Fluid Power Journal

Variable Volume Reservoir

The Variable Volume Reservoir (VVR) is a compact oil reservoir that may replace conventional hydraulic oil reservoirs; it’s a simple improvement and a great solution to weight, space, costs, safety, oil contamination, and environmental issues.

Available with capacities of 7, 14, or 25 liters (1.8, 3.6, or 6.5 gallons), the reservoirs weigh only 36, 67, and 110 lbs. respectively when full of oil and on selected applications can replace conventional hydraulic oil reservoirs that are 40 times larger or more. A VVR can be installed on new equipment or retrofitted to an existing system very simply as it will always use much less space than the reservoir it replaces.


Being sealed, slightly pressurized at 0-0.6 bar (1-9 psig), and airless allows the VVR to dramatically reduce the ingestion of solid contaminants and oxidation-producing moisture, increasing fluid life by up to five times that with conventional reservoirs. The VVR maintains a relatively constant pressure at pump inlets, supercharging the pumps and reducing the possibility of cavitation, whatever the altitude and angle of the reservoir. Unlike gas accumulators, the pressurization is generated mechanically by a spring and won’t be affected by temperature or ambient pressure.

The VVR is not impacted by pump flow or system pressure. The VVR has been tested on a system with a flow rate of 300 lpm (80 gpm) using our 7-liter (1.8-gallon) reservoir with great success. A Hägglunds MB283 motor was powered for six weeks using 5 liters (1.25 gallons) of hydraulic oil instead of the 300 to 450 liters (80 to 120 gallons) normally required based on the pump flow of 150 lpm (40 gpm).

Fluid will enter or leave the VVR only when there is the need for compensation during cylinder motion and/or thermal changes. Otherwise the returning flow goes directly to the pump inlet. Using the VVR is similar to having a hydrostatic, closed-loop system except that the VVR can be used with multiple pumps and multiple actuators.


The reason that the VVR allows the oil to be recirculated many times a minute, unlike other reservoirs requiring a 2-3 minute pause, is due to the fact that the VVR is airless and sealed. When the VVR is filled with oil during commissioning or following the replacement of a component, the air present in the hydraulic network is bled out of the system using the top cover air bleed valve. This simple process, part of the oil fill procedure, will take 15-20 minutes, only a few minutes longer than with a conventional reservoir. Keep in mind that filling the system and adding 7 liters (1.8 gallons) in a VVR will be simpler logistically than with a conventional reservoir.

A simple manifold system is all that is needed when a VVR is used to feed multiple pumps. A collector manifold can be added for multiple drain lines returning to the VVR. VVRs can also be installed in parallel when needed for larger differential volumes.

The appropriate VVR size for an application is determined by two factors: first, the total differential volume of the cylinders and, second, the system fillvolume, including the fluid required in motors, pump housings, cylinder heads, hoses, filters, and coolers. This oil will expand by a maximum of 10% if temperature ranges between -30ºC to 125ºC (-22ºF to 257ºF).


As a general rule, 75% of a VVR capacity will be reserved for the differential volume while the remaining 25% will be kept for fluid thermal expansion. Keep in mind that double rod cylinders or a pair of cylinders working in opposition, as in a steering system, don’t generate a differential volume but need to be included in the fill volume assessment. For applications where only motors are used, all the focus should be put on the fill volume to assess the thermal expansion volume requirement. The VVR has a finite volume and will not expand more than the rated volume. In order to be conservative, our thermal expansion factor is strictly set at 10%. However, not all equipment will be exposed to such extremes, so we can adjust the factor lower for specific applications. We don’t recommend using the VVR when oil temperatures are below -25ºC (-13ºF). For extreme low-temperature uses, we can provide a simple and affordable oil preheating system that will bring the oil above -13°F. Keep in mind that preheating 7 or 14 liters of fluid to working temperature will always be simpler, faster, and cheaper than heating the fluid in a traditional, high-volume reservoir.


VVRs have been used on drill rigs and other complex hydraulic systems for over five years, and therefore are extensively tested both in the laboratory and in real-life conditions. Our first customer, Versadrill Canada, designers and assemblers of drill rigs, went from using a 220-liter (57-gallon) reservoir to our 7-liter (1.8-gallon) VVR with no negative consequences. The weight of the reservoir went from 600 lbs to 36 lbs. This is quite a benefit for a unit that requires being mobile and is occasionally carried by helicopter. This demanding application demonstrates the solid quality of design and manufacturing of the VVR.

Recently, a client used the VVR on his compact tracked vehicle, which uses two hydrostatic drives. The small 7-liter (1.8-gallon) VVR replaced a 200-liter (50-gallon) reservoir, again saving a lot of weight and space while feeding a constant supercharge to pumps regardless of vehicle orientation and altitude.

Another drill rig maker uses our 7-liter (1.8-gallon) VVR on track-mounted drill rigs that have five pumps (rotary head, cylinders, mud pump, and two hydrostats for the tracks) with a total flow of about 380 liters (100 gpm).

Navatek Ltd., a designer of high-performance boats and stability systems based in Hawaii, required a reservoir that was light, compact, sealed, and not affected by rough seas and a salty environment. The VVR was a perfect fit for the application.

variable4 Some of the resistance to the use of the VVR comes from the perception that reservoirs have a function in cooling the oil and that it is necessary to let particles settle at the bottom of the reservoir. Concerning cooling, it is well documented that reservoirs may provide some cooling, but very marginally, and they will always be affected by the ambient temperature, whether exposed to frigid conditions, sitting in the hot sun, or installed next to a combustion engine. New heat exchangers are compact, efficient, and reliable. If they are rightly sized, they will provide 100% of the cooling work regardless of the reservoir size or ambient temperature. With modern filter technology, particle decantation is no longer part of the equation for reservoir size justification. Suspended, micron-sized particles would take days, not minutes, to settle given fluid viscosity. Because the VVR is sealed, contamination ingestion of particles and moisture will drastically be reduced. It would be normal to expect lower filtration costs and longer-lasting hydraulic components.

While the VVR usually costs more than a conventional aluminum or steel reservoir, some basic economic analysis needs to be performed. Depending on the size of the conventional hydraulic oil reservoir replaced by the VVR and the type of oil used, a VVR can provide significant savings within a very short period. Typically when replacing reservoirs of 200 liters (50 gallons) or more, the VVR will pay itself back within the second or third oil fill. For larger reservoirs or users of more expensive biodegradable oil, the payback can be immediate at the first oil fill.

Keep in mind that this simplistic analysis only takes into account the cost of the VVR, the cost of the conventional oil reservoir replaced, and the oil cost. No value is put on the potential important weight savings, the reduction of oil contamination-related issues, oil lasting up to five times longer, the elimination of great environmental risks in the event of a major leak/spill, longer-lasting hydraulic pumps with supercharge effect, etc.

In conclusion, the VVR makes a lot of sense, both technologically and economically. The VVR is unique and very innovative. Its compact size can be a distraction, but it can do a better job than reservoirs many times larger.

For more information, contact Jean Courcelles, VP Business Development, at 514-966-7436 or jean@smartreservoir.com. Visit www.variablevolumereservoir.com.

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3 thoughts on “Variable Volume Reservoir”

  1. Govindaraja Gopalakrishnan says:

    Well thought design feature and adaptable in the industrial hydraulic system. Can it be useful in Aircraft hydraulics? Applications can be better explained if an example of an application with a hydraulic circuit.

  2. Avni Zengin says:

    This application has been very useful to me, I congratulate those who think.
    I hope to meet you during the project phase.
    I want to be constantly informed about developments.
    I would like information on heat problems.
    I wish you good health.

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