Detailed Explanation of Lubrication System for Centrifugal Compressors
Proper lubrication of centrifugal compressors is of vital importance to ensure safe, reliable operation and long service cycles of the unit. The main factors affecting lubrication performance include the structure of the lubrication system, oil supply flow, lubrication mode and quality of lubricating oil.
01 Main Functions of Lubrication for Centrifugal Compressors
A stable lubricating oil film is formed between relatively moving friction surfaces to isolate direct contact between friction surfaces, thereby reducing wear, lowering friction power consumption and extending the service life of friction components.
Protective films are formed on metal surfaces via physical or chemical adsorption to prevent corrosion of friction surfaces.
Circulating lubricating fluid carries away most heat generated on friction surfaces, achieving cooling of friction surfaces.
Flowing lubricating oil washes off contaminants attached to component surfaces and wear debris generated during operation, cleaning friction surfaces.
Sealing the working volume for compressed gas.
02 Composition of Centrifugal Compressor Lubrication System
To form a stable oil film between sliding bearings and main shaft journals, centrifugal compressors generally adopt forced pressure circulating lubrication. The lubrication system mainly consists of lube oil tank, main oil pump, auxiliary oil pump, oil cooler, oil filter, overhead oil tank, valves and pipelines.
During normal operation, lubricating oil from the tank is drawn into the filter by pumps, pressurized by the lube oil pump, and then delivered through oil coolers, oil filters and regulating valves. Clean lubricating oil is supplied to satisfy lubrication of all friction pairs and sealing requirements of working volumes in the compressor unit.
Lube Oil Tank
The lube oil tank serves for supply, recovery, sedimentation and storage of lubricating oil. An internal heater is installed to heat lubricating oil before startup, raising oil temperature to the range of 35~45°C to meet startup requirements of the unit.
The oil return port and pump suction port are generally arranged on two sides of the tank, with a partition baffle in between. It provides time for sedimentation of impurities and release of gas from returned oil so as to guarantee oil quality. A liquid level indicator is mounted on the side wall to monitor variation of lubricating oil level inside the tank.
Lubricating Oil Pumps
Centrifugal compressors are normally equipped with one main oil pump and one standby auxiliary oil pump. The main pump runs during normal operation. When failure of the main pump or oil circuit causes system oil pressure to drop below a set value, the auxiliary pump starts automatically to deliver adequate lubricating oil to all lubrication points.
The configured pump flow rate is generally 200~350 L/min, and outlet pressure shall not be lower than 0.5 MPa. After pressure reduction, lubricating oil enters bearings at a pressure of 0.08~0.15 MPa.
Oil Cooler
Temperature of lubricating oil returning to the tank usually rises. The oil must be cooled before recirculating to lubrication points. Oil coolers cool pressurized oil discharged from pumps to control oil temperature supplied to bearings.
To maintain oil supply temperature steadily within 35~45°C, two oil coolers are generally configured: one in service and the other on standby for switchover when necessary. Both coolers can be operated simultaneously if the operating unit provides insufficient cooling capacity.
Oil Filter
Oil filters are installed at pump outlets to filter lubricating oil entering the compressor, preventing foreign contaminants or wear particles generated inside the machine from entering the system. This is an effective measure to guarantee oil quality. Two filters are configured for unit safety: one operating and one standby.
Overhead Oil Tank
The overhead oil tank is a safety protection facility. When oil supply from main and auxiliary pumps is interrupted, lubricating oil in the overhead tank flows by gravity through pipelines to each lubrication point to maintain lubrication during unit coast-down. The oil storage capacity of the overhead tank shall sustain oil supply for at least 5 minutes.
During normal unit operation, lubricating oil enters the overhead tank from the bottom and overflows from the upper overflow outlet directly back to the main tank. If main and auxiliary pumps fail, oil from the overhead tank flows through oil supply lines to lubrication points and returns to the tank, ensuring lubrication during coast-down and safe shutdown of the unit.
The overhead oil tank is generally installed at a height of no less than 5 m above the unit shaft centerline. Piping from the overhead tank to lubrication points shall be designed with minimum length and bends to minimize flow resistance.
A breather device is fitted on the top of the overhead tank. When oil flows out of the tank, air drawn through the breather replenishes the tank volume to avoid negative pressure that hinders oil outflow.
To ensure oil from the overhead tank can only flow toward lubrication points, a check valve is installed on the main pipeline between pump outlet and the compressor. The check valve closes immediately once pump oil supply fails, forcing overhead tank oil to flow only to lubrication points.
In addition, the performance of lubricating oil directly affects safe, reliable and long-cycle operation of centrifugal compressors. Improper selection or misuse of lubricating oil will impair lubrication and sealing performance and may trigger equipment failures. Therefore, lubricating oil must be properly and reasonably selected strictly according to specific operating conditions of the compressor and performance requirements for lubricants.
