In order to analyse the causes resulting in inner-wall cracking on high-pressure main steam valve's shell of N300-16.7/538/538 type steam turbine in one power plant

the inner wall of the main steam valve has been inspected in the field

and its stress intensity being analysed.Results show that the material's hardness tallies well with that defined in standard

and the material's performance hasn't obviously changed

the cracking occurred is due to thermal stress resulted from temperature difference between inner wall and outer wall during the precess of variation in start-up and shutdown of the unit

as well as in load fluctuation etc.

under effect of altenating thermal stress

the cracking occurred on value shell due to thermal fatigue and then expanded.Hence

it is recommended to control the temperature rise rate of the main steam valve in process of start-up and shutdown not being greater than 25 ℃/min.