Separate Seal Oil and Lube Oil Consoles in Wet-Seal Gas Compressors

A practical maintenance and oil analysis article from Khash’s perspective

In many process gas compressors, especially older centrifugal compressors handling natural gas, refinery gas, synthesis gas, sour gas, or wet process gas, the machine may have two separate oil systems:

1. Lube oil system: for bearings, thrust bearings, gear coupling, gearbox, driver bearings, and sometimes control oil functions.
2. Seal oil system: for wet seals, where oil is used as a liquid barrier to prevent process gas from escaping along the compressor shaft.

This arrangement is common in API-type turbomachinery installations. API 614 covers minimum requirements for lubrication systems, oil-type shaft-sealing systems, oil-control systems, and auxiliaries, excluding dry gas seal systems. (apiwebstore.org

The key point is this:

The lube oil system protects rotating mechanical surfaces.
The seal oil system protects the plant from process gas leakage.
They are both oil systems, but their contamination risks are very different.

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1. Why do some gas compressors have a separate seal oil console?

In a wet-seal compressor, oil is injected into the shaft seal at a pressure slightly higher than the process gas pressure. The oil forms a liquid sealing barrier. A classic explanation from compressor oil-seal literature is that the seal oil is forced into the compressor seal at higher pressure than the process gas to prevent the gas from escaping from the casing.

This oil then divides into two leakage paths:

A. Inboard seal oil leakage

This is the oil that migrates toward the process gas side. It can contact or absorb process gas. In wet gas, sour gas, or hydrocarbon gas service, this is the dangerous side.

Possible contaminants include:

• Dissolved hydrocarbon gas
• H₂S
• CO₂
• Ammonia, depending on process
• Condensate
• Water
• Light hydrocarbons
• Process chemicals
• Fine solids or corrosion products
• Acidic or reactive compounds

B. Outboard seal oil leakage

This is the oil that migrates toward the atmospheric or bearing side. It is usually cleaner than inboard leakage, but it can still carry aged oil, air, water, and fine contamination.

The reason many users prefer a separate seal oil console is simple: it prevents contaminated seal oil from directly contaminating the main lube oil system. Some compressor station designs use completely separate seal oil and lube oil systems specifically to prevent sour-gas contamination of the lube oil.

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2. Difference between lube oil and seal oil duty

Lube oil duty

The lube oil system normally has to provide:

• Hydrodynamic oil film for journal bearings
• Thrust bearing lubrication
• Heat removal from bearings
• Clean oil supply to critical clearances
• Protection against rust and oxidation
• Stable viscosity
• Low foaming tendency
• Good air release
• Low water content
• Low varnish potential
• High cleanliness level

The main enemies of the lube oil system are:

• Oxidation
• Thermal degradation
• Water ingress
• Particle contamination
• Air entrainment
• Additive depletion
• Varnish formation
• Cross-contamination from seal oil or control oil

Seal oil duty

The seal oil system has a more severe contamination environment because the oil is intentionally placed close to process gas.

Seal oil must provide:

• Stable sealing pressure above gas pressure
• Correct differential pressure control
• Low foaming and good air release
• Gas release capability
• Good demulsibility
• Oxidation resistance
• Cleanliness at close seal clearances
• Low tendency to form sludge or varnish
• Compatibility with process gas exposure

The main enemies of the seal oil system are:

• Dissolved gas
• Hydrocarbon contamination
• Water
• Process condensate
• Sour gas contamination
• Low resistivity or high conductivity contaminants
• Fine solids
• Foaming
• Oil degradation due to repeated gas absorption and release
• Varnish and sludge deposition in small seal clearances and control valves

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3. Typical arrangement of separate consoles

A practical separate system may include:

Lube oil console

Usually includes:

• Main oil reservoir
• Main AC pump
• Auxiliary AC pump
• Emergency DC pump or rundown tank, depending on design
• Oil coolers
• Duplex filters
• Pressure control valves
• Temperature control valve
• Bearing supply header
• Return header
• Mist eliminator or demister
• Reservoir heaters
• Level control
• Local and remote instrumentation

Seal oil console

Usually includes:

• Seal oil reservoir
• Main and standby seal oil pumps
• Overhead tank or emergency seal oil supply, depending on design
• Seal oil coolers
• Duplex seal oil filters
• Differential pressure control valves
• Seal oil supply header
• Inboard leakage collection
• Outboard leakage collection
• Sour oil drainers or seal oil drain pots
• Degassing tank
• Gas vent or recovery system
• Level controls
• Pressure control instruments
• Differential pressure transmitters
• Alarms and trips

Wet seal systems often include sour oil drainers or pots to separate process-side seal oil leakage from process gas. Seal-oil degassing systems are also used to separate entrained or dissolved gas from the exiting seal oil before it returns to the tank or recovery system.

