If more air remains in solution in a used oil or if a more stable surface foam occurs than with fresh oil, several different causes should be checked.
There are almost always leaks in pipelines, filters, flange connections or pumps when so-called "false air" enters the oil from the outside. An oil pump sucks in air because, for example, its seals are worn or because the level in the oil tank is too low or too high. The conditions in the tank or upstream of a pump's air intake may have changed, or filtration may be affecting the flow rate.
Increased air content
If too much false air has been sucked into the oil, its excess content will not remain dissolved in the oil. The specifically lighter air rises and separates in the form of small air bubbles, creating a stable foam carpet.
Failure of the defoamer
The antifoam additives added during oil production can, especially in the case of high-viscosity gear oils, be reduced to such an extent by overly thorough oil filtration that renewed controlled addition is advisable.
When oil barrels are stored for several years, the silicone-containing additive may partially "frame up" on the oil. However, the defoamer immediately goes back into solution when the container is moved slightly.
Foam can also be caused by contaminants. In addition to the obvious dust and water particles, other agents such as residues from permanently elastic sealing compounds, steam jet water (with grease solvent), lubricating greases or assembly pastes, residues of corrosion protection agents, metalworking fluids or antifreeze (e.g. from a refill vessel) are often responsible for foaming.
Mixing with other oil types
Even if oil manufacturers confirm the miscibility of oils: Oils are not always truly "compatible" with each other. Especially when a synthetic ester-based bio-oil is mixed with a detergent HLP hydraulic oil, or a saturated ester-based bio-oil is mixed with an unsaturated ester, the surface-active properties of the mixed molecules and the surface tension of the fluids change. The antifoaming agent does not suffice anymore to cause the air bubbles of the foam to burst. A complete oil change or the addition of a defoamer will remedy the situation.
Problem: Foam in the system
Surface foam as a stable layer up to a height of about 5 cm on the surface is no cause for alarm. However, if increased foam formation suddenly occurs or if the foam even penetrates from all openings, this can be accompanied by a loss of oil with a drop in the oil level. In extreme cases, this can lead to insufficient lubrication because the oil pump sucks in air. Foam can also be dangerous if the foam carpet acts like an insulator, adversely affecting heat dissipation through the oil cooler or the housing surface and thus increasing oil oxidation.
In some damage patterns, air-release properties and foam behavior overlap. Therefore, it is advisable to check both air-release properties and foaming behavior in complex installations, as required by the procedural regulation for the analysis of used turbine oils (VGB).