CEPSI2010_Parichart Remedial Action Step of Sub Synchronous Vibration _rev Oct 2010

8/8/2019 CEPSI2010_Parichart Remedial Action Step of Sub Synchronous Vibration _rev Oct 2010

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TableIBearingDescription

No. Type Diameter (Inches) Figure

Bearingno.1

DTP:DoubleTiltPadBearing

(PivottedShoeJournalBearing,ButtonType,Pin

Located,4Pads,1PadDown)

fromKingsburyInc.

12.018

Bearingno.2 16.024

Thiscombinedcyclepowerplant isconsistingof2GasTurbineand1SteamTurbineunits.TheSteamTurbine

unitwasfirstsynchronized in1995andhadbeenexecutedwith5minorinspection(MI)and1majoroverhaul(MO)

outages. TheSteam Turbine has been observed the symthom by condition monitoring which can be analyzed the

trend simplyand speedily.The significantconditionmonitoringdatachosenareOverallShaftVibration,Admission

SteamTurbineInletPressure,LPSteamTurbineInletTemperature,LPSteamExtractionTemperature,CondenserHighVacuumRangePressure,andLubeOilInletTemperature.TheOverallShaftVibrationisrecordedatbearinglocations

andthesetpointacceptedbyGEcriteriaofalarmandtripis150 mppand225mpprespectively.

OnMarch15th

,2008,thevibrationtrendpresentedasthefollowingfigurewasrecordedthetripeventwithout

alarmsignalobservedbyoperator.ThiswasshowntheMVARs(GENVARS)decreasedfrom+10to 10when itwas

broughtaGasTurbineunitdown.ThenSteamTurbinewas trippedby thevibration1X=230 mpp (2X=160 mpp)

within20minutesafter.Moreover,thevibrationatBearingno.1(VIB1X,1YBRG#1)wasnotconsistencyobviously.

TheSubsynchronousvibrationcouldnotbecapturedbyOverallVibrationmonitoring,so itwasneededtomeasure

thespectrumvibration.

Figure2VibrationtrendrecordofTripeventonMarch15th

,2008

Pivoted

shoe


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METHODSAfter the tripeventwith230mpp, thevibrationanalyzing teamhad torecord theVibrationSpectrumofminutes

before trip again. During testing, it was found that the Steam Turbine suddenly had vibrated up to the overall

magnitudeof1X=134 mpp ,1Y=139mppat60MW (52% load)whenMVARchangedandabnormaldifferential

expansionappearedasshowninthefigure3.Asshowninfigure4,theSubsynchronousof812Hzcapturedhadvalue

upto10 mppandthenatthemomentoftripevent itsvalue increased immediatelyupto75mpp.Practicallythe

normalSubsynchronoushadvaluednotover45 mppwhenaSteamTurbineisrunning.Therefore,thisproblemwasimpliedthatthepossiblecausescouldbefluidinducedinstability,theresultofBearingwear,Bearingoverclearance,abnormalofBearingAlignment.

Figure3VibrationtrendrecordofTripeventonMarch17

th

,2008

Figure4SubsynchronousvibrationrecordoftestingonMarch17th

,2008

Thehistoryrecordtoldusthatthevibrationhighproblemwasfoundsince2002.ItishighlyvibratedatBearingno.1

during60MW(52%Load)whileanotherGasTurbinestartedupandSteamwasadmitted;however, Itcouldnotbe

foundany severedamageonanybearings.Therefore,wekeptoncorrecting theproblemby lowspeedbalancing,

field balancing, and bearing replacement depended on which obvious evidences found during such outages.

Furthermore, the root cause analysis diagram in the below figure was the main tool to check the possible causes

duringproblemsolving.

Duringtheevent

Beforetheevent


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Figure4RootcauseanalysisdiagramofVibrationhighproblem

Intheinitialcountermeasure,therewere2options,6dayworkingonCompromisedBalancingand8dayforBearing

Inspection, in order to correct the problem. The 6day working including Generator preparation on Compromised

Balancingwasselected.TheSteamTurbineandVibrationteamworkedtogethertoexecutethethreerunbalancing

andanalyze thevibrationvectors, but the vibration stillhighat1x=200 mpp, instabilityofwhichhad resulted by

Subsynchronousasusual.

ThenwehadthemeetingagainandcomeupwithBearingInspectionfor8days.AftertheBearingno.1disassembly,

the wear of pivoted shoe was shown in the figure 5, so the Bearing had to be replaced. As the following steps,

Realignment (TurbineGenerator Coupling) had to be finished, as well as rubbing reducing by increasing oil seal

clearance.