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4. The most important operating parameter: differential pressure

For wet seals, the most critical control parameter is not simply seal oil pressure. It is:

Seal oil pressure minus reference gas pressure

The seal oil must remain above the process gas pressure by the required differential pressure. Too low differential pressure can allow gas leakage. Too high differential pressure can increase oil leakage into the compressor, raise oil consumption, increase gas absorption into the oil, and overload drain pots or degassing systems.

This is why seal oil systems are highly dependent on:

• Accurate pressure transmitters
• Clean sensing lines
• Properly functioning differential pressure control valves
• Stable seal oil pump performance
• Correct restriction orifices
• Correct seal clearances
• Clean filters
• No blocked drain lines
• No stuck level control valves

A wet seal system with dirty oil can become a control problem, not only a lubrication problem.

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5. How the lube oil system should be maintained

The lube oil system should be treated as a rotating equipment reliability system, not just an oil tank.

A. Daily/shift checks

Operators should record:

• Reservoir level
• Oil supply pressure
• Bearing header pressure
• Oil supply temperature
• Bearing metal temperatures
• Differential pressure across filters
• Cooler inlet/outlet temperatures
• Pump status
• Oil mist or vapor from reservoir vent
• Abnormal foam in sight glass
• Return oil condition
• Water drain observations
• Any sudden color or odor change

B. Weekly checks

Maintenance/reliability team should review:

• Filter differential pressure trend
• Bearing temperature trend
• Oil cooler performance
• Water drain frequency
• Reservoir breathing condition
• Desiccant breather condition, if installed
• Cleanliness trend
• Top-up volume
• Oil consumption rate
• Any abnormal vibration/bearing temperature correlation

C. Monthly or routine laboratory testing

For a critical gas compressor lube oil system, routine tests should include:

Test	Why it matters
Viscosity at 40°C	Confirms correct oil grade and detects dilution or oxidation
Acid number by ASTM D664	Tracks acidic degradation products
Water by Karl Fischer	Detects dissolved/emulsified water before free water appears
Particle count ISO 4406	Critical for bearings and control components
MPC ASTM D7843	Measures varnish potential
RULER ASTM D6971	Measures antioxidant depletion
RPVOT ASTM D2272	Oxidation stability reserve
FTIR oxidation/nitration	Detects chemical degradation trend
Elemental analysis ICP	Wear metals, additive metals, contamination
Foam tendency/stability	Important for reservoir and bearing supply reliability
Air release	Important for high-circulation turbine/compressor oils
Demulsibility	Important if water ingress is possible
Membrane patch photo	Visual confirmation of varnish/sludge character

For critical compressors, MPC and RULER should not be treated as optional. A compressor oil can look visually clean and still have high varnish potential.

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6. How the seal oil system should be maintained

Seal oil maintenance must be more aggressive than normal lube oil maintenance because the oil is exposed to process gas.

A. Daily/shift checks

Operators should monitor:

• Seal oil supply pressure
• Seal oil differential pressure
• Reference gas pressure
• Seal oil temperature
• Seal oil filter differential pressure
• Seal oil pump status
• Seal oil tank level
• Seal oil leakage rate
• Inboard leakage rate
• Outboard leakage rate
• Drain pot level
• Degassing tank behavior
• Gas vent flow
• Abnormal foaming
• Abnormal smell
• Color change
• Unusual oil consumption

B. The most important seal oil trend: leakage rate

The seal oil leakage rate tells a story.

Increasing inboard leakage may indicate:

• Seal wear
• Excessive differential pressure
• Damaged seal rings
• Contaminated seal faces
• Varnish/sludge deposits
• Incorrect oil viscosity
• Process pressure instability
• Drain restriction
• Poor degassing

Increasing outboard leakage may indicate:

• Seal clearance issue
• Pressure imbalance
• Blocked return line
• Degraded oil
• Mechanical seal wear
• Foaming/air entrainment

C. Seal oil drain pots must not be ignored

Drain pots are often treated as “simple accessories,” but they are actually diagnostic points.

They can show:

• Excess gas carryover
• Water contamination
• Hydrocarbon condensate
• Sludge
• Black deposits
• Sticky varnish
• Foaming
• Abnormal level control behavior

Seal oil drain pots should be inspected, drained, and sampled under a controlled procedure. Poor drain pot management can return contaminated oil back into the seal oil reservoir.

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7. Oil analysis program for separate lube oil and seal oil systems

The biggest mistake is to test only the lube oil and ignore seal oil.

