HAZOP method, deviation analysis

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Developed by Marion Chambon

The Hazard and Operability analysis (HAZOP) was developed by Imperial Chemical Industry in 1963. At first, it was established for chemical industries, in order to evaluate risks related to the possible deviations of a project. HAZOP is a systematic and structured method mainly used during the first design stage. It is also regularly updated during the operating stage of the plant. The main theory assumes that deviation forms are triggered by the operating design and bring about risky events. Deviations are defined as departures from an intention, an established course or an accepted standard. A team is created, terminologies are clarified, and a preliminary HAZOP enables members to acquire sufficient knowledge about the studied system. The plant is divided into parts, and study nodes are selected. The plant’s operating parameters (pressure, temperature…) are defined by parameters associated with guide words(more, less, no for instance) so that deviations are revealed and quantified. As a consequence, the plant is reviewed in a safer, more reliable and more efficient way.

Part of a project’s risk management, this method optimises resources and brings about financial benefits to the plant’s owner (and operator). It highlights which control and safety systems are really required to the plant. The quicker HAZOP’s recommendations are applied in the design process, the more benefits there are. Nowadays many companies use this method to enhance their project management. It is particularly well suited to systems that employ material and energy flows. However, it has some limits. Useful for project management, it is not suited for program and portfolio management due to its impossibility to assess hazards resulting in interactions between the different parts of a system. This feature triggers its impossibility to assess hazards brought by a gathering of projects. HAZOP has advantages and limits that are discussed in this article.



HAZOP was initially thought for reviewing a new phenol plant design. The Heavy Organic Chemicals Division of the Imperial Chemical Industry (British and international company) conceived this method in 1963. At the beginning, it was based on a “critical examination” studying the alternatives of a system. Later, the method’s focus was altered in order to study system’s deviations after producing the first detailed design of the plant. The operability study method was created.

The 1st June 1974, a chemical plant’s explosion close to Flixborough village killed 28 people, many of them were from this village. This event might have been avoided by assessing possible deviations of the plant. After this event, HAZOP method became more widely used.[1]

In 1977, ICI published a guide of this method officially named HAZOP. Since that date, the deviation analysis process has been applied in many projects from various types of company. Discussions and research have highlighted its usefulness in project management, but they have also unveiled some limits.

When to perform this method

The Hazard and Operability Analysis should be performed when the project members aim to check a design or running instructions; to decide the location of a construction; to decide whether to buy a piece of equipment; to obtain a list of questions to put to a supplier; or to improve the safety of existing facilities.[2] Even though HAZOP is commonly used when the first design is completed, it can be applied while the system is operating. Nowadays, HAZOP is particularly used in the process industries, such as oil and gas, chemical, petrochemical, fertilizers, power generation, but also in mining, ore extraction, pharmaceutical manufacturing, food additives manufacturing, water and waste water treatment.[3]

This deviation analysis is best suited for assessing hazards in processes, equipment or facilities using energy and material flows. Risks related to a project can be assessed from at least three perspectives thanks to HAZOP[4]:

  • Design
    • Assess the system design capability to meet the user specifications, and to be in line with safety standards and guidelines
    • Identify system’s weaknesses
  • Physical and operational environments
    • Assess the environment of the system to ensure that the location is appropriate in terms of places, thermal constraints, material near the installation (and possible danger with the operation) etc.
  • Operational and procedural controls
    • Assess engineered controls, sequences of operations, procedural controls such as human interactions
    • Assess different operational modes such as the start and the shutdown of the system, the normal operation, the unsteady states of some functions, the emergency shutdown

When a company estimates a project involving mechanical parameters requires a structured risk analysis, HAZOP may be proceeded. Employees, creators of the project and experts external professionals are gathered to enhance the system’s analysis.

HAZOP Process

Team Members

A team performing HAZOP should be composed from four to eight multidisciplinary members. The facilitator (chairperson) must be experienced with HAZOP to ensure that his team doesn't diverge from the procedure. The Responsible Officer and him select the HAZOP team members. These members are from various domains. They will share their knowledge and skills in order to achieve the HAZOP process.

