How Much Hydropneumatic Pressure is Comparable to Hydraulic?

Yes. This is a good way to do it. The only gotcha is that there should be no difference in elevation or you will have to take the weight of the liquid into account. (For water, 1 bar = about 33.4562 feet elevation difference, so 4 inches elevation difference would be about 0.01 bar).

Yes. Pressure is pressure is pressure. Your gage reading it does not care if it is hydraulic or pneumatic unless it is a vibrating crystal pressure sensor, and few of those exist anymore.

Thank you all for the advice.

In respect to Harley's comment: "Pressure is pressure is pressure."

I know he's talking about pressure reading but what about the application of pressures? Can we consider Hydraulic pressure and Pneumatic pressure same in case of forces they generate? I'm testing resistance to pressure of this container and I wonder if there is any preference using Hydraulic pressure over Pneumatic pressure.

I've read about gas compressibility and its output force (which is somehow jumpy).

Do you have any experience in application of Hydraulic pressure and Pneumatic pressure or any reference you could introduce?

In addition, in which category of pressure is the my proposed system assigned? Hydraulic or Pneumatic?

Hydraulic testing is much safer than pneumatic due to the explosive force of a gas compared to a non-compressible liquid...The mixing might mitigate the force somewhat, but I don't see the point...If you're using a gas to raise the pressure it's pneumatic....

..."hydraulic testing should always be used unless there are very good reasons against it. In a similar thread on this forum some time ago I mentioned involvement with pressure testing during my HSE days. This included a discussion with a company about pressure testing of hyperbaric chambers used by divers for North Sea oil industry work. With assistance from one of HSE's senior engineering inspectors it was agreed that hydraulic testing of such chambers was not reasonably practicable in view of i) how any test liquid would affect the electrical and other complex fittings within such chambers and ii) the time and effort, etc., needed to remove such equipment before hydraulic testing and subsequently refit it. However, for pneumatic testing very stringent precautions were required. These included doing it on a large and remote site with blast walls or berms and other measures to protect those controlling the testing remotely from a suitable bunker."...

http://forum.iosh.co.uk/posts/m686613-Pneumatic-pressure-test-vs-Hydraulic-pressure-tests

As you and others have said, hydraulic tests are safer because far less energy is stored in the relatively incompressible hydraulic fluid than compressible pneumatic fluid. Because of this property, one can easily overshoot a pressure level with hydraulics than pneumatics. This minor drawback far outweighs the safety advantages but it is always good engineering practice to know of all drawbacks no matter how frivolous.

That's a good point. That 6bar nitrogen, if released, would want to occupy 6 times its previous volume, maybe taking liquid with it. Hydraulic is much safer. The greater the volume of gas, the more danger there is.

You have a hydropneumatic tank. Most of these have a membrane separating the liquid and gas.

Yours is probably safe if the amount of liquid occupies 99% of the tank volume.

You can claim it.

It would help you to understand the gas laws first.

Gas laws - Wikipedia

The main reason for hydraulic testing vs pneumatic is safety.

With hydraulic testing, the pressure drops to zero almost instantly in the event of a failure. With a pneumatic component added to the system, there can be a lot of stored energy in the form of compressed gas.

Be careful.

If you used a pressure gauge, & it read 6 bar, you could.

Use one of these

Build the pressure with liquid instead of gas

Dangerous: 2.2l disposable nitrogen gas bottle filed to 100bar. Cost £30.00. Provides approx 33 litres at 6bar. Value after tests completed £0.

Safe: Karcher K2™ pressure jet washer, output at 100bar. Cost £45.00 Provides unlimited volume at 6bar. _{No recommendation given or implied}. Value after tests completed £45. Cheaper pressure washers are available.

Do not rely on a single pressure switch to cut off at 6 bar. Always provide a back up pressure switch set at 0.5 bar above test cut off to conduct test in safely.

a fluid, say hydraulic oil only compresses (1/2% / 1000PSI) or 69 BAR and additional to your concern as to the nitrogen inclusion into the water, again in oil, inclusion will only take place above 1000 PSI, which is the basic reason for the bladders in high pressure accumulators.

I regularly used air over oil chambers to operate hydraulic system pressure. The danger is limited to the air over oil cylinder. The rest of the system can be treated as a purely hydraulic system. The only caveat is to ensure that the air over oil cylinder never runs dry and allows air into the hydraulic side of the system. Then the hydraulic system gets squishy and spongey.

How do you isolate the two? If there is a failure on the Hydraulic side, Doesn't the compressed air continue to provide pressure to the system?

In the systems I worked with, no isolation was required. The air/oil vessel was relatively small and if you ran the system out of oil, you blew air into the hydraulic piping.

If isolation was important, then one of two things were done.

First, you got an air/oil reservoir with a diaphragm, like Lyn mentioned. When the unit ran out of oil, the diaphragm contained the air pressure. Some also had a valve in the bottom with a mushroom shaped operator that would shut off the oil flow if the diaphragm pushed down against the mushroom. There were also reservoirs that were little more than a rodless double acting cylinder with hydraulic oil on one side of the piston and air applied to the other side.

Second, you put a float valve in the bottom of the reservoir that shut off the hydraulic fluid outflow if the level went below a minimum level.

For either type, you would lose hydraulic pressure if you ran the reservoir to it's minimum, but would still contain the air and prevent it from going into the hydraulic piping.

Look up "pressure test pump, line leakage, central heating" on E bay. A water hydraulic hand pump capable of delivering up to 30bar, complete with 5L tank, 1.8m hose line and pressure gauge cost about £25 ($32) to purchase.

You say you don’t have the proper facility. Well, if this is in your shed at the bottom of the garden, and you kill yourself, that is one thing. If it is in a workplace that is a completely different matter as your insurance company will walk away when it goes BANG! And it will be you that is up in front of the Beak explaining your defence against prosecution for manslaughter. I think that’s called “Murder 2” in North America, though someone from that island may explain further, I suppose.

So that needs to be understood as well, Mildred.

If you know the difference(s) between hydraulic and pneumatic, next is to learn the sameness(es).

If your test chamber isn't too big, or your testing extremely repetitive, I would be looking for a portable hand operated hydro-static test pump. They don't cost much and are no more difficult to operate than a bicycle tire pump at 6 bars. It is not worth the risk, or the cost of using a hyrdro-pneumatic system safely