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In PVC (polyvinyl chloride) production, the JX—300X DCS control system is employed to automatically control the entire polymerization process, enabling closed-system PVC production and achieving the desired results from the system upgrade. Henan Shenma Chlor-Alkali Chemical Co., Ltd. is a chemical enterprise with an annual production capacity of 50,000 tons of PVC; its original polymerization DCS control system was designed to control only the temperature of the reactor, resulting in a system of limited scale. In November 2003, during the technological upgrade to achieve closed-loop production of polyvinyl chloride, since the original control system was too small and could not be expanded, after thorough research, it was decided to use the JX-300X control system. During the renovation, the control functions for feeding, temperature raising, temperature maintenance, stripping, compression condensation and recovery, as well as centrifugal drying of the 6 polymerization reactors were integrated into a single DCS control system, enabling automatic control over the entire process from polymerization feeding and temperature maintenance to product discharge. After several months of continuous operation, this system has demonstrated stable performance and is safe and reliable. It not only increases the yield and quality of resin while reducing the workload on workers, but also improves the working conditions for polymerization and saves energy, achieving the desired goal of operating 100 reactors without cleaning them or opening their lids. System selection, configuration scale, and structure: By understanding the operation status of DCS systems from other manufacturers and taking into account the actual conditions of Henan Shenma Chlor-Alkali Chemical Co., Ltd., in order to facilitate the network connection between the conversion/distillation DCS system and the polymerization DCS system, it was decided to use Zhejiang University Zhongkong’s JX-300X system for the polymerization DCS. This system not only meets the requirements for production monitoring, process control, operation interfaces, parameter alarms, data recording, and trend analysis, but it also utilizes advanced domestic software and hardware technologies. Its flexible structure and high level of safety make it easy to maintain, thus fulfilling the needs of industrial processes. Based on the actual characteristics of the polymerization production process, the system is equipped with 68 analog input channels, 52 thermal resistance sensors, 28 analog output channels, 210 digital input channels, and 147 digital output channels, for a total of 505 channels. Based on these points, the system configuration is determined to include 4 operation stations (including one engineer station), 1 control station, 2 control cabinets, 1 relay cabinet, 28 control circuits, 7 I/O racks, 4 power supply units, etc. The control station is responsible for collecting and processing signals as well as performing calculations to generate outputs; it features self-diagnosis capabilities. The main control card, data forwarding card, and power supply of the control station are arranged in a hot standby redundant configuration, allowing for automatic and seamless switching in the event of a failure. The operation station provides a human-machine interface that receives real-time process data from the control station, converts it into corresponding process variables, and displays them on a monitor for operators to monitor and control. The engineer station can serve both as an operation station and for system configuration. Data exchange between the operation station and the control station is carried out via the high-speed communication network Scnet. This system features a total of 16 process interfaces, namely: 1) Soft water and stabilizer operation screens ； 2) Monomer operation screen ； 3) Dispersant operation screen ； 4) Initiator operation screen ； 5) Water injection and terminator operation screen ； 6) Coating kettle rinsing water operation screen ； 7) Operation screens for Vessels 1#–6# ； 8) Recycling operation screen ； 9) Stripping operation screen ； 10) Constant temperature control parameters ； 11) Formula list. These images meet the needs of aggregate production control. On-site electrical instrumentation: For this technological upgrade of polyvinyl chloride, in addition to retaining the existing detection, transmission, and measurement devices, automatic control valves, and electrical equipment, 21 additional detection, transmission, and measurement devices were installed, along with 71 programmable valves and automatic control valves, and 15 programmable pumps. The control scheme for the suspension polymerization process of polyvinyl chloride involves feeding soft water, monomer, dispersant, initiator, and other additives in specific proportions in a sealed manner. Subsequently, the temperature is raised using a hot water jacket, constant temperature is maintained, water is added automatically, a terminator is introduced, and the product is discharged automatically for rinsing. The residual monomer is recovered through a stripping and recovery system, water is removed via centrifugal drying, and finally the resin is packaged in bags. For such a complex process, the use of fully automated control can improve production efficiency, reduce consumption, and minimize environmental pollution. Therefore, this control system is mainly composed of the following components: sealed feeding control for 6 reactors ； Temperature rise and constant temperature control for 6 reactors ； Control of boiling and discharge flushing for 6 kettles ； Stirrer tower control ； Compression condensation recovery control and centrifugal drying control, etc. The closed-feed control process is relatively complex and requires high levels of precision; SC control language is used for programming to enable sequential control. It comprises nine subroutines: the soft water subroutine, the stabilizer subroutine, the monomer subroutine, the dispersant subroutine, the initiator subroutine, the mid-process water injection subroutine, the terminator subroutine, the reactor coating subroutine, and the discharge flushing subroutine. Each