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The hazard and operability (HAZOP) study is the most commonly used process hazard analysis (PHA) method in the world today. It is one of the techniques commonly accepted by regulators. The HAZOP method identifies deviations from design intent by applying guide words, such as No, More and Less, to aspects of the design intent (such as flow, temperature, pressure, addition, reaction, etc.) within parts of the process, called nodes, such as lines and vessels. A team of people then brainstorms causes of each deviation within each node and identifies the sequence of events that results, including safeguards that protect against them, and the consequences. Each sequence of events represents a scenario. Also, the team may identify improvements to reduce risk. The flowchart in the figure illustrates the process of conducting a HAZOP study.

hazop flow chart

HAZOP Study Nodes

Processes must be divided into sections for detailed review. In HAZOP studies, nodes are used. Generally, they are defined as pipe sections and vessels in which process chemicals are, or may be, present. This is called the “line-by-line” HAZOP method. Nodes also may be steps in a procedure for a study on procedures, or process functions, e.g. control loops for a study on control systems.

Some companies use combinations of lines and major vessels as nodes, “super nodes”, to speed up studies. This approach provides a broader view of a process but it complicates the analysis and scenarios may be missed.

Typically, size is not an important attribute of a node for piping but it does influence whether a vessel is noded. For example, a sample pot is not normally a node but a knock-out pot could be. Nodes cannot be defined uniquely for a process. However, equivalent HAZOP study results can be expected with alternative subdivisions, although that has not been proven empirically.

Noding starts at the beginning of process. Each major vessel within the study scope is identified. Starting with the first major vessel, each of its inlet lines in the main process flow path to the vessel are noded starting with the main inlet line. Next the vessel is noded. Then each outlet line in the main process flow path is noded. This process is repeated for all vessels in the primary process flow path. Vessels and lines in side streams and other process flow paths are noded at any time in the noding process that makes sense.

Global nodes are used to represent the whole process or certain aspects of it. They serve several purposes:

  • Global nodes allow initiating events that affect more than one node to be addressed

Initiating events may affect the entire process or parts of it, for example, some external events such as flooding, or the global loss of utilities such as electric power. Thus, they may apply to multiple nodes or may not be specific to any particular node.

  • Specific issues that arise in more than one node / system

Examples include facility siting, human factors, and common piping issues.

  • Need to look at hazards from the perspective of the overall process

HAZOP study practitioners must avoid omitting scenarios that may not be identified by focusing on individual nodes. For example, multiple failure scenarios may involve causes originating from more than one node or system.

The study facilitator / leader usually prepares the node list for a process prior to commencement of a study.

HAZOP Study Design Intent

Design intent is the set of required or desired process behaviors, as intended by the process designers. The different aspects of design intent for a node are represented by parameters such as flow and pressure. Generally, numerous parameters are important for each node. The HAZOP method focuses on deviations from design intent because they represent potential problems, for example, lack of flow in a transfer line or overpressuring a vessel, that may result in hazard and operability scenarios.

There is no universal standard for what should be specified as part of the design intent for a process. Conceptually, the definition of design intent for a process may appear to be straightforward but practically it is challenging. Usually, the design intent for a process is complex with some aspects that are subtle. Of course, not all aspects of design intent need to be addressed in a HAZOP study but a determination needs to be made as to which aspects should be included. It is essential that HAZOP studies identify and consider all aspects of design intent for which deviations may result in scenarios within the scope and objectives of the study. Scenarios will be missed if the design intent is not defined fully and a complete set of deviations is not considered.

Unfortunately, HAZOP Study teams often define design intent simply by selecting process parameters from a checklist without full consideration of all key aspects of design intent. This practice likely results in missed scenarios. It is essential that study teams understand the meaning of design intent and consider all important aspects in studies to ensure that scenarios are identified as completely as possible. A preferred approach is to define the design intent for each process node as it is considered and extract parameters from it. This approach encourages a more complete treatment of design intent.

HAZOP Study Deviations

Deviations from design intent are generated by applying guide words to process parameters for each node:

Guideword + Parameter = Deviation

Usually, a standard set of guidewords is used (see table). For example, for an inlet line to a vessel, No + Flow = No Flow, or for a vessel, High + Pressure = High Pressure. The generation of such deviations is the key aspect of HAZOP studies yet mistakes are commonly made by practitioners.

The correct generation of deviations begins with an understanding of what is meant by each guideword. Their meanings are provided in the table.

 Table. HAZOP Guidewords.

