Density is a simple and commonly used physical property index of petroleum and its products. It refers to the mass of substances contained in unit volume at specified temperature. The unit is kg / m3
Because the density will change at different temperatures, the density measured at high temperature is smaller than that measured at low temperature. In order to facilitate comparison, the density of general oil products is often expressed by the density at the specified temperature. China’s GB stipulates that the density at the standard temperature (20 ℃) is the standard, and the density is g / cm3. Density is of great significance in production, storage and transportation, and is used in product measurement and refinery process design. To some extent, it can judge the general quality of oil products, and the density is also used in the measurement of converted quantity and delivery acceptance. Simply judge the nature of oil products and roughly estimate the type of crude oil according to the density. For example, the density of crude oil containing more alkanes is often lower than that of crude oil containing cycloalkanes and aromatics. The more sulfur, oxygen and nitrogen compounds, colloids and asphalt, the higher the density of crude oil Breaing.
In addition, the density can preliminarily determine the oil type: gasoline P = 0.7-0.76g/cm3, aviation kerosene 0.77-0.84g/cm3, lubricating oil 0.87-0.89g/cm3
The density can approximately evaluate the changes of oil quality and chemical composition, especially in the process of storage and transportation. If the density of an oil increases or decreases significantly, it can be judged whether it is mixed with heavy oil or light oil.
Viscosity is an important physical and chemical index of lubricating oil. It is of decisive significance for the classification, quality identification and use of various lubricating oils. It is also an indispensable physical constant in the process of design and calculation. The internal friction or internal resistance between molecules when a liquid or semi fluid substance flows under the action of external force.
In line with international standards, we use kinematic viscosity m2 / s, which is commonly used in actual production. The relationship between the two is 1m2 / S = 106mm2 / S (crude oil)
The degree to which the viscosity of lubricating oil changes with temperature is viscosity temperature property. Generally, when the temperature increases, the viscosity decreases, and when the temperature decreases, the viscosity increases. The viscosity ratio refers to the ratio of the kinematic viscosity at the lower temperature to the kinematic viscosity at the higher temperature measured by the oil at two specified temperatures China is in line with international ISO and adopts 40 ℃ and 100 ℃. Viscosity index refers to the characteristic that oil viscosity changes with temperature. A high viscosity index means that oil changes little with temperature. It can be found out from the table So what is the significance of viscosity to oil production and use?
(1) When the viscosity of the engine increases, it will affect the power. If the viscosity is too low, it will make it difficult to start and reduce the oil film support ability.
(2) Most lubricants are classified according to viscosity, which is a basis for selecting lubricants.
(3) High viscosity and poor cooling effect. Due to the slow circulation speed, the number of times through the filter is less, and the washing effect is poor.
(4) Oil with low viscosity is easy to break.
(5) Poor sealing.
(6) Increase the consumption of lubricating oil.
Oiliness refers to the performance of lubricating oil adsorbed on metal surface to reduce friction, improve oil performance, small wear and low friction coefficient. Such additives are generally polar molecules, which can be directionally adsorbed on the metal surface to form a firm oil film and can withstand high strength. But it cannot play the role of extreme pressure. Extreme pressure lubrication generally has high temperature, which will reduce the adsorption force of polar molecules. Oily agents are usually combined with other additives such as anti-oxidation and anti rust for spindle oil, hydraulic oil, guide rail oil, etc. Therefore, oily agents are generally added under low load to ensure that sufficient lubricating oily agents are effective. At high temperature and high load, oily agents have little effect, while anti-wear extreme pressure agents increase wear under low temperature and low load.
4) Acid value
The number of milligrams of potassium hydroxide required to neutralize acidic materials in Ig petroleum products is called the acid value, expressed in mgKOH / g (generally refers to the measured value without additives). The acidity measured in the determination of oil acid value is the total value of organic acid, inorganic acid and other acidic substances, but mainly organic acid substances.
Function of determining acid value:
(1) The higher the acid value, the more acidic materials contained in the oil and the stronger the corrosion force.
(2) Judge the corrosivity of oil products to metals
The oil contains less organic acid and will not corrode the metal when there is no water. In the presence of water, even a small amount of low molecular organic acid can also interact with metal equipment to corrode the equipment. Organic acid also corrodes metal aluminum or zinc to generate metal soaps, causing accelerated oxidation and deterioration of oil, and soaps accumulate in the oil to form deposits.Introduction to Common Quality Indexes of lubricating Oil (grease)
(3) Judge the degree of oil deterioration
After the lubricating oil is used for a period of time, due to the oxidation and deterioration of the oil under the action of heat and oxygen, the acidic substances increase and corrode the equipment.Introduction to Common Quality Indexes of lubricating Oil (grease)
(4) After some oils are added with additives, the acid value increases due to the acidity of the additives. For example, the acid value of antirust turbine oil is within 0.03mgkoh/g before adding agents and within 0.3mgkoh/g after adding agents. Therefore, the quality of oil cannot be judged from the acid value.
(5) The service life of the oil is mainly estimated by measuring the depth of acid oxidation. Of course, other performance tests, such as rust prevention, can be accurate.
5) Pour point
The low temperature at which the oil can continue to flow when cooled under the conditions specified in the standard is called the pour point.
When the oil is cooled to the high temperature where the liquid level does not move under the conditions specified in the standard, it is the condensation point.
