SIL - Safety Integrity Level
SIL stands for Safety Integrity Level.
It is a numerical rating from 1 to 4 that specifies the required reliability and performance of a Safety Instrumented Function (SIF) within a Safety Instrumented System (SIS). The SIL target indicates how much risk reduction the safety function must deliver to bring a hazard to an acceptable level.
SIL is assigned through formal risk assessment processes such as HAZOP and Layer of Protection Analysis (LOPA). It is defined by the following target average Probability of Failure on Demand (PFDavg) ranges:
- SIL 1: 10⁻² to 10⁻¹
- SIL 2: 10⁻³ to 10⁻²
- SIL 3: 10⁻⁴ to 10⁻³
- SIL 4: 10⁻⁵ to 10⁻⁴
Examples include a high-high pressure trip on a gas separator vessel (typically SIL 2) or a critical emergency shutdown system (often SIL 3).
The SIL concept is widely used in high-hazard process industries such as oil & gas, petrochemicals, and mining to demonstrate compliance with functional safety standards including IEC 61511.