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By Dave Wooton, Gerald L. Munson, and Greg Livingstone
Figure 1. It’s the formation of foam in a close-up picture. You can see the bubbling brown liquid in the top of the picture. Foam can get pulled into the lube circulating system and cause mechanical damage.
Foaming is a fundamental physical property of a lubricating fluid. Foam can degrade the fluid’s life and performance as well as that of the equipment being lubricated. Even though foam performance often is a defined specification for the new fluid, it’s many times ignored on used fluid (Figure 1). The reasons for loss in foam control and the methods of controlling this property in a used fluid should be added to the lubricant specialist’s understanding and knowledge.
Foam in used lubricants has been studied for more than 50 years. It typically forms on a fluid that’s been agitated. The bubbles range from about 10 μm to 1,000 μm in diameter and aren’t uniformly dispersed. They rise to the surface and coalesce to form larger bubbles. As bubble size overcomes surface tension, they break.
Two variables are of interest: foaming tendency and foam stability. The foaming tendency quantifies the amount of foam (in milliliters) that was made during the air introduction and still remains immediately after the cessation of the air flow. Foam stability quantifies the extent to which the bubbles aren’t combining and breaking. In this test, it’s the amount of foam that remains, in milliliters, after 10 min. from the time the air flow is stopped.
Lubricant foaming can lead to serious operational problems. The most common are inadequate lubrication, cavitation and loss of lubrication. However, increased lubricant oxidation, micro-dieseling, control valve losses, spongy hydraulic piston movement and pump oil pressure control losses also can be the results of severe foaming.
There’s often considerable aeration as lubricant circulates through the system and returns to the sump. If the fluid can’t release this air efficiently, it accumulates and causes foam. If foam continues to build, it can cause the oil level to drop until the oil pick-up tube begins to pull foam with the oil.
A fluid’s foaming property is measured using ASTM D892, which measures foam by three sequences that differ only in measuring temperature. Sequence I measures the foaming tendency and stability at 24°C (75°F). Sequence II uses 93.5°C (200°F). Sequence III uses the same conditions as Sequence I, except it’s performed on fluid that has just been measured in Sequence II. The fluid sample from sequence I is not used in sequence II. The fluid sample used in sequence II is carried into sequence III. There are some antifoam additives that this sequence of test measurement shows significant differences. There are many times the fluids will be seeing sequence II operating conditions.
The antifoam additives require significant high-speed blending to disperse them as small particles. Because of this manufacturing operation, the test standard allows the sample to be treated with a Waring-type blender to redisperse the antifoam additive before running the tests.
The results are reported as two numbers for each sequence. For example: 20/0 means 20 ml of foam tendency was measured after 5 minutes of aeration followed by no foam stability (0 ml) after the 10 minute settling time. Most new oil specifications require 10 ml to 50 ml tendency maximum and 0 ml stability.
Lubricant foam is an agglomeration of small bubbles, each like a balloon, where equal forces applied in all directions support the sphere. The bubble’s ability to break is related to surface tension and surface homogeneity.
“Two variables are of interest: foaming tendency and foam stability.”- Dave Wooton, Gerald L. Munson, and Greg Livingstone
If the fluid has strong interfacial surface tension, the bubble will be strong and tend not to break or coalesce. Thus, foam stability will be long lasting. If the fluid lacks surface homogeneity, it will affect the surface of the bubble. Thus, the forces toward the bubble walls now will be unequal — deforming, weakening and breaking the bubble wall.
Foam bubble strength is directly related to bubble size. Smaller bubbles exert less force against the walls and, thus, are more stable. Stokes Law defines foam stability in relation to the bubble size. In the fluid, the gas bubbles rise, separate and move in accordance with Stokes’ Law. Larger bubbles rise faster than smaller ones. Therefore, the stronger, small bubbles are last to reach the surface. When they do, their behavior depends on the nature of the bubble-surface interface. Modifying the interface to aid in breaking bubbles is the antifoaming agent’s job.
The most common way to increase foam in an aqueous soap solution is to add glycerin. It increases the solution viscosity and forms a weak bond with the water, factors that strengthen the bubble. Lubricant additives and contaminants have the same action. Detergents are a form of soap that often acts as a pro-foamant, wreaking havoc with the oil’s foaming characteristics.
Oxidation products are polar components that often increase fluid polarity and viscosity. Contamination products have been shown to produce stable foam. As they accumulate in the fluid, favorable foaming properties deteriorate. When you observe a lube foaming, always check the fluid’s particle count. Particulates can act as nucleation sites on which bubbles grow. Such bubbles often are the small ones, which are slower to dissipate. Any kind of surface-active contaminant can cause foam and air entrainment. Particles smaller than 4 μm have a similar effect. Apply ASTM Standard D4055-04, Standard Test Method for Pentane Insolubles by Membrane Filtration, using a 0.45 μm membrane to evaluate the fine-solid contamination to determine if there’s a problem.