Functions and usage of DCS, ESD, and PLC in production systems

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I. Overview of Automated Control Systems (10) 1. Basics of Automated Control Simple loop control models Diagram (using level control as an example) a) Sensors, which are used for detecting parameters in the production process; also known as measuring and transmitting instruments b) The process being controlled and the system itself c) The controller d) The actuator e) Other components such as alarm systems and interlocks 2. Development of Control Systems There are four stages in its development: instrument control systems, digital direct control systems (DDC), distributed control systems, and fieldbus control systems (FCS). II. DCS (Distributed Control System) A distributed control system is also referred to as a “decentralized control system” or “distributed control system”. English: “Distributed control system” is abbreviated as “DCS”. Characteristics of a distributed control system: it is a next-generation control system based on microprocessors, featuring decentralized control along with centralized display and operation. 1. Basic structure of the DCS system: The DCS system consists of three main components: 1) Distributed process control units: Their function is to convert various variables in the production process into data suitable for monitoring operations. Various operational instructions are sent to the actuators through these control units, which also perform tasks such as A/D and D/A conversions as well as filtering. The components include functional boards such as AI, AO, DI, DO, PI, PO, along with a CPU, power supply, and terminal boards. 2) Operation management units: Operators can use these units to monitor the status of the production process and issue operational commands to it. The components include human-machine interfaces. 3) Communication systems: These systems are responsible for exchanging and transmitting data between the distributed process control units and the operation management units. The components include cables and interfaces. 2. Fieldbus Control System (FCS): Trends in the future development of DCS systems. Fieldbus Foundation: Established in October 1994; it has 120 member organizations. In China, organizations such as the Automation Research Institute of the Ministry of Metallurgy, Beijing Huakong Technology Co., Ltd., and the Chinese Instrument and Control Association are among its members. 　　The Foundation Fieldbus is a network that establishes digital, bidirectional, multi-node communication links between multiple intelligent field devices and automation systems; it replaces the 4–20mA analog signal standard and serves as a bidirectional digital multi-node communication protocol for connecting intelligent fieldbus instruments with control room equipment. The products include the Honeywell TDC3000 used in carbon monoxide air separation, the Zhejiang University Zhongkong JX-300XP used for acetic anhydride, and the Emerson DeltaV used for new carbon monoxide methanol processes, among others. 3. Composition and structural format of DCS systems: 1) Hierarchical structure diagram of DCS; 2) Components of each structural part; 3) Introduction to DCS hardware – main control cabinet (), power supply, CPU, I/O cards, communication cards, safety barriers, relays, etc.; 4) Concept of redundancy. Introduction to 4CS software. DeltaV 5. Comprehensive safety protection (grounding and lightning protection) 6. Application and fault analysis. II. ESD 1. SIS and ESD. “Design Specifications for Safety Instrumented Systems in the Petrochemical Industry” (SH/T 3018-2003). A safety instrumented system is a system that utilizes instruments to achieve safety functions. The system includes sensors, logic operators, final actuation elements, and corresponding software. The Safety Instrumented System (SIS) is also known as the Emergency Shutdown System (ESD), Safety Shutdown System (SSD), Safety Interlock System (SIS), or Safety Protection System (SPS). A Safety Instrumented System should utilize a programmable control system that has been certified by an authoritative body. ” 2. Introduction to ESD: ESD is the abbreviation for Emergency Shutdown Device, which refers to the emergency shutdown system. When certain variables in the manufacturing process deviate from normal levels, the control system commands an emergency and safe shutdown of production to prevent significant financial losses and casualties. The ESD emergency shutdown system, in accordance with the principle of safety independence, operates independently of the DCS distributed control system, and its safety level is higher than that of the DCS. Under normal conditions, the ESD system remains in a static state and requires no human intervention. As a safety protection system, it operates above the production process control to monitor the safety of the devices in real time. Only in the event of an emergency in the production equipment is it not necessary to go through the DCS system; instead, the ESD directly sends out protection interlock signals to safeguard the on-site equipment and prevent the spread of danger from causing significant losses. According to available data, people’s judgment and actions in dangerous situations are often delayed and unreliable; when operators are faced with a life-threatening situation, they must react within 60 seconds, and the probability of making a wrong decision is as high as 99.9%. Therefore, it is highly necessary to install safety interlocks that are independent of the control system; this is an important principle for ensuring safe production. It moves when it should move and stays still when it shouldn’t, which is a notable feature of the ESD system. 　　 Why is it necessary to have an ESD system set up separately? Of course, general safety interlock protection functions can also be implemented by a DCS. However, for larger-scale emergency shutdown systems, they should be installed separately from the DCS in accordance with the principle of safety and independence. There are several main reasons for this: (1) to reduce the probability of both control functions and safety functions failing simultaneously, so that a malfunction in the DCS does not compromise the safety protection system; (2) for large-scale installations or rotating mechanical equipment, the faster the response time of the emergency shutdown system, the better. This helps to protect the equipment and prevent accidents from escalating ； It also helps in identifying the causes of accidents and keeping records of them. Since DCS processes a large amount of process monitoring information, its response speed is difficult to make fast ； (3) The DCS system is a process control system that is dynamic and requires frequent manual intervention, which may lead to human-induced errors ； ESD, on the other hand, is static and does not require human intervention; by setting it this way, human errors can be avoided.