For these compressors, you need two oil analysis programs:

1. Lube oil analysis program
2. Seal oil analysis program

They must be trended separately.

Recommended sampling points

Lube oil system

• Main reservoir
• Bearing supply header, after filters
• Bearing return header
• Before and after kidney-loop filtration, if installed
• Bottom drain, if water/sludge is suspected

Seal oil system

• Seal oil reservoir
• Seal oil supply after filters
• Inboard seal oil leakage
• Outboard seal oil leakage
• Degassing tank outlet
• Drain pot sample
• Return oil before reservoir
• Bottom drain of seal oil tank

Why separate sampling matters

A reservoir sample alone can hide local problems. In wet-seal compressors, the most contaminated oil may be in the inboard leakage path, drain pot, or degassing section — not in the main reservoir sample.

A good diagnosis compares:

• Seal oil supply vs. seal oil return
• Inboard leakage vs. outboard leakage
• Seal oil reservoir vs. drain pot
• Lube oil reservoir vs. seal oil reservoir
• New oil reference vs. used oil

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8. Suggested testing for seal oil

Seal oil needs the normal turbine/compressor oil tests, plus additional contamination-focused tests.

Test	Importance for seal oil
Viscosity	Detects gas dilution, condensate contamination, wrong oil, oxidation
TAN ASTM D664	Tracks acidic contamination and oxidation
Water KF	Very important for wet gas and sour gas systems
Particle count	Protects seal clearances and control valves
MPC	Detects varnish potential from degraded seal oil
RULER	Tracks antioxidant depletion caused by gas/water/thermal stress
FTIR	Detects oxidation, nitration, hydrocarbon contamination, abnormal chemistry
RPVOT	Measures remaining oxidation stability
Foam	Critical because gas/oil contact can increase foaming
Air release	Critical because gas entrainment affects pressure control
Demulsibility	Critical if water/condensate is present
ICP metals	Wear, corrosion, additive trend, contamination
Membrane patch/photo	Shows sludge, varnish, carbon, corrosion debris
GC or hydrocarbon screening, where available	Helps detect process gas/condensate contamination
H₂S/sulfur-related checks, where applicable	Important in sour gas service

For sour gas or wet gas service, the seal oil sample may require special lab handling and safety precautions. The lab must know the oil is from a seal oil system exposed to process gas.

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9. Filtration best practice for the lube oil system

The lube oil system should normally have:

A. Full-flow pressure filters

These protect bearings and control components. They should be duplex type for online changeover.

Key points:

• Monitor differential pressure trend.
• Do not wait until filters are fully blocked.
• Use correct beta-rated elements.
• Avoid cheap elements with poor collapse strength.
• Avoid bypass operation.
• Investigate sudden filter DP increase.

B. Kidney-loop filtration

A kidney-loop unit should run continuously or periodically, depending on oil condition.

It should target:

• Fine particles
• Water
• Oxidation by-products
• Varnish potential
• Sludge precursors

C. Varnish removal technology

For varnish-prone compressor oils, mechanical filters alone are not enough. Mechanical filtration removes particles, but soluble varnish precursors can remain in the oil. Varnish control requires technology capable of removing dissolved and insoluble degradation products.

For critical compressors, the best practice is not only “clean oil” but:

chemically stable oil + low MPC + healthy antioxidants + low water + low particles

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10. Filtration best practice for the seal oil system

Seal oil filtration should be more conservative because contamination can affect sealing reliability and gas leakage.

A. Full-flow seal oil filters

These must protect:

• Seal rings
• Seal bushings
• Differential pressure control valves
• Orifices
• Small clearances
• Drain pot controls
• Level control valves

A small amount of sticky varnish or sludge can cause unstable seal oil control.

B. Offline conditioning

A seal oil reservoir should ideally have offline conditioning for:

• Particles
• Water
• Sludge
• Varnish precursors
• Acidic degradation products
• Dissolved/entrained gas management, where applicable

C. Degassing is not oil purification

This is an important point.

A degassing tank or gas separator removes gas from oil, but it does not fully restore oil health. It does not remove all oxidation products, acidic molecules, varnish precursors, or fine polar degradation products.

So the seal oil system may need:

• Degassing
• Water removal
• Fine filtration
• Varnish removal
• Acid control
• Oil analysis trending

Degassing alone is not oil reliability management.

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11. Why seal oil systems create more varnish risk

Wet-seal compressor seal oil is exposed to a harsher environment than normal bearing oil.

