Is it possible to omit the frequency converter in the constant pressure water supply system? What is the use of the inverter in the constant pressure water supply system? (1)

1. Introduction

For a long time, traditional regional and building water supply systems have been re-pressurized by municipal pipe networks and water towers or sky pools to meet users’ requirements for water supply pressure. In this kind of water supply system, the booster pump usually uses the water pressure requirement of the most unfavorable water point to determine the corresponding head design, and then the pump set is selected according to the change of flow, and the operation mode of the pump is determined. Due to the obvious changes in the season and time of the water use in the community, the daily water supply operation control often adopts the operation mode adjustment of the water pump and the outlet valve opening to adjust the water volume and pressure of the water supply. A lot of energy is wasted in the outlet valve, and there is a pool. “Secondary pollution”.
The application of variable frequency speed regulation technology in water supply pumping stations has successfully solved the two major problems of energy consumption and pollution. In actual operation, the variable frequency and constant pressure water supply technology for the community has obvious advantages over the traditional pressurized water supply system, such as stable water pressure and low maintenance and operation costs.

2. The basic principle of variable frequency and constant pressure water supply

The frequency conversion constant pressure water supply control system uses the measured pressure of the pipe network to adjust the output frequency after the calculation of the built-in PID regulator of the inverter to realize the constant pressure water supply of the pipe network. The frequency overrun signal of the frequency converter (usually used as the pressure limit signal of the pipe network) can inform the PLC to switch the frequency conversion pump logic in a timely manner. In order to prevent the occurrence of water hammer, the start and stop of the pump will link its outlet valve.
Suppose that the entire system consists of three pumps, a frequency converter (with built-in PID regulator and simple programmable controller functions), a pressure transmitter and several auxiliary components. The functions of each part are as follows: the pressure transmitter installed on the water supply pipeline converts the pressure of the pipe network into an electrical signal of 1-5 volts; the frequency conversion governor is used to adjust the speed of the water pump to adjust the flow; PLC is used for logic switching.

In addition, the above-mentioned system is also equipped with peripheral auxiliary circuits to ensure that when the automatic control system fails, the system can be maintained in manual adjustment mode to ensure continuous production.
According to the principle of pump-pipe water supply, there are two methods in principle to adjust the water supply flow
The first is throttling adjustment. When the water supply valve is opened, the flow rate increases; when the water supply valve is closed, the flow rate decreases.
The second is speed regulation. The speed of the water pump increases, and the water supply flow increases; the speed decreases, and the flow decreases. For occasions where the water flow often changes (such as domestic water), the speed adjustment is used to adjust the flow, which has excellent energy-saving effects.

3. Two working modes of variable frequency speed regulation and constant pressure water supply

(1) Working mode of multi-pump parallel variable frequency constant pressure variable water supply: When the water flow is less than the flow of one pump under the condition of power frequency and constant pressure, a variable frequency pump will adjust the speed and constant pressure water supply; when the water flow increases, frequency conversion The speed of the pump automatically rises; when the speed of the variable frequency pump rises to the power frequency speed, in order to further increase the water flow, it is controlled by the variable frequency water supply controller to automatically start a power frequency pump, and the flow rate provided by the power frequency pump is constant (The flow rate under the power frequency speed and constant pressure), the other parallel power frequency pumps are put into operation according to the same principle.
In the case of water supply with multiple pumps in parallel with variable frequency constant pressure variable, when the water flow drops, the speed of the variable frequency speed control pump decreases (the frequency of the inverter power supply decreases); when the frequency drops to zero flow, the frequency conversion water supply controller sends an instruction, Automatically shut down a power frequency pump to exceed the parallel water supply. In order to reduce the impact (hydraulic or current impact) when the power frequency pump is automatically switched on or exceeded. When it is put into use, the speed of the variable frequency pump will automatically decrease, and then slowly increase to meet the requirements of constant pressure water supply. When exceeded, the speed of the variable frequency pump should automatically rise, and then slowly fall to meet the requirements of constant pressure water supply. The above frequency automatically rises, and the fall is controlled by the water supply inverter controller.
(2) Constant pressure variable cycle start and start first stop working mode: In this water supply mode, when the water flow is less than the flow of the variable frequency pump at constant pressure and power frequency, the variable frequency pump will automatically adjust the speed to supply water. When the water flow increases, the speed of the variable frequency pump increases. When the speed of the variable frequency pump rises to the power frequency speed, the variable frequency water supply controller controls the water pump to switch to the power frequency grid directly (not supplied by the frequency converter). The frequency converter additionally starts a parallel pump into operation. As the water flow increases, the remaining parallel pumps are soft-started in the same way as above. This is the cyclic soft start input method.
The variable-frequency constant-pressure water supply system uses a potentiometer to set the pressure (the internal panel pressure can also be used), a pressure sensor (feedback is 4-20mA) to detect the pressure in the pipe network, and the pressure sensor sends the signal to the inverter PID loop After the PID loop is processed, it sends a water increase or decrease signal to control the motor speed. If the pressure is still insufficient or excessive within a certain delay time, another water pump can be started through the frequency converter to switch between power frequency and frequency conversion to make the actual pipe network pressure consistent with the set pressure. With the reduction of water consumption, the frequency converter automatically reduces the output frequency or cuts off the water pump, achieving the purpose of energy saving.

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