Unusual Failure of a Steam Turbine

Fusing of the switch contacts of a boiler feed pump drive motor led to the failure of a turbine. After rubbing of most of the Ni-Cr steel LP wheels had occurred, due to the admission of water carried over with the steam, a copper-rich alloy from the interstage gland rings melted, penetrated the wheel material, and gave rise to radial and circumferential cracking in four of the LP wheels. It was concluded that when the rotor moved axially and the wheels came into contact with the diaphragms there was a tendency for the former to dish, with the development of both radial and circumferential tensile stresses on the side in contact with the adjacent diaphragm. In the presence of the molten copper-rich alloy, these stresses gave rise to severe hot cracking.