A Degrading Element of Safety-instrumented Systems with Combined Maintenance Strategy

Abstract

Safety-instrumented systems (SISs) are widely used to prevent hazardous events. The mechanical actuator subsystem in a SIS can become more vulnerable with time due to progressive degradation mechanisms, such as erosion, corrosion and wear-out etc. Such kind of phenomenon challenges the assumption of constant failure rates or exponentially distributed lifetime that the existing reliability analysis depends on. This study aims to assess the performance of the actuator of a SIS subject to a continuous degradation, which will be modeled by homogeneous gamma process. Periodic tests with the interval т are executed to check the subsystem state. A combining maintenance strategy including corrective maintenances (CMs) and imperfect preventive maintenances (PMs) will be adapted according to the state, which can be evaluated by actual degradation level. Given that maintenances are triggered only at inspection dates, the actuator can experience downtime in cases of failures. The expected downtime in each test interval will be used to estimate the average unavailability of the SIS. A numerical example is shown that the average unavailability of such a SIS sub-system is changing with time rather than keeping as a constant value