Time and time again, we see test specifications with the requirement to control and measure aeration. Unlike other tests, there are no standards or accepted practices to follow. We decided this was an important phenomenon to explore and understand, as it directly affects the performance and reliability of engine lubrication components. This is the first in a series of posts exploring the topic of aeration.
Understanding the problems
Air contamination in oil is a serious condition. There are five problems associated with aerated oil that can be deadly to lubrication components, like oil pumps. By aerated oil, we mean entrained air, foam or both (which is the usual case). The five problems include the following:
Oxidative oil degradation
Impaired heat transfer
Retarded oil supply
Depending on the 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 of oil 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 casing 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:
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.
Foam causes the dampening of important headspace oil movement in components 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 designs 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.
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.
When vapor bubbles become rapidly pressurized, such as in a pump or journal bearing, destructive microjets of oil can collide with 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 (at light oil fractions) as well as from water contamination if present (water vapor).