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.
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.