Circumferential vs Longitudinal Stress of a Cylindrical Shell in ASME VIII Vessels; 

Let’s look at the formula for calculating the Cylindrical Shell thickness of the ASME Section VIII Pressure Vessel. We can see that the Minimum Thickness calculated from Circumferential Stress is approximate twice the thickness calculated from Longitudinal Stress. This is because the Circumferential Stress is twice as much as Longitudinal Stress. Therefore, when calculating Shell thickness, use the formula of Circumferential Stress.

Refer to: What is Hoop Stress in pressure vessel?