How to deal with transformer oil discoloration?

During use, transformer oil may become darker in color (such as yellow or black) due to oxidation, deterioration, or contamination, affecting its insulation performance and life. Decolorization treatment needs to ｓｅｌｅｃｔ appropriate methods according to the cause of discoloration. The following is a systematic treatment plan:

Common causes of transformer oil discoloration

| Color change | Possible causes | Treatment focus |

|--------------|--------------|--------------|

| Dark yellow/amber | Slight oxidation, aging | Adsorption treatment or regeneration |

| Reddish brown/brown | Deep oxidation, increased acid value | White clay adsorption or sulfuric acid regeneration |

| Black | Carbide pollution, arc decomposition | Filtration + adsorption or replacement of new oil |

| Milky white/turbid | Water or particle pollution | Dehydration first, then filtration |

Transformer oil decolorization treatment method

1. Adsorption decolorization method (commonly used)

Principle: Use adsorbents (such as activated white clay, silica gel, activated bauxite) to adsorb pigments, oxidation products and polar compounds in the oil.

Operation steps:

1. Pretreatment:

- Remove water and impurities by vacuum dehydration or centrifugal separation (to avoid clogging the adsorbent).

2. Adsorption decolorization:

- Heat the oil to 50~70℃ (to improve adsorption efficiency).

- Add activated clay or activated alumina at 3%~10% of the oil weight and stir for 30~60 minutes.

- Filter and separate the adsorbent (using a plate and frame filter press or precision filter element).

3. Post-treatment:

- Test the color (ASTM D1500), acid value (GB/T 264) and dielectric loss (tanδ) of the oil.

Applicable scenarios:

- Mild to moderate discoloration (such as yellow-brown oil caused by oxidation).

- Activated clay works best, but trace ash may remain and requires fine filtration.

Sulfuric acid-white clay regeneration method (deep decolorization)

Principle: Sulfuric acid can remove colloids, asphaltene and dark oxides in oil, and white clay further adsorbs residual impurities.

Operation steps:

1. Sulfuric acid treatment:

- Slowly add 1%~3% concentrated sulfuric acid (H₂SO₄) at 20~30℃, stir and let stand to precipitate acid residue.

2. Neutralization and washing:

- Neutralize acidic oil with sodium hydroxide (NaOH) or sodium carbonate (Na₂CO₃), and then wash with hot water until neutral.

3. White clay adsorption:

- Add 5%~10% activated white clay, heat to 80~90℃, stir and filter.

Note:

- The sulfuric acid method may affect additives in oil, so be careful.

- Suitable for oil that is severely oxidized and blackened, but it has high environmental protection requirements and is now mostly replaced by alternative technologies.

Solvent extraction method

Principle: Use solvents (such as propane, furfural) to selectively dissolve pigments and oxidation products in oil.

Operation steps:

1. Mix the oil and solvent in proportion, and recover the solvent after separation.

2. The solvent needs to be distilled and regenerated, and the residual solvent needs to be completely removed.

Advantages: Good decolorization effect, suitable for industrial treatment.

Disadvantages: Complex equipment, flammable and explosive solvents.

Hydrogenation treatment (hydrogenation refining)

Principle: Hydrogen is introduced under high temperature and pressure, and the oxides are reduced to harmless components through catalysts (such as nickel and molybdenum).

Applicable scenarios:

- Deeply degraded oil (such as high acid value and dark black color).

- Professional equipment is required, usually carried out in refineries.

Combined process (recommended)

For severely contaminated oil, multi-step combined treatment can be used:

1. Pretreatment: Centrifugal separation or filtration to remove particles and moisture.

2. Adsorption decolorization: activated clay treatment.

3. Supplementary refining, such as hydrogenation or solvent extraction.

Precautions

1. Adsorbent selection:

- Activated clay: good decolorization effect, but the amount added needs to be controlled (too much may cause the oil viscosity to increase).

- Silica gel: suitable for adsorption of trace pigments, but the cost is relatively high.

2. Temperature control:

- The oil temperature does not exceed 80℃ during adsorption to avoid accelerated oxidation.

3. Environmental protection requirements:

- Waste adsorbents are treated as hazardous waste (such as white clay containing acid residue).

Economic Comparison

| Method | Cost | Decolorization Effect | Suitable Scale |

|------------------|----------|----------|----------------|

| Activated Clay Adsorption | Low | Medium~High | Small and Medium-sized Treatment |

| Sulfuric Acid-Clay Method | Medium | High | Severely Degraded Oil |

| Solvent Extraction | High | Very High | Industrial Regeneration |

| Hydrogenation Treatment | Very High | Optimum | Professional Refinery |

Summary

- Mild discoloration: Activated clay adsorption is preferred.

- Deep deterioration: Sulfuric acid regeneration or hydrogenation treatment is required.

- After treatment, the color, acid value and electrical properties must be tested to ensure that the oil quality is restored to standard!

- Unrepairable oil: If the dielectric loss is >1% or the breakdown voltage is <30 kV, it is recommended to ｒｅｐｌａｃｅ it with new oil.

> Tip: For transformer oil, discoloration only improves the appearance, and the key is still to ensure its insulation performance (such as breakdown voltage and dielectric loss).