Determination of the Safety Integrity Level for a Safety Instrumented Function

A User's View and Experience

• Ian Hitchen

Keywords: Safety Integrity Level, Hazard and Operability Study, SIL Determination Methods, Layer of Protection Analysis, Risk Graphs, Consequence Matrix, ALARP

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Abstract:

The international standards IEC 61508 and IEC 61511 require that the operator of the plant identifies the hazards associated with operating that plant under normal and abnormal conditions. It also requires that any risk to people and/or the environment are estimated. If these risks are greater than the operator’s target values then methods of risk reduction need to be put into place until the target is reached or bettered.

If the non instrumented safety function risk reduction methods do not achieve the target then the remaining risk reduction is assigned to a Safety Instrumented Function (SIF). The amount of risk reduction required of the SIF is the Safety Integrity Level (SIL) required of that function throughout its life cycle.

This paper examines the stages required to meet the determination of the SIL of identified SIFs. The methodology is based on the Institute of Chemical Engineers HazOp process stage 3 and introduces a preliminary SIF + SIL identification stage. The paper then examines different methods for determining the SIL and the barriers to implementation. © 2009 Inside Publishing. All Rights Reserved.

Volume: 2009   Issue: 1

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