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To ensure the safe operation of the compressor unit, centrifugal compressor units require a comprehensive lubrication system. It is used to supply oil to the bearings, gears, speed increase gear, and motor bearings of the compressor unit, enabling friction between the moving and stationary parts through a liquid (oil film), while also removing the heat generated as well as tiny metal particles. In addition, some units use shaft displacement meters that operate based on pressure oil. Therefore, it is very important to maintain and adjust the lubricating oil system of the compressor unit properly. I. Introduction to the lubricating oil system of centrifugal compressor units The entire lubricating oil system consists of the following main components: oil tank, pre-pump filter, oil pump, oil cooler, oil filter, oil-gas separator, exhaust fan, overhead oil tank, valves, and connection pipelines. It is generally installed above and around the fuel tank, forming a centralized fuel supply system. The operations are carried out by the operator through the instrument and electrical control systems. 1. Below, the lubrication system for centrifugal compressors is illustrated using the four product compressors of the No. 2 14,000 capacity oxygen generator at Angang Oxygen Plant as an example. For the relevant process parameters of each unit, refer to the “Oxygen Regulations for Workshop 5”. 2. The various lubrication systems also differ in other aspects, mainly in the following areas: (1) The two oil pumps installed in the Shenmu air compressors and Hangzhou Oxygen compressors are each driven by separate motors with identical power levels ； The two Ingersoll Rand nitrogen compressors are equipped with two oil pumps; one is driven by a separate motor, while the other is driven by the compressor’s main motor. ⑵The main motor bearings of the air and oxygen compressors rely on a lubrication oil system to provide pressurized oil for circulating lubrication ； The main motor bearings of the nitrogen press are lubricated by the circulation of pressure-free oil in the bearing oil tank. ⑶The oil pumps equipped in the air and oxygen compressors, as well as those in the nitrogen compressor, have different instrument and electrical control systems; the differences are mainly evident in the buttons and switches used to start the oil pumps, as well as in the settings and adjustments of parameters. 3. During the more than two years of tuning and maintenance of these four product compressors, we carried out various modifications. It mainly includes: (1) Modification of the fuel filler port on the transparent fuel tank. Through modifications, refueling during the normal operation of the unit will not disrupt the vacuum level in the fuel tank and gearbox. ⑵Modification of the air-gas separator. By replacing the fume exhaust fan with a jet-type oil mist filter, lubricant loss is reduced, electricity consumption is saved, and pollution of the ambient air by kitchen fumes is significantly diminished. ⑶Modification of the heating system for the air-cooled oil cooler. By installing a steam heating unit for the oil in the air-cooled oil cooler, it is possible to significantly reduce the time required to heat the lubricating oil before starting the unit in winter, thereby enabling the air-cooled system to start up promptly. ⑷Modification for excessive vibration of the vacuum pressure gauge. Through this modification, the severe vibration of the vacuum pressure gauge has been effectively prevented, facilitating routine inspections of the unit during operation. ⑸Modification to improve the accuracy of the vacuum oil temperature gauge. Through modifications, the measured oil temperature can accurately reflect the actual oil temperature of the unit, facilitating routine maintenance of the equipment. ⑹Modification of the air and oxygen pressure oil pump start buttons, interlock switches, and interlock logic. By unifying the local panel control buttons and logic control of the two units, the operation of switching oil pumps between each other is simplified, and unnecessary shutdowns caused by operational errors are avoided. ⑺Modification for lubricant sampling analysis in nitrogen compressors (not implemented). By opening oil holes on the side of the bottom of the tank and connecting an oil drain valve, it is possible to take samples of the lubricating oil in the nitrogen compressor for analysis. II. Relevant commissioning of the lubricating oil system before unit startup: After maintenance is completed and before the centrifugal compressor unit is put into operation, a thorough and careful commissioning of the lubricating oil system is necessary to lay a solid foundation for its proper operation during the next cycle. The commissioning work mainly includes the trial operation of the oil pump, the improvement of the instrumentation and control system, tests for the mutual switching of oil pumps, tests related to oil pressure interlock alarms and