Case Study: Remaining Life Analysis of Creep Strength Enhanced Ferritic Steel T91 After 20 Years Operation on 600 MW Coal-Fired Power Plant

Coal-Fired Power Plant Boiler 600 MW has been operated more than 20 years. Secondary superheater outlet (SSH Outlet) tube bursts had been occurred on this unit. Long-term overheated failure mode was observed at failed tubes on three occasions which was on September 2017, April 2018, and December 2019. The design tubes are called SA-213 T91. On April 2018 and December 2019, the powerplant operator has joined hand with two different public university research institute which are called institution-1 and institution-2 to conduct tests to find the root cause of the failure by hardness test, microstructural examination, creep test and the remaining life analysis. The results of the hardness test on the fail tube on April 2018 and December 2019 were 170.84±1.98 Harness Vickers (HV) and 168-219 HV respectively compared to the new material hardness which are on the average of 258HV. On April 2018, creep tests were resulted that expected time to failure on 750°C and 72.93Mpa was 3.6hours; on 750°C and 93.46 Mpa was 0.4hours; on 650°C and 148.73Mpa was 4.3hours; on 650°C and 186.14 Mpa was 0.7hours; 550°C and 261.80Mpa was 9.6hours; 550°C and 316.05Mpa was 1.7hours. On December 2019, creep tests were resulted that expected time to failure on 680°C and 82.1Mpa was 16.290hours; on 650°C and 152Mpa was 0.84hours; last on 550°C and 201Mpa was 0.269hour. In order to operate more than 25 years, the stress should be less than 89.3Mpa according to ASME II PART D 2019. 6.9mm thickness tube has been resulted hoop stress 62Mpa. Welded metal approached resulted that tube remaining life of 570°C evaluated temperature was 54.4years. 600°C evaluated temperature resulted remaining life was 5.98years. In order to ensure that boiler was not overheated, Metal temperature monitoring and Coal blending has been applied to achieve the best result among bad conditions.