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An Autopsy...

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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Unread postAuthor: ALIHISGREAT » Tue Dec 23, 2008 3:58 am

That could have turned nasty and ruined his Birthday :shock:

and i also agree that for a proper autopsy, we need to see the caanon (or whats left of it)
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Unread postAuthor: roboman » Tue Dec 23, 2008 11:53 am

Xxplosive42o wrote:BTW, Where in socal do you live? I'm over in Burbank JFYI.


I'm in Long Beach.


I get the feeling it fractured around the pressure gauge, although there is a crack in the threaded coupler connecting the barrel to the valve.
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Unread postAuthor: Ragnarok » Tue Dec 23, 2008 12:05 pm

MrCrowley wrote:I'm guessing, without seeing pictures, that you have some small diameter fittings on the chamber maybe connecting the chamber to the valve, and that the recoil of the water in the barrel put too much strain on these fittings.

Sounds right to me.
I thought most spudding veterans knew that firing water was a bad idea because the large weight of it could cause damaging recoil. Additionally, if there's any bends between the valve seat and barrel (guaranteed in a sprinkler valve cannon), this can cause a "water hammer" effect.

I'm sure I remember reading these things on Spudtech's forums when I was getting into the hobby. Obviously, the knowledge has been forgotten with the loss of those forums.

Not to say I didn't do it a few times myself when I started out, but I had a swarm of problems with cannon damage and stopped.
I've tried it again a couple times since with cannons which didn't have the design problems that make it damaging (example: Behemoth) and avoided mishap there - but not all cannons have that luck.
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Does that thing kinda look like a big cat to you?
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Unread postAuthor: roboman » Tue Dec 23, 2008 12:13 pm

Yay! Photobucket let me upload my pics! :D

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Fractured barrel connector.

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Threaded hole for pressure gauge.

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About half of the air tank.

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Another section of the pressure gauge threads.

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The remains of the cannon.

Somehow, when the whole thing blew up, the pressure gauge shot out across the street, and I never was able to recover it. :?
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Unread postAuthor: FishBoy » Tue Dec 23, 2008 12:33 pm

The lack of barrel support may have something to do with the broken barrel connection.
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Unread postAuthor: roboman » Tue Dec 23, 2008 12:40 pm

True... It was standing vertically when it accidentally went off.
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Unread postAuthor: JDP12 » Tue Dec 23, 2008 12:46 pm

also... i was right! the pressure gauge wasn't through a double layer and it split almost right down the gauge hole it looks like to me....

looks like a combination of that and small fittings "choking" the flow did your cannon in
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Unread postAuthor: roboman » Tue Dec 23, 2008 12:51 pm

Do you guys think anti-siphon valves are worse than the inline ones, and the use of an inline could possibly have prevented this?
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Unread postAuthor: VH_man » Tue Dec 23, 2008 1:11 pm

Id say it was the fact you didn't put your gauge through double layer fittings.......

the barrel because there was no barrel support.
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Unread postAuthor: roboman » Tue Dec 23, 2008 1:34 pm

I'll fix that on my next one.
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Unread postAuthor: FishBoy » Tue Dec 23, 2008 1:36 pm

Well Doctors, I think we did a fine job, meet me in combustions at 2.00, there's a spray 'n pray with a burnt cleanout.

:D

Anyway, I think that if you look at the last few posts. it's clear what happened to your gun. next time, tap guages through 2 layers of pvc and support your barrel.
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Unread postAuthor: jimmy101 » Tue Dec 23, 2008 5:29 pm

So the gun triggered itself and failed? That would be the first thing you should figure out how to fix, a (potentially) deadly weapon shouldn't be going off by itself.

Folks have to be careful about blaiming the pressure gauge's hole for the failure. Often what looks like the failure point is indeed the exact opposite; it marks the point where the failure stopped. Pressure induced cracks will often propagate towards any holes (plugged or not) in a pressure chamber. The crack may not have started at the hole, it may have ended at the hole.

You also need some barrel supports.

So lets summarize;
1. Your gun fired on it's own. That is very bad.
2. There were no barrel supports so it ripped itself apart. Fortunately, your family jewels are still within your reach.
3. The failure may or may not have had anything to do with the hole in the single layer of PVC. Clearly there were failures in the pressure envelope unrelated to the pressure gauge's hole, for example at the coupling between the bend and the chamber.
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Unread postAuthor: psycix » Tue Dec 23, 2008 7:02 pm

Am I the only one who wonders how it could have cracked at TWO different places at the same time?
And why does the chamber blow when its fired? Chambers usually only explode while filling it with a too high pressure or dropping the gun.
And if you would have dropped it, then the pressure would be gone before the gun hits the ground right?

Now I wonder, which crack blew first and how did it cause a chain reaction to crack somewhere else too?
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Unread postAuthor: roboman » Wed Dec 24, 2008 12:21 pm

It didn't exactly fire on it's own. I think my foot accidentally triggered the manual bleed switch, since it was in the "open" position when I examined it.

I only filled the tank to ~95 psi. The valve is rated to 125, and the pipe is rated to 280. All of my fittings are pressure rated, and 95 psi shouldn't have blown it up (I never dropped it).
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Unread postAuthor: psycix » Thu Dec 25, 2008 7:18 am

Are you sure there was really 95 psi in there?
Maybe the gauge was broken?
Have you ever put the gauge over its limits?
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