Safety integrity level PFDavg

Safety Integrity Level Probability of Failure on Demand (PFDavg.) Low Demand Mode Risk Reduction Factor (RRF)... 

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). 

ANSI/ISA-84.00.01-2004-1, Clause 9.2.3, provides two tables for defining the SIL requirements. Table 3 provides the SIL requirements in terms of PFDavg- Table 4 provides the SIL requirements in terms of Frequency of Failure (e.g., failures per hour) and defines the acceptable hazard rate for the high-demand/continuous SIF. ANSI/ISA-84.00.01-2004-1, Clause 9.2.3, states that when Table 4 is used, neither the proof-test interval nor the demand rate is used in the determination of the safety integrity level. This means that the Table 4 requirements should not be converted into PFDavg requirements, using the proof-test interval or the demand rate. Erroneous results can easily occur if a high-demand mode SIF is treated as a low-demand mode SIF, followed by incorrect use of Tables 3 and 4 (ANSI/ISA-84.00.01-2004-1 Clause 9.2.3). 

PFDavg, probability of failure on demand SIF, safety instrument functions SIL, safety integrity level. 

Table 10.2 Safety Integrity Levels (ISA-84.01 andlEC61511) SIL RRF (Risk Reduction PFDavg (Probability Factor) of Failure on...

In order to get a scale of performance safety practitioners have adopted the concept of safety integrity levels or (SILs). The SILs are derived from earlier concepts of grading or classification of safety systems. The principle is illustrated in the next figure where the layer of protection provided by an SIS is seen to be quantified as a risk reduction factor from which it can be converted to a PFDavg and referenced to an SIL classification table. 

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... 

SIS and SIL for BMS A master fuel trip required by design codes demands multiple actions. The verification results shall confirm that the required risk reduction is achieved. However, the validation can be compromised when an SIF is not defined properly and its functional requirements are poorly specified or when all actions for total shutdown are included in the same functional requirements of the same SIF [8]. From discussions in previous chapters it is clear that the safety life cycle model not only helps with necessary ways and means to avoid systematic failures, but also helps to ensure the required integrity level to prevent random failures. The safety standards (lEC 61508/61511) required to identify a set of parameters and factors for PFDavg calculations are ... 

Essentially the SIL table provides a class of safety integrity to meet a range of PFDavg values. Hence the performance level of safety instrumentation needed to meet the SIL is divided into a small number of categories or grades. 

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