HAZOP vs QRA: Understanding the Key Differences in Process Safety

HAZOP vs QRA are complementary process safety studies that support the safe design, operation and modification of industrial facilities. HAZOP identifies process deviations, causes, consequences and existing safeguards, while QRA evaluates selected hazardous scenarios by estimating their frequency and modelling their effects on people, assets and surrounding areas. Understanding the difference helps project teams select the right study for safer design, regulatory compliance and risk-informed decision-making.

What Is a HAZOP Study?

A Hazard and Operability Study (HAZOP) is a structured Process Hazard Analysis method used to identify hazards and operational problems within a process system. It is conducted by a multidisciplinary team using process drawings, operating information and standard guidewords. The process is divided into sections called nodes, where deviations such as no flow, high pressure, low temperature, reverse flow and high level are reviewed. The team then evaluates their causes, consequences, existing safeguards and any additional risk-reduction measures required.

Key Objectives of a HAZOP Study

A HAZOP is performed to:

• Identify credible process hazards
• Detect operability and reliability concerns
• Review process deviations and abnormal conditions
• Evaluate the adequacy of existing safeguards
• Identify missing or ineffective protection measures
• Recommend practical risk-reduction actions
• Improve plant safety and operational performance

Typical HAZOP Deliverables

The study normally produces:

• HAZOP worksheets
• Node and design-intent register
• Hazard and scenario register
• Risk-ranked recommendations
• Action tracking register
• Marked-up process drawings
• Final HAZOP report

The deliverables provide a traceable record of the scenarios reviewed, the safeguards credited and the actions agreed by the study team.

What Is a Quantitative Risk Assessment?

A Quantitative Risk Assessment (QRA) is a numerical method used to evaluate the risk associated with credible hazardous events. It combines accident-frequency analysis with consequence modelling to assess the potential effects of fires, explosions, toxic releases and other major accident scenarios.

Unlike HAZOP, which identifies what may go wrong, QRA estimates how often an event may occur and how severe its consequences could be. The results are expressed through individual and societal risk, fatality frequencies, consequence distances and risk contours, supporting facility-layout decisions, land-use assessment, comparison with risk criteria and ALARP demonstration.

Key Objectives of a QRA Study

A QRA is performed to:

• Estimate the frequency of credible accident scenarios
• Quantify individual and societal risk
• Evaluate fire, explosion and toxic-release consequences
• Identify the main contributors to overall facility risk
• Compare calculated risk against acceptance criteria
• Assess the effectiveness of risk-reduction measures
• Support an ALARP demonstration
• Provide a numerical basis for engineering decisions

Typical QRA Deliverables

The assessment may include:

• Individual risk contours
• Individual Risk per Annum values
• Societal risk results
• FN curves
• Fire-radiation modelling
• Explosion-overpressure modelling
• Toxic-dispersion modelling
• Accident-frequency calculations
• ALARP assessment
• Risk-reduction recommendations
• Final QRA report

HAZOP vs QRA: Key Differences

The most important distinction is that HAZOP identifies hazards through structured technical discussion, whereas QRA calculates the risk associated with selected hazardous scenarios.

HAZOP vs QRA Workflow Comparison

Although the two studies are connected, their workflows are different.

Step	HAZOP Workflow	QRA Workflow
1	Define the study scope and boundaries	Identify credible major accident hazards
2	Review P&IDs and supporting documents	Develop representative accident scenarios
3	Divide the process into nodes	Assign release cases and failure frequencies
4	Define the design intent of each node	Perform frequency analysis
5	Apply relevant guidewords and deviations	Conduct fire, explosion and toxic modelling
6	Identify causes and consequences	Estimate individual and societal risk
7	Review existing safeguards	Compare results with risk-acceptance criteria
8	Raise recommendations where gaps exist	Evaluate additional risk-reduction measures
9	Record findings and prepare the report	Review assumptions, sensitivities and uncertainties
10	Track actions through formal closeout	Issue the QRA report and periodically update the study

The HAZOP workflow is centred on systematic team discussion. The QRA workflow is based on scenario selection, numerical modelling, frequency calculations and risk evaluation.

Applications of HAZOP

HAZOP is widely used in facilities where process deviations can lead to unsafe conditions, equipment damage or production interruption.

