What primarily governs superheater tube design stress?

• A. Low-temperature properties
• B. Pressure rating
• C. Thermal expansion
• D. High-temperature allowable stress

Answer: (D)

In high-temperature service, the limiting factor for tube stress is the material’s strength at the operating temperature, i.e., the high-temperature allowable stress. Superheater tubes run hot enough that creep and long-term degradation reduce the metal’s strength far more than at room temperature. The design uses the temperature-dependent allowable stress from the code, which accounts for this reduced strength over time, to ensure the tube won’t fail under sustained pressure and heat.

So even though pressure contributes to the stress, the maximum safe stress is set by how much stress the material can safely sustain at the high temperature. Low-temperature properties aren’t governing here, and while the tube must withstand pressure, the critical factor in determining design stress is the high-temperature allowable stress. Thermal expansion influences fit and clearances, not the primary stress limit in this context.