The basic team is composed of[5]:

  • Chairperson: He is an independent person with sufficient knowledge of HAZOP techniques. He doesn’t have any responsibility with the system on which HAZOP is performed, nor with the domain the system will be applied.
  • Project engineer
  • Process engineer: He is usually responsible for the P&Id (explanation later) and the process flow diagram and development.
  • Design engineer: He is usually specialised in mechanical design, has been involved with the design of the system and will be concerned with the project cost.
  • Instrument/electrical engineer: He designs and selects the control systems for the plant.
  • Safety engineer

Depending on the process, the team may also be composed of:

  • Operation manager
  • Maintenance engineer
  • Suppliers representative

Other specialists may also enhance the formation of the team if they are relevant for the method’s application. For instance, if the plant includes chemical processes, a chemical engineer would step up the identification of deviations related to the chemical process.

Once the team is composed, members agree on the focus of the study, on the meeting dates, and they assure everybody knows and will use the right HAZOP terminologies.

HAZOP Terminology

Before applying HAZOP it is important to know the accurate definitions of the terms. The facilitator must ensure that team members have the same terminologies. This agreement on terminologies avoids assessment mistakes generated by misunderstanding.

Part: Subsystem with a specific intent/function (interest in the function needed for the system, not in safety elements)
Element parameter: Physical or chemical property associated with the process.
Guide words: Set of 10-12 words triggering the deviation
Deviation: Departures from the intention which are discovered by systematically applying the guide words to process parameters (e.g. "more pressure")
Cause: Reasons why deviations might occur. Once a deviation has been shown to have a credible cause, it can be treated as a meaningful deviation. These causes can be hardware failures, human errors, an unanticipated process state (e.g. change of composition), external disruptions (e.g. loss of power), etc.
Consequence: Results of deviations (e.g. release of toxic materials).
Safeguards: Engineered system or administrative controls to prevent the causes or mitigate the consequences of deviations (e.g. process alarms, work procedures, vacuum valve)
Actions required (or recommendations): Suggestions for design changes, procedural changes, or areas for further study (e.g. adding a redundant pressure alarm or reversing the sequence of two operating steps.

All those terms are in the final HAZOP worksheet mentioned in Results and record information part. They are the foundation of the HAZOP analysis, and each team member has to acknowledge their definitions. At the same time, information about the plant’s design/operating data are gathered.

Preliminary HAZOP

Most of the time, systems require preliminary analysis. It helps to define the boundaries of the system the team is going to study. HAZOP’s scope is also discussed.

Implementation of HAZOP in a pump design example: The team wants to study a specific pump operating system taking water from a tank, and releasing it in a motor. They could only consider the pump system. If HAZOP’s boundaries also consider the pipes connecting to the pump, the tank before the pump and the motor’s system the scope is completely different and the deviation analysis will include other deviations than just the pump.

Some HAZOP analysis may have already been applied on similar systems. Accidents records and team members’ experiences enhance the deviation assessment. Therefore, the chairperson must organise meeting dates and supply the necessary resources for the meetings. He assures that all the team members have necessary information and tools for applying HAZOP. Tools and diagram define the system and allow the team to understand and overcome the operating system features, such as:

  • Process Flow sheet and diagram (PFS or PFD)

Process flow diagram for the production of benzene by the hydrodealkylation of toluene.PNG Figure 1: Process flow diagram for the production of benzene by the hydrodealkylation of toluene (Source: [6])

It is a flowchart revealing the principal plant’s components. It is mainly used in chemical and process engineering, though its concepts can also be applied to other processes as well. Series of symbols and notations describe the process on the diagram (or sheet). PFD and PFS only show major components, and do not emphasise minor details as the P&ID.
  • Piping and Instrumentation Diagram (P&ID)

Piping and Instrumentation Diagram for the production of benzene by the hydrodealkylation of toluene.PNG Figure 2: Piping and Instrumentation Diagram for the production of benzene by the hydrodealkylation of toluene (source: [6])

This diagram displays all the minor details such as piping details and designations. It is more accurate than the previous diagram. It schematises the relationships between the functions of the piping. The Piping and Instrumentation Diagram is the last step in process design.
  • Process calculations
  • Process Data Sheets
  • Instrument Data Sheets
  • Interlock schedule
  • Layout requirements
  • Description of the process and provisional operating instructions
  • Start-up and emergency shut down procedures
  • Electrical area specification
  • Standards used
  • Maintenance procedures

An accurate knowledge of the plant brings about the next step in HAZOP analysis: dividing the system into parts.

Divide the system in parts

The plant must be divided in parts. This step of the HAZOP analysis is one of the most important. The team members have to be careful not to divide the system too much in order to understand the function of each part and the links between the different parts. Referencing to the definition of the “part” is the best way not to diverge from HAZOP purpose.

