Current standards covering fire and gas systems are prescriptive and focus on commercial applications such as buildings. Many end users in the process industry believe there is a need for a performance based standard for fire and gas systems used in industrial applications. Other performance based standards such as IEC 615081 and 615112 use the term SIL (Safety Integrity Level) to describe system performance. There are many devices used in safety instrumented systems in the process industries that are independently certified for use in certain integrity levels.

SIL RATINGS FOR FIRE & GAS SYSTEM HARDWARE – ARE WE BARKING UP THE
WRONG TREE?

By : Paul Gruhn, PE, CFSE, ICS Triplex, Houston, TX

ABSTRACT

Current standards covering fire and gas systems are prescriptive and focus on commercial applications such as buildings. Many end users in the process industry believe there is a need for a performance based standard for fire and gas systems used in industrial applications. Other performance based standards such as IEC 615081 and 615112 use the term SIL (Safety Integrity Level) to describe system performance. There are many devices used in safety instrumented systems in the process industries that are independently certified for use in certain integrity levels. However, there is considerable debate whether fire & gas system hardware should have SIL ratings at all. Vendors are naturally interested in promoting independently certified hardware in order to differentiate their products. However, considering the differences between safety instrumented systems and fire & gas systems, focusing on the SIL rating or performance of the actual fire & gas hardware alone may be a misleading and questionable practice. This paper reviews a) the differences between safety instrumented systems and fire & gas systems, b) how typical voting of fire & gas sensors not only reduces nuisance trips (which is desirable) but also reduces the likelihood of the system actually responding to a true demand (which is not desirable), and c) why concepts and standards that apply to safety instrumented systems (e.g., SIL ratings) may not be appropriate for fire & gas systems.

INTRODUCTION

Vendors are interested in promoting certified products such as fire & gas sensors as a way to differentiate themselves. For example, a vendor may gain a marketing advantage stating they have a single sensor that is certified for use in SIL 2 applications. However, there has been considerable debate within the industry whether fire and gas hardware should have SIL ratings at all. Some are strongly opposed to the idea. However, there is recognition that current standards such as EN 543 and NFPA 724 do not adequately cover industrial fire & gas applications. Hence the need to consider a potentially new standard and the formation of a new task team within the ISA SP84 committee (covering safety instrumented systems in the process industry).

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