Its all in Asme bpv. Carbon steel has a design stress of about 120Mpa, Ti Gr 2 is about 90ish, Ti Gr 5 is over 220 mpa - from memory though, the book is at work.

There is a specific set of formula to plug the numbers in to which spits out the minimum shell. Asme is conservative, but it is pretty much standard throughout the industry I work in. Other standards may give lower answers but they will be in the same ball park.