Please be careful!
John Wright

edit: I think there are standards or codes governing how to do this correctly for containers that have previously contained flammable liquids or vapors. Hopefully someone will chime in that knows where to point you to that info.

Don,

Here's a start:

http://www.aws.org/technical/facts/FACT-06.PDF

AWS F4.1 applies to your particular situation. Your library should have some or maybe even all of the referenced standards. You can purchase them online by doing a search for them.

Dave

There obviously is a risk in doing the repair. Since it got a hole "ripped" in it, is it worth fixing considering the risk?

First thing to do is have a "change of address" card in your pocket....you may need it.

Hi Don
I'm the guy that ends up with projects like this because the other guys make excuses instead of just getting the job done.
First off, can you make a sound weld? MIG or stick I'd stay away from brazing.
I've made several similar repairs by filling the tank with h2o while positioning the repair area at the top so that as much flammable air as possible is purged from the tank. Ideally you would not want any air but I try to eliminate water at the repair location to eliminate weld contamination.
If the tank is opened up with gaps over 1/8 -1/4" don't try a repair bead. Do it right and remove the damaged area and plate it with a similar thickness mat. Extend your patch past the existing by at least 10x mat. thickness and make a simple lap joint a sound weld around the outside will do. Preparation is everything, clean everthing to be welded with a grinder. Get a tight fit up between plates, tacks every 2", (a single hammer tap to the tack welds immediatly after the tack is made will help). Whatever process your using take time to set the machine up and try a practice piece of similar mat. Don't forget to test it when your done. I wouldn't trust the H2o already in the tank for this.(density) A little deisel fuel will show you any leaks in a hurry and its simpler than a pressure test.
Oh by the way I got one my first welding promotions when a fellow pulled up with a 55 gal drum and said "hey can you make a burn barrel outa this for me"? My coworker drug a torch out the door just before I heard the explosion. The lid peeled right off the can, we found it later a 1/4 mile away. He returned to work after 2months off. That days lesson "always check for gas!!!!!!!"
Thaks and good luck
Randy

There's no risk involved if you take the proper precautions. But are you prepaired to take the proper precautions?

First you need a combustible gas meter. Then blind, disconnect, plug or remove any piping attached to the tank. Flush and purge the tank with water or steam to completely remove any petroleum residue. Ventilate the tank for at least 8 hours with mechanical ventilation.

After that, turn off the ventilator and test the atmosphere inside the tank with the combustible gas meter. Less than 10% LEL is the industry safety standard for welding, cutting and confined space entry. But I've never seen or issued a hot work permit that wasn't 0.0 percent LEL.

Good to go? Not yet. Now you need a fire watch, a fire extinguisher and an emergency action plan in case something goes wrong. A telephone and the number of the fire/EMT agency that can respond in a reasonable amount of time.

I know that sounds like a lot, but we do it all the time and believe me "safety doesn't cost, it pays." We always get the job done and the consequences of not doing it safely are too severe to contemplate.

Edit: Well I liked whiteyford's advice especially the part about tapping the tack welds with a hammer. He gave you the information you were looking for, not a sermon on safety, LOL! Sorry to sound so preachy. Just be safe and try to keep some of my recommendations in mind.

http://www.fortunecity.com/village/lind/247/weld_book/Ch2.htm#s5
I hope this is not to late but this site leads to military meathods of prepairibg tanks with flamable liquids.I dont know how to type this web site so you can just click on it?