How to filter contaminants in lubricating oil?

Lubricating oil pollutants mainly include solid particles, moisture, air, oxidation products (sludge) and chemical pollutants (such as acid, fuel dilution, etc.). Different pollutants require different filtering methods. The following is a systematic lubricating oil purification solution:

Solid particle pollutants (metal chips, dust, etc.)

Filtration method

- Mechanical filtration (most commonly used)

- Filter element type:

- Surface filter element (such as metal mesh, filter paper): intercepts large particles (>10μm).

- Depth filter element (such as glass fiber, sintered metal): can capture tiny particles (1-5μm).

- Filter accuracy selection:

- General industrial equipment: 10-25μm.

- Precision hydraulic system: 1-5μm (such as servo valve oil).

- Centrifugal separation

- Suitable for high-density particles (such as metal grinding chips), but has limited effect on particles <5μm.

Recommended equipment

- Offline oil filter vehicle (with high-precision filter element).

- Duplex filter (filter element can be replaced online).

Water pollution (free water, emulsified water, dissolved water)

Treatment method

|                  Type of water                      |                             Removal method                                     |            Applicable equipment               |             Effect              |

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

| Free water (settled water)                    | Gravity sedimentation, centrifugal separation                    | Sedimentation tank, centrifuge           | Remove most of the free water |

| Emulsified water (tiny water droplets) | Vacuum dehydration, coalescence separation                    | Vacuum oil filter, coalescence oil filter | Reduce to below 50ppm |

| Dissolved water (molecular level)        | Vacuum heating dehydration, molecular sieve adsorption | Vacuum oil filter, desiccant                  | Reduce to below 10ppm |

Recommended equipment

- Vacuum oil filter (best choice, can remove water, degas, and filter particles at the same time).

- Coalescing oil filter (suitable for emulsified water).

Air (foam, bubbles)

Treatment methods

- Vacuum degassing: Use a vacuum oil filter to extract the air dissolved in the oil.

- Static degassing: Allow the oil to naturally precipitate bubbles after shutdown (low efficiency).

- Defoaming agent (use with caution): Only temporary relief, may affect oil performance.

Oxidation products (sludge, colloid)

Treatment methods

- Mechanical filtration: Use an adsorbent filter element (such as an activated carbon filter element) to remove colloids.

- Electrostatic oil purification: Use an electrostatic field to adsorb oxidized colloids (applicable to turbine oil, etc.).

- Oil change: If the sludge is serious (acid value> 2mg KOH/g), it is recommended to ｒｅｐｌａｃｅ the oil with new oil and clean the system.

Chemical pollutants (acids, fuel dilution, etc.)

Treatment methods

|        Pollutants          |                                Removal methods                                            |                              Remarks                                    |

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

| Acidic substances    | Alkaline neutralization (such as silica gel adsorption), oil change  | High acid value oil may have oxidized and failed |

| Fuel dilution             | Vacuum distillation, oil change                                                       | Common in internal combustion engine oil          |

| Additive exhaustion | Add additives or oil change                                                            | Oil testing is required to determine                       |

Comprehensive filtration solution

(1) Offline purification (recommended)

- Use a multi-functional oil filter (such as vacuum + coalescence + fine filtration):

- Steps:

1. Pre-filtration (25μm) to remove large particles.

2. Heating + vacuum dehydration (60-70℃, -0.09MPa) to remove moisture and air.

3. Fine filtration (1-5μm) to remove tiny particles.

- Applicable scenarios: hydraulic oil, transformer oil, turbine oil, etc.

(2) Online filtration

- Install a bypass filtration system (such as a high-precision filter element with β≥200):

- Continuously filter the oil circulating in the system to extend the oil change cycle.

(3) Centrifugal + adsorption combination

- First, centrifuge to remove most of the water and particles, and then use molecular sieves to adsorb trace moisture.

How to choose a filtration solution?

|    Contaminants    |                   Recommended methods                     |    Target cleanliness     |

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

| Solid particles       | Mechanical filtration (1-5μm)                               | ISO 4406 15/13/10       |

| Moisture               | Vacuum dehydration + coalescence                     | ≤100ppm (free water)  |

| Air                         | Vacuum degassing                                                | ≤0.1% volume              |

| Sludge/colloid      | Electrostatic oil cleaning or adsorption filtration | No visible sediment      |

| Acidic substances | Silica gel adsorption or oil change                       | TAN <1.0 mg KOH/g     |

Maintenance recommendations

- Regular oil inspection (every 3-6 months) to monitor the contaminant content.

- ｓｅｌｅｃｔ the appropriate filter element (refer to the equipment manufacturer's requirements).

- Avoid excessive heating (>80℃ will accelerate oil oxidation).

The combination of "mechanical filtration + vacuum dehydration + adsorption purification" can effectively restore the cleanliness of the lubricating oil and extend the life of the equipment. If the oil has deteriorated severely, it is more economical to ｒｅｐｌａｃｅ it directly.