The basic control strategy of the constant pressure water supply control system is: using the motor speed control device and programmable logic controller (PLC) to form the control system to optimize the speed control operation of the pump group, and automatically adjust the number of pumps to complete the closed-loop control of the water supply pressure, so as to achieve the purpose of stabilizing the water supply pressure and saving electric energy when the flow of the pipe network changes. The control target of the system is the outlet pressure of the main pipe of the pump station. The water supply pressure value set by the system is compared with the actual value of the main pipe pressure fed back. After the difference is input into the CPU for calculation and processing, the control command is issued to control the number of pumps and motors in operation and the speed of the variable pump motor, so as to achieve the pressure stability at the set pressure value.

At present, the motor speed control devices used in the automatic constant pressure water supply system all adopt AC frequency conversion technology, and the control device of the system adopts PLC controller or special water supply control system. Because PLC can not only realize the logical control of pump group and valve, but also complete the digital PID regulation function of the system. It can monitor all kinds of operation parameters and control points in the system in real time, and complete the CRT picture display, fault alarm and print report forms of the system operation condition, so it is widely used in some high standard control systems. In the municipal water supply network: the automatic constant pressure water supply system has a standard communication interface, which can be connected with the upper computer of the urban water supply system to realize the optimal control of the urban water supply system, and provide modern dispatching, management, monitoring and economic operation means for the urban water supply system. With the continuous expansion of the scale of urban construction and the improvement of living standards, coupled with the implementation of one meter water supply for each household in residential quarters, the demand for the reliability (pressure and flow) of municipal water supply network is higher and higher, and various decentralized or centralized pressure facilities are also gradually increasing. In these pressurized facilities, the variable water supply mode of regulating pool and frequency control constant pressure water supply system (hereinafter referred to as the system) has shown great advantages. The design and application experience are described as follows:

1、 Basic information of the system

The system is composed of frequency converter, digital regulator, programmable logic controller (PLC), sensor, pump motor and related electrical control equipment. It is a mechatronic intelligent equipment with frequency conversion speed regulation and automatic closed-loop control function.

The system has the function of constant pressure control and variable flow water supply. Through the pressure sensor and flow sensor installed on the main outlet pipe, the system converts the non electric signals of pressure and flow into electrical signals in real time and inputs them to the input module of programmable logic controller (PLC). The signals are processed by CPU and compared with the set signals to obtain the best operating condition parameters. The output module of the system outputs logic control instructions and frequency conversion It can control the number of pumps in operation and the operating conditions of variable displacement pumps, and realize the soft start, soft switching and frequency conversion operation of each pump according to the instructions of CPU. The system can automatically determine the circulating operation of pumps according to the change of water consumption of users, so as to improve the stability of the system and the quality of water supply.

2、 System control function

1. The start and stop of water pump is controlled by PLC, which can have full cycle soft start function;

2. It is equipped with automatic, manual switching and manual operation devices. When the control cabinet is not used, manual operation can be used to make the water pump run directly under the power frequency;

3. Timing rotation function: control water pump (including standby pump) periodic automatic exchange use, so that the pump life is basically the same;

4. Stop pump protection after water shortage in front storage tank, with fault display function;

5. The water supply system is generally equipped with a small pump or pressure tank to adapt to the use of small flow (such as at night), and the system can automatically switch and control (optional function);

6. It has the function of automatically starting the fire pump with power frequency, or automatic frequency conversion to meet the requirements of fire water supply;

7. Electrical protection functions such as phase loss, leakage, overload and instantaneous power-off protection.

3、 Introduction to common functions

1. PID regulation function

   the water pressure signal (4-20mA or 0-5V) fed back by the pressure sensor is directly sent to the input port of PLC. The input signal is the same as the set pressure value. After PID comparison and calculation, the number of pumps to be switched and the operation frequency of variable frequency pumps are determined. The system parameters are constant in the actual operation, so that the system pressure is always stable in the set range.

2. Sleep function

In order to save energy, the system has a special "sleep" function which can make the water pump stop working. When the frequency output of the frequency converter is lower than its lower limit, the main pump stops working (in sleep state), and the sleep water pump starts to make up water. The lower limit frequency of "sleep value" inverter output and "sleep confirmation time" can be set by inverter parameters. When the output frequency of the frequency converter is lower than the sleep value, the frequency converter will continue to work. When the output frequency of the frequency converter is higher than the sleep value, the frequency converter will enter the sleep state. The "wake-up value" is started by the lower limit of water supply pressure. When the water supply pressure is lower than the lower limit, the PLC sends a command to wake up the inverter.

