also known as chromium copper-​nickel (67.2Cu-​ 30Ni-​2.8Cr). Among all the copper alloys, this alloy exhibits excellent resistance to impingement attack, SCC, and corrosion of most acids and waters and steam condensates. It is used in applications involving severe corrosion problems such as power-​plant condensers, feedwater heaters, shipboard heat exchangers and condensers, refinery heat exchangers like overhead condensers and coolers, and after coolers. Because of its lower thermal conductivity than brass, some designers provide 5%–​10% more heat transfer area [187]. 2.23.3.3 Designation of Copper and Copper Alloys Used as Heat Exchanger Materials A detailed list of copper and its alloys and the nominal compositions used in various types of heat exchangers is given in Table 2.38 [188]. 2.23.3.4 Product Forms –​Copper Tubes for Heat Exchanger Copper alloys are available in shapes like plates, sheet and strip, heat exchanger tubes (bare and integral fin), and ferrule stock. ASTM designations for heat exchanger tubes are shown in Table 2.39, and for plates, sheets, and strips in Table 2.40. Some ASTM Specifications for heat exchanger copper tubes are shown below: 1. ASTM B111-​ 98(2004) –​ Standard Specification for Copper and Copper-​ Alloy Seamless Condenser Tubes and Ferrule Stock. 2. ASTM B359/​B359M-​18 –​Standard Specification for Copper and Copper-​Alloy Seamless Condenser and Heat Exchanger Tubes With Integral Fins. 3. ASTM-​B395 –​Standard Specification for U-​Bend Seamless Copper and Copper Alloy Heat Exchanger and Condenser Tubes.

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TABLE 2.38 Copper and Its Alloys Used in Heat Exchangers UNS No.

Common Name

Composition

C10100 (OFE) C10200 (OF) C10300 C10400 C10500 C10700 C10800 C11000 C11300 C11400 C11500 C11600 C12000 (DLP)

Oxygen-​free electrolytic (electronic) copper Oxygen-​free copper without residual deoxidants Oxygen-​free extra-​low-​phosphorus copper Oxygen-​free copper with silver

99.99% Cu min 99.95% Cu min 99.95% Cu min 99.95% Cu min

Oxygen-​free low-​phosphorus copper Electrolytic tough-​pitch copper Silver-​bearing tough-​pitch copper

99.95% Cu min 99.95% Cu min 99.95% Cu min

Phosphorus-​deoxidized copper (low-​residual copper) Phosphorus-​deoxidized copper (high-​residual copper) Phosphorus-​deoxidized copper (high-​residual copper) Fire-​refined copper

99.9% Cu min

Phosphorized, arsenical copper

99.94% Cu min, 0.1%–​0.5% As, 0.015%–​0.04% P

C19200 C19400 C23000 C26000 C26800 C27000 C28000 C36500

Phosphorized copper with 1% iron High-​strength modified copper Red brass Cartridge brass Yellow brass

98.97% Cu, 1.0% Fe, 0.03% P 97.4% Cu, 2.4% Fe, 0.13% Zn, 0.04% P 84%–​86% Cu, remainder Zn 70% Cu, 30% Zn 65% Cu, 35% Zn

Muntz metal Leaded Muntz metal, uninhibited

C36600

Arsenical inhibited leaded Muntz metal

60% Cu, 40% Zn 58%–​61% Cu, 0.4%–​0.9% Pb, 0.25% Sn max, remainder Zn 58%–​61% Cu, 0.4%–​0.9% Pb, 0.25% Sn max, remainder Zn

C36700 C36800 C44300 C44300 C44400 C44500 C46400 C46500 C46600 C46700 C60800 C61300 C61400

Antimonial inhibited Phosphorus inhibited Admiralty brass Arsenical inhibited admiralty brass Antimonial inhibited admiralty brass Phosphor inhibited admiralty brass Uninhibited naval brass Arsenical inhibited naval brass Antimonial inhibited Phosphorus inhibited Aluminum bronze Aluminum bronze, 7% Aluminum bronze D

C12200 (DHP) C12300 (DHP) C12500 C12700 C12800 C12900 C13000 C14200

99.9% Cu min 99.9% Cu min 99.88% Cu min, remainder Ag, As, Sb, and others

70%–​73% Cu, 0.9%–​1.2% Sn, remainder Zn

59%–​62% Cu, 0.5%–​1.0% Sn, remainder Zn

95% Cu, 5% Al 90% Cu, 6%–​7.5% Al, 0.15% Ni, 2%–​3% Fe 91% Cu, 7% Al, 2% Fe

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TABLE 2.38  (Continued) Copper and Its Alloys Used in Heat Exchangers UNS No.

Common Name

Composition

C63000 C63200 C65100 C65500 C68700 C70400 C70600 C71000 C71500 C71640 C71900 C72200

10% Aluminum-​nickel bronze 9% Aluminum-​nickel bronze Low-​Si bronze B High-​Si bronze A Aluminum brass D, arsenical inhibited Copper-​nickel, 5% Copper-​nickel, 10% Copper-​nickel, 20% Copper-​nickel, 30% Copper-​nickel-​iron-​manganese Chromium copper-​nickel Chromium copper-​nickel

82% Cu, 10% Al, 5% Ni, 3% Fe 82% Cu, 9% Al, 5% Ni, 4% Fe 98.5% Cu, 1.5%–​2.0% Si, 0.7% max Mn 97% Cu, 3% Si 77.5% Cu, 20.5% Zn, 2% Al 95% Cu, 5% Ni 90% Cu, 10% Ni 80% Cu, 20% Ni 70% Cu, 30% Ni 29%–​32% Ni, 1.7%–​2.3% Fe, 1.5%–​2.5% Mn 67.2% Cu, 30% Ni, 2.8% Cr 83% Cu, 16.5% Ni, 0.5% Cr

Source:  Compiled from [188] ASM.

TABLE 2.39 ASTM Specifications for Copper Alloy Heat Exchanger Tubes ASTM Spec.

Description

Alloys UNS No.

B111

Copper and copper alloy seamless condenser tubes and ferrule stock

B359

Copper and copper alloy seamless condenser and heat exchanger tubes with integral fins U-​bend seamless copper and copper alloy heat exchanger and condenser tubes Welded copper and copper alloy heat exchanger tube Seamless and welded copper-​nickel tubes for water desalting plants

C10100, C10200, C10300, C10800, C12000, C12200, C14200, C44500, C60800, C61300, C61400, C68700, C70400, C70600, C71000, C71500, C71640, C72200 C10100, C10200, C10300, C10800, C12000, C12200, C14200, C19200, C23000, C44300, C44400, C44500, C60800, C68700, C70400, C70600, C71000, C71500, C72200 C10200, C10300, C10800, C12000, C12200, C14200, C19200, C23000, C44300, C44400, C44500, C60800, C68700, C70400, C70600, C71000, C71500, C72200 C10800, C12200, C19400, C23000, C44300, C44400, C44500, C68700, C70400, C70600, C71000, C71500, C71640, C72200 C70600, C71500, C71640, C72200

B395

B543 B552

TABLE 2.40 Copper Alloy Plates, Sheets, and Strips ASTM Spec.

Description

Alloys UNS No.

B171

Copper alloy plate and sheet for pressure vessels, condensers, and heat exchangers

B569

Brass strip in narrow widths and light gage for heat exchanger tubing Copper-​silicon alloy plate, sheet, strip, and rolled bar for general purposes and pressure vessels

C36500. C36600, C36700, C36800, C44300, C44400, C44500, C46600, C46700, C61300, C61400, C63000, C63200, C70600, C71500, C72200 C26000

B96

C65100, C65400, C65500, C65800

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2.23.4 Copper Corrosion 2.23.4.1 Corrosion Resistance Copper is classed as a noble metal, and therefore copper is corrosion resistant. The formation of a thin protective layer on the surface is the reason for the corrosion resistance of the copper alloys, such as brass, copper-​nickel, and bronze. The presence of oxygen or other oxidizing agent is essential for corrosion to take place. Copper alloys are resistant to neutral and slightly alkaline solutions with the exception of those containing ammonia, which causes SCC [131]. They are resistant to urban, marine, and industrial atmospheres. Copper and most of the copper alloys are sensitive to velocity effects. They are resistant to water and find use in domestic and industrial systems and in seawater applications. They are attacked by oxidizing acids, NH4OH plus O2 can cause SCC, and hydrogen sulfide, sulfur, and its other compounds attack [189]. Important sources on corrosion of copper are Ref. [183] and Cieslewicz and Schweitzer [190]. 2.23.4.2 Galvanic Corrosion Copper and copper alloys occupy a mid position in the galvanic series. Copper is cathodic to aluminum and SS, yet anodic to passive SS, high-​nickel alloys, and titanium. 2.23.4.3 Pitting Corrosion The main drawback of copper alloys is their susceptibility to pitting corrosion in water containing the combination of oxygen and sulfide. Hence, copper is not suitable for coastal water applications. However, alloys C70600 and C71500 display excellent resistance to pitting in seawater. 2.23.4.4 Intergranular Corrosion The most susceptible alloys for this form of corrosion are Muntz metal, admiralty metal, aluminum brasses, and silicon bronzes. 2.23.4.5 Dealloying (Dezincification) Various forms of dealloying, including dezincification, have been covered in Chapter 3, of “Heat Exchangers: Operation, Performance, and Maintenance ”. Dealloying of copper-​base materials is briefly discussed here. Brasses containing more than 15% Zn, such as leaded Muntz metal, admiralty brass, and naval brass, are susceptible to dealloying when subjected to prolonged contact with slow-​moving water or mildly acidic water. Details of trends of dezincification, SCC, and impingement attack with increasing zinc content in copper-​zinc alloys are discussed in Ref. [81]. Method to control dezincification. Small amounts (0.02%–​0.11%) of As, Sb, or P added to admiralty brass (C44300) impart high resistance to dezincification, and the inhibited alloys are called arsenical admiralty brass, antimonial admiralty brass, and phosphorous admiralty brass. Similarly, the addition of As, Sb, or P tends to inhibit dezincification in leaded Muntz metal C36500 and in naval brass (C46400). The uninhibited alloys and the corresponding inhibited alloys are given in Table 2.41.

TABLE 2.41 Uninhibited and Inhibited Alloys Uninhibited Alloys

Inhibited Alloys

C36500 Leaded Muntz metal C44300 Uninhibited admiralty brass C46400 Uninhibited naval brass

C36600, C36700, C36800 –​As, Sb, P inhibited leaded Muntz metal, respectively C44300, C44400, C44500 –​As, Sb, P inhibited admiralty brass, respectively C46500, C46600, C46700 –​As, Sb, P inhibited naval brass, respectively

Material Selection and Fabrication

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2.23.4.6 Dealuminification Dealuminification in the 5%–​8% aluminum bronzes is occasionally reported, but it is not a significant problem until aluminum reaches the 9%–​11% range [191]. Dealloying is rarely seen in all alpha, single-​phase alloys such as aluminum bronzes C60800, C61300, and C61400, and when seen, conditions of low pH and high temperatures are usually present. 2.23.4.7 Denickelification Denickelification is occasionally reported in the higher nickel copper-​nickel alloys, 70Cu-​30Ni (C71500), in feedwater pressure tubes at temperatures over 212°F (100°C), in low-​flow conditions, and in high local heat flux [135], and in refinery overhead condensers, where hydrocarbon streams condense at temperatures above 300°F (149°C) [191]. According to Tuthill [191], denickelification problem appears to be associated with hot spots that develop in the tubing as a result of fouling and thermogalvanic differences that arise. 2.23.4.8 Erosion-​Corrosion Erosion-​corrosion is the common tubeside phenomenon in copper and some copper alloys with water on the tubeside as a result of excessively high velocity and turbulence. To improve resistance to erosion-​corrosion, copper is alloyed with iron, chromium, and titanium [109]. Aluminum brass and copper-​nickels are highly resistant to erosion-​corrosion. 2.23.4.9 Stress Corrosion Cracking Pure copper is immune to SCC, but the higher zinc content brasses with Zn content in the range of 20%–​40% are subject to SCC. Susceptibility increases as zinc content increases from 20% to 40%. Prominent copper alloys susceptible to SCC include C23000, C26000, C26800, C27000, C28000, C36500, C44300 (uninhibited), and C46400. Copper-​nickels and pure copper are more resistant. The combined action of at least three substances usually is necessary to produce SCC in stressed copper-​base alloys [191]: (1) ammonia or ammonia-​producing material (organic and inorganic substances containing nitrogen), (2) moisture (or water), and (3) oxygen. SCC in copper alloys is usually intergranular, whereas admiralty and aluminum brass almost invariably crack transgranularly and so does brass [192]. Cracks can be detected by eddy current inspection. 2.23.4.10 Steam-​side Stress Corrosion Cracking Among the copper alloys typically used for condenser tubes, the brasses (copper-​zinc) are the most susceptible to SCC in environments containing ammonia. Other susceptible materials include arsenical aluminum brass and admiralty brass. The environmental conditions leading to SCC on the steam side of copper alloy condenser tubes are ammoniacal solutions containing dissolved oxygen. The ammonia is derived from the water treatment compounds used for boiler feedwater, which decompose due to thermal effects in the boiler [193–​195]. Test for SCC –​ASTM B154. ASTM B154, mercurous nitrate test, is a standard test method for detecting SCC of copper and copper alloys. This test method is an accelerated test for detecting the presence of residual (internal) stress that might result in SCC of individual parts in storage or in service. 2.23.4.11 Condensate Corrosion Copper alloy tubes, particularly brass, can withstand the action of steam condensate in a very satisfactory manner except where the steam condensate contains high concentrations of ammonia and oxygen [194]. Condensate corrosion, also known as “ammonia attack”, on the steam side of condensers, generally takes place in the air removal section where ammonia and oxygen concentrations are particularly high. Many of the methods of preventing ammonia SCC are also helpful to prevent condensate attack.

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Material Selection and Fabrication

2.23.4.12  Deposit Attack Deposit attack on condenser tubes occurs under conditions of stagnant or low water velocities, generally less than 3.1 ft/​s (1 m/​s). Deposition of waterborne particles on tube surfaces leads to differential aeration and anodic dissolution of the tube material [196]. Deposit attack is overcome by the following measures [194]: • maintain tubeside water velocity above 1 m/​s and avoid stagnant conditions in aggressive media by flushing with treated water during shutdown • use more resistant materials like 70/​30 and 90/​10 copper-​nickels and titanium. 2.23.4.13  Hot-​spot Corrosion Hot-​spot corrosion is a localized form of pitting corrosion that takes place at “hot spots” on the condenser tube wall due to low water velocity and/​or high heat flux [196]. These conditions can be caused by poor steam distribution or the absence of water on the cooling-​water side of the tube. This form of corrosion rarely takes place in main steam condensers. 2.23.4.14  Snake Skin Formation Snake skin formation is the result of deposition of copper corrosion product carried from low-​ pressure feedwaters employing admiralty brass tubes or high-​pressure feedwater heater tubes made of Monel 400 or 70Cu-​30Ni. The deposition results in thin flaky skin formation similar in appearance to skin shed by snake. This snake skin falls from the tubes when dried [194]. 2.23.4.15  Corrosion Fatigue Copper and copper alloys such as beryllium copper, cupronickels, phosphorus bronzes, and aluminum bronzes exhibit resistance to corrosion fatigue in many environments involving corrosiveness and applied stress. 2.23.4.16 Biofouling Copper and copper alloys including copper-​nickel exhibit excellent resistance to marine biofouling such as barnacles, mussels, and marine invertebrates. 2.23.4.17  Cooling-​water Applications The principal constituents of water that affect the performance of copper alloys are dissolved oxygen, nutrients, bacteria, organisms, biofouling, sediment, and residual chlorine from the chlorination practice [191]. Inhibited admiralty brass, aluminum brass, and cupronickels exhibit excellent resistance in seawater and show erosion-​corrosion and biofouling resistance. Hence, these alloys are now extensively used in fossil power-​plant heat exchangers. 2.23.4.18  Resistance to Seawater Corrosion Corrosion resistance of copper and copper alloys in seawater is determined by the nature of the naturally occurring and protective corrosion product film [191]. The main drawback of copper alloys is their susceptibility to pitting corrosion in sulfide-​bearing seawater in the presence of oxygen. This is especially true with polluted coastal water. Sulfide attack is further discussed next. 2.23.4.19  Sulfide Attack Sulfide attack is the accelerated corrosion of copper alloys that occurs when the cooling water, most often brackish water or seawater, is polluted with sulfides, polysulfides, or elemental sulfur, and no copper alloy is resistant to sulfide attack [194]. The major effect of the sulfide is to destroy the existing protective surface oxide film, if any. It can greatly increase general corrosion, and it

Material Selection and Fabrication

257

can induce or accelerate dealloying, pitting, and erosion-​corrosion [194]. The corrosion attack is enhanced in the presence of oxygen. 2.23.4.20 Exfoliation Copper-​nickel alloy feedwater heater tubes are subjected to exfoliation [189, 197]. Exfoliation is due to pronounced schistous oxidation of the materials, and the resulting loss in strength of the feedheater tubes, which is no longer sufficient to withstand the water pressure and hence burst. The factors that cause exfoliation are (1) cyclic and peak loads, that is where the plant was frequently started up and shutdown, and (2) the presence of oxygen, water vapor, and water. Among the copper-​nickels, 70:30 copper-​nickel was the most susceptible, but 80:20 and 90:10 are better. Monel alloy 400, Inconel alloy 600, and titanium retain their “as-​installed” appearance under test conditions [197].

2.23.5 Copper and Aquatic Life Copper is declared poisonous to aquatic life. Hence its discharge into the source of cooling water is prohibited by laws and environmental regulations.

2.23.6 Copper Welding Copper and its alloys are welded by SMAW, GTAW, PAW, SAW, and OAW. Of these, GTAW and GMAW are the most popular, with argon, helium, or nitrogen gas shielding. The primary criteria for choosing between GMAW and GTAW are thickness of the metal to be welded and amount of welding to be performed. GMAW is generally preferred for thicknesses greater than about 1/​4 in. (6.4 mm). The GMAW process provides intense heat generated by the arc, high deposition, dense deposits, low preheat and interpass temperatures, good properties, and minimum distortion [184]. GTAW is used for thickness less than about 0.08 in. (2 mm). Generally, welding is carried out using dcsp with a 3:1 mix of helium and argon. For thickness between 2 and 6 mm, both processes can be used. Other joining methods include resistance welding and induction welding, brazing, and soldering. It is worth bearing in mind that electron beam and friction welding, including friction stir, have been used extensively and very successfully to weld thick section copper without the need for filler metals, high preheat temperatures and expensive shielding gases. Welding of copper and its alloys is discussed in detail by Gaffoglio [184], in Ref. [198], Dawson [199–​201], and in Ref. [202–​204]. 2.23.6.1 Weldability Copper alloys can be welded with most of the conventional welding processes although of the arc welding processes, gas shielded arc methods are the most common. Two characteristics of copper alloys to be considered while welding copper alloys are: 1. high thermal conductivity, meaning that preheat is required for many joints, even at quite modest thicknesses 2. high coefficient of thermal expansion, meaning that distortion can be an issue with root gaps rapidly closing during welding. When welding copper, allowances are necessary for the chilling of the welds because of the very high thermal conductivity of the metal. Preheating is often required. Considerable distortion is possible in coppers because of the higher thermal expansion than other commercial metals (i.e. distortion can be an issue with root gaps rapidly closing during welding). These characteristics pose difficulties that must be overcome for satisfactory welding.

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Material Selection and Fabrication

2.23.6.2 Factors Affecting Weldability Other than the elements that comprise a specific alloy, the principal factors that influence weldability are (1) thermal conductivity, (2) preheating, (3) thermal expansion, (4) alloying elements, (5) surface cleanliness and surface oxide, (6) shielding gas, (7) joint design, (8) welding position, (9) consideration for precipitation-​hardenable (PH) copper alloys, (10) hot cracking, and (11) porosity. 2.23.6.3  Thermal Conductivity Unless adequate measures are taken to counteract the rapid heat sink effect, it is not possible to establish the fully fluid weld pool necessary for good fusion, adequate joint penetration, and deoxidation [184, 202]. Therefore, the type of current and the shielding gas must be selected for maximum heat input to counteract the rapid heat dissipation from the weld region. Make all joint preparation with wide root gaps and tack frequently. 2.23.6.4 Preheating The relatively high thermal conductivities of copper and most copper alloys result in the rapid conduction of heat from the weld joint to the surrounding base metal. Preheating copper is generally necessary for thicknesses above about 1/​8 in. (3 mm) when using argon shielding [202]. Preheating of these alloys will reduce welding heat requirements for fusion. Recommended preheat temperature varies from 302°F to 1292°F (150°C to 700°C) depending on the thickness, shielding gas, welding current, and process used. 2.23.6.5  Thermal Expansion The high thermal expansion causes root gaps to close as welding proceeds, and due allowance must be made when fixturing the assembly for welding. Figure 2.24 shows typical joint design for welding of copper and copper alloys. 2.23.6.6  Hot Cracking Copper alloys with wide liquidus-​to-​solidus temperature ranges, such as copper-​tin and copper-​ nickel, are susceptible to hot cracking. Low-​melting interdendritic liquid solidifies at a temperature

FIGURE 2.24  Typical joint design for welding of copper and copper alloys.

Material Selection and Fabrication

259

lower than the bulk dendrite. Shrinkage stresses produce interdendritic cracking during solidification. Hot cracking can be minimized by reducing restraint on the weldment, preheating to slow the cooling rate, reducing the magnitude of the welding stresses, reducing the root opening, and increasing the size of the root pass. 2.23.6.7 Porosity Elements such as zinc, lead, cadmium, and phosphorus have low boiling points. Vaporization of these elements during welding will result in porosity. Porosity can be minimized by fast weld speeds and use of filler metal low in these elements. 2.23.6.8 Welding Precautions When welding thick copper with preheats of over 250°C and welding currents of more than 350 amp., the health and safety of the welder and personnel working in the vicinity must be considered. Lagging the item being welded with thermal blankets is essential as is the provision of adequate screening from the very powerful TIG or MIG arc. The welder should select a dense filter glass when using welding currents above 300 amps to reduce eye strain. Carbon, stainless steel or ceramic tiles or tape can be used as temporary backing strips and are helpful in controlling root bead shape.

2.23.7 Copper Alloys Specific Weldability Considerations 2.23.7.1 Copper Alloys The most notable feature distinguishing the welding of most copper alloys from the welding of pure copper is the reduction of heat input and preheating requirements due to low thermal conductivities of the copper alloys compared to the pure metal. Copper alloys commonly welded, notably brasses, aluminum bronzes, and cupronickels, are readily weldable by all gas-​shielded arc welding processes. Metallurgical and welding features that may require particular attention are discussed under the individual alloys. 2.23.7.2 Brasses Alloying elements lower conductivity. The alloys most commonly welded are naval brass, admiralty brass, and aluminum brass. Compared to the coppers, brasses require much less preheat and less current to weld. Copper’s problems with hydrogen and oxygen reactions are absent when welding brasses, because brasses have low-​hydrogen solubility limits and zinc deoxidizes [184]. Lead contributes to hot cracking. Evolution of zinc fume causes suffocation to the welder and obstructs the view of the welding. Zinc fume makes welds porous and unacceptable, particularly in autogenous welding [200]. The gas-​shielded arc processes provide a means of reducing problems associated with zinc volatilization to a minimum, and due consideration should be given to the possibility of using a nonmatching filler metal like silicon bronze or aluminum bronze, which will reduce the evolution of zinc fumes by forming a surface film on the weld pool [200, 202]. When using argon-​shielded TIG welding, alternating current is essential, and for direct current working, helium is preferred [202]. 2.23.7.3  Silicon Bronzes Silicon bronzes are the most weldable of the copper alloys. Adding as little as 1.5% silicon reduces conductivity to 15% of that of copper. As a result, preheating is not essential, and little heat input is required so that welding speeds can be high [184]. Both TIG and MIG processes are suited to silicon bronzes. Silicon bronze is hot short in the temperature range of 1472°F–​1742°F (800°C–​950°C), and cooling through this critical temperature range should be as rapid as possible, especially when the

260

Material Selection and Fabrication

welded structure is restrained. This is particularly faced when using the MIG welding process, known for high welding speeds and high deposition rates. However, with rapid cooling, there is a possibility of forming brittle, nonequilibrium phases in the weld metal, which, under conditions of restraint, can cause weldment cracking [200]. 2.23.7.4  Copper-​Aluminum Alloys (Aluminum Bronzes) Due to relatively high thermal conductivity and the ease with which refractory oxide films form on its surface, aluminum bronzes require a highly concentrated heat source with full shielding of the weld pool [203]. The high gas solubility and affinity of oxygen for molten aluminum bronze, its higher thermal expansion and contraction, and its susceptibility for hot shortness, particularly single-​phase aluminum bronzes that contain less than 7% Al, should be considered. Clean welding conditions, a proper weld design and joint preparation, and a welding procedure to accommodate thermal stresses and reduce the tendency for weld metal cracking are suggested [203]. 2.23.7.5  Copper-​Nickel or Cupronickel Alloys Thermal and electrical conductivities approximate those of carbon steels. These properties facilitate welding without preheating, and the interpass temperature during welding should not exceed 150°F (66°C).

2.23.8 PWHT Copper alloys are not frequently post-​weld heat-​treated, but they may require controlled cooling to minimize residual stress and hot shortness. After welding, the HAZ will be softer and weaker than the adjacent base metal. For copper-​zinc alloys, post-​weld stress-​relief heat treatment at 482°F–​ 572°F (250°C–​300°C) is advisable from SCC point of view [202].

2.23.9 Dissimilar Metal Welding The requirement to join copper and copper alloys to other copper alloys or to other nonferrous or ferrous alloys is frequently encountered. Important factors to be considered when welding dissimilar metals include the following [201]: interalloying, electrochemical corrosion, differential thermal expansion, and weld metal dilution.

2.24 NICKEL AND NICKEL-​BASE ALLOYS METALLURGY AND PROPERTIES Nickel is a hard, tough, and malleable white metal. Nickel exhibits unique properties of high-​ temperature strength, toughness, wear resistance, and resistance to corrosion and oxidation. Nickel alloys were developed to withstand corrosives found in the chemical, petrochemical power, marine, and pulp and paper industries. Nickel readily forms alloys with ferrous and nonferrous metals like Fe, Cr, Cu, and Co. The properties that favor the nickel and nickel-​base alloys for construction of process equipment include the following: 1. Ability to withstand a wide variety of severe operating conditions involving high temperature, high stresses, corrosive environments, and a combination of these factors. 2. Nickel-​base superalloys exhibit good resistance to corrosion, creep-​rupture, fatigue, thermal fatigue, thermal shock, and impact. 3. Resistance to elevated-​temperature oxidation, sulfidation, and carburization. 4. Used as an alternative material in place of austenitic SS to combat SCC. For all practical purposes, nickel alloys containing more than 30% nickel are immune to SCC.

Material Selection and Fabrication

261

5. Ability to be formed by conventional methods and to be joined by most conventional welding process such as GTAW, GMAW, and SMAW using coated electrodes. 6. Purity and nontoxic character are exploited in food processing and fine chemical industries. 7. Low-​temperature application. Nickel is an fcc metal that retains good ductility and toughness at subzero temperatures. Among the nickel alloys, Monel K-​450, Hastelloy B, Hastelloy C, Inconel alloy 600, Inconel alloy 706, Inconel alloy 718, Invar-​36, andInconel X-​700 exhibit excellent combinations of strength, ductility, and toughness up to −263°C [205].

2.24.1 Classification of Nickel Alloys Nickel alloys are in general classified into the following groups: 1. commercially pure nickel 2. nickel-​copper alloys and copper-​nickel alloys 3. nickel-​chromium alloys 4. nickel-​iron-​chromium alloys. Important sources on material properties of various nickel, nickel-​copper, Inconel, and Incoloy alloys include Refs. [206–​208]. 2.24.1.1 Commercially Pure Nickel Alloys 200 and 201 are the major examples of this class. Because of its corrosion resistance, nickel is used to maintain product purity in the processing of foods and synthetic fibers. Nickel is highly resistant to various reducing chemicals and is unexcelled in resistance to caustic alkalies. A major area for use of alloys 200 and 201 is in caustic evaporators because of their outstanding resistance to hot alkalies. Thermal conductivity of nickel is relatively high. In reducing environments, such as dilute sulfuric acid, nickel is more corrosion resistant than iron but not as resistant as copper or nickel-​copper alloys. Annealed nickel has a low hardness and good ductility and malleability. These attributes, combined with good weldability, make the metal highly fabricable. 2.24.1.2 Nickel-​Copper Alloys and Copper-​Nickel Alloys Nickel and copper are completely soluble in each other so that a series of alloys has been available. Nickel-​copper alloys are known as Monel alloys. Monel alloy 400, Monel alloy R-​405, Monel alloy 450, and Monel alloy K-​500 offer somewhat higher strength than unalloyed nickel without loss of ductility. Monel alloys resist corrosion in a broader range of environments. 2.24.1.3 90–​10 and 70–​30 Copper-​Nickel Alloys There are two main copper-​nickel alloy grades used in marine service that are generally available in most product forms. These are copper-​base alloys with either 10% or 30% nickel and are described as 90–​10 and 70–​30 copper-​nickel, respectively. Both alloys contain small additions of iron and manganese that have been chosen to provide the best combination of resistance to flowing seawater and overall corrosion resistance. The 30% nickel alloy is stronger and can withstand higher seawater velocities, but for most applications, the 90–​10 alloy provides good service at a lower cost. The nominal composition of commercial pure nickel and Monel (nickel-​copper) is given in Table 2.42 and their products form in Table 2.43. 2.24.1.4 Inconel and Inco Alloy Inconel includes nickel-​ chromium alloys and Inco nickel-​ chromium alloys. The combination of nickel and chromium in the alloys provides resistance to both reducing and oxidizing corrosive solutions. The nickel-​ chromium alloys resist oxidation, carburization, and other forms of

262

Material Selection and Fabrication

TABLE 2.42 Nominal Composition of Major Alloying Elements of Pure Nickel and MONEL (Nickel-​ Copper Alloys) Alloy Name

UNS Designation

Nickel 200 Nickel 201 Monel 400 Monel R-​405 Monel 450 Monel K-​500

N02200 N02201 N04400 N04405 N044… N05500

Ni

Cu

Fe (max)

Mn (max)

Others

99.0 (min) 99.0 (min) 66.0 66.0 31.0 66.5

0.25 0.25 30.0 31.0 68.0 30.0

0.40 0.40 1.5 1.2 0.7 1.0

0.35 0.35 2.0 2.0 —​ 0.8

C0.08 C0.02 C0.12 C0.15 —​ Al 2.8 C0.1

TABLE 2.43 STM Spec. for Various Product Forms of Pure Nickel and Monel Alloys (Nickel-​Copper Alloys) Alloy Name

UNS Designation

Nickel 200 N02200 Nickel 201 N02201 Monel alloy 400 N04400

Sheet/​Plate

Tube

Pipe

Forgings

Fittings

Bar

B/​SB 162 B/​SB160, 162 B127/​SB127

B/​SB163 B/​SB163 B/​SB163

B/​SB161, 725 B/​SB161, 725 B/​SB165, 725

B/​SB160, 564 B/​SB564 B/​SB564

B/​SB366 B/​SB366 B/​SB366

B166 B164 B/​SB164

high-​temperature deterioration. The alloys do not become brittle at cryogenic temperatures, have good tensile and fatigue strengths at moderate temperatures, and display excellent creep-​rupture properties at high temperatures. Inconel alloy 600 (UNS 06600) is a nickel-​chromium alloy with good oxidation resistance at high temperatures and resistance to chloride SCC, corrosion by high-​purity water, and caustic corrosion. Inconel alloy 601 (UNS 06601) is a nickel-​chromium alloy with an addition of aluminum for high resistance to oxidation and other forms of high-​temperature corrosion. It also has high mechanical properties at elevated temperatures. It resists oxidation up to 2300°F and has good resistance to sulfidizing atmospheres. Inconel alloy 617 (UNS 06617), a nickel-​chromium-​cobalt-​molybdenum alloy, exhibits an exceptional combination of metallurgical stability, strength, and oxidation resistance and carburization resistance at high temperatures. Resistance to oxidation is enhanced by aluminum addition. This alloy resists a wide range of corrosive aqueous environments. Inconel alloy 625 (UNS 06625) is a nickel-​chromium-​molybdenum alloy with an addition of niobium that acts with the molybdenum to stiffen the alloy’s matrix and thereby provides high strength without a strengthening heat treatment. Its high strength allows the use of more thin walled vessels and tubing than that is possible with many other materials, thereby saving weight and improving heat transfer. It exhibits strength and toughness from cryogenic temperatures to 1800°F (980°C). It exhibits good oxidation resistance, fatigue strength, and corrosion resistance. The alloy resists a wide range of severe corrosive environments and is especially resistant to pitting and crevice corrosion.

Material Selection and Fabrication

263

Inconel alloy 718 (UNS 07718) is a PH nickel-​chromium alloy containing significant amounts of iron, niobium, and molybdenum along with lesser amounts of aluminum and titanium. It combines corrosion resistance and high strength with outstanding weldability including resistance to post-​weld cracking. The alloy has excellent strength from −423°F to 1300°F (−253°C to 705°C). Oxidation resistance is up to 1800°F (980°C). Inconel alloy X-​750 (UNS 07750) is a nickel-​chromium alloy similar to Inconel alloy 600 but made PH by additions of aluminum and titanium. The alloy has good resistance to corrosion and oxidation along with high tensile and creep-​rupture properties at temperatures to about 1300°F (700°C). Inco alloy G-​3 (UNS 06985) is a nickel-​chromium-​iron alloy with additions of molybdenum and copper. It has good weldability and resistance to intergranular corrosion in the welded condition. The low carbon content helps prevent sensitization and subsequent intergranular corrosion of the weld HAZs. It is used for flue gas scrubbers and for handling phosphoric and sulfuric acids. Inco alloy C-​276 (UNS 10276) is a nickel-​molybdenum-​chromium alloy with an addition of tungsten, having excellent corrosion resistance in a wide range of severe environments. The high molybdenum content makes this alloy especially resistant to pitting and crevice corrosion. The low carbon content minimizes carbide precipitation during welding to maintain corrosion resistance in as-​ welded conditions. It is used in pollution control, chemical processing, pulp and paper production, and waste treatment. Inco alloy HX (UNS 06002) is a nickel-​ chromium-​ iron-​ molybdenum alloy with outstanding strength and oxidation resistance at temperatures to 2200°F (1200°C). Matrix stiffening provided by the molybdenum content results in high strength in a solid-​solution alloy having good fabrication characteristics. 2.24.1.5 Nickel-​Iron-​Chromium Alloys and Inco Nickel-​Iron-​Chromium Alloys for High-​Temperature Applications The Incoloy alloys are based predominantly on the nickel-​iron-​chromium ternary system. Some alloys contain molybdenum and copper for enhanced corrosion resistance, and aluminum, titanium, or niobium for strengthening by heat treatment. The Incoloy alloys are characterized by good corrosion resistance in aqueous environments and by excellent strength and oxidation resistance to high-​temperature atmospheres. At high temperatures, the substantial chromium content provides resistance to oxidizing environments, and the combination of nickel, iron, and chromium results in good creep-​rupture strength. The high nickel content makes the alloys superior to SSs in resisting corrosion, especially chloride SCC. Incoloy alloy 800 (UNS 08800) is a nickel-​iron-​chromium alloy known for its strength, and its excellent resistance to oxidation and carburization in high-​temperature applications. The alloy maintains a stable, austenitic structure during prolonged exposure to high temperatures. It is particularly useful for high-​temperature equipment in the petrochemical industry because the alloy does not form the embrittling sigma phase after long exposures at 1200°F–​1600°F (649°C–​871°C). It is used for process piping, heat exchangers, and nuclear steam generator tubing. High creep and rupture strengths are other factors that contribute to its performance in many applications. Incoloy alloy 800HT (UNS 08811) is a nickel-​iron-​chromium alloy having the same basic composition as Incoloy alloy 800 but with significantly higher creep-​rupture strength in the 1100°F–​1800°F (595°C–​980°C) range.

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Material Selection and Fabrication

TABLE 2.44 Nominal Composition (%) of Major Alloying Elements of Inconel and Inco Alloys Alloy Name

UNS Designation

Ni

Inconel alloy 600 Inconel alloy 601 Inconel alloy 625 Incoloy alloy 800 Incoloy alloy 800H Incoloy alloy 800HT Incoloy alloy 825 Inco alloy 904L Inco alloy C-​276

N06600 N06601 N06625 N08810 N08810 N08811 N08825 N08904 N10276

76 60.5 61.0 32.5 32.5 32.5 42.0 25.5 57.0

Cr

Fe

15.5 23.0 21.5 21.5 21.0 21.0 21.5 21.0 16.0

8.0 14.0 2.5 45.0 45.0 45.0 30.0 45.0 6.0

Mo

9.0

3.0 4.7 16.0

Others Ni. 58 (min), Mn 1 (max) Al 1.4 Nb +​Ti 3.6 Nb 3.6 C0.1(max), Al +​Ti 0.75 C 0.07, Al +​Ti 0.75 C 0.08, Al +​Ti 1.0 Cu 2.2, Ti 1.0 Cu 1.5, Co 0.2 W4.0

Incoloy alloy 825 (UNS 08825) is a nickel-​iron-​chromium alloy with additions of molybdenum and copper and stabilized with titanium. It has excellent resistance to both reducing and oxidizing acids and seawater. It exhibits good resistance to SCC and intergranular corrosion. Nominal compositions of major elements of Inco alloys for high-​temperature applications are given in Table 2.44. Alloy description, product forms, and ASTM/​ASME Code references for Inconel (Nickel-​Alloy) and Incoloy Alloy are given in Table 2.45. The ASTM specifications for nickel-​base tubular products are given in Table 2.46. 2.24.1.6 Magnetic Properties and Differentiation of Nickels Nickel, like steel, is strongly magnetic at room temperature. When the Curie temperature (i.e. the temperature at which the metal loses its magnetic properties) is known, a simple magnetic test can be used to differentiate the nickel alloys. For nickel, the Curie temperature is about 680°F (360°C); nickel-​copper alloy is slightly magnetic at room temperature and has a Curie temperature of 110°F–​ 140°F (43°C–​61.7°C); nickel-​chromium alloy is nonmagnetic at room temperature and has a Curie temperature of −40°F (−40°C).

2.24.2 Nickel and Nickel-​Base Alloys: Corrosion Resistance Nickel and nickel-​base alloys exhibit resistance to general corrosion in aqueous solution, localized attack, SCC, and erosion-​corrosion. 2.24.2.1 Galvanic Corrosion The element nickel (passive) is nobler than iron and copper in the electrochemical series. 2.24.2.2 Pitting Resistance The pitting and crevice corrosion resistance of a nickel-​based corrosion resistant alloy (e.g. Incoloy 825, Inconel G-​3, 622, 625, 686, and C-​276) is primarily a function of the steel composition, with chromium, molybdenum, tungsten, and niobium playing the major roles. The PREN is an index used to rank the resistance of individual steels to pitting and crevice corrosion. The PREN for selected nickel-​chromium-​molybdenum alloy is given in Table 2.47. 2.24.2.3 Intergranular Corrosion Nickel-​chromium alloys with carbon content above 0.2% exposed to temperatures between 1100°F and 1500°F (593°C–​816°C) will be sensitized, resulting in chromium carbide precipitation in

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Alloy Name

UNS Designation

Plate, Sheet, and Strip

Tube

Pipe

Forgings

Fittings

Rod/​Bar

Inconel alloy 600

N06600

B/​SB168

B/​SB163

B/​SB564, 516, 517

B/​SB366

B/​SB166

Inconel alloy 601 Inconel alloy 625 Incoloy alloy 800

N06601 N06625 N08800

B/​SB168 B/​SB443 A/​SA240, 480, B/​ SB409, 906

B/​SB163 B 163 B/​SB163

B/​SB167, B/​SB829, B/​SB775, B/​SB163, B516, B/​SB751 B/​SB167 B444, B704, B705, B751, B775, B829 B/​SB514, 515, 407,751, 775,829

—​ B564 B/​SB564

B/​SB366 B/​SB366 B/​SB366

B/​SB166 B446 B/​SB408

Incoloy alloy 800H Incoloy alloy 800HT Incoloy alloy 825

N08810 N08811 N08825

B/​SB424, B/​SB906

B/​SB163

B/​SB425, B/​SB 564

B/​SB366

B/​SB 425, B/​SB564

Inco alloy 904L Inco alloy C-​276

N08904 N10276

B/​SB625 B/​SB575

B/​SB163 B/​SB163

B/​SB423, B/​SB704, B/​SB705, B/​SB751, B/​SB775, B/​SB829 B/​SB673, 674, 677 B/​SB619, 622

B/​SB649 B/​SB564, 574

B/​SB366 B366

Material Selection and Fabrication

TABLE 2.45 Inconel (Nickel-​Alloy) and Incoloy Alloy Product Forms and Specifications

265

266

Material Selection and Fabrication

TABLE 2.46 ASTM Specification for Nickel Alloy Tubular Products ASTM Spec.

Description

UNS No.

B751

General requirements for nickel and nickel alloy seamless and welded tubes Specification for seamless nickel and nickel alloy condenser and heat exchanger tubes Welded chromium-​nickel-​iron-​molybdenum-​copper, columbium-​stabilized alloy tubes Welded nickel-​iron-​chromium alloy tubes for general corrosion resisting and low-​or high-​temperature service Welded nickel-​iron-​chromium alloy tubes for general corrosive service and heat-​resisting applications Welded nickel and nickel cobalt alloy tube

—​

B163 B468 B515

B516 B626

B720 B924

Seamless cold-​worked tubes of nickel alloy for use in condenser and heat exchanger service Standard Specification for Seamless and Welded Nickel Alloy Condenser and Heat Exchanger Tubes With Integral Fins

N02200, N02201, N04400, N06600, N06690, N08800, N08810, N08825 N08028, N08024, N08026 N08800, N08810

N06600 N10001, N10665, N10276, N06455, N06007, N06975, N08320, N06985, N06002, N06022, N06030, N06059, N08031, N30556, N06230 N08310 -​-​-​-​-​-​

TABLE 2.47 Pitting Resistance Number for Selected Nickel-​Chromium-​Molybdenum Alloy Composition Required to Calculate PREN Alloy Inconel alloy 625 Incoloy alloy 825 Inco alloy 904L Inco alloy C-​276

UNS Designation

Cr

Mo

W

Nb

PRENa

N06625 N08825 N08904 N10276

21.5 21.5 21.0 16.0

9.0 3.0 4.7 16.0

—​ —​ —​ 4.0

3.6 —​ —​ —​

40.4 26.0 28.0 46.0

 PREN =​%Cr +​1.5 (%Mo +​%W +​%Nb).

a

grain boundaries. This leaves areas adjacent to the boundaries low in chromium and susceptible to intergranular corrosion. Such corrosion can be counteracted in three ways [209]: 1. select low-​carbon alloy and consumables 2. select titanium-​and columbium-​alloyed base metal and welding consumables 3. solution anneal the weldment at 2000°F–​2200°F (1093°C–​1204°C) to dissolve precipitated carbides. 2.24.2.4 Stress Corrosion Cracking There is a possibility that welded joints in nickel-​rich alloys will suffer SCC if in contact with strong caustic soda, fluorosilicates, and certain mercury salts [79]. Nevertheless, nickel alloys have very

Material Selection and Fabrication

267

good resistance to SCC in alkalies and solutions containing chloride ions, definitely much better than the austenitic SSs. Important information on SCC of nickel and nickel-​base alloys includes [177] the following: (1) low-​carbon nickel (0.02% C max) is recommended for use in contact with caustics if the service temperature is above 850°F (455°C), (2) nickel-​iron alloys and nickel-​chromium-​ iron alloys containing up to 40% nickel can fail by SCC on exposure to hot chloride solutions, and (3) sensitized Ni-​Cr-​Fe alloys containing higher amounts of nickel may be subjected to polythionic acid SCC at ambient temperature.

2.24.3 Nickel and Nickel-​Base Alloys: Welding In terms of their weldability, these alloys can be classified according to the means by which the alloying elements develop the mechanical properties, namely solid solution alloys and precipitation hardened alloys. A distinguishing feature of precipitation hardened alloys is that mechanical properties are developed by heat treatment (solution treatment plus ageing) to produce a fine distribution of particles in a nickel rich matrix. The solid-​solution-​strengthened (non-​age-​hardenable) wrought nickel and its alloys can be arc welded under conditions similar to those used in the arc welding of austenitic SSs. 2.24.3.1 Weldability Most nickel alloys can be fusion welded using gas shielded processes like TIG or MIG. Of the flux processes, MMA is frequently used but the SAW process is restricted to solid solution alloys and is less widely used. Solid solution alloys are normally welded in the annealed condition and precipitation hardened alloys in the solution treated condition. Preheating is not necessary unless there is a risk of porosity from moisture condensation. It is recommended that material containing residual stresses be solution-​treated before welding to relieve the stresses. 2.24.3.2 Considerations while Welding Nickel While welding nickel and nickel-​base alloys, greater care such as pre-​cleaning and cleanliness is required to prevent contamination-​induced cracking or porosity, or embrittlement due to sulfur and lead [209, 210]. As nickel content increases, the weld puddle viscosity increases; the coefficient of expansion decreases, as does weld penetration [209]. Additional considerations should include effect of minor elements on weldability, susceptibility to cracking in the weld bead caused by high heat input or excessive welding speeds, and SCC of certain welded structures in service. These welding considerations include: 1. Pre-​cleaning and Surface Preparation. 2. Surface Oxide should be thoroughly removed from the surfaces to be welded because they can inhibit wetting and result in lack of fusion. Oxides are normally removed by grinding with an aluminum oxide or silicon carbide wheel or a carbide burr, machining, abrasive blasting, or pickling. 3. Weld Metal Porosity-​Surface contaminations, poor welding techniques, improper shielding, and the presence of hydrogen, carbon dioxide, nitrogen, and oxygen gases often cause porosity in nickel weldments. 2.24.3.3 Joint Designs Molten nickel-​alloy weld metals do not flow and wet the base metal as readily as do carbon-​steel and stainless-​steel weld metals. The operator must place the metal at the proper location in the joint. Therefore, the joint must be sufficiently open to permit proper manipulation of the electrode and deposition of the weld beads. Suggested joint designs for butt joints in nickel alloys are shown in Figure 2.25.

268

Material Selection and Fabrication

FIGURE 2.25  Nickel welding –​(a) joint design and (b) groove in backup bar.

2.24.3.4 Heat Input High heat input during welding may result in constitutional liquation, carbide precipitation, or other metallurgical phenomena. Some degree of annealing and grain growth can take place in the HAZ. These, in turn, may cause cracking or loss of corrosion resistance, or both. 2.24.3.5 Hot Cracking Weldments of nickel-​base alloys are susceptible to liquation cracking or hot cracking. This takes place particularly when welding conditions of high restraint are present, as in circumferential welds that are self-​restraining. Measures to overcome hot cracking include the following: (1) limit heat input during welding, (2) obtain fairly small grain sizes in the microstructures, and (3) minimize the amount of deleterious minor elements such as sulfur and phosphorus that are known to cause hot cracking. 2.24.3.6 Sulfur Embrittlement Nickel combines with sulfur at elevated temperature to form a brittle sulfide. This phenomenon takes place preferentially at the grain boundaries and is exhibited when the material is stressed or bent. Prior to carrying out any operation involving high-​temperature operations such as welding, brazing, hot forming, PWHT, etc. the surfaces should be cleaned thoroughly and should be free from sulfur-​bearing substances. 2.24.3.7 Lead Embrittlement Lead causes embrittlement in all nickel-​base alloys in much the same manner as sulfur. Therefore, lead-​containing fluids or lubricants should be removed from the surfaces. 2.24.3.8 Carbide Precipitation Like some austenitic SSs, nickel-​chromium and nickel-​iron-​chromium alloys with carbon content above 0.2% exposed to temperatures between 1100°F and 1500°F (593°C–​816°C) will be sensitized due to carbide precipitation in the weld HAZ. Such sensitization does not result in accelerated attack in most environment. In general, carbide precipitation is overcome by the following measures [209]: 1. use low-​carbon-​base alloy and consumables 2. select titanium-​or columbium-​stabilized alloy base metal and consumables 3. solution anneal the weldment at 2000°F–​ 2200°F (1093°C–​ 1204°C) to dissolve carbide precipitate.

Material Selection and Fabrication

269

2.24.3.9 Pitting Corrosion of Weldments To resist pitting and crevice corrosion, weldments must have smooth surfaces and be chemically homogeneous on a microscopic level. Molybdenum imparts pitting resistance to nickel-​chromium alloys. Use filler metal with sufficient molybdenum; even in the event of iron dilution, enough molybdenum will impart greater resistance to weld metal than that of base metal. For this reason, many base metals with 4% or 6% Mo have been welded with filler metal such as Inconel 112 (60% Ni) electrode and Inconel 625 filler metal, both containing 9% Mo [209]. 2.24.3.10 Strain Age Cracking Most of the PH alloys are susceptible to strain age cracking. Alloys containing columbium have a better resistance to cracking because of the slower hardening response of the columbium precipitate than the aluminum or titanium precipitate.

2.24.4 Welding Methods Typical arc welding processes employed for welding nickel and nickel-​base alloys are GTAW, GMAW, and SMAW. SAW and PAW have limited applicability. The GTAW process is preferred for welding the PH alloys, although GMAW and SMAW can also be used. Reference [211] details the welding of nickel and nickel-​base alloys.

2.24.5 Post-​weld Heat Treatment Nickel and nickel-​base alloys generally do not experience any metallurgical changes, either in the weld metal or in the HAZ, which affect normal corrosion resistance. Hence, no PWHT is normally required. However, heat treatment may be necessary in circumstances such as (1) to meet specification requirements or (2) for stress relief of a welded structure to avoid age hardening or SCC of the weldment used to contain hydrofluoric acid vapor or caustic soda.

2.24.6 Hastelloy® The Hastelloy family of corrosion-​resistant alloys include B-​3®, C-​4, C-​22®, C-​276, C-​2000®, C-​ 22HS®, G-​30®, G-​35®, G-​50®, etc. Standard forms are bar, billet, plate, sheet, strip, coils, seamless or welded pipe and tubing, pipe fittings, flanges, fittings, welding wire, and coated electrodes. For general guidelines for welding, brazing, hot and cold forming, heat treating, pickling and finishing, refer to Hastelloy − corrosion-​resistant alloy, Haynes Corrosion-​Resistant Alloys International, Global Headquarters, Indiana, USA.

2.25 TITANIUM: PROPERTIES AND METALLURGY Titanium is a reactive, nontoxic, and low-​density metal (density, ρ =​4.4 gm/​cc, whereas the density of steel is 7.8 gm/​cc). Titanium and its alloys are used in major applications for which their inherent properties justify their selection: (1) for corrosion resistance and (2) where specific strength is of major advantage. Titanium “bridges the design gap” between aluminum and steel and offers a combination of many of the most desirable properties of each. Its lightweight with specific gravity lying between aluminum and steel (approx. 60% of steel) gives a high strength-​to-​weight ratio. For example, the strength-​to-​weight ratio for annealed Ti-​6Al-​4V (Grade 5) is about 7.81 (density is 0.160 lb/​in.3 and yield strength is 125 ksi), whereas it is about 1.25 for annealed SSs. Increases in strength, as with other materials, are achieved by the addition of alloying elements such as Al, Cu, Mo, Si, V, and Zr.

270

Material Selection and Fabrication

It is known for its excellent corrosion resistance due to a stable, tenacious oxide film formed on its surface. Corrosion failures with titanium are rare and usually associated with weld defects. All titanium alloys contain minute amount of interstitial elements such as carbon, oxygen, nitrogen, and hydrogen. Titanium and its alloys have several excellent properties for use in many diverse applications. They have an excellent strength to weight ratio, as strong as steel with half the weight. They form a tenacious oxide film giving excellent corrosion resistance particularly in chloride environments and many acids, are non-​magnetic, have good creep resistance and oxidation resistance, excellent biomedical properties and finally will retain good toughness down to –​196°C.

2.25.1 Crystallographic Structures of Titanium Titanium and its alloys can then be divided into four main groups dependent on the microstructure: 1. pure and commercially pure titanium 2. alpha phase alloys 3. beta phase alloys 4. alpha-​beta alloys. The most commonly used titanium alloys are alpha-​beta, among them Ti-​6AL-​4V and Ti-​6Al-​2Sn-​ 4Zr-​2Mo. Ti-​6Al-​4V accounts for about half of all titanium alloy manufactured.

2.25.2 Properties that Favor Heat Exchanger Applications The combination of high strength, stiffness, good toughness at cryogenic temperature, immunity to corrosion in wet chlorine, excellent corrosion resistance to many chemicals, and erosion resistance allows widespread use of titanium and titanium alloys from cryogenic −253°C (−423°F) to moderately high temperature 600°C (1000°F). Titanium is used as heat exchanger material in chemical industries, refinery, absorption refrigeration and air conditioning, refinery heat exchangers, brine heat exchangers, desalination plants, power-​plant surface condensers, and auxiliary exchangers. Titanium finds its widest use in chemical process industries as process equipment, wet scrubbers, heat exchangers, valves, pumps, and piping systems. Almost all nuclear stations the world over have standardized on the use of titanium as tube material. It is available in various product forms including sheets, plates, cladded plates, pipes, tubes, including U-​bend, and enhanced tubes (integral fin tube, applied fin tube, and corrugated and rope tube). The arguments against titanium are its high price, high-​quality standards, sensitivity to fouling, corrosion fatigue, and possibility of hydrogen embrittlement if cathodic protection in excess is applied [109]. For fixed-​baffle spacing, titanium tube heat exchangers are susceptible to flow induced vibration due to its low modulus of elasticity and the use of thin-​walled sections for cost considerations. However, this can be overcome by reducing the baffle spacing.

2.25.3 Alloy Specification Titanium specification and grades include Grade 1, Grade 2, Grade 3, Grade 4, Grade 5, Grade 6, Grade 7, Grade 9, Grade 11, and Grade 12. Grades 1, 2, 3, and 4 apply to commercially pure titanium (98.5%–​99.5%) and differ in the amounts of interstitials like oxygen, nitrogen, carbon, and iron. By varying these interstitial elements, the unalloyed titanium grades can be strengthened. The higher the grade, the greater is the amount of oxygen and the greater is the metal’s strength. Grades

271

Material Selection and Fabrication

TABLE 2.48 Titanium Grades, Composition, and Structure Grade

UNS No.

Description

Phase

Grade 1 Grade 2 Grade 3 Grade 4 Grade 5 Grade 5 Grade 6 Grade 6 Grade 7 Grade 9 Grade 11 Grade 12

R 50250 R 50400 R 50550 R 50700 R 56400 R 56401 R 54520 R 54521 R 52400 R 56320 R 52250 R 53400

Unalloyed titanium Unalloyed titanium Unalloyed titanium Unalloyed titanium Ti-​6Al-​4V Ti-​6Al-​4V ELIa Ti-​5Al-​2.5Sn Ti-​5Al-​2.5Sn ELIa Ti-​0.20 Pd Ti-​3Al-​2.5V Ti-​0.20 Pd Ti-​0.3Mo-​0.8Ni

A A A A α–​β α–​β Α Α Α Near α Α Near α

  Extra-​low interstitials grade. R 52550 and R 52700 are titanium alloy castings.

a

TABLE 2.49 General Characteristics of Titanium Alloys and Applications Alloy

General Characteristics

Applications

Grade 1 (R 50250)

Highest purity, relatively low strength and high ductility; good corrosion resistance among the four ASTM unalloyed grades due to low interstitials The pure titanium; work horse material, i.e. most used. Exhibits the best combination of strength, ductility, weldability, and corrosion resistance. Minimum yield strength of 275 MPa, which is comparable to those of annealed austenitic stainless steels Unalloyed titanium, high-​strength and medium oxygen

Used in continuous service up to 425°C (800°F). Used for PHEs Used in continuous service up to 425°C (800°F) Piping, pressure vessels, and heat exchangers

Grade 2 (R 50400)

Grade 3 (R 50550) Grade 4 (R 50700) Grades 7 and 11 (R 52400 and R 52250) Grade 12 (R 53400)

Highest strength of the pure unalloyed grades. High oxygen. Moderate formability, outstanding corrosion, and fatigue resistance in saltwater Unalloyed titanium plus 0.12% to 0.25% Pd. Medium strength, normal oxygen content. Good corrosion resistance in reducing and oxidizing environments. (Note: Grade 11 is with low oxygen, low strength.) Stronger and more resistant to crevice corrosion at higher temperatures than unalloyed Titanium grades. Less expensive than Grades 7 and 11

Main use in shell and tube heat exchanger Piping, pressure vessels, and heat exchangers Piping, pressure vessels, and heat exchangers and PHEs

Pressure vessels and heat exchangers

5, 7, 11, and 12 are titanium alloys. Various ASTM grades, alloy designations, compositions, and metallurgical phases present are given in Table 2.48; characteristics and application areas are given in Table 2.49. An important source book on titanium is the Titanium Handbook [212]; also see Schutz and Thomas [213] and [214–​218].

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2.25.4 Titanium Grades and Alloys 2.25.4.1 Unalloyed/​Alloyed Grades Unalloyed titanium typically contains between 99%–​99.5% titanium, with the balance being made up of iron and the interstitial impurity elements hydrogen, nitrogen, carbon, and oxygen. Pure titanium has low tensile strength and is extremely ductile. Dissolved oxygen and nitrogen markedly strengthen the metal, and carbon and iron to a lesser extent. Grade 1 is purer and therefore more ductile and finds use where severe forming is required; unalloyed titanium mill products of Grade 2, most widely used and more ductile, find use where severe forming is required; this grade has outstanding resistance to general and other forms of corrosion in many environments, but it is not resistant to crevice corrosion [105]. Grade 3 provides slightly higher strength due to its higher percentage of interstitial elements [215]. Titanium is not as widely used in the alloyed form in chemical process industries, the exception being titanium 0.2% palladium alloy (Grade 7) to enhance corrosion resistance in mildly reducing solutions and high-​temperature chloride solutions. Typical applications extend to hydrochloric, phosphoric, and sulfuric acid solutions and areas of service where the operating conditions vary between oxidizing and mildly reducing conditions [214, 216]. The presence of palladium has no other significant effect on the physical or mechanical properties of titanium and it makes titanium costlier. 2.25.4.2 ASTM and ASME Specifications for Mill Product Forms ASTM and ASME specifications that govern unalloyed titanium and alloyed titanium in various mill product forms such as strip, sheet and plate, seamless and welded pipe, seamless and welded tube for condensers and heat exchangers, bars and billets, and forgings in various grades are given in Table 2.50 and 2.51. For these alloys, the design temperatures should be less than approximately 806°F (430°C) to avoid excessive oxidation and oxygen embrittlement in continuous service. Industry uses titanium in sheet and plate form in addition to tubes and plate used in heat exchangers. Construction can be solid titanium, titanium-​clad steel, or with titanium linings. Solid titanium construction is generally more cost-​effective than clad construction when vessel wall thickness is below approximately 1–​1.5 in. (25.4–​38.1 mm). Commonly used tubesheet materials and their properties are given in the Table 2.50. 2.25.4.3 Titanium tubes for Condensers and Heat Exchangers ASTM B338-​17(2021) –​Standard Specification for Seamless and Welded Titanium and Titanium Alloy Tubes for Condensers and Heat Exchangers. This specification covers 28 grades of seamless and welded titanium alloy tubes for surface condensers, evaporators, and heat exchangers.

TABLE 2.50 Titanium Product Shapes, Grades, and ASTM/​ASME Specifications Product Description

Grades*

ASTM/​ASME

Strip, sheet, and plate Seamless and welded pipe Seamless and welded tube for condensers and heat exchangers Bars and billets Forgings

Grades 1, 2, 3, 4, 5, 7, 9, 11, 12 Grades 1, 2, 3, 4, 7, 9, 11, 12 Grades 1, 2, 3, 4, 6, 7, 9, 11, 12 Grades 1, 2, 3, 4, 5, 6, 7, 11, 12 Grades 1, 2, 3, 4, 5, 6, 7, 11, 12

B265/​SB265 B861/​SB861 B338/​SB338 B348/​SB348 B381/​SB381

*  Partially given. For full details refer to the relevant standard.

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2.25.5 Titanium Corrosion Resistance 2.25.5.1 Surface Oxide Film Titanium exhibits excellent corrosion resistance to general corrosion, pitting, erosion-​corrosion, and galvanic corrosion. The superior corrosion resistance of titanium alloys is derived from the formation of a highly stable, tenacious, protective oxide film on its surface. This film, typically 50–​200 Å thick, is primarily titanium dioxide (TiO2) in rutile (highly crystalline) and/​or anatase (somewhat amorphous) form [218]. Even if the oxide film is damaged, being a reactive metal and with a high affinity for oxygen, the protective oxide films form spontaneously and instantaneously whenever fresh metal surfaces are exposed to traces of oxygen or moisture [213]. Film growth is also accelerated under strongly oxidizing conditions such as in HNO3 and CrO3 media [219]. Another desirable feature is that at low temperatures, this surface oxide film provides a barrier for penetration of hydrogen into titanium surfaces. Conversely, titanium is severely corroded in a reducing environment wherein it is not readily repassivated. Once the breakdown of the passive film occurs, the corrosion process is unhindered. Loss of oxide film could occur due to abrasion, chemical reduction of the oxide, or surface contamination, notably by iron oxide [220]. 2.25.5.2 Resistance to Waters Titanium exhibits good corrosion resistance to freshwater, industrial cooling waters including raw seawater, brackish estuary water, and polluted water. Titanium (like many other metals) is subject to the formation of mineral scales when water temperatures are excessive [220]. It resists all forms of corrosive attack by freshwater and steam to temperatures as high as 600°F (316°C) and corrosion by seawater to temperatures as high as 500°F (260°C) [219]. The presence of sulfides in seawater does not affect the corrosion resistance.

2.25.6 Forms of Corrosion 2.25.6.1 Galvanic Corrosion From its position in the galvanic series, titanium is highly immune to galvanic corrosion but tends to accelerate the corrosion of the other metal of the galvanic couple. The exception is in highly reducing acid environments where titanium may not passivate. Under these conditions, it has a potential similar to aluminum and will undergo accelerated corrosion when coupled to other more noble metals [219]. While using titanium tubes, extreme care should be taken to avoid galvanic corrosion of tubesheets and water boxes of a condenser, which have different corrosion potential. Solid titanium or titanium-​clad tubesheets are preferred in order to eliminate galvanic corrosion of a tubesheet of an anodic material, such as a copper alloy or carbon steel. Water boxes should be coated for protection. If a titanium-​clad tubesheet is used (a minimum of 3/​16 in. or 4.5 mm thick cladding is required), the tube-​to-​tubesheet joints should be welded, but if a solid tubesheet is used, roller expansion can be performed. 2.25.6.2 Hydrogen Embrittlement In a galvanic couple, if titanium is the cathodic member, hydrogen may be evolved on its surface. Normally, the surface oxide film on titanium acts as an effective barrier to penetration by hydrogen. By specification, hydrogen is limited to 150 ppm maximum in all the common tube metal grades [220]. Within the range of pH 3–​12, the oxide film is stable and presents a good barrier to penetration by hydrogen [219]. Under certain conditions, titanium can absorb hydrogen and become embrittled: 1. Dry, nonoxidizing hydrocarbon, or hydrogen stream –​the oxide films break down since there is no mechanism for replacing oxygen lost to the process stream. 2. Disruption of the oxide film allows easy penetration by hydrogen.

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3. If the temperature is above 176°F (80°C), hydrogen may diffuse into the metal and cause hydrogen embrittlement. 2.25.6.3 Crevice Corrosion Titanium alloy as well as commercially pure titanium will be subjected to localized attack in tight crevices exposed to hot (>158°F/​70°C) aqueous solutions, or chloride, bromide, iodide, or fluoride solutions. Crevice corrosion of unalloyed titanium may occur in seawater at temperatures above the boiling point. One major exception is the alloys containing 0.2% palladium [221]. Titanium alloy Grade 12 and Ti-​Pd (Grades 7 and 11) offer resistance to crevice corrosion in seawater at temperatures up to 500°F (260°C) [219]. Both alloys also provide improved resistance to reducing acid conditions, and Grade 12 offers increased strength particularly as the temperature is increased. Where corrosion of titanium is a problem under deposits, molybdenum-​bearing titanium grade-​ Grade 12, R53400 (Ti-​0.3Mo-​0.8Ni), should be used. 2.25.6.4 Erosion-​Corrosion An important property of titanium is its excellent resistance to erosion-​corrosion and its various forms such as cavitation and impingement attack. Titanium is considered one of the best cavitation resistant materials available for seawater service. Titanium has the ability to resist erosion due to high-​velocity seawater up to 30 m/​s [218]. 2.25.6.5 Stress Corrosion Cracking SCC of commercial titanium and titanium alloys may occur in red fuming nitric acid at room temperatures. The possibilities of SCC in natural seawater can be reduced by alloy selection and heat treatment. The presence of defects combined with an unfavorable stress condition can account for almost instantaneous failure of welded structures in contact with trichloroethylene [79]. 2.25.6.6 MIC Titanium is highly resistant to MIC. More than 30 years of extensive titanium alloy use in biologically active process and raw cooling waters, especially seawater, appears to substantiate titanium’s resistance to MIC [218].

2.25.7 Thermal Performance With its low fouling resistance values in combination with thinner tube walls, the overall heat transfer coefficient has been more than adequate in all installations. Titanium’s resistance to erosion-​corrosion permits high flow velocity up to 38 m/​s (120 ft/​s) if pressure drop considerations permit. High velocity means higher heat transfer rates and lower fouling factors. Experience has shown that titanium exchangers handling seawater can be designed with a fouling factor as low as 0.0005 hft2°F/​BTU [219]. 2.25.7.1 Fouling As titanium is perfectly passivated, its surface has no biostatic action and would be fouled biologically. Fouling, mussels, and barnacles are a problem in a power station installed on the seashore. Therefore, tubes must be kept clean by an online tube cleaning system such as a sponge rubber ball system, or by a correct system cooling water velocity, or by chlorination treatment.

2.25.8 Applications 2.25.8.1 Titanium Tubing for Surface Condensers Titanium tubing in steam condensers has proven to be the most reliable of all tubing materials. It is immune to corrosion from chlorides. It returns no harmful metal ions to the environment or to the

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275

condensate feedwater stream. Titanium overcomes the problems of random tube failure caused by erosion-​corrosion at tube inlets due to turbulence or partial blockage by shells, mussels, debris, and ammonia condensate and is highly resistant to erosion attack by steam impingement. In addition to fresh tubing, titanium tubings are used for retubing the leaky tubes. When retubing an existing condenser that has been designed for use with tubes other than titanium, there are six areas to consider [219]. They are as follows: (1) titanium tube heat exchangers require shorter baffle spacings to accommodate thinner tubes and low Young’s modulus to overcome FIV problems; (2) tubesheet material for a possible galvanic couple with titanium as cathodic material (titanium can essentially be welded only to titanium, so that the tubesheets must also be made of titanium or be overlaid with titanium if a welded joint is desired); (3) effect of reduced weight due to the use of thin-​walled tubes and low density; (4) effect of reduced water velocity since the inner diameter has increased due to thin walled tubes; (5) fouling; and (6) if there is already a cathodic protection system in operation, make it compatible with titanium; such measures/​cautions include that hydriding can take place at potentials greater than −0.75 V (SCE) –​do not exceed −0.90 V (SCE) –​that the system must have an automatic potential control and that sacrificial anodes should not be placed closer than 30 in. from the tubesheet. 2.25.8.2 Refinery and Chemical Processing: Service Experience The primary reason for titanium’s expanded application in the refinery industries, particularly in the condenser service, is due to its superior resistance to handle refinery process streams containing hydrogen sulfide, chlorides, dilute hydrochloric acid, ammonia, and hydrogen [220, 222]. Titanium Grade 2 tubes are used in overhead coolers and condensers. Titanium for refinery services is limited to operating temperature of 260°C. If hydrogen is present, temperature should not exceed 175°C (350°F) to prevent hydrogen embrittlement. 2.25.8.3 Chemical Processing The primary basis for titanium’s expanded application in the chemical processing industries has been its superior resistance to the aggressive, mildly reducing, oxidizing, and/​or chloride-​containing process streams. 2.25.8.4 Applications in PHE Due to its inherent corrosion resistance, titanium is used as a PHE material in corrosive applications.

2.25.9 Titanium Fabrication 2.25.9.1 Tubesheet Materials-​Galvanic Consideration Tubesheets can be solid titanium, explosively clad titanium on steel, loose lined titanium on steel, or a dissimilar metal. Solid tubesheets are used primarily in all-​titanium tubed units. Either explosively clad tubesheets or loose liners are used with steel or other dissimilar metal shells. When explosively clad or loose lined tubesheets are used, the tubes must be seal welded to prevent minor tubeside fluid leakage from reaching the shellside steel shell, causing undetectable corrosion. If titanium tubes are to be used with dissimilar metal tubesheets, as is often the case in retubing jobs, the possibility of galvanic corrosion must be considered. 2.25.9.2 Flow Induced Vibration of Titanium Tubes Tube vibration in a heat exchanger occurs when shellside crossflow velocity is too high and there is small baffle spacing. Excessive tube vibration may result in fatigue failures at support plates or in midspan collision damage. Titanium’s hardness and corrosion fatigue resistance act to minimize vibration damage, but its lower Young’s modulus (than steel or copper-​nickel alloys) must be

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considered in design to keep deflection within acceptable limits. Proper baffle spacing should be incorporated into the designs of both new and retrofit titanium tube bundles to avoid flow induced vibration. 2.25.10  Welding Titanium Unalloyed titanium and all alpha-​titanium alloys are weldable. Alpha-​beta alloys, alloy Ti-​6Al-​4V, and weakly beta-​stabilized alloys are also weldable. Beta-​phase alloys and strongly stabilized alpha-​ beta-​phase alloys are difficult to weld and they will embrittle. 2.25.10.1 Welding Methods Titanium and titanium alloys can be arc welded with the gas shielded processes, TIG, MIG or Plasma TIG. The flux shielded processes can be used but are more difficult due to the inherent higher oxygen contents and are not therefore recommended. Commercially pure titanium and weldable alloys can be arc welded successfully by the manual gas tungsten arc, plasma arc, and gas metal arc processes. If economically justified by thickness, run length, or repetitive work, automatic hot wire TIG with special trailing shields is used. Procedures and equipment are generally similar to those used for welding austenitic SS or aluminum, with two important exceptions [215]: (1) titanium requires greater cleanliness –​wear cotton gloves when heating the parts, store parts in a clean dry area; and (2) it requires auxiliary gas shielding. In general, fluxes are not used due to the high-​temperature reactivity of titanium. 2.25.10.2 Weldability Considerations Titanium is a reactive metal and will burn in pure oxygen at 600°C and nitrogen above 800°C. Oxygen, nitrogen and hydrogen readily diffuse into titanium and can quickly cause embrittlement. Titanium does not suffer with any cracking issues other than possible cold cracking of the HAZ from embrittlement. When heated in air, titanium oxidizes rapidly, and at temperatures near its melting point 1800°C (3272°F), (1) molten titanium reacts with most materials, which embrittles the weldment and reduces the corrosion resistance; and (2) it dissolves its own oxide, leaving inclusions in welds. In its solid state, titanium above 1200°F absorbs oxygen, nitrogen, and hydrogen from the atmosphere. Even small amounts of these elements can cause embrittlement in the weld and loss of corrosion resistance [223]. Hydrogen in concentrations exceeding 150 ppm can embrittle the pure metal and also the various commercial alloys available [224]. Consequently, to assure a successful titanium weld, air must be excluded from both the face and the back of the weld metal, and a secondary inert-​gas shield must protect the weld area until it cools to 600°F–​800°F (315°C–​425°C). 2.25.10.3   Shielding Gases In arc welding, this is accomplished by replacing the air surrounding the weld area by argon or helium with argon. No other gases can be used. When titanium parts are heated to more than 500°F (260°C), contact with adhesive tapes, papers, marking crayons, etc. containing more than 50 ppm chlorine or other halogen compounds should be avoided [225]. 2.25.10.4  Welding of Titanium to Dissimilar Metals Satisfactory fusion-​welding of titanium to dissimilar metals, except with vanadium and silver, is not possible because of the formation of brittle intermetallic compounds. Therefore, interconnections between dissimilar metals are made by mechanical means such as by bolting. Lining vessels with titanium cannot be accomplished directly by fusion-​welding. In some instances, silver filler metal is used for seal welds [217].

Material Selection and Fabrication

277

FIGURE 2.26  Joint design for titanium welding.

2.25.10.5  Manufacturing Facilities While it is not always possible to fabricate large titanium assemblies in a closed clean room as demanded for a clean environment, designating areas of the heavy workshop area to be screened off, paying great attention to preventing weld contamination with shop dirt/​dust by screening, and stringent solvent cleaning of weld preparations and filler wire before welding are necessary to achieve the proper weld metal characteristics and corrosion resistance. 2.25.10.6 Joint Design Joint designs for titanium welding are essentially the same as those found in the inert-​gas arc welding of other metals. The joint design should allow for full shielding of both sides or permit the exclusion of air from parts of the joint reaching temperatures of 1000°F (538°C) or greater [226]. For example, argon purge holes are provided in vessel shell nozzle reinforcing plates and tubular stiffeners. Special configurations are necessary on titanium-​clad tubesheet joints to give adequate back shielding. Joint design for welding of titanium sheets are shown in Figure 2.26. 2.25.10.7 Pre-​cleaning and Surface Preparation A good titanium weld begins with cleanliness, both in the immediate weld area and in the shop floor. Do not store parts unless they are wrapped and sealed from the atmosphere. 2.25.10.8  Cleaning Titanium Titanium can be cleaned by steam cleaning, alkaline cleaning, vapor degreasing, and solvent cleaning methods. Cleaning solutions for titanium must maintain passivity and avoid possible hydrogen uptake by the titanium. Titanium cleaning is discussed by Chevalier [225], and ASTM Specification B600 details procedure for descaling and cleaning titanium and its alloys. 2.25.10.9   Degreasing Titanium parts that are free of scale or oxide require only degreasing. Light or medium contamination is degreased by immersion in hot alkaline solutions or by spraying. Avoid using chlorinated

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solvents such as trichloroethylene vapor, alcohols, and methanol in particular [226]. Deposits remaining from solvents of this type may cause cracking in high-​strength alloys if the metal is subsequently heated in stress relief or actual service. 2.25.10.10   Descaling or Oxide Removal Light oxides may be removed by brushing with an SS wire brush, sand blasting, or draw filing. Steel wool or abrasives should never be used because of the danger of contamination. If grinding is required, the use of silicon carbide is preferred. Heavy oxide scales may be removed by grinding, machining, liquid abrasive blasting, salt-​bath descaling, or alkaline cleaning. The use of carbon steel wire brushes to remove deposits from titanium is not recommended. Misapplication of titanium, the use of improper cleaning procedures and other abuses can lead to failure. On the other hand, careful use of some preventive measures, particularly those concerned with corrosion and galling resistance, can significantly extend the useful life of titanium equipment. 2.25.10.11   Rinsing Water rinsing after degreasing or acid pickling must be carried out with deionized water having a resistivity of more than 50,000 ohm/​cc when intending to do an operation such as heat treatment, welding, etc. above 500°F (260°C) [225]. 2.25.10.12 Preheating Titanium welding does not normally require preheating [215]. For very low ambient temperature and low humidity conditions, preheating to 100°F–​150°F (37°C–​65°C) helps moisture removal. 2.25.10.13 Filler Metal Titanium and its alloys can be welded using a matching filler composition; compositions are given in The American Welding Society specification AWS A5.16/​A5.16M: 2013 (ISO 24034:2010 MOD) –​Specification for Titanium and Titanium-​Alloy Welding Electrodes and Rods, or any other specifications. Table 2.51 shows AWS A 5.16 Classifications of Titanium Filler Metal and some filler metal composition is shown in Table 2.52.

TABLE 2.51 AWS A 5.16 Classifications of Titanium Filler Metal AWS Classifications

Composition

ERTi-​1 ERTi-​2 ERTi-​3 ERTi-​4 ERTi-​5 ERTi-​5ELI ERTi-​6 ERTi-​6ELI ERTi-​7 ERTi-​9 ERTi-​9ELI ERTi-​12

Ti-​0.50O Ti-​0.05O Ti-​0.13O Ti-​0.20O Ti-​6Al-​4V Ti-​6Al-​4V Ti-​5Al-​2.5Sn Ti-​5Al-​2.5Sn Ti-​0.05O-​0.2Pd Ti-​3Al-​2.5V Ti-​3Al-​2.5V Ti-​0.3Mo-​0.8Ni

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TABLE 2.52 Recommended Titanium Filler Metals Filler Grade ERTi-​1 ERTi-​2 ERTi-​3 ERTi-​4 ERTi-​6 ERTi-​7 ERTi-​9 ERTi-​11 ERTi-​5ELI ERTI-​23

Base Metal Grade 1 2 3 4 6 7 9 11 23 23

2.25.10.14 Welding Procedures 1. Gas Tungsten Arc Welding. Due to the inert-​gas shielding characteristics and the high degree of arc control, GTAW is ideally suited for titanium. AWS D10.6, Gas Tungsten Arc Welding of Titanium Piping and Tubing, covers various aspects of GTAW of titanium. 2. Shielding. Inert-​gas shielding (either by argon or by helium) of the weld area is accomplished by the following methods: a. A vacuum chamber or special inert-​gas-​filled chamber, which eliminates the need for elaborate jigs and other fixtures that normally would be required to adequately shield complex assemblies in air; it is generally considered that the highest weld quality is obtained by employing vacuum or argon purged chamber welding. b. A backup gas shielding with trailing shield by welding in open air. c. Argon is generally used in preference to helium for primary shielding at the torch because better arc stability is achieved. Argon-​helium mixture can be used if higher voltage, hotter arc, and greater penetration are desired. d. Though Argon is costlier compared to helium, the former is preferred for use in trailing shields and backup devices, because it is denser. AWS D10.6 on gas shielding offers several alternatives for shielding, including work-​mounted trailing shields and torch-​mounted trailing shields. 2.25.10.15 Welding Titanium in an Open-​Air Environment with Three Shielding Gases Techniques The shielding for welding titanium in an open atmosphere can be accomplished by using a combination of three shielding gases techniques. They are as follows [227]: 1. The primary shielding is provided by the welding torch. 2. The primary shielding is supplemented by a trailing shield, which is known as the secondary shielding; the trailing shield permits cooling of the weld deposit and the adjacent HAZ under a blanket of argon gas. 3. The back shielding protects the backside of the weld and its adjacent HAZ. Typical gas shielding for tube-​to-​tubesheet welding is shown in Figure 2.27.

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FIGURE 2.27  Gas shielding for titanium tube-​to-​tubesheet welding.

TABLE 2.53 Weld Colors and their Meanings (Visual Color Acceptance Criteria) Color of Weld Zone

Interpretation

Silver Light straw to dark straw Dark blue Light blue, green Gray, white

Correct shielding. Satisfactory weld Slight contamination (Action: Remove surface oxide by brushing and ensure correct shielding) Heavier contamination, may be acceptable depending on service Heavier contamination, unlikely to be acceptable Very heavy contamination, unacceptable (Action: Remove by grinding, repair, then weld again)

2.25.10.16  MIG Welding The MIG process employs a direct current supply and reverse polarity. The power supply generally consists of a constant-​potential rectifier, although a power supply with a drooping load characteristic is also applicable. 2.25.10.17 Method to Evaluate the Gas Shielding Since titanium readily reacts with air at elevated temperatures to form oxides that exhibit specific colors, the oxide color of the weld surface can be used as an effective method to evaluate the inert gas shielding. Welds made with proper shielding will exhibit a bright, metallic, shiny silvery weld color; the change in color represents increasing amounts of weld contamination, which usually takes place due to faulty or inadequate trailing shielding [227]. Table 2.53 differentiates the acceptance criteria and required disposition for each oxide color category. 2.25.10.18 Weld Defects 1. Titanium and its alloys are not prone to solidification cracking. However, under conditions of severe restraint, solidification cracking has been observed sometimes [229]. 2. Weld metal porosity is probably the most frequently encountered weld defect in titanium weld metals [228]. Porosity is generally caused by hydrogen, which may originate from gas entrapment, filler metal, or base metal. Careful shielding of the weld metal and cleaning of the surfaces will help to reduce the weld metal porosity. Welding at low speeds gives extra time for gas escape, and quality of cut edges also affects the incidence of porosity. 3. Contamination cracking will occur when substantial pickup of interstitial elements occurs. This problem can be solved by careful cleaning of the surfaces and the weld joint area, and proper shielding.

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281

2.25.10.19 Heat Treatment 2.25.10.19.1 Stress Relief Welding generally increases strength and reduces ductility. PWHT can be helpful to improve mechanical properties and to reduce residual welding stress, and the stabilization of the microstructure for elevated-​temperature applications [228]. Ordinarily, low-​strength alloys require no stress relief. It is often required in high-​strength alloys, especially when constructing elaborate titanium welds. Alpha alloys, which include the commercially pure grades of titanium, can be stress relieved at 1000°F (540°C), 1 h/​in. of thickness, in 1/​2 to 4 h. Alpha-​beta alloys welded in the annealed condition but not subsequently heat-​treated can be stress relieved in the same manner [225, 226]. 2.25.10.20 Forming of Titanium-​Clad Steel Plate Titanium reacts with iron at elevated temperatures to form a brittle compound. This alloying may take place when the clad material is subjected to high temperatures while rolling, hot forming, stress relieving, welding, and the like. If the bond deteriorates to a great enough degree, the titanium layer may actually fall off the backing plate.

2.26 ZIRCONIUM Zirconium is considered the mainly corrosion resistant material in almost all acetic acid solutions. Application of Zirconium has been used very successfully while working with sulfuric acid. The benefit of Zirconium is that corrosion rate will be extremely small if properly applied and apparatus life of over 20 years is likely.

2.26.1 Properties and Metallurgy Zirconium is a nontoxic, reactive metal. It is very similar in characteristics to titanium except that its density (6.45 g/​cm3 or 0.235 lb/​in.3) is about 50% higher. The mechanical properties of zirconium are intermediate to those of aluminum and mild steel. It is inert to many chemicals, and it has very low corrosion rates in many corrosive environments. The metal has good ductility and strength and hence can be fabricated in most forms common to other metals. It can be welded comparatively easily and can be used as a structural material in corrosive applications. References [230–​235] provide either specific or general information on zirconium.

2.26.2 Alloy Classification Zirconium and its alloys are available in two general categories: (1) commercial-​grade zirconium, containing hafnium as an impurity –​this includes R60702 (unalloyed zirconium), R60703, R60704 (zirconium-​tin alloy), R60705 (zirconium-​niobium alloy), and R60706; and (2) alloys of zirconium essentially free of hafnium, for nuclear application, commonly called Zircaloys. These include R60001, R60802 (Zircaloy-​2), R60804 (Zircaloy-​4), and R60901 (Zr-​2.5Nb). The purpose of alloying in zirconium is to improve elevated-​temperature strength and corrosion resistance while maintaining low neutron absorption.

2.26.3 Product Forms Zirconium and its alloys are available in plate, sheet, bar, rod, and tubing to a variety of material specifications. ASTM B550 covers four commercial grades of zirconium ingots: R60702, R60704, R60705, and R60706. ASTM B523 covers three grades of zirconium and zirconium alloy (R60702, R60704, R60705) seamless and welded tubes for condensers and heat exchangers.

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FIGURE 2.28  Zirconium pull through tube bundle. (Courtesy of Titan Metal Fabricators Inc., Camarillo, CA.)

ASTM B523/​B523M-​18 –​Standard Specification for Seamless and Welded Zirconium and Zirconium Alloy Tubes.

2.26.4 Applications Zirconium finds its applications in the nuclear industry and chemical process industries. The properties that favor its use as a structural material in nuclear reactors include its remarkably low thermal neutron absorption cross section, a high melting point, fair strength, and good corrosion resistance in water and liquid metals [231]. Because of its inertness, it is an excellent material for equipment used in food processing and pharmaceutical preparations. In chemical industries, it is used in process equipment, heat exchangers, piping, reactor vessels, etc. Figure 2.28 shows zirconium pull through tube bundle. Zirconium-​cladded components are used in heat exchangers exposed to seawater.

2.26.5 Limitations of Zirconium The design of zirconium alloys for elevated-​temperature applications is hindered by two factors: (1) the transformation of zirconium from the hexagonal close-​packed (hcp) structure to a bcc structure at approximately 1585°F (863°C) and (2) the problem of low corrosion resistance in contact with high-​temperature steam.

2.26.6 Corrosion Resistance Zirconium is a reactive metal. It has a high affinity for oxygen. Its surface is covered with a protective oxide film, which is self-​healing in nature. This surface film protects the base metal from corrosion attack. Whenever any fresh zirconium surface is exposed to an oxygen-​bearing environment, an adherent protective oxide film forms on its surface instantaneously. 2.26.6.1 Resistance to Chemicals Zirconium resists corrosion in almost all alkalies, either fused or in solution. Its resistance to alkalies is better than that of tantalum, titanium, and 18:8 SS [232]. It has an excellent resistance to HCl, boiling H2SO4 up to 70%, boiling HNO3 up to 90%, most organics, and all alkaline solutions to boiling temperature, but is attacked by HF. Zirconium is corroded severely in wet chlorine, brine, dilute hydrochloric acid, and seawater that contains chlorine. Zirconium, while resistant to most chloride solutions, is not resistant to ferric and cupric chlorides above 1% [236].

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283

2.26.6.2 Hydrogen Embrittlement of Zirconium Alloys Hydrogen embrittlement is a problem with zirconium and zirconium alloys, which often are used as cladding materials for nuclear reactors. Zircaloy-​2 (a zirconium alloy), which has been used as a fuel rod cladding, may absorb as much as 50% of the corrosion-​produced hydrogen and is subject to hydrogen embrittlement, especially in the vicinity of the surface. Studies have revealed that the nickel in the zircaloy-​2 was responsible for the hydrogen pickup. This has led to the development of zircaloy-​4, which has significantly less nickel than zircaloy-​2 and is less susceptible to embrittlement. In addition, the introduction of niobium into zircaloy-​4 further reduces the amount of hydrogen absorption.

2.26.7 Fabrication Zirconium and its alloys are ductile and workable. In fabrication of zirconium, there are some general considerations that must be taken into account [230]: (1) the purity or composition of the zirconium being fabricated and (2) its tendency to gale under sliding contact with other metals –​hence, while machining zirconium, tools must be kept sharp and avoid light and interrupted cuttings. Zirconium is extremely stable in contact with most common gases at room temperature, but at temperatures of a few hundred degree centigrade, it reacts readily with oxygen, nitrogen, and hydrogen, resulting in embrittlement. Therefore, welding and brazing zirconium requires high-​purity inert-​gas shielding of weld puddle and hot bead from air.

2.26.8 Welding Method Zirconium and zirconium alloys are most commonly welded using the GTAW process. Tungsten electrode for welding zirconium using GTAW should have a 20o–​30o taper and a blunt end as shown in Figure 2.29 [235]. Other arc welding processes are not used because most zirconium alloys are used in applications that require very high weld metal purity and integrity. Electron beam welding can be used for welding thick sections. As the most promising commercial property of zirconium is its high resistance to corrosion, it is essential that the welding should not reduce corrosion resistance [234]. Zirconium has a low coefficient of thermal expansion, which contributes to a low distortion during welding. 2.26.8.1 Weld Metal Shielding At high temperatures, zirconium is extremely reactive. Therefore, the weld metal and the surrounding area must be carefully shielded from air to avoid reaction of weld metal with atmospheric gases and the resulting embrittlement. The shielding gas should be highly pure argon, helium, or a mixture of these two gases. Moisture, oxygen, hydrogen, nitrogen, or carbon dioxide in the shielding gas will be absorbed by the molten metal and will result in weld embrittlement.

FIGURE 2.29  Tungsten electrode for welding zirconium using GTAW –​the tungsten electrode should have a 20o–​30o taper and a blunt end.

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2.26.8.2 Surface Cleaning ASTM Specification B614-​16(2021) –​Standard Practice for Descaling and Cleaning Zirconium and Zirconium Alloy Surfaces, covers the cleaning and descaling procedure of zirconium and zirconium alloys for the removal of shop dirt, oxides, and scales resulting from heat treatment operations and surface contaminants. 2.26.8.3 Filler Metals Zirconium and zirconium alloy electrodes are covered under AWS A5.24-​2014. AWS A5.24/​ A5.24M:2014 –​Specification For Zirconium And Zirconium-​ Alloy Welding Electrodes And Rods.

2.27 TANTALUM Tantalum is a high-​density (16.6 g/​cm3), inherently soft, fabricable metal. It has a high melting point 5432°F (3000°C). It is categorized as a refractory metal. Tantalum and its alloys (Ta-​2.5W and Ta-​ 10W) resist the broadest range of environments, making it a preferred corrosion-​resistant material. The outstanding corrosion resistance of tantalum in aqueous media is attributed to the spontaneous formation of a thin, amorphous, passive oxide film on the surface of the metal. The passive film forms in almost all environments, even in ones of extremely low oxidizing tendency, except for fluorides including HF, strong caustic, and oleum [236]. Tantalum often competes with zirconium, niobium, and titanium, whose corrosion resistance also depends upon an amorphous oxide film. It is inert to practically all organic and inorganic compounds at temperatures under 302°F (150°C). The only exceptions to this are hydrofluoric acid and fuming sulfuric acid. Equipment made of tantalum includes heat exchangers, condensers, spiral coils, U-​tubes, side-​arm reboilers, and distillation columns. References [236–​242] provide either specific or general information on tantalum. ASTM B521-​ 22 –​ Standard Specification for Tantalum and Tantalum Alloy Seamless and Welded Tubes.

2.27.1 Corrosion Resistance Tantalum exhibits excellent resistance to most forms of corrosion. Its general corrosion rate is extremely low. The passive oxide film virtually prevents pitting, crevice and intergranular corrosion, and SCC. Tantalum is cathodic in a galvanic cell with all construction metals and liberates hydrogen. Hydrogen is rapidly absorbed by tantalum with resulting embrittlement. Hydrogen embrittlement is the single most important cause of failure of tantalum [236]. Therefore, it is of utmost importance to avoid applications in which there may be a cathodic reaction [242].

2.27.2 Performance Compared with other Materials Tantalum is often compared to glass in regard to corrosion resistance. Of all the metals, tantalum is considered most like glass in corrosion resistance, and due to this property, it is used in glass and glass-​lined equipment [242]. However, unlike glass, tantalum has good resistance to brittle fracture and failure due to vibration and shock. Its strength and rigidity are similar to that of steel, while its machinability and formability are similar to copper.

2.27.3 Heat Transfer Properties Tantalum and its alloys are ideal for heat exchangers because of their high thermal conductivity (thermal conductivity of tantalum is higher than Zr, titanium, 304/​316 SS, alloys 600 and 625, and Hastelloys C-​276 and B-​2), their ability to be used in thin-​walled tubes, and nonfouling

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characteristics. The wall thickness of tantalum tubes, typically 0.015–​0.20 in. (0.381–​0.508 mm), had become an engineering standard. This compares with 0.035 in. (0.889 mm) for titanium and zirconium, since allowances have to be made for corrosion [236]. The heat transfer rate of tantalum does not change with time, due to the absence of corrosion and deposits.

2.27.4 Welding Similar to titanium and zirconium, tantalum readily reacts with carbon, hydrogen, oxygen, and nitrogen at temperatures above 600°F (315°C). When dissolved interstitially in tantalum, these elements increase the strength properties, but reduce the ductility. Therefore, any fusion-​welding must be performed in air-​free atmosphere. This is achieved either by vacuum or by inert-​gas shielding.

2.28 MATERIALS FOR HIGH TEMPERATURE HEAT EXCHANGERS High temperature heat exchangers are important in many industries, including power generation, waste heat recovery, nuclear power plants, etc. High temperature heat exchangers, operating in the inlet temperature range of 750–​1100°C, require the use of advanced materials, such as nickel-​based alloys, cobalt based alloys, ceramics, etc. Heat exchangers have to be able to function effectively and efficiently in high-​temperature environments. Appropriate materials include: 1. nickel-​based alloys such as Hastelloy and Inconel 617 and Inconel 625 2. HAYNES® alloys such as 214®, 230®, and 556® alloys 3. Alloy 800H, Alloy 800HT, Alloy 330, Alloy 230, Alloy HX, and 253 MA 4. advanced carbon and silicon carbide composites.

2.29 GRAPHITE Graphite is a unique material; it has the most valuable combination of properties. It has properties common to both metals and nonmetals. It is very corrosion resistant to acid but it is porous by nature. In order to seal the graphite pores so they don’t leak pressure they impregnate the graphite with phenolic resin. Impervious graphite is used as a heat exchanger material. It is made by impregnating graphite with a phenolic or furfuryl alcohol resin. Graphite is an allotropic form of carbon. It is used as a heat exchanger material due to the following valuable properties: 1. high thermal conductivity 2. resistance to corrosive fluids 3. stable over wide range of temperature 4. ability to withstand thermal shock 5. low coefficient of friction 6. ability to be fabricated to the desired (a) strength, (b) porosity, (c) density and compactness, (d) grain structure and fineness, and (e) surface finish 7. good machining characteristics and possibility to machine it into desired shapes 8. due to smooth surface finish, the fouling is minimum, and hence there is less deterioration in thermal performance. References [243–​246] provide either specific or general information on graphite.

2.29.1 Impregnated Graphite ASME Code Section VIII Div. 1, Part UIG –​Requirements for Pressure Vessels Constructed of Impregnated Graphite Non-​mandatory

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Graphite is naturally porous so it is impregnated with resin to make it impervious to gases and liquid, therefore, only impregnated graphite is suitable for construction of pressure vessels and components. The use of impregnated graphite for the manufacture of pressure vessels presents unique material considerations for design, fabrication, and testing. Metallic vessels, being made from materials that are normally ductile, are designed using well-established allowable stresses based on measured tensile and ductility properties. In contrast, the parts of impregnated graphite vessels are relatively brittle, and the properties of the parts are dependent upon the fabrication process. Design. Adequacy of specific designs should qualified by compliance with all applicable materials, design, fabrication, examination, inspection testing, certification, and overpressure protection rules contained in the ASME Code. Some of materials consideration as per ASME Code Sec VIII Div. 1 include: 1. Modules of elasticity. The typical modulus of elasticity is 2.0 × 106 psi (14 × 103 MPa) compared with that of ferrous materials, which may be on the order of 30 × 106 psi (207 × 103 MPa) . This is low modulus characteristic requires careful consideration of vessel geometry in order to minimize bending and tensile stresses. 2. Fatigue, like metallic materials, the impregnated graphite material, when stressed at sufficiently low levels, exhibits good fatigue life, 3. Creep and temperature effects. Impregnated graphite material is not subject to creep. The material has nearly constant tensile strength characteristics throughout the specified temperature range. Possible loss of strength at elevated temperatures is related to the maximum permissible temperature of impregnation agent.

2.29.2 Equipment Applications and Service Limitations 1. Impregnated graphite pressure vessels covered by Part UIG of ASME Code Sec VIII Div. 1 are limited to: a. the shell and tube heat exchangers b. bayonet heat exchangers c. cylindrical block heat exchangers d. rectangular block heat exchangers e. plate heat exchangers f. cylindrical vessels. 2. Impregnated graphite pressure vessels have the following limitations: a. maximum external design pressure; 2.4 MPa (350 psi) b. maximum internal design pressure: 2.4 MPa (350 psi) c. minimum design temperature: –73°C (–100°F) ) d. maximum design temperature: 204°C (400°F).

2.29.3 Standard Test Method for Impregnated Graphite (Mandatory Appendix 38) 1. Standard Test Method For Compressive Strength Of Impregnated Graphite (Mandatory Appendix 38). 2. Testing The Coefficient Of Permeability Of Impregnated Graphite (Mandatory Appendix 39). 3. Thermal Expansion Test Method For Graphite And Impregnated Graphite (Mandatory Appendix 40).

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2.29.4 Applications of Impervious Graphite Heat Exchangers Impervious graphite resists a wide variety of inorganic and organic chemicals. But strong oxidizing acids such as nitric acid, concentrated sulfuric acid, and wet chlorine cannot be handled [245]. Heat exchangers with improved resistance to oxidizing agents are being developed. Unlike the ceramic, graphite can handle hydrofluoric acid up to 60% and hot caustics [243]. Graphite heat exchangers are employed as boilers and condensers in the distillation by evaporation of hydrochloric acid and in the concentration of weak sulfuric acid and of rare earth chloride solutions.

2.29.5 Drawbacks Associated with Graphite Drawbacks associated with graphite are the following: 1. The principal limitation in the application of graphite lies in the synthetic resins used for both impregnation and laminate. The resins undergo decomposition at temperatures above 356°F (180°C) and hence graphite heat exchangers are limited to this temperature [245]. 2. Brittleness (poor impact strength), poor abrasion resistance, and low tensile strength are problems; the poor tensile strength is overcome by modifying the design and fabrication. 3. Not recommended for fine chemical industries like pharmaceutical, brewing, and food processing industries. 4. Thermal /​Mechanical shock. Mechanical shock is a common problem with carbon block heat exchangers because the fragile nature of the graphite used and the gasketed mechanical design of a carbon block heat exchanger. 5. Gasket Leaks. A carbon block heat exchanger is designed using multiple graphite blocks stacked on top of one another with gaskets in between to seal the acid. 6. Fouling /​Plugging. The phenolic resin used to impregnate the carbon block has a thermal expansion rate much higher than the graphite it is impregnating. As the heat exchanger heat up and cools down this resin flakes from the surface. This increased roughness causes the acid to stick to the side of the hole. This fouling causes decreased heating capability as well as will fully plug fairly quickly. As the heat exchanger gets older the time in between cleanings greatly shorten causing the heat exchanger cleaning to be more frequent. 7. Heat Exchanger Cleaning. The cleaning of a graphite heat exchanger usually consists of totally disassembling the graphite heat exchanger. Replacing all the gaskets during reassembly. Special care needs to be taken not to over tighten bolts and break graphite blocks /​domes but get gaskets to seal. 8. Handling and Installation. Graphite is a relatively weak material, which can be damaged by mechanical shock during handling and installation. Graphite heat exchanger construction details are discussed by Hills [244] and Schley [246]. 2.29.6  Cubic Graphite Heat Exchangers The graphite cubic block heat exchanger is adapted to the heating, cooling, condensation, and absorption of highly corrosive liquid chemicals. Rows of holes are drilled through graphite blocks both horizontally and vertically to form the process and service channels. Heat is transferred by conduction through the impervious graphite left between the rows of holes which separate the mediums being used. Graphite block heat exchangers consist in a stack of blocks encapsulated in a steel shell. A cubic block graphite heat exchanger is shown in Figure 2.30(1).

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Material Selection and Fabrication

FIGURE 2.30(i)  A cubic block graphite heat exchanger.

2.29.7 Shell and Tube Heat Exchanger The shell and tube design consists of graphite tubes and tubesheets to exploit its noncorrosive property on the tubeside. Graphite heat exchanger block and tube bundle is shown in Figure 2.30(ii); the tube bank is enclosed in a shell made of steel, cast iron, copper, aluminum, or lead, with or without corrosion-​resistant linings. The baffles may be of soft metal or plastic (polytetrafluoroethylene [PTFE]). Due to poor strength of graphite, the operating temperature and pressure are limited to 356°F (180°C) and 5 bar, respectively. The graphite shell and tube heat exchanger occupies a very large space, and hence these units are not suitable to handle large throughput of chemicals like fertilizers, synthetic fibers, and heavy chemicals [244].

2.29.8 Graphite Plate Exchanger Alfa Laval Diabon® plate-​and-​frame heat exchangers are used for highly corrosive media, combining high-​efficiency heat transfer benefits with the exceptional corrosion resistance of graphite material. Compared to other solutions, such as graphite blocks, Diabon provides the additional

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289

FIGURE 2.30(ii)  (a) Graphite cylindrical bloc wIth drilled holes, (b) Tubesheet and tube joint and (c) heat exchanger tube bundle. (Courtesy of Mersen, Paris La Défense, France.)

benefits of reduced fouling and full access to the heat transfer surface. Diabon is a dense, synthetic resin-​impregnated high quality graphite with a fine and evenly distributed pore structure, and can be used with corrosive media up to 390°F. For details on this type of heat exchanger, refer to Chapter 7 of “Heat Exchangers: Classification, Selection, and Thermal Design”.

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Material Selection and Fabrication

2.30 GLASS Low coefficient of thermal expansion, lightweight, inertness to many chemicals, and high compressive strength are the important factors that favor the use of glass. Glass heat transfer equipment finds excellent applications whenever one or more of the following are required: 1. corrosion resistance 2. product purity 3. visibility 4. low maintenance. References [247–​250] provide either specific or general information on glass.

2.30.1 Applications Because of its excellent corrosion resistance and visibility, glass heat transfer equipment proves very desirable in pilot-​plant applications in all industries. Its transparency permits fast and accurate troubleshooting if a flow problem arises [243]. The industries that employ glass heat transfer equipments include [247] the following: 1. chemical and petrochemical (corrosion resistance) 2. pharmaceutical (corrosion resistance and product purity) 3. food and beverage (product purity and inertness) 4. dyestuff (visibility and smooth surface).

2.30.2 Mechanical Properties and Resistance to Chemicals Borosilicate glass has a relatively low coefficient of thermal expansion compared to other glasses. It is subject to thermal shock and is weak in impact strength, although this is improved by thermal tempering [243]. Glass heat transfer equipment can operate at temperatures up to 392°F (200°C). This temperature is limited by the gasket material employed and not by the glass material.

2.30.3 Construction Types There are three types of glass heat exchangers used today. They are [247] as follows: 1. shell and tube heat exchangers 2. coil heat exchangers 3. hybrid heat exchangers.

2.30.4 Glass-​lined Steel Process equipment made of glass-​lined steel offers the corrosion resistance of glass and the structural strength of steel.

2.30.5 Drawbacks of Glass Material The parameters that restrict the use of glass as a heat exchanger material are the following: 1. Glass is sensitive to mechanical shock, thermal shock, thermal stresses, abrasion, and overstressing of nozzles [250].

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2. Corrosion. Glass is not completely inert; acids, alkalies, and even water can corrode glass in varying forms and degrees, but very slowly.

2.31 TEFLON With the introduction of Teflon in 1965, E. I. du Pont de Nemours & Co. Inc. made a significant technical contribution with the design and production of heat exchangers with flexible and noncorroding tubes of Teflon fluorocarbon. Teflon (PTFE), a highly chemically inert, noncorroding material, is well suited for the corrosive applications that have long been a problem in industry. The nonsticking smooth surface of Teflon tubes resists fouling and scale buildup. Exchangers made of Teflon have replaced many exchangers in corrosive services that use construction materials such as SS, impervious graphite, glass, zirconium, titanium, Hastelloy, and tantalum [251]. Shell and tube heat exchangers and immersion coils made of Teflon are used successfully for corrosion-​free heating, cooling, and condensing of many corrosive fluids found in chemical processing, steel, and plating industries. Part of the information on Teflon has been drawn from Ref. [252].

2.31.1 Teflon as Heat Exchanger Material The reasons that favor Teflon as a candidate heat exchanger material are [252] noncorrosive, resistant to fouling, resistant to shock, noncontaminating, temperature and pressure resistance. Heat exchangers of Teflon can handle corrosives up to 400°F (204°C) and pressures up to 125 psig (862 kPag), depending on temperature and good electrical resistance. The electrical resistance of Teflon enhances applications in the plating industry.

2.31.2 Heat Exchangers of Teflon in the Chemical Processing Industry Two different types of heat transfer equipment are being produced. They are shell and tube units and exposed tube bundles. Shell and tube heat exchanger. Shell and tube units with tubing of Teflon are used for heating, cooling, or condensing chemically aggressive process streams. These include sulfuric, hydrofluoric, nitric, hydrochloric, and other acids, caustic and other alkalies, halogenated compounds, salt solutions, and organic compounds [252]. The units are single-​pass exchangers containing flexible tubes of Teflon fused at both ends into an integrated honeycomb structure. All surfaces exposed to the corrosive process stream are made of Teflon. Units are available with Teflon-​lined shells for heat exchange between two corrosive streams.

2.31.3 Design Considerations Teflon tubing exhibits relatively low thermal conductivity, that is 0.11 BTU/​h ft2 °F, and this shortcoming is overcome by increasing the heat transfer area and decreasing the wall thickness of the tubing. With small-​bore tubing, a large surface area is obtained for a given volume. Practical and economic optimization led to the establishment of 0.1 in. outer diameter tubing as the smallest standard product for coils [251].

2.31.4 Size For shell and tube units, shell diameters range from 3 to 10 in. (76.2–​254 mm) for standard units. Nominal tube length is 24–​288 in. (610–​7315 mm), and shells are made of carbon or SS, fiberglass, or other materials. Tubing sizes range from 0.10 to 0.375 in. (2.54–​9.4 mm) for either FEP or Q-​ series. The heat transfer area is 5.1–​1104 ft2 (0.5–​103 m2) in standard units [252].

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Material Selection and Fabrication

2.31.5 Heat Exchanger Fabrication Technology In a unique honeycomb process, the ends of individual tubes of Teflon are fused into an integrated tubesheet. This construction provides the user with an extremely large heat transfer area in a relatively compact unit at a low cost. Reactor coils. Reactor coils are designed for immersion into agitated vessels or storage tanks to heat or cool corrosive fluids.

2.31.6 Fluoropolymer Resin Development Exchangers based on Teflon FEP can be used to 30 psig (207 kPa) at 300°F (149°C), maximum, and these limits have been extended to 50 psig (9345 kPa) at 400°F (204°C) in exchangers made with Q-​series fluoropolymer tubes [252]. Substantially higher-​pressure capabilities are available at lower than maximum temperatures.

2.32 CERAMICS For high-​temperature heat exchangers, material temperature limits are a major constraining factor. For metallic materials in use above 649°C (1200°F), the choice is essentially limited to SSs, nickel-​ and cobalt-​base superalloys, and heat-​resistant cast alloys. Structural ceramics are used to provide mechanical strength at elevated temperatures, usually in the range of 600°C–​1600°C (1110°F–​ 2910°F) [253]. Ceramic materials such as silicon carbide and silicon nitride exhibit excellent high-​ temperature mechanical strength and are used for high-​temperature heat exchanger applications. Advanced-​technology materials such as carbon-​bonded carbon-​filament composites have adequate elevated-​temperature mechanical properties, but their applications are limited because they are not usable in the presence of oxygen [254].

2.32.1 Suitability of Ceramics for Heat Exchanger Construction Because of their high-​ temperature capability and oxidation resistance, ceramics are obvious materials for high-​temperature heat exchangers, particularly in energy and resource conservation [254]. The drawbacks of ceramics for high-​temperature applications are 1. brittleness 2. permeability 3. unsuitable for fabrication by joining techniques 4. irreparability.

2.32.2 Classification of Engineering Ceramics There are three major classifications of engineering ceramics [254] as follows: 1. ceramic oxides such as alumina, beryllia, and zirconia 2. glass ceramics 3. ceramic carbides and ceramic nitrites.

2.32.3 Types of Ceramic Heat Exchanger Construction 1. tubular construction 2. plate fin construction.

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2.32.4 Hexoloy® Silicon Carbide Heat Exchanger Tube An alternative to metals, glass, and other tube materials for enhanced heat transfer, uptime, and reliability for high corrosive environment is Hexoloy silicon carbide (SiC) heat exchanger tubes supplied by Saint-​Gobain. Hexoloy silicon carbide heat exchanger is shown in Figure 2.31a. Hexoloy SiC’s thermal conductivity is almost equal to that of commonly used graphite tubes. Its thermal conductivity is two times that of tantalum, five times that of SS, ten times that of Hastelloy, and 15 times that of glass. The result is higher efficiency while requiring less heat transfer area. Figure 2.31b shows the comparison of thermal conductivity of Hexoloy SiC with some heat exchanger tube materials. 1. High corrosion resistance. Hexoloy SiC tubes have been proven for years in HF, bromine, high concentration nitric acids, mixed acids, bases, oxidants, and chlorinated organics. 2. Extreme hardness and high strength. Hexoloy SiC is one of the hardest high-​performance materials available for heat exchanger tubes.

FIGURE 2.31  Hexoloy heat exchanger tube- (a) Hexoloy silicon carbide heat exchanger unit and (b) comparison of thermal conductivity of Hexoloy SiC with few heat exchanger tube materials. (Courtesy of Saint-Gobain Advanced Ceramics, Niagara Falls, NY.)

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Material Selection and Fabrication

2.33 COMPOSITE Composite pipes and vessels are used in many environments due to their special properties such as high strength-​to-​weight ratio, corrosion resistance and the ability to be tailored to specific design requirements. As properties of the constituent materials and fabrication methods have improved, the use of composite pipe and vessels has increased accordingly. Modern composite materials have found wide use in the chemical process, petrochemical, and pulp and paper industries primarily because of their corrosion resistance compared to steel and other metals. The class of fiber reinforced plastics includes many combinations of matrix and reinforcement materials. In weight sensitive applications such as offshore oil platforms, the primary attribute of interest is the weight savings when compared to exotic metal alloys that would otherwise be required to contain corrosive liquids [255].

2.33.1 Fiberglass Tanks and Vessels The oil industry was one of the first to use fiberglass tanks. Fiberglass tanks were an obvious choice due to their light weight and corrosion resistance as compared to carbon steel tanks. For applications in very corrosive services composite tanks and vessels can be produced with in a dual laminate configuration –​a fiberglass tank shell with a liner constructed of plastic.

2.33.1 ASME Code Section X Fiber-​Reinforced Plastic Pressure Vessels Provides requirements for construction of a fiber-​reinforced plastic pressure vessel (FRP) in conformance with a manufacturer’s design report. It includes production, processing, fabrication, inspection and testing methods required for the vessel. Section X includes three Classes of vessel design: Class I and Class III –​qualification through the destructive test of a prototype; and Class II –​mandatory design rules and acceptance testing by nondestructive methods. These vessels are not permitted to store, handle or process lethal fluids. Vessel fabrication is limited to the following processes: bag-​molding, centrifugal casting and filament-​winding and contact molding. Rules pertaining to the use of the ASME Certification Mark with the RP Designators are also included.

2.34 ALLOYS FOR SUBZERO/​CRYOGENIC TEMPERATURES Steels and nonferrous materials are used for containment, handling, and transporting of liquefied gas and liquefaction of gases. Other applications include stationary structures and mobile equipment exposed to adverse climates or operating conditions or both. Austenitic steels, stainless steels, double standardized and tempered fine grain nickel, steels, copper, and aluminum are excellent materials that can withstand cryogenic Temperatures below −150°C (−238°F) often are identified as cryogenic temperatures. Temperatures for liquefying commonly used types of gases are below 100°C. The temperatures are approximately −162°C for LNG and approximately −184°C for liquefied ethylene gas. The motive for utilizing low-​temperature technology is that at cryogenic temperatures, liquid gases occupy much less volume than their pressurized gaseous state. Therefore, the containment vessels for liquid gases may be smaller, thinner (because of lower pressure), and less costly [256]. Material properties relevant at cryogenic temperature. The most important cryogenic material properties are the following: (1) toughness, (2) DBTT, impact strength, (3) yield strength, (4) plastic deformation, (5) corrosion, (6) thermal conductivity, (7) thermal expansion, (8) specific heat, (9) fatigue behavior, (10) creep behavior, (11) magnetic properties, etc.

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2.34.1 Ductile-​Brittle Transition Temperature Some metals display a marked loss of ductility in a narrow temperature range below room temperature. This is called the DBTT. The application of steels below their NDT is avoided because of the danger created by brittle crack propagation that could lead to the catastrophic failure of a component or entire system [16]. Below the NDT, very little energy is required for crack propagation [114]. The following factors can improve the toughness of steels [17]: (1) ASTM grain size has a strong effect on a given steel’s NDT. As the grain size number increases (i.e. the grains become smaller), the NDT drops. Therefore, fine-​grained steels usually are used for low-​temperature applications. (2) Proper heat treatment can be very effective in increasing a steel’s fracture toughness. Quenching and tempering is an effective heat treatment for improving toughness. (3) Among alloying elements, nickel, titanium, and manganese tend to increase steel’s toughness. (4) Another variable that affects DBTT is thickness.

2.34.2 Requirements of Materials for Low-​Temperature Applications A major requirement of materials for liquefaction equipment and for containment and transport of liquefied gases is toughness at the handling temperatures of the liquid. Additional requirements include minimum weight, weldability characteristics to resist hot cracking, cold cracking, and embrittlement of HAZs, high strength and toughness of the welded joints, and corrosion resistance, low thermal expansion to minimize dimensional changes due to temperature difference between the ambient and service temperature, relatively low specific heats, low thermal conductivity to minimize thermal conduction, etc. Carbon steels have only limited corrosion resistance and must be replaced with corrosion-​resistant alloys when metal loss becomes severe.

2.34.3 Notch Toughness Notch toughness is defined as the ability to resist brittle fracture at high stresses, such as that can be caused by impact loading. Notch toughness is an indication of the capacity of a steel to absorb energy when a stress concentrator or notch is present. In the presence of a flaw, such as a notch or crack, a material will likely exhibit a lower level of toughness. The notch toughness of a steel product is the result of a number of interactive effects, including composition, deoxidation and steelmaking practices, solidification, and rolling practices, as well as the resulting microstructure. Notch toughness is measured by various means. The favored method recommended by the American Petroleum Institute as well as the ASTM and ASME is the Charpy V-​notch test. It is considered the most appropriate because a part or structure will generally fail due to a notch or other stress concentration [257] or a defect such as gouge, weld crack, arc strike, or a sharp discontinuity[258]. Using the Charpy V-​notch test, one can determine the TT at which a material becomes brittle. This information helps the designer to choose a steel that will remain ductile through the range of temperatures or stresses it will be subjected to in service. 2.34.3.1 Notch Toughness: ASME Code Requirements The ASME Code should be consulted for allowable stress at low temperatures and for the testing required of the material to ascertain that it is suitable for low-​temperature service. The ASME Code stress tables designate −20°F as the beginning of low temperature.

