Evaluation of hazardous events and spurious activations for safety instrumented systems in all modes of operation

Modern industrial facilities depend heavily on Safety Instrumented Systems (SISs) to prevent accidents or mitigate their consequences. However, SISs can impose significant costs on facilities due to spurious activations. This paper proposes a probabilistic alternative to the common approach, which is still widely used despite its limitations. The proposed model evaluates both hazardous events and spurious activations and is applicable to safety systems operating in all modes of operation, i.e., low-demand, high-demand, and continuous modes. It incorporates both actual and false demands into the analysis and considers both planned and unplanned proof-test strategies. In the case of spurious activations, minimal repairs are also accounted for. Additionally, the model addresses the precedence of dangerous failures over false demands, accounting for the fact that the occurrence of a dangerous failure prevents further spurious activations due to false demands. The validation through a Markov model and Monte Carlo simulations demonstrates the accuracy and robustness of the proposed model which is applied to a safety system designed to protect a flammable liquid vessel from high-level hazardous situations. A comparative analysis with the common approach shows that, while this approach might serve as an approximation of the proposed model, its accuracy is limited to specific conditions that must be carefully considered.