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Triggered Burst Disk Trouble

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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Triggered Burst Disk Trouble

Unread postAuthor: Zippster » Fri Feb 01, 2008 2:21 pm

So I've finally decided upon a triggered burst disk design for my upcoming howitzer, but I can't seem to get my proof-of-concept rig working. I hooked up the 2 1-1/2" unions - which I'll also be using on the full-size - inline as the valve, like the conventional triggered burst disk design (vent the pilot which is at 1/2 chamber pressure, thus causing the increased pressure differential to break the disks). The problem isn't that they're leaking, its that the pilot can't hold *any* pressure without bursting both of the disks. I figure the cause of this is the amount of shock the disks take when the pilot fills as fast as it does. This has consistently blown both disks 3 trials in a row, so if it would still do it on the full-scale, then it might work as a trigger if ran a hose from the main chamber and had it dump into the pilot.

And both disks are the same burst-strength for those of you who would have to ask.

My questions are:

1. Why do you think it behaves like it does?
2. Would it fill w/o bursting if I filled it a LOT slower?
3. If I did implement this as my trigger on the full-scale, then what variables are of most concern, ie would flow to the pilot or pilot volume make it more consistent?
4. Would performance differ too much between venting the pilot or dumping into the pilot?
5. And for those of you who disagree, rather than flaming: is there a better way to do this?

A second (third, fourth, I don't care...) opinion of someone who has a bit more experience than I would be much appreciated before I empty my wallet on a full-scale. Thank you in advance :D
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Unread postAuthor: Pilgrimman » Fri Feb 01, 2008 6:18 pm

I have a small bore triggered burst disk gun, so I know how to use one. Your first question: Assuming the material is 1 layer, you may be right in thinking you are filling too fast. I fill my gun with a very low volume pump, like less than 5 in.^3. (Not by choice, believe me! :D ) It takes maybe 30 seconds of pumping to get to 50 psi. However, if you are using aluminum foil, your problem might be the same one I had: The cutter on the cardboard container for the roll leaves little pockmarks that severely weaken the foil. I have to go to great lengths to avoid this (Read: take the roll out 8)) . Another possibility is the size of the disks. My gun is .75 caliber. Yours is 1 1/2". That would probably result in MUCH lower pressures needed to burst the disks. For comparison, DYI has his STCBDC (exact order escapes me) (2" bore) that bursts 32 layers of aluminum foil at ~500 psi. (If I'm thinking of the right gun) My gun bursts 3 layers at 60 psi. The ratios of disks to pressure are 1:20 and 1:15 for mine and DYI's guns, respectively. I could see yours having fairly low burst pressures. I don't really get how you plan to trigger the disks, but I fill from the main chamber, then shut off the ball valve to the area between the disks so it is isolated. I finish filling, then I pilot the isolated air with a blowgun. Trust me, use the tried and true method of venting the pilot. It's easier, as it has never failed me once, and has worked since the first try. As I understand, you are trying to fill into the pilot chamber, then into the main chamber.

Three things I want to know: What are you using for disks, how many layers, and what are you using to fill? A schematic would be nice too. For reference, here is my triggered burst disk gun: http://www.spudfiles.com/forums/triggered-burst-disk-by-pilgrimman-t12809.html
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Unread postAuthor: Zippster » Fri Feb 01, 2008 6:51 pm

I've been testing the concept to see if it would work on a MUCH larger cannon if I were to make one, so current setup sucks simply because I didnt want to spend too much cash on a small-scale.

- I'm using aluminum foil for the disks
- I'm still experimenting with the exact number, but my arbitrary tests was at 4 layers to about 50 PSI
- My compressor, which is probably the problem :? But for the larger one, that is to-be-decided

I'm guessing then for the larger one I'd fill from the chamber and etc. like you have it, so as to avoid such a problem.

While were on the topic of burstdisks, would you have any suggestions for further sealing the unions (eg. which side needs more gasket, etc.)? I already have a 1/4" gasket and multiple bike innertube rings, but it still leaks a tiny ammount. I don't doubt that I could pump through the leak, but it would be extremely annoying to deal with the pilot leaking all the time.
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Unread postAuthor: Pilgrimman » Fri Feb 01, 2008 7:10 pm

The unions leak? That's weird. The ones I have use O-Rings and work just fine. The end with the female threaded outer ring thing should attatch to the chamber, and the male threaded bit should be attatched to the barrel.

