HAZOP Study: Guide Words, Worksheet Example and Step-by-Step Process
A HAZOP (Hazard and Operability Study) is a structured team review that finds hazards in a process design. It splits the design into sections called nodes and applies guide words such as NO, MORE, LESS and REVERSE to each process parameter, such as flow, pressure, temperature and level. Each combination creates a deviation to examine, for example "more level" or "reverse flow". The team records the causes, consequences, safeguards and recommendations for every credible deviation in a HAZOP worksheet. The method is described in the international standard IEC 61882:2016. It is one of the process hazard analysis methods accepted under OSHA's Process Safety Management standard.
When the Design Looks Safe but Isn't
On 23 March 2005, a series of explosions at the BP Texas City refinery killed 15 workers and injured 180 others. The US Chemical Safety Board found that a distillation tower, the raffinate splitter, flooded with hydrocarbons during startup and was overpressurized. The relief flow passed through a blowdown drum and an atmospheric vent stack, producing a geyser-like release of flammable liquid that ignited.
This is exactly the kind of scenario HAZOP is designed to question. The guide word MORE applied to the parameter LEVEL forces the team to ask several questions. What happens if the tower level exceeds its design limit? Where does the relief go? What safeguards exist? Do they depend on the same instrument that might be giving a false reading?
HAZOP does not guarantee that all hazards are found. But it provides a structured, systematic framework for asking the right questions about process deviations — the kind of questions that are routinely not asked during normal engineering design reviews.
What Is a HAZOP Study?
A Hazard and Operability Study (HAZOP) is a structured and systematic technique for identifying hazards and operability problems in process systems. It was developed in the late 1960s by ICI (Imperial Chemical Industries) in the UK and has since become the industry-standard methodology for process safety risk identification in chemical, petrochemical, pharmaceutical, oil and gas, and other process industries.
The method is standardized internationally in IEC 61882:2016, Hazard and operability studies (HAZOP studies) – Application guide. The standard defines the study process, the team roles, and the core guide words.
HAZOP works by systematically examining a process design — using engineering drawings, process flow diagrams, and piping and instrumentation diagrams (P&IDs) — and applying a set of standardized guide words to each process parameter at each defined point in the system (called a node) to generate potential deviations, their causes, consequences, and safeguards.
The core question HAZOP answers: What can go wrong with this process design, and what are the consequences?
HAZOP is classified as a qualitative risk identification method. It identifies hazards and operability problems; it does not quantify their likelihood or frequency (that is the role of quantitative risk assessment techniques like Fault Tree Analysis or QRA).
When Should a HAZOP Be Conducted?
| Stage | When | Purpose |
|---|---|---|
| Design HAZOP | During detailed engineering — before construction | Identify hazards while changes are still low-cost |
| Pre-startup HAZOP | Before commissioning a new or modified facility | Verify that safety systems are in place |
| Operational HAZOP (revalidation) | Periodically for existing plants; OSHA PSM requires the process hazard analysis to be updated and revalidated at least every five years | Capture changes since last HAZOP; revalidate against current P&IDs |
| Management of Change HAZOP | Before implementing any significant modification to the process | Assess new hazards introduced by the change |
| Incident-triggered HAZOP | After a serious incident or near miss | Assess whether similar deviations can occur elsewhere in the system |
The HAZOP Methodology: Guidewords and Parameters
HAZOP is built on two foundational concepts: parameters and guide words.
