Page 66 - Fluid Power Engineering

Hydraulic Oils and Theor etical Backgr ound 43

The undissolved air may exist in the hydraulic oil due to the
following reasons:

• Liberation of dissolved air due to local pressure drop
• Air leakages in suction lines, pipe connections, glands, and
others
• Returning fluid, which may contain free air, splashing freely
down into the fluid reservoir
• Low fluid level in reservoir, insufficient residence time
• Bad design of tank
• Incorrect maintenance activity

Fluids should be able to absorb and transport a minimal amount
of air and release it as quickly as possible. This requirement is fulfilled
by using chemical additives.

Water Contamination
It is highly recommended to remove water from the hydraulic
systems, except for the water-based fluids. However, this is not easy to
attain. The following are the main sources of water contamination:

• Condensation from humid air drawn in via the breather. The
ability of a typical mineral hydraulic oil to absorb moisture
from humid air is proportional to the air humidity, as shown
by Fig. 2.24.

FIGURE 2.24 The effect of the relative humidity of the air in the tank on the
concentration of the absorbed water in a typical mineral-based hydraulic oil
at constant temperature.