There were many cases of girdling  loop  failures in the reheater section of the 500 MW boilers of athermal power station after  being  in  service relatively for a short period of a few months. Within seven months of synchron-ization, the  bottom  'U'portion  of  the front reheater

girdling loop  of  the  boiler  of  500 MW thermal power 

plant had developed  a  leak. The  plant  was  restarted

alter replacing the failed protion of the type 304 stain-less steel 'U' tube. Failure occurred again within seven

months of operation. Similar  failures  followed  in the other units of the station

Gnanamoorthy, J B

Jayakumar, T

Raj, Baldev

English

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NitIi,.nnl Wv,rk .ch,,l, , it B„il, ,r Cnrr,,.cinn . 11-1211? Alrril . i 9Q5. ;V,%fJ. Jarrtshcdprrr. INDIAFAILURE ANALYSIS OF A 304 SS GIRDLING LOOP OFTHE BOILER IN A 500 MW THERMAL POWER PLANTJ.B. GNANAMOORTHY T. JAYAKUMAR & BALDEV RAJMetallurgy and Materials GroupIndira Gandhi Centre for Atomic ResearchKalpakkarn - 603 102IntroductionThere were many cases ofgirdling loop failures in the rehcatersection of the 500 MW boilers of athermal power station after being inservice relatively for a short periodof a few months. Within sevenmonths of synchronization, the hot-tonn 'Uportion of the front rehcatergirdling loop of the boiler of 500MW thermal power plant had devel-oped a leak. The plant was restartedalter replacing the failed protion ofthe type 304 stainless steel 'U' tube.Failure occurred again within sevenmonths of operation. Similar failuresfollowed in the other units of thestation. The composition of the tubematerial was 0.059% C. 18.5%7.r. Cr.8.7 "Z e , 1.8% Mn. 0.49% Si. 0.01 %S and 0.029`7 P. Four tube samplesS 1 , S2. S3 and S4 (see fig. 1) cutfrom the failed girdling loop weresent to us for investigation of thefailure. A new reference bend (Fig.2) of the same materials was alsoreceived. The followinginvestigations were carried out.Visual and SteromicroscopicExaminationsVisual examination of thefailed portionshowed a star likeopeining at thebend of the U tube (fig. 1). Thisopening was of smaller size at theinner wall of the tube compared tothat on the outer wall. Both visualand stereomicroscopic examinationsrevealed flow patterns on the tubewall at the opening. suggesting thatthis large opening was formed as aresult of leakage of high pressuresteam from inside of' the tube. Asteam wash dimple was seen on theinner surface of the tube on the sideopposite to the opening. Figures 3aand 3b are stereomicroscopicpictures showing characteristicfeatures of erosion at the edges ofthe opening. The inner surface ofthe tube had two long cracks ex-tending from the hole. stereomicro-scopic examination also showedcracks starting from inner surfaceof the tube and running across thewall thickness. The oxide scale onthe inner surface of the tube hadcracked at many places. Bright.colourless salt crystals were alsoseen embedded in the oxide scale atthe inner surae of the tube.Scanning Electron Microscopic(SEM) ExaminationA small sample cut from theof the girdling loop cracked portion of the tube wasK-1f B GNANAMOORTHY, T. JAYAKUMAR & BALDEV RAJopened out and examined in it SEM.The fracture was of' typicalintergranular morphology ( fig. 4a)and ma11V areas O1 the fracture Sur-face were covered by corrosionproducts (fig. 4h) suggesting it cor-rosion cracking mechanism for thefailure. The SEM examination ofthe inner surface. of the tube showedsevere cracking of the oxide scale(fig. 5a) near the cracked portion ofthe tube. lnterlranular corrosionattack was also noticed I fig. 5h) insome regions where the oxide scalewas damaged. Bright salt crystalsembedded in the oxide scale werealso seen in this