Valve Application, Maintenance and Repair Guide (In Situ State-of-the-Art Valve Welding Repair) (Gate, Globe, & Check Valves), Volume 2
SED R I A L
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Valve Application, Maintenance, and Repair Guide In Situ State-of-the-Art Valve Welding Repair (Gate, Globe, & Check Valves), Volume 2
INTEREST CATEGORIES Valves Repair Welding Hardfacing Repair Nuclear Valve Repair KEYWORDS Maintenance Repair Valves Welding Hardfacing
The Valve Application, Maintenance, and Repair Guide is a three volume series that provides a generic overview of valve application, selection, maintenance, and repair. Volume 2 of this series is a comprehensive guide for in situ weld repair of gate, globe, and check valve components such as bodies, bonnets, discs, and seats. The information in this guide, though directed toward nuclear plant personnel, will assist all power plant engineers, planners, and maintenance personnel responsible for valve maintenance and repair. Volume 1 of this report will contain information on valve design, application, and sizing; while Volume 3 will provide information on welding repair of butterfly, ball, and plug-type valves.
BACKGROUND Several EPRI-sponsored studies and surveys have concluded that valve failures are the single largest contributor to unscheduled shutdowns and lost plant availability. While a majority of these failures are maintenance related and can be addressed in short order, the more time consuming problems are typically related to cracks or wear of the valve body or critical internal components. Replacing a valve or part especially those which are large or safety related, can take in excess of 26 weeks to obtain, if it is available at all. Today with the major emphasis on shorter outages and downtime, utilities are opting for innovative repair techniques which keep the valve either in-line or at the plant site. This guide has been developed to provide utility personnel with the information necessary to identify and evaluate the problem, select the appropriate repair option, establish the repair criteria and procedures, and carry out a successful repair. OBJECTIVES • To provide general design and operating criteria for gate, globe, and check valves. • To present wear and degradation problems that are unacceptable for plant performance and/or safety, and provide repair options. • To provide the most current and effective welding repair techniques and technology to successfully repair valve components in accordance with the requirements of the governing code body. APPROACH EPRI surveyed the industry leaders associated with valve manufacturing, maintenance, repair, and operations to gain a thorough understanding of common valve failures, materials, manufacturing methods, and repair methods typically employed. The survey was also used to identify qualified repair
EPRI TR-105852v2s
Electric Power Research Institute
December 1996
vendors, their in situ repair capabilities, and their experience. The Nuclear Plant Reliability Data System (NPRDS) and License Event Report (LER) databases provided information on failures and their causes by valve type. Plant visits were made to manufacturing and repair centers to gather in-depth knowledge on specific component repair techniques and how remote welding and machining technology was utilized in these repairs. All of this data was then broken down by valve type and component, repair options and procedures, and repair operations (machining, welding, heat treating, and so on). RESULTS This guide presents nuclear utilities with the background information necessary to understand why their valve is not meeting either internal or external leak requirements, and provides detailed repair options and procedures to address the problem with sound engineering practices meeting the requirements of the ASME Boiler & Pressure Vessel Code. The guide is written in such a manner to allow the responsible engineer to understand each step in the repair process, to write repair procedures and specifications, and to follow the work as it is performed by in-house or contractor personnel. Experienced valve repair personnel looking for specific information should refer to specific topical sections or appendices for detailed information. EPRI PERSPECTIVE Since 1980, EPRI has been engaged in the development and evaluation of welding and machining equipment and practices related to in situ valve repair. An extensive effort has been made by valve repair service vendors and equipment manufacturers to specifically address the repair of main steam isolation valve seats and guide ribs. An equal effort has been made to develop hardfacing materials welding techniques for in situ application. This document draws on the results of these efforts and presents alternative repair solutions which are known to, or practiced by few. The repair methods presented in this guide have provided considerable cost savings and plant availability to those utilities which have chosen to look forward. PROJECT Work Order 3887 and 3814 EPRI Project Managers: Michael K. Phillips, Repair and Replacement Applications Center Vic Varma, Nuclear Maintenance Applications Center Nuclear Power Group For ordering information about this report, call the Nuclear Maintenance Applications Center, 800/356-7448. For membership information, call 415/855-2514.
Valve Application,Maintenance,and Repair Guide In Situ State-of-the-Art Valve Welding Repair (Gate, Globe, & Check Valves), Volume 2 TR-105852v2 Final Report December 1996
Prepared by EPRI Repair & Replacement Applications Center (RRAC) Principal Investigators Michael K. Phillips Shane J. Findlan Contributing Author William F. Newell, Jr. W. F. Newell & Associates
Prepared for Nuclear Maintenance Applications Center Repair & Replacement Applications Program 1300 W.T. Harris Boulevard, Charlotte, NC 28262 (P.O. Box 217097, Charlotte, NC 28221)
Operated by Electric Power Research Institute 3412 Hillview Avenue, Palo Alto, CA 94304 (P.O. Box 10412, Palo Alto, CA 94303)
EPRI Project Managers Vic Varma Michael K. Phillips Nuclear Power Group
DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES THIS REPORT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM: A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS REPORT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY’S INTELLECTUAL PROPERTY, OR (III) THAT THIS REPORT IS SUITABLE TO ANY PARTICULAR USER’S CIRCUMSTANCE; OR (B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDING ANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THIS REPORT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS REPORT.
ORDERING INFORMATION PRICE: $35,000 Requests for copies of this report should be directed to the Repair & Replacement Applications Center or the Nuclear Maintenance Applications Center (NMAC), 1300 W.T. Harris Boulevard, Charlotte, NC 28262, 800/356-7448. There is no charge for reports requested by RRAP and NMAC member utilities. Electric Power Research Institute and EPRI are registered service marks of Electric Power Research Institute, Inc. Copyright © 1996 Electric Power Research Institute, Inc. All rights reserved.
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
ACKNOWLEDGEMENTS Volume 2 of the EPRI RRAC/NMAC Maintenance and Repair Guide, In-Situ State-of-the-Art Valve Welding Repair has been developed with the support of numerous organizations and individuals dedicated to the advancement of new repair technology and methods. The authors want to thank the steering committee members of the Repair and Replacement Applications Program and the Nuclear Maintenance Applications Center for their vision to recognize the problems associated with valve repair and support this much needed guide. We want to especially recognize the following utility personnel, valve manufacturers, and repair service vendors who made a contribution by way of technical input, repair experience, or advice that supported the development of this product: Pedro Amador Rafiq Bandukwala Pat Brennan James Cobb Bill Fingrudt Bill Knect Ray Luda Garland Mahan John Polacheck Tim Satterfield Al Silvia Terry Weigel
Welding Services, Incorporated Edward Valves, Incorporated VR-TESCO, Incorporated Edward Valves, Incorporated GE Nuclear Services, Incorporated Anchor Darling Valve Company Anchor Darling Valve Company Entergy/River Bend PCI Energy Services Anchor Darling Valve Company Northeast Utilities Climax Portable Machine Tools, Incorporated
Our thanks also goes to the following companies who hosted on-site visits and provided detailed information on their valves and repair practices: Anchor Darling Valve Company Edward Valves, Incorporated VR-TESCO, Incorporated Entergy/Grand Gulf Finally, we would like to extend a word of special thanks to those who reviewed the final draft: Greg Hartraft John Aikin Bill Pratt Bill Knect Richard Smith John Hosler
GPU Nuclear AECL TVA Anchor Darling Valve Company Altran EPRI iii
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Let it be known that the various alloys or hardfacing materials referred to throughout the document have trademarked names. The following is a list of trademarked alloys or hardfacing: Colmonoy Delchrome Deloro Hastelloy Incoloy Inconel Monel NOREM Nucalloy Stellite Tristelle
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Wall Colmonoy Incorporated Deloro Stellite Limited Stoody Deloro Stellite Incorporated Haynes International Incorporated Inco Alloys International Incorporated Inco Alloys International Incorporated Inco Alloys International Incorporated EPRI Stoody Deloro Stellite Incorporated Stoody Deloro Stellite Incorporated Stoody Deloro Stellite Incorporated
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
CONTENTS
1.0
2.0
3.0
4.0
PROJECT DESCRIPTION .............................................................................................................. 1-1 1.1
Introduction and Application ............................................................................................. 1-1
1.2
Summary ............................................................................................................................. 1-3
CODES AND STANDARDS ............................................................................................................ 2-1 2.1
General ................................................................................................................................ 2-1
2.2
Pressure/Temperature Ratings .......................................................................................... 2.2.1 General .................................................................................................................... 2.2.2 Special Class Valves (Weld-End Valve) ................................................................... 2.2.3 Intermediate Rating Valves (Weld-End and Threaded Valves) ................................
2.3
General Discussion of Pressure Boundary Materials ..................................................... 2-7
2.4
Required Minimum Wall and Corrosion Allowance ......................................................... 2-9
2-4 2-4 2-6 2-7
GENERAL VALVE DESIGN ............................................................................................................ 3-1 3.1
Nomenclature/Glossary of Terms ...................................................................................... 3-1 3.1.1 Introduction .............................................................................................................. 3-1 3.1.2 Glossary of Terms .................................................................................................... 3-1
3.2
Common Valve Construction Features ........................................................................... 3.2.1 Body-to-Bonnet Connections ................................................................................. 3.2.2 Seat and Seat Rings .............................................................................................. 3.2.3 Disc-to-Stem Connection ....................................................................................... 3.2.4 Disc/Stem Guide Arrangements ............................................................................
3-10 3-10 3-14 3-23 3-26
VALVE MATERIALS ........................................................................................................................ 4-1 4.1
Trim Components and Materials ....................................................................................... 4-1
4.2
Material Selection Practices .............................................................................................. 4.2.1 Design Practices to Minimize Corrosion .................................................................. 4.2.2 Design Practices to Minimize Erosion ..................................................................... 4.2.3 Design Practices to Minimize Wear and Galling ......................................................
4.3
Hardfacing ........................................................................................................................... 4-9 4.3.1 Cobalt-Base Alloys (Stellites™) ............................................................................... 4-9 4.3.2 Cobalt-Free Alloys ................................................................................................. 4-10
4.4
Resilient (Soft) Seating .................................................................................................... 4-11
4.5
Pressure-Retaining Materials .......................................................................................... 4-14
4-3 4-3 4-6 4-8
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair 5.0
6.0
7.0
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GATE VALVES ................................................................................................................................ 5-1 5.1
Introduction and Application ............................................................................................. 5-1
5.2
Design .................................................................................................................................. 5-1 5.2.1 General .................................................................................................................... 5-1 5.2.2 Solid-Wedge Gate ................................................................................................... 5-3 5.2.3 Split-Wedge Gate .................................................................................................... 5-5 5.2.4 Flexible-Wedge Gate ............................................................................................... 5-7 5.2.5 Double-Disc Gate .................................................................................................... 5-9 5.2.6 Parallel-Slide Gate ................................................................................................. 5-12 5.2.7 Slab Gate ............................................................................................................... 5-15 5.2.8 Knife Gate .............................................................................................................. 5-17
5.3
Repair Issues .................................................................................................................... 5.3.1 Leakage Past the In-Body Seat ............................................................................. 5.3.2 Leakage Past the Pressure Seal Ring ................................................................... 5.3.3 Valve Stem Leaks .................................................................................................. 5.3.4 Valve Body Damage .............................................................................................. 5.3.5 Valve Bonnet Leaks ............................................................................................... 5.3.6 Wedge Guides .......................................................................................................
5-19 5-19 5-24 5-26 5-28 5-31 5-33
GLOBE VALVES ............................................................................................................................. 6-1 6.1
Introduction and Application ............................................................................................. 6-1
6.2
Design .................................................................................................................................. 6.2.1 General .................................................................................................................... 6.2.2 Horizontal Globe ...................................................................................................... 6.2.3 Y-Globe .................................................................................................................... 6.2.4 Angle Globe ............................................................................................................. 6.2.5 Y-Angle Globe .......................................................................................................... 6.2.6 Control Valves ..........................................................................................................
6.3
Repair Issues .................................................................................................................... 6.3.1 Leakage Past the In-Body Seats ........................................................................... 6.3.2 Leakage Past the Pressure Seal Ring ................................................................... 6.3.3 Valve Stem Leaks .................................................................................................. 6.3.4 Valve Body Damage .............................................................................................. 6.3.5 Valve Bonnet Leaks ............................................................................................... 6.3.6 Poppet (Disc) Guide Ribs ......................................................................................
6-2 6-2 6-3 6-5 6-6 6-7 6-8
6-11 6-11 6-14 6-14 6-15 6-16 6-16
CHECK VALVES ............................................................................................................................. 7-1 7.1
Introduction and Application ............................................................................................. 7-1
7.2
Design .................................................................................................................................. 7.2.1 General .................................................................................................................... 7.2.2 Swing Check ............................................................................................................ 7.2.3 Lift Check ................................................................................................................. 7.2.4 Tilting Disc ...............................................................................................................
7.3
Repair Issues .................................................................................................................... 7.3.1 Leakage Past the In-Body Seats ........................................................................... 7.3.2 Leakage Past the Pressure Seal Ring ................................................................... 7.3.3 Valve Stem Leaks .................................................................................................. 7.3.4 Valve Body Damage .............................................................................................. 7.3.5 Valve Bonnet Leaks ............................................................................................... 7.3.6 Poppet (Disc) Guide Ribs ...................................................................................... 7.3.7 Leakage Past the Hinge Pin Cover ........................................................................
7-2 7-2 7-2 7-5 7-8
7-10 7-10 7-13 7-14 7-14 7-14 7-14 7-15
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair 8.0
VALVE COMPONENT REPAIRS .................................................................................................... 8-1 8.1
Valve Body Repair .............................................................................................................. 8.1.1 Repair Strategy ........................................................................................................ 8.1.2 Repair Prerequisites ................................................................................................ 8.1.3 Flaw Removal .......................................................................................................... 8.1.4 Filler Material Selection ........................................................................................... 8.1.5 Preheat and Post-Weld Heat Treatment (PWHT) Requirements ............................. 8.1.6 Welding Repair ........................................................................................................ 8.1.7 Final Machining ........................................................................................................ 8.1.8 Inspection ................................................................................................................ 8.1.9 Testing .....................................................................................................................
8-1 8-2 8-3 8-4 8-4 8-4 8-5 8-7 8-7 8-8
8.2
Pressure Seal Area Repair ................................................................................................. 8-8 8.2.1 Repair Strategy ........................................................................................................ 8-9 8.2.2 Repair Prerequisites .............................................................................................. 8-10 8.2.3 Flaw Removal ........................................................................................................ 8-11 8.2.4 Filler Material Selection ......................................................................................... 8-12 8.2.5 Preheat and Post-Weld Heat Treatment Requirements ......................................... 8-12 8.2.6 Welding Repair ...................................................................................................... 8-13 8.2.7 Final Machining ...................................................................................................... 8-14 8.2.8 Inspection .............................................................................................................. 8-14 8.2.9 Testing ................................................................................................................... 8-14
8.3
Seat Ring Repair ............................................................................................................... 8.3.1 Repair Assessment and Strategy .......................................................................... 8.3.2 Repair Prerequisites .............................................................................................. 8.3.3 Defect Removal ..................................................................................................... 8.3.4 Filler Material Selection ......................................................................................... 8.3.5 Preheat and Post-Weld Heat Treatment Requirements ......................................... 8.3.6 Repair Welding ...................................................................................................... 8.3.7 Final Machining ...................................................................................................... 8.3.8 Inspection .............................................................................................................. 8.3.9 Testing ...................................................................................................................
8-14 8-16 8-16 8-27 8-28 8-29 8-29 8-32 8-34 8-34
8.4
Seat Ring Replacement .................................................................................................... 8.4.1 Repair Assessment and Strategy .......................................................................... 8.4.2 Repair Prerequisites .............................................................................................. 8.4.3 Seat Ring Removal ................................................................................................ 8.4.4 Filler Material Selection ......................................................................................... 8.4.5 Preheat and Post-Weld Heat Treatment Requirements ......................................... 8.4.6 Seat Ring Installation and Welding ........................................................................ 8.4.7 Machining .............................................................................................................. 8.4.8 Inspection .............................................................................................................. 8.4.9 Testing ...................................................................................................................
8-35 8-39 8-40 8-42 8-45 8-45 8-45 8-50 8-52 8-52
8.5
Integral Seat Repair .......................................................................................................... 8.5.1 Repair Assessment and Strategy .......................................................................... 8.5.2 Repair Prerequisites .............................................................................................. 8.5.3 Defect Removal ..................................................................................................... 8.5.4 Filler Material Selection ......................................................................................... 8.5.5 Preheat and Post-Weld Heat Treatment Requirements ......................................... 8.5.6 Repair Welding ...................................................................................................... 8.5.7 Machining .............................................................................................................. 8.5.8 Inspection .............................................................................................................. 8.5.9 Testing ...................................................................................................................
8-52 8-53 8-54 8-55 8-55 8-55 8-56 8-57 8-59 8-59
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
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8.6
Integral Seat Replacement ............................................................................................... 8.6.1 Repair Assessment and Strategy .......................................................................... 8.6.2 Seat Hardfacing Replacement Prerequisites ......................................................... 8.6.3 Seat Removal and Weld Preparation ..................................................................... 8.6.4 Filler Material Selection ......................................................................................... 8.6.5 Preheat and Post-Weld Heat Treatment Requirements ......................................... 8.6.6 Welding Repair ...................................................................................................... 8.6.7 Machining .............................................................................................................. 8.6.8 Inspection .............................................................................................................. 8.6.9 Testing ...................................................................................................................
8-59 8-60 8-61 8-61 8-63 8-64 8-64 8-65 8-66 8-67
8.7
Bonnet Backseat Repair and Replacement .................................................................... 8.7.1 Repair Strategy ...................................................................................................... 8.7.2 Repair Prerequisites .............................................................................................. 8.7.3 Flaw Removal ........................................................................................................ 8.7.4 Filler Material Selection ......................................................................................... 8.7.5 Preheat and Post-Weld Heat Treatment Requirements ......................................... 8.7.6 Welding Repair ...................................................................................................... 8.7.7 Final Machining ...................................................................................................... 8.7.8 Inspection .............................................................................................................. 8.7.9 Testing ...................................................................................................................
8-67 8-67 8-68 8-69 8-71 8-71 8-72 8-72 8-74 8-75
8.8
Guide Repair and Replacement....................................................................................... 8.8.1 Repair Strategy ...................................................................................................... 8.8.2 Repair Prerequisites .............................................................................................. 8.8.3 Flaw Removal ........................................................................................................ 8.8.4 Filler Material Selection ......................................................................................... 8.8.5 Preheat and Post-Weld Heat Treatment Requirements ......................................... 8.8.6 Welding Repair ...................................................................................................... 8.8.7 Final Machining ...................................................................................................... 8.8.8 Inspection .............................................................................................................. 8.8.9 Testing ...................................................................................................................
8-75 8-76 8-77 8-78 8-79 8-79 8-79 8-80 8-80 8-80
8.9
Guide Rib Repair ............................................................................................................... 8.9.1 Repair Strategy ...................................................................................................... 8.9.2 Repair Prerequisites .............................................................................................. 8.9.3 Flaw Removal ........................................................................................................ 8.9.4 Filler Material Selection ......................................................................................... 8.9.5 Preheat and Post-Weld Heat Treatment Requirements ......................................... 8.9.6 Welding Repair ...................................................................................................... 8.9.7 Final Machining ...................................................................................................... 8.9.8 Inspection .............................................................................................................. 8.9.9 Testing ...................................................................................................................
8-81 8-82 8-83 8-83 8-84 8-84 8-85 8-88 8-88 8-88
8.10
Hinge Pin Repair ............................................................................................................... 8-88 8.10.1 Repair Strategy ...................................................................................................... 8-95 8.10.2 Repair Prerequisites .............................................................................................. 8-97 8.10.3 Flaw Removal ........................................................................................................ 8-97 8.10.4 Filler Material Selection ......................................................................................... 8-98 8.10.5 Preheat and Post-Weld Heat Treatment Requirements ......................................... 8-98 8.10.6 Welding Repair ...................................................................................................... 8-99 8.10.7 Final Machining ...................................................................................................... 8-99 8.10.8 Inspection ............................................................................................................ 8-100 8.10.9 Testing ................................................................................................................. 8-100
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
9.0
8.11
Wedge/Disc Repair (Gate, Swing Check, and TDC) ..................................................... 8.11.1 Repair Strategy .................................................................................................... 8.11.2 Repair Prerequisites ............................................................................................ 8.11.3 Flaw Removal ...................................................................................................... 8.11.4 Filler Material Selection ....................................................................................... 8.11.5 Preheat and Post-Weld Heat Treatment Requirements ....................................... 8.11.6 Welding Repair .................................................................................................... 8.11.7 Final Machining .................................................................................................... 8.11.8 Inspection ............................................................................................................ 8.11.9 Testing .................................................................................................................
8-100 8-102 8-103 8-105 8-107 8-107 8-108 8-110 8-112 8-113
8.12
Poppet Hardfacing Repair .............................................................................................. 8.12.1 Repair Strategy .................................................................................................... 8.12.2 Repair Prerequisites ............................................................................................ 8.12.3 Flaw Removal ...................................................................................................... 8.12.4 Filler Material Selection ....................................................................................... 8.12.5 Preheat and Post-Weld Heat Treatment Requirements ....................................... 8.12.6 Welding Repair .................................................................................................... 8.12.7 Final Machining .................................................................................................... 8.12.8 Inspection ............................................................................................................ 8.12.9 Testing .................................................................................................................
8-113 8-114 8-115 8-116 8-117 8-117 8-117 8-118 8-119 8-120
8.13
Valve Stem Repair ........................................................................................................... 8-120
DEFECT REMOVAL ........................................................................................................................ 9-1 9.1
Grinding ............................................................................................................................... 9-5
9.2
Machining ............................................................................................................................ 9-6
9.3
Lapping ................................................................................................................................ 9-8
9.4
Honing ............................................................................................................................... 9-10
10.0 WELDING PROCESS SELECTION ............................................................................................. 10-1 10.1
Gas Tungsten Arc Welding (GTAW) ................................................................................. 10-2
10.2
Gas Metal Arc Welding (GMAW) ...................................................................................... 10-3
10.3
GMAW-P ............................................................................................................................. 10-4
10.4
FCAW ................................................................................................................................. 10-4
10.5
Shielded Metal Arc Welding (SMAW) .............................................................................. 10-5
10.6
Machine Welding ............................................................................................................... 10-6
10.7
GTAW-P-AU ........................................................................................................................ 10-6
10.8
Efficiency/Cost Comparisons .......................................................................................... 10-6
11.0 WELDING FILLER MATERIAL ..................................................................................................... 11-1 11.1
Types of Filler Metals ....................................................................................................... 11-1
11.2
Filler Metal Requirements ................................................................................................ 11-2
11.3
Filler Metal Selection ........................................................................................................ 11-4 11.3.1 A-Numbers ............................................................................................................. 11-8 11.3.2 F-Numbers ............................................................................................................. 11-9
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair 11.4
Storage of Welding Filler Materials ............................................................................... 11-11
11.5
Handling of Welding Filler Materials ............................................................................. 11-11
11.6
Dissimilar Weldments ..................................................................................................... 11-12
11.7
Filler Metals for Hardfacing Applications ..................................................................... 11-15
11.8
Typical Weld Filler Metal Specifications ....................................................................... 11-15
12.0 SHIELDING AND PURGING GASES ........................................................................................... 12-1 12.1
Purging .............................................................................................................................. 12-1
12.2
Shielding............................................................................................................................ 12-2
13.0 PREHEAT/POST-WELD HEAT TREATMENT REQUIREMENTS ................................................. 13-1 13.1
Preheat............................................................................................................................... 13-3
13.2
Interpass Temperature ..................................................................................................... 13-4
13.3
Post-Weld Heat Treatment ................................................................................................ 13-5
13.4
Guide to Selection of Heating Methods .......................................................................... 13.4.1 Gas Torch Heating ................................................................................................. 13.4.2 Electrical Resistance Heating ................................................................................ 13.4.3 Furnace Heating .................................................................................................... 13.4.4 Induction Heating ...................................................................................................
13.5
Guide for Selection of Preheat and PWHT ..................................................................... 13-7
13.6
Preheat and PWHT Procedures ....................................................................................... 13-8 13.6.1 General .................................................................................................................. 13-8 13.6.2 Temperature Measurement .................................................................................... 13-8 13.6.3 Thermocouples ...................................................................................................... 13-9 13.6.4 Minimum Controlled Band Width ......................................................................... 13-14 13.6.5 Heat Treatment Records ...................................................................................... 13-14
13.7
Preheat and Post-Weld Heat Treatment Considerations—Hardfacing ....................... 13-16 13.7.1 Preheat ................................................................................................................ 13-16 13.7.2 Post-Weld Heat Treatment—Hardfacing .............................................................. 13-16
13.8
Contracted PWHT ........................................................................................................... 13-16
13-7 13-7 13-7 13-7 13-7
14.0 WELDING METHODS AND PROCEDURES ............................................................................... 14-1
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14.1
Manual Base Metal Welding Procedure Guidelines ....................................................... 14-1 14.1.1 Manual SMAW ....................................................................................................... 14-1 14.1.2 Manual GTAW ........................................................................................................ 14-2
14.2
Machine Base Metal Welding Procedure Guidelines ..................................................... 14-2 14.2.1 Machine GTAW ...................................................................................................... 14-2
14.3
Hardfacing Welding Procedure Guidelines .................................................................... 14.3.1 General Requirements—Hardfacing ...................................................................... 14.3.2 Suggested Welding Practices ................................................................................ 14.3.3 Manual GTAW ........................................................................................................ 14.3.4 Localized Hardfacing Repair Practices ..................................................................
14-3 14-3 14-5 14-5 14-6
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair 14.4
Machine Hardfacing Welding Procedure Guidelines ..................................................... 14-8 14.4.1 Machine GTAW ...................................................................................................... 14-8 14.4.2 Machine PTAW ...................................................................................................... 14-9
14.5
Temperbead Welding Repair Guidelines ........................................................................ 14-9 14.5.1 Temperbead Qualification and Demonstration ....................................................... 14-9 14.5.2 Implementation .................................................................................................... 14-13
15.0 INSPECTION AND TESTING METHODS .................................................................................... 15-1 15.1
Nondestructive Examination ........................................................................................... 15-1
15.2
Post-Repair Testing .......................................................................................................... 15-3 15.2.1 Exercise Test .......................................................................................................... 15-3
15.3
Blue Check ........................................................................................................................ 15-4
APPENDIX A: GATE VALVE WEDGE FITTING TECHNIQUES ............................................................. A-1 APPENDIX B: IN SITU WELDING EQUIPMENT ................................................................................... B-1 APPENDIX C: IN SITU VALVE MACHINING EQUIPMENT ................................................................... C-1 APPENDIX D: VALVE MANUFACTURERS ........................................................................................... D-1 APPENDIX E: IN SITU VALVE REPAIR VENDORS .............................................................................. E-1 APPENDIX F: REFERENCES AND ADDITIONAL BIBLIOGRAPHY ................................................... F-1 Additional Bibliography ................................................................................................................ F-6 APPENDIX G: ACRONYMS AND ABBREVIATIONS ............................................................................ G-1
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
LIST OF FIGURES Figure No.
Page No.
Section 3 ____________________________________________________________________________ 3-1
Globe Valve—Typical Valve Nomenclature .................................................................................. 3-2
3-2
Gate Valve—Typical Valve Nomenclature ................................................................................... 3-3
3-3
Screwed Bonnet ........................................................................................................................ 3-11
3-4
Flanged (Bolted) Bonnet ........................................................................................................... 3-12
3-5
Welded Bonnet .......................................................................................................................... 3-12
3-6
Pressure-Sealed Bonnet ........................................................................................................... 3-13
3-7
Seat Joint Mating Surfaces ....................................................................................................... 3-14
3-8
Seat Plane Distortion under Vertical and Horizontal Bending Moments ................................... 3-15
3-9
Typical Globe Valve Seating Configurations .............................................................................. 3-17
3-10
Cross Ring Indentation .............................................................................................................. 3-18
3-11
Soft Seat Retention Methods .................................................................................................... 3-19
3-12
Methods for Attaching Seat to Body .......................................................................................... 3-21
3-13
Flexible Seat .............................................................................................................................. 3-22
3-14
Floating Seat ............................................................................................................................. 3-22
3-15
Spring-Loaded Packing Seals ................................................................................................... 3-23
3-16
Stem Connections ..................................................................................................................... 3-24
3-17
Gate Valve Gate Guide.............................................................................................................. 3-27
Section 4 ____________________________________________________________________________ 4-1
Double-Seated Globe Valve Trim Components ........................................................................... 4-2
4-2
Balanced Disc Cage-Style Valve ................................................................................................. 4-2
xiii
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair Figure No.
Page No.
Section 5 ____________________________________________________________________________ 5-1
Inside Screw Stem Thread Configurations .................................................................................. 5-2
5-2
Rising Stem Design, Outside Screw ........................................................................................... 5-2
5-3
Solid-Wedge Gate Valve ............................................................................................................. 5-4
5-4
Split-Wedge Gate Valve .............................................................................................................. 5-6
5-5
Flex-Wedge Gate Valve ............................................................................................................... 5-8
5-6
Double-Disc Gate Valve ............................................................................................................ 5-10
5-7
Through-Conduit, Double-Disc Gate Valve ............................................................................... 5-11
5-8
Parallel-Slide Gate Valve ........................................................................................................... 5-13
5-9
Through-Conduit, Parallel-Slide Gate Valve .............................................................................. 5-15
5-10
Slab Gate Valve ......................................................................................................................... 5-16
5-11
Knife-Gate Valve ........................................................................................................................ 5-18
5-12
Welded-In Gate Valve Seat Ring ............................................................................................... 5-20
5-13
Gate Valve Wedge Seating Too Low in Seats ........................................................................... 5-21
5-14
Split-Wedge Gate with Spacer Ring .......................................................................................... 5-22
5-15
Typical Pressure Seal Bonnet Configuration ............................................................................. 5-24
5-16
Pressure Seal Bonnet ............................................................................................................... 5-26
5-17
Typical Valve Stem/Bonnet Backseat ........................................................................................ 5-27
5-18
Typical Gate Valve Body ............................................................................................................ 5-29
5-19
Typical Bolted Bonnet Components .......................................................................................... 5-31
5-20
Single Guide Design for Gate Valves ........................................................................................ 5-34
5-21
Parallel Guide Design for Gate Valves ...................................................................................... 5-35
xiv
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair Figure No.
Page No.
Section 6 ____________________________________________________________________________ 6-1
Plug-Type Disc ............................................................................................................................ 6-2
6-2
Cage-Type Disc ........................................................................................................................... 6-3
6-3
Horizontal Globe Valve ................................................................................................................ 6-4
6-4
Y-Globe Valve .............................................................................................................................. 6-5
6-5
Angle Globe Valve ....................................................................................................................... 6-6
6-6
Y-Angle Globe Valve .................................................................................................................... 6-7
6-7
Single-Port Control Valve ............................................................................................................ 6-8
6-8
Double-Port Control Valve ......................................................................................................... 6-10
6-9
Three-Way Valve ....................................................................................................................... 6-11
6-10
Disc Seating Too Low in Seat .................................................................................................... 6-13
6-11
Valve Body ................................................................................................................................ 6-15
Section 7 ____________________________________________________________________________ 7-1
Swing Check Valve ...................................................................................................................... 7-3
7-2
Lift Check Valve, T-Pattern ........................................................................................................... 7-5
7-3
Lift Check Valve, Y-Pattern........................................................................................................... 7-6
7-4
Lift Check Valve, Angle Configuration ......................................................................................... 7-6
7-5
Disc-Type Lift Check Valve .......................................................................................................... 7-7
7-6
Piston-Type Lift Check Valve ....................................................................................................... 7-7
7-7
Tilting Disc Check Valve .............................................................................................................. 7-9
7-8
Disc Seating Too Low in Seats .................................................................................................. 7-12
7-9
Swing Check and Tilting Disc Check Valve, Standard Compression Flange and Gasket Joints ......................................................................................................... 7-16
7-10
Swing Check and Tilting Disc Check Valve, Pressure Seal Joints ............................................ 7-17
xv
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair Figure No.
Page No.
Section 8 ____________________________________________________________________________ 8-1a
Typical Body Repair Locations .................................................................................................... 8-1
8-1b
Recommended First Layer Weld Repair Bead Placement for Valve Body Base Material Repairs .............................................................................................. 8-6
8-1c
Recommended Second Layer Weld Repair Bead Placement for Valve Body Base Material Repairs .............................................................................................. 8-6
8-1d
Recommended Cavity Fill Weld Repair Bead Placement for Valve Body Base Material Repairs .............................................................................................. 8-7
8-2a
Pressure Seal Repair Location ................................................................................................... 8-9
8-2b
Example of Pressure Seal Area Boring ..................................................................................... 8-12
8-3a
Seat Ring Configuration ............................................................................................................ 8-15
8-3b
Blue Check for Seating Location ............................................................................................... 8-17
8-3c
Typical Wedge Dimensions ....................................................................................................... 8-18
8-3d
Gate Valve Seat Measurements ................................................................................................ 8-19
8-3e
Reference Dimensions for Gate/Check Valve Seat Width ......................................................... 8-20
8-3f
Reference Points on the Seat Ring of Swing Check Valve ........................................................ 8-21
8-3g
Seating Location on Tilting Disc ................................................................................................ 8-22
8-3h
Reference Mark Locations on Tilting Disc ................................................................................. 8-23
8-3i
Seat Location on Poppet ........................................................................................................... 8-24
8-3j
Typical Body Dimensions, Globe Valve ..................................................................................... 8-25
8-3k
Typical Body Dimensions, Y-Type Globe and Lift Check Valve .................................................. 8-26
8-3l
Gate Valve Seat Machining ....................................................................................................... 8-27
8-3m
Globe Valve Seat Machining ..................................................................................................... 8-28
8-3n
Gate Valve Hardfacing Application, Machine GTAW ................................................................. 8-30
8-3o
Globe Valve Hardfacing Application, Machine GTAW ............................................................... 8-31
8-3p
Globe Valve Seat Final Machining ............................................................................................ 8-33
8-3q
Typical Globe Valve Seat Angle Differential .............................................................................. 8-34
8-4a
Typical Gate Valve Seat Ring Seal Weld ................................................................................... 8-35
xvi
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair Figure No.
Page No.
Section 8 ____________________________________________________________________________ 8-4b
Typical Seal Weld Locations for Globe-Type Seat Rings ........................................................... 8-36
8-4c
Seat Ring Angle Generated by the Angle in the Seat Pocket ................................................... 8-37
8-4d
Example of Globe Valve with a Threaded Seat Ring ................................................................. 8-38
8-4e
Split View of Valve Body (In-Line) Exhibiting the Seal Weld Fixture .......................................... 8-43
8-4f
Valve Body Preparation for Replacement Seat Ring ................................................................. 8-44
8-4g
Typical Weld Buildup for Repair of Threaded Globe Valve Seat Ring Pocket ............................ 8-46
8-4h
Valve Boring System with Thread Attachment for Restoration of Threads in the Seat Ring Pocket ........................................................................................... 8-47
8-4i
Example of Tapered Seat Ring .................................................................................................. 8-48
8-4j
Split View of Valve Body (In-Line) Exhibiting Seal Weld Head Setup ........................................ 8-49
8-4k
Gate Valve Seat Narrowing ....................................................................................................... 8-51
8-4l
Typical Lapping Fixture for Globe-Type Seat Ring .................................................................... 8-52
8-5a
Turbine Throttle Valve ................................................................................................................ 8-56
8-5b
Swing Check Valve Lapping Machine ....................................................................................... 8-58
8-6a
Seat Removal with Automatic Grinding System ........................................................................ 8-62
8-6b
Base Material Restoration for Seat Ring ................................................................................... 8-65
8-6c
In Situ Machining of MSIV Seat ................................................................................................ 8-66
8-7a
Typical Bonnet Backseat/Stem Configuration for Blueing ......................................................... 8-69
8-7b
Proper Stem to Backseat Seating Configuration ....................................................................... 8-73
8-7c
Lapping Bonnet Backseat to Stem ............................................................................................ 8-74
8-8a
Typical Single Guide Configuration for Gate-Type Valves ......................................................... 8-75
8-8b
Double-Guided Gate Configuration ........................................................................................... 8-76
8-9a
Globe Valve Configuration Showing Guide Rib ......................................................................... 8-81
8-9b
Y-Type MSIV .............................................................................................................................. 8-85
8-9c
Guide Rib Hardfacing Refurbishment Sequence ...................................................................... 8-87
xvii
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair Figure No.
Page No.
Section 8 ____________________________________________________________________________ 8-10a
Swing Check and Tilting Disc Check Valves ............................................................................. 8-89
8-10b
Swing Check Valve, Pin Supported in Cast Journals ................................................................ 8-90
8-10c
Swing Check Valve, Bracket Mounted on Two Pads .................................................................. 8-91
8-10d
Swing Check Valve, Pin Supported in Bracket Attached to Bonnet .......................................... 8-91
8-10e
Hinge Pins and Covers Inserted into Disc Assembly Bushings ................................................ 8-92
8-10f
Pipe Plug Seal Cover ................................................................................................................ 8-93
8-10g
Blind Flange Cover .................................................................................................................... 8-94
8-10h
Pressure Seal Cover ................................................................................................................. 8-95
8-10i
Boring Bar with Flange Facing Capabilities............................................................................. 8-100
8-11a
Typical Wedge ......................................................................................................................... 8-101
8-11b
Removable Valve Disc/Wedges ............................................................................................... 8-101
8-11c
Seat Angle Determinations for Tilting Disc Check Valve ......................................................... 8-105
8-11d
Wedge or Disc Mounted in Fixture .......................................................................................... 8-106
8-11e
Hardfacing Refurbishment Sequence ..................................................................................... 8-109
8-11f
Lapping Table and Portable Lapping Adapter ......................................................................... 8-110
8-11g
Seat Angle Determination for Tilting Disc Check Valve ........................................................... 8-111
8-11h
Lapping Fixture for In-Body Tilting Disc Check Valve Seat and Disc ...................................... 8-112
8-12a
Poppets with Hardfaced Wear Rings ....................................................................................... 8-113
8-12b
Poppet Seat and Wear Ring Dimensions ................................................................................ 8-116
8-12c
Typical Globe Valve Seat Angle Differential ............................................................................ 8-119
Section 9 ____________________________________________________________________________ 9-1
Removal of Surface Indications Where No Weld Repair Is Required .......................................... 9-2
9-2
Excavation for Defect Type Verification and Local Weld Repair .................................................. 9-4
9-3
Gate Valve Seat Machining ......................................................................................................... 9-7
9-4
Milling Machine Removing Hardfacing from Guide Ribs ............................................................. 9-8
9-5
Gate Valve Lapping Machine ...................................................................................................... 9-9
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair Figure No.
Page No.
Section 10 ___________________________________________________________________________ 10-1
Welding Process Versus Deposition Rate at 100% Duty Cycle ................................................ 10-2
10-2
Process Diagram (GTAW) ......................................................................................................... 10-2
10-3
Circuit Diagram (GTAW) ............................................................................................................ 10-3
10-4
Process Diagram (GMAW) ........................................................................................................ 10-3
10-5
Block Diagram (GMAW) ............................................................................................................ 10-4
10-6
Shielded Metal Arc Welding Process ........................................................................................ 10-5
10-7
Circuit Diagram (SMAW) ........................................................................................................... 10-5
Section 11 ___________________________________________________________________________ 11-1
Styles of Consumable Inserts ................................................................................................... 11-7
Section 12 ___________________________________________________________________________ 12-1
Sample Fixtures for Purging the Inside Diameter of Pipe Weldments ....................................... 12-2
Section 13 ___________________________________________________________________________ 13-1
Typical Setup for Preheating Pipe Welds with Electrical Resistance Equipment ...................... 13-2
13-2
Typical Setup for Flexible Pad Heating Elements ...................................................................... 13-4
13-3
Thermocouple Attachment for PWHT Using the Capacitor Discharge Method ...................... 13-10
13-4
Thermocouple Attachment for PWHT Using Clips and a Flattened Tube ............................... 13-11
13-5
Minimum Controlled Band Width (CBW) for Post-Weld Heat Treatment ................................. 13-12
13-6
Required Thermocouple Locations and Minimum Heated Band Width (HBW) for Post-Weld Heat Treatment ................................................................................................. 13-13
13-7a
Heat Treatment Quality Control Specification .......................................................................... 13-15
13-7b
Heat Treatment Quality Control Record .................................................................................. 13-17
Section 14 ___________________________________________________________________________ 14-1
Temperbead Groove Preparation, Geometry, and Weld Bead Sequence ............................... 14-11
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Figure No.
Page No.
Appendix A _________________________________________________________________________ A-1
Blue Check Illustration ................................................................................................................ A-4
A-2
Blue Check Illustration ................................................................................................................ A-4
A-3
Blue Check Illustration ................................................................................................................ A-5
A-4
Blue Check Illustration ................................................................................................................ A-5
A-5
Blue Check Illustration ................................................................................................................ A-6
A-6
Heel and Toe Measurement Parameters ..................................................................................... A-7
A-7
Gauges Used to Determine Seat and Wedge Angles ................................................................. A-8
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
LIST OF TABLES Table No.
Page No.
Section 2 ____________________________________________________________________________ 2-1
Valve Design Codes .................................................................................................................... 2-2
2-2
Typical Valve Standards .............................................................................................................. 2-3
2-3
Safety Classes and Applicable Standards .................................................................................. 2-4
2-4
Pressure/Temperature Ratings for Steel Valves .......................................................................... 2-5
2-5
Materials Commonly Used for Pressure Boundary Parts ............................................................ 2-8
Section 4 ____________________________________________________________________________ 4-1
Chart of Wear and Galling Resistance of Material Combinations ............................................... 4-4
4-2
Metals in Order of Corrosion Rate .............................................................................................. 4-5
4-3
Critical Variables for Accelerated Erosion-Corrosion ................................................................... 4-7
4-4
Properties of Plastics and Elastomers Used for Soft Seats ...................................................... 4-13
Section 11 ___________________________________________________________________________ 11-1
P-Number, Composition, and Specifications for Selected Valve Materials ................................ 11-1
11-2
Welding Filler Material Use by Process..................................................................................... 11-2
11-3
Weld Filler Metal Specifications ................................................................................................ 11-3
11-4
Typical Filler Metals for Various Base Metals ............................................................................ 11-5
11-5
Weld Metal for Joining Selected Stainless Steel Base Materials .............................................. 11-6
11-6
Popular Electrode Product Forms and Available Sizes ............................................................. 11-8
11-7
A-Numbers—Classification of Ferrous Weld Metal Analysis for Procedure Qualification ......... 11-9
11-8
F-Number Grouping of Welding Electrodes and Rods ............................................................ 11-10
11-9
Filler Metal Selection for Different P-Number Combinations ................................................... 11-13
11-10
Selected Weld Filler Metals Used for Hardfacing and Valve Seat Applications ....................... 11-14
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair Table No.
Page No.
Section 13 ___________________________________________________________________________ 13-1
Preheat, Interpass, and PWHT Guidelines ............................................................................... 13-1
13-2
Mandatory Requirements for Post-Weld Heat Treatment of Welds ........................................... 13-5
13-3
AWS Post-Weld Heat Treatment Temperatures for Selected Base Metal .................................. 13-6
13-4
Code References for Preheat & PWHT Requirements ........................................................... 13-16
Section 15 ___________________________________________________________________________ 15-1
xxii
ASTM and ASME Standards for Nondestructive Examination Methods ................................... 15-2
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
1 PROJECT DESCRIPTION
1.1 Introduction and Application Valves play a key role in the operation and performance of power plants. Maintenance and repair of valves have been a leading concern of plant operators as they seek to maximize plant availability and the return on their capital investment. EPRI has initiated numerous studies and surveys aimed at understanding the impact of valve-related problems on plant availability. The results of these studies indicate that valve failures are the single largest contributor to unscheduled shutdowns and lost power generation [1]. EPRI responded positively to the results of these studies by initiating several projects that addressed basic maintenance needs, such as bolting practices, valve stem packing, and motor-operated valve (MOV) training. A costly issue that has not been adequately addressed involves the repair of the valve bodies, bonnets, and internal trim components, which generally requires some degree of welding and machining. Damage to critical valve parts due to wear, erosion, corrosion, cracking, and other destructive mechanisms typically leads to lengthy forced or extended outages when welding is required. Welded repairs to valves have traditionally been performed by the valve manufacturer who attempts a lengthy repair in their manufacturing facility or a “hot” shop facility if the valve is contaminated. Another typical approach has been to replace the valve with a new one. This also has many shortcomings, especially in today’s market when replacement stock is not readily available for most larger valves. Today, with the emphasis on shorter refueling outages and tighter radiation exposure limits, utilities are looking for a faster and more cost-effective approach to getting back on-line quickly. In situ repair of welded-in valves and on-site repair for flanged valves have become the preferred or required approach. Performing the repair in situ avoids cutting the valve out of line and shipping it to the repair facility, eliminates the system hydro testing after reinstallation, and keeps the owner in control of the repair and its impact on the outage schedule. Major in situ repairs are currently being performed by original equipment manufacturers (OEMs), service vendors, and utility maintenance personnel with varied levels of success. The welding and machining technology currently available to make field repairs is
1-1
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
limited, focusing primarily on some large nuclear valves (main steam isolation valves or MSIVs) of the globe-type configuration. In the early 1980s, EPRI sponsored a program to develop in situ machining and welding equipment to address the boiling water reactor (BWR) MSIV seating problems. Several equipment manufacturers and service organizations designed and built MSIV repair systems. These systems were demonstrated at the EPRI NDE Center in 1987 and several are being utilized today [2]. The result of that EPRIsponsored program was a better understanding of the valve repair issues that the nuclear industry faces, which has further influenced the development of repair systems for smaller globe-type valves. In 1994, the subscribing members of the EPRI Repair and Replacement Applications Program (RRAP) initiated a study to identify the capabilities and qualifications of organizations performing in situ valve repair. Three separate surveys were sent out to field service vendors, valve manufacturers, and RRAP utility members. The purpose of the surveys were to: • Identify the most common valve repair issues • Determine the types and sizes of valves that contribute to down time • Determine the capabilities of the manufacturers and service vendors to perform welded in situ repairs • Develop contacts and obtain case histories for these guidelines The results of that survey indicated that the number of utilities and service vendors performing in situ valve repairs was less than ten, and repair capabilities were quite limited. It was also apparent from the survey that there had been very little transfer of information on repair cases and capabilities across the industry. In fact, many utility contacts were unaware of in situ repair capabilities, other than general maintenance grinding/lapping of valve seats. In 1996, the Nuclear Maintenance Applications Center (NMAC) and its member utilities joined with RRAC to develop a comprehensive valve application, repair, and maintenance series. Volume 1 addresses the selection of valves and maintenance issues. This document, Volume 2, addresses the repair of gate, globe, and check valves. Volume 3 will address the repair of ball, butterfly, and plug-type valves. The purpose of the guidelines in this document is to present information on valve problems that require repair practices and applications beyond those typically recognized as maintenance. These guidelines specifically address machining to remove defects in valve components and restoring those materials by welding. These guidelines are aimed at the system, maintenance, and welding engineers responsible for maintaining valves. Secondary audiences are the design engineers and others interested in understanding the types of valves and the techniques needed to repair them. Most important, these guidelines lead the engineer down a path to resolving problems associated with valve base material and trim by offering guidance in understanding the work scope and developing resource requirements. Finally, the guidelines will assist in writing a repair specification and provide the basic knowledge for the engineer to understand the operations he will oversee.
1-2
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
1.2 Summary Numerous EPRI-sponsored studies and development efforts have been performed since the early 1980s to address valve reliability issues in nuclear power plants. The majority of these programs have resulted in applicable products or guidelines aimed at valve maintenance and diagnostics. Some of the more familiar products addressed bolting, on-line leak sealing, motor-operated valve (MOV) diagnostics and maintenance, and several valvespecific maintenance guidelines [3, 4, 5, 6, 7, 8, 9]. This study moves beyond the general maintenance of valves and addresses the repair of valve bodies and internal components, and more specifically, in situ repair. For the purpose of this program, “repair” refers to operations that incorporate welding and/or machining of base materials. Valves that require repair due to cracking, erosion, corrosion, or wear have had significant impact on plant availability, extending outages by days or even weeks. Most of these repairs are unique due to repair location, size, type, and configuration of the valve. In addition to being unique, most of the failures are identified well into the outage or during start up, which adds to the scheduling impact on start up. When a problem is identified, the real challenge begins. Utilities must first search out a qualified in-house staff repair vendor or team of vendors. Together, they must then develop a repair strategy and implement it. The repair program typically requires developing or adapting welding and machining systems, developing a mock-up, demonstrating the repair, qualifying welders and procedures, and finally implementing the repair. Any one of these steps can be a great challenge that results in significant delays. Only recently have cost-effective repair techniques been demonstrated for the repair of large globe-type valves such as the Y-type Atwood Morrill MSIV found in BWR plants [26, 27]. Repairs to smaller globe valves and all gate- and check-type valves have been performed but with varying degrees of success. This guide provides extensive guidance to the user in identifying a specific repair issue, understanding the repair options, walking through the specific repair, understanding the Code requirements, and preparing the valve for system testing. This guide specifically includes the following: • Technical descriptions of gate, globe, and check valves are provided. Individual sections on design and application along with advantages and disadvantages are provided. • Materials of construction are presented including pressure retaining materials and trim such as cobalt-based, nickel-based, and iron-based hardfacing alloys. • A section is provided on each specific type of valve, the typical repair issues, and the repair options. The repair options direct the user to specific repair sections. • An extensive section on specific component repairs is presented, including the component repair list prerequisites; repair strategies; flaw removal techniques; material selection; machining, welding, and heat treatment requirements; and final inspection and testing requirements.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
• Sections are provided on detailed welding material selection, including welding guidelines for specific processes, base material, and filler metals such as gas tungsten arc welding (GTAW) of hardfacing on carbon steel substrates; and preheat and post-weld heat treatment guidelines. • Appendices to the guide provide a listing of contractors and equipment suppliers capable of providing assistance for the repair of valve components and implementation of these guidelines.
1-4
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
2 CODES AND STANDARDS Industry codes and standards provide extensive design rules and guidance for valve design. These codes and standards provide the necessary rules for establishing such design requirements as wall thicknesses for pressure boundary parts, end connection configuration, and accepted materials, along with their allowable stresses. Industry codes and standards do not provide design rules for non-pressure boundary parts that are critical to valve operation, such as valve yokes, gaskets, and packing [8–12]. 2.1 General A clear understanding of the applicable codes and standards that apply to a valve installation is essential to design, evaluate, procure, install, or modify nuclear valves and nuclear balance-of-plant valves, as well as fossil plant valves, where ASME I and ANSI B31.1 apply. The applicable edition of the code or standard should be known as well. There are over 70 industry documents that relate to valve requirements for designing, manufacturing, or testing. The most frequently used valve documents are published by the following organizations: • American Society of Mechanical Engineers (ASME Boiler and Pressure Vessel Code) • American National Standards Institute (ANSI Standards) • Manufacturers Standardization Society of the Valve and Fitting Industry (MSS-SP Standard Practices) • American Water Works Association (AWWA Standards) • American Petroleum Institute (API Standards) • Underwriters Laboratory, Incorporated (UL Standards) • Instrument Society of America (ISA) • U.S. Code of Federal Regulations (CFR), specifically General Design Criteria (GDC) of 10 CFR Part 50
2-1
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
For nuclear plants, the codes and standards most frequently used for valve design are the ASME Boiler and Pressure Vessel Code, Section III, Nuclear Power Plant Components; Section VIII, Rules for Construction of Pressure Vessels; and ANSI B16.34. A chronology of the development of the major codes and standards is presented below and summarized in Table 2-1. Table 2-1 Valve Design Codes [8] (Source: EPRI NP-6516) Valve Type/Function
Code or Standard
Applicable Dates
Alternative Guidelines for Design of Butt-Weld Valves
MSS-SP66
Mid-1960s through 1973
Design of Category I Valves
ASME III ANSI B16.5* ANSI B16.34* (Note 1) (Note 2)
1971 on up to 1977 1977 on
Design of Non-Nuclear Boiler Pressure Boundary Valves
ASME I ANSI B16.5** ANSI B16.34** (Note 1)
1914 on up to 1977 1977 on
Design of Non-Safety, Non-Boiler Valves
ANSI B16.5 ANSI B16.34
Up to 1973 1973 on
* As invoked by ASME III
** As invoked by ASME I
Notes: 1. ANSI B16.34 provides the pressure/temperature rating, as well as requirements for minimum wall thickness, pressure boundary materials, marking, dimensions, and testing. ANSI B16.5 provides the pressure/temperature rating and the minimum wall thickness, and outlines the requirements for testing. 2. ASME III applies to nuclear safety-related valves. The earlier editions of ASME III referred to ANSI B16.5 or MSS SP66 primarily for pressure/temperature ratings and wall thickness, but retained the rules for materials, design, examination, and testing. The current ASME III refers to a large extent to ANSI B16.34 for valve requirements, but still retains design rules, spe cial material requirements, and special nondestructive examination requirements.
The earlier editions of ASME III relied on ANSI B16.5 and/or MSS-SP66 for pressure/ temperature ratings and wall thickness, but retained the rules for materials, design, examination, and testing. Currently ASME III refers to ANSI B16.34 for most valve requirements. ASME III still retains design rules, special material requirements, and special nondestructive examination requirements. In addition to providing rules for nuclear plant valve design, ANSI B16.34 applies to nuclear balance-of-plant valves and to fossil plants. For these applications, other standards also have been used for the design of valves. Table 2-2 identifies other standards that might be applied to the design and/or selection of valves to be used in non-nuclear valve applications. For older plants, ANSI B16.5 provided primary guidelines and MSS-SP66 provided alternative guidelines for the design of butt-weld end valves. When ANSI B16.34 was issued, the thrust of MSS-SP66 was incorporated as special class valves (that is, nondestructive examination such as radiography allowed a higher pressure for a given temperature), and MSS-SP66 was withdrawn. 2-2
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Table 2-2 Typical Valve Standards Standard No.
Title
AWWA-C504
Rubber–Seated Butterfly Valves
API-602
Compact Gate Valves
MSS-SP67
Butterfly Valves
MSS-SP70
Cast Iron Gate Valves
MSS-SP72
Ball Valves
MSS-SP80
Bronze Gate, Globe, Angle, and Check Valves
MSS-SP84
Steel Valves, Socket Welding, and Threaded Ends
MSS-SP66
Pressure Temperature Ratings for Steel Valves
MSS-SP71
Cast Iron Check Valves
MSS-SP85
Bronze Valves
Special consideration should be made if these or any other standards are not included in Table 126.1 of ANSI B31.1. The above referenced standards provide many of the design rules for valves. However, they do not address non-pressure-containing functional components or internal parts for non-nuclear valve applications. For nuclear valves, the requirements for internal parts have been given only a limited formal design approach for Class 1 valves by ASME III, Subarticle 3500. In order to properly address ASME Code Class 2 and Class 3 valves, ASME Code Case N62-4 was issued, which provides rules for materials, design, fabrication, inspection, and examination of internal and external valve parts. Prior to use, the code case should be consulted for the full scope of items covered. Code cases are optional. The rules of a code case become mandatory only if a purchaser invokes its requirements on a manufacturer, and then the entire code case is mandatory. Code cases are periodically reviewed, at which time they are reaffirmed or annulled. Code cases are annulled when the requirements have been incorporated into the code (for example, ASME III) or the code case is no longer needed. The categorization of nuclear safety-related equipment, including valves, is determined by referring to ANSI/ANS-51.1 (formerly ANSI N18.2, Nuclear Safety Criteria for the Design of Stationary Pressurized Water Reactor Plants) and ANSI/ANS-52.1 (Formerly ANSI N2.2, Nuclear Safety Criteria for the Design of Boiling Water Reactor Plants), U.S. Code of Federal Regulations (10 CFR 50.55a), and U.S. NRC Regulatory Guide 1.26 (Quality Group Classifications and Standards for Water, Steam, and Radioactive-Waste-Containing Components of Nuclear Power Plants). It should be noted that ANSI/ANS-51.1 and ANSI/ANS-52.1 are currently undergoing revision and will result in a new, combined standard, ANSI/ANS-50.1. Safety Class 1 is for reactor coolant pressure boundary components. Safety Class 2 is for components that form part of the reactor coolant pressure boundary, but might be excluded from safety Class 1 by provisions of 10 CFR 50.55a, or that are necessary for
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
the safe shutdown of the reactor, or to maintain the reactor in a safe condition. Safety Class 3 is for systems that support safety Class 1 and 2 systems. Consult 10 CFR 50.55a, Regulatory Guide 1.26, and ANSI/ANS-51.2 and 52.1 for complete definitions. The basic standards that apply and their correlation to safety classes are given in Table 2-3. Table 2-3 Safety Classes and Applicable Standards Safety Class 1
10 CFR 50.55a Reactor Coolant Pressure Boundary (referred to as Quality Group A in Reg. Guide 1.26) ASME III Cl 1
2
-
Reg. Guide 1.26
Remarks
-
-
Quality Group B -
-
ASME III Cl 2 3
-
Quality Group C -
-
ASME III Cl 3 NNS*
-
Quality Group D ANSI B31.1
For systems that contain or might contain radioactive material, but are not in Groups A, B, or C
* Not nuclear safety-related
2.2 Pressure/Temperature Ratings
2.2.1 General As previously stated, the pressure/temperature rating of a valve is provided in various codes and standards. The standard that is used depends on the materials selected and the valve style. Typical pressure/temperature ratings are included in the following codes and standards: • Steel, nickel alloy, and other special alloy valves: ASME III, ANSI B16.34 (see Table 2-4) • Cast iron gate valves: MSS-SP70 • Cast iron check valves: MSS-SP71 • Cast iron globe valves: MSS-SP85 • Bronze gate, globe, and check valves: MSS-SP80
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Table 2-4 Pressure/Temperature Ratings for Steel Valves [8] (Source: EPRI NP-6516) RATINGS FOR CARBON STEEL MATERIALS A105(a) A155-KC70(e)
A155-KCF70(e) A216WCB(a)
A350-LF2(d) A515-70(a)
A516-70(a)(g) A537-Cl.1(d)
A675-70 A696-Gr.C(a)
Notes: (a) Permissible, but not recommended for prolonged usage above about 800 °F. (d) Not to be us ed over 650°F. (e) Not to be used over 700°F. (f) Not to be used over 850°F.
Standard Class Valves – Flanged and Butt-Welding Temperature, °F -20 to 100 200 300 400 500 600 650 700 750 800 850 900 950 1,000
Working Pressure by Classes, psig 150 285 260 230 200 170 140 125 100 95 80 65 50 35 20
300 740 675 655 635 600 550 535 535 505 410 270 170 105 50
400 990 900 875 845 800 730 715 710 670 550 355 230 140 70
600 1,480 1,350 1,315 1,270 1,200 1,095 1,075 1,065 1,010 825 535 345 205 105
900 2,220 2,025 1,970 1,900 1,795 1,640 1,610 1,600 1,510 1,235 805 515 310 155
1,500 3,705 3,375 3,280 3,170 2,995 2,735 2,685 2,665 2,520 2,060 1,340 860 515 260
2,500 6,170 5,625 5,470 5,280 4,990 4,560 4,475 4,440 4,200 3,430 2,230 1,420 860 430
4,500 11,110 10,120 9,845 9,505 8,980 8,210 8,055 7,990 7,560 6,170 4,010 2,570 1,545 770
Special Class Butt-Welding End Valves Temperature, °F -20 to 100 200 300 400 500 600 650 700 750 800 850 900 950 1,000
Working Pressure by Classes, psig 150 290 290 290 290 290 275 270 265 240 200 130 85 50 25
300 750 750 750 750 750 715 700 695 630 515 335 215 130 65
400 1,000 1,000 1,000 1,000 1,000 950 935 925 840 685 445 285 170 85
600 1,500 1,500 1,500 1,500 1,500 1,425 1,400 1,390 1,260 1,030 670 430 260 130
900 2,250 2,250 2,250 2,250 2,250 2,140 2,100 2,080 1,890 1,545 1,005 645 385 195
1,500 3,750 3,750 3,750 3,750 3,750 3,565 3,495 3,470 3,150 2,570 1,670 1,070 645 320
2,500 6,250 6,250 6,250 6,250 6,250 5,940 5,825 5,780 5,250 4,285 2,785 1,785 1,070 535
4,500 11,250 11,250 11,250 11,250 11,250 10,690 10,485 10,405 9,450 7,715 5,015 3,215 1,930 965
Prior to determining the rating of a valve, a determination of the ANSI pressure class must be made. The class is based on the design and operating conditions of the system (for example, temperature and pressure). After the ANSI pressure class is determined, it 2-5
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
must be recognized that other conditions can limit the valve’s final rating. Valves with elastomeric or plastic gaskets, packing, or seating elements might not meet the entire range of pressure/temperature conditions for their designated pressure class. ANSI B31.1 rules for non-nuclear valves provide no specific allowance for excursions of operating pressure or temperature above design condition values. The maximum design pressures and temperatures are established by the pressure/temperature tables previously referenced. The user of this document should refer to the codes or standards and addenda applicable to the particular plant to determine the code provisions, if any, that permit allowance for variations from design conditions. Current editions of several codes and standards now permit the operating pressure to exceed the design pressure by not more than 10% under conditions of relief or safety valve operation. In addition, under certain conditions ASME III permits Class 2 and Class 3 valves to operate at a higher pressure than that normally allowed for the attained temperature. If the ASME criteria are allowed for these occasional transients, then other sections of ASME III apply as appropriate. ANSI B16.34 also makes provisions for departure from the standard pressure temperature ratings. The applicable code or standard should always be consulted when selecting a valve to ensure that the system design pressure and temperature are covered by the pressure/ temperature rating allowed by the applicable code or standard. When selecting the pressure class of the valve, other considerations can apply, such as pressure spikes due to dynamic loads (for example, water hammer) or greater strength required to support a heavy operator.
2.2.2 Special Class Valves (Weld-End Valve) A special class valve for which additional nondestructive examination (for example, radiography) is required, thus permitting a higher pressure/temperature rating. Tables of acceptable pressure and temperature are published in ANSI B16.34 for both standard class valves and special class valves. For example, a Class 600 carbon steel valve made from A216 WCB can be used at 1,200 psig at 500˚F (8.2 MPa at 260˚C) as a standard class valve. The same valve, when nondestructive examination is performed to merit the rating of Class 600 special class, can be used at 1,500 psig at 500˚F (10.3 MPa at 260˚C). This option can be valuable when the pressure and temperature allowed by B16.34 standard class do not meet the system requirements, but the special class does meet the system requirements. A special class is sometimes cost-effective and would not have the higher fluid flow pressure drop associated with the higher pressure class valve.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
2.2.3 Intermediate Rating Valves (Weld-End and Threaded Valves) ANSI B16.34 and ASME III specify a minimum wall thickness for each standard pressure class (for example, Class 150 and Class 300) and the inside diameter of a valve. When the actual wall thickness of a valve exceeds the minimum wall thickness specified for the standard pressure class and inside diameter, but is less than the specified minimum wall thickness for the next higher standard pressure class, ANSI B16.34 and ASME III make provisions, and provide formulas for, determining an intermediate pressure rating. This option requires higher hydrostatic test pressures than the next lower standard pressure class and should be exercised by or through the manufacturer. Intermediate-rated valves are used when system pressures and temperatures exceed those allowed for a standard pressure class, and when the wall thickness exceeds that required for the standard pressure class. For example, a manufacturer might provide a Class 1878 valve for a pressurized water reactor (PWR) reactor coolant system where a standard Class 1500 would not suffice, but a standard Class 2500 would far exceed the requirements. This option is different from special class valves in that an additional wall thickness above the minimum is required to allow a higher pressure/temperature rating for intermediaterated valves versus additional nondestructive testing for special class valves. 2.3 General Discussion of Pressure Boundary Materials Pressure boundary parts are defined in ASME III as the body, the bonnet, the disc, and the bolting that joins the body to the bonnet. Stems and seats are not pressure boundary parts. ASME III requires that these parts be made of an ASME III material, except for 2-inch (50 mm) or smaller line valve discs and safety valve discs and nozzles that are internally contained by the external body structure. However, ASME III permits use of material produced under ASTM specifications, provided the requirements of the ASTM specification are identical to or more stringent than the ASME III material. Other valve standards and codes do not specifically identify pressure boundary materials. However, ANSI B16.34 requires the body, bonnet, cover and body bonnet, or body cover bottom to be constructed of material listed in Table I of ANSI B16.34. Materials commonly used for pressure boundary parts (as defined in ASME III) fall into three categories: stainless steels or other corrosion-resistant alloys, carbon steels, and low-alloy steels. See Table 2-5 for commonly used pressure boundary materials.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Table 2-5 Materials Commonly Used for Pressure Boundary Parts Material Type Castings3
Stainless Steel Valves
Carbon Steel Valves
ASTM A351, Gr CF8 (304 SS)
ASTM A216-WCB
ASTM A351, Gr CF8M (316 SS)
ASTM A216-WCC
Low-Alloy Steel Valves ASTM A217-WC6 (1-1/4Cr-1/2Mo) ASTM A217-WC9 (2-1/4Cr- 1Mo)
ASTM A351, Gr CF3 (304 SS) ASTM A351, Gr CF3M (316 low carbon SS) Forgings3
Plate
Bolts, Studs, and Nuts
ASTM A182-F304, F316
ASTM A105
ASTM A182, F11 (1-1/4Cr-1/2Mo)
ASTM A182-F304L, F316L
ASTM A350-LF2
ASTM A182, F22 (2-1/4Cr-1Mo)
ASTM A240-304, 304L
ASTM A515GR70
ASTM A387-1,CL2 (1-1/4Cr-1/2Mo)
ASTM A240-316, 316L
ASTM A516GR70
ASTM A387-2, CL2 (2-1/4Cr-1Mo)
ASTM A193, Gr B7 ASTM A194, Gr 2H
ASTM ASTM ASTM ASTM
ASTM ASTM ASTM ASTM ASTM ASTM ASTM
A193, Gr B71 A194, Gr 2H1 A193, Gr B6 (410 SS) A194, Gr 6 (410 SS) A193, Gr B8 (304 SS)2 A193, Gr 8 (304 SS)2 A564, Gr 630
A193, A194, A193, A194,
Gr B7 Gr 2H Gr 16 Gr 4
Notes: 1. Although sometimes provided, these materials are not appropriate for stainless steel valves due to their potential for corrosion. 2. Not recommended for threading into 304 or 316 bodies, because galling can occur. 3. Typically used for valve bodies, bonnets, and discs or plugs.
