Abstract: Negative pressure-free water supply systems are used for secondary pressurization of domestic water. The term "negative pressure-free" refers to the absence of pressure drop in the municipal water supply network during pump operation. This requires the combined use of multiple technologies, including vacuum suppression, flow stabilization compensation, pre-pressure balance compensation, energy storage and release, variable frequency speed control, and intelligent control. Currently, intelligent controllers include dedicated microcontroller-based controllers and PLC-based intelligent controllers. PLCs, with their flexibility and reliability, are increasingly used in negative pressure-free water supply systems. Hollysys' LM series PLCs, with their powerful data processing capabilities, optimized PID technology, and standard application programs for negative pressure-free water supply systems, are increasingly being used in negative pressure-free water supply equipment.
Keywords: negative pressure water supply; Hollysys LM series PLC; control system standard program
1 Introduction
Negative pressure-free water supply is developed based on variable frequency constant pressure water supply. Its advantage lies in directly connecting the system to the municipal water supply network without the need for water tanks or reservoirs. The water supplied to users is in a completely sealed environment, avoiding secondary pollution of drinking water during the supply process. This water supply method combines tankless water supply with variable frequency constant pressure water supply, achieving a more environmentally friendly and energy-efficient effect than other water supply methods. Furthermore, it fully utilizes the existing pressure of the municipal water supply during the supply process, thus saving more than 50% of electricity. Moreover, the negative pressure-free water supply system incorporates technologies such as vacuum suppression, flow stabilization compensation, pre-pressure balance compensation, energy storage and release, variable frequency speed regulation, and intelligent control. It does not cause pressure drop in the municipal water supply network while supplying water, thereby ensuring the normal operation of the municipal water supply network.
2. Overview of Negative Pressure-Free Water Supply Technology
The core concept of a negative pressure-free water supply control system is to control the operating frequency of water pumps through a frequency converter to achieve energy-saving water supply. The system also incorporates municipal pipeline protection, pump protection, and fault handling functions. To save costs, most systems currently use a single frequency converter to control multiple pumps, commonly known as a one-to-two, one-to-three, or one-to-four control system. In this method, the frequency converter controls each pump in turn.
There are two operating modes for the water pumps: variable frequency (VFD) and fixed frequency (SCFM) operation. The central controller automatically controls the pump's operation based on the user's water consumption. After system startup, the pumps initially operate in VFD mode. When water consumption increases, the VFD pumps switch to SCFM operation, and the next pump starts operating in VFD mode. When water consumption decreases, the SCFM pumps stop operating, and this pump switching process repeats continuously.
When water usage is low or nonexistent, the water pump can enter a dormant state to save energy and extend its lifespan. When water usage increases, the pump will automatically wake up from dormancy. When a pump has been running for an extended period, the system will automatically select the pump with the shortest running time to operate, ensuring balanced operation across all pumps. If a pump malfunctions, the system will automatically skip that pump, without affecting the switching on/off process of other pumps.
3. Structural Design of the Negative Pressure-Free Water Supply Control System
In traditional negative pressure water supply systems, the controllers are mostly designed with microcontrollers, which generally do not allow users to modify their internal programs. If additional functions are desired, the controller supplier must be consulted. This results in poor system flexibility, and these controllers typically haven't undergone reliability testing, potentially leading to various reliability issues.
Using a PLC as the core control unit ensures reliability, and its open programming environment makes system development and maintenance more convenient. Furthermore, Hollysys provides standard example programs for negative pressure-free water supply equipment, which users can directly use to build a negative pressure-free water supply control system, or modify as needed. This greatly improves the efficiency of system setup and the system's openness.
