Reasons for corrosion and leakage of superheater tubes in a subcritical boiler

A tube burst accident occurred in the final stage superheater of a subcritical boiler. The causes of superheater tube bursting were analyzed using macroscopic observation, mechanical property testing, metallographic examination, scanning electron microscopy and energy spectrum analysis, X-ray diffraction analysis. The results show that oxide scale was generated on the inner surface of the superheater tube, causing local overheating of the tube wall. The oxide scale reached a certain thickness and fell off, accumulated at the lower elbow position, resulted in a reduction in steam flow cross-section area and a decrease in flow velocity, further caused an increase in pipe wall temperature. The materials of pipe deteriorated, coupled with a decreased in mechanical properties, even resulted in high temperature creep loss and wall thickness recluction, and ultimately led to tube burst accident of the superheater under the combined effect of high temperature environment and steam pressure.