Process Parameters
A parameter is any measurable property of the process at a given node. Common parameters include:
| Parameter | Examples |
|---|---|
| Flow | Rate, direction, quantity of material flowing in a pipe |
| Temperature | Temperature of process fluid, vessel contents, utilities |
| Pressure | Operating pressure in vessels, pipelines, and equipment |
| Level | Liquid level in vessels, columns, and tanks |
| Composition | Concentration of reactants, products, or contaminants |
| Phase | Liquid vs. vapor phase transition |
| Reaction | Rate of reaction, selectivity, heat of reaction |
| Time | Duration of steps in batch processes |
Guide Words
Guide words are standard words that, when combined with a process parameter, generate a meaningful deviation to examine:
| Guide Word | Meaning | Example Deviation |
|---|---|---|
| NO / NONE | Complete negation of the parameter | No flow (pipe blockage, pump failure) |
| MORE THAN | Quantitative increase | High flow, high temperature, high pressure |
| LESS THAN | Quantitative decrease | Low flow, low temperature, low pressure |
| AS WELL AS | Qualitative increase — something extra | Contamination, impurity, additional phase |
| PART OF | Qualitative decrease — something missing | Missing component, incomplete composition |
| REVERSE | Opposite direction | Reverse flow, back-pressure |
| OTHER THAN | Complete substitution | Wrong material, wrong phase |
| EARLY / LATE | Timing in batch processes | Step initiated too early or too late |
| BEFORE / AFTER | Sequence deviation in batch processes | Step performed out of sequence |
HAZOP Worksheet Example
The worksheet below shows what a completed HAZOP record looks like for one node. The node, equipment tags and findings are illustrative, but the structure is the one used in real studies.
Node 2: Transfer line from day tank TK-101 via pump P-101 to process vessel V-102, including level control valve LV-102 and relief valve PSV-102. Design intent: Transfer flammable solvent at 10 m³/h to maintain V-102 at 50% level, at ambient temperature and a maximum pressure of 6 barg.
| Deviation | Causes | Consequences | Safeguards | Adequate? | Recommendation |
|---|---|---|---|---|---|
| NO FLOW | P-101 trips; suction valve left closed; strainer blocked | V-102 runs down; P-101 runs dead-headed, overheating the seal and risking a solvent leak | Low-flow alarm FAL-101 | Partly: alarm only | R1: Add a low-flow trip on P-101 and assess the required integrity |
| MORE LEVEL (V-102) | LV-102 fails open; level transmitter LT-102 reads falsely low; manual bypass left open | V-102 overfills to PSV-102; liquid solvent released at the vent; flammable vapor cloud | High-level alarm LAH-102, driven by the same LT-102 | No: the alarm shares the transmitter that may have failed | R2: Install an independent high-high level switch that trips P-101. R3: Route PSV-102 to a closed system, not to atmosphere |
| REVERSE FLOW | P-101 stops while V-102 is pressurized | Back-flow into TK-101; possible overfill of the day tank | Check valve NRV-101 | Yes, if maintained | R4: Add NRV-101 to the inspection and test schedule |
| MORE PRESSURE (V-102) | Outlet blocked; external fire | Overpressure; vessel rupture | PSV-102 | To be confirmed | R5: Confirm and document the PSV-102 sizing basis for the blocked-outlet and fire cases |
| AS WELL AS (composition) | Water carried over from the tank bottom | Water in the hot downstream process flashes to steam, causing a pressure surge | Tank water draw-off procedure | Partly: relies on operators | R6: Add the water draw-off to the shift checklist; consider an interface detector |
Three habits make a worksheet like this useful rather than decorative:
- Consequences are written assuming the safeguards fail. This shows how much protection is really needed.
- Safeguards are tested for independence. In the MORE LEVEL row, an alarm driven by the same failed instrument is not a safeguard.
- Every recommendation has a number, so it can be tracked to close-out as a corrective action.
The 7-Step HAZOP Process
Step 1: Define the Scope and Objectives
Before the HAZOP begins, the study leader must define:
- The boundary of the system to be studied
- The objectives (design review, revalidation, MoC assessment)
- The level of detail required
- The documentation to be used (P&IDs, PFDs, operating procedures)
- The acceptance criteria for risk (what level of risk requires a recommendation?)
Step 2: Assemble the HAZOP Team
A HAZOP is a team exercise. The multi-disciplinary team typically includes:
| Role | Contribution |
|---|---|
| HAZOP Leader (Facilitator) | Qualified HAZOP specialist; guides the methodology; ensures systematic coverage |
| Process Engineer | Design intent; process knowledge |
| Instrument/Control Engineer | Control system design; alarm philosophy |
| Operations Representative | Operational experience; real-world deviations |
| Maintenance Engineer | Equipment failure modes; maintenance access |
| Safety Engineer | Consequence assessment; safeguard evaluation |
| Scribe | Records all findings in real time |
Team size of 5–8 people is optimal. Larger teams become difficult to facilitate.
