Requirement 1: tightness

The pump oil in the vacuum pump accessories should have suitable viscosity, which can make the vacuum pump start quickly at low temperature, have good sealing performance at high temperature, and have low temperature rise in the pump oil. It does not contain light volatile components and reduces the return oil rate of vacuum pump during operation.

Requirement 2: saturated vapor pressure

Saturated vapor pressure is one of the key indexes of pump oil. In a closed vessel with constant temperature, when the vapor-liquid two phases reach dynamic equilibrium, the pressure is called saturated vapor pressure.

The saturated vapor pressure of oil products should be kept as low as possible. Under the highest working temperature of the pump, the saturated vapor pressure should be low enough and lower than the limit pressure of vacuum pump. At least 6.5 × 10-5 kPa should be reached at 60 ℃ (the saturated vapor pressure will drop about one order of magnitude for every 20 ℃ temperature rise).

Requirement 3: Ultimate total pressure and ultimate partial pressure

Limit total pressure: the total pressure of all substances (gases) in vacuum pump measured by Pirani meter or thermocouple vacuum gauge. At present, foreign countries pay attention to the test index of full pressure.

Limit partial pressure: the limit air partial pressure of the pump measured by mercury contraction vacuum gauge (Maxwell's meter), the partial pressure of double-stage rotary vane pump shall be ≤ 6 × 10-5kpa, and the difference between the limit total pressure and the limit partial pressure shall not be greater than an order of magnitude. The greater the difference between the two, the more volatile components contained in the pump oil, the worse the properties of the oil.

Requirement 4: smoothness of pump oil in vacuum pump accessories

The friction wear, surface fatigue wear and corrosion wear of the friction surface are all related to the smooth condition. The results show that good performance of the pump oil is conducive to the inhibition of corrosion wear; it can effectively reduce the adhesive wear and external fatigue wear; the fluid lubricant can clean the friction surface and reduce the friction and wear. Reduce friction resistance to save energy, reduce wear and prolong mechanical life.

Requirement 5: cooling performance

Reducing the temperature of friction surface is an important function of smoothness. When the friction surface moves, it is necessary to work against the friction force, and all the work consumed in restraining the friction force 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 of high viscosity is the largest, the heat of low viscosity is the least, and the heat of boundary friction is between the two. It can be seen that the proper viscosity of the pump oil can not only smooth the liquid and reduce the generation of friction heat, but also can take the friction heat out of the pump body in time.

Requirement 6: cleanliness

When friction, it is prevented from contacting with the surrounding media, causing corrosion and corrosion. The pump oil with excellent performance shall clean the scale on the friction area as much as possible.

Requirement 7: oxidation stability of 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 automatic oxidation tendency depending on their chemical composition and external conditions. In the presence of air (or oxygen) and metal catalyst, the acid value and viscosity of the pump oil will change and precipitate will be formed during the oxidation time at a certain temperature. Oxidation will occur along with the application process, so some aldehydes, ketones, acids, resins, asphaltenes and other substances will be gradually generated. Oxidation stability is the performance of inhibiting the formation of the above substances which are not conducive to the use of oil products.

Requirement 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 harsh vacuum application environment will put forward the request of thermal safety of vacuum oil.

Natural mineral oil has its unavoidable defects, and its heat resistance is worse than that of synthetic oil. The composition of natural mineral oil is easy to deteriorate with the thermal oxidation synthesis. The stability of limit pressure is poor, and the pumping speed decreases with the application time. The D943 of ordinary pure mineral oil oxidation test is about several hundred hours, while the D943 of synthetic oil can be extended to more than 3000 hours. It can be seen that its service life has been extended.

Requirement 9: demulsibility

During the operation of the vacuum pump, the water and cold air in the system will be discharged into the pump oil. If the anti emulsifying property is not good, it will form the emulsion with the mixed water, so that the water is not easy to be discharged from the bottom of the circulating oil tank, so as to form the vacuum drop and poor smoothness.

Therefore, demulsibility is an important physical and chemical property of industrial smooth oil. Especially in the pharmaceutical, food, ceramic and other industries in the application of vacuum drying process, anti emulsification performance is more important. Anti emulsification of good pump oil-water separation can be strong, water vapor and condensate gas after opening the gas valve, will discharge local steam. In addition, low viscosity anti emulsification is better than high viscosity pump oil.

Requirement 10: antifoaming

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. During operation, the gas in the vacuum system will be discharged from time to time through the sealed oil layer of the pump. The discharged gas will generate a large number of bubbles on the surface of the oil layer, and overflow from the exhaust hole of the pump, resulting in oil loss and equipment pollution. The anti foam effect of pump oil with antifoaming agent has been improved obviously.

In a word, vacuum pump oil is a part of the vacuum pump accessories, which plays an important role in the equipment, and certain requirements should be followed in the selection.