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PT Notes

Uses of Bow Tie Diagrams in Process Safety

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 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. Bow tie diagrams have various uses. Some principal uses are described in this PT Note.

Bow tie diagrams provide a visually intuitive and readily understood depiction of hazard scenarios that is easy for non-experts to understand. Consequently, they provide a valuable communication tool to show the hazards and the safeguards that protect against them to stakeholders who may not have a deep technical background, such as front-line personnel and managers. Operators and mechanics can readily appreciate the safeguards that ensure process operations stay under control and managers can clearly see the safeguards that must be managed properly to ensure process safety.

Bow tie diagrams also help in identifying safety critical equipment and tasks for prioritization in maintenance, procedures development, and training. The identification of critical equipment supports asset integrity programs and knowledge of safety critical tasks supports personnel competency programs. 

Process safety incidents often involve the failure of safeguards, misplaced reliance on safeguards that were assumed to be functional but without verification, or the unrecognized degradation of safeguards. This emphasizes the need for better management of safeguards. Bow tie analysis provides a powerful tool for doing so. Bow tie diagrams clearly show the barriers and controls that need to be managed. The term, barrier management, is used in bow tie analysis to describe the process of ensuring that the safeguards an organization intends and expects to have in place are capable of preventing losses, properly implemented, in service (not disabled or bypassed), and supported and maintained such that they function as expected when needed.

Bow tie analysis is a useful aid in incident investigation. It helps in considering why safeguards failed and allowed an incident to occur. It produces a better understanding of the existing barriers and degradation controls and the need for new or strengthened ones. Bow tie diagrams help in sharing lessons learned across an organization and they can be used to identifying patterns of safeguard failures across multiple incidents.

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