Implementation of HAZOP in a pump design example:

Pump example.PNG

Figure 3: Pump example[7]

A simple pump system composed of a power supply, receiving water from one side and releasing it by another. Part1 is the part of the pump in contact with water and the pipes conveying water into the pump and releasing it after, whereas Part2 is power supply.

Study node and design intent

After dividing the system, the team selects study nodes for each part. Study nodes define the locations (on the piping and instrumentation drawings and procedures) on which the team will analyse the process parameters deviations.

For instance, if the part selected is a reactor composed of a pump, the pump is a study node. Study nodes could be a line, a vessel, a valve, a tank. The choice of study nodes depends on the chosen scope of HAZOP. If the team decided to analyse a pump in detail, the study nodes may be the power supply and the part in contact with water. If the system is built with a tank, pumps, valves, a motor, pipes, and many other items, the pump could be considered as only one study node and not be defined as two parts. The HAZOP’s scope and the division of the parts of the system lead to the study nodes choices.

When a study node is selected, its design intent is clarified. It is the description of how the system is expected to behave while it is studied. It is a qualitative estimation of the system activity and a quantitative estimation of the process parameters. Regarding the objective of HAZOP (know all the deviations of a plant in order enhance it) the knowledge related to the study node design intent is fundamental. Once the design intent is defined the team can select the process parameters and start to think about their deviations.

Select a process parameter

The most usual process parameters are:[8]

  • Flow
  • Temperature
  • Pressure
  • Level
  • Agitation
  • Mixing

Other examples of process parameters:

  • pH
  • Composition
  • Addition
  • Separation
  • Transfer
  • Reaction
  • Control
  • Particle size
  • Speed
  • Communication
  • Viscosity
  • Time
  • Start/Stop
  • Measure
  • Impurities

Parameters are associated with guide words in order to spot the possible deviations of the study nodes. Each study nodes are characterized by specific process parameters.

Implementation of HAZOP in a pump design: The first part of the pump (where water is flowing) takes into account water’s FLOW and the PRESSURE.

Example 2: Some tank’s process parameters could be PRESSURE in the tank and LEVEL (because the volume of the tank is limited and a minimum level can be needed in some cases).

Apply a guide word

The choice of the guide words is relevant for detecting faults and operability problems. Those guide words are related to previous process parameters. They are estimated during the normal system operation, as well as during its start and its shutdown. The table below show examples of guide words and their common meanings.

NO or NOT Complete negation of the design intent
MORE Quantitative increase LESS Quantitative decrease
AS WELL AS Qualitative modification, increase PART OF Qualitative modification decrease
REVERSE Logical opposite of the design intent OTHER THAN Complete substitution
EARLY Relative to the clock time LATE Relative to the clock time
BEFORE Relative to order or consequences LATER Relative to order or consequences

When a process parameter is combined with a guide word it brings about deviations. For instance:

FLOW + NO = no flow

MORE + PRESSURE = High pressure

Implementation of HAZOP in a pump design: The flow of the part 1 of the pump can be related to the guide words: no, more, less, reverse

When a deviation is noticed, possible causes and consequences have to be determined to avoid them.

Determine possible causes and consequences of the deviation

Deviations may be triggered by various causes, such as:[9]

  • Insufficient outputs
  • Valve closing
  • Obstruction, clogging, foreign material
  • Undesirable liquid input
  • Temperature or pressure variation
  • Operation error
  • Sensor failure, computer failure, actuator failure, valve failure, set point error
  • Excessive valve opening
  • Insufficient supply
  • Leak toward the outside
  • Abnormally high density
  • Vaporization or evaporation
  • Maintenance error
  • Human errors

The multidisciplinary team shares its knowledge related to the system to establish an exhaustive list of possible causes and consequences of a deviation. One deviation can be triggered by multiple events, and may bring about various consequences and level of consequences (high, medium or low impact). Deviations bring about dangerous phenomena such as fire, explosion, toxic dispersion, system shutdown, non-achievement of the system purpose. When a deviation brings about to undesirable and unbearable consequences the team looks for appropriate safeguards.

