Residual life estimation of high temperature tubings based on oxide scale thickness measurement

Super heater and Reheater tubes offossil boilers, wherein creep is the major damage mechanism, are required to be assessed periodically (after one lakh hours) for remaining life. For remaining life estimation of these tubes, knowledge of the operating tube metal temperature forms a very valuable input. Unfortunately , this information is hard to get, since measurement of tube temperature directly or indirectly through the measurement of steam temperature, are carried out only infrequently due to various technical and economic considerations. It is more convenient to estimate the operating tube metal temperature in service by examination of tube samples for thickness ofsteam side oxide scale, hardness and deterioration of microstructure. Since the changes in these parameters are fitnctions of time and temperature their currentvalues may be used to estimate average tube thermal history, for a given operating time. This paper deals with relatively accurate estimations of tube metal temperature using three approaches viz.(i) Microsstructural evoluation (ii) Steam side oxide scale thickness built tip and (iii) Hardness degradation. One time estimate of temperature and stress values have been extrapolated back to the initial conditions using linear growth of steam side scale thickness andf re side wastage of the tube. Total service life has been discretized at a smaller interval to increase the accuracy of estimation. Typical results on remaining life calculation based on above ntodification have been described.