Quick answer: sizing a vapor recovery unit starts from gas flow and concentration, not from tank volume alone. Add up the peak vapor load from unloading, breathing and Stage II returns, apply a safety margin, then match the compressor-condenser train and membrane area to that duty. A unit sized this way runs efficiently at normal load and still holds emissions stable at peak.
Step 1: Add Up the Vapor Sources
- Unloading events: the largest single pulse — tank truck deliveries push saturated vapor out of storage tanks.
- Storage breathing: continuous daily loss driven by temperature swings.
- Stage II returns: vapor pushed back from vehicle fueling, now routed into the Stage III unit.
Step 2: Convert Sources into Design Duty
Peak load is not the sum of averages — it is the worst realistic overlap: a delivery happening while fueling continues on a hot afternoon. Size the condensing train for that peak, then check part-load behavior, because most hours of the year the unit runs well below peak. Compressors with staged or variable control handle this span far better than a single fixed machine cycling on and off.
Step 3: Match the Membrane Stage
| Parameter | What to Check |
|---|---|
| Membrane area | Sized to residual flow after condensation, not to raw load |
| Operating envelope | Temperature and pressure limits the condenser must deliver |
| Permeate routing | Enriched vapor returns upstream — no second vent |
Common Sizing Mistakes
The two we see most: sizing only to storage capacity while ignoring unloading pulses, and oversizing the membrane stage for peak flow so it idles at low vacuum most of the year. Both waste money — one in undersized capacity, the other in idle capital.
FAQ
What site data should I prepare for sizing?
Tank numbers and product types, delivery frequency and volume, dispenser count and throughput, plus the Stage II return configuration if present.
Can a unit be expanded later?
Yes — skid-based designs allow a second condensing stage or additional membrane modules to be added as throughput grows.
Luoyang Wohong Petrochemical Equipment Co., Ltd. (Vohon) designs and manufactures vapor recovery units and membrane separation systems for fuel stations and tank farms.