The effect of partial stroke testing on the reliability of safety valves

Safety instrumented systems (SIS) are used to protect against consequences of hazardous events in the oil and gas industry. It is important to detect failures that may impede the SIS from performing upon demands. Recently, partial stroke testing has been introduced as an automatic means to test the SIS valves. Partial stroke testing is able to detect failures that traditionally have been revealed by function testing without causing process disturbances. Unfortunately, authors have different views on how partial stroke testing contributes to SIS reliability, and how failures detected by partial stroke testing should be classified. In this paper, it is shown that the failure classification may influence the hardware design, whereas the reliability modeling approach is not affected. In addition, a partial stroke test coverage factor has been proposed based on historical data.1