Article written by Riyaz Ali on 10/19/2021 – Read the Original Article
Abstract
Final Control Elements (Control valves or Safety Shut Down Valves) are the key components of any close loop control system, whether it is used for Basic Process Control Systems (BPCS) or for Safety Instrumented Systems (SIS). Financial constraints derive different constructions of valves suitable for throttling vs On-Off applications. However, due to past accidents, reliability has become a key criterion in the valve selection process. Many process industry manufacturers and producers, based on their plant-specific experiences, are tempted to use control valves in safety shutdown applications—specifically smaller size valves, which may not be cost prohibitive. This post will provide clarity on when to assign the SIL suitability for valves used in different scenarios (process control vs. safety shutdown) and establish criteria to assign SIL applicability for the “Final Element”.
Introduction
Safety integrity Level (SIL) is the discrete level for specifying the safety integrity requirements of the safety instrumented functions. It is a quantifiable measurement of risk used to establish safety performance targets of SIS systems. A SIL can be expressed in terms of Probability of Failure on Demand (PFD) or Risk Reduction Factor (RRF). Risk reduction factor is simply the reciprocal of PFD (1/PFD). SIL levels are designated in terms of PFD or RRF as a range of numbers.
PFD is a value that indicates the probability of a system failing to respond to a demand. PFD is a function of test interval time and failure rate of the equipment under control.
In short, to establish an SIL suitability rating for a Safety Instrumented Function (SIF) loop, a PFD value needs to be computed for the components of a loop. A SIF loop consists of a sensor, logic solver and final element. To calculate PFD, an equipment failure rate number is required.
Failure Mechanism
Failures are categorized so that failure data can be organized in a consistent way. ISA Technical report ISA-TR84.00.02-2002 – Part 1 talks about two failure modes – physical (random) failures and functional (systematic) failures…
