The Ability of PHA to Identify Hazard Scenarios

Process hazard analysis (PHA) is used to identify possible hazard scenarios for processes so that the adequacy of existing protective measures can be considered and recommendations made for risk reduction, as necessary. When an incident occurs in a process, one of the first responses is to examine the PHA for the process to see if the incident was identified as a scenario. If the scenario is missing, the PHA is assumed to be deficient in some way. However, this judgment is based on the assumption that PHA can identify all hazard scenarios that actually occur in processes. Notably, this assumption has not been validated.

It is important to understand whether or not particular scenarios can be predicted by PHA. Scenarios that are not amenable to prediction using PHA must be addressed in other ways. Scenarios that are predictable, but have been missed in practice, provide the opportunity to identify PHA improvements to minimize the likelihood of missing scenarios.

A number of representative process safety incidents taken from the literature are analyzed in this paper to assess whether or not they could have been predicted by PHA. Reasons why scenarios were missed are identified. Recommendations are made for ways to address scenarios that were not predictable using PHA, and, for those scenarios that were predictable but that were missed, recommendations are made to address the underlying causes through improved PHA approaches and practices.