Here local dumpsters won't take 20 pound propane cylinders, you know the ones with the old style valves. The valves use 3/4" NPT fitings. If using one or more as an aux. tank, I would place them downstream on piping system, and mounted so outlet is at the bottom. This makes them self draining. They wiil more than stand pressure you are talking about. But still using pancake comp. is like putting lipstick on a pig- It is still a pig!