Mechanism 1: Process gas dissolves into the oil

In wet seals, high-pressure seal oil contacts the process gas. The oil can absorb gas on the inboard side. EPA wet seal documentation describes how high-pressure seal oil circulates around the compressor shaft and absorbs gas on the inboard side. (globalmethane.org)

When this oil returns to lower pressure zones, gas comes out of solution. This repeated absorption and release can cause:

• Foaming
• Microbubbles
• pressure instability
• oxidation acceleration
• additive stress
• oil film disturbance
• poor filtration performance
• varnish/sludge precipitation

Mechanism 2: Hydrocarbon condensate changes oil solvency

If light hydrocarbons or condensate enter the seal oil, the oil’s solvency balance can change.

This may cause:

• Viscosity reduction
• Additive dilution
• Reduced flash point
• Lower film strength
• Poorer varnish suspension
• Sudden precipitation of degradation products
• MPC increase or abnormal membrane patch appearance

Mechanism 3: Sour gas contamination accelerates degradation

In sour gas service, H₂S, CO₂, water, and corrosion products can create a very aggressive environment.

Possible effects:

• Acid formation
• Copper/iron corrosion
• Additive depletion
• Sludge formation
• Dark deposits
• Increased TAN
• Sticky polar degradation products
• Higher varnish potential

Mechanism 4: Seal oil runs through small clearances

Wet seals, seal rings, restriction orifices, and control valves have small clearances. These areas are ideal deposition points for varnish.

Varnish prefers:

• Low-flow zones
• Cooler surfaces
• Servo/control valve lands
• Orifices
• Seal faces
• Drain lines
• Dead legs
• Tank walls
• Heat exchanger surfaces

So even if the reservoir oil looks acceptable, deposits can form locally.

Mechanism 5: High local shear and aeration

Seal oil can experience:

• High differential pressure
• Turbulence
• gas flashing
• aeration
• high shear at restrictions
• rapid pressure reduction

This can increase oxidation stress and create insoluble degradation products.

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12. How seal oil problems can contaminate the lube oil system

Even when seal oil and lube oil consoles are separate, cross-contamination can still happen indirectly.

Possible routes include:

• Wrong oil transfer during top-up
• Shared portable filtration unit without proper flushing
• Shared hoses
• Shared oil drums or containers
• Maintenance error during outage
• Common drain handling
• Wrong valve lineup
• Leakage between systems in combined designs
• Human error during oil purification work

This is why seal oil and lube oil should have:

• Separate oil transfer carts
• Separate filtration hoses
• Separate sampling kits
• Separate labels
• Separate oil analysis reports
• Separate contamination limits
• Separate cleanliness targets
• Separate top-up records

A seal oil system exposed to process gas should never be treated like a clean bearing oil system.

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13. Varnish symptoms in wet-seal compressors

Typical symptoms include:

• Sticky seal rings
• Unstable seal oil differential pressure
• High seal oil filter differential pressure
• Frequent filter changeout
• Dark oil in drain pots
• Sticky deposits in level control valves
• Fouled differential pressure control valve
• Abnormal seal oil leakage
• Foaming in seal oil tank
• Slow gas release from oil
• High MPC
• Rapid antioxidant depletion
• TAN increase
• Dark membrane patch
• Brown/orange/black deposits
• Unexplained seal failures
• Higher bearing temperature if cross-contamination occurs
• Problems after shutdown and restart

The shutdown/restart issue is very important.

When the compressor is hot and running, some degradation products may remain dissolved in the oil. During shutdown, the oil cools down, solvency decreases, flow stops, and varnish can precipitate on seal surfaces or control components. On restart, the system may face sticky valves, restricted flow, unstable seal oil pressure, or seal problems.

This is why varnish in wet-seal compressor systems is often not only an “oil analysis number.” It becomes a start-up reliability risk.

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14. Alarm limits and action limits

Every plant should define its own limits based on OEM, oil supplier, historical trends, and criticality. But practically, the following logic is useful.

Lube oil warning signs

Parameter	Warning sign
Viscosity	±5% from new oil: investigate; ±10% serious
TAN	Increasing trend more important than single number
Water	Any rising trend is important; free water is unacceptable
ISO particle count	Sudden jump indicates ingress or filter bypass
MPC	Rising trend indicates varnish potential
RULER	Rapid antioxidant depletion indicates chemical stress
RPVOT	Falling reserve indicates reduced oxidation stability
Foam/air release	Any deterioration can affect bearing reliability

Seal oil warning signs

Parameter	Warning sign
Seal oil leakage	Increasing trend from baseline
Differential pressure	Instability or control hunting
Filter DP	Fast increase after filter change
Water	Repeated presence means ingress source not controlled
Viscosity	Reduction may indicate gas/condensate dilution
TAN	Increase may indicate sour gas/water/oxidation stress
MPC	Any rising trend is serious because seal clearances are sensitive
Foam/air release	Critical because gas exposure is inherent
Drain pot sludge	Strong warning sign
Dark/sticky deposits	Varnish/sludge risk

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15. Best-practice maintenance philosophy

For this type of compressor, the maintenance philosophy should be:

For lube oil

Maintain oil for bearing life, heat removal, and varnish-free operation.

