Superheater Tube Bulge After 90,000 Hours
A SA-213 T22 superheater tube has a 51 mm outside diameter and a 6 mm wall. It operates at 100 bar steam pressure and a calculated mean metal temperature of 540°C. The design life is 100,000 hours. At 90,000 hours, one tube is found bulged and cracked longitudinally. The failed tube has 0.4 mm of fireside scale and 0.1 mm of steam-side scale. Its hardness is 75 HRB; nearby unfailed tubes read 82 to 85 HRB. Metallography shows aligned carbide particles and creep voids at grain boundaries, with no fatigue striations. The plant wants to restart after replacing only the failed tube and argues that the remaining tubes still have 10,000 hours of design life.
1. Calculate the nominal hoop stress and explain whether it alone explains the failure.
Read the worked answer
Thin-wall hoop stress: sigma_h = pD/(2t), with p = 10 MPa, D = 0.051 m, t = 0.006 m:
sigma_h = 10 x 0.051/(2 x 0.006) = 42.5 MPa.
At 540°C, T22 has useful creep strength, but 42.5 MPa is not a comfortable margin for 100,000 hours, and the calculation uses nominal wall and mean temperature. Local metal temperature can be higher because of the 0.4 mm fireside scale and the 0.1 mm steam-side scale, and the bulge indicates local wall thinning or local overheating. The hoop stress alone does not explain why one tube failed; the localized scale and temperature are the differentiators.
2. Interpret the hardness, scale, and microstructure evidence. Identify the mechanism and rule out fatigue.
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The evidence points to creep damage from long-term overheating:
- Hardness of 75 HRB is below the 82 to 85 HRB of neighboring tubes. T22 softens as carbides coarsen and the tempered martensite/bainite structure degrades; this is consistent with overaging and creep exposure.
- Aligned carbide particles indicate spheroidization and coarsening, a thermal aging effect.
- Grain-boundary creep voids and a longitudinal bulge are direct evidence of creep cavitation and diametral strain.
- The fireside scale of 0.4 mm raises the tube metal temperature; the steam-side scale of 0.1 mm also adds insulation and can indicate poor steam-side conditions.
Fatigue is unlikely: there are no striations, and the crack is longitudinal and associated with a bulge rather than a transverse crack at a restraint. Corrosion fatigue would also usually show striations or a different crack path. The 90,000-hour exposure with a hardness drop is consistent with creep life being consumed faster than the nominal design assumed.
3. Recommend the inspection scope and life-assessment actions before restart.
Read the worked answer
Do not restart after replacing only the failed tube. The neighboring tubes show the same aging trend and may be close to failure.
Inspection scope:
- Hardness survey of the whole superheater bank, with special attention to tubes with hardness below about 80 HRB.
- Measure steam-side and fireside scale on representative tubes; replace tubes with steam-side scale above about 0.2 mm or evidence of blockage.
- Check for fireside slagging, gas-side imbalance, and burner tilt that could create a local hot zone.
- Metallographic replication or cutout samples for creep void classification on the hottest tubes.
Life assessment:
- Use the measured hardness, scale, and metal temperature to estimate the remaining life, not the original 100,000-hour design life. A tube at 75 HRB after 90,000 hours has already accumulated creep damage.
- Replace tubes below the hardness threshold and those with creep voids. Consider upgrading to T91 or increasing wall thickness if the temperature cannot be reduced.
- After restart, monitor metal temperature and install thermocouples on the hottest tubes. Reduce temperature or load if the measured temperature exceeds the design value.