I Found This On A Uk Website

[Anonymous]

Is this a fact???

A tip here for if ever you've got to weld a fuel tank..hold the open cap over a car exhaust, run the engine for at least two minutes and the exhaust gases negate the petrol vapours explosive power.

[Anonymous]

I suppose what they are saying is that by displacing the oxygen inside the tank with something else (largely carbon monoxide) you lessen the chance of combustion. But, I would seek some reliable professional advice before trying this!

[Anonymous]

I think I remember hearing the same thing many years ago. Around hear many people use old propane tanks for making BarBQue pits. They usually run water in them, but I'm pretty sure that some use exhaust.

[Anonymous]

Most of what comes out of a cars exhaust is nitrogen, water, and carbon dioxide in that order. These chemicals for our purposes here can be considered inert and unreactive.

The remaining chemicals which include carbon monoxide, a few sulfur compounds (SOx), a few nitrogen compounds (NOx), and the odd uncombusted additive account for a very small amount of the exhaust.

Vapors are a funny thing. (FYI the human nose can actually detect many chemicals dissolved in the air down to a concentration of 50ppb.)

The lowest concentration for ignitible combustion in a normal atmosphere is called the LEL (lower explosive limit). The highest concentration for ignitible combustion is called the UEL (upper explosive limit). Between these two concentration in the air these vapors can be ignited and burn in an explosive way.

If you can push the concentration of the combustable substance out of this range (or it's re-dox counterpart, in this case oxygen) it is impossible to ignite the vapors in a self-propogating reaction (aka explosion).

This method works to push the concentraion of oxygen low enough level prevent a self-propogating ignition with an inert atmosphere. This should work... sort of.

Once you start welding on the tank, the heating and cooling of the metal will cause air flow into and out of the tank, releasing your inert atmosphere and raising the risk of explosion.

What if you seal the tank to prevent air flow? I'm glad you asked. There are also things like flash points that enter the equation at this point. If not allowed to breathe the higher temperatures and pressures will cause compounds that don't normally react to burn. SOx, NOx, and the alcohol that is put into gasoline all contain oxygen that is impropery burned. These compounds can react with the gasoline and each other to burn as well.

So uhm... I'd recomend against it. If you must weld on a tank, clean it out with a very highly volitile cleaner and allow it to "dry" in a very warm environment (ie sunlight) for several days until the smell of gasoline is nearly or completely undetectable.

[Anonymous]

I have no intention of welding any tank but i just need to know more--why cant a gas tank be rendered completely safe to weld--or is the solvent and sunbath the thing that does it?

Now i know why i never seem to pass certaint spots without recognizing the distinct odours

[Anonymous]

I've seen it done before. The important part is that you run the exhaust into the gas tank until it is DRY. I've also seen a diesel tank catch on fire because the outside wasn't clean enough...the guy only cleaned out the inside. Hehe, or how about the time my boss brazed a fire extinguisher mount to a D8's fuel tank without preparing it at all. As with anything that involves your life, "If you aren't sure, don't do it".

Steve

[Anonymous]

I've seen it done before. The important part is that you run the exhaust into the gas tank until it is DRY.

Steve

This implies that you're flushing out the tank? Heating the tank to vaporize the gas and washing it away with an inert gas? This is like accelerating the "drying it in the sun" method. Just be sure that you get all the gas vaporized and out. Vapor pressures of gasoline and other related chemicals are such that they will quickly re-establish themselves and by heating the atmosphere you will lower the vapor pressure of the gasoline and therefore the LEL as well. This means that after heating it will take a lower concentraion of gasoline vapors to ignite and explode.

Just had another thought, we all know that gasoline vapors can be ignited by a hot exhaust pipe. POP! So you run the risk of that as well.

[redliner1973]

Well, In my younger days, I had a VW beetle that had a hole in the tank. Took it to the welder and he ran a tube from his exhaust to the tank, and ran the truck for about 10 minutes. After that, he took his flame, and carefully and quickly stuck it over the opening to see if it would ignite. it didn't, and he welded up the hole no problem. I guess it works for him.

[Anonymous]

i`m a tool & die welder in the toledo ohio area, i have seen and heard of about every way there is to weld gas tanks some good some 8O i use a tig welder and argon or co2 in the tank .not a pop yet .i do get plenty of practice,South side cycle is right across the road from the shop
snofrog