I've welded a bunch of fuel tanks- - - -even did a braze fix of a fuel fitting leak on top of one about half full of gas one day. The trick is to totally fill the interior of the tank with a non-flammable gas like nitrogen, CO2, or even exhaust gas from a very rich-running engine. No oxygen to support combustion- - - - -no kaboom. Pressure vessels don't have to have a fire to do damage- - - -the fracturing of the metal itself is the dangerous part.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!