Welding diesel fuel tank

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I need to braze a fitting onto the valve at the bottom corner of a 300 gallon diesel tank (empty).  Would it be safe to add five gallons or so of water to the tank and tip it so that the water collects near where I need to braze?  I don't want to deal with getting rid of 300 gallons of oily water.  Is there another safe way, besides filling it with car exhaust or inert gas?  The valve and fitting are on the outside of the tank, but close enough to be worried.  i appreciate any advice.
Reply:I have welded fuel tanks on my brother's tri-axles diesel fuel won't explode unless mixed with air and compressed, as in a engine. It will flame up but not explode. Don't use water it will act as a heat sink and take for ever to heat. Braze or weld away.
Reply:I have welded diesel tanks on trucks with fuel in them. I ran the exhaust in there to be sure but diesel is no problem.Yes I can tack it for you.    I know that won't hold but that is what you asked me to do.  Millermatic 350P XR Aluma-Pro push/pullMillermatic 185/spool gunMiller Dialarc 250 HF Water cooledThermal Dynamics Plasma
Reply:An empty oily tank is worse than a full tank. This is a fitting on a valve tho, so in this case the water as a heat sink just might be the ticket. Purge also if ya want but do not seal up that tank while brazing.
Reply:I have also welded on a few diesel tanks some with fuel still in them. A good friend did have one explode on him. the tank was empty at the time. I have been told to make sure that the tank has never had a fuel additive mixed in before. i always ask the driver and then when they say no or I don't know. Which they always do, I tell them then you won't mind standing beside me while I weld it. I would take every safety measure you can before striking an arc. You never really know if only diesel has been in that tank.
Reply:Originally Posted by Robby T.I have also welded on a few diesel tanks some with fuel still in them. A good friend did have one explode on him. the tank was empty at the time. I have been told to make sure that the tank has never had a fuel additive mixed in before. i always ask the driver and then when they say no or I don't know. Which they always do, I tell them then you won't mind standing beside me while I weld it. I would take every safety measure you can before striking an arc. You never really know if only diesel has been in that tank.
Reply:I've welded a few tanks but they always had fuel in them. I have my concerns on this project. How close to the actual tank is the fitting? Close enough for fire inside the tank kind of worries? 300 gals is a pretty good size tank to have a fire in
Reply:Just curious but why would you braze a fitting to a valve???   I have never seen a valve that was on a fuel tank that was not threaded for NPT.  As long as you have some threads you can adapt fittings to get where you want to go.  Also as soon as you heat up the valve to start brazing you are going to burn the sealling material out of the valve and then you just ruined the valve.  Just my $.02  ~JacksonI'm a Lover, Fighter, Wild horse Rider, and a pretty good welding man......
Reply:I wanted to braze a copper tube through a drilled fitting and into the tank a few inches off the bottom.   I didn't have any fittings that would allow the fuel tube through, so I scrapped the plan and installed the valve and filter  without the sludge-avoider.  So, no brazing this time, but I appreciate your concern and responses.  I'll be keeping my eye out for a hose to squirt car exhaust into the next one.