Assuring Tube-to-Tubesheet Joint Tightness and Strength Stanley Yokell MGT Inc. F201 The Academy, 970 Aurora Avenue, Boulder, CO 80302-7299 e-mail:
[email protected]
This paper describes preparing mockup tubesheet specimens for visual examination using a digital microscope to determine that tube-totub e-to-tube tubeshee sheett join jointt weld weldss are of the specified specified size and that expanded joints are satisfactory for the intended purpose. It discusses nondestructive examinations (NDE) of the tubesheets and tube joints intended to assure achieving sufficient tightness and strength to satisfy the uses to which the exchangers will be put. This Th is pa paper per ref refers ers to th thee ASM ASME E Boi Boiler ler an and d Pre Pressu ssure re Ves Vessel sel Code Co de (Co (Code) de) pa parag ragrap raphs hs tha thatt ap apply ply to tub tubee joi joint nt wel welds ds an and d expan exp anded ded joi joint ntss inc includ luding ing she shear ar loa load d tes testin ting g wh when en the Co Code de requires it [1]. The discussion also addresses the need for manu facturers to have qualified tube joining procedures and personnel qualified to use the qualified procedures. The work concludes with a summary of ways to assure tube joint tightness and strength. [DOI: 10.1115/1.4006123] Keywords: ASME code, contr control ol hole hole,, contr control ol tube, digit digital al microscope, gas-bubble testing, hybrid expanding, hydroexpanding, mock-up, mock-u p, nond nondestruc estructive tive testin testing, g, perce percent nt wall reduc reduction, tion, rolle roller r expanding, tube joint, tube expansion, tube weld, ultrasonic testing, liquid penetrant testing
Introduction Tight, strong tube joints are essential for long life and satisfactory operation of shell-and-tube heat exchangers. To assure tightness and strength requires manufacturers to have and to follow procedures for tubesheet drilling, tube hole preparation, tube joint welding, and tube expanding. In this connection, Table RCB-7.21 and 7.21M and Paragraphs RB 7.24 and RC-7.24 of the TEMA Standards have requirements for tubesheet drilling and preparation with annular grooves; the HEI Standard for Power Plant Heat Exchangers Paragraph 5.72 has standards for drilling and annular grooves; the HEI Standards for Closed Feedwater Heaters Paragraph 3.8.3 and Table V have requirements for tubesheet drilling but are silent on requirements for annular grooves [ 2 – 4]. The vast majority of shell-and-tube heat exchangers in North America and many other locations are designed and constructed in accordance with the rules of Part UHX of Section VIII Section VIII Division Division 1 of the Code. For designs that Part UHX does not cover, Paragraph U-2(g) applies. The text of Paragraph U-2(g) is as follows. This Division of Section VIII VIII does does not contain rules to cover all details of design and construction. Where complete details are not given, it is intended that the Manufacturer, subject to the acceptance of the Inspector shall provide details of design and construction which will be as safe as those provided by the rules of this Division. Manufacturers often use finite element analysis to satisfy U-2(g). Depending upon the service conditions to which the exchanger will be exposed and its design conditions are sometimes advantageous ge ous to des design ign and con constr struct uct ex excha chang ngers ers to Sec Sectio tion n VIII Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNAL OF P RESSURE V ESSEL T ECHNOLOGY. Manuscript received June 27, 2011; final manuscript manuscript receiv received ed Octob October er 25, 2011; publ published ished online October 18, 2012. Assoc. Editor: William J. Koves.
Journal of Pressure Vessel Technology
Division 2. Paragraph 4.18 of Section VIII VIII,, Division 2 has rules for permissible materials, design, and construction of shell-andtube heat exchangers built to this division. Tube Joint Welds. Both Section VIII Section VIII Divisions Divisions 1 and 2 of the Code define full-strength define full-strength welds as those in which the weld design strength is equal to or greater than the axial tube strength. They partial-strength welds as those in which the design strength define partial-strength define is bas based ed on the mechanic mechanical al an and d the therma rmall axi axial al loa loads ds (in either either direction) that are determined in accordance with referred to paragraphs and appendices. They define seal welds as welds used to supplem sup plement ent expa expanded nded joints to ensu ensure re leak tightness tightness with weld sizes not determined based on axial tube loading. Both sections say of full-strength and partial-strength welds, “Such welds do not require qualification by shear load testing,” and “full-strength and partial-s part ial-stren trength gth weld weldss also prov provide ide addi addition tional al tube join jointt leak tightnes tigh tness.” s.” It is note notewort worthy hy that the desi design gn proc procedur edures es of both divisions are the same; however division 2 allows higher allowable stresses than does division 1. Individuals concerned with tubular exchangers should be aware that full-strength and partial-strength tube joint welds may meet all of either division’s requirements but not seal the tubes to the tubesheets if there is a gap in the weld. Similarly, gaps in seal welds prevent sealing.
