A HAZOP, or Hazard and Operability Study is a structured and systematic method used to identify hazards and potential operability problems by examining how a process or operation could deviate from its intended design.
A HAZOP study does not begin by asking only, “What could go wrong?”
Instead, a multidisciplinary team systematically examines sections of a process and asks how specific process parameters could deviate from the design intent. Guide words are used to generate these deviations, after which the team identifies possible causes, consequences, existing safeguards and any actions required.
For example, instead of reviewing a reactor simply as a whole, a HAZOP team may examine whether there could be:
- No flow
- More flow
- Less flow
- Reverse flow
- Higher temperature
- Lower temperature
- Higher pressure
- Lower pressure
The consequences of each credible deviation are then considered.
This structured approach is what makes HAZOP one of the most widely recognised techniques for identifying process hazards and operability issues. IEC 61882 provides an application guide for HAZOP studies using guide words and covers study preparation, examination, documentation and follow-up.
What Does HAZOP Stand For?
HAZOP stands for:
Hazard and Operability Study
The two parts of the name are equally important.
Hazard
A hazard review considers deviations that could result in:
- Fire
- Explosion
- Toxic release
- Loss of containment
- Chemical exposure
- Overpressure
- Equipment damage
- Environmental impact
- Injury or fatality
Operability
HAZOP is not limited to major accident hazards.
It also identifies conditions that could make the process difficult to operate as intended.
Examples may include:
- Unstable process control
- Frequent alarms
- Product quality problems
- Difficult startup or shutdown
- Process interruptions
- Equipment reliability concerns
- Maintenance difficulties
This is why a HAZOP should not be described simply as a checklist for identifying accidents.
It is a structured examination of deviations from design intent and their potential hazards and operability consequences. IEC 61882 identifies both hazards and operability problems as part of the technique.
How Does a HAZOP Study Work?
The fundamental principle of a HAZOP is straightforward.
The process is divided into manageable sections, commonly referred to as nodes.
For each node, the team defines the design intent.
A process parameter is then combined with an appropriate guide word to generate a deviation.
For example:
Parameter: Flow
Guide word: No
Deviation: No Flow
The team then considers:
- What could cause the deviation?
- What could happen if it occurs?
- What safeguards already exist?
- Is the risk adequately controlled?
- Is an action or further study required?
This process is repeated systematically across the defined nodes.
A typical HAZOP worksheet may therefore include:
| Item | Description |
|---|---|
| Node | Section of the process being examined |
| Design Intent | Intended process operation |
| Parameter | Process variable being examined |
| Guide Word | Word used to generate a deviation |
| Deviation | Departure from design intent |
| Causes | Credible causes of the deviation |
| Consequences | Potential outcomes |
| Safeguards | Existing protective measures |
| Recommendations | Required actions or further review |
The exact worksheet format may differ between organisations, but the underlying methodology remains the systematic examination of deviations from design intent.
What Are HAZOP Nodes?
A node is a defined section of a process selected for detailed examination.
Depending on the complexity of the plant, a node may represent:
- A section of piping
- A vessel
- A reactor
- A pump system
- A heat exchanger
- A transfer line
- A storage system
- A defined operating step
Nodes should be selected so that the design intent of the section can be clearly understood and the study can be performed effectively.
A node that is too large can make the discussion unfocused.
A node that is too small can make the study unnecessarily inefficient.
The objective is to define logical sections that allow the team to examine meaningful deviations systematically.
What Are HAZOP Guide Words?
Guide words are one of the defining features of a HAZOP study.
They are applied to process parameters to generate deviations from the design intent.
Common guide words include:
| Guide Word | Typical Meaning |
|---|---|
| No / Not | Complete absence of the design intent |
| More | Quantitative increase |
| Less | Quantitative decrease |
| As Well As | Qualitative addition |
| Part Of | Qualitative reduction |
| Reverse | Logical opposite or reverse direction |
| Other Than | Complete substitution or unexpected condition |
The applicability of individual guide words depends on the process and parameter being examined.
The guide words are not intended to generate an unlimited list of unrealistic scenarios.
They are used to stimulate a disciplined examination of credible deviations.
IEC 61882 is specifically based on the use of guide words to examine systems and provides guidance on their application within the HAZOP procedure.
HAZOP Parameters and Deviations
Guide words are normally combined with process parameters.
Common parameters include:
- Flow
- Pressure
- Temperature
- Level
- Composition
- Phase
- Reaction
- Time
The combination generates a deviation.
Example
Parameter: Temperature
Guide word: More
Deviation: High Temperature
The HAZOP team then asks:
What could cause high temperature?
