Variable Volume Reservoirs
Variable Volume Reservoirs
(OP)
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?
Thanks,
Mike
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?
Thanks,
Mike





RE: Variable Volume Reservoirs
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
RE: Variable Volume Reservoirs
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
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.
Ted
RE: Variable Volume Reservoirs
RE: Variable Volume Reservoirs
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
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:
https://www.parker.com/literature/Hydraulic%20Syst...
RE: Variable Volume Reservoirs
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...