Study on the Development of Ultrasonic Wall Thickness Measurement Apparatus for coal-fired Boiler Tubes

Thermal Power Research Section, and Information and Communication Engineering Research Section

As major equipment in a coal-fired power station, the boiler contains numerous tubes that are prone to abrasion from ash erosion and steam cuts. Stable continuation of power generation needs periodic measurement of the outer diameter and wall thickness of a boiler tube. In fact, conventional inspection of boiler tubes has required considerable expenses and time due to its narrow and deep position.

To solve this technological problem, the Research & Development Department has started the development of an ultrasonic wall thickness measurement apparatus for coal-fired boiler tubes including the deep part in collaboration with Nikko Inspection Service Co., Ltd (located in Muroran city, Hokkaido).

Advantageously, our compact and portable ultrasonic wall thickness measurement apparatus can scan the outer diameter and wall thickness of the tube in a continuous and three-dimensional manner (able to detect surface flaws), by moving an ultrasonic probe on the boiler tube outer surface, both circumferentially and axially. Also, a performance test was conducted in the deep part of the boiler tube in our coal-fired power station to confirm the ability of this apparatus to provide desired results.

1. The apparatus was miniaturized to the dimensions of 430mm(width)265mm(height)30mm(thickness), enabling wall thickness measurement in a narrow boiler tube (tube array space of approx.32mm).
2. By conducting a performance test in an actual boiler, it was confirmed that surface flaws can be detected in the boiler tube with a measurement accuracy of 0.1mm (at 4800 points: 40 circumferential points 120 axial points).
3. Since this apparatus can detect surface flaws at 4800 points around the thickness reduced part of a boiler tube, the minimum thickness can be assuredly found.
4. Due to few practical examples of this type of apparatus for externally detecting surface flaw for the outer diameter and wall thickness of the boiler tube in a narrow part, we applied for a patent (Patent application: 2006-332269).

Photo-1 Ultrasonic wall thickness measurement apparatus

Fig-1 Sample of measurement results