The conformance of a Safety Instrumented Function (SIF) is based on Systematic Capability, Hardware Fault Tolerance and Probability of Failure on Demand (PFDavg). The process of calculating PFDavg is part of the SIL verification. To perform SIL verification the target SIL (PFDavg) is required and this is provided

from the risk analysis (LOPA) study. Layer of Protection Analysis (LOPA) will detail the risk or gap that is required to be closed to meet the risk target frequency. The risk gap (PFDavg) is calculated as Ft (Tolerable frequency) / Fe (Total event frequency) and the inverse of this will provide the required risk reduction also known as the Risk Reduction Factor (RRF).

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Example: Ft = 1E-05, Fe= 2.3E-04: Ft/Fe = 0.0434: PFDavg = 0.0434: RRF = 1/PFDavg = 23.

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Simplified equations can be used as a first pass in the calculation, however to fully analise and calculate PFDavg the following parameters must be considered for each component in the SIF

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• Service Life (also known as Mission Time)

• Proof Test Coverage

• MTTR

• Beta

• Diagnostic Coverage

• Proof Test Interval

• failure rate data (Source)