Typical applications include:

• Oil and gas processing facilities
• Refineries
• Petrochemical plants
• LNG facilities
• Offshore platforms
• Chemical manufacturing plants
• Fertiliser plants
• Pharmaceutical facilities
• Hydrogen-production projects
• Power-generation plants
• Water and wastewater treatment systems
• Food-processing facilities

It can be applied to new designs, existing plants, package units, expansion projects and modifications managed through Management of Change.

Applications of QRA

QRA is generally used where the magnitude and geographical extent of major accident risk must be quantified.

Common applications include:

• Major-hazard facilities
• LNG terminals and storage sites
• Oil refineries
• Offshore installations
• Petrochemical complexes
• Pipeline networks
• Tank farms
• Hydrogen-production and storage facilities
• Carbon-capture projects
• Bulk chemical-storage sites
• Hazardous-material transportation
• Facility siting and layout studies
• Regulatory and land-use planning assessments

QRA results may also support emergency planning, occupied-building assessment, risk-based separation distances and evaluation of proposed expansion projects.

Standards and Guidelines for HAZOP

HAZOP studies may be performed using recognised process safety and risk-management guidance.

Standard or Guidance	Application
IEC 61882	Guidance for applying the HAZOP methodology
IEC 61511	Functional safety requirements for the process industry
CCPS Guidelines	Good practice for Process Hazard Analysis and process safety
OSHA 29 CFR 1910.119	Process Safety Management requirements
API process safety guidance	Industry practices for managing process hazards
ISO 31000	General principles and framework for risk management

The exact standards applied depend on the project location, industry sector, client requirements and regulatory framework.

Standards and Guidelines for QRA

QRA methodology may be aligned with international risk-management, functional-safety and facility-siting guidance.

Standard or Guidance	Application
ISO 31000	General risk-management principles
IEC 61508	Functional safety of electrical and programmable systems
IEC 61511	Functional safety for Safety Instrumented Systems
API RP 752	Management of hazards associated with permanent process plant buildings
API RP 753	Management of hazards associated with portable buildings
CCPS Risk Analysis Guidelines	Methods for consequence, frequency and risk analysis
UK HSE Guidance	Risk criteria and ALARP principles

These references help establish consistent modelling methods, assumptions, risk criteria and documentation practices.

Which Study Should Be Conducted First?

In most projects, HAZOP is conducted before QRA because it systematically identifies process deviations, equipment failures, loss-of-containment events and other credible hazards. Selected HAZOP scenarios can then be carried forward into the QRA for frequency analysis, consequence modelling and calculation of individual and societal risk.

However, early QRA or consequence modelling may also be performed during concept or FEED stages to support facility layout and major design decisions. The final study sequence should therefore be selected according to the project stage, available information and required risk-based decisions.

How HAZOP and QRA Work Together

HAZOP vs QRA should not be treated as competing methods. They address different parts of the same risk-management process.

HAZOP provides:

• Structured identification of process deviations
• Cause-and-consequence scenarios
• Review of alarms, trips and mechanical safeguards
• Operability findings
• Engineering recommendations

QRA provides:

• Accident-frequency estimates
• Fire, explosion and toxic-release effects
• Individual and societal risk calculations
• Comparison against acceptance criteria
• Quantitative support for ALARP decisions

For example, a HAZOP may identify that a vessel could overpressure following a blocked outlet. QRA may then assess the frequency of loss of containment, model the resulting fire or explosion and determine the risk to personnel inside and outside the facility.

Together, the studies provide a stronger technical basis for design, operations and emergency preparedness.

Benefits of Conducting Both HAZOP and QRA

Using both methodologies can provide:

• More complete identification of process hazards
• Numerical understanding of major accident risk
• Better prioritisation of risk-reduction measures
• Improved facility-layout decisions
• Stronger support for ALARP demonstration
• Improved regulatory and stakeholder confidence
• Better emergency-response planning
• More effective allocation of safety investment
• Reduced likelihood of major accident events
• Improved design and operational reliability

HAZOP identifies where problems may occur, while QRA helps determine which scenarios make the greatest contribution to overall risk.

Conclusion

HAZOP identifies process hazards, deviations and safeguards through a qualitative team review, while QRA quantifies accident frequencies, consequences and overall risk levels. Used together, they provide a stronger basis for safer design, effective risk reduction and regulatory compliance.

Stepin Engineering provides practical process safety training to help engineers understand and apply HAZOP, QRA and related risk assessment methodologies.