Hazard rate of a plant equipped with a two-channel protective system subject to a high demand rate

In this paper, a Markovian approach is used to evaluate the hazard rate (accident frequency) of a plant equipped with a two-channel protective system. Within the Markovian approach, it is possible to account properly for the dependence of the unavailability of the protective system on the frequency of accident initiators (demands on the protective system). Two models are analyzed here. In one model, only the performance of the overall protective function is monitored: the failure of a single channel is not revealed by a demand. In the other model, failure of a single channel is revealed by a demand, and repair of the failed channel is initiated with the plant still on line. In the latter case, for fixed rates of channel failure and repair, the plant hazard rate initially increases as a function of demand rate, reaches a maximum, and then actually decreases with increasing demand rate. The effects of common cause failures are also taken into account. Results of previous work can be shown to be approximations to the results obtained here.