(18d) The Interrelationship of Risk Criteria and Frequency Modifiers

Guidelines for Quantitative Safety Risk Criteria (CCPS) discussed the various risk criteria used to judge the acceptability of process risk. The criteria applied during a specific process hazard analysis (PHA) depend on the definition of the analysis boundary. For example, the risk assessment associated with many PHAs, such as HAZOP and What-if, focuses on preventing loss of containment and uses criteria related to maximum hazardous event frequency. Alternatively, for significant process hazards, some owner/operators and regulatory authorities require the assessment of direct harm (i.e., injury or fatality rate) and use criteria related to harmful event frequency, such as maximum individual risk or societal risk.

Depending on the analysis boundary and risk criteria, the factors considered during the risk assessment change. For hazardous event criteria, the assessment considers factors that affect the potential for loss of containment and release of hazardous chemicals. These factors may include the control, preventive and mitigative equipment that stop the propagation of the event and enabling conditions that contribute to the initiating event. When harmful event criteria are used, additional factors that reduce the potential harm may be considered. These factors may include enabling events affecting the hazard presence, mitigative equipment and structures that impede the progression of the hazardous situation, and conditional modifiers affecting the probability of harm, such as occupancy and flammability.

The above considerations are mentioned but not clearly linked in current CCPS publications including HEP, Safety Risk Criteria, and LOPA. This paper uses a theoretical incident sequence to explain the application of the risk criteria and the appropriate factors used to support risk assessments relying on these different criteria.