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Safety Integrity Level (SIL) is determined during the risk assessment process as part of the safety lifecycle to ensure that identified hazards are mitigated to a tolerable level. The process begins with identifying potential hazards using techniques like HAZOP or FMEA. Once hazards are identified, the associated risks are evaluated by analyzing the severity of consequences, the likelihood of occurrence, and the ability to avoid the hazardous event. The next step involves defining the organization’s or industry’s tolerable risk criteria. Based on this, it is determined how much risk reduction is required. This helps identify whether a Safety Instrumented Function (SIF) is necessary and, if so, what level of reliability it must achieve. Methods such as Risk Graphs, Risk Matrices, Fault Tree Analysis (FTA), or Layer of Protection Analysis (LOPA) are commonly used to determine the required SIL. For example, LOPA evaluates existing layers of protection and calculates the risk reduction needed from the SIF. The SIL level corresponds to the probability of failure on demand (PFDavg), with SIL 1 offering the least and SIL offering the highest risk reduction. The selected SIL ensures that the safety system performs reliably enough to reduce the risk of the hazardous event to an acceptable level.
https://www.sigma-hse.co.in/safety-integrity-level-sil
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