Here are some links hte explain the mathematical formula's for different situations where hoop stress can build up and how to calculate for design as well as a link to a software tool to figure out all kinds of stresses as well as other types of situations when one is designing a piping system.. In short, most if not all of the links cover Hoop stress as it relates to metallic piping and some will explain it better than others. ;0
I have more but first, I'm going to post about a book I just found that will definitely be of everyone's interest in here so, I apologize for not being as thorugh as I usually am. Rest assured that when I complete the new post, I will return to give you some more links to look at so you vcan have as much resources available in order to get a better understanding what Hoop Stress is all about and how it relates to your application. ;)
This one is from a company in Germany that makes cladded thick wall pipe, and shows how the effects of PWHT as well as a technique named "impansion can alter the hoop & Axial stress at the seam of the pipe in comparison of the two methods of stress relief, and shows a distinct difference in outcomes depending on hte alloy being stress relieved using the two techniques:
Well, that's about all I've got for now, so enjoy the rest of the readings and take a good look at the article from Walter Sperko of Sperko Engineering! ;)