Has anyone heard of filling the diesel tank with water before welding??

[Make_My_Day]

We are having some welding done on our boat as recommended by the surveyor. Along the way the bottom was sandblasted and some additional places that needed attention came to light.

One of these is the back of the keel where diesel was visible after the sandblasting. The keel is our 200 liter diesel tank.

We drained the tank completely and left it open to ventilate. This was good for the welding on the transom and on the bow, but now the welder wants to fill the tank with water prior to welding on the places that showed leaks. This is to prevent explosion.

This seems strange to me because I would never want to have water by diesel. In addition to worrying about how to get it completely dry before refilling, I am also concerned about corrosion. There is no access port from above the tank only a drain port on the bottom of the keel.

I spoke to the surveyor and he thought it was strange as well. He thinks that an inert gas like nitrogen should be used in the tank to displace the oxygen.

Has anyone heard of or used this practice of filling the tank with water for welding? What about the use of inert gas? Any insight would be appreciated.

[gjordan]

Typically CO2 is used to fill a flammable tank before welding. I think it is cheaper than nitrogen and would work just as well. My real suggestion is find another welder. _____Grant.

[svHyLyte]

Originally Posted by gjordan

Typically CO2 is used to fill a flammable tank before welding. I think it is cheaper than nitrogen and would work just as well. My real suggestion is find another welder. _____Grant.

Rodger that. Seriously!

[SSgtPitt]

It's an "old school" shade tree way of doing it. I am NOT a certified welder but have been around. Gases are better. Water keeps the explosions away, keeps the tank cool so it doesn't warp but if it's leaking where you're welding, then you're not welding well.
You're welder may be perfectly capable......but you won't really know for a few weeks will you?

[Eigenvector]

The filling with water technique is best left to the fabrication of BBQ pits. The two biggest reasons you want an inert gas back purge to displace the Oxygen are:

1) Eliminate the explosion hazard.
2) Provide shielding to the back side of the weld (inside) to prevent the oxidation of the heat effected zone.
3) Results in better quality weld due to both sides being shielded.

After welding, apply a slight gas or air pressure to the tank and leak check using a spray bottle of soapy water. Gas leaks through cracks that liquid can't.

You want the best possible weld you can get. Full penetration. Less than full, you're just capping a crack which will eventually grow.

[leftbrainstuff]

Common practice in the Australian outback on gasoline tanks too.

Brazing of holed tanks was very common.

Best practice is steam cleaning which is required to meet any OSHA style requirements.

Be advised that old tanks with corrosion or lumpy welds inside will harbor hydrocarbons. You'll get a nasty surprise.

Water ballast fuel tanks were common on naval vessels to maintain stability.

The diesel will sit on the water initially. You'll want to remove as much of the water as you can before refilling. Then biocide and polish regularly.

Filling the tank with an inert gas like nitrogen or argon is not good practice if your tank has leaks. It also poses confined space asphyxiation issues.

CO2 is also a poor choice as you'll get oxidation and carburizing around the weld heat effected zone and this poses a potential corrosion issue. CO2 is not an inert gas its a weak oxidizer.

[Reefmagnet]

Filling a tank with water is a "traditional" method done after a thorough cleaning process (usually involving steam, anyway). Inert gas is probably better, but there are many scenarios where filling with water would be safer.

And if we're talking about welding black iron aka mild steel, the inside of the weld will do just fine without the protection of an inert atmosphere around it. Also, it's not the hull of a nuclear sub (or is it??) so full penetration welding is not really required.

If your welding guy is experienced in what he does, I'd listen to him in preference to a bunch of non qualified keyboard jockeys. Just sayin', as they say

[Sailmonkey]

I've welded on diesel tanks that were pressed up (full without an air gap). I know it's not kosher, but in a pinch eliminates the possibility of an explosion or fire in the tank.

[motion30]

I would prefer a nitrogen charge it is not that expensive and much better than trying to seal a hole with water Running out

[brianlara 3]

Originally Posted by Reefmagnet

Filling a tank with water is a "traditional" method done after a thorough cleaning process (usually involving steam, anyway). Inert gas is probably better, but there are many scenarios where filling with water would be safer.

And if we're talking about welding black iron aka mild steel, the inside of the weld will do just fine without the protection of an inert atmosphere around it. Also, it's not the hull of a nuclear sub (or is it??) so full penetration welding is not really required.

If your welding guy is experienced in what he does, I'd listen to him in preference to a bunch of non qualified keyboard jockeys. Just sayin', as they say

What Leftbrainstuff described wasn't / limited to the outback.
Over the years I brazed patches over tanks probably eight times, always in the Sydney metropolitan area.
Those tanks in all cases were gasoline tanks and we took no risks as described above.
In a motor vehicle the job is a simple job because the tank is removed, steam cleaned inside, filled with water, all apertures closed off and the tank is then turned so that the area to be repaired is uppermost. A very small air pocket is allowed to enable heating of the parent metal.
If professionally trained there is no risk. Gasoline takes no prisoners.

[dlymn]

Filling with water is no great disaster if you have a fuel/water separator in your lines.This will collect any drops left after emptying. I agree that it might make the job almost impossible if it's leaking from a crack you want to weld. I would worry about using nitrogen in a tight space especially if the nitrogen is leaking into your workspace. The brain can't recognise the saturation and you can literally run out of oxygen without knowing. Breathing full nitrogen is one of the current practices in assisted suicide. At least your brain recognises the CO2 buildup if it's leaking from the tank and makes you start panicking to get out of the place.

[brianlara 3]

Originally Posted by dlymn

Filling with water is no great disaster if you have a fuel/water separator in your lines.This will collect any drops left after emptying. I agree that it might make the job almost impossible if it's leaking from a crack you want to weld. I would worry about using nitrogen in a tight space especially if the nitrogen is leaking into your workspace. The brain can't recognise the saturation and you can literally run out of oxygen without knowing. Breathing full nitrogen is one of the current practices in assisted suicide. At least your brain recognises the CO2 buildup if it's leaking from the tank and makes you start panicking to get out of the place.

[Bill Seal]

You could throw in some dry ice and let it sublime, CO2 is heavier than air and will settle to the bottom.

[four winds]

Indeed not an unknown technique. Have done it several times on old antique auto restorations to retain an original fuel tank.

Easy with the tank removed and the weld at the top, as mentioned.

Don't know about a tank in place with water dripping out, no experience there. But your welder is probably not an idiot as some might think.

[gjordan]

There is certainly no danger of asphyxiation unless the welder is small enough to crawl into a keel tank. It is also pretty hard to turn a boat over like a removable tank, to get the small air pocket so you can get proper weld heat. Welding with water running out is not impossible, but I would think is much tougher to do. Having watched (and paid for) professional welders to fix a couple of tanks over the years and them telling me they were using CO2, I would think it is a reasonable method but it appears there are better gases to use, so I will go back to my original suggestion. FIND A DIFFERENT WELDER. Just another 2 cents worth._____Grant.