The selection of materials is dependent on such factors as resistance to corrosion and erosion, and to some extent, the pressure/temperature rating for the various materials. It is common practice for the valve body to match the piping material. Fluid system conditions, including the environment, primarily dictate material selection. For example, the boric acid content of a pressurized water reactor coolant system leads to the selection of a stainless steel body, bonnet, and bolting. The superior erosion resistance of stainless steel is another reason for its selection for this high-velocity system. Further, the required retention of water purity in a demineralized water system requires the use of stainless steel, where small amounts of corrosion products that could result from the use of carbon steel cannot be tolerated. Carbon and low-alloy steel valves are used in the steam, feedwater, extraction steam, and condensate systems, where the water chemistry can be controlled to restrict the corrosion rate.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Carbon steels and stainless steels have yield strengths that are about equal at room temperature; however, low-alloy steels generally have a significantly higher yield strength than carbon or stainless steels. At the higher operating temperatures of a water-cooled reactor [500˚F to 600˚F (260˚C to 316˚C)], the yield strength of stainless steel is less than that of carbon steel. Carbon steel is not recommended for prolonged usage above 800˚F (427˚C) because of its potential graphitization damage and creep damage at elevated temperatures. The low-alloy steels have the highest yield strength at 500˚F to 600˚F. For valve bodies and bonnets, the same material or product form is not required to be used for both parts. The rating applied, however, must be based on the valve body, with the bonnet designed and material selected accordingly. All materials should be selected based on specific service conditions. For example: (a) a stainless steel valve in corrosive service conditions should have stainless steel bolting to preclude bolting corrosion due to leakage, (b) for steam service, which has a high moisture content and which might result in erosion, 2 !f Chrome (Cr) 1 Molybdenum (Mo) or 1!!f Cr-!s Mo material should be used for the valve body and bonnet, even though the temperature would permit carbon steel. In addition, for high velocity service, 2!f Cr-1Mo is superior to carbon steel, and stainless steel is vastly superior. Several other materials are available for valves, such as cast iron (ASTM A-126), ductile iron (ASTM A-395), and bronze (ASTM B-62). Note that ASME III does not permit cast iron or ductile iron valves. Other alloys are also used for service environments such as seawater, where aluminum-bronze valves are often used. 2.4 Required Minimum Wall and Corrosion Allowance One of the most basic parameters of valve design specified by the various codes for pressure boundary integrity is the required minimum wall thickness. The minimum wall thickness is, simply, the required thickness of the valve body wall to safely contain fluid at the design pressure and temperature. This required thickness is a function of the valve diameter, the design pressure of the valve, and the allowable stress (which is determined by the material used, the design temperature, and the code class of the valve). ANSI B16.34 provides tables of minimum wall thickness as a function of valve inside diameter and pressure rating, and formulas for interpolating the required wall thickness for pressure ratings that are not listed in the tables. ASME III Subsections NB, NC, and ND provide formulas for required minimum wall thickness as a function of B16.34 thicknesses with certain modifications. If the actual wall thickness of a valve falls below the minimum required wall thickness, it can be resolved in one of several ways. If the wall thinning is over a large area, the valve should either be replaced or rerated. If the wall thinning is restricted to a small local area, the area falling below the required minimum wall thickness can be weldrepaired to restore the original thickness, or the valve can be rerated. 2-9
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Rerating is possible if the maximum pressure allowed for the pressure class of the valve exceeds the actual design pressure of the valve. For instance, a Class 300 valve, with a maximum allowable pressure of 655 psig at 300˚F (4.5 MPa at 149˚C), was used in a system with a design pressure of 250 psig at 300˚F (1.7 MPa at 149˚C) . Using the formulas for interpolation between pressure classes (in B16.34 or ASME III, NB, NC, or ND, as applicable), the required minimum wall thickness for the actual design conditions can be determined. Rerating a valve requires the concurrence of the valve manufacturer and also requires that the reduced rating be incorporated into the plant design and licensing documents. A corrosion allowance should also be included for valves that are cast or forged, although the casting or forging process normally dictates that the final wall thickness of the valve will be in excess of that required by ANSI B16.34. In the smaller sizes and lower pressure ratings, the required wall thickness is often far less than the minimum practical thickness of a casting. In addition, some foundries produce their castings at least 1/8-inch (3 mm) thicker for each inch of metal thickness, compared to the specified wall thickness. Corrosion allowance, as used herein, is defined as additional wall thickness beyond that required by ANSI B16.34, to compensate for corrosion loss over the life of the valve. Certain product lines of some manufacturers were originally designed to meet the required wall thickness in American Petroleum Institute (API) standards. API standards require a wall thickness of excess of ANSI B16.34, thus providing a corrosion allowance when used in ANSI B16.34 applications. ANSI B16.34 has provided some excess in their tabulated wall thickness. When comparing these values against the required wall thickness determined by calculation, annex F, paragraph F1.4 of ANSI B16.34, states, in part that “. . . the actual values in Table 3 (of ANSI B16.34) are approximately 0.1 inches (2.5 mm) heavier than those given by the equation . . .” Some users take this to mean a corrosion allowance, although it does not specifically say this nor should it be interpreted that way. Total compliance with ANSI B16.34 would require wall thickness in accordance with Table 3 (of ANSI B16.34) for the life of the valve.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
3 GENERAL VALVE DESIGN
3.1 Nomenclature/Glossary of Terms
3.1.1 Introduction This section covers commonly used valve terminology and nomenclature. As examples, Figures 3-1 and 3-2 show a globe and a gate valve, and the typical nomenclature used for these valve types. References are given, where appropriate, to figures found in later sections that depict the term being defined. Many terms used in this document are defined in the following standards and technical textbooks [1]. •
Glossary of Valve Terms, Grove Valve Regulator Company, Oakland, CA, 1980.
• ASME Standard 112, Diaphragm Actuated Control Valve Terminology, American Society of Mechanical Engineers, New York, NY. •
ISA Handbook of Control Valves, Instrument Society of America, Pittsburgh, PA, 1971.
•
Control Valve Handbook, Second Edition, Fisher Control Company, Marshalltown, Iowa, 1977.
• ANSI B95.1, Terminology for Pressure Relief Devices.
3.1.2 Glossary of Terms active valve. A valve that must perform a mechanical motion while performing its intended function. (Note: The scope of this definition is currently under discussion by ASME.) actual discharge area. The minimum net area that determines the flow through a valve. actuator spring (diaphragm actuator) (see Figure 3-l). A spring that moves the actuator stem in a direction opposite to the direction created by diaphragm pressure.
3-1
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair Diaphragm Plate Diaphragm Case Diaphragm
Boss Diaphragm Actuator Actuator Spring Actuator Stem
Stem Connector
Yoke Packing Valve Stem
Bonnet
Stem Guide Body
Backseat Disc/Plug
Seat Ring
Figure 3-1 Globe Valve—Typical Valve Nomenclature actuator stem (diaphragm actuator) (see Figure 3-l). A rod-like extension (usually the valve stem) of the diaphragm plate to permit convenient external connection. backpressure. Pressure on the downstream side of the valve. backseat (see Figure 3-1). A shoulder on the stem disc of a valve that seals against a mating surface inside the bonnet to act as a backup seal to the packing to limit stem seal leakage.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
block and bleed. The capability of obtaining a pressure seal across the upstream and downstream seats of a valve, usually a gate valve, when the body pressure is bled off to the atmosphere through blowdown valves or vent plugs. This is useful in testing the integrity of seat shut-off and in accomplishing minor repairs under pressure; it is also useful in keeping different process fluids separated. See double block and bleed. bluing. The act of applying a fine non-drying film of blue paste on a finished component, such as a valve disc, and placing it against its matching component for the purpose of identifying contact areas. body (see Figures 3-1 and 3-2). The principal pressure-containing part of a valve where the closure element and seats are located. Stem Movement
Handwheel Rotation Handwheel (Actuator) Yoke Nut Yoke Stem
Packing Bonnet Stem Guide Body-Bonnet Flanged Joint Body Disc/Gate
Process Fluid Flow End Connection Valve Seats
Figure 3-2 Gate Valve—Typical Valve Nomenclature
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
bonnet (see Figures 3-l and 3-2). The separable portion of the valve pressure boundary that permits access to the internals. The major part of the bonnet assembly, excluding the sealing means. The top pressure-containing part of a valve, which is attached to the body, guides the stem, and adapts to extensions or operators. bonnet assembly. An assembly that includes the part through which a valve plug stem moves and is a means for sealing against leakage along the stem. It usually provides a means for mounting the actuator. bore (or port). The inside diameter, or other control configuration, of the flow passage through a valve (for example, the diameter of the hole in the ball of a ball valve, the inside diameter of seat rings). boss. A reinforced area for the attachment of accessories or small connections. bubble-tight shutoff. A phrase used in describing the sealing ability of a valve. During air pressure testing of a valve in the closed position, leakage past the seats is bubbled through water. To qualify as “bubble-tight,” no bubbles should be observed in a prescribed time span. bypass. A system of pipes and valves intended to permit the diversion of flow or pressure around a line valve or to equalize the body cavity pressure to either the upstream or downstream side. cage (see Figure 4-2). A hollow cylindrical trim element that is a guide to align the movement of a valve disc with a seat ring and also to retain the seat ring in the valve body. Often the walls of the cage contain openings that determine the flow characteristics of a control valve. capacity. The rate of flow through a valve under stated conditions of pressure drop and fluid density. chatter. Rapid reciprocating or vibrating motion of the valve disc during which the disc contacts the seat. closure element (see Figures 3-1 and 3-2). The moving part of a valve, positioned in the flow stream, that controls the flow through the valve. Ball, gate, plug, clapper, disc, etc., are specific names for closure elements. Cv (valve flow coefficient). The capacity factor of a valve, also called the valve flow coefficient; specifically, the number of gallons of water at 60˚F that will flow through a given valve within 1 minute, with a pressure drop (loss) of 1 psi. corrective maintenance. The repair and restoration of equipment that has failed or is malfunctioning and is not performing its intended function. design pressure. The pressure used in the design of a valve and other pressure-retaining components for the purpose of determining the minimum permissible thickness. 3-4
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
When applicable, static head shall be added to the design pressure to determine the thickness of the pressure-retaining components. There are slight differences in the exact definition of the design pressure used by different codes; therefore, the definition from the applicable code, such as ASME, must be used. diaphragm (see Figure 3-1). A flexible pressure-responsive element that transmits force to the diaphragm plate and actuator stem. diaphragm actuator (see Figure 3-1). An assembly utilizing fluid pressure acting on a diaphragm to develop a force to move the actuator stem. It might, or might not, have a spring for the positioning and return of the actuator stem. diaphragm case (see Figure 3-1). A housing, consisting of top and bottom sections, used for supporting a diaphragm and establishing one or two pressure chambers. diaphragm plate (see Figure 3-1). A plate concentric with the diaphragm for transmitting force to the actuator stem. disc (see Figures 3-1 and 3-2). The closure element of a gate, globe, check, butterfly, safety, or relief valve. The disc in different valve designs can be referred to as the gate, wedge, poppet, or plug. discharge area. The minimum net area that determines the flow through the valve. double-acting actuator. An actuator that utilizes a switching mechanism to achieve powered operation in either direction, extending or retracting the actuator stem as dictated by the controller. double block and bleed. The capability of a valve to isolate the body cavity from line pressure when the valve is in either the fully closed or fully open position. (See block and bleed for this operation with the valve in only the closed position.) In the open position, pressure-energized, seat-ball valves and through-conduct gate valves can effectively shut off the system pressure from entering the valve body cavity from either the upstream or downstream side, permitting the integrity of the seats to be checked with the closure member in the open position. dynamic unbalance. The net force produced on the valve disc in any stated open position by the fluid pressure acting upon it. extension bonnet. A bonnet with an extension between the packing box assembly and bonnet flange to thermally isolate the stem packing from the process fluid. fail-as-is. A characteristic of a particular type of actuator that, upon loss of power supply, causes the valve plug, ball, or disc to remain in the position attained at the time of the loss of external actuating power. fail-closed. A condition wherein the valve disc moves to the closed position upon loss of external actuating power. 3-5
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
fail-open. A condition where the valve disc moves to the open position upon loss of external actuating power. fail-safe. The selection of fail-as-is, fail-closed, or fail-open action, which avoids an undesirable consequence in a fluid system. field serviceable. A statement indicating that normal repair of the valve or replacement of operating parts can be accomplished in the field without returning the valve to the manufacturer. fire safe. A statement associated with a valve design that is capable of passing certain specified leakage and operational tests during and after exposure to fire of specified conditions. flow characteristic. Relationship between flow through the valve and percent rated travel as the latter is varied from 0% to 100%. This is a special term; it should always be designated as either inherent flow characteristic or installed flow characteristic. flow coefficient. See Cv. flutter. Rapid reciprocating motions of the valve disc during which the disc does not contact the seat or body. gate (see Figure 3-2). The closure element of a gate valve. globe valve (see Figure 3-1). A basic control valve type that gets its name from the globular shape of its body. It normally uses the basic valve disc as its valve closure member. hardfacing. A surface preparation in which an alloy is deposited on a critical valve surface (for example, seat, guide, and disc), usually by weld overlay or spray coating techniques, to increase resistance to galling, abrasion, and corrosion. high-recovery valve. A valve design that dissipates relatively little flow stream energy due to streamlined internal contours and minimal flow turbulence. Therefore, pressure downstream of the valve vena contracta recovers to a high percentage of its inlet value. (Straight-through flow valves, such as rotary-shaft ball valves, are typically high-recovery valves.) inlet size. The nominal pipe size of the inlet of a valve, unless otherwise designated. in situ repair. A statement associated with performing a machining or welding operation on a valve body or component that is still in-line rather than in a shop. lapping. The final finishing of a hardsurfaced sealing face. The finishing is performed with a fine grit paper or paste and an iron plate to obtain a polished or near-polished surface. It can be performed manually with bench-type or portable machines.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
leak test pressure. The specified inlet static pressure at which a quantitative seat leakage test is performed in accordance with a standard procedure. leaking. A quantity of fluid passing through an assembled valve when the valve is in the closed position under stated closure forces, with pressure differential. (ANSI B16.104 leakage classifications are defined in Tables 2-1 and 2-2 of that document.) live loading. A term used in reference to stem packing stuffing box arrangements to denote that the packing gland follower is loaded through springs to minimize loss of packing load due to packing consolidation and wear. low-recovery valve. A valve design that dissipates a considerable amount of flow stream energy due to turbulence created by the contours of the flowpath. Consequently, pressure downstream of the valve vena contracta recovers to a lesser percentage of its inlet value than is the case with a valve having more streamline flowpath. (Although individual designs vary, conventional globe-style valves generally have low pressure recovery capability.) lower valve body. A half housing for internal valve parts having one flow connection. For example, the half housing of a split body valve. maximum allowable working pressure. The maximum pressure permissible in a pressure-retaining component at a designated temperature. This pressure is based on the nominal thickness of the component, exclusive of allowances for corrosion and thickness required for loadings other than pressure. Maximum allowable working pressure is also used as the basis for the pressure setting of the pressure-relieving devices protecting the component. non-rising stem (see Figure 5-1B). A gate valve having its stem threaded into the gate. As the stem turns, the gate moves (for example, from the closed to the opened position), but the stem does not rise. Stem threads are exposed to line fluids. outlet size. The nominal pipe size of the outlet of a valve, unless otherwise designated. outside screw and yoke (0S&Y) (see Figure 5-2B). A valve in which the fluid does not come in contact with the stem threads. The stem sealing element is between the valve body and the stem threads. packing (stuffing) box assembly (see Figure 5-19). The part of the bonnet assembly used to seal against leakage around the valve plug stem, including various combinations of all or part of the following: packing gland, packing nut, gland follower, lantern ring, packing spring, packing flange, packing flange studs or bolts, packing flange nuts, packing ring, packing wiper ring, and felt wiper ring. packing gland (see Figure 5-19). The piece that compresses the packing. passive valve. A valve that is not required to perform a mechanical motion while performing its intended function. 3-7
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
plug. See closure element. port-guided (see Figure 6-8). A design on which the valve plug is aligned by the body port or ports only. pressure-containing member. A part of the component that is in actual contact with the pressure media. pressure-retaining member. A part of the component that is stressed due to its function in holding one or more pressure-containing members in position. push-down-to-close construction. A globe-style valve construction in which the valve plug is located between the actuator and the seat ring, so that extension of the actuator stem moves the valve plug toward the seat ring, finally closing the valve. push-down-to-open construction. A globe-style valve construction in which the seat ring is located between the actuator and the valve plug, so that extension of the actuator stem moves the valve plug away from the seat ring, opening the valve. quick opening flow characteristic. An inherent flow characteristic in which there is maximum change in the flow coefficient with minimum stem travel. rated C. The value of Cv at the rated full-open position. rated travel. Linear movement of the valve plug from the closed position to the rated full-open position. (The rated full-open position is the maximum opening recommended by the manufacturer.) rising stem (see Figure 5-1A). A valve stem that rises as the valve is opened. seat (see Figures 3-1 and 3-2). That portion of the valve internals parts that is contacted by a valve closure member to achieve a shutoff. seat angle. The angle between the axis of the valve stem and the seating surface. A flatseated valve has a seat angle of 90˚. seat area. The area determined by the inside and outside diameters of the seat. seat diameter. The smallest diameter of contact between the fixed and moving portions of the pressure containing element of a valve. seat load. The contact force between the seat and the valve plug. seat ring (see Figure 3-1). A separate piece inserted in a valve body to form a valve seat. separable flange. A removable flange that fits over a valve body flow connection, generally held in place by a retaining ring.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
static unbalance. The net force produced on the valve disc in its closed position due to the differential pressure on it. stem (see Figures 3-1 and 3-2). A rod or shaft transmitting force from an operator to the closure element of a valve to change its position. stem connector (see Figure 3-1). A fitting to connect the actuator stem to the valve stem. stem-guided (see Figure 3-1). A special case of top-guided construction in which the valve disc is aligned by a guide acting on the valve disc stem. stem unbalance. The net force produced on the valve disc stem in any position by the fluid pressure acting upon it. stuffing box (see Figure 5-19). The annular chamber provided around a valve stem in a sealing system into which deformable packing is introduced. tapping. The motion of a check valve disc element that causes it to impact against the full disc open stop, making a “tapping” noise. through-conduit. An expression characterizing valves which, in the open position, present a smooth uninterrupted interior surface across the seat rings and through the valve port, thus affording minimum pressure drop. There are no cavities or large gaps in the bore between seat rings and body closures, or between seat rings and the ball/gate. top-guided (see Figure 3-1). A design in which the valve plug is aligned by a single guide in the body, adjacent to the bonnet or in the bonnet. top- and bottom-guided (see Figure 7-5). A design in which the valve plug is aligned by guides in the body or in the bonnet, and in the bottom flange. The plug is guided above and below the seat. top- and port-guided. A design in which the valve plug is aligned by a guide in the bonnet or body, and the body port. trim. The internal parts of a valve that are in contact with line fluid other than the body and bonnet (usually consisting of the seat ring, valve plug, stem, valve plug guide, guide bushing, and cage). upper valve body. A half housing for a split-body type valve. valve body assembly. An assembly of a body, bonnet assembly, and bottom flange. valve closure member. The part of a valve that is positioned to close, open, or control the amount of flow. valve plug (see Figure 3-1). A movable part that provides a variable restriction in a port.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
valve-plug guide. The portion of a valve plug that aligns its movement in a seat ring, bonnet, bottom flange, or any two of these. valve-plug stem (see Figure 3-1 and 3-2). A rod extending through the bonnet assembly to permit positioning of the valve plug. vena contracta. The location where the cross-sectional area of the flow stream is at its minimum size, where fluid velocity is at its highest level, and fluid pressure is at its lowest level. wire drawing. Erosion caused by small high velocity jets in closely spaced surfaces, or by cavitation or liquid droplet impingement. It usually occurs when the disc is closed but some unintentional gap due to local damage or particulates causes the surfaces not to be in intimate contact. yoke (see Figures 3-1 and 3-2). A structure by which the valve actuator assembly is supported rigidly on the bonnet assembly. 3.2 Common Valve Construction Features Details of construction common to most valves are related to the minimum required components to achieve pressure integrity, seating integrity, and to actuate the valve. Although some variance can be found between manufacturers, these common construction features serve the same basic functions of connecting the body and bonnet, shutting off pressure, connecting the stem to the disc, and sealing around the movable stem.
3.2.1 Body-to-Bonnet Connections The bonnet can be a removable portion of a valve connected to the body by screwing, flange bolting, welding, or a pressure-sealing mechanism. In some cases, the bonnet can be an integral part of the valve body. Removal of the bonnet generally provides access to the valve trim, except in end-entry valves, such as ball and butterfly valves. For the end-entry valves, access to the trim is through the inlet or outlet ports or through the body joint. 3.2.1.1 Screwed Bonnet The screwed-in bonnet-type valve, shown in Figure 3-3, is one of the simplest and least expensive constructions; it is commonly limited to valve sizes up to 3 inches. In valves larger than 3 inches, the tools and torque required to tighten the joint become too cumbersome. Threaded joints should be avoided where thread corrosion or galling can make disassembly difficult. There are two variations of the threaded joint: one where the bonnet is screwed directly onto the body, the other where a union is used.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Stem
Bonnet Body
Threaded Connection
Figure 3-3 Screwed Bonnet Screwing the bonnet directly onto the body requires that the gasket or ground joint accommodate itself to rotating faces, and frequent unscrewing of the bonnet can damage the joint faces. Another disadvantage of these joints is the variability in the circumferential alignment between the bonnet and body, because the final assembly position is dependent on the number of turns required during threading. Threaded joints, however, offer the advantage of being easily seal-welded to provide a redundant seal or to eliminate the joint seal altogether. Joining the bonnet to the body using a union ring offers the advantage of preventing motion between the joint faces as the two are being made up, thereby permitting repeated unscrewing of the bonnet without damaging the joint faces or seals. Unions also prevent accidental unscrewing of the bonnet by the operator. 3.2.1.2 Flanged (Bolted) Bonnet Flanged bonnet joints, such as those found in valves shown in Figure 3-4, have an advantage over the screwed joint in that smaller tools and lower torque are required to tighten the joint. Flanged joints can be used on any size valve, under any operating pressure, but they become very bulky and heavy when used on very large valves and under high operating pressures. At temperatures above 650˚F, creep relaxation can, in time, noticeably lower the bolt load and allow the joint to leak. If the application is critical, the flanged joint can be seal welded.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Stem
Packing
Bolts Bonnet Flange
Gasket
Body Flange
Figure 3-4 Flanged (Bolted) Bonnet 3.2.1.3 Welded Bonnet Welding the bonnet to the body effectively provides a very economical and long-term seal regardless of size, operating pressure, and temperature. This arrangement can be used to achieve both a sealing function and a load carrying function, as shown in Figure 3-5. When coupled with screwed or flanged joints, the weld joint is designed to seal only against pressure and requires minimal weld material. Except for cast iron, welding can be performed on most materials.
Bonnet
Groove Weld Body
Figure 3-5 Welded Bonnet This arrangement is used where the valve is expected to be maintenance-free for long periods, where the valve is a throw-away design due to its relative cost to replace versus repair, or where the required sealing reliability of the valve far outweighs the difficulty of gaining access to valve internals, such as in bellows-sealed stem valves.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
3.2.1.4 Pressure-Sealed Bonnet The pressure sealed bonnet design, shown in Figure 3-6, provides the advantage of reduced weight and size over flanged connections, and allows the internal bonnet pressure to increase the joint sealing contact stress, instead of unloading it as in bolted designs. This joint is most attractive in larger valves and high pressure applications where the pressure forces are high enough to generate the required contact stress to seal at the metal-to-metal joint. This type of bonnet seal is usually available only on valves of pressure Class 600 or higher. It is particularly suited to high temperature applications (660˚F) where bolted bonnet joints can loosen due to bolt creep. One of the disadvantages of this type of bonnet joint is that it provides no positive mechanical location between the bonnet and body and often allows misalignment to occur, which can cause stem binding. Binding can lead to stem galling, leakage through the stem packing, and potential valve inoperability. In large valves, proper assembly of the bonnet usually requires installation of the valve with the stem vertical and pointing upward (see Figure 3-3). Bonnet Capscrews Bonnet Retainers
Gasket Retainer Spacer Ring Pressure Seal Gasket
Bonnet
Valve Body
Figure 3-6 Pressure-Sealed Bonnet Another drawback of the pressure seal bonnet joint is that it can start to leak in applications where frequent pressure or temperature fluctuations are experienced; therefore, the bonnet cannot be safely tightened under pressure when a leak occurs because of the possibility of making the leakage more severe when attempting corrective action. In addition, if a leak should occur, it is more difficult to repair and reassemble the valve than with a bolted bonnet due to the required careful alignment and tightening sequence procedures during assembly.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
3.2.2 Seat and Seat Rings The seat is the fixed, pressure-containing portion of a valve that comes in contact with the closure member of the valve. The seat can be all metal construction, or it can incorporate soft conforming seat inserts, such as elastomers or plastics, to make a tighter seal or to reduce the required load to seat. For seat tightness, the objective is to block off, or minimize, the path formed by the “valleys” on the seating surface. An enlarged view of the valleys in mating surfaces is shown in Figure 3-7. Filling in the valleys requires the compressive stresses in the mating surfaces to be of sufficient magnitude to elastically or plastically deform the mating surfaces until the leak path is blocked off.
A. Low Seating Force
Elastic or Plastic Deformation B. High Seating Force
Figure 3-7 Seat Joint Mating Surfaces In addition to the basic design of the seat itself, other factors that directly affect seating and operability are distortions that can occur at the disc/seat interface due to pressure, thermal gradients, and mechanical loads transmitted to the valve body by the adjacent piping. As shown in Figure 3-8, applied bending moments on gate valve bodies cause the seat plane to tilt and distort, which can result in leakage and gate pinching in wedge-type valves. In globe valves, body distortions produce ovality in the seat, which lead to mismatch with the circular seating area on the tapered seated plugs. Distortions caused by line loads become more severe when venturi-type valves or valves that are smaller than the pipeline size are installed with upstream and downstream reducers. 3-14
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Seat Plane Distortion
Vertical Moment
Vertical Moment
Original Seat Face Distorted Seat Face
Horizontal Moment
Horizontal Moment
Seat Plane Distortion
Figure 3-8 Seat Plane Distortion under Vertical and Horizontal Bending Moments To avoid leakage or binding problems caused by line loads, valves should not be located at points of large line loads. Also, the section modulus of the valve body should be significantly greater than the pipe to keep the stresses and distortions within acceptable limits. Axisymmetric-type valves, such as ball and butterfly, tend to be stiffer and are less sensitive to line loads. 3.2.2.1 Metal-to-Metal Seating When using metal-to-metal seating, the high compressive stresses required to produce surface conformance between the two seating surfaces are achieved by making narrow line-to-line contact between the disc or plug and the seat. Narrow line-to-line contact should provide a certain minimum width in order to establish a tight seal and prevent indentation damage caused by the plug on the seat. In addition, the seat should have enough base width to provide an adequate backup cross-section that is capable of supporting the high compressive stress at the disc-seat interface without yielding the base material. In control valves, seat loading is usually expressed as pounds of force
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
per linear inch of mean seat joint circumference. For globe-type control valves using line-to-line contact, loading can vary from 25 pounds per inch to 600 pounds per inch, and most manufacturers rely on their own tests to develop specific magnitudes. Based on the ISA Handbook of Control Valves, typical values are: 1. Twenty-five pounds per inch (4.35 kN/m)—Low pressure drop service, leak-tight shutoff is not required. 2. Fifty pounds per inch (8.7 kN/m)—Moderate pressure drop service, slight leakage expected (0. 1% Cv maximum). 3. One hundred pounds per inch (17.5 kN/m)—Near drop-tight service (0.01% Cv maximum) seals 3,000 psi pressure drop on 0.015-inch width, 30˚ joint of 316 SS. 4. Three hundred pounds per inch (52.0 kN/m)—Drop-tight service (seals 6,000 psi on 0.025-inch width, 20˚ joint of AISI 440-C SS, hardness 55 Rockwell c scale (Rc). 5. Six hundred pounds per inch (105 kN/m)—Pressure service greater than 6,000 psi. Although the apparent average compressive seating stress on items 3 and 4 are 13,000 psi (89 MPa) and 35,000 psi (241 MPa), which is less than the yield strength of the material, localized contact stresses at the peaks of the surface irregularities are much higher, thus providing the surface yielding needed to accomplish a seal. The required degree of seat tightness and accompanying stem thrust should be reasonably selected. Specifying high seat tightness increases the size and cost of the actuator needed to develop the higher loads. As an alternative to using high contact forces, the mating surfaces can be “superfinished” to achieve a good seal. However, this superfinish can degrade quickly in applications where fluid contaminants are present that can get trapped between the mating surfaces during opening and closing action. Another common method used to accomplish a seal is to lap the disc and seat during assembly; however, lapping the surfaces should be limited so that a wide contact band does not develop. Developing high compressive stress to achieve good seating should be weighed against potential damage due to galling or gross surface yielding. Surfaces that slide under load, such as the disc of a gate valve, should be sized so that contact stress is maintained below the galling threshold for the material combination. Depending on the mating materials and the details of the actual geometry, the calculated average contact stress to cause galling can vary from as low as 2,000 psi (13.8 MPa) for some stainless steels to as high as 50,000 psi (344.7 MPa) for cobalt-based materials such as Stellite 6. In reality, the contact stress at the surface is not uniform, due to the irregular and uneven loading encountered in actual application; therefore, the average contact stress should be limited to lower values. Typically, the contact stress for Stellite 6 is limited to 20,000 psi (137.9 MPa) to avoid galling in sliding applications.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Typical seating configurations employed in globe valves are shown in Figure 3-9. The seat design shown in Figure 3-9a, used in low pressure globe applications, provides the advantage of not requiring precise alignment between the disc and seat, and it eliminates galling because the surfaces move perpendicular to each other during loading. The seat design, shown in Figures 3-9b, 3-9c, and 3-9d, allows higher contact stress to be developed due to the narrower contact band between the mating surfaces; however, it requires better control of alignment between the disc and seat. As shown in Figures 3-9c and 3-9d, taper angles (half-cone angles) between 15˚ and 45˚ are in use in various disc designs. Even though taper angles as small as 15˚ have been used in some valves, they should be avoided because it has been found that for reliable nonsticking operation of the disc, magnitudes of 30˚ and higher should be used. A
B
Seat Angle 45° Disc
Disc
Seat
Seat
C
D 15°
Disc
Disc 45˚
Seat
Seat
Figure 3-9 Typical Globe Valve Seating Configurations When selecting the disc/seat contact geometry and materials, the potential for cross ring indentation-type damage should also be considered. Seat ring indentation, as shown in Figure 3-10, is caused when a hard narrow surface and a soft wider surface contact, and
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
the softer material yields. The indentation left on the softer component can create leakage during a subsequent shutoff if the normal clearances present in the assembly of the plugto-seat components allow the new seating band to cross the previous indentation. Cross ring indentation damage and its adverse effect on shutoff can be prevented by making the component of a softer material narrower than the wider component.
Plug
Impression of Disc on Soft Seat Leak Path
Seat
Figure 3-10 Cross Ring Indentation 3.2.2.2 Soft Seating Soft seats are used to accomplish good seating with much lower contact force than in metal-to-metal seats. It is easier to deform the softer materials and fill out the valleys in the mating surfaces with considerably lower forces. In most designs, the soft seat rings provide the primary seating, with metal-to-metal closure acting as a secondary seal in case of damage or failure of the soft seal material. This secondary metal-to-metal contact also makes the seats fire safe and allows some degree of seat tightness, should the resilient seat ring fail. Whenever the temperature, radiation, and pressure environment permit, soft seals should be strongly considered because of the ease in accomplishing good seating with soft seals. Because seat ring materials do not have the required strength and stiffness to resist rupture against pressure, and blowout against differential pressure, they must be securely clamped in the seat. Several methods of restraining the seat ring in globe valves are shown in Figure 3-11. The same restraint methods are employed in gate and check valves.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Primary Elastomer Seal Ring Backup Metal Seal Ring
Valve Disk
Clamp Elastic Seal
Clamp Bolts A. Seal Clamped in Plug
B. Seal Clamped in Seat Ring Unvented Gland, 150 PSI ∆P Limit Vented Gland, 1,500 PSI ∆P Limit
O-Rings
Seated C. O-Rings on Double V-Port Plug (for Low ∆P)
D. O-Ring in Dove-Tail Gland (for Higher ∆P)
Figure 3-11 Soft Seat Retention Methods The material properties to be considered in the selection of a soft seat ring are: • Fluid compatibility including chemical reaction, swelling, loss of hardness, permeability, and degradation • Room for thermal expansion • Hardness • Permanent set and extrusion under load • Rate of recovery upon removal of the load 3-19
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
• Tensile and compressive strength • Radiation resistance • Abrasion resistance • Wear resistance • Thermal resistance 3.2.2.3 Seat Attachment The method of attachment and sealing of the leakage path between the seat and the body is as important as the seat itself. Methods of attaching fixed seats to the body are shown in Figure 3-12, which shows screwing, welding, interference fitting the seat ring into the body or seat pocket using press or shrink fits, bolting, clamping between two pieces, and simply welding and machining the seat face into the body. Sealing at the body is achieved using elastomers, gaskets, soft metals, metal-to-metal sealing by interference, and seal welding. The seat-to-body restraint should be independent of the seat loading and should not depend on the seat load to achieve a seal. Inherent in fixed seat designs is the problem that body distortions, caused by pressure, thermal gradients, and line loads, are transmitted directly to the seat. These distortions create leakage paths between the disc-to-seat mating surfaces in metal-to-metal seating unless some flexibility is designed into the disc (as in flex disc gate valves) or the globe valve seat, as shown in Figure 3-13. The type of attachment to the body should also consider maintenance that might be required on the seat.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair Valve Body with Seat Ring Bore Seat Ring Seal Weld
Valve Body Seat Ring
Hardfacing
Thread
A. Screwed
D. Welded-In Seat Ring
Retainer Bolt Gasket
Valve Body Seat Ring
Seat Ring
Brazing Material Valve Body
B. Bolted Gasket
Flexible O-Ring
E. Brazed Valve Body Welded Hard Facing
Seat Ring
Valve Body
C. Integral Hardfacing
F. Axially Clamped
Figure 3-12 Methods for Attaching Seat to Body
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Valve Body Seat Clamp
Flexible Seat
Disc
Figure 3-13 Flexible Seat When using gaskets, the seat should incorporate a metal-to-metal stop as shown in Figure 3-11B to limit the amount of compression applied to the gasket because repeated stress cycling of the gasket will lead to relaxation of the joint seal and eventual leakage. Metal type gaskets should not be reused unless explicitly permitted by the gasket vendor. Floating seats, such as those used in trunnion-mounted ball valves, do not require independent restraints, but are held in place by the ball itself. Sealing of the floating seat in the body is accomplished using elastomer or packing seals, as shown in Figure 3-14. One ball valve design for high temperature service applications uses spring-loaded packing seals as shown in Figure 3-15. Spring
Body Cavity
O-Ring
Seat Ring Seat Ring Insert Sealant Channel
Figure 3-14 Floating Seat
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Stem Closure Element Bonnet Body
Seat Packing Spring
Figure 3-15 Spring-Loaded Packing Seals
3.2.3 Disc-to-Stem Connection The disc-to-stem connection joint, which transmits the load from the actuator to the disc, should be designed to have equal or greater strength than the stem itself. However, only the API Code imposes this requirement, and several valve manufacturers supplying valves to the power industry do not follow this guideline. Depending on the valve type, the joint can be fixed, free to rotate, or allow freedom for the disc to float laterally, as shown in Figure 3-16.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Stem
Stem Disk (Poppet)
Poppet Collar
Body Seat
Poppet Body Seat Ring
Body
A. Fixed, Integral
B. Free-to-Rotate
Stem Bonnet Disk
Stem Disc Articulated Link Guide Slot Guide Bar/Cam Downstream Seat
Tee
Square Head
Valve Body C. Cam-Guided, Articulated-Link Stem-to-Disc Connection
D. Laterally Floating
Figure 3-16 Stem Connections
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
3.2.3.1 Fixed Joints Typical fixed joint disc-to-stem connections commonly used are integral, welded, and screwed. These connections are normally used in non-rotating stem applications where the stem and seat maintain their axial alignment. When this joint is used in a globe valve application, an external means to prevent stem rotation should be provided. This is necessary to prevent galling at the disc-seat interface. When used in wedging-type gate valves, precautions should be taken to ensure that excessive lateral displacement of the stem will not cause binding of the stem in the bonnet stuffing box area. 3.2.3.2 Free-to-Rotate Connections Free-to-rotate connections should be used in rotating stem applications or when disc-toseat rotation is undesirable. These joints are frequently found in globe valves and nonrising stem gate valves. In non-rising stem gate valves, the disc-to-stem joint is threaded so that the rotation of the stem in the disc opens and closes the valve. Most free-to-rotate connections provide some limited lateral disc displacement to prevent stem binding and allow the disc to align itself with respect to the seat face. Free-torotate connections should incorporate some means of preventing the disc from spinning. Asymmetric flow created by multiple elbows upstream can cause the disc to spin; in fact, this has occurred in some swing check valve designs. Spinning discs can damage the disc and seat upon contact, and can cause premature failure of the connection due to excessive wear. 3.2.3.3 Laterally Floating Connections Floating connections are generally T-slot designs that permit assembly of the joint by simply sliding the parts together in a lateral direction. The T-slot is usually oriented in the direction of the flow (that is, in line with the expected disc displacement) to permit sliding to occur without causing stem binding. These joints are most commonly found in gate valve applications where the gate receives its alignment from guides in the body during the complete open-to-close cycle. These joints incorporate an anti-rotation feature, such as a square head, to prevent stem rotation. As shown in Figure 3-16c, another type of design found in power plants uses a double articulated link-type stem-to-disc connection to allow the wedge to float freely in the lateral direction without causing side loads on the stem or uneven loading of the wedge-to-seat face.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
3.2.4 Disc/Stem Guide Arrangements Guides are required for certain valve types to provide proper alignment of moving parts and prevent poor valve performance or inoperability. These guides are manufactured from soft materials or from very hard anti-wear or anti-galling materials, depending on the application and service. 3.2.4.1 Wedge Gate Guides Gate guides (see Figure 3-17) are provided specifically on wedge gate valves to keep the gate away from the seat faces, except for a small distance very near the fully-closed position, in order to minimize wear on the seating faces. Typically, the sliding surfaces on the gate and guide are overlaid with hardfacing materials to prevent galling of the sliding interfaces, due to the load generated by the differential pressure acting across the gate as the valve is being closed or due to the gate weight when the valve is in a position other than with the stem vertical. The clearance between the gate slots and the guide should not be so excessive that gate tilting occurs, thus causing accelerated wear or deterioration of the seating faces.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Gate Gate Guide
Figure 3-17 Gate Valve Gate Guide
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
3.2.4.2 Stem Guides Stem guides (see Figure 4-1) are most commonly found in globe valves. Stem guides, which provide alignment for the plug, are typically manufactured from softer materials to provide some lubricity and to prevent galling of the stem. Stem guides should be provided where significant side loads on the plug are present. These forces can be generated by side discharge, such as in angle globe valves. Stem guides are also provided when the stem is relatively long and flexible, such as in extended bonnet globe valves and in lift check valves (see Figure 7-5). 3.2.4.3 Disc Guides Disc guides (see Figures 6-9 and 6-11) are most commonly found in control, relief, and safety valves. These guides provide alignment between the disc and seats, and offer lateral support for uneven fluid discharge forces.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
4 VALVE MATERIALS
4.1 Trim Components and Materials Components of the valve considered to be trim consist of the removable or in-line repairable internal parts that contact the flowing fluid. As an example, in globe-type control valves, the plug or disc, seats, stem, guides, bushings, and cages are trim components (see Figures 4-1 and 4-2). Other components considered trim, but that do not come into direct contact with the fluid, are the components that make up the stuffing box: the packing gland, spring, lantern ring, and packing retainer ring. Secondary trim parts are stem-to-disc attachment, seat retaining rings, seat-to-body seals, spacers, and so on. Parts not included as trim are components that define the valve pressure boundary: the body, bonnet, body closures, and bonnet and body bolts and nuts. Closure gaskets and seals are neither pressure boundary components nor trim components, but maintain the leak-tight integrity of the valve [1, 2].
4-1
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair Valve Plug Stem Packing Flange Gland Follower Packing
Packing Spring
Packing Box Ring
Wiper Ring
Stem Bushing Seat Ring
Valve Plug/ Disc
Seat Ring
Stem Bushing
Figure 4-1 Double-Seated Globe Valve Trim Components Bonnet
Cage Gasket
Sliding Seal Balancing Hole
Cage/Disc Guide
Seat Valve Disc (Plug)
Figure 4-2 Balanced Disc Cage-Style Valve
4-2
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Strained-hardened 316 SS, quenched and tempered 410 SS, and precipitation-hardened 17-4 PH SS are the most commonly used materials for valve stems and other valve trim materials. Although not as corrosion resistant as 316 SS, the higher strength and correspondingly higher allowable stresses make the 410 SS and 17-4 PH materials much more attractive for larger sizes and higher pressure rated valves, because smaller diameter stems can be used. Hardsurfacing trim materials are discussed in Section 4.3. 4.2 Material Selection Practices In general, trim material selection should consider all of the important factors discussed below, in addition to mechanical strength considerations.
4.2.1 Design Practices to Minimize Corrosion Corrosion in valves can be minimized or eliminated by selecting materials that do not react with the fluid or with the material around them. There are different types of corrosion and the corrosion type dictates the material required. Corrosion control is especially important in valves that are subjected to fluids that pose a hazard if allowed to leak into the environment. Corrosion is the deterioration of a metal by reaction with the environment. Corrosion is generally controlled by selecting corrosion-resistant materials. Corrosion resistance of a component can be improved by covering the wetted surface with a plating, cladding, or by heat-treating. The rate of corrosion is influenced by the fluid velocity and media temperature. Table 4-1 lists some commonly used trim materials and their suitability for power plant applications.
4-3
4-4
F
P
P
P
F
P
P
P
F
F
F
F
F
F
F
Bronze
Inconel
Monel
Hastelloy B
Hastelloy C
Titanium 75A
Nickel
Alloy 20
Type 416 Hard
Type 440 Hard
17-4 PH
Alloy 6 (Co- Cr)
ENC*
Cr Plate
Al Bronze
F
F
F
F
F
F
F
P
P
P
F
P
P
P
F
P
P
316 SS
F
F
F
F
F
F
F
S
S
S
S
S
S
S
S
F
F
Bronze
S—Satisfactory F—Fair P—Poor *—Electroless Nickel Coating
P
316 SS
Key:
P
304 SS
304 SST
S
F
F
F
F
F
F
F
F
P
F
P
P
P
S
P
P
Inconel
S
F
F
S
F
F
F
F
F
F
F
P
P
P
S
P
P
Monel
S
S
F
S
F
F
F
F
S
F
F
P
P
P
S
P
P
Hastelloy B
S
S
F
S
F
F
F
F
F
F
F
F
F
F
S
F
F
Hastelloy C
S
F
F
S
F
F
F
F
F
P
F
F
F
P
S
P
P
Titaniu m 75A
Table 4-1 Chart of Wear and Galling Resistance of Material Combinations
S
F
F
S
F
F
F
P
P
F
F
S
F
F
S
P
P
Nickel
S
F
F
S
F
F
F
P
P
F
F
F
F
F
S
P
P
Alloy 20
S
S
S
S
F
S
F
F
F
F
F
F
F
F
F
F
F
Type 416 Hard
S
S
S
S
S
F
F
F
F
F
F
F
F
F
F
F
F
Type 440 Hard
S
S
S
S
P
S
F
F
F
F
F
F
F
F
F
F
F
17-4 PH
S
S
S
S
S
S
S
S
S
S
S
S
S
F
F
F
F
Alloy 6 (Co- Cr)
S
S
P
S
S
S
S
F
F
F
F
F
F
F
F
F
F
ENC*
S
P
S
S
S
S
S
F
F
F
S
S
F
F
F
F
F
Cr Plate
P
S
S
S
S
S
S
S
S
S
S
S
S
S
F
F
F
Al Bronze
EPRI Licensed Material
Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Galvanic corrosion is often found in valves due to the use of dissimilar materials for the body and trim. The galvanic series (see Table 4-2) is used to give an indication of the rate of corrosion between different metals or alloys when they are in contact in an electrolyte, such as seawater. Metals close to the active end of the table will behave as an anode and corrode, while metals close to the noble end will act as a cathode and be preserved. The distance between two metals in the chart is usually an indication of corrosion rate. This process is referred to as cathodic protection, with the cathodic material receiving material deposits from the sacrificial anode. Table 4-2 Metals in Order of Corrosion Rate Corroded End (Anodic or Least Noble) •
Aluminum
•
Mild Steel (Carbon)
•
Cast Iron
•
Ni-resist
•
Type 410 stainless steel (active)
•
Lead-tin solder
•
NOREM™ hardfacing alloys
•
Types 304 and 316 stainless steel (active)
•
Cobalt-base hardfacing alloys
•
Lead and tin
•
Muntz metal, manganese-bronze, and naval brass
•
Nickel-base hardfacing alloy
•
Nickel 200 (active) and Inconel™ Alloy 600 (active)
•
Yellow brass, admiralty brass, aluminum bronze, red brass, copper, silicon bronze, 70-30 copper nickel, comp. G-bronze, and comp. M-bronze
•
Nickel 200 (passive) and Inconel Alloy 600 (passive)
•
Monel™ Alloy 400
•
Types 304 and 316 stainless steel (passive) and Incoloy™ Alloy 825
•
Incoloy Alloy 625, Hastelloy™ Alloy C and titanium
Protected End (Cathodic or Most Noble)
Area differences also affect galvanic corrosion. A larger anodic area, compared to the cathodic area, is preferred because it reduces the amount of corrosion. As an example, a stainless steel bolt in a carbon steel body usually causes the carbon steel to corrode at only a slightly increased rate, whereas, a carbon steel bolt in a stainless steel body corrodes at a rapid rate because the stainless steel acts as a large cathode.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
4.2.2 Design Practices to Minimize Erosion Erosion is wear damage in which loss of material occurs due to the action of moving particles carried in a fluid stream. This action is most severe when the velocity of the fluid is high, such as during valve throttling or closing and opening under high pressure drops. Entrained sand, slurries, catalyst fines, and liquid particles in flashing flow are sometimes associated with this type of wear. The selection of materials for the pressure containment parts, that is, the body and bonnet, is rather limited from the standpoint of their ability to withstand erosion, but the use of sacrificial liners at the areas of impingement has been successful. There are four principal types of erosion: (1) abrasive particle, (2) cavitation, (3) erosion-corrosion, and (4) high velocity fluid impingement. These types of erosion and specific guidance regarding how to improve the resistance of the trim materials to their effects are discussed in the following paragraphs. 4.2.2.1 Abrasive Erosion In abrasive erosion, small particles that are harder than the trim surface are carried at high velocity in the fluid stream and impinge upon and scour away the trim metal. Resistance of materials to impingement erosion varies with the angle of impingement. At low impingement angles (< 15˚ with respect to the surface), hardfacing materials with large amounts of carbides, such as Stellite 1, are recommended. At high impingement angles (> 80˚), hardfacing alloys with large amounts of relatively ductile matrix, material such as cobalt in Stellite 21, are recommended. Stellite 6, however, has been found to provide the best combination of erosion resistance and wear resistance as a trim material for the widest range of valve geometries that have large variations in impingement angles. However, these cobalt-based materials will be activated if they are in fluids that are transported through the reactor core region, thus creating a radiation concern in that the cobalt may plate out on the interior walls of a piping system or be captured in crevices known as “crud-traps.” Cobalt-free alternatives, which are discussed in Section 4.3.2, have been found to provide very good abrasive erosion resistance. 4.2.2.2 Cavitation Erosion Cavitation occurs as the result of vapor bubbles forming when the pressure of a liquid flowing in the restricted passages of a valve becomes less than the vapor pressure of the liquid at that temperature. The bubbles then collapse as the flow area enlarges and the pressure recovers. The implosion of bubbles produces shock waves and very high localized stresses at the surface of the metal, causing the material to fail and detach from the surface. Because no known material can withstand continuous severe cavitation service without failure, ease of trim replacement should be a strong consideration for service in cavitating conditions. Cobalt-based hardfacing alloys, such as Stellite 6 and Stellite 21, have found extensive use for resisting cavitation erosion. Other materials used are Type 440/C, No. 6 Colmonoy, hardened tool-steel, and sintered tungsten carbide with a nickel binder. Recent testing has shown the EPRI-developed NOREM alloys to be more resistant to cavitation erosion than the Stellite Alloys 6 and 21 [10]. 4-6
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Cavitation erosion can be reduced by system design; by selecting the hardest trim material that will not crack from the impact of repeated valve closure and thermal shock; by using multiple valves to distribute the total pressure drop by providing back pressure; by using valves that incorporate multiple pressure drop stages designed to prevent cavitation through any one stage. 4.2.2.3 Combined Erosion and Corrosion Both erosion and corrosion can occur in a piping system; although not simultaneously. The erosion strips away the protective coating of corrosion, thus allowing additional corrosion to occur, repeating the cycle. Accelerated failures of carbon-steel piping and fittings have occurred in feedwater service due to a combined erosion-corrosion phenomenon. Valves and other components installed in these systems are subjected to the same degradation mode. The failures are attributed to a single phase erosion-corrosion phenomenon that happens to plain carbon steel when exposed to flowing water having a low dissolved oxygen content (less than 10 ppb) in combination with a pH value less than about 9.3. Erosion-corrosion is essentially a flow-assisted dissolution process of the magnetite corrosion film normally present under deoxygenated feedwater conditions. This phenomenon results in much higher metal corrosion rates than would normally be encountered. Loss rates can be greater than 1 mm (about 0.040 inch) per year in severe cases. The worst attack occurs in areas of the feedwater system where temperatures are between 260˚F and 400˚F (127˚C and 204˚C). The phenomenon is critically dependent on a number of variables, particularly flow velocity, temperature, pH, and oxygen content of the feedwater, and the elemental composition of the steel. A comparison of the critical operational variables to typical PWR feedwater conditions is provided in Table 4-3. Table 4-3 Critical Variables for Accelerated Erosion-Corrosion Critical Operational Accelerated Variables for Erosion-Corrosion
Typical PWR Feedwater Condition
pH less than 9.3
pH between 8.8 and 9.6
Temperatures between 212 ˚F (100˚ C) and 525 ˚F (275˚ C), with worst attack between 260 ˚F (127˚ C) and 400 ˚F (204˚ C)
Varies depending on location in system, typically between 100˚F (38˚C) and 450˚F (232˚C)
Dissolved oxygen content less than 10 ppb
Dissolved oxygen content less than 5 ppb in hot standby and less than 3 ppb in power operation
Turbulent hydrodynamic conditions (high fluid flow velocities)
Fluid flow velocity varies throughout the system. High localized velocities in fitting and valves are common.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Valve parts intended for PWR feedwater applications, particularly parts exposed to highly turbulent flow, should not be constructed of plain carbon steel. The use of low-alloy steel, with at least 0.5% chrome, has been shown to significantly reduce erosion-corrosion attack and should be used as a replacement material whenever possible. Typical replacement materials would be !s Cr-!s Mo Plate (A-387, Type 2), !s Cr-!s Mo Plate(A-387, Type 12), !s Cr-!s Mo Plate Forging (A-182, Type F12 and A-336, Class F12), 1!f Cr-!s Mo Casting (A-217, Type WC6) and 1!f Cr-!s Mo Bar (A-739, Grade B11). 4.2.2.4 High Velocity Fluid Impingement Erosion High velocity fluid impingement erosion occurs when extremely high velocity fluid jets turn abruptly, bouncing off one surface to impinge and erode the adjacent part. Impingement erosion can be a form of erosion-corrosion, whereby the high velocity fluid jet blasts away the protective surface coating as rapidly as it forms. Average fluid velocity must exceed several hundred feet per second for impingement erosion to occur. Fluid impingement erosion can be prevented or reduced using the same techniques and materials for improving resistance to abrasive erosion and cavitation erosion.
4.2.3 Design Practices to Minimize Wear and Galling Wear and galling of seating materials is responsible for many check valve problems, especially those involving operability. Depending on the extent of the damage, the valve might require more force than normal to actuate it, or damage might even make the valve inoperable. In some cases, the damage can be so severe that even the structural integrity of the valve is compromised. Galling is a condition that occurs on the rubbing surfaces of mating parts where material transfer results in localized welding, with subsequent spalling and a further roughening of the surfaces. Galling causes the damaged material to: • Impede the valve during stroking • Degrade the seat joint • Increase the operating force • Make the valve completely inoperable (in the worst case) Factors affecting galling include the type of materials in contact, temperature, surface finish, hardness, contact pressure, and the line fluid. Higher temperatures will generally anneal or soften the metals, increasing their galling potential. Test data show that hardness is the most significant factor affecting wear; the harder the material, the less the wear. Galling like wearing, can be prevented by: • Using hard materials for one or preferably both parts • Avoiding excessive contact stresses • Providing adequate operating clearances 4-8
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
• Using an appropriate lubricant between the sliding surfaces • Selecting pairs of material with low galling potential; using different materials for components in contact, rather than the same material • Breaking in these components by cycling under low load before subjecting them to the full load 4.3 Hardfacing One of the most common methods used to prevent galling and excessive wear is hardfacing. Hardfacing is the process of applying—by welding, plasma-spraying, or flame-spraying—a point, edge, or layer of wear-resistant metal onto another metal to increase its resistance to abrasion, erosion, or galling. In some cases, hardfacing is applied to impart some corrosion resistance to the base metal. For wear resistance, hardness is required only on the surface of the metal. Hardfacing is generally used when external lubrication is not feasible or is inadequate to give the desired service life, and it is usually applied only to the critical surfaces. In contrast to heat treatment to achieve high surface hardness, hardfacing can be used effectively in very large components where the contact area is small and heat treatment of the entire component would be impractical. Also, because hardfacing is a welding technique, it can be used for in-line repair or to refurbish large components without dismantling. Hardfacing on valves is typically used on the plug or disc-to-seat joint (both on the disc and on the seat) to maintain a tight seal. Other areas that are often overlaid include the stem, bushing, and disc or plug guides. No particular restrictions are imposed when using a base metal of carbon steel, but there are some restrictions when using other metals, including stainless steels. For most metals, it is desirable to preheat the base metal to prevent cracking of the hardfacing, as cooling occurs. In some cases, hardfacing can be achieved by case-hardening techniques, such as carburizing and nitriding. These superficial hardness treatments usually produce casehardened depths of less than 0.025 inch that are normally not detected by conventional hardness measurement, such as Brinell and Rockwell tests, but require microhardness testing methods.
4.3.1 Cobalt-Base Alloys (Stellites™) Cobalt-chrome and tungsten-carbide are the principal materials used in hardfacing. These materials consist of hard tungsten and chromium particles in a softer cobalt matrix. Stellite™ is a trade name typically associated with these types of materials. They are often referred to as “AMS (American Materials Society) 5387” or “CoCr” (Cobalt-Chrome). Stellite 6 (CrCo-A) is used on valve seats, while the slightly harder, but more brittle, Stellite 12 (CoCr-B) can be used on plugs. For field repair of worn surfaces, Stellite 21 is 4-9
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
less hard, more ductile, and has lower cracking tendencies, making its use more practical, even though its wear properties are not as good as Stellite 6. For smaller valve parts, the disc or plug and seats can be made of solid Stellite material. Stellites have a reputation for being hard to apply. In particular, they are susceptible to stress or shrink cracking during cooling. To a large extent, this is an unavoidable sideeffect of being a brittle material, and similar problems occur with all hardfacing materials. Generally, the harder the deposited material, the more susceptible it is to cracking. These problems can be reduced or eliminated by utilizing good welding practices, preheating the base material, and minimizing the deposited thickness to less than 0.200 inch (5 mm). Additional suggestions are provided in Section 14. In contrast to the cobalt-chrome materials, which are supplied in a wire form and applied using conventional welding techniques, tungsten-carbides are usually applied to all trim shapes by the spray-welding process and are then fused to give a nonporous surface. Service temperature, thermal shock, and the required corrosion-resistance must be considered when using tungsten-carbide. Loading of the valve seat must be uniform, and impact forces during closure should be low to prevent cracking of tungsten-carbide. Refer to Section 11 for chemistry specifications.
4.3.2 Cobalt-Free Alloys Cobalt-60 has been identified as a major isotope responsible for out-of-core radiation contamination problems in the nuclear power industry. Cobalt-60 is an activation product of natural cobalt, which is found in cobalt-based alloys. Surfaces subject to high wear do wear over time, and metallic particles are introduced into the pipeline due to seat lapping and other valve maintenance. Valve repair, such as seat lapping to decrease seat leakage, produces significant amounts of cobalt grinding debris. Naturally occurring cobalt in these alloys is 100% Cobalt-59. If it is in a pathway to the reactor vessel, alloy particles will pass through the core, become exposed to thermal neutron flux, and become activated to Cobalt-60. This isotope has a half life of 5.25 years and emits 1.3 megaelectronvolt (MeV) gamma rays. These small cobalt particles accumulate in the piping system in crevices and cracks where the flow velocity is low, and in stagnant pockets or “crud traps” that are inherent in the design of some valve bodies. The accumulation of this radioactive contamination increases with time and becomes a major hindrance to access for maintenance work, raising ALARA (radiation levels as low as reasonably achievable) concerns. A number of years ago, the high price and uncertain availability of cobalt-based alloys led to efforts by manufacturers to develop alternate hardfacing materials using nickelbased, cobalt-free alloys such as ASTM A565 Grade 616, Deloro™ 40 and 50, and Colmonoy™ 4 and 5 (NiCr-A and NiCr-B, respectively).
4-10
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
These alloys exhibit lower mechanical and corrosion-resistance attributes than cobaltbased alloys and initially had limited use and acceptance. However, their overall properties are sufficiently good that their potential for use in nuclear valves should not be ignored. More recently, there have also been renewed efforts to develop new low-cobalt or cobalt-free alloys (nickel-based or iron-based) that reduce the exposure of service personnel to radiation due to Cobalt-60. Several EPRI-sponsored efforts have been conducted to evaluate existing low-cobalt alloys or cobalt-free hardfacing materials and to develop new cobalt-free alloys as a valid alternative for hardfacing. Further evaluation of cobalt-free alloys by EPRI, material manufacturers, and valve manufacturers continues, along with the development of new iron-based cobalt-free alloys and other cobaltfree alloys. These evaluations include testing for mechanical properties and corrosion resistance and some in situ valve testing. EPRI evaluation and testing has led to the development of the NOREM™ cobalt-free, iron-based hardfacing alloy. Test results confirm that the NOREM 02 and 02A Alloys provide galling wear, cavitation erosion resistance, corrosion resistance, hardness, and friction characteristics comparable to Stellite 6, and weldability characteristics equal to Stellite 21 [10]. NOREM has been used extensively in nuclear applications by valve manufacturers in cast form (for manufacture of seat rings or discs) and in weld material form for application by welding. Several utilities and service vendors have also applied the welding products in situ. Other cobalt-free materials being evaluated or marketed include: • Valloy™ 1 • Delchrome™ 91 • Tristelle™ 5183 • Nucalloy™ 453 and 488 4.4 Resilient (Soft) Seating Resilient seats are used to accomplish good seating with much lower contact force than in metal-to-metal seats. It is easier to deform the softer materials and fill out the valleys in the mating surfaces. In most designs, the soft seat rings provide the primary seating, with metal-to-metal closure acting as a secondary seal in case of damage or failure of the soft seat material. For some designs with stringent leakage rate requirements, such as those required for containment isolation, the soft seat is very beneficial. It can be designed to control leakage at low pressure differentials. As the pressure increases, the seating relies on metal-to-metal contact.
4-11
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
This secondary metal-to-metal contact also makes the seats fire safe and allows some degree of seat tightness, should the resilient seat ring fail. Whenever the temperature, radiation, and pressure environment permit, soft seals should be strongly considered because of the ease in accomplishing good seating with soft seals. Because resilient seat ring materials do not have the required strength and stiffness to resist rupture against pressure and blowout against differential pressure, they must be securely clamped in the seat or installed in a grooved pocket. The material properties to be considered in the selection of a soft seat ring are: • Room for thermal expansion • Hardness • Permanent set and extrusion under load • Rate of recovery upon removal of the load • Tensile and compressive strength • Radiation resistance • Abrasion resistance • Wear resistance • Thermal resistance • Halogen content and stability under high temperature and radiation conditions • Fluid compatibility including chemical reaction, swelling, loss of hardness, permeability, and degradation Common elastomeric materials used in resilient valve seats and some of their respective properties are listed in Table 4-4.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Table 4-4 Properties of Plastics and Elastomers Used for Soft Seats [8] (Source: EPRI NP-6516) TEFLON (Halon, TFE, and Fluon)
Radiation resistance: maximum 10 4 rads Low coefficient of friction High chemical resistance Temperature limit of 400 °F (204° C) Susceptible to abrasion
TEFLON (Glass Filled)
Radiation resistance: maximum 10 4 rads Low to moderate coefficient of friction High chemical resistance Temperature limit of 450 °F (232° C) Susceptible to abrasion, but better than unfilled teflon
NYLON (Zytel, Nypel, and Fosta)
Radiation resistance: 104 rads High coefficient of friction Mode rate to low chemical resistance Temperature limit of 250 °F (121° C) Not susceptible to abrasion
KEL-F (CTFE)
Radiation resistance: 107 rads Low coefficient of friction Good chemical resistance Temperature limit of 300 °F (149° C) Susceptible to abrasion
TEFZEL
Radiation resistance: 107 rads Low coefficient of friction High chemical resistance Temperature limit of 300 °F (149° C) Moderate resistance to abrasion
POLYETHYLENE
Radiation resistance: 108 rads Low to moderate coefficient of friction High chemical resistance Temperature limit of 180 °F (82 °C) Not susceptible to abrasion
NATURAL GUM RUBBER
Radiation resistance: 107 rads High coefficient of friction Mode rate to low chemical resistance Temperature limit of 130 °F (54 °C) Good abrasion resistance
BUNA-N
Radiation resistance: 104 rads High coefficient of friction Mode rate to low chemical resistance High resistance to petroleum products Temperature limit of 210 °F (99 °C) Not susceptible to abrasion
VITON
Radiation resistance: 107 rads High coefficient of friction Good chemical resistance Temperature limit of 400 °F (204° C) Not susceptible to abrasion
ETHYLENE,
Radiation resistance: 108 rads High coefficient of friction Mode rate to low chemical resistance Temperature limit of 300 °F (149° C) Not susceptible to abrasion
PROPYLENE, TERPOLYMER
4-13
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
4.5 Pressure-Retaining Materials Materials commonly used for pressure-boundary parts (as defined in ASME III, Code Case N62, and ANSI B16.34) fall into three categories: stainless steels or other corrosionresistant alloys, carbon steels, and low-alloy steels. The selection of materials is dependent on such factors as resistance to corrosion and/or erosion, and to some extent, the pressure/temperature rating for the various materials. All materials should be selected based on specific service conditions. It is common practice for the valve body to match the piping material. Typical valve pressure-boundary materials and their advantages and disadvantages are discussed below. Table 2-5 depicts specific material specifications and grades commonly used for valve pressure-boundary parts. The same material or product form is not required to be used for the valve bonnet as for the body. The rating applied, however, must be based on the valve body, with the bonnet designed and material selected accordingly. Stainless steel—SA/A351 CF3 and CF8, SA/A182 F304, F316 and F347 The superior erosion resistance of stainless steel is a common reason for its selection for a high-velocity system. Further, the required retention of water purity in a demineralized water system requires the use of stainless steels, because the small amounts of corrosion products that could result from the use of carbon steel cannot be tolerated. The boric acid content of a pressurized water reactor coolant system also leads to the selection of a stainless steel body and bonnet. Advantages: • Good high temperature/pressure performance • Good general corrosion resistance Disadvantage: • High initial cost Carbon steel—SA/A 216, WCB and WCC, SA/A 105, or SA/A 350 Carbon steel valves are used in the steam, feedwater, extraction steam, and condensate systems, where the water chemistry can be controlled to restrict the corrosion rate. Carbon steels and stainless steels have yield strengths about equal at room temperature. At the higher operating temperatures of a water-cooled reactor (500˚F to 600˚F) (260˚C to 316˚C), the yield strength of carbon steel is greater than that of stainless steel. Carbon steel is not recommended for prolonged usage above 800˚F (427˚C) because of its potential graphitization and creep damage.
