Safety Integrity Level (SIL) evaluation of safety instrumented systems considering competing failure modes and subsystem priorities

Safety Integrity Level (SIL) is a crucial measure of the safety performance of Safety Instrumented Systems (SISs), reflecting their ability to reduce risk. However, SIL analysis has often overlooked the impact of competing failure modes and subsystem priorities within SISs. This paper introduces a novel probabilistic model for evaluating the SIL of safety functions that incorporates these critical aspects. The model calculates the time-dependent Probability of (dangerous) Failure on Demand (PFD) and Probability of Failing Safely (PFS) at the component, subsystem, and system levels. The average PFD (PFDavg) and SIL are calculated considering both planned and unplanned proof tests. The proposed model is validated through Monte Carlo simulations and applied to a safety system designed to protect a process vessel from high-pressure hazards. A comparative analysis with existing models demonstrates that competing failure modes and subsystem priorities significantly influence PFD, PFS, PFDavg, and consequently SIL, especially in systems with longer proof test intervals and higher Safe Failure Fractions (SFFs).