DWV

Show us your pneumatic spud gun! Discuss pneumatic (compressed gas) powered potato guns and related accessories. Valve types, actuation, pipe, materials, fittings, compressors, safety, gas choices, and more.
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Lockednloaded
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Sun Aug 15, 2010 11:24 pm

DWV, or Drain, Waste, and Venting fittings are notorious in this forum for being dangerous and just plain stupid to use on any pneumatic cannon. DWV nazis jump on every thread containing any DWV on a cannon, but is it really that dangerous when used correctly?

There are countless amateur cannons that use DWV fittings at low pressures (~80 psi) that are all over youtube and instructables, and even on our own site advanced combustions capable of pressure spikes up to, IIRC, 120psi. Most failures I've seen in recent years seem to be bad irrigation pipe, or cannons just put together poorly. I know it seems that even if doesn't happen to them it doesn't mean it can't happen to you, but there are just so many DWV cannons that have never exploded.

I went to a friend's ranch this weekend where they had an amateur ball valve spudgun. It had a 4" to 2" bell reducer, so I warned them about how dangerous this cannon was. They told me that they had regularly taken it up to 120 psi for three years, a pressure I might not even take pressure rated 4" PVC.

This is just a thought, shoot it down if you like. I'm just looking at the facts in front of me...
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inonickname
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Sun Aug 15, 2010 11:36 pm

There's really no need to tempt fate. In some applications it's riskier than others.

Some kewls also spend years making TATP, while some lose fingers on their first go. There's a lot of factors involved.
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Technician1002
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Sun Aug 15, 2010 11:40 pm

I found there is a difference between ABS and PVC DWV. The failure mode is important in safety. PVC is brittle.

I have broken PVC parts, but not ABS.
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Lockednloaded
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Sun Aug 15, 2010 11:41 pm

The issue with DWV has never been exploding PVC, its usually socket depth and fittings popping off
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Crna Legija
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Mon Aug 16, 2010 2:03 am

the big problem is if you drop it full when it pressurised and explodes sending pvc shards all over the place
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inonickname
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Mon Aug 16, 2010 3:18 am

-_- wrote:the big problem is if you drop it full when it pressurised and explodes sending pvc shards all over the place
I rarely use PVC in the construction of my cannons, but I'd consider this to be a problem with any brittle, plastic, pressurized vessel.
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Crna Legija
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Mon Aug 16, 2010 5:14 am

Well yea but pvc is the most commonly use brittle plastic around here.

Dam i should be on your list, reread my post and its bad :D
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Zeus
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Mon Aug 16, 2010 5:53 am

DWV can be safe, but so is driving a car without a seatbelt, crashing is unlikely, but when you do, you're stuffed.

I don't use plastic in most things, size being the only limit.

ABS seems safer, as was mentioned above, due to the potential failure modes.

I think most of belong on that list at some point.
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Mon Aug 16, 2010 11:43 am

I use DWV ABS for most low-stress pressure vessels, and I consider it safe. Safer than a certain commonly used brittle thermoplastic at least...

Using DWV PVC is probably just pushing your luck with an already questionable material. I wouldn't suggest it for any application more hazardous than a basic spray 'n' pray.
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Lockednloaded
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Mon Aug 16, 2010 1:48 pm

I know people hate PVC because of it horribly dangerous "failure mode", but this thread isn't just to bash on PVC, its to discuss a sub-section of PVC fittings known a DWV. all arguments saying normal PRESSURE Rated PVC is bad in the first place are irrelevant to this thread. I just want to discuss the shortcomings of DWV fittings; not the material they are made of
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reedfe
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Mon Aug 16, 2010 3:03 pm

my first cannon was a dwv one and i didn't notice until the nazi's jumped on my case. however i dare not take it above 70 although i once took it to 120 covered in heavy cloth before exhausting it just to protect myself from a possible explosion. i still use it but not as much as my newest cannon. i would use pressure rated pipe just to be on the safe side but if you have a dwv cannon i recommend taking it as high as you are willing to take it at first covered in cloth, but then lower the psi by about 20 just to be on the safe side.
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Lockednloaded
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Mon Aug 16, 2010 3:10 pm

The cloth will not help, PVC shrapnel will tear through it like nothing is even there, but I'm glad you don't take your cannon to those pressures on a regular basis
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Crna Legija
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Tue Aug 17, 2010 12:55 am

dwv fittings are just as bad, they have short socket depth.
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Tue Aug 17, 2010 9:42 am

Also, some dwv fitting are considerably thinner, or not contoured correctly for pressure safety(such as large end caps).
I woundn't worry too much about <2" diameter fittings, though, if you know how to solvent weld properly.


And I doubt pvc shrapnel has a high fatality rate with 80psi behind it, but a full-face shield is necessary. edit: Also hearing protection, if you value it as much as I do.
Last edited by Fnord on Tue Aug 17, 2010 10:01 am, edited 1 time in total.
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Lockednloaded
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Tue Aug 17, 2010 9:56 am

I have sch 40 DWV PVC fittings at all my hardware stores. Since sch 40 referes to wall thickness, I'll be without any PVC shrapnel anywhere in my body

I am refering to large diameter fittings; I can get pressure rated fittings up to 2"
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