4-14
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Advantages: • Widely used (available) • Moderately priced Disadvantages: • Not appropriate for prolonged service over 800˚F (427˚C) • Poor chemical resistance to most corrosives • Poor resistance to erosion by high velocity vapor droplets, such as flashing condensate or wet steam • Never appropriate for any trim parts, except base material for a disc that is overlaid with corrosion-resistant material at the seat Chrome-moly low- (carbon) alloy steel—SA/A217 WC6 and WC9, SA-182 F11 and F22 For steam service that has a high moisture content and that might result in erosion, 2!f Cr-1Mo or 1!!f Cr-!s Mo material should be used for the valve body and bonnet, even though the temperature and water chemistry would permit carbon steel. In addition, for high-velocity service, 2!f Cr-1Mo is superior to carbon steel. Low-alloy steels generally have a significantly higher yield strength at higher operating temperatures than carbon or stainless steels. Advantages: • Can operate continuously at high temperatures • Price is reasonable, considering its superior characteristics • Has better resistance to corrosion than carbon steel • Gives additional erosion resistance and is, therefore, recommended on flashing or erosive service Disadvantage: • Welding must be followed by post-weld heat treating for thicker cross-sections. Many other materials are available for valves, such as cast iron (A-126), ductile iron (A395), and bronze (B-62). ASME III permits the use of some, but not all, cast iron or ductile-iron materials for valves. There are many more cast alloys available that are generally used because of their own particular resistance to various fluid chemistries, such as aluminum-bronze (B-148), bronze (B-61), Alloy 20 stainless steel (A-35a, CN7M), Monel (A-494, M-35), and Inconel (A-494, CYAO). Alloys such as aluminum-bronze or Monels are often used for service environments such as seawater.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Cast iron—A-126 Advantages: • Low cost • Good for general service Disadvantages: • Limited pressure and temperature rating • Brittle, can crack easily • Not allowed by ASME III Bronze—SB/B-61, SB/B-62 Advantages: • Low cost • Good for general service (air and water) Disadvantages: • Limited in temperature and pressure
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
5 GATE VALVES
5.1 Introduction and Application Gate valves are the most commonly used isolation valves in steam and process service. They provide reliable sealing performance over a wide range of temperatures and pressures. Because gate valves are designed to take advantage of differential pressure, they are very effective in high pressure and high temperature systems. Gate valves are available in sizes ranging from 1/4 inch (6.3 mm) to 48 inches (1,200 mm) and larger. One of the major benefits of the gate valve design is the low pressure drop realized across it, almost approaching that of a straight length of pipe. One of the major drawbacks of the gate valve design is the difficulty in maintaining, repairing, or replacing its seats [8, 13–18]. 5.2 Design
5.2.1 General Gate valves can be either a rising stem or non-rising stem design (see Figure 5-1). Rising stem designs, utilizing an outside screw and yoke (OS&Y) (see Figure 5-2), provide the advantage of having the power threads outside the fluid, thus minimizing thread damage from exposure to the fluid. Rising stem action allows the incorporation of an optional backseating feature to assist in isolating the packing from the process fluid by pulling up the stem against the inside of the bonnet. Another rising stem option utilizes a power screw inside the valve body, which exposes the threads to the fluid. A non-rising stem configuration requires the power screw to be inside the valve disc or wedge. Because the stem rotates in the packing without axial motion, packing wear and damage resulting from abrasive contaminants and undesirable materials being dragged across the packing is avoided. The disadvantages of the non-rising stem are that the threads are exposed to the fluid, the stem cannot be backseated, and the disc position cannot be judged by the stem position from the outside. Additionally, inside screw and non-rising stem configurations are usually limited to low pressure and low temperature applications.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Packing
Packing
Disc Stem
A. Rising Stem Design
B. Non-Rising Stem Design
Figure 5-1 Inside Screw Stem Thread Configurations [8]
Handwheel
Packing Yoke Stem Packing
A. Non-Rising Handwheel
B. Rising Handwheel
Figure 5-2 Rising Stem Design, Outside Screw [8]
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Most gate valve designs are offered with metal-to-metal seating at the gate-to-seat interface. Metal-to-metal seating allows valves to operate at a much higher temperature than would be possible with elastomeric- or polymeric-type seat materials. Metal-tometal seating also makes the valve fire safe for most applications. Five versions of the gate valve are widely used: solid-wedge, split-wedge, flex-wedge, double disc, and parallel slide. There are many variations of each of these five types available today, including the conduit, slab gate, and knife gate. When a system designer specifies a type of gate valve for an application, one should remember that not all designs offer the same level of performance. A design that is perfectly acceptable for one service might not perform reliably in another. These guidelines are intended to be used only as general criteria. For a specific service and all critical service applications, the system designer should consult with a valve engineer who is familiar with both the application and the valve before selecting a specific design.
5.2.2 Solid-Wedge Gate Solid-wedge gates (see Figure 5-3) are of a simple, one-piece construction characterized by a V-shaped wedge that converts the axial stem thrust to a high seat load, normal to the seat faces. The seats can be separate pieces held firmly into the valve body by either press fitting, welding, or threading; or they can be welded and machined into the body. Typically, the body of a wedge gate valve has gate guides on the sides (as shown in Figure 3-17) that mate with guide slots on the sides of the disc. These guides support the load due to differential pressure across the wedge, and keep the wedge away from the seat faces, except for a small distance very near the fully closed position, to minimize wear. In small valves, the seat loading caused by the closing force applied through the stem to the gate is much higher than the seat loading created by the pressure differential across the gate. Therefore, seating effectiveness is not significantly increased by increasing the differential pressure across the gate. In larger valves, the differential pressure acting on the gate provides the primary load against the seat, and the mechanical force from the stem is used to enhance the seating action.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Bonnet
Stem
Solid Wedge Seat Ring
Body
Figure 5-3 Solid-Wedge Gate Valve [13] The solid-wedge design is suitable only in smaller sizes where the stiffness of valve body and disc is much higher than that of the adjacent pipe. The increased stiffness minimizes seat distortion, which can create increased seat leakage or gate pinching due to pipe loads transmitted to the valve ends. The solid-wedge gate design is not suited for large valves, especially in high temperature applications where differential expansion and distortion of the gate, body, and seats, due to mechanical and thermal loads, can cause loss of seat tightness and/or binding of the gate, which can either increase the operating thrust required or, in some cases, cause complete inoperability. Solid-wedge valves are not generally recommended for use with power actuators. 5-4
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
The following list identifies some of the advantages and disadvantages of solid-wedge gate valves: Advantages: • Seating effectiveness can be increased by the application of additional force to the stem. • Solid-wedge gate valves have simple construction. • They have bi-directional operation due to symmetrical construction. Disadvantages: • Solid-wedge gates are sensitive to line loads: bending moment, torsion, and axial loads, which are transmitted by the adjacent pipe to the valve ends. The sensitivity increases with the size of the valve. • Seating is sensitive to thermal distortions because solid-wedge gates do not have the ability to easily conform to the seat face plane distortion. • Lack of flexibility in solid-wedge gates can cause the wedge to stick closed during heatup or cooldown cycles when the gate is left in the closed position. The sticking can be caused by the expansion of the stem during heatup, pushing the wedge down, or by the differential thermal expansions that exist between the gate and the body materials that can create an interference at the seat during cooling. For example, gates made of higher coefficient of thermal expansion materials (such as 410 stainless steel), when used in conjunction with valve bodies made of lower coefficient of thermal expansion materials (such as carbon steel or 316 stainless steel), can create this unfavorable situation during cooldown. • It is difficult to perform in-line repair because of the problem in achieving an accurate matching of seat angles during lapping. • Depending on the clearance in the gate area, the gate could tilt under flow forces and create galling or high wear at the disc/seat faces.
5.2.3 Split-Wedge Gate Split-wedge gates (see Figure 5-4) are similar to solid-wedge gates, but are comprised of two separate pieces. The split-wedge construction permits the gate assembly to more easily tolerate line loads and temperature transients by allowing each wedge piece to align with its mating seat. This feature is used in larger gate valves to overcome sticking problems encountered with the solid wedge. Because of the ability of each gate wedge to align itself independently against its respective seat, this type of construction allows both wedge pieces to seat simultaneously, consequently fluid pressure can be trapped in the body. Under a temperature increase, the thermal expansion of this trapped fluid can cause buildup of very high pressures in the body, which can damage the pressure boundary. The trapped fluid also increases the thrust required to open the valve, occasionally resulting in complete inoperability. Provisions to relieve the body pressure must be made in such valves to eliminate these problems. 5-5
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Yoke
Pressure Seal Bonnet Stem
Split Wedge Seat Ring Body
Figure 5-4 Split-Wedge Gate Valve [13]
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
The following list identifies the advantages and disadvantages of split-wedge gates: Advantages: • Split-wedge gates have better immunity to line loads than the solid-wedge design, minimizing sticking and leakage. • They can provide simultaneous shut off against pressure on both the upstream and the downstream seats (block and bleed). • Seating effectiveness can be increased by the application of additional force to the stem. • Simultaneous seating of both discs can be used to check body seat integrity without line pressure by pressurizing the body between the seats. • It is easier to repair seat faces in-line than with the solid wedge because the splitwedge design can tolerate more angular mismatch. Disadvantages: • Both wedge pieces can seat simultaneously, thus trapping pressure in the body. This can cause inadvertent overpressure in the body during thermal transients and an increase in the thrust required to open the valve due to the combined friction from the two wedge pieces, in some cases rendering the valve inoperable. This condition is often referred to as “double disc drag.” • The two-piece construction is more expensive and somewhat more complex than a solid wedge. It also has the potential for allowing disengagement between the gate pieces and the stem. • Depending on the clearance in the gate guide area, it is possible for the gate to tilt under flow forces and create galling or high wear at the disc/seat faces.
5.2.4 Flexible-Wedge Gate Flexible-wedge gate design (see Figure 5-5) was introduced to overcome leakage or gate binding and sticking problems caused by distortion of the valve body due to thermal and pipeline stresses transmitted to the valve ends. The flexible-wedge design, a simple variation of the solid wedge, is constructed in one piece composed of two discs that are connected with an integral boss that permits independent flexure of the discs. Because the flexible wedge is simple and has no separate components that could become loose inservice, it is widely used in power plants.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Yoke
Pressure Seal Bonnet
Stem
Seat Ring
Flexible Wedge
Body
Figure 5-5 Flex-Wedge Gate Valve [13]
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
The following list identifies some of the advantages and disadvantages of flexiblewedge gates: Advantages: • Flexible-wedge gates have better immunity to line loads than the solid-wedge design, minimizing sticking and leakage. • They are simpler in construction than split-wedge gates. • Seating effectiveness can be increased by application of additional force to the stem. • Simultaneous seating of both discs can be used to check body seat integrity, without line pressure, by pressurizing the body between the seats. • It is easier to repair seat faces in-line than with the solid wedge because the flexiblewedge design can tolerate more angular mismatch. Disadvantages: • Both wedge pieces can independently seat simultaneously, thus trapping pressure in the body. This can cause inadvertent overpressure in the body during thermal transients and an increase in thrust required to open the valve due to the combined friction from the two wedge pieces, in some cases rendering the valve inoperable. This condition is often referred to as “double-disc drag.” • Depending on the clearance in the gate guide area, it is possible for the gate to tilt under flow forces and create galling or high wear at the disc/seat faces.
5.2.5 Double-Disc Gate Double-disc gate valves (see Figure 5-6) are of multiple-piece construction with the faces of the gate pieces contacting the seat parallel to each other. When going from the open to closed position, the wedge pieces move down together as an assembly without any relative motion between them until, at the very end of the stroke, one of the pieces contacts the bottom stop. Continued motion of the stem after this position causes a climbing action of one wedge piece on the other at the inclined plane interface between them, which in turn expands them laterally against their respective seats. Double-disc gate valve design can provide simultaneous seating against both the upstream and downstream side pressures. This can be an advantage because of the redundancy in seating available in such a design. However, the double seating feature can also be a disadvantage because the body can trap fluid, which can cause inadvertent high pressure during thermal transients.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Stem Upper Wedge Mechanism Wedge Pin
Disk Retainer
Lower Wedge
Disk
Wedge Spring
Figure 5-6 Double-Disc Gate Valve [13] Some double-disc expanding gate valve designs employ special mechanisms that kinematically prevent premature gate expansion when the gate assembly is in the mid-travel position. Expansion of the gate before reaching the end of the stroke can prevent the gate from closing completely. This design is also made in a through-conduit, double-wedge arrangement that permits expansion of the gate in the open position, as well as the closed position (see Figure 5-7). The design is extensively used in applications where a “pig” is run through the pipeline to isolate a pipe section for testing or repair.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Stem
Body
Seat
Segment
Gate
Figure 5-7 Through-Conduit, Double-Disc Gate Valve [8] The following list identifies some of the advantages and disadvantages of parallel expanding gates: Advantages: • Double-disc gates can provide a positive simultaneous shut-off against pressure on both the upstream and the downstream seats. • Through-conduit, double-wedge design can double block and bleed, that is, provide block and bleed in the closed position and also prevent the line pressure from entering the body cavity through both seats simultaneously in the open position. • Seating effectiveness can be increased by the application of additional force to the stem. • Double-disc seating can be used to check the integrity of both seats simultaneously by pressurizing the body between the seats.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Disadvantages: • Depending on the actual gate construction and its stiffness, parallel-expanding gate design can be very intolerant of line loads and thermal transients. • This type of gate is normally unidirectional or has a preferred flow direction for best performance. The two wedge pieces are usually asymmetrical, and one of the two pieces has the better ability to self-align with respect to the seat face. • Both wedge pieces can independently seat simultaneously, thus trapping pressure in the body, causing inadvertent overpressure in the body during thermal transients and an increase in thrust required to open the valve due to the combined friction from the two wedge pieces, in some cases rendering the valve inoperable. This condition is often referred to as “double-disc drag.” • This type of gate is more complex and requires a special mechanism to prevent inadvertent mid-travel expanding movement of the discs towards the seating surfaces. • Depending on the clearance in the gate guide area, it is possible for the non-throughconduit gate designs to tilt under flow forces and create galling or high wear at the disc and seat faces. The through-conduit gate design, shown in Figure 5-7, is not susceptible to this problem.
5.2.6 Parallel-Slide Gate The parallel-slide gate, also called a parallel-expanding, double-disc gate (see Figure 5-8), is constructed in two pieces, with each disc allowed to float independently and mate with its seat. The individual pieces are not mechanically wedged against their respective seat, but are pre-loaded by a spring between them that provides initial seating force. The flexibility of the spring allows distortion and changes in dimensions between the seat faces to be easily accommodated without pinching the gate, which provides complete immunity from sticking and binding under line loads and thermal transients. The pressure differential across the gate increases the seat contact force and provides a tighter seal. Parallel-slide, double-disc gates can provide only a downstream seat and are most effective in the larger valve sizes in applications where at least moderate differential pressure exists.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Stem
Disc Retaining Pins
Disc Disc Carrier
Seat Preload Spring
Figure 5-8 Parallel-Slide Gate Valve [8] Self-wiping action of the gate against the seat during operation keeps the seat faces clean of any foreign material and provides good sealing action over a long period time, especially in clean fluid service. The following list identifies some of the advantages and disadvantages of the parallelslide, double-disc gate valve: Advantages: • Of all the gate valve designs discussed, parallel-slide double-discs are the most tolerant of line loads due to the ability of the spring between the gate pieces to absorb large seat deflections with virtually no change in seat contact force. • They are tolerant of temperature changes during operation. The gate does not bind due to the differential thermal expansion effects because of the resilient spring between the discs.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
• They are tolerant of a lack of parallelism between the two seat faces because of the ability of the two independent gate pieces to align themselves. This feature also provides a good shut-off under bending moments transmitted to the valve ends by the adjacent pipe, which causes tilting of the seat faces. The ability to absorb large variations in dimensions between the seat faces, without any adverse effect on the valve performance, allows more economical fabrication tolerances to be used than in wedge gate valves. • There is less tendency for galling, due to smaller changes in seat loading under line loads and thermal transients. • These gates can be used bi-directionally, due to their symmetrical design. • The dual-disc seating can be used to check body seat integrity without line pressure by pressurizing the body between the seats. Disadvantages: • Seating effectiveness cannot be increased by the application of additional force to the stem as in wedge-gate valves. • Parallel-slide, double-disc gates provide downstream seating only; the upstream disc does not seat against line pressure. • Floating gate pieces can trap body pressure and affect double-disc seating, allowing inadvertent overpressure in the body during thermal transients. • This type of gate has a constant spring load over the entire stroke, creating a nominally higher running torque. • Depending on the clearance in the gate guide area, it is possible for the non-throughconduit gate designs to tilt under flow forces and create galling or high wear at the disc and seat faces. The through-conduit gate design, shown in Figure 5-9, is not susceptible to this problem.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Stem
Body
Seat
ThroughConduit Slide Gate
Figure 5-9 Through-Conduit, Parallel-Slide Gate Valve [8]
5.2.7 Slab Gate The slab gate design features a very simple one-piece parallel gate (see Figure 5-10), which is matched flat on both sides. The slab type of gate requires axially movable seats between the seat and body, and a soft-type seat insert to allow seating without the high contact stresses required in metal-to-metal seats. Seating between the gate and seat faces is accomplished in both upstream and downstream locations. Both seats are designed to float freely in their respective seat pockets which are machined into the valve body, and the seats are forced against the gate by springs. When the gate is closed, the upstream seat is axially forced against the gate by the springs, and the differential pressure is
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
acting on the unbalanced annular area of the seat. The downstream seating is achieved by floating the gate against the downstream seat, due to the differential pressure acting across the entire area defined by the seat bore. Preload Spring
O-Ring Seal Seat Ring Insert
Stem
Preload Spring
Movable Seat Gate
Body
Figure 5-10 Slab Gate Valve [8]
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
The following list identifies some of the advantages and disadvantages of slab gate valves: Advantages: • Slab gates can tolerate line loads better than wedge-type gate valves without binding or seating degradation. They are virtually immune to line loads if sufficient clearances are present, due to the spring-loaded seat design that can absorb large changes in dimensions of the seat pocket area caused by line loads transmitted to the valve ends by the adjacent pipe. • These gates are easy to maintain because of the removable seat design. • Slab gates will self-relieve body overpressure to the low pressure side by pushing the spring energized seats away from the gate. This eliminates the high pressure buildup in the body cavity associated with most of the other gate valves under temperature increases. Disadvantages: • Seating effectiveness cannot be increased by applying additional force to the stem as in wedge gate valves. • Service conditions are limited to 400˚F with conventional soft seating materials made of elastomers and plastics. Some designs utilize higher temperature packing materials, for example, carbon, graphite, or asbestos for higher temperature applications. • Slab-gate valves cannot compete with butterfly valves in size and cost.
5.2.8 Knife Gate Knife-gate valves (see Figure 5-11) are similar to conventional gate valves, except that they use a relatively thin plate for a disc, similar to a guillotine. Only the plate, which extends through the bonnet, is exposed to the fluid. In this design, the stem and threads are protected from the fluid medium.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Body
Knife Gate
Figure 5-11 Knife-Gate Valve [8] Knife-gate valves can be provided with metal seats or soft seats, and can be made in almost any size, because these valves lend themselves to economical fabrication from plates. They are normally available in pressure ratings no higher than Class 125/150. Knife-gate seat tightness can be good, but would not be expected to be comparable to a wedge gate, ball, or butterfly valve. Because of the rectangular-shaped packing gland geometry and its high stuffing box cross-sectional area, packing maintenance/tightness is not expected to be as good as a conventionally packed stem. Knife-gate valves can be used in low pressure service at moderate and low temperatures, where abrasive particulates are in the flow stream, or where clogging of piping/ valve might be expected due to a high percentage of solids in the fluid.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
The following list identifies some of the advantages and disadvantages of knife-gate valves: Advantages: • Knife-gate valves have an economical construction for low pressure service. • They are suitable for contaminant-laden (dirty) service. • These valves are compact and lightweight. • The stem and thread are not exposed to the fluid medium. Disadvantages: • Knife-gate valves are less reliable in preventing leakage to the environment due to a large rectangular geometry stuffing box design. • They are not suitable for high pressure service because of the flat body design, which is not an efficient geometry for pressure containment. 5.3 Repair Issues
5.3.1 Leakage Past the In-Body Seat Leakage past the seats can be the result of corrosion products on the disc/wedge and seat, incomplete contact between the seat and disc, scratches, pits, cuts, and erosion on the disc and/or seat, or an obstruction lodged across or between the seats preventing the gate from closing. This section addresses the typical problems that require more attention than the typical maintenance activities, such as cleaning, inspection, and lapping or polishing. The two types of seat rings discussed herein include integral-type seats and welded-in seat rings (Figure 5-12).
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Gate Valve Body
Seat Ring
Seal Weld
Figure 5-12 Welded-In Gate Valve Seat Ring 5.3.1.1 Damage Assessment (DA) and Repair Options (RO) DA: Valve disc and seat have foreign material and corrosion products that are preventing full closure. RO: Polish the seat and disc with emery cloth in accordance with standard maintenance procedures. If the corrosion products are still attached or surface scratches are present, lap the seat and disc per Section 9.3. Seat contact can then be verified by performing a bluing evaluation to ensure 360˚ contact on the disc face, as discussed in Section 8.3. DA: A bluing test reveals less than 360˚ surface contact between the wedge and seats. RO: Verify that sufficient stem stroke and pressure are available to fully close the valve. RO: Verify that the wedge is not seating too low in the seats. RO: Verify that the clearance between the wedge guide and wedge guide slots are sufficient to allow for proper movement of the wedge. Repair in accordance with Section 8.8. RO: Lap the seats and wedge per Section 9.3 to ensure that they are flat. Perform a blue check. If 360˚ contact is not achieved, a well qualified machinist/valve technician is needed to properly shim and fit the disc per Section 8.11. 5-20
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
DA: A bluing test reveals 360˚contact between the seats and wedge/disc, but the valve is still leaking. RO: Verify that sufficient stem stroke and pressure are available to fully close the valve. RO: Verify that the width of the seat contact surface is appropriate for the size and pressure rating of the valve. If the contact surface is too wide, the seating surface should be narrowed by lapping the relief angle of the seat per Section 8.3. The suggested seat width should be obtained from the valve manufacturer or design drawings. DA: A bluing test reveals that the wedge is seating too low in the seats (see Figure 5-13).
Wedge
Seat
Normal Seating Position
Figure 5-13 Gate Valve Wedge Seating Too Low in Seats [16] RO: Procure a new wedge and fit it as discussed in Section 8.11. RO: On double-disc valves (Anchor Darling Valve Company), a new disc can be procured. RO: On Equiwedge Valves (Edward Valve Company), a thicker spacer ring can be installed between the gate halves (see Figure 5-14).
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Stem
Gate Half
Gate Half
Spacer Ring
Two-Piece Wedge Note: Halves held together by parallel guide ribs
Hardfaced Wedge Guide Rails
Wedge Halves
Body Groove
Body
Hardfaced Wedge Seat
Figure 5-14 Split-Wedge Gate with Spacer Ring [17] RO: Re-hardface the wedge and fit per Section 8.11. DA: The seat ring has cuts and/or erosion damage beyond what can be lapped out.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
RO: If the damage is isolated to some small local areas and does not extend well into the seat ring base material, a repair to the seat ring can be performed per Section 8.3 provided that the existing hardfacing is of a repairable type. This is a challenging repair that requires very skilled welding personnel who are experienced in welding hardfacing materials. RO: If the damage to the seat ring is severe, the seat ring should be replaced in accordance with Section 8.4. In situ seat ring replacement is generally restricted to valves 12 inches (305 mm) and larger due to the limited development of welding and machining systems. RO: Except for some very specific circumstances, valves less than 12 inches (305 mm) should be removed from the line and repaired in a shop. Generally, valves less than 6 inches (152 mm) can be replaced at a cost that is comparable to the repair cost. DA: The integral hardfacing seat has cuts and/or erosion beyond what can be lapped out. RO: If the damage is isolated to some small local areas and does not extend well into the valve body base material, a repair to the hardfaced seat can be performed per Section 8.5 if the existing hardfacing is of a repairable type. This is a challenging repair that requires very skilled welding personnel who are experienced in welding hardfacing materials. RO: If the damage to the hardfacing is extensive or protrudes into the base material, the hardfaced seat should be replaced in accordance with Section 8.6. In situ replacement of integral seats is generally restricted to valves 12 inches (305 mm) and larger, due to the limited development of welding and machining systems. RO: Except for some very specific circumstances, valves less than 12 inches (305 mm) should be removed from the line and repaired in a shop. Valves less than 6 inches (152 mm) can generally be replaced at a cost that is comparable to the repair cost. DA: The wedge/disc is found to have cuts and/or erosion beyond what can be lapped out. RO: Procure a replacement wedge or pair of discs. The new components will have to be fitted to the valve as detailed in Section 8.11. RO: If the wedge or disc has an isolated crack or cut and the valve technician determines that the wedge has enough material on it so that it will not drop through the seat after repair, a localized repair can be attempted as described in Section 8.11. RO: Remove the existing hardfacing and repair per Section 8.11. This approach is generally more effective than the localized repair option.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
5.3.2 Leakage Past the Pressure Seal Ring Leaks past the pressure seal ring (see Figure 5-15) are commonly the result of insufficient bolt tension on the bonnet. The bolt tension can be lost due to vibration, temperature and pressure fluctuations, or by creep in high temperature applications. Corrosion products and other foreign material between the body bore and seal ring can also contribute to leaking. Other contributors to leaking body-to-bonnet seals include cuts, scratches, other defects in the mating surfaces of the body seal area, the bonnet seal surface, and the replaceable seal ring. 1 – Packing Flange Nut 2 – Packing Flange Washer 3 – Packing Flange 4 – Packing Flange Eye Bolt 5 – Gland Bushing 6 – Gland Bolt Clamp 7 – Clamp Bolt 8 – Clamp Jam Nut
9 – Bonnet Retaining Ring 10 – Packing Ring 11 – Yoke 12 – Half Dog Set Screw 13 – Bonnet 14 – Seal Ring 15 – Stem
Figure 5-15 Typical Pressure Seal Bonnet Configuration [18] The type and extent of damage can be determined after the valve bonnet is removed and the seal areas inspected utilizing standard maintenance practices. Critical dimensions should be compared to manufacturer drawings and specifications prior to initiating any repair activity.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
5.3.2.1 Damage Assessment and Repair Options DA: Minor scratches and cutting are found in the body sealing surfaces of the body and/or bonnet seal area. RO: Determine the depth of the imperfections. Flaws less than .010 inch can be removed by polishing with a fine emery cloth or buffing wheel. RO: If flaws are deeper than .010 inch (.25 mm) on the body seal, the complete inside diameter of the contact area should be honed per Section 9.4 until the flaw is removed. The bore should then be measured and an oversized seal ring selected, as discussed in Section 8.7. RO: If flaws are deeper than .010 inch (.25 mm) on the bonnet angular surface, the sealing surface can be repaired by taking a skim cut with a lathe as described in Section 8.7. The critical angle must be maintained. DA: Deep cuts, porosity, and erosion are found on the body sealing area. RO: If the flaw is isolated, a localized repair can be performed by grinding out the defect area, rewelding, and finish machining and lapping as detailed in Section 8.7. An oversized seal ring might be required based on the finished inside diameter measurements. RO: If the flaws are numerous about the diameter of the bore, the complete inside diameter of the seal area should be machined to remove all of the cladding, and the seal area should be rewelded, machined, and honed to blend with the surrounding bore diameter as detailed in Section 8.2. DA: Deep cuts, porosity, and erosion are found on the bonnet sealing surface (see Figure 5-16).
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Pressure Seal Bonnet Sealing Surface
Pressure Seal Gasket
Bonnet Backseat
Figure 5-16 Pressure Seal Bonnet [18] RO: Procure a replacement bonnet. RO: Verify the dimensional tolerances from the manufacturer and remove the defects by machining. RO: Rebuild the sealing surface and large diameter by welding and remachining it to the manufactured dimensions.
5.3.3 Valve Stem Leaks Two areas on the valve stem contribute to leakage of steam or process. The valve stem provides a sealing surface for the packing to seal against in the bonnet, preventing process leakage to the atmosphere, and a stem backseat that seals against the bonnet backseat to allow for packing replacement under system pressure (see Figure 5-17). Valves stems are generally made of hardened steels such as quenched and tempered 410 stainless or precipitation-hardened superalloys—materials that are not suitable for weld repair. Except for cosmetic repairs, damaged or bent stems should be replaced.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Valve Stem
Hardfaced Bonnet Backseat
Figure 5-17 Typical Valve Stem/Bonnet Backseat 5.3.3.1 Damage Assessment and Repair Options DA: The valve stem has scratches and pitting. RO: If the scratches and pitting are just at the surface, they can be removed by polishing the surface with an emery cloth or buffing wheel. RO: If the scratches and pitting are numerous, the stem can be chucked in a lathe and turned while applying an emery cloth. RO: If the scratches or pits are few but less than .005 inch (.13 mm), they can be ground out and blended. RO: If the scratches, pits, or cuts are less than .005 inch (.13 mm), they can be removed by taking a machine cut with a lathe about the diameter of the stem, and then blended and polished to remove the edges and cold work. Total removal should not exceed .010 inch on the diameter measurement. RO: If any defects are determined to be deeper than .005 inch (.13 mm), the stem should be replaced. DA: The stem backseat has scratches, cutting, erosion, or pitting. RO: If the damage is very minimal, the stem backseat should be lapped against the bonnet backseat as presented in Section 8.7. RO: The stem can be chucked into a lathe and skim cuts taken on the seat angle until the flaw is removed per Section 8.13. The manufacturer should be consulted for the proper angle and tolerances. The mating seats should then be lapped together as presented in Section 8.7.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
RO: If the stem backseat is extensively damaged, the stem should be replaced. It should be noted that the stem designs generally provide a considerable amount of extra material in the stem backseat area to accommodate extensive sealing face material removal. DA: The bonnet backseat has scratches, cutting, erosion, or pitting. RO: If the damage is very minimal, the stem backseat should be lapped against the bonnet backseat as presented in Section 8.7. RO: The bonnet can be chucked into a lathe and skim cuts taken on the hardfaced seat angle until the flaw is removed. The manufacturer should be consulted for the proper angle and tolerances. The mating seats should then be lapped together as presented in Section 8.7. RO: If the integral hardfaced bonnet backseat is extensively damaged, the seat should be weld-repaired per Section 8.7. RO: If the replaceable hardfaced seat ring is extensively damaged, the seat ring should be replaced per Section 8.7.
5.3.4 Valve Body Damage The need for valve body repairs is generally the result of trim components improperly installed or foreign materials not allowing the internal sealing components to fully close. Except for cases of casting defects that lead to through wall weeping, most body repairs are performed in conjunction with trim repairs, such as seats or pressure seal bores. The valve body (see Figure 5-18) is a pressure boundary and thus must be repaired in accordance with the rules of the jurisdictional code. Generally, this is either the ASME or NBIC.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair Body Flange
Pressure Seal Hardfacing/Bore
Guide Rib
Seat Ring
Flow Bore
Figure 5-18 Typical Gate Valve Body 5.3.4.1 Damage Assessment and Repair Options DA: A deep groove is found in the body flow path just beyond the integral seat ring. RO: If the defect is away from the seat and sufficient wall thickness is available after the flaw has been removed, the area can be blended into the surrounding base material by grinding and flapping per Section 9.1. RO: Determine if the erosion is related to a crack in the hardfaced seat that is contributing to high velocity flow. If the seat is damaged, the base material flaw should be repaired in accordance with Section 8.1, and then the seat repaired in accordance with Section 8.5 or 8.6, depending on the extent of the damage. RO: If erosion damage is confined to the body flow area, the body should be repaired in accordance with Section 8.1. Except for cases where the repair area is easily accessible from the bonnet bore, the valves will have to be removed from the line to allow access from the butt-welded end.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
DA: A crack or deep groove cuts into or through the integral hardfaced seat and into the body material. RO: If the flaw is a narrow crack, the flaw could be ground out manually, and the crack weld-repaired as discussed in Section 8.5, utilizing a butter material followed by a hardfacing repair. RO: If, after removing a narrow crack, it was determined that sufficient minimum wall was not available, a base metal repair would be required per Section 8.1, followed by a hardfacing repair per Section 8.5. RO: If a large section of the seat is damaged due to erosion, the integral hardfaced seat should be removed, the base material repaired or restored in accordance with Section 8.1, and the seat restored in accordance with Section 8.6. DA: A deep groove is found in the body flow path just beyond the seat ring. RO: If the defect is away from the seat and sufficient wall thickness is available after the flaw has been removed, the area can be blended into the surrounding base material by grinding and flapping per Section 9.1. RO: Determine if there is a crack in the seat hardfacing or a flaw in the seat ring seal weld that is contributing to high velocity flow and erosion. If the seat ring is damaged, the base material flaw should be repaired in accordance with Section 8.1, and then the seat repaired in accordance with Section 8.5 or 8.6, depending on the extent of the damage. RO: If erosion damage is confined to the body flow area, the body should be repaired in accordance with Section 8.1. Except for cases where the repair area is easily accessible from the bonnet bore, the valves will have to be removed from the line to allow access from the butt-welded end. DA: A deep groove from erosion is found under the seat ring and protruding into the downstream base material. RO: If accessible, the eroded area can be repaired by welding from the downstream side of the seat ring. This option presents a risk of the seat warping. The more welding required, the greater the risk. RO: The seat ring should be removed per Section 8.4. The eroded base material should then be restored per Section 8.2. The weld prep for the replacement seat ring should be established and the seat ring installed per Section 8.4. DA: A deep groove or cut penetrates into or through the seat ring and into the downstream base material. RO: The seat ring should be removed per Section 8.4. The eroded base material should then be restored per Section 8.2. The weld prep for the replacement seat ring should be established and the seat ring installed per Section 8.4. 5-30
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
DA: The bonnet bore has pitting, erosion, and corrosion (below the pressure seal area). RO: The defects should be removed by grinding or machining, and blended into the surrounding base material. If the minimum wall thickness is violated, the wall thickness should be restored by welding per Section 8.1. DA: The bonnet bore is cracked below the pressure seal area. RO: The crack should be completely removed by machining or grinding. If the minimum wall thickness is not violated, the area should be blended into the surrounding base material. RO: The defects should be removed by grinding or machining. If the minimum wall thickness is violated, the wall thickness should be restored by welding per Section 8.1.
5.3.5 Valve Bonnet Leaks Leaks to the atmosphere because of bonnet problems can generally be prevented by good maintenance practices. Adherence to recommended bolting, packing, and gasket selection guidelines developed by EPRI and others have led to substantially improved reliability of valves. Repairs to body-to-bonnet flanges, packing chambers, and backseats (see Figure 5-19) are quite uncommon but are addressed here along with casting defects. Gland
Gland Nut Gland Flange
Upper Packing End Ring Packing Ring
Lower Packing End Ring
Gland Bolt Gland Rretainer Capscrew
Body/Bonnet Stud and Nut
Bonnet Body Body/Bonnet Gasket Backseat Stem
Figure 5-19 Typical Bolted Bonnet Components [8] 5-31
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
5.3.5.1 Damage Assessment and Repair Options DA: A weep hole in the bonnet casting is allowing process to escape to the atmosphere. RO: If the valve is less than 6 inches (152 mm), it might be more cost effective to replace the bonnet or the complete valve than to repair it. RO: Remove the bonnet from the valve and repair it as if it were a valve body crack per Section 8.1. The flaw should be ground out to approximately mid-wall and welded out, and then ground from the opposite side of the bonnet until weld metal is revealed. The second side should then be welded out and blended. DA: Inspection of the packing chamber reveals steam cuts into the base material. RO: If the valve is less than 6 inches (152 mm), it might be more cost effective to replace the bonnet or the complete valve than to repair it. RO: If the cut is less than .010 inch (.25 mm) deep, a burr grinder can be utilized to remove the defect and blend into the surrounding base material. RO: If the flaws are deeper than .010 inch (.25 mm) on the packing chamber, the complete inside diameter of the chamber should be honed per Section 9.4 until the flaw is removed. The bore should then be measured and larger packing rings installed. RO: If the defect must be weld-repaired, it should be performed in the same manner as a pressure seal bore repair discussed in Section 8.2. DA: Erosion is found on the body or bonnet flange. RO: Determine the minimum flange thickness to see if the flange can be machined to remove the defect area per Section 9.2. Care must be taken to ensure that all critical dimensions are within tolerance. This is extremely critical to valves with ring gaskets. RO: If the minimum flange thickness cannot be maintained by re-machining the flange face, a localized weld repair should be performed by grinding and welding per Section 9.2, and the final surface remachined to obtain flatness for gasket sealing. RO: On very critical valves where flange thickness must be restored, the complete flange face can be built up by welding and remachining per Section 8.1. DA: The bonnet backseat has scratches, cutting, erosion, or pitting. RO: If the damage is very minimal, the stem backseat should be lapped against the bonnet backseat as presented in Section 8.7. RO: The bonnet can be chucked into a lathe and skim cuts taken on the hardfaced seat angle until the flaw is removed. The manufacturer should be consulted for the proper
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
angle and tolerances. The mating seats should then be lapped together as presented in Section 8.7. RO: If the integral hardfaced bonnet backseat is extensively damaged, the seat should be weld-repaired per Section 8.7. RO: If the replaceable hardfaced seat ring is extensively damaged, the seat ring should be replaced per Section 8.7.
5.3.6 Wedge Guides Wedge guides accurately control the alignment of the wedge assembly into the seats to minimize unnecessary contact stresses and galling wear between the hardfaced surfaces. Repair to the guides requires precision machining to maintain the critical tolerances commonly ignored. If the guides are too tight, they can restrict movement and possibly sealing; if they are too loose, the wedge can flutter when the valve is almost closed, causing severe damage to the wedge and seats. Two designs of guides are used by gate valve manufacturers, the single guide (see Figure 5-20) and the parallel guides (see Figure 5-21). Neither has a real performance advantage over the other, but maintenance and repair of the single guide is somewhat easier, due to the accessibility of both contact faces.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Gate Gate Guide
Figure 5-20 Single Guide Design for Gate Valves [8]
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair Guide Slot Guide Rib
Figure 5-21 Parallel Guide Design for Gate Valves 5.3.6.1 Damage Assessment and Repair Options DA: Inspection reveals that the guide rib is preventing the wedge from fully closing. RO: The guide(s) should be blued and the wedge assembly installed and stroked to determine if and where contact is being made between the wedge slots and the guide. When the area is determined, the guide should be machined or ground to provide clearance. DA: The wedge is fluttering while opening/closing and during operation, causing severe vibration and damage to the sealing faces. RO: Tighten the clearances by building up the hardfacing on the wedge slot faces and machining per Section 8.8. RO: Tighten the clearances by building up the hardfacing on the guide(s) and machining per Section 8.8. NOTE: Valve manufacturers vary as to which surfaces of the wedge guide(s) and guide slots they hardsurface. Check with the valve manufacturers for the specific valve type. DA: The seal weld that connects the guide rib to the body is cracked. RO: The crack should be ground out and a repair weld performed in accordance with Section 8.8. DA: A critical section of the guide is missing. RO: Fit a replacement section into the valve and weld per Section 8.8. RO: Remove the valve from the line and install a replacement guide. RO: Place the wedge in the seats and fit a replacement guide into the valve body per Section 8.8, using the wedge as a fit-up tool. 5-35
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
6 GLOBE VALVES
6.1 Introduction and Application Globe valves are the most extensively used valves for modulating (throttling) service, due in part to the adaptability of the basic design to accommodate difficult conditions, such as high pressure, temperature, and differential pressure. To a lesser extent, globe valves can be used in shutoff applications. The streamlined surface that the plug presents to the flow minimizes turbulence and permits reliable control of the flow rate. Compared to ball and butterfly valves, globe valves present a high flow resistance. Flow capacity of globe valves is about one third that of low resistance valves such as ball and butterfly. However, as flow capacity decreases, resistance to cavitation and noise increases [8, 13, 19]. Globe valves are unidirectional. When used in throttling applications, they should be installed with flow under the seat. The configuration of the disc is such that only the bottom presents a smooth profile to the flow. The sharp profile of the top of the disc, when exposed to high velocity fluids, causes turbulence. This not only creates problems with obtaining reliable throttling but also can damage the valve. For shutoff applications where a tight seal is desired, the valve should be installed with flow over the disc. With flow under the disc, the actuator has to provide a force equal to the full differential plus an additional amount to provide a sufficient unit load in the seat to achieve a seal. This force must be maintained if the valve is to remain tight. Any relaxation of the stem thrust due to material deformation or actuator tolerances will cause the valve to leak. With flow over the disc, the pressure acts as the sealing force and the problem is eliminated. In this situation, the actuator is required only to overcome the differential pressure force when the valve is operated. The unidirectional nature of the globe valve for throttling and shut off is the source of many problems. In an attempt to combine two functions in one valve, globe valves are frequently required to seal and throttle in the same direction. When this occurs, one function or the other is not going to be properly taken care of and damage to the disc or seat can occur due to cavitation or erosion. When the throttling function is required in one direction and shutoff is desired in the other, a globe valve is an ideal choice.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
6.2 Design
6.2.1 General In contrast to the gate valve’s disc-to-seat sealing action, which is accompanied by sliding and friction, the globe-valve plug or disc approaches, or moves away from, the seat in a direction perpendicular to the seat plane without sliding. Thus, relatively high seat contact stresses can be developed to get very tight shutoff without galling the seating surfaces. Globe valve stems are either a rising and nonrotating design or a rising and rotating design. Some rotating and rising stem globe valves have an integral stem-to-disc connection that causes sliding at the seat face during the final closing action. Globe valves are available in a wide variety of materials with both soft seating options and metal-to-metal seating components. Due to their relatively short stroke to achieve the full open position (as compared to gate valves), globe valves can easily incorporate diaphragm or bellows-type stem seals to provide zero external leakage design. Metalto-metal seating allows valves to operate at higher temperatures than elastomeric- or polymeric-type seat materials. Variations of two basic disc (poppet) designs are utilized in globe-type valves. Plugtype discs (see Figure 6-1) are generally supplied for normal service applications on standard globe valves. These include isolation and limited throttling service. For severe throttling service, a special throttling, or cage, disc (see Figure 6-2) should be used. A severe application is any application where the pressure is reduced across the valve by 20% or more. For example, for an upstream pressure of 100 psi, any throttling that would reduce downstream pressure to less than 80 psi should be done with a special disc. The actual configuration of the disc and the sophistication of the design depend on the size of the pressure drop.
Figure 6-1 Plug-Type Disc [19]
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Figure 6-2 Cage-Type Disc [19] Aside from the different types of discs supplied, standard globe valves are supplied in four different body configurations. The most common is the standard pattern (also called the horizontal or tee pattern). The other configurations include the Y-pattern (or wye), the angle pattern, and the Y-angle (or elbow down). When a system designer specifies a globe valve for a particular application, careful consideration must be given to the specific function and operating conditions. A valve that is perfectly suited for one application might not perform reliably in another. These repair guidelines are intended to be used as general criteria. The system designer should consult with a valve engineer prior to selecting a valve for a particular service. This is extremely important for critical service and throttling applications.
6.2.2 Horizontal Globe The horizontal globe (see Figure 6-3) is most commonly used, although it has the greatest flow resistance of any of the globes. When it is installed with the stem in the vertical position, it is easiest to maintain. It can be installed in either horizontal or vertical lines with flow in either direction.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Operator
Yoke
Stem
Pressure Seal Bonnet
Disk/Poppet
Body
Seat Ring
Figure 6-3 Horizontal Globe Valve [13]
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
6.2.3 Y-Globe The Y-globe (see Figure 6-4) is designed for use where throttling capability is required, but lower flow resistance is desired. The inclined stem angle makes maintenance more difficult than with a regular globe. It can also be installed in either horizontal or vertical lines. In vertical lines, it is not recommended for installation with the stem angled down. The Y-globe has been widely used for main steam isolation service, providing low pressure drop capability under full flow with no throttling.
Pressure Seal Bonnet
Disk/Poppet Operator
Seat Ring Yoke
Body
Figure 6-4 Y-Globe Valve [13]
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
6.2.4 Angle Globe The angle globe (see Figure 6-5) allows a designer to take advantage of a 90˚ bend in the piping and replace the elbow with a throttling valve. This eliminates extra welds, reduces system pressure drop, and permits the most efficient use of available space. When used as a throttling valve, the flow in the vertical leg should be upward under the disc. For isolation service, the flow should be downward over the disc.
Operator
Yoke Bonnet
Disk
Seat Ring
Body
Figure 6-5 Angle Globe Valve [13] 6-6
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
6.2.5 Y-Angle Globe The Y-angle globe (see Figure 6-6) combines the installation versatility of the angle valve with the lower pressure drop of the Y-globe. The installation limitations of the Yangle globe are the opposite of those for a regular angle valve. When used for throttling, the flow in the vertical leg should be downward, and in isolation application the flow should be upward or over the disc.
Operator
Stem
Pressure Seal Bonnet
Yoke
Disk
Seat Ring
Body
Figure 6-6 Y-Angle Globe Valve [13]
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
6.2.6 Control Valves In order to meet the demands of fluid control, several styles of globe valves have been developed. The following sections describe the globe-type valves; most are of the horizontal body style, with modifications to the inlet ports, discs, and guides. 6.2.6.1 Single-Port Valve The most common control valve is a single-port, top-guided design in which the valve disc is guided with the lower portion of the valve bonnet (see Figure 6-7). The singleport valve is generally specified for applications where tight shutoff is required. Diaphragm Actuator
Actuator Stem
Packing Valve Stem
Bonnet
Body Plug
Seat
Figure 6-7 Single-Port Control Valve [8]
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Because the high pressure fluid acts across the entire area of the disc defined by the seat port diameter, the resultant unbalanced force on the disc can be quite large and is the dominant component in sizing the actuator. Because of relatively high actuator thrust requirements, single-port globe valves are most commonly used in a smaller size [3 inches (76 mm) or less], even though they can also be used in 4-inches to 8-inches (102-mm to 203-mm) sizes with high thrust actuators. 6.2.6.2 Double-Port (Double-Seated) Valves Double-port valve bodies, shown in Figure 6-8, are used to balance the forces acting on the disc because high pressure fluid tends to exert opening force on one seat, and closing force on the other. The net force is lower than in single-port valves, which permits a smaller actuator to be used for a given size valve. The smaller actuator also provides for more stable control operation due to the absence of large plug force versus travel gradients. Double-port valves are most commonly used in sizes of 6 inches (152 mm) or larger, and are generally of top and bottom guided construction. Because it is difficult to close the two seats simultaneously, particularly due to differential thermal expansion effects in operation, double-port valves should not be required to perform a shutoff function. It should be pointed out that the double-seated valves use slightly different diameters for the top and bottom seat to allow assembly and removal of the smaller disc through the larger seat.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Seat Ring
Valve Disc/Plug Seat Ring
Guide Bushing
Body Bottom Cap
Figure 6-8 Double-Port Control Valve [8] 6.2.6.3 Three-Way Valves Three-way valves, as shown in Figure 6-9, use a double-port body construction for diverting or mixing service, and require three pipeline connections. Because the pressure differentials across the two seats are different, actuator selection requires careful consideration, especially when unbalanced valve construction, as shown in Figure 6-7, is used. To remove the disc, the upper valve seat, which is generally threaded-in, must be removed.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Stem
Bonnet
Seat
Disc Body Seat
Figure 6-9 Three-Way Valve [8] 6.3 Repair Issues Repair issues addressed in this section pertain to fixed components such as valve bodies, bonnets, valve seats, and pressure seal areas. Repair of control valve disc packs and actuators should be referred to the OEM.
6.3.1 Leakage Past the In-Body Seats Leakage past the seats can be the result of corrosion products on the poppet and seat; incomplete contact between the seat and disc; scratches, pits, cuts, and erosion on the disc and/or seat; or an obstruction lodged across or between the seats preventing the gate from closing. This section addresses the typical problems that require more attention than the typical maintenance activities, such as cleaning, inspection, and lapping or polishing. The three types of seat rings discussed herein include integral-type seats, threaded-in seat rings, and welded-in seat rings.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
6.3.1.1 Damage Assessment and Repair Options DA: Valve disc and seat has foreign material and corrosion products that are preventing full closure. RO: Polish the seat and disc with emery cloth in accordance with standard maintenance procedures. If the corrosion products are still attached or if surface scratches are present, lap the seat and disc per Section 9.3. Seat contact can then be verified by performing a bluing evaluation to ensure 360˚ contact on the disc face, as discussed in Section 8.3. DA: A bluing test reveals less than 360˚ surface contact between the disc and seats. RO: Verify that sufficient stem stroke and pressure are available to fully close the valve. RO: Verify that the disc is not seating too low in the seat ring by measuring the inner diameter (ID) of the seat ring and the outer diameter (OD) of the poppet. RO: Verify that the guide ribs are in alignment with the seat and that clearances between the poppet wear rings and the guide ribs are in tolerance. Excessive clearance can result in improper seating. Repair in accordance with Section 8.9. RO: Lap the seats and disc per Section 9.3. Perform a blue check. If 360˚ contact is not achieved, a well qualified machinist/valve technician is needed to properly fit the disc per Section 8.12. RO: Measure the poppet and seat angles. The poppet angle should be a minimum of 1˚ less than the seat angle. If the dimensions are not in tolerance, a well qualified machinist/valve technician is needed to properly fit the disc per Section 8.12. DA: A bluing test reveals 360˚ contact between the seats and poppet/disc, but the valve is still leaking. RO: Verify that sufficient stem stroke and pressure are available to fully close the valve. RO: If the seat ring is a threaded-in type with a gasket under it, tighten the seat ring or remove the seat ring and replace the gasket. RO: If the seat ring is welded-in, check to see if the valve is leaking past the seal weld of the seat ring. If so, repair in accordance with Section 8.3. DA: A bluing test reveals that the disc is seating too low in the seats (see Figure 6-10). RO: Procure a new poppet and fit it, as discussed in Section 8.12. RO: Re-hardface the disc/poppet and fit it, per Section 8.12.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Body Seat Disk Seat
Disk
Body
Body Seat
Figure 6-10 Disc Seating Too Low in Seat DA: The seat ring has cuts and/or erosion damage beyond what can be lapped out. RO: If the damage is isolated to some small local areas and does not extend well into the seat ring base material, a repair to the seat ring can be performed per Section 8.3 if the existing hardfacing is of a repairable type. RO: If the damage to the seat ring is severe, the seat ring should be replaced in accordance with Section 8.4. In situ seat ring replacement is generally restricted to valves 6 inches (152 mm) and larger due to the limited development of welding and machining systems.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
RO: Except for some very specific circumstances, valves less than 6 inches (152 mm) should be removed from the line and repaired in a shop. Generally, valves less than 6 inches (152 mm) can be replaced at a cost that is comparable to the repair cost. DA: The integral hardfacing seat has cuts and/or erosion beyond what can be lapped out. RO: If the damage is isolated to some small local areas and does not extend well into the valve body base material, a repair to the hardfaced seat can be performed per Section 8.5 if the existing hardfacing is of a repairable type. This is a challenging repair that requires very skilled welding personnel experienced in welding hardfacing materials. RO: If the damage to the hardfacing is extensive or protrudes into the base material, the hardfaced seat should be replaced in accordance with Section 8.6. In situ replacement of integral seats are generally restricted to valves 6 inches (152 mm) and larger, due to the limited development of welding and machining systems. RO: Except for some very specific circumstances, valves less than 6 inches (152 mm) should be removed from the line and repaired in a shop. Valves less than 6 inches (152 mm) can generally be replaced at a cost that is comparable to the repair cost. DA: The poppet is found to have cuts and/or erosion beyond what can be lapped out. RO: Procure a replacement poppet. The new components must be fitted to the valve as detailed in Section 8.12. RO: If the poppet has an isolated crack or cut and the valve technician determines that the poppet has enough hardfacing material on it so that it will not drop through the seat after repair, a localized repair can be attempted as described in Section 8.12. RO: Remove the existing hardfacing and repair the poppet per Section 8.12. This approach is generally more effective than the localized repair option.
6.3.2 Leakage Past the Pressure Seal Ring Damage and associated repair of pressure seal bodies are the same for gate and globe valves. See Section 5.3.2.
6.3.3 Valve Stem Leaks Damage and associated repair of valve stems is the same for gate and globe valves. See Section 5.3.3.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
6.3.4 Valve Body Damage The need for valve body repairs is generally the result of trim components improperly installed or foreign materials not allowing the internal sealing components to fully close. The result is erosion or wire drawing in leakage areas to high velocity flow in a concentrated area. Except for cases of casting defects that lead to through-wall weeping, most body repairs are performed in conjunction with trim repairs, such as seats or pressure seal bores. The valve body (see Figure 6-11) is a pressure boundary and thus must be repaired in accordance with the rules of the jurisdictional code. Generally, this is either ASME or NBIC.
Bonnet
Disc
Pressure Seal Area Guide Rib
Body
Flow Bore
Seat Area
Figure 6-11 Valve Body [19] 6.3.4.1 Damage Assessment and Repair Options See Section 5.3.4.1.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
6.3.5 Valve Bonnet Leaks Damage and associated repair of gate and globe valve bonnets are the same. See Section 5.3.5. 6.3.6 Poppet (Disc) Guide Ribs Guide ribs accurately control the alignment of the poppet assembly into the seat to minimize potential hardfacing damage or bending of a stem. Repair to the guide ribs and poppet guide rings require precision machining to maintain the critical tolerances. If the guides are too tight, movement and sealing can possibly be restricted. If they are too loose, the poppet can hit low in the seats and give a false closure, or the poppet can vibrate during operation, causing severe damage to the guide ribs. Guide ribs are most commonly found in Y-type globe valves, where alignment is hampered by the heavy disc sliding at an angle. Guide ribs are also found in critical service, high flow applications, where high side loads can effect the seating in horizontal or teetype globe valves. 6.3.6.1 Damage Assessment and Repair Options DA: Inspection of the valve reveals that the guide rib is preventing the poppet from fully closing. RO: The guide rib should be blued, and the poppet assembly installed and stroked to determine if and where the contact is being made between the poppet wear rings and the guide ribs. After determining the clearance problem, a decision should be made about machining the poppet wear rings per Section 8.12 or the guide ribs per Section 8.9. DA: The poppet is fluttering while opening/closing and during operation, causing severe vibration and damage to the sealing faces. RO: Tighten the clearances by building up the hardfacing on the poppet wear rings and machining per Section 8.12. RO: Tighten the clearances by building up the hardfacing on the guide rib and machining per Section 8.9. NOTE: The first option is to repair the poppet. DA: The fillet weld that connects the guide rib to the body is cracked. RO: The crack should be ground out and repair weld performed, as presented in Section 8.9 for guide repair. DA: A section of the guide rib hardfacing is missing.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
RO: Perform a guide rib hardfacing repair per Section 8.9 by removing all of the remaining hardfacing material and rewelding new material. DA: Cracks are found in the guide rib hardfacing. RO: If the cracks are shallow and isolated, they can be ground out and blended to a 3:1 minimum taper. RO: If the cracks are extensive or in a critical area that can impact the position of the poppet as it approaches the seat, the crack should be removed and hardfacing repair performed per Section 8.9.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
7 CHECK VALVES
7.1 Introduction and Application Check (or non-return) valves are designed to automatically prevent the reversal of flow in a piping system without mechanical assistance. Where the flow in a line is in one direction and isolation is required in the opposite direction, the ability of a check valve to close independently can be very useful. When time is critical or complex control systems are impractical, the automatic operation of a check valve offers a significant advantage. The primary functions of a check valve are: • Minimizing reverse flow • Keeping lines full of fluid • Preventing fluid loss when the system is not in operation • Preventing reverse rotation of pumps • Preventing fluid outflow from vessel • Preventing water column separation The operation of the check valve is dependent on the type of check valve, flow velocity, flow turbulence, fluid medium, operating conditions, and upstream components. Check valves are available in many types and sizes and are selected based on size, closure speed, flow conditions, pressure drop, durability and ease of maintenance, seat tightness, and process. EPRI Application Guidelines for Check Valves in Nuclear Power Plants, NP-5479, Revision 1, provides a comprehensive discussion of check valve application disturbances. Typical applications of check valves are at the discharge of multiple pumps that provide flow and pressure head to a common manifold. If one of the pumps ceases to produce flow and pressure head, the non-return valve at its discharge prevents a flow reversal through that pump caused by the pressure head produced by the other pumps. Other applications include feedwater lines to boilers and, in general, a means to minimize the loss of process media in the event of a pipeline rupture.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
One of the major drawbacks to check valves is their susceptibility to seat damage as a result of water hammer and high flow reversal. Because the disc impacts directly on the seating surface during closure, high force impacts (slamming) can quickly destroy the sealing surfaces. Other repair issues of concern are corrosion, hinge wear, and hinge pin wear. 7.2 Design
7.2.1 General Non-return valves are designed to function by swinging a disc or lifting a poppet. These valves are best installed in a horizontal line and are opened by the velocity head of the flowing fluid. In almost all cases, the impetus to close the disc is initiated by the weight of the valve disc or by springs with the primary seating force generated by the system differential pressure. In some instances, auxiliary external weights, springs, dashpots, or other actuation means are used to aid closing, decrease slamming action against the seats, or to prevent closing when servicing. Because the sealing force in a check valve is obtained solely as a result of pressure, problems can be encountered in obtaining a tight seal at low differentials. This is particularly true with metal-seated valves. The large unit loading necessary for a tight seal is not present at low pressures in a check valve. When environmental conditions permit the use of a resilient seat material, the weight of the disc can sometimes provide enough force to seal. Because resilient seating materials require very little unit stress to provide tight shutoff, they perform well in low pressure applications. Most check valves are available with either metal or resilient seats. There are four basic types of check valves available: swing, lift, tilting disc, and in-line checks. There are many varieties and options available for each of these four types. Selecting the best design for a particular application tends to be challenging to even the most experienced engineer. Basically, check valves are selected for their ability to close quickly before a significant reverse flow velocity develops. The consensus among check valve manufacturers, architect engineers, and users regarding the closure speed of check valves—in order of performance from slowest to fastest—lists lift, tilting disc, and swing. However, this is a rule of thumb and does not take system characteristics into account. As with other types of valves, the perfect valve for one application might not be the correct valve for a somewhat similar application. For specific applications, the system designer should consult with a valve engineer qualified in valve selection prior to specifying a valve.
7.2.2 Swing Check The swing check valve (see Figure 7-1) is the most widely used type of check valve in power plant applications due to its relatively simple design, low pressure drop, good seat tightness, ease of maintenance, and low cost. It offers the least flow resistance and is generally provided with a bonnet so that the internals are accessible when the valve is welded in-line. 7-2
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Arm
Hinge Pin
Back Stop Disc Assembly Normal Flow
Seat
A
A
62˚ REF
Checking Member
Figure 7-1 Swing Check Valve [9, 13]
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
The checking member in a swing check valve (see Figure 7-1) is a disc supported by a hinge arm, which in turn is free to swing about a hinge pin pivot point located outside the diameter of the disc. The clearances at the hinge pin and the hinge arm to disc connection are critical to providing proper articulation for the disc to align itself with respect to the seat face, thus providing a good seat. The seat face is usually inclined at a certain angle with respect to vertical to ensure disc contact against the seat face and provide sufficient contact force to initiate a seal. The angle through which the disc swings to go from the fully closed to fully open position ranges from about 45˚ to 80˚ for design variations supplied by different manufacturers. In certain designs, the disc moves completely out of the main flow area when the valve is in the fully open position. Such designs are called “clearway” swing check valves, which offer the lowest pressure drop during fluid flow conditions. However, clearway design swing checks are also most susceptible to accelerated wear and degradation of the internals because their discs are more unstable and experience higher fluctuation when flow disturbances are present upstream of these valves. In some designs, the complete disc, hinge pin, and hinge arm assembly are hung from the bonnet for ease of maintenance. Variations in the alignment of the bonnet with the valve body could, however, adversely effect the performance of the valve. In a majority of the designs similar to that depicted in Figure 7-1, the disc stud connecting the disc to the hinge arm serves as a full-open stop by contacting a stop pad either on the valve body or the bonnet. The common variations in disc stop arrangement are (a) a hinge arm extension beyond the disc stud, which hits a stop pad in the valve body, (b) an extension on the hinge arm near the hinge pin location, which hits a stop pad in the valve body, and (c) an area near the outer diameter of the disc, which directly hits a stop pad in the valve body. Advantages and disadvantages of the basic swing check valve configuration are: Advantages: • Swing check valves are the simplest and most economical to construct of all check valves. • They are available in line sizes exceeding 48 inches (122 mm). • Swing check valves have the least resistance to flow and cause the lowest pressure drop of all check valves. • Seating is easily done due to the self-aligning design of the disc-to-hinge arm connection. • They are easy to maintain and repair seat face in situ due to relatively low accuracy requirements in seat plane orientation.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Disadvantages: • Swing check valves have the slowest dynamic response due to the long pendulum length (the distance between the hinge pin and the disc assembly center of gravity) and relatively long stroke. This creates higher pressure surges (resulting from a flow reversal) than other types of check valves. • They require more frequent maintenance if used for continuous operation at velocities less than the velocity required for full opening. Oscillation of the disc in the flow stream, in some cases accompanied by tapping against the backstop, can result in accelerated wear or fatigue of the hinge pin and the disc stud area. This can lead to premature failure of the valve if the maintenance frequency is not adequate. • They have higher wear than other types due to disc oscillation under variable flow conditions.
7.2.3 Lift Check Lift check valves are available in T-pattern (see Figure 7-2), Y-pattern (see Figure 7-3), or angle (see Figure 7-4) configurations. The closure element has a relatively short distance to travel to its seat, so lift check valves are less likely to slam or cause water hammer compared with a swing check.
Figure 7-2 Lift Check Valve, T-Pattern [13]
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Figure 7-3 Lift Check Valve, Y-Pattern [13]
Figure 7-4 Lift Check Valve, Angle Configuration [13] 7-6
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Two of the most common types of lift check valves are disc-type (see Figure 7-5) and piston-type (see Figure 7-6). Ball-type lift check valves are sometimes used in small sizes. In disc-type lift check valves, the disc is guided by a smaller diameter pin, whereas guiding in piston-type lift check valves is done at the outside diameter of the piston, which provides more guiding surface area, resulting in less wear when operating in a less than full open position.
Seat
Disc Guide Pin
Figure 7-5 Disc-Type Lift Check Valve [8]
Spring Body
Piston
Figure 7-6 Piston-Type Lift Check Valve [8] 7-7
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
The lift check valve can also be equipped with auxiliary devices, such as springs to accelerate closure and dashpots to slow the valve down in the final stages of travel, just before seat contact. Advantages and disadvantages of lift check valves compared to other types of check valves are: Advantages: • Lift check valves have fast response due to the short stroke from the fully open to closed position. • They are suitable for operation at less than full open velocities. • Small size spring-assisted designs can be mounted either horizontally or vertically. • Ball-type valves are good for viscous fluids. • Piston-type valves are good for pulsating flow, because dashpots can be incorporated. • Piston-type valves are also available in Y-pattern body, which has a lower pressure drop. Disadvantages: • Lift check valves have a higher pressure drop under fluid flow conditions than other types of check valves. • Piston-type valves should not be used where solid contaminants are present, which could cause sticking.
7.2.4 Tilting Disc The tilting disc check valve (see Figure 7-7) is similar in operation to the swing check valve, except the disc pivots about an axis located between the disc centerline and the outside diameter of the disc. The pivot axis (the distance between the pivot point and center of gravity of the disc) is axially offset from the face of the seat, which minimizes the pendulum length and provides faster closing action than the swing check. Due to its faster response and shorter travel, the tilting disc valve is less susceptible to slamming than the swing check valve because it closes before significant reverse flow velocity is allowed to develop. Some larger size tilting disc check valve designs incorporate torsional springs inside the hinge pin to assist the disc closing action.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Disc
Disc Stop
Seat
Body
Figure 7-7 Tilting Disc Check Valve [8] The tilting disc designs come in several disc shapes. Smoothly contoured disc profiles have been developed by some manufacturers to minimize turbulence in the valve body and to improve disc stability. Because the entire disc remains in the flow stream, even in the fully open position, the stability of the disc and the resulting hinge pin wear is dependent upon the actual shape of the disc and flow path through the valve body. Advantages and disadvantages of tilting disc check valves, compared to other check valves, are: Advantages: • Tilting disc check valves eliminate fatigue problems associated with the disc-tohinge arm connections in swing check valves. • They have faster response than swing check valves. • They are less susceptible to degradation caused by variable flow than swing check valves. • They are less susceptible to slamming than swing check valves. Disadvantages: • Tilting disc check valves have a somewhat higher pressure drop than swing check valves. • They are more expensive than swing check valves. • They are more difficult to repair because of the need for tighter dimensional control for proper operation.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
• Seat tightness is hardest to achieve due to the precise alignment required between the disc and seat because of the lack of articulation. • High hinge pin wear might prevent full disc closure into its seat. 7.3 Repair Issues Repair issues addressed in this section pertain to fixed components, such as valve bodies, bonnets, valve seats, and pressure seal areas.
7.3.1 Leakage Past the In-Body Seats Leakage past the seats can be the result of corrosion products on the disc/poppet and seat; incomplete contact between the seat and disc; scratches, pits, cuts, and erosion on the disc and/or seat; or an obstruction lodged across or between the seats that prevents the gate from closing. This section addresses the typical problems that require more attention than the typical maintenance activities, such as cleaning, inspection, and lapping or polishing. The three types of seat rings discussed herein include integraltype seats, threaded-in seat rings, and welded-in seat rings. 7.3.1.1 Damage Assessment and Repair Options DA: Valve disc and seat has foreign material and corrosion products that are preventing full closure. RO: Polish the seat and disc with emery cloth in accordance with standard maintenance procedures. If the corrosion products are still attached or if surface scratches are present, lap the seat and disc per Section 9.3. Seat contact can then be verified by performing a bluing evaluation to ensure 360˚ contact on the disc face as discussed in Section 8.3. RO: For disc-type lift check valves with a center guide pin, check the pin and guide bore for corrosion, debris, or galling. Polish or machine the guide pin, and hone the guide bore per Section 9.4. RO: For piston-type check valves, check the guide rings and guide ribs for interference. Repair the poppet per Section 8.12 and the guide ribs per Section 8.9. DA: A bluing test reveals less than 360˚ surface contact between the disc and seats. RO: Verify that sufficient stem stroke and pressure are available to fully close the valve. RO: Verify that the disc is not seating too low in the seat ring by measuring the ID of the seat ring and the OD of the poppet.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
RO: Verify that the guide ribs are in alignment with the seat and that clearances between the poppet wear rings and the guide ribs are in tolerance. Excessive clearance can result in improper seating. Repair in accordance with Section 8.9. RO: Lap the seats and disc per Section 9.3. Perform blue check. If 360˚ contact is not achieved, a well qualified machinist/valve technician is needed to properly shim and/ or fit the disc per Section 8.11 for swing check valves and Section 8.12 for tilting disc and lift check valves. RO: Measure the poppet and seat angles of the lift and tilting disc-type check valves. The poppet angle should be a minimum of 1˚ less than the seat angle. If the dimensions are not in tolerance, a well qualified machinist/valve technician is needed to properly fit the disc per Section 8.12. RO: Tilting disc—Examine the hinge, hinge pin, and hinge pin bushings for wear. If excessive wear is found, repair as directed by the manufacturer. Machine the new bushing with a portable boring bar per Section 8.10. RO: Swing check—Check the alignment of the disc with the seat. Make adjustments as necessary per the manufacturer’s directions. DA: A bluing test reveals 360˚ contact between the seats and poppet/disc, but the valve is still leaking. RO: Verify that sufficient stem stroke and pressure are available to fully close the valve. RO: If the seat ring is a threaded-in type with a gasket under it, tighten the seat ring or remove the seat ring and replace the gasket. RO: If the seat ring is welded-in, check to see if the valve is leaking past the seal weld of the seat ring. If so, repair in accordance with Section 8.3. RO: Verify that the width of the seat contact surface is appropriate for the size and pressure rating of the valve. If the contact surface is too wide, the seating surface should be narrowed by lapping the relief angle of the seat per Section 8.3. The suggested seat width should be obtained from the valve manufacturer or design drawings. DA: A bluing test reveals that the disc is seating too low in the seats (see Figure 7-8).
