[Abstract] The chemical industry plays a pivotal role in China, and chemical products have an irreplaceable function in various fields. The production status of the entire chemical industry directly affects the development of the national economy and people's daily lives. This paper designs a DCS control system for the production line of high-tower compound fertilizer using an advanced control system. This is of great significance for improving product quality and output, reducing raw material loss, and increasing the economic benefits of enterprises.
Keywords: DCS; Automation; PLC; WinCC; Chemical Industry
1 Introduction
The level of development of electronic technologies, represented by integrated circuits and electronic computers, and the depth and breadth of their application in various fields of social production and life, have become important indicators for measuring a country's technological progress and level of modernization.
The world is currently in an era of technological revolution, with electronic information technology at the forefront of this global industrial revolution. In terms of microcomputer applications and their scope, the fastest-growing area is the monitoring, display, and control of automated production processes, particularly in the development of traditional products and energy conservation. Driven by urgent demands for energy conservation and environmental protection, achieving energy-efficient and civilized production, and improving economic efficiency, both domestic and international efforts are placing great emphasis on the technological implementation of industrial automation.
Advanced integrated automation technology for industrial processes is a comprehensive process control technology system that integrates automation control technology, computer information technology, integrated control theory, intelligent control engineering, and high-efficiency energy-saving technology. This project successfully applies advanced integrated automation technology to industrial production processes, using an advanced control system to automatically monitor, display, and control the production line processes of chemical products, achieving intelligent control (DCS), improving product quality and output, reducing raw material loss, saving energy, improving economic efficiency, and effectively integrating energy conservation with environmental protection and civilized production.
2 System Overall Design Scheme
2.1 System Design Principles and Achievable Performance
The principle of practicality
Based on specific circumstances, this system is designed to provide practical and technologically advanced production control functions. The goal is to enable manual operation only when required by the process, while allowing the DCS system to achieve fully automated control of the production process, thereby improving product quality and output.
Reliability principle
High reliability is crucial for chemical production systems. Firstly, the hardware system is reliable, implementing a dual-CPU, dual-CP hot-standby redundancy connection mechanism. Bus redundancy further enhances system reliability. Secondly, the software system possesses high reliability and fault tolerance capabilities.
System mean time between failures (MTBF): 15 years
Mean Time Between Failures (MTBF) of the module: 50 years
System availability (MTBF/MTBF+MTTR): greater than 99.99%.
Safety principles
The system employs distributed control, and all selected I/O modules can be hot-swapped during operation, possessing comprehensive fault detection and diagnostic functions. The host computer features complete alarm display, fault handling, and operational alarm functions. Interlocking and emergency stop functions ensure excellent system safety.
Principle of Advanced Technology
The system design should grasp the development trends of advanced and stable sensor technology, the latest electronic control technology, computer technology, network and communication technology, and adopt an advanced architecture. It should select advanced and mature technologies and equipment in the same industry as much as possible, choose advanced software and hardware platforms, and create a high-starting-point system development and application environment. The system should have an advanced architecture, advanced control algorithms, advanced communication functions, advanced network functions, and advanced hot standby redundancy functions. It should not only have control functions, but also complete information processing and management functions, so that it has a fully integrated and automated control mode, which is at the leading level in China.
Principle of Flexibility
The system adopts a distributed I/O structure. The ET200 acts as a slave station, connecting field signals to the automation system via the PROFIBUS-DP fieldbus. It can be centrally installed indoors or distributed in the control field, giving the system configuration great flexibility. It also has flexible software configuration functions, allowing for flexible modification of control algorithms and easy tuning of control parameters.
Extensibility principle
The system features an open, modular architecture and software system, facilitating system expansion and interconnection with management information systems. It is open, possessing good control over functionality, configuration, and communication interfaces, and can be smoothly upgraded as science and technology advance.
Maintainability principle
The system has a self-diagnostic function, which can locate and alarm faults in the software and hardware systems, record fault handling, and has a mean time to repair (MTTR) of 3 minutes. It can also perform online maintenance.