NO (NOT, NONE) Negation of design intent No part of the intention is achieved but nothing else happens.
MORE (MORE OF) Quantitative increase The intention occurs in a way that is quantitatively greater. Usually applies to quantities, properties and activities.
LESS (LESS OF) Quantitative decrease The intention occurs in a way that is quantitatively lesser.
AS WELL AS (ALSO) Qualitative increase All of the intention is achieved together with something else.  
PART OF Qualitative decrease   Some of the intention is achieved but some is not.
REVERSE Logical opposite  The opposite of the intention happens. Often applies to activities.
OTHER THAN Complete substitution No part of the intention is achieved and something quite different happpens.

Deviations should not just be selected from a standard set in a rote manner because important deviations likely will be missed. The generation of deviations should be part of the creative process of HAZOP studies. The purpose of using guide words is to facilitate creative exploration of deviations from design intent which helps to increase the chances of study completeness.

It is important to understand that any conceivable deviation from design intent can be generated by applying one of the standard guidewords to a process parameter, which is the power of the HAZOP study method. The challenge is to ensure that all important deviations are considered for each node by fully defining the design intent and generating a complete set of deviations from it.

In generating deviations, most practitioners do not have a problem in applying the guidewords No, More, and Less to common parameters such as Flow and Pressure. They generate deviations that are obvious. However, the combination of some guidewords and parameters may not produce an obvious deviation. For example, while No Flow is an obvious deviation, As Well As Flow is not meaningful as it stands. Here, practitioners must ask “What else can happen as flow is occurring?” One answer is a chemical reaction (e.g. polymerization, a chemical reaction, is a concern for flowing monomers as it may cause pipe blockages). Thus, “As Well As Flow” can rephrased as the more meaningful “Chemical Reaction”.

Similarly, “As Well As Composition” can be rephrased to produce the more meaningful “Contamination”. The logic in this case is that in addition to whatever materials are intended to be present in a node, additional, unintended materials are present, i.e. contaminants, hence the deviation, “Contamination”. It is also possible to combine Other Than with Flow to generate “Chemical Reaction”. In this case, a chemical reaction occurs instead of flow rather than in addition to it.

By way of counterexample, More Flow would not be an appropriate way to generate Chemical Reaction because the guideword “More” implies a quantitative increase, not a qualitative change. Note that Flow and some other parameters have multiple characteristics which can be important. Thus, Flow may be Flow Rate or Flow Quantity depending on the circumstances in the process.

Some practitioners confuse deviations with causes. For example, in a procedural PHA study, a maintenance step may involve replacing a check valve. Consider the application of the guideword “Reverse” to this action. What deviation might be generated by applying Reverse to Replace Check Valve. A clear contender would be “Backwards Installation of the Check Valve”. Novice practitioners may suggest “Improper Maintenance” as an appropriate deviation in this situation but that is the cause of the backwards installation of the check valve, not a deviation.

Some practitioners confuse deviations with consequences. For example, in a procedural PHA study, a maintenance step may involve replacing a gasket. Consider the application of the guideword “Other Than” to this action. What deviation might be generated by applying “Other Than” to “Replace Gasket”? One important characteristic of a gasket is its specification. Thus, Incorrect Gasket Specification would be a meaningful deviation in this case. Novice practitioners may suggest “Leak” as an appropriate deviation in this situation is but that would be a consequence of an incorrect gasket being installed, not a deviation.

Some practitioners confuse deviations with other deviations. For example, in the case of the check valve replacement, the practitioner may be thinking in terms of the consequences of reverse installation and believe that the result would be to obstruct flow. They then theorize that the appropriate deviation is No Flow. As we have seen, this is incorrect. Deviations are departures from the aspect of design intent expressed by the parameter, not the consequences of a deviation. The correct treatment of this situation in a HAZOP study would be to identify reverse installation of a check valve in a line as a cause of No Flow in the node containing the check valve.

Another example of the incorrect application of Reverse to Replace Check Valve would be to assign the deviation “Backflow” to this combination. However, backflow is a deviation in its own right, typically generated by applying Reverse to Flow.

A key test in deciding which combination of guideword and parameter makes sense for a deviation is to identify the attribute or aspect of the process that is addressed by the deviation. The parameter should then be clear and the appropriate guide word can be confirmed by reviewing their meanings. For example, if Missing Component is being considered as a deviation, on reflection, it should be obvious that the attribute of the process that is involved is composition. If a component is missing, some of the intention is achieved but some is not. Thus, Part Of is the clear choice as the most appropriate guideword.

