Foam may seem harmless, but if unaccounted for, it can lead to serious issues in a machine. Foam finds its way into the oil volume’s headspace after the oil has initially become aerated. Small amounts of air in oil isn’t uncommon, particularly if the oil is expected to become agitated, such as in a gear system. However, as oil becomes more aerated, air bubbles will rise to the surface. Depending on the surface tension, these bubbles will either burst or stay intact in the form of foam.
Additives such as defoamants help to destabilize the foam bubbles and keep overall foam levels low. If air entrainment is not controlled, the defoamants may not be sufficient. The surface tension can be compromised by contaminants such as water, solid particles, grease or other soaps and detergents. The system’s design can also encourage further aeration.
Small amounts of foam may not be cause for concern. However, when foam levels rise and do not return to normal or the foam leads to a drop in the oil level, there may be a bigger issue brewing. Once these concerning foam levels are observed during an inspection, report it and then be sure to follow up. When investigating the root cause, three possibilities should be considered: contamination, defoamant additive levels and mechanical issues.
Visual observation or oil analysis should be performed to determine if there has been an increase in any specific contaminant. Rising water content or particle counts are often the No. 1 reason why foam propagates. Trending this data over time will pinpoint if an increase in the contamination levels correlates to the foaming issue.
The defoamant additive levels may also have depleted or become ineffective. This can be measured through oil analysis by comparing the results from air release and foam stability tests to those from a baseline sample. Please note that defoamant additives are carefully homogenized in the original oil’s formulation, so additive levels that are too high or low can lead to an increase in foam.
Finally, keep in mind that air entrainment is worsened when oil is heavily agitated. Suction lines positioned ahead of a hydraulic pump can also pull in air from connector leak points, suction inlet points or any other ingression point.
Once you have determined the causes of foam in your oil, you can then work to mitigate it and reduce its impact on your equipment.