Abstract: In order to meet Class I A emission standards, the second sewage treatment plant in Shandong Yucheng City needs to implement centralized management of the entire plant, adopting high-performance, high-quality PLC products to ensure the safe and efficient operation of the control system. The control system based on LK series PLC from Hollysys was applied in the wastewater treatment process based on A2/O biological phosphorus removal and denitrification, and can fully meet the requirements of process control, providing a high reliability, low cost, More optimized control scheme.
Keywords: LK series PLC; control system
1 Introduction
Shandong Yicheng No. 2 Sewage Treatment Plant Project is one of the 20 major livelihood projects in Licheng City in 2011. The total design scale is 90,000 m3/d, of which the first-stage engineering design scale is 30,000 m3/d. The water quality of the designed effluent meets the Class A standard of the “Emission Standard of Pollutants for Municipal Wastewater Treatment Plantsâ€. After the project is completed, it will effectively solve the problem of wastewater treatment in industrial enterprises in the High-tech Development Zone. The control system of the wastewater treatment plant is based on the LK series PLC from Hollysys, providing a reliable guarantee for the safe and stable operation of the entire system.
2 Design of wastewater treatment control system
Sewage treatment plant control system design based on modern advanced control concepts, the use of decentralized control, centralized monitoring and management of control methods, and Hollysys LK series PLC as the core of the control system. Based on LK series PLC applied to sewage treatment plant, it can fully meet the requirements of A2/O biological phosphorus and nitrogen removal treatment process control, and provides a high reliability, low cost and more optimized control scheme.
The sewage treatment plant adopts a computer control system combining LK210 as the main PLC and a centralized and decentralized computer control system. It has a central control room and four on-site PLC control stations. The structure of the entire control system is shown in Figure 1. The on-site control station conducts decentralized control of the various processes of the sewage treatment plant, and the central control room implements centralized management of the entire plant. The data communication between the field control station and the central control room is carried out by Industrial Ethernet. The transmission medium is optical fiber. The ring topology network structure is adopted and the transmission rate is 100 Mbps.
Figure 1 Structural schematic
3 Functions of Sewage Treatment Control System
Sewage treatment plant upgrade control system is divided into three levels of management, including production management level (central control room), on-site control level (PLC control station) and local control level. The management functions at all levels are as follows:
3.1 Production Management Level (Central Control Room)
It mainly completes the management, dispatching, centralized operation, monitoring, system function configuration, on-line modification and setting of control parameters, recording, report generation and printing, and fault alarming of the sewage treatment plant. The system provides a friendly man-machine interface with the following features:
1) Acquisition and real-time monitoring of process parameter values, electrical parameter values, and operating status information of production equipment at all sections of the plant.
2) Adjustment loop display, including all control loops, can modify set value, control mode, adjustment parameters, etc., and there is a real-time trend graph to facilitate PID loop tuning.
3) Based on the collected information, establish various types of information databases and make trend curves (including historical data) for various types of process parameter values ​​for analysis and comparison by operators.
4) The alarm system provides faults during the process, operating conditions and comprehensive information in the automated process to help detect dangerous situations in a timely manner to reduce serious accidents and failures during the operation of the sewage treatment plant. This information informs the operator in a visible and audible manner that if a monitoring circuit fails, the circuit part in the corresponding monitoring screen in the system will change color and flicker, accompanied by sound and alarm information to remind the operator to pay attention, and at the same time the alarm Information storage and printouts. The system has different information types and information levels to help operators identify the most important alarm information at the fastest speed.
5) Guide the operation with the "man-machine" dialogue. In the automatic state, the device can be manually operated (such as on/off operation) with the keyboard or mouse.
6) Show the profile of the whole plant plane and dozens of process flows.
7) Dynamic graphics and real-time data display. The graphics system can handle input and output signals on all screens. According to the user's needs, the use of graphical tools on the process diagrams, dynamic curves, historical trend charts, bar graphs and tables for dynamic or static display.
8) Automatically record changes to important equipment operations and important parameters for management and accident analysis.
9) The system is set up for multi-user, multi-regional approach to strict security management.
10) Adopt the operating mode of graded operation and maintenance. All personnel enter the system operation, first login, login includes the user name and password, the system according to the level of the login person to open the corresponding function: for the general operator can only perform a simple, normal system operation; and for system maintenance This should be done by the system administrator.
11) Networking with factory management systems to achieve resource sharing and integrated management.
12) Communication with other manufacturers' control systems via the communication bus.
3.2 Field Control Level (PLC Control Station)
According to the distribution of production structures in the factory area and the requirements of the technological process, each site PLC station is linked through the backbone network. As a control layer, it has the following functions:
1) According to the control procedure, the process and electrical equipment in the section under the jurisdiction of the company are controlled automatically. At the same time, the process parameters, electrical parameters and operating status of the electrical equipment are collected.
2) Communication with the monitoring and management system of the central control room through the communication network. Data is transmitted to the monitoring management system, and the on-off command issued by the monitoring management system is accepted.
3) The field control station provides convenient automatic control, critical alarm and continuous loop control.
4) No response alarm: At any time, if the field device does not respond to the output of the PLC of the control station (on/off or on/off timeout does not respond). The control station PLC will send an error signal.
5) The preset value for the PLC program of the control station can be adjusted by the central monitoring station.
3.2.1 coarse grid and lifting pump station control station
Mainly used for coarse grid and lifting pump room equipment control and data acquisition. The main features include:
1) The level difference between the front and the back of the coarse grid is continuously measured by the ultrasonic liquid level difference meter, and compared with the set value, the opening and the up and down operation of the mechanical grid are controlled for slagging. The set time can also be used to control the slag discharge cycle.
