DETERMINING THE REQUIRED SAFETY INTEGRITY LEVEL FOR YOUR PROCESS

A Safety integrity level (SIL) analysis is the initial step in the Saf ety system design process. Where the HAZOP process normally discovers potential hazards and provides general recommendations, the SIL is a s pecific analysis which defines the Safety criteria and mitigation of h azards which can lead to a significant economic, safety and environmen tal consequences. There are three SILs utilized by ISA S84 and four by IEC 1508/1511 for Risk Classification, as defined in terms of Probabi lity 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 consequence s and the Process demand rate. The methodology can be customized to co mply with existing company standards, and should satisfy many of the c ritical OSHA 29 CFR - 1910.119 requirements. In addition, the paper ad dresses 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 SI Ls of 2 or higher. (C) 1998 Elsevier Science Ltd. All rights reserved.