Fundamentals of Hydraulic Reservoirs

How to Size a Hydraulic Reservoir 

The first variable to resolve when sizing a hydraulic reservoir is determining volume. A rule of thumb suggests that the reservoir's 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.

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Keep in mind that this is only a rule of thumb for initial sizing. In fact, the National Fluid Power Association's (NFPA) 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 hydraulic 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.

For applications where space conservation is important, heat exchangers can reduce hydraulic 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.

The Pros and Cons of Smaller Hydraulic Reservoirs

A trend toward specifying smaller hydraulic reservoirs 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 hydraulic 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.

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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.

READ MORE: Cylindrical reservoir breaks from convention

Understanding Reservoir Design Configurations

Hydraulic reservoirs are available in various design configurations. Rectangular reservoirs are a common type which traditionally have a hydraulic power unit comprised of a pump, electric motor, and other components mounted on top of the hydraulic reservoir tank. Therefore, the top of the reservoir must be structurally rigid enough to support these components, maintain alignments, and minimize vibration. An auxiliary plate may be mounted on top of the reservoir 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 as seen in Figure 2 below. This conserves space, because the hydraulic 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.

The &#;generator&#; side of the system is the pump which brings in a fixed or regulated flow of oil to the bottom side of the cylinder &#; to move the piston rod upwards. Hydraulic cylinders transform the pressure and oil flow in a hydraulic system into work or mechanical force. They are used where linear motion is required to move something.

Also known as &#;hydraulic jacks&#;, &#;hydraulic rams&#; or &#;actuators&#;, they convert fluid power into mechanical energy. A hydraulic cylinder differs from a hydraulic motor as it carries out a linear (translatory) rather than rotary movement, hence the term &#;linear motor&#;.

Used at high pressures, hydraulic cylinders produce large forces and precise movement; they are therefore constructed of strong materials, such as steel that is capable of withstanding the large forces involved.

There are primarily two styles of hydraulic cylinder construction used in industry: tie rod and welded body cylinders. Beyond this, other broad types of cylinder design include: telescopic, plunger, differential, re-phasing and single and double-acting hydraulic cylinders.

Hydraulic cylinders are usually double-acting: oil under pressure can be applied to either side of the piston to provide movement in either direction. Single-acting cylinders are sometimes used where the weight of the load is used to return the cylinder to the closed position.

Hydraulic cylinders enable more flexibility in design and structure when transferring force between two different points. Different sized cylinders make it possible to create a system that can pull, push and lift weights; bends and corners can be incorporated into the system design &#; useful if there are real space constraints.

However, a hydraulic cylinder should only be used for linear pushing and pulling. No bending moments or side loads should be transmitted to the piston rod or the cylinder. For this reason, a cylinder should ideally be connected by using a single clevis with a spherical ball bearing. This allows the cylinder to move and allow for any misalignment between it and the load it is pushing.

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