With the rapid development of industry today, hydraulic systems are widely used in the industrial field as drive or lubrication devices. However, the problem of water ingress into the working medium of hydraulic systems is often not easily detected or is most easily overlooked by users. This phenomenon is also highly concealed during operation and inspection. The causes, hazards, and remedies of water ingress into hydraulic oil are briefly discussed below.
I. Current Status of Hydraulics in China's Industrial Sector
Examples include vertical mill roller loading systems and clinker cooler hydraulic systems:
1. When the pressure-holding performance of a hydraulic system decreases or lubrication becomes poor, maintenance personnel tend to habitually look for problems with pumps and valves. In most cases, they believe that after valves or pumps are worn, the clearances increase, causing pressure-holding capability to decrease. After replacing valves or pump spare parts, they find that pressure-holding capability improves and lubrication temporarily recovers. This further reinforces the mistaken belief that the problem lies with the pumps and valves.
2. After some time, pressure-holding of the hydraulic system deteriorates again, and the pump unit frequently re-pressurizes, causing oil temperature to rise; in some cases, the hydraulic control system shuts down due to an oil temperature alarm, disrupting continuous production.
Users generally purchase new valves or pumps again and replace them to maintain pressure-holding. The problem is that, for example, the wear condition of the seals at the piston and shaft-end gland of the cylinder has not improved, until obvious external leakage occurs or internal leakage causes poor pressure holding, only then are improvement measures such as replacing spare parts and repairing cylinders proposed. Under such circumstances, maintenance costs are relatively high and the time required is relatively long. Few users regularly sample and test the viscosity and cleanliness of the oil, i.e., the degree of impurity and water content, to determine whether they are within the range permitted by national standards.
II. How Water Enters the Hydraulic System
A hydraulic system must be an open system. For the pump unit to draw oil, it must remain open to atmospheric pressure. At this point, the air breather/filter acts like a person's "nose." To maintain hydraulic viscosity and fluidity, hydraulic systems require heaters and relay control to keep the working medium at a suitable operating temperature. Generally, heaters with automatic start/stop heating are installed. The hydraulic oil operating temperature is generally maintained at about 35–45°C.
In southern regions with a long rainy season, there are many days with high temperature, low air pressure, and high humidity. If the outside air temperature is lower than this range, the air in the space above the oil surface liquefies into water droplets due to the temperature difference and adheres to the sidewalls and top of the reservoir. When the gravity of the water droplets becomes great enough to overcome surface tension, they begin to slide down or drip into the working hydraulic oil. In northern China, the air is relatively dry in winter, but in summer and autumn when there is more rain, humidity can also be high, and the same problem occurs. The liquefaction of moisture in the air also involves forward and reverse processes. The schematic diagram is shown below:
Figure: Schematic diagram of condensation in a conventional hydraulic system reservoir
Labels: Air in/out; Air breather/filter; Hydraulic oil at about 45°C; Electric heater; Suction oil filter; To pump unit; 3 × 1 kW, 220 V AC; Y-connection.
III. States of Water in Hydraulic Oil
The phenomenon of water entering a hydraulic system is highly concealed and difficult to detect, and is therefore often overlooked by maintenance personnel. The reasons are as follows:
1. The solubility of water in hydrocarbons is very low. Hydrocarbon molecules are much larger than water molecules, have particularly low dipole moments, and are very weakly polarizable. There is no hydrogen bonding between water and hydrocarbon molecules; the attraction between them relies only on induction and dispersion forces (aromatic and unsaturated hydrocarbons are exceptions). Their molecules can associate with water molecules only to a limited extent. Water dissolved in hydrocarbons does not dissociate into ions but resides between hydrocarbon molecules in the form of individual molecules. In most hydrocarbons, it does not associate until saturation is reached. The solubility of water in hydraulic oil is mainly affected by external factors such as temperature, humidity, and pressure. Water solubility generally increases with the above parameters.
