How do you ensure a Safety Instrumented System in operation will maintain its original design integrity for a lifetime of 15+ years?
The IEC 61511 (2016) safety life-cycle provides some brief guidance for operation and maintenance of a safety instrumented system (SIS) in clause 16. The stated objectives are to ensure that the validated system’s safety integrity is not compromised in any way, and that the SIL for each safety instrumented function (SIF) is sustained over the whole system lifetime.
Once an SIS reaches the operation stage, it's important that equipment is regularly inspected and maintained. Proof test procedures should have been developed for each safety function. The frequency of carrying out these inspections and tests should already have been determined by probability of failure calculations.