DCS upgrade and renovation for 300kt/a ammonia synthesis plant

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In 1988, Dahuahua Group in Cangzhou, Hebei, carried out a comprehensive technical upgrade of its synthesis system, making a one-time transition from the disk-based instruments of the old system to a computer-controlled system, thus setting a precedent for chemical industry automation at that time. During this renovation process, a great deal of valuable experience was gained, and the results earned it the Second Prize for Scientific and Technological Progress under the **\"8·5\" Plan, as well as the First Prize for Provincial Scientific and Technological Progress. In recent years, computer components have been updated rapidly; spare parts for older systems are no longer produced, and the original spare parts have run out. Moreover, the hardware of industrial computers has now reached the end of its useful life and is beyond its designed lifespan. Occasional failures in such hardware can lead to serious flaws in production control systems, threatening the safe and stable operation of production processes. The group company has resolved to upgrade the existing DCS systems, designating it as the key project for the major maintenance in 2004. 1 Introduction to the advantages and disadvantages of old and new systems: Modern DCS systems are evolving toward being general-purpose systems with good hardware compatibility, user-friendly software, powerful functions, and open databases, whereas older DCS systems are highly specialized with independent components in various aspects, but they offer high reliability. 1.1 Characteristics of the old system: For the first DCS upgrade at our plant, we used Yokogawa’s CENTUM A control system. The application software was developed by our factory’s instrumentation and process engineers through repeated discussions and practical testing; it represents a control solution with a very high degree of automation. Practice has shown that the automatic control rate of this ammonia synthesis system exceeds 95%, the utilization rate of automatic control instruments is over 99%, and the utilization rate of interlocks is 100%. The independently developed water-to-carbon ratio control system, hydrogen-to-nitrogen ratio control system, anti-surge control systems for the four main units, and others have enabled full automation of the entire ammonia synthesis production process. This is unique in the national fertilizer industry to date, and the complexity of its control schemes is also rare; examples are given below. The water-carbon ratio control system utilizes 33 internal computer instruments, employing methods such as ratio control, cascade control, two-way cross selection, and logical control. Achieved: ① Automatically adjusting the steam volume based on the analysis results of total carbon to maintain a stable water-to-carbon ratio ； ②When the system increases or decreases load, it does so automatically in accordance with the requirements of the production process sequence ； ③Automation such as transmitting signals to other systems to integrate the entire ammonia synthesis system, and so on. Provide the appropriate parameters, and the production process will proceed automatically! The hydrogen-to-nitrogen ratio control system utilizes 22 internal computer instruments, employing control methods such as ratio control, feedforward control, cascade control, and pure lag compensation. Achieved: ① Overcoming the serious problem of large time lag in this system’s control ； ②It automatically adjusts according to changes in load. The above are two representative examples out of the 18 complex control systems independently developed by our factory, which is sufficient to demonstrate the level of advancement in our automation technology. However, the hardware limitations of the old system result in longer data acquisition times, which affects the control performance of some control systems ； It doesn’t integrate well with modern data communication ； Poor scalability ； The software lacks strong functionality ； The lack of mathematical models has hindered the development of some control systems that require a large number of such models ； The software is difficult to configure and the graphical interface is poor, which prevents process engineers from operating on the flow diagrams ； Issues such as poor online maintenance have turned it into an automated \"island\" that is unable to meet the requirements of 21st-century ETS (Enterprise Technology Solutions); therefore, its upgrading is imperative. 1.2 Features of the new system: Since current production operation and maintenance personnel rely heavily on the DCS system, and the control scheme of the original system is quite mature, Yokogawa’s DCS products are still used to maintain the existing style and facilitate a smooth transition; only the software and hardware capabilities have been enhanced. This time, the Yokogawa CS3000 FIO (Field Network I/O) system was used, which incorporates the latest \"OPEN Network Technologies\" as well as hardware and software that have proven their high reliability in practical use. 1.2.1 Communication Network? The CENTUM-CS3000 system features a hierarchical network structure that separates process real-time data and operational monitoring data from non-real-time information as well as shared resource information, with each type being transmitted over a different network. It effectively improves communication efficiency and reduces communication load: ① V-NET, a redundant real-time control network used for operation monitoring and information exchange ； ②E-NET, a data transmission network used between operation stations, engineer stations, and higher-level computers ； ③The ESB bus is a redundant real-time communication bus used to control data transmission between the CPU in the station and local I/O nodes. 1.2.2 Hardware System? The system’s FCS adopts a “Pair&Spare”“Fail Safe” design with 4 CPUs, enabling a sampling time of up to 50ms and ensuring high reliability in data acquisition ； Multiple anti-interference techniques are employed, including optoelectronic isolation, a high common-mode rejection ratio, as well as appropriate grounding measures (with a ground resistance of less than 100 ohms) and shielding; as a result, it possesses strong anti-interference capabilities and can operate in environments with high levels of electronic noise, radio frequency interference, and vibration ； A large range of I/O modules tailored to various needs are available; these modules are highly versatile, compatible with older systems, offer easy scalability, and feature flexible and simple wiring options ； HIS is a standard PC, which **facilitates the installation and maintenance of hardware and software ； The flexible wiring options make wiring simple ； The unique KS cable connection method keeps the control station uncluttered, enables more rational wiring, and allows for online plugging and unplugging. The high-speed RISC processor VR5432 is capable of performing 64-bit floating-point operations, and its powerful computing and processing capabilities enable the implementation of various advanced control strategies such as multi-variable control, model predictive control, fuzzy logic control, and more. 1.2.3 Software System: The operating platform used is WINDOWS 2000. Software developed on this platform boasts high versatility, and its configuration is flexible and convenient, **reducing the configuration time ； Offline authentication and online debugging functions enhance the accuracy and reliability of configuration, while the user-friendly and powerful graphical interface enables operations on flowcharts; process operators and maintenance staff no longer need to memorize lengthy station numbers ； The open database technologies DDE (Dynamic Data Exchange) and OPC (OLE for Process Control) provide direct data services for our plant’s MIS system. Multiple programming languages: MS-Excel, Visual C++, Visual Basic, etc., meet the needs for developing application software. 