2.3 Design Basis and Standards
The main standards and specifications for process control systems are as follows:
Definitions and specifications adopted in monitoring, data acquisition and automatic control systems (ANSI/IEEE C37.1-1987)
Guidelines for Lightning Protection of Electronic Equipment (GB1450)
Computer Grounding Technical Requirements GB2887
Basic Standards for Low-Voltage Electrical Appliances
Standard for Construction and Acceptance of Automation Instrumentation Engineering (GB50093-2002)
Standard for Construction and Acceptance of Electrical Installations GBJ232-90, 92
Security Engineering Procedures and Requirements (GAT75-94)
Functional Indicators and Graphical Symbols for Process Measurement and Control Instruments (HG/T20505-2000)
Regulations for the Selection and Design of Automated Instruments (HG/T20507-2000)
Design Specifications for Signal Alarm and Safety Interlock Systems (HG/T20511-2000)
Specifications for Instrumentation Piping and Wiring Design (HG/T20512-2000)
"Design Specifications for Grounding of Instrument Systems" HG/T20513-2000
Design Specifications for Heat Tracing and Insulation of Instrumentation and Pipelines (HG/T20514-2000)
Instrument Isolation and Flushing Design Specifications (HG/T20515-2000)
Design Specifications for Automatic Analyzer Chambers (HG/T20516-2000)
"Common Terminology in Automatic Control Design" HG/T20699-2000
Specifications for Engineering Design of Programmable Logic Controller Systems (HG/T20700-2000)
"General Instructions for Automatic Control Installation Drawings, Specifications for Graphic Symbols and Material Library" HG/T21581-95
"Code for Construction and Acceptance of Electrical Installation Engineering Panels, Cabinets and Secondary Circuit Wiring" (GB50171-92).
The latest standards include IEEE 802.3 network technology standard, IEC 870-5-103, and related standards such as GB, SD, JIEC, IEEE, IEC, ISO, ANSI, and DIN.
2.4 System Overall Structure Diagram
Redundant system architecture diagram
2.5 Functional Description of Each Unit
2.5.1 Supervisory Operator Station
The host computer in the main control room monitors the DCS system below via the PROFIBUS bus. The host computer operator station mainly performs the following functions:
Real-time display of detection points: Displays the actual values of various status points in real time.
The detection point bar display can be set with upper and lower alarm limits.
Various PID adjustments: PID parameters can be set, and manual/automatic switching can be achieved.
Alarm function: Any malfunction will trigger an alarm in a timely manner (including alarm display on the monitoring screen and alarm signal).
Error message diagnosis: Based on system faults, generate a diagnostic report.
Historical curve real-time recording: Displays the curve status of various important parameters in real time, facilitating monitoring and future review.
The operation log shows that it can record and archive all the operations of all relevant personnel in real time, so that when an accident is caused by an erroneous operation, the cause of the accident and the person responsible can be quickly identified.
Animated demonstrations: Each process screen has a certain animation effect, which enhances the visual appeal of the monitoring screen.
2.5.2 Host Computer Engineer Station
The engineer workstation can also function as an operator workstation, and in addition, it provides the following functions:
User Management: Set administrator usernames and passwords, including creating and deleting administrator and operator users.
Permission settings: Different operator permissions can be set for different on-site operation buttons, and then different permissions can be set for operators.
Program and configuration download: User programs and configuration data can be downloaded to the DCS controller online or offline.
Online fault diagnosis: It can perform hardware diagnosis on the DCS controller in real time. When a fault occurs, it can perform fault diagnosis and location in real time.
2.5.3 DCS Process Controller
Siemens' S7315-2DP process controller communicates with the host computer via the PROFIBUS bus and enables redundant configuration between the two process controllers. When one controller malfunctions due to a power outage or other fault, the system can automatically switch to the other redundant controller to operate.
The controller is connected to the two control substations below via the Profibus bus, and performs high-speed acquisition and processing of the acquisition signals from the control substations.
2.5.4 DCS Process Control Substation
The system consists of two process control substations, which complete the real-time acquisition and output control functions of all analog and digital signals required by the factory.
Real-time performance is of paramount importance in industrial monitoring. The requirements of this communication system are as follows:
• Digital acquisition cycle < 0.05s;
• Analog signal acquisition period: Electricity < 0.1s, Non-electricity < 0.2s;
☐ Real-time database update cycle: <1 second;
Control command response time: ① Control command response time
Hardware configuration and performance of 3DCS system
The system incorporates computer technology, control technology, network communication technology, fieldbus technology, etc., and connects computers, PLCs, DCSs, and field intelligent instruments into an organic whole through a network bus, allowing users to monitor and control the status of field controllers from a remote monitoring room.
