A three-stage vapor recovery unit is the piece of gas station equipment that turns tank breathing emissions into recovered fuel instead of atmospheric pollution. It is installed at the storage tank’s breather valve and starts automatically whenever tank pressure rises, processing vapor through a combination of compression, condensation, and membrane separation. For anyone planning to buy, install, or operate one, the practical questions come down to four things: how it works, how to size it, how to install it properly, and how to keep it running. This guide covers all four.
How the Unit Works: Compression, Condensation, and the Membrane
The unit processes vapor in a fixed sequence. A compressor draws vapor from the tank breather valve and raises its pressure. The pressurized vapor then passes through a heat exchanger where the heavier hydrocarbons condense into liquid fuel, which returns to the tank by gravity. What remains after condensation is a lean mixture of air and light hydrocarbons, and this flows into a membrane module. Inside the membrane, hydrocarbon molecules pass through to a central collection region under vacuum and are returned to the tank, while air stays behind and is vented as clean exhaust. When tank pressure drops below the shutdown threshold, the unit returns to standby automatically.
This staging exists because condensation is efficient at recovering the bulk of the vapor but cannot economically reach very low outlet concentrations on its own. The membrane does that final polishing. The result is that the exhaust concentration of non-methane hydrocarbons is held below 10 grams per cubic meter, regardless of how high the breathing spikes get.
Sizing the Right Unit
Sizing starts with the tank and unloading pattern. The processing capacity is usually expressed in cubic meters per hour, and the WHSH series is available from 4 to 20 cubic meters per hour. To pick the right size, estimate the peak instantaneous vapor load — mainly from the largest tank on site during an unloading event and from the daily breathing cycle — and add a margin so the unit is not running flat out at every peak.
An undersized unit backs up tank pressure under heavy loading; an oversized one spends most of its life idling and running up electrical costs for nothing. Under-loading that leaves the compressor cycling constantly is just as wasteful as over-loading. Match the capacity to the station’s real unloading frequency and tank volume, not to an optimistic number in a brochure.
Beyond capacity, confirm the electrical supply. The unit requires three-phase AC power — typically 380 volts at 50 Hz — with correct phase sequence, plus a proper grounding system, and the installation site must suit the unit’s environmental limits of temperature and humidity. It is worth confirming these before purchase so nothing is discovered at the foundation stage.
Correct Installation Practices
Installation begins with the site. The unit is skid-mounted, so the location needs a flat, well-ventilated outdoor area above the surrounding grade with drainage around the perimeter, clear of heavy foot traffic, and positioned so the intake can connect to the tank’s breather pipe. The skid should be secured to its foundation with the mounting bolts provided, so mechanical vibration cannot move it.
Piping ties the unit to the station. Install a shut-off valve and a flame arrester on the intake line, and a shut-off valve with a check valve on the liquid return line. The return line must slope continuously toward the tank so the recovered liquid flows by gravity instead of pooling. All gas and liquid lines must pass a hermetic pressure test before being placed in service — do not skip this. Make the electrical connections to the rated supply, seal every cable entry properly, and complete the secondary grounding with a resistance meeting the site specification.
Because the unit operates in an area with flammable gases, the explosion-proof enclosure is designed so that its cover must not be opened under normal operation, and all explosion-proof surfaces must stay sealed. Only trained personnel equipped with combustible-gas detection instruments should carry out any work that requires access, and always after the combustible gas concentration is confirmed safe and the power is disconnected.
Commissioning Before Regular Operation
Commissioning follows a fixed sequence. First confirm the environment, pipe connections, and valves are in order. Then start with a power check: the phase sequence must be correct, or the-phase protection will alarm and refuse to start. After power-on, the unit enters automatic mode after a short delay, and the cycle is verified by observing a real breathing event or a controlled test: compressor starts, pressure builds, condensation returns liquid to the tank, membrane separates the remaining vapor, and exhaust concentration stays within limits.
Once commissioned, the unit runs unattended for routine operation. It starts and stops automatically based on tank pressure, so operators do not need to intervene during normal unloading. The control interface shows live system pressure, temperatures, processing status, and alarm history, so a quick glance confirms the unit is behaving.
Routine Operation and Maintenance
Day-to-day operation is largely about monitoring rather than touching. Watch that the unit starts automatically when expected, that the compressor runs without unusual noise, that recovered liquid returns to the tank, and that no alarms are showing. The control display logs daily processing time, total processed volume, and return volume, which give a clear picture of how hard the unit is working.
Monthly checks deal with the mechanical and electrical essentials: confirm the power supply and that the switches on the panel are secure, verify the grounding wires including the secondary ground are intact, and check for looseness or debris at pipe connections. After a start, confirm the suction and exhaust ports run smoothly with no abnormal noise and that nothing is piled around the unit. Always switch off the main power before any maintenance, and never work on live equipment.
Two consumable items deserve special attention. The membrane module is the highest-value component and its life depends on keeping the incoming gas clean, so the filters and the gas-liquid separator must be cleaned on schedule. The compressor needs its oil level and condition checked and the oil changed at the recommended intervals. A vacuum-pump seal or the membrane pressure system are the places problems most often announce themselves — high system pressure or abnormal exhaust usually trace back to a solenoid valve, a pipeline, or the membrane pressure control.
Keeping the Unit Reliable Over Its Life
A three-stage vapor recovery unit repays good management. Size it to the station’s real peak load, install it with correct piping slope and grounding, commission it through a complete cycle, and inspect the filters, the membrane feed, and the compressor oil on schedule. Those habits keep the exhaust concentration within limits, protect the highest-cost components, and let the unit run unattended for years between service calls. When a fault does appear, the unit’s own alarm and diagnostic display points straight at the cause, so the correction is fast and the downtime is short.