Vapor Recovery Units
Vapor recovery unit compressor lubricants are selected for compressors that recover low-pressure, hydrocarbon-rich vapors from storage tanks, production facilities, loading operations and other vented gas sources.
Recovered vapors contain methane, ethane, propane, butane, pentane and heavier condensable hydrocarbons, along with water vapor, carbon dioxide (CO₂), hydrogen sulfide (H₂S) and other components. Where the process gas contacts the lubricant, hydrocarbons dissolve into it or condense into it and reduce its operating viscosity substantially.
NEXT Lubricants supplies mineral, polyalphaolefin (PAO), polyalkylene glycol (PAG) and polyethylene glycol (PEG) lubricants for vapor recovery units. Mineral and PAO lubricants suit lighter, less dilution-prone vapors; PAG, water-soluble PAG (PAG-WS) and PEG formulations resist viscosity reduction in richer, more condensable hydrocarbon streams.
For calculated tank-vapor cases and the complete lubricant-chemistry ladder, see Severe VRU Compressor Oil Dilution: Causes and Solutions.
A vapor recovery unit captures low-pressure hydrocarbon vapor that would otherwise be vented or flared and compresses it back into a sales, fuel, or process system. The vapor it handles is richer than almost any other gas stream on the site.
Heavier hydrocarbons dissolve into the lubricant far more readily than lighter ones. Propane, butane, and pentane reduce in-service viscosity, while methane and ethane have a much smaller effect.
Recovered vapor is condensable, making vapor recovery the most dilution-aggressive duty in gas compression. Heavy components can enter the lubricant as liquid rather than only as dissolved gas.
Low suction pressure does not cause dilution. Solubility increases with pressure, so gas composition—not suction pressure—determines how much hydrocarbon the lubricant retains.
Dissolved gas and condensed liquid are different failure modes. Lubricant chemistry addresses dissolved-gas dilution. Effective separation and adequate superheat prevent liquid carryover, which no lubricant can continuously compensate for.
A unit that pulls its tanks into a vacuum draws in air, creating both a safety hazard and an oxidation problem. Under these conditions, lubricant viscosity can increase rather than decrease.
Recommended NEXT Vapor Recovery Unit Compressor Lubricants
NEXT GPL PAG-EO
Very Heavy / Severe-Dilution Vapor Recovery
Base Oil: PEG
ISO Range: 32 – 220
NEXT GPL MIN
Wet / Light Hydrocarbon Vapor Recovery
Base Oil: Mineral
ISO Range: 32 – 680
How Vapor Recovery Units work and the compressor's role
Crude Oil & Condensate Tank Vapor Recovery
Vapor recovery units recover flash, working and breathing vapors from crude-oil and condensate storage tanks operating at or near atmospheric pressure. Crude-oil and condensate tanks rank among the largest sources of recoverable vapor.
Tank Battery & Wellsite VRUs
Upstream production facilities capture vapors from tank batteries and other low-pressure production sources for recovery instead of venting or flaring.
Loading & Terminal Vapor Recovery
Truck, rail and terminal loading generates displacement vapors as liquid hydrocarbons enter storage or transport vessels. Vapor-recovery systems collect these streams for processing or recovery.
Flash Gas Recovery
Pressure reduction of hydrocarbon liquids releases dissolved gases as flash vapor. VRU compressors recover these streams and return them to a process, fuel-gas or sales-gas system.
Process & Low-Pressure Vent Recovery
Vapor recovery units also recover vapors from compressors, dehydrators and other low-pressure sources within oil-and-gas and process facilities.
Factors affecting lubricant selection
Vapor recovery combines hydrocarbon-rich vapor, low suction pressure, variable flow and the possibility of condensation. The following factors determine the lubricant chemistry and viscosity required.
- Vapor composition and molecular weight Indicates the concentration of methane, ethane, propane, butane, pentane and heavier components that interact with the lubricant.