Code Requirements for Welded Joints Tube joint weld requirements for tube joints of exchangers built to Section VIII Section VIII Division Division 1 are in Paragraph UW-20 and subparagraphs UW-20.1–20.7 of the current edition of the Code. Rules for tube joint welds of exchangers built to Division 2 are in Paragraph 4.18.10. To ensure leak tightness, the author’s criterion is that the thickness of the weld through the root shall be at least as great as the thickness of the tube wall. Welded joint tightness depends upon the welds being continuous, without cracks or gaps. Full-strength and partial-strength tube joint welds must meet the Code sizes. Because there is no simple way to determine whether the welds meet the Code requirements, it is prudent to validate the procedures by preparing specimens (tubeshe (tub esheet et moc mockups kups)) and exam examinin ining g them unde underr mag magnific nification ation.. For this purpose, the specimens must be sectioned and polished. The purpose of examining the welds is to determine that the weld sizes meet the Code requirements and are not flawed with cracks or porosity. Weld Procedu Procedures, res, Procedu Procedure, re, and Personn Personnel el Qualifi Qualifications. cations. It is the manufacturer’s responsibility to prepare and qualify welding procedure specifications (WPSs), maintain procedure qualification records (PQRs) for welded tube joints, and to qualify and maintain records of the workers’ qualifications in the use of the qualified qual ified procedures procedures (WPQ (WPQs). s). The Cod Code’s e’s Sect Section ion IX has suggested forms for these purposes. The tubesheet and tube materials used in preparing mockup specimens must very closely match the materials of production exchangers. The report of examination of the mockups should include the mill test reports for the mockup tubesheet and tubes. Specimen thicknesses of mockup tubesheets must be reasonably close to the thickness of the production tubesheet except in cases where tubesheets are very thick. Many specimens that the author has examined examined have been as thic thick k as 280280-mm mm (approximat (approximately ely 11-in.). Where tubesheets are clad with weld metal, weld metal on mockups must be applied using the production weld procedure, and liquid penetrant (LP) and UT examined in the same manner as that of the production tubesheet. Any labo laborator ratory y exam examinat inations ions shou should ld iden identify tify the spec specimen imenss with the manufacturer’s job number and cite the mill test report identification for the materials of the mockup along with the manufacturer’s procedure identification and the name of the welder. It is desirable to also include the PQR number and the welder(s)’
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qualification for using the procedure in the report of the examination.