This system is equipped with two control cabinets to control the production equipment.
For each production unit, two ET200M units are set up as PROFIBUSDP slave stations to send distributed I/O signals to the controller.
In this system, we have configured two CPU315-2DP units as PROFIBUS DDP masters to process field signals from DP slave units. Either unit can control two production units, with one serving as a backup during normal operation. In the event of a master CPU failure, a dedicated redundant program ensures that control tasks automatically switch to the backup system.
In the host computer section of the control station, we set up operator and engineer stations.
The host computer and the control station, as well as the main system CPU and the backup system CPU, communicate via the PROFIBUS bus.
The hardware configuration of the DCS system will be introduced below.
3.1 Specific configuration of operator station and engineer station
The system consists of two industrial control computers, one of which serves as an operator station and the other as an engineer station.
We used Advantech IPC610 for the two industrial PCs. They are reliable, with a CPU clock speed of Pentium 2.4G, a 160G hard drive, and 2G of memory.
1. Operator Station
The operator station must meet the following requirements:
• There should be no fewer than 100 dynamic process flow diagrams, which can be displayed in multiple windows (4 diagrams per screen).
• Accessing any screen generally only requires one keystroke, and at most no more than three.
• The number of dynamic tags on the operation station is ≤3000.
• It has a trend curve recording function, with a total of ≤100 trend curves, a 1-minute time interval, and more than 80 trend curves in 24 hours. Each trend curve chart allows more than 6 curves to be displayed overlaid.
• It has functions such as production report, event report, and alarm report.
• The operator station and the field control station are connected via a PROFIBUS bus, which serves as the transmission medium and has a communication rate of >10 Mbit/sec.
• The time for flowchart status updates, screen transitions, or refreshes is adjustable, with a minimum value of less than 100ms (including dynamic and static data).
• Power supply: 220VAC±15%, 50Hz.
2. Engineer's Station
The engineering station has all the functions of the operator station. In addition, the engineering station also has functions such as programming, configuration, diagnosis, debugging and modification.
The engineering workstation must meet the following requirements:
• The engineer workstation can bring up any predefined system display screen. Any screens and trend charts generated on the engineer workstation can be loaded onto the operator workstation via the communication bus.
• The engineering station can retrieve system configuration information and related data from any processing unit within the system via the communication bus, and can also download configuration data from the engineering station to each distributed processing unit and operator station. Once new configuration data is confirmed, the system can automatically refresh its memory.
• The engineering workstation includes a workstation processor, a graphics processor, and main memory and peripheral devices required to hold all databases, various displays and configuration programs in the system, and can provide a historical trend buffer required for system trend display.
Power supply: 220VAC±15%, 50Hz
3.2 Controller
1. CPU
For the controller, we use the CPU315-2DP from SIEMEN SPLC's high-performance S7-300 series.
The CPU315-2DP has the following design features:
• High-performance processor
The CPU315-2DP executes a binary instruction in just 100ns, meeting higher requirements for program size and instruction processing speed.
• Expanded storage capacity
The CPU315-2DP has 128KB of RAM.
• Flexible scalability
The CPU315-2DP can expand to up to 32,768 digital or 2,048 analog I/Os, and its second interface can connect to a total of 96 DP slaves, demonstrating strong scalability.
• PROFIBUS-DP interface
The integrated PROFIBUS-DP interface enables the CPU315-2DP to connect directly to the PROFIBUS-DP fieldbus as a master station, establishing a high-speed distributed automation system and greatly simplifying operation, with a maximum transmission rate of 12 Mbit/s.
• Multi-point Interface (MPI)
The CPU315-2DP can establish a simple network of up to 32 stations using MPI, and can establish connections between stations on the communication bus (C bus) and MPI, with a maximum data transfer rate of 12 Mbit/s.
• Function block protection:
Using password protection in user programs can prevent unauthorized access.
• Diagnostic cache:
The last fault and interrupt events of the 315-2DP are stored in a FIFO buffer for diagnostic purposes. The number of entries can be determined through parameterization.
• Real-time clock:
The diagnostic information from the CPU will be labeled with the date and time.
3.3ET200M
1. Overview of ET200M:
• The ET200M is a high-density modular I/O station with the capability to handle all the needs of distributed automation problems and has an IP20 protection rating.
• The explosion-proof analog input and output modules that conform to the HART protocol mean that the ET200M is suitable for process engineering and is also suitable for use with redundant systems.
