risk assessment toolsHAZOP

What is HAZOP?

A Hazard and Operability Analysis (HAZOP) is a risk management technique used to identify potential hazards and functional flaws in existing or planned plant systems. Its purpose is to reduce risk and ensure the safety of workers in plant environments.It is primarily used to study complex operational hazards and functions in chemical processing plants but is also used in nuclear, water, sewage, and treatment plants.

A HAZOP study is performed by an interdisciplinary team of experts including engineers, chemists, facilities managers and safety officers to identify procedural risks, process hazards, and design flaws.

This article covers: 1) how HAZOP is used as a risk assessment tool; 2) HAZOP examples and sample HAZOP study report; 3) software and tools to help streamline HAZOP reporting; and 4) free featured HAZOP template you can download and customize.

HAZOP as a Risk Assessment Tool

HAZOP is based on the assumption that hazards happen because elements of design and operation can deviate from their original intention.

The HAZOP team discusses possible deviations and different scenarios where the system or process could fail. The unit can then propose recommendations for safeguards and improvements to lower the risk of identified hazards and operational failures from occurring.

A HAZOP study primarily assesses the following three parts of a plant function:

1. Design

To assess the design’s capability to fulfill its intended function and identify its weaknesses e.g. the composition of the chemical batch reactor.

2. Physical environment

To assess the environment where the system or design will operate and ensure that it is ideal e.g. is there enough space for the chemical batch reactor to operate as intended?

3. Procedure

To assess the engineered controls such as automation, sequence of steps, human interactions e.g. the steps in producing the target chemical concentration.

HAZOP Example Case Study

Here is an example of how a HAZOP study can be used by an expert team to assess the procedure involved in chemical production:

Setting: Chemical Mixing Plant

Objective: To produce a batch of 10% concentration HCl (Hydrochloric acid)


  1. Reverse Osmosis (RO) water is added to the batch mixer via centrifugal pump
  2. Additives are mixed with the RO water
  3. A 33% solution of HCl is added to the mixture of RO water and additives, resulting in 10% concentration HCl

Guide word/Scenario: No RO water added.

Possible cause(s): Centrifugal pump loses priming or valve closed totally.

Consequence(s): Solution will be highly concentrated.

Safeguard(s): Proper monitoring of the centrifugal pump (hydraulic pressure).

Comments: Hydraulic pressure gauge should be reviewed and maintained on a regular basis.

Recommendation(s): Review and do maintenance work on the hydraulic pressure gauge of the centrifugal pump.

Hypothetical Scenarios Using Guide Words

After identifying the objective and steps involved in the procedure, the HAZOP team will then explore different scenarios where the procedure could deviate from its intended function.

A HAZOP study encourages the use of “guide words” to help explore all potential deviations. Here is an example of how the team could use common guide words to explore deviations involving the input and mixture of RO water:

  • No or not (Design intent negated completely) – No RO water added
  • More (Quantitative increase) – RO water added is more than intended
  • Less (Quantitative decrease) – RO water added is less than intended
  • As well as (Qualitative modification/increase) – RO water mixed with UV water is added
  • Part of (Qualitative modification/decrease) – Tap water is added instead of RO water
  • Reverse (Logical opposite of design intent) – RO water is omitted from the mixture
  • Other than (Complete substitution) – UF water is used instead of RO water
  • Early (Relative to the clock time) – RO water is added too early
  • Late (Relative to the clock time) – RO water is added too late
  • Before (Relating to order or sequence) – RO water is added before the additives and 33% HCl are ready
  • After (Relating to order or sequence) – RO water is added after the additives and 33% HCl

In the first scenario where “No RO water is added” the team will deliberate and identify the possible causes, consequences, safeguards and ultimately provide a recommendation.

Examples of risk are the probability of all-engine failure in an aircraft during a given flight, the frequency of toxic release from a chemical plant, the loss of power steering in a car when cornering or the probability of a major radioactive release from a nuclear power station.As noted by HSE4 risk should not be reduced to a single quantity and its components need to be separately identified. There are examples in the literature, however, when the term “risk” is specifically defined by combining consequences with their probability.

