Everyone here has proposed bits and pieces of the complete process to SAFELY purge a tank. Unfortunately, without a gas tester there is no way to know you are 100% sure of no reaction when you start putting fire into the propane vessel.
I researched this topic a few years ago when I had to convert a 12,000 gallon propane tank into a compressed air tank. I needed to cut the tank open to have a 24" manway welded in, an 8" air port, and some low point water drains. It was empty, but had a few lbs of pressure on it from a bit of propane when we received it. I had to find a method to guarantee that when the welder put the torch to it we wouldn't be finding him a ride home from the moon.
If you are interested in reading a thread about it and all the details on an engineering forum, it's here.
The long and short of it is you have to purge the tank to get the residual propane gases and any other bad stuff out. As TopShot points out, he receives tanks that have had their valves removed for an unspecified time- and has lucked out that they have been opened long enough to have naturally vented/purged all the gases. Since propane is a product, it has specifications and rarely has much in the way of oils in it. They are a different animal to deal with when purging tanks. (Our tank was dry as a bone when we opened it.)
Purging is a process where you put a gas, like nitrogen, into the tank, via a tube, into the furthest corner from the vent opening. You want the flow to be carrying the propane residual OUT of the tank.
There's been lots of talk about purging with all sorts of gases...I don't know who wants to use helium, its bloody expensive. Nitrogen makes up 70-odd % of our atmosphere and is cheap.
So you need to put a gas sniffer on the purge discharge when you want to test to see if you are good to go. It should read 0 hydrocarbons when you are good to go. Nothing.
With our 12K gallon tank, we first did a purge using nitrogen. Used enough bottles to fully purge the tank, 20 or so. Hardly changed anything. Scratched our heads, and stuck a compressed air hose in it over a weekend. Two days later, did another test. MUCH better. Did more compressed air and finally we sniffed zero. Purged once more with nitrogen and checked on the morning of the cutting. Still zero. Home free.
With tanks containing oil or diesel, there is a phenomena called "bounce" where the tank may be purged clean but the residual hydrocarbons on the surface of the metal start recontaminating the air if the tank sits for any time. I do not know how this is properly dealt with.
So cutting a tank containing flammable fuels is something like dealing with refrigeration...you can do it without the fine knowledge and special tools, and 99 times out of 100 you won't get caught. Get the learning, the right tools, and do the job safely.
Jon.
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