This component is constructed from several parts that normally include:
1. a metal fill and drop tube
2. a float mechanism
3. flapper valve with hinge assembly
When the level of product in the tank is less than 90% the flapper valve is in the open position. However, whenever a transfer of product is made to the tank, the level of product in the tank begins to rise.
If the level of product reaches the float it will begin to rise with the product. As product continues to be transferred into the tank, the float will continue to rise and begin to move the flapper valve toward the closed position. As the flapper valve begins to close it moves into the stream of product being transferred into the tank.
When the level of product reaches 90% of the maximum volume of the tank the flapper is forced into the closed position by the product being transferred into the tank. This restricts the flow of any further product into the tank to a rate slow enough to prevent an overfill and allow the transfer operator to stop the transfer process.
Some flapper valves have the added feature that if the product level reaches 95% of the maximum tank volume the valve assembly fully closes and completely stops any further delivery of product into the tank.
Flapper valves must be compatible with the product stored in the UST, and have a COMM 10 Material Approval for use with USTs installed in the State of Wisconsin.