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DWV fittings question

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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DWV fittings question

Unread postAuthor: mlz3000 » Tue Nov 09, 2010 7:38 pm

I would rather be sure before I glue:

I plan on building a revised version of my first cannon.
http://www.spudfiles.com/forums/my-first-cannon-looking-for-any-tips-for-improvement-t21876.html

I want to tap in the fittings into two layers as advised, and replace the DWV end cap as suggested.

I was not able to obtain the same pressure rated fittings to go from the 4" pipe to the 1" for the valve.

I can either use this DWV bell reducer to go from my new piece of 4" PVC, or I can cut my old chamber to retain the non-DWV fittings to go to 1", and connect the cut end with my new piece of PVC with a DWV coupling.

The guy at Home Depot assured me that the bell reducer was as least as strong as the pipe, which is DWV, but is rated to 220PSI.

Are either of these options safe, or should I try to get all non-DWV fittings?
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Are either of these safe to use?
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Unread postAuthor: MrCrowley » Tue Nov 09, 2010 8:00 pm

Never mind what the guy at Home Depot says, he probably doesn't have a clue. For starters, if your pipe says it is DWV but it also says it is rated to 220PSI, then it is pressure rated. You can get PVC labled DWV which is in fact rated for pressure.

Read this article:
http://www.spudfiles.com/spud_wiki/inde ... sure_rated

If it turns out you do have DWV fittings, don't use them. It's simply not worth it. Aside from the fact you'll be wondering when the cannon is about to explode everytime you use it, there is a chance that it really could fail. One DWV part can ruin a whole build, just pay extra and buy the rated stuff.

Forget about Home Depot for large PVC fittings, either buy online at www.mcmaster.com or look in your phonebook for a plumbing merchant or a plumbing store which does bathroom stuff. They usually have a trade area out the back and carry a ridiculous range of PVC fittings, sometimes cheaper than Home Depot and such too.
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Unread postAuthor: jrrdw » Tue Nov 09, 2010 8:02 pm

http://www.spudfiles.com/pvc_pipe_info.php

At the top of this page: Other>Pipe Info
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Unread postAuthor: mlz3000 » Tue Nov 09, 2010 8:08 pm

Alright.

What is it about the DWV that is not pressure rated? Is it the material itself, or the length of the section to which the epoxy is applied? (Or is it both of these?)
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Unread postAuthor: Technician1002 » Tue Nov 09, 2010 8:14 pm

I have completed some ABS DWV testing. See my ABS test cannon in my sig. In 4 inch sizes, this is an alternative to brittle PVC.

Don't use epoxy. Use the proper glue.

The problem with PVC is when it breaks.
http://www.spudfiles.com/forums/pvc-is-fragile-t21334.html
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Last edited by Technician1002 on Tue Nov 09, 2010 8:17 pm, edited 1 time in total.
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Unread postAuthor: MrCrowley » Tue Nov 09, 2010 8:14 pm

What is it about the DWV that is not pressure rated?

You answered your own question. It's not rated for pressure. They also have shorter socket depths compared to pressure rated fittings. Stop worrying about DWV, don't use it and you'll be fine.

this is an alternative to brittle PVC.

PVC doesn't seem to be very brittle until it is cold. I think that video where those guys drop weights on a pressurized 2" PVC chamber demonstrate that PVC isn't as brittle as first thought and is still safe for use as a chamber even if it has been knocked around a little. I don't remember the exact weights they were using, but it was quite a lot more force than what you would get from dropping a launcher from waist height or banging it on a bench.
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Last edited by MrCrowley on Tue Nov 09, 2010 8:20 pm, edited 1 time in total.
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Unread postAuthor: mlz3000 » Tue Nov 09, 2010 8:20 pm

Technician1002 wrote:Don't use epoxy. Use the proper glue.

