introduction
With the rapid development of control technology, computer technology, communication technology, and network technology, the scope of information exchange and communication is rapidly expanding from the field equipment level in factories to all levels of control and management, covering workshops, factories, enterprises, and even markets worldwide. Science and technology are the primary productive forces, and the level of scientific and technological development determines a country's future and destiny. Currently, some developed countries in Europe and America have reached a considerably high level of automation. In contrast, a large number of enterprises in my country currently require modernization to adapt to social development. Industrial Ethernet plays a crucial role in the field of automation control. If industrial control networks adopt Ethernet, they can be brought into the mainstream of computer network technology development; however, certain technologies are also needed to improve the network's real-time performance and reliability to meet the requirements of industrial control sites.
Boilers are widely used equipment in industrial production and daily life. Boiler water treatment is crucial for ensuring safe and economical boiler operation and saving fuel; it is a fundamental technical aspect of boiler operation. Improper or untreated boiler feedwater can shorten boiler lifespan, waste fuel, and even cause major equipment accidents and casualties. Currently, the most widely used process flow designs for boiler water treatment systems are ion exchange (chemical treatment) and osmosis (physical treatment). Traditional water treatment processes are often controlled by relays or manually, leading to frequent system failures, high labor intensity for workers, and high operating costs. This article introduces a PLC-based water treatment process control system for chemical treatment.
The network structure of industrial automatic control systems is becoming increasingly decentralized, and the systems are becoming more complex, with increasingly high-speed and tightly integrated connections between components. While fieldbus control systems offer numerous advantages in industrial control, the significant differences between various fieldbus communication protocols present numerous challenges in achieving full-system automation integration across the entire project. Therefore, industrial control networks that facilitate the full-system automation integration of factories are gradually becoming a research hotspot. Ethernet, due to its wide application and advanced technology, has gradually monopolized the communication field of commercial computers and the upper-level information management and communication in process control, and there is a growing trend towards its direct application in industrial settings.
1. Water treatment process flow
Figure 1. Schematic diagram of the process flow
The main method of chemical water treatment is ion exchange, which uses ion exchange resins to absorb ions from dissolved salts in the water. After a certain period of operation, the ion exchange resins become ineffective, at which point operation needs to be stopped and the resins regenerated (reduced) so that they can be reused (cation exchange resins are regenerated using acid; anion exchange resins are regenerated using alkali). To ensure an uninterrupted water supply, the chemical water treatment workshop is equipped with three sets of ion exchangers, which are regenerated in rotation.
The system schematic diagram is shown in Figure 1. The system consists of multiple devices, including three series of filters, cation exchangers, CO removal devices, anion exchangers, and demineralized water tanks. The programmable control system controls various valves, pumps, and fans in the process flow. It selects the operation of series 1, 2, and 3 based on the running time, displays the actual flow direction of the process on the process simulation board, and reflects the operating status of valves, pumps, fans, and dosing pumps.
This wastewater treatment system is divided into three stages. Primary treatment uses mechanical methods, such as screens, sedimentation, or flotation, to remove stones, sand, grease, and oils from the wastewater. The principle is to achieve solid-liquid separation through physical methods, separating pollutants from the wastewater. Secondary treatment is biological treatment, where pollutants in the wastewater are degraded and transformed into sludge by microorganisms. The process configurations are diverse, including activated sludge, AB, A/O, A2/O, SBR, oxidation ditch, stabilization pond, and land treatment methods, among others. This system uses the activated sludge process. Tertiary treatment is advanced wastewater treatment, including nutrient removal and disinfection through chlorination, ultraviolet radiation, or ozone technology. Depending on the treatment objectives and water quality, some wastewater treatment processes may not include all of the above steps.
2. Control System Design
According to operational requirements, the control device needs to control the commissioning, shutdown, and regeneration of three series of filters, cation exchange beds, and anion exchange beds. The operating modes are divided into Series 1 operation, Series 2 operation, Series 3 operation, Series 1 regeneration, Series 2 regeneration, and Series 3 regeneration. The operation and regeneration of the three series can be performed via jogging or automatic control using the selection keys. Jogging operates according to the programmed sequence, with the jogging button used to advance the steps. Automatic control automatically advances the steps at a pre-set time based on the operator's start command. When all three sets of equipment are running simultaneously in automatic control mode, all equipment produces water. Each set of equipment automatically performs regeneration and operation according to a pre-set operating water volume periodically. During operation, the PLC automatically compares the already running water volume parameters. When the already running water volume of two or three sets of equipment is close, the system will automatically issue an alarm, prompting the equipment management personnel to adjust the inlet water flow to prevent two or three sets of equipment from simultaneously entering regeneration mode.
Each set of equipment determines whether to regenerate based on the set operating water volume and the measurement value of the anion exchange bed intelligent conductivity meter. Under normal operating conditions, the equipment's transition from operating to regeneration mode is controlled by the preset operating water volume. When the equipment reaches the set water volume endpoint, it will automatically switch from operating mode to regeneration mode. During normal operation, in the event of a sudden power outage, water outage, high liquid level shutdown in the pure water tank, or low air pressure at the pneumatic valve inlet, the PLC's internal metering device will automatically hold the value. Upon restarting, it will resume measuring from the previously held flow rate, ensuring that the system does not make measurements during system shutdowns, water outages, or air outages. In addition, the anion exchange bed outlet of the equipment is equipped with an intelligent conductivity meter to prevent water quality from exceeding standards. When the pure water quality reaches the preset conductivity, the conductivity meter will automatically output an alarm signal. The PLC will automatically receive the signal and force the equipment to switch from operating mode to regeneration mode, thus ensuring that the effluent water quality will not exceed standards due to changes in the raw water quality.
