The HAZOP team focuses on one section of the process at a time, called a ‘node'. A process parameter, such as Flow, is selected and a series of guidewords is combined with the ‘Flow' parameter to suggest possible deviations. For example, the guideword ‘No' combined with ‘Flow' might suggest pump stopped, valve left closed, etc. The team agrees the consequences, decides which are significant hazards or operating problems and determines if there are adequate safeguards. This process is repeated for all guidewords until the node is completed, and the team moves on to repeat the process for the next node.

Parameters Guideword Meaning of Guideword
Operating Mode
As Well
AsPart Of
Other Than
No part of the intention occurs
More of the intention occurs
Less of the intention occurs
The opposite of the intention occurs
The intention occurs but is unstable
Something in addition to the intention occurs
Only some of the intention occurs
Something else happens

There are three basic types of causes of deviations.

1. Human error, which includes acts of omission or commission

2. Equipment failure, which includes mechanical, structural or control failures

3. External events, which includes loss of utilities, adjacent plant, weather, etc.

Consequences should be understood in terms of the immediate effects, i.e. specific plant failure modes, as well as the wider potential impact on personnel, plant and the environment. Sometimes, consequences may develop over time, e.g. a small leakage into a vessel may not activate an alarm or trip for several hours.

Safeguards are procedures and hardware devices such as pressure relief valves, non-return valves, shutdown valves and instrument trips, which prevent a hazardous event, or detect and give warning of a hazardous event, or mitigate the consequences of a hazardous event once it has occurred.