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PT Notes

Relationship of Bow Tie Analysis (BTA) to Process Hazard Analysis (PHA)

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.

Bow tie analysis (BTA) involves the construction of diagrams that depict how prevention and mitigation barriers and controls (i.e. safeguards) protect against threats (i.e. initiating events) that can cause hazardous events, and the adverse consequences that can arise from them.

The goal of PHA studies, such as hazard and operability (HAZOP) studies, is to identify all possible hazard scenarios for a process. In contrast, BTA depicts those scenarios that produce a single hazardous event in a single diagram. Thus, a bow tie diagram displays only a small set of the hazard scenarios for a process. BTA does not identify unknown hazard scenarios. PHA is needed first to accomplish that objective.

PHA studies list scenarios in a tabular format that is not conducive to the visualization of scenarios in terms of their contributing elements. In contrast, bow tie diagrams are visually intuitive, clearly depict the elements of scenarios, and are readily understood. Thus, BTA can be a useful tool to support PHA studies by providing a graphical representation of the output of PHA studies.

The focus in BTA is on the identification of prevention and mitigation barriers and degradation factors and controls that impact the barriers while the focus in PHA is on scenario identification. Barriers and controls (safeguards) are addressed in PHA but not always in the same detail as in BTA. In particular, barrier degradation factors and controls often are explored in more detail in BTA.

Bow tie diagrams can be constructed informally to depict scenarios from PHA or formally to support barrier management and other uses, such as incident investigation. Formal use involves the application of a set of rules in their construction. In applying rules, it is useful to be aware that bow tie analysis is a less formal variation of Cause Consequence Analysis which combines high-level fault and event trees. The prevention side of a bow tie diagram corresponds to a fault tree while the mitigation side corresponds to an event tree. Consequently, rules for fault and event tree analysis apply when constructing formal bow tie diagrams.

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