Discrimination of low- and high-demand modes of safety-instrumented systems based on probability of failure on demand adaptability

Probability of failure on demand is commonly used to measure the performance of safety-instrumented systems in the low (frequency) demand mode, while probability of failure per hour is taken as the measure in the high-demand mode. In current IEC 61508, once per year of the demand frequency is regarded as the borderline between two modes. However, few explanations can be found about why the borderline is here. This study focuses on the intermediate area of the two demand modes, examines the adaptability of probability of failure on demand with demand rates and then proposes a discrimination criteria of demand modes based on the analysis for the adaptability of probability of failure on demand with the Markov method. According to these criteria, in the high-demand mode where probability of failure on demand is not an effective measure, the equipment has higher probability to fall in hazard before a proof test when the safety-instrumented system is unavailable, and otherwise, the safety-instrumented system runs in the low-demand mode. The mean downtime of the safety-instrumented system before its failure is revealed in a proof test can help to locate the borderline, which is thus influenced by the configurations of safety-instrumented systems.