Introduction

The OSHA process safety management (PSM) and EPA risk management program (RMP) dictate that a process hazards analysis be used to determine the protective measures necessary to protect workers, the community and the environment. A compliant program will incorporate "good engineering practice," which means that the program follows the codes and standards published by such organizations as the American Society of Mechanical Engineers, American Petroleum Institute, American National Standards Institute, National Fire Protection Association, and American Society for Testing and Materials.

In February 1996, the Instrument Society of America published a standard ANSI/ISA S84.01-1996, "Application of safety instrumented system for the process industries" ⁽¹⁾ . This standard became an American National Standards Institute (ANSI) standard in March 1997. With its acceptance as an ANSI standard, it will be enforceable under OSHA PSM and EPA RMP.

The new ANSI/ISA S84.01-1996 standard and the draft IEC 61508 ⁽²⁾ standard require that a target safety integrity level (SIL) be assigned for any new or retrofitted safety instrumented systems (SIS). The SIS consists of the instrumentation or controls that are installed for the purpose of mitigating the hazard or bringing the process to a safe state in the event of a process upset. A SIS is used for any process in which the process hazards analysis (PHA) has determined that the mechanical integrity of the process equipment, the process control, and other protective equipment are insufficient to mitigate the potential hazard.