Stage 1, Stage 2, Stage 3: How the Three-Stage Vapor Recovery Chain Works

A service station vapor recovery system is not one device; it is a chain of three stages, each capturing vapors at a different point. A stage-3 unit is only part of the picture, and its performance depends on the other stages working correctly.

Stage-1: unloading recovery

When a tanker unloads fuel into an underground tank, the incoming liquid displaces an equal volume of vapor. In a stage-1 system, that vapor is routed back into the tanker through a separate vapor line, so it is not released into the air. The vapor line connection and the tanker’s vapor recovery equipment make this a closed loop.

Stage-2: refueling recovery

During refueling, vapors displaced from the vehicle tank are captured at the nozzle. The system uses a vapor-tight nozzle and a vacuum to pull vapors back into the underground tank. The key setting is the air-to-liquid ratio — typically 1:1 to 1.2:1, meaning for every liter of fuel dispensed, roughly one liter of vapor is captured. If the ratio is set too low, vapors escape; if too high, air is drawn into the tank, increasing tank pressure.

Stage-3: tank vapor treatment

Even with stages 1 and 2 working well, vapors still accumulate in the underground tank. Sources include: vapor that is drawn into the tank during stage-2 capture, temperature-driven tank breathing (the tank inhales and exhales as temperature rises and falls), and pressure variations from filling. These accumulated vapors must be handled — a stage-3 vapor recovery unit is installed on the tank vent line to treat them.

Stage-3 is where the WHSH-M fits. Its job:
– Keep tank pressure near atmospheric (start at the preset pressure, stop at the preset stop pressure)
– Recover hydrocarbons from the tank vapor and return them to the tank as liquid
– Discharge the remaining air at a concentration far below the emission limit
– Operate automatically and continuously without staff attention

Practical notes for correct operation

The pressure/vacuum valve settings matter. Typical settings are +2 to +3 kPa on the pressure side and -1.5 to -2 kPa on the vacuum side. These settings coordinate the tank’s breathing with the stage-3 unit’s start/stop points.

The unit’s start and stop pressures are configurable in the system settings. In practice, stations commonly use a start pressure around 200–300 Pa and a stop pressure around 100 Pa, so the unit runs only when the tank actually needs pressure relief. During tanker unloading, the unit continues to operate normally — no action is required from the staff.

One point worth repeating: stage-3 performance cannot compensate for a poorly maintained stage-1 or stage-2 system. The three stages are designed as a chain. A leak in the stage-2 system, for example, draws air into the tank, which increases the vapor load the stage-3 unit must process.

For more information: +86 135 2699 0215 / lixiang@vohonoilpipe.com

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