2 CONFIGURATION OF THE CENTUM CS-3000 SYSTEM The CENTUM CS-3000 software system includes a real-time operating system and applications, among other things. The engineer station provides a standard graphical configuration method. All configurations are carried out in a graphical environment through an interactive menu interface, which is convenient and fast, requiring no extensive computer knowledge from the user. Design and configuration are carried out simultaneously, simplifying the workflow. The rich user-defined macro functionality makes configuration easier and more efficient. Moreover, the configuration files can be modified and downloaded online without shutting down the control station, which greatly facilitates online maintenance. 2.1 Control chart configuration: In the configuration process, we simply need to place the required solidification modules (feedback control module, sequential control module, calculation module, SFC module, unit instrument module, etc.) in the appropriate positions, enter the corresponding data (tag numbers, ranges, positive/negative action, etc.), and define the logical starting point for the data; thereafter, the logical data lines are connected automatically, making the creation of control charts simple and fast. To ensure that the control principle remains exactly the same as that of the old system, we carefully studied and understood the function and characteristics of each fixed module during the configuration process. To make proper use of each module, we reviewed a large amount of English-language literature and conducted numerous experiments, thereby ensuring the safe and reliable operation of all chemical processing equipment. The control chart function of the CENTUM CS-3000 also facilitates our rapid configuration and logical reasoning. Thanks to previously proven control solutions and our own strong skills, it took us a month to configure 77 control charts as well as nearly 200 sets of logic and control systems; of course, this is also thanks to the powerful configuration capabilities of the CENTUM CS-3000. 2.2 Creating flowcharts: Thanks to the WINDOWS 2000 operating system, its graphic capabilities have been enhanced; it enables the creation of visually rich graphics. Additionally, features such as dynamic processing and data integration mean that flowcharts are no longer merely static images. Their ease of use and ability to facilitate data monitoring make them a standard tool for process operations. During the production process, we carefully observed the shape of the equipment on site and drew it based on its actual appearance. We made full use of the powerful graphic functions to create a three-dimensional effect that accurately reflected the real situation on site, thereby compensating for the shortcomings of the original flowchart. We consulted a large amount of information to draw diagrams of the internal structure of the key equipment ； Inches of monitoring and control instruments are installed in the appropriate locations, providing all the functions required for daily operation ； The dynamic integration of data and graphics allows the graphics to change in response to the data from the relevant instruments (such as changes in liquid level, flame size, valve opening and closing, etc.) ； Combined with other diagrams to form a comprehensive flowchart. Drawing on years of practical experience, we have fully developed its functions and created 270 perfect flowcharts that allow the data from various instruments to be updated and displayed in real time on these flowcharts ； Relevant operations (changing SVs and MVs, performing CAL adjustments, etc.) and monitoring can be carried out on the corresponding instruments. The color-changing function provides a three-dimensional dynamic effect to the display, thereby **simplifying the operator’s work** ； Rich graphic processing tools and application menus make it possible to create various types of graphics, while the ruler feature enables very precise positioning of graphics. The information bar below clearly shows the compilation process and any issues that arise. 2.3 Report Generation The CENTUM CS-3000 utilizes the well-known Excel program. By taking advantage of Excel’s features, along with the macro functions developed by Yokogawa Electric and our own custom programs, we have designed report formats that allow us to generate satisfactory shift reports, daily reports, monthly reports, and annual reports simply by entering the required data in the appropriate fields. These reports can be printed on schedule or at any time as instructed by the operators, which makes it easy to store and retrieve them and enables their long-term preservation. 2.4 Features of the Operation Interface A powerful and user-friendly operation interface enables optimal use of flowchart operations. In addition to regular operations, the processing of system data information also simplifies daily maintenance; it can store 100,000 historical records, providing a reliable basis for analyzing problems and identifying their causes ； Each control station can store up to 2,560 historical trends, simplifying maintenance ； The management of alarm information through classification and grading makes monitoring more efficient ； The visual system maintenance interface displays in detail every station, every node, every card, and so on. Well-designed operation tools, along with nearly perfect flowcharts, enable operators to work as if they are talking to their own family – skillfully and warmly. Our goal is to make it not just an operation tool, but also a vivid online learning resource. 3 Summary In general, this DCS upgrade not only retained the mature control schemes of the old system but also addressed the shortcomings of the existing system through improvements and innovations. The DCS upgrade for our plant’s 300 kt/a ammonia synthesis facility has not only raised the level of automatic control in our plant to a leading position but also provided valuable experience for other manufacturers carrying out DCS upgrades.