During the filling process, the rising liquid displaces the vapors present in the upper portion of the tank. These displaced vapors have to escape. If there is not a tight connection between the delivery hose and the tank, some vapors flow out around the hose nozzle while others are expelled through the vent pipe. Unless captured, the escaping vapors can make a major contribution to an air pollution problem.
To deal with the problem, the petroleum industry developed Stage I gasoline vapor recovery systems. The process was referred to as Stage I because it addressed capture of vapors during the first phase of the gasoline transfer process at a motor fuel station: transfer of gasoline from the tanker truck to the storage tanks. The capture of vapors at the point where fuel later flows into automobile and truck tanks is referred to as Stage II.
In a typical Stage I system, the gasoline transport driver connects two hoses between the delivery truck and the storage tank to which the delivery is being made. Gasoline from the truck flows through one hose into the storage tank. Displaced vapors, pushed out of the storage tank by the rising liquid, flow into the second hose. These vapors are pulled into the storage-tank compartment on the delivery vehicle by the vacuum created as a result of the emptying of product from the compartment.
After completion of his gasoline deliveries, the driver returns to the terminal. There, the gasoline vapors that have been collected in his or her truck compartments are removed when the truck is refilled with product. Some may be incinerated. Most, however, are processed through compression, adsorption, or refrigeration. In this process, most of the vapors are converted back to a liquid gasoline state and returned to storage. More important, however, is the fact that the use of Stage I systems prevents vapors displaced during the filling of storage tanks from escaping into the atmosphere.
See also Stage II vapor recovery.