Hydraulic V/S Pneumatic Pressure

Air (pneumatic) is a compressible fluid, water and oil (hydraulic) fluids are incompressible.

That wraps it up.

Mike

All three are compressible. THAT wraps it up.

You're not an engineer, are you? For all practical purposes, my statement stands.

All three are compressible...technically. For PRACTICAL applications in everyday life, oil and water are not.

Thanks standarded. I feel somewhat vindicated.

Mike

Dear Friend,

Please compare between:

Compressibility of Air = 200 times, i.e. it can be compressed to its 1/200th volume.

And

Compressibility of liquids = 1/1600 times, i.e. that, however the pressure you apply the difference in volume will not be more than 1/1600 of actual volume.

Therefore, the liquids under pressure act instantly like solids, but at the same time shift the force in all directions.

Air and oil (i.e. gases and liquids) have the tendency to flow and are commonly known as fluids. Pneumatic actuators dampen the action under compression.

Regards.

corollary to the above: With pneumatic test, safety is a major concern

But do not neglect safety with a hydrotest as well:

http://www.youtube.com/watch?v=NDEISLyxwZc

(there are many videos/incidents with deaths/injuries using water)

"incompressible" is not entirely true.

Here is another video of a hydro failure - to illustrate that they are not perfectly safe.

http://www.sperkoengineering.com/html/pipe%20burst%20test.mov

Just imagine a failure on a small branch/coupling connection ............... which actually happened recently, where a pressure gauge blew off and hit a guy underneath the chin. He did not survive.

"Just imagine a failure on a small branch/coupling connection ............... which actually happened recently, where a pressure gauge blew off and hit a guy underneath the chin. He did not survive."

In the hydraulics shop I was employed at a couple of years ago (hydraulic pumping units and accumulators for oil drilling Blowout Preventer systems) a system was being tested... 1 experienced guy, and 1 new guy.. they noticed a hydraulic union leaking.. the pressure in the 1" sch80 lines was holding steady at 3000 psi... and the accumulator bottle iso valves are open, so there is a lot of oil under pressure, and, with nitrogen pushing back in the accumulators... the older guy grabs a pipewrench... they are both basically leaning over the fitting... and he proceeds to attempt to tighten the union.... turns it the wrong way... and booooom

a jet of hydraulic oil straight up, blows the safety glasses off both their faces.. and up another 25 feet and effectively removes the vapour barrier and insulation off the ceiling of the shop.. no one hurt.. just hydraulic oil in their noses and eyes.... and 40+ gallons of fluid everywhere...

safety procedures and testing procedures were subseqently changed... (ya think?)

Chris

Further explanation:

As I remember, ASME hydrostatic tests are conducted at 1.5 x MAWP, which is a structural strength test. If a vessel fails such a test, it bulges a bit and then splooshes out a dab of test fluid (water or oil as the case may be). This is rather uneventful.

Pneumatic tests are usually at 1.25 x MAWP. They are meant more as a leak test, typically under water so that leaking bubbles can be seen. A small leak just generates some bubbles, but a failure may consist of a substantial rupture that blows fragments of the vessel for some distance.

I Agree that pneumatic test are for leak detection and hydraulic test are conducted for strength testing. Density of the test medium also matters for leak test. Air being thinner than oil or water will leak through smallest crack or other opening where as oil or water may not leak that easily or may take longer time for detection.

pcchatur; use helium, for testing for leaks perry

Helium is ok where volume of the piece under test is small and the area to be tested is also limited. You have to enclose the piece or the area under test in in some flexible cover like polyethylene. But when test piece is quite big like Turbo Generator casing helium test may not be feasible.

pressure = pressure = force over an area [full stop]

Stored Energy in pressurized containers is a whole different ball game.

Pneumatic failures are very explosive, hydraulic failures are more tame, but nevertheless, still dangerous.

I am an older engineer from Penn State. In my fluid flow text book, air is considered a fluid.

PEbobimm

Pneumatic is more dangerous because of the expansion ratio. If you have compressed air at 120 psi, and it blows, then the entire volume of air that can escape from the fault will equalize the pressure, which means it will occupy 120 volumes. This expansion can drive projectiles far and hard.

