The WHSH-M series three-stage vapor recovery unit is installed at the breather valve of an underground fuel storage tank. When an unloading operation starts, or when tank pressure rises above the preset threshold, the unit activates automatically and runs through a continuous process of compression, air-cooled condensation, gas-liquid separation and membrane separation. This article walks through each stage of the working cycle as described in the equipment manual, so that station operators and maintenance engineers understand what happens inside the skid.
Automatic Start and Pressure Control
The unit does not run continuously. It stays in standby until the tank pressure sensor detects a rise above the start threshold, which typically happens during fuel unloading or when thermal breathing expands the vapor headspace. Once triggered, the compressor starts and the unit runs until tank pressure drops below the shut-off threshold, then returns to standby. This pressure-driven on-demand operation keeps power consumption low and reduces mechanical wear compared to units that run on a fixed schedule.
Compression and Air-Cooled Condensation
Vapor drawn from the tank headspace enters the compressor, which raises the pressure and temperature of the vapor stream. The compressed vapor then passes through an air-cooled heat exchanger, where ambient air cools it down. As the vapor cools, a portion of the hydrocarbon fraction condenses into liquid. The cooled mixture flows into a gas-liquid separator mounted on the skid.
Gas-Liquid Separation and Liquid Return
In the separator, the condensed liquid settles to the bottom and is routed back to the storage tank through a dedicated pipeline. The residual gas stream, which still contains light hydrocarbon vapors that did not condense at the air-cooled temperature, moves on to the membrane module. Because the liquid return line connects directly to the tank, recovered fuel goes straight back into storage rather than being collected in a separate drum.
Membrane Separation Stage
The membrane module sits at the heart of the system. Hydrocarbon molecules in the residual gas have a higher affinity for the membrane material than air molecules do. As the gas flows across the membrane surface under vacuum, oil molecules adhere to the membrane and permeate through to the central tube, from where they are drawn back into the tank. Air molecules cannot pass through the membrane and remain on the feed side, exiting the skid through the vent stack as clean air.
Explosion-Proof Design and Safety
The entire unit is rated for operation in flammable vapor environments. The explosion-proof junction box inside the enclosure must not be opened during operation, and all flame-path surfaces must remain properly sealed. Maintenance personnel are required to use combustible gas detectors before opening any electrical compartment, and work may only proceed when vapor concentration is below the applicable safety limit.
Operating Cycle Summary
In a typical day at a busy station, the unit cycles on and off many times — once per unloading, plus additional cycles during temperature swings. Each cycle follows the same sequence: pressure trigger, compression, air-cooled condensation, separation, membrane polishing, then automatic shut-down when pressure normalizes. Understanding this cycle helps operators recognize abnormal behavior, such as the unit running continuously (which may indicate a tank leak or a stuck pressure sensor) or never starting (which may indicate a failed compressor or blocked vapor line).