TL;DR

Low pressure output is most often worn inlet valve plates allowing blow-by. Check separator delta-P first, then do a compression efficiency test.

What you might see

system pressure below setpoint
compressor runs continuously without reaching cut-out
tool or process receiving insufficient pressure
VSD running at high frequency with no pressure gain

Likely causes

Tap to mark as ruled out

Worn inlet valve plates or piston rings causing blow-by and reduced compression efficiency
Blocked air/oil separator element driving excessive pressure drop between compressor and receiver
System demand exceeding compressor capacity at current VSD speed cap
Pressure setpoint or relief valve set too low after maintenance

Required tools

Elektronikon controller (access to delta-P reading)
Replacement separator element (Atlas Copco OEM part)
Replacement inlet valve plate set
Torque wrench and socket set
LOTO kit

Safety first

Lock out and depressurize the compressor fully before opening any internal covers. The pressure vessel and pipework hold lethal stored energy.
The air/oil separator and oil system remain hot after shutdown. Allow 30 minutes to cool before handling.

Procedure

7 steps · ~60 min

1
Note the differential pressure across the air/oil separator element on the controller display. Values above 0.7 bar indicate a blocked separator that must be replaced.[1]
2
Replace the separator element if delta-P is high, then retest pressure output before continuing the diagnosis.
3
Verify the pressure setpoint in the Elektronikon controller. Confirm it matches the system design value.
4
Check the VSD output frequency on the controller. If the drive is at its maximum programmed speed cap and pressure is still low, the compressor is undersized for current demand.
5
Lock out the compressor. Inspect the inlet valve plates for visible wear or carbon buildup. Worn or pitted plates cannot seat properly.[1]
6
Replace the inlet valve plate assembly as a matched set. Replacing only the valve plate without the seat often results in rapid re-failure.
7
After reassembly, re-commission the compressor and verify it reaches setpoint within the expected run time.

Sources

Atlas Copco GA VSD+ Air Compressor User Manual, Atlas Copco
Atlas Copco GA VSD+ IOM, separator element differential pressure limits and valve plate replacement procedures (general)
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