Christopher Tidy wrote:
My plan was to take an "officially empty" cylinder out into our field
(we have no neighbours nearby) on a cool day, make sure it was away from
any sources of ignition and slowly open the valve to vent the propane to
the atmosphere. When the propane inside was at atmospheric pressure, I
intended to remove the valve and fill the tank with water from a hose to
expel the remaining propane. Not having removed a tank valve, I'm not
sure how easy it would be to fill the tank with water, but it seems like
a neat way to remove all the remaining propane. What do you think? I'd
be interested to hear your views.

The bleeding off of the gas I did slowly i.e. I didn't open the valve fully, I
just cracked it and let it hiss slightly for several hours. When it stopped
hissing then I brought it inside to warm it up (I did it on a cold day) and then
repeated it. Then I opened the valve fully and turned it upside down and let it
get as aired out as it would (I'm not sure this actually does anything). Then I
pressurized the tank to about 140psi with compressed air and vented it again. I
repeated the pressurize/vent cycle 3 times. By that time I figure what was left
inside wouldn't burn, too diluted.

Then I clamped the tank, heated the neck to release the thread locker, and,
using a big wrench, I unscrewed the valve. No point at all in trying to get
water into one of those valves, gotta get it out fully. With the valve out, then
I ran it mostly full of hot water and poured in a cup or so of household bleach,
then plugged it and shook it all up (for a larger tank, I'd have rolled it
around for a couple of minutes) then drained out the solution. No more stink.

It isn't easy to remove the valve without a way to rigidly clamp the tank. With
the tank clamped, however, it's pretty easy as long as you use heat.

Grant