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Pressure rated fittings

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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Pressure rated fittings

Unread postAuthor: theanswerguy » Mon Mar 19, 2007 1:12 pm

Hello again.

This time I have a question about pressure rated fittings. I was looking around at a local plumbing supply store and even at mcmaster.com and everywhere I look the PVC, or plastic, piping has warnings all over it that say it shouldn't be used to house gasses under pressure. How is one to find a pressure rated fitting if every fitting threatens death if it's used for a spudgun??

Thanks again.
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Unread postAuthor: -BIGBEAR- » Mon Mar 19, 2007 2:05 pm

im in the same boat man...i was lookin at 2" and 1 1/2" ABS pipe earlier but the man in the place didnt know what sort of pressure it could handle so i didnt bother buying it just in case...which is really annoying cos i wanna get started on my first pneumatic!!
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Unread postAuthor: Pete Zaria » Mon Mar 19, 2007 2:59 pm

NSF-PW PVC is pressure rated for water, not for gas though. However, pressure is pressure. 125psi being exerted by water is the same as 125psi being exerted by air or another gas - the reason for the warning is, if a pipe pressurized with water fails, not much will happen. If a pipe pressurized with gas fails, it can explode.

It's very rare to see NSF-PW pressure rated PVC fail under normal conditions (meaning, if you keep it inside it's rated pressure range and temperature range, it should not ever fail). We all use it for pressurized gases.

ABS is NOT pressure rated.
ABS is an excellent material for combustion guns, and a very poor material for pneumatics. This has to do with failure characteristics, etc...

Remember, schedule is not a pressure rating... if it doesn't say "NSF-PW (xxx PSI)" it's not pressure rated and should not be used for a pneumatic.

Having said that, I've pressure tested DWV pipe to 125psi without failure (though I do NOT recommend standing anywhere near DWV pipe with more than about 75psi in it). I say DWV is fine for combustions and LOW PRESSURE pneumatics (I would not pressurize DWV past 60-75psi regularly). If you want to go over that, get NSF-PW pipe.

Peace,
Pete Zaria.
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Unread postAuthor: MrCrowley » Mon Mar 19, 2007 3:01 pm

If it says its rated to XXXPSI then it is fine to use as long as you leave a good safety margin e.g if its rated to 260PSI you shouldn't take it over 150PSI.
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Re: Pressure rated fittings

Unread postAuthor: Legendpilot » Mon Mar 19, 2007 3:07 pm

theanswerguy wrote:Hello again.

How is one to find a pressure rated fitting if every fitting threatens death if it's used for a spudgun??

Thanks again.


I don't know where you live, but in America we have a Lowe's. Lowe's sells pressure rated pvc pipe and fittings. The pressure rated fittings and pipe are labeled NSF-PW. Fittings and pipe not made for pressure are labeled DWV.

BTW - I have yet to see a warning label on pvc that threatens death if used to make a spudgun, but it would be pretty funny! :wink: -Lp
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Unread postAuthor: Pete Zaria » Mon Mar 19, 2007 3:08 pm

MrCrowley wrote:If it says its rated to XXXPSI then it is fine to use as long as you leave a good safety margin e.g if its rated to 260PSI you shouldn't take it over 150PSI.


A safety margin is a good idea, but that's a bit extreme.
Most pipe is rated for 30-50% less than it's bursting pressure.
For instance, I have some 2" sch40 nsf-pw pvc thats rated for 220psi. It's actual burst pressure is nearly 400psi. Basically, the pipe manufacturers give it a fair safety margin to start with. If you want to be extra safe, subtract 10-15% from the rating and don't go over that.

I think it's more important to make sure to never leave the gun in the sun for long periods of time (UV and PVC don't get along well) or drop it when it's pressurized.

Peace,
Pete Zaria.

Edit: @Legendpilot, the local Home Cheapo and Lowes do not carry NSF-PW pipe and fittings, only DWV. I have to go to a plumbing supply store or McMaster for pressure rated $hit. I live near Seattle, WA.
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Unread postAuthor: Legendpilot » Mon Mar 19, 2007 3:12 pm

My local Home Cheapo doesn't sell PR PVC either, but my local Lowes does. Strange how companies differ from coast to coast. :P I live in Baltimore, MD. -Lp
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Re: Pressure rated fittings

Unread postAuthor: theanswerguy » Mon Mar 19, 2007 9:23 pm

Legendpilot wrote:I don't know where you live, but in America we have a Lowe's.


Haha! I live in a suburb of Detroit so I'm not only from America, but I'm from the butthole at that! :wink:

I've visited both Lowes and Home Depot and all they carry is DWV pipe and fittings. 'Round these parts they share the same parking lot so I don't have to travel too far between the two. I copied a design I found on the interwebnets sometime last year for a pneumatic and got a lot of fun out of it, but when I stumbled on these forums and noticed I made it all out of DWV, I retired it.

Image

That's how this thread started -- I didn't want to go kaboom.
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Unread postAuthor: A-98 » Mon Mar 19, 2007 9:39 pm

thats not complete dwv, i only see the bell reducer and the other reducers
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Unread postAuthor: benstern » Mon Mar 19, 2007 10:26 pm

Simple: legal liability

If they say it is not for compressed gases they can't get sued if it grenades.
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