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PT Notes

Barrier Management Using 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.

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. These situations arise for various reasons including operational or commercial pressures, dependence on humans who easily forget, and complacency. This emphasizes the need for better management of safeguards.

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 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.

Barrier management makes reliance on implicit barriers explicit, provides clarity on those relied upon to prevent incidents, documents their important characteristics, specifies requirements for functionality throughout their lifetime, provides continuous monitoring of their status, and ensures they are not degraded. These items are captured in a written performance standard for each barrier.

A barrier management plan is used to ensure the performance standards for barriers are met for both engineered and human barriers. The plan must cover each barrier at a facility. It is used to monitor and measure performance. Leading indicators are used to detect degradation in performance.

Barrier management should be part of a safety management system. Typically, engineered barriers are managed in an asset integrity (mechanical integrity) program. Historically, human barriers have not been managed rigorously and much more attention needs to be paid to them.

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