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A Close Encounter: The Explosion of a Valve

A place to ask general spud cannon related questions.
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Unread postAuthor: Velocity » Sun Apr 15, 2007 9:27 am

rmich732 wrote:The 2"x3/4" galvanized bushing is nice, because it distributes some of the weight of the gun to the back (making it easier to hold), as well as prevents an explosion of the back end. However, now the female adapter is the most likely to fail... oh well, better than the plug!


I wrote this on spudtech on November 7th, 2006... Seeing into the future perhaps?

So I basically am sure that the combination of tightening the bushing too much, using a metal bushing, having a less than sufficient bumper, and the use of a very heavy piston caused this to happen.

In my new version of this, I will use a SCH 40 plug sealed with an oring and secured by a few screws. The pilot valve will only be 1/4", because if I used a 3/4" pilot valve, the nipple which would screw into the back of the SCH 40 plug would extend too far and not give the piston enough room.

I am not sure what type of piston to use. Sure, 1.25" fittings piston would be lighter, but I was really happy with this one (until yesterday...). As of now, I plan to use the same piston, cut down a bunch (because the valve will be overall shorter), and hollowed out. This will be a tiny bit heavier than the 1.25" fittings piston, but will be much stronger too.

I could post pictures of the bumper, but it wouldn't matter. The bumper was in the metal bushing, and did not extend out of it. The piston only came in contact with the edges of the metal bushing, and therefore the whole assembly was useless (it did cut down a bit on pilot space though...

jjrrw: I thought all fittings had a taper (they are all called NPT fittings, which stands for National Pipe Taper)...
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Unread postAuthor: noname » Sun Apr 15, 2007 10:33 am

You're lucky in two ways:
1. You weren't hurt.
2. The whole thing didn't blow when it cracked, probably because the air was already leaving the barrel. I am a witness to exploding PVC (though everyone seems to doubt it) and it blasts shards everywhere. I was finding fragments in my basement 2 weeks after the explosion. You would've gotten at least 5 itsy-bitsy chunks in your back, I guarantee it.
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Unread postAuthor: mark.f » Sun Apr 15, 2007 10:39 am

Close call, rmich, close call. It makes me reconsider the piston valve I built for our robot! I used a metal bushing on the back as well, but I didn't tighten it very much. Maybe I'll consider replacing it. I also used a pretty thick bumper, (half a tennis ball).

<img src="http://www.markfh11q.net/images/roboticsvalve/valve.jpg">

Glad to see you're not hurt, and yeah, noname, PVC goes everywhere when it pops. Didn't fail it with air, (I filled it with some "stuff" and then stuck a "thing" in it and then threw it in a hole), but the fragmentation was evident. Steel fails much better though, without as much shrapnel. :wink:
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Unread postAuthor: Velocity » Sun Apr 15, 2007 12:16 pm

I believe the fact that it cracked and did not explode indicated that it was not caused by too much pressure, but instead was caused by the stress from the bushing. This is good to know, because I would be really pissed if it was a solvent welding problem (I would probably have to cut apart all of my older cannons and start again, cause I wouldn't want to put my safety on the line :cry: ).

Later today, I am going to check my garage for any fragments or other odd things (I doubt that I will find any), just because I was so freaked out yesterday that I just posted it and forgot about the garage.

EDIT: Forgot to say, nice piston valve Mark :P
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Unread postAuthor: PVC Arsenal 17 » Sun Apr 15, 2007 5:01 pm

There's no reason not to use a metal bushing. The chance of failure due to over-tightening is there no matter what material the bushing is.

I try to thread in fittings as little as possible. It helps to use plenty Teflon tape and a thick coat of Teflon paste. That way you can eliminate leaks without over stressing the fitting.


Rmich, if you can find my Omega Project thread, (buried somewhere on this forum) take a look at the pictures I took of fittings that had "knit lines" Look at the remnants of your valve and see if you can find any knit lines. (they're usually easy to spot on fittings 1 1/2" and larger) They could very well be the cause of your valve's failure. Although I was assured by several PVC manufacturers that knit lines do not decrease a fitting's ability to withstand pressure.

If the cracks are along a knit line then I might want to reconsider even building my Omega cannon.


*edit: found the pics.

Image

Image
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Unread postAuthor: joe blogs » Sun Apr 15, 2007 7:40 pm

[quote="frankrede"]You also used the pipe for a purpose of which it was designed.
That is your fault Joe Blogs, you should have checked the pipe yourself.
[quote]

I was new to spudgunning and i asked the employee a specific question "can this pipe take 120 psi" his reply was "it can take pressures way over that".
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Unread postAuthor: Bubba05 » Fri Apr 20, 2007 11:16 am

These things we build aint ment to last forever! It's only a mater of time befor they pop! Good to hear your ok thoe!

Bubba
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Unread postAuthor: Velocity » Sat Apr 21, 2007 10:03 pm

I personally believe that piston valves do not need to crack. If they were only temporary things which were guareenteed to break, I would not build them. It was just a mistake on my part, and therefore I know on my next one (which is being built within the next few weeks) some things to improve to make it safer.

PVC Arsenal: There were no knit lines
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Unread postAuthor: Bubba05 » Sun Apr 22, 2007 2:24 am

Good spirit mate thats the way to think!

Bubba
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Unread postAuthor: XxtriviumxX » Sun Apr 22, 2007 2:39 am

that was a strange yet desturbing story, the good thing is that your ok
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