TURBINE OVERSPEED TRIP PROTECTION

1 CharlesR.(Charlie)RutanisSeniorEngineeri ngAdvisor(EngineeringFellow),apositionth atisasharedresourcewithLyondell/Equistar ,inAlvin, ,hottapping/plugging, ,Conoco,DuPont,Cain, ,hottapping, (MechanicalEngineering,1973) ,ASME,AIChE,PumpingSystems,VibrationInst itute,HoustonBusinessRoundTable,TexasA&M sTurbomachineryandInternationalPumpUser sSymposia,SouthernPowerMachineryandGasCo mpressionConference, ,TexasTechAcademyofEngineers,HydraulicIn stitute/ANSIPumpStandardsReviewCommittee , ,history,currentstandardsanddesigns, failsafe ,turbinebucketsorasectionofawheelcanbrea kfreeoftherotorand,inaworstcasescenario, ,Chemical,andGasIndustryServices,APIS tandard612(1995),FourthEdition,June1995, : Rotorsshallbecapableofsafeoperationatmom entaryspeedsupto127%oftheratedoperatings peedatnormaloperatingtemperature.

2 : Overspeedtripdevicesshallbecheckedandadj usteduntilvalueswithin1% ,whensupplied,shallattainthreeconsecutiv enontrendingtripvaluesthatmeetthiscriter ion. General-PurposeSteamTurbinesforRefineryS ervice,APIS tandard611(1988,Reaffirmed1991),ThirdEdi tion, ,states: Rotorsshallbecapableofoperatingwithoutda mageatmomentaryspeedsupto110% (inrev-olutionsperminute) ( ),ParameterTripspeedClassperNEMASM23 Ais115%ofratedspeedandforSM23 Bis110%ofratedspeed. Atthemaximumof127percentofratedspeed, (2000) (ISO/DIS)10437(1993)forpetroleumandnatur algasindustries special-purposesteamturbinesisslightlydi fferentfromtheAPIS tandard612(1995) : , Additionally, : Theoverspeedsystemshallincludebutnotlimi tedtothefollowing:a)electronicoverspeedd etectionsystem(speedsensorsandlogicdevic es),API670;b)electro-hydraulicsolenoidva lves;c)emergencytripvalve(s)/combinedtri pthrottlevalve(s).

3 OVERSPEEDI fyouhaveacarwithatachometer, (revolutionsperminute)thattheengine,turb ine, , , ,LPAlvin,TexasELECTRONICTRIPSYSTEMAPI recommendstwospeedsensorswhere: A or B tripsignalisseen,thentheturbinetrips. A or B lossofsignalorpower,analarmisgivenbutthe turbineremainsrunning. A and B lossofsignalorpower, specialpurposesteamturbine. Thissystemisadequateifthelossoftheturbin eandtheequipmentdrivenbytheturbinedoesno tresultinasignificantupsetand/ , : Anycombinationoftwotripsignalswillresult inaturbinetrip, , A and B, B and C, or A and C. Thelossofpowersignalorpowerofanyoneofthe speedsensorswillresultinanalarm. Thelossofpowerorsignalofanytwoofthespeed sensorswillresultinaturbinetrip.

4 (Note:Thisisforade-energizedtotripconfig urationaspreferredbyAPI.) , , speedpickupwheel thatisbeingheldinpositionbyanonferrousmo untingbracket(Figures3and4).Inadditionto thespeedsensors,tworadialvibrationprobes ,twothrust(axialpositionprobes),thrustbe aringtempera-tureindicators, , ,aplunger,andaspringasshowninFigures5,6, 7,8, , stopperdisk andtheadjustablebushingwheretheplungerex trudesfromthebody(Figure5). ,typicallybolted, ,centrifugalforcepullstheplungertotheout side, , (SideView). (EndView).thecentrifugalforceincreasesfr omthespeed,rpm, ,setwitharelativelytightclearance,isposi -tionedsuchthatwhentheplungermovesout, ,thehydraulicoilisdumpedtothedrain, ,theturbinewouldruntodestruc-tionwhenthe load(compressor,pump,orgenerator) (refertoFigure10).

