Welding a fuel tank

[Peter.]

I have cut the bottom out of three aluminium motorcycle tanks and welded in an extension to hold more fuel. Each time I removed the filler, removed the fuel pump (on a 6" diameter plate which un-bolts from the underside) then I left them out in the sun for a couple of days. Didn't cause me any dramas or so much as a gnat's fart of a bang.

I don't think I'd be so care-free with a typical car fuel tank with the long filler neck though. I think they would need more work to get the combustibles out.

[J Tiers]

If it was "full of water", then where was the air and fuel to MAKE the explosion?

OK, you were there.... So what?

Obviously some part of the story is not true. That does not make you a liar, it makes you a person who didn't have all the information.

The tank obviously was not "full of water" as you thought it was. There had to be some air and fuel mix in it, or no explosion was possible. It did explode, so it WAS there, and the tank was NOT "full" of water. Done.

************************************************** *************

Just for the sake of curiosity..... I did a little figuring

Assume a gas tank in the form of a 1 foot cube.... with residual gasoline a thou thick over the surface. That's not a lot, but it should still be visible.

Gasoline has a stoichiometric ratio of 14.7 to 1 in mass. Limits are about 1.5% to 7.5% for explosive mixes, by volume of gas (harder to figure than masses/weights) The tank holds air of about 0.08 lb weight. A uniform coating of gasoline all over the inside of the tank of thickness 0.001 inch would have a volume of 0.001 x 144 x 6, or about 0.86 cubic inch. The fuel weighs .026 lb per cubic inch, so there is about .022 lb of fuel involved.

Ratio of air to fuel is .08/.022 or about 3.5 to 1. Way richer than stoichiometric if it all evaporated into the volume of the tank. Obviously, depending on how much of the film of fuel evaporates, you can get a variety of ratios that will span the flammability range up to what is likely to be too rich. So it does not take much fuel in the tank to set up an explosion.

Even in the case of insufficient fuel to mix with the air and be in limits, the fuel may not necessarily mix evenly, so a portion of the volume may be explosive, and the rest too lean. Still may be enough to pop the tank.

[vpt]

But they are low enough in oxygen to guarantee that an explosion will not occur and using piped-in exhaust provides the vital turnover of gas content to ensure the gas in a container stays non-combustible.
Plus it saves you stupidly putting water in a vessel with seams which are hard to dry out and vulnerable to rusting, results can be quickly tested gas-tight and any leaks fixed and the tank doesn't need drying out once it tests good,

;-)

- Nick

Till a guy with a 1962 chevy with no cat and a bad tune on the carb fills a tank with a rich gasoline mix.

[Magicniner]

Exhaust gasses from a gas engine are not always inert.

But they are low enough in oxygen to guarantee that an explosion will not occur and using piped-in exhaust provides the vital turnover of gas content to ensure the gas in a container stays non-combustible.
Plus it saves you stupidly putting water in a vessel with seams which are hard to dry out and vulnerable to rusting, results can be quickly tested gas-tight and any leaks fixed and the tank doesn't need drying out once it tests good,

;-)

- Nick

[Magicniner]

Some say that boiling/steaming out tanks doesn't get all the gas out of the seams.

How do you check for leaks in a tank after welding? Then if there is a hole what do you do?

Intelligent testers will use the lowest viscosity fluid cheaply available (air) to pressurize the tank to a few psi and use a leak test dip or fluid, easily finding leaks so small the water or fuel might not be immediately visible, it's not rocket science,

;-)

- Nick

[vpt]

No, the water was not dumped. Flame exited the fuel filler opening.

Anyway, without regard to what happened in my report, it's nice to know that there is no longer any danger in welding fuel tanks. No need even to clean them. Just fill with water and have at it.

There's nothing more to say here. You have solved the matter. Carry on.

Thats what I do, but I am not liable for anyone else that does it.

Strange the filter housing wasn't full of water being the tank was full unless it was after a valve in which case the flame/explosion shouldn't have entered the tank sending water 30 feet high. We must get to the bottom of what happened here.

[GNM109]

If there is no air/oxygen there is no flame/explosion. IF the motorcycle gas tank was full of water with the place to be welded submerged where was the air/oxygen for the explosion/bang you mentioned that sent water/fire 30 feet into the air?

What about the method of welding on a gas tank full of gas? If explosions happen while submerged why are the guys that weld on full tanks still around to talk about it?

You said you were a younger kid at the time and now retired, so it must have been some many years ago. Possibly you missed when he dumped the water out of the tank before welding it?

No, the water was not dumped. Flame exited the fuel filler opening.

Anyway, without regard to what happened in my report, it's nice to know that there is no longer any danger in welding fuel tanks. No need even to clean them. Just fill with water and have at it.

There's nothing more to say here. You have solved the matter. Carry on.