shutdown, as well as static and dynamic tests of the high-level oil tank. 1. Oil pump trial operation: Before starting the oil pump, it is necessary to carry out all the preparatory tasks required before its startup. In particular, it should be noted that for oxygen-permeation and nitrogen-permeation units, sealing gas must first be introduced, and its pressure must be adjusted to the appropriate level as specified. To ensure safe oil supply from the oil pumps, the two pumps should be started in rotation. After starting the oil pump, relevant inspections should be carried out: (1) Check whether the vibration and noise of the pump during operation are normal, so as to address any issues with the pump itself, as well as those related to its installation and calibration, in a timely manner. For example: excessive vibration and high noise caused by loose foundation screws, poor alignment between the pump body and the motor, or an inappropriate opening degree of the pressure control valve in the pump itself. ⑵Check whether the motor current is overloaded, verify that the motor’s configuration is appropriate, and check the motor’s rotation direction. ⑶Adjusting oil temperature and oil pressure: a. The oil temperature can be adjusted by controlling the amount of water in the oil cooler and by turning the heater on or off, so as to reach the parameters specified in the design. It is important to note that when starting the electric heater, the oil pump must also be started, to prevent poor heat transfer around the heater, which could lead to saponification and damage to the quality of the oil. During normal operation of the oxygen permeation system, the oil temperature can also be automatically regulated by a self-acting temperature control valve. b. The total oil supply pressure of the unit can be adjusted and controlled through the manual return valve, low-pressure safety valve, self-acting pressure regulator provided in the oil circuit system, as well as the pressure regulator built into the pump itself. For example, during normal operation of the oxygen permeation system, the automatic pressure regulation of the oil pressure is carried out by a self-acting pressure regulator ； The air permeability system is equipped with a screw-type oil pump, which comes equipped with a pressure regulating valve; by adjusting this valve, it is possible to control the oil pressure of the oil pump. c. The oil supply pressure at each lubrication point of the compressor unit can be adjusted by throttles installed separately on each oil supply pipeline in the lubricating oil system. Before starting the unit, the oil pressure at each lubrication point should be adjusted to the required \"set operating pressure + 60 kPa\" as specified in the manual, using the throttle valve. It should be noted, however, that after the unit is started, final adjustments should be made based on the actual oil pressure during the unit’s normal operation. Additionally, when the temperature of a particular bearing is slightly high and cannot be addressed immediately, the oil pressure at that location can be temporarily increased to stabilize the bearing temperature and maintain the normal operation of the unit. However, after making adjustments, close attention should be paid to changes in the shaft temperature; if the shaft temperature continues to rise, it is necessary to report this immediately to higher authorities and recommend that an inspection be carried out. 2. Improvement of the electrical control system: As the stability of various systems in oxygen generators continues to improve, higher demands are placed on the automatic control system for centrifugal compressors as well. Given the importance of the lubricating oil system, its instrumentation and electrical control systems must be meticulously designed to be safe, reliable, and meet the process requirements. Before the compression unit is started up, it is necessary to carefully verify and check relevant parameters such as the interlocking operation of the lubricating oil system pumps and the interlock controls, in accordance with the unit’s manual, in order to avoid accidents caused by incorrect parameter settings. 3. Interlocking test of oil pumps: The interlocking operation of oil pumps refers to an electrical and mechanical interlock control system that ensures that the backup oil pump can be activated promptly in case the operating oil pump fails or there is a loss of oil supply while the unit is running normally. The safety and reliability of the mutual switching between oil pumps are directly related to the safety of the unit, as they can prevent accidents such as bearing damage caused by a lack of oil supply to the unit. Therefore, before the unit is started, systematic and comprehensive tests must be conducted on the interlocking operation of the oil pumps in the lubrication system to ensure safety during the unit’s normal operation. Next, I will illustrate this with an example of the experiments conducted to evaluate the effects