The HAZOP guide words at a glance
- Seven basic guide words: NO or NOT, MORE, LESS, AS WELL AS, PART OF, REVERSE, OTHER THAN.
- Time and sequence (IEC 61882): EARLY, LATE, BEFORE, AFTER.
- Deviation = guide word × parameter, e.g. MORE + pressure = high pressure, REVERSE + flow = reverse flow.
- Further guide words are allowed if they are defined before the study starts. Not every combination is meaningful – the team pursues the credible ones.
The basic HAZOP guide words
| Guide word | Meaning | Example deviations |
|---|---|---|
| NO or NOT | Complete negation of the design intent | No flow, no agitation, no cooling |
| MORE | Quantitative increase | More flow, higher pressure, higher temperature, higher level |
| LESS | Quantitative decrease | Less flow, lower pressure, lower level, lower temperature |
| AS WELL AS | Qualitative modification or increase – something happens in addition | Contamination, second phase, external leak, corrosion products |
| PART OF | Qualitative modification or decrease – only part of the intent is achieved | Missing component, wrong concentration, incomplete reaction |
| REVERSE | Logical opposite of the design intent | Reverse flow, reverse reaction |
| OTHER THAN | Complete substitution | Wrong substance, wrong destination, different operating mode |
In practice, teams often use combined forms such as “high flow”, “low level” or “no flow”. They are simply a guide word applied to a parameter (MORE + flow, LESS + level, NO + flow).
Guide words for time and sequence
| Guide word | Meaning | Example deviations |
|---|---|---|
| EARLY | Relative to clock time: too early | Valve opens too early, feed starts before the reactor is at temperature |
| LATE | Relative to clock time: too late | Cooling starts too late, alarm acknowledged too late |
| BEFORE | Relative to order: a step happens before it should | Heating starts before the vessel is filled |
| AFTER | Relative to order: a step happens after it should | Vent valve closed after purging has already started |
These guide words matter most for batch processes, start-up and shutdown and for procedures. In the classic German PAAG method, deviations in time fall under OTHER THAN and a reversed sequence under REVERSE.
Parameters: what the guide words are applied to
The design intent of a node is expressed through parameters. Typical parameters in process plants are flow, pressure, temperature, level and composition, plus – depending on the plant – pH, reaction, phase, viscosity, agitation, time, sequence, utilities, maintenance and sampling.
| Parameter | NO / NOT | MORE | LESS | REVERSE | AS WELL AS | OTHER THAN |
|---|---|---|---|---|---|---|
| Flow | No flow | High flow | Low flow | Reverse flow | Contaminated flow | Wrong substance |
| Pressure | – | High pressure | Low pressure, vacuum | – | – | – |
| Temperature | – | High temperature | Low temperature | – | – | – |
| Level | Empty vessel | High level, overfill | Low level | – | – | Wrong vessel filled |
| Composition | Component missing | High concentration | Low concentration | – | Impurity, water, air | Wrong material |
| Reaction | No reaction | Faster reaction | Slower reaction | Reverse reaction | Side reaction | Different reaction |
Not every cell yields a credible deviation. Many studies pre-select the meaningful combinations for each node; the facilitator makes sure that nothing relevant is skipped.
A practical HAZOP deviation list
Many companies keep a standard list of deviations as a starting point for process nodes:
- No, high, low and reverse flow
- High and low pressure, vacuum
- High and low temperature
- High and low level, overfill
- Contamination, wrong composition, wrong substance
- Loss of containment: leaks, ruptures, corrosion, erosion
- Loss of utilities: power, cooling water, instrument air, nitrogen, steam
- Start-up, shutdown and emergency shutdown
- Maintenance, isolation, draining and purging
- Sampling and manual operations
Guide words for procedures and human error
HAZOP can also examine operating procedures, where each node is a step. IEC 61882:2016 includes an improved case study of a procedural HAZOP. The guide words are then applied to actions:
| Guide word | Deviation of a step |
|---|---|
| NO / NOT | Step omitted |
| MORE / LESS | Too much or too little of an action, e.g. valve opened too far |
| AS WELL AS | An additional, unintended action |
| PART OF | Step only partly carried out |
| REVERSE | Opposite action, e.g. valve closed instead of opened |
| OTHER THAN | Wrong action, wrong equipment, wrong material |
| EARLY / LATE | Step carried out too early or too late |
| BEFORE / AFTER | Steps carried out in the wrong order |
German guide words (PAAG)
In Germany HAZOP is also known as the PAAG method. Its seven classic guide words (Leitworte) correspond to the basic IEC guide words:
| IEC 61882 | German (PAAG) |
|---|---|
| NO / NOT | NEIN / NICHT |
| MORE | MEHR |
| LESS | WENIGER |
| AS WELL AS | SOWOHL ALS AUCH |
| PART OF | TEILWEISE |
| REVERSE | UMKEHRUNG |
| OTHER THAN | ANDERS ALS |
Tips for applying guide words
- Agree the set in advance: define the guide words, including any additional ones, before the first session.
- Choose an order and keep it: parameter first or guide word first – both are allowed in IEC 61882; consistency helps the team.
- Pre-select combinations: skip combinations that make no physical sense instead of discussing them.
- Write a precise design intent: without design values, MORE and LESS cannot be applied meaningfully.
- Close with overview topics: at the end of a P&ID, many teams review topics that apply to the whole drawing, such as maintenance, utilities failure, start-up and shutdown or sampling.
Our free HAZOP template includes the guide words as a drop-down list and a guide word sheet. Worked deviations for six process units are in HAZOP examples.
Frequently asked questions
What are the seven HAZOP guide words?
NO or NOT, MORE, LESS, AS WELL AS, PART OF, REVERSE and OTHER THAN. IEC 61882 adds EARLY, LATE, BEFORE and AFTER for time and sequence.
What is the difference between a guide word and a deviation?
A guide word is a prompt such as MORE. A deviation is the guide word applied to a parameter, such as more flow or higher pressure.
Can I add my own guide words?
Yes. IEC 61882 allows further guide words if they are defined before the study begins.
Are HIGH and LOW guide words?
In practice they are used as shorthand for MORE and LESS combined with a parameter, for example high flow or low level.
Which guide words are used for procedures?
The same set, applied to steps: NO (step omitted), MORE or LESS, AS WELL AS (extra action), PART OF (incomplete step), REVERSE (opposite action), OTHER THAN (wrong action), EARLY or LATE and BEFORE or AFTER (wrong order).