Impact of common cause failure on reliability performance of redundant safety related systems subject to process demand

Common Cause Failures (CCFs) can compromise reliability performance of safety related systems and hence configurations with identical redundant units receive special attention in many industries, including in automotive, aviation and process applications. This paper introduces a new reliability model for redundant safety related systems using Markov analysis technique. The proposed model entails process demand in conjunction with CCF and established system failure modes such as dangerous undetected failures for the first time and evaluates their impact on the reliability performance of the system. The reliability of the safety related systems is measured using the Probability of Failure on Demand (PFD) for low demand systems. The safety performance of the system is also appraised using Hazardous Event Frequency (HEF) to quantify the frequency of system entering a hazardous state that will lead to an accident if the situation is not controlled accordingly. The accuracy of the proposed Markov model is verified for a case study of flammable liquid storage tank overpressure protection system. It is demonstrated that the proposed approach provides sufficiently robust results for all demand rates, demand durations, dangerous undetected and CCF frequencies and associated repair rates for redundant safety related systems utilised in low demand mode of operation.