Code Requirements for Expanded Joints When Code tubesheet thickness calculations take advantage of the stiffening effect of the length of tube expanded into the tubesheet, tubes in expanded joints must have continuous, intimate, hydraulically tight contact with the tube hole surface. When tube holes are prepared with annular grooves, tube metal must penetrate and make intimate contact with the bottom of the grooves. In the Code’s Division 1, nonmandatory Appendix A provides a basis for establishing allowable loads for tube joints. It is important to be aware of Paragraph A-1(b) in Appendix A which states, The rules in this appendix are not intended to apply to U-tube construction. In its Division 2, normative Annex 4.C of the current edition provides a basis for establishing allowable loads for tube joints of exchangers built to that division. Paragraph 4.C.1.2 in Annex 4.C has an identical paragraph as in Appendix A of Division 1. It states, The rules in this appendix are not intended to apply to U-tube construction. The word normative in the title of Annex 4.C indicates that it is expected that design and construction will follow the rules of the annex. Tube Joint Efficiencies. Tables A-1 of Section VIII Division 1 and 4.C.1 of Division 2 list tube joint efficiencies. These efficiencies are not based on any published experimental or analytical work but were established by Code Committee members with much experience with tube joints. With few changes and additions, the listed efficiencies have been successfully used for decades. Tube Expansion Procedures and Personnel Qualifications. The ASME Code does not have requirements to certify tube expanding procedures and to certify the qualifications of the personnel authorized to use the certified procedures. TEMA and HEI Standards are also silent about tube expanding procedures. In order for the ASME Code certified manufacturers’ heat exchangers to be acceptable for export to members of the European Community, in addition to meeting the Code requirements, they must meet the requirements of the European Pressure Vessel Directive (PVD [5]). The PVD requires heat exchanger manufacturers to have certifications of expanding procedures and qualifications of workers who use the procedures. The PVD requirements parallel their requirements for welding procedures and workers who use them. In the Code’s Division 1, nonmandatory Appendix HH establishes requirements for tube joint expanding procedure specifications. The text and accompanying forms parallel the text and forms for WPSs, PQRs, and WPQs of Section IX of the Code. Appendix HH has definitions for various types of tube expanding and the equipment used in doing it. Paragraph HH-4 has requirements for tube expanding procedure specifications (TEPS); paragraph HH-5 has requirements for tube expanding procedure qualifications; paragraph HH-6 has requirements for tube expanding performance qualification; paragraph HH-7 subdivides tube expanding variables to be described in the procedures into essential and nonessential variables, paralleling the system used for WPSs. Form QEXP-1 provides a form for manufacturers to record their TEPS. It is accompanied by Table QEXP-1 that has instructions for filling out the TEPS form. Division 2 does not have an appendix similar to that of Appendix HH. However for design and construction to either division, it is prudent for specifying engineers to require manufacturers to have and qualify procedures for tube expanding using the forms suggested in Section VIII Division 1 Appendix HH. Most reputable North American heat exchanger shops have such written procedures and workers qualified in their use. But except for shops that have met the PVD requirements, the procedures are not certified by an Authorized Inspection Agency. Similarly, except for
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shops that meet the PVD, there are no certifications of personnel in the use of the procedures. The strength and tightness of expanded joints, and the efficiencies listed in Tables A-1 and 4.C.2 assume that there will be continuous intimate contact between the tubes and holes and that where the holes are grooved with annular grooves, tube metal will substantially fill the grooves. Measurements and Settings. Procedures for production tube expanding should include measuring a representative number of tube holes and measuring a representative number of tubes to be expanded into the measured tube holes. These should be designated control holes. The measuring tools for making these measurements must be of recent calibration. The holes and tubes should be measured for the depth of expansion at 45 deg intervals around the circumference and at 25-mm ( 1-in.) intervals along the depth. Percent tube wall reduction measurements of expansions in the control holes are used to set hydrostatic expansion pressures (or if explosive expanding is used, explosive content) and torque settings for roller expanding. After trial expansions achieve appropriate settings for the desired percent wall reduction, the manufacturer should verify by measurement the percent wall reduction every 50 expansions for tube end counts of 500 or greater. When the tube end count is less than 500, the manufacturer may adjust the intervals accordingly. Examining Tube Expansion in Mockups. The purposes of examining tube expansions in mockups are (1) to determine that expansion begins at an appropriate distance from the root of the front face welds, (2) to see whether there is continuous interfacial contact, and (3) to make sure there is penetration of tube metal into the grooves.
Shear Load Testing When the manufacturer builds a heat exchanger using joint efficiencies listed in Tables A-1 of Section VIII Division 1 and 4.C.2 of Section VIII Division 2 that requires shear load testing, the fixture used for testing must conform to Figure A-3 for construction to Division 1 and 4.C.2 for construction to division 2. It is noteworthy that although the division 1 Appendix A and Division 2 Annex C do not apply to U-tube construction, it is a common practice for specifications for U-tube closed feedwater heaters to require shear load testing specimens for intermediate and high pressure heaters. The reason for including this requirement is the assumption that shear load tested joints that equal tube strength will meet the tightness requirements of the heater. This is a fallacious assumption because it is possible to have a tube joint as strong as the tube that has a discontinuity in the weld or if welded and expanded, a leak path through the expanded tube length and a discontinuity in the weld. Where the Code requires shear load testing, manufacturers should test an appropriate number of tube joints. Be aware that push-out shear load tube testing welded and expanded joints causes some loss of the interfacial pressure between the tube and hole surface because of the Poisson effect. Yokell illustrated this phenomenon in a paper on hybrid expanding that showed failures in the weld before the tubes yielded [ 6].
