Abstract: During the normal operation of the steam turbine of a natural gas compressor unit of a synthetic ammonia plant with an annual output of 450,000 t, the opening signal of the speed control valve output by the control system shows a gradual decline. However, the actual opening position of the steam turbine control valve and its rotational speed remain stable, while the compressor discharge pressure remains within normal range without abnormal fluctuations. Based on the working principle of the speed control system of the steam turbine, the relevant systems and components of the unit were investigated. It was determined that the gradual decline in the elasticity of the inner spring of the pilot valve or the wear of the hydraulic servomotor feedback rod components was the main reason for the continuous slow decline in the speed control valve opening signal of control system of the steam turbine. And the wear of the needle roller bearing on the hydraulic servomotor feedback rod was found in the subsequent maintenance, which has validated the accuracy of the fault cause analysis.The hydraulic servomotor that was replaced during previous maintenance and repaired for backup use was reinstalled and commissioned. Over prolonged operation, the speed control valve opening signal of the control system has remained stable and no abnormal changes have occurred.

Key words: steam turbine, speed control system, speed control valve, hydraulic servomotor, pilot valve