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How fast does your burst disc leak?

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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How fast does your burst disc leak?

Unread postAuthor: CS » Tue Jan 08, 2013 6:47 am

At 30 psi I lose 3-5 psi in one minute. I'm wondering if you all are having better luck then me? Tommorow I'm going to try and use a gasket insted of an oring, They may be more soft and fix the potential problem of subtle mis alignment.
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Unread postAuthor: Zeus » Tue Jan 08, 2013 6:53 am

It depends on your disc material, and which way you hook up the union too, they're unidirectional.

Mine seals perfectly with a cable grommet and aluminium tape.
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Unread postAuthor: cowscankill » Tue Jan 08, 2013 11:29 am

They leak? :| :?:

My burst disks never leak, until they burst. Maybe it's the way I have mine set up? Using a PVC union and a rubber o-ring with aluminum disks:
Image

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Unread postAuthor: PaperNinja » Tue Jan 08, 2013 11:44 am

I use a rubber gasket and aluminum foil in a 1/4 steel union, with the "male" union piece ground down. It holds perfectly to 150 psi, and once I get my spark plug I'll be able to test it up to 600 (I don't have a great way to seal where the spark plug goes, so the shock pump is too slow for the leak there)
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Unread postAuthor: MrCrowley » Tue Jan 08, 2013 2:39 pm

Mine never leaks either, if it leaks I'll re-assemble the burst disk for another shot as the leak will through the mix off.

I've used a variety of combinations over the years. Gaskets are great but tend to get damaged after awhile and o-rings are pretty good too but are prone to being shot out the barrel. My current setup has been the most successful yet: I grounded down the union half with the cone so that it is flat and threaded that part to the barrel; I stack o-rings inside the union half attached to the chamber (make sure the o-rings are the same diameter as inside the union fitting) until the top o-ring is half inside the union and half poking out; put the disks on top of this o-ring and thread on the union half attached to the barrel.

Because the o-rings inside one union half are much larger in diameter than the hole through the middle of the union attached to the barrel, the o-rings have no place to go and aren't shot down the barrel.
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Unread postAuthor: jrrdw » Tue Jan 08, 2013 5:23 pm

What are you using for your fitting? Screw together 2 piece union like cowscankill pictured or cam lock like Sgort87 commonly uses?
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Unread postAuthor: D_Hall » Wed Jan 09, 2013 1:59 am

Mine will lose about 1/2 psi every 30 minutes.
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Unread postAuthor: Zeus » Wed Jan 09, 2013 2:04 am

For a chamber that size, that's a hell of a leak
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Unread postAuthor: jackssmirkingrevenge » Wed Jan 09, 2013 2:08 am

D_Hall wrote:Mine will lose about 1/2 psi every 30 minutes.


When you say "mine" do you mean VERA?
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Unread postAuthor: CS » Wed Jan 09, 2013 4:28 am

Very similar to this configuration, just PVC.

Image

One problem is that the male peice rotates when in fact all you need is compression. The rotation will score the burst disk as it is tightened and means it'll rupture at 20 psi instead of 60 psi.

So I got to make the pipe float inside of the male adapter so the burst disc won't get chewed up. Just how a union works...

A union is a fitting that allows a threaded joint without requiring the joined pipes to be rotated.
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Unread postAuthor: Zeus » Wed Jan 09, 2013 4:51 am

Cut the tapered part off another male fitting, so you lose no length in the cut, face them both off, stone both faces to a mirror finish, then lubricate one face, and reassemble it. They slide against each other, and you have no problems at all.
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Unread postAuthor: jrrdw » Wed Jan 09, 2013 8:21 am

O-ring>burst disk>flat washer>male piece. That way nothing is spinning against the burst disk and you have a nice flat sealing face...
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Unread postAuthor: D_Hall » Wed Jan 09, 2013 11:30 pm

jackssmirkingrevenge wrote:
D_Hall wrote:Mine will lose about 1/2 psi every 30 minutes.


When you say "mine" do you mean VERA?


Yes.

It's not really an issue though... She doesn't get fueled until she's ready to fire and she gets fired within seconds of being fueled (a process itself that only takes a couple minutes).

And honestly? I'm not even 100% sure she's leaking. Experience in other endeavors tells me that when you pump up a pressure vessel, the pressure will drop a small amount simply due to cooling of the oh-so-recently compressed gas.

But I've not done the proper experiments so I'm willing to call it a leak, even if it isn't (I don't really know).
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Unread postAuthor: CS » Thu Jan 10, 2013 2:39 am

Well I cut a rubber sheet out like this:
It is only leaking 1 psi every 5 minutes at 30 psi. Yeah!

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(russian zorbing)
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