5 Iftheturbinevalveschangetheirpositionins tantaneously,thismeasure-mentoftimeisnow asthetimeconstant, ,andthenthemathematicalequationis:(1)whe re:n=Perunitchange,speede= ,secondsTC=Timeconstant,secondsFromEquat ion(1),whent=TC,thenn=121/eorn= /Then, spositionchangesarefoundbydifferentiatin gEquation(1) :(2)FromEquation(2),dn= , ,thedefi-nitionischangedto:therotortimec onstantgivesthelengthoftimeitwouldtaketh erotortoreachtwicethespeedduetoa100perce ntinstantaneousdropinturbineload, :(3)where:t=Time,secondsn=Speedchangeint ime t, percentTC=Rotortimeconstant,secondsL=Ins tantaneousloadchange,percentFromEquation (3) , , :(4)where:TC=Rotortimeconstant,secondsN= Ratedspeed,rpmWR2=Rotorinertia,lbs-ft2HP =RatedhorsepowerTheonlyfactorinEquation( 4) ,therotorinertia, , ,TC, (s),tripvalve(s),solenoidvalve(s),electr onicand/orhydrauliccontrolsorrelays, (s) ,priortothis, ,whatisthedifferencebetweentheturbinerot orspeedresponsetoaninstantaneouslossoflo adasopposedtoasuddenlossofload?

6 ,acoupleofcycles, (s) (s)betweentheturbineandthegeneratorand,i nsomeinstallations, , ,turbinesthatdrivecompres-sor(s),pumps,f ans,blowers,etc., Athrottlingofthesuction, Acouplingfailure, ;however,inanalysisofafewcouplingfailure s,thecompletefailurewouldtakeonesecondto twoseconds. , ,fromthediscussionabove, , , ,nozzlebox,valvechest, , (5) (5)isbasedontheassumptionthattheturbineh asanefficiencyof60percentatthedesignrate dloadandsteamconditionsasthestoredenergy isused.(5)PROCEEDINGSOFTHETHIRTY-SECONDT URBOMACHINERYSYMPOSIUM 2003112dndtTC=()() nLtTortnLTCC==()()TNWRHPC=22()()()()NNBT UWRNftt222=+ where:Nf=MaximumspeedNt=Tripspeed,rpmBTU =TotalstoredenergyWR2=Rotorinertia,lbs-f tTheamountofstoredenergyinthesteamchest, nozzlebox, ,Equation(5) ,thefeetofinletpiping(P)betweenthetripth rottlevalveandsteamchestrequiredtoreacha noverspeedcanbecalculated.

7 (6)where:P=Lengthofinletpipingbetweentri pthrottlevalveandturbinesteamchest,feetT C=Turbinerotortimeconstant,secondsV=Stea mvelocityininletpiping,ft/secWithasteami nletvelocityof150ft/secanda1/2secondtime constant, , ,however, , , ,theturbinehas15percentexcesspower, , : Determinetheactualcriticalspeed Defineminimumgovernorsetpoint(lowspeedst op) Determinemaximumgovernorsetpoint(highspe edstop) TesttheemergencytripsystemsIncludedinthe testingoftheemergencytestsystemsisthetes tingoftheoverspeedtripsetpoint(s).Aslong asthegovernorfunctionsproperly,theoperat ionofrunninguptotheoverspeedsetpoint(s) ,directlyorasanonpaidconsultant, , ,frequency,exceptions,maintenanceactivit ies,safetyrequirements, ,inadditiontothewrittenprocedure,agraphi caloutlineoftheprocedureforeachindividua lsteamturbinewasdevelopedandprovidedtoal ltheindividualsinvolvedintheoverspeedtri ptesting(Figure13).

