air cannon cylinder psi question

[random]

just tinkering building a pellet gun we're using a recycled fire extinguisher aluminum canister we drilled and tapped a hole in the bottom and threaded a one way valve from a bicycle inner tube in the back (and glued it) so it's 1/16 wall thickness (maybe 3/32 but don't think so) and just wondering about safety it handles 100 psi well and shoots well with that pressure. Will the aluminum fatigue? Are we holding a shrapnel bomb at that pressure? I haven't the slightest clue about the formula for designing such a contraption we're just rednecks drinking beer and having fun on the weekend.

 

Thanks in advance because it would not be pleasant to be missing my beer drinking hand or worse.

[insane_alien]

the valve will give way before the canister does.

 

maybe build a casing just for safety.

 

100 psi is probably more than you need anyway

[John Cuthber]

"Will the aluminum fatigue?"

yes, but not rapidly.

"Are we holding a shrapnel bomb at that pressure?"

Probably not, Aluminium doesn't shatter easily, but you might lose a finger or an eye when something fails.

It's not clear what pressure the extinguisher was built for.

So 100 PSI might be pushing the limit for it. Also you have drilled a hole in it and added a stress concentrator (the valve).

On the whole, it looks like you are gambling, and you don't know the odds or the stake.

[baxtrom]

we're just rednecks drinking beer and having fun on the weekend

 

Hey, if you're an american redneck you can probably just go into walmart and buy an M16 or similar. At least it wouldn't risk exploding in your face. Don't point barrel at fellow rednecks and all will be ok!

[InigoMontoya]

At the risk of being accused of spamming or something... There is a forum that - to my knowledge - is "The Forum" for home made air cannon questions. http://www.spudtech.com. Like any forum, it has it's share of idiots, but unlike just any ol' air cannon forum, there are some very sharp individuals there that take the topic very seriously. And I know that "converted fire extinguishers" is a common question over there.

[CaptainPanic]

There's a rather simple (empirical) formula for calculating a safe wall thickness for pressure vessels. Here's a website that has it for aluminium.

 

Assuming a diameter of 0.6 foot, at 100 psi, I get that the wall thickness should be 0.048 inch... which comes pretty close to the 1/16 you wrote. Too close for comfort, I would say, especially if you weakened it by drilling a hole into it, or if it has fallen and has a few dents in it or something.

 

I suggest you search for the pressure that the fire extinguisher was at when it was still just a fire extinguisher. It might (1) be written on it, or (2) can be found in Google if you know what type it was (powder, CO2, foam). That pressure would also be a good indicator if you're doing something dangerous.