At present, countries all over the world use pour point to represent low-temperature performance.
Pour point and freezing point are important indexes of low temperature fluidity of lubricating oil. The lubricant used for machinery at low temperature is generally lubricating oil with a pour point of 10-20 ℃ lower than the service temperature. It is not necessary to use low pour point lubricating oil in high temperature area, because the lower the pour point of the oil, the deeper the dewaxing and the higher the cost. Low temperature fluidity and viscosity of lubricating oil are affected. For oil products with little or no wax, when the lubricating oil is reduced to a certain temperature, the viscosity will greatly increase and the lubricating oil will lose fluidity. Therefore, when selecting low-temperature oil for sampling, in addition to the pour point, the low-temperature viscosity should also be considered. This is sticky temperature solidification. In addition, it is structural solidification. When the temperature of waxy oil gradually decreases, the wax contained in the oil will gradually crystallize and precipitate when it reaches the melting point, and then continue to cool. The wax forms a crystallization network and makes the whole oil lose fluidity, which is structural solidification. What is the significance of measuring pour point and freezing point?
(1) Due to the different freezing point of petroleum products, the fluidity is lost during use, and the low-temperature fluidity is judged according to the different temperature
(2) For waxy oil, the freezing point can be used as an index to estimate the paraffin content, because the more paraffin content in the oil, the easier it is to solidify
6) Rust prevention
It means that a certain amount of additives are added to the lubricating oil to make the oil have the performance of preventing metal corrosion. Generally, steam and water often exist in turbine oil under working conditions. A large amount of water vapor will not only emulsify the oil, but also seriously rust the equipment. The adhesion ability of lubricating oil to metal is easy to be damaged in the presence of water vapor. A certain amount of extremely strong organic compounds should be added to make it tightly adsorbed on the metal surface and make the water out of contact with the metal, which can play the role of rust prevention.
The evaluation method of rust resistance is generally liquid phase corrosion test. According to the survey of Japanese machinery industry, the amount of corrosion loss accounts for about 2% of GDP, while the cost for rust prevention is 0.06% of the direct loss. It can be seen that rust prevention is of great significance. Generally, antirust oil (grease) is used for long-term antirust, and coating, electroplating and rinsing are used for permanent antirust. Most of the antirust grease is petroleum lubricating oil as the base oil, which is made by adding antirust agent.
This kind of antirust grease is soaked, sprayed and applied under normal temperature and heating conditions. It is coated on the metal surface for protection. It can be washed off when it is stored for a period of time. It is also used in the lubricating oil system (such as internal combustion engine, turbine and hydraulic system) to seal and operate the general antirust lubricating oil, which plays a role in antirust during sealing. When unsealing starts, it will be put into operation directly without changing new oil. In addition, some packaging and storage materials contain long-term chronic volatile gas antirust agents. Generally, the viscosity of finished antirust oil is mostly below 15-20mm2 / s at 40 ℃.
There are three states of water in oil
(1) Suspended water
Water is suspended in oil in the form of fine droplets to make it become emulsion, which can be removed by vacuum drying.Introduction to Common Quality Indexes of lubricating Oil (grease)
(2) Dissolved water
Water is evenly dispersed in hydrocarbon molecules in molecular state, which is called dissolved water. Its dissolution depends on the chemical composition and temperature of the oil. The higher the temperature, the more melting. Due to the small amount of melting, it can be ignored.Introduction to Common Quality Indexes of lubricating Oil (grease)
(3) Free water
The precipitated fine water particles gather in large water droplets, settle down from the oil and exist in the state of oil-water separation
Generally, in oil analysis, anhydrous (trace below 0.03%) means that there is no free water and suspended water, and the dissolved water is difficult to remove. So what’s the harm of water in oil?Introduction to Common Quality Indexes of lubricating Oil (grease)
A. Water in the oil freezes in winter, blocking pipes and filters
B. The presence of water increases the corrosivity and emulsification of lubricating oil
C. Reduce the dielectric property of oil products, seriously cause short circuit and burn down the equipment
D. The lubricating oil has water, which is easy to produce bubbles and reduce the strength of oil film
E. Water accelerates oil oxidation
F. Low temperature sediment can form oil and impurities
G. The high temperature of lubricating oil and water produces steam and destroys the oil film
H. For ester oil, the additive will be hydrolyzed and precipitated. In this case, even if the water is removed, the original performance of the additive cannot be restored
M. Experiments show that adding 1 drop of water into the oil can shorten the bearing fatigue life by 48%. When the moisture content exceeds 0.10 /, the pipeline may produce cavitation. When the moisture content exceeds 0.5%, it will cause serious wear. If the moisture content exceeds 3%, the service life will be shortened by 85%, which is generally controlled below 0.10%.Introduction to Common Quality Indexes of lubricating Oil (grease)
Generally, water in lubricating oil means bearing death. If the oil contains 0.2% water, the service life of the bearing is reduced by half. 3% water leaves only 15%. Once the oil is emulsified, it must be replaced with a new oil. Therefore, the oil filler must be tightened and the oil barrel must be placed in a dry place. If it is stored in the open air, it should be stacked horizontally to reduce the ponding at the bucket mouth and avoid the bucket “breathing” to absorb water.Introduction to Common Quality Indexes of lubricating Oil (grease)