• The ET200M is a passive station (slave) on the PROFIBUS-DP fieldbus with a maximum data transfer rate of 12 Mbit/s.
• The ET200MI/O station includes an IM153 interface module and up to eight S7300 programmable controller modules, which connect to the automation system via PROFIBUS DIP fieldbus.
• The ET200M can be configured using an active bus module, allowing for hot-swappable module replacement during operation. Other modules continue to operate (hot-swappable).
2. IM153-2 Interface Template:
• Connect the ET200M as a slave to PROFIBUS-DP (copper wire).
• Suitable for redundant PROFIBUS-DP systems.
• It has timestamp functionality and time synchronization.
3. Regarding hot-swapping
In SIEMENS' PLC control system:
The S7-300, acting as a PROFIBUS P master with DP slave stations ET200M, ET200S, and ET200iS, supports hot-swapping (requires an active bus backplane). However, a soft redundancy system using the S7-300 as the master cannot fully implement hot-swapping functionality. When a module is removed from the ET200M slave, the CPU does not shut down; the SF light on the master CPU and the standby CPU illuminates, and the BUSF light flashes. The SF light on the two IM153-2 modules on the ET200M slave illuminates, and the BF light flashes. All I/O values of the modules on the ET200M slave are cleared to 0, and the S7-300 master loses control over the ET200M slave. When the module is reinserted into the ET200M slave, the system switches from the master CPU to the standby CPU, the SF, BUSF, and BF lights turn off, and the soft redundancy system returns to normal operation.
As described above, the S7-300 CPU, which acts as the PROFIBUSDP master, is configured with hard redundancy, and then a DP slave station ET200M is connected to it. This enables the hot-swappable function of the entire system (an active bus backplane is required).
4. Software Configuration and Performance of DCS System
4.1 Upper-level monitoring software WinCC
The supervisory control and data acquisition (SCADA) software used is SIEMENS' latest WinCC V6.0. SIMATIC WinCC (Windows Control Center) is a product combining Siemens' advanced automation technology with the powerful functions of Microsoft. It offers various effective functions for process automation and is a human-machine interface and SCADA system for personal computers, categorized by price and performance. It can easily combine standards and user programs to create human-machine interfaces that accurately meet actual requirements. Compared with other monitoring systems, WinCC has the following system characteristics:
1. SIMATIC WinCC is a general-purpose system.
WinCC can be used for all operator control and monitoring tasks in the field of automation. WinCC clearly displays events occurring in processes and production. It displays the current status and records data sequentially. Recorded data can be displayed in full or in a simplified format, edited continuously or as required, and can be exported. WinCC provides various function blocks for these functions, as well as graphical displays, combined with user programs, information processing, measurement value processing, recipe parameters, and reports.
2. SIMATIC WinCC functionality can be added as needed for the task.
Specialized software features are offered as optional software packages, which customers can purchase separately to accommodate data and functionality expansions. For example, an existing single-user configuration system can be expanded into a multi-user system using the optional server software package.
3. SIMATICWinCC provides security guarantees.
WinCC can report early signs of crises during production, displaying these signals on the screen or aloud via the sound card. WinCC supports troubleshooting with help functions and operation guides. A WinCC workstation can be dedicated to process control to ensure critical process information is not obscured. Software-assisted operation strategies protect processes from unauthorized access and provide error-free operation in industrial environments. Worldwide after-sales support further enhances security.
4. SIMATIC WinCC ensures data integrity.
WinCC provides continuous document data selection and system operation security through two redundant workstations. If one server is compromised, the system switches clients to the other server to ensure continuous operation. When the failed client restarts, documents on both servers are automatically matched to ensure uninterrupted document data transmission.
5. SIMATIC WinCC is fully open-source software.
WinCC is a first-class, object-oriented 32-bit application that runs on PCs under Microsoft Windows 98/2000 or Windows NT 4.0/5.0 operating systems. WinCC can enter the Windows world as an ideal communication partner through the OLE and ODBC Windows standard mechanisms. Therefore, it can be easily integrated into a company's data processing systems. WinCC is not only open in terms of data, but also in terms of system functionality. This means that system developers can use WinCC as a foundation to develop related application software or write extended functions.
