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Lubricant Varnishing and Varnish Mitigation
Matthew G. Hobbs, Ph.D. [email protected] John Evans, C.Eng. M.I.Mech.E. [email protected] Peter T. Dufresne Jr. [email protected] Aaron Hoeg [email protected]
LUBMAT 2014
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Who Has Varnish Problems?
Systems/Industries affected: • Turbines, compressors, Pulp & Paper, Al/Steel Mills. GE Technical Information Letter: • All users are expected to have varnish-related
problems over time. Exxon Mobil Survey of 192 US Power Plants: • 40% reported recent varnish-related problems.
Source: Exxon Mobil, General Electric
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Varnish Hurts Your Bottom Line
Varnish detrimental to equipment reliability: • Decreased clearances. • Increased wear. • Filter plugging and restricted oil flow. • Poor heater/cooler performance. • Valve sticking and unit trips. Unit trips result in down-time and are especially costly: • Typical single cycle GT produces: 100 – 400 MW. • Over 24 hours: 9,600 MW hours. • At $50/MW hour, outage costs you: $ 480,000/day.
€ 350,000/day. £ 286,000/day.
Source: General Electric
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Varnish Defined
Varnish, noun: • “A thin, hard, lustrous, oil-insoluble deposit,
composed primarily of organic residue.” • “It is not easily removed by wiping” and “is resistant
to saturated solvents.” Formed by thermal/oxidative breakdown of lubricant. • Chemical change – irreversible and unavoidable.
Source: ASTM D7483-12
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Varnish REDefined
Varnish, noun: • “A thin, hard, lustrous, oil-insoluble deposit,
composed primarily of organic residue.” • “It is not easily removed by wiping” and “is resistant
to saturated solvents.”
• Varnish begins as an oil-soluble degradation product. • Soluble varnish converts to insoluble deposits
• This conversion is physical and reversible.
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Forms of Varnish
Varnish exists in 2 forms: • Soluble (dissolved). • Insoluble (suspended or deposited). Dynamic Equilibrium. Forms constantly interconvert. • Dependent on balance between 2 varnish states. No net change under constant conditions: • Every equivalent of soluble varnish which becomes
insoluble is balanced by an equivalent of insoluble varnish which dissolves.
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Equilibrium All systems strive to reach balance/equilibrium. Le Chatellier’s Principle: Changing conditions upset balance and shift equilibrium. • Everyday example: reversible phase change - evaporation of water.
Condition change: new dynamic equilibrium featuring more vapor and less liquid. • Equilibrium shifts to favor vapor state at high temperature.
Room Temperature (20°C) ↑ Temperature New Temperature (60°C)
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Temperature and Phase Changes Matter exists in 3 phases/states: • Solid: most condensed, most interaction between neighbors. • Liquid: intermediary. • Gas: least condensed, least interaction between neighbors. Water example: • Less condensed phase (gas) favored as temperature ↑. • ↑ temperature, ↑ energy of water molecules: disrupts
interactions between neighboring molecules. • Less interaction, molecules move apart: favors disperse state. Equilibrium between 2 phases will always shift to favor more disperse state when temperature ↑. • Reversibility: as temperature ↓, equilibrium shifts to favor
more condensed state.
Like other equilibria, that between soluble and insoluble varnish is temperature-dependent. Gas Turbine Application: • Fluid is hot at the bearing – favors disperse soluble varnish state. • Fluid cools elsewhere – shifts equilibrium to favor insoluble varnish.
Peaking plants especially susceptible: whole system allowed to cool.
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Varnish Phase Equilibrium
Bearing Temperature (120°C) ↓ Temperature Reservoir Temperature (40°C)
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Equilibrium and the Varnish Cycle Varnish life cycle: 1. Degradation begins: soluble varnish formed – equilibrium established.
Degradation continues: soluble varnish levels ↑. • Equilibrium shifts – soluble varnish more likely to convert to insoluble form.
2. Beyond saturation point: varnish no longer soluble – particles and deposits. • Reversible physical change (cf. evaporation of water).
Varnish Levels
Saturation Point
Step 1: Soluble Varnish
Step 2: Insoluble
Varnish
Step 3: Varnish Deposit
Chemical Change
Physical Change
Physical Change Lubricant
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Fighting Back: Varnish Mitigation
Strategies:
1) Particle Removal Systems 2) Soluble Varnish Removal
(SVR) Systems
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Particle Removal Systems
Examples: • Conventional Filtration. • Depth Media Filtration. • Electrostatic Filtration (ECR™, EOC™, BCA™ etc.). Methods rely upon the removal of insoluble varnish.
• Well-suited to cooler applications. • Relatively poorly suited to hot applications:
• Gas Turbines. • Steam Turbines.
• Cannot remove soluble varnish.
