SIL selection is the process of assiging a SIL (Safety Integrity Level) to a safety function in order to close the gap between residual risk (evaluated in HAZOP) and tolerable risk (as per Client Risk Matrix).
SIL (or its inverse – Risk Reduction Factor RRF) is defined in IEC 61508/61511 for low demand operation, as follows:
|SIL||PFD (avg)||PFD (power)||RRF|
|1||0.1–0.01||10−1 – 10−2||10–100|
|2||0.01–0.001||10−2 – 10−3||100–1000|
|3||0.001–0.0001||10−3 – 10−4||1000–10,000|
|4||0.0001–0.00001||10−4 – 10−5||10,000–100,000|