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Safety integrity level
In functional safety, safety integrity level (SIL) is defined as the relative level of risk-reduction provided by a safety instrumented function (SIF), i.e. the measurement of the performance required of the SIF.
In the functional safety standards based on the IEC 61508 standard, four SILs are defined, with SIL4 being the most dependable and SIL1 the least. The applicable SIL is determined based on a number of quantitative factors in combination with qualitative factors, such as risk assessments and safety lifecycle management. Other standards, however, may have different SIL number definitions.
Assignment, or allocation of SIL is an exercise in risk analysis where the risk associated with a specific hazard, which is intended to be protected against by a SIF, is calculated without the beneficial risk reduction effect of the SIF. That unmitigated risk is then compared against a tolerable risk target. The difference between the unmitigated risk and the tolerable risk, if the unmitigated risk is higher than tolerable, must be addressed through risk reduction of provided by the SIF. This amount of required risk reduction is correlated with the SIL target. In essence, each order of magnitude of risk reduction that is required correlates with an increase in SIL, up to a maximum of SIL4. Should the risk assessment establish that the required SIL cannot be achieved by a SIL4 SIF, then alternative arrangements must be designed, such as non-instrumented safeguards (e.g, a pressure relief valve).
There are several methods used to assign a SIL. These are normally used in combination, and may include:
Of the methods presented above, LOPA is by far the most commonly used in large industrial facilities, such as for example chemical process plants.
The assignment may be tested using both pragmatic and controllability approaches, applying industry guidance such as the one published by the UK HSE. SIL assignment processes that use the HSE guidance to ratify assignments developed from risk matrices have been certified to meet IEC 61508.
There are several problems inherent in the use of safety integrity levels. These can be summarized as follows:[citation needed]
These lead to such erroneous statements as the tautology "This system is a SIL N system because the process adopted during its development was the standard process for the development of a SIL N system", or use of the SIL concept out of context such as "This is a SIL 3 heat exchanger" or "This software is SIL 2". According to IEC 61508, the SIL concept must be related to the dangerous failure rate of a system, not just its failure rate or the failure rate of a component part, such as the software. Definition of the dangerous failure modes by safety analysis is intrinsic to the proper determination of the failure rate.[citation needed]
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Safety integrity level
In functional safety, safety integrity level (SIL) is defined as the relative level of risk-reduction provided by a safety instrumented function (SIF), i.e. the measurement of the performance required of the SIF.
In the functional safety standards based on the IEC 61508 standard, four SILs are defined, with SIL4 being the most dependable and SIL1 the least. The applicable SIL is determined based on a number of quantitative factors in combination with qualitative factors, such as risk assessments and safety lifecycle management. Other standards, however, may have different SIL number definitions.
Assignment, or allocation of SIL is an exercise in risk analysis where the risk associated with a specific hazard, which is intended to be protected against by a SIF, is calculated without the beneficial risk reduction effect of the SIF. That unmitigated risk is then compared against a tolerable risk target. The difference between the unmitigated risk and the tolerable risk, if the unmitigated risk is higher than tolerable, must be addressed through risk reduction of provided by the SIF. This amount of required risk reduction is correlated with the SIL target. In essence, each order of magnitude of risk reduction that is required correlates with an increase in SIL, up to a maximum of SIL4. Should the risk assessment establish that the required SIL cannot be achieved by a SIL4 SIF, then alternative arrangements must be designed, such as non-instrumented safeguards (e.g, a pressure relief valve).
There are several methods used to assign a SIL. These are normally used in combination, and may include:
Of the methods presented above, LOPA is by far the most commonly used in large industrial facilities, such as for example chemical process plants.
The assignment may be tested using both pragmatic and controllability approaches, applying industry guidance such as the one published by the UK HSE. SIL assignment processes that use the HSE guidance to ratify assignments developed from risk matrices have been certified to meet IEC 61508.
There are several problems inherent in the use of safety integrity levels. These can be summarized as follows:[citation needed]
These lead to such erroneous statements as the tautology "This system is a SIL N system because the process adopted during its development was the standard process for the development of a SIL N system", or use of the SIL concept out of context such as "This is a SIL 3 heat exchanger" or "This software is SIL 2". According to IEC 61508, the SIL concept must be related to the dangerous failure rate of a system, not just its failure rate or the failure rate of a component part, such as the software. Definition of the dangerous failure modes by safety analysis is intrinsic to the proper determination of the failure rate.[citation needed]