Is a safety instrumented system really required to meet my plant's safety objectives?

With the introduction of ISA standard ISA-S84.01-1996, Application of Safety Instrumented Systems for the Process Industries, many companies are beginning to assess the effects that this standard has on new and existing interlocks in their facilities. (ISA-S84.01 refers to interlocks as safety instrumented systems [SISs].) Several critical steps must be performed to determine if an SIS is necessary, and, if so, what safety integrity level (SIL) is required to meet corporate or plant risk acceptance levels. However, these critical steps are only mentioned in the standard (i.e., detailed guidance is not provided); therefore, additional clarification and guidance are necessary to effectively implement them. Because these critical steps involve making risk-based decisions, companies need to develop and implement systematic risk assessment methodologies to provide the information and basis for each decision. Applying these methodologies helps companies determine if risk objectives are being met and helps effectively and economically implement the necessary SISs. This paper provides the following: An overview of issues to consider when establishing corporate or plant risk acceptance levels A description of a qualitative/quantitative methodology that can be utilized as an effective, risk-based decision-making tool An approach for combining information from a process hazard analysis (PHA) with a risk-based decision-making tool to determine the need for and the level of SIS to implement Using the method described in this paper will help companies better understand the adequacy of the process safeguards installed in their facilities and will help identify areas that should be addressed to meet the requirements in ISA-S84.01.