Analysis of the causes of leakages and preventive strategies of boiler water-wall tubes in a thermal power plant

摘要

The reasons for the failures of a boiler water-wall tube used in the thermal power plant were investigated in this paper. The material of water-wall tube was made of medium-carbon steel, SA-210 Grade C, which is pearlitic heat-resistant steel with a working temperature lower than 500 degrees C and is widely used in boilers tubes. To investigate the causes of failure, various techniques including macroscopic inspection, chemical composition analysis, mechanical performance testing, optical microscopy observation and Energy Dispersive Spectroscopy (EDS) were carried out. Corrosive products were found on the inner wall of the tube's bulge positions. The thickness of the tube wall covered by the corrosive products reduced due to the corrosion. The boiler water quality was analyzed and it was found that there was excessive Sodium Hydroxide (NaOH), which was the main corrosion-induced substance. Metallographic examination results showed that the microstructure in the corroded area has aged. Based on the analysis results, failure causes can be drawn: excessive NaOH in the boiler water was the main reason for the corrosion. The characters of alkaline corrosion products provided the condition that corrosion can proceed continuously which caused the thinning of the water-wall tube. As the process proceeds, it reduces performance of the tube and creates bulge which finally causes leakage.