1. Introduction In chlor-alkali enterprises, the liquid level in the pressurized dissolved air tank of the brine process is usually controlled by regulating valves. The valve size is used to stabilize the liquid level. However, due to the lag in manual adjustment, the liquid level fluctuates wildly, severely impacting the stability of the production process. Secondly, frequent starting and stopping of the motor causes significant damage to the control devices and motor due to the flow rate of the regulating valve. Thirdly, it increases the failure rate of the regulating valves, increases the labor intensity of workers, and leads to economic losses. To address this issue, last year our institute, in the design of the 100,000 T/year brine project for Jiangsu Northern Chlor-Alkali Group, adopted frequency converter control for the brine booster pump motor. By adjusting the motor speed, the flow rate of the booster pump is changed, thereby stabilizing the liquid level in the pressurized dissolved air tank. After more than a year of operation, it has stabilized production and achieved good economic benefits. 2. Original Control Scheme The original control scheme typically used a large-diameter DN150 regulating valve to adjust the liquid level based on changes in the liquid level. Due to the large valve diameter, there was a certain amount of leakage and a lag in manual adjustment, severely affecting the quality and accuracy of the adjustment, making it difficult to stabilize the liquid level in the pressurized dissolved air tank. Furthermore, the regulating valve frequently broke down. To save energy and improve control stability, variable frequency speed control technology was adopted to control the speed of the booster pump motor to change the inlet flow rate, thereby achieving stable liquid level and energy saving. 3. Control Scheme Design 3.1 Selection of Variable Frequency Drive The Siemens MicroMetroSTER430 variable frequency drive was selected. This drive is suitable for various variable frequency drive devices, especially water pumps and fans in industrial production. Its features include user-centric performance, ease of use, more input-output terminals, an optimized operation panel with manual/automatic switching functions, and adaptive software. In addition, the mechanical parts adopt a modular structure, making configuration particularly flexible. Detailed parameters are shown in Table 1. 3.2 Closed-Loop Automatic Control System Design 3.2.1 Control System Block Diagram (Figure 1) 3.2.2 Automatic Control Operation According to the process control requirements, a level transmitter is installed on the pressurized dissolved gas tank. The transmitter compares the measured values ​​of the incoming and outgoing gas with the setpoint to generate a deviation. Based on the magnitude and direction of the deviation, PID calculations are performed, and a 4-20 mA DC signal is output to the frequency converter. The frequency converter performs frequency conversion control based on this signal, changing the motor speed, thereby changing the liquid supply of the pressurized pump. Since liquid is still being discharged from the outlet, this change, in turn, affects the liquid level. The change in liquid level is then fed back to the regulator through the level transmitter measurement. The regulator, after calculation, sends another signal to the frequency converter, again realizing frequency regulation, speed regulation, and flow regulation, thus achieving the purpose of automatically controlling and stabilizing the container liquid level, realizing closed-loop control. 3.2.3 Anti-Interference Design Since the frequency converter integrates high-voltage and low-voltage control, the harmonic current of the output main circuit interferes with the control circuit, affecting the input PID signal and causing the frequency converter to malfunction. Therefore, during the design and installation, the technical specifications were strictly followed. The control signal line was laid with shielded cable, and the wiring was spaced a certain distance from the main circuit. The shielding layer was reliably grounded, and R≤4Q was required. The actual operation results showed that the transmission signal and control signal were not affected by the radiation and large current interference of the frequency converter. 4. Implementation effect of the scheme (1) Energy saving The power consumption of the motor is proportional to the cube of the speed. When the motor speed is slightly reduced, the power consumption will drop significantly, with a maximum power saving of 30%. (2) Good closed-loop control Since the frequency converter has the function of receiving standard current signals, it can adjust the speed and control the liquid level according to the liquid level setting value. In addition, the output frequency of the frequency converter can be manually adjusted remotely to control the speed to stabilize the liquid level. (3) High control accuracy The adjustment accuracy of the regulating valve is 3% to 5%, and it can reach 1% to 1.5% with the valve positioner, but the adjustment accuracy of the frequency converter is 0.1%, and the control will be more stable. (4) High reliability The frequency converter is currently operating normally, and its reliability is beyond doubt. (5) Good starting and speed regulation performance. The frequency converter has a soft start, a small starting current, and a small impact on the power grid. The frequency conversion speed regulation is stepless, and the speed regulation is smooth, with a small mechanical impact on the pump, which can extend the service life of the pump. The above conclusions can be verified by the pump start-up status. (6) Complete protection functions. The frequency converter has functions such as overcurrent limiting, overcurrent stall prevention, short circuit and ground fault protection, and input undervoltage protection, so it has high reliability. 5. Energy Saving and Investment Effects 5.1 Energy Saving Effects Table 2 shows the test data of the booster pump motor before and after the installation of the frequency converter. As can be seen from the table, the difference in active power of the motor before and after the installation is 13.9 kW. Since chemical production is continuous, and the equipment is calculated to run for 8,000 hours per year, the annual electricity cost savings (electricity price is 0.58 yuan/kW·h) are: 13.9 × 8,000 × 0.58 = 64,500 yuan. 5.2 Investment Effects 5.2.1 Investment Amount: See Table 3. 5.2.2 Investment Payback Period: Based on the data calculated in Table 3, without considering savings in valves, labor, etc., the static investment payback period is 1.6 years. 6. Conclusion Based on the above analysis of energy saving effects, investment amount, and static investment payback period, the application of closed-loop control schemes and the use of frequency converter speed regulation technology to replace traditional energy-consuming regulating valve control schemes is an inevitable trend. Frequency converters deserve vigorous promotion and application.