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In addition to holding in reserve enough fluid to supply a hydraulic system's varying needs, a reservoir, Figure 1, 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.

These are the traditional roles of reservoirs; new trends may present deviations from the norm. For example, new designs for hydraulic systems often call for reservoirs that are much smaller than those based on traditional rules of thumb. Because most systems warrant some special consideration, it is important to consult industry standards for minimum guidelines. Recommended Practice NFPA/T3.16.2* addresses basic minimum design and construction features for reservoirs.

Reservoir sizing

Although the considerations just discussed may be important, the first variable to resolve is, indeed, reservoir volume. A 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. Keep in mind that this is only a rule of thumb for initial sizing. In fact, 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 or not you choose to adhere to the traditional rule of thumb or follow the trend toward smaller reservoirs, be aware of parameters that may influence the reservoir 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.

Systems exposed to high ambient temperatures require a larger reservoir unless they incorporate a heat exchanger. Be sure to consider the substantial heat that can be generated within a hydraulic system. This heat is generated when the hydraulic 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. In any event, NFPA/T3.16.2 requires that maximum fluid capacity of the reservoir be marked permanently on its top plate.

A trend toward specifying smaller reservoir has emerged as a means of reaping economic benefits. A smaller reservoir is lighter, more compact, and less expensive to manufacture and maintain than one of traditional size. Moreover, a smaller reservoir reduces the total amount of fluid that can leak from a system - important from an environmental standpoint.

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, because a smaller reservoir has less surface area for heat transfer, a heat exchanger may be necessary to maintain fluid temperature within requirements. Also, contaminants will not have as great an opportunity for settling, so high-capacity filters will be required to trap contaminants that would otherwise settle in the sump of 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. When specifying a small reservoir, consider installing a flow diffuser, which reduces 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 with others to form large bubbles that readily rise to the fluid's surface.

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