| Summary: | 碩士 === 國立臺灣科技大學 === 機械工程系 === 91 === Boiler tubes in the steam unit of fossil power plant are always immersed in high flame during operation. In high temperature zone the oxide scale, caused by the flow of hot steam under high pressure, is easily built up along the inside of tubes to reduce the heat transfer capability. The thickness of this oxide scale is one of important parameters required for the computer program to estimate the remaining life of the tubes. At present, the sacrificial tube removal, an inaccurate and time consuming method, is the only way to measure the thickness of this oxide scale. This study is intended to develop a nondestructive method using ultrasonic technique to measure the thickness of this oxide scale from the outside.Extensive metallurgical analyses (OM, X-ray diffraction analysis, EPMA) were performed on nine sacrificial tubes, removed from selected locations, to study the ultrasonic characteristics of this oxide scale and to serve as a standard for thickness measurement. A standard operation procedure (SOP) for this new ultrasonic testing method was set up in laboratory, and the reading accuracy is within ±10%. Field tests were also conducted to verify the accuracy of oxide scale thickness measurement. Oxide scale thickness values measured in field using this new method were compared with the actual thickness values measured from removed tubes. The comparing result showed that the accuracy of thickness measurement is above 90%.
This study has demonstrated that the newly developed nondestructive method can successfully measure the inside oxide scale thickness from the outside of tube in a much shorter time without tube removal. In addition to the saving of money, more data can be acquired during outage to refine the remaining life assessment so that any necessary measures can be taken to prevent premature failure.
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