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2" Union Burst Disks

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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2" Union Burst Disks

Unread postAuthor: whoa044 » Fri Sep 24, 2010 11:51 pm

I was wondering, since I never got the opportunity to test out a soda can cutout to see its bursting pressure, has anybody ever done so with a 2" union?

The new hybrid design will be a large 2" hybrid. I've heard that problems may arise when so many burst disks are required. It's been told you must grind the edges off the inside of a union to fit a gasket, and that it will make fitting so many soda can disks airtight.

The thickness of 10 pieces is 0.97 millimeters. Will such a gap in the union be overcome by simply torquing the union a little more? The predicted mixes I may achieve (depending on burst disks), will be anywhere from 5x to 20x.
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Unread postAuthor: SpudBlaster15 » Sat Sep 25, 2010 12:13 am

For calculating the failure pressure of the disk, see here.

Hardware store unions have a tapered face on both the male and female components, which makes them very poor for holding burst disks. You will need to modify the union by grinding/machining the taper down to a flat surface, and using a rubber washer to form a seal between the union face and the burst disk.
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Unread postAuthor: whoa044 » Sat Sep 25, 2010 12:25 am

Do all unions have tapered faces? What's required is to make the inside surface level, and to have a gasket on both sides of the disk? Would it be safe to glue the gasket to the union? Thanks so far btw
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Unread postAuthor: SpudBlaster15 » Sat Sep 25, 2010 10:44 am

Every hardware store union that I've examined has had a tapered sealing surface. The only off the shelf units that don't are the 3kpsi rated forged steel unions from McMaster.

What's required is to make the inside surface level, and to have a gasket on both sides of the disk?


Yes, but you only need a gasket on the chamber side of the disk.

Would it be safe to glue the gasket to the union?


I did just that with my UPHL-38 several years ago. The adhesive used was PC Super Epoxy, and it worked well.

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Unread postAuthor: whoa044 » Sat Sep 25, 2010 11:23 am

I ordered a 2" 2kpsi union from Murray Equipment Inc just a two days ago.

Would it be possible to just grind off the barrel side of the union, so the other tapered side holds the gasket? Or if both sides are grinded off, the pipe on the chamber side will hold the gasket inside the union.
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Unread postAuthor: Crna Legija » Sat Sep 25, 2010 6:46 pm

you could give them a call and ask if the union you ordered has tapered faces
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Unread postAuthor: whoa044 » Sat Sep 25, 2010 9:46 pm

Or I could just wait a few days. If it isn't tapered, then I guess I'm all good, only need a rubber washer.

I'm thinking of buying a few 1/4 thick neoprene squares, and then cut disks out using an X-Acto knife. That just gave me a great idea:
Instead of using rubber to make seals in the union. Couldn't we use thick rubber to be the burst disk itself?
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Unread postAuthor: Crna Legija » Sun Sep 26, 2010 8:28 am

rubber wouldn't burst very well, it would make a small hole in the middle and the heat from the mix will melt it away a bit more but it wont be very fast like say layered aluminium foil and rubber disks would be expensive
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Unread postAuthor: whoa044 » Sun Sep 26, 2010 9:24 am

Yeah, rubber splits, rather than just falling apart like thin metal.
Does anybody know where I can get 2" circular punches?
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Unread postAuthor: Technician1002 » Sun Sep 26, 2010 10:58 am

whoa044 wrote:Yeah, rubber splits, rather than just falling apart like thin metal.
Does anybody know where I can get 2" circular punches?


Make one. Use a piece of iron pipe and cut a cutting edge on it. I launched a piece of conduit with a sharpened edge at a washing machine and it performed well. A washing machine is a little thicker than a pop can.
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Unread postAuthor: whoa044 » Sun Sep 26, 2010 12:55 pm

Good idea. Shouldn't be any trouble, except making sure that that is matches the diameter of the union. I'm assuming you also use 2" unions?
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Unread postAuthor: ramses » Sun Sep 26, 2010 1:56 pm

He uses his QDV, which is a type of piston valve.

Measure the diameter inside of the area in the union where the BD will go, and find something round and metal at the hardware store with the same diameter.
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Unread postAuthor: whoa044 » Sun Sep 26, 2010 3:16 pm

Yeah I figured that's what I would do. It seems tapered steel pipe will be easier (and a little more consistent) to make a sharpened edge
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Unread postAuthor: whoa044 » Tue Sep 28, 2010 7:09 pm

So I finally got my forged steel fittings in the mail from MEI. The box weighed, by my guess, around 30 pounds. This included all 2" fittings: 2, 6" nipples. a tee, union, and three bushings.

I swear, that union was huge:
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I can see how the inside is tapered, if anybody is interested, I have a short video showing how large the thing is, and what the insides look like.
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Unread postAuthor: Gun Freak » Tue Sep 28, 2010 7:17 pm

Holy hell! That thing is a monster. Good luck on the launcher, can't wait to see it!
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