Abstract: This paper describes the application of variable frequency drive (VFD) technology in water pumps, oil pumps, and fans in oil refining enterprises, achieving good energy-saving results, generally exceeding 30%. VFD technology is reliable, beneficial for safety and environmental protection, and extends the service life of motors, making it widely applicable in the oil refining industry. Keywords: VFD; oil refinery; energy saving In recent years, with the rapid development of electronic technology, the increasing maturity of VFD technology, and the relative decrease in VFD prices, the significant energy-saving effect has led to the widespread application of VFDs in the oil refining industry. Larger domestic oil refineries have used VFDs to varying degrees in atmospheric and vacuum distillation, catalytic cracking, delayed coking, ketone-benzene dewaxing, furfural refining, and gas fractionation units, all achieving energy savings of 40%–60%. In the early 1990s, our company first installed FRNP7 series frequency converters on the sewage lift pumps in the drainage workshop. After a year of operation, it proved that the frequency conversion speed regulation technology was reliable and had a significant energy-saving effect, making it suitable for widespread application. Subsequently, we purchased more than 40 more frequency converters and installed them in atmospheric and vacuum distillation, catalytic cracking, gas fractionation, gas storage and transportation, oil storage and transportation, boilers, heating furnace fans, and water supply pumps. Some adopted closed-loop control, while others adopted open-loop control, all adjusting flow rate by speed. With the pump outlet valve and regulating valve fully open, significant energy-saving effects were achieved, with annual energy savings exceeding 400 × 10⁴ kW•h. 1. Energy-saving principle of frequency converter 1.1 Working principle of frequency converter The basic working principle of frequency converter circuit is as follows: The three-phase AC power supply outputs a constant DC voltage through the diode rectifier bridge. The inverter is composed of six groups of high-power transistors. Utilizing its switching function, the high-frequency pulse width modulation (PWM) driver outputs pulse signals according to a certain pattern to control the base of the transistor, so that the transistor outputs a set of rectangular pulse waveforms with equal amplitude but unequal width. The amplitude is equal to the DC side voltage Vd of the inverter, while the width varies according to a sine law. This set of pulses can be equivalent to a sine wave. This pulse voltage is used to drive the motor to run. By controlling the amplitude and frequency of the output waveform of the PWM driver, the frequency and voltage of the transistor output waveform can be changed to achieve the purpose of frequency conversion speed regulation. 1.2 Energy Saving Principle In production, the energy consumption of many devices is related to the unit's speed, with oil and water pumps being the most prominent examples. These devices are generally selected based on the maximum load conditions that may occur during production, such as maximum flow rate and head. However, the actual flow rate required in production is often much smaller than the designed maximum flow rate. If the motor used is not speed-adjustable, the flow rate can usually only be controlled by adjusting the valve opening, resulting in significant energy loss at the valve. If the motor speed is adjusted instead of using valves, then when the required flow rate decreases, the motor speed decreases, and the energy consumption will be significantly reduced. It is known that when the speed decreases by 1/2, the flow rate decreases by 1/2, the pressure decreases by 1/4, and the power decreases by 1/8, that is, the power decreases in a cubic relationship with the speed. If the motor speed is reduced instead of closing the valve, then with the decrease in the pump's output head, the power previously consumed by the valve can be completely avoided while maintaining the same output flow rate. This is the principle behind speed regulation for energy saving. In simple terms, without a variable frequency drive (VFD), the pump's outlet discharge is adjusted by the outlet valve, which can easily overload the motor. When the flow rate is low, the valve needs to be closed slightly, increasing pipeline resistance and wasting energy on the pump outlet valve. Installing a VFD reduces the pump's speed, head, and motor power consumption, effectively eliminating the energy wasted on the outlet valve. Because of the constant torque characteristic, the motor torque remains constant after the speed reduction, ensuring consistent discharge and thus saving energy. 2. Application of VFDs 2.1 Application of VFDs in Water Pumps Our company's first VFD was used on a sewage lift pump, a Japanese Fuji product, model FRN75-P74. At the time, we didn't fully understand the performance and reliability of VFDs, so the application of VFDs on the sewage lift pump was experimental. After the first frequency converter was put into operation, the motor operating current decreased by nearly 50%, the motor operating temperature dropped significantly, mechanical wear decreased, and maintenance workload was reduced. During operation, it was found that the frequency converter's various protection functions were reliable, eliminating motor burnout due to overload or single-phase operation, ensuring safe operation. Based on the positive results of using the frequency converter on the sewage lift pump, our company also installed frequency converters on the domestic drinking water and hot water pumps. The power of the drinking water and hot water pump motors are 55 kW and 37 kW respectively. During the day, water consumption is high, requiring the pump outlet valve to be fully open, while at night, water consumption is lower, requiring the outlet valve to be closed appropriately. However, closing the outlet valve partially can cause pump pressure buildup, and leaks in the pipeline network can occur frequently. To solve these problems, frequency converters were installed on the drinking water and hot water pumps, adopting a constant pressure closed-loop control scheme. After being put into operation, the operation was stable without any malfunctions, solving the pressure buildup problem and achieving good energy-saving results. 