Foaming and air entrainment in lubricating oil circulation systems are more common than you might think. Left unchecked, they can lead to serious operational issues such as fluctuating oil pressure, oil pump cavitation, excessive oxidation, and even machine breakdowns. Understanding the causes of foaming and how to manage it effectively is crucial for maintaining optimal lubrication and system performance.
Why Does Lubricating Oil Foam?
Most lubricating oils are formulated with antifoam additives to minimize foaming. However, contrary to what their name suggests, foam inhibitors don’t prevent air bubbles (aeration) from forming in circulating oil. Instead, these additives (often silicone-based) reduce the surface tension of air bubbles, causing them to rupture and merge into larger bubbles that rise quickly to the oil’s surface and dissipate, as illustrated below:
Measuring Foam in Lubricating Oil
The foaming tendency and stability of a lubricant are typically tested using the ASTM D892 test method, which consists of three sequences:
• Sequence I: Measures foaming tendency and stability at 24°C.
• Sequence II: Measures foaming tendency and stability at 93.5°C.
• Sequence III: Conducted at 24°C, but on the same fluid that was tested in Sequence II.
The results are reported in a two-number format, e.g., 20/0, where the first number indicates foam tendency (in milliliters) after five minutes of aeration, and the second number represents foam stability after a ten-minute settling time. Ideally, new oils should have a maximum foam tendency of 10 to 50 mL and 0 mL foam stability after the settling period.
This photo of two oils was taken during the five-minute aeration period of the ASTM D892 Foam Test. Excessive foam formation can be seen on the surface of the oil on the left which contained no antifoam additive. The oil on the right was fortified with a foam inhibitor and exhibits negligible foam formation. The photo also shows the larger oil bubbles (that migrate to the surface more readily) in the oil with the antifoam additive.
Although the majority of lubricating oils are formulated with antifoam additives, foam and air entrainment problems are quite common and are usually hard to treat. Traditionally the standard procedure was to run an ASTM D892 foam test on the offending oil, and then indiscriminately add an aftermarket antifoam additive. Generally, foam went away quickly, only to return shortly afterwards. More antifoam was added, and the cycle was repeated until the system became so overloaded with foam inhibitor that the oil had to be dumped. Today, there are more practical methods of establishing and treating the source of foam problems and it is therefore usually unnecessary to use aftermarket antifoam additives.
What Causes Foaming and How to Fix It?
Even though most lubricants contain antifoam additives, persistent foaming issues can still occur. The root causes vary but often include:
✅ Water Contamination: Even small amounts of water in oil can promote foaming. ✅ Solids Contamination: Dirt and debris disrupt oil flow and encourage foam formation. ✅ Depleted Foam Inhibitor: Excessive fine filtration can strip oils of their foam inhibitors. ✅ Mechanical Issues: Leaks, excessive turbulence, and air leaks in pumps can aerate the fluid. ✅ Incorrect Oil Level: Overfilling or underfilling the sump can create foaming problems. ✅ Cross-Contamination: Mixing different lubricants can lead to foaming. ✅ Grease Contamination: Grease entering the oil can alter its foaming properties. ✅ Too Much Antifoam Additive: Overuse of aftermarket foam inhibitors can cause more harm than good.
How to Troubleshoot Foaming Issues
Traditionally, foaming issues were tackled by repeatedly adding aftermarket antifoam additives—only for the foam to return soon after. Today, a more strategic approach is recommended:
- Test the Oil: Conduct a foam test to assess the severity of the issue.
- Identify the Root Cause: Use a process of elimination to determine the likely culprit.
- Take Corrective Action: Address contamination, mechanical faults, or fluid compatibility issues.
- Avoid Overuse of Antifoam Additives: Instead of continuously adding foam inhibitors, resolve the underlying cause.
Need Expert Advice?If you’re facing foaming issues or any other lubrication challenges, our experts are here to help. Drop us an email at info@bcl.co.za, and we’ll provide tailored solutions to keep your equipment running smoothly.