Vapor recovery units fail in predictable patterns. Knowing the common modes shortens diagnosis time and prevents secondary damage. The failures below are the ones we see most often in adsorption-absorption and compressor-based packages at tank terminals, loading racks and tank batteries.
Two habits make diagnosis faster than any single reading: log suction and discharge pressure across several complete cycles rather than one number, and trend bed differential pressure and outlet concentration.
Symptom-to-Cause Lookup
Start with the alarm or the odor complaint.
| Symptom | Likely cause | Check first |
|---|---|---|
| Compressor cycles every few minutes | Setpoint too close to the tank pressure band | Suction pressure across several cycles |
| High discharge pressure trip | Fouled coalescer, closed valve, overfilled recovery tank | Tank level and every valve downstream |
| High discharge temperature | Low suction pressure raising compression ratio | Suction pressure against the design point |
| Knocking or slugging | Liquid carryover from tank or loading line | Knockout drum level, demister, line slope |
| Rising VOC reading at the bed outlet | Breakthrough, channeling, or missed rotation | Delta-P and outlet concentration trends |
| Bed delta-P climbing at normal flow | Dust, polymer or heavy hydrocarbon loading | Inlet filtration, moisture, last regeneration |
| Readings steady while the process moves | Transmitter or level gauge drift | Compare against an independent gauge |
| PLC trip with no process upset | Marginal setpoint, relay wear, moisture in air | Input value at the moment of trip |
| Trips that appear only in winter | Moisture freezing in positioners, impulse lines | Air dewpoint and heat tracing status |
| Failures after a few years on the coast | Salt-laden air corroding terminals and stems | Enclosure rating, terminal condition |
1. Excessive Compressor Cycling
- Symptoms – rapid start and stop, motor overheating, contactor wear, short seal life.
- Root cause – suction setpoint too close to the tank pressure band, or small vapor space relative to capacity.
- Check first – suction pressure across several cycles against the tank band, and vent valve response.
- Fix and prevention – widen the deadband, add receiver volume. Commissioning setpoints go marginal as throughput changes. See the vapor recovery unit compressor and floating roof tank vapor recovery pages.
2. High Discharge Pressure Trips
- Symptoms – trip on discharge pressure, relief lifting, restarts that fail after a short run.
- Root cause – a downstream restriction: fouled coalescer, partly closed manual valve, full recovery tank.
- Check first – the level logic: a transmitter reading low keeps filling a full tank until the relief lifts.
- Fix and prevention – walk the discharge path valve by valve, clean the coalescer, calibrate the level instruments.
3. Liquid Carryover Into the Unit
- Symptoms – slugging, knocking, damaged compressor valves, flooded carbon beds.
- Root cause – sloshing tanks and foam-prone crudes entrain liquid into vapor lines, and loading pushes liquid into the header.
- Check first – knockout drum level, demister and its support, drains at line low points.
- Fix and prevention – correct demister sizing, consistent slope and drainage, separation of loading vapor from breathing vapor. See vapor recovery for oil transfer and piping and maintenance considerations.
4. Carbon Bed Breakthrough
- Symptoms – emissions rise gradually; without outlet monitoring the first sign is an odor complaint or an LDAR reading.
- Root cause – saturation, or breakthrough arriving early when a bed channels or heavies load the inlet face.
- Check first – delta-P and outlet concentration together: rising delta-P at steady outlet means loading, rising outlet means saturation.
- Fix and prevention – rotate or regenerate on the trend, and record outlet concentration after every regeneration. Media and components come through VRU spare parts and service.
5. Freeze-Related Control Failures
- Symptoms – positioners and solenoids that fail to stroke, valves stuck at one position, trips only in cold snaps.
- Root cause – moisture in instrument air freezing in orifices and pilot passages; frozen impulse lines misread.
- Check first – air dewpoint, heat tracing and insulation, energized before the first cold night.
- Fix and prevention – dry air, heat tracing on critical lines, drained impulse lines, scheduled receiver draining.
