Effect of steamside oxidation and fireside corrosion degradation processes on creep life of service exposed boiler tubes

Abstract

In conventional steam plants, the efficiency mainly depends on the outlet steam temperature of boiler. The outlet steam temperature is maintained by subjecting different sections of boiler to high temperatures. In these harsh conditions boiler tubes undergo different degradation processes viz creep, fatigue, steamside oxidation, fireside corrosion and erosion. Therefore, life evaluation of boiler components is required at regular intervals of time for better functionality of power plant. Various life assessment techniques like microstructural evaluation, hardness measurements, Oxide scale measurements, Accelerated temperature tests are used for assessing the residual life of boiler tubes. In this paper, the creep life is estimated using destructive accelerated temperature tests on several T22 tubes from various sections of Platen Superheater, Final Superheater and Reheater of boiler. Accelerated temperature tests are carried out at higher temperatures under the nominal steam load and remaining life is estimated by extrapolating the results for the constant service temperature. But, the service temperature of tube never remains constant during a regular plant operation due to insulation effect of steamside oxidation and fireside corrosion. Further, during start up and shut down of plant, tubes undergo low cycle fatigue damage. Though, the life consumption of boiler tube due to fatigue is infinitesimal compared to creep because of thinner section of tubes. Therefore, the effect of cyclic operation is not considered, but the effect of steamside oxidation, fireside corrosion and erosion are incorporated in the accelerated temperature test results in estimating the residual life of boiler tubes. Further, these results are compared with the life obtained from Oxide scale thickness measurements carried out by Non-destructive Ultrasonic technique. In both the methods, metal temperature calculation using steady state conductive heat transfer equations play a crucial role in estimating the residual life. Mainly, these methods are based on thermal properties of oxide scale and short term stress rupture tests. Therefore, these methods can be applied for different grades of boiler materials such as T23, T24, T91 and T92 etc for residual life estimation.