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New burst disk idea ?

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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Unread postAuthor: trigun » Sat Feb 14, 2009 12:48 am

Well i went and bought the pan liner or pvc sheet whatever you prefer to call it. It works very well at a single layer i used it at 5x 2 times and 1 7x before it got to cold for the propane to get enough pressure in it. The only problem i had was that its " burst " properties were not so good. It simply had a small rip in it. >>> Image

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Unread postAuthor: starman » Sat Feb 14, 2009 1:48 am

OK that's interesting. My initial thoughts are that it doesn't blow out nearly enough to get the flow you will want. While that crack will let out the pressure, it will also act like a flow restricter.

Ideally, you want the disk to completely disintegrate. If you have seen aluminum foil or masking tape burst disk shots, you'll see that the whole 2" opening has been mostly blown clear...like this...

Image

This was blown out by compressed air. My combustion ignitions blows the opening out utterly and entirely.
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Last edited by starman on Sat Feb 14, 2009 2:10 am, edited 1 time in total.
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Unread postAuthor: Lentamentalisk » Sat Feb 14, 2009 1:58 am

you may want to score lines into it, across the diameter of it, to make it split open. Given stress concentration points, a crack or tare will propagate much more easily.

the other possiblity is that you score a circle slightly smaller than the I.D. of the union, and see if that will rip out entirely.
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Unread postAuthor: trigun » Sat Feb 14, 2009 2:42 am

I will probally just stay with my aluminum can burst disks they blow out completely.
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Unread postAuthor: spudamine » Sat Feb 14, 2009 4:29 am

I got the same effect when I tried using rubber sheet for burst disks, wasn't impressed. And it failed at a pathetic pressure, something like 20psi.
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Unread postAuthor: starman » Sat Feb 14, 2009 4:37 am

It is amazing how difficult it is to find decent burst disk material. Many people have gone down this road and always seem to drift back toward aluminum. The masking tape is workable up to about 7-8 layers, but only practical on a camlock setup. Aluminum still seems to be the winner for union holders.
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Unread postAuthor: inonickname » Sat Feb 14, 2009 6:59 am

Too true starman. I'm going to be testing things like anti-stick baking paper, different tapes, glad wrap, cellophane and rose petals for ultra minis (I know these will work, you can use them like a burst disk just with your hands and the sound is impressive)
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Unread postAuthor: FishBoy » Sat Feb 14, 2009 8:37 am

starman wrote:Aluminum still seems to be the winner for union holders.


I have realized that as well; you have any tips for using aluminum burst disks? I made a few a while back that were either 6 layer or 8 layer to use in my hybrid at 6x-8x (3/4" union). The problem was none of them sealed at all.
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Unread postAuthor: trigun » Sat Feb 14, 2009 10:52 am

In order for the union to seal i recomend adding a o-ring to the union. A 1.5" union can use a 2" PVC union O-ring just a fun fact. I think next thing on my list to try is ..... well back to aluminum
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Unread postAuthor: starman » Sat Feb 14, 2009 12:36 pm

FishBoy wrote:
starman wrote:Aluminum still seems to be the winner for union holders.


I have realized that as well; you have any tips for using aluminum burst disks? I made a few a while back that were either 6 layer or 8 layer to use in my hybrid at 6x-8x (3/4" union). The problem was none of them sealed at all.


Maybe try some vaseline gunked in between the first layer and the union seal. Trigun's O-ring is probably the ultimate answer.

I've been meaning to try those cardboard beer coasters from restaurants...it's hard to find them though, except the ones you get there. I drove all over town one day looking for them, came up dry. Undoubtedly, a restuarant supply company would have access to them, just no consumer level places I could find.
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Unread postAuthor: psycix » Sat Feb 14, 2009 1:00 pm

I got the same effect when I tried using rubber sheet for burst disks

It seems like that burst disk material has to be brittle.
Soft materials just make a small tear and bend open a little. Brittle and hard stuff cracks to pieces and is completely destroyed.
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Unread postAuthor: FishBoy » Mon Feb 16, 2009 9:33 am

When I use milk bottle plastic, I open the union and there is always a perfectly circular hole, fully open :D . I like trigun's idea of soda can metal but what pressure does it burst at?
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Unread postAuthor: psycix » Mon Feb 16, 2009 12:01 pm

No material bursts at a set pressure.
You need to specify the diameter too.
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Unread postAuthor: FishBoy » Mon Feb 16, 2009 6:37 pm

Well I believe coke cans are made fairly consistently; lets say we cut a burst disk out of the side of one and put it in a 3/4" union. What would be a reasonable estimate of burst pressure?
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Unread postAuthor: Combustion Monkey » Mon Feb 16, 2009 6:58 pm

starman wrote:It is amazing how difficult it is to find decent burst disk material. Many people have gone down this road and always seem to drift back toward aluminum. The masking tape is workable up to about 7-8 layers, but only practical on a camlock setup. Aluminum still seems to be the winner for union holders.


Just as a side note, I've been using starmans tape setup for golf balls. It works great and I don't use it on a camlock, but apply it directly to my coaxial breach face. I use three layers of tape for my 1x combustion and the results are impressive.
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