To the hydraulic system maintenance engineer, it can be both normal and alarming that hydraulic oil contains air. Nonetheless, it’s part of hydraulic fluid’s natural composition to contain between 6% - 12% of dissolved air. The trouble starts if this air transforms into being a form that is not dissolved, and it can lead to serious trouble! If the air makes its way into the pump intake, then air can then transform into entrained air. The problem with this is that it can spoil the stiffness of the fluid, which will decrease its efficiency. It can also increase the levels of noise. But that’s not all. Supposing the entrained oil reaches the pump outlet and gets compressed, you can then expect very high peak temperatures to develop. Air mixing with the oil film will oxidize it. The cracking noise is an indication of oxidisation. As we all know, it’s oxidisation that degrades hydraulic fluid. The air bubbles will then start to pop after smashing themselves against the valve plate of the pump and any other areas that they touch. This is when cavitation takes place and there’s erosive wear going on. The only way to stop this occurring is to prevent it. Air will come out of the solution in certain conditions. For example, when the temperature of the hydraulic oil increases or there is a decrease in static pressure, then the solubility of the air is reduced and that’s when the fluid can host the formation of bubbles. The release of the air is referred to as gaseous cavitation. It can come about from: · Suction strainers or inlet filters becoming clogged · The intake-line isolation valves causing turbulence · An inlet that has been poorly designed such as it being too long, having multiple bends or the diameter of it being too small · The intake line being restricted or collapsed · There being too much lift between the minimum fluid level and the pump intake · The reservoir being clogged or undersized The bottom line is that although you will have air in your hydraulic fluid, it’s key to keep an eye on what condition it is in. it can be a serious and costly error to allow air to contaminate a hydraulic system.