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Figure 7-8 Disc Seating Too Low in Seats [21] RO: Procure a new poppet and fit it as discussed in Section 8.12. RO: Re-hardface the disc/poppet and fit it per Section 8.12. DA: The seat ring has cuts and/or erosion damage beyond what can be lapped out. RO: If the damage is isolated to some small local areas and does not extend well into the seat ring base material, a repair to the seat ring can be performed per Section 8.3 if the existing hardfacing is of a repairable type.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
RO: If the damage to the seat ring is severe, the seat ring should be replaced in accordance with Section 8.4. In situ seat ring replacement is generally restricted to valves 6 inches (152 mm) and larger due to the limited development of welding and machining systems. RO: Except for some very specific circumstances, valves less than 6 inches (152 mm) should be removed from the line and repaired in a shop. Generally, valves less than 6 inches (152 mm) can be replaced at a cost that is comparable to the repair cost. DA: The integral hardfacing seat has cuts and/or erosion beyond what can be lapped out. RO: If the damage is isolated to some small local areas and does not extend well into the valve body base material, a repair to the hardfaced seat can be performed per Section 8.5 if the existing hardfacing is of a repairable type. This is a challenging repair that requires very skilled welding personnel who are experienced in welding hardfacing materials. RO: If the damage to the hardfacing is extensive or protrudes into the base material, the hardfaced seat should be replaced in accordance with Section 8.6. In situ replacement of integral seats is generally restricted to valves 6 inches (152 mm) and larger, due to the limited development of welding and machining systems. RO: Except for some very specific circumstances, valves less than 6 inches (152 mm) should be removed from the line and repaired in a shop. Valves less than 6 inches (152 mm) can generally be replaced at a cost that is comparable to the repair cost. DA: The poppet is found to have cuts and/or erosion beyond what can be lapped out. RO: Procure a replacement poppet. The new components will have to be fitted to the valve as detailed in Section 8.12. RO: If the poppet has an isolated crack or cut, and the valve technician determines that the poppet has enough hardfacing material on it so it will not drop through the seat after repair, a localized repair can be attempted as described in Section 8.12. RO: Remove the existing hardfacing and repair per Section 8.12. This approach is generally more effective than the localized repair option.
7.3.2 Leakage Past the Pressure Seal Ring Damage and the associated repair of pressure seal bodies are the same for gate- and check-type valves. See Section 5.3.2.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
7.3.3 Valve Stem Leaks Valves stems are found on stop check valves. Damage and the associated repair of valve stems are the same for gate and globe (stop check) valves. See Section 5.3.3.
7.3.4 Valve Body Damage The need for valve body repairs is generally the result of trim components improperly installed or foreign materials not allowing the internal sealing components to fully close. The result is erosion or wire drawing in leakage areas to high velocity flow in a concentrated area. Except for cases of casting defects that lead to through-wall weeping, most body repairs are performed in conjunction with trim repairs, such as seats or pressure seal bores. The valve body is a pressure boundary and thus must be repaired in accordance with the rules of the jurisdictional code. Generally, this is either the ASME or NBIC. 7.3.4.1 Damage Assessment and Repair Options See Section 5.3.4.1.
7.3.5 Valve Bonnet Leaks Damage and the associated repair of gate and check valve bonnets are the same. See Section 5.3.5.
7.3.6 Poppet (Disc) Guide Ribs Guide ribs accurately control the alignment of the poppet assembly into the seat to minimize potential hardfacing damage or bending of a stem. Repair to the guide ribs and poppet guide rings requires precision machining to maintain the critical tolerances. If the guides are too tight, they can restrict movement and possibly sealing. If they are too loose, the poppet can hit low in the seats and give a false closure, or the poppet can vibrate during operation, causing severe damage to the guide ribs. Guide ribs are most commonly found in Y-type lift check and stop check valves where alignment is hampered by the heavy disc sliding at an angle. Guide ribs are also found in critical service, high flow applications where high side loads can effect the seating in horizontal lift and angle lift check valves. 7.3.6.1 Damage Assessment and Repair Options DA: Inspection of the valve reveals that the guide rib is preventing the poppet from fully closing.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
RO: The guide rib should be blued, and the poppet assembly installed and stroked to determine if and where contact is being made between the poppet wear rings and the guide ribs. After determining the clearance problem, a decision should be made as to machining the poppet wear rings per Section 8.12 or the guide ribs per Section 8.9. DA: The poppet is fluttering while opening/closing and during operation, causing severe vibration and damage to the sealing faces. RO: Tighten the clearances by building up the hardfacing on the poppet wear rings and machining per Section 8.12. RO: Tighten the clearances by building up the hardfacing on the guide rib and machining per Section 8.9. NOTE: The first option is to repair the poppet. DA: The fillet weld that connects the guide rib to the body is cracked. RO: The crack should be ground out and a repair weld performed as presented in Section 8.9 for guide repair. DA: A section of the guide rib hardfacing is missing. RO: Perform a guide rib hardfacing repair per Section 8.9 by removing all of the remaining hardfacing material and rewelding new material. DA: Cracks are found in the guide rib hardfacing. RO: If the cracks are shallow and isolated, they can be ground out and blended to a 3:1 minimum taper. RO: If the cracks are extensive or in a critical area that can impact the position of the poppet as it approaches the seat, the crack should be removed and hardfacing repair performed per Section 8.9.
7.3.7 Leakage Past the Hinge Pin Cover Swing check and tilting disc check valves are designed with standard compression flange and gasket joints (see Figure 7-9) and pressure seal joints (see Figure 7-10). Leaks are typically the result of insufficient gasket or seal ring compression as a result of reduced bolt tension lost due to vibration, temperature, and pressure fluctuations. Other contributors to leaking joints include improper bolting, corrosion and foreign materials between the valve body and gaskets, scratches, cuts, erosion, and steam cuts.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Spiral Wound Gasket
Hinge Pin
Roll Pin
Cover
Cover Nuts
Figure 7-9 Swing Check and Tilting Disc Check Valve, Standard Compression Flange and Gasket Joints [21]
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Hinge Pin Bonnet Studs
Hinge Pin
Metal Gasket
Hinge Pin Cover
Nuts
Figure 7-10 Swing Check and Tilting Disc Check Valve, Pressure Seal Joints [21] The type and extent of damage can be determined after the hinge pin cover is removed and the seal areas inspected utilizing standard maintenance practices. Critical dimensions should be compared to manufacturer drawings and specifications prior to initiating any repair activity.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
7.3.7.1 Damage Assessment and Repair Options DA: Minor scratches and cutting are found in the body sealing surfaces of the body and/or bonnet seal area. RO: Determine the depth of the imperfections. Flaws less than .010 inch (.25 mm) can be removed by polishing with a fine emery cloth or buffing wheel. RO: If flaws are deeper than .010 inch (.25 mm) on the body seal, the complete inside diameter of the contact area should be honed per Section 9.4 until the flaw is removed. The bore should then be measured, and an oversized seal ring selected as discussed in Section 8.10. RO: If flaws are deeper than .010 inch (.25 mm) on the bonnet angular surface, the sealing surface can be repaired by taking a skim cut with a lathe as described in Section 8.10. The critical angle must be maintained. DA: Deep cuts, porosity, and erosion are found on the body sealing area. RO: If the flaw is isolated, a localized repair can be performed by grinding out the defect area, rewelding, and finish machining and honing as detailed in Section 8.10. An oversized seal ring might be required, based on the finished inside diameter measurements. RO: If the flaws are numerous about the diameter of the bore, the complete inside diameter of the seal area should be machined to remove all of the cladding, and the seal area rewelded, machined, and honed to blend with the surrounding bore diameter as detailed in Section 8.10. DA: Deep cuts, porosity, and erosion are found on the hinge pin bonnet sealing surface. RO: Procure a replacement bonnet. RO: Verify the dimensional tolerances from the manufacturer and remove the defects by machining. DA: Deep cuts, porosity, and erosion are found on the valve body flange sealing surface. RO: Machine the surface to the suggested finish, removing the defects as presented in Section 8.10. RO: Weld buildup the damaged area and re-machine the pocket and sealing surface as presented in Section 8.10.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
8 VALVE COMPONENT REPAIRS
8.1 Valve Body Repair This section presents the practices necessary to repair flaws in the pressure-retaining bodies, bonnets, and flanges of in-line welded valves (see Figure 8-1a). Body Flange
Pressure Seal Area Bonnet Bore
Guide Ribs
Seat Hardfacing
Seat Ring
Flow Bore
Figure 8-1a Typical Body Repair Locations [13]
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Cracks in the cast or forged bodies of valves are generally the result of manufacturing defects. The most common type of crack in a valve body is the result of a shrink cavity or other void in a casting. These types of cracks can also be seen in bonnets and flanges. Cracks are also found in the heat-affected zones (HAZ) of fabrication welds that join bonnet flanges to the body and weld joints that join the valve to the process line. Repairs to valve bodies can be performed with the same practices and procedures utilized to repair piping and pressure vessels of similar materials. The restricted access due to size and configuration makes repairs to valve internals quite challenging. The use of cast materials can also present welding challenges. Crack repairs are successfully completed when skilled personnel are provided the proper training and tools and the repair area is accessible. The biggest challenge is making repairs on P-Nos. 4 and 5 valve bodies (see Table 11-1 for P-Number material classification), which require elevated preheats and post-weld heat treatment (PWHT). Current EPRI research is aimed at reducing these high PWHT temperatures through the use of temperbead welding techniques. The valve body, bonnet, and flanges are recognized as pressure-retaining materials. As a result, these components must be repaired in accordance with the rules of ASME and ANSI, and by ASME or NBBI (National Board of Boiler & Pressure Vessel Inspectors) manufacturer or repair certificate holders. All welding must be performed by qualified welders and procedures in accordance with the rules of ASME Section IX.
8.1.1 Repair Strategy A thorough evaluation of the crack or defect should be made prior to performing an in situ repair of a valve body. Access to the defect location, the size of the defect, the materials involved, the final machining tolerances, and the repair window (schedule) should be reviewed. As a result of this review, the owner must decide whether to perform the repair with a vendor or with in-house personnel, to replace the valve, or to remove it from the line and send it to a shop for repair. The following questions should be asked in the process of developing a repair strategy: • Can the indication be removed by grinding without encroaching upon the design minimal wall, thus avoiding a weld repair? • Does the indication or crack go through the wall requiring a system hydrostatic test after completion? • Will the repair encroach upon an area that requires precise tolerances and finishes such as hardfaced seats or pressure seal inlay areas, which would be altered by welding stresses and heat from the repair? • Is the repair area accessible through the bonnet bore for manual repair techniques? • Is machining and welding equipment available to perform the repair? • Is PWHT equipment available, along with knowledgeable personnel?
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
• If the indication was found by UT or RT, are the equipment and personnel available to perform this inspection after the repair is completed? • Are sufficient qualified and experienced personnel available to perform this replacement? • Are sufficient rigging points and space available to move equipment in and out of the valve? • What are the preheat and PWHT requirements for performing this replacement? If required, can the welding and machining equipment withstand these elevated temperatures? • Is a replacement valve available in the time frame necessary, without impacting plant availability?
8.1.2 Repair Prerequisites The following should be considered prior to initiating a repair: • Record the manufacturer’s name, size, pressure rating, valve type, body style, body material, and trim material data from the valve nameplate. • Obtain drawings and material specifications from the manufacturer. • Obtain and review the maintenance and repair history to see if there have been any changes to the original design. • Procure a replacement pressure seal ring and an oversize ring if the valve has a pressure seal bonnet. • Prepare a repair “traveler” or work instruction that outlines the repair sequence and inspection “hold points.” All measurements and findings should be recorded for a final report. • Install clean plugs in the upstream and downstream bores of the valve to prevent chips and foreign materials from getting down the line. • Install a protective liner over the pressure seal bore area and in-body seats. • Qualify the necessary welding procedures for hardfacing and base material buildup and repairs. • If the repair requires unique welding or machining applications or if it might have adverse effects on seat dimensions, the full repair sequence should be demonstrated on a mockup. This includes both machining and welding operations. • Measure and record critical dimensions such as guide bores, distances between seats, pressure seal bores, etc., to monitor any distortion due to welding and PWHT.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
8.1.3 Flaw Removal The method of flaw removal is based on the size, depth, and location of the flaw. If a small localized area is to be repaired inside the valve due to a casting flaw, erosion, or a crack, the area can be manually ground out as discussed in Section 9.1. Examples of repairs in which manual grinding might be employed to remove localized flaws include casting pores, a pin hole or minor erosion on the down stream side of a seat ring, steam cuts at turns in globe and control valves, or steam cuts across flange faces. A portable boring bar or milling machine would be utilized to remove a circumferential crack in a seat ring-to-body weld or a flange-to-body weld. If a seat ring is removed and there is major damage to the seat ring pocket, a boring bar or gate valve seat repair machine similar to those shown in Section 9.2 would be utilized. Although quite rare, a flange facer can be utilized if a flange has numerous cuts that would require weld buildup of the flange to maintain acceptable thickness. After removing the flaw, the repair area should be tapered away from the flaw to allow for easy access and welder visibility. Material removal should always be kept to a minimum. A liquid penetrant or magnetic particle examination must be performed after machining to ensure that no unacceptable indications remain. The acceptance criteria for rounded indications can found in ASME Section XI, Article 4000.
8.1.4 Filler Material Selection Filler material selection is dictated by base material compatibility, process selection, and position as presented in Section 11. The filler material should match the base material chemistry with properties comparable to the base material. See Table 11-4 for guidance in selecting the proper filler materials.
8.1.5 Preheat and Post-Weld Heat Treatment (PWHT) Requirements Preheating and PWHT should be performed as presented in Section 13. Preheating provides very positive benefits to the welding application and final product. Preheating is employed to reduce the tendency for cold cracking (or hydrogen-assisted cracking), to reduce the hardening of the heat-affected zone (HAZ), to reduce the residual stresses, and to decrease component distortion. Although preheating is not a regulatory requirement, the benefits far outweigh the potential problems of not using it as shown in Section 13.1. When welding on low carbon steel P-No. 1 materials greater than 1-1/2 inches (38-mm) thick, a PWHT exemption can be gained by utilizing a 200˚F (93°C) preheat. When welding on low alloy steels such as the P-No. 4 (WC6) and P-No. 5 (WC9), 300˚F (482˚C) and 400˚F (662˚C) respective preheats are suggested. A minimum preheat of 100˚F (38˚C) is suggested for welding on P-No. 8 (CF3 & CF8) substrates.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
When welding in confined locations, the recommended preheats might not be conducive to productivity. In these cases, the preheat should be lowered to a workable level. Unless previously qualified at the lower temperatures, a new welding procedure will be required for the lower preheat. Castings that have been in service might have water in the defect and surrounding area. This water/moisture needs to be driven off with local heating to eliminate the potential for porosity. This localized preheating can be performed with an oxyacetylene torch. Post-weld heat treating is required for all weld repairs performed on P-Nos. 4 and 5 substrates (see Table 11-1). As mentioned earlier, exemptions to PWHT are available for PNo. 1 materials based on the thickness and preheat temperature as provided in Table 13-4.
8.1.6 Welding Repair Valve body welding repair can be performed with the gas tungsten arc welding (GTAW), shielded metal arc welding (SMAW), gas metal arc welding (GMAW), or fluxcored arc welding (FCAW) processes. The process should be selected based on accessibility, quality of weldment required, and productivity. The various processes are described in Section 10. The GTAW process is generally used where critical control is needed around a valve seat or where radiation levels require remote welding. Very localized repairs or configurations that are not readily achievable by automated systems are normally repaired with the SMAW process. Large repairs can be performed with manual or automated GMAW processes. Repairs to flanges can be performed with any of the processes, and should be selected based on the amount of material to be deposited. Figure 10-1 presents a productivity comparison of the various welding processes. Valve body repairs should be completed with the following steps: • After excavating the repair cavity, the repair area should be thoroughly cleaned and inspected using either the MT or PT examination method. The flaw must be totally removed from the repair cavity. • Preheat the valve body or localized area as described in Section 13. • Perform the repair weld with the selected process, monitoring frequently for distortion. • If required, perform a post-weld heat treatment using the guidelines established in Section 13. • Cool to the ambient temperature. Because of the variety of valve locations and types of damage possible to a valve body, it would be difficult to provide definitive repair techniques and sequences. Other techniques utilizing sound welding practices and minimal heat input have been used with much success. The following temperbead repair concept provides some general guidance pertinent to a valve body repair: • Establish welding parameters prior to initiating a weld in the component. • Lay out the weld sequence such that the first layer of weld metal covers the total repair cavity and onto the finished surface as shown in Figure 8-1b. 8-5
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Figure 8-1b Recommended First Layer Weld Repair Bead Placement for Valve Body Base Material Repairs [22] • If SMAW welding is employed, a 3/32-inch (24-mm) electrode should be used on the first two layers. If additional layers are required, larger electrodes can be used. • Stringer beads should be employed to reduce heat input and lower residual stresses. • Welding of second and subsequent layers should be deposited in a manner that aims to control residual stresses. Weld beads should tie into previously deposited weld metal and not onto base material. See Figure 8-1c.
Figure 8-1c Recommended Second Layer Weld Repair Bead Placement for Valve Body Base Material Repairs [22] • Cap out the weld approximately 1/16 inch to 1/8 inch (1.6 mm to 3.2 mm) above the parent metal surface as checked with a straight edge. Do not add excessive layers as this contributes to higher residual stresses. See Figure 8-1d.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Figure 8-1d Recommended Cavity Fill Weld Repair Bead Placement for Valve Body Base Material Repairs [22]
8.1.7 Final Machining The purpose of final machining is to restore the valve body to its original dimensions so that the valve can function as designed. Manufacturer’s design drawings should be available to refer to body bore clearances or flange face dimensions and finishes. The final machining step should include a thorough comparison of critical dimensions with those taken before the welding and PWHT operations. It is easy to remove material but very difficult to put it back. All machinists must know where they are and what their machining plan is at all times. In most cases involving valve body repair, the final machining step is a grinding operation. If machining with a boring bar, flange facer, or valve seat repair machine is required to restore critical dimensions, minor grinding might be employed to reduce the major buildup. When the bonnet bore has been repaired near the pressure seal area or the poppet is guided by the bonnet bore, the bore should be checked by taking ID measurements at various locations in the bore. If there is any question as to concentricity, the bonnet bore should be honed as presented in Section 9.4. As a final check, the poppet should be installed and moved up and down to make sure that it is free. The typical clearance is .020 inches to .030 inches (0.5 mm to 0.8 mm) on the diameter. It is recommended that the seat and disc be checked by bluing for distortion. If necessary, they should be lapped as presented in Section 9.3. If weld repairs have been made to the valve bonnet, a blue check should be performed between the stem and backseat as discussed in Section 8.7.7.
8.1.8 Inspection The ASME Section XI Boiler and Pressure Vessel Code and the ANSI/ASME B31.1 Power Piping Code provide guidance for the repair and inspection of valve materials defined as pressure-retaining. ASME Section XI, Subarticle IWA-4400 requires that the
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
repair welding be performed by qualified welders and procedures in accordance with Section IX. It also requires that the repair cavity (pressure-retaining base material) shall be examined by the liquid penetrant (PT) or magnetic particle (MT) method. A final examination of the finished repair must be performed and documented utilizing the inspection process that was used to find the indication originally.
8.1.9 Testing Pre-service testing shall be performed in accordance with the requirements of ASME/ ANSI OM, Part 10, when the owner determines that the repair activity effects the valve’s performance or designed function. This could include a local leak rate test (LLRT), an integrated leak rate test (ILRT), or a operability/functional test. If the pressure boundary has been penetrated, a hydrostatic test is required. 8.2 Pressure Seal Area Repair This section presents the practices necessary to repair the body bore area, which serves as a corrosion-resistant sealing surface for the valve bonnet seal ring. This section also addresses the repair of the valve bonnet seal area (see Figure 8-2a).
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Figure 8-2a Pressure Seal Repair Location [18]
8.2.1 Repair Strategy Leaking of a pressure seal joint can be the result of insufficient load on the pressure seal gasket; corrosion, chips, or other foreign matter on the sealing surfaces; damage to the pressure seal gasket itself; or surface imperfections in the body wall in the form of pin holes, cracks, erosion, or indentations from assembly. Only the surface imperfections require repair techniques. The other issues can be resolved with standard maintenance practices. An evaluation of the body bore and bonnet sealing surface is required to understand why the bonnet is leaking. If the defects in the body bore are less than .010-inch (.25-mm) deep, the bore can be honed using a portable hone (such as equipment
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
available from Sunnen) as shown in Section 9.4. If the defects are greater than .010 inch (.25 mm), a weld repair should be performed. If the leak is the result of a flaw on the bonnet sealing surface, the flaws can be removed by machining the angled sealing surface in a lathe and polishing it with an emery cloth. Larger defects up to .020 inch (.20 mm) might be scalable using a graphite pressure seal ring. The following questions should be asked in developing a repair strategy for the repair of the pressure body bore: • Does the bore have a welded inlay of corrosion-resistant weld metal? Many manufacturers do not deposit an inlay on forged stainless steel valves but do on cast stainless steel. • Is the flaw or crack localized such that it could be removed by grinding? If so, machining might not be required. • Does the flaw or crack encroach upon the retaining ring groove such that machining of the groove will be required after repair? If so, a boring bar will be required. • Is the flaw or crack deep enough that weld repair would cause distortion to the valve body bore? If so, a boring bar will be required. • Will removal of the crack or flaw violate minimum design wall thickness? If so, can a qualified base metal repair be performed? The base metal repair should be performed as presented in Section 8.1. • Is machining and welding equipment available to perform the repair? • Are sufficient qualified and experienced personnel available to perform this repair? • Does the owner or repair vendor possess the proper NBIC repair certifications and ASME Section IX qualified welding procedures? • Are sufficient rigging points and space available to move equipment in and out of the valve? • Is equipment available to preheat the repair area? If required, can the welding and machining equipment withstand elevated temperatures?
8.2.2 Repair Prerequisites The following should be considered prior to initiating a repair: • Record the manufacturer’s name, size, pressure rating, valve type, body style, body material, and trim material data from the valve nameplate. • Obtain drawings and material specifications from the manufacturer. • Obtain the original design minimum wall thickness from the manufacturer. • Obtain and review the maintenance and repair history to see if there have been any changes to the original design. • Procure a replacement pressure seal ring and an oversize ring.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
• Prepare a repair “traveler” or work instruction that outlines the repair sequence and inspection “hold points.” All measurements and findings should be recorded for a final report. • Install clean plugs in the upstream and downstream bores of the valve to prevent chips and foreign materials from getting down the line. • Install a protective liner over the in-body seats. • Qualify the necessary welding procedures for hardfacing and base material buildup and repairs. • Demonstrate the machining and welding sequence on a full-scale mockup to familiarize personnel with the work sequence and requirements. • Measure and record critical dimensions, such as guide bores, distances between seats, retainer ring groove diameter and depth, diameter at the top of the body bore, and the pressure seal bore, to monitor distortion due to welding.
8.2.3 Flaw Removal The flaw should be removed by grinding or machining. If grinding is being utilized to remove a flaw, liquid penetrant should be applied to the area to help identify the crack location. After completely removing the flaw, the area should be smoothed and tapered to provide good welding visibility and access. If the flaw distribution is extensive or if orbital welding equipment is going to be utilized for welding, a boring bar should be utilized to remove material about the complete bore area as shown in Figure 8-2b. A 2:1 or 3:1 transition should be employed at the upper and lower end of the weld preparation groove.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Chuck Module
End View Tool Head
Stand Off
Figure 8-2b Example of Pressure Seal Area Boring [23] After completing the grinding or machining operation, an MT or PT examination of the weld preparation area must be performed to ensure that no unacceptable indications remain prior to welding.
8.2.4 Filler Material Selection An austenitic stainless steel wire or electrode is the filler material for pressure seal inlays that is specified by the valve manufacturers. Alloys typically referred to by valve manufacturer include 18-8, Types 302, 304, 308, 309, and 316. The Type 309L is most commonly used by the valve manufacturers.
8.2.5 Preheat and Post-Weld Heat Treatment Requirements Preheating and PWHT should be performed as presented in Section 13. Preheating provides very positive benefits to the welding application and final product. Preheating is employed to reduce the tendency for cold cracking (or hydrogen-assisted cracking), to reduce the hardening of the heat-affected zone (HAZ), to reduce the residual stresses, and to decrease component distortion.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Because of the differences in coefficient of thermal expansion between the stainless steel filler materials and the carbon or low-alloy steel substrates, high residual stresses, which promote distortion and cracking, are very common. As a result, a 400˚F (204˚C) preheat is strongly recommended when performing this repair to P-Nos. 1, 4, and 5 valve bodies (see Table 11-1). If necessary, the preheat can be lowered to the temperatures suggested in Table 13-1. If a repair is made to a cast stainless steel valve body (CF3 or CF8), a minimum preheat of 100˚F (38˚C) is suggested. Castings that have been inservice might have water in the defect and surrounding area. This water/moisture needs to be driven off with local heating to eliminate the potential for porosity. This localized preheating can be performed with an oxyacetylene torch. Except as required by Code, stress relieving should be avoided. Post-weld heat treating is required for all weld repairs performed on P-Nos. 4 and 5 substrates. Exemptions to PWHT are available for P-No. 1 materials, based on the thickness and preheat temperatures, as provided in Table 13-4.
8.2.6 Welding Repair Repair welding of pressure seal bores falls into two categories; local cladding and base metal repair, and bore buildup. Local repairs are generally performed with the GTAW and SMAW processes where weld quality and control is most important. The GTAW, SMAW, GMAW, and FCAW processes have been used for full diameter bore buildups. The automatic GTAW and SMAW processes are generally used for large repairs, or where radiation levels require remote welding. Several automated systems, as shown in Appendix B, are commercially available to perform overlay welding. Localized base metal repairs and deposition of corrosion-resistant cladding require special attention in order to ensure a successful repair. The following steps should be considered when making these high stress weldments: 1. Establish welding parameters prior to initiating a weld in the component. 2. Establish the weld sequence such that the first layer of weld metal covers the total repair cavity. 3. If SMAW is employed, use a 3/32-inch (2.4-mm) electrode on the first two layers. If additional layers are required, larger electrodes can be used. 4. Use thin stringer beads to reduce heat input and lower residual stresses. 5. Deposit welding on second and subsequent layers in a manner that controls residual stresses. Weld beads should tie into previously deposited weld metal and not onto base material. 6. Cap out the weld approximately 1/16 inch to 1/8 inch (1.6 mm to 3.2 mm) above the parent metal surface as checked with a straight edge. Do not add excessive layers because this contributes to higher residual stresses.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
8.2.7 Final Machining The extent of the weld repair dictates the type of machining to be performed. In the case of small localized repair, the controlled grinding approach can be utilized. A hand grinder and a thin template cut from shim stock of .010-inch (.25-mm) thickness or less, having the same radius as the body bore diameter, can be used to reduce the weld buildup to the height of the shim stock above the bore. A flapper wheel can then be used to blend the weld buildup to just below the body bore. A straight edge and feeler gauges can ensure that the repair area is no more than .010 inch (.25 mm) below the surrounding material. A hone can then be used to finish the bore and blend the repair. Where major welding has been performed, or where the welding has impacted critical dimensions at the top of the bore or retainer ring, a boring bar similar to that shown in Figure 8-2b should be used. Regardless of the machining technique, a hone should be used to finish the sealing area (see Section 9.4). A finish of 32 rms or finer is recommended. Dimensional tolerances of .005 inch (.13 mm) on the diameter are required to obtain a good seal with an iron seal ring. The recent development and use of graphite seal rings have provided tolerances of up to .010 inch (.25 mm). These new rings also provide some tolerance for finish and small scratches up to about .010-inch (.25-mm) deep.
8.2.8 Inspection The ASME Section XI Boiler and Pressure Vessel Code and the ANSI/ASME B31.1 Power Piping Code provide guidance for the repair and inspection of valve materials defined as pressure-retaining. ASME Section XI, Subarticle IWA-4400 requires that the repair welding be performed by qualified welders and procedures in accordance with Section IX. It also requires that the repair cavity (pressure-retaining base material) shall be examined by the liquid penetrant or magnetic particle method. A final PT must also be performed after final machining.
8.2.9 Testing Pre-service testing shall be performed in accordance with the requirements of ASME/ ANSI OM, Part 10, when the owner determines that the repair activity affects the valves performance or designed function. This could include a local leak rate test (LLRT), an integrated leak rate test (ILRT), or a operability/functional test. If the pressure boundary has been penetrated, a hydrostatic test is required. 8.3 Seat Ring Repair This section focuses on the repair of welded seat rings and the hardfacing material applied to them. See Figure 8-3a. Replacement of seat rings is covered in Section 8.4.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Gate Valve Body
Seat Ring
Seal Weld
Figure 8-3a Seat Ring Configuration [11] Seat rings are found in most gate, swing check, and tilting disc check valves in which the seat is 90˚ to the bonnet bore. They are also found in globe, angle, lift checks, and Y-type valves. However, the globe-type configurations also utilize the integral seats in many applications. Most repairs performed on seat rings are the result of cracks or wear in the hardfacing materials. Left unrepaired, erosion will eventually penetrate completely through the seat ring and into the pressure-retaining base material. Repairs to seat rings can be completed successfully with the proper knowledge, tools, and trained personnel. Machining and welding technology for in situ repair of gate valve seats has advanced significantly since the early 1990s. Several field service companies can repair or replace seat rings of pressure Class 300 and above with a minimum nominal diameter of eight to ten inches (203 mm to 254 mm). Smaller gate valves should be removed from the line and repaired in the shop or replaced. Under extreme circumstances, seat ring repairs have been performed on valves as small as 2-inches (51-mm) nominal diameter. Repairing globe valve seat rings (parallel to the bonnet bore) is easier than repairing gate valve seat rings (perpendicular to the bonnet bore), due to the accessibility of the seat straight down the bonnet bore. Machine and lapping tools are available for all sizes and pressure ratings. Because of the straight-in approach to the seat, welding can be performed with either machine or manual welding processes. 8-15
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
8.3.1 Repair Assessment and Strategy A thorough evaluation should be performed prior to committing to a seat ring repair. Minimal tolerances are often employed, making material removal or distortion from welding very critical to obtaining a leak-free seat. In many cases, seat ring removal and replacement is a more cost-effective repair option. When hardfacing materials such as Stellite 1, 6, and 12, or the nickel-based alloys are encountered, strong consideration should be given to removing the existing hardfacing material and completely rewelding or replacing the seat ring. In evaluating the repair/replacement option, the following questions should be asked: • Can the defect be removed by grinding without impacting the sealing area, thus avoiding a weld repair? • Does the crack or defect penetrate into the seat ring base material such that a welded repair activity could significantly deform the ring? If so, the ring should be replaced. • Does the crack or defect penetrate into the valve body such that a welded repair of the pressure boundary material will be required? If so, the ring should be replaced in conjunction with the valve body repair. • Review the valve inspection report and determine if enough hardfacing material exists on the seat ring to perform a repair and still achieve acceptable seating contact. If not, can the disc or wedge be built up or adjusted to compensate for the material loss? If the hardfacing on the seat ring must be built up, the owner should also consider the option of seat ring replacement. • Is machining and welding equipment available to perform the repair? • Are sufficient rigging points and space available to move equipment in and out of the valve? • Is the existing hardfacing material repairable without the use of elevated preheat temperatures?
8.3.2 Repair Prerequisites 8.3.2.1 Generic Prerequisites The following should be considered prior to initiating a repair: • Record the manufacturer’s name, size, pressure rating, valve type, body style, body material, and trim material data from the valve nameplate. • Obtain drawings and material specifications from the manufacturer. • Obtain and review the maintenance and repair history to see if there have been any changes to the original design. • Procure a replacement pressure seal ring and an oversize ring if the valve has a pressure seal bonnet.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
• Prepare a repair “traveler” or work instruction that outlines the repair sequence and inspection “hold points.” All measurements and findings should be recorded for a final report. • Install clean plugs in the upstream and downstream bores of the valve to prevent chips and foreign materials from getting down the line. • Install a protective liner over the pressure seal bore area. 8.3.2.2 Gate Valve Specific Prerequisites • Establish match marks on both sides of the wedge with respect to the matching seat, prior to removing the wedge from the valve body. This ensures that the wedge is reinstalled in the same direction. • Perform a blue check (see Section 15.3) and record the seating location on the wedge as shown in Figure 8-3b.
Figure 8-3b Blue Check for Seating Location [16]
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
• Measure and record the wedge dimensions as shown in Figure 8-3c.
Dia. C
Dim. A
Dim. B
Figure 8-3c Typical Wedge Dimensions [16]
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
• Measure and record the minimum distances between the valve seats at the locations shown on Figure 8-3d.
Gate Valve Body Dim. A
Seat Ring
Dim. C
Ref. Point Seal Weld
Dim. B
Figure 8-3d Gate Valve Seat Measurements [11]
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
• Measure and record the seat width on both seats as shown in Figure 8-3e. Partial Penetration Groove Weld Seat Ring
Seat Contact Width
15˚
Figure 8-3e Reference Dimensions for Gate/Check Valve Seat Width [13] • Utilizing a low stress punch, establish two reference points on the seat ring as shown in Figure 8-3d and record the reference dimensions. These are used later to establish the final hardfacing thickness. 8.3.2.3 Swing Check Valve Specific Prerequisites • Remove the disc assembly and record any damage. Particular attention should be given to the seat, hinge pins, and hinge pin bushings. • Perform a blue check and record the seating location of the disc. • Measure and record the seat width as shown in Figure 8-3e.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Seat Contact Width
15˚
Reference Marks
62˚ Ref.
Figure 8-3f Reference Points on the Seat Ring of Swing Check Valve [13] • Utilizing a low stress punch, establish three reference points on the seat ring as shown in Figure 8-3f and record the reference dimensions. These are used later to establish the final hardfacing thickness.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
8.3.2.4 Tilting Disc Check Valve Specific Prerequisites • Remove the disc assembly and record any damage. Particular attention should be given to the seat, hinge pins, and hinge pin bushings. • Perform a blue check and record the seating location on the disc as shown in Figure 8-3g. When working with large valves, a light check might be just as appropriate and work equally welI. Bonnet
Disc
Body
Disc
Seat Ring
Seating Diameter
Figure 8-3g Seating Location on Tilting Disc [24, 28] 8-22
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
• Utilizing a low stress punch, establish three reference points on the seat ring as shown in Figure 8-3h and record the reference dimensions. These are used later to establish the final hardfacing thickness. A 0˚
240˚
120˚
A Reference Mark
Figure 8-3h Reference Mark Locations on Tilting Disc [28] 8.3.2.5 Horizontal Globe, Angle Globe, and Lift Check Valve Specific Prerequisites • Remove the poppet (plug) and record any damage. Particular attention should be given to the seat ring, poppet seat, body guides, and poppet guides. • Perform a blue check and record the seating location on the poppet as shown in Figure 8-3i. This should be a thin line of contact.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Seating Location
Original Stem-Disc Assembly
Figure 8-3i Seat Location on Poppet [13, 16] • Measure and record the inside diameter of the seat ring, the outside diameter of the seat angle, the distance from the top of the seat to the bonnet flange surface, and the body bore diameter as shown in Figure 8-3j.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair Poppet
OD
ID Body Seat
Bore Diameter Flange To Seat
Figure 8-3j Typical Body Dimensions, Globe Valve [28] • Measure and record the ID and OD of the poppet seat and the OD of the poppet wear ring. See Figure 8-3j.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
8.3.2.6 Y-Type Globe and Lift Check Valve Specific Prerequisites • Remove the poppet (plug) and record any damage. Particular attention should be given to the seat ring, poppet seat, body guides, and poppet guides. • Perform a blue check and record the seating location of the poppet. This should be a thin line of contact. • Measure and record the inside diameter of the seat ring, the outside diameter of the seat angle, the distance from the top of the seat to the bonnet flange surface, and the body bore diameter as shown in Figure 8-3k.
Flange to Seat
Body Bore Diameter
OD Seat Ring
ID
Figure 8-3k Typical Body Dimensions, Y-Type Globe and Lift Check Valve [13] • Measure and record the ID and OD of the poppet seat and the OD of the poppet wear ring.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
8.3.3 Defect Removal The method of flaw removal is based on the number of flaws, their distribution, and the overall amount of material to be removed. If a small localized area is going to be repaired due to a corrosion pit, stress crack across the seat, or a deep wear scar, the defect should be removed by manual grinding as discussed in Section 9.1. In applications where the repair to the seat ring hardfacing is extensive, all of the hardfacing should be removed from the face of the seat ring by machining with a portable lathe as shown in Figure 8-3l for gate-type valves and Figure 8-3m for globe configurations. If the flaws propagate into the seat base material, a manual grinding method should be employed to remove the remaining flaw leaving a 3:1 tapered weld preparation cavity (see Section 9.1). A verification should then be performed with an etchant to verify complete removal of all hardfacing material and a liquid penetrant examination performed to verify that no cracks or defects exist in the area to be repair welded.
Guide Rib
Drive Motors
Hardfacing Material
Cutting Tool
Figure 8-3l Gate Valve Seat Machining [26] 8-27
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair Drive Motor
Upper Chuck
Boring Bar
Lower Chuck
Clean Plug
Cutting Tool Inflatable Bladder
Figure 8-3m Globe Valve Seat Machining [26]
8.3.4 Filler Material Selection Filler material selection is dictated by base material compatibility and position as presented in Section 11. Except for the rare case where a repair is made to seat ring base material, the filler material will be a hardfacing alloy such as ASME Section II, SFA 5.13, RCoCr-A (Alloy 6), Alloy 21, or NOREM 02A (see Table 11-10 ). If the hardfacing is a repairable type, such as Alloy 21 or NOREM 02A, the repair can be performed with matching hardfacing material. If the existing hardfacing is a much harder material, such as Alloy 1 (RCoCr-C), Alloy 6 (RCoCr-A), or one of the nickel alloys (RNiCr-A or B), a softer repair material should be utilized. If a base metal repair or buildup is required, a Type E309 or ER309 for P-No. 1 or P-No. 8 substrates, or ER70S-3 for A106 (P-No. 1) seat rings, can be selected.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
8.3.5 Preheat and Post-Weld Heat Treatment Requirements As presented in Section 13, PWHT is not required (see Table 11-1 for P-Number specifications) for depositing hardfacing on P-Nos. 1 and 8 substrates, but is required for P-Nos. 4 and 5 substrates; however, different rules apply to the repair of the base materials. The PWHT requirements and exemptions are listed in Tables 13-2, 13-3, and 13-4. While not required, a moderate preheat is suggested when depositing any hardfacing material on a uniform surface and is strongly recommended when performing localized repair. The preheat reduces the potential for microcracking in the fusion zone of the dissimilar materials. A 250˚F (121˚C) minimum preheat is suggested when applying hardfacing materials to forged P-No. 1 seat rings, and a 300˚F (149˚C) preheat for cast P-No. 1 substrates. A 200˚F (93˚C) preheat is suggested for both forged and cast P-No. 8 substrates. When performing localized welding repairs of existing hardfacing deposits on seat rings, a 250˚F (121˚C) to 400˚F (204˚C) preheat is recommended. The higher preheat temperatures might not be feasible due to their effect on remote equipment or personnel.
8.3.6 Repair Welding Repair welding of seat rings in gate-type configurations is generally performed with the machine GTAW process (see Figure 8-3n) due to the difficult welding positions encountered and the lack of accessibility. The accessibility is much improved in check valves, but the variety of welding positions make this repair also challenging. Machine GTAW also offers superior welding controls for bead placement and heat input, which are critical to the successful application of a crack-free hardfacing deposit.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Control Handle Weld Head Wire Spool Video Camera AVC Unit Weld Wire
Mounting Chuck
Figure 8-3n Gate Valve Hardfacing Application, Machine GTAW [26] Repair welding of seat rings in globe-type valves is routinely performed with the machine GTAW process (see Figure 8-3o). The most common applications have been the large Y-type main steam isolation valves (MSIV) and feedwater regulator valves. The manual GTAW process can also be utilized in applications where the valve is large enough and the seat close enough to the valve opening to allow the welder to get both hands (arms) into the valve.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair Wire Spool
Guide Track
Frame
Wire Guide
Video Camera
Weld Head
Figure 8-3o Globe Valve Hardfacing Application, Machine GTAW [26] The shielded metal arc welding (SMAW) process with coated electrodes has been successfully utilized in both gate and globe applications, especially for localized repairs. Welding control can be compromised, compared to the machine process, resulting in stress cracking. Spatter is another concern for the SMAW process, which requires protection of other hardfaced surfaces prior to welding. The shielded metal arc process can be employed as a repair process for GTA-welded hardfacing deposits. A welding sequence should be developed prior to initiating the weld repair. Because the seat ring is a thin cylinder, extreme care should be exercised to minimize distortion from the heat input. A minimum of two hardfacing layers should be applied. If the repair requires more than three layers or 0.250 inch (6.3 mm) of deposited filler material, a butter layer should be utilized prior to depositing the hardfacing. After welding, a measurement should be taken to ensure that sufficient material has been applied to
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
allow for machining and lapping to final dimensions. If a stress crack is encountered at any time during the hardfacing application, a repair should be completed prior to applying additional material. Localized hardfacing repairs are quite challenging and should be performed as presented in Section 14.2. The defect should be completely removed and the repair area preheated to a minimum of 250˚F (121˚C). Thin layers should be deposited until the deposit is approximately 1/16 inch (1.6 mm) above the original surface.
8.3.7 Final Machining 8.3.7.1 Gate-Type Configurations Remachining of seat rings to their original dimensions can be quite challenging in the machine shop and even more challenging in situ. This step is by far the most difficult and critical step in repairing or resurfacing a seat ring. Obtaining accurate information and dimensions from the OEMs and having good references and data prior to welding will pay great dividends in the final machining operation. The data recorded prior to machining and welding, along with the manufacturer’s drawings, should be used to develop the finished seat thickness and width. Once the seat has been re-established, the final adjustments necessary to get a 360˚ blue check are made by machining, grinding, or lapping the wedge as presented in Section 8.11. In situ machining of the seat ring is performed with special boring bars developed specifically for gate valve-type seats. These machines are designed to align off the seat ring bore, off the guide ribs (see Figure 8-3p), or off the opposite seat and bore (gate valves). Throughout the machining operation, visual examinations should be performed to verify that the hardfacing is free of surface cracks. After machining, there should be sufficient material to finish the surface by lapping. For gate and swing check valve seats, the machining operation should also put the relief angle on the seat to narrow the seat width to that recommended by the valve manufacturer. The seat surface should be finished with a gate valve lapping machine such as that shown in Section 9.3.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Radial Feed
Mounting Fixture
Chuck Module
Tool Head
Figure 8-3p Globe Valve Seat Final Machining [23] For local seat repair applications, the seat can be remachined with a machine tool and lapped as in the same manner as the total resurfacing repair method, or the repair can be ground and blended with a flapper wheel, followed by lapping. 8.3.7.2 Globe-Type Configurations The final machining step plays the biggest role in achieving minimal leakage past the seat. The primary goal of the final machining operation is to have a finished seat that is very accurately aligned with the bore, perpendicular to the bore, and has a precise and consistent angle. Maintaining the proper seat angle within tolerance is extremely critical to achieving a good seal. The seat angle is typically 1˚ different than that of the poppet as shown in Figure 8-3q. The data recorded prior to machining and welding, along with the manufacturer’s drawings, should be used to establish the target dimensions needed to locate the seat. After machining, the seat should be lapped to blend any remnant tool marks. Lapping should be minimized in order to maintain the sharp angle and concentricity resulting from the machining operation. The acceptable blue check will be a 360˚ fine line around the poppet seat. 8-33
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Figure 8-3q Typical Globe Valve Seat Angle Differential [13]
8.3.8 Inspection The ASME Section XI Boiler and Pressure Vessel Code and the ANSI/ASME B31.1 Power Piping Code do not provide repair and inspection requirements for the seat ring and hardfacing because they are not defined as pressure-retaining components. However, ASME Section XI does require that the repair welding be performed by qualified welders and procedures in accordance with Section IX. As a matter of good practice, the repair cavity should be examined by the liquid penetrant or magnetic particle method. These same methods should be used after completing the repair. It is essential that the sealing surface of the hardfacing be free of any linear indications to ensure leak-tightness across the seat.
8.3.9 Testing Testing shall be performed in accordance with plant design requirements. This could include a local leak rate test (LLRT), an integrated leak rate test (ILRT), or in some cases no leak rate test at all. Because the pressure boundary has not been penetrated, no hydrostatic test is required.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
8.4 Seat Ring Replacement This section focuses on the replacement of welded-in and threaded seat rings. Repair of seat rings is addressed in Section 8.3. Seat rings are found in most gate, swing check, and tilting disc check valves in which the seat is 90˚ to the bonnet bore. Seat rings are found in about half of the globe valves, angle valves, lift checks, and Y-type valves, with the balance being manufactured with integral seats. Welded seat rings are installed with a partial penetration seal weld joining the seat ring to the valve body. In gate-type configurations, the weld is located in the flow stream area of the valve as shown in Figure 8-4a. In globe-type valves, the seal weld is found either above or below the seat at the discretion of the manufacturer (see Figure 8-4b). These welds are typically 1/8 inch to 3/8 inch (3.2 to 9.6 mm) deep, providing a seal to the leak path under the seat ring. This weld is not considered a structural weld.
Seal Weld
Figure 8-4a Typical Gate Valve Seat Ring Seal Weld [27]
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Seal Weld
Figure 8-4b Typical Seal Weld Locations for Globe-Type Seat Rings [28] Threaded seat rings are generally found in control and safety valves of all pressure classes and in Class 150 and Class 300 pound gate, globe, and check valves. Threaded seat rings are designed to stay in by preloading like a bolt thread or by welding. These can be either tack welds or a full diameter seal weld located on the upstream side of the seat ring.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
There are two basic seat ring configurations for gate valves. The most common configuration has the seat ring pocket that is machined parallel to the flow bore as shown in Figure 8-4a. If the seat is at an angle from the bonnet bore centerline to accommodate a wedge, the angle is machined into the seating end of the ring prior to applying hardfacing. In the second seat ring configuration, the seat ring pocket is machined into the valve body at an angle to the flow bore (see Figure 8-4c). This design allows for a uniform length seat ring, which makes in situ machining more difficult but makes alignment of the new ring much easier. All threaded-type gate valve seat rings are also made in this configuration.
Gate Valve Body Seat Ring Angle (Typically 5˚)
Seat Ring Pocket
Figure 8-4c Seat Ring Angle Generated by the Angle in the Seat Pocket [14] Globe valve seats rings, welded-in and threaded (see Figure 8-4d), are installed parallel to the bonnet bore. This is true for horizontal, angle, Y, and, Y-angle designs.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Lower Stem
Packing Stuffing Box Packing Top Closure (Bonnet) Gasket Guide Bushing
Inner Valve Seat Ring
Body
Guide Bushing Gasket
Bottom Closure
Drain Plug
Figure 8-4d Example of Globe Valve with a Threaded Seat Ring (Courtesy of Anchor/Darlington Valve Company) In situ seat ring replacement is quite common in most globe-type valve applications. Only recently, with the advances of in situ machining and welding equipment, have some of the valve manufacturers and service companies been able to replace welded-in gate valve seat rings in a cost-effective manner. Several field service companies can replace seat rings in most gate-type valves 8 inches to 10 inches (203 mm to 254 mm) nominal size and larger. Smaller gate valves should be removed from the line and repaired in the shop or replaced. Under extreme circumstances, welded-in seat ring repairs have been performed on valves as small as 2 inches (51 mm) nominal diameter. 8-38
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Replacing globe valve seat rings (perpendicular to the bonnet bore axis) is easier than replacing gate-type valve seat rings (parallel to the bonnet bore axis), due to the accessibility of the seat straight down the bonnet bore. Welding, machining, and lapping equipment is available for all sizes and pressure ratings. Because of the straight-in approach to the seat, welding and machining can be performed with either machine or manual repair processes. The replacement of threaded seat rings is much more common, even in situations where the valve is welded in-line. Simple replacement due to a damaged ring can be performed with grinders and special spanner wrenches. The repair challenge for valves with threaded-in seat rings is when the body threads have been damaged or eroded away. Like that of valves with welded-in seat rings, the globe-type configuration is quite repairable with the portable boring bars currently available. In situ repair of gate valve body threads is limited to very special “one-off” applications above 12-inches (305-mm) diameter. This section simply addresses the replacement of welded-in and threaded seat rings. After the seat ring is replaced, the mating disc or poppet must be fitted to the new seat ring to achieve the desired seal. In many cases, particularly with the tilting disc and swing check valves, the fitting operation can be just as difficult as the seat ring repair.
8.4.1 Repair Assessment and Strategy In cases where the hardfacing material is thin, has multiple indications, or is of a hardenable type, seat ring replacement is generally the proper approach. This approach avoids the potential welding problems associated with hardfacing materials. The replacement ring is already hardfaced and, therefore, the only welding required is that of the ring to the valve body, utilizing a compatible filler material. A thorough evaluation should be performed prior to committing to a seat ring replacement. Minimal tolerances are often employed, making material removal or distortion from welding in the seat ring critical to obtaining a leak-free seat. This operation requires significant resources and management support to ensure that mockup training is performed and that the necessary equipment is available. When evaluating the option of seat ring replacement versus valve replacement, the following questions should be asked: • Can the flaw be removed by grinding without impacting the sealing area, thus avoiding a seat ring repair or replacement? • Does the crack or flaw penetrate into the valve body or threaded area such that a welded repair of this pressure boundary material will be required? If so, are the proper welding procedures qualified and materials available to do a base metal repair? • Is there enough hardfacing material on the seat ring, based on the valve inspection report, to allow for reconditioning to the proper seat configuration? If so, can the disc or wedge be built up or adjusted to compensate for the material loss and thus avoid seat ring replacement? • Is machining, welding, and lapping equipment available to perform the replacement? 8-39
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
• Are qualified and experienced welding, machining, and valve repair personnel available to properly support the repair? • Are sufficient rigging points and space available to move equipment in and out of the valve? • Are replacement seat rings available? • If needed, are replacement wedges, discs, or poppets available? • If body threads are damaged, can the valve design be changed to accomodate a drop-in seat ring that is seal-welded?
8.4.2 Repair Prerequisites 8.4.2.1 Generic Prerequisites The following should be considered prior to initiating a repair: • Record the manufacturer name, size, pressure rating, valve type, body style, body material, and trim material data from the valve nameplate. • Obtain drawings and material specifications from the manufacturer. • Obtain and review the maintenance and repair history to see if there have been any changes to the original design. • Procure a replacement pressure seal ring and an oversize ring if the valve has a pressure seal bonnet. • Procure replacement seat rings and have the supplier ready to finish machining them after taking final measurements in the field. If the plant has machining capability, the rings can be shipped to the job site for final machining. • Procure a replacement wedge or disc if required. • Prepare a repair “traveler” or work instruction that outlines the repair sequence and inspection “hold points.” All measurements and findings should be recorded for a final report. • Install clean plugs in the upstream and downstream bores of the valve to prevent chips and foreign materials from getting down the line. • Install a protective liner over the pressure seal bore area. • Determine if PWHT equipment and personnel are available for replacing seat rings in valve bodies fabricated from P-Nos. 4 or 5 materials. 8.4.2.2 Gate Valve Specific Prerequisites • Establish match marks on both sides of the wedge with respect to the matching seat prior to removing the wedge from the valve body. This will ensure that the wedge is re-installed in the same direction.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
• Perform a blue check and record the seating location on the wedge as shown in Figure 8-3b. The blue check will provide the valve repair technician with the information necessary to determine what type of repair or adjustments will be required for the wedge to seat properly upon re-installation. • Measure and record the wedge dimensions as shown in Figure 8-3c. • Measure and record the minimum distances between the valve seats at the locations shown as shown in Figure 8-3d. • Utilizing a low stress punch, establish two reference points behind each seat ring as shown in Figure 8-3d. and record the reference dimensions from the seat ring face to the reference mark. These will be used to re-establish the seat face location when fitting the new seat rings. • Locate and establish the seal weld centerline. Compare with the manufacturer’s drawings and establish the cut line. 8.4.2.3 Swing Check Valve Specific Prerequisites • Remove the disc assembly and record any damage. Particular attention should be given to the seat, disc, hinge pins, and hinge pin bushings. • Perform a blue check and record the seating location of the disc. The blue check will provide the valve repair technician with the information necessary to determine what type of repair or adjustments will be required for the disc to seat properly upon re-installation. • Measure and record the seat width as shown in Figure 8-3f. • Utilizing a low stress punch, establish three reference points on the seat ring 120˚ apart as shown in Figure 8-3f and record the reference dimensions. These will be used later to re-establish final seat face location when fitting the new seat ring. • Locate and establish the seal weld centerline. Compare with the manufacturer’s drawings and establish the cut line. 8.4.2.4 Tilting Disc Check Valve Specific Prerequisites • Remove the disc assembly and record any damage. Particular attention should be given to the seat, hinge pins, and hinge pin bushings. • Perform a blue check and record the seating location on the disc as shown in Figure 8-3g. Measure the poppet seat angle and verify with the manufacturer’s drawing. The blue check will provide the valve repair technician with the information necessary to determine what type of repair or adjustments will be required for the disc to seat properly upon re-installation. • Utilizing a low stress punch, establish three reference points on the seat ring as shown in Figure 8-3h and record the reference dimensions. These will be used later to establish the final hardfacing thickness. • Measure the disc seat angle and verify with the manufacturer’s drawings.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
• Locate and establish the seal weld centerline. Compare with the manufacturer’s drawings and establish the cut line. 8.4.2.5 Horizontal Globe, Angle Globe, and Lift Check Valve Specific Prerequisites • Remove the poppet (plug) and record any damage. Particular attention should be given to the seat ring, poppet seat, body guides, and poppet guides. • Perform a blue check and record the seating location on the poppet as shown in Figure 8-3i. This should be a thin line of contact. • Measure and record the inside diameter of the seat ring, the outside diameter of the seat angle, the distance from the top of the seat to the bonnet flange surface, and the body bore diameter as shown in Figure 8-3j. • Measure and record the ID and OD of the poppet seat, and the OD of the poppet wear ring. • Measure the poppet seat angle and verify with the manufacturer’s drawings. • Locate and establish the seal weld centerline. Compare with the manufacturer’s drawings and establish the cut line. 8.4.2.6 Y-Type Globe and Lift Check Valve Specific Prerequisites • Remove the poppet (plug) and record any damage. Particular attention should be given to the seat ring, poppet seat, body guides, and poppet guides. • Perform a blue check and record the seating location of the poppet. This should be a thin line of contact. • Measure and record the inside diameter of the seat ring, the outside diameter of the seat angle, the distance from the top of the seat to the bonnet flange surface, and the body bore diameter as shown in Figure 8-3k. • Measure and record the ID and OD of the poppet seat, and the OD of the poppet wear ring. • Measure the poppet seat angle and verify with the manufacturer’s drawings. • Locate and establish the seal weld centerline. Compare with the manufacturer’s drawings and establish the cut line.
8.4.3 Seat Ring Removal Seat ring removal is performed by several methods. When selecting a removal process, issues such as accessibility, time/schedule constraints, radiation levels, and air quality should be considered. The methods most commonly used include: • Manually grinding out the seal weld • Grinding out the seal weld with automatic systems • Machining out the seal weld with a single point tool (see Figure 8-4e)
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Rotational Drive Weld Torch Mounting Fixture
Figure 8-4e Split View of Valve Body (In-Line) Exhibiting the Seal Weld Fixture [27] • Machining out the complete seat ring • Removing the threaded-in seat ring by grinding out the tack or seal weld and unscrewing it with a special spanner wrench Grinding is a time-consuming process that also presents radiation exposure and air quality concerns. Several companies have developed manipulators that get the maintenance personnel out of the valve (thereby reducing the overall radiation exposure) and provide some time savings over the manual method. Grinding can also be used when equipment or qualified machinists are not available. Machining out the seal weld or complete seat ring is quick, clean, and precise when performed by a qualified machinist with well-designed equipment. Globe valve seat rings can be removed with a variety of commercially available boring bars, as well as some special bars developed by repair organizations. Machining out gate and check valve seat rings is performed very effectively with some of the special machines developed by service companies specializing in field machining and welding. Machines designed and manufactured for gate valve seat ring repair were commercially available by mid-1996. The sequence for removing a seat ring is the same for both gate and globe valves. Extreme care should be taken to establish the cut location. This can be performed visually, by etching, or by UT. The machine tool should be centered to the bore and aligned to the cut line to minimize material removal. Review the drawings to verify the seal weld depth. The seal welds are generally 3/16-inch to 1/4-inch (4.7-mm to 6.3-mm) deep. While
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
machining, the machinist should look for a crack in the machined groove indicating that the seal weld has been penetrated. If the machined groove reaches a depth of 3/8 inch (9.5 mm) and a crack is not visible, a determination should be made about going deeper or adjusting the cut location. It is extremely important that the seat ring pocket depth is not increased by cutting into the base material because this alters the seat ring reference. After severing and removing the ring, the valve body should be prepped as shown in Figure 8-4f to install the new ring.
Valve Body
Flow Bore
Finished Prep
Figure 8-4f Valve Body Preparation for Replacement Seat Ring [11] During service, the crevice between the seat ring pocket and the seat ring (or the seat ring threads) can fill with corrosion particulate and other materials resulting in a stuck seat ring. Methods that can be used to remove the seat ring include: • Welding lugs on the ID bore and pulling and rotating the ring • Run weld beads along the ID of seat ring to shrink the ID • Heat valve body around seat ring to expand the pocket
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
• Cool the seat ring with dry ice • Sever axially with a grinder to remove segments (extreme care should be taken to minimize the damage to the in-body threads) In those applications where the body threads of a threaded-in seat ring have been damaged, the threads should be removed completely with a boring bar. Care must be taken to ensure that all base metal defects have been removed by performing a PT of the machined surface. If a design change is authorized for a welded-in seat ring, the new pocket diameter and depth must be achieved by precision machining.
8.4.4 Filler Material Selection Filler materials selection is dictated by base material compatibility and position as presented in Section 11. For stainless steel valves, a Type E309 or ER308 filler material should be selected for stainless steel (P-No. 8) substrates, and a 7018 or ER70S-3 for carbon steel (P-No. 1) substrates.
8.4.5 Preheat and Post-Weld Heat Treatment Requirements As presented in Section 13, PWHT is not required for joining P-No. 8 substrates; is conditional, based on thickness and carbon levels in the P-No. 1 substrates; and is required for P-Nos. 4 and 5 substrates (see Table 11-1 for P-Number specifications). The PWHT requirements and exemptions are listed in Tables 13-2, 13-3, and 13-4. Preheat and PWHT should be applied by placing heating mats on the outside surface of the valve body in the area opposite the weld. The heating mats should be wrapped with insulation and the temperature controlled as described in Section 13.
8.4.6 Seat Ring Installation and Welding Installation of seat rings should be performed by a qualified valve technician or under the direction of a manufacturer’s representative. Proper seat ring installation significantly reduces the time and effort needed to fit the disc or wedge to the new seat. Except for some gate valves and check valves with tapered seats, seat ring installation for both gate- and globe-type configurations is basically the same. If the damaged threads have been machined out and are going to be restored, the machined area should be built up with weld metal per Section 14.1 or 14.2 as shown on Figure 8-4g. If required by the qualified welding procedure, preheat the repair area of the valve prior to welding. The new threads can then be machined into the body with a special threading attachment available for some portable boring bars (see Figure 8-4h). If the new seat ring must be tack or seal welded, follow steps 5 through 8 of the Seat Ring Installation Sequence on page 8-49.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Weld Buildup
Seat Ring
Figure 8-4g Typical Weld Buildup for Repair of Threaded Globe Valve Seat Ring Pocket
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Radial Feed Control
Axial Feed Control
Machine Tool Body
Rotating Bar
Mounting Plate
Valve Body
Tool Head
Figure 8-4h Valve Boring System with Thread Attachment for Restoration of Threads in the Seat Ring Pocket [23] Seat Ring Installation Sequence 1. Verify the critical dimensions of the replacement seat ring and pocket. The new ring should match the OD and length of the removed ring. 2. If required, machine the OD and length of the replacement seat ring. It should be noted that the valve manufacturers produce these components to very strict tolerances; therefore, field machining is a rare occurrence and should be avoided. 3. Rig the seat ring into the valve body and fit it into the pocket. The seat ring should bottom out in the pocket. Extreme care should be taken to ensure that the pressure seal bore is not nicked or scratched. 4. Verify that the seat ring is properly seated in the bottom of the pocket and properly aligned with the bore. 8-47
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
NOTE: If the welded-in type seat ring is tapered (see Figure 8-4i), it must be “clocked in” to the proper orientation. Some manufacturers provide a match mark at the 12 o’clock position for alignment. Valve manufacturers can also provide critical reference measurement locations to ensure proper seat ring orientation.
Body Match Mark
Seat Ring Match Mark
Figure 8-4i Example of Tapered Seat Ring [16] 8-48
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
5. Tack weld the seat ring into position per Section 14.1. 6. Install the welding system into the valve body and align the weld joint. 7. If required by the qualified welding procedure, preheat the seal weld area of the valve to the required temperature. 8. Perform the seat ring to body seal weld (partial penetration groove weld) per Section 14.1 (manual process) or 14.2 (machine process). This is generally a two- or three-pass weld. CAUTION: Welding heat input should be minimized to reduce the potential for seat ring distortion.
In situ seat ring replacement in gate-type configurations is generally performed with the machine GTAW process (see Figure 8-4j) due to the difficult welding positions encountered and the lack of accessibility. The accessibility is much improved in check valves but the variety of welding positions make this repair challenging.
Weld Head
Seal Welds
Torch Extended To Perform Weld
Figure 8-4j Split View of Valve Body (In-Line) Exhibiting Seal Weld Head Setup [27] Replacement of seat rings in globe-type valves is performed routinely with the machine GTAW process (see Figure 8-3o). The manual GTAW process can also be utilized in applications where the valve is large enough and the seat close enough to the valve opening to allow the welder to get both hands (arms) into the valve.
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The shielded metal arc welding (SMAW) process is frequently utilized to install seat rings in globe valves, but is extremely difficult to use in gate applications. Welding control can be compromised, compared to the machine process resulting in stress cracking. Spatter is another concern for the SMAW process, which requires protection of finished hardfaced and sealing surfaces prior to welding. The shielded metal arc process can be employed as a repair process for GTA-welded hardfacing deposits. A welding sequence should be developed prior to initiating the weld repair. Because the seat ring is a thin cylinder, extreme care should be exercised to minimize distortion due to heat input. If hardfacing repairs are required after installing the seat ring, they should be performed as presented in Section 8.3 and the welding guidelines provided in Section 14.3. The defect should be completely removed and the repair area preheated to a minimum of 250˚F (392˚C). Thin layers should be deposited until the deposit is approximately 1/16 inch (1.6 mm) above the original surface.
8.4.7 Machining No machining is required of the seal weld. Because welding stresses and heat can affect the seat ring, post-weld lapping is required. Machining is required only in cases where the seat ring has warped considerably due to excessive heat input. Machining of the seat ring hardfacing should be performed as described in Section 8.3.7. Because replacement seat rings come from the manufacturer with a ground, not lapped, sealing surface, a minimal amount of lapping as described in Section 9.3 is necessary. The condition of the installed seat ring on gate valves can be determined by applying a thin layer of bluing to the hardsurfaced seat and lightly lapping to locate low spots on the ring. If low spots are found, continue to lap until all bluing is removed from the sealing surface. The last step is to establish the correct seat width on the gate and swing check seats by grinding the relief angle on the outer edge of the hardfaced face (see Figure 8-4k). Refer to the valve manufacturer’s specification and/or drawings to obtain this dimension, which is based on ring size and pressure rating.
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Grind/Machine Outside Angle To Narrow Seat Width
Figure 8-4k Gate Valve Seat Narrowing [13] For globe-type valves, the seat or seat ring should be blued and a plug-type lap machined to the exact conical angle of the seat ring, and used to check for concentricity and low spots (see Figure 8-4l). Lapping should continue until all of the bluing is removed. The ideal seat shall be concentric to the bore, perpendicular to the bonnet flange, with a crisp angle at the intermost seat dimension. The well-defined angle provides a thin, 360˚ contact line between the poppet and seat.
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Plug O.D. to Fit Seat I.D.
Figure 8-4l Typical Lapping Fixture for Globe-Type Seat Ring [13]
8.4.8 Inspection The ASME Section XI Boiler and Pressure Vessel Code and the ANSI/ASME B31.1 Power Piping Code require that the seal weld be inspected by either the liquid penetrant (PT) or magnetic particle (MT) process as discussed in Section 15.1. A PT should also be performed on the hardfaced seat to verify that welding stresses have not cracked the seat.
