(21b) PHAs at Identical Plants: Why Aren't They the Same?

There is a common misunderstanding about the nature of HazOps and other process hazard analyses, that they are objective studies. That is, that different teams, working at different times and different places, but looking at the same process, will get the same results. In fact, HazOps are not objective. Their virtue is not in their absolute objectivity, but in the discipline they impose on a review team and how they document the relative subjectivity of the team.

There are several reasons that HazOp teams may get different results for nominally identical processes: structural differences in methodology, differences in assessments of likelihood, and differences in assessments of consequences. Even accounting for these differences, different PHA teams also can and do make different recommendations.

How different can they be? In addition to a review of why HazOps can yield different results, this study looks at two separate processes, each operated at several different sites, and quantifies the differences between the HazOps conducted at those sites. It is just as likely that only one facility will recognize a hazard as it is that all facilities will recognize a different hazard. The study then goes on to discuss what these differences tell us about the potential for overlooking hazards and mischaracterizing the risk of hazards that are discovered. The study concludes with recommendations for increasing the rate at which “identical” facilities discover the same hazards, while at the same time acknowledging that facilities are rarely identical.