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Chamber sizes and pressures

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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Chamber sizes and pressures

Unread postAuthor: niglch » Thu Feb 21, 2008 10:25 am

Can anyone give a rough estimate as to the maximum PSI a pneumatic cannon made out of pressure-rated SCH-40 PVC can handle with a good degree of safety? I'm trying to figure out the most reasonable chamber size for my barrel-sealing piston cannon. Obviously, the #1 factor affecting performance is pressure, and the #2 factor is probably chamber volume. However, according to GGDT, a 2"x24" chamber filled to 100psi gives roughly the same performance as a chamber of half that volume filled to 120psi. It seems to me that the latter would take much less time and effort to fill especially with a manual pump. However, filling to higher pressures can also be more dangerous, so I'm not sure what to do here.
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Unread postAuthor: Fnord » Thu Feb 21, 2008 12:09 pm

Most people stay below 135 psi with pvc, so what you are planning is acceptable.
Your major limiting factor is the strength of your valve.

If you want to be completely safe with the chamber, just get 12" of steel pipe. It'll probably cost less than $20 for the pipe and fittings. The only real downside is the extra weight.
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Unread postAuthor: Jared Haehnel » Mon Feb 25, 2008 1:36 pm

I though it was rated to 220psi... but a good margin of safety doesn't hurt.
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Unread postAuthor: Jared Haehnel » Mon Feb 25, 2008 1:45 pm

So I just checked nothing better to do the store is so dead right now...

Heres the results

4" = 220 psi
3" = 240 psi
2" = 260 psi

Thats what its rate to. Thats is all sch 40
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Unread postAuthor: niglch » Mon Feb 25, 2008 8:29 pm

Yeah, but I don't think it's recommended that you bring PVC up to those pressures. Like _Fnord said, the valve is going to be the weak point because of increased stress from the movement of the piston. Holes must also be drilled in the valve to empty/fill the pilot volume which creates another potential weakness. Best to be safe.

However, I am wondering what a reasonable chamber volume would be for a gun designed for manual filling (shock/bike/homemade pump). I was thinking about 1 liter, but I have no real frame of reference since I haven't built a pneumatic before. At least according to GGDT, this volume should give fairly good performance to my design at safe pressures. My goal is ~400fps with a 50g projectile.
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Unread postAuthor: koolaidman » Mon Feb 25, 2008 9:18 pm

http://www.harvel.com/tech-specs-pvc-pipe-40.asp
At the bottom of that page theres a chart that gives all the specs on sch40.
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