8 , , ,unbalance,damagedbearings,andimproperly installedbearings,etc., :TURBINEOVERSPEEDTRIPPROTECTION113 PTVC= Signalgeneratorisusedtotesttheoverspeeds etpoints,ifanelectronicgovernorisusedand /oranindependentelectronicoverspeedprote ctionsystemisused. Oiltrips. Emergencytrips,mechanical,electronic,and manual,ontheturbineplatformandinthecontr olroom. ,theturbinerotorcanbebroughtuptoaslowrol lstate, , (highspeedstop) ,thetestingis,orishopedtobe, ,thentheonlywaytotestthesetpointiswithas ignalgenerator, ,adjustmentnut,plunger, , ,springforce,speed, ,theweightoftheplunger,andthedistancethe plungermusttraveltostrikethetripleverare fixed, ,orrepositioningthespringcompressionadju stmentnut, ,tolistafew: Tripplungerimproperlymachined,aphonograp hfinishontheboreoftheplungerguidebushing Theplungerandguidebushinghadabuildupofva rnish.

9 Theinstalledspringwastoostrong. Theinstalledspringwastooweakfromeitheran oldspringthathadlostsomeofitsforceoraspr ingwithtoolowaspringconstant. , ,3600rpmunitoverspedtoatleast5400rpmasar esultofthediskandbolt,fromanupstreamextr actionnonreturnvalve,becamelooseandmoved intoapositionthatresultedinthenonreturnv alvestickingopen(Figures14,15,and16). (Incident1).Incident2 Anincidentshowingaturbinedrivingagenerat orwithaninstantaneouslossofloadisshownin Figures17,18,19, (Incident2).Incident3On02/24/01, ,thehighestloggedspeedwas4988rpm, ,3600rpm,boilerfeedwaterpumpdriveturbine occurredon05/30 (A,Incident2). (B,Incident2). (C,Incident2). , , , , ,theoperatorreportedthereedtachometerrea dingwas4900rpmandheardthetripmechanismst artto clatter ; , ,theoverspeedtripmechanismmalfunctioned, ,about15yearsago,wasputtingasteamturbine drivenrefrigerationcompressoronline,andh ewasreadingthespeedwithareedtachometerse ttingontheauxiliaryoilpumpturbine, , (Clark,2002) ,whenpossible,the: Yearoftheincident Sizeoftheturbine Drivenobject Whythegovernorfailedtocontrolthespeed Whytheoverspeedtripfailed Otherfactors/informationThisisanexcellen tdatabase;however, ,theincidentmaynotbereported.

10 (RefertoAPPENDIXA forasampleincidenttable.)CONCLUSIONA safe,reliable, ,atspeed, , ,theentireprotectionsystem electrical,mechanical,andhydrauliccompon ents , ContributingFactors(05/02/97) #1through# >5000hpT/TValveStuckOccurredduringnormal operation; <5000hpT/TValveStuckOccurredduringnormaloperation; <5000 > ; ; , ; > ; ; > , > > < ; ; < , , ; < , < < > , ;shouldhaveclosedautomatically, < ; , , , < , , , ,turbinecentrifugallyexploded; > ,governorsensedfallingspeedandopenedgove rnorvalve100%Tripvalvehungopen; ,thegovernorvalvehadbeenstickingduetoste amdeposits, , > > ;stuckrelaywasnotdiagnosedinfirstinciden t, > , ContributingFactors(05/02/97) #21through# 2003118 INCIDENTNUMBERYEARSIZEDRIVENOBJECTWHYGOV ERNORFAILEDTOCONTROLSPEEDWHYOVERSPEEDTRI PFAILEDOTHERFACTORS/INFORMATION21< "flimsy" , >5000 , , > ; ; >5000hpGeneratorStuckopen; ; ; , > ; > ; < , , >5000hpGeneratorDuringstartup,firstvalvei nracksystem"popped" ,itiscommonforfirstrackvalveto"pop" > , , >5000hpPapermilllineshaftValvestuckopen; ,switchintripcircuitwaswired"normallyope n"insteadof"normallyclosed"perOEM ,whengovernorvalvestuckopen, > ;wasbeingsetatmanufacturer < , ContributingFactors(05/02/97) #35through# > ; <5000hpDuringuncoupledtest, , < (butterflytype).