Crossing the Retirement Finish Line With High Temperature Steam Turbine Rotors - Energy-Tech...
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Crossing the retirement finish line with high temperature steam turbine rotors By Stephen R. Reid, PE, and Rachel Sweigart, TG Advisers Inc. | Posted: Wednesday, May 27, 2015 5:23 pm Background New environmental regulations and the installation of renewable energy sources have resulted in many retirements of older legacy steam turbine generating units. Most of these older units have spent the majority of their life in operation “base loaded.” As a result, they have logged significant operating hours with few on/off cycles. Longterm, temperature dependent damage mechanisms of these turbine rotors must be considered to ensure successful operation to the desired retirement dates. This article highlights two case studies and some operational strategies to extend life to the desired retirement date. Consider high temperature, life limiting mechanisms HP and IP rotors with material properties similar to ASTM A470 typically operate with steam inlet temperatures of 1,000°F. With time, creep cracks can initiate in highly stressed areas. In addition, the material can suffer significant embrittlement with time, reducing the material ductility that is an important property for remaining life analysis. These mechanisms are at play in the rotor bore and blade attachment sections. Creep Creep is a time, temperature and stress dependent mechanism that must be considered for units with a significant number of service hours. For creep cracking, rotor operating temperatures in general must be above 800°F and a significant stress level must be present at all times to be concerning. Creep life targets of 30 years are frequently quoted by OEMs for high temperature rotor designs. Design life is typically determined from a “Larson Miller” curve, which relates time, temperature and stress for crack initiation (see Figure 1). The LMP approach does not account for crack propagation and/or the interaction of creep and fatigue from on and off cycling. The industry has utilized a fracture mechanics approach to assess life once cracks have initiated, and/or to assess preexisting cracks in high temperature zones of steam temperature rotors. Figure 1. Larson Miller Parameter Curve
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Transcript of Crossing the Retirement Finish Line With High Temperature Steam Turbine Rotors - Energy-Tech...
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6/19/2015 CrossingtheretirementfinishlinewithhightemperaturesteamturbinerotorsEnergyTechMagazine:TurbineTech
http://www.energytech.com/columns/turbine_tech/article_f07e4bfa04be11e5bcdfbba68ac16c1a.html?mode=print 1/4
CrossingtheretirementfinishlinewithhightemperaturesteamturbinerotorsByStephenR.Reid,PE,andRachelSweigart,TGAdvisersInc.|Posted:Wednesday,May27,20155:23pm
Background
Newenvironmentalregulationsandtheinstallationofrenewableenergysourceshaveresultedinmanyretirementsofolderlegacysteamturbinegeneratingunits.Mostoftheseolderunitshavespentthemajorityoftheirlifeinoperationbaseloaded.Asaresult,theyhaveloggedsignificantoperatinghourswithfewon/offcycles.Longterm,temperaturedependentdamagemechanismsoftheseturbinerotorsmustbeconsideredtoensuresuccessfuloperationtothedesiredretirementdates.Thisarticlehighlightstwocasestudiesandsomeoperationalstrategiestoextendlifetothedesiredretirementdate.
Considerhightemperature,lifelimitingmechanisms
HPandIProtorswithmaterialpropertiessimilartoASTMA470typicallyoperatewithsteaminlettemperaturesof1,000F.Withtime,creepcrackscaninitiateinhighlystressedareas.Inaddition,thematerialcansuffersignificantembrittlementwithtime,reducingthematerialductilitythatisanimportantpropertyforremaininglifeanalysis.Thesemechanismsareatplayintherotorboreandbladeattachmentsections.
