Inert Gas System
Crude oil is a volatile and gaseous cargo, and the C O W system causes a large quantity of Aammable vapours to be released.To prevent this, tankers operating COW must inert the tank spaces by keeping oxygen content below 5% from the plant and 8% in the tanks. 1)
For fire or explosion to occur three íactors must be present: fuel, oxygen and ignition. Hydrocarbons vvill not bum in air unless they are mixed in the correct proportions.The lower and upper explosive limits are, respectively, I % and 1 0% hydrocarbons in air In a tanker carrying cargo it is clearly not possible to remove the fuel from the equation, non despite strenuous efforts, has it proved possible to eliminate all sources of ignition.This leaves oxygen as the only element that can be eliminated in normal operations, provided the proper procedures are carried out. 2)
Air consists of 21 % oxygen and 79% nitrogen, plus traces of other gases. There must be a minimum of 11% oxygen present before combustion can take place.The International Maritime Organization (IMO) therefore requires that oxygen be reduced to a maximum of 8% by volume in the atmosphere of a ships cargo tanks.4)
To achieve this, an inert gas, such as carbon dioxide or nitrogen, is introduced into the tanks. There is a plentiíul supply in the form of exhaust gases produced by the ships main engines and auxiliaries, although if more inert gas is needed, an auxiliary boiler may have to be used or a special inert gas generator fitted. The inert gases are scrubbed and cooled beíore being injected into the tanks. As the cargo level falls, more inert gas is pumped in to prevent cargo gases vaporising and being replaced by air, which might raise oxygen levels above the danger threshold. 5)