How to Change an Air/Oil Separator on a Rotary Screw Compressor
Your air/oil separator plays a big role in keeping your rotary screw compressor running clean and efficiently. When the separator starts to clog or fail, you can end up with oil carryover, higher operating temperatures, pressure drop issues, and unnecessary wear on the machine.
The good news? Replacing an air/oil separator is a straightforward preventative maintenance task when done safely and correctly.
In this guide, we’ll walk through the basic process of changing an air/oil separator on a rotary screw air compressor, along with the tools, safety precautions, and common mistakes to avoid.
What Does an Air/Oil Separator Do?
In an oil-injected rotary screw compressor, compressed air leaves the airend mixed with lubricant. The air/oil separator removes the oil from the compressed air before it leaves the sump tank.
A properly functioning separator helps:
- Reduce oil carryover into your air system
- Protect downstream equipment
- Maintain compressor efficiency
- Prevent excessive pressure drop
- Extend lubricant life
Most rotary screw compressors require separator replacement around every 4,000 hours, although operating conditions and OEM recommendations can vary.
Signs Your Air/Oil Separator May Need Replacement
Common symptoms include:
- Excessive oil carryover
- High separator differential pressure
- Increased oil consumption
- Higher operating temperatures
- Reduced compressor efficiency
- Separator maintenance interval reached
If your compressor has reached its recommended maintenance interval, replacing the separator before problems occur is usually the safest move.
Before You Begin: Safety First
Working on a pressurized air compressor can be dangerous if proper lockout/tagout procedures are skipped.
Before performing any maintenance:
- Wear proper PPE
- Lock out and tag out electrical power
- Isolate the compressor from the air system
- Verify zero pressure inside the unit
- Confirm the machine cannot restart
Never assume the machine is depressurized without physically checking.
Tools & Materials Needed
Before starting, gather the following:
- Gloves
- Clean cloths
- Oil pan
- Lockout/tagout device
- O-ring grease or moly grease
- Comprehensive tool kit
- Combination wrenches
- Channel lock pliers
- Ratcheting wrench
- Impact driver
- Torque wrench
Having everything ready before you begin helps reduce contamination risk and downtime.
Step 1: Lockout/Tagout and Depressurize the Compressor
Lockout/tagout is the most important step of the entire process.
Disconnect Power
Lock out the compressor disconnect and verify it cannot be energized.
A good practice is to:
- Turn away from the panel when operating the disconnect
- Use one hand when possible
- Verify the disconnect is securely locked
After locking out the machine:
- Attempt to start the compressor
- Confirm the control panel has no power
Isolate the Air System
Shut off the air supply valve to isolate the compressor from the system.
Verify Zero Pressure
Using a wrench, loosen the sump pressure monitoring line carefully.
Before proceeding:
- Tap the fitting to confirm no residual pressure remains
- Verify the pressure gauge reads zero
Do not continue until all pressure has been relieved.
Step 2: Remove the Old Air/Oil Separator
Once the machine is safely isolated and depressurized, you can begin disassembly.
Disconnect Tubes and Control Lines
Loosen and disconnect:
- Air lines
- Control lines
- Blowdown valve connections
- Check valve piping
Combination wrenches and channel lock pliers are commonly used during this step.
Disconnect the Discharge Pipe
Remove the coupling connecting the discharge pipe to the minimum pressure check valve.
Remove the Sump Tank Lid Bolts
Remove the bolts securing the sump tank lid.
Many technicians use:
- Impact drivers for accessible bolts
- Channel lock pliers for tight areas
Set the bolts aside carefully to avoid contamination.
Disconnect the Scavenge Line
Remove the scavenge line from the top of the sump tank lid.
Remove the Lid and Separator
Carefully lift the sump tank lid off the tank.
Then remove the old separator element.
Be extremely careful not to allow dirt, dust, or debris into the sump tank during this process.
Step 3: Install the New Separator
Install the Replacement Separator
Place the new replacement air/oil separator into the sump tank carefully.
Again, cleanliness matters here. Any contamination inside the tank can create bigger problems later.
Clean Mating Surfaces
Use clean cloths to wipe:
- Sump tank surfaces
- Lid sealing surfaces
- O-ring grooves
Lubricate the O-Ring
Apply O-ring grease or moly grease before positioning the O-ring.
This helps:
- Prevent pinching
- Improve sealing
- Make future removal easier
Reinstall the Lid
Carefully lower the sump tank lid back into position.
While reinstalling:
- Align the discharge pipe properly
- Check minimum pressure check valve alignment
- Start all bolts by hand first
Tighten the Lid Bolts Properly
Snug the bolts evenly using a ratchet.
Then use a torque wrench to tighten bolts to the manufacturer’s specification.
Use a star-pattern tightening sequence to distribute force evenly across the lid.
Important Note About Torque Specs
Torque values vary by compressor model and sump tank design.
Always follow your OEM service manual for the correct torque specifications. Avoid overtightening, which can damage the lid, gasket, or tank.
Step 4: Reconnect Lines and Components
Reconnect and tighten:
- Discharge pipe coupling
- Scavenge line
- Air lines
- Blowdown valve connections
- Control lines
Double-check all fittings before restoring pressure.
Step 5: Restart the Compressor Safely
Before powering the unit back on:
- Reconnect the sump pressure monitoring line
- Slowly reopen the air supply valve
- Check for air leaks around the sump tank lid and fittings
Remove Lockout/Tagout
Once everything checks out:
- Remove lockout/tagout devices
- Restore power carefully
- Slowly return the compressor to service
Monitor the machine during startup for:
- Air leaks
- Abnormal sounds
- Oil leaks
- Pressure issues
Common Mistakes to Avoid
Skipping Proper Depressurization
Never loosen fittings on a pressurized sump tank.
Allowing Dirt into the Tank
Cleanliness is critical during separator replacement.
Overtightening Bolts
Always use proper torque procedures.
Forgetting the Scavenge Line
A disconnected scavenge line can cause major oil carryover problems.
Reusing Damaged O-Rings
Inspect seals carefully and replace when needed.
Final Thoughts
Replacing an air/oil separator is one of the most important preventative maintenance tasks on a rotary screw compressor. Taking the time to follow proper safety procedures, maintain cleanliness, and torque components correctly can help prevent leaks, oil carryover, and unnecessary downtime.
For most stationary rotary screw compressors, separator replacement is part of a regular preventative maintenance schedule and should never be ignored once intervals are reached.
Frequently Asked Questions
How often should an air/oil separator be replaced?
Most rotary screw compressors require separator replacement around every 4,000 hours, depending on operating conditions and OEM recommendations.
What happens if an air/oil separator becomes clogged?
A clogged separator can increase pressure drop, raise operating temperatures, reduce efficiency, and cause oil carryover into the air system.
Can I replace an air/oil separator myself?
Many maintenance teams handle separator replacement in-house, but proper lockout/tagout and depressurization procedures are critical.
Why is cleanliness important during separator replacement?
Dust or debris entering the sump tank can contaminate the lubricant and damage compressor components.
Do all rotary screw compressors use the same separator?
No. Separator designs vary by manufacturer and model. Always verify the correct replacement separator for your compressor.
Where do your products ship from?
Everything ships from our warehouse in Greenville, South Carolina, and our support team is based here too, ensuring fast shipping and real help when you need it.
Watch the Full Separator Replacement Video
For a visual walkthrough of the separator replacement process, watch the full video below.