One unlikely possibility might be that you are not making the disk as flush as possible. If one layer is wrinkled too much, the one before it could bear the brunt of the pressure, and prematurely burst, and leave you with fewer layers. I had this happen once (You wont notice it when it happens, as the pressure drop is virtually nil, but the foil is still weakened by one layer. (That is difficult to describe) Make sure the foil is taut when you put the disks in. Another question: do the disks actually tear, or do they simply slip out of the union? If the union leaks air, it sure will have a harder time holding a disk at pressure. Some pictures would help, if you can.
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Unread postAuthor: Zippster » Fri Feb 01, 2008 7:30 pm

The leak is REALLY small - about 1PSI/7seconds is my guess. That might sound horrible, but the chamber I'm using to test is only about 40 cubic inches, and its even slower if I really cinch it down. As far as I can tell, the disks aren't wrinkled and none of them are breaking prematurely, but it might be a wrinkled layer of bike innertube. Would adding even more layers of innertube make it seal better? Or should I just live with it?
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Unread postAuthor: DYI » Fri Feb 01, 2008 8:21 pm

I use ONE layer of homemade butyl rubber gasket on the back union of the SCTBDC, and it holds 500 psi (more than 3 times what the union is rated for :roll: ). I used to pump it to 160psi with a bike pump, and it sealed nearly perfectly (probably lost about 0.5psi/min). This was with 16 layer foil disks, and 1 layer of butyl rubber gasket in a 1-1/2" union.

So you really should be using less gasket material than you are now. With a well made gasket and a finely cut disk, and all aligned properly, it should seal perfectly, for any of our purposes.

The only thing I can think of is that maybe the union isn't tight enough. I know it's like asking if you plugged in your modem when you say your internet doesn't work, but you need to tighten the unions with 18" or longer pipe wrenches until you can't move them anymore. I'm not trying to insult your intelligence by saying that, just making sure.
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Unread postAuthor: Zippster » Fri Feb 01, 2008 9:36 pm

I've been tightening them as hard as humanly possible, but thast w/o leverage. I guess I'll rig up some easy way to leverage it as I've been getting by on what little tools my dad has (I had to borrow a channel-lock from our neighbors...) Beyond that, there's no 'Eureka!' point or anything else that would help it (like orientation)? If not, just say so.
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Unread postAuthor: DYI » Fri Feb 01, 2008 9:47 pm

As long as you're sure that nothing is getting caught in the threads, and that the disks and gaskets don't have any defect, it probably just needs to be tightened. Putting the female tapered part of the union on the higher pressure side helps too.

The main problem that I see is that the union generally needs to be quite a bit more than hand tight to get a proper seal. The situation with the SCTBDCv1.5 isn't that bad, because I just jam the stationary wrench between bars of the stand.

If my experience has taught me anything, it's that there are very few "eureka" moments with TBD guns. Sometimes it feels like they just fix themselves. The only eureka moment I've ever had was when I realised that I could get 32 layer disks to seal, allowing me to use the SCTBDC at its full pressure potential. This was after months of failure at getting sheat alu. disks to seal...
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Unread postAuthor: Zippster » Fri Feb 01, 2008 10:48 pm

For lack of desire to start a new thread, I've got a question about going full-scale

I figured for the real thing, I'd get a 10' stick of 3" PVC for the chamber, and a 10' stick of 2" PVC for the barrel. I know this topic has been beat to death waaaaaaay to much, but the C:B ratio works out to be 2.25 to 1. Do you think thats adequate for a 10' barrel? I'd hate to get build this thing to find that the chamber is insufficient. If you've got anything to say there, I'td be appreciated.
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Unread postAuthor: DYI » Fri Feb 01, 2008 10:53 pm

2.25:1 is massive overkill on chamber volume. The SCTBDC when it was used at 160 psi ran a 0.2:1 ratio. I'm not saying this is a bad thing. Since you have access to PVC pipe, pulling massive overkill on the chamber is fine, whereas I, having only steel, have to conserve as much on chamber volume as possible.

Essentially, if your valve, pressure, and barrel size are limited, increasing chamber volume is the only was to increase performance. Just make sure your neighbours aren't picky about noise, because this is going to be way louder than my gun.
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Unread postAuthor: Zippster » Sat Feb 02, 2008 1:25 am

Oh boy do I have plans for this thing! It'll have a HUGE range, and hey, the highschool is across the highway...

How do you pull of a chamber a fifth the size of your barrel? By the time the air makes it out the barrel wouldn't it be at one sixth the starting pressure?

Where do you live that you don't have PVC? Australia?
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Unread postAuthor: DYI » Sat Feb 02, 2008 7:58 pm

How often do you use GGDT? It quotes a muzzle velocity of 365fps with a 1 pound projectile with your current design at 100 psi. At 150 psi, the number rises to 436 fps with the same projectile. Fairly powerful, although rather weak by my standards for a gun that big.

You are correct in assuming that the system pressure when the projectile left the barrel on the old SCTBDC was 1/6 of the starting chamber pressure.

You must remember, however, that as long as the pressure behind the projectile is higher than the pressure in front of it, it will continue to accelerate. In the case of the old SCTBDC (before HP upgrades), the pressure behind the projectile would be slightly less than 29 psia, while the pressure in front of it would only be 14.7 psia. Barrel friction was low enough to be negligible in this case, because the projectiles were loose fitting and wadded, and the whole thing was sprayed down with PAM before firing.

I live in Ontario, Canada, far enough north that temperatures in the winter drop below what could be considered safe for PVC pressure piping. In a recent revelation, I discovered that my local hardware store can actually order in steel pipe and fittings up to 6" nominal size. This makes more volume available, but due to the cost of high ID steel pipe/fittings, efficiency is still imperative.
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