Reliability effects of test strategies on safety-instrumented systems in different demand modes

Periodic proof testing reveals hidden faults and improves the availability of a safety-instrumented system (SIS). Three testing strategies are applied: simultaneous testing, sequential testing, and staggered testing. Staggered testing is generally found to give a higher safety availability than the two other strategies, but at a higher cost. The testing strategy will also influence the safety availability through the demand rate for the SIS. This paper uses Petri net modeling to examine how the three testing strategies, under different demand rates, influence a SIS subsystem of two identical channels that are voted 1-out-of-2. The Petri net models are verified by comparing the calculated results with those obtained by traditional methods. The study shows that the SIS unavailability reduction gained by staggered testing does not depend on the demand rate. When common cause failures (CCFs) are considered, however, the study shows that the unavailability reduction by staggered testing increases with the fraction of CCFs.