What a HAZOP study is
A HAZOP (Hazard and Operability) study is a structured review in which a team examines a process part by part and applies guide words such as "no", "more" and "less" to parameters such as flow, pressure and temperature. The goal is to find deviations from the design intent, their causes and their consequences.
It started in the chemical industry and is now used in refining, petrochemicals, food, pharmaceuticals and any process with fluids, energy and automation. IEC 61882 is the application standard. The question behind it is simple: what happens if the process does not behave as intended?
A HAZOP works on the design (process flow diagram, P&ID), so it is strongest before construction, but it is also used on existing plants and on modifications.
Visual map of the HAZOP
The map shows the study flow, the guide words, the worksheet and the example of six deviations with calculated risk.
Guide words and parameters
The study crosses each guide word with each parameter of a node to produce a deviation:
| Guide word | Meaning | Example deviation |
|---|---|---|
| No / Not | Complete negation of the intent | No flow |
| More | Quantitative increase | More pressure, more temperature |
| Less | Quantitative decrease | Less level |
| As well as | Something extra happens | Contamination in the line |
| Part of | Only part of the intent is achieved | Incomplete composition |
| Reverse | Opposite of the intent | Reverse flow |
| Other than | Complete substitution | Wrong product in the tank |
Typical parameters: flow, pressure, temperature, level, composition and, in some studies, time and sequence. Not every combination makes sense: "reverse temperature", for instance, is discarded.
Step by step
- Define scope and documents: up-to-date P&IDs, process description, product safety data, balances.
- Divide into nodes: sections with a clear design intent, such as "transfer solvent from tank to reactor".
- State the design intent and the parameters of the node.
- Apply each guide word to each parameter and ask whether the deviation is credible.
- List causes, consequences and existing safeguards for every credible deviation.
- Estimate the risk (severity × likelihood) and decide whether to recommend something.
- Record recommendations with an owner and a due date, and track them to closure.
A little arithmetic helps plan the sessions. With 4 nodes, 5 parameters and 7 guide words there are 4 × 5 × 7 = 140 combinations to consider. Most are dismissed quickly as not credible; the time goes to the few deviations that matter.
Worked example: solvent transfer
Fictional example. Node: the line from flammable-solvent tank T-101, through pump P-101, to reactor R-201. Design intent: transfer solvent at a steady flow to the reactor. Severity (S) and likelihood (L) are scored 1 to 5; risk R = S × L. Criterion adopted: R of 8 or more requires a recommendation.
R = Severity × Likelihood; recommend if R ≥ 8
| Deviation | Causes | Consequences | Safeguards | S × L = R | Recommendation |
|---|---|---|---|---|---|
| No flow | Pump trip, closed valve | Reactor without solvent, heating with no heat removal | Low-flow alarm | 4 × 3 = 12 | Low-flow interlock that shuts off heating |
| More flow | Control valve fails open | High reactor level, off-spec batch | High-level alarm | 2 × 2 = 4 | None |
| More pressure | Outlet blocked with pump running | Flange rupture and flammable release | Local pressure gauge | 5 × 2 = 10 | Relief valve on the discharge, returning to the tank |
| Less level (tank) | Tank not refilled | Pump runs dry, seal damage | Manual check | 3 × 3 = 9 | Low-level switch that stops the pump |
| Reverse flow | Pump stops with reactor pressurized | Product flows back to the tank, contamination | None | 4 × 2 = 8 | Check valve on the pump discharge |
| Other than (wrong product) | Wrong product loaded into the tank | Unwanted reaction in the reactor | Labeled connections, unloading procedure | 5 × 1 = 5 | None; keep the procedure |
With the R ≥ 8 criterion, four deviations produce a recommendation: no flow (12), more pressure (10), less level (9) and reverse flow (8). The other two (4 and 5) stay with current safeguards. Note that the worst consequence (wrong product, S = 5) triggers no action because the likelihood is low and a procedure exists; that decision must be recorded and justified, not forgotten.
Who takes part and how to run it
- Study leader: knows the method and facilitates without arguing about the design.
- Secretary (scribe): records deviations, causes and recommendations in the worksheet.
- Process/design: explains the design intent.
- Operations and maintenance: know how the plant really runs and fails.
- Instrumentation and safety: assess alarms, interlocks and existing protection.
Run short sessions with breaks, one node at a time. Long, tiring sessions make the team skip deviations. Also record open questions, each with an owner.
HAZOP, FMEA, FTA and PHA: when to use each
HAZOP starts from process deviations and suits plants with fluids, pressure and automation. FMEA starts from components and failure modes. Fault tree analysis starts from an unwanted event and works down to causes. The preliminary risk analysis is lighter and fits early phases or simple tasks. HAZOP results feed the risk register of the occupational risk management program.
Use HAZOP when the risk comes from how the process operates and interacts, not from a single part. In maintenance, RCM complements the study by defining how to prevent the failures already identified.
After the study: recommendations and changes
The value of a HAZOP is in closing the recommendations. Each one needs an owner, a due date and verification. Any later change to the process goes through change management and, when it alters the risk, a review of the study. Risky operating activities, such as breaking into a line carrying hazardous product, need a work permit and lockout of energy sources.
Common mistakes
- Outdated P&ID: the study analyzes a plant that no longer exists.
- Nodes that are too large: the team loses focus and deviations become generic.
- Counting on an untested safeguard: an alarm nobody checks does not protect.
- Turning it into bureaucratic paperwork: filling every combination without discussion.
- Recommendations with no owner: the list ends in the minutes and never reaches the plant.
- Ignoring the human factor: operating errors, start-up, shutdown and maintenance are also deviations.
FAQ
What does HAZOP stand for?
Hazard and Operability Study: a structured examination of process deviations.
What are the HAZOP guide words?
The main ones are no/not, more, less, as well as, part of, reverse and other than, applied to parameters such as flow, pressure, temperature and level.
What is the difference between HAZOP and FMEA?
HAZOP starts from deviations of process parameters in nodes of the diagram; FMEA starts from components and their failure modes. They complement each other.
How many people should be in a HAZOP team?
A multidisciplinary team, usually five to eight people, with a leader, a secretary and representatives of process, operations, maintenance and safety.
When should a HAZOP be done?
At the design stage, before starting up a new plant, on significant modifications and in periodic reviews of existing plants.
Which standard describes HAZOP?
IEC 61882 is the application guide for the method.
Sources
- IEC 61882: Hazard and operability studies (HAZOP studies): application guide.
- KLETZ, T. A. HAZOP and HAZAN: identifying and assessing process industry hazards. Rugby: Institution of Chemical Engineers.
- CCPS. Guidelines for Hazard Evaluation Procedures. Hoboken: Wiley.
- CROWL, D. A.; LOUVAR, J. F. Chemical Process Safety: Fundamentals with Applications. Prentice Hall.
Keep studying: trusted sources and links
- ISO: ISO 31000, Risk management
- ABEPRO: Associação Brasileira de Engenharia de Produção (in Portuguese)
- Wikipedia: Risk management
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About the author
Lean Manufacturing & Behavioral Management Specialist
Over 20 years in the automotive and metalworking industries (GM and Dana), Lean Manufacturing practitioner since 2006. SENAI instructor and mentor in Brazil’s Brasil Mais Produtivo program, delivering consulting, training and audits for 50+ companies, combining quality, productivity and people development.