How to use a Risk Matrix?

LikelihoodVery LikelyUnlikelyUnlikelyHighly Unlikely
Major InjuriesHighHighMediumMedium
Minor InjuriesHighMediumMediumLow
Negligible InjuriesMediumMediumLowLow

A risk matrix is often used during a risk assessment to measure the level of risk by considering the consequence/ severity and likelihood of injury to a worker after being exposed to a hazard. The two measures can then help determine the overall risk rating of the hazard. Two key questions to ask when using a risk matrix should be:

  1. Consequences: How bad would the most severe injury be if exposed to the hazard?
  2. Likelihood: How likely is the person to be injured if exposed to the hazard?

How to Assess Consequences?

In assessing the consequences of a hazard, the first question should be asked “If a worker is exposed to this hazard, how bad would the most probable severe injury be?”. For this consideration we are presuming that a hazard and injury is inevitable and we are only concerned with its severity.

It is common to group the injury severity and consequence into the following four categories:

  • Fatality – leads to death
  • Major or serious injury – serious damage to health which may be irreversible, requiring medical attention and ongoing treatment
  • Minor injury – reversible health damage which may require medical attention but limited ongoing treatment). This is less likely to involve significant time off work.
  • Negligible injuries – first aid only with little or no lost time.

To illustrate how this can be used in the workplace we will use the example of a metal shearing task. A hazard involved could include a piece of metal flying out of the equipment while in use. In this example the probable most severe injury would be “Major or Serious Injury” with the possibility of bruising, breakage, finger amputation.

How to Assess Likelihood?

In assessing the likelihood, the question should be asked “If the hazard occurs, how likely is it that the worker will be injured?”. This should not be confused with how likely the hazard is to occur. It is common to group the likelihood of a hazard causing worker injury into the following four categories:

  • Very likely – exposed to hazard continuously.
  • Likely – exposed to hazard occasionally.
  • Unlikely – could happen but only rarely.
  • Highly unlikely – could happen, but probably never will.

How to Implement Control Measures?

After identifying and assigning a risk rating to a hazard, effective controls should be implemented to protect workers. Working through a hierarchy of controls can be an effective method of choosing the right control measure to reduce the risk.

OSHA recommends the following guidelines to accomplish hazard control

  • Eliminate or control all serious hazards immediately.
  • Use interim controls while you develop and implement longer-term solutions.
  • Select controls according to a hierarchy that emphasizes engineering solutions (including elimination or substitution) first, followed by safe work practices, administrative controls, and finally personal protective equipment.
  • Avoid selecting controls that may directly or indirectly introduce new hazards.
  • Review and discuss control options with workers to ensure that controls are feasible and effective.
  • Use a combination of control options when no single method fully protects workers.


Coffee morning kettle example risk assessment
HazardWhat could happen?Who could be hurt?Action taken to minimise risk
Kettle sides becoming very hot when boiledSomeone could touch the sides and burn their handsVolunteersThe kettle will only be used in the kitchen, and only by volunteers. The kettle has a rubber handle, which does not heat up.
Hot water being spilledScaldingVolunteers, attendeesThe kettle must not be carried from the kitchen when it is full of hot water – the water should be poured into cups, teapots or coffee pots before being taken elsewhere. Volunteers should take extra care when carrying hot drinks.
Children pulling hot kettle on themselvesScaldingChildrenChildren will not be allowed to use the kettle at any time. The kettle will only be used in the kitchen, where children are not allowed to go.
Damage to electric power cableAnybody who touches the damaged cable could be electrocuted.Volunteers, caretaker.The caretaker will check the full length of the cable, plus plugs and sockets, once a week. This should be noted in the caretaker’s records. This should be done when the kettle is unplugged.
OverflowingBoiling water could land on people, scalding them. Overflowing could also lead to water getting into plug sockets, which can cause electrocution.VolunteersThe maximum level must be clearly marked on the kettle. If this wears off after time, it should be drawn back on with permanent ink.

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