Ranking every HAZOP row with severity (S) and likelihood (L) gives a quick picture of where the risk is. Two rules keep it useful.
Rank before and after safeguards, or say which
Rank the unmitigated case (consequence without safeguards, likelihood of the cause) if the aim is to find rows for LOPA; rank the mitigated case if the aim is to show residual risk. Mixing the two in one study makes the ranking meaningless.
Use the company matrix
Use the same severity and likelihood definitions as the risk matrix in the company's risk criteria (First Out AI's Risk calibration tab holds it).
Go to LOPA when
- Severity is in the top two categories, or
- The safeguards include a SIF or an alarm credited as protection, or
- The team cannot agree that existing safeguards are adequate.
In First Out AI
S and L from 1 to 5 give the risk S × L, coloured low (below 8), medium (8 to 14) and high (15 and above). The action list sorts recommendations by risk so the team can see the priorities. Rows that mention a SIF tag in the safeguards appear in that SIF's SRS draft as hazard references.
Written for First Out AI as engineering guidance. It supports engineering work and does not replace it; check every value against your standards, certificates and company rules with a competent functional safety engineer.