Average probability of failure on demand

The targets for average probability of failure on demand or frequency of dangerous failures per hour apply to the safety instrumented function, not to individual components or subsystems. A component or subsystem (for example, sensor, logic solver, final element) cannot have a SIL assigned to it outside its use in a specific SIF. However, it can have an independent maximum SIL capability claim. 

The proof test interval should be selected to achieve the average probability of failure on demand as required in the safety requirements specification. 

Safety integrity level (SIL) Target average probability of failure on demand (PFDa, g) Target risk reduction Factor (RRF) Target frequency of dangerous failures to perform the SIF (per hour)... 

Results of the evaluation typically include a number of safety integrity and availability measurements. Most important, the average probability of failure on demand (PFDavg) and the safe failure fraction (SFF) is calculated for low demand mode. Probability of failure per hour is calculated for high demand mode. From charts, the SIL level that the... 

PFD average (PFDavg) is a term used to describe the average probability of failure on demand. PFD will vary as a function of the operating time interval of the equipment. It will not reach a steady state value if any periodic inspection, test, and repair is done. Therefore, the average value of PFD over a period of time can be a useful metric if it assumed that the potentially dangerous condition (also called hazard) is independent from equipment failures in the safety instrumented function. 

That information is needed along with other parameters to perform an accurate average probability of failure on demand calculation. 

Safety Integrity Level Average probability of failure on demand (Low demand mode of operation)... 

The probability of any one well failing on demand has been calculated. If five are present and any one can cause system failure then the approximate average probability of failure on demand can be obtained by adding the PFDavg of each well. 

Since many safety evaluations are done using average probability of failure on demand (PFDavg), the equation for PFDavg should be derived. The average approximation is given by ... 

The RPS is a sort of SIS (Safety Instrumented System) (Torrres et al., 2009). A SIS is defined as an instrumented system used to implement one or more safety instrumented control functions. A SIS is composed of any combination ofsensors, logic solver and final elements" (lEC 61511, 2003). The standard lEC 61508 requires every safety function to achieve a determined Safety Integrity Level (SIL). For low demand operating systems the SIL levels are defined in terms of average probability of failure on demand (PFDavg, see Table 2). 

Safety integrity level (SIL) Target average probability of failure on demand Target risk reduction... 

NOTE 4 The target failure measure may be a specified value of average probability of failure on demand or dangerous failure rate derived from a quantitative analysis or the specified range associated with the SIL if it has been determined by qualitative methods... 

Table V/4.2-1 Safety Integrity Level and Average Probability of Failure on Demand...

A qualitative and quantitative failure analysis of the hardware—failure mode and effects analysis with documentation. It is necessary to carry out a probabilistic reliability calculation to determine the average probability of failure on demand of the safety function. 

The following is one example of how the average probability of failure on demand of a safety function for a given system may be derived and is based upon Annex B in BS EN 61508-6. 

For the example given in Figure 37 (loot architecture) it can be shown that the average probability of failure on demand for a system with a very low failure rate is ... 

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