Measurement Best Practices for Safety Instrumented Systems

ANSI/ISA S84-1996 is migrating to a new international standard – the IEC 61511. Over the past few years, global CPI and HPI users have started to adopt this new standard, particularly for new installations. From these installations, “best practices” have started to emerge. Some of these, related to measurement, are documented here. The objective is to allow the user to comply with the new standards, while maximizing real-world safety and availability, and minimizing life cycle cost. INTRODUCTION STANDARDS AND TERMINOLOGY The ANSI/ISA S84-1996 standard has guided North American users for the past decade in designing safety instrumented systems. This standard is migrating to a new international standard – the IEC 61511 – with the new name ANSI/ISA 84.00.01-2004. Even where application of the new standard is not yet mandatory – for example, in jurisdictions governed by OSHA – global CPI and HPI users in particular have already started to apply the new standard to new installations, for three key reasons. First, global users recognize that adopting a single standard at all of their sites provides consistent engineering and maintenance practices, potentially reducing design, procurement and documentation costs. Second, users in most jurisdictions recognize that the standard will eventually become mandatory. For example, OSHA considers the standard to be “Recognized and Generally Accepted Good Engineering Practice” (RAGAGEP). As with the eventual adoption of S84-1996 by OSHA, complying with the standard now will minimize the need for future re-engineering. Finally, the new standard potentially provides users with flexibility to improve safety and availability, while reducing maintenance costs. For example, as will be detailed later, with the right equipment and installation, the new standard may allow the user to significantly stretch out inspection intervals. For existing safety systems that were designed to comply with ANSI/ISA S84-1996, the new standard includes a “grandfather clause” "For existing SIS designed and constructed in accordance with codes, standards, or practices prior to the issue of this standard (e.g., ANSI/ISA-84.01-1996), the owner/operator shall determine that the equipment is designed, maintained, inspected, tested, and contents