How to filter used industrial oil?

To filter industrial oil (such as hydraulic oil, gear oil, turbine oil, etc.) cleanly, it is necessary to use appropriate filtering processes and equipment according to the type of oil, degree of contamination and usage requirements. The following are systematic filtering methods and steps:

Core goals of industrial oil filtration

1. Remove solid particles (metal chips, dust, etc.) → Prevent equipment wear

2. Remove moisture (free water, emulsified water) → Avoid oil oxidation and corrosion

3. Remove gas/air → Improve oil compressibility and stability

4. Purify colloid/acidic substances → Extend oil life

Process flow of industrial oil filtration

1. Pretreatment (coarse filtration)

- Purpose: Remove large particle impurities (>50μm).

- Method:

- Screen filtration: Use 100~50μm filter to intercept visible impurities.

- Magnetic filtration: Adsorb iron chips (applicable to gear oil, rolling mill oil).

- Equipment: Plate and frame oil filter, self-cleaning filter.

2. Main treatment (deep purification)

Choose one or a combination of the following processes according to the type of pollution:

| Type of pollution | Recommended method | Technical principle | Treatment effect |

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

| Solid particles | Precision filtration | Multi-layer filter element (10μm→5μm→1μm) intercepts particles | Cleanliness reaches NAS 1638 level 6 or below |

| Free water/emulsified water | Coalescing dehydration + vacuum drying | Coalescing filter element combines small water droplets → vacuum evaporation | Water ≤100ppm (hydraulic oil standard) |

| Dissolved water/gas | Vacuum degassing | Heating (60℃) under negative pressure (≤0.1kPa) to promote the volatilization of water and gas | Gas content ≤0.1% |

| Colloid/oxidation products | White clay adsorption or electrostatic purification | Activated white clay adsorbs polar substances; high-voltage electrostatic field adsorbs colloidal particles | Acid value ≤0.1mgKOH/g |

3. Post-treatment (fine filtration and performance recovery)

- Fine filtration: Final polishing of 1~3μm filter element.

- Additive supplement: Add anti-wear agent (such as ZDDP) and antioxidant as needed.

Examples of filtration solutions for different industrial oils

1. Hydraulic oil filtration

- Problem: Particle contamination causes the valve core to stick.

- Solution:

A[coarse filtration 100μm] --> B[coalescence dehydration] --> C[vacuum degassing] --> D[fine filtration 3μm]

- Effect: Cleanliness ISO 4406 15/13/10, moisture ≤50ppm.

2. Gear oil filtration

- Problem: Metal wear particles + water emulsification.

- Solution:

A[magnetic filtration] --> B[centrifugal separation] --> C[white clay adsorption] --> D[5μm filtration]

- Effect: iron content ≤10mg/kg, moisture ≤0.05%.

3. Turbine oil (steam turbine oil) filtration

- Problem: moisture and fine particles affect lubricity.

- Solution: vacuum oil filter circulation treatment (parameters: 65℃, 0.5kPa, β₅≥200 filter element).

- Effect: moisture ≤10ppm, NAS level 5.

Operation Precautions

1. Temperature Control:

- Heating temperature ≤ 70℃ (mineral oil)/60℃ (synthetic ester) to prevent thermal aging.

2. Flow matching:

- Oil filter flow ≥ 1/5 of the system tank volume (e.g. 2m³/h equipment for a 10m³ tank).

3. Safety regulations:

- Explosion-proof design of vacuum equipment (ATEX certification is required when handling fuel).

Acceptance criteria (taking ISO/GB as an example)

| Indicators | Hydraulic oil standards | Gear oil standards | Test methods |

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

| Cleanliness (ISO 4406) | 15/12/9 | 18/15/12 | ISO 11171 |

| Moisture content | ≤100ppm | ≤200ppm | ASTM D6304 |

| Acid value | ≤0.2mgKOH/g | ≤0.5mgKOH/g | ASTM D664 |

Solutions to common problems

- Still turbid after filtration: Check whether the emulsification is not broken (add 0.1% demulsifier).

- Frequent clogging of the filter element: pre-coarse filtration or centrifugal separation to reduce the load.

- Insufficient vacuum: Check for seal leakage or vacuum pump oil contamination.

Through the above methods, industrial oil can be efficiently filtered to a level close to that of new oil, and the equipment wear rate can be reduced by more than 50%. For key equipment (such as wind power gearboxes), it is recommended to equip it with an online oil filtration system to achieve real-time purification.