Demulsibility

Why is it important?

Demulsibility refers to how well an oil sheds water.

‘New’ circulating lube oils shed water very easily, providing a natural protection from the negative effects of water. However, over time, these oils can start to start to lose their demulsibility characteristics, particularly if the system has been exposed to high amount of water throughout its life. When an oil starts to lose its demulsibility characteristics, whatever water is present is more likely to become emulsified and circulate through the system. This means that the same amount of water will cause disproportionately more damage, and at a faster rate. Therefore, this is a condition that should be investigated.

How is it measured?

Demulsibility is measured using ASTM D1401. The procedure involves mixing and agitating equal volumes of water and oil, and measuring how long it takes for the two layers to separate. The faster the oil and water separate, the better. The test concludes after 30 minutes, and any stable emulsion remaining is considered a fail, which should be investigated.

Issues with water contamination

Water should be treated as a contaminant in circulating lube oil systems. Elevated water levels can lead to:

• Premature Oil Degradation (viscosity thickening / hydrolysis / oxidation / acidity / foaming / air entrainment / sludge / varnish)
• Elevated Wear (rust / corrosion / hydrogen embrittlement)
• Operational Issues (pump cavitation / poor filterability / valve stiction)

Corrective actions you can take

In practical terms, knowing the oil’s demulsibility characteristics helps you to decide on a corrective action plan:

• If water is present and demulsibility is good -> remove the water and continue running
• If water is present and demulsibility is poor ->  an oil change may be needed

Additional Resources

Article: How Much Water is Too Much: That Depends
Whitepaper: Impact of Water Contamination on Lubricants