Turbine Oil Life Assessment in a 7-Turbine Power Plant
A 10-Year Data Analysis Using TAN Trend (ASTM D4378), Antioxidant Depletion, and MPC
1. Objective
This study evaluates the remaining turbine oil life across:
- 5 Gas Turbines (GT-1 to GT-5)
- 2 Steam Turbines (ST-1, ST-2)
over a 10-year period, using three key condition indicators:
- Acid Number (TAN) trend (interpreted per ASTM D4378)
- Antioxidant depletion (AO %)
- MPC varnish potential
The analysis is strictly based on:
- measured values,
- calculated rates,
- and remaining-life projections
2. Baseline and Input Data
Baseline (Year 0 after 24h circulation)
- TAN = 0.08 mg KOH/g
- Antioxidant = 100%
- MPC = 5
Year-10 Data
| Unit | Type | TAN | TAN increase | TAN slope | AO % | MPC |
|---|---|---|---|---|---|---|
| GT-1 | GT | 0.27 | 0.19 | 0.019 | 61 | 16 |
| GT-2 | GT | 0.34 | 0.26 | 0.026 | 49 | 23 |
| GT-3 | GT | 0.43 | 0.35 | 0.035 | 34 | 29 |
| GT-4 | GT | 0.52 | 0.44 | 0.044 | 19 | 36 |
| GT-5 | GT | 0.22 | 0.14 | 0.014 | 72 | 12 |
| ST-1 | ST | 0.23 | 0.15 | 0.015 | 68 | 13 |
| ST-2 | ST | 0.31 | 0.23 | 0.023 | 52 | 19 |
3. Calculation Methodology
3.1 TAN Slope
TAN slope per year =
(Current TAN − Baseline TAN) ÷ 10 years
3.2 Antioxidant Depletion Rate
AO depletion per year =
(100 − Current AO %) ÷ 10
3.3 MPC Growth Rate
MPC growth per year =
(Current MPC − 5) ÷ 10
3.4 Remaining Life Based on Antioxidant
AO remaining life =
(Current AO % − 25) ÷ AO depletion per year
3.5 Remaining Life Based on MPC
MPC remaining life =
(30 − Current MPC) ÷ MPC growth per year
3.6 Final Remaining Life
Remaining life =
Minimum of (AO remaining life, MPC remaining life)
4. Derived Results
| Unit | AO loss/yr | MPC rise/yr | AO life (yrs) | MPC life (yrs) | Remaining Life |
|---|---|---|---|---|---|
| GT-5 | 2.8 | 0.7 | 16.8 | 25.7 | 16.8 |
| ST-1 | 3.2 | 0.8 | 13.4 | 21.3 | 13.4 |
| GT-1 | 3.9 | 1.1 | 9.2 | 12.7 | 9.2 |
| ST-2 | 4.8 | 1.4 | 5.6 | 7.9 | 5.6 |
| GT-2 | 5.1 | 1.8 | 4.7 | 3.9 | 3.9 |
| GT-3 | 6.6 | 2.4 | 1.4 | 0.4 | 0.4 |
| GT-4 | 8.1 | 3.1 | <0 | <0 | 0 |
5. Acid Number Trend Analysis (ASTM D4378 Context)
ASTM D4378 uses trend-based interpretation:
- Warning threshold ≈ 0.10 mg KOH/g/year
- Severe ≈ 0.15 mg KOH/g/year
Observed TAN slopes:
- Range = 0.014 to 0.044
Interpretation:
- All units are below ASTM D4378 warning threshold
- No unit shows abnormal TAN acceleration
- Relative differences exist:
- GT-4 highest
- GT-5 lowest
6. Antioxidant Depletion Analysis
Ranking (best to worst):
- GT-5 (72%)
- ST-1 (68%)
- GT-1 (61%)
- ST-2 (52%)
- GT-2 (49%)
- GT-3 (34%)
- GT-4 (19%)
Observations:
- Gas turbines show higher depletion rates
- GT-3 and GT-4 show accelerated consumption
7. MPC Analysis
Ranking (best to worst):
- GT-5 (12)
- ST-1 (13)
- GT-1 (16)
- ST-2 (19)
- GT-2 (23)
- GT-3 (29)
- GT-4 (36)
Threshold comparison:
- MPC >30 → exceeded by GT-4
- MPC near 30 → GT-3
- MPC 20–30 → GT-2, ST-2
8. Cross-Parameter Relationships
8.1 TAN vs AO
- Higher TAN slope corresponds to lower AO
- Indicates consistent oxidation progression
8.2 AO vs MPC
- Strong inverse relationship
- Lower AO → higher MPC
8.3 TAN vs MPC
- Moderate correlation
- MPC increases faster than TAN
9. Remaining Life Ranking
Best → Worst
- GT-5 → 16.8 years
- ST-1 → 13.4 years
- GT-1 → 9.2 years
- ST-2 → 5.6 years
- GT-2 → 3.9 years
- GT-3 → 0.4 years
- GT-4 → 0 years
10. Distribution of Limiting Parameter
| Unit | Limiting Factor |
|---|---|
| GT-5 | AO |
| ST-1 | AO |
| GT-1 | AO |
| ST-2 | AO |
| GT-2 | MPC |
| GT-3 | MPC |
| GT-4 | AO + MPC |
11. Statistical Observations
11.1 Spread
- AO range: 19% to 72%
- MPC range: 12 to 36
- TAN slope range: 0.014 to 0.044
11.2 Turbine Type Effect
- Gas turbines:
- higher TAN slope
- higher AO depletion
- higher MPC
- Steam turbines:
- lower degradation rates
11.3 Limiting Mechanism Distribution
- AO-driven: 4 units
- MPC-driven: 2 units
- Combined: 1 unit
12. Final Data-Based Conclusions
- TAN trend remains within ASTM D4378 acceptable range across all units
- Antioxidant depletion shows clear degradation hierarchy
- MPC differentiates high-risk units
- AO and MPC show strong inverse correlation
- MPC becomes limiting before TAN
- Gas turbines degrade faster than steam turbines
- Remaining life varies significantly despite identical baseline
Final Statement
The dataset shows that:
- TAN trend reflects oxidation progression
- Antioxidant depletion governs chemical life
- MPC governs operational instability
And the remaining life is controlled by the earliest limiting parameter between antioxidant depletion and MPC escalation.
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