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"What does demulsibility mean and how does it relate to oil?"
Demulsibility is the ability to release water. This is important when the equipment is operating in humid climates or in a plant atmosphere that is wet or humid. Paper mills, steel mills and food-processing operations have significant exposure to water-based process fluids.
Because oil is hygroscopic, meaning that it absorbs water, it seems that water is destined to get soaked into the oil. Water enters through thermal breathing (hydraulic systems and mechanical systems that heat and cool), through the high-pressure blast of the operator’s cleaning hose, through the gravity draining of condensate and indirect water spray, and through any number of yet undiscovered ways. It must be removed. Generally, it will settle out if the reservoir is large enough and the flow cycle is low enough.
When demulsibility is lost, the oil will cloud or foam, and the loaded components will wear rapidly. If the plant operates in the absence of water and is climate controlled, then demulsibility may not be a key factor to consider. If water is present, then demulsibility must be monitored.
If you have time, your lab can run a specific test that can be performed to measure the remaining demulsibility potential.
For a quick pass/fail test, measure an equal amount (1 pint) of new lubricant and water, and fill a blender. Heat to 100 degrees F (38 degrees Celsius) or whatever temperature closely resembles the operating temperature of the sump. Thoroughly mix the oil with the water. Measure the amount of time it takes to separate. Also measure how much of the mixture does not separate (the emulsion is called the “cuff”). Now repeat the test with the aged lubricant.
If it takes more than 20 percent longer to separate, then consider having a lab run the test according to the lab procedure. If the results indicate that the demulsibility capacity is diminished, then consider how to best address the problem, particularly if the sump is prone to moisture contamination.