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How To Deal With Air In Hydraulic Oil

The word ‘contaminant’ refers to any substance that will interfere with the performance of the fluid. Air is one of these contaminants and it can cause considerable damage in the hydraulic system. There are 4 different forms that air can take in the system:
   Free air - a pocket of air that is trapped in the system
   Air that has been dissolved in the oil
   Entrained air - bubbles in the oil that are less than 1mm in diameter
   Larger air bubbles of over 1mm that are on the surface of the oil

Of the above, entrained air is the most concerning. Performing a bleed will usually rid the system of free air. It’s normal to have some amount of foam and this will not usually affect the system. However, if there is a substantial amount of foam present, for example, enough to result in an overflow of the reservoir, this should ring warning bells of there being serious oil degradation or air contamination issues.

Why should entrained air be considered dangerous?

Entrained air can cause issues such as:
· erosion caused by cavitation
· increased heat
· increased noise
· decreased thermal conductivity

Gaseous cavitation

When the air is in the hydraulic oil it can be dissolved by increased heat and pressure decreases. Dissolved air being released is known as gaseous cavitation. The pump inlet can be a problem area for dissolved air due to:
Suction strainer or inlet filters being clogged
Intake-line isolation valves causing turbulence
An inlet that has been poorly designed with a diameter that it too small, has multiple bends or excessive length
Excessive vertical lift

As with other problems of the hydraulic systems, it’s important to maintain equipment regularly so as to prevent air contaminating the fluid. If your system has air contamination, it would be a costly mistake to not recognise it and fix it.


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