Possible causes may include loss of cooling, excessive heat input or an uncontrolled reaction, depending on the specific process.
What could happen?
Possible consequences may include product degradation, excessive pressure, thermal decomposition or, in some processes, thermal runaway.
What safeguards are present?
The answer may include process control, alarms, shutdown functions or pressure relief, depending on the actual design.
Is further action required?
The team records an action where existing safeguards or information are not considered adequate.
The important point is that a real HAZOP study evaluates the actual plant design and operating conditions. Generic example causes should never simply be copied into a HAZOP worksheet without technical review.
A Simple HAZOP Example
Consider a pump transferring a flammable liquid from a storage tank to a downstream process.
Design Intent
Transfer liquid from Tank A to Reactor B at the required flow rate.
Parameter
Flow
Guide Word
No
Deviation
No Flow
Possible Causes
Depending on the actual system, credible causes could include:
- Pump failure
- Power failure
- Closed isolation valve
- Blocked pipeline
- Empty upstream tank
Possible Consequences
The consequences depend on the wider process.
For example, loss of feed could disrupt downstream operation, cause a process upset or contribute to another hazardous deviation.
Existing Safeguards
Potential safeguards could include:
- Low flow alarm
- Pump monitoring
- Interlocks
- Operator response
Recommendation
If the team identifies a credible consequence that is not adequately addressed, it may recommend a design change, further analysis or an additional safeguard.
This example demonstrates an important limitation of simplified online HAZOP examples:
The same deviation can have very different consequences depending on the process.
A HAZOP study must therefore be based on actual process information rather than generic templates.
What is the HAZOP Methodology?
A HAZOP study is typically performed through four broad stages:
1. Definition
The scope and objectives of the study are established.
This may include defining:
- Process boundaries
- Systems included in the study
- Interfaces
- Objectives
- Assumptions
- Required documentation
- Study team
2. Preparation
Relevant technical information is collected and reviewed.
Depending on the project, this may include:
- Piping and Instrumentation Diagrams
- Process Flow Diagrams
- Process descriptions
- Design basis documents
- Cause and Effect diagrams
- Operating procedures
- Equipment data
- Relief system information
- Control narratives
The quality of the HAZOP is strongly influenced by the quality and maturity of the information being reviewed.
3. Examination
The multidisciplinary team systematically examines each node.
Guide words and parameters are used to identify deviations.
For each deviation, the team evaluates:
- Causes
- Consequences
- Existing safeguards
- Required actions
This is the main examination stage of the HAZOP.
4. Documentation and Follow Up
The study findings are documented.
Recommendations should then be assigned, tracked and formally closed through an appropriate action management process.
A HAZOP that identifies actions but does not ensure appropriate follow-up is incomplete from a practical risk management perspective.
IEC 61882 specifically addresses HAZOP study definition, preparation, examination sessions, resulting documentation and follow-up.
Who Should Be Part of a HAZOP Team?
A HAZOP is intended to benefit from multidisciplinary knowledge.
The team composition depends on the facility and process being studied, but may include representatives from:
- Process engineering
- Operations
- Process safety
- Instrumentation and control
- Mechanical engineering
- Electrical engineering
- Maintenance
- Project engineering
- Other relevant technical disciplines
The HAZOP should normally be led by a competent and appropriately experienced facilitator or chair.
The value of a multidisciplinary team is that different participants understand different aspects of how the plant operates and how failures may occur.
For example:
The process engineer may understand the intended process behaviour.
The operator may identify practical operating deviations.
The instrumentation engineer may understand control and shutdown arrangements.
The maintenance representative may identify failure mechanisms or maintenance-related concerns.
A HAZOP should therefore not be treated as a document review completed by one individual in isolation.
The effectiveness of the study depends significantly on competent facilitation, appropriate technical participation and accurate process information.
When Should a HAZOP Study Be Performed?
HAZOP can be applied at different stages of a process or facility lifecycle.
During Process Design
A HAZOP can identify hazards and operability issues before the plant is constructed or commissioned.
The study is often performed when the design has sufficient technical definition to allow meaningful examination.
Before Commissioning
A review may be required to confirm that relevant HAZOP actions and safety requirements have been appropriately addressed before startup.
During Process Modification
Changes to process conditions, equipment, piping, control systems or operating philosophy can introduce new hazards.
The appropriate level of HAZOP or other hazard review should be determined through the organisation’s Management of Change process.
The UK’s HSE notes that the degree of HAZOP required for plant modifications should take account of the extent of the change, associated hazards and previous studies.
