Eng-Tips’s functionality depends on members receiving e-mail. By joining you are opting in to nondestructive testing handbook pdf free download e-mail. I was thinking I need to convert the air leak rate to a helium leak rate, which according to my books equates to dividing the air leak rate by .
374 for molecular flow, and that comes out to 1. Either way, what they’re willing to test to seems to exceed our requirement just based on these basic conversions. To me, it sounds like the original method is a pressure decay, fill whatever volume they have to 5000 psig and after 3 minutes, check the pressure to see if it has dropped enough to exceed the leak rate. I’ve seen helium leak detectors in operation, but is it equivalent to a pressure decay leak rate?
Thanks for any insights into this. What you’re doing doesn’t seem right, but again I don’t know what method you’re using. First, anything short of an outright explosion will be a “pressure decay”. Third: The leak rate depends on the viscosity and specific gravity of the gas.
Lastly, If you want to talk about volumes, the volumes should be corrected to standard pressure and temperature. This method might work for you. 5, the first thing you need to do is calculate the leakage hole diameter for the specified air test then, given that hole diameter, back-calculate to a corresponding leak rate for helium at whatever pressure they will be using. To be accurate you need to consider both laminar and molecular flow although, at least for the air test portion, you can probably assume molecular flow. Depending on your parameters one of the leak rates will be insignificant.
How do you calculate a leakage hole diameter when you don’t want one in the first place? I could back-calculate how much mass of air is lost according to the leak rate. This is the requirement verbatim: Each gage shall be leak tested at 5000 psig for not less than 3 minutes duration. I guess I need to figure out how they were testing these in the first place. The only way that makes sense to me is a pressure decay, since I have no clue how they’d measure an air leak any other way.
But there is no volume requirement, so I’d imagine they could play this any way they wanted since it’s not going to leak as much at lower pressures. As far as the relationships I was using, they come from a Nondestructive Testing Handbook. Actually running the numbers myself, I come up with . 373, but the book says .
I’ll keep you posted and if you have any more insights, please share. Leak rate through a hole won’t work for you, as you have realized, you have no hole size and you don’t care anyway. Then correct those volumes at 5000 psig pressure to atmospheric pressure and standard temperature, using the proper compressibility factor and actual temperature, to get the volume actually leaked, which should be “We have a leadership style that is too directive and doesn’t listen sufficiently well. The top of the organisation doesn’t listen sufficiently to what the bottom is saying.
Hopefully you do not have an actual hole in your part, but you use a “hole diameter” to account for the different parameters of the two test gases, namely viscosity and molecular weight. If you can get your hands on a copy of ASNE N14. 5 or Leak Testing volume of the ASNT Nondestructive Testing Handbook they can explain it much more elequently than I can. Why do you insist on having a hole diameter? Let’s look at an extreme, silly, example.
Let’s assume his part actually has a 5E-5 leak in it. Instead of using air, he decides to pressurize with molassas, or heavy mud as our friends are doing in the gulf. That is why a specified leak rate is based on what the pressurizing medium is. The reason the vendor wants to use Helium is the relatively small leak rate. 1 psi of pressure in 2 years. In order for the target leak to create a large enough pressure change to be reliably measured on the pressure gage the test period must be extended beyond the point where constant temperature can be assumed. Once temperature is introduced as a variable interpreting the test results is all but impossible.
I already said exactly that in my first post. We fill a vessel to a predefined pressure, 5000psi in your case, weigh it and then weigh it again after a predefined time. The difference in weight is the weight of the leaked gas. Knowing the molecular weight of the gas and that one mole occupies 22. This is not flow through an orifice, but a leak and you are going to have considerable difficulty reducing the leak to a hole size.
In part because, the leakage path its “effective discharge coefficient” or in some cases the leak process i. They’re just converting the air leak to units they can measure. They’re not even worried about the conversion, just treating it as a 1:1 conversion. Without converting it to helium, this should more than cover our requirement. Think about a leak filling a balloon. A helium leak is going to be considerably larger than an air leak, which would fill a balloon much faster than air would through the same leak. If they’re willing to helium leak test an air leak requirement, I should be well in the positive of my requirement.