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Two Trane water-cooled screw chillers were in operation in a building for a year without professional maintenance to save money. During this period, the 2# unit experienced abnormal compressor noise, a sharp increase in noise (measured at 105dB, much higher than the normal 78dB) and severe vibration. The following is a detailed explanation of the in-depth fault analysis and effective handling process.
The unit uses R22 refrigerant, this model does not have a built-in oil pump, and relies on the pressure difference between the suction and exhaust air and the evaporator level to complete the oil supply, oil return and lubrication. The preliminary analysis shows that the abnormal noise, increased noise and increased vibration of the compressor may be caused by the following aspects:
Insufficient refrigerant: The system leakage leads to the difference between high and low pressure, and the evaporator liquid level and oil tank oil level are low, which affects the oil supply and the suction and exhaust lubrication of the screw compressor. In addition, it causes the difficulty of oil return in the oil circuit, the oil level of the oil tank is lower, the oil film strength decreases, and the dry friction and abnormal noise of the rotor are aggravated.
Oil circuit problems: low system differential pressure and low evaporator liquid level exacerbate the difficulty of oil return, low oil level in the oil tank and decrease in oil supply, resulting in poor lubrication, dry friction and abnormal noise of the yin and yang rotors. Oil filter blockage, oil deterioration, and main oil solenoid valve failure can also lead to an increase in oil pressure difference, a decrease in oil supply, and induce abnormal noise.
Bearing problems: the system pressure difference is often low and the oil pressure is low, which reduces the lubrication and cooling oil supply of the bearing, and the bearing is prone to dry friction due to lack of oil, which causes equipment damage in serious cases. The decrease of oil cooling effect causes the bearing to heat up, the friction to increase, and the abnormal noise to aggravate. If not treated in time, the bearing may jam due to continuous dry friction, and the failure will expand.
A comprehensive inspection of the operating parameters and status of the 2# unit found many abnormalities:
- The evaporation temperature fluctuates between 0--1.5°C, (the normal range should be 3--7°C);
- 蒸发压力过低(350--422KPAG,正常值为448--517KPAG);
- 冷凝压力过低(约850KPAG,应为898--~1397KPAG);
- The pressure setting of the differential oil pressure relay deviates from the standard value (non-50PSIG);
- The approximation temperature of the evaporator is too high (the temperature difference between the chilled water outlet and the evaporation temperature is up to 8°C);
- The opening of the electronic expansion valve is abnormally high (95--100%, normal is 30-50%);
- The exhaust temperature of the compressor is too high, and the exhaust superheat is too large;
- The temperature of the motor windings of the compressor is on the high side.
The troubleshooting steps are as follows:
System leakage inspection: smear the parts that may leak (such as condenser, evaporator temperature sensor, pressure transmitter, maintenance angle valve, safety valve and other thread connection ports) with soapy water, and find that there is obvious leakage (the interface has rusted) at the connection between the condenser temperature sensor and the shell. No other leakage points were found during the comprehensive investigation.
Leakage repair: For the leakage without maintenance angle valve and located at the condenser and temperature sensor thread buckle, PSI rapid pressure plugging glue is used to repair it. Clean the leakage, treat it into a rough surface, allocate PSI material according to the leakage range, squeeze it into the hole crack, finish the surface, and remove the excess part. After curing at 25°C for 16 hours (extremely hard), no leakage was confirmed.
Refrigerant replenishment and adjustment of operating parameters: After repairing the leak and confirming that there is no leakage, start the unit and replenish the refrigerant. First of all, adjust the control value of the oil pressure difference to normal (50PSIG), reduce the setting value of the chilled water temperature in the control center, set the load percentage and current percentage to 60% of the manual control, and set the chilled water reduction rate to 0.5°C/min to prevent the rapid growth rate when the heat load is too large. Considering that the unit has been in operation for more than 3 years and has been operating for a period of time in a state of insufficient refrigerant, replenish the refrigeration oil to replenish the oil level of the oil tank. After the setting is completed, the unit is started and the refrigerant is replenished, and the operating parameters such as evaporation pressure, evaporation temperature, exhaust temperature, and condensation pressure are closely monitored.
Operation and commissioning: 3 bottles of 68.0kg refrigerant were supplemented and continuously operated for 1 hour, the state and working conditions of the unit tended to be stable, but the parameters did not meet the standard. After about half an hour of operation, the working conditions and parameters basically returned to normal, and the compressor had no abnormal noise and a calm sound. At this time, the control center is restored to the automatic control, the load percentage and current percentage are restored to 95%, and the chilled water reduction rate is set to 1.0°C/min.
After half an hour of operation, the parameters are recorded as follows:
- Compressor load: 95%
- Compressor average line current: 276A
- The average temperature of each winding of the motor: 20°C
- Evaporation pressure: 495kPAG
- Saturation temperature: 5.8°C
- 冷凝压力:1253 kPAG
- Saturation temperature: 35.5°C
- Compressor discharge temperature: 54°C
- Exhaust superheat: 18.5°C
- Electronic Expansion Valve Opening: 45%
- Cooling water inlet temperature: 29.0°C
- Outlet temperature: 33.9°C
- Chilled water inlet temperature: 11.7°C
- Outlet temperature: 7.9°C
So far, the unit has been successfully repaired and put into normal use. Through the diagnosis and elimination of the fault of the Trane screw chiller, the following conclusions are drawn:
Neglect of maintenance: This type of failure often stems from the user's lack of attention to the condition of the equipment, even if there are problems such as leakage, insufficient refrigerant, low oil level and poor lubrication, the failure continues to be used, resulting in aggravated failure.
Lack of professional maintenance: The long-term lack of professional maintenance and poor quality of operation management personnel is an important cause of such failures. Regular professional maintenance and improved operation management can help prevent similar failures.
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