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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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