Underground Storage Tanks

Ball float valves (also known as float-vent valves) are fitted on the bottom of the vent line and hang down several inches below the top of the UST. When the product level is below the cage, the ball rests at the bottom of the cage and the vent line is open (see illustration below, left). As the level of the product rises above the bottom of the cage, the ball floats on the product and rises in the cage. As the delivery continues, the ball eventually seats in the vent line (see illustration below, right) and restricts vapor flowing out the vent line BEFORE the tank is full. If all tank fittings are tight, the ball float valve can create enough back pressure to restrict product flow into the tank--which can notify the driver to close the truck's shutoff valve. However, if the UST has loose fittings, sufficient back pressure may not develop and will result in an overfill.

NOTE: Manufacturers do not recommend using ball float valves with suction piping, pressurized delivery, or coaxial Stage I vapor recovery.

Ball Float Valve with the ball at the bottom of the cage and the vent line open. This indicates the product is below the cage and the vent line is open.

Ball Float Valve rises as the product rises. The ball eventually seats in the vent line and restricts vapor flowing out of the vent before the tank is full.

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