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PT Notes

PHA Practices to Avoid - Undocumented and invalid assumptions

PT Notes is a series of topical technical notes on process safety provided periodically by Primatech for your benefit. Please feel free to provide feedback.

Critical process safety decisions are made using the results of PHA studies so worksheet entries must be accurate and defensible. Entries should not be based on undocumented, hidden, unsupported, or invalid assumptions. For example, study teams should not simply assume that relief valve sizing is adequate. There should be confirmation that the adequacy of relief valve sizing has been addressed.

Study team members must be alert to assumptions that are made by the team and challenge them when they do not have a sound foundation. There is little point in performing a high quality study on a process that does not actually exist because invalid assumptions have been made.

Sometimes assumptions are made during the performance of a PHA study to avoid suspending a study session to seek their confirmation, for example, assumptions regarding the accuracy of documents or how the process operates. They should be documented and confirmation obtained promptly to avoid possible study re-work if they prove to be unfounded.

PHA is a subjective process and there are uncertainties in the worksheet entries made by the team. It may be necessary for the team to make assumptions that are not capable of verification, for example, on the credibility of causes, or the severity and likelihood of hazard scenarios. In such cases where there may be some doubt about worksheet entries, conservative assumptions should be made. Similarly, worst-case assumptions should be made regarding variable factors that influence PHA scenarios, such as process and meteorological conditions at the time of a release, the presence of people, etc.

Assumptions that are made during the performance of a study should be identified in the PHA report so that reviewers are aware of them and can seek independent verification, as needed.

PHA worksheet entries should be based on facts or sound engineering judgment. Undocumented, hidden, unsupported, or invalid assumptions should be avoided.

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