Adsorption Bed Management
Beds are the part of the unit most often managed on habit rather than data. Three measurements support a replacement decision.
- Bed differential pressure – a slow rise at constant flow means loading. A sudden drop suggests channeling, which is worse, because vapor has found a path around the carbon.
- Outlet concentration – track it weekly. The trend, not the absolute number, shows how much capacity remains.
- Moisture and temperature – capacity falls when the bed is wet or hot, and a wet bed desorbing through a hot afternoon shows a trend that cycles with ambient.
Steam regeneration suits heavier hydrocarbons where steam and condensate handling exist. Vacuum regeneration suits sites without steam and beds that should not be wet. Incomplete regeneration is the recurring problem with both – a bed back in service before it is dry shortens the next cycle.
Compressor and Instrument Health
- Discharge temperature – a compression ratio problem more often than a cooling problem; find out why suction is low first.
- Valves – worn valves cut capacity, raise temperature and are the first casualties of carryover.
- Oil condition – rising level, milky oil or hydrocarbon dilution in the analysis points to liquid entering the machine.
- Instrument drift – a drifted transmitter looks like a process change until compared with a second source; keep pressure transmitters on a calibration schedule.
- Nuisance PLC trips – read the input value at the moment of trip, not the alarm text; marginal setpoints, relay wear and moisture cause most of these. See VRU control panel and instrumentation.
Seasonal and Coastal Effects
Winter brings moisture and viscosity problems: frozen instrument air, ice in impulse lines, condensate freezing at low points. Summer brings thermal ones: higher suction and discharge temperature, and beds that release part of what they adsorbed in the hottest hours.
Coastal and offshore sites add corrosion as a wear mechanism rather than an event. Salt-laden air attacks terminals, stem packing, enclosures and conduit entries. Match enclosure ratings to the environment, seal conduit, and inspect terminals routinely. Installation and commissioning set the baseline those inspections measure against.
Diagnostic Checklist
When a unit misbehaves, start here. Most field calls resolve before the bottom of the list.
- Confirm tank levels and blanketing gas supply – a full or over-blanketed vapor space explains many trips.
- Log suction and discharge pressure across several cycles rather than one reading.
- Walk the discharge path for closed manual valves, then check coalescer differential pressure.
- Inspect the demister, knockout drum level and vapor line drains.
- Test level transmitter calibration against an independent measurement.
- Compare bed delta-P and outlet concentration against the last regeneration date.
- If seasonal, check air dewpoint, heat tracing and solenoid response.
- Read the trip log for values at the moment of trip, not just the alarm text.
Frequently Asked Questions
Why does my VRU compressor keep cycling?
The control band is narrower than the pressure movement the unit sees: the suction setpoint sits too close to the tank pressure band, or the vapor volume is small enough that pressure crosses the deadband quickly. Widening the deadband, adding receiver volume or confirming vent valve response resolves most cases.
What causes high discharge pressure trips on a VRU?
A restriction between the compressor and the recovered liquid destination. Fouled coalescer elements, a manual valve left partly closed and an overfilled recovery tank are the most common, and a full tank is usually a level indication problem.
How often should carbon beds in a VRU be replaced?
It depends on the service, the hydrocarbon mix and the moisture the bed sees, so there is no universal interval. The usable indicators are the outlet concentration and bed delta-P trends: when outlet concentration rises steadily after each regeneration, the media is near the end of its useful life.
Is steam or vacuum regeneration the better choice?
It depends on what the site has and what the bed is loaded with. Steam suits heavier hydrocarbons where steam is available and condensate can be routed correctly. Vacuum avoids the steam system and the water that comes with it.
How do we tell a real instrument failure from normal drift?
By comparing the reading against a second source. A transmitter that reads steadily while the process moves is a candidate for drift, and calibration records are what separate drift from a process change.
Talk to an Engineer
Send us the symptoms, whatever trend data you have and the site conditions. We will tell you what we would check first and whether the fix is a setpoint, a component or a package change. Contact our engineers.