Risk criteria, protection layers, and conditional modifiers



Risk analysis assesses the likelihood and consequence of events. The acceptability of the identified risk is determined by comparing it to a specified risk tolerance. The criteria applied depend on the analysis boundary, which may be the hazardous event or extend to the harm posed by the hazardous event. Risk analyses generally begin with a determination of the likelihood that the hazardous event occurs. This is where the process deviation exceeds the safe operating limit of the process resulting in loss of containment, release of hazardous materials, or other undesirable consequence. These analyses require estimation of the likelihood that the initiating event occurs and the probability that the proactive protection layers do not operate as required, allowing the hazardous event to occur. Reactive protection layers and conditional modifiers are considered when the analysis is evaluating the likelihood that harm is caused by the hazardous event.

Various methods for performing risk analyses are discussed in several CCPS publications including Chemical Process Quantitative Risk Analysis [CCPS/AIChE, Guidelines for Chemical Process Quantitative Risk Analysis, 2000], Hazard Evaluation Procedures [CCPS/AIChE, Guidelines for Hazard Evaluation Procedures, 2008], and Layers of Protection Analysis [CCPS/AIChE, Layer of Protection Analysis: Simplified Process Risk Assessment, 2001]. However, the link between the selected risk criteria as described in Guidelines for Developing Quantitative Safety Risk Criteria [CCPS/AIChE, Guidelines for Developing Quantitative Safety Risk Criteria, 2009] and the factors considered in the analysis is not clearly described in these texts. Recognizing this opportunity, this article begins with a brief introduction to risk analysis concepts to provide a foundation for a discussion of the typical analysis boundaries and associated risk criteria. Then, it discusses how the analysis boundary and risk criteria affect the consideration of protection layers, enabling conditions, and conditional modifiers. © 2011 American Institute of Chemical Engineers Process Saf Prog, 2011