8.4.9 Testing Testing shall be performed in accordance with plant design requirements. This could include a local leak rate test (LLRT), an integrated leak rate test (ILRT), or in some cases no leak rate test at all. Because the pressure boundary has not been penetrated, no hydrostatic test is required. 8.5 Integral Seat Repair This section presents the operations and requirements necessary to perform a repair of integral in-body valve seat. An integral seat is hardfacing material welded directly onto the pressure-retaining valve body. Replacement of the complete integral seat is presented in Section 8.6. Integral seats are primarily found in globe, angle, lift check, and Y-type valves. Integral seats are rarely found in gate and swing check valves with a pressure Class of 600 and
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higher. The exception is an occasional tilting disc check valve and some 2 inches (51 mm) nominal pipe size (NPS ) and smaller gate valves. Because integral seats are basically non-existent in nuclear valves of the gate-type configuration, this discussion is limited to the globe-type valves. In cases where a utility does come in contact with one of these gate-type valves, the repair can be addressed similarly to that of a gate valve seat ring hardfacing repair. Welding would be performed in the same manner with a butter layer of weld material that is compatible with the base material applied over the base material. This ensures a sound substrate for the hardfacing material. Integral seat repair is extremely challenging to the most experienced valve repair experts. However, they can be completed successfully with the proper knowledge, equipment, welding materials, and trained personnel. Machining and welding technology for in situ repair of globe valve seats has advanced significantly since the early 1980s. Machining and lapping tools, as well as automatic GTAW and GMAW systems, are commercially available for all sizes and pressure ratings. Because of the straight-in approach to the seat, welding can be performed with either machine or manual welding processes. In situ hardfacing repairs are readily performed by field service companies for globe valves 10 inches (254 mm) and larger. Under extreme circumstances, seat repairs have been performed on valves as small as 2 inches (51 mm) nominal diameter.
8.5.1 Repair Assessment and Strategy Developing a repair strategy with backup positions is essential prior to committing to a seat ring repair. Due to the inherent high residual stresses and low ductility of the hardfacing materials, a small pit can become long multiple cracks after grinding or welding is initiated. Because of the tight tolerances required to obtain a leak-free seal, excessive material removal or distortion from welding could result in the need for a larger scope repair or complete replacement. The type of hardfacing on the seat, as well as the base material, must also be considered. The less forgiving hardfacing materials, such as those commonly referred to as Stellite 1 (ASME SFA 5.13, RCoCr-C), 6 (ASME SFA 5.13, RCoCr-A), and 12 (ASME SFA 5.13, RCoCr-B), and the nickel-based alloys, such as Colmony 4 and 5, and Deloro 40 and 50 (ASME SFA 5.13, RNiCr-A and B), are extremely difficult to repair. If the base material is a P-No. 4 or P-No. 5 (see Table 11-1) and the hardfacing is thin, a preheat and post-weld heat treatment is required. When evaluating the hardfacing repair/replacement option, the following questions should be asked: • Can the flaw be removed by grinding without impacting the sealing area, thus avoiding a weld repair? • Can the flaws be removed by machining a thin layer off the complete seat surface and maintain required hardfacing thickness, thus avoiding a weld repair? • Does the crack or flaw penetrate into the valve body such that a welded repair of this pressure boundary material will be required?
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• Is there enough hardfacing material on the seat ring, based on the valve inspection report, to perform a repair and still achieve acceptable seating contact? If not, can the disk or wedge be built up or adjusted to compensate for the material loss? If the hardsurfacing on the complete seat diameter must be built up, the owner should also consider the option of complete seat replacement. • Is machining and welding equipment available to perform the repair? • Are sufficient rigging points and space available to move equipment in and out of the valve? • Is the existing hardfacing material repairable without the use of elevated preheat temperatures? Can the welding and machining equipment withstand these elevated temperatures?
8.5.2 Repair Prerequisites The following should be considered prior to initiating a repair: • Record the manufacturer’s name, size, pressure rating, valve type, body style, body material, and trim material data from the valve nameplate. • Obtain drawings and material specifications from the manufacturer. • Obtain and review the maintenance and repair history to see if there have been any changes to the original design. • Procure a replacement pressure seal ring and an oversize ring if the valve has a pressure seal bonnet. • Prepare a repair “traveler” or work instruction that outlines the repair sequence and inspection “hold points.” All measurements and findings should be recorded for a final report. • Install clean plugs in the upstream and downstream bores of the valve to prevent chips and foreign materials from getting down the line. • Install a protective liner over the pressure seal bore area. • Qualify the necessary welding procedures for hardfacing and base material repairs. • Demonstrate the full repair sequence on a mockup. This includes both machining and welding operations. • Remove the poppet (plug) and record any damage. Particular attention should be given to the in-body seat, poppet seat, body guides, and poppet guides. • Perform a blue check and record the seating location of the poppet as shown in Figure 8-3i. This should be a thin line of contact. • Measure and record the inside diameter of the seat, the outside diameter of the seat angle, the distance from the top of the seat to the bonnet flange surface, and the body bore diameter as shown in Figure 8-3j. • Measure and record the ID and OD of the poppet seat, and the OD of the poppet wear ring(s). 8-54
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8.5.3 Defect Removal The method of flaw removal should be based on the number of flaws, their distribution, and the overall amount of material to be removed. If a small localized area is going to be repaired due to a corrosion pit, stress crack across the seat, or a deep wear scar, the defect should be removed by manual grinding as discussed in Section 9.1. In applications where numerous shallow indications are on the seating surface and a sufficient hardfacing thickness remains, a surface cut across the seat at the proper angle should be performed with a portable lathe as shown in Figure 8-3p. If the flaws propagate into the base material, a manual grinding method or a portable lathe should be employed to remove the remaining flaw leaving a 3:1 tapered weld prep (see Section 9.1). A liquid penetrant (PT) examination should be performed to verify that no cracks or indications exist in the area to be repair welded.
8.5.4 Filler Material Selection Filler materials selection is dictated by base material compatibility, welding position, and process as presented in Section 11. Except for the case where a repair or buildup of the base material is required, the filler material is a matching hardfacing alloy such as Alloy 21. If the existing hardfacing material is one of the more hardenable materials, such as Alloy 1 (ASME SFA 5.13, RCoCr-C), 6 (ASME SFA 5.13, RCoCr-A), or 12 (ASME SFA 5.13, RCoCr-B), a more forgiving material, such as Alloy 21, NOREM 02A, should be selected. In some cases, a stainless material, such as ASME SFA 5.9, ER309 or ASME SFA 5.9, ER308, has been selected because the repair area is just a small portion of the overall seat area and would have little effect on the distribution of load across the seating surface. When a base metal repair or buildup is required or if the weldment is for structural restoration (minimum wall), the selection must be based on the base material and operating temperatures. For structural restoration of all base materials and for high temperature applications on P-Nos. 4 and 5 substrates, a compatible filler material must be utilized (see Table 11-4). For non-structural buildups, a Type 309 stainless filler material can be utilized. An austenitic filler material, such as ER309, provides a more forgiving substrate for depositing hardfacing than the carbon steel filler materials, due to their similar coefficients of thermal expansion.
8.5.5 Preheat and Post-Weld Heat Treatment Requirements As presented in Section 13, PWHT is not required for depositing hardfacing on P-Nos. 1 and 8 substrates, but is required for P-Nos. 4 and 5 substrates (see Table 11-1 for PNumber specifications). However, different rules apply to the repair of the base materials. The PWHT requirements and exemptions are listed in Tables 13-2, 13-3, and 13-4. While not required, a minimum 200˚F to 400˚F (93˚C to 204˚C) preheat is essential to performing any localized repair. The lower preheats can be utilized when repairing Alloy 21 or NOREM 02A on stainless substrates, and increasing to 450˚F to 550˚F (232˚C to 288˚C) for P-No. 5 substrates. Successful repairs to the more hardenable hardfacing
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materials often require preheats in excess of 800˚F (427˚C). The suggested preheat temperature also varies based on the quality and technique of the welder, the quality of filler material, and the method of base metal fabrication.
8.5.6 Repair Welding Localized welding repair of an integral seat in globe-type valves is generally performed with the machine GTAW process and the manual SMAW process. The most common applications have been the large Y-type main steam isolation valves (MSIV) and turbine throttle valves (see Figure 8-5a). The manual GTAW process can also be utilized in applications where the valve is large enough and the seat close enough to the valve opening to allow the welder to get both hands (arms) into the valve.
Figure 8-5a Turbine Throttle Valve 8-56
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
The shielded metal arc welding (SMAW) process with coated electrodes has been successfully utilized on large and small valves where radiation exposure is not a concern. A drawback to the SMAW process is that it produces spatter and compromises welding control; however, both can be overcome with good practices. In some cases, the position or weldment configuration dictates the SMAW process as the only viable repair process. Localized hardfacing repairs are quite challenging and should be performed as presented in Section 14.2. Experience has shown that a tapered repair cavity that provides access and visibility to the welder, a well planned and proven welding sequence, and a welder experienced with welding hardfacing materials greatly enhance the potential for a crack-free repair. After welding, a measurement should be taken to ensure that there is a minimum of 1/16 inch (1.6 mm) of hardfacing material above the original surface and surrounding hardfacing to allow for machining and lapping to the final dimensions. If a stress crack is encountered at any time during the hardfacing application, the crack should be ground out prior to applying additional material. The finished weld should be wrapped in insulation to allow for slow cooling.
8.5.7 Machining For local seat repair applications, the seat can be remachined with a machine tool and lapped, or the repair can be ground and blended with a flapper wheel, followed by additional grinding and lapping with a lapping machine, such as that shown in Figure 8-5b.
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Mounting Plate
Valve Seat
Lapping Plate
Figure 8-5b Swing Check Valve Lapping Machine [Courtesy of EFCO] If the entire seat was built up with additional hardfacing, which is not recommended, remachining with an in situ boring bar is required. These machines are designed to align off the guide ribs and flange face (see Figure 8-3p) in order to maintain concentricity with the bore. This step is critical in performing a successful seat repair. Obtaining accurate information and dimensions from the OEMs and having good references and data prior to welding will pay great dividends in the final machining operation. The data that are recorded prior to machining and welding, along with the manufacturer’s drawings, should be used to develop the finished seat thickness and width. Throughout the machining operation, visual examinations should be performed to verify that the hardfacing is free of surface cracks. After machining, the seat should be lapped to blend any remnant tool marks. Lapping should be minimized to maintain the sharp angle and concentricity resulting from the machining operation.
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After the in-body seat has been re-established, the final adjustments necessary to get a 360˚ blue check are made by machining and lapping the poppet as presented in Section 8.12.
8.5.8 Inspection The ASME Section XI Boiler and Pressure Vessel Code and the ANSI/ASME B31.1 Power Piping Code do not provide repair and inspection requirements for the seat ring and hardfacing because they are not defined as pressure-retaining components. However, ASME Section XI does require that the repair welding be performed by qualified welders and procedures in accordance with Section IX. As a matter of good practice, the repair cavity should be examined by the liquid penetrant or magnetic particle method. These same methods should be used after completing the repair. It is essential that the sealing surface of the hardfacing be free of any linear indications to ensure leak-tightness across the seat.
8.5.9 Testing Testing shall be performed in accordance with plant design requirements. This could include a local leak rate test (LLRT), an integrated leak rate test (ILRT), or in some cases no leak rate test at all. Because the pressure boundary has not been penetrated, no hydrostatic test is required. 8.6 Integral Seat Replacement This section presents the operations and requirements necessary to replace integral inbody valve seats. An integral seat is hardfacing material welded directly onto the pressure-retaining valve body. Repair of integral seats is presented in Section 8.5. Full seat replacement has become quite common in the nuclear industry as utilities replace their cobalt bearing hardfacing alloys with the non-cobalt bearing types. Other typical reasons for seat replacement include insufficient thickness of the remaining hardfacing due to several repair or maintenance operations, and cracking of the hardfacing types that are very difficult to repair. This would include the cobalt-based Alloys 1, 6, and 12 (ASME SFA 5.13, RCoCr-C, A, B, respectively), and the nickel-based alloys, Deloro 40 and 50 and Colmonoy 4 and 5 (ASME SFA 5.13, NiCr-A and B). Integral seats are primarily found in globe, angle, lift check, and Y-type valves. Integral seats are rarely found in gate and swing check valves with a pressure Class of 600 and higher. The exception is an occasional tilting disc check valve and some 2 inches (51 mm) NPS and smaller gate valves. Because integral seats are basically non-existent in nuclear valves of the gate-type configuration, this discussion is limited to the globetype valves. In cases where a utility does come in contact with a gate-type valve, the repair is very similar to that of a gate valve seat ring hardfacing replacement. Welding would be performed in the same manner with a butter layer of weld material that is compatible to the base material applied over the base material. This ensures a sound substrate for the hardfacing material.
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Integral seat replacement is extremely challenging to the most experienced valve repair experts. However, it can be completed successfully with the proper knowledge, equipment, welding materials, and trained personnel. Machining and welding technology for in situ repair of globe valve seats has advanced significantly since the early 1980s. Machining and lapping tools, as well as automatic GTAW and GMAW systems, are commercially available for all sizes and pressure ratings. Because of the straight-in approach to the seat, welding can be performed with either machine or manual welding processes. In situ hardfacing repairs are readily performed by field service companies for globe valves 10 inches (254 mm) and larger. Under extreme circumstances, seat replacements have been performed on valves as small as 2-inches (51-mm) nominal diameter.
8.6.1 Repair Assessment and Strategy In cases where the hardfacing material is thin, has multiple indications, or is of a hardenable type, complete seat replacement is generally the most cost-effective approach. Developing a repair strategy is essential prior to initiating a integral seat replacement. The potential for schedule setbacks is high due to the difficulty in machining hardfacing materials and the sensitivity of these materials to cracking. When the schedule is critical, mockup training in every aspect of the replacement should be performed. The activity that is taken for granted or overlooked will become a lesson learned for others to avoid. An equally important issue essential to that of a repair strategy is management support. The successful replacements have been those that had quality equipment, experienced machining and welding personnel, manufacturer’s support, training, and an organization with recognized, responsible, and empowered personnel. When evaluating an in situ seat hardfacing replacement, the following questions should be asked: • Can the flaw be removed by grinding without impacting the sealing area, thus avoiding a weld repair? • Can the flaws be removed by machining a thin layer off the complete seat surface and maintain required hardfacing thickness, thus avoiding a weld repair? • Does the crack or flaw penetrate into the valve body such that a welded repair of this pressure boundary material will be required? • Is there enough hardfacing material on the seat, based on the valve inspection report, to perform a repair and still achieve acceptable seating contact? If not, can the disk or wedge be built up or adjusted to compensate for the material loss? • Can the indication be repaired more easily with a localized repair method? Should that be the initial repair method with replacement as a backup approach? • Is machining and welding equipment available to perform the repair? • Are sufficient qualified and experienced personnel available to perform this replacement? 8-60
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• Are sufficient rigging points and space available to move equipment in and out of the valve? • What are the preheat and PWHT requirements for performing this replacement? Can the welding and machining equipment withstand these elevated temperatures?
8.6.2 Seat Hardfacing Replacement Prerequisites • Record the manufacturer’s name, size, pressure rating, valve type, body style, body material, and trim material data from the valve nameplate. • Obtain drawings and material specifications from the manufacturer. • Obtain and review the maintenance and repair history to see if there have been any changes to the original design. • Procure a replacement pressure seal ring and an oversize ring if the valve has a pressure seal bonnet. • Prepare a repair “traveler” or work instruction that outlines the repair sequence and inspection “hold points.” All measurements and findings should be recorded for a final report. • Install clean plugs in the upstream and downstream bores of the valve to prevent chips and foreign materials from getting down the line. • Install a protective liner over the pressure seal bore area. • Qualify the necessary welding procedures for hardfacing and base material buildup and repairs. • Demonstrate the full repair sequence on a mockup. This includes both machining and welding operations. • Remove the poppet (plug) and record any damage. Particular attention should be given to the in-body seat, poppet seat, body guides, and poppet guides. • Perform a blue check (see Section 15.3) and record the seating location of the poppet as shown in Figure 8-3i. This should be a thin line of contact. • Measure and record the inside diameter of the seat, the outside diameter of the seat angle, the distance from the top of the seat to the bonnet flange surface, and the body bore diameter as shown in Figure 8-3k. • Measure and record the ID and OD of the poppet seat, and the OD of the poppet wear ring(s). • Based on the measurements of the poppet seat, determine whether the poppet seat hardfacing will be replaced.
8.6.3 Seat Removal and Weld Preparation Seat removal can be performed by several methods. When selecting a removal process, issues such as accessibility, time and schedule constraints, amount of material to be
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removed, size of valve, radiation levels, and air quality should be considered. The methods most commonly used include: • Manually grinding out the seat • Grinding out the seat with automatic systems (see Figure 8-6a)
Figure 8-6a Seat Removal with Automatic Grinding System • Machining out the seat with a single point tool (see Figure 8-3p) • Milling out the seat Grinding is a time consuming process that also presents radiation exposure and air quality concerns. Several companies have developed manipulators that get the maintenance personnel out of the valve (thereby reducing the overall radiation exposure) and provide some time savings over the manual method. Grinding can also be used when machining equipment or qualified machinists are not available. Machining the seat out is quick, clean, and precise when performed by a qualified machinist with well-designed equipment. Globe valve seats can be removed with a variety of commercially available boring bars, as well as some special bars developed by repair organizations. Several service companies have developed globe valve machining systems that mill out most of the material and then finish the job with a single point turning method.
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The machining sequence incorporates both hardfacing removal and weld preparation for the build up material and hardfacing. The repair procedure should explicitly detail the machining operation. It should include drawings of the prep configuration, with dimensions, of the weld prep for the buildup or butter layer, and the weld prep for the hardfacing deposit if a two-step method is selected. Because of dilution into the original base material when the hardfacing was deposited initially, as much as 1/8 inch to 3/16 inch (3.1 mm to 4.8 mm) of additional material has to be removed to get rid of the hardfacing material. This additional material removed must be replaced with buildup layers if minimum wall thickness is violated or seat support is reduced, or with a butter layer if the original base material dimensions must be re-established. It is extremely crucial that the machine tool be centered to the bore and aligned to the cut line to minimize material removal. Axial and radial reference points must be established for monitoring material thickness removal, for position referencing when tooling is broken, and for final machining. Hardfacing removal is a time consuming operation. Because the material is extremely hard, slow travel speeds and feed rates are utilized. Tooling wears quickly and tool bits have a tendency to break, thus requiring frequent tool bit changeouts. This can be reduced by utilizing carbide tooling and very rigid machine tools. A liquid penetrant (PT) exam must be performed on the pressure-retaining base material after machining or prior to welding. A second PT should be performed on the buildup or butter layer prior to depositing the hardfacing material.
8.6.4 Filler Material Selection Filler material selection is dictated by the purpose of the weldment, base material compatibility, process, and welding position as presented in Section 11. As stated previously, seat replacement requires that some of the original base material be removed in order to remove the hardfacing material that was diluted for some depth into the original base material. Because hardfacing thickness should be minimized, the base material must be replaced to re-establish the base material dimensions prior to depositing the hardfacing material. In cases where minimum wall thickness has been encroached upon or even violated, the ASME Section XI requires that filler material matching the base material chemistry and properties be used (see Table 11-4). If the weld is being treated as a non-structural buildup or butter layer, a filler material more compatible with the hardfacing alloy can be selected. As an example, filler metal ASME SFA 5.9, ER309 (stainless steel) is often used on the P-No. 1 carbon steel substrates because of its similar coefficient of expansion to the cobalt and iron-based hardfacing alloys. When a base metal repair or build up is required, or if the weldment is for structural restoration (minimum wall), the selection must be based on the base material and operating temperatures. For structural restoration of all base materials and for high temperature applications on P-Nos. 4 and 5 substrates, a compatible filler material must be utilized. 8-63
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The less hardenable Alloy 21 or the NOREM 02A hardfacing materials should be selected for utilization on most globe-type seat applications. These materials are easier to apply over most substrates with very little preheat (see Section 13) and provide the properties necessary for impact and erosion applications. The galling wear properties of the very hard materials are unnecessary in globe valves. For more information on hardfacing selection, refer to the EPRI Cobalt Reduction Guidelines [29].
8.6.5 Preheat and Post-Weld Heat Treatment Requirements As presented in Section 13, PWHT is not required for depositing hardfacing on P-Nos. 1 and 8 substrates, but is required for P-Nos. 4 and 5 substrates (see Table 11-1 for PNumber specifications). However, different rules apply to the repair of the base materials. The PWHT requirements and exemptions are listed in Tables 13-2, 13-3, and 14-4. While not required, a minimum 200˚F to 400˚F (93˚C to 204˚ C) preheat is essential when welding hardfacing materials. The lower preheats can be utilized when welding Alloy 21 or NOREM 02A on stainless substrates, and increasing to 450˚F to 550˚F (232˚C to 288˚C) for P-No. 5 substrates. Successful applications of the more hardenable hardfacing materials often require preheats in excess of 800˚F (427˚C). The suggested preheat temperature also varies based on the quality and technique of the welder, the quality of filler material, and the method of base metal fabrication.
8.6.6 Welding Repair Welding an integral seat into globe-type valves is generally performed with the machine GTAW process and the manual SMAW process. The most common applications have been the large Y-type main steam isolation valves (MSIV) (see Figure 8-3o) and turbine throttle valves. The manual GTAW process can also be utilized in applications where the valve is large enough and the seat close enough to the valve opening to allow the welder to get both hands (arms) into the valve. Suggested welding practices are presented in Section 14.2. The shielded metal arc welding (SMAW) process with coated electrodes has been successfully utilized on large and small valves where radiation exposure is not a concern. A drawback to the SMAW process is that it produces spatter and compromises welding control; however, both can be overcome with good practices. In some cases, the position or weldment configuration dictates the SMAW process as the only viable repair process. When welding on large valves or valves where a significant quantity of material was removed to eliminate the original hardfacing material, a two-step welding application is recommended. The base metal should be restored to its original position by building up the area as shown in Figure 8-6b. This buildup can then be machined with the desired weld prep or cavity to ensure that the hardfacing deposit does not exceed approximately
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3/16 inch (4.8 mm). The hardfacing is then deposited into the prepared cavity. This methodology, similar to that used by the original manufacturer, ensures proper location and thickness of the new seat, as well as ensuring a quality hardfacing deposit. After completing the hardfacing weldment, it should be wrapped in insulation to allow for slow cooling.
Figure 8-6b Base Material Restoration for Seat Ring
8.6.7 Machining Machining the seat to its original dimensions can be quite challenging in the machine shop and even more challenging in situ. A good machining operation results in minimal leakage past the seat at the completion of the job. In situ machining to obtain a finished seat is performed with the same boring bar utilized to remove the hardfacing material and provide the weld prep in which the hardfacing was deposited (see Figure 8-6c). The primary goal of the final machining operation is to have a finished seat that is very accurately aligned with the bore, is perpendicular to the bore, and has a precise and consistent angle. The data recorded prior to machining and welding, along with the manufacturer’s drawings, should be used to establish the target dimensions needed to locate the seat.
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Figure 8-6c In Situ Machining of MSIV Seat After machining, the seat should be lapped to blend any remnant tool marks. Lapping should be minimized in order to maintain the sharp angle and concentricity resulting from the machining operation. After the in-body seat has been re-established, the final adjustments necessary to get a 360˚ blue check are made by machining and lapping the poppet as presented in Section 8.12.
8.6.8 Inspection The ASME Section XI Boiler and Pressure Vessel Code and the ANSI/ASME B31.1 Power Piping Code address repair of the base material but do not provide repair and inspection requirements for the seat hardfacing because it is not defined as pressureretaining. ASME Section XI, Subarticle IWA-4400 requires that the repair welding be performed by qualified welders and procedures in accordance with Section IX. It also requires that the repair cavity (pressure-retaining base material) shall be examined by liquid penetrant or magnetic particle method. As a matter of good practice, PT or MT inspection should be used after completing the hardfacing deposit and final machining. It is essential that the sealing surface of the hardfacing be free of any linear indications to ensure leak-tightness across the seat.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
8.6.9 Testing Testing shall be performed in accordance with plant design requirements. This could include a local leak rate test (LLRT), an integrated leak rate test (ILRT), or in some cases no leak rate test at all. Because the pressure boundary has not been penetrated, no hydrostatic test is required. 8.7 Bonnet Backseat Repair and Replacement This section provides the operations and requirements necessary to repair or replace backseats in the bonnet of valves. Like in-body seats, manufacturers provide both integral seats and replaceable seat rings. Both types are addressed here. Because backseats are on the removable bonnet, these repairs should be performed in a shop with either stationary or portable machining and welding equipment. As with any hardfacing repair, proper training, equipment, and experienced personnel are needed to complete a successful repair or replacement.
8.7.1 Repair Strategy A thorough review of options should be considered prior to committing to a repair approach. Very tight tolerances are required, making material removal or distortion from welding very critical to obtaining a leak-free seat. In applications where there is a seat ring, seat ring removal and replacement can be more cost-effective than the repair option for a multitude of reasons. When cracks or flaws are encountered on integral seats, and the hardfacing material is Alloy 1, 6, and 12 (ASME SFA 5.13, RCoCr-C, A, and B, respectively), strong consideration should be given to removing the existing hardfacing material and applying a new hardfacing deposit. When evaluating the repair and replacement option, the following questions should be asked: • Can the flaw be removed by grinding, thus avoiding any type of weld repair? • Can the flaw be removed by machining a thin layer off the complete seat surface and still obtain a good blue check, thus avoiding a repair weld? • Does the crack or flaw penetrate into the seat ring base material such that a welded repair activity could significantly deform the ring? If so, the ring should be replaced. • Is there enough hardfacing material on the seat ring, based on the valve inspection report, to perform a repair and still achieve acceptable seating contact? If the hardsurfacing on the seat ring must be built up, the owner should consider seat ring replacement. • Does the crack or flaw protrude into the valve body such that a welded repair of the pressure boundary material is required? If so, the integral seat or seat ring, as appropriate, should be replaced in conjunction with the valve body repair described in Section 8-1.
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• If this is a seat ring type, is a replacement seat ring available? • Is machining and welding equipment available to perform the repair in the machine shop? • Is portable machining and welding equipment available to perform the repair if the shop is not available or equipped? • Is the existing hardfacing material repairable without the use of a elevated preheat temperature that might distort the bonnet or bonnet flange? • Is the base material a P-No. 4 or P-No. 5 Alloy (see Table 11-1) that requires a PWHT? If so, machining of the bonnet flange is probably required due to distortion.
8.7.2 Repair Prerequisites The following should be considered prior to initiating a repair: • Record the manufacturer’s name, size, pressure rating, valve type, body style, body material, and trim material data from the valve nameplate. • Obtain drawings and material specifications from the manufacturer. • Obtain and review the maintenance and repair history to see if there have been any changes to the original design. • Procure a replacement pressure seal ring and an oversize ring if the valve has a pressure seal bonnet. • Prepare a repair “traveler” or work instruction that outlines the repair sequence and inspection “hold points.” All measurements and findings should be recorded for a final report. • Qualify the necessary welding procedures for hardfacing and base material repairs. • Remove the valve stem from the bonnet and record any damage. Particular attention should be given to the stem backseat shoulder. • Measure and record the bonnet seat inside diameter, outside diameter and seat angle. • Measure and record the stem backseat major diameter, minor diameter, and seat angle. • Perform a blue check and record the seating location on the stem backseat shoulder as shown in Figure 8-7a. This should be a thin line of contact.
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Packing Ring Bonnet Stem
Figure 8-7a Typical Bonnet Backseat/Stem Configuration for Blueing [16]
8.7.3 Flaw Removal 8.7.3.1 Localized Repair If a small localized area is going to be repaired due to a corrosion pit, stress crack across the seat, or a deep wear scar, the defect should be removed by manual grinding as discussed in Section 9.1. The finished repair cavity should have a 3:1 taper into the surrounding area to provide access and visibility to the welder. A liquid penetrant (PT) examination should be performed to verify that no cracks or indications exist in the area to be repair welded. 8.7.3.2 Integral Seat Replacement Seat hardfacing removal can be performed by several methods. When selecting a removal process, issues such as machine tool availability, amount of material to be removed, material hardness, and radiological concerns should be considered. The methods most commonly used to remove bonnet backseats include: • Manually grinding out the seat hardfacing • Machining out the seat with a lathe • Machining out the seat with a portable boring bar Manual grinding is a time consuming process that also presents radiological concerns when the valve is contaminated. Grinding can be used successfully when machining equipment or qualified machinists are not available.
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Machining the seat out is quick, clean, and precise when performed by a qualified machinist on a stationary lathe or mill. Seats can be removed with a variety of commercially available boring bars, as well as some special bars developed by repair organizations. Several service companies have developed globe valve machining systems that mill out most of the material and then finish the job with a single point turning method. The machining sequence incorporates both hardfacing removal and weld preparation for the build up material and hardfacing. The repair procedure should explicitly detail the machining operation. It should include drawings of the prep configuration, with dimensions, of the weld prep for the buildup or butter layer, and the weld prep for the hardfacing deposit if a two-step method is selected. Because of dilution into the original base material when the hardfacing was deposited initially, as much as 1/8 inch to 3/16 inch (3.2 mm to 4.8 mm) of additional material has to be removed to get rid of the hardfacing material. This additional material removed must be replaced with buildup layers if minimum wall thickness is violated or seat support is reduced, or with a butter layer if the original base material dimensions must be re-established. Hardfacing removal is a time consuming operation. Because the material is extremely hard, slow travel speeds and feed rates are utilized. Tooling wears quickly and tool bits have a tendency to break, thus requiring frequent tool bit changeouts. This can be reduced by utilizing carbide tooling and very rigid machine tools. A liquid penetrant (PT) exam must be performed on the pressure-retaining base material after machining or prior to welding. An informational PT should be performed on the buildup or butter layer prior to depositing the hardfacing material. 8.7.3.3 Seat Ring Removal Bonnet backseat ring removal is performed by several methods. When selecting a removal process, issues such as machine tool availability, amount of material to be removed, material hardness, and radiological concerns should be considered. The methods most commonly used to remove bonnet backseat rings include: • Manually grinding out the seal weld • Machining out the seal weld with a lathe or portable boring bar • Machining out the complete seat ring with a lathe, mill, or portable boring bar • Machining of lock welds and unthreading the seat ring Grinding is a time consuming process that also presents radiation exposure and air quality concerns. Grinding can be used when machining equipment or qualified machinists are not available. Machining out the seal weld or complete seat ring is quick, clean, and precise when performed by a qualified machinist on a lathe or mill. Seat rings can be removed with a variety of commercially available portable boring bars, as well as some special bars developed by repair organizations.
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Extreme care should be taken to establish the cut location. The machine tool should be centered to the bore and aligned to the cut line to minimize material removal. Review the drawings to verify the seal weld depth. The seal welds are generally 3/16-inch to 1/4-inch (4.8-mm to 6.4-mm) deep. While machining, the machinist should look for a crack in the machined groove indicating that the seal weld has been penetrated. If the machined groove reaches a depth of 3/8 inch (9.5 mm) and a crack is not visible, a determination should be made about going deeper or adjusting the cut location. It is extremely important that the seat ring pocket depth is not increased by cutting into the base material because this will alter the seat ring reference. After severing and removing the ring, the valve body should be prepped to install the new ring.
8.7.4 Filler Material Selection Filler material selection is dictated by the purpose of the weldment, base material compatibility, process, and welding position, as presented in Section 11. As stated previously, seat replacement requires that some of the original base material be removed in order to remove the hardfacing material that was diluted for some depth into the original base material. Because hardfacing thickness should be minimized, the base material must be replaced to re-establish the base material dimensions prior to depositing the hardfacing material. In cases where minimum wall thickness has been encroached upon or even violated, the ASME Section XI requires that filler material matching the base material chemistry and properties be used (see Table 11-4). If the weld is being treated as a non-structural buildup or butter layer, a filler material more compatible with the hardfacing alloy can be selected. As an example, filler metal ASME SFA 5.9, ER309 (stainless steel) is often used on the P-No. 1 carbon steel substrates because of its similar coefficient of expansion to the cobalt- and iron-based hardfacing alloys. When a base metal repair or buildup is required, or if the weldment is for structural restoration (minimum wall), the selection must be based on the base material and operating temperatures. For structural restoration of all base materials and for high temperature applications on P-Nos. 4 and 5 substrates, a compatible filler material must be utilized (see Table 11-4). The less hardenable Alloy 21 or the NOREM 02A hardfacing materials should be selected for utilization on most globe-type seat applications. These materials are easier to apply over most substrates with very little preheat (see Section 13) and provide the properties necessary for impact and erosion applications. The galling wear properties of the very hard materials are unnecessary in backseat applications. For more information on hardfacing selection, refer to the EPRI Cobalt Reduction Guidelines [29].
8.7.5 Preheat and Post-Weld Heat Treatment Requirements As presented in Section 13, PWHT is not required for depositing hardfacing on P-Nos. 1 and 8 substrates, but is required for P-Nos. 4 and 5 substrates (see Table 11-1 for 8-71
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P-Number specifications). With the exception of P-No. 1 materials, the PWHT requirements are the same for the partial penetration seal welds and the repair or buildup of the base materials. The PWHT requirement can be eliminated on P-No. 1 substrates provided a 200˚F (93˚C) preheat is utilized. The requirements and exemptions are listed in Tables 13-2, 13-3, and 14-4. While not required, a minimum 200˚F to 400˚F (93˚C to 204˚C) preheat is essential when welding hardfacing materials. The lower preheats can be utilized when welding Alloy 21 or NOREM 02A on stainless substrates, and increasing to 450˚F to 550˚F (232˚C to 288˚C) for P-No. 5 substrates. Successful applications of the more hardenable hardfacing materials often require preheats in excess of 800˚F (427˚C). The suggested preheat temperature also varies based on the quality and technique of the welder, the quality of filler material, and the method of base metal fabrication.
8.7.6 Welding Repair Localized repair of bonnet backseat is performed with manual GTAW or SMAW processes. The GTAW process is preferred due to the welding control and quality desired for this small but challenging application. The SMAW process provides a higher deposition rate but compromises control. A comprehensive discussion of seat ring repair welding is presented in Section 8.3.6 and integral seat welding in Section 8.5.6. Integral bonnet backseat repair in which all of the hardfacing material is replaced can be performed quite efficiently with the manual GTAW or SMAW processes. Orbital GTAW machines, as presented in Appendix B, have also been utilized to build up the substrate and apply the hardfacing filler materials. A detailed discussion of integral seat replacement welding, which applies directly to this application, is presented in Section 8.6.6 Seat ring replacement of bonnet backseats is almost exclusively performed with the manual GTAW process. This process provides the precision bead placement and heat input controls that are necessary to control distortion to the ring. The drawback to the SMAW process is the need to protect the hardfaced surface from the weld spatter generated. A detailed installation sequence and welding discussion is provided in Section 8.4.6. If the SMAW process is selected, 3/32-inch (2.4-mm) diameter electrodes should be utilized for the first two layers to minimize heat input and distortion.
8.7.7 Final Machining The final machining step is critical to achieving minimal leakage past the seat. The primary goal of the final machining operation is to have a finished seat that is very accurately aligned with the stuffing box bore, is perpendicular to the bonnet face, and has a precise and consistent angle. The seat angle must be held to very strict manufacturer’s tolerances of less than 1/2 degree to ensure that the stem seat makes contact at the bottom of the bonnet backseat as shown on Figure 8-7b.
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Bonnet Backseat
Stem Seat
Stem
Figure 8-7b Proper Stem to Backseat Seating Configuration [13] Machining of the seat must be performed on a lathe or with a portable boring bar that has independent alignment capability. The data recorded prior to machining and welding, along with the manufacturer’s drawings, should be used to establish the target dimensions needed to locate the seat. After machining, the backseat should be lapped with the stem as shown on Figure 8-7c. The bonnet and stem should be placed upside down and two or three packing rings installed in the packing chamber. A very fine lapping compound should be placed between the mating surfaces and the stem gently rotated circularly around the backseat. Lapping should be minimized in order to maintain the sharp angle and concentricity resulting from the machining operation. The acceptable blue check will be a 360˚ fine line around the stem seat.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair Rotate Back & Forth
Stem Seat Lapping Compound Between Seats Stem Bonnet Packing Rings
Figure 8-7c Lapping Bonnet Backseat to Stem [16] On bolted bonnet-type valves that have a flange, a check should be made to see if the flange has distorted due to welding and stress relieving. If the flange has distorted, it must be machined back to the manufacturer’s tolerances and the requirements of ANSI/ASME B16.5, Steel Pipe Flanges and Flanged Fittings.
8.7.8 Inspection The ASME Section XI Boiler and Pressure Vessel Code and the ANSI/ASME B31.1 Power Piping Code address repair of the base material but do not provide repair and inspection requirements for the seat hardfacing because it is not defined as pressureretaining. ASME Section XI , Subarticle IWA-4400 requires that the repair welding be performed by qualified welders and procedures in accordance with Section IX. It also requires that the repair cavity (pressure retaining base material) shall be examined by liquid penetrant or magnetic particle method. As a matter of good practice, PT or MT inspection should be used after completing the hardfacing deposit and final machining. It is essential that the sealing surface of the hardfacing be free of any linear indications to ensure leak-tightness across the seat.
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8.7.9 Testing Testing shall be performed in accordance with plant design requirements. This could include a local leak rate test (LLRT), an integrated leak rate test (ILRT), or in some cases no leak rate test at all. Because the pressure boundary has not been penetrated, no hydrostatic test is required. 8.8 Guide Repair and Replacement This section presents the guidelines necessary to repair or replace the body wedge guides of gate valves. Valve manufacturers produce gate valves with two typical styles of wedge guides. Flex wedge, solid wedge, double disk, and parallel-slide gate valves use a single guide on each side of the wedge, which protrudes from the valve body as shown in Figure 8-8a. These guides are either cast with the valve body and machined to size, or are bar stock machined to size and welded to the valve body. Although rare, some valve manufacturers machine grooves in the valve body. The second type of guide style used by manufacturers is a pair of guides on each side of the valve, which protrude from the valve body forming a groove or slot for the wedge tabs to slide between (see Figure 8-8b). These parallel guides are used primarily in split wedge and slab gate-type valves.
Guide Rib
Figure 8-8a Typical Single Guide Configuration for Gate-Type Valves [16]
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Guide Slot
Guide Rib
Figure 8-8b Double-Guided Gate Configuration [17] Welding repairs to gate valve guides are rarely necessary. When buildup is required to tighten tolerances, the welding is generally performed on the wedge because it can be removed from the valve and machined and welded in the repair shop. The most common guide repairs are limited to grinding and machining to remove gouges on the mating surfaces, or to build up or replace guides due to erosion. Welded repairs to guides must be performed with the same practices and procedures as the pressure-retaining valve body (see Section 8.1). The lone exception is when the repair is restricted to the repair or build up of a guide that is welded to the valve body and the body is not a part of the repair. Like other valve repairs, skilled personnel with the proper training and equipment are necessary to be successful. The biggest challenge is making repairs on P-Nos. 4 and 5 valve bodies that require elevated preheats and PWHT (see Table 13-1). The valve body and integral guides are recognized as pressure-retaining materials. As a result, these components must be repaired in accordance with the rules of ASME and ANSI, and by an ASME or NBBI manufacturer or repair certificate holder. All welding must be performed by qualified welders and procedures in accordance with the rules of ASME Section IX.
8.8.1 Repair Strategy The objective of any type of guide repair is to provide a low friction, non-interfering path with lateral structural support for the wedge as it is directed into the seats. Guide location and clearances between the guide slot and the wedge are critical.
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A thorough understanding the valve guide design and the immediate problem must be understood prior to developing a repair strategy. The following questions must be answered: • Are the guides cast into the body? • Are the guides eroded or damaged on the contact surface requiring buildup, or are they distorted or bent? • Are the guides welded in bar stock that can be replaced if eroded or bent, or if the contact surface is cracked? If an attachment weld is cracked, what additional work is required? • Can the indication be removed by grinding without encroaching upon the design minimal wall, thus avoiding a weld repair? • Does the indication or crack go through the wall, requiring a system hydrostatic test after completion? • Will the repair encroach upon tolerance-sensitive guide contact surfaces, which would be altered by welding stresses and heat from the repair? • Is the repair area accessible through the bonnet bore for manual repair techniques? • Is machining and welding equipment available to perform the repair? • Is PWHT equipment available, along with knowledgeable personnel? • If the indication was found by UT or RT, are the equipment and personnel available to perform this inspection after the repair is completed? • Are sufficient qualified and experienced personnel available to perform a guide repair or replacement? • Are sufficient rigging points and space available to move equipment in and out of the valve? • What are the preheat and PWHT requirements for performing this replacement? If required, can the welding and machining equipment withstand these elevated temperatures? • Is a replacement valve available in the time frame necessary, without impacting plant availability? As a result of this review, the owner must decide whether to perform the repair with inhouse personnel, use contract valve repair services, replace the valve, or remove it from the line and send it to a shop for repair.
8.8.2 Repair Prerequisites The following should be considered prior to initiating a repair: • Record the manufacturer’s name, size, pressure rating, valve type, body style, body material, and trim material data from the valve nameplate.
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• Obtain drawings, material specifications, and clearance tolerances between the wedge and guides from the manufacturer. • Obtain and review the maintenance and repair history to see if there have been any changes to the original design. • Procure a replacement pressure seal ring and an oversize ring if the valve has a pressure seal bonnet. • Prepare a repair “traveler” or work instruction that outlines the repair sequence and inspection “hold points.” All measurements and findings should be recorded for a final report. • Match mark the wedge to the valve seat to ensure that the wedge is reinstalled in the same direction. • Install clean plugs in the upstream and downstream bores of the valve to prevent chips and foreign materials from getting down the line. • Install a protective liner over the pressure seal bore area and in-body seats. • Qualify the necessary welding procedures for base material and/or guide repairs, and for hardfacing repairs if the guide has hardfacing material on the contact surface. • If the repair requires unique welding or machining applications or if weld stresses might have adverse effects on alignment, the full repair sequence should be demonstrated on a mockup. This includes both machining and welding operations. • Measure and record critical dimensions, such as guide bores, distances between seats, pressure seal bores, etc., to monitor distortion due to welding and PWHT.
8.8.3 Flaw Removal If a small localized area is going to be repaired due to erosion, stress cracks across the hardfacing, or a deep wear scar, the defect should be removed by manual grinding as discussed in Section 9.1. The finished repair cavity should have a 3:1 taper into the surrounding area to provide access and visibility to the welder. A liquid penetrant (PT) examination must be performed to verify that no cracks or indications exist in the area to be repair welded. If a welded-in guide is to be replaced, it can be removed by grinding out the fillet welds that attach it to the valve body, or it can be milled out with a portable milling machine. A PT or MT examination must be performed to verify that no cracks or indications exist in the area to be repair welded. In those valves that have hardfacing applied to the contact surfaces, the hardfacing can be removed with a portable milling machine. After machining, the surface should be etched to verify that the hardfacing material has been removed and then PT or MT inspected to verify base material integrity.
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8.8.4 Filler Material Selection Please refer to Section 8.7.4.
8.8.5 Preheat and Post-Weld Heat Treatment Requirements Please refer to Section 8.1.5.
8.8.6 Welding Repair Repair welding of gate valve guides can be performed with the GTAW, SMAW, GMAW, or FCAW processes The process should be selected based on accessibility and the application (repair, replacement, or refurbishment). The various processes are described in Section 10. The GTAW process is generally used for localized repairs or refurbishment of integral guides where size permits and where radiation levels require remote welding. Replacement of welded-in guides, localized repairs in small or confined areas, or configurations that are not readily achievable by automated systems are normally repaired with the SMAW process. Large repairs, material refurbishment, or guide replacement can be performed with semi-automatic GMAW processes. Figure 10-1 presents a productivity comparison of the various welding processes. Guide repairs and refurbishment should be completed with the following steps: • After excavating the repair cavity, the repair area should be thoroughly cleaned and inspected using either the MT or PT examination method. The flaw must be totally removed from the repair cavity. • Preheat the valve body or localized area as described in Section 13. • Perform the repair weld with the selected process, monitoring frequently for distortion. • Welding should be performed in a manner that aims to control residual stresses. • Cap out the weld approximately 1/16 inch to 1/8 inch (1.6 mm to 3.2 mm) above the parent metal surface as checked with a straight edge. Do not add excessive layers because this contributes to higher residual stresses. • If required, perform a post-weld heat treatment using the guidelines established in Section 13. • Cool to the ambient temperature. • Perform a MT or PT examination of the repair area. For replacement of welded-in guides, the following sequence is recommended: • After excavating the repair cavity, the repair area should be thoroughly cleaned and inspected using either the MT or PT examination method. The flaw must be totally removed from the repair cavity. • Preheat the valve body or localized area as described in Section 13. • Install the wedge into the valve body. Ensure proper orientation and seating contact. 8-79
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• Fit the replacement guides into the wedge guide slots and shim them equally with the clearances specified by the valve manufacturer. Tack weld the guides to the valve body. • Remove the wedge from the valve body, verifying clearances and alignment during the removal operation. Refit as necessary. • Install protective covers on the seats and pressure seal bore. • Perform installation welding, monitoring frequently for distortion. • If required, perform a post-weld heat treatment using the guidelines established in Section 13. • Cool to the ambient temperature. • Perform a MT or PT examination of the repair area.
8.8.7 Final Machining Final machining of the guides must be coordinated with the repair or refurbishment effort on the wedge to ensure suggested manufacturing dimensions and tolerances between the guides and guide slots. For localized repairs or replacement of welded-in guides, the repair or attachment fillet welds can be ground and blended to the required configuration. For larger repairs or restoration of integral guides, a milling machine should be used to establish consistent dimensions. In many cases, the edges of the guides are gound with a radius to reduce the possibility of the wedge getting hung on a sharp corner.
8.8.8 Inspection The ASME Section XI Boiler and Pressure Vessel Code and the ANSI/ASME B31.1 Power Piping Code provide guidance for the repair and inspection of valve materials defined as pressure-retaining. ASME Section XI, Subarticle IWA-4400 requires that the repair welding be performed by qualified welders and procedures in accordance with Section IX. It also requires that the repair cavity (pressure-retaining base material) shall be examined by liquid penetrant or magnetic particle method. A final examination of the finished repair must be performed and documented utilizing the inspection process that was used to find the indication originally.
8.8.9 Testing Pre-service testing shall be performed in accordance with the requirements of ASME/ ANSI OM, Part 10, when the owner determines that the repair activity effects the valve’s performance or designed function. This could include a local leak rate test (LLRT), an integrated leak rate test (ILRT), or a operability/functional test. If the pressure boundary has been penetrated, a hydrostatic test is required.
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8.9 Guide Rib Repair This section presents the guidelines necessary to repair hardfaced guide ribs in large Ytype, and critical application globe valve configurations (see Figure 8-9a). Hardfaced guide ribs are not necessary in most globe valves because the poppet is directed straight down the body bore and the flow is from under the poppet. Globe valves designed for flow over the seat or with side loads against the poppet often have hardfaced guide ribs.
Guide Rib
Figure 8-9a Globe Valve Configuration Showing Guide Rib [19] Guide ribs are used to reduce body wear from the poppet moving up and down, and to reduce the friction coefficient of heavy poppets in large Y-type valves such as lift checks and MSIVs. In Y-type designs, the ribs are built up above the bore, and positioned to carry the load of the poppet. Welding repairs to globe valve guides are rarely necessary. When buildup is required to tighten tolerances, the welding is generally performed on the bottom rib (when looking down the valve bore) because it carries the majority of the load. The most common guide repairs are limited to grinding and machining to remove gouges on the surfaces. In those valves that have a rib in the flow path, buildup of the base material and hardfacing on the edges of the rib are also common due to erosion. Welded repairs to guide ribs must be performed with the same practices and procedures as the integral seat repair (see Section 8.5). The single exception is when the repair is restricted to the repair or build up of a guide rib that has a remaining hardfacing or corrosion-resistant cladding thickness of 1/8 inch (3.2 mm) or greater. Like other valve repairs, skilled personnel with the proper training and equipment are necessary to be successful. The biggest challenge is making repairs to guide ribs that have a P-Nos. 4 and 5 substrate (see Table 11-1). These substrate materials require elevated preheats and PWHT temperatures as shown in Table 13-1. 8-81
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Because the hardfacing alloy is welded directly to the pressure-retaining body, the guide ribs must be repaired in accordance with the rules of ASME and ANSI and by an ASME or NBBI manufacturer or repair certificate holder. All welding must be performed by qualified welders and procedures in accordance with the rules of ASME Section IX.
8.9.1 Repair Strategy Developing a repair strategy with backup positions is essential prior to committing to a guide rib repair. Due to the inherent high residual stresses and low ductility of the hardfacing materials, a small pit can become long multiple cracks when grinding or welding is initiated. The type of hardfacing on the guide rib, as well as the base material, must also be considered. The less forgiving hardfacing materials, such as those commonly referred to as Stellite 1 (ASME SFA 5.13, RCoCr-C), 6 (ASME SFA 5.13, RCoCr-A), and 12 (ASME SFA 5.13, RCoCr-B), and the nickel-based alloys such as Colmony 4 and 5, and Deloro 40 and 50 (ASME SFA 5.13, RNiCr-A and B) are extremely difficult to repair. Although they are more welder friendly, repair with Stellite 21 (cobalt-base) and NOREM (iron-base) can be challenging to repair. If the base material is a P-No. 4 or P-No. 5 and the hardfacing is thin, a preheat and post-weld heat treatment are required. The following questions should be asked when faced with a guide rib repair: • Can the flaw be removed by grinding without impacting the contact area, thus avoiding a weld repair? • Can the flaws be removed by machining a thin layer off the complete guide rib surface and still maintain the required bore dimensions and hardfacing thickness, thus avoiding a weld repair? • Are the guide ribs eroded or damaged on the contact surface requiring buildup, or are they distorted or bent? • Are the guide ribs welded in bar stock that can be replaced if eroded or bent, or if the hardfacing is cracked? If an attachment weld is cracked, what additional work is required? • Does the crack or defect penetrate into the valve body such that a welded repair of the pressure boundary material would be required? • Does the indication or crack go through the wall, requiring a system hydrostatic test after completion? • Is machining and welding equipment available to perform the repair? • Are sufficient rigging points and space available to move equipment in and out of the valve? • Is the existing hardfacing material repairable without the use of elevated preheat temperatures? Can the welding and machining equipment withstand these elevated temperatures? • Are sufficient qualified and experienced personnel available to perform a guide repair or replacement? 8-82
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
8.9.2 Repair Prerequisites The following should be considered prior to initiating a repair: • Record the manufacturer’s name, size, pressure rating, valve type, body style, body material, and trim material data from the valve nameplate. • Obtain drawings, material specifications, and clearance tolerances between the poppet and guide ribs from the manufacturer. • Obtain and review the maintenance and repair history to see if there have been any changes to the original design. • Procure a replacement pressure seal ring and an oversize ring if the valve has a pressure seal bonnet. • Prepare a repair “traveler” or work instruction that outlines the repair sequence and inspection “hold points.” All measurements and findings should be recorded for a final report. • Install clean plugs in the upstream and downstream bores of the valve to prevent chips and foreign materials from getting down the line. • Install a protective liner over the pressure seal bore area and in-body seats. • Qualify the necessary welding procedures for base material repair and hardfacing repairs. • If the repair requires unique welding or machining applications, such as a guide rib which is in the flow path or on spiraling rib designs, the full repair sequence should be demonstrated on a mockup. This includes both machining and welding operations. • Measure and record critical dimensions, such as guide bore and pressure seal bore diameters, to monitor distortion due to welding and PWHT.
8.9.3 Flaw Removal The flaw should be removed by grinding or machining. If grinding is being utilized to remove a crack, liquid penetrant should be applied to the area to help identify the crack location. After completely removing the crack, the area should be smoothed and tapered to provide good welding visibility and access. If the flaw distribution is extensive and a decision is made to replace all of the hardfacing on a rib or ribs, a boring bar or milling rail should be used to remove the hardfacing material. Boring bars are available with adapter kits that will machine designated segments of a complete diameter. A 2:1 or 3:1 transition should be employed at the upper and lower end of the weld preparation groove. After completing the grinding or machining operation, an MT or PT examination of the weld preparation area must be performed to ensure that no unacceptable indications remain prior to welding.
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8.9.4 Filler Material Selection Filler material selection is dictated by the purpose of the weldment, base material compatibility, process, and welding position, as presented in Section 11. Replacement of the guide rib hardfacing requires that some of the original base material be removed in order to remove the hardfacing material that was diluted for some depth into the original base material. Because hardfacing thickness should be minimized, the base material must be replaced to re-establish the base material dimensions prior to depositing the hardfacing material. In cases where minimum wall thickness has been encroached upon or even violated, ASME Section XI requires that filler material matching the base material chemistry and properties be used (see Table 11-4). If the weld is being treated as a non-structural buildup or butter layer, a filler material more compatible with the hardfacing alloy can be selected. As an example, filler metal ASME SFA 5.9, ER309 (stainless steel) is often used on the P-No. 1 carbon steel substrates because of its similar coefficient of expansion to the cobalt- and iron-based hardfacing alloys. When a base metal repair or build up is required, or if the weldment is for structural restoration (minimum wall), the selection must be based on the base material and operating temperatures. For structural restoration of all base materials and for high temperature applications on P-Nos. 4 and 5 substrates, a compatible filler material must be utilized (see Table 11-4). The less hardenable Alloy 21 or the NOREM 02A hardfacing materials should be selected for utilization on most globe-type seat applications. These materials are easier to apply over most substrates with very little preheat (see Section 13) and provide the properties necessary for impact and erosion applications. The galling wear properties of the very hard materials are unnecessary in backseat applications. For more information on hardfacing selection, refer to the EPRI Cobalt Reduction Guidelines [29].
8.9.5 Preheat and Post-Weld Heat Treatment Requirements As presented in Section 13, PWHT is not required for depositing hardfacing on P-Nos. 1 and 8 substrates, but is required for P-Nos. 4 and 5 substrates (see Table 11-1 for PNumber specifications). With the exception of P-No. 1 materials, the PWHT requirements are the same for the partial penetration seal welds and the repair or build up of the base materials. The PWHT requirement can be eliminated on P-No. 1 substrates provided a 200˚F (93˚C) preheat is utilized. The requirements and exemptions are listed in Tables 13-2, 13-3, and 14-4. While not required, a minimum 200˚F to 400˚F (93˚C to 204˚C) preheat is essential when welding hardfacing materials. The lower preheats can be utilized when welding Alloy 21 or NOREM 02A on stainless substrates, and increasing to 450˚F to 550˚F (232˚C to 288˚C) for P-No. 5 substrates. Successful applications of the more hardenable hardfacing
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materials often require preheats in excess of 800˚F (427˚C). The suggested preheat temperature also varies based on the quality and technique of the welder, the quality of filler material, and the method of base metal fabrication.
8.9.6 Welding Repair Welding a guide rib in globe-type valves is generally performed with the machine GTAW process and the manual SMAW process. The most common applications have been the large Y-type main steam isolation valves (MSIV) (see Figure 8-9b) and feedwater check valves. The manual GTAW process can also be utilized in applications where the valve is large enough and the seat close enough to the valve opening to allow the welder to get both hands (arms) into the valve. Suggested welding practices are presented in Section 14.1. Body Flange
Pressure Seal Area Bonnet Bore
Guide Ribs
Seat Hardfacing
Seat Ring
Flow Bore
Figure 8-9b Y-Type MSIV [13] The shielded metal arc welding (SMAW) process with coated electrodes has been successfully used on large and small valves where radiation exposure is not a concern. A drawback to the SMAW process is that it produces spatter and compromises welding 8-85
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control; however, both can be overcome with good practices. In some cases, the position or weldment configuration dictates the SMAW process as the only viable repair process. When welding on large valves or valves where a significant quantity of material was removed to eliminate the original hardfacing material, a two-step welding application is recommended. The base metal should be restored to its original thickness by building up the area with a matching filler material or butter layer (see Figure 8-9c). The buildup can then be machined with the desired weld prep or cavity to ensure that the hardfacing deposit has a uniform thickness and does not exceed approximately 3/16 inch (4.8 mm). The hardfacing is then deposited as described in Section 14. This methodology, similar to that used by the original manufacturer, ensures the proper location and thickness of the new guide rib wear surface, as well as ensuring a quality hardfacing deposit. After completing the hardfacing weldment, it should be wrapped in insulation to allow for slow cooling.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair Original Base Metal (Prior To Welding) Hardfacing Dilution
Hardfacing Deposit
New Machined Surface
Typical Guide Rib 1
2 Build-Up Material
New Machined Weld Prep Machined Surface
Build-Up Fusion Line 4
3 Machined Weld Prep
Hardfacing Deposit
Finished Machined Hardfacing
Hardfacing Fusion Line
Hardfacing
Build-Up Fusion Line 5
6
Figure 8-9c Guide Rib Hardfacing Refurbishment Sequence
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8.9.7 Final Machining Final machining of the guide ribs must be coordinated with the repair or refurbishment effort on the poppet wear rings to ensure the suggested manufacturing dimensions and tolerances between the components. For localized repairs, the repair welds can be ground and blended to the required configuration. For larger repairs or restoration of integral guide ribs, a boring bar should be utilized to establish consistent dimensions parallel to the bore and seat. This ensures proper seating of the poppet into the seat. After machining, the hardfaced contact surface should be ground or polished to remove any major tool marks.
8.9.8 Inspection The ASME Section XI Boiler and Pressure Vessel Code and the ANSI/ASME B31.1 Power Piping Code provide guidance for the repair and inspection of valve materials defined as pressure-retaining. ASME Section XI, Subarticle IWA-4400 requires that the repair welding be performed by qualified welders and procedures in accordance with Section IX. It also requires that the repair cavity (pressure-retaining base material) shall be examined by liquid penetrant or magnetic particle method. A final examination of the finished repair must be performed and documented utilizing the inspection process that was used to find the indication originally.
8.9.9 Testing Pre-service testing shall be performed in accordance with the requirements of ASME/ ANSI OM, Part 10, when the owner determines that the repair activity effects the valve’s performance or designed function. This could include a local leak rate test (LLRT), an integrated leak rate test (ILRT), or an operability/functional test. If the pressure boundary has been penetrated, a hydrostatic test is required. 8.10
Hinge Pin Repair
This section focuses on the repair of the hinge pin body bore penetrations, hinge pin cover pressure seal bores, and gasket seal faces of swing check and tilting disc check valves (see Figure 8-10a). Repair of removable parts such as brackets, bushings, hinge pin covers, and hinge pins should be repaired in the machine shop or the parts replaced with original manufacturer’s equipment.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair Bonnet Stops Lifting Lugs
Internal Disc Hanger
Weld Ends
Body
Seat Ring
Disc Stop
Hinge Pin
Disc
Arm Back Stop
Disc Assembly
Normal Flow
Seat
Figure 8-10a Swing Check and Tilting Disc Check Valves [9] Manufacturers provide swing check valves with three typical hinge pin designs. In the first design (see Figure 8-10b), the pin is supported in journals that are integrally cast in the body neck above the seat ring. One journal is drilled through the body wall to permit insertion of the pin and sealed with one of three gasket and flange configurations. A modified version of this design incorporates a single hinge pin passing through both journals and having a flange on both ends. Repair of journal bores and gasket sealing face could be performed in situ. The second design consists of a bracket
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mounted on two pads located above the seat ring (see Figure 8-10c). No in situ hinge pin repair is necessary for this design. The third design also requires no in situ repair of the hinge pin assembly. In this design, the pin is supported in a bracket attached to the bonnet (see Figure 8-10d). Cover Plate
Pin
Hanger
Disc Nut
Journal
Bonnet Flange
Disc
Figure 8-10b Swing Check Valve, Pin Supported in Cast Journals [21]
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Bracket Pinned Bracket Bolt Bracket Pad Disc
Figure 8-10c Swing Check Valve, Bracket Mounted on Two Pads [21] Bonnet Cover
Bracket Disc Hangar Disc
Figure 8-10d Swing Check Valve, Pin Supported in Bracket Attached to Bonnet [21] 8-91
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The hinge pin design is quite similar for all the manufacturers of tilting disc check valves. Hinge pins and covers are installed on either side of the valve and inserted into the bushings of the disc assembly (see Figure 8-10e). Repair of the hinge pin body penetration bore and gasket sealing face can be repaired in situ. Hinge Pin
Hinge Pin
Hinge Pin Cover Bolt
Disc
Hinge Pin Bushing
Section A-A
Disc
A
A
Figure 8-10e Hinge Pins and Covers Inserted into Disc Assembly Bushings [13] Three types of hinge pin cover joints are provided by check valve manufacturers. They include a pipe plug seal cover (see Figure 8-10f), a blind flange cover (see Figure 8-10g), and a pressure seal cover as shown in Figure 8-10h.
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Spiral Wound Gasket
Body Hinge Pin
Figure 8-10f Pipe Plug Seal Cover [16]
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair Hinge Pin Assembly Hinge Pin
Cover Spiral Wound Gasket
Bushing
Disc
Body
Hinge Pin
Figure 8-10g Blind Flange Cover [16]
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair Hinge Pin Hinge Pin Bonnet
Metal Gasket
Hinge Pin Cover
Nut
Stud Hinge Pin (Alternate Design)
Figure 8-10h Pressure Seal Cover [16] Because of the critical alignment needed for the disc to seal against the seat, repair of hinge pin bores and gasket surface requires extremely knowledgeable and experienced personnel. A repair should not be attempted without good drawings with dimensions and tolerances. Repairs to tilting disc check valves (TDC) are extremely difficult and should not be attempted without the manufacturer’s support. Where possible, critical body bores, pressure seal bores, and bushings should be ground or honed and a new hinge pin machined to size, instead of performing a welded repair. Small automatic GTAW systems and line boring equipment is commercially available to perform bore and gasket sealing face repairs. Accessories necessary to machine threads are also available as attachments to boring bars. Once again, this equipment must be used by qualified technicians to successfully complete these repairs. The valve body and hinge pin covers are recognized as pressure-retaining materials. As a result these components must be repaired in accordance with the rules of ASME and ANSI and by an ASME or NBBI manufacturer or repair certificate holders. All welding must be performed by qualified welders and procedures in accordance with the rules of ASME Section IX.
8.10.1 Repair Strategy Two specific repairs typical to hinge pins are addressed here: leakage at the hinge pin cover and excessive clearances in the body bores due to wear. A brief discussion of hinge pin repair is also presented. Leakage of the hinge pin cover can be the result of insufficient load on the gasket, corrosion, wire drawing or steam cutting, foreign material between the sealing surface and the gasket, and damage to the gasket. In the case of a pressure seal gasket configuration,
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surface imperfections in the body bore, such as pin holes, cracks, or erosion, could be the cause for a leak. Of all the possibilities, only surface imperfections such as cracks, erosion, or pitting would be cause for repair. The other issues can be resolved with good maintenance and assembly practices. In cases where the pressure seal bore is damaged, refer to Section 8.2 for specific guidance. Seating misalignment and play in the hinge pin can be the result of wear to the hinge pin, bushings, or the body bores. A thorough inspection should be made to specifically identify the out-of-tolerance component and, where possible, repair it with non-welding and machining practices. If a hinge pin bore requires repair, the following questions should be asked in developing a repair strategy: • Are detailed drawings and a manufacturer’s representative available for consultation? This is extremely important when repairing tilting disc check valves. • Is the flaw or crack localized such that it could be removed by grinding? If so, machining might not be required. • Is the bore or bushing worn out of round such that a new or oversized bushing could be installed to resolve the problem? If there is a bushing already in place, is it machined on center or was it machined after it was placed in the valve? • Is the flaw or crack deep enough that weld repair would cause distortion to the valve body bore? If so, a boring bar is required. • Will removal of the crack or flaw violate minimum design wall thickness? If so, can a qualified base metal repair be performed? The base metal repair should be performed as presented in Section 8.1. • Is machining and welding equipment available to perform this repair? • Are sufficient qualified and experienced personnel available to perform this repair? • Does the owner or repair vendor possess the proper NBIC repair certifications and ASME Section IX qualified welding procedures? • Are sufficient rigging points and space available to move equipment in and out of the valve? • Is equipment available to preheat the repair area? If required, can the welding and machining equipment withstand elevated temperatures? • Is a replacement valve available in the time frame necessary, without impacting plant availability? When considering the repair of a gasket seal area other than pressure seal-type joints, the following additional questions should be asked: • Can the flaw or defect be removed by grinding or machining? If so, will a thicker gasket or spacer be required for the hinge pin to eliminate the axial movement in the pin? • Is the flaw or crack deep enough that weld repair would cause distortion to the valve body bore? If so, a flange facer and boring bar is required. 8-96
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When considering a hinge pin welded repair, refer to Section 8.13 and follow the guidelines presented there.
8.10.2 Repair Prerequisites The following should be considered prior to initiating a repair: • Record the manufacturer’s name, size, pressure rating, valve type, body style, body material, and trim material data from the valve nameplate. • Obtain drawings and material specifications from the manufacturer. • Obtain the original design minimum wall thickness from the manufacturer. • Obtain and review the maintenance and repair history to see if there have been any changes to the original design. • Procure a replacement pressure seal ring and an oversize ring for the valve bonnet and hinge bores if the valve is so equipped. • Prepare a repair “traveler” or work instruction that outlines the repair sequence and inspection “hold points.” All measurements and findings should be recorded for a final report. • Clearly identify both sides of the hinge pin bores and all parts associated with each side. • Install clean plugs in the upstream and downstream bores of the valve to prevent chips and foreign materials from getting down the line. • Install a protective liner over the in-body seats. • Qualify the necessary welding procedures for hardfacing and base material buildup and repairs. • Demonstrate the machining and welding sequence on a full scale mockup to familiarize personnel with the work sequence and requirements. • Measure and record critical dimensions such as hinge pin bore diameters and length, and distances between hinge pin bores, to monitor distortion due to welding. • Verify seating alignment prior to performing any final boring or facing operations.
8.10.3 Flaw Removal The flaw should be removed by grinding or machining. If grinding is being utilized to remove a crack, liquid penetrant should be applied to the area to help identify the crack location. After completely removing the crack, the area should be smoothed and tapered to provide good welding visibility and access.
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If the flaw distribution is extensive, the bore is worn out of round, or orbital welding equipment is going to be utilized for welding, a boring bar should be used to remove material about the complete bore area as shown in Figure 8-2b. A 2:1 or 3:1 transition should be employed at the upper and lower end of the weld preparation groove. After completing the grinding or machining operation, an MT or PT examination of the weld preparation area must be performed to ensure that no unacceptable indications remain prior to welding.
8.10.4 Filler Material Selection Filler material selection is dictated by base material compatibility, process selection, and position as presented in Section 11. The filler material should match the base material chemistry with properties comparable to the base material. See Table 11-4 for guidance in selecting the proper filler materials.
