Confirming the SIS Layer of Protection

The Safety Instrumented System (SIS) is a complex protection layer with high reliability requirements. Assuring the effectiveness of the SIS requires monitoring the effectiveness of the Safety Instrumented Function (SIF) as well as the Independent Protection Layer(s) (IPL) that reduce demand on the SIF. A SIF tripping more frequently than anticipated may be due to flaws in demand reducing IPLs. A slower than expected or failed SIF trip may be due to flaws in the SIF components, the proof testing program, the maintenance program, or inadequate SIF design. This paper will identify parameters that should be routinely monitored, illustrate how data can be captured and analyzed detect weaknesses in the SIS IPL, and provide recommendations for routine monitoring and audit activities.