Why This Page Exists And Why Inert Gas Is Not “Just a Tanker System”
Inert Gas Systems are often taught as:
“Keep oxygen below 8%.”
That simplification has:
• killed crew
• destroyed tankers
• caused massive pollution
• led to criminal prosecutions
Inert gas is not just about fire prevention.
It is about pressure control, gas chemistry, human factors, and discipline.
This page treats IGS as what it truly is:
A life-critical atmosphere control system operating at the edge of explosion physics.
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Table of Contents
1. What an Inert Gas System Really Does
2. The Fire Triangle — And How IGS Breaks It
3. How an Inert Gas System Works (Step-by-Step, Real World)
4. Major Components — Function, Failure & Consequences
5. Types of Inert Gas Systems
6. Oxygen, Pressure & Chemistry — The Numbers That Matter
7. Operational Phases (Loading, Discharge, Ballast, Tank Cleaning)
8. Failure Modes & Accident Pathways
9. Human Factors & Historical Disasters
10. Regulations, SOLAS & Port State Reality
11. Final Engineering Takeaway
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1. What an Inert Gas System Really Does
An Inert Gas System:
• Reduces oxygen concentration
• Controls tank pressure
• Prevents explosive mixtures
• Prevents air ingress
• Protects tank structure
It does not:
• remove hydrocarbons
• make tanks “safe to enter”
• eliminate toxicity
• replace gas-freeing
IGS is a preventative containment system, not a cleaning system.
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2. The Fire Triangle — And Where Inert Gas Attacks It
Fire requires:
1. Fuel (hydrocarbon vapour)
2. Oxygen
3. Ignition source
On tankers:
• fuel is unavoidable
• ignition sources are always possible
So IGS removes oxygen.
Critical Thresholds
• Air: ~21% O₂
• Flammable range: ~11–21% O₂
• Safe inert condition: < 8% O₂
• Target onboard: 5–7% O₂
Above this, one spark is enough.
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3. How an Inert Gas System Works (Reality, Not Diagrams)
Step 1 – Gas Production
Flue Gas Source
• Boiler uptake gas
• Rich in:
• Nitrogen (N₂)
• Carbon dioxide (CO₂)
• Low oxygen by nature
Typical flue gas O₂: 2–5%
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Step 2 – Scrubbing & Cooling
Hot flue gas enters the IG scrubber:
• seawater sprayed counter-flow
• removes:
• soot
• particulates
• sulphur compounds
• cools gas to safe temperature
Failure here = acidic gas entering tanks
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Step 3 – Blower Pressurisation
IG blowers:
• provide flow
• maintain tank overpressure
• prevent air ingress
Typical delivery pressure:
• 200–300 mmWG above atmosphere
Blowers must:
• start automatically
• trip safely
• never overspeed
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Step 4 – Oxygen Analysis
Oxygen analyser continuously samples IG:
• if O₂ > 8%:
• alarms activate
• system shuts down
• IG supply is blocked
This is the most falsified sensor on ships.
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Step 5 – Deck Seal
The deck seal:
• prevents backflow of tank vapours
• uses a water column as a flame barrier
If deck seal fails:
• hydrocarbon vapours can reach:
• engine room
• boiler uptake
• hot surfaces
Several historical explosions started here.
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Step 6 – Pressure/Vacuum (PV) Breakers
PV breakers:
• protect tank structure
• open on:
• overpressure
• vacuum collapse
Failure modes:
• seized pallets
• frozen drains
• incorrect settings
PV breaker malfunction = structural failure risk.
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Step 7 – Distribution
IG flows via:
• master riser
• branch lines
• tank isolation valves
Poor distribution causes:
• stratification
• local oxygen pockets
• false confidence
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4. Major Components — Function, Failure & Consequences
4.1 Inert Gas Generator / Source
Failure:
• boiler flame instability
• poor combustion
• high oxygen content
Consequence:
• unsafe IG delivered
• system shutdown during cargo ops
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4.2 Scrubber
Failure:
• poor wash water flow
• corrosion
• sulphur carryover
Consequence:
• acidic condensation
• tank coating damage
• toxic atmosphere
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4.3 IG Blowers
Failure:
• bearing failure
• vibration
• loss of capacity
Consequence:
• loss of positive pressure
• air ingress
• explosive mixture formation
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4.4 Oxygen Analysers
Failure:
• sensor drift
• blocked sampling lines
• deliberate bypass
Consequence:
• system believes gas is safe when it is not
This is where criminal liability often begins.
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4.5 Deck Seal
Failure:
• low water level
• frozen seal
• incorrect alignment
Consequence:
• flashback path created
Deck seal failure is unforgiving.
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5. Types of Inert Gas Systems
5.1 Flue Gas Inert Gas Systems
Advantages
• simple
• low capital cost
• proven
Disadvantages
• depends on boiler operation
• sulphur contamination
• less precise oxygen control
Common on:
• older crude/product tankers
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5.2 Nitrogen Inert Gas Systems
Advantages
• high purity nitrogen
• independent of boilers
• precise oxygen control
Disadvantages
• higher cost
• power consumption
Common on:
• chemical tankers
• LNG/LPG carriers
• modern product tankers
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5.3 Membrane Inert Gas Systems
Advantages
• compact
• energy efficient
• modular
Disadvantages
• limited capacity
• sensitive to contamination
Used on:
• smaller vessels
• offshore units
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6. Oxygen, Pressure & Chemistry — Numbers That Matter
Parameter Typical Limit
Oxygen in IG < 8%
Target O₂ in tanks 5–7%
IG temperature < 65°C
Tank pressure +200 mmWG
CO₂ content 12–14%
Small deviations = large risk.
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7. Operational Phases & IGS Behaviour
Loading
• IG must maintain overpressure
• oxygen trending critical
• failure = loading stop
Discharge
• IG replaces cargo volume
• prevents vacuum
• stabilises atmosphere
Ballast Voyage
• tanks remain inerted
• oxygen creep monitored
Tank Cleaning
• IG must be isolated
• ventilation required
• many fatalities occur here
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8. Failure Modes & Accident Pathways
8.1 Air Ingress During Discharge
Cause:
• blower failure
• PV breaker malfunction
Result:
• explosive mixture forms unnoticed
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8.2 False Oxygen Readings
Cause:
• blocked sampling
• sensor poisoning
• deliberate tampering
Result:
• unsafe gas accepted as safe
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8.3 Deck Seal Loss
Cause:
• poor maintenance
• freezing
• incorrect filling
Result:
• flashback potential
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8.4 Human Error During Tank Entry
Cause:
• misunderstanding “inert”
• bypassed permits
• time pressure
Result:
• asphyxiation or explosion
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9. Human Factors & Historical Lessons
Most IGS accidents involve:
• complacency
• poor training
• misunderstood alarms
• “temporary” bypasses
IGS does not forgive shortcuts.
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10. Regulatory Framework & Enforcement
Core Regulations
• SOLAS Chapter II-2
• IBC Code
• IGC Code
• Class rules (DNV, LR, ABS)
Port State Focus
• oxygen analyser calibration
• deck seal condition
• alarm functionality
• crew knowledge
Failures can:
• detain vessel
• stop cargo ops
• void insurance
• trigger criminal investigation
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Final Engineering Takeaway
Inert Gas Systems do not prevent explosions by luck.
They prevent them by continuous discipline.
The system assumes:
• sensors tell the truth
• valves are aligned correctly
• crew understand invisible risks
When any one of those fails:
the ship becomes a bomb.
IGS is not a background system.
It is a constant guardian that must never be ignored.