Page 63 - Fluid Power Engineering

40 Cha pte r T w o

the fluid increases or the pressure decreases, the dissolved air is liber-
ated. The liberated air exists in the fluid as discrete bubbles. When the
fluid containing the entrained air returns to the reservoir, the air rises
to the surface and causes foam. The bad design of a tank and oil
return line may also lead to the formation of air bubbles and foam. If
the foam builds up, it can cause severe problems in the hydraulic
system. Therefore, most hydraulic fluids contain foam-depressant
additives that cause the rapid breakdown of the foam.

2.2.11 Cleanliness
Cleanliness in hydraulic systems has received considerable attention.
Some hydraulic systems, such as aerospace hydraulic systems, are
extremely sensitive to contamination. Fluid cleanliness is of primary
importance because the contaminants can cause component malfunc-
tion, prevent proper valve seating, cause wear in components, and
may increase the response time of servo valves. Air, water, solvent,
and other foreign fluids are in the class of fluid contaminants.
Air Contamination
According to Henry’s law, the amount of a given gas dissolved in a given
type and volume of liquid is directly proportional to the partial pressure of
that gas in equilibrium with that liquid, at a constant temperature. Hence,
the solubility of air in the hydraulic fluid is linearly proportional to
the absolute pressure above the liquid surface, and normally decreas-
es with rising temperature.
The solubility is often evaluated by the Bunsen coefficient, defined
as the volume of a gas at 0°C and standard atmospheric pressure which dis-
solves in a unit volume of a solvent. Figure 2.21 shows the solubility of
air in a typical mineral-based hydraulic fluid. The solubility of air in
mineral oil is relatively high compared to the other typical hydraulic
media.
The entrained air is of minor significance as long as it remains
dissolved in the hydraulic fluid. However, the undissolved air results
in serious problems, mainly
• Reduction of the bulk modulus of the oil-air mixture
• Reduction of the density of the fluid by an amount
corresponding to the volume fraction of entrained air
• Slight increase in the viscosity of a hydraulic fluid
• Excessive aeration in a hydraulic system results in unreliable
operation, high noise levels, and the possible damage of
pumps and other com ponents

Free air bubbles passing through a hydraulic pump are subjected
to sudden compression under adiabatic conditions, thus raising the
temperature of the compressed air bubbles. Assume that the initial