SIL calculation in gas processing plants based on systematic faults and level of maturity

Abstract

Since Safety Instrumented Systems (SIS) are of great importance in terms of safety and protection in industrial plants, many efforts have been made for appropriate use of these systems. As it is vital not to sacrifice hazards, potential dangers, and people’s safety and health, for economic and financial reasons, device selection and configuration in SISs should be pursued with sufficient care to achieve a certain Safety Integrity Level (SIL). Today, many SIL calculation methods use pure random failures, while systematic failure mechanisms are still active in every plant and may cause serious damages and injuries. Meanwhile, the available standards have not explicitly suggested “specific requirements relating to 100% avoidance of systematic faults and errors, which generally lead to systematic failures”. In this paper, the failures which led to shutdown in four units of a gas refinery, including a total number of 352 safety instrumented functions (SIFs), during 10 years of operation are investigated. Based on the obtained practical results, systematic error contribution to the studied SIFs failures is determined. Moreover, amateur decision making profound impacts on dangerous failures have been analyzed. The developed closed-form formula for calculation of systematic error’s weight in various levels (e.g., management level, engineering phase, etc.) has been verified, using different case-studies of an in-service gas plant. Furthermore, a practical guideline is proposed to improve the decreased actual SIL value, while taking into account the impacts of systematic failures.