Abstract: Generally, the heat consumption rate will be 3% to 7% higher than the designed value when the unit is operated at the range of 50%~80% of the rated load. Aiming at the low efficiency issue under partial loads, the causes of the sharp decrease of unit thermal efficiency are analyzed. Through comparison between theoretical analysis and field testing data, it is figured out that the low efficiency is mainly attributed to the increase of throttling loss of the main steam valve as the load is reduced. The technical outline for designing the area of the first-stage stationary blades is then proposed. In addition, the necessity and feasibility of improving the area of governing stage nozzles for existing units are presented with the implementation schemes put forward. Through practical operation the improvement is proved to be successful.

Key words: governing stage, nozzle area, unit performance, improvement scheme, implementation result