Selection of DCS

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Summary of DCS selection: Based on the actual conditions of our company, this article analyzes the key considerations for selecting a DCS system from aspects such as technical expertise, economic factors, technological advancement, the strength of the supplier, and after-sales service. Keywords: DCS system, selection. Our company is a pulp and paper manufacturing enterprise under construction. The main production facilities include an 80,000 T/year bamboo pulping plant (the first of its kind in the world), a 50,000 T/year waste paper pulping plant, and a 120,000 T/year cardboard production line. These facilities are supported by an 18 MW boiler and a wastewater treatment system with a capacity of 20,000 m3. Due to the large scale of the entire plant and the large number of control points, a DCS control system is used throughout the plant for control. There are over 10 automation integration providers available for us. It’s quite difficult to choose the right one that suits our factory’s actual needs among such a large number of options. I was directly involved in the selection of the DCS system. Next, I would like to share my personal views on the selection of the DCS system for my factory. DCS stands for Distributed Control System, and it is primarily composed of four main components: I/O cards, controllers, operator stations, and communication networks. When selecting a DCS, several factors should be taken into consideration: First, in theory, DCS can be used in various process applications, making it a universal solution; however, the manufacturers of DCS systems specialize in certain areas. For example, HOMEYWELL is mainly used in the petrochemical industry, while BAILEY Company is primarily used in the power sector. In China, Hangzhou’s Huazhang Electric mainly focuses on paper machine drives and pulp and paper manufacturing ； Hangzhou HollySys has a strong advantage in the field of electricity ； Sichuan Gada has strengths in pulp and paper production, transmission systems, QCS, and heat pump systems ； Shaanxi University of Science and Technology has strengths in pulp and paper manufacturing as well as the control of headbox systems ； Shaanxi Keda and Chengdu Tuoshida have strengths in transmission technology. However, different processes have various specific requirements; therefore, when selecting a DCS system, we should try to choose one whose manufacturer’s expertise aligns with our needs. II. Cost-effectiveness should be considered from the perspective of the DCS’s price and the expected benefits it will generate. There are domestic and imported DCS systems. For the same category, imported DCS systems have more powerful control functions, featuring advanced control algorithms such as Smith prediction and 3D matrix operations. Domestic DCS systems are much cheaper than imported ones, yet they can still meet the technical requirements. Structurally, the controllers of foreign DCS systems vary little among different manufacturers; the preset algorithms of these controllers differ slightly, and the way in which the controllers are connected to the I/O cards also varies. The operation stations, on the other hand, differ significantly. Some are based on PCs, while others are based on minicomputers; those using PCs are much cheaper than those using minicomputers. They are much cheaper in China than abroad; domestic industrial PCs cost only a few ten thousand yuan, while those imported from abroad cost tens of thousands of dollars. There are UNIX and NT operating systems; systems using NT do not crash frequently, and they are also cheaper. There are dedicated monitoring software solutions as well as general-purpose ones; general-purpose monitoring software offers better openness. Users should choose the appropriate option based on the scale of the project and the available budget. Based on our current situation, it is recommended to use foreign-made DCS controllers, as they offer superior control functions; for the operation stations, domestic PCs can be used with the NT operating system. Third is the technical advancement of DCS, which refers to the use of proven cutting-edge technologies in the system, as well as its potential for further development and viability. This includes the openness and interconnectivity of DCS systems, the use of fieldbuses, and support from third-party software. The considerations are mainly based on the following aspects: (1) All factories need to establish a proper LAN (local area network). As users, we must pay close attention to the openness of the DCS system. Not only the network layout of the current control system must be considered, but also the networking with expanded production lines, in order to achieve data sharing. The main technical parameters to consider in terms of networking are: a) topology, which refers to the actual logical representation of a network’s layout. Typical topologies include bus, ring, star, and tree topologies; in our factory, a bus network topology is recommended. b. Data transmission methods: baseband transmission and broadband transmission. Transmission media include twisted pair, baseband coaxial cable, broadband coaxial cable, and optical fiber, among others. Our factory uses baseband transmission for data transfer from the field sites to the control center, with baseband coaxial cables as the medium; other applications employ broadband transmission, using optical fibers as the medium. c. Medium access control methods and medium access protocols, which enable effective channel utilization and ensure fairness among various users on the network. (2) Operation station: When considering the operation station, the following aspects should be taken into account: a. Functions of the windows: The functions of windows are mainly divided into process windows and system windows. The process window primarily takes into account aspects such as the overall view, grouping, alarm displays, trend curves, flowcharts, the ability of the operation guidance displays to convey information, capacity, switching capabilities, and corresponding speeds. The system window primarily reflects the system’s technical parameters, structure, and configuration methods. Factors to consider include the manufacturer, model, speed of the operation station CPU, RAM capacity, word length, cycle time, etc. b. Reporting function: It mainly evaluates whether the operation station has the capabilities for report types, formats, historical data tables, data retrieval, synthesis, and archived reports, in order to determine if it meets our requirements. C. The name and version of the operating system; Microsoft’s NT2000 is generally used. (3) Process control station: a. Control capability, which is the fundamental function of a process control station; it is assessed mainly in terms of the capabilities of the control blocks, their quantity, the ease and flexibility of configuration, as well as the adaptability of the modules. Complex control functions, self-tuning capabilities, the methods for constructing various control loops, and the ability to switch between different modes all need to be taken into consideration. Additionally, the manufacturer must provide us with calculations of the CPU load and the headroom for each CPU, which is generally required to be between 10% and 20%. b. The type of control station and the response time of interlocks vary; depending on the characteristics of each project and the contents of measurement and control, as well as the number of I/Os, the response time of interlocks also changes. When selecting the DCS system for our plant, it is necessary to choose one that suits the specific control objectives; for example, in the control of thermal power plants, a system with fast response times should be selected. C. Reliability of the control system and adaptability to the environment. The reliability of control systems is already very high, and it can be increased further through redundant configurations. In our factory, it is recommended to use CPU redundancy for PLCs in the bamboo pulping and cardboard production workshops as well as in the thermal power plants. It is also necessary to see whether the system can adapt to this industrial environment; this can be determined by considering the track record of the control integration manufacturer. (4) I/O: For I/O, the key factors are the type of signals, their range, resolution, load capacity, wiring method, and the layout of remote I/O devices. We should consider I/O blocks with a signal type of 4-20ma and detachable terminals. Use a remote I/O layout. (5) Engineer station and software configuration tool. For the engineer station, the main considerations are hardware configuration and basic software. The configuration tool focuses primarily on the configuration methods, as well as whether the capabilities and reliability of online and offline configuration, along with the flexibility and convenience for real-time online monitoring, meet the required standards. The fourth point is the technical capabilities of the contractor, that is, how well the contractor is familiar with specific process steps and the DCS itself. When choosing a manufacturer for the DCS system, we should select one with strong technical capabilities and expertise that matches our requirements. There are manufacturers that achieve excellent performance in the aspects we require, and whose control systems operate well. The fifth point is after-sales service; foreign manufacturers usually charge high prices for accessories and spare parts, and fail to provide them in a timely manner. DCS users should choose manufacturers with strong backing, robust technical capabilities, and excellent domestic technical support. Computer technology is developing rapidly, and DCS manufacturers also continuously update their products; compatibility between old and new products must also be taken into account. By selecting our plant’s DCS system based on these five key aspects, it should be possible to ensure that the system meets the requirements of advancement, reliability, rationality, and cost-effectiveness. It will be able to fulfill various control requirements effectively and serve production well.