Air can be present in your hydraulic system in four forms:
Free air – a pocket of air trapped in part of a system.
Dissolved air – hydraulic fluid contains between 6 & 12 percent by volume of dissolved air.
Entrained air – air bubbles typically less than 1 mm in diameter dispersed in the fluid.
Foam – air bubbles typically greater than 1 mm in diameter which congregate on the surface of the fluid.
Of these four forms, entrained air is the most problematic.
Pre-filling components and proper bleeding of the hydraulic system during start-up will largely eliminate free air.
Small amounts of foam are cosmetic and do not pose a problem. However, if large volumes of foam are present, sufficient to cause the reservoir to overflow for example, this can be a symptom of a more serious air contamination and/or fluid degradation problem.
Negative effects of entrained air include:
– Reduced bulk modulus, resulting in spongy operation and poor control system response
– Increased heat-load
– Reduced thermal conductivity
– Fluid deterioration
– Reduced fluid viscosity, which leaves critical surfaces vulnerable to wear
– Cavitation erosion
– Increased noise levels
– Decreased efficiency
As pointed out above, hydraulic fluid can contain up to 12 percent dissolved air by volume. Certain conditions can cause this dissolved air to come out of solution, resulting in entrained air.
When fluid temperature increases or static pressure decreases, air solubility is reduced and bubbles can form within the fluid. This release of dissolved air is known as gaseous cavitation.
Decrease in static pressure and subsequent release of dissolved air can occur at the pump inlet, as a result of:
-Clogged inlet filters or suction strainers
-Turbulence caused by intake-line isolation valves
-Poorly designed inlet
-Collapsed or otherwise restricted intake line
-Clogged or undersized reservoir breather
Air entrainment can also occur through external ingestion. Like gaseous cavitation, this commonly occurs at the pump as a result of:
-Loose intake-line clamps or fittings
-Porous intake lines
-Low reservoir fluid level
-Faulty pump shaft seal
Like other hydraulic problems, proper equipment maintenance will prevent the occurrence of most air contamination problems.