Air can exist in oil in four different states of coexistence. They include the following:
1. Dissolved – not visible (no clouding present); can represent as much as 10 percent of the total volume.
2. Entrained – unstable microscopic air bubbles (clouding present).
3. Free – trapped pockets of air in dead zones, high regions and standpipes.
4. Foam – highly aerated tank and sump fluid surfaces (more than 30 percent air).
Some the problems associated with aerated oil include: Oxidation, Thermal degradation, Cavitation
Oxidation: Oil can oxidize when its molecules come into contact with oxygen. It stands to reason that the more oxygen exits from the oil, the faster the oil will oxidize.
Thermal Degradation: Aerated oil generates heat by the following mechanisms:
Adiabatic compression of air bubbles (localized heat generation)
Aeration-induced oil flow resistance in piping and components (energy is converted to heat)
Loss of bulk modulus (air makes oil compressible, which generates heat)
Cavitation: When vapor bubbles become rapidly pressurized, such as in a pump or journal bearing, destructive microjets of oil can collide with machine surfaces at extremely high velocities. Some have estimated that the velocities may approach the speed of sound. The result is a progressive, localized erosion of these surfaces. Note that vapor bubbles cause most erosive damage from cavitation, not air bubbles. Vapor bubbles form from the oil itself (light oil fractions) as well as from water contamination (water vapor).