I just acquired a large propane tank that I plan to turn into a BBQ pit, any safe suggestions on how to cut it, the tank has had propane in it for twenty years. I have been told by one person that you have to fill the tank completely full of water and let it sit six months before taking a torch to it, is this true and is there a better safe way to do this? All help appreciated.
On gas tanks, when I was a kid, I was told by a welder to "wash them out with soap and water" and so have always done that. It's a pain to rinse all that soap out. ;)
Yep, that's what he said too. ;)
Solder, weld, cut whatever just have it so full of (rinse;) water that if it does pop (from the torch being rich if nothing else) it won't be big, so won't be anything. Wear you ear plugs so you won't have to stop and go change your shorts?
BTDT :) with a dremel and my pickup's gas tank. The filler pipe went all the way to the bottom of the tank and was a pain to fill (rear tank '75 F150) so I washed it out with soap and water and with water running out of the tank reached inside the filler tube and cut a couple slots into the "roof" of the filler pipe. It doesn't fill as good as the front tank where the filler pipe ends 1" inside the tank but it sure as hell a lot better now.
Also on a hot day gasoline would leak out of the cap for cryin out loud, the slots stopped that business too.
Hey, it can't catch fire if there's nothin to burn, right? ;)
Problem is some people don't realize, just because they don't see it, there might still be something there to burn. Like -new- invisible vapors created from the heat of soldering or even directly from the torch.
So that's the trick don't give it any -space- to burn and blow up. Might as well use water huh? ;)
Even if it's a brand spankin new gas tank or propane bottle... using a torch on it can still blow your ass up if you give it space see? ;)
Open every tap on the tank. Fill the tank with water until it runs out every tap. Keep pumping. Cut with torch. I figure it's a farmhouse sized tank, or you wouldn't be wanting to make a smoker out of it, so there should be a tap on the top centerline somewhere. Water comes out there and you're probably safe to cut anywhere below that point. Yeah, it's messy. You'll need to turn up the cut pressure to get in against the water pressure, so be ready to use a lot of oxygen.
This same thing was asked over on THEFOREGE listserv a while back and below is one of the responses:
Remove all the valves and fittings and invert the tank over a coffee can to drain out the scented oil that collects in old ones. I refill with hot water and detergent and let soak...drain, rinse, refill with the openings at the very top...no air space. Chip away lead based paint ( on old tanks)in a smallish area right on top and torch cut a port wet . I try to make it about
4" on a side. Next, cut your door free and drain....
I posted my big list of steps for doing this to sci.engr.joining.welding ages ago (groups.google will have it).
You need to vent it, then fill it with water. You can't remove _all_ the propane, because it's absorbed into the rusty inner surface . So instead you must reduce the ullage space (gas space above the water) such that even an inflammable mix of that small volume is no hazard.
Don't use an inert gas fill. Although it might work, you can't verify that it _will_ be safe. Water / air has an obvious waterline, CO2/air doesn't.
There's also the slightly hairy step of knocking the valve off, so that you can get the filling hose in. These valves rarely unscrew, so you're often having to saw the valve base off below the valve. There's still a risk of fire at this point, but as the tank was already emptied of all liquid gas, the flame can't be too big and there's no risk of BLEVE.
The mercaptan stenching agent stinks, so you might want to wash the tanks out too. Add detergent to a filled tank, not an empty one and then add water, otherwise you'll have a yard full of foam before you get any water into the tank. You'll still need to heat the opened tank to burn off the paint and the smell.
Been out of town, you've probably already done this. I have researched this problem and am confident in my solution, in fact I've cut several propane tanks. People who have never done this say silly things like "fill it with water" - easy to say but very difficult or impossible to do! I wrote up what I do to safely prepare old propane tanks for reuse: