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)’
C 2012 by ASME Copyright V
DECEMBER 2012, Vol Vol.. 134 / 064 064502 502-1 -1
qualifi qual ifica cati tion on fo forr us usin ing g th thee pr proc oced edur uree in th thee re repo port rt of th thee 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, shee t, tube tubess in expa expanded nded joints must have continuous continuous,, inti intimate mate,, 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 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 established by Code Committee Committee members members with much experience with tube joints. With few changes and additions, tio ns, the lis listed ted effi efficie cienci ncies es hav havee bee been n suc succes cessfu sfully lly use used d for decades. Tube Expans Expansion ion Proce Procedure duress and Perso Personnel nnel Quali Qualificati fications. ons. The ASM The ASME E Cod Codee doe doess not hav havee req requir uireme ements nts to cer certif tify y tu tube be 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 ord order er for the ASM ASME E Co Code de cer certifi tified ed man manufa ufactu cturer rers’ s’ hea heatt 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 [5]). The PVD requires heat exchanger manufacturers to have certifications of expanding procedures and qualificati ca tion onss of wo work rker erss wh who o us usee th thee pr proc oced edur ures es.. Th Thee PV PVD D requirementss parallel their requirement requirement requirementss 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. tion s. The text and accompanyi accompanying ng forms paral parallel lel 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 expa expandin nding g proc procedur eduree spec specificat ifications ions (TEPS (TEPS); ); parag paragraph raph HH-5 has requirements for tube expanding procedure qualifications; paragraph HH-6 has requirements for tube expanding perform fo rman ance ce qu qual alifi ifica cati tion on;; pa para ragr grap aph h HH HH-7 -7 su subd bdiv ivid ides es tu tube be expanding variables to be described in the procedures into essential and none nonessen ssential tial variables, variables, para paralleli lleling ng the syst system em 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 prud prudent ent for spec specifyin ifying g engi engineer neerss to requi require re manu manufactu facturers rers to have and qualify procedures for tube expanding using the forms suggested in Section VIII VIII Division 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 Vol.. 134, DECEMBER 2012 064502 064 502-2 -2 / Vol
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 tinu ous intimate intimate con contact tact between the tube tubess 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 desig control holes. holes. The measuring tools for making these measnated control nated uremen ure ments ts mu must st be of rec recent ent cal calibr ibrati ation. on. The ho holes les and tub tubes es 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 sett settings ings for roll roller er expa expandin nding. g. After trial expa expansio nsions ns achieve appropriate settings for the desired percent wall reduction, the manufacturer should verify by measurement the percent wall reduct red uction ion every 50 exp expans ansion ionss for tu tube be end counts counts of 50 500 0 or greater. When the tube end count is less than 500, the manufacturer may adjust the intervals accordingly. Examining Tube Expans Examining Expansion ion in Mocku Mockups. ps. The purposes 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 VIII Division Division 1 and 4.C.2 of Section VIII Section VIII Division 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 weld ed and expanded, expanded, a leak path through the expa expanded nded tube length and a discontinuity in the weld. Wheree the Code requires shear load testing, Wher testing, manu manufactu facturers rers should test an appropriate number of tube joints. Be aware that push-ou pus h-outt shea shearr load tube test testing ing weld welded ed and expa expanded nded 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 pape paperr “Pres “Pressure sure Testing Feedwater Feedwater Heate Heaters rs and Power Plant Auxiliary Heat Exchangers” pointed out that the purpose of hydrost hyd rostatic atic testing pressure vessels vessels is to stres stresss the structure 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 demonst dem onstrates rates by math mathemat ematical ical anal analysis ysis that grad graduati uations ons on the test gages customarily used to measure hydrostatic test pressure
