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Here is what I was told by the propane folks when I re-purposed some tanks.  They definitely had propane in them, until I emptied them.

Their instructions require getting the valve screw out.  I could not, so I attached a hose to the tank to purge all gas and keep the valve open while I removed valve.  We won't discuss removing the valve of the first tank.

The propane folks said to fill the tank with water a few times and empty it.  The water pushes the gas out so fill to over flowing.  You will have to shake the bottle a little to get the air bubbles out of the upper curves.  They have gas in them.

Cut with with your preferred method.  I used a disk on one and a plasma cutter on the other.

When I got one open there was a thick oily goo in it.  No idea what it was but it did not ignite when I burned it out with a torch.  The other tank did not have anything in it.

Now I have a propane tank forge in the making and a propane tank for a foundry when I get around to building it.

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  • 5 months later...

Just to add what I had been told, the process is essentially what natenaaron said, except that liquid detergent should be added to the water solution. I was also told that, after the valve has been removed but prior to the rinses, it should be hung upside down for about a month. It just seems like too much hassle; I bought a helium tank from Target for $20, emptied the helium, and used it immediately. That tank is a temporary tank as per the USDOT specification, but I actually got in touch with a materials engineer at the manufacturer and he said that (very unofficially) it is fine for foundry use as he used one for the same purpose. 

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Since you said "brand new tank", check the label:  Most brand new tanks (never filled or used) have been nitrogen purged so they don't get condensation on the inside and rust while waiting to be purchased.  The label usually says so.  In that case, the risk is low...BUT....

NEVER ASSUME!  so it pretty much makes the point moot.  You don't want to be put on the "Darwin Award" list, even as a runner up.

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I always test the valve, I have in the past got a full tank on which the valve spindle turned freely, it would not have been a good idea to unscrew that valve.

to test it I hold an air line on it with the valve open and listen for the air going in, then remove the airline and you will hear and feel the air coming out, no air movement means the valve is closed and the tank MAY BE FULL, DO NOT USE THIS TANK.

if ok remove valve, fill with water, shake, leave a while, empty, fill with water and washing up liquid ( dish soap ), shake, leave for a few days, empty and cut

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Since you have or will be purchasing the connector fittings, regulator, etc for your burner attach it to the tank that is going to be cut up and open it up and see if anything comes out.  Be aware that in many places the government requires the new fool resistant valves on the tank which shut down if it detects a leak.  It used to be you could just hook up the connector and open the valve.  The new valves will drastically slow the flow, maybe even stop it, if it senses a leak.  Thus you need to have a valve before the burner and then open the tank valve slowly, once open, open the second valve.  This all should a lot more complicated than it is, you just want to make sure it is really empty.  Once the tank is confirmed empty just unscrew the valve and flush out the tank.

I have never had a tank with goo in it.  Maybe that is a regional thing only found in places like San Francisco or Indiana. 

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