A failure analysis and remnant life assessment of boiler evaporator tubes in two 250 MW boilers

Abstract

It has been established from (a) the tube thermal flux calculations, (b) the failure surface fractographic features, (c) the data sheets on evaporator tube temperatures from both Mitsubishi and Seimens, (d) the reported data from the literature and (e) to a certain extent, the tube strain gauge data that the recent waterwall tube failures in two 250 MW boilers were the result of high stresses being active at certain boiler locations. At start-up these stresses were estimated from fractographic features to be around 370–420 MPa in magnitude, which resided between the tube materials yield and ultimate tensile strengths. The defect extension process in the present waterwall tube failures was shown to be fatigue crack growth, which is the most common failure mode in waterwall boiler tubes. By adopting two separate approaches, viz., an “upper bound” ASME XI design line and an influence diagram model approach which was designed specifically for waterwall tubes, detailed remnant life values could be estimated for a number of different situations. In the case of new replacement tubes, both approaches estimated a remnant life of about 9 years. Note that the failure probability of the upper bound ASME XI predictions was demonstrated to be around 10 −5. Also the influence diagram model approach was extremely helpful in identifying the probable root cause of corrosion fatigue failures in waterwall boiler tubes. With respect to the remnant life estimates two separate cases were considered, viz., (1) tubes with defect populations as recorded during the recent NDT study and (2) new replacement evaporator tubes which contained zero defects.