| name | hazop-study |
| description | HAZOP study methodology — guidewords (No/More/Less/Reverse/Other), nodes, deviations, consequences, safeguards, action items, LOPA layers of protection, IEC 61882, risk matrix. |
| metadata | {"priority":7,"promptSignals":{"phrases":["HAZOP study","hazard and operability","HAZOP guideword","HAZOP deviation","process hazard analysis","LOPA layers of protection","IEC 61882"],"minScore":3}} |
HAZOP Study — Complete Skill
Definition and Purpose
HAZOP (Hazard and Operability Study): structured technique to identify hazards and operability problems in a process design
- Originated: ICI (UK) 1960s; Kletz; formalized in IEC 61882:2016
- Applicability: new designs (during P&ID stage); modifications; periodic revalidation
- Output: action items to reduce risk to ALARP (As Low As Reasonably Practicable)
HAZOP Team
Team leader (facilitator): trained HAZOP leader; independent of design; neutral
Scribe: records deviations, consequences, safeguards, actions
Process engineer: knows design intent and operating conditions
Instrument/control engineer: knows SIS, control loops
Operations representative: knows actual operating practice
Safety engineer: knows risk standards
Others as needed: maintenance, commissioning, vendor
HAZOP Process Structure
Step 1 — Define Study Nodes
Divide process into manageable sections (nodes)
A node typically contains one major unit operation or equipment item
Examples: pump suction line, distillation column feed section, heat exchanger, reactor vessel
Step 2 — Define Design Intent
For each node: what should happen, at what conditions (T, P, flow, composition)
Step 3 — Apply Guidewords to Parameters
Parameters: flow, temperature, pressure, level, composition, time, pH, viscosity, electrical supply, utility supply
Guidewords:
| Guideword | Meaning | Example |
|---|
| No/None | Complete negation | No flow (pump failure, blocked line) |
| More | Quantitative increase | High pressure (control valve fails open) |
| Less | Quantitative decrease | Low temperature (heater failure) |
| Reverse | Opposite direction | Reverse flow (pump trip + backflow) |
| Part of | Qualitative decrease | Wrong composition (contamination) |
| As well as | Qualitative increase | Additional component (wrong material in) |
| Other than | Complete substitution | Wrong material charged (startup error) |
| Early/Late | Time | Early startup (before purge complete) |
| Before/After | Sequence | Steps out of order |
Deviations: Guideword + Parameter = Deviation
Example: MORE + PRESSURE = High Pressure
Step 4 — For Each Deviation, Identify:
- Causes: what could cause this deviation (1 cause at a time; independent causes separated)
- Consequences: what could happen as a result
- Existing safeguards: controls, alarms, trips, procedures, containment, layout
- Risk assessment: severity × likelihood → risk level
- Recommendations (action items): additional safeguards or design changes needed
Risk Matrix (Typical)
Severity (Consequence) categories:
S1 — Minor: minor injury, minor environmental; no community impact
S2 — Moderate: lost time injury, significant spill; limited community impact
S3 — Major: permanent injury/fatality, large release; community impact
S4 — Catastrophic: multiple fatalities, large uncontrolled release
Likelihood categories:
L1 — Unlikely: < 10⁻⁴ /yr
L2 — Possible: 10⁻³ /yr
L3 — Probable: 10⁻² /yr
L4 — Frequent: > 10⁻¹ /yr
Risk = Severity × Likelihood → High/Medium/Low/Negligible
High risk → action required; medium → consider action; low → acceptable with existing safeguards
HAZOP Recording Format
| Node | Deviation | Cause | Consequence | Safeguards | Risk | Action | Assigned to |
|---|
| HE-101 Shell Side | High Temperature | Steam pressure high (PIC-101 fails) | Tube rupture, shell overpressure | High temp alarm TAH-101; PRV PSV-101 | Medium | Verify PRV sized for steam breakthrough | J. Smith |
Severity Classification vs. Risk Tolerance
ALARP principle: risk reduced to ALARP; cost of further reduction grossly disproportionate to benefit
Typical individual risk tolerance:
Maximum tolerable: 10⁻⁴ fatalities/year (UK HSE — workers)
Broadly acceptable: 10⁻⁶ fatalities/year
ALARP region: 10⁻⁶ to 10⁻⁴ (must justify by cost-benefit)
Layers of Protection Analysis (LOPA)
Semi-quantitative SIL determination after HAZOP
Frequency after all IPLs:
f_mitigated = f_initiating × Π(PFD_i)
f_initiating = initiating event frequency [events/yr]
PFD_i = probability of failure on demand for each IPL i
Independent Protection Layers (IPLs):
Process design (e.g., inherently safer): PFD = 0.1
BPCS (basic process control system): PFD = 0.1
Human response > 10 minutes: PFD = 0.1
Safety instrumented system (SIS): PFD = 0.1 (SIL 1) to 0.001 (SIL 3)
PSV (relief valve): PFD = 0.01
Dike/bund: PFD = 0.01 (for pool fire; 0.1 for vapor)
SIL determination:
Required PFD_SIS = f_target / (f_initiating × Π(other IPLs))
Then SIL from PFD:
SIL 1: PFD = 0.1–0.01
SIL 2: PFD = 0.01–0.001
SIL 3: PFD = 0.001–0.0001
SIL 4: PFD = 0.0001–0.00001
Common HAZOP Findings (Patterns)
- No flow — blocked discharge: pump cavitation, motor overload → add LO alarm
- High pressure — cooling failure: relief valve undersized for fire case → review PRV
- Reverse flow — pump trip: check valve missing or failed → add check valve
- High temperature — heater runaway: no TIC backup → add independent high-temp trip
- Wrong material — startup: no composition analysis online → add composition alarm or analyzer
Operability Issues (Non-safety)
HAZOP also identifies operability problems (not just safety)
- Startup difficulties, controllability at turndown, instrument rangeability, maintenance accessibility
- Documented in same record but may have different severity scale
Revalidation Requirements
API RP 750 / OSHA PSM (29 CFR 1910.119): PHA revalidation every 5 years
Following major changes: Management of Change (MOC) triggers HAZOP of affected nodes
After incident: incident investigation may expand HAZOP scope
IEC 61882:2016 Scope
Full HAZOP methodology; recording format; team roles; reporting; batch/software HAZOP extensions
Batch HAZOP: use time-based guidewords (early, late, out of sequence, too long, too short)
Software HAZOP (CHAZOP): for control systems; deviations in software logic, data communication
Output
Provide: HAZOP action items (numbered), consequence severity and likelihood before/after safeguards, safeguard type (BPCS/SIS/PRV/admin), LOPA PFD calculation, target mitigated frequency vs. risk tolerance, SIL assignment if applicable, node definition with design intent, applicable standard (IEC 61882, API RP 750, OSHA 1910.119).
