What is SIL and how is it determined for safety functions?

SIL (Safety Integrity Level) rates the reliability of safety instrumented functions, from SIL 1 (lowest) to SIL 4 (highest). Determination process: HAZOP identifies hazards, risk assessment (LOPA, risk graph, or matrix) determines required risk reduction, SIL target assigned based on risk reduction needed. SIL 1: risk reduction 10-100x, PFD 0.1-0.01; SIL 2: 100-1000x, PFD 0.01-0.001; SIL 3: 1000-10000x, PFD 0.001-0.0001. Design must meet target through architecture (redundancy), component selection, proof test frequency, and systematic capability per IEC 61511.