2. Communication function

   the system has the communication function of computer, PLC and frequency converter are provided with RS232 or 485 interface, PLC can choose a computer to communicate with one or more sets of systems, using the computer can monitor: current, voltage, frequency, speed, pressure, etc., can also control all kinds of parameters of frequency converter. In addition, the system also has manual / automatic operation, fault alarm, operation status, current, voltage, frequency status display, water shortage protection and other functions.

4、 Engineering application

In engineering design, we should first determine which pumps should be controlled by frequency conversion, so as to set the program in advance on PLC. Because the coefficient of variation of water supply in residential quarters is large, the selection of variable frequency control pump should be considered in the following situations:

1. Water consumption changes little with time: water is supplied continuously 24 hours a day, and the maximum flow of the working pump is still large when the water consumption is low. At this time, the same type of pump with the same specification can be selected. According to the peak water consumption, the scheme of one for use and one for standby or multiple for use and one for standby can be selected to control all pumps by frequency conversion.

2. When the residential area is large in scale, constant speed pump and variable frequency speed pump can be used to supply water. At this time, the soft starter is used to start and stop the constant speed pump, and the "variable frequency" is used in the whole control system+ The mode of "soft start fixed control" can reduce the number of control points on PLC, which is better in terms of technology, economy and operation and maintenance.

3. Water supply mode for living and fire pumps: in some residential areas where fire pumps are installed, due to long-term standby of equipment, the reliability will be reduced, which not only increases the project investment, but also reduces the resource utilization rate. At this time, living and fire pumps can be used together. Electrical control should be: life and fire two kinds of working pressure values are set in advance on PLC, normal life water supply is usually made, the equipment runs in low water pressure state, when a fire occurs, the system automatically switches the water pressure to the fire high pressure state, and there are devices on the pipeline that can ensure the pressure and flow required by the fire state. In order to consider the economy and rationality of pump selection, this method is generally adopted when the difference between high and low water pressure is not big.

5、 Economic benefit analysis

From the previous frequency conversion energy-saving principle, we know that the relationship between pump power P1, water supply Q1 and pump speed n1 is as follows:

P1/P=(n1/n)3 Q1/Q=n1/n

Where: Q - rated flow, Q1 - actual flow, P - shaft power of rated flow, P1 - shaft power of actual flow, N - rated speed of water pump, N1 - actual speed of water pump

When the rated flow is q = 100%, n = 100%, P = 100%; if N1 = 90% N, Q1 = 90% Q, P1 = 72.9% P, the power can be saved by 27.1%; if N1 = 80% N, Q1 = 80% Q, N1 = 51.2% N, the power can be saved by 48.8%.

In practice, the selection of pumps in most systems is designed according to the peak of daily actual demand, and the flow margin of general selection is between 5% and 15%. After the transformation of frequency conversion system and the use of dormant small pump, it is very easy to save more than 20% energy. Because of its obvious power-saving effect, the system can recover investment quickly, benefit for a long time, and the social benefit is also very huge.

6、 Other advantages of the system

Compared with other water supply modes, frequency conversion and constant pressure water supply system has the following obvious advantages besides remarkable energy saving effect:

1. The constant pressure water supply technology adopts the frequency converter to change the power frequency of the motor, so as to adjust the speed of the water pump and change the outlet pressure of the water pump. Compared with the way of controlling the outlet pressure of the water pump by adjusting the valve, it has the effect of reducing the pipeline resistance and greatly reducing the closure loss.

2. Because the variable displacement pump works in the frequency conversion condition, when its outlet flow is less than the rated flow, the pump speed is reduced, the wear and heat of the bearing are reduced, and the mechanical service life of the pump and motor is prolonged.

3. The pump motor adopts the soft start mode, and accelerates according to the set acceleration time, so as to avoid the impact of the current impact on the grid voltage fluctuation when the motor starts, and avoid the surge of the pump system caused by the sudden acceleration of the motor. Eliminate water hammer completely.

4. Constant pressure automatic control can be realized without frequent operation of operators, which reduces labor intensity and saves manpower.

7、 Application fields of constant pressure water supply

It can also be used in hot water supply, constant pressure spray and other systems.