How a deviation is produced
A deviation is a parameter multiplied by a guide word: no or none, more, less, as well as, part of, reverse, other than, early, late, before, after. The parameters are flow, pressure, temperature, level, composition, reaction, phase and the operating states.
Column mapping
| Worksheet column | Place in the bowtie |
|---|---|
| Possible causes | Candidate threat (left) |
| Consequences | Consequence (right), or the top event itself |
| Safeguards | Candidate barrier |
| Actions required | Missing barrier, open item |
The filter is essential
A study produces hundreds of rows. Moving all of them to the left makes the bowtie unreadable and kills its only real advantage — the shared picture. The test does not change: a deviation that can reach the top event on its own is a threat; one that only matters together with another condition is an escalation factor on the relevant barrier.
Same consequence, different top events
Deviation consequences usually gather under a handful of headings: loss of containment, fire, explosion, equipment damage. Those gathering points are often good candidate top events — which is how the number of bowties a site needs gets measured rather than guessed.
The column that must not be lost
"Actions required" is the column most often dropped in the transfer. Yet it is the most valuable one in a bowtie: every open action is a barrier that does not exist yet, and it should sit on the diagram as an empty place.