2.34.4 Selection of Material for Low-​temperature Applications Selecting materials for low-​temperature and cryogenic applications calls for thorough understanding of the application and knowledge of the mechanical properties that each grade of metal provides. Since various low-​temperature materials are available, the designer must consider the merits of each material according to the application.

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2.34.5 Materials for Low-​temperature and Cryogenic Applications Aluminum, copper, titanium, nickel-​base alloys, ferritic steels including 9% Ni steels, and SSs offer designers a wide choice and have been used successfully for liquefaction, containment, and transport of liquefied gases. The selection of aluminum, copper, nickel-​and cobalt-​base alloys, titanium, low-​alloy ferritic steels including 9% Ni steels, and SSs is discussed next. Table 2.54 shows a list of materials for low-​temperature application along with the minimum temperature applicability. 2.34.5.1 Aluminum for Cryogenic Applications Aluminum and aluminum alloys have fcc crystal structures and retain good ductility at subzero temperatures. Aluminum can be strengthened by alloying and heat treatment while still retaining good ductility along with adequate toughness at subzero temperatures. Among the aluminum and aluminum alloys, 1100, 2014 to 2024, 2195, 2219-​T87, 3003, 5052, 5083-​O, 5086 and 5456, 6061-​ Tb, 7005, 7075, 7079, and 7178 are recommended for cryogenic applications. The salient features of these alloys are discussed next. Before that, the factors that favor aluminum use in cryogenics are listed [259, 260] in the following: 1. Unlike other metals, aluminum has no DBT transition regardless of the direction of stress. 2. Inertness to cryogenics like methane, ethylene, argon, helium, neon, O2, N2, H2.

TABLE 2.54 Materials for Low-​Temperature Application Alloy Designation Low-​carbon steel

Alloy steel

Stainless steel Aluminum Copper Nickel Titanium

Specification A 442, A 516 A 537 A 662, A 724 A517 A203, Gr. A and B, 2.25% Ni A203, Gr. D, E, and F, 3.50% Ni A645, 5.0% Ni A353 A553, Type II 8% Ni A553, Type I 9% Ni A543 A736 A844 (9% Ni) 304, 304L, 316, 316L, 347 1100, 2014 to 2024, 2219-​T87, 3003, 5083-​0, 5456, 6061-​Tb, 7005 C10200, C12200 (DHP), C17200, C22000, C26000, C51000, C70600, C71500 Monel-​K, Hastelloy B, Hastelloy C, Inconel alloy 600, 706, Inconel alloy 718, Invar-​36, Inconel alloy X-​700 Ti-​5Al-​2.5Sn, Ti-​6Al-​4V (ELI) Ti-​5Al-​2.5Sn (ELI)

Note:  Refer to ASTM Standards or National Codes for applicable lowest temperature.

Approximate Lowest Temperature −50°F (−46°C) −75°F (−59°C) —​ −75°F (−59°C) −90°F (−68°C) −150°F (−101°C) −320°F (−196°C) −320°F (−196°C) −275°F(−170°C) −320°F (−196°C) −180°F (−118°C) −50°F (−46°C) −320°F (−196°C) −452°F (−269°C) −452°F (−269°C) −325°F (−198°C) −452°F (−269°C) −441°F (−263°C) −320°F (−196°C) −423°F (−253°C)

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3. Absence of corrosion at cryogenic temperatures. Protective coatings are rarely necessary, but the designer should be cautious of galvanic couple. 4. The tensile strength increases proportionately more than yield strength in the: 2000 series: aluminum alloys such as 1100, 2014 to 2024, 2195, and 2219-​T87. 3000 series: aluminum alloy 3003 is used in the fabrication of brazed PFHEs and other equipment in gas liquefaction plants. It is available as tubing (including finned tubing), pipe, sheet, and plate. It is readily joined by brazing or welding. Alloy 3003 meets the requirements of the ASME Boiler and Pressure Vessel Code for working temperatures up to −321°F (−196°C). 5000 series: alloys such as 5052, 5053, 5083, 5086, and 5456 exhibit a combination of properties that make them popular for most applications. Their moderate strength, good toughness, and good weldability have resulted in their selection for oceangoing tankers for carrying oxygen, LNG, and other cryogenic gases, and for tank trailers, stationary storage containers, and processing equipment. For PFHEs, 5083-​O is used. 6000 series: alloy 6061 offers the advantage listed for 5000 series alloys except that in the as-​ welded condition, its strength is low. 7000 series: alloys such as 7005, 7039, 7075, 7079, and 7178 display the highest strength of all aluminum alloys. But they lose toughness below −320°F (−196°C). They are generally nonweldable and found only in limited application. Two newer alloys, 7039 and X7007, show promise for cryogenic service because they are readily welded and retain adequate toughness at all temperatures [256]. 2.34.5.2 Copper and Copper Alloys Copper and copper alloys have fcc crystal structures similar to those of aluminum and retain a high degree of ductility and toughness at subzero temperatures, down to −423°F (−253°C). Copper alloys that might be considered for use at subzero temperatures are C10200 oxygen-​free copper, C12200 (DHP), C17200, C22000, C26000, C51000, C70600, and C71500 [205]. The development of lightweight brazed aluminum heat exchangers for cryogenic applications caused copper to be replaced by aluminum for many of these components. 2.34.5.3 Titanium and Titanium Alloys Commercially pure titanium may be used for tubing and small-​scale cryogenic applications that involve only low stresses in service. For temperatures down to −320°F (−196°C), the normal interstitial-​grade alloys Ti-​5Al-​2.5Sn and Ti-​6Al-​4V are suitable. Interstitial impurities such as iron, oxygen, carbon, nitrogen, and hydrogen reduce the toughness of these alloys at both room and subzero temperatures. For temperatures below −196°C, ELI grades of Ti-​5Al-​2.5Sn and Ti-​6Al-​4V are used [205]. The lower strength, all alpha Ti-​5Al-​2.5Sn ELI is used down to −423°F (−253°C), the temperature of liquid hydrogen. Titanium and titanium alloys are not recommended for containment or other use with either liquid or gaseous oxygen in cryogenic service, because any fresh surface caused due to abrasion or impact exposed to oxygen will cause ignition and hence possible explosion. 2.34.5.4 Nickel and High-​Nickel Alloys Nickel is an fcc metal that retains good ductility and toughness at subzero temperatures. Unalloyed nickel is low in strength and has only limited applications at subzero temperatures. However, several nickel-​base alloys, including some superalloys, exhibit excellent combinations of strength, ductility, and toughness up to −441°F (−263°C). Typical nickel-​base alloys for cryogenic applications include

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Monel K-​450, Hastelloy B, Hastelloy C, Inconel alloy 600, Inconel alloy 706, Inconel alloy 718, Inconel X-​700, and Invar-​36 (36% Ni-​iron) [205]. 2.34.5.5 Carbon Steels and Alloy Plate Steels ASTM specifications A203, A353, A442, A516, A517, A537, A553, A612, A645, A662, and A724 describe steel plates with minimum Charpy V-​notch energy or lateral expansion requirements at testing temperatures from −15°F to −320°F (−26°C to −196°C). 2.34.5.5 1 Carbon Steels Carbon steels provide service to −75°F. Less costly than alloy steels, they combine better weldability with low coefficients of thermal expansion and thermal conductivity. In carbon steels, the principal means of improving notch toughness is through changes in composition of C, Mn, Si, and Al contents. Carbon lowers toughness, whereas Mn increases it. Si and Al are added as deoxidizers. Silicon-​killed steel has slightly better notch toughness than semikilled steel, and silicon-​aluminum killed steel has still higher toughness [258]. 1. ASTM A516 The major advantage of A516 steels is their low initial cost. But they feature the lowest ASME stresses, 13,750–​17,500 psi. Thus, a given design strength requires heavier gauges than are needed with high-​strength steels. A516 steel is used widely in air liquefaction plants, refrigerating plants, transport equipment, and containment vessels operating down to −50°F (−46°C) [257]. For these applications, the steel is normally made to meet impact test requirements of ASTM A300 Class 1 specification, which calls for plates to be normalized and to meet a Charpy keyhole minimum of 15 ft-​lb at −50°F. 2. ASTM A517 Of the low-​temperature alloy steels, A517 Grade F has the highest allowable stresses. At −50°F, its impact strength (Charpy V-​notch) is 40 ft-​lb, and its notch and crack resistance are sufficient to encourage wide usage. 3. ASTM A537 Grades Higher strength with good notch toughness is available in carbon steels such as the two classes listed in ASTM A537 grades, normalized (Class A) or quenched and tempered (Class B), which provides 60,000 min psi yield strength plus 15 ft-​lb of impact strength (Charpy V-​notch) at −75°F. 4. ASTM A203/​A203M-​17 As a steel making practice, the steel shall be killed and shall conform to fine grain size requirements. The heat treatment requirements for all plates are presented, and all plates under Grades A, B, D, and E shall be normalized as required. The steel shall conform to the required chemical compositions. Two mechanical test requirements are presented that includes, tension test requirements and impact test requirements. 5. 2.5% Nickel Steels ASTM specification A203 Grades A and B are used in service down to −90°F (−68°C). The low temperature requirements are given in ASTM specification A300.

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6. 3.5% Nickel Steels ASTM specification A203 Grades D and E are used in service down to −148°F (−100°C). Forgings and bolting materials are also covered in ASTM specifications. The low-​temperature requirements are given in ASTM specification A300. 7. 5% Nickel Steel ASTM A645/​A645M-​10(2022) –​Standard Specification for Pressure Vessel Plates, 5 % and 5.5 % Nickel Alloy Steels, Specially Heat Treated. This is used as a wrought material for service down to approximately −185°F (−120°C), for the fabrication of welded vessels for handling and storage of liquid ethylene in land-​based plant and marine tankers. Quenched, temperized, and reversion annealed ASTM A645 specifies 5% nickel steel designed for LNG service. Armco Cryonic 5 is covered by ASTM A645. 8. ASTM A353 Alloy Steel ASTM A353/​A353M-​17(2022) –​Standard Specification for Pressure Vessel Plates, Alloy Steel, Double-​Normalized and Tempered 9 % Nickel. ASTM A553/​A553M-​17e1 –​Standard Specification for Pressure Vessel Plates, Alloy Steel, Quenched and Tempered 7, 8, and 9 % Nickel. This is one grade 9% Ni steel, normalized and tempered for subzero use. Yield strength is 75 ksi; specification for longitudinal and transverse impact (Charpy V-​notch) energy is 20 ft-​lb minimum at −320°F (−196°C). ASTM A553 steels contain 8% or 9% nickel and are essentially quenched and tempered to 85 ksi yield strength (minimum). Impact energy minimum is 25 ft-​lb (longitudinal) and 20 ft-​lb (transverse) at −196°C (−320°F) for Type I, 9% Ni, or −170°C (−275°F) for 8% Ni steel (Type II). The welding considerations are the same for both types of steel regardless of heat treatment. The 9% nickel steels have long been used for ethylene, methane, LNG, oxygen, and nitrogen applications. One of the benefits of nickel steels in design of LNG vessels is volume and weight savings due to their relatively high strengths. Selection criteria and fabrication aspect of 9% nickel steel are discussed in detail later. 9. 36% Ni-​Iron Alloy 36% nickel-​iron alloy possesses a useful combination of low thermal expansion, moderately high strength and good toughness at temperatures down to that of liquid helium, –​452 ºF (−269 ºC). These properties coupled with good weldability and desirable physical properties make this alloy attractive for many cryogenic applications. A modified form of 36 per cent nickel-​iron alloy known as INVAR* M 63 has been used for LNG membrane tanks. It is available as plate, strip, sheet, pipe, tubing, bars, billets, forgings, and wire. 2.34.5.6 Products Other than Plate A partial list of ASTM specifications for other than plate steel products for subzero service is in the following: • • • • •

A333 Seamless and welded steel pipe A334 Seamless and welded carbon and alloy steel tubes A350 Forged or rolled carbon and alloy steel flanges, fittings, and valves A352 Ferritic steel castings A420 Piping and fittings of wrought carbon steel and alloy steel

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• A522 Forged or rolled 8% and 9% nickel steel flanges, fittings, and valves • A671 Electric fusion-​welded steel pipe • A757 Carbon and alloy steel castings. In addition to the steels listed, a number of proprietary ferritic steels have been developed by several steel producers to meet certain requirements for service at subzero temperatures. 2.34.5.7 Austenitic Stainless Steel In the last decade, there has been a considerable increase in the use of austenitic SSs for cryogenic services at temperatures between −240°F and −452°F (−151°C and −268.9°C). They play an important role in LNG ships for containment tanks, storage tanks, cargo piping systems, and a variety of ancillary equipment. The AISI 300 type steels such as 304, 304L, 310S, 316, 316L, 321, and 347 offer a fine combination of toughness and weldability for service down to −452°F (−269°C). Among these alloys, Types 304 and 304L are the most commonly used alloys. Consequently, they have the largest service experience and coverage in design codes. These grades have moderate strength and excellent toughness, and they are selected for their formability, fabricability, and ready availability in a variety of product forms [14, 261]. Among the modified varieties, nitrogen-​containing, high-​proof-​strength stainless (e.g. Types 304N, 316L +​N) is used for cryogenic processing plants and in liquid oxygen and nitrogen storage and transportation applications. The addition of 0.2% N raises the proof strength by about 40% and moderately increases the tensile strength without sacrificing ductility or fracture toughness [14].

2.34.6 Cryogenic Vessels The cryogenic storage vessel is designed to have two distinct shells, one is referred to as the inner shell or product container and the other is referred to as the outer shell and is often referred to as the vacuum jacket. The air between the holes is sucked out with the aid of the air separation unit and it is vacuumed for better insulation. In large-​capacity cryogenic tanks, multilayer insulation, powder isolators, and fabric materials are used as insulators. Linde, Germany is one of the leading manufacturers of cryogenic vessels.

2.34.7 Fabrication of Cryogenic Vessels and Heat Exchangers Most of the cryogenic materials are formable and weldable. Most of the units are welded. Brazing is limited to the manufacture of PFHE [256]. Important considerations for fabrication of cryogenic plants are discussed in detail in Ref. [262]. Some of the considerations are the following: 1. All raw materials and consumables should undergo rigorous quality and specification tests. 2. Edge preparation is considered to be of the utmost importance for all types of welding, and clear specifications should be laid down and strictly adhered to for all joints. 3. Welding plants should be checked for proper functioning. 4. It is essential that the weld metal and HAZ achieve a fracture toughness greater than or equal to that of the base metal. The weldment should resist hot and cold cracking and embrittlement of the HAZ.

2.34.8 9% Nickel Steel Low-​carbon 9% nickel steel was developed in the United States by the International Nickel Company, Inc. The fundamental mechanical properties of 9% Ni steel must meet the minimum

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requirements of ASTM 353 or ASTM 553 Type I specifications. The steel is used for vessels and plant for processing, transportation, and storage of liquefied gases down to −196°C. The excellent low-​temperature toughness of this steel results from the alloying addition of approximately 9% nickel and the presence of stable retained austenite at cryogenic temperature. Nickel also suppresses the formation of ferrite/​pearlite high-​temperature transformation products; thus, a microstructure is produced that is higher in strength and notch toughness [263]. 2.34.8.1 Merits of 9% Nickel Steel Materials previously used, such as copper and austenitic corrosion-​resistant steels, presented no particular difficulties in construction, but the cost of these materials was high [264]. The use of 9% nickel steel has increased because it is a cheaper material than those materials previously used for operation down to −321°F (−196°C). The higher strength of this steel compared with aluminum, copper, or even austenitic steels makes possible the use of higher design stresses and hence offers high strength-​to-​weight ratio. The major factors that favor 9% nickel steel include [14] the following: 1. High design stress coupled with good fracture toughness characteristics. 2. Relatively low thermal expansion compared with austenitic SS and aluminum alloys. 3. Chemically resistant to liquid oxygen and nitrogen, producing no corrosion products that would hamper the operation of valves and meters or cause unsafe conditions [265]. 4. Low thermal conductivity. 5. Good weldability by a variety of processes, including shielded metal arc, gas metal arc, and SAW [266]. 2.34.8.2 Welding Procedures The welding procedures most widely used are MMA welding with covered electrodes, automatic or semiautomatic SAW with continuous wire, MIG welding, cored wire welding, and for the root pass TIG welding and PAW. Employ MIG welding with a spray mode of metal transfer to overcome lack of fusion problems, to obtain clean welds while keeping the heat input to relatively low levels [269, 270]. 2.34.8.3 Guidelines for Welding of 9% Nickel Steel As is well known, 9% Ni steel derives its properties through double normalizing and tempering or quenching and tempering. The welding processes and the weld metal must be such as not to alter, beyond acceptable limits, the structural characteristics of the parent metal in the HAZ. To ensure tensile strength and toughness in the fusion zone, and to avoid welding flaws, a few practical rules are suggested by Pozzolini [268] and Thorneycroft and Heath [267]. 2.34.8.4 Welding Problems with 9% Nickel Steel Conventionally, welding of 9% Ni steel is made using high-​nickel alloy of the austenitic type. This process presents some critical problems [271]: 1. The yield strength of the weld metal is low compared with that of the base metal. 2. During welding, there is a high susceptibility to various forms of hot cracking such as longitudinal bead cracking and crater cracking of the weld metal. 3. The high-​alloy filler material is relatively expensive and results in higher construction cost. 2.34.8.5 Post-​weld Heat Treatment Since the 9% nickel steel retains its toughness after welding, there is no need for post-​weld stress-​ relieving treatment. In October 1960 (see Operation Cryogenics), a number of welded pressure

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vessels in 9% nickel steel were tested to destruction in the United States in the as welded condition to illustrate that the 9% steel would behave in a tough and ductile manner at −196°C without stress relief [272]. ASME Code Case 1308-​S allows use of the steel in the as-​welded condition in thicknesses up to 2 in. (50.8 mm). Fabricators of vessels for some chemical applications may for sections above a certain thickness, typically above 50 mm, wish to apply a stress-​relieving operation at a temperature of 1050°F. Further, in some instances, it may be necessary to produce large components by welding the plate to required size followed by hot forming; in such work, a double-​normalizing and tempering treatment will normally be required on the welded and formed material [273].

2.34.9 Welding of Austenitic Stainless Steels for Cryogenic Application The austenitic SSs with or without nitrogen strengthening are readily welded by all of the common welding processes, provided appropriate procedures and consumables are used. The welding processes include SMAW, TIG, and MIG welding processes. The GTAW process in conjunction with ER308L or ER316L wire produces the cleanest weld metal and excellent toughness even at −196°C (−320°F). Guidelines on welding of SSs for cryogenic applications are discussed in Ref. [261]. “Guide to the Welding and Weldability of Cryogenic Steels” (IIS/​IIW-​844-​87), issued by the IIW, is principally devoted to the consideration of welding of fine-​grain aluminum-​killed steels and nickel-​alloyed ferritic steels up to 9% nickel. 2.34.9.1 Charpy V-​Notch Impact Properties Due to service conditions, the selection of welding consumables is guided by the mechanical property requirements for the weld metal. The most important mechanical property is Charpy V-​notch impact at −450°F (−268°C); the minimum energy absorption is 20 ft-​lb, and lateral expansion is 0.015 in minimum. 2.34.9.2 Problems in Welding The following four main factors affect the strength and toughness of the as-​deposited weld metal and take on added significance at cryogenic temperatures [274]: 1. sensitization 2. ferrite content 3. nitrogen pickup 4. oxide inclusions. These four phenomena are discussed in detail in Ref. [261]. The following discussion is based on that reference. 2.34.9.2.1 Sensitization The grain boundary precipitation of chromium carbides reduces weld metal toughness at low temperatures. Weld metal toughness can be improved by using very low carbon fillers such as 308L (0.04% C max) and 316L (0.03% C max). For SSs with carbon content higher than 0.03%, annealing of the welds at temperatures greater than 950°C, followed by rapid cooling, dissolves the carbides and improves the toughness. 2.34.9.2.2 Ferrite Content To avoid microfissuring, for a wide range of SS, the weld metal is usually balanced to provide 4–​8 FN. However, for cryogenic service, higher ferrite reduces toughness at −196°C and lower

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temperature. To obtain a good combination of strength and toughness of the weld, ferrite content should be maintained in the range of 0–​2 FN [275]. 2.34.9.2.3 Nitrogen Pickup Nitrogen increases the yield strength and decreases the toughness of SS weld metals [275, 276]. When GTAW and GMAW processes are followed, careful gas coverage should be provided to avoid nitrogen pickup in the weld metal. While employing SMAW, lime coatings of the electrode generally give better coverage and less nitrogen pickup than titanium coatings. 2.34.9.2.4 Oxide Inclusion Content Oxide inclusions form sites for the initiation of microvoids. The toughness at cryogenic temperatures increases with decreasing inclusion contents [276]. To reduce inclusions, use basic-​coated electrodes, since they generally provide better toughness than rutile ones, due to lower oxygen content and oxide inclusions than the rutile coatings [277].

2.34.10 Safety in Cryogenics 2.34.10.1 Checklist The following list is a brief compilation of various principles and measures for safety in cryogenics. It can be used as a reminder when designing cryogenic equipment [278]. 1. A thorough knowledge of the chemical, physical, and toxic properties of the cryogens to be used. 2. Knowledge of the construction material properties especially embrittlement, hydrogen embrittlement, and combustibility. Materials susceptible to hydrogen attack and hydrogen embrittlement such as titanium must not be used in hydrogen service. 3. Knowledge of the possible low-​temperature effects. 4. Maintaining safe distance for adjacent structures. 5. Testing of each component and safety device before commissioning. 6. Directives such as onsite emergency plan, clear definitions of work tasks, operation manuals for all possible operating conditions as well as for all possible accident conditions. 7. Training of the operation staff in order to minimize the risk of incidents and accidents. 8. Safety precautions –​oxygen monitoring, safety garments, and ventilation.

2.35 CLADDING A clad plate is a composite plate consisting of a base metal and a cladding of corrosion resistant alloy (CRA), resistant metal, or a plastic (polyvinyl chloride plastic-​clad steel plate). Cladding is required on the process side to provide corrosion resistance against highly severe corrosive service fluid or to increase wear resistance of a component . It is well known that various grades of austenitic SSs of Types 304, 304L, 308, 316, and 347, nickel, Monel, Inconel, cupronickel, aluminum, copper, zirconium, titanium, etc. exhibit excellent corrosion resistance in many corrosive environments. However, the construction of large assemblies such as pressure vessels and heat exchangers in corrosion-​resistant metal involves costs. Consequently, increasing use is being made of clad materials to achieve the optimum balance of strength and surface properties to overcome corrosion by an economical means. The principal cladding techniques include hot roll bonding, cold roll bonding, explosive bonding, centrifugal casting, brazing, and weld overlaying, although adhesive bonding, extrusion, and hot isostatic pressing have also been used to produce clad metals. Apart from cladding, the other methods of protecting the base metals are lining, which refers to sheet

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or strip attached internally to the component by mechanical or intermittent fusion techniques, and sheathing by external attachment.

2.35.1 Clad Plate In a clad plate, the base material is being selected for economic reasons and strength purposes and the cladding layer for one or a combination of the following: corrosion and erosion resistance, cryogenic properties, elevated temperature properties, wear-​resistance properties, etc. The selection of clad steel or any other clad metal requires decisions to be made regarding [279]: 1. the optimum choice of CRA/​backing steel combination 2. the selection of manufacturing method appropriate to the part which is clad 3. the approach to the fabrication. 2.35.1.1 Backing Materials Some examples of steel grades for backing metal include [280]: C-​Mn Steel –​ A/​SA516 Gr 60-​65-​70 A/​SA537 Cl 1-​2 C-​Mn-​Mo Steel –​ A/​SA204 Gr A-​B-​C Cr-​Mo –​ A/​SA387 Gr 11-​12-​22 Cl 1-​2 Cr-​Mo-​V –​ A/​SA542 Gr D C-​Mn-​Mo-​Ni –​ A/​SA533 type B-​C-​E. 2.35.1.2 Corrosion Resistance Cladding Grades Corrosion resistance cladding grades include [280]: Ferritic stainless steel 410S Duplex SS Cr-​Ni austenitics 304L, 321, 347 Cr-​Ni-​Mo austenitics 316L, 316LMo, 316Ti, 316Nb, 317L Aluminum metal Copper, Bronze, and Brass Nickel based alloys “825”–​N08825, “625”–​N06625, “C276”–​N10276, “C22”–​N06022 Titanium. Illustrations of clad tubesheet of shell and tube heat exchanger is shown in Figure 2.32.

2.35.2 Cladding Thickness The thickness of the clad layer required is usually small relative to that of the base material, because the latter is designed to take the majority of the load. The thickness of the clad material may vary from 5% to 50% of the base plate thickness, but normally it is held in the range of 10%–​20%.

2.35.3 Methods of Cladding The term cladding covers a wide range of processes including the following: 1. 2. 3. 4.

loose lining resistance cladding lining using plug welding thermal spray cladding

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FIGURE 2.32  Cladded tubesheet of shell and tube heat exchanger. (Note: item (b) is unclad tubesheet).

5. weld overlay cladding 6. hot roll bonding 7. explosive bonding 8. centricast pipe for cladding of pipe 9. coextruded pipe or duplex tubing 10. hot isostatic process. Loose linings improve either new or existing structures. Cladding by weld overlay and thermal spraying works for new and existing components. Hot roll bonding and explosive bonding offer corrosion resistance to new construction. Centricast pipe involves centrifugal casting of cladding on the base metal. Duplex tubings are coextruded. All cladding methods have some limitation, either economic or practical. These cladding processes, except hot isostatic process, are discussed next.

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2.35.3.1 Loose Lining Loose lining refers to the installation of a thin corrosion resistance lining inside a process vessel. The cladding liner is about 0.3–​2.0 mm thickness. Early titanium cladding attempts were based on loose lining. Use of thin titanium layers loose clad to steel is limited to process systems where [281]: 1. heat transfer between the shell and process medium is not critical 2. loss of pressure or vacuum will not collapse the liner 3. temperatures are low 4. there is no problem in suspending vessel internals on the lining. If these four factors are not critical, a loose-​clad vessel may be economically practical. 2.35.3.2 Thermal Spraying Thermal spraying is accomplished by heating the cladding metal to a molten state and spraying it on the prepared surface of the base metal. The thickness normally ranges from 0.2 to 2.5 mm. One of the advantages of this process is that the temperature of the base metal normally does not exceed 302°F–​392°F (150°C–​200°C), which normally does not affect the base metal. Various methods of cladding by lining are given in Figures 2.33 and 2.34.

FIGURE 2.33  Various clad lining techniques for plates.

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FIGURE 2.34  Various clad lining techniques for vessel components.

2.35.3.3 Weld Overlaying or Weld Surfacing Definition: A weld overlay is a type of cladding that uses a welding process to melt a material onto the surface of another, different material. The surface to be overlaid must be cleaned of oxide and dirt. Weld overlaying by a fusion process may be applied only when the base metal and the weld metal deposit are compatible. While weld surfacing, an important consideration is weld dilution, which is discussed next. 2.35.3.4 Weld Dilution To ensure an overlay of specified composition for the intended purpose, the filler metal must be enriched sufficiently to compensate for dilution. For any given filler metal composition, changes in welding procedures such as maintaining approximately 50% bead overlap, use of small-​diameter electrodes, low heat input, and directing the arc onto the previously deposited bead minimize dilution. The welding procedure specification should contain the acceptable limits of chemical

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FIGURE 2.35  Estimation of weld metal dilution (schematic).

composition of the deposit. Methods of calculating weld dilution and the approximate weld metal content of any element is shown in Figure 2.35.

2.35.4 Weld Overlay Cladding Methods Various welding methods have been adapted to overlaying. Some of the methods include the following: • • • • • • • • • • • •

GMAW FCAW spray transfer CO2 welding manual TIG TIG hot wire TIG cold wire (mechanized) plasma hot wire SMAW electroslag strip welding submerged arc –​single or multiple wire submerged arc strip cladding plasma transferred arc welding.

The selection of any one technique is dependent upon [279] access, welding position (downhand or positional), alloy type and dilution specified and economics. Cladding Processes. While most of the existing arc and electro slag welding processes can be utilized for weld cladding, strip cladding with submerged arc and electro slag welding process are the most attractive choices for applications that require large surface area coverage due to their substantially higher deposition and surface area coverage rates. AWS specification for various corrosion-​resistant weld-​surfacing alloys is covered by, for SS, AWS A5.4, A5.9, and A5.22; for copper base, AWS A5.6 and A5.7; for nickel-​base surfacing alloys, AWS A5.11 and A5.14; and for cobalt base, AWS A5.2. MMA process. The MMA process represents the most flexible means of weld overlaying. This process is economical for small areas and uneconomical for large areas. Submerged arc and electroslag welding are proven economical methods for large areas. GMAW or Metal Inert Gas (MIG) Welding. MIG welding is like SMAW welding in that it uses a consumable electrode that melts to form an overlay. However, the electrode does not contain flux, so the shielding gas must be added separately. GMAW process with spray transfer, pulse transfer, or spot welding is normally used. The Inco method involves intermittently spot welding a thin (0.6–​ 2.0 mm) copper-​nickel lining to the head with an MIG torch and a wire fed [282].

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Tungsten Inert Gas (TIG) Welding. In TIG welding, a separate wire is used to add filler material. Due to the growing demand for improved welding quality and tighter tolerances, mechanized TIG welding is increasingly applied to complete delicate tasks, especially after recent process developments which led to higher melting rates and enhanced productivity. The use of TIG welding in the clad welding process has certain advantages [283]: 1. The accuracy of the process when welding overlay complex surfaces, especially when welding overlay zones with corners, curves or inside or outside edges, avoids the need for manual touch-​ups. 2. In TIG weld overlay, programming of precise parameters: cladding speed, currents, voltage, wire feed speed and preheating temperature, allows for greater control of the dilution rate (characterized by the mix of molten filler material with parent metal) thereby guaranteeing correct chemical composition of deposits. Plasma Transferred Arc (PTA) Welding. PTA welding is an inert-​gas process that uses a non-​ consumable electrode like TIG welding. The major difference is that the cladding material is added directly to the arc as a powder. Submerged Arc Welding. As its name suggests, submerged arc welding is distinct from other processes because the arc stays hidden –​or submerged –​beneath the flux blanket. In other respects, submerged arc welding closely resembles SMAW. Laser Welding. Typically an automated process, laser welding uses a focused beam of light to quickly melt the cladding into the parent metal. Laser offers high efficiency and excellent results with a smaller heat-​affected zone than is possible with conventional arc welding. Laser cladding also produces smooth, flat cladding free from surface imperfections. Laser clad items require little to no finishing, further improving the efficiency of the process [284]. Strip cladding process In the strip cladding process, a strip is substituted for a solid wire. Advantages claimed for the process are relatively high deposition rates, low dilution and flexibility, 100% bonding, and good surface finishing [285]. Several variations of the strip cladding process exist, mainly subdivided into single-​and double-​strip techniques. Features of strip cladding [286]: 1. Arc-​less process –​uses conductive flux and works on Joule’s resistance heating principle. 2. The strip current passes through the molten slag. The resulting resistance heating effect melts the strip and deposits the molten weld pool onto the base material. 3. Low dilution level (9–​12%). Process has significant advantages over SAW. Submerged arc strip cladding. The high deposition rates achieved by SAW are well suited to large area surfacing applications. Both single and multiple electrode SAW methods are used for surfacing. The productivity of this process can be improved further by the use of higher welding currents and wider strips. The associated problems are arc blow, increased penetration and poor bead characteristics, and dilution. Magnetic steering reduces penetration, and hence dilution and arc blow control [287]. Feature of Submerged arc strip cladding include [286]; 1. Utilizes an arc that runs back and forth at high speed along the strip. 2. The arc causes more penetration into the base material, resulting in dilution levels of ~20%. 3. Deposition rate: 12–​14 kg/​h for 60x0.5mm strip. 4. Current range restricted to limit dilution.

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Material Selection and Fabrication

Electroslag strip cladding process. Compared to conventional cladding processes, such as pulsed GMAW (GMAW-​P) and strip submerged arc surfacing, the ESS process is known to provide both high deposition rate and low dilution level (9–​12%) [288]. There is no arc between the strip electrode and the parent material. ESW uses higher welding currents than SAW strip cladding so the welding heads used are more heavy duty [289]. Other features include [286]: 1. Arc-​less process –​uses conductive flux and works on Joule’s resistance heating principle. 2. Low dilution level (9–​12%). Process has significant advantages over SAW. 2.35.4.1 Stainless Steel Strip Cladding Consideration for stainless steel strip cladding. When it is required to clad a thick-​walled carbon steel vessel with an austenitic steel by the strip cladding process, three factors must be considered [285, 290]. These are the following: 1. Dilution. In general, the effects of dilution have been overcome either by depositing more than one layer or by using a dual-​strip process. 2. A guaranteed minimum thickness of cladding. 3. Suitable deposit microstructure and mechanical properties (e.g. 4%–​10% free ferrite in an austenitic matrix and sufficient ductility at the clad metal interface to satisfy a 3T side bend test). Metal powder additions to control ferrite. Ferrite-​stabilizing metal powders are used to control the ferrite content of the cladding and transition zone [291]. Metal powder additions have already been used to enhance deposition rates in many conventional welding processes. According to Oh and Devletian [291], the other benefits of metal powder additions include good control over weld penetration, HAZ size, and improved fracture toughness of the weld. 2.35.4.2 Procedure and Welder Qualification The general requirements for the qualification of cladding are in accordance with the ASME Code, Section IX. 2.35.4.3 Inspection of Overlays Soundness of cladding is usually tested by these methods [285]: (1) liquid penetrant to reveal any pinhole porosity; (2) ultrasonic inspection for lack of fusion and to detect large slag inclusions; (3) soundness of bond and ductility by side bend tests –​excessive iron dilution or irregular penetration patterns usually fail bend tests; (4) corrosion test for resistance to corrosion; (5) chemical analysis to assure the specified composition; (6) metallography for studying the microstructure; and (7) hardness profile across the overlays. For SS, additional tests include the measurement of delta ferrite in the weld metal. 2.35.4.4 Nickel Alloy Cladding Submerged arc process, GMAW spray transfer, and SMAW are the preferred processes. Nickel alloy weld metals are readily applied as overlays on carbon and low-​alloy steels and other materials. Inconel or Monel cladding can be applied to the tubesheet with inert-​gas metal arc (MIG) process.

2.35.5 Roll Cladding Roll cladding involves metallurgically fusing and rolling of corrosion-​resistant thin plate to the base metal at the rolling mill, as shown in Figure 2.36. The bond formed is part mechanical and part metallurgical; consequently, metallurgically incompatible materials normally cannot be produced. In the roll cladding process, a rectangular plate pack of compatible base and cladding metals is assembled. The plate pack consists, in this order, of (1) base metal, (2) a layer of cladding metal,

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311

FIGURE 2.36  Roll cladding process.

(3) a parting compound, (4) a layer of cladding metal, and (5) base metal. The facing surfaces of cladding and base metal are precleaned, and surface oxides are removed.

2.35.6 Materials and Standards of Clad Steel Plates Generally, the base metal of clad plates are usually carbon steels or low alloy steels, which may be made in accordance with ASTM A516 (ASME SA-​516), ASTM A285 (ASME SA-​285), ASTM A387 (ASME SA-​387), ASTM A36, ASTM A283, etc. The cladding metal may be selected from a variety of special metals based on different classifications [292]: • Stainless steel clad plate: the cladding metal may be austenitic stainless steel or duplex stainless steel such as SS 304/​304L, SS 316/​316L, SS 321, SS 347, SS 310S, Duplex 2205, Duplex 2507, UNS S31803, 904L, AL6XN, etc.

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Material Selection and Fabrication

• Standard specification: ASTM A240 (ASME SA-​240), ASTM A263 (ASME SA-​263), ASTM A264 (ASME SA-​264). • Nickel and nickel alloy clad plate: the cladding metal may be in accordance with ASTM B127, ASTM B162, ASTM B168, ASTM B333, ASTM B409, ASTM B424, ASTM B443, ASTM B463, ASTM B575, ASTM B582, ASTM B625. Typically, these materials include Inconel 600, Inconel 617, Inconel 625, Incoloy 800/​800H, Incoloy 825, Incoloy 925, Monel 400, Monel K-​500, Hastelloy C-​276, Hastelloy C-​22, Nickel 200, Nickel 201, Alloy 20, Alloy 31, etc. The clad plate manufacture standard is ASTM A265/​ASME SA-​265. • Copper and copper alloy clad plate: the cladding metal may be copper and copper alloy of ASTM B96, ASTM B152, ASTM B171, ASTM B846. The copper clad steel plate shall be manufactured in accordance with ASTM B432-​19. 2.35.6.1 Stainless Steel Clad Metal Stainless steel-​clad metals can be produced in plate, strip, tube, rod, and wire form. One of the most common clad plates are carbon or low-​alloy steels clad with 300-​series austenitic grades. The types of austenitic stainless steel cladding commonly available in plate forms are [293]: Type 304 (18-​8), Type 304 L (18-​8 low carbon), Type 309 (25-​12), Type 310 (25-​20), Type 316 (17-​12Mo), Type 316 Cb (17-​12 Nb stabilized), Type 316 L (17-​12 Mo low carbon), Type 317 (19-​13 Mo), Type 317 L (19-​13 Mo low carbon), Type 321 (18-​low Ti), Type347(18-​11Nb). Grades 321 and 347 are the basic austenitic 18/​8 steel (Grade 304) stabilized by Titanium (321) or Niobium (347) additions. These grades are used because they are not sensitive to intergranular corrosion after heating within the carbide precipitation range of 425-​850°C. Backing materials A285, A201, A212, A204, A 302. 2.35.6.2 Titanium Clad Steel Plate The titanium-​clad steel plate is a typical metallic laminar composites. It is also known as titanium cladding or titanium-​steel bimetallics. Usually, they are produced by deformation bonding: either hot/​cold-​roll bonding or explosive bonding. The titanium clad steel plate obtains the low cost and high strength of steel with the outstanding corrosion resistance of titanium. 2.35.6.3 Standard and Specification The titanium-​clad steel plate shall be manufactured to ASTM B898. The base metal may be a variety of carbon steel plates manufactured to ASTM A516, ASTM A515, ASTM A266, ASTM A572, ASTM A709, ASTM A387, ASTM A240, etc. The cladding metal may be ASTM B265 titanium Gr.1, Gr.2, Gr.3, Gr.7, Gr.9, Gr.12, and Gr.16, etc. The cladding techniques can be either hot rolling, cold rolling, explosive bonding, or a combination of them. Titanium-​Carbon Steel Clad Plate [294] 1. ASME SB265 Gr.2 +​ASME SA516 Gr.70 HIC Clad Steel Plate 2. ASME SB265 Gr.7 +​ASME SA516 Gr.70 Clad Steel Plate 3. ASME SB265 Gr.11 +​ASME SA283 Gr. C Clad Steel Plate 4. ASME SB265 Gr.1 +​ASME SA516 Gr.70 HIC Clad Steel Plate. 2.35.6.4 ASTM Specification for Clad Plate Some of ASTM specifications for clad plate are listed below: 1. ASTM A263-​12(2019) –​Standard Specification for Stainless Chromium Steel-​Clad Plate 2. ASTM A264-​ 12(2019) –​Standard Specification for Stainless Chromium-​ Nickel Steel-​ Clad Plate

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3. ASTM A265-​12(2019) –​Standard Specification for Nickel and Nickel-​Base Alloy-​Clad Steel Plate 4. ASTM B432-​19 –​Standard Specification for Copper and Copper Alloy Clad Steel Plate 5. ASTM B898-​20 –​Standard Specification for Reactive and Refractory Metal Clad Plate. The cladding metal may be titanium and titanium alloy (ASTM B265/​ASME SB-​265), niobium and niobium alloy (ASTM B393), zirconium and zirconium alloy (ASTM B551/​ASME SB-​551), tantalum and tantalum alloy (ASTM B708) –​this specification covers the standard requirements for tantalum and tantalum alloy plate, sheet, and strip in the following grades: R05200, R05400, R05255, R05252, and R05240.

2.35.7 Test and Inspection [295] The processed clad plate will be tested as given below: 1. Chemical composition. Ladle analysis of base metal and cladding material. 2. Mechanical tests. Test items are in accordance with specified standard and customer’s request. 3. Bond integrity. Ultrasonic flaw detection test. Ultrasonic inspection is performed as per ASTM A264/​A265. 4. Dimension measurement: The thickness, width and length are measured for each plate. 5. Tensile Test. The tensile properties shall be determined by a tension test of the composite plate or base plate for evaluation of strength of material. 6. Bend Test. Bend tests with the cladding metal outside indicate the strength of the bond. 7. Shear Strength. The ASTM specification requires minimum shear strength of 140 MPa. 8. Corrosion Test. ASTM A262 Practice E for stainless steel cladding material ASTM G48 method A for nickel alloy . 9. Quality check –​(1) dimensional accuracy, (2) interface of the cladding and base metal, (3) shear strength, (4) weldablility, (5) workability, (6) corrosion resistance, and (7) storage and handling. 2.35.7.1 Inspection of Stainless Steel Cladding Clad qualities are evaluated by the following tests [287]: 1. corrosion test according to ASTM A262 2. side bend test (ASTM E 190) for the ductility of clad metal and for fusion between clad and base metal 3. ferrite content test to ensure resistance to hot cracking or microfissuring 4. microprobe analysis to determine the distribution of Cr and Ni across the depth of cladding 5. microstructure examination.

2.35.8 Explosive Cladding The explosive cladding process utilizes explosive energy to create a metallurgical bond and to produce cost effective clad plate of both conventional and unique metal combinations. In this process, the cladding plate is accelerated by means of an explosive charge to a high velocity of the order of 1000 ft/​s (322 m/​s), before impacting the base plate. This process makes available a range of metal combinations and many materials that are normally considered incompatible and hence cannot be produced by conventional methods. For example, titanium can be bonded to mild steel, copper to SS, SS to brass, and many other combinations. Explosive welding is principally associated with the fabrication of large clad plates, shells, tubes, nozzles, tubesheet, the plugging of defective

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Material Selection and Fabrication

tube joints of shell and tube heat exchangers, etc. Most of the clad tubesheet applications have consisted of nickel and nickel alloys, copper and copper alloys, and SS clad on SA 516–​70 [296]. Table 2.55 presents a list of alloys commonly supplied as explosion clad [297]. 2.35.8.1 Explosion Cladding Process Sequence 1. backer material is primarily carbon steel or stainless steel but can be any material 2. cladder material 3. mating surfaces are ground clean 4. assemble backer /​cladder /​explosive 5. detonate explosion 6. flattening 7. testing and inspection. 2.35.8.2 Welding Geometries Explosive welding geometries are mostly (1) parallel cladding and (2) angular cladding. Large-​area clad plates for fabrication of plates are made using the parallel arrangements. Figure 2.37 illustrates the principles of parallel explosion cladding. Figure 2.38 shows a clad head and Figure 2.39 shows the photomicrograph of a typical explosion weld. 2.35.8.3 Angular Geometry In angular geometry arrangement (Figure 2.40), the two surfaces to be welded are at an angle to each other, and explosive is sited on the reverse side of one of the components. On initiation of the explosion, the two plates are forced together, colliding intimately to form a junction. Small-​area cladding such as tube-​to-​tubesheet joint expansion (and plugging of leaking tubes (Figure 2.40)) is

TABLE 2.55 List of Clad Alloys and Base Metals Cladding Metals

Base Metals

SS alloys: A410S, A304/​304L, A316/​316L, A317/​317L, A321, A347 Nickel and nickel alloys: 200, 201, 400, 600, 625 800, 904L, 825, 625, C22, C4, Titanium Zirconium Tantalum

A 203, A387Gr 11 Cl2, A 516–​70 A 387Gr 12 Cl2; A 516 Gr 60, 65, 70; A537 Cl 1, Cl 2; A553 Type 1, A 738 Gr A, B

FIGURE 2.37  Explosive cladding principle. (Courtesy of Dynamic Materials Corporation, CO.)

Material Selection and Fabrication

315

FIGURE 2.38  Clad head. (Courtesy of Dynamic Materials Corporation, CO.)

FIGURE 2.39  Photomicrograph of a typical explosion weld. (Courtesy of Dynamic Materials Corporation, CO.)

FIGURE 2.40  Angular geometry explosion cladding: (a) tube-to-tubesheet expansion and (b) plugging of leaking tube—Dynafusion® explosively welded plug.

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Material Selection and Fabrication

made using the angular arrangements. This setup is appropriate in view of the short bond lengths of approximately 1 in. that are normally required [298]. Metal combinations that are welded commercially include carbon steel to carbon steel, titanium to SS, and 90-​30 copper-​nickels. The principle of geometry applied for tube-​to-​tubesheet joint expansion and plugging of leaking tubes is explained next. 2.35.8.4 Tube-​to-​Tubesheet Welding In most instances, the weld is located near the front of the tubesheet and has a length of approximately 0.5 in. (12.7 mm) or three to five times the tube wall thickness [299]. Most applications of explosion welding in tube-​to-​tubesheet joints involve tube diameters of 0.5–​1.5 in. (12.7–​38.1 mm). The angular disposition of the component surfaces is achieved in this instance by machining a countersink at the outer end of the tubesheet hole. The countersink depth is usually 0.5–​0.6 in. at an included angle between 10° and 20° [300]. The detonator is placed in the bore of a polyethylene insert to form a composite cartridge, which is placed within the tube hole. On ignition, the shock waves emanating from the detonator are transmitted by the polyethylene insert to the tube, thus imparting to it the required radial velocity. Tubes may be welded individually or in groups. While determining the choice of explosive welding for tube-​to-​tubesheet joining, one should consider (1) the thickness of the tubesheet, (2) ligament width, and (3) tube diameter and wall thickness [299]. End effect. Because of energy losses at the end of the tube, the velocity at the tube extremity is lower than elsewhere in the system, thereby producing an end effect [298]. This area may well have a velocity below that required for welding. The tube is therefore initially positioned with its end projecting some short distance from the face of the tubesheet. The end effect area thus lies outside the tubesheet, and a reasonably uniform tube velocity is thereby achieved over the intended weld zone along the machined angle, as shown in Figure 2.40. 2.35.8.5 Plug Welding Explosive welding of plugs in leaking tubes Figure 2.40(b) is an effective technique for conventional heat exchangers and nuclear heat exchangers where there is a problem of nuclear radiation [301]. These areas may be inaccessible due to hotness, corrosiveness, radiation, etc. Plug welding, being a maintenance operation, is usually carried out on-​site. The only operation that remains to be carried out within the confines of the exchanger tubesheet is machining of a countersink, similar to tube-​to-​ tubesheet explosion welding. 2.35.8.6 Inspection of Joint Quality The usual methods of testing explosive welds are discussed in Refs. [299, 300]. Typical inspection methods in addition to visual inspection include the following: (1) pulse-​echo ultrasonic technique (ASTM A578) to assess the bond integrity –​an ultrasonic frequency in the range of 2.5–​10 MHz usually is adequate; (2) radiography applicable to welds between metals with significant density variation and an interface with a large wavy pattern; (3) metallographic examination of the weld interface on a plane parallel to the detonation front and normal to the surface –​a well-​formed wave pattern without porosity generally is indicative of a good joint; and (4) the bond strength by various destructive tests like chisel test, tension-​shear test, and tension test. 1. Chemical Composition and Mechanical Properties The base metal and cladding metal plates shall conform to its chemical and tensile requirements respectively in accordance with relative standard specifications. 2. Bonding Integrity, Ultrasonic Test on Clad Plate The ultrasonic examination shall be performed in accordance with ASTM A578/​A578M.

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317

2.35.9 Processing of Explosion Clad Plates Clad plates usually are distorted somewhat during explosion cladding. This requires straightening to meet standard flatness requirements. Also it is customary to supply explosion-​clad plate in the as-​cladded condition because the hardening that occurs immediately adjacent to the interface usually does not significantly affect the bulk properties of the base plate. If some service requirements demand PWHT, this requirement may be complied with. Clad steels are always flame-​cut from the backing side.

2.35.10 ASME Code Requirements in Using Clad Material ASME Code Section VIII, Div. 1, places the responsibility on the owners and users for determining cladding material that will be suitable for the intended service. 2.35.10.1 Clad Tubesheets Tube-​to-​tubesheet welds in the cladding of either integral or weld metal overlay clad tubesheets may be considered strength welds (full or partial), provided the welds meet the design requirements of UW-​20. In addition, when the strength welds are to be made in the clad material of integral clad tubesheets, the integral clad material to be used for tubesheets shall meet the requirements in (1) and (2) for any combination of clad and base materials. The shear strength test and ultrasonic examination specified in (1) and (2) are not required for weld metal overlay clad tubesheets. (1) Integral clad material shall be shear strength tested in accordance with SA-​263. One shear test shall be made on each integral clad plate or forging and the results shall be reported on the material test report. (2) Integral clad material shall be ultrasonically examined for bond integrity in accordance with SA-​578. 2.35.10.2 Processing of Clad Steel Plates 1. Cutting Clad steel plate can be sheared by shearing or punching, cut by a planer, etc. Cold and hot forming operations can be carried out on clad plate for the manufacture of heads and shells. In both cases the clad surface should be protected from damage or contamination. 2. Shaping Shaping of clad steel plate can be made by roll-​bending, pressing and spinning. To take advantage of cladding material features, cold working is recommended to the maximum extent possible. Pressure vessel heads, shells, tubesheets, and other components can be made from explosion-​clad plates by conventional hot-​or cold-​forming techniques. A differently stressed condition exists at the bond interface, and this governs the thicknesses and diameters that can be successfully bent [302]. Hot forming, welding, or heat treatment must take into account the metallurgical properties of the materials, grain growth, and the possibility of undesirable diffusion that may occur at the interface [299, 300]. Various precautions to be taken while fabricating clad plates are discussed by Ellis [303]. 2.35.11 Welding of Stainless Steel Clad Plate The aim of welding clad steel is to maintain a continuous fully corrosion resistant layer across the joint. In the case of single side welds in internally clad pipe, the weld preparation is generally a “J” bevel

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with the nose of the bevel being entirely within the clad layer. Good cleanliness and dryness is essential to prevent contamination of the deposit with sulfur or hydrogen which could lead to cracking. The root can then be welded entirely within the clad layer using GTAW with a filler metal of matching or over-​matching corrosion resistance. For example, for cladding of 304L or 316L, a 309MoL filler metal would be preferred; for 904L, 825 or 625 cladding, 625 filler would be preferred [279]. When welding clad steels, the dilution effects should be given consideration. The backing steel should be welded first; however, if the welding does not present any great difficulty, the clad layer can be welded first with a suitable electrode, back chipped, and the base plate then welded [299, 303]. Joint preparation for clad steels is shown in Figure 2.41. A typical welding procedure for Hastelloy clad steel plate is shown in Figure 2.42. Joining clad steel to unclad steel sections normally requires making the butt weld and restoring the clad section in a fashion similar to joining two clad plates. Other considerations for welding SS clads are carbide precipitation, sigma-​phase precipitation, and delta ferrite content. 2.35.11.1 Welding Clad Plate by SMAW Process The welding of steel integrally clad with other metals and alloys is more exacting than the welding of either one individually. Special attention shall be paid to (a) the accuracy of joint preparation and fit-​up, (b) selection of electrodes for individual beads, (c) bead placement, (d) sequence of welding operations, and (e) welding technique. 2.35.11.2 Selection of Filler Metals Stainless-​ steel-​ clad carbon or low-​ alloy steel plates are sometimes welded with SS filler metal throughout the whole plate thickness. But it is enough to use carbon or low-​alloy steel filler metal on the unclad side, followed by removal of a portion of the cladding and completion of the joint with stainless filler metal. For the clad side, Type 309L filler metal could be used for base metal such as Types 405, 410, 430, 304, and 304L; Type 309 Cb for 321 or 347; and 309 Mo for 316. The user should consider the manufacturer’s recommendations in choosing filler metals. Reference [304] gives guidance for selection of filler metals for welding clad metals of austenitic SSs, ferritic SS, nickel alloys, and copper alloys, and guidance for the butt joint designs for welding clad steels from both sides, respectively.

FIGURE 2.41  Joint design for welding of clad steel plate. (Adapted from Kearns, W.H. (ed.), Welding Handbook, Vol.4, Metals and Their Weldability, 7th edn., AWS, Miami, FL, 1982, Chapter 12.)

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319

FIGURE 2.42  Method of welding of steel plate clad with Hastelloy.

PWHT. If stress-​relief annealing is required, use either stabilized or low-​carbon SSs to avoid carbide precipitation. Care should be taken to ensure absence of sigma-​phase precipitation. Differential thermal expansion between mild steel backing and SS cladding will induce stresses in the cladding. 2.35.11.3 Titanium Clad Steel Repair Titanium (Ti) clad steel is widely used for large pressure vessels and other equipment in different industries to take advantage of the corrosion resistance of Ti, but at a lower cost than solid Ti construction. Titanium has not been successfully fusion welded directly to steel because it has limited solubility for Fe. To avoid welding Ti directly to steel, the most common method of joining clad plates is the Batten Strip technique. The Ti cladding material is stripped back 15 to 20 mm from the weld joint, after which the steel is welded and inspected. Next, the space where the cladding was removed is filled with Cu, Ti, or steel filler strips. Finally, a Ti cover strip or Batten Strip about 50mm wide is welded over the joint using fillet welds and gas tungsten arc welding (GTAW) techniques. This method is illustrated in Figure 2.43. As an alternative, titanium-​clad steel is produced by roll bonding, usually with an interlayer, direct explosive bonding (usually without an interlayer), or by a combination of explosive bonding and roll bonding. Interlayers are used to improve the bond

FIGURE 2.43  Titanium clad steel plate repair method (Schematic).

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Material Selection and Fabrication

strength of the clad steel or to overcome metal plasticity compatibility restrictions encountered in roll bonding. Industrial-​grade pure iron (Fe), ultralow-​carbon steel, niobium (Nb) alloys, tantalum (Ta) alloys, copper (Cu) alloys, and nickel (Ni) alloys have been used as interlayers in the cladding process[305]. Typical thickness of Ti-​clad ranges from 2.0 to 19.0 mm (0.08 to 0.75 in.) depending on the application.

2.36 POST-​WELD HEAT TREATMENT OF WELDED JOINTS IN STEEL PRESSURE VESSELS AND HEAT EXCHANGERS Though many welded structures and components perform satisfactorily without PWHT, there are certain well-​established conditions where application of PWHT is either mandatory or traditional [306]. For example, 1. Heavy thickness metals and alloy steels as specified in applicable fabrication code and customer requirement. 2. PWHT of pressure vessel and heat exchanger is mandatory for certain applications (e.g. lethal service). 3. With some welding processes like electroslag and electrogas, PWHT is required. The high heat input and the long thermal cycle inherent in the electroslag welding process produce a large HAZ, which is subject to grain coarsening and a loss of fracture toughness. The coarse-​ grained weld metal should be refined by a normalizing heat treatment to develop required properties [307, 308]. Normalizing removes nearly all traces of the cast structure of the weld metal and nearly equalizes the properties of the weld metal and the base metal. 4. It is important to note that vessels operating under creep conditions should be stress relieved before being put into service. If the vessels are not stress relieved, residual stresses may be superimposed on the applied stress, and this may lead to premature creep failure.

2.36.1 Objectives of Heat Treatment PWHT is intended to relieve the residual stresses generated by thermal contraction after welding and thereby to minimize the risk of subsequent distortion or cracking or to produce improvements in metallurgical structure or properties of the weld metal.

2.36.2 Types of Heat Treatment 1. Annealing is a heat treatment process normally applied to material after cold working or before welding (air-​hardenable materials). The process increases ductility and reduce the hardness to make it more workable. The annealing process requires the material to be heated above its recrystallization temperature for a set amount of time followed by an extended slow furnace cooling. The cooling rate depends upon the types of metals being annealed. Annealing also removes stresses that can occur when welds solidify. 2. Normalizing is a heat treatment process applied to ferrous materials. The objective of the normalizing heat treatment is to restore the original microstructure after hot forming or severe cold working. The cold working operations such as forging, bending, hammering hardens the materials and make it less ductile. Normalizing works in three stages –​the recovery stage, recrystallization stage and the grain growth stage. For normalizing of steel, the material is cooled in still air after heating to temperatures above the upper critical temperature or A3 temperature. The normalizing temperature for carbon and alloy steels is in the range of 1600°F–​ 1800°F, depending on the type of steel. 3. Quench and temper. Steel is rapidly cooled from above its upper critical temperature or A3 temperature to a temperature far below this range. Water or oil is used to accelerate the cooling.

Material Selection and Fabrication

321

In the as-​quenched condition, the product is not suitable for most commercial applications because of its lower ductility and high hardness. Therefore, steel must be tempered in order to soften to improve its ductility and toughness and relieve internal stresses. Tempering is a reheating treatment done at lower temperatures, usually in the range between 400°F and 1300°F. 4. Quench annealing. This is normally adopted for sensitized austenitic SS. Sensitization causes precipitation of chromium carbide in the HAZ. This precipitation causes the steel to lose chromium below 11% and makes the zone susceptible to corrosion. Also during cold work, SS work hardens to a very high degree. In order to dissolve the carbides in case of welding and to bring down the hardness in cold working, austenitic steels are heated above 1050°C and quenched in water. Quench annealing is also employed to heat treat 6Mo superaustenitics. 5. Stress relieving is a heat treatment designed to relieve residual stresses in the metal structure. The residual stresses may result from cold-​working operations, from machining, or from welding. The treatment consists of slowly heating the part to about 1150°F–​1350°F and, after holding at this temperature for a period of time, slowly cooling the steel in the furnace or still air. With ferritic steels, the treatment preferably consists of heating the complete unit in a furnace to a temperature of 600°C–​700°C. This heat treatment is sometimes called process anneal, subcritical anneal, or post heat treatment after welding [309, 310].

2.36.3 Effects of Changes in Steel Quality and PWHT IIW experience [306] shows that steels that are microalloyed with any element (or combination) of Al, Ti, Nb, or V and that have a CE value ≤0.41% and carbon content value ≤0.15% are susceptible both to HAZs and to degradation of the weld zone by plastic strains and hence can better accommodate the presence of residual stresses induced in the weld zone. Under certain conditions, use of a suitable filler metal and proper welding procedures can result in satisfactory weld joints without the application of PWHT, provided there is no mandatory code requirement for PWHT. If stress-​relief treatment is still required, it may be possible to achieve this with mechanical stress relief or with lower stress-​relief temperatures than that are usually required. The carbon content in steel should be lowered to meet the HAZ hardness requirements, if PWHT is not employed [311].

2.36.4 ASME Code Requirements for PWHT The requirements for PWHT are dictated by most of the codes as a function of base metal composition and thickness. The ASME Code Div. 1 [312] requires specific heating rates, holding times, temperatures, and cooling rates. Heat treatment is the process of heating metal without letting it reach its molten, or melting, stage, and then cooling the metal in a controlled way to select desired mechanical properties. Heat treatment is used to either make metal stronger or more malleable, more resistant to abrasion or more ductile.

2.36.5 PWHT Cycle When a PWHT is specified, the operations should be monitored and documented by an inspector. Items of importance in heat treatment may include the following: 1. area to be heated 2. heating and cooling rates

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FIGURE 2.44  Heat treatment cycle-Schematic.

3. holding temperature and time 4. temperature measurements and distribution 5. equipment calibration. There are three stages of heat treatment: (1) heat the metal slowly to ensure that the metal maintains a uniform temperature, (2) soak, or hold, the metal at a specific temperature for an allotted period of time, and (3) cool the metal to room temperature. Schematic of PWHT cycle is shown in Figure 2.44. Alloy steels in general require more severe heat treatment than mild steel, but temperature is usually kept below the critical range, i.e. below about 700°C, unless normalizing at a temperature around 900°C is adopted [310]. The rate of heating varies with the quality, specification, geometry, and size of jobs and will be as low as 100°C/​h/​in. for the case of high-​alloy steel and 220°C/​h/​in. for the case of medium carbon steel. Soaking temperature, soaking time, and rate of cooling depend upon the grade and specifications of materials. Heat treatment guidelines for carbon and low alloy steels, refer to UCS 56 tables of ASME Section VIII, Div. 1.

2.36.6 Quality Control During Heat Treatment Heat treatment schedules should be based on the code, welding procedure specification, and material specification requirements. The heat treatment must be carried out in accordance with the approved procedures. Such heat treatment procedures should indicate the following: 1. rate of heating 2. soaking time and temperature 3. rate of cooling 4. location of thermocouple 5. for local heat treatment, the heating band width. For every heat treatment cycle, a coupon test plate should be made. While heat treating, metals temperature must be measured and recorded. Thermocouple location and number of thermocouples are to be provided as per fabrication code and approved procedure. The time-​ temperature cycle for heat treatment is recorded on a continuous recorder, which is made

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TABLE 2.56 Heat Treatment Details Part

Heat Treatment

Temp.

Holding Time

Holding Rate

Cooling Rate

Remarks

available to the authorized inspector for verification. The time-​temperature chart and heating diagram are the records of the heat treatment operation. All thermocouple and temperature measuring and recording instruments are calibrated periodically. For all instruments and gauges, a serial number is allotted. The details of the heat treatment, such as PWHT or heat treatment after cold forming, local heat treatments, and annealing on a job site or part/​partial annealing − but not the heat treatment of purchased parts (materials, dished heads) − for the equipment or its parts should be prepared as given in Table 2.56.

2.36.7 Methods of PWHT The methods of PWHT include furnace heating, in situ heating, and local heating. If the whole assembly is to be heat treated, this is usually carried out in a gas-​fired or oil-​fired furnace. While heat treating in a furnace, care has to be taken to ensure that the entire exchanger is brought uniformly to the holding temperature to avoid introducing unnecessary thermal stresses. This is ensured by the use of thermocouples on the vessel interior as well as the exterior to verify adequate through-​wall heating [39]. When local heat treatment is carried out, electric resistance heating with resistance elements or braided heaters or induction heating or gas burners may be used.

2.36.8 Effectiveness of Heat Treatment The level of residual stress after PWHT depends on the material composition, stress-​relieving temperature and duration, and stress level before heat treatment. The efficiency of stress relieving can be determined by [310] the following: 1. bent bar tests 2. tests of creep relaxation type 3. determination of stresses in welds.

2.36.9 Defects Arising due to Heat Treatment 1. Overheating, which will cause scaling and surface reaction. 2. Distortion due to poor PWHT cycle temperature program. 3. Sagging of the tube bundle or the shell due to inadequate support or because the existing supports are widely spaced. 4. For weld overlays on steel plates, prolonged overheating will cause carbon diffusion from the base metal into the overlay and will weaken the base metal, and this may cause intergranular corrosion in corrosive environments. Carbide-​forming elements like Mo, V, and Cr slow down diffusion. Other effects include sensitization and sigma-​phase precipitation if the heat treatment temperature is high.

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2.36.10 Possible Welding-​Related Failures These are possible welding-​related failures during stress-​relief annealing [76]: 1. temperature-​induced embrittlement 2. elongation-​induced embrittlement 3. stress relief cracking, SRC or reheat cracking, RC. Stress relief cracking is defined as intergranular cracking in the heat-​affected zone (HAZ) or weld metal that occurs during PWHT or during high temperature service. Stress relief cracking during PWHT, also known as reheat cracking, is essentially grain boundary embrittlement due to carbide precipitation during the PWHT. The main purpose of the PWHT is to relieve the stresses built up during welding. Reheat cracking may occur in low alloy steels containing alloying additions of chromium and molybdenum or chromium, molybdenum and vanadium when the welded component is being subjected to post weld heat treatment, such as stress relief heat treatment, or has been subjected to high temperature service (typically in the range 350 to 550°C). Cracking is almost exclusively found in the coarse grained regions of the heat affected zone (HAZ) beneath the weld, or cladding, and in the coarse grained regions within the weld metal.

2.36.11 NDT After PWHT A surface examination after PWHT is recommended to confirm flaws have not developed near the surface during PWHT. NDT inspections that are normally required after PWHT is hardness testing. Certain materials are prone to cracking during PWHT. This is known as reheat cracking (RC) or stress relief cracking (SRC). HAZs of welds of such materials are to be subjected to NDT after PWHT.

REFERENCES 1. Henthorne, M., Understanding corrosion, Chem. Eng., Deskbook Issue, 4 December, 19–​33 (1972). 2. Kobrin, G., Materials selection, in Metals Handbook, Korj, L. J. and Olson, D. L., (eds.), vol. 13, Corrosion, 9th edn., American Society for Metals, Metals Park, OH, 1987, pp. 321–​337. 3. Moore, R. E., Selecting materials to meet environmental conditions, in Materials Engineering T. Selecting Materials for Process Equipment (K. J. McNaughton, ed.), McGraw-​Hill, New York, 1980, pp. 31–​33. 4. Hildebrand, E. L., Materials selection for petroleum refineries and petrochemical plants, Mater. Protect. Perform., July, 19–​22 (1972). 5. Elliott, P., Design details to minimize corrosion, in Metals Handbook, Vol. 13, Corrosion, 9th edn., American Society for Metals, Metals Park, OH, 1987, pp. 338–​343. 6. Guide to selecting engineered materials, metals, polymers ceramics, composites, Adv. Mater. Process., 137(6), (1990). 7. Designing and fabricating aluminum weldments, Weld. Design Fabr., February, 53–​62 (1977). 8. Daemen, R., Welding heavy gauge high tensile steels, Brown Boveri. Rev., 52, 512–​524 (1965). 9. Langer, B. F. and Harding, W. L., Material requirements for long life pressure vessels, Trans. ASME, J. Eng. Power, October, 403–​410 (1964). 10. Hattangadi, A. D. and Seth, B. B., Lamellar tearing in fillet weldments of pressure vessel fabrication, Weld. J., April, 89-​s–​96-​s (1983). 11. Frew, J. A., Lloyd’s approach to quality assurance of welded structures, Weld. Metal Fabr., July–​ August, 256–​260 (1974). 12. Clarke, J. S. and Northup, M. S., Design of vessels for petroleum refining and petrochemicals service, ASME J. Eng. Power, October, 411–​418 (1964).

Material Selection and Fabrication

325

13. Bravenec, E. V., Why steel fracture, Chem. Eng., July 10, 100–​108 (1972). 14. Swales, G. L. and Haynes, G. A., Nickel steels in LNG tankers, in Source Book on Materials Selection, Vol. 1, (R. B. Guina, compiler), ASM, Metals Park, OH, 1977, pp. 469–​474. 15. Climax Molybdenum Co. Ltd., Guide to the Choice and Fabrication of Pressure Vessel Steel (an affiliate of AMAX American Metal Climax, Inc.), Climax Molybdenum Co. Ltd., London, U.K. 16. Schillmoller, C. M. and Craig, B. D., Nickel steels in Arctic service, Mater. Perform., October, 46–​49 (1987). 17. Benzer, W. C., Trends in materials application: Steels, Chem. Eng., Deskbook Issue, October 12, 101–​ 110 (1970). 18. Nippon Steel Corporation, Low alloy steel plates for high temperature service, Nippon Steel Corporation, Cat. No. EXE 396, April 1976. 19. Schade, J. P. and Ross, R. W., Control corrosion with nickel base alloys, Adv. Mater. Process., 7, 37–​40 (1994). 20. Jana, S. R. and Honavar, D. S., A study in designing of weld metals for high austenitic stainless steels, in Stainless Steels –​Processing and Metallurgy, Proceedings of the International Conference on Stainless Steels (P. Krishna Rao, M. K. Asundi, N. B. Ballal, and H. S. Gadiyar, eds.), INCOSS-​89, Bombay, Omega Scientific Publishers, New Delhi, India, 1989, pp. 149–​158. 21. Swindeman, R. W. and Gold, M., Developments in ferrous alloy technology for high-​temperature service, ASME J. Pressure Vessel Technol., 113, 133–​140 (1991). 22. Kane, R. D. and Cayard, M. S., Improve corrosion control in refining processes, Hydrocarb. Process., November, 129–​142 (1995). 23. NACE, Corrosion Abstracts, National Association of Corrosion Engineers, Houston, TX. 24. NACE, Corrosion Data Survey –​Metals Section, 6th edn., 1985, and Corrosion Data Survey –​ Nonmetals Section, 1975; National Association of Corrosion Engineers, Houston, TX. 25. Warren, D., Hydrogen effects on steel, Mater. Perform., January, 38–​46 (1987). 26. Richardson, H. T., Application of welding requirements and standards for oil refinery equipment, Weld. Metal Fabr., October, 367–​374 (1968). 27. Kar, R. J. and Todd, J. A., Alloy modification of thick section 2.25Cr-​1Mo steel, in Application of 2.25 Cr-​1 Mo Steel for Thick Wall Pressure Vessels, ASTM STP 755 (G. S. Sangdahl and M. Semchyshen, eds.), American Society for Testing and Materials, Philadelphia, PA, 1982, pp. 228–​252. 28. Wilten, H. M., Microscope speeds diagnosis of refinery corrosion, Hydrocarb. Process.Petrol.Refiner., 40, 151–​280 (1961). 29. Hagel, W. C. and Miska, K. H., How to select alloy steels for pressure vessels –​I, Chem. Eng., July 28, 89–​91 (1980). 30. API, Steels for hydrogen service at elevated temperatures and pressures in petroleum refineries and petrochemical plants, API Publication 941, Washington, DC, 1997. 31. Kirchner, W. L., Innovations in vessel fabrication, Mater. Protect. Perform., December, 20–​22 (1972). 32. Hoffman, H. C., Pressure vessel primer, Mach. Design, October 9, 168–​171 (1980). 33. Sattler, F. J., Forrer, G. R., and Parker, W. O., Jr., Inservice inspection of nuclear plants, Mater. Eval., November, 18A–​22A, 27A–​29A (1972). 34. Landrum, R. J., Designing equipment for corrosion resistance, Chem. Eng., Deskbook Issue, October 12, 75–​82 (1970). 35. NACE, Equipment service life factors, Mater. Perform., September, 79 (1988); R. J. Landrum, Fundamentals of Designing for Corrosion Control, National Association of Corrosion Engineers, Houston, TX. 36. ASTM, Annual Book of ASTM Standards for Metal Products, Test Methods and Analytical Procedures 36a. ASTM Volume 01.01: Steel –​Piping, Tubing, Fittings, Volume 01.02: Ferrous Castings; Ferroalloys, Volume 01.03: Steel –​Plate, Sheet, Strip, Wire; Stainless Steel Bar, Volume 01.04: Steel –​Structural, Reinforcing, Pressure Vessel, Railway, Volume 01.05: Steel –​Bars, Forgings, Bearing, Chain, Tool, Volume 01.06: Coated Steel Products, Volume 01.09: Fasteners; Rolling Element Bearings, ASTM, Philadelphia, PA, 2021. 36b. ASTM Volume 02.01: Copper And Copper Alloys, Volume 02.02: Aluminum And Magnesium Alloys, Volume 02.03: Electrical Conductors, Volume 02.04: Nonferrous Metals –​Nickel, Titanium, Lead, Tin, Zinc, Zirconium, Precious, Reactive, Refractory Metals And Alloys: Materials For Thermostats,

326

37.

38. 39. 40. 41. 42. 43. 44. 45. 46.

47. 48. 49. 50.

51.

52. 53. 54. 55. 56.

57. 58. 59. 60. 61. 62.

Material Selection and Fabrication Electrical Heating And Resistance Contacts, And Connectors, Volume 02.05: Metallic And Inorganic Coatings; Metal Powders And Metal Powder Products, ASTM, Philadelphia, PA, 2021. Hudgell, R. J. and Tickle, H., Current practices for ultrasonic and radiographic examination of tubes, tubeplates and tubeplate welds of tube bundles in heat exchangers, in Developments in Pressure Vessel Technology –​2, Inspection and Testing, Applied Science Publishers, London, U.K., 1979, pp. 77–​100. Orlando, J. S. and Ballantine, W., Materials for corrosion resistant fasteners, in Metals Handbook, desk edition, American Society for Metals, Metals Park, OH, 1985, pp. 15.29–​15.30. Danis, J. I., Material selection, fabrication and inspection of refinery heat exchangers, Weld. J., June, 25–​30 (1986). Schlunder, E. U. (Editor-​in-​chief), Mechanical design codes: Expansion joints, in Heat Exchanger Design Handbook, Hemisphere, Washington, DC, 1983, Section 4.16. Yokell, S., A Working Guide to Shell and Tube Heat Exchangers, McGraw-​Hill, New York, 1990. Weymueller, C. R., ASTM specs guide users of plate steels I –​ Structural steels, Weld. Design Fabr., February, 42–​48 (1992). Weymueller, C. R., ASTM specs guide users of plate steels II –​ Pressure vessel steels, Weld. Design Fabr., March, 47–​52 (1992). Smallwood, R. E., Heat exchanger tubing reliability, Mater. Perform., February, 27–​32 (1977). BSI, Specification for steel tubes for heat exchanger, BS 3606:1978, British Standards Institution, London, U.K., pp. 1–​14. Foxall, D. H. and Gilbert, P. T., Selecting tubes for CPI heat exchangers –​III, in Materials Engineering I: Selecting Materials for Process Equipment (K. J. McNaughton, ed.), McGraw-​Hill, New York, 1980, pp. 172–​174. Engle, J. P., Chemical cleaning of feedwater heaters, Mater. Perform., July, 30–​33 (1974). Brosilow, R., Weldability—​What it means to fabricators, Weld. Design Fabr., June, 74–​80 (1978). Nippes, E. F. (ed.), Principles of joining metallurgy, in Metals Handbook, Vol. 6, Welding, Brazing, and Soldering, 9th edn., American Society for Metals, Metals Park, OH, 1983, pp. 21–​49. Nippes, E. F. (ed.), Arc welding of carbon steels: Shielded metal arc welding, in Metals Handbook, Vol. 6, Welding, Brazing, and Soldering, 9th edn., American Society for Metals, Metals Park, OH, 1983, pp. 75–​95. Nippes, E. F. (ed.), Arc welding of carbon steels: Submerged arc welding, in Metals Handbook, Vol. 6, Welding, Brazing, and Soldering, 9th edn., American Society for Metals, Metals Park, OH, 1983, pp. 114–​151. Nippes, E. F. (ed.), Gas metal arc welding (MIG welding), in Metals Handbook, Vol. 6, Welding, Brazing, and Soldering, 9th edn., American Society for Metals, Metals Park, OH, 1983, pp. 152–​22. Nippes, E. F. (ed.), Plasma arc welding, in Metals Handbook, Vol. 6, Welding, Brazing, and Soldering, 9th edn., American Society for Metals, Metals Park, OH, 1983, pp. 214–​224. Nippes, E. F. (ed.), Electroslag welding, in Metals Handbook, Vol. 6, Welding, Brazing, and Soldering, 9th edn., American Society for Metals, Metals Park, OH, 1983, pp. 225–​237. Nippes, E. F. (ed.), Electrogas welding, in Metals Handbook, Vol. 6, Welding, Brazing, and Soldering, 9th edn., American Society for Metals, Metals Park, OH, 1983, pp. 238–​244. Nippes, E. F. (ed.), Arc welding of low alloy steels and other ferrous metals, in Metals Handbook, Vol. 6, Welding, Brazing, and Soldering, 9th edn., American Society for Metals, Metals Park, OH, 1983, pp. 245–​370. Nippes, E. F. (ed.), Weld discontinuities, in Metals Handbook, Vol. 6, Welding, Brazing, and Soldering, 9th edn., American Society for Metals, Metals Park, OH, 1983, pp. 829–​855. Brosilow, R., How can fabricators control weld properties? Weld. Design Fabr., March, 46–​50 (1985). Bailey, N., The establishment of safe welding procedures for four low alloy steels, Weld. J., 51, 169s–​ 177s (1972), Welding Research Supplement. Yurioka, N., Suzuki, H., Ohshita, S., and Saito, S., Determination of necessary preheating temperature in steel welding, Weld. J., June, 147-​s–​153-​s (1983). Gross, J. H., Pressure vessel steels: Promise and problems, in Source Book on Materials Selection, Vol. 1 (R. B. Guina, compiler), American Society for Metals, Metals Park, OH, 1977, pp. 271–​276. Farrar, J. C. M. and Dolby, R. E., Lamellar Tearing in Welded Steel Fabrication, Welding Institute, Abington Hall, Abington, Cambridge, U.K., 1972.

Material Selection and Fabrication

327

63. Wormington, H., Lamellar tearing in silicon-​ killed boiler plate, Weld. Metal Fabr., September, 370–​373 (1967). 64. Gross-​Wordemann, J. and Dittrich, S., Prevention of temper embrittlement in 2.25 Cr-​1Mo weld metal by metallurgical actions, Weld. J., May, 123-​s–​128-​s (1983). 65. Complementary information test for lamellar tearing, Weld. World, 19, 47–​53 (1981). 66. Lippold, J. C. and Savage, W. F., Solidification of austenitic stainless steel weldments: Part III –​The effect of solidification behavior on hot cracking susceptibility, Weld. J., December, 388-​s–​396-​s (1982). 67. Goodwin, G. M., Hot cracking, measurement, mechanisms and modelling, Weld. J., February, 26–​31 (1990). 68. Lin, W., Lippolod, J. C., and Baseslack, W. A., III, An evaluation of heat affected zone liquation cracking susceptibility, Part I: Development of a method for quantification, Weld. J., April, 135-​s–​153-​ s (1993). 69. Dhooge, A. and Vinckier, A., Reheat cracking –​ Review of recent studies (1984–​1990), Weld. World, 30, 44–​71 (1992). 70. Nakamua, H., Naiki, T., and Okahayaski, H., Fracture in the process of stress relaxation under constant strain, in Proceedings of the First International Conference on Fracture, Vol. 2., Sendai, Japan, September 12–​17, pp. 863–​878, and Stress Relief Cracking in HAZ, IIW-​IX-​648–​69 and IIW-​X-​531–​69. 71. Onodera, S., Improved quality of heavy steels and their welds as related to the integrity of reactor pressure components, Nucl. Ener.Design, 85(3), 305–​313, (1985). 72. Lippold, J. C., Baeslack, W. A. III., and Varol, I., Heat affected zone liquation cracking in austenitic and duplex stainless steels, Weld. J., January, l-​s–​5-​s (1992). 73. Lin, S. and Indacochea, J. E., Weldability of steels, in Metals Handbook, Vol. 1, Properties and Selection; Iron, Steels, and High Performance Alloys, 10th edn., American Society for Metals, Metals Park, OH, 1990, pp. 603–​62. 74. Connor, L. P. (ed.), Welding Hand Book, Vol. 1, Welding Technology, 8th edn., 1987, pp. 102–​121. 75. Datasheet No. 111b, Weldability testing –​Weld cracking tests, Weld. J., February, 73 (1990). 76. Kautz, H. R. and Herbert, E. D., Modern welding techniques and component performance, Weld. World, 30, 337–​341 (1992). 77. U. S. Steel Group, U. S. Steel Handbook of Plate Products, U. S. Steel Group, Pittsburgh, PA. 78. Lohmeier, A. and Reynolds, S. D., Jr., Carbon steel tube-​to-​tubesheet joints for high pressure feedwater heaters, Westinghouse Eng., September, 138–​143 (1966). 79. Jarman, R. A. and Shreir, L. L., Corrosion of jointed structures –​Part II, Weld. Metal Fabr., November/​ December, 445–​449 (1987). 80. Kirby, G. N., Corrosion performance of carbon steel, in Materials Engineering I: Selecting Materials for Process, Chemical Engineering, March 12, 1979, pp. 72–​84. 81. Uhlig, H. H., Corrosion and Corrosion Control, John Wiley & Son, Inc., New York, 1963. 82. Campbell, H. C., The ABCs of welding metallurgy for plain carbon steels, Weld. J., August, 55–​57 (1994). 83. Nippes, E. F. (ed.), Metals Handbook, Vol. 6, Welding, Brazing, and Soldering, 9th edn., American Society for Metals, Metals Park, OH, 1983, pp. 238–​244. 84. Bargett, W. E. and Reeve, L., High strength weldable steel, Iron Steel, October/​November, pp. 277–​ 294, (1954). 85. Irvine, K. J., The development of high strength structural steels, in Proceedings of the ISI/​BISR, Joint Conference, ISI Publication 104, ISI, London, U.K., 1967. 86. Kearns, W. H. (ed.), Carbon and low-​alloy steels, in Welding Handbook, Vol. 4, Metals and Their Weldability, 7th edn., American Welding Society, Miami, FL, 1982, pp. 2–​74. 87. Gutzeit, T., Corrosion in petroleum refining and petrochemical operations, in Metals Handbook, Vol. 13, Corrosion, 9th edn., American Society for Metals, Metals Park, OH, 1987, pp. 1262–​1287. 88. Wordemann, G. J. and Dittrich, S., Prevention of temper embrittlement in 2.25Cr-​1Mo weld metal by metallurgical actions, Weld. J., May, 123-​s–​128-​s (1983). 89. Craig, W. R., Refineries test their metals, Weld. Design Fabr., April, 81–​85 (1977). 90. Wilson, A. D., Tougher steels improve pressure vessel performance, Adv. Mater. Process., 4, 39–​42 (1993).

328

Material Selection and Fabrication

91. Murakami, Y., Nomura, T., and Watnabe, J., Heavy section 2¼ Cr-​1Mo steel for hydrogenation reactors, in Application of 2.25 Cr–​Mo Steel for Thick Wall Pressure Vessels, ASTM STP 755 (G. S. Sangdahl and M. Semchyshen, eds.), American Society for Testing and Materials, Philadelphia, PA, 1982, pp. 383–​417. 92. Baumert, K. L. and Polk, C. J., Temper embrittlement studies on 2.25Cr-​Mo steel, Mater. Perform., September, 39–​45 (1976). 93. Swift, R. A., Effects of composition and heat treatment on the mechanical properties of 300 mm gage 2.25 Cr-​1Mo steel plate, in Application of 2.25 Cr–​1Mo Steel for Thick Wall Pressure Vessels, ASTM STP 755 (G. S. Sangdahl and M. Semchyshen, eds.), American Society for Testing and Materials, Philadelphia, PA, 1982, pp. 166–​188. 94. Imgram, A. G. and Swift, R. A., Pressure vessel, piping and welding needs for coal conversion systems, J. Mater. Ener.Syst., December, 212–​221 (1985). 95. Lukens Steel Company, Lukens Plate Steel Specification Guide, Lukens Steel Company, Coatesville, PA, 1993. 96. Yamamoto, S. and Natsume, S., Improvement of toughness of Cr-​Mo steel weld metal at low temperature, Weld. Metal Fabr., April, 110–​115 (1986). 97. Climax Molybdenum Company, Modified 9 Cr-​1 Mo steel for power plants, Molybdenum Mosaic, 6, 17, (1982), Climax Molybdenum Company of Michigan. 98. Ritche, R. O., Parker, E. R., Spencer, P. N., and Todd, J. A., A new series of advanced 3Cr-​Mo-​Ni steels for thick section pressure vessels in high temperature and pressure hydrogen service, J. Mater. Ener.Syst., December, 151–​162 (1964). 99. Brown, R. S., How to select the right stainless steel, Adv. Mater. Process., 4, 20–​24 (1994). 100. Debold, T. A., Select the right stainless steel, Chem. Eng. Prog., November, 38–​43 (1991). 101. Redmond, J. D. and Miska, K. H., The basics of stainless steels, Chem. Eng., October, 89(10), 79–​93 (1982). 102. Linstroth, R. L., Check for atmospheric corrosion when using stainless steels, Chem. Eng. Prog., July, 49–​51 (1991). 103. Metallurgy of fusion welding, Pt 3: Stainless steels, Weld. Metal Fabr., October, 376–​378 (1994). 104. Llewellyn, D. T., Steels, Metallurgy and Applications, Butterworth-​Heineman, Oxford, U.K., 1992. 105. AL-​6XNR Alloy, Allegheny Ludlum Steel Corporation, Pittsburgh, PA, 2002, pp. 1–​56. 106. Kearns, J. R. and Deverell, H. R., The use of nitrogen to improve the corrosion resistance of FeCrNiMo alloys for the chemical process industries, Mater. Perform., June, 18–​28 (1987). 107. Davison, R. M., Corrosion of stainless steel, in Metals Handbook, Vol. 13, Corrosion, 9th edn., American Society for Metals, Metals Park, OH, 1987, pp. 546–​575. 108. Davison, R. M., Rahoi, D. W., and Gemmel, G., Stainless steels for heat exchanger service, in Industrial Heat Exchangers Conference Proceedings (A. J. Hayes, W. W. Liang, S. L. Richlen, and E. S. Tabb, eds.), American Society for Metals, Metals Park, OH, 1985, pp. 371–​380. 109. Forchhammer, P., Results of damage research on corrosion failures in heat exchangers, Heat Trans. Eng., 5, 19–​32 (1984). 110. Paul Mueller Company, Product Literature, Mueller Accu-​Therm Plate Heat Exchangers, Paul Mueller Company, Springfield, MO, 2006. 111. Blom, K. J. and Degerbeck, J., Low manganese austenitic stainless steels has improved resistance to pitting and crevice corrosion, Mater. Perform., July, 52–​54 (1983). 112. Slaughter, G. M., Franco-​Ferreira, E. A., and Patriarca, P., Welding and brazing of high temperature radiators and heat exchangers, Weld. J., January, 15–​22 (1968). 113. Redmond, D. J. and Miska, K. H., High performance stainless steels for high chloride service –​Part I, Chem. Eng., July 25, 93–​96 (1983). 114. Schillmoller, C. M. and Althoff, J. H., How to avoid failures of stainless steels, Chem. Eng., May, 119–​122 (1984). 115. Pollock, W. I., What’s hot in metal alloys, Chem. Eng., October, 78–​88 (1992). 116. Kane, R. D., Super stainless steels resist hostile environments, Adv. Mater. Process., 7, 16–​20 (1993). 117. Borenstein, S. W., Microbiologically influenced corrosion failures of austenitic stainless steel welds, Mater. Perform., August, 62–​66 (1988). 118. Odegard, L., Welding of stainless steels, corrosion in welds, effect of oxides, slag and weld defects on the pitting resistance, Weld. World, 36, 153–​160 (1995).

Material Selection and Fabrication

329

119. Brosilow, R., Can you weld to avoid corrosion, Weld. Design Fabr., September, 67–​74 (1982). 120. Standard Recommended Practice for Examination and Evaluation of Pitting Corrosion, G 46, Annual Book of ASTM Standards, American Society Testing and Materials, Philadelphia, PA, G46-​94, 2005. 121. Garner, A., Materials selection, for bleached pulp washers, Pulp Paper Canada, 82, 1 (1981). 122. Bandy, R., Lu, Y. C., Newman, R. C., and Clayton, C. R., Roles of nitrogen in improving the resistance to localised corrosion in austenitic stainless steels, in Equilibrium Diagrams and Localised Corrosion, Electrochemical Society, New Orleans, LA, 1984, p. 471. 123. Dexter, C. S., Localized corrosion, in Metals Handbook, Vol. 13, Corrosion, 9th edn., American Society for Metals, Metals Park, OH, 1987, pp. 104–​122. 124. Uhlig, H. H., Environmental effects on properties of materials, Mater. Protect. Perform., July, 23–​27 (1972). 125. Gooch, T. G., Stress corrosion cracking of welded austenitic stainless steel, Weld. World, 22, 64–​76 (1984). 126. Copson, H. R., Effects of composition on stress corrosion cracking of some alloys containing nickel, in Physical Metallurgy of Stress Corrosion Fracture, Interscience, New York, 1959, p. 247. 127. Pitcher, J. H., Trends in CPI materials –​ Steels, Chem. Eng., Deskbook Issue, December 4, 39–​42 (1972). 128. Espy, R. H., Coping with stress corrosion in stainless steel welds, Weld. Design Fabr., October, 82–​ 84 (1977). 129. Nishida, H., Nakamura, K., and Takahashi, H., Intergranular stress corrosion cracking of sensitized 321 SS tube exposed to polythionic acid, Mater. Perform., April, 38–​41 (1984). 130. NACE, Protection of austenitic stainless steels in refineries against stress corrosion cracking by use of neutralizing solutions during shutdown, NACE RP-​01-​70, National Association of Corrosion Engineers, Houston, TX. 131. Fontana, M. G. and Greene, N. D., Corrosion Engineering, 2nd edn., McGraw-​Hill, New York, 1978. 132. Bangs, S., Gas turbine regenerator uses TIG, laser beam, Weld. Design Fabr., March, 81–​88 (1981). 133. Jones, R. L., Prevention of stress corrosion cracking in boiling water reactors, Mater. Perform., February, 70–​75 (1991). 134. Gooch, T. G. and Willingham, D. C., Weld decay in austenitic stainless steels, Welding Institute, Abington Hall, Abington, Cambridge, U.K., 1975. 135. Steigerwald, R., Metallurgically influenced corrosion, in Metals Handbook, Vol. 13, Corrosion, 9th edn., American Society for Metals, Metals Park, OH, 1987, pp. 123–​135. 136. Fenn, R., Welding metallurgy of stainless steel, Weld. Metal Fabr., October, 382–​386 (1987). 137. Banks, B., Process selection and filler materials for welding stainless steels, Weld. Metal Fabr., March, 107–​110 (1969). 138. Allen, J. S., Fabrication of heat exchangers and specialised vessels, Weld. Metal Fabr., July, 240–​245 (1968). 139. Weymueller, C. R., Welding the austenitic steels, Weld. Design Fabr., June, 82–​87 (1978). 140. Beigay, J. M. and Lovejoy, P. T., How to arc weld the stainless steels, Weld. Design Fabr., August, 57–​63 (1981). 141.1 Damian Kotecki and Frank Armao, Stainless Steels Properties –​How to Weld them Where to use them, Publication C6.4000, Issue Date 10/​14, Lincoln Global Inc., THE LINCOLN ELECTRIC COMPANY, Cleveland, OH, pp. 1–​37. 141.2 www.linc​olne​lect​ric.com/​ass​ets/​US/​EN/​lit​erat​ure/​c64​000.pdf 142. www.hob​artb​roth​ers.com/​resour​ces/​techni​cal-​artic​les/​fil​ler-​met​als-​for-​weld​ing-​aus​teni​tic-​stainl​ ess-​steel-​tube-​and-​pipe/​ 143. Castner, H. R., What you should know about austenitic stainless steels, Weld. J., April, 53–​59 (1993). 144. Smith, D. V., A practical approach to ferrite in stainless steel weld metal, Weld. J., May, 57–​60 (1989). 145. Lundin, C. D., Lee, C. H., Menon, R., and Osorio, V., Weldability evaluation of modern 316 and 347 austenitic stainless steels: Part I –​Preliminary results, Weld. J., 35-​s–​46-​s (1988). 146. Kotecki, D. J., Standards and industrial methods for ferrite measurement, Weld. World, 36, 161–​169 (1995). 147a. Schaeffler, A. L., Constitution diagram for stainless steel weld metal, Metal Prog., 56(11), 68 (1949). 147b. DeLong, W. T., Ferrite in austenitic stainless steel weld metal, Weld. J., 53(7), 273-​s–​286-​s (1974).

330

Material Selection and Fabrication

147c. Kotecki, D. J. and Siewet, T. A., WRC–​1992 constitution diagram for stainless steel weld metals: A modification of the WRC–​1988 diagram, Weld. J., May, 171-​s–​178-​s (1992). 148. The Welding Institute, Datasheet no. 82a, Effects of residual, impurity, and microalloying elements on weldability and weld properties, Weld. J., December, 53–​54 (1986). Excerpted from Effects of Residual, Impurity, and Microalloying Elements on Weldability and Weld Properties, The Welding Institute, 1985; adapted by Hallock C. Campbell. 149. Polland, B., The effects of minor elements on the welding characteristics of stainless steel, Weld. J., 67(9), 202-​s–​213-​s (1988). 150. Vackar, B. K., Satisfying the codes, clean dry materials, good techniques & right chemistry, Weld. Design Fabr., June, 86–​87 (1977). 151. Lula, R. A., Introduction, in Source Book on the Ferritic Stainless Steels (R. A. Lula, compiler), American Society for Metals, Metals Park, OH, 1982, pp. ix–​xii. 152. Nichol, T. J., Franson, I. A., and Moller, G. E., Applications of new high chromium ferritic stainless steels in the chemical process industries, in Corrosion/​81, Paper No. 117, NACE, 1981, pp. 357–​401. 153. Wock, W. C., Selecting steel tubing for high temperature service, in Materials Engineering I: Selecting Materials for Process Equipment (K. J. McNaughton, ed.), McGraw-​Hill, New York, 1980, pp. 252–​255. 154. Davison, R. M. and Steigerwald, R. F., The new ferritic stainless steels, Metal Prog., 116(6), 40–​46 (1979). 155. Kaltenhauser, R. H., Improving the engineering properties of ferritic stainless steels, Amer. Soc. Metals, Metals Eng. Q., 11(2), 41–​47 (1971). 156. Demo, J. J., Structure, Constitution, and General Characteristics of Wrought Ferritic Stainless Steels, ASTM STP 619, McGraw-​Hill, New York, 1977. 157. Paul Bond, A., Mechanisms of intergranular corrosion in ferritic stainless steels, Trans. Metall. Soc. AIME, 245(8), 2127–​2134 (1969). 158. Nichol, T. J. and Davis, J. A., Intergranular Corrosion Testing and Sensitization of Two High Chromium Ferritic Stainless Steels, ASTM STP 656, American Society for Testing and Materials, Philadelphia, PA, 1978, pp. 179–​196. 159. Walker, R., Duplex and high alloy stainless steels –​Corrosion resistance and weldability, Mater. Sci. Technol., 4, 78–​84 (1988). 160. Practical guide to using duplex stainless steels, A Guide To The Use Of Nickel-​Containing Alloys No. 10044, Nickel Institute, 2020, pp. 1–​21. 161. www.twi-​glo​bal.com/​techni​cal-​knowle​dge/​job-​knowle​dge/​dup​lex-​stainl​ess-​steel-​part-​1-​105 162. Stephenson, N., Welding status of duplex stainless steels for offshore application –​Part I & II, Weld. Metal Fabr., June, 159–​164, and July, 235–​238 (1987). 163. Faucher, D. and Gilbert, D., in Proceedings of the International Conference on Duplex Stainless Steels ‘86, Netherlands Institute voor lastechniek, The Hague, the Netherlands, October 1986. 164. Tuthill, A. H., Usage and performance of nickel-​containing stainless steels in both saline and natural waters and brines, Mater. Perform., July, 47–​50 (1988). 165. Mats, L., Development of superaustenitic stainless steels, Weld. World, 36, 55–​63 (1995). 166. Rabensteiner, G., The welding of fully austenitic stainless steels with high molybdenum contents, Weld. World, 27, 2–​13 (1989). 167. Davison, R. M. and Redmond, J. D., Practical guide to using 6 Mo austenitic stainless steels, Mater. Perform., December, 39–​43 (1988). 168a. Cary, H. B., Modern welding technology, in Welding the Nonferrous Metals, Prentice-​Hall, Engle Wood Cliffs, NJ, 1979, pp. 431–​460. 168b. Kaiser Aluminum and Chemical Sales, Inc., Welding Kaiser Aluminum, Kaiser Aluminum and Chemical Sales, Inc., Kaiser Center, Oakland, CA, 1967. 168c. Aluminum Company of America, Welding Alcoa Aluminum, Aluminum Company of America, Pittsburgh, PA, 1967. 168d. Reynolds Metals Company, Welding, Soldering and Brazing, Reynolds Metals Company, Richmond, VA, 1958. 169. Swaney, W., Trace, D. E., and Winterbottom, W. L., Brazing aluminum automotive heat exchangers in vacuum: Process and materials, Weld. J., May, 49–​57 (1986).

Material Selection and Fabrication

331

170. Taylor, M. A. (ed.), Plate-​Fin Heat Exchangers, Guide to Their Specification and Use, HTFS (Harwell Laboratory), Oxon, U.K., 1980. 171. Aluminum Association, Aluminum Brazing Handbook, 4th edn., Aluminum Association, Washington, DC, 1990. 172. Irving, B., Welding the four most popular aluminum alloys, Weld. J., February, 51–​55 (1994). 173. ASM, Corrosion behavior, in Aluminum: Properties and Physical Metallurgy (J. E. Hatch, ed.), American Society for Metals, Metals Park, OH, 1984, pp. 242–​319. 174. Pourbaix, M., Atlas of Electrochemical Equilibria in Aqueous Solution, National Association of Corrosion Engineers, Houston, TX, 1974. 175. ASM, Corrosion of aluminum and aluminum alloys, in Metals Handbook, Vol. 13, Corrosion, 9th edn., American Society for Metals, Metals Park, OH, 1987, pp. 583–​609. 176. Lancaster, J. F., Fabrication of aluminum pressure vessels, Weld. Metal Fabr., December, 455–​459 (1958). 177. Logan, H. L., The Stress Corrosion Cracking of Metals, John Wiley & Sons, New York, 1966. 178. Henthorne, M., Metals selection for corrosion control, Chem. Eng., March 6, 113–​118 (1972). 179. Darden, J. W., Triebel, C. A., Maes, J. P., and VanNeste, W., Monoacid/​diacid combination as corrosion inhibitors in antifreeze formulations, SAE 900804, pp. 135–​139, (1990). 180. Schwerdtfeger, W. J., Effects of cathodic protection on the corrosion of an aluminum alloy, J. Res. Nat. Bureau Stand., 68C, 283 (1964). 181. McGill, W. A. and Weinbaum, M. J., Aluminum vapour diffused steels resist refinery corrosion, Mater. Protect. Perform., July, 28–​32 (1972). 182. Young, J. G., Quality control in aluminum welding, Weld. Metal Fabr., January, 10–​14, 62–​67 (1964). 183. ASM, Corrosion of copper and copper alloys, in Metals Handbook, Vol. 13, Corrosion, 9th edn., American Society for Metals, Metals Park, OH, 1987, pp. 610–​640. 184. Gaffoglio, C. J., Copper takes a lot of heat, Weld. Design Fabr., October, 90–​99 (1979). 185. Copper Development Association, Inc., Standards Handbook, Wrought Products, Alloy Data/​2, Copper Development Association, Inc., Greenwich, CT, 1985. 186. Peters, D. T. and Kundig, K. J. A., Selecting copper and copper alloys –​Part I: Wrought products & Part II: Cast products, Adv. Mater. Process., 2, 26–​32, and 6, 20–​26 (1994). 187. Bridgeport Brass Company, Condenser Tube Handbook, Bridgeport Brass Company, U.K. 188. ASM, Copper in Metals Handbook, Vol. 2, Nonferrous Metals, 9th edn., American Society for Metals, Metals Park, OH, 1979. 189. Baumann, G., Materials for tubes used in the manufacture of high pressure feed heaters, Brown Boveri Rev., 54, 700–​702 (1967). 190. Cieslewicz, J. M. and Schweitzer, P. A., Copper and copper and copper alloys, in Corrosion and Corrosion Protection Handbook, 2nd edn. (P. A. Schweitzer, ed.), Marcel Dekker, New York, 1983, pp. 125–​152. 191. Tuthill, A. H., Guidelines for the use of copper alloys in seawater, Mater. Perform., September, 12–​22 (1987). 192. Thompson, D. H., How to prevent stress-​corrosion cracking of copper alloys, Chem. Eng., February, 130–​134 (1961). 193. Syrett, B. C. and Coit, R. L., Causes and prevention of power plant condenser tube failures, Mater. Perform., February, 44–​49 (1983). 194. Syrret, B. C. et al., Corrosion in fossil fuel power plants, in Metals Handbook, Vol. 13, Corrosion, 9th edn., American Society for Metals, Metals Park, OH, 1987, pp. 985–​1010. 195. Spencer, E., The effect of material choice on heat transfer in steam surface condensers, Mater. Perform., March, 29–​32 (1979). 196. Woodward, A. R., Howard, D. L., and Andrews, E. F. C., Condensers, pumps and cooling water plant, ­chapter 4, in Modern Power Station Practice, Vol. C, Turbines, Generators, and Associated Plant (D. J. Littler, E. J. Davies, H. E. Johnson, F. Kirkby, P. B. Myerscouh, and W. Wright, eds.), Pergamon Press, New York, 1991. 197. Hopkinson, B. E., Copper-​nickel alloys for feedwater heater service, ASME Publication No. 62-​ WA.294, ASME, New York. 198. Kearns, (ed.), Copper alloys, in Welding Handbook, Vol. 4, Metals and Their Weldability, 7th edn., American Welding Society, Miami, FL, 1982, pp. 270–​316.

332

Material Selection and Fabrication

199. Dawson, R. J. C., Inert gas shielded arc welding of copper and copper alloy –​Dissimilar metal joint, Weld. Metal Fabr., February, 60–​66 (1968). 200. Dawson, R. J. C., Inert gas shielded arc welding of copper and copper alloys, Part 2, Weld. Metal Fabr., February, 59–​63 (1967). 201. Dawson, R. J. C., Fusion Welding and Brazing of Copper and Copper Alloys, Newness-​Butterworths, London, U.K., 1973. 202. Callcut, V. and Brown, L., Joining copper and copper alloys, Weld. Metal Fabr., June, 232–​236 (1996). 203. Modern methods for vessel fabrication –​ Developments at the Works of Daniel Adamson, Weld. Metal Fabr., January, 4–​15 (1963). 204. www2.ador​fon.com/​produ​cts-​and-​servi​ces/​weld​ing-​ele​ctro​des-​wires/​fil​ler-​rods-​wire-​tig-​mig-​saw/​ cop​per-​cop​per-​all​oys/​ 205. Campbell, E. J., Alloys for structural applications at subzero temperatures, in Metals Handbook, Vol. 3, Properties and Selection: Stainless Steels, Tool Materials and Special Purpose Metals, 9th edn., American Society for Metals, Metals Park, OH, 1980. 206. Inco Alloys International, Inc., Product Handbook, Publication No. IAI-​38, 4th edn., Inco Alloys International, Inc., Huntington, W. VA, 1988. 207. Inco Alloys International, Inc., Quick Reference Guide to Nickel and High Nickel Alloys, Inco Alloys International, Inc., Huntington, W. VA. 208. Inco Alloys International, Inc., High Performance Alloys for Thermal Processing, Publication No. IAI-​48-​1, 4th edn., Inco Alloys International, Inc., Huntington, W. VA, 1988. 209. Kiser, S. D., Sound welds in nickel alloys, Weld. Design Fabr., October, 86–​88 (1988). 210. Conaway, H. R., Welding the nickel alloys, Part 1: Cleaning and joint design, Weld. Design Fabr., February, 73–​75 (1977). 211. Conaway, H. R., Welding the nickel alloys, Part 2: SMAW and GMAW, Part 3: GTAW, SAW and OAW, Part 4: Plasma welding, nickel overlays, Weld. Design Fabr., March, 75–​79; April, 106–​109; May, 70–​73 (1977). 212. ASM, Material Properties Handbook: Titanium Alloys, ASM International, Materials Park, OH. 213. Schutz, R. W. and Thomas, E. D., Corrosion of titanium and titanium alloys, in Metals Handbook, Vol. 13, Corrosion, 9th edn., American Society for Metals, Metals Park, OH, 1987, pp. 669–​706. 214. Data sheet, summary table of titanium alloys, Adv. Mater. Process., 6, 123–​126 (1994). 215. Inskeep, J., Mcmaster, J. A., and Trumbell, C. A., Move over for titanium, Weld. Design Fabr., April, 88–​94 (1977). 216. Seagle, S. R. and Bruce, B. P., Titanium: Its properties and uses, Chem. Eng., March 8, 111–​113 (1982). 217. Titanium Fabricators Ltd. –​A Profile, Welding and Metal Fabrication, October 1988, pp. 346–​349. 218. Schutz, R. W., A case for titanium’s resistance to microbiologically influenced corrosion, Mater. Perform., January, 58–​61 (1991). 219. TIMET, TIMET CodeWedl® Titanium Tubing, TIMET, Denver, CO. 220. Jacobs, R. L. and McMaster, J. A., Titanium tubing: Economical solution to heat exchanger corrosion, Mater. Protect. Perform., July, 33–​38 (1972). 221. Gleekman, L. W., Trends in CPI materials: Nonferrous metals –​ II, Chem. Eng., Deskbook Issue, December 4, 47–​49 (1972). 222. McMaster, J. A., Selection of titanium for petroleum refinery components, Mater. Perform., April, 28–​33 (1979). 223. Cooley, J., How to weld titanium without a trailing shield, Weld. Design Fabr., March, 65–​66 (1982). 224. Hull, W. G., Fusion welding of titanium and its alloys, Weld. Metal Fabr., January, 24–​28 (1961). 225. Chevalier, R., General methods for cleaning titanium, Weld. Metal Fabr., April, 134 (1974). 226. RMI Titanium Company, Facts about Welding Titanium, RMI Titanium Company, Niles, OH. 227. Crement, D. J., Narrow groove welding of titanium using the hot-​wire gas tungsten arc process, Weld. J., April, 71–​76 (1993). 228. Baeslack, W. A. and Decker, D. W., Substructural characteristics in titanium alloy weldments, Metal Trans., 11A, 605–​612 (1980).

Material Selection and Fabrication

333

229. Ellis, M. B. D. and Gittos, M. F., Tungsten inert gas welding of titanium and its alloy, Weld. Metal Fabr., January, 9–​12 (1995). 230. Yau, T.-​L., Zirconium, in Corrosion and Corrosion Protection Handbook, 2nd edn. (P. A. Schweitzer, ed.), Marcel Dekker, New York, 1983, pp. 231–​282. 231. Lacy, C. E. and Keeler, J. H., Zirconium –​ Fabrication techniques and alloys development, Mech. Eng., October, 875–​878 (1955). 232. Cox, F. G., Zirconium, Weld. Metal Fabr., October, 358–​365 (1958). 233. Webster, R. T. and Yau, T.-​L., Zirconium in sulfuric acid applications, Mater. Perform., February, 21–​25 (1986). 234. Rollason, E. C., Welding of zirconium, Weld. Metal Fabr., July, 230–​234 (1956). 235. Richard Sutherlin, Welding zirconium and zirconium alloys Part I, The Fabricator, 2004. www. thefab​rica​tor.com/​tube​pipe​jour​nal/​arti​cle/​tube​pipe​fabr​icat​ion/​weld​ing-​zircon​ium-​and-​zircon​ium-​all​ oys-​part-​i 236. Burns, R. H. and Prabhat, K., Conquer corrosion in harsh environments with tantalum, Chem. Eng. Prog., March, 32–​35 (1996). 237. Schweitzer, P. A., Tantalum, in Corrosion and Corrosion Protection Handbook, 2nd edn. (P. A. Schweitzer, ed.), Marcel Dekker, New York, 1983, pp. 213–​230. 238. Keeler, J. H., Four refractory metals –​W, Ta, Cb and Mo, Mech. Eng., November, 41–​45 (1965). 239. Schussier, M., Tantalum –​Its properties and applications for the chemical industry, Refractory Hard Metals, June (1983). 240. Hampel, C. A., Corrosion resistance of titanium, zirconium and tantalum used for chemical equipment, Corrosion, 17, 9–​17 (1961). 241. Yan, T.-​L. and Bird, K. W., Know which reactive and refractory metals work for you, Chem. Eng. Prog., 88(2), 65–​69 (1992). 242. Gleekman, L. W., Trends in CPI materials: Nonferrous metals –​ I, Chem. Eng., Deskbook Issue, October 12, 111–​118 (1970). 243. Sayers, J. A., Trends in CPI materials: Brittle materials, Chem. Eng., Deskbook Issue, December 4, 51–​56 (1972). 244. Hills, D. E. G., Graphite heat exchangers –​I, Chem. Eng., December 23, 80–​83 (1974). 245. Hills, D. E. G., Graphite heat exchangers –​II, Chem. Eng., January 20, 116–​119 (1975). 246. Schley, J. R., Impervious graphite for process equipment, Chem. Eng., February 18, 144–​150 (1974). 247. Muoio, J. M., Glass as a material of construction for heat transfer equipment, in Industrial Heat Exchangers Conference Proceedings (A. J. Hayes, W. W. Liang, S. L. Richlen, and E. S. Tabb, eds.), American Society for Metals, Metals Park, OH, 1985, pp. 385–​390. 248. Perry, R. H. and Chilton, C. H., Chemical Engineering Handbook, 5th edn., McGraw-​ Hill, New York, 1973. 249. Heat exchangers, in QVF2002, QVF Process system GmbH, a member of De Dietrich Process Systems GmbH, Hattenbergstraße 36, Mainz, Germany, pp. 5.1–​5.25 (2007). (www.qvf.com/​en/​ Compan​y_​5/​index.shtml) 250. Lederman, S., Types and prevention of glass lined equipment failures, Mater. Perform., March, 34–​41 (1983). 251. Wenner, C. W., Jr., Corrosion-​free heating and cooling using Teflon heat exchangers, Mater. Perform., September, 55–​58 (1988). 252. Ametek Heat Exchangers of Teflon, M/​s. Ametek, Haveg Division, USA. 253. Lehman, R. L., Premier on engineering ceramics, Adv. Mater. Process., June, 31–​41 (1992). 254. McDonald, C. F., The role of the ceramic heat exchanger in energy and resource conservation, Trans. ASME, J. Eng. Power, 102, 303–​315 (1980). 255. Jack E. Helms, Pressure Vessels and Piping Systems–​Composite Materials for Pressure Vessels and Pipes, Encyclopedia of Life Support Systems (EOLSS), Department of Mechanical Engineering, Louisiana State University, Baton Rouge, Louisiana. 255.1 www.eolss.net/​sam​ple-​chapt​ers/​c05/​e6-​165-​03-​00.pdf 256. Schwartzberg, R. F., Selecting structural materials for cryogenic service, in Source Book on Materials Selection, Vol. 1 (R. B. Guina, compiler), ASM, Metals Park, OH, 1977, pp. 25–​30. 257. Marshall, E. G., Steels for low temperature service, in Source Book on Materials Selection, Vol. 1 (R. B. Guina, compiler), ASM, Metals Park, OH, 1977, pp. 216–​220.

334

Material Selection and Fabrication

258. Vanderbeck, R. W., Special carbon steels –​Tough ductile at subzero temperature, Chem. Eng., May, 103–​106 (1960). 259. Aluminum in hot pursuit of cryogenics, Weld. Design Fabr., September, 45–​49 (1974). 260. Johnson, E. W., Aluminum alloys, Chem. Eng., August 8, 133–​135 (1960). 261. Welding of stainless steels for cryogenic application, Weld. World, 30, 100–​107 (1992). 262. Fabrication of cryogenic plant, Weld. Metal Fabr., March, 84–​89 (1969). 263. Nippes, E. F. and Balaguer, J. P., A study of weld heat affected zone toughness of 9% nickel steel, Weld. J., September, 237-​s–​243-​s (1986). 264. Fieldhouse, A. B., New electrode for welding 9% nickel steel, Weld. Metal Fabr., April, 149–​150 (1964). 265. Johnson, R. J., Nickel steel alloys for liquids at 320°F, Chem. Eng., July, 115–​118 (1960). 266. Tharby, R. H., Welding consumables for 9% nickel steel, Weld. Metal Fabr., February, 51–​61 (1973). 267. Thorneycroft, D. R. and Heath, D. J., Further aspects of the welding of 9% nickel steel, Weld. Metal Fabr., February, 59–​70 (1963). 268. Pozzolini, P. F., The role of 9% nickel steel in the transport of LNG, Weld. Metal Fabr., February, 40–​46 (1973). 269. Coulson, K. J., Shop manufacturing experience with 9% nickel steel, Weld. Metal Fabr., February, 47–​50 (1973). 270. Machin, R., Welding aspects of 9% nickel steel, Weld. Metal Fabr., July, 266–​269 (1966). 271. Watanabe, M., Tanaka, J., and Watanabe, I., Ferritic filter for gas shielded metal arc welding 9% nickel steel, Weld. Metal Fabr., May, 167–​176 (1973). 272. Operation Cryogenics-​9% nickel steel vessels burst and impact tests, International Nickel Co., Inc., Chicago Bridge & Iron Co., and United Steels Corp, Publication AUDCO A-​61 (1961). Destructive tests of 9% nickel steel vessels at–​320°F, ASME-​62-​273, Engineering dividion, ASME annual meeting, New York, Nov. 25–​30 (1960). 273. Jordan, D. E., High strength filler material for welding 9% nickel steel, Weld. Metal Fabr., August, 335–​339 (1967). 274. McHenry, H. T., The properties of austenitic stainless steels at cryogenic temperatures, in Austenitic Steels at Low Temperature, Plenum Press, New York, 1983. 275. Ogawa, T. and Koseki, T., Weldability of newly developed austenitic alloys for cryogenic service: Part II. High nitrogen stainless steel weld metal, Weld.J., 67, 8-​s–​17-​s (1988). 276. Ogawa, T. and Koseki, T., Weldability of newly developed austenitic alloys for cryogenic service: Part I. Up-​to-​date overview of welding technology, Weld. J., 66, 332-​s–​341-​s (1987). 277. Lancaster, J. F., The Metallurgy of Welding, Brazing and Soldering, American Elsevier, New York, 1965. 278. Perinic, G., A small course on safety in cryogenics, 2003. http://​cem.ch/​gperi​nic/​kryok​urs/​en/​kryok​_​ 99.htm 279. Liane Smith, Engineering with CLAD STEEL, Nickel Institute Technical Series No 10 064, Nickel Institute, pp. 1–​23. 280. https://​indust​eel.arcelo​rmit​tal.com/​fich​ier/​clad-​pla​tes-​broch​ure/​ 281. Gleekman, L. W., Trends in CPI materials: Nonferrous metals –​ II, in Materials Engineering I: Selecting Materials for Process Equipment (K. J. McNaughton, ed.), McGraw-​Hill, New York, 1980, pp. 92–​94. 282. Firth, K. and Heath, D. J., Nickel alloys provide protective lining and overlays, Weld. Metal Fabr., April, 195–​203 (1980). 283. www.polyso​ude.com/​weld-​over​lay/​ 284. www.titano​vala​ser.com/​blog/​what-​is-​weld-​over​lay/​ 285. Bush, A. F. and Cohin, P., The successful application of the strip cladding process with austenitic strips, Weld. Metal Fabr., June, 234–​241 (1969). 286. The New Dimension In Strip Cladding, Lincoln Electric, pp. 1–​23. 287. Mallya, U. D. and Srinivas, H. S., Effect of magnetic steering of the arc on clad quality in submerged arc strip cladding, Weld. J., July, 289-​s–​293-​s (1993). 288. Devletian, J. H., Koch, A., and Buckley, E. N., Unique application introduces electroslag cladding to US industry, Weld. J., January, 57–​60 (1992). 289. www.ozm​etal​san.com/​ima​ges/​cata​log/​55608ESA​B_​St​rip_​Clad​ding​_​El_​Kita​bi_​-​_​EN.pdf 290. Marshall, A. B., Jordan, M. F., and Aston, J. L., Stainless steel strip cladding, Weld. Metal Fabr., August, 292–​293 (1973).

