Air vent head valves are used in ships, navy tanks, oil tanks helps to have airflow in and out of the ship at the same time prevent seawater from entering in. Seawater is prevented by the ball lock in the valve heads from flow in. The ball lock is released and the air flows through the valve head, once there is no sea water over the surface of the head. There is also a box type air vent head valve available which work in a different principle