Tightness Testing Specimens The paper “Pressure Testing Feedwater Heaters and Power Plant Auxiliary Heat Exchangers” pointed out that the purpose of hydrostatic testing pressure vessels is to stress the structure to show that it is capable of resisting the loads due to pressure [ 7]. It states that, although the Code does not permit leaks during hydrostatic testing, such testing does not disclose minute leaks through tube joints when the back face of the tubesheet is not visible. It demonstrates by mathematical analysis that graduations on the test gages customarily used to measure hydrostatic test pressure
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Fig. 1 Ultrasonic testing feedwater heater tubesheet after weldwire cladding and machining
Fig. 2 Liquid penetrant examining a feedwater heater tubesheet cladding after machining
Fig. 3 Gas leak bubble testing feedwater heater tube-to-tubesheet joints
Fig. 4 Typical layout of mockup tubesheet specimen for feedwater heater with two tube thicknesses
and declines in hydrostatic test pressure are too coarse to indicate minute leaks. This is especially of concern when the tubeside pressure is higher than that of the shellside. Consequently, purchasers should specify and manufacturers should use other nondestructive means to assure tube joint tightness. These are ultrasonic testing tubesheets after weld metal cladding and machining (Fig. 1), LP examining the tubesheet after weld cladding and again after tube joining by welding (Fig. 2), followed by gas-bubble testing (Fig. 3), and, where tightness is of extreme importance, helium leak sniffer testing with the helium air mixture in the shell and the tube joints sniffed. Such nondestructive test must conform with the requirements of Section V of the Code. The workers administering the tests should be qualified to level 2 or level 3.
A Typical Preparation of a Mockup for Examination Under the Digital Microscope Figure 4 is a typical layout of a set of specimens cut from a mockup that a feedwater heater manufacturer prepared. The numbers indicate the tubes selected for examination under the digital microscope. In the specimen shown in Figs. 5 – 8, the tubes were first full-strength welded, then hybrid expanded after welding. The manufacturer performed the welding using a qualified WPS for autogenous gas tungsten arc welding (GTAW). Expanding
Journal of Pressure Vessel Technology
Fig. 5 Photograph of specimen A 0.035 wall tubes
began approximately 12-mm (approximately 1/2-in.) beyond the weld root. Because Appendix A of Division 1 and Annex 4.C of Division 2 of the Code do not apply to U-tube feedwater heaters, DECEMBER 2012, Vol. 134 / 064502-3
Fig. 6 Photograph of specimen B 0.035 wall tubes
Fig. 8 Photograph of specimen B 0.049 wall tubes
tubes the roll gage setup was Airetool #1214 with a torque setting of 2–4. After welding the tubes to the mockup tubesheet and subsequent expanding, the manufacturer filled the tubes with a plastic medium that neither shrunk nor expanded upon hardening. The manufacturer sawed the specimen on the axial centerlines of the tubes and polished the halves to close to a mirror finish. The reason for sawing along the tube axes was to minimize the possibility of loosening the tubes. Tables 1 and 2 tabulate the hole measurements and expansions of five tubes each of 5/8 in. OD 0.035-in. ( 16-mm 0.089) and 5/8-in. OD 0.049-in. (16-mm 1.24-mm) tubes for a 5-7/8-in. (150-mm) thick mockup tubesheet specimen.
