OT, Hydraulic accumulators

I didn't want to hijack the thread about e-stops and hydraulic pumps. Below is a link to an article in Hydraulic and Pneumatics about accumulators. Be patient. The site is slow.
http://www.hydraulicspneumatics.com/200/Issue/Article/False/43954/Issue

If you use hydraulic accumulators at all then you should read this article.

Aha. It is kind of like a hydrofor.

On our systems we use a kind of pump that has several pistons that are actuated by a rotating cam. The flow of the pump is adjusted very accurately by changing the angle by which the cam actuates the pistons.
One advantage is that the flow control is very accurate, and a minor advantage is that the pump has in principle zero work when the angle is neutral, so it saves energy.
I dont remember the exact type, but there should be only two major manufacturers of this kind of pump. One of them is Vickers, dont remember the other one.

Because there is no pressure accumulation anywhere, when you stop the pump there is for sure no pressure in the system.

Wikipedia calls it "axial piston pump, bent axis principle":
http://en.wikipedia.org/wiki/Hydraulic_pump
(look at the bottom of the page).
This is principal diagram:
http://en.wikipedia.org/wiki/Image:Bend_axis_pump.jpg

JesperMP said:

Aha. It is kind of like a hydrofor.

On our systems we use a kind of pump that has several pistons that are actuated by a rotating cam. The flow of the pump is adjusted very accurately by changing the angle by which the cam actuates the pistons.
One advantage is that the flow control is very accurate, and a minor advantage is that the pump has in principle zero work when the angle is neutral, so it saves energy.
I dont remember the exact type, but there should be only two major manufacturers of this kind of pump. One of them is Vickers, dont remember the other one.

Because there is no pressure accumulation anywhere, when you stop the pump there is for sure no pressure in the system.

Click to expand...

Jesper
Just to make sure we are not getting messed up with terminology.
The pump you described is an axial. IF you run the pump and pressurize the system then turn the pump off AND there is no demand (ie valve open) AND no leaks the system should stay pressurized.

Now for the accumulator. A common design is a cylinder with a piston. On one side of the piston is nitrogen (or nitrogen filled bladder) and on other side of piston is hydraulic fluid. So increasing hydraulic pressure compresses the nitrogen and conversely when hydraulic pressure drops then the higher nitrogen pressure pushes the piston to supply fluid to the system.

The accumulator stores a "reserve" of pressurized fluid so that when a large load is activated, it supplies fluid IF the pump cannot keep up. This reduces pressure surges and shocking of the system.

Dan Bentler

IF you run the pump and pressurize the system then turn the pump off AND there is no demand (ie valve open) AND no leaks the system should stay pressurized.

Click to expand...

I have to disagree. Maybe there is pressure, but there will not be enough stored energy to start a movement. The only way that energy can be stored in a pressure system is if the pressure compresses something. In the accumulator you compress a gas. You cannot compress a liquid. It is the same reason you can control hydralics much more precisely than pneumatics.

Another factor is that the pump will not be 100% sealed between the two sides. Maybe 99.99% but not 100%. The assumption that there is no leak is not realistic.
I know from experience from looking at a manometer. When you cut the pump, the pressure goes instantly.

JesperMP said:

You cannot compress a liquid. It is the same reason you can control hydralics much more precisely than pneumatics.

Click to expand...

You can, in fact, compress a liquid; just not as much as a gas.

Check out this thread if you want to learn more than you probably care to about the compressibility of oil:
http://www.patchn.com/SMF/index.php?topic=612.0

It is all relative

JesperMP said:

Aha. It is kind of like a hydrofor.

On our systems we use a kind of pump that has several pistons that are actuated by a rotating cam. The flow of the pump is adjusted very accurately by changing the angle by which the cam actuates the pistons.

Click to expand...

These types of pumps are not that accurate and their response is usually very slow.

I know from experience from looking at a manometer. When you cut the pump, the pressure goes instantly

Click to expand...

As long as there is nothing to store the energy you are correct for all practical purposes.

I know from experience from looking at a manometer. When you cut the pump, the pressure goes instantly

Click to expand...

That is true but as I keep reminding the hydraulic guys it isn't oil that makes things move. It is force. Other forces like gravity can still make things move or even pressurize a system, not much but sometimes enough to move something. If you look at the thread below you will see that oil will lose pressure rapidly after moving the actuator just a few thousandths of an inch or leaking just a little fluid.

[OT]

Check out this thread if you want to learn more than you probably care to about the compressibility of oil:
http://www.patchn.com/SMF/index.php?topic=612.0

Click to expand...

It is amazing how even simple problems have complex answers.
A student asked essentially the same question on this forum about using hydraulics for shock absorbers.
http://forums.hydraulicspneumatics.com/eve/forums/a/tpc/f/8641063911/m/9031010042
The student was told to look up the equations on the internet. I knew it was not easy to find information on this topic and I also knew he would never find the formulas. This is not an easy topic and beyond the knowledge of those telling the student to study. I am posting the answer slowly. One post every time the thread is not at the top. I will do this until I answer oddball's question. I hope to make a point one post at a time. Take a look if you are interested. The next post is calculating how much the oil is compressed if a 1000 lb weight is drop from 10 inchs on the column of oil. The calculations are not trivial. There is lots of calculus and differential equations. I haven't even got to the hole yet. FUN!!!
[/OT]

Most of the hydraulics I deal with involves pressures between 15,000 and 20,000 PSI, and occasionally 40,000 psi, operating on large cylinders, so compressibility of oil becomes a major design consideration. With expereience its not too bad but for the first timer it can be daunting.

