Which factor governs the high-temperature design stress for superheater tubes?

• A. Long-term creep strength
• B. Thermal expansion rate
• C. Fatigue resistance
• D. High-temperature allowable stress

Answer: (B)

At elevated temperatures, the stress in a superheater tube is mainly driven by how much it wants to expand and how much its ends are restrained. When a tube heats up, it tends to lengthen, but if its ends are fixed to headers or supports that don’t allow that expansion, thermal restraint creates axial stress. The magnitude of that stress depends on the rate and amount of expansion the tube experiences as temperature rises and on how tightly the system constrains the tube during heat-up and cool-down. So the rate at which the tube expands with temperature directly governs the design stress you must allow for to avoid overstressing during transient thermal events. Long-term creep strength and fatigue resistance matter for life and durability, but they don’t set the immediate high-temperature design stress in the way that expansion restraint does. High-temperature allowable stress is the limit used in design, not the factor that governs the stress itself.