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Fighting Back: Varnish Mitigation
• Even if insoluble varnish is removed, soluble varnish will continue to build up. • As long as soluble varnish is present, damaging particles/deposits can still form.
The key to successful varnish mitigation: • Remove soluble varnish before harmful deposits and varnish particles form.
Varnish Levels
Saturation Point
Step 1: Soluble Varnish
Step 2: Insoluble
Varnish
Step 3: Varnish Deposit
Chemical Change
Physical Change
Physical Change
Lubricant
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Soluble Varnish Removal (SVR) Systems
• Makes use of specialized polar media (ICB™ etc.). • Polar media attracts and removes soluble varnish
from oil on the molecular level. • Effective at operating temperatures. • Removes soluble varnish feedstock so that varnish
deposits are not produced during outages or cool down periods.
Image Source: Argonne National Laboratory
SVR removes soluble and insoluble varnish by positive feedback: • Balance between varnish states upset – equilibrium shifts to compensate. • Insoluble varnish dissolves to restore balance. • Newly dissolved varnish continually removed – equilibrium continues to shift.
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Exploiting the Varnish Equilibrium
Soluble varnish removed by SVR:
Balance between varnish states upset.
Normal varnish equilibrium:
No net change
Deposited varnish dissolves to restore
balance: New varnish equilibrium
Soluble varnish removed by SVR:
Balance between varnish states upset.
Deposited varnish dissolves to restore
balance: New varnish equilibrium.
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SVR Case Study
0
10
20
30
40
50
60
MPC
ΔE
Sample Date
Trending MPC Data – GT Lubricant Reservoir
SVR Installed High Risk of
Varnish-Related Failure.
Target
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Equilibrium in Action: SVR Demonstration
• Left: untreated.
• Right: treated with SVR
>50°C: Soluble Varnish
<20°C: Insoluble
Varnish
Oil sample: no varnish apparent at operating temperature. Sample split in half:
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Summary
• Current varnish definition ignores the fact that varnish exists in 2 distinct forms: • Soluble varnish. • Insoluble varnish.
• Soluble and insoluble varnish exist in dynamic equilibrium with one another. • Changing conditions (temperature, varnish levels etc.) upset balance between
soluble and insoluble varnish. • Equilibrium will shift to restore balance.
• Particle removal systems effective in cooler applications where equilibrium favors insoluble varnish state.
• Ineffective for hot applications – cannot remove soluble varnish which will continue to deposit out.
• SVR systems remove soluble varnish as it forms at operating temperatures. • SVR shifts varnish equilibrium – previously deposited varnish removed as well.
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Matthew G. Hobbs, Ph.D. [email protected]
John Evans, C.Eng. M.I.Mech.E. [email protected]
Thank you!
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Varnish: Polarity Solubility is dependent upon the molecular polarity.
Polarity, noun: • Distribution of charge within a molecule.
Non-Polar: • Little or no separation of charge.
Polar: • Molecular charges well-separated.
Image Source: Molfield.org
Basic axiom: “Like dissolves like.”
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Solubility: Polarity Basic axiom: “Like dissolves like.”
• Non-polar molecules will dissolve readily in other non-polar molecules.
• Example: gasoline in diesel. • Polar molecules will dissolve readily in other polar molecules.
• Example: sugar in water. • Polar molecules will not dissolve appreciably in non-polar molecules.
• Example: water in oil. Mineral oil base stocks are non-polar but breakdown to form polar products. • These polar degradation products are the precursors to varnish. • Polar degradation products prefer polar metal surfaces to non-polar base stock. • Group II base stocks are more strongly non-polar than the Group I oils that they
have replaced. • Varnish is less soluble in Group II oils than older Group I oils.
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Turbine Lubricant Formulation
1) Base Stock Selection Group I: • Solvent refined. • Contains polar aromatics. • Superior solvency – keeps polar
varnish dissolved. Group II/III: • Hydrotreated/hydrocracked. • No polar aromatics. • Superior oxidation resistance at the
cost of varnish solvency.
95 – 99.5% Base Stock 0.5 – 5% Antioxidant
Additive
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Turbine Lubricant Formulation
2) Antioxidant Additives Antioxidants: • Synergistic mixture of amines and
phenols. • Oxidize preferentially to base stock. • Sacrificial: consumed as they
protect the lubricant. • Delays oxidation; cannot prevent it. • May react with degraded base
stock to produce varnish deposits.
95 – 99.5% Base Stock 0.5 – 5% Antioxidant
Additive
NR R
HNR R
ROO
ROOH
. .
HOH2C OH
OH2C OH.
Peroxy FreeRadical
StableHydroperoxide Amine Antioxidant (e.g. NDPA)
Phenol Antioxidant (e.g. MBDTBP)
Amine Radical
Phenol Radical - STABLE
