What-If and HAZOP: How to Choose the Right Method
In the field of process safety, both What-If Analysis and HAZOP are established methods for hazard identification.
When working in process safety, one of the most practical questions we face is not just which method is better — HAZOP or What-If Analysis— but how to use each method effectively without losing sight of real hazards.
From my point of view, the two techniques are not in competition. If implemented correctly, both are capable of revealing critical deviations. What truly matters is the quality of the brainstorming and the engineering judgment behind the analysis.
This article presents real-world considerations based on field experience, and an example of a deviation that would have been hard to detect through a classic HAZOP approach alone.
Using guide words is valuable — but it must not replace critical thinking.
In HAZOP, the structured use of deviations can support a systematic analysis, but it should never lead engineers to limit themselves to predefined combinations.
On the contrary, asking questions outside the guide word structure is sometimes essential to uncover potential hazards that would otherwise be missed.
This is where the What-If Analysis technique can offer flexibility. By encouraging open-ended brainstorming, it often fosters a broader exploration of abnormal scenarios — especially useful when analyzing process changes or non-standard conditions.
This article explores how to choose between What-If and HAZOP for process safety, highlighting their strengths and limitations.
Before we examine this example, let’s briefly recall the definitions of HAZOP and What-If analysis — to understand their scope and limitations.
What is a HAZOP?
HAZOP (Hazard and Operability Study) is a structured and systematic technique used to identify potential hazards and operability problems in industrial processes. It is particularly suited for complex, continuous processes and is most effective when applied to well-defined systems, such as detailed P&IDs.
The HAZOP methodology is based on the use of guide words (e.g., NO, MORE, LESS, REVERSE) that are applied to each process parameter (such as flow, pressure, or temperature) to explore possible deviations from normal operation. For each deviation, the team identifies:
- Possible causes
- Potential consequences
- Existing safeguards
- Recommendations for further risk reduction
A multidisciplinary team is typically involved in a HAZOP session, including process, instrumentation, operations, and safety experts.
HAZOP is widely recognized by international standards such as IEC 61882 and is often required in high-risk industries like chemicals, oil & gas, and pharmaceuticals.
A HAZOP requires a complete and validated P&ID (Piping and Instrumentation Diagram) and, ideally, a Process Flow Diagram (PFD).
These documents serve as the technical basis for the analysis and must reflect the actual configuration and instrumentation of the system.
For more real-world examples of how HAZOP reveals hidden issues, see this HAZOP case study on piping material compatibility
What is a What-If Analysis?
What-If Analysis is a qualitative hazard identification technique based on structured brainstorming. It involves a team of knowledgeable individuals asking open-ended questions such as “What if this valve fails?” or “What if power is lost?” to explore potential deviations, failures, and their consequences.
Unlike HAZOP, which follows a fixed set of guide words to systematically challenge each process parameter, the What-If approach is inherently more flexible. It allows the team to formulate any relevant question, including those that do not easily fit within traditional guide words.
For example:
“What if the relief valve discharge line is not connected?”
This type of deviation — although critical — may fall outside standard guide-word logic, making it harder to identify in a conventional HAZOP. In contrast, the What-If Analysis technique encourages open-ended thinking based on experience, procedural knowledge, and engineering judgment.
The strength of What-If lies in its adaptability to explore scenarios that go beyond parameter deviations — especially procedural errors, omissions, or installation issues.
What-If Analysis is commonly used in the following situations:
- During early project phases, when P&IDs are not yet finalized and a preliminary hazard assessment is needed.
- For modifications to existing systems, such as the addition of a new line, pump, or adjustment of set points.
- In utility and ancillary systems, like nitrogen, water, or steam lines, where a full HAZOP may be excessive.
- When technical documentation is limited, especially in older or poorly tracked installations.
- As a preliminary screening tool, to determine whether a more detailed hazard analysis is necessary.
However, based on my field experience, the What-If method can also be effectively applied to complex industrial systems, provided that the analysis is conducted in a structured manner by a competent team.
Regulatory frameworks do not mandate the use of a specific technique based on system complexity, but require that the chosen method is proportionate to the risk level and appropriately documented.
This technique encourages a wide range of questions, making it suitable for capturing non-standard or scenario-based hazards. Its effectiveness depends on the competence, experience, and active contribution of the team.
A detailed and updated P&ID or process description should always be used — regardless of the chosen methodology.
Can What‑If Analysis miss hazards that HAZOP would catch?
Not necessarily. Both methods depend heavily on how they are applied. While HAZOP offers a structured approach through guide words, it may sometimes lead teams to focus too narrowly on predefined deviations.
From an experienced engineer’s perspective, important hazards may go undetected in a HAZOP if the discussion does not extend beyond guide words or if generic questions (e.g., about relief locations or human exposure) are not explicitly addressed.
