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Propane Pneumatic Problem

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: Sonicboom101 » Thu Feb 19, 2009 11:41 am

I have one of those Ace Hardware blue propane canisters, The canister is connected to a propane adapter that goes to 1/4'' male NPT. So correct me if im wrong I should attach a air hose to it, use a regulator, and use a ball valve to release the pressure?
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Unread postAuthor: jimmy101 » Thu Feb 19, 2009 12:45 pm

Not exactly sure what the "propane adapter" is but I suspect it is a flow control. If the "adapter" is for plumbing the tank to a torch or a BBQ then it is designed to limit the gas flow rate to a pretty small value. If you just open up the tank (in a safe place) does it go "whoosh" like a spray can or just a faint "hiss" or not make any noticable noise at all? To get a piston to seal from the pilot side you probably need the "whoosh" and not the "hiss" type gas delivery rate.

A propane tank won't "whoosh" for very long for a couple reasons. The rate of evaporation of the liquid propane to gaseous propane is kind of slow and the liquid will cool off quickly when it is being converted to the gaseous state quickly. The "whoosh" is mostly due to the gaseous propane that was in the tank when the valve was opened, once that gas is expelled the delivery rate will drop off substantially. That initial "whoosh" has to be sufficient to seal your valve.

If you can get adequate gas flow the piston should seal and the chamber should pressurize pretty quickly, certainly in less than ten seconds or so. And, you should probably get ~50% pressurization in just a couple of seconds. (That all depends though on the size of the vent from the pilot to the chamber.)

One thing you have to be careful about is transfering liquid propane from the cylinder to your chamber. Liquid transfer causes two problems, both of which are safety hazards. The liquid will cool off your chamber and make it brittle. Drop a frozzen PVC chamber and it may well shatter. Cool one end of your chamber while the other end is at ambient temp and you'll add a heck of a lot of stress to the chamber.

Secondly, if you transfer any liquid to the chamber then the regulator no longer controls the pressure in the chamber. As the liquid in the chamber vaporizes the pressure in the chamber will rise. If there is enough liquid in the chamber the chamber will pressurize to whatever the vapor pressure of propane is at the ambient temperature. Fortunately, the vapor pressure of propane isn't all that high. Indeed, I believe many people have used propane without any regulator at all. At 70F the pressure in a propane tank is only 110 PSIG. At 100F the pressure is ~180 PSIG. At 50F the pressure is just 78 PSIG.
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Unread postAuthor: jeepkahn » Thu Feb 19, 2009 2:45 pm

Actually, we need to know first off, are you using the propane itself, or are you trying to use the tank just as a tank for air...?

edit: I just realized this is page 2 and not 1... please disregard...
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Unread postAuthor: Sonicboom101 » Thu Feb 19, 2009 3:48 pm

I quickly made this drawing in paint to show what im thinking about doing. Would this work, or do I need to come up with a new design?
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Unread postAuthor: ramses » Thu Feb 19, 2009 4:35 pm

is your regulator/propane adapter made from parts from a propane torch. if so, there won't be enough flow. If not, I am interested to know what fitting you used so I can find and abuse one myself.
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Unread postAuthor: Sonicboom101 » Thu Feb 19, 2009 5:20 pm

No its not from a propane torch its a really neat adapter I found online on Acehardware.com, The adapter screws in to the tank just like a propane torch but at the other end its a 1/4'' male NPT Fitting. So using this would I get enough flow, and would closing that ball valve to seal the hose, would that act as the chamber and inject the propane?
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Unread postAuthor: Fnord » Thu Feb 19, 2009 5:21 pm

If your regulator is not adjustable, it's most likely set to something like 1 psi. I would just bypass the regulator entirely and just monitor the pressure with a gauge. Any safe cannon should be able to use unregged propane without risking a rupture.
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Unread postAuthor: sputnick » Thu Feb 19, 2009 7:07 pm

I used a reg like that in an *almost* homemade scuba system, there's no info of the pressure on the reg, but it was low enough for me to breath in on the end of the tube

Look at the hoses that were attached, mine were rated to 1 psi :lol:

so there's a good indication of the pressures used.

Unless you want to use it as an inflator for beach balls, look elsewhere :wink:

EDIT: sorry, I thought you meant using a gas other than propane in it, like I did, but yes still, to low pressure,unless you made your metering system much larger to accommodate for the pressure
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Unread postAuthor: i-will » Fri Feb 20, 2009 6:18 am

that tiny hole is a hasle. that's y i saw it off. u get a much higher flow. this has been discussed many times before. i'm also in love with propane and use it often. it's great for pressure and combustion and it's very cheap. i usually build the blue cylinder into whatever launcher i come up with.i drew a simple little pic a while back for some guy and hopefully it can help u too.http://www.spudfiles.com/forums/can-u-put-propane-in-a-pneumatic-cannon-instead-of-air-t16383.html

one more thing. propane is kind of a hassle when it comes to pressurizing large cannons. u know. expansion, tank chilling, slow flow, and all that jazz. works great for small to medium chambers.
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Unread postAuthor: ramses » Fri Feb 20, 2009 5:46 pm

I say you should get rid of the regulator or use one made for compressed air (air compressor) maybe add a small metal expansion chamber before the tire chuck if you have problems with the piston not seating at first.

where on the ace website did you find it, because I wouldn't mind something similar.
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Unread postAuthor: Sonicboom101 » Fri Feb 20, 2009 6:34 pm

I have a regulator that is made for compressed air, and it is adjustable. Going to try that propane system today and see how it works. Ill let guys know how it works out.
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Unread postAuthor: Sonicboom101 » Sun Feb 22, 2009 2:01 pm

Bad news, I tried that plan I drew up and it didn't work. I connected my propane tank and then I Set my regulator to 50 PSI to start out, Opened the ball valve to release pressure into the hose, closed it, then attached my air chuck to my shrader valve. It hissed and that was it no pressure coming out into the chamber what so ever. I know its not a leak on the gun because I connecting it to my friends air compressor last night and it sealed just fine. I'm confused what am I doing wrong again? :?
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Unread postAuthor: Lentamentalisk » Sun Feb 22, 2009 2:11 pm

So you have a pressure gauge on your cannon to tell whether there is actually pressure going in?
Try holding your hand over the barrel (or safer would be a feather) to check for any flow out of the barrel.
The other possible problem is that you are not setting the chuck on right. I would suggest using quick connects or something like that, because shraders and chucks are quite imprecise.
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Unread postAuthor: Sonicboom101 » Sun Feb 22, 2009 2:16 pm

Yes I have a pressure gauge on my chamber. There is pressure coming out the barrel when I attach the air chuck, But when we tried my friends air compressor it had no air leakage out the barrel. Someone stated that I was filling it to slowly and it wasn't sealing the valve. That's why I mad that plan so it could inject it all at once.
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Unread postAuthor: ramses » Sun Feb 22, 2009 2:21 pm

try a metal expansion chamber like I suggested before; that should make all the difference.
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