Boilers are the most troublesome components of electric power, chemical and processing plants, generating high costs in unscheduled shutdowns, repairs and power replacement. Every occurrence of ruptured tubes leads to emergency shutdown of the entire plant. Past data show that a combination of wall thinning and overheating (creep) is a major damage mechanism contributing to the failure of boiler tubes. This paper describes a joint international effort to demonstrate the value of non-destructive testing (NDT) in evaluating the condition of a boiler. The main question is how to treat the combined effect of overheating and wall thinning, which determines the way NDT results are utilised. There is a broad literature base dealing with this problem as well as guidelines (for example from the Electric Power Research Institute (EPRI)), but they mainly pertain to low-alloy steels, such as chromium-molybdenum (Cr-Mo), which are prevalent in boilers used in most Western countries. However, the research described here concentrates on Central Asian locations, where the majority of furnace tubes are made of grade 20 steel. This paper attempts to develop recommendations for the combined treatment of different types of tube degradation in low-carbon steel boiler tubes, whereas existing reject/removal criteria treat each damage mechanism separately. The method described for predicting the remaining tube life is based on using two NDT ultrasonic techniques: electromagnetic acoustic transducer (EMAT) for wall thickness measurement and specialised ultrasonic testing (UT) to detect internal oxide scale. It is concluded that NDT methods offer an attractive solution to remaining life assessment (RLA) in power boilers due to their ability to accurately determine wall thickness and hoop stress in tubes and to indirectly detect the degree of overheating by measurement of the internal oxide thickness.
