Verification of Dangerous and Safe Behaviors Independence in Safety Instrumented Systems

Safety instrumented systems (SIS) are designed to prevent the occurrence or mitigate the consequences of potentially hazardous events. The failure of SISs to achieve their assigned functions could result in huge consequences with respect to both (i) the safety of the monitored system (dangerous failures) as well as (ii) its production availability due to false trips (safe failures). The aim of this paper is to propose multi-phase Markov models for some useful SIS configurations. Each of these models considers both safe and dangerous behaviours, and thus allows the assessment of the whole SIS performance with respect to safety and availability of the monitored system. In addition, the exploitation of these models enables us to verify the assumption according to which safe and dangerous behaviors can be considered as independent. An illustrative example is treated, where Petri Nets models are also developed aiming to overcome the Markov models drawback related to the states combinatorial explosion.