A unified model for evaluating the safety integrity level of safety instrumented systems

This paper proposes a new figure of merit (FOM) for evaluating safety integrity levels (SIL) for safety instrumented systems (SIS). Currently, SIL ratings are based on two separate tables - one for low process demands and one for high process demands. The proposed FOM, probability of an accident as a function of time, PAC(t), unifies the two separate tables into a single table and extends the concept of risk reduction factor (RRF), which is currently only defined for low demand applications, to high demand applications as well. Using PAC(t) as the new FOM explicitly includes the process demand rate in the model and therefore, permits the effects of different demand rates on the safety performance of a specific SIS to be quantified. The model also allows for the inclusion of diagnostic coverage and on-line repair so that the effects of these parameters can also be quantified. Finally, using PAC(t), the maximum time of periodic inspection (TI) permitted before the SIS moves to a lower SIL rating can be easily calculated. A number of examples illustrate the application and usefulness of PAC(t) as the defining FOM for SIL evaluation.