Average probability of failure on demand estimation for burner management systems

Abstract

Proper estimation of Safety Integrity Level (SIL) depends largely on accurate estimation of Safety performance in terms of average Probability of Failure on Demand, (PFDavg). For complex architectures of logic solvers, sensors, and valves, this can be calculated by distinguishing combinations of subsystems with basic (K-out-of-N) KooN approach for identical components. In the case of the typical configurations of valves for a burner management systems with non-identical subsystem configurations the KooN approach does not apply. Hence, it becomes an issues to calculate the correct safety performance since some of the established methods give too optimistic results due to lack of Common cause Failure information and data on non-identical components or sub-systems. This paper formulates a Markov model for determination of average probability of failure on demand for non-identical components and also proposes a more conservative lowest failure rate approach and maximum beta factor contrary to pragmatic minimum or average beta for correct estimation of average probability of failure on demand. It can be deduced that the measure of safety performance for components or subsystems with unequal failure rates depends largely on common cause failure, but a single beta factor is not appropriate to model the commonality of the failure. The result revealed that both geometric mean and lowest failure rate approaches result in different PFDave values with the lowest failure rate being the most conservative and optimistic result.