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:

  • Oxidative Oil Degradation
  • Thermal Degradation
  • Impaired Heat Transfer
  • Retarded Oil Supply
  • Cavitation
Acronyms Used
in this Article

ZDDP - Zinc dialkyldithiophosphate

RPVOT - Rotating pressure vessel oxidation test

FTIR - Fourier transform infrared spectroscopy

RULER - An instrument which determines the rate of depletion of antioxidant additives to be determined

ASTM - American Society for Testing Materials


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:

1. 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.

2. Thermal Degradation. Aerated oil generates heat by the following mechanisms:

  • adiabatic compression of air bubbles (localized heat generation)
  • aeration-induced oil flow resistance in piping and components (energy is converted to heat)
  • loss of bulk modulus (air makes oil compressible which generates heat)

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:

  • Aerated oil is not a good thermal conductor.
  • Restricted oil flow from aeration impedes convection (movement of the heat from movement of the fluid).

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.

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

  • Oil Compressibility. Aerated oil is hard to pump. It’s like trying to pump against a sponge. The actual delivered oil volume (oil flow rate) may be only a fraction of what the pump normally supplies without the aeration condition.
  • Dampening. Foam causes the dampening of important headspace oil movement in machines that depend on oil lifting (throwing) mechanisms, including splash lubrication, paddle gears, flingers and slingers. The foam retards the oil travel (toss) through the air, resulting in it failing to reach critical zones of the machine, including bearings and gears.
  • Reduced Oil Density. Many machines depend on oil flowing efficiently by gravitational forces. A bubbly oil has very low density and gravitational pull. For instance, a ring oiler my lift some foamy oil to the upper port of the journal bearing, however its low density (and increased apparent viscosity) impair its ability to penetrate downward into the bearing’s channels and grooves for lubrication. The same is true in gravity oil drains and headers from bearings and gears in circulating oil systems.
  • Air-Lock. Foamy, low-density oil can cause air-lock resulting in a complete cessation of oil flow (restricted oil drains, loss of pump prime, redirected oil flow, etc.). An aerated oil has an apparent viscosity sharply greater than that of the oil alone which compounds the problem.
  • Reduced Oil Level. Foam robs liquid-phase oil from the reservoir or sump which means the working oil level falls. This often brings the oil level below what is needed to adequately prime pumps (head), supply oil to lifting devices (ring, collars, paddles, flingers, slingers, etc.) and supply oil to bath/splash-lubricated gears and bearings. Low oil level is a circular problem causing more aeration, more heat and less air-release residence time.

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).

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:

  • Plunging oil returns (free falls that cause churning and aeration) versus return lines with diffusers that ooze the oil back to tank
  • Suction leaks or pump seal leaks that entrain air
  • Over agitation of tanks (low oil level or poor tank design) causing turbulence, vortexes and/or lapping (folding in air on tank surfaces)
  • Vented drains where air and oil mix coming down drain lines before returning to tank

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:

  • Ensure healthy condition of defoamants additives
  • Keep air bubbles large and buoyant by keeping oil interfacial tension high. The smaller the air bubble, the longer it takes to reach the surface and detrain. Factors that reduce interfacial tension:
  • emulsified water
  • oxides
  • polar additives or contaminants

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:

  • Large-volume reservoirs
  • Reservoirs with flow-directed baffling to avoid short circuiting
  • Use of settling and air detrainment tanks

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

  • Apply slight reservoir vacuum.
  • Use an off-line vacuum chamber for both air release and dehydration purposes.
  • Install vortex deaeration technologies.
  • Install wire-cloth air coalescers/separators in the reservoir.

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:

  • Depleted silicone defoamants additive - C
  • Prematurely oxidized/thermally degraded oil (RPVOT, viscosity, FTIR, RULER, etc.) - C and S
  • Elemental analysis of contaminants - C S Interfacial tension analysis - C S Ultracentrifuge/total insolubles analysis - C and S S ASTM air release and foam tendency/stability tests - C

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.