Safety instrument system manuals—a need to balance reliability and safety

Abstract

With the recently approved IEC 61511 and ANSI/ISA-84.00.01 standards, process industry companies are being challenged to determine the level of performance required from their safety instrumented functions (SIF) and to verify that the design, installation, operation, and maintenance employed is sufficient to ensure that the required performance is sustained until the SIF is decommissioned. As a practical matter, user companies must self-certify or user approve their SIF equipment, since they are responsible for the correct operation of the SIF in the operating environment, not just individual components as manufactured. To accomplish this, user companies need lifecycle assistance from equipment manufacturers, irrespective of hardware or software certification status related to IEC 61508. One of the best means to communicate the necessary information is via a safety manual for those involved in instrument system design installation and operation. IEC 61508 requires that equipment manufacturers provide a safety manual with any product claimed to comply with IEC 61508. Although required for product certification, it is the authors' current experience that quality and consistency in safety manuals is lacking. Part of this is due to IEC 61508 not providing a clear concise template with minimum requirements. The safety manual requirements are scattered throughout the rather complex standard. This article seeks to provide a user's perspective of what essential information is needed from equipment manufacturers to not only comply with IEC 61508, but also to provide enough information to the user of the product to allow the essential balance of reliability and safety in both SIF and instrumented protective function applications.

Using the ISA 84/HAZOP/LOPA procedure to design a safety instrumented system for a fumed silica burner

“…When an ISA 84 analysis is complete, it has defined the distributed control system (DCS) architecture, identified the safety critical instrument loops, defined the reliability and design of safety critical loops, and has defined the requirements to maintain these safety critical loops. The ISA 84 methodology addresses the complete safety life cycle of the SIS, including: performing a hazard and risk assessment; evaluating the layers of protection; design, installation, operation, maintenance, and decommissioning [7, 14, 15].…”