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Pneumatic vs. Hydrostatic Leak Testing
6

Pneumatic vs. Hydrostatic Leak Testing

Pneumatic vs. Hydrostatic Leak Testing

(OP)
Does anyone have a reference chart that compares equivalency of pressures to detect leaks. For example if I am using a testing medium of Helium @ 150psi to detect leaks, how much hydro pressure would be needed to detect the same leak?

RE: Pneumatic vs. Hydrostatic Leak Testing

Helium has a molecular weight of 4. Water has a molecular weight of 18. A leak point that would hold water but not hold helium would not leak at very very high water pressure, the molecule is too big to fit through the gap at 0 psig, 150 psig, or 10,000 psig.

We often see thread leaks on pneumatic tests that would not leak on a hydrostatic test at twice the pressure.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Pneumatic vs. Hydrostatic Leak Testing

It is indeterminate as indicated by zdas04.

RE: Pneumatic vs. Hydrostatic Leak Testing

Helium gas works nice for leak checks because it is easy to detect and it readily passes thru even the smallest gaps in a sealed joint. The fluid being sealed and the seal material compatibility can have a large influence on the leak rate.

RE: Pneumatic vs. Hydrostatic Leak Testing

If you assume the leakage path is an orifice, it's an easy enough calculation to compare the leakage rate between various fluids at a given pressure difference.

If you assume leakage is by diffusion, that too is an easy enough calculation based on constant concentration difference.

However, real leaks are more complex than that.

Detecting a leak consists of two distinct things: 1) getting enough material to leak and 2) having a way to detect what has leaked, and where it's leaking. So equivalent leakage rates are only meaningful if your only means of detection is by watching the pressure drop in a filled system over a period of time.

The difference between water and any gas as a test medium is huge- water is a very dense and vastly more viscous fluid with a huge surface tension (absent entirely from consideration for gases)

RE: Pneumatic vs. Hydrostatic Leak Testing

Actually a couple of psi of He will find leaks that water never will.
If we assume that the leak is large enough that molecular diffusion isn't the limiting factor,just think of the ratio of viscosity between the two media. That ratio will be more than 100, and then think of how much water you can detect compared to He.

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P.E. Metallurgy, Plymouth Tube

RE: Pneumatic vs. Hydrostatic Leak Testing

I would be more worried about the molecular size than the equivalence of pressure.

RE: Pneumatic vs. Hydrostatic Leak Testing

Ed: we agree that a helium leak test is doubly sensitive because a) it has a high tendency to leak at an appreciable rate through small leakage paths and b) it can be detected in very small quantity, such that you can easily find leaks that a loss in pressure versus time test would not find in a reasonable period of time. Consider the mass of helium in a 1mm bubble that you find with Snoop, to the mass in a 0.1 mL droplet of water...

RE: Pneumatic vs. Hydrostatic Leak Testing

It might be cheaper to use Nitrogen instead of Helium, but still conservative compared to the leak rate of water.

STF

RE: Pneumatic vs. Hydrostatic Leak Testing

Nitrogen, being ~78% of air can not be detected directly, you have to use a flow meter or pressure decay. However, helium leak testing is much more expensive and difficult. We do lots of production leak testing with either air or nitrogen. All depends on what you are testing and how tight the spec is.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Pneumatic vs. Hydrostatic Leak Testing

We use nitrogen and air, with soapy water. You just look for the bubbles...

RE: Pneumatic vs. Hydrostatic Leak Testing

We manufacture (in Europe) concrete pressure pipes according to U.S.A. AWWA C301 Standards (for water mains). Immediately after laying pipe in trench we test with a small compressor (at a pressure of approx 4 atm)the joint as per attached doc : the air will fill the gap between the two rubber seals. The test is lasting only 20-30 seconds : if there is no pressure drop, the operation of laying pipes goes on, if not the pipe is withdrawn, the rubber seals checked and replaced (if necessary) and the spigot and socket rings cleaned so to repeat the test. Such a procedure (very quick) to control that the pipe has been well laid and so the job can continue. Actually, every 600-800 meters, the pipeline is checked under pressure with water (600-800 meters of pipeline is filled up with water and tested according to the real working pressure).

