This is a questions that has been asked many times in this discussion forum - please learn to use the search function at the top of this page (find the "post a poll" button and just below it in the middle is the "search" hypertext). When you use the search function you can read all the wisdom of the many people who contribute to this forum. Also, you should go to the COADE "Mechanical Engineering News" site and download and read all the past issues as they present a wealth of practical piping engineering. (I love "cut-and-paste).