HAZOP guidewords are simple prompts, but the quality of a HAZOP depends on how intelligently they are applied. A workshop that mechanically combines every guideword with every process parameter can become repetitive and slow. A workshop that uses too few prompts can miss credible deviations. The facilitator’s job is to use guidewords to challenge the design intent in a disciplined way and help the team identify realistic causes, consequences and safeguards. For related engineering support, see our HAZOP workshop facilitation.
This guide explains common HAZOP guidewords, how they translate into deviations, how to avoid unproductive combinations and how to document scenarios so the study remains useful after the workshop.
What is a HAZOP deviation?
A HAZOP study divides the process into nodes and compares actual or possible operation with the intended design. A deviation is a meaningful departure from that intent. It is commonly generated by combining a guideword with a process parameter.
For example, the guideword No combined with the parameter Flow produces the deviation No Flow. The team then asks what could cause no flow in that node, what would happen if it occurred, what safeguards would respond and whether further action is required.
The guideword is therefore not the hazard by itself. It is a structured prompt that helps the team discover scenarios.
Common HAZOP guidewords
No or Not
This represents complete absence of the design intent. Typical examples are no flow, no cooling, no agitation or no electrical power. Causes may include a closed valve, blocked line, stopped pump, equipment trip, instrument failure or utility loss.
More
More indicates a quantitative increase. Common deviations include more flow, higher pressure, higher temperature or higher level. The team should distinguish the deviation from the cause. “High pressure” is the deviation; causes might include blocked outlet, control valve failure, thermal expansion or external fire depending on the node.
Less
Less is a quantitative decrease. Examples include low flow, low pressure, low temperature or low level. A low-flow scenario may have very different consequences from no flow, particularly where minimum flow is required for equipment protection, heat transfer or reaction control.
Reverse
Reverse is particularly important for flow. Reverse flow may occur after pump trip, pressure reversal between systems, failed check valve, incorrect line-up or depressurisation of one side of an interface. Consequences can include contamination, overpressure, equipment damage or transfer of hazardous material to an unintended system.
Other Than
Other Than captures substitution, incorrect composition or the wrong material. It can be useful for contamination, incorrect chemical addition, wrong utility connection, air ingress, liquid carryover, gas breakthrough or incorrect feed composition.
As Well As
This guideword considers an additional material, phase or event occurring together with the intended condition. Examples include water with hydrocarbon, solids with liquid, gas entrainment or simultaneous transfer from another source.
Early, Late, Before and After
Sequence-related guidewords are particularly useful for batch processes, regeneration cycles, start-up and shutdown activities. They help the team identify hazards caused by steps occurring in the wrong order or at the wrong time.
Process parameters used with guidewords
Common HAZOP parameters include flow, pressure, temperature, level, composition, phase, reaction, mixing, utility supply and time. The relevant parameters depend on the node. A compressor suction node may focus on flow, pressure, temperature and liquid carryover, while a chemical dosing system may require composition and sequence prompts.
The facilitator should select combinations that make physical sense. “Reverse temperature” is generally not useful, while “reverse flow” may be essential. The objective is completeness without meaningless repetition.
How to identify credible causes
Once a deviation is selected, the team identifies credible initiating causes. These should be sufficiently specific to understand the scenario. Rather than recording “equipment failure,” the worksheet may identify pump trip, control valve failing closed, blocked strainer or upstream isolation inadvertently closed.
Causes can include equipment failure, control system failure, utility loss, human error, incorrect line-up, corrosion, fouling, external events and interactions with other systems. Existing operating experience and incident history are valuable inputs.
How to describe consequences
Consequences should be recorded before taking credit for safeguards. This helps the team understand the unmitigated scenario and prevents a safeguard from being used to dismiss the consequence prematurely.
A strong consequence description explains the physical progression. For example: blocked outlet causes pressure rise in the vessel, pressure exceeds design pressure if relief is unavailable, loss of containment occurs, and a flammable release may lead to fire or explosion if ignited. The detail should match the workshop purpose without turning each entry into a full quantitative analysis.
Safeguards versus recommendations
Safeguards are existing or committed features that reduce the likelihood or consequence of a scenario. They can include control functions, alarms, trips, relief devices, mechanical design features and procedures. The workshop should record them accurately, including tag numbers where practical.
A recommendation is different. It addresses a gap, uncertainty or opportunity for improvement. Recommendations should be clear enough that someone outside the workshop can understand what needs to be resolved and why.
When a HAZOP scenario should go to LOPA
Some HAZOP scenarios require more detailed risk assessment. Where consequences are severe and the adequacy of safeguards is uncertain, the team may refer the scenario to Layer of Protection Analysis.
LOPA tests whether safeguards qualify as independent protection layers and whether their combined risk reduction is adequate. Our guide to IPLs and risk reduction factors explains the next step.
Guidewords and functional safety
HAZOP is also an important input to functional safety. A deviation may identify a hazardous scenario requiring an instrumented protective function. If LOPA determines that a Safety Instrumented Function is needed, the target risk reduction may lead to a SIL classification.
This is why the HAZOP worksheet should be traceable. Later lifecycle activities need to understand the initiating cause, hazardous event, credited safeguards and design assumptions.
Avoiding common HAZOP guideword mistakes
Common problems include treating safeguards as causes, combining unrelated scenarios in one row, recording vague consequences, applying every guideword mechanically and spending too much time on low-value combinations. Another frequent issue is allowing the workshop to redesign the facility in real time rather than recording a clear action for engineering resolution.
Good facilitation keeps the team focused on hazard identification while allowing enough discussion to understand the scenario.
Preparing the node before applying guidewords
Guidewords work best when the node has a clear design intent and boundaries. The facilitator should understand normal operating conditions, upstream and downstream interfaces, equipment purpose and key controls before starting deviation review.
Our companion guide on HAZOP node selection and workshop preparation covers how to structure nodes and prepare drawings for an efficient study.
HAZOP support for Australian projects
Pro Engineers provides HAZOP facilitation together with HAZID, LOPA, functional safety and process engineering support. The objective is a study that is systematic enough to identify credible hazards but practical enough to support real design decisions.
For a HAZOP workshop, revalidation or focused review of a modification, contact Pro Engineers.