Hydraulic Reservoirs

The article provides a concise account of the constructional details of hydraulic reservoirs. As you are aware, a hydraulic reservoir is an essential part of a hydraulic power pack. 

Hydraulic Power Pack

A hydraulic power pack transforms the power conveyed by its prime mover to hydraulic power at pressures and flow rates as required for all the system actuators. It is usually a compact, portable, and pre-configured assembly that contains components required to store and condition a given quantity of fluid, and push part of the fluid into the system.

Hydraulic Reservoirs

Any hydraulic system requires a sufficient amount of high-quality fluid at all times for its efficient operation. The fluid is drawn and pushed by the pump, circulated continuously through various intermediate components to system actuators, and then carried back to the reservoir. The circulating fluid is liable to accumulate contaminants and absorb heat from the system. Therefore, the fluid must be serviced before it is pumped again into the system.

A well-designed reservoir:

  • allows a reasonable dwell time for the fluid,
  • allows most of the contaminants to drop out,
  • assists in dissipating the heat,
  • allows air bubbles to come to the surface and dissipate,
  • compensates for the fluid volume changes,
  • provides a convenient mounting place for the pump-motor unit and valves

The reservoir must be located where there is good air circulation for the quick dissipation of the heat. The servicing parts (sight glasses/ filters/ filler breather/ drain cock) must easily be accessible.

Standard Features

A well-designed reservoir should be completely enclosed and self-contained. It may be provided with a filler-cum-breather opening with a suitable air filter, temperature controller, level indicator, and tank heater. The bottom plate of the reservoir is usually inclined from side to side or ‘V’-shaped. A drain plug must be provided at the lowest point of the bottom plate.

Constructional Features 

 A typical reservoir is provided with many components, such as:


  • Baffle plate
  • Suction line
  • Return line
  • Breather with air filters
  • Suction strainer/filter
  • Fluid-level sight-windows/indicators
  • Pressure gauges
  • Removable cover
  • Drain plug
  • Magnetic tank cleaners,
  • Diffuser
  • Thermometers

Baffle Plate

A baffle plate is fitted lengthwise through the middle of the reservoir. Its purpose is to separate the suction chamber from the return chamber. The baffle plate ensures that the return fluid takes a circuitous path through the reservoir, avoiding the same part of the fluid circulating continuously. This also provides more time for the contaminants to settle within the reservoir and assists the reservoir in dissipating heat as quickly as possible.

Suction Line

The suction line is used to carry the fluid to the inlet of the pump. Its bottom end should be located a distance above the bottom floor, so as to prevent the settled contaminants from entering the pump again.

Return Line

The return line is used to carry the return fluid from the system back to the reservoir. The return line must terminate below the fluid level and up to a height two to four times the pipe diameter above the base plate to reduce the turbulence and foaming.


The opening provides a path for filling the reservoir during the fluid replacement time. It also allows a passage for the air to breathe in and out of the reservoir during the operating time to equalize the interior and exterior pressures. An air filter of five microns (or better) prevents the ingress of airborne contaminants into the reservoir. The breather may include a quantity of desiccant material (silica gel) for the dehumidification of the inflowing air.

Strainer/ Suction Filter

A strainer and/or suction filter are fitted to the suction line to prevent dirt, grit, sludge, rust, and other contaminants from entering the pump. A suction filter must be fitted in a service-friendly manner so that it is easy to maintain and replace the filter.

Fluid Level Indicators

The fluid level monitoring is assisted by a sight window or a fluid level indicator or by using a level gauge.

Pressure Gauge

The use of the pressure gauge is a safety measure as it monitors over-pressures and assists in troubleshooting.

Removable Covers

A reservoir must be designed for easy access to clean out all the residues and rust that may have accumulated in the reservoir, and for flaking paint.

Drain Plug

The bottom part of the reservoir is usually provided with a downward gradient and a drain plug at its lowest point so that the system fluid can be drained completely without any difficulty.  


It is used in combination with return-line filter to slow down the return fluid. The reduced velocity prevents foaming and re-suspension of deposited dirt. It reduces turbulence/noise.

Magnetic Tank Cleaners

Tank cleaners with permanent magnets can be used for attracting and holding the abrasive ferrous particles.

Sizing of Reservoirs

The size of the reservoir must ensure that the fluid in circulation has a reasonable dwell time, dissipate heat quickly, suppress turbulence in the fluid, and release any entrained air. For a hydraulic system, where mineral fluids are used, and medium-to-high frequency demands are expected, a reservoir with a capacity of three to five times the volume flow rate of the system fluid is adequate.

Reservoir size, m3          = (3 to 5) x Q m3/min

Reservoir size, litre        = (3 to 5) x Q lpm

Reservoir size, gallons = (3 to 5) x Q gpm

With this general rule, the fluid returned to the reservoir has three to five minutes of dwell time in the reservoir before it circulates again.

Heat Exchangers

If the cooling effect from the reservoir is insufficient, a heat exchanger (or cooler) must be fitted to increase the heat dissipation rate. Types: (1) air-cooled heat exchangers and (2) water-cooled heat exchangers.  When the heat to be removed is comparatively small, an air-cooled heat exchanger can be used. When the heat to be removed is high, or its surrounding atmosphere is liable to be very hot, a water-cooled heat exchanger should be used.

Reference: JOJI PARAMBATH, Industrial Hydraulic Systems – Theory and Practice, Universal Publishers, Boca Raton, USA, 2016.

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Bangalore, India

Fluidsys Training Centre, Bangalore offers long-term and short-term training courses in the field of pneumatics and hydraulics.

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Hydraulics: Hydraulic safety, Hydraulic fundamentals, Fluids, Filters, Power packs, Pumps, Pressure relief valves, Linear actuators, Rotary actuators, Hydrostatic transmission (HST), Directional control valves, Non-return valves, Flow control valves, Pressure control Valves, Accumulators, Seals, Fluid conductor system, Electro-hydraulics, Proportional valve system, Servo valve system, Basics of hydraulic cartridge valve systems, Load Sensing Systems, Applications of hydraulic systems, Preventive maintenance & troubleshooting, and Design of hydraulic systems.

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2 thoughts on “Hydraulic Reservoirs

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  1. Great informative article with easy to understand words and sentences good work. Can you please tell me difference between close loop and open loop circuit please don’t be confused it’s circuit


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