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The DCS system used in the ammonia synthesis unit of Henan Zhongyuan Dahua Group Co., Ltd. is the Yokogawa CENTUM V from Japan. It has been in operation continuously for 18 years since the plant was built in 1987, and it is now entering a period with a high incidence of failures; the stability of the system is thus severely threatened. Coupled with difficulties in obtaining spare parts, the company decided to upgrade the DCS system during the annual maintenance in 2005. 1 Design concept: Simplify the existing structure, increase system redundancy, maintain and expand existing functions to facilitate operational procedures, while also taking into account system expansion. To this end, the Japanese Yokogawa CENTUM CS3000 system was selected to upgrade and transform the existing DCS system. The modification covers the control cabinet (I/O cabinet); the original system terminal cabinet and safety barrier are retained, while the previously processed and isolated field signals are introduced into the new CS3000 terminal cabinet via the newly added CN terminal blocks. The original DCS had 6 control stations, 2 monitoring stations, and 1 communication station, resulting in a complex structure. The new DCS system integrates them into 2 control stations, fully retaining all the original functions. The previously used 01 station has been transformed into a remote I/O controller without any control station required; communication between the old DCS and the PLC is handled by communication cards. The hardware configuration of the new system consists of the engineer station HIS0164 and the operator stations HIS0163, HIS0162, HIS0160, HIS0159, HIS0158, as well as the field control stations FCS0101 and FCS0102, as shown in Figure 1. http://www.nmtech.com.cn/jishuwang/upload1/080526938152044.jpg 1) The engineer station HIS0164 is used to run the standard configuration software for the CS3000 system, flowchart configuration software, standard operation and monitoring software, OPC Server interface software, and reporting software. It is primarily used for system configuration, online maintenance, deployment of engineering programs, and report generation. Operator stations such as HIS0163 and HIS0162 are used for process operation and monitoring. A V-Net interface card VF701 is installed on each engineer station and operator station to connect to the V-Net network. The Ethernet card built into the PC is used to connect to the E-Net network. V-Net is a real-time system control network that adopts a dual-redundancy architecture to connect devices such as operator stations, field control stations, and bus converters, for transmitting process control information. The V network supports the IEEE802.4 communication protocol, as well as the 10Mbps token passing network protocol. E-Net is an information and communication network used by the engineer station to transmit configuration databases to various operator stations. It forms a local area network that enables the stations within it to share peripheral resources. It complies with the IEEE802.3 standard, uses TCP/IP as its communication protocol, and has a communication speed of 100Mbps. 2) The process control stations FCS0101 and FCS0102 are both composed of AFG40D units. The AFG40D adopts a dual-redundancy control structure, with 2 independent CPUs in each control unit; all 4 CPU units operate simultaneously. The system compares the results produced by the 2 CPUs in each module to ensure consistency, and if the results are consistent, the controller of that module outputs the result, thereby achieving complete redundancy and fault tolerance. 2 Technical challenges to overcome 1) Internal system communication. The CS3000 system uses 2 control stations; the FCS0102 station is equipped with 1 remote I/O node (NODE). Through the ER-BUS, the EB401 of the FCU (Field Control Unit) communicates with the EB501 in the remote nodes, enabling the transfer of process data between the control unit and the remote I/O cards, thus facilitating the processing and control of I/O process data in the compression plant. The transmission medium between the controller and the compressor is a 50Ω coaxial cable, employing a dual-redundancy structure. The CS3000 system also provides an RS485 communication interface for communicating with PLCs on the MODBUS bus. The entire interlock system of the ammonia synthesis plant is controlled by redundant \"Siemens S7-400H\" PLCs. To facilitate the exchange of process data between these two systems, the MODBUS communication protocol is used. For communication with the Siemens PLCs, Yokogawa’s serial communication card ALR121 is employed in a redundant configuration, with communication established using the \"K1-4-3MODBUS\" protocol. 2) Temperature compensation of the thermocouple. The temperature compensation for some of the thermocouples in the original synthesis unit, including K-type and S-type thermocouples, is achieved by providing a constant temperature terminal at 70 °C in the on-site junction box; subsequent compensation signals are then added within the DCS using the special functions of the cards. The CS3000 system has modified some of the functions of the original cards as a result of an upgrade; these functions differ significantly from those of the original DCS temperature compensation method. It is no longer possible to implement temperature compensation using the original method, so the internal function blocks of the DCS are used during configuration to carry out temperature compensation for thermocouples. First, create an empty PVI table (T00000 is used for the set value of 70 °C, which is the constant temperature of the field junction box), and then use the ‘ADD’ operation block to combine the field temperature with the set value. After final inspection and testing, it was confirmed that the function worked perfectly; adopting this approach can **reduce the costs associated with using compensation wires. 3) DCS programming and configuration. In terms of internal configuration, the new DCS differs significantly from the old system in terms of implementation methods. Many of the control algorithms used in the original design lack relevant documentation, including the cascade control of P03048 and P02025 for regulating the speed of S02001, the control of the combustion ratio in the auxiliary boiler, the control of the pressure reduction stations P03054 and H09001, the conversions performed by H03003 and P03029, as well as the calculation of the water-to-carbon ratio and air-to-carbon ratio. To ensure that the new system could perform the same functions as the old one, testing was carried out while configuration was in progress. Individual programs such as those for controlling the pressure relief stations, the sequential control of Unit 12, and the communication between the main DCS and PLCs (in a redundant manner) were rewritten, until all these important algorithms and functions were implemented. The new functions added to the 3-system DCS include more user-friendly menu interfaces, while continuing to retain all the existing functions such as control rod diagrams, trend charts, alarm messages, and operation guidance information. Additionally, a logic diagram function has been introduced; the interlock logic programs that were previously run in the PLC are now presented in the form of flowcharts within the DCS, allowing operators to easily view them and understand the various interlock states of the system at a glance. The operator can also open multiple windows simultaneously, using one to view the alarm screen and another to view the flowchart, which makes operations more convenient. The new system adds interconnection capabilities between various operation stations, enhancing the system’s security, facilitating its maintenance, and enabling data transfer between these stations. Taking into account the future modification needs of the unit vibration and displacement monitoring system in the compression plant, the new system adds an additional operation station in the compression plant, while reserving sufficient channels at the control station. This ensures that the operators are aware of the status of the production process system as well as the operational requirements imposed by load fluctuations on the units. It also creates the conditions necessary for removing the YS-80 system in subsequent modifications, and permission settings are used to ensure the reliability of operations. 4 Summary The renovation and testing of the new system were completed before the start of the annual maintenance period. Once the maintenance began, it took only 1 week to switch from the old system to the new one, ensuring that all important functions were implemented simultaneously. Since the transformation and commissioning of the synthetic DCS control system, there have been no production unit shutdowns caused by issues with the control system itself, and the new system has also received unanimous praise from operators for its excellent user interface.