HAZOP
Hazard & Operability Study Guide-word
ICI · IEC 61882 · CCPS · Process- and procedure-based

HAZOP is a structured, multi-disciplinary workshop technique that walks through a design or procedure, node by node, applying a fixed set of guide words ("no", "more", "less", "as well as", "part of", "reverse", "other than") to each design parameter to provoke imagination of credible deviations. Each deviation is then traced to causes, consequences, existing safeguards and recommended actions. The strength of HAZOP lies in the discipline of the prompt list combined with the imagination of the assembled experts.

Overview of the technique

HAZOP was developed at Imperial Chemical Industries in the 1960s under Trevor Kletz and codified by the Chemical Industries Association in 1977. It became the global standard for chemical process-safety review and is now formalised in IEC 61882:2016. The team — typically a chair, scribe, designer, operator, maintainer and an external expert — examines a piping-and-instrumentation diagram (P&ID) or procedure broken into discrete nodes (line segments, equipment items, procedure steps).

For each node the team identifies the design intent (what should happen) and applies guide words to relevant parameters (flow, pressure, temperature, level, composition, time, sequence, pH, viscosity). The cross-product of guide word and parameter generates deviations — "no flow", "more pressure", "reverse flow", "high temperature", "step performed early". The team then captures plausible causes, consequences, existing safeguards and recommendations on a worksheet. HAZOP variants extend the method to procedures (PHAZOP), software (CHAZOP) and human factors. The output feeds Layer-of-Protection Analysis (LOPA), bow-tie barrier definition, and SIL determination per IEC 61511.

HAZOP — guide word × parameter generates deviations on each node Guide words No / none More Less As well as Part of Reverse Other than Early / late Parameters Flow Pressure Temperature Level Composition Time / sequence Action / step Operating mode × Deviation "More pressure" on Node 14 (transfer line) Causes downstream block · runaway reaction · PSV failure Consequences vessel rupture · release to atmosphere · ignition Safeguards / actions PSV (existing) PRZ alarm + interlock SIS trip (proposed) Recommendations Add SIL-2 interlock Update operator SOP Re-rate piping spec → LOPA / SIL study
Figure 1. The HAZOP grammar — for each node the team multiplies guide words by relevant parameters, captures deviations, and ends with safeguards and actions feeding LOPA / SIL determination.

When to use it

Typical applications

  • Process plant design and major modification reviews
  • Procedure validation (start-up, shutdown, batch operations)
  • Software-controlled process review (CHAZOP)
  • Pre-start-up safety review under OSHA PSM / Seveso III
  • Input to LOPA, SIL determination and bow-tie analysis

Aviation relevance

  • Used on fuel-handling, hydraulic and de-icing systems
  • Procedural HAZOP for cabin / fuelling / pushback operations
  • Software HAZOP supports DO-178C / ARP 4754A activities
  • Input to ATS-system safety cases under SESAR / EASA
  • Recurrent on aerodrome fuel farms and ground-service depots

Benefits

Disciplined imagination

The fixed prompt list of guide words combined with subject-matter expertise produces broad, repeatable deviation coverage that ad-hoc brainstorming rarely achieves.

Multi-disciplinary insight

Bringing designer, operator and maintainer into one room often surfaces interface failures and procedural mismatches invisible from any single perspective.

Auditable record

The structured worksheet — node, deviation, cause, consequence, safeguard, action — produces an evidence trail that satisfies regulators (OSHA PSM, Seveso, IEC 61511).

Foundation for further analysis

HAZOP outputs feed naturally into LOPA, SIL, bow-tie and FTA, sequencing qualitative hazard discovery before quantitative protection-layer assessment.

Limitations

Resource intensive

A complete HAZOP on a complex plant runs into hundreds of hours of expert time; under cost pressure scope is often narrowed in ways that compromise coverage.

Quality depends on team

Without an experienced chair, a balanced multi-disciplinary team and good preparation, a HAZOP becomes a procedural tick-box that misses subtle hazards.

Single-deviation focus

Classical HAZOP examines deviations one at a time; complex interaction effects, common-cause failures and emergent system behaviour need complementary methods (FTA, STAMP).

Software & human limits

Adapting guide words to software, cyber and human-factor parameters is non-trivial; CHAZOP and procedural HAZOP variants help but do not fully resolve the abstraction mismatch.

In short

HAZOP turns expert imagination into a disciplined search by walking design intent through a fixed set of guide words and parameters. It is the foundation of process-safety review and an upstream input to bow-tie, LOPA, SIL and FTA studies.

References (APA 7)

Kletz, T. A. (1999). HAZOP and HAZAN: Identifying and assessing process industry hazards (4th ed.). IChemE.

International Electrotechnical Commission. (2016). Hazard and operability studies (HAZOP studies) — Application guide (IEC 61882:2016). IEC.

Crawley, F., & Tyler, B. (2015). HAZOP: Guide to best practice (3rd ed.). Elsevier.

Center for Chemical Process Safety. (2008). Guidelines for hazard evaluation procedures (3rd ed.). Wiley.

International Electrotechnical Commission. (2016). Functional safety — Safety instrumented systems for the process industry sector (IEC 61511). IEC.

Further reading

Dunjó, J., Fthenakis, V., Vílchez, J. A., & Arnaldos, J. (2010). Hazard and operability (HAZOP) analysis: A literature review. Journal of Hazardous Materials, 173(1–3), 19–32.

Khan, F. I., & Abbasi, S. A. (1998). Techniques and methodologies for risk analysis in chemical process industries. Journal of Loss Prevention in the Process Industries, 11(4), 261–277.

Reddy, K. (2017). Software HAZOP: A systematic approach. Journal of System Safety, 53(3), 18–24.