A Kingston valve is a venting device fitted on ships and submarines, typically at keel level, used to fill or purge ballast, fuel, or water tanks. This action allows tanks to be flushed and cleaned, or, in the case of submarine ballast tanks, for the vessel to adjust its buoyancy in preparation for submerging. The Kingston valve works in conjunction with vents at the top of the tank that allow air trapped in the tank to escape. The valves are typically automated, but will normally feature a manual override. Kingston valves are also sometimes used to scuttle, or purposely sink, a vessel.
Most large ships and submarines have tanks build into the keel area, or the lowest part, of their hulls used to hold fuel, fresh water, or ballast water. In the case of fresh water and fuel tanks, periodic flushing and cleaning is necessary and requires an access point to open water. The ballast tanks in submarines are used to adjust the overall buoyancy of the vessel to dive and surface. In both cases, one common method of opening these hull cavities to the open sea in a controlled fashion is the use of a Kingston valve arrangement.
Typically located along the keel line of the vessel, the Kingston valve allows water to enter the tanks at a rate dictated by the valve actuation level. For this system to work effectively, the tank must be fitted with purge risers and vent valves at the top of the tank that allow air to escape, and thereby permitting the water to enter. The actuation levels, or the extent to which these valves are opened, can be carefully balanced to exercise full control of the flood rate of the tank in question. This is typically achieved remotely from the bridge, although most Kingston valve systems feature an additional manual override.
Once the cleaning or diving operations are complete, the seawater in the tanks is removed by closing the vent valves, opening the Kingston valves, and pumping air into the tank. The air displaces the water, pushing it back out of the open Kingston valves and emptying the tank. Although simple in theory, a Kingston valve arrangement can be used to maintain very fine buoyancy control of vessels such as submarines. The valves are also used on occasion to scuttle ships earmarked for destruction by allowing the tanks and hull to flood unchecked. One famous vessel sunk in this fashion was the German World War Two aircraft carrier the Graf Zeppelin.