Abstract: A new fact is introduced which may lead to unstable shaft torsional oscillation of steam turbine. In a power plant experiments it was found that the operation of governing system would cause the pressure oscillation in turbine living steam inlet pipeline. For steam turbine with fast response governor, if the natural frequency of pressure oscillation is equal or close to the shaft torsional frequency, the unstable torsional oscillation would occure, and may lead to the damage of shaft system.

Key words: Steam pipe Pressure oscillation, Steam turbine, Torsional oscillation