Question ... If you build a pressure vessel entirely from piping components such as reducers, end caps, flanges and tees ... is it a vessel or is it a bunch of piping elements ? Answer ... depends mostly on the classification (insurance) company and the owner. Generally if it has a piping line number, it's a pipe, if it has a vessel number, the vessel design code applies. It's not a pipe stress engineer's job to be defining or guessing these things.