The primary condition for the proper functioning of a hydraulic system is the cleanliness of the system's interior. During the manufacturing, assembly, and usage of the hydraulic system, contamination is inevitable. Even new hydraulic fluid may sometimes contain unexpected contaminants. Therefore, measures must be taken to filter out contaminants as quickly as possible. Otherwise, the equipment may experience failures once it is put into operation. Early failures are often severe, with some components, such as pumps, valves, and motors, potentially suffering fatal damage.

The purpose of system cleaning is to eliminate or minimize early failures of the equipment. The goal of cleaning is to improve the cleanliness of the hydraulic fluid, ensuring that the fluid's cleanliness level remains within the contamination tolerance of the critical hydraulic components in the system. This is to guarantee the operational reliability of the hydraulic system and the service life of its components.

1.Common Cleaning Methods for Hydraulic Systems

1.1 Solvent Heating Immersion Cleaning

Immerse the parts to be cleaned into a cleaning tank equipped with a heating device (the heating temperature is generally between 35 to 85℃). Compressed air or steam is introduced into the cleaning solution to keep it in motion. The immersion time is 0.5 to 2 hours. To enhance the cleaning effect, common additives, as shown in Table 1, can be added to the cleaning solution to improve rust prevention, decontamination, and cleaning capabilities.

Table 1: Common Additives for Cleaning Fluids

1.2 Spraying and Washing

Using a pressure jet machine for cleaning is suitable for continuous operation in medium to large factory workshops. A corrosion-resistant pump is used to spray a heated aqueous solution at a pressure of 0.3 MPa for cleaning. Generally, the parts to be cleaned undergo a continuous three-stage spraying process through a pre-wash chamber, a wash chamber, and a hot water rinse chamber.

Additionally, airflow generated by compressed air can be used to blow off contaminants, with pulsating airflow being the most effective.

1.3 Motorized Scrubbing

Soft bristles can be used to remove dirt to maintain the precision and low roughness of components. For instance, wire mesh oil filters often use hard steel wire brushes, which can sometimes damage the filter element or alter the filtration precision. Similarly, for high-precision, low-roughness hydraulic valve bodies, a deburring brush with abrasive balls is used to polish the ends of valve holes, junctions of holes, and sediment grooves. This deburring brush consists of black nylon filaments with diameters ranging from 0.3 to 0.6 mm and green silicon carbide abrasives of M20 specification bonded together.

1.4 Ultrasonic Cleaning

Ultrasonic waves of appropriate power are directed into the cleaning liquid, creating tiny point-like cavities. When these cavities expand to a certain extent, they suddenly collapse, forming local vacuums. The surrounding fluid rapidly fills these vacuums, generating powerful sound pressure and mechanical shock waves (known as cavitation), which can reach several thousand atmospheres. This force causes contaminants on the surfaces of parts submerged in the cleaning liquid to be dislodged. This method offers short cleaning times and high-quality results, and it can clean complex-shaped parts that are difficult to clean manually. Compared to manual cleaning, this method increases efficiency by more than tenfold and reduces costs. However, for porous materials like oil filters, the absorption of sound waves may affect the cleaning effectiveness.

1.5 Heating and Volatilization Method

Some contaminants can be removed by heating them to volatilize. However, this method cannot remove carbon, ash, and solid residues inside hydraulic components.

1.6 Acid Treatment Method

This method uses different acid solutions for different metal materials. After removing surface contaminants, the parts are immersed in a solution composed of CrO3, H2SO4, and H2O, forming a corrosion-resistant film on the surface.

2. Cleaning of Hydraulic Components

Hydraulic components are crucial parts of a hydraulic system, and their cleanliness directly impacts the operational reliability of the system after assembly. Therefore, before assembling the hydraulic system, it is necessary to check if the components' cleanliness meets the required standards. The ISO standard cleanliness levels for typical hydraulic components are shown in Table 2.

Table 2: Typical Hydraulic Component Cleanliness Levels

In the process of machining and manufacturing mechanical parts, various pollutants and impurities can be introduced. Therefore, purification measures must be taken before assembly to remove residual contaminants from the machining process. During cleaning, suitable methods should be selected based on the structure, size, type, nature of contaminants on the cleaned surfaces, and cleanliness requirements.

For removing oxidation, sand particles, burrs, rust, etc., from the workpieces, methods such as pressure jetting or acid cleaning can be employed. To eliminate lubricants, stearic acid, paraffin wax, dirt, and mechanical impurities, cleaning agents are used. Metal cleaning agents are effective for rust removal, oxide removal, and enhancing rust resistance of the workpieces. The selection of cleaning agents depends on their ability to dissolve contaminants on the surfaces without causing corrosion or generating harmful gases.

To improve the cleaning effectiveness and rust resistance, appropriate cleaning additives can be added. Vibratory deburring machines or abrasive nylon brushes can be used for removing burrs. For complex geometric parts, ultrasonic cleaning is suitable. After cleaning, a comprehensive inspection should be conducted to ensure all contaminants are removed before assembly.

Once all parts are cleaned, they should be stored in a clean, enclosed area that is easy to sweep and maintain cleanliness. The air inside the facility should be filtered, and the pressure should be kept higher than outside to prevent dust from entering. If necessary, parts can be wrapped in polyethylene plastic for handling and short-term storage. The humidity in the facility should be maintained at 35% to 45%, and the temperature at around 20°C to prevent rusting of the parts.

