Quick answer: Sizing a VRU starts with an accurate estimate of the vapor volume — working losses, breathing losses, and flash gas — at the peak rate, not the average. From there you set the suction and discharge pressures, choose a compressor type that fits the flow and pressure ratio, size the driver and VFD for the required turndown, and account for gas composition (including H2S and lubricant compatibility) and site constraints like power, footprint, and noise. Undersizing means venting; oversizing means short-cycling and vacuum risk. Get the vapor estimate right first.
Key figures at a glance
- Sizing basis: peak vapour generation rate (flashing, breathing, working losses)
- Key inputs: vapour composition, flow, pressure, temperature
- Recovery choice: adsorption vs absorption vs condensation by duty
- Turndown: unit must handle variable/low-flow periods
- Design goal: meet the VOC limit across the full operating range
Most VRU problems trace back to the design basis, not the machine. A unit sized on average vapor rates trips on peaks; one sized only for flash misses breathing losses. This checklist covers the decisions that determine whether a VRU performs, framed as the questions to answer before you buy.
Estimating vapor volumes and flash gas
Everything hinges on the vapor estimate. You need three contributions: working losses (vapor displaced during filling), breathing losses (vapor expelled by daily thermal cycling), and flash gas (light ends released when live liquid drops to tank pressure). Flash is estimated from the liquid composition and the pressure drop across the last stage of separation; working and breathing losses depend on throughput, tank size, colour, and climate. Size the compressor for the peak combined rate, and use the range between minimum and peak to set the required turndown.
Gas composition, H2S and lubricant selection
The vapor composition drives material and lubricant choices. Sour vapor containing H2S requires H2S-compatible metallurgy and seals and affects where the recovered gas can go. The gas can also strip or dilute the compressor lubricant — which is why lubricant selection matters. In some vapor recovery duties a water-soluble PAG (polyalkylene glycol) lubricant is preferred over mineral oil because it resists dilution by hydrocarbon condensate that would thin a conventional oil and cause it to wash off bearing surfaces; the right lubricant depends on the gas and the compressor type, and it is a design decision, not an afterthought.
Selecting compressor type and drive
Match the machine to the duty, as set out in our components guide: rotary vane for small, steady loads; rotary screw for larger, fluctuating flows; reciprocating for higher discharge pressures; liquid-ring for wet or dirty vapor. For the driver, choose an electric motor where power is available (simpler, quieter, lower-maintenance) or a natural-gas engine on remote sites, accepting more maintenance and its own emissions.
Pressure settings and vacuum protection
The suction pressure set point holds the tank at a small positive pressure — enough to draw vapor into the unit without letting it vent. The discharge pressure must meet the sales-line or fuel-gas pressure the recovered gas has to enter. The single most important safety setting is vacuum protection: the control logic must slow, recycle, or shut down the unit before it can pull the tank below atmospheric pressure, because a vacuum draws air into the tank and creates an explosive mixture.
Turndown and VFD sizing
Vapor rates swing widely between quiet periods and active filling. A VRU with narrow turndown short-cycles — starting and stopping repeatedly — which wears the machine and destabilises tank pressure. A VFD lets the compressor slow to match low flow, widening turndown, and a recycle loop covers the gap below the compressor’s minimum. Size the VFD and recycle for the actual minimum-to-peak ratio you calculated.
Site constraints: power, footprint, noise
Practical limits shape the final design: available electrical power (and its reliability), the skid footprint the site can accommodate, ATEX area classification, and noise limits at the boundary. These often decide between electric and engine drive and between compressor types before performance does.
Design mistakes that cause VRU failures
The recurring failures are predictable: sizing on average rather than peak vapor (venting on peaks), ignoring flash gas (or double-counting it), too-narrow turndown (short-cycling), an undersized inlet scrubber (liquid carryover and compressor damage), the wrong lubricant for the gas (lube washout), and pressure settings that risk vacuum. An independent design review before purchase catches these on paper, where they are cheap to fix.
The data a VRU sizing exercise actually needs
A sound VRU design starts from a vapour load calculation, and the quality of that calculation depends on the inputs gathered up front. The dominant number is the peak vapour generation rate, which combines working losses (vapour displaced as tanks fill), breathing losses (vapour expelled as tanks warm and cool), and flash losses (light ends breaking out as pressure drops). Each is estimated from tank sizes, throughput, product volatility (true vapour pressure), and the site’s temperature swings.
Alongside the peak, the design needs the vapour composition and the turndown range — how low the flow can fall during quiet periods — because a unit sized only for the peak may cycle or lose efficiency at minimum flow. Suction pressure and temperature, available utilities, footprint, and the applicable VOC limit complete the picture.
Getting these inputs right is what separates a VRU that holds tank pressure and meets its limit across the whole operating range from one that is technically installed but practically struggling. It is also why an independent vapour assessment before purchase pays back: it grounds the specification in the site’s real numbers rather than a supplier’s default assumptions.
Frequently asked questions
How do you size a vapor recovery unit?
Estimate the total vapor volume — working, breathing, and flash — at the peak rate, then select a compressor and driver for that peak while providing enough turndown to handle the minimum flow. Sizing on average rates is the most common error.
When should a water-soluble PAG lubricant be used in a vapor recovery unit?
A water-soluble PAG lubricant is used when the recovered vapor would dilute or wash off a conventional mineral oil — for example where hydrocarbon condensate is present — because the PAG resists that dilution and keeps a protective film on the compressor’s bearing surfaces. The gas composition and compressor type determine the choice.
What happens if a VRU is oversized?
An oversized VRU short-cycles and can pull the tank toward vacuum at low vapor rates, drawing in air. Correct turndown, a VFD, and a recycle loop prevent this, but the cleaner fix is sizing to the real vapor profile.
Related Rodanco resources: See our vapour recovery unit page and vapor recovery unit consulting page, or get in touch through the Rodanco contact page.
Where Rodanco fits: Rodanco provides independent VRU design review and consulting so the specification matches the real vapor load before capital is committed. See our Vapour Recovery Unit and VRU consulting pages, or arrange a design review through the Rodanco contact page.