The key to achieving Functional Safety for a process is development of a robust process for identifying required levels (Safety Integrity Levels, SIL) of risk reductions per IEC 61508 / 61511.
After specific hazardous scenarios are identified and requirements for risk reduction are established, a SIL Determination method can be used to determine required SILs. Multiple evaluation methods are available, the suitability of which is dependent upon several factors:
Risk reduction required (the “gap” between estimated risk and target risk), and
Boundaries of the risk under consideration.