How to purify transformer oil and insulating oil?

The purification of transformer oil (insulating oil) aims to remove moisture, gas, solid particles, acidic substances and other pollutants in the oil and restore its insulation, cooling and arc extinguishing performance. The purification method needs to be selected according to the degree of oil deterioration and the type of pollutants, and is usually divided into physical purification, chemical treatment and combined process. The following is a detailed purification process and technology:

Purification goals

1. Remove pollutants: moisture, gas, particles, sludge, acidic compounds.

2. Recovery performance:

- Dielectric strength (≥60kV/2.5mm, GB/T 507 standard)

- Water content (≤15ppm, IEC 60422 standard)

- Acid value (≤0.1mg KOH/g)

- Gas content (such as oxygen ≤3000ppm).

Purification methods and equipment

1. Mechanical filtration (removal of solid particles)

- Equipment: plate and frame oil filter, centrifuge, precision filter.

- Principle: Separate metal chips, fibers, dust, etc. through filter paper (1~5μm pore size) or centrifugal force.

- Applicable scenarios: pretreatment of new oil or slightly contaminated oil.

Vacuum dehydration and degassing (core process)

- Equipment: vacuum oil filter (integrated heating, atomization, high vacuum).

- Process:

1. Heat the oil to 50~70℃ (reduce viscosity and promote water evaporation).

2. Atomize the oil or form a film to increase the evaporation area.

3. Evaporate water and gas under high vacuum (residual pressure <50Pa).

4. The condenser captures water vapor and refluxes the dry oil.

- Effect:

- Water content can be reduced to below 10 ppm.

- Simultaneously remove dissolved gases such as oxygen and nitrogen.

Adsorption purification (deep treatment)

- Adsorbent type:

- Molecular sieve (3A/4A): selectively adsorbs water.

- Activated clay/silica gel: adsorbs acidic substances and colloids.

- Ion exchange resin: remove polar compounds.

- Method:

- Offline treatment: oil circulates through an adsorption tower.

- Online treatment: A thermosiphon adsorption tank is built into the transformer (such as a silica gel respirator).

Chemical regeneration (severely deteriorated oil)

- Method:

- Alkali treatment: neutralize acidic substances with sodium hydroxide, then wash with water.

- Sulfuric acid-clay method: sulfuric acid removes impurities, and clay adsorbs residues (professional operation is required).

- Note: Chemical regeneration may change the composition of the oil, and subsequent fine filtration is required.

Electrostatic purification (high-efficiency removal of microparticles)

- Principle: high-voltage electrostatic field charges impurities and adsorbs them.

- Applicable: precise purification of ultra-high voltage transformer oil.

Typical purification process (combined process)

1. Pretreatment: centrifugation or plate and frame filtration to remove large particles.

2. Vacuum dehydration and degassing: remove moisture and gas.

3. Adsorption treatment: deep dehydration and deacidification through an olecular sieve or white clay tower.

4. Precision filtration: A 1μm filter element intercepts residual particles.

5. Inspection and acceptance: Determination of dielectric strength, moisture, acid value, and other indicators.

Special pollution treatment

- Metal particles: magnetic filter adsorbs iron filings.

- Carbide: activated carbon or electrostatic filtration.

- Emulsified water: coalescence separation + vacuum dehydration.

Precautions

1. Temperature control: heating does not exceed 80℃ to prevent oil oxidation.

2. Vacuum degree: the lower the residual pressure, the higher the dehydration efficiency (oil splashing must be prevented).

3. Adsorbent management: Molecular sieves need to be activated before use (baking at 300℃) to avoid oil contamination and failure.

4. Environmental protection requirements: Waste oil and waste adsorbents are treated as hazardous waste.

Online vs offline purification

- Online purification: circulating treatment during transformer operation (such as live oil filtering) to reduce downtime.

- Offline purification: centralized treatment after oil extraction, suitable for severe pollution.

Standard reference

- International: IEC 60422 (Guidelines for Maintenance of Insulating Oil).

- Domestic: GB/T 14542 (Guidelines for Maintenance and Management of Transformer Oil).

By properly selecting purification technology, the life of transformer oil can be extended by more than 10 years and the risk of equipment failure can be significantly reduced. For deeply deteriorated oil (acid value > 0.5mg KOH/g or dielectric strength < 40kV), it is recommended to ｒｅｐｌａｃｅ it with new oil.