Glossary · Functional safety
SIL allocation
Also known as: SIL determination, SIL assignment
German: SIL-Zuordnung
In functional safety under IEC 61511, SIL allocation is the process of allocating risk reduction to protection layers and determining the safety integrity level required for each safety instrumented function, based on the hazard and risk analysis.
- Functional safety
In one sentence
SIL allocation assigns risk reduction to protection layers and determines the SIL each safety instrumented function must achieve.
Example
A LOPA study credits a relief valve and an operator alarm, and the remaining risk gap for the overpressure scenario leads to a SIL 1 requirement for the pressure trip.
How it applies
- Methods: Common methods are layer of protection analysis (LOPA), risk graphs and calibrated Risk matrix approaches. The method and its calibration should be defined before the study begins.
- Protection layers: Other layers, such as relief devices, alarms with operator response or mechanical design, may be credited only if they are independent of the initiating cause and of each other.
- Results: The output is a required SIL per SIF, recorded as a Safety integrity requirement in the SRS. A result above SIL 3 is usually a sign that the process design should be reconsidered.
- Documentation: Record scenarios, frequencies, credited layers and assumptions in a traceable way. The documentation team should make sure that operator actions credited in the analysis appear in the operating instructions and training material.
SIL allocation vs. SIL verification
SIL allocation decides which SIL is required. SIL verification checks whether the designed function achieves it. Mixing the two, for example choosing the SIL to fit a device, undermines the analysis.