Automatic constant pressure control of water supply system based on Delta PLC and VFD (Variable-frequency Drive)

Abstract: This paper introduces the automatic constant pressure control of Delta PLC and VFD (Variable-frequency Drive) in the water supply system of the beverage factory, and designs the automatic constant pressure control method of the water supply system. The structure of the control system, the working process, the control flow of the PLC ladder diagram and the design of the human-machine interface of the touch screen are analyzed. The system is systematic, reliable, easy to operate, highly automated, intuitive man-machine interface, and has high application value.

Keywords: PLC touch screen water supply system

?1 Introduction

The conventional water supply system adopts the constant speed control of the water pump, and adjusts the flow and pressure by changing the size of the valve to achieve constant water pressure. This method has the following problems in operation.

(1) There is adjustment lag in manual operation, the stability of the whole system is poor, and the degree of automation is low, which makes the overflow pipe drain frequently and cause waste of resources;

(2) The constant speed operation of the water pump not only causes a waste of electric energy, but also causes damage to the bearings due to the long-term high-speed operation of the pump, which affects the service life of the pump, and the spare water pump is rusted to death;

(3) The water pressure cannot be guaranteed during the peak water consumption period in summer every year. When a sudden power grid failure occurs, the insufficient water volume will cause inconvenience to the production of the enterprise;

In order to provide constant pressure water supply, it is very important to transform the water supply system of the beverage company. Programmable logic controller (referred to as PLC) is an electronic device specially designed for digital operations in industrial environments. It has a series of advantages such as good versatility, high reliability, flexible installation, convenient expansion, and high cost performance, and its bus and network capabilities are getting stronger and stronger. It can easily form a control system with the host computer to achieve high cost performance and high performance of the system. operate.

In this paper, an automatic constant pressure water supply control system was developed by using Delta's PLC and VFD (Variable-frequency Drive) products, which can effectively reduce the burden on the staff, improve the optimal operation of the water supply system, and enhance the system's anti-interference performance. Avoid hardware aging losses.

2 System introduction

In order to improve the production environment, a large beverage company invested in a clean water technical renovation project and built a water supply system with a daily output of 50,000 tons of water. It also built a water pump system, a pressurized pump system and a high-level pool. According to the characteristics of the company's water demand, part of the water from the pump system is directly supplied to the production water department, and part of it needs to be transported to the high-level pool through a booster pump, and the water pressure supplied to the production water department is quite different from the water pressure supplied to the high-level pool. . At the same time, the high-level pool is more than ten kilometers away from the pump room. It is also difficult to solve the liquid level of the high-level pool, the design of the booster pump system, and how to "link" with the pump system.

In view of the above characteristics, we have designed an automatic constant pressure control water supply system that combines PLC control and VFD (Variable-frequency Drive) control from the perspective of technical reliability and economical practicality. It realizes the control purpose of "remote linkage" between the booster pump system and the suction pump system.

3 System scheme

The control of the system uses Delta's DVP-EH type DVP64EH00R PLC as the technology platform, DOP-A57CST touch screen as the operation interface, and VFD110B43A VFD (Variable-frequency Drive) as the execution component. The touch screen is connected with the COM port of DVPEHPLC through the COM2 port, and adopts the MODBUS protocol. PLC controls VFD (Variable-frequency Drive) through RS485 port (easy to expand), and supports MODBUS protocol. In addition, it also includes Schneider's soft starter, motor protector, data acquisition and its auxiliary equipment (see Figure 1).

Figure 1 The block diagram of the system

3.1 Suction pump system

The whole pumping system has four 150KW deep well pump motors and two 90KW deep well pump motors. It adopts the VFD (Variable-frequency Drive) circulation mode, and the six motors can be set to work under the frequency conversion mode. Use a 150KW and a 90KW soft starter 150KW and 90KW motors. When the VFD (Variable-frequency Drive) works at 50HZ and the pressure of the pipe network is still lower than the lower limit set by the system, the soft starter will automatically start a motor and put it into power frequency operation. When the pressure reaches the upper limit, it will automatically stop the work frequency running motor.

The system equips each motor with a motor protector because the power of the motor is large. In order to ensure the good use conditions of the motor under the conditions of overload, undervoltage, overvoltage, overcurrent, phase sequence imbalance, phase loss, and motor idling, etc., To achieve the purpose of prolonging the service life of the motor.

The system is equipped with a water level display instrument, which can perform high and low level alarms. At the same time, the PLC can ensure the water quality monitoring at a reasonable water level, and also protect the normal operation of the electric mechanism.

The system is equipped with a flow meter, which can not only display the cumulative flow rate for a period of time, but also display the instantaneous flow rate, and can carry out the statistics of the water output and the monitoring of the water output flow of each pump.

3.2 Solutions to different pressure water supply demands in the company

In order to stably and reliably meet the requirement that the water supply pressure (0.4~0.45Mpa) in some areas of the company is lower than the water pressure (0.8~0.9Mpa) of the main network, it is equipped with a pressure-stabilizing and pressure-reducing valve to adjust, and the adjustable range is 0.1~0.8Mpa .

3.3 Booster pump system

Since the pump room is far away from the high-level pool, the head of the pump that directly supplies water to the high-level pool is insufficient. Therefore, a pressurized pump room is designed at a height difference of 36 meters from the high-level pool, equipped with two vertical centrifugal pumps (one for one and one for the other) Preparation) The power of the motor is 75KW, and the head is 36 meters. The control system of the booster pump needs to consider the following conditions:

(1) If the water level in the high-level pool is low and there is water in the main pipe, open the water inlet electric butterfly valve and start the booster pump to supply water to the high-level pool;

(2) If the water level of the high-level pool is full and there is water in the main pipe, an alarm signal will be given and the booster pump and water inlet electric butterfly valve will be turned off;

(3) If there is no water in the main pipe, it means that the water consumption has increased or the water supply of the pump room has stopped, and the electric butterfly valve must be opened to replenish water from the high-level pool to the main pipe.

Like the water pump, we equip the booster pump with a soft starter and motor protector to ensure the long-term reliable operation of the booster pump. At the same time, it is equipped with a water level sensor, a digital display and a water shortage sensor for the high-level pool.

In order to ensure the constant pressure supply of the entire main water pipe network, when the high level pool is full and there is water in the main water pipe, the booster pump stops, at this time the pressure in the main pipe will be "suppressed", which will eventually cause the pressure in the main pipe to rise, and this pressure will be transmitted to the In the water pump room, the control system of the water pump detects this pressure and performs constant pressure frequency conversion control, so as to achieve constant pressure water supply of the entire main network.