What Is SIL Determination?
The most common technique used for SIL determination is layer of protection analysis (LOPA). See “what is a LOPA?” for more information.
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:
| SIL | PFDavg | Risk Reduction Factor (RRF) | ||
| From | From | To | To | |
| 1 | 0.1 | 0.01 | 10 | 100 |
| 2 | 0.01 | 0.001 | 100 | 1,000 |
| 3 | 0.001 | 0.0001 | 1,000 | 10,000 |
| 4 | 0.0001 | 0.000001 | 10,000 | 100,000 |
A key task during “allocation” is therefore to carry out “SIL determination”; in other words to decide what SIL is required from each SIF.
What =Method can do for you
Provide experienced LOPA study facilitators.
Train and give mentoring support to aspiring LOPA study leaders.
Technical author
David Sparkes
Technical Director, Method Process Safety
David is an experienced process engineer with a solid background in plant support, capital projects and control system design.
4 x Recent =Method Process Safety SIL Determination projects
LOPA following earlier HazOp to establish the effectiveness of existing and intended protection layers and safety functions at a hydro electricity facility.
Industry: Water + Waste
Topic: Layer of Protection Analysis (LOPA)
Layer of Protection Analysis for Hydrogen Energy Management System (EMS).
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Industry: Water + Waste
Topic: Layer of Protection Analysis (LOPA)
Layer of Protection Analysis (LOPA) study fora distillery expansion project.
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Topic: Layer of Protection Analysis (LOPA)