Because fluids are incompressible (except in black holes: no I'm not the wierdo that said fluids are compressible) they do not expand upon release. They will rush out the fault until the pressure is equalized, which takes but an instant. In that instant, a jet of fluid can cut you in half easily.

It should go without saying that the higher the pressure with compressed gases, the danger goes up proportionally. Gas cylinders under 2200 psi have been known to blow through concrete walls as projectiles and hardly slow down. Objects can be blown hundreds of feet as though blown by dynamite. In fact, they are similar in nature due to the expansion ratio of hot gases when dynamite goes off.

be warned.

Chris

actually I'm wrong on the volumes numerically, but the principle is the same.

If atmospheric pressure is 14.7 psi at sea level, then the volume expansion ratio is 120/14.7 = 8.16:1 so 8.16 because the escaping volume of air is displacing atmospheric air, not vacuum. If the air were escaping into vacuum space, then the volumetric expansion ratio would be 120:1

2200/14.7 = 149.66 : 1

Chris

ps.. agreed; Gases are compressible fluids.

Basicly, they are the same... both fliuds, both brought up to the same pressure to test a unit. But way different in the amount of each you would need to consume to reach similar pressures in similar volumes, way different in the devices you need to use to manage them, and both way different it how they will react if the test subject fails because one is compressable and one is not... well...

Everything is compressible, but the degree of compressibility as compared to application determines if it is relevant or not.

For example, oil IS compressible but in aspects of fluid power systems, not by much. So for 'most' applications it is considered non-compressible. However, if your oil fluid power system is for a precision servo system, or has long piping runs, or is a precision motion control system, then oil compression should be accounted for.

Base estimate values we used when I use to design high-speed precision robotic arms are:

Up to 2,000 psi oil will compress about 0.5% per 1,000 psi

At 3,000 psi oil will compress about 1.2%

Base estimates we used when I use to design hydrostatic testers for large diameter pipe:

Up to 2,000 psi water will compress about 0.3% per 1,000 psi

At 3,000 psi water will compress about 0.75%

(note: all final data was calculated using actual fluid properties, we used the above as initial design estimates.)

Bottom line is you want to use a fluid with the least amount of compressability to perform pressure tests with... it makes design of the testing system simpler and minimizes the amount of damage incured should the test fail.

I mean, you should see the damage cuased when a weld seam splits during hydrostatic testing of a 60' lenght of 36" OD pipe and water that was previously contained at 3,000 psi blows out ceiling panals in the plant. Which is why we point the weld seam up before testing. BUT, could you imagin what the damage woul be like if we had that volume of 3,000 PSI air escape all of a sudden?

I'll take the water anyday.

Java Head, I'd give you a GA if I could vote - well explained and supported.

If I may add - degassing of test "fluid" (and test vessel) is a need to do, often not done.

Also elasticity in lines and test subject are major factors in how much liquid is expelled on failure. Remember such tests are above 'safe working pressure'

A smart test set-up also has a flow sensitive shutoff close to the test subject, especially where hoses and/or long feed lines are involved. Also one at accumulators when such are the pressure provider.

Much drama above is about the lack of these (and dissolved gas).

Not all people speak in terms of states of matter.

3 states are "fluids" - but have their own 'state names' due to obviously demarcate characteristics. Clearly the OP meant "liquid" and "gas" - (not plasma)

Then again, I've just read a diatribe on "concrete is PLACED - not POURED!" - which beggars the question of how do you get it "PLACED"? So forgive my "sensitivity" to "distorted semantic based positions".

Again, Nice post - I hope others will award what I can't.

Kyzine

Answering just the OP: One of my friends feel that the hydraulic pressure and pneumatic pressure are different.

No. Pressure is independent of the medium. Stresses induced on the tested part are depending on the pressure (not the medium). 100 psi of any fluid induce the same stresses in the same part.

Another question is the safety concerns of using gases or liquids to make pressure tests.

Regards

If water or liquid under pressure ruptures a vessel you may get wet. If air (compressible gas) ruptures a vessel, size matters. If the tank it small you could have it in a barrel with padding, something to catch flying pieces. If the tank is large you might dig a pit, put the tank in it and cover the hole before you start.

To a gauge 1000 psig is the same for liquid or gas but before you pressure test with a gas ask yourself, "why?"