The traditional method of adding gasoline to, or withdrawing gasoline from, an aboveground tank or a compartment of a tank truck is through an opening in the top of the tank. With a highly volatile liquid like gasoline, however, the splashing and turbulence which accompanied the loading procedure created an excess of vapors, vapors that escaped into the atmosphere, creating an environmental problem.
Confronted with the problem, engineers determined that one way to deal with it was to load gasoline into an aboveground tank or tank vehicle through an opening in the sidewall, near the bottom. In the loading process, the gasoline would thus enter below the usual level of the liquid already in the tank.
This, in turn, would materially reduce the volume of vapors generated in the filling process.
Although the bottom loading principle is easy to understand, creation of a coupling and valve system that would permit a tight connection between the delivery hose and the tank required a sophisticated design. The tank portion of the bottom loading valve had to be built so that it would not open until the delivery hose was securely in place, and so that it would close before the delivery hose was disconnected.
Bottom loading of aboveground tanks and tank vehicles is commonplace today. To minimize problems in the use of bottom-loading systems, API has adopted a recommended practice that specifies the location of bottom loading valves, as well as the configuration of connective hardware to be used in a bottom-loading hookup.
See also Top loading.