Flue Gas IG System

What makes a flue gas inert gas system different from a dedicated IG generator

A flue gas inert gas system draws its gas directly from the ship's boiler uptake rather than burning fuel specifically to produce inert gas. Boiler flue gas already has an oxygen content low enough, typically around 2 to 4 percent, to inert cargo tanks once it has been cleaned and cooled, so a flue gas system avoids the extra fuel consumption a dedicated combustion-type generator needs. The trade-off is dependency: flue gas systems only produce usable gas while the boiler is actually firing at a sufficient rate, which is straightforward on a steam tanker running cargo pumps off the boiler but can be a limitation on vessels where boiler load varies independently of inerting demand.

Main components

Scrubber tower

Flue gas is drawn through a water spray scrubber that cools the gas and removes sulphur dioxide and particulates before it goes anywhere near the cargo tanks, since raw flue gas is both too hot and too dirty to inject directly.

Gas blower

One or more blowers, sized with standby capacity, raise the scrubbed gas to the pressure needed to push it through the deck main and maintain positive pressure in the cargo tanks against any inward leakage.

Deck water seal

A water-filled seal on deck, positioned between the blower and the cargo tanks, that lets gas flow forward to the tanks under normal operation but automatically blocks any flashback of cargo vapour toward the machinery space if pressure conditions reverse.

Non-return valve and deck isolating valve

A mechanical non-return valve backs up the water seal as a second line of defence against reverse flow, with an isolating valve allowing the whole system to be shut off from the cargo tanks for maintenance.

Oxygen analysers

Continuous oxygen monitoring at the scrubber outlet and, separately, in the cargo tank atmosphere itself, since the gas leaving the scrubber and the gas actually present in a tank some distance away are not always the same reading.

Selection and sizing

Capacity is set by the volume of the largest cargo tank or tank group that has to be inerted or gas-freed within the operational time allowed, plus the topping-up flow needed to hold tank pressure positive during discharge as cargo is pumped out and gas space expands. Because output depends on boiler firing rate, the system's practical capacity has to be checked against realistic boiler load during cargo operations, not against the boiler's maximum rated output.

Regulations and class

SOLAS Chapter II-2 requires inert gas systems on the tanker types and sizes specified in the regulation, with oxygen content in the cargo tank atmosphere kept at or below 8 percent by volume, and typically maintained closer to 5 percent or less in practice as a working margin. Class surveys check the deck water seal, non-return arrangements and oxygen analyser calibration on a running schedule, since these are the components that prevent flammable vapour from reaching an ignition source outside the tank.

Typical faults

• Deck water seal running dry or with insufficient water level, removing the physical barrier against flashback
• Scrubber water supply interrupted, allowing hot, corrosive, unscrubbed gas toward the blower and deck main
• Oxygen analyser drift or fouling, giving a false low reading while actual tank oxygen content has risen
• Blower capacity insufficient to keep pace with cargo discharge rate, letting tank pressure fall and drawing in outside air

What to look for in a supplier

• Scrubber and blower capacity matched to the vessel's actual boiler output under realistic cargo operation conditions, not a nameplate best case
• Deck water seal design proven for the vessel's expected list and trim range, since a seal that only works upright is a real limitation at sea
• Oxygen analyser systems with independent calibration gas supply and a maintenance record that can be checked at survey
• Documented flashback protection philosophy covering both the water seal and the mechanical non-return valve as separate, redundant barriers

Check the deck seal water level before every cargo operation, not on a calendar schedule; it is the one component standing between a routine inerting job and flammable vapour reaching the engine room side of the system.