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Jesper, if you are looking for an electronic analogue to the accumulator it would be a capacitor. A pump analogue would be a generator. The generator creates current flow, likewise a pump creates fluid flow. An accumulator stores fluid under pressure (energy) to provide brief flow to smooth out pump pulsastions or for brief demand flow above the pump flow rate. Likewise, a capacitor does not generate electricity, it is a temporary storage device. The comparison obviously fails if pushed to the extreme but it conveys the general idea.

What Are You Doing?

Alaric said:

Most of the hydraulics I deal with involves pressures between 15,000 and 20,0000 PSI, and occasionally 40,000 psi, operating on large cylinders, so compressibility of oil becomes a major design consideration. With expereience its not too bad but for the first timer it can be daunting.

Click to expand...

Most valves can handle pressures that high. Are you sure you aren't off by a factor of 10? The only applications I have heard of that use pressures like that are hydroforming or lifts (jacks).

Accumulators can be used to smooth out the "pulses" of a positive diplacement type pump, like a 3 cylinder piston pump.

Also, hydraulic accumulators are used to absorb shocks. One of our Caterpillar loaders has ride control, the other does not. It has nitrogen accumulators on the boom lift cylinders, and when cruising over rough terrain, you can see the boom move up and down a few inches. The difference in ride is absolutely phenomenal. Going from a loader with it to the one without ruins your day. In some places you can double the speed and still be smoother with the ride control.

Of course you can turn the feature off for times when you need really accurate boom control.

Peter, those pressures are correct, Oil pressure of Fifteeen Thousand to Twenty Thousand and Fourty Thousand (though I see I did make a typo and put one too many zeros after the comma on 20). Did you mean to say that "most valves cannot handle pressures that high" because I know of only a couple that can? But we cheat. (sorry, trade secret)

Check your PMs for more info about the application.

Peter Nachtwey said:

The only applications I have heard of that use pressures like that are hydroforming or lifts (jacks).

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Some large interference fit bearings use oil under these pressures (15,000 to 40,000 psi)to "hydroplane" the inner race over the shaft. The oil is forced in between the race and the shaft by a special port on the shaft.









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Alaric said:

Did you mean to say that "most valves cannot handle pressures that high" because I know of only a couple that can? But we cheat. (sorry, trade secret).

Click to expand...

That isn't cheating. That is engineering. Good show.

Just so everyone knows, hydraulic valves typically go to 350 bar or 5145 psi. Most people just round that to 5000 psi. Making the valve bigger would help it withstand the pressures but the trick is to do that and still have fine conrol over the flow. The spools must be made of special material. That those high pressures the flow would be very high and almost corrosive.

Some of my hydroforming customers go up to 90000 psi but they use intensifiers to do this.

Accumulators can be used to smooth out the "pulses" of a positive diplacement type pump, like a 3 cylinder piston pump.

Click to expand...

Yes, but I tell my customers to use them because pressure is like the voltage on a power supply. If one varies the power supply voltage it causes problems. In the hydraulic case the gains change because the flow is proportional to the square root of the pressure drop. If the pressure drop is relative constant then the actuator gain will be relatively constant. This keeps the tuning constant. It depends on what you are trying to do.

In the hydraulic case the gains change because the flow is proportional to the square root of the pressure drop. If the pressure drop is relative constant then the actuator gain will be relatively constant. This keeps the tuning constant. It depends on what you are trying to do.

Click to expand...

So true. We use an accumulator to make up for pressure dips from the pump. When you start a move the system requires flow. Just like in electronics when current goes up voltage can dip. We see this on our pumps it takes time before the pump can react to the flow to maintain the pressure. This is where the accumulator discharges to maintain the pressure until the pump has time to react.

pressure drop is relative constant then the actuator gain will be relatively constant

Click to expand...

It seems like this would be true for a pressure increase as well. But I have never had much trouble with tuning as the pressure increases.

For example I maintain a 1000psi working pressure. When I need high tonnage the pressure can go up to 5000psi. I use the same tuning unless the cylinder size changes. If I am using more than one cylinder on the valve I will have one set of gains for the quick move (small) cylinders and another when the larger high tonnage cylinders are connected to the valve.

I think I know the answer but I am far from a hydraulic expert.

Peter Nachtwey said:

These types of pumps are not that accurate and their response is usually very slow.

Click to expand...

What I mean is when comparing it with regulating any other kind of pump. For sure if you compare with a servo valve system, then the servo valve wins on speed an accuracy.
You posted about the accumulator being used to save energy, and my point was that this kind of pump system works in alternative way to also save energy. Rather than using an intermediary buffer, this system only uses just the energy needed to create the required flow and pressure at any given time. So for our application this pump fits the bill partly because it also saves energy.

About the compression of fluids. Yes I know, my statement that fluids cannot be compressed was too simplistic. If you go high enough, anything can be compressed.

For information we operate at max 300 Bar.