Hi Grant,

I spent a while yesterday looking at people's project pages and came
across your propane tank degassing page. I've considered reusing propane
tanks on several occasions, but had a different method of degassing in
mind. Before I get to the details I should point out that our propane
tanks (in the UK) might be a little different from yours, and that I'm
talking about the largest (47 kg) variety.

I'm not thinking of trying this right now, so this post is just for your
interest really. These tanks appeal to me because, having built a few
high voltage machines, I want a source of large conductive metal spheres
(or near spheres) to use as discharge terminals. The custom made
aluminium spheres cost hundreds of pounds each, whereas as you've
already pointed out, these tanks are often free. Our tanks have a convex
top and bottom, and my idea was to remove two tank bases and stick weld
them together to made a large conductive dome. The base ring and
handle/valve protector could be removed from the cylinders easily. The
resulting terminal would be damn near indestructible, unlike the usual
spun aluminium spheres which are easily dented.

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.

Best wishes,

Chris