The approach to SIL assessment is based on a risk analysis where some absolute risk criteria has been established. Each safety function at the plant/system/equipment/tag is allocated reliability targets based on this criteria. In SIL assessment the safety unavailability is quantified based on IEC 61508 standard (Functional safety of electrical / electronic / programmable electronic safety related systems). The standard requires that SIL shall be determined for each safety function, e.g. based on a risk analysis. SIL is the discrete level for specifying the safety integrity requirements of the safety functions to be allocated to the E/E/PE safety related systems, where SIL 4 has the highest level of safety integrity and SIL 1 has the lowest.
The SIL definitions of IEC 61508 for a safety function operating in low demand mode of operation are presented in the table.
Safety Integrity Level (SIL) |
Probability of Failure on Demand (PFD) |
Risk Reduction Factor (RRF) |
| 4 |
≥ 10-5 to < 10-4 |
> 10,000 to ≤ 100,000 times |
| 3 |
≥ 10-4 to < 10-3 |
> 1,000 to ≤ 10,000 times |
| 2 |
≥ 10-3 to < 10-2 |
> 100 to ≤ 1,000 times |
| 1 |
≥ 10-2 to < 10-1 |
> 10 to ≤ 100 times |
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| Safety Integrity Level (SIL) for low demand mode |
Here PFD is 'Probability of Failure on Demand' (This is a measure of safety unavailability and accounts for the unavailability due to hardware failures). AGT's approach is in line with the cross industry standards IEC 61508 and industry specific standards like IEC 61511 for process systems.
AGT provides SIL assessment services using the qualitative & quantitative methods in accordance with IEC 61508/61511.
- Risk Graph Method
- Layer of Protection Analysis (LOPA)
- Based on 'The Norwegian Oil Industry Association OLF guideline no. 070 (2001)'
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