Similarly, in considering the application of “Part Of” to “Composition”, a practitioner may suggest “Incorrect ratio of materials” as the deviation. Certainly, composition is related to the ratio of materials. However, the actual parameter in this case is the ratio of materials, not composition. Thus, it should be clear that this deviation is best viewed as resulting from the combination of “Other Than” with “Ratio of Materials”. It is also possible that “More” or “Less” could be applied if the concern is with adjusting the ratio of materials upwards or downwards, and even “Reverse” if the concern is with reversing the ratio of two materials.

In generating deviations, it is important to understand that not all guidewords generate meaningful deviations for all parameters, for example, No Temperature is not meaningful. Also, the same deviation can be generated by applying different guidewords to different parameters, for example, As Well As Flow and Other Than Flow to generate Chemical Reaction. Furthermore, multiple deviations may exist for the same guide word / parameter combination, for example, As Well As Flow can generate both Chemical Reaction and Foaming.

Deviations should be generated logically and consistently in HAZOP studies to alleviate confusion and they should be generated completely to reduce the chances of missing scenarios and producing an incomplete study.

HAZOP Study Team

A HAZOP study is conducted by a team of individuals led by a person knowledgeable in the HAZOP technique. The interaction of the team members results in a more complete review than would be accomplished by each individual working separately on the same project.

The composition of HAZOP study teams is critical to the successful performance of studies. Typical teams are composed of people who together can provide the information needed to define the design intent for operating the process. A team typically consists of 3 - 8 individuals. One member is a person trained and knowledgeable in the HAZOP method. The other members usually are selected for their knowledge of the process operation and/or technical contribution to the team. There is no single perfect combination of team members. However, since the team members need to be knowledgeable of the process design and/or operation, most should come from the operating facility. A typical team may consist of people from these disciplines:

  • Team facilitation
  • Design
  • Process engineering
  • Operations
  • Maintenance
  • Control systems
  • Process safety

Both operations and maintenance personnel should participate in HAZOP studies. Deviations from maintenance intent are as important as deviations from operations intent. Both can result in hazard scenarios that may be missed without the knowledge provided by operations and maintenance personnel. Furthermore, a case can be made for having more than one person on the team from the same discipline to reflect different types of work, levels of experience, ways of performing jobs, attitudes, and behaviors which can have a major impact on the results of a HAZOP study. Sufficient people should be present during a study to provide complete and accurate information about the process under all conditions it experiences.

HAZOP studies benefit significantly from the contributions of a Control Systems Engineer who has detailed knowledge of the process instrumentation, controls, alarms and interlocks. The control scheme is a critical aspect of design intent and such knowledge is essential for the identification of deviations from control intent.

Chemical processes may pose hazards involving reactive chemicals. Identification of reactivity hazard scenarios requires that one or more team members have expertise in chemistry, a knowledge of the chemical processes being studied, and an understanding of chemical reactivity hazards. The challenge is greater than for the other major hazards of toxicity, flammability, and explosivity because chemical reactivity hazards are not as well recognized or understood, hazard scenarios involving them are more involved and harder to identify.

Specialty Team Members with technical expertise beyond that of regular team members may be needed when some topics are addressed, certain issues arise, or a particular part of the process is examined, for example, an electrical engineer, structural engineer, fire protection engineer, human factors specialist, or emergency response planner.

As HAZOP study practice has evolved, the value and importance of participation by personnel with other roles has emerged. A Quality Assurance (QA) Manager can take responsibility for alerting the facilitator to deviations from the purpose, scope, and objectives of the study and performance requirements established for the study so that corrective action can be taken at the time they occur. Also, the QA Manager is charged with being alert to omissions by the study team.

HAZOP studies rely on subjective judgment. Consequently, a Devil’s Advocate can be designated to challenge and debate the views of team members in order to help determine their validity. The Devil’s Advocate should encourage teams to look not just for evidence to confirm expressed views but also evidence to the contrary and also focus attention on differences rather than similarities between situations in order to reach more objective decisions.

HAZOP study teams should not consist entirely of people who know the process well. Groupthink can be a problem. It can be addressed by the participation of an Independent Senior Engineer who does not have any prior experience with the process being studied. Such a person can challenge assumptions made by other team members and contribute knowledge that may not be possessed by them.