of these subroutines can be started independently or in a interlocked manner. When the interlock button for the feeding main program is set to “IN”, the entire feeding process operates automatically. In addition, there are three custom single-loop controls for monomer return, wash water return, and intermediate water injection return. (The details of the specific controls are omitted.) Additionally, other controls such as compressed condensation exhaust pressure control, reflux control, and centrifuge feed control all employ single-loop PID control. System debugging: To avoid disrupting production, most of the debugging work is carried out during the intervals between polymerization reactions and during shutdowns for maintenance. ■ Hardware device calibration: For the instruments used for on-site monitoring, standard signals are first applied on-site; the DCS is used to check whether the displayed values of temperature, pressure, and flow rate fall within the allowable error ranges. Signals are then sent from the control station to verify whether the operation of the valves on-site meets the required standards, as well as to ensure that the feedback status and contact signals function properly. Using this method, 68 automatic valves, 10 level transmitters, 6 flow meters, and 2 weight transmitters were calibrated. The results of these calibrations showed that all devices met the process requirements, ensuring the accuracy of the instruments. ■ Software debugging: After the on-site instruments are properly tuned for each circuit, debugging of the various subroutines begins. Water is added to the polymerization reactor to carry out water-driven experiments, and subroutine simulation tests for each unit are initiated to check whether the sequence of valve operations follows the specified requirements. Practice has shown that this cumbersome task laid the foundation for the joint debugging of the main program, providing a basis for the successful final feeding. Problems and Solutions Some issues were identified during the initial debugging and commissioning phases, and they were addressed. ■ During the process of feeding soft water and monomers into the tank, a significant vibration in the process pipelines could be heard at the moment either the startup procedure or the shutdown procedure was initiated. Initially, it was suspected that the pipelines were not properly secured; however, upon closer inspection, it was found that at the moment of startup, the pump, the main valve, and the valve on the tank started operating simultaneously, resulting in resonance and thus significant vibration. By modifying the sequence in which the pump and valves are activated, this vibration was resolved, eliminating the potential safety hazard. ■ It is recommended that a single set of limit switches be shared by both the electrical and instrumentation systems for the limit switches on the bottom valves of each kettle spray valve. During practical use, it was found that the limit switches of the spray valve and bottom valve often caused feedback signals to return at different times, which disrupted the continuation of the process. By adjusting the wiring, it was possible to achieve consistent performance from both the electrical system and the instruments. It met the requirements of both parties, and the results were satisfactory. ■ The pressure gauge at the outlet of the flushing water pump keeps breaking. At the time of commissioning, the pressure gauge at the outlet of the flushing pump kept breaking. By modifying the existing program to prevent the flushing pump from starting frequently and to keep the backflow at the pump outlet under automatic control, the stability of the pressure at the pump outlet was ensured, thereby eliminating the safety risks associated with frequent startups of the large motor. Application Conclusion ■ Since the JX—300X DCS system was put into operation over half a year ago, it has operated stably and reliably, with simple operation, which has enabled further optimization of process parameters and fully utilized the production capacity of the existing equipment. ■ It enables fully automated control of the polymerization process from feed to discharge, improving the yield and quality of the resin, stabilizing process parameters, simplifying on-site operations, reducing the workload for operators, and increasing labor productivity. ■ It enables polymerization production to be carried out in 100 reactors without the need to clean them or open their lids; the smell of monomers is hardly detectable at the polymerization stations. **This improves the working conditions in the polymerization process and reduces the harm caused by toxic gases to human health. ■ A communication interface is provided, allowing connection to remote computers via a hub, which creates the conditions for future centralized monitoring and control of the process. ■ It offers high reliability, reduces the workload associated with instrument maintenance. Moreover, thanks to the localization and versatility of the system components, it lowers the inventory levels of spare parts as well as the capital tied up in them, thereby reducing production costs. ■ The original control systems mostly relied on conventional instruments for control; there were numerous control loops and detection points, as well as many control instruments. The control cabinets were also numerous and installed in dispersed locations. With the adoption of the JX—300X system, the entire system requires only 2 control cabinets, 1 relay cabinet, and 4 operation stations, **reducing the number of control devices and saving costs and space, thus fully demonstrating the advantages of the DCS system. ■ Due to its comprehensive functions, advanced technology, safety and reliability, as well as low failure rate, this system saves a significant amount of money on maintenance, thereby generating excellent economic and social benefits. Practice has proven that the application of this system is highly successful. This article is provided by Zhejiang Zhongkong Technology Co., Ltd. Liu Yueju and Huang Jinbao work at Henan Shenma Chlor-alkali Chemical Co., Ltd., while He Xiang and Liang Yalin work at Zhejiang Zhongkong Technology Co., Ltd. (end)