2. Free water appears as a second phase in hydraulic oil. Free water is in equilibrium with dissolved water in the oil. Water droplets vary in size, are highly unstable, and under various forces may be suspended, associate with one another, gradually settle, or become sediment water. When the hydraulic pump discharges hydraulic oil and the oil and water are thoroughly mixed, free water droplets become surrounded by an emulsifying film, forming emulsified water. Emulsified water can exist relatively stably. However, through the oil sight glass on the sidewall of the reservoir, a transparent water layer is often seen at the bottom and slightly yellow, bright hydraulic oil above it.
IV. Hazards of Water to Hydraulic Systems
When the water content of hydraulic oil exceeds the standard, the hazards are as follows:
1. The viscosity of hydraulic oil gradually decreases, causing the hydraulic system to fail to hold pressure. After pressure-holding performance decreases, the system frequently re-pressurizes, causing oil temperature to rise. When the alarm threshold is reached, the system shuts down due to a fault. This affects the continuous and stable normal operation of the hydraulic system and the entire system, thereby reducing the working stability of the equipment driven by the hydraulic system and lowering output.
2. When the viscosity of hydraulic oil drops below the specified limit, its lubrication effect on pumps and valves decreases. This aggravates wear of pumps and valves. The particles worn off enter the hydraulic oil and cause contamination, which further aggravates wear of hydraulic working components such as pumps, valves, and cylinders. This greatly shortens the service life of hydraulic components, increases the frequency of spare parts replacement, and increases the labor and spare parts costs for replacement.
3. When the viscosity of the lubricating oil decreases to a certain level, its lubrication effect gradually deteriorates. For example, in a reducer this manifests as an increase in oil temperature, creating the false impression that the water cooler is not providing sufficient cooling. In a vertical mill, for example, wear of the planetary gears and thrust pads/bushings is aggravated. The color of the lubricating oil gradually changes from colorless and clear to yellow, then darker, turbid, and finally black. Another example is the temperature rise of the grinding roller bearings; local overheating may cause the bearings to spall, break into pieces, or crack. This greatly shortens the normal service life of the reducer or grinding roller bearings, disrupts normal production, increases labor and spare parts costs, and reduces product output.
4. As the amount of water entering the hydraulic system increases, corrosion occurs at the pump inlet or other low-pressure locations, seriously affecting the hydraulic pressure of the working device and causing faults such as local loss of power and oil becoming waste/unusable. A large amount of water entering the hydraulic system will also rust metal components. The floating rust produced dissolves in the oil, further contaminating the hydraulic oil and further threatening the safety of the hydraulic system. This is the root cause of why, when overhauling hydraulic valves or disassembling cylinders, corrosion is found on the inner walls of valve bodies, the bottom of the reservoir, or the bottom of cylinder pistons.
5. As metal particles (in the form of magnetite, Fe₃O₄) in the hydraulic oil increase, the oil color changes from yellow to black; rust (ferric oxide, Fe₂O₃) can also cause changes to yellow or iron-red.
V. Remedies
The solutions to the problem of water ingress into hydraulic oil are roughly as follows:
1. Shut down the machine and let the hydraulic oil stand to allow stratification. Drain the water layer from the bottom drain valve of the reservoir. For hydraulic oil in other locations, remove it by disassembling ports and purging, then remove residual moisture by drying.
2. Hydraulic oil with a small amount of water ingress can also be cleaned using a filter containing a water-removal agent.
3. The most economical and practical method is to use a high-vacuum oil purifier, which uses vacuum to thoroughly remove water to within the national standard range. At the same time, the high-vacuum oil purifier also has the function of fine-filtering particulate matter in the hydraulic oil to within the range specified for the hydraulic system, such as NAS 1638 Class 5, 9, and 10, to ensure that the cleanliness of the hydraulic system working medium is within the allowable range.
2026 October 2nd Week VAFEM Product Recommendation
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2026-10-16