As far as I know, if the design pressure is below 15 psig, it is not possible to use section VIII "U" stamp"
Also, above a certain diameter ratio ( which relates to design pressure), the code recommends you use special rules or use an alternate code for ultra thick vessels. This upper limit may come into play for diameter ratios greater than 1.6:1
For very thick vessels, it may be wiser to not use a monolithic wall and instead use the special rules for ribbon-wound multi layer pressure vessels ( common in ammonia reforming work)
As the prior message indicates, it is best to read the preamble to each code to confirm its scope.