Risk Reduction Through Partial Stroke Testing
The process industry uses a variety of trip valves that provide emergency shutdown functions and, thus, play a very significant role in the safety of process plants. The reliability of such valves must be commensurate with the criticality of the process they safeguard and must comply with good engineering practices, industry standards, and regulatory requirements. Trip valves are tested periodically with a full closure test, usually known as the full-stroke test to ensure the operational reliability of the trip valve system. This type of test may present practical difficulties related to process disruption and associated cost and safety issues. Therefore, a partial stroke test of safety trip-valves has been proposed, which reduces the implications of the full-stroke test and provides an indication of the reliability/availability of the trip-valve system response on actual process demands.
The basic question we are attempting to address in this paper is the impact of periodic partial-stroke testing upon the overall reliability of the trip-valve systems. First we address the need for partial stroke testing and explain the problems of safety valves. Next, a modern partial stroke test setup is described, as it is currently used in industry. The effects of partial and full stroke testing are quantified. Using reported industry data in a case study we present the effects that different combinations of partial and full stroke testing have on the performance of the safety valve in terms of safety availability.
KeywordsSteam Turbine Control Valve Programmable Logic Controller Safety Valve International Electrotechnical Commission
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