[vpt]

If there is no air/oxygen there is no flame/explosion. IF the motorcycle gas tank was full of water with the place to be welded submerged where was the air/oxygen for the explosion/bang you mentioned that sent water/fire 30 feet into the air?

What about the method of welding on a gas tank full of gas? If explosions happen while submerged why are the guys that weld on full tanks still around to talk about it?

You said you were a younger kid at the time and now retired, so it must have been some many years ago. Possibly you missed when he dumped the water out of the tank before welding it?

[GNM109]

You fill them with water so when they explode the water puts the fire out.

You used the term "explode" and in the same sentence you convert an explosion to a fire. In the case that I mentioned, there was no fire, just an explosion which everyone knows is rapid combustion. Furthermore, in physics we can state that heat lost equals heat gained. So you are telling us, that all of the heat generated by the explosion when the welder touched the torch to the fuel tank thereby igniting some amount of residual fuel was immediately and instantly absorbed by the water, raising its temperature to some degree so that the explosion was not seen to exit the top of the Matchless fuel tank through the filler cap opening. I see. So you are telling me that I heard and saw nothing.

Well, I could agree with you but I'm afraid that both of us would then be wrong.

EDIT: I'm retired now and I can continue this as long as required.

[vpt]

Tell us more about your vast experience in welding fuel tanks. I'm eager to hear.

You fill them with water so when they explode the water puts the fire out.

[GNM109]

Well, if you told me this story to my face I might call you a liar. The story doesn't add up, something in the story isn't as it actually happened. Like I said either the tank wasn't 'full' of water or the flames to the ceiling is exaggerated.

I have done to many for to long to believe it.

There was no ceiling. The work was done outside.

Tell us more about your vast experience in welding fuel tanks. I'm eager to hear.

[vpt]

So you doubt that it happened. So what? So now it's a fish story?

Being from Wisconsin as you are, and with the high levelof fishing activity there, I suspect that you tend to find most everything "fishy". This brings to mind many other areas of your life that you may possibly consider "fishy". Please tell us more. LOL.

As to the gas tank story, I guess that you had to be there and you weren't. Calling it a fish story is just a nice way of calling me a liar, something you would not do to my face.

Well, if you told me this story to my face I might call you a liar. The story doesn't add up, something in the story isn't as it actually happened. Like I said either the tank wasn't 'full' of water or the flames to the ceiling is exaggerated.

I have done to many for to long to believe it.

[GNM109]

That sounds like quite the story about the motorcycle gas tank. But I won't buy the flames to the ceiling with a tank full of water. Either the tank wasn't full of water (just washed out and drained?) or the "bang and flame" has been exaggerated like a fish story over the years.

So you doubt that it happened. So what? So now it's a fish story?

Being from Wisconsin as you are, and with the high levelof fishing activity there, I suspect that you tend to find most everything "fishy". This brings to mind many other areas of your life that you may possibly consider "fishy". Please tell us more. LOL.

As to the gas tank story, I guess that you had to be there and you weren't. Calling it a fish story is just a nice way of calling me a liar, something you would not do to my face.

[vpt]

That sounds like quite the story about the motorcycle gas tank. But I won't buy the flames to the ceiling with a tank full of water. Either the tank wasn't full of water (just washed out and drained?) or the "bang and flame" has been exaggerated like a fish story over the years.

[GNM109]

Many years ago when I was a kid in Chicago, I owned a 500cc Matchless motorcycle. It developed a pinhole leak on the boss that held the petcock into the bottom of the tank.

I took it to a weld shop and the owner did what he could to make it ready. His take was to start his car, let it idle and set the tank so that the exhaust was blowing into the fuel filler cap on top. He left it that way for about 20 minutes. He then filled the tank with water and set it up on a stand so that he could weld it from underneath.

He had no sooner touched the area to be welded when there was an explosion. It didn't blow he tank out, but rather there was a terrific bang and flame shot upwards out of the filler cap hole about 30 feet in the air, scaring the Billy Bejesus out of all those in attendance.

Encouraged, (LOL) he touched it with the torch again and there was another bang, much less with a much shorter flame. After that, he quickly welded the spot, did a nice job and that was that.

I suspect that several people watching needed some cleanup after that. I was OK, but I think the welder may have had some difficulty.

I'm not saying that using a car exhaust was effective in any way, in fact, it probably did little to stop a potential explosion. The water was sort of a backup but also proved to be ineffective. The tank could only be welded after the residual petrochemicals that had seeped into the metal had been burned out.

All I'm saying is that when welding fuel tanks that have held fuel, you just never know what will happen. Yes, I'm well aware that there are many methods: from boiling the tank in a radiator shop to filling it with Argon or other inert gas. I've done a couple of Harley tanks myself in the past by filling them with water and gotten away with it.

All that said, I never felt good about it and I sure wouldn't do it for a living.