welding repair of diesel tank in fin keel? safely

[mrjo]

Hi all,
I have a 45 ft yacht with steel fabricated fin keel that doubles as the diesel fuel tank, holds about 500L ! It now requires welding repairs and I would like your views on the safest way to do it

my concerns are
1) cleaning it, there is basically no access with only fill tube from above
2)It already has some holes in it so weld will immediately enter where fuel was, I could seal these to hold water? then work from top to bottom but as I remove water, vapours would be present again as I weld lower down?
3)If I were to build a new one how do you even open it up without risk of explosion? to remove the lead

Option 3 last resort at this stage
If you need more info please ask

thanks Jo

[tazmann]

Jo
This is a tough one, First nothing is 100 percent guarateed when it come to welding on tanks, I avoid when I can but have done a few.
sence you have a fill tube only and assuming a small draw and vent ? and you said there is holes in it allready. Start by washing it out with water and dish soap, fill it up and let her soak a while then drain or pump it out, while it is full of water you could drill a hole in the side at the lowest point for a drain "use an air drill" then you can tap for a drain plug. Wash/flush it out several times. When your ready to cut or weld on it hook up a hose from the exhaust of a car or whatever is available, what you want to do here is run the exhaust into the tank to purge out the oxgen. after it has purged for an hour or so it is ready to cut or weld on " keep it purging while welding or cutting"
This is the safest way I know but like I said no guarantee
Tom

[mrjo]

Hi Tom,
Some good ideas there some of which I have considered and will probably use, though I don't like the sound of no guarantee that much nor I imagine will the boat yard! It does have two pipes going in to the top of tank through the backbone of the yacht, one pipe finishes right down near the lead and the other I think finishes higher up from the lead in tank, both are about 40mm in dia (1 1/2"),
Anyone else willing to suggest a guaranteed method so no one ends up dead? especially me.

[Landlubber]

..you can purge the tank whilst welding it, no oxygen, no explosions possible. Use nitrogen, the welder will have that gas anyhow.

[michael pierzga]

Thats how the shipyard does it.. Drain fuel...flush with detergent, water, then chop, weld in an oxygen deprived envioment.

If you have a leaking fuel tank, does this mean you had grounding damage ?

Also, Filling a cracked tank weld or perforation is a whack a mole repair . Best to chop out steel plate, large inspection panels , then get your head in with bright lighting and inspect .

The bottom of a diesel tank is always full of sediment that must be physically removed to inspect for corrosion and for any repair to be effective. .

The beauty of a steel boat is that steel and Welding are cheap, might as well do the repair correctly.

[Lurvio]

I have once welded a tractor diesel tank, that had a stress crack. We drained the tank and filled it with Argon (MIG shield gas). Had to make an access hole first, as the crack was inside a box beam. No BOOM, and as far as I know, the weld has held up to this day.

BTW, this was in school, so plenty of precautions were made.

Lurvio

[mrjo]

Thanks for the input, sounds like purging with some inert gas makes sense, Michael I should clarify this is a timber yacht with steel fin, it hasn't suffered any grounding damage just poor maintenance, no anodes = corroding from outside. I was planning on doing the welding myself so to ensure an oxygen free environment is there a best option between Argon, Nitrogen or the exhaust from a combustion engine? obviously engine exhaust is the cheapest and easiest to organise.
Jo

[SeaJay]

In my industry, the guys are welding on live high pressures natural gas lines day in and day out. While always a very hazardous operation, the worst danger starts when oxygen (air) has been introduced into the pipe. The typical procedure is to purge with nitrogen. I've seen a half a mile of 12" pipe purged this way, and I'm sure that was just child's play compared to other situations. Noisy and expensive but when done correctly, safe. No oxygen, no boom.

[Landlubber]

...and it contains oxygen....do you really want to save a few bob to risk blowing yourself to the next place

[mrjo]

Hi landlubber,
I think my first post shows I don't want to make an early exit, just asking the question if the exhaust option is just as effective as the other 2 choices. when you say "and it contains oxygen" I assume you mean the exhaust does?
So it seems general consensus is Nitrogen, will find out how to purchase it.

Thanks, Jo

[Landlubber]

mate, any inert gas if OK, the cheapest will be nitrogen, but just go for whatever is easiest to use.