Figure5ThepivotedshoeofBearingno.1Condition

WhentheunitwasstartedupagainonApril12nd

,2008,itwasfoundthesamesignalthatwehadhavenbefore.

However,theSteamTurbinewasabletosupplyhigherload,longerasitcouldrun.Inthemeanwhile,of90MW(78%

Load)loadandtheVibrationatBearingNo.2shown130 mpp,theSteamTurbineteamtriedtoincreaseviscosityof

inletlubeoilinorderthattheBearingmechanismwouldbeincreaseditsstiffness.Thetemperaturewasadjusted

from49oCto46

oC,buttheVibrationwasstillhigh.Thenthetemperaturewasbroughtbacktotheprevious.It

eventuallytrippedatVibration2X=300 mpp(1X=220 mpp)whileMVARwasraised.

Followingourexperiencestoresolvethisdifficulty,itcouldbesummarizedthatSubsynchronouscouldbeselfexcited

from every single source of rotating behaviour such as Load exciting, Load increasing, MVAR increasing, Steam

Admission,Startupmodeswitching,abnormaldifferentialexpansion,etc.TheSteampathwas likelytobeseriously

clogged,soithadnosimpleremedyforseveraldays.Ifwecontinuedthecorrection,wewouldneedmoreinputdata

SteamTurbineHighVibration

AABBNNOORRMMAALL

BBEEAARRIINNGG LLOOAADD


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from theOEM (OriginalEquipmentManufacturer)Engineering,accuratehistoryrecord,besidesour technical team

experiences.

WithGErecommendations, Bearingboreswereadjustedandtherotorpositionwasmovedtoproperposition,butthe

result was that the Steam Turbine was heavily vibrated at 5001,000 rpm. Consequently, the discussing meeting

betweenthemanufacturer,GEanduser,EGAThadtheadditionalissueaboutgenerator2poleshortturnproblem.GE

technicaladvisor(TA)recommendedthatthefirstprioritywasimprovingtheSteamTurbineconditiontobereadyto

testrunforthegeneratorsolvingissue.

Inthesummaryresultofthemeeting,itwasshownthatwecouldnotreducethevibrationwithoutSteamTurbineinternalpartcorrectionanymore.Hencethat45dayMajorOverhaulwasperformedtorehabilitatetheentireoftheSteamTurbinerotor. Infactofany

unforeseen evidences enhancing Subsynchronous vibration, all parts of rotor obstructed the Steam path, working

fluid,hadtobecloselytakencarebyGETAandEGATteam.

Afterupperhalfcasing removal, the formeractivitiesperformedweredividing flange level inspection, frontcasing

weightunbalancingtest(horndroptest),andBearinglevelcomparedwithdesignlevel.Theinspectionwasfoundthat

theLeftsideofdividingflangelevelwaslowerthantherightsideof3mm.,69%offrontcasingweightunbalancing,andBearingno.1 level is lowerthandesignof2.7mils.Notonlywastheevidencefound,thewater inductiontrace

alsoappeared.On June3rd

,2008, the front casingweightunbalancingwasadjusted tobewithin10%as shown in

figure6,causingTurbinecasingLifting.Sothefollowingstepwas InternalAlignment,alldiaphragmstagecentering,

andthelatteractivitiesarerubbingdecreasing,realignment,expandingtheanchorboltholes,Bearinginspectionand

setting. In spite of foundation deformation found, the corrective action could not be done because of time

constraints.Ithascontinuouslymonitoredtoprotecttheproblemintime.

Figure6HydraulicliftingtheFrontcasingintheprocessofHorndroptest


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Figure7WaterInductionDamageandRubbingRepairing

Figure8ExpandingtheanchorboltholesofBaseplate

Figure9BearingInspectionandSetting


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RESULTSExecuting most of remedial action steps, we started up the unit again on July 20

th, 2008. Notwithstanding

Subsynchronousappearingofthe100MW(87%load),theTurbinecouldbearonhigherSubsynchronousmagnitude

of80mppcomparedwith10mppoftheprevioustripevent.Infact,theSteamTurbineconditionwasbetter,butit

furtherneeded fine tunning in thestartupprocessbyCompromisedbalancing tohandleTurbinecarefully.Because

theBearingno.1 isverysensitive, itcanbestimulatednatural frequencyeasily.Thereforesuitableoilviscositywas

adjustedbyinletLubricationoiladjustingtohelpsteadilyrotatablebearings.TheresultwasthattheSubsynchronousmagnitudereducedgradually.Itwasfirstlydecreasedfrom49

oCto46.5

oC,thentheSubsynchronouswasreduced as

shown infigure10.Nextthetemperaturewasreducedto45oC,andtheSubsynchronouswasreduceddramatically.

