Rotary screw compressor runs and loads, but isn't producing rated capacity: usually a separator, inlet valve, or air-end issue.
What this problem usually means
When the compressor isn't making the air it should: pressure builds slowly, or the unit runs at full load but the system can't hold pressure, the issue MIGHT be on the compressor side, OR in the distribution.
It is actually pretty rare for a compressor to just suddenly have lower output capacity, it is mostly an 'all or nothing' kind of thing (compressor not loading, not producing ANY air in that case).
It is also not very easy to diagnose, if the compressor has lower output capacity, or we are simply using more air then the compressor is rated for. The result is the same: lower pressure. But it doesn't tell use WHY the pressure is lower.
Most of the time, it is because of increased air usage (extra machines, etc) or simply more air leaks.
How it IS possible for a screw compressor to output less then the rated capacity. Main culprits are:
- a clogged oil separator
- a partially-opening inlet valve
- wear in the air-end
These are 'mechanical' causes. A last cause of lowered compressor capacity could be because of hot inlet air conditions, but in general this would cause overheating problems sooner then low capacity problems, and if it results in lower capacity it would be a few percent, so it would only be noticeable if the whole system was in a 'critical balance' anyway, something you'd like to avoid.
Again: lower output capacity IS possible, but if the symptom is "low pressure" I would always first look at the consumption side.
For system-wide pressure problems (leaks, demand, distribution), see [Low Pressure](/troubleshooting/general/low-pressure).
Check these first
5–10 minute checks before diving deeper
- Compare actual output (m³/min or cfm) against the unit's rated capacity (easiest way is to do a pump up test)
- Check separator differential pressure—high DP means clogged separator
- Listen at the inlet valve when loaded—is it fully open? (hard to do)
- Check for excessive bearing or air-end noise (worn air-end) (consult manufacturer)
Common root causes
Why this happens in rotary screw compressors
- Separator element clogged Dirty separator creates a high pressure drop across the element. Sump pressure runs high but outlet pressure stays low — air can't get through.
- Inlet valve not fully opening Inlet valve opens partway but not completely. The compressor makes air, but not enough — throttled by a dirty or sticky valve.
Still stuck?
If the checks above haven't pointed at the cause, post your symptoms in the Q&A. Real-world answers, no sales pitch.