Condition Monitoring of T91 Superheater Tubes through Measurement of Internal Oxide Scale Thickness

Abstract

The T91 Superheater tubes are one of the critical components in the supercritical boiler of power generation plants. The tubes normally operate at a high temperature which could accelerate the degradation of material after long-time service exposure. Therefore, the tube condition needs to be assessed periodically to ensure its fit for the next service duration. The common non-destructive inspection method to monitor the tube condition is through measurement of metal thickness using ultrasonic testing. However, this method was found less suitable to address overheating issues due to the thick steam oxide scale on internal tube surface. In this study, a method to measure the internal oxide scale on the T91 superheater tubes surface was developed. Measurement on T91 superheater tube which was operated for 60,000 hours at 605ºC, shows that the thickness of the internal oxide scale is thicker than 230µm. Since some of the oxide scales were found intact and does not exfoliate, the tube was subjected to localize overheating which can cause in-service failure. Unfortunately, there is no specific allowance of oxide scale thickness stated in the design code for boiler construction. Therefore, oxidation data from this study was extrapolated and plotted as an oxide life curve which then use to estimate life consume and metal temperature during the condition monitoring process.