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Some background for context only

I am in the process of designing a pressure system where my pressurized fluid needs to be kept incredibly clean, with typical contaminants at the ppt or below level. The fluid in question is more compressible than a typical hydraulic fluid, so I need to build in an expansion volume for when I cycle to lower pressure. This expansion volume is going to look very much like a hydraulic accumulator.

The most common hydraulic accumulators are pneumatic, and the gas charge is separated from the fluid by a bladder, diaphragm, piston, or metal bellows. All of these are problems for my cleanliness requirements, as the moving parts will tend to shed metal shavings, etc.

The question

My question is, why is the separation of the two volumes necessary? In my case, this is for a fixed piece of equipment, so I can guarantee that the accumulator will always be vertical. Can I just have a nitrogen-filled cylinder with the fluid connected at the bottom and rely on the fluid to act as my piston? If this does work, why isn't it more common?

Edit to address some of the comments:

I'm not asking for advice on how to design a clean accumulator; I only want to know the fairly general question of whether it is possible to not separate the charge gas from the fluid.

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  • $\begingroup$ How would a bladder/diaphragm shed metal shavings? $\endgroup$
    – JMac
    Oct 25 '17 at 19:13
  • $\begingroup$ It would shed rubber bits (hence the "etc."). More relevant to my case, rubber tends to emanate radon at a relatively high rate which is even worse for me. $\endgroup$ Oct 25 '17 at 19:25
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    $\begingroup$ perhaps because at various pressures gas can get absorbed into liquid... see the "bends" for divers. $\endgroup$
    – Solar Mike
    Oct 25 '17 at 19:25
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    $\begingroup$ why do you assume the bladder / diaphragm is rubber? : it could be silicon ,... $\endgroup$
    – Solar Mike
    Oct 25 '17 at 19:26
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    $\begingroup$ Why don't you check out some sources such as boschrexroth.com/ics/cat/… , and then refine your question appropriately. $\endgroup$
    – Solar Mike
    Oct 25 '17 at 19:28
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gases are soluble in oils. this means that as your oil enters the gas-filled accumulator and the pressure in it builds up, gas will probably dissolve into the oil. Then, when you release the pressure on the system, the gas will boil out of solution and cause the oil to foam up vigorously. the foamy oil then enters your plumbing and creates big problems, especially if it is ingested by a pump.

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  • $\begingroup$ Just to add what I think is a critical detail, the solubility increases with pressure, so the decrease in solubility is what causes the boiling. I think that means in my case I may be able to get away with it if either my pressure change is small and slow or I can find an oil with a very flat solubility curve $\endgroup$ Oct 26 '17 at 18:59
  • $\begingroup$ good idea to put a foam trap in the plumbing circuit in case something goes wrong. I had a $600 repair bill when ink from a degassing station i designed made it into the vacuum pump! $\endgroup$ Oct 26 '17 at 19:41

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