As fuel level lowers, air flows into the tank, and it only flows out when the tank is filled. To maintain atmospheric pressure, all tanks should be fitted with vents (or vent filters) to allow sufficient air movement in and out of the vessels. As tanks are emptied, clean air should flow inside the tank, and when it is filled, it should escape. The tank’s vent cleans the air of its dirt and microbial contaminates that can damage the content in the tank.