SIL Determination

In the Functional Safety Lifecycle, the next step after hazard and risk assessment is “allocation of safety functions to protection layers”. This is a process whereby suitable control measures are specified for each of the potential hazardous events found during the H&RA Hazard & Risk Assessment (usually a HazOp) . This includes mechanical safeguards such as pressure relief valves, and also Safety Instrumented Systems (SIS).

The process of “allocation” is deciding which protective measures will be put in place, and how reliable they need to be. If a safety instrumented function (SIF) is to be used, the reliability required from it will be expressed as a safety integrity level, or SIL. There are four SILs – 1, 2, 3 and 4, which give increasing levels of reliability, as follows:

A key task during “allocation” is therefore to carry out “SIL determination”; in other words to decide what SIL is required from each SIF.

The most common technique used for SIL determination is layer of protection analysis (LOPA).  See “what is a LOPA?” for more information.

What =Method can do for you

• Provide experienced LOPA study facilitators
• Train and give mentoring support to aspiring LOPA study leaders