What is Safety Integrity Level (SIL) and how is it determined?

Safety Integrity Level (SIL) rates the required reliability of a Safety Instrumented Function (SIF). Levels 1-4 with increasing reliability (SIL 4 = highest). Determination methods: Risk graph - qualitative assessment of consequence, exposure, avoidance possibility. LOPA (Layer of Protection Analysis) - quantitative, calculates risk reduction required based on initiating event frequency and consequence severity. Independent Protection Layers (IPLs) are credited. SIL requirement = risk gap not covered by non-SIS protections. Higher SIL requires: redundancy, more frequent testing, rigorous design. SIL 3+ is rare in process industry. IEC 61511 provides guidance for process sector. SIL assignment affects instrumentation specification, voting logic, and lifecycle management costs.