Machinery Lubrication

Machinery Lubrication July - August 2016

Machinery Lubrication magazine published by Noria Corporation

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he formation of foam is a common problem with lubri- cants, particularly those with more system turbulence. Foam becomes an issue when it is out of control or when it affects a machine's operation. For this reason, lubricants are formulated with certain additives known as anti-foam agents or defoamants. These additives are larger in size and may be prone to separa- tion in certain cases. As system cleanliness becomes more stringent, it raises the ques- tion of whether these additives can be filtered out of the oil. What Are Defoamants? Defoamants primarily exist as one of two chemical compositions: methyl silicone and polymethacrylate. They are blended in oil at different concentrations based on the viscosity and other physical properties of the lubricant in which they are designed to work. Sometimes the application also makes a difference in the type of anti-foam agent to be used. For instance, gear oils with higher viscosities have more of a tendency to foam, due to the likelihood of air bubbles residing in the fluid longer, than a low viscosity turbine oil in which the air bubbles can rise more rapidly, accumulate on the fluid's surface and break on their own. Most lubricating oils are formulated with defoamants to minimize the risk of a stable foam occurring on the oil level's surface. For the lubricant to have the best chance of performing its intended purpose, there must be a delicate balance between defoamants and all other additives blended in the lubri- cant. If the concentration of defoamants is too low, the machine may experience issues. If the concentration is too high, it could lead to excess foam due to a change in the oil's surface tension, which impairs the oil's air-handling characteristics. How Defoamants Work As air becomes entrained in oil, air bubbles attempt to rise to the surface. While a bubble is rising through the oil, it passes through and picks up any number of defoamant additives that are blended into the lubricant. Defoamants are a little different than most additives in that they are suspended in oil rather than dissolved. This is important because defoamants would lose their ability to minimize foam if they were in a dissolved state. Once an air bubble traps some of these additives in the bubble wall and finally reaches the oil's surface, the defoamant works to impair the film strength of the bubble wall. Think of it as providing a weak spot in a chain. The interfacial tension of oil is relatively high, but the interfacial tension between the oil and the defoamant droplet is much lower. At this point, the additive spreads and ruptures, allowing air to spill into the atmosphere as the bubble bursts and the stable foam on the oil's surface is minimized. In a perfect world, defoamants would do their job, and foam would never be a problem. However, these additives can lose T C o n t a m i n a t i o n C o n t r o l Can DEFOAMANT Additives Be FILTERED? IN THE TRENCHES We s ca sh | Nori a Corpor at ioN 46 | July - August 2016 | www.machinerylubrication.com How temperature and viscosity influence the foam tendency for mineral oils

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