Fatigue lifetime assessment on a high-pressure heater in supercritical coal-fired power plants during transient processes of operational flexibility regulation

•A method for identifying heater lifetime during transient processes is proposed.•Changes of fluid pressure and temperature in heaters at four measures are analyzed.•Tube sheet of heaters is the most probable region for thermo-mechanical fatigue.•Allowable number of cycles of tube sheet at four flexible measures are obtained. The operational flexibility of coal-fired power plants has become a priority objective; however, the lifetime of main devices is reduced due to thermomechanical fatigue during transient processes. This study focuses on the safety of high-pressure heaters at operational flexibility measures. The dynamic characteristics of thermal parameters in the 1# HP heater were obtained and analyzed based on transient models of a coal-fired power plant. The change trends of pressure/temperature on the steam/feedwater side of the 1# HP heater at three throttling the extraction steam measures were different with that at the feedwater bypass measure. Subsequently, finite element analysis method was used, and the thermomechanical stresses under the four measures were calculated and analyzed. The time to reach the maximum mechanical stresses was shorter than that of thermal stresses. Finally, the fatigue lifetimes of the heater were estimated. The maximum fatigue damage ratio at the per load cycle was 0.0069%, and the minimal allowable number of cycle was 14413 in the upper region of the tube sheet when the extraction steams of #1, #2, and 3# HP heaters are regulated simultaneously. The study could offer data guidance to the safety and maintenance of coal-fired power plants during operational flexibility regulations.