Combination Non-Destructive Test (NDT) Method for Early Damage Detection and Condition Assessment of Boiler Tubes

Boilers, the most troublesome components of electric power, chemical and processing plants generate high costs in unscheduled shutdowns, repairs and power replacement. Every occurrence of ruptured tubes leads to emergency shutdown of the entire plant. This paper describes the joint international effort to develop faster and more efficient methods for condition assessment and remaining life prediction for boiler tubes. The work was performed under the grant from Kazakhstan Ministry of Education and Science. The authors have visited a number of coal-fired electric plants throughout Central Asia and found that a combination of wall thinning and overheating were major damage mechanisms contributing to boiler tube failures. The periodic inspection of boiler tubes include ultrasonic measurement of remaining wall thickness and in many cases, it involves cutting tube segments and performing metallurgical analysis for loss of original strength due to overheating. Systematic research was undertaken with the objective to correlate the results of combined non-destructive testing (NDT) with condition assessment of boiler tubes. The evaluation included non-contact wall thickness measurement with EMAT technology plus internal oxide layer measurement with specialized ultrasonics. The first method shows the remaining tube wall thickness, thus allowing to calculate total stress, and the latter one has the potential to indirectly characterize microstructure degradation, which up to now could only be determined by destructive analysis. The existing tube removal criteria are treating each damage mechanism separately while in reality, a combined effect of wall thinning and the "degree of overheating" decides about true condition of a tube. The procedure that utilizes the results of both described NDT methods was developed for improved methodology to assess tube condition and to predict its remaining life.