What Is a Stage-3 Vapor Recovery System and Why Do Service Stations Need One?
Every time a gasoline tank is filled, vapors are displaced. Every time a car is refueled, vapors escape from the nozzle area. And even when nothing is being dispensed, the tank “breathes” as temperature and pressure change through the day. These vapors are volatile organic compounds (VOCs) — they represent product loss, they contribute to air pollution, and they create a flammable risk around the tank farm.
Vapor recovery at a service station is normally divided into three stages.
Stage-1 recovery captures vapors displaced from the underground tank during tanker unloading, returning them to the tanker. Stage-2 recovery captures vapors at the dispenser during refueling, using a vacuum-assisted nozzle and an air-to-liquid ratio of roughly 1:1 to 1.2:1. Stage-3 recovery handles what the first two stages cannot: the vapors that accumulate inside the storage tanks from normal breathing and from incomplete capture. A stage-3 vapor recovery unit is installed at the tank vent line and treats this tank vapor so that it is not simply vented to atmosphere.
How a stage-3 unit works, in brief
The unit is connected to the tank vent (the breather valve). When tank pressure rises above the preset start value, the unit starts automatically. Vapors are drawn into the system and passed through a membrane separation module. Hydrocarbon molecules permeate the membrane preferentially and are returned to the tank, where they re-condense into liquid fuel. Clean air, which does not permeate the membrane, is discharged. When tank pressure drops below the stop value, the unit returns to standby automatically.
Key design figures for the WHSH-M unit
The WHSH-M is a membrane-based stage-3 unit with the following design parameters:
– Vapor processing capacity: 4–20 m³/h
– Power: 1.3 kW (membrane-only model) or 2.7 kW (air-cooled + membrane model)
– Noise: ≤63 dB(A) (membrane-only) or ≤75 dB(A) (air-cooled + membrane)
– Ambient operating range: -40 °C to +70 °C
– Enclosure rating: IP55
– Footprint: 1300 × 900 × 1160 mm
– Weight: 390 kg (membrane-only) or 410 kg (air-cooled + membrane)
What a stage-3 unit actually delivers
For the station owner, the measurable results are:
– Recovered liquid fuel returned to the tank and resold — the recovered oil contains no water and needs no secondary dewatering;
– Tank pressure kept near atmospheric, reducing breathing losses and vapor accumulation;
– Discharge concentration far below the national emission limit, with NMHC treatment efficiency above 99% in normal operation;
– Unattended automatic operation: the unit starts and stops on tank pressure, so no staff time is needed.
The economics are straightforward: the vapors that used to leave the tank are now liquid fuel again. Over a year of normal operation, the recovered volume is often enough to make the unit self-funding, before counting the compliance and safety benefits.
For more information: +86 135 2699 0215 / lixiang@vohonoilpipe.com
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