2.2 Application of Variable Frequency Drives in Oil Pumps Our company has hundreds of oil pumps of various types, with a total installed capacity of nearly 10,000 kW, of which the largest motor capacity is 500 kW. In the process of technological transformation in recent years, in order to meet the needs of future production development, the spare capacity of pumps and motors is relatively large. Taking the atmospheric and vacuum distillation unit as an example, the capacity of the main pumps can reach 250 × 10⁴ t of crude oil per year. However, in actual production, the actual annual processing volume is (100～140) × 10⁴ t. The survey results show that the power consumption of pumps is 30%～60% in the pump outlet valve and control valve. Therefore, the application of variable frequency drives in oil pumps will have great energy-saving potential. (l) Application of Variable Frequency Drives in Refining Units Based on the application effect of variable frequency drives in water pumps and the current operating status of oil pumps, variable frequency drives were first selected to be installed in the first and second intermediate reflux oil pumps of the atmospheric and vacuum distillation unit. For safety and stability, a control scheme was adopted that allows for switching between open-loop, closed-loop, and regulating valve control. After implementation, the effects were significant. The speed of the first-stage pump decreased from 2950 r/min to 990 r/min, saving 35% of electricity; the speed of the second-stage pump decreased from 2950 r/min to 650 r/min, saving 43% of electricity. Operational results show that the closed-loop control scheme better leverages the advantages of the frequency converter. Closed-loop control was subsequently adopted for the frequency converters on the continuously operated pumps and the first, second, and third-stage pumps, as well as the reduced-voltage pump. With the widespread application of frequency converters, the energy-saving effect is significant, with annual energy savings of approximately 100 × 10⁴ kW•h. The heavy oil catalytic cracking unit was designed to process 80 × 10⁴ t/a, but the actual processing volume is less than 70 × 10⁴ t/a, resulting in a relatively large margin in the pump design. This unit is a major power consumer for our company. To reduce power consumption, frequency converters were installed on the raw material, recycle oil, top circulation, absorber feed, and liquid hydrocarbon delivery pumps, resulting in power savings of 30% to 60%, amounting to 80 × 10⁴ tkW•h annually. This also resolved equipment issues such as leaking liquid hydrocarbon pumps and frequent malfunctions. The gas fractionation unit was completed and put into operation in November 1994, with a designed processing capacity of 12 × 10⁴ t/a. However, due to insufficient raw materials, the processing volume was only maintained at around 6 × 10⁴ t/a at that time. Although the trays were plugged and the measuring orifice plates were adjusted to maintain production, the pumps were too powerful for the production volume at that time, and even with control valve adjustments, normal production could not be maintained. To solve this production problem, frequency converters were installed on the reflux pump of the ethane stripper, the intermediate pump of the propylene refiner, and the top reflux pump of the propylene refiner. After being put into use, the adjustment accuracy met the production requirements, the adjustment was accurate and timely, the operation was stable, the product quality was qualified, and the power saving effect was 30% to 60%, with an annual power saving of 60 × 10⁴ tkW•h, achieving the goal of both solving the production problem and saving power consumption. (2) Application of frequency converters in oil storage and transportation systems. Our company produces (50-70) × 10⁴ tons of gasoline and diesel oil annually. To improve loading speed, the motors of the gasoline and diesel loading pumps have large capacities, both 132 kW. At the beginning of loading, the pumps can operate at full load. As loading continues, the amount of oil being transported needs to be gradually reduced, and the pump outlet valve is gradually closed to adjust the flow rate. When loading the last few cars, the outlet valve is closed to the minimum, causing pump pressure buildup, which leads to pump leakage and serious exceedance of oil and gas concentration in the pump room. Therefore, the pump room is listed as a major safety hazard by the company. To solve this problem, frequency converters were installed for both gasoline and diesel oil. After being put into use, the voltage and current have decreased, the pressure buildup phenomenon has been eliminated, and the flow rate adjustment is flexible. Employees in the control room can adjust the flow rate in a timely manner according to the loading situation on the platform, which greatly reduces the labor intensity. The same phenomenon exists in the liquid hydrocarbon storage and transportation system. 18.5kW frequency converters were installed on both the gas separation feed pump and the superposition feed pump, solving the pressure buildup problem and ensuring uniform feeding, thus creating conditions for the stable operation of the gas separation and superposition units. 2.3 Application of Frequency Converters in Blowers Since the boiler is the main power source for oil refining enterprises, and the boiler blower is the main power-consuming equipment in the boiler system, vibration and excessive shaft temperature are common during operation. Therefore, installing frequency converters on blowers is of great significance. The selected variable frequency speed control device should have multiple protection functions to better ensure the safety and reliability of boiler operation. After investigating and carefully analyzing the boiler operation status, it is believed that the following technical issues must be addressed when installing frequency converters on boiler blowers. (l) The safe operation of the boiler is the fundamental guarantee of the company's power. Although the variable frequency speed control device is reliable, it is necessary to ensure the safe steam supply to the boiler in case of problems. Therefore, automatic switching between power frequency and variable frequency speed control operation must be achieved. (2) The capacity of the drive motor of the boiler blower is 135kw. For such a