Hydraulic systems are the "lifeblood" of industrial equipment — core machinery such as injection molding machines, die-casting machines, forging presses, and metallurgical rolling mills all rely on hydraulic oil to transmit power and control actuation. However, during fabrication, installation, and long-term operation, various contaminants accumulate on the inner walls of hydraulic piping. Statistics indicate that over 75% of hydraulic system failures are directly related to oil contamination. Contaminants not only accelerate wear on hydraulic pumps, valve spools, and seals but can also cause servo valve sticking, sluggish response, and even system loss of control. For newly installed equipment and aging systems that have operated for years without cleaning, systematic chemical cleaning of oil and water circuits is the essential prerequisite for ensuring reliable hydraulic system operation.
I. Sources and Hazards of Hydraulic System Contaminants
Hydraulic system contaminants can be categorized into three types by source, each posing distinct damage mechanisms:
Fabrication and Installation Residues are the primary contamination source in newly built hydraulic systems. During welding, bending, and assembly of hydraulic piping, weld slag, oxide scale, metal cutting chips, and rust inevitably remain on inner pipe walls. If steel pipes are not thoroughly neutralized and rinsed after acid pickling, residual pickling solution crystals slowly dissolve after oil filling, corroding the piping and hydraulic components. Based on engineering experience, a single 6-meter DN50 newly installed hydraulic steel pipe can carry 5–15 grams of solid particles on its inner walls when unwashed — once these particles enter a servo valve with the hydraulic oil (valve spool clearances typically only 2–5μm), they can cause irreversible spool scoring.
Wear Debris Generated During Operation is a continuous source of contamination. Moving pairs in hydraulic pumps, motors, and actuators generate metal wear particles (iron, copper, aluminum) during extended operation. These particles themselves act as an abrasive "grinding compound" — circulating in the oil, they continuously cut new metal surfaces, forming a vicious cycle of "wear → debris generation → accelerated wear." Simultaneously, hydraulic oil oxidizes under high temperature and pressure, generating sludge and gum that adhere to valve spools and orifice surfaces, causing sluggish hydraulic valve actuation and reduced response speed.
Externally Ingressed Contaminants include dust and moisture entering through reservoir breather ports, impurities introduced during hydraulic oil changes, and seal fragments and fibers falling in during maintenance disassembly. Moisture ingress is particularly lethal — when hydraulic oil water content exceeds 0.1%, the oil emulsifies and turns white, drastically reducing lubrication performance, while acidic substances generated from reactions between water and oil additives corrode metal surfaces. For hydraulic equipment with independent water cooling systems (such as die-casting machines and large injection molding machines), scale and corrosion in cooling water piping are equally critical factors affecting heat dissipation efficiency — every 10% decrease in water-side heat exchange efficiency raises hydraulic oil temperature by 3–5°C, further accelerating oil oxidation.
II. Principles and Methods of Hydraulic System Chemical Cleaning
Hydraulic system chemical cleaning differs from conventional "oil change flushing" — it is a systematic process: specialized cleaning agents circulate through the piping to dissolve or disperse oil sludge, oxides, weld slag, and rust adhered to pipe walls and component cavities, followed by high-flow filtration to carry contaminants out of the system, ultimately restoring pipe inner walls to bare metal and achieving required oil cleanliness standards.
Cleaning Agent Selection is the core of chemical cleaning. Hydraulic system piping is primarily carbon steel and stainless steel; cleaning agents must satisfy the dual requirements of "efficient oil and rust removal" and "no substrate damage." Water-based cleaning agents are the mainstream approach — with non-ionic surfactants as the base, compounded with penetrants, dispersants, and corrosion inhibitors, pH controlled in the mildly alkaline range of 9–11, offering excellent emulsification and stripping capability for oil sludge and carbonized oil deposits, while maintaining extremely low corrosion rates on carbon steel and copper alloys. For heavily rusted piping, appropriate amounts of citric acid or glycolic acid (3%–5%) can be added to the cleaning agent to dissolve rust through chelation without damaging pipe walls. The use of hydrochloric acid in hydraulic system cleaning is strictly prohibited — even trace residual Cl⁻ ions pose pitting risks to precision servo valve mating components during subsequent operation.
