Abstract: A method for collecting and maintaining oil at elevated pressures eliminates the undesirable discharge of volatile components to the atmosphere. At least two tanks are interconnected by a valved liquid inlet manifold and a valved gas outlet. While one tank is being filled with pressurized petroleum, the second tank is in valved vapor communication with the first tank and both tanks are isolated from atmosphere. This configuration permits the filling and storage of the first tank at pressures above atmospheric. When the first tank is full to capacity, the inlet manifold directs liquid into the second tank. Before unloading the first tank to a tank truck or pipeline, any excess vapor pressure is vented to the atmosphere. The process is repeated by introducing the second tank to an emptied first tank.