Quantifying the impact of partial stroke valve testing of safety instrumented systems

Abstract The ISA S84 and IEC 1508/1511 standards, along with the AIChE CCPS Guidelines on safety instrumented (interlock) systems are performance oriented, not prescriptive. They do not tell people what logic system to use, what field device configuration to use, or how often to test a system. They merely list the performance requirements for the system. In other words, the greater the level of risk of the process, the greater the performance needed of the safety system. These standards, along with OSHA PSM requirements, state companies need to “determine and document that equipment is designed, maintained, inspected, tested and operating in a safe manner”. Engineering tools are currently available which can model (i.e. determine and document) the performance of different types of systems. When one quantifies the performance of the overall system, from sensor to final element, one quickly comes to the conclusion that the valves represent the “weak link” in most of today’s systems. The typical failure mode of discrete shutoff valves is being stuck. The only way to test for such a condition is to stroke the valve, but closing the valve completely and stopping production is not desirable. One does not need, however, to fully stroke the valve in order to test its functionality. If one were able to partially stroke the valve (in a simple, reliable, and secure manner), on-line, without stopping production, a dramatic improvement in safety can result. When one quantifies the safety impact of this test method, the result is typically an improvement by one order of magnitude.