Vapor recovery units fail in predictable patterns. Knowing the common modes shortens diagnosis time and prevents secondary damage. The items below cover the failures we see most often in adsorption-absorption and compressor-based VRU packages.
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1. Excessive Compressor Cycling
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The compressor starts and stops rapidly when suction pressure setpoint sits too close to the tank pressure band, or when the vapor space is small relative to unit capacity. Consequences include motor overheating and premature seal wear. Fix: widen the deadband, verify pilot-operated vent valve response, or add a receiver buffer.
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2. High Discharge Pressure Trips
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Downstream restrictions – fouled coalescers, closed manual valves, or overfilled recovery tanks – push discharge pressure into trip territory. Check the liquid level logic first; a failed level transmitter that reads low keeps filling until relief lifts.
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3. Liquid Carryover Into the Unit
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Sloshing tanks and foam-prone crudes entrain liquid into vapor lines. Slugging damages compressor valves and floods carbon beds, which then release hydrocarbon slugs downstream. Demister sizing and slope-consistent piping prevent most cases.
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4. Carbon Bed Breakthrough
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Emissions rise gradually as adsorption beds saturate. Without periodic outlet monitoring, the first sign may be an odor complaint or LDAR reading. Regenerate or rotate beds on schedule; track bed delta-P and outlet concentration trends.
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5. Freeze-Related Control Failures
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Cold-climate sites see instrument air moisture freezing in positioners and solenoids. Heat tracing critical lines and dry instrument air solve this seasonally recurring issue.
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Diagnostic Checklist
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When a unit misbehaves: confirm tank levels and blanketing gas supply, log suction/discharge pressures across several cycles, inspect the demister, then test the level transmitter calibration. Most field calls resolve in this order.