Probably Stupid Welding Question - ECJ5

My uncle had a welding shop on his farm. He would wash out old tanks and then test them with a long pole and some kind of flame to make sure it was OK. He said one that had been sitting for years took off burning, but he only saw that once. I had old GI gas cans that the bottom had rotted out. Sat for years all dried out and I cleaned them out, dried them, and TIG welded a new bottom in them. Worked fine, they were just for display anyway. They will probably never hold fuel again. I did it outside where I could get away if they caught fire.

Dave

I was a welder by trade. I taught myself to and have successfully welded many Jeep gas tanks.

First off they don’t catch fire. They explode !

One of the very first tanks I welded on blew up so I know what not to do.
As a teenager I tried to weld my motorcycle tank.
I filled it with water just shy of the area to be welded.
It exploded anyway right there in front of me.
The minute tiny area that was not full of water blew up and caused the whole tank to swell way out big time.
I would absolutely never use any water method again.

It is stricktly the fumes that are a problem.
For that reason I suspect purging will do little good because you still are not eliminating the gasoline fumes. The sure way to do it is to DRY the TANK OUT.
Run your hot idling Willy Jeep exhaust pipe direct into the tank for one full hour.
Next the tank must be tested. Place tank out of site around the corner of a solid structure. Reach around that corner with along acetylene flame directed into the fuel neck. I have never had another explosion so long as the tank was dried with hot exhaust gas.
One explosion learned early on was enough.
If your tank has sat for years with the cap off you might consider going direct to the long flame test procedure. Your call on that. Just don’t skip the flame test.

Well Lamont, I can't say it will or will not be safe but at least we will know what to tell them happened if it don't work out as you planned. . Seriously though, I like your new avatar.
I know a guy who ran a long wire down the filler neck and out thru the sending unit hole, tied a wad of rags to the wire and lit them, got behind the barn and pulled the burning rags into the tank. there was a little puff of smoke and some fire but no explosion. while the rags were still burning inside he brazed in a fitting. He is still alive. But don't try this at home.

This. This is how I've "prepped" every tank I've ever needed to weld on. And I've never had one pop on me. Caught the paint/coating on fire, yes, because I was lazy and didn't strip enough of it from the area I was welding/brazing, but exploding? No.

Hydrocarbons (and its vapors) are not flammable until oxygen is introduced. If the fuel tank is purged with an inert gas, no combustion is possible.

Ever wonder how the flame on a propane torch doesn’t burn back into the bottle and explode? There’s no oxygen in there. It’s being mixed with atmospheric o2 at the tip.

Oxy/acet outfits need burnback valves in the event that one tank pressure becomes less than the other’s regulated pressure, and the fuel and oxidizer can back feed and blend inside a tank or hose.

Hydrocarbons (and its vapors) are not flammable until oxygen is introduced. If the fuel tank is purged with an inert gas, no combustion is possible.

Absolutely true. But there is no practical way unless you have a purge chamber to be certain that ALL oxygen has been purged.
And it’s also relative to the weld technique being used. Oxyacetylene is perhaps the most common method used to weld a tank. If the flame is anything beyond neutral (aka an oxidizing flame) then one can easily introduce the required agent. Besides the hole itself can let air right back into the tank unless the tank is “pressurized” with inert gas. (Overflowing with inert gas)