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Valve CV for a 1/2 inch bore cannon?

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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Valve CV for a 1/2 inch bore cannon?

Unread postAuthor: High-PSI » Mon May 16, 2011 8:01 am

Hey Guys,

I am building an all metal 1/2 inch pipe (.60" bore) cannon with my son. It will be firing Delrin slugs through a 30 inch barrel at 300 psi. I have two valves here I can use. The smaller of the two has a CV of 12 and the larger valve has a CV of 16. Do I need the higher CV for this cannon? Will it even matter at this bore size? I would prefer saving the higher CV valve for a larger bore cannon. But, I wanted your opinion.

Thanks.

Matt
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Unread postAuthor: jackssmirkingrevenge » Mon May 16, 2011 8:08 am

Run GGDT with a generic valve, give it a valve seat diameter of 0.5" and see what it tells you for a CV of 12 - if it says valve coeffciicent is "optimistic" then it's not going to matter.

What kind of valves are they?
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Unread postAuthor: High-PSI » Mon May 16, 2011 8:17 am

These are the same Granzow diaphram valves my big cannon uses.

They are both over 1/2 inch bore. One is 1 inch and the other is 1-1/4 inch. They will be reduced down to 1/2 inch pipe for this application. I am assuming the 1 inch valve will be plenty.

My GGDT does not work for some reason. I just have a list of files that will not open.........

Matt
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Unread postAuthor: jackssmirkingrevenge » Mon May 16, 2011 8:32 am

I don't have GGDT here but I can assure you that the smaller valve is already overkill. A 1 inch tube has 400% the cross-sectional area of a 1/2" tube, you'll be fine.

I predict you're going to have some pretty good performance with those specs, even with a relatively small chamber.
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Unread postAuthor: High-PSI » Mon May 16, 2011 8:45 am

I actually already fired a bench test rig with this valve and slug (though with a 4 foot barrel). It fired a 18 gram metal tipped slug clear through one sheet of 3/4 inch plywood and deeply embedded into a second at 160 PSI. That was at point blank range, though.

Anyway, what I am shooting for (pun intended) is the kick of a rear firearm with OK accuracy and good power so my boys can have some fun.

I figure the small chamber this thing will have (compared to me big gun) will only take 20 seconds or so to hit 300 psi.

I already have a decent scope and all materials on-hand to build it.

This should be a fun cannon, and easy to build.

Matt
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