Recommend actions and safeguards

There are five types of safeguards:

  • Identify the deviation
  • Compensate for the deviation
  • Prevent the deviation from occurring
  • Prevent further escalation of the deviation
  • Relieve the process from the hazardous deviation

The protection, detection and indicating mechanisms are part of safeguards. Their efficiency impact the estimation of the risk triggered by the deviation. Therefore, they can be improved or replaced with safer, more efficient and more reliable devices.

Results and record information

During the HAZOP process results are put on a sheet, as the one below. It is important to write every part of the HAZOP process on it. If the team estimates that a deviation should be less considered, it is recommended to write it in the remarks column. Writing remarks enables future teams working with HAZOP, as well as other workers having a look on the results to understand the decisions made.

HAZOP worksheet.PNG

Figure 4: HAZOP worksheet model

The results are recorded, as well as the scope of this study, the description of the facilities, the methodology and the action plan recommendations. During a project, HAZOP can be repeated after a change in the process mechanism or design. It is useful for evaluating the deviations of an actual system. This method gathers a lot of information about various plants, systems, mechanisms that can be used during a project consideration.

Advantages and Limits of the method

HAZOP was created in the 70s and is still used by many companies in order to manage technical projects. Related to energy and material flow, this method has some advantages:[2] [8]

-HAZOP is a systematic, simple and comprehensive process .
-It detects design errors at an early stage of the system conception.
-It enhances the hazard analysis of a project.
-It is useful for difficult hazards to quantify, to detect, to isolate, to count, to predict.
-It covers human errors, confronts hazards resulting in human performances and behaviours.
-The method doesn’t oblige to accurately rate all the deviations, their probability, occurrence, impact, severity.
-The study is led by an independent chairperson.
-There is a multidisciplinary team doing brainstorming.
-Results are recorded on a clear and easy to take in hand sheet.
-HAZOP covers safety as well as operational aspect.
-It is simpler and more intuitive than some other project risk management tools.

Even though the Hazard and Operability analysis method can allow companies to detect possible deviation, it has some limits:[2] [8]

-An accurate knowledge of the system is required.
-HAZOP is not efficient for systems putting into play electronic or mechanisms. It is more focused on systems using fluids.
-It cannot assess hazards resulting in interactions between different parts of a system or a process.
-It could be difficult to find out all the causes of a deviation. Therefore, some causes are triggered by another deviation’s consequences.
-There are no risk ranking or prioritisation safety measures resulting from this method.
-It doesn’t assess the effectiveness of existing or proposed safeguards.
-The brainstorming can lead to endless discussions and details of the system.
-Most of the time, some team members dominate the discussion.
-HAZOP is not an audit in order to enhance the design or the procedure of the system.
-This method is not suited for program and portfolio management because it cannot assess interaction between systems, particularly if the link is not physical (cables, pipes). If HAZOP is applied on a program or a portfolio, the parts would be so much divided that it would result in analyzing each project in the program or in the portfolio.


  1. CLIFTON A. ERICSON II. Hazard Analysis Techniques for System Safety. John Wiley & Sons, 2016
  2. 2.0 2.1 2.2 RAUSAND Marvin and HØYLAND Arnjolt. System Reliability Theory: Models, Statistical Methods, and Applications. Wiley, 2nd Edition. January 2004
  3. ANONYMOUS. Example. Last view: 30 September 2017. www.hazop-study.com
  4. PQRI. Manufacturing Technology Committee –Risk Management Working Group. Risk Management Training Guides. Last modification: 3 December 2015. Last view: 22 September 2017. http://pqri.org/wp-content/uploads/2015/08/pdf/HAZOP_Training_Guide.pdf
  5. NSW Government. HAZOP Guidelines, Hazardous Industry Planning Advisory Paper No 8. January 2011. www.planning.nsw.gov.au
  6. 6.0 6.1 MANCA, David. BFD, PFD, P&ID. Power point Lab 7 of “Process Systems Engineering” Politecnico di Milan. Last modification: 26 November 2014. Last view: 22 September 2017. pselab.chem.polimi.it
  7. FLAUS, Jean-Marie. B4. La méthode HAZOP. Lecture slides. 2015
  8. 8.0 8.1 8.2 DAS, Idrajit. HAZOP ANALYSIS. Last modification: 3 March 2012. Last view: 22 September 2017. https://fr.slideshare.net/AnandKumar87/hazop-analysis
  9. FLAUS, Jean-Marie. Risk analysis, Chapter 10: Deviation analysis using the HAZOP method. ISTE, Wiley. 2013
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