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Burst disk problem.

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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Unread postAuthor: Crna Legija » Thu Oct 13, 2011 12:33 am

try lube the O-rings inside.
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Unread postAuthor: warhead052 » Thu Oct 13, 2011 12:41 pm

Ok, I solved part of the problem. You guys know the little Lay's single chip bags? Those managed to hold back 100 psi for me, without leaks. :D And it obliterates the bag when it bursts, so its an excellent burst disk. I will try and post some pictures of the mortar when the weather clears up a bit.
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Unread postAuthor: ramses » Thu Oct 13, 2011 3:22 pm

Have you mounted a burst disk, then taken apart the union without pressurizing?

You may still be tearing the foil so it shears out of the union at 25 PSI.
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Unread postAuthor: warhead052 » Thu Oct 13, 2011 6:38 pm

Yeah, tried that, and no tears at all. I dunno, its weird. It works sometimes, and other times it does not, which leaves a huge garbage pile of broken disks...
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Unread postAuthor: warhead052 » Fri Oct 14, 2011 11:37 am

Sorry for the double post, but here are the pics.

Image

Full picture.

Image

Ruptured disk. It was the only one that worked today. It was three layers of plastic.
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Unread postAuthor: DYI » Fri Oct 14, 2011 1:32 pm

The flow from a disk like that is going to be far less than optimal. I suggest trying to maximize the consistency of your production process for the Al foil disks, as follows:

Lay out a length of foil on a floor, preferably carpeted, with no debris present. Cut from the roll, and fold to half the length. It won't fold cleanly by itself, typically - use a small CPVC pipe or similar inside the fold area to push out the section where it tries to wrinkle. After it's folded and as straight as you can get it, go to the other end and cut all along it with scissors, making absolutely certain that there are NO remaining sections only one layer thick. This also crimps the end together and keeps the foil lined up during subsequent folds. Repeat this process for ALL folds.

To cut the disks: lightly press the layered foil sheet you just made over the male side of the union, enough to shape it to the union's contours and get an outline. It may wrinkle, but that should be outside of the foil section you're using. Cut just barely inside the resulting line with *sharp* scissors. Done properly, the cutting will crimp the edges and crimp all layers together so they don't get out of line later, just like it did during each step while you were making the layered sheet.

In total, I've folded perhaps one thousand square feet of (Our Compliments brand) foil like this into disks from 8 to 256 layers thick, and I have never observed the consistency issues you're talking about, leading me to suspect poor workmanship on producing the disks.

Also, neither of the pictures you posted are particularly useful, because they do not show the sealing face of your union. If my disk production method doesn't work for you, please take pictures of both sides of the sealing face and post them here.

EDIT: as to thickness, I seem to recall getting roughly 10psi/layer for the disks made of standard thickness foil in a 2" malleable iron union. Try eight layers for this one. Disks that thin (~0.004") are quite fragile, so be careful.
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Unread postAuthor: warhead052 » Fri Oct 14, 2011 1:39 pm

Ok. That's a good bit of info, so thank you! I will try, but it just started to get windy so I won't be able to shoot anything till it clears up a bit (don't want anything to go astray when firing)....
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