Calculate proper preload for accumulator?

I'm installing a new hydraulic accumulator on a machine. It's coming in empty because of shipping regs and I have to pressurize it. I can't find a factory spec for the nitrogen pressure. The spec from the accumulator mfg is a max of 4:1 ratio between operating pressure and preload. The pump pressure is on at 140 bar and off at 180 bar, so it will see a max of 180 bar hydraulic pressure. The accumulator is a diaphragm type and has a capacity of 1.4 liter.

If I preload it to 45 bar I will be right at the 4:1 ratio. Will this be enough to keep the diaphragm from "bottoming out" at 180 bar? How much does the nitrogen volume compress under a given hydraulic load?

Will it harm anything or reduce function by increasing the preload? Is anything gained by increasing the preload?

If I recall correctly from 20 some years ago at Elliot Turbomachinery, compressor and turbine mfgs., halve the volume and double the pressure. So, you would halve twice, first time would be 90 BAR, second would be 180. You would have 1/4 the gas volume at 4 times the pressure.

Anyone know if that is wrong?

Cheers,

George

Boyle's law says PV/T = a constant. That means that if temperature stays the same, pressure X volume stays the same number. Halve the volume, double the pressure as gmatov says. Multiply 45 X 1.4 = 63. Divide the pressure into 63 and you will have the volume of the compressed gas. Lowering the charge pressure until the diaphragm is just short of bottoming at max pressure will give you a little more active volume, but probably not enough to be worth the worry. Boyle's law is for an ideal gas and is not perfectly accurate in the real world, but close enough for must applications. Remember that the nitrogen will also heat from compression, so you will reach the max pressure sooner. If you do not use fluid and just hold, the nitrogen will cool and the pressure will drop. You will get a large increase in the initial run up but once that temperature equalizes, you won't notice the change as it cycles.

Bill

Thanks Bill, that helps tremendously.

One more detail

When you pressurize the accumulator, the increase will produce a certain amount of heat. It would seem like the nitrogen would cool when expanded out of the bottle and regain some of the heat as pressure builds in the receiver, largely balancing things out, but apparently it picks up enough heat from the piping that you end up with a warm receiver. When it cools, the pressure will drop, so you need to bring it up to pressure, close the valve, and do something else for a while. After it cools, refill it to the desired pressure or even a little bit more, then wait again. After a couple of repetitions, you will be close enough.

Bill

Mud said:

I'm installing a new hydraulic accumulator on a machine. ... Will it harm anything or reduce function by increasing the preload? Is anything gained by increasing the preload?

Click to expand...

What is the purpose of the accumulator? Is it a pulsation dampener, energy storage, peak pressure limiter, or none of the above?

Energy storage.

It's too bad that all the real "experts" on accumulators haven't answered your
question. Quickest way to get them to show up johhny on the spot would be for
me to post on this thread.

Curiosity, what is this machine that has such a small accumulator? The Bullards I worked on were in the neighborhood of 2 gallons, possibly larger, I would have to get out the manual to check.

9100,

Part of my answer was because an Engineer there told me that each stage of our centrifugal compressors doubled the pressure. That is the most that a centrifugal can do. Hence, our 10kPSI model had to have about 12 stages, and they were balanced, 12 on each end of the shaft. I would have to dig into those books, too, but I think they were large center wheels and small outer wheels, just the opposite of turbines with small impulse wheels for HP steam, and larger wheels down stream for the reduced steam power to impinge upon.

I am, by NO means, an Hydraulic Engineer. i have worked on this, but to the book.

Cheers,

george

Weird, no idea what the drama in a few prior posts are about, not my business.

The majority of hydraulic energy storage accumulators in industry are precharged to one half the highest working pressure they experience in service. That's 90 bar for Mud's question.

Yup, it sounds simple but math and physics back it up. Can do but not required here.

It will most assuredly not harm anything by charging to that pressure, it was designed for it if the operating pressure rating of the accumulator goes to 180 bar (which we know it does). Yes, you'll gain by charging to that pressure instead of the lower pressure: more energy storage which is the stated goal.

With some sort of cycling on and off pump going from 140 to 180 bar charge the accumulator to 90 bar and forget about it unless it has to operate to the very highest standards and requires detailed analysis.

I don't understand why accumulators would be charged to half pressure. If you want the maximum volume of fluid available regardless of pressure, you would pressurize to a low one so it would take the maximum volume change to reach the max pressure. If you want the maximum volume between two pressures, you charge to the lower one, giving the largest volume change to reach the max pressure, but the downside is that when you get down to the min, it will suddenly drop to zero. Half charge sounds like a simple compromise, not max storage.

Personally, I never wanted to pay the price for accumulators. On my molding setups I use inexpensive hydraulic cylinders from Grainger and pressurize the operating rod end with a strut pump. I have brackets extending out on the rod end holding pump start and stop switches. The fluid goes in the other end. Transfer molding needs two shots, mold closing and injection, with only holding pressure in between, so I can use a small pump instead of a much larger one idling most of the time. By adjusting the pressure and switch positions, I can set any range and I don't have to pay for accumulators or pressure switches.

What is all the vituperation? I know what adiabatic and isothermal mean, but this doesn't require that level of analysis.

Bill

Hydac (HYDAC Literature, click on "operating in installation instructions") recommends nitrogen charging to 90% of minimum working pressure.

Other tips are included in the instructions.

No connection to Hydac.