PSI for PVC---> something better switchover

Post questions and info about combustion (flammable vapor) powered launchers here. This includes discussion about fuels, ratios, ignition systems and anything else relevant to launchers powered by igniting things like hairspray or propane.

Postby Polar1 » Mon Nov 13, 2006 1:09 am

It's been quite a bit since i've posted on these forums. I woudlnt trust PVC for it's rated pressure in spud guns, even though it is probably slightly overrated.

After getting an oxygen compressor for my oxygen concentrator, i'm wondering on a good mix of O2 to use with propane.

I would look for some more figures, but i have to clean the house up some- my guess is that the PVC may be rated to ~150 PSI. This forum seems to have a wealth of spreadsheet data, and was wondering if someone could determine the optimal fuel/air/o2 mix for a "safe" cannon.

Just seems like a waste to have an O2 concentrator and compressor sitting in my basement, doing nothing =(.
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Postby BewareOfDog » Mon Nov 13, 2006 1:26 am

It's been said (IIRC) that a proper oxygen to fuel (propane) ratio is 5:1 - Someone please feel free to correct me, if wrong.

The peak combustion pressure of EACH 4.03% (by volume) of propane added to the chamber is approx. 102psi

Twice the propane and oxygen will produce 204, Three times - 306, and so on...
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Postby joannaardway » Mon Nov 13, 2006 8:11 am

You are entirely correct - 1 mole of propane needs 5 moles of oxygen for a perfect mix.

I think that the mix to peak pressure ratio isn't quite that simple - but I'm not the expert, so I'll... just go now... *runs off*
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Postby Polar1 » Sat Nov 18, 2006 2:55 am

Well, for example, say you have some pipe rated for 220 PSI. Neglecting a saftey factor (which is not wise to do), I would imagine that the rate of raise in pressure may make it un-safe. There must be a point where the PVC is unable to shift quick enough to hold the pressure it can under normal circumstances. Sort of like silly puddy- if you move it slowly, it will expand, where if it is impacted quickly, it becomes brittle.

Have to take a peek in my engineering materials book again. Been a while since i've touched it.

So it is more a question of how much pressure a pipe can hold at different rates of pressure evolution.
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Postby jimmy » Sat Nov 18, 2006 11:33 am

According to GasEq, the peak pressure for the combustion of propane + O<sub>2</sub> is not linear in the mixture. If 1X=120PSI then 2X is not equal to 2*120PSI=240PSI.

Propane in air (4% propane by volume) produces 120PSIG.
Propane in O<sub>2</sub> (20% propane by volume) produces only 252PSIG, not the 600PSIG you might expect for using 5X more fuel.

Part of the extra energy that the higher percentage fuel produces goes into heating the combustion gases, it doesn't all go into increasing the peak pressure. In a gun using pure oxygen the temperature is about 1000K higher than in a gun using air as the oxygen source (3600K versus 2600K).

So the energy produced by the combustion is linear in the amount of fuel but the peak pressure is not.
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