A process design review checks whether a design is technically correct and complete, while a HAZOP study asks what could go wrong with a design that has already been developed in enough detail to examine. The design review is a verification activity that tests the design basis, calculations, equipment selection, drawings and compliance with codes and project requirements. The HAZOP is a structured hazard and operability study that applies guide words to parameters, node by node, to find deviations from design intent, their causes, their consequences and the adequacy of safeguards. A design review asks "is the design right?", and a HAZOP asks "what happens if the process does not behave as intended?"

The two are often confused because both involve engineers gathering around the same drawings, and both can produce recommendations for design changes. They serve different purposes, use different methods and happen at different points in a project. Treating one as a substitute for the other leaves gaps: a design can pass a review of its calculations and still contain an unexamined hazard, and a HAZOP cannot reliably catch an error in a heat balance or a mis-sized line.

This article explains what each activity is, how they differ in purpose, method, timing, team and output, how they work together, and where practitioners disagree. The term "process design review" is used in several ways across industry, so the definition used here is stated explicitly. The guidance is general and should be adapted to company procedures and any applicable standards.

What Is a Process Design Review?

A process design review is a structured examination of the process design to verify that it meets the design basis, applicable codes and standards, and project requirements. It is usually led by the design organisation, often with input from other disciplines and the client.

The term covers a family of activities, sometimes carried out under different names such as design verification, engineering review, P&ID review or peer review. In this article, it means a review focused on technical correctness and completeness of the design, not a hazard identification workshop.

What Does a Process Design Review Typically Examine?

A process design review typically examines the design basis and operating philosophy, process flow diagrams, heat and material balances, equipment sizing and specification, piping and line sizing, control philosophy, relief and blowdown basis, utilities, materials of construction, and the consistency between drawings and data sheets. It also checks compliance with codes, standards and project specifications.

Who Takes Part in a Design Review?

Participants are usually discipline engineers, checkers, lead engineers and sometimes client or operations representatives. Reviews may be internal to the design team or carried out with the client at defined stages. The format can range from a drawing check by one engineer to a formal multi-discipline review meeting.

When Do Design Reviews Happen?

Design reviews typically happen at several points in a project, such as concept, front-end engineering design and detailed design, and again before issuing drawings for construction. They are repeated as the design matures, which is different from a HAZOP, which is usually timed for when the design is detailed enough to examine line by line.

What Is a HAZOP Study?

A HAZOP study is a structured, team-based examination of how a process could deviate from design intent, what could cause the deviation, what the consequences would be and what safeguards exist. It is described in the IEC 61882 application guide and in CCPS hazard evaluation guidance.

The team divides the process into nodes, states the design intent of each, and applies guidewords such as no, more, less, as well as, part of, reverse and other than to parameters such as flow, pressure, temperature and level. The result is a worksheet of deviations, causes, consequences, safeguards, risk ratings and recommendations.

Who Takes Part in a HAZOP?

A HAZOP is led by a trained facilitator with a scribe and a multidisciplinary team that typically includes process or design engineering, operations, instrumentation and control, maintenance or mechanical engineering and safety. Operations input is particularly important, because it brings real-world knowledge of how the plant behaves.

When Does a HAZOP Happen?

A HAZOP is typically carried out once the P&IDs and other process safety information are mature enough to be examined in detail, usually during detailed design. It is repeated or revalidated for existing plants and after significant modifications.

What Is the Main Difference Between HAZOP and a Process Design Review?

The main difference is purpose. A design review verifies that the design is correct against requirements, whereas a HAZOP investigates hazards and operability problems that arise when the process departs from its intended behaviour. One checks conformity, and the other probes failure and deviation.

How Do They Differ in Method?

A design review typically follows checklists, calculation checks, drawing mark-ups and comparison against specifications and standards. A HAZOP follows a guideword-driven method with node selection, design intent statements and a worksheet that records causes, consequences and safeguards. The design review is largely about checking against known requirements, while the HAZOP is about imagining what could go wrong.

How Do They Differ in Focus?

A design review focuses on whether the design meets the basis and requirements, including sizing, compliance and constructability. A HAZOP focuses on process hazards and operability: abnormal conditions, equipment and instrument failures, human response, start-up, shutdown and interactions between systems.

How Do They Differ in Team and Atmosphere?

Design reviews are often led by the design organisation, with discipline engineers who know the design well. HAZOPs benefit from an independent facilitator and from participants who can challenge the design, including operations staff. A HAZOP discussion deliberately asks uncomfortable questions about the design, which is a different dynamic from verification.

How Do They Differ in Output?

A design review produces comments, mark-ups, corrected calculations and an action list for the design team. A HAZOP produces a worksheet and report with deviations, causes, consequences, safeguards, risk ratings and recommendations, with named owners and dates for follow-up.

How Do They Differ in Timing?

Design reviews are repeated across project stages and often continue until drawings are issued. A HAZOP is typically scheduled once, at a defined maturity point, and then updated for changes. Running a HAZOP on immature drawings wastes time because the design keeps changing, and running design reviews only after the HAZOP is too late to catch basic errors cheaply.

