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It is well known that controlling pipeline pressure in gas transmission and distribution is one of the key criteria for ensuring a stable gas supply; however, how to maintain this pressure at an appropriate level to enable steady gas delivery remains something that requires research and practical exploration. In 2000, Shenyang Jintong Automobile Company, a joint venture between Shenyang Jinbei Automobile Manufacturing Company and General Motors of the United States, requested Shenyang Gas Corporation to provide a gas supply with stable pressure via the \"Jintong gas pipeline\" to meet production needs. This posed a challenge for us in terms of automatically regulating pipeline pressure, but it also offered us an opportunity to explore such solutions. Shenyang Jintong Automobile Company requires that the pressure in the \"Jintong gas pipeline\" be maintained between 80-90 kPa. The pressure in Shenyang’s urban gas network is generally around 50 kPa, which clearly does not meet the gas supply requirements of Jintong Company. From June to November 2000, the Shenyang Gas Company and the Shanghai Public Utilities Research Institute developed the \"Automatic Pressure Monitoring System for Gas Pipelines\" based on the existing operating conditions and taking local circumstances into account. After more than half a year of actual operation, this system met the gas supply requirements of Jin Tong and achieved the expected results. I. Introduction to the Automatic Pressure Monitoring System for Gas Pipelines 1. Process design of the automatic pressure monitoring system for gas pipelines (1) Actual operating conditions: The storage and distribution station of Shenyang Gas Corporation is equipped with a dry gas tank with a capacity of 150,000 cubic meters ； Four conveyors, two with a capacity of 12,000 cubic meters per hour and two with a capacity of 7,200 cubic meters per hour ； 2200-kilometer urban pipeline network ； The pipeline network pressure is below 50 kPa during off-peak periods of gas usage, and remains above 50 kPa during peak usage periods. “The \"Jintong gas pipeline\" is 0.8 kilometers long, with a diameter of DN300; it is connected to the outlet of the pump and also linked to the urban gas network ； (2) Process design scheme: Since the “Jintong gas pipeline” has a small diameter, is short in length, and requires a low volume of gas flow, it still needs high pressure ； Given that urban pipeline networks have large diameters, long lengths, and high gas consumption, a valve can be installed between the \"Jintong Gas Pipeline\" and these urban pipelines. By operating a compressor under normal conditions, it is easy to increase the pressure in the \"Jintong Gas Pipeline\", with any excess pressure being released into the urban pipeline network through the valve ； (3) Advantages of the design scheme: First, the investment required is low; merely adding an electric valve that can automatically adjust its opening degree based on pressure is sufficient. If methods such as variable-frequency motors are used to regulate pressure, the investment needed will be much higher than that of this scheme ； Second, urban pipeline networks have a high degree of capacity; releasing pressure through these networks does not cause sharp changes in their pressure levels ； Third, operating just one compressor at normal times is sufficient to maintain the pressure in the \"Jintong gas pipeline\"; the excess pressure can be released into the urban gas network through valves, which can slightly increase the pressure in that network and reduce the number of compressors that need to be operated during peak usage periods ； 2. System composition and functions of various components: The automatic pressure monitoring system for gas pipelines, developed based on the process plan, consists of a compressor, electric valves, pipeline pressure transmitters, an automatic pressure monitor, and auxiliary electrical devices. (1) The compressor is the equipment used to increase the pressure in JinTong pipelines; it pushes the gas from the gas storage tank into these pipelines, thereby raising their pressure ； (2) The electric valve is an actuator used to regulate the pressure in the pipeline. When the pressure in the Jintong pipeline is high, the electric valve opens under control, allowing the pressure to be released into the urban pipeline network. When the pressure in the Jintong pipeline is low, the electric valve closes under control, thereby increasing the pressure in that pipeline. The pressure in the Jintong pipeline is regulated by changing the degree of opening of the valve ； (3) The pipeline pressure transmitter is a primary instrument for monitoring pipeline pressure in the system; it not only detects and displays the pipeline pressure but also provides a basis for the automatic pressure monitor to carry out pressure monitoring ； (4) The pressure automatic monitor is the core of the pipeline pressure automatic monitoring system; it receives signals from pressure transmitters and operates based on the operating parameters preset by the user, controlling the opening degree of electric valves to keep the pipeline pressure within