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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 barriers and controls 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. Barriers protect against hazard scenarios. There are two types of barriers. Prevention barriers act to prevent a hazard scenario by terminating the scenario before the top event occurs. Mitigation barriers act to reduce the consequences of a scenario after the top event occurs. Prevention barriers are shown in purple in the figure and mitigation barriers are shown in green. A high level shutdown system for a tank is an example of a prevention barrier. A dike (bund) around a tank is an example of a mitigation barrier (see Figure).

The function of a barrier may be compromised by a situation, condition, defect, or error that defeats it or reduces its effectiveness. These are known as degradation factors , or sometimes, escalation factors , because they act to increase risk. A degradation factor may be prevented from impairing a barrier by measures known as degradation controls , or controls, for short. For the prevention barrier of a high level shutdown system, a control for the degradation factor “Loss of electric power”, might be “Backup power supply” (see Figure). For the mitigation barrier of a dike (bund), a control for the degradation factor “Loss of structural integrity due to aging” might be “Periodic inspection” (see Figure).

A distinction is made between barriers and controls in a bow tie diagram because, by definition, controls may not meet the more rigorous validity requirements for barriers. Thus, the term degradation control, or control, is used rather than degradation barrier.

Process safety practitioners use the term safeguards to describe both barriers and degradation controls. A source of possible confusion is that the term controls is also used as a synonym for safeguards in process safety.

Careful specification of barriers and controls helps to make bow tie construction more efficient by minimizing iterations and revisions.