Transactions Tr ansactions of the ASME
Fig. 1 Ult Fig. Ultras rasoni onic c tes testin ting g fee feedw dwat ater er hea heater ter tub tubesh esheet eet aft after er weldwire cladding and machining
Fig. 2 Liqu Fig. Liquid id penetrant penetrant examining examining a feed feedwat water er heater tubesheet cladding after machinin machining g
Fig. 3 Gas leak bubble bubble testing feedwate feedwaterr heater tube-to-t tube-to-tubeubesheet joints
Fig. 4 Typi Typical cal layout layout of mockup mockup tubesheet specimen specimen for for feedwater heater with two tube thicknesses
and declines in hydrostatic test pressure are too coarse to indicate minute min ute leaks. This is espe especial cially ly of conc concern ern when the tube tubeside side pressure is higher than that of the shellside. Consequ Con sequentl ently, y, purc purchase hasers rs shou should ld spec specify ify and man manufact ufacturers urers should use other nondestructive means to assure tube joint tightness. These are ultrasonic testing tubesheets after weld metal cladding din g and mach machinin ining g (Fig (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 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 man manufact ufacturer urer perfo performed rmed the weld welding ing usin using g a qual qualified ified WPS for autogenous gas tungsten arc welding (GTAW). Expanding
Journal of Pressure Vessel Technology
Fig. 5 Photogra Photograph ph of specimen A 0.035 wall tubes 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 Vol.. 134 / 064 064502 502-3 -3
Fig. 6 Photogra Photograph ph of specimen B 0.035 wall tubes tubes
Fig. 8 Photogra Photograph ph of specimen B 0.049 wall tubes 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 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. 5/8-in. OD 0.049-in. (16-mm 1.24 1.24-mm -mm)) tube tubess for a 5-7/8-in. (150-mm) thick mockup tubesheet specimen.
Examination of Specimens Using the Digital Microscope
Fig. 7 Photogra Photograph ph of specimen A 0.049 wall tubes 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 hydroexpan ex pandin ding g was performe performed d in the inner rows wit with h tu tube be IDs in 0.520-in. ( 13.2-mm) range using a HydroPro, Inc. mandrel p/n 7130-74 713 0-74023 023-130 -1300 0 with 41,8 41,800–4 00–42,10 2,100 0 psi ( 288,200–290,269 kPa) expanding pressure. The remaining rows with tube IDs in 0.548-in. ( 13. 13.9-mm 9-mm)) rang rangee were expa expanded nded using a Hydr HydroPro oPro,, 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 intended to prod produce uce a final total percent wall reduction reduction of 6–8%. 6–8 %. Airet Airetool ool manufacture manufactured d the #1214 gun and tool for 6-in 6-in.. (152 152-mm) -mm) reach with 2-in 2-in.. (52-m 52-mm) m) roll dept depth. h. The tool is fitted fitt ed wit with h a thr thrus ustt col collar lar.. The roll gag gagee set setup up for 0. 0.049 049-in -in.. (1.24-mm) minimum wall tubes was Airetool number 2330 with torqu to rquee set settin ting g 2–6 2–6.. For 0. 0.035 035-in -in.. ( 0.89 0.89-mm -mm)) mini minimum mum wall Vol.. 134, DECEMBER 2012 064502 064 502-4 -4 / Vol
The author examined the specimens shown in Figs. 5 – 8 using the digital microscope shown in Fig. 9 9.. He marked the specimens as Specimens A and B, 0.035 with the tubes numbered 6–10, and specimen spec imenss A and B 0.04 0.049 9 with the tubes numbered 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 10 illustrates 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 Figure 11 illustrates 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 edge ed gess an and d th thee pe pene netr trat atio ion n an and d bo bott ttom omin ing g ou outt of th thee me meta tall deformed defo rmed into the groo grooves. ves. Figure 12 shows the contact of the tube tu be OD and hole hole ID at th thee lan land. d. Figure Figure 12 shows shows the land between the grooves with the tube in intimate contact with the tube hole. Figure 13 Figure 13 shows shows the second groove with tube metal bottoming out in the groove. The examination of the expanded length of tub tubes es bey beyon ond d the grooves grooves was at 2-i 