2) Measure the liquid level data with the ultrasonic liquid level gauge and turn on the lift pump according to the liquid level.
3) Real-time acquisition of level gauge, level difference, inlet pump line pressure and other parameters.
3.2.2 fine grid control station
Mainly used for fine grid, grit chamber equipment control and data acquisition. The main features include:
1) Continuously measure the difference between the liquid level before and after the fine grid by the ultrasonic liquid level difference meter, and compare with the set value to control the opening and up and down operation of the mechanical grid to discharge slag. The set time can also be used to control the slag discharge cycle.
2) Real-time acquisition of liquid level difference, influent COD, PH, SS, flow and other parameters.
3) Control the operation of other equipment on the grit chamber according to the set period.
3.2.3 SBR Control Station
It is mainly used for the control and data collection of the hydrolysis acidification pool and equipment. The main features include:
1) Automatically control the continuous operation of all devices in working condition.
2) The equipment is periodically cycled according to process requirements, and the operating time can be adjusted on site.
3) Collect pressure, level and other parameters in real time.
4) Complete networking with other devices.
3.2.4 Dewatering Machine Room Station
Mainly used for dehydration room equipment control and data acquisition. The main features include:
1) The dehydration engine room is controlled by the dehydration engine room manufacturer, and the PLC site only performs monitoring operations.
2) Control the agitator in the mud storage tank and control it according to the timing.
3) Real-time collection of parameters such as liquid level in sludge storage tanks and excess sludge flow.
4) Control blower duct valve opening
3.3 Local control level
The "local/remote" knob on the field control box is switched to the "local" position and the local manual control is achieved via the "Start/Stop" button on the box.
The sewage treatment plant upgrading and control system is designed as three control modes. The levels of the three modes from high to low are in-place manual, remote manual, and remote automatic.
4 Characteristics of Sewage Treatment Control System
Based on LK series PLC in the sewage treatment plant processing control system has the following characteristics.
1) High performance. The design of the system components is in line with the true industrial grade. The control system can operate in a strict industrial environment for a long time and stably, minimize the risk of failure of the control system, ensure the energy income, and ensure the continuous water supply service of the water plant for 7×24 hours.
LK series PLC is equipped with industrial-grade processor, its main frequency can reach 533MHz, at the same time, it has nanosecond processing speed, single instruction can reach the processing speed of 13ns at the fastest; with large-capacity memory, program capacity can reach 16M, The data capacity can reach 64M, while the power-down data storage can reach 1M; also has a very large system I / O capacity, digital I / O up to 57,444 points, analog I / O is up to 3,584 points; LK series PLC timers and counters are extremely long and unlimited, as long as the program capacity allows users to freely use.
2) High reliability. PLC control stations are generally located in high electromagnetic interference environments, such as pumping rooms and blower rooms. The control system based on LK series PLC has strong electromagnetic compatibility. The system adopts various isolation and anti-interference designs to ensure that the system can operate stably in a strong electromagnetic interference environment.
The water treatment environment may be contaminated and accelerate the aging of the electronic components in the equipment. Based on the LK series PLC, special protection process (protection coating) modules can be provided to increase the service life of components in corrosive chemicals and corrosive environments. Avoid unplanned downtime and reduce maintenance costs.
3) Ease of use and ease of maintenance. Once the PLC control system is installed, the control system can enter the automatic operation state. In the automatic operation process, the PLC control system is required to perform module replacement and system maintenance without affecting the entire system operation.
The LK series PLC module supports hot plugging. When the module is replaced, there is no need to interrupt the system operation. The new module will automatically initialize the data and establish communication with the CPU quickly. The backplane is designed with anti-aliasing to prevent the wrong module from being inserted. The flexible SD memory card allows for engineering recovery and backup copying, making system maintenance more convenient and faster.
4) Openness and compatibility. The communication network based on the standard protocol can be very convenient for accessing third-party equipment, such as dosing system, UV disinfection, and sludge dewatering machines, allowing a large amount of process feedback information transmission, supporting device parameter access functions, and improving system performance. And diagnostic capabilities.
The control system not only supports communication with a variety of superior monitoring software, such as iFIX, INTOUCH, Kingview, MCGS, force control, etc., but also supports communication with mainstream brand touch screens, such as Pro-Face, HITECH, eView, Weinview, nTouch . As long as the HMI supports a standard communication protocol, it can be easily connected.
The powerful extension function reserves the interface and capacity for future expansion and upgrade.
5) Process diagnostics and online maintenance. The powerful diagnostic functions provided can conveniently and efficiently access related information and discover the root cause of the fault and the process parameters that need to be corrected. This helps detect problems in the production process in advance and avoid unplanned downtime of the equipment, thereby reducing operating costs.
The operating station installs host monitoring configuration software HollyView6.53 and LK programming software PowerPro V4, with powerful features, can be easily and intuitively configured host computer and PLC programming, and supports online download, online modification and off-line simulation and debugging functions .
5 Conclusion
Based on the design and implementation of the LK series PLC in the control system of the sewage treatment plant, the LK series PLC has taken full advantage of flexible configuration, reliable control, strong openness, and easy on-line maintenance, providing a reliable guarantee for the safe and stable operation of the entire system.
references:
[1] LK Large Programmable Controller Hardware Manual [M]. Beijing: Beijing Hollysys Group, 2011
[2] LK series programmable controller selection sample [M]. Beijing: Beijing Helvshi Group, 2011
[3] Automation Solutions for Water Treatment Industry [M]. Beijing: Beijing Helv Group, 2010
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