Air/Oil Separator Failures caused by Minimum Pressure Check Valve Failures

The failure of a minimum pressure check valve in a lubricated rotary screw compressor can cause the following problems:

If it fails to open properly on start-up:
The minimum pressure check valve is designed to start opening at approximately 35 PSIG in the air/oil sump. As the pressure builds, the valve should be completely open at approximately 70 PSIG. If it is varnished and sticks completely closed until the pressure in the air/oil sump reaches 75 to 100 PSIG, it will finally open quickly and allow a large volume of air to pass through the air/oil separator rapidly and cause the separator to collapse inward. Separator elements are typically designed to withstand a 35 PSID. A sudden burst of high pressure air through the separator element could cause a momentary 75+ PSID.

If it fails to close properly on shut-down:
The minimum pressure check valve should close quickly when the compressor is shut down. This will prevent any sudden backward flow of air through the machine. If it sticks in the open position and allows high velocity reverse flow, this will cause the air/oil separator to collapse outward. Some compressor manufacturers use a positively sealing inlet valve that will help prevent this in the event that the minimum pressure check valve fails open.

Most operator manuals recommend that the minimum pressure check valve be serviced once a year.