Oil-injected rotary screw compressors are designed to have minimal oil carryover—typically 2-5 ppm after the separator. When you're seeing visible oil in the compressed air, pooling in lines, or contaminating downstream equipment, something has gone wrong.
The most common culprits are the oil separator element, overfilling with oil, high operating temperatures, or problems with the scavenge (return) line that recycles separated oil back to the sump.
Rotary Screw Troubleshooting
Oil in Compressed Air – Rotary Screw Air Compressor Troubleshooting
Excessive oil carryover contaminating downstream air—usually a separator, oil level, or temperature problem.
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Safety Notice
Oil contamination in compressed air can damage downstream equipment, ruin products, and create slip hazards. For food, pharmaceutical, or breathing air applications, oil carryover is a critical quality issue.
What this problem usually means
Check these first
5–10 minute checks before diving deeper
- Check oil level—overfilled oil causes carryover (level should be between min/max marks)
- Check separator differential pressure—high DP indicates a clogged separator element
- Check discharge temperature—high temps (above 100°C) break down the separator
- Inspect scavenge line and check valve—plugged line means separated oil can't return to sump
- Verify correct oil type is installed—wrong viscosity causes separation problems
- Check when separator was last replaced—typical life is 4,000-8,000 hours
- Look for oil in the moisture drain at the tank or aftercooler—indicates carryover problem
- Check if problem started after an oil change or service—could indicate overfill or wrong oil
Common root causes
Why this happens in rotary screw compressors
Separator element worn or damaged
Separator media breaks down over time. Typical replacement interval is 4,000-8,000 hours. Collapsed or torn element allows oil through.
Oil level too high
Overfilling causes oil to flood the separator and carryover into the discharge. Always fill to the correct level mark, not "full".
Scavenge line blocked
The scavenge (return) line returns separated oil from the separator bottom back to the sump. If plugged, oil backs up and carries over.
High discharge temperature
Excessive heat breaks down separator media and thins the oil, increasing carryover. Address overheating issues first.
Wrong oil type or degraded oil
Using the wrong viscosity or oil that has broken down causes poor separation. Always use manufacturer-specified oil.
Minimum pressure valve not working
If the minimum pressure valve doesn't hold proper pressure during operation, the separator can't work efficiently, allowing oil to carry over.
What NOT to do
Don't simply add more filtration downstream to "catch" the oil—this treats the symptom, not the cause. Excessive oil carryover will quickly saturate coalescing filters, increasing costs and still allowing contamination. Fix the source problem.
Full Rotary Screw Troubleshooting Manual
Step-by-step diagnostics, root cause logic, and practical fixes for oil-injected rotary screw compressors. Save time and reduce downtime with proven methods.
- Step-by-step diagnostics for common failures
- Root cause analysis techniques
- Practical fixes with parts notes
- Works across all major brands