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The ammonia synthesis plant of Henan Zhongyuan Dahuahua Group Co., Ltd. originally had two Siemens S5 PLC control systems; one was located in the central control room and was responsible for the interlock protection and sequential control of the plant’s components other than the compression units – this PLC was known as the main control PLC； Another set is located in the on-site compression control room and is responsible for the interlock protection of the six units; it is called the compression PLC. The physical distance between the two is nearly 500m. The two systems are connected to each other via a communication card CP525 and a communication cable, which transfers the real-time data from the six units to the main control PLC; from there, this data is sent on to the DCS control system. The entire system operates as a stand-alone unit, with no redundant design. 1 Necessity of renovation: The system has been in operation for nearly 20 years since its commissioning in 1989, and it is now in a period with a high incidence of failures, primarily due to aging of the system and an increasing rate of hardware failures. Since 2001, accidental failures have occurred: power cards, CPU cards, communication cards, and output cards have all caused the entire system to stop operating due to localized faults, resulting in huge direct economic losses and posing a serious threat to the safe and long-term operation of the equipment. Additionally, the original PLC used was from Siemens’ S5 series, and production of this series has been gradually discontinued, making it extremely difficult to obtain spare parts. In light of this, it has been decided to carry out a comprehensive upgrade in two major maintenance cycles, upgrading the current S5 series PLCs directly to the latest S7-400H redundant control system. The overall requirement for this upgrade is that the new system be more reliable, have redundancy, and provide some room for expansion. 2 Modification details: Two sets of Siemens S7-400H redundant systems are selected for the modification; the original S5-135U systems are replaced with Siemens S7-400H PLCs. Each system consists of one engineer station, one set of S7-400H systems, and field I/O devices. A circular redundant industrial Ethernet (Ethemet) is used to form the communication network. The renovation covers the PLC system cabinets, I/O interface card cabinets, and auxiliary operation consoles. The terminal cabinets and digital I/O safety barriers of the original system are retained, while analog I/O safety barriers are added; the processed and isolated field signals are introduced into the new S7-400H system through the newly added CN terminal blocks. Taking the main control PLC as an example, the new S7-400H system uses 1 control station and 4 I/O interface card cabinets (IP1–IP5). The control station is equipped with redundant control units, including 2 S7-414H CPU cards and an Ethernet communication card CP443-1; it facilitates data communication with the upper-level PC through a dual-ring fiber optic Ethernet network, and provides interfaces for establishing a factory-level Ethernet LAN. The lower-level control network uses the Siemens PROFIBUS-DP bus to connect the CPU to various ET-200 (field I/O units), thereby enabling data communication at the lower level. The IP4 cabinet is equipped with a serial communication card CP341 for communicating with the compression PLC and the main DCS. A redundant system is employed, with redundancy in power supply, CPU, communication network, and I/O; the I/O cards feature hot-swapping capability, and the system has a first-level signal alarm function. The engineer station uses WINDOWS 2000 as its system software, while the configuration software is STEF7 5.2 and the monitoring software is WINCC 5.02. The entire system can be configured, parameterized, and developed online ； Industrial control software can be used to monitor in real time changes in process parameters across the entire plant, equipment operation, and the occurrence of faults. Insert a SIMENS Ethernet card at the engineer station and connect it to the Ethernet network, allowing for reading and writing data from PLCs on the network as well as from other stations. The configuration of the compression PLC is similar; it is also equipped with a CP341 for communicating with the main PLC. The main PLC and the compression PLC establish redundant communication via a serial bus, ensuring secure and reliable communication. The entire system is essentially fully redundant, including the control unit, power supplies, communication processors at various levels, and most of the cards. The system configuration after the modification is shown in Figure 1. 3 Implementation process and technical challenges addressed: Upgrades were carried out during two major overhauls in 2004 and 2006. Since PLC renovation is a large-scale systematic project with a lengthy process and substantial workload, it is necessary first to understand and grasp the original design intent of the PLC in order to ensure that its existing functions are maintained while new functions can be implemented. After comparing and selecting various PLC control systems currently in use, Siemens’ S7-400H PLC was chosen as the target for renovation and upgrading. Firstly, the Siemens S7 series of PLCs exhibits good compatibility with the S5 series; the interlock programs written for S5 can be fully or partially converted directly into S7 programs using the professional software provided by Siemens. Such conversion is of great importance in ensuring consistency in the functionality of the PLCs before and after the upgrade. Secondly, hardware