Finally,theunitcouldbebroughtbacktodispatchingcontrolcentertoThailandnationalelectricgridonAugust1st

,

2008.

Figure10TheSubsynchronous reducingfrominletLubeoiltemperatureAdjustingDuring2010minorinspectionoutage,wehadtoanalysebythepreviousrootcauseanalysisdiagramalongwiththe

correctingactivitiesinoutage.Thelastpossiblecausewhichisabnormalbearingloadwasfoundbyanalysingtheshaft

positionplotofallbearingpositions.ItwascorrectedbyliftingtheBearingno.3supporttodistributetheloadfrom

Bearingno.2tootherbearingsandthentheSubsynchronouswasdisappearedcompletely.

AccordingtotheexperiencesinSteamTurbineproblemanalysisandcorrection,mostlyinternalturbinepartcouldbe

abletorepairinourEGATworkshop,soourteammustconsiderthemosteconomicaloptionwithtimeandmethod

tradeoffdependingonourworkshopresourcesand teamcapabilities. Even though thechecklistof therootcause

analysisfishbonediagramforthiscomplicatedproblemcouldbeapplied, inrealitywecounteredthebarriersduring

countermeasureinsite.Themostimportantfactormustbeconsideredisselectingtherightmethodintherighttime.

Forthisreason,theremedialactionstepofSubsynchronousvibrationtripcouldbesummarizedasfollowings.For Bently Nevada vibration monitoring system, this steam turbine is observed the rotating condition by Overall

vibrationwhichhadthe150 mppand225 mppasalarmandTripsetpointsrespectively.

IftheTripeventishappenedimmediatelyafteranymanuallyorautomaticallyprocessesoccur,measurethespectrum

vibration and observe whether Subsynchronous is shown. If it is appeared, then indicate the magnitude of

Subsynchronousvibration.TheprocessescanbeloadreducingbyshuttingdownoneofGasTurbine,loadincreasingbystartinguponeoftheGas

TurbineandSteamadmission,MVARsraising,loadexciting,startupmodeswitching,etc.

Incase,themagnitudeofSubsynchronousisnotover10 mpptobeartripevent.TheSteamTurbineconditionismost

severe.TheinternalpartsofSteamTurbinehavetobeinspectedandcorrected.Checkthehorndroptestandcorrect

ifneeded,centering,rubbingPartsreducing,Bearingsandpivotedshoesinspection,couplingalignmentetc.


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In case, the magnitude of Subsynchronous is about 80 mpp to bear trip event. It is meant that Steam Turbine

conditionisabnormal.Thereare3optionswithoutupperhalfcasingremovaltochoose.

1. Adjustinginletlubricationoilfor12dayduringrunningCaution: Bearing metal temperature at other location might be high because of viscosity change, so it is

neededcloselymonitoringduringadjustment.

2. Compromisedbalancingfor6day:This isalsodependingon theposition to insertrequiredweights. If it isinsertedatTurbineGeneratorcoupling,thebalancingisonlyneeded2daycorrection.

Caution:ThismethodwillsupporttheTurbinerunningatfull load,but itmightsuddenlyvibratedhighlyat

75%load.Hencetheoperatorhastocarefullycarrytheturbinepassingthroughthisconditionsmoothly.

3. Bearinginspectionfor8dayCaution:Thedisadvantageofthismethodisthatrealignmentduringhighturbinemetaltemperatureisquite

difficult.

Normally, if it does not have trip event, but the Subsynchronous signal sometimes appears during operation. The

Steam Turbine Condition is quite normal. Keep monitoring the data: Bearing Metal Temperature, Lube Oil Inlet

Temperature,Noiseat laststagebladeposition,BackPressure,StatorCoolingTemperature,MWandMVARChange

comparedwithSubsynchronoustrend.RecordSubsynchronous andcomparewiththepreviousandpreparetoinspect

andcorrectpossiblerubbingpartsand BearingsinspectionduringscheduledOverhaul.

CONCLUSIONSIn Conclusion, the Subsynchronous vibration problem analysis needed to consider the entire area of rotating

behaviour.TheEGATteamalsohadtomakedecisionundertimelimitandlesseffortrestriction.Moreover,themost

importantbeforefollowingtheActionStep,theSteamTurbineconditionandaccuratemaintenancerecordhadtobe

considered.TheobviousexampleisthatthefirstactionofCompromisedBalancingandLubeoiltemperatureadjusting

wasnotsucceededbecauseoftheseriousdamagefromwaterinduction.Therefore,itprimarilyneededtocorrectall

possiblerubbinginternalparts.