The composition of PHA study teams is critical to the successful performance of studies. There is little value in compromising team composition for PHA studies when so much depends on their results. The actual composition of a team for a particular study will depend upon the study charter and the type of process being studied.

HAZOP Study Facilitators / Leaders

Study facilitators / leaders have many responsibilities. Increasingly, professional certifications are being required for professional skills. HAZOP study facilitators / leaders must know how to address many technical issues including:

  • Full definition of HAZOP design intent
  • Optimal subdivision of processes into nodes
  • Level of detail needed to ensure appropriate recommendations for risk reduction
  • How to capture human factors in hazard scenarios
  • Management of interpersonal interactions of team members during brainstorming
  • Impact of cognitive and motivational biases and fallacious reasoning by team members

Certification helps to ensure HAZOP study facilitators / leaders know how to handle such issues and are competent to conduct studies. Certification provides assurance that facilitators have met a defined standard of education, experience, practice, training and knowledge in leading HAZOP studies.

HAZOP Study Scribes

Often, HAZOP studies are recorded by an individual designated as a scribe or technical secretary. Theirs is an important role. Under the guidance of the team leader, they record PHA sessions and may draft the study report.

Many HAZOP study team leaders act as their own scribe. Use of HAZOP study recording software gives leaders a subtle but powerful form of control over the team. They can direct the team's attention to highlighted entries in the worksheet and readily display checklists and other documents. Of course, leaders who scribe should have reasonable keyboard skills and be comfortable with all the other responsibilities of leading studies so that scribing does not impair their performance as a leader.

Some HAZOP study team leaders prefer to use a separate scribe. Ideally, scribes should be technically oriented. They need a knowledge of processes in order to understand the discussions that occur during HAZOP studies. Also, they need to understand the process that is followed in performing a HAZOP study and be familiar with the technical terms and acronyms used. Sometimes, the role is filled by a junior engineer. Administrative personnel may be challenged by the role although administrative personnel with appropriate experience have acted successfully as scribes.

Scribes can help team leaders by noting suggestions made by team members and reminding the team leader, assisting with quality control checks, and managing checklists used in the study.

Requirements for scribes include:

  • Competency with HAZOP study recording software, including both use of the software and knowledge of its capabilities.
  • Ability to establish a good working relationship with the team leader. A good scribe can be a helper, not just a recorder.
  • Sufficient skill to be able to start typing as soon as a team consensus emerges without waiting for instruction or dictation by the team leader. However, some team leaders prefer to instruct the scribe to make entries.
  • Ability to record studies without slowing the progress of the study or interfering with the creative flow of discussion.

Attributes of good scribes include:

  • Attention to detail
  • Responsiveness
  • Listening skills
  • Typing skills
  • Spelling / grammar skills

Guidelines should be provided for scribes to ensure they do not diminish the quality of studies:

  • Listen to the team leader and the team discussions.
  • Follow the team leader's instructions.
  • Respond only to directions from the team leader.
  • Do not make worksheet entries or edits unless directed to do so by the team leader.
  • Try to anticipate needed entries.
  • Capture key parts of the discussions.
  • Do not try to be the leader.
  • Attend PHA training before scribing, if possible.
  • Know and understand guidelines for worksheet entries.
  • Learn to use recording software.
  • Do not play with the recording software.
  • Do not move around the worksheet, project, or software unnecessarily.

Recording is the first responsibility of a scribe. Participation as a team member is a secondary responsibility. Individuals whose undivided attention is needed in the study should not be assigned the role of scribe.

HAZOP Study Sessions

Processes often require many hours of study time. Therefore, HAZOP teams typically meet over a number of sessions. Sessions typically last from 3 - 6 hours with breaks included.

The actual time required for a HAZOP study depends on many factors including:

  • Skill and experience level of the team and the facilitator / leader
  • Size of the process
  • Complexity of the nodes and process
  • Purpose, scope and objectives of the HAZOP study
  • Team dynamics
  • Fluency of team members in a common language

Usually, the first few nodes take longer and novice teams typically take longer for the first few sessions. General estimates of the time taken for nodes depending on their complexity are:

Small / simple: 2 hours

Medium / moderate: 2 – 4 hours

Large / complex: 4 – 6 hours (or more)

HAZOP Study Documentation

At each session, scenarios that are identified by the study team are recorded in worksheets, often using custom software, such as PHAWorks RA Edition.

