Our NDT Technicians measured via UT the thickness of an in-service vessel shell wall to be around 0.350" thick.
The U-1A lists the Nominal Thickness as 0.250".
I calculated the min required wall thickness per ASME BPVC Section VIII para UG-27 to be 0.158". [ri=17.75", P=150 psi, S=20ksi...