Safety Integrity Level (SIL) Assessment
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Overview
The basic purpose of the safety instrumented level (SIL) classification is to determine whether a Safety Instrumented Function (SIF) was needed or not, and if the other independent protection layers were sufficient for controlling risk to As Low As Reasonably Practicable (ALARP), and then classify the SIL rating for each SIF.
The concept of SIL rating has been developed over time and is now an integral part of the engineering design in most process related industries. The accepted methods are laid out in various standards including:
- IEC 61508 Functional Safety of Electrical/Electronic/Programmable Electronic Safety-Related Systems; and
- IEC 61511 Functional Safety: Safety Instrumented Systems for the Process Industry Sector.
These documents set out the minimum requirements for Safety Instrumented Systems (SIS) and the individual SIF that makes up the SIS.
SIL Classification Methodology
The sequential steps used to conduct the SIL classifications are shown as follows:
- Select and confirm the safety instrumented function (SIF);
- Identify the location and design intent of this SIF;
- Decide the Target Mitigated Event Likelihood (TMEL) for this SIF with respect to personnel safety (S), environment (E), and commercial asset (C);
- Identify all demand scenarios where the SIF needed to be activated;
- Identify all major consequences occurring when the SIF was not successfully activated;
- Estimate the frequency of each scenario – the frequency assessment was in line with the agreed Risk Matrix;
- Estimate the frequencies of all IPLs and OPLs that were guarding against the demand scenario;
- Calculate the PFD;
- Determine the SIL of given SIF – the final SIL was the highest SIL identified from the various scenarios related to this SIF;
- Repeat above steps from Item 2 for all other SIFs.