Material Selection and Fabrication

335

291. Oh, Y. K. and Devletian, J. H., Electroslag strip cladding of stainless steel with metal powder additions, Weld. J., January, 37–​44 (1992). 292. www.metal​spip​ing.com/​clad-​plate.html 293. Stainless Steel Cladding and Weld Overlays, ASM Specialty Handbook: Stainless Steels, 06398G J.R. Davis, Davis & Associates, 1994 ASM International®, pp. 107–​119. 294. https://​lkal​loy.com/​titan​ium-​all​oys/​titan​ium-​car​bon-​steel-​clad-​plate/​ 295. www.jind​alst​ainl​ess.com/​proc​essi​ndus​try/​pdfs/​clad-​plate-​broch​ure.pdf 296. Hix Hugh, B., Explosion bonded metals offer diversification in vessel design, Mater. Protect. Perform., December, 28–​31 (1972). 297. Banker, J. G., Try explosion clad steel for corrosion protection, Chem. Eng. Prog., July, 40–​44 (1996). 298. Hardwick, R., Methods for fabricating and plugging of tube to tubesheet joint by explosion welding, Weld. J., April, 238–​244 (1975). 299. Linse, V. D. and Temple, P. I., Explosion welding, in Welding Hand Book, Vol. 2, 8th edn., 1991, pp. 766–​781. 300. Nippes, E. F. (ed.), Explosion welding, in Metals Handbook, Vol. 6, Welding, Brazing, and Soldering, 9th edn., American Society for Metals, Metals Park, OH, 1983, pp. 705–​718. 301. Johnson, W. R., Explosive welding: Plugs into heat exchanger tubes, Weld. J., January, 22–​32 (1971). 302. Peacock, G. A., Fabrication of thick plates, Weld. Metal Fabr., June, 242–​252 (1963). 303. Ellis, D., Fabrication in clad steels, Weld. Metal Fabr., September, 359–​365 (1967). 304. Kearns, W. H. (ed.), Welding Handbook, Vol. 4, Metals and Their Weldability, 7th edn., AWS, Miami, FL, 1982, Chapter 12. 305. https://​cdn.ymaws.com/​titan​ium.org/​resou​rce/​res​mgr/​02_​jens​_​fol​der/​indus​tria​l_​co​mmit​tee_​webp​ age/​map_​proc​eedi​ngs/​ban​kerj​ohn2​018_​map_​ti_​c​lad.pdf 306. Recommendation of PWHT of welded joints in steel pressure vessels and other heavy duty structures, by Joint Working Group, IIW Commissions IX and X, Weld. World, 29, 341–​362 (1991). 307. Ellis, D. J. and Gifford, A. F., Application of electroslag and consumable guide welding, Weld. Metal Fabr., April, 112–​119 (1973). 308. Ricci, W. S. and Eagar, T. W., A parametric study of the electroslag welding process, Weld. J., December, 397-​s–​405-​s (1982). 309. Carpenter, O. R. and Floyd, C., Heat treatment of carbon and low alloy pressure vessel steel, Weld. J., (1957). 310. Watson, S. J., Post weld treatment of welded units for the relief of stress, Weld. Metal Fabr., September, 318–​327 (1958). 311. Ohshita, S., Yurioka, N., Mori, N., and Kimura, T., Prevention of solidification cracking in very low carbon steel welds, Weld. J., June, 129-​s–​136-​s (1983). 312. ASME, Boiler and pressure vessel code, Section VIII, Division 1, Pressure vessels, The American Society of Mechanical Engineers, New York, 2021.

Suggested Readings Akshat kumar Sanghvi, Aakash Patel, Shantanu Rangnekar and Vaibhav Rane, EasyChair Preprint No 5060, Designing and Analysis of Cryogenic Storage Vessel, February 27, 2021. ASM, Corrosion of nickelbase alloys, in Metals Handbook, Vol. 13, Corrosion, 9th edn., American Society for Metals, Metals Park, OH, 1987, pp. 641–​657. ASM, Weldments, brazed assemblies, and soldered joints, in Metals Handbook, Vol. 17, Nondestructive Evaluation and Quality Control, 9th edn., American Society for Metals, Metals Park, OH, 1989, pp. 582–​609. ASM, Metals Handbook, Vol. 1, Properties and Selection: Iron, Steels, and High Performance Alloys, 10th edn., American Society for Metals, Metals Park, OH, 1990. ASM, Metals Handbook, Vol. 2, Properties and Selection: Nonferrous Alloys, and Special Purpose Materials, 10th edn., American Society for Metals, Metals Park, OH, 1992. Charles, J., Gagnepain, J. C., Dupoiron, F., Jobard, D., and Soulignac, P., Austenitic-​ferritic stainless steels and clad plates: Properties and applications to pressure vessels, WRC Bull., 374, pp. 25–​37. Carpenter, D. D., A guide for determining post weld heat treatment requirements for ASME Code Section VIII pressure vessels, Trans. ASME, J. Pressure Vessel Technol., 106, 115–​123 (1984).

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Cary, H. B., Modern Welding Technology, Prentice Hall, Englewood Cliffs, NJ, 1979. Connor, L. P. (ed.), Welding Handbook, Vol. 1, Welding Technology, Vol. 2, Welding Processes, Vol. 3, Materials and Application, 8th edn., American Welding Society, Miami, FL, 1987. Castro, R. and DeCadenet, J. J., Welding Metallurgy of Stainless and Heat Resisting Steels, Cambridge University Press, London, U.K., 1975. Dulicu, D., Practical welding aspects of welding stainless steel, Weld. Metal Fabr., July, 277–​280 (1995). Farrar, J. C. M., Developments in stainless steel welding consumables, Weld. Metal Fabr., January, 27–​32 (1990). McNaughton, K. J. (ed.), Materials Engineering, Part I: Selecting Materials for Process Equipment; Part II: Controlling Corrosion in Process Equipment, McGraw-​Hill, New York, 1980. www.mate​rial​weld​ing.com/​how-​to-​weld-​9-​nic​kel-​steel/​ Mejzak, G. J., An introduction of glass for the chemical process industries, in ASME National Congress on Pressure Vessel and Piping Technology, 1983. www.neonic​kel.com/​techni​cal-​resour​ces/​gene​ral-​techni​cal-​resour​ces/​pren-​pitt​ing-​res​ista​nce-​equ​ival​ent-​ num​ber/​ Newcombe, G., The bimetal concept, Weld. Metal Fabr., November, 379–​406 (1974). Rayner, R. E., Better metallurgy for process equipment, Hydrocarb. Process., January, 53–​60 (1994). www.twi-​glo​bal.com/​techni​cal-​knowle​dge/​job-​knowle​dge/​weld​ing-​of-​ferri​tic-​cryoge​nic-​ste​els-​100 www.twi-​glo​bal.com/​techni​cal-​knowle​dge/​job-​knowle​dge/​weld​abil​ity-​of-​materi​als-​nic​kel-​and-​nic​kel-​all​ oys-​022 www.twi-​glo​bal.com/​techni​cal-​knowle​dge/​job-​knowle​dge/​dup​lex-​stainl​ess-​steel-​part-​2-​106 www.twi-​glo​bal.com/​media-​and-​eve​nts/​press-​relea​ses/​2021/​sigma-​phase-​embrit​tlem​ent-​of-​super-​dup​lex-​ stainl​ess-​ste​els www.langle​yall​oys.com/​knowle​dge-​adv​ice/​what-​is-​sigma-​phase-​in-​dup​lex-​stainl​ess-​steel/​#:~:text=​ Sigma%20ph​ase%20is%20a%20c​hrom​ium,its%20res​ista​nce%20to%20pitt​ing%20co​rros​ion. www.neonic​kel.com/​techni​cal-​resour​ces/​fabr​icat​ion/​weld​ing-​titan​ium-​titan​ium-​all​oys/​ https://​cdn.ymaws.com/​titan​ium.org/​resou​rce/​res​mgr/​02_​jens​_​fol​der/​indus​tria​l_​co​mmit​tee_​webp​age/​ map_​proc​eedi​ngs/​ban​kerj​ohn2​018_​map_​ti_​c​lad.pdf www.carp​ente​rtec​hnol​ogy.com/​hubfs/​PDFs/​All​oysf​orCo​rros​iveE​nvir​onme​nts.pdf www.cor​roth​erm.co.uk/​hubfs/​resour​ces/​cor​roth​erm-​intro​duct​ion-​super-​dup​lex-​stainl​ess-​ste​els.pdf www.smt.sand​vik.com or contact your local Sandvik sales office. Sandvik duplex stainless steels, Sandvik Materials Technology SE-​811 81 Sandviken, Sweden,2009, pp. 1–​18. www.materi​als.sand​vik/​4a2​16d/​globa​lass​ets/​glo​bal-​cont​ent/​downl​oad/​pro​duct​s_​do​wnlo​ads/​tubu​lar-​produ​ cts/​s-​120-​eng​_​10.pdf https://​nick​elin​stit​ute.org/​media/​8dab​1e8a​1825​480/​nickelpub10044_​a​prac​tica​lgui​deto​usin​gdup​lex.pdf www.steelp​ipes​fact​ory.com/​wp-​cont​ent/​uplo​ads/​2021/​05/​Datash​eet-​For-​Stainl​ess-​Steel-​Super-​Dup​lex-​ 2507.pdf https://​eng​inee​ring​libr​ary.org/​refere​nce/​pro​pert​ies-​of-​met​als-​doe-​handb​ook www.azom.com/​arti​cle.aspx?Articl​eID=​104 Khlefa A. Esaklul, Hydrogen damage, in Trends in Oil and Gas Corrosion Research and Technologies, 2017 www.twi-​glo​bal.com/​techni​cal-​knowle​dge/​job-​knowle​dge/​high-​temp​erat​ure-​hydro​gen-​att​ack-​htha-​143 www.twi-​glo​bal.com/​techni​cal-​knowle​dge/​faqs/​what-​is-​high-​temp​erat​ure-​hydro​gen-​att​ack-​htha-​hot-​hydro​ gen-​att​ack www.osti.gov/​bib​lio/​5995​543 www.metal​spip​ing.com/​heat-​exchan​ger-​tub​ing.html Rodriguez, P., Selection of Materials for Heat Exchangers, HEB 97, Alexandria, Egypt Apri l5–​16, 1997, pp. 59–​72 www.finetu​bes.co.uk/​-​/​media/​amet​ekfi​netu​bes/​files/​produ​cts/​materi​als/​fine​_​tub​es_​-​_​al​loy-​6mo.pdf?la=​en www.esmagr​oup.com/​produ​cts/​Proc​ess%20Inst​rume​ntat​ion/​Centra​vis/​6MO%20D​ata%20Sh​eet.pdf www.nsall​oys.com/​produ​cts/​stainl​ess-​steel-​bar/​dup​lex/​254-​smo.html

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Quality Control, Inspection, and Nondestructive Testing

3.1 QUALITY CONTROL AND QUALITY ASSURANCE Product quality refers to how well a product satisfies customer needs, serves its purpose, and meets industry standards. Product quality is rapidly becoming an important competitive issue. Competitiveness in the market, safety and the consequences of product failure, and the developing legislation related to product liability known as consumer protection acts have become important factors in the pursuit of high quality. For heat exchangers and pressure vessels, the overriding reasons are to avoid the consequences of failure, which can be catastrophic in human, monetary, and environmental terms. Flaws in critical components may increase the likelihood of failure.

3.1.1 Quality Management in Industry It has often been said that quality cannot be inspected into a component; it must be built into it. This is certainly true in heat exchanger fabrication also [1, 2]. In the past, quality control (QC) processes relied on separate QC staff to undertake inspection of the finished products, accepting those that conformed to the specification and rejecting the others. Under this method, higher quality is equated with higher cost. This method is also known for the high rejection of finished products, since control was not exercised at all vital stages of the manufacturing process. Because of this, management faced a conflict between improving quality of the product and reducing product cost. Therefore, a new concept of addressing a more total control of quality of a product/​process became known as quality assurance programs (QAPs) and provided systems that could cover all aspects, from raw material to finished product, including design, planning, material selection, purchasing, fabrication, and dispatch. With the introduction of QAP, the traditional “final inspection and test” approach, particularly that involving third-​party inspection, has shifted toward total quality assurance (QA); in which responsibility for quality is placed firmly with the fabricator [2].

3.1.2 Quality and Quality Control The term quality can be defined as conformance of a material or finished product to specifications and drawings. The definition for quality from ISO 8402-​1994 is “The totality of characteristics of an entity (product or service) that bear on its ability to satisfy stated and implied needs”. Another definition states that “The QC of a product is the degree to which it meets the requirements of the customer. With manufactured products, it is a combination of both quality of design and quality of manufacture [3]”.

DOI: 10.1201/9781003352051-3

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3.1.2.1 Aim of Quality Control The aim of QC is to provide quality that is satisfactory, adequate, reliable, and economical. The overall quality system involves integrating the quality aspects of several related steps, including the proper specification, fabrication, and inspection [4]. In order to achieve the quality, a planned and systematic QAP has been established.

3.1.3 Quality Assurance Quality assurance (QA) is the establishment of a QC system to guarantee the desired quality level of a product from design raw materials through fabrication, final assembly, and dispatch. QA comprises two aspects [5]: (1) QC and (2) quality administration. The term quality control was defined already. Rao [5] defines quality administration as the systematic organizational effort to effectively implement the quality standards by trained and qualified personnel and to establish documents as proof of quality achieved. 3.1.3.1 Need for Quality Assurance It is accepted that inspection and nondestructive testing (NDT) applied at appropriate stages during manufacture constitutes an integral and vital part of QC. There are a number of disadvantages in using inspection to control quality. (1) It is costly; (2) it is operator dependent; (3) it is too late; and (4) it only concentrates on the product, and not the processes. However, there is a growing recognition that quality characteristics and reliability cannot be guaranteed by inspection and tests alone. Such features must be “designed into” and then “built into” the products [2]. QA techniques have been developed to ensure that critical activities such as material procurement, design, and manufacturing activities such as welding and heat treatment are properly controlled. 3.1.3.2 Essential Elements of Quality Assurance Program A good QAP consists of five basic elements [6]: (1) prevention, (2) control, (3) assurance, (4) corrective action, and (5) quality audit. QA is based on systematic planning, testing, analysis, and documentation in all phases during the manufacture of a product. It begins with the formulation of the specifications and extends through planning, design, development, production, review, testing, commissioning, after-​sales service, and evaluation of the product’s performance [7]. In a well-​organized company with an established QA system, each manager in the organization knows his or her responsibility and has the required competence. 3.1.3.3 Requirements of Quality Assurance Programs for Success To be effective, QA programs should be sufficiently flexible to cater not only for “high-​quality” products but also to avoid the unnecessary imposition of high-​quality standards on “low-​quality” products. This will enhance the quality cost. Without such flexibility, a company cannot remain competitive [2]. 3.1.3.4 Quality Assurance in Fabrication of Heat Exchangers and Pressure Vessels Much of the QA literature is concerned with high-​volume mass production industries with emphasis on statistical QC, quality circles, total quality management (TQM), automation, etc. aimed principally at reducing the rejection, scrap, and rework, and hence at minimizing the product cost [8]. As pressure vessels, boilers, and heat exchangers are usually manufactured for a specific purpose on a “one-​off” basis, statistical methods of QC are not usual, although this applies where justified. Most of the QA literature on fabrication of pressure vessels and heat exchangers lie in standards, codes, and the technical literature of the individual industry.

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3.1.3.5 Contents of Quality Assurance Program (QAP) for Pressure Vessels and Heat Exchangers The QA in fabrication of pressure vessels comprises certain activities to be carried out by the pressure vessel manufacturer to assure the quality of design and construction for the safety and reliability of the pressure vessel. The activities that form the QAP for pressure vessels and heat exchangers are discussed in Refs. [2] and [4]. 1. Codes and interpretation 2. Approval of designs by the client/​third party/​consultant 3. Material control 4. Calibration of testing and inspection equipments 5. Qualification of welding procedures and welder performance 6. Qualification of welding inspectors and NDT personnel 7. Production methods and production control 8. QC during forming and shaping of components 9. QC during assembly of parts 10. QC during production welding 11. NDT of welds 12. Directing and expediting nonconformance reports (NCRs) 13. QC during post-​weld heat treatment (PWHT) 14. NDT after PWHT 15. Leak testing (LT) of pressure vessels 16. Hydraulic testing or proof testing of pressure vessels 17. Documentation 18. Auditing 19. Periodic preparation of budget reports on inspection costs.

3.2 QUALITY CONTROL SYSTEM (QC) To assure that the manufacturer has the ability and integrity to build pressure vessels and heat exchangers that meet the code requirement, it should have and demonstrate a QC system [9]. This system must include a written description, known as a quality manual. To be effective, the QC system must have managerial direction, technical support, and resources that are made available for its implementation. Basics of QC system for pressure vessel manufacture are discussed in detail by Chusu [9] and in mandatory appendix 10 quality control system, ASME Code Section VIII Div. 1[10a] and related code sections-​welds must conform to Section IX of the ASME code [10b], welding consumables must be stored as per Section II, Part C of the code [10c], and NDT examinations as per ASME Code Section V [10d].

3.2.1 Features of Quality Control System The salient features of QC system for manufacture of pressure vessels/​heat exchangers are hereunder [9]: 1. Authority and responsibility. The authority and responsibility of the QC manager shall be clearly established through management’s statement of authority, signed by competent authority in the top management. 2. Organization. An organization chart showing the relationship between management, engineering, purchasing, manufacturing, QC, and inspection shall be presented. Because a QC

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program often crosses departmental boundaries within an organization, a company must fix responsibility for the program [11]. 3. Design specifications, drawings, design calculations, etc. 4. Material procurement control. This system covers material requirements, indenting, and purchasing. Only code-​permitted materials shall be indented. 5. Material control. This system covers material receipt, inspection, and identification of materials, storage, and issue, including welding consumables and vendor evaluation. Procedures shall be established for examination of incoming materials like plates, pipes, forgings, etc. 6. Process control, examination, and inspection. This system ensures that the various stages of fabrication are carried out under controlled conditions and defines work instructions that include criteria for workmanship, requirements for equipment, and inspection after each work operation affecting quality. The system establishes written procedures for controlling fabrication and testing, welding, hot forming, NDT, LT, PWHT, hydrostatic testing, etc. At appropriate manufacturing stages, hold points are established so that inspections can be made on the material before commencing the next operation. QC personnel are involved in inspection at hold points, including nondestructive tests and interpretation of these tests. 7. Correction of nonconformities. This system covers control and disposal of nonconforming materials and components (i.e. items not conforming to code requirements) and ensures conditions adverse to quality are identified and corrected. 8. Welding QA. This system covers the quality of welding of components with specific reference to welding process parameters design, like pre-​cleaning and surface preparation, joint design, edge preparation and fit-​up, base metal preheat, maintenance of interpass temperature and postheating, gas shielding, filler metal selection and preheating welding electrodes, welding procedures, and qualification of welders and welding machine operators. Specify methods to monitor welding before, during, and after welding. Welds must conform to Section IX of the ASME Code [10b], Welding consumables must be stored as per Section II, Part C of the code [10c] and NDT examinations as per ASME Code Section V [10d]. Documents must cover issue and return of consumable. Proper choice of materials, welding process, qualified personnel, etc. is necessary to ensure building the quality in the product. 9. Nondestructive examination. This system covers nondestructive examination and inspection carried out in all stages of manufacture, written procedures for various NDT techniques as per code requirements, and qualification of personnel, and provides documentary evidence that all code requirements are met. 10. Heat treatment. This system covers the requirements of heat treatment and calibration of temperature measuring and recording instruments. 11. Maintenance and calibration of measuring instruments. This system ensures that inspection and test equipment and gauges are calibrated and maintained properly. 12. Documentation. An extensive system of documentation is required to provide QC data records. Send written procedures that define responsibility and assign specific responsibilities to the management, supervisors, and QC staff. Keep records showing that each has done their work for follow-​up when necessary [11]. 13. Authorized inspector. Authorized inspectors, or third-​party inspection agencies are involved in design approval and materials selection, followed by inspection and testing during various stages of fabrication. 14. Retention of records. This system covers the preservation and retention of records of the product manufactured to meet the code requirement. Such records include mill test reports, NDT reports, radiographs, impact test results, heat treatment charts, etc.

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15. Audits. An audit is an independent examination of quality to provide information; it is carried out by an independent organization called an audit team [12]. The aim of the audit is to establish that the procedures of the company’s quality manual are being followed. Management should establish a schedule for audits and adjust it on the basis of previous results.

3.2.2 ASME Code: Quality Control System Mandatory “QC system” requirements are stated in Appendix 10 of ASME Code Section VIII, Div. 1 [10a]. As per ASME Code, the manufacturer or assembler shall have and maintain a QC system that will establish that all code requirements, including those for material, design, fabrication, examination of vessels and vessel parts, and inspection, will be met. The QC system shall cover the following features: (1) authority and responsibility, (2) organization, (3) drawings, design calculations, and specification control, (4) material control, (5) examination and inspection program, (6) correction of nonconformities, (7) welding, (8) NDT, (9) heat treatment, (10) calibration of measuring and test equipment, (11) records retention, (12) sample forms, (13) inspection of vessels and vessel parts, (14) inspection of pressure relief valves, and (15) certifications. 3.2.2.1 Material Control The material control shall include a system of receiving control which will ensure that the material received is properly identified and has documentation including required Certificates of Compliance or Material Test Reports to satisfy code requirements as ordered. The material control system shall ensure that only the intended material is used in code construction. 3.2.2.2 Correction of Nonconformities There shall be a system agreed upon with the Inspector for correction of nonconformities. A nonconformity is any condition which does not comply with the applicable rules of the code. Nonconformities must be corrected or eliminated in some way before the completed component can be considered to comply with this Division. 3.2.2.3 Records Retention The Manufacturer or Assembler shall have a system for the maintenance of radiographs, Manufacturer’s Data Reports, and Certificates of Compliance/​Conformance as required by this Division. 3.2.2.4 Inspection of Vessels and Vessel Parts 1. Inspection of vessels and vessel parts shall be by the Inspector as defined in ASME Code Section VIII Div. 1 UG-​91. 2. The written description of the QCS shall include reference to the Inspector. 3. The Manufacturer shall make available to the Inspector, at the Manufacturer’s plant or construction site, a current copy of the written description of the Quality Control System. 4. The Manufacturer’s QCS shall provide for the Inspector at the Manufacturer’s plant to have access to all drawings, calculations, specifications, procedures, process sheets, repair procedures, proof test reports, records, test results, and any other documents as necessary for the Inspector to perform his duties in accordance with the code.

3.3 QUALITY MANUAL To meet the requirements of the codes and standards, the manufacturers develop a quality system and a QAP, which is explained in the company’s quality manual or QA manual. It states the company’s quality policy and details the quality systems, organization, and the responsibilities of the interrelated

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functions to meet the objectives of the QA program. It contains all the sample documents (forms and formats) used in the quality management systems and procedures.

3.3.1 Contents of Quality Assurance Manual The QA manual gives the following details [13]: • • • • • • • • • • • • •

the company’s quality policy organizational charts duties of QA personnel, supervisors, and technicians disposition of drawings, documents, and facsimile record-​keeping forms material control testing, inspection, and QC procedures qualification of welding procedures and qualification of welding personnel training and qualification of NDT personnel marking of nonconforming products heat treatment procedures calibration of measuring and testing equipment documentation possession and use of the code stamp.

3.3.2 Main Documents of the Quality System The main documents used to control the fabrication, examination, and inspection are the QC plan, process sheet, and checklist. These documents are discussed next. 3.3.2.1 Quality Assurance Program Based on approved drawings and material registration issued by the client, a QAP will be prepared that shall indicate: • • • • •

stages of manufacture at which examination and inspection are to be carried out details of examination to be carried out reference documents and acceptance standards witness/​hold points for client/​third party records to be maintained.

Before the commencement of manufacture, the QC plan shall be made available to the authorized inspector, who shall review the same and indicate the approval. 3.3.2.2 Operation Process Sheet Based upon the QC plan, approved drawings, and the hold points indicated by the authorized inspector, the production engineering department in consultation with the QA section shall prepare the operation process sheet for various components. The operation process sheet will: 1. Describe the stages of manufacture and list them sequentially. 2. Indicate procedures such as welding procedure specification (WPS), heat treatment, etc. to be used for that operation at appropriate columns and contain special instructions to the operator wherever required. 3. Provide space to record and make reference to reports such as NDT, NCR, etc.

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4. Provide space for signature by the QC inspector after carrying out necessary examination. 5. Provide space for indicating the hold points and witness points as specified on the QC plan. Operation process sheets for various operations are given later. 3.3.2.3 Checklist Based upon the QC plan with the hold and witness points indicated by the authorized inspector, approved drawing, and process sheet, prepare a checklist detailing the examination and inspection to be carried out during the various operations. Provision shall be made for the signature of the authorized inspector. Hold and witness points indicated in the QC plan shall be transferred to the checklist and submitted to the authorized inspector for verification.

3.4 ELEMENTS OF QUALITY COSTS Cost of quality is the costs companies incur ensuring that products meet quality standards, as well as the costs of producing goods that fail to meet quality standards. The cost of quality can be categorized into four categories –​prevention cost, appraisal cost, internal failure cost, and external failure cost [14a, 14b]:

3.4.1 Prevention Costs Prevention costs are the costs of all activities that are designed to prevent quality deviation or non-​ conformance from arising in products or services.

3.4.2 Appraisal Costs Appraisal costs are costs that are incurred to ensure the conformance to quality standards and performance requirements.

3.4.3 Internal Failure Costs Internal failure costs are the costs that are associated with defects found within the organization before the customer receives the product or service. The examples of the internal failure costs include, in-​process scrap, rework, design changes, product liability, etc.

3.4.4 External Failure Costs External failure costs are the costs that are associated with defects found after the customer receives the product or service. The examples of the external failure costs include processing customer complaints, customer returns, warranty claims, product recalls, etc.

3.4.5 Optimum Cost of Quality Elements of quality cost and quality cost model are shown in Figure 3.1 [15]. Production costs rise disproportionately as designers tighten tolerances whereas reduced quality may result in low reliability, rework, rejections which may give rise to customer complaint, hazards to human beings and the environment, etc. One way to decide where tolerances should be tight is to make a Pareto analysis of the dimensions and tolerances of the work [11]. This method classifies the elements of a total unit in the order of their importance to the performance of that unit. Such a classification of

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FIGURE 3.1  Quality cost vs measure of quality.

dimensions includes functionally critical, significant, and significant only for good appearance and for good workmanship. Veith [11] cites the Pareto analysis of 5000 dimensions of a pressure vessel, which showed that only 5% of the dimensions are critical, 18% are significant, and the remaining 77% dimensions require only good workmanship and acceptable appearance. Keeping in mind that under a QC system, every out-​of-​tolerance dimension is considered nonconforming, decide just exactly which dimensions are critical to the vessel operation and be more selective in assigning tolerances.

3.5 QUALITY REVIEW AND EVALUATION PROCEDURES Review is an important activity in a quality system to ensure that compliance with a set of procedures and agreed standards takes place all the time at all stages of the production process. Reviews must be methodical, and depending on the nature of the organization, they may take the form of (1) management audits, (2) system audits, or (3) project or product audits. Reviews are carried out periodically.

3.5.1 Auditing Audits should be conducted at specified intervals in accordance with written procedures and a checklist, to verify by investigation that applicable elements of the quality system and QAPs have been effectively implemented and documented. The four steps in conducting audit are [16] (1) planning and preparation, (2) performing the audit, (3) reporting the results, and (4) follow-​up and closeout.

3.6 DOCUMENTATION The quality system requires extensive and minute documentation of every procedure related to the manufacture of a product. The documentation serves as a proof of the activities carried out and the quality achieved by the manufacturer. The following certificates and records should be documented where applicable, and a copy of these should be sent to the customer:

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1. customer’s order 2. vessel detailed drawing and a materials list 3. material certificates of chemical and physical properties 4. welding procedure specification (WPS) and procedure qualification records (PQRs) 5. welder qualification records 6. records of welding consumables used 7. certificate of radiography and other NDT operations 8. reports of nonconformities 9. heat treatment charts 10. leak test report 11. hydrostatic test report 12. reproduction of vessel markings and stamping 13. manufacturer’s data report and associated partial data report, if any.

3.7 QUALITY MANAGEMENT 3.7.1  Quality Gurus –​their Contribution for Quality Management Many prominent stalwarts or Quality Gurus have emerged within the quality field, but some have stood out as key figures of quality. In no particular order, some of the top Quality Gurus who have shaped industrial quality management is given below [17]. 3.7.1.1 Dr. Walter Shewhart Dr Shewart was responsible for the modern process control and the concepts that were developed by his student, W. Edward Deming based on Shewart’s original “Plan, Do, Check and Act” cycle. Dr. Walter Shewhart who developed the Plan, Do, Check, Act (PDCA) cycle (also known as “Plan-​Do-​Study-​Act”) as well as theories of process control and the Shewhart transformation process. 3.7.1.2 Dr. W. Edwards Deming Dr. Deming developed his complete philosophy of management, which he encapsulated into his 14 key principles for management to follow to significantly improve the effectiveness of a business or organization and the “seven deadly diseases of management”. Deming greatly helped to focus the responsibility of quality on management and popularized the plan-​do-​check-​act (PDCA) cycle, which led to it being referred to as the “Deming Cycle”. 3.7.1.3 Dr. Joseph M. Juran Juran is best known for the following contributions to the quality philosophy, (a) Three Basic Steps to Progress, (2) Ten Steps to Quality Improvement, and (3) the Juran Trilogy. Quality triology consist of –​quality planning, quality improvement, and quality control. 3.7.1.4 Armand V. Feigenbaum Mr. Feigenbaum developed the idea of total quality control based on three steps to quality consisting of quality leadership, modern quality technology, and an organizational commitment to quality. Armand V. Feigenbaum is known for his concept of the hidden plant. In every factory, a certain proportion of its capacity is wasted by not getting it right the first time. According to him, quality is the customer’s perception of what quality is, not what a company thinks it is and “Total quality control” is an effective system for integrating the quality development, quality maintenance, and quality improvement efforts of the various groups in an organization so as to enable production and service at the most economical levels which allow full customer satisfaction.

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3.7.1.5 Dr. Kaoru Ishikawa Dr. Ishikawa developed the Ishikawa diagram, also known as the fishbone or cause-​effect diagram. He was known for popularizing the seven basic tools of quality. 3.7.1.6 Dr. Genichi Taguchi Dr. Taguchi developed the “Taguchi methodology” of robust design, which focused on making the design less sensitive to variation in the manufacturing process, instead of trying to control manufacturing variation. 3.7.1.7 Shigeo Shingo Shigeo Shingo developed lean concepts such as Single Minute Exchange of Die (SMED) or reduced set-​up times, as well as Poka-​Yoke (mistake proofing) to eliminate obvious opportunities to avoid mistakes from occurring. 3.7.1.8 Philip B. Crosby Philip B. Crosby developed the idea of Cost of Poor Quality (COPQ) to explain how “quality is free”. He popularized “zero defects” to define the goal of a quality program as the elimination of all defects and not the reduction of defects to an acceptable quality level

3.7.2 Quality Philosophies of Deming, Juran, and Crosby 3.7.2.1 Deming Deming created 14 Principles for Management that summarized his business philosophy. The principles became a basis for transformation of industry. The 14 principles apply anywhere, from small organizations to large ones, to the service industry as well as to manufacturing. They apply to any division within a company [18, 19]. 3.7.2.2 The Deming PDCA Cycle The Deming Cycle (or Plan-​Do-​Check-​Act (PDCA)) is a four-​step iterative technique used to solve problems and to improve organizational processes. Dr. Walter A. Shewhart developed the original concept during the 1920s. 3.7.2.3 Juran’s Contribution to Concepts of Quality Juran is widely credited for adding the human dimension to quality management. He advocated for the education and training of managers. For Juran, human relations problems were the ones to isolate. Resistance to change –​or, in his terms, cultural resistance –​was the root cause of quality issues. Juran is best known for the following contributions to the quality philosophy [20]: 1. Three Basic Steps to Progress 2. Ten Steps to Quality Improvement 3. The Juran Trilogy. 1. Juran Trilogy Juran’s Trilogy summarizes that managing for quality consists of three basic quality-​oriented processes: quality planning, quality control, and quality improvement. The role of quality planning is to design a process that will be able to meet established goals under operating conditions. 2. Juran’s Ten Steps to Quality Improvement 1. build awareness of both the need for improvement and opportunities for improvement

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2. set goals for improvement 3. organize to meet the goals that have been set 4. provide training 5. implement projects aimed at solving problems 6. report progress 7. give recognition 8. communicate results 9. keep score 10. maintain momentum by building improvement into the company’s regular systems. 3.7.2.4 Philip B. Crosby Philip Crosby is best known for [21]: Quality is Free Zero Defects Getting it right first time Conformance to requirements The Four Absolutes of Quality The Crosby Vaccine –​for management to prevent poor quality The Fourteen Steps of Quality Improvement. 3.7.2.4.1 Crosby Four Absolutes of Quality Crosby’s response to the quality crisis was the principle of “doing it right the first time” (DIRFT). The four major principles are: 1. quality has to be defined as conformance to requirements, not goodness 2. quality comes from prevention, not detection 3. the quality performance standard is zero defects, not acceptable quality levels 4. quality is measured by the price of nonconformance, not by indexes. 3.7.2.4.2 The Crosby Vaccine In the Crosby style, the “Vaccine” is explained as medicine for management to prevent poor quality. It is in five sections that cover the requirements of Total Quality Management, Integrity, Systems, Communication, and Policies. 3.7.2.5 What do the Philosophies of Deming, Juran, and Crosby have in Common? 1. Customer focused. 2. Commitment and leadership from top management. 3. Continuous improvement based on facts. 4. Team based.

3.8 QUALITY TOOLS AND QUALITY IMPROVEMENTS METHODS Good quality management can enhance an organization’s brand and reputation, protect it against risks, increase its efficiency, boost its profits and reduce waste, and position it to keep on growing. It takes less energy and material when quality improves. Low-​quality products and services deter customers from making a purchase. As a result, it is essential for all businesses to develop and maintain quality standards and insights. In an organization, quality is every employee’s responsibility.

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The quality concepts that support an organization in pursuing improvements and quality excellence are briefly discussed below: 1. Seven Quality tools 2. Quality Management Standards, ISO 9001 3. PDCA Cycle 4. PSCA Cycle 5. MBNQA 6. Total Quality Management (TQM) 7. Lean tools 8. Kaizen 9. 5S 10. TPM 11. Six Sigma 12. Lean-​Six Sigma.

3.8.1 The Seven Quality Control (7-​QC) Tools Quality Management tools play a crucial role in improving the quality of products and services. With their help, employees can easily collect the data as well as organize the collected data which would further help in analyzing the same and eventually come to solutions for better quality products. In 1974, Dr. Kaoru Ishikawa brought together a collection of process improvement tools in his text Guide to Quality Control. They are known as the seven quality control (7-​QC) tools, histogram, check sheet, Pareto chart, cause-​and-​effect diagram (also called Ishikawa diagram or fishbone diagram), control chart, scatter diagram, and stratification [22–​25]. They’re frequently implemented in conjunction with today’s most widely used process improvement methodologies, such as Six Sigma, TQM, continuous improvement processes, and Lean management. These seven basic tools are described below. 3.8.1.1 Histogram A histogram is a graphical representation (bar chart) of the distribution of data. It is also a representation of tabulated frequencies, shown as adjacent rectangles, erected over discrete intervals, with an area equal to the frequency of the observations in the interval. The height of a rectangle is also equal to the frequency density of the interval, i.e. the frequency divided by the width of the interval. The total area of the histogram is equal to the number of data. Histograms are created by dividing the range of data into equally-​sized segments. A histogram is shown in Figure 3.2.

FIGURE 3.2 Histogram.

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3.8.1.2 Pareto Analysis (80-​20 Rule) The Pareto Principle is called 80-​20 rule or rule of vital few. This rule assumes that in any process, 80% of a process’s or system’s problems are caused by 20% of major factors, often referred to as the “vital few”. The remaining 20% of problems are caused by 80% of minor factors. Therefore, focus on efforts on the problems that have the greatest potential for improvement. In quantitative terms, 80% of the problems come from 20% of the causes (machines, raw materials, operators, etc.). Therefore effort aimed at the right 20% can solve 80% of the problems. Pareto Chart The credit for Pareto Chart goes to Italian economist, Wilfredo Pareto. A Pareto Chart depicts the frequency with which certain events occur. A combination of a bar and line graph, the Pareto chart depicts individual values in descending order using bars, while the cumulative total is represented by the line. A Pareto chart is shown in Figure 3.3. 3.8.1.3 Fishbone Diagram or Cause and Effect Diagram Cause and effect analysis diagram is also known as Ishikawa, herringbone or fishbone diagram. The fishbone diagram is a graphical method for finding the root causes of an effect. It is used to solve quality problems by brainstorming causes and logically organizing them by branches. Causes are normally grouped into six major categories to identify these sources of variation. These six categories are as follows: 1. man/​staff/​operator –​it include any person involved with the process 2. methods –​methods include how the process is performed for executing the job or work or manufacturing a component or assembly 3. machines 4. materials 5. measurements 6. environment. A cause and effect diagram for analyzing roller bearing failure is shown in Figure 3.4.

FIGURE 3.3  Pareto analysis.

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FIGURE 3.4  Cause and effect/​Ishikawa /​Fishbone diagram for a roller bearing failure analysis.

3.8.1.4 Check Sheets Check sheets are tools for collecting data. They are designed specific to the type of data to be collected. Some examples of check sheets are daily maintenance check sheets, attendance records, production log books, etc. Check sheets are simple data gathering devices. Check sheets are used to collect data effectively and efficiently.

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FIGURE 3.5  Stratification of data (illustration).

3.8.1.5 Stratification A technique that separates data gathered from a variety of sources so that patterns can be seen. When data from a variety of sources or categories have been lumped together, the meaning of the data can be impossible to see. The technique separates the data so that the pattern can be seen. The technique is used in combination with other data analysis tools. An example of data stratification is given in Figure 3.5. 3.8.1.6 Scatter Diagram Scatter diagram is a quality management tool which helps to analyze relationship between two variables. In a scatter chart, data is represented as points, where each point denotes a value on the horizontal axis and vertical axis. If the variables are correlated, the points will fall along a line or curve. A scatter diagram is used when a variable exists that is below the control of the operator. An example of a Scatter diagram is shown in Figure 3.6. 3.8.1.7 Statistical Quality Control and Control Chart Statistical process control uses sampling and statistical methods to monitor the quality of an ongoing process such as a production operation. Control charts can be classified by the type of data they contain. For instance, an x–-​chart is employed in situations where a sample mean is used to measure the quality of the output. Process variability can be monitored using a range or R-​chart. In cases in which the quality of output is measured in terms of the number of defectives or the proportion of defectives in the sample, a p-​chart can be used. The control chart is a graph used to study how a process changes over time it shows whether a sample of data falls within the common or normal range of variation. Data are plotted in time order [26, 27]. A control chart always has a central line for the average, an upper line for the upper control limit, and a lower line for the lower control limit. It has upper and lower control limits (UCL and LCL) which separate common from assignable causes of variation. Control chart gives signal before the process starts deteriorating. It aids the process to perform consistently and predictably. It gives a good indication of whether problems are due to operation faults or system faults. Examples of control charts include the following: – • mean or X or X-​bar chart –​it is used to monitor changes in the mean value or shift in the central tendency of a process

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FIGURE 3.6  (a) A scatter diagram, (b) pressure vs volume relation at constant temperature, and (c) volume vs temperature at constant pressure of a real gas.

• range (R) chart –​this chart monitors changes in the dispersion or variability of the process • p-​chart –​this chart is used to measure the proportion that is defective in a sample. – Mean or X control chart is shown in Figure 3.7.

3.8.2 ISO 9000 ISO is the International Standards Organization, headquartered at Geneva, Switzerland. One of its purposes is to develop and promote worldwide quality standards. The standards define minimum requirements for quality assurance systems that directly influence product quality and customer satisfaction [28]. 3.8.2.1 ISO 9000 Series The ISO 9000 series of quality standards defines the basic management systems that a manufacturer should have to ensure that the end product consistently conforms to the order requirements. The ISO 9000 series contains these standards:

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– FIGURE 3.7  Typical mean or X control chart.

1. ISO 9000:2015: Quality Management Systems –​Fundamentals and Vocabulary (definitions). 2. ISO 9001:2015: Quality Management Systems –​Requirements. 3. ISO/​TS 9002:2016: Quality Management Systems –​Guideline for the application of ISO 9001:2015. 4. ISO 9004:2018: Quality Management –​Quality of an Organization –​Guidance to Achieve Sustained Success (continuous improvement). 5. ISO 19011:2018: Guidelines for Auditing Management Systems. 3.8.2.2 Benefits of ISO 9000 Major benefits of ISO 9000 are the following: It introduces a systems concept. It brings about changes that are organizational, procedural, and operational. It focuses on prevention of nonconformities. It improves communication in the system. It satisfies quality levels and facilitates improvements. 3.8.2.3 ISO 9001 Quality Management System ISO 9001 is the international standard that specifies requirements for a quality management system (QMS). Organizations use the standard to demonstrate the ability to consistently provide products and services that meet customer and regulatory requirements. It is the most popular standard in the ISO 9000 series and the only standard in the series to which organizations can certify. 3.8.2.4 ISO’s Quality Management Principles This standard is based on a number of quality management principles (QMP) including a strong customer focus, the motivation and implication of top management, the process approach, and continual improvement. The seven quality management principles are: 1. customer focus 2. leadership 3. engagement of people 4. process approach

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5. improvement 6. evidence-​based decision making 7. relationship management. 3.8.2.5 Scope of ISO 9001 Standards ISO 9001 is based on the plan-​do-​check-​act methodology and provides a process-​oriented approach to documenting and reviewing the structure, responsibilities, and procedures required to achieve effective quality management in an organization. Specific sections of the standard contain information on many topics, such as: 1. requirements for a QMS, including documented information, planning and determining process interactions 2. responsibilities of management 3. management of resources, including human resources and an organization’s work environment 4. product realization, including the steps from design to delivery 5. measurement, analysis, and improvement of the QMS through activities like internal audits and corrective and preventive action. 3.8.2.7 Benefits of ISO 9001 Certification The potential benefits to an organization of implementing a quality management system based on this International Standard are: 1. the ability to consistently provide products and services that meet customer and applicable statutory and regulatory requirements 2. facilitating opportunities to enhance customer satisfaction 3. addressing risks and opportunities associated with its context and objectives 4. the ability to demonstrate conformity to specified quality management system requirements.

3.8.3 Plan-​Do-​Check-​Act (PDCA) Plan-​Do-​Check-​Act (PDCA) is a scientific method used to manage change, and is also known as the Deming Cycle. It was developed by Dr. W Edwards Deming in the 1950s. It is also known as PDSA, where the “S” stands for “study”. The PDCA cycle is a simple four-​stage method as given below [29, 30]: 1. Plan –​recognize an opportunity or process that needs improvement opportunity and plan a change. 2. Do –​implement the changes. 3. Check –​evaluate the results in terms of performance. 4. Act –​standardize and stabilize the change or begin the cycle again, depending on the results. PDCA is the foundation of continuous improvement or kaizen. This ensures the improvement is stable. PDCA cycle is illustrated in Figure 3.8.

3.8.4 PDSA Technique Dr. Deming emphasized the PDSA cycle, not the PDCA cycle, with a third step emphasis on Study (S), not Check (C). The PDSA or the Plan-​ Do-​ Study-​ Act technique is a quality improvement tool that helps organizations enhance the quality of their products and services [31–​33].

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FIGURE 3.8  PDCA cycle.

3.8.5 MBNQA The Malcolm Baldridge National Quality Award recognizes U.S. organizations for performance excellence. The award has a set of requirements against which a company could design and assess a QMS built around the criteria for promoting business excellence. Apart from external assessments to attain the award, there is no ongoing certification against these requirements.

3.8.6 Total Quality Management Total Quality Management (TQM) is a management approach focusing on the improvement of quality and performance in all functions, departments, and processes across the company to provide quality services which exceed customer expectations. TQM expands the scope of quality of every department from top management to lower level employees. The term “total” means the entire organization –​all teams, departments and functions is involved in quality management. 3.8.6.1 Elements of Total Quality Management In a TQM effort, all members of an organization participate in improving processes, products, services, and the culture in which they work. The eight principles of total quality management [34–​39] include (1) customer focused, (2) total employee involvement, (3) process centered, (4) integrated system, (5) strategic and systematic approach to achieving an organization’s vision, mission, and goals, (6) continual improvement, (7) fact-​based decision making, and (8). effective communications. These elements are considered so essential to TQM that many organizations define them, in some format, as a set of core values and principles on which the organization is to operate. The principles of total quality management is depicted in Figure 3.9 3.8.7 5S 5S is a system to reduce waste and optimize productivity through maintaining an orderly workplace and using visual cues to achieve more consistent operational results. Implementation of this method “cleans up” and organizes the workplace basically in its existing configuration, and it is typically the first lean method which organizations implement [40, 41]. 5S originated as five Japanese words: Seiri, Seiton, Seisou, Seiketsu, and Shitsuke, i.e. 5S is a five step methodology for creating a more organized and productive workspace. In English these have come to be known as: • • • • •

Sort: eliminate anything that is not truly needed in the work area. Straighten: organize the remaining items. Shine: clean and inspect the work area. Standardize: create standards for performing the above three activities. Sustain: ensure the standards are regularly applied.

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FIGURE 3.9  Elements of total quality management (TQM).

At their core, 5S activities build the discipline needed for substantial and continuous improvement by creating (and sustaining) efficient and effective work areas. The 5S Foundation 5S serves as a foundation for deploying more advanced lean production tools and processes. The goal of 5S is to create a work environment that is clean and well-​organized. Adherence to 5S standards is considered the foundation of Total Productive Maintenance (TPM).

3.8.8 Total Productive Maintenance Total Productive Maintenance (TPM) is an organization-​wide strategy to increase the effectiveness of production environments, especially through methods for increasing the effectiveness of equipment. It is an approach to equipment maintenance that strives to achieve perfect production: 1. no breakdowns 2. no small stops or slow running 3. no defects or production rejects 4. it creates a safe working environment: 5. no workplace accident. Getting operators involved in maintaining their own equipment, and emphasizing proactive and preventive maintenance will lay a foundation for improved production (fewer breakdowns, stops, and defects). The implementation of a TPM program creates a shared responsibility for equipment that encourages greater involvement by plant floor workers.

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FIGURE 3.10  Illustration of eight Pillars of TPM.

3.8.8.1 TPM Pillars and 5S The traditional TPM model consists of a 5S foundation (Sort, Set in Order, Shine, Standardize, and Sustain) and eight supporting activities known as eight pillars. The goal of 5S is to create a work environment that is clean and well-​organized. Each TPM pillar will have their own unique role in improving the plant’s performance. The ‘eight pillars of TPM’ are [42–​47]: 1. Focused Improvement (Kobetsu Kaizen) 2. Autonomous Maintenance (Jishu Hozen) 3. Planned Maintenance (Keikaku Hozen) 4. Early Management (Product and Equipment) 5. Quality Maintenance (Quality Hozen) 6. Education & Training 7. Office TPM (TPM in Administrative and Support Departments) 8. Safety, Health, & Environment (SHE). TPM pillars are depicted in Figure 3.10. 3.8.8.2 Goals of TPM TPM emphasizes proactive and preventative maintenance to maximize the operational efficiency of equipment. TPM aims for zero accidents, zero defects, zero breakdowns and zero loss through the creation of a perfect manufacturing system. The three goals of TPM include: (1) zero unplanned failures, (2) zero product defects, and (3) zero accidents

3.8.9 KAIZEN™ “KAIZEN™ means improvement. When applied to the workplace KAIZEN™ means continuous improvement involving everyone –​managers and workers alike. Continuous improvement is the process of constantly making things better than they were before. In lean manufacturing, Kaizen is

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the practice of continually making small, incremental improvements for a safer, more productive, and efficient workplace. 3.8.9.1 The Core of KAIZEN™ There are five Fundamental KAIZEN principles that are embedded in every KAIZENTM tool [48, 49]: 1. know your customer 2. let it flow 3. go to workplace (Gemba) 4. empower people 5. be transparent.

3.8.10 Lean Lean is the set of “tools” that assist in the identification and steady elimination of waste. Any use of resources that do not create value for the end customer is considered a waste and should be eliminated. The Toyota Production System originally detailed seven wastes. Lean projects might focus on eliminating or reducing anything a final customer would not want to pay for scrap, defect/​ rework, excess inventory, queuing or wait time, unnecessary transportation of materials or products, overproduction, redundant motion, and other non-​value-​added process steps. The seven wastes are shown in Figure 3.11

FIGURE 3.11  Seven wastes as per lean manufacturing.

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FIGURE 3.12  Six Big Losses as per lean manufacturing.

A core element of this framework is the distinction between value-​adding and nonvalue adding activities. Value-​adding activities contribute to what the customer wants of the product or service and that they would be willing to pay for. 3.8.10.1 Lean Manufacturing Lean manufacturing or lean production is a systematic method for the elimination of waste (“Muda”) within a manufacturing system. Lean also takes into account waste created through overburden (“Muri”) and waste created through unevenness in workloads (“Mura”). It involves identifying and eliminating non value adding activities in design, production, supply chain management, and dealing with the customers. Lean initiatives opt to improve production, quality, and lead-​time of and the end goal is to provide the highest quality product while continuing to meet demand and reducing waste in the process. As waste is eliminated quality improves while production time and cost are reduced [50].

3.8.11 Six Big Losses Identification of the Six Big Losses is one of several lean tools used by manufacturers today to understand the most common forms of waste within manufacturing operations. In addition to other tools, such as root cause analysis, 5S, and JIT production, detection of the Six Big Losses will streamline production and expose bottlenecks in production. The Six Big Losses are breakdowns, setup and adjustments, small stops, reduced speed, startup rejects and production rejects. However, the Toyota Production System originally detailed seven wastes. Figure 3.12 shows six big losses in manufacturing processes as per lean manufacturing.

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3.8.12 Lean Tools The new terms of lean tools as per Lean Lexicon, 5th Edition are [51]: • • • • • • • • • • • • • • • • • • • • • • • • •

5S (Seiri, Seiton, Seisou, Seiketsu, and Shitsuke) Andon Bottleneck Analysis Continuous Flow Gemba (The Real place) walk Heijunka (Level Scheduling) Hoshin Kanri (Policy Deployment) Jidoka (Autonomation) Just-​in-​Time(JIT) Kaizen (Continuous Improvement) Kanban (Pull System) KPI (Key Performance Indicator) Muda (Waste) Overall Equipment Effectiveness (OEE) PDCA (Plan, Do, Check, Act) Poka-​Yoke (Error Proofing) Root Cause Analysis Single Minute Exchange of Die (SMED) Six Big Losses SMART Goals Standardized Work Takt Time Total Productive Maintenance (TPM) Value Stream Mapping Visual Factory.

3.8.13 Six Sigma Quality Management Methodology The term Six Sigma was coined by Motorola as its methodology for improving business processes by minimizing defects and refers to the statistical measurement indicating there are only 3.4 defects

FIGURE 3.13  Six Sigma Principle.

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FIGURE 3.14  Six Sigma methodologies.

out of every one million opportunities to produce a defect. In the case of a normal distribution, 68.26% of the data points are within ± 1 σ from the mean, 95.46% are within ± 2σ and 99.73% are within, ± 3 σ. Six sigma concept is shown in Figure 3.13. 3.8.13.1 Six Sigma (6σ) Definition Six Sigma (6σ) method is a set of techniques and tools for process improvement. Basics of six sigma methods is discussed in reference [52, 53] among others. 3.8.13.2 Six Sigma Methodologies Six Sigma projects follow two project methodologies inspired by Deming’s PDCA Cycle: • DMAIC (Define –​Measure –​Analyze –​Improve –​Control) which is used for projects aimed at improving an existing business process. • DMADV (Define –​Measure –​Analyze –​Design –​Verify) which is used for projects aimed at creating new product or process designs. Design for Six Sigma (DFSS) –​is used to develop new processes or products at Six-​Sigma-​quality levels. Two of the above Six Sigma Methodologies are shown in Figure 3.14. 3.8.13.3 Six Sigma Tools and Methods Six Sigma is a system of statistical tools and techniques focused on eliminating defects and reducing process variability. Some of the basic tools include brainstorming, root cause analysis/​The 5 Whys, voice of the customer, the 5S System, Kaizen, benchmarking, Poka-​yoke (Mistake Proofing), value stream mapping, etc. [53]. The objective is to remove waste and inefficiencies in the value stream and create leaner operations. 3.8.13.4 Lean vs Six Sigma Six Sigma focuses on reducing process variation and enhancing process control, whereas lean drives out waste (non-​value-​added) and promotes work standardization and flow. The two initiatives approach their common purpose from slightly different angles [53, 54]: 1. Lean focuses on waste reduction, whereas Six Sigma emphasizes variation reduction. 2. Lean achieves its goals by using lean tools such as KAIZEN, 5S-​workplace organization, and visual controls, whereas Six Sigma tends to use quality tools like statistical data analysis, design of experiments, hypothesis tests, etc.

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FIGURE 3.15  Concept of Lean Six Sigma method.

3.8.13.5  Lean Six Sigma Lean Six Sigma is a synergized managerial concept of Lean and Six Sigma. Lean focuses on eliminating the eight kinds of wastes and Six Sigma focuses on improving process quality by identifying and removing the causes of defects and minimizing variability in processes. Lean Six Sigma blends these two approaches. The Lean Six Sigma method improves both the process and the quality. Lean strives for more flow and generating value. Six Sigma strives for stable and effective processes. In combination they reinforce each other and are completely complementary [54–​57]. The concept of Lean Six Sigma is shown in Figure 3.15.

3.9 INSPECTION 3.9.1 Definitions The term inspection can be defined as the process of examination of a material or a finished product using sensory organs, tools, equipments, instruments, and gauges to assess the quality of a product. The term inspection, as far as ASME Code Section V and other referencing code sections are concerned, applies to the functions performed by the authorized inspector, whereas the term examination applies to the QC functions performed as stipulated in the quality manual by QC staff of the company or the manufacturer.

3.9.2 Objectives of Inspection During manufacture, inspection has three principal objectives [58]: 1. To provide assurance that there are no defects, indicating manufacturing was above the standard required in the specification. 2. To provide assurance that no defects are present that could impede subsequent processing inspections. 3. To provide assurance that no defects are present in the completed component that will pose safety problem.

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3.9.3 Design and Inspection In the preliminary design stage, performance criteria and material selection should be made compatible with NDT, and the aim should be to provide maximum accessibility for inspection both during fabrication and in service. The design should include inspection in critical areas where the fabrication process is likely to introduce defects or where service conditions will impose critical stresses [6].

3.9.4 Inspection Guidelines Some general rules for organization of inspection are as follows: 1. Prepare inspection briefs in clear and concise formats. Assemble and incorporate into the inspection brief the setting plan, assembly drawings, tools, and instruments required for inspection [59]. 2. Collect the required documents. 3. Define the techniques of inspection. 4. Establish the sequence of inspections. 5. Specify inspection of materials and components as soon as possible. 6. Establish safety regulations for inspections.

3.9.5 Scope of Inspection of Heat Exchangers During the various stages of production process, inspectors examine work and document their observations. The scope of inspection of heat exchangers is dealt in Chapter 4. 3.9.5.1 Material Control and Raw Material Inspection One of the important duties of inspectors is to inspect the raw materials like plates, forgings, and tubes, and certify that they are as per purchase order and free from defects. The code requires goods to be accompanied by documents that detail the mill test reports and certifications. A designated receiver should have the receiving procedure, the purchase order, and a checklist covering component dimensions, and examine for laminations, surface defects, transshipment damage, identification and verification of heat numbers, and test records supplied by vendors [11]. An easy way to avoid using the wrong materials in parts is to check composition using a portable spectroscope, which quickly identifies all major elements [60]. The receiver signs in acceptable material for storage, and tags and segregates nonconforming materials, followed by identification, which includes a job number, serial number, and heat number, if relevant. Throughout production, identification follows the material or component, assuring traceability. All this work is carried out according to the QC procedures laid down in the quality manual [13]. 3.9.5.2 Positive Material Identification Positive material identification (PMI) is a process used to determine the elemental composition of materials. The test identifies the alloys that make up a particular material. The purpose of alloy verification is to ensure that only materials that are specified as part of the design requirement are supplied and used. PMI is typically a field testing method with portable analyzer. Measurements results are shown either in form of elemental concentration and/​or by specific alloy name.

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3.9.6 Detailed Checklist for Components Before conducting the inspection, prepare a checklist of what, how, and when to inspect. Instruct the inspector to record deviations and their resolutions in an inspector’s log [59]. Checklists detail steps in inspection of shell, channel, tubesheet, flanges, expansion joint, tube bundles, final assemblies, etc. A typical checklist for tubesheet, taken from a manufacturer’s work, is given next.

3.9.7 TEMA Standard for Inspection TEMA [61] inspection policy is specified in paragraph G-​2. G-​2.1 specifies manufacturer’s inspection, and G-​2.2 specifies the purchaser’s inspection.

3.9.8 Master Traveler During fabrication, a traveler that accompanies the part or component is sometimes the easiest way to carry out QC procedure. A drawing with all the welds numbered and marked with the points to be examined is a convenient and careful way to handle QC of welds. This document is known as a master traveler [11, 62]. The traveler tabulates operations for manufacture of any part or component. The traveler can continue to cover the complete vessel. From a production standpoint, however, it is often better to divide a complex vessel into components, with a traveler for every component and a separate sheet for the final assembly.

3.9.9 Third-​Party Inspection In addition to a firm’s own QAP, acceptance inspection is carried out by specialists and experts from various international inspection organizations (known as third-​party inspection) such as TÜV Nord, TÜV SÜD, TÜV Rheinland, Lloyd’s Register, Stoomwezen, ISPESL, Bureau Veritas Quality International, Det Norske Veritas, etc. Third-​party inspection agencies normally stipulate an involvement starting with approval of design, materials, and fabrication, followed by inspection and testing. Lloyd’s approach to QA of welded structures is discussed by Frew [63]. The approach is briefly described here. The efficacy of the fabricator’s quality system is evaluated, and suitable NDT methods are considered in relation to the joint geometries involved. Approval of fabrication takes into consideration the welding quality design, welder procedure qualification and production tests, PWHT, etc. 3.9.9.1 Hold Points and Witness Points Consideration should be given to the establishment of hold points and witness points, where an examination is to occur prior to the accomplishment of any further fabrication steps [64]. This is of vital importance for fabrication of pressure vessels and heat exchangers as per code and/​or standard. Through hold points and witness points, authorized code inspectors exercise control over the activities such as design calculations, drawings, receipt of materials and welding consumables, qualification of welding procedures and welders, joint design, work preparation before welding, examination during welding and after welding, NDT, NCR, PWHT, hydrostatic tests, etc. A typical scheme for hold points/​witness points and verification points is furnished in Chapter 4.

3.10 WELDING DESIGN 3.10.1 Parameters Affecting Welding Quality Three important parameters, as shown in Figure 3.16, that contribute to the quality of a welded products are [58] the following:

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FIGURE 3.16  Components of fabrication quality assessment.

1. material –​composition, product form, and mechanical properties 2. welding –​procedures, welders, equipment, and supervision 3. inspection –​stage inspection and NDT. Elements of welding design for building quality into the product are discussed in this part.

3.10.2 Welding Quality Design The highest standards of weld quality demand QC or NDT or both. Not all welds can be inspected by common NDT methods. The achievement of consistent high-​quality welds made by manual or automated welding processes requires the following [11, 58]: 1. well-​established and qualified welding procedures 2. strict implementation of all the weld procedures 3. trained and qualified welders, and welding machine operators 4. well-​maintained and calibrated equipments, instruments, and gauges 5. proper supervision and control of welding operations 6. recording of all information and measurements during welding 7. detailed NDT 8. welding consumables must be stored as per Section II, Part C, of the code or as per quality manual 9. documents must cover issue and return of consumables. In addition to these, more attention must be paid to the already proven techniques of QC particularly in the following areas [3]: • component design –​eliminate sites for fatigue or brittle fracture, improve access for welding and inspection • weld detail design –​use butt welds, rather than fillets or seal welds • material selection –​weldability considerations • operator selection –​test the ability of the welder or welding machine • supervision and production control –​adequate supervision or monitoring in the case of machines.

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TABLE 3.1 Welding Parameters that Control Quality of Weld Joint Figure Base No. No. Materials

Thickness (mm)

Welding Process

No. Weld Filler Runs Material

Shld. Gas/​ Preheating PQR WPS PTP Flux (°C) No. No.

Note: If necessary, several tables of this form may be used, having the same revision number. A blank space below the table may be left for notes such as weld details and for numbered sketches of the joint.

For better control of weld quality of major and critical joints, tabulate the various welding parameters as shown in Table 3.1.

3.10.3 Variables Affecting Welding Quality A wide range of welding variables affect the weld quality. They include [58] welding process, weld preparation, weld bead sequence, parent material compositions, weld metal composition, fluxes, electrodes and filler wires, preheat and interpass temperature, welding speed, heat input, cleaning between passes, PWHT, post-​weld cleaning, and surface treatment.

3.10.4 Scheme of Symbols for Welding A welding symbol is the graphical representation of the specifications for producing a welded joint. It contains all the necessary information, viz. welding position, dimensions and geometry of the weld, details of groove/​fillet, welding process, etc. A basic weld symbol consists of three parts namely, (1) arrow line, (2) reference line, and (3) tail. Refer to AWS A2.4-​2020 Standard Symbols for Welding, Brazing and Nondestructive Examination. AWS A2.4-​2020 –​Standard Symbols for Welding, Brazing, and Nondestructive Examination. This standard presents a system for indicating welding, brazing, and nondestructive examination requirements including the examination method, frequency, and extent. Typical AWS welding symbols are shown in Figure 3.17.

3.10.5 Standard for Welding and Welding Design Welding and welding design shall be as per ASME Code Section IX. The scope of Section IX is given here. 3.10.5.1 ASME Code Section IX ASME BPVC –​Section IX Section IX Welding, Brazing, and Fusing Qualifications. Section IX is a “service code” to other BPVC Sections, providing requirements relating to the qualification of welding, brazing, and fusing procedures. It also covers rules relating to the qualification and requalification of welders, brazers, and welding and brazing operators in order that they may perform welding or brazing in component manufacture. There are, in addition, tables of P-​ numbers that have been assigned to each material specification, so that the various examinations can be classified together. Similarly, F-​numbers are assigned to electrodes and welding rods to utilize coverage of several different electrodes or welding rods within one qualification test. Welding, brazing and fusing data cover essential and nonessential variables specific to the welding, brazing or fusing process used. The different material joining processes covered in Section IX are: 1. welding, brazing, and fusing procedures 2. welders and welding operators

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FIGURE 3.17  AWS welding symbols.

3. brazers and brazing operators 4. plastic fusing operators. For some comments on the contents of Section IX refer to [65–​67]. Procedure specification: for each material joining process there should be a procedure specification, these procedure specifications are termed as: • Welding Procedure Specification –​WPS • Brazing Procedure Specification –​BPS • Fusing Procedure Specification –​FPS.

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Contents of a Procedure specification: the procedure specification, i.e. a Welding Procedure Specification (for welding) contains parameters related to the welding process along with the values or range (as applicable) required to produce a sound weld. These parameters have been termed as variables by ASME. Variables are of three types; • essential variables • non-​essential variables • supplementary essential variables. 3.10.5.2 Nondestructive Testing(NDT) of Weldment Welding NDT is the use of nondestructive testing to inspect a weld. Welds are one of the most common parts of industrial assets that inspectors test. Using nondestructive weld testing equipment, inspectors can determine whether a weld is strong or has potential defects that could compromise its integrity. The most common welding NDT methods include ultrasonic testing, magnetic particle inspection, acoustic emission, dye penetrant, radiography, and eddy current. Some common types of destructive weld testing are: 1. Guided bend weld test. Bending a sample section of the weld to predetermined radius to make determinations about its internal structure. 2. Macro etch weld testing. Removing a small sample from the weld, polishing the samples, then etching on the samples with an acid mixture in order to test the internal makeup of the weld. 3. Transverse tension test. Testing the tensile properties of the base metal, the weld metal, and the bond between them. 3.10.5.3 Selection of Consumables While selecting the consumables, the requirements for hydrogen control must be satisfied to avoid hydrogen-​induced cold cracking. Electrodes and fluxes must be heated before use to ensure low hydrogen contents. 3.10.5.4 P Numbers All materials used for pressure vessel manufacture have been grouped under different P numbers. The P number grouping of materials is based essentially on comparable metal characteristics such as composition, weldability, and mechanical properties. The object of grouping the base materials is to reduce the number of qualifications required. Thus a single qualification may be adequate for several material specifications. The P number groupings are given in Table 3.2. 3.10.5.5 Filler Metals The filler metals are grouped under both F numbers and A numbers.

TABLE 3.2 P Number and F Number Groupings Ferrous metals Aluminum and its alloys Copper and its alloys Nickel and its alloys Unalloyed titanium

P1–​P11 P21–​P25 P31–​P35 P41–​P45 P51–​P52

Fl–​6 F21–2​ 4 F31–3​ 6 F41–4​ 4 F51

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3.10.5.6 F Numbers All the electrodes and fillers are grouped under different F numbers. The F number grouping is based essentially on their usability characteristics, and the object of the grouping is to reduce the number of welding procedures and performance qualifications. The F number grouping is giving in Table 3.2. 3.10.5.7 A Numbers As well as classifying the filler metals under F numbers, they are classified under A numbers. The A number classifications of the filler metals are based on the weld metal composition, whereas the F number classifications are based on the usability characteristics.

3.10.6 Welding Procedure Specification A Welding Procedure Specification, or WPS, is a document that serves as a guide for the effective creation of a weld that meets all applicable code requirements and production standards. A WPS contains details that are necessary to create the desired weld. This includes information such as base metal grade, filler metal classification, range of amperage, shielding gas composition, and pre-​heat and inter pass temperatures. The idea is that if a group of welders adhere to all the details on a WPS, they should each be able to produce welds with reasonably similar mechanical properties [68]. 3.10.6.1 Contents of Welding Procedure Specification A WPS shall describe all of the essential/​nonessential and supplementary essential variables (when required) for each process used. It will contain information regarding joint details, joint preparation, base metals, type and size of filler metal, welding position, electrical characteristics (type of current, range of current, and voltage), welding techniques, shielding gas, preheat, PWHT, interpass temperature, not preparation prior to welding from second side, welding speed, etc.

3.10.7 Procedure Qualification Record, PQR A PQR is a record of the range of welding parameters used to qualify a welding procedure. The procedures for creating and testing the sample welds, as well as the final results, are documented on a Procedure Qualification Record, or PQR. If the test results are acceptable, the PQR is approved and can then serve as the foundation on which one or more WPSs are drafted. In short, a PQR serves as evidence that a given WPS can, in fact, be used to produce an acceptable weld [68]. The WPS and the PQR are required to prove that the weldment has desired properties and not to prove the skill of the welder or the welding machine operator. The welder or operator performance qualification is to prove the ability of the welder to deposit good weld metal; the welding operator performance qualification is to show if the operator can operate the welding equipment.

3.10.8 Welder’s Performance Qualification The object of the welder’s performance qualification is to determine the ability of the welders to make sound welds. The welders may be qualified based on the results of the mechanical tests or by radiographic examination (RT) of the test coupon. For qualifying welders, only the essential variables (as applicable to welders’ skill) are considered. Any change in those variables requires requalifying the welder. In the case of welding machine operator qualifications, ability to operate the welding machine is tested. A welding machine operator is usually qualified along with a procedure test.

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FIGURE 3.18  Welding positions for qualification –​(a) welding positions for plates and pipes and (b) fillet weld in plate.

3.10.9 Welding Positions and Qualifications Welding positions is the arrangement of the welding part or welding test coupon based on its axis slope and face rotation relative to its horizontal or vertical plane. Normally, there are four types of welding positions namely horizontal, flat, vertical, and overhead. And the most common types of welds are groove and fillet welds. Welders can perform these two welds in all four positions. ASME Code has classified five welding positions for qualifications: flat, horizontal, vertical, overhead, and horizontal fixed. Some positions are shown in Figure 3.18 with designated notations for groove and fillet welds.

3.10.10 Weld Defects and Inspection of Weld Quality 3.10.10.1 Weld Defects (Discontinuities) In the correct sense of the word, defect is a rejectable discontinuity or a flaw of rejectable nature. A defect is definitely a discontinuity, but a discontinuity need not necessarily be a defect. Technically,

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discontinuities are not defects unless they exceed the limits set by the codes or standards have specified [69]. Discontinuities may be found in weld metal, heat-​affected zones, and base metal. 3.10.10.2 Causes of Discontinuities Defects are caused due to inferior design, poor workmanship, wrong welding procedure, poor weldability of the materials, etc. 3.10.10.3 General Types of Defects and their Significance Defects in weldments in general can be classified as follows: 1. Welding process defect: lack of fusion, incomplete penetration, porosity, slag, etc. 2. Foreign inclusions: slag, copper, oxide films, tungsten, etc. 3. Geometric defects: overlap, undercut, excessive reinforcement, burn through or excessive penetration, distortion, improper weld profile, etc. 4. Metallurgical defects: cracks, gas porosity, embrittlement, structural notches, hydrogen induced cold cracking, liquation cracking, solidification cracking, ductility-​dip cracking, stress relief cracking, lamellar tearing, low-​delta ferrite, etc. Planar defects, that is two-​dimensional (2D) defects, like cracks, lack of fusion, lack of penetration, severe undercut, etc. are critical in nature, involve lack of bonding, and are not tolerated. Three-​ dimensional (3D) defects, like slag inclusion, cavities, and pores, are tolerated to a certain extent, depending on the length of the inclusion and product class. Notches are not allowed for low temperature and cryogenic applications. Sharp notches, which act as stress raisers, are smoothed out wherever accessible to avoid stress concentration. Some weld defects that are schematically shown in Figure 3.19 and 3.20 show radiographic interpretation of welding defects. Faults in fusion welds, causes, and detection are given later. 3.10.10.4 Faults in Fusion Welds in Constructional Steels 1. Cracks: solidification cracking, hydrogen-​induced HAZ cracking, lamellar tearing, and reheat cracking; longitudinal, transverse, threat, crater, toe, and roof cracks. 2. Porosities: porosities include uniformly scattered porosity, cluster porosity, linear porosity, elongated porosity, or piping porosity, and wormhole porosity. 3. Solid inclusions: solid inclusions include linear slag inclusions and isolated slag inclusions, tungsten inclusions, or infused filler metal. 4. Lack of fusion and penetration: this type of defect tends to the subsurface and is therefore detectable only by ultrasonic or x-​ray methods. Lack of side wall diffusion, which penetrates the surfaces, could be detected using magnetic particle or dye/​fluorescent penetrant inspection. 5. Imperfect shape: detection: All shape defects are detectable by visual examination (VT). 6. Arc strike: accidental contact of electrode or weld torch with plate surface remote from weld. These usually result in simulated hard spots just beneath the surface, which tend to corrode and thus should be avoided. a. Miscellaneous faults like spatter is caused by the incorrect weld conditions or contaminated consumable or preparations to explosive conditions. Globules of molten metal are thrown out from the weld pool and adhere to the parent metal. b. Copper pickup is the melting of copper duct tube in the MIG weld due to incorrect conditions. In general, detection is possible only by x-​ray or ultrasonic techniques.

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FIGURE 3.19  Weld defects –​schematic. (Note: (a) gas porosity or worm holes, (b) distributed porosities, and (c) slag inclusion. Others are defined in the figure itself.)

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FIGURE 3.20  Radiographic interpretation of welding defects: (a) transverse/​longitudinal root crack, (b) slag inclusion, (c) oxide inclusion, and (d) porosity. (From NDT Education Resource Center, Developed by the Collaboration for NDT Education, Brian, F.L., ed., Center for Nondestructive Evaluation, Iowa State University, Ames, IA.)

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3.10.10.5 Approach to Weld Defect Acceptance Levels Two quality levels are proposed for weld defect acceptance [70]: 1. The first, the fitness-​for-​purpose level, is such that larger defects might cause failure of the structure and they must, therefore, be repaired. Suitable fitness-​for-​purpose analyzes the true significance of the defects in place of the old acceptance levels. Fracture mechanics assessment can be used to demonstrate that many defects are not significant in terms of service conditions and that continued safe operation can be assured [71]. 2. The second is a QC level. Repairs are not attended out where welds fall below this, but the cause of the loss of quality is examined so that it may be maintained well above the fitness for purpose level.

3.11 NONDESTRUCTIVE TESTING METHODS Nondestructive testing (NDT) is a testing and analysis technique used by industry to evaluate the properties of a material, component, structure or system, welding defects and discontinuities without causing damage to the original part. NDT also known as nondestructive examination (NDE), nondestructive inspection (NDI) and nondestructive evaluation (NDE). Without effective means of NDT, it would probably be impossible to build many of the major high-​integrity structures [71]. The most commonly used NDT methods are VT, dye penetrant testing (PT), magnetic particle testing (MT), radiography testing (RT), UT, acoustic emission testing (AET), eddy current testing (ET), and LT. Besides these, there are other NDT methods, such as radioscopy, thermal imaging, computer tomography, and holography, which are employed for special applications.

3.11.1 Scope of NDT NDT principally involves surface examination and volumetric examination, material identification and composition, quality characteristics of castings and forgings, welding defects such as cracks, voids and inclusions, porosities, lack of penetration, lack of fusion, lack of bond, undercut, laminations in rolling and forging, laminar inclusions and delamination in components and environmental assisted cracking such as hydrogen induced cracking and stress corrosion cracking, etc. which can be evaluated by NDT methods.

3.11.2 Destructive Testing (Mechanical Testing) In contrast to NDT, destructive testing causes damage to the test specimen. The purpose of destructive testing, also known as mechanical testing, is to reveal material properties when external forces are applied dynamically or statically. Important material properties of interest include tensile strength, elasticity, elongation, hardness, fracture toughness, fatigue, and resistance to impact. Common mechanical tests that provide information about those properties include tensile testing, compression testing, torque testing, bend testing, hardness testing, charpy impact testing, and shear testing

3.11.3 Nondestructive Testing Standards For list of ASTM standards on NDT methods, refer to [72]. 3.11.3.1 Codes and Standards NDT is often prescribed by codes and standards for the fabrication of components, safety critical parts, and in-​service equipment. Therefore, it is critical for all refinery, chemical plant, gas plant,

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and pipeline owners to have a thorough understanding and experience in the interdisciplinary field of NDT. 3.11.3.2 ASNT Standards ASNT SNT-​TC-​1A. This recommended practice establishes a general framework for a qualification and certification program for NDE technicians. In addition the standard provides recommended educational requirements and training requirements for different test methods. 3.11.3.3 ASTM’s Nondestructive Testing Standards ASTM’s nondestructive testing standards provide guides for the appropriate methods and techniques used to detect and evaluate flaws in materials and objects without destroying the specimen at hand. Detected flaws are evaluated for possible rejection due to nonconformance to set acceptance criteria. 3.11.3.4 Section V Nondestructive Examination Section V is a “service code” to other BPVC Sections, providing requirements and methods for nondestructive examination. It also includes manufacturer’s examination responsibilities, duties of authorized inspectors and requirements for qualification of personnel performing inspections and examination. Examination methods are intended to detect surface and internal discontinuities in materials, welds, and fabricated parts and components. 3.11.3.5 ASTM E1316-​21 Standard Terminology for Nondestructive Examinations This standard defines the terminology used in the standards prepared by the E07 Committee on Nondestructive Testing. These nondestructive testing (NDT) methods include: acoustic emission, electromagnetic testing, gamma-​and x-​radiology, leak testing, liquid penetrant testing, magnetic particle testing, neutron radiology and gauging, ultrasonic testing, and other technical methods. 3.11.3.6 AWS B1.10, 1999 –​Guide for the Nondestructive Examination of Welds This standard provides a reference guide for the kinds of nondestructive examination methods that are used to verify that welds meet the requirements of a code or specification. 3.11.3.7 AWS B1.10M/​B1.10:2016 Guide for the Nondestructive Examination of Welds

3.11.4 NDT Techniques There are several techniques used in NDT that can be categorized into conventional and advanced techniques [73]: 3.11.4.1 Conventional NDT Techniques Conventional methods are techniques that have matured over the course of decades and in this time, have become well-​documented in codes, standards, and best practices. The setup and procedure of a conventional technique is typically simpler in comparison to advanced methods. 3.11.4.2 Advanced NDT Techniques Generally, the setup, procedure, and data interpretation of advanced methods are more complicated and can require specialized understanding and experience from a properly trained technician. Advanced NDT (ANDT) method are much efficient than conventional NDT methods. These methods have great amount of automation which help in understanding and comparing the data of different reading. These data can be used for the future references and can be saved and stored. Advanced nondestructive testing (ANDT) inspection methods often require more advanced training

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and technician skill than traditional NDT inspections, but offer greater insight into asset damages [73]. Below is a partial list of ANDT inspection methods [74]: 1. Acoustic Pulse Reflectometry 2. Automated and Remote Ultrasonic Testing 3. Advanced Ultrasonic Backscatter Technique (AUBT) 4. Electromagnetic Acoustic Transducer (EMAT) 5. Alternating Current Field Measurement (ACFM) 6. Computerized Radiography 7. Digital Filmless Radiography 8. Digital Radiography (DR) 9. Radioscopy 10. Flash Radiography 11. Electromagnetic Testing (ET) 12. Electro-​Magnetic Acoustic Transducer (EMAT) 13. Guided Bulk Wave (GBW) 14. Guided Wave Ultrasonics Testing (GWT/​GUL) 15. Internal Rotary Inspection System (IRIS) 16. Infrared Thermography 17. Holographic Testing 18. Phased Array UT(PAUT) 19. Pulsed Eddy Current (PEC) 20. Time of Flight Diffraction (TOFD) 21. Magnetic Flux Leakage (MFL) 22. Laser Testing Methods (LM) 23. Laser Profilometry 24. Laser Shearography 25. Long Range Ultrasonic Testing (LRUT) 26. Near Filed Testing (NFT) 27. Phased Array Ultrasonics Inspection 28. Pulsed Eddy Current (PEC) 29. Remote Field Electromagnetic Testing (RFET) 30. Replication.

3.11.5 Nondestructive Testing Methods Examination Procedure –​General Requirements 1. Scope 2. General 3. Equipment and Materials 4. Calibration 5. Examination 6. Valuation 7. Documentation.

3.11.6 Qualification(s) There are many different aspects for qualification in NDT. The most common qualifications are: 1. procedures 2. inspection equipment

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3. inspection personnel 4. documentation.

3.11.7 Selection of NDT Methods There are five main parameters to be considered when selecting an inspection method: (1) anticipated type and size of discontinuity, (2) capabilities and limitations of the inspection method, (3) acceptance standards, (4) cost, (5) personnel qualification, and (6) equipment. 3.11.7.1 Capabilities and Limitations of Nondestructive Testing Methods In fabrication of heat exchangers and pressure vessels, NDT is commonly used for the detection of flaws in welds, forgings, castings, plates, tubes, etc. Each NDT method has its own flaw detection characteristics, and therefore no individual NDT method can replace another one. No single test or series of tests will give 100% assurance of quality. To use NDT technique effectively, one must be aware of the limitations of the different methods. NDT selection, applications, advantages, and limitations are tabulated in Refs. [6, 75–​82], among others, and a typical example is given in Table 3.3.

3.11.8 Acceptance Criteria For each NDT technique, acceptance criteria are an integral part of most codes and standards. Acceptance or rejection of flaws is based on different factors, and a vital few are (1) accessibility for repair and cost of repair, (2) safety, with hazards and consequences of failure, (3) desired life, (4) type of materials used, (5) material thickness, (6) design stress, and (7) nature of service environment (corrosive or noncorrosive).

3.11.9 Cost of NDT Different NDT methods have different costs in any particular situation. Two basic cost factors that should be considered in the selection of an NDT method are the initial equipment cost and the cost of performing the inspection [76]. The cost of performing NDT tests includes the labor cost, training cost, and cost of consumables such as radiographic film, dyes, magnetic particles, etc. VT is almost always the least expensive. In general, the costs of radiographic, ultrasonic, and eddy current inspections are greater than those of visual, magnetic particle, and liquid penetrant techniques. 3.11.9.1 The Economic Aspects of NDT [81, 82] 1. Direct Costs. The true costs of nondestructive testing include such obvious components such as inspection equipment, consumable materials, and employee time, etc. 2. Consumables costs. 3. Manpower costs. 4. Other Costs. Consideration must be given to the cost and time required for access, scaffolding, surface preparation, transport and insurance, etc. 5. Indirect Factors. In addition to the identified areas where the benefits of NDT can be quantified, and the substantial benefits from disaster prevention, there are some indirect benefits or costs that must be taken into consideration. 6. Cost of not doing NDT. The cost of not doing NDT in many situations would be colossal and even unbearable.

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TABLE 3.3 Comparison of Common NDE Methods Principle

Applications

Advantages

Limitations

Visual inspection

Visual inspection is normally performed by using naked eyes. Its effectiveness may be improved with the aid of special tools. Tools include fiberscopes, borescopes, magnifying glasses, and mirrors Liquid penetrant is drawn into surface defects by capillary action. Visible or fluorescent dye reveals flaws

To determine such things as the surface condition of the part, reinforcement, and undercutting of welds, alignment of mating surfaces, shape, or evidence of leaking Used for the detection of surface discontinuities such as cracks, seams, laps, cold shuts, laminations, and porosity that are open to surface

Cheapest NDT method, applicable at all stages of construction or manufacturing, do not require extensive training, capable of giving instantaneous results

Limited to only surface inspection. Require good lighting. Require good eyesight

Inexpensive, portable. Very sensitive. Independent of magnetic and electrical properties of material. Simple to perform

Magnetic particle

Detect surface and subsurface flaws and discontinues which distort applied magnetic field, causing leakage fields that attract iron power on surface. Flaws up to 1/​ 4 in. beneath a surface can be detected

Cracks, laps, voids, porosity, and inclusions and other discontinuities on or near surface of ferromagnetic materials

Inexpensive, equipments are portable, suitable for large, immovable objects. Equipment easy to operate. Provide instantaneous results, sensitive to surface and subsurface discontinuities

Ultrasonic

Flaws reflect sound waves traveling through material; elapsed time before echo is received determines locations of flaw. Computerized equipment can produce images of flaws (C-​scan)

Applicable for thickness measurement, detection of discontinuity, cracks, voids, porosity, inclusions, laminations, and delaminations, lack of bonding between dissimilar materials, etc.

Capable of detecting internal defect, can provide the size of discontinuity detected, very sensitive to planar type discontinuity, suitable for automation, equipment are mostly portable and suitable for field inspection, immediate results.

Limited to detection of surface breaking discontinuity. Not applicable to porous material, require access for pre-​and postcleaning, irregular surface may cause the presence of nonrelevant indication, penetrant must wet surface Applicable only to ferromagnetic materials. The sensitivity of this method decreases rapidly with depths below the surface being examined. Insensitive to internal defects. Require demagnetization of materials after inspection. Coating may mask indication. Material may be burned during magnetization Trained inspectors required, reference standards required. Require the use of couplant to enhance sound transmission. Require calibration blocks and reference standards. Require highly skillful and experience operator/​inspector. Not so reliable for surface and subsurface discontinuity

Liquid penetrant

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Method

Metals absorb x-​rays and gamma rays. Flaws and thin sections absorb less, so that more radiation strikes film. Flaws appear as dark shadows

Can detect internal discontinuities such as voids, porosity, inclusions, and cracks in castings and weldments in a wide range of materials, sizes, and shapes.

Eddy current

An alternating current is made to flow in a coil (probe), which, when brought into close proximity of the conducting surface of the material to be inspected, induces an eddy current flow in the material. Any discontinuity that appreciably alters the normal flow of eddy current can be detected by eddy current inspection Materials and structures/​components emit acoustic energy during crack growth and plastic deformation. Sensor detects noises due to crack growth

Conductive materials with constant cross section; especially pipe/​ tube inspection. Defects variations in metal type and microstructure as well as flaws

AE

Online monitoring of pressure vessels. Monitoring of weld overlays and thermal shock. Aircraft structures. Creeps. Welds defects. Stress corrosion cracking

Capable of detecting surface, subsurface, and internal discontinuities. Permanent record including evidence of proper test procedures. Defects flaws at any depth. Applicable to almost all materials. Many equipments are portable The inspection system can easily be automated, it is a noncontact method, equipment are portable and suitable for field application, some equipment are made dedicated for specific measurement (e.g. conductivity, crack depth, etc.). Fit for continuous online inspection

Radiation is hazardous to workers. Expensive method, incapable of detecting laminar discontinuities, some equipment are bulky, it needs electricity, require both film side and source side accessibility, results of film RT are not instantaneous –​it requires film processing, interpretation, and evaluation.

Pressurized-​vessel flaws are detected before failure. All loaded areas are tested, regardless of sensor location. AE monitoring on production lines is faster than ultrasonic

Test cannot pinpoint source of emission. Cannot detect defects that may be present and also that do not move or grow. This is not a quantitative technique but gives a qualitative assessment of the condition of the tank.

Often too sensitive to unimportant parameters or minor dimensional fluctuations, applicable only to conducting materials. If it is to be used for ferromagnetic material, the item must be magnetically saturated to minimize effect from permeability. Require highly skilled and experienced operator

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3.11.9.2 The Benefits of NDT for a Business NDT represents a small percentage of the overall costs of manufacturing. The benefits of NDT for a business include the following [83]: 1. Improved safety and accident prevention. 2. Reduced downtime. 3. NDT keeps assets up and running while testing procedures are under way. 4. NDT can be used to ensure the highest levels of quality, right from the raw materials stage through to the finished product. This ensures consistent product reliability and compliance with client and industry requirements. 5. Cost savings. 6. Mitigates environmental risk. 7. Enables regulatory compliance.

3.11.10 Personnel NDT tends to be rather dependent for its effectiveness on the capabilities of the persons performing it. The first requirement for good NDT is the properly trained and experienced personnel. NDT personnel must be able to evaluate and record test results with accuracy and consistency. Poorly run tests or incorrect interpretation of the results leads to rejection of good parts and acceptance of defective parts [84]. 3.11.10.1 NDT Personnel Qualifications In order to promote uniformity in performing and interpreting NDT tests, the American Society for Nondestructive Testing has drawn up some minimum qualification requirements for personnel, who are graded as Level I, Level II, and Level III. Each NDT level has its own requirements. 3.11.10.2 Level I, II and III Qualifications as per SNT-​TC-​1A-​2020 Level I Qualified to perform specific calibrations, specific NDT, specific evaluations, and record results. Should receive specific instruction or supervision from a NDT Level II or III. Level II Qualified to set up and calibrate equipment and interpret and evaluate results as per applicable codes, standards and specifications. Should be familiar with technique limitations. Organize and report the results. Should exercise assigned responsibility for on-​the-​job training and guidance of trainees and Level I personnel. Level III Qualified to develop, qualify and approve procedures, establish techniques, interpreting codes and standards. Should have sufficient practical background in applicable materials, fabrication and product technology. Should be capable of training and examining Level I and II personnel. 3.11.10.3 Training of NDT Personnel The manufacturer may conduct a training program to meet the requirements of SNT-​TC-​1A through a designated employee who meets Level III requirements. Qualification and training of NDT personnel are discussed in detail in Ref. [9].

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3.11.11 Inspection Equipment Only high-​performance flaw-​detecting equipment should be used. NDT equipment is to be handled properly in order to assure continued accuracy of examination. The measuring instruments, tools, and gauges are to be calibrated periodically with reference standards.

3.11.12 Reference Codes and Standards ASME Code Section V is the most widely used code for NDT throughout the world. The basic coverage of ASME Code Section V on nondestructive examination is given here. 3.11.12.1 ASME Code Section V: Nondestructive Examination This section contains requirements and methods for nondestructive examination, which are referenced and required by other code sections. Also included are manufacturer’s examination responsibilities, duties of authorized inspectors, and requirements for qualification of personnel, inspection, and examination.

3.11.13 NDT Symbols NDT symbols are also used on the engineering drawing to specify the method of examination, NDT locations, number of examinations, partial examination, etc. NDT symbols may be combined with welding symbols by using an additional reference line or by specifying the test method in the tail of the welding symbol. Figure 3.21 depicts the AWS symbols for NDT examination and Figure 3.22 shows some examples of NDT symbols.

FIGURE 3.21  AWS symbols for NDT examination and standard location of elements.

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FIGURE 3.22  Some examples of NDT examination symbols.

AWS A2.4-​98 –​Standard Symbols for Welding, Brazing and Nondestructive Examination. Type of test Symbol: Acoustic emission AET, Electromagnetic ET, Leak LT, Magnetic particle MT, Neutron radiographic NRT, Penetrant PT, Radiographic RT, Ultrasonic UT, and Visual VT. 3.11.13.1 Specify NDT Locations To specify a number of examinations to be conducted on a joint or part at random locations, the number of required examinations shall be placed in parentheses either above or below the letter designation away from the reference line. NDT symbols may be combined with welding symbols by using an additional reference line or by specifying the test method in the tail of the welding symbol.

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3.11.14 Written Procedures NDT should be carried out in accordance with written procedures to achieve consistency and accuracy. Without a written procedure, there is no guarantee as to the effectiveness or repeatability of the test. Detailed NDT procedures/​instructions are essential for several reasons [2]: (1) they demonstrate that a fabricator fully understands code and standard, and contractual requirements; (2) they provide comprehensive instructions at the place of test and mitigate against difficulties arising from different operator/​inspection authority interpretation; and (3) facilitate surveillance and constitute a record of how testing was carried out. 3.11.14.1 Content of NDT Procedures NDT should be carried out in accordance with written procedures irrespective of whether or not it is a code requirement [2]. A procedure usually specifies items given in Table 3.4 [85]. 3.11.14.2 General Details of Requirements in the NDT Procedure Document NDT procedures should not be allowed to become lengthy “training manuals” for inexperienced personnel or “educational texts” for the benefit of the customer [2]. However, they must contain sufficient detail to ensure effective examination of the material or component under consideration [86, 87]. 3.11.14.3 Deficiencies in NDT Procedures The most common deficiencies in NDT procedures fall into three main categories [2]: 1. They lack detail and simply state “refer to” BS Specifications, ASME Code, and so on, in relation to calibration standards, acceptance standards, etc. They fail to identify which of the options contained in the specifications or codes apply to the component under inspection. 2. The method is identified correctly, but the techniques and test parameters specified are not specifically tailored to the actual part to be examined. 3. They omit technical requirements specific to the particular procedure.

TABLE 3.4 Content of NDT Procedures 1. Scope 2. Applicable documents such as codes and standards 3. Inspecting personnel 4. Inspection equipment –​calibration and reference standards 5. Surface preparation 6. Test method/​techniques 7. Evaluation of indications and reporting 8. Acceptance criteria 9. Report 10. Records Source:  Adapted from [85] Hamlet, R.A. and Lavender, D.H.

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3.11.15 Auditing the NDT Procedures Auditing will make sure that the correct NDT methods and techniques are being implemented and executed within quality department [88]: 1. Ensuring compliance with applicable industry codes, standards and regulatory requirements. 2. Assessing the correct NDT techniques are being used. 3. Ensuring the correct NDT test pieces are in place. 4. Checking staff are following NDT test procedures. 5. Verification of personnel’s NDT qualifications.

3.11.16 Third-​Party Inspection in Nondestructive Testing By drafting precise inspection plans, third-​party NDT companies can help utilities enhance maintenance plans, prolong asset life spans, and streamline operations. Third party inspection services include: 1. auditing of inspection facilities 2. validation of inspection procedures and personnel 3. witnessing of inspection processes. Some of the third party inspection services agencies are given below: Applus+​ A-​Star Testing & Inspection Pte Ltd, Singapore A-​Tech N.D.T. Limited, Alberta, Canada Ashtead Technology Ltd. Dacon Inspection Technologies Eddyfi Fischer Technologies Inc. General Electric Mistras Group, Inc. Magnetec Inspection Inc., Chicago, Illinois 20/​20 Ndt Inc GRANDE Prairie, Alberta, Canada Intertek’s global services Ndt Global Inc, Canada Nikon Metrology Inc. Olympus Corporation, Japan Sonatest Ltd., England TWI Global, UK Welding Inspection Services Ltd, United Kingdom YXLON International GmbH Zetec Inc., Washington.

3.11.17 Discontinuities The goal of nondestructive examination is to identify anomalies or irregularities for evaluation; irregularities as distinguished from the overall examination area. A discontinuity is not necessarily a “defect”. It defines a defect as “a discontinuity or discontinuities that by nature or accumulated effect render a part or product unable to meet minimum applicable acceptance standards or specifications”.

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Discontinuities may be found in the weld metal, heat-​affected zones, and base metal zones of weldments made in the five basic weld joint types: butt, T-​, corner, lap, and edge. 3.11.17.1 Defect Detection Defect detection techniques fall into two categories: 1. those that can only detect defects on or near to the surface of a component (surface techniques) 2. those which can detect both surface and embedded defects (volumetric techniques). 3.11.17.2 Surface Techniques 1. Dye Penetrant Inspection (PT) 2. Eddy Currents 3. Magnetic Particle Inspection (MPI or MT). 3.11.17.3 Volumetric Techniques 1. Radiography 2. Ultrasonics. 3.11.17.4 Defect Detection Capability The types of defect /​flaw and degradation that can be detected using NDT are summarized as [89]: 1. Planar defects –​these include flaws such as fatigue cracks, lack of side-​wall fusion in welds, environmental assisted cracking such as hydrogen cracking and stress corrosion cracks; cold shuts in castings etc. 2. Laminations –​these include flaws such as rolling and forging laminations, laminar inclusions and de-​laminations in composites. 3. Voids and inclusions –​these include flaws such as voids, slag, and porosity in welds, and voids in castings and forgings. 4. Wall thinning –​through life wall loss due to corrosion and erosion. 5. Corrosion pits –​these are localized and deep areas of corrosion. 6. Structural deformities such as dents, bulges, and ovality.

3.12 VISUAL EXAMINATION Visual Inspection (VI), or visual testing (VT) is the primary evaluative method of inspection, and it is the oldest method of NDT. Visual methods are of necessity restricted to surface examination, but they may sometimes be useful for a detailed examination of an imperfection after other methods have determined its position [90]. In addition to locating surface defects and raw material identification, VT can be an excellent process control technique during various stages of fabrication and can identify subsequent problems revealed during PWHT and hydrostatic testing/​LT. VT requires three basic conditions to be in place. These are: good vision, to be able to see what you are looking for good lighting, the correct type of light is important experience, to be able to recognize problems.

3.12.1 Importance of Visual Inspection The criticality of inspection in manufacturing and production becomes evident when the potential consequences of missed defects are examined. In some cases, missed flaws can have serious

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consequences ranging from injury to fatality. In general, there are five basic steps that must be completed during a visual inspection task [91]: 1. Set up: obtain procedures or work instructions, items to be inspected, and equipment needed for inspection. 2. Present: prepare item for inspection (e.g. by installing in holding fixture or microscope). 3. Search: examine item for possible defects. 4. Decide: compare potential defect against standards to determine if it exceed the standards. 5. Respond: accept or reject the item based on the decision made in Step 4, mark the item as needed, and complete required paperwork.

3.12.2 Parameters that Impact Inspection Performance The factors that impact inspection performance fall into the following categories [91]: 1. Task Factors. Task factors involved in inspection are associated with the physical nature of the inspection task. 2. Environmental Factors. Environmental factors that impact inspection performance include physical components such as lighting, noise, temperature, and the design of the workplace. 3. Individual Factors. Individual factors refer to physical, mental, and personality characteristics of the inspector such as age, intelligence, and extraversion. 4. Organizational Factors. Organizational factors refer to the larger structural, administrative, and political environment in which the inspection tasks occur.

3.12.3 Principle of Visual Testing The basic principle used in visual NDT is to illuminate the specimen with light, usually in the visible region. The specimen is then examined with the eye under adequate illumination, either naturally or by light-​sensitive devices such as photocells and phototubes. The VT is classified as direct vision technique and remote VT.

3.12.4 Direct Vision Examination Direct vision technique is applicable when access is sufficient to place the eye within 24 in. (609.6 mm) of the surface to be examined and at an angle not less than 30° to the surface to be examined. Mirrors as well as aids such as magnifying lens may be used to assist examination.

3.12.5 Remote Visual Examination Remote VT is conducted with aids such as mirrors, telescopes, borescopes, fiber optics, cameras, and other suitable equipment. These optical aids supplement direct vision.

3.12.6 Translucent Visual Examination Translucent visual examination is a supplement of direct visual examination. The method of translucent visual examination uses the aid of artificial lighting, which can be contained in an illuminator that produces directional lighting.

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3.12.7 Merits of Visual Examination VT is easily applied, quick, and often requires no special equipment other than good eyesight and some relatively simple and inexpensive equipment. Optical aids such as illuminators, mirrors, borescopes, etc. can be used to enhance one’s capability of visually inspecting equipment. Cameras, computer systems, and digital image analyzers can also be used to further the capabilities and benefits of visual inspection.

3.12.8 Visual Testing Written Procedure Written procedure for VT is detailed in ASME Code Section V. The written procedure should contain at a minimum content as shown in Table 3.5.

3.12.9 Reference Document Codes and standards or the purchaser’s specification is needed. An important document for visual inspectors is the AWS Guidebook on Visual Examination [64] and ASME Code Section V.

3.12.10 Visual Examination: Prerequisites As with any other nondestructive inspection method, there are various prerequisites that should be considered to perform VT. Some of the more common attributes to consider are [64] the following: 1. The visual examiner should have sufficient visual acuity to perform an adequate inspection. 2. Visual inspectors should have sufficient knowledge on welding procedures and safety practices. There are many potential safety hazards present. 3. Much of the success of visual inspection depends on the surface condition and the lighting arrangements, i.e. the inspector should have adequate illumination, either natural or artificial.

TABLE 3.5 Written Procedure for VT 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Essential Variable Nonessential Variable Personnel qualifications Product forms (pipe, plate, forgings, etc.) to be inspected How VT is to be performed, i.e. sequence of examination Surface condition and criteria for surface cleaning Surface cleaning procedures including tools to be used Method or tool for surface preparation, if any Lighting intensity Whether direct or remote viewing is used Special illumination or optical devices to be used, if any Sequence of performing examination when applicable Data to be tabulated, if any Reports or general statements to be completed.

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3.12.11 Visual Examination Equipment Certain tools are sometimes necessary for some aspects of visual weld inspection. Various measuring tools are used for checking the dimensions of the welds. There are many different types of fillet weld gauges used to determine the size of fillet welds. Other gauges can be used to verify root opening, weld reinforcement, and weld bevel angle. Measuring devices are used to check root openings, clearance dimensions of backing materials and alignment and fit-​up of the workpieces. Temperature indicators verify preheat and interpass temperature. These tools are specified in Refs. [64, 92]. Some of the tools and gauges most frequently used in visual welding inspection are the following: 1. ammeters to measure current during welding or inspection by MT 2. temperature-​sensitive crayons for surface markings 3. surface contact thermometers and pyrometers 4. weld gauges, measuring scales, fillet gauges, and devices 5. multipurpose gauges capable of performing many measurements such as measuring convex and concave fillet welds, weld reinforcement, and root opening 6. taper gauge to measure root opening (gap) 7. hi-​lo gauge, also known as a mismatch gauge, to measure the internal alignment of a pipe joint 8. fiber-​optic devices such as borescopes, fiberscopes, etc. and video probes 9. Ferrite gauges.

3.12.12 Visual Testing Technique Applications Visual inspection is used to detect defects and verify quality in four primary areas: 1. dimensional quality 2. surface quality 3. correct assembly 4. accuracy or correct operation. Visual examination is generally used to determine the surface condition of the part, alignment of mating surfaces, shape, or evidence of leaking. In addition, visual examination is used to determine a composite material’s subsurface conditions.

3.12.13 NDT of Raw Materials VT of the base materials, such as plates, tubes, pipes, forging, and castings, prior to fabrication can detect scabs, seams, loose rusts, scale, or other harmful surface conditions that tend to cause weld defects. Plate laminations may be observed on cut edges. Dimensions should be confirmed by measurements. Base metal should be identified by type, grade, and heat number. Even though plates and forged pressure parts are ultrasonically tested by material suppliers, these products for critical applications warrant ultrasonic inspection in the manufacturer’s premises.

3.12.14 Visual Examination During Various Stages of Fabrication by Welding The effectiveness of visual inspections is improved when a QC system is instituted that provides for coverage at all phases of the welding process –​before, during, and after welding. Visual inspection used before, during, and after welding finds 90% of defects that would be detected later using more costly methods such as UT or radiography [93]. The items to be checked by visual inspection prior to welding, during welding, and after welding are detailed in Refs. [64, 76]. Some of the important points are given next.

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3.12.14.1 Visual Examination Before Welding Review drawing and specifications check machine settings, qualification of welding procedures and personnel, joint preparations, dimensions, cleanliness, and surface preparation, review welding process and consumables to be used, establish checkpoints and check fit and alignment of weld joints: groove angle, root openings, joint alignment, etc. 3.12.14.2 Visual Examination During Welding Treatment of tack welds, quality of the root pass and the succeeding weld layers, joint root preparation prior to welding the second side, preheat and interpass temperatures, sequences of weld passes, interpass cleaning, checking the welding procedure parameters, that is voltage, amperage, and speed, parameters pertaining to shielding gases and distortion. 3.12.14.3 Visual Examination After Welding This covers final weld inspection like (1) weld size, appearance, color, contour, and surface roughness; (2) extent of welding; (3) dimensions; (4) distortion; and (5) visible external weld defects such as cracks, undercut, overlap, exposed porosity and slag inclusions, unacceptable weld profile, arc strike, weld spatter, reinforcement, concavity, and burn through.

3.12.15 Developments in Visual Examination Optical Instruments 3.12.15.1 Remote Visual Inspection Remote visual inspection (RVI) is a nondestructive testing (NDT) technique that uses visual aids to inspect areas of infrastructure from a distance that are too dangerous, remote, or inaccessible for direct visual inspection. RVI technologies include remotely operated cameras, borescopes, videoscopes, and fibroscopes. Special TV cameras are available that can be inserted into openings as small as 1.5 in. (38.1 mm) diameter [94], and a closed-​circuit transmission system can then be arranged to give an image of the internal surface of butt welds in pipelines and bores [90]. The three basic remote VT aids are borescopes, fiberscopes, and video borescopes. One of the leading visual inspection equipments/​aids providers is Olympus NDT Inc., Waltham, MA (www.olym​pus-​ims. com/​en/​rvi-​produ​cts/​). In recent years, unmanned aerial vehicles (UAVs), commonly known as drones, have seen increased adoption and usage for remote visual inspections of structures that are difficult to reach by traditional means, such as flare stacks, elevated pipe trays, and cooling towers. For long-​term traceability of inspection results, a good archive is necessary. One particular variety of RVI system, the videoscope is well-​suited to capturing, storing, and archiving everything found in an inspection. A videoscope is a versatile system equipped with an interface with a viewing screen and control system, out of which comes an insertion tube. The insertion tube is long and thin (typically less than 0.5 in.) and contains the camera and lens system. The camera sends the video signal through the insertion tube to the screen, where the inspector views it [95]. 3.12.15.2 Borescopes Borescopes, video scopes, flashlights, and mirrors are used in areas of limited accessibility. Flexible fiber optic examination systems enable the inspector to perform remote visual examination of some areas that are not accessible for direct visual examination or to rigid borescopes. Basics of borescopes. As the name implies, a borescope is an optical instrument designed to enable an observer to inspect the inaccessible areas such as inside of a narrow tube, bore, or a chamber. One can insert them into very small openings, extending the vision to welds far inside the darkest

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recess in process piping, heat exchangers, pressure vessels, and other equipment. Illuminating the weld and magnifying it 3× or 4×, borescopes are easy to use, relatively inexpensive, and extremely effective [96]. Flexible fiberscopes. Flexible fiberscopes, in contrast to the stiff borescope, can be inserted into curved pipes and cavities. The light in the fiberscopes is transmitted via a bunch of ultrathin optical fibers with a diameter as small as 7 μm (0.007 mm). A photo of an industrial fiberscope is shown in Figure 3.23a. Stiff borescopes. The objective and the ocular are connected by means of one or more removable extension tubes. The borescope’s length can thus be varied as required. Where straight-​line access is available, rigid borescope offer many advantages over fiberscopes, most notably in resolution, image brightness, design flexibility, and price. A photo of a rigid borescope is shown in Figure 3.23b.

FIGURE 3.23  Remote VT aids. (a) Industrial fiberscope, (b) rigid borescope, and (c) industrial videoscope. (Courtesy of Olympus NDT Inc., Waltham, MA.)

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3.12.15.3 Video Imagescopes Videoimagescopes employ video technology for RVI. At the tip of the flexible videoimagescope probe is a miniature charge-​coupled device (CCD) sensor (like a tiny video camera), which sends high-​resolution images in full, natural color to be displayed on a TV monitor. Diameters of probes are as small as 8 mm; lengths range from 1.5 to 22 m. A photo of an industrial videoscope is shown in Figure 3.23c. 3.12.15.4 Video Microscopes Handheld or fixtured video microscopes see where traditional microscopes cannot. They feature high-​resolution CCD image sensors, fiber-​optic self-​illumination, special objective lenses or HI-​ Mag borescopic lenses, shutter speeds to 1/​10,000 s, freeze frame, and memory, with multiwindow display of 4, 9, 16, or 25 images simultaneously. They can overlay images for comparison or even measurement. Nominal magnifications are from 20× to 1000×. 3.12.15.5 Combining Computers and Visual Inspection Combining visual inspection with computer analysis eases job tracking and planning by storing information and radiographs in databases. Computers that collect and analyze data, update standards, and expand certification programs enable the visual inspector to concentrate on data collection, leaving tedious calculations to the computer [93]. 3.12.15.6 High-​Speed Video High-​speed video uses digital cameras that capture events too fast for the eye to see by recording a very large number of images within a short period of time and then playing them back slowly. High-​speed visual inspection with automated output is used for the inspection of the surface of sheet material and television techniques may use enhanced image and pattern recognition methods. Remote photography of inaccessible surfaces, such as inside a radioactive environment, is also possible. High-​speed cine is also used for studying fast events. Arrays of optical diodes can be used instead of television cameras [97].

3.13 LIQUID PENETRANT INSPECTION 3.13.1 Principle Liquid penetrant inspection (PT) is a means to find discontinuities that are open to the surfaces by bleedout of liquid penetrant medium that is applied over the test surface. The penetrant enters the discontinuities under the influence of capillary action. If the discontinuity is significant, penetrant will be held in the cavity when the excess is removed from the surface. Upon application of a developer, blotter action draws the penetrant from the discontinuity to provide a contrasting indication on the surface, indicating the presence and location of discontinuities. The indications are examined either in natural daylight or in adequate artificial illumination or ultraviolet light, depending on colored or fluorescent penetrant particles used. Various stages of the PT process are depicted in Figure 3.24. For generic information on PT examination refer to Ref. [98].

3.13.2 Techniques Either a color contrast (visible) penetrant or a fluorescent penetrant shall be used with one of the following three penetrant processes: 1. water washable 2. post-​emulsifying 3. solvent removable.

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FIGURE 3.24  PT. (a) Schematic and (b) stages of PT.

The visible and fluorescent penetrants used in combination with these three penetrant processes result in six liquid penetrant techniques.

3.13.3 Applications Liquid penetrant inspection can be effectively used to detect surface discontinuities such as cracks, porosity, seams, laps, cold shuts, laminations, or lack of bond on nonporous metallic materials, both ferrous and nonferrous, and on nonporous nonmetallic materials such as plastics and glass. The method is particularly useful on nonmagnetic materials, since magnetic particle inspection (MT) cannot be applied to test them.

3.13.4 Merits of PT Its simplicity and low cost make it a popular test. If properly used, it readily detects minute surface openings. The basic steps involved in the application of the test method are relatively simple. Except for VT, it is perhaps the most commonly used nondestructive test for surface examination. Penetrant inspection is reasonably fast. The method is easy to learn and can be applied properly.

3.13.5 Limitations The success of penetrant inspection, like most other inspection methods, depends on the visual acuity of the inspector [76]. The parts must be thoroughly cleaned to allow the penetrant to enter flaw openings, and flaws must be open to the surface [99]. Experience has shown that very slight variations in performing the penetrant process and subsequent inspection can invalidate the findings by failing to indicate all flaws [100]. Therefore, it has become accepted practice that all penetrant inspections be carried out exclusively by trained and certified personnel. It is not suitable for inspecting porous materials.

3.13.6 Written Procedure A written procedure for PT examination is given in Table 3.6.

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TABLE 3.6 Written Procedure for PT 1.0 2.0 3.0 4.0 5.0 6.0 7.0 7.1 7.2 7.3 7.4 8.0 9.0 10.0

Scope: materials, shapes, and sizes to be examined Reference document Extent of examination Approved method and material: type of penetrant, penetrant removal, emulsifier, and developer Surface preparation Test environment and lighting condition Procedure Penetrant application, dwell time, and the testing of the surface and penetrant during the examination if outside is 60°F–​125°F Excess penetrant removal Developer application Examination Acceptance criteria Personnel certification List of approved penetrant materials

Liquid penetrant examination procedure deficiencies include incorrect penetrant material selection, excessive penetrant removal, and incorrect developer application [2].

3.13.7 Standards ASTM E 165/​165M-​2018 –​Recommended practice for PT. ASTM E433-​71(2018) –​Reference photographs for PT. ASME Section V, Articles 6 and 24. ASME Section VIII, 1 Div., Appendix 8.

3.13.8 Test Procedure Six basic steps make up the liquid penetrant inspection procedure. They are [101] the following: 1. pre-​cleaning the part to be inspected with a solvent and drying 2. application of the penetrant to the part to form a film over the surface and allowing sufficient time for it to enter into the opening 3. removal of excess penetrant with water wash, solvent, or emulsifier and drying 4. application of a thin coating of developer over the surface under observation 5. VT under adequate light and interpretation of indications 6. post inspection cleaning.

3.13.9 Penetrants The penetrants are mixtures of organic solvents, which are characterized by their ability to wet materials, spread rapidly, and penetrate into minute defects and dissolve dyes, so that the indications produce a definite red color as contrasted to the white background of the developer.

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3.13.9.1 Types of Penetrants Depending upon the type of dye used, penetrants are divided into visible penetrants, which are usually red in color to provide a contrast against the white background from the developer under white light, and fluorescent penetrants, which provide a greenish yellow indication against a dark background when viewed under a black light (ultraviolet). The fluorescent penetrants are most generally used for the detection of fine cracks, but these are not recommended for use on rough surfaces because of the difficulty in removing the excess penetrant. While selecting chemicals, only those chemicals that are compatible with the base material are to be chosen. According to the manner in which the excess penetrants are removed from the surface, that is the rinse process, they are classified into three classes: (1) water washable, (2) solvent removable, and (3) postemulsifiable, which are not in themselves water washable, but are made so by applying an emulsifier after the penetration is completed.

3.13.10 Method There are two varieties of penetrant method, both using a similar principle: the visible dye method and the fluorescent dye method.

3.13.11 Selection of Developer The developer is applied to get the penetrant in the discontinuities back to the surface so that it can form an indication of the discontinuity. The penetrant is drawn out by the capillary action but in the reverse direction. It also serves as a color contrast background for dye material.

3.13.12 Penetrant Application The penetrant should be applied by brushing or spraying. The surface temperature of the part should be between 60°F and 125°F (16°C and 52°C). 3.13.12.1 Penetration Time or Dwell Time The period of time during which the penetrant is permitted to remain on the specimen is a vital part of the test. The minimum time required for the penetrant to enter into the discontinuity is called the dwell time. The recommended dwell times are as follows: • for carbon steel 10–​20 min • for alloy and stainless steel 15–​20 min. During the dwell time, the penetrant shall not be allowed to dry up, and fresh penetrant shall be applied as often as required. 3.13.12.2 Approved Material Penetrant must be odorless, nontoxic, nonflammable, and stable in storage. The developer should be wet and nonaqueous. The penetrant and developer must be compatible with each other and should be procured from the same manufacturer. For the examination of nickel-​based alloys, sulfur content in the penetrant shall not exceed 1% of the residue by weight, whereas for the examination of the austenitic stainless steels or titanium, the chlorides shall not exceed 1% of the residue by weight.

3.13.13 Surface Preparation The surface to be inspected shall be clean and free from oil, grease, sand, rust or scale, welding flux and spatter, etc. The presence of surface contaminants prevents spreading of the penetrant and affects

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penetrability mechanical operations such as shot peening, machining, buffing, and sand blasting are strictly prohibited prior to the examination to avoid closing of discontinuities. Weld surface shall be ground free of ripples that might mask the indications of unacceptable discontinuities. Sufficient air circulation is necessary during testing.

3.13.14 Excess Penetrant Removal Excess solvent-​removable penetrants shall be removed by wiping with a cloth or absorbent paper, repeating the operation until most traces of penetrant have been removed. The remaining traces shall be removed by lightly wiping the surface with cloth or absorbent paper moistened with solvent. To minimize removal of penetrant from discontinuities, care shall be taken to avoid the use of excess solvent.

