Bubbles in a hydraulic or lubrication system can undermine operating efficiency, inhibit heat removal, increase wear, and ultimately, lead to higher maintenance and component-replacement costs. Too often, equipment operators dismiss air contamination as inevitable. But you can take steps to reduce or eliminate the harmful effects of air contamination.
Air intrusion can take many forms, but foam and entrained air represent the biggest threats for equipment operators. Entrained air refers to bubbles suspended below the fluid’s surface that create air pockets trapped in the fluid. Foaming, on the other hand, results when those bubbles rise to the surface of the fluid. In both cases, the air pockets can impede flow and leave equipment vulnerable to wear.
Surface foam in a well-designed and properly filled hydraulic reservoir rarely causes issues during equipment operation. However, foaming may indicate more fundamental problems, such as oil contamination or degradation. Excessive foaming can cause the oil level to drop so much that the pump’s suction line is exposed at the surface, resulting in aeration of the hydraulic fluid. Excessive foaming can even cause oil to overflow out the reservoir breather, where it becomes hazardous.
Although foam is more visible and tends to draw the most concern from maintenance people, entrained air actually causes the most damage. Entrained air within a hydraulic or lubrication system can be harder to identify because it has few external or visual indicators. Air becomes entrained through normal engine and machine vibration, flow surges from retracting cylinders, leaks, incorrect oil level, and from working on slopes. Entrained air can cause cavitation, microdieselling, and increased noise or vibration. The ultimate result is excessive component wear. It may also reduce power, responsiveness, or efficiency.
The nature and extent of foaming results from the properties of a lubricant’s base oil and the additives used in it to provide the desired lubrication. As a result, antifoaming agents typically have been used to control excessive foaming. Unfortunately, most of these agents rely on large silicon particles, which work by bridging the foam layers from their lower surface tension, eventually causing bubbles to rupture. Modern systems, however, usually have filtration systems that screen out such large particles, undermining the effectiveness of these additives.
Shell Lubricants has studied the causes and effects of air contamination and has developed an understanding of foaming and entrained air and how they interact with a wide variety of systems. Working with equipment manufacturers and researchers, we have identified and tested more than ten potential antifoaming components that can pass through fine filters without compromising their effectiveness. The most promising additives—which have a silicon backbone structure but are modified to yield good foaming characterisitcs—provide excellent foam control, high material compatibility, and, most importantly, remain effective after filtration.
Our tests found that adding these silicon-based antifoam agents to an industrial lubricant could reduce foaming by 50%, even when the fluid was cycled several times through a fine filtration system, which generally traps particles greater than 3 µm. Using a supplementary antifoaming agent often helped suppress the foaming upon fine filtration and retained the performance characterisitics of the oil for a longer period of time.
Shell’s team of scientists, working with researchers at Stanford University, studied bubble stability, rupture dynamics, and other properties of foaming to better understand the causes and most effective solutions for foam mitigation. Among the findings, the team conducted single-bubble rupture studies on a range of base oil systems using Stanford’s technique and recorded the time for each bubble to coalesce. The bubble rupture rates correlated well with the bulk foam measurements done using the industry standard tests such as ASTM D892 (Standard Test Method for Foaming Characteristics of Lubricating Oils).
Further, the team observed that multicomponent base oil systems stabilized the bubbles more than single-component systems did. As lighter components evaporated in multicomponent systems, the surface tension of the oil increased and created small flows, like wine along the side of a glass. These flows drew more oil to the top of the bubble, thickening its walls, which made it less likely to burst. However, in single-component systems, such chemically driven flows were missing, resulting in faster bubble rupture from gravitational drainage. This study was reported in a recent article in Proceedings of National Academy of Sciences.
The research team is now developing mathematical models for determining antifoam distribution and evaporation effect on foam stability that will allow them to simulate how pure or blended oils will perform, both before and after filtration. They will then apply those findings in designing formulations that reduces foaming.
Entrained air refers to bubbles suspended below the fluid’s surface that create air pockets trapped in the fluid, whereas foam results when those bubbles rise to and remain at the surface of the fluid.
Tackling Air Entrainment
To improve fuel and energy efficiency, manufacturers are making equipment for higher loads and pressures with smaller reservoirs. This means the oil spends less time in the sump, and as a result, the oil must have greater air- and water-separation properties. At the same time, most equipment operators demand longer intervals between oil drains to reduce maintenance and lubricant costs. The oil, in other words, must work harder for longer, resulting in higher temperatures that can affect its ability to release air. All these factors mean that lubricants with good air-release properties are more important than ever.
Creating a lubricant that controls both foaming and air entrainment is complex. Silicone-based additives, for example, are excellent antifoaming agents but fall short for air release. To address this dilema, Shell’s statistics and chemometric group helped find, map, and screen multiple base oil combinations of the same viscosity to determine the best formulas for faster air release. We found that gas-to-liquid (GTL) base oils had exceptional air release properties compared to mineral-base oils of the same viscosity. We then developed fully formulated, prototype GTL hydraulic fluids using an optimized base oil mixture and performance additive package. The fully formulated GTL hydraulic fluid had a much quicker air-release time than fluid with a standard base oil and the same additive package.
In our tests, hydraulic fluid with synthetic base oils, such as GTLs, combined with a performance-additive package, had bubbles with much larger diameters than fluids with mineral-base oils. The larger bubbles ruptured more quickly, allowing the air to escape more rapidly.
Although we incorporate many of these findings into our product development, we continue to research this vital issue. We believe these ongoing studies will have a significant impact on the development of air-resistant lubricants and prolong the life of a lubricant in our machinery.
Abhishek Kar, Ph. D., is a research engineer, Lubricants Discovery Hub, and Sravani Gullapalli, Ph.D., is a research engineer, Hydraulics, Industrial Oil and Tech Services, both at Shell Global Commercial Technology. For more information, visit www.shell.com.