The required pressure in the reservoir is maintained by means of compressed air. Compressed air is generally introduced into the reservoir from the top at a pressure specified by the manufacturer. In order to control this pressure, a pressure control device such as a pressure regulator is provided in the airline entering the reservoir. The function of this pressure regulator is to maintain a constant pressure in the reservoir, irrespective of the level and temperature of fluid in the reservoir.
A pressurized reservoir will only have a single entry point for filling up the fluid in the tank. Since the reservoir is always maintained at a pressure, it becomes important to have a foolproof system with safety relief valves, for the filling of fluid in the reservoir. Sufficient guidelines are provided by all manufacturers of such pressurized reservoirs.