3.13.15 Standardization of Light Levels for Penetrant and Magnetic Inspection Proper illumination of indications from PT and MT is essential if an acceptable level of probability of detection is to be achieved with either test method. In order to control the illumination for inspection of workpieces, four separate types of measurement must be specified precisely [102]. These are: 1. white light illumination for color contrast processes 2. ultraviolet illumination for fluorescent processes 3. ambient visible light in darkened inspection rooms 4. visible light from ultraviolet sources. The fluorescent light is more sensitive, due to the fact that the human eye can more easily discern a fluorescent indication.

3.13.16 Evaluation of Indications Prior to the evaluation of test results, it is necessary to interpret the indications. Identifying indications requires much practice. Indications can be classified as (1) false indications, (2) nonrelevant indications, and (3) true or relevant indications. Only those indications are to be studied while evaluating the results of the examination.

3.13.17 Acceptance Standards For welds and materials as per ASME Section VIII Div. 1, Appendix 8, all surfaces to be examined shall be free of (1) relevant linear indications, (2) relevant rounded indications greater than 3/​16 in. (4.8 mm), and (3) four or more relevant rounded indications in a line separated by 1/​16 in. (1.6 mm) or less edge to edge.

3.13.18 Postcleaning Penetrants are difficult to remove completely from discontinuities, and if they are corrosive to the material, or otherwise not compatible with the product application, they should be avoided.

3.13.19 Developments in PT PT accessories are available from Magnaflux, a Division of ITW, Inc., USA. They include Ni-​ Cr test panels (Japanese) and TAM panels. Ni-​Cr test panels are used for evaluating penetrant system performance and sensitivity; they are available in four types of flaw depths, 10, 20, 30, or

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50 μm. A TAM panel is a fast and reliable means of monitoring the proper functioning of a liquid penetrant system. A stainless-​steel panel with five star-​shaped flaws of decreasing size is on one half, with a rough sand-​blasted surface on other half, for checking sensitivity and removability of penetrants.

3.14 MAGNETIC PARTICLE INSPECTION Magnetic particle inspection MT or MPI is an NDT technique to locate surface and subsurface discontinuities such as cracks, inclusions, pores, shrinkages, laps, folds, seams, etc. arising out of manufacturing operations and service constraints in ferromagnetic materials. In principle, this method involves magnetizing an area to be examined, and applying ferromagnetic particles (the examination’s medium) to the surface. MT is also used for testing of weld preparations, weld cutbacks on double-​sided welds, roots of welds made from one side only, stub, branch, and nozzle welds and attachment welds to shell, load-​carrying non-​pressure parts like supports and lifting lugs, and areas where temporary attachments have been removed. The latter areas are particularly vulnerable to cracking [103].

3.14.1 Principle MT is based on the principle that if a ferromagnetic object is magnetized, the surface or subsurface discontinuities lying at an angle to the magnetic lines of force cause an abrupt change in the path of magnetic flux flowing through the object (Figure 3.25). This results in local flux leakage at the surface over the discontinuity. When finely divided ferromagnetic particles are applied over the surface, some of the particles will be attracted to the regions of leakage field and will pile up and bridge the discontinuity. The piling up of magnetic particles along the discontinuity indicates its location, shape, and extent (Figure 3.25a). While using this method, it should be seen that the magnetic field must be in a direction that will intercept the principal plane of the discontinuity (Figure 3.25b). Whichever technique is used to produce the magnetic flux in the part, maximum sensitivity will be to linear discontinuities oriented perpendicular to the lines of flux. For optimum effectiveness in detecting all types of discontinuities, each area is to be examined at least twice, with the lines of flux during one examination being approximately perpendicular to the lines of flux during the other.

3.14.2 MT Techniques There are many different techniques and combinations of techniques of MT. The ASME Boiler and Pressure Vessel Code, Section V, Article 7, recognizes five different techniques of magnetization: 1. prod technique 2. longitudinal magnetization technique 3. circular magnetization technique 4. yoke technique 5. multidirectional magnetization technique. There are two different ferromagnetic examination media: dry particles and wet particles. Both forms can be either fluorescent or non-​fluorescent (visible, color contrast) and come in a variety of colors to contrast with the tested material. Two of the most-​used methods are the stationary horizontal system, using longitudinal and circular magnetization techniques, and the very portable yoke technique.

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FIGURE 3.25  MT principle –​(a) disruption of local flux leakage at the surface over discontinuity, (b) magnetizing current and crack detectability and (c) and (d) magnetising current and crack indications.

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3.14.3 Reference Documents 1. ASTM specifications E 709-​2021 –​Standard Guide for Magnetic Particle Testing. This guide describes the use of the following magnetic particle method techniques: a. dry magnetic powder b. wet magnetic particle c. magnetic slurry/​paint magnetic particle d. polymer magnetic particle. 2. E 1316-​2022 –​Standard Terminology for Nondestructive Examinations. The nondestructive testing (NDT) methods include: acoustic emission, electromagnetic testing, gamma-​and X-​ radiology, leak testing, liquid penetrant testing, magnetic particle testing, neutron radiology and gauging, ultrasonic testing, and other technical methods. 3. ASME Section V, Article 7, and ASME Section VIII, Div. 1, Appendix 6 and 7 (for steel castings) are also used.

3.14.4 Test Procedure There are several basic steps in applying MT: 1. Generally inspect the part and review the testing criteria. 2. Surface preparation-​Clean the part well. 3. Magnetization of the test object under evaluation. 4. Application of magnetic particles. 5. Inspect for indications, interpretations of the patterns formed. 6. If required, demagnetization of the items being tested.

3.14.5 Factors Affecting the Formation and Appearance of the Magnetic Particles Pattern There are many factors that can affect the formation and the appearance of the magnetic particles on the surface of the component being tested. The most common include [101, 104] (1) discontinuity size, shape, distance below the surface, and its orientation; (2) direction and strength of the magnetic field; (3) method of magnetization employed; (4) intensity of current used; (5) size, shape and mobility of the magnetic particle and the method of application; and (6) shape of the component.

3.14.6 Merits of Magnetic Particle Inspection For detection of surface defects, liquid PT and MP are being very widely used. In the case of ferromagnetic materials, magnetic particle technique will also detect subsurface flaws that are not open to surface. Important advantages of the MT method are [101] (1) it is rapid and simple to operate, (2) indications are produced directly on the surface of the component, and (3) a skilled operator can make a reasonable estimate of crack depth with suitable magnetic powders and proper technique.

3.14.7 Limitations of the Method MT has certain limitations, like (1) the method is not applicable for nonmagnetic metal, (2) the sensitivity to detect subsurface flaw decreases exponentially with distance below the surface, (3) exceedingly heavy currents are required at times to examine very large units, and (4) current imparted by electrodes to induce magnetic fields can cause burns, which must be avoided because they can be a source of corrosion attack.

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TABLE 3.7 Written Procedure for MT 1.0 2.0 3.0 4.0 5.0 6.0 7.0 7.1 8.0 9.0 10.0 11.0 12.0 13.0

Scope: materials, shapes, and sizes to be examined Reference document Magnetizing technique Examination equipment Surface preparation Examination medium: type of ferromagnetic particles to be used, manufacturer, color, wet or dry powder, etc. Magnetization current (type and amperage) Magnetizing field adequacy Application of examination medium Evaluation of indication Acceptance criteria Demagnetization Reports Records

3.14.8 Written Procedure The written procedure should contain the items given in Table 3.7 as minimum requirement. 3.14.8.1 Magnetic Particle Examination Procedure Deficiencies Deficiencies may include (1) incorrect method, e.g. coil method specified for specimens having L/​D ratios less than two, (2) incorrect magnetizing currents, (3) failure to perform examination at more than one current on complex sections, and (4) incorrect sequence of examination or failure to demagnetize when the second operation produces a lower flux density than the first [2].

3.14.9 Magnetizing Current One of the primary requirements for detecting a defect in a ferromagnetic material is that the magnetic field induced in the part must intercept the defect at 45°–​90° angle. Flaws that are normal to magnetic field will produce the strongest indications because they disrupt more of the magnet flux. For the purpose of magnetizing parts for MT, either alternating current (which has relatively little penetrating power and hence is good for surface flaw detection) or direct current (good for the detection of subsurface discontinuities) or half-​wave rectified alternating current, which has the merits of DC and AC, is widely used. Since it is difficult to correlate the results obtained with AC with the result obtained using DC, it is important to mention in the specifications or drawings whether AC or DC is to be used. In the event that this is not specified, DC shall be used.

3.14.10 Surface Preparation for Testing Surfaces to be inspected shall be clear and free from oil, grease, sand, loose rust, scale, ripples, welding spatters, or any other surface contaminants that could interfere with the interpretation of the magnetic particle indications. Surface preparation by grinding or machining may be necessary where surface irregularities could mask indications due to discontinuities. Magnetic particle test shall not be performed if the surface temperature of the part exceeds 600°F. The test method includes: 1. Preclean inspection area. 2. Place yoke on test piece perpendicular to direction of suspected cracks.

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3. Energize yoke to form a magnetic field. 4. Apply magnetic powder or prepared bath while yoke is energized and indications will form immediately.

3.14.11 Equipment for Magnetic Particle Inspection The magnetic particle test equipment essentially consists of (1) a means to magnetize the component, (2) a device for application of magnetic particles, and (3) demagnetization of components (if required) after inspection.

3.14.12 Magnetizing Technique As per ASME Code Section V, one or more of the following five magnetization techniques shall be used: (1) prod technique, (2) longitudinal magnetization technique, (3) circular magnetization technique, (4) yoke technique, and (5) multidirectional magnetization technique. Coil magnetization technique is shown in Figure 3.26a, prod magnetization in Figure 3.26b, and yoke magnetization in Figure 3.26c. 3.14.12.1 Coil Magnetization The use of coils and solenoids is one of the methods of indirect magnetization. When the length of a component is several times larger than its diameter, a longitudinal magnetic field can be established

FIGURE 3.26  Magnetizing techniques. (a) Coil magnetization, (b) prod magnetization, and (c) yoke magnetization.

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in the component by use of a coil or solenoid. The component is placed longitudinally in the concentrated magnetic field that fills the center of a coil or solenoid. This magnetization technique is often referred to as coil magnetization. 3.14.12.2 Prod Magnetization Prod magnetization produces a circular magnetic field. This will detect defects that are lengthwise in the test piece. Prods are portable current-​carrying copper conductors, which are used to magnetize localized areas as shown in Figure 3.26b. Magnetization is accomplished by pressing the prods against the surface area to be examined. When current flows through the prods, a circular magnetic field is created in the test part. The magnetic flux is perpendicular to the line joining the two prods, and the flux density is greatest along this line. During prod magnetization, the prods are spaced 150–​ 200 mm apart and in line with the suspected discontinuities. AC, DC, or half-​wave DC (HWDC) can be used to suit the job requirement. 3.14.12.3  Direction of Magnetization At least two separate examinations shall be performed on each area. During the second examination, the lines of magnetic flux shall be approximately perpendicular to those used during the first examination. Examination shall be done by the continuous method. 3.14.12.4  Prod Spacing Prod spacing should not exceed 8 in. (203.2 mm). Shorter spacing may be used to accommodate the geometric limitations of the area being examined or to increase the sensitivity, but not less than 3 in. (76.2 mm). 3.14.12.5 Yoke Magnetization Yoke magnetization introduces a longitudinal field to show up defects perpendicular to the flaw, or across the surface of the part. Both permanent and electromagnetic yokes are used. Permanent magnetic yokes have magnetic strength limitations but are most useful for spot inspection of welds. They can be used when electricity is not available. Electromagnetic yokes are U-​shaped with a coil to carry current as illustrated in Figure 3.26c. Longitudinal magnetic fields are set up in the test specimen when the coil is energized by either AC or DC current. Alternating current electromagnetic yokes should have a lifting power of at least 10 lb at the maximum pole spacing. Directions of magnetization should be the same as that for the prod technique. Pole spacing should be limited to 2–​8 in. (50.4–​203.2 mm). 3.14.13  Examination Coverage All the examinations should be conducted with sufficient overlap to ensure 100% coverage at the required sensitivity.

3.14.14 Inspection Medium (Magnetic Particles) The inspection medium shall consist of finely divided ferromagnetic particles. They must have good mobility, very fine size and shape, high permeability, and low retentivity. The particles may be in dry powder form (for the dry method) or suspended in a suitable liquid medium such as water or kerosene (for the wet method). The size of the dry particles is about 60 μm, and a blend of elongated and spherical particles is used to attain good sensitivity and mobility. The wet particle size is about 40–​60 μm.

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3.14.15 Application of Examination Medium The magnetic particles should be applied in such a manner that a thin uniform dust-​like coating settles upon the surface of the test part while the part is being magnetized. Specially designed powder blowers are used for this purpose.

3.14.16 Inspection Method According to the condition of the magnetic particles used for inspection, the inspection techniques are known as the dry method and the wet method. 3.14.16.1 Dry Method In the dry method, dry magnetic particles are used for examination. The dry powder method is preferred for surface and subsurface defects [104]. The powders are available in colors that will contrast with the background of the part being examined. Inspection by the dry method is carried out with portable magnetizing equipment. 3.14.16.2 Wet Method In the wet method, the magnetic particles are suspended in a liquid base of oil or water. The magnetic particles used in this method are finer than those used for the dry method, which makes the wet method more sensitive to detection of fine surface defects. Two types of wet magnetic particles are used. They are (1) visible magnetic particles, either reddish or black, observed under normal white light, and (2) fluorescent magnetic particles, which fluoresce when exposed to near-​ultraviolet (black) light. The inspection bath is mostly flowed, sprayed, or brushed over the specimen surface. Most MT by the wet method is done with stationary-​type equipment, although portable equipment may be used.

3.14.17 Evaluation of Indications The types of indications obtained by MT due to “piling up” of the magnetic particles at the lines of flaws are as follows: 1. Relevant indications are those that result from mechanical discontinuities. 2. Linear indications are those indications in which the length is greater than three times the width. 3. Rounded indications are circular or elliptical with the length equal to or less than three times the width. 4. Nonrelevant indications may arise due to metallurgical discontinuities and magnetic permeability variations. These nonrelevant indications shall be re-​examined by other suitable NDT methods such as PT and ET.

3.14.18 Demagnetization When the residual magnetism in the part could interfere with subsequent processing or usage, the part shall be demagnetized after completion of the test.

3.14.19 Record of Test Data The information such as material, method used (dry or wet), type of magnetization, type of current, amount of current, and nature of defects should be recorded at the time of each test for future reference.

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3.14.20 Interpretation 1. Surface defects appear sharp and distinct. 2. Subsurface flaws appear rough and fuzzy. 3. Width of surface flaw indication varies with depth.

3.14.21 Acceptance Standards For pressure vessels, heat exchangers, fittings, and components manufactured to ASME Section VIII Div. 1, as per Appendix 6, all surfaces to be examined shall be free of (1) relevant linear indications, (2) relevant rounded indications greater than 3 16 in. (4.8 mm), and (3) four or more relevant rounded indications in a line separated by 116 in. (1.6 mm) or less edge to edge.

3.14.22 MT Accessories Various MT accessories are marketed by M/​s Magnaflux, a Division of ITW, Inc., USA, and by others. They include residual field indicators, ASME field indicators, quantitative quality indicators, centrifuge tube and stand for ascertaining bath concentration, easy-​to-​use puffer bulb for applying dry method powders, copper braided pads to prevent burning by ensuring good electrical contact between test part and contact plates, test bar with artificial surface and subsurface discontinuities for testing with yoke method, and test block with subsurface discontinuities for testing with the prod method and coil method. For testing the magnetic particles, a Betz ring is used. The Betz ring has holes drilled at different depths. As a magnetic field is induced in the Betz ring, magnetic particles are sprinkled to the surface. The number of visible line indications determines the sensitivity of the particles. Typically, specifications require visual detection of the fifth hole, which produces a broad indication [99].

3.15 MAGNETIC RUBBER TECHNIQUE (MRT) The magnetic rubber technique was developed for detecting very fine cracks and is capable of revealing finer cracks than other magnetic techniques. The technique uses a liquid (uncured) rubber containing suspended magnetic particles. The rubber compound is applied to the area to be inspected on a magnetized component as shown in Figure 3.27. Inspections can be performed using either an applied magnetic field, which is maintained while the rubber sets (active field), or the residual field from magnetization of the component prior to pouring the compound. A dam of modeling clay is often used to contain the compound in the region of interest. The magnetic particles migrate to the leakage field caused by a discontinuity. As the rubber cures, discontinuity indications remain in place on the rubber [105–​107].

FIGURE 3.27  Schematic diagram of magnetic rubber particles testing method.

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3.16 RADIOGRAPHIC TESTING Radiographic Testing (RT) is a nondestructive testing (NDT) method which uses either x-​rays or gamma rays to examine the internal structure of manufactured components identifying any flaws or defects. The term radiography usually implies a radiographic process that produces a permanent image on a film. The technique utilizes electromagnetic radiation to penetrate an object to reveal information about its internal conditions, particularly internal flaws and surface flaws/​discontinuities [108].

3.16.1 Principle of Radiography In radiography testing the test-​part is placed between the radiation source and film. The material density and thickness differences of the test-​part will attenuate (i.e. reduce) the penetrating radiation through interaction processes involving scattering and/​or absorption. The differences in absorption are then recorded on film(s) or through an electronic means. When a radiation beam passes through a specimen, some of the radiation energy is absorbed and the intensity of the beam is reduced. To make an RT of the object, the intensities of the radiation that pass through the specimen must be recorded and compared. Variations in beam intensity are seen as difference in shading that are typical of the types and sizes of the flaw present. The recording is being achieved either by letting the rays strike a photographic film (Figure 3.28) or by letting the rays fall on a fluorescent screen that absorbs some of the radiation and converts it to visible light, thus producing a visible image known as fluoroscopy.

3.16.2 RT Methods In industrial radiography there are several imaging methods available, techniques to display the final image, i.e. film radiography, real time radiography (RTR), Computed Tomography (CT), radioscopy, digital radiography (DR), and Computed Radiography (CR).

FIGURE 3.28  Radiographic testing principle.

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3.16.3 Application Surface flaws that are detectable by radiography include undercuts, concavity at the weld root, incomplete filling of the grooves, excessive reinforcements, overlaps and electrode spatter, and internal flaws such as blow holes, gas inclusions, porosity, piping, slag inclusion, cracks, incomplete penetration, incomplete fusion, and tungsten inclusions. More information on flaw-​detecting capabilities is discussed later in this section.

3.16.4 Radiation Sources (X-​Rays and Gamma Rays) Two types of radiation sources are commonly used in weld inspection. They are x-​ray machines and radioactive isotopes. X-​rays are produced by machines that range from portable, low-​energy units in the range of 50–​400 kV used for inspection of relatively thin objects, say, metal thickness up to 7.5 cm steel equivalents, to betatrons and linear accelerators up to 30 meV, capable of radiography of thick objects up to 20 in. (508 mm) of steel. Gamma-​ray radiation is emitted by radioisotopes, the two most common being cobalt-​60 and iridium-​192. Cobalt-​60 will effectively penetrate a thickness up to approximately 8 in. (203 mm) of steel, whereas iridium-​192 is effectively limited to a steel thickness of about 3 in. (76.2 mm). The minimum thickness of steel on which x-​rays may be normally used is up to and including 19.05 mm, and gamma-​ray sources may normally be used for thicknesses greater than 19.05 mm. 3.16.4.1 Comparison of X-​Ray and Gamma-​Ray Radiography Most industrial radiography is performed with x-​rays. It is highly effective and versatile, and with modern techniques, it is very accurate. The principal advantage of gamma-​ray radiography is lower cost, the source can be very small so that transportability is easy, and it is economical for low volume test pieces. However, gamma rays are not as sensitive to small defects as are x-​rays in thickness less than 1 in. (25.4 mm), cracks are the most difficult to detect and generally require a longer exposure time than with x-​rays, and they do not provide as sharp a definition [103].

3.16.5 Merits and Limitations Merits. Radiography has been the standard for volumetric inspection of weldments for decades. In spite of high cost, hazards, and the time-​consuming nature of the process, radiography stands out as the NDT method that gives a permanent record of test results. Among other advantages, it has surface and subsurface inspection capability, and radiographic images aid in characterizing (identifying type) discontinuities [76]. Limitations. A significant limitation of radiography is that discontinuities must be favorably aligned with the radiation beam. This is usually not a problem for discontinuities round in cross section, such as slag inclusions and porosity [76, 90]. On the other hand, cracks and planar defects that diverge considerably from the direction of the radiation beam would nearly miss detection, and therefore ultrasonics is sometimes employed in addition to radiography when very critical examination of a weld is required [90]. Angular notch-​type discontinuities such as lack of fusion, incomplete penetration, and cracks must be oriented to the radiation beam so as to show up on the x-​rays film [108]. Coarse grain structure, especially in large-​pass welding, can cause problem due to diffraction mottling effects [109].

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3.16.6 Requirements of Radiography Essential requirements for producing a radiograph are the following [64]: 1. a source of radiation 2. x-​ray film 3. a trained operator 4. a means of processing the exposed film 5. a trained person capable of interpreting the radiographic images.

3.16.7 Radiographic Test Written Procedure Radiographic examination shall be performed in accordance with a written procedure. Each procedure shall include at least the following information, as applicable: 1. material type and thickness range 2. isotope or maximum x-​ray voltage used 3. source-​to-​object distance 4. distance from source side of object to film 5. source size 6. film brand and designation 7. screens used. The radiographic test written procedure is given in Table 3.8. Radiographic examination procedure deficiencies may include incorrect exposure geometry, film placement, source selection, penetrameter selection, incomplete film identification, and failure to submit a technique sheet with sketch [2].

TABLE 3.8 Written Procedure Radiographic Test 1.0 Scope 2.0 Techniques 3.0 Surface preparation 4.0 Identification system 5.0 Location markers 6.0 Radiation sources 7.0 Film 8.0 Screens 9.0 Focus to film or source to film distance 10.0 Penetrameter 10.2 Number of penetrameters 10.3 Placement of penetrameter 11.0 Density 12.0 Sensitivity 13.0 Processing of films 14.0 Processing technique 15.0 Personnel certification 16.0 Evaluation of radiographs 17.0 Reporting 18.0 Retention of radiographs

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3.16.8 ASTM Standards 1. ASTM E1742/​E1742M-​18 –​Standard Practice for Radiographic Examination. 2. Test Methods E1030, E1032, and E1416 contain information to help develop detailed technique/​procedure requirements. 3. ASTM E94-​00 –​Standard Guide For Radiographic Examination. 4. Others a. E94/​ E94M-​ 17 –​Standard Guide for Radiographic Examination Using Industrial Radiographic Film. b. E746-​ 18 –​Standard Practice For Determining Relative Image Quality Response Of Industrial Radiographic Imaging Systems. c. E1000-​16 –​Standard Guide for Radioscopy. d. E1695-​20 –​Standard Practice for Radiographic Examination. e. E1814-​14 –​Standard Practice for Computed Tomographic (CT) Examination of Castings. f. E1815-​18 –​Standard Guide for Computed Radiography. g. E2033-​17 –​Standard Practice for Radiographic Examination Using Computed Radiography.

3.16.9 General Procedure in Radiography The steps of performing an RT are as follows: 1. Surface preparation. 2. Selection of radiation source, depending upon the thickness of the part and location of the component/​assembly. 3. Selection of film and exposure parameters for desired quality (i.e. sensitivity, density) and selection of intensifying screens. 4. Processing of exposed films. 5. Interpretation of radiographs. 6. Record keeping and preservation of radiographs.

3.16.10 Reference Documents Radiography is one of the most widely established methods for the detection of internal as well as surface defects and has been written into many national codes and standards. ASTM Standards E94, E142, E155, E272, E390, E592, E586, and E747 cover RT. ASME Code Section V and VIII, Div. 1, Appendix 4, covers RT.

3.16.11 Safety One of the most important considerations in x-​ray or gamma-​ray work is the exercise of adequate safeguards for personnel.

3.16.12 Identification Marks The identification marks such as pressure vessel number, job number, seam number, segment number, or spot number, date, and manufacturer’s symbol should appear in each film as radiographic images.

3.16.13 Location Markers Location markers should appear as radiographic images and should be placed on the part, and their locations shall be hard stamped or engraved on the surface of the part being radiographed. Lead

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marker accessories containing sets of alphabets and numbers including arrow and oblique are available in the market.

3.16.14 Processing of X-​Ray Films All the radiographs should be free from fogging and processing defects. While processing, radiograph details should not be lost due to scratches, finger marks, etc. The Welding Engineer Data Sheet provides guidance for troubleshooting radiographs [110].

3.16.15 Surface Preparation Weld surfaces should be ground free of ripples so that the resulting radiographic image cannot mask or be confused with the image of discontinuities. Prior dressing of the weld surface ensures interpretation of films with a minimum amount of referring back to the welds themselves. In the case of submerged arc welding, no surface preparation is required provided the weld bead is smooth and free from valleys or ridges.

3.16.16 Radiographic Techniques for Weldments of Pressure Vessels A single-​wall exposure technique shall be used for radiography whenever practical. This technique is employed for plates, shell welds, and castings of larger sizes. In the single-​wall technique, the radiation passes through only one wall of the weld (material), which is viewed for acceptance on the radiograph. In panoramic technique, the radiation source is positioned at the centre of cylindrical component such as a pipe or vessel with the film wrapped around the surface of the weld so that the entire length of circumferential weld joint can be examined with one exposure. When it is not practical to use a single-​wall technique, a double-​wall technique shall be used. An adequate number of exposures shall be made to demonstrate that the required coverage has been obtained. 1. Single-​Wall Viewing. For materials and for welds in components, a technique may be used in which the radiation passes through two walls and only the weld (material) on the film-​side wall is viewed for acceptance on the radiograph. When complete coverage is required for circumferential welds (materials), a minimum of three exposures taken 120o deg to each other shall be made. Single wall technique is shown in Figure 3.29, Ref.(10d). 2. Double-​Wall Viewing. For materials and for welds in components 31/​2 in. (89 mm) or less in nominal outside diameter, a technique may be used in which the radiation passes through two walls and the weld (material) in both walls is viewed for acceptance on the same radiograph. For double-​wall viewing, only a source-​side IQI shall be used. Double wall technique is shown in Figure 3.30, Ref.(10d). Common techniques for various fillet welds are shown in Figure 3.31. A diagrammatic representation of the film and source is shown in Figure 3.32.

3.16.17 Panoramic Radiography with Isotopes One of the important advantages of using isotopes is that by a single exposure, the entire circumference of a weld can be radiographed, provided there is accessibility to keep the source at the center of the pipe [111].

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FIGURE 3.29  Single wall RT technique.

3.16.18 Full Radiography ASME Code Section VIII, Div. 1, UW-​11 lists the conditions for which the full RT for the welded joint is mandatory. Some of these conditions are as follows: 1. All butt welds in the shell and heads of vessels used to contain lethal substances. 2. All butt welds in vessels in which the least nominal thickness at the welded joint exceeds 1.5 in. (38.1 mm). 3. All butt welds in the shell and heads of unfired steam boilers having design pressures exceeding 50 psi. 4. All butt welds in nozzles, communicating chambers, etc. attached to vessel sections or heads that are required to be fully radiographed.

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FIGURE 3.30  Double wall RT technique.

3.16.19 Spot Radiography Spot radiography should be in accordance with ASME Code Section VIII, Div. 1, UW-​52. Spot radiography means that one spot is examined on each vessel for each 50 ft increment of the weld or part thereof for which a joint efficiency from column (b) of Table UW-​12 is selected. For vessels

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FIGURE 3.31  Common RT techniques for corner fillet welds.

FIGURE 3.32  Diagrammatic representation of the film and radiation source.

FIGURE 3.33  Spot radiography.

with less than 50 ft of weld, 50 ft increments of weld may be represented by one spot examination. Length of each radiography shall be at least 6 in. (152.4 mm). Further, all T joints must be radiographed, and at least one shot must be taken on each longitudinal and circumferential seam (Figure 3.33).

3.16.20 Radiographic Quality The quality of the radiographs that decides the efficiency of flaw detection is determined by two factors: sensitivity and density.

3.16.21 Radiographic Sensitivity The effectiveness of a radiographic technique to bring out the smallest details in a given thickness of an object is referred to as radiographic sensitivity. The more the exactness, the better is the sensitivity. Radiographic sensitivity is affected by two factors: (1) contrast –​the magnitude of change in density produced on the film because the defect and sound material absorb different amounts of radiation –​and (2) definition –​the amount of blurring of the projected shape of the defect produced on the film. The terms and definitions that best represent (1) and (2) are thickness sensitivity and unsharpness, respectively [112]. In quantitative terms, sensitivity is expressed as a percentage ratio of the smallest size of an artificially produced defect shown on the radiograph to the thickness of the object under examination. The smaller the numerical value of sensitivity, the better is the radiographic sensitivity. The artificial defect in the form of a hole, step, or wire of various dimensions is called as image quality indicator (IQI) or penetrameter.

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3.16.22 Density of Radiographs The density of a radiograph refers to its darkness. Areas exposed to relatively large amounts of radiation are known as areas of high density, and areas exposed to less radiation are known as areas of light density. The density of the radiograph should be minimum of 1.8 for x-​rays and 2.0 for gamma rays. When the density of the radiation varies by more than −15% or +​30% from the density through the body of the penetrameter, additional penetrameters should be placed in the exceptional areas.

3.16.23 Image Quality Indicators An IQI, also called a penetrameter (or penny), is a simple geometric form used to judge the quality of a radiograph. It is made of the same material as or a similar material to the component being examined. The dimensions of the IQI bear some numerical relation to the thickness of the part being examined. They are placed on the test piece during setup and are radiographed at the same time as the test piece. IQIs are preferably located in regions of maximum test piece thickness and greatest test piece to film distance, and near the outer edge of the central beam of radiation. The degree to which the image of the IQI is visible in the developed image is a measure of the quality of that image. IQI design. A number of IQI designs are used by different authorities. There are American standards, British standards, and French and German standards. Three basic types of IQI are available. They are the wire type, step-​wedge type, and strip-​hole type. The most widely used wire-​type IQI is of the DIN type (German), conforming to DIN 54109 Fe, Cu, and Al. It specifies a set of three parameters, each containing seven equidistant wires. The diameter of the wires varies in geometric progression. Each diameter is represented by a whole number. The three sets are 1–​7, 6–​12, and 10–​16. The ASTM standard for wire-​type IQI is E747-​2018 Wire-​ type IQI is shown in Figure 3.34a. ASTM E747-​18 –​Standard Practice for Design, Manufacture and Material Grouping Classification of Wire Image Quality Indicators (IQI) Used for Radiology. This practice is applicable to x-​ray and gamma-​ray radiography. The step-​wedge type is the simplest IQI. This type of penetrameter is commonly used in UK-​BS 3971, and in France-​AFNOR Code. It consists of small steps, whose thickness (0.5%–​5% of specimen thickness) increases in geometric progression. Step-​wedge-​type IQI is shown in Figure 3.34b. Step-​hole type is step-​wedge type with holes. For the strip-​hole type IQI, the most common and widely used version is the ASTM or ASME penny. Strip-​hole IQI is also known as plaque-​type IQI, whose thickness is about 2% of the specimen thickness. This consists of a small rectangular piece of metal with appropriate material with three holes. The diameters of the holes are multiples of thickness: IT, 2T, and 4T, where T is the thickness of IQI. Plaque type IQIs are described in ASTM E 94/​E94M-​2017. Strip-​hole type IQI is shown in Figure 3.34c. 3.16.23.1 Number of IQIs One IQI should be used for each radiograph, generally. In case of radiography using a panoramic exposure, three IQIs placed at 120° apart are accepted. 3.16.23.2 How to Calculate IQI Sensitivity An IQI is used to indicate the quality of radiographic technique and not to measure the size of a defect that is shown. Sensitivity for wire-​type IQI is based on the smallest size of wire diameter visible, and for strip-​ hole type IQI, it is based on diameter of the smallest hole:

Percent of sensitivity =

smallest size of penny visible ×100 thickness of the object

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FIGURE 3.34  IQI types. (a) Wire, (b) step wedge, and (c) strip hole.

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For the strip-​hole type or plaque-​type IQIs, the IQI image and the specified hole are the essential indications of sensitivity. The thickness of the IQI and the hole diameter to be seen are generally specified in the code. The following are the different sensitivity levels of inspection with strip-​hole type pennies: 1–​1T 2–​1T 1–​2T 2–​2T 1–​4T 2–​4T

4–​1T 4–​2T 4–​4T

Normally, the image of 2–​2T hole should be visible in the radiograph. Critical components require a level of 1–​2T or 1–​IT, and less critical components may need only a quality level of 2–​4T or 4–​4T.

3.16.24 Examination of Radiographs The examination of radiographs of welds should be made on the original films using a viewing device of suitable illuminating power. The standard of acceptance shall be made with respect to the reference chart of x-​rays given in ASME Code Section VIII, Div. 1, Appendix 4.

3.16.25 X-​Ray Image Clues to Welding Discontinuities When interpreting radiographs of weldments, one must look for different types of discontinuities (undercut, porosity, lack of fusion, etc.). All discontinuities, however, must be considered before deciding to accept or reject and to correct welding practices. Some of the common weld defects and their typical radiographic appearances are given in Table 3.9.

TABLE 3.9 Weld Defects and Radiographic Appearances Defect

Image

Crack

Cracks can be detected in a radiograph only when they are propagating in a direction that produces a change in thickness that is parallel to the x-​ray beam. Appears as a fine, dark, zig zag irregular line, may be branched Very narrow, straight dark line or lines oriented in the direction of the weld seam along the weld preparation or joining area Continuous or intermittent dark straight line at the center of the weldment. Also known as incomplete penetration (IP) Since tungsten has a higher atomic number than the iron or aluminum, the radiographic indications are white or lighter area with a distinct outline on the radiograph Looks rounded and black, and readily recognized with well-​defined outlines Nonmetallic solid material entrapped in weld metal. Look less distinct than voids. Dark indications and irregular shapes. May appear singly or be linearly distributed or scattered within the weld or along the weld joint areas Comes in clusters or in random fashion. The image will be normally dark round (since all porosity is void) or irregular spots or specks appearing singularly, in clusters, or in rows Sometimes porosity is elongated and may appear to have a tail. This is the result of gas attempting to escape while the metal is still in a liquid state “Wire like” indications Dark spots, which are often surrounded by light globular areas (icicles) Oxide inclusions are less dense than the surrounding material and, therefore, appear as dark irregularly shaped discontinuities in the radiograph

Lack of fusion Lack of penetration (LOP) Tungsten inclusions Void Slag inclusions

Porosity or gas inclusion or blow hole Wormhole porosity Whisker Burn-​through Oxide inclusions

Source:  Adapted from [6] Bryant, L.E. and McIntire, P.; [113] Garrett, W.R.

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3.16.26 Acceptance Criteria For porosity, radiographs are often compared with charts showing arbitrary arrays of pores. The charts are intended to limit the porosity to a certain percentage of the area on the radiograph. Slag inclusions are usually accepted on the basis of length (the only dimension readily measured by radiographs). Planar defects (cracks, lack of fusion, etc.) are normally not permitted [112, 113]. An interpretation is complete and a better conclusion is arrived at only after a thorough VT of the part. For acceptance criteria regarding image density, relevant indications, maximum size of rounded indications, aligned rounded indications, spacing, clustered indications, etc. refer to ASME Code Section VIII, Div. 1, Appendix 4, and Section V, or the International Institute of Welding (IIW) collection of reference radiographs.

3.16.27 Documentation When reporting and documenting the results of radiographic film interpretation, complete and accurate information must accompany the radiographs. This is required for subsequent customer review, and possibly regulatory agency review. As per ASME Code Section V, the films shall be retained up to five years, by the manufacturers.

3.16.28 Other Methods in Radiography Over the last few years, several new radiographic techniques have been developed. Important developments in the field of radiography are discussed in Ref. [75] and Hellier [114]. Some of the developments follow.

3.16.29 Computed Tomography Computed tomography (CT) in radiography is either a 2D or a 3D inspection technique that can be applied for various kinds of objects and materials. This method was adapted from the medical computerized axial tomography scanner. The method involves the repeated scanning of a specimen with a narrow beam of radiation in a series of steps around the specimen. The radiation impinges on a linear array of detectors whose output is digitized and provides a reconstruction of an image or slice, which allows a view of the specimen perpendicular to the direction of the radiation [115]. The principal advantage of this method is that it produces an image of a thin slice of the specimen under examination.

3.16.30 Microfocus Radiography Micro-​focus radiography facilitates to observe the minute details of the object through production of magnified x-​ray images, which in turn enhances the flaw detection capability which improves reliability in comparison to conventional radiography. Microfocus radiography or the use of fine focus x-​ray tubes is finding an important role in industrial radiography. The microfocus x-​ray source can produce extremely sharp radiographs at exceedingly short film-​to-​focus distances. The reason for this is that microfocus x-​ray machines with focal spots as small as 1 mm can provide an enhanced flaw detection capability with greater reliability than it is attainable with conventional radiographic equipment. Figure 3.35 shows applications of Microfocus radiography –​Figure 3.35(a) shows inspection of nozzle-​shell welded joint and 3.35(b) shows boreside tube-​to-​tubesheet joint weld [75]. The rod anode tube heads range in diameters from 4 to 18 mm and available in various lengths up to 1500 mm.

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FIGURE 3.35  (a) Inspection of nozzle-​shell welded joint and (b) boreside tube-​to-​tubesheet jointweld.

3.16.31  Digital Radiography Unlike conventional radiography, digital radiography doesn’t require film. Instead, it uses a digital detector to display radiographic images on a computer screen almost instantaneously. It allows for a much shorter exposure time so that the images can be interpreted more quickly. The four most commonly utilized digital radiography techniques in the oil and gas and chemical processing industries are computed radiography, direct radiography, real-​time radiography, and computed tomography. These methods are briefly discussed below [116]: 1. Computed Radiography Computed radiography (CR) uses a phosphor imaging plate that replaces film in conventional radiography techniques. This technique is much quicker than film radiography but slower than direct radiography. 2. Direct Radiography Direct Radiography (DR) is also a form of digital radiography and very similar to computed radiography. The key difference lies in how the image is captured. In DR, a flat panel detector is used to directly capture an image and display that image on a computer screen. Although this technique is fast and produce higher quality images, it is more costly than computed radiography. 3. Real-​Time Radiography Real-​time radiography (RTR), like it’s name suggests, is a form of digital radiography that occurs in real time. RTR works by emitting radiation through an object. These rays then interact with either

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a special phosphor screen or flat panel detector containing micro-​electronic sensors. The interaction between the panel and the radiation creates a digital image that can be viewed and analyzed in real time. Compared to conventional radiography, RTR has a lower contrast sensitivity and limited image resolution. Images created via RTR often suffer from uneven illumination, limited resolution, a lack of sharpness, and noise. These factors have a major impact on image quality.

3.16.32 Neutron Radiography Neutron Radiography is an imaging technique which provides images similar to x-​ray radiography. The difference between neutron and x-​ray interaction mechanisms produce significantly different and often complementary information. While x-​ray attenuation is directly dependent on atomic number, neutrons are efficiently attenuated by only a few specific elements. For example, organic materials or water are clearly visible in neutron radiographs because of their high hydrogen content, while many structural materials such as aluminum or steel are nearly transparent [117]. Some applications of neutron radiography are inspections of manufactured components of many types such as those engineered for the aircraft, aerospace, and automotive industries, including (1) precision in manufacture of turbine engine blades, (2) corrosion, (3) flaws in adhesive bindings, (4) internal flaws, (5) missing or misplaced O-​rings or other components, and (6) cracks, inclusions, and voids or other types of internal defects in materials, etc. [118, 118.1].

3.16.33 Radioscopy Radioscopy is similar to radiography; the only difference is that an image receptor replaces the radiographic film. The receptor is an image intensifier coupled to a low-​light-​level camera. The image shows up on a high-​resolution monitor. An operator views the image when it appears on the screen; no film processing is involved. This feature provides online examination of longitudinal and circumferential pipe and shell welds. Also real-​time radioscopy provides immediate response imaging with the capability to follow motion of the inspected part. This includes radioscopy where the motion of the test object must be limited (commonly referred to as near real-​time radioscopy). Real-​time radioscopy may be performed on materials including castings and weldments. This radioscopic methodology may be used for the examination of ferrous or nonferrous materials and weldments. Radioscopy is nearly ten times as fast as film radiography, considering time to develop film and interpret radiographs, and the image can be recorded on videotape [119]. Radioscopy is included in ASME Code Section V. As with conventional Radiography, radioscopy is broadly applicable to any material or object through which a beam of penetrating radiation may be passed and detected. Different types of detection device are now available, like systems with digital detector arrays, or analog component such as an electro-​optic device or an analogical camera. Recent technology advances in the areas of camera techniques, and digital image processing provide acceptable sensitivity for a wide range of applications [120]. 3.16.33.1 ASME Code Section V Mandatory appendix real-​time radioscopic examination Real-​time radioscopy provides immediate response imaging with the capability to follow motion of the inspected part. This includes radioscopy where the motion of the test object must be limited (commonly referred to as near real-​time radioscopy). Real-​time radioscopy may be performed on materials including castings and weldments. This radioscopic methodology may be used for the examination of ferrous or nonferrous materials and weldments.

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3.16.34 X-​Ray Fluoroscopy X-​ray images can be made visible instantaneously by use of a fluorescent screen instead of a photographic film. Thus, this provides a process of real-​time image viewing. For permanent records, fluoroscopy images can be photographed (fluorography).

3.16.35 Gammascopy Gammascopy systems with image processing facility are now commercially available for inspection of thick structural parts with iridium-​192 or cobalt-​60 sources. The sensitivity and resolution obtainable in the gammascopy are inferior to those of film radiography.

3.16.36 Digitization of Radiographs and Laser Scanner System Although radiographic film remains the main image capture medium in many applications, it has disadvantages in long-​term storage of images because of the possibility of image quality loss over time, difficulty of access and retrieval, and strict storage environment requirements [114]. NDT Scan, a recent development of DuPont NDT Systems, converts high-​quality film radiographs from analog to digitized form, which offers the prospects of image storage in a digital format on optical media, analysis, recall, and simultaneously providing solutions to the problems associated with film.

3.16.37 Imaging Plate This is a high-​sensitivity imaging plate that essentially replaces conventional radiographic film while utilizing standard radiographic equipment. It is estimated that the imaging plate can be reused about 40,000 times [114].

3.16.38 High-​Energy Radiography Radiation sources such as synchrotrons and betatrons are available.

3.16.39 Gamma Radiography The development of modern gamma cameras permits safe and reliable remote operations. Apart from the commonly used isotopes, a number of other sources as europium, ytterbium, etc. are finding increasing applications [75].

3.17 ULTRASONIC TESTING In ultrasonic testing (UT), high-​frequency sound waves are transmitted through the material being inspected. The sound waves travel through the material with some loss of energy (attenuation) and are reflected at interfaces or discontinuities such as cracks, flaws, inclusions, and seams [77, 121]. The reflected sound beam from the flaws or interface is detected and analyzed to define the presence and location of discontinuities or the thickness of the material. It is also especially suited for determining characteristics of engineering materials such as elastic moduli, study of metallurgical structure, grain size, and density variation. Figure 3.36 and Figure 3.37 show conventional UT method. There are three basic components in UT, viz., (1). pulser/​receiver, a device that emits the ultrasonic waves/​pulses in waves, (2) when discontinuities or flaws are detected in the wave path, the energy is reflected and converted into an electric signal by the transducer, and (3) data display/​visualization. The transducer is passed over the object being inspected, which is typically coupled to the test object by gel, oil, or water. This couplant is required to efficiently transmit the sound energy from

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FIGURE 3.36  UT principle –​conventional method.

the transducer into the part, however. This couplant is not required when performing tests with non-​contact techniques such as electromagnetic acoustic transducer (EMAT) or by laser excitation. Imperfections in the material reduce the amount of sound that is received, allowing the location of flaws to be detected [122].

3.17.1 Test Method The primary UT techniques and their applications are [123] the following: 1. Pulse echo: most effective for cracks, inclusions, and thickness measurements. 2. Through transmission: most effective for porosity, grain size, and density measurements. 3. Resonance: most effective for detection of thickness and laminar discontinuities. There are three modes of ultrasonic vibrations that are frequently used in UT of materials. These are (1) longitudinal waves, (2) transverse waves, and (3) surface or Rayleigh waves. Most ultrasonic weld testing is done manually by an operator manipulating an ultrasonic probe over the test object and visually monitoring the screen of an oscilloscope. The pulse-​echo method with A-​scan data presentation is most commonly used for inspecting welds. This system utilizes a cathode ray tube (CRT) screen to display the test information.

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FIGURE 3.37  UT principle –​conventional method.

3.17.1.1 Pulse Echo Technique This technique introduces a sound beam into the test material surface. The sound will travel through the part, either reaching the rear wall of the material and then returning to the transducer or returning early when reflected from a discontinuity within the part. If the acoustic velocity is known, the time interval recorded is then used to derive the distance travelled in the material. With pulse echo testing, the same transducer emits and receives the sound wave energy. 3.17.1.2 Through Transmission Testing Through transmission testing uses an emitter to send the ultrasound waves from one surface and a separate receiver to receive the sound energy that has reached the opposite side of the object. Through-​transmission testing method is shown schematically in Figure 3.38.

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FIGURE 3.38  UT – Through-​transmission testing (schematic).

3.17.1.3 Contact and Immersion Testing Ultrasonic testing can also be split into two main types: contact or immersion testing. Contact ultrasonic testing is typically used for on-​site inspections accessibility or portability. Immersion ultrasonic testing is a laboratory-​based or factory-​based nondestructive test that is best suited to curved components, complex geometries and for ultrasonic technique development. In this method, the component or material is submerged in a water, which acts as a couplant in place of the gels used for contact ultrasound.

3.17.2 Air Coupled Testing Certain inspections and materials cannot tolerate the application of wet coupling and so in certain circumstances air coupled ultrasound testing may be performed. This entails the application of sound through an air gap. This typically entails the use of lower frequency inspection.

3.17.3 Presentation An amplitude scan, or A-​scan, is the most basic presentation of waveform data and represents individual sound pulses sent through the material tested. An A-​scan is a one-​dimensional graph where ultrasonic echo amplitude is plotted as a function of time. In a healthy A-​scan, the first, and tallest

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peak, is the interphase reflection. The second and subsequent peaks are back wall echoes. The signal amplitude represents the intensities of transmitted or reflected beams. This may be related to flaw size, sample attenuation, or other factors. B-​scans are two-​dimensional, cross-​sectional views of the inspected material depicting material thickness measured at different positions over time. C-​scan. This presentation offers a plane view of the defect in the part. A good estimate of the size and shape of the flaw is obtained, but only a poor estimate of flaw depth. A combination of the B-​scan and C-​scan will give a 3D image of the defect. D-​scan (nonparallel scan). Scan that shows the data collected when scanning the transducer pair perpendicular to the direction of the sound beam along a weld. Figure 3.39 shows A-​, B-​and C scan presentations.

FIGURE 3.39  A-​, B-​, and C scan presentations (schematic).

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3.17.4 ASTM Standard for UT ASTM E164-​2019 –​Standard Practice for Contact Ultrasonic Testing of Weldments. This practice covers techniques for the ultrasonic A-​ s can examination of specific weld configurations joining wrought ferrous or aluminum alloy materials to detect weld discontinuities.

3.17.5 Application of Ultrasonic Technique in Pressure Vessel Industry This technique’s primary application is the detection and characterization of internal discontinuities [76]. It is also used to determine effectiveness of weld overlay bonding/​cladding and to measure thickness [90]. The information possible to obtain from the UT data includes defect location, orientation, size, and type. Only ultrasonics and radiography can substantially reveal subsurface flaws.

3.17.6 Written Procedure Minimum information required for written procedure for UT is detailed in ASME Code Section V. A partial list of information to be contained in a UT written procedure is given in Table 3.10. 3.17.6.1 Ultrasonic Examination Procedure Deficiencies These may include insufficient examination, incorrect scanning sensitivity versus calibration sensitivity, failure to describe recalibration, recording, and failure to verify instrument linearity [2].

3.17.7 ASME Code Coverage ASME Code Section V/​ASTM Specifications are as follows: (a) SA 388/​ASTM, A 388/​A 388 M, (b) SA 435/​SA 435 M, identical to ASTM A 435/​A 435 M, (c) SA 577/​SA 577 M, identical to ASTM A577/​A 577 M, (d) SA 578/​SA 578 M, identical to ASTM A 578/​A 578 M, and (e) SA 609, identical to ASTM A 609/​A 609 M except for some deletions.

TABLE 3.10 Written Procedure for UT 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Scope: materials and product form (casting, forging, plate, etc.), weld types, thickness, etc. Surface preparation Couplant Technique (straight beam or angle beam, contact or immersion) Angles and mode(s) of wave propagation in the material Search unit details Ultrasonic instrument details like frequency, screen height, amplitude control linearity Calibration and reference blocks Extent of scanning, maximum scan speed, and scanning pattern Description of the demonstration or qualification of the procedure used to detect and size flaws Evaluation Acceptance criteria Postcleaning Reporting

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3.17.8 Advantages of Ultrasonic Inspection The conventional ultrasonic flaw detector is well known for its portability, simplicity of operation, superior penetrating power, and high sensitivity, and allows testing from one surface. The method provides almost instantaneous indications of discontinuities. Ultrasonic method is particularly sensitive for the detection of 2D defects such as fine cracks and laminar-​type defects, which are not easily found by other methods including radiography [90, 124]. UT is sometimes employed in addition to radiography when critical examination of a weld is required [90]. The greater ability of UT techniques to describe the shape and size of a flaw is basic for fracture mechanics analysis, which has led to a growing need for UT inspection in pressure vessel industries [62]. With some electronic systems, a permanent record of inspection results can be made for future reference. By combining angulation, amplitude, signal character, signal movement, signal shape, and frequency analysis, it is possible to accurately determine the discontinuity type [108]. UT presents no health hazards such as radiation or chemical hazard.

3.17.9 Limitations of Ultrasonic Inspection Some important limitations of this method include the following: 1. Accuracy and reproducibility of this method depends largely on the skill of the operator to interpret results. 2. All results are based on subjective evaluation, and objective records are not possible to obtain [90]. 3. Discontinuities that are present in a shallow layer immediately beneath the surface may not be detectable. 4. Misleading signals due to grain size. For example, the columnar grain structure of stainless-​ steel welds generally makes UT impracticable, although UT of stainless steel plate presents no problems [103]. 5. The ultrasonic signal background that arises from the clad/​metal interface of a clad plate can make it difficult to detect defects up to 10 mm deep just below the clad surface and therefore for critical applications machine clad faces [62]. 6. Weld root conditions pose problems. For example, weldment containing internal concavity and an improperly located counterbore, improper weld capping, sharp reentrant angles, and high-​low conditions of vertical fusion zones can cause radiation diffraction and diametrical shrinkage [125]. 7. Reference standards made from a similar material as that being tested are required for calibration. 8. UT is less suitable than RT for determining porosity in welds, because round gas pores respond to UTs as a series of single-​point reflectors [77].

3.17.10 Examination Procedure 3.17.10.1 Pulse-​Echo Technique This system is the most versatile and the most employed for inspection. In pulse-​echo technique, flaws are detected by measuring the amplitude of signals reflected from a flaw and also the time required for these signals to travel between specific surface and the flaw. Depending on test piece shape and inspection objectives, pulse-​echo technique can be accomplished with longitudinal, shear, or surface waves, and with straight beam or angle beam techniques. Since it is important to intercept the discontinuity at or near 90°, it is common for more than one angle search unit to be used to inspect a particular weld [76]. Data can be analyzed in terms of type, size, location, and orientation of defects, or any combination of these factors.

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3.17.11 Components of a UT Instrumentation The basic components of a UT instrument include the following: 1. an electronic signal generator or pulser 2. an electronic clock 3. a transmitting transducer or search unit 4. couplant to transfer acoustic energy to specimen and back to the receiver 5. a receiving transducer 6. an echo signal amplifier 7. a display device.

3.17.12 Angle Beam Technique Angle beam testing is by far the most commonly used technique in ultrasonic flaw detection. While straight beam techniques can be highly effective at finding laminar flaws, they are not effective when testing many common welds, where discontinuities are typically not oriented parallel to the surface of the part. This situation can occur in many types of welds, in structural metal parts, and in many other critical components. Angle beam probes consist of a transducer and a wedge, which may be separate parts or built into a single housing. They use the principle of refraction and mode conversion at a boundary to produce refracted shear or longitudinal waves in a test piece. Most commonly used angle beam probes generate a refracted shear wave at standardized angles of 45, 60, or 70 degrees in the test material. In typical inspections the sound beam will travel at the generated angle down to the bottom of the test piece and then reflect upward at the same angle. Moving the probe back and forth causes the sound beam to sweep across the full height of a weld. This scanning motion enables inspection of the entire weld volume and detection of discontinuities both at the fusion lines and within the weld body [126]. A perpendicular crack will reflect angled sound energy along a path that is commonly referred to as a corner trap, as seen in the illustration Figure 3.40. The angled sound beam is highly sensitive to cracks perpendicular to the far surface of the test piece (first leg test) –​Figure 3.40a –​or, after bouncing off the far side, to cracks perpendicular to the coupling surface (second leg test) as shown in Figure 3.40b.

3.17.13 Surface Wave Technique To detect defects that are very close to the surface, surface waves are used. For its effectiveness, the surface condition should be good.

3.17.14 Surface Preparation The material surfaces to be used for the ultrasonic scanning must be smooth to allow the free movement of the probes and provide satisfactory conditions for the transmission of the ultrasonic waves. The surface should be free, on each side of the weld for a minimum of one skip distance, from weld spatter, loose rust and scale, grinding particles, dirt, paint, or other foreign matter. It may also be necessary to remove gross weld surface irregularities, undercut, sharp ridges, or valleys that will interfere with the interpretation of the test.

3.17.15 Probes Generation and detection of ultrasonic waves for inspection are accomplished by means of a transducer element, which is contained within a device known as a search unit or a probe. The active element in a search unit is a piezoelectric crystal.

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FIGURE 3.40  Angle beam technique. (a, b) First leg test shows a perpendicular crack reflecting sound wave and second leg test –​the angled sound beam after bouncing off the far side, to cracks perpendicular to the coupling surface (second leg test). (Courtesy of Olympus NDT Inc., Waltham, MA.)

3.17.16 Couplant Where the probes are in direct contact with the sample, a thin layer of viscous medium is used as a couplant between the sample and the probe. Couplants are needed to provide for effective transfer of ultrasonics between search units and parts being inspected. It may be a viscous material, liquid, semiliquid, or paste. Couplant having good wetting characteristics, such as SAE 30, machine oil, grease, gel, or water, should be used. The couplant should be noncorrosive and nontoxic. Couplants may not be comparable to one another, and the same couplant should be used for calibration and examination.

3.17.17 Ultrasonic Testing of Welds To detect all possible defects, the weld is to be examined over its entire cross section and along the length specified. In fixed position, the ultrasonic beam is directed at only a part of the weld; to examine the entire weld, the probe is moved over the scanning zone (Figure 3.41) in the following ways: lateral motion, traversing motion, swiveling motion, and orbital motion. Use frequencies of at least 2 MHz. Some UT schemes are shown in Figure 3.42 [127].

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FIGURE 3.41  UT scanning pattern.

FIGURE 3.42  UT techniques for defects detection.

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FIGURE 3.43  Position of probes for inspection of Butt welded surfaces –​tandem technique.

TABLE 3.11 Recommended Angle of Probe for Plate Thickness Plate Thickness (mm) 5–​15 15–​30 30–​60 Greater than 60

Angle of Probe Recommended 80° 70° 60° 45°

3.17.17.1 Plate Thickness and Angle of Probe Recommended The angles of probe recommended for different plate thickness are shown in Table 3.11. Thickness over 100 mm. When using a single-​probe technique, the path length for a full skip distance in thickness over 100 mm (approx. 4 in.) becomes too great to examine the entire cross section of the weld from one surface. In this case, the weld should be scanned from four sides, that is from both sides of the weld on each surface of the plate. Applications to Butt-​welded curved surfaces. While inspecting the curved surface, the probe may be suitably adopted to match the curved surface so as to comply with this requirement. The examination should be carried out with shear wave probes at a frequency of 2 MHz. At least two different probe angles (45° and 60° or 70°) should be used. Tandem technique scan should be made with 45° probes from the four sides of the weld. Figure 3.43 shows the arrangement of transmitter-​receiver probes for detection of longitudinally and transversely oriented defects in welds of butt-​welded curved surfaces. 3.17.17.2 Defect Location The accurate determination of the position of a defect in a welded joint is important to carry out repairs or from fracture mechanics point of view. The distance and the depth can be easily calculated from the path length, probe angle, and thickness.

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3.17.17.3 Sensitivity and Resolution Defect detectability is dependent on the sensitivity, resolution, and noise discrimination of the equipment. Sensitivity is the ability of the equipment to detect the minute amount of sound energy reflected from a defect. Resolution measures the ability to separate the indication resulting from multiple defects that are close together. 3.17.17.4 Examination Coverage The test object shall be examined by moving the search unit over the examination surface so as to scan the entire examination volume. As a minimum, each pass of the search unit shall overlap a minimum of 10% of the transducer dimension perpendicular to the direction of the scan. 3.17.17.5 UT Calculators To determine locations of subsurface indications, inspectors rely on ultrasonic calculators. Calculators come in models for common inspection angles, 45°, 60°, and 70°, used to determine sound path, depth, or surface distance when one variable is known. 3.17.17.6 Acceptance Criteria According to ASME Code Section VIII, Div. 1, Appendix 12, imperfections that produce a response greater than 20% of the reference level shall be investigated and evaluated in terms of the acceptance standards. Indications characterized as cracks, lack of fusion, or incomplete penetration are unacceptable regardless of length. For additional acceptance criteria, consult the referencing code section. 3.17.17.7 Reference Blocks Reference blocks are needed for calibrating the UT equipment. The standard reference flaws in calibration blocks serve as a base of comparison for real flaws. The standard flaws are cylindrical holes, flat-​bottom holes, grooves, or edges [128]. The most widely used reference blocks are IIW reference block and the distance sensitivity calibration (DSC) block (Figure 3.44a). There are other types of reference blocks, like the ASME block (Figure 3.44b), BWRA block, Dutch block, Sulzer’s block, ASTM block, miniature angle beam block, etc. 3.17.17.8 Calibration The calibration of the instrument is done to check parameters such as time base, linearity, resolution, sensitivity, beam spread, beam angle, probe index, etc. The ultrasonic probes must be calibrated accurately for angle of propagation, probe index, and beam form.

3.17.18 Phased Array Ultrasonic Testing Phased array ultrasonic testing (PAUT), or PA, is a specialized type of UT that uses multiple element array transducers and powerful software to steer high-​frequency sound beams through the test piece and map returning echoes, producing detailed images of internal structures. Two and three dimensional views can be generated showing the sizes and locations of any flaws detected. Conventional ultrasonic transducers for NDT commonly consist of either a single active element that both generates and receives high-​frequency sound waves, or two paired elements, one for transmitting and one for receiving. Phased array probes, on the other hand, typically consist of a transducer assembly with from 16 to as many as 256 small individual elements that can each be pulsed separately in a programmed pattern. These elements are pulsed in such a way as to cause multiple beam components to combine with each other and form a single wave front traveling in the desired direction. Similarly, the receiver function combines the input from multiple elements into a single presentation. Conventional UT and PA is shown schematically in Figure 3.45.

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FIGURE 3.44  UT reference block. (a) DSC block and (b) ASME block.

Figure 3.46 displays the concept of phased arrays, principle of testing weld, and example of defect image detected in welds. Time delays to the eight elements control focusing and beam sweep. Focal spot size is controlled by beam spread. Checking of phased array probe resolution is shown in Figure 3.47.

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FIGURE 3.45  Conventional UT and PA comparison (schematic).

FIGURE 3.46  Phased array technique. (a) Principle and (b) technique for welds. (Courtesy of Anmol Birring, Houston, TX.)

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FIGURE 3.47  Checking phased array probe resolution. (Courtesy of Anmol Birring, Houston, TX.)

3.17.18.1 Industry Applications Weld inspections Crack detection due to hydrogen damage (HIC, SOHIC, and SCC) Inspecting composite materials Corrosion mapping Flaw sizing for remaining life calculations. 3.17.18.2 Merits of PAUT Unlike conventional and automated UT, which is performed for fixed angles of 45°, 60°, and 70°, phased array testing can cover all angles in this range. This is significant as a single phased array inspection can cover all angles from 40° to 75° and displays the image in real time. 3.17.18.3 What do the Images Look Like? In most typical flaw detection and thickness gauging applications, the UT data will be based on time and amplitude information derived from processed RF waveforms. These waveforms and the information extracted from them will commonly be presented in one or more of four formats: A-​scans, B-​scans, C-​scans, and S-​scans. 3.17.18.3.1 A-​Scan Displays A-​scan is a simple waveform presentation showing the time and the amplitude of an ultrasonic signal, as commonly provided by conventional ultrasonic flaw detectors and waveform display thickness gauges. An A-​scan waveform represents the reflections from one sound beam position in the test piece. Generalized beam profile and direction of motion and image showing hole/​defect position by angle beam UT method as per A-​scan is shown in Figure 3.48. 3.17.18.3.2 B-​Scan Displays B-​scan is an image showing a cross-​sectional profile through one vertical slice of the test piece, showing the depth of reflectors with respect to their linear position. Generalized beam profile and direction of motion and image showing hole/​defect position by conventional UT method as per B-​ scan is shown in Figure 3.49.

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FIGURE 3.48  Generalized beam profile and direction of motion and image showing hole/​defect position by angle beam UT method as per A-​scan displays. (Courtesy of Olympus NDT Inc., Waltham, MA.)

FIGURE 3.49  Generalized beam profile and direction of motion and image showing hole/​defect position as per B-​scan displays. (Courtesy of Olympus NDT Inc., Waltham, MA.)

3.17.18.3.3 C-​Scan Displays C-​scan is a 2D presentation of data displayed as a top or planar view of a test piece, similar in its graphic perspective to an x-​ray image, where color represents the gated signal amplitude at each point in the test piece mapped to its x-​y position. Generalized beam profile and direction of motion and image showing hole/​defect position by conventional UT method as per C-​scan is shown in Figure 3.50, and generalized beam profile and direction of motion and image showing hole/​defect position by phased array method as per C-​scan is shown in Figure 3.51. 3.17.18.4 Notable Disadvantages of PAUT 1. Compared to conventional ultrasonics, PAUT may require a greater initial investment in equipment and experienced technicians, however these costs are frequently offset by the increased flexibility and a reduction in the time required to perform a given inspection. 2. PAUT often requires additional operator training to ensure the effectiveness and accuracy of the inspection results.

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FIGURE 3.50  Generalized beam profile and direction of motion and image showing hole/​defect position by conventional C-​scan displays. (Courtesy of Olympus NDT Inc., Waltham, MA.)

FIGURE 3.51  Generalized beam profile and direction of motion and image showing hole/​defect position as per phased array C-​scan displays. (Courtesy of Olympus NDT Inc., Waltham, MA.)

3.17.19 Application of Ultrasonic Technique for Thickness Measurement This technique’s primary application is the 3.17.19.1 Ultrasonic Plate Tester Ultrasonic plate testers (Figure 3.52) are available to perform edge-​to edge testing before cutting or welding operations in order to check quickly the presence of laminations, gross internal discontinuities such as pipes, ruptures, or slag inclusions. This is to avoid any likelihood of lamellar tearing due to stress introduced by welding [103]. 3.17.19.2 Ultrasonic Thickness Gauges Using the speed of sound through the test material, the material thickness is determined by calculating the time elapsed between peaks one and two. The delay is how long it takes for the ultrasound

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FIGURE 3.52  Ultrasonic plate tester.

FIGURE 3.53  Ultrasonic thickness measurement (schematic).

wave to pass through the interface and it can be used to determine the distance between the probe and material [130–​132]. Figure 3.53 shows ultrasonic thickness measurement of plate thickness. Ultrasonic thickness measurement by using drones. When a tall asset required non-​destructive testing (NDT) in the form of ultrasonic thickness measurement, it used to require a human and lifts, scaffolding, ladders, elevated baskets, catwalks, or other solutions. But now, drones are being deployed with ultrasonic testing (UT) devices to save humans from the height. Drones equipped with ultrasonic thickness gauges –​known as ultrasonic testing (UT) drones –​offer a safer alternative by eliminating the need for human personnel to perform the testing directly[133.1–​133.3, 134]. 3.17.19.3 Ultrasonic Coating Thickness Gages Ultrasonic testing works by sending an ultrasonic vibration into a coating using a probe (transducer) with the assistance of a couplant applied to the surface. The vibration travels through the coating until it encounters a material with different mechanical properties –​typically the substrate but perhaps a different coating layer. The vibration, partially reflected at this interface, travels back to the transducer. Meanwhile, a portion of the transmitted vibration continues to travel beyond that interface

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FIGURE 3.54  Ultrasonic coating thickness measurement (schematic).

and experiences further reflections at any material interfaces it encounters [132]. Figure 3.54 shows ultrasonic coating thickness measurement. 3.17.19.4 Quantitative Wall Thickness Measurements Quantitative wall thickness measurements can be made around the entire circumference of the pipe and it is possible to distinguish external and internal metal loss. In addition to ultrasonic tools for measuring wall thickness, intelligent pigs can be fitted with systems that use angular ultrasonic shear waves for detecting cracks. For inspection of geothermal piping it would first be necessary to remove any scale prior to using an intelligent pig and to select transducers with appropriate temperature range and ability to withstand corrosive environments. 3.17.19.5 Ultrasonic Examination of Nozzle Welds Ultrasonics is of prime importance for examining the attachment welds of nozzles and branches where the wall thickness involved is in the order of 25 mm or greater [103]. Various automated and semiautomated equipment has been developed for double-​fillet type through nozzles. The branch weld is normally subject to 100% examination before and after weld heat treatment.

3.17.20 Fracture Mechanics The basic premise of fracture mechanics is that every structural component, including pressure vessels, contains defects or discontinuities. Three major factors influence the performance of a fabricated product: stress, material, and defects [129]. Fracture mechanics allows calculation of the critical defect size, that is the limiting size between failure and no failure of the product. If the defect is larger than the critical defect size, then the failure is likely to occur. If the defect is smaller than the critical defect size, then the risk of failure is small. Before attempting any remedial action, particularly with thick welds, it is essential that the depth of any imperfection be determined so as to assess its importance and to assist in its removal, should that be considered necessary [90]. 3.17.20.1 Crack Evaluation Fracture mechanics analysis requires a knowledge of both the length and the extent in depth of a crack, with the latter being the more important factor. Radiography cannot easily indicate crack depth, since this requires continuous density measurement (with a light beam of the order of 10−2 mm wide) across the crack image [112]. However, the well-​established radiographic tube shift method may be used for depth location [90]. The UT technique provides actual defect size and position, which are then treated mathematically to determine under what service conditions failure may

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occur or the discontinuities need to be repaired. Alternately, AET can provide the location of the defect and UT can be used to determine its size. However, fracture mechanics and AET have limits to their uses, especially for tough, low-​strength steels.

3.17.21 Other Developments in UT? Some important developments in the field of UT examination are the following: 1. An ultrasonic probe, installed and left in place or attached to a moving fixture, provides a continuous scan of a test part. 2. Automated ultrasonic inspection of weld defects. Computer-​ based ultrasonic imaging system and automatic scanners for automated inspection of defects give permanent record, postprocessing of signal for detailed analysis, online imaging, and 3D imaging of defects, etc. 3. Ultrasonic plus eddy current. Southwest Research Institute, San Antonio, TX, has successfully combined UT and eddy current in order to simultaneously inspect the surface of a reactor vessel. It is capable of precise location of small cracks [114]. 4. Weld scan probes. Several new weld scan probes were developed whose benefits include instant results that couplant is not necessary, the effects of probe lift-​off that permeability variations and natural conductivity variations are minimized, and that whether welds have reinforcement, flush ground, or are of dissimilar metal, there is a probe type for the application [114].

3.17.22 Acoustical Holography Acoustical holography is an NDT technique that uses ultrasound to evaluate the interior integrity of a weld or a test object. Ehlman [135] describes the principle and applications of acoustical holography. Acoustical holography is mainly used for thick materials used for pressure vessels, castings, forging, etc. and inspection of welds in thick and thin materials, contouring of the surfaces, and inspection of nozzle welds. At present, two types of acoustical hologram systems are available: the liquid surface type and the scanning type. Scanning-​type equipment is mostly used. 3.17.22.1 Merits and Comparison of Acoustical Holography with Radiography and Ultrasonic Testing In conventional UT, it is difficult to obtain exact information concerning the location, shape, and size of the flaws. Using acoustical holography, a single scanning operation provides top, side, and 3D images that completely characterize weld defects [135]. 3.17.22.2 Holographic and Speckle Interferometry Advanced NDT techniques such as laser holography interferometry and speckle interferometry have demonstrated their usefulness for inspecting nuclear pressure vessels and piping. The techniques are suited to measuring deformation resulting from residual stresses or mechanical behavior of materials or fatigue.

3.17.23 Automated Ultrasonic Examinations A technique of ultrasonic examination performed with equipment and search units that are mechanically mounted and guided, remotely operated, and motor-​controlled (driven) without adjustments by the technician. The benefit of automation is achieved by the integration of NDT sensors with standard commercially available industrial robots as well as collaborative robots, also known as “cobots”. Custom written software for acquiring and visualising data creates a seamless and intuitive user experience that can be adapted to specific needs [136].

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3.17.24 Automated Ultrasonic Testing Methods Automated Ultrasonic (AUT) inspection services are advanced, accurate, and effective techniques to monitor discontinuities over successive inspection intervals, calculate growth rates of discontinuities, and plan repair or replacement activities. Automated ultrasonic testing equipment allows visualization and documentation of [137]: weld inspection, crack detection, production control, thickness and corrosion measurement and corrosion mapping.

3.17.25 Corrosion Mapping Automated ultrasonic straight beam corrosion mapping, as its name suggests, maps out and measures any flaws in the base material. This data is plotted to show top (C-​scan), side (B-​scan) and end views (D-​scan) as shown below. These views show the position, extent and depth of any defects and allow identification of individual flaws such as pitting as well as general loss of thickness. The scans will also show laminar defects in the plate including inclusions, plate laminations and blistering. An important feature of AUT is the high degree of repeatability.

3.17.26 Phased Array Corrosion Mapping The Phased Array unit is designed for internal and external corrosion-​erosion mapping on piping. The probe unit works with semi-​automated and full mechanized scanners or it can be used manually to produce encoded line scans. The scanner effectively creates a localized immersion bath with the PA probe. This allows the probe to scan rough and uneven surfaces while maintaining consistent coupling of the ultrasonic beam through the water column [137].

3.17.27 Hydrogen Damage Scans using multiple shear and straight beam probes or a single tri-​element probe is used to detect both blistering and stepwise cracking. A straight beam probe can detect laminar flaws but does not allow detection of stepwise linking of blisters. With the tri-​element probe, the straight beam probe is combined with two angle beam probes set at 90o to each other detect any out of plane reflectors such as cracking at the edges of blisters.

3.17.28 Weld Inspection (Pulse-​Echo and TOFD) The GPS and GPA scanners are capable of performing both conventional pulse-​echo (shear wave angle beam) and Time of Flight Diffraction (TOFD) scans. Complete weld volume and HAZ coverage is assured by collecting data points at every 1mm perpendicular to the weld axis (y-​direction) before indexing the scanner 4mm parallel to the weld axis (x-​direction).

3.17.29 Different Techniques of Automated Ultrasonic Testing The following are some popular automated ultrasonic testing techniques used in different industries [138]: 3.17.29.1 Rapid Ultrasonic Gridding Rapid ultrasonic gridding is a computerized ultrasonic inspection technique that creates thickness grid maps to identify areas that are affected (wall-​thinning) because of corrosion and other damage mechanisms. It can cover a large surface area ten times faster than manual ultrasonic testing or typical automated ultrasonic testing techniques.

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3.17.29.2 Phased Array Ultrasonic Testing PAUT (Phased Array Ultrasonic Testing) is a type of automated ultrasonic testing that uses electronic phased array probes to induce pulses to determine damage mechanisms at multiple depths. PAUT technique is utilized to inspect welds, high-​temperature zones and areas that are likely to be cracked. 3.17.29.3 Rapid Automated Ultrasonic Testing Rapid AUT (RAUT) is used to gather high-​definition data utilizing the PAUT probes. Therefore, it is an ideal technique to comprehend a particular damage mechanism required to determine maintenance needs. In other words, RAUT uses a phased array probe that can perform 94,000 thickness scans on each square foot. The RAUT technique combines the hardware motions. 3.17.29.4 Full Matrix Capture (FMC) FMC is an evolution of the PAUT technique and uses the same probes. Its main advantage is that there is no need to focus or steer the beam as the entire area of interest is in focus. It is also relatively tolerant of misaligned flaws and structural noise. This makes it very easy to set up and use. The disadvantage is that the file sizes are very large and the acquisition speed can be slower than with PAUT.

3.17.30 Automated and On-​Line Ultrasonic Testing Instrumentation exists to perform automated A-​, B-​and C-​scans of wall thickness and provide digital, hard copy and video images. Numerous other improvements over manual testing are available. It is also possible to permanently install ultrasonic transducers for continuous monitoring in a particular location.

3.17.31 Other methods 1. Encoded manual ultrasonic examinations (EMUT). A technique of ultrasonic examination performed by hand with the addition of an encoder, and may or may not include a guiding mechanism (i.e. a wheel or string encoder attached to the search unit or wedge). 2. E-​scan (also termed an electronic raster scan). A single focal law multiplexed, across a grouping of active elements, for a constant angle beam stepped along the phased array probe length in defined incremental steps. 3. Semiautomated ultrasonic examinations (SAUT). A technique of ultrasonic examination performed with equipment and search units that are mechanically mounted and guided, manually assisted (driven), and which may be manually adjusted by the technician.

3.18 ADVANCED UT METHODS Ultrasonic testing includes many different types some of which are given below. 1. Automated Ultrasonic Backscatter Technique (AUBT) 2. Rapid Ultrasonic Gridding (RUG) 3. Dry-​Coupled Ultrasonic Testing 4. Long Range Ultrasonic Testing (LRUT) 5. Internal Rotary Inspection System (IRIS) 6. Time-​of-​Flight-​Diffraction (TOFD).

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FIGURE 3.55  Advanced ultrasonic backscatter technique (AUBT).

3.18.1 Advanced Ultrasonic Backscatter Technique (AUBT) Developed by Shell in the early 1990’s, AUBT is a reliable method for detecting and quantifying damage in pressure vessels and piping from high temperature hydrogen attack (HTHA). The technique uses conventional UT probes and a digital oscilloscope to provide both an A-​Scan display and frequency analysis. AUBT procedures cover both the parent material and welds (HAZ) [139–​141]. Ultrasonic Backscatter Technique is shown schematically in Figure 3.55 [139]. AUBT incorporates ultrasonic backscatter detection with velocity measurement and spectral analysis. The examinations incorporate: 1. Spectrum analysis helps determine the degree of HTHA, is sensitive to fissures and is independent of the measurement system. 2. By analyzing the difference in ratio between sound velocities in materials both affected and unaffected by HTHA, the presence and extent of HTHA damage can be determined.

3.18.2 Rapid Ultrasonic Gridding (RUG) Rapid Ultrasonic Gridding (RUG) is a method of nondestructive testing (NDT) used to gather thickness measurements of industrial infrastructure in the power generation, oil and gas, chemical, and paper industry. It utilizes multiple ultrasonic thickness probes simultaneously, to rapidly gather thickness measurements in a predefined or ad hoc space. RUG can be utilized for a quantitative inspection of tank floors by providing thickness corrosion mapping of the floor as opposed to traditional qualitative inspections performed using MFL floor scanners [142, 143].

3.18.3 Dry-​Coupled Ultrasonic Testing (DCUT) During UT inspection, due to the impedance mismatch between the piezoelectric transducer and the material inspected, a liquid couplant is typically used to transmit the vibrations from the transducer into the part and receive the vibrations back into the transducer. An alternative method to using liquid couplant is Dry-​Coupled Ultrasonic Testing (DCUT) [144]. DCUT is a low-​cost method that does not require a liquid couplant to inspect metallic and nonmetallic material. Additionally, DCUT transducers are capable of withstanding high voltages. DCUT is a versatile method that can

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be performed using flexible, contact, wheel, or remote transducers. The inspection is cleaner, easier, and does not contaminate the material inspected or the environment.

3.18.4 Long Range Ultrasonic Testing (LRUT) or Guided Wave Ultrasonic Testing Long Range Ultrasonic Testing (LRUT) or Guided Wave Ultrasonics (GWUT) refers to the use of guided waves for inspection of hollow cylindrical structures, such as pipes and tubes, using a ring of transducers placed around the structure. The interaction of all the beams from the different transducers contained within the tube geometry create a unified wave front that fills up the tube geometry and permits travelling long distances. Depending on the tube material and interfacial boundary conditions (e.g. tube coatings, surrounding soil /​cement, liquids or solids inside the pipe), LRUT rings can be used to inspect up to approximately 100 m (310 ft) in front and behind the inspection ring. Inspection method. This method is performed by placing a ring of transducers around a pipe. The sound waves emitted from these transducers travel down both directions of the pipe. If they come in contact with corrosion or damage they will be reflected back towards the transducers, which then collect the data automatically. What differentiates this from more traditional methods of UT is that, with LRUT, a liquid couplant between transducers and the surface is not required. For this reason LRUT is one of the fastest inspection tools for carrying out pipeline surveys for corrosion and other damage mechanisms [145–​149]. GW travelling in a pipe is shown in Figure 3.56a [146] and Figure 3.56b [147] shows the instrumentation for GW inspection method.

Insonified area Normal-beam excitation

α

Insonified area Angle-beam excitation

λ Combined excitation

FIGURE 3.56  (a) GW inspection method-​principle. (b and c) GW inspection method of a pipe.

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FIGURE 3.56 (Continued)

3.18.5 Internal Rotating Inspection Systems The ultrasonic internal rotating inspection system (IRIS) option is used to inspect a wide range of materials including ferrous, nonferrous, and nonmetallic tubing. This technique detects and sizes wall loss resulting from corrosion, erosion, wear, pitting, cracking, and baffle cuts [150–​152]. Olympus digital IRIS inspection technology is used extensively as a prove-​up technique for remote field testing (RFT), MFL, and eddy current inspections. IRIS relies on a transducer to generate an ultrasonic pulse parallel to the axis of the tube under test. It also relies on a rotating mirror that directs the ultrasonic wave into the tube wall. The mirror is driven by a small turbine powered by the pressure of water pumped into the tube. The system works by inserting a probe into a flooded pipe. The probe then move through the pipe, scanning as it goes. IRIS method involves inserting and pulling a probe through a tube to be inspected where it directs an ultrasonic beam towards the internal tube wall. By measuring these sound waves, the probe can detect things like internal and external erosion, corrosion, pitting, denting, and fretting. It can also detect bulges, restrictions, remaining wall thickness, channeling, and tubesheet defects. IRIS is an ultrasonic technique and hence it requires a couplant. In this case, water. Tubes under test must therefore first be flooded to use this technique. IRIS testing method is shown in Figure 3.57a and Figure 3.57b [153, 154].

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FIGURE 3.57a  Tube inspection with IRIS for ferrous and nonferrous materials. (Courtesy of Olympus NDT Inc., Waltham, MA.)

FIGURE 3.57b  IRIS testing method [153].

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3.18.6 Time of Flight Diffraction (TOFD) Time of Flight Diffraction (TOFD) is an inspection method used to detect flaws in welds. In time-​ of-​flight diffraction (ToFD) systems, a pair of ultrasonic probes are used, positioned on opposite sides of a weld-​joint or area of interest. A transmitter probe emits an ultrasonic pulse which is picked up by the receiver probe on the opposite side. In an undamaged part, the signals picked up by the receiver probe are from two waves: one that travels along the surface (lateral wave) and one that reflects off the far wall (back-​wall reflection). When a discontinuity such as a crack is present, there is a diffraction of the ultrasonic sound wave from the top and bottom tips of the crack. Using the measured time of flight of the pulse, the depth of the crack tips can be calculated automatically by trigonometry application. Figure 3.58 [155] shows the two probes TOFD inspection method. For more details, refer ASME Code Section V. TOFD can be used independently or in conjunction with other ultrasonic techniques. Some of the most common techniques are [156]: single group TOFD, multiple TOFD, TOFD with pulse echo/​creeping waves, and TOFD with phased array.

3.19 ACOUSTIC EMISSION TESTING Acoustic emission testing (AET) offers a new method for analyzing various physical phenomena and behavior of materials, and for performing NDT of materials, manufacturing processes, and structural components [157–​160]. AE is commonly defined as transient elastic waves within a material, caused by the rapid release of localized stress energy. Hence, an event source is the phenomenon which releases elastic energy into the material, which then propagates as an elastic wave. Acoustic emissions can be detected in frequency ranges under 1 kHz, and have been reported at frequencies up to 100 MHz, but most of the released energy is within the 1 kHz to 1 MHz range [160].

3.19.1 Principle of Acoustic Emission Acoustic emission (AE) is the phenomenon of transient elastic wave generation due to a rapid release of strain energy caused by a structural alteration in a solid material. The release of AE is illustrated in Figure 3.59a and 3.59b [158]. When a discontinuity approaches a critical size, the AE count rate increases markedly, warning of impending instability and failure. The emission of transient elastic waves or sound waves can be detected by placing a sensor on the surface of the material. The amplitude of an AE is proportional to local strain in the base material at the AE site. A time-​amplitude noise signature generated during the crack growth can be used to indicate the failure process likely to emanate from the site. Since AE reveals its existence during its growth, this leads to one of the advantages of the method: real-​time monitoring.

3.19.2 Source of Acoustic Emission Generally, the structural alterations are the results of either internally generated or externally applied load or due to various physical or failure modes. Physical phenomena and failure modes that can release AE are described in Refs. [157, 161] and are given in Table 3.12.

3.19.3 Emission Types and Characteristics Basically, the AE signals have been classified into two different types [162]: burst type and continuous type. The characteristics of these wave forms are discussed by Baldev Raj et al. [163]. Both the emission types are generated by discrete processes. The difference between these two types is in the average repetition rate.

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FIGURE 3.58  Basic TOFD two-​probe testing configuration.

3.19.4 Kaiser Effect An important feature of AE is its irreversibility. If a material is loaded to a given stress level and then unloaded, usually no AE will be released upon immediate/​reloading until the previous load has been exceeded. This is known as the Kaiser effect. This property has an important practical

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FIGURE 3.59  (a and b) Release and propagation of AE in a material and the principle of AET. ((a) Adapted from [157] Lenain, J.-​C.)

TABLE 3.12 Written Procedure for AET (1)  the equipment to be used (2)  the couplant, sensor type and location, frequency, and the location/​placement of AE sensors (3)  the process for stressing the component (4)  the data to be recorded and reported (5)  the qualifications of the personnel operating the equipment and interpreting the results (6)  system calibration.

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implication because it can be used in the detection of subcritical growth of flaws, such as stress corrosion cracking, fatigue crack growth, and hydrogen embrittlement.

3.19.5 Physical Phenomena that Can Release AE Primary sources of acoustic emission in materials that undergo fracture are: 1. plastic deformation development and fracturing of hard inclusions in metals 2. fiber breakage, matrix cracking and delamination in composites 3. welding defects 4. aggregate fracture, voids closure, etc. in concrete.

3.19.6 Reference Code 1. ASME Code Section V covers AE examination of metal vessels and fiber-​reinforced vessels. 2. ASTM E2374-​16(2021) –​Standard Guide for Acoustic Emission System Performance Verification.

3.19.7 AE Methods Applications The three major applications of AE techniques are [160]: 1. Source location –​determine the locations where an event source occurred. 2. Material mechanical performance –​evaluate and characterize materials/​structures. 3. Health monitoring –​monitor the safety operation of a structure, i.e. bridges, pressure containers, and pipe lines, etc. 3.19.7.1 AE Methods Applications as per ASME Code Section V 1. Acoustic Emission Examination of Fiber Reinforced Plastic Vessels. 2. Acoustic Emission Examination of Metallic Vessels During Pressure Testing. 3. Continuous Acoustic Emission Monitoring of Pressure Boundary Components. 4. Continuous acoustic emission (AE) monitoring of metallic components in nuclear plant systems. 5. Continuous acoustic emission (AE) monitoring of non-​nuclear metal components. 6. Continuous monitoring of nonmetallic (fiber reinforced plastic) components.

3.19.8 Written Procedure The written procedure to be applied during an AE examination usually specifies [161] (1) the equipment to be used; (2) the couplant, sensor type and location, frequency, and the location/​ placement of AE sensors; (3) the process for stressing the component; (4) the data to be recorded and reported; (5) the qualifications of the personnel operating the equipment and interpreting the results; and (6) system calibration.

3.19.9 Equipment The AE monitoring system consists of sensors, preamplifiers, amplifiers, filters, signal processors, and a data storage device together with interconnecting cables or wireless transmitters and receivers. Simulated AE source(s) and auxiliary equipment such as pressure gauges and temperature sensors are also required.

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3.19.9.1 AE Testing Instrument The important items of the instrumentation are as follows: (1) a sensor (piezoelectric crystal device) with frequency response between 20 and 600 kHz, (2) a preamplifier to convert signals from 10 μV to 1–​10 V DC, (3) high-​pass or band-​pass filters to eliminate mechanical and electromagnetic noises, (4) an amplifier, (5) a threshold detector, and (6) a counter.

3.19.10 Signal Analysis To use AE monitoring, the noise generated by the flaw growth should be picked and the extraneous background noise should be suppressed. One can sort out sound by three characteristics: frequency, amplitude, and rate of occurrence [164]. There are several ways to process the AE signal. The following are the more common ways by which signals are processed: ring-​down counting analysis, energy analysis, amplitude distribution analysis, and frequency analysis [157], However, the most widely used method for the analysis of AE signals is ring-​down counting, which involves the counting of the number of threshold crossings, called counts. Ring-​down counting has the major advantages of simplicity and practical effectiveness. An AE signal is characterized by parameters like [157] (1) peak amplitude, (2) signal duration, (3) number of counts per event, (4) rise time, and (5) energy.

3.19.11 Factors Influencing AE Data The factors that influence AE data include [163] source characteristics, structural characteristics of the medium, frequency pass band, and gain of amplifiers and threshold level.

3.19.12 Applications: Role of AE in Inspection and Quality Control of Pressure Vessels and Heat Exchangers AE techniques can be successfully used for the following applications [161]. Vessel testing and examination. The hydrostatic test performed during vessel acceptance testing is one of the best times for an AE examination. The AE examination process is sensitive to environmental conditions (rain, blow dusting, etc.), motors and rubbing cables, piping, and mechanical noises from adjacent pumps. No examination should be run when sensor noise is too high. Proof testing of pressure vessels and heat exchangers. Pressure vessel testing has been one of the most successful areas of application of AE technique. This method is used extensively for preservice proof testing, periodic requalification testing, and continuous online monitoring of pressure vessels [161]. When discontinuity approaches critical size, the AE count rate increases markedly, warning of impending instability and failure. Online monitoring. AE is produced when the component undergoes a dynamic change such as deformation, crack growth, phase change, or leakage of a fluid from a pressurized compartment. Hence, continuous online monitoring is possible [166]. Monitoring of welds. AE can monitor and detect failure modes that generate AE, such as phase transformations, lack of fusion, lack of penetration, voids and porosity, inclusions and contamination, and cracking. The technique offers great promise for preliminary weld screening prior to NDT and for welder training [164]. Environmental cracking. AE is used for detecting environmental cracking phenomena such as corrosion fatigue, hydrogen embrittlement, and stress corrosion cracking.

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Thermal shock. Frequently, vessels subject to thermal stress cycles are monitored during temperature cycling, during start-​up, or during shutdown. Weld overlays. AE can monitor separation and delamination of weld overlays. Detection of creep failure at welds. AE has been used to study reheat cracking, a form of brittle creep rupture that occurs in the heat-​affected zones of welds during stress relief [165]. Leak detection. Leak detection by AE testing method provides rapid quantitative information. The method is as follows: in the absence of any leak, the AE activity will only be a minimum, caused by the background noise. In the presence of a leak, pressure release from inside the vessel results in the generation of AE that can be detected by the sensor.

3.19.13 Merits of Acoustic Emission Testing The advantages of AET are discussed in Ref. [161]. Some of the advantages are as follows: 1. In AE monitoring, there is no input energy to the structural system/​material as in the case of UT or radiography. 2. AE has the capacity to locate and evaluate discontinuities in the entire structure at one time, rather than selectively testing localized regions. 3. AET may be used as a means for confirming the location and concentration of discontinuities determined by techniques such as RT and UT. 4. The use of AET with fracture mechanics and UT may allow the complete characterization of a pressure vessel and its life can be predicted. 5. AE monitoring on production lines is faster than ultrasonics because its sensors need not be moved over the entire surface being inspected.

3.19.14 Acousto-​Ultrasonics (AU) A nondestructive examination method that uses induced stress waves to detect and assess diffuse defect states, damage conditions, and variations of mechanical properties of a test structure. The AU method combines aspects of acoustic emission (AE) signal analysis with ultrasonic materials characterization techniques.

3.20 EDDY CURRENT TESTING ET or ECT is a noncontact method widely used for the NDT of tubular products. The basis of ET is the detection of quality problems by observation of the interaction between electromagnetic fields and metals. In this method, a small electric current, known as an eddy current, is induced in a material, and any changes in the flow of this current due to a flaw or inhomogeneities in the material are detected by a nearby coil and subsequently processed electronically. Conventional eddy current and remote field eddy current are the two commonly used techniques for heater tube inspections. Nonferromagnetic tubes such as austenitic stainless steel, copper-​nickel alloys, brass are inspected by conventional eddy current method. Ferromagnetic materials are inspected by the remote field eddy current method.

3.20.1 Eddy Current Testing Principle Eddy currents are formed by an alternating magnetic field as an energized A/​C coil nears a conductive material. When there is a defect in the material, the flow of the currents changes and can be

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FIGURE 3.60  Eddy current testing principle.

detected by measuring the impedance changes that occur in the A/​C coil. This method of testing is a very efficient nondestructive method for finding defects in the heat exchanger and condenser tubing. Eddy Current testing principle is shown in Figure 3.60 [167].

3.20.2 Eddy Current Techniques The eddy current technique encompasses several branches of inspections, including [168]: 1. eddy current testing (EC) 2. pulsed eddy current (PEC) 3. pulsed eddy current array (PECA) 4. surface scan eddy current (SECT) 5. partial saturation eddy current (PSEC). 3.20.3  Common applications Eddy current NDT can examine large areas very quickly, and it does not require the use of coupling liquids. In addition to finding cracks, eddy current testing can also be used to check metal hardness and conductivity and to measure thin layers of nonconductive coatings, such as paint on metal parts. At the same time, eddy current testing is limited to materials that conduct electricity and thus cannot be used on plastics. In heat exchanger and pressure vessel applications, the areas of applications of ET include [169]: 1. Production-​line inspection of tubular products during manufacture to detect seams, laps, cracks, voids, inclusions, etc. on conductive metal surfaces. 2. In-​service inspection of tubes for service-​induced defects such as loss of tube wall thickness due to corrosion, erosion, pitting, fretting wear, baffle cuts, and growth of manufacturing defects. 3. For surface, and in some cases subsurface, inspection of welds for discontinuities. 4. Material sorting. 5. Clad overlay measurement.

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6. 7. 8. 9.

Paint thickness measurement. Coating thickness measurement. Electrical conductivity measurements. Bolt hole inspection. Cracking inside bolt holes can be detected using bolt hole probes, often with automated rotary scanners. 10. Thickness measurements of thin material. Eddy current techniques can be used to perform a number of dimensional measurements. The type of measurements that can be made include [170]: a. thickness of thin metal sheet and foil, and of metallic coatings on metallic and nonmetallic substrate b. cross-​sectional dimensions of cylindrical tubes and rods c. thickness of nonmetallic coatings on metallic substrates.

3.20.4 Tube Inspection Eddy current inspection is often used to detect corrosion, erosion, cracking and other changes in tubing. Heat exchangers and steam generators, which are used in power plants, have thousands of tubes that must be prevented from leaking. The eddy current test method and the related remote field testing method provide high-​speed inspection techniques for these applications [171]. ET tube inspection principle is shown in Figure 3.61a [172].

3.20.5 Eddy Current Testing Method In ET or ECT, an alternating current is made to flow in a coil (probe), which, when brought into close proximity of the conducting surface of the material to be inspected, induces an eddy current flow in the material (Figure 3.61b). The presence of a defect, a discontinuity, or a metallic object other than the specimen under test disturbs the eddy current flow. The test coil is placed in a unit called a measuring probe. The process lends itself to automated production-​line testing. ECT signals are interpreted using the ASME depth curve to determine their depth. The ASME depth curve is based on the phase of the ECT signal. Signals with a phase angle in the 0°–​40° range represents

FIGURE 3.61a  Eddy current inspection principle.

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FIGURE 3.61b  Principle of ET of tube. (a) Interaction between electromagnetic fields and metals and (b) flow of eddy currents on a normal tube and around a defect. (Courtesy of Olympus NDT Inc., Waltham, MA.)

inner diameter defects while signals with a phase angle of greater than 40° represent outer diameter defect. The signal from a hole is set to 40°. The ET principle explained in this section is based on Granville [173] and Ref. [174].

3.20.6 Eddy Current Examinations Methods as per ASME Code Section V Some of Eddy current examinations methods as per ASME Code Section V: 1. Eddy Current Examination of Nonferromagnetic Heat Exchanger Tubing. 2. Eddy Current Examination on Coated Ferromagnetic Materials. 3. External Coil Eddy Current Examination of Tubular Products. 4. Eddy Current Measurement of Nonconductive-​Nonferromagnetic Coating Thickness on a Nonferromagnetic Metallic Material. 5. Eddy Current Detection and Measurement of Depth of Surface Discontinuities in Nonferromagnetic Metals With Surface Probes. 6. Eddy Current Examination of Ferromagnetic and Nonferromagnetic Conductive Metals to Determine If Flaws Are Surface Connected.

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TABLE 3.13 Written Procedure for Eddy Current Testing 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Scope of examination: for tubing examination, tube material, diameter, wall thickness… Size and type of probes Examination frequencies Eddy current equipment model and manufacturer Scanning direction and speed during examination Inspection technique, e.g. hand probe, mechanized probe, remote-​control fixtures Calibration procedure and standards Description of data-​recording equipment and procedure Signal processing and acceptance criteria Reporting results

7. Alternative Technique for Eddy Current Examination of Nonferromagnetic Heat Exchanger Tubing, Excluding Nuclear Steam Generator Tubing. 8. Eddy Current Array Examination of Ferromagnetic and Nonferromagnetic Materials for the Detection of Surface-​Breaking Flaws. 9. Eddy Current Array Examination of Ferromagnetic and Nonferromagnetic Welds for the Detection of Surface-​Breaking Flaws.

3.20.7 Written Procedure ASME Code Section V furnishes the minimum information necessary in the written procedure. Written Procedure for Eddy current testing is shown in Table 3.13

3.20.8 Reference Standards for Eddy Current Testing An eddy current system consisting of an instrument and a probe must always be calibrated with appropriate reference standards at the start of a test. A sample reference sample of tube inspection is shown in Figure 3.62 [175]. In thickness measurement applications, the reference standards would consist of various samples of known thicknesses. The operator observes the response from the reference standards and then compares the indications from test pieces to these reference patterns to categorize parts. Proper calibration with appropriate reference standards is an essential part of any eddy current test procedure.

3.20.9 ASTM Specifications E 426-​16(2021) for electromagnetic (eddy current) testing of seamless and welded tubular products of austenitic stainless steel and similar alloys. E 215-​22 for testing of aluminum alloy tubes. E 309-​16 for steel tubular products with magnetic saturation. E 243-​18 for seamless copper and copper alloy tubes. ASTM B903-​15(2022) –​Standard Specification for Seamless Copper Heat Exchanger Tubes With Internal Enhancement. ASTM E2884-​22 –​Standard Guide for Eddy Current Testing of Electrically Conducting Materials Using Conformable Sensor Arrays.

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FIGURE 3.62  A sample reference tube for ECT.

3.20.10 Petrobes Probes can be classified into four categories [91, 176]: 1. Surface probes, which are used for crack detection and corrosion measurements. 2. Encircling probes, which are used primarily for production control of wire or tubular products (Figure 3.63a). 3. Internal bobbin probes, which are used to check tubings (Figure 3.63c). 4. Rotating scanning probe, also known as ID surface riding “pancake coil” (RPC) as shown in Figure 3.63c; it can locate very small defects and determine their exact location on the circumference. 3.20.10.1 Probe Configuration Two basic probe coil configurations are used for tube inspection. They are (1) the absolute probe configuration that consists of a single coil as shown in Figure 3.63a, and its test setup is shown in Figure 3.63b and (2) differential probe configuration, which consists of two coils connected in opposition as shown in Figure 3.63c. Figure 3.64 shows differential probe with single encircling coil. The absolute probe is sensitive to gradual changes in tube dimensions such as gradual tube thinning, but relatively poorly sensitive for small, sharp defects such as pitting. In differential probe configuration, the net induced voltage is canceled out when the coils experience identical conditions. Because of this, differential probes are highly sensitive to sharp defects such as pitting, but not sensitive to defects such as gradual tube thinning [69]. Differential coils: two or more coils electrically connected in series opposition such that any electric or magnetic condition, or both, that is not common to the areas of a specimen being electromagnetically examined will produce an unbalance in the system and thereby yield an indication.

3.20.11 Eddy Current Test Equipment Eddy current test equipment covers a range from simple units to fully automatic systems. In general, the eddy current equipments will incorporate these elements: 1. A source to provide an alternating current of required frequency to excite the test coil. 2. A modulating device consisting of a test coil and test object combination, which brings out desired information in the form of an electrical signal.

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FIGURE 3.63  ET probes: (a) single coil encircling probes, (b) typical test setup with single coil probe, and (c) internal bobbin probes.

FIGURE 3.64  Eddy current differential probe configuration.

3. Signal processing. 4. Signal display.

3.20.12 Signal Processing Eddy current signals are vector quantities and have both amplitude and direction (phase). The amplitude and phase of the signals can be displayed on a cathode-​ray oscilloscope. In the case of tube inspection, the phase of the signal can give information regarding the depth and defect origin [173].

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3.20.13 Inspection or Test Frequency and its Effect on Flaw Detectability Generally, test frequencies used in eddy current inspection range from 200 Hz to 6 MHz or more. Frequency has a direct bearing on the ability to penetrate the component wall thickness. At lower frequencies, penetration is greater, but the sensitivity to flaw detection decreases, whereas at high frequencies, the depth of penetration is lower, and small flaws remain undetected as the depth increases. An optimum frequency should be selected so that the penetration is sufficient to reach any subsurface flaw. When detecting flaws at some considerable depth below the surface, that is a thicker part, very low frequencies must be used and sensitivity is sacrificed [174].

3.20.14 Operating Variables The principal operating variables associated with eddy current inspection include coil impedance, electrical conductivity, magnetic permeability, lift-​off and fill factors, edge effect, and skin effect. Fill factor, edge effect, skin effect, depth of penetration, and frequency are discussed next [173]. 3.20.14.1 Depth of Penetration and Frequency The depth of penetration of eddy current is a critical factor. For example, in the case of tube inspection, if the eddy currents do not penetrate the wall thickness, then it will miss the defects. The depth of penetration of eddy current (τ) is given by the following equation [173]:



τ=

500

σµ f

where σ is the conductivity (m/​Ω · mm2) μ is the magnetic permeability (one for nonmagnetic materials) f is the test frequency (Hz) Depth of penetration is shown in Figure 3.65.

FIGURE 3.65  Depth of penetration of eddy current.

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The standard depth of penetration is generally taken to be that depth at which the eddy current field intensity drops to 37% of the intensity at the conductor surface. For tube inspection, the test frequency is often the frequency at which the depth of penetration is equal to the tube wall thickness. This is given by the equation [173]

f =

250 kHz σt 2

where t is the tube wall thickness (mm). When phase analysis is being applied, the f90 frequency, that is the frequency at which there is a 90° phase difference between signals from the inside wall of the tube and signals from the outside wall, will be used. The f90 frequency is given by the equation [173]

f90 =

3ρ kHz t2

where ρ is the resistivity (μohm · cm). 3.20.14.2 Fill Factor and Probe Size Requirements The probe size requirements for eddy current tube testing are determined by the “fill factor”, given by the expression [173] 2



d  Fill factor =  1   d2 

where d1 is the diameter of the probe d2 the inside diameter of the tube. For an internal or bobbin-​type coil, the fill factor indicates how well the inspection coil fills the inside of the tube being inspected. Ideally, the fill factor should be close to 1.0. A fill factor of 1.0 will not allow smooth travel of the probe inside the tube. Therefore, as a thumb rule, the optimum fill factor for tube testing can be about 0.70 [173]. This allows reasonable sensitivity to be achieved while still maintaining adequate clearance when dirt or dents may be present in the tube. 3.20.14.3 Edge Effect When an inspection coil approaches the end or edge of a part being inspected, the eddy currents are distorted, because they are unable to flow beyond the edge of the part. The distortion of eddy current results in an indication known as “edge effect”. In general, it is not advisable to inspect any closer than 118 in. (3.2 mm) from the edge of a part [174]. 3.20.14.4 Skin Effect Eddy currents are not uniformly distributed throughout a part being inspected. They are densest at the surface immediately beneath the coil and become progressively less dense with increasing distance below the surface. This phenomenon is known as “skin effect”. At some distance below the surface of a thick wall, there will be essentially no current flowing.

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3.20.15 Inspection Method for Tube Interior The probe, usually an internal bobbin type, is introduced into the tube under test and passed along the length of the tube to the far end, usually not only by means of compressed air, but also by other means. It is then withdrawn from the tube at a constant speed, typically in the range of 0.5–​1.0 m/​ s, using a winch unit [91, 173]. During the probe withdrawal, the eddy current signals are recorded on a chart and analyzed further.

3.20.16 Inspection of Ferromagnetic Tubes The highly ferromagnetic nature of carbon steel and ferritic stainless steels severely limits the application of standard eddy current technique. Nonferromagnetic materials have a relative permeability of approximately one, whereas ferromagnetic materials have typical relative permeabilities of 500–​2000. Since the higher the permeability is, the lower is the depth of penetration, eddy currents in ferromagnetic materials are concentrated at the surface and hence buried or back-​wall defects remain undetected. Also small variations in permeability give rise to relatively high noise levels [173]. This limitation is overcome by saturating the ferromagnetic tubes magnetically with a direct current, so that the material reacts as a nonmagnetic material when inspecting [91] or by remote field slit eddy current (RFEC) technique. The drawbacks of these two methods are discussed by Bergander [177]. Such drawbacks are as follows: (1) Since magnetic saturation uses high-​amperage current for saturation, there is a need for probe cooling, the possibility of false indications due to variations in permeability, difficulty in detecting gradual tube thinning, and complicated probe design. (2) To make the RFEC technique sensitive to gradual wall thinning, the probes have to be complex. To overcome these difficulties, Bergander [177] suggests using a magnetic flux leakage (MFL) method to examine ferritic tubes.

3.20.17 Testing of Weldments Because of the circumferential orientation of eddy current flow, this method is effective in detecting most types of longitudinal weld discontinuities such as open welds and weld cracks. On the other hand, difficulty arises for the detection of a thin planar discontinuity that is oriented substantially perpendicular to the axis of the cylinder.

3.20.18 Calibration In order to provide accurate and repeatable results, artificial defects of known size are introduced into a specimen piece of tube of the same material and dimensions as the tubes to be inspected. Reference defects can be of various types. The most commonly used reference standard consists of a number of through holes in a tube along with their signal pattern [175].

3.20.19 Merits of ET ET is attractive because it offers both very high sensitivity and relatively high scanning speeds. Direct contact with the test material is not required and a coupling medium is not necessary; the process is repeatable, the test method is relatively fast, and it can be easily automated.

3.20.20 Limitations of Eddy Current Testing The major limitations of ET are the following: 1. Skin effects usually restrict the depth of inspection. It is generally limited to 0.25 in. (6.4 mm) for nonferromagnetic materials and 0.010 in. (0.25 mm) for ferromagnetic materials [176].

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The limitation due to high permeabilities of ferromagnetic materials may be significantly overcome by magnetic saturation of the area being inspected or by flux leakage method. 2. Since many variables, namely, permeability, conductivity, probe position, and weld contour, can affect an eddy current signal, an accurate measurement of one property requires the ability to eliminate or at least considerably reduce interference from other properties [176]. Multifrequency techniques are used in practice to increase the information content of signals [175]. A classical solution, due to recent developments in ET, which can reduce but not eliminate this problem, is the application of different ET probes [176]. Other limitations include [173] the following: The signal is more closely related to volume of material lost than to wall thickness lost. Defects at the tubesheet, and at the tube and baffle plate interface, can be difficult to detect. For critical applications, results may need to be verified by an alternative technique. 3.20.21  Eddy Current Arrays Eddy current array testing, or ECA, is a technology that provides the ability to simultaneously use multiple eddy current coils that are placed side by side in the same probe assembly. Each individual coil produces a signal relative to the phase and amplitude of the structure below it. This data is referenced to an encoded position and time and represented graphically as a C-​scan image showing structures in a planar view. In addition to providing visualization through C-​scan imaging, ECA enables coverage of larger areas in a single pass while maintaining high resolution. ECA can permit use of simpler fixturing and can also simplify the inspection of complex shapes through custom probes built to fit the profile of the test piece [178].

3.20.22 Automated Surface Inspection Using Eddy Current Array Technology Inspecting the surface of long bars and tubes has been a requirement in the metal manufacturing industry for many years. Material quality and performance requirements are continuously evolving with respect to the various types, orientations, and sizes of surface indications. While different technologies can be used to detect surface defects, eddy current array (ECA) stands out due to its ability to adapt to various geometries and deviations on both ferromagnetic and nonferromagnetic products [179].

3.21 TUBE INSPECTION WITH MAGNETIC FLUX LEAKAGE Magnetic flux leakage (MFL) is a tube-​testing technique primarily designed for the rapid testing of ferromagnetic tubes with non-​ferromagnetic fins wrapped around them, such as in air fin coolers. Two strong magnets generate a static magnetic field that saturates the tube wall . When a flaw (pitting, wall loss, etc.) is located between the two magnets, the magnetic flux in the tube wall is disturbed and a small amount of flux will leak into the inner tube. This leakage of flux is detected by the coils placed between the magnets. The variation of the flux leakage induces current in the coils, thereby causing a signal output. This signal output can be used to provide information on any wall-​thickness reduction in the tube. MFL is effective for aluminum-​finned carbon steel tubes because the magnetic field is almost completely unaffected by the presence of such fins [180, 181]. Magnetic flux leakage (MFL) is mainly applied in the inspection of air fin coolers, but it can also be used for inspecting bare tubes with diameters of one inch (2.5cm) and above [182]. Principle of MFL inspection of ferrous tube and MFL probe and flow of eddy currents is shown in Figure 3.66.

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Sensors Tube

Eddy current instrument

FIGURE 3.66  (a) Principle of MFL inspection of ferrous tube and (b) MFL probe and flow of eddy currents. (Courtesy of Olympus NDT Inc., Waltham, MA.)

3.22 REMOTE FIELD EDDY CURRENT TESTING The remote field eddy current testing (RFEC) inspection technique is similar to the ET method, which uses low-​frequency alternating current and through-​wall transmission to inspect pipes and tubes from the inside. The method is most preferred to apply to ferromagnetic materials because

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conventional ET techniques are not suitable for detecting opposite-​wall defects in such materials unless the material is saturated [183]. The exciter coil generates eddy currents at low frequency in the circumferential direction. The electromagnetic field transmits through the thickness and travels on the outer diameter. A receiver coil that is placed in the remote field zone of the exciter picks up this field. In this zone, the wall current source dominates the primary field directly from the exciter. Anomalies anywhere in this indirect path cause changes in the magnitude and phase of the received signal, and can therefore be used to indicate defects. This method has been used primarily for inspecting well casing, small-​diameter boiler tubes, and heat exchanger tubes. The separation between the two coils is between two and five times the tube ID. Remote field testing (RFET) is being used to successfully inspect ferromagnetic tubing such as carbon steel or ferritic stainless steel. This technology offers good sensitivity when detecting and measuring volumetric defects resulting from erosion, corrosion, wear, and baffle cuts. The principle of RFEC is explained in Ref. [183] and by Atherton et al. [184]. The method is illustrated schematically in Figure 3.67a and 3.67b. The probe uses a relatively large internal exciter coil, which is driven with a low-​frequency alternating current. A detector coil is placed near the inside of the pipe wall, but axially displaced from the exciter coil by about two pipe diameters. Two distinct coupling paths exist between the exciter and the detector coils: (1) the direct path inside the tube, which is attenuated rapidly by circumferential eddy currents induced in the tube wall, and (2) the indirect coupling path that originates in fields that diffuse outward through the pipe wall in the vicinity of the exciter. Merits and demerit. An important advantage of this method is that the method has higher sensitivity to axially and circumferentially oriented flaws in ferromagnetic materials. This allows detecting corrosion damage due to erosion-​corrosion, tube wall thinning, pitting, and stress corrosion cracking [183]. The major disadvantage is that when applied to nonferromagnetic material, it is not generally as sensitive or accurate as conventional ET method.

3.23 TUBE INSPECTION WITH NEAR FIELD TESTING The near field testing (NFT) technology is a rapid method intended specifically for fin-​fan carbon steel tubing inspection. This new technology relies on a simple driver-​pickup eddy current probe design (Figure 3.68a) providing very simple signal analysis. NFT is specifically suited for the detection of internal corrosion, erosion, or pitting on the inside of carbon steel tubing. The NFT probes measure lift-​off or “fill factor” and convert it to amplitude-​based signals (no phase analysis). Because the eddy current penetration is limited to the inner surface of the tube, NFT probes are not affected by the fin geometry on the outside of the tubes [185–​189]. Indirect energy flow path Detector coil

Drive rod

Exciter coil

Wheel support

FIGURE 3.67a  Tube inspection with remote field ET technique (REFC or RFT).

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FIGURE 3.67b  Remote field eddy current test probe. (Courtesy of Olympus NDT Inc., Waltham, MA.)

Near field testing (NFT) is used for the inspection of fin-​fan carbon steel tubing in heat exchangers tubes, specifically designed to detect internal corrosion, erosion, or pitting on the inside of carbon steel tubing [189]. Because the eddy current penetration is limited to the inner surface of the tube, NFT probes are not affected by the fin geometry on the outside of the tubes. NFT is also much more sensitive to defects close to structures such as support plates and tube sheets. NFT technology that uses two coils –​a transmitter and a receiver. Typically the receiver coil is close to the transmitter coil, taking advantage of the transmitter’s near-​field zone, that is, the zone where the magnetic field from the transmitter coil induces strong eddy currents, axially and radially, in the tube wall. NFT is also much more sensitive to defects close to structures such as baffle plates and tubesheets. The NDT probes for near field testing measure lift-​off or ‘fill factor’ and convert it to amplitude-​based signals (no phase analysis). Because eddy-​current penetration is limited to the inner surface of the tube, NFT probes are not affected by the fin geometry on the outside of the tube. Figure 3.68b shows finned tube inspection with eddy current NFT [189].

3.24 PULSED EDDY CURRENT (PEC) Pulsed eddy current (PEC) technique, a variant of the EC technique uses high amplitude short duration pulses having a spectrum of frequencies. Hence, PEC enables wider depth of investigation compared to the conventional EC technique. A pulsed eddy current (PEC) tool can be used to test pipes for damage without the need for removal of insulation or coatings. In this case, the test probe

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FIGURE 3.68a  Tube inspection with eddy current NFT. (Courtesy of Olympus NDT Inc., Waltham, MA.)

FIGURE 3.68b  Finned tube inspection with eddy current NFT

FIGURE 3.68c  Finned tube inspection with eddy current NFT[189].

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FIGURE 3.69  Pulsed eddy current testing (schematic).

consists of a transmitter and receiver coil and instrumentation to send step current pulses to the transmitter coil. In PEC, the standard pulsed eddy current array (PECA) probe is specifically engineered to maximize productivity for the detection of corrosion under insulation (CUI) and corrosion under fireproofing (CUF) in pipes, vessels, sphere legs, and more [190–​194]. Inspection technique. In pulsed eddy current (PEC) inspection, a probe induces eddy currents in a component, and the probe measures wall thickness by tracking the amount of time it takes the eddy currents to decay. The thicker the wall, the longer it takes for the eddy currents to decay to zero. Pulsed eddy current testing (schematic) is shown in Figure 3.69 [193] Advantages. PEC can be done without need for contact with the surface of the material. Applications. PEC can be applied to in-​service assets, and can detect damages through insulation and fireproofing, so it is an effective tool for corrosion-​under-​insulation (CUI) and flow-​accelerated corrosion (FAC) assessments.

3.25 HEAT EXCHANGER TUBE INSPECTION METHODS EDT method is commonly used to inspect tubing in heat exchangers, condensers, air coolers, and other appliances. Eddy current testing is a high-​speed method that can be performed to inspect through painting and coatings and is used to assess the condition and lifespan of tubes [197]. For heat exchanger tube inspection, there are five inspection methods: 1. ECT –​Eddy Current Testing of heat exchanger tubes 2. RFT –​Remote Field Testing 3. NFT –​Near Field Testing (for fin fan cooler inspection) 4. IRIS –​Internal Rotary Inspection System 5. MFT –​Magnetic Flux Leakage Testing. Choosing the appropriate inspection method depends on tube material and specific inspection needs.

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3.25.1 Eddy Current Testing of Chiller Tubes Chiller tubes undergo daily stress, which can lead to rust and corrosion. This rust can also infiltrate the evaporator tubes of the chillers, potentially leading to further damage to the compressor and preventing the water from chilling effectively. Common defects to look out for include pitting, freeze ruptures, external corrosion, and internal pitting [197].

