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Understanding barriers with Bowtie Diagrams

Barriers are never 100% reliable. They can fail, and history has shown that they do fail. Process Safety is not the absence of incidents; it is the presence of effective barriers. In tabular risk assessments, it can be difficult to understand the combination of barrier failures that make an incident more likely. On a Bowtie Diagram, barriers are grouped together by threat and consequence, making the links between them far easier to understand.

Bowtie Diagrams can take our understanding of barrier failure even further. Escalation factors allow us to focus on what can cause a barrier to fail. They don’t cause an incident to occur directly, but the deterioration of barrier performance can make one more likely.

An escalation factor can be looked on in the same way as a threat – it is an event that can cause barrier failure. In the example below, the ‘Emergency Exits’ barrier may be impaired by the exits being blocked by equipment. This is the escalation factor. To prevent this, there could be clear, enforceable house rules around blocking exits, as well as regular inspections carried out to make sure that exits remain clear. These are the escalation barriers.

Barrier failure

Escalation barriers usually require us to carry out certain actions or activities. Effective risk management requires us not just to carry these out, but also to check that they are being done and to act upon any omissions. It’s much easier to visualise this whole process by constructing accident scenarios on a Bowtie Diagram.

Escalation factors are a valuable Bowtie element, but they should be used sparingly and only be used to represent failure modes which constitute a major threat to the effectiveness of a barrier.

Process Safety management requires that barriers continue to remain effective throughout the entire lifetime of a major hazard site. Escalation Factors are a great tool for focusing on what can reduce barrier effectiveness, and what we can do about it.