For Existing Facilities
Existing plants may require HAZOP review where:
- Significant modifications occur
- New chemicals are introduced
- Operating conditions change
- Process knowledge changes
- Previous assumptions are no longer valid
- A major incident or near miss identifies previously unrecognised hazards
What Is the Difference Between HAZOP and HIRA?
HAZOP is one technique within the broader field of Hazard Identification and Risk Analysis.
CCPS describes HIRA as a collective term covering activities used to identify hazards and evaluate risk throughout a facility’s lifecycle. It includes techniques ranging from qualitative hazard identification methods to detailed quantitative risk analysis.
In simple terms:
HIRA: The broader discipline and range of activities used to identify hazards and evaluate risk.
HAZOP: A structured qualitative technique used to identify hazards and operability problems through deviations from design intent.
HAZOP is therefore not a replacement for every other risk assessment technique.
The appropriate method depends on the decision being supported.
HAZOP vs LOPA
HAZOP and LOPA are closely connected, but they have different purposes.
HAZOP
HAZOP identifies deviations, causes, consequences and potential hazards.
It asks:
What could go wrong if the process deviates from its design intent?
LOPA
LOPA evaluates a defined cause consequence scenario and examines whether sufficient risk reduction is provided by qualifying protection layers. Learn more about Layer of Protection Analysis (LOPA).
It asks:
Is the identified risk adequately controlled, and is additional risk reduction required?
CCPS places HAZOP within qualitative hazard identification techniques and LOPA within more quantitative or semi quantitative risk analysis tools.
A natural relationship is therefore:
HAZOP identifies a hazardous scenario
↓
Further analysis determines whether additional risk assessment is required
↓
LOPA may evaluate the risk and protection layers
↓
A required SIL may be determined where an additional Safety Instrumented Function is needed
This does not mean every HAZOP deviation automatically requires a LOPA or SIL assessment.
The need for further analysis depends on the scenario and the organisation’s risk assessment process.
HAZOP vs What If Analysis
Both HAZOP and What If analysis can be used to identify hazards.
However, the approaches differ.
HAZOP
Uses a structured combination of:
- Nodes
- Design intent
- Parameters
- Guide words
- Deviations
What If Analysis
Uses structured questions such as:
- What if the pump fails?
- What if the wrong material is added?
- What if cooling is lost?
What If analysis can be efficient for certain applications, but a conventional HAZOP provides a more systematic guide word based examination of deviations.
The appropriate technique depends on the scope, process complexity and objective of the study.
What Are the Main Benefits of a HAZOP Study?
A well conducted HAZOP can help organisations:
Identify Hazards Before an Incident Occurs
The objective is to identify credible hazardous scenarios before they result in an incident.
Improve Process Operability
The study can identify conditions that may lead to:
- Difficult control
- Unstable operation
- Startup problems
- Shutdown problems
- Frequent process interruptions
Improve Design Decisions
Recommendations can identify opportunities to improve:
- Process design
- Instrumentation
- Alarms
- Interlocks
- Shutdown arrangements
- Operating procedures
Support Further Risk Assessment
HAZOP findings can provide the basis for:
- LOPA
- QRA
- SIL determination
- Consequence analysis
- Chemical reaction hazard assessment
Improve Cross Functional Understanding
The HAZOP process brings different technical disciplines together to examine how the plant behaves under abnormal conditions.
Common HAZOP Mistakes
Starting the Study with Incomplete Design Information
A HAZOP performed on poorly defined or outdated drawings can produce unreliable conclusions.
The technical information should be sufficiently mature and representative of the process being examined.
Treating HAZOP as a Checklist Exercise
A HAZOP is not simply about filling every row in a worksheet.
The team needs to understand the process and meaningfully evaluate credible deviations.
Including Only One Discipline
Process hazards often arise from interactions between process conditions, equipment, control systems and human actions.
A multidisciplinary team is therefore important.
Treating Every Safeguard as Independent Protection
The HAZOP may record safeguards, but whether a safeguard qualifies for risk reduction credit in a subsequent LOPA is a separate question.
Independence and other IPL criteria need to be considered.
Writing Recommendations Without Clear Ownership
Actions should be specific enough to be understood, assigned and tracked.
Closing the HAZOP Without Closing the Actions
The purpose of a HAZOP is not merely to identify issues.
Appropriate actions need to be managed through completion and closure.
The UK’s HSE emphasises the importance of implementing actions arising from HAZOP and other safety reviews.
What Information Is Required for a HAZOP Study?
