fig. 2. this modular power unit demonstrates a trend in design: mounting the electric motor vertically with the pump submerged in hydraulic fluid. this technique reduces leakage, noise, and floor space required.Traditionally, the pump, electric motor, and other components of a hydraulic power unit mount on top of a rectangular reservoir. The reservoir top, therefore, must be structurally rigid enough to support these components, maintain alignments, and minimize vibration. An auxiliary plate may be mounted on the reservoir's top to meet these objectives. A big advantage of this configuration is that it allows easy access to the pump, motor, and accessories.

A current design trend has the electric motor mounted vertically, with the pump submerged in hydraulic fluid, Figure 2. This conserves space, because the reservoir can be made deeper and take up less floor space than one with traditional "bathtub" proportions. The submerged-pump design also eliminates external pump leakage, because any fluid leaking from the pump flows directly into the reservoir. In addition, the power unit is quieter, because the hydraulic fluid tends to damp pump noise.

An alternate configuration positions the reservoir above the pump and motor, Figure 3. This overhead configuration provides the advantage of combining atmospheric pressure and the weight of the fluid column to flood (force fluid into) the pump inlet, which helps prevent cavitation. The reservoir's top cover can be removed to service internal components without disturbing the pump and motor.

fig. 3. this industrial hydraulic power unit consists of five pump-motor assemblies supplied by an overhead reservoir. the overhead mounting provides pressurized fluid to each pump's inlet, and mounting pump-motor assemblies offset from reservoir provides access for lifting pump-motor assemblies from overhead. The overhead reservoir may cause a problem with gravity-return drain lines, so an auxiliary pump may be needed to route fluid up to the reservoir. When noise is a problem, overhead tanks provide the most convenient way to enclose the pump and electric motor within a noise suppression chamber.

Many applications use reservoirs that combine characteristics of the different configurations. For example, an L-shaped reservoir, Figure 4, combines the advantages of top- and base-mounted reservoirs - a flooded pump inlet and easy accessibility of components.

Reservoirs can also be pressurized to flood the pump. This pressure can come from an external source or from trapped air and fluid thermal expansion. A pressure-control valve allows filtered air to enter the reservoir when the fluid cools but prevents its release unless air inside reaches a threshold pressure.

Shape and construction

There is no standard reservoir shape. Geometrically, a square or a rectangular prism has the largest heat-transfer surface per unit volume. A cylindrical shape, on the other hand, may be more economical to fabricate. If the reservoir is shallow, wide, and long, it may take up more floor space than necessary and does not take full advantage of the heat-transfer surface of the walls.

Theoretically, because heat rises, the reservoir top holds the greatest potential for heat transfer to the atmosphere. However, in particularly dirty environments, contaminants often collect on the reservoir top and act as insulation. This reduces the effective heat transfer from the top of the reservoir, so reservoir sides could actually be the most effective heat transfer area in some instances. On the other hand, a tall and narrow geometry conserves floor space and provides a large surface area for heat transfer from the sides. Depending on the application, however, this shape may not provide enough area at the top surface of the fluid to let air escape.

The reservoir should be strong and rigid enough to allow lifting and moving while full. Appropriate lift rings, lugs, or forklift provisions should be included.


Reservoir accessories are used for:

  • straining new fluid as it enters a system
  • filtering air drawn into the reservoir as hydraulic fluid level rises and falls during system operation
  • indicating fluid level in the reservoir
  • indicating fluid temperature
  • routing return fluid to minimize potential pump cavitation and improve heat transfer
  • heating cold or low-viscosity fluids to necessary operating temperature, and
  • removing ferrous contaminant particles from the fluid.

Fluid must be added to the reservoir at startup, after cleanout, and to make up for losses. Two filler openings should permit reasonably rapid filling (at least 5 gpm each), intercept large contaminant particles from the new fluid, and either seal when closed or filter incoming air if vented as a breather. The openings should be on opposite sides or ends of the reservoir. Metal strainer screens of 30-mesh or finer should have internal metal guards and be attached so tools are necessary for removal. The filler cover should be permanently attached, and if it does not include a breather, a separate breather should be specified. In either case, 40-µm air filtration should be provided.

In addition to slowing down fluid returning to the reservoir, reducing foaming and pump cavitation from flow disturbances at the inlet, and providing fluid mixing without agitation, flow diffusers also reduce noise and the need for baffling. They are especially effective in small reservoirs with high flows and in deep reservoirs with a small floor area.

A fluid-level indicator should be located at each filler. Indicators should have high and low levels marked against a contrasting background to help maintain appropriate fluid level. An electronic level indicator can serve as a more sophisticated alternative. These devices use a variety of means to measure liquid level. Transducers produce a continuous output, and switches signal when liquid reaches a predetermined high or low level.

Fluid temperature measurement is not required by the NFPA standard, but a selection of thermometers is available, many in the same housing as the fluid-level indicator. (If high fluid temperature is a continuing problem, the heat source in the circuit should be identified and removed.) As with level indicators, a variety of electronic temperature indicators are available.

In either case, signals generated by these devices are routed to a display or control panel to provide operators with an indication of fluid status. Wiring a level or temperature switch into the machine's control can prevent equipment damage by shutting down the machine if fluid reaches a dangerously low level or high temperature.

After shutdown, or when the reservoir is exposed to colder temperatures, the fluid may be too cold for immediate operation. Cold fluid may become viscous or thick enough to prevent it from being drawn into the pump, causing pump cavitation or other problems that can damage components or cause system malfunctions. A thermostatically controlled heater to warm fluid until its viscosity becomes compatible with the system solves this problem. Again, by wiring this thermostat into the system control, machine operation can be prevented until fluid reaches a minimum temperature.

Magnets can be placed in the reservoir to capture and remove metallic particles from the fluid stream. Fluid returning to the reservoir should be routed past in-tank magnets to collect as many ferrous particles as possible. Magnets should be checked periodically and cleaned to ensure continued maximum performance.

Although hydraulic filters are usually not considered reservoir accessories, almost all pump inlet strainers are located within the reservoir, and many other filters mount on or through reservoir surfaces. Because the inlet strainer is out of sight, a pressure gage will help indicate when cleaning is necessary.

Integral reservoirs

In some systems, the hydraulic reservoir is built as an integral part of the equipment it serves. Because of the diversity of designs and special design practices, integral reservoirs are not addressed in the NFPA/ANSI standard. They are used most often with mobile equipment, and their placement often is an afterthought, which necessitates custom-designed shapes for irregular areas.

A number of potential problems exist with integral reservoirs that require special consideration. These include:

  • available space may limit size. Because heat transfer capacity is a function of size, external oil coolers or heat exchangers may be needed
  • irregular shape may require special baffling to properly route fluid
  • surrounding equipment may limit convectional heat transfer
  • service accessibility may be poor, and
  • special heat shielding may be needed to isolate components or the operator from reservoir heat.