Introduction to the risk based design of safety instrumented systems for the process industry

This paper introduces issues related to the design of safety-instrumented systems (SIS) using a risk-based approach. The paper does not aspire to give an exhaustive guidance to actually designing such systems. The design and realisation of safety systems is a highly specialised skill, this paper only aims to introduce the various concepts and terminologies to the reader. Ever since the IEC 61508 was being drafted, the risk based design as opposed to deterministic designs, is becoming more and more accepted. Since the publication of ISA/ANSI SP84.01 and later IEC 61508, authorities started to require compliance or at least regard compliance as the best practice to compliance with authority regulations with regards to the design and maintenance of safety systems that use instruments to perform the functions, e.g., OSHA regards ISA/ANSI SP84.01 as the benchmark for compliance to their 29 CPR 1910.119. Authorities in Europe regard compliance to IEC 61508/61511 as benchmark for compliance to the Seveso 2 directive. Risk based design of safety instrumented systems (SIS) aims to establish the risk reduction that the SIS is to provide to arrive at an acceptable or at least tolerable remaining risk. If the risk without the SIS is already acceptable, no SIS would be required. If the initial risk without SIS is high, the risk reduction factor needs to be high and hence the integrity requirements for the SIS are high. This paper outlines how initial risks are established, how integrity requirements for SIS are defined and how those requirements are achieved both in the SIS design and in the life-cycle management.