The Useful Life of a SIF component is the period within which the constant failure rate assumption — fundamental to all PFDavg calculations — remains valid. Per IEC 61508-2 (§7.4.9.5, Note 3), once the useful life limit is exceeded, the results of probabilistic SIL Verification calculations are no longer valid. This limit is derived from the underlying failure rate ‚bathtub curve‚, which describes the three phases of a component’s life: early failures, constant failure rate, and wear-out.
Manufacturers publish useful life limits in the component Safety Manual or SIL-certificate. Typical values for common SIF components are:
- Sensors: up to 50 years
- Barriers / isolators: ~10 years
- Logic Solvers (fail-safe PLC): ~15 years
- Solenoid valves: 8–12 years
- Actuator / valve assemblies: ~10 years
Where a component’s useful life is shorter than the SIF Mission Time, the component must be replaced or overhauled before the end of its useful life, or the Operator must justify an extension via a formal ‚Prior Use‘ assessment per IEC 61511-1 (§11.5.3). See our blog Mission Time vs Useful Life for a worked example