In today's rapid industrial development, hydraulic systems are widely used in industrial fields as driving or lubricating devices; however, the problem of water intrusion into the working medium of hydraulic systems is often not easy to find or is most easily ignored by the majority of users. This phenomenon is also highly concealed during operation and inspection. The following is a brief discussion on the causes, hazards and treatment methods of hydraulic oil water intrusion.
1. Current status of hydraulics in my country’s industrial field
For example, vertical grinding roller loading system and grate cooler hydraulic system:
1. When the pressure-holding performance of the hydraulic system decreases or the lubrication effect is poor, maintenance personnel often habitually look for problems with pumps and valves. In most cases, it is believed that after the valve or pump is ground, the gap becomes larger, causing the pressure holding capacity to decrease. After replacing new valves or pump spare parts, it is found that the pressure holding capacity is improved and the lubrication effect is temporarily restored. This led to the misconception that it was a problem with the pump valve;
2. After a period of time, it was discovered that the pressure maintaining of the hydraulic system was poor again, and the pump set of the hydraulic system frequently compensated for pressure, causing the oil temperature to rise; even the oil temperature alarm of the hydraulic control system shut down, delaying the continued production.
Users usually repeatedly purchase new valves or replace pumps to maintain pressure maintaining conditions. Then the problem arises. For example, the grinding condition of the seal of the cylinder piston and shaft end gland has not improved. It is not until the external leakage of the cylinder is obvious or the internal leakage pressure is not maintained well that improvement measures such as replacing spare parts and repairing the cylinder are proposed. In this case, the maintenance cost is relatively high and the time cycle is relatively long. Few users regularly sample and test the viscosity and cleanliness of the oil, that is, whether it contains impurities and moisture, and whether it is within the range allowed by the national standard.
2. How water enters the hydraulic system
The hydraulic system must be an open system. In order for the pump unit to absorb oil, it must be kept open to atmospheric pressure. At this time, the air filter is similar to a human "nose". In order to maintain hydraulic viscosity and fluidity, the hydraulic system requires heater and relay control to keep the working medium at a suitable operating temperature. They are generally equipped with heaters that start and stop heating automatically. Maintain the working temperature of hydraulic oil generally around 35~45°.
In southern regions with long rainy seasons, there are many days with high temperatures, low atmospheric pressure and high humidity. When the outside air temperature falls below this range, the temperature difference causes the air in the space above the oil surface to condense into water droplets on the tank walls and top. Once the droplets' weight exceeds what surface tension can support, they slide or drip down into the working hydraulic oil. Winter air in northern regions is relatively dry, but humidity can also be high during rainy summer and autumn periods, causing the same problem.
Moisture in the air liquefies in both forward and reverse directions. The schematic diagram is as follows:
3. The existence of water in hydraulic oil
The phenomenon of water entering the hydraulic system is very hidden and difficult to detect, so it is ignored by maintenance personnel. Here’s why:
1, The solubility of water in hydrocarbons is very low. Because hydrocarbon molecules are much larger than water molecules, their dipole moments are extremely low and their polarizability is very weak. There are no hydrogen bonds between water and hydrocarbon molecules, and the attraction between them relies only on induction and dispersion forces (except for aromatic hydrocarbons and unsaturated hydrocarbons). Their molecules can associate with water molecules within certain limits. Water dissolved in hydrocarbons does not dissociate into ions, but is located between hydrocarbon molecules in a single molecular state. There is no association in most hydrocarbons until saturation is reached.
The solubility of water in hydraulic oil is mainly affected by external factors such as temperature, humidity and pressure. The solubility of water generally has an increasing relationship with the above parameters.
2. Free water forms a second phase in hydraulic oil. It is in equilibrium with the water dissolved in the oil. Water droplets vary in size and are highly unstable. Under various forces, they may remain suspended, coalesce, gradually settle or form a separated water layer. When the hydraulic pump delivers oil and the oil and water mix thoroughly, free water droplets surrounded by an emulsifying film form emulsified water. Emulsified water can remain relatively stable. However, through the sight glass on the side of the tank, a transparent lower water layer and a bright, slightly yellow upper layer of hydraulic oil can often be seen.
4. The harm of moisture to hydraulic systems
When the moisture content of hydraulic oil exceeds the standard, there are several hazards:
1, the viscosity of the hydraulic oil gradually decreases, causing the hydraulic system to fail to maintain pressure; after the pressure-maintaining performance is reduced, the system will frequently replenish pressure, which will cause the oil temperature to rise. When the alarm critical value is reached, the system will malfunction and shut down; affecting the continued stable and normal operation of the hydraulic system and the entire system, thereby affecting the stability of the equipment driven by the hydraulic system and reducing the output;
2, hydraulic oil whose viscosity is reduced to exceed the standard value will reduce the lubrication effect of pump valves; cause the grinding of pump valves to intensify, and after the ground particles enter the hydraulic oil and cause pollution, they will continue to intensify the grinding of hydraulic working components such as pump valves and cylinders; greatly shorten the working life of hydraulic parts, increase the frequency of spare parts replacement, and increase the cost of replacement manpower and spare parts costs;
3, the lubrication effect of lubricating oil whose viscosity is reduced to a certain level gradually becomes worse. For example, the oil temperature of the reducer increases, which shows that the cooling effect of the water cooler cannot be achieved. For example, the grinding of planetary gears and thrust tiles of vertical mills is intensified. The color of the lubricating oil gradually changes from colorless and bright to yellow, gradually darkens, turbid to black. Another example is the temperature rise of the grinding roller bearing, which may cause the bearing to peel off, fall off pieces or burst due to excessive temperature.
This greatly shortens the normal service life of the reducer or grinding roller bearing. Delay the normal production of equipment. Increased labor and spare parts costs, and lost product production output;
4. As more water enters the hydraulic system, corrosion develops at the pump inlet or other low-pressure locations, severely affecting hydraulic pressure in the working equipment and causing faults such as local loss of power and wasted oil. Large quantities of water entering the hydraulic system also corrode metal components. The resulting surface rust dissolves into the oil, further contaminating the hydraulic fluid and threatening system safety. This is the fundamental reason rust is found on the inner walls of valve body cavities, at the bottom of oil tanks, or beneath hydraulic cylinder pistons when hydraulic valves are serviced or cylinders disassembled.
5. As metal particles (in the form of ferric oxide) increase in the hydraulic oil, the color of the oil changes from yellow to black. There are also changes to yellow or iron red caused by rust (ferric oxide).
5. Solutions
The solutions to the problem of hydraulic oil water intrusion are generally as follows:
1, stop the machine and let the hydraulic oil stand to allow stratification. Release the water layer from the drain valve at the bottom of the fuel tank; the hydraulic oil in other locations should be purged and removed by disassembling the port, and then drying and removing it.
2, hydraulic oil that has a little water in it can also be removed using a filter with a water remover.
3, the most economical and practical way is to use a high vacuum oil filter, which uses vacuum to completely remove moisture within the national standard range. At the same time, the high vacuum oil filter also has the function of finely filtering the particles in the hydraulic oil to within the range specified by the hydraulic system. For example, NAS1638-5, 9, 10 levels ensure that the cleanliness of the working medium of the hydraulic system is within the allowable range.
(Source: Spare Parts Network)