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 t ube circuits and/or entire bank has been a difficult job in the past. Even thermal calculation methods are generally ineffective, since the failures i n SH/RH tubes are often the result of localised conditions arising from wid e variations in temperature distribution in the tube bank. A methodology adopted in BHEL has helped utilities overcome these limitatio ns by judiciously combining non-destructive and destructive evaluations. Th e oxide scale thickness values are used in conjunction with operating hours , stress, geometry etc. of the tubes. for predicting the tube metal tempera ture 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 th e equivalent metal temperature. The metal temperature is greatly enhanced due to the reduction in heat-tran sfer because of lower thermal conductivity of the magnetite oxide scale wit hin the SH/RH tubes. Due to the increase in the tube metal temperature beyo nd the designed operating temperature, the creep life of the tube is drasti cally reduced. Hence, it is imperative to predict the temperaturee mapping of the coils and the residual life of SH/RH coils extensively by oxide scal e measurement technique. In this paper, the non-destructive technique employed in assessing the meta l temperature distribution and remaining life calculation for a few utility boilers is presented.