HAZOP Guide Words Reference Card

📊 Quick Reference Table

Guide Word	Meaning	Common Parameters	Example Deviation
NO / NONE	Complete absence	Flow, Power, Signal, Level	No flow through cooling pump
MORE	Quantitative increase	Flow, Pressure, Temp, Level	High pressure in reactor vessel
LESS	Quantitative decrease	Flow, Pressure, Temp, Level	Low temperature in distillation
AS WELL AS	Something extra present	Composition, Phase, Contaminants	Water contamination in acid
PART OF	Something missing	Composition, Components, Steps	Reactant B missing from batch
REVERSE	Opposite direction	Flow, Rotation, Sequence	Reverse flow in return line
OTHER THAN	Complete substitution	Material, Operation, Mode	Wrong chemical loaded
EARLY	Action before intended	Sequence, Timing, Operations	Valve opens before safe state
LATE	Action after intended	Sequence, Response, Alarms	Interlock activates too late
SOONER/LATER	Duration deviation	Batch time, Residence, Process	Reaction completes early

📋 Detailed Guide Word Application

∅

NO / NONE / NOT

Complete negation of design intent — nothing happens

Flow No flow → Pump failure, closed valve, blockage

Pressure No pressure → Leak, open vent, supply failure

Level No level → Drain open, massive leak

Power No power → Outage, breaker trip

Facilitator Prompt

"What could cause this process parameter to be completely absent? What happens if there's zero flow, zero pressure, zero signal?"

↑+

MORE / MORE OF

Quantitative increase beyond normal operating range

Flow High flow → Control valve fails open, pump over-speed

Pressure High pressure → Blockage, runaway, overcharging

Temperature High temp → Cooling failure, exothermic reaction

Level High level → Outlet blocked, overfilling

Facilitator Prompt

"What if this parameter increases significantly above normal? Consider control system failures, operator errors, and upstream upsets."

↓−

LESS / LESS OF

Quantitative decrease below normal operating range

Flow Low flow → Pump wear, partial blockage

Pressure Low pressure → Small leak, relief valve open

Temperature Low temp → Heater failure, excess cooling

Mixing Low mixing → Agitator fault, baffles missing

Key Difference: LESS vs NO

LESS = reduced but still present (pump running slowly). NO = completely absent (pump stopped).

⟲

REVERSE / OPPOSITE

Logical opposite of design intent — direction reversal

Flow Reverse flow → Check valve failure, pressure reversal

Rotation Reverse rotation → Phase error, wiring mistake

Reaction Reverse reaction → Equilibrium shift

Sequence Reverse sequence → Operator error, PLC fault

⚠️ Most Overlooked Guide Word

REVERSE is frequently skipped. Always ask: "What if flow direction reverses? What if the sequence runs backward?"

AS WELL AS / PART OF

Qualitative change — extra present or component missing

AS WELL AS Water, air, impurities, additional phase present

PART OF Missing reactant, skipped step, absent phase

Facilitator Prompt

"What contaminants could enter this stream? What component of the mixture could be missing?"

≠

OTHER THAN / INSTEAD

Complete substitution — something entirely different

Material Wrong chemical loaded, mislabeled container

Operation Wrong procedure followed, different mode

Destination Material routed to wrong location

Mode Operating in wrong state (startup vs normal)

Human Error Focus

OTHER THAN scenarios often involve human error. Ask: "What if an operator makes a mistake? What if the wrong material is delivered?"

MMRisk Reference Card

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✅ HAZOP Facilitator Checklist

Before the Workshop

Distribute reference cards to all participants
Explain each guide word with facility examples
Clarify LESS vs PART OF difference
Emphasize REVERSE is mandatory
Review node definitions and P&IDs

During the Workshop

Apply all guide words systematically
Consider startup, shutdown, abnormal ops
Don't combine multiple deviations
Trace consequences to worst credible outcome
Document all safeguards, even obvious ones

Before Moving to Next Node

All 7 primary guide words applied
Timing deviations considered (if relevant)
At least one cause per deviation
Risk rankings assigned
Recommendations specific and actionable

Pro Tips for Effective HAZOP Sessions

Keep sessions under 3 hours — fatigue reduces effectiveness after this point
One deviation at a time — don't let the team jump ahead or combine scenarios
Consider all operating states — startup and shutdown often have different hazards
Never dismiss a deviation — document it even if quickly ruled out
Use the "What if" extension — "What if this happened during a power outage?"
Track action items live — assign owners before moving on

⚠️ Common Mistakes to Avoid

Mistake	Solution
Skipping "obvious" deviations	Apply all guide words systematically, even if outcome seems unlikely
Combining multiple deviations	Consider each deviation separately; combined scenarios come later
Only considering equipment failure	Include human error, external events, and process upsets as causes
Stopping at immediate consequence	Follow the consequence chain to the worst credible outcome
Ignoring existing safeguards	Document all protective measures before ranking risk

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NO / NONE / NOT

Facilitator Prompt

MORE / MORE OF

Facilitator Prompt

LESS / LESS OF

Key Difference: LESS vs NO

REVERSE / OPPOSITE

⚠️ Most Overlooked Guide Word

AS WELL AS / PART OF

Facilitator Prompt

OTHER THAN / INSTEAD

Human Error Focus

✅ HAZOP Facilitator Checklist

Before the Workshop

During the Workshop

Before Moving to Next Node

Pro Tips for Effective HAZOP Sessions