Ultrasonic and EMAT - Important Tools to Analyze a Combined Effect of Multiple Damage Mechanisms in Boiler Tubes

Boilers, used in many industrial applications generate high costs to national economy in unscheduled shutdowns, repairs and power replacement. Every occurrence of ruptured tubes usually 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. The past experience shows that wall thinning due to corrosion and erosion as well as overheating causing loss of strength are two major damage mechanisms contributing to boiler tube failures. This is well recognized by the industry and many preventive maintenance procedures have been developed. Some of the most common involve the use of nondestructive testing and especially ultrasonics, namely wall thickness measurement and mapping, and indirect testing for overheating and creep damage by measuring the thickness of the internal oxide scale in tubes. This latter UT technique eliminates the need for destructive metallographic structure analysis by recognizing that even a thin scale can seriously impede heat transfer causing elevation of temperature in tube wall. A long-term effect of elevated temperature includes degrading steel grain structure and consequent loss of strength. Despite all these developments, the results of both UT tests are usually considered separately and consequently, separate tube reject/replacement criteria for thinning and for overheating are used. However, tubes are failing due to combined effect of many damage mechanisms. Authors have undertaken a systematic research with the major objective to develop the method for assessing tube condition that would analyze the results of all available NDT tests. Specifically, for wall thinning, an EMAT method was used and for overheating and creep damage – the high frequency UT method with a delay line. The results of EMAT were used, first to determine thinning rates and then to calculate the hoop stress in tube wall. The data from oxide thickness measurement were correlated to the degree of creep damage by destructive metal structure analysis. The new method for condition assessment was then proposed that recognizes the combined effect of various damage mechanisms as detected by ultrasonic testing. It was concluded that nondestructive test methods and especially ultrasonics offer an attractive solution to overall evaluation of boiler condition providing that test results are treated as an input for further analysis.

References

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