Vapor recovery units are sized for the busy case: full delivery rates into large tanks on a hot afternoon. Real stations also spend time in the opposite corner, processing a slow trickle of vapor from small deliveries, overnight breathing or partial offloads. This partial load region behaves differently from design conditions, and operators who understand it stop misreading normal behavior as faults. Efficiency, control response and condensate handling all shift together when the feed slows down.
What Partial Load Means for the Machine
At full load the vapor stream is strong, steady and rich, and the unit settles into the operating point its controls were tuned for. At partial load the stream is weak and intermittent: the vacuum side waits for vapor that arrives in pulses, temperatures drop because less compression heat is generated, and condensate flows thin out. The machine still works correctly, but every stage operates away from its design center. Separation efficiency can drift, cycling becomes more frequent, and readings that look alarming on a dashboard are simply the signature of a light feed.
Where the Efficiency Story Gets Subtle
Separation efficiency is a ratio, and ratios behave oddly when the numerator shrinks. At low flow, a slightly larger share of hydrocarbon may slip through the stages that at full load capture it easily, because residence times and concentration profiles change. The absolute losses remain small, since the total vapor volume is small, but the percentage can look worse. Control systems tuned to respond to concentration may hunt or cycle as the pulses of vapor arrive unevenly. None of this indicates a defect; it indicates the unit is being asked to idle along, which it does differently from sprinting.
Full Load and Partial Load Compared
| Aspect | Full load | Partial load |
|---|---|---|
| Vapor feed | Strong, steady, rich | Weak, pulsed, variable |
| Control behavior | Settled at design point | More cycling, hunting possible |
| Condensate flow | Continuous and predictable | Thin and intermittent |
| Efficiency reading | Matches design band | Ratio can dip while absolute losses stay small |
| Operator watch point | Capacity and alarms | Cycle frequency and drain condition |
Operating Habits That Help at Low Load
- Read cycle frequency rather than single events; occasional short cycles at light feed are normal behavior.
- Keep the condensate path warm and clear, because thin flows freeze or dry out faster than full ones.
- Avoid tuning alarms to full-load expectations; alarm bands should tolerate the partial load signature.
- When efficiency figures look low, check the delivery log first: a small offload explains most dips.
- Log partial load episodes so the service partner can distinguish behavior from wear at the next visit.
Why Designers Care About This Region Too
Equipment makers design for the whole envelope, and partial load is where component choices prove themselves. A vacuum source that holds stable vacuum at low flow, controls that tolerate pulsed feeds without hunting, and drains that function on thin condensate flows are all partial load qualities. Buyers comparing units for a station with many small deliveries, such as busy urban sites with frequent partial offloads, should weigh this region explicitly instead of comparing headline capacity alone.
When Partial Load Behavior Stops Being Normal
The line between behavior and fault is trend-based. Cycling that grows steadily more frequent at the same feed pattern, efficiency drift that no longer returns on the next full delivery, or drains that stop passing liquid at all deserve a service visit. The habit that catches these early is the same: a short log entry per delivery, noting feed size and any behavior worth remembering. Patterns become visible in a week of entries, and the service partner arrives with history instead of guesses.
Frequently Asked Questions
Is lower efficiency during small deliveries a problem?
Usually not. Absolute losses are small because the vapor volume is small, and the ratio often returns to the design band at the next full load. A persistent dip across all feed sizes is the case worth investigating.
Why does the unit cycle more at partial load?
The vapor arrives in weak pulses, so controls reach their start and stop conditions more often. Growing cycle frequency at unchanged feed patterns is the symptom to watch.
Can alarms be tuned for light deliveries?
Yes, alarm bands should cover the whole operating envelope, including partial load signatures, so the unit does not cry wolf during normal slow periods.
Luoyang Wohong Petrochemical Equipment Co., Ltd. supplies vapor recovery units engineered for the full operating envelope, from full tanker deliveries to light partial offloads.