Examination of Specimens Using the Digital Microscope
Fig. 7 Photograph of specimen A 0.049 wall tubes
there was no requirement for the manufacture to prepare specimens for shear load testing and the manufacturer did not prepare such specimens. The first stage of the hybrid expansion was by hydroexpanding intended to produce approximately 3% wall reduction. The hydroexpanding was performed in the inner rows with tube IDs in 0.520-in. ( 13.2-mm) range using a HydroPro, Inc. mandrel p/n 7130-74023-1300 with 41,800–42,100 psi ( 288,200–290,269 kPa) expanding pressure. The remaining rows with tube IDs in 0.548-in. ( 13.9-mm) range were expanded using a HydroPro, Inc. mandrel p/n 7130-74023-1375 at a pressure of 40,000–41,000 psi (275,790–282,685 kPa) expanding pressure. The HydroPro system used was a p/n 6100-10020-60702 unit with 0–60 ksi pressure capability and a transducer p/n 84754 that had recently been calibrated. The second stage of the hybrid expanding was by roller expanding intended to produce a final total percent wall reduction of 6–8%. Airetool manufactured the #1214 gun and tool for 6-in. (152-mm) reach with 2-in. (52-mm) roll depth. The tool is fitted with a thrust collar. The roll gage setup for 0.049-in. (1.24-mm) minimum wall tubes was Airetool number 2330 with torque setting 2–6. For 0.035-in. ( 0.89-mm) minimum wall
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The author examined the specimens shown in Figs. 5 – 8 using the digital microscope shown in Fig. 9. He marked the specimens as Specimens A and B, 0.035 with the tubes numbered 6–10, and specimens A and B 0.049 with the tubes numbered 1–5. Tube walls are identified on the microphotographs as L for left side and R for right side and with the tube number. Figure 10 illustrates the microphotograph of the welds taken at the ligament between tubes numbers 6 and 7 and shows the weld measurement and the measurement of the unexpanded gap behind the weld root. Upon complete examination of the welds of the specimens a small number had leak paths through the welds smaller than specified. Figure 11 illustrates the microscopic examination of the expansions in the region of the first annular groove. It shows the tube/ hole interference at the intersection of the tubes with the groove edges and the penetration and bottoming out of the metal deformed into the grooves. Figure 12 shows the contact of the tube OD and hole ID at the land. Figure 12 shows the land between the grooves with the tube in intimate contact with the tube hole. Figure 13 shows the second groove with tube metal bottoming out in the groove. The examination of the expanded length of tubes beyond the grooves was at 2-in. ( 51-mm intervals). Figure 14 shows intimate contact of the tube OD with the hole ID at 2 in. Figure 15 indicates that there were no discontinuities over the entire expanded length. Complete examination of all the expanded tubes in the specimens indicated that all tubes bottomed out in the grooves and corner discontinuities were insignificant in all grooves examined. All but two expanded lengths showed intimate hole/tube contact. Figure 16 shows one microphotograph where there are discontinuities. The conditions shown in Fig. 16 prevailed through the expanded length which led to its rejection. The author’s
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Table 1 Tubesheet Mockup Specimen First Stage Hydroexpanding/Second Stage Roller Expanding: 0.035 Minimum Wall Tubes Two Ring Grooves 1/4in wide 3 1/64in Deep, Trapezoidal Tubesheet thickness 5–7/8in
Job No. deleted Percent wall reduction 3% initial statge, 8% final. Expansion depth 5-3/4in Measurements and calculations prior to tube expansion for 0.035 MW tubes
Holes for calculations Tube row/hole No. per figure 1 Hole ID Tube OD Tube clearance (a and b) Tube ID Tube ID þ clearance 2 wall thickness (b–d) Wall reduction factor % Wall reduction/100 Wall reduction (Rw f) Calculated expanded ID
1
2
3
4
5
0.632 0.624 0.008 0.549 0.557 0.075 0.08/0.03
0.632 0.624 0.008 0.549 0.557 0.076 0.08/0.03
0.632 0.624 0.008 0.549 0.557 0.076 0.08/0.03
0.632 0.624 0.008 0.549 0.557 0.076 0.08/0.03
0.632 0.624 0.008 0.549 0.557 0.076 0.08/0.03
0.006/0.002 0.563/0.559
0.006/0.002 0.562/0.558
0.006/0.002 0.562/0.558
0.006/0.002 0.563/0.559
0.006/0.002 0.563/0.559
(a) (b) (c) (d) (e) (f) Rw (g) (h)
