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Question about valve and barrel

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Question about valve and barrel

Unread postAuthor: blackosprey » Sun Feb 10, 2008 8:34 pm

Hey, I'm in the process of making an over/under pneumatic, and I keep getting barrel choke error from GDDT. I'm using 1" for the chamber, a 1" Watermaster Sprinkler with a blowgun for a valve, and 1/2" for the barrel. Will this create a problem?

By the way, I have five feet of both 1" and 1/2", because I didn't want to buy ten feet of something bigger.
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Unread postAuthor: MrCrowley » Sun Feb 10, 2008 11:02 pm

http://www.thehalls-in-bfe.com/GGDT/index.html

Read that.

WRN: Barrel potentially choking flow
This warning rears it's head anytime the valve's effective oriface is greater than 65% of the cross sectional area of the barrel breech. If the valve to breech interface is blunt (ie, not tapered), then a 65% flow field is about the best one can expect in the barrel. Thus, the limiting factor for massflow is not the valve, but the breech of the gun. There is nothing wrong with such a design; and in fact, such designs are excellent performers. However, this warning may be an indicator that you're paying too much for your valve (ie, buying/building a bigger valve than you need). However... Since it is possible to get higher than 65% efficiency in the barrel with the use of a tapered interface or constant cross sectional system (read: large burst disc system), I had a decision to make. Do I allow the excessively large flow knowing that in many systems it will overestimate performance or do I restrict the flow knowing that in other systems it will underestimate performance? In the end, I split the difference between those two extremes. If this warning is triggered, GGDT uses the average value between the 65% barrel breech and the stated valve oriface.

WRN: Barrel choking flow
This warning is similar in cause to the previous warning; the difference is in the matter of degree. In this case, the valve oriface area has exceeded the barrel breech area. Even a perfectly tapered breech interface will not be able to pass all the gas the valve is capable of delivering. In this case, GGDT sets the valve oriface to equal to that of the barrel breech area.
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Unread postAuthor: blackosprey » Mon Feb 11, 2008 11:42 pm

Thanks, I guess I missed that part of his website. But I do have another question. I want to make the barrel the same length as the chamber (60"), but at 53.3" I start getting the gas liquification error. I read the description here, but is it really anything worth worrying about? As in, what happens if the gas actually does liquefy?
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