Remaining life assessment for boiler tubes affected by combined effect of wall thinning and overheating

Abstract

Boilers, the most troublesome components of electric power, chemical and processing plants generate high costs in unscheduled shutdowns, repairs and power replacement. Every occurrence of ruptured tubes leads to emergency shutdown of the entire plant. This paper describes the joint international effort to develop faster and more efficient methods for condition assessment and remaining life prediction for boiler tubes made of low-carbon steel. Authors have undertaken a systematic research with the major objective to correlate the results of combined nondestructive testing (NDT) with condition assessment of boiler tubes. The evaluation included non-contact wall thickness measurement with EMAT technology plus internal oxide layer measurement with specialized ultrasonics. While the first method shows the remaining tube wall thickness, thus allowing calculating total stress, the latter one has the potential to characterize microstructure degradation, which up to now could only be determined by destructive analysis. The special attention was directed towards identification and analysis of creep damage due to overheating. In recent years, techniques were developed to identify heat damage by measuring the thickness of internal oxide scale because even a thin scale can seriously impede heat transfer causing elevation of temperature in tube wall. A combined effect of wall thinning and the "degree of overheating" on tube remaining life was investigated. The uniqueness of this work lies in one of the first attempts to develop and validate a tool for methodology for condition assessment and remaining life prediction, for Steel20 tube material, while most of previous authors had concentrated on Cr-Mo steels. Another contribution is the combined treatment of two different damage mechanisms and practical utilization of two various NDT techniques. To-date, both results are treated separately, and consequently separate reject criteria exist for overheating and separate for wall thinning. As a result of work presented in this paper, a procedure was recommended to calculate the tube remaining life based on the results of two ultrasonic tests.

Failure Investigation of Superheater Through Investigate the Nearest Component

“…Therefore, efforts to identify the cause of this failure and its maintenance are critical. Superheater tube failures have been damaged in several similar power-generating units in the same area [7][8][9]. The superheater tube leaked and was partially bent during the maintenance shutdown, as shown in the picture below.…”

Measurement of Thinned Water-Cooled Wall in a Circulating Fluidized Bed Boiler Using Ultrasonic and Magnetic Methods

“…However, it is still difficult to measure the thickness changes of the tube due to the damages. The combination of the ultrasonic and electromagnetic field has been developed in the electromagnetic acoustic transducer (EMAT) system for the inspection of the water-cooled tubes [19,20]. The magneto-elastic phenomenon and Lorentz force help the EMAT inspect a deeper damage without the need for coupling material.…”