Leakage test with dry air
Leakage test with dry air
(OP)
Hi Colleagues,
We are trying to design a Water/Waste System to an aircraft and ground test phase is coming soon. After the installation of system to aircraft, ground test team will test the system with pressurized dry air for leakages. Test duration is 15 minutes and pressure will be measured after this duration to see if system leaks or not.
Ground test team warned me that they are used to have a permitted leakage rate, which does not count as installation failure. They say that since all the system connections are not welded, there will be negligible leakages from connections, couplings, even from ball valves when dry air is used.
On the other hand, our supplier insists that pressure drop during the test should be zero. I beleive that ground test team is right, but I can't calculate a leakage rate for them. Is there a rule of thumb for this? If you know, can you suggest me any references/books that I can find more information?
We are trying to design a Water/Waste System to an aircraft and ground test phase is coming soon. After the installation of system to aircraft, ground test team will test the system with pressurized dry air for leakages. Test duration is 15 minutes and pressure will be measured after this duration to see if system leaks or not.
Ground test team warned me that they are used to have a permitted leakage rate, which does not count as installation failure. They say that since all the system connections are not welded, there will be negligible leakages from connections, couplings, even from ball valves when dry air is used.
On the other hand, our supplier insists that pressure drop during the test should be zero. I beleive that ground test team is right, but I can't calculate a leakage rate for them. Is there a rule of thumb for this? If you know, can you suggest me any references/books that I can find more information?





RE: Leakage test with dry air
If they test with air, but the actual system will be handling liquid, or some 2 phase and only a liquid leak is important, you could develop a method based on the difference between theoretical gas leak rates vs liquid leak rates. Given their different characteristics, a measurable leak rate for a gas during a leak test would correspond to a much lower leak rate of a liquid emanating during actual use. As long as the liquid leak rate predicted from a gas leak test is less than some "allowable" liquid leak rate (whatever that might be I don't know), you could say it passes the test.
http://virtualpipeline.spaces.msn.com
"We can't solve problems by using the same kind of thinking we used when we created them." -Albert Einstein
RE: Leakage test with dry air
You're right: our medium is water, hence a liquid. My theoretical leak rate for water is zero. the problem is, I don't know how to calculate a theoretical gas leak rate.
Test group says that in every connection, valve etc. that are designed for water, there are extremely small gaps, through which water cannot flow but dry air can. They estimated a standard leak rate of 12 cm3/min for three differend subsystems, which seems illogical to me since our subsystems have different scales, different numbers of equipment, so should have different leak rates. I'm trying to find leak rate of each connection/valve/motor etc. for dry air for a range of pressures and I thought that maybe there existis a simpler rule/equation/database that I don't know but you experts use.
haven't you ever encountered some calculation like this?
RE: Leakage test with dry air
12 cm3/min looks like a lot of leakage. I think that such a leak rate is not valid for joint and fitting leakage testing.
Actually for a 15 minute test, I would't expect ANY leakage caused by anything other than for a connection not properly completed at all. Is that what the test is for?
http://virtualpipeline.spaces.msn.com
"We can't solve problems by using the same kind of thinking we used when we created them." -Albert Einstein
RE: Leakage test with dry air
RE: Leakage test with dry air
If you set up an air test on one end of the system at some test pressure, how much flow would you get out of the farthest end (at atomospheric outlet pressure), using something like an average diameter of tubing in the system? For lack of anything better, try something like 1/100th of that flowrate as an allowable to prove all connections were made or not made. What's that flowrate? How does it compare with the 12 cm3/m?
http://virtualpipeline.spaces.msn.com
"We can't solve problems by using the same kind of thinking we used when we created them." -Albert Einstein
RE: Leakage test with dry air
RE: Leakage test with dry air
however, you need to specify any leak rate contracturally first.
also, in my experience with air tests, temperature fluctuations during testing can cause psi drops without any leaks.
RE: Leakage test with dry air
It takes care of all the problems with straight pressure decay.
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RE: Leakage test with dry air
API Specification 6D, January 2002
ISO 14313:1999,
See Annex C
And ISO 5208, Industrial valves—Pressure testing of valves.
Also see API 598
Table 5 Maximum Allowable Leakage Rates for Closure Tests
a) For the liquid test, 1 millimeter is considered equivalent to 16 drops.
b) There shall be no leakage for the minimum specified test duration (see Table 4). For liquid test, O drop means no visible leakage per minimum specified test
duration. For gas test, O bubble means less than 1 bubble per minimum specified test duration.
c)The maximum permissible leakage rate shall be O. 18 cubic inch (3 cubic centimeters) per minute per inch of nominal pipe size.
d) The maximum permissible leakage rate shall be 1.5 standard cubic feet (0.042 cubic meter) of gas per hour per inch of nominal pipe siz