Inert Gas System
An Inert Gas System (IGS) keeps the atmosphere in the cargo tanks of a tanker below the oxygen level at which the hydrocarbon vapour could ignite. By supplying inert gas — flue gas from the boiler, or gas from an inert-gas generator, with a low oxygen content — it makes the tank atmosphere too oxygen-poor to support combustion.
Inert gas is used during cargo discharge, tank cleaning and ballasting, and the tanks are kept under a slight positive pressure of inert gas. The IGS is mandatory on oil and chemical tankers above a certain size and is one of the most important safety systems on a tanker, since an oxygen-rich tank atmosphere with cargo vapour is explosive.
The space above the cargo in a tanker is full of flammable vapour; the only thing reliably keeping it from exploding is the lack of oxygen maintained by the inert gas system. Loss or misuse of the IGS has led to catastrophic tanker explosions, which is why it is heavily regulated and closely operated.
Why do tankers use inert gas?
To keep the oxygen level in the cargo tanks below the point at which the hydrocarbon vapour could ignite, preventing explosions during discharge, tank cleaning and ballasting.