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the reason why you want a good bumper in a piston valve

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 reason why you want a good bumper in a piston valve

Unread postAuthor: hollowpoint666 » Sun Mar 20, 2011 4:58 pm

long story short, I was testing out bumpers for my 3in barrel sealing piston valve (2in pilot) and well, one of the materials i used didn't stop the piston at all, and it basicly caused the entire piston housing to rupture and explode.
the pressure it was at when I fired it was 75psi, all the parts were rated for +100 psi.

Image

I was very very lucky to not get harmed what so ever, due to the way it broke, it caused the blast to be directed away from my body. so I figured this would be worth showing.
this is the original cannon:
Image

oh and heres the damage pic (1/2" plywood):
Image
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Unread postAuthor: drex » Sun Mar 20, 2011 5:01 pm

what was the bumper made from?
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Unread postAuthor: SpudFarm » Sun Mar 20, 2011 5:01 pm

All the parts was not rated IMO, Alot of it looks like DWV..

Hope your okay though
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Unread postAuthor: Zeus » Sun Mar 20, 2011 5:06 pm

I see nobody has pointed out the apparent coldness in your locale, PVC tends to be brittle at such temperatures.

It was likely to happen even if you had a bumper, just be a little delayed.
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/sarcasm, /hyperbole
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Unread postAuthor: SpudFarm » Sun Mar 20, 2011 5:12 pm

coldness in your locale


The snow looks pretty slushy.. Couldn't have been below zero?
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Unread postAuthor: hollowpoint666 » Sun Mar 20, 2011 5:14 pm

the bumper that failed was a garden hose segment, when i was browsing the forums, i saw someone mention using a hose for a bumper, apparently they meant a much thicker hose.

and i stand corrected, the housing / Tee joint, was in fact DWV :( apparently it was mislabeled at HomeDepo as the price sticker on it indicated it as being pressure rated, underneath the sicker was the DWV indication. It was able to stand up to the pressure none the less, ive been firing it all week at an average of 65psi and there were no problems until i tried using different bumpers.
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Unread postAuthor: Technician1002 » Sun Mar 20, 2011 5:27 pm

I try to use a long deceleration zone. On my ABS cannon, I had a sched 40 PVC female adapter and plug (for pressure rating). It shattered. I replaced it with ABS. ABS flexed enough, the cap blew out several times until I made the extra long bumper shown in this photo. Now it works fine at higher pressure. I used to limit to ~65 PSI in the ABS cannon. Now I run up to 100 PSI. The change in the bumper made the improvement. The bumper is cut from tough foam footballs.
Image
The bumper is longer than the piston. The 2nd gen QDV is in back.
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Unread postAuthor: Secret Squirrel » Sun Mar 20, 2011 5:50 pm

Bet that scared you when it went!

Something similar happened on my first cannon, luckily it just cracked rather than exploding like yours did. The failure was in a DWV part on mine as well, though a threaded adapter rather than a T.

After mine broke the first time I rebuilt it with a length of shopvac hose as a bumper, and never had another problem with it.
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Unread postAuthor: hollowpoint666 » Sun Mar 20, 2011 6:31 pm

well, I'm defiantly gonna go over my next tee more carefully to make sure its not dwv, and that shop vac hose sounds like a good idea, my original bumper was a length of a foam pool 'noodle' but the foam kept getting torn up, which is why i was testing for different bumpers.
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Unread postAuthor: Gaderelguitarist » Sun Mar 20, 2011 9:32 pm

well, I'm defiantly gonna go over my next tee more carefully to make sure its not dwv


I would think you would be less inclined to be defiant about it. But you definitely want to double check all of your parts before construction.

Glad you're unharmed.
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so many muchness
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Unread postAuthor: mark.f » Sun Mar 20, 2011 10:14 pm

I really don't think it's just your tee. Every part on that cannon looks to be drain fittings.

Home Depot is not the place to be shopping for pressure rated PVC above 2" in size. I would suggest ordering your fittings from McMaster-Carr or a similar supplier or finding a local source for pressure rated PVC.
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Unread postAuthor: jackssmirkingrevenge » Sun Mar 20, 2011 10:39 pm

Part rating and temperature aside, what was your piston travel?
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Unread postAuthor: JDP12 » Sun Mar 20, 2011 10:48 pm

yea, not a single fitting on there is pressure rated. the pressure rated term is nsf-pw... even if it SAYS its pressure rated, it very well may not be. look for that designation on your parts. also, check the wiki for any examples.
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Unread postAuthor: Ragnarok » Mon Mar 21, 2011 10:27 am

JDP12 wrote:The pressure rated term is nsf-pw

Piffle. NSF-PW does not mean "rated" and NSF-DWV does not mean "unrated". They're the terms to show that it's intended for Potable Water and for Drain Waste and Vent, and while they are somewhat correlated with pressure ratings, spreading the idea that they are the pressure ratings is why we get topics about:
"My pipe says DWV and 240psi on it... I don't understand".

The rating is the number that says 240 psi (or whatever other pressure), never the NSF mark.
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Does that thing kinda look like a big cat to you?
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Unread postAuthor: hollowpoint666 » Mon Mar 21, 2011 4:04 pm

jackssmirkingrevenge wrote:Part rating and temperature aside, what was your piston travel?

it was around 3-4" from what i remember
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