Circulation Flushing Process is critical to ensuring cleaning effectiveness. Hydraulic system piping cannot rely on the system's own hydraulic pump for cleaning — normal operating flow rates are far insufficient to scour and strip deposits from pipe walls. An independent high-flow flushing pump station must be connected, achieving oil flow velocities 2–3 times the normal operating velocity (typically ≥6–8 m/s), using high-velocity fluid shear forces to strip contaminants from pipe walls. The flushing pump station's filter should achieve 5–10μm precision (β value ≥200), ensuring stripped particles are immediately captured rather than circulated. For branch lines and dead ends, pulse flushing must be employed — periodically varying pressure to generate instantaneous high-velocity surges that dislodge deposits in dead zones for removal.
III. Oil Circuit Cleaning Process Flow
The standard hydraulic oil circuit chemical cleaning process includes the following five stages:
Stage 1: Oil Drain and Piping Inspection. Completely drain all old hydraulic oil from the system. Remove precision components such as servo valves, proportional valves, and hydraulic cylinders, replacing them with flushing bypass pipes — precision components themselves do not participate in circulation flushing, as flushed-out contaminants would directly damage valve spools. Inspect piping welds and flange connections for integrity, confirming no loosening or leakage in any piping section.
Stage 2: Alkaline Cleaning and Degreasing. Circulate a 2%–3% specialized water-based cleaning agent solution at 50–60°C for 4–6 hours, using the emulsification and penetration action of surfactants to thoroughly strip oil film, rust preventive oil, and cutting fluid from pipe walls. The alkaline cleaning solution's color progressively changes from initial milky white to yellow-brown to deep black — the color change intuitively reflects the degree of oil stripping. Alkaline cleaning endpoint determination: when two consecutive samples (30-minute intervals) show no further color deepening by visual comparison, the endpoint is reached. After draining the alkaline solution, rinse with clean water until effluent pH ≤8.
Stage 3: Acid Cleaning for Rust Removal (As Needed). For piping with significant rust, circulate a 3%–5% citric acid solution (pH 3.5–4.5, temperature 50–60°C) for 3–6 hours to chelate and dissolve pipe wall rust. Citric acid's advantage lies in its mild acidity, avoiding over-corrosion of piping, with biodegradable waste. During acid cleaning, sample every 30 minutes to test Fe³⁺ concentration; when the concentration no longer increases, rust removal is complete. After acid cleaning, rinse with clean water until pH ≥5.
Stage 4: Passivation Treatment. After acid cleaning, pipe walls are in an active state and highly susceptible to secondary atmospheric rusting. Circulate 0.5%–1% citric acid passivation solution (pH 9–10, temperature 60–70°C) for 2–4 hours to form a dense protective oxide film on pipe wall surfaces.
Stage 5: Drying and Restoration. After draining the passivation solution, purge the piping with dry, clean compressed air or nitrogen to thoroughly remove residual moisture — any water remaining in the piping will cause oil emulsification after refilling. Once piping dryness is confirmed, reinstall the removed hydraulic components and fill with new hydraulic oil to the standard level.
IV. Water Circuit Cleaning Process
In the cooling water systems of hydraulic equipment (such as die-casting machine mold cooling water circuits and injection molding machine oil cooler water-side piping), water-side scaling is the primary cause of reduced heat dissipation efficiency after long-term operation. Cooling water is typically industrial circulating water or groundwater with high hardness; Ca²⁺ and Mg²⁺ deposit on heat exchange surfaces to form CaCO₃ scale, with thermal conductivity only 1/50 that of carbon steel — 1mm of scale can reduce heat exchange efficiency by 10%–15%, causing sustained elevation of hydraulic oil temperature.