Assembly should be conducted on a clean workbench, and the personnel involved in assembly must also maintain cleanliness. After assembly, all openings on the assembled hydraulic components should be plugged, wrapped with polyethylene plastic for transportation. Hydraulic cylinders can be installed in a separate pre-cleaning oil tank system for circulation flushing with pre-cleaning oil. The required oil volume should be at least 5 times the capacity of the hydraulic cylinder. Typically, after 5 repeated flushes, it can be considered clean.

The oil tank should be manually cleaned with cheesecloth or polyurethane resin sponge; do not use cotton yarn or cloth to wipe the tank. Welding slag and iron filings in the corners of the tank can be removed with adhesive or dough made from flour. Additionally, particle blowing, vacuum cleaning, and steam cleaning can be performed. After cleaning, acid washing should be conducted to completely remove surface oxides, followed by treatment with rust inhibitor on the inner surface of the tank. Acid washing can be done by immersion in a tank or by flushing.

3.Cleaning of Hydraulic System During Assembly

(1) Selection of Cleaning Oil

When the system piping and reservoir are relatively clean, cleaning oil with the same viscosity as the operating fluid can be used. If the system is not clean inside, a cleaning oil with slightly lower viscosity can be chosen. The cleaning oil should be compatible with the system's operating medium and all sealing materials. The amount of cleaning oil used is typically 60% to 70% of the standard oil volume of the reservoir. Before adding the cleaning oil, ensure that the reservoir is cleaned thoroughly.

(2) Heating of Cleaning Oil

Heating the oil helps dislodge deposits inside the system more easily. Generally, the oil should be heated to 50-60°C. The cleaning pressure should be maintained at 0.1-0.2 MPa, and the flow rate of the cleaning oil should be as high as possible to facilitate the removal of contaminants.

(3) Installation of Oil Filters

In the cleaning circuit, install a coarse oil filter (50-100 µm) at the inlet and a fine oil filter (10-50 µm) at the return outlet. Initially, use the coarse filter, and after a period of flushing, gradually switch to finer mesh filters for subsequent filtering. Initially, clean the filter every half hour, then adjust based on conditions, gradually extending the cleaning interval. Typically, the cleaning time for the entire hydraulic system is 8 hours.

(4) Intermittent Operation of Hydraulic Pump

To achieve effective cleaning, intermittently operate the hydraulic pump. During the cleaning process, gently tap the oil pipes with a wooden hammer several times to encourage contaminants to detach quickly. Pay more attention to tapping at bends and welded areas of the pipes, but avoid excessive force to prevent damage. The tapping time should be about 15% of the total cleaning time.

(5) Ensure Turbulent Flow of Cleaning Fluid

To ensure effective cleaning, the cleaning fluid must flow in a turbulent state, with a Reynolds number above 4000 to guarantee good cleaning results. Otherwise, increase the flow rate of the cleaning fluid, raise the temperature, and lower the viscosity. Cleaning at low flow rates and room temperature generally yields poorer results.

(6) Draining of Cleaning Oil

Upon completion of cleaning, thoroughly drain the cleaning oil, including from pumps, valves, pipelines, and oil coolers. Loosen the drain plugs of the pipelines and use compressed air to blow out residual oil, or introduce operating fluid to flush out the cleaning oil.

(7) Addition of Hydraulic Oil

After draining the cleaning oil, promptly add hydraulic oil and conduct a short-term trial operation to prevent rusting of the pipelines, etc.

4.Precautions during Cleaning

(1) During general hydraulic system cleaning, hydraulic oil or test oil used for operation should be employed. Avoid using kerosene, gasoline, alcohol, steam, or other liquids to prevent corrosion of hydraulic components, pipelines, reservoirs, and seals.

(2) During the cleaning process, ensure the hydraulic pump operates and the cleaning medium heats up simultaneously. Rubber residue inside the system is easier to remove when the cleaning oil temperature is maintained between 50 to 80°C.

(3) Non-metallic hammers can be used to tap on oil pipes during cleaning. Tapping can be continuous or intermittent to facilitate the removal of deposits inside the pipelines.

(4) Intermittent operation of the hydraulic pump contributes to improving cleaning effectiveness, with intervals typically lasting 10 to 30 minutes.

(5) Install filters or screens in the return line of the cleaning oil circuit. Initially, use an 80-mesh screen due to higher impurity levels, switching to a 150-mesh screen or finer during later stages of cleaning.

(6) The cleaning duration generally ranges from 48 to 60 hours, depending on factors such as system complexity, required filtration precision, and degree of contamination.

(7) To prevent rust caused by external moisture, continue operating the hydraulic pump until temperatures return to normal at the end of cleaning.

(8) After cleaning, thoroughly drain the cleaning oil from the circuit.

Hydraulic system cleaning is absolutely necessary for hydraulic systems being assembled or newly put into operation. Any negligence in cleaning due to a sense of luck will lead to fatal damage to the equipment. After rigorous cleaning, it can reduce and avoid faults during system commissioning and early operation, shorten the debugging period of the system, and reduce unnecessary losses. However, contamination control of the system is an ongoing process that cannot be accomplished once and for all. During operation, it is necessary to regularly inspect the condition of the hydraulic fluid and control contaminants to ensure cleanliness within the allowable range specified by the system.

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