Demulsibility is a term commonly used in the language of lubrication. Demulsibility is the ability of oil to release or separate from water. In other words, it is a measure of how well the lubricant can resist emulsification. A high demulsibility rating means that the lubricant will resist forming an emulsion with water, while a low number indicates that it will not.
Oil and water separate readily because similar molecules attract each other, i.e. oil sticks with oil, water sticks with water. However, when a mixture of oil and water is agitated, an emulsion is formed. Three oils with various degrees of demulsification and emulsification are shown on the right:
In the sample on the left all the oil and water are demulsified.
There is an interface (emulsion) of oil and water in the middle sample,In the sample on the right all the oil and water are emulsified.
In most instances the oil and water will separate completely when left to settle for an adequate period of time. The water drops to the bottom since water is denser than oil.
In lubrication the demulsification property of oil is a benefit since water shedding is important for systems that have the potential to become contaminated with water. Water that enters a circulating system and emulsify can increase wear and corrosion, reduce load-carrying capacity, promote oil oxidation, deplete additives and plug filters. In hydraulic systems it can also adversely affect the operation of valves, servos and pumps. Although highly refined oils permit water to separate readily, demulsibility can be affected negatively by the presence of impurities and contaminants in the oil. Some oil additives such as rust inhibiters and dispersants can actually promote emulsion formation.
The impact of demulsibility depends on the level of contamination and the residence time (the time the oil spends ‘resting’ in the reservoir) of the system. When the resident time is sufficient the demulsified water can be removed by engineering solutions such as drain valves, suction, etc.
Demulsibility testing can show failure in the lab, but with sufficient residence time, the oil may shed water at an acceptable rate that does not impact oil performance. Small oil reservoirs with lower residence times require better demulsibility performance than larger sumps. It is recommended that testing for demulsibility should be conducted on a regular basis if the oil system is exposed to water or if demulsibility performance is suspicious.
The ASTM D1401 demulsibility test method is commonly used to determine the ability of oil to separate from water. A 40 ml sample of the test specimen and 40 ml of distilled water are stirred for 5 minutes in a graduated cylinder at a specified temperature. The time required for the separation of the emulsion is recorded after every 5 minutes. If complete separation or emulsion reduction to 3 ml or less does not occur after standing for 30 minutes, the volumes of oil, water, and emulsion remaining are reported.
Regardless of what is said above, a few equipment manufacturers do not recommend hydraulic oils that shed water. Caterpillar hydraulic oil is for instance formulated to hold water in dispersion.
The oil contains emulsifiers specifically designed to disperse water. Caterpillar does not recommend oils that “separate,” “shed,” or “release” water. They claim that separated water drawn through the hydraulic system can damage pumps and other components. If the water freezes, it can also cause serious damage to hydraulic systems. Notwithstanding this many Caterpillar machines in mixed fleet operations are operating satisfactorily with lubricants that do demulsify or shed water.
Q8Oils offer a comprehensive range of demulsifiable lubricants for a wide variety of automotive, construction, industrial, mining, agricultural and other applications. For more information phone 011 462 1829, email us at info@bcl.co.za or visit www.bcl.q8oils.co.za.
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