3.26 TUBESHEET DIAGRAM FOR WINDOWS The tubesheet diagram (TSD) for Windows is a computer software intended for use by inspection groups in the oil, gas, and power industries to inspect tubes periodically in heat exchangers, boilers, steam generators, chillers, and similar multitube equipment and to award a classification based on user-​defined criteria such as corrosion severity, material type, defect position, etc. TSD is shown in Figure 3.70.

3.26.1 Automated Tube Inspection System Automated tube inspection system (ATIS) is a software package that carries out automated analysis of the data from eddy current and electromagnetic in-​service tube inspection systems in real time. The system discriminates between baffle support plate signals and defect signals and can be utilized on a wide variety of tube materials. The results are classified in terms of defect severity and stored in a hard disk, from which they can be retrieved and summarized at any time.

3.27 ALTERNATING CURRENT FIELD MEASUREMENT (ACFM) Alternating current field measurement (ACFM) is an electromagnetic technique used for the detection and sizing of surface breaking cracks in metallic components and welds. It combines the advantages of the alternating current potential drop (ACPD) technique and Eddy Current Testing (ECT) in terms of defect sizing without calibration and ability to work without electrical contact respectively [198]. The ACFM probe introduces an electric current locally into the structure and measures the associated electromagnetic fields close to the surface. The presence of a defect disturbs

FIGURE 3.70  Tubesheet mapping diagram. (Courtesy of Powerfect, Brick, NJ.)

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FIGURE 3.71  Alternating current field measurement.

the associated fields and the information is graphically presented to the system operator. ACFM inspection can be performed through paint and coatings, hence it is considered to be a faster and economic technique than others methods. The sensitivity is greatest for surface discontinuities and rapidly diminishes with increasing depth below the surface. The inspection method is shown in Figure 3.71 [199, 200].

3.27.1 Applications Alternating current field measurement is used for [201, 202]: 1. inspection of welds in offshore platforms and rigs 2. detection and sizing of fatigue cracks and hydrogen induced cracking 3. detection of cracks and corrosion in vessels, storage tanks and piping in oil and gas and power generation industries.

3.28 ACOUSTIC PULSE REFLECTOMETRY (APR) Acoustic pulse reflectometry (APR) is based on the measurement of one-​dimensional acoustic waves propagating in tubes. Any change in the cross sectional area in the tubular system creates a reflection, which is then recorded and analyzed in order to detect defects. The APR examination method is used for the detection of discontinuities open to or on the internal surfaces of tubes and piping. Typical types of discontinuities that can be detected by this method are cracks, corrosion pits, through-​wall holes, wall loss, and blockages [203].

3.28.1 ASTM E2906/​E2906M-​18 Standard Practice for Acoustic Pulse Reflectometry Examination of Tube Bundles. This practice describes use of Acoustic Pulse Reflectometry (APR) technology for examination of the internal surface of typical tube bundles found in heat exchangers, boilers, tubular air heaters, and reactors, during shutdown periods. The purpose of APR examination is to detect, locate and identify flaws such as through-​wall holes, ID wall loss due to pitting and/​or erosion, as well as full or partial tube blockages. APR may not be effective in detecting cracks with tight boundaries.

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3.29 BARKHAUSEN NOISE ANALYSIS Barkhausen noise analysis is a nondestructive method involving the measurement of a noise like signal induced in a ferromagnetic material by an applied magnetic field and the principle is shown in Figure 3.72 [204]. There are some main material characteristics that will directly affect the

FIGURE 3.72  Areas of applications of Barkhausen noise analysis.

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intensity of the Barkhausen noise signal include, hardness, stress-​the presence and distribution of elastic stresses and microstructure. Compressive stresses will decrease the intensity of Barkhausen noise while tensile stresses increase it. This fact can be exploited so that measuring the intensity of Barkhausen noise can provide a signal for the amount of residual stress. Measurements commonly require a small sensor to come in contact with or very near to the sample surface. Acquisition and processing of the signal is completed in real-​time resulting in immediate feedback and the capability to meet high throughput scenarios. Various dynamic processes such as creep and fatigue similarly involve changes in stress and microstructure and can also be monitored with Barkhausen noise [205–​ 209]. Areas of applications of Barkhausen noise analysis method is shown in Figure 3.72 [205].

3.29.1 Applications Barkhausen Noise Analysis is useful for evaluation of residual stresses in steels, evaluation of microstructural changes, evaluation of hardness, grinding damage detection, heat treatment defect detection, case depth analysis, hardness and residual stresses, microstructure-​case depth, testing surface defects, etc. [206].

3.30 AUTOMATED CORROSION MAPPING The use of Automated Corrosion Mapping systems working with robotic scanners have been employed by many sectors of Industry for rapid overview of equipment condition. This is often referred to as AUT Corrosion Mapping or Ultrasonic Corrosion Mapping. The systems are capable of acquiring actual wall thickness over large areas of vessels and piping while the system remains in operation. Typical Automated Corrosion mapping systems can inspect 20–​30 sq. m per standard work day. The benefit of using the automated imaging systems allows a picture (C-​Scan image) to quickly identify any significant reduction in wall thickness. These automated corrosion mapping scans can then be superimposed into development drawings of equipment and accurately indicate location of problem regions [210].

3.31 DRONES USE IN NONDESTRUCTIVE TESTING Common nondestructive testing (NDT) inspection methods at heights involve scaffolding, man lifts, or ropes to inspect the surface of a structure –​yet these methods can be unsafe, time-​consuming, and expensive. This requirement can be overcome by unmanned aerial systems (more commonly known as drones) which provide NDT experts across industries with a unique aerial perspective. This viewpoint can enable easy access to remote or inaccessible areas without compromising the pilot’s safety. Drones give NDT experts a fast, accurate way to perform inspections while reducing operational costs and minimizing safety risks. Paired with photography and video recording, a drone can help NDT experts perform a broad range of surveying and inspection services safely and sustainably [211–​213].

3.32 DYNAMIC NDT METHODS Dynamic NDT involves application of a known vibration to an object or structure and observation of its vibrational response. The dynamic response is sensitive to the presence of flaws. It is also possible to use dynamic methods to determine variations in material properties. Two different methods are used in dynamic testing: (1) measurement of natural (resonant) frequency and (2) measurement of rate of attenuation. Objects or structures can vibrate at different natural frequencies. These frequencies are a function of geometric parameters, physical constants (e.g. elastic modulus, density, Poisson’s ratio) and end constraints. Modes of vibration include flexural, torsional, longitudinal, radial, diametrical and annular. Modal analysis refers to study of the natural frequencies, damping values and mode shapes of physical systems [214].

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3.33 ELECTROMAGNETIC SORTING OF FERROUS METALS The procedure is for sorting ferrous metals using the electromagnetic (eddy-​current) method. The procedure relates to instruments using absolute or comparator-​type coils for distinguishing variations in mass, shape, conductivity, permeability, and other variables such as hardness and alloy that affect the electrical or magnetic properties, or both, of the material. In the absolute-​coil method, the equipment is calibrated by placing standards of known properties in the test coil. The value of the tested parameter (for instance, hardness, alloy, or heat treatment) is read on the scale of an indicator. In the comparative-​coil method, the test piece is compared with a reference piece and the indication tells whether the piece is within or outside of the required limits [215].

3.33.1 ASTM E566-​19 Standard Practice for Electromagnetic (Eddy Current/​ Magnetic Induction) Sorting of Ferrous Metals.

3.34 ELECTROMAGNETIC ACOUSTIC TRANSDUCERS In order to overcome temperature limitations of conventional ultrasonic transducers, it is possible to use electromagnetic acoustic transducers (EMATs) to detect flaws and measure thickness. These produce ultrasonic acoustic waves by electromagnetic interaction with an electrical conductor and are used on the external surface of the object to be tested. EMAT works by generating ultrasonic waves into a test object using electromagnetic induction with two interacting magnetic fields. A relatively high frequency field generated by electrical coils interacts with a low frequency or static field generated by magnets to generate a Lorentz force in a manner similar to an electric motor. This disturbance is transferred to the lattice of the material, producing an elastic wave. In a reciprocal process, the interaction of elastic waves in the presence of a magnetic field induces currents in the receiving EMAT coil circuit [216–​219]. The schematic of EMAT inspection technique is shown in Figure 3.73 [216]. The primary advantages of EMATs are

FIGURE 3.73  Schematic of EMAT inspection technique.

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that couplants are not required and that elevated temperatures can be withstood. Other advantages include tolerance of rough surfaces and ability to conduct high speed scans.

3.35 OPTICAL HOLOGRAPHY NDT Optical holography NDT (HNDT) is an imaging method, which records the amplitude and phase of light reflected from an object as an interferometric pattern on film. It thus allows reconstruction of the full 3D image of the object. In HNDT, the test sample is interferometrically compared in two different stressed states –​before testing and after testing. Stressing can be mechanical, thermal, vibration, etc. The resulting interference pattern contours the deformation undergone by the specimen in between the two recordings. Surface as well as sub-​surface defects show distortions in the otherwise uniform pattern. HNDT is widely applied in aerospace to find impact damage, corrosion, delamination, debonds, and cracks in high performance composite aircraft parts as well as turbine blades, tyres, and air foils [220, 221].

3.35.1 Holographic Interferometry in Crack Detection Holographic interferometry can be used to observe surface deformation caused by the presence of cracks. This can form the basis of an inspection scheme for locating small cracks in structural members, or other objects.

3.35.2 Real-​time Holographic Interferometry Real-​time holographic interferometry can be used to monitor the growth of cracks in materials by observing the temporal change of surface deformation of the specimen. Real-​time holographic interferometry has three particularly attractive features for this application [222]: 1. It is sufficiently sensitive to allow detection of cracks before they have grown to a readily observable macroscopic size. 2. Cracking can be continuously monitored without removing the specimen from the corrosive environment for inspection. 3. The entire specimen surface can be examined without the need for point-​by-​point inspection.

3.36 MAGNETIC FLUX LEAKAGE Magnetic flux leakage (MFL) technique is a nondestructive testing method for ferromagnetic materials. It uses strong permanent magnets to magnetize the object of interest to near saturation flux density. MFL utilizes magnets to temporarily magnetize the part and, if flaws are present, the magnetic field created will show distortions, signaling the presence of things like corrosion, pitting, and wall loss. Defects such as corrosion or erosion damage, or pitting result in magnetic flux leakage. The flux leakage is detected by magnetic field sensors and is proportional to the volume of metal loss. The MFL technique does not require contact and can be automated for high speed testing [223, 224]. MFL is usually regarded as a qualitative technique, although some estimates of defect size can be made. Thus, MFL is largely a screening tool which can be followed by ultrasonic inspection for determination of defect size. MFL test methods are used for the detection of outer surface, inner surface and subsurface discontinuities in magnetic steel tubular products of uniform cross section, such as seamless and welded tubing. MFL pigs are available for a range of internal diameters, including small diameter heat exchanger tubes. Figure 3.74a [223] and Figure 3.74b shows the MFL test method [224].

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FIGURE 3.74  (a) A representation of a general MFL setup and the effects of magnetic flux on a tube with a flaw and (b) A representation of a general MFL setup for testing a pipe.

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3.37 MICROWAVE NONDESTRUCTIVE TESTING Microwave nondestructive testing (MNDT) of materials is an important method for detection of flaws, cracks, defects, voids, inhomogeinities, moisture content (MC), etc. by means of microwaves. The term microwaves refer to electromagnetic waves with frequencies between 300 MHz and 100 GHz. Since the penetration of microwaves in good conducting materials is very small, MNDT techniques are mainly used for nonmetallic materials. For MNDT techniques, the measured parameters are reflection coefficients, transmission coefficients, dielectric constants, loss factors, and complex permeabilities as a function of frequency (microwaves) and temperature. These measured parameters can be related to material parameters of interest (e.g. flaws, inhomogeinities, moisture content, etc.) by suitable modeling and calibration [225].

3.38 SMART PIG A smart pig is a pipeline inspection device, and it is a critical part of keeping pipelines operating safely. In-​line inspection using “intelligent pigs” is used to detect corrosion in large diameter pipelines transporting oil and gas. It applies the magnetic flux leakage principle (MFL) and thus can detect metal loss at the internal and external pipe surface. This is a fully self-​supported “intelligent pig” that does not require cable support and that is pumped through the pipeline by either the product or water. It provides cleaning and inspection services at the same time, saving companies both time and money [226]. There are several different pigs for the in-​line inspection of pipes and each performs a different function. After cleaning of the pipe of debris and checked for any possible obstructions, the smart pigs can be sent through to provide valuable information and locations for potential metal loss, disbonded coating and small cracks or dents in the steel [227]. There are three types of smart pigs, viz. [228–​230]: 1. Magnetic Flux Leakage (MFL). This technology creates a strong magnetic field in the steel of the pipe, through magnets located circumferentially around the pig. 2. A caliper tool. This technology suspends little ‘fingers’ on springs that feel along the inside of the pipe looking for dents. 3. Electromagnetic Acoustic Transducer (EMAT). This technology looks for small cracks in the pipe. Figure 3.75 shows pipeline inspection arrangement using intelligent pigs [231].

3.39 REPLICATION METALLOGRAPHY Metal components, structures, and systems can be subject to some of the harshest safety critical operating conditions and environments. Replication metallography is a nondestructive testing (NDT) method of duplicating the microstructure of a component in order to observe grain structure

FIGURE 3.75  Pipeline inspection technique using Smart pig.

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FIGURE 3.76  A sample replica for a test surface.

at high magnification using a light microscope or Scanning Electron Microscope (SEM) for subsequent analysis. The application of this technique can facilitate the identification of a number of degradation mechanisms enabling their management, thus minimising unplanned outages and avoiding catastrophic failures [232]. Figure 3.76 shows a sample replica for a test surface.

3.39.1 Applications Replication metallographic techniques can support a wide variety of analysis methods, including creep damage assessment, thermal degradation of the materials, failure analysis/​crack analysis, graphitization, hydrogen embrittlement (HE), stress corrosion cracking (SCC), intergranular corrosion (IGC), etc. [233] ASTM E1351-​01(2020) –​Standard Practice for Production and Evaluation of Field Metallographic Replicas. Replication is a nondestructive sampling procedure that records and preserves the topography of a metallographically prepared surface as a negative relief on a plastic film (replica).

3.40 SHEAROGRAPHY Shearography NDT method is used for measuring and detecting a range of different defects on metallic and composite materials. The method, also known as speckle pattern shearing interferometry is an optical interferometric technique that is finding increasing application for composite inspection. The interferometric images produced by shearography can help detect disbonds, delaminations, impact, porosity, wrinkles, or other damage. Composite mechanical damage is typically in the form of delaminations or disbonds, broken fibers due to impact, fatigue damage that affects the zone of composite material via micro cracking, fiber delaminations, fiber breaks and overall loss of mechanical modulus, or can be caused by thermal damage from prolonged exposure to heat above resin cure temperatures as well as combination of effects due to extreme operational conditions [234]. This nondestructive inspection technique uses coherent laser light in a similar manner to holographic interferometry to create a visual representation of a test object, for nondestructive testing (NDT), strain measurement and vibration analysis.

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3.40.1 Digital Shearography Digital shearography also called digital speckle pattern shearing interferometry has been widely accepted by industry as an efficient nondestructive testing (NDT) tool for composite material. The gradient of a deformation field is measured directly in digital shearography, and near-​surface defects are found by recognizing specific ‘butterfly’ patterns on the phase map created by phase-​shift digital shearography [235].

3.41 THERMOGRAPHY Thermography or Infrared thermography (IRT) is a nondestructive testing method used to detect and measure small temperature differences to help find deterioration in assets and plant sites. Thermograms are produced and flaws are indicated by changes in temperature contrasts as a result of modifications in heat flow. IR testing is commonly used to locate hot spots and anomalies such as voids and inclusions. IR thermography is also capable of detecting corrosion damage, delaminations, voids, inclusions, and other flaws that affect heat transfer. However, in order to detect these anomalies, there must be sufficient temperature difference between the component and its surroundings. Thermography in geothermal applications would be limited to uninsulated piping. It is advantageous over traditional visual inspection and other tools because infrared technology does not have to be in contact with the equipment being monitored [236, 237].

3.42 PAIRT Pulsed active infrared thermography (PAIRT) for inspection of uninsulated piping. This technique involves transient thermal perturbation of the object. The resultant sequence of temperature distribution is monitored with an infrared camera and the data is recorded digitally. PAIRT can be used either internally or externally [238].

3.43 LEAK TESTING Leak Testing (LT) is an NDT technique used for the detection and location of leaks and for the measurement of fluid leakage in either pressurized or evacuated vessels and components [77]. A leak may be a crack, fissure, hole, or passage that admits any fluid or lets fluid escape. For pressure vessels and heat exchangers, LT is used to ensure the leak-​tightness of various welded joints, flanged joints, tube-​to-​tubesheet joints, mechanical joints, enclosures, etc.

3.43.1 Written Procedure LT procedures should encompass one or a combination of the following [239, 240]: • locating leaks • determining the leakage rate • monitoring for leakage. ASME Code Section V specifies the minimum information to be provided for the LT procedure, which include the following: 1. scope and extent of the examination 2. type of equipment to be used for detecting leaks or measuring rate of leakage 3. surface cleanliness and surface preparation 4. LT method 5. temperature, pressure, gas, and percentage concentration to be used.

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3.43.2 Methods of Leak Testing The basic principle of LT involves identification and location of the leaks or the measure of the flow of fluid or the testing medium quantitatively through the leak. This could be the actual flow rate, drop in pressure or vacuum level, etc. Various LT methods are detailed in ASME Code Section V, Yokell [59], Refs. [239–​246].

3.43.3 LT Methods as per ASME Code Section V 1. Bubble Test –​Direct Pressure Technique. 2. Bubble Test –​Vacuum Box Technique. 3. Halogen Diode Detector Probe Test. 4. Helium Mass Spectrometer Test –​Detector Probe Technique. 5. Helium Mass Spectrometer Test –​Tracer Probe Technique. 6. Pressure Change Test. 7. Thermal Conductivity Detector Probe Test. 8. Helium Mass Spectrometer Test –​Hood Technique. 9. Ultrasonic Leak Detector Test. 10. Helium Mass Spectrometer –​Helium Filled Container Leakage Rate Test, Non-​mandatory.

3.43.4 Requirements of Leak Testing As per ASME Code Section V, leak testing method(s) or technique(s) specified by the referencing code, the referencing code section shall then be consulted for the following: 1. personnel qualification/​certification 2. technique(s)/​calibration standards 3. extent of examination 4. acceptable test sensitivity or leakage rate 5. report requirements 6. acceptance criteria 7. retention of records.

3.43.5 ASTM Standards for LT Some of the ASTM Standards for LT of heat exchangers and pressure vessels are hereunder: 1. ASTM E493/​E493M-​11(2022) –​Standard Practice for Leaks Using the Mass Spectrometer Leak Detector in the Inside-​Out Testing Mode. 2. ASTM E498/​E498M-​11(2022) –​Standard Practice for Leaks Using the Mass Spectrometer Leak Detector or Residual Gas Analyzer in the Tracer Probe Mode. 3. ASTM E498/​E498M-​11(2022) –​Standard Practice for Leaks Using the Mass Spectrometer Leak Detector or Residual Gas Analyzer in the Tracer Probe Mode. 4. ASTM E499/​E499M-​11(2017) –​Standard Practice for Leaks Using the Mass Spectrometer Leak Detector in the Detector Probe Mode. 5. ASTM E515-​11(2018) –​Standard Practice for Leaks Using Bubble Emission Techniques. 6. ASTM E908-​98(2022) –​Standard Practice for Calibrating Gaseous Reference Leaks. 7. ASTM E1002-​11(2018) –​Standard Practice for Leaks Using Ultrasonics. 8. ASTM E1003-​13(2018) –​Standard Practice for Hydrostatic Leak Testing. 9. ASTM E1211/​ E1211M-​ 17 –​ Standard Practice for Leak Detection and Location Using Surface-​Mounted Acoustic Emission Sensors.

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10. ASTM E1603/​E1603M-​11(2022) –​Standard Practice for Leakage Measurement Using the Mass Spectrometer Leak Detector or Residual Gas Analyzer in the Hood Mode. 11. ASTM A1047/​A1047M-​05(2019) –​Standard Test Method for Pneumatic Leak Testing of Tubing. 12. ASTM E2930-​13(2021) –​Standard Practice for Pressure Decay Leak Test Method.

3.43.6 Leak Test Methods The basic principle of LT involves identification and location of the leaks or the measure of the flow of fluid or the testing medium quantitatively through the leak. This could be the actual flow rate, drop in pressure or vacuum level, etc. 3.43.6.1 Bubble Leak Testing Bubble leak testing is used to find leaks in many different components. The two most common forms of bubble leak testing are the direct-​pressure technique and the vacuum-​box technique. 3.43.6.2 Water Immersion Bubble Test Method Bubble testing, direct pressure technique. The water immersion bubble test consists of immersing a pressurized part, usually with high pressure dry air or nitrogen, in a water tank and watching for escaping bubbles. Soap solution bubble test. Instead of submersing the part in water, the pressurized unit to be tested is sprayed with a soap solution and the operator is able to see the bubbles formed by gas escaping from where the leak is. This method has a higher sensitivity than water immersion. It allows detection of leaks up to 10-​5 mbar . l/​s and is suitable for very large systems. 3.43.6.3 Gas Leak Lake Testing For this method, pressurize the shellside of the heat exchanger with air or nitrogen to the maximum allowable working pressure at room temperature, set the unit vertically. With a ring of about 2 in. height (50.8 mm), make a dam on the tubesheet face. Fill the tubeside with water to the level of the dam and observe for bubbles. 3.43.6.4 Acoustical Leak Detection Acoustical leak detection uses the sonic or ultrasonic signal generated by gas as it expands through the leak orifice. The intensity and frequency of the signal are function of the differential pressure, the size and geometry of the hole. Acoustical leak detection technique requires a sensitive microphone; it is simple and fast, but is limited to about 10-​3 mbar·L/​s. This technique is not recommended for on-​line leak detection in heat exchangers because of industrial noises and interferences. 3.43.6.5 Ultrasonic Leak Detection The leakage of a gas through small openings, even for small pressure differences, generates noise in the ultrasonic range. Therefore, it is possible to detect the leaks by listening for them using an ultrasonic listening device, which can record low noise levels produced in the high frequencies (40–​50 kHz) that occur due to turbulent flow [34]. This type of leak testing can be performed in an open or enclosed area and can be calibrated to isolate the sound produced by the leak. Two methods of leak testing using acoustic waves. first method is based on an active arrangement, which detects

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leaks by analyzing acoustic data generated by the gas leakage, which is measured by an ultrasound transducer. The second one is also based on ultrasonic signals, but it uses a passive arrangement in which an ultrasound emitter is placed inside the vessel. The leakage is then detected because the fluid from inside the vessel conducts the sonic signal to an external ultra-​sound detector through the discontinuity. Examples of acoustical leak detection applications include testing plumbing lines, vessels, HVAC system ductwork, and medium to large non-​sealed components to determine if they are leaking under pressure or vacuum [245]. 3.43.6.6 Dye Penetrant Method Dye penetrant method is an adaptation of a technique used to find cracks in metals and defects in welds. It uses a low viscosity fluid that exhibits a high rate of surface migration. This fluid is painted on one side of a suspected leak site, and after a time, it is detected on the other side of the wall. The test is simple, low cost, it leaves records, the sensitivity can be as high as 10-​6 mbarl/​s [247]. 3.43.6.7 Pressure Change Testing Pressure change testing is conducted to determine the allowable leakage rate across the boundaries of a closed component or system at a specific pressure or vacuum. By monitoring the change in pressure over a period of time, the leakage rate can be determined, either by the loss of pressure in a pressurized system or through the increase in pressure in a system under vacuum. 3.43.6.7.1 Pressure Decay Test This method consists of pressurizing the system with a high pressure gas, usually dry air or nitrogen. Then the part is isolated from the gas supply and, after a stabilizing period, its internal pressure is monitored over time. From the amount of pressure loss and the length of time for the loss to occur, the leak rate can be calculated. If the pressure remains the same, that component is leak-​free. 3.43.6.7.2 Vacuum Decay Test or Pressure Rise Test A vacuum decay test or pressure rise test works in the opposite way of the pressure decay test. This method involves evacuating the part to suitably low pressures and, after stabilizing the pressure, measuring the increase in pressure caused by test media entering the part. Only parts that are able to withstand external pressure can be tested in this way. 3.43.6.8 Inside-​out Helium Vacuum Chamber Leak Testing In “inside-​out” techniques, known, the component is placed inside an airtight chamber or hood with a vacuum pumping and a mass spectrometer and the test unit is pressurized with helium. Helium molecules escaping from the component are conveyed and measured by the leak detector. The leak detector finds the leak and gives the total measurement. 3.43.6.9 Outside-​in Helium Leak Testing In the “outside-​in” helium leak testing technique, the part to be tested is placed in a containment hood suitable to contain helium and connected to a vacuum group equipped and mass spectrometer. The test consists of evacuating the test unit and flooding the hood with helium. Helium, due to its atomic characteristics, has a high penetration capability. So a mass spectrometer can detect the helium leaked into the component through cracks and porosities not detectable using other systems. The test method is shown in Figure 3.77.

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FIGURE 3.77  Inside-​out and outside-​in helium leak test –​hood method.

3.43.6.10 “Bombing” Test The “bombing test” is used to check the tightness of components which are already hermetically sealed and which exhibit a gas filled internal cavity. The components examined include IC housings, transistors, laser diodes, reed contacts, and quartz oscillators. For testing, they are placed in a pressure vessel which is filled with helium. Operating with the test gas at relatively high pressure (5 to 10 bar) and leaving the system standing over several hours the test gas will accumulate inside leaking test objects. This process is called “bombing” [243]. 3.43.6.11 Radiotracer Technique Radiotracers are the most sensitive method mostly used for on-​line leak detection in heat exchangers. Radiotracers allow an early detection of small leakages before these develop into major pollution incidents. The benefits using radiotracer methods are: reducing shutdown time, ensuring safe operation, protecting environment from pollution, and saving money [248, 249]. Figure 3.78 shows the principle of radiotracer method for leak detection in shell and tube type heat exchanger. For example, a very small amount of a compatible radioisotope is injected as a sharp pulse into the higher pressure process stream entering the heat exchanger. Normally, a minimum of two radiation detectors are monitoring radiotracer movement through the heat exchangers. The injection detector mounted at the tube side inlet (high pressure) monitors the injection peak and time. The output detector mounted at the shell outlet (low pressure) detects radiotracer infiltrated into the lower pressure side from the higher pressure side showing the presence of a leak (if any).

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FIGURE 3.78  Principle of radiotracer method for leak detection in shell and tube heat exchanger.

3.43.6.12 Acoustic Emission Leak Testing Acoustic ultrasonic leak detectors are sensitive to airborne sound and inaudible sound waves due to escaping gas. Acoustic emission techniques, on the other hand, detect high frequency sounds that travel within the envelope itself. The acoustic emission technique relies on the fact that escaping gas or liquid through a small breach creates a high frequency sound wave that travels through the enveloping system via acoustic leak path. It is the capture and recording of these waves that make up the acoustic emission technique. The acoustic emission technique is best used where there is a direct sound path between the suspected leak location and the location of the sensor. The received signal is highly amplified and transmitted to the monitor where the number of events, the event rate and changes in the event rate are observed. 3.43.6.13 Tracer Gas Leak Testing The term “tracer gas leak testing” describes a group of test methods characterized to detect and measure a tracer gas flowing through a leak. The most commonly used tracer gases are halogen gas (CFC, HCFC and HFC refrigerant), helium, and a mixture of nitrogen 95% hydrogen 5%. But halogens are losing their appeal as a tracer gas, due to environmental protection rules (ozone depleting substances) following Montreal and/​or Kyoto protocols. On the other hand, helium and especially hydrogen/​nitrogen mixture are gaining more interest. The most suitable helium detector is based on a mass spectrometer. There are two ways to carry out leak testing with tracer gas: 1. detection of tracer gas escaping from leaks of a filled unit (inside-​out method) 2. detection of tracer gas entering due to leaks (outside-​in method). 3.43.6.14 Halogen Diode Detector Probe Test Method The Halogen diode detector probe test is a method to conduct a leak inspection by using a tracer gas and a detector probe to detect the presence of halogen. The detection of halogen across a pressure boundary would indicate the presence of a leak. It requires that the system be pressurized with a gas containing an organic halide, such as one of the freons. The exterior of the system is then scanned with a sniffer probe sensitive to traces of the halogen bearing gas [250]. Halogen diode detector probe testing can be used for leak inspection in a variety of industries. One of the more common tests is to pressurize the shell side of a heat exchanger with the tracer gas (normally halogen) and then probe the tubesheet welds to detect the presence of halogen, which would indicate a leak. This type of leak test can be conducted on new components to ensure they

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are leak tight before being put into service or on existing components to determine the location of a leak. 3.43.6.14.1 Leak Testing Method One of the more common tests is to pressurize the shell side of a heat exchanger with the tracer gas (normally halogen) and then probe the tubesheet welds to detect the presence of halogen, which would indicate a leak. Traverse the tube ends with a probe at a distance of about 1/8 in. (3.2 mm) and at a predetermined scanning rate using a standard leak. Also insert the probe into the tube ends for about ¼ in. (6.35 mm). Check for cracked tube ends by inserting the probe into each tube end to a minimum depth of ¾ in. (19 mm) and holding it there for about three seconds. T he halogen diode detector LT method is a semiquantitative method to detect and locate leakages and should not be considered quantitative. The sensitivity of this method is 10−6 Pa · m3/​s. This type of leak test can be conducted on new components to ensure they are leak tight before being put into service or on existing components to determine the location of a leak. Figure 3.79 shows halogen-​diode leak detector of a STHE.

3.43.7 Helium Mass Leak Detection Methods LT using helium as a tracer gas is one of the mandatory code and customer requirements. Helium leak detection can be done either by pressurizing the vessel with helium and sniffing (detector) the welds or joints from outside or by evacuating the vessel and spraying helium on the welds from outside by a tracer probe or enclosed in a vacuum mask. In each case, the helium atoms passing through a leak are connected to a mass spectrometer, where they are ionized. The current proportional to the quantum of ions is amplified and measured as the leak rate. A helium mass leak test can be used to find through-​wall leaks in pressure equipment and welds. A vessel or exchanger is first filled with 5–​10 psi of helium and sealed. This can be conducted in one of three ways [239, 246]: 1. detector-​probe technique 2. tracer-​probe technique 3. vacuum technique or hood method. For details refer ASME Code Section V.

FIGURE 3.79  Halogen-​diode leak detector testing of a STHE.

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3.43.7.1 Helium Mass Spectrometer Test –​Detector Probe Technique The detector probe technique, as shown schematically in Figure 3.80a, involves pressurizing the vessel to be leak tested with certain percentage of pure helium and sensing the leaking helium using the detector probe, which is moved at a definite speed over all the joints to be tested. Any leakage from the pressurized system is detected by the mass spectrometer leak detector. The detector probe is a semiquantitative technique to detect and locate leakages and should not be considered quantitative. The sensitivity of this method is of the order of 10−7 Pa · m3/​s. Instrument A helium mass spectrometer leak detector capable of sensing and measuring minute traces of helium shall be used. Leakage shall be indicated by one or more of the following signaling devices: 1. Meter: a meter on, or attached to, the test instrument. 2. Audio Devices: a speaker or set of headphones that emits audible indications. 3. Indicator Light: a visible indicator light. Auxiliary Equipment Transformer. A constant voltage transformer shall be used in conjunction with the instrument when line voltage is subject to variations. Detector Probe. All areas to be examined shall be scanned for leaks using a detector probe (sniffer) connected to the instrument through flexible tubing or a hose. 3.43.7.2 Helium Mass Spectrometer Test –​Tracer Probe Technique In the helium tracer probe method as shown in Figure 3.80b, the mass spectrometer leak detector is connected to the internal volume of an evacuated vessel or heat exchanger while helium is sprayed from the probe at each joint and is moved over the external surface to detect the specific locations of leaks. Any leakage into the vacuum side is detected by the mass spectrometer leak detector. The tracer probe is a semiquantitative technique to detect and locate leakages and should not be considered quantitative. 3.43.7.3 Helium Mass Spectrometer Vacuum Test –​Hood Technique This test is commonly used to check for leaks in tube-​to-​tubesheet joint welds on a heat exchanger. The test should be performed after all tube end welding but before tube expansion so that only the welds are being tested. Before testing, the shellside shall be evacuated. A hood (usually a plastic bag) is used to enclose the end of the exchanger so that any leaking helium is collected in the bag and all tube end welds on one side can be tested at once. The test arrangement is shown in Figure 3.80c and 3.80d. The hood test is conducted by placing the component under a vacuum and connected to the mass spectrometer. A “hood” or “envelope” is then established around a portion of the component under test, such as the tubesheet of a heat exchanger. The hood, which is normally made of a plastic material or bag, is then filled with helium to test a large area at one time. If a leak is present, the helium will be drawn into the part due to the differential pressure. The mass spectrometer is monitored to verify the presence of helium leakage. The test arrangement is shown in Figure 3.80c. Vacuum technique or hood method. The vacuum technique or hood method involves evacuating the vessel to be tested and filling with almost 100% helium or air-​helium mixture in the enclosing mask and detecting the inflow of helium to the vessel under vacuum through leaks if any, using the

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FIGURE 3.80  Helium mass spectrometer testing. (a) Tracer probe technique, (b) detector probe technique, and (c) vacuum technique or hood method. (Adapted from [239] McMaster, R.C.)

instrumentation as shown schematically in Figure 3.80c. This is a quantitative measurement technique. The high sensitivity of the hood technique makes possible the detection and measurement of total helium gas flow from the high pressure side. The sensitivity of this method ranges from 10−7 to 10−11 Pa · m3/​s. A typical test setup is shown in Figure 3.80d.

3.44 IN-​SERVICE EXAMINATION OF HEAT EXCHANGERS FOR DETECTION OF LEAKS In-​service examination of leaks, using either pressurized air or vacuum technique or bubbler, is shown schematically in Figure 3.81. Fractured tubes at the interior section can be plugged using tube

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FIGURE 3.81  In-​service examination of leaks: (a) by vacuum technique using bubbler, (b) by vacuum technique using soap foam, (c) pressure testing using soap water, (d) leak testing with helium pressurised on the shellside, by sniffing method with helium detector, and (e) pressure testing with vacuum type helium detector.

stabilizers (Figure 3.82), marketed by M/​s Expando Seal Tools. Additionally, individual tube can be tested by the following two methods: 1. Individual near end tube testing plugs/​kits as shown in Figure 3.83. 2. Individual through the tube testing method as shown in Figure 3.84.

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FIGURE 3.81 (Continued)

FIGURE 3.82  Plugging fractured tube using tube stabilizer.

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FIGURE 3.83  Individual near end tube testing plugs/​kits. (Courtesy of Maus Italia F. Agostino & C.s.a.s., BagnoloCremasco (Cr), Italy.)

FIGURE 3.84:  Individual through the tube testing. (Courtesy of Maus Italia F. Agostino & C.s.a.s., BagnoloCremasco (Cr), Italy.)

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REFERENCES 1. Danis, J. I., Material selection, fabrication and inspection of refinery heat exchangers, Welding J., June, 25–​30 (1986). 2. Nicholson, S., The role of NDT within Q. A. programmes applied to pressure vessels and other process plant, Br. J. NDT, May, 121–​127 (1982). 3. Burgess, N. T., Quality control –​What’s it all about? Weld. Metal Fabr., December, 464–​468 (1966). 4. Silk, M. G., Developments in ultrasonics for sizing and flaw characterization, in Developments in Pressure Vessel Technology –​ 2, Inspection and Testing (R. W. Nichols, ed.), Applied Science Publishers, London, U.K., 1979, pp. 101–​139. 5a. Rao, B. S. C., Quality assurance in fabrication of pressure vessels, in Welding Engineering Handbook, Vol. I (S. Sundarrajan, S. VijayaBhaskar, and G. C. Amarnath Kumar, eds.), Welding Research Group, Radiant Publications, Secunderabad, India (1990). 5b. Rao B,S,C, Quality Assurance in Fabrication of Boilers and Pressure Vessels, HEB 97, Alexandria, Egypt April 5-​6, 1997, pp. 1–​18. 6. Bryant, L. E. and Mcintire, P., eds., Nondestructive Testing Handbook, 2nd edn., Vol. 3, Radiography and Radiographic Testing, American Society for Nondestructive Testing, Columbus, OH, 1985. 7. Blatter, H. and Frey, H., System effectiveness and quality assurance of electronic systems. Brown Boveri Rev., 11–​78, 739–​743. 8. Dale, B. G. and Plunkett, J. J., A study of audits, inspection and quality costs in the pressure vessel fabrication sector of the process plant industry, Proc. Inst. Mech. Eng., 198B, 49–​54. 9. Chuse, R., Pressure Vessels, the ASME Code Simplified, McGraw-​Hill, New York, 1984. 10a. ASME Boiler and Pressure Vessel Code, Section VIII, Division 1, Pressure Vessels, American Society of Mechanical Engineers, New York, 2021; 10b. ASME Boiler and Pressure Vessel Code, Section IX, Welding and Brazing Qualifications, American Society of Mechanical Engineers, New York, 2021; 10c. ASME Boiler and Pressure Vessel Code, Section II, Material Specifications, Part A –​ Ferrous Materials. Part –​ B Nonferrous Materials, Part –​ C Welding Rods, Electrodes and Filler Metals, Part –​ D Material Properties, American Society of Mechanical Engineers, New York, 2021; 10d. ASME Boiler and Pressure Vessel Code, Section V. Nondestructive Examination, American Society of Mechanical Engineers, New York, 2021. 11. Veith, C. J., Developing a quality control program, Welding Design Fabr., September, 71–​73 (1975). 12. Lavender, D. H. and Lavender, J. D., An NDT audit –​What to expect, Br. J. NDT, July, 264–​265 (1987). 13. Weymueller, C. R., NDT builds safety into a nuclear plant, Welding Design Fabr., September, 54–​57 (1982). 14a. https://​asq.org/​qual​ity-​resour​ces/​cost-​of-​qual​ity 14b. NDT feature-​testing triggers big savings, Welding Design Fabr., February, 31–​39 (1975). 15. BS 6143-​1:1992, Guide to the economics of quality –​Process cost model, British Standards Institution, 1992. 16. Zairi, M., Total Quality Management for Engineers, Aditya Books, New Delhi, India, 1992. 17. www.biz​ m anu​ a lz.com/​ i mpr​ o ve-​ q ual​ i ty/​ w ho- ​ a re- ​ t he- ​ t op- ​ q ual ​ i ty- ​ g urus.html1 ​ 9 99- ​ 2 022 Bizmanualz, Inc. 18. Deming, W. Edwards. Out of The Crisis, MIT Press, pp. 23–​24. 19. https://​asq.org/​qual​ity-​resour​ces/​total-​qual​ity-​man​agem​ent/​dem​ing-​poi​nts 20. www.juran.com/​blog/​the-​juran-​tril​ogy-​qual​ity-​plann​ing/​ 21. www.bl.uk/​peo​ple/​phi​lip-​cro​sby 22. www.ifm.eng.cam.ac.uk/​resea​rch/​dsto​ols/​tqm-​tools/​ 23. https://​asq.org/​qual​ity-​resour​ces/​stat​isti​cal-​proc​ess-​cont​rol 24. www.ispatg​uru.com/​stat​isti​cal-​qual​ity-​ cont​rol 25. www.luc​idch​art.com/​blog/​what-​are-​the-​7-​basic-​qual​ity-​tools 26. https://​asq.org/​qual​ity-​resour​ces/​cont​rol-​chart 27. www.itl.nist.gov/​div​898/​handb​ook/​pmc/​secti​on3/​pmc31.htm 28. https://​qual​ity-​one.com/​iso-​9001/​ 29. www.lean.org/​lexi​con-​terms/​pdca/​

Quality Control, Inspection, and Nondestructive Testing

487

30. https://​asq.org/​qual​ity-​resour​ces/​pdca-​cycle 31. www.dem​ing.org/​the​man/​theor​ies/​pdsacy​cle 31a. https://​dem​ing.org/​expl​ore/​pdsa/​ 32. https://​asq.org/​qual​ity-​resour​ces/​pdca-​cycle 33. www.isixsi​gma.com/​dic​tion​ary/​dem​ing-​cycle-​pdca/​ 34. www.isixsi​gma.com/​meth​odol​ogy/​total-​qual​ity-​man​agem​ent-​tqm/​eight-​eleme​nts-​tqm/​ 35. Christopher M. Lowery, et.al, “TQM’s Human Resource Component,” Quality Progress (February 2000), pp. 55–​58. 36. www.juran.com/​blog/​what-​is-​total-​qual​ity-​man​agem​ent/​ 37. www.isixsi​gma.com/​meth​odol​ogy/​total-​qual​ity-​man​agem​ent-​tqm/​intro​duct​ion-​and-​imp​leme​ntat​ion-​ total-​qual​ity-​man​agem​ent-​tqm/​ 38. https://​qual​ity-​one.com/​tqm/​ 39. https://​asq.org/​qual​ity-​resour​ces/​total-​qual​ity-​man​agem​ent 40. www.epa.gov/​sus​tain​abil​ity/​lean-​think​ing-​and-​meth​ods-​5s 41. Applying 5S in the Manufacturing Industry | Lean Production 42. www.fiixs​oftw​are.com/​blog/​build​ing-​a-​tpm-​prog​ram-​where-​to-​start/​ 43. www.lea​npro​duct​ion.com/​tpm/​ 44. https://​tpm.jmac.co.jp/​servi​ces/​ 45. www.grap​hicp​rodu​cts.com/​infog​raph​ics/​pill​ars-​of-​tpm/​ 46. www.lea​npro​duct​ion.com/​tpm/​ 47. Introduction to Total Productive Maintenance, Student Study Guide, TPM100, Revision0, April 2006, Denso, TPM100-​SG-​001-​00.doc, pp. 1.1–​1.21 48. www.kai​zen.com/​what-​is-​kai​zen 49. www.5sto​day.com/​kai​zen/​ 50. www.twi-​glo​bal.com/​techni​cal-​knowle​dge/​faqs/​faq-​what-​is-​lean-​manufa​ctur​ing 51. Lean Lexicon, 5th edition, Lean Enterprise Institute, Inc., 2014. 52. www.sixs​igma​coun​cil.org/​wp-​cont​ent/​uplo​ads/​2018/​08/​Six-​Sigma-​A-​Compl​ete-​Step-​by-​Step-​ Guide.pdf 53. https://​corpor​atef​i nan​cein​stit​ute.com/​resour​ces/​man​agem​ent/​six-​sigma/​ 54. www.simp​lile​arn.com/​what-​is-​six-​sigma-​a-​compl​ete-​overv​iew-​arti​cle 55. www.edu​cba.com/​six-​sigma-​conce​pts/​ 56. https://​asq.org/​qual​ity-​resour​ces/​six-​sigma 57. www.simp​lile​arn.com/​what-​is-​six-​sigma-​a-​compl​ete-​overv​iew-​arti​cle 58. Nicholson, R. D. and Hemsworth, B., Assessment of nuclear reactor fabrication: A regulatory view, Weld. World, 30, 126–​129 (1992). 59. Yokell, S., A Working Guide to Shell-​and-​Tube Heat Exchangers, McGraw-​Hill, New York, 1990. 60. Craig, W. R., Refineries test their metals, Welding Design Fabr., April, 81–​85 (1977). 61. TEMA, Standards of Tubular Exchanger Manufacturers Association, 10th edn., Tubular Exchanger Manufacturers Association, Tarrytown, NY, 2019. 62. Nichols, R. W., Golden rules for nondestructive examination, Metal Constr. 12, May (1980). 63. Frew, J. A., Lloyd’s approach to quality assurance of welded structures, Welding Metal Fabr., August, 256–​260 (1974). 64. Guide for the Visual Inspection of Welds, ANSI/​AWS B1.11:2000, American Welding Society, Miami, FL, 2000, p. 30. 65. www.weldin​gand​ndt.com/​summ​ary-​of-​asme-​bpvc-​sect​ion-​ix-​part-​1/​ 66. www.fly​abil​ity.com/​ndt 67. www.twi-​glo​bal.com/​what-​we-​do/​servi​ces-​and-​supp​ort/​asset-​man​agem​ent/​non-​dest​ruct​ive-​test​ing/​ ndt-​tec​hniq​ues/​micro-​focus-​x-​ray 68. www.aws.org/​resour​ces/​det​ail/​wps-​and-​pqr-​demy​stif​i ed 69. Vackar, B. K., Satisfying codes, Welding Design Fabr., June, 86–​87 (1977). 70. Harrison, J. D. and Young, J. G., A rational approach to weld defect acceptance levels and quality control, Welding Metal Fabr., October, 367–​371 (1973). 71. Mercer, W. L., NDT –​A vital force in engineering, Br. J. NDT, January, 11–​14 (1987). 72. www.astm.org/​produ​cts-​servi​ces/​standa​rds-​and-​publi​cati​ons/​standa​rds/​non​dest​ruct​ive-​test​ing-​ standa​rds.

488

Quality Control, Inspection, and Nondestructive Testing

73. www.sli​desh​are.net/​aman1​312/​adva​nce-​ndt-​meth​ods-​82474​341, Aman kumar, Advance NDT Methods, Nov. 21, 2017 74. www.ircl​ass.org/​ind​ustr​ial/​advan​ced-​non-​dest​ruct​ive-​test​ing/​ 75. Raj, B. and Subramanian, C. V., Nondestructive testing –​Welding industry, in Welding Engineering Handbook, Vol. I (S. Sundarrajan, S. VijayaBhaskar, and G. C. Amarnath Kumar, eds.), Welding Research Group, Radiant Publications, Secunderabad, India (1990). 76. Guide for the Nondestructive Inspection of Welds, ANSI/​AWS B1.10–​99, American Welding Society, Miami, FL, 1999. 77. Hayes, C., The ABC’s of nondestructive weld examination, Welding J., May, 46–​51 (1997). 78. Robert, R. L. and Crane, W. D., Nondestructive inspection, Chapter 24, in Handbook of Materials Selection (M. Kutz, ed.), John Wiley & Sons, Inc., New York, 2002. 79. www-​pub.iaea.org/​MTCD/​publi​cati​ons/​PDF/​TCS-​13.pdf 80. Training Course Series No. 13 Guidebook for the Fabrication of Non-​Destructive Testing (NDT) Test Specimens International Atomic Energy Agency, 2001 81. Guidebook For The Fabrication Of Non-​Destructive Testing (Ndt) Test Specimens IAEA, VIENNA, 2001 IAEA-​TECDOC-​TCS-​13 ISSN 1011–​4289 © IAEA, 2001 Printed by the IAEA in Austria June 20, pp. 1–​55. 82. Non-​destructive Testing: A Guidebook for Industrial Management and Quality Control Personnel, International Atomic Energy Agency, Vienna, 1999, pp. 1–​287. 83. www.min​ing-​tec​hnol​ogy.com/​cont​ract​ors/​expl​orat​ion/​nex​xis-​min​ing/​pressr​elea​ses/​eight-​benef​i ts-​ndt/​ 84. Dunn, F. W., Magnetic particle inspection fundamentals, Mater. Eval., December, 42–​47 (1977). 85. Hamlet, R. A. and Lavender, D. H., Standardization within the writing of the NDT procedure, Br. J. NDT, 359–​360 (1986). 86. https://​eclips​esci​enti​fic.com/​Textbo​oks/​Dev​elop​ing%20Qu​alif​ied%20NDT%20Pro​cedu​res%20 &%20The%20Te​chni​cal%20Ju​stif​i cat​ion%20Proc​ess%20-​%20Sam​ple.pdf 87. Developing Qualified NDT Procedures & The Technical Justification Process and Technical Justification Process, Eclipse Scientic, 1st Edition, pp. 1–​233. 88. NDT Methods –​Applied Technical Services (atslab.com) 89. www.hse.gov.uk/​comah/​sragt​ech/​tech​meas​ndt.htm 90. Litting, B. G., N.D.T. methods and assessment of weld defects, Welding Metal Fabr., 435–​440 (1967). 91. SANDIA Repror SAND2012-​8590, October 2012 Visual Inspection: A Review of the Literature Judi E. See, pp. 1–​76. 92. Boving, K. G., ed., NDE Handbook, Nondestructive Examination Methods for Condition Monitoring, Butterworths, London, U.K. 93. Bauer, L., Visual inspection: Looking ahead, Welding Design Fabr., September, 31–​32 (1991). 94. Miller, J. S., Inspect tubes and pipes with mini-​TV cameras, Welding Design Fabr., October, 84–​86 (1979). 95. www.thefab​rica​tor.com/​tube​pipe​jour​nal/​arti​cle/​testi​ngme​asur​ing/​tak​ing-​a-​close-​look-​at-​the-​vis​ual-​ ins​pect​ion-​proc​ess-​for-​pip​ing-​syst​ems 96. Lang, W. J., Eyeballing welds with a borescope, Welding J., June, 61–​62 (1990). 97. www.bindt.org/​What-​is-​NDT/​Vis​ual-​ins​pect​ion/​ 98. Betz, C. E., Principles of Penetrants, Magnaflux Corp., Chicago, IL, 1963. 99. Selner, R., Dye penetrant and magnetic particle inspection, Welding J., February, 28–​31 (1982). 100. (a) Kevin, W., Guide to liquid penetrant testing, Welding J. (Suppl), Nondestructive Examination, October, 21–​23 (1997); (b) Lebowitz, C. A., A primer on nondestructive examination methods for weld inspection, Welding J. (Suppl), Nondestructive Examination, October, 29–​31 (1997). 101. Heine, H. J., Liquid penetrant and magnetic particle inspection, Foundry Manage. Technol., July, 36–​39 (1991). 102. Lovejoy, D. J., Standardization of light levels for magnetic and penetrant inspection, Br. J. NDT, 36, 8–​9 (1994). 103. Brown, F. and Howes, I., NDT methods and equipment for the quality fabrication shop, Welding Metal Fabr., 611–​618 (1977). 104. Jefferson, T. B., Welder certification: What does it mean? Welding Design Fabr., June, 66–​68 (1981). 105. https://​apps.dtic.mil/​sti/​pdfs/​ADA620​200.pdf 106. www.nde-​ed.org/​NDETec​hniq​ues/​MagP​arti​cle/​Testi​ngPr​acti​ces/​Mag​neti​cRub​ber.xhtml

Quality Control, Inspection, and Nondestructive Testing 107.

489

Principles and Application of Magnetic Rubber Testing for Crack Detection in High-​Strength Steel Components: I. Active-​Field Inspection S K Burke, M E Ibrahim and G R Hugo Maritime Division Defence Science and Technology Organization DSTO-​TR-​3032, Commonwealth of Australia 2015 AR-​016-​112 December 2014, pp. 1–​46. 108. Hellier, C. J., What’s new in nondestructive testing of welds and heat affected zones? Welding J., March, 39–​43 (1993). 109. Banks, B., Problems of quality control in the manufacture of pressure vessels, Welding Metal Fabr., July, 241–​246 (1972). 110. Welding Engineer Data Sheet, No. 468 –​Troubleshooting guide for radiographs, Welding Design Fabr., September, 79 (1979). 111. Litting, B. G., Quality control of welded components, Welding Metal Fabr., July, 294–​297 (1967). 112. Carson, H. L. and Feaver, M. J., An improved means for the measurement of weld radiograph image quality, Welding Metal Fabr., January, 29–​36 (1973). 113. Garrett, W. R., Reading radiographs takes time and patience, Welding Design Fabr., September, 77–​79 (1979). 114. Hellier, C., Weld inspection –​Today and tomorrow, Welding Design Fabr., September, 78–​81 (1977). 115. Murray, S. H., Computed tomography used in weld inspections at NASA, Welding J., February, 33–​36 (1990). 116. https://​inspe​ctio​neer​ing.com/​tag/​radi​ogra​phy 117. www-​llb.cea.fr/​neutr​ono/​nr1.html 118. Michael F. L’Annunziata, Introduction: Radioactivity and Our Well-​Being in Radioactivity, 2007 118.1. www.scienc​edir​ect.com/​top​ics/​earth-​and-​planet​ary-​scien​ces/​neut​ron-​radi​ogra​phy 119. Whritnour, R., Radioscopy speeds inspection, Welding Design Fabr., September, 24–​25 (1992). 120. https://​cameo.mfa.org/​wiki/​Rad​iosc​opy 121. www.twi-​glo​bal.com/​techni​cal-​knowle​dge/​faqs/​ult​raso​nic-​test​ing 122. Carl, R. W., Ultrasonic testing for the fabricator, Welding Design Fabr., October, 25–​33 (1994). 123. Anjard, R. P., Fundamentals of ultrasonic testing, Heat Treating, April, 34–​38 (1978). 124. Fortunko, C. M. and Schramm, R. E., Ultrasonic nondestructive evaluations of butt welds using electromagnetic-​acoustic transducers, Welding J., February, 39–​46 (1982). 125. Watson P. D., Design for welding examination, Welding J., February, 32–​35 (1982). 126. www.olym​pus-​ims.com/​en/​ndt-​tutori​als/​flaw-​detect​ion/​weld-​ins​pect​ion/​ 127. Ultrasonic signals, document no. 11S/​11W-​850–​86, The Welding Institute, Abington, Cambridge, U.K. 128. Meyer, H.-​J., Estimating flaw size in pressure vessel welds, Welding Design Fabr., November, 78–​ 81 (1977). 129. Hansen, B., A rational approach to NDT of welds, Welding Design Fabr., May, 64–​67 (1977). 130. www.olym​pus-​ims.com/​en/​ndt-​tutori​als/​thickn​ess-​gauge/​intro​duct​ion/​ 131. www.bindt.org/​What-​is-​NDT/​Ult​raso​nic-​thickn​ess-​gaug​ing/​ 132. www.defel​sko.com/​resour​ces/​using-​ult​raso​nic-​coat​ing-​thickn​ess-​gages 133.1. https://​innova​teen​ergy​now.com/​resour​ces/​dro​nes-​being-​used-​for-​ut-​on-​ene​rgy-​ass​ets 133.2. Drones Being Used for Ultrasonic Testing (UT) on Energy Assets July 05, 2021 133.3. www.ndt.net/​sea​rch/​docs.php3?id=​26051 134. www.fly​abil​ity.com/​ult​raso​nic-​test​ing 135. Ehlman, W. G., Acoustical holography looks through welds, Welding Design Fabr., February, 68–​70 (1977). 136. What is Automated Ultrasonic Testing (AUT)? (modsonic.com) 137. www.iris​ndt.com/​uk/​in-​serv​ice-​ins​pect​ion/​automa​ted-​ult​raso​nic-​test​ing-​aut/​ 138. www.twi-​glo​bal.com/​what-​we-​do/​servi​ces-​and-​supp​ort/​asset-​man​agem​ent/​non-​dest​ruct​ive-​test​ing/​ ndt-​tec​hniq​ues/​time-​of-​fli​ght-​diff​ract​ion 139. www.iris​ndt.com/​uk/​in-​serv​ice-​ins​pect​ion/​advan​ced-​ult​raso​nic-​back​scat​ter-​techni​que-​aubt/​ 140. www.ans​ndt.com/​advan​ced-​ndt-​servi​ces/​htha_​high_​temp​erat​ure_​hydr​ogen​_​att​ack/​ 141. www.mistr​asgr​oup.com/​how-​we-​help/​spec​ial-​empha​sis/​htha/​ 142. https://​inspe​ctio​neer​ing.com/​tag/​rapid+​ult​raso​nic+​gridd​ing 143. https://​en.everyb​odyw​iki.com/​Rapid_​Ultr​ason​ic_​G​ridd​ing 144. www.inners​pec.com/​news/​dry-​coup​led-​ult​raso​nic-​test​ing-​nde-​met​hod

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145. A. Demma, Ondes Guidées: Aspects Favorables Et Limitations Guides Waves: Opportunities And Limitations, –​Guided Ultrasonics Limited Unit 2 Reynard Business Park –​Windmill Road Brentford –​TW89LY (UK), pp. 1–​6. 146. www.gwultr​ason​ics.com/​knowle​dge/​pipe/​ 147. www.gwultr​ason​ics.com/​knowle​dge/​gw-​intro/​ 148. www.mistr​asgr​oup.com/​how-​we-​help/​field-​insp​ecti​ons/​advan​ced-​ndt/​automa​ted-​ult​raso​nic-​test​ing/​ gui​ded-​wave-​test​ing/​ 149. www.inners​pec.com/​knowle​dge/​gui​ded-​waves-​tec​hnol​ogy 150. www.ndt.net/​arti​cle/​wcnd​t00/​pap​ers/​idn​627/​idn​627.htm 151. www.edd​yfi.com/​en/​tec​hnol​ogy/​inter​nal-​rot​ary-​ins​pect​ion-​sys​tem-​iris 152. www.zener-​group.com/​knowle​dge-​base/​inter​nal-​rot​ary-​ins​pect​ion-​sys​tem-​iris/​ 153. www.iris​ndt.com/​uk/​in-​serv​ice-​ins​pect​ion/​tube-​ins​pect​ion-​inter​nal-​rot​ary-​ins​pect​ion/​ 154. https://​war​dves​sela​ndex​chan​ger.com/​non-​dest​ruct​ive-​test​ing-​meth​ods-​inter​nal-​rot​ary-​ins​pect​ion-​sys​ tem-​iris/​ 155. www.ndt.net/​arti​cle/​tofd/​hecht/​hecht.htm 156. www.olym​pus-​ims.com/​en/​appli​cati​ons/​intro​duct​ion-​to-​time-​of-​fli​ght-​diff​ract​ion-​for-​weld-​ins​ pect​ion/​ 157. Lenain, J.-​C., General principles of acoustic emission, Mater. Eval., October, 1000–​1002 (1981). 158. www.ndtt​echn​olog​ies.com/​produ​cts/​aco​usti​c_​em​issi​ons.html 159. www.one​stop​ndt.com/​ndt-​artic​les/​what-​is-​acous​tic-​emiss​ion-​test​ing 160. www.aru​dra.co.in/​aet.html 161. Miller, R. K. and McIntire, P., eds., Nondestructive Testing Handbook, Vol. 5, Acoustic Emission Testing, 2nd edn., American Society for Nondestructive Testing, Columbus, OH, 1987. 162. Debel, C. P., Proceedings of the Fifth Riso International Symposium on Metallurgy and Material Science, Riso National Laboratory, Roskilde, Denmark, 1984, p. 19. 163. Raj, B. and Jha, B. B., Fundamentals of acoustic emission, Br. J. NDT, 36, 16–​23 (1994). 164. Prine, D. W., Inspect as you go with acoustic emission, Welding Design Fabr., January, 74–​77 (1977). 165. Clark, J. N., The detection of creep failure at welds by acoustic emission, Part 1: Brittle creep rupture, Br. J. NDT, January, 9–​21 (1982). 166. Kalyansundaram, P., Raj, B., Kasiviswanathan, K. V., Jayakumar, T., and Murthy, C. R. L., Leak detection in pressure tubes of a pressurised heavy water reactor by acoustic emission technique, Br. J. NDT, 34(11), 539–​544 (1992). 167. www.merr​ickg​roup​inc.com/​artic​les/​what-​is-​eddy-​curr​ent-​test​ing/​ 168. www.mistr​asgr​oup.com/​how-​we-​help/​field-​insp​ecti​ons/​trad​itio​nal-​ndt/​eddy-​curr​ent/​ 169. www.twi-​glo​bal.com/​techni​cal-​knowle​dge/​faqs/​faq-​what-​are-​the-​appli​cati​ons-​and-​capab​ilit​ies-​of-​ eddy-​curre​nts 170. www.inter​tek.com/​non-​dest​ruct​ive-​test​ing/​ndt-​spe​cial​ist/​eddy-​curr​ent-​tubes/​ 171. www.edd​yfi.com/​en/​tec​hnol​ogy/​eddy-​curr​ent-​test​ing-​ect 172. www.ndtt​echn​olog​ies.com/​produ​cts/​eddy_​curr​ent_​test​ing.html 173. Granville, R. K., In-​service eddy current examination of nonferrous industrial heat exchanger tubing, Br. J. NDT, 33, 403–​409 (1991). 174. Cecco, V. S., Franklin, E. M., Houserman, H. E., Kincaid, T. G., Pellicer, J., Hagemier, D., Eddy current inspection, Lampman, S. R. and Zorc, T. B., (eds.), in Metals Handbook, Vol. 17, Nondestructive Evaluation and Quality Control, 9th edn., ASM, Metals Park, OH, 1989, pp. 164–​194. 175. www.nde-​ed.org/​NDETec​hniq​ues/​Eddy​Curr​ent/​Appli​cati​ons/​tub​eins​pect​ion.xhtml 176. Clark, W. G., Multiple-​element eddy current probes for enhanced inspection, Mater. Eval., July, 794–​802 (1993). 177. Bergander, M. J., Flux leakage examination of ferrous heat exchanger tubing, Mater. Eval., June, 811–​812 (1986). 178. www.olym​pus-​ims.com/​en/​ndt-​tutori​als/​eca-​tutor​ial/​intro/​ 179. www.olym​pus-​ims.com/​en/​appli​cati​ons/​automa​ted-​surf​ace-​ins​pect​ion-​using-​eddy-​curr​ent-​array-​tec​ hnol​ogy/​ 180. www.edd​yfi.com/​en/​tec​hnol​ogy/​magne​tic-​flux-​leak​age-​mfl 181. www.antins​pect​ion.com/​nft-​near-​field-​test​ing 182. www.app​lus.com/​glo​bal/​en/​what-​we-​do/​sub-​serv​ice-​sheet/​magne​tic-​flux-​leak​age-​tube-​ins​pect​ion

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183. Fisher, J. L., Remote field eddy current inspection, in Metals Handbook, Vol. 17, Nondestructive Evaluation and Quality Control, 9th edn., American Society for Metals, Metals Park, OH, 1989, pp. 195–​201. 184. Atherton, D. L., Frey, P., and Guo, X., Remote field slit defect responses, Br. J. NDT, 36, 4–​7 (1994). 185. www.kont​roll​tech​nik.com/​meth​ods/​tube-​ins​pect​ion/​c-​mec-​rem​ote-​field-​ec 186. www.aries​mar.com/​in/​en/​nearfi​eld-​test​ing-​nft 187. www.ixar.in/​serv​ice/​near-​field-​test​ing/​ 188. www.app​lus.com/​glo​bal/​en/​what-​we-​do/​sub-​serv​ice-​sheet/​near-​field-​test​ing-​and-​rem​ote-​field-​ test​ing 189. www.antins​pect​ion.com/​nft-​near-​field-​test​ing 190. www.mistr​asgr​oup.com/​how-​we-​help/​field-​insp​ecti​ons/​advan​ced-​ndt/​pul​sed-​eddy-​curr​ent 191. https://​inspe​ctio​neer​ing.com/​tag/​pec 192. www.edd​yfi.com/​en/​tec​hnol​ogy/​pul​sed-​eddy-​curr​ent-​pec 193. www.com​sol.fr/​paper/​fin​ite-​elem​ent-​model-​based-​optim​izat​ion-​of-​pul​sed-​eddy-​curr​ent-​exc​itat​ ion-​rise-​t-​31633 194. www.mistr​asgr​oup.com/​how-​we-​help/​field-​insp​ecti​ons/​advan​ced-​ndt/​pul​sed-​eddy-​curr​ent-​pec/​ 195. Neumaier, P., Testing heat exchanger tubes using eddy current techniques with computerised signal analysis, Br. J. NDT, September, 233–​237 (1983). 196. Stepinski, T. and Maszi, N., Conjugate spectrum filters for eddy current signal processing, Mater. Eval., July, 839–​844 (1993). 197. www.olym​pus-​ims.com/​en/​ndt-​tutori​als/​eca-​tutor​ial/​appli​cati​ons/​test​ing-​tubes/​ 198. www.twi-​glo​bal.com/​what-​we-​do/​servi​ces-​and-​supp​ort/​asset-​man​agem​ent/​non-​dest​ruct​ive-​test​ing/​ ndt-​tec​hniq​ues/​alte​rnat​ing-​curr​ent-​field-​meas​urem​ent 199. www.aris​eglo​bal.com/​advan​ced-​ndt-​ins​pect​ion-​servi​ces/​alte​rnat​ing-​curr​ent-​field-​meas​urem​ent/​ 200. https://​crad​pdf.drdc-​rddc.gc.ca/​PDFS/​unc​199/​p802​467_​A1b.pdf 201. www.dexon-​tec​hnol​ogy.com/​ins​pect​ion-​servi​ces/​advan​ced-​ndt-​servi​ces/​other-​advan​ced/​alte​rnat​ing-​ curr​ent-​field-​meas​urem​ent-​acfm/​ 202. www.twi-​glo​bal.com/​what-​we-​do/​servi​ces-​and-​supp​ort/​asset-​man​agem​ent/​non-​dest​ruct​ive-​test​ing/​ ndt-​tec​hniq​ues/​alte​rnat​ing-​curr​ent-​field-​meas​urem​ent 203. www.aru​dra.co.in/​aeti.html 204. www.ndt.net/​arti​cle/​v11​n06/​wong/​wong.htm 205. www.str​esst​ech.com/​str​esst​ech-​bulle​tin-​2-​the-​prop​erit​ies-​of-​bar​khau​sen-​noise/​ 206. www.str​esst​ech.com/​knowle​dge/​non-​dest​ruct​ive-​test​ing-​meth​ods/​bar​khau​sen-​noise-​analy​sis/​ 207. www.indust​rial​heat​ing.com/​artic​les/​93722-​bar​khau​sen-​noise-​ndi-​for-​heat-​treatm​ent-​defe​cts-​and-​ case-​depth-​analy​sis 208. www.ndt.com.au/​what-​is-​bar​khau​sen-​noise-​analy​sis/​ 209. Hill Engg, Cordova, CA. https://​hill-​engi​neer​ing.com/​bar​khau​sen-​noise-​analy​sis/​ 210. www.ans​ndt.com/​advan​ced-​ndt-​servi​ces/​ultra​soni​c_​co​rros​ion_​mapp​ing/​ 211. www.ndt.net/​sea​rch/​docs.php3?id=​26051 212. www.ndt.com.au/​3-​benef​i ts-​of-​using-​dro​nes-​in-​non​dest​ruct​ive-​test​ing/​ 213. www.fly​abil​ity.com/​artic​les-​and-​media/​dro​nes-​ndt-​benef​i ts 214. www.osti.gov/​servl​ets/​purl/​777​718 215. www.total​mate​ria.com/​page.aspx?ID=​Check​Arti​cle&site=​kts&NM=​193 216. www.inners​pec.com/​knowle​dge/​emat-​tec​hnol​ogy/​ 217. www.olym​pus-​ims.com/​en/​ult​raso​nic-​tran​sduc​ers/​emat/​ 218. https://​inspe​ctio​neer​ing.com/​tag/​emat 219. www.bindt.org/​What-​is-​NDT/​Index-​of-​acron​yms/​E/​EMAT/​ 220. www.ndt.net/​ndtaz/​cont​ent.php?id=​621 221. www.ndt.net/​arti​cle/​ndt-​slove​nia2​009/​PDF/​P28.pdf 222. www.total​mate​ria.com/​page.aspx?ID=​Check​Arti​cle&site=​kts&NM=​203 Date Published: Jun-​2007 223. https://​inspe​ctio​neer​ing.com/​tag/​mfl 224. www.ins​pect​ech.ca/​Techni​cal/​Tec​hniq​ues/​Flux​Leak​age/​index.html 225. www.ndt.net/​arti​cle/​wcnd​t00/​pap​ers/​idn​251/​idn​251.htm 226. https://​inspe​ctio​neer​ing.com/​tag/​pigg​ing

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227. https://​inspe​ctio​neer​ing.com/​tag/​ili 228. www.tcene​rgy.com/​about/​expl​ore-​ene​rgy/​unders​tand​ing-​how-​it-​works/​how-​we-​see-​ins​ide-​our-​ pipes-​with-​inte​llig​ent-​machi​nes/​ 229. www.dexon-​tec​hnol​ogy.com/​pipel​ine-​servi​ces/​inte​llig​ent-​pigg​ing/​ 230. https://​petrow​iki.spe.org/​Pipel​ine_​pigg​ing 231. www.nde-​ed.org/​About​NDT/​Selec​tedA​ppli​cati​ons/​Pip​elin​eIns​pect​ion/​Pip​elin​eIns​pect​ion.htm 232. www.r-​techma​teri​als.com/​field-​repl​icat​ion-​metall​ogra​phy/​ 233. www.elem​ent.com/​nucl​eus/​2016/​repl​icat​ion-​metall​ogra​phy-​a-​non-​dest​ruct​ive-​solut​ion 234. www.twi-​glo​bal.com/​techni​cal-​knowle​dge/​faqs/​what-​is-​shear​ogra​phy 235. https://​spie.org/​news/​5180-​fast-​non-​dest​ruct​ive-​test​ing-​under-​dyna​mic-​load​ing?SSO=​1 236. https://​inspe​ctio​neer​ing.com/​tag/​infra​red+​ins​pect​ion 237. www.twi-​glo​bal.com/​what-​we-​do/​servi​ces-​and-​supp​ort/​asset-​man​agem​ent/​non-​dest​ruct​ive-​test​ing/​ ndt-​tec​hniq​ues/​man​ual-​ult​raso​nic-​test​ing 238. www.inter​tek.com/​non-​dest​ruct​ive-​test​ing/​materi​als-​test​ing/​on-​site-​met​allu​rgy/​ 239. McMaster, R. C., ed., Nondestructive Testing Handbook, Vol. 1, Leak Testing, 2nd edn., American Society for Nondestructive Testing and American Society for Metals, Columbus, OH. 240. Anderson, G. L., Leak testing, in Metal Handbook, Vol. 17, Nondestructive Evaluation and Quality Control, 9th edn., American Society for Metals, Metals Park, OH, 1989, pp. 57–​70. 241. Hans Rottländer Walter Umrath Gerhard Voss, Fundamentals of leak detection, Leybold GmbH Cat. No. 199 79_​VA.02, Leybold GmbH Bonner Str. 498 · D-​50968 Köln, pp. 1–​47. 242a. Leak Detection Methods: A Comparative Study of Technologies and Techniques, Short version, VTech, pp. 1–​24. 242b. https://​cdn.thomas​net.com/​ccp/​20106​502/​32497.pdf 243. www.leyb​oldp​rodu​cts.com/​media/​pdf/​90/​c7/​87/​Fundam​enta​ls_​o​f_​Le​ak_​D​etec​tion​_​EN.pdf 244. https://​cont​ent.leyb​old.com/​leak-​detect​ion-​funda​ment​als 245. Phoenix Industrial Solutions, 2016 by Phoenix Industrial Solutions. 245.1. www.pho​enix​indu​stri​also​luti​ons.net/​logo 246. www.app​lus.com/​glo​bal/​en/​what-​we-​do/​sub-​serv​ice-​sheet/​mass-​spect​rome​ter-​hel​ium-​leak-​test 247. www.cor​rosi​onpe​dia.com/​7-​best-​meth​ods-​for-​detect​ing-​leaks-​in-​press​ure-​vess​els/​2/​6826 248. Training guidelines in non-​destructive Testing Techniques: Leak Testing at Level 2, IAEA Training Course Series No. 52, International Atomic Energy Agency Vienna, 2012, pp. 1–​171. 249. Leak detection in heat exchangers and underground pipelines using radiotracers, Training Course Series No. 38, International Atomic Energy Agency, Vienna International Centre, Austria, pp. 1–​65. 250. Halogen Diode –​Leak Inspection (applus.com)

Suggested Readings BSI Handbook 22: 1983 Quality Assurance, British Standards Institution, London, U.K. Connor, L. P., ed., Chapter 11, Weld quality, Chapter 12, Testing for evaluation of welded joints, Chapter 13, Codes and standards, in Welding Handbook, Vol. 1, Welding Technology, 8th edn., American Welding Society, Miami, FL, 1987. Emerson, W. F., Technology reinvents visual inspection, Welding Design Fabr., May, 31–​32 (1995). Erickson, K. D., Defining the role of a certified visual inspector, Welding J. Suppl. Nondestruct. Examination, October, 17–​19 (1997). Hingwe, A. K., Compiler, Quality Control Source Book –​Application of QC to Ferrous Metalworking, American Society for Metals, Metals Park, OH. Betz, C. E., Principles of Magnetic Particle Testing, Magnaflux Corp, Chicago, IL, 1966. https://​inspe​ctio​neer​ing.com/​tag/​paut Jackson, D., ISO 9000: Key to Europe’s door, Welding Design Fabr., April, 59–​60 (1992). Kirwillian, A., The true cost of quality, Welding Metal Fabr., January/​February, 5–​8 (1987). Krautkramer, J. and Krautkramer, H., Ultrasonic Testing of Materials, Section 8.6, 4th edn., Springer-​Verlag, New York, 1990. Lebowitz, C. A., A primer on nondestructive examination methods for weld inspection, Welding J. Suppl. Nondestruct. Examination, October, 29–​31 (1997). Libby, H. L., Introduction to Electromagnetic Nondestructive Test Methods, Wiley-​Interscience, New York, 1971.

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McGonnagle, W. J., Nondestructive Testing, McGraw-​Hill, New York, 1961. Mikulak, J., Viewpoints on quality, codes, conversions, and pipelines, Welding Design Fabr., March, 70–​73 (1977). www.modso​nic.com/​types-​of-​ult​raso​nic-​test​ing/​ Radiography in Modern Industry, 4th edn., Eastman Kodak Co., Rochester, NY, 1980. Richardson, H. D., Industrial Radiography Manual, Government Printing Office, Washington, DC, 1968. Reprinted in 1979 by the American Society for Nondestructive Testing. Ultrasonic inspection, in Metals Handbook, Vol. 17, Nondestructive Evaluation and Quality Control, 9th edn., American Society for Metals, Metals Park, OH, pp. 231–​277, 1989. Waker, K., Guide to liquid penetrant testing, Welding J. Suppl. Nondestruct.Examination, October, 21–​23 (1997). Welding Engineer Data Sheet No. 414 –​Effectiveness of nondestructive testing methods, Welding Design Fabr., February, 41 (1975). Weldments, brazed assemblies, and soldered joints, in Metals Handbook, Vol. 17, Nondestructive Evaluation and Quality Control, 9th edn., American Society for Metals, Metals Park, OH, pp. 582–​609, 1989. Weymueller, C. R., Ultrasonic testing for the fabricator, Welding Design Fabr., October, 25–​33 (1994).

4

Fabrication, Brazing, and Soldering of Heat Exchangers

4.1 INTRODUCTION This chapter discusses fabrication of shell and tube heat exchangers (STHE) and brazing of compact heat exchangers, such as automobile radiators, and heat exchanger coils and soldering of radiators. Several different methods are used to construct pressure vessels, STHEs, and compact heat exchangers. Most pressure vessels are constructed with welded joints. STHE fabrication involves the construction of items from different parts using at least one of a range of processes such as cutting, bending, welding, heat treatment, and assembling processes. The materials to be used in pressure vessels must be selected from code approved material lists. Fabrication uses semi-​finished or raw materials to make components from start to finish. This work is typically completed by a fabrication shop. The actual making of the heat exchanger could include a huge number of different manufacturing processes and techniques. Heat exchanger fabrication process requires the usage of the latest technology, highly skilled staff, and top quality equipment and lifting quality. On occasions where the heat exchanger requires non-​core parts, those parts are sourced from outside companies. The commonly used metal joining processes are welding, brazing, and soldering. The plate fin heat exchanger (PFHE) type without gaskets, tube-​fin exchangers, especially automobile radiators, micro channel and microgrooved tube fin heat exchangers, and some highly compact metal rotary regenerators are brazed. Soldering is widely used manufacture of for copper tube-​copper fin radiators. Fabricated heat exchanger units are subjected to inspection in addition to stage inspection, NDT, testing and shipment.

4.2 FABRICATION OF THE SHELL AND TUBE HEAT EXCHANGER With advances in material and welding technology, fabrication processes today call for high standards of understanding and workmanship. Equipment made with materials of correct choice and quality and adequate design may fail in service if the workmanship is sacrificed during fabrication [1]. This section details standard shop floor practices for fabrication of shell and tube heat exchangers. The detailed methods of shop floor practices are left to the discretion of the manufacturer in conformity with applicable codes/​standards such as: 1. Standards of the Tubular Exchanger Manufacturers Association (TEMA), 10th edn., 2019, Tubular Exchanger Manufacturers Association, Inc., Tarrytown, NY. 2. ASME Boiler and Pressure Vessel Code, Section VIII, Division 1–​Pressure Vessels, American Society of Mechanical Engineers, New York, 2021. 3. ASME Boiler and Pressure Vessel Code, Section II Parts A, B, C and D for Materials specification, 2021. 494

DOI: 10.1201/9781003352051-4

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4. ASME Boiler and Pressure Vessel Code, Section V Nondestructive Examination, 2021. 5. ASME Boiler and Pressure Vessel Code, Section IX Welding, Brazing, and Fusing Qualifications, 2021. 6. Standards for Steam Surface Condensers, 12th Edition, 2017, HEI, Cleveland, Ohio. 7. Standards for Closed Feedwater Heaters, 9th Edition, 2015, HEI, Cleveland, Ohio. 8. API 660, Shell and Tube Heat Exchangers, Ninth Edition, 2015, American Petroleum Institute, Washington, DC. 9. National and local laws or regulations as applicable. 10. Others. Codes and standards in their latest editions shall form basis for fabrication, inspection, testing, and acceptance of documents. The manufacturer shall submit for purchaser’s approval three prints of an outline drawing showing overall dimensions, nozzle sizes and locations, supports, and weight. The purchaser’s approval of drawings does not relieve the manufacturer of responsibility for compliance with TEMA standard and applicable code.

4.2.1 Manufacturing and Testing After the material selection has been completed, proper raw materials are supplied whether in form of plates, wires, rods, or pipes. Later on, they are cut and machined if required. Machined parts are then assembled by welding in the required geometry. Depending on the material type and thickness at the weld point, some heat treatment procedures as preheating, and post-​weld heating may be required. At each step, products should be inspected by the authority. During construction, some nondestructive test methods are applied to check whether the quality of the test piece satisfies the standards or not. For detecting weld surface flaws, magnetic particle or dye penetrant methods are utilized, so that discontinuities on the surface, or near the surface can be identified. X-​ray inspection may be an alternative to detect subsurface cracks, however since it is a highly expensive method, it is not commonly preferred unless a very critical weld point is not present. Ultrasonic inspection is a more common and relatively easy way to detect both surface and subsurface defects. For guidance on pressure vessels and heat exchanger manufacture and inspection refer to [2] and for stage wise inspection during fabrication of shell and tube heat exchanger refer to [3].

4.2.2 Quality Assurance Plan (QAP) Assurance for safety and reliability of boilers, pressure vessels and heat exchangers is a systematic approach involving various stages right from material receipt and identification to final stages of testing and shipment. Details of quality assurance plan or quality assurance program is discussed in Chapter 3. A typical format for QAP is shown in Table 4.1 and a detailed QAP for the manufacture of a STHE is shown in Annexure Table 4.11. 4.2.2.1 Inspection and Test Plan As part of quality assurance and control measures, inspection, testing and measurement activities should be planned, defined and carried out in a documented manner, i.e. in ITP –​Inspection and Test Plan. In manufacturing, an Inspection and Test Plan (ITP) is an inspection checklist that includes those characteristics that should be checked at each stage of the process. It could cover all aspects of production process, from receiving inspection, when materials are received to the pre-​shipment inspection, i.e. the ITP has a tabular format and the content extracted from the construction code. In each row of the table, main operations, there is a quality control and inspection requirement and this determines which party is responsible for control and inspection. It should include reference to

newgenrtpdf

496

TABLE 4.1 Quality Assurance Plan for the Fabrication of PV/​Heat Exchanger Fabricator Name and Address: Purchaser Name and Address:

1. 2 …. ….

Component/​ Operation

Characteristics

Type/​Mode of Inspection

Reference document

Acceptance standard

Format of recording

Inspection by

Reviewed by

Witnessed by

Remarks

Fabrication, Brazing, and Soldering of Heat Exchangers

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support processes, documents needed such as work instructions or engineering specifications, and other resources required to complete an inspection. The exact inspection details should be specified on the ITP format sheet, these could include things like visual inspections, product measurements, etc. A typical ITP for manufacture of an air-​cooled heat exchanger is shown in Annexure Table 4.12. 4.2.2.2 Features of ITP There are three parties in the ITP, the manufacturer, the Third Party Inspector (TPI), and the Client or purchaser. The ITP is one of the most useful working documents. Used in a key monitoring and control role, it summarizes the activities of manufacturer, contractor, and inspector or inspecting agency. The ITP identifies all inspection points for the purchaser’s inspector. Then the manufacturer needs to prepare the project quality control plan based on this inspection and test plan. For details on ITP refer to [4]. 4.2.2.3 Hold Points and Witness Points Consideration should be given to the establishment of hold points and witness points, where an examination is to occur prior to the accomplishment of any further fabrication steps. Through hold points and witness points, authorized code inspectors exercise control over the manufacturing activities. Hold points. The use of “hold points”, where a manufacturer must stop manufacture until an inspector or Notified Body completes an interim works inspection, should be shown. In general, raw material inspection and identification, post weld heat treatment review, hydrostatic test, performance test, run-​out test, and final inspection are hold points. Witness Point (W). The manufacturer shall notify the client and the Inspector, but there is no hold on the production. Review (R).Review means review document, which includes the review of quality control records, test reports, etc. A sample “Hold Points and Witness Points” plan for a STHE is shown in Table 4.2.

4.3 VENDOR’S RESPONSIBILITIES 4.3.1 Scope of Supply Supply of following items is in vendor scope: 1. vessel/​exchanger as per design data sheet, drawings, specification and code 2. fasteners for all nozzles and their companion flanges 3. gaskets for all nozzles.

4.4 DETAILS OF MANUFACTURING DRAWING Manufacturing drawings for heat exchanger fabrication should contain some additional details than any other engineering drawing. Such additional details for the fabrication of heat exchangers are listed by Rao [5]. Some of the details pertaining to welding and testing plan are [5] as follows: 1. Name plate location and details. 2. Weld line orientations and locations, weld profiles, and sizes. 3. Testing plan indicating the tests to be conducted on welds, and assemblies such as NDT test, destructive tests, hydrostatic testing/​pneumatic testing, etc.

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TABLE 4.2 Hold Points and Witness Points Number

Stages of Manufacture

H

W

1. 2. 3. 4. 5. 6. 7. 8. 9. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

Drawing and design calculations approved Raw material for pressure parts clearance WPS/​POR/​welder qualifications Forming dimensions for shell and dished ends U-​bends, qualification report Heat treatment chart for forming (if applicable) Fit up of pr. welds incl. attachments on pr. parts Air test for RF pads Shell ID check by template Radiography for pressure part welds MT/​PT of nozzle and pressure parts attachment welds PWHT (if applicable) clearance for pressure part assembly Review of heat treatment charts NDT after heat treatment if applicable Destructive testing of production test plate Tubesheet and baffle inspection after drilling Tube bundle skeleton before tubing Tube-​to-​tubesheet joints, NDT/​LT Pneumatic and hydro test for tube-​to-​tubesheet joints Shellside and tubeside hydrostatic test NCR clearance Painting of unit satisfactory Stamping and document folder clearance

X X X X —​ —​ X —​ —​ X —​ X —​ X X —​ —​ —​ —​ X X X X

—​ —​ —​ —​ X X —​ X X —​ X —​ X —​ —​ X X X X —​ —​ —​ —​

Note:  Hold Point (H), Verification of records(W) and X applicable.

4. Welding plan to show WPS reference to each weld. 5. Technical delivery conditions of materials. 6. Edge preparation and method employed. 7. Electrodes with equivalent AWS specifications. 8. Welding methods to be employed. 9. Code certification and stamping requirements as indicated in the purchase order. 10. Maximum allowable working pressure (MAWP). 11. Design pressure. 12. Hydrostatic test pressure. 13. Operating temperature. 14. Design temperature. 15. Minimum design metal temperature.

4.4.1 Additional Necessary Entries Rao [5] recommends the following additional entries in the drawing: • code requirements • volume of pressure chambers • special loadings

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• • • • • •

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corrosion allowance joint efficiency joining method other than welding (e.g. brazing, adhesive bonding, rolling-​in of tubes) surface treatment (e.g. painting, pickling, coating, lining) inspection authority instructions for the jobsite (e.g. assembly weld).

4.4.2 Fabrication Requirements It shall be entirely the vendor’s responsibility to ensure compliance to codes and standards specified for design, fabrication, testing, etc. and to all requirements of the requisition including specifications and drawings after the order is placed. Stage wise and final inspection shall be carried out as per approved ITP. All vessels/​heat exchangers shall be offered for inspection to purchaser or his/​her authorized inspector. Inspectors shall have free access to all workshops of Vendors or sub-​Vendors. In addition to final inspection and certification by Inspector, Inspector’s written approval shall be obtained by the Vendor at all stages of fabrication. Others include: 1. Fabricator shall ensure the materials properties of both ferrous and nonferrous metals retained and not deteriorated during fabrication. 2. The fabricator shall submit the fabrication drawings with bill of materials, G.A drawings, layout, detailed manufacturing procedure, NDT procedures and agency for carrying out inspection and documentation formats along with time schedules for approval before fabrication. Raw material shall be offered for inspection via third party inspection agencies like TUV Sud, TUV Rheinland, DNV, Lloyds, BV, etc. and inspection report shall be submitted. 3. The fabricator shall submit all test certificates for all the raw materials before using them for fabrication. 4. Manufacturing plan shall include the detailed procedure of manufacture, the equipment used and the tolerances that are maintained at various stages of manufacture to achieve the final required dimensions. 5. Tube bundle fabrication drawings will include details such as tube layout, number tubes in each pass, number of baffles, type and description (for segmental baffles include cross-​baffle cut, layout, and orientation in a view that shows the cuts), details and locations of all sealing and sliding strips, tie-​rods and spacers, support plates, tubesheet and tube holes, including cladding or weld overlay if required, pass-​partition plates, impingement protection device details, if applicable, U-​tube bend schedule, if applicable. 6. Lifting lugs shall be provided as per standard for lifting of channels, channel covers, floating heads, shell covers, test rings, test flanges, etc. 7. In case of removable bundles the following are required: a. Stationary tubesheets shall be drilled and tapped at vertical (0º) position for attaching eyebolts or a lifting lug of adequate thickness shall be welded with 25 mm (minimum) diameter hole. b. Drilling and tapping as above shall be carried out on floating tube sheet. c. Last support plate shall be drilled (25 mm diameter hole) for lifting the assembled bundle. Local stiffening may be necessary to withstand the bundle and load, which shall be designed by the Vendor. 8. Pulling eyebolts shall be provided for all removable bundles. These shall be sent loose along with the spares. Insert piece shall be provided for all clad tube sheets. Material of insert piece shall be same as that of cladding material. 9. Stationary tubesheet shall be drilled and tapped for tie rods. Care should be taken to ensure that holes are not drilled through.

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10. Jack screws shall be provided on all girth flanges. 11. Fabrication tolerances and finish. The dimensional tolerances shall be within the limits indicated on the specification. Where tolerances are not specified, these shall be in accordance with the requirements of TEMA Section 2, F-​1and Code. 12. All edges and corners including baffle holes shall be de-​burred (or rounded, if shown). 13. All the consumables used in NDT and Inspection shall be from approved sources. 14. All the welding consumables used for welding shall confirm to ASME Section II C and shall be of reputed make. 15. The welder and welding procedure shall be qualified as per ASME Section IX. 16. The edges/​surfaces to be welded shall be cleaned thoroughly before welding. After completion of welding the inside weld reinforcement of all the welds shall be ground flush with the parent material. 17. Repairs. If the number of repairs on any weld during entire fabrication is more than three or as agreed, the welder and welding procedure shall be re-​qualified. The vendor shall not perform any weld repairs without the prior approval of the repair procedure from the purchaser. 18. Inspection shall be carried out during every stage of fabrication both during fabrication and before delivery as per approved ITP and also for sub-​ordered materials, if any. 19. The fabrication of a heat exchanger and pressure vessels involves operations like machining, drilling, forming, drawing, stretching, rolling, bending, shearing, grinding, welding, brazing, etc. in order to manufacture all of the parts, which are to be assembled to produce the final heat exchanger. All these methods can introduce residual stresses, which may impair the service life of the component or system during operation. Hence good engineering practices are to be used to keep the residual stress to the minimum.

4.4.3 Quality Control During Production Welding The following welding precautions and controls are to be exercised by the manufacturer to achieve the desired quality [5]: 1. Weldments fit-​up configuration control. 2. Cleanliness of the weldments and welding consumables to be free from rust, oil, moisture, etc. 3. Proper storage and baking of electrodes. 4. Purging and shielding gas quality and gas flow control in the Gas Tungsten’ Arc Welding (GTAW) process. 5. Pre-​heating and interpass temperature control. 6. Welding parameters control such as welding current, arc voltage, and heat input rate. 7. Weld profile control. 8. Weld spatter control. All the above mentioned controls during production welding will greatly help achieving the desired quality of the welds in a pressure vessel. 4.4.3.1 Dimensional Check –​Required Tolerances The inspector should conduct dimensional checks of pressure vessels to ensure that they are within the required tolerances of the specification. At a minimum these checks consist of the following [6]: • • • •

mill under-​tolerance of plates and pipes tolerances for formed heads out-​of-​roundness of shell nozzles and attachments orientation, projection and elevation

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• nozzles and attachments levelness • weld mismatch • weld reinforcement. 4.4.3.2 Shell and Tube Heat Exchanger Fabrication and Inspection Guidelines The heat exchanger still is a pressure vessel and all requirements for ASME pressure vessel does apply for heat exchanger as well. There are several inspection requirements that only apply to the heat exchanger and not regular pressure vessel. The various stages of fabrication, inspection and testing shall include but not be limited to the following: 1. Raw material identification and verification of mill test certificate. 2. Edge preparation plate for welding, including visual cheek for lamination. 3. Rolling of shell sections, tack welding, and alignment for welding of longitudinal seams (LSs). 4. Shell alignment of longitudinal and circumferential seams. 5. Rolling tolerances on individual shell section. 6. Alignment and fit up of shell sections and components. 7. Root pass clearance before further welding and cleaning. 8. Welding of shells, checking the dimensions, and subjecting pieces to radiography. 9. Welding of expansion bellows or joints, nozzles, if required. 10. Checking the circularity and the assembly fit ups, including nozzles. 11. Profile and thinning of dished ends after forming. 12. Marking, drilling, cleaning and checking of tubesheet after drilling holes for correctness of tube layout patterns, tube holes diameter, ovality, scoring marks if any in tube holes, pass partition grooves, visual check for lamination, etc. 13. Baffles drilling, removal of burr, inspection. 14. Tubesheet to shell set-​up for fixed tubesheet exchanger, prior to welding. 15. Cleaning of tube ends. 16. Inspect mock-​up of tube to tubesheet joint. 17. Tube to tubesheet joint expansion or welding. 18. Visual check of shell inside and tube bundle before insertion. 19. Review of NDT reports and heat treatment charts. 20. Final visual and dimensional inspection. 21. Witnessing of hydro test and stamping. 22. Assembly of channel with shell assembly. 23. Preparation for shipment. 24. Preparation of data folder. Components of STHE and its assembly is shown in Figure 4.1, a flow chart for manufacture of a fixed tubesheet heat exchanger is shown in Figure 4.2 and an inspection plan for heat exchanger components in Figure 4.3. A detailed quality assurance program is given in Annexure Table 4.11.

4.5 DETAILS OF MANUFACTURE OF STHE A heat exchanger consists of various components (as shown in Figure 4.1). The methods for the manufacture of the components must be in accordance with the requirements specified in the specification for manufacture of heat exchanger and should meet code requirements. The major components are 1. Heads or dished ends. 2. Shell (body of the vessel).

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FIGURE 4.1  Major components of STHE. (Note: (e) Flexible expansion joint is used for piping applications.)

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FIGURE 4.2  Flow chart for manufacture of a fixed tubesheet heat exchanger. (Courtesy of Universal Heat Exchangers Ltd., Coimbatore, India.)

3. Cones or reducers. 4. Attachments to the shell including tubesheet(s), nozzles, expansion joint, saddle supports, lifting lugs, etc. Manufacture of these components are discussed below.

4.5.1 Quality Control During Assembly of Parts After forming the shells and dished ends, and other pressure retaining parts, the manufacturer shall examine to ensure they conform to the prescribed shape and meet the thickness requirements after forming. During assembly of shells, dished ends, nozzles, manhole frames, nozzle reinforcement pads, and supports, etc. the manufacturer has to take certain precautions and exercise controls to ensure that the assembly of parts should not result in unacceptable mismatch in weld fit-​ups, improper orientation of parts in assembly, unacceptable dimensions of the pressure vessel and its appurtenances and should ensure that all nozzles, manhole frames, nozzle reinforcement, and other appurtenances to

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FIGURE 4.3  Inspection and test plan for heat exchanger components.

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the inside and outside of the vessel properly fit the vessel curvature. The codes specify the tolerance on weld joint configurations finished weld shape and various dimensions of the vessel. Many a time, the shells of the vessels are rolled and welded before the dished ends are received from the sub-​contractor. The shells would have been made with negative tolerance on diameter and dished ends with positive tolerance on diameter resulting in mismatch in the joint between dished end to shell leading to undesirable stress concentration at the joint and difficulty in welding. Before sizing and forming of the shells, the manufacturer should first get the dished ends formed, in order to get the circumference data of the dished ends. Using these values the manufacturer should decide the sizes of the shell course in order to keep the mismatch between shells and dished ends within the tolerances. The manufacturer should generate records of the dimensions of each shell, dished end, nozzles, and the appurtenances as a proof of achieving the dimensions and shape within the limits [7].

4.5.2 Identification of Materials All materials used for the construction of pressure vessels should have traceability. For example, acceptable methods of traceability for plates used in the fabrication of vessels include [8]: 1. plate number 2. lot number 3. heat number. Identification includes a job number, serial number, and heat number, if relevant. Throughout production, identification follows the material or component, assuring traceability [9]. Figure 4.4 shows scheme of plate marking and transfer of identification [8].

4.5.3 Positive Material Identification (PMI) Positive material identification (PMI) is an essential nondestructive testing (NDT) method utilized to verify that supplied materials conform to the specifications with reference to material composition. The material testing can be performed on-​site before the parts go into the production process or for final products. PMI is a portable method of analysis and can be used in the field on components. There are several nondestructive examination (NDE) methods that can be used for PMI. Two of the more popular are X-​Ray Fluorescence (XRF) and Optical Emission Spectroscopy (OES). Handheld x-​ray fluorescence (XRF) is the most common PMI method and the portability of the hand-​held equipment allows to perform PMI on-​site at the production plant or factory floor.

4.5.4 Edge Preparation and Rolling of Shell Sections, Tack Welding, and Alignment for Welding of Longitudinal Seams 4.5.4.1 General Discussion on Forming of Plates In general, the forming of plates in the pressure vessel industry covers the bending of plates into shell courses/​belts and spinning and pressing of circular plates to form vessel heads. Before forming the plate, it shall be inspected by UT plate tester. The most critical factors governing the cold forming of a shell course in a given material are (1) the plate thickness and (2) the final diameter of the finished cylinder. Therefore, the radius of bending is limited to a maximum percentage strain in the outer fiber in the range of 3%–​4.5% without an intermediate annealing or stress relief [10]. According to Peacock [10], since local straining during cold bending may cause higher values of strain in localized

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FIGURE 4.4  Plate marking and transfer of identification-Illustration.

areas, in practice, a factor of safety is adopted to limit the strain in the outer fibers to 2%–​2.25%. The strain ε on the outer fibers of the shell has been calculated by Carpenter and Floyd [11] as follows:

ε=

R1 R2 − 0.5 R1t R1 R2 − 0.5 R2 t

− 1 (4.1)

where R1 is the final radius R2 is the initial radius (for a flat plate use a value of 1000 in.) t is the thickness of plate. After forming the shell/​dished ends, the manufacturer should examine them to make certain that they conform to the prescribed shape/​contour and meet the thickness requirements after forming. Details of shell and vessel heads manufacture, quality control and inspection are outlined in Ref. [8].

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4.5.4.2 Manufacturing Processes The basic manufacturing process used in the fabrication of pressure vessel is forming. Forming is the process by which the size or shape of the part is changed by application of force. Depending on the temperature during fabrication, it is categorized as hot, warm, or cold forming. The manufacturing processes fall under the following headings: • pressing • spinning • bending. The manufacture of heads and cones is considered to be more difficult than that of shells due to difficulty in controlling the dimensions of these two items. For this reason, they should be made first (within the code tolerances), and shells are made later to suit these heads and cones. The fabrication of shell is discussed next and the heads and end closures are discussed later. 4.5.4.3 Fabrication of Shell –​General Refer to TEMA Section RCB-​3 for shells and shell covers. The heat exchanger shell should be fabricated from either commercially available pressure vessel quality seamless or welded wrought pipe or from plate rolled into a longitudinally fusion-​welded cylinder. In the case of rolled shells, the certified plates are put into inspection after marking the sizes as per drawing requirements. The inspector will stamp the plates with a monogram and allow for cutting. If the length of the plate is sufficient to accommodate the full circumference of the shell, this is the preferred condition because the shell will have only one longitudinal seam. All longitudinal and circumferential welds of shells for other than kettle-​type heat exchangers shall be finished flush with the inner contour for ease of tube bundle insertion and withdrawal. For kettle-​type heat exchangers, this requirement shall not apply to welds in the enlarged section, unless they are in the bottom quadrant of the shell. Openings and attachments should clear weld seams. All longitudinal and circumferential welds of shells for other than kettle-​type heat exchangers shall be finished flush with the inner contour for ease of tube bundle insertion and withdrawal. For kettle-​type heat exchangers, this requirement shall not apply to welds in the enlarged section, unless they are in the bottom quadrant of the shell. For removable-​bundle heat exchangers, the permissible out-​of-​roundness of a completed shell, after all welding and any required heat treatment, shall allow a metal template to pass through the entire shell length without binding. The template shall consist of two rigid disks (each with a diameter equal to the diameter of the transverse baffle or support plate) rigidly mounted perpendicularly on a shaft and spaced not less than 300 mm (12 in.) apart. After cutting the plates to size and edge preparation, the edges of plates should be examined for defects like cracks and lamination. The plates are then rolled to the required dimensions. Edge preparation is a critical factor in the production of high-​quality joints and in the making of sound welds. Inadequate preparation, resulting in irregular plate edge or rough V formation, prevents close fit and makes the task of the welder more difficult [12]. For LS welding, the rolled section is held in position by tack welding. The width, length, and thickness of the tack welds are critical. A very thin tack has an effect similar to arc strike, and this local position will reveal cracks from high carbon martensite due to rapid cooling [1]. Therefore, tacks should have sufficient metal deposition. For satisfactory welding, cleanliness of the edges to be welded is essential.

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FIGURE 4.5  (a–​c) Rolling of shell plate (schematic). (d) Rolling of shell plate. (Courtesy of Edmonton Exchanger & Manufacturing Ltd, Edmonton, Alberta, Canada.)

4.6 PLATE BENDING 4.6.1 Roll Bending Rolling of plates should be in the transverse direction of the plates (as shown in Figure 2.6 of Chapter 2). The direction should be marked on the plates. The usual practice of bending is by passing the plate through either a three-​or a five-​roll plate bending machine of adequate capacity to bend the plate to the required diameter. The most commonly used bending machine in fabrication industry is the three-​roll plate bending machine (Figure 4.5). A four-​roll bending machine offers production advantages, although it is not particularly suited to hot work [13].

4.6.2 Vertical Plate Bending Machine The vertical plate bending machine can be used for all ASME Code pressure vessel work, and results are claimed to be as good as or better than those obtained with horizontal plate bending rolls. Some of the advantages claimed for vertical roll bending include the following [13]: (1) absence of camber in the rolled shell, (2) higher capacity and hence thicker plates can be rolled, (3) less power consumption, (4) the weight of the plate has very little effect on the bending accuracy, and (5) the mill scale experienced on many plates drops clear during the bending operation, whereas in horizontal machines it tends to get rolled into the plate surface.

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4.6.3 Manipulative Equipment In the fabrication of heat exchangers, rotator sets or turning rolls are utilized to manipulate the shell for various welding positions. They ensure smoother welds. Circumferential welding is carried out by providing for the components to be rotated on the roller manipulator on the support bed. Basically, these comprise two types [14]: (1) self-​aligning rotators and (2) conventional rotators. Self-​aligning rotators consist of rubber-​tired wheels that automatically align themselves to the change from one diameter of vessel to another. In the case of conventional rotators, the rolls have to be manually positioned across the frames to suit the diameter of the vessel.

4.7 WELDING OF SHELLS, CHECKING THE DIMENSIONS, AND SUBJECTING PIECES TO RADIOGRAPHY Certain advance preparation is necessary while fabricating the shell. For correcting the ovality and weld distortion, and for baffle gauge checking, suitable fixtures, spiders, and innovative devices for specific type of exchanger are to be developed and kept in advance to avoid unnecessary hammering, heating, and grinding, which reduce the quality of material and weldment. In a big company manufacturing large numbers of exchangers and pressure vessels, standardization of components like elbows, flanges, nozzles, reinforcement (RF) pad designs, and edge preparation is required. This leads to smooth flow of material. After tack welding, the shell section is ready for longitudinal seam (LS) welding. During shell welding, spiders are tacked internally to avoid distortion of the shell. The welding is first done from one side. The other side is back chipped or gouged and offered for inspection. The inspector examines the area for the weld metal penetration, oxidized metal removal, and the weld free from weld defects. In very large or thick-​walled vessels, the shell course may be fabricated from two or more curved plates and joined by several LS welds [15]. Before the assembly of circumferential joints, each shell course should be checked for shell thickness, inside diameter, and circularity, and shell sections may be rerolled if necessary. If any rerolling is required, it should be done before radiography, and all LSs should be dye penetrant tested after rerolling. The welding of shell courses is shown in Figure 4.6 and a welded shell course is shown in Figure 4.7. Check the curvature using a sweep board as sown in Figure 4.8. Circumferential welding is carried out by providing for the components to be rotated on the roller manipulator. While welding two shell sections circumferentially, the LSs of the adjacent shell courses should not be aligned. After welding the various shell courses, the circumferential seams are subjected to radiography. If any portion of the weld is found defective, it should be repaired and inspected. (Girth, longitudinal, and nozzle attachment welds in exchanger shall be full penetration weld.)

4.7.1 Dimensional Check The following dimensions of shell are checked [8]: 1. outside circumference at both ends as well as at the center of the shell 2. inside or outside diameter at four angles on either side of the shell 3. straightness of the shell through center lines at 0, 90, 180 and 270o 4. length of the shell at four center lines. Shell circumference and straightness check is shown in Figure 4.9.

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FIGURE 4.6  Welding of shells.

FIGURE 4.7  Welded shell courses (CS, circumferential seam and LS, longitudinal seam).

4.7.2 Welding of Nozzles After completion of the shell, mark the shell for the nozzle locations, cut open, prepare edges, fit the reinforcement (RF) pad and the nozzle, weld and subject the welds of RF pad for leak test, and examine by penetrant/​magnetic particle test. Inspection of back-​gouged welded joints by magnetic particle or liquid penetrant examination techniques is especially important for joints of set-​in nozzles, which are not readily radiographed following welding. Most shops use a special-​purpose submerged arc welding machine. The welding is through the entire thickness of the shell plate. If the component to which nozzles are attached is subsequently stress relieved, it is the fabricator’s responsibility to maintain true gasket faces by machining or otherwise. A typical welding of a set-​through nozzle is shown in Figure 4.10.

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FIGURE 4.8  Rolled shell ovality check with a sweep board.

FIGURE 4.9  Rolled shell circumference and longitudinal dimensions check.

511

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FIGURE 4.10  Welded set-​through nozzle.

4.7.3 Reinforcing Pads and Testing Reinforcing pads whenever required as per drawings or code shall be of the same material or equivalent as the heat exchanger component to which it is welded. All reinforcing pads shall be provided with two 1/​8“ (3 mm) NPT tapped holes located 180 apart for air soap solution test with a pressure of 1.25 kg/​cm2. This test shall also be required to be carried out for slip on flanges. Higher test pressures are not recommended because of accompanying risks and also because the soap bubbles have a chance to blow off. Tell-​tale holes in the reinforcing pads shall be plugged with hard grease unless otherwise indicated after the hydro test of the exchanger.

4.7.4 Operations Process Sheet for Welding of Nozzles on Shell/​Head Prepare an operations process sheet as given below: 1. Hole marking and clearance. 2. Hole cutting and inspection by VT/​PT/​MT/​UT. 3. Edge preparation and inspection by VT/​PT/​MT. 4. Tack weld nozzle on shell/​dish. 5. Set up clearance. 6. First side welding. 7. Second side gouging. 8. Second side welding. 9. Dress up of weldment. 10. Visual examination/​UT/​PT/​MT. 12. Air Leak test of reinforcement pads. 13. Clearance for further process.

4.7.5 Flanges Unless otherwise indicated, dimensions, drilling, facing and tolerances for nozzle flanges (and blind covers if required) shall be as per ASME B 16.5 (for size up to 24“ NB) and ASME B 16.47 series B (for sizes above 24” NB) for the respective class.

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4.7.6 Bolts, Studs, and Tapped Holes All inch bolting and threading shall conform to ASME B 1.1 except for size 1“ and above where it shall be eight threads per inch. Metric bolting and threading shall conform to ISO-​R261 except for size M24 and above, where it shall have 3 mm pitch.

4.7.7 Supports Supports for horizontal exchangers shall comply with the standard drawing attached or as shown in the equipment drawing. When it is important, the consignee will indicate which support shall be fixed and which support shall be sliding, otherwise the vendor shall indicate this on his proposal drawing supports welded to the heat exchanger shall not interfere with inspection of heat exchanger circumferential seams.

4.7.8 Attachment of Expansion Joints Fabrication, inspection and pressure test, stamping, and reports for flanged-​and-​flued or flanged expansion joints shall conform to the requirements of Part UG and those of (a) through (g) of Appendix CC of ASME Code Section VIII, Div. 1. Some of the ASME Code requirements for the attachment of expansion joint are the following: 1. Flexible elements shall be attached by full-​penetration circumferential welds. 2. Attachments such as nozzles and backing strips shall not be located in highly stressed areas such as inner and outer tori, and annular plate of the expansion joint, unless the welds are ground flush and fully radiographed. 3. The circumferential attachment welds between the expansion joint and the shell shall be examined 100% on both sides by penetrant/​magnetic particle test. Welding of an expansion joint and its inspection are shown in Figure 4.11.

4.7.9 Checking the Circularity of the Shell and the Assembly Fit, with Nozzles and Expansion Joints Welded The ovality of the shell should be kept as low as possible, and the shell should be free from flat spots and projections from weld seams and nozzles. Check the positions of connections, supports, expansion joints, and all appurtenances relative to the working points [16]. The clearance between the baffles and the shell shall be as specified in the drawings. Excessive clearances would decrease the heat transfer efficiency of the exchanger; inadequate clearance will pose a problem when inserting tube bundle. Measure the shell length at the quarter points and the nozzle and support base distances to the shell centerline. To avoid difficulty in insertion of the bundle, a pull-​through gauge as shown in Figure 4.12 is passed through the shell [17].

4.7.10 PWHT of Shells Use spider bracing for adequate support during the stress-​relieving operation, and thermocouples are fastened to the vessel internally and externally. Temperatures are closely monitored and controlled during the period of temperature raising, soaking, and relaxing. Test plates inserted both inside and outside the vessel during stress relieving are later tested to ensure that the plate has been returned to its proper ductility and tensile strength [18].

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FIGURE 4.11  Welding of expansion joint with a heat exchanger shell. (a) Closer view (Courtesy of Mueller, Springfield, MO), (b) welding of the expansion joint before insertion of tube. (Courtesy of U.S. Bellows, Inc., Houston, TX, www.usbell​ows.com), and (c) inspection of expansion joint weld with heat exchanger shell. (Courtesy of U.S. Bellows, Inc, Houston, TX, www.usbell​ows.com.)

4.8 TUBESHEET AND BAFFLE DRILLING 4.8.1 Tubesheet Drilling The main process for tube sheet is drilling due to the large number of holes. Improving the process efficiency of drilling is one of the keys to improving productivity. Tubesheet drilling is the proper drilling of holes in tubesheets, complying with predetermined hole configurations to suit device requirements. In other words, correct tube sheet drilling is accurately calculated so that the tubes that tube sheets will support can operate as specified. Tubesheet drilling is an important aspect of tubesheet fabrication because getting the holes wrong can mean improper heat circulation. To avoid delays in this process and to achieve better tolerances on hole sizes and ligaments, use of numerical control drilling machine is recommended. Lubricating the holes is sometimes not allowed because the lubricant contaminates the tube-​to-​tubesheet joint. Holes must be reamed to get smooth finish. After reaming, examine the tubesheets for the tube count, pitch, and the tube layout pattern, which should match the drawings. Tube hole diameters and tolerances shall conform to TEMA Table RCB-​ 7.2.1. Permissible tubesheet ligaments, drill drift, for the heaviest tube wall thicknesses shall conform to TEMA Table RCB-​7-​2.2. Tubesheet machining is shown in Figure 4.13, tube layout pattern is shown in Figure 4.14 and tube hole marking as per tube layout pattern is shown in Figure 4.15.

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FIGURE 4.12  Pull-​through gauge to check the circularity of heat exchanger shell. (a) Schematic and (b) gauge passing through the shell assembly.

FIGURE 4.13  Tubesheet blank machining.

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FIGURE 4.14  Tube layout pattern. (Note: item (b) is to be read as rotated triangular pattern.)

FIGURE 4.15  Tube hole marking as per tube layout pattern.

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4.8.2 Checklist for Tubesheet Inspection After Fabrication For inspection prepare an ITP as given below: Outer diameter of the tubesheet. Diameter of the raised face shellside. Depth of the raised face shellside. Diameter of the raised face channel side. Depth of the raised face channel side. Thickness of the tubesheet. Width of the pass partition groove. Depth of the pass partition groove. Number and orientation of pass partitions. Size of the tube holes. Number and layout of tube holes. Holes marking. Drilling. Finish reamer. Outermost tube periphery diameter (OTL). Ligament: nominal ligament number of holes below nominal ligament. Minimum ligament near pass partition. Depth of the welding groove. Tube hole expansion groove –​width, depth and location. Number of pulling eyes and sizes. Number of tie rods and sizing. Number of bolt holes and sizes. Neutral line marking. Obtain clearance.

4.8.3 Tube Hole Finish In general, tube hole finish ranges from somewhat smoother than 32 rms to the roughest permissible finish. The range of commercially supplied feedwater heater, heat exchanger, and condenser tubing surface finishes is usually 60–​100 rms [16]. Inspect the holes using a high-​intensity light and a magnifying lens. The surfaces of the holes should be uniform throughout and free of scores, longitudinal scratches, spiral, annular, or axial tool marks. The inside edges of tube holes should be free from burrs to prevent cutting of tubes. A drilled tubesheet for shell and tube heat exchanger is shown in Figure 4.16. Schematic of tubesheet holes with radial and longitudinal imperfections is shown in Figure 4.17.

4.8.4 Preparation of Tube Holes Preparation of tube holes is as follows: 1. Drill and ream tubesheet holes. 2. Be certain that the ligaments are sufficient to guarantee a safe and permanent tube joint. 3. When conditions permit, utilize a sizing or burnishing tool to further assure a good finish in the tube hole. This will also increase the tensile strength of the ligament.

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FIGURE 4.16  Drilled tubesheet. (a) A section of drilled tubesheet and (b) a tubesheet under tube rolling-​in progress. (Courtesy of Edmonton Exchanger & Manufacturing Ltd, Edmonton, Alberta, Canada.)

FIGURE 4.17  Surface imperfections in machined tubesheet. (Courtesy of Elliott Tool Technologies Ltd, Dayton, OH.)

4. The serrations or grooves to be used will determine the holding power of the tube. 5. It is extremely important during this operation that there are no longitudinal scorings left in the tubesheet. 6. After tube holes have been prepared they are usually coated with a rust preventative compound. Before inserting any tube it is important to remove all traces of this oil coating. It is extremely important that great care be taken in handling the tubes for insertion in all of the vessels discussed above. Be certain that the tube ends are clear of any foreign material. 7. Tubesheet inspection –​the tubesheet shall be inspected before welding to the shell. The hole diameter, grooves dimensions, drilling pattern with respect to inlet nozzle fluid entry, the thickness of the tubesheet and surface finish shall be measured and checked against acceptance tolerances in the approved drawing.

4.8.5 Drilling of Baffles The complete set of baffles or part is bundled, and the bunch is held tightly with the help of clamps or by other suitable means as shown in Figure 4.18. The holes in baffles are drilled as per TEMA

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FIGURE 4.18  Baffles holes drilling.

Standards. Reject baffles in which more than 2% of the holes exceed the size limits of the standard [19]. Oversized holes can enhance fretting and wear due to flow-​induced vibration and can also reduce the thermal performance. Make sure that the cuts match the drawing dimensions and are free from burrs.

4.9 TUBE BUNDLE ASSEMBLY 4.9.1 Tube Bundle Assembly Methods Tube bundle assembly consists of tubes, baffles, spacers, and tie rods. Before insertion, the ends of the tubes must be fine sandblasted to remove all traces of oxide and scale. Tube ends are trued by cutting and deburring of the excess length. The tubes must be degreased by a strong solvent for a distance of 1.5 times the thickness of the tubesheet [19]. For a fixed tubesheet exchanger, the tube bundle assembly is carried out by two methods: (1) assembly of tube bundle outside the exchanger shell and (2) assembly of tube bundle inside the shell. These methods are discussed next.

