Hydraulic reservoirs are storage tanks that hold liquids or gases used in fluid power applications. They are usually rectangular, cylindrical, T-shaped, or L-shaped and made of steel, stainless steel, aluminum, or plastic. Hydraulic reservoirs vary in terms of capacity, but need to be large enough to accommodate the thermal expansion of fluids and changes in fluid level due to normal system operation. Large hydraulic reservoirs provide cooling and reduce recirculation. Top-mounted or side-mounted fillers or breathers permit the entry of filtered air and prevent the ingress of contaminants. Typically, the filler or breather is located at the opening of the reservoir where fluid is added. Some reservoirs include stationary or removable baffles to direct fluid flow and ensure proper circulation. Integrated level gauges such as dipsticks and floats enable users to monitor fluid levels. Some hydraulic reservoirs are wall-mounted. Others include a mounting bracket at the base of the reservoir.
Hydraulic reservoirs are available with a variety of features. Pressurized devices are used in hydraulic systems where atmospheric pressure is not sufficient to maintain a net positive suction. Both air-pressurized and liquid-pressurized hydraulic reservoirs are available. Some hydraulic reservoirs are equipped with a relief valve to keep the device at a safe operating pressure. Others are coated with a rust-inhibiting agent or feature a hinged or fully removable top. Hydraulic reservoirs with internal temperature gauges are commonly available. As a rule of thumb, hydraulic systems that are exposed to high ambient temperatures require a larger reservoir unless they incorporate a heat exchanger. Capacity, an important specification, is usually measured in gallons or a similar volumetric unit. To prevent foaming or fluid blow-over, foam-retarding diffusers are sometimes built into the reservoir.
Many hydraulic reservoirs are manufactured in accordance with standards established by organizations such as the Joint Industrial Council (JIC), the National Fluid Power Association (NFPA), the American Society of Lubrication Engineers (ASLE), and Deutsche Industrie Norm (DIN). These organizations also provide information about sizing requirements and other technical considerations. For example, the NFPA recommends that that reservoir volume equal at least three times the rated output of the system’s fixed-displacement pump, or the mean flow rate of its variable-displacement pump.