NON-DESTRUCTIVE METHOD OF EVALUATING THE BEHAVIOUR OF HIGH TEMPERATURE HIGH PRESSURE TUBES IN STEAM GENERATORS

Creep rupture failure of superheater (SH) and reheater (RH) tube is a major cause of the forced outage of utilities. Deciding when to replace select tube circuits and/or entire bank has been a difficult job in the past. Even thermal calculation methods are generally ineffective, since the failures in SH/RH tubes are often the result of localised conditions arising from wide variations in temperature distribution in the tube bank. A methodology adopted in BHEL has helped utilities overcome these limitations by judiciously combining non-destructive and destructive evaluations. The oxide scale thickness values are used in conjunction with operating hours, stress, geometry etc. of the tubes, for predicting the tube metal temperature and the remaining useful life with the help of software programmes. The growth of oxide scale is a function of temperature and time of exposure in steam. By measuring the thickness of oxide scale and with the knowledge of oxide growth kinetics for a given material, it is possible to deduce the equivalent metal temperature. The metal temperature is greatly enhanced due to the reduction in heat-transfer because of lower thermal conductivity of the magnetite oxide scale within the SH/RH tubes. Due to the increase in the tube metal temperature beyond the designed operating temperature, the creep life of the tube is drastically reduced. Hence, it is imperative to predict the temperaturee mapping of the coils and the residual life of SH/RH coils extensively by oxide scale measurement technique. In this paper, the non-destructive technique employed in assessing the metal temperature distribution and remaining life calculation for a few utility boilers is presented.