propane tanks do not explode all the time like on the movies.. i forget the ratio exactly but it is like 2.? to 9.? parts per million to ignite. i routinely service lp tanks and on asme tanks when i have to replace a part, and the tank is mostly full, i open the valve (in a ventilated area) wide open until the lp freezes. then i do what i need to.
dot tanks i would connect a hose and burn off or expel the gas, remove the valve and follow some of the suggestions above.
years back i have welded on partially full gas tanks, with a torch, after inducing halon. however our govt says halon is bad. like coffee or beer