2-in. n. ( 51-mm intervals). Figure 14 Figure 14 shows shows intimate contact of the tube OD with the hole ID at 2 in. Figure 15 Figure 15 indicates indicates that there were no discontinuities over thee ent th entire ire ex expan panded ded len length gth.. Com Compl plete ete exa exami minat nation ion 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 examined.. All but two expanded lengths showed intimate hole/tube contact. Figure 16 16 shows one microphotograph where there are discontinuities. The conditions shown in Fig. 16 16 prevailed through thee ex th expan panded ded len length gth wh which ich led to its rej reject ection ion.. The aut author hor’s ’s
Transactions Tr ansactions of the ASME
Table 1 Tu Table Tubesheet besheet Mockup Specimen First Stage Hydroexpanding/S Hydroexpanding/Second econd Stage Roller Expanding: 0.035 Minimum Wall Tubes 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 calculatio calculations ns 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 Tu Table Tubesheet besheet Mockup Specimen First Stage Hydroexpanding/S Hydroexpanding/Second econd Stage Roller Expanding: 0.049 Minimum Wall Tubes 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 calculatio calculations ns 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 digital microscope microscope used used to examine examine the specimens specimens shown in Figs. 10 Figs. 10– –16
Journal of Pressure Vessel Technology
Fig. 10 Tub Fig. ube-t e-too-tub tubesh esheet eet welds welds at 6L and 7R Lea Leak k pat paths hs 0.0364 in. and 0.0364 in. Unexpanded gaps behind weld roots 0.387 in. and 0.375 in.
DECEMBER 2012, Vol Vol.. 134 / 064 064502 502-5 -5
Fig. 11 7R and 6l Gro Fig. Groove ove 1. Discontinu Discontinuitie ities s are insi insignifi gnifican cant. t. Grooves are trapezoidal trapezoidal..
Fig. 12 7R and 6L Land between between grooves. grooves. No discontinui discontinuities. ties.
Fig. 13 6R an Fig. and d 7L Gro Groove ove 2. Ins Insign ignific ifican antt dis discon conti tinui nuitie ties. s. Grooves are trapezoidal 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 inti intimate mate continuous continuous cont contact. act. For thin thinner ner tube tubeshee sheets, ts, the author’s criterion is 100% intimate contact. Vol.. 134, DECEMBER 2012 064502 064 502-6 -6 / Vol
Fig. 14 Tu Tube be 6L and 7R at 2 in. No discontinuities discontinuities..
Fig. 15 Tub Fig. ube e 6L and 7R at 5 in. No dis discon contin tinuit uities ies over over the expanded length.
Fig. 16 Tube 7R and 8L 8L at 2 in. Tube Tube 8L is not in intima intimate te contact with the tubesheet and the expansion is unacceptab unacceptable. le.
Summary and Conclusions Assuri Ass uring ng the att attain ainmen mentt of sat satisf isfact actory ory lea leak k tig tight htnes nesss and strength of tube-to-tubesheet connections requires nondestructive test te stin ing g tu tube besh shee eets ts an and d tu tube be jo join intt we weld ldss du duri ring ng an and d af afte ter r
Transactions Tr ansactions of the ASME
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 us used ed if lea leakag kagee of the tubesid tubesidee str stream eam into the sh shell ell is not tolerable. Examinin Exam ining g secti sectioned oned and poli polished shed moc mockup kup tube tubeshee sheett spec speciimens me ns at ma magn gnific ificati ations ons of 24 24X X usi using ng th thee VAX di digit gital al mi micro cro-scop sc opee ca can n re reve veal al we weld ld qu qual alit ity, y, we weld ld si size ze co conf nfor ormi mity ty to specifica spec ification tion and Cod Codee requ requireme irements. nts. Illus Illustrati trations ons show showed ed how such su ch exa exami minat nation ion rev reveal ealss tub tube/h e/hol olee sur surfac facee con contac tactt and lac lack k there th ereof of and pen penetr etrati ation on of def deform ormed ed tu tube be me metal tal int into o ann annula ular r grooves. groo ves. The VAX micro microscop scopee allo allows ws much higher magn magnifica ifica-tions tio ns to exa exami mine ne the specimen specimen when the there re is sus suspic picion ion 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, 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 Standards for Close Closed d Feedwa Feedwater ter Heate Heaters rs,, 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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