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Membranes, Molecules and the Science of Permeation |
The term "vehicle refueling vapors" refers to the vapors displaced from the vehicle fuel tank during refueling. Storage tank evaporative vapors, on the other hand, are vapors that are created as gasoline undergoes a change from liquid phase to vapor phase. This change must occur to re-establish an equilibrium vapor concentration in the space above the liquid in fixed roof storage tanks (USTs or ASTs). The vapor space concentration is driven below natural levels by the ingestion of lean vapors or air into the storage tank during vehicle refueling. If the natural equilibrium vapor concentration is momentarily reduced, liquid gasoline will evaporate until the equilibrium concentration level is re-attained. One gallon of liquid gasoline will expand to approximately 520 gallons of vapor at 40 percent hydrocarbon concentration. Therefore, storage tank pressure will increase rapidly as relatively small amounts of liquid evaporate. This increased pressure can result in vapor emissions from pressure/vacuum relief valves or through any leaks in the vapor piping. |
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