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Hydrocarbon dilution and operating viscosity
Determines whether the formulation maintains sufficient film thickness after hydrocarbons dissolve into the lubricant. -
Condensability and dew-point conditions
Determines whether heavier hydrocarbons remain as vapor or condense into the suction system, compressor or lubricant. -
Suction and discharge conditions
Set the pressure ratio, compressor loading, discharge temperature, gas density and the amount of hydrocarbon the lubricant retains. -
Lubricant and gas temperature
Govern hydrocarbon solubility, in-service viscosity, oxidation rate and the likelihood of condensation. -
Condensate and liquid carryover
Disturbs the lubricating film and dilutes the lubricant, which makes effective suction separation and drainage essential. -
Variable flow and cycling duty
Create changing thermal and lubrication demands as tank pressure, production rate and vapor generation fluctuate. -
Water, CO₂ and H₂S
Determine corrosion risk, lubricant stability, material compatibility and whether sour-gas requirements apply.
What NEXT needs to recommend a lubricant
A lubricant recommendation is based on the information below. Provide whatever information is available; NEXT will identify whether any additional details are required.
- Compressor Details Manufacturer, model, serial number and compressor type, such as reciprocating, rotary screw or centrifugal.
- Current Lubricant and Performance Current oil, oil volume, operating hours and any problems with viscosity, deposits, consumption, carryover or lubricant life.
- Gas Composition Main hydrocarbon components and any water, CO₂, H₂S, nitrogen, oxygen or other substances present. An existing gas-composition report also serves.
- Operating Conditions Suction and discharge pressures and temperatures, oil temperature and relevant operating limits for each compression stage.
- Application and Specific Requirements How the compressor is used and any purity, catalyst, material-compatibility, minimum-viscosity or other application-specific requirements.
- Final Recommendation NEXT evaluates the available information and confirms the recommended product, lubricant chemistry and ISO viscosity grade, with compatibility and changeover guidance where required.
Operational Benefits of Correct Lubricant Selection
Selecting the lubricant according to the compressor, gas composition and operating conditions can provide several operational benefits.
- Stable Operating Viscosity Maintains sufficient film strength after hydrocarbons dissolve into and dilute the lubricant.
- Reliable Variable-Load Performance Sustains lubrication across changing suction pressure, vapor flow, compressor load and cycling.
- Reduced Unplanned Downtime Prevents lubrication-related wear, overheating, valve problems, deposits and unexpected compressor shutdowns.
- Extended Component Life Protects cylinders, piston rings, packing, bearings, rotors and other lubricated components.
- Clean Running and Deposit Control Minimizes varnish, carbon and deposit formation for cleaner valves, cylinders and lubricant-system components.
- Longer and More Predictable Service Intervals Resists oxidation, viscosity loss and contamination for controlled maintenance planning.
Our technical team can help identify the right product.
Lubricant Selection, Technical Support and Compatibility Documentation
NEXT combines application knowledge, laboratory data and an extensive internal cross-reference database to support lubricant selection, conversions and compressor troubleshooting.
Depending on the application, we can provide:
- Application-Specific Product Recommendations Lubricant recommendations based on gas or refrigerant composition, compressor design, operating conditions and current performance issues.
- Lubricant Cross-Referencing Identifies suitable NEXT alternatives by comparing base-oil chemistry, viscosity, application, specifications and operating requirements.
- Compatibility and Conversion Documentation Provides chemistry comparisons, mixture-test data, material compatibility, flushing requirements and top-off or changeover guidance.
- Dilution Data and PVT Graphs Shows how gas or refrigerant concentration, pressure and temperature affect lubricant dilution, density and operating viscosity.
- Troubleshooting and Root-Cause Support Supports investigations into foaming, oil carryover, dilution, deposits, corrosion, high lubricant consumption and reduced oil life.
What type of oil is used in a vapor recovery unit compressor?
Usually a polyalkylene glycol (PAG) or a polyethylene glycol (PEG), because most recovered vapor is rich enough that mineral and polyalphaolefin (PAO) lubricants lose too much viscosity.
The reason is chemical similarity. Mineral oils and PAO are non-polar, as are the hydrocarbons in the vapor, so they mix readily and dissolved gas removes operating viscosity. PAG carries oxygen in its backbone, which makes it polar and hydrocarbons correspondingly less soluble in it.
Mineral and PAO remain appropriate on genuinely lean recovery duties — compressor seal vents, some fuel-gas recovery. They are not the default here.
Why can VRU vapors reduce compressor oil viscosity?
Because of where the vapor came from.
Tank vapor is generated when hydrocarbon liquid flashes as it drops from separator pressure to atmospheric storage. It therefore inherits the heavy tail of that liquid, and heavier hydrocarbons dissolve into a lubricant far more readily than light ones.