When applied with strong process knowledge and effective brainstorming, a What-If analysis may highlight scenarios that a rigid HAZOP structure could overlook.
Can HAZOP catch hazards that What‑If might miss?
Not necessarily. While HAZOP is designed to systematically explore process deviations through guide words, this structure does not guarantee that all relevant hazards will be identified. If the team applies the method mechanically, some critical scenarios — especially those not linked directly to a process parameter — may be missed.
A well-led What-If Analysis, based on deep process knowledge and active brainstorming, can surface hazards that might be overlooked in a conventional HAZOP.
Ultimately, the effectiveness of any PHA method depends on how critically the team engages with the analysis, not on the method itself.
Hazard Analysis Example — PSV Vent Line Not Connected
Let’s now consider a simple yet insightful example — a case where the deviation would be difficult to identify through the standard application of guide words.
The image (Fig. 1) shows a reactor (RX-01) supplied with nitrogen from a distribution header operating at 6.0 barg. A pressure-reducing regulator (PR) lowers the pressure to 2.5 barg before entering the reactor.
Downstream of the regulator, a pressure safety valve (PSV) is installed and set to open at 3.0 barg, providing protection in the event the PR fails in the open position. In this case, the downstream system—including the reactor and piping—could be exposed to full line pressure (6.0 barg), and the PSV would lift to prevent overpressure.
A standard HAZOP would typically capture this scenario through the Guide Word “MORE pressure inside a Reactor”, identifying the PSV as a safeguard. However, an additional and often overlooked hazard may arise from where the PSV discharges.
If the discharge line is missing or not routed to a safe location, the release of nitrogen into a confined or occupied area may cause oxygen displacement, leading to a significant asphyxiation risk.
The next image (Fig. 2) illustrates the formation of an asphyxiant nitrogen gas cloud, highlighted in yellow, discharging into an enclosed area inside the chemical plant, where operators may be present. This scenario shows the potential for oxygen displacement and personnel exposure in the absence of proper discharge routing.
This configuration introduces a potential asphyxiation hazard, which may not be identified in a HAZOP performed using a standard parameter-based approach, unless the team also evaluates relief discharge locations and the associated human exposure risk.
The use of guide words like “MORE pressure” can prompt a structured discussion on mechanical risks affecting downstream equipment and connections, such as overpressure of vessels, piping integrity, or PSV lift., but this should not limit the scope of the analysis.
The effectiveness of HAZOP depends on the team’s ability to look beyond the guide words and raise critical questions about the broader implications of each deviation — including the destination of relief flows and the potential impact on human health.
In some HAZOP practices, a dedicated section is included to address general topics such as relief systems, vent locations, or utility interfaces. However, these discussions are sometimes treated as secondary or are not explored with the same level of detail as process deviations. It is essential to recognize that guide words are a tool — not a substitute for engineering judgment, particularly when worker safety is at stake.
This example underscores the importance of adopting a complete process perspective in safety studies. Effective risk identification requires not only evaluating control failures (e.g., overpressure), but also considering where substances may be released and how they could affect people or the environment. This mindset is essential, regardless of whether a HAZOP or What-If methodology is used.
From this perspective, a What-If approach may be better positioned to capture such hazards, particularly when the team is encouraged to raise scenario-driven questions like: “What if the relief valve discharges into a confined area?” This flexibility can lead to early identification of risks that might otherwise be overlooked if the analysis remains constrained within standard guide-word logic.
Whether you apply HAZOP, What-If, or other PHA techniques, the method must be integrated into a broader safety management framework.
Ing. Ivet Miranda
Other Articles You May Find Useful
• The 4 Safety Management System Pillars in a Chemical Plant
• HAZOP Example: Material Compatibility Failure
• Safety Relief Valves vs Rupture Discs: Key Differences
• Rupture Disc Activation in Process Safety
• Vent Header Design: Why Top Tie-Ins Are Safer
• Rupture Disc Positioning: Best Practices
⬆️ Back to TopFAQ
When should I use What‑If Analysis instead of HAZOP?
The What‑If method is often used during early design phases, for minor plant modifications, or when system documentation is limited. It is also effective as a preliminary screening tool.
However, this does not mean it is limited to simple systems. When applied by an experienced team using a structured approach, What‑If Analysis can be successfully used even for complex industrial processes.
HAZOP remains the preferred choice when a comprehensive and systematic hazard review is required, especially for high-risk systems with well-defined P&IDs.
Can the same risk matrix be used for both HAZOP and What-If?
Yes. Both HAZOP and What-If are types of Process Hazard Analysis (PHA), and they can, and often should, use the same risk matrix to ensure consistency in evaluating severity and likelihood across different studies.
What other methodologies exist besides HAZOP and What‑If Analysis?
Other methods include Checklist, FMEA, Fault Tree Analysis, LOPA, and Bow-Tie. The choice depends on the complexity of the process, project phase, and specific objectives.