Acid Number (TAN) in Turbine Oils – Everything You Need to Know

1. What Acid Number (TAN) Really Represents

The Total Acid Number (TAN) is the amount of potassium hydroxide (KOH) required to neutralize acidic species in oil, expressed in mg KOH/g.

From a turbomachinery perspective, TAN is not just acidity—it is a proxy for oxidation progression, additive depletion, and varnish precursor formation.

Key mechanisms generating TAN:

• Oxidation → organic acids, peroxides
• Hydrolysis (especially in synthetics)
• Additive degradation products
• Contamination (water, gases, combustion products)

👉 TAN is therefore a lagging indicator of degradation, but still critical when interpreted correctly.

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2. Acid Number of NEW Turbine Oils (Group I → IV + EP Oils)

2.1 Base Oil Chemistry Impact on TAN

New oil TAN is not zero. It depends on:

• Base oil polarity
• Additive package chemistry
• Refining severity

Typical Ranges (Practical + Field-Based)

Oil Type	Typical New TAN (mg KOH/g)	Key Reason
Group I (Solvent refined)	0.05 – 0.20	More polar compounds, residual aromatics
Group II	0.02 – 0.10	Cleaner, lower sulfur/aromatics
Group III	0.01 – 0.05	Highly hydroprocessed, very low polarity
Group IV (PAO)	~0.00 – 0.03	Essentially non-polar base
Ester-containing (Group V blends)	0.05 – 0.20	Ester polarity + hydrolysis susceptibility
EP Turbine Oils (if used)	0.5 – 2.0	Sulfur-phosphorus additives contribute acidity (Bureau Veritas)

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Critical Insight (Your Style):

👉 The cleaner the base oil (Group II/III/IV)
➡ the lower the starting TAN
➡ BUT the higher sensitivity to oxidation-induced TAN increase

This is why:

• Group II/III oils show faster TAN rise trend once antioxidants are depleted
• Group I oils show slower but masked degradation

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3. TAN in USED Turbine Oil – What It Really Means

3.1 TAN Is Measuring More Than “Acids”

TAN captures:

• Organic acids (oxidation)
• Weak acidic oxidation intermediates
• Additive degradation by-products
• Some varnish precursors

👉 It is not selective—it measures all acidic species together.

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3.2 TAN vs Oxidation vs Varnish

Sequence in turbine oils:

1. Antioxidants active → TAN stable
2. Antioxidants depleting (RULER drop)
3. Peroxides + soluble varnish → slight TAN increase
4. Rapid oxidation → TAN accelerates
5. Insoluble varnish + deposits form

👉 TAN increase is already late-stage chemistry in many systems.

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4. TAN and Remaining Antioxidants (Critical Relationship)

From a chemistry standpoint:

• Antioxidants (phenols, amines) suppress oxidation
• Once depleted:
  • Free radicals form
  • Oxidation accelerates
  • Acidic compounds increase sharply (wearcheck.com)

Correlation in Practice

Parameter	Early Stage	Mid Stage	Late Stage
RULER	High	Dropping	Low
MPC	Low	Increasing	High
TAN	Stable	Slight rise	Rapid increase

👉 Key message:
TAN increase = confirmation of oxidation AFTER antioxidant depletion

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Your Strong Position (Correct):

TAN alone is NOT predictive → it is confirmatory.
RULER + MPC + TAN together = full chemistry picture.

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5. ASTM D4378 & OEM Philosophy on TAN Limits

ASTM D4378

This standard clearly states:

• TAN must be trended, not judged as absolute
• A baseline from new oil is mandatory (machinerylubrication.com)

Typical Industry/OEM Limits:

Criterion	Limit
Absolute TAN	~1.0 mg KOH/g (typical guideline)
Increase from new oil	+0.2 to +0.5 mg KOH/g
OEM-specific	Often stricter (0.3 increase)

👉 Many OEMs (GE, Siemens, etc.) prefer:

• ΔTAN (increase) rather than absolute value

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Why?

Because:

• Additive chemistry changes baseline
• Different base oils behave differently
• Absolute TAN alone is misleading

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6. Why ASTM D664 Is Recommended Over ASTM D974

ASTM D664

ASTM D974

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6.1 Methodology Difference

Aspect	ASTM D664	ASTM D974
Detection	Potentiometric (electrode)	Color change
Endpoint	Objective (electrical)	Subjective (visual)
Accuracy	High	Moderate
Repeatability	Excellent	Operator-dependent

D664 uses electrical potential change to detect endpoint, eliminating human interpretation

D974 relies on color change (orange → green), which is operator-dependent

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6.2 Why D974 Is Problematic for Turbine Oils

1. Color Interference

• Dark oil (oxidized, varnished) → endpoint unclear
• Additives interfere with indicator

2. Weak Acids Not Fully Detected

• D974 misses weak organic acids
• Underestimates degradation

3. Human Error

• Operator judgment varies

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6.3 Why D664 Is Preferred (Critical Points)

• Detects all acidic species (strong + weak)
• Works well for modern Group II/III oils
• Essential for low TAN oils (<0.1)
• High repeatability for trending

👉 This is why:

• OEMs
• Laboratories
• Critical industries

➡ Standardize on D664 for turbine oils

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7. Critical Interpretation Mistakes (You Should Highlight)

❌ Mistake 1: Using Absolute TAN Only

→ Always compare with new oil baseline

❌ Mistake 2: Ignoring Antioxidant Status

→ TAN alone misses early degradation

❌ Mistake 3: Comparing Different Oils

→ TAN is formulation-dependent

❌ Mistake 4: Using D974 for Modern Oils

→ Leads to false confidence

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8. Advanced Practical Insights (Khash-Level)

🔥 1. TAN Increase vs Varnish

• TAN rise correlates with soluble varnish formation
• By the time TAN rises → varnish already forming

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🔥 2. Hot vs Cold Sampling

• Hot oil → captures soluble degradation products
• Cold oil → underestimates TAN trend

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🔥 3. TAN Plateau Misinterpretation

• TAN may plateau while varnish continues forming
  → because acids convert to insoluble deposits

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🔥 4. Group II/III Oils Trap

• Low starting TAN → gives false sense of stability
• But oxidation can accelerate rapidly after depletion

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9. Final Engineering Summary

• TAN measures total acidic degradation products
• It is:
  • Essential for confirmation
  • Not sufficient for prediction

Correct approach:

• TAN (D664) → confirm oxidation
• RULER → antioxidant life
• MPC → varnish potential

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Closing Statement

If TAN is increasing, the oil is already chemically compromised.
If you wait for TAN to rise, you are already late.
The real control point is before TAN moves — at antioxidant depletion and soluble varnish formation.

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