What primarily determines the required thickness of a steam drum under internal pressure?

What primarily determines the required thickness of a steam drum under internal pressure?

• A. Longitudinal stress formula
• B. Circumferential (hoop) stress formula
• C. Weld strength
• D. Corrosion allowance considerations

Answer: (B)

The circulating, membrane stress around the shell due to internal pressure is the circumferential (hoop) stress, and it is the largest stress the shell experiences. For a thin-walled cylinder, hoop stress is sigma_h = p r / t (or p D / 2t). To keep this stress within the material’s allowable stress, the thickness must satisfy t ≥ p r / S_allow. Since hoop stress is twice the longitudinal stress, it controls the minimum required thickness. Weld strength and corrosion allowance are important considerations, but the value that sets the required thickness under internal pressure is the hoop stress.

The circulating, membrane stress around the shell due to internal pressure is the circumferential (hoop) stress, and it is the largest stress the shell experiences. For a thin-walled cylinder, hoop stress is sigma_h = p r / t (or p D / 2t). To keep this stress within the material’s allowable stress, the thickness must satisfy t ≥ p r / S_allow. Since hoop stress is twice the longitudinal stress, it controls the minimum required thickness. Weld strength and corrosion allowance are important considerations, but the value that sets the required thickness under internal pressure is the hoop stress.