There are ten requirements for vacuum pump oil selection, do you know?
The pump oil in the accessories of vacuum pump shall have suitable viscosity, which can make the vacuum pump start quickly at low temperature, have good sealing property at high temperature, and have low temperature rise in the pump. It does not contain light volatile components, which can reduce the oil return rate of vacuum pump during operation.
2： Saturated vapor pressure
Saturated steam pressure is one of the key indexes of pump oil. In a closed vessel with constant temperature, when the vapor-liquid phase reaches dynamic equilibrium, the pressure is called saturated vapor pressure.
The saturated vapor pressure of oil should be as low as possible. Under the high working temperature of the pump, the saturated steam pressure should be low enough and lower than the limit pressure of the vacuum pump. At least 6.5 × 10-5 kPa should be reached at 60 ℃ (for every 20 ℃ rise in temperature rise, the saturated vapor pressure will drop by about one order of magnitude).
3： Limit full pressure and limit partial pressure
Limit total pressure: the total pressure produced by all substances (gas) in the vacuum pump is measured by a series of thermograms or a thermocouple vacuum gauge. At present, foreign countries pay attention to the test indexes of full pressure.
Limit partial pressure: the limit air partial pressure of the pump measured by mercury compact vacuum gauge (Macintosh gauge). The partial pressure of the two-stage rotary vane pump shall be ≤ 6 × 10-5kpa. The difference between the limit full pressure and the limit partial pressure shall be no more than one order of magnitude. The greater the difference between the two, the more volatile components contained in the pump oil, and the worse the properties of the oil.
4： Smoothness of vacuum pump oil
Friction and wear of friction surface, surface fatigue and corrosion are all related to smooth conditions. The pump oil with good performance can prevent corrosion wear, effectively reduce adhesion wear and surface fatigue wear, and the fluid smooth agent can clean the friction surface and reduce friction wear. Reduce the friction resistance to save energy, reduce wear and prolong the mechanical life.
5： Cooling performance
Reducing the temperature of friction surface is an important function of smoothness. When the friction surface moves, work must be done by restraining the friction. All the work consumed in restraining the friction is converted into heat, which will cause the temperature of the friction surface to rise. The heat of friction is related to the smooth state. The heat with high viscosity is the largest and the heat with low viscosity is the smallest, while the heat of boundary friction is between them. It can be seen that the pump oil with appropriate viscosity can not only smooth the liquid, reduce the generation of friction heat, but also take the friction heat out of the pump in time.
In case of friction, it shall be prevented from contacting with surrounding media, causing corrosion, corrosion and damage. The pump oil with good performance shall clean the scale on the friction area as much as possible.
7： Oxidation stability of vacuum pump oil
Oxidation stability clarifies the anti-aging properties of smooth oil, and has special performance requirements for pump oil in industrial assembly line. All smooth oils have different levels of autoxidation tendency according to their chemical composition and external conditions. In the presence of air (or oxygen) and metal catalyst, the pump oil will show the change of acid value, viscosity and precipitate formation in the oxidation time at a certain temperature. The oxidation will occur along with the application process, so some aldehydes, ketones, acids, resins, asphaltenes and other substances will be gradually generated. The oxidation stability is the performance of inhibiting the formation of the above substances that are not conducive to the use of oil products.
8： Heat stability
It indicates the high temperature resistance ability of oil, that is, the resistance ability of smooth oil to thermal synthesis, that is, the thermal synthesis temperature. Some severe vacuum application environment will put forward the request of thermal safety for vacuum oil.
Natural mineral oil has its inevitable defects. Its heat-resistant performance is worse than synthetic oil. Its composition is easy to deteriorate with thermal oxidation synthesis. Its ultimate pressure stability is poor, and its pumping speed also drops with application time. D943 of common pure mineral oil oxidation method is about several hundred hours, while D943 of synthetic oil can be extended to more than 3000 hours. It can be seen that its service life is prolonged.
During the operation of the vacuum pump, the water and air conditioner in the system will be discharged into the pump oil. If the demulsification resistance is not good, it will form an emulsion with the mixed water, making it difficult for the water to be discharged from the bottom of the circulating oil tank, which may cause the vacuum drop and poor smoothness.
Therefore, demulsibility is an important physical and chemical property of industrial smooth oil. Especially in the application of vacuum drying process in medicine, food, ceramics and other industries, the demulsibility is more important. The separation of oil and water of pump oil with good demulsibility can be strong. Water vapor and condensate gas will discharge local water vapor after opening the gas valve. In addition, low viscosity anti emulsification oil is better than high viscosity pump oil.
During the operation of the vacuum pump, because of the presence of air, bubbles often occur, especially when the oil contains active additives, and it is more prone to foam, and the foam is not easy to disappear. The use of foam in smooth oil will destroy the oil film, not only cause the friction surface to attack or increase wear, but also promote the oxidation and transformation of smooth oil. The gas in the operating vacuum system will be discharged from time to time through the sealed pump oil layer, and the discharged gas will generate a large number of bubbles on the surface of the oil layer, and overflow from the pump exhaust hole, forming oil loss and equipment pollution. The pump oil with antifoaming agent has obvious progress in antifoaming effect.
In a word, vacuum pump oil is a part of vacuum pump accessories, which plays an important role in the equipment. Certain requirements should be followed in the selection.