Abstract: In a 300 MW steam turbine of a power plant, for shorter time after it was put into operation, some abnormal phenomena, such as oil leakage of hydraulic servomotor mechanism of high pressure regulating valve, the degradation of the oil quality, the jam of servo valve, the decrease of system oil pressure and others, were occurred; and these abnormal phenomena influenced on safe operation of the unit seriously. Based on analysis, it was found that the reason of the abnormal phenomena is dissolving of the seal ring of hydraulic servomotor; and premature aging and failure of the seal ring were caused by the excessive temperature of the cylinder body of the hydraulic servomotor. Some measures, such as adding a cooling block on the hydraulic servomotor and reducing the temperature of the cylinder body of the servomotor, were adopted; and the oil leakage of hydraulic servomotor and degradation of oil quality were solved effectively.

Key words: leakage of hydraulic servomotor, degradation of fire-resistant oil, analysis, treatment measure