A tail gas treating process is one of pollution control processes in refineries and natural gas processing plants required to manage process safety. Based on IEC 61511 safety lifecycle, the safety must be maintained during all design, operation, and maintenance activities. This paper addresses an allocation of specific functions performed by a safety instrumented system (SIS) to protection layers in analysis phase of the safety lifecycle for the studied tail gas treating process. An application

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... of layer of protection analysis (LOPA) process to evaluate three scenarios identified in a prior hazard and risk assessment by using a hazard and operability (HAZOP) study is proposed. The results obtained from the proposed assessment are the specifications of the safety function requirements and the safety integrity requirements. The safety integrity level (SIL) determination results can be utilized as the criterions for the safety instrumented function (SIF) design and engineering in order to conform the required SIL.