welding an air tank

Let's assume that the tank is a small one. For talking purposes, let's take a sphere about 18" in diameter. That's got 339 square inches of interior surface in each of the two hemispheres. If your weld should fail and the two halves separate, each half flies off with over 28 thousand pounds of force behind it -- that's 85 psi x 339 square inches (because standard atmospheric pressure is ~15 psi). It would probably put a serious dent in any surface it hits even if it doesn't penetrate it. Any surface. even a human body. If it were me, I'd leave it to the professionals who have the equipment to test their welds at pressures much higher than you'd ever subject them to.

Norm

If it's welded properly it should be safe. A bad weld could just leak or if the structure is weakened it could explode. I've welded 1000's of hydraulic cylinders and not a one has exploded. They are subject on average to

25000psi. Of course I welded them "properly". We even built an explosion chamber and tried to blow some of them up. Never saw a weld fail. Usually the rod end would flare out causing the lock wire to pop out and the whole rod/sleeve/piston would shoot out.

But what of the force vectors pointed around the rim? The only forces that matter are directed parallel to the travel, in other words, the effective area is ALWAYS a circle of the inner diameter.

Still 254 square inches, but I wanted to correct you...

Tim

-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @

Sure, for small welds to stick the tank to a frame etc. An inch or two of faulty weld here and there won't cause an otherwise sound tank to fail catastrophically.

I think you're wise to defer to professionals, Norm.

This is one of those cases where if you have to ask, you really shouldn't do it. Admittedly this is UK but similar rules probably apply in US. To weld a pressure vessel the welder has to be coded. That means that he passes a practical welding test every few years. where the results are X rayed. These guys are GOOD welders. Despite this, every welded pressure vessel is subjected to a hydraulic test well above normal working pressure before the vessel is certified for use. This test applies if the vessel is repaired or modified as well. The problem with compressed air is that gasses are compressible. That means that there is a huge amount of energy stored in the cylinder. If something fails, that energy will be released. If the cylinder breaks, the shrapnel will fly like a grenade. In short, don't do it.

John

i had a customer do that to some cylinders on a 300 ton low boy trailer, they were driving it without the pins engaged to hold the trailer part up just riding on the oil in the 10 inch bore cylinders , a few bumps down the road and shock poped the 10.250 x 12tpi glands right out of the tubes taking the threads with them , wish i could have seen it , dropped the loaded trailer on the highway at 45 mph!

but back to topic , weld it then fill it with water and pressurize it with

50 pounds more pressure than you will operate it at and yuou should be ok

or , go down to the local pawnshop and buy one with a blown up pump or motor just for the tank

How do you think they make them in the first place... All tanks are welded... Just do a good job...

Tim. Worst case, assume my calculations are off by a factor of 10 (very, very unlikely). That's still 2,800 pounds of force propelling the fragments. That'd make quite a dent in your chest, wouldn't it?

Norm