What is a HAZOP study?
HAZOP was developed by ICI in the 1960s and is the most widely used hazard identification method in the chemical, pharmaceutical, oil and gas and food industries. Its power is in the systematic use of guidewords, which push the team to imagine deviations they would otherwise miss. A HAZOP usually follows a preliminary hazard analysis and is repeated for significant changes under management of change.
Guidewords and deviations
| Guideword | Meaning | Example deviation |
|---|---|---|
| No / none | Complete negation of the intent | No flow |
| More / less | Quantitative increase or decrease | More pressure, less level |
| As well as | Something in addition | Water as well as solvent |
| Part of | Only part of the intent achieved | Part of the composition |
| Reverse | Logical opposite | Reverse flow |
| Other than | Complete substitution | Wrong material charged |
| Early / late | Timing | Late addition of catalyst |
HAZOP example
The example covers two nodes of a batch reactor unit: the feed line from storage tank T-101 to reactor R-201, and the reactor itself. More flow (control valve fails open) and more temperature (cooling failure) can both lead to a runaway reaction; with only alarms and a relief valve as safeguards they rank high, and the team recommends an independent trip and an automatic feed cut-off. Reverse flow into the storage tank is also high, protected by a single check valve. More pressure in the reactor ranks medium because the relief valve and alarm are considered adequate.
Running an effective HAZOP
- Use up-to-date P&IDs and operating procedures, and define the design intent of every node.
- Keep sessions short (3 to 4 hours) to keep the team sharp.
- Record, don't design: note recommendations and solve them after the session.
- Close the loop: track every recommendation to a documented resolution.
How to use this HAZOP tool
- Prepare. Name the study, the drawings (P&IDs) and the team, and divide the process into nodes.
- Apply guidewords. For each node, combine parameters and guidewords; the deviation is filled in for you.
- Record causes and consequences. For each credible deviation, record causes, consequences and existing safeguards.
- Rank the risk. Rate severity and likelihood with the existing safeguards, in the table or on the matrix.
- Recommend actions. Write recommendations and assign them, then check the coverage grid for gaps.
- Export the record. Export the HAZOP worksheet and matrices to Excel or PDF.
Frequently asked questions
What is a HAZOP study?
A hazard and operability study (HAZOP) is a structured team review of a process design. The process is divided into nodes, and for each node the team applies guidewords (no, more, less, reverse…) to process parameters (flow, pressure, temperature…) to find deviations from the design intent, their causes and consequences, and whether the safeguards are adequate. IEC 61882 is the standard reference.
What are the HAZOP guidewords?
The basic guidewords are No (none), More, Less, As well as, Part of, Reverse and Other than, often with Early, Late, Before and After for time and sequence. Combined with a parameter they form a deviation, for example no flow, more pressure or reverse flow.
What is a node?
A node is a section of the process with a clear design intent, such as a feed line from a tank to a reactor, or a vessel. Studying one node at a time keeps the review systematic. Nodes are usually marked on the P&ID.
Who should be in a HAZOP team?
An independent, experienced chair (facilitator), a scribe, and specialists who know the design and its operation: process engineering, operations, maintenance, instrumentation and control, and safety. Typically 5 to 8 people.
Should HAZOP rate risk?
Classic HAZOP is qualitative, but many companies add a risk ranking (severity × likelihood) with the existing safeguards to prioritize the recommendations. Scenarios with high risk often go on to a LOPA (layer of protection analysis) or SIL assessment.
What is the coverage grid?
The coverage grid shows, for each parameter, which guidewords have been studied, coloured by the highest risk found. It helps the chair check that meaningful deviations have not been skipped.