Pressurized Reservoirs

installations. A typical reservoir for use with ground and ship installations is shown in figure 9-1. This type of reservoir is made of hot rolled steel plates and has welded seams. The ends extend below the bottom of the reservoir and serve as supports. The bottom of the reservoir is convex, and a drain plug is incorporated at the lowest point. Nonpressurized reservoirs are also used in several transport-, patrol-, and utility-type aircraft. These aircraft are not designed for violent maneuvers and, in some cases, do not fly at high altitude. Those aircraft that have nonpressurized reservoirs installed and that fly at high altitudes have the reservoirs installed within a pressurized area. (High altitude in this situation means an altitude where atmospheric pressure is inadequate to maintain sufficient flow of fluid to the hydraulic pumps.) Most nonpressurized aircraft reservoirs are constructed in a cylindrical shape (fig. 9-2). The outer housing is manufactured from a strong corrosion-resistant metal. Filter elements are normally installed internally within the reservoir to clean returning system hydraulic fluid. Some of the older aircraft have a filter bypass valve installed to allow fluid to bypass the filter if the filter becomes clogged. Reservoirs that are filled by pouring fluid directly into them have a filler (finger) strainer assembly installed in the filler well to strain out impurities as the fluid enters the reservoir. Figure 9-2.—Nonpressurized aircraft reservoir. The quantity of fluid in the reservoir is indicated by either a glass tube, a directing gauge, or a float-type rod, which is visible through a transparent dome installed on the reservoir. PRESSURIZED RESERVOIRS A pressurized reservoir is required in hydraulic systems where atmospheric pressure is insufficient to maintain a net positive suction head (NPSH) to the pump. There are two common types of pressurized reservoirs—fluid-pressurized and air-pressurized. Fluid-Pressurized Reservoir Some aircraft hydraulic systems use fluid pressure for pressurizing the reservoir. The reservoir shown in figure 9-3 is of this type. This reservoir is divided into two chambers by a floating piston. The piston is forced downward in the reservoir by a compression spring within the pressurizing cylinder and by system pressure entering the pressurizing port of the cylinder. The pressurizing port is connected directly to the pressure line. When the system is pressurized, pressure enters the pressure port, thus pressurizing the reservoir. This pressurizes the pump suction line and the reservoir return line to the same pressure. The reservoir shown in figure 9-3 has five ports—pump suction, return, pressurizing, overboard drain, and bleed. Fluid is supplied to the pump through the pump suction port. Fluid returns to the reservoir from the system through the return port. Pressure from the pump enters the pressurizing cylinder in the top of the reservoir through the pressurizing port. The overboard drain port is used to drain the reservoir while performing maintenance, and the bleed port is used as an aid when servicing the reservoir. Air-Pressurized Reservoirs Air-pressurized reservoirs, such as the one shown in figure 9-4, are currently used in many high-performance naval aircraft. The reservoir is cylindrical in shape and has a piston installed internally to separate the air and fluid chambers. Air pressure is usually provided by engine bleed air. The piston rod end protrudes through the reservoir end cap and indicates the fluid quantity. The quantity indication may be seen by inspecting the distance the piston rod protrudes from the reservoir end cap. The reservoir is provided with 9-2