Abstract: This paper briefly introduces the important role of fieldbus systems in the petrochemical industry, analyzes their advantages in practice, and presents personal views on their application and development.
Keywords: FCS control system; structural characteristics; advantages analysis;
Looking ahead, the national policy of China is "informatization drives industrialization, and industrialization promotes informatization." Currently, the informatization of petroleum and chemical enterprises is structured in three layers: The first layer is the basic automation layer of the production process, represented by PCS (Process Control System), which mainly includes advanced control systems (DCS, FCS), advanced control software, soft measurement technology, and real-time databases. The second layer is the production process operation optimization layer, represented by MES (Manufacturing Execution System), which mainly includes cash planning and scheduling technology (APS), real-time production tracking technology, dynamic quality control and management technology, and material balance technology. The third layer is the production process management optimization layer, represented by ERP (Enterprise Resource Planning), which mainly includes enterprise resource management (ERP), supply chain management, product quality data management, equipment resource management, and enterprise e-commerce platforms. While there is some overlap in the functional division of these three layers, each has its own focus. Currently, China National Petroleum Corporation (CNPC) is promoting and implementing advanced management systems on the MES and ERP platforms. Although the PCS layer is at the bottom, the effectiveness of the various control systems and technologies applied at this layer directly affects the speed and outcome of the informatization process. Currently, DCS (Distributed Control System) remains the mainstream, with nearly 800 systems in use domestically. However, Fieldbus Control System (FCS) represents the current development direction of control technology and DCS, and has entered the industrial application stage. There are various comments on FCS, ranging from praise for the new technology to confusion about its current state. Despite these differing opinions, this author will briefly offer the following personal views on the application and development of FCS, using examples from our company's equipment:
I. Characteristics of the (FCS) Fieldbus Control System
According to the IEC/ISA definition, a fieldbus is a digital, bidirectional, multi-branch communication network that connects intelligent field devices and automation systems. It is an interconnection network for the lowest-level field devices and instruments in process automation, integrating field communication networks and control systems. It features system openness, interoperability and compatibility, intelligent and autonomous field devices, highly decentralized system architecture, and adaptability to field environments. A structural comparison between an FCS and a traditional control system (such as a DCS) is shown in the figure.
II. Advantages of the (FCS) Fieldbus Control System
For safety reasons, the distance between refining and chemical plants and the operation and control personnel is generally far, making FCS (Fieldbus Control System) more advantageous than DCS (Distributed Control System). Taking our company's 200,000 tons/year catalytic reforming and 100,000 tons/year diesel hydrotreating combined unit as examples, we will analyze the advantages of using a fieldbus system. Currently, this combined unit uses a DCS system, with a distance of approximately 450m between the unit and the control station. The I/O points of the DCS system field control station are as follows (excluding ESD system points): Analog input points (AI) with control: 106 points, indication: 208 points; Temperature indication (T/C), RTD: 340 points; Analog output (AO): 93 points; Digital input (DI): 50 points; Digital output (DO): 22 points. Based on these points, the minimum scale of the distributed control system (considering a 20% redundancy margin) should include at least 4 control cabinets, including: 57 8-point AI cards, 68 6-channel T/C cards, 27 8-point AO cards, 4 16-point DI cards, and 2 16-point DO cards. Four safety barrier cabinets are included, each equipped with safety shields, relay terminals, etc., for all inputs and outputs. One power supply cabinet provides power to the control cabinet and safety barrier cabinets. There are four operator stations and one engineer station. Cable laying follows a route from field instruments to junction boxes, then through multi-core cables to the control room terminals. This requires approximately 41 km of 8-core cable and approximately 41 km of 2-core cable. If an FCS system is used, its advantages will be evident in everything from control system design, installation, commissioning, to routine maintenance.
(I) Savings in Initial Investment and Installation Costs: Taking PROFIBUS-DP as an example, a multi-master control system is constructed. Since the intelligent devices in the field have multiple functions, separate controllers, alarm setters, and various cards and barriers of the DCS system are no longer needed. Therefore, all cards, barriers, and other transmitters, setters, and other control equipment in the four control cabinets and four safety barrier cabinets of this combined unit can be eliminated. However, compared to Type III electrical instruments, the price of field intelligent instruments is slightly higher, but overall, the hardware investment is lower than that of the DCS system. Due to the reduction in equipment in the control room and the simplicity of the fieldbus system wiring (multiple devices can be connected to a single twisted pair), the quantity and specifications of cables, cable trays, and cable management systems are greatly reduced, saving investment and reducing design and installation workload. The reduced cable tray width also reduces the difficulty of civil engineering construction. Overall, initial investment can be reduced.
