1. Introduction The air compressors at a certain petrochemical plant's air compressor station mainly provide purified air for instrument control to three important production units: the petrochemical plant, the fertilizer plant, and the power engineering department. If an air compressor stops operating and cannot be restored quickly, it will cause a complete shutdown of the three production units. The reliability of the air compressor's electrical control system directly affects the completion of various production tasks and plays a crucial role. 2. Analysis of the Original Air Compressor System 2.1 Background of the Modification Before the modification, the air compressor station's electrical control system was used from 1989 to 2005, a period of 16 years. The electrical control system used conventional relays without system fault indication. The electrical equipment was aging, the wiring was chaotic, and the instrument system interlocking was complex, resulting in complex and difficult equipment management, high technical requirements, and the inability to quickly resolve faults. The five air compressors experienced an average of 10 failures per year, causing large fluctuations in air pressure and greatly affecting the stable and reliable operation of the instrument air system. 2.2 Basis for Modification The original system control loop is shown in Figure 1. The working principle can be understood through analysis of Figure 1. When the air compressor starts, pressing the start button SB2 activates the oil and water pressure start circuit, closing the 1C AC contactor and locking the AC contactor, starting the motor. After 20 seconds, 1SJ activates, and 1ZJ closes to engage the thermal protection system, enabling normal operation. If the oil and water pressure are abnormal during startup, the compressor cannot start. During normal system operation, if thermal protection, oil pressure, or water pressure protection activates, a trip will occur. It can also be seen that the system uses a conventional relay interlocking system, making the system mechanism relatively complex. [align=center] Figure 1 Original System Control Circuit[/align] 3 PLC Retrofit Application 3.1 Hardware Retrofit Principle The principle of the main circuit of the retrofitted system is shown in Figure 2. The principle of the control circuit of the retrofitted system is shown in Figure 3. [align=center] Figure 2 Retrofitted System Main Circuit Principle[/align] [align=center] Figure 3 Retrofitted System Control Circuit Principle[/align] 3.2 PLC Ladder Diagram As shown in Figure 4. [align=center]Figure 4. PLC Ladder Diagram of the Modified System[/align] 4. Conclusion Advantages of the System Modification: The system adopts the internal soft logic function of a high-reliability PLC (Programmable Logic Controller), greatly simplifying the external wiring of the air compressor control circuit. The external wiring is clear and concise. Furthermore, the PLC supports online editing, facilitating on-site debugging. Fault indication and fault interlocking functions have been added, making it easier to find and handle air compressor faults, greatly shortening system recovery time. Even non-electrical professionals can identify the fault location based on the fault display, allowing for timely inspection and handling, thus greatly improving system reliability. The system program design employs a mutually interlocking contact fault detection design to avoid contact faults in the main AC contactor, thermal relay, water pressure gauge, and oil pressure gauge. While meeting process requirements, the program design statements are practical, concise, and functionally clear. This system has been in use since early 2005, operating stably and reliably, with clear fault displays, enabling targeted maintenance work, greatly shortening system recovery time, reducing the workload of staff, and achieving good economic and social benefits.