There are several ways to purge a tank but here is the only one that regular guys cannot mess up.
1. Take the tank to the local propane dealer and ask them to remove the valve.
2. Fill the tank totally full with very hot water.
This is because there are heavy distillates that hold propane like CO2 in a soda pop. You need to dislodge all the gas from the oils that could be left in the tank.
3. Pour all the water out.
4. Fill the tank again with any water (hot/cold no matter).
This will be the final purge just to be extra safe.
5. Pour off just enough water to expose about 2" air space and cut out a big hole or remove the very top completely.
This will ensure that if any gas or flammable dregs possibly remaining will be ventilated to oblivion making it non flammable.
6. Pour out water and cut away to your hearts content.
After all that, there is no way any gas could still be in the tank unless you put it there by letting your torch pop out of something like that.