Direct answer: Frequent VRU cycling usually indicates poor capacity turndown, unstable tank pressure or incorrect control settings. High suction pressure may come from excessive vapor flow, blocked piping or insufficient compressor capacity. Liquid carryover usually points to poor header drainage, an undersized knockout vessel or failed level control. Troubleshooting should use pressure, flow, temperature and liquid-level trends instead of repeated manual restarts.
VRU troubleshooting safety
Hydrocarbon vapor can be flammable, toxic or oxygen deficient. Follow site isolation, gas testing, electrical classification and lockout procedures. Do not open equipment or bypass shutdowns without an approved safety procedure.
Problem 1: Frequent start-stop cycling
Possible causes
- Compressor minimum capacity is higher than the normal vapor rate.
- Start and stop pressure settings are too close.
- Pressure transmitter impulse line or reading is unstable.
- Several tanks connect and disconnect rapidly.
- Air ingress changes gas volume and control behavior.
Checks
Trend suction pressure, run status, valve position and vapor flow. Review compressor turndown, recycle control and available storage volume in the header or suction vessel. Adjustments should remain within safe tank pressure limits.
Problem 2: High tank or suction pressure
Possible causes
- Peak vapor flow exceeds unit capacity.
- Filter, carbon bed, hose or piping is restricted.
- Liquid has accumulated in a low point.
- A control valve is not fully open.
- Discharge pressure is too high for the compressor.
Compare pressure at the tank, header, knockout vessel and equipment inlet to locate the restriction. Review simultaneous loading or production events. Never raise tank relief settings as a shortcut for inadequate capacity.
Problem 3: Low suction pressure or excessive vacuum
Possible causes include oversized capacity, poor recycle control, closed source valves or low vapor production. Excessive vacuum can damage storage tanks or pull air into the system. Confirm pressure calibration and verify that the VRU control range matches the connected equipment.
Problem 4: Liquid carryover
Check header slope, drains, knockout vessel level, mist eliminator condition and high-level shutdown. Sudden liquid slugs may follow tank overfill, condensation or poor piping layout. Liquid can damage compressors and reduce activated carbon performance.
Problem 5: High discharge temperature
High compression ratio, low suction pressure, inadequate cooling, fouled heat exchangers or mechanical problems can raise temperature. Compare current readings with the equipment curve and historical data. Check cooling flow, ambient conditions and compressor condition.
Problem 6: Low hydrocarbon recovery
Confirm actual vapor composition and flow before blaming equipment. Air leakage can dilute vapor. Worn valves, compressor leakage, saturated carbon, inadequate cooling or incorrect solvent circulation can reduce recovery. Reconcile recovered product with inlet load over a representative operating period.
Preventive maintenance checklist
- Inspect knockout vessel, drains and level switches.
- Check filters and pressure drop.
- Verify pressure, temperature and flow instruments.
- Inspect compressor lubrication, seals, valves and cooling.
- Test shutdowns and alarms.
- Check carbon bed or absorbent performance where applicable.
- Inspect vapor headers for leaks, corrosion and liquid low points.
- Review operating trends and abnormal events.
Read the detailed VRU piping and maintenance guide.
When the VRU is undersized
Capacity may be inadequate if high pressure occurs during repeatable peak events even when piping, valves and controls are clear. Recalculate vapor flow from tank filling, flashing, breathing and simultaneous sources. A lead-lag unit, larger compressor or separate source header may be required.
Frequently asked questions
Why does my VRU keep shutting down?
Common shutdown causes include high liquid level, high discharge temperature, high discharge pressure, low lubrication pressure, motor protection or unsafe suction conditions. The first-out alarm should be recorded before reset.
Why does a VRU pull air into the tanks?
The unit may be drawing more gas than the sources produce, the pressure setting may be too low or tank hatches and seals may leak. Capacity control and leak inspection are both required.
How often should a VRU be inspected?
Use the manufacturer and site maintenance plan, then adjust frequency based on operating hours, liquid contamination, pressure-drop trends and alarm history.
Can the VRU be replaced by a vapor combustion unit?
Yes in some applications, but this sacrifices product recovery and changes fuel, emissions and safety requirements. Compare lifecycle performance before changing technology.
VRU technical resources
Review how a VRU works, the tank vapor recovery design guide and PollutionCtrl oil and gas vapor recovery equipment. Contact PollutionCtrl with operating trends and gas data for a troubleshooting review.