Step 3: Divide the System into Nodes
A node is a defined section of the process where the design intent is uniform — a length of pipe between two pieces of equipment, a vessel, a heat exchanger. The study leader divides the P&ID into nodes before the study begins.
Node selection is a critical facilitator skill. Nodes that are too large miss specific hazards. Nodes that are too small make the study unnecessarily slow.
Step 4: Apply Guide Words to Each Node
For each node, the team works systematically through each relevant parameter and applies each guide word:
- The leader states the parameter and guide word: "FLOW — MORE THAN"
- The team identifies plausible causes for that deviation at that node
- For each credible cause, the team identifies consequences
- The team identifies existing safeguards that prevent, detect, or mitigate the consequence
- The team assesses whether the existing safeguards are adequate
- If safeguards are inadequate, a recommendation (action item) is recorded
Step 5: Record Findings in the HAZOP Worksheet
The HAZOP worksheet is the formal record of the study. Standard worksheet columns:
| Column | Content |
|---|---|
| Node | Reference number and description of the node being studied |
| Guide Word + Parameter | The deviation being examined |
| Causes | Plausible causes of the deviation |
| Consequences | Outcomes if the deviation occurs and safeguards fail |
| Safeguards | Existing controls, alarms, interlocks, procedures |
| Likelihood / Severity | Qualitative risk rating (if used) |
| Recommendations | Actions required to reduce risk |
| Action Party | Named owner for each recommendation |
Step 6: Follow Up on Recommendations
The HAZOP study generates recommendations — proposed design changes, additional interlocks, procedural modifications, further study requirements. The HAZOP is not complete until all recommendations have been:
- Reviewed and accepted, modified, or rejected by engineering management
- Assigned to a named owner with a completion date
- Tracked to close-out
A HAZOP that generates 200 recommendations and tracks none of them to completion has not reduced risk — it has created a false sense of security.
Step 7: Document the Study
The HAZOP report is the permanent record of the study. Auditors, regulators and the next revalidation team will rely on it. A complete report follows this structure:
| Section | Content |
|---|---|
| 1. Executive summary | System studied, dates, number of nodes, and number of recommendations by risk ranking |
| 2. Scope and boundaries | What was included and excluded, and the design basis |
| 3. System description | Process description and design intent for each node |
| 4. Methodology | Guide words and parameters used, risk ranking matrix, reference to IEC 61882 |
| 5. Team and attendance | Names, roles and disciplines, attendance per session |
| 6. Documents reviewed | P&IDs, PFDs, cause-and-effect charts and procedures, with revision numbers |
| 7. Node list | Node numbers, descriptions and marked-up P&IDs |
| 8. Worksheets | The complete worksheets, including deviations found not credible |
| 9. Recommendations register | Each recommendation with its owner, due date and status |
| 10. Open items | Questions parked for later resolution |
| 11. Sign-off | Study leader and engineering management approval |
Revision numbers matter. A HAZOP is valid only for the drawings it reviewed, and when the P&IDs change, the report shows exactly which parts must be revisited.
