LNG Plants
LNG plant compressor lubricants are selected for the specific compressor duty, gas composition and lubrication system. LNG facilities contain natural-gas booster compressors, refrigerant compressors, boil-off gas compressors and flash-gas compressors, each with different operating requirements.
Propane and mixed-refrigerant circuits expose lubricants to hydrocarbon dilution, which reduces operating viscosity. Boil-off gas and flash-gas compressors handle methane-rich vapour under low-temperature conditions, but the lubricant temperature depends on the compressor design and its separation from the process gas.
NEXT Lubricants supplies PAO and PAG formulations for lubricated compressors across LNG liquefaction plants, export terminals, storage facilities and import terminals.
An LNG plant cools natural gas to approximately −162 °C (−260 °F) so it can be stored and transported as a liquid. Reaching that temperature requires several compressors—feed-gas boosters, refrigerant-circulation compressors, boil-off-gas compressors, and flash-gas compressors—and they do not share one lubrication requirement.
The compressor’s position in the process determines the lubricant requirement, not the fact that the site produces LNG. Refrigerant, boil-off-gas, and feed-gas compressors handle different fluids at different temperatures, so each machine must be assessed independently.
Refrigerant circuits present the greatest dilution risk. Propane and mixed refrigerants dissolve readily into conventional lubricants and reduce operating viscosity, creating the same fundamental problem found in hydrocarbon-refrigeration service.
Cold gas does not automatically mean cold lubricant. Boil-off vapor arrives near storage temperature, but the lubricant remains where the compressor design places it. Suction temperature alone is therefore a poor indicator of the temperature the lubricant experiences.
Some compressors keep the lubricant entirely separate from the gas. Oil-free cylinders with separately lubricated crankcases are common in boil-off-gas service, shifting the requirement from dilution resistance to bearing and gear protection.
Two lubricant chemistries cover most LNG-plant duties for opposite reasons. Polyalkylene glycol resists hydrocarbon dilution where refrigerant contacts the lubricant. Polyalphaolefin provides low-temperature fluidity and low oil carryover where the gas is methane-rich or physically separated from the lubricant.
products
Recommended NEXT LNG Compressor Lubricants
NEXT GPL PAG
Hydrocarbon Refrigeration & LNG Process Compressor Lubricant
Base Oil: PAG
ISO Range: 32 – 680
NEXT PAO
Cryogenic LNG & Process Gas Compressor Lubricant
Base Oil: PAO
ISO Range: 15 – 320
Compression process
LNG Plants: Process, Applications & Compressor Role
Natural Gas Booster Compression
Compression of treated natural gas where additional pressure is required before or within the liquefaction process.
Propane Refrigerant Compression
Propane pre-cooling cycles reduce the temperature of the natural gas and other refrigerant streams before final liquefaction. Large LNG plants use centrifugal compressors, while smaller and modular plants also use oil-injected rotary screw compressors.
Mixed-Refrigerant Compression
Mixed-refrigerant compressors circulate combinations of methane, ethane, ethylene, propane, nitrogen and other components through the liquefaction cycle. The refrigerant composition and direct gas–oil contact determine the dilution effect on the lubricant.
Boil-Off Gas Compression
BOG compressors recover methane-rich vapor generated during LNG storage and transfer for reuse, fuel gas or reliquefaction.
Flash Gas Compression
Compressors recover vapor generated during pressure reduction or LNG handling and return it to the process or fuel-gas system.
LNG Import & Regasification Terminals
Compression is primarily associated with boil-off and vapor management at receiving, storage and regasification terminals rather than the LNG vaporisation step itself.
Selection
Factors Affecting Lubricant Selection
LNG compressor lubricant selection is influenced by the compressor position, gas or refrigerant characteristics and the overall operating envelope.
- Gas Composition Methane and heavier hydrocarbons have different solubility characteristics in compressor lubricants. Increasing concentrations of ethane, propane, butane, pentane and heavier components generally increase dilution and reduce in-service viscosity.
- Suction and Discharge Conditions Pressure, compression ratio and the number of compression stages influence gas solubility, mechanical loading, discharge temperature and the viscosity required to maintain an effective lubricant film.
- Oil and Gas Temperature Temperature affects gas solubility, lubricant viscosity and oxidation rate. Lower oil temperatures can increase the amount of hydrocarbon dissolved in the lubricant, while elevated temperatures increase thermal and oxidative stress.
- Gas and Load Variability Changes in well conditions, gas composition, flow rate and compressor load can alter dilution, discharge temperature and lubricant-film requirements throughout the operating cycle.
Process
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 can also be provided.
- 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.
Benefits
Operational Benefits of Correct Lubricant Selection
Selecting the lubricant for the specific LNG compressor duty provides several operational benefits.
- Stable Operating Viscosity Helps maintain sufficient film strength after hydrocarbons dissolve into and dilute the lubricant.
- Reduced Unplanned Downtime Helps prevent lubrication-related wear, overheating, valve problems, deposits and unexpected compressor shutdowns.
