Quick answer: A VRU package is a self-contained skid built around a gas compressor. The essential parts are an inlet (suction) scrubber to remove liquid, the compressor itself, a driver (electric motor or gas engine), a variable frequency drive to match speed to vapor flow, a suction pressure controller with a recycle/bypass valve to protect the tanks from vacuum, and an after-cooler with condensate separation to recover liquids. The compressor type — rotary screw, rotary vane, reciprocating, or liquid-ring — is chosen for the flow, pressure ratio, and gas conditions.
Key figures at a glance
- Core components: compressor/blower, scrubber, skid, controls, recovery unit
- Scrubber: removes liquids ahead of the compressor
- Compressor/blower: draws vapour under low pressure and boosts it
- Skid: pre-engineered, pre-piped package for site install
- Controls: pressure control tie-in to tank/VRT blanket
A VRU is more than its compressor, but the compressor choice drives everything else on the skid. This guide walks through each component, the four compressor families used in vapor recovery, and how the controls keep the whole package safe and efficient.
What a VRU package / skid includes
Packaging a VRU on a single skid means it arrives pre-engineered, tested, and ready to tie in. A standard package contains: the inlet scrubber, the compressor and its driver, the VFD and motor control (or engine controls), the after-cooler, the condensate handling, the suction pressure control loop, the recycle line, instrumentation and a PLC, and safety shutdowns. On a compact site the whole thing fits within an ATEX-rated skid footprint with local noise attenuation.
Compressor types used in VRUs — and where each fits
Four families dominate. All of them do the same thing — squeeze gas into a smaller volume to raise its pressure — but they suit different duties:
Rotary screw
Two intermeshing rotors continuously reduce gas volume. Screw compressors handle high and variable flows well, tolerate wet gas better than some alternatives (especially oil-flooded screws), and pair naturally with a VFD for wide turndown. They are a common default for larger, fluctuating vapor loads.
Rotary vane
Sliding vanes in an eccentric rotor make a simple, robust machine that performs well at the low pressure ratios typical of tank-vapor recovery. Rotary vane units are popular on smaller tank-battery VRUs for their reliability and steady low-flow performance.
Reciprocating
Pistons deliver high compression ratios and are efficient across a range of duties, making them suited to applications needing higher discharge pressure. They have more wearing parts and are more sensitive to liquid carryover, so scrubber performance matters even more.
Liquid-ring
A liquid (often glycol or a hydrocarbon) forms a rotating ring that seals and compresses the gas. Liquid-ring compressors are inherently tolerant of liquid slugs and are chosen where the vapor is wet or dirty, or where the sealing liquid also helps handle the stream.
Inlet scrubber and liquid handling
Every compressor needs a clean feed. The inlet scrubber knocks out entrained droplets and any condensate before the gas reaches the machine — a single liquid slug can damage a reciprocating or screw compressor. Recovered liquid is drained and collected. On wet streams a well-sized scrubber is the difference between a reliable unit and constant trips.
Controls: pressure sensing, VFD, bypass and shutdown logic
The control system is what makes a VRU safe. A suction pressure transmitter watches tank/header pressure. The PLC modulates compressor speed through the VFD to match the incoming vapor rate — this is what gives a VRU its turndown and stops it short-cycling. When vapor flow drops below what the compressor can pull, the recycle/bypass valve returns gas to suction so the unit keeps running without pulling the tank toward vacuum. Critically, the logic shuts the unit down before it can pull a vacuum, because vacuum would draw oxygen into the tank and create an explosive mixture.
Drivers: electric motor vs natural-gas engine
Electric motors are the low-maintenance, low-noise choice where power is available and are simplest to permit. Natural-gas engines suit remote, off-grid locations and can burn a slipstream of gas, at the cost of more maintenance and their own emissions. The site’s power availability and emissions targets usually settle the choice — see our design and sizing guide for how this fits the wider specification.
Cooling and condensate recovery
Compression heats the gas; the after-cooler then removes that heat so the heavier hydrocarbons condense. The condensed NGLs are separated and sent to storage or blending, while the residual gas goes to sales or fuel. Effective cooling directly raises how much liquid you recover.
Common spare parts and wear items
Typical consumables and wear parts include compressor seals and bearings, valves (on reciprocating machines), lubricant and filters, scrubber level controls, VFD components, and instrumentation. Keeping critical spares on hand and following a preventive-maintenance schedule is the practical route to uptime — covered in our repair-and-maintenance guide.
How the package components work together as a system
A VRU package is more than a compressor on a skid; its reliability comes from how the components are matched. Vapour first enters a scrubber or knockout that removes entrained liquid, because liquid carry-over is the single most common cause of compressor damage and recovery-media fouling. The compressor or blower then draws the vapour under the slight vacuum that keeps tank pressure controlled, and boosts it to the pressure the recovery stage needs.
The recovery stage — adsorption, absorption, or condensation — captures the hydrocarbons, and the control system ties the whole package back to the tank or VRT pressure, starting, modulating, and stopping the unit to hold that pressure within a tight band. The skid packaging pre-integrates all of this with the instrumentation and safety devices so it can be installed and commissioned as a unit.
Because the components are interdependent, undersizing or neglecting any one of them undermines the rest: a marginal knockout shortens compressor life, an oversized compressor pulls air and raises the explosion risk, and a poorly tuned control loop lets tank pressure — and emissions — swing. Specifying the package as a matched system, not a collection of parts, is what delivers sustained recovery.
Frequently asked questions
What is a VRU package?
A VRU package is a skid-mounted vapor recovery unit that combines the compressor, scrubber, driver, VFD, controls, and cooling into one pre-engineered, tested unit ready to tie into a tank battery or vapor header.
Which compressor is best for a VRU?
It depends on the duty. Rotary vane suits small, steady tank-battery loads; rotary screw suits larger, fluctuating flows; reciprocating suits higher discharge pressures; and liquid-ring suits wet or dirty vapor. The gas conditions and flow decide it.
What is the VFD for on a VRU?
The variable frequency drive changes compressor speed to match the incoming vapor volume. That widens turndown, prevents short-cycling, and helps keep tank pressure stable without pulling a vacuum.
What are the main components of a vapor recovery unit?
A VRU package typically combines an inlet scrubber to remove liquids, the compressor and its driver (electric motor or gas engine), a VFD and pressure controls tied to tank or VRT pressure, a recycle or bypass valve, and an after-cooler with condensate separation to collect the recovered liquids.
Related Rodanco resources: See our vapour recovery unit page and vru performance assessment page, or get in touch through the Rodanco contact page.
Where Rodanco fits: Rodanco assesses VRU packages in the field to confirm the compressor, scrubber, and controls are matched to the real vapor load — a frequent cause of underperformance. See our Vapour Recovery Unit overview or arrange a VRU performance assessment through the Rodanco contact page.