This design focuses on a 1-to-3 negative pressure-free water supply control system. The system mainly consists of a PLC, frequency converter, centrifugal pump, pressure sensor, water level sensor, phase loss protector, and fault alarm lights. The PLC is responsible for the switching control algorithm of the three pumps, dynamically adjusting the output frequency of the frequency converter based on the set pressure at the pipeline outlet, and implementing functions such as pump reversal, sleep mode, fault handling, and negative pressure compensation. This system uses a Hollysys LM series PLC as the system controller, and connects to a field touchscreen HT6720T via the LM series PLC's built-in RS232 interface. The touchscreen program has functions such as system parameter setting, displaying system operating status, and querying system alarm records. To facilitate user access to the control system's operating status and setting parameters such as outlet pressure, an SMS data transmission module is also connected via the LM series PLC's built-in RS485 interface, allowing users to check the pump's operating status and set the pipeline outlet pressure at any time via their mobile phones. The structure of the negative pressure-free water supply control system is shown in Figure 1.
3-1 Structure diagram of negative pressure-free water supply control system
4. Software Design of Negative Pressure-Free Water Supply Control System
According to the process requirements, the flowchart of the negative pressure-free water supply control system is shown in Figure 2.
Figure 2. Flowchart of the negative pressure-free water supply control system
The user interface of the negative pressure-free water supply control system is made using an HT6720T touchscreen. The interface primarily monitors the operating status of the control system, sets system operating parameters, displays real-time alarms, and displays historical alarms. The touchscreen's operating status interface, as shown in Figure 3, displays the operating status of each pump and several important output parameters, including inlet pressure, target pressure, and outlet pressure.
Figures 4 and 5 show the parameter setting interface, where system parameters can be configured. All settings are retained even after power failure. Data in the power-off retention area of the LM series PLC is permanently stored, eliminating concerns about losing system settings due to power outages. Figure 6 shows the touchscreen alarm query screen, allowing users to view real-time and historical alarm information.
Figure 3 Touchscreen operating interface
Figure 4 Touchscreen parameter setting interface 1
Figure 5 Touchscreen parameter setting interface 2
Figure 6. Touchscreen alarm query screen
5. System Characteristics
The negative pressure-free water supply system based on the LM series PLC has the following characteristics:
ì Reliability
Reliability is crucial for negative pressure water supply systems. Compared to traditional microcontroller controllers, the LM series PLC offers superior stability, interference resistance, and performance under harsh conditions. The input and output points of the LM series PLC are isolated from the field, providing a solid hardware foundation for system reliability. The incremental PID control block provided by the system allows for rapid adjustment of the water pump's operating frequency based on user water consumption, quickly stabilizing the water pressure at the set value and ensuring stable system operation.
ease of use
The LM series PLC is compact, highly integrated, fast in operation, and has a large logic control capacity. It supports both online and offline debugging and features powerful viewing capabilities, significantly reducing programming and debugging workload and shortening the debugging cycle. Its flexible programming functions greatly facilitate future system upgrades. The system uses a 7-inch TFT touchscreen, providing a clear overview of the entire system's operation. Various control parameters can be adjusted according to different operating conditions, and real-time and historical alarm lists can be generated. Furthermore, the LM series PLC utilizes its built-in RS485 interface to connect to an SMS data transmission module, allowing users to directly check the system's operating status and set parameters such as pipeline outlet pressure via their mobile phones at any time.
Functional
This system is built upon the standard program for negative pressure-free water supply using the LM series PLC. Its functions include automatic pump switching, pump hibernation/reversal, pump priority determination, optimized fault handling, multi-time period setting, automatic low water level protection, negative pressure-free operation, SMS communication, and PLC locking. The entire system was built with almost no modifications to the standard program, enabling a convenient and rapid construction of the control system while achieving powerful functionality.
References
[1] Tang Xiaomeng, Technical Analysis of Pressure-Free Water Supply Equipment [J], Journal of Guangzhou University
[2] Yao Hong, Tian Sheng, A brief analysis of the current operation status and energy-saving renovation measures of secondary booster pump stations [J], Energy Saving Technology
[3] Ma Shuhuan, Technical conditions for the approval of negative pressure water supply equipment and pipeline network, water supply and drainage
[4] Gu Zhansong, Principles and Applications of Programmable Controllers, National Defense Industry Press
[5] Hollysys, HOLLiAS LM Small Programmable Controller Hardware Manual, 2008
About the author:
Liu Yong, male, holds a master's degree in automatic control and currently serves as an industry technical support engineer at Beijing Hollysys Automation Drive Technology Co., Ltd.
