HYDRAULICS 18 HYDRAULICS & PNEUMATICS March 2023 www.hpmag.co.uk based antifoaming agents to an industrial lubricant could reduce foaming by 50%, even when the fluid was cycled several times through a fine filtration system that screens out particles greater than 3 ?m. Optimising the antifoaming chemistry with a supplementary antifoaming agent also helped to suppress foaming after fine filtration, although it often needed a mixture of antifoaming agents to achieve the desired results. Understanding air entrainment and foaming To improve fuel and energy efficiency, manufacturers are making equipment designed for higher loads and pressures with smaller reservoirs and tanks. This means that the oil spends less time in the sump, so must have better air and water separation properties. At the same time, most equipment operators demand longer oil-drain intervals to reduce maintenance and lubricant costs. Consequently, the oil must work harder for longer, which results in higher temperatures that can affect its ability to release air. All these factors mean that lubricants with excellent air-release properties are more important than ever. In tests conducted at RWTH Aachen University, Germany, hydraulic fluids using synthetic base oils, such as gas-toliquids (GTL) fluids, combined with a performance additive package, had bubbles with much larger diameters than fluids using mineral base oils. These larger air bubbles in GTL fluids rise to the surface quicker than smaller air bubbles, which results in GTL fluids having superior air release performance. However, balance needs to be struck in a lubricant formulation for both good foaming and good air entrainment properties. Silicone-based additives, for example, are excellent anti-foaming agents but are poor for air release. To address this problem, Shell’s base-oil systems using a technique developed at Stanford and recorded the time for each bubble to coalesce. The bubble rupture rates correlated well with the bulk foam measurements from industry-standard tests such as ASTM D892. Furthermore, the team observed that multicomponent base-oil systems stabilised the bubbles more than the single-component systems. In multicomponent systems, as the lighter components evaporated, the surface tension of the oil increased and created small flows on the bubble surface like wine droplets clinging to the side of a glass. These flows drew more oil to the top of the bubble, thereby thickening its wall, which made it less likely for it to burst. However, in single-component systems such chemically driven flows were missing, which resulted in faster bubble rupture through gravitational drainage. The research team is now developing mathematical models for determining the effects of antifoaming agent distribution and evaporation on foam stability that will enable them to simulate how pure or blended oils might perform before and after filtration. They will then apply their findings to designing formulations that reduce foaming. Shell is incorporating many of these findings into its product development and continuing to research this vital issue. It believes that these ongoing studies will have a significant impact on the development of foam-resistant lubricants and combat the unseen enemy of air contamination in lubricants. www.shell.com/lubricants *Shell Lubricants refers to the various Shell companies engaged in the lubricant business. statistics and chemometric group, working with researchers at Shell Technology Centre Houston and Milwaukee School of Engineering, USA, helped to find, map and screen multiple base-oil combinations of the same viscosity to determine the best formulas for faster air release. They found that GTL base oils had exceptional air-release properties compared with mineral base oils of the same viscosity. The group developed fully formulated, prototype GTL hydraulic fluids using an optimised base-oil mixture and performance additive package. The fully formulated GTL hydraulic fluid had a much better airrelease time than a fluid with a standard base oil and the same additive package. Shell scientists also undertook to understand the fundamental mechanics behind foam rupture by carrying out a joint collaborative project with Stanford University, USA to study bubble rupture dynamics. The team conducted singlebubble rupture studies on a range of
RkJQdWJsaXNoZXIy MjQ0NzM=