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The 2MCL525+2MCL457 type air compressors are auxiliary units for our company’s ammonia synthesis production process, responsible for supplying instrument air and air for production throughout the plant. Since its installation and commissioning in 1998, this unit has been operating well, with only one scheduled major overhaul of the air compressor carried out in 2002. However, as the operating cycle of this unit extended, we observed that the temperature of the bearing shells on the high-speed shaft of the air compressor kept rising, the mechanical vibration levels also increased, and many instrument control points were damaged; as a result, the unit could no longer operate properly. Therefore, the company decided to shut down the unit for a major overhaul, analyzed the causes of the failure, and implemented preventive measures. The 2MCL525+2MCL457 type air compressors (whose main technical parameters are shown in Table 1) are manufactured by Shenyang Blower Factory. It consists of a low-pressure cylinder (2MCL525) and a high-pressure cylinder (2MCL457). The prime mover is a three-phase asynchronous motor produced by Shanghai Electric Machinery Factory, which drives the centrifugal compressor through a gearbox and a gear coupling; the compressor has four stages with three intermediate coolings. The casing is of a horizontally split type. The compressor mainly consists of a stator (casing, partition, bearings, seals, etc.) and a rotor (shaft, impeller, sleeve, balance disc, half coupling, etc.). The compressor is mounted on an integral steel plate base; all bearings are sliding bearings (with forced lubrication), and the sealing between compressor stages uses a labyrinth seal. 1 Fault Diagnosis and Handling 1.1 High Temperature of the Compressor’s High-Speed Shaft Bearing The operating temperature of the bearings on the compressor’s speed increaser shaft should generally be around 45–50°C; the maximum temperature should not exceed 65°C. Typically, 65°C is set as the alarm temperature, while 75°C is set as the temperature at which the machine should shut down automatically. Generally, the reasons for high bearing temperature are as follows: (1) The clearance between the bearing and the shaft journal is too small ； (2) The bore diameter of the throttle ring at the bearing lubricant inlet is small, resulting in insufficient oil supply ； (3) The oil inlet temperature is too high, or the oil contains moisture and is dirty or degraded ； (4) Dirt enters the bearing, wearing out the bearing shells ； (5) Damage to bearing shells, etc. We have conducted an inspection regarding the high temperature of the high-speed shaft bearing. After opening the cover of the speed increaser and removing the large and small gears from it, we found that both the upper and lower bearing shells of the high-speed shaft were damaged to varying degrees. Preliminary analysis suggested that the increased temperature of these bearing shells was caused by excessive vibration and improper clearance, which led to their damage. As a result, these bearing shells were replaced and their clearance was adjusted so that all bearings in the speed increaser met the following technical requirements: (1) The interference between the bearing shell and the bearing cover should be 0.03–0.05 mm ； (2) Bearing clearance: 0.30–0.37 mm for the large gear, and 0.144–0.24 mm for the small gear ； (3) Thrust clearance: 0.30–0.35 mm for the large gear, and 0.25–0.30 mm for the small gear. We inspected the bearing shell using the shaft lifting method and found that its clearance was 0.14 mm, which is at the lower limit of the required clearance. As the temperature of the shaft and the bearing shell rises during operation, and their expansion coefficients differ, the clearance of the bearing shell may become even smaller. Therefore, it was decided to adjust the clearance of the newly installed bearing shell to 0.20 mm, so that it falls within the upper limit of the normal requirements ； The oil inlet of the bearing blocks was polished, and the oil wedge openings of the shaft bearings and bearing blocks were adjusted. During the compressor testing, the temperature of the bearing shells decreased but remained high; the temperature during operation was close to the upper limit of the normal operational parameters, which is not ideal. However, the clearance of the bearing bushings was good, and both the oil inlet temperature and the quality of the oil were normal. So why was the temperature of the bushings still high? To address this issue, we inspected the oil circuit of those bearing bushings; upon opening the throttle valve for the lubricating oil, we found a small piece of broken rubber blocking the throttle hole, which caused obstruction in the oil flow and reduced the amount of oil entering, thereby leading to an increase in the temperature of the bushings. We enlarged the throttle aperture of the lubricating oil inlet throttle ring and increased the lubricating oil inlet pressure from 0.15 MPa to 1.8 MPa. We then inspected the lubricating oil circuit and suddenly discovered a problem with the oil filter, so we replaced it with a new one. After testing, the bearing pad temperatures were normal, and the unit was operating within acceptable parameters. 1.2 Excessive vibration levels in the air compressor and motor (especially high vibration in the motor) 1.2.1 Cause analysis The centrifugal compressor is a high-speed, highly precise machine; excessive vibration can lead to serious mechanical failures. Generally, the vibration value of each bearing section shall not exceed 0.03 mm. There are many reasons for compressor vibration, with the main ones being as follows: (1) Bending of the main shaft ； (2) The bearing cover and bushing are not sealed tightly together ； (3) Contact friction between the rotor and the gas seal occurs ； (4) The foundation is not solid, and the anchor bolts are loose ； (5) Damaged bearings, excessive bearing shell clearance ； (6) Poor rotor dynamic balance, etc. 1.2.2 Fault Handling To address this issue, we checked the clearance of the bearing shells in the compressor and motor. It was found that the clearance in the motor’s bearing shells was above the specified range; the required clearance was 0.18–0.22 mm, whereas the actual clearance was 0.25 mm. We adjusted this clearance by grinding the split surfaces so that it fell within the 0.20 mm range, and we also adjusted the compression force on the back of the bearing shells to keep it within a reasonable level. Additionally, during the inspection of the motor, friction was detected on the rotor, and the balance adjustment components were found to be loose. We deemed it necessary to recheck and adjust the motor’s dynamic balance; therefore, the rotor was sent to Wushihua Petrochemical for balance