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.
1. G. Ya. Bezlyud’ko, et al., “Portable Electromagnetic-Acoustic Thickness Meters (EMAT)”, Russian J. of NDT, Vol. 40, No. 4, 004, pp. 239–245. Translated from Defektoskopiya, Vol. 40, No. 4, 2004, pp. 28–35.
2. Boynard C., et al. “Influence of Superficial Scale in Signal Variation generated by EMAT on Boiler Tube Inspection”, Proc. of ECNDT (2006), paper Tu. 3.8.2
3. Alers G.A. et al., "ASNT Nondestructive Testing Handbook", Vol. 6, Ultrasonic Testing, Section 10: Other Ultrasonic Techniques, December (1990)
4. Kumar D., Mishra S., “High Temperature Ultrasound Testing for In-situ Condition Monitoring of Superheated Steam to Improve Creep Life of Platen Superheater Tubes in Ultra Super Critical Power Plants”, Int. J. of Scientific Res . Vol. 4, Issue 5, May 2015, ISSN No. 2277-8179
5. D.N. French, Metallurgical Failures in Fossil Fired Boilers, John Wiley & Son , New York, New York, 1983. pp. 143-152.
6. K. Lee, T. Nelligan, “The Use of Magnetostrictive EMAT Transducers on Oxide Scaled Boiler Tubes”, Internal Communication by Panametrics-NDT , Waltham, Massachusetts, USA, 2005
7. G.J. Nakoneczny, R.D. Murphy, “Application of EPRI/B&W Developed EMAT Systems for Assessing Boiler Tubes”, ICOLM Intern. Conference on Life Mgmt and Extension of Power Plant , May 2000, Xi’an, P.R. China
8. Bergander M., Pechacek R., “Advancements in EMAT Technology for Boiler Tube Inspection”, Proc. of ASNT Fall Conference , Indianapolis, November 2000.
9. Bergander M., “EMAT Thickness Measurement for Tubes in Coal Fired Boilers”, Applied Energy , 2003, Vol. 74, (3-4) pp.439-444.
10. Bergander M., et al., “EMAT Thickness Measurement for Tubes in Coal Fired Boilers”, Proc. of ASNT Fall Conference , Columbus, October 2001.
11. Chaudhuri S., “High Temperature Boiler Tube Failures - Case Studies”, Proc. COFA-1997 , pp. 107-120.
12. Paterson, I.M., Rettig T.W., “Remaining Life Estimation in Boiler Pressure Parts – 2.25Cr/1Mo Superheater and Reheater Tubes”, EPRI, Project RP2253-5, Final Report , Palo Alto, CA 1987
13. Toft L.H., Mardsen R.A., “The Structure and Properties of 1%Cr-0.5%Mo Steel After Service in CEGB Power Stations”, Conf. of Structural Processes in Creep, JISI/JIM, London (1963), 275
14. “Diagnostics of New Generation Thermal Power Plants”, Edited by T. Chmielniak and M. Trela, Polish Academy of Sc. Inst.of Fluid-Flow Machinery , IFFM Publishers, Gdansk 2008 pp.253-338
15. Babcock&Wilcox Co., Power Generation Group, “Plant Service Bulletin 1994”
16. EPRI Carbon Steel Handbook, EPRI, Palo Alto, CA, 2007
17. Sankhala K., et al. “Study of Microstructure Degradation of Boiler Tubes Due to Creep for Remaining Life Analysis”, J. of Eng. Res ., Vol. 4, Issue 7 (v.2), July 2014, pp.93-99
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