Tech Tip

The Role Of The Hydraulic Reservoir In A System

The reservoir wears many hats in a hydraulic system. The main function of a reservoir is to hold system hydraulic fluid in a convenient location for the pump inlet. In addition to system requirements, the reservoir also holds excess fluid needed when the hydraulic system is in operation. This excess fluid is needed when an accumulator is being charged or a cylinder is being extended.

The reservoir performs many roles in the operation of the hydraulic system. One of its primary jobs is for heat dissipation (cooling the hydraulic fluid) and fluid conditioning (dissipation of contaminants and aeration). Most hydraulic reservoirs incorporate an internal baffle that is used to circulate turbulent fluid that is hot, dirty and aerated from the system return side of the reservoir to the quiet and cooler pump inlet side. This movement of the fluid around and through the baffle creates time for fluid contaminants to settle out to the bottom of the tank and for air entrapped in the fluid to separate and rise to the fluid surface.

With most industrial hydraulic systems, the reservoir also serves as the mounting surface for system components such as pump/motor assemblies, filters, accumulators, manifolds and electrical control panels.

Hydraulic reservoir sizing

When sizing a hydraulic reservoir, many factors must be considered. A well designed reservoir offers much more than just fluid storage. A poorly designed or sized reservoir can compromise an otherwise well designed hydraulic system.

There are two basic methods for sizing an hydraulic reservoir. The first and more traditional method is the “rule-of-thumb” means of calculating the reservoir size by using 3 to 5 times the pump discharge rate (GPM). This method may work well for most hydraulic systems. The second method of sizing the reservoir relates to the heat dissipation capacity of the tank. In order to use this method the heat balance of the hydraulic system will need to be determined. This is done by first calculating the amount of heat that will be generated in the system via pressure drops and other sources, then by calculating the amount of heat that can be dissipated through the reservoir. Heat dissipation is determined as follows:

Heat Dissipation: HD = 0.001 x (T1 - T2) x

T1 = Max. allowable fluid temperature (Degree F)

T2 = Max. ambient air temperature (Degree F)

A = Area of the tank in contact with the fluid (Sq.Ft.)

In calculating the heat balance of the hydraulic system, the designer can determine if a heat exchanger is needed or if the tank size can be increased to dissipate the excess heat

By incorporating a larger reservoir, the design engineer may be able to reduce excess energy, labor and component costs by eliminating the need to include an air-to-oil or water-to-oil heat exchanger circuit.

If the heat balance method is used, it is recommended that the reservoir be mounted above the ground to help ensure adequate air flow across the bottom and all four sides of the tank. Finally, it is recommended that an air space be provided within the tank and above the oil level that is approximately 10% of the reservoir’s fluid capacity. This is necessary to allow for thermal expansion of the fluid and provide a free fluid surface for de-aeration.

Hydraulic reservoir styles

The hydraulic reservoir style can take on many different shapes and sizes which may include machine bases, transmission housings and stand alone tanks. However, most reservoirs used in industrial hydraulic systems normally follow one of four basic design styles…..conventional, vertical, overhead or L-shaped.

There are many items to consider when determining the reservoir style that best fits the hydraulic system being designed. Below are just a few:

A. Ease of maintenance and serviceability

B. Unit location (floor space requirements and air flow around the unit for


C. Fluid type (some hydraulic fluids require a flooded suction application)

D. Pump inlet configuration (flooded suction applications can prolong pump life as well as reduce pump suction line noise)

Conventional reservoir style

The conventional reservoir configuration incorporates a rectangular, V-bottom tank with the pump/motor assembly mounted horizontally on the reservoir cover. Service access to both the hydraulic pump and electric motor is normally unlimited and the pump can be removed and replaced without draining the fluid from the tank. The pump suction lift requirements can be kept to a minimum by routing the inlet line down through the cover and into the fluid.

Vertical reservoir style

The vertical reservoir configuration incorporates a tall, square tank with the pump and motor mounted vertically. The electric motor is mounted above the tank cover and the hydraulic pump is mounted below the cover, inside the reservoir. This arrangement allows for the overall space requirements to be reduced. Also, the pump is protected and with the inlet line extended to within 1-1/2" of the bottom, the suction lift requirements are kept to a minimum. Service to the pump is very limited. The reservoir cover and any attached components must be removed to permit access to the pump. However, the pump can be removed and replaced without draining the fluid from the tank.

Overhead reservoir style

The overhead reservoir configuration incorporates a rectangular, V-bottom tank that is mounted above the pump/motor assembly on a base frame. Service access to the hydraulic pump and electric motor is only limited by the reservoir location. The inlet line enters the tank through a connection in the bottom providing a positive feed or flooded suction for the pump. A shut-off valve is normally mounted at the tank suction connection so the pump can be removed and replaced without draining the fluid from the tank. Return lines enter the tank above the fluid level and an anti-siphon hole is drilled in the line just inside the tank to permit removal of the external line or component without fluid siphoning from the tank.

L-shaped reservoir style

The L-shaped reservoir configuration incorporates many of the same features as the overhead style. The L-shaped reservoir is a tall, narrow rectangular tank with the pump/motor assembly mounted beside the tank on a common base frame. This arrangement provides full access to the hydraulic pump and electric motor for service. The pump inlet line enters the tank through a connection in the sidewall, near the bottom, providing a positive feed or flooded suction for the pump. As with the overhead reservoir design, a shut-off valve in the suction line and anti-siphon holes in the return lines allow the pump to be removed and replaced without draining fluid from the tank.

Reservoir accessories

Whatever the size or style of reservoir used, it is recommended that some or all of these commonly used accessories are incorporated into the reservoir construction:

A. Visual fluid level / temperature gauge

B. Air breather / filler cap assembly

C. Clean-out cover (bolt-down top)

D. Fluid level switch

E. Fluid temperature switch

F. Magnet rod assembly

G. Fluid drain / sampling valve

H. Corrosion resistant interior coating

Reservoir maintenance

All industrial hydraulic systems should incorporate the use of pressure line filters, return line filters and reservoir air breathers to reduce the amount of fluid contaminants entering the tank. Some systems should also include suction strainers or filters. Filter indicators or switches and a maintenance schedule should be incorporated to ensure the timely replacement of dirty filter elements. Finally, it is recommended that the reservoir be drained and the interior cleaned once a year.

Note: “Tech Tips” offered by Flodraulic Group or its companies are presented as a convenience to those who may wish to use them and are not presented as an alternative to formal fluid power education or professional system design assistance.

Experts in fluid power, electrical and mechanical technologies.