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Authors: Operation Support Department, Changqing Petrochemical Company; Zhou Donghao, Feng Guangzhou. Please indicate the authors and copyright when reprinting this article. The vacuum distillation units in oil refineries typically employ vacuum pumping to reduce the operating pressure in the vacuum column. The vacuum pumping system for the vacuum tower of the 500×10^4 t/a atmospheric and vacuum distillation unit at Changqing Petrochemical Company mainly consists of two-stage steam ejectors, a condenser, a vacuum unit, and corresponding connecting pipelines. The vacuum unit utilizes the 2BW4 303-2HC0 water-ring vacuum pump set manufactured by Nashim Industrial Co., Ltd.; the main equipment in this unit is the water-ring vacuum pump. The level control system for the gas-liquid separator of the vacuum pump consists of a magnetic level gauge, level switches, and solenoid valves. It plays a crucial role in the normal operation of the vacuum pump, directly affecting the efficiency of decompression and vacuum generation. According to process requirements, the liquid level in the separator should be at the mid-level; either too high or too low a level will affect the vacuum pumping efficiency. Since its operation began in 2005, the liquid level control system of the reduced-head vacuum pump has suffered from issues such as large control deviations, frequent equipment failures, and recurrent control malfunctions; it is unable to meet the process requirements, and manual adjustments to the control liquid level are often necessary, which has severely affected the stable operation of the facility. During the major maintenance in August 2011, upgrades were carried out on the separator level control system to address issues such as inaccurate level measurement, corrosion of the solenoid valve diaphragms, and malfunctioning level switches. Nearly 5 years of operational experience have shown that level control has been effectively improved, the system’s failure rate has decreased, the level of automatic control has risen, and this has ensured the stable operation of the production facility over extended periods of time. 1 Current status and problems of the level control system. The water ring vacuum pump unit consists of a water ring vacuum pump, a gas-liquid separator, a cooler, and internal connection pipes (Figure 1); it is a closed-loop system. The reduced-pressure gas and working water are sent to the gas-liquid separator via a vacuum pump; the separated gas is discharged from the top of the separator, while the water, under the effect of pressure difference, is cooled before being fed back into the vacuum pump. The working water can thus circulate automatically. http://www.huoyumi.com/d/file/ywj/2018-04-13/ca2adaa59e68012e94187e7b4b788957.jpg A water ring vacuum pump must be filled with working fluid before it is started; without this fluid, the pump cannot create a vacuum (but this will not cause any mechanical damage) ； Excessive working fluid increases the starting load, to the point where the load becomes too heavy and normal starting is not possible ； Insufficient working fluid can lead to a decrease in vacuum pumping capacity. Reduced head gas usually contains steam, and condensate may form, causing the liquid level in the separator to rise and leading to an increase in the power consumption of the vacuum pump motor. 1.1 Current status of the liquid level control system The vacuum pump liquid level control system consists of a magnetic level gauge, liquid level limit switches, and solenoid valves. Three proximity switches mounted on the body of the SanChang magnetic flap level gauge via bundling are used to detect the low, normal, and high liquid levels respectively. This enables control of the solenoid valves on the water replenishment and drainage lines, ensuring that the level in the separator remains close to the normal level at all times (Figure 2). http://www.huoyumi.com/d/file/ywj/2018-04-13/71c24df3b265e74e4eae03f98e9a0334.jpg When the liquid level is at position ①, the low-level switch activates (the contacts change from closed to open). The inlet solenoid valve is energized and opens, thereby starting the water replenishment process ； When the liquid level is between ① and ②, the low-level switch contacts close, while the inlet solenoid valve remains energized and open ； When the liquid level rises to position ②, the normal level switch activates (the contacts change from open to closed), the inlet solenoid valve is powered off and closes, thereby stopping the supply of water ； When the liquid level is between ② and ③, the contacts of the normal level switch open ； When the liquid level rises to position ③, the high-level switch activates (the contacts change from open to closed), the outlet solenoid valve is energized and opens, allowing drainage to begin ； When the liquid level drops between ③ and ②, the contacts of the high-level switch open, while the outlet solenoid valve remains powered and open ； When the liquid level drops to position ②, the normal level switch activates (the contacts change from open to closed), causing the outlet solenoid valve to de-energize and close, thus stopping the drainage ； When the liquid level is between ② and ①, the normal level switch contacts open ； When the liquid level drops to position ①, the low-level liquid switch activates (the contacts change from closed to open), causing the inlet solenoid valve to be energized and open, thus starting the water replenishment process. The entire control process repeats continuously to ensure that the liquid level remains at the proper level. Taking the level control of P1028A as an example, the operating states of relevant instruments during the makeup and drainage processes are introduced (Table 1). When the liquid level is between ①–②, ②–③, and ③–②, the status of the proximity switches remains the same. In the absence of remote indication, there may be misjudgments regarding the liquid level. Furthermore, if the normal-position proximity switch (220N) fails to function properly, it can cause the inlet solenoid valve (221) or outlet solenoid valve (222) to remain in the open state continuously. This results in loss of control over the liquid level, thereby affecting the vacuum pumping efficiency of the vacuum pump. http://www.huoyumi.com/d/file/ywj/2018-04-13/7343330bec5fd8db15958461ca6da0e9.jpg 1.2 Problems with the level control system In 2005, the level control instruments for the vapor-liquid separator of the vacuum pump were put into use, and level control was carried out in accordance with the control scheme provided by the manufacturer. After years of operation in the field, the level control system has exhibited various problems, such as large control deviations, numerous equipment failures, and frequent instances of control malfunction. These issues severely affect the long-term operation of the vacuum distillation unit. In particular, when the processing load on the unit is high, unstable level control causes the vacuum pump to shut down or leads to excessively high exhaust temperatures, resulting in significant fluctuations in the process control of the vacuum distillation unit. 1.2.1 Magnetic flap level gauge The SanChang magnetic flap level gauge is a local display instrument. The level data (range values rather than exact values) must be confirmed by operators on site. However, there is no actual level indication on the DCS screen in the control room, preventing operators from accurately knowing the actual level on site. 1.2.2 Level switches: There are three magnetic level switches at the liquid level detection points. They are secured to the outer cylindrical wall of the Sanchang magnetic flap level gauge using clamps; the fixing positions determine the locations of the liquid level measurement points. In practical use, the sensitivity of the contactless detection is affected by the cleanliness of the float itself in Sanchang magnetic level gauges. Additionally, since the level switch is fixed to the cylinder of the SC magnetic level gauge using straps, the contact area is small; as a result, vibrations generated during equipment operation can cause delays in detection, affecting the response speed and leading to the generation of incorrect signals, which in turn prevents the solenoid valve from closing or opening properly. The insulation of the proximity switch terminals was poor; during wire removal and reconnection, the insulation got damaged. This caused the output signal wire to be grounded, resulting in the output signal remaining in a closed state at all times, thereby rendering the liquid level control ineffective. The level detection in the liquid level make-up and drainage system is accomplished by 3 proximity switches. With so many measurement points, high requirements are placed on the overall integrity of these 3 proximity switches; if one of them malfunctions, it will result in an inability to control the liquid level. 