Diesel Tank Bomb

Discussion in 'Boat Design' started by Katoh, Mar 9, 2011.

  1. Katoh
    Joined: May 2010
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    Katoh Senior Member

    I am in the middle of a full strip down and restoration of an aluminum 20' boat, I have removed the under floor fuel tank which sat in its own fully water tight well. The tank was bolted and foamed in which has now caused me a little problem, The tank was under water when removed and were in some places the foam had really taken to the aluminum sides, an amount of crevice corrosion is present.
    I have removed the tank and cleaned up the outside with high pressure washer to expose all the corrosion spots, while hosing down the inside I also found a leek on the bottom of the tank, maybe others. The tank is out of the boat, all fittings have been removed including float and sender unit. I plan to fill the tank with hot soapy water and let it sit for a couple of days, then rinse with clean water.
    I only have a mig with a spoolgun, that I was going to use to fill all the crevices with, and fix the crack, I know tig maybe better but I don't own one, nor do I have access to one.
    The last thing I want to do is turn the tank into a bomb, being diesel I cant see it being explosive but you never know.
  2. Bruce46
    Joined: Jul 2006
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    Location: Stuart, Fla.

    Bruce46 Junior Member

    The best advise I can give is throw that tank away and start with a new one. You might want to consider one of the new plastic tanks, no corrosion problems.
  3. Katoh
    Joined: May 2010
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    Katoh Senior Member

    The big problem is that tank was custom built to fit in that fuel well, there's no way I'm going to go altering that because of the hull design, just not going to happen. There is little chance of finding anything with that configuration and size pre-made, new tank would have to be custom, now were talking serious $. Just looking at the plastic tank OK you buy for 250-300 dollars but shipping to Au add an extra 1.5K, Ill fix the original first!
  4. michael pierzga
    Joined: Dec 2008
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    michael pierzga Senior Member

    Cut a good size inspection port on the top and carefully inspect the inside of the tank...tanks also corrode from the inside...no use wasting time.
  5. MarEng2
    Joined: Mar 2011
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    Location: Tejas

    MarEng2 New Member

    Based on personal experience you should be fine welding this tank. I have had empty 3,000bbl tanks with diesel residue (clingage and scale) catch fire before. They burn sedately and are easily extinguished. With the washing you've already done I doubt you'd be able to get your tank to light.

    You state that the access covers and sending unit are removed - keep them off while welding. I know I'm stating the obvious but you can't have a bomb if you can't build up pressure. Also, you need to have a relief path for the heated air from welding.

    I speak only about the safety side of the repair - I don't weld aluminum and have no comments on the efficacy of the proposed repairs.
  6. bntii
    Joined: Jun 2006
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    bntii Senior Member

    Maybe get throw a little pressure into tank to look for pin holes.
    Soapy water outside and look for bubbles- look carefully along all the welds.
    As mentioned a peek inside is worth the effort.
  7. tazmann
    Joined: Aug 2005
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    tazmann Senior Member

    Last aluminum truck tank I repaired I did the same washed it out several times then when I went to weld on it I ran an exhaust hose in it and let it purge for a while. then I welded it up with spool gun with the exhaust still blowing in there. took a couple passes because the exhaust does not enhance the weld on the first pass.
    The best and safest bet would be build a new tank.
  8. DianneB
    Joined: Jan 2010
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    Location: Manitoba

    DianneB Junior Member

    I have done numerous fuel tanks and I usually start with a wash ("Marine Clean" is good). If I am concerned about vapors, I put the hose on the discharge end of the shop vac and leave it blowing into the tank while I am working.

    There is a great product called "US Standard Fuel Tank Sealer" that puts a good fuel-proof coating over the inside of the tank - it looks like the old enameled cookware when you are done! It seals any small leaks and provides a nice clean surface and stops any dirt or rust from getting in the fuel. Every time I remove a fuel tank, it gets a coat of sealer as a matter of routine and I have never had a problem after.
    1 person likes this.
  9. pdwiley
    Joined: Jun 2008
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    pdwiley Senior Member

    Standard practice when I did my welding tickets 20+ years ago was to rotate a tank so the weld was uppermost and fill the tank with water to within 25mm or so of the top.

    No or minimal air space, no fuel-air mix, no possibility of a big bang.

    Back-purging with inert gas would work too.

    There's a company on Ebay Australia advertising polyethylene diesel tanks, don't know if they have a suitable size/shape for your needs though.

  10. Katoh
    Joined: May 2010
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    Katoh Senior Member

    Thanks for all the feed back, I used a plumbers camera or snake camera to have a look in the tank, and it looks in good shape, the pitting is all on the outside, were the foam was, I should elaborate its not all over the tank but a bit here and there, actually there's no pitting on the base at all.
    I was planning to used compressed air to find the leaks, one I know off, and any others I dont. When I removed the tank from the boat there was no fuel leak present, I must have just popped it now with the high pressure washer. I do like the sounds of that "US Standard Fuel Tank Sealer" I have to research it little more.
    As for plastic tanks I don't think so in my case, The tank is 4' x 2' x 10" approx, It bolts from either end into the well it has L shaped stringers welded to the top as supports for the checker plate floor above, the filling points are side mounted that run into their own well leading to the side of the boat. Its that tightly fit I cant even fit a well pump in there anywhere.
    Many thanks
  11. Poida
    Joined: Apr 2006
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    Poida Senior Member

    Watch out with the compressed air leaking method.

    Most compressors will pump to at least 6 bar where a fuel tank will only need to withstand 0.1 bar it most probably will withstand more than that but not much more. I would certainly suggest a pressure regulator in line and not to exceed 1 bar.
  12. Katoh
    Joined: May 2010
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    Katoh Senior Member

    You are dead right! I was not planning on more than 20psi, don't want to create more holes than needed.
  13. El Sea
    Joined: Aug 2006
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    Location: St Petersburg, Florida

    El Sea Junior Member

    UL recommends round tank to be tested at 5 PSI & rectangle tanks at 2-3 PSI.
    We find most alum. fuel tanks that are approaching 20 years of age to be problematic if encased in foam.
  14. rcwade
    Joined: Mar 2011
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    rcwade New Member

    Por 15 has saved me many a headache on restoring auto tanks. Not sure on the size of this tank but it may be something to look into. Seals the tank and you don't have to worry about welding up the holes.

    Joined: Oct 2002
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    FAST FRED Senior Member

    "You are dead right! I was not planning on more than 20psi, don't want to create more holes than needed."

    5 psi is enough. 20 will blow every marine fuel tank!

    Fill the tank with water cap it , and take a hose 10-11 ft up filled with water .

    That's all the "test" you need.

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