My question is : is it possible to calculate a max allowed air pressure drop whilst the water test will be satisfied ?

Any comment will be appreciated. Thanks

RE: Pneumatic vs. Hydrostatic Leak Testing

We did some hydro test with fluorescein in the water, and used black lights to search for leaks.
Much more sensitive than standard hydro.
This was a case where we didn't want to use air because of the stored energy involved.

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P.E. Metallurgy, Plymouth Tube

RE: Pneumatic vs. Hydrostatic Leak Testing

When we use air we use a vacuum, not positive pressure

RE: Pneumatic vs. Hydrostatic Leak Testing

You will have a lot of difficulty obtaining 150 psi of vacuum.
Read the OP, boo1.

STF

RE: Pneumatic vs. Hydrostatic Leak Testing

Locating leaks when doing vacuum testing isn't easy. It's not like you can Snoop it and look for concave bubbles...(grin)

RE: Pneumatic vs. Hydrostatic Leak Testing

Well, yeah, you just check it from inside :)

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

RE: Pneumatic vs. Hydrostatic Leak Testing

You actually kinda can. Have found leaks in vacuum systems before by spraying vacuum oil on all the joints. When the vacuum improves for a few seconds, you've found the leak. Not super applicable to a PV mind you, but you can sorta snoop test vacuum systems...

Not particularly relevant though. A properly conducted helium test (using a mass spec) will find leaks at very small fractions of the hydrotest pressure that would NEVER leak with water. Don't really think you can make an equivalency..

RE: Pneumatic vs. Hydrostatic Leak Testing

Before the days of the Nanny Society control I used a bee smoker to find leaks. Then I had to get a hot work permit to use the bee smoker. Then there was a policy that hot work permits wouldn't be issued except by a VP. Then the fear and superstition around vacuum operations eliminated the problem (and increased unrecoverable gas by a few trillion standard cubic feet). The bee smoker worked great.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Pneumatic vs. Hydrostatic Leak Testing

You can check engine intakes for vacuum leaks with propane - CAREFULLY.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Pneumatic vs. Hydrostatic Leak Testing

When bee smokers started being a problem I proposed running a hose from a small ice chest with a chunk of dry ice in it and using a hose off the drain to direct the "smoke" of sublimation/condensation to indicate any leaks (same way propane works), and some idiot said "we can't purposely release CO2 into the atmosphere". I mumbled "then stop exhaling" and left the meeting never to return.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Pneumatic vs. Hydrostatic Leak Testing

When I ran vacuum furnaces, and we had one with issues we first pulled a vacuum and tested the joints using butane (a cigarette lighter), after we fixed those we moved on to He.
We never told anyone one that we were testing with butane, the other engineer was a smoker.

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P.E. Metallurgy, Plymouth Tube

RE: Pneumatic vs. Hydrostatic Leak Testing

While I perhaps sense a little cynicism in this thread, as a matter of fact one can effectively "see" leaks with some applied vacuum, and even e.g. with vacuum applied on the outside e.g. of piping etc. It has reportedly been done, and perhaps even to some extent is codified at least for some purposes in weld inspection (e.g. in AWWA C206 Field Welding of Steel Water Pipe standard), with the use of a contraption referred to and described therein as a "look-box".
While I don't know of the specific correlations you seek, as others have already stated I suspect at least for many practical circumstances a quite low level of air pressure or vacuum, with careful view or other sensors, may well reveal some VERY small, even if quite high applied hydro-pressure, leak paths.

RE: Pneumatic vs. Hydrostatic Leak Testing

JUST MY OBSERVATION HERE PUDDyCAT.

hydrostatic testing I have used for high pressure testing, so if it fails it will not explode. safety factor.
helium works great for testing for leaks with a sniffer. testing welds, bellows, that require sealed leak proof containment.

RE: Pneumatic vs. Hydrostatic Leak Testing

The OP mentioned "helium leak testing at 150psi". I would assume this means a 150psi delta. If the leak being detected is in a welded component, using an inert gas is nice because it won't contaminate the surfaces. On the other hand, using hydrostatic is much safer than using pneumatics for leak testing at high pressures. There are also some leak test fluids that can be easily detected using UV light.

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