Incorporating process demand into models for assessment of safety system performance

The paper presents a simple model of a safety instrumented system (SIS) that explicitly incorporates process demand in addition to including both the detected and undetected modes of the fail dangerous state, and shows the impact of process demand on SIS performance. The research concludes that the explicit incorporation of process demand is necessary to assess SIS safety performance appropriately, and that a simple arbitrary division between low demand and high demand is insufficient. The study also suggests a method of using the model to determine an appropriate value of T_I, the time to periodic inspection and maintenance of the SIS, so as to achieve a suitable level of SIS safety performance. The paper notes the limitations of the present work, and provides suggestions for future research in this area