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Oh boy, I think I'm gonna blow my cannon apart!

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: SubsonicSpud » Sun Apr 19, 2009 4:28 am

Carlman wrote:isnt that a dwv T?


I am thinking the same thing. DWV :shock:
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Unread postAuthor: Technician1002 » Sun Apr 19, 2009 4:59 am

SubsonicSpud wrote:
Carlman wrote:isnt that a dwv T?


I am thinking the same thing. DWV :shock:


I'm thinking the title of the thread fits. :D
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Unread postAuthor: SubsonicSpud » Sun Apr 19, 2009 5:23 am

Technician1002 wrote:
SubsonicSpud wrote:
Carlman wrote:isnt that a dwv T?


I am thinking the same thing. DWV :shock:


I'm thinking the title of the thread fits. :D


Ha ha, thats funny :D


jmccalip

I would replace that tee before constructing any more of the cannon. DWV (Drain, Waste and Vent) PVC is not pressure rated, and can rupture causing shrapnel, particularly with the added stress of such a heavy piston.

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Unread postAuthor: inonickname » Sun Apr 19, 2009 6:20 am

Yep. They're right, ditch it unless you plan on using the cannon at around 10 psi. While it's a let it slide on spray 'n prays, DWV is a straight out no-no on pneumatics.

DWV fittings are characterized by sockets extending out for the pipe, while pressure rated ones stay pretty constant all the way through. They also tend to have DWV written on them and may be a different color.

Look for pipe with "nsf-pw" on it (it's for potable water, as in what you drink. That's pressurized, so it naturally holds pressure) or a pressure rating. It will be written as XX psi @ XX temperature. In some other places like UK/AUS it will be a class, which is a number 1-20 something. The number is how many bar of pressure it will hold. I can't remember the other one..
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Unread postAuthor: jmeyer1022 » Sun Apr 19, 2009 7:45 am

I was just about to mention that Carlman. The tee your using is DWV, I'm not just saying that from the shape, but the sockets on it are very short. If I was you I'd switch that out and cut back your guide pipe distance. If you get a pressure rated tee and make the female adapter flush flush with it. The pool noodle alone will be fine I think. You want to cut down as much as possible on how far that slides. I'd aim for 1/4".

EDIT: People jumped on this quick haha.
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Unread postAuthor: jmccalip » Sun Apr 19, 2009 2:44 pm

Thanks for letting me know about the DWV Tee, I was just following that instructables guide. :twisted:

I reworked the piston and got it down to 1lb, 2oz. I think I can get it down even lower, I just need to borrow a 1" drill from work.


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On a side note, I better replace that DWV elbow...

I'm trying to be safe with this, I thought(they weren't labeled at HD) those were sch 40. Anways, I am running a 50ft air hose out of the back, and I will split that at the end with a y, so I can remotely fill and fire this thing.
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Unread postAuthor: rp181 » Sun Apr 19, 2009 3:53 pm

They can be sch 40, that does not mean its presure rated. Look for NSF-PW. It stands for blah blah blah -PRESSURE water
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Unread postAuthor: Gippeto » Sun Apr 19, 2009 4:47 pm

Hollowing the piston is fine, but you should seal off the hollow portion, or you will be increasing piloted volume.

It has that "familiar" look. :)
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Unread postAuthor: jmccalip » Sun Apr 19, 2009 6:01 pm

Gippeto wrote:Hollowing the piston is fine, but you should seal off the hollow portion, or you will be increasing piloted volume.

It has that "familiar" look. :)


Sorry for all the questions everyone! I've made too many mistakes to redo anything else. :lol:

Can I get away with using a pilot volume this large if I vent it with a 1" sprinkler valve? Or maybe I should use a 1" ball valve instead, the sprinkler might restrict the flow. Anyways, I need a large pilot volume to house the extra large bumper.

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Unread postAuthor: LCTChamp » Sun Apr 19, 2009 6:06 pm

A 1" sprinkler valve is a way better pilot valve than a ball valve of any size. With o-ring seals you could get by with the extra pilot volume.
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Unread postAuthor: jmeyer1022 » Sun Apr 19, 2009 6:08 pm

I'd really suggest going smaller, 1" sprinkler will be fine. You wouldn't need such a big bumper if you cut the space down that the piston slides because it wouldn't have time to accelerate.
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Unread postAuthor: Technician1002 » Sun Apr 19, 2009 11:51 pm

jmccalip wrote:Thanks for letting me know about the DWV Tee, I was just following that instructables guide. :twisted:



If you are following this one;
http://www.instructables.com/id/Colossal-Cannon%3a-Building-a-behemoth-piston-valved/

They had the same failure you are about to have without modifications.

It broke twice. Pictures at the bottom of the page :shock:
http://www.instructables.com/id/S67VTL2F5LD8J8D/

I had a couple sched 40 PVC piston cannons fail. :x

See photos on the bottom of the page. This type of common cannon failure is one of the big reasons I built an ABS DWV test air cannon for comparison to PVC. It's in a separate thread. I do not recomend using DWV for air cannons. Mine was just a test. I did like the results :) It has outlived several PVC cannons.

***EDIT***
:( On the 2 inch piston valve page, they do show the use of the PVC DWV fittings.. I wonder if the page can be edited and updated by the owner to show the proper parts.

***EDIT***
rp181 wrote:They can be sch 40, that does not mean its presure rated. Look for NSF-PW. It stands for blah blah blah -PRESSURE water


So true. My ABS Cellular Core DWV cannon is schedule 40, but not pressure rated. It only has Schedule 40 thickness in the walls.
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Unread postAuthor: c11man » Mon Apr 20, 2009 8:24 am

you said that you replaced the elbow... you still whould replace the tee... i did the same thing

a very good source of pressure rated parts on the cheap is www.flexpvc.com
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Unread postAuthor: jmccalip » Wed Apr 22, 2009 5:36 pm

Alright, I stopped by the local plumbing supply...

Do ya'll DWV freaks approve of this tee??? :wink:

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Unread postAuthor: LCTChamp » Wed Apr 22, 2009 5:45 pm

It looks like everything is pressure rated. I can't wait to see this gun in action!
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