Thread Content
1 What are the configuration and features of the automatic control system? The WebField control system for the \"18·30\" purification process consists of four operation stations, one engineer station, two control stations, and a process control network. ? In the overall design, we focused on the following aspects. ? 1.1 Ensuring safe operations? Given that raw materials and products are flammable, explosive, and toxic, and that leaks or other incidents during the production process can pose threats to production equipment and human safety, it is necessary to consider safety aspects not only in the design of production facilities and processes but also when selecting automation equipment and designing control systems. This ensures that both hardware and software contribute to a safe production process by enabling rapid responses to unexpected events and providing effective solutions. For this project, we took the following considerations and designs into account regarding security. ? (1) Analog signals between the control system and field devices are isolated using safety barriers, while digital input signals (DI) and output signals (DO) are isolated using relays. ? (2) During software design, restrictions were imposed on operational parameters and behaviors, and a design was implemented to hide certain user interfaces; in other words, different levels of operators are assigned distinct user interfaces along with varying levels of operational permissions. ? (3) Different alarm sounds are set for different work sections, while voice alarms are used for important process parameters. ? (4) Tagging operation: When instrument maintenance personnel are performing repairs on a device, they use software-based tagging to alert operators that the corresponding circuit (or measurement point) is under maintenance and is in a frozen state. In the case of a control circuit, it is set to manual operation mode; only the output of the control circuit can be changed, while all other parameters remain at their values before freezing, until the tagging is removed. 1.2 Is operation reliable? Under normal operating conditions, if the system cannot function accurately and reliably, it can pose a risk to production. Especially when the production equipment is not operating under normal conditions, it is even more necessary for the control system to carry out reliable interlock actions in a timely manner; otherwise, shutdowns and safety accidents may occur. For this project, we have made the following considerations and designs with regard to reliability. ? (1) High-quality, high-precision equipment and components were selected for the design of key on-site devices, such as online hydrogen/oxygen analyzers, flow meters, shut-off valves, and relays. ? (2) Dual redundancy measures have been adopted in the design of the control system’s main control card, data forwarding card, key components, system power supply, network, and system power supply circuits. ? (3) The emergency stop interlock system adopts a combination of hardware and software; operators can operate the buttons on the auxiliary control panel as well as the soft buttons on the computer interface. ? (4) The operation stations are designed in a peer-to-peer manner; any one of them can serve as a hot redundant operation station for the others, allowing operations to continue using another station in case one of them fails. ? 1.3 Full automation? This device has numerous detection points; the actual number of usage points is 360 ； There are numerous instrumentation devices on site: 123 detection and transmission devices, 40 pneumatic control valves, and 6 online analyzers ； It features a high degree of automation; there are 40 control loops in total, with conventional control being the primary method, along with some proportional-integral control and variable ratio control ； The safety interlock design is comprehensive. The system scale is shown in Table 1. ? 2 Implementation of the main control circuits and resolution of practical challenges: During the operation of the methanation tower, changes in the volume of gas passing through it can occur due to adjustments in the output of alcohol and ammonia within the entire ammonia synthesis system, as well as changes in the system’s operating conditions. An increase in reaction heat can also result from excessive carbonization emissions; all these factors cause rapid fluctuations in the reaction heat within the methanation tower. As a result, conventional control methods cannot be used to regulate the temperature at key points within this tower. ? Under the premise of heat balance, two streams of cooling gas with different temperature differences are used to regulate the fluctuations in the hotspot temperature of the catalytic bed caused by minor changes in gas composition and flow rate in the methanation reactor; a recirculation line is employed to prevent sharp increases in the temperature of the catalytic bed resulting from issues such as excessive carbonization exhaust gases. When self-heating cannot maintain equilibrium, an electric heater is used to keep the temperature of the catalytic bed at a constant level. The control circuit is shown in Figure 1. Table 1: I/O table for the purification process unit? http://www.nmtech.com.cn/jishuwang/upload/0602151448405020.jpg http://www.nmtech.com.cn/jishuwang/upload/0602151449435183.jpg Figure 1: Schematic diagram of the control circuit. (1) During the heating and reduction phase, the electric heater is controlled based on the heating and reduction curve to increase the temperature of the catalytic bed; this curve is adjusted as needed, and it is recorded for future reference. ? (2) During normal production, the CO concentration remains within normal levels, the gas flow rate is fairly stable, and the fluctuation range of the hotspot temperature in the catalytic bed of the methanolization reactor stays within the control limits set by the cooling bypass system. The stepwise regulation provided by valves 1 and 2 in this bypass system enables higher precision in control and more stable operation. ? (3) Excessive CO levels in the exhaust gases; at this point, the temperature of the catalyst bed in the methanation reactor rises sharply, prompting the control system to activate the bypass circuit. (4) When the hotspot temperature drops, the control selector automatically activates the electric heater. ? 3 Operation status of the device and suggestions for upgrading similar devices: Since its commissioning in May 2005, this system has been operating well; its control system fully meets the requirements of the process, and it has the following main features. (1) The control system is designed to be reliable, featuring comprehensive accident detection and rapid response capabilities; it can handle various unexpected situations in a correct and effective manner. Its interlock functions are timely and accurate, playing a significant role in protecting the process equipment and ensuring safety. ? (2) Since its operation began, the system has been functioning properly; users are satisfied with its technical specifications, and the automatic control operation rate is above 95%. ? (3) The hot spot temperature in the catalytic bed of the methanolization tower is kept under stable control; when the operation conditions in the dimethyl ether production section are stable, the hot spot temperature can be maintained within a range of ±5°C of the set value. Manual intervention is required only when the CO content in the exhaust gas doubles or when the ammonia synthesis system experiences significant load increases or decreases. ? (4) Using different zones and representation methods for various alarms facilitates operators’ intuitive identification of them. ? (5) The user interface is clear, **reducing the time required for operator training, as well as the time needed for on-site system debugging and commissioning of the process.