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PT Notes

What is the Difference Between a Hazard and a Top Event in Bow Tie Diagrams?

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.
The value of bow tie diagrams is increasingly being recognized in process safety. This PT Note addresses the important distinction between the hazard and the top event in a bow tie diagram.
Bow tie diagrams graphically depict a set of hazard scenarios for a process (see Figure). Scenarios begin with a threat and end with a consequence. The scenarios all result from the same hazard which is a potential source of harm. In process safety, hazard is typically defined as an operation, activity or material with the potential to cause harm to people, property, the environment, or a business.
The hazard is shown in the bow tie diagram to provide clarity as to the source of risk. Hazards exist; they don’t happen, and they exist even when the process is in its controlled state. For example, a flammable material contained in a process is a hazard, a fire involving the material is not. Hence, “Hexane in Tank” is the hazard in the figure.
A well-defined hazard makes it easier to properly define the top event for the bow tie diagram.
The top event represents the moment when there is loss of control over the hazard and the potential for harm is realized, for example, when containment of a hazardous material is lost. The term derives from fault tree analysis where the undesired event lies at the top of the tree that is used to show its causes. In process safety, the terms hazardous event and loss event are sometimes used synonymously with top event. For the hazard of “Hexane in Tank” the top event might be “Spill from Hexane Tank”, or “Fire at Pump”. The latter is shown in the Figure.
Careful specification of the top event helps to make bow tie construction more efficient by minimizing iterations and revisions.

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