1.3 Solenoid Valve: The solenoid valve is a direct-acting spring-loaded diaphragm type; the diaphragm is made of rubber reinforced with fabric, offering dual-sided pressure bearing capacity, equal pressure on both sides, low hysteresis, and no slippage. The diaphragm directly affects the flow capacity of the solenoid valve. When the water used for replenishment is of poor quality or contains many impurities, these impurities accumulate on and below the diaphragm, causing blockages in the balance holes within the valve body and affecting the sealing performance of the diaphragm ； Over time, accumulated impurities can corrode and abrade the diaphragm, causing it to age and shortening its service life. This may lead to leaks or failure of the solenoid valve to open. Therefore, after a certain period of use, the solenoid valve must be disassembled in order to clean the diaphragm. The spring of the solenoid valve may also lose its elasticity over time as a result of prolonged use, which prevents the solenoid valve from operating properly. Typically, the solenoid valve needs to be replaced multiple times per year, and it also lacks a remote manual control function. 2 Renovation of the vacuum pump level control system: To address the issues associated with the existing level control system and to ensure accurate and efficient control of the separator’s level, while reducing the consumption of instrumentation materials and minimizing failure rates, it was decided to renovate the level filling and drainage system (Table 2). ① Since there is no space in the separator for additional instrumentation, the magnetic flap level gauge was replaced with one that provides a 4–20mA signal for remote transmission, to be installed in its original location ； ② The level switch is removed; the original signal wire is retained, and its function is fulfilled by the output of the magnetic flip-level gauge ； ③Replace the solenoid valve, opt for a pneumatic ball valve, use the solenoid valve as the control element for the ball valve, and add remote control functionality ； ④ Lay the air supply piping to the pneumatic ball valve; utilize the existing level switch cables for instrument signal wiring. http://www.huoyumi.com/d/file/ywj/2018-04-13/43450a9948fcfb5a8f66863c34eab412.jpg After the modification, the liquid levels ①, ②, and ③ are all set through system configuration. To ensure that the make-up and drain water levels remain in the middle range of the level gauge, the inlet and outlet cylinder valves operate accordingly (Figure 3). When the liquid level drops to position ①, the ball valve of the inlet make-up water cylinder (controlled by the air supply solenoid valve) is energized and opens, initiating water replenishment ； When the liquid level is between ① and ②, the inlet cylinder ball valve remains energized to allow water replenishment ； When the liquid level rises to position ②, the ball valve of the inlet water supply cylinder shuts off due to loss of power, thereby stopping the water supply ； When the liquid level rises between ② and ③, the inlet and outlet cylinder ball valves remain de-energized and closed ； When the liquid level rises to position ③, the ball valve of the outlet drain cylinder (controlled by the air supply solenoid valve) is energized and opens, initiating drainage ； When the liquid level drops between ③ and ②, the ball valve of the outlet cylinder remains energized to drain water ； When the liquid level drops to ②, the solenoid valve of the outlet cylinder de-energizes, halting drainage ； When the liquid level drops between ② and ①, the ball valves of the inlet and outlet cylinders remain closed due to power loss; when the liquid level drops to position ①, the ball valve of the inlet cylinder opens under power supply, and water replenishment begins. The entire control process repeats continuously to ensure that the liquid level remains at the proper level. http://www.huoyumi.com/d/file/ywj/2018-04-13/ab1b6250f50c0241528e0bb0ff07935b.jpg 3 Comparison of liquid level control performance before and after the modification. As a result of these modifications, the shortcomings of the original liquid level control instruments were largely eliminated, enabling the liquid level control system to be simpler, more accurate, and require less maintenance, thus ensuring the long-term operation of the vacuum pump. Through operation over two maintenance cycles, approval was obtained from the staff in the process unit (Table 3). http://www.huoyumi.com/d/file/ywj/2018-04-13/e4e4877c300088bc802011ec84f8447d.jpg 4 Conclusion The level control system consisting of a magnetic flap level gauge and 2 small cylinder ball valves (controlled by solenoid valves in the air circuit) provides more accurate and effective control; the failure rate of the valves is significantly reduced, and it is also possible to monitor the operating status of these valves. Since its inspection, modification and renovation in 2011, the level control system has been operating stably with remarkable results. The upgraded Sanchang magnetic flap level gauge provides accurate measurements, and the small-cylinder ball valve ensures sensitive and effective control, keeping the liquid level at 50%–55% to meet the process control requirements.