Improvement of high-profile door control method for feed-water pump steam turbine

Abstract : The improvement of MEH high-pressure control gate control mode has been implemented in #1 and #2 units of Changxing Power Generation Co., Ltd. This project is used as an example to introduce the improvement method and the improvement in the debugging process in detail. It is proposed to be promoted on the same type of units. application.

The water supply system of No. 1 and No. 2 300MW units of Changxing Power Generation Co., Ltd. uses two 50% capacity small turbines to drive two half-capacity boiler feed pumps, and a 30% capacity motor-driven boiler feed pump serves as a backup. The water-supply turbine uses the NK50/56/0 steam turbine from Hangzhou Steam Turbine Works. The small turbine control system uses the MEH-IIIA system of Xinhua Control Engineering Co., Ltd.

1. The original control system design The steam pump speed control system is a MEH system driven by high-pressure anti-fuel oil. During normal operation, the working steam is extracted from the engine's four-stage extraction. The standby steam comes from the reheater cold-end steam.

The half-capacity water-supply pump steam turbine has only a single steam chamber, and the variable-speed steam source is switched using an external switching method.

External switching principle: When the host load is higher than the switching point, the feed pump steam turbine is driven by the steam source from the four extraction steam source, and the high-pressure control door is closed; when the load of the host machine drops to the switch point, the high-pressure control door opens, and the higher pressure from the cold is then The steam enters the water-supply-pump turbine after throttling at a high pressure, and at the same time, the four pumping-off to the water-supply-turbine branch check valve is closed, and the feed-water pump turbine automatically switches from the low-pressure steam source to the high-pressure steam source, since the steam source switch is In the external realization of the water pump turbine, it is called outside switching.

Small turbine adjustment system block diagram (before improvement) see Figure 1.

The thermal calculation data of the original design conditions are shown in Table 1, Table 2.

Original MEH control system schematic

The overlap between low-key and high-profile doors is 10%, as shown in the following figure:

According to the above-mentioned thermal calculation data and MEH design, it should be able to meet the requirements of the feedwater turbine to ensure the quality of water supply regulation under the following conditions, but it is actually difficult to achieve.

1) The actual operation of the unit is that it starts with an electric pump first. Since the capacity of the electric pump is 30% and the leakage of the recirculation bypass valve of the electric pump is taken into account, the first steam pump is put into operation when the load of the unit rises to about 70 MW. The four-stage extraction pressure at the time of operation is 0.13 MPa and the cold pressure is 1 MPa. Because the pressure of the four pumps is very low, the feed pump steam turbine maintains the required output of the system. The low pressure control valve is often fully open. When the high pressure control valve participates in the adjustment, the control response of the feed pump steam turbine is very slow, causing the water level of the steam drum to run out of control.

2) During the start-up process, under the load of 80 to 90 MW, when the second pulverizer is started, the pressure of the main steam is greatly disturbed, and the pressure of the steam drum rises sharply. After the low-pressure control valve of the water-supply pump turbine is forced to open, the speed still cannot be reached. The water supply automatic control system commanded the target value, and the high-profile door was turned on, but the feed-water pump steam turbine responded slowly, but after a while (20s or so), the speed immediately soared, exceeding the target value, in order to suppress the rise of the drum water level. The water supply automatic control system instruction requires that the speed be reduced rapidly, and the speed is not to come down. Manual intervention is significant or not, and the result of the protection of the drum water level is the MFT.

The above phenomenon is analyzed as follows: When the working conditions change during operation, the rotational speed after the low-pressure regulating door is fully opened cannot reach the required rotational speed because the intake steam amount of the feed-water pump steam turbine is not enough. The pumping of the pump to the water pump turbine is relatively long and the system is relatively large. After the operation of the high-profile door, the steam that comes from the cold first raises the pressure of the inlet steam pipe of the feed pump steam turbine. The control signal of the high-profile door is taken from the speed of the small steam turbine. As a result, the speed of regulation rises slowly, and when the speed is required to be reduced, even if the high-speed power is turned off, the capacity of the system's capacity cannot be relieved in time. The low-voltage control door is in a fully open state and the speed is slow, making the adjustment characteristic of the entire system. Very poor. In addition, at the time of the unit RB, the four-supply pressure dropped rapidly. Due to the above reasons, the out-of-control of the small turbine speed also occurred, resulting in the drum water level MFT.

2, improve the idea:

In view of this type of feed water pump steam turbine variable speed steam source switching mode for external switching, and MEH control logic is based on the principle of internal switching design, feeling unreasonable, decided to feed water turbine steam turbine high pressure control from the speed to control the speed to close the valve before the steam pressure .

The actual operation found that when the host load 250MW, the four pumping pressure is maintained at 0.63MPa, the regulation characteristics of the small machine under this pressure is ideal, the actual speed is consistent with the required speed of the CCS, the response time is very short, the opening of the low-key door is maintained At 50%, there are allowances for various fluctuations. Therefore, it is envisaged that the high-speed door control speed will be changed to control the low-key door pressure, and the pressure regulator will be used to maintain a stable pressure in front of the low-key door; the low-speed door will control the speed and the low-key door will not be adjusted.

For the low-profile door after the transformation of the specific pressure of the automatic set value of 0.315MPa, in different operating conditions and then adjusted according to the actual situation to determine.

The control logic of the improved MEH high-profile gate is as follows:

Since the original low-turbine low-key door inlet pressure is not transmitted to the MEH signal, it is necessary to increase the signal. In order to improve the reliability of the control signal, three pressure transmitters are installed in front of the low-profile door of the small computer to access the MEH. The signal processing is performed in three steps, and the software is modified in the MEH control loop. Separate the high-profile door control from the low-profile door control and add a separate PID regulator to drive the high-profile door, which has reached the goal of controlling low-key door pressure. Another one is added to the MEH screen for easy operation.

Small turbine adjustment system block diagram (after improvement) see Figure 2.

3, the solution to the safety problem Since the transformation of the small machine high-profile door into a pipeline pressure regulating valve, so we still face safety problems in the transformation. When the unit is under rated load, the cold pressure is 3.7 MPa. If the high-profile door jams under this operating condition, the cold steam will be directly sent to the four pumping, and the four-pipe valve and the deaerator The safe operation has a great danger. For this we replaced the manual isolation door of the cold and the small machine with an electric isolation door, introduce the DCS control, add two pressure switches behind the high-profile door, and send the signal to the DCS, if the high-profile When the pressure behind the door exceeds 0.8MPa, it will delay 5 seconds by DCS protection action to automatically signal the cooling and then to the small electric isolation door, and the other will be pumped to the deaerator. The electric isolation door logic is modified to high pressure when the time delay is 5 seconds automatically Closed, its value is 0.9MPa. The above logic changes are completed in the DCS and the corresponding control screen is added. Three pressure transmitters are installed for the low-profile pressure measurement in front of the door, and software is used to determine the three pressures automatically to achieve the purpose of improving reliability.

4. The improved effect After improving the control loop of the small turbine, it improves the system's ability to respond to emergencies, increases the reliability and safety of the system, effectively reduces the labor intensity of the operating personnel, and achieves very good results. The MFT is avoided because the water level of the boiler drum is out of control due to the loss of control of the speed of the small machine, which is also a good guarantee for further improving the reliability of the unit operation in the future. From the standpoint of the transformation of the high-profile door control mode of the small machine into operation, the system has a reasonable design and is stable and reliable.

5. It is proposed to popularize and apply the improvement based on the high-profile gate control of the MEH system of Changxing Power Generation Co., Ltd. It has been successful, so it is recommended to promote the MEH system of the same and similar units.

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