An inerting system increases the safety of a fuel tank, ball mill, or other sealed or closed-in tank that contains highly flammable material. Inert in scientific terminology means ‘not readily reactive with other elements; forming no chemical compounds or something that is not chemically reactive.’ An inert fuel tank is non-combustible. The inerted space may be on land, or aboard ship, or airborne.
A fire requires three elements: heat (ignition source), fuel and oxygen (or air) to initiate and sustain. A fire can be prevented by removing any one of the three elements. If presence of an ignition source can not be prevented in a fuel tank then a fuel tank can be made inert by (1) reducing the oxygen content of the ullage (space above the fuel that contains air and fuel vapors) below the threshold required for combustion, or (2) by reducing the air-fuel ratio of the ullage below the minimum threshold (Lower Flammability Limit) required for combustion, or (3) increasing the fuel air ratio above the maximum threshold (Upper Flammability Limit) that can support combustion.
At present, fuel tanks are rendered inert by adulterating the ullage with an inert gas such as nitrogen, nitrogen enriched air, steam or carbon dioxide. This reduces the oxygen content of the ullage below combustion threshold. Without sufficient oxygen in the tank, the fuel vapors in the ullage cannot ignite, and an explosion does not occur. Alternate methods based on reducing the ullage fuel air ratio below Lower flammable limit (LFL) or increasing the fuel air ratio above the Upper flammable limit (UFL) have also been proposed.
Read more about Inerting System: Inerting Gas Systems in Aircraft, Current FAA Rules On Inerting in Aircraft, Other Methods of Inerting Fuel Tanks
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