What is the effect on required wall thickness when pressure increases while S and E remain constant?

• A. First increase then decrease
• B. Increase
• C. Decrease
• D. Stay the same

Answer: (B)

The essential idea is that the wall must carry the internal pressure without exceeding the allowable stress of the material. For a cylindrical vessel under internal pressure, the hoop stress is roughly proportional to P and D divided by the wall thickness, so keeping stress below the allowable limit (S with the weld efficiency E) requires t to increase as P increases. With S and E fixed, increasing pressure raises the required thickness (t grows roughly in proportion to P). Therefore, the correct result is that the required wall thickness increases as pressure increases.