Inert Gas System: What It Means
The Inert Gas System (IGS) is a fire and explosion prevention system that fills cargo tanks with non-flammable gas, displacing oxygen below the level needed to support combustion.
Why Tankers Need IGS
Crude oil and petroleum products give off hydrocarbon vapors in the empty (ullage) space above the cargo. If this vapor mixes with oxygen in the right proportions (roughly 1–10% hydrocarbon in air), the result is an explosive mixture — a single spark can cause catastrophic explosion.
IGS removes this risk by keeping oxygen levels below 8%, making the atmosphere in the tank non-explosive regardless of hydrocarbon vapour concentration.
How IGS Works
Source of inert gas: On most crude tankers, inert gas comes from the main boiler flue gas. The exhaust from burning fuel in the boiler is approximately 14–15% CO₂, 3–5% O₂, and 80%+ N₂ — much lower oxygen than the atmosphere (21% O₂).
Process:
- Flue gas drawn from boiler
- Passed through scrubber tower (cooled to ~32°C, SO₂ and soot removed by sea water wash)
- Sent by blowers through the deck seal and into cargo tanks
- Tanks maintained at slight positive pressure (100–1,000mm WG above atmospheric)
On LNG carriers and some modern product tankers: Nitrogen generators produce high-purity nitrogen (99.9% N₂, <0.1% O₂) — even safer and no dependency on boilers running.
Critical IGS Operations
| Operation | IGS Action |
|---|---|
| Before loading | Purge with IG to below 8% O₂ |
| During loading | Maintain positive pressure as cargo rises |
| During discharge | Supply IG to replace outgoing cargo |
| Tank cleaning | Maintain IG until washing complete |
| Gas freeing | Controlled removal of IG before dry-docking |
Common IGS Failures and Consequences
- Deck water seal failure: Gas can flow back from deck into IGS system — leading to fire near flue gas outlets
- O₂ analyser failure: Tank O₂ levels may rise without detection — cargo operations must stop
- Positive pressure loss: Air ingress into tanks — explosive mixture can form
- Scrubber failure: Unscrubbed hot acidic gas can damage pipes and seals
Every incident with IGS failure has potential for catastrophic explosion. This is why IGS maintenance is treated as a critical safety priority and Port State Control inspectors check IGS records carefully.
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