Lubricating oil demulsibility test

Lubricating oil demulsibility test

1. Basic concepts

Emulsification is the phenomenon in which one liquid is tightly dispersed in another liquid to form an emulsion. It is a mixture of two liquids rather than dissolving each other. When the lubricating oil comes into contact with water during use, it will produce different degrees of emulsification under certain conditions.

The lubricity resistance or demulsification degree of lubricating oil refers to the ability of oil to emulsify when exposed to water, and quickly realize oil-water separation after heating and standing.

The main factors affecting the separation performance of lubricating oil and water are the refining degree of base oil, the degree of oil pollution and the compatibility of oil additives. For the blended refined oil, the mechanical impurities, sludge and other pollutants generated during use will seriously affect the water separation or demulsification degree of the oil.

2. Test methods and analytical instruments

This performance index of lubricating oil is mainly tested according to GB/T 7305 “Determination of separation between petroleum and synthetic liquid water”, which is equivalent to ASTMD1401. When determining the water separation of oil with kinematic viscosity less than 90mm²/s at 40°C, the determination temperature is 54±1°C; When determining the water separation of oils with kinematic viscosity greater than 90mm²/s at 40°C, the determination temperature is 82±1°C.

This is done by filling the specimen and 40mL each of distilled water into the same graduated cylinder. At the specified temperature, stir the mixture at 1500r/min for 5min. Stop stirring and lift the stirring blades, and observe the number of milliliters of oil, water, emulsion layer volume in the graduated cylinder and the time of response every 5 min from the side. The results are reported in the following way: (oil layer ml-aqueous layer ml-emulsion layer mL) time min, for example, (40-37-3) 15 min.

The measuring device for water separation in GB/T 7305 is mainly composed of a measuring cylinder, a water bath, an electric motor agitator and a stopwatch. Among them, the measurement simple is made of heat-resistant glass, the scale is in the range of 5~100mL, the index is 1mL, the inner diameter of the measuring simple is 27~30mm, the height is 225~260mm, and the scale error should not be greater than 1mL. The water bath has sufficient size and depth, and the self-control accuracy of the water bath temperature is ±1 °C. The agitator consists of stainless steel blades and connecting rods with a length of (120±1.5) mm, width ± (190.5) mm, thickness of 1.5 mm, and a connecting rod diameter of about 6 mm. The motor speed is (1500±15)r/min.

In addition, there is GB/T 8022 “Lubricating oil demulsibility determination method”. This method is mainly used to determine the demulsibility performance of high viscosity lubricating oils. The method is to add 405mL of sample and 45mL of distilled water to a special separating funnel, stir and separate at a speed of (4500±500)r/min at a temperature of 82°C for 5min, and after standing for 5h, measure and record the volume of water separated from the oil, the volume of emulsion and the percentage of water in oil.

In daily oil monitoring work, the commonly used method is GB/T 7305. During the test, it should be noted that the refined oil is tested with distilled water, but for monitoring the demulsification performance of the oil, it is best to use the water quality that may enter the lubricating oil at the equipment site to test. For example, when testing the demulsification performance of steel mill bearing oil, the difference between distilled water and steel mill cooling water is large, because there are more pollutants in the steel mill cooling water, which strengthens the emulsification performance of the oil. Therefore, in order to accurately judge the demulsibility of steel mill bearing oil under specific conditions, the anti-demulsification performance of the oil should be tested with steel mill bearing cooling water.

3. Purpose of detection

1) The demulsibility of lubricating oil largely depends on the degree of refinement of the base oil. The detection of the demulsibility of new oil can reflect the quality of new oil, and is one of the indicators to identify the quality of real and fake oil.

2) Lubricating oil products in some occasions inevitably enter a lot of moisture, which requires that the water in the oil can be quickly separated from the oil, otherwise the emulsion formed by water and oil will reduce the lubricating performance of lubricating oil, increase wear, produce corrosion, generate sludge, and hinder the normal circulation of lubricating oil. Therefore, lubricating oil products are required to have good anti-demulsification performance.

3) Industrial lubricating oil used in the circulating system, such as hydraulic oil, gear oil, turbine oil, bearing oil, etc., must be in contact with cooling water or water vapor during use, and the amount is so large that it is necessary to regularly discharge the mixed water from the bottom of the circulating tank. Therefore, there are high requirements for the demulsibility of these oils, especially turbine oils and bearing oils.

4) With the extension of the use time, the degree of oxidation and acidic substances contained in the oil increase, which will produce more mechanical impurities. These substances will make the demulsibility of the oil worse. Therefore, demulsibility is also an important quality indicator to judge whether the oil needs to be replaced.

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