IGS (Inert gas system). After the Convention on Safety of Life at Sea 1974 and that on Prevention of Marine Pollution from Ships 1973 entered into force oil tankers had to be fitted with such a system.
Inert gas is produced and piped to the cargo tanks in order to reduce the oxygen content of the yap ours in the tank. This makes the atmosphere in the cargo tanks non-flammable when the tanks are not completely gas free. Hydrocarbon gas in oil tanks cannot burn in an atmosphere containing less than 11 per cent oxygen by volume. If the oxygen content is kept below, say, 5 per cent by volume, fire or explosion should not occur in the space.
The inert gas is produced in a fixed IGS. There are usually two methods of producing inert gas in an oil tanker. Ships with main or auxiliary boilers (for example, in steam turbine ships) can use the gas from the engine flue which contains only about 2 to 4 per cent oxygen. This is “scrubbed” with sea water in a scrubbing tank to remove sulphur dioxide (a very corrosive substance) and other harmful substances and then blown into the tanks through a fixed piping system. Alternatively, a vessel without such boilers (for example, a diesel engine vessel, the most common type) cannot use the exhaust from the diesel engine. The oxygen content will be far too high. An inert gas generator is used in which diesel or light fuel oil is burned to produce the gas. The gas is then scrubbed and cooled and piped to the tanks as described above. The IGS also contains barriers to prevent any petroleum gas from the tanks returning to the engine room where the IGS plant is located. These barriers take the form of a deck water seal and various nonreturn valves.