q&aAPrimeronSISUnderstandingProcessSafetySystemsDesignbyMikeBoudreaux22July2009FlowControlQ:Howdidtheconceptofsafetyinstrumentedsystemscometobe?HowhasSISdesignstrategyevolvedsinceitsinceptiontowhereitstandsnow?A:Industryincidents,suchasthosethathaveoccurredinFlixborough,England,Seveso,Italy,Bhopal,India,andPasadena,Texas,aswellasothers,haveledtoanincreasedinterestinprocesssafety.Muchofthefocushasbeentoreduceprocessriskthroughinherentlysafedesignandindependentlay-ersofprotection(IPL).Safetyinstrumentedsystemsareoneofthemanylayersofprotectionthatareusedtodeliverincreasedprocesssafety.ModernsafetyinstrumentedsystemsarebasedonfunctionalsafetydesignconceptsthatareprovidedbyIEC61508andIEC61511.Overthepast25years,SISdesignconceptshavemirroredprocesscontrolsystemdevelopments.Controlsystemshaveevolvedfrompneumaticsandhardwiredpanelboardstocentral-izedDCSstodigitalplantarchitectures.Similarly,SISshavepro-gressedfromrelaysandswitchestoPLCswithredundantarchi-tecturestologicsolverswithadvanceddiagnosticscapabilities.SISdesignhasevolvedfromusingrulesofthumbandprescrip-tiverequirementstodesigningsafetyloopsbasedonthefunction-alsafetyrequirementsoftheprocess.Q:Fromageneralprocesssafetyperspective,whyaresafetyinstrumentedsystemsimportant?WhatcapabilitiesdoSISsgenerallyoffertheend-userforprocesssafetyimprovement?A:Whenaprocesscannotpracticallybedesignedtobeinherentlysafe,anSIScanbeusedtoreduceriskstoanacceptablelevel.AnSIScanbedesignedtodeliveraspecifiedsafetyintegritylevel(SIL)ofriskreduction.IEC61508definesSIL1throughSIL4,witheachSILdesignatingarelativelevelofriskreductionprovidedbyasafetyinstrumentedfunction(SIF)byanadditionalorderofmagnitude.Q:WhatroledostandardsplayintheworldofSIS?Whatshouldend-usersknowaboutstandardsrelatedtoSIS?A:ThemodernconceptforSISintheprocessindustriesisbasedonIEC61508andIEC61511.IEC61508isagenericfunctionalsafetystandardthatcanbeappliedacrossallindustries.IEC61511isafunctionalsafetystandardthatappliesspecificallytotheprocessindustrysector.ISA(www.isa.org)hasadoptedIEC61511asANSI/ISA84.00.01-2004(ISA84),withtheadditionofagrandfatherclause.OtherindustrysectorshavestandardsbasedonIEC61508,suchasIEC62061formachinerysafetyandIEC61513forthenuclearpowerindustry.IntheUnitedStates,OSHA(www.osha.gov)hasstatedthatISA84isrecognizedandgenerallyacceptedasgoodengineeringprac-ticeforSIS.ThismeansthatifaprocessmanufacturerusesISA84asabasisforSISdesign,thismanufacturerwillbeconsideredincompliancewithOSHAPSMrequirementsforSIS.IEC61511hassimilarrecognitionasabestpracticeundertheSEVESOIIDirectiveintheEuropeanUnion.Someothercountrieshavesimi-larregulationsthatrecognizeIEC61511.Q:Whataresomeofthecommonpitfallsend-usersneedtobeawareofwhendevisingtheirSISdesignandimplementationstrategies?A:Duringtheanalysisandimplementationphasesofthesafetylifecycle,therearetwomajoractivitiesthatcanhaveasignificanteffectontheperformanceoftheSIS.Whendevelopingasafetyrequirementsspecification(SRS),processmanufacturerssome-timesgooverboardandmaketheSRStoocomplextobepracti-cal,ortheygointheoppositedirectionanddon’tprovideacon-sistentsetofdocumentationwherethesafetyrequirementsareclearlyspecified.Clause10ofIEC61511containsanitemizedlistofinformationthatshouldbeincludedinaSRS,butatthemostbasiclevel,theSRSshouldprovideafunctionaldescriptionandtheintegrityrequirementsforeachSIF.TheSRSisthedocumentagainstwh...
优质VIP
