A three-stage vapor recovery unit is the piece of site equipment that sits between the storage tank and the atmosphere, quietly turning what would otherwise be vented hydrocarbon vapor back into liquid. Getting it right involves more than unpacking a skid and plugging it in. This guide walks through four things every project team needs to nail — the operating principle, the correct way to select the unit, the installation steps that protect it from day one, and the maintenance routine that keeps it performing.
How the Unit Works: Condensation First, Membrane Second
The core of a WHSH-series three-stage vapor recovery unit is a combination of two processes. First, vapor from the tank breather line enters a compressor where it is pressurized; as pressure rises the temperature climbs, and the hot, pressurized gas is then passed through a cooler. Cooling drops the gas below the dew point of most hydrocarbon fractions, and the condensed liquid is separated out and returned to the tank through a recovery line. This condensation step does the heavy lifting on recovery efficiency.
What condensation cannot handle is the low-molecular-weight fraction that stays gaseous. That is where membrane separation takes over. The membrane module uses the fact that hydrocarbon molecules pass through the membrane material more slowly than the small nitrogen and oxygen molecules of air. A vacuum pump pulls negative pressure on one side of the membrane, drawing the hydrocarbon molecules across and returning them to the tank, while air passes out through the exhaust. The result is a unit that returns liquid to the tank and discharges only cleaned air.
Selecting the Right Unit: Sizing and Site Criteria
Selection starts with the vapor-load calculation. The amount of vapor a tank breathes out depends on tank volume, local temperature swings, and unloading frequency. Adding up the station’s tank capacity and its daily delivery pattern gives a peak vapor-flow estimate, and that number should fall inside the unit’s throughput range. WHSH-series units cover roughly 4 to 20 cubic metres per hour, which matches most station footprints.
Beyond throughput, confirm the site can support the unit. Check that three-phase power of the required voltage and power draw is available, and that the footprint and clearance around the installation point meet the skid’s dimensions. Confirm the availability of a suitable flat, slightly raised foundation with drainage, and plan the route for the vapor inlet and liquid return lines. Selection is as much about the site as the equipment — a well-chosen unit still fails if the site cannot feed it or protect it.
Installation: The Steps That Protect the Unit
Correct installation is what makes a vapor recovery unit last. The skid goes on a flat, level concrete base that sits above the surrounding grade with drainage so water cannot pool under it. The vapor inlet line from the tank breather should carry a shut-off valve and a flame arrester, and the liquid return line back to the tank should carry a shut-off valve and a non-return valve. After all pipe connections are made, the lines must be pressure-tested for tightness before the unit goes into service.
Electrical installation is equally important. The unit requires three-phase power with the correct phase sequence — a phase-protection device watches this and blocks startup if an error is present. Grounding must meet the electrical resistance requirement for the site, and all electrical connections in the hazardous area must be made into the unit’s explosion-proof fittings. Never open an explosion-proof enclosure while the unit is live; isolate power and verify the atmosphere is safe first.
Startup and Commissioning
At first power-up the unit runs through a startup delay of about ten seconds before it enters operating mode. If the phase-protection indicator stays red, the phases are wrong or the supply is off-specification; switch off, swap any two phases, and power up again. Once the unit is in running mode, check that exhaust concentration holds within its normal band and that the system starts and stops automatically as tank pressure crosses the set points. Commissioning is also the time to confirm the touchscreen data — inlet pressure, membrane pressure, exhaust concentration, and run time — are all reading correctly, because those are the numbers you will rely on for months of maintenance decisions.
Maintenance: A Routine That Keeps Performance
Daily, check the exhaust concentration and the membrane pressure against the previous day’s readings and look for drift. The exhaust number is the clearest signal of the unit’s health — if it climbs, the likely causes are a membrane reaching the end of its service life, a valve not switching cleanly, or liquid carryover into the membrane. Because liquid and debris are the fastest ways to shorten membrane life, confirm the gas-liquid separator ahead of the membrane is working and keep it clean.
On a routine schedule, check the vacuum-pump oil through its sight glass and top it up or change it with the specified oil; an overcurrent alarm on the pump usually points to low oil, overheating, or an overdue service. Check the grounding and the tightness of terminal connections. When the unit’s alarm history shows a recurring pattern, review the logged timestamps before calling the supplier — a pattern tied to an unloading event or a time of day is far easier to diagnose than a one-off alarm.
Some maintenance is beyond a station field team and should go to the supplier’s service support rather than being forced. If a fault cannot be cleared on site with the checks above, stop the unit, keep the area safe, and call for remote or on-site service.
Keeping the Whole Lifecycle in View
A three-stage vapor recovery unit is reliable when operating principle, selection, installation, and maintenance are all done deliberately. Understand that it is condensation-first then membrane-second; size it to the station’s actual vapor load and site conditions; install it on a proper base with the right valves, pressure tests, phase sequence, and grounding; and run the daily and scheduled checks that catch problems early. Follow that path and the unit will keep the tank near atmospheric pressure, return liquid where it belongs, and keep the exhaust where it should be — for years of unattended, dependable operation.