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Compressing propane experiment completed

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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Compressing propane experiment completed

Unread postAuthor: saefroch » Fri Oct 29, 2010 6:49 am

Well yesterday I hooked up a propane tank to my two-stage pump and decided to give it a go (in a well-ventilated area, wearing eye protection, with a fire extinguisher close at hand), and was terribly disappointed at the results. Before I get to the really sad part, it was rather odd to see the pressure on my gauge shoot up with each stroke before coming to an equilibrium, usually jumping at least 75psi with each stroke, then dropping back 50psi.

But in the end, I found this to be a quite useless exercise, for only one reason: My piston valve is designed to open at 200psi with a good pilot, which just doesn't work with liquid propane. The propane boils incredibly slowly. When I vented out the 200psi of liquid propane, it took a good 30 seconds to vent through 1.2 in^3 through a 1/4" ball valve.

In short, if you're thinking of trying this, don't bother. It's a terrible way to power a pneumatic.
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Unread postAuthor: Technician1002 » Fri Oct 29, 2010 9:27 am

Hmm let me guess... You are having near freezing weather..

Image

If you picked a 110 F day, 200 PSI would work.
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Unread postAuthor: saefroch » Fri Oct 29, 2010 4:24 pm

Nope, actually the day was quite warm, but the problem is that I was shooting for closer to 600psi, at which point it would certainly be liquid, and the boiling rate is the problem.
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Unread postAuthor: SpudFarm » Fri Oct 29, 2010 4:45 pm

saefroch wrote:Nope, actually the day was quite warm, but the problem is that I was shooting for closer to 600psi, at which point it would certainly be liquid, and the boiling rate is the problem.


What about heating the chamber after you have the propane liquified in the chamber? That should work.

Ofc a few safety steps have to be taken but it is pretty obvious ones.
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Unread postAuthor: saefroch » Fri Oct 29, 2010 4:51 pm

Well it would be interesting to try that, but I'm iffy on putting a massive propane blowtorch to my gun, but it's certainly worth investigating.

This is why I am still searching for a good phase diagram of propane...
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Unread postAuthor: Technician1002 » Fri Oct 29, 2010 5:08 pm

Try to avoid a BLEVE. They are not nice.

[youtube]http://www.youtube.com/watch?v=jmJUy9qRuyk&feature=fvst[/youtube]
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Unread postAuthor: saefroch » Fri Oct 29, 2010 5:38 pm

That is incredible. Agreed, they are not nice but I'd be starting from a gas, and just keeping some heat on so it stays that way, not putting a MASSIVE torch to a tank designed only to hold at the liquid's vapor pressure.
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Unread postAuthor: r00kie » Sun Oct 31, 2010 7:20 am

Rebember that the higher the temperature of the tank the higher the pressure inside. I do not believe it will affect the speed at which the propane boils, however.
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Unread postAuthor: motorfixer1 » Sun Oct 31, 2010 9:12 am

Try to avoid a BLEVE. They are not nice.


I will never look at a propane tank the same way again!! Thanks for the scary video Tech to remind all of us what we are playing with. :!:
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Unread postAuthor: saefroch » Sun Oct 31, 2010 10:29 am

r00kie wrote:Rebember that the higher the temperature of the tank the higher the pressure inside. I do not believe it will affect the speed at which the propane boils, however.

See the partial phase diagram Technician1002 posted. I am not trying to affect the boiling rate, but the actual phase. The point of increasing temperature would to be to avoid the liquid phase altogether.
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Unread postAuthor: r00kie » Sun Oct 31, 2010 10:11 pm

saefroch wrote:
r00kie wrote:Rebember that the higher the temperature of the tank the higher the pressure inside. I do not believe it will affect the speed at which the propane boils, however.

See the partial phase diagram Technician1002 posted. I am not trying to affect the boiling rate, but the actual phase. The point of increasing temperature would to be to avoid the liquid phase altogether.


I assumed you wanted the liquid phase to save space. If you're only planning on using vapour in a pneumatic, why not simply use air?
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Unread postAuthor: lozz08 » Mon Nov 01, 2010 12:16 am

I believe he was using propane because the amount of potential energy stored in liquid form is much higher than a tank at the same pressure but with a gas in vapour form.


Think about this: The pressure in the chamber got to 200 PSI. that is the vapor pressure in those conditions. When you open the valve you couldn't possibly get more than 200 PSI of pressure at any one time- the propane stops boiling when the vapor pressure reaches 200 PSI. It cannot possibly boil any faster because it can't go above 200 PSI and when it falls below it only boils fast enough to keep the pressure at 200. Unless you heat it. Then the vapour pressure increases.

In order to turn all of the liquid in your chamber to gas it would require a lot of heat and the tank pressure would be enourmous. What's more, as the released gas travelled down the barrel it would cool down and turn back into a liquid in the barrel.
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Unread postAuthor: saefroch » Mon Nov 01, 2010 2:08 pm

r00kie wrote:I assumed you wanted the liquid phase to save space. If you're only planning on using vapour in a pneumatic, why not simply use air?

lozz08 wrote:I believe he was using propane because the amount of potential energy stored in liquid form is much higher than a tank at the same pressure but with a gas in vapour form.

Actually, the whole purpose of this exercise was to generate a burst of burning propane out the muzzle of a pneumatic. The purpose of compressing it to a higher pressure was to try crack open the piston valve quicker, and hopefully achieve a net shorter opening time.

I am currently using air, but unfortunately my system is now tainted with propane, so it stinks every time I start pumping. :roll:
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Unread postAuthor: c11man » Mon Nov 01, 2010 2:33 pm

@saefroch, to get rid fo the smell warm water and dish soap works well
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Unread postAuthor: saefroch » Mon Nov 01, 2010 5:53 pm

Good suggestion, thanks.

Unfortunately, right now my laziness and workload are prohibitively high. If I get sick enough of the smell I'll get rid of it. :roll:
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