During shutdown of the subcritical 600MW unit turbines

the main steam valve should be closed tightly and rapidly within 300ms

thus to quickly cut down the high pressure steam source

slow down the turbine speed

and prevent turbine over speed accidents.However

since the commissioning of subcritical600MW steam turbine unit in Taishan Power Plant

the closing time of high pressure main steam valve always exceeded 300ms during the shutdown.Especially

this phenomenon aggravated when the steam turbine ran at high load.Therefore

the reason was analyzed

and the valve stem bending

insufficient spring force and complicate connection force of horizontal lever were considered as the major causes.Finally

several countermeasures were put forward

such as transforming the mode that the lever is connected to the main steam valve into the one that the servomotor is directly connected the main steam valve

meanwhile

increasing the spring force.