A simplified quantitative analysis methodology for the determination of required Safety instrumented Systems (STS) and the associated target Safety integrity Levels (SIL) is presented. As prerequisites, the company policy for risk acceptability and a hazard and operability (HAZOP) study are needed. A risk acceptability criterion for large commodity chemical, petrochemical, or refining companies is discussed. The methodology starts with the selection of high potential risk scenarios from the HAZOP study. Then the effectiveness of the relevant process safeguards is evaluated based on layers of protection analysis, to assess if there are adequate and sufficient safety protection layers in the chemical process so that the actual risk of the process is at an acceptable level. The method allows the user to determine first if a SIS is required and then, what SIL is required for each function it performs. If a SIS already exists in the process, the methodology can be used to verify the required SIL for each safety instrumented function. (C) 2009 American Institute of Chemical Engineers Process Saf Prog 29: 22-31, 2010