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HAZOP Guide Words and Deviations Reference Table

A HAZOP study works by applying guide words such as no, more and less to process parameters such as flow, pressure and temperature, then asking what could cause that deviation and what would follow. This reference lists the standard combinations with chemical plant examples, so facilitators and team members always have prompts at hand.

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Guide word	Parameter	Deviation	Example cause	Example consequence
No / None	Flow	No flow	Blocked line, closed valve, pump trip, empty source tank	Reactor starved of feed, loss of cooling, pump running dry
More	Flow	More flow	Control valve fails open, wrong pump speed, bypass left open	Overfilling, off-ratio feed, exotherm from excess reagent
Less	Flow	Less flow	Partial blockage, fouled strainer, worn pump	Poor mixing or heat removal, off-spec product
Reverse	Flow	Reverse flow	Check valve failure, pressure higher downstream	Contamination of upstream equipment, reaction in wrong vessel
Other than	Flow	Wrong material	Incorrect hose connection, mislabeled tote	Unplanned reaction, toxic release, product loss
More	Pressure	High pressure	Blocked outlet, runaway reaction, external fire, blocked vent	Vessel overpressure, relief device lifts, loss of containment
Less	Pressure	Low pressure or vacuum	Condensing vapors, pump-out with blocked vent, steam condensation after cleaning	Vessel collapse, air ingress into flammable systems
More	Temperature	High temperature	Cooling failure, heater control failure, fouled exchanger	Runaway reaction, decomposition, boil-over
Less	Temperature	Low temperature	Heating failure, cold ambient, wrong utility	Freezing, crystallization or blocked line, reaction accumulation
More	Level	High level	Level transmitter fault, outlet blocked, overcharge	Overflow, carry-over into vent or downstream equipment
Less	Level	Low level	Leak, drain left open, instrument fault	Pump cavitation, heater exposed, loss of liquid seal
More	Concentration	High concentration	Overcharge of reagent, evaporation, dosing pump fault	Faster reaction, corrosion, exotherm
Less	Concentration	Low concentration	Dilution with water, dosing failure, wrong recipe	Incomplete reaction, off-spec product, unreacted raw material
As well as	Composition	Contamination or extra component	Cross-connection, residual from previous batch, water ingress	Side reaction, quality failure, hazardous byproducts
Part of	Composition	Component missing	Charging step skipped, empty hopper, ratio controller fault	Incomplete reaction, unstable intermediate
More	Reaction	Faster or larger reaction	Catalyst overcharge, accumulation then sudden start, agitator restart	Thermal runaway, gas evolution, overpressure
Less	Agitation	Poor mixing	Agitator motor trip, shaft or coupling failure	Stratification, hot spots, delayed reaction then surge
No / None	Utilities	Loss of cooling water, steam, nitrogen or instrument air	Utility header fault, power failure	Loss of temperature control, loss of inerting, valves moving to fail position
Early / Late	Time or sequence	Step too early or too late	Operator action, batch controller logic, delayed sample result	Wrong addition order, incomplete hold time, reaction accumulation
Other than	Operating mode	Startup, shutdown or maintenance state	Procedure not followed, bypassed interlock, abnormal operation	Exposure to hazards not present in normal operation

Guide words are prompts, not a complete list of deviations. Tailor parameters and causes to your node, and record safeguards, risk ranking and actions in your own HAZOP worksheet.

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How to use it in a session

Divide the process into nodes, such as a reactor with its feed lines and vent, then walk each node with the table open. Filter by parameter, read the deviation aloud and ask the team for credible causes. Record the consequence, existing safeguards and any actions in your HAZOP worksheet. The examples in the table are starting points. Not every combination is meaningful for every node, and discarding a combination with a reason is part of a good study.

Filter by parameter when working through a node line by line
Use the 'Other than' and 'Early / Late' rows to cover startup and batch sequence
Export to Excel to build your study template

Where HAZOP fits in process hazard analysis

HAZOP is one of the accepted methods for process hazard analysis, alongside what-if, checklists and FMEA. In the United States, OSHA process safety management for covered processes requires a hazard analysis that is revalidated at least every five years. HAZOP suits continuous processes and complex batch units well because it is systematic. For simple utility systems a what-if review may be enough, so match the method to the risk and the available process knowledge.

Common mistakes

Studies fail when the team lists only the normal operating mode, accepts 'operator will notice' as a safeguard, or skips batch sequencing and human factors. Another common problem is stopping at the consequence without judging whether safeguards are independent and adequate. Keep the drawings current, include operations and maintenance staff, and track every action to closure.

Do not count alarms as independent of the control system they depend on
Review the P&IDs against the plant before the session
Track actions to closure in a system, not an email thread

Frequently Asked Questions

What are the standard HAZOP guide words?

The commonly used set is no or none, more, less, as well as, part of, reverse and other than. Many teams add early, late, before and after for timing and sequence deviations. They combine with parameters such as flow, pressure, temperature and level.

How often should a HAZOP be repeated?

For processes covered by OSHA 29 CFR 1910.119 the hazard analysis must be updated and revalidated at least every five years. Changes to the process, incidents and new information may require earlier review under your management of change procedure.

Is HAZOP suitable for batch processes?

Yes, but add time-based prompts and sequence steps because many hazards come from the wrong order or timing of additions. Treat each recipe phase as a node or use a sequence-based variant of the method.

Do I need a facilitator?

A trained facilitator and a dedicated scribe greatly improve quality and speed. The facilitator manages the method and discussion while the scribe records causes, consequences, safeguards and actions.

Netray can help link HAZOP actions and management of change items to maintenance and quality records in your ERP so that follow-up does not depend on spreadsheets.

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