In brief
HAZOP is a team-based analysis that systematically questions how a process could deviate from its design intent, using guide words and process parameters. A sound result depends on an up-to-date P&ID, well-chosen nodes and an independent team leader. Unless the findings are linked to the risk assessment document and the recommendations are closed out, the study stays on paper.
What is a HAZOP study, and in which plants is it used?
What happens if the flow control valve of a pump feeding solvent to a reactor fails and sticks fully open? Does the reactor overflow, can the relief valve handle this flow, and from which alarm does the operator find out? This is the shortest answer to the question what is a HAZOP study: HAZOP (Hazard and Operability Study) is a team exercise that systematically asks, for every part of a process, "what happens if it does not work as designed?", with the help of guide words and process parameters. The principles of the method are described in the international standard IEC 61882.
HAZOP is powerful in plants running continuous or batch processes with pipelines, tanks, reactors, heat exchangers and pressure equipment. Chemicals, paints and varnishes, plastic raw materials, and production plants using adhesives and solvents are typical examples. In chemicals and plastics businesses in Bursa and the surrounding provinces, especially in Kocaeli and Bursa, HAZOP often comes up when a new line is being commissioned or a change is being made to an existing process.
From a legal point of view, the Risk Assessment Regulation does not make any particular method mandatory; it requires the method used to be stated in the document, the pages to be initialled and the last page signed. Upper-tier establishments falling within the scope of the Regulation on the Prevention and Mitigation of Major Industrial Accidents because of the quantity of dangerous substances prepare a safety report describing their processes and safety management system (Art. 11) and update this report when the process changes (Art. 12). A systematic process hazard analysis forms the technical basis of these documents.
HAZOP does not replace scoring methods such as FMEA or Fine-Kinney; it answers a different question. Scoring methods rank known hazards, while HAZOP finds deviations that nobody has thought of yet.
Preparation: the P&ID, design intent and node selection
The answer to how to carry out a HAZOP study starts not in the meeting room but in the preparation. Before the sessions, the team leader gathers these documents:
- Piping and instrumentation diagrams (P&IDs) reflecting the current situation on site (as-built),
- The process flow diagram, heat and mass balance, and operating instructions,
- The alarm and trip list, and relief valve set points,
- The safety data sheets of the substances used,
- Past accident and near miss records, and the zone classification in the explosion protection document.
The P&ID is then divided into nodes. A node is a section of the process whose design intent can be written in one sentence, such as "feed solvent from tank T-101 to reactor R-201 at ambient temperature at the specified flow rate". Boundaries are usually drawn at changes of equipment (pump, heat exchanger, control valve), and each node is marked on the P&ID with a coloured pen.
| Example node | Design intent | Point to watch |
|---|---|---|
| Solvent storage tank | Storage within a set level range under a nitrogen blanket | Overfilling, vacuum, loss of nitrogen |
| Feed line and pump | Transfer from tank to reactor at the specified flow | Reverse flow, pumping against a closed valve |
| Reactor | Reaction at a set temperature and pressure | Loss of cooling, runaway reaction |
| Product cooling heat exchanger | Cool the product to a set temperature | Tube rupture, loss of cooling water |
If nodes are too large, deviations are missed; if too small, the study drags on and the team repeats the same discussion. In batch processes, nodes are the steps of the recipe rather than equipment: charging, heating, reaction, discharging and cleaning.
HAZOP guide words and parameters
HAZOP guide words are short expressions describing a deviation from the design intent. Each word is combined with a parameter to generate a deviation: "More" combined with "pressure" gives the deviation "high pressure", and "Reverse" combined with "flow" gives "reverse flow".
| Guide word | Meaning | Example deviation |
|---|---|---|
| No | The design intent is not achieved at all | No flow: pump stopped, valve closed |
| More | Quantitative increase | High flow, high temperature, high level |
| Less | Quantitative decrease | Low flow, low pressure, inadequate cooling |
| As well as | Something else happens in addition to the intent | Water or contaminant in the line |
| Part of | Only part of the intent is achieved | One component of the mixture under-fed |
| Reverse | The opposite of the intent | Back flow from the reactor to the tank |
| Other than | Something completely different happens | Wrong substance fed from the wrong tank |
| Early / Late, Before / After | Deviations in timing and sequence (batch processes) | Catalyst added before heating |
The basic parameters are flow, pressure, temperature, level, composition and mixing. Operating states such as start-up, shutdown, sampling, maintenance and cleaning should be added to these; these transitional moments are less familiar than normal production, and situations where valves and sensors are in unusual positions come to light here. Not every combination of word and parameter is meaningful; the team passes over meaningless combinations with a short note.