(II) Savings in Installation and Routine Maintenance Costs: When additional field control equipment is needed later, there is no need to install new cables; they can be connected to the existing cables nearby, saving on later cable investment and corresponding construction work. It is generally believed that installation costs can be reduced by 60%. Because the field control equipment itself has self-diagnostic and fault-handling capabilities, and the control network can transmit the status and diagnostic information of the field control equipment to the control room, operators and managers can quickly and accurately understand and query the operating status of the production site and the automatic control equipment. As intelligent field devices that act as network nodes, they possess digital computing and digital communication capabilities. Signals are directly transmitted between the field measurement transmitters and actuators via the network, thus control functions can be completed directly on-site, realizing a completely decentralized control system function, fundamentally improving the reliability and effectiveness of the control system operation.
2.3 Users have a high degree of initiative in system integration.
The control network overcomes the shortcomings of the closed nature of dedicated networks in DCS systems. It adopts an open and standardized solution, allowing users to select a bus system based on widely recognized and open protocols, without worrying about incompatible protocols and interfaces during system integration.
III. Problems in Domestic Engineering Applications
(I) Price Issues The main feature of FCS is its savings in installation and maintenance costs. However, its hardware price is not lower than traditional DCS or PLC, and may even be slightly higher. Overseas FCS pilot application reports have illustrated this point, showing that the total cost of FCS is low. But in China, on the one hand, users are very sensitive to hardware prices, and imported FCS hardware is shockingly expensive; on the other hand, due to the current low labor costs in China, the costs of engineering design, management, installation, and commissioning are far lower than abroad. This is a major reason why FCS is not widely used in China.
(II) Redundancy Issues According to the design philosophy of fieldbus, low-speed components do not require redundancy because the risks are already dispersed; local faults will not lead to global failures. The intelligent instruments in the fieldbus also have the function of predicting maintenance, allowing for proactive prevention. Furthermore, in the event of a field instrument failure or control loop malfunction, the host computer can intervene. When the host computer fails, the field instruments can autonomously adjust themselves by forming their own control loops in the field. However, in some special applications in actual production, control system users (especially production enterprises that have experienced factory accidents due to control system failures) place particular emphasis on redundancy, some even requiring complete redundancy (including power supply, network, etc.). Currently, the international fieldbus standards FF's FL and PROFIBUS's PA do not offer redundancy. Of course, complete redundancy is not entirely impossible technically, but it is often economically impractical.
(iii) Debugging and operation and maintenance are more difficult than expected. Because fieldbus technology includes many new technical contents, especially fieldbuses like FF are quite complex, and various difficulties are often encountered in the debugging process and operation and maintenance.
IV. Future Prospects
Currently, most process industries in my country use DCS as the mainstream control system and fieldbus-based instruments as the dominant instruments. However, due to differing perceptions and the fact that fieldbus intelligent instruments are not yet the dominant products, FCS cannot be widely adopted. From the perspective of utilizing existing resources, the disappearance or complete replacement of DCS systems in the short term is unreasonable. We should focus on existing DCS systems and fully explore the potential of existing equipment (e.g., installing gateways between DCS and FCS to enable information transmission), ensuring that existing investments are not wasted. Furthermore, DCS is a continuously evolving control system, and it will inevitably be modified to adopt fieldbus technology, enabling it to connect with fieldbus intelligent sensors (intelligent instruments) and local FCS systems. Currently, all these factors contribute to the coexistence of FCS and DCS. It is estimated that it will take approximately 10 years for FCS to truly become the dominant system.
References:
[1] Industrial Data Communication and Control Networks, Yang Xianhui, Tsinghua University Press.
[2] Fieldbus and Industrial Ethernet and their Application System Design, Li Zhengjun, Posts & Telecom Press.
For details, please click: Application and Development of Fieldbus Control Systems in the Petrochemical Industry
