Average probability of failure on demand PFDavg

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... 

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). 

Introducing the average probability of failure on demand...PFDavg... 

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. 

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. 

Below some simplified formulas are given for calculating the average probability of failure to perform the design function on demand PFDavg for the entire safety-related system reahzing given safety function. 

SIL Average probability of dangerous failure of the safety function on demand (PFDavg)... 

In the present paper, only the low demand mode of operation is assumed (i.e. the safety function is only performed on demand, and not more than one per year). The target failure measure is then defined by the average probability of dangerous failure on demand of the safety function (PFDavg), and the associated SIL is determined by Table 1. Note that these definitions stem from lEC 61508, 2009 revision, which are more precise than in the first edition. 

Figure 5. Probability of dangerous failure on demand at time t, PFD t), and on average, PFDavg-...

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