Crack cause analysis of pulverizing wheel in fan mill of 600 MW steam turbine unit

• Failure analysis of cracks recently occurred in pulverizing wheels of fan mills.
• Modal and transient analysis in acquiring dynamic features of assembled wheel.
• Nonlinear stress/strain with tested material property in operational temperature.
• Increased failure risk resulting from large growth of stress/strain in worn condition.
• Failure prevention approach by optimized repairing before critical wearing quantity.

Compared with other types of coal mills, fan mill presents advantages of compact structure and less construction cost. Therefore, fan mill is widely applied in thermal plants. This work carried out root cause analysis of crack failures occurred in pulverizing wheels of fan mills, which are applied in 600 MW steam turbine units of a power plant in China. The cracks are observed in dovetail grooves with beaters installed, and they are with substantial depths and critical to the safety of the fan mills. To find out the failure cause, Finite Element Analysis (FEA) is carried out including: (I) modal analysis and transient analysis of the assembled pulverizing wheel to determine the dynamic characteristics; (II) stress and strain computation of the local structure to obtain the concentrated stress and strain, where operational loads and contact behavior between different components are taken into account. The results indicate that the initial stress and strain exceed the yield limit under the normal operational condition, which reduces the margin of safety. Moreover, the stress and strain increase significantly when the pulverizing wheel is worn, which greatly increased the risk of failure. To avoid crack failures caused by wearing, analysis with different wearing quantities is carried out to determine the optimal repair strategy.