Someone posted somewhere a couple of photographs of the destruction wrought by a CO2 cylinder that burst in a brewery. I think it was a 50 pound syphon bottle but the holes it left in the building were pretty impressive.
Anyway, you don't want to let any air get into the cylinders so store them with valves closed. Moisture inside will turn into corrosive carbonic acid when they are refilled if not thoroughly purged first and, of course, moisture leads to the possibility of rusting. I stored beyond 5 years past the last hydro check that will, of course, have to be repeated.


