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A method used in gasoline vapor recovery which permits vapors in the tank being filled to be transferred to another tank without intervention of any external power source.
In Stage II vapor recovery systems, vapors present in the vehicle fuel tank must be removed from the tank without being allowed to escape into the atmosphere. When the balance refueling process begins, the vapor recovery nozzle is inserted into the fill pipe of the vehicle’s tank. The nozzle bellows makes a tight connection with the fill pipe. As gasoline flows through the nozzle and into the tank, the rising level of liquid forces the vapors, present in the tank, into the bellows that surround a vapor port on the nozzle. The continuing pressure created by the rising liquid pushes the displaced vapors back through the bellows, through the vapor-return port of the nozzle, and then on through the vapor-return portion of the dispenser hose.
The same pressure source continues to push the vapors on through the piping in the dispenser and then through the vapor-return pipe under the driveway, all the way back to the underground tank from which the liquid is flowing.
This arrangement constitutes a closed system. Vapors displaced while filling one tank (the vehicle tank) will follow the path of least resistance and flow back through the system into an underground storage tank.
Thus, vapors in the vehicle tank are transferred to the underground storage tank without the help of any type of secondary power source. The transfer is accomplished entirely as a result of the normal in-flow of liquid into the vehicle tank and the offsetting liquid out-flow from the storage tank. Because of this balance–liquid in, vapor out and liquid out, vapor in–the method is referred to as a balance system.
Stage II balance systems, employed for most underground tank installations, are less efficient when used with aboveground tanks because of the thermal effects of the sun and the higher pressure required to force vapors upward into aboveground tanks.