Accumulator Charging in a Sinusoidally Varying Hydraulic System

[JoyceA]

Hello,
This may be a very basic question regarding the workings of accumulators, but I have been unable to find the answer elsewhere.
If the pressure and flow within a hydraulic system is varying sinusoidally (peak pressure = 200 bar, period = 10 s) due to the motion of a piston, and you want to use an accumulator to store the excess pressure and flow when it is above 100 bar say, does the accumulator continue to charge once it has reached 200 bar and the system pressure falls back down to 100 bar and lower?

 

[FredGarvin]

I'm not quite understanding your question, but I think it depends on how you have it plumbed into the system. You usually specify the pressure limit in the accumulator. At that point there is a relief that should stop the flow to the accumulator and, say, return to tank. The accumulator should be sized so that your pump doesn't see the oscillation in pressure demand.

 

[JoyceA]

Hopefully this is a little more precise, if the accumulator charges as the system pressure increases from 100 bar to 200 bar (peak pressure), will the accumulator continue to charge when the system pressure drops to 199 bar? I still want to store the excess pressure and flow that is over 100 bar, but as the system pressure drops, can it overcome the 200 bar that is now stored in the accumulator?

To give a better overview of the system, it is a piston-and-cylinder sending fluid to a hydraulic motor, via an accumultor which I want to use to smooth the pressure and flow variations.

 

[FredGarvin]

I think I see what you are saying now. No. The accumulator will not store that pressure since they are usually just teed into the line pressure. So it sees the same pressure as the line. It will if there is some kind of shut off valve on the outlet. What will happen is that excess pressure is used to move a weight or a pressure loaded diaphragm. If you wanted to store that, you would need at least some kind of valve to close off the accumulator and hold that pressure. It will dampen out the pressure fluctuations though.

 

[JoyceA]

Thank you Fred, that is as I thought but I wasn't fully sure. So is there no way to store the excess pressure as the system pressure falls from 199bar to 100bar?

I have some other questions too, if you wouldn’t mind taking a look over them please.

When the system pressure rises over 100 bar, and this is the level at which is the accumulator is pre-charged to, does the flow split? With a certain amount of flow already at 100 bar continuing on to the motor and the excess pressure and flow going into the accumulator? And in this case, does the pressure level in the fluid going to the motor rise as the pressure level in the accumulator rises? Or can the flow in the system only go to either the motor or the accumulator at the one time and not to both simultaneously?

Is it possible for an accumulator to have two ports? Both a separate in-flow and out-flow port? Do you have information on or can you point me in the right direction of an energy or storage tank or device that satisfies the same need for an accumulator, but does so by possibly taking in ALL the varying flow from the cylinder and outputting smooth flow and pressure to the motor.

 

[FredGarvin]

Thank you Fred, that is as I thought but I wasn't fully sure. So is there no way to store the excess pressure as the system pressure falls from 199bar to 100bar?

There is, but it depends on how you have the accumulator plumbed into your system. To store pressure like that, you need some way to shut off the accumulator from the system or else it will simply match the system pressure at all times. I use them like this all the time for simply dampening out pressure fluctuations.

When the system pressure rises over 100 bar, and this is the level at which is the accumulator is pre-charged to, does the flow split? With a certain amount of flow already at 100 bar continuing on to the motor and the excess pressure and flow going into the accumulator? And in this case, does the pressure level in the fluid going to the motor rise as the pressure level in the accumulator rises? Or can the flow in the system only go to either the motor or the accumulator at the one time and not to both simultaneously?

I would really like to see a schematic before really answering most of that. I am a bit confused because you make it sound like the accumulator is upstream of the pump. I have attached a couple of very basic diagrams showing a simple accumulator installation.

Is it possible for an accumulator to have two ports? Both a separate in-flow and out-flow port? Do you have information on or can you point me in the right direction of an energy or storage tank or device that satisfies the same need for an accumulator, but does so by possibly taking in ALL the varying flow from the cylinder and outputting smooth flow and pressure to the motor.

I can't say I have ever seen an accumulator with two ports, but that doesn't mean that someone out there doesn't make one.

 

[JoyceA]

Fred,

Please find attached a very basic schematic of a section of the system, apologies for it being so crude! I have not drawn valves on the cylinder, but it is a double-acting cylinder so that the system always receives positive pressure and flow. The input force is from ocean waves, which at this stage we are assuming to be sinusoidal, although they are of course irregular waves.

Ideally, we would like the motor to receive constant or at least smoothed flow so that the power produced is of higher quality. Without smoothing, the peaks in flow and pressure (and hence in the power) are 10 times the average, which is unacceptable. We are looking to confirm a concept that accumulators (or now possibly some other form of pressure storage device) can be used to store the excess pressure over/up to 10 cycles (1 wave period = 10s => 1 extension & retraction of the cylinder) and fed back into the system during the pressure troughs to maintain constant pressure and flow to the motor.

I hope that this helps you to help me with my previous queries!

 

[FredGarvin]

OK. Now I have a better idea. I was thinking in terms of a standard hydraulic system.

You have the accumulator in the right place and this is how they are used for dampening applications. What you will need to look at is the sizing of the accumulator and the precharge required (or weight if going with a dead weight type). These will be based on your system volume changes as well as the mean pressure you want your system to see. I would assume that you would want a pretty high precharge in your accumulator so that you don't have a huge lag in your system's response.

Ideally you would have a constant volume system but we know that that will not be the case. My guess is that you will only get a certain amount of dampening and not eliminate it completely. I would recommend that you talk with a hydraulics rep to size out an accumulator to meet your needs. They have a lot of experience and can do it quickly.

Here are some links to look through that may help you:

http://www.engineersedge.com/hydraulic/accumulator_equations.htm
http://www.engineersedge.com/wwwboard/posts/5200.html
http://www.hydraulicspneumatics.com/200/Issue/Article/False/43954/Issue