10 BasicReliabilityEng

©ABB-Page1ExtendedAutomationSystem800xAFunctionalsafetyEngineering-BasicReliabilityEngineering©ABB-Page2FunctionalsafetyBasicReliabilityEngineering„Failurerate„Reliability/Unrealiability„ReliabilityEngineringTerms„ConstanfFailureRate„PFavg©ABB-Page3FunctionalsafetyFailures„Randomfailure„Afailureoccuriongatarandomtime,whichresultfromoneormoredegradationmechanism„Canbeinfluencedbytemperature,humidity,vibrations„Systematicfailure„Afailurerelatedinadeterministicwaytoacertaincause,whichcanonlybeeliminatedby:„Amodificationofthedesignorofthemanufactoringprocess,„Operatorprocedures„Documentation„Otherrelevantfactors©ABB-Page4FunctionalsafetyFailurerate„Failurerate–numberoffailuresperoperatinghours„Failureratethatvariedwithtime„Constantfailurerate„Averagefailurerateoveralongperiod„Ex:„100solenoidsareplacedintooperation„Duringfirstyear–7fail„Whatistheavgfailurerateduringtheyear?„λ=7/(100units*8760hrs/year)=7,99E-6failure/year„Mostconservative:„λ=7/(93units*8760hrs/year)=8,6E-6failure/year©ABB-Page6FunctionalsafetyTerms„Failurerate–thenumberoffailuresperunitoftime„Failureratethatvarieswithtime„Constantfailurerate„Averagefailurerateoveralongperiodoftime„Probabilityofsuccess–thechancethatasystemwillperformitsintendedfunctionwhenoperatedwithinitsspecifiedlimits.©ABB-Page7FunctionalsafetyReliability/Safetyengineeringterms„Reliability„theprobabilityofsuccessduringanintervaloftime„Example„Theprobabilityofsuccessfuloperationfor1hour=0,999„Whatistheprobabilityofsuccessfuloperationin1day?„PS(24)=PS(1hour)*PS(1hour)….24„PS(24)=(1hour)24=(0,999)24=0,976©ABB-Page8FunctionalsafetyReliability/SafetyEngineeringTerms„FailureRate–Failureperunittimeperdevice„MeanTimeToFail(MTTF)–Theaveragesuccessfuloperatingtimeofasystem©ABB-Page10Functionalsafety©ABB-Page11FunctionalsafetyRepairableSystems„Whataboutrepairablesystems?„Themeasurement“reliability”requiresthatasystembesuccessfulforanintervaloftime.„Whatisneededforarepairablesystemisameasurethatgivesustheprobabilitythatitwillworksuccessfullyinthesituationwhererepaircanbedone©ABB-Page12FunctionalsafetyMTTR–MeanTimeToRestore„MeanTimeToFailure(MTTF)–„Theaveragesuccessfuloperatingtimeintervalofasystem„MeanTimetoRestore(MTTR)„Theaveragefailuretimeintervalofasystem.„Appliesonlytorepairablesystems„Averagetimetodetectthatafailurehasoccurred+averagetimetoactuallymaketherepair„RestoreRate(μ)-NumberofrestorespertimeperiodMTTR1=μμ1=MTTR©ABB-Page13FunctionalsafetyMeanTimetoRestore„Averagetimetodetectthatafailurehasoccurred+averagetimetoactuallymaketherepair„Ex:„Iffailuresonlyaredetectedbyperiodicinspectionandtest„TI=Testinterval,RT=RepairTime„MTTR=TI/2+RT©ABB-Page14FunctionalsafetyMeanTimeBetweenFailures(MTBF)„MeanTimeBetweenFailures„Theaveragetimeintervalofonefailure/restorecycleofasystem„Appliesonlytorepairablesystems„MTBF=MTTF+MTTR©ABB-Page15FunctionalsafetyAvailability/Unavailability„ProbabilityofSuccess–thechancethatasystemwillperformitsintendedfunctionwhenoperationalwithinitsspecifiedlimits„AVAILABILITY„Theprobabilityofsuccessatamomentoftime(allowsforpastfailures)„UNAVAILABILITY„Theprobabilityoffailureatamomentintime„A=1-U©ABB-Page16FunctionalsafetyAvailabilityvsreliability„Availability–theprobabilityofsuccessatamomentoftime„Reliability–theprobabilityofsuccessforanintervaloftime(nofailureandrepairduringinterval)0or1©ABB-Page17FunctionalsafetyReliability/Safetyengineeringterms„Reliability„theprobabilityofsuccessduringanintervaloftime„Example„Theprobabilityofsuccessfuloperationfor1hour=0,999„Whatistheprobabilityofsuccessfuloperationin1day?„PS(24)=PS(1hour)*PS(1hour)….24„PS(24)=(1hour)24=(0,999)24=0,976©ABB-Page18FunctionalsafetyAvailablevsreliabilty„Asystemorcomponentisreliableevenifithashadeasafeshutdown(PFS)orspurioustrip.Butasystemisonlyavailableifitisupandrunningandperformsthetasksitisintendedtodo.Therelationbetweenavailabilityandreliabilityisasfollow:„Reliabilityistheprobabilityofacomponent,orsystem,functioningcorrectlyoveragivenperiodoftimeunderagivensetofoperatingconditions.„Reliabilityexample:ASIFisoperatinginaPSDsystemonanoilinstallation.ThisSIFiscontrollingapressurereliefvalveonagasvessel.ItiscriticalthatthisSIFisabletorespondonanaction.Itisonlypossibletodomaintenanceonceayear.Thentheprooftestintervalis1year.ThereliabilityforthisSIFistheprobabilitythattheSIFwillreliefthepressureifahighpressureshouldoccur,orthattheSIFisinafailsafemode.„Theavailabilityofasystemistheprobabilitythatthesystemwillbefunctioningcorrectlyatanygiventime.„Availabilityexample:ThesameSIFasmentionedinthereliabilityexamplealsohaveanavailabilityfigure.TheavailabilityforthisSIFistheprobabilitythattheSIFisoperational.OperationalmeanthattheSIFcontinuouslyperformsitsintendedfunctionality,i.e.supervisingthepressureincludingreliefofthepressureuponahighpressuredemand.ThisisnotpossiblewhentheSIFisinthefailsafemode.„A:Availability,R:Reliability(0or1),PFS:ProbabilityofFailingSafely(spurioustrio);„A=R–PFS(0&1)©ABB-Page19Functionalsafety©ABB-Page20Functionalsafety©ABB-Page21FunctionalsafetyIEC61508Part6举了好几个系统结构例子。但描述得很粗略和含糊，加上其名字和体