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A type of valve used in the piping of a gasoline station.
The distance between dispensers and storage tanks in a modern gasoline station is 60 feet or more. The systems are designed so that the piping gradually slopes back toward the tanks. If there was an open pipe between the dispenser and the tank, however, each time a pump was turned off all product then present in the piping would flow back into the storage tank. Each time a pump was turned back on it would have to start afresh in moving product all the way from the storage tank, through the empty pipe, into the dispenser.
This would cause a delay in dispensing. It would also mean that the liquid seeking to flow through the piping would have to sputter its way through the air and vapor that had accumulated in the piping after the pump had been turned off from its previous use.
To prevent this from happening, engineers have designed a number of valves that can be positioned at various points in the piping–valves whose function is to check the backward flow of product when a pump is turned off. The valves hold product in the line, and are thus said to keep the system “primed.”
These various valves are referred to as union check valves, angle check valves, vertical check valves, and foot valves. Even though a variety of check valves are available, only one such valve is required in a typical piping system and it can be placed anywhere along the piping.
All check valves operate in much the same fashion. Inside the metal body of each valve is a disk–or two metal disks. These disks are called “poppets.” When the action of the pump begins, the disks are pulled or pushed in the direction of the dispenser, allowing liquid from the tank to flow around the poppets. When the pumping action stops, however, the poppets drop back down to their normal position, seating themselves on a lip that runs around the inside of the valve.
Once the poppets seat themselves inside the valve, liquid in the piping above the valve–toward the dispenser–is trapped. It cannot flow back into the tank. The system thus remains “primed” until pumping action is next resumed.
Although all check valves operate on the same principle, different locations in the piping system call for somewhat different design configurations. Angle check valves are used at the top of the storage tank. Vertical check valves are used at the dispenser location. Foot valves are used inside the storage tank, at the lower end of the suction stub. Union check valves are used inside a union, usually at the pump.
Check valves are not required in the piping from aboveground storage tanks. However, if a check valve should be installed in aboveground storage tank piping, it should be accompanied by a thermo-relief valve to allow product expansion caused by solar heating to be relieved back to the storage tank. Thermo-relief valves are also often required for use with submersible-pump piping systems.
See also Suction system.