the desired range. The setting parameters of the automatic pressure monitor allow for the definition of four levels – upper limit, upper-middle limit, lower limit, and lower-middle limit – all within the range of the pressure transmitter (0–160 kPa). For pressures above 1 kPa, the response time is 1 second ； 3. Basic principles of the system’s automatic control operation: When the pump is running, the pressure monitoring device detects that the pressure in the Jintong pipeline is below the lower limit; it then commands the electric valve to close. As the electric valve closes slowly, the pressure in the Jintong pipeline rises. Once the pressure reaches the lower limit, the device commands the valve to stop closing. Since the pump continues to apply pressure, the pressure in the pipeline keeps rising. When it reaches the upper limit, the pressure monitoring device commands the electric valve to open. As the valve opens slowly, the pressure in the pipeline decreases, and once it reaches the upper limit, the device commands the valve to stop opening. The conveying volume per hour of the conveyor remains relatively stable. If the gas flow rate in the Jintong system is also stable, then after a few adjustments, the opening degree of the valve will stabilize at a certain value, and the pressure in the Jintong pipeline will remain within the originally set range ； If the gas flow rate at Kimungun fluctuates, the automatic control system adjusts itself to reach a new balance ； II. Technology of Automatic Pressure Monitoring System for Gas Pipelines 1. Pressure transmitters: EJA pressure transmitters manufactured by Yokogawa Instruments, a Sino-Japanese joint venture, are used. Their main advantages include high precision (±0.075%), good stability, low requirements regarding operating conditions, and maintenance-free operation; they also feature an LED display with four digits, making them suitable for long-term continuous use ； 2. Electric valve: A dual-plate valve designed specifically for gas use, equipped with an explosion-proof electric actuator; it features functions such as limit control, over-torque control, operation indication, and output of opening degree signals ； 3. Pressure monitoring cabinet: A vertical cabinet that combines detection and control functions; its main functions include A. Collecting signals from the pressure transmitters in the JinTong pipelines ； B. Collect signals for full open, full closed, over-torque, and opening degree of the valve ； C. Automatically control the opening, stopping, and closing of the electric valve according to the set requirements ； D. Power source for opening and closing the output solenoid valve ； E. Connect the RS232 interface to the host computer ； F: The LED display shows the pressure of the JinTong pipeline and the opening degree of the electric valve; the lights indicate whether the valve is on, off, in operation, or stopped, as well as when the pressure reaches the upper limit, lower limit, or other specified levels. An audible alarm is emitted when the pressure reaches the upper or lower limits ； 4. Key system technical aspects: A. The system utilizes single-chip microcomputer technology, with software developed using a combination of assembly language and MBASIC; it is suitable for design requirements with relatively specific functions, offering both cost efficiency and affordability ； The basic programming procedures are stored in EPROM, which enhances the stability and reliability of operation. Parameters can be set according to operating conditions using function keys; the system utilizes erasable E2PROM chips, offering flexibility to meet various requirements ； B. The signal acquisition, setting, and calculations carried out by the pressure automatic monitor involve low-voltage electricity, while operating electric valves requires high-voltage electricity. Controlling high-voltage systems with low-voltage ones is theoretically feasible, but in practice various problems arise, primarily interference issues. To address interference issues, a combination of various measures is adopted, including relay isolation, adding absorption circuits to the sources of interference, placing reactive components away from low-voltage sections, enhancing the instrument’s resistance to interference, and utilizing the intelligence of software to filter out interference ； C. In its overall design, the system takes into account the actual requirements of operational conditions by incorporating various functions such as one-in-use and one-backup configuration, manual and automatic switching, as well as light indication and sound alarms ； III. Postscript 1. After more than half a year of operation, this system has met the design requirements, which indicates that the original design approach was correct and feasible; a summary is now being prepared to further improve the system’s performance ； 2. The technology of this system can be applied in areas that involve intelligent control of stronger systems by weaker ones, such as maintaining or adjusting pipeline pressure over long distances and monitoring the start and stop of pressure pumps ； 3. This system has its shortcomings; any criticism and suggestions are welcome.