Air-release properties & foaming tendency

Year of publication: 2002


Air present in the oil in dissolved form or foam occurring in the form of surface foam or as dispersed bubbles in the oil can cause significant operational problems. Larger hydraulic or turbine oil fillings are most commonly affected. However, the user can also be confronted with this problem in the case of gear and circulating oils.

Table of contents

  1. The foaming tendency
  2. Foaming: Possible reasons
  3. Air-release properties
  4. Too much air remains dissolved in the oil - Serious consequences:

The foaming tendency

The values for foam behavior describe the tendency of the oil to form surface foam or provide information on the functioning of foam inhibitors (antifoam additives). Foam occurs when air or gas bubbles with a diameter of 15 micrometers to a few millimeters float up to the surface from the interior of an oil and do not break down. The ASTM D 892 method is used to determine whether an oil is prone to foam formation.

Pure, unalloyed oils do not tend to foam. If foam nevertheless occurs in fresh oils, it is often the cafe for oils with a high additive content. To improve their foaming behavior, so-called foam inhibitors, usually based on high-viscosity silicone oil, are added to them during production. Oil company employees often reach into their bag of tricks and add foam-breaking silicone additives to a highly foaming used oil. The effect is amazing, but it often results in deteriorated air-release propertes and a lack of corrosion protection.


Foaming: Possible reasons

If more air remains in solution in a used oil or if a more stable surface foam occurs than with fresh oil, several different causes should be checked.

"False air"

There are almost always leaks in pipelines, filters, flange connections or pumps when so-called "false air" enters the oil from the outside. An oil pump sucks in air because, for example, its seals are worn or because the level in the oil tank is too low or too high. The conditions in the tank or upstream of a pump's air intake may have changed, or filtration may be affecting the flow rate.

Increased air content

If too much false air has been sucked into the oil, its excess content will not remain dissolved in the oil. The specifically lighter air rises and separates in the form of small air bubbles, creating a stable foam carpet.

Failure of the defoamer

The antifoam additives added during oil production can, especially in the case of high-viscosity gear oils, be reduced to such an extent by overly thorough oil filtration that renewed controlled addition is advisable.
When oil barrels are stored for several years, the silicone-containing additive may partially "frame up" on the oil. However, the defoamer immediately goes back into solution when the container is moved slightly.


Foam can also be caused by contaminants. In addition to the obvious dust and water particles, other agents such as residues from permanently elastic sealing compounds, steam jet water (with grease solvent), lubricating greases or assembly pastes, residues of corrosion protection agents, metalworking fluids or antifreeze (e.g. from a refill vessel) are often responsible for foaming.

Mixing with other oil types

Even if oil manufacturers confirm the miscibility of oils: Oils are not always truly "compatible" with each other. Especially when a synthetic ester-based bio-oil is mixed with a detergent HLP hydraulic oil, or a saturated ester-based bio-oil is mixed with an unsaturated ester, the surface-active properties of the mixed molecules and the surface tension of the fluids change. The antifoaming agent does not suffice anymore to cause the air bubbles of the foam to burst. A complete oil change or the addition of a defoamer will remedy the situation.

Problem: Foam in the system

Surface foam as a stable layer up to a height of about 5 cm on the surface is no cause for alarm. However, if increased foam formation suddenly occurs or if the foam even penetrates from all openings, this can be accompanied by a loss of oil with a drop in the oil level. In extreme cases, this can lead to insufficient lubrication because the oil pump sucks in air. Foam can also be dangerous if the foam carpet acts like an insulator, adversely affecting heat dissipation through the oil cooler or the housing surface and thus increasing oil oxidation.
In some damage patterns, air-release properties and foam behavior overlap. Therefore, it is advisable to check both air-release properties and foaming behavior in complex installations, as required by the procedural regulation for the analysis of used turbine oils (VGB).

Air-release properties

The air-release properties (DIN 51 381) of an oil indicate the time in min. required to allow air blown into the oil under defined conditions to outgas again to a value of 0.2% by volume of the initial value. The value of the air-release properties does not indicate the absolute air content of the oil under test. The air-release properties inform about the ability of the oil to release injected or dispersed air.

The air-release properties of an oil, in contrast to the foaming behavior, cannot be improved by the addition of additives, but only worsened. The actual air content in the oil, which is in the order of 7 to 10 % by volume, depends on the type and additivation of the base oil, the age of the oil charge, any mixing with substances of a different type, and design details of the system.

Unlike foam, which usually collects on the surface of the oil, air dissolved in the oil is not so easy to detect. Sometimes a slight cloudiness is a clue.

Too much air remains dissolved in the oil - Serious consequences:

Just as tap water contains about 1.2% by volume of "invisible" air, i.e., air dissolved in water, a fresh oil always contains dissolved air. In the case of hydraulic oils, the proportion is a considerable approx. 9 % by volume. This substance-related dissolved air does not usually cause any operating faults. Only when more air enters the oil from the outside and is kept in solution in the oil like CO2 in mineral water, does it become critical: The oil becomes compressible, air bubbles separate at hot spots and influence the formation of a hydrodynamic lubricating film. Significant disturbances often occur in the oil-bearing system.

Air in hydraulic oils

An increased proportion of dissolved air in hydraulic systems can cause the oil filling to "spring". Precise control and positioning is no longer possible. An increased amount of dissolved air is deposited at the hydraulic pump, the component with the highest temperature in the circuit. This is usually the cause of the dreaded cavitation of hydraulic pumps or hydraulic motors. In addition, the air bubbles in the oil also cause the "diesel effect", which is often noticeable in the advanced stage with a dark coloration of the oil due to carbon particles (soot). The effect occurs when instead of about 9% air, up to about 15% air is suddenly dissolved in the cold oil due to oils that are incompatible with each other, such as mixing an HLP hydraulic oil with an engine oil. The more than 5% "excess" air no longer remains dissolved in the oil. It rises in small bubbles.
These form especially at the hottest parts of the system and virtually bubble out at these points. Since high or rapid pressure fluctuations occur in hydraulic systems, the resulting compression temperatures between the oxygen from these air bubbles and the surrounding hydrocarbons in the oil cause small, explosive combustions. These occur in a similar way to combustion in a diesel engine. Since the air bubbles that are the cause of the "diesel effect" contain only a few oxygen molecules, combustion is incomplete, with the formation of soot flakes that can turn the oil or even filter inserts black. Since the soot particles are capable of lubrication, the "diesel effect" usually has no effect on the lubricity of the oil. However, since the effect can also occur in the area of seal lips, seals should be inspected for pitting-like material removal, especially if the oil has poor air-release properties.

Air in bio-oils

The air-release properties of bio-oils are negatively affected by mixing with mineral oil. In particular, additives containing calcium, which impart detergent properties to the mineral oil, often deteriorate the air-release properties of the bio-oil from the just permissible 10 minutes to more than 20 minutes. Similar poor results can be observed with blends of different bio-oils (saturated/unsaturated). With significantly deteriorated air-release properties, cavitation damage and the diesel effect must be expected.

Air in turbine oils

In turbine systems, oil with excessively poor air-release properties leads to oil pressure fluctuations. An increase in bearing temperatures due to poorer thermal conductivity of the oil-air mixture can also be observed. If turbine bearings (plain bearings) are supplied with an oil-air dispersion, the hydrodynamic lubrication on which these bearings "live" is severely disrupted. Bearing failures are bound to happen. The cause of deteriorated air-release properties, which should be part of the standard scope of inspection for turbine oil fillings, is aged oil or oil contaminated with a different type of oil (oil from the control circuit or the variable-speed coupling, EP turbine oil). The only remedy here is an oil change.