Issues and precautions regarding temperature and pressure rise during steam generator startup

How is the boiler startup speed regulated? Why can’t the pressure increase speed be too fast?

The pressure increase speed at the initial stage of boiler start-up and during the entire start-up process should be slow, even, and strictly controlled within the specified range. For the startup process of high-pressure and ultra-high-pressure steam drum boilers, the pressure increase speed is generally controlled to be 0.02~0.03 MPa/min; for imported domestic 300MW units, the pressure increase speed should not be greater than 0.07MPa/min before grid connection, and should not be greater than 0.07 MPa/min after grid connection. 0.13MPa/min.
In the early stage of boosting, because only a few burners are put into operation, the combustion is weak, the furnace flame is poorly filled, and the heating of the evaporation heating surface is relatively uneven; on the other hand, because the temperature of the heating surface and furnace wall is very low, Therefore, among the heat released by fuel combustion, there is not much heat used to vaporize the furnace water. The lower the pressure, the greater the latent heat of vaporization, so there is not much steam generated on the evaporation surface. The water cycle is not established normally, and heating cannot be promoted from the inside. The surface is heated evenly. In this way, it is easy to cause greater thermal stress in the evaporation equipment, especially the steam drum. Therefore, the temperature rise rate should be slow at the beginning of the pressure increase.

In addition, according to the change between the saturation temperature and pressure of water and steam, it can be seen that the higher the pressure, the smaller the value of the saturation temperature changing with the pressure; the lower the pressure, the greater the value of the saturation temperature changing with the pressure, thus causing a temperature difference Excessive heat stress will occur. So to avoid this situation, the duration of the boost should be longer.

In the later stage of pressure increase, although the temperature difference between the upper and lower walls of the drum and the inner and outer walls has been greatly reduced, the pressure increase speed can be faster than that in the low pressure stage, but the mechanical stress caused by the increase in working pressure is greater, so the pressure in the later stage The boost speed should not exceed the speed specified in the regulations.

It can be seen from the above that during the boiler pressure boosting process, if the pressure boosting speed is too fast, it will affect the safety of the steam drum and various components, so the pressure boosting speed cannot be too fast.

What issues should be paid attention to when the unit starts to heat up and pressurize?

(1) After the boiler is ignited, soot blowing of the air preheater should be strengthened.
(2) Strictly control the temperature rise and pressure rise speed according to the unit startup curve, and monitor the temperature difference between the upper and lower drums and the inner and outer walls to not exceed 40°C.
(3) If the reheater is dry-fired, the furnace outlet smoke temperature must be strictly controlled not to exceed the allowable temperature of the tube wall, and the superheater and reheater tube walls must be closely monitored to prevent overheating.
(4) Closely monitor the drum water level and open the economizer recirculation valve when water supply is stopped.
(5) Strictly control the quality of soda drinks.
(6) Close the air door and drain valve of the steam system on time.
(7) Regularly monitor the furnace fire and oil gun input, strengthen the maintenance and adjustment of the oil gun, and maintain good atomization and combustion.
(8) After the steam turbine is overturned, keep the steam temperature at a superheat level above 50°C. The temperature difference between the two sides of the superheated steam and reheated steam should not be greater than 20°C. Use desuperheating water carefully to prevent large fluctuations in steam temperature.
(9) Regularly check and record the expansion instructions of each part to prevent obstruction.
(10) When an abnormality is found in the equipment that directly affects normal operation, the value should be reported, the pressure increase should be stopped, and the pressure increase should be continued after the defects are eliminated.