Pneumatic vs. Hydrostatic Leak Testing 6

[Puddycat]

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?

 

[zdas04]

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

 

[weldstan]

It is indeterminate as indicated by zdas04.

 

[tbuelna]

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.

 

[moltenmetal]

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)

 

[EdStainless]

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

 

[dicer]

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

 

[moltenmetal]

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...

 

[Sparweb]

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

STF

 

[dgallup]

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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[boo1]

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

 

[robyengIT]

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

 

[EdStainless]

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

 

[boo1]

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

 

[Sparweb]

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


STF

 

[moltenmetal]

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

 

[SnTMan]

Well, yeah, you just check it from inside

Regards,

Mike

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

 

[NBrink]

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..

 

[zdas04]

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

 

[dgallup]

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.