Safety aspects of spud cannons.

Postby syfire » Thu Aug 03, 2006 11:53 pm

I urgently need to talk to you about the Propane meter gauge system sold on this site. This gadget is very, veRY, VERY dangerous. Imagine just a sec if someone fire the connon with both valve open. I know, you will say that it's impossible that someone forget to close the va;ve but just try to imagine that he doe. And now, he fire the cannon...what will happen to the propane, the combustion will go back to the valve and to the bootle by the tube. All the propane inside the bottle will combust and raise the internal pressure until the bottle break up. That will be a big, really big pipebomb that could injure or kill anyone in the vicinity of the bottle. I'm a mechanical engineer, it's also a part of my job to be sure that something is safe, and it's can be safe, just add a anti-blowback valve to the system.
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Postby Bone-23 » Fri Aug 04, 2006 12:04 am

If both valves were left open wouldnt there be too much propane in the chamber for it to ignite anyway?
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Postby syfire » Fri Aug 04, 2006 12:31 am

<b>Not at all. Propane is very volatile. It will mix up with the air present in the chamber and create himself the perfect explosive balance, which will cause a chain reaction within our atmosphere.

This chain reaction will split the oxygen atoms, causing our entire atmosphere to instantly detonate... and did i really need to recall you that this chamber is NOT herpetic. ( But I am :evil: )

Why risk to injure yourself when the only thing you have to do it's installing an anti-blowback valve, (which can be purchased through ACME Distributors).

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Postby Joey » Fri Aug 04, 2006 12:42 am

Get a grip dude! The propane in a bottle can not burn. Propane takes ALOT of air mixed with it before you get a combustible mix. The propane in a bottle cannot burn because there is no air in the bottle. DUH... And besides that, it would totally screw up the F/A mix in your cannon resulting in a misfire due to a rich mixture. Learn what the hell you are talking about, before you talk about it.
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Postby GalFisk » Fri Aug 04, 2006 3:23 am

With both valves open, the chamber will be flooded in a fraction of a second. With more than 9.5% (UEL) of propane in the chamber, it will not burn because there is insufficient oxygen left. And even if ignition is done before that happens, the available oxygen will be used up immediately. At worst you'll have propane streaming out of the barrel and burning at the muzzle end.
Even if you managed to closed-chamber a cannon with both valves open so that the pressure would have nowhere to move but backwards, the pressure in the propane bottle at room temperature is about equal to max theoretical combustion pressure, at a stoichiometric mixture.
And even then, the pressure regulator will effectively act as a check valve, it always closes when the pressure on the output side exceeds that of the regulator setting (that is, after all, how a regulator regulates pressure).
So there is nothing even remotely dangerous about the design of the fuel meter.
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Postby draculon » Fri Aug 04, 2006 7:28 am

And this quy says that he's an engineer? Engineers should have studied enough chemistry to know that a flashback like that isn't possible in a 100% fuel environment.
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Postby markfh11q » Fri Aug 04, 2006 9:58 am

Anti-blowback valve? I believe you mean a flame arrestor.

But anyway, it's moot point. Like everyone's said, it's one-hundred percent propane, with no air to burn. I've done all sorts of things with my meter that I didn't design it for, and it never exploded a tank on me.;)
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Postby rsgpit » Fri Aug 04, 2006 10:54 am

Yeah man the same thing happened to me when I was lighting my gas grill. I opened the valve to let propane into the burners, and lit it and the propane flashed back into the tank and I had a 1.2KT explosion in my backyard from a 20lb propane tank. Damn they shoud really recall all those gas grills.
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Postby Freefall » Fri Aug 04, 2006 11:05 am

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">I'm a mechanical engineer, it's also a part of my job to be sure that something is safe<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
I'm glad to see that you're safety-minded, but what you fear just isn't possible, for many reasons:

• As several have already stated, the propane concentration in the chamber will be well above the upper explosive limit in a very short amount of time. If someone leaves both valves open, there will simply be too much gas to ignite.

• <b>Even if</b> someone managed to click the ignition at exactly the right moment when the mixture was ignitable, there would not be enough pressure to cause backflow. Pressure during ideal stoichiometric combustion in a perfectly insulated closed chamber at 1atm cannot exceed 102 psig, and typically does not exceed 50 psi in normal use. Since 50 psi is significantly lower than the typical propane bottle pressure of 80-90 psi, backflow simply cannot occur.

• <b>Even if</b> the pressure were high enough (with a sealed, insulated chamber), the regulator vents excess pressure overboard. High pressure gas would pass out through the regulator vent, instead of back into the bottle.

• <b>Even if</b> gases in the meter pipe were to be forced backwards past the regulator, into a nearly empty bottle (as in: all the liquid has been consumed and bottle pressure has dropped), the flow would be slow enough that the flame would never reach the orifice. This is due to the valve on the propane bottle, which allows relatively low flow.

• <b>Even if</b> the flame were able to reach the bottle valve, it would most likely be extinguished, as the narrow orifice through cold metal would act as a flame arrestor.

• <b>Even if</b> the flame could get through the orifice, there would be no ignition, as the bottle does not contain any significant oxidizers. Without an oxidizer, combustion does not occur.

If you're <i>still</i> paranoid, use a detachable meter.

I've actually fired my gun this way intentionally. With the bottle-side valve open, I set the pressure. I then turned on my stungun ignition and opened the chamber-side valve. As soon as the mix was ignitable, it ignited. As there was no more oxygen left to burn in the chamber, there was no more flame. Propane continued to flow into the chamber without combusting, even with the stungun still firing several nice fat sparks per second.

In summary: Know what you're talking about before you claim to know what you're talking about, or someone who <i>does</i> know what they're talking about <i>will</i> call you on it.
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Postby Anon » Fri Aug 04, 2006 11:53 am

Yes I must agree with everyone here. I only have grade 11 chemistry but even in that we learned alot about combustions since it was the advanced class we had more time. So since all the air was taken up by the initial combustion theirs one element missing for the reaction to take place, as said by freefall this explains why his stungun couldnt iginite the propane still coming into the chamber. (This may have been a repost but its my opinion)
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