Abstract: This paper introduces the shutdown accident caused by the imbalance of steam/carbon ratio in the unit resulting from the sudden rise of rotational speed, initial steam flow rate and steam extracting amount and then a large drop because the speed governing system in the steam turbine of syngas compressor was out of control. According to the governing mechanism, control components structure and working principles of steam turbine and combining with the characteristics of the accident, the paper analyzes the causes of the accident. It excluded the impacts of steam turbine high pressure steam pressure, exhaust steam pressure, extraction control valve and compressor load on the accident and confirmed that the main stop valve opening was out of control, which was the main cause of the accident. Through further analysis, the paper excluded the impact of oil particles in the pilot valve on the main steam valve. Based on the analysis results and through on-site static debugging, it is found out that the feedback lever and the feedback transmission rod connecting bolt were disengaged. After fastening the connecting bolt and carrying out static test on the control mechanism of the main stop valve, the fault was eliminated. In order to prevent similar faults from happening again in the steam turbine, corresponding requirements have been made in management.

Key words: steam turbine, speed governing system, connecting bolt, disengage, out-of-control, causes, countermeasures