How to filter dirty hydraulic oil?

When hydraulic oil becomes dirty (contains particles, water, colloid and other pollutants), it is necessary to restore its cleanliness through a systematic filtering method. The following are specific filtering steps and precautions that apply to industrial equipment and construction machinery:

Preparation before filtering

1. Detect the oil state

- Visual inspection: Observe the color of the oil (normally transparent or light yellow). If it is black, turbid, or has suspended matter, it means serious pollution.

- Simple test:

- Water content detection: ｄｒｏｐ oil on a hot steel plate, if there is a "crackling" sound, it means that it contains water.

- Viscosity test: Twist the oil with your hands; if the viscosity decreases significantly, it may have been oxidized.

- Professional testing (recommended):

Send oil samples to the laboratory to test particle contamination (ISO 4406), water content (ppm) and acid value.

2. Prepare tools and equipment

- Filtration equipment: oil filter cart (with precision filter element), centrifugal oil filter, vacuum dehydrator.

- Auxiliary tools: clean oil storage barrels, funnels, filter paper (for emergency use), protective gloves.

- Consumables: matching filter elements (ｓｅｌｅｃｔ 5-25μm accuracy according to the degree of contamination).

Filtration method (operation by scenario)

1. Professional equipment filtration (recommended)

Applicable scenarios: online or offline circulation filtration of hydraulic systems.

Steps:

Connect the oil filter:

- Connect the oil inlet of the oil filter to the bottom of the oil tank (suck out dirty oil) and the oil outlet to the top of the oil tank (refill clean oil).

- If the system allows, disconnect the main circuit and connect the oil filter to the hydraulic system to form a bypass cycle.

ｓｅｌｅｃｔ the filter element accuracy:

- Coarse filtration: first, use a 20-50μm filter element to remove large particles.

- Fine filtration: ｒｅｐｌａｃｅ with a 5-10μm filter element for secondary filtration.

- If it contains water:

- Add a coalescing separation filter element (to separate water) or enable the vacuum dehydration function.

Start filtration:

- Run the oil filter and circulate the oil until the cleanliness meets the standard (detected by a particle counter).

- Typical cycle time: about 2-4 hours for a small system (50L), 8-12 hours for a large system (more than 500L).

2. Simple emergency filtration (no professional equipment)

Applicable scenarios: temporary treatment or remote areas.

Steps:

Silent sedimentation:

- Pour the dirty oil into a clean container and let it stand for 24 hours. After the large particles and water are precipitated, extract the upper layer of oil.

Multi-layer filter paper filtration:

- Use a funnel + filter paper (or cotton cloth) to filter layer by layer. You can stack filter papers of different mesh sizes (such as 100 mesh and then 10μm filter paper).

- Note: This method can only remove some solid particles and cannot handle water and colloids.

3. Centrifugal filtration (for oil with high water content)

Steps:

Use a centrifugal oil filter to separate water and heavy particles in the oil through high-speed rotation (3000- 6000 rpm).

The treated oil needs to pass through a precision filter to remove light pollutants.

Key points

1. Prevent secondary pollution

- All tools and containers must be thoroughly cleaned (rinse with new oil, avoid using water or alcohol).

- Keep the environment dust-free during filtration, and cover the oil tank port to prevent dust.

2. Control temperature

- It is recommended to keep the oil temperature at 40-60℃ (which can improve fluidity, facilitate water evaporation and impurity separation).

3. Filter element replacement rules

- When the oil filter pressure gauge shows that the pressure difference exceeds 0.3MPa, ｒｅｐｌａｃｅ the filter element immediately to avoid filter element rupture and pollution backflow.

4. Fire safety

- Open flames are prohibited at the filtration site, and the temperature of the oil when heated shall not exceed 80℃ (the flash point is usually above 160℃).

Post-filtration treatment

1. Oil testing

- Test cleanliness (such as ISO 4406 standard: 18/16/13 or less is qualified) and water content (<500ppm).

2. System flushing

- If the oil tank or pipeline has been seriously contaminated, it is necessary to flush the system with new oil and then inject the filtered oil.

3. Additive supplementation

- If the oil's antioxidant property decreases, an appropriate amount of antioxidant or anti-wear agent (which must be compatible with the oil) can be added.

Situations where filtration is not possible (oil change required)

If the oil has the following conditions, new oil must be replaced:

- Severe oxidation: the oil is black and viscous, and the acid value is greater than 1.0mgKOH/g.

- Additive exhaustion: anti-foaming and anti-emulsification properties are significantly reduced.

- Irreversible contamination: too much chemical solvent or metal debris is mixed in.

Daily maintenance recommendations

1. Regular filtration: Perform offline circulation filtration on the hydraulic oil every 500 working hours.

2. Source control:

- Use a refueling machine equipped with a filter when refueling.

- Clean the pipe interface when replacing hydraulic components.

3. Monitoring tools:

- Install an online particle sensor or a moisture alarm to monitor the oil status in real time.

The above methods can effectively restore the cleanliness of the hydraulic oil and avoid equipment failures caused by oil contamination. If the contamination is serious or the system has extremely high requirements for cleanliness (such as aviation hydraulics), it is recommended to ｒｅｐｌａｃｅ the new oil and thoroughly clean the system.