a heavy liquid such as water in the bottom of a large tank is separated from a light liquid such as fuel which is on top of the water by the heavy liquid displacing a float actuated valve off its seat to open a drain . when the heavy liquid has drained from the tank , the float of the float actuated valve which has a specific gravity less than the heavy liquid and greater than the light liquid will sink with the level of the heavy liquid until the valve moves back to its seat to prevent the light liquid from draining out of the tank .