ACKNOWLEDGMENTAsintheprevioussection,IhavereliedoncommentsandinformationfromanumberofengineerswithwhomIhave

workedandcooperatedonanumberofprojectsoverthepastyears.Theircommentsandobservationshavebeenof

considerable assistance in identifying areas where more detailed information is required and in bringing to my

attention subjects that I may not havebeen, at that time, addressing in sufficientdetail,oreven atall. For these

reasons,theirassistanceandadvicehasbeeninvaluable.

As in this paper, my special thanks go to my colleagues Supaniti Songsawas, Manassavee Thuphom, and Cheewin

Youngngern for allowing me to interrupt his work. Their review and input from their experience have added

considerably toanyvalueotherplantengineersmayfindinthispaper.Theirconceptandthetimetakentoreadand

commentonthetextandthewayithasbeenarrangedhavebeeninvaluable.

Iwouldalso like toacknowledge the input from the followings.Eachhasmade informationavailable tomeorhas

been prepared to discuss certain aspects of units and our joint experiences. Santhiti Chanthauthai, head of

EngineeringSectioninSteamTurbineDepartment,SupawatKladthong,KobchaiWasuthalainan, ApichatPaugsopatai,

Sompong Pantmuk of Steam Turbine Department, Sahathep Joipradit of other departments in Mechanical

Maintenance Division, Pramote Kajornpol of South Bangkok Power Plant, EGAT, Athikom Jumrutsri, GE Technical

Advisor.

Tomakethetextclearer,Ihaveusedseveraloffiguresandtables.Someofthesewerepresentedatthepowerplant

site,conferences,andseminarsIattended.

Specialthanksgotomyofficestaff inSteamTurbineDepartment,fortheirassistance,especiallyWasuthepSinmayo

andPhipobChomboonchoy.Theirhelpinparticularwasinvaluable.


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REFERENCES[1] General Electric Company, Electrical Generating Authority of Thailand: South Bangkok Combined CyclePlant, South Bangkok, Thailand, File No. 62.1001 Steam Turbine Generator 103, Maintenance and SystemDescription,TurbineNo.270T238,GeneratorNo.290T238,Schenectady,NewYork:GE,1992[2] Grissom,B(2002),FundamentalsofRotatingMachineryDiagnostics,Minden,Nevada[3] H.P.Bloch,andM.P.Singh,SteamTubineSecondEdition:Design,applicationsand rerating, theUnitedStatesofAmerica,2009.[4] ElectricityGeneratingAuthorityofThailand,Rootcauseanalysisreport:SteamTurbineVibrationHighTrip,SBC10,Thailand,2008[5] Electricity Generating Authority of Thailand, Learning organization knowledge management report:RubbingCorrectionofSubsynchronousVibrationSteamTurbine,SBC10,Thailand,2008[6] GeneralElectric,SteamTurbineVibrationReport:ForcedOutageVibration,Thailand,2008

Miss Parichart Suttiprasit obtained a Bachelors degree of Mechanical Engineering fromChulalongkornUniversity,Bangkok,Thailand in1999.SincejoiningElectricityGeneratingAuthorityof Thailand (EGAT) as a mechanical engineer in steam turbine department, she had carried outseveral works regarding problem solving on steam turbine operation and maintenance includingauxiliaries inEGATpowerplants.Currently,sheworksasaplanningengineer inbusinessplanningdepartmentofgenerationdeputy.Mr.ChaiyantTibdeeobtainedaBachelorsdegreeofMechanicalEngineeringfromSongklanakarin University, Hatyai, Thailand. In 1999, he initially worked as mechanical engineer in boilerdepartment;hehadcarriedoutseveralworksregardingproblemsolvingonboileroperationandmaintenance.Afterjoining the testingengineeringdepartment,heworksasamechanical testingengineerandhehasbeenexecutedmanyvibrationanalysistestingprojectsinEGATpowerplants.Mr.NaritWongsaobtainedahighvocationalcertificateinmechanicalareaandhasworkedassteamturbinesitemanagerinsteamturbinedepartment.SincejoiningEGATin1980,hehasexecutedalotof steam turbine outages in EGAT power plants as technical skill, foreman, and site manager.Regardingtohisworkexperiences,heexecutedtheissuesonsteamturbineandauxiliariessuchasproblemsolving,functiontesting,sitemanaging,etc.andhealsosuppliedthedetailsofthistopicinhisresponsibleareas.

CEPSI2010_Parichart Remedial Action Step of Sub Synchronous Vibration _rev Oct 2010

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