8.10.5 Preheat and Post-Weld Heat Treatment Requirements Preheating and PWHT should be performed as presented in Section 13. Preheating provides very positive benefits to the welding application and final product. Preheating is employed to reduce the tendency for cold cracking (or hydrogen-assisted cracking), to reduce the hardening of the heat-affected zone (HAZ), to reduce the residual stresses, and to decrease component distortion. Although preheat is not a regulatory requirement, the benefits far outweigh the potential problems of not using it as shown in Section 13.1. When welding on low-carbon steel P-No. 1 materials greater than 1-1/2 inches (38-mm) thick, a PWHT exemption can be gained by utilizing a 200˚F (93˚C) preheat. When welding on low-alloy steels such as the P-No. 4 (WC6) and P-No. 5 (WC9), 300˚F and 400˚F (149˚C to 204˚C) respective preheats are suggested. A minimum preheat of 100˚F (38˚C) is suggested for welding on P-No. 8 (CF3 and CF8) substrates. When welding in confined locations, the recommended preheats might not be conducive to productivity. In these cases, the preheat should be lowered to a workable level. Unless previously qualified at the lower temperatures, a new welding procedure is required for the lower preheat. Castings that have been inservice might have water in the defect and surrounding area. This water/moisture needs to be driven off with local heating to eliminate the potential for porosity. This localized preheating can be performed with an oxyacetylene torch. Post-weld heat treating is required for all weld repairs performed on P4 and P5 substrates (see Table 11-1). As mentioned earlier, exemptions to PWHT are available for P1 materials based on the thickness and preheat temperatures as provided in Table 13-4.
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8.10.6 Welding Repair Hinge pin, body bore, and gasket seal face welding repair can be performed with the GTAW, SMAW, GMAW, or FCAW processes The process should be selected based on accessibility and the quality of the weldment required. The various processes are described in Section 10. Repair welding of the hinge pin bores is generally performed with the machine GTAW process (see Figure 8-3n) due to the difficult welding positions encountered and the lack of accessibility. Machine GTAW also offers superior welding controls for bead placement and heat input. Very localized repairs or configurations that are not readily achievable by automated systems are normally repaired with the SMAW process. Repairs to flanges can be performed with any of the processes and should be selected based on the amount of material to be deposited. Figure 10-1 presents a productivity comparison of the various welding processes. Hinge pin body bores and flange repairs should be completed with the following steps: • After excavating the repair cavity, the repair area should be thoroughly cleaned and inspected using either the MT or PT examination method. The flaw must be totally removed from the repair cavity. • Preheat the valve body or localized area as described in Section 13. • Perform the repair weld with the selected process, monitoring frequently for distortion. • If required, perform a post-weld heat treatment using the guidelines established in Section 13. • Cool to the ambient temperature.
8.10.7 Final Machining Final bore diameters, shoulder depths, and location are extremely critical to obtaining the hinge pin location that results in proper seat alignment. Final machining must be performed in conjunction with seat fitting. The extent of the weld repair dictates the type of machining to be performed. In the case of small localized repair, the controlled grinding approach can be utilized. A hand grinder and a thin template cut from shim stock of .010-inch (.25 mm) thickness or less, having the same radius as the body bore diameter, can be used to reduce the weld buildup to the height of the shim stock above the bore. A flapper wheel can then be used to blend the weld buildup to just below the body bore. A straight edge and feeler gauges can ensure that the repair area is no more than .010 inch (.25 mm) below the surrounding material. A hone can be used to finish the bore and blend the repair. Where major welding has been performed, unmachined bushings installed, or where the welding has impacted critical dimensions, a boring bar with flange facing capability similar to that shown in Figure 8-10i should be used. Regardless of the machining technique, a Sunnen hone or the equivalent should be used. 8-99
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Figure 8-10i Boring Bar with Flange Facing Capabilities [Courtesy of Climax Portable Machine Tools]
8.10.8 Inspection The ASME Section XI Boiler and Pressure Vessel Code and the ANSI/ASME B31.1 Power Piping Code provide guidance for the repair and inspection of valve materials defined as pressure-retaining. ASME Section XI, Subarticle IWA-4400 requires that the repair welding be performed by qualified welders and procedures in accordance with Section IX. It also requires that the repair cavity (pressure-retaining base material) shall be examined by liquid penetrant or magnetic particle method. A final PT must also be performed after final machining.
8.10.9 Testing Pre-service testing shall be performed in accordance with the requirements of ASME/ ANSI OM, Part 10, when the owner determines that the repair activity affects the valve’s performance or designed function. This could include a local leak rate test (LLRT), an integrated leak rate test (ILRT), or a operability/functional test. If the pressure boundary has been penetrated, such that a full thickness weld repair was performed, a hydrostatic test is required. 8.11
Wedge/Disc Repair (Gate, Swing Check, and TDC)
This section focuses on the base material and hardfacing repair or replacement of removable valve discs or wedges. See Figures 8-11a and 8-11b. These repairs are performed in conjunction with the seat ring repairs and replacements discussed in Sections 8.3 and 8.4.
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Dia. C
Dim. A
Dim. B
Figure 8-11a Typical Wedge [16]
Hangar
Disc
Disc Nut
Figure 8-11b Removable Valve Disc/Wedges [21]
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Discs and wedges are found in most gate, swing check, and tilting disc check valves in which the seat is 90˚ to the bonnet bore. Most repairs performed on discs and wedges are the result of cracks or wear in the hardfacing materials. Left unrepaired, erosion will eventually penetrate completely through the wedge and into the base material. Repairs to discs and wedges are performed in a machine shop with the proper tools and knowledgeable personnel. In many cases, replacement of the disc or wedge is far more cost effective than performing a repair. This especially true of discs and wedges in valves smaller than 8 inches (203 mm) nominal and a pressure Class of less than 600 pounds. Repairs to discs are also much easier than wedges because machining and welding are performed on one side of the component.
8.11.1 Repair Strategy An evaluation of valve size and pressure class, available replacement parts, and personnel should be performed prior to committing to a disc or wedge repair. Welding and final machining is difficult and most challenging to even the most experienced valve personnel. Minimal tolerances are often employed, making material removal or distortion from welding very critical. In many cases, valve disc replacement is a more costeffective repair option. Unless the disc has an abundant amount of hardfacing on it and a flaw or crack is very small, the existing hardfacing material should be completely removed and rewelded. When hardfacing materials such as Stellite 1, 6, and 12 or the nickel- and iron-based alloys are encountered, they should always be completely removed and rewelded. When evaluating the repair/replacement option, the following questions should be asked: • What is the hardfacing material? • What is the nominal valve size and pressure class? • Can the flaw or crack be removed by grinding without impacting the sealing area, thus avoiding a weld repair? • If the flaw or crack is in the sealing area, can it be removed by surface grinding and maintain adequate thickness to permit proper seating? This is most critical for wedges. • Does the crack or defect penetrate into the wedge or disc base material such that a welded repair activity could significantly deform the component? If so, the forged or cast component wedge should be replaced. • Is the disc or wedge being built up or adjusted to compensate for the material loss of the inbody seat or seat ring? Disc or wedge repair is much easier than seat repair. A report detailing critical seat dimensions must be taken prior to performing a base metal buildup and hardfacing deposit on the disc or wedge. • Is there machining, special fixturing, measuring, and welding equipment available to perform the repair?
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• Does the owner or repair vendor posses the proper NBIC repair certifications and ASME Section IX qualified welding procedures? • Is preheat and post-weld heat treatment equipment available? • Are trained and qualified valve specialists familiar with the welding of hardfacing and fitting of discs or wedges available? If proven specialists are not available, this repair should not be performed.
8.11.2 Repair Prerequisites 8.11.2.1 Generic Prerequisites The following should be considered prior to initiating a repair: • Record the manufacturer’s name, size, pressure rating, valve type, body style, body material, and trim material data from the valve nameplate. • Obtain drawings and material specifications from the manufacturer. • Obtain and review the maintenance and repair history to see if there have been any changes to the original design. • Procure a replacement pressure seal ring and an oversize ring if the valve has a pressure seal bonnet. • Prepare a repair “traveler” or work instruction that outlines the repair sequence and inspection “hold points.” All measurements and findings should be recorded for a final report. 8.11.2.2 Gate Valve Specific Prerequisites The following should be considered prior to initiating a repair: • Establish match marks on both sides of the wedge with respect to the matching seat, prior to removing the wedge from the valve body. This ensures that the wedge is reinstalled in the same direction. • Perform a blue check (see Section 15.3) and record the seating location on the wedge as shown in Figure 8-3b. • Measure and record the wedge dimensions as shown in Figure 8-3c. • Measure and record the minimum distances between the valve seats at the locations shown in Figure 8-3d. • Verify that the seat width is acceptable on both seats as shown in Figure 8-3e. • If the in-body seats were reworked or replaced, measure and record the dimensions between the seats at the locations shown in Figure 8-3e. These dimensions must be taken to perform the rough machining and fitting of the wedge.
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8.11.2.3 Swing Check Valve Specific Prerequisites The following should be considered prior to initiating a repair: • Perform a blue check and record the seating location of the disc. • Measure and record the seat width as shown on Figure 8-3f. • Mark each of the hinge pin components in relation to each side so that the assembly can be reassembled correctly. • Final machining of the disc is performed after all in-body seat repair or replacement is completed. 8.11.2.4 Tilting Disc Check Valve Specific Prerequisites The following should be considered prior to initiating a repair: • Remove the disc assembly and record any damage. Particular attention should be given to the seat, hinge pins, and hinge pin bushings. Mark each of the hinge pin components in relation to each side so that the assembly can be reassembled correctly. • Perform a blue check and record the seating location on the disc as shown in Figure 8-3g. • Record the seat bore inside diameter, outside seat diameter, and depth as shown in Figure 8-11c. If the in-body seat was repaired or replaced, these dimensions need to be recorded after final seat finishing. These dimensions are used to machine the final seat dimensions on the disc.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair Inner Dia. “A” Outer Dia. “B” Seat Depth “C”
θ θ = arctan [(B - A)/2]C
Figure 8-11c Seat Angle Determinations for Tilting Disc Check Valve
8.11.3 Flaw Removal Prior to removing any hardfacing material, the wedge or disc should be mounted in a fixture similar to that shown in Figure 8-11d and the machine tool squared to the
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existing face. An index surface should then be machined on the casting or forging inside the hardsurfacing material to use for future indexing operations.
Wedge Hardfacing
Mounting Clamp Top View
Lathe Adaptor Plate
Side View
Figure 8-11d Wedge or Disc Mounted in Fixture [26] The method of flaw removal is based on the number of flaws, their distribution, and the overall amount of material to be removed. If a small localized area is going to be 8-106
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repaired due to a corrosion pit, stress crack, or a deep wear scar, the defect should be removed by manual grinding as discussed in Section 9. In applications where the repair to the hardfacing is extensive, all of the hardfacing should be removed from the face of the disc by machining on a lathe or boring mill. If the flaw propagates into the base material, a manual grinding method should be employed to remove the remaining flaw leaving a 3:1 tapered weld preparation cavity (see Section 9.1). A verification should then be performed with an etchant to verify complete removal of all hardfacing material, and a liquid penetrant examination performed to verify that no cracks or defects exist in the area to be repair welded.
8.11.4 Filler Material Selection Filler material selection is dictated by base material compatibility and position as presented in Section 11. Except for the rare case where a repair is made to the disc or wedge base material, the filler material will be a hardfacing alloy, such as ASME Section II, SFA 5.13, RCoCr-A (Alloy 6), Alloy 21, or NOREM 02A (see Table 11-10 ). If the hardfacing is a repairable type, such as Alloy 21 or NOREM 02A, the repair can be performed with matching hardfacing material. If the existing hardfacing is a much harder material, such as Alloy 1 (RCoCr-C), or Alloy 6 (RCoCr-A), or one of the nickel alloys (RNiCr-A or B), a softer repair material should be utilized. When refurbishing a disc or wedge, the original base metal thickness lost to remove all of the hardfacing should be restored with a filler material compatible with the base material and operating temperature. For structural restoration of all base materials and for high temperature applications on P-Nos. 4 and 5 substrates, a compatible filler material must be utilized (see Table 11-4). For non-structural buildups, a Type 309 stainless filler material can be utilized. An austenitic filler material, such as ER309, provides a more forgiving substrate for depositing hardfacing than the carbon steel filler materials, due to their similar coefficients of thermal expansion. Hardfacing materials should not be deposited directly on cast steel substrates.
8.11.5 Preheat and Post-Weld Heat Treatment Requirements As presented in Section 13, PWHT is not required for depositing hardfacing on P-Nos. 1 and 8 substrates, but is required for P-Nos. 4 and 5 substrates (see Table 11-1 for PNumber specifications); however, different rules apply to the repair of the base materials. The PWHT requirements and exemptions are listed in Tables 13-2, 13-3, and 13-4. While not required, a minimum 200˚F to 400˚F (93˚C to 204˚C) preheat is essential to performing any localized repair. The lower preheats can be utilized when repairing Alloy 21 or NOREM 02A on stainless substrates, and increasing to 450˚F to 550˚F (232˚C to 288˚C) for P-No. 5 substrates. Successful repairs to the more hardenable hardfacing materials often require preheats in excess of 800˚F (427˚C). The suggested preheat temperature also varies based on the quality and technique of the welder, the quality of filler material, and the method of base metal fabrication.
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8.11.6 Welding Repair Localized welding repair of valve discs and wedges is generally performed with the manual GTAW process and the manual SMAW process. The GTAW process provides the superior control and quality that are critical to depositing hardfacing materials in confined repair cavities. The shielded metal arc welding (SMAW) process with coated electrodes has been successfully utilized on large and small repair applications. A drawback to the SMAW process is that it produces spatter and compromises welding control; however, both can be overcome with good practices. Localized hardfacing repairs are quite challenging and should be performed as presented in Section 14.3. Experience has shown that a tapered repair cavity that provides access and visibility to the welder, a well planned and proven welding sequence, and a welder experienced with welding hardfacing materials greatly enhance the potential for a crack-free repair. After welding, a measurement should be taken to ensure that there is a minimum of 1/16 inch (1.6 mm) of hardfacing material above the original surface and surrounding hardfacing to allow for machining and lapping to the final dimensions. If a stress crack is encountered at any time during the hardfacing application, the crack should be ground out prior to applying additional material. The finished weld should be wrapped in insulation to allow for slow cooling. When total hardfacing refurbishment is performed, the manual or machine GTAW, SMAW, GMAW, or PTAW processes can be utilized. In most applications, the selection criteria are based on deposition rate, weld quality, and minimal heat input. The PTAW process is most widely used by valve manufacturers but is rarely found in non-production facilities. The GMAW process is the second most common hardfacing method used in production. However, welding control and quality is not as good as the GTAW and PTAW processes. The GTAW process provides superior quality but has slow deposition rates and high dilution. When welding on discs and wedges where a significant quantity of material was removed to eliminate the original hardfacing material, a two-step welding application is recommended. The base metal should be restored to its original thickness by building up the area with a matching filler material or butter layer. The buildup can then be machined with the desired weld prep or cavity to ensure that the hardfacing deposit has a uniform thickness and does not exceed approximately 3/16 inch (4.8 mm) (see Figure 8-11e). The hardfacing is then deposited as described in Section 14. This methodology, similar to that used by the original manufacturer, ensures the proper location and thickness of the new guide rib wear surface, as well as ensuring a quality hardfacing deposit. After completing the hardfacing weldment, it should be wrapped in insulation to allow for slow cooling.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair Original Base Metal (Prior To Welding) Hardfacing Dilution
Hardfacing Deposit New Machined Surface
Disc/Wedge Half
Machined Surface
Build-Up Material New Machined Weld Prep
Build-Up Fusion Line
Machined Weld Prep
Hardfacing Deposit Finished Machined Hardfacing
Hardfacing Fusion Line
Build-Up Fusion Line
Hardfacing
Figure 8-11e Hardfacing Refurbishment Sequence
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8.11.7 Final Machining Final machining and lapping of the disc or wedge must be performed after all in-body seat work is completed, as discussed previously in the prerequisites. Final machining is generally performed on a lathe or vertical boring mill. Lapping is performed with a lapping table or portable lapping machine as shown in Figure 8-11f. A final blue check as discussed in Section 15.3 should be performed to verify 360˚ seat contact.
Figure 8-11f Lapping Table and Portable Lapping Adapter (Courtesy of EFCO) 8.11.7.1 Gate Valve Wedges Final machining and fitting of wedges require the skill of an experienced technician. Detailed fitting techniques are presented in Appendix A, Gate Valve Wedge Fitting Techniques.
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8.11.7.2 Parallel Disc and Swing Check Valve Discs Final machining of these flat disc designs requires that the hardfacing surface is square to the reference face and has a total thickness as specified by the valve manufacturers. Discs can be fitted by adjusting mechanisms (that is, spacers, clearances between pins and bushings, and so on) typical of the specific valve type. Refer to the manufacturer’s instructions. EPRI/NMAC Report TR-100857, Check Valve Maintenance Guide, provides details for fitting swing check valves. 8.11.7.3 Tilting Disc Check Valve Disc Because of the seat angle, which is similar to a globe valve seat, critical diameters and angles must be machined on the disc to obtain line contact between the seat and disc. This is accomplished by machining an angle on the disc and seat, with the steeper angle on the disc as shown in Figure 8-11g. After machining the proper seat configuration on the disc, both the disc and the seat should be lapped with the lapping discs shown in Figure 8-11h. Final fitting of the disc and hinge pin assembly is accomplished through the hinge pin adjustments.
Disc
1˚
Seat Ring Valve Body
Figure 8-11g Seat Angle Determination for Tilting Disc Check Valve
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Plug O.D. to Fit Seat I.D.
Lapping Fixture for Seat
Bore to Fit Disc O.D.
Lapping Fixture for Disc
Figure 8-11h Lapping Fixture for In-Body Tilting Disc Check Valve Seat and Disc [13]
8.11.8 Inspection The ASME Section XI Boiler and Pressure Vessel Code and the ANSI/ASME B31.1 Power Piping Code provide guidance for the repair and inspection of valve materials defined as pressure-retaining. ASME Section XI, Subarticle IWA-4400 requires that the repair welding be performed by qualified welders and procedures in accordance with Section IX. It also requires that the repair cavity (pressure-retaining base material) shall be examined by liquid penetrant or magnetic particle method. A final examination of
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the finished repair must be performed and documented utilizing the inspection process that was used to find the indication originally.
8.11.9 Testing Pre-service testing shall be performed in accordance with the requirements of ASME/ ANSI OM, Part 10, when the owner determines that the repair activity effects the valve’s performance or designed function. This could include a local leak rate test (LLRT), an integral leak rate test (ILRT), or a operability/functional test. 8.12
Poppet Hardfacing Repair
This section focuses on hardfacing repair and replacement of globe-type poppets (also referred to as plugs). The hardfacing is located on the seat. If the poppet has wear rings, they are also hardfaced (see Figure 8-12a). These repairs are performed in conjunction with the seat ring repairs and replacements discussed in Sections 8.3 through 8.6, and the guide rib repairs discussed in Section 8.9.
Wear Rings
Hardface Deposit
Seat
Figure 8-12a Poppets with Hardfaced Wear Rings [16] Poppets are found in globe, lift check, stop check, and control valves in which the seat is perpendicular to the bonnet bore. Most repairs performed on poppets are the result of cracks or wear in the hardfacing materials. Left unrepaired, erosion will eventually penetrate completely through the hardfacing and into the base material.
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Repairs to poppets are commonly performed in a machine shop with the proper tools and knowledgeable personnel. In some cases, replacement of the poppet can be more cost-effective than performing a repair. This is especially true of standard poppets in valves smaller than 6-inches (152 mm) nominal size.
8.12.1 Repair Strategy An evaluation of valve size, type, poppet design, availability of replacement parts, and personnel should be performed prior to committing to a poppet seat or wear ring repair. Hardfacing welding of these big parts and final machining is difficult and most challenging to even the most experienced valve personnel. Minimal tolerances are often employed, making material removal or distortion from welding very critical. Except in cases where a minor flaw is to be repaired, the existing hardfacing material should be completely removed and rewelded. When hardfacing materials such as Stellite 1, 6, and 12, or the nickel-based alloys are encountered, they should always be completely removed and rewelded. Welding over or building up hardfacing material that has been inservice should be avoided because it is very susceptible to stress cracking. In evaluating the repair/replacement option, the following questions should be asked: • What is the hardfacing material? • Can the flaw or crack be removed by grinding without impacting the sealing area, thus avoiding a weld repair? • If the flaw or crack is in the wear ring, can it be removed by grinding without impacting operation of the poppet, thus avoiding a weld repair? • If the flaw or crack is in the sealing area, can it be removed by taking a light cut across the contact and still maintain adequate hardfacing thickness? • Does the crack or defect penetrate into the poppet base material such that a welded repair activity could significantly deform the component? If so, the forged or cast poppet should be replaced. • Are the wear rings being built up or adjusted to compensate for the material loss of the in-body guide ribs? Poppet repair is much easier than guide rib repair. A report detailing the finished bore and seat diameters must be taken prior to performing a base metal buildup and hardfacing deposit on the poppet. • Is machining, special fixturing, measuring, and welding equipment available to perform the repair? • Does the owner or repair vendor possess the proper NBIC repair certifications and ASME Section IX qualified welding procedures? • Is preheat and post-weld heat treatment equipment available? • Are trained and qualified valve specialists familiar with welding of hardfacing and fitting of poppets available?
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8.12.2 Repair Prerequisites The following should be considered prior to initiating a repair: • Record the manufacturer’s name, size, pressure rating, valve type, body style, body material, and trim material data from the valve nameplate. • Obtain drawings and material specifications from the manufacturer. • Obtain and review the maintenance and repair history to see if there have been any changes to the original design. • Procure a replacement pressure seal ring and an oversize ring if the valve has a pressure seal bonnet. • Prepare a repair “traveler” or work instruction that outlines the repair sequence and inspection “hold points.” All measurements and findings should be recorded for a final report. • Remove the poppet (plug) and record any damage. Particular attention should be given to the seat ring, poppet seat, body guides, and poppet guides. • Perform a blue check and record the seating location on the poppet as shown in Figure 8-3i. This should be a thin line of contact. • Measure and record the inside diameter of the seat ring, the outside diameter of the seat angle, the distance from the top of the seat to the bonnet flange surface, and the body bore diameter as shown in Figure 8-3j. • If the in-body seat has been repaired or replaced, new measurements should be recorded as shown in Figure 8-3j and those dimensions used to establish new poppet seat dimensions. • Measure and record the ID and OD of the poppet seat and the OD of the poppet wear ring prior to initial machining (see Figure 8-12b).
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I.D. Seat
O.D. Seat
Wear Ring Dia.
Figure 8-12b Poppet Seat and Wear Ring Dimensions [16]
8.12.3 Flaw Removal The method of flaw removal is based on the number of flaws, their distribution, and the overall amount of material to be removed. If a small localized area is going to be repaired due to a corrosion pit, stress crack, or a deep wear scar, the defect should be removed by manual grinding as discussed in Section 9. In applications where the repair to the hardfacing is extensive, all of the hardfacing should be removed from the poppet seat or guide ring by machining on a lathe. If the flaw propagates into the base material, a manual grinding method should be employed to remove the remaining flaw leaving a 3:1 tapered weld preparation cavity (see Section 9.1). A verification should then be performed with an etchant to verify complete removal of all hardfacing material, and a liquid penetrant examination performed to verify that no cracks or defects exist in the area to be repair welded.
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8.12.4 Filler Material Selection Filler material selection is dictated by base material compatibility and position as presented in Section 11. Except for the rare case where a repair is made to the disc or wedge base material, the filler material is a hardfacing alloy, such as ASME Section II, SFA 5.13, RCoCr-A (Alloy 6), Alloy 21, or NOREM 02A (see Table 11-10). If the hardfacing is a repairable type, such as Alloy 21 or NOREM 02A, the repair can be performed with matching hardfacing material. If the existing hardfacing is a much harder material, such as Alloy 1 (RCoCr-C), Alloy 6 (RCoCr-A), or one of the nickel alloys (RNiCr-A or B), a softer repair material should be utilized. When refurbishing a poppet seat or wear ring, the original base metal thickness that was lost to remove all of the hardfacing should be restored with a filler material compatible with the base material and operating temperature. For structural restoration of all base materials and for high temperature applications on P-Nos. 4 and 5 substrates, a compatible filler material must be utilized (see Table 11-4). For non-structural buildups a Type 309 stainless filler material can be utilized. An austenitic filler material such as ER309 provides a more forgiving substrate for depositing hardfacing than the carbon steel filler materials, due to their similar coefficients of thermal expansion. Hardfacing materials should not be deposited directly on cast steel substrates.
8.12.5 Preheat and Post-Weld Heat Treatment Requirements As presented in Section 13, PWHT is not required for depositing hardfacing on P-Nos. 1 and 8 substrates, but is required for P-Nos. 4 and 5 substrates (see Table 11-1 for PNumber specifications); however, different rules apply to the repair of the base materials. Because of the mass and heat sink of a poppet, preheat is strongly recommended. The PWHT requirements and exemptions are listed in Tables 13-2, 13-3, and 13-4. While not required, a minimum 200˚F to 400˚F (93˚C to 204˚C) preheat is essential to performing any localized repair. The lower preheats can be utilized when repairing Alloy 21 or NOREM 02A on stainless substrates, and increasing to 450˚F to 550˚F (232˚C to 288˚C) or P-No. 5 substrates. Successful repairs to the more hardenable hardfacing materials often require preheats in excess of 800˚F (427˚C). The suggested preheat temperature also varies based on the quality and technique of the welder, the quality of filler material, and the method of base metal fabrication.
8.12.6 Welding Repair Localized welding repair of valve discs and wedges is generally performed with the manual GTAW process and the manual SMAW process. The GTAW process provides the superior control and quality that are critical to depositing hardfacing materials in confined repair cavities. The shielded metal arc welding (SMAW) process with coated electrodes has been successfully utilized on large and small repair applications. A drawback to the SMAW process is that it produces spatter and compromises welding control; however, both can be overcome with good practices. 8-117
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Localized hardfacing repairs are quite challenging and should be performed as presented in Section 14.2. Experience has shown that a tapered repair cavity that provides access and visibility to the welder, a well planned and proven welding sequence, and a welder experienced with welding hardfacing materials greatly enhance the potential for a crack-free repair. After welding, a measurement should be taken to ensure that there is a minimum of 1/16 inch (1.6 mm) of hardfacing material above the original surface and surrounding hardfacing to allow for machining and lapping to the final dimensions. If a stress crack is encountered at any time during the hardfacing application, the crack should be ground out prior to applying additional material. The finished weld should be wrapped in insulation to allow for slow cooling. When total hardfacing refurbishment is performed, the manual or machine GTAW, SMAW, GMAW, or PTAW processes can be utilized. In most applications, the selection criteria are based on deposition rate, weld quality, and minimal heat input. The PTAW process is most widely used by valve manufacturers but is rarely found in non-production facilities. The GMAW process is the second most common hardfacing method used in production. However, welding control and quality is not as good as the GTAW and PTAW processes. The GTAW process provides superior quality but has slow deposition rates and high dilution. When welding on poppet seats and wear rings where a significant quantity of material was removed to eliminate the original hardfacing material, a two-step welding application is recommended. The base metal should be restored to its original thickness by building up the area with a matching filler material or butter layer (Figure 8-11e presents a series of steps that would be the same for wear rings and poppet seats). The buildup can then be machined with the desired weld prep or cavity to ensure that the hardfacing deposit has a uniform thickness and does not exceed approximately 3/16 inch (4.8 mm). The hardfacing is then deposited as described in Section 14. This methodology, similar to that used by the original manufacturer, ensures the proper location and thickness of the new seat or wear ring surface, as well as ensuring a quality hardfacing deposit. After completing the hardfacing weldment, it should be wrapped in insulation to allow for slow cooling.
8.12.7 Final Machining Final machining of the poppet seat must be performed after all in-body seat work is completed, as discussed previously in the prerequisites. Final machining is generally performed on a lathe. This final machining step is extremely critical to obtaining a 360˚ thin contact line. A very consistent conical seat with a minimum 1˚ steeper angle than the in-body seat angle will ensure a leak-free valve (see Figure 8-12c). A final blue check as discussed in Section 15.3 should be performed to verify 360˚ seat contact.
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Figure 8-12c Typical Globe Valve Seat Angle Differential [13] Final machining of the poppet wear ring must also be performed after all in-body repair to the guide ribs are completed. The wear rings should be machined on a lathe to the diameter that provides the manufacturer’s suggested clearances. After machining, the wear rings should be finished with a light emery cloth to remove the tool marks.
8.12.8 Inspection The ASME Section XI Boiler and Pressure Vessel Code and the ANSI/ASME B31.1 Power Piping Code provide guidance for the repair and inspection of valve materials defined as pressure-retaining. ASME Section XI, Subarticle IWA-4400 requires that the repair welding be performed by qualified welders and procedures in accordance with Section IX. It also requires that the repair cavity (pressure-retaining base material) shall be examined by liquid penetrant or magnetic particle method. A final examination of the finished repair must be performed and documented utilizing the inspection process that was used to find the indication originally.
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8.12.9 Testing Pre-service testing shall be performed in accordance with the requirements of ASME/ ANSI OM, Part 10, when the owner determines that the repair activity effects the valve’s performance or designed function. This could include a local leak rate test (LLRT), an integrated leak rate test (ILRT), or a operability/functional test. 8.13
Valve Stem Repair
Repairs to valve stems are generally limited to light machining and grinding to remove corrosion, pitting, and score marks. Welding is not recommended because stems are typically machined from high strength, precipitation-hardened 17-4PH stainless steel, quenched and tempered 410 stainless steel, or strained-hardened Type 316 stainless steel. To weld these materials, the stem must first be annealed, then welded, then retempered or hardened. These operations would be time consuming and expensive, and would most likely result in a warped stem. Light machining and grinding can be used to remove minor surface defects. If the stem is machined in a lathe and the machined area passes through the packing gland, the entire length that passes through the packing should be machined. The machined area should be finished with a fine grit emery cloth to remove the tool marks and the ends tapered. All machining operations should be followed by a liquid penetrant (PT) examination. If surface defects must be repaired by welding, the owner should contact the manufacturer for design requirements and recommendations. If minor surface defects, such as score marks or pits, are encountered, methods such as plasma spray, metal spray, or liquid metal have been successfully used, followed by light machining.
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9 DEFECT REMOVAL Repair of valves or any other component almost always requires metal removal as part of the remedial activity. Several defect removal methods can be used and should be chosen based on such variables as location, access, time and equipment availablility, skilled manpower, amount of material to be removed, and the repair method to follow [22]. Defect removal methods generally used for valve repair include: grinding, machining, honing, lapping, and, in rare cases, carbon arc or plasma arc gouging. Grinding is the most common method of removing defects, but hand grinding is laborious and very time consuming. In situations where defects are found to be extensive and away from critical sealing surfaces, a more efficient technique, such as carbon arc gouging or plasma arc gouging, can be used. In situations where weld preparation has to be precise, such as around pressure seal bores or hardfaced seats and seat rings, machining provides both defect removal and weld preparation in a single step. Honing and flapping are material finishing methods that are used to finish machined surfaces for metalto-metal sealing. Localized Defect Removal—No Weld Repair Required. If unacceptable indications are found using liquid penetrant testing (PT), magnetic particle testing (MT), or ultrasonic testing (UT), they must be removed. Typical acceptance criteria can be found in ASME Section V and B31.1. Indications that are shown to be near the surface of the piping component by NDE methods can be removed by surface grinding and their removal verified by PT or MT. If the remaining wall thickness of the component, measured using an ultrasonic thickness gauge, is greater than the minimum required wall thickness, the area can be blended to a minimum 3:1 (width:depth) profile and returned to service. All blending should leave a smooth transition from ground areas to the original component wall thickness as shown in Figure 9-1. Periodic MT or PT inspection should be performed to minimize material removal.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair Surface Indications
t Minimum
tActual
A
Surface indications found by MPI
Blend Out Indications
B
Remove indications by grinding
W
C
D
W:D > 3:1
Areas of indication removal blended to a minimum 3:1 (W:D) ratio and all areas must be greater than t minimum at maximum grind out depth.
Figure 9-1 Removal of Surface Indications Where No Weld Repair Is Required
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Some instances might exist where the actual wall thickness of the piping or fittings is significantly thicker than the minimum required wall thickness. In these cases, when a grindout is performed and more than 15% of the actual wall is removed, a weld repair should be performed to build the wall up to the original thickness and profile. This eliminates relatively deep grindouts that could act as stress concentrations when piping is returned to service. Localized Defect Removal—Weld Repair Required. A weld repair is required when the depth required to remove unacceptable defects results in the remaining wall thickness being below code-acceptable limits. The following are the steps for performing such a repair: 1. Excavate the defect (surface indications will already be exposed; UT indications must be exposed). 2. Remove the defects by grinding, gouging, or machining. 3. Surface grind and verify the defect removal by PT or MT. 4. Weld repair the cavity as described below. Following a local repair, the area should be examined by PT or MT and the inspection results of the localized repair recorded for future reference. Excavation Shape for Local Weld Repairs. When the depth of material excavated necessitates a weld repair, the cavity must be prepared for welding after final defect removal. Either during or following defect removal, the excavation should be shaped to provide adequate access for the welding operation to ensure proper fusion. An approximately 2:1 width:depth profile is typical. Figure 9-2 illustrates a typical defect excavation and profile for weld repair. Other profiles can be used if there is sufficient access to deposit the root pass and prevent lack of sidewall fusion. Also, there must be sufficient access to ensure that there is no slag entrapment.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair Indications found by inspection
tActual
t Minimum
A
Defect location as determined by surface or ultrasonic inspection
B
Excavation for defect type verification
C
Complete defect removal and verified by WFMPI
W
D
D
Excavation profiled to 2:1 (W:D) for weld repair
Figure 9-2 Excavation for Defect Type Verification and Local Weld Repair
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If the excavation is performed using a thermal removal process (carbon arc or plasma arc gouging), a 1/16 inch (1.6 mm) minimum of material must be removed by grinding prior to the final inspection and before welding begins to remove any carbon pick-up. When the weld preparation has been completed, MT or PT should be performed on the entire cavity prior to weld repair. 9.1 Grinding Grinding is the most commonly used material removal method for valve repair. The process is used for the removal of localized cracks in hardfacing, pitting and casting defects, removal of seat rings, and blending of valve repair deposits. Equipment used for grinding purposes is extensive and varied. Pencil or small disc grinders are generally used to remove defects, but if large amounts of metal are to be removed, large disc grinders can be used. The grinders are electric or air-driven portable types with wheels of various shapes, sizes, and abrasive characteristics. Three American National Standard Institute (ANSI) standards list the generic specifications for grind wheels as follows: • ANSI B74.13, Markings for Identifying Grinding Wheels and Other Bonded Abrasives • ANSI B74.2, Specifications for Shapes and Sizes of Grinding Wheels, and Shapes, Sizes, and Identification of Mounted Wheels • ANSI B7.1, Safety Requirements of the Use, Care, and Protection of Abrasive Wheels Every grinding wheel has two constituents: the abrasive that does the cutting and the bond that holds the abrasive component. Variations of these components can be selected to give a wide range of grinding characteristics. It is essential that proper practices are followed so that grinding does not produce any harmful effects that could result in future problems. Grinding wheels should never be forced, which can cause excessive wheel breakdown and, more important, areas of localized high temperature. This type of grinding can result in reduced fatigue strength and failure due to the introduction of induced residual tensile stresses from improper grinding operations. Also, the intense localized heating can result in overheating the material, which can cause the formation of brittle untempered martensite, or the development of tight, shallow surface grinding cracks. These problems can be avoided by using grinding practices that produce a smooth surface that is free of induced residual stresses or sites for the initiation of fatigue cracks. It is also good practice to ensure that abrasive cutting or grinding consumables are identified for use on carbon and low-alloy steels versus stainless steels. Grinding or cutting consumables that are impregnated with carbon or low-alloy steel residues when used on stainless steel induce contamination of the stainless steel surface and most likely result in surface corrosion. Conversely, it is a good practice to restrict stainless steel grinding wheels for use on stainless steels and not for use on carbon or low-alloy steels to avoid future mix-ups [22].
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9.2 Machining Machining using mechanized cutting equipment is used to remove defects or internal components cleanly and with precision. Machining equipment used to repair valves include boring bars, milling machines, magnetic drills, flange facers (portable lathes), and pipe cutting/beveling machines. Machining has the advantage of cutting and forming the weld preparation with a single piece of equipment, and dimension tolerances are closely controlled. Boring bars are mounted to the inside bore with expanding chucking systems or are attached with special stand-off brackets outside the valve. Smaller boring bars are also available for machine hinge pin penetrations and bonnet packing boxes. Portable boring bars have been used where circumferential machining of a bore is required, such as when removing valve seats, removing valve seat hardfacing, preparing seat areas for new seats, and finishing the new hardfacing to the required location and angles. Specialized boring bars are available that turn 90˚ to the bonnet bore for gate valve refurbishment as shown in Figure 9-3. Other bars are available with accessories to grind or mill seats and guide ribs, and to drill and tap stud holes.
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Guide Rib
Drive Motors
Hardfacing Material
Cutting Tool
Figure 9-3 Gate Valve Seat Machining [26] Portable milling machines can be used to remove more localized materials at a much faster rate than turning machines. Small milling machines have been developed to remove hardfacing from guide ribs as shown in Figure 9-4. Milling machines have also been used to repair gate valve guides and remove the bulk of the hardfacing from integral globe valve seats.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Figure 9-4 Milling Machine Removing Hardfacing from Guide Ribs Flanged sealing surfaces require a flat spiral or phonographic finish in order to maintain leak tightness. Portable flange facers are used to develop the proper finish for flat flexitalic gaskets or for special ring-type grooves of body-to-bonnet joints in critical service valves. Flange facers are also used to develop proper sealing surfaces for threaded-in seat rings and for some types of hinge pin covers. Machining should always be performed by skilled machinists, both in the field and in shop applications. 9.3 Lapping Lapping is a polishing procedure that is used to finish hardfaced wedges, discs, poppets, and seats. Lapping is not used to remove deep defects. A general rule of thumb is that defects less than .010 inch (.25 mm) are removed by lapping. Lapping provides a seat surface finish of from 16 rms down to 2 rms, depending on the grit of the lapping compound or paper and the seat material [9]. Lapping machines are available for most valve types in both portable and bench-type systems. Gate valve lapping machines (see Figure 9-5) are used by field service groups in the field and manufacturers in the shop to finish in-body seats and discs or wedges. The same can be said for globe, check, and safety relief valves from 1/4 inch to 36 inches (6.3 mm to 914 mm) nominal. For globe-type valves, lapping should be used to remove tool marks as the point of contact is only at the bottom of the seat.
9-8
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
T
A
DN
Figure 9-5 Gate Valve Lapping Machine In many cases, lapping is performed with special discs made specifically for one application, such as tilting disc check valve seats or gate valve seats that need to be narrowed. These plates should be very thick to maintain dimensionally stability. The seat should be lapped to the finish and dimensions specified by the manufacturer. The seat angle and width is critical to obtaining a leak-free valve.
9-9
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
9.4 Honing Honing is an abrasive process that is used to obtain a very cylindrical and fine finish. The process is used to eliminate inaccuracies due to machining by providing a very straight and round bore with very tight tolerances. Honing tools are placed in the bore with a radial load applied such that the honing stones are in contact with the bore. The hone is then driven (rotated) in the bore, generating a high quality surface. Finishes range from 3 rms to 50 rms [9]. Honing stones consist of aluminum oxide, silicon carbide, or diamond abrasive grit, held together in stick form by vitrified clay, resin, or a metal bond. The grain and grade of abrasive to be used is based on the amount of stock to be removed and the surface finish desired. Silicon carbide is generally used for cast iron, and aluminum oxide is recommended for use on steel. As with grinding discs and lapping compound—the finer the grit, the finer the surface finish. For valve repair, hones are used on bonnet packing bores and on body hinge pin penetrations and pressure seal bores in preparation for pressure seal ring installation. The hone produces a very straight and cylindrical surface for the pressure seal ring to seat against. Material Removal Rates for Honing. In general, the honing process can be used to remove material from bore diameters at a rate of .005 inches per minute (ipm) to .012 ipm (.13 mm/min. to .30 mm/min.) on steel parts, based on parts having a length equal to three or four times the diameter [30]. For many pressure seal areas, this rate will be much faster as a result of the length-to-bore ratio being much less. Recommended rotational surface speeds are 50 feet per minute to 110 feet per minute, (15.2 m/min. to 33.5 m/min.) for steels. In general, the harder the material to be honed, the slower the speed.
9-10
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
10 WELDING PROCESS SELECTION The selection of a welding process is influenced primarily by the quantity and extent of repairs required. The inherent difficulties associated with cleaning and the requirements for preheat and post-weld heat treatment on some of the materials are additional important considerations in selecting the welding process. Only arc welding processes that are appropriate for manufacturing or in situ repair or replacement are discussed herein [10, 31]. Localized repairs of limited scope are normally conducted with manual shielded metal arc welding (SMAW). When several components are to be installed or when repair areas encompass a substantial portion of the surface area of a component, use of semiautomatic processes, including pulsed gas metal arc welding (GMAW-P) or flux-cored arc welding (FCAW), should be considered. These processes have the ability to rapidly deposit quality weld metal (see Figure 10-1). Care should be observed if these processes are to be used on austenitic stainless steel or the other high-alloy components. The wires and techniques available might not produce deposits of sufficient quality without some development or procedure evaluation.
10-1
EPRI Licensed Material
Welding Process @ 100% Duty Cycle
Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
SMAW (E7018) 1
1
9— 2
1— 2
GTAW 6
0
(Cold Wire Feed) PAW 0
7
GMAW (Spray) 24
2
FCAW 2
0
36
10
20
30
40
Deposition Rate, LB/HR
Figure 10-1 Welding Process Versus Deposition Rate at 100% Duty Cycle [22] 10.1
Gas Tungsten Arc Welding (GTAW)
GTAW is an arc welding process that produces coalescence of metals by heating them with an arc between a tungsten (nonconsumable) electrode and the work. Shielding is obtained from a gas or gas mixture. Pressure might or might not be used, and filler metal might or might not be used. This process is sometimes called tungsten inert gas welding (TIG). In Europe, it is called WIG welding, using Wolfram, the German word for tungsten. The process is illustrated in Figures 10-2 and 10-3. Direction of Travel
Shielding Gas Molten Weld Metal
Welding Torch Tungsten Electrode Arc Filler Rod
Solidified Weld Metal Base Metal
Figure 10-2 Process Diagram (GTAW) [32]
10-2
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Cooling Water Supply
Torch
Power Source
Inert Gas Supply
Filler Metal Water Drain Gas Base Metal Foot Pedal (Optional)
Work Lead
Electrode Lead
Figure 10-3 Circuit Diagram (GTAW) [32] This welding process is suitable for welding virtually any material used for power plant piping. Advantages include the high quality of the weld deposit, freedom from slag, availability of skilled personnel, effective use on localized repairs and the ability to be automated. The major disadvantage of this process is the slow rate of deposition. This factor severely limits the size of the job that can be attempted economically. 10.2
Gas Metal Arc Welding (GMAW)
GMAW is an arc welding process that produces coalescence of metals by heating them with an arc between a continuous filler metal (consumable) electrode and the work. Shielding is obtained entirely from an externally supplied gas or gas mixture. Some variations of this process are called MIG (manual inert gas) or CO2 welding. See Figures 10-4 and 10-5. The pulsed-current variation of this process is typically used for depositing high-alloy materials.
Shielding Gas Molten Weld Metal Solidified Weld Metal
Direction of Travel Nozzle Electrode Arc Base Metal
Figure 10-4 Process Diagram (GMAW) [32] 10-3
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Control System
Feed Control Wire Spool
Gas Out Gun Control Gas In
Hand Held Gun
Voltage Control
Cable (Power, Gas, and Coolant) Work Lead
Wire Feed Drive Motor
Contractor Control 110V Supply
Shielding Gas Source
Power Source
Figure 10-5 Block Diagram (GMAW) [32] This process is typically a semi-automatic process that is suitable for the welding of carbon, low-alloy, and many high-alloy steel components in all positions. This approach requires thorough evaluation using pulsed current for all-position welding on high alloys, such as P-Nos. 4 and 5 chrome-moly steels (see Table 11-1). The process also offers a very high rate of deposition and the ability to be used for localized repairs, as well as the capability of being automated. The disadvantages of the process include the lack of craft familiar with the process and the need for specialized equipment and further evaluation. 10.3
GMAW-P
The pulsed gas metal arc (GMAW-P) process is a semi-automatic technique that is suitable for welding carbon and low-alloy steel materials in all positions. Although it has been used for austenitic stainless steel and other high-alloy materials, it is not at a stage of demonstrated practice where it can be recommended for out-of-position welding without further and extensive evaluation by the user. The advantages of this process include high deposition rates, lack of slag, and the ability to be used for localized repairs. Disadvantages include the lack of skilled craft familiar with GMAW-P and the need for specialized equipment. 10.4
FCAW
Flux-cored arc welding (FCAW) is a semi-automatic process that is suitable for the welding of carbon and low-alloy steel components in all positions. As with GMAW-P, FCAW approaches require thorough evaluation for all-position welding of high alloys at this time. This process offers a very high rate of deposition and the ability to be used 10-4
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
for localized repairs. The disadvantages of the process include the lack of craft familiar with the process, slag removal after each weld pass, and the need for specialized equipment and further evaluation. 10.5
Shielded Metal Arc Welding (SMAW)
SMAW is an arc welding process that produces coalescence of metals by heating them with an arc between a covered metal electrode and the work. Shielding is obtained from decomposition of the electrode covering. Pressure is not used and filler metal is obtained from the electrode. See Figures 10-6 and 10-7.
Covering Electrode Arc Solidified Slag Shielding Gases Weld Metal Base Metal
Figure 10-6 Shielded Metal Arc Welding Process [33] Electrode Holder
Power Source
Electrode
Electrode Lead
Base Metal
Work Lead
Figure 10-7 Circuit Diagram (SMAW) [32]
10-5
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
This process is a widely used manual welding process that is suitable for joining most power plant materials. Advantages include high rate of deposition, availability of skilled personnel, and the ability to be used for localized repairs. However, the need to remove slag from completed weld passes and the frequent starts and stops to change electrodes can reduce productivity and increase the potential for defects. 10.6
Machine Welding
Machine welding offers high quality weld deposits and has gained greater acceptance in recent years. Machine approaches have been used quite successfully in both fossil and nuclear repair applications. It has proven efficient where only several welds were involved and on repairs where an entire piping system was replaced. This process has been generally applied by service vendors who possess both portable machining and machine or automatic welding capabilities. Several utilities who own the appropriate equipment (machining and welding) are beginning to use this approach for in-house repair, even on activities of small to moderate scope. NOTE: Most of the welding conducted to date in an “automatic” manner is now defined as “machine” welding, in accordance with code revisions. Even work done with GTAW-P-AU using a video/remote technique is now defined as “machine” welding. Welding that does not require any operator interface or adjustment is defined as “automatic” [32]. 10.7
GTAW-P-AU
The machine pulsed gas tungsten arc welding (GTAW-P-AU) process is the most widely accepted machine process for in situ valve repair where high quality and reproducibility are required. It provides reasonable deposition rates when properly used and very high quality weld deposits without the need for slag removal. Primary applications include the joining of any of the candidate pipe/component materials. This process requires the use of highly trained welding operators and specialized equipment. 10.8
Efficiency/Cost Comparisons
Deposition rate ranges for the commonly applied repair welding processes are shown in Figure 10-1. Where only a few welds are involved, the selection of process for weld repair is not terribly important. However, if many welds are involved, the GTAW-P-AU process should be strongly considered. Other processes such as SMAW, GMAW and FCAW exhibit higher individual deposition rates, but the GTAW-P-AU process has demonstrated a greater ability to produce defect-free welds on a consistent basis, even in the plant environment.
10-6
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
11 WELDING FILLER MATERIAL Many factors enter into the selection and use of welding filler materials. The composition of the base material, the welding process to be used, post-weld heat treatments, and mechanical and service requirements all effect what and how it can be used. Of particular interest are factors pertaining to whether original versus existing material properties will be matched or if under-matching approach will be considered. Use of conventional, offthe-shelf material manufactured to existing standards might be appropriate, or special non-standard or proprietary filler metals might provide superior results. These factors, as well as the availability of the chosen material, effect selection decisions [12, 32–54]. The most common base metals encountered in valve repair are provided in Table 11-1. Table 11-1 P-Number, Composition, and Specifications for Selected Valve Materials [10] P-No.
Nominal Composition
1
Forging
Casting
Plate
Bolts, Studs, and Nuts
Carbon Steel
SA-105 SA-181 SA-350-LF2
SA-216 WCA SA-216 WCB SA-216 WCC
SA-285 C SA-515 Gr 60,70 SA-516 Gr 60,70 SA-299
SA-193, Gr B7 SA-194, Gr 2H
3
Carbon Molybdenum
SA-182 F1 SA-182 F2
SA-217 WC1
SA-204 A SA-387 Gr
SA-193 Gr B7, 16 SA-194 Gr 2H, 4
4
1-1/4 Cr 1/2 Mo
SA-182 F11
SA-217 WC6
SA-387 Gr 11
5
2-1/4 Cr-1 Mo
SA-182 F22
SA-217 WC9
SA-387 Gr 22
8
Austenitic Stainless Steel
SA-182 F304 SA-182 F316 SA-182 F347
SA-351 CF3 SA-351 CF8 SA-351 CF8C SA-351 CF8M SA-351 CF3M
SA-240 Type 304 SA-240 Type 316 SA-240 Type 347
SA-193 Gr B7 1 , B6, B82 , 8 2 SA-194 Gr2H 1 , 6 SA-564 Gr 630
Notes: 1. Although sometimes provided, these materials might not be appropriate for stainless steel valves due to their potential for corrosion. 2. These materials are not recommended for threading into 304 or 316 bodies, because galling can occur.
11.1
Types of Filler Metals
Several types or product forms of weld filler metal are available for repair or replacement. The three major product forms are covered electrodes, bare wire, and consumable insert material. The type used is welding process-specific and is shown in Table 11-2. 11-1
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Table 11-2 Welding Filler Material Use by Process [22] Welding Process
Covered Electrode
Bare Straight
Bare Spooled
Comsumable Insert
X
N/A
N/A
N/A
GTAW & PAW
N/A
X
X
X
GTAW-P-AU
N/A
N/A
X
X
GMAW
N/A
N/A
X
N/A
SAW
N/A
N/A
X
N/A
SMAW
The FCAW process and some submerged arc welding (SAW) and SMAW covered electrodes use tubular core fabricated wires filled with alloy powder and/or flux. Covered electrodes and bare wire are available in a variety of diameters, lengths, and spool sizes. The welding process, amount of welding, accessibility, and equipment to be used all effect the selection of product form and size. Where a uniform inside weld root configuration is required for erosion or other design considerations or where the GTAW or GTAW-P-AU welding processes are to be used, a consumable insert is generally specified. Consumable inserts are shape dependent with the K-style, 1/8-inch x 5/32-inch (3.2-mm x 4.0-mm), rectangular (radiused corners) style gaining favorable recognition because of its ease and tolerance to fit-up variations. 11.2
Filler Metal Requirements
Weld filler metals are manufactured and supplied in accordance with predetermined specifications. Table 11-3 lists those weld filler metal specifications that might apply to welding the variety of candidate valve repair and replacement materials. The most current (at this writing) American Welding Society (AWS) A5.XX-XX specifications are listed. Although ASME SFA5.XX-XX specifications are usually identical to AWS specifications, AWS issues new additions before ASME.
11-2
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Table 11-3 Weld Filler Metal Specifications [37–52] AWS Specification
Specification Title
A5.01-87
“Filler Metal Procurement Guidelines”
A5.1-81
“Specification for Covered Carbon Steel Arc Welding Electrodes”
A5.4-81
“Specification for Covered Corrosion-Resisting Chromium and Chromium-Nickel Steel Welding Electrodes”
A5.5-81
“Specification for Low-Alloy Steel Covered Arc Welding Electrodes”
A5.6-84
“Specification for Covered Copper and Copper Alloy Arc Welding Electrodes”
A5.7-84
“Specification for Copper and Copper Alloy Bare Welding Rods and Electrodes”
A5.9-81
“Specification for Corrosion-Resisting Chromium and Chromium-Nickel Steel Bare and Composite Metal Cored and Stranded Welding Electrodes and Welding Rods”
A5.11-83
“Specification for Nickel and Nickel Alloy Covered Welding Electrodes”
A5.13-80
“Specification for Solid Surfacing Welding Rods and Electrodes”
A5.14-89
“Specification for Nickel and Nickel Alloy Bare Welding Electrodes and Rods”
A5.16-90
“Specification for Titanium and Titanium Alloy Welding Electrodes and Rods”
A5.18-79
“Specification for Carbon Steel Filler Metals for Gas-Shielded Arc Welding”
A5.20-79
“Specification for Carbon Steel Electrodes for Flux-Cored Arc Welding”
A5.21-80
“Specification for Composite Surfacing Welding Rods and Electrodes”
A5.22-80
“Specification for Flux Cored Corrosion-Resisting Chromium and Chromium-Nickel Steel Electrodes”
A5.28-79
“Specification for Low-Alloy Steel Filler Metals for Gas-Shielded Arc Welding”
A5.29-80
“Specification for Low-Alloy Steel Electrodes for Flux-Cored Arc Welding”
A5.30-79
“Specification for Consumable Inserts”
These filler metal specifications list chemical, mechanical property, packaging, labeling, and safety criteria. Ranges are provided, where technically acceptable, to enable a variety of vendors to manufacture products within the stated criteria. Each specification is accompanied by an appendix that offers additional practical information, such as a short commentary on each weld metal contained in the specification, its possible uses, and precautions where necessary [37–52]. Regardless of the weld filler metal, certain requirements beyond the AWS or ASME specifications need to be incorporated into the procurement documents. Packaging plus physical condition and cleanliness are very important. Although normally required to be in hermetically sealed containers, covered electrodes should be in an undamaged state with no evidence of broken or cracked flux coating. Damaged covered electrodes should be discarded as scrap or returned to the vendor. Bare wire in straight lengths and on spools should be free of dirt, grease, or any other foreign substance that can be either seen visually or observed by use of the “white glove test.” 11-3
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Wire exhibiting such deleterious material should be cleaned, scrapped, or returned to the vendor—but never used as-is. Even when bare wire appears clean, it should be wiped down with an approved solvent immediately prior to welding. A requirement to have spooled bare wire individually wrapped in sealed plastic bags can be very helpful as a moisture protection measure, especially if long term storage is anticipated. These precautionary measures usually cost a few cents per pound, but are well worth the investment. It is not unusual for remedial measures that are needed to correct problems resulting from dirty wires to require the expenditure of person hours and dollars that exceed by many times the original purchase price of the weld filler metal. 11.3
Filler Metal Selection
Weld metals are typically chosen to provide compatible as-welded chemistry and mechanical properties. Within this criteria, the weldments generally exhibit acceptable inservice performance. Because repair or replacement has been determined necessary, emphasis must be placed on the base metals and performance of the completed weldment. The as-welded chemistry of the weld deposit and base metal heat-affected zone (HAZ) are now of primary importance [37–56]. Weld filler metal chemical compositions chosen for original fabrication and installation were usually selected to provide a deposit with overmatching properties to compensate for dilution with the base metal. This might not always be the case in repairs, because existing system materials, although providing adequate service, might not be in their originally installed condition. Thus, alternate weld material choices might be necessary to consider. HAZ concerns, however, can be addressed only through control of the welding process and technique. In most cases, a variety of weld filler metal options exist within any alloy category, and the actual choice is dependent on specific performance and ability-to-weld factors. Tables 11-4 and 11-5 provide some of the options available in filler metal selection. These choices should be considered only as starting points in the development of welding procedures. A brief commentary for those filler metal specifications typically used is provided in Section 11.8.
11-4
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Table 11-4 Typical Filler Metals for Various Base Metals [22]
P-No.
Base Metal
C-Steel
C-Mo
1/2 to 1-1/4 Cr+Mo
2 and 2-2/4 Cr+Mo
3 and 5 Cr+Mo
7 and 9 Cr+Mo
300 Series Stainless
1
C-Steel
A
A
A
A
A
A
G
3
C-Mo
A
B
B
B
B
B
G
4
1/2 to 1-1/4 Cr+Mo
A
B
C
C
C
C
G
5
2 and 2-1/4 Cr+Mo 3 and 5 Cr+Mo 7 and 9 Cr+Mo
A A A
B B B
C C C
D D D
D E E
D E F
G G G
8
300 Series Stainless
G
G
G and H
H
H
H
I
Filler Metal Descriptions Table Designation
Composition
Examples for GTAW or GMAW
Examples for SMAW
A=
Carbon steel
E7016 or E7018
ER70S-2 or ER70S-3
B=
Carbon-1/2 moly
E7016-A1 or E7018-A1
ER80S-B2 or ER80S-B2L4
C=
1-1/4 chrome-1/2 moly
E8018-B2 or E8018-B2L4
ER80S-B2 or ER80S-B2L4
D=
2-1/4 chrome-1 moly
E9018-B3 or E9018-B3L4
ER90S-B3 or ER90S-B3L4
E=
5 chrome-1/2 moly
E502-15 or E502-16
ER502
F=
9 chrome-1 moly
E505-15 or E505-16
ER505
G=
Stainless steel
E309 or E309L
ER309 or ER309L
H=
Nickel-chromium-iron
ENiCrFe-2 or ENiCrFe-3
ERNiCr-3
I=
See Table 11-5
Notes: 1. If both base metals are 1/2 chrome-1/2 moly, the filler metal for SMAW can be E8018-B1. 2. For noncyclic thermal service below 600 °F (315 °C) and when post-weld heat treatment is not used, Type 309 stainless steel is acceptable. 3. See the appropriate AWS filler metal specifications for filler metals for other welding processes. 4. Recent ASME Code changes have restricted the use of low carbon (L-grade) electrodes for certain applications, due to their reduced creep rupture properties (ASME Section I, 1992, PW-5.6; ASME B31.1, 1992, 127.2).
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Table 11-5 Weld Metal for Joining Selected Stainless Steel Base Materials [64] Base Metal B Base Metal A
304L
316L
347
308L
308L 316L
308L 347L
316L/CF-3M
308L
316L
347L
347/CF-8C
308L
347L
347L
304/CF-8 304/CF-3 316/CF-8M
Note: The 300-series stainless weld filler metal should be procured to exhibit an as-welded ferrite level of 8 FN to 15 FN. The range should be determined by magnetic measurement in accordance with AWS A4.2-74, “Standard Procedures for Calibrating Magnetic Instruments to Measure the Delta Ferrite Content of Austenitic Stainless Steel Weld Metal.”
Covered electrodes and bare wire are available in a variety of diameters, lengths, and spool sizes. Consumable inserts are shape-dependent, with the K-style, 1/8 inch x 5/32 inch (3.2 mm x 4.0 mm), gaining favorable recognition (see Figure 11-1). Table 11-6 illustrates common product form sizes available and those that are the most popular for piping and valve work.
11-6
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Shape "A" (EB)
Shape "J"
(a)
(b)
Shape "K"
Shape "G"
(c)
(d)
Shape "Y" (e)
Figure 11-1 Styles of Consumable Inserts [51]
11-7
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Table 11-6 Popular Electrode Product Forms and Available Sizes [22] Available Sizes (Note 1) Product
Popular Sizes
Diameter
Length
Spool
Diameter
Length
Spool
Covered Electrode
1/16– 5/32
14– 16
N/A
3/32, 1/8, & 5/32
14
N/A
Bare Wire (GTAW)
0.020 –5/32
18– 36
N/A
1/16, 3/32, & 5/32
36
N/A
Bare Wire (GTAW-AU)
0.020 –1/16
N/A
2–60 lb.
0.035 & 0.045
N/A
#2 & 10
Bare Wire (GMAW)
0.020 –1/16
N/A
2–60 lb.
0.035 & 0.045
N/A
All
Bare Wire (SAW)
1/16– 1/4
N/A
25– Max. lb.
All
N/A
All
Cored Wire (FCAW)
0.30– 1/16
N/A
14– 60 lb.
0.045 & 1/16
N/A
All
Form
Note: 1. All sizes presented in inches. To convert to SI units multiply by 25.4 mm.
11.3.1 A-Numbers Weld metals are grouped into classes known as A-Numbers, depending on their chemical composition. A-Number designations are shown in Table 11-7. It should be noted that the A-Numbers do not always correspond to the P-Numbers. The A-Numbers for the SMAW and GTAW filler materials most commonly used for piping and associated repairs can be summarized as follows [12, 53]: SMAW
GTAW
A-Number
Nominal Composition
E7018
ER70S-2
1
Mild carbon steel
E7018-A1
ER80S-D2
2
Carbon-0.5% Mo
E8018-B2
ER80S-B2
3
0.4%–2% Cr-Mo
E9018-B3
ER90S-B3
4
2%–6% Cr-Mo
E505
ER505
5
6%–10% Cr-Mo
E316H
ER316H
8
Austenitic stainless steel
A new procedure qualification is required when there is a change in the A-Number. However, qualification with A-Number 1 also qualifies for A-Number 2, and vice versa.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Table 11-7 A-Numbers—Classification of Ferrous Weld Metal Analysis for Procedure Qualification [22] Analysis, % 1 A-No.
Types of Weld Deposit
C
Cr
Mo
Ni
Mn
Si
1
Mild Steel
0.15
--
--
--
1.60
1.00
2
Carbon-Molybdenum
0.15
0.50
0.40– 0.65
--
1.60
1.00
3
Chrome (0.4% to 2%)Molybdenum
0.15
0.40– 2.00
0.40– 0.65
--
1.60
1.00
4
Chrome (0.2% to 6%)Molybdenum
0.15
2.00– 6.00
0.40– 1.50
--
1.60
2.00
5
Chrome (0.6% to 10.5%)-Molybdenum
0.15
6.00– 10.50
0.40– 1.50
--
1.20
2.00
6
Chrome-Martensitic
0.15
11.00 –15.00
0.70
--
2.00
1.00
7
Chrome-Ferritic
0.15
11.00 –30.00
1.00
--
1.00
3.00
8
Chromium-Nickel
0.15
14.50 –30.00
4.00
7.50– 15.00
2.50
1.00
9
Chromium-Nickel
0.30
25.00 –30.00
4.00
15.00 –37.00
2.50
1.00
10
Nickel to 4%
0.15
--
0.55
0.80– 4.00
1.70
1.00
11
Manganese-Molybdenum
0.17
--
0.25– 0.75
0.85
1.25– 2.25
1.00
12
Nickel-ChromeMolybdenum
0.15
1.50
0.25– 0.80
1.25– 2.80
0.75– 2.25
1.00
Note: 1. Single values shown above are maximum.
11.3.2 F-Numbers Electrodes and welding rods are grouped, according to their usability characteristics, into F-Number designations. This grouping is made to reduce the number of welding procedures and performance qualifications. The numbers are tabulated in Table 11-8 for ferrous alloys that correlates F-Number, AWS/ASME Specification number, and AWS Classification Number. The most commonly used filler materials and their corresponding F-Numbers can be summarized as follows [12, 53]: F-Number 1 2 3 4 5 6 43
Filler Material E7020, E7024 E6013, E7014 E6010, E6011 E7018, E8018-B2, E9018-B3 E7016, E8016-B2, E9016-B3 E308-15, E308-16, E309-15, E309-16 E316-15, E316-16, E347-15, E347-16 ER70S-2, ER80S-B2, ER90S-B3 ER308, ER309, ER316, ER347 ENiCrFe2, ENiCrFe3, ERNiCr3 11-9
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
A new procedure is required when there is a change from one F-Number to any other F-Number, for example, two different procedures are required to join carbon steel using E6011 and E7018 electrodes. This is because there is a change in F-Number, E6011 is F-3 and E7018 is F-4. The other essential variables have to be considered because it takes only one variable change to require separate qualification. For example, if only F-Number was considered, E7018 or E9018-B3 could be used interchangeably because they have the same F-Number. However, the weld metal analysis or A-Number is another essential variable that has to be considered. Because E7018 and E9018-B3 have different A-Numbers, they cannot be used for the same applications. Alternatively, E7016 and E7018 have both the same A- and F-Numbers, so the same procedure can be used for both filler materials. Table 11-8 F-Number Grouping of Welding Electrodes and Rods [22] (SOURCE: ASME Section IX, 1990 QW 432) F-No .
ASME Specification No.
