American Mobile Power ASV-0500 - Anti-spill Vent - 1/2" NPT

Anti-spill vents move air and vapor from inside the tank to the outside and also move air from outside the tank to inside. When the fuel inside the tank heats up it expands; tanks heat up from the sun and from hot aerated fuel returning to the tank from the engine. The vent releases, to atmosphere, the air that is displaced when the fuel expands and prevents pressure build up inside the tank. When fuel cools inside the tank it shrinks in volume and the vent prevents a vacuum occurring inside the tank. It also helps remove the air when the tank is filled with fuel.

Anti-spill vents contain a float to seat and seal in the vent body when the vent is inverted from the vertical position. In the inverted position, the steel ball presses the float against the seat in the body. This prevents fuel from leaking in the event of a truck rollover. The float also seals and stops fuel sloshes from exiting the tank.