I highly doubt that vapor recovery allows fuel to return to the underground tank. I’ve never heard that.
Makes. Most of the excess fuel resides in the fuel pump coaxial filler hose. Have you ever gone to a gas pump, put the nozzle in, hit the lever and it clicks after one flush? Vapor recovery bumped the fuel in the hose against a check valve. Wait a second and all the fuel from the last guy who tried to fill up went into your tank.
For further reading, see Phase I, Phase II, and Vacuum Assisted Vapor Recovery.
Gas stations operate in a semi-closed system to keep vapors out of the air. When you put fuel in the car, the vapors that come out have to go somewhere. Returning to the ground (with any liquid shock that vapor recovery can handle) they condense and drain into the next machine. When the tanker truck fills the underground tanks at the gas station, the hoses on the truck work similarly to the filler nozzle on the pump. The fuel comes out of the truck, the steam comes out of the underground tank and back into the truck. The truck returns to the fuel farm, dumps the vapor into a condensation tank, where it is recycled and resold. Something, emissions and recovering profits that were lost when Exxon/Shell/BP decided it was a good idea to throw oil on the baby ducks.
There is always ignorance. The guy who pulls the nozzle up to the edge of the tank filler neck to try to get the best “wash”. This is how you deliberately overflow a canister with coal. For the rest of us who just want to get to work or back on the road, when it clicks, stop. It’s full.