adjustment, after confirming that its curvature met the required standards. After rechecking, it was found that the dynamic balance of the original rotor at Wuhu Petrochemical was poor, with a deviation value of 50g; the required standard is an imbalance level of less than 10g. At Wuhu Petrochemical, we reduced this value to 4g. After assembly, the center of the coupling between the motor and the compressor speed increaser was checked again, and it was found that there was a significant centering error: the horizontal deviation between the motor and the speed increaser was 0.25 mm, while the axial deviation was 0.10 mm. We believe that the cause of the central deviation is the movement of the motor resulting from long-term operation and vibration of the unit, and the allowable deviation between the motor and the gearbox is generally within 0.05 mm, whether it is a horizontal, vertical, or axial deviation. Therefore, the centers of the motor and the gearbox were adjusted to stay within the allowable range (the horizontal deviation was adjusted to 0.04 mm, and the axial deviation was adjusted to within 0.03 mm). After testing, the operation was satisfactory, and the vibration level of the motor was within the specified range. We also checked the vibration issue of the low-pressure cylinder. During the inspection, the bearing shell clearances of each bearing as well as the tightening force on the back side of the bearing shells were rechecked. The bearing shell clearances were all within normal limits, but it was found that there was no tightening force on the back side of the east bearing shell in the low-pressure cylinder. The technical specifications require a gap of 0.03–0.05 mm at the back of this tile, but in reality there is only a gap of 0.01 mm; therefore we adjusted this gap to 0.04 mm. In addition, a recheck of the dynamic balance of the rotor of this low-pressure cylinder was carried out, reducing its imbalance level (which should be 5g) from 10g to 4g. After maintenance and alignment, a trial run was conducted; the vibration levels of the low-pressure cylinder improved, with the amplitude dropping from 0.05 mm to 0.03 mm. Thus, the vibration levels were at a satisfactory level, but not yet at an excellent one. Analysis shows that the issue lies in alignment; that is, when the machine is operating under load, thermal expansion and the loads on the gear box teeth cause the shafts to shift vertically and horizontally. As a result, the machine automatically adjusts itself to achieve proper alignment during operation. The center of the compressor shaft remains unchanged horizontally when the temperature rises, whereas the two gear shafts of the gearbox move horizontally. Under load, due to the increase in temperature of the motor and the gearbox, these two gear shafts move to either side as a result of forces and thermal expansion. If the shafts of these two devices are aligned horizontally at room temperature, the center of the gearbox’s pinion shaft will shift northward during operation. Taking this into account, during cold-state alignment, the center of the compressor is intentionally shifted northward, so that during operation, the centers of the shafts of both devices end up aligned horizontally due to temperature compensation. Therefore, during alignment in the cold state, we intentionally adjusted the axis center of the motor horizontally to be 0.04 mm further south than that of the gearbox ； Similarly, when adjusting the compressor gearbox in the vertical direction, temperature factors must also be taken into account. At the same temperature, the casing of the gearbox is much higher than that of the compressor’s low-pressure cylinder; as a result, during normal operation of the unit, the height to which the axis of the gearbox rises vertically is significantly greater than that of the compressor. Therefore, during alignment in the cold state, the axis of the compressor is intentionally adjusted to be 0.10 mm higher than that of the gearbox. When the unit reaches its normal operating temperature, the heights of the speed increaser and the low-pressure cylinder in the vertical direction become essentially the same. After adjustment, testing showed that the vibration value was 0.006 mm, achieving an excellent level. 2 Regarding the issue of damaged temperature sensors at many control points: After the compressor was turned on, it was found that the temperature sensors at various bearings were damaged, with the main problem being broken wires in the thermocouples used for measuring temperature. Our analysis suggests that there are two main reasons for this issue. On one hand, it was caused by excessive vibration of the unit during operation some time ago; on the other hand, it resulted from the thermocouple breaking due to an insufficiently gentle bending radius at the points where the instrument wires bend at right angles at the bearing shells. For these two reasons, we believe that during this maintenance work we resolved the issue of high vibration in the unit; therefore, we only made improvements to the method of installing the instrument wires, rounding the right angles at the bends on the back side of the bearing shells to ensure a smooth transition for the wires. After the improvements, tests showed good operating conditions, and during the following months of operation, the problem of frequent damage to the temperature control points on the instruments that had existed before no longer occurred. 3 Conclusions (1) A high temperature in the speed increaser is generally caused by an overly small clearance between the bearing shells; of course, there may also be issues related to oil supply, such as poor oil quality and insufficient amount of lubricating oil. Regarding the issue of high temperature in the speed increaser, the clearance of the bearing bushing was adjusted first, and later the amount of lubricating oil supplied was increased, which brought the temperature of the speed increaser back to normal levels. (2) The main factors causing unit vibration are generally improper bearing clearance, poor dynamic balance of the equipment’s rotor, and an inappropriate alignment between the units. This recent maintenance of the unit illustrates this point: first, the amount of bearing shims was adjusted, then the balance of the rotor was adjusted, and finally the centering of the unit was rechecked and adjusted – only through these steps was the problem of excessive vibration in the unit and motor resolved. (3) To address the issue of damaged temperature control points on instruments, it is generally resolved along with the solution to vibration problems ； On the other hand, insufficiently gentle curvature of the instrument wire is also a factor that causes the wire to break; rounding off the bends where the wire passes around obstacles can resolve this issue.