Gasoline vapors accumulate in automobile and truck tanks, above the liquid level. When the tanks are filled, the rising liquid forces these vapors to seek an escape route. When a conventional nozzle is used, the displaced vapors flow out around the nozzle spout and into the air. When thousands of vehicles are refueled, in an urban area, the escaping vapors add significantly to the region’s air pollution.
In areas where air pollution is serious, environmental regulations require the use of Stage II gasoline vapor recovery systems. Such systems are referred to as Stage II because they address the second phase in the capture of vapors at a gasoline station. Stage I refers to the capture of gasoline vapors when fuel is delivered to the station’s storage tanks.
There are two basic types of Stage II systems. One is the balance system, which transfers vapors from the vehicle tank to the station storage tank without the assistance of an external force, such as a vacuum. The second type is a system that relies on a vacuum source to help move the vapors out of the vehicle tank and into the storage tank.
A key feature in the balance system is a hose nozzle that makes a tight connection with the fill pipe on the vehicle fuel tank. The nozzle spout is fitted with an accordion-like bellows that presses snugly against the fill pipe lip. Balance systems are designed with controls that prevent fuel from flowing into the vehicle tank unless there is a tight connection between the nozzle bellows and the fill pipe.
When fuel delivery begins, gasoline flows from the dispenser into the vehicle fuel tank. Vapors displaced by the rising liquid seek an escape route. They find that route in an open port in the nozzle bellows. The vapors flow into the port, through the nozzle bellows, through a coaxial hose that connects the nozzle to the dispenser, and finally on through a vapor-return pipe back into a storage tank–typically, the same tank from which gasoline is being pumped.
This movement of vapors is accomplished without use of any external force. The withdrawal of vapors from the vehicle tank is balanced by the simultaneous addition of the vapors to the underground tank from which liquid is being withdrawn–hence the name, balance system.
To improve the efficiency and convenience of Stage II systems, it is often deemed desirable to assist the transfer of vapors. This is accomplished through use of a vacuum. Vacuum systems are of two types: vacuum-assist systems and aspirator-assist systems.
Although use of vacuum systems is usually optional in conjunction with underground storage tanks, such systems are required in facilities where aboveground tanks are installed. Vapors displaced from a motorist’s tank that condense may encounter resistance to flow “uphill” to the aboveground storage tank without assistance. This assistance is provided through creation of a vacuum that has the effect of propelling the vapors back through the vapor return line into the storage tank.
In vacuum-assist systems, a vacuum pump provides the vacuum power. In aspirator-assist systems, the vacuum is created through an aspirator device.
See also Stage I vapor recovery.