of the modification for the mutual operation of DH90-6 air compressor oil pumps during the annual maintenance in Workshop 5 in November 2003: The DH90-6 air compressor is equipped with two oil pumps, A and B; the operation buttons and switches on the local control panel are shown in the figure below. We carried out the following experiments to assess the impact of this modification for mutual oil pump operation: (1) Experiments showed that when the mutual operation switch on the local control panel is set to the “automatic” position, the start and stop buttons for the oil pumps on that panel do not function. ⑵Experiments have shown that when the standby switch on the local control cabinet is in the “manual” position, neither the oil pump interlocking function nor the oil pressure interlock function on the microcomputer operates. ⑶Through experiments, verify whether the mutual switching of oil pumps on the microcomputer is effective. a. Manually start pump A; once the oil pressure is normal, switch the standby mode from \"manual\" to \"automatic\". b. On the microcomputer, modify the PVLOTP value for the standby oil pump PIAS1045 (the condition under which the previously operating oil pump stops automatically after the standby oil pump starts) from 0.2 MPa to 0.38 MPa (higher than the pressure of pump A), in order to increase the PVLLTP value. c. Then, on the computer, the PVLLTP value for PIAS1045 (the condition for starting the backup oil pump) was changed from 0.12 MPa to 0.32 MPa (which is higher than the actual oil pressure value of Pump A). At this point, pump B starts automatically and operates simultaneously with pump A. d. After verifying on-site that pump B was operating normally, the PVLLTP value of PIAS1045 was modified from 0.32 MPa to 0.12 MPa using a computer, in order to reduce the PVLOTP value. e. Then, the PVLOTP value for PIAS1045 was revised from 0.38 MPa back to 0.2 MPa. At this point, the pump that was running previously stops automatically, completing the pump swap operation. Perform the above operations on each of the two oil pumps separately. ⑷Assume the running oil pump loses power and stops operating; check whether the mutual switching is effective. a. Manually start pump A; once the oil pressures in all sections are normal, switch the standby mode from \"manual\" to \"automatic\", cut off the power supply to pump A via electrical control, and confirm that pump B can start automatically. b. Manually start pump B; once the oil pressures in all sections are normal, switch the standby mode from the \"Manual\" position on the control panel to the \"Automatic\" position, thereby cutting off the power supply to pump B via electric control, and confirm that pump A can start automatically. ⑸The simulated operator performs a normal pump reversal for the oil pump. During the long-term, normal operation of the unit, in accordance with the requirements outlined in the air separation lubricant manual, the oil pump needs to be switched over regularly every three months, so as to allow for inspection and maintenance of the operating oil pump, as well as to verify that the backup pump is ready for use. a. Conduct a simulated experiment by the operator to perform normal pump switching, with Pump A in operation, Pump B in standby mode, and the mutual standby switch set to the “automatic” position. b. Switch the standby switch from “automatic” to “manual”, then start pump B manually; at this point, both oil pumps will operate simultaneously ； c. Then, switch the standby switch from “manual” to “automatic”, so that Pump A will shut down automatically as per the instrument control interlock. This method of operation is safe and reliable; it prevents accidents where low oil pressure or no oil supply occurs when the standby pump is started up again after being idle for a long time, thereby causing the unit to stop operating due to interlocks. Of course, at this time it is also possible to manually stop pump A first, and then switch the standby switch from the \"manual\" position to the \"automatic\" position in order to complete the pump swap operation. The pump switching procedure when Pump B is in operation and Pump A is in standby can follow the steps above. ⑹Simulate the operations performed by the operator after the oil pump maintenance to verify the effectiveness of the inter-switching modification of the oil pumps, thereby preventing accidents caused by pump switching that lead to interlock shutdown of the air compressor. a. Conduct the simulation experiment when pump A is in operation, pump B is in standby mode, and the mutual standby switch is set to the “automatic” position. b. Switch the standby switch from \"automatic\" to \"manual\" mode, manually start pump B, and then manually stop pump A. Simulate the maintenance of oil pump A; once the maintenance is complete, start oil pump A and check its operation. During this process, regardless of how many times oil pump A is started or stopped, it will continue until its normal operation is