helium leak rate based on 5% helium fill

[falcon4]

I'm trying to develop a Mass spec leak rate for a high pressure valve. (3000-3200 psig)

We are allowed a mass loss rate of 0.0015 grams/hr. (nitrogen)

I would like to convert that to scc/sec nitrogen
and then to scc/sec Helium leak rates.

Once that is accomplished I need to develope a relative Helium leak rate when the system uses a 5%He and 95%N2 mix.
I have been given values internally but would like to independently confirm these numbers. ( or come reasonably close)

 

[israelkk]

0.0015 grams/hr is 1.2 scc/hr = 0.0003333 scc/sec of Nitrogen. Helium volumetric leak is 4.38 greater than Nitrogen leak = 0.001426 scc/sec. If the tank has 5% Helium then the partial pressure of the Helium is approximately 5% of the 3000 psi. Therefore, the leak rate is only 5% of 0.001426 = 0.000071 scc/sec

 

[falcon4]

i have heard of Helium ratios ranging from 1.4 to 2.6 in relation to Air. I assumed a value of 2.0 and came up with 3.35e-5 scc/sec.

I feel more confident now to review these figures with the current source who calculated the leak rate to 9.96e-6. I don't want to build this test any tighter than we need. The safety factor is built into the original mass loss rate and we need not compound it.

Thanks
gary

 

[israelkk]

The ratio depends on the pressure because Helium and Nitrogen are not behave as an ideal gas at 3000psi and each behave differently. Sorry, I calculated the leak/flow rate again and at 3000 psi the volumetric ratio is 2.8 so the Helium leak will be even less. The theoretical/ideal gas theory will give a volumetric ratio of the square root of 7 = 2.646 because the molecular weight of Nitrogen is 28 and of Helium is 4 giving a ratio of 7 for the molecular weights.

 

[NGiLuzzu]

Just for more information: you can find useful formulas and theoretical considerations for this kind of conversions in paragraph 7 of the European standard EN 1779 (August 1999) about Non-destructive testing - Leak testing - Criteria for method and technique selection.


falcon4,
may ask which method/technique will you use to detect helium leakage from your high pressure valve?


Thanks and Regards, 'NGL

 

[falcon4]

Since we will have high pressure involved we plan to first proof test the valve at 150%WP. Then we need to install a burst disk into the valve. We will then perform the HE /N2 filling operation with the valve still in the proof fixture.
Once pressure and temp stabilize we'll do a 60 second pressure decay to check for gross leakage. Once acceptable we'll close the chamber and start pump down and subsequent leak test.

Do you have other suggestions?

Gary

 

[NGiLuzzu]

Gary,
as far as I can understand, you do not use a sniffer probe (local, semi-quantitative method), but you build a vacuum chamber around the valve under test (global, quantitative method).

Am I right?

Thanks, 'NGL

 

[falcon4]

That is correct,
Since the leak rate is for the cummulative rates of all possible leak paths, we are interested in the "global" as you put it, value. If we find a non-conforming valve, we can either "probe" it, use leak detection fluid, or immerse it in alcohol or DI water, to locate the leak points.

Gary