A disk inside a pipe or tube that is capable, when in the “closed” position, of restricting the flow of liquid.
An example of a flapper-valve application in petroleum marketing operations is offered by certain types of mechanical over fill prevention devices used in an underground storage tank. These devices are installed in the drop tube, near the top of the tank. They are held in place by a hinge on one side of the disk. Outside the tube there is a float mechanism, which is attached to the flat disk, the flapper valve, located inside the tube.
Normally, the float mechanism is in a relaxed position, resting vertically along the side of the drop tube. When the float is in this position, the flapper disk inside is also held in a vertical position against the side of the tube. Liquid entering the tank, through the drop tube, falls, unimpeded, past the flapper valve.
During a filling operation, however, the liquid level in the tank rises. When the level nears the top of the tank, it causes the float mechanism to begin moving upward. This movement causes the flapper to move into the stream of liquid flowing past. The pressure of the liquid snaps the flapper shut, shutting off the flow of liquid through the drop tube.