保护层分析-LayersOfProtAnalysis.ppt
Functional safety Engineering - Layers of Protection,Layers of Protection Analysis,Fault Propagation Context Event Tree Method Layers of Protection Definition LOPA Event Tree Initiating Evnents and Failure Rates Example of Layers of Protection,Fault Propagation modelling,Analyze the Chain of events that leads to an accident,Control System Fails,Operator does not respond properly,Mechanical relief failed,Overpressure Event,Initiating event,Layers of Protection Analysis - LOPA,A variant of Event Tree Analysis More structures Like event tree analysis, the initiating event start the chain of events Branches are layers of protection Consider only two outcome Accident No Event,LOPA Version of Event Tree,PL2, fails,PL1, fails,PL3, fails,Accident,Quantify using multiplications; all logical ANDs,Ex, Column Rupture LOPA,Draw the LOPA diagram for following situations Consequence of the accident = fire due to distillation column rupture has a root cause of loss of cooling water Layers of protection Operator respond to alarms and stops the process Process generally designed to withstand loss of cooling water The column has a pressure relief valve Source of ignition are controlled in the process area,Ex: Column rupture LOPA,LOPA Calculations,Proceed as with event tree, but only need to calculate the frequency of accident Resulting accident frequency is initiating event frequency multiplied by PFD of all protection layers Example Cooling water failure frequency is 0,5/year Protection layers Operator response failure PFD = 0,15 Relief valve failure PFD = 0,07 Process design inadequate PFD = 0,01 Ignition source contacted PFD = 0,3,Ex: Column Rupture, LOPA,F=0,5/y * 0,15 * 0,01* 0,07 * 0,3 = 1,58E-5 per year,Failure Rate Quantification,Historical reliability data specific to your installation is best, but often difficult to get Plant maintenance and SIS function test data by equipment type Industry average data grouped by equipment type Some expert judgment is still inevitable,Using Maintenance and Function Test Data,Companies usually keep maintenance logs IEC 61511 requires function testing and documentation of results Function test data used to approximate failure rate: Simple Equation for point estimate of failure rate: = # Failures / Total Unit of Operation = Failure rate,PFD from Failure Rate,PFD depends on failure rate, failure mode and test interval Failure rate is divided into failures that can cause a false trip versus those that failure on demand Most databases list the failure mode for an equipment item An untested devices PFD gets larger as the operational time interval increases For devices subject to periodic inspection and test the average PDF can be used,PDFavg ( t)/2,Typical Protection Layers,BPCS The BPCS and SIS are physically separated devices, including sensors, logic solvers and final elements Failure of the BPCS is not responsible for initiating the unwanted accident BPCS has the proper sensor and actuators available to perform a function similar to the one performed by the SIS,PDF 0,1 (By definition),Typical Protection Layers,Operator Response Operator always Present Operator has indication of Problem Operator has time to Act Operator is Trained in the Proper Response,PDF 0,1, if all condition is met PDF = 1,0, if condition not met,Typical Protection Layers,Use factor (Time at Risk) Hazard is not always present Ex Batch production,Typical Protection Layers,Mechanical Integrity of Vessel Is vessel designed to withstand the pressure and temperature generated as a result of the initiating event? Mechanical Relied Devices Relief Valves Rupture Disk Fusible Plugs PFD calculated based on failure rate statistics found in database,Typical Protection Layers,External Risk Reduction Water Spray Curtains Dual Walled piping Enclosures with Scrubing Only consider LOPA if protection system will COMPLETELY ELIMINATE the CONSEQUENCE,Typical Protection Layers,Ignition Probability Most plants are designed to limit sources of Ignition Function of release size and released material P 0,3 for flammable gases P 0,1 - 0,3 for volatile gases P 0,1 for heavy liquid,Typical Protection Layers,Explosion Probability Probability that explosion will occur given ignition has already occurred Not typically used because flash fire will occur if explosion does not, so consequences not prevented Use with CAUTION ! In most cases explosion probability should be ignored,Typical Protection Layers,Occupancy Fraction of time that effect zone of incident outcome in question is occupied Not typically used because occupancy is accounted for in consequence analysis,Layers of Protection Analysis,Fault Propagation Context Event Tree Method Layers of Prote tion Definition LOPA Event Tree Initiating Evnents and Failure Rates Example of Layers of Protection,