HAZOP vs Other Risk Assessment Methods
HAZOP is one of several process safety risk identification methods. Understanding when to use HAZOP versus alternatives is essential:
| Method | Type | Best For | Not Suitable For |
|---|---|---|---|
| HAZOP | Qualitative, systematic | Complex continuous process systems with P&IDs | Simple systems; batch processes without detailed procedures |
| Fine-Kinney | Semi-quantitative | General workplace risk assessment; non-process hazards | Detailed process system analysis |
| FMEA | Qualitative, systematic | Equipment failure mode analysis; mechanical systems | Process chemistry hazards |
| What-If Analysis | Qualitative, less structured | Early concept stage; simpler systems | Complex interlinked process systems |
| Fault Tree Analysis (FTA) | Quantitative | Frequency estimation for specific top events | Broad hazard identification |
| Bow-Tie Analysis | Qualitative/quantitative | Barrier visualization for identified hazard scenarios | Primary hazard identification |
| JSA/JHA | Qualitative | Task-level hazard analysis for specific operations | System-level process hazard analysis |
When HAZOP is the right choice:
- You are designing or modifying a continuous chemical or process plant
- The system has complex interactions between process streams, equipment, and control systems
- Detailed P&IDs are available
- The consequences of a process deviation could be severe (fire, explosion, toxic release, loss of containment)
When HAZOP is not the right choice:
- The system is simple and does not have complex process interactions
- No P&IDs exist (early design concept stage — use What-If instead)
- The hazards are primarily task-based rather than process system-based (use JSA instead)
Common HAZOP Failures
A poorly executed HAZOP can be more dangerous than no HAZOP — it creates a documented record that hazards were reviewed, while missing significant risks.
Incomplete P&IDs: The HAZOP is only as good as the drawings it reviews. Out-of-date P&IDs, missing instrument details, or drawings that don't reflect as-built conditions produce incomplete findings.
Wrong team composition: A HAZOP conducted without operational experience misses real-world deviations. A HAZOP without instrument engineering expertise misses control system failure scenarios.
Superficial consequence assessment: Teams that accept "operator response" as a safeguard without questioning whether the operator can realistically respond in time and with the right information will systematically underestimate risk.
Recommendations not tracked: The most common HAZOP failure. Recommendations are generated, presented to management, and then not tracked. Six months later, none have been implemented.
Insufficient time: A complex process unit cannot be adequately reviewed in a single day. Rushed HAZOP studies systematically skip difficult deviations and underestimate consequence severity.
HAZOP and ISO 45001 / Process Safety Standards
HAZOP satisfies several key requirements of ISO 45001 and major process safety standards:
| Standard / Clause | Requirement | How HAZOP Satisfies It |
|---|---|---|
| ISO 45001 Clause 6.1 | Hazard identification and risk assessment | HAZOP is a systematic hazard identification method for process hazards |
| OSHA PSM 29 CFR 1910.119 | Process Hazard Analysis (PHA) for highly hazardous chemicals | HAZOP is one of the accepted PHA methodologies |
| EU Seveso III Directive | Major accident hazard identification | HAZOP is widely used to meet this requirement at major-hazard sites |
| IEC 61882:2016 | Application guide for HAZOP studies | Defines the HAZOP process, roles and guide words |
| API RP 14C | Safety analysis of offshore surface safety systems (SAFE charts) | A separate, prescriptive analysis; HAZOP is commonly used alongside it for broader process review |
| IEC 61511 | Safety instrumented systems (SIS) | HAZOP identifies the initiating events that SIS design must address |
How FindRisk Supports Process Safety Reviews
While HAZOP is a specialist workshop-based methodology, FindRisk supports the broader process safety ecosystem that surrounds it:
Pre-HAZOP hazard screening: Before commissioning a full HAZOP, use FindRisk's AI-assisted assessment to conduct a preliminary hazard identification. This helps scope the HAZOP, identify the highest-priority nodes, and ensure the team comes to the workshop with a shared understanding of the system's hazard profile.
Post-HAZOP recommendation tracking: HAZOP recommendations are corrective actions. Use FindRisk to track each recommendation to close-out — with assigned owners, deadlines, and verification evidence — ensuring the study's findings are actually implemented.
Operational risk assessment: For operational tasks associated with the process plant (vessel entry, hot work, chemical handling), FindRisk's Fine-Kinney methodology provides task-level risk quantification that complements the system-level HAZOP findings.
Frequently Asked Questions
What does HAZOP stand for?
HAZOP stands for Hazard and Operability Study. "Hazard" refers to scenarios that could harm people, the environment or assets. "Operability" refers to problems that stop the plant from running as intended, even if they are not dangerous. The method was developed at ICI in the UK and is now standardized in IEC 61882.
