introduction
With the continuous development of 3C technologies (computer, communication, and control) and frequency conversion technologies, water supply systems in residential communities, industrial water supply, various waterworks, large factories and mines, and fire-fighting water supply systems have traditionally relied on equipment such as water towers and elevated water tanks. These systems not only occupy large areas and require significant investment, but are also increasingly unable to meet modern water supply requirements. Furthermore, traditional methods struggle to guarantee real-time water supply, and because water pumps are designed for maximum capacity, peak water usage periods are short, easily leading to energy waste and pipeline damage due to excessive pressure. This paper takes the control system of the Yongcheng Coal and Power Group water plant as an example to introduce a constant pressure water supply system based on PLC and touchscreen control.
1. System Introduction
The variable frequency constant pressure water supply system of Yongcheng Coal and Power Group's water plant consists of a PLC controller, two variable frequency speed controllers, a soft starter, a touch screen display, an AC contactor, a thermal relay, a pressure transmitter, a water level transmitter, a flow transmitter, and other electrical control equipment, as well as four 75kW water pumps and one 45kW small water pump, as shown in Figure 1.
Figure 1. Diagram of constant pressure water supply system
A water level transmitter is installed in the water storage tank of the water supply system, and a pressure transmitter and a flow transmitter are installed on the main outlet pipe to detect the water level, pressure, and flow rate. These signals are converted into 4~20mA signals and input to the analog input module of the PLC. The detected pressure signal and the pressure set via the touch screen are processed by PID calculation and controlled by the PLC.
The system adjusts the pump motor speed by controlling the output frequency of the frequency converter to maintain constant water pressure. Simultaneously, water level detection determines the number of water source wells opened and the reverse osmosis system's water production capacity adjusted based on the water level. The touchscreen display shows the current, frequency, water level, water pressure, operating time at both mains and variable frequency, and the operating status of each pump. System information can also be published to the company's intranet for remote system diagnostics and control.
2 Controlled Objects
Based on the real-time situation of the water supply system at Yongcheng Coal and Power Group's water plant, the system controls the opening, closing, stopping, and operation of on-site equipment (pumps, valves, etc.); the opening and closing of solenoid valves; the opening and stopping of each water source well; and the starting/stopping of the frequency converter, in order to achieve automatic control of constant pressure water supply at the water plant.
3. System Working Principle
The system has both manual and automatic operation modes:
3.1 Manual Operation Mode
When manual operation is selected, each water pump can be controlled by pressing the start and stop buttons as needed. This mode should only be used when a system malfunction occurs.
3.2 Automatic Operation Mode
(1) The pump is also faultless. Start the vacuum pump to draw a vacuum. If the requirements are met, the AC contactor of pump #1 will engage, and the motor and the frequency converter will be connected. At the same time, the solenoid valve of pump #1 will be opened. By detecting the pressure, the PLC will perform PID calculation. At this time, the output frequency of the frequency converter will start to rise from 0 Hz. If the pressure is not enough, it will rise to 50 Hz. After a delay, the soft starter will switch pump #1 to the power frequency and then start pump #2. This process will continue until the outlet water pressure reaches the set pressure.
(2) Pump switching procedure
Based on the flow rate detected by the flow sensor, if the water output decreases and the water pressure is too high, the PLC controls the inverter's output frequency to reduce the water output and stabilize the water pressure. If the inverter's output frequency is 20Hz, the PLC starts timing. If the water pressure decreases, the timing stops. If the water pressure remains higher than the set pressure, after a certain time, according to the principle of first-in, first-out, the PLC will first close the solenoid valve of the pump that has been running the longest, and then shut down that pump, until the water pressure reaches the set value.
(3) Start the small power pump
For the residential water supply of Yongcheng Coal and Power Company, which is a system with highly seasonal water usage, pumps 1-4 are 75kW, and pump 5 is 45kW. Therefore, during periods of low water usage, a single 45kW pump is sufficient to maintain water pressure. In such cases, pump 5 can be operated via frequency converter to operate at a low frequency, thus maintaining stable water pressure in the water supply system.
(4) Remote control and fault diagnosis
The control system sends information to the host computer located in the logistics department of the bureau via the PLC's communication module CP340. Communication between the two uses RS-485. When the host computer detects a fault, such as a low water level in the reservoir, it can notify the activation of more water source wells and reverse osmosis units to raise the water level and prevent pumps from running dry due to low water levels. Because the host computer can publish information over the network, when a fault occurs that on-site personnel cannot resolve, the manufacturer can assist with troubleshooting through remote diagnostics.
4. Control System Hardware Configuration and Software Programming
4.1 Hardware Configuration
(1) PLC configuration
The programmable controller uses a Siemens S7-300 series CPU-314 main unit, one power supply module, two 32-bit I/O modules, two 16-bit output modules, two 12-bit AD modules, one 12-bit DA module, and one CP340 module. The CP340 communicates with the host computer via RS-485.
(2) Touch screen
The touchscreen uses the Siemens TP270 series, which has a rich operating system and can easily communicate with a programmer via serial communication or with the PLC via MPI network. The touchscreen can display real-time motor current, pipeline pressure, water level, flow rate, operating status of each water source well, and various fault information; it also allows for online control of the PLC.
(3) Selection of frequency converter and control method
In this system, based on the principle of redundancy design, two Mitsubishi dedicated water supply frequency converters are used for mutual backup in case of failure. The start/stop of the frequency converters is controlled by PLC digital output.
4.2 Software Programming
(1) PLC programming
From a software configuration perspective, a PLC has a system program and a user program. The system program is installed on the CPU module and comes with the hardware. The user program is programmed by a programmer and input into the programmable controller's storage module. The program adopts a block-based structure with five forms: organization block (OB), program block (PB), function block (FB), data block (DB), and sequence block (SB) (not used in this system). Therefore, in the PLC software, the various functional program modules are organically combined through the main program. Thus, the PLC program mainly handles the startup and switching programs of various pumps in the field, the startup of small-power pumps, analog signal processing, and the processing of communication data with the host computer and touch screen.
(2) Host computer program
The host computer uses VB to write communication programs and establish databases. The command frame format issued by the communication program must fully conform to the RS-485 communication protocol of Siemens PLCs; the host computer must disassemble and identify the response frames sent back by the PLC in order to correctly separate the exchanged data and useful status information.
(3) Touchscreen program
The touch screen program mainly consists of screens such as the main screen, parameter settings, power supply circuit, reverse osmosis, real-time curves, report statistics, and fault maintenance, as shown in the figure below.
5. Conclusion
The water supply system employs a variable frequency constant pressure water supply system based on PLC and touch screen control. This system can automatically adjust the pump speed or accelerate/decelerate according to actual water pressure changes, achieving constant pressure water supply and reducing energy consumption; it can also extend the service life of the main pump motor. Because it uses dual frequency converters, the system can automatically switch to the other frequency converter if one fails, essentially maintaining uninterrupted constant pressure water supply, demonstrating a certain level of advanced technology.
Currently, after more than a year of operation at the Yongcheng Coal and Power Group's water plant, the system is performing well, meeting the expected design requirements, demonstrating significant energy-saving effects, and receiving high praise from users. It also shows great promise for future applications.