Water circuit cleaning primarily uses acid descaling. Carbon steel water circuits employ 5%–8% hydrochloric acid or 5%–10% sulfamic acid in circulation for 4–8 hours; stainless steel water circuits switch to a citric acid system. The cleaning solution is pumped from the water circuit inlet and returned from the outlet to the cleaning tank, with acid concentration and Ca²⁺ concentration tested every 30 minutes. When acid concentration no longer decreases and Ca²⁺ stabilizes, scale removal is essentially complete. After acid cleaning, rinse with clean water until pH ≥6, then circulate 1% NaNO₂ passivation solution for 2 hours to protect pipe walls. After water circuit cleaning, a flow test must be conducted — measure flow at rated water pressure, comparing pre- and post-cleaning data; flow recovery to ≥90% of design value indicates pass.
V. Cleanliness Testing and Acceptance Standards
The ultimate criterion for hydraulic system cleaning is not "looking clean" but cleanliness grades based on quantitative test data.
Oil Sampling: Collect oil samples from the sampling port on the flushing pump station return line, first draining a quantity of oil to flush the sampling valve. Sampling containers must be clean bottles filtered through 0.45μm membrane filters. Samples must be submitted for testing within 2 hours.
Testing Method: Use an automatic particle counter (APC) to detect the size distribution and count of solid particles in the oil. Testing standards reference NAS 1638 (National Aerospace Standard) or ISO 4406. NAS 1638 defines 14 cleanliness grades from 00 to 12, with lower numbers indicating cleaner oil. ISO 4406 uses a three-code system to indicate the particle count levels for ≥4μm, ≥6μm, and ≥14μm particles, e.g., 17/15/12.
Acceptance Standards: For newly built or thoroughly cleaned hydraulic systems, oil cleanliness should achieve NAS 1638 Grade 6–7. High-precision systems with servo valves (such as injection molding machines, CNC hydraulic stations) require NAS Grade 5–6; conventional industrial hydraulic systems (such as forging presses, shearing machines) may relax to NAS Grade 7–8. Acceptance requires three consecutive sampling tests (20-minute intervals), with all three meeting the standard — a single sample may exhibit non-uniform particle distribution by chance.
Water circuit system acceptance criteria are simpler: post-cleaning water flow recovery to ≥90% of design value; discharge water clear with no visible rust particles; water sample testing: total iron ≤5 mg/L, suspended solids ≤20 mg/L.
VI. Common Problems and Engineering Experience
Based on hundreds of hydraulic equipment cleaning projects, the following recurring issues deserve special attention:
Rapid Oil Re-contamination After Cleaning — The most common cause is incompletely cleaned branch lines or dead ends; residual contaminants are flushed out after oil filling and operation. The solution is to add a pulse flushing stage and individually connect each branch line to the flushing pump for "branch-by-branch cleaning," eliminating dead zones.
Hydraulic Valve Sticking After Cleaning — Usually caused by failure to seal valve block oil ports with clean plugs when removing hydraulic components; contaminants from the piping entered the valve block's internal oil passages during flushing. The correct practice is to seal all removed hydraulic component oil ports with clean plastic plugs and flush valve block oil passages with clean hydraulic oil before reinstallation.
Persistent Local Overheating After Water Circuit Cleaning — Possibly due to some cooling water branch pipes being completely blocked by scale, preventing circulation of cleaning solution through those sections (cleaning solution always follows the path of least resistance). Suspected blocked branch pipes should be individually disassembled and cleaned or cleared with high-pressure water jetting, ensuring every cooling water pipe participates in circulation cleaning.
Recommended Cleaning Intervals: Newly built hydraulic systems must undergo chemical cleaning before commissioning (known in the industry as "pre-startup acid cleaning and passivation"); operating hydraulic systems are recommended for systematic chemical cleaning every 3–5 years or during major overhauls — the specific interval is determined based on periodic oil testing results; cleaning should be considered when the oil NAS grade drops by more than 2 levels. Water circuit cleaning intervals depend on water quality and scaling rate, generally every 1–2 years.
Danyang Blue Star Cleaning brings 20 years of hydraulic system cleaning experience, equipped with independent high-flow flushing pump stations (maximum flow 400 L/min) and online particle counting instruments, providing one-stop services from piping disassembly, chemical cleaning, and passivation treatment to cleanliness testing for injection molding machines, die-casting machines, forging equipment, and metallurgical hydraulic stations. All projects receive NAS 1638 cleanliness test reports, ensuring documented verification of acceptance.
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