Vapor recovery is usually described in three stages, and the numbering confuses people because the stages describe where vapor is captured, not which one is most important. Stage I collects vapor displaced when fuel is delivered into a storage tank. Stage II collects vapor displaced at the dispenser when a vehicle is refuelled. Stage III collects the vapor that remains in the tank, or is generated during storage, and processes it back into liquid product. A vapor recovery unit belongs to Stage III.
What each stage captures
The three stages form a chain rather than three independent systems. Stage I works during deliveries, when a tanker discharges fuel and the rising liquid level pushes vapor out of the tank. A return line carries that vapor back to the tanker instead of releasing it. Stage II works during vehicle refuelling, capturing the displacement vapor at the nozzle and returning it to the station tank. Stage III then deals with what accumulates in the tank, including vapor generated by temperature swings and the residual pressure that builds up in the tank ullage.
How the stages connect
Stage I and Stage II keep vapor inside the system; Stage III processes it. If Stage I and Stage II are working properly, much of the vapor that would otherwise escape is returned to the tank. That improves the environmental result but it also means the tank ullage pressure tends to rise over time, which is precisely why a Stage III unit is needed to bring the pressure back down and recover the hydrocarbon as liquid.
The practical consequence is that Stage III performance depends on the stages ahead of it. A leaking Stage II return path or a poorly sealed Stage I connection changes the composition and volume of vapor reaching the Stage III unit, and the unit will look as though it is not performing even when its own components are sound.
Where a VRU fits
| Stage | When it operates | What it captures | Where the vapor goes |
|---|---|---|---|
| Stage I | During tanker delivery | Displaced vapor from the storage tank | Returned to the tanker via a vapor return line |
| Stage II | During vehicle refuelling | Displacement vapor at the dispenser | Returned to the station storage tank |
| Stage III | Continuously in operation | Tank ullage vapor and generated vapor | Processed by the VRU and recovered as liquid |
A Stage III unit normally combines condensation with a membrane stage. Condensation liquefies the bulk of the hydrocarbon; the membrane handles the dilute residual and returns an enriched stream back to the condenser inlet. The recovered liquid is returned to storage, and the treated gas leaves the unit within the required limit.
What to check first when performance drops
- Confirm the site is actually losing vapor, rather than the monitoring system reporting incorrectly;
- Inspect Stage II nozzles and the return path for leaks or blocked lines;
- Check Stage I hose connections and the tank vapor return line;
- Verify tank pressure behaviour over a full day, not a single reading;
- Only then review the Stage III unit, starting with condensation temperature and inlet flow.
Common questions
Does a station need all three stages?
Requirements depend on the jurisdiction and the site. Many stations operate Stage I and Stage II only, with Stage III added where the tank pressure balance or local emission requirements call for it. The three stages are complementary rather than alternatives.
Is Stage III the same as a VRU?
Broadly yes, in the sense that the vapor recovery unit is the equipment that performs Stage III duty. Some sites refer to the unit itself as the Stage III system, while others use Stage III to mean the whole function of processing tank vapor.
Why does tank pressure rise if the other stages are working?
Because Stage I and Stage II return vapor to the tank instead of releasing it. Over time, temperature cycling and continued operation increase the amount of vapor in the ullage, and a Stage III unit is what brings that pressure back down by recovering the hydrocarbon as liquid.
Luoyang Wohong Petrochemical Equipment Co., Ltd. designs and manufactures vapor recovery units and membrane separation skids for gasoline stations and fuel depots. Contact us for system sizing, site survey and after-sales service.