Depending on the project, the HAZOP team may require:
- P&IDs
- Process Flow Diagrams
- Process descriptions
- Equipment data
- Design basis information
- Cause and Effect diagrams
- Control philosophy
- Alarm philosophy
- Operating procedures
- Relief system information
- Chemical and process safety information
- Previous hazard studies
- Relevant incident history
- Proposed modification details
The exact requirements depend on the stage and scope of the study.
Is HAZOP a Risk Assessment?
HAZOP is primarily a structured hazard identification and operability analysis technique.
It identifies deviations, causes and consequences and records safeguards and actions.
However, a conventional HAZOP does not necessarily provide a quantitative estimate of incident frequency or consequence risk.
The UK’s HSE notes that conventional HAZOP identifies potential hazards but does not by itself provide incident likelihood or loss estimates, and further hazard or risk analysis may be required to determine whether identified risk is acceptable.
This is why HAZOP may be followed by other techniques such as:
- LOPA
- QRA
- Fault Tree Analysis
- Event Tree Analysis
- SIL determination
The required level of analysis should be appropriate to the hazard and the decision being made.
How INDSAFE Can Support HAZOP Studies
INDSAFE can support organisations with structured process safety studies aimed at identifying process hazards, operability issues and potential improvement actions.
Depending on project requirements, HAZOP activities can be integrated with:
- Process Hazard Analysis
- Layer of Protection Analysis
- SIL determination
- Quantitative Risk Assessment
- Chemical Reaction Hazard Assessment
- Process Safety Management
- Hazardous Area Classification
The objective is not simply to produce a HAZOP worksheet.
The objective is to help organisations understand credible deviations, evaluate the resulting consequences and identify appropriate actions for risk reduction and improved operability.
The scope, methodology and supporting studies should always be tailored to the process, lifecycle stage and project requirements.
Frequently Asked Questions About HAZOP
What is a HAZOP study?
A HAZOP study is a structured and systematic technique used to identify hazards and operability problems by examining deviations from the intended design or operation of a process.
What does HAZOP stand for?
HAZOP stands for Hazard and Operability Study.
What are HAZOP guide words?
HAZOP guide words are words used with process parameters to generate deviations from design intent. Common examples include No, More, Less, As Well As, Part Of, Reverse and Other Than.
What is a HAZOP node?
A HAZOP node is a defined section of a process selected for systematic examination.
Who participates in a HAZOP?
A HAZOP is typically conducted by a multidisciplinary team that may include process engineering, operations, process safety, instrumentation and control, maintenance and other relevant disciplines.
Is HAZOP the same as LOPA?
No.
HAZOP is primarily used to identify hazards and operability problems.
LOPA evaluates defined scenarios and examines whether sufficient risk reduction is provided by qualifying protection layers.
Does every HAZOP deviation require a recommendation?
Not necessarily.
Recommendations are typically raised where further action, clarification, analysis or improvement is required. Many deviations may already be adequately addressed by the design or existing safeguards.
Is HAZOP required after every modification?
Not necessarily.
The appropriate level of review should be determined through the organisation’s Management of Change process, taking account of the nature and extent of the modification and associated hazards.
Is HAZOP a quantitative risk assessment?
No.
HAZOP is generally used as a qualitative hazard identification and operability analysis technique. Further analysis may be required where quantitative or semi quantitative risk evaluation is necessary.
Key Takeaway
A HAZOP study is a structured way of examining how a process can deviate from its intended design and what could happen as a result.
Its strength comes from the combination of:
Clear design intent
Logical process nodes
Guide words and parameters
Multidisciplinary expertise
Systematic examination
Effective action follow-up
HAZOP is not a substitute for every type of process safety analysis. Instead, it is often a foundational hazard identification study that can support further assessments such as LOPA, QRA and SIL determination.
For organisations handling hazardous processes, a well planned and competently facilitated HAZOP can identify both major process hazards and practical operability problems before they develop into incidents or costly process disruptions.
Technical References
International Electrotechnical Commission
IEC 61882:2016, Hazard and Operability Studies (HAZOP Studies), Application Guide
IEC 61882 provides guidance on HAZOP studies, including guide words, application, study preparation, examination sessions, documentation and follow-up.
Center for Chemical Process Safety, AIChE
Guidelines for Hazard Evaluation Procedures
A recognised CCPS reference covering process hazard evaluation methodologies.
Center for Chemical Process Safety, AIChE
Introduction to Hazard Identification and Risk Analysis
CCPS places HAZOP within the broader range of hazard identification and risk analysis techniques.