Pipe caps are not designed, per se, by the ASME VIII rules. ASME VIII, UG-11 allows for standard pressure parts that are pressure-rated and meet the requirements of one of several standards. A quick Google search yielded a number of websites that indicate ANSI B16.3 is the standard for forged steel pipe caps. I'm sure that if you check further you will find a website with the pressure rating information too. Hope this helps.
