welding tanks, new and used

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I hear a lot of conflicting advice on welding tanks.  I would appreciate any advice.  I have several tanks that need to be welded, all steel at the moment but could see aluminum and stainless some day.  some tanks are new, some have been used for water, gasoline or oil.  i realize the thickness of the metal dictates how hard the welding will be, but my main concern is safety.  i would rather error on the side of safety than to have an accident.all of the tanks can be taken out of service.  all can be de-pressurized and have an opening in them.  all of them could be filled with water during welding if need be.  i can certainly clean the welding area very well. i will likely pressure wash as best possible first inside and out.the reason for welding is one of two usually, repair or add an inlet/outlet.i have received advise ranging from, never do it to just fill it with water.  the simplest one is a really big water tank, hundreds of gallons and has a huge opening on top.  i was thinking that one can just be drained and welded and refilled after cooling.  the most complicated one is a 20 gallon tank that has gas in it now and a single 2.5" opening on top.  i want to add a bottom drain on that one if possible.  i was thinking to power-wash as best possible, then fill with water, close, turn upside-down, grind the welding area, drill the hole, and weld a female pipe fitting of some kind, then empty, power-wash again and let it dry in the texas sun for an afternoon.  i just don't want to do it without hearing opinion from others since again some people say don't weld used tanks ever, and others say don't weld tanks even new ones.
Reply:To error on the side of safety, is to not weld the tanks, but . . .For one thing, the tank must be vented.  A sealed tank will build up extreme pressure as the gas heats up during welding.  Search around for the video of a tire exploding when its rim was welded on.For another, I'm assuming you're not going to be inside the tank when it is welded.  That's a whole different story.The liquid fuel isn't particularly dangerous.  The vapor is, but only when oxygen is available.Any liquid fuel left behind (even residue on the surface, or sludge in crevices) can emit flammable vapors.  Steam cleaning is better at removing this than pressure washing.  The heat thins out hydrocarbons, and helps them wash away better than detergents.  Once it's clean, you want to purge the air if you can.  I recently welded the tank neck on my roto-tiller (although I admit that was plastic welding on polypropylene), and I left a hose connected to a CO2 regulator slowly blowing CO2 into the dry tank.  Stainless and aluminum do better with an argon purge (especially if you're welding with GTAW).I'm not recommending any specific procedures, but just some pointers to keep you in the safer direction.
Reply:appreciate the advice.  i will never be in the tank.  the really big tank has a really big opening, big enough to throw a basketball through.  since it had just water in it, i plan to just drain it, weld it (from the outside) and fill it back up after cooling.for the gas tank, i will look into a steam cleaner, but right now i don't have one.  I keep feeling like it should be ok if i pressure wash it and then have it completely filled with water when i weld it so that there is no air inside.  just to be sure i would find some way to hold the water in but still have a very weak cap holding it so that no pressure buildup could take place.
Reply:Safe or not I have welded on many gasoline vessels after rinsing well with H2O for 15-20 minutes. Hose in the bottom flow out the top type of thinking. I am not saying to use this just that I have. Remember it is the vapors on most hydrocarbon fuels that cause sudden injuries. Many a time I have set a blaze something on or near the weld with out suffering from some big boom event.Last edited by Fat Bastard; 01-10-2011 at 06:24 PM.