Dry Gas Seals vs. Wet Seals in Centrifugal Compressors — The Critical Role of Turbine (Lube) Oil
1. How Lube Oil Interacts with Seal Systems (All Designs)
In centrifugal compressors, the seal system is not isolated—it is deeply integrated with the lubrication system, especially in configurations where lube oil and seal oil share common reservoirs or conditioning systems. Even in modern “dry” systems, indirect dependencies remain.
A. Wet Seal Systems (Oil-Flooded / Oil Film Seals)
Wet seals rely directly on turbine oil as a functional sealing medium:
- Hydrodynamic sealing principle: Oil is injected at a pressure higher than process gas → creates a barrier film preventing gas leakage.
- Seal oil console (per API 614):
- Pumps (main + standby)
- Pressure control valves
- Degassing drum
- Filters and coolers
- Critical parameters:
- Seal oil pressure margin above gas pressure (~1.5–2 bar)
- Oil viscosity stability (ISO VG 32/46 typical)
- Cleanliness (ISO 4406 typically ≤ 16/14/11 or tighter)
- Air release and foaming characteristics
👉 In these systems, lube oil = sealing medium = hydraulic control fluid.
B. Dry Gas Seal (DGS) Systems
Dry gas seals are designed to operate without oil in the sealing interface, but lube oil still plays indirect roles:
- Separation seals (labyrinth or carbon ring):
- Prevent lube oil migration into the dry gas seal cavity
- Use buffer gas (usually nitrogen or air)
- Bearing housing proximity:
- Seal cartridges are adjacent to oil-lubricated bearings
- Pressure balance is critical to avoid oil ingress
- Shared systems (in many legacy or integrated designs):
- Common reservoirs for lube oil and seal oil (especially retrofits)
- Contamination pathways via vents, drains, or degassing systems
👉 In DGS: oil is not the sealing medium, but it is a contamination threat and a system dependency.
2. Failure Pathways: How Lube Oil Affects Seal Integrity
Across both seal types, turbine oil influences sealing through:
1. Pressure Control Stability
- Seal oil pressure control valves (wet seals) or buffer gas regulators (DGS separation seals) depend on clean, varnish-free actuation
- Instability → loss of differential pressure → gas leakage or oil ingress
2. Cleanliness & Particle Transport
- Sub-micron varnish + hard particles:
- Scratch seal faces (DGS)
- Erode control valve internals
- Plug orifices and restrictors
3. Thermal Behavior
- Oil degradation → increased TAN → reduced heat transfer efficiency
- Elevated temperatures at seal faces accelerate:
- Carbon ring wear (wet seals)
- Face distortion (DGS)
4. Air Release & Foaming
- Poor air release → entrained air in seal oil → compressibility → unstable film
- Foam → cavitation in seal oil pumps → pressure fluctuations
3. Varnish Mechanisms and Their Impact on Seal Performance
A. Varnish in Wet Seal Systems
Wet seals are extremely sensitive to varnish because they depend on precision hydraulic control.
1. Control Valve Stiction
- Varnish deposits on spool and sleeve surfaces
- Leads to:
- Delayed response
- Hysteresis in pressure control
- Oscillating seal oil pressure
➡ Result: Loss of pressure margin → gas leakage or seal collapse
2. Restriction of Orifices & Flow Paths
- Varnish + insolubles deposit in:
- Restrictors
- Flow control valves
- Degassing lines
➡ Result:
- Reduced seal oil flow
- Inadequate cooling
- Local overheating → accelerated oil degradation (feedback loop)
3. Filter Plugging & ΔP Instability
- Varnish causes:
- Rapid differential pressure increase across filters
- Frequent bypass activation
➡ Result:
- Unfiltered oil reaching seals → wear + erosion
4. Degassing Inefficiency
- Varnish alters oil surface tension
- Reduces gas separation efficiency in seal oil tanks
➡ Result:
- Gas entrainment → unstable oil film → seal leakage
B. Varnish Impact on Dry Gas Seals
Even though DGS should be oil-free, varnish introduces catastrophic contamination risks.
1. Oil Migration Through Separation Seals
- Varnished oil → increased viscosity + tackiness
- Easier adherence to shaft and migration past labyrinths
➡ Result:
- Oil reaches DGS faces
- Carbonization under high temperature
2. Face Contamination & Loss of Gas Film
- Dry gas seals rely on micron-level gas film (non-contact operation)
- Oil/varnish deposits:
- Block spiral grooves
- Disrupt lift-off mechanism
➡ Result:
- Face contact → rapid wear → seal failure
3. Fouling of Secondary Seals (O-rings)
- Varnish deposits reduce elasticity and mobility
➡ Result:
- Loss of axial flexibility
- Inability to accommodate shaft movement
4. Instrumentation and Gas Control Issues
- Varnish affects:
- Pressure transmitters (plugging impulse lines)
- Flow control valves for barrier gas
➡ Result:
- Incorrect readings → improper seal gas control → failure risk
4. Cross-System Risks (Critical in Real Plants)
In many installations:
- Common oil reservoir for lube + seal oil
- Shared filtration systems
- Thermal degradation zones (bearings) feeding seal systems
👉 This creates a systemic varnish loop:
- Oxidation → varnish precursors
- Transport through system
- Deposition in cool zones (valves, seals)
- Functional failure → temperature rise → more oxidation
5. Practical Engineering Insights (Field Reality)
- A compressor trip due to dry gas seal failure is often wrongly attributed to “seal design,” while the root cause is:
- Oil contamination
- Varnish-driven control instability
- Wet seal systems with:
- Stable TAN but high MPC are at high risk
- Because soluble varnish affects control before TAN rises significantly
- In DGS systems:
- Even ppm-level oil carryover is unacceptable
- Varnish makes this carryover more adhesive and destructive
6. Engineering Conclusion
- Wet seals: Oil is the sealing medium → varnish directly destroys hydraulic stability
- Dry gas seals: Oil is a contaminant → varnish enables migration and adhesion → catastrophic face failure
👉 In both cases, turbine oil is not just a lubricant—it is a reliability driver for seal integrity.
7. Strategic Reliability Message
- You do NOT solve seal reliability by:
- Changing seals frequently
- Adjusting pressures blindly
- You solve it by:
- Controlling oil chemistry (acid + varnish precursors)
- Maintaining ultra-clean oil
- Managing temperature and oxidation
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