compatibility is also very important. The original compressed PLC utilized point-to-point communication via CP525 with the main control PLC; after the modification, the CP341 communication card from the S7 series can replace the original CP525 to enable serial communication. Additionally, among the S7 series of PLCs, the S7 400 series was chosen; within this series, the redundant S7 400H PLC was selected, as it represents the most advanced PLC control system in the S7 series. In terms of hardware, its CPU, power supply, I/O modules, communication networks, etc., can all be made fully redundant. Additionally, there are specialized redundancy software packages available in terms of software, all of which contribute to an **improved stability**. To ensure the successful completion of the renovation in one go, preparations for the upgrade began in April 2003. Efforts were made to further understand the existing PLC control system, the modified parts of the original PLC’s schematic diagrams and wiring diagrams were rechecked, knowledge of the new S7 400 PLC was acquired, and relevant PLC suppliers were visited on multiple occasions. Throughout the entire PLC renovation process, over 1,200 meters of cable were laid, nearly 2,000 terminal connections were made, and 700 channel checks were conducted. The circuit inspections took place in various locations such as the site, the 304A electrical building, the compression building, and the control room. Under the challenging conditions of limited space in the control cabinet rooms, tight deadlines, and heavy workload, the renovation tasks were completed successfully, with numerous difficulties overcome. 1) Compress communication between the controller and the main PLC. The original communication method was point-to-point communication between S5 CP525s. During the first renovation, it was changed to point-to-point communication between S7 CP341s and S5 CP525s. There were issues related to the different communication methods and parameter settings between the S7 and S5 systems (as the manufacturer had not designed for such communication). After a month of experimentation, the parameters were finally set correctly. The 2nd modification involves the point-to-point communication from CP341 to CP341 for the two S7 systems after the modification. 2) Communication between the main PLC and the main DCS; the DCS was also upgraded to the Yokogawa CS3000 system during the same period. Although communication was required to be carried out using the MODBUS protocol, setting up communication between the S7 CP341 and the MODBUS communication card ALR121 of the CS3000 presented a significant challenge. 3) Add a host computer, draw a logic diagram, and edit the monitoring program; the original PLC did not have a host computer or monitoring program, let alone any display interface. 4) The original interlock program contained sections that could not be converted, such as the Graph blocks in S7, which required manual coding. 5) Difficult to debug; channel and circuit testing involves instrumentation, electrical systems, and process engineering. 6) The integration of the old and new systems after the modification was carried out during the major repair. Although most of the equipment was shut down, the plant air and instrument air, which are part of the utility systems, could not be interrupted. This involved the PLCs that control these air supplies; ensuring a seamless transition of the system was a critical task. After analysis, it became clear that it was necessary to maintain the normal operation of the instrument air control units. The SB61 program for controlling the drying of plant air in the original PLC had to continue to function properly. The 4 cooling water pumps and 2 air compressors involved in this process needed to be connected in series by the electrical team to enable their operation on-site, as required by the process requirements. During the removal of the original terminals, the PLC program was modified to retain the signals from the instrument air control unit, ensuring that the SB61 program and the several solenoids involved in the wiring continue to receive a 24V DC supply. After the new system is powered on, first restore the wiring of the instrument air control unit to minimize the duration during which instrument air supply is interrupted. Thus, the handover between the old and new systems was completed. 4 Comparison before and after the renovation and economic benefits: Since the PLC upgrade was completed in 2004, the system has started up successfully each time, with no failures in the PLC itself to date; this has ensured its long-term operation and also provided experience for the subsequent upgrades of the main control PLC. Before the renovation, starting in 2001, the plant stopped operating at least twice a year due to failures in the PLC, such as problems with the power card, CPU card, communication card, and output card; each such shutdown resulted in direct economic losses of over 1 million yuan, meaning that the annual direct economic losses caused by PLC failures amounted to more than 2 million yuan. After the modification, it has been in operation for 3 years without a single shutdown caused by the compressed PLC. Furthermore, the successful transformation of the main control and compression PLCs is significant because the group company also has desalination units and boiler systems that use S5 PLCs; the programs for these two sets are more complex, with more control elements involved, making the transformation process more challenging. Successfully upgrading the main control and compression PLCs can cultivate talent for future PLC upgrades and accumulate valuable experience.