6. SIMATIC WinCC is easy to use.
WinCC's configuration environment is consistent with Microsoft Windows and incorporates ergonomic design principles. Even operators with varying levels of knowledge can master the system in a short time. The system includes multimedia self-learning software, detailed online help, and a unique configuration wizard to aid learning.
WinCC's creation process screen function can be used to display overview screens, group screens, single-point screens (adjustment screens), trend screens, graph screens, bar charts, and other screens.
Given the aforementioned superior features, using WinCC on both the engineering station and operator station easily meets the functional requirements of this device, including production monitoring, process control, operation screens, parameter alarms, data logging and trend analysis, data archiving, and report display and printing. The engineering station allows for user management and permission settings, and can access any predefined system display screen. Any screens and trend charts generated on the engineering station can be loaded onto the operator station via the communication bus, and configuration data can be downloaded from the engineering station to each distributed processing unit and operator station.
4.2 STEP7 process controller programming software
The PLC control software selected is SIEMENS STEP7 V5.4. STEP7 is the standard software for creating programmable logic control programs for SIMATIC S7-300/400 stations. It is easy to use, object-oriented, has an intuitive user interface, configuration replaces programming, has a unified database, and its programming language conforms to IEC1131-3.
In STEP7, projects are used to manage the hardware and software of an automation system. STEP7 uses SIMATIC Manager for centralized project management, which allows for easy browsing of data from SIMATIC S7, M7, C7, and MinAC. All the SIMATIC software tools required to implement various functions of STEP7 are integrated into STEP7.
STEP7 can use ladder logic (LAD), function block diagrams (FBD), or statement lists (STL). STEP7 is a powerful system with key functions including hardware configuration and parameter setting, communication configuration, programming, testing, startup and maintenance, documentation, operation, and diagnostics. All STEP7 functions have extensive online help; simply open the object with the mouse or select it and press F1 to access its online help. Hardware configuration allows for system configuration and parameter setting of the CPU and individual modules; communication configuration allows for setting communication methods between stations; and system diagnostics provides the status of the automated system, enabling quick viewing of CPU data and program fault causes, as well as graphical display of hardware configuration and module faults, thus achieving online fault diagnosis.
4.3 Software Redundancy
This system employs redundancy to meet redundancy requirements. Soft redundancy enables redundancy in: main unit power supply, backplane bus, etc.; PLC processor redundancy; PROFIBUS fieldbus network redundancy (including redundancy of communication interfaces, bus connectors, and bus cables); and redundancy of the IM153-2 communication interface module of the ET200M station.
The soft redundancy system consists of two PLC control systems, A and B. Initially, system A is the primary system and system B is the backup. If any component in the primary system A fails, the control task automatically switches to the backup system B. At this point, system B becomes the primary system and system A becomes the backup. This switching process involves a complete switchover of the power supply, CPU, communication cables, and IM153 interface module. During system operation, even if no components fail, operators can manually switch between the primary and backup systems by setting control words. This manual switching process is very useful for adjusting, replacing, and expanding the hardware and software of the control system, i.e., Altering Configuration and Application Program in Run Mode.
When the soft redundancy system is operating, the A and B control systems (processor, communication, I/O) run independently, with the master system's PLC controlling the I/O in the ET200 slave stations. The PLC programs in systems A and B consist of non-duplicated and redundant user program segments. The master system PLC executes all user programs, while the backup system PLC only executes the non-duplicated user program segments, skipping the redundant ones.
If the CPU experiences a shutdown, power failure, or other malfunction, the fault diagnosis takes approximately 100–1000 milliseconds.
In this system, we chose PROFIBUS as the synchronization method between the master and backup systems. Under this method, the time required to synchronize 1K of data is approximately 250ms.
The switching time of a DP slave station is related to the number of slave stations. In PROFIBUS synchronization mode, the switching time between two PROFIBUS slave stations is approximately 20ms.
After installing the software for soft redundancy, you can find example programs and function block libraries in STEP7. Different network connections require calling different function block packages. By calling different function block packages and selecting the appropriate connection method in the network configuration in STEP7, you can easily meet the system's redundancy requirements.
5. Conclusion
The design process of the DCS control system for high-tower compound fertilizer follows the concept of distributed control and centralized management. Redundancy is configured in the process control layer to improve system stability. It operates advanced control strategies and methods as well as mainstream control technologies, meeting the high requirements of stability and safety in the chemical industry. Its control system construction concept has certain reference value for the chemical industry.
