Air Entrainment and Foaming Hydraulic Fluid

Air entrainment and foaming. Air enters a hydraulic system through the reservoir or through air leaks within the hydraulic system. Air entering through the reservoir contributes to surface foaming on the oil. Good reservoir design and use of foam inhibitors usually eliminate surface foaming.

Air entrainment is a dispersion of very small air bubbles in a hydraulic fluid. Oil under low pressure absorbs approximately 10 percent air by volume. Under high pressure, the percentage is even greater. When the fluid is depressurized, the air produces foam as it is released from solution. Foam and high air entrainment in a hydraulic fluid cause erratic operation of servos and contribute to pump cavitation. Oil oxidation is another problem caused by air entrainment. As a fluid is pressurized, the entrained air is compressed and increases in temperature. This increased air temperature can be high enough to scorch the surrounding oil and cause oxidation.

The amount of foaming in a fluid depends upon the viscosity of the fluid, the source of the crude oil, the refinement process, and usage. Foam depressants are commonly added to hydraulic fluid to expedite foam breakup and release of dissolved air. However, it is important to note that foam depressants do not prevent foaming or inhibit air from dissolving in the fluid. In fact, some antifoamants, when used in high concentrations to break up foam, actually retard the release of dissolved air from the fluid.

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