Air is normally found in hydraulic fluids, a fact that seems surprising initially. In fact the 6 to 12% of air that is present in hydraulic fluids is dissolved air, which means it does not appear as individual air bubbles. If that air comes out of solution, or air enters the fluid in any other way, through the intake, for example, then it becomes problematic.
The biggest issue is that air, unlike hydraulic fluids, is compressible. When air bubbles are present they cause noise, vibration and an increase in temperature at the outlet, as the compression on the air produces heat. Although this heat does not transfer directly to the fluid, it can cause oxidation and nitration of the film around the air bubble, increasing the degradation of the fluid more rapidly than expected and thereby shortening the active life of the hydraulic fluid. The noise and vibration causes unnecessary wear and tear on the components involved; and if the hydraulic equipment is noisy anyway, then an extra few decibels could take it above safe working limits, even with ear protection.
When needed, air that is present in a hydraulic system, but not in the fluid itself, can be bled from the system. It should be borne in mind that the effect of air on hydraulic fluids is degradation, so the fluid should be replaced as a caution anyway. Bleeding air from a hydraulic system should only be attempted by a trained engineer, as there are many dangerous factors involved. Many manufacturers often advise that air does not need to be bled from the system, as it will naturally work its way out or mix with the fluid and cause air bubbles, which is not ideal as we know.
Once small air bubbles have formed the fluid should be replaced, as it cannot be easily re-dissolved. Of course, the cause of the air bubbles should also be investigated to prevent it happening again. There are some common causes of trapped air that should be ruled out first; Air entering the system from outside is usually the result of porous lines or tubes or a hole in one of these parts. Similarly, poor seals and clamps connecting lines to reservoirs and chambers allow air to enter the system, and in some cases a low reservoir level may attract air from the outside. These things will be on any decent maintenance schedule, so if the maintenance is being carried out properly and regularly then any issues with these components can be addressed before they cause major issues.
Dissolved air from the hydraulic fluid coming out of solution happens through a process known as gaseous cavitation. This occurs when static pressure drops, or when oil temperature increases. A decrease in static pressure can be caused by clogged filters and valves at the inlet or in the reservoir altering the pressure inside the machinery, or by an inlet that is not fit for purpose. Inlets that are too small, or of a length that is too great in comparison to the size of machinery, or even those with too many bends can cause a decrease in static pressure and this is one of the first places to look, especially if the equipment has been modified at any stage.
If there are any suspicions that air has entered a hydraulics system, either via gaseous cavitation or from the outside, steps should be taken to drain and replace the fluid and check components for damage if vibrations and excess noise has been noticed. An inspection of the seals, valves and lines should then be carried out to determine the location of air ingress, and then repairs should be made. Any repairs should be noted on a maintenance report to track the frequency with which this issue occurs – it may be the case that modifications need to be made in order to stop this happening again.