Focus on:

• Cleanliness
• Dryness
• oxidation stability
• antioxidant health
• varnish potential
• correct viscosity
• no cross-contamination

For seal oil

Maintain oil for sealing reliability, gas safety, and deposit-free control.

Focus on:

• Differential pressure stability
• gas release
• water/condensate removal
• drain pot health
• sludge/varnish control
• filtration
• clean control valves
• oil chemistry stability

The seal oil system should be treated almost like a process-contaminated oil system, not just a lubrication system.

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16. Recommended field inspection checklist

Lube oil console

Check:

• Reservoir level and condition
• Oil color and clarity
• Foam on return
• Water at bottom drain
• Filter DP
• Cooler performance
• Pump vibration/noise
• Bearing supply temperature
• Bearing return temperature
• Bearing metal temperature trend
• Breather condition
• Reservoir vent mist
• Oil top-up history
• Oil sample trend

Seal oil console

Check:

• Seal oil tank level
• Seal oil supply pressure
• Reference gas pressure
• Differential pressure
• Seal oil temperature
• Filter DP
• Pump status
• Inboard leakage
• Outboard leakage
• Drain pot level
• Drain pot sludge/water
• Degassing tank performance
• Vent line condition
• Foam
• Abnormal odor
• Gas carryover
• Oil consumption
• Seal oil sample trend

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17. Filtration and purification recommendations

For critical wet-seal gas compressors, I would not rely on one technology only.

Minimum expectation

• High-quality full-flow filters
• Proper beta rating
• Duplex arrangement
• Clean oil transfer procedure
• Routine reservoir cleaning
• Correct sampling points
• Trend-based oil analysis

Better practice

• Dedicated offline kidney-loop filtration for lube oil
• Dedicated offline conditioning for seal oil
• Water removal capability
• Fine particle removal
• Varnish removal capability
• Acid/degradation product control
• Separate equipment for seal oil and lube oil

Best practice

• Permanent oil conditioning on both consoles
• MPC/RULER/RPVOT/FTIR-based oil health program
• Seal oil-specific contamination tracking
• Drain pot sampling
• Root cause analysis for every abnormal seal oil trend
• Written procedure for top-up, filtration, sampling, and oil transfer
• No sharing of hoses or carts between contaminated seal oil and lube oil service unless fully flushed and verified

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18. Why wet-seal compressor oils need stronger varnish management

Wet-seal compressor systems are varnish-sensitive because they combine all the bad actors:

• Hot oil
• High pressure
• Process gas exposure
• gas absorption and release
• aeration
• water risk
• condensate risk
• small clearances
• control valves
• drain pots
• dead legs
• shutdown/startup temperature cycling
• low-flow zones
• potential sour gas contamination

This is why two compressors using the same oil brand can behave very differently:

• A normal bearing lube oil system may remain stable.
• A wet-seal oil system may degrade faster because the oil is in direct contact with process gas.

The oil is not failing only because of the lubricant brand. It may be failing because of the application severity.

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19. Practical Khash conclusion

A gas compressor with separate lube oil and seal oil consoles should not be managed as “one oil system with two tanks.”

It is better to think like this:

The lube oil console is a bearing reliability system.
The seal oil console is a process-contaminated sealing reliability system.

For the lube oil system, the main goal is to protect bearings and prevent varnish-related temperature, trip, and reliability issues.

For the seal oil system, the main goal is to keep the wet seal clean, stable, gas-tight, and free from sticky deposits.

The biggest mistake is to test only viscosity, TAN, water, and particles. That is not enough for critical wet-seal compressors.

A serious oil analysis program must include:

• MPC
• RULER
• RPVOT
• FTIR
• particle count
• water by Karl Fischer
• viscosity
• TAN
• foam
• air release
• demulsibility
• membrane patch photo
• separate drain pot and seal oil return samples

And the biggest reliability lesson is this:

Seal oil contamination can become varnish.
Varnish can become seal control instability.
Seal instability can become compressor availability risk.

For wet-seal gas compressors, clean oil is not enough. The oil must be chemically controlled, gas-managed, water-controlled, varnish-controlled, and tested from the right sampling points.