The most useful field indicator is heating value. Pipeline-quality gas sits in one range; recovered tank vapor commonly runs well above it, and heating value tracks molecular weight. An operator with only a Btu figure from the sales meter already knows enough to be concerned.
Does low suction pressure cause oil dilution in a VRU?
No — and this is the most persistent misconception in the application.
Gas dissolves into a lubricant in proportion to its partial pressure, and solubility rises with pressure. The near-atmospheric suction side is therefore the least diluting condition anywhere in the machine. Dilution happens at the discharge and in the oil separator, where pressure is highest and the sump sits at discharge pressure.
Low suction pressure matters for three indirect reasons, all real. It fixes a high compression ratio, so discharge temperature is high and a large volume of gas passes each unit of oil. It identifies a vapor that came off a liquid surface, which is the actual diagnostic. And a unit that pulls its tanks into vacuum draws air in, which accelerates oxidation.
None of those is dilution. The dilution mechanism is composition.
Is oil dilution always worse in a VRU than in other gas compressors?
Not automatically, and the honest answer is that it depends on the stream — but the odds are unusually poor.
Three conditions raise the risk of hydrocarbon condensation in any compressor: a warm inlet, a heavy gas, and the presence of water and acid gas. Recovered tank vapor tends to meet all three at once, because it comes off an outdoor vessel warmed by the sun, it is heavy by origin, and it left a liquid containing water.
That coincidence is what makes vapor recovery a demanding duty rather than the suction pressure. It is still worth evaluating the specific stream: a lean vent-recovery compressor and a condensate tank VRU are both called vapor recovery and are not the same problem.
What is the difference between dissolved gas and condensed liquid?
They are different failures and they need different responses, which is why the distinction is worth making.
Dissolved gas enters the lubricant in proportion to partial pressure. It is calculable from composition and operating conditions, reversible when conditions change, and it thins the charge uniformly. Chemistry selection handles it.
Condensed liquid does not merely thin the oil — it displaces it from surfaces and washes the film away. It cannot be filtered out once mixed into the sump, and it arrives unevenly: at night when ambient falls, at start-up before the machine reaches temperature, and after a slug from an upstream vessel. A sample taken at the wrong moment can look normal.
A more dilution-resistant lubricant holds viscosity better when condensate arrives. It does not stop it arriving.
When should PAG be used instead of PAO in a VRU?
On most tank vapor duties, and the question is usually which PAG rather than whether.
PAO and mineral are non-polar and dilute heavily in C3+ rich vapor, so they are appropriate for genuinely lean recovery only. Water-insoluble PAG covers lighter vapor and vent recovery. Water-soluble PAG covers crude tank vapor and wetter streams. PEG covers condensate tanks, loading vapor and anything running near its dew point.
One constraint governs the change: PAG and hydrocarbon-based lubricants are not compatible, so converting from a mineral or PAO charge requires a full drain, flush and filter change rather than a top-up.
When should NEXT GPL PAG-EO be used in a vapor recovery unit?
Where dilution has to be nearly eliminated rather than merely reduced, and where a heavier viscosity grade cannot achieve it.
Its hydrocarbon solubility below 3 wt% means the operating viscosity stays close to the neat viscosity at temperature, so grade selection becomes a straightforward viscosity-at-temperature calculation without a large dilution correction.
That matters most when a grade increase has run out of room. Compensating for severe dilution by moving up two or three ISO grades produces a lubricant too viscous to feed at start-up and too viscous to return. At that point the correct move is to stop losing viscosity rather than to start with more.
On a lean vent-recovery duty it buys resistance the machine does not need.
Can one lubricant be used for several VRUs on the same site?
Only after comparing the streams, because units on one lease can differ more than units in different industries.
A crude tank VRU, a condensate tank VRU and a loading rack compressor face substantially different C3+ content, and the difference is large enough to change the chemistry rather than just the grade.
Where consolidation is wanted, the practical approach is to specify for the most severe stream rather than the average — a lubricant adequate for the condensate unit will be more than adequate elsewhere, and it avoids holding two incompatible chemistries on one site.
Explore Other Gas Compression Applications
Natural Gas Processing & NGL
Natural Gas Gathering
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Flare Gas Recovery
Lubricants for compressors recovering flare-header gas for return to fuel gas, process or sales-gas systems.