Water wall tubes failure analysis in a 25-megawatt steam power plant

The failure of water wall tubes in a boiler steam power plant has been analysed. When the first inspection was conducted on the 25-Megawatt steam power plant, leakage was found on several water wall circulating fluidized boiler (CFB) tube. Visual examination, chemical composition analysis, hardness measurement, and metallographic examination on the tube samples cut from the water wall were used to analyse the cause of the failure. Visual examination showed that there is a thick weld joint on the tube inner side that could induce overheat and cause the tube to fracture and, in some locations, there were thick deposits on the tube inner side. A Hardness measurement of the tube near the fracture location showed an increase in the hardness value ranged from 191 – 206 HV. Metallographic examinations showed that there has been some micro-crack along the grain boundary. Composition analysis on the deposit at the inner side of the tube showed a trace of magnesium and calcium indicate that the water quality of the boiler doesn’t meet the standard. To resolve the problem, it is recommended to fix the weld procedure and kept the water quality to meet the standards.