02/07/2026 by Jay Hatton
Demulsibility is a common term in the language of industrial lubrication. It is a critical characteristic for ensuring that lubricants like hydraulic oil, gear oil and turbine oil do not combine with excess moisture to make stable, milky emulsions when they encounter water in equipment.
In the following sections, we explore the demulsibility of industrial oils in depth.
What is demulsibility?
Demulsibility describes the ability of an industrial oil to separate or release from water. It measures how well the lubricant resists mixing with water (emulsification). If an oil has a high demulsibility rating, it can resist forming an emulsion, while a lower number indicates it will not.
Why does demulsibility matter?
During machine operations, industrial oils and water can mix together to create an emulsion. However, if a lubricant has good demulsibility, the water naturally drops out of the industrial oil, so it can then be drained off. In cases of poor demulsibility, the negative consequences can be manifold.
When an emulsion forms, it can completely change the viscosity of the oil and compromise the protective film it offers to parts. This lubrication failure results in increased metal-on-metal contact, causing friction, wear, heat and damage to components and bearings, leading to mechanical breakdown.
While separated oil can continue to protect metal parts from wear, if water is suspended within, it can actively promote corrosive forces like rust. Also referred to as iron oxide, rust impacts the structural integrity of metal parts, making them brittle and more susceptible to breaking.
The presence of water in oils also accelerates fluid oxidation. This can deplete crucial chemical additives in the lubricant’s formula and negatively affect its protective and performance properties.
While oils are engineered to help clean systems of unwanted contaminants, emulsions can easily become mixed with varnish, grit and dust particles and form sludge. This thick detritus can cause valves to stick and clog oil filters. If lubricants can’t pass through filtration measures, they bypass them without being cleaned and push contaminated oil through the system, severely impacting equipment operations.
What causes poor demulsibility?
As an industrial lubricant ages, it starts to lose its inherent ability to shed moisture. Common contaminants degrading demulsibility include solid particulates like metals, dirt and dust that attract water and stop the oil from separating cleanly. Degraded oil byproducts can also include polar contaminants that serve as emulsifying agents, supporting water and oil combining.
Standardised demulsibility testing
Demulsibility is measured in laboratories using standardised testing methods like ASTM D2711 and ASTM D140. The tests involve mixing a set volume of water and oil vigorously together at a specific temperature for five minutes. Lab technicians must then monitor the formed mixture to record how much time passes before the oil, water and emulsion layers separate. If the liquids separate swiftly into their distinctive phases with minimal to no emulsion remaining, technicians give lubricants a perfect demulsibility rating.
How do you maintain system demulsibility?
To maintain system demulsibility, machine operators must proactively control contamination. This is mainly achieved by effectively preventing water ingress and physically removing water found in the system. Minimising oxidation by-products is also vital, along with monitoring filtration systems to ensure continuous operations. Adding aftermarket chemicals is not advisable as they can impact the integrity of industrial lubricants in use.
Today, leading lubricant makers like Kluber and Fuchs create industrial oils designed to cope with operating environments where risk of water contamination exists; for instance, in the paper manufacturing industry, where water is integral and acts as the main medium for breaking down wood, transporting cellulose fibres and forming sheets of paper. To support effective lubrication and prevent emulsification, demulsifies are included in oil formulations to promote effective separation.
For applications involving risk of water contamination, careful lubricant selection is critical. Oils in service should be regularly tested to ensure they remain uncontaminated and are fit for purpose.
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