Gas Dilution in Compressor Lubricants
What Gas Dilution Changes
Gas dilution is not a contaminant in the conventional sense. It does not produce wear particles, it does not change colour, and it does not appear on a standard used-oil analysis report. What it changes is the lubricant’s working condition inside the compressor — and through that, the conditions the bearings, rotors and seals actually run on.
Matching a lubricant to a gas compressor means selecting for the operating viscosity inside the machine — not for the rated viscosity on the data sheet.
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Operating Viscosity
Dissolved gas lowers the in-service viscosity below the rated grade — often substantially in hydrocarbon service. -
Bearing Film Thickness
Reduces the hydrodynamic film at journal bearings and rotor contacts, where the OEM clearance was set for the rated viscosity. -
Density and Carryover
Changes lubricant density in the sump, affecting separator efficiency and oil consumption rates. -
Foaming and Air Release
Dissolved-gas behaviour alters foam stability and air-release performance, which additives alone cannot correct.
Technical Background
WHAT GAS DILUTION IS
How much process gas dissolves into the lubricant depends on three variables: gas composition, operating pressure, and sump temperature. Henry’s law applies — solubility rises with pressure and falls with temperature. In a screw compressor running at, for example, 25 bar discharge on a hydrocarbon-rich feed, several percent of the oil mass in the sump can be dissolved gas.
A lubricant with an ISO VG 68 grade has that viscosity measured at 40 °C, at atmospheric pressure, in clean condition. Inside the compressor, the same fluid is at operating temperature, at operating pressure, and saturated with dissolved process gas. The combination of all three drops the effective viscosity, and the drop is the viscosity that determines bearing film thickness — not the data-sheet value.
THE BASE OIL CHEMISTRY EFFECT
Different base oils dissolve different gases to very different extents. For hydrocarbon process gas, the spread between chemistries is wide. Mineral oils and PAOs share strong mutual solubility with hydrocarbons. Polyalkylene glycols — particularly PAG-EO and EO/PO copolymer (water-soluble) grades — have markedly lower solubility with hydrocarbon gas, and the operating viscosity drop in the same service is consistently smaller.
This is one of the engineering reasons PAG chemistries are specified in heavy hydrocarbon, sour gas and natural gas pipeline service. The chemistry is not being chosen for additive load or thermal margin alone — it is being chosen for what the lubricant looks like at the bearing after the gas has dissolved into it.
Variables That Drive Gas Dilution
Four variables determine the operating viscosity inside the compressor. The PVT dilution tool takes each as an input and returns the resulting operating viscosity.
Gas Composition
Different gas species dissolve into base oils to very different extents. Heavy hydrocarbons dissolve far more than methane; CO₂ behaves differently again.
Operating Pressure
Solubility rises with pressure (Henry's law). Discharge pressure is usually the dominant driver of dilution magnitude.
Operating Temperature
Solubility falls with temperature. The sump and discharge temperature set how much dissolved gas the lubricant carries in operation.
Base Oil Chemistry
The largest single lever. PAG, PAO, mineral, POE and diester base oils dissolve the same gas to very different levels at the same conditions.
BASE OIL CHEMISTRIES
How Base Oils Behave in Gas Compression Service
Base oil chemistry is the largest variable in gas dilution behaviour. The ranges below reflect general engineering expectation — exact behaviour depends on the gas, pressure and temperature.
Mineral
High mutual solubility with hydrocarbon gas. Suitable for lower-pressure service where dilution stays modest. Typically the lowest-cost option.
PAO
Moderate-to-high hydrocarbon solubility. Strong thermal and oxidative stability; widely used in process gas where gas loading is manageable.
PAG-WI
Lower hydrocarbon solubility than mineral or PAO. Long-established in heavy hydrocarbon and pipeline gas service for that reason.
PAG-WS (EO/PO copolymer)
Markedly low solubility with hydrocarbon gas, including sour gas and CCUS streams. Used where dilution would otherwise drive operating viscosity below bearing requirements.
PAG-EO (PEG)
Pure ethylene oxide chemistry with very low hydrocarbon solubility. Strong choice for heavy hydrocarbon, sour gas and petrochemical service where holding operating viscosity is the priority.
Diester Blend
Used in specific hydrocarbon and food-grade applications. Generally moderate solubility behaviour; specification depends on the full duty profile.
Related Technical Resources
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Frequently Asked Questions
What is gas dilution in a compressor lubricant?
Gas dilution is the dissolution of process gas into the lubricant under operating pressure. The dissolved gas lowers the lubricant’s operating viscosity below its rated value at 40 °C, affecting bearing film thickness, sealing and oil consumption inside the running machine.
How much does gas dilution reduce operating viscosity?
It depends on the gas composition, pressure, temperature and base oil chemistry. In heavy hydrocarbon service, reductions of 30% or more compared to the rated grade are well documented. The PVT dilution tool estimates the magnitude for a specific set of conditions.
Why doesn't standard oil analysis detect gas dilution?
Because dissolved gas flashes out of a lubricant sample almost immediately once it is no longer at operating pressure. By the time the sample reaches the lab bench, the gas is gone and the viscosity measured at 40 °C reads close to fresh-oil reference.
Which base oil chemistry handles hydrocarbon gas dilution best?
PAG chemistries — particularly EO/PO copolymer (water-soluble) and PAG-EO grades — have markedly lower solubility with hydrocarbon gas than mineral or PAO. For heavy hydrocarbon, sour gas and natural gas pipeline service, PAG-based lubricants typically hold a higher operating viscosity at the same conditions.
How do I calculate gas dilution for my system?
Enter the gas composition, operating pressure and sump temperature into the NEXT Lubricants PVT dilution tool. The tool returns the predicted gas dilution and the resulting operating viscosity for the lubricant chemistry you select, and supports side-by-side comparison between chemistries.