Troubleshooting of Excessive Temperature of Compressor Thrust Bearings

I. Fault Description and Root Causes
Fault Overview

The compressor supplies air for smelting processes, with a flow rate of 285 m³/min, working pressure of 444 kPa, rotational speed of 11,250 r/min and rated power of 1,125 kW. 

The thrust bearing at measuring point TE218 adopts line-supported tilting pad structure. When the lubricating oil temperature was 45°C, the bearing temperature reached 98°C, 

close to the alarm threshold of 100°C. To ensure safe operation, the lubricating oil temperature was lowered to 40°C, and the thrust bearing temperature dropped to 93°C.

After half a year of operation, the thrust bearing temperature rose continuously and exceeded the alarm value, triggering automatic shutdown of the unit. To sustain production, 

the compressor interlock protection was disabled, and the thrust bearing kept running above the alarm temperature. During major overhaul, disassembly inspection revealed seizure and burning of the thrust bearing, 

severe scratches on the thrust disc surface, and the thrust bearing was beyond repair.

After grinding the thrust disc, replacing the thrust bearing and the balance drum air seal, the unit resumed operation. Nevertheless, the thrust bearing temperature exceeded the alarm threshold again within three months.
Root Cause of Thrust Bearing Overheating

Potential triggers include insufficient surface finish of the thrust disc and excessively high linear velocity on the bearing surface. However, the primary factor is excessive bearing load. 

Although the damaged thrust disc was repaired during overhaul, it could not restore the original technical standard. In addition, high thrust bearing temperature already existed during commissioning when the thrust disc remained intact.

According to structural analysis, the compressor rotor consists of four impellers and a balance drum mounted on the main shaft. A balance hub air chamber is arranged on the casing. 

The thrust bearing at measuring point TE218 is installed at the end of the bearing housing, while the adjacent thrust bearing in the same housing (measuring point TE201) only runs at 81°C. 

This indicates that the balancing force generated by the balance hub is smaller than the axial force produced by the four impellers.

The large clearance of the air seal on the balance hub leads to air leakage, reducing the pressure difference before and after the balance hub and weakening the balancing force.

After replacing the air seal, the temperature at TE218 stopped rising and stabilized at 98~99°C (same as commissioning status), still close to the 100°C alarm limit, which proves the bearing is still under excessive load.

Unreasonable Oil Return Structure

Inspection of oil supply and return pipelines shows lubricating oil enters the bearing housing from both sides. Driven by centrifugal force generated by the rotating thrust disc, 

oil flows outward along the pad surface to lubricate and cool the thrust bearing and thrust disc.

However, no dedicated drain outlet is arranged at the thrust bearing and thrust disc position inside the bearing housing. Most lubricating oil flows outwards along the shaft, 

forming a stagnant internal circulation zone at the matching surface between thrust bearing and thrust disc. Heat generated in this zone cannot be fully carried away, 

resulting in heat accumulation and carbon deposition adhering to the thrust bearing and thrust disc, which further elevates bearing temperature.

II. Fault Resolution Measures
Increase Balancing Force to Reduce Bearing Load
The most direct method to improve balancing force is to enlarge the balance hub diameter, yet this modification involves heavy workload and high cost.

After research, the pressure inside the rear air chamber of the balance hub was reduced to negative pressure to widen the pressure difference before and after the hub and enhance balancing force. 

Holes were drilled on the volute of the rear balance air chamber, and a pipeline with an inner diameter of 65 mm was connected to the first-stage air inlet of the compressor. 

The balance air chamber is connected to the primary inlet to form negative pressure, expanding the pressure difference across the balance hub and boosting balancing force, thereby lowering the load on the thrust bearing.

Test results: at lubricating oil temperature of 45°C, the thrust bearing temperature at TE218 was 93°C, and TE201 remained at 81°C.
Utilize Misalignment of Motor Magnetic Center to Reduce Bearing Load
Although the balance pipeline partially improves balancing force, full axial force balance requires additional vacuum equipment, which brings operational inconvenience.

During unit alignment, the axial dimension of the coupling was adjusted to the maximum allowable value. The axial clearance between the compressor and the gearbox coupling was set to the upper limit of 1.7 mm. 

The motor magnetic center was offset by 4 mm relative to the rotor centerline. During operation, the electromagnetic force of the motor acts opposite to the bearing axial load and transmits force via the flexible diaphragm coupling, reducing the thrust bearing load.

After alignment and trial run, the bearing temperature at TE218 dropped to 90°C, and TE201 was 83°C, achieving basically balanced bearing load.

Nevertheless, the temperature of both thrust bearings increased to varying degrees after long-term operation (TE218: 93°C; TE201:84°C). Disassembly inspection in overhaul found blackened bearing pads, 

together with yellow and black carbon deposits on the middle area of the thrust disc and bearing housing, requiring further optimization.

Optimize Oil Return Structure

Two holes were drilled on both sides at the middle of the bearing housing. Two semicircular grooves were machined outward on the bearing cover adjacent to the holes to add dedicated drain outlets. 

Lubricating oil in the stagnant zone can flow out through the side holes in the bearing housing and the semicircular oil drain grooves on the bearing cover to form effective circulation, 

improving cooling performance for the thrust bearing and thrust disc.

After reconstruction, under normal operating oil temperature of 45°C, the thrust bearing temperature at TE218 stabilized at 86°C and TE201 at 80°C with no continuous temperature rise, and carbon deposition was significantly reduced.
III. Precautions

After modifying the oil return structure, the flow resistance of the oil supply and return circuit around the thrust bearing decreases, 

increasing oil supply to this bearing while reducing oil supply for other bearings and triggering their temperature rise.

Therefore, the oil pressure of each oil distribution branch must be readjusted. Specifically, the oil pressure of the thrust bearing distribution point was adjusted from 280 kPa to 250 kPa, 

ensuring all other bearing branches operate at designed pressure. Finally, the thrust bearing temperature at TE218 stabilized at 87°C.

The unit has operated safely for two years with stable bearing temperature at measuring point TE218.