I successfully devalved a little propane tank today, the kind they don't want you to use any more. I bled it off for a couple of days, with the tank set at the top of a largish sloped driveway so the gas would run down the hill and diffuse. When it finished hissing I turned it upside down with the valve open for a few hours. Then I took it into the shop and plumbed an air hose to it and pressurized it to 120psi and vented it, then repeated the air venting twice. I tried to get the valve to admit water but it plain didn't work. I cobbled up a chain clamp setup like Ted Edward's, heated the threaded area with a handheld propane torch until the paint barely started to smoke, and turned the valve out easily with a large crescent wrench.
It was a lot easier than I'd thought it would be based on all the horror stories. I filled it with hot water and a little dishwashing detergent for a first cut at de-stinking it (I don't much like the garlicky smell of methyl mercaptan). If anyone has any bright ideas as to how best to get the stink out of the tank I'd like to know about it.
I'm going to make a little forge.
Grant Erwin