Definitely don't use anything other than a round shape for a pressure vessel. The stresses are pretty impressive, even at a relatively low pressure. I'd suggest 3" pipe if you're planning to go that route, as the cross-sectional area of a pipe increases by a factor of six as the diameter doubles. You'll restrict your volume too much by using 1" pipe, and when you use enough of it to get the volume you want, it will weigh more than a piece of 3". Welded schedule 40 pipe will hold all the pressure you need.
Jerry