4.9.2 Assembly of Tube Bundle Outside the Exchanger Shell Assembly of tube bundle outside the shell has better control on tie rod and baffles setup, and it is easy to insert the tubes. The drawback of this method is that tubes in the bundle are likely to be damaged while lifting and inserting. The core assembly consisting of baffles, tie rods, and spacers with sealing strips, if any, is known as baffle cage assembly. A typical baffle cage assembly of Helixchanger® is shown in Figure 4.19a. Figure 4.20 shows a heat exchanger under tube insertion and a completed tube bundle. Guide bullets in the leading end of the tube (as shown in EMbaffle heat exchanger, Figure 4.21) greatly assist the tubing operation [16, 20]. Figure 4.22 shows U-​tube assembly outside the heat exchanger shell and completed U-​tube bundle assemblies(to be discussed later). Assembly of the tube bundle is carried out on a fixture, or tube bundle rolls (Figure 4.23) and the completed tube bundle is lifted using bundle lifter as shown in Figure 4.24 before insertion into a shell.

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FIGURE 4.19  Heat exchanger assembly. (a) Helixchanger baffle cage assembly (Courtesy of Vermeer Eemhaven B.V., Rotterdam, the Netherlands) and (b) shell and tube heat exchanger under fabrication –​baffle assembly is shown adjoining shell. (Courtesy of Festival City Fabricators, div. of CSTI, Stratford, Ontario, Canada.)

FIGURE 4.20  Tube bundle assembly. (a) Insertion of straight tubes through tubesheet and (b) tube bundle ready for insertion into the shell. (Photo courtesy of Mueller, Springfield, MO.)

4.9.3 Assembly of U-​Tube Bundle Assembly of a U-​tube bundle is always done outside the shell. The following procedure is normally adopted: 1. Insert the innermost row of the tubes through the baffle and tie rod assembly with the tubesheet. 2. After getting the inner row inspected, tubes are tacked or slightly expanded into the tubesheet or tightly held into position by any other means. 3. Insert tubes from the center toward the outside and get each row inspected before proceeding to the next row.

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FIGURE 4.21 EMbaffle® tube bundle under assembly. The figure shows tubes with pilots. (Courtesy of Bronswerk Heat Transfer B.V., Nijkerk, the Netherlands. EMbaffle® is a registered trademark of EMbaffle B.V., Alphen a/​d Rijn, the Netherlands. EMbaffle® Technology was developed by Shell Global Solutions International B.V. and since July 2012 owned by Brembana & Rolle S.p.A. (www.embaf​fle.com)).

FIGURE 4.22  (a) Insertion of U-​tubes through tubesheet (Courtesy of Powerfect, Brick, NJ) and (b) U-​tube bundle heat exchangers –​on the extreme right, tube bundle is on a movable trolley. (Courtesy of Titan Metal Fabricators Inc, Camarillo, CA.)

4. After all tubes are inserted, the bundle is carefully lifted with cradles and inserted inside the shell.

4.9.4 Sequence of Fixed Tubesheet Heat Exchanger Tube Bundle Assembly Outside the Shell Operation list for fixed tubesheet heat exchanger tube bundle assembly outside the shell. Prepare a QAP format as in Annexure Table 4.12.

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FIGURE 4.23  Completed tube bundle assembly placed on rolls/​trolley. (Courtesy of Allegheny Bradford Corporation, Bradford, PA.)

1. Set one tubesheet on the fixture. 2. Assemble the baffles, tie rods and spacer tubes or sealing strips. 3. Insert the tubes. 4. Tighten the nuts, lock nuts, and weld. 5. Set and weld the impingement plate. 6. Set and weld the sliding/​sealing strips, if any. 7. Check tube bundle with ring gauge. 8. Check shell inside with push through gauge. 9. Obtain clearance for tube bundle insertion. 10. Insert the tube bundle into shell. 11. Set the second tubesheet and draw the tubes through. 12. Weld the second tubesheet with shell. 13. Level the tubesheet side and obtain clearance. 14. Complete the tube-​to-​tubesheet root run welding and inspect by PT. 15. Complete light/​strength expansion and obtain clearance. 16. Trim the excess portion in tubes and obtain clearance. 17. Machine the gasket seating surface, if distortion is noticed.

4.9.5 Impingement Plate Attachment When impingement plate are welded to tie rod or spacers, they shall be supported by at least two tie rods or spacers. Minimum thickness of impingement baffles shall be 6 mm for carbon and low alloy steel and 3 mm for high-​alloy steel and nonferrous or as per standard.

4.9.6 Cautions to Exercise While Inserting Tubes Install the tubes carefully such that the tubes do not suffer any damage. When tubes are at their upper size tolerance and tubesheet holes are at their lower size tolerance, it is difficult to insert tubes.

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FIGURE 4.24  Tube bundle handling devices. (a) Tube bundle support rolls and (b) tube bundle lifter. (Courtesy of Powermaster Ltd, Mumbai, India.)

Misalignment of baffles and supports increases the difficulty. It is most important to set as a hold point the beginning of the tube loading and to witness this operation [16].

4.9.7 Tube Bundle Insertion Inside the Shell Many times, the tube bundle is moved on trolley (or rolls) before inserting into a shell (Figure 4.24). If the tube bundle is heavy, about 10% of the tubes are inserted on the periphery of the tubesheet to make the baffle assembly true vertical, and the partially assembled tube bundle is inserted into the shell. Remaining tubes are inserted after welding the tubesheet to the shell. For easy insertion of tube bundle inside the shell, devices such as tube bundle inserter are readily available in the market. Typical tube bundle inserter kit is shown in Figure 4.25. A tube bundle insertion fixture for shell and tube type heat exchangers is provided wherein a tube bundle (75% weight of the heat exchanger unit) is stationary and kept in horizontal position. The shell (15 % weight of the unit) is clamped on the fixture and which is aligned with tube bundle. Slowly the fixture, supported on trolley wheels, is pulled on to the tube bundle by means of pulley and sling mechanism which clamped on tubesheet side of tube bundle as shown in Figure 4.26 and Figure 4.27.

4.9.8 Assembly of Tube Bundle Inside the Shell To avoid the risk of tubes getting damaged in the case of the bundle assembled outside, some fabricators prefer to assemble tubes inside the shell. Normally, this method is possible when the

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FIGURE 4.25  Tube bundle inserter for heat exchanger. (Courtesy of Powermaster Ltd, Mumbai, India.)

FIGURE 4.26  Tube bundle insertion in to shell (schematic).

shell inside diameter is sufficiently large for a person to go inside and arrange tie rods with tubesheet and baffles. The assembly procedure is shown schematically in Figure 4.28, and the sequence of operations is [17] as follows: 1. The operator will go inside the shell and arrange the baffle assembly by fixing the tie rods with the tubesheet. Insert a few tubes on the periphery to true up the baffle assembly. 2. Inspect the assembly and tack the nuts wherever required. 3. Insert more tubes on the periphery of the baffles to make these true vertical and insert remaining tubes by starting from bottom to top. Use pilots initially to guide the tubes. 4. After insertion of all tubes, the other tubesheet is fixed in its position. Push a few tubes out at the center and at the peripheral ends of the tubesheet and check the projection of the tubes before tacking and welding of the tubesheet with shell.

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FIGURE 4.27  Tube bundle insertion in to a shell. (Courtesy of Villa Scambiatori Srl Italy.) (Note- The shell ends are fitted with grith flange.)

FIGURE 4.28  Scheme for assembly of tube bundle inside the shell.

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4.9.9 Tube Nest Assembly of Large Steam Condensers Clean conditions are established within the condenser shell prior to the commencement of tubing. Tubing is usually started at the bottom rows, working progressively upward. This allows personnel to exit easily.

4.10 TUBESHEET TO SHELL WELDING Tubesheets are welded to the shell by various methods [16, 21, 22]. Some configurations of tubesheet to shell welding are shown in Figure 4.29. Welding of tubesheet to shell causes distortion of the tubesheet. The distortion will increase the distance between two tubesheets at some places and reduce at some other places. This will buckle the tubes, and the effect is more prominent in stainless steel material because of its higher thermal expansion coefficient and poor thermal conductivity. Additionally, the distortion will not give a true gasket seating surface, thus causing the joint to leak unless the surface is machined for its trueness. Also, the pass partition plate has to be machined as per the bow to get proper seating [16]. If the tubesheet is to be machined to suit pass partition, the effective thickness of the tubesheet will be reduced by the amount of the bow. A number of methods are used to control distortion of tubesheets as follows: 1. Put a few long tie rods through the tubes and hold the tubesheets at various positions. 2. Put a thick plate at the face of tubesheet and tighten it. Weld this joint well with a trained and qualified welder. 3. Weld the tubesheet with shell while keeping the exchanger vertical. The tubesheet is clamped to the bed plate on which exchanger is resting. 4. Use of hubbed tubesheets with a method of joining is known as lip welding (Figure 4.29d). 5. Bolt with a channel or closure to be assembled with it. 6. Use an end ring. The end ring method is explained next.

FIGURE 4.29  Configurations of tubesheet to shell welding –​(a–​c) both shell and channel welded to tubesheet, (d) tubesheet with a stub is welded to shell and (e–​f) tubesheet is welded to shell and channel side in bolted design.

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FIGURE 4.30  Tubesheet with end ring welded. (a) Before machining and (b) after machining.

FIGURE 4.31  Tubesheet to shell welding –​root run by TIG welding.

Use of an end ring. In this method, an end ring of about 50 mm width is welded on the tubesheet step. Suitable restraint is applied to reduce welding distortion on the tubesheet. Whatever distortion appears on the tubesheet and on the end ring will be machined prior to drilling operation. Figure 4.30 shows the tubesheet with end ring welded, before machining and after machining. Caution while welding shell to the tubesheet. Before putting the welder on the actual job, some mockup test piece should be welded and root fusion should be checked by macroetching. It is a common practice of good fabricators to give a root run by the TIG process as shown in Figure 4.31 after purging the exchanger with argon gas [23]. The TIG process gives better root fusion and penetration. In all cases, after welding the tubesheet with the shell, the joint is inspected for welding defects by RT/​UT.

4.11 TUBE-​TO-​TUBESHEET JOINT FABRICATION 4.11.1 Tube Expansion The reliability of a shell and tube heat exchanger depends upon the integrity of all tube-​to-​tubesheet joints, each of which must be virtually free from defects. An effective tube-​to-​tubesheet joint must

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be leak-​tight under the operating conditions of expansion and contraction, pressure or vacuum, and corrosion attack. The tubes of most tubular exchangers are connected to the tubesheets by only expanding or by welding and expanding. There are several techniques for expanding tubes into the tubesheets. Three main techniques used for tube expansion are (1) rolling-​in, (2) explosive joining, and (3) hydraulic expansion.

4.11.2 Tube-​to-​Tubesheet Joints Four types of tube-​to-​tubesheet joints (Figure 4.32) are hereunder [24]: 1. Roll or expand only (without grooves). The joint has few applications because, without grooves and without welds, it has poor strength and leak resistance. Figure 4.32a. 2. Roll or expand only with grooves. This joint relies on the rolling or expanding of the tubes into the grooves to provide some strength and leak resistance. This type is quite popular when stresses and the possible consequences from developing a leak are low. If it becomes necessary, these low-​cost joints provide for easy tube replacements. Figure 4.32b. 3. Roll or expand and seal weld with grooves. If a seal weld has been added to the joint to increase leak resistance. This design can also be used to repair leaking joints that have not been welded. Figure 4.32c. 4. Strength weld without grooves. This design option relies upon the weld to provide both strength and leak resistance. It is usually applied where mechanical and /​or thermal stresses are high and leaks are unacceptable. Figure 4.32d. 5. Strength weld and expanded.

4.11.3 Tube-​to-​Tubesheet Joint Expansion and/​or Welding Sequence In industry the most debated event in making the tube-​to-​tubesheet joint is the sequence in which the expansion and welding is done. One camp says to weld, test the weld and then expand; another says expand, weld and then test the weld. The purchaser must ultimately make the decision for which sequence to use. The steps to follow if “weld tube ends-​and then expand or” “expand tube ends-​and then weld” is followed, the steps [25, 26] listed in Table 4.3 [25] should be followed.

FIGURE 4.32  Four types of tube-​to-​tubesheet joints [24].

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TABLE 4.3 Tube-​to-​Tubesheet Joint Expansion and Welding Sequence, Ref. [25] Expand tube–​weld

Weld tube–​expand

• • • • • • • •

• • • • • • • • • •

Clean tubesheet holes Clean tube ends Insert tubes into tubesheet holes Expand tube ends, i.e., rolling-in Air test rolled joint; re-​expand as needed Weld tube ends Helium leak test weld joints; repair as needed Hydrostatic test

Clean tubesheet holes Clean tube ends Insert tubes into tubesheet holes Weld tube ends Air test welds; repair as needed Preliminary helium leak test of welds; repair as needed Expand tube Penetrant test welds; repair as needed Final helium leak test welds Hydrostatic test

4.11.4 Preferred Method of Making the Tube-​to-​Tubesheet Joint The preferred method of making the tube-​to-​tubesheet joint is first to expand and then seal weld. When this sequence is followed a higher quality expanded joint is possible and the risk of cracking the seal weld during the expansion step is eliminated. In addition a higher quality seal weld can be made.

4.11.5 Quality Assurance Program for Tube-​to-​Tubesheet Joint The tube-​to-​tubesheet joint quality assurance program shall include (1) tube-​to-​tubesheet mock-​up (approval), (2) welder qualifications, (3) welding procedures qualification, (4) tube hole and tube end cleaning procedures, (5) procedure for tube-​to-​tubesheet joint welding, (6) procedure for tube-​ to-​tubesheet joint roller expansion (known as tube-​expanding procedure specification as per ASME Code Section VIII, Div. 1), (7) checking of tube-​to-​tubesheet joint leak tightness, and (8) reference to TEMA Table RCB-​7.2.1 for tube hole tolerances or any other standards or codes as stipulated by the vendor [23].

4.11.6 Mock-​up Test The most important step for accomplishing a quality tube-​to-​tubesheet joint are the production of a good mock-​up (and adherence to the parameters established). Before undertaking the actual tube expansion work, prepare a mock-​up model as shown in Figure 4.33. It is made using the same material and the design that is proposed for the full-​size heat exchanger and tested for the reliability of the tube-​to-​tubesheet joints. The test specimens are full-​size tube and a test block that models the tubesheets. Process parameters were established by mock-​up trials. The joining process was qualified by dye penetrant (DP) test, helium or pneumatic leak test, pullout test, and microetching tests of the welds to ascertain minimum leak path. The test measures tube-​to-​tubesheet joint strength, that is pullout or pushout (shear load) strength and leak tightness, and anchorage for grooved tubesheets. Pullout strength is the axial force required to break the “bond” between the tube and the tubesheet. The shear load test subjects the specimens to axial loads until either the tube or the joint fails. Control unit settings of the expanding unit, which show the amount of torque applied, are noted for individual cases. The best possible torque giving the leak tightness of the joints is then used on actual exchanger.

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FIGURE 4.33  Mock-​up model for tube-​to-​tubesheet expansion. (a) Square layout and (b) 60° layout.

4.11.7 Mock-​up Test-​ASME Code Requirements 1. Shear Load Test a. The shear load test subjects a full-size specimen of the tube joint under examination to a measured load sufficient to cause failure. b. The test block simulating the tubesheet may be circular, square or rectangular in shape, essentially in general conformity with the tube pitch geometry. The test assembly shall consist of an array of tubes such that the tube to be tested is in the geometric center of the array and completely surrounded by at least one row of adjacent tubes. The test block shall extend a distance of at least one tubesheet ligament beyond the edge of the peripheral tubes in the assembly. The test block is shown in Figure 4.34.

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FIGURE 4.34  Tube-​to-​tubesheet mock-​up for shear load test.

c. All tubes in the test block array shall be from the same heat and shall be installed using identical procedures. i. The finished thickness of the test block may be less but not greater than the tubesheet it represents. For expanded joints, made with or without welding, the expanded area of the tubes in the test block may be less but not greater than that for the production joint to be qualified. ii. The length of the tube used for testing the tube joint need only be sufficient to suit the test apparatus. The length of the tubes adjacent to the tube joint to be tested shall not be less than the thickness of the test block to be qualified. d. The procedure used to prepare the tube-to-​tubesheet joints in the test specimens shall be the same as used for production. e. The tube---to-tubesheet joint specimens shall be loaded until mechanical failure of the joint or tube occurs. The essential requirement is that the load be transmitted axially. 2. Tube Expanding Procedure Specification (TEPS) A TEPS is a written document that provides the tube expander operator with instructions for making production tube-​to-​tubesheet joint expansions in accordance with code requirements. 3. Tube Expanding Procedure Qualification The purpose for qualifying a TEPS is to demonstrate that the expanded joint proposed for construction will be suitable for its intended application. The tube expanding procedure qualification establishes the suitability of the expanded joint, not the skill of the tube expander operator. 4. Tube Expanding Variables Variables are subdivided into essential variables that apply to all expanding processes, and essential and nonessential variables that apply to each expanding process such as roller expanding and hydroexpanding. Essential variables are those in which a change, as described in specific variables, is considered to affect the mechanical properties of the expanded joint, and shall require requalification of the TEPS. Nonessential variables are those that may be changed at the Manufacturer’s discretion and are included in the TEPS for instruction purposes.

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4.11.8 Requirements for Expanded Tube-​to-​Tubesheet Joints The connection of the tubes to the tubesheets is a critical operation. The requirements for creating the desired interfacial pressure at the tube-​to-​tubesheet joints are as follows [27]: (1) the tube deformation must be fully plastic; (2) the surrounding tubesheet must deflect under the pressure that the tube applies; and (3) upon pressure release, tubesheet recovery must be greater than tube recovery. Expanding pressure beyond that required for the tube-​to-​hole contact must be applied to meet these conditions. 4.11.8.1 Major Causes of Tube to Tubesheet Joint Leaks 1. As a general rule the smaller the clearance the better, from the expanding point of view, no matter what expanding method is used. 2. Tube rolling leakage is usually caused by one of the following: under-​rolling, over-​rolling, improper preparation of tubesheet and differential thermal expansion. 3. Improper preparation of tube holes. Improper preparation of tube holes is major cause in tube leakage. The smoother the tube seat or tube hole the easier it is to roll an optimum tube joint. 4. Differential thermal expansion. Differential thermal expansion can result with thicker tubesheets. When the expansion due to heat varies noticeably between the thinner tube and tubesheet, a shift of the tube results. 5. One of the most important steps for ensuring a safe and permanent tube joint is to thoroughly clean the surfaces of the tube end and the tube hole wall. These two surfaces must be clean and free of all dust, mill scale and pits or scratches. 6. It is extremely important to eliminate any longitudinal scoring in the tubesheet hole wall. These longitudinal lines will cause leaky tubes.

4.11.9 Tube-​to-​Tubesheet Joint Expansion Methods Tube end expansion may be done either by use of a conventional three roller expander or hydraulic means or explosion welding. Welding may be done manually, with or without filler metal, or by use of semiautomatic/​automatic welding equipment. Before the fabrication step is begun, a mock-​up tube-​to-​tubesheet joint should be assembled using the same materials and design that is proposed for the full size heat exchanger. The proposed tube end expansion method and welding procedure should be employed and the mock-​up cross-​sectioned for macro examination in order to proof test all the parameters and establish the weld quality and weld penetration. In some unusual cases micro examination may also be employed to check weld quality. 4.11.9.1 Hydraulic Expansion Hydraulic expansion of tubes into tubesheets can be done by either the bladder or the O-​ring technique (also known as direct hydraulic expanding technique) [28]. In the bladder technique (Canadian Patent 1152876), a bladder is inserted into the tube and then pressurized hydraulically to expand the tube. In the O-​ring technique, a mandrel with two O-​rings at appropriate locations is inserted into the tube. Hydraulic pressure applied between the O-​rings causes the tube in that region to expand. Compared to rolled joints, hydraulically expanded joints are weaker than rolled joints, and they are further weakened by thermal cycling [28]. Hydraulic tube expansion process is further discussed later. 4.11.9.2 Rolling-​in Process Rolling is by far the most commonly used technique. Tube expanding, known also as tube rolling or “rolling” of tubes, is the art of cold-​working (plastic deformation) the ends of tubes

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FIGURE 4.35  Cold-​working principle of tube expanding. (a) Tube positioned in the tubesheet hole, (b) roller expansion in progress, and (c) expansion completed. (Tube deforms plastically where as tubesheet ligaments deform elastically.) (Courtesy of Elliott Tool Technologies Ltd, Dayton, OH.)

into intimate contact with the walls of tubesheet holes, and the nature of deformation is shown schematically in Figure 4.35. If properly executed, the rolling-​in process produces pressure-​ tight joints to ensure strength and stability. Tube rolling-​in has been explained in detail by Sharma [1], Syal [17], Yokell [27], Fisher and Brown [29], Sonnenmoser [30], Dudley [31], and Yokell [32]. A mechanically expanded joint by roller expansion may be acceptable when: 1. service temperatures are below 200°C 2. tubesheet is sufficiently thick to allow rolling-​in a suitable length of tube 3. design pressures are relatively low. 4.11.9.3 Explosive Joining Explosive welding is a solid state welding process, which uses a controlled explosive detonation to force two metals together at high pressure. The resultant composite system is joined with a durable, metallurgical bond. Explosive joining has been successfully used to expand tubes into tubesheets by detonating a carefully sized explosive charge inside each tube hole, causing it to impact with the tubesheet and make a metal-​to-​metal joint with the tubesheet. Details of explosive joining have been covered in Chapter 2 in detail. Advantages of explosive welding include [33]: 1. It can bond many dissimilar, normally unweldable metals. 2. No pre-​or post-​weld heat treatment is required. 3. Welds are not affected by thermal cycling therefore the process is ideally suited for heat exchangers such as HP feed water heaters. 4. Suitable for most materials commonly used in heat exchanger manufacture. 5. Tube wall thinning, work hardening and lengthening of the explosively expanded sections of tubes are normally negligible. All of these expansion processes involve deforming the tube plastically. The residual stresses resulting from the deformation differ in both orientation and magnitude between the various methods. In general, rolling processes, which produce surface burnishing (cold work) and non-​ axisymmetric deflections, are considered to produce higher inside surface residual stresses than hydraulic or explosive expansion processes for the same tubing properties.

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4.11.10 Tube Expansion by Rolling The tube ends can be mechanically sealed to the tubesheet without welding, using a process called expansion. Roll expansion involves inserting a tool into the tube and rotating and pressing on an inner tapered mandrel of the tool, which in turn moves upward and pushes three to five tapered rollers into contact with the tube wall. The tube is expanded into contact with the tubesheet until a specified torque level is reached. Table 4.3 shows the recommended amounts of wall reduction for various materials. Although tube expansion is the most economical option, it is also the least robust type of tube-​end joint design. This joint type should be selected for noncritical applications or for those where tubes may need to be replaced in the future. For additional leak protection, the tube ends can be seal welded and expanded. There is no calculated size for seal welds. There is, simply, a minimal weld that provides a seal at the joint. This type of weld depends on the additional step of expansion to provide the full strength of the joint. 4.11.10.1 Mechanical Rolling Methods In mechanical rolling, a set of hardened rolls in a cage rotate around a tapered mandrel. The rolls travel up the mandrel causing an increasing radial force exerted at the contact point between the rolls and the tube. This increasing force moves the tube material outwards until it contacts the ID of the tubesheet hole and continues until supposedly the tubesheet material is just below its yield point. 4.11.10.2 Rolling Equipment A range of electric, electronic, pneumatic, or hydraulically driven torque controlled tube-​expanding machines are used for the expansion of heat exchanger tubes. There are several standard types of expanders, as well as expanders built for specific purpose, each of which may be of different size and roller length. However, all work on the same principle and all have three essential parts, namely, a tapered central mandrel or pin; three, four, or five rollers of suitable length spaced equally around the mandrel; and a cylindrical roller cage that constrains the loosely held rollers in their respective positions. Most expanders are self-​feeding type. The roller cage may be fitted with a collar mounted on a ball bearing to prevent it from being drawn into the tube. Pneumatic tube-​ expanding machines are shown in Figures 4.36 and 4.37. A pneumatic driven torque controlled tube-​expanding machine and an electric driven torque controlled tube-​expanding process in action are shown in Figure 4.38. In addition to the basic tube expander, a few similar tube expanders such as tack rolling tube expander, step-​by-​step tube expander for thick tubesheet, tube expander for thin tubesheet, step expander designed to avoid destructive axial stress in welded joints, etc. are used on shop floor. A list of different types of tube expanders used on the shop floor for tube-​expanding purposes is shown in Figure 4.39. For successful roller expansion, in addition to tube expanders, a large number of accessories such as tubesheet hole brush, tube guide, serrating/​grooving tool, tube end facer, internal tube cutter, special tool for flaring tube ends, standard reversible thrust collar, tube end beveling machine, drift to remove stub, hydraulic ram for gripper-​type tube puller, hydraulic operated gripper-​type tube puller for condenser tubes and hydraulic ram for spear-​type tube puller, tube

FIGURE 4.36  Tube expander. (Courtesy of Elliott Tool Technologies Ltd, Dayton, OH.)

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FIGURE 4.37  Pneumatic tube-​to-​tubesheet rolling machines. (a) Mini torque controlled pneumatic rolling machines for tubes diameter 6.3–​4.1.9 mm (1/​4″–​5/​8″), (b) Macrol –​torque controlled pneumatic rolling machines for tube diameters 12.7–​50.8 mm (1/​2″–​2″), and (c) step-​by-​step expander for thick tubesheet. (Courtesy of Maus Italia F. Agostino & C.s.a.s., Bagnolo Cremasco (Cr), Italy.)

leak testing gun set, hole gauge, and internal measuring gauge on the shop floor are shown in Figures 4.40 , 4.41, 4.42, 4.43, and 4.44. An automatic grooving double-​axis work center is shown in Figure 4.45. 4.11.10.3 Basic Rolling Process When rolling tubes, an expander as shown in Figure 4.36 is inserted into the tube end and the tapered mandrel is rotated. Feeding the mandrel inward causes the expander rollers to be forced apart; by rolling over the inside tube surface, cold-​work the tube metal. When the tube hole is enlarged and contacts the tubesheet hole surface, the tubesheet acts as a restraint barrier, and further expanding deforms the tube metal and forces it into more intimate contact with the tubesheet metal. Since not all displaced tube metal can escape radially, it flows from the center to each end of the rolled joint. The tubesheet metal is also affected and the hole is slightly enlarged [29]. The net result of the expanding operation is a joint condition similar to shrink-​fitted shaft coupling. The basics of tube rolling process is shown in Figure 4.46.

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Fabrication, Brazing, and Soldering of Heat Exchangers

FIGURE 4.38  (a) Pneumatic tube expansion drive and (b) electrical expansion system with torque controller and a telescopic shaft. (Courtesy of Powermaster Ltd, Mumbai, India.)

FIGURE 4.39  Tube expanders. (a) Five roll tube expander, (b) TACK rolling tube expander (conical expander), (c) step-​by-​step tube expander for thick tubesheet, (d) tube expander for thin tubesheet, and (e) step expander designed to avoid destructive axial stress in welded joints. (Courtesy of Teco Tube Expanders Company, Leiderdorp, the Netherlands.)

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FIGURE 4.40  Accessories for tube expansion. (a) Tubesheet hole brush, (b) tube guide, (c) serrating/​grooving tool, (d) tube end facer, and (e) internal tube cutter. (Courtesy of Powermaster Ltd, Mumbai, India.)

FIGURE 4.41  Accessories for tube expansion. (a) Tube end facers, (b) special tool for flaring tube ends, (c) grooving tool, (d) standard reversible thrust collar, (e) tube end beveling machine, and (f) drift to remove stub. (Courtesy of TECO Tube Expanders Company, Leiderdorp, the Netherlands.)

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FIGURE 4.42  Accessories for inspection of rolled tube. (a) Tube testing gun set and (b) gauges used for checking holes –​hole gauge and internal measuring gauge. (Courtesy of TECO Tube Expanders Company, Leiderdorp, the Netherlands.)

FIGURE 4.43  Tube puller. (a) Hydraulic ram for gripper-​type tube puller, (b) hydraulic operated grippertype tube puller for condenser tubes, and (c) hydraulic ram for spear-​type tube puller. (Courtesy of TECO Tube Expanders Company, Leiderdorp, the Netherlands.)

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FIGURE 4.44  Tube rolling-​in accessories. (a) Mafix system dedicated gun for tube locking, (b) probes FDP and VDP with quick-​release coupling, (c) semiautomatic continuous hydraulic tube puller, (d) automatic continuous hydraulic tube puller, and (e) flue brushes for tubesheet holes cleaning. (Courtesy of Maus Italia F. Agostino & C.s.a.s., Bagnolo Cremasco (Cr), Italy.)

4.11.10.4 Factors Affecting Rolling Process The factors that affect the quality of the rolling process are discussed in detail by Fisher et al. [29]. Some of the important factors are the following: 1. cleanliness of the tube, tubesheet, and rollers 2. conditions of the cage, rollers, and mandrel 3. lubrication and cooling 4. tube and hole dimensions 5. torque cutoff control monitoring and maintenance 6. rolling technique, roller rotation speed, number of rolls, angle of rolls relative to the tube axis, shape of the rolls 7. worker fatigue.

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FIGURE 4.45  Automatic grooving double-​axis working center. (Courtesy of Maus Italia F. Agostino & C.s.a.s., Bagnolo Cremasco (Cr), Italy.)

FIGURE 4.46  Basics of tube rolling process. (a) A tube inside the tubesheet hole, (b) tube contacting the tubesheet, and (c) permanent deformation of tube completed joint. (After [29] Fisher, F.F. and Brown, G.J.)

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FIGURE 4.47  Effect of overrolling of tube on adjoining tube –​schematic. (Courtesy of Elliott Tool Technologies Ltd, Dayton, OH.)

It is best to avoid the use of any lubricant during the roller expansion. But if, because of undue wear or heating of the rolls, lubrication is thought necessary, then it must be applied with discretion [34]. On completion of assembly and expansion, the localized areas around the tube-​ to-​tubesheet joint should be cleaned again with the solvent so as to remove the final traces of lubricant. 4.11.10.5 Optimum Degree of Expansion The most important task on any tube rolling job is to determine the optimum degree of expansion for a particular tube and tubesheet material combination. With the introduction of new materials and higher pressures and temperatures, the need for a control method for the rolling-​in of the tubes becomes more apparent [31]. The technique used for expanding the tubes should be one that, to the greatest possible degree, avoids both under-​and overrolling [32]. Underrolling results in inadequate plastic flow at the tube-​tubesheet interface. This will result in leaks and require rerolling. On the other hand, overrolling will distort the tubesheet, leading to unseating of adjacent tubes and excessive tubesheet radial expansion and/​or over hardening of tubes. The effect of overrolling is shown schematically in Figure 4.47. Therefore, it is important to roll every joint uniformly to the optimum degree. To achieve this, the rolling technique must be one that utilizes automatic control of the tube expansion rather than depending on the skill of the operator. 4.11.10.6 Methods to Check the Degree of Expansion The methods employed for checking the desired degree of expansion had been achieved are these [29]. One is based on measuring the changes in tube dimensions like reduction in tube wall thickness, or methods that limit or measure the mandrel travel or simulating wall reduction by interference or pullout strength versus torque curve. In order to avoid the undesirable features inherent in rolling methods that employ empirical formulas or that measure changes induced by the tube rolling process, many designers prefer methods that measure the torque applied to the expander mandrel. Various rolling methods based on measure of torque employ methods like the manual “feel” method, mechanical feel method, controlled feel rolling methods, elongation or extrusion measurement, etc. Rolling by “feel” may be the most practical method of retubing, since there will generally be considerable variation in the existing tube holes [35].

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4.11.10.7 Criterion for Rolling-​in Adequacy There are many tools designed to actuate the expanding equipment, most with torque controlled by feed and speed limits and some with infinite variable controls. Optimum tube rolls may be predetermined by calculation on the basis of tube wall reduction. 4.11.10.8 TEMA Guidelines for Tube Wall Reduction RGP-​RCB-​7.3 The optimum tube wall reduction for an expanded tube-​to-​tubesheet joint depends on a number of factors. Some of these are tube hole finish, presence or absence of tube hole serrations (grooves), tube hole size and tolerance, tubesheet ligament width and its relation to tube diameter and thickness, tube wall thickness, tube hardness and change in hardness during cold working, tube o.d. tolerance, type of expander used, type of torque control or final tube thickness control, function of tube joint, e.g. strength in resistance to pulling out, minimum cold work for corrosion purposes, freedom from leaks, ease of replacement, etc. length of expanded joint, and compatibility of tube and tubesheet materials. 4.11.10.9 Tube Hole Grooving, TEMA RB-​7.2.4 1. Tube holes for expanded joints for tubes 15.9 mm(5/​8”) OD and larger shall be machined with annular ring groove(s) for additional longitudinal load resistance. 2. For strength welded tube to tubesheet joints, annular grooves are not required. 3. For roller expanded tube joints when tubesheet thickness exceeds 1” (25.4 mm) at least two grooves shall be used, each approximately 1/​8” (32 mm) wide by 1/​64” (0.4 mm) deep. Tubesheets with thickness less than or equal to 1” (25.4 mm) may be provided with one groove. For hydraulic or explosive bonded tube joints, at least one groove shall be used. Minimum groove width, “w” shall be calculated as w=​1.56 rt where r =​tube mean tube radius t =​tube wall thickness, except groove width need not exceed 1/​2” (12.7 mm). Groove depth shall be at least 1/​64” (0.4 mm). 4. When integrally clad or applied tubesheet facings are used, all grooves should be in the base material unless otherwise specified by the purchaser. 5. Higher pin-​count expanders should be considered for titanium tubes when size permits. 6. Other methods for tube expansion can include hydraulic and explosive expansion procedures. Acceptance criteria and verification for these methods are to be agreed upon between the manufacturer and owner. 7. The target tube inside diameter after expansion can be calculated as follows: ID of rolled tube =​(tubesheet hole –​OD tube) –​ID of tube (2 × tube thickness) × (% wall reduction) 8. Percentage thinning is defined as follow:  dth − dif  % thining =​1 − (4.2)  ×100 do + di   where do =​outside diameter of tube before expansion di =​inside diameter of tube before expansion dth =​tube hole diameter in tubesheet dif = inside diameter of tube after complete expansion. 9. Length of expansion. Tubes shall be expanded into the tubesheet for a length no less than 2” (50.8 mm) or the tubesheet thickness minus 1/​8” (32 mm), whichever is smaller. In no case shall the expanded portion extend beyond the shellside face of the tubesheet. When specified by the purchaser, tubes may be expanded for the full thickness of the tubesheet.

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10. Contour of the expanded tube shall be substantially uniform expansion throughout the expanded portion of the tube, without a sharp transition to the unexpanded portion. 11. Tube projection. Tubes shall be flush with or extend by no more than one half of a tube diameter beyond the face of tubesheet, except that tubes shall be flush with the top tubesheet in vertical exchangers to facilitate drainage (avoid crevice corrosion under deposits) unless otherwise specified by the purchaser. 12. Welded tube-​to-​tubesheet joints. When both tubes and tubesheets, or tubesheet facing, are of compatible materials, the tube joints may be welded. 12.1 Fabrication and testing procedures. Welding procedures and testing techniques for either seal welded or strength welded tube joints shall be by agreement between the manufacturer and the purchaser. 13. Explosive bonding and/​or explosive expanding may be used to attach tubes to the tubesheets where appropriate. 4.11.10.10 Wall Reduction as the Criterion of Rolling-​in Adequacy The amount of wall reduction is estimated by measuring hole and tube dimensions before rolling and tube internal diameter after rolling. The following equation may be used for calculating the percent wall reduction [16]:



Percent tube wall reduction =

(

)

 di′ − di + clearance  100   (4.3) 2 ( measured unrolled tube wall thickness, t )

where di′ is the tube internal diameter after rolling dj is the tube internal diameter before rolling d is the measured tube outside diameter, and clearance is the measured tubesheet hole diameter D minus d. From Equation 4.3, the desired tube inside diameter after rolling is given by

di′ = D − 2t (1 − x ) (4.4)

where di′ is the tube inside diameter after rolling D is the diameter of the hole in the tubesheet t is the unrolled tube wall thickness x is the percent of tube wall thinning. In other words, expanded tube ID is given by: 1. Hole diameter in tubesheet − Tube OD =​Play between tubesheet and tube 2. Play between tubesheet and tube +​Tube ID +​Percentage of tube wall reduction =​Expanded tube ID A go and no-​go gauge is made to ascertain that the tubes are properly expanded. While calculating tube dimensions after expansion, tolerances on tube outer diameter and tubesheet hole diameter and tube wall thickness must be taken into account.

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TABLE 4.4 Possible Tube Wall Thinning as per Tube Material Carbon steel Stainless steel Alloy steel Titanium Copper alloy Admiralty brass 3003/​4004 aluminum 6061T aluminum

5%–​8% 3%–​5% 4%–​6% 4%–​5% 4%–​8% 7%–​8% 5% maximum 10%–​12%

4.11.10.11 Amount of Thinning of Tubes The amount of thinning of the tubes for full expansion after metal-​to-​metal contact depends upon the tube and tubesheet material. Percentages of thinning for some common tube materials are given in Table 4.4. The setting of the torque control unit is established on a mock-​up model. 4.11.10.12 Correct Tube Wall Reduction Tube expanding is the art of reducing a tube wall by compressing the OD of the tube against a fixed container such as rolling tubes into tubesheets, ferrules, etc. To assure a proper tube joint, the tube wall must be reduced by a predetermined percentage. Elliott Tool Technologies Ltd, Dayton, OH, recommends Figure 4.48 to use for determining the correct tube wall reduction. This figure shows a typical 3/​4″–​16 gauge tube. Before rolling this tube, find the proper rolling dimension as follows: 1. First determine the tube hole size. 2. Then determine the tube outside diameter. 3. Subtract the tube outside diameter from the tube hole dimension. 4. With a tube gauge, determine the inside diameter of the tube before rolling. 5. By adding the dimension found in “4” to the clearance between the tube OD and the tube hole, find out tube’s inside diameter at metal-​to-​metal contact. 6. Roll the tube-​to-​tubesheet and check the ID of the tube with a tube gauge. 7. By subtracting “5” from the rolled diameter, determine the actual amount of expansion (tube wall reduction) on the inside diameter of the tube. Figure 4.48 can be used for predetermining both the % of wall reduction required and the actual inside diameter that should be rolled. Since the amount of wall reduction greatly determines the quality of the tube joint, one should arrive at the % required for the application prior to tube rolling. By subtracting the tube inside diameter “4” from “2”, you determine actual wall thickness. Dial setting test chart for determining proper amount of tube expansion with automatic torque control unit is shown in Figure 4.48. 4.11.10.13 Determining Three, Four, or Five Roll Expander Design Generally, a three roll expander is well suited but four or five roll expander is needed in the following applications: 1. Tube material. Stainless steel, titanium, work hardening materials, minimum four-​roll design is desired.

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FIGURE 4.48  Rolled tube inner diameter calculation procedure. (a) A tube positioned inside the tubesheet hole and (b) expanded tube. (Courtesy of Elliott Tool Technologies Ltd, Dayton, OH.)

2. Wall thickness. 20 gauge (0.035″) and thinner For stainless steel or titanium –​five roll preferred; and carbon steel, brass, copper, aluminum –​four roll preferred. 3. Tube pitch. Tubesheets with thin ligaments in a triangular tube pitch pattern may be disrupted using a standard three roll expander. 4.11.10.14  Length of Tube Expansion According to TEMA, RCB-​7.3.1 for R and B classes, tubes may be expanded into the tubesheets for a length not less than 2 in. (50.8 mm) or the tubesheet thickness minus1/8 in. (3.2 mm), whichever is smaller, and for C class, the length not less than two tube diameters or 2 in. (50.8 mm) or the tubesheet thickness minus 1/8 in. (3.2 mm), whichever is smallest. Importance of Sealing Full Length of Tubesheet. Seal the full length of tubesheet as shown in Figure 4.49. (1) If less than full length, medium is condensed and trapped between tube and tubesheet, and this will lead to premature joint deterioration and tube decay. (2) If tube is expanded beyond tubesheet thickness, tube bulging creates a sharp edge that weakens the tube.

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FIGURE 4.49  Sealing full length of tubesheet. (Courtesy of Elliott Tool Technologies Ltd, Dayton, OH.)

4.11.10.15 Common Causes of Tube Joint Failure 1. Not enough tube expansion (underrolling). 2. Too much tube expansion (overrolling). 3. Dirty, scratched tubes or tubesheets. 4. Dents or other imperfections of the tube. 4.11.10.16 Full-​depth Rolling Full-​depth rolling on thicker tubesheets may be desirable to close the clearance between the tube and the tubesheet hole; however, in some cases, this may tend to weaken the joint due to differential thermal expansion. For most stringent conditions, welding and rolling may be desirable. When this is considered necessary, the holes should be grooved near the weld face to minimize differential thermal expansion effects [35]. 4.11.10.17  Joint Cleanliness Cleanliness is one of the basic requirements for making good joints. Both tube holes and tube ends should be well cleaned from scale, rust, and dirt before assembly, and care should be taken to keep oil, soap, and other lubricants out of the unrolled joint. 4.11.10.18 Phenomenon of Tube End Growth During Rolling and the Sequence of Tube Expansion While rolling, the rolls are always pushing a wave of metal ahead of them. As with the uniform pressure model, any pressure greater than , where σ1 is the tube yield strengths at the manufacturing temperature will cause tube end extrusion. This is the reason for the well-​known phenomenon of “tube end growth during rolling” [27]. The length of elongation of the rolled tubes is of the order of a few thousandths of an inch to 1/​16 in. (3.2 mm) or more, depending on the size, material, and tube wall thickness, as well as the length of the joint to be rolled [29]. Tube end growth of fixed tubesheet exchanger. When rolling U-​tubes or tubes having sufficient flexibility to absorb the elongation, no difficulty will be experienced when rolling the second or closing end of the tubes. They should, however, be rolled uniformly in order to maintain the proper tube alignment. However, this is not true for rolling the second end of the tubes that connect second tubesheet of a fixed tubesheet exchanger. If the tubes are expanded sequentially, the tube extrusion can cause a tubesheet to bow and tilt out of perpendicularity. This problem in the fixed tubesheet

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FIGURE 4.50  Sequence of tube rolling. Tack tubes.

FIGURE 4.51  Effects of rolling sequence on tubesheet and tubes. (From [29] Fisher, F.F. and Brown, G.J.)

exchanger is overcome by judiciously spacing and rolling in a sufficient number of so-​called tack tubes consisting of a few tubes at six or eight equally spaced at the peripheral locations and at the center as shown in Figure 4.50 [31, 32]. On a finished bundle, bowed and dished tubesheets may indicate faulty tube expanding. Figure 4.51, taken from Fisher et al. [29], depicts in a somewhat exaggerated form the effects of elongation created by various rolling sequences. Figure 4.51a shows the effect without tack tubes or other restraining means when rolling operations are carried on successively across the tubesheet. Figure 4.51b and c illustrate the stress conditions created by the rolling sequenced as enumerated. Figure 4.51d depicts the behavior of slender tubes when rolled into two fixed tubesheets. The bow shown, to a certain extent, relieves the load on the tubesheets. 4.11.10.19 Size of Tube Holes The clearance between tube and tube hole has a definite influence on the tube-​expanding operation. Excess clearance lengthens the expanding operation considerably and increases the plastic deformation and work hardening of the tube material, although the actual joint forming time remains the

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same [29, 30]. Minimum play before rolling is to advantage. Tube hole diameters and tolerances are specified in TEMA Standard paragraph RCB-​7.4.1. 4.11.10.20 Hydraulic Expansion Hydraulic expansion is the direct application of a high internal hydraulic pressure within a tube or sleeve in order to form a tight joint between the tube and the tubesheet or a tight seal between the sleeve and the tube. Specifically, high-​pressure water is injected inside a tube but within the tubesheet. As pressure increases, the tube goes into plastic deformation until it makes contact with the tube hole. Additional pressure stresses the ligament but always within the ligament’s elastic range. When pressure is released, the result is a tight interfacial fit between the OD of the tube and the ID of the hole. Figure 4.52 shows sequences of tube expanding by HydroSwage® system (Figure 4.53). Figure 4.54 shows Hydex-​hydraulic tube expansion system and Figure 4.55 shows hydraulic system for tack expanding unit. The main benefits of hydroexpanding are the following. 1. No step expanding –​any thickness of tubesheet can be expanded in one operation. 2. Minimal work hardening of the tube material during expansion –​even difficult tube materials such as titanium or super duplex stainless can be easily expanded. 3. Less wall reduction and less stress concentration from the expanded to the unexpanded portion of the tube which results in longer tube life.

FIGURE 4.52  Sequences of tube expanding. (a) Tube-​Loc™ drawbar in tube, (b) drawbar pulls, positions, and sets tube in place, (c) HydroSwage mandrel in tube, (d) high pressure expansion of tube, and (e) smooth transition from expanded to unexpanded areas groove. (Courtesy of Haskel International, Inc., Burbank, CA. All rights reserved.)

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FIGURE 4.53  Mark V HydroSwage system for tube expanding. (Courtesy of Haskel International, Inc., Burbank, CA. All rights reserved.)

FIGURE 4.54  Hydex hydraulic tube expansion system. (Courtesy of Powermaster Ltd, Mumbai, India.)

4.11.10.21 Strength and Leak Tightness of Rolled Joints In general, the joint strength and leak tightness of tubes expanded into bare holes, that is without grooves, are functions of the surface area in contact between the tube and the hole, residual interfacial pressure at the tube-​to-​tubesheet interface produced by the expansion process, static coefficient of friction, and Poisson’s ratio [29]. Individual factors that affect the joint strength are discussed next.

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Fabrication, Brazing, and Soldering of Heat Exchangers

FIGURE 4.55  Hydraulic system for tack expanding. (Courtesy of Teco Tube Expanders Company, Leiderdorp, the Netherlands.)

Expanders’ condition. Well-​made/​maintained expanders having hardened and ground mandrels and rollers are essential for making good rolled joints [35]. Expanders having rollers with sharp or improperly shaped entrance ends are believed responsible for many joint failures [29]. Feed and speed. In general, faster machine rolling produces better results but with a tendency to greater tube hole deformation, while slower rolling tends to increase tube wall thinning [35]. Length of tube expansion. The strength of a rolled joint increases with an increase (not proportional) in length and an increase in the tube wall thickness [29]. Excessive joint length is more expensive and may cause premature failure if the tube and tubesheet materials have a high difference in coefficients of thermal expansion as, for example, copper tubes and steel tubesheets. Short joint lengths in thick tubesheets have an advantage; that is in case of joint failure or leaks, the unrolled portion of the joint may be utilized. Tube and tubesheet materials. Ideally, the tubesheet material should match the mechanical properties of the tubes. As the ratio of elastic limits of tube and tubesheet materials rises, the strength of the joint decreases. Therefore, the elastic limit of the tube material should be relatively low [30]. A tube and tubesheet combination consisting of steel tube and steel tubesheet produces joints having a high holding strength, whereas a steel tube rolled into a copper tubesheet would have low strength. Experience has shown that for the best overall results, the hardness of the tube should be less than that of the tubesheet material. A reversed condition usually results in distorted or greatly enlarged tube holes [29]. For materials with limited ductility, like ferritic stainless steel tubes, rolling should be done carefully. Friction. Expanded joints are primarily friction joints. Thus, all other factors being equal, increase in friction or resistance to sliding will tend to increase the strength of a rolled joint.

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551

Wall reduction. Rolled joint pullout strength increases approximately linearly with wall reduction, but rolling must not proceed to the point at which adjacent holes are distorted and tubes dislodged. Factors that decide the optimum tube wall reduction. Factors that decide the optimum tube wall reduction for an expanded tube-​to-​tubesheet joint are specified in TEMA Section RGP-​RCB-​7.5. 4.11.10.22 Joint Leak Tightness Joint leak tightness depends on factors such as quality of tube, tube ends, and tube hole condition in tubesheets, tube wall thickness, and joint RFs by tube hole annular grooves. Quality of tube holes (tube hole ovality and finish). It must always be remembered that strength of joint alone is not enough to guarantee a good joint. Consideration must be given to joint leak tightness, which is influenced greatly by the degree of roughness of the joint interfaces [29] and tube hole ovality [30]. In general, a leak-​tight joint can be made much more easily if the contacting surfaces of tube and tubesheet hole are smooth, but such a joint has, because of this smoothness, a lower joint strength whereas rough tube holes make strong joints. For tubes whose hardness is roughly equal to that of the tubesheet material, as in boiler and refinery heat exchangers, the conventional “drill and ream” tube hole finish is in order [29]. Spiral machining marks and lateral grooves have a detrimental effect on the quality of the joint. This can be avoided by paying proper attention while drilling/​reaming the bore [30]. The effect of tube hole finish on the mechanical strength and leak tightness of an expanded tube-​to-​tubesheet joint is discussed in TEMA Section RGP-​RCB-​7.43. Baffles, tube ends, and tube hole condition. Baffle holes and tubesheet holes shall be free from burrs to avoid longitudinal scratches on tube ends which is one of the main causes for leaky expanded tube-​to-​tubesheet joints. Should such scratches or tool marks occur, they should be removed completely by grinding. All tube holes should be truly circular and of cylindrical shape. Tube wall thickness. The thicker the tube wall, the better is the seal, because of the increase in the difference in mean tangential plastic strain. It is shown that within certain limits, thick-​walled tubes are more readily sealed by expansion [30]. 4.11.10.23 Joint Reinforcements Joint reinforcements by tube hole annular grooves. In order to meet the demands of higher pressures, most tube joints are strengthened by the machining of suitably shaped circumferential grooves into the walls of the tubesheet hole (Figure 4.56). During the rolling-​in process, a portion of the tube metal is extruded into these grooves. In this manner, additional anchorage and shear strength are provided for the rolled joint, hence increasing the pullout strength. Rectangular grooves are

FIGURE 4.56  Tube-​to-​tubesheet joint reinforcements by annular groove. (a) Tubesheet with single groove, (b) Tubesheet with double grooves, (c) Tubesheet with double grooves and clad tubesheet.

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Fabrication, Brazing, and Soldering of Heat Exchangers

commonly used for RF purposes, and they increase the holding strength and stability of rolled joints by more than 40%–​50% [32]. Discussion of tube hole annular grooves provision in TEMA. TEMA RB Class require at least two grooves, each approximately 1/8 in. wide by 1/​64 in. deep (3.2 mm wide × 0.4 mm deep). TEMA C class requires grooving for 5/8 in (4.1.875 mm) and larger diameter tubes for design pressures over 300 psi (2068 kPa) and/​or temperatures in excess of 350°F (176.7°C). When integrally clad or applied tubesheet facings are used, all grooves should be in the base material unless otherwise specified by the purchaser. When an exchanger is built with small-​diameter tubes and with grooving in the tubesheet, the designer should consider [16] (1) the percentage of ligament width removed by the grooving and (2) the percentage of tubesheet thickness that the grooves occupy. For such tubes, better results might be achieved by using a coarse tube hole finish in the range of 250 rms or grooves similar to those desired for thin, high-​strength tubes. More than two annular grooves. The two 1/8 in. wide × 1/​64 in. deep (3.2 mm wide × 0.4 mm deep) grooves of the TEMA are adequate for most rolled joints. However, for thin-​walled, high-​strength tubes, a series of smaller, shallower grooves of the same total width are also followed. 4.11.10.24 Step Rolling In general, the maximum length of tube that can be roller expanded in one rolling tool application is about 2 in. (50.8 mm). For larger joint lengths, rolling is done in steps with overlapped rolled lengths. If the step-​rolled lengths do not overlap, there is a series of transitions between the rolled and unrolled lengths [29]. 4.11.10.25 Roller Expander for Tube Extending Beyond the Tubesheet For tubes that extend beyond the open face of the tubesheet, a thrust collar that fits over the tube must be used. The thrust collar positioned against the tubesheet face will accurately locate the expanding rolls within the tubesheet hole. 4.11.10.26 Expanding in Double Tubesheets The recommended sequence of expanding for the double-​tubesheet exchangers can be adopted, starting with number one as shown in Figure 4.57 [31]. Tube expansion is carried out at the inlet end inner tubesheet first. Particular care is taken in setting up the expanders to ensure that expansion beyond the edge of the tubesheet does not occur.

FIGURE 4.57  Recommended sequence of tube expansion for the double-​tubesheet exchanger. (After [31] Dudley, F.E.)

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4.11.10.27 Leak Testing This is discussed in a later section. 4.11.10.28 Residual Stresses in Tube-​to-​Tubesheet Joints All tube-​expanding methods leave residual stresses in the tube wall. If these stresses are tensile and above 100 MPa, the tube is susceptible to stress corrosion cracking. Some steam generator manufacturers do a thermal stress relief after expansion to reduce these residual tensile stresses. However, in some situations, this is impractical if not impossible [29]. Compared to rolled joint and explosively expanded joint, hydraulically expanded joint is weaker. The level of residual stress is maximum in roller expanded and minimum in hydraulically expanded joint [36]. The susceptibility of tube-​to-​tubesheet joints formed by these three methods to stress corrosion cracking is shown in Figure 4.58 [36].

4.12 TUBE-​TO-​TUBESHEET JOINT WELDING Expanded joints are adequate for many applications and are still used extensively where pressure differences are not high enough to cause leakage. However, higher temperature and pressure requirements on heat exchangers make expanded joints inadequate, and for certain services, the mixing of the shellside and tubeside fluids is prohibited. For these conditions, welding of tube-​to-​ tubesheet joints must be carried out. At high operating temperatures, the expanded joint undergoes stress relief. The interfacial joint stresses, which give the joint its strength, are relaxed, the tube wall contracts, and leakage develops [37]. Weld joints have no such limits at high temperatures. According to Brosilow [37], tube-​to-​tubesheet welding is preferred for the following circumstances: 1. Tube pitch is too small to permit an expanded joint. 2. Thermal cycling is severe and operating temperatures are relatively high. 3. In critical applications where the danger of corrosion is high. The expanded joint is not continuous, since crevices between tube wall and tubesheet lead to crevice corrosion. 4. When access for maintenance is limited, as in nuclear and in some chemical process exchangers.

FIGURE 4.58  Stress corrosion due to residual stresses in the tube of tube-​to-​tubesheet joint. (Adapted from [35] Kong, C.-​S., Lee, B.-​I., and Shin, S.-​H.)

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If welding is used, either it serves as a seal (known as a seal-​welded joint) or the weld is used as the principal means of load-​bearing connection (known as a strength-​welded joint).

4.12.1 Methods of Tube-​to-​Tubesheet Joint Welding The following means of tube-​to-​tubesheet welded joints are generally followed, depending on the service involved: 1. expand in plain holes and seal welding 2. expand in grooved holes with seal welding 3. expand in plain holes and strength welding. All compatible weldable materials, namely, tubes, tubesheet, and weld filler metal, may be joined at tube-​to-​tubesheet joints with conventional arc welding methods and gas tungsten arc (TIG) welding.

4.12.2 Merits of Sequence of Completion of Expanded and Welded Joints Merits and demerits of these methods are as follows. 1. First expansion then welding. Merits a. Welds are free from strains induced by rolling operation. b. The integrity of the tube expansion can be tested. Demerits a. Welds may contain porosity due to the presence of lubricants used during expansion, if cleaning is not done properly. b. Expansion may get loosened under the influence of heat during welding. Such tubes would require rerolling. 2. First welding then expansion. Merits a. Welds are free from porosity due to the absence of lubricants. b. If the tube-​to-​tubesheet joint requires PWHT, there is no point in expanding first. The heat treatment will relax the tubes. c. Tube projection for welding is uniform. Demerits a. Welds may be damaged due to strain caused by expansion. b. There is no foolproof check for the leak tightness of the expansion carried out, as the seal welds may help in shadowing the under expansion of the tubes.

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c. Weld shrinkage may reduce the tube inner diameter so that it is difficult to insert expanders. This problem is overcome by reaming. 3. Ebert [24] discusses four types of tube-​to-​tubesheet joints (a) expanding without grooves, (b) expanding with grooves, (c) expanding with grooves and seal welding, and (d) strength welding. Characteristics of such joints are hereunder: a. Roll or expand only (without grooves): strength is poor leak resistance is poor tube replacement is very easy application is limited (i.e. low-​pressure water or air). b. Roll or expand only (with grooves): strength is limited (can be improved by explosive expansion) leak resistance is limited (can be improved by explosive expansion) tube replacement is easy applicable for low stresses and low consequences of leakage. c. Roll or expand and seal weld (with grooves): commonly used by the oil industry strength is limited (can be improved by explosive expansion) leak resistance is good (can be improved by explosive expansion) fabrication problems are faced tube replacement is more difficult applicable where stresses are not too high and where risks of leakage is low. d. Strength weld (no grooves required): full strength leak resistance is good (maximum leak resistance can be achieved if two pass welds are used) some fabrication problems, especially if there are grooves tube replacement is more difficult, especially if there are grooves applicable where mechanical and thermal stresses are high and leakage is unacceptable. 4. Sequences to be followed in the case of welding tube ends and then expand [25]: a. clean the tubesheet holes b. clean and deburr the tube ends c. insert tubes into tubesheet d. tack weld some tubes on tubesheet at four places e. weld tube ends f. air leak test welds; repair as needed g. expand the tube ends h. penetrant test welds; repair as needed i. leak test welds using helium. 5. Sequences to be followed in the case of expand tubes and then weld [25]: a. clean the tubesheet holes and deburr the ends b. clean and deburr the tube ends c. insert tubes into tubesheet d. expand tubes e. leak test the rolled joints; re-​expand as needed f. weld tube ends (weld tube ends in a random order to avoid concentration of heat in a localized area) g. perform helium leak test of the welded joints.

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Fabrication, Brazing, and Soldering of Heat Exchangers

4.12.3 Full-​depth, Full-​strength Expanding after Welding When the tube welding is to be done at or near the tubesheet front face, it is desirable to expand the tubes to full strength for approximately the whole tubesheet thickness for the following reasons [27]: 1. Expanding the tubes into the holes isolates the welds from the effects of tube vibration. 2. Contact between the tube and the hole permits heat to flow between the tube and tubesheet. 3. For full-​strength expanded tubes, the ligament efficiency is much greater than when there is no contact as given by the formulas. For example, the expression for minimum ligament efficiency μ is given by [38, 39]:

µ=

µ=

p−d for tubes welded to the tubesheet p p − (d − t ) d

(4.6)

for tubes expanded more than 90% of the tube sheet thickness

where d is the tube outer diameter p is the tube pitch t is the tube wall thickness.

4.12.4 Requirements for the Welding and Testing of Tube-​to-​Tubesheet Joints The connection between the tube and the tubesheet, called the tube-​to-​tubesheet joint, is the most critical aspect of a heat exchanger. Factors to consider include joint design, welding equipment, proper cleaning of the tube and tubesheet prior to welding, the welding-​expansion sequence, and testing and inspection. Even seemingly minor mistakes in design or fabrication can lead to costly leaks or other types of failures. 1. applicable codes and standards 2. welding process and joint detail 3. welding procedure qualification 4. welders qualification 5. preparation of tubes and tubesheet holes 6. welding 7. examination of joints including leak test.

4.12.5 Certain Preparation for Tube-​to-​Tubesheet Welding There are three phases in the process of welding tubes to tubesheets where problems can occur [40]: 1. the preparation of the tubes 2. the preparation of the tubesheet 3. tube and tubesheet welding. 4.12.5.1 Preparation of the Tubes The cut must be perfectly perpendicular and without burrs, since the next step requires the tube to pass through the tubesheet. The quality of the tubes, the tolerances of ovality, and the variation of thicknesses are also of major importance.

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Trimming. In this case, after the tube has passed through the tubesheet, a squaring machine with an internal grip is used to repeatedly bring the tubesheet overhang to the right value. Indeed, on great lengths, a precise positioning can be difficult. The Preparation of the Tubesheet. Tubesheets are machined and therefore the holes are very accurate and free of scoring marks. The end of the hole can be beveled before the weld to ensure a stronger connection under extreme circumstances (temperature-​pressure). In such cases, the welding will be done with filler wire and will require a multi-​pass approach. Placement of the tube. The positioning of the tube on the tubesheet depends on the intended application and the strain that the heat exchanger will be undergoing. For example, in a typical configuration where the end of the tube is aligned with the tubesheet, three steps are essential for a welded connection: 1. the centering 2. the weld 3. the rolling-​in after the weld. Centering. This step allows a perfect repeatability and avoids welding defects that can result in serious consequences. The main situation where a defect can occur is when there is a gap between the tube and the tubesheet. This often happens towards the end of the welding process and can create fusion defects or holes. An exaggerated configuration with excessive gap between the tube and tubesheet hole of the assembly before welding is shown in Figure 4.59 Centering guarantees zero slack and full contact between the end of the tube and the tube sheet. Centering creates a contact only at the end and gases can therefore easily escape from the weld bath. Once the preparation ensures contact between tubesheet and tube and the weld bath can evacuate the gases, the orbital weld will be perfectly repeatable so optimal quality is guaranteed. The contact zone between the tube and the tubesheet must be clean. Grease, oil or other residues from the tube manufacturing or machining can cause the formation of unacceptable blowholes, with outlets on the surface or enclosed in the welds. Compared to manual welding, orbital tube to tubesheet welding requires more detailed attention to such things as the tube type, the material, the end preparation, and the dimensions of the required weld thickness, etc. To use orbital welding equipment, it is strongly recommended to choose seamless tubes (or those without a flattened weld), concentricity faults between the inner and the

FIGURE 4.59  An exaggerated configuration with excessive gap between the tube and tubesheet hole of the assembly before welding.

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Fabrication, Brazing, and Soldering of Heat Exchangers

outer diameter must be limited to a minimum in order to allow the repeatability of the electrode positioning. If good thermal conduction is requested during welding, the gap between the tube and the bore must be eliminated by a slight expansion of the tube. 4.12.5.2 Automated or Manual Welding decision Another important decision to make in the early stages of a heat exchanger project is whether to specify manual or automated welding of the tube end joints. Both manual and automated welding have advantages, but some drawbacks need to be considered carefully in order to select a process that balances speed with the ability to customize and provide reliability. If a fabricator is required to build the same heat exchanger type and configuration repeatedly, an automated welding process may be the best choice for an assembly-​line production process. Orbital welding, which is an automated gas tungsten arc welding (GTAW) process, requires more initial setup time and test runs. Manual welding –​the process is more time intensive than automated welding and can be more expensive. A skilled tube-​joint welder can adjust the amount of filler being fed, the volts and amps being used, travel speed of the torch and the distance of the torch to the tubesheet while welding the joint. 4.12.5.3 Tube to Tubesheet Joint Welding and Expansion Welding creates off-​gases that can damage welds if not vented properly. When tubing is expanded into a tubesheet hole first, and that region is effectively sealed, the only way these gases can escape is to pass back through the weld itself, also known as blowback. This can lead to porosity and other defects in the weld, especially as the weld is completed or on multiple-​pass welds. Blowback also can cause equipment damage and safety issues. For these reasons, many fabricators recommend welding tubes to the tubesheet prior to expansion. It is important for end users and fabricators to discuss details such as the tube welding and expansion sequence early in the process to avoid delivery delays and quality problems.

4.12.6 Welding Methods 4.12.6.1 Orbital Welding Welding tubes to tubesheet can be done using either manual TIG or orbital TIG. The difference is significant since –​compared to manual welding –​one operator can pilot two orbital welding heads simultaneously which increases the welding output significantly. Orbital welding therefore saves time and improves the quality of the weld. All welds are documented to ensure optimal traceability of the welding process. Compared to manual welding, orbital tube to tubesheet welding requires more detailed attention to such things as the tube type, the material, the end preparation, and the dimensions of the required weld thickness, etc. To use orbital welding equipment, it is strongly recommended to choose seamless tubes (or those without a flattened weld), concentricity faults between the inner and the outer diameter must be limited to a minimum in order to allow the repeatability of the electrode positioning. 4.12.6.2 Tube to Tubesheet Joint Welding Machine The orbital tube to tubesheet weld head is design for fabrication of various heat exchangers and boiler industry. It can be used for recessed, projected and flush tubing with or without wire feed. Tungsten inert gas (TIG) /​Gas tungsten arc welding (GTAW) head that efficiently welds tube to tubesheet with the highest precision and accuracy while consistently producing high quality welds.

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FIGURE 4.60  Commonly adopted tube-​to-​tubesheet joint configuration for welding. Note: (a) Tube flush with the tubesheet, (b) projecting tube, (c) trepanned tubesheet, (d) recessed tube, (e) rear side welding, (f) added ring weld type, and (g and h) bell-​mouthed and fused welding. (Courtesy of Polysoude, S.A.S, Nantes, France.)

4.12.6.3 Tube-​to-​Tubesheet Joint Configuration for Welding In general, the tube-​to-​tubesheet connections have been made by passing the tubes through the tubesheet and fillet welding on the face side, recess welding, and internal bore welding using one of the joint geometries as discussed later. 1. Figure 4.60 shows most commonly adopted tube-​ to-​ tubesheet joint configuration for welding. Note: a. In Figure 4.60b, projection of 1 mm is preferred because it allows better welding and is suitable for welding without filler metal. b. Figure 4.60c, incorporating a trepan in the tubesheet. It is used for materials in which the welds may be subjected to cracking when made under conditions of restraint. A trepanned joint is more expensive to machine, and the ligament length between tubes is a governing factor in use of such a joint [41]. c. In Figure 4.60d, a recessed welded type is employed for avoiding stagnation of the fluid on the tubesheet and to reduce turbulence on the tubeside. d. Figure 4.60g shows tube end bell-​mouthed and fused. e. Added ring weld type –​Figure 4.60f shows fillet welds of the tube-​to-​tubesheet joint with a ring of special alloy placed on the protruding end and melted by an argon arc process. 2. Figure 4.61 shows various forms of tube-​to-​tubesheet joint configuration for welding of flush tubes. 3. Figure 4.62 shows various forms of tube-​to-​tubesheet joint configuration for welding of protruding tubes. 4. Figure 4.63 shows forms of tube-​to-​tubesheet joint configuration for recessed tubesheet welding. 5. Figure 4.64 shows various forms of tube-​to-​tubesheet joint configuration for internal bore welding behind the tubesheet. 6. Figure 4.65 shows various forms of tube-​to-​tubesheet joint configuration for welding of clad tubesheet.

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Fabrication, Brazing, and Soldering of Heat Exchangers

FIGURE 4.61  Tube-​to-​tubesheet joint configuration for welding of flush tube. (Courtesy of Polysoude S.A.S, Nantes, France.)

FIGURE 4.62  Tube-​to-​tubesheet joint configuration for welding of protruding tube.

FIGURE 4.63  (a–d) Tube-​to-​tubesheet joint configuration for welding of recessed tubesheet. (Courtesy of Polysoude S.A.S, Nantes, France.) (e) Recess welded joint. (Courtesy of Mueller, Springfield, MO.)

Successful tube-​to-​tubesheet welding depends critically on the accurate machining of holes, joint preparation of the tubesheet, and cleaning of all surfaces prior to welding. The tubesheet should be cleaned immediately after drilling, and compressed air should not be used to blow off the cleaning solution. Dry nitrogen is suggested in place of compressed air.

Fabrication, Brazing, and Soldering of Heat Exchangers

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FIGURE 4.64  Tube-​to-​tubesheet joint configuration for internal bore welding behind the tubesheet. (Courtesy of Polysoude S.A.S, Nantes, France.)

FIGURE 4.65  Tube-​to-​tubesheet joint configuration for welding of clad tubesheet.

Recessed tube cladding fusion: The recessed tube cladding fusion technique makes possible mechanical cleaning of all surfaces to be fused (Figure 4.65c). This technique requires no addition of filler metal because the cladding provides the filler, and it melts only the corner of the clad hole and fuses only to the top of the tube [42].

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Fabrication, Brazing, and Soldering of Heat Exchangers

4.12.6.4 Considerations in Tube-​to-​Tubesheet Joint Welding There are certain factors to be considered when planning a tube-​to-​tubesheet joint [43]: 1. Is the tubesheet an air-​hardening material that will require PWHT? 2. Are the tube ends and the tubesheet holes clean? 3. Welding before expansion or expansion before welding? Other factors to be considered include the following [27]: 1. granular structure 2. the coefficient of thermal expansion of the tubes and tubesheet materials must be close enough. These factors are discussed next. Is the tubesheet an air-​hardening material that will require PWHT? While PWHT is possible under shop conditions, it is often difficult to accomplish under field maintenance conditions. This difficulty is overcome by these measures: (1) Frequently, a non-​air-​hardening austenitic or Inconel overlay can be specified for the tubesheet [42], thus eliminating the need for PWHT. (2) If an austenitic overlay is not possible due to corrosion problems, the tubesheet can often be overlaid with a low-​carbon filler metal of the same nominal composition as the tubesheet. While PWHT would likely be required after overlaying to decrease the HAZ hardness, the lower carbon content of the overlay would not harden as readily during tube-​to-​tubesheet welding, often eliminating the need for a subsequent PWHT [43]. This aspect is discussed with reference to a feedwater heater with carbon steel tubes welded to carbon steel forging tubesheet in Ref. [42]. Are the tube ends and the tubesheet holes clean? To achieve an absolute seal with good mechanical properties, the welded joint must be free of voids. The tubes, holes, and weld rings should be carefully cleaned to eliminate the danger of voids forming due to evaporation of surface contaminants such as grease, oil, and cutting fluids. It may be necessary to solvent clean the tube ends, the tubesheet should be thoroughly degreased, and the tubesheet holes should be steam cleaned or sandblasted to remove any scale or dirt that could interfere with the tube-​to-​tubesheet joints. Titanium tube holes and tubesheets should be cleaned with a solvent such as acetone or methyl ethyl ketone. Methanol or chlorinated solvents such as trichloroethylene should not be used [44]. After cleaning, both the tube ends and tubesheet holes should be visually examined for longitudinal scratches, which could create a leakage path. Welding before expansion or expansion before welding? A third consideration for achieving successful tube-​to-​tubesheet joints is the proper application of rolling. The sequence for making a tube-​to-​tubesheet joint is determined by the following factors [27]: (1) the requirement that the surfaces be very clean; (2) the need for a path of escape of welding generated gases –​otherwise the gases will cause porosity in the weld; (3) the maximum desirable root gap for the material being welded; and (4) the need to repair welds that have failed in service. Many purchasers recommend to do seal welding first and then roll the joint. The reason that seal welding is done first is to allow the welding gases behind the weld to escape along the tube and to avoid the situation where the presence of oil and fluids used during expansion do not have the opportunity to contaminate the weld pool. If significant rolling is performed prior to welding, a sealed annular space between the tube and the tubesheet is formed. The air in this annular region will expand due to welding heat. Since the gas cannot escape, it expands into the tube-​to-​tubesheet weld, creating pinhole porosity. Except for joining thin-​walled titanium tubes to titanium tubesheets, these requirements indicate that tubes should be welded to the tubesheets prior to expanding [27]. Light expansion followed by seal welding and followed by full expansion is most preferred, since it has the merits of the two sequences just discussed. Prior to tube-​to-​tubesheet welding, it is often desirable to expand the tube into the tubesheet by a “light roll” to ensure that the tube is centered

Fabrication, Brazing, and Soldering of Heat Exchangers

563

in the hole for good tracking in automatic welding. A “hard roll” prior to welding increases the chance of producing a weld defect from escaping gas as the weld is being made. Experience has shown that the best welding results are obtained when tubes are only anchored into the tubesheet prior to welding by these means [27, 43]: (1) center punching, (2) a tapered drift pin, and (3) a light expansion (presumably to create tube/​hole contact that is not hydraulically tight). Another method, suggested by Yokell [26] is to use one of the commercially available expanders that compresses a polymer in the tube end to produce radial pressure. These can be set to lock the tube in place without creating a hydraulically tight seal. While performing rolling operation, lubricant should not be used, to avoid contamination. However, without lubricant, there is a possibility that heat due to friction will cause some of the hardened roller and cage material to flake off [27]. Proper rolling techniques following welding are required. Some fabricators have found it necessary to start rolling at least 12.5 mm (0.5 in.) away from the weld, proceeding toward the shellside of the tubesheet [43]. Such a procedure minimizes the amount of mechanical loading on the weld. Refer to tube hole finish RCB-​7.2.3 and 7.2.4, tubehole grooving RB-​7.2.4 and C-​7.2.4, RCB-​7.3 tube to tubesheet joint, RCB-​7.3.1 expanded tube-​to-​tubesheet joint, and RCB-​7.3.2 welded tube-​ to-​tubesheet joint. Granular structure. Tubesheets must have a granular structure fine enough to permit consistently uniform weld metal deposits. Carbon steel tubesheets should always be grain refined [27]. Coefficient of thermal expansion of the tubes and tubesheet materials. Consider, for example, an austenitic stainless steel tube, TIG welded to a carbon steel tubesheet with high-​alloy filler metal. The coefficient of thermal expansion of the austenitic stainless steels (17.8 × 10−6 cm/​cm °C) is about 50% greater than that of the carbon steel (12.1 × 10−6 cm/​cm °C). The fusion temperature is in the range of 2800°F (1730°C). As the welds solidify rapidly, the difference in the rates of expansion engenders high levels of thermal stress [27]. Stresses due to operating pressure superimposed on the thermal stresses may cause premature failures. The clearance between the tube and tubesheet hole. When the tube-​to-​tubesheet hole clearance is excessive, the required root penetration cannot be achieved and the weld root will fracture. 4.12.6.5 Mock-​ups for Tube-​to-​Tubesheet Welding To determine correct welding parameters, use mock-​ups to scrutinize tube-​to-​tubesheet weldments. Welded mock-​ups are sectioned and prepared for review of the root area and dimension checks of the weld. Examinations of welded mock-​ups are discussed in detail by Syal [17]. An expanded and welded tube-​to-​tubesheet mock-​up is examined by the following methods: 1. Radiography examination: The weld shall be free from porosity, cracks, or other defects that would result in leakage in service. 2. Micro-​and macro-​examination: to detect defects, metal flow in the annular grooves (if grooves are present in the tubesheet), fusion, penetration, etc. 3. Minimum leak path. Leak Path is defined as the shortest way in any direction on a macro/​ micro specimen between the inside of the tube and the gap between tube and tubesheet as shown in Figure 4.66. It is the distance from the root of the weld to the surface nearest the root. For strength-​welded joints, the minimum leak path shall be the tubesheet thickness, and for seal-​welded joints 0.7 times the tubesheet thickness. 4. Hardness examination: tubesheet, tube, weld, and HAZ; normal hardness limits are for carbon steel 220 HV, low-​alloy steel 260 HV, and stainless steels 250 HV or as specified in the specification [17].

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Fabrication, Brazing, and Soldering of Heat Exchangers

FIGURE 4.66  Illustration of minimum leak path.

FIGURE 4.67  Welded tube-​to-​tubesheet model pressure test setup.

5. Tear test for strength welded joint. 6. Pulling test. Tubes shall be pulled on a tensile testing machine with a pulling load not less than 0.1 × L × d × 3000 (kgf) for expanded tubes without seal welding, where L is the length of expansion joint (cm) and d tube outer diameter (cm) [17]. 7. For welded mock-​up, pressure test as shown in Figure 4.67.

4.12.7 Welding Process GTAW of tube-​to-​tubesheet welding. The TIG welding process is used extensively for the welding of nonferrous and austenitic tubes and to a limited extent for carbon steel tubes. This process is particularly useful in the welding of small-​diameter tubes where the small ligaments are not suited to the metal arc process [34]. The joint is either autogenous or made with wire feed. Much of the joining of tube-​to-​tubesheet is done manually, especially on small units, and automatic equipment has been developed for use in large heat exchangers where thousands of tubes are used.

Fabrication, Brazing, and Soldering of Heat Exchangers

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Manual TIG welding. Manual welding requires accurate control over torch movement, especially when the tubes used for weldments have thin walls and small diameters. Although the welds are acceptable, they may lack uniformity and production rate is low; weld size and contour depend largely on the skill of the operator in rotating the torch at a constant speed for proper fusion of the filler wire. Orbital/​automatic TIG welding. Using fully automatic equipment, it is possible to obtain uniform high-​quality welds with good reproducibility, and production rate is high. Defects that require rework are few. The automatic GTAW equipment consists of orbital welding setup, in which the welding head is automatically rotated. These orbital welding machines often use a mandrel for self alignment on the tube to be welded. Once locked into position, a GTAW torch rotates around the face of the tube and joins it to the tubesheet, frequently without the addition of supplemental filler material. Figures 4.68 and 4.69 show the automatic TIG welding heads in action, and Figure 4.70 shows a CNC controlled automatic tube-​to-​tubesheet welding machine. In a cold wire method, a cold filler wire is fed to the arc from a spool, through a wire feed drive and an adjustable wire guide tip, all of which rotate with the welding head. Argon shielding gas is automatically fed to the orbiting head. In the actual welding sequence, welding at each end of the

FIGURE 4.68  Tube-​to-​tubesheet joint welding head. (a) Magnatech welding head and (b) multi welding heads in action. (Courtesy of Magnatech Limited Partnership, East Granby, CT.)

FIGURE 4.69  Tube-​to-​tubesheet TIG orbital welding head. (a) Weld head and (b) power source and teach pendant. (Courtesy of Maus Italia F. Agostino & C.s.a.s., Bagnolo Cremasco (Cr), Italy.)

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Fabrication, Brazing, and Soldering of Heat Exchangers

FIGURE 4.70  CNC controlled automatic tube-​to-​tubesheet welding. (Courtesy of Vermeer Eemhaven B.V. Bunschotenweg, Rotterdam, the Netherlands.)

tube is staggered to ensure that gas leak rate and pressure are not affected by both ends of the tubes being sealed simultaneously [45]. If TIG welding is to be used, the tube wall should be expanded to intimate contact with the tube hole. This may be done by roller expanding or pinning with a tapered pin; however, the tapered pin method is preferred for two reasons [35]: (1) The tube will be exposed to a minimum chance of contamination and (2) there will be only a line contact around the periphery of the tube-​to-​tube hole. This allows the gases generated during welding to escape behind the weld puddle, reducing weld porosity, and the incidence of blow holes as joints are completed. If preheating and post-​weld heat treating are required, gas heating can be used or modern equipment is available to carry out the operation in situ. The equipment consists of a computer-​ controlled temperature monitor and clamp-​on cartridge heating elements [45]. Good-​quality welds between tube and tubesheet require a high degree of operator skill and concentration. Cleanliness at the joint is always desirable. 4.12.7.1 Orbital Welding Whenever high-​quality results are required, orbital welding is the first choice for the joining of tubes. The welding torch –​in most cases, the TIG welding process is used –​travels around the tubes to be joined, guided by a mechanical system. The name orbital welding comes from the circular movement of the welding tool around the workpiece. Generally, orbital welding technique covers two main fields of application: (1) tube-​to-​tube/​pipe-​to-​pipe joining and (2) tube-​to-​tubesheet welding. Two kinds of current are applied in the TIG welding technique: (1) direct current (DC) is most frequently used to weld nearly all types of materials and (2) alternating current (AC) is preferred to weld aluminum and aluminum alloys.

Fabrication, Brazing, and Soldering of Heat Exchangers

567

FIGURE 4.71  Orbital TIG welding head –​welding of recessed tube-​to-​tubesheet. (Courtesy of Polysoude S.A.S, Nantes, France.)

4.12.7.2 Enclosed Orbital Tube-​to-​Tubesheet Welding Heads Without Filler Wire Welding head shown in Figure 4.71 is designed for TIG welding (GTAW) of tube-​to-​tubesheet applications, if they can be accomplished without filler wire. With these weld heads, flush or slightly protruding tubes with a minimum internal diameter of 9.5 mm (3/​8″) can be welded, the maximum diameter being 33.7 mm (1 1/​3″). The weld is carried out in an inert atmosphere inside a welding gas chamber, providing very good protection against oxidation. For clamping, a mandrel is inserted into the tube to be welded and expanded mechanically. 4.12.7.3 Open Tube-​to-​Tubesheet Welding Heads With or Without Filler Wire Open orbital tube-​to-​tubesheet weld heads that can be used with filler wire cover the whole range of applications from tubes with an ID of 10 mm (13/​32″) up to tubes with a maximum OD of 60 mm. The TIG torch travels around the tubes, which can be protruding, flush, or recessed. The welding heads are equipped with a TIG torch with gas diffuser (Figure 4.72). A sufficient gas protection is achieved only at the zone around the torch that is covered by the shielding gas streaming out of the gas lens. If oxygen-​sensitive materials need to be welded, the gas protection can be improved by installing a gas chamber. The welding heads can be equipped with an integrated wire feeder. A pneumatic clamping device can be used to hold the weld head in working position on the tube plate, enabling several welding heads to be operated by just one person. Welding lances allow the operator to carry out internal bore welding with gapless joints behind a tubesheet or a double tubesheet. 4.12.7.4 Arc Voltage Control (AVC) Options As for most orbital welding applications, a pulsed current is applied. 4.12.7.5 Welding Equipments In most cases, the welding equipment used for tube-​to-​tubesheet welding is strictly adapted to the kind of application and the desired level of automation:

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Fabrication, Brazing, and Soldering of Heat Exchangers

FIGURE 4.72  Orbital TIG welding head –​welding of flush tube-​to-​tubesheet joint using a welding head with tube locking arrangement. (Courtesy of Polysoude S.A.S, Nantes, France.)

1. Welding equipment featuring three controlled axes (gas, current, rotation) is composed of a stationary installed power source and an enclosed welding head. This equipment allows for the execution of fusion welding without addition of filler wire. 2. The welding equipment, including four controlled axes (gas, current, rotation, wire), is composed of a stationary installed power source and an open welding head. The equipment is suitable for single-​pass welding; two passes must be welded in two separate steps. 3. The welding equipment fitted with five controlled axes (gas, current, rotation, wire, arc voltage control (AVC)) is composed of a power source designed to control six axes and a welding head of the type TS 2000 or TS 60 with AVC configuration. The equipment allows the chaining of two passes with filler wire; the raising of the torch between the first and the second pass can also be programmed and is carried out without interruption of the weld cycle. 4. Welding equipment furnished with six controlled axes (gas, current, rotation, wire, AVC, oscillation), comprises a PC power source and a welding head of the type 20/​160. The equipment allows multi-​pass welding (two or more passes); the torch can be displaced in radial direction. (This text is based on “Polysoude: The art of welding, The Orbital Welding Handbook, Polysoude S.A.S, Nantes, France”.) 4.12.7.6 Specific Requirements of Tubes and Weld Preparations Compared to manual welding, the planning of the orbital tube-​to-​tubesheet welding requires some more specific attention: 1. The tubes have to be seamless; concentricity faults between the inner and the outer diameter must be limited to a minimum to allow the repeatability of the electrode positioning. With standard applications (flush, protruding, or recessed tubes), the torch is aligned at the inside of the tube whereas the welding is carried out at the external diameter. Concentricity faults would cause unacceptable variations of the distance between workpiece and electrode and thus directly after the arc length. 2. As with V-​joints, it is virtually impossible to ensure reliable melting of the base of the tube edge, especially in the vertically down position (fusion defects are to be seen on micrographic sections); these joints have to be replaced by J-​preparations.

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4.12.7.7 Welding of Flush Tubes Depending on the application, orbital welding of flush tubes with or without filler metal is possible. Different joint designs (Figure 4.60, 4.61, and 4.62) are shown as follows: 1. standard preparations 2. J-​preparations 3. V-​preparations 4. relief groove. 4.12.7.8 Welding of Flush Tubes with Addition of Filler Wire Welding equipment fitted with four or five controlled axes can be used for this application; the open tube-​to-​tubesheet welding head should be configured with devices adapted to the requirements: 1. integrated or external wire feeder 2. with or without AVC 3. with or without shielding gas chamber (for the welding of titanium or zirconium) 4. torch angle of 0° or 15°. Depending on the dimensions, and the required weld thickness, one or two passes are necessary. One pass of the torch is always applied on the first pass for tightness; layers needed for mechanical strength and wear resistance will often require a second pass. 4.12.7.9 Welding of Protruding Tubes Protruding tubes are always welded with addition of filler wire, but in some cases, the weld is beginning with a fusion pass. As shown in Figure 4.62, different joint designs are possible. 1. standard preparation without groove 2. J-​preparations 3. V-​preparations. Welding equipment fitted with four or five controlled axes can be used for this application. Depending on the pitch and the protruding distance, the torch inclination may be varied. Standard torch angle is 15° or 30°. 4.12.7.10 Welding of Recessed Tubes Different joint designs –​(d) standard preparation without groove, (e) J-​preparation, (f) V-​preparation, and (g) welding the tubesheet –​are shown in Figure 4.63. The preparation of the type g is frequently used in the petrochemical industry; welding equipment with six controlled axes and a 20/​160 welding head with separate clamping device have to be used. This type of application generally requires a specific project to study the best adaptation of clamping tools and welding procedures. Depending on the dimensions, and the required weld thickness, one or two passes are necessary. One pass of the torch is always applied on the first pass for tightness; layers needed for mechanical strength and wear resistance will often require a second pass. 4.12.7.11 Internal Bore Welding Face-​side welding techniques shown in Figure 4.61 are economical and generally reliable, but they produce a weld that is difficult to inspect by radiography, and they introduce a long crevice between the tube and the tubesheet [45]. Localized corrosion may start in this crevice. Even if the width of the crevice is made as small as possible by expansion of the tube, inside hole cracks or corrosion

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still poses a danger. These problems are overcome by internal bore welding methods. In the internal weld joint shown in Figure 4.64 u and v, the tube does not pass through the tubesheet. It is welded to a short stub machined on the shellside of the tubesheet. This joint has the important advantages of easy inspectability by radiography and eliminating the crevice. However, it is much more difficult to weld than the face-​side design because of the proximity of the tubes. There is no room for an orbiting arc welding head on the outside of the tube. The weld in an internal bore weld is made by welding from the bore side of the tube. 4.12.7.12 Internal Bore Welding Behind the Tubesheet To avoid crevice corrosion between the tube and the tubesheet, gapless joints are welded from the inside of the tubes at the backside of the plate as shown in Figure 4.73. This type of application requires extended accuracy of the workpiece preparation and welding. Some possible joint designs are, x. standard without groove; y. preparation with relief groove, without recess; and z. preparation with relief groove, with recess. Unlike conventional tube-​to-​tubesheet applications, the internal bore welding operations behind the tubesheet require a gas protection of the root (at the outside of the tube). Only with a preparation of the type x, where the tube end is positioned sufficiently deep in the bore (e.g. half of tube wall thickness), a root protection is not necessary. The protection can be provided by flooding the entire apparatus with inert gas or if the backside of the tubesheet is accessible by a local protection applied tube after tube. With a tube ID of more than about 35 mm, the use of welding tools with filler metal is possible. If relatively thick-​walled tubes of 3–​3.6 mm (depending on the base material) are to be welded, a horizontal weld position with the plate at the bottom with the weld head also, horizontally positioned, is recommended. Since the operator cannot see the torch position inside the tube, the distance from the face of the tubesheet to the welding joint must be very precise. Welding equipment fitted with three or four controlled axes can be used for this application; in the case of a joint preparation of type x, five controlled axes are necessary. The welding heads must be equipped with a peculiar lance for internal bore welding. By means of a weld lance that mounted at the front of the weld head, internal bore welding can be carried out at tube ID between 10 and 33.7 mm (13/​32″ and 1 1/​3″).

FIGURE 4.73  Internal bore welding behind the tubesheet by TIG welding. (Courtesy of Polysoude S.A.S, Nantes, France.)

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4.12.7.13 Welding of Sections of Unequal Thickness In SMAW, as in other welding processes, special techniques are required while welding a thick member to a very thin member due to large difference in heat-​dissipating capacities. An application involving components of unequal section thicknesses, namely, the welding of heat exchanger tubes having 0.093 in. (3 mm) wall thickness to a tubesheet as thick as 10 in. (254 mm), is discussed in Ref. [46]. In this situation, the welding current needed to obtain good penetration into the thick tubesheet is sometimes very high for the thin tube wall and results in undercutting of the tube and a poor weld. If an adequate amount of current for the tube is used, the heat is not sufficient to provide adequate fusion in the thick tubesheet and again results in a poor weld. Industrial practices to minimize heat-​sink differential are [46] as follows: 1. The usual method of avoiding difficulty is to cut a 0.25 in. (6.35 mm) deep circular groove in the upper surface of the tubesheet around the tube hole (Figure 4.60c); the groove restricts heat transfer. 2. To place a copper backing block against the thin member during welding. The block serves as a chill, or heat sink, for the thin member. Yet another method is to preheat the thick member. Normally, 150°C is preferred for carbon steel tubesheets; for stainless steel, it is not required since the thermal conductivity is very much lower. Aluminum tubesheet-​to-​tube welding. For good heat balance, trepan a groove in the tubesheet, making a welding lip of the same thickness as the tube wall, and then edge weld the lip and the wall. Better still, extend the tube one wall thickness beyond the lip to improve melting and fusing [47]. 4.12.7.14 Seal-​welded and Strength-​welded Joints Tube welds at the tubesheet front face are classified as either seal welding or strength welding. Strength-​welded joints. Strength-​welded joints in lieu of full expansion and seal welding are adopted for higher temperatures and pressures, as expanded joints lose their leak tightness at higher temperatures. Tube-​to-​tubesheet strength welds are used to transfer all longitudinal mechanical and/​ or thermal loads from the tubes to the tubesheet. The suggested definition of strength weld leads to the following considerations [27]: (1) Using the joint efficiencies specified in the Paragraph UW-​15 (c) of ASME Code Section VIII, Div. 1, the total weld cross-​sectional area times the joint efficiency must equal the tube cross-​sectional area; (2) weld strength must be based upon the lower of the tube or tubesheet allowable stress; and (3) differential thermal expansion between the tube, tubesheet, and weld metal must be considered. The strength weld, which usually has the weld throat 1.4 times the tube wall, is considered to be superior to other joint types in both mechanical strength and sealing ability. When the tube wall thickness is thinner than 1.6 mm (1/​16 in.), special weld-​groove geometry and welding technique is required. Almost all such strength welds are fillet welds, either at the front face of the tubesheet or within the hole; or, in a very few cases, the tube holes may be drilled to match the tube inside diameter and the tubes butt-​welded to the tubesheet secondary face (Figure 4.64u, v, and y), known as internal bore welding. Figure 4.74 shows tubes under insertion and tubes welded to tubesheet. Typical strength-​welded joints are shown in Figure 4.74 and 4.75 and some acceptable seal welded and strength welded joints as per ASME Code Section VIII, Div. 1 is shown in Figure 4.76. Seal welds: Seal welds (Figure 4.76a and b)are used to prevent fluid leakage between the shellside and the tubeside. Although seal welds confer additional strength on the weld joint, the incremental capacity of the joints to withstand pressure and temperature imposed by loads is neglected in calculating the load-carrying capacity of the joint [26]. Nevertheless, the code requires a qualified welding procedure to be used. Seal welded joints are normally used for service pressure up to 80 kg/cm2 (g) and temperature up to 350°C. It is recommended to expand tubes in grooved holes for better holding strength for higher service pressures.

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FIGURE 4.74  Tube-​to-​tubesheet joint –​tubes under insertion and tubes welded to tubesheet. (Courtesy of Festival City Fabricators, div. of CSTI, Stratford, Ontario, Canada.)

FIGURE 4.75  Tube-​to-​tubesheet strength welded join. (Courtesy of Vermeer Eemhaven B.V., Rotterdam, the Netherlands.)

4.12.7.15 Welding of Titanium Tubes-​to-​Tubesheet For highly reliable leak tightness, many nuclear and fossil plant heat exchangers demand welded tube joints. Since titanium cannot be successfully welded to other materials, a titanium tubesheet, either solid or explosively clad titanium, is required. For cladding, at least 3/​16″ thick is preferred. The tube should be roller expanded before welding. Either a light tack roll or a full expansion can

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be performed. The expansion is required to keep air from the back face from contaminating the weld and to prevent vibration of the tube that could result in weld cracking. i.e. the contact roll eliminating the clearance between the tube and tube hole is a very important operation in obtaining a sound weld. Both the tube holes and the tube ends should be cleaned with a solvent such as acetone before welding. Methanol or chlorinated solvents such as trichloroethylene should not be used [44]. Since the extrusion of the tube during conventional rolling would lead to problems with a clad-​type tubesheet potentially causing the clad layer and interstitial layers to separate, it is suggested that the tubes are hydroswaged as the extrusion forces are deemed to be lower with this application. 4.12.7.16 Ductility of Welded Joint in Feedwater Heaters For applications such as feedwater heaters, the tubesheets are subjected mainly to bending stresses due to high feedwater pressure, which may be as high as 5000 psi in modern plant cycles. The tube-​ to-​tubesheet joints must remain impervious to water under this high pressure while sustaining the cyclic stresses due to tubesheet flexure, and temperature variations between the tube and tubesheet caused by such operating conditions as start-​up, load shedding, shutdown, and possible abnormal transient operations [42]. According to Lohmeir et al. [42], due to these reasons, the flexural and thermal stresses can be sufficiently large, even exceeding the elastic limit, to propagate leakage paths from weld metal flaws. In order to cope with such high stresses without suffering damage, the welds must have consistent mechanical properties and high ductility. The main factors to be controlled to ensure the ductility of the joints are [48] the following: 1. combination of tube and tubesheet materials 2. welding parameters 3. heat treatment. The carbon content of the basic material and welding alloy (less significant factor). The following factors ensure welded joints of even quality and high ductility [48]: 1. Tubes and tubesheets are always of the same material, with a maximum carbon content of 0.20%. The filler is of a special alloy whose main feature is an extremely low percentage of carbon. 2. All joints and the electrode are preheated before welding. 3. When all welding is completed, the entire tubesheet is stress relieved.

FIGURE 4.76  Some acceptable weld geometries as per ASME Code Section VIII, Div. 1. (a and b) Seal weld joints and (c and d) Strength weld joints.

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4.12.7.17 Inspection of Tube-​to-​Tubesheet Joint Weld 1. The completed tube-​to-​tubesheet joint weld should be inspected visually for defects. Use magnifying glasses to check and mark cracks, pinholes, etc. 2. A dye penetrant inspection is also adopted. 3. In some tube-​to-​tubesheet joints, a radiographic inspection can be made of selected areas. 4. Defects such as cracks or porosity should be ground out and repaired by GTAW with filler metal. 5. UT inspection of tube-​to-​tubesheet joint welds. UT inspection on GTAW tube-​to-​tubesheet welds for critical applications is well established to verify the weld leg length and throat thickness and to investigate the welds for typical defects, Ref. [49, 50]. The ultrasonic probe is positioned in the scanner’s hollow shaft and inserted into the inspected tube in the heat exchanger. The probe is rotated and moved axially simultaneously, thus performing a spiral movement. Inspection starts and stops a couple of millimeters before and after the weld, respectively. A centering mechanism is keeping the inspection head centered in the tube. the weld inspection principle as shown in Figure 4.77. 6. Radiographic inspection of tube-​to-​tubesheet joint welds. For very critical applications, the acceptance standards for tube-​to-​tubesheet welds like the seal, spigot, and butt/​fillet weld are generally based on high-​definition panoramic radiography. The small size of the welds and their shape necessitate a very short source to film distance [50]. This requires a highly specialized equipment such as the microfocus rod anode set and miniature low-​energy gamma sources, a unique film cassette for use on small-​diameter welds to implement radiography on these welds [50, 51]. Figure 4.78 illustrates the principles of the radiographic testing technique to test tube-​to-​tubesheet weld. 4.12.7.18 Leak Testing of Tube-​to-​Tubesheet Joint Leak testing of exchangers will allow one to determine if tubes have been adequately rolled or welded into the exchanger tubesheets. The simplest and inexpensive test is gas bubble testing also known as soap bubble testing. Such testing can be accomplished by pressurizing the shellside with air to a pressure of 1.0–​1.25 ksc (gauge) and “soaping” the face of the tubesheet and looking for bubbles, or by filling the shell with water, pressurizing the channel with air, and looking for bubbles through the shellside nozzle. Various leak testing methods are discussed in Chapter 3. Figure 4.79 shows a vacuum gun for rolled joint leak testing gun. 4.12.7.19 Testing of Tube-​to-​Tubesheet Joints Tube-​to-​tubesheet welds are to be tested using the manufacturer’s standard method. Any special testing using halogens or helium will be performed by agreement between manufacturer and purchaser. If there is a potential hazard due to a leak in service or when the tubeside design pressure is substantially higher than that of the shellside, it is desired to specify halogen or helium sniffer leak testing to satisfy no-​leak requirements. For high-​pressure feedwater, small leaks through the tube-​ to-​tubesheet joints lead to wire drawing (worm holing), which can be extremely expensive to repair while in service [32]. Hence, typical feedwater heater procurement specifications require helium leak sniffer testing (mass spectrometer method) of the tube-​to-​tubesheet joints. For tubes welded to the tubesheet and subsequently expanded, in addition to such leak testing, the welds should be examined by dye penetrant test or UT or RT. 4.12.7.20 Brazing Method for Tube-​to-​Tubesheet Joints Brazing is used for tube-​to-​tubesheet joints that are free from stress concentration and crevice corrosion problems. For the fabrication of heat exchangers, three methods of brazing are in common use: torch brazing, furnace brazing, and dip brazing. ASME Code Section VIII, Div. 1, prohibits brazed joints for lethal service and for unfired steam boilers.

Fabrication, Brazing, and Soldering of Heat Exchangers

FIGURE 4.77  UT inspection of tube-​to-​tubesheet joint welds.

4.12.7.21 Heat Treatment Heat treatment of fixed tubesheet exchangers may be done by either of these methods: 1. with tubes welded in one tubesheet and left free in the holes of other tubesheet 2. both ends of the tubes welded with tubesheets. Salient features of these methods are discussed next.

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FIGURE 4.78  Radiographic inspection of tube-​to-​tubesheet weld joint by Microfocus rod anode x-​rays set. (a) Front end, (b) rear side fillet weld, and (c) spigot weld.

FIGURE 4.79  (a and b) Rolled tube-​to-​tubesheet joint testing vacuum gun. (Courtesy of Maus Italia F. Agostino & C.s.a.s., Bagnolo Cremasco (Cr), Italy.)

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4.12.7.22 With Tubes Welded in One Tubesheet and Left Free in the Other Tubesheet The procedure adopted is as follows: 1. After welding the tubes on one tubesheet, stress relieve the joints by leaving the tubes free in the second tubesheet. 2. Weld the tubes to the second tubesheet and stress relieve the second tubesheet by placing only part of the exchanger in the furnace. 4.12.7.23 Both Ends of the Tubes Welded with Tubesheets During heat treatment, the rate of heating and cooling should not exceed 30°C per hour. The temperature gradient between the outer skin of the shell and the innermost tube should be narrowed down to an acceptable limit by soaking the exchanger during the heating stages. 4.12.7.24 Heat Treatment: General Requirements Prior to PWHT, the exchanger should be thoroughly examined by visual inspection and NDT methods to avoid welding repair after completion of heat treatment. Consider the following points while heat treating heat exchangers: 1. The complete exchanger should be thoroughly cleaned before charging into the furnace. 2. Exchangers must be heat treated in a furnace that is heated electrically or by gas burners. 3. Sufficient numbers of thermocouples should be provided on the exchanger to verify uniform heating; their locations are also important. 4. Rate of heating, holding temperature, and rate of cooling are generally governed by material used and the construction code. 5. Ensure that the tubes are adequately supported to prevent sagging of the tube bundle. Similarly, during PWHT of the exchanger, it may be necessary to support the shell, if its supports are widely spaced and sagging is likely. 6. Heat treatment should always be done before hydrostatic test. 7. After heat treatment, the tubesheet and tubes shall be examined to ensure that its surfaces are free from scale, oxidation, decarburization, fine cracks, distortion, dimensional accuracy, etc. Details on QC during PWHT of welded components are discussed in Chapter 2 on material selection and fabrication.

4.13 ASSEMBLY OF CHANNELS/​END CLOSURES WITH SHELL ASSEMBLY Where a fully pressed end is required, the end closure is purchased from the trade as a pressing or can be fabricated from a series of petals on the shop floor. (Forming of various types of heads are discussed later.) Assemble the channels/​end closures with the shell assembly and measure the flatness. Measure tubesheet flatness across the diameter of the gasket ring for conformity with the specifications. Inspect fixed tubesheets extended as flanges for deformation due to weld shrinkage.

4.13.1 Bolt Tightening After the heat exchanger assembly, tighten the bolts in a controlled sequence crisscross pattern similar to that shown in Figure 4.80 except for special high-​pressure closures when the instructions of the manufacturer should be followed. It is important that all the flange bolts carry approximately equal load. For exchangers in critical services where it is essential to avoid leakage, use the ultrasonic bolt tensioning devices to ensure that the bolts are tightened uniformly [43].

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FIGURE 4.80  (a and b) Two types of bolt tightening sequence.

4.13.2 Hydrostatic Testing The hydrostatic test shall be carried out after PWHT and complete assembly and all NDT performed, prior to applying painting, insulation, coating, and packing, under the supervision of a QC engineer and/​or third-​party inspection, by observing the state and local pressure vessel code. Care has to be taken for safe supporting and complete venting of the pressure vessel. Welded joints are to be cleaned prior to testing the exchanger to permit proper inspection during the test. Exchangers which are to be stacked in service shall be stacked in the shop and pressure tested as a combined equipment. Shellside Hydrostatic Testing. After manufacturing completion, the heat exchanger shellside will be subjected to the hydrostatic testing. The test pressure amount shall be consistent with the value indicated in the approved drawing. The holding time shall be based on the supplier approved test procedure. The pressure gages calibration and range shall be controlled. Tubeside Hydrostatic Testing. After successful completion of shellside hydrostatic testing, the bonnets are assembled, and tubeside is subjected to hydrostatic testing. Similarly, the amount of test pressure shall be as the one indicated in the approved drawing. Other requirement is similar to the shell test. No leakage and pressure drop shall be observed while the tube side is under pressure. 4.13.2.1 TEMA Standard Requirement RCB-​1.3 For the shell and tube heat exchanger built to the TEMA Standard, as per paragraph RCB 1.3.1, the exchanger shall be hydrostatically tested with water. The minimum hydrostatic test pressure at room temperature shall be as per code. The test pressure shall be held for at least 30 min. The shellside and the tubeside are to be tested separately in such a manner that leaks at the tube-​to-​tubesheet joints can be detected from one side. When the tubeside design pressure is higher than the shellside pressure, the tube bundle shall be tested outside of the shell only if specified by the purchaser and the construction permits. In case of stacked assembly, these tests will be done in the stacked condition. 4.13.2.2 Standard Test The exchanger shall be hydrostatically tested with water. The test pressure shall be held for at least 30 minutes. The shellside and the tube side are to be tested separately in such a manner that leaks at the tube joints can be detected from at least one side. Welded joints are to be sufficiently cleaned prior to testing the exchanger to permit proper inspection during the test. The minimum hydrostatic

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test pressure and temperature shall be in accordance with the code. Liquids other than water may be used as a testing medium if agreed upon between the purchaser and the manufacturer. 4.13.2.3 Pneumatic Test As per RCB-​1.3.2, when liquid cannot be tolerated as a test medium the exchanger may be given a pneumatic test in accordance with the code. It must be recognized that air or gas is hazardous when used as a pressure testing medium. The pneumatic test pressure and temperature shall be in accordance with the code. 4.13.2.4 Hydrostatic Test Fluid The hydrostatic pressure test is in general carried out with drinking-​quality water free from any corrosive and suspended substances, especially, chlorides and microorganisms. Do not test austenitic stainless steel exchangers with water that has a chloride ion concentration high enough to cause stress corrosion cracking. Because most potable water supplies are chlorinated, it is necessary to check the chloride levels. The chloride level in testing water is limited to 25 ppm for austenitic stainless steel types 304, 310, and 321, Incoloy 800, and for aluminum, and 100 ppm for Cr-​Ni-​Mo austenitic steel types 316 and 317. Alternatively, use conditioned demineralized or distilled water for hydrostatic testing. 4.13.2.5 Use of Fluorescent or Visible Tracer Dyes in Hydrostatic Test Fluids Fluorescing dye indicators can be added to the water used in hydrostatic pressure tests for improving the visibility for locating leaks. 4.13.2.6 Cyclic Hydrostatic Testing of Feedwater Heater When the tubeside design pressure is very high in the order of 1500 lb/​in.2 and above as in the case of closed feedwater heater, it is suggested for cyclic testing. High-​pressure feedwater heater requires to bring the tubeside to the hydrostatic test pressure followed by dropping the pressure to atmospheric pressure for each cycle, and the cycle is to be repeated for ten cycles. Such testing has beneficial effects on the structure in addition to possibly opening subsurface porosity bubbles and disclosing cracks in the welds [27]. 4.13.2.7 Improved Method for Hydrostatic Testing of Welded Tube-​to-​Tubesheet Joint of Feedwater Heaters The integrity of tube-​to-​tubesheet joints is judged by hydrostatic tests and leak testing techniques. The basic limitations in these tests are that they reveal only those defects that provide, at the time of testing, a flow path completely through the weld. Experience with carbon steel tubesheet welds has shown that the leaks that can occur after service operation give no indication of leakage during shop floor leak testing or hydrostatic testing [42]. One method that has been successful in detecting subsurface defects involves cycling the heat exchanger to hydrostatic test pressure a number of times and providing a thermal shock to the tubesheet weld surface in the shop floor. The high tube-​to-​ tubesheet joint stresses due to pressure cycling have been particularly successful in completing the potential leakage paths that require a short propagation distance. 4.13.2.8 HydroProof™ HydroProof is the easiest, most advanced system for the hydrostatic testing of individual tube-​to-​ tubesheet joints for boilers, condensers, feedwater heaters, heat exchangers, and air heaters. Using water under pressure, this lightweight and durable tool can hydrotest up to 2000 psi for sizes from 5/​8″ to 2.5″ OD. With the addition of the full tube length HydroProof, it is now possible to test the integrity of a tube along its entire length, with the same ease of use and setup offered by the standard HydroProof. HydroProof unit is shown in Figure 4.81.

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Fabrication, Brazing, and Soldering of Heat Exchangers

FIGURE 4.81  HydroProof for the hydrostatic testing of individual tube-​to-​tubesheet joint. (Courtesy of HydroPro, Inc., San Jose, CA.)

4.13.2.9 Pneumatic Tests As per ASME Code paragraph UG-​100, vessels that cannot be filled with water or cannot be readily dried and where traces of water are not tolerated can be tested pneumatically at 1.25 times the MAWP to be stamped on the vessel multiplied by the ratio of the stress values of the test temperature of the vessel to the stress values of the design temperature, except where other code requirements govern however the maximum test pressure shall not exceed 150 psig. Additionally, pneumatic test shall be carried out for the following: 1. To check the leak tightness of tube-​to-​tubesheet joint, pneumatic test shall be conducted from shellside. Test pressure shall as per applicable code. 2. Reinforcement pads, slip-​on flanges, and linings at a pressure of 2 bar(g). 4.13.2.10 Pneumatic Testing Procedure Pneumatic testing is hazardous and should be used with caution under the following conditions: 1. Test pressure shall not exceed 1.25 times the design pressure as per ASME Code Section VIII, Div. 1 however the maximum test pressure shall not exceed 150 psig. 2. The pressure shall be increased gradually and in increments of 10 psig, which enables detection of any major leaks in the system. The pressure should be held for 10 min. 3. A safety valve should be installed to prevent over pressurization. 4.13.2.11 Stamping The vessel shall be stamped as follows: Serial no. Shellside design Pr Shellside design temp. Shellside hydrotest Pr. Inspection by (Inspector Name/​Inspection Agency) Hydrotested on (Date)

Tubeside design Pr. Tubeside design temp. Tubeside hydro test Pr.

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4.14 PREPARATION OF HEAT EXCHANGERS FOR SHIPMENT During plant manufacture, storage, transport to site, and site erection, special precautions are to be taken to ensure that all the components remain clean and reasonably protected. 1. Internal and external surfaces are to be free from loose scale and other foreign matter that are readily removable. 2. Oil, water, or other liquids used for cleaning and hydrostatic testing should be drained before shipment. The shellside may be dried out by vacuum pump and the tubeside by blowing through with hot air. 3. All exposed machined contact surfaces shall be coated with a removable rust preventive and protected by suitable cover against damage. Rust-​preventive compounds must be regarded as contaminants to be removed before the heat exchanger is put into service [51]. 4. All threaded connections are to be suitably plugged. 5. The exchanger and any spare parts are to be suitably protected to prevent damage during shipment. 6. All telltale holes shall be packed with hard grease. In the event of alloy material, the grease shall be low lead and should not contaminate the material. 7. Tie-​rods or tie-​bars installed on shell expansion joints for protection during shipping shall be painted in a contrasting color and clearly tagged to specify their removal before commissioning. 8. The purchaser shall specify if inert gas (e.g. nitrogen, argon) purge and fill is required. Positive pressure shall be indicated by a pressure gage. Gages shall be suitably protected from damage during transportation. The purchaser shall maintain the positive pressure of the inert gas during storage. 9. When an inert gas fill is used, the vendor shall apply a label or wired metal tag on all openings that states, “Contents are under pressure and must be depressurized before opening.”

4.14.1 Other Protection Considerations 1. If water residues cannot be tolerated, equipment should be dried by one of the following methods. a. Blowing dry air or nitrogen, of relative humidity less than 15% (usually dehumidified), through the heat exchanger and monitoring the outlet air until the relative humidity falls below 30%. b. Evacuating the heat exchanger with a vacuum pump to an absolute pressure of between 0.4 kPa (0.06 psi) and0.5 kPa (0.075 psi). 2. After draining and drying, internal surfaces can be protected against corrosion by the addition of a desiccant (e.g. silica gel), by the addition of a volatile corrosion inhibitor, or by blanketing with an inert gas such as nitrogen [typically at gauge pressures up to 100 kPa (15 psi)]. TEMA guidelines for the preparation of heat exchangers for shipment are discussed next.

4.14.2 TEMA Guidelines G-​6 –​Preparation of Heat Exchangers for Shipment 1. Cleaning. Internal and external surface are to be free from loose scale and other foreign material that is readily removable by hand or power brushing. 2. Draining. Water or other liquids used for cleaning or hydrostatic testing are to be drained from all units before shipment. 3. Flange Protection. All exposed machined contact surfaces shall be coated with a removable rust preventative and protected against mechanical damage by suitable covers. 4. Threaded connection protection. All threaded connections are to be suitably plugged.

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5. Damage protection. The exchanger and any spare parts are to be suitably protected to prevent damage during shipment. 6. Expansion joint protection. External thin walled expansion below shall be equipped with a protective cover which does not restrain movement.

4.14.3 Painting External surfaces shall be painted as per drawings/​specifications. Use paint that will withstand the surface temperature while in operation. Stainless steels do not require painting.

4.14.4 Nitrogen Filling If desired by the purchaser, the heat exchanger internals may be filled with dry nitrogen to protect the internal parts against corrosion.

4.15 MAKING UP CERTIFICATES Final Documentation. Manufacturers compile the documentation folder for a vessel after that vessel has successfully passed its visual/​dimensional inspection and hydrostatic test. Vendor shall complete requisite number of copies of final document folder as required in purchase order. This folder shall contain the following information duly certified by Inspector: 1. Vendor’s Code Certificate. 2. Sketches of Exchanger showing “As Built” dimensions and the plates used with their cast and test numbers. 3. Vessel detail drawing together with a materials list. 4. Material test and analysis certificates. 5. Welding procedure specification and procedure qualification records. 6. Welder qualification reports. 7. Radiographic results. 8. Ultrasonic, Magnetic particle, Dye-​penetrant test results (if applicable). 9. Leak test records. 10. Hydrostatic test report. 11. Charts showing complete heat treatment cycle (if applicable). 12. Production test coupon results (if applicable). 13. Charpy V-​notch test results (if applicable). 14. Job inspection report. 15. Stage wise inspection chart. 16. Final inspection report. 17. Guarantee certificate.

4.16 FOUNDATION LOADING DIAGRAMS/DRAWINGS Foundation loading diagrams/​drawings shall show the following: 1. Empty weight, operating weight including all possible loads and weight full of the possible content of the equipment. 2. Forces and moments due to seismic and wind loads (if applicable). 3. Dimensions of base/​support plate and sliding plate (if applicable) complete with diameter, number and location of hold down bolts or anchors, and thickness of metal through which bolts must pass through.

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4.16.1 Schematics or Flow Diagrams Where a vendor supplies equipment which includes any instrumentation, instrument connections, valves, safety devices, or control schemes, these components shall be shown on a schematic diagram identifying vendor and type and listing respective operating conditions or settings.

4.16.2 Installation, Maintenance, and Operating Instructions When specified on “Vendor Data Requirements” form, the instructions shall be in a completely self contained manual including the following as a minimum: 1. equipment description with outline, cross section and details 2. installation instructions 3. pre-​commissioning check list 4. details of periodical maintenance and repair procedure.

4.17 HEADS AND CLOSURES 4.17.1 Pressure Vessel Heads Types of Pressure Vessels Heads. There are four basic types of pressure vessels heads or dish ends is most widely used in fabrication of pressure vessels. 1. flat heads 2. ellipsoidal heads 3. toripherical heads or flange and dished heads 4. hemipherical heads. Closures for heat exchangers and pressure vessels are either in the form of flat cover or formed head. In contrast to a flat cover which resists pressure only by bending, a formed head resists pressure primarily by developing membrane (in-​plane) stresses. Therefore, the thickness of a formed head can be less than that of a flat cover. The most common variety is the so-​called “torispherical” head which is characterized by four geometric quantities: head inside diameter Di, crown radius L; knuckle radius r; and head thickness th. Major categories of pressure vessel heads are shown in Figure 4.82 4.17.2  Flat Heads These heads have flat surfaces, which makes them ideal for applications that demand flat inside surfaces (4.82a). The required thickness of flat head, t, is given by t = 0.4D



P + C (4.7) σ

where P is the design pressure D internal diameter σ is allowable tensile stress C is corrosion allowance. 4.17.3  ASME Flanged and Dished (ASME F&D) Heads As the name suggests, these heads are ideal for pressure vessels designed for moderate pressures. The F&D heads are shallower than 2:1 heads, which means they need not be dished as much and as deep as a 2:1 head.

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FIGURE 4.82  Major categories of pressure vessel heads single piece vs multi pieces fabrication.

