Fatigue life prediction of regulating valves on the intermediate-pressure section of a 400 MW steam turbine

Fatigue life prediction of regulating valves on the intermediate-pressure (IP) section of steam turbine is presented in this paper. The transient temperature fields and stress fields of the valve body are calculated using finite element method, and the operating records of a 400 MW steam turbine located in Hubei Province of China, is used as input, and all practical operating processes including cold-startup, hot-startup, extreme-hot-startup, shutdown and emergence shutdown are considered in the FEM computations. The load patterns for the valve are determined using rain-flow counting method and the local stress–strain approach, furthermore, the cyclic stress–strain loop of the key point enduring maximum stress range is plotted. Prediction of fatigue crack-initiation life for the turbine valve is given, and the life and crack prediction are agreed with the data from practical industry inspection. It is found in the computation that the maximum stress range appears near the stiffening rib in the valve body during turbine service. This result indicates that the circumferential thermal stress concentration region is near the stiffening rib, and also implied that crack may emerge in this location. Some cracks are actually witnessed near the rib when we did inspection on the turbine valves since 2000. The fatigue life prediction is also roughly accord with our investigative data. It is concluded that the damage during turbine emergence shutdown contributes the main part to the damage fatigue capability which is defined as the number of cycles to crack-initiation in the valve due to cyclic operating conditions. The method for comprehensive study on transient temperature and stress fields and life estimation of the turbine valve is also proposed in this paper.