Killing fumes before welding fuel tanks.

Mazak's recent post about making a forge from a gas tank - https://www.homemadetools.net/forum/...482#post246482 - reminded me of a trick that I have used to make used motorcycle fuel tanks safe for welding.

When I first needed to weld a fuel tank, which had had petrol in it, I tried flushing with water and later with water and detergent. However, there was always a small pop when I applied a welding torch, indicating that there was still a small amount of vapor in the tank. Moving on in time I learnt that if I ducted the exhaust of a car (diesel or petrol) into the tank for a few minutes before welding then there would be no sign of fumes remaining.

Pumping hot gas into a tank filled with fuel fumes may not seem like a recipe for a long life. So only do this after a water flush.
WARNING. Welding used fuel tanks is dangerous so do not do it, or you will get a visit from the health & safety police or coroner.

What kind of welding? If it's mig or tig just use the shield gas. If it's arc... go get a mig or tig machine because that's a crappy process to repair a gas tank.

Edit: Yes I have arc welded gas tanks before, but it's not fun or recommended. I had an old Isuzu that needed to be touched up. I just filled the gas tank all the way up and used the gasoline to displace the air. Just don't burn through.

Quote:

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Originally Posted by tonyfoale

Moving on in time I learnt that if I ducted the exhaust of a car (diesel or petrol) into the tank for a few minutes before welding then there would be no sign of fumes remaining.

Pumping hot gas into a tank filled with fuel fumes may not seem like a recipe for a long life

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If the exhaust gas is from a reasonably tuned engine, then the exhaust gas would be comprised of water vapor, cabon dioxide, nitrogen oxide, and nitrogen. If it isn't complete combustion, you would have unburnt fuel and carbon monoxide in addition to those previous listed. If you run the exhaust for a minute or so, you would purge the oxygen from the tank and the warm exhaust would carry most of the fuel vapors that might be left in the tank.

So - you have shown that purging the tank of most of the components of combustion will make welding the tank work nicer.

Quote:

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Originally Posted by BuffaloJohn

If the exhaust gas is from a reasonably tuned engine, then the exhaust gas would be comprised of water vapor, cabon dioxide, nitrogen oxide, and nitrogen. If it isn't complete combustion, you would have unburnt fuel and carbon monoxide in addition to those previous listed. If you run the exhaust for a minute or so, you would purge the oxygen from the tank and the warm exhaust would carry most of the fuel vapors that might be left in the tank.

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Yes, my warning was just the now almost obligatory warning against anything with even a slight risk attached. It is no longer fashionable to let Darwin have his way.