Determining The Required Safety Integrity Level For Your Process

There are three SILs utilized by ISA S84 and four by IEC 1508/1511 for Risk Classification, as defined in terms of Probability of Failure on Demand (PFD). This paper provides a methodology to evaluate and classify risk in terms of Consequences, and to determine the SIL for the process under consideration based on these Consequences and the Process Demand Rate. The methodology can be customized to comply with existing company standards, and should satisfy many of the critical OSHA 29 CFR - 1910.119 requirements.

In addition, the paper addresses the configuration of the Safety Instrumented System (SIS) and the impact of field devices on the system SIL, including the necessity of redundancy and testing of field devices to achieve and maintain SILs of 2 or higher.