Inert gas is produced on board crude oil carriers (above 20000 tonnes) by using either a flue gas system or by burning kerosene in a dedicated inert gas generator. The inert gas system is used to prevent the atmosphere in cargo tanks or bunkers from coming into the explosive range.[5] IG keeps the oxygen content of the tank atmosphere below 8% (on crude carriers, less for product carriers and gas tankers), thus making any air/hydrocarbon gas mixture in the tank too lean to ignite. IG is most important during discharging and during the ballast voyage when morehydrocarbon vapour is likely to be present in the tank atmosphere. Inert gas can also be used to purge the tank of the volatile atmosphere in preparation for gas freeing - replacing the atmosphere with breathable air - or vice versa.
The flue gas system uses the boiler exhaust as its source, so it is important that the fuel/air ratio in the boiler burners is properly regulated to ensure that high quality inert gas is produced. Too much air would result in an oxygen content exceeding 5%, too much fuel oil would result in carryover of dangerous hydrocarbon gas. The flue gas is cleaned and cooled by the scrubber tower. Various safety devices prevent overpressure, return of hydrocarbon gas to the engine room, or supply of IG with too high oxygen content.
Gas tankers and product carriers cannot rely on flue gas systems (because they require IG with O2 content of 1% or less) and so use inert gas generators instead. The inert gas generator consists of a combustion chamber and scrubber unit supplied by fans and a refrigeration unit which cools the gas. A drier in series with the system removes moisture from the gas before it is supplied to the deck. Cargo tanks on gas carriers are not inerted, but the hold space around them is. This arrangement allows the tanks to be kept cool using a small heel of cargo while the vessel is in ballast while retaining the explosion protection provided by the inert gas.
