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the truth about dwv, and pw pvc pipe

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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the truth about dwv, and pw pvc pipe

Unread postAuthor: covey12 » Sat Apr 25, 2009 9:31 pm

so i had accidently picked up a dwv pipe fitting, and got a handful of complaints about it, i have used one of these before on an old cannon before i even knew what it was, and had no problems at all, so how unsafe is it really? my chamber was pw but i dont see how 40 psi can blow up a fitting even if it is dwv
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Unread postAuthor: jmeyer1022 » Sat Apr 25, 2009 9:38 pm

40 PSI won't unless its a bad seal. If it's DWV in a piston cannon though the short socket depths and the stress from the recoil could easily break it. The thickness isn't the problem its the socket depths. On a pressure rated piece the socket depths are twice the depth of non pressure rated fittings.
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Re: the truth about dwv, and pw pvc pipe

Unread postAuthor: Technician1002 » Sat Apr 25, 2009 9:39 pm

covey12 wrote:so i had accidently picked up a dwv pipe fitting, and got a handful of complaints about it, i have used one of these before on an old cannon before i even knew what it was, and had no problems at all, so how unsafe is it really? my chamber was pw but i dont see how 40 psi can blow up a fitting even if it is dwv


The thin PVC seems to be brittle and tends to grenade a lot. Schedule 40 ABS Cellular core DWV seems to be more robust. I tested it a little. The results are in another thread.
http://www.spudfiles.com/forums/abs-cellular-core-test-cannon-t17968.html

If it is PVC, I would highly recommend exchanging it if possible. The failure mode for PVC is lots of smaller shards.
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Unread postAuthor: Moonbogg » Sat Apr 25, 2009 10:28 pm

Yeah man, the cellcore ABS is good to go for combustions at least. I have an advanced combustion and used it quite a bit and it seems solid. But without a barrel support I wouldn't give it more than a day's use before the barrel joint cracked.
I haven't used the PVC stuff but I know how thin and flimsy that stuff is. I wouldn't use that for any cannon. It might make a good plastinade though.
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Unread postAuthor: inonickname » Sat Apr 25, 2009 10:54 pm

If you're going to use a non pressure rated pipe then I'd use abs. Simply because it has so much better failure characteristics.

DWV will probably handle very low pressures fine if it's treated well. Could always be reinforced with chicken wire, fiberglass tape or any manner of things.
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Unread postAuthor: Cosmic Muffin » Sun Apr 26, 2009 2:25 am

how would you use chicken wire? :?
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Unread postAuthor: Ragnarok » Sun Apr 26, 2009 4:03 am

inonickname wrote:Could always be reinforced with chicken wire, fiberglass tape or any manner of things.

The chicken wire theory was debunked years ago - mostly, it just added extra bits to be thrown around if the pipe did go.
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Unread postAuthor: inonickname » Sun Apr 26, 2009 4:11 am

Ragnarok wrote:
inonickname wrote:Could always be reinforced with chicken wire, fiberglass tape or any manner of things.

The chicken wire theory was debunked years ago - mostly, it just added extra bits to be thrown around if the pipe did go.


Really? It would seem to me that if wrapped tightly enough it could limit expansion which is one of the things that leads to the pipe going. Of course depends on the gaps between the wire I suppose.
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Unread postAuthor: Technician1002 » Sun Apr 26, 2009 9:56 am

inonickname wrote:
Ragnarok wrote:
inonickname wrote:Could always be reinforced with chicken wire, fiberglass tape or any manner of things.

The chicken wire theory was debunked years ago - mostly, it just added extra bits to be thrown around if the pipe did go.


Really? It would seem to me that if wrapped tightly enough it could limit expansion which is one of the things that leads to the pipe going. Of course depends on the gaps between the wire I suppose.


Expansion isn't the failure mode. Cracking and the resulting fragmentation is the problem. Chicken wire does nothing to limit the initial stressing that leads to failure. By the time the wire contains the expansion, it has already failed.
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Unread postAuthor: psycix » Sun Apr 26, 2009 3:19 pm

Really? It would seem to me that if wrapped tightly enough it could limit expansion which is one of the things that leads to the pipe going. Of course depends on the gaps between the wire I suppose.

If the pipe is made of a very soft material like rubber, so that it balloons when stressed, then you could use the chicken wire to reinforce it.
This is how steel braided hose works.

But since the PVC barely stretches before failing, you will need to wrap the chicken wire perfectly, and pre-tension it well.
Even then, this does not guarantee a secure pressure chamber.
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Unread postAuthor: Biopyro » Sun Apr 26, 2009 3:25 pm

I'm not sure about PVC, but I know PET (soda bottles) fail by expansion.
Wrapped in duct or fibreglass tape they are much stronger, because they can't swell to produce a split.
They can still rupture at either end though, although my pump didn't go high enough to find out what that pressure was.

I assume that PVC isn't so brittle it doesn't swell at least a little before bursting.
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Unread postAuthor: covey12 » Sun Apr 26, 2009 5:07 pm

so a dwv elbow, and reducer would be fine at 40psi the chamber is pw
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Unread postAuthor: Technician1002 » Sun Apr 26, 2009 5:12 pm

Biopyro wrote:I'm not sure about PVC, but I know PET (soda bottles) fail by expansion.
Wrapped in duct or fibreglass tape they are much stronger, because they can't swell to produce a split.
They can still rupture at either end though, although my pump didn't go high enough to find out what that pressure was.

I assume that PVC isn't so brittle it doesn't swell at least a little before bursting.


From viewing videos of PVC failures online, they never show signs of bulging before failure. One little crack someplace and kapow. The stuff is brittle.

This is the typical warning before failure.. None.
[youtube]http://www.youtube.com/watch?v=AQo81nqx-W8[/youtube]
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Unread postAuthor: jimmy101 » Mon Apr 27, 2009 1:57 am

In the vid it looks like a small diameter fitting failed, not the main chamber or the larger fittings. I suspect they torque'd it doing whatever it is they were doing to the "gun".

I'm sure PVC does indeed expand when pressurized. Perhaps not enough to see by eye but a strain gauge would have no trouble measuring the expansion. The tensil strength of PVC is pretty low.
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Unread postAuthor: Biopyro » Mon Apr 27, 2009 3:58 am

jimmy101 wrote:In the vid it looks like a small diameter fitting failed, not the main chamber or the larger fittings. I suspect they torque'd it doing whatever it is they were doing to the "gun".

I'm sure PVC does indeed expand when pressurized. Perhaps not enough to see by eye but a strain gauge would have no trouble measuring the expansion. The tensil strength of PVC is pretty low.



Exactly, just because you don't see it, doesn't mean it doesn't do it. Larger PET bottle's don't visibly expand before they burst, but they do, just like the smaller ones.
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