5 Ways Aerated Oil Wrecks Machines

Jim Fitch, Noria Corporation
Tags: contamination control

We’re told machines should not be allowed to swallow air. But what if they do?

What harm could be caused by this bubbly stuff anyway? Do we really have to make the machine burp? Will a few pats on the back do the trick?

For many of you, air contamination is no laughing matter. Why? Because air contamination is a serious condition.

There are five deadly problems associated with aerated oil. By aerated oil, I’m referring to entrained air, foam or both, which is the usual case. The five problems include the following:

Depending on the machine design, application and aeration severity, it is possible that all five of these conditions could be happening at the same time. Let’s discuss each of these killers in more detail:

Oxidative Oil Degradation

Aeration exposes oil to oxygen. The bubbles produce a high surface area interface between the air and the oil. The interface serves as reaction sites for oil oxidation to initiate, particularly when the oil is hot and moist.

Thermal Degradation

Aerated oil generates heat by the following mechanisms:

The heating problem is compounded by impaired cooling, as described below. The building heat leads not only to oil oxidation but also to thermal degradation (such as from microdieseling) forming varnish, sludge and carbon insolubles. Additives such as zinc dialkyldithiophosphate (ZDDP) will also deplete prematurely due to the heat.

Impaired Heat Transfer

Aeration degrades the heat transfer properties due to the following reasons:

While foam retards the oil’s ability to release heat in the reservoir, entrained air also interferes with heat transfer (and movement) in coolers and through machine casing, piping and other thermally conductive surfaces. When oil runs hot, viscosity runs thin which degrades film strength in frictional zones leading to wear. Of course, impaired heat transfer properties compounds the problems described in numbers 1 and 2 above.

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Retarded Oil Supply

Many factors contribute to oil supply problems associated with air. Some of these factors include:


When vapor bubbles become rapidly pressurized, such as in a pump or journal bearing, destructive microjets of oil can collide with machine surfaces at extremely high velocities. Some have estimated that the velocities may approach the speed of sound. The result is a progressive localized erosion of these surfaces. Note that vapor bubbles cause most erosive damage from cavitation, not air bubbles. Vapor bubbles form from the oil itself (light oil fractions) as well as from water contamination (water vapor).

Now that we know the harm caused by aeration and foam, let’s direct our attention to what can be done to prevent its occurrence. I’ve broken the strategies to control aeration into four plans labeled A through D, meaning you move sequentially through the plans until you find a strategy that works.

Ideally, aeration should be held in check by deploying a Plan A strategy which conforms to proactive maintenance. However, because of frequent deficiencies in machine design and the difficulty of performing proactive fixes on the run (to control root causes), other strategies may be left as the only remaining options.

Below is a brief description of the four plans or strategies and how they can be implemented to control aeration:

Plan A - Stop air from becoming entrained. When you control entrained air, by default, you also control foam. Below are the top four ways air becomes entrained in lubricating oils and hydraulic fluids:

Plan B - Keep air buoyant to aid its rapid detrainment from your oil. If air does become entrained, the following are strategies for rapid release to the atmosphere without forming foam:

When entrained air passes through oil filters, pumps, bearings, etc. air bubbles are crushed to such an extent that they don’t release quickly. In extreme cases, the air/oil mixture has the consistency of whipping cream.

Plan C - Give air detrainment sufficient residence time. Given enough time, even finely crushed air bubbles can migrate out of the oil. Strategies for accomplishing this include:

Plan D - Deploy air detrainment practices and technologies to accelerate separation time. Options include:

Role of Oil Analysis

While routine oil analysis is not effective at detecting or measuring the actual presence of air in oil, it can pick up common properties associated with root causes (C) and symptoms (S) of air-related problems, such as those in following:

In summary, managing aeration and the air-handling ability of lubricants is no insignificant matter. Air is a real contaminant that requires thoughtful monitoring and control. Perhaps your contamination control program began with dirt, then progressed to moisture, but now it’s time to give your machine a gentle burp.