confirmed. c. Finally, regardless of which of the two oil pumps the operator wishes to retain, switching the standby switch from \"manual\" to \"automatic\" allows the microcomputer to identify that pump as the main operating pump at that time. When Pump B is in operation and Pump A is in standby, to simulate the operator’s actions after the oil pump maintenance, refer to the steps above. During the above mutual switching tests, it is essential to pay special attention to the following: the time interval between each operation of the mutual switching switch and the start/stop buttons must be at least two seconds; otherwise, such operations will not be recognized by the microcomputer, which will result in the relevant instrument control interlocks failing to function, and in some cases, both oil pumps may stop operating simultaneously. 4. Tests related to oil pressure interlock alarms and shutdown: The purpose of these tests is to ensure that, when the oil pressure drops during normal operation of the compressor, the microcomputer can immediately issue audible and visual alarms as well as provide informational alerts ； After the standby oil pump starts up in an interlocked manner, can the oil pressure stabilize immediately and rise to normal levels, thereby preventing the shutdown of the unit? ； If the oil pump fails to start in a timely manner when switching between them, can the unit be shut down promptly to protect it? For example, after this maintenance work, when we conducted simulation tests on the interlock alarm and shutdown procedures related to the oxygen transfer oil pressure, we found that when the oil pressure dropped to 0.15 MPa, although the backup oil pump could start up in time, the oil pressure would still drop inertially to 0.12 MPa, which is the value that triggers the interlock shutdown for oxygen transfer, thereby causing the shutdown of the oxygen transfer system. Thus, with the plant manager’s approval, we increased the interlock value for the oil pump switching from 0.15 Mpa to 0.17 Mpa, thereby avoiding this potential risk. 5. Static and dynamic tests of the high-level oil tank: The high-level oil tank is installed to ensure lubrication of the rotor and bearings during the idle operation of the unit after the compressor stops due to a fuel shortage, thereby preventing damage to the bearing shells. The high-level oil tank is generally installed at a height of 6 to 9 meters above the centerline of the compressor unit’s rotating shaft. A transparent sight glass is installed on the oil return pipeline to facilitate checking whether the high-level oil tank is functioning properly. After the initial installation of the compressor unit or after annual maintenance, corresponding \"static\" and \"dynamic\" tests should be conducted on the oil supply from the high-level oil tank. In principle, the time during which the high-level oil tank supplies oil should be more than three times the idling time of the compressor. III. Maintenance of the lubrication system: 1. Inspection of the oil tank: Oil level – Ensure that during the operation of each unit, the oil level in the main oil tank is above 2/3. For the two nitrogen compressors, since the bearings of the main motors are lubricated by oil at no pressure in the bearing oil tank, it is necessary to top up the oil level in the motor oil tank promptly when it becomes low. Oil quality: According to regulations, the oil quality is tested once every 3 months. 2. Oil pump inspection: No abnormal noises should be present, and the vibration speed should be < 2.8 mm/s. 3. Inspection of the oil cooler: The oil temperature can be adjusted within the specified range; the oil cooler is operating properly, with no leaks or spills, and it should be cleaned during annual maintenance. 4. Oil filter inspection: The resistance of the oil filter should be < 0.15 MPa; the filter element should be cleaned or replaced during the annual maintenance. 5. Pay attention to how seasonal changes and differences between day and night affect the temperature of the lubricating oil; make adjustments gradually to avoid causing significant impacts on the vibration of the compression unit. 6. Pay attention to the impact of leaks in the lubricating oil system on operating parameters. For example: Lubricant contaminates the vibration sensing connector, resulting in no vibration measurement values being displayed ； Lubricant contamination of the vibration measurement connector causes fluctuations in the vibration measurement values, which can lead to an automatic shutdown. In short, among the auxiliary devices of a compressor unit, the lubricating oil system plays an important role that cannot be ignored. As personnel responsible for operating and maintaining compression units, it is necessary to carry out a thorough and careful tuning of the lubrication system after the unit has been overhauled ； During normal operation of the unit, conduct thorough inspections, strengthen maintenance, and operate in accordance with the procedures.