Ball Float Valves
Ball float valves (also known as float-vent valves) are fitted on the bottom of the vent line inside the tank. 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. Flow restrictors must begin restricting flow when the tank is 90 percent full or 30 minutes before overfilling. If the tank top is tight, the ball float valve can create enough back pressure to restrict product flow into the tank, which can notify the driver to stop delivery to the tank. However, if the UST has loose fittings or other nontight tank top components, flow will not be restricted and an overfill will occur.
NOTE: Manufacturers do not recommend using ball float valves with suction piping, pressurized delivery, or coaxial Stage I vapor recovery.