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

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.

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.