Here is my 2 cents
I work at a natural gas compressor station, and we deal with gas in pipes that need cut and welded very often. feel free to PM me with any questions
Here is a quote from my build of what i did just a few days ago(the next one I will just fill with water then drain and cut)
On the topic of propane tank flush witch is always a hot topic around here I will describe my process
1- release any leftover gas
2 - remove valve from tank (be careful there may be some pressure left so remove SLOWLY)
3- I "purged the" tank with compressed air (this should be done with great caution because you are adding air to the tank and creating a gas to air mixture that is explosive)
4- I added about a quart of Super Clean degreaser and filled tank with water and allowed to overflow for a few minutes
5- Drained the tank and refilled with water, and then an immediate drain of the water, after this step it still had a heavy smell of propane, but all of the gas is gone, the smell is ethyl mercaptan an
oderant added to the gas so you and I can smell it.
ethyl mercaptan is very hard to get rid of
6- then I began to cut the tank
this entire process took me about an hour, from a tank with a little gas in it to torching the top off