Is there a maximum thickness a vessel head/shell can be under the rules of ASME Section VIII, Div.1? If so, is it further limited by material selection (ie; P1 may be thicker than P8)?

I don't know why, but I am under the impression that 8" is the maximum permitted for butt welding (forged vessels may be different) and you would have to consider going to a Div.2 or Div.3 design beyond that.

If anyone has a code paragraph to steer me towards will be appreciated.

maximum thickness is governed mainly due to availability of the material and limitation on fabrication processes (specially rolling of high thickness). Finally it comes to economics of material & fabrication cost.

Refer to section UG 27 for the limits given based on t/d ratio but appendix 1 provide supplimentary rules if this ratio exceeds.

Thank you for your reply, Sanjeev.

I found my answer elsewhere, although not entirely reflective of my original post. Section II, Part A, SA-516, paragraph 1.3, limits the thickness to 8" for grade SA-516-70. Although not properly identified in my original post, as a person who has worked primarily with carbon steel, this is the number that was stuck in my mind.