Impact of oxide layer on performance and creep life of a superheater tube in a coal-fired power plant

Abstract

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 h 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 h, 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 h, 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 h when accounting for tube thinning. Without accounting for the reduction in thickness, the estimated life was 210,650 h, which was approximately 1.2 years longer.