Implementation of Safety Instrumented Functions in Petrochemical Facilities

A summary of contemporary practices and approaches implementing Safety Instrumented Functions (SIFs) within ethylene production facilities will be presented. The practices and approaches will be discussed in the context of ANSI/ISA-84 and IEC 61508 These two bodies of knowledge have been recognized by OSHA as RAGAGEP in the U.S. on requirements that govern the design, installation, configuration, testing and evaluation of Safety Instrumented Systems (SISs). For fired burner applications, NFPA 85 and API RP 556 would also serve as RAGAGEP for specialized, additional requirements that would have to be taken into account for any SIF in such an application, but these ancillary details will not be explored to any great level of detail.

The primary topics that will be discussed include the practical application aspects of implementing SIFs. The discussion will include considerations of hazards appropriate for SIS mitigation, technology selection, level of redundancy, implementation and functionality testing and how these factors can affect cost, schedule and quality. A brief summary of SIF fundamentals will also be included, primarily to highlight the basic requirements that must addressed in any SIF implementation.

SIFs are becoming increasingly popular in both new installations and upgrades of vintage plants, but not properly implementing a SIF can lead to a false sense of security and result in devastating consequences. This paper will walk readers through a practical means of assessing the suitability for service for a SIF, properly specifying hardware and software requirements for new designs, and apposite maintenance practices for preserving the Safety Integrity Level (SIL) for the life of the equipment.