How Do HAZOP and Process Design Review Work Together?

They work together in sequence: design reviews help ensure the design is mature and correct enough for a productive HAZOP, and HAZOP findings feed back into design changes that are then reviewed again. Each strengthens the other.

How Does a Design Review Prepare for a HAZOP?

A good design review improves the quality of the drawings, resolves obvious errors, confirms line numbers, equipment data and relief information, and helps the team agree the design intent for each node. This reduces the time a HAZOP team wastes on drawing discrepancies.

How Does a HAZOP Feed Back into Design?

HAZOP recommendations may require additional instrumentation, changes to relief systems, revised control logic or procedural changes. These changes should go through the project's change process and be reviewed by the design team to confirm that they are correct and do not introduce new problems.

Does a HAZOP Replace Design Verification?

No. A HAZOP is not designed to check calculations, confirm equipment sizing or verify code compliance. Relying on a HAZOP to catch design errors is a common misunderstanding, and it can leave gaps in verification.

Does a Design Review Replace a HAZOP?

No. A design review that confirms the design meets requirements does not necessarily explore deviations, multiple causes, operator response or interactions between systems. Those are the strengths of a HAZOP.

What Other Reviews Sit Alongside HAZOP and Design Review?

Several other studies complement both, and the project's safety and engineering plan should say which will be used and when. Their names and scopes vary between organisations.

What Is a HAZID?

A hazard identification study, often called HAZID, is typically carried out earlier in a project at a higher level, to identify major hazards, concept-level risks and design options before detailed drawings exist. It is broader and less detailed than a HAZOP.

What About Safety Integrity and Protection Layer Studies?

Where safety instrumented functions are used, studies such as SIL determination, and layer of protection analysis for higher-risk scenarios, may follow from HAZOP findings. These studies quantify or assess the adequacy of protection in more detail.

What About Constructability, 3D Model and Operability Reviews?

Projects often include model reviews, constructability reviews and operability or maintainability reviews. These check layout, access, isolation, maintainability and human factors, and they complement both the design review and the HAZOP.

What Are the Common Mistakes When Comparing the Two?

Common mistakes include treating the HAZOP as a design check, scheduling it too early, ignoring operations input, or treating design review sign-off as proof that hazards have been examined. Avoiding these mistakes improves both activities.

Why Is Running a HAZOP Too Early a Problem?

Because immature drawings change, and the team ends up re-examining nodes or analysing a design that no longer exists. Wait until the P&IDs are stable enough for a line-by-line review, while leaving time to implement recommendations.

Why Is Skipping Operations Input a Problem?

Because operators and maintenance staff understand real behaviour, workarounds and failure modes. Their absence reduces the realism of the study and the credibility of safeguards.

Why Is Treating Review Sign-Off as Proof of Safety a Problem?

Because conformity with requirements does not mean that every hazard has been examined. Requirements themselves may not anticipate every scenario.

What Are the Different Views on HAZOP and Design Review?

Practitioners agree that both are necessary, but they differ on how to sequence and resource them. These views reflect project size, risk and organisational culture.

What Do Advocates of Strong Early Design Review Say?

Advocates argue that investing in thorough design reviews before the HAZOP reduces rework, shortens HAZOP sessions and allows the team to concentrate on real hazards. They see the design review as the foundation of quality.

What Do Advocates of Early and Repeated Hazard Studies Say?

Others argue that hazard studies, including HAZID and preliminary HAZOP, should begin early so that fundamental design choices reflect safety considerations, since late changes are costly. They support multiple hazard review stages through the project.

What Do Pragmatists Say?

Pragmatists argue that the labels matter less than clarity: define what each review is for, who attends, what inputs are needed and how outputs are tracked. A well-defined review plan avoids duplication and gaps.

FAQ: Common Questions About HAZOP and Process Design Review

Is a HAZOP a Type of Design Review?

Not exactly. Both examine the design, but a HAZOP is a hazard and operability study, while a design review is mainly a verification of correctness and compliance.

Which Comes First?

Design reviews occur throughout the project and should mature the design before the HAZOP, which is typically carried out once the P&IDs are detailed enough.

Can a HAZOP Find Design Errors?

It may reveal some, but it is not designed to verify calculations or code compliance, so it should not replace design verification.

Who Should Lead Each?

Design reviews are usually led by the design organisation or a lead engineer, while a HAZOP should be led by a trained, preferably independent, facilitator.

Does the Client Need to Attend Both?

Client and operations representatives often attend design reviews at key stages and should participate in the HAZOP, where their operating knowledge is particularly valuable.

Are There Standards for Each?

IEC 61882 describes HAZOP studies. Design review practices are usually defined by company or project procedures and applicable engineering codes.

Conclusion

A process design review and a HAZOP answer different questions and need each other. The design review makes sure the design is correct and complete, and the HAZOP tests whether the design stays safe and operable when things deviate.

Projects that plan both, sequence them sensibly, involve the right people and track recommendations to closure get better designs and safer plants. Define each review's purpose in your project plan, confirm the requirements of any standards or clients involved and adapt this guidance to your own procedures.