Quick answer: Most VRU downtime comes from a short list of causes — liquid carryover damaging the compressor, oxygen ingress from a unit pulling vacuum, seal and bearing wear, scrubber level faults, and VFD or control trips. Nearly all are preventable with a maintenance routine that keeps the scrubber working, verifies pressure settings and vacuum protection, monitors lubricant and seals, and checks the controls. When a unit trips repeatedly or recovers less than it should, a performance assessment usually finds a fixable root cause rather than a worn-out machine.
Key figures at a glance
- Common failures: compressor/seal wear, valve issues, control/pressure faults
- Symptom: rising emissions or tank pressure = recovery underperforming
- Maintenance: seals, valves, filters, and control calibration
- Root causes: liquid carry-over, fouling, and cycling
- Goal: sustained VOC capture and reliable uptime
A VRU that trips keeps venting the vapor it was bought to recover — losing product and breaching emissions limits until it is back online. The good news is that VRU failures repeat: the same handful of problems account for most downtime. Knowing them turns firefighting into prevention.
Top VRU failure modes
The recurring failures are liquid carryover into the compressor, oxygen ingress caused by the unit pulling a vacuum, compressor seal and bearing wear, scrubber level control faults, and VFD or PLC trips. Each has clear symptoms and a clear fix — covered below.
Liquid carryover and scrubber problems
The compressor needs a dry feed. If the inlet scrubber is undersized, its level control fails, or a liquid slug arrives from the header, liquid reaches the compressor and can cause serious mechanical damage — especially on reciprocating and screw machines. Symptoms include vibration, knocking, and rapid lubricant contamination. Prevention: verify scrubber capacity against the real liquid load, keep the level control and drain working, and add slug protection where the header can deliver liquid.
Oxygen ingress and tank vacuum events
If a VRU pulls the tank below atmospheric pressure, it draws air — and therefore oxygen — into the tank. That is both a safety hazard (an explosive mixture) and a process problem (oxygen in the recovered gas). It happens when the suction set point is wrong, the recycle/bypass valve fails, or turndown is inadequate at low vapor rates. Prevention: confirm the vacuum-protection logic actually shuts the unit down before vacuum, test the recycle valve, and make sure turndown matches the minimum vapor flow.
Compressor and seal wear
Seals and bearings wear, and worn seals leak vapor — losing recovery and, on the shaft, releasing emissions. Wear accelerates with liquid carryover, the wrong lubricant, or running outside the design envelope. Prevention: use the specified lubricant for the gas, monitor seal condition and lubricant analysis, and keep the machine within its designed pressure and flow range.
Control and VFD faults
The controls are what keep a VRU safe and efficient, so control faults show up as trips, short-cycling, or a unit that will not modulate. Common causes are a drifting suction pressure transmitter, VFD faults, or PLC configuration that no longer matches the real vapor profile. Prevention: calibrate the pressure instrumentation, keep VFD cooling and connections clean, and re-check the control set points against current operating conditions.
Preventive maintenance checklist
A practical routine covers:
- Scrubber level control and drain — verify operation, check for carryover.
- Suction pressure set point and vacuum-protection shutdown — test the logic.
- Recycle/bypass valve — confirm it operates at low load.
- Compressor lubricant — analyse for contamination and dilution; top up or change on schedule.
- Seals and bearings — inspect for leaks and wear; monitor vibration.
- After-cooler and condensate system — check cooling and liquid recovery.
- VFD and controls — calibrate instruments, review set points against current vapor rates.
- Safety devices and ATEX integrity — inspect on schedule.
When to call for a performance assessment
If a unit trips repeatedly, recovers less than expected, short-cycles, or the tanks still vent despite a running VRU, the issue is usually a mismatch between the unit’s settings and the real vapor load — not a failed machine. A field performance assessment measures the actual vapor profile and pressures and pinpoints the root cause, which is almost always cheaper to fix than to replace.
Diagnosing the root cause of VRU underperformance
When a vapour recovery unit stops meeting its recovery target, the symptom is usually rising tank pressure or increased measured emissions — but the root cause sits in one of a few places. Compressor or vacuum-pump issues (worn seals, valve wear, lubrication problems) reduce the unit’s ability to draw and boost vapour. Liquid carry-over from an undersized or fouled knockout floods and fouls the recovery media or the compressor. Control and instrumentation faults — a drifting pressure transmitter or a stuck control valve — leave the unit responding to the wrong signal.
A structured diagnosis works back from the emission or pressure symptom: confirm the pressure/vacuum control setpoints, check the knockout and any filters for liquid and fouling, verify compressor performance against its curve, and inspect the recovery stage (carbon condition, absorbent quality, or condenser duty). Because these failures often compound — carry-over accelerating compressor wear, for example — fixing only the visible symptom tends to let the problem return.
Preventive maintenance built around the failure modes above — scheduled seal and valve service, knockout draining, media checks, and control calibration — keeps recovery efficiency stable and avoids the emissions excursions that draw regulatory attention.
Turning failure modes into a maintenance schedule
The most effective way to keep a VRU recovering is to convert its known failure modes into a preventive schedule rather than waiting for the emissions or pressure symptom. Compressor seals and valves wear predictably, so scheduled inspection and replacement pre-empt the performance loss they cause. The liquid knockout needs regular draining and checking, because carry-over is both a direct efficiency loss and the accelerant of compressor wear. Recovery media — carbon condition or absorbent quality — degrades over time and is checked and replaced on a cycle.
Instrumentation is part of the schedule too: pressure transmitters drift and control valves stick, so periodic calibration keeps the unit responding to the true tank pressure rather than a false signal. Because these failure modes compound — carry-over damaging the compressor, a drifting control loop letting pressure swing — a schedule that addresses them together prevents the cascade that a single reactive repair does not.
The payoff is stable recovery efficiency and fewer emissions excursions, which matters both for compliance and for the recovered-product revenue a well-running unit returns. Preventive maintenance built on the failure modes is what keeps a VRU quietly meeting its target instead of drifting toward the next breakdown.
Frequently asked questions
What is the most common cause of VRU failure?
Liquid carryover into the compressor is among the most common and most damaging, usually caused by an undersized or malfunctioning inlet scrubber. Oxygen ingress from a unit pulling vacuum is the most safety-critical.
How often should a VRU be serviced?
Follow a scheduled preventive-maintenance routine covering the scrubber, lubricant, seals, controls, and safety devices, with the interval set by the compressor type, duty, and gas conditions. Continuous monitoring of pressures and trips flags problems between services.
Why does my VRU keep tripping?
Repeated trips usually mean the unit’s settings do not match the real vapor load — inadequate turndown causing short-cycling, a failing recycle valve, or drifting pressure instrumentation. A performance assessment measures the actual profile and finds the cause.
Related Rodanco resources: See Rodanco’s vapour recovery unit overview, and consider a VRU performance assessment to benchmark a unit that is drifting off target.
Where Rodanco fits: Rodanco runs VRU performance assessments that measure the real vapor load and find the root cause of trips and lost recovery. See our Vapour Recovery Unit and VRU performance assessment pages, or arrange an assessment through the Rodanco contact page.