Abstract : The stability of gas pressure is crucial to ensuring the quality of ductile cast iron pipes during annealing. This paper addresses the frequent tripping of the gas compressor during pressurization and the unstable pressure downstream of the gas pipeline due to large pressure fluctuations. It introduces the principle and implementation method of using an S7-200 PLC to retrofit the system with a frequency converter. Practice has proven that this technical solution is effective. Keywords : Frequency converter, PLC, constant pressure gas supply KEYWORD : Transducer PLC, constant pressure gas supply. In the production process of centrifugal ductile iron pipes, the mechanical properties of the ductile iron pipes are closely related to the heat treatment curve of the annealing furnace, making the accuracy of temperature control in each section of the annealing furnace crucial. Blast furnace gas is a byproduct of the blast furnace smelting process. To fully utilize blast furnace gas, the company uses a gas compressor to pressurize the blast furnace gas before supplying it to the annealing furnace. The accuracy of annealing furnace temperature control depends on the stability of the gas pressure. If the gas pressure fluctuates too much, with a constant air-to-gas ratio (air-to-gas ratio), the furnace temperature in each section will continuously change with the gas pressure (i.e., the flow rate), potentially causing the entire batch of cast pipes to be scrapped. [b][align=center]For details, please click: Renovation of the Gas Pressure Control System for Cast Iron Pipe Annealing Furnace[/align][/b]