Life assessment of a superheater tube in coal-fired power plant

The failure of a superheater tube in a coal-fired power plant, designed to resist creep damage at high temperatures and pressures, was investigated after it ruptured following 115,360 hours of operation, which was significantly shorter than its expected 200,000-hour life. An oxide layer measuring 106 μm was discovered on the inner side of the tube, which acted as a thermal insulator, raising the tube's temperature and reducing its thickness. This led to increased stress, decreased creep resistance, and a shortened tube life. The oxide layer's growth, tube temperature, and stress were estimated. After 115,360 hours, at a flue gas velocity of 22 m/s, the oxide layer reached an estimated thickness of 120 μm. The outer and inner tube surface temperatures were 522 °C and 510 °C, respectively, and the stress was estimated at 79.9 MPa. To extend tube life to the designed 200,000 hours, it was determined that reducing the flue gas velocity to 15.8 m/s or lower would be required. At this velocity, the tube’s creep life was estimated at 200,070 hours when accounting for tube thinning. Without accounting for the reduction in thickness, the estimated life was 210,650 hours, which was approximately 1.2 years longer.