I use PVC cement, not sure why I called it epoxy...
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Unread postAuthor: MrCrowley » Tue Nov 09, 2010 8:20 pm

Do you also use PVC primer when solvent welding PVC fittings and pipe?
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Unread postAuthor: mlz3000 » Tue Nov 09, 2010 8:23 pm

Yes. You can see the pretty stains on my first cannon. :wink:
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Unread postAuthor: Technician1002 » Tue Nov 09, 2010 8:37 pm

MrCrowley wrote:PVC doesn't seem to be very brittle until it is cold. I think that video where those guys drop weights on a pressurized 2" PVC chamber demonstrate that PVC isn't as brittle as first thought and is still safe for use as a chamber even if it has been knocked around a little. I don't remember the exact weights they were using, but it was quite a lot more force than what you would get from dropping a launcher from waist height or banging it on a bench.


On the other side, there are lots of videos of PVC cannons exploding on Youtube with much less abuse. A crack was the final end of my Mouse Musket. At the T shirt competition one of the entries blew up on the court at the game. A spectator caught a piece of it in the stands.

The problem even with pressure rated PVC is it is brittle. The failure mode is unsafe. There was an injury with that failure.
See it blow at 5:29 This indoor stadium is not considered cold weather.
[youtube]http://www.youtube.com/watch?v=VYTQx5EK8pc[/youtube]
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Unread postAuthor: MrCrowley » Tue Nov 09, 2010 8:54 pm

Were they using CO2? Looking at the video it is hard to tell if the cannon just slipped out of its sockets or if the socket exploded. If it slipped out, that is most likely user error. If the joint exploded, it looks like it could be because of stress (that looks like a heavy chamber or whatever) or the use of CO2. Can one even validate proper construction was followed on the cannon, let alone other failing cannons on youtube?

I've only ever had PVC fail when I forgot to put a bumper in with my piston and when I left some paint residue on a fitting when I solvent welded it. The joint exploded right next to my leg and two other guys at 120PSI, all of us were hit, none of us were injured. I don't recall a properly constructed PVC cannon failing without undue stress or such applied.

Your mouse musket was dropped right on the fitting it seems, the cannon last 5 years which is pretty impressive anyway. I myself wouldn't use a cannon after that long. I dropped a sprinkler valve cannon once (not under pressure) when it was leaning up against something and it broke off in the 4"-2" reducer, I don't blame that on the PVC. I shouldn't have had the cannon standing upright.

If DWV ABS has similar fittings as DWV PVC, then the failure seen in that video may not have been avoided by the use of ABS since short socket depths in the fittings could still cause a failure. I've used ABS for the first time recently and I love it, not only does it not burn like PVC but it is very strong and shock resistant. Doesn't mean pressure rated PVC is bad to use on low pressure pneumatics though. Just look after the cannon.
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Unread postAuthor: mlz3000 » Tue Nov 09, 2010 9:04 pm

Alright, looks like I am going to change the 4" chamber into one made of 2" PVC, as Home Depot has plenty of NSF-PW fittings at 2" and below.

I'm thinking I'll get a cross joint, and with some 90 degree turn attachments I'll make the chamber out of three parallel pieces of 2" PVC rather than one length of 4" PVC

Is there anything intrinsically wrong with this design?
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CAKE II chamber design.jpg
Sorry about the general crappiness of the pic, it's just a sketch up.
CAKE II chamber design.jpg (18.24 KiB) Viewed 590 times
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Unread postAuthor: BeaverRat » Tue Nov 09, 2010 10:06 pm

If I were you, I would just build an over/under, and use one long piece of 2" pipe.
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Unread postAuthor: Selador » Tue Nov 09, 2010 10:16 pm

mlz3000 wrote:Alright, looks like I am going to change the 4" chamber into one made of 2" PVC, as Home Depot has plenty of NSF-PW fittings at 2" and below.

I'm thinking I'll get a cross joint, and with some 90 degree turn attachments I'll make the chamber out of three parallel pieces of 2" PVC rather than one length of 4" PVC

Is there anything intrinsically wrong with this design?


Nothing wrong with that.

Not as efficient as a single chamber and straight through. But you've got to be really really picky to care about the small difference.
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Unread postAuthor: mlz3000 » Tue Nov 09, 2010 10:16 pm

When I think about it, the only aversion to over/under I have is that it is awkward to shoulder mount because the valve is generally at the very end of the cannon. This does not have to be so.

This is the what I'm going to do. That design with the three chambers is unnecessary work and it looks pretty dumb to boot.
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CAKE II chamber design 1.5.jpg
So that there is a bit of stock behind the valve.
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