Table 2 Tubesheet Mockup Specimen First Stage Hydroexpanding/Second Stage Roller Expanding: 0.049 Minimum Wall Tubes Two Ring Grooves 1/4in wide 3 1/64in Deeep, Trapenzoidal Tubesheet thickness 5–7/8in
Job No. deleted Percent wall reduction 3% initial stage, 8% final. Expansion depth 5-3/4in Measurements and calculations prior to tube expansion for 0.035 MW tubes
Holes for calculations Tube row/hole No. per figure 1 Hole ID Tube OD Tube clearance (a–b) Tube ID Tube ID þ clearance 2 wall thickness (b–d) Wall reduction factor % Wall reduction/100 Wall reduction (Rw f) Calculated expanded ID
1
2
3
4
5
0.633 0.626 0.007 0.520 0.527 0.106 0.08/0.03
0.633 0.626 0.007 0.520 0.527 0.106 0.08/0.03
0.633 0.626 0.008 0.520 0.528 0.105 0.08/0.03
0.633 0.626 0.008 0.520 0.528 0.105 0.08/0.03
0.633 0.626 0.007 0.520 0.527 0.106 0.08/0.03
0.008/0.003 0.536/0.531
0.008/0.003 0.535/0.530
0.008/0.003 0.536/0.531
0.008/0.003 0.536/0.531
0.008/0.003 0.536/0.531
(a) (b) (c) (d) (e) (f) (Rw) (g) (h)
Fig. 9 VHX digital microscope used to examine the specimens shown in Figs. 10–16
Journal of Pressure Vessel Technology
Fig. 10 Tube-to-tubesheet welds at 6L and 7R Leak paths 0.0364 in. and 0.0364 in. Unexpanded gaps behind weld roots 0.387 in. and 0.375 in.
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Fig. 11 7R and 6l Groove 1. Discontinuities are insignificant. Grooves are trapezoidal.
Fig. 12 7R and 6L Land between grooves. No discontinuities.
Fig. 13 6R and 7L Groove 2. Insignificant discontinuities. Grooves are trapezoidal.
criterion for acceptance of discontinuities in contact between the tube and hole surfaces in tubesheets 50-mm (approximately 2-in.) or thicker is that a minimum of 90% of the expanded length shall be in intimate continuous contact. For thinner tubesheets, the author’s criterion is 100% intimate contact.
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Fig. 14 Tube 6L and 7R at 2 in. No discontinuities.
Fig. 15 Tube 6L and 7R at 5 in. No discontinuities over the expanded length.
Fig. 16 Tube 7R and 8L at 2 in. Tube 8L is not in intimate contact with the tubesheet and the expansion is unacceptable.
Summary and Conclusions Assuring the attainment of satisfactory leak tightness and strength of tube-to-tubesheet connections requires nondestructive testing tubesheets and tube joint welds during and after
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construction. Although the Code-required hydrostatic testing verifies that the exchanger strength is adequate, it does not disclose minute leaks through the tubesheet when the tubeside pressure is higher than that of the shellside. Other means of leak testing must be used if leakage of the tubeside stream into the shell is not tolerable. Examining sectioned and polished mockup tubesheet specimens at magnifications of 24X using the VAX digital microscope can reveal weld quality, weld size conformity to specification and Code requirements. Illustrations showed how such examination reveals tube/hole surface contact and lack thereof and penetration of deformed tube metal into annular grooves. The VAX microscope allows much higher magnifications to examine the specimen when there is suspicion of a potential indication. The combination of applying appropriate nondestructive testing methods and microscopic examination of sectioned mockup speci-
Journal of Pressure Vessel Technology
mens along with helium leak sniffer testing assures that tube-totubesheet connections will be tight and strong enough for the service of the exchanger.
References [1] The ASME Boiler and Pressure Vessel Code, The American Society of Mechanical Engineers, New York. The current edition of the Code is the 2011 edition. The Code is published at two-year intervals. [2] Standard for Power Plant Heat Exchangers, 4th ed., 2004, The Heat Exchange Institute, Cleveland, OH. [3] Standards of the Tubular Exchanger Manufacturers Association , 9th ed., 2007, The Tubular Exchanger Manufacturers Association, Tarrytown, NY. [4] Standards for Closed Feedwater Heaters, 8th ed., 2008, The Heat Exchange Institute, Cleveland, OH. [5] Directive 97/23/EC of the European Parliament. [6] Yokell, S., 2007, “Hybrid Expansion Revisited,” ASME J. Pressure Vessel Technol., 129, pp. 482–487. [7] Yokell, S., 2011, “Pressure Testing Feedwater Heaters and Power Plant Auxiliary Heat Exchangers,” ASME J. Pressure Vessel Technol., 133, 054502.
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