Worksheets serve several purposes including:

  • Checking by the facilitator / leader and team members after each session
  • Generation of necessary actions during a study
  • Reference by team members during the study
  • Quality control review by peers and/or third parties
  • Generation of actions on recommendations on completion of the study
  • Review by interested parties on completion of the study, e.g. regulators
  • Revalidating studies

HAZOP Study Recommendations

HAZOP studies are intended to enable the development of recommendations for risk reduction to tolerable levels. These action items typically are enhancements to existing safeguards or new safeguards. Typically, the need for them is based on determining the risk of scenarios using risk matrices or layers of protection analysis considering existing safeguards.

In some cases, information may be unavailable to decide if a potential problem exists. In such cases where an information need exists, the HAZOP study team should note the issue, assign someone to collect the needed information, and continue with the study. Once the information becomes available, the study team can resolve the issue.

Latest HAZOP Study Developments

Some of the latest developments with the HAZOP method are described in these publications:

Analytical Methods in Process Safety Management and System Safety Engineering – Process Hazards Analysis, in Handbook of Loss Prevention Engineering, Wiley-VCH, 2013.

The validity of engineering judgment and expert opinion in hazard and risk analysis: The influence of cognitive biases. Process Safety Progress, Volume 37, pages 205-210, Issue 2, June 2018.

Key issues in performing hazard and operability (HAZOP) studies, Loss Prevention Bulletin, Issue 257, pages 26-28, October, 2017.

Simultaneous Operations (SIMOPS) Review: An important hazard analysis tool, Process Safety Progress, Volume 36, Issue 1, pages 62–66, March 2017.

Cognitive biases in process hazard analysis, Journal of Loss Prevention in the Process Industries, Volume 43, pages 372-377, September, 2016.

Guidelines for addressing limitations in the performance of HAZOP studies, Loss Prevention Bulletin, Issue 250, pages 21 - 24, August, 2016.

Design intent for hazard and operability (HAZOP) studies, Process Safety Progress, Volume 35, Issue 1, pages 36–40, March 2016.

PHA Team Member Roles That May Be Overlooked, Loss Prevention Bulletin, Issue 247, February 2016.

The treatment of domino effects in PHA, Process Safety Progress, Volume 34, Issue 3, pages 220–227, September 2015.

Chemical Reactivity and Hazard and Operability (HAZOP) Studies, Loss Prevention Bulletin, Issue 244, August, 2015.

The importance of defining the purpose, scope, and objectives for process hazard analysis studies, Process Safety Progress, Volume 34, Issue 1, pages 84 - 88, March, 2015.

Consider Chemical Reactivity in Process Hazard Analysis, Chemical Engineering Progress, Vol. 111 (1), pages 25 - 31, January 2015.

Competency requirements for process hazard analysis (PHA) teams, Journal of Loss Prevention in the Process Industries, Volume 33, pages 151-158, January 2015.

A critique of the hazard and operability (HAZOP) study, Journal of Loss Prevention in the Process Industries, Volume 33, Pages 52-58, January 2015

Requirements for improved process hazard analysis (PHA) methods, Journal of Loss Prevention in the Process Industries, Volume 32, Pages 182–191, November 2014.

Initiating events, levels of causality, and process hazard analysis, Process Safety Progress, Vol. 33, Issue 3, pages 217–220, September 2014.

The role of people and human factors in performing process hazard analysis and layers of protection analysis, J. of Loss Prevention in the Process Industries, Vol. 26, pages1352-1365, 2013.

Treatment of multiple failures in process hazard analysis, Process Safety Progress, Vol. 32, Issue 4, pages 361–364, December 2013.

On the Validation of Safeguards for Process Hazard Analysis, Process Safety Progress, Volume 32, Issue 2, pages 165–169, June 2013.

What risk reduction measures should be credited in process hazard analysis?, Process Safety Progress, Vol. 31, No. 4, pps 359–362, December, 2012.

Process hazard analysis for phases of operation in the process life cycle, Process Safety Progress, Vol. 31, No. 3, pages 279–281, September, 2012.

Conducting process hazard analysis to facilitate layers of protection analysis, Process Safety Progress, Vol. 31, No. 3, pps 282–286, September, 2012.

Of Interest:  

Primatech offers consulting services for HAZOP studies including facilitation, scribing, reviewing, and mentoring and coaching.

Primatech offers multiple training courses that provide instruction in different aspects of performing HAZOP studies including:

Primatech offers software products that assist in the use of the HAZOP method including:

Primacert offers certification of competency for HAZOP facilitators.

Primatech can assist you with all your needs for HAZOP studies. Please contact us for further information.

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