保护层分析-LayersOfProtAnaly.ppt
ABB-Page 1E x t e n d e d A u t o m a t i o n S y s t e m 8 0 0 x AFunctional safety Engineering-Layers of Protection ABB -Page 2FunctionalsafetyLayers of Protection AnalysisnFault Propagation ContextnEvent Tree MethodnLayers of Protection DefinitionnLOPA Event TreenInitiating Evnents and Failure RatesnExample of Layers of Protection ABB -Page 3FunctionalsafetyFault Propagation modellingnAnalyze the Chain of events that leads to an accident Control SystemFailsOperator does not respond properlyMechanical relief failedOverpressure EventInitiating event ABB -Page 4FunctionalsafetyLayers of Protection Analysis-LOPAnA variant of Event Tree AnalysisnMore structuresnLike event tree analysis,the initiating event start the chain of eventsnBranches are layers of protectionnConsider only two outcomenAccidentnNo Event ABB -Page 5FunctionalsafetyLOPA Version of Event TreePL2,failsPL1,failsPL3,failsAccidentQuantify using multiplications;all logical ANDs ABB -Page 6FunctionalsafetyEx,Column Rupture LOPAnDraw the LOPA diagram for following situationsnConsequence of the accident=fire due to distillation column rupture has a root cause of loss of cooling waternLayers of protectionnOperator respond to alarms and stops the processnProcess generally designed to withstand loss of cooling waternThe column has a pressure relief valvenSource of ignition are controlled in the process area ABB -Page 7FunctionalsafetyEx:Column rupture LOPA ABB -Page 8FunctionalsafetyLOPA CalculationsnProceed as with event tree,but only need to calculate the frequency of accidentnResulting accident frequency is initiating event frequency multiplied by PFD of all protection layersnExamplenCooling water failure frequency is 0,5/yearnProtection layersnOperator response failure PFD=0,15nRelief valve failure PFD =0,07nProcess design inadequate PFD=0,01nIgnition source contacted PFD=0,3 ABB -Page 9FunctionalsafetyEx:Column Rupture,LOPAF=0,5/y*0,15*0,01*0,07*0,3=1,58E-5 per year ABB -Page 10FunctionalsafetyFailure Rate QuantificationnHistorical reliability data specific to your installation is best,but often difficult to getnPlant maintenance and SIS function test data by equipment typenIndustry average data grouped by equipment typenSome expert judgment is still inevitable ABB -Page 11FunctionalsafetyUsing Maintenance and Function Test DatanCompanies usually keep maintenance logsnIEC 61511 requires function testing and documentation of resultsnFunction test data used to approximate failure rate:nSimple Equation for point estimate of failure rate:=#Failures/Total Unit of Operation=Failure rate ABB -Page 12FunctionalsafetyPFD from Failure RatenPFD depends on failure rate,failure mode and test intervalnFailure rate is divided into failures that can cause a false trip versus those that failure on demandnMost databases list the failure mode for an equipment itemnAn untested devices PFD gets larger as the operational time interval increasesnFor devices subject to periodic inspection and test the average PDF can be usedPDFavg (t)/2 ABB -Page 13FunctionalsafetyTypical Protection LayersnBPCSnThe BPCS and SIS are physically separated devices,including sensors,logic solvers and final elementsnFailure of the BPCS is not responsible for initiating the unwanted accidentnBPCS has the proper sensor and actuators available to perform a function similar to the one performed by the SIS PDF 0,1(By definition)ABB -Page 14FunctionalsafetyTypical Protection LayersnOperator ResponsenOperator always Present nOperator has indication of ProblemnOperator has time to ActnOperator is Trained in the Proper ResponsePDF 0,1,if all condition is metPDF=1,0,if condition not met ABB -Page 15FunctionalsafetyTypical Protection LayersnUse factor(Time at Risk)nHazard is not always presentnEx Batch production ABB -Page 16FunctionalsafetyTypical Protection LayersnMechanical Integrity of VesselnIs vessel designed to withstand the pressure and temperature generated as a result of the initiating event?nMechanical Relied DevicesnRelief ValvesnRupture DisknFusible PlugsnPFD calculated based on failure rate statistics found in database ABB -Page 17FunctionalsafetyTypical Protection LayersnExternal Risk ReductionnWater Spray CurtainsnDual Walled pipingnEnclosures with ScrubingnOnly consider LOPA if protection system will COMPLETELY ELIMINATE the CONSEQUENCE ABB -Page 18FunctionalsafetyTypical Protection LayersnIgnition ProbabilitynMost plants are designed to limit sources of IgnitionnFunction of release size and released materialnP 0,3 for flammable gasesnP 0,1-0,3 for volatile gasesnP 0,1 for heavy liquid ABB -Page 21FunctionalsafetyLayers of Protection AnalysisnFault Propagation ContextnEvent Tree MethodnLayers of Prote tion DefinitionnLOPA Event TreenInitiating Evnents and Failure RatesnExample of Layers of Protection ABB -Page 22Functionalsafety
