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pneumatic burst disk

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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pneumatic burst disk

Unread postAuthor: kydavies » Fri Jun 25, 2010 12:55 pm

i am using a 1 1/4" sch 80 union for my burst disk but it leaks like a sive. ive noticed that the burst disks (pieces of pop can and cardboard) wrinkle as a result of the center bulging. then i switched to duct tape which does not wrinkle but it still leaks through the union. other than wrapping the treads in teflon tape i have no other solution.
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Unread postAuthor: MRR » Fri Jun 25, 2010 1:29 pm

Try o-ring -> burst disk -> washer.

The 1 1/4 union probably damages the burst disk when you screw it on.
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Unread postAuthor: SpudFarm » Fri Jun 25, 2010 1:33 pm

Wich side of the union is connected to the chamber?
The O-ring should be on the chamber side but you probably understand that.

Also. You can look at Captain's hybrid to see what he did to his union.
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Unread postAuthor: kydavies » Fri Jun 25, 2010 1:50 pm

MRR il give that a try

also Spudfarm the non threaded side is connected to the chamber
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mikemurph wrote:yea, the ammo has been changed to a more aero dynamic projectile. it consists of the back half of a dart, and a hot glue ball
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Unread postAuthor: MrCrowley » Fri Jun 25, 2010 6:40 pm

I grinded down one side of the union to help with it sealing.
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I use a double gasket to help add thickness so the union can be tightened more without running out of threads. The gaskets go on the flat side of the union and the burst disk on the threaded side.
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Unread postAuthor: DYI » Fri Jun 25, 2010 7:35 pm

With those cheap unions bought at hardware stores, sealing issues do pop up occasionally. My best solution to the problem was to face the tapered part sticking out so that it's simply a flat surface, and then to use butyl rubber gaskets on both sides of the disk to seal and prevent tearing. Got me a nearly leak-proof seal at 150psi. Chuck a pipe nipple in the lathe then thread the union onto it to get a good centered grip.

If you don't have a lathe, I did have some success doing it *very* carefully on a bench grinder (that's what I used for SCTBDC, and it didn't leak enough to be worrisome at 500 psi).

If you're not using a triggered burst disk (and thus not trying to get a seal on both sides), facing the tapered part isn't really necessary, but it's still preferable to use gaskets on both sides of the disk.
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Unread postAuthor: kydavies » Fri Jun 25, 2010 9:36 pm

i am VERY limited to tools and supplies (drill, hack saw, 1 file, chop saw for wood/pvc and some of my dads tools) but i should be able to get a hold of some gaskets after i move in about 1 week or 2
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Unread postAuthor: DYI » Fri Jun 25, 2010 9:49 pm

I've never bought gaskets for unions. Cutting them from old car tire inner tubes works just fine for any union I've used.
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Unread postAuthor: c11man » Sat Jun 26, 2010 11:37 am

kydavies, is this a pvc or a metal burstdisk, becuse im pretty sure you have a pvc one and everybody is talking about metal ones
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Unread postAuthor: Lockednloaded » Sat Jun 26, 2010 1:53 pm

c11man wrote:kydavies, is this a pvc or a metal burstdisk, becuse im pretty sure you have a pvc one and everybody is talking about metal ones


kydavies wrote:i am using a 1 1/4" sch 80 union


I know he doesn't specifically say it is sch 80 PVC, any sch 80 metal would probably be overkill. Also I remember his old build thread for this and am pretty sure it is PVC. Your burst disk material may be the problem here, especially the cardboard. you should use aluminum foil of aluminum tape for your burst disk, neither of them crumple, and neither of them leak in my personal experience.
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Unread postAuthor: kydavies » Sat Jun 26, 2010 4:53 pm

sorry about the mix uo im using a sch 80 PVC union and ive tried aluminum foil but it bursts at a very low pressure and i dont have aluminium foil tape. also the sides of pop cans crumple. even when i use aluminium foil the union still leaks
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Unread postAuthor: DYI » Sat Jun 26, 2010 9:44 pm

...ive [sic] tried aluminum foil but it bursts at a very low pressure...


You mean to say that any thickness of aluminum foil which will fit in your union (that would be anything under about 1000 layers, by my estimation) will burst at a "very low pressure"? Considering your use of a PVC construction, I'm rather worried that you regard approximately 24000 psi as being "very low pressure". I suggest you switch to something a bit more substantial. 6AL-4V titanium perhaps, or maraging steel.

I'd also like to know where you got that PVC union :roll:
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Last edited by DYI on Sat Jun 26, 2010 10:44 pm, edited 2 times in total.
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Unread postAuthor: kydavies » Sat Jun 26, 2010 10:33 pm

k, i think i figured it out there is to much surface area on my burst disk so it is bursting at low pressure. So there is a larger total force acting on the aluminium foil. i used 3 layers and it burst almost immediatly after the first pump (my chamber is 5 inches of 32mm PVC)

Also the end this, i have i have a 1 1/4 sch 80 PVC union that i bought from home hardware for $5.49 (would you like the recipt and the store number as well? cause its not going to happen). do you really expect me to cut 1000 aluminium foit disks for 1 shot!?. and where the hell are you getting 24000 PSI from? if i typed it it was a typing error... :x
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Unread postAuthor: Lockednloaded » Sat Jun 26, 2010 10:38 pm

hes saying that aluminum foil can hold pressure in large quantity, so you should not give up on it.. it takes me 6 layers of foil to hold 90 psi on my 1" union. how much pressure do you need your disks to hold?
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Unread postAuthor: jhalek90 » Sun Jun 27, 2010 7:34 am

Well, i dont understand why you would even use cardboard...

I would guess that the cardboard is causing the leak. Try it without that first and see what happens.
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