Root cause failure analysis of a division wall superheater tube of a coal-fired power station

Abstract

Coal-fired boiler tubes are generally exposed to high internal pressure and high temperature of steam and flue gas. The common cause of any metallurgical failure of a superheater tube is due to the tube metal temperature higher than that as originally specified [1]. Tube metal temperature may increase gradually over many years due to the growing oxide scales inside tube or elevate rapidly caused by loss of internal steam or water-coolant flow. Internal pressurized tubes are critical components in water-tube boiler and steam superheater elements. Tubes in such application are vulnerable to high temperature upset condition, undergoing severe creep deformation or even final rupture. Therefore, boiler tubes in power plants have finite life because of prolonged exposure to high temperature, stress, aggressive environment, corrosive degradation, etc. However, uses of suitable boiler tube material in thermal power plants are required to ensure that the materials are safely used under higher temperatures and pressures for a long period of operation [2]. Many works concerning the failure problems in water-tube boiler have been reported. The North American Electric Reliability Council (NERC) reported that the coal-fired boilers are among the highest economic risk components in any other power plant. By far, the greatest number of forced outages in all types of boiler is caused by failures [3]. Elimination of boiler tube failure could save the electric power industry about $5 billion a year [4]. Metallurgists from French, Inc. [1] published data of the top 10 causes of failures where creep (long-term overheating) is 23.4%, followed by fatigue (13.9%) (thermal 8.6%, corrosion 5.3%), ash corrosion (12.0%), hydrogen damage (10.6%), weld failures (9.0%), high temperature (short-term overheating) (8.8%), erosion (6.5%), oxygen pitting (5.6%), caustic attack (3.5%) and stress corrosion cracking (2.6%). In general, 30% of all tube failures in boilers and reformers are caused by creep [5–7].