Protection-driving method of a feedwater heater and the device thereof

A steam turbine plant including at least one feedwater heater means, the steam turbine plant comprising:

• a boiler means having a superheater and a reheater therein, a high pressure steam turbine means driven by steam generated in the superheater and supplied through a main steam pipe means, an intermediate pressure steam turbine means driven by reheat steam heated in the reheater means and conducted through a hot reheat steam pipe means, a condenser means for condensing a steam exhausted from the intermediate pressure turbine means, a cold reheat steam pipe means connecting an outlet of the high pressure steam turbine means with an inlet of the reheater, a feedwater pipe means for connecting the condenser means with an upstream side of the superheater, the at least one feedwater heater means is disposed in the feedwater heat pipe means, and extracting steam pipe means is disposed between the high pressure steam turbine means and the feedwater heater means for introducing an extracting steam into the feedwater heater means, a control valve means is disposed in the extracting pipe means for controlling an amount of the extracting steam,
• means are provided for calculating an allowable thermal stress in accordance with a predetermined thermal stress,
• means are provided for calculating an allowable feedwater temperature variation ratio based upon an output of the allowable thermal stress calculating means,
• means for calculating an amount of the extracting steam to be introduced into the feedwater heater means, and
• means for controlling the extracting value in accordance with the output of the amount of the extracting steam calculating means.

A method of controlling at least one feedwater heater means in a steam turbine plant when the plant is starting and stopping, the method comprising the steps of:

• setting an allowable thermal stress value of the feedwater heater means,
• calculating a remaining working life of the feedwater heater means under a condition of a predetermined thermal stress,
• calculating an allowable thermal stress value under a specific remaining working life of the feedwater heater means based on outputs of the calculated remaining working life and the set allowable thermal stress,
• calculating an allowable feedwater temperature variation ratio based on the value of the allowable thermal stress under a predetermined specific remaining working life,
• calculating an actual feedwater temperature variation ratio of the feedwater heater means,
• calculating a feedwater temperature ratio deviation in accordance with the calculated values of the feedwater temperature variation ratios,
• calculating an amount of the extracting steam to be introduced into the feedwater heater means based on the value of the calculated feedwater temperature ratio deviation and a condition of the extracting steam, and
• controlling an extracting valve in accordance with an output of the calculated amount of the extracted steam.