Abstract: In order to prevent boiler tube explosion and leakage, it is necessary to detect the inner surface cracks of small-diameter boiler tubes using the ultrasonic. This paper introduces the ultrasonic inspection method focusing on the probe selection principles, making of the reference testing blocks, choosing of the sensitivity and scanning sensitivity and the examing standard setting. Taking the water wall tubes as an example, the determination of the false indication of inner tube surface threads and the wave characteristics of the inner surface cracks of small-diameter tubes are briefly described. Practice shows that the given method is helpful to improve the detection rate of the inner surface cracks of small-diameter boiler tubes.

Key words: ultrasonic inspection, power plant, boiler, small-diameter tube, inner surface crack