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The start mode of the feed water pump is variable frequency soft start

05 Nov 2020Email

The start mode of the feed water pump is variable frequency soft start

 

 


1. Boiler water supply system
All the piping systems from the deaerator feed water tank through the boiler feed water pump, the high pressure heater to the boiler feed water console are called the boiler feed water piping system. The boiler water supply piping system can be divided into low pressure and high pressure water supply piping systems according to its pressure. The pipes and valves from the downcomer of the deaerator feedwater tank to the inlet of the feedwater pump are called low-pressure feedwater piping systems. The pipes and valves from the outlet of the feedwater pump through the high-pressure heater to the front of the boiler feedwater operation table are called the high-pressure water supply piping system. The water supply pipeline of our 250MW unit adopts mother control. Three electric feedwater pumps with 100% capacity are installed in the system, two of which are in operation and one is standby. On the outlet pressure side of each feedwater pump, a check valve and an electric gate valve are installed according to the water flow direction. The function of the check valve is to prevent pressure water from flowing back into the feedwater pump and reverse the feedwater pump when the feedwater pump stops working. The function of the gate valve is to cut off the connection with the high pressure side when the feed water pump stops working. The boiler feedwater console includes two parallel pipelines, 100% main feedwater and 30% bypass feedwater. The two parallel feedwater pipelines are equipped with main feedwater electric gate valve, electric main control valve, bypass electric gate valve and bypass electric control. valve.

 


2. Water supply control method
The task of steam drum boiler feed water control is to adapt the feed water volume to the boiler evaporation capacity and keep the water level in the steam drum within a certain range. The water level of the steam drum is an important factor affecting the safe operation of the boiler. If the water level is too high, it will destroy the normal operation of the steam-water separation device. In severe cases, it will cause the steam to carry more water, thereby increasing the scale on the superheater tube wall and the turbine blades. Even the water impact of the gas turbine will damage the blades; if the water level is too low, the water circulation will be destroyed and the water wall will be broken. Range of water level fluctuation during normal operation: (30~50)mm Abnormal situation: 200mm The current water supply process control system uses a fixed-speed water supply pump, which can be changed by changing the opening of the regulating valve to change the resistance of the water supply pipeline. Water supply. In the stage of boiler heating and pressure increase, steam turbine impulse rotation and 30% load, the single impulse system is used to maintain the drum water level within a given range by adjusting the opening of the feedwater bypass valve. In the 30%~100% stage, a three-impulse system is used to adjust the opening of the main feedwater regulating valve. The drum water level is measured by a redundant transmitter, and the optimal value is taken as the measured signal after the drum pressure is corrected; the feed water flow is corrected by the feed water temperature as the total feed water flow; the main steam flow is corrected by the main steam pressure as the main steam flow .

 


3. Raising the question
The control system can not only accomplish the task of liquid level control well under more frequent load changes, but also overcome the disturbance caused by the change of water supply. The system has fast response, good stability and strong anti-interference ability, but there are some problems. The input current of our measured motor is 65A, the input power is about 552.5kW, and the difference between the input power and the actual power consumption of the pump is 267.46kW. This power loss mainly includes:
(1) Loss of the motor itself.
(2) Loss of water pump and pipeline.

 


4. Application of frequency conversion speed regulation technology
4.1 Analysis The frequency converter converts the AC to DC smoothly, and then passes through the inverter circuit after smoothing and filtering. The DC is converted into AC of different frequencies, so that the motor can obtain the voltage, current and frequency required for stepless speed regulation. According to the basic laws of fluid mechanics, we know that fans and pumps are all square torque loads, flow is proportional to speed, pressure is proportional to the square of speed, and power is proportional to the cube of speed. We know that when the frequency converter adjusts the speed downward, it belongs to the constant torque characteristic. However, due to the structural characteristics of a general three-phase asynchronous motor, it is difficult to reach its rated output torque when its operating frequency is below 50 Hz. The torque required for fan and pump loads is also reduced when running at low speeds, which just adapts to the characteristics of reduced output torque when the three-phase asynchronous motor is driven by a frequency converter at low speeds.
4.2 System composition According to the above analysis, we can use the regulator's output tracking signal 0~10V to send to the frequency converter to synchronously change the speed of the water pump and the motor while the water flow is controlled by the regulating valve. This can not only achieve the purpose of controlling the size of the water flow, but also reduce the mechanical wear of the water pump. While ensuring the constant level of the boiler drum, it can also achieve the effect of saving electricity. Because when the flow rate drops, its speed will decrease in proportion to the flow rate, and the shaft power of the water pump will decrease in proportion to the third power of the speed. Therefore, changing the speed and adjusting the flow rate has a significant energy saving effect. The output frequency of the inverter will be proportional to the signal input to the inverter, that is, if the input signal is large, the frequency of the inverter output is also high, and the signal is small, and the output frequency is also small.
4.3 Precautions for inverter selection The upper limit frequency of the inverter cannot exceed 50Hz. If it exceeds, the power will increase sharply, and the mechanical strength of the motor and the capacity of the inverter will not meet the requirements. Generally, pump loads should not be operated below 15Hz, otherwise the temperature rise of the motor must be considered.

 


5. Effect
(1) After the feedwater pump adopts frequency conversion speed regulation technology, as the speed of the feedwater pump decreases, the power consumption is also reduced, and the power saving effect is significant.
(2) The starting method of the feed water pump is a frequency conversion soft start, which effectively reduces the impact of large current when the motor starts, and extends the service life of the motor.
(3) When the load is low, the speed and output of the pump will decrease, thereby reducing bearing wear and heating, and prolonging the service life of the feedwater pump.
(4) The water supply pressure is stable, and the frequency conversion system can adjust the water output of the feed water pump according to the change of the drum water level to ensure the stability of the main pipe water pressure.
(5) The automation degree of the equipment is improved, which reduces the labor intensity of the operators.

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