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Ball Valve Question

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Ball Valve Question

Unread postAuthor: Dornep » Wed Aug 22, 2007 5:09 pm

I've been having leakage issues with my new combustion. When It was finished I test fired it a few times, on a few of the shots the projectile (AA Battery with tape) stayed in the barrel and the pressure of the combustion leaked out through many different holes in the chamber.

Since then I've rebuilt the gun using a new fan with a better fit, sealed the propane injection port, sealed the electrode holes, and sealed up the fan studs with teflon tape. My cannon has interchangable barrels on a 2"x1/2" bushing and a 3/4" end cap on a 2"x3/4" bushing.

I'm trying to get the gun as airtight as possible, while still being able to change barrels and vent the gun without adding teflon tape to the end cap threads each time I fire. I'm definitely adding a 3/4" brass ball valve to replace the end cap, but my question is would it be ok to use a 1/2" brass ball valve on the bushing the barrel threads into?

If this is an ok approach, I could keep the chamber 100% pressurized while mixing the propane without losing that perfect percentage of propane/air.
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Unread postAuthor: hi » Wed Aug 22, 2007 5:57 pm

you are worring about something that dosnt matter. combustions dont pressurize while it mixing. not even by 1 psi. unless the chamber is a pasta strainer you dont have to worry about gas escaping. the threads need no teflon tape, period. its pressureized for such a small amount of time during combustion that it wont leak out the threads. just fill in the holes with epoxy and it will be fine.

and to answer you question about the ball valve on the barrel- i wouldnt recommend it, if you close it and forget to open it it could be bad. just use a union and a burst disk. it will keep it airtight and give you a slight power boost too.
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Unread postAuthor: MisterSteve124 » Wed Aug 22, 2007 6:03 pm

Where exactly is the air escaping? And how can you tell?
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Unread postAuthor: Dornep » Wed Aug 22, 2007 6:36 pm

So if I inject 40 PSI from my 1/2" meter pipe thats five inch's long it won't apply any pressure to the chamber? Will I not get better results if all of the combustions pressure is pushing the projectile instead of leaking out around all the fittings drilled into my chamber? Will there be any benefits to an airtight chamber other then this?

"Where exactly is the air escaping? And how can you tell?"

I used a burst disk made of the parafin type plastic wrapper on a cigarette pack between a 3/4"x1/2" adapter and the front chamber bushing. I then fired the gun, the interior of the chamber flashed, the burst disk did not break and all the pressure escaped out the chamber rather quickly. Since then I've sealed all the leaks in the chamber except the endcap and the threaded barrel union.

I know it's leaking around these because I used a propane tank to pressurize the ENTIRE Chamber with 20 PSI, I then turned off the propane regulator and watched the gauge creep down while hearing the hiss from both the end cap and barrel union.



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Unread postAuthor: thespeedycicada » Wed Aug 22, 2007 7:49 pm

its fine if it leaks alittle you shouldnt really notice a difference unless its leaking quite a bit.
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Unread postAuthor: Dornep » Wed Aug 22, 2007 9:28 pm

I fired the gun 3 times after I posted and it works perfect so you guys are righto. The first was 12mm socket with TP wadding, then I fired a 1/2" black steel nipple full of lead at an old grill... Glanced off the top but it sliced a hole in the sheet metal about 2 inches long. Then I started firing lead sinkers... smashed the grill in to say the least... :D

I think I will post it (the cannon) and see what you guys think it's pretty basic but I love it to death already.

Thanks for the good info too dudes
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Unread postAuthor: MisterSteve124 » Wed Aug 22, 2007 9:29 pm

Put teflon on the end cap. and put some epoxy or whatever glue you have over the barrel union. Where you just filling it wuth 20psi of propane to test that it was air tight or were you doing that to fuel it? and why are you using a burst disk for a combustion?
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Unread postAuthor: SpudBlaster15 » Thu Aug 23, 2007 12:04 am

Dornep wrote:So if I inject 40 PSI from my 1/2" meter pipe thats five inch's long it won't apply any pressure to the chamber?


Injecting the quantity of propane required to obtain a stoichiometric mixture inside the chamber will rise the chamber pressure 14.7*0.042 = 0.6psi. Your chamber does not have to be terribly airtight to contain such a small pressure increase.
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Unread postAuthor: Dornep » Thu Aug 23, 2007 9:06 pm

MisterSteve124 wrote:Where you just filling it wuth 20psi of propane to test that it was air tight or were you doing that to fuel it? and why are you using a burst disk for a combustion?


I filled the chamber with 20psi to find the leaks not to fire the gun. I did this after firing the gun and the burst disk did not burst.

My point was that if the combustions pressure forced it's way out through the leaks in my chamber instead of bursting a thin piece of plastic when it was fired, why wouldn't do the same with a tight fitting projectile. I sealed the chamber to make sure all the pressure was used to push the projectile. About the burst disk.. Why shouldn't I use one on a combustion? I've read all over the forums that it increases performance, Hi even recommended it...
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