flowrate from a sealed container

[craigmj]

Hello Group,

I'm creating a test device. This test device will be used to detect a leak in a sealed tank (of air). There is a vacuum inside the tank (low pressure, like under 5"Hg).

What I know is the pressure differential, the minimum allowed hole size, the volume of the tank, and the temperature (considered constant).

What Im trying to work out is a way to indicate whether the size of the leak "hole" in the tank is above a certain critical equiv diameter. Right now I am thinking that I could measure the change in pressure over some period of time. If the hole were too large the pressure would drop too quickly over some alotted period of time.

Its likely that the airflow through this small hole (0.0005") will probably have a velocity large enough that I can not treat it as incompressible (Mach#>0.3). The air on the inside of the "sealed" container is obviously changing density (growing as the vacuum drops).

I would be interested to hear if anyone has done this type of calculation before, and what kinds of things to think about that could affect the results.

Cheers
CMJ

 

[BigInch]

This has been discussed many times in this forum over the past year, generally in the context of "orifice flow" or "flow through an orifice". Try an advanced search for those terms.

You can also google "orifice flow" and get tons of references.

If you don't find what you need, come back here.

http://virtualpipeline.spaces.msn.com

 

[craigmj]

Im not asking to calculate flowrate through an orfice, that is not difficult, but rather to understand how to think about my particular problem.

The air denisty inside the tank is increasing in proportion to the pressure decrease (ideal gas law). Therefore the mass flowrate through the crack (or orfice) is changing with time.

My hope to to write a mathmatical expression that plots multiple curves of pressure vs time for a partiular volume tank and a max crack diameter (orifice).

I have not been able to find any resources regarding this paticular topic online, as of yet. Thanks for the suggestion though.

 

[Zapster]

http://www.eng-tips.com/viewthread.cfm?qid=177444

 

[HEC]

craigmj,

Looks like you are trying to determine pass/fail criterial for a vaccum test rig, for casting porosity or the like. There are pressure decay testers on the market these suppliers have much experience in the field. Try google pressure decay.
Cheers

Mark Hutton

 

[HEC]

Just a PS, if integrity is the only issue rather than size of leak, the use of sonic (sound) may be an alternative. There are products available that "listen" and locate compressed air leaks.

Mark Hutton

 

[mbeychok]

craigmj:

This online article discusses and explains two different claculation methods for determining the mass flow rate versus time for accidental pressurized gas releases. The article also includes an example calculation for each method that shows they both produce the same answers:

www.air-dispersion.com/feature2.html

Milton Beychok
(Visit me at www.air-dispersion.com)
.