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leaky burst disc

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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leaky burst disc

Unread postAuthor: JDP12 » Tue Dec 11, 2007 12:31 pm

Hey all, i was pressure testing my hybrid for leaks today, but i noticed something bad- air was escaping past my soda can burst disk. I hadn't tightened the union very much, so i think that might be part of the problem. My union is orientated the same way as SB15's on his handheld rifle if that is a piece of information you need.

also, i was PMin SB15 and he recommended adding an aluminum washer after the burst disc for more clamping power. I was just wondeirng since i don't have access to large metal drill bits or a milling machine, and i don't want to spend $30+ for one bit that i'll never use again, if i could make that out of plexiglass? I think i should be able to, but i'm not sure. I think the combustion would be quick enough so that the plexiglass doesn't warp, but what do you guys think?

Thanks for your help with both of my questions
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Unread postAuthor: SpudBlaster15 » Tue Dec 11, 2007 7:02 pm

Sorry about that, I read your PM about the plexiglass washer, but didn't have time to respond at that particular instant. I then completely forgot about it.

Plexiglass may work, though I would be concerned with the potential for it to shatter when exposed to the shock loading of 300 - 700psi suddenly acting upon its surface. The only way to find out for certain is to cut out a washer, and test it.

Provided it does not shatter, melting should not be an issue.
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Unread postAuthor: DYI » Tue Dec 11, 2007 7:06 pm

Using aluminum foil instead of soda can disks will probably be easier, as well as cheaper. In a 1.5" union, 32 layers of aluminum foil will usually survive 400-450 psi. In a 1" union, more than twice that. And with only a butyl rubber gasket they seal nearly perfectly, even at 400+ psi.
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Unread postAuthor: JDP12 » Tue Dec 11, 2007 7:26 pm

hmm yea i just can't figure out why its not sealing, SB15, do you think its somethin to do with the union orientation?

UPDATE: K i tested it with aluminum foil and with the union flipped, still leaked. do you think adding that disk would help SB15?
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Unread postAuthor: noname » Tue Dec 11, 2007 10:43 pm

If you don't have a rubber washer in the union, it's obviously gonna leak. If you do have one, try getting another so you have one on each side of the disc.
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Unread postAuthor: JDP12 » Tue Dec 11, 2007 10:49 pm

oh i want one on each side?

I have one on one side, but not on the other... Hmm i'll add a rubber piece to my 1/4" plexiglass washer im making.. I think 1/4" should hold up fairly well (plexiglass that is)
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Unread postAuthor: SpudBlaster15 » Wed Dec 12, 2007 1:07 am

Using a gasket on the opposite side of the union would be pointless, as this surface does not form a seal between the disk and union face.

Increased clamping surface area is the answer.
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Unread postAuthor: spanerman » Wed Dec 12, 2007 5:30 pm

^seconded, theres a oring and leather washer in mine....and its grinded flat which helps no end
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Unread postAuthor: JDP12 » Wed Dec 12, 2007 6:46 pm

which helps no end? what do you mean by that
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Unread postAuthor: noname » Wed Dec 12, 2007 11:59 pm

The rubber washer should be on the chamber side of the burst disc, it's pretty much useless on the other side.
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Unread postAuthor: spanerman » Thu Dec 13, 2007 4:47 am

ilovetoblowthingsup wrote:which helps no end? what do you mean by that



sory (term used over here in uk) it means it helps alot
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Unread postAuthor: BigGrib » Thu Dec 13, 2007 6:03 am

i used a cam lock right at the end of my chamber for an interchangeable barrel system and i unscrewed my barrel from the male cam lock, put a burst disk in the female and put the male in it and locked it down tight worked like a charm. cam locks are really easy to come by where i live and really cheap too (aluminum is even cheaper than poly cam locks) I would personally use that cam locks i think it would make replacing the burst disks easier after each shot.
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