Creep
Creepisatime,temperatureandstressdependentmechanismthatmustbeconsideredforunitswithasignificantnumberofservicehours.Forcreepcracking,rotoroperatingtemperaturesingeneralmustbeabove800Fandasignificantstresslevelmustbepresentatalltimestobeconcerning.Creeplifetargetsof30yearsarefrequentlyquotedbyOEMsforhightemperaturerotordesigns.DesignlifeistypicallydeterminedfromaLarsonMillercurve,whichrelatestime,temperatureandstressforcrackinitiation(seeFigure1).TheLMPapproachdoesnotaccountforcrackpropagationand/ortheinteractionofcreepandfatiguefromonandoffcycling.Theindustryhasutilizedafracturemechanicsapproachtoassesslifeoncecrackshaveinitiated,and/ortoassesspreexistingcracksinhightemperaturezonesofsteamtemperaturerotors.
Figure1.LarsonMillerParameterCurve
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6/19/2015 CrossingtheretirementfinishlinewithhightemperaturesteamturbinerotorsEnergyTechMagazine:TurbineTech
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AsnotedinFigure1,therecanbeasignificantscatterofcreepmaterialdata.TheOEMswilladdadditionalsafetymarginsanddefineaDesignCurve,whichisalwayslowerthanthecurveproducedfromtheminimumtestpoints.Asaresult,unitsusuallyfarexceedtheir30yearcreepdesigntarget.Rotorlifeisoftenlimitedbypreexistingflawsthataredevelopedfromtheforgingprocess.Thisisespeciallytrueofrotorsproducedpriortoforgings,whichweremanufacturedusingvacuumdegasingtechnology(i.e.airmeltprocess).
Embrittlement
Dr.SwamiSwaminathanofTurboMetInternational,whileundercontractbyElectricPowerResearchInstitute(EPRIproject24815)performedtestingonaretiredrotortosupportneededmaterialsdataforlifeassessmentsofserviceexposedrotors.Testingprovidedmanyconcerningchangesinmaterialpropertieswithtimeandtemperature.Amainareaofconcernwasthelargedegreeofrotorembrittlementduringtherelativelyshortoperatingperiodoflessthan200Khours.ThedegreeofembrittlementwasquantifiedbyashiftFractureAppearanceTransitionTemperature(FATT).TheFATTdefinesthetemperatureatwhichtherotorsteelexhibits50percentbrittleandductilebehavior.Theresultingcriticalcracksizesthatdefinetheburstconditionforarotoraredirectlyrelatedtothematerialsductility.Thisparameterisimportantsinceithelpstodefinetherequiredrotorsoakperiodtoensurethematerialisductilewhenitisexposedtohighstressesduringcoldstarts.
Swaminathancompletedtestingindifferentsteadystateoperatingtemperaturezonesattheboreandperipherallocations.TheworkprovedlongtermexposuretotemperaturecouldincreasetheFATTbyapproximately200F.Anotherinterestingfactwasthatembrittlementdidnotoccurinthehighestoperatingtemperaturezone.Embrittlementwasmorepronouncedinthezonewheretemperaturewasinthe800Frange.
Asaresultoftheseefforts,preexistingflawsinrotorshaverequiredamoreconservativematerialpropertyboundaryconditiontoassessrisksofbrittlefracture.Inmanycases,embrittlementhaslimitedthenumberofstopsandstartsand/orhasresultedinprematureretirementofagingrotors.
Notablecasestudies
IProtorembrittlement
AsaresultofarecentfailureofanIPstage2dovetail,TGAdvisershadanopportunitytoevaluatetheembrittlementconditiononarotorthatloggedmorethan425Kofoperatinghours.TheresultingshiftinFATTwassignificantlygreaterthanexpected.TheFATTforthestageofconcernhadincreasedtomorethan600Fwhichwasapproximately300Fhigherthananonembrittledrotorsteelofthesamechemistry.
