Can the filtered hydraulic oil still be used?

Whether the hydraulic oil can continue to be used after filtration depends on the following key factors:

1. Degree of contamination

- Mild contamination: If the hydraulic oil contains only a small amount of solid particles or water, it can usually be restored to cleanliness and meet the standards for continued use after precision filtration (such as using a high-precision filter element, centrifugal separation or vacuum dehydration).

- Severe contamination: If the oil has been oxidized, emulsified (appearing milky white), contains metal debris or chemical impurities (such as fuel or solvents), it is difficult to restore performance even after filtration, and it needs to be replaced.

2. Oil aging state

- Physical and chemical index detection: Viscosity, acid value (TAN), water content, and particle contamination (as per the ISO 4406 standard) must be tested. If the viscosity changes by more than ±10%, the acid value increases significantly (>1mg KOH/g), or the water content exceeds the standard (>0.1%), it means that the oil has aged and filtration cannot reverse chemical degradation.

- Appearance and odor: Dark brown, turbid or burnt, indicating severe oxidation and should be replaced.

3. Filtration technology effect

- Filtration accuracy: Hydraulic systems usually require particle size control (such as NAS 1638 Class 8 and below), and high-efficiency filter elements with β≥200 (such as 3μm absolute accuracy) are required.

- Water removal capacity: If the system contains water, coalescence separation or vacuum dehydration is required to make the water content ≤500ppm (sensitive system requires ≤200ppm).

4. System requirements

- High-pressure/precision systems (such as servo valve systems): have extremely high cleanliness requirements (ISO 14/12 level), and slight contamination may also cause valve core jamming. It is recommended to strictly follow the equipment manual for judgment.

- Ordinary system: If the oil product index meets the standard after filtration, it can be reused with caution.

Recommended operation process

1. Sampling and testing: Send to a professional laboratory to analyze key indicators.

2. Matching filtering equipment: ｓｅｌｅｃｔ a filtering device for the type of pollutant (such as electrostatic adsorption for colloids and cellulose filter elements for particles).

3. System flushing: The pipeline needs to be flushed before changing the oil to avoid secondary contamination of residual pollutants.

When must it be replaced?

- Base oil oxidation (large viscosity change, high acid value).

- Additive depletion (such as anti-wear agent ZDDP content less than 0.1%).

- Irreversible contamination (such as mixing with foreign oil, coolant).

Conclusion: If the physical and chemical indicators of the filtered oil are qualified and there is no irreversible deterioration, it can continue to be used; otherwise, it needs to be replaced. Regular monitoring (sampling every 500 hours) is the key to extending the life of the oil. For critical equipment, it is recommended to handleit  conservatively and ｒｅｐｌａｃｅ new oil to avoid risks.