Long-term high-temperature exposure of reheater and superheater tubes in fossil-fired steam boilers results in the growth of iron oxide scale (magnetite) on the inner tube surface. This internal oxide layer on the water side of the tube acts as a thermal insulator, reducing heat transfer through the tube wall into the internal water vapor. Over time, this insulating effect limits heat transmission into the water vapor inside the tube, which, in turn, causes chronic overheating of the tube wall. The prescience of an oxide scale layer contributes to a reduction in boiler efficiency and promotes accelerated metallurgical failure.
Specialized ultrasonic measurement techniques are used for the assessment of internal oxide scale thickness. Traditionally, these specialized techniques involved the use of high frequency ultrasonic transducers that are designed to simultaneously measure the thicknesses of the tube’s remaining wall and the internal oxide scale.
The application of specialized ultrasonic techniques permits the evaluation of a large number of tubes in a short time, resulting in better decisions concerning the identification of unusual unit operating conditions and component replacements