Withretirementplannedinthenearfuture,theutilitydecidedtoremovethestageandevaluate
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6/19/2015 CrossingtheretirementfinishlinewithhightemperaturesteamturbinerotorsEnergyTechMagazine:TurbineTech
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methodstoensurereliableoperationuntiltheunitsretirementdate.Duringtheremainingoperatingwindow,theunitwasexpectedtohavejustafewcoldstarts,whicharethemostconcerningforanembrittledrotor.AdecisionwasmadebytheevaluatingteamtoassessmodificationsoftheexistingrotorcoldstartproceduretoaccountfortheFATTshift.Multiplestartupsimulationswererunvaryingspeed,holdtimesandrampratestoensurehighstressesoccurredonlyaftertherotorachievedaductilestate.Theevaluationalsoincludedanassessmentofdifferentialexpansion,whichwassignificantlychangedwithlongersoakperiods.
Theresultsoftheanalysisprovedtheembrittledrotorscouldachieveductilepropertiesbyextendingsoakperiodsatlowerspeedswithoutdetrimentallyaffectingdifferentialexpansion.Theplotbelowshowsapotentialimprovementof400percentincriticalcracksizeforamodifiedcoldstart.Adaptingtherevisedstartupprocedureshasallowedthisutilitytostayontrackwithitsretirementplan.Followupanalysisoftheactualrevisedstartuppracticehasshownaclosematchtotherecommendedcurves.
Controlstagefailure
AnothernotablefailurewasexperiencedonaninletstageofanHPturbinethathadaccumulatedmorethan300Khoursofoperation(seeFigure2).
Thefailureprovidedanopportunityfortheutilitytotestlargersectionsofthecontrolstagedisctodetermineifaweldrepairwasfeasible.Thetestprogramincluded:RotorBoresonicTests(MT,UT,visual)MaterialTests(actualrotormaterial)TensileTests(detectcreepsoftening)ImpactTests(estimatefracturetoughness)AcceleratedCreepRuptureTest
Testingprovedthefailuretobeattributedtohightemperaturecreepnotchsensitivityinthestressconcentratedareaoftherotordovetails.Followinganassessmentoftherotormaterialpropertiesatvariousradiallocations,acutlinewasestablishedandtherotorwasbuiltup,stressrelieved,remachinedandnewbladesinstalled.Therepairwassuccessfulandtheunitachieveditsdesiredretirementdateofapproximately10yearsaftertherepair.
Acknowledgements
TGAdviserswouldliketothanktheConsumersEnergyteamofMatthewT.Helms,VladimirTrbulinandMarkE.WittbrodtfortheirtechnicalsupportanddirectionwiththeHPandIProtorassessments.TGAalsowouldliketothankDaveSheasley,RogerKarnandMarkMillerofNRGforthetechnicalsupportwiththeHProtorcontrolstageassessmentefforts.
StephenR.Reid,P.E.,ispresidentofTGAdvisersInc.andhasmorethan30yearsofturbineand
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6/19/2015 CrossingtheretirementfinishlinewithhightemperaturesteamturbinerotorsEnergyTechMagazine:TurbineTech
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rotatingmachineryexperience.Reidandhisteamprovideturbinetroubleshooting,healthassessmentsandexpertwitnessservicestomajorenergycompaniesintheU.S.andhaveprovidedconditionassessmentevaluationsonmorethan100turbinegeneratorsintheU.S.ReidalsoisashortcourseinstructorforEPRI,ASME,ElectricPowerandPOWERGEN,hasnumerouspatentdisclosuresandawards,andpublishedmorethan20technicalpapersandarticles.Reidwastherecipientofthe1993ASMEGeorgeWestinghouseSilverMedalAwardforhiscontributionstothepowerindustryandispastchairmanoftheASMEPowerGenerationOperationsCommittee.HeisaregisteredprofessionalengineerinthestateofDelaware.Youmaycontacthimbyemailing
RachelSweigartjoinedTGAdvisersin2014asaconsultingengineer.Shehasprovidedlifeassessment,analyticalmodelingandtroubleshootingservicesformainturbinegeneratorslocatedthroughoutthecountry.SweigartisamechanicalengineeringgraduatefromLafayetteCollege.Youmaycontactherbyemailing [email protected]