01 Main Functions of Lubrication for Centrifugal Compressors
A stable lubricating oil film is formed between relatively moving friction surfaces to isolate direct contact between friction surfaces, thereby reducing wear, lowering friction power consumption and extending the service life of friction components.
Protective films are formed on metal surfaces via physical or chemical adsorption to prevent corrosion of friction surfaces.
Circulating lubricating fluid carries away most heat generated on friction surfaces, achieving cooling of friction surfaces.
Flowing lubricating oil washes off contaminants attached to component surfaces and wear debris generated during operation, cleaning friction surfaces.
Sealing the working volume for compressed gas.
02 Composition of Centrifugal Compressor Lubrication System
To form a stable oil film between sliding bearings and main shaft journals, centrifugal compressors generally adopt forced pressure circulating lubrication. The lubrication system mainly consists of lube oil tank, main oil pump, auxiliary oil pump, oil cooler, oil filter, overhead oil tank, valves and pipelines.
During normal operation, lubricating oil from the tank is drawn into the filter by pumps, pressurized by the lube oil pump, and then delivered through oil coolers, oil filters and regulating valves. Clean lubricating oil is supplied to satisfy lubrication of all friction pairs and sealing requirements of working volumes in the compressor unit.
Lube Oil Tank
The lube oil tank serves for supply, recovery, sedimentation and storage of lubricating oil. An internal heater is installed to heat lubricating oil before startup, raising oil temperature to the range of 35~45°C to meet startup requirements of the unit.
The oil return port and pump suction port are generally arranged on two sides of the tank, with a partition baffle in between. It provides time for sedimentation of impurities and release of gas from returned oil so as to guarantee oil quality. A liquid level indicator is mounted on the side wall to monitor variation of lubricating oil level inside the tank.
Lubricating Oil Pumps
Centrifugal compressors are normally equipped with one main oil pump and one standby auxiliary oil pump. The main pump runs during normal operation. When failure of the main pump or oil circuit causes system oil pressure to drop below a set value, the auxiliary pump starts automatically to deliver adequate lubricating oil to all lubrication points.
The configured pump flow rate is generally 200~350 L/min, and outlet pressure shall not be lower than 0.5 MPa. After pressure reduction, lubricating oil enters bearings at a pressure of 0.08~0.15 MPa.
Oil Cooler
Temperature of lubricating oil returning to the tank usually rises. The oil must be cooled before recirculating to lubrication points. Oil coolers cool pressurized oil discharged from pumps to control oil temperature supplied to bearings.
To maintain oil supply temperature steadily within 35~45°C, two oil coolers are generally configured: one in service and the other on standby for switchover when necessary. Both coolers can be operated simultaneously if the operating unit provides insufficient cooling capacity.
Oil Filter
Oil filters are installed at pump outlets to filter lubricating oil entering the compressor, preventing foreign contaminants or wear particles generated inside the machine from entering the system. This is an effective measure to guarantee oil quality. Two filters are configured for unit safety: one operating and one standby.
Overhead Oil Tank
The overhead oil tank is a safety protection facility. When oil supply from main and auxiliary pumps is interrupted, lubricating oil in the overhead tank flows by gravity through pipelines to each lubrication point to maintain lubrication during unit coast-down. The oil storage capacity of the overhead tank shall sustain oil supply for at least 5 minutes.
During normal unit operation, lubricating oil enters the overhead tank from the bottom and overflows from the upper overflow outlet directly back to the main tank. If main and auxiliary pumps fail, oil from the overhead tank flows through oil supply lines to lubrication points and returns to the tank, ensuring lubrication during coast-down and safe shutdown of the unit.
The overhead oil tank is generally installed at a height of no less than 5 m above the unit shaft centerline. Piping from the overhead tank to lubrication points shall be designed with minimum length and bends to minimize flow resistance.
A breather device is fitted on the top of the overhead tank. When oil flows out of the tank, air drawn through the breather replenishes the tank volume to avoid negative pressure that hinders oil outflow.
To ensure oil from the overhead tank can only flow toward lubrication points, a check valve is installed on the main pipeline between pump outlet and the compressor. The check valve closes immediately once pump oil supply fails, forcing overhead tank oil to flow only to lubrication points.
In addition, the performance of lubricating oil directly affects safe, reliable and long-cycle operation of centrifugal compressors. Improper selection or misuse of lubricating oil will impair lubrication and sealing performance and may trigger equipment failures. Therefore, lubricating oil must be properly and reasonably selected strictly according to specific operating conditions of the compressor and performance requirements for lubricants.
