Effect of isolators on SIL of a Safety Instrumented System

A common concern in using intrinsic safety barriers and isolators as part of an SIS (Safety Instrumented System) is whether these devices will deteriorate the overall SIL (Safety Integrity Level). Two factors must be addressed to answer this concern: system architecture, and failure rate. Fault trees are developed for several Safety Logic Solver architectures, including simplex, duplex, and triple modular redundant, and the effect of adding barriers and isolators is evaluated. The paper shows how the SILs for these combinations may be derived and demonstrates that the addition of suitable barriers or isolators need not have an adverse effect on the overall SIL.