Edited by Mary C. Gannon, senior associate editor

Figure 1. Vibration bars, like this bolt-down style, serve as an alternative to auxiliary plates and reduce vibration on reservoirs. Photo courtesy of Trilex Fluid Power

When it comes to reservoir design, bigger is not necessarily better. In fact, the trend is to provide smaller reservoirs.

In addition to holding in reserve enough fluid to supply a hydraulic system’s varying needs, a reservoir provides:
• a large surface area to transfer heat from the fluid to the surrounding environment
• enough volume to let returning fluid slow down from a high entrance velocity. This lets heavier contaminants settle and entrained air escape
• a physical barrier (baffle) that separates fluid entering the reservoir from fluid entering the pump suction line
• air space above the fluid to accept air that bubbles out of the fluid
• access to remove used fluid and contaminants from the system and to add new fluid
• space for hot-fluid expansion, gravity drain-back from a system during shutdown, and storage of large volumes needed intermittently during peak periods of an operating cycle, and
• a convenient surface to mount other system components, if practical.

New designs for hydraulic systems often call for reservoirs that are much smaller than those based on traditional design rules. Because most systems warrant some special consideration, it is important to consult industry standards. NFPA Recommended Practice T3.16.2 addresses basic minimum design and construction features for reservoirs.

Reservoir sizing
The first variable to consider when sizing is reservoir volume. A traditional rule of thumb for sizing a hydraulic reservoir suggests that its volume should equal three times the rated output of the system’s fixed-displacement pump or mean flow rate of its variable-displacement pump. This means a system using a 5-gpm pump should have a 15-gal reservoir. The rule suggests an adequate volume to allow the fluid to rest between work cycles for heat dissipation, contaminant settling, and deaeration. NFPA’s Recommended Practice states, “Previously, three times the pump capacity had been recommended. Due to today’s system technology, design objectives have changed for economic reasons, such as space saving, minimizing oil usage, and overall system cost reductions.”

Whether adhering to the traditional rule of thumb or following the trend toward smaller reservoirs, be aware of parameters that may influence the size required. For example, some circuit components — such as large accumulators or cylinders — may involve large volumes of fluid. Therefore, a larger reservoir may have to be specified so fluid level does not drop below the pump inlet regardless of pump flow.

High fluid or ambient temperatures may require a heat exchanger or larger reservoir to dissipate heat. Be sure to consider the substantial heat that can be generated within a hydraulic system. This heat is generated when the system produces more power than is consumed by the load. A system operating for significant periods with pressurized fluid passing over a relief valve is a common example.

Reservoir size, therefore, often is determined primarily by the combination of highest fluid temperature and highest ambient temperature. All else being equal, the smaller the temperature difference between the two, the larger the surface area (and, therefore, volume) required to dissipate heat from fluid to the surrounding environment. Of course, if ambient temperature exceeds fluid temperature, a water-cooled or remote-mounted heat exchanger will be needed to cool the fluid. In fact, for applications where space conservation is important, heat exchangers can reduce reservoir size (and cost) dramatically. Keep in mind that the reservoir may not be full at all times, so it may not be dissipating heat through its full surface area. The reservoir should contain additional space equal to at least 10% of its fluid capacity. This allows for thermal expansion of the fluid and gravity drain-back during shutdown, yet still provides a free fluid surface for deaeration. NFPA/T3.16.2 requires that maximum fluid capacity of the reservoir be marked permanently on its top plate.

Specifying smaller reservoirs has emerged to save costs. A smaller reservoir is lighter, more compact, and less expensive to manufacture and maintain than one of traditional size. A smaller reservoir also reduces the total amount of fluid contained in a system.

But specifying a smaller reservoir for a system must be accompanied by modifications that compensate for the lower volume of fluid contained in the reservoir. For example, a smaller reservoir with less surface area for heat transfer may require a heat exchanger to maintain fluid temperature.

Also, contaminants will not have as great an opportunity for settling, so high-capacity filters may be required to trap contaminants that would otherwise settle in the sump of the reservoir. However, many industry experts prefer this technique because they explain that it is better to actually remove contaminants using filters than it is to let contaminants remain in the reservoir.

