Overcoming challenges in using layers of protection analysis (LOPA) to determine safety integrity levels (SILs)

A procedure is provided in this paper for SIL determination using LOPA in which facility risk tolerance criteria are used meaningfully and justifiably. The procedure entails the calculation of overall facility risk and an examination of contributions to the risks to individuals and groups of people from sources of risk such as different types of hazards, different modes of process operation, and different units and processes. The procedure is illustrated with an example.