Final answer:
The plates of a pressure vessel are rolled around the circumference to align the material's grain structure with the circumferential stress from internal pressure, providing optimal resistance to the forces exerted by the contained fluid.
Step-by-step explanation:
The plates for the shell of a pressure vessel are usually formed with the rolling direction around the circumference. This construction technique aligns the grain structure of the material in a way that optimally resists the circumferential stress exerted by the internal pressure. Fluid pressure is exerted in all directions, and the grain of the material provides strength where the vessel experiences the greatest stress, which is in the circumferential direction. The forces due to fluid pressure are always exerted perpendicularly to any surface, and designing the plates in this manner ensures that the structural integrity of the vessel is maintained under pressure.