Air Contamination and Its Effects on a Machine's Reliability

Jeremy Wright, Noria Corporation
Tags: contamination control

In my previous editorial columns, I described the two most infamous killers of machines: particle and water contamination. While instructing a machinery lubrication seminar recently, I was a bit taken back by the lack of knowledge about air contamination and its effects on lubricants and machine components. Thus, I have decided to make it the topic of this discussion.

Air can exist in oil in four different states of coexistence. They include the following:

1. Dissolved – not visible (no clouding present); can represent as much as 10 percent of the total volume.

2. Entrained – unstable microscopic air bubbles (clouding present).

3. Free – trapped pockets of air in dead zones, high regions and standpipes.

4. Foam – highly aerated tank and sump fluid surfaces (more than 30 percent air).

The problems associated with aerated oil include:

Depending on the machine design, application and aeration severity, it is possible that all five of these conditions could be happening concurrently.

1. Oxidation
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.

2. 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.

3. 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 and leads to wear. Of course, impaired heat transfer properties compound the problems described in numbers 1 and 2 above.

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

5. Cavitation
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).

So, now that we know the effects of aeration on the lubricant and machine components, we can discuss ways of stopping it from becoming a problem. What follows are some steps to take if you notice an air contamination problem.

Step 1. Stop air from becoming entrained. When you control entrained air, by default, you also control foam. Here are the top four ways air becomes entrained in lubricating oils and hydraulic fluids:

Step 2. Keep air buoyant to aid its rapid detrainment from your oil. If air does become entrained, utilize the following strategies for rapid release to the atmosphere without forming foam:

When entrained air passes through oil filters, pumps and bearings, 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.

Step 3. 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:

Step 4. Deploy air detrainment practices and technologies to accelerate separation time. Options include:

Managing aeration and the air-handling ability of lubricants is no insignificant matter. Air is a real contaminant that requires thoughtful monitoring and control, and left unattended, it can destroy your machines. Perhaps your contamination control program began with particle control, then progressed to moisture – much like discussed in my last few articles. But without a program that considers air as a contaminant, you will suffer losses.

I hope this article sheds some light on the subject for you and that you can now make an informed decision on how to best alleviate your air problems.

Jim Fitch. “The Perils of Aerated Oil - Let Your Machine Burp.” Practicing Oil Analysis magazine, January 2005.