- Extended Component Life Supports the protection of cylinders, piston rings, packing, bearings, rotors and other lubricated components.
- Lower Lubricant Consumption Supports appropriate cylinder lubrication and helps reduce excessive oil use, carryover and unnecessary top-ups.
- Clean Running and Deposit Control Helps minimise varnish, carbon and deposit formation for cleaner valves, cylinders and oil-system components.
- More Consistent Compressor Efficiency Supports effective sealing and lubrication without creating unnecessary viscous resistance or friction.
- Longer and More Predictable Service Intervals Improves resistance to oxidation, viscosity loss and contamination for more controlled maintenance planning.
Cross reference tool
Our technical team can help identify the right product.
TECHNICAL SUPPORT
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.
Frequently Asked question
What type of oil is used in LNG compressors?
Polyalkylene glycol (PAG) or polyalphaolefin (PAO), and the choice follows what the compressor handles rather than the fact that it sits in an LNG plant.
PAG goes where hydrocarbon refrigerant or process gas contacts the oil, because it resists dilution. PAO goes where the gas is methane-rich, where the lubricant is separated from the process stream, or where low-temperature fluidity and low carryover matter most.
A single site normally needs both. Refrigerant compressors and process gas machines pull one way; boil-off, flash gas and separately lubricated machines pull the other.
Why is PAG used in LNG compressors?
Because refrigerant circuits are a hydrocarbon dilution problem, and PAG is the chemistry that resists it.
Propane pre-cooling and mixed refrigerant circuits run propane, ethane, ethylene and butane through the machine. Those dissolve readily into mineral and PAO lubricants, which are chemically similar to them, and dissolved refrigerant reduces operating viscosity.
PAG carries oxygen in its molecular backbone, which makes it polar. Hydrocarbons are correspondingly less soluble in it, so it holds grade where a non-polar base stock will not.
The heavier the refrigerant, the more this matters. A propane pre-cooling circuit dilutes more than a methane-rich stream at the same pressure.
Why is PAO used in LNG compression?
For three properties that matter at the cold and clean end of the plant.
Low-temperature fluidity. Anywhere the lubricant can be chilled by proximity to cryogenic equipment, it still has to flow and return. NEXT PAO reaches a pour point of −68 °C (−90 °F).
Low volatility and low carryover. Lubricant leaving as vapor passes coalescing filters that only capture droplets, and in an LNG plant the downstream path leads into cold boxes, molecular sieves and product streams that carry specifications.
Oxidation stability. These machines run continuously for long periods, so service life is set by oxidation rather than by dilution on the methane-rich duties.
Where the stream is largely methane, dilution is mild and these three properties become the deciding ones.
Does an LNG boil-off gas compressor lubricant operate at −162 °C (−260 °F)?
No, and this is the most common misconception about the duty.
Boil-off vapor can approach LNG storage temperature at the suction, but the lubricant is not at the suction. Where it sits depends entirely on the compressor’s architecture — a crankcase charge separated from the gas path runs near ambient, and even a cylinder lubricant is warmed by the heat of compression.
Some boil-off compressors go further and use oil-free cylinders specifically to keep lubricant out of the gas altogether, leaving only the crankcase and running gear to lubricate.
So the suction temperature on the datasheet is not the lubricant’s operating temperature. Establish the oil temperature separately — it is the number that decides the viscosity grade, and it is the one most often left off an enquiry.
What is the difference between LNG refrigerant and boil-off gas compression?
They sit at opposite ends of the selection logic despite being on the same site.
Refrigerant compressors circulate the fluids that do the cooling — propane in a pre-cooling circuit, or a mixed refrigerant containing methane, ethane, ethylene, propane and nitrogen. Those hydrocarbons dissolve into the lubricant, so dilution resistance governs and PAG is normally correct.
Boil-off compressors recover methane-rich vapor from storage and transfer. Methane dissolves far less readily than the heavier components, so dilution is mild and the constraints become low-temperature behavior, carryover and oxidation life — which points to PAO.
Same plant, same operator, opposite requirements. That is why the page carries two chemistries rather than one.
Does lubricant carryover matter more in an LNG plant?
Yes, because of what sits downstream.
Carried-over lubricant in an LNG process stream travels toward cryogenic heat exchangers, cold boxes and molecular sieve beds. Heavy components that condense or freeze at process temperatures foul those surfaces, and cleaning them is not a routine maintenance task.
That makes low volatility and effective separation a design requirement rather than an efficiency preference on this application — and it is a stronger argument for a synthetic base stock on the methane-rich duties than the temperature argument usually is.
It is also the reason some operators specify oil-free compression on boil-off service in the first place.
Related Applications
Explore Other Gas Compression Applications
Natural Gas Processing & NGL
Industrial & Specialty Gas
Compression and liquefaction of nitrogen, hydrogen, helium and other industrial gases.
Petrochemical Gas Compression
Lubricants for hydrocarbon, cracked, recycle and reactive gas streams in petrochemical processing.
Natural Gas Transmission & Storage
Compression of treated natural gas in pipeline and storage infrastructure.