Foam is a dispersion of an entrapped gas in a liquid where air bubbles form a separate layer on the surface of the fluid and are separated by relatively thin fluid films. Foam stability can vary in a hydraulic system, Figure 2 and Figure 3, depending on surface area, surface tension, viscosity, and the concentration of the contaminant.

In the absence of a defoamer, all foam will eventually end up as a stable high surface area foam. However, if the foam continues to grow, displacing the liquid phase, it may reach the pump inlet, causing cavitation.

The only way to break this high surface area foam is to introduce a defoamant (i.e. a silicone) into the surface film that stabilizes the foam, Figure 4(A). The defoamant then spreads throughout the surface of the film, surrounding the air pockets, Figure 4(B). As the defoamant spreads, the shearing force causes a flow of the stabilizing film away from the gas bubble interface, resulting in a thinning of the interfacial film, Figure 4(C). This continues until the bubble ruptures, finally resulting in a release of the gas contained in the bubble, Figure 4(D).

Antifoam composition varies from single to multiple component systems. Single component systems are typically water-insoluble and surface active, since they must displace the pre-foaming surfactant in the interfacial film stabilizing the gas bubbles. Examples of single component systems include: fatty acids and their glycerides or ethoxylates and polypropylene glycols. Typically, this class of antifoams is used at concentrations of 0.1 to 0.4%.

A multicomponent antifoam system typically contains a dispersion of a mineral oil-based material, hydrophobic silica, and a surfactant, such as a fatty acid or alcohol ethoxylate composition in the fluid through- out the hydraulic system.

A bubble eliminator?

A bubble eliminator is a tapered-tube type device with a chamber of circular cross-section that becomes smaller (in relation to its length), and is connected to a cylindrical shaped chamber. Fluid with bubbles flows tangentially into the larger end of the tapered tube from an inlet port and forms a swirl flow that circulates fluid through the flow passage. The swirl flow accelerates downstream, and the fluid pressure along the central axis decreases downstream. From the end of the tapered tube, the swirl flow decelerates downstream and the pressure increases towards the out- let port. Bubbles are trapped in the vicinity of the central axis and collected near the area where the pressure is lowest. When backpressure is applied by a check valve or an orifice located at the downstream side of the bubble eliminator, the bubbles are ejected through a vent port. The dissolved gas in the fluid is also eliminated by extracting bubbles at the pump’s suction side under negative pressure.

Use of such a device may allow the hydraulic designer to gain the following benefits:
● a reservoir with lighter weight, smaller space, and lower cost
● slower fluid degradation, which extends fluid’s useable life
● prevent pump cavitation and noise
● requiring less fluid in reservoir, which reduces cost and increases safety
● shorter heating time in cold weather
● decrease in fluid compressibility
● easier contamination control, and
● simpler configuration of reservoir, with no baffle plate needed.

George E. Totten is senior research scientist and Roland J. Bishop, Jr. is a project scientist in UCON fluids and lubricants for the Dow Chemical Co., Tarrytown, N.Y. Ryushi Suzuki is president of Opus System, Inc., Tokyo. Yutaka Tanaka is a professor of engineering at Hosei University, Toyko. For a more detialed discussion, refer to SAE Technical Papers, 982037 — Bubble Elimination in Oil for Fluid Power Systems, by Suzuki/Tanaka/ Arai/Yokata; and 972789 — Hydraulic Fluids: Foaming, Air En- trainment, and Air Release — A Review, by Totten/Sun/Bishop. Either can be ordered from SAE by calling 412/776-4841 or visiting www.sae.org.