region.X-Ray P/lotoelc utroli Spectroeopv(XPS)The oxide scale on the innersurface of the tube was analysedusing XPS in order to understand thenature of the salt deposits on thetube surface. Apart From Cr. Ni. Feand C (whose spectra are not givenhere). elements such as Na, (TI andCa were detected indicating thepresence of Na and Ca salts possiblyin the form of their chlorides on theinner surface of the tube. Figures hand 7 `give the XPS spectra showingpeaks of" Na and Cl respectively.Et'(1/11(itiun of the Microstrlr("Illcc ofthe Tube MaterialSince intergranular crackingOf austenitic stainless steels ist)e:nerally associated with the extentoI- sensitization in the steel.evaluation was carried out using theASTM standard A 262 severelysensitize(] whereas the new tube witsonly slightly sensitir.ed at the innersurface of the tube.Metallographic examinationof the failed tube was carried outafter Cleo: troclie nlical aching in 1(1MKOH. No sigma phase was detectedby optical microscopic examinationof the etched sample. This is in linewith the available information thatprecipitation oi' sigma phase is notexpected in type 30-1 SS during therelatively short period ol' exposurein the temperature range of 520-570"C as experienced by the failedtube.Resiclnul Stress Alva.suremewsVisual examination of the tubesegments cut I"ronl the failed loop atthe t'-bend region Showed ovalityand thickness variations which areexpected during cold hendink-1.Residual stress measurements werecarried out using XRI) method. Theequipment used w as it portable X-ray stress arlalyser ( Rigaku StrainllexMSF). Each point of measurementfable - IX-Ray DilTraction ConditionsMethod ParallelBeam MethodChar=acteristic X-rayDi Iiracti'on planeDiffraction angleFilterTube voltageTube CurrentIrradiated areax-ray fixed timeK-2Cr K beta(211)156.5V3() kVlUn1A2x4mill0 Sec.J. B. GNANAMOORTHY. 1'. JAYAKUMAR & BALDEV RAJwas ci cc ho po Ii shed beforerilcastiremef t to avoid any surfaceaitefacts. T he conditions oI* X-ravdiffraction are given in table 1. Theangles psi used were 0. 10.20 and 30degrees.All the peaks were correctedfor background and I'or absorptionhe fore peak location . Peak locationwas done by fitting a parabola at thelop 201/ of' the corrected peak andtop of the parabola was taken as peakIop.The measurements werecarried out Of the outer surface ofthe tube segment S4 (Fig. 1) of the.failed tube and also on the outersurface of the new tube. Residualstress on the inner surface wasmeasured only in the failed tubesection near the steam wash dimple(point 5 in fig. I ). The measuredvalues of residual stress are presentedin figs. I & 2. At one location one thefailed tube segment S4. residualtensile stresses of I (4 MPa(Circumferential) and 7 MPa (axial )Were Measured. Similarly a residualtonsil stress o1' 133 MPa(circumlcrcntial)and acompressivestress of, 180 MPa (axial) weremeasured at another point. At allother locations on the Outer surfaceonly compressive stresses werepresent both in the failed tubesegment and in the new tube.Measurements were not carried outon the inner surface as this requiresthe sectioning of the tubes whichwill change the extent and pattern oI'residual stresses in the tube. Theonly measurement of residual stressdone on the inner stirlace of thefailed tube section has given it conl-pressive stress value of 17(1 (axial)near the steam wash dimple. Di.1'-cil.'.s-IollThe Failure of the V!