4.17.4  Semi-​elliptical (SE) Heads This elliptical shape head is designed in a 2:1 ratio, and it is also known as 2:1 head. In this head, the width to the depth ratio is 2:1. It means, the head depth is four times the head width. SE head features a half ellipse, so its head depth is usually quarter of its diameter. This head shape is economical,

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and is perfect for high pressures owing to its height to weight ratio. Semi-​elliptical head has radius varying between the minor and major axis, in the ratio of 2:1.

4.17.5 Torispherical Head For torispherical (ASME flanged and dished heads), L =​Di and r =​0.06Di. These heads have a dish with a fixed radius and a transition between the cylinder and the dish, called the “knuckle” with a toroidal shape. These heads have a dish with a fixed crown radius (CR), the size of which depends on the type of torispherical head. The transition between the cylinder and the dish is called the knuckle. The knuckle has a toroidal shape. Most common types of torispherical heads: 1. Klöpper head. This is a torispherical head. The dish has a radius that equals the diameter of the cylinder it is attached to. The knuckle has a radius that equals a tenth of the diameter of the cylinder. 2. Korbbogen head. This is a torispherical head also named semi ellipsoidal head (DIN 28013). The radius of the dish is 80% of the diameter of the cylinder (CR =​0.8 × D0.). The radius of the knuckle is (KR =​0.154 × D0).

4.17.6 Hemispherical Head (Hemi) The hemispherical head has a simple radial geometry, the depth of the head is half the diameter and the radius of the head equals the radius of the cylindrical part of the vessel. 4.17.7 Conical With the shape of concentric cone and a toroidal knuckle, the conical head is typically applied as the bottom head of a pressure vessel to facilitate the collection and removal of internal materials and for the connection of two-​stage vessels of varying diameters.

4.18 HEADS AND CLOSURES FORMING METHODS For thin plates, the cheapest method is by cold spinning and hot spinning of greater thickness. For still greater thickness, a forming press is used. Where the dimensions of the head are such that a single forming operation is not possible, several plates are pressed to the required contour and then welded together to a cap. This is known as the crown-​and-​segment technique. Forming heads and quality control and inspection is discussed in details in Ref. [8].

4.18.1 Flat Heads Blank diameter, Db, for the pressure vessel head flat can be determined from the welding engineer data sheet nomograph [52] or from the formula given therein:

D b = 2 RH s + 1.33R 2 (4.8)

where R is internal radius (D/​2) Hs is the straight portion of the head. Flat heads are usually made by forging; it is a bought out item and is subjected to raw material inspection at the receiving department. The main dimensions that are to be checked are the diameter

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FIGURE 4.83  Dimensional check of flat head.

and thickness of the flat head. The diameter is measured at least at 0, 45, 90, and 135o (in four directions) as shown in Figure 4.83.

4.18.2 Spinning Spinning of ends is applied to a wide range of materials. The forming is done by either hot or cold spinning. Salient features of spinning of various materials are discussed by Peacock [10] and include the following: 1. Mild steel ends up to 1/8 in. (3.2 mm) thick are cold spun with intermediate anneals, and ends greater than this thickness are hot spun at 1150°C. 2. Low-​alloy steels are successfully hot spun. 3. For stainless steels with ends up to 5/8 in. (4.9 mm) thickness, cold spinning with intermediate annealing at 1050°C is preferred. Ends greater than this thickness are satisfactorily hot spun in the temperature range 900°C–​1200°C; due care is to be taken to avoid carbide precipitation. 4. Ends can be made from explosion-​clad plates by conventional hot or cold forming techniques. For more details on forming of clad plates, refer to the section on cladding in Chapter 2. The spun heads will have a constant thickness in the hoop direction but will vary in the meridional direction. The thickness will be a maximum at the crown and a minimum in the knuckle region. The amount of thinning in the knuckle can be as high as 20%–​30%. Under internal pressure, the direct hoop stresses are compressive and hence the heads are susceptible to buckling [53].

4.18.3 Dished Heads Dished heads are made by cold or hot pressing and in combination process of pressing and spinning. Heads of comparatively smaller size and lower thickness are made either by cold pressing or hot pressing, whereas larger sized heads are made by combination process of both pressing and subsequent spinning. For spinning the dished heads, a hole of approximately 7/​8″ to 1 1/​8″ in diameter has to be provided at the center of the blank for holding during spinning. In case any nozzles are to be attached at the center of the head, the opening is enlarged to fit the nozzle. If not, the hole

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FIGURE 4.84  Dimensional check of dished head.

has to be plugged, either by welding alone, or by welding in a plug whose thickness will match the thickness of the head at that place. The dimensions to be checked for dished heads are shown in Figure 4.84 [8].

4.18.4 Pressing Pressing, either hot or cold, is usually employed for the smaller diameter ends and is not as flexible in operation as spinning, because slight changes in end dimension require an alteration to the ring and pressing die. In cold pressing, due to effect of work hardening, the chances for surface cracks on the outer surface is very high and hence calls for liquid penetrant testing (LPT) during examination of surfaces. In hot pressing, utmost care shall be taken to see that the heating, the maintenance of heat during the process, and the subsequent cooling shall be strictly as per the procedure, as they have a great bearing on the grain structure of the material. Hence, the majority of ends are hot pressed for two main reasons [10]: (1) the metal to be pressed undergoes less work hardening and is less prone to cracking, and (2) the load needed to press the material to the required dimensions is less. This process is usually done in stages and it is a comparatively slow process. Shapes thus produced can have local deformation and hence the process calls for thorough inspection. For forming of heads by pressing, two major kinds of equipment are required. They are (1) the cold forming and dishing press and (2) the flanging and knuckling machine [54]. The manufacturing process of heat exchanger heads is shown in Table 4.5. Figures 4.85 and 4.86 show hot forming of heads, Figure 4.87 shows cold pressing of a head, and Figure 4.88 shows flanging of a head.

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TABLE 4.5 Sequence of Manufacturing of Vessel Heads 1. Blank cutting 2. Edge preparation and welding of blanks (for multipiece construction) 3. Preheating of blank 4. Cold or hot pressing by means of deep drawing 5. Heat treatment 6. Edge preparation 7. Mechanical testing (hardness, tensile test, impact test) and nondestructive examinations (PT, MT, UT, RT, PMI) 8. Descaling (pickling and shot blasting) and polishing 9. Dimensional control and visual inspection 10. Dispatch Source:  König +​Co., GmbH, Netphen, Germany, www. Koenig-​co.de.

FIGURE 4.85  Hot forming of a head –​hot spinning of an elliptical head, Øi4500 × 80 +​4 made of SA-​516 Gr. 70 +​316L. (Courtesy of König +​Co., GmbH, Netphen, Germany.)

4.18.5 Crown-​and-​Segment (C and S) Technique Large hemispherical heads and torispherical heads are fabricated by the crown-​and-​segment (C and S) technique as shown in Figure 4.89a. Hemispherical vessel heads are fabricated from a series of hemispherical orange-​peel sections and one dish-​shaped section to form a hemisphere, whereas the torisphere is fabricated by welding a spherical cap to a toroidal portion made of several segments (or gores) that have been welded together. Two methods are followed for making the petals [54]. In one method, the required form is marked around the template on the flat plate; the petal is flame cut and then hot formed. In the more common method, the plate is rolled to the required petal shape, and a template made to the form of the finished petal is used to mark out the lines to be cut. Then flame cutting is carried out manually, and the edges are beveled and examined. All the plates are assembled on a supporting structure and welded. Caps for the hemispheres/​torispheres are welded into position using suitable welding methods. At the various welds, there may be local variations in geometry, although these will be reduced in magnitude by hammering and spinning after the

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FIGURE 4.86  Hot forming of an ellipsoidal head. (Courtesy of Voestalpine Grobblech GmbH, Linz, Austria.)

FIGURE 4.87  Cold forming of a head. (Courtesy of Voestalpine Grobblech GmbH, Linz, Austria.)

FIGURE 4.88  Fabrication of a head –​flanging. (Courtesy of Edmonton Exchanger & Manufacturing Ltd Edmonton, Alberta, Canada.)

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FIGURE 4.89  (a) Multipiece head (fabrication by crown-​and-​segment technique). (b) Multipiece head and cone under fabrication by crown-​and-​segment technique. (Courtesy of Voestalpine Grobblech GmbH, Linz, Austria.)

welding operation. The thickness in the C and S heads will be essentially the same as that of the base plate material [46]. Figures 4.89b and 4.90 show fabrication of heads and cones by C and S technique.

4.18.6 Cones Cones are produced by two methods. When the thickness is comparatively small and the diameter is comparatively large, it can be made in plate bending machine by independently adjusting the pressure rollers at both ends of the bending machine. This work needs expert operators compared to the shell bending. When the thickness of the cone is higher and the diameter is smaller, cones are made by pressing. The pressing is carried out using matching male and female dies either in full length or in pieces depending on the machine capacity. The dimensions are checked as shown in Figure 4.91 [8]:

4.18.7 PWHT of Dished Ends Dished ends shall be stress relieved if cold formed, and normalized if hot formed. A stress-​relieving/​ normalizing chart duly certified by an inspection agency shall be furnished. For hot-​formed dished ends, mechanical properties shall be proved on a test coupon after subjecting the coupon to the same heat treatment as of the dished end, including the final normalizing. Problems faced during heat treatment of heads include [55] the following: (1) when the head is quenched, reasonable escaping of gases and steam should be provided to avoid origination of a steam cushion hindering uniform cooling, and (2) the heat treatment is associated with the risk of distortion. A head under heat treatment is shown in Figure 4.92.

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FIGURE 4.90  Crown and petal head fabrication. (a) A petal is under fabrication and (b) a head at finishing stage. (Courtesy of König +​Co., GmbH, Netphen, Germany.)

FIGURE 4.91  Dimensional check of cone.

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FIGURE 4.92  Heat treatment of a head. (Courtesy of Voestalpine Grobblech GmbH, Linz, Austria.)

FIGURE 4.93  Shape control by automatic laser measurement. (Courtesy of Voestalpine Grobblech GmbH, Linz, Austria.)

4.18.8 Dimensional Check of Heads Heads distorted during the operations after pressing out have to be checked for reduction or enlargement of diameter, ovality, and increase in height [55]. Use of laser for checking of dimensions of formed head is shown in Figure 4.93.

4.18.9 Purchased End Closures Purchased end closures should be accompanied by the following certificates: 1. mill certificates of raw material and test coupons 2. process of manufacture

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3. raw material plate thickness used for pressing 4. minimum thickness achieved after pressing 5. type of heat treatment carried out 6. stress-​relieving/​normalizing chart (time and temperature chart) 7. as-​built dimensions of dished end 8. mechanical test results on test coupons 9. NDT reports.

4.19 BRAZING 4.19.1 Definition and General Description of Brazing and Soldering Process The commonly used metal joining processes are brazing, welding and soldering. Brazing and soldering are methods for joining two or more pieces of material –​primarily metals. The key difference among these processes is the temperature used to create the joint. Brazing and soldering, in essence, are the same in that they both melt the filler metal (braze or solder) only, not the base materials. The liquid filler metal wets the base materials through capillary action. When the liquid filler metal solidifies, it is bonded to the base materials, creating a joint. What differentiates soldering and brazing is the melting temperature of the filler metal. To be defined as brazing, the braze alloy liquidus temperature must be below the melting point of the base material but above 450ºC (842ºF).

4.19.2 Brazing Most of the PFHEs, some tube-​fin exchangers, especially automobile radiators, micro channel and microgrooved tube fin heat exchangers, and some highly compact metal rotary regenerators are brazed. Brazing joins two similar or dissimilar metals/​nonmetals by heating them in the presence of a filler metal having a liquidus temperature above 840°F (450°C) but below the solidus temperature of the base materials. Heating may be provided by a variety of processes. The molten filler metal distributes itself between the closely fitted surfaces of the joint by capillary action. Base materials suitable for brazing include aluminum, copper, gold, nickel, silver and steel. Primary filler metals used in brazing include aluminum, cobalt, copper, gold, nickel or silver. These primary filler metals are often alloyed with other elements to obtain desirable properties and performance.

4.19.3 Soldering Soldering differs from brazing –​in soldering filler metals have a liquidus temperature below 840°F (450°C). A flux is used in soldering to clean the metal surfaces, allowing easier flow of the liquid filler metal over the base material. Base materials suitable for joining by soldering include brass, copper, iron, gold and silver. Filler metals used in soldering were once lead based, however, owing to regulations, lead-​based solders are increasingly replaced with non-​lead versions, which may consist of antimony, bismuth, copper, indium, tin, or silver. Soldering is widely used for copper tube-​copper fin radiators, in the electronics industry for making electrical connections, such joining copper to printed circuit boards (PCB), plumbers also use the process to join copper pipes, etc. For in depth knowledge on brazing refer to Ref. [56–​64].

4.19.4 Characteristics of Brazing The materials being joined, referred to as the base or parent materials, are positioned so only a small gap separates the pieces. A braze alloy is positioned close to the gap and upon melting, the braze alloy is drawn into the gap by capillary action. Thereby a solid joint is formed upon cooling. Features of brazing include [56],

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1. braze alloy, also called filler or filler metal, has liquidus above 450°C (840°F) 2. typical processing temperatures: 540–​1620°C (1000–​2950°F) 3. forms metallurgical bond with base material by dissolving a very small amount of surface material 4. base metal remains solid 5. performed in air by adding fluxes, or without the use of fluxes in inert, reducing or vacuum atmospheres.

4.19.5 Characteristics of Soldering [56] 1. solder alloy, called solder, has liquidus below 450°C (840°F) 2. forms bond with base material mainly by mechanical adhesion 3. base metal remains solid 4. performed in air or nitrogen using a flux to break down surface oxides. There are different brazing and soldering methods. All these methods are discussed next. The objective of this section is to provide comprehensive details on the fundamentals of brazing, brazing processes, and brazing of important heat exchanger materials including aluminum, stainless steel, and nickel.

4.19.6 Brazing Advantages Strong, uniform, leak-​proof joints are made rapidly, inexpensively, and even several joints simultaneously. It is unrivaled for assembling thin or delicate components or assemblies that are being produced in large quantities with the facility to produce many joints simultaneously [57–​59]. Precise joining is comparatively easy without the application of intense local heat to small areas. There is no heat-​affected zone in brazing. Brazing requires little operator skill. Since the base materials do not melt at the brazing temperature, practically any two materials can be joined together in spite of a difference in composition, melting point, or thermal expansion. This includes similar and dissimilar metals. However, not all combinations of dissimilar metals can be brazed.

4.19.7 Disadvantages of Brazing The brazing process is often considered an art today Shah [60]. According to Shah [60], brazing requires considerable expenditure and capital cost, as well as development time, before ideal brazed joints can be manufactured, particularly for complicated assemblies. If there is any change in any one of the brazing process variables, such as flux, filler metals, temperature, brazing atmosphere including vacuum, or equipment and fixture, in general one needs to redefine the brazing process to obtain ideal joints.

4.19.8 Brazing Codes and Standards A list of some AWS Brazing and Soldering documents are shown in Table 4.6. 4.19.8.1 ASME Code Section IX, Welding, Brazing, and Fusing Qualifications Section IX is a “service code” to other BPVC Sections, providing requirements relating to the qualification of welding, brazing, and fusing procedures. It also covers rules relating to the qualification and requalification of welders, brazers, and welding and brazing operators in order that they may perform welding or brazing in component manufacture. Welding, brazing and fusing data cover essential and nonessential variables specific to the welding, brazing or fusing process used.

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TABLE 4.6 AWS Brazing and Soldering Documents/​Specification C3.2M/​C3.2:2019 C3.3:2008 (R2016)

Standard Method for Evaluating the Strength of Brazed Joints Recommended Practices for the Design, Manufacture, and Examination of Critical Brazed Components C3.4M/​C3.4:2016 Specification for Torch Brazing C3.5M/​C3.5:2016 Specification for Induction Brazing C3.6M/​C3.6:2016-​AMD1 Specification for Furnace Brazing C3.7M/​C3.7:2011 (R2022) Specification for Aluminum Brazing C3.8M/​C3.8:2020 Specification for the Ultrasonic Pulse-​Echo Examination of Brazed Joints C3.9M/​C3.9:2020 Specification for Resistance Brazing C3.11M/​C3.11:2011 Specification for Torch Soldering C3.12M/​C3.12:2017 Specification for Furnace Soldering C3.14M/​C3.14:2020 Standard Method for Evaluation of Brazed Joints Using Visual and Metallographic Techniques BRH, 5th Edition 2007 Brazing Handbook SHB, 3rd Edition2000 Soldering Handbook Guideline for Hand Soldering Practices

4.19.8.2 AWS A2.4:2020 –​Standard Symbols for Welding, Brazing, and Nondestructive Examination This standard presents a system for indicating welding, brazing, and nondestructive examination requirements.

4.20 ELEMENTS OF BRAZING The proper use of brazing for a given application requires that due consideration be given to several factors: 1. joint design 2. filler metal selection 3. pre-​cleaning and surface preparation 4. fluxing 5. fixturing 6. heating method/​heat sources 7. post-​braze treatment and removing flux residues. These factors are discussed in detail next.

4.20.1 Joint Design Joint design should consider the following factors: 1. Joint clearance. Joint clearance determines capillary force. The capillary force draws the molten filler metal deeply into every joint clearance. The joint clearance must be within specified limits. Suggested joint clearances as per joint width for various brazing processes are tabulated in Ref. [58]. 2. Avoid flux entrapment. 3. When joining different metals, their differing coefficients of thermal expansion become vitally important.

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4. When brazing foil is used, the mating surfaces should be held in contact. The foil thickness will then determine the joint clearance. 5. The brazing symbol on the engineering drawing designates the location, class, and configuration of the brazed joint. 5.1 Symbols shall be as per ANSI/​AWS A2.4, Standard Symbols for Welding, Brazing, and Nondestructive Examination.

4.20.2 Joint Gap The joint gap is of great importance for establishing a high-​strength joint. Generally, joint strength will increase with decreased joint gap. The correct dimensioning of joint gaps (gap width) and cleanliness of the brazing surfaces are crucial factors for determining the capillary attraction. Excessively large gaps and improperly cleaned surfaces reduce the capillary pressure and lead to brazing gaps being only partially filled. The high capillary pressure in gaps smaller than 0.05 mm is exploited for brazing operations in protective gas atmospheres or vacuum. For mechanized brazing with fluxes, the gap range is between 0.05 and 0.2 mm. Up to a gap size of 0.2 mm, the capillary attraction is enough to assure adequate penetration and filling of the gap with filler metal. Wider gaps are difficult to fill and are therefore not suited for mechanized brazing. The range up to 0.5 mm is still suited for manual brazing. For gap widths exceeding 0.5 mm, the low level of capillary pressure prevents reliable and uniform filling of brazing gaps with filler metal.

4.20.3 Joint Types In common with all forms of brazing, lap joints should be used in preference to butt joints. Joint types can be butt, lap, and T. A joint’s strength depends largely on the bonding area, which can be thought of as the overlap zone where two surfaces of base material rest against each other. The greater the bonding area, the higher the strength. Figure 4.94 shows types of brazing joint.

4.20.4 Brazing Alloy or Filler Metals The brazing alloy, often called filler metal, is the material used to bridge the gap between the two pieces being joined. Forming an effective bond requires the braze alloy to wet, spread, and be drawn in to the joint where it metallurgically bonds with the mating surfaces. If some of the braze alloy is soluble in or reacts with some constituent of the parent material, it is called alloying. The molten braze alloy may dissolve the base metal, forming an effective bond. Brazing filler is a nonferrous metal or alloy that melts at a temperature lower than that of the melting point of the base metal. Most filler metals are alloys, so they melt through a range of temperatures. When molten, the filler metal must wet the base metals, flow, and spread into joints to form brazed joints. The desired characteristics of molten filler metal are a low contact angle as shown in Figure 4.95, high liquid surface tension, and low viscosity.

FIGURE 4.94  Types of brazing joint.

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FIGURE 4.95  Molten filler metal contact angle.

4.20.5 Capillary Attraction and Joint Clearance Braze alloys come in a wide variety of forms, including powder, foil, paste, and solid metal pre-​ forms. Pre-​forms can be put into place as part of the assembly process and may allow complete assembly of a component prior to brazing. Pre-​forms may offer a cleaner application and be less labor-​intensive than pastes, but they can be difficult to place correctly in the assembly. The braze alloy must be fully liquid at the brazing temperature to flow and enter the joint through capillary action. Brazing temperature therefore, as a general guideline, should be 20–​30°C (68–​86°F) higher than the braze alloy liquidus.

4.20.6 Selection of Filler and Flux Typical considerations when choosing a filler and flux material are [57, 65]: • • • • •

corrosion and environmental issues melting range and thermal effect on material and process mechanical strength of brazed joint joint gap size and geometry (cost).

4.20.7 Composition of Filler Metals The American Welding Society lists, in AWS 5.8-​2019, the specifications for brazing filler metal. Conventional fillers that find use in brazing of heat exchangers are in general based on aluminum, copper, nickel, cobalt, silver, and gold. Vacuum-​grade fillers are silver-​based, gold-​palladium, aluminum-​silicon, and copper-​based alloys. The selection of filler metals is discussed by Weymuller [66] and Birchfield [67]. 4.20.7.1 Specification for Filler Metals for Brazing AWS A5.8M/​A5.8:2019 –​Specification for Filler Metals for Brazing and Braze Welding AWS A5.8M/​A5.8:2019 prescribes the requirements for the classification of brazing filler metals for brazing and braze welding. 4.20.7.2 Aluminum Filler Metals Aluminum filler metals (BA1Si series) are used for brazing aluminum. To reduce the possibility of galvanic corrosion, brazing filler metals are aluminum alloys rather than dissimilar metals [68]. Most of the brazing alloys are based in the aluminum-​silicon eutectic system, containing between 7% and 12% silicon with a melting point of 1070°F (577°C). Occasionally, other elements are added. They are available as filler metal or as clad brazing sheet. They are used to join the wrought

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FIGURE 4.96  Maximum permissible brazing temperatures for various aluminum alloys.

grades such as 1100, 3003, 3004, 3005, 5005, 5050, 5052, 6053, 6061, 6062, 6063, 6951, and 7005 [66, 69]. Figure 4.96 shows maximum permissible brazing temperatures for various aluminum alloys [58]. 4.20.7.3 Cladding Alloys The cladding alloy (filler metal) is the thin layer of the Al-​Si alloy that is bonded to the core alloy. This thin cladding alloy, usually making up 5% to 10% of the total thickness of the brazing sheet, melts and flows during the brazing process, to provide upon cooling a metallic bond between the components. The addition of Si lowers the melting point of Al. Commercial filler metals may contain form 6.8% to 13% Si. Some types of cladding alloys are given below [61]: AA4343: 6.8 to 8.2% Si, Melting point range: 577 to 605°C. This alloy has the lowest Si content and consequently the longest freezing range. It is the least fluid of the filler alloys and the least aggressive at dissolving the core alloy. AA4045: 9.0 to 11.0% Si, Melting point range: 577 to 590°C. This is the most common of the filler alloys. Its properties are between AA4343 and AA4047. AA4047: 11.0 to 13.0% Si, Melting point range: 577 to 580°C. This alloy has the highest fluidity because of its extremely narrow melting range (eutectic composition). AA4047 flows rapidly on melting and is the most aggressive at dissolving the core alloy. Because of these properties, it is not used as a cladding alloy (although it may), but more in flame brazing applications where these properties are in fact desirable. 4.20.7.4 Copper Fillers Copper filler (BCu) metal is used to braze ferrous and nickel-​based alloys. Copper-​phosphorus fillers (BCuP series) apply mostly for joining copper and its alloys and stainless steel. Avoid using these fillers on ferrous alloys, nickel-​based alloys, and copper alloys containing more than 10% nickel [66, 69]. These fillers are relatively low in cost, since they lack silver or gold. Copper-​based filler alloys are not compatible with sulfur-​bearing fuels [66, 70]. Copper-​zinc fillers (BCuZn series) are used to braze steels, stainless steels, copper and its alloys, and nickel-​based alloys.

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TABLE 4.7 Typical Filler Metal Applications Primary Metal in Filler

Base Materials

Aluminum Copper Copper-​phosphorus

Aluminum-​based alloys Copper-​based alloys, ferrous metals, nickel-​based alloys Copper-​based alloys; avoid using on ferrous alloys, nickel-​based alloys, and copper alloys containing more than 10% nickel Steels, stainless steels, copper and its alloys, and nickel-​based alloys SS types 300 and 400, nickel-​based, and cobalt-​based alloys Ferrous; nonferrous except aluminum and magnesium and their alloys, which melt at low temperatures Thin sections of SS, nickel-​based, or cobalt-​based alloys requiring high corrosion and oxidation resistance

Copper-​zinc Nickel Silver Gold

4.20.7.5 Nickel-​based Filler Metals Nickel-​based filler (BNi series) metals are used primarily to braze heat-​and corrosion-​resistant alloys, most commonly nickel-​and cobalt-​based alloys and the AISI 300 and 400 series stainless steels. These filler metals provide joints that have excellent corrosion resistance and high-​temperature strength [71]. 4.20.7.6 Silver-​based Filler Metals Silver-​based brazing alloys (BAg series) find a multitude of uses, because they cover a wide range of brazing temperatures, 1145°F–​1900°F. They are used to join ferrous and nonferrous metals and alloys, except low-​melting metals such as aluminum, magnesium, and their alloys [66]. Their advantages are free flow, relatively low brazing temperature, and that they make ductile and smooth joints [69]. 4.20.7.7 Gold-​based Fillers Gold-​based fillers (BAu series) are preferred for specialized applications such as aerospace heat exchangers to braze stainless steel, nickel, and cobalt-​based alloys that require oxidation and corrosion resistance at elevated temperatures, and compatibility with sulfur-​bearing fuel. These fillers are primarily used in furnace brazing applications. Because gold interacts little with base metals, gold-​based fillers will not alter properties of the base metal [67]. Various filler metals and their applications are given in Table 4.7. 4.20.7.8 Forms of Filler Metal To control the amount of filler metal requirement, the filler metals are available in predetermined amounts and shapes known as preforms. Various preforms are wire, strip, sheet, rod, powder, and shapes that fit specific joints. Preforms suit well with automatic brazing. Aluminum brazing sheet is a standard, commercial product. The constructional features and metallurgy of aluminum brazing sheets are explained next. 4.20.7.9 Placement of Filler Metal Placement of filler metal is an important design factor. Usually, the filler material is preplaced near the joints to be brazed. Either the braze metal is metallurgically clad to a thin flat structural member known as a brazing sheet, sandwiched between the parts to be joined, or base metal is coated with a slurry of braze alloy powder and binder. For a typical joint, placement of filler metal is shown in Figure 4.97.

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Fabrication, Brazing, and Soldering of Heat Exchangers

FIGURE 4.97  Placement of filler metal in a joint.

4.20.7.10 ASME Code Specification for Filler Metals Pressure vessel braze metals conform to ASME Section II, Part C. Tables in Part C identify filler metal specifications that are identical to AWS filler metal specifications. The code addresses fluxes or brazing atmospheres indirectly through its requirements to successfully qualify a brazing procedure.

4.20.8 Pre-​cleaning and Surface Preparation 4.20.8.1 Cleanliness Wetting is negatively affected by the presence of surface oxides and contaminants such as dirt, grease, lubricants and detergents. Poor cleanliness results in lack of wetting, incompletely filled joints, and low joint strength. 4.20.8.2 Pre-​cleaning Clean, oxide-​free surfaces are essential to ensure sound brazed joints. Grease, oil, dirt, marking crayons, and oxides prevent the wetting, uniform flow, and bonding of the brazing filler metal, and they impair fluxing action, resulting in voids and inclusions. Pre-​cleaning of components may be accomplished by degreasing with organic solvents or vapors or by a mild chemical etch, such as dilute caustic soda solution [58]. Cleaning should always be carried out before assembly. Otherwise, if an assembled component is immersed in a chemical cleaner, residues will inevitably be trapped in the joints [72]. Mechanical cleaning may be performed by wire brushing or blasting. Grinding or machining requires the use of chemical cleaning to remove the machining fluids. Chemical cleaning methods will be determined by the type of surface contaminants likely to be found on the surface. Degreasing agents will remove hydrocarbon deposits from cutting oils, while aqueous cleaners work well to remove water-​based machining fluids. Plasma cleaning and blasting with CO2 pellets would not leave any residues if developed to be technically and economically viable cleaning methods. 4.20.8.3 Scale and Oxide Removal Good performance, which characterizes the soundness (leak tightness, joint strength, and fillet shape) and integrity of the brazed joint, is possible only when the surface oxide film on the metal surface is dispersed sufficiently for wetting and flow of filler metal to occur. 4.20.8.4 Chemical Cleaning Scale and oxide removal can be accomplished mechanically or chemically. Descaling can be done with any of the following solutions [69]: 1. acid cleaning 2. acid pickling –​sulfuric, nitric, and hydrochloric acid 3. salt bath pickling. Prior to descaling, degreasing is recommended.

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4.20.8.5 Protection of Precleaned Parts After cleaning, the cleaned parts should be assembled and brazed without delay to avoid oxidation and buildup of contamination. The cleaned surfaces should not be hand touched after cleaning. They should be stored and transported to the braze preparation areas in dry, clean containers, such as plastic bags.

4.20.9 Fluxing Conventional brazing processes performed in air or other oxygen-​bearing atmosphere require fluxing. In other words, vacuum brazing does not require flux, and furnace brazing performed in inert or strongly reducing atmosphere usually does not require flux. Fluxes are necessary to displace the oxide layers at the bonding sites and to allow the wetting by the molten filler metal. Without flux, molten filler forms a ball even though the surface it contacts is clean and the temperature is high. Additionally, the flux protects the bonding sites against reoxidation of the cleaned surface during brazing. However, fluxes are not intended to perform the primary function of removal of oxides, scales, coatings, and surface contaminants. Fluxes come as paste, powder, slurry, or liquid. 4.20.9.1 Wetting and Spreading A braze joint is formed when the braze alloy (filler metal) is drawn into the space between two surfaces that are close and parallel. This movement is via capillary action and occurs in three steps: adhesion, wetting, and spreading of the braze alloy. Good wetting and spreading of the liquid filler metal on the base metal are necessary in brazing. If the conditions within the capillary space of the joint do not promote good wetting, the filler metal is not drawn into the space by capillary attraction. 4.20.9.2 Selection of a Flux Flux selection is influenced by the following factors [69, 71]: base material, filler metal, brazing process, joint configurations, and flux dispensing methods such as brush, dip, syringe, spray, etc. which determine flux forms and ease of cleaning. Typical considerations when choosing a filler and flux material are –​corrosion and environmental issues, melting range and thermal impact on material and process, mechanical strength, joint gap size and geometry, and cost. 4.20.9.3 Composition of the Flux Fluxes contain one or more heavy metal chlorides in addition to active fluoride salts along with other chemicals. Fluxes are generally mixed with water to form a thick slurry, which is applied to the precleaned parts to be brazed. 4.20.9.4 Varieties of Flux [57] 1. NOCOLOK® flux NOCOLOK® flux is a white powder consisting of a mixture of potassiumfluroaluminate salts of the general formula K1-​3 Al‑F4-​6. The flux has a defined melting point range of 565°C to 572°C, below the melting point of the Al-​Si brazing alloy. The flux is noncorrosive and non-​hygroscopic and is only very slightly soluble in water(0.2 % to 0.4 %). The shelf and pot life of the flux is therefore indefinite. The flux does not react with Al at room temperature or at brazing temperature and only becomes reactive when molten. 2. Electrostatic fluxing Dry fluxing is a technology whereby the flux is electrostatically charged and applied to grounded heat exchanger or individual heat exchanger components. The electrostatic attraction causes a layer

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of flux to be deposited on the work piece. Dry fluxing lends itself best to parts with simple geometries [57, 61]. 3. NOCOLOK® Sil flux NOCOLOK® Sil Flux is particularly suitable for condenser manufacturing, this technique uses a mixture of flux and elemental silicon powder, sprayed on the microchannel tubes using a binder/​ carrier. Similar to pre-​fluxing, spraying or coating the tubes is carried out prior to assembly. 4. NOCOLOK® Li flux Controlled Atmosphere Brazing (CAB) with potassium fluoroaluminate fluxes continues to be a worldwide standard technology used for the production of all-​aluminum heat exchangers. It has been demonstrated, by means of corrosion testing methods (e.g. SWAAT, CASS and NSS), that a residual flux layer improves the corrosion resistance when compared with bare parts. The new NOCOLOK® Li Flux shows the same outstanding properties and brazing performance of standard potassium fluoroaluminate flux –​with further enhancement of the post-​braze flux residue characteristics [57]. 5. NOCOLOK® flux paste NOCOLOK® Flux Paste is a flux paste specifically formulated for providing a viscous homogeneous mixture of NOCOLOK® Flux (15–​50 %) in an organic carrier for brazing aluminum cladded interfaces where traditional water based fluxing is not suitable. Typical applications include but not limited to: interior tube seams snap over joints (clinch tubes), tube-​to-​header joints, internal turbulators. The paste can be dispensed manually by brush or syringe or with automatic dispensing equipment. The paste can be used in both furnace and flame braze process [57].

4.20.10 Fluxing Methods Methods of Fluxing are given below [57, 61, 62]: 1. Flux painting Using a flux paint (flux +​carrier +​binder) allows certain heat exchanger components to be pre-​ fluxed and is helpful in the case where pre-​fluxing internal components, baffles, side supports and even radiator headers and condenser manifolds is desired. 2. Dry-​fluxing This technique makes use of powder painting equipment modified to work with the flux properties. As the flux is applied dry, there is no need to mix flux slurries, to measure flux slurry concentration and there is no wastewater. NOCOLOK® Dry-​static flux, with a unique particle size characteristic, was specially developed for this application. Some care is required with the handling of dry-​fluxed components as the pre-​braze flux adhesion is less that of wet fluxing. 3. Pre-​fluxing or binder fluxing The concept here is to pre-​flux certain heat exchanger components such as microchannel tubes, headers and manifolds in a paint-​line like fashion. The flux is mixed with a suitable binder/​carrier and the components are cleaned, sprayed with the flux mixture and dried/​cured.

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4. Preparation of surfaces and joints The successful use of the novel brazing technique requires a uniform coating of Si-​flux powder on the metal surfaces prior to brazing. In the present work, the uniform coating could be deposited from a water-​based slurry after cleaning the surfaces chemically prior to dipping. 4.20.10.1 Demerits of Brazing Using Corrosive Fluxes Though several advantages can be claimed for the brazing techniques using fluxes [73], brazing fluxes are chemically active, and their residues are highly corrosive, especially in the presence of moisture. Therefore, after brazing, flux residuals must be removed by a cleaning process to avoid both corrosion and contamination problems.

4.20.11 Fixturing As far as possible, components to be brazed should be self-​fixturing. Fixtures, being in contact with the part, extend the heating time to bring the components up to the brazing temperature. Slow heating due to attachment of fixturing will change the characteristics of the filler metal [72]. Examples of self-​fixturing construction include aluminum screws, rivets, resistance welds, argon arc tack welds, piercing, and tagging. For external fixtures, austenitic stainless steels or heat-​resisting nickel alloys are recommended. Mild steel can also be used for low-​volume production. Use clean and dry fixtures to avoid pumpdown delays. Steel banding. An alternative to permanent fixtures is the use of disposable steel banding. The steel bands are used only once and are disposed of after brazing [57]. Note: heating method and post-​braze treatment and removing flux residues are discussed later.

4.20.12 Heating Method Brazing, which utilizes a wide variety of heat sources, is often classified by the heating method used. Different heat sources/​heating method can be used for brazing including induction, resistance heaters, ovens and furnaces and torches/​flame. Out of these methods, some methods heat locally (only the joint area), others heat the entire assembly (diffuse heating). These two techniques of heat addition is discussed below [74]. 4.20.12.1 Local Heating 1. Torch Brazing In this method, the heat required to melt and flow filler metal is supplied by a fuel gas flame. The fuel gas can be acetylene, hydrogen, or propane and is combined with oxygen or air to form a flame. This process is readily automated and requires low capital investment. Torch brazing requires the use of a flux, so a post-​braze clean is often required. 2. Induction Brazing Heat is created by a rapidly alternating current which is induced into the workpiece by an adjacent coil. High frequency induction heating for brazing is clean and rapid, giving close control of temperature and location of heat. 3. Resistance Brazing

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This is a process in which heat is generated from resistance to an electrical current (as for induction brazing) flowing in a circuit which includes the workpieces. The process is most applicable to relatively simple joints in metals which have high electrical conductivity. 4.20.12.2 Diffuse Heating Techniques 1. Furnace Brazing Furnace brazing is the process by which metal components are joined using a dissimilar lower melting filler metal. Furnace brazing is accomplished using a variety of techniques. They include different furnace designs, which include batch as well as continuous furnaces. Furnace types used for brazing, include: vacuum, controlled atmosphere, continuous/​mesh belt and reducing atmosphere. Furnace brazing offers two prime advantages: protective atmosphere brazing (where high purity gases or vacuum negate the need for flux) and the ability to control accurately every stage of the heating and cooling cycles. Heating is either through heating elements, or by gas firing. 2. Dip Brazing This involves immersion of the entire assembly into bath of molten braze alloy or molten flux. In both cases the bath temperature is below the solidification point of the parent metal, but above the melting point of the filler metal.

4.20.13 Post-​braze Treatment and Removing Flux Residues 4.20.13.1 Removal or Cleaning of Post-​braze Flux Residue The most common methods for post-​braze flux removal are: soaking/​wetting –​use hot water with agitation in a soak tank to remove excess flux immediately following the braze operation, and then dry the assembly. When soaking is not possible, use a wire brush along with a spray bottle or wet towel. Methods of post-​braze cleaning are hereunder [75]. 4.20.13.2 Mechanical Cleaning Usually, removal of flux residue can only be done by mechanical means. From solid surfaces and from robust joints, as well as from stainless steel fixtures, the flux residues can be mechanically removed by sand or grit blasting. Wire brushing is a second alternative for flux residue removal. 4.20.13.3 Chemical Cleaning A solution of hot boric acid (10 to 15%, 75–​80°C) can be used to remove some of the flux residue from brazed assemblies. Aluminum dissolution by boric acid is relatively moderate. Handling (preparation and usage) and waste disposal of spent chemical solutions can be problematic and expensive –​due to their corrosive properties and the subsequently necessary waste water treatment. 4.20.13.4 Ultrasonic cleaning Ultrasonic treatment may be effective in removing flux residues, provided that the parts to be cleaned fit into the ultrasonic dipping bath. A detergent (cleaning agent) can be added to the solution to improve the cleaning activity.

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4.21 QUALITY CONTROL AND QUALITY ASSURANCE SYSTEM FOR BRAZING OF HEAT EXCHANGERS Designing a quality brazement. The braze joint should be designed as part of the overall component design. The joint configuration plays a critical role in the mechanical and fatigue resistance, corrosion resistance, thermal and electrical properties and physical appearance of the final assembly. Certainly, consideration must be given to cost-​effective manufacture. The principles of QC, quality assurance, inspection, and NDT techniques have been dealt with in Chapter 3. Quality assurance for aluminum brazing is discussed in ANSI/​AWS C3.7-​93, Specification for Aluminum Brazing [76].

4.22 BRAZING METHODS Brazing processes are classified according to the sources or methods of heating. There are numerous brazing methods, including the following: 1. torch brazing 2. dip brazing 3. furnace brazing 4. vacuum and controlled atmosphere brazing (CAB) 5. induction brazing 6. resistance brazing 7. infrared brazing. All these brazing methods include the same brazing procedures. The prime difference lies in the way the parts are heated and the way flux and filler metals are applied. Except for vacuum and controlled atmosphere brazing, all methods require flux. In this section, the first four brazing methods are discussed. To braze, surfaces must be cleaned, free of excess oxide, coated with flux, and spaced a few thousandths of an inch apart. Brazing filler is placed in or near the joint to be formed, assembled, and fixtured. When heat is applied, the flux displaces oxide and shields the metal from air. As the filler flows, it displaces flux and wets the base metal, adapting to submicroscopic irregularities and dissolving the small high points it encounters.

4.22.1 Six Fundamentals of Brazing to Follow There are six fundamentals of brazing that every brazer should follow to ensure consistent and repeatable joint quality, strength, hermeticity, and reliability. Following are the “six rules of brazing” [77]: 1. The provision of a clean surface at the joint interface at brazing temperature. Keep components free from grease, oil, dirt and foreign matter. 2. The need to heat the components of the joint evenly to brazing temperature. Brazer skill is important. Let the braze filler flow through the joint with help of a small temperature increase in one end. 3. The selection of the “right” alloy for the job in question. Avoid cadmium alloys. Minimize zinc content but use as low melting range as possible to avoid thermal impact. 4. The selection of the most appropriate method of removing the oxide skin from the surfaces of the joint. 5. The use of an appropriately dimensioned joint gap. 6. The application of the filler material to the appropriate part of the joint.

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4.22.2 Torch or Flame Brazing Flame brazing implies the use of aflame as the heat source to accomplish what is described above. It is also known as torch brazing. Any joint that can be reached by a torch and brought to brazing temperature can be readily brazed by this technique. Flame brazing lends itself well to joining components with simple configurations such as tube-​to-​tube, tube-​to-​fitting and joints having large thermal mass differences. Handheld manual torch brazing is most frequently used for repairs, one-​of-​a-​kind brazing jobs, short production runs, and as an alternative to gas or arc welding. For example, repairs or low-​production copper-​to-​copper joints in return bends of an evaporator or a condenser of an air conditioner are preformed by torch brazing. Most commercial torch brazing is carried out by use of oxyacetylene flame in which the torch is adjusted to produce slightly reducing flame for most application. In automatic torch brazing, the assembly is moved automatically in relation to the torch, or vice versa. It is used for mass production. Flame brazing lends itself well to joining components with simple configurations such as tube-​to-​tube, tube-​to-​fitting and joints having large thermal mass differences. Since much faster heating rates are possible than in furnace brazing, flame brazing is versatile [78]. 4.22.2.1 Torch Brazing of Aluminum Flame (torch) brazing of aluminum involves locally applied heat typically generated by a slightly reducing oxy-​acetylene, oxy-​hydrogen or oxy-​natural gas flame. As with other aluminum brazing processes, close temperature control is important. Care must be taken to ensure even heat distribution [61, 77, 78]. 4.22.2.2 Process Parameters Apart from the selection of flux and filler material, important process parameters are the cleanliness and proper geometrical alignment of the individual components. Filler metal may be pre-​placed or added during brazing using a brazing rod [61]. Post-​braze cleaning to remove chloride flux residues is required. Provided proper filler metals and fluxes are selected, flame brazing can also be used for brazing aluminum to copper. 4.22.2.3 Consumables –​ Gas Most commercial gas mixtures are acceptable for flame brazing aluminum [78]: • • • •

oxygen-​propane oxygen-​methane oxygen-​natural gas oxygen-​acetylene (oxyacetylene).

Oxyacetylene combination produces the hottest flame and may be used, but with extreme care to avoid overheating and burn-​through. 4.22.2.4 Joint Clearances The recommended gap tolerances for flame brazing range from 0.1 mm to 0.15 mm. Larger gap clearances can be tolerated, but capillary action is reduced, gravity activity is increased and more filler metal may be required. Friction fits should also be avoided as this will restrict filler metal flow and result in discontinuities in the brazed joint area. 4.22.2.5 Equipment for Flame Brazing with Hand-​held Filler or Pre-​placed Filler Recommended equipment for flame brazing [61]:

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1. Hardware torch. It is critical that the joint area is brought up to temperature uniformly. For this reason a dual headed torch capable of heating the joint from two sides is recommended. 2. Torch tip. A multi-​orifice tip generates a broader flame at the exit of the tip. 4.22.2.6 Flame Brazing Process with Hand-​held Filler or Pre-​placed Filler All methods are constituted by following key process steps [78]: 1. cleaning the parts prior to brazing 2. fluxing (when appropriate) and application of filler (if pre-​placed filler) 3. assembling the parts 4. heating for brazing and brazing (applying handheld filler if appropriate) 5. post-​braze cooling 6. post-​braze cleaning. 4.22.2.7 Brazing of Inlet/​Outlet Tubes to End Plate of Brazed Plate Heat Exchangers A brazed plate heat exchanger (BPHE or BHE) consists of stainless steel plates brazed together in a vacuum furnace process with a copper filler. It is common to use either copper based or stainless tubes for connections, i.e. the brazed pipe/​tube joints can be either [77]: • copper to copper • stainless to stainless • stainless to copper. Brazing of inlet/​outlet tubes to end plate of brazed plate heat exchanger is shown in Figure 4.98. 4.22.2.8 Flame Brazing of Aluminum with Copper Flame brazing aluminum to copper is common in the refrigeration industry where copper tubes are brazed to aluminum fins. In flame brazing the inter-​diffusion of copper and aluminum can be halted rapidly by simply removing the heat source –​in this case simply removing the flame is sufficient to allow the joint to cool quickly. During the brazing process, the flame should never be directly applied to the joint, because the heat should be transferred by conduction through the parts to be brazed. As soon as the filler metal begins to melt, the flame must be quickly removed. A second issue with brazing copper to aluminum is that the aluminum has a much lower melting point than copper (Al: app. 650°C; and Cu: above 1000°C). Therefore, the flame is usually directed on the copper [79]. Filler metal in flame brazing of aluminum to copper. Use of Al-​Si filler alloy (Al, 88%–​Si, 12%–​ A A4047). Standard procedure like in flame brazing of aluminum to aluminum –​just a little bit faster to avoid burn-​through. 4.22.2.9 Induction Brazing of Return Bends of Heat Exchanger Coil There are “U” return bends that are brazed to the receiving tubes on the heat exchanger coils. The current process for joining is (1) either flame or torch brazing and (2) induction brazing. Induction brazing is faster, safer, greener and more repeatable than other brazing methods. The reasons for using induction heating are [62, 80–​83]: 1. “Unlike torch brazing, induction brazing is noncontact and minimizes the risk of overheating.” It is also more efficient, with no wasted energy and almost no increase in ambient air temperature. 2. The induction coils will heat only that portion of an assembly that needs to be heated for brazing.

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FIGURE 4.98  Brazing of inlet/​outlet port tubes to end plate of brazed plate heat exchanger.

4.22.2.10 The Induction Brazing Process The standard induction brazing system consists of three components –​the power supply, the workhead with attached induction coil, and a chiller or cooling system. The process sequence of induction brazing process include the following steps: 1. clean the base materials by removing residues, oxides, etc. 2. ensure the correct gap between the parts 3. apply flux to the joint area 4. position, and if necessary clamp the parts to be brazed 5. apply the brazing filler alloy 6. induce the desired heat in the joint area 7. after brazing, remove any remaining oxides or flux residue.

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FIGURE 4.99  Induction brazing of return bends of a heat exchanger coil.

The power supply connects to the workhead, and the coil is custom designed to fit around the joint. Figure 4.99 shows a typical example of an induction heating setup for brazing a heat exchanger coil, in which a small region of a copper-​tube-​to-​fitting assembly is being heated inside a copper coil.

4.22.3 Dip Brazing In the dip brazing process, often referred to as salt-​bath brazing, the part or assembly being joined is held together and immersed in a bath of molten salt, which flows into the joints when the parts reach a temperature approaching that of the bath. The molten flux, at a temperature slightly above the liquidus of the filler metal, serves as a heat source and partially supports the submerged metal parts. In addition to providing the heat for brazing, many of the salts have fluxing properties. Dip brazing has been extensively used in industry for daily production of aluminum and other alloy parts. It has the unique advantages that [58, 60, 61] (1) the time required for heating is about one-​fourth that required for furnace brazing, (2) distortion due to self-​weight is less due to buoyancy, and (3) the parts reach the brazing temperature uniformly. Choice of flux. Commercial dip brazing fluxes are generally similar to fluxes used with other brazing methods. Their main ingredients include the combinations of sodium chloride, potassium chloride, aluminum fluoride, and lithium chloride. Dip brazing requires water-​free flux to avoid spattering of moisture from the bath. Dip bath heating means. Heating may be by gas for lower temperature systems or by one of the two forms of electrical heating: either resistance elements mounted around the pot or electrodes immersed in the molten salt bath.

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TABLE 4.8 Dip and Furnace Brazing Steps Dip Brazing

Furnace Brazing

Pre-​cleaning and surface preparation Preparation of flux bath Assembly and fixturing Preheating Brazing in flux bath Cooling or quenching Hot water washing Flux removal Finishing and inspection

Pre-​cleaning and surface preparation Flux preparation and applicationa Assembly and fixturing Brazing in furnace Cooling or quenching Hot water washinga Flux removala Finishing and inspection

  Not applicable for vacuum brazing.

a

Flux pot. Flux pots comprise steel-​reinforced vessels lined with high-​alumina, acid-​proof fire brick. Pot covers are well insulated. Large covers may be mounted on rollers and power operated. Preheating the assemblies. Dip brazing generally requires preheating. This helps in a flux preplaced joint, dries the flux, and vaporizes moisture from the assembly before being immersed into the salt bath and thereby prevents accidents from steam explosions. It also can reduce brazing time, minimize the salt residue that forms on the part, and reduce distortion, as well as reduce bath size [69]. Preheating is achieved either by the flue gases or by electrical heating in a suitable furnace to a temperature 50°F–​100°F (30°C–​60°C) below the solidus temperature of the brazing filler metal until the entire assembly has attained this temperature. For aluminum brazing, the assembly is preheated to approximately 1000°F (538°C). Dip brazing procedure. Table 4.8 shows comparison of dip brazing process with furnace brazing process. Immersion time. The time needed to form the joints depends on the mass of the assembly and its temperature at the moment it enters the molten salt bath. Dip time varies from as little as a few seconds to as long as 10 or 20 min. There is no formula to arrive at the immersion time, but it is arrived by trial-​and-​error practice [58]. Maintaining the flux bath. A properly maintained flux bath will produce a workpiece that is bright and shiny, with fillets well formed and complete [69]. To achieve these properties, the bath should be relatively free of sludge and surface film, slightly acidic with a pH between 6.4 and 7.0 (for aluminum, a pH between 5.3 and 6.9), and relatively constant chemical composition. The bath is periodically checked for moisture, acidity, chemical composition, temperature, and other physical characteristics by the laboratory and corrective action made as required. Scum and sludge removal. Contaminants in and on top of the hot flux interfere with the quality of brazing. Contaminants that float to the surface are called scum, and they are readily removed by skimming the bath’s surface with a sheet of aluminum. There are a number of commercial preparations that, when added to the bath, cause the particles to coagulate, making their removal easier [58]. Contaminants that sink to the bottom of the pot are called sludge. The sludge is removed periodically by ladling with a perforated tool.

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Ventilation. Salt-​bath dip brazing can emit toxic or noxious gas, fumes, and dust. Ventilation to remove air emissions from above the bath is a must. Limitations of dip brazing. Some of the limitations of dip brazing are as follows: 1. Dip brazing generally requires preheating to prevent accidents from steam explosions. 2. The shape of the part must be designed to avoid trapping air or salt and to be drained completely. 3. Maintenance problem due to power shutdown for internally electric-​heated salt bath. 4. A different salt will probably be required for the next application if either the parent metals or the brazing alloy has been changed; frequent changing is not economical.

4.22.4 Furnace Brazing The term furnace brazing can be applied to any brazing process where a furnace is used as the heat source to raise the parts to be joined to their brazing temperature. Furnace brazing is extensively used in industry for brazing heat exchangers and joining complex parts. It offers uniform heating, and hence accurate temperature control is possible. The finished product has excellent quality. The process is economically attractive for brazing heavy assemblies in which multiple brazed joints are to be formed simultaneously or many assemblies are to be joined simultaneously [56, 61]. 4.22.4.1 Brazing Furnace Selection A thorough knowledge of the product to be brazed, material requirements, production volumes and schedule is vital when a company is procuring a furnace or looking for a commercial brazing company. The types of brazing furnaces are generally classified as continuous, batch or vacuum furnaces. Processing requirements of the base material and the filler alloy with respect to temperature and atmosphere are the most critical aspects of furnace selection [56, 57, 61]. 4.22.4.2 Brazing Furnace Selection: Vacuum vs. Continuous-​belt Those who create a brazing infrastructure, have to take the decision to determine if it is better to purchase a vacuum furnace or a continuous-​belt furnace for the particular brazing needs. This important decision (for any brazing company) involves understanding primarily three key factors about their production [84]: 1. the quantity of brazed components that they need to produced. 2. the sensitivity to oxygen of any of those base metals that they are planning to braze 3. do any of those base metals contain elements that will easily and readily outgas when heated. Furnace heating. The furnaces may be heated by gas, oil, or electricity, and provided with temperature controls. Fluxes or specially controlled atmosphere that performs fluxing functions must be provided. Filler metal must be preplaced in the form of sheet, wire, rings, powder, or other suitable forms. Furnace Atmospheres. Furnace brazing takes place in a vacuum or in a controlled atmosphere of high purity inert or reducing gas. The brazing atmosphere, whether gaseous or vacuum, should be free from harmful constituents such as sulfur, oxygen, and water vapor. When brazing in a gaseous atmosphere, it is a common practice to monitor the water vapor content of the atmosphere as a function of dew point. In general, a furnace atmosphere having a dew point less than about −60°C (−75°F) produces better quality. Many furnaces are equipped with dew point and oxygen

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measurement devices. It is important that the measurements are taken in the critical brazing zone of the furnace because this is where these impurities will reach their lowest concentrations. Brazing furnaces. In general, brazing furnaces are classified as (1) batch furnace, (2) semicontinuous or locked furnace, or (3) continuous furnace. Single-​chambered, batch-​type units are used for low-​ volume production, while multichambered semicontinuous or continuous furnaces are employed for high-​volume production applications. Selection of a particular type of production furnace depends on the size and geometry of the parts to be brazed, as well as production rates required. 4.22.4.3 Fundamentals of Brazing Process Control It is generally accepted that the most important variables involved in the brazing process include the brazing temperature, time at temperature, brazing atmosphere, and the rate and mode of heating. Some of these factors are discussed next. 1. Heating Rate Geometry and the distribution of mass within the part, heating element configuration relative to part location, and vacuum environment characteristics will determine the heating rates needed to realize a good braze. 2. Brazing Temperature Brazing temperature is mostly governed by the base metal characteristics and the type of filler metal. In general, the filler metals with the higher melting points correlate with higher strength joints. Various base metals and their melting temperature range and brazing temperature range are shown in Table 4.9. 3. Brazing Time Brazing time will depend somewhat on the mass/​thickness of the parts and the amount of fixturing necessary to hold them. It has become industry practice to minimize the holding time at brazing temperatures to minimize excessive filler metal flow and/​or erosion, evaporation, and oxidation. Brazing times in the order of less than a minute are common. 4. Temperature Uniformity Producing consistent braze joint quality throughout the part/​furnace load/​temperature uniformity in all of the workpieces depends on the configuration of the work, its placement, how it interrelates with the hot zone, etc.

TABLE 4.9 Base Metals and Their Melting Temperature Range and Brazing Temperature Range Base Metal

Melting Temperature

Brazing Temperature

Aluminum and aluminum Alloys Stainless steels

588°C–​657°C (1090°F–​1215°F) 1370°C–​1532°C (2500°F–​2790°F)

571°C–​621°C (1060°F–​1150°F) 618°C–​1232°C (1145°F–​2250°F)

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FIGURE 4.100  Vacuum-​type batch furnace. (Adapted and modified from [88] Patrick, E.P., Weld. J., October, 159, 1983).

5. Control of Distortion During The Furnace Cycle Control of distortion is vitally important for successful furnace brazing, particularly complex parts. Distortion can result from fast heating, fast cooling, stresses that develop during heating, residual stresses in a workpiece, phase transformation, and properties of dissimilar base materials [85]. According to Tennenhouse [85], distortion can be minimized or eliminated by measures like uniform heating and cooling of the part, reduced and controlled heating rates, use of dummy weights, heat shields and lightened fixtures, and adequate supports to the parts. 4.22.4.4 Batch Furnaces In a batch furnace, every batch or load is processed separately. Depending on how the furnace is loaded, it is called a pit furnace (loaded from the top), a bell furnace (loaded from the bottom), or a box or retort furnace. The furnaces are sealed using either a gasket or seal made of sand or oil to keep the oxygen and moisture levels low. Classification of batch furnaces. Batch furnaces are classified as follows [69, 86]: 1. direct-​combustion furnaces 2. muffle furnaces or hot-​wall furnaces 3. retort-​bell-​type combustion furnaces 4. vacuum brazing furnaces: single-​pumped retort furnaces, double-​pumped retort furnace, batch-​type vacuum furnace (hot or cold wall design). For more details, see Refs. [69, 86]. A batch type vacuum braze furnace is shown in Figure 4.100 4.22.4.5 Brazing Thermal Cycle The main step in the brazing process is the brazing cycle itself. Because of its direct influence on the final product, the time-​temperature cycle has to be carefully adjusted. Equally important are the furnace conditions, i.e. temperature profile, temperature uniformity, and atmospheric conditions [56, 61]. A typical brazing process cycle is pre-​heat (optional) including holding, ramp to temperature, brazing, cool-​down and exit, as illustrated in Figure 4.101. Pre-​heating is done up to a temperature which is below the filler metal solidus. Part thickness and the amount of fixture determine the

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Fabrication, Brazing, and Soldering of Heat Exchangers

FIGURE 4.101  Typical furnace brazing cycle (schematic).

pre-​heat and holding time. Enough time should be allowed for the temperature to stabilize. The parts should heat quickly but uniformly. The heating rate is therefore limited by the risk of creating distortion due to thermal stress build-​up. Brazing time should be the minimum time that allows the braze alloy to flow through the joint at the coldest part of the work piece. Extended processing times can lead to excessive interactions between the filler and base metal, grain growth, and recrystallization. Cool-​down and exit. A controlled cool-​down allows the joint to solidify. Cooling under a protective atmosphere down to a temperature of about 150 °C (300 °F) for steels will avoid discoloration of parts exiting the furnace. 4.22.4.6 Brazing Process Cycle in a Batch Furnace A typical batch furnace cycle would include the following steps [56]: Step 1: the cycle begins by loading and closing the furnace. Step 2: the load is heated to an intermediate temperature to allow any surface contamination to off-​gas and allow the load to come to a uniform temperature. Step 3: if the atmosphere is inert, it may be introduced at this point. If a flammable atmosphere will be used, the chamber may be filled with an inert gas that will later be replaced with the flammable atmosphere. Step 4: the furnace will ramp to temperature, hold at the selected brazing temperature, and then ramp down to cool the load. During the cool-​down, flammable atmospheres must be purged from the furnace for safety. Step 5: the load may be removed when it is below a temperature that will cause discoloration. In a semicontinuous vacuum furnace, the brazing chamber is divided into two or more heating stations so that part movement from station to station brings about the braze. The number of hot zones is determined by the parts to be brazed and the desired production rate. For example, in a three-​zone brazing chamber, the braze is completed in three stages, and three carriers of parts, each at a different heating station, are contained in the brazing chamber environment at all times. Cycle times for a three-​zone chamber are approximately one-​third that for the batch process [87]. A semicontinuous or locked furnace is shown schematically in Figure 4.102. It is developed to avoid contamination while loading or unloading work [88]. The entrance vestibule is generally used to preheat and outgas the parts prior to transfer into the brazing zone. The exit vestibule is used for the initial phase of cooling and filler metal solidification under an inert or dry air environment [87]. For more details on batch-​type and continuous-​type vacuum furnaces, see Ref. [88].

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FIGURE 4.102  Semicontinuous or locked furnace.

4.22.4.7 Continuous Furnaces Continuous brazing is suitable for production with a steady flow of incoming parts. The parts move through the furnace, either as individual components or in trays or baskets. Mesh belt, humpback or pusher furnaces are generally used. To allow rapid heating to temperature, the heat input in the first section of the furnace must be high. A short dwell time at the brazing temperature is followed by a cool-​down, fast enough to avoid formation of precipitates and intermetallics which may cause embrittlement. The cooling section of the furnace must be long enough for the parts to reach a low enough temperature, typically below 150°C (300°F) for steels, to avoid oxidation upon exiting the furnace. Nitrogen is introduced at the entrance and exit to purge the sections. Brazing processes without corrosive flux. Various processes have been developed for reducing or eliminating corrosive flux in the brazing of heat exchangers. Three industrial processes are followed to overcome the requirement of corrosive flux. They are 1. controlled atmosphere brazing 2. use of noncorrosive flux under the trade name Nocolock 3. vacuum brazing. All three methods are commercially used for the manufacture of heat exchangers and high-​volume automotive heat exchangers, such as radiators, air-​conditioning evaporators, heater cores, and turbo air coolers. In general, the fluxless processes require furnace atmosphere having a dew point less than about −60°C (−75°F) to produce better quality. Controlled atmosphere brazing. In controlled atmosphere brazing (excluding a vacuum atmosphere), a continuous flow of a certain gas is maintained in the work zone to avoid contamination from outgassing of the metal parts and dissociation of oxides. Controlled atmospheres for furnace brazing include combusted fuel gas, dissociated ammonia, cryogenic or purified N2 +​H2, deoxygenated and dried hydrogen, and purified inert gas as specified by AWS designations. The controlled atmosphere such as pure dry hydrogen or inert gases inhibits the oxidation and scaling of the surfaces. Inert gases, such as helium and argon, do not form compounds with metals and inhibit vaporization of volatile elements. A dry-​nitrogen atmosphere is excellent for production volume brazing with a fluoride flux. With controlled atmosphere brazing, the need for a flux is eliminated or the flux requirement is considerably reduced.

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Use of noncorrosive flux under the trade name Nocolock. Use of noncorrosive flux under the trade name Nocolock pertains to aluminum brazing. Therefore, it is covered in the next part, while discussing aluminum brazing.

4.22.5 Vacuum Brazing Vacuum brazing is a form of furnace-​brazing process for brazing assemblies in a vacuum environment without the use of flux. Vacuum brazing is being used commercially to produce automotive and aircraft heat exchangers. Large prototype industrial heat exchangers and massive cryogenic heat exchangers have been successfully vacuum brazed. Commercial vacuum brazing generally is done at pressures varying from 10−5 to 10−1 torr, depending on the materials brazed and the filler metals being used. A vacuum braze furnace is shown in Figure 4.100. Parent metals brazed. Vacuum brazing is most suitable to brazing aluminum alloys, stainless steels, superalloys, titanium alloys, zirconium, or other reactive elements with particularly stable oxides. 4.22.5.1 Brazing Process Cycle in a Vacuum Furnace A typical brazing process in a vacuum furnace would include the following steps [56]: Step 1: The load is charged into the furnace. A vacuum pump (rough pump) removes much of the air that entered the furnace with the load. Another pump (normally a diffusion pump) reduces the pressure further. Step 2: Inert gas (nitrogen) is let into the furnace and the pressure is increased to a pressure in the range 0.9–​1.5 bar to allow for convection heating (if the furnace is equipped with a fan for improving convective heating). Step 3: Heating of the load begins. The heating rate should allow the parts to heat uniformly to minimize distortion. Step 4: At the end of the holding time, vacuum pumping is activated to give a low pressure. Step 5: The temperature is increased to the brazing temperature. The braze alloy starts to melt and is drawn into the joint. Step 6: The pressure is increased if the base alloy has alloying elements that tend to vaporize. The soak time at brazing temperature should be just long enough to assure melting of all braze alloy in the entire load; too long may lead to embrittlement. Step 7: In this step, the temperature is lowered until the braze alloy is completely solidified. Step 8: Fast cooling down to room temperature is achieved by introducing inert gas into the furnace up to a certain elevated pressure. The proper cooling rate depends on the risk of distortion and if there are certain requirements on the cooling rate for obtaining the best material. Merits of vacuum brazing. Merits of vacuum brazing are as follows [56]: 1. The method eliminates the need for fluxes and post-​braze cleaning. 2. It is nonpolluting and possesses high braze performance capabilities. 3. Vacuum prevents oxidation of metals by removing air from around the assembly and also removes volatile impurities and gases from the metals. 4. The method imparts a high standard of cleanliness to the work that could not be achieved by any other method. 5. The overall production time is minimized since there is no fluxing action and subsequent post-​ braze cleaning.

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4.22.5.2 Furnace Brazing –​Safety Awareness Safety is a key concern when working with industrial processes. Therefore, any person working in the heat treatment industry including brazing should be aware of the hazards from processes and equipment and apply appropriate safeguards to control the risk at an acceptable level. 4.22.5.3 Gases Used in the Process Gases used in the brazing process are mainly nitrogen, dissociated methanol and hydrogen. Hydrogen may be produced by dissociation of ammonia. Hydrocarbon gases are in certain cases used for carbon potential control. The main hazards related to the used gases and to methanol are [56]: • • • •

explosions/​flammability/​fire toxicity and asphyxiation cold burn hazards pressurized piping and the gas expansion hazard.

4.22.5.4 Post-​braze Cleaning Brazing fluxes are chemically active, and their residues are highly corrosive, especially in the presence of moisture. Therefore, after brazing, flux residuals must be removed to avoid both corrosion and contamination problems. Use boiling water, which usually removes chloride-​type flux residue, or a strong acid such as nitric, followed by water rinsing. Oxidized areas adjacent to the brazed joint can be cleaned by pickling, wire brushing, or blast cleaning. Post-​braze cleaning is further discussed later. 4.22.5.5 Braze Stop-​offs When filler metal flow must be restricted to definite areas, “stop-​offs” are employed to outline the areas that are not to be brazed. Usually a stop-​off is applied as a thin, continuous line around critical areas on a metal’s surface where BFM must not be allowed to flow. Braze stop-​off materials, if used, must be compatible with base metal, filler metal, fluxes, and furnace atmosphere. Stop-​off residue must be removed from the joint after brazing. Wire brush, air blast, or water flush will remove the stopoff residue.

4.23 BRAZING OF ALUMINUM Brazed aluminum assemblies are all aluminum with excellent corrosion resistance when properly cleaned of any residual corrosive flux. Brazed aluminum assemblies conduct heat uniformly. Therefore, brazed aluminum heat exchangers are long-​lasting and highly efficient. Techniques for brazing aluminum are similar to those used for brazing other metals. Commercially, the same equipment is often used. But there are some important differences between aluminum and other brazeable metals that affect the standard brazing techniques. These differences stem from the refractory nature of the surface oxide film on aluminum, from the oxide that forms on aluminum having a high melting point (3722°F), and from the low melting point of aluminum [68]. Other characteristic features of aluminum relevant for brazing include the following: it conducts heat quickly, surface oxides form rapidly, thermal expansion is greater than many other common metals, and it does not change color as its temperature changes [58].

4.23.1 AWS C3.7M/​C3.7-​2022: Specification for Aluminum Brazing This specification presents the minimum fabrication and quality requirements for brazing of aluminum and aluminum alloys.

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Fabrication, Brazing, and Soldering of Heat Exchangers

4.23.2 Need for Closer Temperature Control Aluminum alloys are brazed with aluminum filler metals that are similar to the base metals. Filler metals have liquidus temperatures closer to the solidus temperature of the parent metals, and hence close temperature control is very important [65]. This can be easily understood from Table 4.9. The brazing atmosphere should be approximately 70°F (39°C) below the solidus temperature of the base metal, but if temperature is accurately controlled and the brazing cycle is short, the difference can be as close as 10°F (5.5°C) [90].

4.23.3 Aluminum Alloys that can be Brazed A majority of the non-​heat-​treatable aluminum alloys like 1100, 3003, 3004, and 5005 and many of the heat-​treatable alloys like 6053, 6061, 6063, 6951, 7005, and 7072 can be brazed. Aluminum alloys containing more than 2.5% magnesium are difficult to braze. Common brazeable wrought alloys, their composition, and approximate melting range. Approximate brazing temperatures for common aluminum alloys are shown in Figure 4.96. In addition to these, aluminum can be brazed to many dissimilar metals and alloys. A partial list includes the ferrous alloys, nickel, titanium, Monel, and Inconel [58, 61].

4.23.4 Elements of Aluminum Brazing 4.23.4.1 Joint Clearance/​Select Capillary Size (Gap) A gap between the two components to be joined is necessary to allow the molten flux to be drawn into and clean and dissolve the oxides and allow the filler metal to be drawn in freely and evenly. The size of the gap determines the strength of the capillary pull. Making a perfect joint requires the components to have the right capillary gap. Only if the gap is correct will the filler alloy spread when molten, by capillary action. For Controlled Atmosphere Brazing (CAB), gap clearances of 0.10 mm to 0.15 mm are recommended for non-​clad components (when the filler metal is fed externally. For clad components such as in a tube to header joint where the tube is clad, the clearance is provided by the thickness of the cladding layer and so intimate contact is recommended. Larger gap clearances reduce capillary action while smaller gaps may restrict filler metal flow causing discontinuities in the joint. More details on joint clearances for aluminum brazing are furnished in Ref. [58]. 4.23.4.2 Pre-​cleaning Brazing aluminum requires that the metal be free from surface contaminants and its oxide layer thin enough to be displaced by the brazing flux. Surface contaminants such as greases, oil, fatty acids, and marking crayons can be removed by vapor degreasing using inhibited trichloroethylene or ultrasonic degreasing. 4.23.4.3 Surface Oxide Removal As highly reactive material, aluminum and its alloys are covered with an oxide. This oxide can be an important hinderant in brazing process, where the oxide film acts as a barrier to the wetting and flow of filler metal [87]. Hence, oxide barrier dispersal is a prerequisite for successful brazing. For non-​ heat-​treatable aluminum alloys, vapor degreasing or ultrasonic degreasing is adequate [72]. Overly thick oxide layers on heat-​treatable alloys can be reduced by either mechanical means or chemical means [58]. Chemical means will consist of a degreasing either in trichloroethylene or a chemical cleaner followed by an acid or alkaline etch. The parts will normally need desmuting in nitric acid, after which they are washed in clean water and dried [72]. Chemical cleaning is not recommended for fluxless vacuum brazing [90].

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FIGURE 4.103  Clad aluminum brazing sheet-​(a) Single side and (b) Both sides cladded.

4.23.4.4 Aluminum Filler Metals In the case of aluminum, the major elements in the filler metal are the same as the base metal except for some additional melting-​point depressants. Complex parts can be formed and assembled using clad brazing sheet to eliminate the need for separate filler wire or shim. Various flux brazing filler alloys, vacuum brazing filler alloys, and clad aluminum flux brazing sheet are dealt with in Ref. [58]. Aluminum brazing sheet. Aluminum brazing sheet consists of a base metal core (typically 3003 or 6951 alloy) and a filler metal, typically either Al-​7.5Si (4343) or Al-​10Si (4045), clad roll bonded to either one or both sides as illustrated in Figure 4.103. The core provides structural integrity, while the clad plays the role of filler metal. An oxide surface layer is depicted to emphasize the importance of this barrier to the wetting and flow of the filler metal and the requirement that the barrier be dispersed for a successful braze [91]. There are many different filler alloys available: furnace brazing uses mainly filler alloys with 6.8 to 8.2% Si (AA4343) and also 9 to 11% Si (AA4045). NOCOLOK® Flux, a typical filler alloy is a fusible alloy of aluminum and silicon. 4.23.4.5 Fluxing Except for vacuum brazing or controlled atmosphere (without oxygen) brazing, or salt-​bath brazing processes where the salt bath itself consists of flux, all surfaces must be fluxed to break down the oxide film on the surface of the work and to promote wetting, and the filler itself must be fluxed. Fluxes for aluminum brazing contain fluorides and chlorides of the alkali metals. To the base are added “activators” such as fluorides, lithium compounds, and sometimes zinc chloride [68].

4.23.5 Aluminum Brazing Methods The main processes used commercially to braze aluminum heat exchangers are: 1. torch brazing 2. flux dip brazing 3. furnace brazing a. controlled atmosphere brazing b. vacuum brazing. Torch brazing and dip brazing employ a flux to break down the oxide film on the surface of the work and promote wetting. Furnace brazing with controlled-​gas atmosphere requires less flux or no flux,

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Fabrication, Brazing, and Soldering of Heat Exchangers

FIGURE 4.104  Heat exchanger assembly brazing sequence.

whereas vacuum brazing does not require a flux and the mechanism of oxide breakdown is quite different. 4.23.5.1 Brazing of Radiators and Condensers The process sequence really depends on the type of heat exchanger being manufactured. For heat exchangers such as radiators and condensers, the most common process sequence for ease of manufacturing is as follows: core assembly-​fixturing→ degreasing → fluxing → drying→ brazing and the procedure is shown schematically in Figure 4.104 and the most common process sequence is as follows [56, 57]: 1. Core assembly. 2. Fixturing. 3. Degreasing/​cleaning. 4. Fluxing: the appropriate flux is applied, usually by means of spray nozzles that ensure a homogeneous distribution of the flux on the heat exchangers. 5. Blowing off excessive flux. 6. Drying: the temperature is increased to around 250°C in order to dry the heat exchanger. 7. Heating: the temperature of the heat exchanger moving through the heating section is raised uniformly to the target braze temperature (around 600°C). 8. Brazing: the flux melts and dissolves the oxide film on the aluminum just prior to the filler metal melting and forming their joints. 9. Cooling: solidification of flux and filler metal occurs. The flux residue forms a thin, adherent film. 4.23.5.2 Aluminum Dip Brazing Aluminum dip brazing has been practiced commercially for many years. The molten salt bath contains chemically active salts that promote brazing. These help to displace the adherent oxide film on aluminum surfaces so that the molten filler metal wets the surfaces and forms the brazed joints. Because of the stability of surface oxide film, and because of low brazing temperatures that have to be used, a highly active flux is required for brazing aluminum. Important parameters pertaining to

Fabrication, Brazing, and Soldering of Heat Exchangers

621

FIGURE 4.105  Manufacturing procedure of dip brazing with flux.

dip brazing aluminum, includes (1) bath compositions such as AIF, NaCl, KCl, etc. (2) bath melding point, 900°C–​1000°F (482°C–​538°C), (3) preheating temperature (approx.), 1000°F (538°C), and (4) brazing temperature 1030°F–​1190°F (555°C–​643°C) [69]. Manufacturing procedure for dip brazing of PFHE is shown in Figure 4.105.

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Fabrication, Brazing, and Soldering of Heat Exchangers

4.23.5.3 Furnace Brazing Furnace brazing is the second most popular method of brazing aluminum in use today. Furnace brazing’s popularity derives from the comparatively low cost of equipment, from the ease with which the existing furnaces can be adapted to aluminum brazing, and from the minimal fixturing required [58]. Furnace brazing of aluminum can be performed in air or nitrogen or as vacuum brazing. Controlled atmosphere brazing (CAB) has grown more than vacuum brazing mainly due to its adaptability to high-​volume serial production. Vacuum brazing is very demanding as regards brazing gap clearances, which must be within close limits to result in good brazements. Vacuum brazing has clear environmental advantages in the absence of hazardous and aggressive fluxes and in the cleanliness of the brazed parts. Vacuum brazing is performed without a flux. No post-​cleaning is therefore required. An additional benefit with the absence of flux residues is that there is no corrosion associated with entrapped flux. The aluminum parts to be brazed are clad with a high-​Mg-​concentration braze alloy, which acts as the filler metal. Mg in the cladding alloy acts as an oxygen getter, thereby improving wetting. Aluminum is brazed at temperatures between 555 and 645°C (1030–​1195°F). Any strengthening, whether by heat treatment or cold working, will be lost at these temperatures. Aluminum poses particular brazing challenges since aluminum oxide is an adherent and readily forms oxide, even at room temperatures. 4.23.5.4 Inert-​gas or Controlled Atmosphere Brazing of Aluminum For inert-​gas brazing of aluminum, the process variables that affect moisture and oxygen levels in the brazing chamber must be controlled. This requires that the workpiece along with the fixture and the carrier be adequately outgassed to remove adsorbed moisture and oxygen prior to entering the inert-​gas chamber. This is achieved by either repeated heating and evacuations, or a vacuum heating of aluminum with magnesium brazing filler metal clad to getter the contaminants [63, 92–​96]. 4.23.5.5 Key aspects of Controlled Atmosphere Brazing (CAB) Many automotive manufacturers use the CAB process in critical automotive applications because it ensures superior joint strength. When brazing with aluminum, the CAB process incorporates the following key aspects: 1. Filler Metal: most commonly an aluminum-​silicon alloy that will flow quickly when melted and create strong joints post-​braze. 2. Furnace: inert/​controlled atmosphere-​retort and continuous (straight and humpback) furnaces. 3. Flux: generally non-​corrosive for aluminum applications; helps to reduce oxides on the metal surface which allows for filler metal to flow. 4. Furnace atmosphere: either nitrogen (N2) which displaces air/​oxygen in the furnace atmosphere or inert gases –​helium and argon; used in brazing metals and ceramics. 4.23.5.6 Controlled Dry Air Brazing The amount of moisture in the brazing atmosphere influences the amount of flux required for successful brazing. When the moisture is not controlled, thick flux slurries, about 50%–​75% by weight, are required. Only a small amount of flux serves the intended purpose, and much of it becomes ineffective because it is hydrolyzed by reaction with moisture present in the air, before the brazing alloy melts [97]. In a dry-​air atmosphere of −40°F (−40°C) dew point or lower, thin slurries containing 20%–​25% flux will serve the purpose. This brazing process also eases the postbrazing cleaning requirements.

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FIGURE 4.106  Basic diagram of a controlled atmosphere furnace brazing line.

4.23.5.7 Controlled Atmosphere Brazing Process CAB of aluminum using a noncorrosive flux is the preferred process for manufacturing aluminum heat exchangers. The process involves brazing in a dry and inert-​gas atmosphere. Mostly nitrogen is used. Argon and helium can also be used, but they are more expensive. The gas is introduced into the critical brazing section of the furnace flows toward the entrance and exit. This prevents contamination of the atmosphere in the braze section. The process sequence of CAB depends on the type of heat exchanger being manufactured. Many automotive manufacturers use the CAB process in critical automotive applications because it ensures superior joint strength. 4.23.5.8 Brazing Process A general procedure for a CAB process in an automotive application would be as such: aluminum parts –​heat exchangers for ­example –​are preloaded with a braze preform such as ring. The furnace as shown in Figure 4.106 [96] utilizes a conveyor that holds the parts in place while moving slowly enough to allow the parts to reach temperature and complete a proper braze. Continuous belt furnaces do not operate in a vacuum. Instead, they are fed non-​reactive gases to replace the oxygen and prevent oxidation of the heated parts. In other words, the atmosphere is carefully controlled rather than evacuated. These continuous belt-​fed furnaces are efficient and effective, offering a less expensive option for any materials that can be brazed outside of a vacuum. Due to relatively high investment costs and relative limited flexibility in running production, CAB is most suitable for large quantity manufacturing. 4.23.5.9 CAB Process Advantages This process allows for a number of benefits. CAB’s advantages include the following [92–​97]: 1. First, the slow and even heat that the furnace provides helps prevent localized overheating and damaging of base materials. 2. The protective atmosphere helps to keep further oxidation from occurring which allows for the braze alloy to flow more freely and form higher quality joints. 3. While CAB process is a slower heating process than torch or induction, it is very efficient in mass production as large quantities can be run continuously. 4. By using a CAB process that provides even heat for every joint, variation from part to part is decreased and the final joints formed are much more robust. CAB Furnaces Seco/​Warwick, USA, CAB furnaces provide the pure nitrogen atmosphere and temperature profile necessary to promote the formation of braze fillets between the fin and tubes and the tube-​to-​header joints of aluminum heat exchangers. A radiation braze CAB furnace is an ideal method for brazing

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Fabrication, Brazing, and Soldering of Heat Exchangers

FIGURE 4.107  Radiation CAB furnace (schematic) (entrance chamber, braze chamber, water jacket cooling chamber, exit chamber, air blast drive table). (Courtesy of Seco/​Warwick Corporation, Meadville, PA.)

FIGURE 4.108  A fully configured CAB furnace system. Adapted and modified from Ref. [64].

similar-​sized products in a continuous flow environment. A schematic of typical radiation CAB furnace system is shown in Figure 4.107. Furnace brazing of aluminum with a noncorrosive flux –​Nocolock flux (Nocolock owned by Alcan Aluminum Ltd). It was discovered that a brazing method using a nonhydroscopic, noncorrosive, potassium fluo-​aluminate flux material offered the benefits of flux brazing without a requisite post-​braze cleaning. A fully configured CAB furnace system includes an aqueous washer or thermal degreaser, a fluxer unit, a dry off oven, and the CAB furnace with heating, brazing, and cooling zone. When used in an inert-​gas atmosphere, it has a significant tolerance to impurities in the furnace atmosphere [63, 98]. The trade name for the noncorrosive flux is known as Nocolock. Heat exchangers such as automotive radiators, heater cores, condensers, evaporators, oil coolers, and charge air coolers are being mass produced using the Nocolock flux brazing process [99]. The flux is a fine white powder mixture of potassium fluo-​aluminates. It is mixed with water to form a dilute slurry, applied by spray or other methods to cleaned aluminum assemblies, and dried. Parts are normally brazed in a nitrogen atmosphere tunnel furnace. Brazing temperatures of 590°C–​620°C (1095°F–​1150°F) are suitable. A fully configured CAB furnace system is shown in Figure 4.108 [63, 64]. Post-​braze treatments Generally speaking, no post-​braze treatments are required. The flux residue is noncorrosive, strongly adherent to the aluminum surface, and provides good corrosion protection. However, in recent years, there continues to be a demand for air-​oxidized or blackened surface radiators and condensers [99]. These are based on the controlled introduction of dry air for surface oxidation or carbon dioxide for surface blackening into an inert atmosphere brazing furnace soon after the completion of the braze. These after-​treatments are required to improve corrosion resistance of aluminum heat exchangers.

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4.23.5.10 Vacuum Brazing of Aluminum In vacuum brazing, aluminum assemblies are heated in a vacuum environment by radiation. The principal differences between vacuum brazing and furnace brazing are that (1) the brazing is done in a vacuum of the order of 10−4 to 10−6 torr, (2) no flux is used, (3) clad brazing sheet and other forms of filler materials usually contain 0.1%–​2.0% magnesium as braze promoter, and (4) heating is by radiation rather than convection. Typical procedures for vacuum brazing aluminum heat exchanger. These include the following: 1. pre-​cleaning of components 2. assembly and fixturing 3. heat/​braze in vacuum 4. back-​fill the furnace: dry nitrogen is often used, although dry air can be used under some circumstances 5. remove the brazed assemblies, which should be air quenched with a fan or allowed to stand until cool. Typical manufacturing procedure for vacuum brazing of PFHE is shown in Figure 4.109. Important parameters for vacuum brazing aluminum. Important process parameters that control brazing and braze quality are (1) vacuum environment, (2) brazing assembly temperature and temperature uniformity, (3) oxide dispersion and wetting, (4) special vacuum brazing filler and brazing sheet alloy, and (5) vacuum brazing furnace parameters. Some or all of these parameters are discussed in Refs. [57, 58, 61, 87, 88, 91]. Oxide dispersion and wetting. It is well known that oxygen and water vapor are the important process contaminants. In fluxless vacuum brazing, the presence of a promoter, either a metal or reactive gas, is needed to getter oxygen and water vapor from the furnace and to suppress reoxidation of the aluminum at the joints, once the oxide film is removed. The fluxless processes use either elemental magnesium, magnesium containing filler metal, or magnesium bearing base metals to getter oxygen [58, 60]. Alternatively, brazing also can be achieved in a high vacuum by prolonged exposure at elevated temperatures without using metal activators [97]. Magnesium as a braze promoter. Many metals can fulfill the function of a braze promoter, but magnesium is the best, because magnesium has the highest vapor pressure of the metals that promote vacuum brazing [97], ease of alloying with aluminum, and low cost [88]. Vacuum brazing filler metals. Generally, vacuum brazing filler metals and brazing sheets are aluminum alloys containing 7.5%–​12% silicon and up to 1.5%–​2.5% magnesium [66]. Since gravity adversely affects vertical capillary filler metal flow in vacuum brazing, preplace filler metal in vertical joints or use a brazing sheet. Alloys of the lxxx, 3xxx, 5xxx, 6xxx, and 7xxx series can be vacuum brazed using No. 7, 8, 13, and 14 brazing sheets. Vacuum brazing furnaces. Vacuum furnaces capable of reaching at least 1200°F ± 5°F (650°C ± 3°C) temperature uniformity in the normal brazing range of 1080°F–​1140°F (582°F–​616°F) are suitable for brazing aluminum [88]. Systems for vacuum brazing of aluminum are discussed in detail in Ref. [87] and Ashburn [100]. Vacuum furnaces are invariably heated by electricity, in any one of a number of forms. A typical electric-​heated vacuum furnace has the heating system surrounded by radiation shields and mounted within a water-​cooled steel shell, which is known as cold-​wall design. Vacuum furnaces for brazing aluminum are equipped with mechanical roughing and oil vapor diffusion pumps to achieve the required 10−4 to 10−6 torr vacuum.

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Fabrication, Brazing, and Soldering of Heat Exchangers

FIGURE 4.109  Manufacturing procedure of flux-​free brazing in a vacuum. (From Linde AG, Engineering Division.With permission.)

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627

Non-​uniform mass and temperature uniformity. PFHEs are relatively uniform in mass and usually present no particular problems in heating to brazing temperature in a furnace atmosphere. However, tube-​fin exchangers such as the automotive radiator, with different thicknesses of parts like fins, tubes, header sheet, and side supports, require special consideration due to their non-​uniform mass distribution. This type of radiator can be heated uniformly using one of these two methods [88]: (1) use of heat shields to slow heating of the core area and (2) orienting the radiator in the furnace with heavier sections facing the heating elements. While both methods provide ±5°F temperature uniformity, the latter method is more amenable to production use. 4.23.5.11 Post-​braze Cleaning and Finishing The brazed aluminum alloys should be free from residual flux to avoid corrosion during service. The post-​braze cleaning methods include ultrasonic means or using chemical solutions, or using one of the chemical brighteners. Cleaning of a fluoride-​type flux. Even though residual fluoride flux is noncorrosive, for aesthetic reasons and for surface coating, the fluoride flux is cleaned using a caustic etch or chemical brighteners, since fluoride fluxes are not water washable [58]. Finishing. When a brazing fillet has been properly formed, no finishing is required except flux removal by post-​braze cleaning. If necessary, the fillet, like the brazed metal, may be ground, filed, or polished. Caution: the brazed assembly should not be cleaned in caustic solution.

4.24 MICROCHANNEL HEAT EXCHANGERS A typical aluminum microchannel heat exchanger (MCHE) has three major components: header, multiport microchannel tube, and louvered fin. These components are joined together by controlled atmosphere brazing. Heat transfer technology based on microchannel coils offers the largest potential for improving energy efficiency, reducing refrigerant volumes required, while also providing a number of other benefits. Microchannel tubes have sub-​millimeter diameters, hence, it is important to avoid particles entering the heat exchanger. For this purpose, it is recommended to install a mesh strainer with a sieve size of 0.25mm or less.

4.24.1 Brazing of MCHE The traditional way of manufacturing finned tube heat exchangers by mechanical tube expansion have the disadvantage of lack of sufficient contact between tubes and fins [64, 101]. On the other hand, the brazing process that metallurgically bonds fins and tubes in microchannel coils (MCHE) eliminates the drawback of contact resistance. Flux, a potassium aluminum fluoride salt, is then used to dissolve the oxide film barrier and prevent further oxidation during the brazing process. MCHE assembly for brazing is shown in Figure 4.110 [64] and a brazed heat exchanger is shown in Figure 4.111 [101].The whole fabrication process for the microchannel coils consists of the following steps [101]: • • • •

Assembling heat exchanger core. Thermal degreasing. Fluxing. Flux is applied to the coil as an aqueous suspension. Drying. The next phase is the removal of residual moisture from the fluxing stage before brazing. Drying is carried out at coil surface temperatures between 200 and 250°C. • Brazing. Brazing process is carried out in an inert (nitrogen) atmosphere of brazing furnace (Controlled Atmosphere Brazing, CAB).

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Fabrication, Brazing, and Soldering of Heat Exchangers

FIGURE 4.110  Microchannel heat exchanger assembly for brazing. Adapted and modified from Ref. [64].

• Cooling. Solidification of the filler metal takes place at the cooling stage whereby a metallurgical bond is formed between all parts of the heat exchanger assembly. The flux residue remains on the heat exchanger surface as a thin film (1 to 2µm). • Testing. This last stage consists of a series of checks, including leakage tests, pressure tests, geometry checks, brazing quality control, and the coil becomes ready for packaging or further manufacturing stages such as bending or coating.

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FIGURE 4.111  Brazed microchannel heat exchanger.

4.25 CUPROBRAZE HEAT EXCHANGER For decades, copper-​brass (flat rectangular fin with flat tube made by lock-​seam method) soldered radiators were used in heavy-​duty diesel engines to avoid annealing effects if brazing is adopted to manufacture radiators. The CuproBraze® process is a technique for manufacturing heat exchangers, particularly automotive radiators. Developed by the International Copper Association, CuproBraze technology enables the manufacture of light, strong, efficient and compact heat exchangers, including radiators, oil coolers, heater cores, charge air coolers and condensers, from high strength and high conductivity copper and copper alloys. These alternative materials and technology offer several benefits over existing systems, including a 10% cost saving over conventional aluminum heat exchangers while offering better energy efficiency. Other features that favor CuproBraze radiators include better corrosion resistance, repairability, higher shock and vibration withstanding capacity. Manufacturing processes are now being applied globally in the manufacture of advanced heat exchangers using the new brazing process, known as CuproBraze [102–​107]. Brazing furnaces have been developed for all levels of production including batch, three-​chamber (semicontinuous), and continuous furnaces. CuproBraze heat exchangers are made using special anneal-​resistant alloys of copper and brass. Tubes are fabricated from brass strip and coated with a brazing filler material. The copper fins, coated tubes, headers and side supports made of brass are fitted together into a core assembly as shown in Figure 4.112 which is then brazed in a furnace. CuproBraze technology is flexible and scalable. The International Copper Association licenses CuproBraze technology free of charge to heat exchanger manufacturers.

4.25.1 Tube Fabrication CuproBraze® heat exchangers are made using special anneal-​resistant alloys of copper and brass. Tubes are fabricated from brass strip and coated with a brazing filler material. The copper fins,

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Fabrication, Brazing, and Soldering of Heat Exchangers

FIGURE 4.112  Cuprobraze of a heat exchanger.

coated tubes, headers and side supports made of brass are fitted together into a core assembly, which is then brazed in a furnace. Brass and copper alloys offer high strength as well as excellent retention of strength at elevated operating temperatures. Several types of brass tubes can be used to manufacture CuproBraze heat exchangers. The drawback of the lock-​seam fold is that the seam becomes an irregularity on the tube surface, which makes it difficult to achieve a uniform gap in brazing the tube to the header. New tube designs offer advantage over the lock-​seam design. The overfold design (called snap-​over) and B-​fold design are just two of many folds being tested for CuproBraze heat exchangers [102].

4.25.2 Brazing Process The brazing of CuproBraze radiators uses a non-​toxic low temperature melting alloy that works well in a nitrogen based controlled atmosphere furnace. The parts are then brazed in an atmospheric continuous mesh belt furnace or in a backfilled vacuum furnace under nitrogen protection. The brazing alloy melts at 590°C (1110°F) and has a melting range of 20°C(68°F). The core material is specially designed to withstand the high temperature. After brazing, the brazed copper-​brass joints are

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631

significantly stronger than the solder metal and would not suffer from galvanic corrosion. Developed for this process, anneal resistant header, fin and tube materials assure the strength of the products.

4.25.3 High-​performance Coatings An uncoated radiator has approximately 30% longer lifetime than a cosmetically spray-​coated radiator. Electrophoretic coating is the best technical solution to increase corrosion resistance. It increases the lifetime by 2.5–​3 times compared to an uncoated radiator. A new option is powder coating that gives good results with respect to corrosion resistance and thermal performance and has a lower cost compared to an electrophoretic coating [102].

4.25.4 Round Tube Versus Flat Tube Conventional soldered copper-​brass radiators uses flat tube. Though round tubes provide a strong joint at the header plates, because of their low surface-​to-​volume ratios, round tubes are inefficient at transferring heat from the tubeside fluid. The surface-​to-​volume ratio can be dramatically increased using flat tubes. Again, the best size and shape of tubes depend on the heat exchanger design and on the fluid, including whether it is a liquid or a gas, and the pressure, temperature, and flow rate of the fluid. Flat channels offer improved heat transfer on both refrigerant and air sides. The first reason is the more favorable section/​surface ratio, which affects the efficiency of heat exchange on the air side and on the refrigerant side. Energy efficiency and refrigerant emissions reduction are the key elements of sustainable refrigeration. On the air side, flat tubes reduce the surface in the shadow of the air stream, where the flow becomes turbulent. The shade of the tubes not only causes inefficient heat-​transfer, it is also the cause of a lot of noise [64]. Air flow pattern around a round tube and flat tube is shown in Figure 4.113, Ref. [63]. The argument in favor of round tubes is that they can provide a strong joint at the header plates. On the other hand, because of their low surface-​to volume ratios, round tubes are inefficient at transferring heat from the internal fluid to the inside surface of the tube walls. The surface-​to-​ volume ratio can be dramatically increased using flat tubes. Again, the best size and shape of tubes is dictated by the special applications. Much depends on the heat exchanger design and on the fluid, including whether it is a liquid or a gas, and the pressure, temperature, and flow rate of the fluid. Young Touchstone, USA/​UK offers its customers the best of both types with its patented FLAT-​ ROUND technology. It uses brass tubes that are flat in the core and round at the headers. This

FIGURE 4.113  Air flow pattern around a round tube and flat tube. Adapted and modified from Ref. [63].

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Fabrication, Brazing, and Soldering of Heat Exchangers

FLAT-​ROUND design combines the superior airflow and heat transfer of flat tubes with reliable tube-​ to-​ header mechanical bonding for exceptional durability. Combining the proven FLAT-​ ROUND technology with CuproBraze technology results in the toughest, most efficient cooling systems in the industry. Pressure Drop Reduction. Comparing the two designs it is evident that flat is beneficial: the reduction in resistance is up to three-​fold under typical operating conditions.

4.26 BRAZING OF HEAT-​RESISTANT ALLOYS, STAINLESS STEEL, AND REACTIVE METALS Heat-​resistant superalloys are either nickel-​based or cobalt-​based alloys, with the alloying elements are chromium, iron, tungsten, and molybdenum. These alloys exhibit strength, oxidation resistance, and resistance to corrosion at elevated temperatures. Some of the nickel-​based wrought alloys used as heat exchanger materials are Hastelloy X, Inconel 625, and Inconel 718, and cobalt-​based alloys, etc. Elements of brazing of nickel-​based and cobalt-​based alloys are discussed in this section.

4.26.1 Brazing of Nickel-​based Alloys Requirements for brazing of nickel-​based alloys. The success of brazing of nickel-​based alloys depends on the careful consideration of the following parameters [108]: 1. physical metallurgy of the alloys 2. surface preparation 3. thermal cycles 4. brazing atmosphere. Physical metallurgy of the alloys. The surfaces of precipitation-​hardenable alloys containing more than 1% Al and Ti are covered with hard, tenacious oxide films. These surface films hinder the wetting of molten filler metal, and the films are in general impossible to reduce, either in a controlled atmosphere or in a vacuum brazing atmosphere. However, these difficulties are not normally encountered with solid-​solution-​strengthened alloys. Surface preparation. Successful brazing requires removal of surface oxide films. To improve the wettability by filler metals, additional surface treatment followed by pre-​cleaning may be required. The surface treatment consists of electroplating of nickel on the surfaces, known as nickel flashing. Thermal cycles. The effects of high-​temperature thermal cycles are [108] as follows: (1) Brazing temperature above their hardening temperatures of 1100°F–​1500°F may alter the alloy properties; (2) at the high brazing temperature of 1850°F, the precipitation-​hardenable alloys may be subjected to grain growth and decrease in stress rupture properties, which cannot be recovered by subsequent heat treatment; and (3) the liquid metal is subjected to embrittlement when the molten metal is under the influence of tensile stress. Hence, the residual or applied tensile stress should be relieved before brazing. Brazing atmospheres. For successful brazing of nickel-​based alloys, either a dry, oxygen-​free reducing atmosphere or a vacuum is preferred. The effects of various controlled atmospheres have been covered earlier. Vacuum brazing in the range of 10−4 torr has proved adequate for brazing most of the nickel-​based alloys. The brazing atmosphere, whether gaseous or vacuum, should be free from harmful constituents such as sulfur, oxygen, and water vapor. In general, a gaseous atmosphere having a dew point less than about −60°C (−75°F) will produce better quality.

Fabrication, Brazing, and Soldering of Heat Exchangers

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4.26.1.1 Brazing Filler Metals Generally, nickel-​based alloys are brazed with nickel-​based alloys containing boron and/​or silicon, which serve as melting-​point depressants. Phosphorous is another effective melting-​point depressant, and it also aids good flow in applications of low stress; their temperatures do not exceed 1400°F (760°C). Sometimes, chromium is present to provide oxidation and corrosion resistance. Other filler metals include copper and silver brazing alloys for low-​temperature applications and proprietary filler metals for service temperature above 1800°F (982°C).

4.26.2 Brazing of Cobalt-​based Alloys Brazing of cobalt-​based alloys can be accomplished by adopting the same techniques used for brazing of nickel-​based alloys but with less stringent brazing requirements [108]. This is due to the absence of appreciable amounts of aluminum and titanium in the base metals. The filler metals are usually cobalt based, nickel based, or gold-​palladium compositions. Most cobalt-​based filler metals contain boron and/​or silicon as melting-​point depressants and Cr, Ni, and W for improved strength, corrosion, and oxidation resistance.

4.26.3 Brazing of Stainless Steel In recent years, there has been a demand, particularly from the aircraft and nuclear energy industries, for complex heat exchangers that operate under arduous conditions of temperature and pressure and adverse environments. To meet these requirements, heat exchangers have been fabricated from stainless steel, usually the 18:8 type stabilized with titanium. Other wrought stainless steels such as of AISI types 316, 316L, 347, and 430 are also extensively used as heat exchanger materials for high-​temperature applications. Many aircraft heat exchangers are of modular construction [109]. Typical brazed forms of stainless steel heat exchangers include PFHEs, shell and tube heat exchangers, and regenerators. To save weight in aircraft and for greater efficiency in heat transfer, the heat exchangers have been made from thin sheet and thin-​wall tubing; typical sizes are sheet 0.005 in thick and tubing of 0.006 in wall thickness. One of the problems in fabricating structures from these thin materials has been to make completely sound joints without any distortion in the heat exchanger matrix. 4.26.3.1 Brazeability of Stainless Steel Brazing of stainless steels is carried out at high temperatures. Among the various factors, the quality of stainless steel brazing depends on the following [70]: 1. Ability to create a proper brazing atmosphere to eliminate oxidation, scaling, and surface reaction that forms sulfides and nitrides on its surface. 2. Ability to arrest the sensitization of austenitic stainless steel during brazing or proper solution treatment of sensitized steels to restore their corrosion resistance. Brazing process. Stainless steel can be brazed by all conventional brazing processes including torch, furnace, dip, and induction brazing. Stainless steel heat exchangers are mostly furnace brazed, either in a controlled reducing atmosphere such as dry hydrogen, dissociated ammonia, or argon, or in vacuum. Filler metals. In addition to brazeability, for most applications, filler metals are selected for mechanical properties, corrosion resistance, service temperature, and compatibility with process fluids. Filler metals for brazing stainless steels include copper alloys, silver alloys, gold and gold-​palladium alloys, and nickel alloys. Nickel-​based filler metals alloy with stainless steel and form secondary

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Fabrication, Brazing, and Soldering of Heat Exchangers

phases with two undesirable characteristics [108]: (1) low ductility and hence susceptibility to hot cracking and (2) higher melting point than the base metal and hence likely to freeze and retard the filler metal flow. To achieve flow in deep joints, diametral clearances of as much as 0.004–​0.008 in. (0.1–​0.2 mm) are necessary. Selection of brazing filler metals for brazing stainless steels is also discussed by Amato et al. [110]. For guidance on selection of brazing filler metals for stainless steels brazing, Welding Engineer Data Sheet No. 493 [111] is useful. Fluxes. Brazing of stainless steel in a controlled atmosphere or in vacuum does not require flux. In some furnace-​brazing applications, flux is necessary. But flux is required for torch brazing, dip brazing, or any other conventional brazing process carried out in an uncontrolled atmosphere. 4.26.3.2 Brazing of Reactive Metals Metals such as titanium and zirconium which react readily with oxygen, nitrogen or hydrogen are preferably brazed either in a dry and clean inert argon or helium atmosphere or in a vacuum. Vacuum brazing at a temperature in the range 900–​950°C (1650–​1740°F) is increasingly preferred as the method which yields the best brazing result.

4.27 POST-​BRAZE CLEANING AFTER LUCASMILHAUPT [112–​114] Depending on the brazing method/​process, there is a need to perform post-​braze joint cleaning to remove residual flux. This step is crucial for several reasons; including the corrosive nature of most fluxes and the possibility that excess flux could contribute to joint failure. The most common cleaning methods involve water soaking/​wetting and quenching.

4.27.1 Cleaning Methods for Post-​braze Flux Removal 1. Soaking/​wetting –​use hot water with agitation in a soak tank to remove excess flux immediately following the braze operation, and then dry the assembly. 2. Quenching –​quench only after the braze filler metal has solidified to avoid cracks or rough braze joints. Once the flux and oxides are removed from the brazed assembly, further finishing operations are seldom needed. The assembly is ready for use, or for the application of an electroplated finish. .

4.28 INSPECTION AND TESTING OF BRAZED JOINT 4.28.1 Quality of the Brazed Joints The quality of brazed joint shall be such that the brazed assemblies are suitable for the intended purpose and that surfaces are free of excess braze filler material. Quality problems of brazed joints of aluminum heat exchangers are discussed by Shah [60, 115, 116]. Some of these problems may also be common to other metals. An imperfect brazed joint is shown in Figure 4.114 and a good brazed joint has large smooth fillets, absence of discontinuities and voids, and the width of the joint is sufficient as shown in Figure 4.115. Imperfect brazed joints include the following [62]: 1. Poor fillets: a braze joint with narrow-​width fillets, due to insufficient time at the braze temperature. 2. Partial joint (underbrazing): due to low brazing temperature, low amount of melting-​point temperature depressant elements in filler metal, or improper heating rate. 3. Overbrazing: a braze joint that is a result of too high brazing temperature or the brazing time; filler metal may dissolve the base metal and reduce its strength.

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FIGURE 4.114  Brazed joint- (a) Poor quality fillet and (b)Desirable quality.

FIGURE 4.115  Plate fin heat exchanger brazing joint details. (a) Schematic, (b) before brazing, (c) after brazing for single fin passage, and (d) after brazing for two layers of fin stack. (Courtesy of Fives Cryo, Golbey, France.)

4.28.2 Discontinuities Discontinuities result in all brazing processes. Some may be process specific. Discontinuities may be associated with structural discontinuities or associated with the braze metal or the brazed joint. Typical discontinuities include [58, 116, 117, 69] the following: (1) lack of fill, (2) flux entrapment, (3) intermittent bond, (4) noncontinuous fillets, (5) voids, (6) porosity, (7) cracks, (8) base metal erosion, (9) unsatisfactory surface appearance, (10) discoloration, and (11) distortion. All discontinuities reduce joint strength. Acceptance limits for discontinuities must identify shape, orientation, location in the brazement, and relationship to other discontinuities. Inspection of brazed joints may be conducted on test specimens or by tests of the finished brazed assembly. The tests may be nondestructive or destructive methods, such as peel tests, tension or shear tests, fatigue test, impact tests, torsion tests, and metallographic examination or proof testing. Various NDT methods include PT, RT, and UT. The scheme of NDT techniques is shown in Figures 4.105 and 4.109.

4.29 NONDESTRUCTIVE TESTING METHODS Nondestructive testing methods for checking quality and specification conformance include [116]:

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Visual Examination. Visual examination with or without magnification –​for evaluating voids, porosity, surface cracks, fillet size and shape, discontinuous fillets, plus base metal erosion (not internal issues such as porosity and lack of fill). Radiographic examination. Useful in detecting internal flaws, large cracks and braze voids, if thickness and x-​ray absorption ratios permit delineation of the brazing filler metal-​cannot verify a proper metallurgical bond. Proof testing. Subjecting a brazed joint to a one-​time load greater than the service level-​applied by hydrostatic methods, tensile loading, or spin testing Ultrasonic examination. A comparative method for evaluating joint quality, in immersion mode or contract mode-​involves reflection of sound waves by surfaces, using a transducer to emit a pulse and receive echoes. Liquid penetrant examination. Dye and fluorescent penetrants may detect cracks open to the surface of joints; not suitable for inspection of fillets, where some porosity is always present. Acoustic emission testing. Evaluating the extent of discontinuity –​using the premise that acoustic signals undergo a frequency or amplitude change when traveling across discontinuities. Thermal transfer examination. Detects changes in thermal transfer rates due to discontinuities or unbrazed areas-​images show brazed areas as light spots and void areas as dark spots.

4.29.1 Leak Testing To make certain no leaks exist, heat exchangers are leak tested. The choice of pressure medium will depend to some extent upon the maximum acceptable leak aperture. Thin oil such as kerosene, heated oil, water, air, or Freon may be used. For critical services, where it is necessary to identify the presence of very minute leaks, helium mass spectrometer leak detection technique may be employed. Pressure (or bubble leak) testing involves the application of air at greater-​than-​ service pressures. Vacuum testing is useful for refrigeration equipment and detection of minute leaks, employing a mass spectrometer and a helium atmosphere.

4.30 DESTRUCTIVE TESTING METHODS There are also several destructive and mechanical testing methods, often used in random or lot testing [116]: 1. Peel testing. Useful for evaluating lap joints and production quality control for general quality of the bond plus presence of voids and flux inclusions –​where one member is held rigid while the other is peeled away from the joint. 2. Metallographic examination. Testing the general quality of joints, detecting porosity, poor filler metal flow, base metal erosion and improper fit. 3. Tension and shear testing. It determines strength of a joint in tension or in shear –​used during qualification or development rather than production. 4. Fatigue testing. Testing the base metal plus the brazed joint –​a time-​consuming and costly method. 5. Impact testing. It determines the basic properties of brazed joints –​generally used in a lab setting.

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6. Torsion testing. Is used on brazed joints in production quality control –​for example, studs or screws brazed to thick sections.

4.31 SOLDERING OF HEAT EXCHANGERS Soldering is defined as a process of joining two similar or dissimilar metals with a filler metal in molten state. Soldering joins materials by heating them in the presence of a filler metal having a liquidus temperature below 840°F (450°C). The filler metal is called solder. Heating may be provided by a variety of processes. The filler metal distributes itself between the closely fitted surfaces of the joint.

4.31.1 Elements of Soldering Most of the elements of soldering, such as joint design, solder selection, fluxing, pre-​cleaning and surface treatment, cleaning after soldering, and inspection, have lot of similarities with brazing process. Therefore, without going deep into these aspects, only salient features of certain elements relevant to radiator design are described here. Subsequently, two versions of the soldering process are described. One involves the conventional soldering of automobile or locomotive radiators, and the other involves the fluxless ultrasonic soldering of all-​aluminum core for room air conditioners. 4.31.1.1 Joint Design Soldered radiator joints are required to perform in a severe environment. They are subjected to fluctuating temperatures, mechanical vibration, immersion under a water-​based fluid, and contact with dissimilar metals. Added to that is the frequent neglect of the quantity and quality of coolant solution [118]. 4.31.1.2 Tube Joints Conventionally, brass flat plates are solder coated and formed into tubes, lock-​seam jointed in a soldering machine. Recently, tubes with a butt-​welded joint have been used for the construction of tubes. This type of radiator tube is stronger and more leak proof than the conventional roll-​formed brass lock-​seam product and provides a material savings of about 25% through elimination of the lock seam [119]. 4.31.1.3 Tube-​to-​Header Solder Joints Solder alloys generally have lower strength properties than the base materials. Joints between the tubes and the header plates are essentially shear loaded in both directions as the radiator is heated, pressurized, and subsequently cooled [120]. Failure of the tube-​to-​header solder joint occurs because of a variety of problems: weak solder joints due to poor mechanical assembly and fit-​up problems, brittle fracture, creep and fatigue, and external or internal corrosion. Possible approaches for increasing the durability of the tube-​header solder joints are discussed by Webb et al. [121]. 4.31.1.4 Solders Solders generally have melting points or melting ranges generally below 800°F (425°C). There is a wide range of commercially available solders designed to work with most industrial metals and alloys. The solders were used to bind copper fins to brass tubes and brass tubes to headers, which are the essential steps in the radiator assembly. These methods are still widely employed today to make heavy-​duty radiators for truck and off-​road applications. The basic process consists of melting, flowing, and solidifying the solder at the joint, typically forming a metallic bond with the soldered surfaces (or parent metals). Typically, the filler metal flows into the joint gap by capillary force, solidifies, and forms a bond. Tin-​lead solders, improved tin-​lead solders, and zinc-​aluminum solders

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Fabrication, Brazing, and Soldering of Heat Exchangers

TABLE 4.10 Solder Compositions Sn–​Pb

Sn–​Pb–​Ag

Zn–​Al–​Cu

Sn

Pb

Sn

Pb

Ag

Zn

Al

Cu

5 10 15 20 25 30

95 90 85 80 75 70

5.5 5.0 3.8 2.5 1.0

94.0 94.5 95.0 95.0 97.5

0.5 0.5 1.2 1.2 1.5

95 89

5 7

—​ 4

are discussed in the following section and their composition is shown in Table 4.10. More details on solder selection can be found in Ref. [117]. Lead-​based solders. Tin-​lead solders are most widely used in the joining of metals, particularly conventional automotive radiators. Most commercial fluxes, cleaning methods, and soldering processes may be used with tin-​lead solders. Mechanical properties. The requirements of a lead-​based solder for tube-​to-​header radiator joints include the following [118]: 1. It must have a minimum amount of tin so that sufficient tin oxide film is formed to passivate the surface. 2. It must have excellent mechanical properties, particularly with regard to creep resistance, to minimize damage to the protective film. 3. It must have good solderability properties. 4. It must be cost-​competitive with alternate joining materials. All lead-​based solders have a very low yield point. Under almost any load, they are subjected to some plastic deformation. These alloys are particularly sensitive to creep deformation. Improved lead-​tin solder alloy. For years, the standard radiator solder had been 70:30 lead/​tin. The 70:30 solder is relatively brittle and prone to stress cracking as a result of vibrational stresses built up in the radiators. This led to solder-​joint failures. This solder has been replaced almost entirely by high-​lead solders containing up to 97% lead [122]. This solder, called soft solder, could better withstand engine vibration; it has the following approximate composition: 97% lead, 1.5%–​2.5% tin, and 0.5%–​1.5% silver. The tin content was reduced both to save money and to improve mechanical properties. Solders containing less tin will form less eutectic, which will improve the solder’s high-​temperature mechanical properties [118]. At the same time, a certain concentration of tin in the solder is necessary. If sufficient tin is present in the solder, a tenacious and stable oxide film forms over the solder, which improves the corrosion resistance of the solder joint. Hence, it is necessary that the solder contain a minimum of 5% tin. In high-​lead solders, where mechanical strength is considered very important, for example, for tube-​to-​header joints, silver is added. Typical of these high-​lead solders are 95Pb-​5Sn and 97.0Pb-​0.5Ag-​2.5Sn (Modine). Zinc-​based solders. Zinc-​aluminum solder, the most standard grade 95Zn-​5Al is specifically used for ultrasonic soldering of aluminum. It develops joints with high strength and good corrosion resistance.

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4.31.1.5 Cleaning and Descaling An unclean surface prevents the solder from flowing and makes soldering difficult or impossible. Solvent or alkaline degreasing is recommended for cleaning oily or greasy surfaces. For removing rust or scale, pickling or mechanical cleaning may be used. 4.31.1.6 Soldering Fluxes Soldering fluxes are divided into three main groups based on their residues [120]: corrosive, intermediate, and noncorrosive. Whenever possible, use the least corrosive flux to eliminate the potential postsoldering corrosion problems stemming from the residual flux. Various compositions of fluxes for copper/​brass radiator manufacture include the following: • zinc chloride, ammonium chloride, and water to make • zinc chloride, sodium chloride, ammonium chloride, hydrochloric acid, and water to make • zinc chloride, ammonium chloride, petroleum jelly, and water to make.

4.31.2 Soldering Processes Soldering processes or the heating methods available for heat exchanger manufacture are as follows: Flame or torch soldering. Handheld manual torch soldering is most frequently used for repairs to tube-​to-​tube-​plate joints. Furnace or oven soldering. Furnace soldering is considered when an entire assembly (e.g. radiator) is to be brought to the soldering temperature and when the assembly is complicated in nature, making other heating methods impractical. Dip soldering. The dip soldering method uses a molten bath of solder to supply both the heat and the solder necessary to join the workpieces. Tube-​to-​header joints of radiators are made by this process. Ultrasonic soldering. Ultrasonic soldering can be considered a form of dip soldering, in which the workpiece is immersed in a tank of molten solder. Ultrasonic soldering is discussed separately. Infrared soldering. This involves focusing infrared light (radiant energy) by means of an optical system. Typical applications include soldering pretinned interference fit return bends by Reynolds interference fit. 4.31.2.1 Various Stages of Radiator Manufacture by Conventional Soldering Process The solders are used to bind copper fins to brass tubes and brass tubes to headers, which are the essential steps in the radiator assembly. These methods are still widely employed today to make heavy-​duty radiators for truck and off-​road applications. The basic process consists of melting, flowing, and solidifying the solder at the joint, typically forming a metallic bond with the soldered surfaces (or parent metals). Typically, the filler metal flows into the joint gap by capillary force, solidifies, and forms a bond. In the soldered design, a radiator core is formed by soldering the plain fins or corrugated fins to flattened heat exchanger tubes, and the core is connected to the upper and the lower tanks via a pair of header plates. Radiator tubes are normally made by fabricating a lock seam and joining this seam continuously in a soldering machine. Then the core is formed in a fixture, wherein the fins are stacked and inserted with flat tubes. To insert tubes into the fins stack, a piercing tool is used. The header plates are inserted and pressed onto tubes for making tube-​to-​header joints. These stages are listed in detail as follows:

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Fabrication, Brazing, and Soldering of Heat Exchangers

1. Tube forming (lock seam), fluxing, solder coating, and cutting to size. 2. Burr removal at the tube ends. 3. Fin rolling. 4. Fin assembly in a fixture. 5. Insertion of tubes. 6. Fitment of header plates. 7. Pre-​cleaning. 8. Backing in a furnace. 9. Squaring of the core assembly and straightening of the blunt fins in the hot condition. 10. Expanding the tube ends using a chisel or a blunt formed tool. 11. Dip soldering of header plate with tubes. 12. Blowing off with air to clear solder blockages. 13. Hot water cleaning and rinsing. 14. Leakage test with a dummy water tank. 15. Fitment of the metal water tanks and soldering with a tube plate or fitment of plastic/​metal water tank and crimping with the tube plate; soldering of inlet, outlet, drain connections, etc. 16. Fitment/​soldering of side mounting/​side sheet assembly. 17. Checking the overall dimensions of the assembly. 18. Leak testing. 19. Drying and painting. 20. Packing and dispatching. 4.31.2.2 Flux Residue Removal Zinc chloride-​based flux residue is best removed by hot water washing containing 2% of concentrated hydrochloric acid, followed by hot water rinse. The silver nitrate test can be used to determine whether all of the salts have been removed [117]. Additional tests are discussed later.

