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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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Burst disk problem.

Unread postAuthor: warhead052 » Tue Oct 11, 2011 2:02 pm

I made a burst disk mortar with a 2 inch union, and my disks keep breaking at only 25 psi. I would like them to break at around 50-75 psi, but no matter what I try, it always pops and 25. Any suggestions on what I should do? I have tried everything from shopping bags to aluminum foil tape (which is a pain to peel off...). Any help would be appreciated.

Edit, forgot to mention that I checked the search, but it only showed stuff relating to 1 1/2" and below, and non of which helped.
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Unread postAuthor: Labtecpower » Tue Oct 11, 2011 2:28 pm

Do they break, or do they pop out of the union?
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Unread postAuthor: warhead052 » Tue Oct 11, 2011 2:56 pm

Just break. And I can't get them to seal either. I screw the union all the way down, and sometimes tight enough to where I have to use pipe wrenchs to get it apart, so I don't see why its not sealing, and when it does finally seal, why it breaks at 25 psi. The chamber is 2 feet of 2 inch pipe.
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Unread postAuthor: saefroch » Tue Oct 11, 2011 2:59 pm

Did you put gaskets into the union to seal the disc?

Layer whatever, and use aluminium foil, not the tape.
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Unread postAuthor: jackssmirkingrevenge » Tue Oct 11, 2011 3:02 pm

Try a single layer of soda can ali
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Unread postAuthor: matti » Tue Oct 11, 2011 3:03 pm

use aluminium foil, and just put so many layers that you get to the pressure you want.

if the union sealing faces are those cone shaped ones, then its very hard to get good seal.. use pipe wrench :D
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Unread postAuthor: Crna Legija » Wed Oct 12, 2011 12:31 am

idk if pvc unions have them but you might need to grind the tapers off to leave a wider flat surface it grips better that way, also use gaskets like sae said.
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Unread postAuthor: jakethebeast » Wed Oct 12, 2011 2:28 am

If the union is the cone shaped one, you should be able to sand the cone away to a flat surface, then just use o-ring to get a good seal
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Unread postAuthor: warhead052 » Wed Oct 12, 2011 9:09 am

Sae, I didn't know I had to.

Jake, Crna, and matti, it is a smooth union, there are no tapers/cones on it, and it has an o ring pre installed.
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Unread postAuthor: pneumaticcannons » Wed Oct 12, 2011 12:30 pm

a while back, I made a burst disk launcher where you twisted the union to shoot. Every time the burst disks would get shredded. So even if your disks are getting shredded every time, it could still mean that your union isn't grabbing your burst disk hard enough
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Unread postAuthor: warhead052 » Wed Oct 12, 2011 2:02 pm

Its not that, its the pressure. It seals once I tighten it down to where I have to use a pipe wrench, but then it pops at 25 psi. I am wondering if it has to do with the chamber volume, considering this is a 2' piece of 2".
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Unread postAuthor: danielrowell » Wed Oct 12, 2011 2:16 pm

So it pops at 25 psi, no matter how many layers of foil you use? That's weird. Can you provide a picture of the disks after they've burst and maybe a pic or two of your union? Maybe that'll give us a better idea of what the problem is.
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Unread postAuthor: warhead052 » Wed Oct 12, 2011 2:22 pm

I will try. My camera is kind of bugging out at the moment, but I will do my best.
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Unread postAuthor: DYI » Wed Oct 12, 2011 2:48 pm

Does the burst disk look deformed in any way after you tighten it in the union? The surface should be as smooth as it was before you tightened the union. Ripples and other deformities act as stress concentrators because of their sharp edges, and may be indicative of tearing in the sealing area of the disk which you can't see.

From my experience with burst disks, something like eight layers of standard thickness (~0.01mm) aluminum foil should fail in the range you're looking for, although this may vary by brand of foil. Also, I didn't really do any testing at below 160psi for 2" unions.

If tearing seems to be a problem, adding a thin gasket on the muzzle side of the union may solve it. Removing the o-ring and replacing it with a gasket (butyl rubber from inner tubes has always worked reasonably well for me) is another option you may want to try.
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Unread postAuthor: warhead052 » Wed Oct 12, 2011 7:51 pm

DYI wrote:Does the burst disk look deformed in any way after you tighten it in the union? The surface should be as smooth as it was before you tightened the union. Ripples and other deformities act as stress concentrators because of their sharp edges, and may be indicative of tearing in the sealing area of the disk which you can't see.

From my experience with burst disks, something like eight layers of standard thickness (~0.01mm) aluminum foil should fail in the range you're looking for, although this may vary by brand of foil. Also, I didn't really do any testing at below 160psi for 2" unions.

If tearing seems to be a problem, adding a thin gasket on the muzzle side of the union may solve it. Removing the o-ring and replacing it with a gasket (butyl rubber from inner tubes has always worked reasonably well for me) is another option you may want to try.


In answer to the first question, thats a negative. Its about as smooth as a section of pvc, and no deformations on it at all. I will try the gasket soon, just need to take a trip to lowes or home depot.
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