Vent (submarine)

In submarine technology a vent is a valve fitted to the top of a submarine's ballast tanks to let air escape from the top of the ballast tank and be replaced by water entering through the opening(s) called "flood ports" or "floods" at the bottom of the tank.

The vents which are used to allow water to enter the submarine's main ballast tanks when it submerges are the main vents and it is air escaping through these that accounts for the spray sometimes seen when submarines dive.

Replacing ballast water with lighter air reduces the vessel's total mass.

The displacement of a submerged submarine with full ballast tanks is greater than the displacement on the surface with empty ballast tanks where buoyancy equilibrium is restored.

Once on the surface, in order to conserve supplies of compressed air, a lower-pressure compressor is then used to remove the remainder of the water from the tanks.