How to filter used insulating oil?

Purifying used insulating oil (such as transformer oil, switch oil, etc.) requires a systematic approach to restore its dielectric strength, chemical stability and oxidation resistance to near-new oil standards. The following is a professional and efficient insulating oil regeneration process:

Main types and hazards of insulating oil pollution

| Pollutants | Source | Hazards |

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

| Moisture | Moisture intrusion, oxidation byproducts | Every 100ppm of moisture ↓ breakdown voltage 30%, accelerated cellulose aging |

| Dissolved gas | Partial discharge, thermal decomposition | Accumulation of flammable gases such as H₂, C₂H₂ causes explosion risk |

| Particles | Equipment wear, fiber shedding | Formation of conductive channels leading to partial discharge (>5μm particles are significantly harmful) |

| Oxidation products | High temperature operation, oxygen contact | When the acid value is >0.1mgKOH/g, the copper winding is corroded and sludge is generated to block the cooling channel |

Insulating oil deep purification process

1. Pretreatment (preliminary purification)

- Mechanical filtration:

- Use a 20μm filter to remove large particles (metal chips, fibers).

- Configure a strong magnetic rod to adsorb ferromagnetic particles (efficiency > 90%).

- Sedimentation separation:

- Stand for 24 hours to allow free water to settle (only suitable for mild pollution with a water content of < 0.1%).

2. Core purification process

(1) Vacuum dehydration and degassing (key step)

- Equipment: two-stage vacuum oil filter (roots pump + rotary vane pump combination)

- Process parameters:

- Vacuum degree: ≤50Pa (absolute pressure)

- Oil temperature: 60-65℃ (avoid >70℃ to accelerate oxidation)

- Atomization method: three-dimensional flash evaporation technology to increase the surface area of the oil film

- Effect:

- Water ≤8ppm (IEC 60814 standard)

- Gas content ≤0.05% (volume ratio)

(2) Precision filtration

- Filter element grade:

- β₅≥200 (corresponding to NAS 1638 level 4)

- Absolute accuracy 1μm (intercept carbon particles, resin precipitates)

- Special design:

- Use a glass fiber filter element (oil-resistant, high temperature resistant)

(3) Adsorption refining (optional)

- Applicable conditions: acid value > 0.05 mgKOH/g or dielectric loss factor tanδ > 0.02 (90°C)

- Method:

- Silica gel/activated alumina: packed adsorption tower treatment (addition amount 3-5% oil weight)

- Molecular sieve: selective adsorption of trace moisture (can be reduced to below 5 ppm)

3. Post-treatment (performance recovery)

- Vacuum standing: The purified oil is left to stand at 50Pa vacuum for 12 hours to remove residual microbubbles.

- Additive supplement:

- Add 0.3% T501 antioxidant (2,6-di-tert-butyl-p-cresol)

- Add 0.1% benzotriazole (metal passivator)

Equipment selection recommendations

1. Vacuum oil filter:

- Processing capacity ≥1.5 times the oil tank volume/hour (e.g.: 15m³/h equipment for a 10m³ oil tank)

- Equipped with an online moisture monitor (capacitive sensor, accuracy ±1ppm)

2. Adsorption system:

- Dynamic circulation white clay treatment tower (contact time ≥30 minutes)

3. Safety design:

- Explosion-proof heater (surface temperature ≤80℃)

- Oxygen content monitoring (working environment O₂＜8%)

Operation precautions

1. Temperature control:

- The heating rate in the heating stage is ≤5℃/min to prevent thermal shock from causing oil cracking.

2. Vacuum system maintenance:

- Regularly ｒｅｐｌａｃｅ vacuum pump oil (every 200 hours or when the water content is ＞100ppm).

3. Environmental compliance:

- Waste clay is treated as HW08 hazardous waste, and waste gas is recovered by condensation.

Acceptance process of recycled oil

1. Laboratory testing:

- Full analysis: chromatography (DGA), PQ index (particle contamination), rotating oxygen bomb (anti-oxidation).

2. On-site test:

- 72-hour cycle test: monitor breakdown voltage fluctuation <5%.

3. Documentation:

- Generate oil passport (data comparison before and after processing, by IEC 60422).

Through the above process, the insulating oil can be reused 3-5 times (depending on the initial contamination level). For oil used in ultra-high voltage equipment above 500kV, it is recommended to entrust a professional regeneration plant to process it to ensure that the dielectric strength is restored to ≥65kV. In daily maintenance, the oil change cycle can be extended to more than 10 years with the online oil filter device.