-bendstainless steel tube of the girdlingloop is not due to the ei»hrittleinentof the material produced hr.nlicrostructural changes during theoperation of the plant. as is evidentfrom the fact that the cracking whichwas initiated from the inner surfaceof the tube (lid not propagate forlonger distances or led to anycatastrophic failure. Instead. as itresult of erosion of" the tube wall byleaking, steam the crack wastransformed into it large holeshowing that the Material hadsufficient toughness to withstandhigh stress without fracture. Also.no emhrrtlllrlg sigma phase wasdetected in the failed material.Measurements of sensitisationin the failed as well as in the newtube have shown that the tubematerial has got severely sensitisedduring the operation of the plantbecause ofexposure to temperaturesin the rang e of 520 to 570"C. AI-thou,,h no intergranular stress cor-rosion Cracking IIC'SC'(') oI, sensi-tised stainless steel is expected inthe high temperature steam duringthe operation of the boiler. IGSCCis possible during the low tempera-ture phase when residual steam inthe tubes gets condensed on the tubewalls. This condensate wiil-dissolve-all the salts deposited on the tubewalls front thephase and will getcollected in the bottom parts of thetube where the residual stresses alsoK-3J. B. GNANAMOORTHY. T. JAYAKUMAR & BALDEV RAJwill he hit-flh near the kends. thuscrcatin_, favourable. conditionsfor IGS('("-_`. As the numberof shutdown/start-uh cycles increasesthe amount of salt deposited in thelower part of the tube also increases.Stcreunlicroscopie and SEMexaminations of the inner surface ofthe Ial let] tube have shown thepresence of salt deposits and thesehave heel) shown to he chlorides ofCa and Na by XPS analysis. Thus thebasic reason for the Failure of 31)4SS tube kends of the girdling loopsappears to he I(iSCC of sensitizedsteel in the presence of concentratedaqueous chloride solution during theperiod(s) of shut-down and start-upof the plant.The only yu cstit n that remainsis that v by no failure has occurred inother plants operating under similarconditions. It is reasonable toexpectthat variations in water chemistryand ()ther operating parallleters ofdiI' ferenl plants arc mainlyresponsible for this.ConclusionThe available evidence showsthat the failure of 3U4 11 girdlingtube is due to in1er2ranular stresscorrosion cracking of the sensitizedsteel in the presence of'chloride saltsdeposited on the tube surface. duringthe low temperature phase of'operation oI the repeaters.Remedial m easure.sintergranular stress corrosioncracking ol'stainless steels takes place ifthe Following conditions are met:I. Sensitized Ill icrostructtirePresence of impurities such aschlorides in the solution phaseTensile stress (applied or re-sidual)It is not possible to avoidsensitization in the present casesince the operating temperature of520-57(Y'C is well within the sensi-tisation temperature regime. flow-ever. sensitized steel is not prolleto IGSCC in the steam phase ofoperation: but only in the conden-sate phase. Therefore. el'forts needto he made to reduce the cold shut-down period to it nllilllllllm. It willalso he advisable to keep thereheater tubes dry during the shut-down so that no condensation ofwater takes place on the tube walls.Strict control of feed water chem-istry by keeping impurities such aschloride to the minimum possiblelevels will reduce.. the accumula-tion of chloride salts on the innersurfaces of the tube. thus reducingthe potential for IGSCC. Since noapplied stress seems to he presentduring the cold shut-down phase.residual stresses are likely to heresponsible for the cracking pro-cess. The residual stresses may bereduced by furnace cooling (vis-a-vis the presently adopted air cool-ing) following solution annealing:however. furnace cooling is likelyto sensitize the material exces-sively which can lead to IGSCCduring precommissioning. Thusthere seems to he not much scopefor the reduction of residualstresses.K-4J. B. GNANAMOORTHY_ T. JAYAKUMAR & BALDEV RAIIn the present case. detailedhistory of operation