Recently, it was required to investigate the distribution of scale deposits on the inner surface of boiler waterwall tubes in a thermal power plant, because the excess scale deposits are the major concerns causing a decline in thermal efficiency and damage by overheating. To avoid these problems tube sampling and metallurgical examinations are carried out and appropriate periods of chemical cleaning are determined. Under the circumstance non-destructive methods to measure the thickness of scale deposits have been needed. The ultrasonic method can be used to estimate the thickness of scale deposits by using the time difference from interface signal between base metal and deposit to the back surface signal of deposits, when the thickness is 100 micrometers or more. However, the thickness can’t be estimated when the thickness is up to 100 micrometers because these two signals are overlapped and superimposed. In this study advanced measurement technology by analyzing the superimposed waveform was developed. As the results of experiments, the thickness of scale deposits can be estimated with accuracy of 10 micrometers. Also this technology can be applied to evaluate the effect of chemical cleaning.

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