AWS Classification No. Steel and Steel Alloys
1
SFA-5.1 and 5.5
EXX20, EXX24, EXX27, EXX28
2
SFA-5.1 and 5.5
EXX12, EXX13, EXX14
3
SFA-5.1 and 5.5
EXX10, EXX11
4
SFA-5.1 and 5.5
EXX15, EXX16, EXX18, EXX48
4
SFA-5.4 (ferritic)
EXX15, EXX16
5
SFA-5.4 (austenitic)
EXX 15, EXX16
6
SFA-5.2
RX
6
SFA-5.17
FXX-EXX
6
SFA-5.9
ERXX
6
SFA-5.18
ERXXS-X
6
SFA-5.20
EXXT-X
6
SFA-5.22
EXXXT-X
6
SFA-5.23
FXX-EXXX-X, FXX-ECXXX-X, and FXX-EXXX-XN, FXX-ECXXX-XN
6
SFA-5.28
ER-XXX-X and E-XXX-X
6
SFA-5.29
EXXTX-X
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
11.4
Storage of Welding Filler Materials
In view of the adverse effects of hydrogen in welding and because a potential source of this hydrogen is the moisture in electrode coatings, the proper storage and treatment of electrodes is essential to obtain crack-free welds. All low hydrogen electrodes should be purchased in hermetically sealed containers, which reduce the risk of hydrogen pick-up. The low hydrogen electrodes, used in the SMAW process for mild steel and low-alloy steels, (that is, those with designations EXX15, 16, or 18 where XX is 70, 80, 90, and so on) pick up moisture very rapidly if left exposed to the atmosphere. These types of inorganic covered electrodes are designed and developed to contain the very minimum of moisture in their coatings (typically less than 0.5%). Therefore, maintaining a low moisture content for the low hydrogen electrodes is most critical, and precautions need to be taken to avoid picking up moisture, as follows: • Ensure that the delivered hermetically-sealed containers are not damaged. • After a can has been opened, all the electrodes must be stored in holding ovens at a temperature of 250˚F (121˚C) minimum. It is important to check the oven temperature because if it is too low, moisture can be picked up; and if it is too high [above 450˚F (232˚C)], the coating can become brittle. • Portable heating ovens should be used at the weld repair area so that the electrodes are not left exposed to the atmosphere. • Any electrodes that are left out in the atmosphere for over four hours (this time can be reduced for humid conditions and for the higher alloy rods, for example, one hour for E9018 electrodes) must not be used unless they are rebaked. If the electrodes are exposed to water, they must be discarded. Baking involves heating the E70/80/90-15, -16, -18 rods to 700˚F–800˚F (371˚C–427˚C) for at least one hour in a drying oven. The rods must be transferred directly from the drying oven to the holding ovens. A clean dry place is sufficient to store E6010 or E6011 electrodes because they do not absorb moisture like the low hydrogen rods. However, care should be exercised in the rebaking of these rods because the coverings are designed to have a moisture level of 3% to 7%, and excessive drying can substantially effect their operation. They should not be redried unless noticeably wet or if they have been exposed to very high humidity for a long period of time. The electrode suppliers should be consulted for the exact rebaking conditions. The GTAW and GMAW filler metals are supplied in boxes. They should be stored in a dry atmosphere and at normal temperature. The surface of the wire should be free of rust, oil, dirt, and moisture. 11.5
Handling of Welding Filler Materials
A utility should have an effective control program for purchasing, receiving, inspection, storage, and handling of welding materials. It is essential that all welding filler materials are correctly identified and not mixed. For example, separate areas are required in the holding ovens so that the different electrodes are not placed in the same location. 11-11
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Bare GTAW filler wires that are different compositions, but are not stamped or tagged, can easily be mixed up. A system is required so that the correct filler materials are issued for a repair. 11.6
Dissimilar Weldments
The need to join dissimilar alloys might be more prevalent in repair and replacement activities. Caution must be observed when selecting weld filler metals for dissimilar welds to ensure that the connection will satisfy corrosion, mechanical, and physical property criteria [44–52, 54–56]. A transition in mechanical and physical properties from one alloy to another is also important. Tensile and yield strength of the weld filler metal should be complementary to the two alloys being joined and not severely overmatched. Other factors including thermal coefficients of expansion/contraction are also important such that unnecessarily high operational stresses can be avoided or taken into consideration in the design. Caution must always be observed in making dissimilar weldments. Minimization of dilution is usually required to avoid excessive changes to the as-deposited weld filler metal composition. This is especially true where nickel filler metals are used. Excessive dilution with iron (> 20%) usually results in cracking. However, the nickel alloy weld filler metals are often chosen for dissimilar weldments and can usually be deposited ontop-of most of the carbon and stainless steels. Conversely, stainless steel and carbon steel weld metals do not usually produce acceptable crack-free deposits on-top-of nickel or its alloys. Tables 11-4, 11-5, and 11-9 illustrate viable candidate weld filler metals for joining dissimilar alloys. Various hardfacing weld filler metals are included in Table 11-10. Again, these are only recommended starting points for procedure evaluation and qualification. Actual conditions dictate which materials are appropriate.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Table 11-9 Filler Metal Selection for Different P-Number Combinations [22] Filler Materials P-Number Combinations
GTAW
SMAW
P-Nos. 1 to 1
ER70S-2
E7018
P-Nos. 1 to 3
ER70S-2 or ER80S-D2
E7018 or E7018-A1
P-Nos. 1 to 4
As above or ER80S-B2
As above or E8018B2
P-Nos. 1 to 5
As above or ER90S-B3
As above or E9018B3
P-Nos. 1 to 8
ER309 or ERNiCr-3
E309, ERNiCrFe-2 or 3
P-Nos. 3 to 3
ER80S-D2
E7018-A1
P-Nos. 3 to 4
ER80S-D2 or ER80S-B2
E7018-A1 or E8018-B2
P-Nos. 3 to 5
As above or ER90S-B3
As above or E9018-B3
P-Nos. 3 to 8
Same as P-Nos. 1 to 8
Same as P-Nos. 1 to 8
P-Nos. 4 to 4
ER80S-B2
E8018-B2
P-Nos. 4 to 5
ER80S-B2 or E9018-B3
E8018-82 or E8018-83
P-Nos. 4 to 8
Same as P-Nos. 1 to 8
Same as P-Nos. 1 to 8
P-Nos. 5 to 5
ER90S-B3
E9018-B3
P-Nos. 5 to 8
Same as P-Nos. 1 to 8
Same as P-Nos. 1 to 8
P-Nos. 8 to 8
ER308, ER316, or ER347
E308, E316, or E347
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Table 11-10 Selected Weld Filler Metals Used for Hardfacing and Valve Seat Applications [57] Typical Hardness Rc (2 layers)
Alloy
Typical Chemical Composition
GTAW
SMAW
Product Forms Bare Rod
Covered Electrodes
#6 RCoCr-A (Bare) ECoCr-A (Coated)
C 1.1 Cr 28 W4 Cobalt Base
40
39
1/8 5/32 3/16 1/4 5/16
1/8 5/32 3/16 1/4
#12 RCoCr-B (Bare) ECoCr-B (Coated)
C 1.4 Cr 29 W8 Cobalt Base
47
40
Same as #6
Same as #6
#21 (No AWS Classification)
C 0.25 Cr 27 Mo 5 Ni 2.8 Cobalt Base
24 (45**)
26 (45**)
Same as #6 plus .045” on spools
Same as #6
NOREM ™ 02A (No AWS Classification)
C 1.25 Cr 25 Mo 2 Ni 4.5 Iron Base, Cobalt Free
38
36
.045 (Solid, Tubular) 1/8 5/32 Strip
1/8
410 ER410 (Bare) E410-15/16 (Coated)
C .12 Cr 12 Mo .75 Ni .6
31
30
0.35, .045 3/32 1/8 5/32 3/16
3/32 1/8 5/32 3/16
Deloro ™ 40 Colmonoy ™ 4 RNiCr-A (Bare)
C 0.45 Cr 10 Si 2.25 Nickel Base
38
N/A
1/8 5/32
N/A
Deloro 50 Colmonoy 5 RNiCr-B (Bare)
C 0.35 Cr 13 Si 3.5 Nickel Base
48
N/A
1/8 5/32
N/A
5/32
**Work hardened
Transition pieces containing two different alloys joined in the same piece of material can simplify dissimilar connections. Such items can be fabricated by buttering one material with selected weld metal, by joining two different materials with the hot isostatic processing (HIP) technique, or by using a flanged transition assembly.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
11.7
Filler Metals for Hardfacing Applications
Selection of weld filler metal for hardfacing applications is influenced by the substrate material (valve body, and so on), service media/environment, temperature, and whether mechanical influences such as galling, friction, or erosion resistance exist. Table 11-10 lists some of the more popular hardfacing weld filler metals [57]. 11.8
Typical Weld Filler Metal Specifications [37–52]
AWS A5.1-91, “Specification for Covered Carbon Steel Arc Welding Electrodes.” The first two digits designate minimum tensile strength of the deposited metal in the aswelded condition. The third digit indicates the position in which the electrode is capable of making satisfactory welds. All positions are 1; 2 is for flat and horizontal fillet welds; 4 is for vertical down and other positions. The last two digits indicate current and polarity for use of the electrode, and coating type. Example: E6012 indicates an electrode (E) depositing 60 ksi (kips per square inch), (413 MPa) tensile-strength weld metal (60), for use in all positions (1) with ac or direct current, straight polarity (dcsp) (2). AWS A5.4-92, “Specification for Covered Corrosion-Resisting Chromium and Chromium-Nickel Steel Welding Electrodes.” The first three digits designate the composition. The letter L indicates low-carbon modifications; H denotes high carbon. Other modifications are indicated by adding the appropriate chemical symbol to the designation. Two final digits indicate usability: 15, direct current, reverse polarity (dcrp); 16, dcrp; and ac. Example: E316L-15 is a covered electrode of 316 stainless steel having a low carbon level, to be used with dcrp. AWS A5.5-96, “Specification for Low-Alloy Steel Covered Arc Welding Electrodes.” The first two or three digits designate minimum tensile strength of deposited weld metal. The third or fourth digit indicates electrode position: 1, all; 2, flat and horizontal fillet. The last digit indicates the current to be used and the type of electrode covering. The suffix, with or without a numeral, designates the chemical composition of deposited weld metal. Example: E7027-A1 is a covered electrode (E), depositing a weld of 70 ksi (482 MPa) tensile strength (70), useful for flat and horizontal fillet (2) welds, ac or dc, sp or rp, having an iron powder coating (7). A1 indicates the weld metal composition. AWS A5.9-93, “Specification for Corrosion-Resisting Chromium and Chromium-Nickel Steel Bare and Composite Metal Cored and Stranded Welding Electrodes and Welding Rods.” Chemical composition classifies these alloys, as indicated by the numeric designation for the stainless steel type. Si indicates high silicon (0.65% by weight to 1.00% by weight); other symbols indicate a high content of those elements. Example: ER316L-Si is an electrode/rod of 316 stainless steel, low carbon (L), and high silicon (Si).
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
AWS A5.11-90, “Specification for Nickel and Nickel Alloy Covered Welding Electrodes.” Classification is based on the chemical composition of undiluted weld metal. The symbol Ni identifies the electrode as a nickel-base alloy; succeeding chemical symbols identify the principal alloying elements. Numbers following hyphens identify a modification within an alloy series. Example: ENiMo-3 is a nickel-base (Ni), molybdenumalloy (Mo) electrode (E), containing 23% to 27% molybdenum (-3). AWS A5.13-80, “Specification for Solid Surfacing Welding Rods and Electrodes.” Bare surfacing welding rods and electrodes are classified on the basis of their chemical composition as manufactured. Covered surfacing electrodes are classified on the basis of the chemical composition of their undiluted weld metal. AWS A5.14-89, “Specification for Nickel and Nickel Alloy Bare Welding Electrodes and Rods.” The symbol Ni identifies the rods as nickel-based, other symbols identify principal alloying elements. Numbers following hyphens indicate a specific alloy within a group. Example: ERNiMo-2 is an electrode (E) or rod (R), nickel with molybdenum-alloy (NiMo), having 15% to 18% molybdenum and other limits on composition (-2). AWS A5.17-89, “Specification for Carbon Steel Electrodes and Fluxes for Submerged Arc Welding.” Electrodes are classified by as-manufactured chemical composition. The letters L, M, or H following E indicates low, medium, or high manganese; the number following indicates the nominal carbon content; and the letter K indicates silicon-killed steel. Fluxes are classified by the tensile and impact strength of a weld made in combination with a particular electrode. The letter F indicates a flux; the number following indicates the minimum tensile strength in 10 ksi (69 MPa). The letter A or P designates the condition of heat treatment, as-welded or post-weld treated. The number following indicates the lowest temperature at which the impact strength meets or exceeds 20 ft lb (27J). Example: F7A6-EM12K refers to a flux that produces a weld metal which, in the aswelded condition, has a tensile strength no lower than 70,000 pounds per square inch (psi) (482 MPa) and Charpy V-notch impact strength of at least 20 ft lb (27J) at -60˚F (-51˚C) when deposited with an EM12K electrode. AWS A5.18-93, “Specification for Carbon Steel Filler Metals for Gas-Shielded Arc Welding.” Chemical composition and mechanical properties of the weld metal are the basis for classification. Example: ER 70S-4 indicates an electrode or rod (ER) weld metal with 70 ksi minimum tensile strength (70), solid wire (S), covered by a required soundness and tension test (-4), and no impact requirement. AWS A5.20-95, “Specification for Carbon Steel Electrodes for Flux-Cored Arc Welding.”
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
The first number following E indicates the minimum tensile strength of the weld metal; the second number indicates the welding position; the letter T indicates a tubular (fluxcored) electrode; a letter or number following a hyphen indicates other characteristics. Example: E70T-5 is an electrode (E); weld metal with a tensile strength of 70 ksi (482 MPa) minimum (7); flat and horizontal position (0); tubular (T) wire; used with CO2 shield (-5). A5.21-80, “Specification for Composite Surfacing Welding Rods and Electrodes.” Composite surfacing welding rods and electrodes are classified on the basis of the chemical composition of the weld metal with the exception of the composite tungstencarbide welding rods and electrodes. Composite tungsten-carbide surfacing welding rods and electrodes are classified on the basis of the chemical composition of the carbon steel tube, the chemical composition of the tungsten-carbide granules, and the mesh size distribution and percent by weight of the tungsten-carbide granules contained in the carbon steel tube. AWS A5.22-95, “Specification for Flux-Cored, Corrosion-Resisting Chromium and Chromium-Nickel Steel Electrodes.” Classification is based on chemical composition of the deposited weld metal and on the shielding medium required. Numbers following the E indicate the American Iron and Steel Institute (AISI) stainless steel grade; the letter T indicates a flux-cored electrode. The numbers following the hyphen indicates the gas shield: 1, CO2; 2, Ar + 2% O2; 3, none; G, not specified. Example; E308T-3 is a flux-cored (T) electrode (E) of 308 stainless steel requiring no shielding gas (-3). It is a self-shielding electrode. AWS A5.23-90, “Specification for Low-Alloy Steel Electrodes and Fluxes for Submerged Arc Welding.” Bare solid electrodes are classified by chemical composition of deposited weld metal. The letters EC indicates a tubular electrode. Fluxes are classified according to certain mechanical properties and by the chemical composition of a weld deposit made by using the flux with a particular electrode. See A5.17. AWS A5.28-79, “Specification for Low-Alloy Steel Filler Metals for Gas-Shielded Arc Welding.” These are classified according to chemical composition and mechanical properties of weld metal deposited by GMAW, GTAW, and PAW (plasma arc welding) procedures might be used. The numbers following E or ER indicates the required minimum tensile strength in ksi, the letter S designates a bare solid electrode or rod; and the letter C represents a cored electrode. A suffix following a hyphen designates a specific asmanufactured chemical composition. Example: ER80S-Ni1 is a solid (S) nickel steel (Ni) electrode-rod (ER) that deposits 80 ksi (551 MPa) tensile weld metal (80) of a specific composition (1). AWS A5.29-80, “Specification for Low-Alloy Steel Electrodes for Flux-Cored Arc Welding.”
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
These are classified according to chemical composition and mechanical properties of the weld metal, type of current, position of welding, and whether CO2 is used as a separate shielding gas. The letter E designates an electrode. The first number following indicates the minimum tensile strength; the second indicates the primary welding position, flat and horizontal (0) or all positions (1). The letter T designates a flux-cored electrode. The number following T indicates usability and performance capability. A suffix following a hyphen shows as-manufactured chemical composition. AWS A5.29-80, “Specification for Low-Alloy Steel Electrodes for Flux-Cored Arc Welding.” These are classified according to chemical composition and mechanical properties of the weld metal, type of current, position of welding, and whether CO2 is used as a separate shielding gas. The letter E designates an electrode. The first number following indicates minimum tensile strength; the second indicates the primary welding position, flat and horizontal (0) or all positions (1). The letter T designates a flux-cored electrode. The number following T indicates usability and performance capability. A suffix following a hyphen shows as-manufactured chemical composition. AWS A5.30-79, “Specification for Consumable Inserts.” The basis of the classification is chemical composition. The prefix IN designates a consumable insert. Numbers indicate as-manufactured chemical composition. Specific shapes and cross-sections must be selected and specified when ordering.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
12 SHIELDING AND PURGING GASES Delivery of gases is very important and can be accomplished in several different ways: single or manifold compressed gas bottles, single or multiple liquid gas dewars, or a bulk system inherent to the plant or temporarily installed for repair or replacement outages. Many opportunities for contamination exist between the gases’ source and the welding torch and weldment. Shielding and purge gases used for welding operations must be free of moisture and other contaminants (foreign gases, and so on). For this reason, gas originating from a liquid source is generally preferred and temporary bulk systems are often installed to support major repair or replacement outages [58–65]. Purity and moisture content concerns can be reduced by specifying a dew point of -60˚F (-51˚C) maximum at the bottle for compressed gas cylinders or the gaseous form after leaving a dewar. The dew point of the gas at the weld should not exceed -40˚F (-40˚C) if possible. Additional moisture removal can be achieved by adding a molecular sieve (for example, Hydro-Purge) to the purge or shielding gas lines where single composition gases are used [65]. These relatively inexpensive devices provide an extra measure for ensuring that foreign gases do not contaminate the shielding or purging operations. Contaminated gas or lack of shielding usually promotes the formation of deleterious oxides or even heavily oxidized surfaces (sugaring). When this occurs, the only prudent remedy requires that the disturbed material be completely removed. Attempts of removal with manual or power wire brushing are usually unsatisfactory. Power grinding or machining to sound, unaffected base or weld metal is usually necessary. 12.1
Purging
Where the system design prohibits the use of backing rings and complete joint penetration with a continuous root surface is needed, purging the inside diameter of the weldment with an inert gas is required. Purging is generally required for all stainless and nonferrous piping system welds, particularly for open root and consumable insert groove configurations. Certain thin wall socket and fillet welds might also require inside diameter or backside purging. Welding grade argon (99.995% pure) is the purge gas that is used most often. Other gases or mixtures can be used, but their suitability requires determination on a case-by-case basis. Sample purge dams and fixtures typical of those in general use are illustrated in Figure 12-1.
12-1
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair Hinges Approx. 6 in. (150 mm)
Retrieval Cord
Weld
Retrieval Cord
Hinged Disks Hinged Collapsible Purging Disk
Vent Hole
Chain or Linkage Pull Cord
Gas Inlet Hose Inlet Opening Rubber Gasket
Figure 12-1 Sample Fixtures for Purging the Inside Diameter of Pipe Weldments [58] Although means for calculating the existence for a proper purge via volume exchanges can be found in references such as AWS D10.11, “Recommended Practices for Root Pass Welding of Pipe Without Backing,” the best and most efficient method is to measure the oxygen content (< 1%) and/or dew point (< -40˚F) at the weld joint just prior to welding. Various devices are commercially available for measuring these variables. 12.2
Shielding
Welding grade argon is also the gas of choice for shielding the arc with GTAW. Where torch designs permit, gas lenses should be used to promote enhanced nonturbulent gas coverage. Use of molecular sieves is encouraged where single gases are employed. Argon-helium mixtures (up to 50%/50%) are also used because of their ability to increase penetration and decrease lack of fusion problems. The addition of active gases such as oxygen (0.5% to 5%) and CO2 (5% to 25%) to argon or argon-helium mixtures is also particularly useful in stabilizing FCAW or GMAW. The use of 100% CO2 shielding gas is not generally used for piping or heavy component repair applications.
12-2
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
13 PREHEAT/POST-WELD HEAT TREATMENT REQUIREMENTS The candidate materials for piping and valve repairs generally require elevated preheating prior to welding to obtain satisfactory joints. Table 13-1 provides examples of the nominal minimum recommended preheat temperatures required for the noted materials. The applicable code should be reviewed to obtain specific requirements, as appropriate [12, 32, 53, 55, 56, 59, 61, 66]. Table 13-1 Preheat, Interpass, and PWHT Guidelines [49, 54, 56] Material
Preheat (° F)
Interpass ( °F)
PWHT
Carbon Steel P-No. 1 P-No. 3 P-No. 4 P-No. 5
50 min.2 200 min. 2 300 min. 2 400 min.2
500 (260 °C) max. 500 (260 °C) max. 650 (343 °C) max. 750 (400 °C) max.
Notes 1&3 Note 1 Note 1 Note 1
Copper & Alloys Copper Alloys
Note 4 50 min.
N/A Note 5
None None
Nickel & Alloys
50 min.2
500 (260 °C) max.
None
Stainless Steels 300 Series AL-6XN
50 min. 50 min.
350 (177 °C) max. 200 (93° C) max.
None 6 None 7
Notes: 1. See Table 13-4 for exemptions to post-weld heat treat. 2. Preheat of 200 °F to 400°F (93 °C to 204°C) is helpful on large castings and fittings. 3. Thickness of material, t < 0.750 in (19 mm). 4. Preheat for commercially pure copper varies as follows: Thickness (in) Preheat °F(min.) 3/16-1/4 (4.8 mm–6.4 mm) 200 (93 °C) 1/4-3/8 (6.4 mm–9.5 mm) 300 (149 °C) over 3/8 (9.5 mm) 500 (260 °C) 5. Maximum interpass temperature for copper-zinc, phosphor-bronze, aluminum-bronze, and copper-nickel is not applicable. Silicon-bronze is established at 200 °F (93 °C) maximum. 6. If required for re-establishing corrosion resistance, solution anneal at 1 ,800°F to 1,900°F (982 °C to 1,038°C) followed by quench in water or other rapid means. 7. If required for re-establishing corrosion resistance, solution anneal at 2 ,050°F (1 ,121°C) minimum followed by quench in water or other rapid means.
Sound workmanship practice would normally result in applying some preheat prior to welding to ensure an absence of moisture or other deleterious matter. When the work is 13-1
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
conducted in cool temperatures or areas of high humidity, warming the weld groove area to prevent condensation is prudent. These low “warm to the touch” preheats generally range from 100˚F to 150˚F (38˚C to 66˚C). After welding has begun, maintenance of preheat is usually required on heavy sections. However, maximum interpass temperatures should not be exceeded. Preheating to the “warm to the touch” temperature range is generally not controlled or even monitored. Where precise control is required, electrical resistance heating is preferred. Unless sophisticated torch arrangements are used, flame heating is usually employed only for general preheating. In either case, preheat temperatures can be monitored with temperature-indicating crayons, direct reading pyrometers, or thermocouples wired to precision meters or chart recorders. Use of electrical resistance heating that is monitored and controlled with thermocouples provides the most accurate preheat approach (see Figure 13-1). Power Lead
Stainless Band
See Note 2
See Note 1
Insulation
Insulating Pad
Notes: 1. Use temperature-indicating crayons here for proper temperature reading at the weld bevel. 2. Minimum distance without interfering with welding operation.
Insulation Banding Thermocouple
Figure 13-1 Typical Setup for Preheating Pipe Welds with Electrical Resistance Equipment [66] Elevated preheats are employed to reduce thermal shock to the component, minimize distortion, and reduce the propensity for cracking. Whereas a post-weld heat treatment (PWHT) is normally applied to reduce residual stresses, post-weld heating or soaking at or above the preheat temperature is applied to assist in removal of hydrogen from the weld and HAZ. 13-2
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Thermal post-weld treatments are generally used to enhance or transform the metallurgical structure or to reduce tensile residual stresses created from fit-up and/or welding operations. Caution must be observed in the implementation of thermal treatments to avoid creating one problem while solving another, for example, reducing residual tensile stresses by stress relieving but ending up with a structure that is severely distorted and unfit for service. This is especially true with valves and internals. Manufacturer maintenance and installation recommendations should be consulted. Additional information can be found in AWS D10.10-90, “Recommended Practices for Local Heat Treatment of Welds in Piping and Tubing” [66]. Preheat and PWHT are both essential variables of the welding procedure. A decrease of more than 100˚F (38˚C) in the preheat temperature qualified requires a new welding procedure specification (WPS). For PWHT, a separate procedure qualification record (PQR) is required for each of the following conditions: • No PWHT • PWHT within a specified temperature range relative to the upper and lower transformation temperatures • For a test coupon Procedure Qualification Record (PQR) receiving a PWHT in which the upper transformation temperature range is exceeded, the maximum qualified thickness for production welds is 1.1 times the thickness of the test coupon. 13.1
Preheat
The main purposes of preheat are: • Retarding the cooling rates of the weld metal and heat-affected zone. This can result in a HAZ with a lower hardness and lower cracking tendency. • Reducing the magnitude of shrinkage stresses • Reducing the risk of hydrogen cracking in the weld metal and HAZ. Higher preheat temperatures permit hydrogen to diffuse away from the weld metal and HAZ. • Evaporating moisture (another source of hydrogen) that might be present at the joint area The minimum preheat temperature shall not be less than that specified in the WPS. Only minimum preheat requirements are addressed in the codes. This is based on the need and the temperature for preheat, which depend upon a number of factors such as base metal composition, degree of restraint of the parts being joined, and material thickness. Therefore, different preheat temperatures can be required for the same materials, depending on these factors. Generally, for low-alloy steels, the need for preheat increases with higher alloy contents, greater restraints, and material thickness. Typical preheat temperatures for different materials are shown in Table 13-1. Normally, when materials of two different PNumbers are joined by welding, the preheat used is the preheat for the material with the higher preheat temperature requirement. For example, if P-No. 1 material is welded to PNo. 5, the preheat temperature selected will be that for the P-No. 5 material, because a higher preheat temperature is required for P-No. 5 compared to P-No. 1. 13-3
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
For low alloy steels (P-Numbers 3, 4, and 5), preheating should not be interrupted until the greater of 3/8 inch (9.5 mm) or 25% of the groove thickness is filled. The required preheat must be re-applied prior to the resumption of welding. If the welding procedure specification requires PWHT, it is good practice to apply it to the joint immediately following welding so that the joint does not cool down to ambient temperature. It should be noted that if the WPS specifies preheating for welding, then it must also be used for the following: • Tack welding • Any thermal cutting or gouging, for example, air carbon-arc gouging Figures 13-1 and 13-2 illustrate typical electrical resistance preheat setups.
Figure 13-2 Typical Setup for Flexible Pad Heating Elements [66] 13.2
Interpass Temperature
Besides specifying preheat temperature, the welding procedure also indicates an upper temperature limit known as the maximum interpass temperature. Therefore, it is equally important to check this temperature after depositing a weld bead. If the maximum interpass temperature is exceeded, welding must stop until the temperature drops to the specified limit. In practice, interpass temperature restrictions are a problem only where multiple pass sequences on small diameter or light wall pipe or fittings are used. Rarely are the temperatures noted in Table 13-1 exceeded for Schedule 40 or greater piping and large component materials with a heavy mass.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
13.3
Post-Weld Heat Treatment
Post-weld heat treatment (PWHT) is performed mainly to relieve residual welding stresses introduced from the welding operation; however, it can also improve ductility, toughness, and corrosion-resistance. The need for PWHT depends on both the material and thickness. For steels (with the exception of austenitic stainless steels), the higher the P-Number, the greater the hardness and brittleness in the HAZ following cooling from the welding temperature. The thicker the material and higher the P-Number (1 to 6), the need for PWHT becomes more critical. PWHT involves slowly raising the temperature of the completed weld joint, holding at a specified temperature for a defined time period, followed by slow cooling. Each step in the procedure is critical and must follow the requirements of the WPS and ASME Code. Typical PWHT temperatures and holding times for different P-Numbers are shown in Tables 13-2 and 13-3. The governing code or alternate criteria must be consulted for each specific situation. The thickness specified is the thickness of the weld, pressure retaining material, or the thinner of the sections being joined, whichever is least. Table 13-2 Mandatory Requirements for Post-Weld Heat Treatment of Welds 1 (Ref. ASME Section III, Table NB-4622.1-1) [12] P-No. (QW-420, Sect. IX)
Holding Temperature Range, ˚F 2
Minimum Holding Time at Temperature for Weld Thickness (Nominal) 1/2” (13 mm) or less
Over 1/2” to 2” (13 to 51 mm)
Over 2” to 5” (51 to 127 mm)
Over 5” (Over 127 mm)
I, 3
1,100 –1,250 (593 ˚C– 680 ˚C)
30 min.
1 hr/in
2 hr plus 15 min. each additional inch over 2” (51 mm)
2 hr plus 15 min. each additional inch over 2” (51 mm)
4
1,100 –1,250 (593 ˚C– 680 ˚C)
30 min.
1 hr/in
1 hr/in
5 hr plus 15 min. each additional inch over 5” (127 mm)
5
1,250 –1,400 (680 ˚C– 760 ˚C)
30 min.
1hr/in
1 hr/in
5 hr plus 15 min. each additional inch over 5” (127 mm)
P-Nos. 8, 34, 42, 43, 45 and hard surfacing on P-No. 1 base metal whose reported carbon content is not more than 0.30%.
PWHT neither required nor prohibited.
Notes: 1. Exemptions to the mandatory requirements of this table are defined in Table NB-4622.7. 2. All temperatures are metal temperatures. Specific information is provided in ASME Section I, Table PW-39, or ASME Section VIII, Division I, Table UCS-56, or ASME B31.1, Table 132.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Table 13-3 AWS Post-Weld Heat Treatment Temperatures for Selected Base Metal [66] Base Metal
1/2 to 1-1/4 CrMo
2 to 3 CrMo
5 and 9 CrMo
300 Series Stainless
C-Steel
C-Mo
C-steel
A
B
C
D
E
A
C-Mo
B
B
C
D
E
B
1/2 to 1-1/4 CrMo
C
C
C
D
E
C
2 to 3 CrMo
D
D
D
D
E
D
5 to 9 CrMo
E
E
E
E
E
E
300 series stainless
A
B
C
D
E
*
A
=
1,100 °F to 1 ,200 °F (590˚C to 650 °C). Not required if wall thickness does not exceed 3/4 inch (19 mm) or for socket joints.
B
=
1,125 °F to 1 ,250 °F (607° C to 680 °C). Not required if wall thickness does not exceed 5/8 inch (16 mm) or for socket joints.
C
=
1,175 °F to 1 ,275 °F (635° C to 690 °C) where the need for high creep and rupture strength is the primary consideration. 1,275 °F to 1 ,350 °F (690° C to 730 °C) where resistance to corrosion or to hydrogen embrittlement is the primary consideration. Not required for: (1) valves 2 inches (51 mm) and smaller, (2) piping or tubing with a diameter of less than 4 inches (102 mm) and wall thicknesses under 1/2 inch (13 mm), and (3) socket joints.
D
=
1,275 °F to 1 ,375 °F (690° C to 745 °C). Not required for: (1) valves 2 inches (51 mm) and smaller, (2) piping or tubing with a diameter of less than 4 inches (102 mm), and (3) socket joints.
E
=
1,300 °F to 1 ,400 °F (705° C to 760 °C). Cool to preheat temperature or below before applying post-weld heat treatment.
* Not considered in this document. See AWS D10.4. “Recommended Practices for Welding Austentic Chromium-Nickel Stainless Steel Piping and Tubing” [56].
Heating and cooling rates can be different, depending on the governing criteria. For example, PWHT heating and cooling rates are different in ASME B31.1 for power piping than they are for components in ASME Section III. ASME B31.1 stipulates that above 600˚F (316˚C), the rate of heating and cooling shall not exceed 600˚F (316˚C) per hour divided by 1/2 the maximum thickness of material in inches at the weld, but in no case shall the rate exceed 600˚F (316˚C) per hour. In ASME Section III, above 600˚F (316˚C), the rate shall not be greater than 400˚F (204˚C) per hour divided by the maximum thickness in inches; regardless of thickness, the rate shall not exceed 400˚F per hour and need be no slower than 100˚F (38˚C) per hour. The use of lower PWHT temperatures with longer holding times at temperature can be used, where permitted. The governing code or criteria should be consulted for the application of alternate PWHT criteria. Specific guidance for the application of alternative approaches is provided in the National Board Inspection Code, ANSI/NB-23, ASME B31.1, and ASME Sections III and VIII. When pressure parts of two different P-Number groups are joined, the PWHT for the material requiring the higher PWHT shall be used. For example, if a P-No. 3 material is joined to P-No. 5 material, the PWHT requirements for the P-No. 5 material shall apply. Therefore, the joint has to be PWHT at a temperature of 1,250˚F (675˚C), even though the P-No. 3 material requires only a temperature of 1,100˚F (593˚C). In PWHT pressureto-non-pressure parts, the PWHT requirement of the pressure part shall apply. 13-6
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Again, it should be noted that the PWHT requirements differ slightly according to different codes. Therefore, it is important to determine the applicable code and apply the PWHT variables as specified by that code. 13.4
Guide to Selection of Heating Methods
13.4.1 Gas Torch Heating Oxyacetylene or oxy-propane heating should be used only for preheating components less than 2-inches thick. With torch heating, the flame must be moved around to avoid localized hot or cold spots. Also, the flame must be neutral and care taken that the reducing (carburizing) center of the flame does not impinge on the surface of the component. For thicker components, it is difficult to introduce and maintain the preheat temperature uniformly over the entire surface. Electrical resistance heating should be used for these applications. Gas torch heating should not be used for PWHT because it is impossible to control the heating and cooling rates, and hold the temperature in the specified range for the required period of time.
13.4.2 Electrical Resistance Heating This is the most common method used for PWHT. The heat can be applied by a variety of equipment, for example, resistance pads, finger elements, braided heaters. If this method is to be used for PWHT, then it can also be applied for the preheat. In this way, the joint can be heated to the PWHT temperature straight from the preheat temperature without allowing the joint to cool down to ambient temperature.
13.4.3 Furnace Heating This method can be used only when the component is portable and can be removed from the site; Therefore, it is seldom used for plant repairs. However, it is economical and produces a uniform temperature in the component.
13.4.4 Induction Heating Induction heating is not widely used for field repairs but is ideally suited for PWHT of heavy-walled components. It produces a very uniform temperature distribution. 13.5
Guide for Selection of Preheat and PWHT
Preheat and PWHT of weld joints is dependent on the base metal composition, dimensions, type of joint, type of welding filler material, and—for PWHT—the preheat applied during welding. It is essential to know the above variables so that the heat treatment requirements for the weld can be selected.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
The mandatory requirements are specified in the different codes. It should be noted that the requirements vary according to which code is being applied. For these cases, the preheat and PWHT are slightly different among the various codes, for example, different heating and cooling rates, definition of nominal thickness, and PWHT exemptions. 13.6
Preheat and PWHT Procedures
13.6.1 General A standard procedure should be prepared for the preheat and PWHT of welded joints. This should describe how the heat treatment is to be performed and will ensure that standard procedures are used. The procedure or specification should include most of the details discussed above: • Purpose of preheating and PWHT • Guide to selection and details of heating methods • Installation of heating equipment • Specific requirements, that is, the heat treatment cycle for different codes It should also include the following topics, which are covered below: • Thermocouple attachment and location • Width of heated band • Documentation Specific procedures can then be prepared for different P-Number materials for the required code application. The procedure tabulates the actual preheat and PWHT parameters, for example, applicable code, base metals, nominal thickness, preheat temperature, PWHT heating rate, holding temperature, holding time, and cooling rate. This procedure can also be used to check and review a contractor’s procedure if the PWHT is to be performed by a heat treatment company.
13.6.2 Temperature Measurement Temperature-indicating crayons or contact pyrometers should be used to check the preheat temperature and to ensure that it is applied for a distance that is equal to the thickness of the part being welded, but not less than 3 inches (76 mm) in all directions from the point of welding. Another crayon should be used to ensure that the joint does not exceed the maximum interpass temperature during welding. Select a crayon that melts at the minimum preheat temperature and apply just outside the weld joint area [no closer than 1/2 inch (13 mm) from the expected weld toe]. The temperature should be checked periodically to ensure that the preheat temperature is still above the minimum value during the whole welding operation.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
For PWHT, the temperature must be monitored using thermocouples, the output of which is recorded on a calibrated, multi-channel, strip chart recorder. The entire PWHT cycle needs to be recorded on the chart. It is also important to note on the strip chart the specific scales of the temperature and time increments, for example, 1 inch (25 mm) is equivalent to 30 minutes on the vertical axis. Each thermocouple recording point must be identified, that is, identify each thermocouple number on the chart with its location so that the heat cycle at each point can be determined.
13.6.3 Thermocouples These are normally chromel-alumel Type K, in accordance with ANSI MC96.1. This standard provides information on temperature measurement and thermocouples, including terminology, fabrication, wire sizes, installation, and temperature-millivolt tables. Thermocouples can be attached using capacitor discharge, thermocouple clips, or flattened tube methods. Figures 13-3 and 13-4 show the three attachment methods. A minimum of two thermocouples for piping < 8 inches (203 mm) in diameter for the purpose of temperature recording are placed 180˚ apart on each pipe weld. For components over 8 inches (203 mm) in diameter, four thermocouples are normally required and they are placed 90˚ apart at the 12, 3, 6, and 9 o’clock positions. As a minimum, thermocouples should be placed at the area of highest and lowest anticipated temperatures. Thermocouples should be located at the outside edge of the minimum controlled band width (see Figure 13-5). A spare thermocouple should be attached in case one fails during PWHT. The thermocouple should also run along the surface of the pipe for at least 8 inches (203 mm) before being brought out through the insulation. Typical placement areas and number of thermocouples are shown in Figure 13-6.
13-9
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Twisted T/C Wire Application Pliers
Workpiece Lead
Figure 13-3 Thermocouple Attachment for PWHT Using the Capacitor Discharge Method [66]
13-10
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Type "K" Thermocouple
Weld Tack Welded Thermocouple Attachment Plate
Thermocouple Leads
1/4" Dia. Tube x 3/4" Long, Flattened to Grip Thermocouple Leads Porcelain Beads
Tack Welded Notes: Tube holding thermocouple should be of same material as weldment.
Figure 13-4 Thermocouple Attachment for PWHT Using Clips and a Flattened Tube [66]
13-11
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair CBW = a t 1 = Run Pipe Wall Thickness t 2 = Branch Pipe Wall Thickness W = Width of Weld Cover Pass
W Run Pipe or Vessel Shell Butt Weld b
CBW = c
D
CBW = D + 2b Branch or Nozzle Connection
b
L
CBW = L + 2b
W
Attachment Weld
b
CBW
a
D Dimension
a
Pipe Header Branch 3 W Where t1 ≤ 1 1/2" 3 t 1 Where t1 > 1 1/2"
b c
2 t1 2 t2
Vessel
6 t1 + W 3 t1 2 t2
CBW = D + 2b Butt Weld in Branch or Nozzle (Where there is less than 2 D between the heating pads, on the branch pipe, and the surface of the run pipe.)
Figure 13-5 Minimum Controlled Band Width (CBW) for Post-Weld Heat Treatment [53]
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
HBW = Minimum Heated Band Width (Area Under the Heating Pads)
Not Less Than 2" (Typ. All Figures)
CBW = = = =
Minimum Controlled Band Width Thermocouple Location (Near Side) Thermocouple Location (Far Side) Typical Locations for Extrathermocouples
CBW HBW Pipe 8" Dia. or Less
If Diameter is > 8"
CBW If Diameter is > 8"
HBW
Pipe Over 8" Dia.
CBW
CBW HBW
HBW
CBW HBW
If Diameter is > 8"
If Diameter is > 8"
CBW Note: Heated zones on branch pipe must be controlled separately from heated zones on run pipe.
HBW
Figure 13-6 Required Thermocouple Locations and Minimum Heated Band Width (HBW) for Post-Weld Heat Treatment [53]
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
13.6.4 Minimum Controlled Band Width Local PWHT consists of heating a circumferential band around the component. The minimum width of the heated band will include any nozzle, attachment, branch connections, and the additional weld width as follows: Circumferential Butt Joints in Pipe
Three times the width of the weld cover pass for wall thicknesses ≤ inch (13 mm). For pipe wall thicknesses > inch (13 mm), three times the pipe wall thickness.
Branch, Nozzle or Attachment Connections
Two times the run pipe wall thickness plus the run pipe thickness under the connection weld on either side of the area under the connection. Along the branch pipe or nozzle away from the weld for a distance of at least two times the branch pipe or nozzle wall thickness.
Typical examples of heated band widths are shown in Figure 13-6.
13.6.5 Heat Treatment Records Before PWHT is performed, a data sheet should be prepared that indicates all the variables of the method. Typical forms that can be completed and kept as a records are shown in Figures 13-7a and 13-7b.
13-14
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair SPECIFICATIONS (Completed by Technical Representative) SPEC. NO. DEPT.
UNIT
DR/CHANGE PROP. NO.
USI
LINE/EQUIP. NO.
WORK PLAN NO.
DATE
JOB DESCRIPTION:
MATERIALS:
PROGRAM PROFILE: (Define heating rates, dwells, heating levels, and cooling rates to be used.) Heating Rate:
Soak Temperature:
Dwell Time:
Cooling Rate: SAMPLE PROGRAM PROFILE Heating Rate
Dwell
Cooling Rate
˚C/Hr
Min.
˚C/Hr
TEMP (˚C)
TIME (min.)
ORIGINATOR (Print)
EXT. NO.
ORIGINATOR (Signature)
DATE
VERIFIED BY
DATE
Figure 13-7a Heat Treatment Quality Control Specification [53] Also, a copy of the temperature chart should be kept as a record and checked by the utility inspector or engineer to ensure that the specified temperature cycle has been used.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
13.7
Preheat and Post-Weld Heat Treatment Considerations—Hardfacing
In most cases, application of hardfacing alloys requires a clean substrate surface and some preheat. Preheating from “warm to the touch” up to and including code minimums is used to remove moisture and contaminants, and to reduce the cooling rate of the molten weld pool for reducing hot or cold cracking tendencies.
13.7.1 Preheat Hardfacing operations on carbon steel substrates typically yield better results when accomplished with preheat. This is especially true for the martensitic stainless steel (410, and so on) and cobalt alloys. Code minimums of course apply, but 200˚F (93˚C) minimum is typical. Prior to EPRI’s development of the NOREM family of alloys, non-cobalt bearing hardfacing alloys typically required the application of 600˚F–800˚F (316˚C–427˚C) preheats. The NOREM alloys were formulated to require reduced preheat but to benefit from the use of up to approximately 200˚F preheats, especially for localized repairs.
13.7.2 Post-Weld Heat Treatment—Hardfacing Post-weld heat treatment requirements vary for different substrates and code sections that a component is being repaired to. In general, PWHT is not required for P-Numbers 1, 8, 34, 42, and 45. The PWHT requirements for P-Nos. 3, 4, and 5 vary from code to code. Table 13-4 is provided to assist in finding the requirements for the specific application. Table 13-4 Code References for Preheat & PWHT Requirements Code Jurisdiction
Code Section
ASME B&PVC, Section I
NB-, NC-, ND-4600
ASME B&PVC, Section III ASME B&PVC, Section VIII ASME Power Pipe Code B 31.1
13.8
Sections 131, 132
Contracted PWHT
PWHT is contracted out by many utilities to specialty contractors or specialist heat treatment companies. Before any work is performed, the contractor must submit the PWHT procedure to be used. This should include, as a minimum, the requirements shown in Figures 13-7a and 13-7b. This informs the utility how the PWHT will be performed, that is, the method, how the joint will be wrapped (heaters and insulation), temperature measurement, and so on, and the heat treatment cycle to be used. This must be reviewed and approved by the utility prior to the repair.
13-16
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair RECORD OF HEAT TREATMENT STRIP RECORDER NO.
CHART NO.
TEMPERATURE AXIS (˚C/Division)
TIME AXIS (Minutes/Division)
SKETCH (weld, numbered thermocouple locations, etc.)
WELD NO., DESCRIPTION, AND LOCATION
THERMOCOUPLE POINT NO.
STRIP CHART COLOR
OPERATOR NAME (Print)
POINT
OPERATOR SIGNATURE
VERIFIED BY:
ACCEPTED BY:
OPERATOR'S SUPERVISOR
ONTARIO HYDRO INSPECTOR'S SIGNATURE
MATERIAL THICKNESS AT THERMOCOUPLE LOCATIONS
DATE
DATE
Figure 13-7b Heat Treatment Quality Control Record [53]
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
The contractor should supply a signed and dated heating chart with the thermocouple locations marked and the time and temperature scales. It is essential that the utility check the charts to ensure that the correct heat treatment cycle has been applied to the joint.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
14 WELDING METHODS AND PROCEDURES Base metal weld repair of most valve components, except hardfacing, can be performed with existing qualified welding procedure specifications (WPS) initially developed for fabrication or repair of standard piping components. Particular attention must be given to the effects of preheat, post-weld heat treatment, and weld residual stresses on the entire valve to minimize distortion. Excessive distortion can cause significant performance problems for internal components and sealing surfaces. Peening should be considered as a means for minimizing distortion on large repairs. The manufacturer’s recommendations should always be consulted. No attempt has been made to provide actual electrical and mechanical parameters in this guideline because of the variations due to thickness, material, and individual utility/service vendor practices. 14.1
Manual Base Metal Welding Procedure Guidelines
The orientation, sequence, and placement of each weld bead is critical to making a localized repair with minimal residual stress. The following information will assist in making manual repairs. In general, WPSs used for construction of major piping sytems are generally adequate for base metal repairs.
14.1.1 Manual SMAW The following guidelines will assist in making manual SMAW repairs: • Cleanliness—Remove all grease, oil, cutting lubricant, inspection paints, and dyes with a solvent such as acetone. If contaminants are not removed, they can cause porosity, lack of fusion with the base material, and cracking. • Prior to welding, the new surface should be examined by liquid penetrant (PT) or magnetic particle (MT) testing to ensure that the surface is free of all cracks and base metal defects. • When possible, utilize a minimal 200˚F (93˚C) preheat when making repairs on carbon steel valve materials. • Use stringer beads of very minimal oscillation. • Wrap heavy sections in insulation immediately after welding to slow the cooling rate.
14-1
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
14.1.2 Manual GTAW The following guidelines will assist in making manual GTAW repairs: • Cleanliness—Remove all grease, oil, cutting lubricant, inspection paints, and dyes with a solvent such as acetone. If contaminants are not removed, they can cause porosity, lack of fusion with the base material, and cracking. • Prior to welding, the machined/ground surface should be examined by liquid penetrant (PT) or magnetic particle (MT) testing to ensure that the surface is free of all cracks and base metal defects. • When welding on cast substrates, deposit a matching butter layer to seal the base material and provide a sound substrate. • Use a torch gas cup as large as possible. • Make the bead width no wider than the gas cup diameter. • Clean the weld deposit thoroughly between passes, grinding and brushing as necessary. • Avoid autogenous welding or washing of deposited repair weld material. 14.2
Machine Base Metal Welding Procedure Guidelines
Most repairs to valve bodies or base material are performed with manual welding approaches unless the repairs are extensive. ALARA considerations can, however, dictate the use of machine welding.
14.2.1 Machine GTAW The following guidelines will assist in making machine repairs: • Cleanliness—Remove all grease, oil, cutting lubricant, inspection paints, and dyes with a solvent such as acetone. If contaminants are not removed, they can cause porosity, lack of fusion with the base material, and cracking. • Prior to welding, the machined/ground surface should be examined by liquid penetrant (PT) or magnetic particle (MT) testing to ensure that the surface is free of all cracks and base metal defects. • When welding in remote applications where interpass cleaning is not practical, use a solid wire product. • On heavy sections, a minimum preheat of 250˚F (121˚C) for carbon steel and 200˚F (93˚C) for stainless is recommended. Preheat should be maintained throughout the welding application. • Minimize the weld torch oscillation when welding directly on cast valve body material. • For machine GTAW applications, use a 2% lanthanated or 2% cerium tungsten electrode with a point sharpened to a 14˚ angle. • Avoid autogenous welding or washing the deposit. 14-2
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
14.3
Hardfacing Welding Procedure Guidelines
14.3.1 General Requirements—Hardfacing Depositing a hardfacing alloy of any type requires thorough planning. Careful consideration should be given to surface preparation, overlay configuration, preheat requirements, and buildup thickness. These considerations, along with sound welding practices, generally result in a crack-free weld [10]. 14.3.1.1 Surface Preparation Remove all grease, oil, cutting lubricant, inspection paints, and dyes with a solvent such as acetone. If contaminants are not removed, they can cause porosity, lack of fusion with the base material, and cracking. Prior to welding, the machined/ground surface should be examined by liquid penetrant (PT) or magnetic particle (MT) testing to ensure that the surface is free of all cracks and base metal defects. The surface should also be etched for the remains of existing hardfacing. If found, these materials should also be removed by grinding or machining. Gouges and depression areas should be built up with a matching filler material and blended to the contour of the substrate surface. 14.3.1.2 Hardfacing Configuration The deposit configuration should be designed in such a way as to minimize distortion and residual stress due to shrinkage. The weld direction and deposited thickness should remain consistent at all times. Varying the deposit thickness or the number of layers creates an unequal distribution of stresses and promotes the potential for cracking. The first deposited layer of hardfacing should cover enough base material or butter material to ensure that subsequent hardfacing layers do not contact base material. This reduces the potential for cold-lap, lack-of-fusion, and crack initiation. Consideration should be given to the design of the weldment for in-process thickness maintenance and post-weld machining. This is especially important to in situ remote welding. Machined reference surfaces or pockets should be provided so that the deposited thickness and width can be measured. Sufficient hardfacing width should be provided such that final machining can remove and contour the edges of the deposit, thus eliminating potential “toe” cracks, typical of hardfacing weldments. 14.3.1.3 Preheating Hardfacing material generally requires preheating depending upon the welding process, base material, and weld configuration. The purpose of preheating is to slow down the cooling rate of the weldment to reduce stress, distortion, and—ultimately—cracking. There can be no specific guidelines for preheating due to the many variables involved. The best guide is experience with the specific application through the use of mockups.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
This is essential when the aim is to weld at room temperature on heavy sections or when only a limited preheat can be used. Based on the experience of EPRI evaluators and valve manufacturers, the following minimum preheat recommendations are provided as a starting point. It should be noted that higher preheats might be required for massive and complex parts. Higher preheats might also be necessary for hardfacing deposits exceeding three layers. 14.3.1.4 Hardfacing Preheat Recommendations for Machine and Manual Gas Tungsten Arc Welding (GTAW) and Shielded Metal Arc Welding (SMAW) • P1 and P8 wrought substrates, three layers or less, min. 150˚F (66˚C). • P1 and P8 cast substrates with butter layer(s), three layers or less, min. 100˚F (38˚C). • P1 substrate, more than three layers, min. 250˚F (121˚C). • P8 substrate, more than three layers, min. 200˚F (93˚C). 14.3.1.5 Preheat Recommendations for Plasma-Transferred Arc Welding (PTAW) • P1 wrought substrate, three layers or less, min. 250˚F (121˚C). • P8 wrought substrate, three layers or less, PTAW process, min. 200˚F (93˚C). • P1 cast substrate, three layers or less, PTAW process, min. 400˚F (204˚C). • P8 cast substrate, three layers or less, PTAW process, min. 300˚F (149˚C). It is a good practice to warm any part to 100˚F (38˚C) to drive off any moisture from pores and crevices of the substrate material. Once the preheat temperature is established, monitoring should be maintained throughout the welding proces. After welding is completed, the part should be wrapped in insulation to slow the cooling rate. 14.3.1.6 Buildup Thickness The thickness of the hardfacing deposit has a direct effect on distortion, residual stress, and preheat requirements. Hardfacing serves as a protective wear surfacing material to preserve the structural material under it. To perform this function, it must be hard and is subsequently more brittle than the substrate. As a recommended practice, all weld deposits should be limited to three layers or 1/4 inch, whichever is less. The total finished deposited thickness should be approximately 1/8 inch (3.2 mm), which will require 3/16 inch to 1/4 inch (4.8 mm to 6.4 mm) welded thickness prior to machining. This thickness includes the height above the base material plus the depth of penetration and mixing below the original surface. If the specific application requires more than the recommended thickness due to the removal of existing hardfacing materials, the base material should be built up with a butter material so that the new hardfacing thickness is not excessive.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
14.3.1.7 Cooling Rate Controlling the cooling rate of the hardfacing deposit is one of the most critical variables to produce a crack-free weld without preheating. Although this topic has been mentioned in several other areas, its importance warrants special attention and general consideration. A rapid cooling rate can lead to cracking, spalling or underbead cracking at the base metal interface, and distortion. Methods that have been found to be effective in slowing the cooling rate include: • Preheating • Insulating the work piece during and after the welding operation • Providing higher heat input during the welding operation Providing higher heat input to reduce the cooling rate of the weld puddle is an effective way to slow the cooling rate where little or no preheat is used in field applications. These practices are contrary to standard welding methods employed to weld carbon and stainless steels but are quite typical for most hardfacing alloy and other high alloy steel welding. The key variable that must be maintained is travel speed. The importance of controlling this variable must be emphasized to welding personnel. It is extremely important to keep the travel speed at 3 inches per minute (ipm) to 4 ipm (75 mm/min. to 100 mm/min.) Faster travel speeds produce thin narrow beads, fast cooling rates, and potential stress (shrinkage) cracks.
14.3.2 Suggested Welding Practices In addition to the general requirements presented previously, depositing hardfacing materials without elevated preheat requires welding practices that are generally known only by experienced hardfacing welders. Many lessons have been learned as a result of several hundred weldability evaluations performed at the EPRI RRAC and information shared by valve manufacturers, service companies, and secondary utility evaluators. Selected welding practices are provided by process.
14.3.3 Manual GTAW The following welding practices have been found to provide the most consistent results: • When welding on cast substrates, deposit a matching butter layer to seal the base material and provide a sound substrate. • A minimum preheat of 250˚F (121˚C) for carbon steel and 200˚F (93˚C) for stainless steel is recommended when more than three layers of hardfacing is required. Preheat should be maintained throughout the welding application. • The torch gas cup should be as large as possible.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
• Keep the bead width no wider than the gas cup diameter. • Use a 2% lanthanated or 2% cerium tungsten electrode with the point sharpened to a 14˚ angle. • Maintain a consistent travel speed. If the optimum travel speed is 3 ipm (75 mm/min.), then 3 ipm should be maintained along the entire length of the weld bead and not be an average speed from 2 ipm to 4 ipm (51mm/min.–100 mm/min.). • Maintain the heat input between 25 KJ/in (9.8 KJ/cm) and 40 KJ/in (15.7 KJ/cm) when welding without preheat. • Avoid increasing the heat input above that used on the first layer. • Use a 50% bead overlap. • Use care when introducing the weld rod. The rod should not be fed into or dipped into the molten puddle. The rod should be located such that the leading edge of the molten puddle moves toward the welding rod. • Avoid actual feeding or dipping of the rod into the puddle, which can cause premature freezing, resulting in a sluggish puddle, and rapid solidification, leading to cracking. • Clean the weld deposit thoroughly between passes, grinding and brushing as necessary. • Avoid autogenous welding or washing of deposited hardfacing material. • At the end of a bead, continue to feed the rod into the molten puddle and down slope as quickly as possible.
14.3.4 Localized Hardfacing Repair Practices Keeping residual stresses to a minimum is the key to a successful localized repair. The elements necessary to complete a repair are: repair cavity preparation/profile, preheat, and weld bead sequence. These repair practices are generally the same with either the manual or machine process. 14.3.4.1 Repair Cavity Preparation/Profile Cracks in a hardfacing deposit generally propagate to the fusion line of the base material. The crack must be completely removed by machining or grinding and verified by liquid penetrant (PT) testing. If removing the crack requires excessive gouging into the substrate material, building up the area to the original base material height with a similar filler material is recommended. After completing the local base material buildup, the repair cavity is prepared. In general, the repair cavity should be profiled such that the welding can be performed in the direction of the original weld. When performing machine repairs, minimum 3:1 taper on both ends of the cavity is recommended to allow a machine welding torch to move into and out of the cavity without obstruction. This gentle slope helps to reduce
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
stress buildup and to aid welder visibility. After grinding and cleaning, a final PT should be performed to verify that no further defects are visible. 14.3.4.2 Preheat Repairs introduce very high localized residual stresses due to differential coefficients of thermal expansion between the base material and the hardfacing material. Demonstrations have shown that repairs on stainless steel substrates are less sensitive to cracking than on carbon steel substrates. This is due to the thermal coefficients between the hardfacing alloy and stainless materials. As an example, localized repairs were successfully demonstrated with the NOREM 02 chemistry on stainless steel substrates at ambient temperatures. Repairs on carbon steel substrates were sometimes successfully performed at ambient temperature. A minimum preheat of 200˚F to 300˚F (93˚C to 149˚C) was consistently successful and is recommended. When repairs are required on very complex parts or on parts of variable base material thickness, additional preheat of up to 500˚F (260˚C) has been successfully demonstrated. Preheats for repair demonstrations were generally applied with a pad to the substrate material and checked frequently during welding to ensure that the minimum temperature was maintained. The preheat was maintained until the repair was completed and then for the duration of the welding activity. The parts were then insulated to provide slow cooling. 14.3.4.3 Repair Welding The orientation, sequence, and placement of each weld bead is critical to making a localized repair of a hardfacing deposit. The following welding practices are suggested for a successful hardfacing repair: • Cleanliness—Remove all grease, oil, cutting lubricant, inspection paints, and dyes with a solvent such as acetone. If contaminants are not removed, they can cause porosity, lack of fusion with the base material, and cracking. • Provide a repair cavity that has a good transition at the ends and sides for visibility and access. • Utilize a minimal 250˚F (121˚C) preheat when making repairs on carbon steel substrates. • Avoid autogenous welding of the existing hardfacing material. • At the end of a bead, maintain the wire feed into the molten puddle and down slope as quickly as possible. • Use stringer beads or very minimal oscillation. • Maintain consistent travel speed. • Wrap finished parts in insulation immediately after welding to slow the cooling rate. 14-7
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
14.4
Machine Hardfacing Welding Procedure Guidelines
General requirements, suggested welding practices, and repair practices are identical with those listed in Sections 14.3.1, 14.3.2, and 14.3.4.
14.4.1 Machine GTAW The following welding practices have been found to provide the most consistent results: • When welding in remote applications where interpass cleaning is not practical, use a solid wire product. • When welding on cast substrates, deposit a matching butter layer to seal the base material and provide a sound substrate on which to deposit NOREM. • When more than three layers of hardfacing is required, use a minimum preheat of 200˚F (93˚C) for carbon steel and 150˚F (66˚C) for stainless. • Avoid increasing the heat input above that used on the first layer. • Maintain the heat input between 25 KJ/in (9.8 KJ/cm) and 40 KJ/in (15.7 KJ/cm) when welding without preheat. • Minimize weld torch oscillation. • Use a 50% overlap of the adjacent bead. • Do not permit the second and subsequent beads/layers to overlap; do not place them so that they touch the original substrate or butter layer as this increases the potential for lack of fusion. • For remote applications, use a 2% lanthanated or 2% cerium tungsten electrode with the point sharpened to a 14˚ angle. • Use care when introducing the weld wire. The wire should be fed into the molten puddle at a point between the tungsten and the leading edge of the molten puddle. If the wire entry point and leading edge of the molten puddle occur at the same point, wire feed, travel speed, or both are too great. • Avoid feeding excessive wire into the puddle, which can cause premature freezing, resulting in a sluggish puddle, and rapid solidification, leading to cracking. • Maintain a consistent travel speed, even when operating in the orbital mode. • Use a slow travel speed of 4 ipm (100 mm/min.) or less to slowly heat the substrate material and to provide a slow cooling rate. • Avoid autogenous welding or washing the deposit. • At the end of a bead, maintain the wire feed into the molten puddle and down slope as quickly as possible.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
14.4.2 Machine PTAW The following welding practices have been found to provide the most consistent results: • When welding on cast substrates, deposit a matching butter layer to seal the base material and provide a sound substrate on which to deposit the hardfacing material. • Use a minimum preheat of 200˚F (93˚C) when more than three layers of hardfacing is required. Preheat should be maintained throughout the welding application. • Avoid increasing the heat input above that used on the first layer. • Clean the weld deposit thoroughly between passes, grinding and brushing as necessary. • Use a 50% bead overlap. • Use a slow travel speed of 5 ipm (126 mm/min.) or less to slowly heat the substrate material and to provide a slow cooling rate. • Avoid autogenous welding or washing of the deposited hardfacing material. • At the end of a bead, maintain powder feed into the molten puddle and down slope as quickly as possible. • Wrap finished parts in insulation immediately after welding to slow the cooling rate. 14.5
Temperbead Welding Repair Guidelines
ASME Sections III, VIII, XI and ASME/ANSI B31.1 recognize repairs to P-Number 1 and 3 materials without the application of post-weld heat treatment if certain conditions are met. These codes do not recognize repairs to cast P-Number 4 or 5 (Ref. ASME Section IX) materials without the application of post-weld heat treatment as does the National Board Inspection Code, NB-23. Temperbead repair techniques have been developed to meet such repair needs. The goal when using the temperbead technique is to produce equal or greater properties in the repair weld heat-affected zone as those found in the parent base material. Creep rupture is not a factor in repairs conducted under ASME Section XI because operating temperatures will not exceed 600˚F (316˚C) [12, 56, 61, 67–74].
14.5.1 Temperbead Qualification and Demonstration A successful temperbead repair relies upon the welder’s ability to temper the heataffected zone of the base metal by careful placement and deposition of repair weld beads and layers. This ability must be demonstrated and documented in the form of the qualification of a welding procedure specification. Specific criteria for alternate repair of P-Numbers 1 and 3 are outlined in ASME Section XI, IWA-4600 (SMAW and GTAW only) and NB-23, Paragraph RD-1040, Welding Method 2 (1995). Also, all requirements specified in ASME Section IX for the qualified process must be observed. Selected highlights and additional information are provided below.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
14.5.1.1 Qualification Considerations Process. The gas tungsten arc (GTAW), gas metal arc (GMAW), flux-cored arc (FCAW), and shielded metal arc (SMAW) welding processes can be used. In most cases, either GTAW or SMAW are used. RRAC has conducted tests with both processes plus FCAW and obtained successful results. The bulk of work has been directed at the SMAW process because most repairs are localized and do not lend themselves to semi-automation or machine welding. Qualification Material. Test coupon material for the welding procedure qualification shall be of the same material specification (including specification type, grade, class, and condition of heat treatment) as the original material for the repair. If the original material specification material is obsolete, the test material used should conform as much as possible to the original material used for construction, but in no case shall the material be lower in strength. Most mockups and qualifications are conducted with new material. RRAC has conducted reference evaluations on service-aged components [piping and valves that have operated in the 850˚F to 1,050˚F (454˚C to 575˚C) range] and has shown that even material that has seen significant service (> 100,000 hours) provides acceptable tensile, yield, and toughness results. Technique. Technique factors, including geometry and position of the repair cavity, percentage of bead overlay, number of layers, applied preheat and interpass temperature, and whether peening is utilized, affect the ability to produce a satisfactory repair. The figure provided in ASME XI, IWA-4600, Figure IWA-4623.1-1 (see Figure 1 of the document) is typically used. (Note: All of the work conducted in the EPRI/RRAC Lundin and Friedman studies omitted Step 2, removal of the weld bead crown of the first layer.) Groove depth is determined by repair depth, and the width of the qualification groove normally is about one inch at the root area. See Figure 14-1.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
30° (Typ)
Step 1: Butter cavity with one layer of weld metal using 3/32-inch diameter coated electrode.
1/8" R
Max Depth
3/4" (Typ)
Reinforcement Weld Temper Bead Layer
Step 2: The second layer shall be deposited with a 1/8inch diameter electrode. Subsequent layers shall be deposited with welding electrode no larger than 5/32-inch maximum diameter. Bead deposition shall be performed in a manner as shown. Particular care shall be taken in the application of the temper bead reinforcement weld at the tie-in points as well as its removal to ensure that the heat-affected zone of the base metal and the deposited weld metal is tempered and the resulting surface is substantially flush.
