Analysis of Causes of Valve Rod Fracture of Steam Turbine High Pressure Main Steam Valve

A 300MW sub-critical coal-fired thermal power plant steam turbine generator set # 1 high pressure steam valve stem is fractured. The turbine unit has been running 46000 hours before the crack happened. The chemical component was examined which was in the standard range. they observed the metallographic structure of the broken stem to find the nitriding treatment of fine grain martensite structure at surface area and lath martensite at central section, healthy structure condition. they also found the existing of circumferential oxidized cracks with white corrosion production. According to that, they believe it was caused by residual stress when grinding. Finally, SEM was used to observe the fracture to further confirm our ideas. (paper)