What are the HAZOP guide words?
The core guide words in IEC 61882 are NO (or NOT), MORE, LESS, AS WELL AS, PART OF, REVERSE and OTHER THAN, plus EARLY, LATE, BEFORE and AFTER for time and sequence. Each is combined with a process parameter to create a deviation, for example NO + FLOW = no flow, or MORE + PRESSURE = high pressure. Teams skip combinations that make no physical sense.
Which standard covers HAZOP?
The international standard is IEC 61882:2016, Hazard and operability studies (HAZOP studies) – Application guide. It describes the study process, team roles, guide words and documentation. Regulations such as OSHA PSM (29 CFR 1910.119) and the EU Seveso III Directive do not prescribe HAZOP by name as the only method, but accept it as a recognized process hazard analysis technique.
Who should lead a HAZOP study?
The HAZOP leader (facilitator) must be a trained specialist in the HAZOP methodology — not simply someone with process engineering experience. The leader's role is to facilitate the systematic application of guide words, manage team dynamics, ensure all deviations are examined, and maintain the pace and quality of the review. Many organizations use external independent HAZOP facilitators for critical studies to ensure objectivity and specialist expertise. The leader should hold formal training in HAZOP facilitation — typically from organizations such as the IChemE or equivalent.
How long does a HAZOP study take?
Duration depends entirely on the complexity of the system and the level of detail in the P&IDs. A small system with a handful of nodes may take a day or two of workshop time, while a complete refinery or chemical process unit can take several weeks. Keep workshop days short enough to stay sharp, because fatigue is a common reason for skipped deviations. Attempting to compress a HAZOP into less time than the system warrants is a systematic way to miss hazards.
What is the difference between HAZOP and HAZID?
A HAZOP is a detailed, systematic review of a process design using guide words and P&IDs. A HAZID (Hazard Identification Study) is a less structured, higher-level brainstorming session used at early project stages — before detailed P&IDs exist — to identify major hazard categories. HAZID typically precedes HAZOP in a project's safety lifecycle.
Does HAZOP identify all hazards?
No — HAZOP is a structured search, but it is not exhaustive. It systematically examines the design against a defined set of guide words and parameters, but the quality of the output depends critically on the quality of the team, the completeness of the P&IDs, and the adequacy of time allocated. HAZOP is best understood as the primary hazard identification method in a suite of complementary techniques — not a guarantee that all hazards have been found.
Is HAZOP required by law?
In the United States, OSHA's Process Safety Management (PSM) Standard (29 CFR 1910.119) requires a Process Hazard Analysis (PHA) for facilities handling highly hazardous chemicals above threshold quantities. HAZOP is one of the accepted PHA methodologies (along with What-If, Checklist, What-If/Checklist, FMEA, Fault Tree Analysis, or an appropriate equivalent), and the PHA must be revalidated at least every five years. In the EU, Seveso III Directive requirements effectively require equivalent systematic hazard identification for major-hazard facilities. HAZOP is one of the most widely used methods for this in European process industries.
Conclusion
HAZOP is the most rigorous and widely used tool available for identifying hazards in process systems. When conducted properly — with the right team, adequate time, complete P&IDs, and systematic follow-through on recommendations — it identifies scenarios that would not emerge from engineering judgment alone.
The disasters that HAZOP is designed to prevent — overpressure events, loss of containment, runaway reactions, toxic releases — are not rare accidents. They are predictable consequences of process deviations that were either not identified or identified but not adequately controlled.
The cost of a HAZOP team for a few weeks is small compared with the cost of a single major incident. That remains true only if every recommendation is tracked to close-out.
Download FindRisk to support your process safety workflow — from preliminary hazard screening through post-HAZOP recommendation tracking — with AI-assisted assessment, mobile documentation, and professional report generation.
Try FindRisk
Ready to modernize your safety workflow?
Conduct AI-powered risk assessments, generate reports instantly, and keep your team safe — anywhere, anytime.