Removing Oil From a Semi-Hermetic Compressor

Occasionally a technician may need to temporarily add oil to a semi-hermetic reciprocating compressor to maintain an adequate oil level, if the oil has been trapped out in the evaporator or suction line. This additional oil should not be left in the system.

The cause of the oil trapping should be identified and repaired in a timely fashion. If the additional oil returns to the compressor, it could cause damage as a result of oil slugging or excessive oil pumping.


There are several methods for removing oil from a semi-hermetic reciprocating compressor. The easiest is through an oil drain plug. If the compressor is equipped with an oil drain plug, all a technician must do is reduce the pressure in the crankcase by running the compressor and front seating its suction service valve.

1. Pump the system down to 1 to 2 psig.

2. Shut down the compressor.

3. Once the compressor is off, front seat its discharge service valve, which will completely isolate the compressor from the system.

4. Carefully loosen the oil drain plug and allow the oil to seep out of the compressor around the thread of the plug until enough oil has been removed. Note: Do not totally remove the plug from the compressor. Only allow the oil to seep around its threads.


If the compressor is not equipped with an oil drain plug, or if it is not convenient to use, oil can be removed from its oil fill plug.

Use the following procedure:

1. Attach a gauge manifold to the access ports of the suction and discharge service valves.

2. Front seat the suction service valve and run the compressor.

3. Continue running the compressor until the crankcase pressure has reached 0 psig.

4. Shut down the compressor.

5. Front seat its discharge service valve.

6. Remove the oil fill plug from the compressor.

7. Insert a 1/4-inch O.D. copper tube with a shutoff valve into the oil fill plug opening. Position the 1/4-inch copper tube so that one end is near the bottom of the crankcase and the other end, which is external to the compressor, is positioned below the oil level of the compressor and into an approved container.

8. Place a rag around the 1/4-inch O.D. copper tube where it exits the compressor.

9. Using dry nitrogen, pressurize the compressor crankcase through the suction service valve's access port to about 5 psig.

10. The oil should drain from the compressor. If necessary, repressurize the crankcase with dry nitrogen.

11. Once a sufficient amount of oil has been removed, remove the 1/4-inch O.D. copper tube.

12. Reinstall the fill plug.

13. Evacuate the system to 500 microns.

14. Backseat both the suction and discharge service valves.

15. Restore power to the compressor and place it back in operation.

This method is more time consuming, but it will allow oil to be removed from a compressor. Before using this procedure, make sure the compressor does not have an oil drain plug. Removing oil from a compressor using its oil drain plug is normally a much easier process.

Joe Marchese is owner of Coldtronics of Pittsburgh. He can be reached at 412-734-4433,, or

Publication date: 04/03/2006

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