of the boilershas not been provided. Informationsuch as total period of shut-down.number of start-up/shut-down cyclesduring; the service of the Failed tubeand also it schedule of the start-upand shut-down operation areessential for a better understandingof the case of the Failure. It will alsohe helpful it (lie chemistry of theFeed water and the condensate duringthe service of the Failed tube areprovided.3-, .,--2(4)S1 S2References1. Modern power station practice.Chemistry and metallurgy.Volume E. British ElectricityInternational . Pergamon Press.p. 191 ( 1990).2. Metals Handbook, Ninth Edition.Volume 13, Corrosion. ASMInternational. p.992 ( 1987).S3FAILED TUBEO.D ', 55m.mTHK.: 5 m.mMat I.: S S-304 HRESIDUAL STRESS t111';ttLOCATION AXIAL CIRCUMFERENTIAL1 - 18(1 + 1332 + 7 + 1643 - 343 - 954 - 200 - 3195 - 17(1 -Notes : I. Points 1 & 3 are at diametrically opposite locatiosn similarly 2 & 4 are dia1101 11 111)opposite point.Points I to 4 are on the outer surface of the tube.3. Pint 5 is on the inner surface of the tailed section of the tube, near the steam-washdimpleFig. I : A schematic of the failed U-bend showing the pipe sections Si.S2, S3 and S4 and the locations where residual stress measurements werecarried out. The measured values of residual stress acre also tabulated.K-5.1 B. GNANAMOORTHY. 1 . JAYAKUMAR & BALDEV RAJ2 13 ! . i/f4 5 6NEW TUBENFTl:BEO1) : 55 ni.nrfHK : S minMall. : S.S. 104 HRFSIDF'AL S ' I'R[ : SS (MPa)LOCATION AXIAL . C'IR(AVM1:1-'R 1:NTIALI - 12O +170 - 153110 O.4- 14() - 765 - 154 - 140( - 1.114 - 1212 : ,l .,r Irr rucrtir u/ thc• rli'li. whitioll ctllllecllec / U-bend.cIiowint,thc, /0( rltic ,rl \ where /-(, .\ iclrrcrl Stress rllc u.^rrrc^nlc ut.^ w ere c crrric cl out.A lcrrsrrrre • cl + rlluc .^ of t/H' rc'.\ielrrcrl .ctre s.^ rrre cll.cu talrlrlcrtrcl.J. B. GNANAMOORTHY, T. JAYAKUMAR & BALDEV RA IFig. 3 : S tele ni('rl ).1Y opic I)i'tilre . C of the o/)eIi iiig III the foi ledtu lle it (Iif f Brent iii(flViif i(Wtu)II S showing e rosion (if t h e to be Iv//J. B. GNANAMOORTHY. T. JAYAKUMAR & BALDEV RAJFig 4 : SEAM pictures of the.fr(lctt(re s(!rj(rcc : ((t) showing purely intcrKrcmrrlarfructrrr(. (h) .^h(! ►► i11L (! r( oil covered /» c(rrr(rsi( ►Ji products and (r region ofi/1te1:L'JYrl11Jl(!r" cracking m4acent to It.K-KJ. B. GNANAMOORTHY, T. JAYAKUMAR & BALDEV RAJFig. 5 : SEM pictures of'the inner .su face of the tube near the failed region(a) showing cracking of the oxide scale. ( b) a region of'intergranular attackand a crack initiatin g . from there : bright salt crystals are also seen in the picture.K-9J. B. GNANAMOORTHY. T. JAYAKUMAR & 13ALDEV RAJX=S S- 'um C: ^Vy2\P!!3 ..:0?U^.^TS 15-04-1994All-, -YS== C.._ 20eV SE^ ceV SCANS 5 TIME Sm 40.009SOURCE A' K-alphaLAaE- _:eel specimen frcn -: a', '5-32 min. etching F!sons Sunfa=e SEien:cR79 - AV-3.5 -11C?0 _O 2 1074 1076 1075 1051Binding Energy (e'vI.\'I'S spec tra tcikeil on t/tc• inner 'iirface oftilt` tithe N /tow'iiI ' a peak mires/ )oncling to poi/iunlr%P5 Cr'_crlde C:\N 2 = C ==._TOE. q S SE C4 1994ANA_-'SE= CAE - 20e'r STEP - meV SCANS . 5 TINE - im 40.005A' K-alpeaSt sce= !,er - a` 5-30 1 :n. etching =: sons S ra[ S- -frc757 -550 ^ ^ -550010540PS 5300I I520,-5000 -150 15 154 156 155 190 2 194 195 195 200 202 204 205 205 210Binding Energy (eV)Fi,i,'. 7 : XPS spectra taken on the inner mrfac•e ofthe tithe s/ owiiig a /ii'iik c'orre.vponellng to chlorideK-1O^m7r7-"' "1" -11111111"71771 `77 1 1 1'$% 'T --"' I l

Failure Analysis of A 304 SS Girdling Loop of the 

Boiler in a 500 MW Thermal Power Plant

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Failure Analysis of A 304 SS Girdling Loop of the Boiler in a 500 MW Thermal Power Plant

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