4.31.3 Ultrasonic Soldering of Aluminum Heat Exchangers All-​aluminum coils for the air-​conditioning industry have been fabricated by flux soldering for many years. Use of flux requires flux cleaning equipment and wastewater treatment facilities. Potential problems from incomplete flux removal and new environmental legislation necessitated development of a fluxless joining process [123], which uses ultrasonic waves to agitate the soldering bath. As ultrasonic waves produce cavitation in the molten bath, they cause a scrubbing action that removes the surface oxide film, thus permitting the solder to wet the aluminum surfaces [124]. Being a fluxless process, ultrasonic soldering is becoming more widely used for the fabrication of all-​aluminum coils for the air-​conditioning industry. Detailed discussions on the subject can be found from [125–​127]. 4.31.3.1 Material that can be Ultrasonically Soldered By this method, hard-​to-​wet metals can be successfully soldered in a very few seconds. Aluminum combined with various other materials, stainless steel, and even ceramic materials has been wetted rapidly and successfully [126]. The ultrasonic soldering process has been used for joining 1100, 1200, 3003, 6061, 6063, Alclad 7072/​3003, and some 2xxx aluminum alloys. 4.31.3.2 Basic Processes for Soldering All-​aluminum Coils Two basic processes that make use of ultrasonic soldering are (1) the “dip” or Alcoa 571 process and (2) the Reynolds interference fit process. Of the two, the Alcoa process is widely used, while the Reynolds method shows promise [127]. Alcoa 571 process. The Alcoa 571 process is a fluxless ultrasonic soldering process specifically developed for the tubular joints in all-​aluminum tube-​fin air conditioner coils. In simplest terms,

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641

FIGURE 4.116  Heat exchanger coil socket configuration.

FIGURE 4.117  Fluxless ultrasonic soldering by the Alcoa 571 process. (Adapted from [128] Jenkins, W.B.)

the process requires (1) a heated solder, (2) immersion of the part to be soldered, and (3) cavitation of the solder by ultrasonics. The requirements are met by the following four main components of the ultrasonic equipment [127]: (1) an ultrasonic power supply, (2) a set of transducers, (3) a heated vessel, and (4) a heat control console. Soldering procedure. The operations for fabrication and soldering of a coil can be divided into four general areas [125]: (1) coil preparation, (2) coil transport, (3) coil preheat, and (4) coil soldering. The coil preparation includes pre-​cleaning the return bends and the tube ends, preferably by ultrasonic vapor degreasing. The coils or core is made by assembling U-​tubes and fins and expanding the tubes in the conventional fashion. The ends of the U-​tubes are flared to a prescribed socket geometry (Figure 4.116) to provide a joint and a friction fit for the inserted return bends [125]. After vapor degreasing the return bends and core tubes ends, the return bends are simply tapped into position. The joints are preheated to soldering temperature in the inverted position by hot air or gas flame. The coil is then quickly transferred and lowered into a molten bath of 95% Zn-​5% Al solder and exposed for a few seconds to ultrasonic cavitation. The actual soldering of a coil requires only a few seconds of ultrasonic exposure. The sequence of operations is depicted in Figure 4.117. Solder composition and maintenance of soldering bath. The solder composition recommended is 95Zn-​5Al, which is maintained at a temperature of 780°F–​800°F. Since each coil processed through the soldering bath takes out some solder with it, the bath should be replenished with fresh solder to maintain its depth. During the tinning process, some aluminum will be eroded off the parts [128].

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Fabrication, Brazing, and Soldering of Heat Exchangers

FIGURE 4.118  Ultrasonic pot –​round bottom. (Adapted from [126] Hunicke, R.L.)

FIGURE 4.119  Socket geometry of heating/​cooling coils for ultrasonic cleaning. (Adapted from [125] Schuster, J.L. and Chilko, R.J.)

Therefore, the replenisher is slightly lean in aluminum, typically 98Zn-​2Al, to keep the bath at 5% Al [124]. Process control. There are four factors that have significant effects on the joint quality [117]: (1) ultrasonic solder pot design, (2) socket design, (3) preheat, and (4) depth of immersion. Another important factor is the soldering time. Ultrasonic soldering pot. The heart of the Alcoa 571 process is the ultrasonic soldering pot, which is the container for the molten zinc-​based solder [126]. Generally, the pot is made from AISI 304-​type stainless steel. These pots come in rectangular (flat bottom), circular, or contoured shape. Bottoms of rectangular pots usually have one or two rows of transducers. Circular and contoured pots are better for soldering large coils because they have room for more transducers [124]. Figure 4.118 shows a sketch of the ultrasonic pot. In general, rectangular tanks have not performed satisfactorily when compared to round tanks [127]. Socket design. Joints of tube sockets are designed for strength and resistance to corrosion. The ends of the U-​tubes are flared to a prescribed socket geometry (Figure 4.119) to provide a joint area and a friction fit for the inserted return bends. Solder penetrates the annular space between the return bend and the socket, wets the surfaces, and completes the joint.

Fabrication, Brazing, and Soldering of Heat Exchangers

643

Preheat temperature. The assembled coil must be preheated before it is dipped in the molten solder, to drive out all the moisture. Preheating reduces the pot size and the soldering time. The preheating should be uniform to ensure that all sockets are above the solder temperature. Preheating time should be minimum to reduce the amount of oxidation. Excessive oxide formation due to longer preheating time makes it difficult for the cavitating solder to break up the oxide film [123, 124]. At the same time, the preheating temperature must be high enough to keep the solder from solidifying on parts entering the ultrasonic bath. Preheating sockets at 840°F–​950°F (450°C–​510°C) ensure that all are above the soldering temperature of 780°F (415°C) without being overheated. For preheating, oscillating spear flame burners are used on nearly every production. Depth of immersion. In an ultrasonic soldering process, the most important factor is the depth to which the joint is submerged [125], which is shown schematically in Figure 4.119. Soldering does not take place until the joint goes deep enough for its interior surfaces to become wet, even though its outer surfaces may start to wet at shallower depths. From the laws of acoustic physics, it is known that the bath develops a gradient of cavitation intensity, reaching a maximum of 1.5–​2 in. (38.50–​ 50.8 mm) beneath the bath surface [124]. Soldering time. Aluminum parts to be tinned are preheated to 800°F, inserted in the solder pot, and the ultrasonics is activated for approximately 3–​6 s, depending on the solder temperature, coil complexity, and solder pot characteristics [87]. At least 3 s is usually required for good joints. For copper and steel, the time should be increased to 5–​8 s. Longer duration of soldering time lengthens the joint length, but causes erosion of return bends. Reynolds process. The Reynolds Metals Company process (U.S. Patent 3,633,266) or interference process comprises four basic steps [128]: 1. Ultrasonic tinning: pretinning of the ends of return bends by immersion in a molten solder bath, which is agitated by ultrasonic energy. 2. Assembly of the bare and pretinned tubular components. 3. Heating of the preassembled tubular components to melt the solder film. 4. Final assembly by the mechanical pressing of the “cork-​fit” tube joints together to disrupt the oxide on the bare member and thereby provide a metallurgically bonded leak-​free joint. The sequence of the Reynolds process is depicted in Figure 4.120.

FIGURE 4.120  Sequence of ultrasonic soldering of heat exchanger coils by Reynolds process. (Adapted from [128] Jenkins, W.B.)

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Fabrication, Brazing, and Soldering of Heat Exchangers

Merits of the Reynolds process. Because only the return bend ends need tinning, large ultrasonic pots are not required, and the ultrasonic equipment cost is also very much less. Depth of insertion of parts to be tinned in the pot is not critical [121].

4.32 NONDESTRUCTIVE TESTING OF SOLDERED HEAT EXCHANGER In this section, QC, inspection, and testing of soldered heat exchangers are discussed. Some or most of the part of the discussion may not be relevant for ultrasonically soldered heat exchangers. QC during various stages of flux-​soldered radiator manufacture should consider at a minimum the following points: 1. fin and tube dimensions, thickness of solder coating 2. solder bath temperature and composition 3. tube washing tank temperature and pH 4. soldering furnace temperature 5. core washing tank pH 6. leak testing and recording the number of leakages in tank, pipe, plate, tube, and others 7. core dimensions.

4.32.1 Visual Inspection Nearly all soldered joints are visually inspected for defects. A soldered joint surface should be shiny, smooth, and free from cracks, porosity, or holes. Ordinarily, no flux residues should remain.

4.32.2 Discontinuities Some types of discontinuities common for soldering are dewetting, nonwetting, and dull or rough solder.

4.32.3 Removal of Residual Flux Corrosivity of residual flux deposits is important. A water extract method of measuring corrosivity through conductivity has been developed. Extracts from flux residues can also be evaluated. The three basic methods of flux residue detection are radioactive tracers, standard chemical analysis, and fluorescent dye evaluation[120]. Minute amounts of dye in the remaining flux readily fluoresce under ultraviolet light.

4.32.4 Pressure and Leak Testing Pressure testing and leak testing can be used to check the integrity of the soldered joints.

4.32.5 Destructive Testing Destructive testing involves sectioning of soldered joints for determining the quality of joints in the interior. Normal destructive testing techniques include mechanical tests, corrosion evaluation, and metallographic examination.

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645

4.33 PROPERTIES OF BRAZED JOINTS 4.33.1 The Requirements of Properties of Brazed Joints The requirements of properties of brazed joints include [129]: 1. mechanical properties 3. microstructure 3. corrosion resistance. 4.33.1.1 Mechanical Properties The thermal cycle of the brazing process will change the mechanical properties of both types of alloys, non-​heat-​treatable and heat-​treatable. The effect depends on the alloy and its original temper. Provided a fast cooling after brazing can be accomplished heat treatable alloys can be naturally or artificially aged. 4.33.1.2 Microstructure The changes of the mechanical properties are due to alterations of the submicrostructure (recovery, recrystallization, dissolution and re-​precipitation of phases). Furthermore in particular the base metal in contact with the liquid filler metal will change its microstructure. This and the formation of a fillet will give some indications about the quality of the brazed joints. 4.33.1.3 Corrosion Resistance Brazing involves joining of two similar or dissimilar metal pieces by a filler metal with composition different from the base metals or the same as the base metals. Therefore, galvanic corrosion is a possibility. In addition to galvanic corrosion, corrosion attack is possible due to entrapment of residual corrosive flux. Brazed radiator joints are required to perform in a severe environment. They are subjected to fluctuating temperatures, mechanical vibration, immersion under engine cooling water, and in contact with dissimilar metals, engine exhaust gases, etc. Added to that is the frequent neglect of the quantity and quality of coolant solution [118].

4.34 CORROSION OF BRAZED JOINTS AND CORROSION CONTROL METHODS Brazing involves joining of two similar or dissimilar metal pieces by a filler metal with composition different from the base metals or the same as the base metals. Therefore, galvanic corrosion is a possibility. In addition to galvanic corrosion, corrosion attack is possible due to entrapment of residual corrosive flux. Brazed radiator joints are required to perform in a severe environment. They are subjected to fluctuating temperatures, mechanical vibration, immersion under engine cooling water, and in contact with dissimilar metals, engine exhaust gases, etc. Added to that is the frequent neglect of the quantity and quality of coolant solution [118].

4.34.1 Factors Affecting Corrosion of Brazed Joints All forms of corrosion can take place in brazed joints. Parameters that contribute to the corrosion of brazed joints are the following: 1. galvanic couple between the base metal and the filler metal 2. joint design 3. brazing method and brazing environment 4. brazing filler metal

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Fabrication, Brazing, and Soldering of Heat Exchangers

5. flux material and retention 6. post-​braze cleaning 7. thermal stresses induced during brazing 8. process related causes.

4.34.2 Corrosion of the Brazed Aluminum Joint 4.34.2.1 Galvanic Corrosion Resistance The corrosion resistance of a brazed aluminum joint, completely free of corrosive flux, is directly related to the solution potential difference that may exist between the alloys in contact. The lower the potential difference, the lower is the rate of corrosion. Potential differences of less than 0.013 V are usually considered insignificant [58, 90]. Since the aluminum brazing fillers are aluminum alloys rather than dissimilar metals, they have very slight potential differences. Therefore, the possibility of galvanic corrosion is low. The common filler alloys 4343, 4045, and 4047 produce a solution potential of −0.82 V in the standard test solution and commonly brazed alloys 1100, 3003, 6061, and 6063 show −0.83 V. The difference in solution potential between these two groups of alloy is 0.01 V, which is insignificant, and this is responsible for the excellent corrosion resistance of the brazed joint. 4.34.2.2 Influence of Brazing Process Process related causes The service life of a heat exchanger may be shortened due to corrosion caused by process related events. Some examples are listed below: 1. Excessively high brazing temperature or time at temperature will lead to excessive Si diffusion in the core. 2. It was proposed by Naruki et  al. [130] that the furnace-​brazed radiator was superior to the vacuum brazed radiator, as far as corrosion resistance is concerned, since the former incorporated a zinc diffusion layer in its surface, which afforded the sacrificial corrosion. This aspect is further discussed next, and suggestions are given to overcome the problem. 3. Zinc retention: pitting corrosion property of vacuum-​brazed 7072 clad aluminum alloy. To attain high pitting corrosion resistance, aluminum heat exchanger tubes in cooling water system are made of 7072 clad aluminum alloy, which contains a nominal concentration of 1% zinc. In the vacuum brazing process, the 7072 clad tube loses its alloying element zinc by vaporization and diffusion [131]. As a result, pitting resistance properties may be changed following vacuum brazing. This is especially true for lighter gauges, 0.02 in. (0.5 mm) or less. However, for heavier gauges, 0.05 in. (1.3 mm), there is sufficient retained zinc in the post-​ braze composite [132]. In inert-​gas brazing, the evaporation of the zinc is minimized [89]. Four factors influence the degree of zinc loss in the vacuum brazing process [133]: a. initial zinc content in the cladding b. cladding thickness c. furnace thermal cycles d. vacuum level. Others 1. Development of K805: Kaiser Aluminum Co. developed a new cladding alloy, one in which the active alloying element would be nonvolatile in the vacuum brazing process. This cladding alloy has been designated K805.

Fabrication, Brazing, and Soldering of Heat Exchangers

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FIGURE 4.121  Multilayer clad aluminum brazing sheet.

2. A multilayer-​clad aluminum material with improved brazing properties [134]. Multiclad consists of a normal heat-​treatable core alloy of the 6000 type clad with commonly used braze cladding of the 4000 type. As an intermediate layer between the core and the braze cladding is an alloy of either pure aluminum or aluminum-​manganese such as 3003, as shown schematically in Figure 4.121. This intermediate layer acts as a diffusion barrier and significantly reduces silicon penetration into the core during brazing, with reduced risk of intergranular corrosion. The melting range of the interlayer is higher than that of the core material. 4.34.2.3 Forms of Corrosion Aluminum is affected by the following forms of corrosion: atmospheric corrosion, uniform corrosion, galvanic corrosion, pitting corrosion and erosion corrosion [135, 136].

4.34.3 Corrosion Protection in All-​aluminum Microchannel Coil Heat exchangers of microchannel coil technology utilize several aluminum alloys in with a metallic coating after brazing. The alloys are carefully chosen to extend the life of the coil. Furthermore, the coil has been designed so that any galvanic couple within the coil has been carefully chosen to provide the maximum life possible for the coil. To prevent corrosion, proper aluminum alloy selection, surface treatment, i.e. surface coating, and field maintenance are necessary. There are several corrosion prevention options and methods that can be adopted for all-​aluminum heat exchangers. These include the application of protective coatings and regular surface cleaning. The durability of aluminum enables its use in many applications, and in doing so it may come into contact with aggressive environments. To achieve strength, aluminum is alloyed with other elements to improve its mechanical and corrosion properties. aluminum alloys of series 3xxx, 4xxx, 7xxx, and 9xxx offer long life. Corrosion protection methods are discussed in References [135–​138].

4.34.4 Corrosion Protection Cooling water system pH control: To prevent corrosion of the aluminum which may be caused by an oxide layer dissolving at extreme pHs (in both acidic and basic environments), the typical pH range for microchannel heat exchangers is 6.0 to 8.5. Controlling the pH within the desired range is critical for controlling aluminum corrosion. Kaltra, Germany one of the leading MCHE

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Fabrication, Brazing, and Soldering of Heat Exchangers

FIGURE 4.122  Illustration of galvanic corrosion of copper tube-​aluminum fin heat exchanger. Adapted from Ref. [137].

manufacturers recommends a well-​proven mechanism of corrosion inhibition of aluminum alloys by sodium molybdate. Unlike many other inhibitors, sodium molybdate doesn’t contribute nutrients, which can exacerbate microbiological activity in closed-​circuit water systems [135, 136]. 4.34.4.1 Multi-​metal Closed-​loop Systems In most HVAC application, refrigerant circuits are multi-​metal, and the points of contact between dissimilar metals are subject to galvanic corrosion. One method of preventing galvanic corrosion is the elimination of direct bi-​metalalic contacts. For closed-​loop chilled water systems, it is recommended that pH-​neutral, molybdate based multi-​metal corrosion inhibitors be added to the cooling medium which is compatible with glycols and suitable for all water qualities. 4.34.4.2 Protective Coatings For working in corrosive environments such as sea coasts, industrial zones, and highly polluted areas, microchannel coils may require additional protection. Electrocoating is a process that uses an electrical current to deposit an organic coating from a paint bath onto a heat exchanger body. Figure 4.122 shows galvanic corrosion of copper tube-​aluminum fin heat exchanger coil and Figure 4.123 shows precoated heat exchanger coil, Ref.(137). 4.34.4.3 Sacrificial Zinc Coatings Optimized aluminum alloy composition contributes to a high strength after brazing and provides sacricial layer to improve the long-​term corrosion resistance for microchannel tubes. Putting a zinc layer on top of an aluminum alloy protects the core of the tube by providing a preferred path for corrosion to spread. 4.34.4.4 Trivalent Chromium Process Coating Trivalent chromium process (TCP) conversion coating is a type of conversion coating used to passivate aluminum alloys as a corrosion inhibitor. Unlike hexavalent chromium, trivalent chromium is non-​toxic and fully complies with RoHS (Restriction of Hazardous Substances) requirements. 4.34.4.5 Corrosion Tests [138] 1. ASTM G85-​A3 –​Acidified Synthetic Sea Water Testing (SWAAT test) 2. Salt Spray Test 3. ASTM B117-​19 –​Standard Practice for Operating Salt Spray (Fog) Apparatus 4. ASTM G85-​19 –​Standard Practice for Modified Salt Spray (Fog) Testing.

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FIGURE 4.123  Precoated heat exchanger coil. Adapted and modified from Ref. [137].

4.35 CORROSION OF SOLDERED JOINTS The basic corrosion mechanism on high lead solder in a radiator is galvanic. With a galvanic attack, solder is the more active metal and hence corrodes, whereas the more noble metal, the brass, acts as the cathode. Corrosion resistance of various solders in automobile coolant systems is compared by Falke et al. [139].

4.35.1 Solder Bloom Corrosion SBC can be defined as a corrosion mode of the copper/​brass automotive radiator’s internal solder joints [140]. Park [140] uses the term bloom to describe the flowering appearance of corrosion by products. SBC, in general, leads to the blockage of radiator tube passages when the internal surface is solder coated or of tube openings when the tube outer surfaces are solder coated. Two different field repair methods are employed to clean the blocked tubes. They are (1) roding by utilizing small-​size ice picks and thin steel rods and (2) caustic rinse. Roding involves removal of corrosion products.

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Fabrication, Brazing, and Soldering of Heat Exchangers

4.35.2 Manufacturing Procedures to Control Solder Bloom Corrosion To reduce the SBC potential of copper/​brass radiators, the radiator materials and manufacturing process can be adjusted as recommended [140]: • Introduce air blow-​off operation immediately after solder dip to minimize the solder build up at the opening and inside surfaces of the tube ends. • Change the header dip process to the air-​side soldering process to minimize the solder coating inside the tube surface. • Use low-​lead solders (Pb below 90%), in place of high-​lead solders like 97/​2.5/​0.5 (Pb/​Sn/​ Ag), 94/​5.5/​0.5 (Pb/​Sn/​Ag). • Avoid solder coating inside tube surface. • Use a properly inhibited coolant. • Use an appropriate radiator rinse cleaning system.

4.36 EVALUATION OF DESIGN AND MATERIALS OF AUTOMOTIVE RADIATORS In general, mechanically assembled aluminum radiators, vacuum-​brazed aluminum radiators, and copper/​brass radiators are evaluated by a number of accelerated tests designed to build confidence in design before large-​scale fleet trials. Some of these tests are designed to represent worst field conditions [141]. These tests are primarily used to assess (1) the mechanical durability and (2) the internal and external corrosion resistance, inhibitors evaluation, etc. The mechanical durability tests and corrosion evaluation tests carried out for the development and field service history of Ford automotive aluminum radiators are discussed by Barkely et al. [141]. Some of these tests are briefly discussed here.

4.36.1 Mechanical Durability Tests Some of the tests developed to evaluate the mechanical durability of the aluminum radiator are given in the following: 1. combined pressure cycle and vibration test 2. vibration test 3. coolant fill test 4. thermal shock test 5. vehicle durability test.

4.36.2 Tests for Corrosion Resistance 4.36.2.1 External Corrosion Tests The aluminum radiator is susceptible to two major types of external corrosion: galvanic and localized pitting. In general, uniform corrosion is not an important durability problem for the aluminum radiator, since it results from exposure to very high or very low pH solutions in the absence of inhibitors [141]. Some of the laboratory tests for evaluating external corrosion resistance are the following: 1. Salt spray (fog) test –​ASTM Salt Spray (fog) Testing B-​117. This involves internally pressurizing the radiators to 145 kPa (21 psig) and then subjecting them to 1000 h of salt spray testing.

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Fabrication, Brazing, and Soldering of Heat Exchangers

2. Humidity test. Several thousand hours of testing in a noncondensing humidity chamber, maintained at 38°C (100°F) and approximately 95% relative humidity. 3. SWAAT test. This is a severe acidified seawater fog corrosion test designed to induce rapid attack. 4.36.2.2 Internal Corrosion Tests The major internal corrosion concerns are the possibility of intergranular corrosion, erosion corrosion, crevice corrosion, and pitting attack. Internal corrosion tests used to evaluate materials and coolants are as follows: 1. laboratory glassware corrosion tests like static pitting tests and polarization diagrams 2. simulated service circulation test.

ANNEXURES ANNEXURE TABLE 4.11 QAP (partial) for STHE Fabrication Sl.No. 1.0 2.0 3.0 4.0

Component/​Operation/​ Inspection

10.0

Review of drawing Review of quality assurance program Review of WPS, PQR and WPQR Material receipt inspection (including bought outs) Examination of marking of materials Material attestation Examination of cutting –​by flame for carbon steel and by plasma for others/​ nonferrous material Examination of prepared plate edges Cold forming of shell, rolled neck, dished head and RF pad Stress relieving of cold formed rolled neck

11.0

Normalizing of cold formed dished heads

11.1

Testing of coupon test plate

12.0

Examination of formed dished head

13.0

Dished end to plug piece welding

14.0

Weld deposition on floating head flange, shell flange and pass partition plate

5.0 6.0 7.0

8.0 9.0

Characteristic —​ —​ -​-​ As per material specification and technical delivery condition Dimensions Verification of transferred identification Preheating to 149°C for carbon steel of thickness above 32 mm

Bevel preparation Profile Stress relieving cycle: specify rate of heating, soaking temperature, soaking time and rate of cooling Normalizing cycle (with coupon test plate): specify soaking temperature, soaking time and cooling in still air a. Tensile b. Micro a. Profile b. Minimum thickness c. PT on both inside and outside of knuckle portions and on edges a. Set up b. 1st side welding as per WPS c. 2nd side welding groove d. 2nd side welding as per WPS e. Full radiography Weld deposition as per WPS, PT on barrier and final layer (Continued)

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Fabrication, Brazing, and Soldering of Heat Exchangers

ANNEXURE TABLE 4.11  (Continued) QAP (partial) for STHE Fabrication Sl.No.

Component/​Operation/​ Inspection

15.0

Longitudinal seam/​circular seam/​seam in test flange, floating head to flange welding, seam in rolled neck

16.0

Nozzle neck to WNRF flange

17.0

Nozzle opening on shell, nozzle

18.0

Nozzle on shell/​nozzle (with reinforcement pad –​wherever applicable)

19.0

External/​internals

20.0

Tubesheet/​baffle holes marking

21.0

Examination of drilled tubesheet

22.0

Examination of drilled baffles

23.0

Assembly of tube bundle

24.0

Tube-​to-​tubesheet joint

25.0

Examination of shell before bundle insertion

Characteristic a. Set up b. 1st side welding as per WPS c. 2nd side welding groove d. 2nd side welding as per WPS e. Spot radiography (at least 10% of total weld length must be radio graphed, length of each RT be 250 mm) –​except for floating head to flange welds, test flange seams. f. PT/​MT on floating head to flange weld a. Set up b. 1st side welding as per WPS (PT on root run for single side welding) c. 2nd side welding groove if accessible d. 2nd side welding as per WPS e. Spot radiography for nozzle connection 10 inch and above a. Marking b. Cutting c. Bevel preparation a. Set up b. Inside welding as per WPS (PT on root run for single side welding) c. Second side welding groove (if accessible) d. Second side welding as per WPS e. PT/​MT on welds f. Air leak test on reinforcement pad at a pressure of 1.25 kg/​cm2 (g) a. Set up b. Welding as per WPS a. Hole diameter b. Pitch c. Number of holes d. The tie rod holes in baffle e. Tie rod/​pulling eye/​sliding strip fixing hole in tubeheet a. Outer tube limit, facing, bolt circle dia., ligament, etc. b. Hole diameter using go/​no go gauge c. Taped hole for the rod/​pulling eye/​slot for sliding strip d. Pass partition grooves e. Expansion grooves (if any) a. No. of holes and hole diameter using go/​no go gauge b. Tie rod holes c. Slots for sliding strip a. Tie rod fixing b. Baffle, supporting plate and spacer tube arrangement c. Sliding strip to supporting plate/​baffle welding d. Tube insertion a. Tube projection b. Tube expansion/​welding Out of roundness checking using pull through gauge

Fabrication, Brazing, and Soldering of Heat Exchangers

653

ANNEXURE TABLE 4.11  (Continued) QAP (partial) for STHE Fabrication Sl.No. 26.0 27.0

Component/​Operation/​ Inspection Pneumatic test at a pressure of 1.25 kg/​ cm2 g or as per code Examination before hydrostatic test

28.0

Hydrostatic test at a pressure of 1. Shellside 2. Tubeside

29.0 30.0

Examination after hydrostatic test Cleaning

31.0 32.0

Painting Marking

33.0

Final examination

34.0

Stamping and shipping

35.0 35.1

Documents To be retained by vendor

35.2

To be sent to customer

Characteristic a. Minimum two calibrated pressure gauges shall be used. b. Leak lightness a. Verification of NDE reports b. Overall dimension a. Minimum two calibrated pressure gauge shall be used b. Leak tightness c. Water used for hydrostatic shall have approx 2% (by volume) of an approved wetting agent in addition to 0.2% of sodium nitrate or any other corrosion inhibitor Visible and dimensional a. Equipment shall be dried by passing dry air b. Equipment shall be purged by dry nitrogen of three times the volume or as specified in the specification As per drawing a. Tangent lines/​main axis b. Owners name and address c. Item no and PO No. d. Inspection by a. Inspection of equipment b. Verification of documents, spares and loose items c. Blanking and rust prevention d. Test hole plugging by hard grease a. Inspectors stamp b. Rub off of details c. Dry N2 filling at 0.25 bar a. Material test certificates b. Material and data report c. Time temp chart d. NDE and non-​conformity report e. Pressure test reports f. Rub off of name plate g. Final dimensional report h. As made drawing a. Inspection certificates b. Material and data report c. Heat treatment chart d. NDE report e. Pressure test reports f. Rub off of name plate and stamping details g. Final dimensional report h. As made drawing i. Others

Note: RF, pad-​reinforcement pad; RT, radiographic; MT, magnetic particle; PT, liquid penetrant; PQR, procedure qualification record and WPS, welding procedure specification.

654

Fabrication, Brazing, and Soldering of Heat Exchangers

ANNEXURE TABLE 4.12 ITP for Air-​Cooled Heat Exchanger Inspection and Test Plan for Fin-​Tube Heat Exchanger or Air-​Cooled Heat Exchanger: • All plates need to be identified against mill test certificates before commencement of fabrication. • Checking test certificates of all materials for pippins, accessories, motors, gear box, etc. • Ensuring that welding procedure and welders are qualified before commencement of fabrication • Where applicable, selection spots for radiography and other nondestructive tests. • If specified witnessing any crack detection, hardness checks, ultrasonic test, etc. • Reviewing of radiographs. • Witnessing of hydrostatic test • Dimensional check and carrying out final inspection of complete bundles for quality of workmanship. • Witness balancing of fans • Witness of fans to undergo an assembly running test (fan gear & motor). • Dimensional check of fan rings. • Inspection of motors including checking of test certificates to ensure that performance tests have been carried out in accordance with applicable specifications. • Witness running tests on gearboxes and gearing to be examined on completion. • Structural steel work to be checked for quality of workmanship, painting and spot-​checked for dimensional accuracy. Ref. www.inspection-​for-​industry.com/​inspection-​and-​test-​plan-​for-​fin-​tube-​heat-​exchanger.html

REFERENCES 1. Sharma, D. D., Heat exchanger fabrication techniques, Lecture Notes of Course No. 490-​SPL, Energy Efficient Heat Exchangers Design, Continuing Education Department, University of Roorke, Roorke, India, 1990. 2. General Engineering Specification for Pressure Vessels, BPCL-​CCR, Uhde India Private Limited, Document No. 6509-​MQ-​EB-​00001, A Company of ThyssenKrupp Technologies, 2009. 3. www.ins​pect​ion-​for-​indus​try.com/​heat-​exchan​ger-​ins​pect​ion.html 4. www.ins​pect​ion-​for-​indus​try.com/​ins​pect​ion-​and-​test-​plan-​for-​press​ure-​ves​sel.html 5. Rao, B. S. C., Quality assurance in fabrication of pressure vessels, in Welding Engineering Hand-​ hook –​Vol. 1 (S. Sundarrajan, S. Vijaya Bhaskar, and G. C. Amarnath Kumar, eds.), Welding Research Group, Radiant Publications, Secunderabad, India. 6. https://​lets​fab.in/​press​ure-​vess​els-​heads/​ 7. Rao B. S. C., Quality Assurance in Fabrication of Boilers and Pressure Vessels, HEB 97, Alexandria, Egypt April 5–​6, 1997, pp. 1–​18. 8 Volume XXIV: A to Z of Pressure Vessels –​Part I | Boardman LLC (boardmaninc.com) 9. Weymueller, C. R., NDT builds safety into a nuclear plant, Weld. Design Fabr., September, 54–​57 (1982). 10. Peacock, G. A., Fabrication of thick plates, Weld. Metal Fabr., June, 242–​252 (1963). 11. Carpenter, O. R. and Floyd, C., Heat treatment of carbon and low alloy pressure vessel steel, Research Suppl., J. Am. Soc. Naval Eng., 69(3), 515–​526, (August 1957). 12. Davies, S., Equipment for boiler fabrication, Weld. Metal Fabr., August, 306–​309 (1969). 13. Aldridge, M., Plate bending and forming, the choice between 3-​roll and 4-​roll bending rolls, Weld. Metal Fabr., November, 555–​566 (1977). 14. Bode, C. F., Manipulators and positioners for welding, Weld. Metal Fabr., January/​February, 26–​30 (1987). 15. Ellis, D. J. and Gifford, A. F., Application of electroslag and consumable guide welding, Weld. Metal Fabr., April, 112–​119 (1973). 16. Yokell, S., A Working Guide to Shell and Tube Heat Exchangers, McGraw-​Hill, New York, 1990. 17. Syal, P. K., Fabrication of Heat Exchangers, Specialised Publication, Cooperative Industries, Mumbai, India.

Fabrication, Brazing, and Soldering of Heat Exchangers

655

18. Trawsfynydd Power Station, Part 1: Heat exchanger fabrication, Weld Metal Fabr., September, 350–​ 361 (1961). 19. Luthy, A., The role of welding in the fabrication of machines and equipment, Brown Boveri Rev., 52, 495–​511 (1965). 20. Woodward, A. R., Howard, D. L., and Andrews, E. F. C., Condensers, pumps and cooling water plant, Chapter 4, in Modern Power Station Practice, Vol. C, Turbines, Generators, and Associated Plant (D. J. Littler, E. J. Davies, H. E. Johnson, F. Kirkby, P. B. Myerscough, and W. Wright, eds.), 3rd edn., Pergamon Press, New York, 1991. 21. Singh, K. P. and Soler, A. I., Mechanical Design of Heat Exchangers and Pressure Vessel Components, Arcturus, Cherry Hill, NJ, 1984. 22. Bevevino, J. W., Tube-​to-​tubesheet welding techniques, Chem. Eng. Prog., 70, 71–​73 (1974). 23. Olson, D. L., Chaney, E., Dallam, C B., Hellner, R. L., Jones, J. E., Liu, S., North T. H., Sabo, R. S., and Sims, J. E., Submerged arc welding, in Metals Handbook, Vol. 6, Welding, Brazing, and Soldering, 9th edn., (E. F. Nippes, ed.), American Society for Metals, Metals Park, OH, 1983, pp. 114–​151. 24. Ebert, H. W., Improving the reliability of tube-to-tubesheet joints, Welding J. Am. Welding Soc., 79(9), 47–49 (2000). 25. Sanders, B. J., Tube-to-tubesheet joints: The many choices, ATI Wah Chang, pp. 111–118. https://www. titanmf.com/wp-content/uploads/docs/Tube-to-tubesheet-joints-The-Many-Choices-BJ-Sanders.pdf 26. https://www.jetvisionengineering.com/newsdetail/418.html 27. Yokell, S., Expanded, and welded-and-expanded tube-to-tubesheet joints, Trans. ASME J. Pressure Vessel Technol., 114, 157–165 (1992). 28. Scott, D. A., Wolgemuth, G. A., and Aikin, J. A., Hydraulically expanded tube-to-tubesheet joints, Trans. ASME J. Pressure Vessel Technol., 106, 104–109 (1984). 29. Fisher, F. F. and Brown, G. J., Tube expanding and related subjects, Trans. ASME, 76(4), 565–575 (1954). 30. Sonnenmoser, A., Expanded joints for austenitic steel tubes in feedheaters for BWR nuclear power station, Brown Boveri Rev., 7/8–73, 360–370 (19). 31. Dudley, F. E., Electronic control method for the precision expanding of tubes, Trans. ASME, 76, 577–584, (1954). 32. Yokell, S., Heat exchanger tube-to-tubesheet connections, in The Chemical Engineering Guide to Heat Transfer, Volume 1: Plant Principles (K. J. McNaughton, ed.), Hemisphere Publishing Corporation, McGraw-Hill Publications Co., New York, 1986, pp. 76–92. 33. https://tei.co.uk/services/explosive-engineering/ 34. Cotton, H., Seal welding of heat exchanger tubulars by the metal arc process, Br. Weld. J., May, 250–257 (1963). 35. Spencer, T. C., Mechanical design and fabrication exchangers in the United States, Heat Transfer Eng., 8, 58–61 (1987). 36. Kong, C.-S., Lee, B.-I., and Shim, S.-H., The characteristics and application of explosive joint on the tubes-to-tubesheet in the heat exchangers of power plants, Transactions of the 15th International Conference on Structural Mechanics in Reactor Technology (SMiRT-15), Seoul, Korea, August 15–20, 1999, pp. IV-81–IV-87. 37. Brosilow, R., Making the most of tube-tubesheet joints, Weld. Design Fabr., May, 54–56 (1976). 38. ASME Boiler and Pressure Vessel Code, Section VIII, Division 1—Pressure Vessels, American Society of Mechanical Engineers, New York, 2021. 39. CODAP: French Code for the Construction of Pressure Vessels, A SNCT, 1985. 40. www.axx​air.com/​en/​blog/​tube-​prep​arat​ion-​for-​tube-​to-​tube-​sheet-​weld​ing 41. Slaughter, G. M., Welding and Brazing of Nuclear Reactor Components, AEC Monograph, Row-​man and Littlefield, New York, 1964. 42. Lohmeier, A. and Reynolds, S. D., Jr., Carbon steel tube-​to-​tubesheet joints for high pressure feedwater heaters, Westinghouse Eng., September, 138–​143 (1966). 43. Danis, J. I., Material selection, fabrication and inspection of refinery heat exchangers, Weld. J., June, 25–​30 (1986). 44. TIMET CodeWeld® Titanium Tubing, TIMET(now Valtimet, Inc.), Denver, CO, 1984. 45. Viri, D. P. and Iceland, W. F., Clinch River modular steam generator tube-​to-​tubesheet and shell closure welding, Weld. J., July, 18–​21 (1984).

656

Fabrication, Brazing, and Soldering of Heat Exchangers

46. Serangeli, G. L., Bebernitz, A. R., Hemzacek, R. T., and Lesnewich, A., Arc welding of carbon steels: Shielded metal arc welding, in Metals Handbook, Vol. 6, Welding, Brazing, and Soldering, 9th edn. (E. F. Nippes, ed.), American Society for Metals, Metals Park, OH, 1983, pp. 73–​244. 47. Designing and fabricating aluminum weldments, Weld. Design Fabr., February, 53–​62 (1977). 48. Sonnenmoser, A., Design and operation of large high pressure feedheaters, Brown Boveri Rev., 7/​8–​73, 352–​359. 49. Marino, A. and Carminati, M., Enhanced inspection of tube-​to-​tubesheet welds in heat exchanger and JFTA for defect characterization, ECNDT 2006-​Tu.3.8.3, Berlin, Germany, 2006, pp. 1–​5. 50. Hudgell, R. J. and Tickle, H., Current practices for ultrasonic and radiographic examination of tubes, tubeplates, and tubeplate welds of tube bundles in heat exchangers, in Developments in Pressure Vessel Technology –​2, Inspection and Testing (R. W. Nichols, ed.), Applied Science Publishers, London, U.K., 1979, pp. 77–​100. 51. Engle, J. P., Chemical cleaning of feedwater heaters, Mater. Perform., July, 30–​33 (1974). 52. Ganapathy, Welding engineer data sheet, Weld. Design Fabr., October, 104–​105 (1979). 53. Galletly, G. D., Plastic buckling of torispherical and ellipsoidal shells subjected to internal pressure, Proc. Inst. Mech Eng., 195, 329–​345 (1981). 54. Vessel fabrication at Old Park, Weld. Metal Fabr., November, 392–​397 (1991). 55. Jilek, L., Technology of pressing large heads, Weld. Metal Fabr., March, 50–​53 (1987). 56. Furnace atmospheres no. 4. Brazing of metals, Linde AG, Gases Division, Unterschleissheim, Germany, pp. 1–​51. 57. NOCOLOK® Encyclopedia, Solvay Special Chemicals, pp.1–​87. 58. Aluminum Brazing Handbook, 4th edn., Aluminum Association, Washington, DC, 1990(Reaffirmed December 2010). 59. BRH, 5th Edition, American Welding Society, Miami, FL 1991. 60a. Shah, R. K., Brazing of compact heat exchangers, in Compact Heat Exchangers –​A Festschrift for A. L. London (R. K. Shah, A. D. Kraus, and D. Metzger, eds.), Hemisphere, Washington, DC, 1990, pp. 491–​529. 60b. Shah, R. K., The art of brazing, in Thermophysical Aspects of Manufacturing and Materials Processing (R. K. Shah et al. eds.), Hemisphere Publishing Corporation, Washington, DC, 1992, pp. 193–​213. 61. EAA Aluminium Automotive Manual –​Joining, 6. Brazing, Version 2015, European Aluminium Association ([email protected]), pp. 1–​25 62. GH The Brazing Guide, GH Induction Atmospheres, Rochester, New York, pp. 1–​8. 63. NOCOLOK® Flux Brazed Aluminum Heat Exchangers for the Refrigeration and Air Conditioning Industry, A Solvay Special Chemicals Aluminum Heat Exchangers, pp. 1–​7. 64. NOCOLOK® flux brazing process Solvay Fluor GmbH, Hannover, Germany, pp.1–​7. 65. Jones, N. H., Brazing: Its contribution to modern industry, Weld. Metal Fabr., January, 4–​9 (1964). 66. Weymuller, C. R., How to choose filler metals, Weld. Design Fabr., July, 33–​39 (1984). 67. Birchfield, J. R., Paste fillers, automation take pain out of brazing, Weld. Design Fabr., June, 78–​80 (1980). 68. Singleton, O. R., A look at the brazing of aluminum –​Particularly fluxless brazing, Weld. J., November, 843–​849 (1970). 69. Brazing, Reference Issue, Weld. Design and Fabr., 66(6), 15–​71, June (1993). 70. Moorhead, A. J. and Whalen, S. J., Brazing of stainless steels, in Metals Handbook, Vol. 6, Welding, Brazing, and Soldering, 9th edn. (E. F. Nippes, ed.), American Society for Metals, Metals Park, OH, 1983, pp. 1001–​1013. 71. Miller, F. M., How to choose nickel base filler metals for vacuum brazing, Weld. Design Fabr., October, 87–​88 (1977). 72. Harvey, J., The brazing of aluminum: Part 1, Weld. Metal Fabr., September, 454–​457 (1977). 73. Schultze, W. and Schoer, H., Fluxless brazing of aluminum using protective gas, Weld. J., October, 644–​651 (1973). 74. TWI Ltd (Head Office), Granta Park, Great Abington, Cambridge, CB21 6AL, UK www.twi- ​ g lo​ b al.com/​ t echni​ c al-​ k nowle​ d ge/ ​ f aqs/​ f aq-​ w hat-​ a re-​ t he-​ d iffer​ e nt-​ m eth​ o ds-​ o f-​ b raz​ ing#:~:text=​Torch%20Braz​ing,and%20r​equi​res%20low%20capi​tal%20inv​estm​ent.

Fabrication, Brazing, and Soldering of Heat Exchangers

657

75. www.alumin​ium-​braz​ing.com/​2010/​12/​22/​remo​val-​or-​clean​ing-​of-​post-​braze-​flux-​resi​due/​ 76. ANSI/​AWS C3.7–​93, Specification for Aluminum Brazing, American Welding Society, Miami, FL, 1993. 77. Philip M. Roberts, The brazing of connecting tubes to stainless steel copper-​brazed plate heat exchangers, Alfa Laval, 2010. (Also “Copper Brazed and Fusion Bonded Compact Plate Heat Exchangers in Water Applications-​Material aspects and life time limiting factors-​“, edited by Jens Rassmus for Alfa Laval 2008). 78. Flame Brazing with NOCOLOK® flux, pp. 1–​11. 79. www.alumin​ium-​braz​ing.com/​2010/​03/​19/​braz​ing-​alumin​ium-​and-​cop​per/​ 80. https://​indu​ctio​nhea​ting​expe​rts.com/​induct​ion-​heat​ing-​appli​cati​ons/​braz​ing/​heat-​exchan​ger-​tubes/​ 81. https://​indu​ctio​nhea​ting​expe​rts.com/​induct​ion-​heat​ing-​appli​cati​ons/​braz​ing/​heat-​exchan​ger-​tubes/​ 82. Fast Induction Brazing of Heat Exchangers, eldec, Auburn Hills, MI 83. Dan Kay, Using Induction Brazing in Manufacturing Operations (Part 1), Kay & Associates, Simsbury, CT. www.indust​rial​heat​ing.com/​artic​les/​92822-​using-​induct​ion-​braz​ing-​in-​manufa​ctur​ ing-​ope​rati​ons-​part-​1 84. www.kay​braz​ing.com/​braz​ing-​artic​les/​1000​929-​braz​ing-​furna​ces-​vac​uum-​vs-​con​tinu​ous-​belt.html 85. Tennenshouse, C. C., Control of distortion during the furnace cycle, Weld. J., October, 701–​704 (1971). 86. (a) Boughton, J. D. and Roberts, P. M., Furnace brazing, a survey of modern processes and plant, Weld. Metal Fabr., March, 85–​92 (1973); (b) Boughton, J. D. and Roberts, P. M., Furnace brazing—​ 2, A survey of modern process and plant, Weld. Metal Fabr., April, 121–​138 (1973). 87. Winterbottom, W. L., Process control criteria for brazing aluminum under vacuum, Weld. J., October, 33–​39 (1984). 88. Patrick, E. P., Vacuum brazing of aluminum, Weld. J., October, 159–​162 (1983). 89. Schmatz, D. J. and Winterbottom, W. L., A fluxless process for brazing aluminum heat exchangers in inert gas, Weld. J., October, 31–​38 (1983). 90. Lentz, A. H., Brazing of aluminum alloys, in Metals Handbook, Vol. 6, Welding, Brazing, and Soldering, 9th edn. (E. F. Nippes, ed.), American Society for Metals, Metals Park, OH, 1983, pp. 1022–​1032. 91. Swaney, W., Trace, D. E., and Winterbottom, W. L., Brazing aluminum automotive heat exchangers in vacuum: Process and materials, Weld. J., May, 49–​57 (1986). 92. www.sol ​ vay.com/​ s ites/​ g /​ files/​ s rpend​ 2 21/​ files/​ t rid​ i on/​ d ocume​ n ts/​ N OCO​ L OK-​ B raz​ i ng-​ P roc​ ess-​2018-​02.pdf 93. Lucas-​ Milhaupt Brazing Experts, Benefits of Controlled Atmosphere Brazing in Automotive Manufacturing, Oct 31, 2018 93.1. https://​blog.lucasm​ilha​upt.com/​en-​us/​about/​blog/​benef​i ts-​of-​con​trol​led-​atm​osph​ere-​braz​ing-​in-​aut​ omot​ive-​manufa​ctur​ing 94. www.seco​warw​ick.com/​en/​produ​cts/​cab-​con​trol​led-​atm​osph​ere-​braz​ing-​furna​ces/​ 95. Controlled Atmospheric Aluminium Brazing (CAB) System, Seco/​Warwick, Inc., 2023, Świebodzin, Poland 96. https://​ampc​omp.com/​news-​upda​tes/​con​trol​led-​atm​osph​ere-​furn​ace-​braz​ing-​for-​cost-​effect​ive-​ assem​bly/​ 97. Terrill, J. R., Cochran, C. N., Stokes, J. J., and Haupin, W. E., Understanding the mechanisms of aluminum brazing, Weld. J., December, 833–​839 (1971). 98. Cooke, W. E., Wright, T. E., and Hirschfield, J. A., Furnace brazing of aluminum with a noncorrosive flux, Weld. J., December, 23–​28 (1978). 99. McCubbin, J. and Lemay, R., A modification of the NOCOLOK™ flux brazing process to produce superior corrosion resistance or black surfaces, SAE-​900408, pp. 416–​425. 100. Ashburn, L. L., Furnace design considerations for aluminum brazing under vacuum, Weld. J., October, 45–​54 (1983). 101. www.kal​tra.com/​sin​gle-​post/​2020/​02/​24/​micro​chan​nel-​heat-​exc​hang​ers-​manufa​ctur​ing-​proc​ess 102. Executive Report, High efficiency and extreme durability result from ICA’s CuproBraze technology and Young Touchstone’s FLATROUND technology, International Copper Association, Ltd, New York.

658 103. 104. 105. 106. 107. 108.

109. 110. 111. 112. 113. 114. 115. 116. 117. 118. 119. 120.

121. 122.

123. 124. 125. 126. 127. 128. 129. 130.

131. 132.

Fabrication, Brazing, and Soldering of Heat Exchangers https://​cup​robr​aze.com/​overv​iew/​cup​robr​aze-​tec​hnol​ogy/​ www.army-​tec​hnol​ogy.com/​cont​ract​ors/​vehic​les/​finnr​adia​tor/​ https://​cup​robr​aze.com/​wp-​cont​ent/​uplo​ads/​2014/​11/​Cup​roBr​aze-​ER-​33.pdf Bulletin CUB-​ 2013, International Copper Association, Ltd. 260 Madison Avenue, 16th Floor, New York, N www.seco​warw​ick.com/​en/​tech-​spo​tlig​hts/​cup​robr​aze/​ Kelly, T. J., Andreano, G. A., Cove, E. J. Flynn, J., Gerken, J. M., Ryan, E. J., and Sponaugle, J. C., Brazing of heat resistant alloys, in Metals Handbook, Vol. 6, Welding, Brazing, and Soldering, 9th edn. (E. F. Nippes, ed.), American Society for Metals, Metals Park, OH, 1983, pp. 1014–​1021. Lindsay, L. B. and Wireman, D. F., Continuous hydrogen furnace brazing of aircraft heat exchangers, Weld. J., October, 737–​739 (1975). Amato, I., Cappelli, P. G., and Fenoglio, G., Brazing of stainless steel heat exchangers for gas turbine applications, Weld. J., October, 338-​s–​343-​s (1975). Welding Engineer Data Sheet No. 493 –​Selection guide for brazing filler metals, Weld. Design Fabr., March, 82 (1982). Flux Removal: Post-​Braze Cleaning, Lucas-​Milhaupt, Cudahy, WI, 2016 https://​lucasm​ilha​upt.com/​EN/​Resou​rce-​Libr​ary/​Techni​cal-​and-​Saf​ety-​Data-​She​ets.htm https://​lucasm​ilha​upt.com/​EN/​Braz​ing-​Acad​emy/​Braz​ing-​Funda​ment​als.htm https://​blog.lucasm​ilha​upt.com/​en-​us/​about/​blog/​ins​pect​ing-​bra​zed-​joi​nts Inspecting Brazed Joints, Lucas-​Milhaupt Brazing Experts on Jul 11, 2014 (blog) Lucas, M. J., Peaske, R. L., Commen, W. H., and Tomsic, M. J., Brazing, in Welding Hand Book, Vol. 2, 8th edn., American Welding Society, Miami, FL, 1991, pp. 279–​422. Lane, J. W. and Brennan, D. T., Improved solder alloy enhances auto radiators, Weld. J., October, 41–​45 (1984). Mill welds thin wall aluminum radiator tubes at 600 fpm, Weld. J., October, 52 (1982). Beal R. E., Bud, P. J., Smith J. F., and White, C.E.T., Soldering, in Metals Handbook, Vol. 6, Welding, Brazing, and Soldering, 9th edn. (E. F. Nippes, ed.), American Society for Metals, Metals Park, OH, 1983, pp. 1071–​1101. Webb, R. L. and Farrell, P. A., Improved thermal and mechanical design of copper/​brass radiators, SAE-​900724, 1990, pp. 737–​748. Mitchell, W. A., Statistical treatment of laboratory data for ASTM D 1384–​70, using soft solder, in Engine Coolant Testing: Slate of the Art, ASTM STP 705 (W. H. Ailor, ed.), American Society for Testing and Materials, Philadelphia, PA, 1980, pp. 220–​232. Haddon, R. C., Ultrasonics dispenses with need for soldering flux, Weld. Metal Fabr., January/​ February, 50–​54 (1979). Gunkel, R. W., Solder aluminum joints ultrasonically, Weld. Design Fabr., September, 90–​ 95 (1979). Schuster, J. L. and Chilko, R. J., Ultrasonic soldering of aluminum heat exchangers, Weld. J., October, 711–​717 (1975). Hunicke, R. L., Ultrasonic soldering pots for fluxless production soldering, Weld. J., March, 191–​194 (1976). Denslow, C. A., Ultrasonic soldering equipment for aluminum heat exchangers, Weld. J., February, 101–​107 (1976). Jenkins, W. B., Fluxless soldering of aluminum heat exchangers, Weld. J., January, 28–​35 (1976). R. Mundt, Hoogovens, Koblenz, Introduction to Brazing of Aluminium Alloys, TALAT Lecture 4601, pp. 1–​24. Naruki, K. and Hasegawa, Y., Corrosion testing of furnace and vacuum brazed aluminum radiators, in Engine Coolant Testing: State, of the Art, ASTM STP 705 (W. H. Ailor, ed.), American Society for Testing and Materials, Philadelphia, PA, 1980, pp. 109–​132. Hattori, T. and Sakamoto, A., Pitting corrosion property of vacuum brazed 7072 clad aluminum alloy, Weld. J., October, 339-​s–​342-​s (1982). McNamara, P. and Singleton, O. R., Zinc distribution in vacuum brazed Alclad brazing sheet, Weld. J., January, 7-​s–​11-​s (1979).

Fabrication, Brazing, and Soldering of Heat Exchangers

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133. Scott, C. A., A new cladding alloy for coolant side corrosion protection of vacuum brazed aluminum radiators, in Engine Coolant Testing: Second Symposium, ASTM STP 887 (R. E. Beal, ed.), American Society for Testing and Materials, Philadelphia, PA, 1986, pp. 27–​43. 134. Engstrom, H. and Gullman, L. O., A multilayer clad aluminum material with improved brazing properties, Weld. J., October, 222-​s–​226-​s (1988). 135. www.kal​tra.com/​wp-​cont​ent/​uplo​ads/​2020/​04/​SG_​M​CHE-​Pro​tect​ive-​Coati​ngs_​Ver.1.3_​EN.pdf 136. www.kal​tra.com/​sin​gle-​post/​2017/​09/​14/​corros​ion-​pro​tect​ion-​for-​micro​chan​nel-​heat-​exc​hang​ers 137. Selection Guide: Environmental Corrosion Protection Condenser Coils and Cooling/Heating Coils for Commercial Products, December 2012, Carrier Corporation Syracuse, New York, pp. 1–15. https://www.shareddocs.com/hvac/docs/1001/Public/08/04-581061-01.pdf 138. www.kal​tra.com/​sin​gle-​post/​2019/​05/​16/​ele​ctro​coat​ing-​for-​micro​chan​nel-​heat-​exc​hang​ers 139. Falke, W. L., Schwaneke, A. E., and Lee, A. Y., Corrosion of solders in automobile coolant systems, Weld. J., October, 460-​s–​465-​s (1973). 140. Park, K. H., Solder bloom corrosion analysis based on field survey data, potential causes, and resolution, in Engine Coolant Testing: Second Symposium, ASTM STP 887 (R. E. Beal, ed.), American Society for Testing and Materials, Philadelphia, PA, 1986, pp. 123–​143. 141. Barkley, J. M., Wiggle, R. R., and Winterbottom, W. L., The development and field service history of Ford automotive aluminum radiators, in Engine Coolant Testing: Second Symposium, ASTM STP 887 (R. E. Beal, ed.), American Society for Testing and Materials, Philadelphia, PA, 1986, pp. 11–​26.

Suggested Readings Allen, J. S., Fabrication of heat exchangers and specialised vessels, Weld. Metal Fabr., July, 240–​245 (1968). www.bru​ker.com/​en/​appli​cati​ons/​ind​ustr​ial/​met​als/​posit​ive-​mater​ial-​ide​ntif​i cat​ion-​pmi.html Brooker, H. R. and Beatson, E. V., Industrial Brazing, Newnes-​Butterworths, London, U.K., 1975. https://​inspe​ctio​neer​ing.com/​tag/​posit​ive+​mater​ial+​ide​ntif​i cat​ion Cold spinning dished ends, Weld. Metal Fabr., January, 30–​32 (1969). Dawson, R. J., Silver brazing copper and its alloys, Weld. Metal Fabr., February, 54–​59 (1971). Douglass, M., Feedwater heating systems, in Modern Power Station Practice, Vol. C, Turbines, Generators, and Associated Plant (D. J. Littler, E. J. Davies, H. E. Johnson, F. Kirkby, P. B. Myerscouh, and W. Wright, eds.), Pergamon Press, New York, Chapter 3. Franklin, H. N., Roper, D. R., and Thomas, D. R., Internal bore welding method repairs condenser leaks, Weld. J., December, 49–​51 (1986). Foster, B. E. and McClung, R. W., Study of x-​ray and isotopic techniques for boreside radiography of tube-​to-​tubesheet welds, Mater. Eval., American Society for Nondestructive Testing, July 1977, pp. 43–​51. Haslinger, K. H. and Hewitt, E. D., Leak tight, high strength joints for corrosion resistant condenser tubing, ASME Paper No. 83-​JPGC-​Pwr39, ASME, New York. Hatch, J. E., Properties of Commercial Wrought Alloys, in Aluminum: Properties and Physical Metallurgy, American Society for Metals, Metals Park, OH, 1984, pp. 354–​355. Hidemasa, I., Consideration on tube-​to-​tubesheet joint of zirconium and titanium tubular heat exchangers, The ATI Wah Chang, www.atimet​als.com/​bus​ines​ses/​busin​ess-​units/​…/​1999​011.pdf Illingworth, A., Testing and inspection, in Heat Exchanger Design Handbook, Vol. 4 (E. U. Schlunder, editorin-​chief), Hemisphere, Washington, DC, 1983, Section 4.7. Koman, P. and Thiemal, K., Heat exchanger in industry, 3rd International Symposium on Orbital Welding in Heat Exchanger Industry, Organized by Polysoude, Paris, France, 1999, pp. 1–​20. https://​lucasm​ilha​upt.com/​EN/​Braz​ing-​Acad​emy/​Braz​ing-​Funda​ment​als.htm McGill, W. A. and Weinbaum, M. J., Alonized heat exchanger tubes give good high temperature service, Mater. Perform., January, 16, 18–​20 (1978). McIntyre, D. R., How to prevent stress corrosion cracking in stainless steels—​Part I, in The Chemical Engineering Guide to Corrosion Control in the Process Industries (R. W. Greene, ed.), McGraw-​Hill Publications Co., New York, 1986, pp. 4–​7. Moorehead, A. J. and Red, R. W., Internal bore welding of 2.25Cr-​1Mo steel tube-​to-​tubesheet joints, Weld. J., January, 26–​36 (1980).

660

Fabrication, Brazing, and Soldering of Heat Exchangers

Schwartz, M. M., Brazing, ASM International, Metals Park, OH, 1987. Taylor, M. A., ed., Plate-​Fin Heat Exchangers, Guide to Their Specification and Use, HTFS (Harwell Laboratory), Oxon, U.K., 1980. Way, M., Willingham, J. and Goodall, R. orcid.org/​0000-​0003-​0720-​9694 (2019) Brazing filler metals. International Materials Reviews. Susanne Hattingh, Bilfinger Power Africa, J. Peter, N. Paine and Ulla E. Gustafsson, Steam Generator Reference Book, Revision 1 Volume 1, EPRI Perspective, Project RP2858; RP4004, Project Managers: J. P. N. Paine, based on work sponsored by The Steam Generator Owners Groups I and II, The Steam Generator Reliability Project, and Electric Power Research Institute, December 1994.

661

Index A Acoustical holography, 437 merits, 437 Acoustic emission testing (AE/​AET), 444–​449 acousto-​ultrasonics (AU), 449 AE methods applications, 447 AE methods applications as per ASME code sec V, 447 applications: role of AE in inspection and quality control of pressure vessels and heat exchangers, 448 emission types and characteristics, 444 equipment, 447–​448 factors influencing AE data, 448 Kaiser effect, 445 merits of acoustic emission testing, 449 physical phenomena that can release AE, 447 principle of acoustic emission, 444 reference code, 447 signal analysis, 448 source of acoustic emission, 444 written procedure for AET, 446 Acoustic pulse reflectometry (APR), 466 ASTM E2906/​E2906M-​18, 466 Advanced UT methods 439–​444 advanced ultrasonic backscatter technique (AUBT), 440 dry-​coupled ultrasonic testing (DCUT), 440 internal rotating inspection systems(IRIS), 442 long range ultrasonic testing (LRUT)/​guided wave long range UT, 441 rapid ultrasonic gridding (RUG), 440 time of flight diffraction (TOFD), 444 Alloys for subzero/​cryogenic temperatures, 294–​302 cryogenic vessels, 300 ductile–​brittle transition temperature (DBTT), 295 fabrication of cryogenic vessels and heat exchangers, 300 materials for low-​temperature and cryogenic applications, 295–​300 aluminum for cryogenic applications, 296–​297 austenitic stainless steel, 300 carbon steels and alloy plate steels, 298–​299 copper and copper alloys, 297 nickel and high-​nickel alloys, 297–​298 products other than plate, 299–​300 titanium and titanium alloys, 297 9% nickel steel, 300–​301 guidelines for welding of 9% NI steel, 301 merits of 9% nickel steel, 301 postweld heat treatment, 301–​302 welding problems with 9% NI steel, 301 notch toughness, 295 notch toughness: ASME code requirements, 295 requirements of materials for low-​temperature applications, 295 safety in cryogenics, 303

selection of material for low-​temperature applications, 295 welding of austenitic stainless steels for cryogenic application, 302–​303 charpy V-​notch impact properties, 302 ferrite content, 302–​303 nitrogen pickup, 303 oxide inclusion content, 303 sensitization, 302 Alternating current field measurement (ACFM), 465–​466 applications, 466 Aluminium alloys: metallurgy, 237 corrosion prevention and control measures, 245–​247 alclad alloys, 246 alloy and temper selection, 245 aluminum diffused steels in petroleum refinery heat exchangers, 246 cathodic protection, 246 design aspects, 245 inhibitors, 246 modification of the environment, 246 thickened surface oxide film and organic coating, 246 corrosion resistance, 241–​243 chemical nature of aluminum: passivity, 242 resistance to waters, 242 seawater, 243 surface oxide film on aluminum, 241 fabrication, 247–​249 joint geometry, 248 parameters affecting aluminum welding, 247 preheating, 248 shielding gas, 248 surface preparation and surface cleanliness, 248 welding filler metals, 249 welding methods, 249 forms of corrosion, 243–​245 corrosion fatigue, 245 corrosion of aluminum in diesel engine cooling water system, 245 crevice corrosion, 245 erosion corrosion, cavitation, and impingement attack, 245 exfoliation corrosion, 244 galvanic corrosion, 243 intergranularcorrosion, 244 pitting corrosion, 243 stress corrosion cracking, 243 uniform corrosion, 243 properties of aluminum, 238 aluminum for heat exchanger applications, 238 product forms and shapes, 240–​241 temper designation system of aluminum and aluminum alloys, 240 wrought alloy designations, 238–​240 classification of wrought alloys, 239

661

662

662 Assembly of channels/​end closures, 577–​580 bolt tightening, 577 hydrostatic testing, 578–​580 cyclic hydrostatic testing of feedwater heater, 579 heaters, 579 Hydroproof™, 579 hydrostatic test fluid, 579 improved method for hydrostatic testing of welded tube-​to-​tubesheet joint of feedwater, 579 pneumatic test, 579 pneumatic testing procedure, 580 stamping, 580 standard test, 578 TEMA Standard requirement RCB-​1.3, 578 use of fluorescent or visible tracer dyes in hydrostatic test fluids, 579 Austenitic stainless steel, 194–​219 alloying elements and microstructure, 196 composition of wrought alloys, 196 alloy types and their applications, 197 type 304, 197 type 310, 197 type 316, 197 austenitic stainless steel fabrication, 206 properties and metallurgy, 194–​195 types of austenitic stainless steel, 194 alloy development, 194 welding, 206–​219 control of distortion, 216 corrosion resistance of stainless steel welds, 219 ferrite content, 207 filler metal for various grades of stainless steel, 208 filler metal selection, 207 gas coverage, 216 hot cracking, 207 joining stainless and other steels, 215 joint design, 208 lining 219 mechanical properties at cryogenic temperature and elevated temperature, 196 mechanism of corrosion resistance, 197–​198 passive versus active behavior, 198 resistance to chemicals, 198 sigma phase, 198 stainless steel in seawater, 198 microfissuring or liquation cracking in austenitic stainless steel weld, 209 nitrogen pickup, 212 porosity: beginning and end, 217 postweld cleaning 219 postweld heat treatment, 218 PWHT cracking, 218 properties of austenitic stainless steels, 195 protecting the roots of the welds against oxidation, 216–​217 gas shielding, 216 root flux, 217 resistance to various forms of corrosion, 198–​205 caustic SCC, 201 comparison of pitting and crevice corrosion of stainless steels, 200

Index crevice corrosion, 200 critical crevice corrosion temperature, 200 critical pitting corrosion temperature, 200 evaluation of sensitization of austenitic SS to PASCC, 203 factors influencing susceptibility to weld decay, 203 galvanic corrosion, 198 influence of nickel content on SCC, 201 intergranular corrosion, 203 knifelineattack, 205 laboratory tests to determine SCC, 203 localized forms of corrosion, 199 measures to overcome weld decay, 204 methods to prevent SCC of SS, 203 pitting corrosion, 199 pitting resistance number, 199 polythionic acid stress corrosion cracking (PASCC) , 202 prediction of IGC by laboratory tests, 205 prevention of scc in boiling water reactors, 203 SCC of welded austenitic stainless steels, 201 stress corrosion cracking, 201 sensitization (weld decay) and corrosion resistance, 215 shielding gases, 208 TIG welding techniques to overcome carbide precipitation, 216 variable weld penetration, 214 welding considerations, 208 welding consumables, 211 welding defects, 217 welding fumes, 216 welding practices to improve the weld performance, 216 welding procedure variations, 212 welding processes, 207, 212 welding stainless steels to dissimilar metals, 218–​219 filler metals for welding SS with dissimilar metal, 219 methods to overcome dilution problems, 218 Automated corrosion mapping, 468 Automotive Radiators, see Evaluation of Design and Materials of Automotive Radiators

B Barkhausen noise analysis, 467–​468 applications, 468 Bolted flange joint (BFJ), 70–​97 BFJ technical requirements, 72 bolting design, 94–​97 bolt area at the root of the threads, 95 determination of bolt loads, 94 flange bolt load W, 96 gasket seating conditions, 94 load concentration factor, 97 maximum recommended bolt spacing, 96 minimum bolt size, 96 minimum recommended bolt spacing, 96 operating conditions, 95

663

663

Index pitch circle diameter, 96 relaxation of bolt stress at elevated temperature, 97 bolting material, 80 design of bolted flange joints, 87–​93 design procedure, 87 gasket dimensions, 92 gasket factor, m, 92 gasket materials, 89 gasket or joint contact surface unit seating load, y, 92 gasket parameters, 89 gasket profile, 90 gasket size, 91 gasket width and diametral location of gasket load reaction, 93 selection of gasket material, 89 flange, 72–​80 ASME code approved flange materials, 78 ASTM standards for flange material, 78 flange classes and types, 73 flanged joint construction, 73 flange face finish definition and common terminology, 74 flange facings, 73 flange finish on gasket performance, 73 flange material selection, 78 flange standards, 74 flange standards-​ASME specifications, 76 flange types, 72 selection of flange, 78 flange design, 97–​99 bolting procedures, 99 flange moments, 97 flange thickness, 98 flanged joints in shell and tube heat exchanger, 85–​87 collar bolts in shell and tube heat exchanger, 86 flange types found in shell and tube exchangers, 86 girth flange, 86 heat exchanger gaskets, 85 forces acting on the bolted flange connection, 70 gaskets, 80–​85 effecting or creating a seal, 80 gasket categories, 80 gasket material considerations, 80 gasket seating, 81 gasket standards, 81 gasket types based on material, 81 guidelines for bolted flanged joint assembly procedure, 82 maintaining the seal, 71 step-​by-​step procedure for integral/​loose/​optional flanges design, 99 Brazing, 593–​595 brazing, 593 brazing advantages, 594 brazing codes and standards, 594–​595 ASME code section IX, welding, brazing, and fusing qualifications, 594 AWS A2.4:2020: standard symbols for welding, brazing, and nondestructive examination, 595 characteristics of brazing, 593

characteristics of soldering, 594 definition and general description of brazing and soldering process, 593 disadvantages of brazing, 594 Brazing, elements of-​595–​604 Brazing alloy or filler metals, 596 capillary attraction and joint clearance, 597 composition of filler metals, 597–​600 aluminium filler metals, 597 ASME Code specification for filler metals, 600 cladding alloys, 598 copper fillers, 598 forms of filler metal, 599 gold-​based fillers, 599 nickel-​based filler metals, 599 placement of filler metal, 599 silver-​based filler metals, 599 specification for filler metals for brazing, 597 fixturing, 603 fluxing, 601–​602 composition of the flux, 601 selection of a flux, 601 varieties of flux, 601 fluxing methods, 602–​603 demerits of brazing using corrosive fluxes, 603 heating method, 603–​604 diffuse heating techniques, 604 local heating, 603 joint gap, 596 joint design, 595 postbraze treatment and removing flux residues, 604 chemical cleaning, 604 mechanical cleaning, 604 ultrasonic cleaning, 604 properties of brazed joints, 645 corrosion resistance, 645 mechanical properties, 645 microstructure, 645 precleaning and surface preparation, 600–​601 chemical cleaning, 600 protection of precleaned parts, 601 scale and oxide removal, 600 selection of filler and flux, 597 Brazing methods, 605–​617 dip brazing, 609 furnace brazing, 611–​616 batch furnaces, 613 brazing furnace selection, 611 brazing furnace: vacuum vs. continuous-​belt, 611 brazing process cycle in a batch furnace, 614 brazing thermal cycle, 613 continuous furnaces, 615 fundamentals of brazing process control, 612 six fundamentals of brazing to follow, 605 torchor or flame brazing, 606–​609 brazing of inlet/​outlet tubes to end plate of brazed plate heat exchangers, 607 consumables–​gas, 606 flame brazing of aluminium with copper, 607

664

664 flame brazing with hand-​held filler or pre-​placed filler, 606, ​607 induction brazing of return bends of heat exchanger coil, 607 the induction brazing process, 607 joint clearances, 606 process parameters, 606 torch brazing of aluminum, 606 vacuum brazing, 616–​617 braze stopoffs, 617 brazing process cycle in a vacuum furnace, 616 furnace brazing-​safety awareness, 617 gases used in the process, 617 postbraze cleaning, 617 Brazing of aluminum, 617 AWS C3.7M/​C3.7-​2022: specification for aluminum brazing, 617 aluminum alloys that can be brazed, 618 aluminum brazing methods, 619–​627 aluminum dip brazing, 620 brazing of radiators and condensers, 620 brazing process, 623 controlled-​atmosphere brazing (CAB) process, 623 controlled dry air brazing, 622 CAB process advantages, 623 furnace brazing, 622 inert-​gas or controlled-​atmosphere brazing of aluminum, 622 key aspects of controlled atmosphere brazing (CAB), 622 postbraze cleaning and finishing, 627 vacuum brazing of aluminum, 625 elements of aluminium brazing, 618–​619 aluminum filler metals, 619 fluxing, 619 joint clearance/​select capillary size (Gap), 618 need for closer temperature control, 618 precleaning, 618 surface oxide removal, 618 Brazing of heat-​resistant alloys, stainless steel and reactive metals, 632–​634 brazing of cobalt-​based alloys, 633 brazing of nickel-​based alloys, 632–​633 brazing filler metals, 633 brazing of stainless steel, 633–​634 brazeability of stainless steel, 633 brazing of reactive metals, 634

C Carbon steels, 180–​182 carbon steel tubes for feedwater applications, 181 corrosion resistance, 181 fabrication, 182–​183 arc welding of carbon steels, 182 hardness limitation for refinery service, 182 plate cutting, 182 weldability considerations, 182 welding defects, 183 welding processes, 182

Index product forms, 180 refinery operations, 181 steel making process improvements, 179 steels, 179 types of steel, 180 use of carbon steels, 180 Cast iron, 178 Ceramics, 292–​293 classification of engineering ceramics, 292 Hexoloy® silicon carbide heat exchanger tube, 293 suitability of ceramics for heat exchanger construction, 292 types of ceramic heat exchanger construction, 292 Chromium–​molybdenum steels, 187–​189 applications, 189 composition and properties, 188 creep strength, 189 welding metallurgy, 189–​192 advanced 3 Cr–​Mo–​Ni steels, 192 control of temper embrittlement of weld metal, 191 CVN impact properties, 191 filler metal, 190 Larson–​Miller tempering parameter, 191 modified 9 Cr–​1Mo steel, 192 postweld heat treatment (stress relief), 191 preheating, 190 reheat cracking in Cr–​Mo and Cr–​Mo–​V steels, 192 step-​cooling heat treatment, 191 temper embrittlement of weld metal, 191 temper embrittlement susceptibility, 190 welding processes, 190 Cladding, 303 ASME Code requirements in using clad material, 317 clad tubesheets, 317 forming of clad steel plates, 318 cladding thickness, 304 clad plate, 304 backing materials, 304 corrosion resistance cladding grades, 304 explosive cladding, 313–​316 angular geometry, 314, 316 explosion cladding process sequence, 314 inspection of joint quality, 316 plug welding, 316 tube-​to-​tubesheet welding, 316 welding geometries, 314 materials & standards of clad steel plates, 311–​313 ASTM specification for clad plate, 312–​313 stainless steel clad metal, 312 standard & specification, 312 titanium clad steel plate, 312 methods of cladding, 304–​308 loose lining, 306 thermal spraying, 306 weld overlaying or weld surfacing, 307 weld dilution, 307–​308 roll cladding, 310–​311 inspection of overlays, 310

665

665

Index nickel alloy cladding, 310 procedure and welder qualification, 310 stainless steel strip cladding, 310 weld overlay cladding methods, 308–​310 processing of explosion clad plates, 317 test and inspection [295], 313 inspection of stainless steel cladding, 313 welding of stainless steel clad plate, 317–​320 selection of filler metals, 318–​319 titanium clad steel repair, 319–​320 welding clad plate by SMAW process, 318 Composite, 294 ASME code section X fiber-​reinforced plastic pressure vessels, 294 fiberglass tanks and vessels, 294 Conventional double tubesheet design, 60 conventional double tubesheet design TEMA guidelines, 60 Copper, 249–​260 copper alloy designation-​UNS system, 250 wrought alloys, 250 copper and aquatic life, 257 copper corrosion, 254–​257 biofouling, 256 condensate corrosion, 255 cooling-​water applications, 256 corrosion fatigue, 256 corrosion resistance, 254 dealloying (dezincification), 254 dealuminification, 255 denickelification, 255 deposit attack, 256 erosion–​corrosion, 255 exfoliation, 257 galvanic corrosion, 254 hot-​spot corrosion, 256 intergranular corrosion, 254 pitting corrosion, 254 resistance to seawater corrosion, 256 snake skin formation, 256 steam-​side stress corrosion cracking, 255 stress corrosion cracking, 255 sulfide attack, 256 copper heat exchanger applications, 250–​253 copper in steam generation, 250 designation of copper and copper alloys used as heat exchanger materials, 25 product forms–​copper tubes for heat exchanger, 251 copper welding, 257–​259 brasses, 259 copper alloys, 259 copper–​aluminum alloys (aluminum bronzes), 260 copper–​nickel or cupronickel alloys, 260 factors affecting weldability, 258 hot cracking, 258 porosity, 259 preheating, 258 PWHT, 260 silicon bronzes, 259 thermal conductivity, 258

thermal expansion, 258 weldability, 257 weldability considerations, 259–​260 welding precautions, 259 welding with dissimilar metal, 260 Corrosion of brazed joints and corrosion control methods, 645–​649 corrosion protection, 647–​649 corrosion tests [138], 648 multi-​metal closed-​loop systems, 648 protective coatings, 648 sacrificial zinc coatings, 648 trivalent chromium process coating, 648 corrosion protection in all aluminum microchannel coil, 647 forms of corrosion, 647 galvanic corrosion resistance, 646 influence of brazing process, 646 factors affecting corrosion of brazedjoints, 645 Corrosion of soldered joints, 649–​650 manufacturing procedures to control solder bloom corrosion, 650 solder bloom corrosion, 649 Cuprobraze heat exchanger, 629–​632 brazing process, 630 high-​performance coatings, 631 round tube versus flat tube, 631 tube fabrication, 629 Curved tubesheets, 60 Cylindrical shell, end closures, and formed heads under internal pressure, 61–​64 cylindrical shell under internal pressure, 62 design for external pressure and/​or internal vacuum, 63 end closures and formed heads, 64–​69 ASME F&D head, 67 conical, 66 ellipsoidal, 65 elliptical head 2:1, 67 flat cover, 64 hemispherical, 64 Klöpper head, 67 Korbbogen head, 67 minimum thickness of heads and closures, 67 torispherical head (or flanged and dished head), 67 thick spherical shells, 63 thin cylindrical shells, 62

D Design methods and design criteria for heat exchanger, 19–​22 allowable stress, 21 ASME code section VIII design criteria, 20 combined-​thickness approach for clad plates, 21 design by analysis (DBA), 20 design by rule (DBR), 20 design criteria, 20 design loads, 19 strength theories, 21 stress categorization, 20

666

666 welded joints, 22–​23 joint categories, 22 welded joint efficiencies, 22 weld joint types, 23 Destructive testing methods, 636–​637 Details of manufacture of STHE, 497–​509 details of manufacturing drawing, 497–​501 fabrication requirements, 499 edge preparation and rolling of shell sections, tack welding, and alignment for welding of longitudinal seams, 505–​508 fabrication of shell–​general, 507 identification ofmaterials, 505 positive material identification (PMI), 505 plate bending, 508–​509 roll bending, 508 quality control during assembly of parts, 503 quality control during production welding, 500–​501 shell and tube heat exchanger fabrication and inspectionnes, 501 tube bundle assembly, see Tube bundle assembly tubesheet and baffle drilling, see Tubesheet and baffle drilling welding of shells, checking the dimensions, and subjecting pieces to radiography, 509–​514 attachment of expansion joints, 513 checking the circularity of shell and the assembly fit, with nozzles and expansion joints welded, 513 dimensional check, 509 flanges, 512 operations checklist list for nozzles on shell/​dish welding, 512 PWHT of shells, 513 reinforcing pads and testing, 512 supports, 513 welding of nozzles, 510 Drones use in nondestructive testing, 468 Duplex stainless steels, 225–​234 advantages over the common austenitic stainless steels, 227 categories of duplex stainless steel, 226 characteristics of duplex SS, 226 comparison of duplex SS with austenitic and ferritic stainless steels, 228 composition, 227 corrosion resistance, 229–​230 intergranular corrosion, 230 pitting and crevice corrosion, 229 PREN of duplex stainless steels, 229 resistance to chemical environments, 229 expansion of tube to tubesheet joints, 234 metallurgy of duplex stainless steels, 227 Norsok standard, 233–​234 process applications, 230 products, 228 welding methods, 231–​233 balancing the austenite and ferrite phases, 231 ferrite in duplex stainless steels, 233 gas shielding, 232 heat input, 231

Index liquation cracking, 232 postweld stress relief, 233 precipitation of chromium nitrides, 232 sigma phase embrittlement and 475oC embrittlement, 232 weldability, 231 welding consumables, 232 welding practices to retain corrosion resistance, 233 welding methods for modern duplex stainless steels, 234 Dynamic NDT methods, 468

E Eddy current testing(ET/​ECT), 449–​459 ASTM specifications, 453 automated surface inspection using eddy current array technology, 459 calibration, 458 common applications, 450 eddy current arrays, 459 eddy current examinations methods as per ASME code Sec. V, 452 eddy current techniques, 450 eddy current test equipment, 454 eddy current testing, 451 eddy current testing principle, 449 inspection method for tube interior, 458 inspection of ferromagnetic tubes, 458 inspection or test frequency and its effect on flaw detectability, 456 limitations of eddy current testing, 458 merits of ET and comparison with other methods, 458 operating variables, 456–​457 depth of penetration and frequency, 456 edge effect, 457 fill factor and probe size requirements, 457 skin effect, 457 probes, 454 probe configuration, 454 reference standards for eddy current testing, 453 signal processing, 455 testing of weldments, 458 tube inspection, 451 written procedure for eddy current testing, 453 Electromagnetic acoustic transducers (EMAT), 469–​470 Electromagnetic sorting of ferrous metals, 469 ASTM E566-​19, 469 Equipment design features, 151–​152 access for inspection, 151 equipment life, 151 fail safe features, 151 field trials, 152 maintenance, 151 safety, 151 Evaluation of design and materials of automotive radiators, 650–​651 Mechanical durability tests, 650 tests for corrosion resistance, 650–​651 Expansion joints, 102–​103 bellows or formed membrane, 111–​119

667

667

Index applications, 113 ASME code sec VIII div 1 bellows expansion joints article 26, 117 bellows design: circular expansion joints, 115 bellows materials, 115 construction, 111 cycle life, 115 EJMA standards, 115 end fittings, 114 flow turbulence, 114 limitations and means to improve the operational capability of bellows, 117 movement capabilities, 113 classification of expansion joints, 103 flexibility of expansion joints, 103 formed head or flanged-​and-​flued head, 103–​111 ASME code and TEMA procedure for design, 108 construction, 105 design method as per ASME code sec VIII div 1, 110 design of formed head expansion joints, 106 finite element analysis, 107 Kopp and Sayre model, 106 Singh and Soler model, 106 TEMA procedure, 108 TEMA RCB–​8.1.1 analyis sequence, 110

F Fabrication of the shell and tube heat exchanger, 494–​497 manufacturing and testing, 495 quality assurance plan (QAP), 495–​497 hold points and witness points, 497 inspection and test plan, 495 features of ITP, 497 Ferritic stainless steels, 220 conventional ferritic stainless steels, 220 corrosion resistance, 224 intergranular corrosion, 224 fabricability, 224 “new” and “old” ferritic and austenitic stainless steels, 220 superferritic stainless steel, 220–​224 applications, 222 characteristics, 222 ductile–​brittle transition, 224 physical properties, 223 strength, 223 superferriticsalloy composition, 221 toughness and embrittling phenomena, 223 welding, 224–​225 Flanged tubesheets: TEMA design procedure A.1.3.3, 58 tubesheet extended as flange, 58 Foundation loading diagrams drawings, 582–​583 installation, maintenance, and operating instructions, 583 schematics or flow diagrams, 583 Fundamentals of tubesheet design, 35–​38 fixed tubesheet heat exchanger, 35 floating head heat exchanger, 38 tubesheet connection with the shell and channel, 35 u-​tube tubesheet, 36

G Glass, 290–​291 applications, 290 construction types, 290 drawbacks of glass material, 290–​291 glass-​lined steel, 290 mechanical properties and resistance to chemicals, 290 Graphite, 285–​289 applications of impervious graphite heat exchangers, 287 cubic graphite heat exchangers, 287 drawbacks associated with graphite, 287 equipment applications and service limitations, 286 graphite plate exchanger, 288 impregnated graphite, 285 shell and tube heat exchanger, 288 standard test method for impregnated graphite (mandatory appendix 38), 286

H Heads and closures, 583–​585 ASME flanged and dished (ASME F&D) heads, 583 cones, 590 conical, 585 crown-​and-​segment (C and S) technique, 588 dimensional check of heads, 592 dished heads, 586 flat heads, 585 forming methods, 585–​593 hemispherical head, 585 pressing, 587 pressure vessel heads, 583 purchased end closures, 592 PWHT of dished ends, 590 semi-​elliptical (SE) heads, 584 spinning, 586 torispherical head, 585 Heat exchanger tube inspection methods, 464–​465 eddy current testing of chiller tubes, 465 Hot cracking, 172 factors responsible for hot cracking, 172 susceptible alloys, 172 types of hot cracking, 172–​176 chevron cracking, 176 crater cracks, 176 ductility dip cracking, 176 heat-​affected zone liquation cracking, 173 reheat cracking or stress-​relief cracking, 174–​176 avoidance of reheat cracking, 175 susceptible alloys, 175 underclad cracking, 175 solidification cracking, 173 elements contributing to solidification cracking, 173 welding procedure-​related factors responsible for solidification cracking, 173 Hydrogen damage, 147–​150 detecting hydrogen damage, 149 fabricability, 150 high temperature hydrogen attack, 148 hydrogen embrittlement, 147

668

668 MR 0175/​ISO 15156, 149 Nelson curves, 148 prevention of hydrogen attack, 148 sources of hydrogen, 147 types of hydrogen damage, 147

I In-​service examination of heat exchangers for detection of leaks, 482–​485 Inspection, 362–​364 definitions, 362 design and inspection, 363 detailed checklist for components, 364 inspection guidelines, 363 master traveler, 364 objectives of inspection, 362 scope of inspection of heat exchangers, 363 material control and raw material inspection, 363 positive material identification, 363 TEMA Standards for inspection, 364 third-​party inspection, 364 hold points and witness points, 364 Inspection and testing of brazed joint, 634–​635 discontinuities, 635 quality of the brazed joints, 634

K Key terms in pressure vessel and heat exchanger design, 24–​25 corrosion allowance, 25 design pressure, 24 design temperature, 24 maximum allowable working pressure, 24 operating pressure or working pressure, 25 operating temperature or working temperature, 25

L Laboratory tests for determining susceptibility to cracking, 176–​178 multitask varestraint weldability testing system, 177 varestraint (variable restraint) test, 177 weldability tests, 176 Leak testing (LI), 474–​482 ASTM standards for LT, 475 helium mass leak detection methods, 480–​482 helium mass spectrometer test—​detector probe technique, 481 helium mass spectrometer test—​tracer probe technique, 481 helium mass spectrometer vacuum test—​hood technique, 481 leak test methods, 476–​480 acoustical leak detection, 476 acoustic emission leak testing, 479 “bombing” test, 478 bubble leak testing, 476 dye penetrant method, 477 gas leak lake testing, 476 halogen diode detector probe test method, 479

Index inside-​out helium vacuum chamber leak testing, 477 outside-​in helium leak testing, 477 pressure change testing, 477 pressure decay test, 477 vacuum decay test or pressure rise test, 477 radiotracer technique, 478 tracer gas leak testing, 479 ultrasonic leak detection, 476 water immersion bubble test method, 476 LT methods as per ASME code Sec V, 475 requirements of leak testing, 475 written procedure, 474 Lifting devices and attachments, 128 Liquid penetrant inspection (PT), 391–​396 acceptance standards, 395 applications, 392 developments in PT, 395 evaluation of indications, 395 excess penetrant removal, 395 limitations, 392 merits of PT, 392 method of inspection, 394 penetrants, 393 approved material, 394 penetrant application, 394 penetration time or dwell time, 394 postcleaning, 395 principle of inspection, 391 selection of developer, 394 standards, 393 standardization of light levels for penetrant and magnetic inspection, 395 surface preparation, 394 techniques, 391 test procedure, 393 written procedure, 392 Low-​alloy steels, 183–​185 applications of low-​alloy steel plates, 184 carbon–​manganese steels, 184 carbon–​manganese–​molybdenum steels, 185 carbon–​molybdenum steels, 184 low-​alloy steels for pressure vessel constructions, 184 selection of steels for pressure vessel construction, 183

M Magnetic flux leakage technique, 470–​471 Magnetic particle inspection (MT), 396–​403 acceptance standards, 403 application of examination medium, 402 demagnetization, 402 equipment for magnetic particle inspection, 400 evaluation of indications, 402 examination coverage, 401 factors affecting the formation and appearance of the magnetic particles pattern, 398 inspection medium (magnetic particles), 401 inspection method, 402 dry method, 402 wet method, 402 interpretation of indications, 403

669

669

Index limitations of the method, 398 magnetizing current, 399 magnetizing technique, 400–​401 coil magnetization, 400 prod magnetization, 401 yoke magnetization, 401 merits of MT, 398 MT techniques, 396 principle, 396 record of test data, 402 reference documents, 398 surface preparation for testing, 399 test procedure, 398 written procedure, 399 magnetic particle examination procedure deficiencies, 399 Magnetic rubber techniques (MRT), 403 Making up certificates, 582 Material selection principles, 135–​136 ASME code material requirements, 136–​137 Section II materials, 137 Section VIII div.1, 137 Section VIII div.1 requirements for pressure vessels constructed of nonferrous materials, 137 cost, 150 cost-​effective material selection, 150 desired material requirements features, 135 evaluation of materials, 150 functional requirements of materials, 138–​146 brittle fracture, 139 corrosion failures, 146 corrosion resistance, 145 creep, 141 fatigue strength, 139 heat and corrosion, 144 hydrogen damage, see Hydrogen damage strength, 138 temperature resistance, 142 toughness, 140 international material specifications, 138 material selection factors, 136 possible failure modes and damage in service, 150–​151 review of operating process, 136 sources of material data, 146 unified numbering system, 137 Materials for heat exchanger and pressure vessel construction, 155–​156 metals, 155 materials for high-​temperature heat exchangers, 285 non-​metals, 156 Mechanical design of pressure vessels and heat exchangers, 5–​15 ASME codes, 10–​15 Code interpretations, 15 comparison of ASME code section VIII div. 1 versus div. 2, 14 scope of the ASME code section VIII pressure vessels, 12 section X fiber-​reinforced plastic pressure vessels, 14 section XIII, rules for overpressure protection, 14

structure of section VIII, division 1, 13 submittals, 15 codes, 9–​10 introduction to few international codes for unfired pressure vessels, 10 structure of the codes, 9 design standards used for the mechanical design of heat exchangers (STHE), 7–​9 differences among TEMA classes R, C, and B, 8 other standards for STHE, 8 TEMA standards (section 5, RCB-​1.1.1), 7 Standards and Codes, 5–​6 benefits of standardization, 6 national standards, 6 trade or manufacturer’s association standards, 6 Mechanical design of STHE, 30–​35 ASME code sec VIII div 1 part UHX rules for STHE, 31 content of mechanical design of STHE, 33 design loadings, 35 design of STHE components, 35 mechanical design and pressure vessel codes and standards, 31 mechanical design procedure, 33 required information for mechanical design, 31 sequence of decisions to be made during mechanical design, 32 software for mechanical design of heat exchanger, 33 STHE types, 30 Microchannel heat exchangers (MCHE), 627–​629 brazing of MCHE, 627 Microwave non-​destructive testing, 472

N Nickel and nickel-​base alloys: metallurgy and properties, 260–​269 classification of nickel alloys, 261–​264 commercially pure nickel, 261 Inconel and inco alloy, 261 magnetic properties and differentiation of nickels, 264 nickel–​copper alloys and copper–​nickel alloys, 261 nickel–​iron–​chromium alloys and Inco nickel–​ iron–​chromium alloys for high-​temperature applications, 263 90-​10 and 70-​30 copper–​nickel alloys, 261 corrosion resistance, 264–​267 galvanic corrosion, 264 intergranular corrosion, 264 pitting resistance, 264 stress corrosion cracking, 266 Hastelloy® 269 welding, 267–​269 carbide precipitation, 268 considerations while welding nickel, 267 heat input, 268 hot cracking, 268 joint designs, 267 lead embrittlement, 268 pitting corrosion of weldments, 269 strainage cracking, 269

670

670 sulfur embrittlement, 268 weldability, 267–​269 postweld heat treatment, 269 welding methods, 269 Nondestructive testing methods, 374–​385 acceptance criteria, 377 auditing the NDT procedures, 384 cost of NDT, 377–​380 the benefits of NDT for a business, 380 the economic aspects of NDT, 377 destructive testing (mechanical testing), 374 discontinuities, 384–​385 defect detection, 385 defect detection capability, 385 surface techniques, 385 volumetric techniques, 385 examination procedure-​general requirements, 376 inspection equipment, 381 level I, II and III qualifications as per SNT-​TC-​1A-​ 2020, 380 NDT personnel qualifications, 380 personnel, 380 qualification(s), 376 reference codes and standards, 381 ASME code section V: nondestructive examination, 381 training of NDT personnel, 380 NDT standards, 374–​375 ASME Code section V nondestructive examination, 375 ASNT standards, 375 ASTM E1316-​21 standard terminology for nondestructive examinations, 375 ASTM standards, 375 AWS B1.10M/​B1.10:2016 guide for the nondestructive examination of welds, 375 NDT symbols, 381–​382 specify NDT locations, 382 NDT techniques, 375–​376 advanced NDT techniques, 375 conventional NDT techniques, 375 scope of NDT, 374 selection of NDT methods, 377 capabilities and limitations of NDT methods, 377 third-​party inspection in NDT, 384 written procedures, 383 content of NDT procedures, 383 deficiencies in NDT procedures, 383 general details of requirements in the NDT procedure document, 383 Nondestructive testing methods of brazed joints, 635–​636 Nondestructive testing of soldered heat exchanger, 644 destructive testing, 644 discontinuities, 644 pressure and leak testing, 644 removal of residual flux, 644 visual inspection, 644 Nozzles, 120–​123 design of pressure vessel nozzles, 122 nozzle openings reinforcement, 123

Index standards for nozzle design, 123–​124 WRC 107/​537 analysis, 124 types of nozzles, 121

O Opening and nozzles, 119–​120 openings, 119 reinforcement pad, 120 reinforcement pad and air–​soap solution testing, 120 Optical holography NDT, 470 holographic interferometry in crack detection, 470 real-​time holographic interferometry, 470

P PAIRT, 474 Pipefittings, 100 Pipes and tubes, 160 selection of tubes for heat exchangers, 160–​163 ASTM specifications for ferrous alloys tubings 162 corrosion tests 161 defect detection 160 dimensional tolerance tests 161 hydrostatic pressure testing 161 pneumatic test 161 specifications for tubes 160 standard testing for tubular products 161 tubing requirements, 160 Plate steels, 156–​160 classifications and designations of plate steels: carbon and alloy steels, 156–​158 ASTM specifications on plate steels used for pressure vessel fabrications and heat exchangers, 157 how do plate steels gain their properties?, 156 mill scale, 159–​160 processing of plate steels, 158–​159 Plate steels weldability problems, 163 cold cracking, 163 hydrogen-​induced cracking, 163–​168 avoiding hydrogen cracking, 163 preheating, interpass temperature, and postheating, 164 lamellar tearing, 168–​172 conditions that promote lamellar tearing 169 detection of lamellar tearing after welding 172 prevention of lamellar tearing 170 structures/​locations prone to lamellar tearing 170 underbead cracking, 168 fish-​eye cracking 172 hot cracking, see Hot cracling Post braze cleaning, 634 cleaning methods for post-​braze flux removal, 634 Postweld heat treatment (PWHT) of welded joints in steel pressure vessels and heat exchangers, 320–​324 ASME code requirements for PWHT, 321 defects arising due to heat treatment, 323 effectiveness of heat treatment, 323 effects of changes in steel quality and PWHT, 321 methods of PWHT, 323 NDT after PWHT, 324

671

Index objectives of heat treatment, 320 possible welding-​related failures, 324 PWHT cycle, 322 quality control during heat treatment, 322–​323 types of heat treatment, 320–​321 Preparation of heat exchangers for shipment, 581–​582 nitrogen filling, 582 other protection considerations, 581 painting, 582 TEMA guidelines G-​6, 581–​582 Pressure vessels, 1–​5 fired and unfired pressure vessel, 1–​5 heat exchangers, 3 hazards due to failures of pressure vessels, 5 unfired pressure vessels, 2 types of pressure vessels, 1 Pressure vessels design, 25–​30 common causes of failures and explosions in pressure vessels, 30 compliance with ASME code, 30 construction details of pressure vessels, 25 data required for a pressure vessel design, 27 design considerations, 27 design parameters, 28 geometry definition, 27 materials of construction, 28 methods of construction of pressure vessels, 29 minimum wall thickness, 28 outlets and drains, 26 pressure equipment devices, 26 pressure vessel design codes, 26 pressure vessel shapes, 25 types of pressure vessel heads, 25 venting and relief devices, 26 Pulsed eddy current (PEC) examination, 462–​464

Q Quality and quality control, 337–​338 aims of quality control, 338 Quality assurance program (QAP), 338–​339 contents of QAP for pressure vessels and heat exchangers, 339 essential elements of QAP, 338 need for quality assurance, 338 quality assurance in fabrication of heat exchangers and pressure vessels, 338 Quality control and quality assurance system for brazing of heat exchangers, 605 Quality control (QC) and quality assurance(QA), 337 Quality costs, 343–​344 appraisal costs, 343 external failure costs, 343 internal failure costs, 343 optimum cost of quality, 343 prevention costs, 343 Quality management, 345 quality Gurus–​their contribution for quality management, 345–​346

671 quality philosophies of Deming, Juran, and Crosby, 346–​347 Crosby four absolutes of quality, 347 Crosby vaccine, 347 Deming PDCA cycle, 346 Juran’s contribution to concepts of quality, 346 Quality management in industry, 337 Quality manual, 341–​343 contents of manual, 342 main documents of the quality system, 342–​343 checklist, 343 operation process sheet, 342 quality assurance program, 342 Quality review and evaluation procedures, 344 auditing, 344 Quality system, 339–​341 ASME code: elements of quality control system, 341 correction of nonconformities, 341 inspection of vessels and vessel parts, 341 material control, 341 records retention, 341 features of QC system, 339 Quality tools and quality improvements methods, 347–​362 5S, 355–​356 ISO 9000, 352–​354 benefits of ISO 9000, 353 ISO 9000 series, 352–​353 ISO 9001 quality management system, 353 Kaizen™, 357–​358 the core of Kaizen™, 358 lean, 358–​359 lean manufacturing, 359 lean tools, 360 lean-​six sigma, 362 lean vs. six sigma, 361 MBNQA, 355 plan-​do-​check-​act (PDCA) cycle, 354 PDSA technique, 354 7 quality control (7-​QC) tools, 348–​352 check sheets, 350 fishbone diagram or cause and effect diagram, 349–​350 histogram, 348 Pareto analysis (80-​20 rule), 349 scatter diagram, 351 statistical quality control and control chart, 351–​352 stratification, 351 six big losses, 359 six sigma quality management methodology, 360–​362 six sigma tools and methods, 361 total quality management, 355 elements of total quality management, 355 total productive maintenance(TPM), 356–​357 TPM pillars and 5S, 357 goals of TPM, 357 Quenched and tempered steels, 185–​187 ASTM specifications, 185 compositions and properties, 185 joint design, 187 postweld heat treatment, 187

672

672 preheat, 187 stress-​relief cracking, 187 weldability, 186 welding processes, 187

R Radiographic testing, 404–​418 acceptance criteria, 415 application, 405 ASTM standards, 407 computed tomography, 415 density of radiographs, 412 digital radiography, 416–​417 digitization of radiographs and laser scanner system, 418 documentation, 415 examination of radiographs, 414 full radiography, 409 gamma radiography, 418 gammascopy, 418 general procedure in radiography, 407 high-​energy radiography, 418 identification marks, 407 image quality indicators, 412 how to calculate IQI sensitivity, 412–​414 number of IQIs, 412 imaging plate, 418 location markers, 407 merits and limitations, 405 microfocus radiography, 415 neutron radiography, 417 other methods in radiography, 415 panoramic radiography with isotopes, 408 principle of radiography, 404 processing of X-​ray films, 408 radiographic quality, 411 radiographic sensitivity, 411 radiation sources (X-​rays and gamma rays), 405 comparison of X-​ray and gamma-​ray radiography, 405 radiographic techniques for weldments of pressure vessels, 408 radioscopy, 417 ASME code sec V reference documents, 407 requirements of radiography, 406 RT methods, 404 safety in RT, 407 spot radiography, 410 surface preparation, 408 written procedure, 406 X-​ray clues to welding discontinuities, 414 X-​ray fluoroscopy, 418 Raw material forms used in the construction of heat exchangers, 152–​154 castings, 152 forgings, 152 handling of materials, 154 material selection for pressure boundary components, 154–​155 baffles, 155 shell, channel, covers, and bonnets, 154

Index testing and inspection, 155 tubes, 154 tubesheet, 155 tubing forms, 154 tubing materials, 154 materials selection for bolted joints, 153 rods and bars, 153 Rectangular tubesheet design, 58–​59 methods of tubesheet analysis, 59 Remote field eddy current testing((RFEC), 460–​461 Replication metallography, 472–​473 applications, 473

S Selection of carbon steels, 179–​180 Selection of cast iron, 178–​179 Service-​oriented cracking, 178 temper embrittlement or creep embrittlement, 178 Shearography, 473–​474 digital shearography, 474 Smart PIG, 472 Soldering, definition, 593 Soldering of heat exchangers, 637–​644 elements of soldering, 637–​639 cleaning and descaling, 639 joint design, 637 soldering fluxes, 639 solders, 637 tube joints, 637 tube-​to-​header solder joints, 637 soldering processes, 639–​640 flux residue removal, 640 stages of radiator manufacture, 639 ultrasonic soldering of aluminum heat exchangers, 640–​644 basic processes for soldering all-​aluminum coils, 640 material that can be ultrasonically soldered, 640 Stainless steels, 192–​195 ASTM specification for stainless steels, 193 classification and designation, 192 designations, 193 guidance for stainless steel selection, 193 martensitic stainless steel, 193 newer stainless steels for heat exchanger service, 195 stainless steel for heat exchanger applications, 195 Stress analysis, 15–​16 classes and categories of stresses, 15 membrane stress, 16 primary stress, 16 stress categories, 15 stress classification, 15 Stress categories, 16–​19 discontinuity stresses, 19 failure modes, stress limits and stress categories, 18 fatigue analysis, 19 local membrane stress, Pl, 17 peak stress, F, 18 primary bending stress, Pb, 17 primary membrane stress, Pm, 16 secondary stress, 17

673

Index stress intensity, 19 thermal stresses, 18 Superaustenitic stainless steels, 235–​237 applications, 236 corrosion resistance, 236 4.5% Mo superaustenitic steels, 235 6% Mo superaustenitic stainless steel, 235 welding, 237 postweld heat treatment, 237 Super duplex stainless steel, 234–​235 difference between duplex and super duplex stainless steels, 235 properties and characteristics, 234 Supports, 124–​125 design basics, 125 design loads, 125 horizontal vessel supports, 125–​127 leg supports, 127 ring supports, 127 saddle supports, 125 zickstress, 126 procedure for support design, 128 ASME code, 128 TEMA rules for supports design (G-​7.1), 128 vertical vessels, 127–​128 lug supports, 128 skirt supports, 127

T Tantalum, 284–​285 corrosionresistance, 284 heat transfer properties, 284 performance compared with other materials, 284 welding, 285 Taper-​Lok® heat exchanger closure, 100–​102 zero-​gap flange, 101–​102 Teflon, 291–​292 design considerations, 291 fluoropolymer resin development, 292 heat exchanger fabrication technology, 292 heat exchangers of teflon in the chemical processing industry, 291 teflon as heat exchanger material, 291 Titanium: properties and metallurgy, 269–​281 alloy specification, 270 applications, 274–​275 applications in PHE, 275 chemical processing, 275 refinery and chemical processing, 275 titanium tubing for surface condensers, 274 corrosion resistance, 273 resistance to waters, 273 surface oxide film, 273 crystallographic structures of titanium, 270 fabrication, 275–​276 tubesheet materials-​galvanic consideration, 275 tube vibration and rigidity, 275 forming of titanium-​clad steel plate, 281 forms of corrosion, 273–​274

673 crevice corrosion, 274 erosion–​corrosion, 274 galvanic corrosion, 273 hydrogen embrittlement, 273 MIC, 274 stress corrosion cracking, 274 properties that favour heat exchanger applications, 270 thermal performance, 274 fouling, 274 titanium grades and alloys, 272 ASTM and ASME specifications for mill product forms, 272 titanium tubes for condensers and heat exchangers, 272 unalloyed and alloyed grades, 272 welding of titanium, 276–​281 cleaning titanium, 277 degreasing, 277 descaling or oxide removal, 278 filler metal, 278 heat treatment, 281 joint design, 277 manufacturing facilities, 277 method to evaluate the gas shielding, 280 MIG welding, 280 precleaning and surface preparation, 277 preheating, 278 rinsing, 278 shielding gases, 276 weldability considerations, 276 weld defects, 280 welding methods, 276 welding of titanium to dissimilar metals, 276 welding procedures, 279 welding titanium in an open-​air environment with three shielding gases, 279 Thermography, 474 Tube bundle assembly, 519–​526 assembly of tube bundle inside the shell, 523 assembly of tube bundle outside the shell, 519 assembly of U-​tube bundle, 520 cautions to exercise while inserting tubes, 522 impingement plate attachment, 522 tube bundle assembly methods, 519 tube bundle insertion inside the shell, 523 tube nest assembly of large steam condensers, 526 Tube inspection, 459–​465 automated tube inspection system, 465 tube inspection with magnetic flux leakage, 459–​460 tube inspection with near field testing, 461–​462 Tubesheet and baffle drilling, 514–​519 checklist for tubesheet inspection after fabrication, 517 drilling of baffles, 518 preparation of tube holes, 517 tube hole finish, 517 tubesheet drilling, 514 Tubesheet design as per ASME code Sec VIII div 1, 46–​47 design considerations, 46 general conditions of applicability for tubesheets, 46

674

674 design of fixed tubesheets of fixed tubesheet heat exchanger, 47 conditions of applicability, 47 design considerations, 47 design of floating head heat exchanger tubesheets, 48 conditions of applicability, 48 design considerations, 48 design of U-​tube heat exchanger tubesheets, 47–​48 calculation procedure for simply supported U-​tube tubesheets, 48 design considerations, 48 tubesheet characteristics, 46 tubesheet extension, 49 design considerations, 49 Tubesheet design as per TEMA Standards (appendix A-​nonmandatory section), 49–​58 compressive stress induced in the tubes located at the periphery of the tube bundle, 57 determination of effective design pressure, 53 differential pressure design, after Yokell, 56 effective differential design pressure, P, 56 merits of differential pressure design, 56 equivalent differential expansion pressure, Pd, 53 longitudinal stress induced in the shell and tube bundle, 56 in the tubes located at the periphery of the tube bundle, σt,l, 57 maximum allowable joint loads, 58 minimum tubesheet thickness as per TEMA, 52 parameter F, 50 shear formula, 51 shell longitudinal stress, σs,l (A.2.2), 56 stress category concept in TEMA, 53 tubesheet formula for bending, 49 tube-​to-​tubesheet joint loads (A.2.5), 58 Tubesheet design procedure: historical background, 38–​40 assumptions in tubesheet analysis, 38 basis of tubesheet design, 40–​46 analytical treatment of tubesheets, 41 deflection, slope, and bending moment, 43 design analysis, 41 factors that control tubesheet thickness, 45 parameter Z, 45 supported tubesheet and unsupported tubesheet, 45 Tubesheet diagram for windows, 465 Tubesheet to shell welding, 526–​527 Tube-​to-​tubesheet joint fabrication, 527–​553 mock-​up test, 529 ASME code requirements, 530 preferred method of making the tube-​to-​tubesheet joint, 529 quality assurance program for tube-​to-​tubesheet joint, 529 requirements for expanded tube-​to-​tubesheet joints, 532 major causes of joint leaks, 532 tube expansion, 527 tube expansion by rolling, 534–​553 amount of thinning of tubes, 544 basic rolling process, 535 common causes of tube joint failure, 546 correct tube wall reduction, 544

Index criterion for rolling-​in adequacy, 542 determining 3, 4, or 5 roll expander design, 544 expanding in double tubesheets, 552 factors affecting rolling process, 539 full-​depth rolling, 546 hydraulic expansion, 548 joint cleanliness, 546 joint leak tightness, 551 joint reinforcements, 551 leak testing, 553 length of tube expansion, 545 mechanical rolling methods, 534 methods to check the degree of expansion, 541 optimum degree of expansion, 541 phenomenon of tube end growth during rolling, 546 residual stresses in tube-​to-​tubesheet joints, 553 roller expander for tube extending beyond the tubesheet, 552 rolling equipment, 534 size of tube holes, 547 step rolling, 552 strength and leak tightness of rolled joints, 549 TEMA guidelines for tube wall reduction RGP-​RCB-​ 7.3, 542 tube hole grooving, RB-​7.2.4, 542 wall reduction as the criterion of rolling-​in adequacy, 543 tube-​to-​tubesheet joint expansion and /​or welding sequence, 528 tube-​to-​tubesheet joint expansion methods, 532–​533 explosive joining, 533 hydraulic expansion, 532 rolling-​in process, 532 Tube-​to-​tubesheet joint welding, 553–​577 certain preparation for tube to tubesheet welding, 556–​558 automated or manual welding decision, 558 preparation of the tubes, 556 tube welding and expansion, 558 full-​depth, full-​strength expanding after welding, 556 methods of tube-​to-​tubesheet joint welding, 554 requirements for the welding and testing of tube to tubesheet joints, 556 sequence of completion of expanded and welded joints, 554 welding methods, 558–​564 considerations in tube-​to-​tubesheet welding, 562 mock-​ups for tube-​to-​tubesheet joint welding, 563 orbital welding, 558 tube-​to-​tubesheet joint configuration, 559 welding machine, 558 welding process, 564, 577 ARC voltage control (AVC) options, 567 both ends of the tubes welded with tubesheets, 577 brazing method for tube-​to-​tubesheet joints, 574 ductility of welded joint in feedwater heaters, 573 enclosed orbital tube-​to-​tubesheet welding heads without filler wire, 567 heat treatment, 575, 577 inspection of tube-​to-​tubesheet joint weld, 574 internal bore welding, 569

675

675

Index internal bore welding behind the tubesheet, 570 leak testing of tube-​to-​tubesheet joint, 574 open tube-​to-​tubesheet welding heads with or without filler wire, 567 orbital welding, 566 seal-​welded and strength-​welded joints, 571 specific requirements of tubes and weld preparations, 568 testing of tube-​to-​tubesheet joints, 574 welding equipments, 567 welding of flush tubes, 569 welding of flush tubes with addition of filler wire, 569 welding of protruding tubes, 569 welding of recessed tubes, 569 welding of sections of unequal thickness, 571 welding of titanium tubes to tubesheet, 572 with tubes welded in one tubesheet and left free in the other tubesheet, 577

U Ultrasonic testing (UT), 418–​439 advantages of ultrasonic inspection, 424 air coupled testing, 421 angle beam technique, 425 application of ultrasonic technique for thickness measurement, 434–​436 application of ultrasonic technique in pressure vessel industry, 423 ASME code coverage, 423 ASTM standard for UT, 423 automated and on-​line ultrasonic testing, 439 automated ultrasonic examinations, 437–​438 components of a UT instrumentation, 425 corrosion mapping, 438 couplant, 426 quantitative wall thickness measurements, 436 ultrasonic coating thickness gages, 435 ultrasonic examination of nozzle welds, 436 ultrasonic plate tester, 434 ultrasonic thickness gauges, 434 different techniques of automated ultrasonic testing, 438–​439 full matrix capture (FMC), 439 phased array ultrasonic testing, 439 rapid automated ultrasonic testing, 439 rapid ultrasonic gridding, 438 examination procedure, 424 pulse-​echo technique, 424 fracture mechanics, 436–​437 crack evaluation, 436 hydrogen damage detection, 438 limitations of ultrasonic inspection, 424 other developments in UT, 437 other methods, 439 phased array corrosion mapping, 438 phased array ultrasonic testing(PAUT), 429–​434 industry applications, 432 merits of PAUT, 432 notable disadvantages of PAUT, 433 what do the images look like, 432–​433

A-​scan displays, 432 B-​scan displays, 432 C-​scan displays, 433 presentation, 421 probes, 425 surface preparation, 425 surface wave technique, 425 test method, 419–​421 contact and immersion testing, 421 pulse echo inspection, 420 through transmission testing, 420 ultrasonic testing of welds, 426–​429 acceptance criteria, 429 calibration, 429 defect location, 428 examination coverage, 429 plate thickness and angle of probe recommended, 428 reference blocks, 429 sensitivity and resolution, 429 UT calculators, 429 weld inspection (by Pulse-​Echo and TOFD methods), 438 written procedure for UT, 423 ultrasonic examination procedure deficiencies, 423

V Vendors responsibilities, 497 scope of supply, 497 Visual examination (VT), 385–​391 developments in visual examination optical instruments, 389–​391 borescopes, 389 combining computers and visual inspection, 391 high-​speed video, 391 remote visual inspection, 389 video image scopes, 391 video microscopes, 391 direct vision examination, 386 importance of visual inspection, 385 merits of visual examination, 387 NDT of raw materials, 388 parameters that impact inspection performance, 386 principle of VT, 386 reference document, 387 remote visual examination, 386 translucent visual examination, 386 visual examination during various stages of fabrication by welding, 388–​389 visual examination after welding, 389 visual examination before welding, 389 visual examination during welding, 389 visual examination equipment, 388 visual examination: prerequisites, 387 VT technique applications, 388 written procedure, 387

W Welding design 364–​374 parameters affecting welding quality, 364 procedure qualification record, PQR, 369

676

676 scheme of symbols for welding, 366 standard for welding and welding design, 366–​369 A numbers, 369 ASME code section IX, 366 filler metals, 368 F numbers, 369 NDT of weldment, 368 P numbers, 368 selection of consumables, 368 variables affecting welding quality, 366 weld defects and inspection of weld quality, 370–​374 approach to weld defect acceptance levels, 374 causes of discontinuities, 371 faults in fusion welds in constructional steels, 371 general types of defects and their significance, 371 weld defects (discontinuities), 370 welder’s performance qualification, 369 welding positions and qualifications, 370 welding procedure specification, 369 welding qualitydesign, 365

Index welding-​related failures, 178 Written procedure, 383, 387, 392, 399, 406, 423, 453, 474

Z Zirconium, 281–​284 alloy classification, 281 applications, 282 corrosion resistance, 282–​283 hydrogen embrittlement of zirconium alloys, 283 resistance to chemicals, 282 fabrication, 283 limitations of zirconium, 282 product forms, 281 properties and metallurgy, 281 welding method, 283–​284 filler metals, 284 surface cleaning, 284 weld metal shielding, 283