RESIDUAL LIFE ASSESSMENT OF HIGH TEMPERATURE BOILER COMPONENT SUBJECTED TO CREEP AND FATIGUE

Abstract

Generally the reheater provided in the boiler of a thermal power plant is subjected to high temperature operation which results in usage factors / damages due to creep and fatigue during its lifetime. The creep damage is caused when the reheater is running at its operating pressure and temperature conditions and the fatigue damage is caused due to the number of start / stop during its service time like cold start, warm start and hot start conditions causing the temperature differences between the header and the inlet steam. The reheater outlet header after its service of 15 years clocking nearly 1.23 lakh hours can be assumed to have spent 70% of the time at the rated pressure and temperature conditions and balance at lower temperature conditions. But to be realistic, there will also be some amount of overloading of the component resulting in overshooting of temperature beyond the rated temperature. The fatigue damage is again caused by planned and unplanned shut down of the plant during the 15 years of operation. In order to assess the remaining life of the component, the individual damages due to creep and fatigue must be computed for estimating the cumulative damage. In one of the reheater headers, some minor cracks were observed in the stub to header weld (crotch) portion after running for nearly 15 years and some doubts were raised about the residual life of the component. The detailed analysis taking into account the operating conditions revealed that the cumulative damage is well within limits, which means that the component can be safely operated for 5 more years. The analysis also indicated that the cracks were not specific to creep. The recommendation was given in this regard to the customer and was validated after the component ran for 5 more years. The details of the analysis with the operating conditions are presented in this paper.