SOME ASPECTS OF CREEP AND LIFE ASSESSMENT OF ENGINEERING COMPONENTS IN THERMAL POWER PLANTS
Keywords:
Creep Life Assessment Technology, Microstructure Study of Ageing FailureAbstract
In thermal power plants, engineering components made of Cr-Mo/ Cr-Mo-V steels operate extensively at high temperature and pressure in corrosive environments to perform specific functions for a minimum specified period of time. In operation, several natural ageing processes such as creep, corrosion, fatigue etc. are responsible for accumulating micro structural damages thereby limiting the lives of the components. This paper describes some aspects of creep life assessment technology and a few case studies related to changes in microstructure due to ageing, failure and remaining creep life of components in thermal power plants. The failure investigation of final super-heater tubes showed that the failure took place due to short term overheating. The temperature excursion due to overheating as estimated from the kinetic data on oxide scale growth was about 830oC. The circumferential expansion in this tube was about 19%. The wall thinning due to oxidation and creep are primarily responsible for failure of this tube. It had been established considering the influence of wall thinning that irrespective of operating temperature, pressure and damage development, modified 9Cr-1Mo steel exhibits longest life among various grades of Cr-Mo steels. The influence of prolong service revealed that un failed re-heater tubes exhibit higher tensile properties than that of platen super heater tubes. In contrast the creep rupture properties of both the tubes at 50 MPa meet the minimum creep rupture properties when compared with NRIM data. The remaining creep life of platen super heater tube as estimated at 50 MPa and 570oC is more than 10 years and that of re heater tube at 50 MPa and 580oC is 9 years.
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References
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