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Choosing an SIL System The identification of risk tolerance is subjective and site-specific. The owner / operator must determine the acceptable level of risk to personnel and capital assets based on company philosophy, insurance requirements, budgets, and a variety of other factors. A risk level that one owner determines is tolerable may be unacceptable to another owner.
When determining whether a SIL 1, SIL 2, or SIL 3 system is needed, the first step is to conduct a Process Hazard Analysis to determine the functional safety need and identify the tolerable risk level. After all of the risk reduction and mitigation impacts from the Basic Process Control System (BPCS) and other layers of protection are taken into account, a user must compare the residual risk against their risk tolerance. If there is still an unacceptably high level of risk, a risk reduction factor (RRF) is determined and a SIS / SIL requirement is calculated. The RRF is the inverse of the Probability of Failure on Demand for the SIF / SIS (see table below).
Selecting the appropriate SIL level must be done carefully. Costs increase considerably to achieve higher SIS / SIL levels. Typically in the process industry, companies accept SIS designs up to SIL 2. If a Process Hazard Analysis indicates a requirement for a SIL 3 SIS, owners will usually require the engineering company to re-design the process to lower the intrinsic process risk.
Safety Integrity Level Risk Reduction Factor Probability of Failure on Demand
SIL 4 100,000 to 10,000 10-5 to 10-4
SIL 3 10,000 to 1,000 10-4 to 10-3
SIL 2 1,000 to 100 10-3 to 10-2
SIL 1 100 to 10 10-2 to 10-1

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