The HAZOP team and session arrangements
A good HAZOP team brings together at the same table the people who designed, operate and maintain the process. The team definition in the Risk Assessment Regulation fits this too: the employer or their representative, the OHS specialist, the workplace physician, the employee representative, support staff and knowledgeable employees representing the departments.
| Role | Duty |
|---|---|
| Team leader | Runs the method and steers the discussion; preferably independent of the design |
| Scribe | Records deviations, causes, consequences, safeguards and recommendations during the session |
| Process engineer | Explains the design intent and the mass and heat balance |
| Operations representative | Describes what is actually done on shift; shows the difference between instructions and the site |
| Maintenance and instrumentation | Know the failure modes of valves, pumps, sensors and trip systems |
| OHS specialist | Links the findings to the risk assessment and the emergency plan |
| Workplace physician (when needed) | Assesses health consequences in toxic exposure scenarios |
In practice, a team of five to eight people works efficiently. Limit sessions to a few hours a day; attention drops in long sessions and the last nodes get rushed. Value the operator's participation most: information such as "this valve is usually left open by hand on the night shift" is not written in any document.
Example: a HAZOP record for a solvent feed line
The table below is a simplified example of the record kept for the feed line node above. In a real record, a number, a responsible person and a target date are added to each line.
| Deviation | Possible cause | Consequence | Existing safeguard | Recommendation |
|---|---|---|---|---|
| No flow | Pump failure, suction valve closed | Under-dosing, pump running dry | Flow indicator | Low flow alarm and pump trip |
| High flow | Control valve stuck fully open | Reactor overfilling, release of flammable vapour | Level indicator | Independent high level trip |
| Reverse flow | Reactor pressure when the pump stops | Tank contaminated with reactor contents, unwanted reaction | None | Non-return valve and periodic testing |
| High pressure | Pump running with the outlet valve closed | Flange leak, solvent fire | Relief valve | Verify that the relief valve is sized for this scenario |
| Other composition | Feeding from the wrong tank | Mixing of incompatible substances | Operating instruction | Line and tank labelling, locked selector valve |
Fill in the "existing safeguard" column honestly. An alarm that does not work on site, a relief valve that is not maintained or a trip that has been bypassed does not count as a safeguard. Topics such as static electricity, chemical storage compatibility and sampling also often come up in this discussion.
Linking the findings to the risk assessment
HAZOP is a qualitative method; it finds every deviation but does not prioritise on its own. So score the consequences in the record using your business's risk scale; the L-type matrix is practical for this. For deviations with severe consequences, the adequacy of the existing layers of protection should be questioned separately. The hierarchy of controls is the basis for choosing measures: design changes and automatic protection come before instructions and personal protective equipment.
Process safety risk analysis only becomes meaningful when the recommendations are closed out. Give each recommendation a responsible person and a date, verify completed ones on site and enter them in the record. Under the regulation, the risk assessment is renewed at least every 2 years in very hazardous and at least every 4 years in hazardous workplaces; but when technology, substances, equipment or production methods change, and after near misses, the period is not waited for. Set up a management of change procedure to handle every change to the process together with the HAZOP record of the relevant node. You can track renewal dates with the OHS calendar tool.
In our risk assessment service, we link HAZOP findings to your workplace's risk assessment document, emergency plan and training programme. For sector-specific hazards see our chemicals and plastics page, and for plants in Kocaeli our Kocaeli OSGB page.
Frequently Asked Questions
What is the difference between HAZOP and FMEA?
FMEA starts from the failure modes of equipment or work steps and scores the risks. HAZOP starts from deviations of process parameters from the design intent and produces a qualitative record. HAZOP is more suitable for processes involving fluids, pressure and temperature, and FMEA for sequential assembly and machining lines.
Is carrying out a HAZOP a legal requirement?
The Risk Assessment Regulation does not make any particular method mandatory; HAZOP is one of the methods that can be chosen. The method used should be stated in the document. In establishments within the scope of major industrial accident legislation, studies that systematically analyse processes form the technical basis of the safety report.
How many sessions does a HAZOP take?
The time depends on the number of nodes and the complexity of the process. A small feed unit may be finished in a few sessions; a whole plant may take weeks. Keeping sessions short and regular gives better results than long marathons in one go.
When should a HAZOP be repeated?
When there is a change to the process, substances or equipment, the relevant nodes are re-examined. A serious incident or near miss is also a trigger. Even without changes, the currency of the HAZOP record should be reviewed when the risk assessment is renewed.
A risk assessment that identifies and scores hazards on site, links each measure to an owner and a date, and has a clear renewal schedule.
Bursa SİSTEMER OSGB Technical Team — written from the field experience of our OHS specialists and workplace physicians. Legal information is valid as of the publication/update date; contact us for an assessment specific to your business.