The controller shows the unit loaded (100% or max speed on VSD) yet pressure sits below target. Either the compressor is producing less air than rated, or the plant is using more air than the compressor can make. Separate the two with a closed-valve test or a flow meter.
1
Demand exceeds capacity (leaks or added equipment)
medRotary screw
CheckClose the discharge isolation valve (or a downstream valve) briefly — if the unit then builds to set point and unloads, it's healthy and the system is over-consuming. Do a leak audit on a weekend with production off: time how fast pressure decays.
FixFix leaks (typically 20–30% of plant air), remove inappropriate uses (blow-offs, cabinet coolers), or add capacity/storage.
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2
Inlet valve not fully opening
medRotary screw
CheckWatch the inlet valve/butterfly linkage when loaded; check the control solenoid gets voltage and the actuator gets control air. Inlet vacuum gauge (if fitted) showing high vacuum when loaded = restriction.
FixClean/rebuild inlet valve; replace load solenoid; replace actuator diaphragm.
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3
Dirty air intake filter
lowRotary screw
CheckFilter restriction indicator in the red, or controller 'air filter' warning.
FixReplace element; never run without a filter.
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4
Blowdown valve leaking while loaded
medRotary screw
CheckAir heard venting from blowdown/silencer while loaded; sump pressure below line pressure + expected drop.
FixRebuild or replace the blowdown valve.
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5
High separator differential pressure
medRotary screw
CheckCompare sump pressure to package discharge pressure. More than ~10–15 psi difference means the separator element is plugged.
FixReplace separator element; check oil condition (varnish plugs elements prematurely).
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6
Worn airend / slipping drive / low VSD maximum speed
highRotary screw
CheckMeasure actual output with a flow meter vs FAD rating. On belt-drive units check tension and pulley size. On VSD, check max frequency parameter and that motor reaches it.
FixCorrect drive issues; airend rebuild if rotor clearances worn.
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Safety
Lockout/tagout before opening enclosures. Verify zero energy. Bleed system pressure before opening any pneumatic connection.