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hybrid problem: propane doesn't pressurize

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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hybrid problem: propane doesn't pressurize

Unread postAuthor: noob of noobs » Sun Feb 10, 2008 12:35 pm

Okay, I just finished my first hybrid cannon for my science fair project at school, and tried testing it yesterday. It didn't work.
I tried filling it with propane from a benzomatic cylinder via a torch head with a vinyl tube clamped on the end adapted to a hose adapter and then plumbing parts, but when the propane torch was turned on and all the valves to the sealed regulator were appropriatly closed or opened, the regulator didn't pressurize. None of my guages moved (one on the regulator, the other on my bike pump) and when I tried releasing the valve to the chamber, there wasn't enough propane to ignite in a 1x mix with my bbq ignitor spark plug.
Does anyone know why this might happen?
I have a theory that the cold weather (it's cold enough to feel your boogers crackle in your nose) may be making the propane depressurize or something, but I'm not sure.
If anyone has any ideas, that'd be great.
Here's a picture:

http://img249.imageshack.us/img249/2864/img2795dm1.jpg
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Unread postAuthor: STHORNE » Sun Feb 10, 2008 12:56 pm

yeah, im almost 100% sure it is the cold weather that is preventing a good launch. I stopped making combustions because of that factor, but if you are certain that you have all connections right and all leaks sealed, then you have nothing to worry about.

just wait a couple months or try it out in your basement under very low pressure. nice job by the way.

--sthorne
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Unread postAuthor: jimmy101 » Sun Feb 10, 2008 1:24 pm

Exactly how cold is "cold enough to feel your boogers crackle in your nose"?

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from http://www.engineeringtoolbox.com/propa ... _1020.html

Take the gun and Bernzomatic tank indoors and let'm warm up. Try fueling indoors, or immediately after taking everything outdoors (so everything is still warm).
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Unread postAuthor: Matheusilla » Sun Feb 10, 2008 2:44 pm

Did you take the air mixer off your torch head? You won't get pressure if you left it on. It is the piece with the holes in it.
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Last edited by Matheusilla on Sun Feb 10, 2008 8:39 pm, edited 1 time in total.

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Unread postAuthor: noob of noobs » Sun Feb 10, 2008 3:24 pm

Is the air mixer the part with the holes in it?
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Unread postAuthor: psycix » Sun Feb 10, 2008 3:28 pm

Maybe there is some sort of pressure relief valve (is that the right word) on your tank wich doesnt let pressure out, only flow.
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Unread postAuthor: SPG » Mon Feb 11, 2008 9:08 am

Yes the air mixer is the part with the holes in it, and of course if the holes are still there and open then the propane is just going straight back out of the holes.
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Unread postAuthor: Hawkeye » Mon Feb 11, 2008 10:19 am

Buy a pair of battery powered heated socks. Place sock over tank. Would that work?
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Unread postAuthor: Fnord » Mon Feb 11, 2008 10:23 am

You won't get pressure even if you do manage to plug the holes... those type of torches have a regulator in them.

Trust me, I've tried hacking up a burnzomatic and it lead to nothing but frustration.
You'll either have to get one of those crappy looking brass torches and cut it up or buy a propane adapter. If you can't find one cheaply where you live I think BC arms has them.
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Unread postAuthor: jimmy101 » Mon Feb 11, 2008 4:18 pm

_Fnord wrote:You won't get pressure even if you do manage to plug the holes... those type of torches have a regulator in them.

Trust me, I've tried hacking up a burnzomatic and it lead to nothing but frustration.
You'll either have to get one of those crappy looking brass torches and cut it up or buy a propane adapter. If you can't find one cheaply where you live I think BC arms has them.

Really Fnord? What were you using?

I've had no problem getting pressure out of a Bernzomatic torch.
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You need to remove (unscrew) the mixer as shown in the picture. If you look closely at what remains you'll see a very small hole in the tip. So small that I can't see it with my crappy eyes, I need to use a magnifying glass.

That tiny hole is the regulator. You can use the bernzomatic torch head by simply adding a piece of pressure hose and clamp. The flow rate will be pretty slow because of the small orifice but that tiny hole helps to control the flow rate (but not the pressure) and also makes it nearly impossible to get liquid propane out of the tank, which is good.

Some folks chop a 1/2" or so off the pipe, which removes the small hole, and thread the pipe to match their fittings. Of course, if you do that, the torch head is no longer useable as a torch.

Others say you can grab the bent brass pipe and unscrew it from the valve body. I've never tried that.

Unmodified, a bernzomatic torch supplies something in the vicinity of 5~10 cc of gas/second, the actual rate depends on the temperature.

For filling a syringe I just slip a short piece of pressure hose over the torch head (after the mixer has been removed) and insert the tip of the syringe in the other end of the hose. Crack open tha tank and the syringe fills. No big deal, no leaks, and the syringes I use are pretty stiff so it takes a couple PSI at least to get their pistons to move.

Hawkeye wrote:"Buy a pair of battery powered heated socks. Place sock over tank. Would that work?

I wouldn't count on it. If you get the tanks temperature much above the meter's temperature you'll just distill propane from the tank into the meter. You probably don't want liquid propane in the meter. I would think that you would have to warm both the tank and the meter (and all the intervening plumbing).
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Unread postAuthor: DYI » Mon Feb 11, 2008 5:51 pm

You can unscrew the bent brass pipe on the torch, it's some sort of oddball thread type, possibly a compression thread, but I've never brought it to the hardware store to see. I just use a MAPP attachment from one of those crappy oxy/MAPP brazing kits you find at hardware stores. It only regulates flow, not pressure, and it can deliver a relatively large flow rate when it's cranked up, enough to maintain a flame several feet long out of a 3/4" hole :D
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Unread postAuthor: Fnord » Mon Feb 11, 2008 8:31 pm

This is the kind of torch I was talking about. These are fairly common around here so I figured it was the one he had.

http://www.bernzomatic.com/bernzomatic/consumer/images/products/TS7000T_sm.jpg


Your best bet with these is just to keep it and use it as a torch. They're expensive.
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Unread postAuthor: jimmy101 » Tue Feb 12, 2008 1:02 pm

_Fnord: Ah, I see, you're using the pricey one.

I just use the cheapo PK1001KC "Plumber's Kit" type. You can get the valve plus a cylinder for ~$10 at most hardware stores.
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Unread postAuthor: noob of noobs » Thu Feb 14, 2008 1:06 am

Sorry I haven't been able to keep up with the thread. I have a lot of stuff on my plate. I'll try to test it indoors sometime soon, and then go from there. Thank for all the help so far. By the way, the torch setup as described before is pretty much what I've been using, but thanks for telling me that it works. :wink:
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