Evaluation of the required SIL level of Safety Instrumented Functions (SIF) – SIL level allocation

The aim of a RIS is to reduce a risk. To define the level of SIL to be allocated (SIL required) to a RIS, a rigorous approach is needed.

To determine the required SIL of a RIS, IEC 61511 introduces three methods which are therefore the most widely used in the process industry sector:

• risk graph ;

• criticality matrix ;

• LOPA.

In practice, these three methods have common steps which, depending on the method, may be explicit or implicit:

• estimation of the criticality of the "initial" risk (i.e. without taking into account the RIS for which we are trying to determine the required SIL);

• comparing the criticality of the risk with acceptability criteria, to determine the risk reduction factor (RRF) that the RIS must introduce to make the risk acceptable;

• estimation of the required SIL level based on the FRR.