Reasons and solutions affecting the foaming performance of hydraulic oil

Foam characteristics are specifically divided into foam stability and foam tendency. The national standard of hydraulic oil GB11118.1-2011 has specific requirements for this indicator, that is, foam stability/foam tendency values: procedure I (first 24 ℃) ≯ 150ml/0ml, procedure II (93.5 ℃) ≯ 75ml/0ml, procedure III (second 24 ℃) ≯ 150ml/0ml. The new hydraulic oil from the regular manufacturer will pass the strict foam characteristic test before leaving the factory, and can be delivered to customers only after passing the test.

The reason for the formation of hydraulic oil foam is, on the one hand, the mechanical agitation of the hydraulic oil in the recycling use, and on the other hand, the air dissolved in the oil is released when the pressure drops. At normal pressure, the amount of air that can be dissolved in mineral oil accounts for about 8% -11% of its volume, and the amount of air dissolved in lubricating oil increases with increasing pressure. When the pressure is reduced, excess air is separated from the oil in the form of bubbles, bringing it to a new equilibrium. Especially when there is too much oil or when the oil pressure drops sharply under high pressure, the foaming situation becomes more severe. In addition, during the startup of the equipment, when the operating temperature is low or there are some polar compounds in the lubricating oil, a stable foam will be generated, and the foam is not easy to break.

The excessive foam in the oil will have adverse effects on the oil itself or equipment operation. Air retention in the oil greatly improves the compressibility of the oil, slows down the transmission response, reduces the accuracy of the hydraulic system, and leads to control system failure; Under high pressure, it is compressed, and under low pressure, it suddenly expands, causing strong mechanical vibration and increased noise; Reduced the density of the oil and increased its viscosity, resulting in poor driving of the hydraulic system. Below 0 ℃, the starting performance of the hydraulic device deteriorates; Accelerating the rate of oil oxidation leads to the formation of precipitates, accelerating the corrosion and wear of mechanical system components, while also shortening the service life of the oil itself; Reduced the efficiency of device usage.

The method to reduce the foaming property of oil products is generally to add anti foam additives to the oil products. At present, the widely used anti foam agent is dimethyl silicone oil. Adding anti foam agent to hydraulic oil can not prevent the tendency of foaming of lubricating oil, but can only reduce the stability of foam adsorption film and shorten the time of foam existence. The surface tension of silicone oil is very small. Due to the decrease in surface energy, silicone oil molecules aggregate at the interface between oil and air, which can easily cause the rupture of the bubble film. Therefore, silicone oil has good defoaming effect, low dosage, low volatility, and good oxidation and high temperature resistance. However, silicone oil is insoluble in lubricating oil, and the emulsion of silicone oil and lubricating oil is an unstable system that easily separates and loses its defoaming effect. Therefore, anti foam additives are generally used after being diluted with kerosene, so that the anti foam agent can be more stable suspended in the hydraulic oil, playing the due defoaming effect.