What happens to required thickness when increasing internal pressure while holding S and E constant?

• A. Increases nonlinearly
• B. Increases linearly
• C. Remains the same
• D. Decreases

Answer: (A)

Increasing internal pressure while keeping the allowable stress and weld efficiency fixed makes the minimum required thickness grow in a non-linear way. The design relation for a cylindrical vessel with internal pressure is t = (P D) / (2 S E − P) (ignoring corrosion allowance). With S and E constant, thickness is a rational function of P: as P rises, the denominator (2 S E − P) decreases, so t increases more rapidly than a straight line. In fact, as P approaches 2 S E, the required thickness would grow without bound, highlighting the nonlinearity. That’s why the correct choice is that the required thickness increases nonlinearly.