large inertia load blower, there may be torsional resonance. Once resonance occurs during operation, it will seriously damage the blower and the drive motor. Therefore, it is necessary to calculate the critical speed of torsional vibration of the blower-motor connection shaft system and take corresponding technical measures. The following measures were taken to address the above problems. (1) A fast instantaneous switching function was set up for switching from power frequency to variable frequency and vice versa. After on-site testing and actual application, the switching function ensured the fast instantaneous switching between power frequency and variable frequency, which can meet the switching requirements during the operation of the blower. (2) In order to further analyze whether there may be torsional resonance in the blower-motor system, after consulting relevant data and structural diagrams, the torsional vibration natural frequency of the blower-motor shaft system was obtained as follows: the first order torsional vibration natural frequency is 1222.4Hz, and the second order is 3867.4Hz. It can be concluded that torsional resonance will never occur when the speed is adjusted from 0 to 50Hz. After the boiler blower was equipped with a frequency converter, the following effects were achieved. (1) Appropriately reducing the furnace pressure can significantly reduce the power consumption of the blower. When the boiler load is 50 t/h and the combustion conditions and operating conditions are similar, the motor current before and after the installation of the frequency converter is reduced from 180A to 90A. Based on 8000h per year, the annual power saving is 30×10⁴kw•h. (2) Due to the improvement of the operation control and regulation system, the combustion conditions tend to be low-oxygen combustion, which improves the boiler thermal efficiency. Before the installation of the frequency converter, the boiler air volume control method is to adjust the inlet damper of the blower and the nozzle damper. The adjustment method of the inlet damper is inflexible and very rough, and it is impossible to accurately adjust the air volume. Therefore, the air volume is often too large during operation. After the implementation of frequency conversion speed regulation, the control of oxygen content during boiler combustion is very convenient and accurate. In the control room, the air volume required for boiler combustion can be slightly adjusted to make the combustion condition tend to low oxygen combustion, the excess air coefficient is reduced from 1.2 to 1.1, the boiler thermal efficiency is increased by 0.8 percentage points, fuel consumption is reduced, and 200-500 tons of fuel oil are saved annually. (3) The power factor is improved. A DC reactor accessory is added to the variable frequency speed control device, which can prevent the high-order harmonics generated by the frequency converter from polluting the power supply, and also prevent inrush current and protect the rectifier module. And the power factor is increased to above 0.95. 3 Conclusion and issues to be noted Through the application of frequency converters in oil refineries, it can be seen that frequency converters have many advantages, which are summarized as follows. (1) Variable frequency speed control technology is reliable and can be widely used in asynchronous motors such as fans, water pumps, and oil pumps in oil refineries. The power saving effect is obvious, and the power saving is generally more than 30%. (2) Using a variable frequency drive to drive an asynchronous motor allows for smooth starting from zero speed to any speed, with low current, fast acceleration, and no inrush current during startup. Furthermore, it offers comprehensive protection, convenient and accurate operation, high adjustment precision, and eliminates valve control, achieving closed-loop control. (3) With a variable frequency drive, the absence of the 5-7 times rated current inrush current during motor startup reduces the likelihood of electrical switch overload damage, reduces transformer impact, improves transformer efficiency, and avoids the impact on the power grid during large motor startup. (4) It extends the service life of pumps and motors. Because the motor operates at speeds below the rated speed for extended periods, the bearings of the motor and fan are less prone to damage, and the motor's heat generation is reduced. Downtime is reduced, saving significant maintenance costs. (5) It reduces noise pollution. Since variable frequency speed control technology adjusts the motor speed to control flow, motors using variable frequency typically operate at speeds below the rated speed, significantly reducing noise compared to before using a variable frequency drive, thus contributing to improved industrial hygiene. (6) Pneumatic control valves are in direct contact with the process medium. Corrosive media can corrode and scour the control valves. However, frequency converters control the motor speed and regulate the flow rate through electrical signals, which does not have this problem and is much more stable than control valves. After replacing control valves with frequency converters, the amount of instrument maintenance is greatly reduced. Although frequency converters have many advantages, the following issues should be noted during use. (1) When selecting a frequency converter, it is necessary to meet the process requirements. In certain specific environments, the pump may not be suitable for frequency conversion due to the limitations of head and flow rate. Frequency converters are not suitable for pumps with high loads and infrequent changes in operating conditions. (2) The power of the frequency converter matched with the motor should generally be selected to be equal to or higher than the motor power. The selection should be of a higher grade and be able to match the device and instruments and implement closed-loop control. (3) Frequency conversion speed regulation technology requires close cooperation among professionals in electrical, instrumentation, process, and equipment departments to ensure installation quality. (4) Most production equipment instrument control valves are air-closing valves. After installing frequency converters, the air-closing is changed to air-opening regulation. This needs to be noted on the instrument panel or an electronic module should be added to unify them to avoid accidents.