In a certain water treatment control process, an industrial Ethernet control system consisting of an industrial computer and an S7-300 PLC was designed and debugged. The iFIX configuration software was used to realize the system's real-time monitoring, real-time alarm, historical curves, historical reports and other functions. The control principle is shown in Figure 2.
Figure 2 Control principle diagram
3. Hardware configuration of the control system
The configuration of hardware and software is an important parameter for evaluating the quality of a system. The hardware components of this system consist of two industrial PCs (one running, one hot standby), one S7-300 PLC, and one switch, as shown in Figure 3. Because there are many points that need to be controlled during the production process, and they are far apart, an S7-300 or higher PLC must be selected, and industrial Ethernet communication must be used.
The lower-level machine uses an S7-300 PLC from Siemens, Germany, and its main hardware configuration is as follows:
One CPU315-2DP module
PS307-1E power supply module (1 piece)
6 SM32232 24V digital output modules
8 SM32132 24V digital input modules
3 SM3318 channel analog input modules
One CP341-1C communication module with RS422/RS485 interface.
Based on control requirements, there are 290 DI (Distribution) quantities, 130 DO (Distribution) quantities, 32 AI (Input/Output) quantities, and 16 AO (Action/Input) quantities. A Quantum PLC system is selected, including one master station and four substations.
Figure 3 System Hardware Configuration
4. Human-computer interface design
The primary function of the host computer is to provide a human-machine interface, allowing operators to intuitively understand various process parameters on-site and issue corresponding control commands according to production needs. Additionally, it can use a large-capacity memory to record historical data, enabling managers to understand the production status over a period of time and providing a reliable basis for developing new production plans to improve efficiency. To achieve these functions, configuration software is the optimal choice. The configuration software used here is STEP7V5.0 and Kingview5.0. It integrates control technology, database technology, network technology, human-machine interface technology, and graphics technology, including components such as dynamic display, alarms, controls, trends, and network communication. It provides a user-friendly interface, requiring only a small amount of code to generate a high-quality control system. The system block diagram is shown in Figure 4.
Figure 4 Block diagram of the control system
There are many popular configuration software options available, such as INTOUCH, iFIX, KingSCADA, and LABview. iFIX, with its high cost-performance ratio, is the preferred choice. iFIX is powerful, easy to control, facilitates programming, and has a rich graphics library. Connecting iFIX to Ethernet is crucial in HMI design; otherwise, all monitoring within the HMI is meaningless. The connection method is as follows: Before starting iFIX, install the MBE (Modbus Ethernet) driver software. After starting iFIX, double-click the system configuration icon to display the system configuration diagram. Then, click the SCADA icon to display the SCADA configuration diagram. Click the question mark to the right of the I/O driver, select Modbus Ethernet v7.17, and click OK to return to the iFIX editing interface. Figure 5 shows the host computer startup screen.
Figure 5. Startup screen of the host computer
Double-clicking the I/O driver will display the MBE icon. Double-clicking the MBE icon again will bring up the IP configuration interface, where you can set the IP address of the PLC you want to control. For example, if the PLC master's IP address is 128.128.1.105, set the system IP address to 128.128.1.105. After setting the IP address, the communication settings between the PLC and the iFIX software are complete. If you need to communicate with multiple PLCs, simply add more devices.
5. Communication methods between PC and PLC
When using Ethernet to enable communication between the host computer and the PLC, the setup is simple and convenient. The host PC and the PLC only need to be set up within the same network segment. For example, assuming the host PC1 address is set to 128.128.1.64, the host PC2 address to 128.128.1.65, and the PLC address to 128.128.1.105, they can communicate with each other. Expansion is also easy; simply add new devices to the same network segment and assign them an IP address.
Figure 6 IP address settings
6 Programming
The programming of the chemical water treatment system can be designed according to the programming instructions of the S7300 PLC [3]. This control system includes the operation and shutdown program control of the 1# series cation bed and anion bed, the operation and shutdown program control of the 2# series cation bed and anion bed, the operation and shutdown program control of the 3# series cation bed and anion bed, the regeneration program control of the 1# series cation bed and anion bed, the regeneration program control of the 2# series cation bed and anion bed, and the regeneration program control of the 3# series cation bed and anion bed. The program design is illustrated below using the operation program control of the 1# series cation bed as an example. The flowchart of the operation program of the 1# series cation bed is shown in Figure 3. When the No. 1 series is put into operation, the No. 1 cation bed inlet valve, the top exhaust valve, and the No. 1 clean water pump are opened simultaneously to perform a cation bed forward wash and exhaust for 1 minute. Then, the cation bed forward wash water valve is opened, the top exhaust valve is closed, and forward wash is performed for 3 minutes. After that, the cation bed forward wash water valve is closed, the No. 1 cation bed outlet valve is opened, and the No. 1 CO and degassing blower are put into operation. This is the cation bed commissioning process.
6. Conclusion
This system is designed primarily for time-sequential control, while incorporating the assistance of an intelligent on-site conductivity meter, giving this chemical water treatment PLC control system unparalleled advantages over other controllers. Field operation results demonstrate that the system offers flexible and convenient control, powerful functionality, and meets production needs. The system has received affirmation and praise from water treatment engineers and instrumentation engineers and is already in use in the boiler water preparation system of a stainless steel plant.
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