The gasoline sold at service stations is stored underground in buried tanks. Each holds several thousand gallons of gas. There are at least two of these tanks per station and each tank usually holds a different grade of gas. Having the gas tanks underground presents an obvious problem: If the gas must get to a dispenser (and your car's gas tank) located above ground, it has to defy gravity in order to get there -- like a waterfall flowing uphill. But moving the gas from its subterranean hideaway up to street level isn't as difficult as you might think.
Most service stations do the job using one of two types of pump -- a submersible pump or a suction pump:
- A submersible pump, as its name implies, is submerged below the surface of the liquid, where it uses a propellerlike device called an impeller to move the fuel upward. Slanted blades on the rotating impeller push the water the way the blades on an electric fan push air.
- A suction pump moves the gas using the principle of unequal pressure. A pipe is inserted in the water. A motor above the fluid level removes enough air from the pipe to decrease the air pressure above the gasoline. The motor continues to remove air until the air pressure above the gasoline is lower than the air pressure pushing down on the gas outside the pipe. The weight of the surrounding air forces the gas inside the pipe upward even as gravity tries to pull it back down. When the air pressure inside the pipe is low enough, the gas simply climbs up into the aboveground dispenser.
The major advantage of a submersible pump over a suction pump is that the impeller can push water over longer vertical distances. However, because the gas tanks at most service stations are located only a few feet below the dispenser, a suction pump is usually more than adequate for the task at hand. There's more to this process, though, and we'll explore it further on the next page.