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Variable Volume Reservoirs

Variable Volume Reservoirs

Variable Volume Reservoirs

Hello All,

I have an application wherein we are considering using a variable volume reservoir (http://www.sobacor.ca/VVR_English.pdf) to replace a conventional reservoir. Our customer has pressed us on weight and consequently fluid volume which lead us to find the above linked product.
The application is mobile and fairly rugged. The oil could be subject to violent sloshing and consequently to high levels of entrained air in the oil. The VVR appears to solve several of these problems.

Does anyone have any experience with these types of reservoirs? What are some of the considerations/trade offs that we need to look at?



RE: Variable Volume Reservoirs

They have really stretched a few truths there.

Some of the volumes they quote are a bit exaggerated to say the least.

It is possible to dramatically reduce the fluid volume, but it needs to be kept at relatively high pressure to suppress the bubbles. Bigger volume reservoirs allow the air to rise out or coalesce into bigger bubbles. Smaller reservoirs can't do that, so they have to work at higher pressure.

Your machine will also pitch and roll, as well as accelerate and decelerate, that can leave big gap in the oil that the pump won't like.

They can be used, but they are not the magic bullet that the link suggests.

RE: Variable Volume Reservoirs

Filling against the ~9 psig spring pressure can be an issue in the field (unless you supply a pump for the purpose). Another issue for field service is bleeding of air when replacing a remote component (yes, they provide a bleed valve, but how much oil will spill when the bleed is used, how well will returning air separate in the reservoir, etc.).

RE: Variable Volume Reservoirs

Thanks for the responses.
As far as maintainability, the technicians would need to be trained to deal with a pressurized system vs a traditional open reservoir. It does make field service a little more difficult.

These type of devices are not intended to behave the same as a traditional reservoir. They are more like a spring loaded accumulator rather than a reservoir. The system would go from an open loop system to a closed loop system that is bled free of air (as best as can be achieved give the system architecture). At least that's what I interpret from their literature.

As far as accelerating and decelerating the fluid in the system, this problem would exist in either system though it may be more difficult to contend with in the open loop system as you do not have pressure to help keep the fluid moving toward the pump suction. Does this statement seem reasonable?

RE: Variable Volume Reservoirs

mikedee42, if the vrr is not well purged it is conceivable that an air bubble could exist in the reservoir. If the system were to be in an attitude that puts the bubble at the pump intake, the pump will draw air. The fact that the reservoir is pressurized will not ensure the pump intake is always flooded and drawing in fluid. For earth-bound systems, gravity determines where the fluid rests and if the pump intake is not in the fluid no fluid will be drawn in.
For the mobile systems used in products with which I have had experience, the reservoir was sized to 1x the maximum system flow rate. It could have been smaller. The reservoir was tall rather than flat so that the pump suction at the bottom of the reservoir was always flooded. Fluid temperature was managed with an air-to-oil cooler fitted with a thermostatic valve.


RE: Variable Volume Reservoirs

i think they are rather optimistic on what this kind of reservoir can achieve. apart from the fact that a pressurized system calls for better trained maintenance personnel that may not always be available and thus can possibly result in nasty incidents, working with the minimum amount of fluid also introduces quite a few problems. you will have more trouble getting entrained air out of the fluid, although careful positioning of return lines might help to prevent it to some extend. you also will have more wear particles in the oil (in terms of ppm) and thus it might well be difficult to keep the system in a cleanliness class as required for sophisticated hydraulic components with the result that you will more often need to exchange costly components. a third factor is oil life: if due to the much smaller volume the oil temperature rises substantially, it will oxidize and degrade faster which will call for more frequent oil drains. a cooler may help to keep the temperature down, and thus keep the circuits working with a fluid of sufficient viscosity, but a cooler will not prevent oxidation totally since the oil will reach higher temperatures before the cooler actually does something. thus, thickening of the fluid due to oxidation and thermal degradation will be more prominent then with a standard setup and degradation products may lead to lacquer and sludge. lacquer will tend to adhere to the surrounding metal in places where there is not much throughput of the oil when the system cools down, e.g. spool and control valves that may subsequently stick...thus, it might well look like you solve one problem and introduce quite a few others.

RE: Variable Volume Reservoirs

I would also like to hear from someone who has actually used one. Have followed the magazine articles introducing it. It has real appeal to me if it actually works.

I think some of the negatives expressed above are carryover from the 'old rules of thumb' where a large reservoir did many duties, that can now be done much more efficiently and with less volume required, by other circuit components:
-Dirt removal: Realistically now, the settling of dirt in a reservoir is I think irrelevant. System requirements today demand removal of much smaller particles than will quickly settle out in a reservoir. So now we use filtration to do a much better job.
-Cooling: Large surface and volume do cool, but a small cooler is so much more efficient at this task.
-Pump inlet suction: Design of the piping and location of the reservoir handles this.

-De-aeration and dwell time in the reservoir. This is the big question mark in my mind. Baffling and screens and oil additives help, but there doesn't seem to be any real substitution for simple dwell time in the reservoir to let the air combine and move upwards. (From that standpoint, a long wide shallow reservoir with minimal distance upward to the free surface is much better than a narrow tall reservoir. Of course wide and shallow has other issues too.)
Given that mobile equipment usually runs higher pump and motor speeds, higher pressures, much more motion and entraining air in the tank surface, has very small tanks, my biggest question is how will this small pressurized V V R device deal with release of entrained air? Purging of free air pockets is a maintenance issue that can be dealt with, but how will the entrained air be removed? This would be especially important for motion control circuits where the air affects fluid and system stiffness and stability.

Is there any information combining the pressurized VVR with the spiral round reservoir concept that removes more air by rotational motion of the return fluid?

Interesting concept to see, I want to watch actual experiences and results.

RE: Variable Volume Reservoirs

I think this type of reservoir is common in aircraft. I know that there was one in the F-16, although my memory is getting pretty dim of that job. I do remember that it used hydraulic pressure on a very small piston that was connected to the big piston to pressurized the reservoir (it was referred to as a "bootstrap reservoir"). There was also a spring, I believe. The big piston/little piston led to some safety issues in the service shops--there was an incident where someone applied a little too much pressure to the big side, which resulted in waaaaaaayyy too much on the little side. I think it might have punched a hole in the building.

But aside from that, I think that kcj's points are very good. We don't, in mobile systems, rely on the reservoir for cooling or settling of crappage. Coolers and filters are much better at those jobs anyway.

I'm not sure how much of an issue the de-aeration is. I've always assumed that the air got there in the first place because the banging and sloshing uncovered the suction line. If the pressurized reservoir has no air space, it should be hard to get air in the system. But keep in mind that I have no actual experience with pressurized reservoirs beyond troubleshooting one particular system many, many years ago.

Here's a picture of the Parker unit:


RE: Variable Volume Reservoirs

The subsea insustry uses these and similar. Google "hydraulic compensator". Because, well, hydraulic systems operate under water do not do well when sea water enters the system. the biggest thing a land based system would need to consider is cooling the fluid- an issue even for some systems operating in cold water. bleeding the system is not that big of an issue, though certainly important. Also, you would need to monitor and replace the oil more often than a larger system because wear components will build up faster with lower volumes. Use a synthetic oil like Royal Purple. and don't skimp on filtration.

It will reduce pump cavitation problems due to the positive pressure, so you have more leeway in placement of the unit farther away from the pump if necessary.

Regarding field service, we used to convert pressureized pesticide sprayers- easy and pretty cheap. (search "compression sprayers" on mcmaster). just remember to drain the system before cracking a hose...

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