Perhaps the greatest challenge to using a smaller reservoir lies with removing air from the fluid. A traditional reservoir provides the opportunity for air to escape from fluid before it is drawn into the pump inlet. Providing too small a reservoir could allow aerated fluid to be drawn into the pump. This could cause cavitation and eventual damage or failure of the pump. On small reservoirs, a flow diffuser can reduce the velocity of return fluid (typically to 1 ft/sec), helps prevent foaming and agitation, and reduces potential pump cavitation from flow disturbances at the inlet. Another technique is to install a screen at an angle in the reservoir. The screen collects small bubbles, which join wi th other s to form large bubbles that readily rise to the fluid’s surface.

Figure 2. In L-shaped reservoirs, oil feeds the inlet side of the pump, reducing cavitation. This 300-gal L-shaped reservoir features a diesel engine driving a variable displacement piston pump and associated circuitry mounted on the apron. Photo courtesy of Devine Hydraulics.

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.

To prevent aerated fluid from being drawn into the pump, prevent aeration of fluid in the first place by paying careful attention to fluid flow paths, velocities, and pressures when designing the hydraulic system.

Design configurations
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, while also allowing easy access to the pump, motor, and accessories.

Motor vibration bars or damping bars reduce vibration and noise coming from the electric motor, and serve as an alternative to auxiliary plates, says Will Bisson of Trilex Fluid Power. The motor vibration bar consists of a steel plate on the top and bottom with rubber in between. The rubber is bonded to both pieces of steel. The damping bars are machined to accommodate different sizes of NEMA motors. They are available in a bolt-down and weldable style, Figure 1.

Drip lips on reservoirs and drip trays have recently grown in use because of environmental concerns. Some drip trays must be manufactured to hold 90-110% of the entire capacity of oil in the reservoir. The drip lip captures any fluid lost during maintenance or malfunctions.

A current design trend has the electric motor mounted vertically, with the pump submerged in hydraulic fluid. This conserves space, because the reservoir can be made deeper and take up less floor space than one with traditional “bathtub” proportions. The submergedpump 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 suppress pump noise.

An alternate configuration positions the reservoir above the pump and motor. 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. Also, the reservoir’s top cover can be removed to service internal components without disturbing the pump and motor.

The overhead reservoir may prevent fluid from flowing properly in 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 Lshaped reservoir, Figure 2, 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 check 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.

Figure 3. This floor model for a reservoir required custom dimensions and filter placements. Photo courtesy of SunSource.

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.

Accessories
Reservoir accessories are used to:
• strain new fluid as it enters a system
• filter air drawn into the reservoir as hydraulic fluid level rises and falls during system operation • indicate fluid level in the reservoir
• indicate fluid temperature, and
• heat cold or low-viscosity fluids to necessary operating temperature.

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. Another current trend is to use coalescing filters to keep moisture content out of the reservoir’s interior because many environmental fluids tend to absorb moisture, which can degrade their performance, says Derek Saunders of Devine Hydraulics.

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.

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 gauge can help indicate when cleaning is necessary.

Integral reservoirs
Integral reservoirs are not addressed in the NFPA/ANSI standard. They are used most often with mobile equipment and often require customdesigned shapes for irregular areas, as seen in Figure 3.

Several 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.

Many off-highway machines contain two or more separate hydraulic systems: a closed system serving as a hydrostatic drive, and an open system for serving auxiliary functions. A combination return and suction intank filter could prove beneficial for some of these applications, says Sun- Source’s Jade Paulseth. These combination suction-return filters replace the suction or pressure filters previously required for the charge pump of the hydrostatic circuit as well as the return filter for the open circuit. Although each circuit operates independently with separate filters, the combination of the closed and open circuits via the in-tank suction-return filter causes interaction between the circuits. Because the charge pump is always fed with pressurized oil, the risk of cavitation is minimized and full performance is available even during the critical cold startup.

Applying the combination filter to the reservoir ensures that the mobile equipment’s system performs reliably even under extreme operating conditions.

This article is based on information appearing in the 2008-2009 Fluid Power Handbook & Directory. Will Bisson of Trilex Fluid Power, wbisson@trilexfluidpower.com; Derek G. Saunders, Devine Hydraulics Inc., dsaunders@dhict.com; and Jade Paulseth, CFPS, SunSource, Mobile Technology Services, jpaulseth@sunsrce.com all contributed to this updated version.