Figure 14-1 Temperbead Groove Preparation, Geometry, and Weld Bead Sequence [12] Position influences bead shape and size, but is affected more by the shape of the repair cavity. The geometry of the repair cavity must be such that multiple beads and layers can be deposited. The bottom and remainder of the cavity must permit at least two beads per layer to be deposited. The cavity must also be of sufficient width to allow the first layer to be deposited without tie-in of the opposite sidewalls. Tie-in of side walls on the first layer prohibits or severely limits the ability to temper the HAZ with subsequent passes or layers. Weld layers composed of beads that exhibit 50% overlap seem to provide the best results and are easily achieved by the welder. Electrodes of increasing diameter are used; the first layer with 3/32-inch (2.4-mm) diameter, second layer with 1/8-inch (3.2-mm) diameter, and third plus remaining layers with no larger than 5/32-inch (4-mm) diameter. Although permitted, many repair cavities are more easily accomplished by using only the 3/32-inch (2.4-mm) and
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
1/8-inch (3.2-mm) diameter electrodes. Cavities are often not wide enough or the position hinders accommodating 5/32-inch (4-mm) electrodes where SMAW is used. Various studies by Alberry, Lundin, and Friedman have proposed methods for ensuring that beads at the edge of the final weld reinforcement receive proper tempering. Although methods including run-off tabs have been evaluated, careful bead placement and sequence provide the most flexible means for proper tempering under varying conditions of cavity geometry, position, and component geometry. Peening can be used, if it is implemented during the qualification and documented. This method for mitigation of shrinkage and residual stresses can be effective, but should never be used where only a thin section (< 1/4 inch) (6.4 mm) remains or on the root/ hot passes or on the final weld reinforcement layer. Peening should be implemented only on the second or later intermediate layers. Preheat/Post Bake. Preheat and interpass temperature are stipulated for P-Numbers 1 and 3. The values stated for P-Number 3 have been shown to be adequate for P-Number 4 and should be effective for P-Number 5, as well. Mass, geometry, and location of the repair cavity dictate the amount of preheat maintenance that is required. Normally on test assemblies, much delay is experienced waiting for the assembly to cool down between weld beads or layers. Conversely, full size components in a power plant generally exhibit large heat sinking that effectively minimizes or reduces interpass temperature delays or concerns. In either case, accurate monitoring via thermocouples or contact pyrometers is essential. A two-hour post-weld bake is required by code for P-Number 1 materials and a four-hour bake for P-Number 3 materials. This step is to ensure that any dissolved hydrogen evacuates to acceptable levels (typically < 5 ml/100 g). This step, although required by code, is very conservative due to modern electrode manufacturing methods that enable production of moisture-resistant electrode coatings. Most domestic electrode manufacturers offer moisture-resistant (MR) low hydrogen electrode coatings (< 5 ml/100 g) that maintain their MR condition for up to 8 to 10 hours of exposure. At least one European source offers MR coatings that maintain < 3 ml/100 g. The EPRI/RRAC work on P-Number 4 materials used typical low hydrogen electrode coatings and observed a two-hour postweld bake at 500˚F ± 50˚F (232˚C to 288˚C) with successful results. 14.5.1.2 Qualification Results As a minimum, tensile, bend, and toughness tests must be performed to demonstrate the ability of the repair procedure to produce satisfactory mechanical properties.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
14.5.2 Implementation 14.5.2.1 Repair Sequence Identification and Layout of Repairs. It is important for repair areas to be identified and characterized via one or more of the various nondestructive examination (NDE) methods. When the location and extent of repairs are known, maps should be developed and the scope of work or repair can be defined. The location and extent of repair required affect the welding procedure qualifications and the number of welders required, as well as practical factors including scaffolding, weld metal consumables, welding machines, and preheating equipment. Excavation of Defects. Unacceptable indications and defects must be removed to sound metal. Removal can be achieved by mechanical means, such as grinding or machining, or by thermal methods, including flame and air arc gouging. Usually no preheat is required when using mechanical methods, but at least some (warm to the touch) is advisable when air arc gouging is used. Where multiple repair cavities are encountered, it can be advantageous to connect them to gain better access for welding and bead placement. If repairs will involve approximately 1/2 of a weld’s length or circumference, consideration should be given for a complete cut-out and reweld. This would typically require full PWHT, but past operations have shown this to normally be cost-effective, as well as to reduce the large unbalanced shrinkage and distortion from the repair of a large area. Preparation and Geometry of Repair Cavity. The geometry of the repair cavity must complement the repair operation. A temperbead repair cavity must have enough width and overall volume to permit deposition of the proper number of weld layers. In particular, the root or bottom of the repair cavity must be able to accept multiple beads to ensure that tempering of the base metal HAZ and subsequent weld beads can be accomplished. NDE of Repair Cavity. Unacceptable indications and defects must be removed prior to repair. NB-23 requires that the excavated area to be repaired be examined by either magnetic particle (MT) or liquid penetrant (PT) testing to establish that no defects exist. Installation of Preheat, Temperature Monitoring, and Insulation. Preheat equipment must be installed to permit both temperature control of the component to be repaired and provide access to the repair cavity for the welder. Where precise control is required, electrical resistance heating is preferred. Unless sophisticated torch arrangements are used, flame heating is usually only employed for general preheating. In either case, preheat temperatures can be monitored with temperature-indicating crayons, direct reading pyrometers, or thermocouples wired to precision meters or chart recorders. Use of electrical resistance heating monitored and controlled with thermocouples provides the most accurate preheat approach. Besides specifying preheat temperature, the welding procedure also indicates an upper temperature limit known as the maximum interpass temperature. Therefore, it is
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
equally important to check this temperature after depositing a weld bead, and if the maximum interpass temperature is exceeded, welding must stop until the temperature drops to the specified limit. In practice, interpass temperature restrictions are a problem only where multiple pass sequences on small diameter or light wall pipe or fittings are used. Rarely are the temperatures exceeded for Schedule 40 or greater piping and large component materials with a heavy mass. Insulation must complement the operation. Most successful operations place the majority of insulation in a temporary but non-removable (short term) fashion and have only removable-type insulation directly over the repair cavity. After repair welding is completed, removable insulation must be replaced for the post-weld bake. Depositing the First and Second Layer. Depositing the first and second layer of repair weld metal is the most important portion of a temperbead repair operation because of their effect on the base metal HAZ. Weld beads must be placed and deposited such that approximately 50% overlap is achieved. Electrode size, heat input, and bead thickness are typically minimized for the first layer to enable the heat of welding from the second bead/layer to provide a level of tempering to the preceding bead and its HAZ. Heat input for the second layer must be maximized to further provide tempering without sacrificing the quality of the deposit. Provided that the prescribed overlap, preheat, interpass temperature, other WPS parameters, and average workmanship standards are observed, repairs with acceptable and reproducible results should be obtained. Depositing the Remaining Fill Layers. The technique for depositing the remaining layers is rather flexible, if the WPS is followed. Maintenance of preheat and not exceeding the maximum qualified interpass temperature is very important. In most cases, manual processes are used due to the localized nature of repairs, and interpass temperature concerns are minimized. Depositing the Final Layer/Weld Reinforcement. The final weld reinforcement is normally made flush with the surface of the component. Care must be observed to avoid creating an additional HAZ in the base material that is not tempered. Observing careful bead placement and sequence can ensure that beads at the edge of the weld reinforcement are properly tempered. Post-Weld Bake. A post-weld bake operation at 500˚F ± 50˚F (232˚C to 288˚C) for a minimum of two hours is required for welds made with the SMAW and FCAW processes. This step is implemented to ensure that hydrogen evacuation from the weldment is complete. The post-weld bake must commence after welding and without permitting the weldment to cool below the minimum preheat temperature. 14.5.2.2 Interim and Final NDE Interim NDE is not required or routinely performed unless cracking, fusion, or other problems are suspected. When the repair is finished and the component cools to room temperature, MT or PT, as a minimum, must be performed. On repairs with a thickness greater than 3/8 inch (9.5 mm) or on welds that originally required radiographic testing (RT), re-testing with RT is normally required by code. 14-14
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14.5.2.3 Documentation Documentation varies, depending upon the extent of the repair and local jurisdictional requirements, but the following would typically be required: • Welding procedure and welder performance qualification records • Weld filler metal information • Repair location information/maps • Weld history record (data sheet, form to record information, and so on) • NDE results • Certificate of authorization, R-stamp • Code data reports
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
15 INSPECTION AND TESTING METHODS
15.1
Nondestructive Examination
Nondestructive examination (NDE), as applicable to valves, refers to the following types of inspection/examination techniques: • Volumetric examination –
Radiographic testing (RT)
–
Ultrasonic testing (UT)
• Surface examination –
Liquid penetrant testing (LP or PT)
–
Magnetic particle testing (MP or MT)
• Visual examination As one might infer from the names of the technique categories: • Volumetric techniques provide information about the entire volume of the part being examined and can detect internal flaws and defects, although there are limits on the thickness of parts that can be successfully internally examined by these methods. • Surface techniques provide information only about the surface of the part being examined (but can detect surface flaws and defects that cannot be easily seen without the use of these techniques). • Visual examination consists of looking at the surface of the part being examined without the benefit of additional surface examination techniques. The requirements for what types of NDE must be done for a given part is a function of its size, function, product form, material, applicable design code, and safety classification. These requirements are given in or invoked by the applicable design code. As discussed in Section 2, this is ASME Boiler and Pressure Vessel Code, Section III for nuclear safety-related valves and ANSI B16.34 for most other valves. The American Society for Testing and Materials (ASTM) and the American Society of Mechanical Engineers (ASME) have developed several standards and recommended
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
practices for each of the inspection methods listed above. Table 15-1 lists the applicable standards according to number and title for each NDE method. Table 15-1 ASTM and ASME Standards for Nondestructive Examination Methods Liquid Penetrant Testing ASTM E 165 ASTM E 270 ASTM E 433 ASME Section V
“Standard Practice for Liquid Penetrant Inspection Method” “Standard Definitions of Terms Relating to Liquid Penetrant Inspection” “Standard Reference Photographs for Liquid Penetrant Inspection” “Article 6”
Magnetic Particle Testing ASTM E 709 ASTM E 269 ASME Section V
“Standard Recommended Practice for Magnetic Particle Examination” “Standard Definitions of Terms Relating to Magnetic Particle Examination” “Article 7”
Radiographic Testing ASTM E 94 ASTM E 142 ASTM E 586 ASME Section V
“Standard Practice for Radiographic Testing” “Standard Method for Controlling Quality of Radiographic Testing” “Standard Definitions of Terms Relating to Gamma and X Radiography” “Article 2”
Ultrasonic Testing ASTM E 500 ASTM E 428 ASTM E 213 ASTM E 164 ASTM E 797 ASTM E 114 ASME Section V
“Standard Definitions of Terms Relating to Ultrasonic Testing” “Standard Recommended Practice for Fabrication and Control of Steel Reference Blocks Used in Ultrasonic Inspection” “Standard Practice for Ultrasonic Inspection of Metal Pipe and Tubing” “Standard Practice for Ultrasonic Contact Examination of Weldments” “Standard Practice for Measuring Thickness by Manual Ultrasonic Pulse-Echo Contact Method” “Standard Recommended Practice for Ultrasonic Pulse-Echo Straight-Beam Testing by Contact Method” “Article 4”
Testing Agency Qualification ASTM E 543
“Standard Practice for Determining the Qualification of Nondestructive Testing Agencies”
After the requirements for the NDE that must be done are determined as discussed above, the rules for how to perform the examinations and the acceptance criteria are given by Section V of the ASME Boiler and Pressure Vessel Code, “Nondestructive Examination.” Both ANSI B16.34 and ASME Section III invoke ASME Section V for this purpose. Obviously, the most serious defects in valves are cracks. Cracks can initiate from casting defects, weld repair defects, improper design, severe operation, or a combination of the above. Cracks can also form in otherwise sound material as a result of elevated stresses. Most cracking can be detected by a thorough visual inspection of the valve body and internals. Cracking is most frequently found in hardfaced trim components; however, it would be prudent to perform a complete inspection to ensure that cracking has not occurred in other locations. Particular attention should be paid to areas that have been previously repaired. Additional cracking can occur adjacent to the repaired area or can initiate as a result of improper repair welds. Although many welds can be located
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
visually, it might be necessary to thoroughly clean and etch the area to delineate the weld. Surface casting defects, such as unfused chaplets or chills, can also be verified by etching or magnetic particle examination. Post-repair visual, liquid penetrant, and magnetic particle inspections should be performed to verify a sound weld. Significant repairs should also be inspected using a volumetric method, such as radiography or ultrasound prior to post-weld stress relieving, in case rework is required. Again, ultrasonic testing should be performed with caution to avoid misinterpretation of the indications from part geometry. 15.2
Post-Repair Testing
Article IMV-3200 of ASME Section XI states: “When a valve or its control system has been replaced or repaired or has under gone maintenance that could affect its performance, and prior to the time it is returned to service, it shall be tested to demonstrate that the performance parameters which could be affected by the replacement, repair, or maintenance are within acceptable limits.” The repair or replacement of valve internals has the potential of binding up a disc, poppet, or stem so that it would not open and close as designed. As a result, a function test must be performed to verify the unobstructed operation of the valve. Prior to performing the exercise test, a final visual inspection of the valve internals should be performed to check for nicks, gouges, or scratches that might have been caused during the process of working inside the valve. Also, verify that no foreign materials, such as grinding dust, metal shavings, or tools, have been left inside the valve.
15.2.1 Exercise Test Valve exercise is performed with the bonnet bolted to the body. The valve should be manually stroked to the fully-closed and then to fully-open position. A spring or load cell device can be used to quantify the force. The disc should travel freely throughout the stroke. On gate and globe valves, verify that the disc or poppet does not bind in the guides or against the valve body, and that it seals properly in the in-body seats. The stem should seat against the bonnet backseat in the fully-retracted position. On check valves, verify that it is not possible for the disc or swing arm assembly to contact the valve body except at the design backstop contact points. Also verify that the backstop contact point is square to the stop. If the valve exercise test is successful, the bonnet should be torqued to the design specifications, and local leak rate tests (LLRT) performed as presented in ASME/ANSI OM, Part 10 [75].
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
15.3
Blue Check
A blue check is a simple method used to detect surface contact between two seat surfaces. Blue checks are performed on valve seats to determine whether 360˚ contact is being made between the disc and seat, and the seating location. Several repair cases utilizing blue checks are presented in Appendix A. Blue checks are performed with an oil-based dye commonly referred to as “prussianblue.” For cases where water is present, a waterproof dye called “neolube” can be used. The blue is applied as a very thin coating to the wedge/disc or poppet. The disc is then placed back in the valve, closed, and then reopened. The line on the wedge/disc or poppet indicates the areas of contact. An experienced valve technician can look at the results and determine the amount of maintenance or repair required to obtain a leakfree joint.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
APPENDIX A
Gate Valve Wedge Fitting Techniques (Courtesy of Jon A. Vanier, EFCO USA, Inc.) The process of fitting a gate valve wedge (disc) to the in-body seats is a much discussed and argued topic among valve technicians. Those in the trade with the knowledge to efficiently produce a proper fit have a tendency to guard their methods closely, leaving those less experienced to use the dreaded “trial and error” method of valve repair, often with time consuming and costly results. Actually, obtaining a proper wedge fit is not a complicated process. The following principles will serve as a guide. They combine some simple techniques with common sense to achieve the desired result of a 360˚ continuous, thin line of contact on both the upstream and downstream sides of the valve. While nothing replaces hands-on experience, the following theory and principles will lead to a proper seal. The most important step is the blue check, which needs to be repeated often. The blue check is a simple method of detecting surface contact between two seating surfaces. For this discussion, the blue check will serve as the guide to obtain a proper fit between the wedge and the in-body seats of the gate valve. Blue checks are performed with an oilbased dye. In cases where excessive water is present, Neolube can be substituted, due to its water-repellent properties. The importance of the blue check and the ability of the technician to correctly interpret its meaning cannot be understated. Upon initial valve disassembly and only after match-marking the wedge orientation, the technician will obtain an as-found blue check. This blue check, if done properly, will be an indication of the time, materials, equipment, and person hours needed to ensure a leak-proof component. The criteria for ensuring a leak-proof seal is determined by many different variables, such as system pressure, system function, valve design, and so on. However, as previously stated, a narrow, continuous line of contact on both seating surfaces is desirable. A look at the theory behind obtaining a leak-proof seal in a gate valve supports a narrow versus wide contact area, regardless of whether the contact is the desired 360˚. The sealing force of the wedge to the in-body seats must be a magnitude higher than the pressure differential across those seats (upstream versus downstream pressure) in order to maintain a seal. The stem and actuator act in unison to exert the sealing force; the actuator (manual, motor, or hydraulic) creates the rotational force that determines the amount of linear force with which the wedge maintains a seal. The stem, via the inclined acme threads, transmits the force of the actuator to the wedge. Actuators are
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
chosen according to the valve’s specific requirements for thrust or torque and for stroke length or angle. While valve size and type influence the stroke length or angle, the dynamic effects of flowing fluid and friction are the main factors influencing thrust and torque. For a gate valve, the main opening force to be overcome by an actuator is the frictional force on the seat, caused by upstream pressure. In most instances, the actuator has been chosen with regard to the above factors. It is essential that this force not be exceeded (for example, a pipe wrench should never be used on a valve to aid in closure) or damage to valve internals could result. The actuator, chosen to exert a specific amount of force, logically exerts more force on a smaller area (narrow seat) than a larger area (wide seat). The sealing force of the gate valve will be greater if that force is applied to fewer square inches on the seating area. Thus, a narrow line of contact provides a better seal For most gate valves, in situ repair is cost-effective. The most prevalent cause of valve failure is leakage; therefore, most repair work focuses on the seat/wedge seal. The technician must always attempt reconditioning of the in-body seats first to determine the cost effectiveness of repairing a valve. In-body seats are attached to the valve body in one of three ways—screwed, welded, or pressed. Due to the difficulty encountered in replacing these seats, evaluating their condition and their prospects for reconditioning is crucial. Wedges, on the other hand, are relatively easy to transport, recondition, or replace, if necessary. While in-body seat replacement might be a viable option, often complete valve replacement is the only alternative, but skilled valve technicians with the proper equipment can prevent valve replacement in many instances. In reconditioning the in-body seats, the focus should be on two primary principles of gate valve repair. The first principle is to ensure a narrow versus wide seating area as determined by the blue check method. Generally, a seating area over .125-inch width is too wide. In that instance, apply a 3˚ taper to the in-body seats with automatic grinding/lapping equipment. When setting up a grinding/lapping machine to achieve a 3˚ taper, the grinding discs must be centered perfectly. A recommended method is to mark the seats with a permanent marker to aid in centering; the technician can adjust the machine more easily by grinding for a few seconds and then checking the wear on the marks. Use it as a guide until it is centered. This will bring the seats back in and ensure the desired narrow contact area. The second principle in reconditioning gate valve seats is to achieve flat seating surfaces. If the blue check is not a continuous line (broken), the in-body seats, the wedge face, or both, are not flat . In most instances, it is the in-body seats that are not flat due to gradual wear and the ring-shaped design. Therefore, after a narrow seat is established, the technician must ensure that the seat is flat. Putting a flat on a narrow seat is accomplished much faster and with greater ease than on a wider seating surface. Bear in mind that grinding on a seat widens the seating surface (recall that tapering can regain a narrow seat) and, if over-grinding occurs, the potential for “dropping” the wedge is present. This is a condition in which too much of the in-body seating surface is reflected on the blue check. Wedge replacement or weld buildup of the wedge faces are the only options at this point—both expensive propositions. Over-grinding can be
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
avoided by this general rule of thumb: For every .001 inch removed from each seat or wedge face, the wedge will drop about .010 inch further into the seats. The ultimate goal is to produce a .062 inch to .126 inch land (flat) on the center of the seat face. The commonly encountered problem of over-grinding can be avoided through careful manipulation of air pressure, correct pressure of the grinding discs, and correct grit of grinding abrasives. Material used for seating surfaces in gate valves ranges from bronze and brass for low temperature, low pressure applications to high grade steel (stainless, 13 chrome, nickelchrome, and cobalt-chromium alloys). When hardfacing alloys are used, they are welded thin and evenly onto a base metal, and then machined to specifications. The thickness of the hardfacing can lead to minimum tolerance problems. The base metal behind the hardfacing overlay is shiny in appearance, in contrast with the dull gray appearance of the stellite. If the seating surfaces appear shiny in some areas and dull in others after cleaning, hardfacing is missing in the shiny areas. A simple acid etch test can determine if this is the case. Apply a 95-part ethanol to 5-part nitric acid solution to the in-body seats (or wedge face); after a few minutes the milder steel (13 chromium) will appear to oxidize while the stellite hardfacing remains the same. If insufficient stellite remains on the in-body seats and it is not practical to replace the seats, complete valve replacement might be necessary. Although a hardfacing weld buildup is possible, it is usually reserved for wedge faces. The final step in reconditioning in-body seats to obtain a leak-proof seal is achieving the proper surface finish. Use a fine grit abrasive (250 grit to 500 grit) and light grinding pressure to obtain a finish of 16 rms (root mean square) or better. When the in-body seats have been reconditioned to maintain a leak-proof seal, the wedge is ready for reconditioning. Do another blue check to redefine the scope of work. The same principles outlined in working the in-body seats also hold true for the wedge faces (that is, ensure that the hardfacing material is intact, .001 inch removal of seating surface results in .010-inch drop in the seat, and so on). When reworking a wedge, keep in mind that various wedge-types exhibit different characteristics. Solid wedges require the tightest tolerances and the most patience to fit. Flex wedges will “give” enough to compensate for slight differences in fit. Easier still to fit are double-disc and paralleldisc wedges, due to the amount of line-up variance incorporated into the valve design. Note that removing any more of the seating surface than is absolutely necessary drops the wedge further into the valve seats and might necessitate an angle change on the wedge. Askilled technician will usually make three or more cuts to fit a wedge. The blue check illustrations shown in Figures A-1 through A-5 help clarify the proper machining steps for an efficient wedge fit-up. Although the possible scenarios shown by blue checks are varied, the concept of machining the wedge angle to attain a fit remains the same.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Side A: 360° Blue / Side Side B: 260° Blue Missing at Toe
Dial in So This Is The Heaviest Cut Area
B
A
Front
Side
Back
SOLUTION: Do Not Machine Side A. Machine Side B, Removing Metal 180° from Center of Gap Area Using Gauges to Determine The Amount of Metal to Be Removed.
Figure A-1 Blue Check Illustration Missing Blue on both Sides at the Same Location. Wedge Needs Slight Angle Change. Angle Change Can Be Done on One Side or Both Sides. Use Plastic Gauge, Feeler Gauge or MIC Lead Wire at Location to Aid in Determining The Amount to Machine of Wedge.
A
Front
B
Side SOLUTION: Machine One Side Amount Indicated by Gauge to Find Gap in Bluing. It’s Much Faster to Remove Metal from Only One Side. Make Sure Guide Clearance Will Allow Wedge to Lean.
Figure A-2 Blue Check Illustration A-4
Back
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair Missing Blue at Toe: Blue Check Shows Continuous Wide Line 270° Around Heel of Wedge Indicating Too Wide of Sears And Improper Wedge Angle on Each Side. Blue Too Wide
Reference 180° For Machine Setup
A
B
Missing Blue *Using Plastic-Gauge or Lead Wire, Find Gap Between Seat/Wedge
Side
Front
Back
Solution: Taper Seats 3° to Narrow Line of Wedge to Seat Contact. Correct Wedge Angle by Machining 180° Opposite of Missing Blue on Each Side.
Figure A-3 Blue Check Illustration Blue Running Off Heel of Wedge: Wedge Has Fallen Through Valve Seats. Need Wedge of Larger Thickness. Measure Heel and Toe of Wedge to Reference Size of Wedge Needed. X
HEEL
A
B
TOE
Y
Front
Side
Back
Solution: (1) Machine Wedge to Base Metal And Weld Build-up With Manufacturer’s Recommended Wedge Face Material. Easiest to Strip, Weld and Remachine One Side at a Time to Keep Wedge Angle Close. (2) Purchase Oversized Wedge Usually 0.050" Larger And Fit New Wedge.
Figure A-4 Blue Check Illustration A-5
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Blue Only in Center of Wedge: Wedge too Wide.... This is Found When Fitting a New or Weld Built-up Wedge.
HEEL
B
A
TOE
Front
Side
Back
Solution: Take and Old Wedge Heel and Toe Measurements to Machine Wedge to Fit Further into Valve. Always Machine Conservatively so as to Not Remove Too Much.
Figure A-5 Blue Check Illustration Using the blue check, heel and toe measurements (see Figure A-6), and feeler gauge/ plastic gauge measurements (see Figure A-7) as guidelines, determine the wedge angle. Then, using the above guidelines, rework the wedge in a mill, lathe, or surface grinder using a dial indicator to remove the least amount of material necessary. Figure A-1 presents a typical wedge fitting case. Side B needs material removed 180˚ from the gap in the blue check print to bring in the material at the gap area. Therefore, the dial indicator would be set at .0005 inch at the center of the gap area and + .0035 inch 180˚ from the center of the gap area. Plus .003 inch was arrived at because the plastic gauge had a reading of .004 inch. After machining (if using a single point application), lap or grind the wedge face to remove any tooling marks. The grit of lapping compound or grinding abrasive should correlate to the roughness of the cut; in general, start with a coarser grit of 240 and work up to a finer grit of 500. It is important to use good lapping techniques and a flat lapping block. A grinding table (such as the EFCO KS-6), drill press adapters, and portable gate lapping machines (EFCO, LarsLapp, or Unislip) can achieve the desired finish at a much faster rate.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
CL
Heel
Typical Gate Valve Seats
(Toe-Heel)/2=X X/Face = Sin of Angle Sin of Angle x 2 = Included Angle
Face
Toe
CL
Figure A-6 Heel and Toe Measurement Parameters
A-7
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair Gauge for Determining The Included Angle for Gate Seats. Instrument Must Be Sized in Relation to Valve Sizes.
CL
Slot Lock Nut (Swivel Joint) Typical
Typical Gate Valve Seats
CL
Preceision Bevel Protractor
Flat Surface Included Angle of Wedge Gauge (Seats)
Figure A-7 Gauges Used to Determine Seat and Wedge Angles
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
When a satisfactory blue check is achieved, the technician should reassemble the valve and perform a final blue check with all valve components present, including the actuator. For example, gate valves fitted with flex, double-disc, or parallel-wedges might not show 360˚ on the blue check without the additional force exerted by their respective actuators. Conversely, in rare instances, a good blue can turn unsatisfactory with this additional force. The technician must also be aware of other variables that could prevent a satisfactory blue, such as wedge guide clearances or binding. If a 360˚ blue cannot be achieved on both sides, the downstream seating surface should be given priority. This seat receives the system pressure as the wedge is forced downstream. In some instances, the wedge can be reversed (turned 180˚ to obtain a good downstream seal). In summary, the principles outlined facilitate the efficient repair of in-line gate valves: always recondition the in-body seats first; use the blue check as a guide to material removal and proper seat contact; taper seats with a 3˚ relief, then grind seats flat; watch for loss of hardfacing; apply a fine finish (16 rms or better); fit the wedge in gradually, using several cuts; and cautious lapping or grinding. Keep in mind that any gate valve identified as leaking-by will have the full pressure drop from upstream to downstream occurring at some time across the narrow orifice of the leak, in most cases. Depending on the system fluid, a leak effects range from relatively minor thermal gradients for gases to the severely destructive results of hot water, steam, or solids. Ensuring that a gate valve can maintain a leak-proof seal is time and effort well spent. The alternative—complete valve replacement—is not a cost-effective option.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
APPENDIX B The data in this appendix is for reference only and in no way constitutes EPRI endorsement of their products or services. There could be other manufacturers/suppliers not listed here for similar equipment and services.
In Situ Welding Equipment These companies either manufacture equipment for sale or design and manufacture equipment especially for their own use. This data is based on a 1994 survey of valve manufacturers, repair vendors, and utilities, and might unintentionally exclude companies of which the authors were unaware. Please refer to the corresponding numbers shown on the address list. Bore welding (pressure seal and globe valve seats) 1, 2, 4, 10, 11, 12, 14, 16, 17, and 18 Gate valve seat face welding 1, 14, 16, and 17 Globe valve seat ring welding 1, 2, 14, 16, and 17 Orbital pipe welding 1, 2, 4, 6, 7, 8, 11, 12, 13, 14, 15, 16, and 17 NOTES: 1. Gate includes tilting disc check valves. 2. Globe includes control, lift check, Y-type, stop, and angle valves. 1. Anchor/Darling Valve Company 701 First Street P.O. Box 3428 Williamsport, PA 17701 717/327-4800 717/327-4805 FAX 2. Arc Machines, Incorporated 10280 Glenoaks Boulevard Pacolma, CA 91331 818/896-9556 818/890-3724 FAX B-1
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
3. Atwood & Morrill Company 385 Canal Street Salem, MA 01970 508/744-5690 508/741-3626 FAX 4. BW/IP International, Incorporated Pump Division 2300 East Vernon Avenue Vernon, CA 90058 213/589-6171 213/589-2080 FAX 5. Climax Portable Machine Tools 2712 East Second Street P.O. Box 230 Newberg, OR 97132 503/538-2185 503/538-7600 FAX 6. Continental Field Systems, Incorporated 23 Westgate Boulevard Savannah, GA 31405 912/232-8121 912/232-0116 FAX 7. Dimetrics P.O. Box 339 Davidson, NC 28036 704/892-8872 704/892-4713 FAX 8. E. H. Wachs Wachs Technical Service 100 Shepard Street Wheeling, IL 60090 847/537-8800 847/520-1168 FAX 9. Edward Valves, Incorporated 1900 South Saunders Street Raleigh, NC 27603 919/831-3311 919/831-3369 FAX 10. EFCO USA, Incorporated 1611 Telegraph Avenue, Suite 1600 Oakland, CA 94612 510/272-0481 800/EFCO USA 510/272-0483 FAX B-2
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
11. Framatome 3315 Old Forest Road P.O. Box 10935 Lynchburg, VA 24506-0935 804/832-3473 804/832-3177 FAX 12. GE Nuclear Energy 640 Freedom Business Court King of Prussia, PA 19406 610/992-6394 13. Magnatech, Incorporated Bradley Park East Granby, CT 06026 203/653-2573 203/653-0486 FAX 14. PCI Energy Services One Energy Drive P.O. Box 3000 Lake Bluff, IL 60044 847/680-8100 847/362-6441 FAX 15. Tri Tool, Incorporated 3806 Security Park Drive Rancho Cordova, CA 95742-6990 916/351-0144 800/345-5015 916/351-0372 FAX 16. VR-TESCO 803 Albion Avenue Schaumburg, IL 60193 800/248-2082 847/893-2433 FAX 17. Welding Services 2225 Skyland Court Norcross, GA 30071 770/452-0005 770/729-8242 FAX 18. Westinghouse/Elliott Valve 5436 Clay Street Houston, TX 77023 713/926-8318
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EPRI Proprietary Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
APPENDIX C The data in this appendix is for reference only and in no way constitutes EPRI endorsement of their products or services. There could be other manufacturers/suppliers not listed here for similar equipment and services.
In Situ Valve Machining Equipment These companies either manufacture equipment for sale or design and manufacture equipment especially for their own use. This data is based on a 1994 survey of valve manufacturers, repair vendors, and utilities, and might unintentionally exclude companies of which the authors were unaware. Please refer to the corresponding numbers shown on the address list. Boring bars (pressure seal bores, globe valve seats, hinge pin bores) 1, 4, 5, 6, 9, 10, 11, 12, 14, 16, 17, and 18 Boring bars (gate valve seat facing) 1, 4, 5, 14, and 16 Boring bars (gate valve seat removal) 1, 4, 5, and 16 Flange facers 1, 4, 5, 6, 8, 9, 10, 11, 12, 14, 16, and 17 Lapping machines 10 Milling machines 1, 5, 6, 14, and 17 NOTES: 1. Gate includes tilting disc check valves. 2. Globe includes control, lift check, Y-type, stop, and angle valves.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
1. Anchor/Darling Valve Company 701 First Street P.O. Box 3428 Williamsport, PA 17701 717/327-4800 717/327-4805 FAX 2. Arc Machines, Incorporated 10280 Glenoaks Boulevard Pacolma, CA 91331 818/896-9556 818/890-3724 FAX 3. Atwood & Morrill Company 385 Canal Street Salem, MA 01970 508/744-5690 508/741-3626 FAX 4. BW/IP International, Incorporated Pump Division 2300 East Vernon Avenue Vernon, CA 90058 213/589-6171 213/589-2080 FAX 5. Climax Portable Machine Tools 2712 East Second Street P.O. Box 230 Newberg, OR 97132 503/538-2185 503/538-7600 FAX 6. Continental Field Systems, Incorporated 23 Westgate Boulevard Savannah, GA 31405 912/232-8121 912/232-0116 FAX 7. Dimetrics P.O. Box 339 Davidson, NC 28036 704/892-8872 704/892-4713 FAX 8. E. H. Wachs Wachs Technical Service 100 Shepard Street Wheeling, IL 60090 847/537-8800 847/520-1168 FAX C-2
EPRI Proprietary Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
9. Edward Valves, Incorporated 1900 South Saunders Street Raleigh, NC 27603 919/831-3311 919/831-3369 FAX 10. EFCO USA, Incorporated 1611 Telegraph Avenue, Suite 1600 Oakland, CA 94612 510/272-0481 800/EFCO USA 510/272-0483 FAX 11. Framatome 3315 Old Forest Road P.O. Box 10935 Lynchburg, VA 24506-0935 804/832-3473 804/832-3177 FAX 12. GE Nuclear Energy 640 Freedom Business Court King of Prussia, PA 19406 610/992-6394 13. Magnatech, Incorporated Bradley Park East Granby, CT 06026 203/653-2573 203/653-0486 FAX 14. PCI Energy Services One Energy Drive P.O. Box 3000 Lake Bluff, IL 60044 847/680-8100 847/362-6441 FAX 15. Tri Tool, Incorporated 3806 Security Park Drive Rancho Cordova, CA 95742-6990 916/351-0144 800/345-5015 916/351-0372 FAX 16. VR-TESCO 803 Albion Avenue Schaumburg, IL 60193 800/248-2082 847/893-2433 FAX
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
17. Welding Services 2225 Skyland Court Norcross, GA 30071 770/452-0005 770/729-8242 FAX 18. Westinghouse/Elliott Valve 5436 Clay Street Houston, TX 77023 713/926-8318
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
APPENDIX D The data in this appendix is for reference only and in no way constitutes EPRI endorsement of their products or services. There could be other manufacturers/suppliers not listed here for similar equipment and services. Valve Manufacturers Anchor/Darling Valve Company 701 First Street P.O. Box 3428 Williamsport, PA 17701 717/327-4800 717/327-4805 FAX Contact: Frank Velez Atwood & Morrill Company 385 Canal Street Salem, MA 01970 508/744-5690 508/741-3626 FAX Contact: Jeff LeBlanc BW/IP International, Incorporated Pump Division 2300 East Vernon Avenue Vernon, CA 90058 213/589-6171 213/589-2080 FAX Contact: Matt Sweeny Circle Seal Controls, Incorporated 2301 Wardlow Road Corona, CA 91720 909/277-6200 909/277-6201 FAX Control Components 22591-T Avenida Empress Rancho Santo Margarita, CA 92688 714/858-1877 714/858-1878 FAX
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Conval, Incorporated 265 Field Road Somers, CT 06071-1049 203/749-0761 203/749-2680 FAX Contact: Michael Farnsworth Copes-Vulcan, Incorporated Vulcan Building (Erie County) Lake City, PA 16423 814/774-1500 814/774-1681 or 774-2646 FAX Crane Valves—Romeoville, Ill. 104 North Chicago Street Joliet, IL 60431 815/727-2600 815/727-4246 FAX Contact: John Carlson Crosby Valve & Gage Company 43 Kendrick Street P.O. Box 308 Wrentham, MA 02093 508/384-3121 508/384-8675 FAX DeZurik 250 Riverside Avenue Sartell, MN 56377 Contact: Jack Herold Edward Valves, Incorporated 1900 South Saunders Street Raleigh, NC 27603 919/831-3311 919/831-3369 FAX Contact: Lemar Green Fisher Controls 205 South Center Street P.O. Box 190 Marshalltown, IA 50158 515/754-3011 515/754-2830 FAX Contact: Larry Fleetwood
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Hammel Dahl, Jamesbury Controls Company 175R Post Road Warwick, RI 02888 401/781-6200 401/781-6204 FAX Henry Vogt Machine Company, Incorporated P.O. Box 1918 Louisville, KY 40201 502/634-1500 502/637-7344 FAX Kerotest Manufacturer Corporation 2525-T Liberty Avenue Pittsburgh, PA 15222 412/392-4300 412/392-4251 FAX Leslie Controls, Incorporated 12503 Telecom Drive Tampa, FL 33637 800/253-7543 813/978-0984 FAX Lunkenheimer Company Beekman Street at Waverly Avenue Cincinnati, OH 45214 513/921-3400 513/244-5228 FAX Manager, Valves and Controls Sulzer USA, Incorporated 200 Park Avenue New York, NY 10166 Contact: Jean-Claude Braendle Masoneilan Valve Company 85 Bodwell Street Avon, MA 02322 508/941-5421 Contact: Larry Swartz Masoneilan-Dresser 1040 South Vail Avenue Montebello, CA 90640 213/723-9351 Contact: Pat Patnaik
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Neles-Jamesbury, Incorporated, Northeast Service Center 1 Polito Drive Shrewsbury, MA 01545 800/255-2669 Contact: Tom Hughes Target Rock Corporation 1966 Broad Hollow Road East Farmingdale, NY 11735 516/293-3800 Contact: Jim White Teledyne Fluid Systems, Farris Engineering—N.J. 400 Commercial Avenue Palisades Park, NJ 07650 201/944-6300 Contact: Paul Papa The William Powell Company Spring Grove at Draper Cincinnati, OH 45214 513/852-2000 513/852-2997 FAX VALCOR Engineering Corporation 4 Lawrence Road Springfield, NJ 07081 201/467-8400 800/241-2159 201/467-8382 FAX Velan, Incorporated 550 McArthur Street Laurent, Quebec Canada H4T 1X8 514/748-7743 Contact: Zoltan Palko Walworth Company 10190-T Harwin Drive Houston, TX 77036 713/777-7788 713/981-1246 FAX WKM, Cooper Industry, Incorporated P.O. Box 4446 Houston, TX 77251 713/939-2211 (Division Headquarters) 713/939-2620 FAX
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Contact: Dean Hannam 713/499-8511 (Cooper Cameron Valves) 713/499-5213 FAX YARWAY Corporation 480 Norristown Road Blue Bell, PA 19422 800/220-8070 215/825-1235 FAX
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
APPENDIX E The data in this appendix is for reference only and in no way constitutes EPRI endorsement of their products or services. There could be other manufacturers/suppliers not listed here for similar equipment and services.
In Situ Valve Repair Vendors ABB CE Nuclear Power Department 9428-1206 1000 Prospect Hill Road Windsor, CT 06095 Contact: Edward Kurdziel Anchor/Darling Valve Company 701 First Street P.O. Box 3428 Williamsport, PA 17701 717/327-4800 717/327-4805 FAX Contact: Frank Velez Atlantic Valve Corporation Turnpike Industrial Park Westfield, MA 01085 413/568-3366 Contact: Joseph L. Casey Atwood & Morrill Company 385 Canal Street Salem, MA 01970 508/744-5690 508/741-3626 FAX Contact: Jeff LeBlanc Babcock & Wilcox Nuclear Services 3110 Odd Fellows Road Lynchburg, VA 24506 Contact: Terry L. Jamerson
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
BW/IP International, Incorporated Pump Division 2300 East Vernon Avenue Vernon, CA 90058 213/589-6171 213/589-2080 FAX Contact: Matt Sweeny Carter Chambers Supply, Incorporated—Monroe, La. P.O. Box 486 Monroe, LA 71211 800/256-4671 Contact: Grant Walker Chalmers & Kubeck, Incorporated 150 Commerce Drive Aston, PA 19014 215/494-7030 Contact: Joe O’Shea Continental Field Systems, Incorporated 23 Westgate Boulevard Savannah, GA 31405 912/232-8121 912/232-0116 FAX Contact: Charles Blakewood Conval, Incorporated 265 Field Road Somers, CT 06071-1049 203/749-0761 203/749-2680 FAX Contact: Michael Farnsworth Copes, Vulcan, Incorporated—Pa. Martin & Rice Avenue P.O. Box 577 Lake City, PA 16423 800/756-4456 Contact: Joseph Dougher Crane Valves—Romeoville, Ill. 104 North Chicago Street Joliet, IL 60431 815/727-2600 815/727-4246 FAX Contact: John Carlson
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
DeZurik 250 Riverside Avenue Sartell, MN 56377 Contact: Jack Herold Edward Valves, Incorporated 1900 South Saunders Street Raleigh, NC 27603 919/831-3311 919/831-3369 FAX Contact: Lemar Green GE Nuclear Energy 640 Freedom Business Court King of Prussia, PA 19406 610/992-6394 Contact: Bill Fingrudt Groth Corporation 1202 Hahlo Street P.O. Box 15293 Houston, TX 77220-5293 713/675-6151 Contact: Robert L. Wood Henry Vogt Machine Company P.O. Box 1918 Louisville, KY 40201-1918 502/634-1500 Contact: Guy A. Jolly In-Place Machining Company 1929 North Buffum Street Milwaukee, WI 53212 414/562-2000 414/265-1000 FAX Contact: Clifford G. Redwine, Jr. Leslie Controls, Incorporated—Calif. 11144 Business Circle Cerritos, CA 90701 213/860-0463 Contact: Dave English Masoneilan Valve Company 85 Bodwell Street Avon, MA 02322 508/941-5421 Contact: Larry Swartz
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
PCI Energy Services One Energy Drive P.O. Box 3000 Lake Bluff, IL 60044 Contact: John Polacheck Dezurik 250 Riverside Avenue Sartell, MN 56377 Contact: Jack Herold Velan, Incorporated 550 McArthur Street St. Laurent, Quebec Canada H4T 1X8 514/748-7743 Contact: Zoltan Palko VR-TESCO 803 Albion Avenue Schaumburg, IL 60193 800/248-2082 708/893-2433 FAX Welding Services 2225 Skyland Court Norcross, GA 30071 Contact: Pedro Amador Westinghouse Electric Corporation 4400 Alafaya Trail, MC 200 Orlando, FL 32826-2399 Contact: Kevin P. Hazel Westinghouse/Elliott Valve 5436 Clay Street Houston, TX 77023 713/926-8318 Contact: Colbert Pearson Westinghouse-Movats 2825 Cobb International Avenue Kennesaw, GA 30144 404/424-6343 404/429-4753 FAX Contact: Wayne Prokop
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APPENDIX F REFERENCES AND ADDITIONAL BIBLIOGRAPHY 1.
Key Valves Prioritization Study. Electric Power Research Institute, Palo Alto, CA: October 1984. Report NP-3611.
2.
Valve Maintenance Equipment Reference Guide. EPRI/Maintenance Equipment Application Center, Charlotte, NC: 1988.
3.
Good Bolting Practices, A Reference Manual for Nuclear Power Plant Maintenance Personnel, Volume 1: Large Bolt Manual. Electric Power Research Institute, Palo Alto, CA: May 1987. Report NP-5067.
4.
On-line Leak Sealing, A Guide for Nuclear Power Plant Maintenance Personnel. Electric Power Research Institute, Palo Alto, CA: July 1989. Report NP-6523-D.
5.
Improvements in Motor-Operated Valves. Electric Power Research Institute, Palo Alto, CA: November 1985. Report NP-4254, Project 2232-2, Interim Report.
6.
Main Feedwater Isolation Valve Maintenance Guide. Electric Power Research Institute, Palo Alto, CA: May 1991. Report NP-7212, Project 2814-27, Final Report.
7.
Main Steam Isolation Valve Maintenance Guide. Electric Power Research Institute, Palo Alto, CA: May 1991. Report NP-7211, Project 2814-27, Final Report.
8.
Guide for the Applications and Use of Valves in Power Plant Systems. Electric Power Research Institute, Palo Alto, CA: August 1990. Report NP-6516. (This document will be revised and reissued as EPRI TR-105852v1, 1997.)
9.
Check Valve Maintenance Guide. Nuclear Maintenance Applications Center, Charlotte, NC: August 1995. Report TR-100857.
10. NOREM Applications Guidelines, Procedures for GTA and PTA Welding of NOREM Cobalt-Free Alloys. Electric Power Research Institute, Palo Alto, CA: November 1995. Report TR-105816. 11. In Situ Valve Repair Capabilities Survey. Electric Power Research Institute Repair and Replacement Applications Center, Charlotte, NC: August 1995. 12. ASME Boiler and Pressure Vessel Code, Sections II, III, IX, and XI; 1995 Edition, The American Society of Mechanical Engineers, New York, NY.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
13. “Anchor/Darling Valve Seminar.” Anchor/Darling Valve Company, Albuquerque, NM: 1995. 14. Maintenance Manual for Flexible Wedge-Type Gate Valves. Anchor/Darling Valve Company. 15. Maintenance Manual for Double Disc-Type Gate Valves. Anchor/Darling Valve Company. 16. Maintenance Manual for 2-1/2”–24” Forged Pressure Seal Gate, Globe and Check Valves. Velan. 17. Equiwedge Gate Valve Maintenance Manual. Edward Valves, Incorporated. 18. Maintenance Manual for Edward Pressure-Seal Valves. Edward Valves, Incorporated. 19. Maintenance Manual for Globe Valves. Anchor/Darling Valve Company, Williamsport, PA: January 1988. 20. Application Guide for Check Valves in Nuclear Power Plants, Revision 1. Electric Power Research Institute, Nuclear Maintenance Applications Center, Charlotte, NC: June 1993. Report NP-5479. 21. Maintenance Manual for Swing Check Valves. Anchor/Darling Valve Company, Williamsport, PA: January 1988. 22. State-of-the-Art Weld Repair Technology for High Temperature and Pressure Parts Repair Guideline. Electric Power Research Institute, Palo Alto, CA: June 1994. Report TR103592. 23. Model 1572 MSIV Repair Machine. Climax Portable Machine Tools, Incorporated, Newberg, OR: August 1987. 24. Maintenance Manual for Tilting Disc Check Valves. Anchor/Darling Valve Company, Williamsport, PA: December 1987. 25. Gate Valve Wedge Fitting Techniques. EFCO USA, Incorporated 26. Fermi II Approach to Cobalt Reduction in Valves. PCI Energy Services, Lake Bluff, IL. 27. “Maintenance and Repair Welding in Power Plants V,” presented at the American Welding Society and Electric Power Research Institute Jointly Sponsored Conference, Orlando, FL (November 30–December 2, 1994). 28. Cobalt Reduction Seminar. Anchor/Darling Valve Company, May 1993. 29. Cobalt Reduction Guidelines, Revision 1. Electric Power Research Institute.
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
30. Erik Oberg, et al. Machinery’s Handbook, 22nd Revised Edition. Industrial Press Incorporated, New York, NY 1984. 31. “Welding Terms and Definitions.” AWS A3.0-80. 32. H. B. Cary. Modern Welding Technology, 2nd Edition. Prentice Hall, 1989. 33. The Procedure Handbook of Arc Welding. The Lincoln Electric Company, Cleveland, OH: 1973. 34. BWR Recirculation Piping System Replacement, Final Report, Volumes 1–3. J. A. Jones Applied Research Company, June 1990. Report NP-6723-D. 35. The Procedure Handbook of Welding, 12th Edition. The Lincoln Electric Company, 1973. 36. “Welding and Brazing,” Metals Handbook, Volume 6, 9th Edition. The American Society for Metals, Metals Park, Ohio. 37. AWS A5.01-93, “Filler Metal Procurement Guidelines.” 38. AWS A5.1-91, “Specification for Covered Carbon Steel Arc Welding Electrodes.” 39. AWS A5.4-92, “Specification for Covered Corrosion-Resisting Chromium and Chromium-Nickel Steel Welding Electrodes.” 40. AWS A5.5-96, “Specification for Low-Alloy Steel Covered Arc Welding Electrodes.” 41. AWS A5.9-93, “Specification for Corrosion-Resisting Chromium and ChromiumNickel Steel Bare and Composite Metal-Cored and Stranded Welding Electrodes and Welding Rods.” 42. AWS A5.11-90, Specification for Nickel and Nickel Alloy Covered Welding Electrodes.” 43. AWS A5.14-89, “Specification for Nickel and Nickel Alloy Bare Welding Electrodes and Rods.” 44. AWS A5.17-89, “Specification for Carbon Steel Electrodes and Fluxes for Submerged Arc Welding.” 45. AWS A5.18-93, “Specification for Carbon Steel Filler Metals for Gas-Shielded Arc Welding.” 46. AWS A5.20-95, “Specification for Carbon Steel Electrodes for Flux-Cored Arc Welding.”
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47. AWS A5.22-95, “Specification for Flux-Cored Corrosion-Resisting Chromium and Chromium-Nickel Steel Electrodes.” 48. AWS A5.23-90, “Specification for Low-Alloy Steel Electrodes and Fluxes for Submerged Arc Welding.” 49. AWS A5.28-79, “Specification for Low-Alloy Steel Filler Metals for Gas-Shielded Arc Welding.” 50. AWS A5.29-80, “Specification for Low-Alloy Steel Electrodes for Flux-Cored Arc Welding.” 51. AWS A5.30-79, “Specification for Consumable Inserts.” 52. AWS “Filler Metal Comparison Chart,” 1989. 53. Guidelines on Fossil Boiler Field Welding Practices. Electric Power Research Institute, Palo Alto, CA: November 1991. Research Project 2504-1, Final Report. 54. AWS D10.4-86, “Recommended Practices for Welding Austenitic Chromium Nickel Stainless Steel Piping and Tubing.” 55. AWS D10.8-86, “Recommended Practices for Welding Chromium-Molybdenum Steel Piping and Tubing.” 56. ASME/ANSI B31.1, ASME Code for Pressure Piping, “Power Piping.” 1995 Edition. 57. Stoody Corporation Product Information Guide. 58. AWS D10.11-87, “Recommended Practices for Root Pass Welding of Pipe Without Backing.” 59. AWS D10.12-89, “Recommended Practices and Procedures for Welding Low Carbon Steel Pipe.” NOTE: The referenced American Welding Society (AWS) standards are also approved by the American National Standards Institute (ANSI).
60. AWS D1.1-92, “Structural Welding Code—Steel,” The American Welding Society, Miami, FL. 61. ANSI/NB-23, A Manual for Boiler and Pressure Vessel Inspection. National Board Inspection Code, 1995 Edition. 62. AWS A3.0-89, “Standard Welding Terms and Definitions.”
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
63. “Welding & Fabricating Data Book 1990/91,” Welding Design & Fabricating. 64. Guidelines for the Repair/Replacement Welding of Nuclear Service Water Systems. EPRI NDE Center, Charlotte, NC: July 1992. Report TR-100386. 65. Hydro-Purge. Coast Engineering Laboratories, Cardena, CA. 66. AWS D10.10-90, “Recommended Practices for Local Heat Treatment of Welds in Piping and Tubing.” 67. R. Viswanathan. World Repair of Aged Piping—A Review of International Experience. Electric Power Research Institute, Palo Alto, CA: 1995. Report RP-3484-01, Draft Report. 68. Temperbead Welding Repair of Low-Alloy Pressure Vessel Steels: Guidelines. Electric Power Research Institute, Palo Alto, CA: December 1993. Report TR-103354, Project C104-02, Final Report. 69. W. J. Childs, S. J. Findlan, and D. W. Gandy. “Repair Welding of SA-508 Cl. 2 Steel Utilizing the 3-Layer Temperbead Approach,” Fatigue, Fracture, & Risk. PVPVoume 215. 70. W. J. Childs, S. J. Findlan, D. W. Gandy, and R. E. Smith. “A Better Way to Control GTA Weld Dilution,” Welding Design & Fabrication. August 1992. 71. W. J. Childs, S. J. Findlan, D. W. Gandy, and R. E. Smith. “Pressure-Vessel Steel Weld-Repair with CLTT,” Welding Design & Fabrication. September 1992. 72. C. Lundin and M. Prager, “Repair and Fabrication Practices for 1-1/4Cr-1/2Mo Steel,” Second International Conference on Interaction of Steels with Hydrogen in Petroleum Industry Pressure Vessel and Pipeline Service, Vienna, Austria (October 19–21, 1994). 73. P. P. Holz. “Half-Bead Weld Repairs for Inservice Applications.” Oak Ridge National Laboratory, 1978. 74. L. Friedman. “EWI/TWI Controlled Deposition Repair Welding Procedure for 1-1/4Cr-1/2Mo and 2-1/4Cr-1Mo Steels.” PVRC Weld Repair Workshop, San Diego, CA (January 31–February 1, 1996). 75. ANSI/ASME S/G OM-10, Inservice Testing of Valves in Light-Water Reactor Power Plants, 1988.
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Additional Bibliography 1.
General Design Criteria 54 through 57 of 10 CFR Part 50: • GDC 54: Piping systems penetrating containment • GDC 55: Reactor coolant pressure boundary penetrating containment • GDC 56: Primary containment isolation • GDC 57: Closed system isolation valves
2.
Code of Federal Regulations, 10 CFR 50 Appendix J, “Primary Reactor Containment Leakage Testing for Water-Cooled Power Reactors,” 1973, as amended in 1976, 1980, 1986, 1988, and 1992.
3.
Code of Federal Regulations, 10 CFR 50.65, “Requirement for Monitoring the Effectiveness of Maintenance at Nuclear Power Plants,” 1991.
4.
ANSI B16.10, Face-to-Face and End-to-End Dimensions of Valves.
5.
ANSI B16.11, Forged Steel Fittings, Socket Welding, and Threaded.
6.
ANSI B16.25, Butt-Welding Ends.
7.
ANSI B16.34, Valves—Flanged and Butt-welding End.
8.
ANSI B16.5, Steel Pipe Flanges and Flanged Fittings.
9.
ANSI B16.9, Factory-Made Wrought Steel Butt-welding Fittings.
10. ANSI B2.1, Pipe Threads. 11. ANSI N18.2/ANS 51.8, Nuclear Safety Criteria for the Design of Stationary Pressurized Water Reactor Plants. 1973. 12. ANSI N271-1976/ANS-56.2-1984, Containment Isolation Provisions for Fluid Systems. 13. ANSI/ANS N18.2a, Supplement to N18.2. 1975. 14. ANSI/ANS-51.1, Nuclear Safety Criteria for the Design of Stationary Pressurized Water Reactor Plants. 1983, revised 1988. 15. ANSI/ANS-52.1, Nuclear Safety Criteria for the Design of Stationary Boiling Water Reactor Plants. 1983. 16. ANSI/ANS-58.1, Nuclear Safety Design Criteria for Light Water Reactors. 17. ANSI/ASME Boiler and Pressure Vessel Code, Section V, “Nondestructive Examination.” 1995. F-6
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18. ANSI/ASME Code for Pressure Piping, B31.1, “Power Piping.” 1995. 19. ANSI/ASME OM (Code)–1990, Code for Operation and Maintenance of Nuclear Power Plants.” 1990. 20. ANSI/ASME OM (Standard)–1987, “Operation and Maintenance of Nuclear Power Plants,” 1987. 21. ANSI/ASME Oma-1988, “Addendum to ASME/ANSI OM-1987.” 1988. 22. ANSI/ASME Omc-1990, “Addendum to ASME/ANSI OM-1987.” 1990. 23. API 598, “Valve Inspection and Test.” 24. ASM Handbook, American Society for Metals, Volume 6. Ohio 1983. 25. ASME Boiler & Pressure Vessel Code, Section III Code Case N62, “Internal and External Valve Items, Division 1, Classes 1, 2, and 3.” 26. ASTM E 114, “Standard Recommended Practice for Ultrasonic Pulse-Echo StraightBeam Testing by Contact Method.” 27. ASTM E 142, “Standard Method for Controlling Quality of Radiographic Testing.” 28. ASTM E 164, “Standard Practice for Ultrasonic Contact Examination of Weldments.” 29. ASTM E 165, “Standard Practice for Liquid Penetrant Inspection Method.” 30. ASTM E 213, “Standard Practice for Ultrasonic Inspection of Metal Pipe and Tubing.” 31. ASTM E 269, “Standard Definitions of Terms Relating to Magnetic Particle Examination.” 32. ASTM E 270, “Standard Definitions of Terms Relating to Liquid Penetrant Inspection.” 33. ASTM E 428, “Standard Recommended Practice for Fabrication and Control of Steel Reference Blocks Used in Ultrasonic Inspection.” 34. ASTM E 433, “Standard Reference Photographs for Liquid Penetrant Inspection.” 35. ASTM E 500, “Standard Definitions of Terms Relating to Ultrasonic Testing.” 36. ASTM E 543, “Standard Practice for Determining the Qualification of Nondestructive Testing Agencies.”
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37. ASTM E 586, “Standard Definitions of Terms Relating to Gamma and X Radiography.” 38. ASTM E 709, “Standard Recommended Practice for Magnetic Particle Examination.” 39. ASTM E 797, “Standard Practice for Measuring Thickness by Manual Ultrasonic Pulse-Echo Contact Method.” 40. ASTM E 94, “Standard Practice for Radiographic Testing.” 41. AWWA C510, “Standard for Double Check Valve Backflow-Prevention Assembly,” 1993. 42. Bergmann, C. A. and E. I. Landerman. Cobalt Release from PWR Valves, Electric Power Research Institute, Palo Alto, CA: July 1984. Report NP-3445. 43. Bergmann, C. A. and L. A. Lamantia. Valve Performance in PWR Chemical and Volume Control Systems, Electric Power Research Institute, Palo Alto, CA: June 1988. Report NP-5796. 44. Cobalt Release from PWR Valves, Electric Power Research Institute, Palo Alto, CA: July 1984. Report NP-3445. 45. Dihne, Bertil (Vattenfall). Valve Seat Repair: Stellite Replaced with Inconel, April 22, 1988, forwarded by letter to EPRI April 27, 1988. 46. A Guide for Developing Preventive Maintenance Programs in Electric Power Plants. Electric Power Research Institute, 1984. Report NP-3416. 47. Good Bolting Practices: A Reference Manual for Nuclear Power Plant Maintenance Personnel, Volume 2: Small Bolts and Threaded Fasteners. Electric Power Research Institute, 1990. Report NP-5067. 48. Valve Stem Packing Improvements. Electric Power Research Institute, 1988. Report NP-5697. 49. Guide for the Application and Use of Valves in Power Plant Systems. Electric Power Research Institute, 1990. Report NP-6516. 50. Welding of NOREM Iron-base Hardfacing Alloy Wire Products: Procedures for Gas Tungsten Arc Welding. Electric Power Research Institute, 1992. Report TR-101094. 51. Application Guidelines for Check Valves in Nuclear Power Plants, Electric Power Research Institute/NMAC, 1993. Report NP-5479 Revision 1. 52. Heard, D. B. and R. J. Freeman. Cobalt Contamination Resulting from Valve Maintenance, Electric Power Research Institute, Palo Alto, CA: August 1983. Report NP-3220.
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53. In Situ Application of Hardfacing Materials in Main Steam Isolation Valves, Electric Power Research Institute, Palo Alto, CA: March 1985. Report NP-3926. 54. INPO Good Practice MA-305 (INPO 87-028), “Postmaintenance Testing,” 1987. 55. INPO Good Practice MA-319 (INPO 92-014), “Preventive Maintenance Program Enhancements,” 1992. 56. INPO SOER 86-03, “Check Valve Failures or Degradation,” 1986. 57. “Maintenance of Pressure Seal Bonnets.” Fifth Valve Technology Symposium, Albuquerque, NM, June 1995. 58. Moore, T., et al., “The Long View for Nuclear Plant Maintenance,” EPRI Journal. (October/November 1991). 59. MSS SP6, “Standard Finish for Contact Faces of Pipe Flanges and Connecting-End Flanges of Valves and Fittings.” 60. MSS SP25, “Standard Marking System for Valves, Fittings, Flanges, and Unions.” 61. MSS SP44, “Steel Pipe Line Flanges (26 inches and larger).” 62. MSS SP53, “Quality Standard for Steel Castings and Forgings for Valves, Flanges and Fittings, and Other Piping Components—Magnetic Particle Examination Method.” 63. MSS SP54, “Quality Standard for Steel Castings for Valves, Flanges and Fittings and Other Piping Components—Radiographic Examination Method.” 64. MSS SP55, “Quality Standards for Steel Castings—Visual Methods.” 65. MSS SP61, “Pressure Testing of Steel Valves.” 66. MSS-SP66, “Pressure Temperature Ratings for Steel Valves.” 67. MSS-SP67, “Butterfly Valves.” 68. MSS-SP70, “Cast Iron Gate Valves.” 69. MSS-SP71, “Cast Iron Check Valves.” 70. MSS-SP72, “Ball Valves.” 71. MSS-SP80, “Bronze Gate, Globe, Angle, and Check Valves.” 72. MSS SP84, “Steel Valves—Socket Welding and Threaded Ends.” 73. MSS-SP85, “Bronze Valves.”
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74. Nolin, L. Cobalt Replacement in Primary Valves, Virginia Power, Electric Power Research Institute, Palo Alto, CA: August 1, 1988. Report NP-1284. 75. Ocken, H., Project Manager, “Implementing Cobalt-Free Hardfacing Technology,” In Seminar Proceedings, Charlotte, NC, Electric Power Research Institute, Palo Alto, CA (April 18, 1989). 76. USNRC GL 86-17, “Technical Findings Related to Generic Issue C-8; BWR MSIV Leakage and Leakage Treatment Methods.” 77. USNRC GL 87-06, “Testing of Pressure Isolation Valves.” 78. USNRC IE Circular 78-15, “Tilting-Disc Check Valve Failed to Close.” 79. USNRC IE Information Notice 80-16, “Shaft Seal Packing in Main Steam Swing Disc and Isolation Valves.” 80. USNRC IE Information Notice 80-29, “Broken Studs on Terry Turbine Steam Inlet Flange.” 81. USNRC IE Information Notice 80-41, “Failure of Swing Check Valves in the Decay Heat Removal System at Davis-Besse Unit No. 1.” 82. USNRC IE Information Notice 82-20, “Check Valve Problems.” 83. USNRC IE Information Notice 82-26, “RCIC and HPCI Turbine Exhaust Check Valve Failures.” 84. USNRC IE Information Notice 84-74, “Isolation of Reactor Coolant System From Low-Pressure Systems Outside Containment.” 85. USNRC IE Information Notice 86-01, “Failure of Main Feedwater Check Valves Causes Loss of Feedwater System Integrity and Water Hammer Damage.” 86. USNRC IE Information Notice 86-57, “Operating Problems with Solenoid-Operated Valves at Nuclear Power Plants.” 87. USNRC IE Information Notice 88-85, “Broken Retaining Block Studs on Anchor Darling Check Valves.” 88. USNRC IE Information Notice 89-62, “Malfunction of Borg-Warner Pressure Seal Bonnet Check Valves Caused by Vertical Misalignment of Disc.” 89. USNRC IE Information Notice 90-79, “Failure of Main Steam Isolator Check Valves Resulting in Disc Separation.” 90. USNRC IE Information Notice 95-14, “Susceptibility of Containment Sump Recirculation Gate Valves to Pressure Locking.”
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91. USNRC NUREG 1482, “Guidelines for Inservice Testing at Nuclear Power Plants,” Draft for Comment published 1993. 92. USNRC NUREG/CR-4302 (ORNL 6193), “Aging and Service Wear of Check Valves Used in Engineered Safety-Feature Systems of Nuclear Power Plants,” 1991. 93. USNRC RG 1.84, “Code Case Acceptability, ASME III Design and Fabrication.” 94. USNRC RG 1.85, “Code Case Acceptability, ASME III Material.” 95. USNRC SRP 3.9.6, “Inservice Testing of Pumps and Valves.” 96. USNRC TI 2515/114, “Inspection Requirements for Generic Letter 89-04, Acceptable Inservice Inspection Programs,” 1992. 97. “Valve Application and Maintenance—An NMAC Study.” Fifth Valve Technology Symposium, Albuquerque, NM, June 1995.
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APPENDIX G ACRONYMS AND ABBREVIATIONS ac
alternating current
AISI
American Iron and Steel Institute
ALARA
radiation levels as low as reasonably achievable
AMS
American Materials Society
ANII
Authorized Nuclear Inservice Inspector
ANS
American Nuclear Society
ANSI
American National Standards Institute
ANSI B31.1
American Society of Mechanical Engineers and American National Standards Institute Power Piping Code
API
American Petroleum Institute
ASA
American Standards Association
ASME
American Society of Mechanical Engineers
ASME III
Section III of the ASME Boiler and Pressure Vessel Code
ASTM
American Society for Testing and Materials
AWS
American Welding Society
AWWA
American Water Works Association
B&PV
Boiler and Pressure Vessel
BWR
boiling water reactor
CBW
controlled band width
CFR
U.S. Code of Federal Regulations
CM
corrective maintenance G-1
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
CoCr
cobalt-chrome
Colmonoy
trade name for nickel-based hardfacing alloys
CrMo
chrome-moly steel
CS
carbon steel
Cv
valve flow coeffecient
DA
damage assessment
dc
direct current
dcrp
direct current, reverse polarity
dcsp
direct current, straight polarity
EPRI
Electric Power Research Institute
EQ
environmental qualification
˚F
degrees Fahrenheit
FCAW
flux-cored arc welding
FCI
Fluid Controls Institute
FMECA
failure modes and effects criticality analysis
FN
ferrite number
F&T
float and thermostatic
GDC
general design criteria
GL
USNRC Generic Letter
GMAW
gas metal arc welding
GMAW-P
pulsed gas metal arc welding
GTAW
gas tungsten arc welding
GTAW-P-AU machine-pulsed gas tungsten arc welding HAZ
heat-affected zone
HBW
heated band width
HIP
hot isostatic process
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
ID
inside diameter
IE
USNRC Inspection and Enforcement Branch
IEB
USNRC IE Bulletin
IEN
USNRC IE Information Notice
ILRT
integrated leak rate test
INPO
Institute of Nuclear Power Operations
ipm
inches per minute
ISA
Instrument Society of America
ISI
inservice inspection
IST
inservice testing
ksi
kips per square inch
LER
licensee event report
LLRT
local leak rate tests
LP
liquid penetrant testing
MeV
megaelectronvolt
MIC
microbiologically induced corrosion
MIG
manual inert gas
Mo
molybdenum
MOV
motor-operated valve
MP
magnetic particle testing
MR
moisture resistant
MSIV
main steam isolation valve
MSS
Manufacturers Standardization Society of the Valve and Fitting Industry
MT
magnetic particle testing
NBBI
National Board of Boiler and Pressure Vessel Inspectors
NBIC
National Board Inspection Code G-3
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
NDE
Nondestructive Examination
NEI
Nuclear Energy Institute
NER
Nuclear Experience Review
Ni
nickel
NIC
Nuclear Industry Check Valve Group
NMAC
Nuclear Maintenance Applications Center
NOREM
EPRI-developed cobalt-free hardfacing alloy
NPAR
Nuclear Plant Aging Research Program
NPRDS
Nuclear Plant Reliability Data System
NPS
nominal pipe size
NRC
United States Nuclear Regulatory Commission
NUMARC
Nuclear Management and Resources Council (now known as NEI)
O&MR
INPO Operation and Maintenance Reminder
OD
outside diameter
OE
INPO Operating Experience Report
OEM
original equipment manufacturer
O&M
Operation and Maintenance
OS&Y
outside screw and yoke
PAW
plasma arc welding
pH
acid/base level
PH
precipitation hardened
PM
preventive maintenance
P-No.
ASME base material designation system. Number based on like chemistry and weldability.
PQR
procedure qualification record
psi
pounds per square inch (pressure)
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EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
PT
liquid penetrant testing
PTAW
plasma-transferred arc welding
PWHT
post-weld heat treatment
PWR
pressurized water reactor
RBI
risk-based inspection
RC
Rockwell C-scale
RCM
reliability-centered maintenance
RCPB
reactor coolant pressure boundary
RG
USNRC Regulatory Guide
rms
root mean square
RO
repair option
rp
reverse polarity
RRAC
Repair & Replacement Applications Center
RRAP
Repair & Replacement Applications Program
RT
radiographic testing
SAW
submerged arc welding
SC
safety class
S/G
standards and guides
Si
silicon
SMAW
shielded metal arc welding
SOER
Significant Operating Experience Report
sp
straight polarity
SP
standard practices
SRP
USNRC Standard Review Plan
SS
stainless steel
SSCs
structures, systems, and components G-5
EPRI Licensed Material Valve Application, Maintenance, and Repair Guide: In Situ State-of-the-Art Valve Welding Repair
Stellite
trade name for cobalt-based hardfacing alloys
TDC
tilting disc check valves
TI
USNRC Temporary Instruction
TIG
tungsten inert gas welding
UFSAR
Updated Final Safety Analysis Report
UL
Underwriters Laboratory, Incorporated
USAS
United States of American Standards Institute
USNRC
United States Nuclear Regulatory Commission
UT
ultrasonic testing
WPS
welding procedure specification
G-6
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Program:
TR-105852-V2
Nuclear Power
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