How Can Liquid SHEAR Affect a Pump?

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In this edition of The Pump Report - Chad walks through the importance of understanding a liquid's shear sensitivity. To learn more visit our website at vikingpump.com

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Shear is relative motion between adjacent layers of a moving liquid. At rest, a liquid experiences no shear, but when put into motion shear starts to occur. For some liquids, shear has no effect on the
viscosity. These liquids are classified as Newtonian liquids. Water is a great example of this. Water retains its viscosity independent of the rate of shear. Other examples include fuel oil, alcohol, and glycerin. The majority of Viking Pump applications fall into a second category of non-Newtonian liquids. While there are a variety of each, let's keep this fairly simple. Some liquids decrease in viscosity as shear is applied. These are shear thinning liquids. Another class of non-Newtonian liquids increases in viscosity as shear is applied. These are shear thickening liquids. Ketchup is a great example of a shear thinning liquid. In the bottle it behaves as a viscous liquid, resistant to flow. But once it's sheared by motion from shaking or stirring its viscosity is reduced and it flows more easily. This shear thinning behavior is often a characteristic desired in a liquid. Take paint for example. When applied with a brush to a surface, it shears and thins to be be more easily spread. But at rest it thickens up, preventing it from running back down. The opposite of shear thinning liquids are shear thickening liquids - and there aren't as many examples of those. But I've mixed up one here. What I have is a mixture of about one part water and one part corn starch. When slowly stirred, it behaves like a liquid, flowing very easily. But when I apply more shear through increased velocity you can see that it becomes very viscous and behaves almost like a solid. So what does this have to do with pump selection? Well, first shear sensitive liquids, especially those that can be damaged by shear, should never be handled by a centrifugal pump. Slow running positive displacement pumps are better suited to the gentle handling of shear sensitive, non-Newtonian liquids. Secondly, understanding the true viscosity of the liquid when it is sheared by flow through the system is critical to the appropriate sizing of pipes, pumps, drive equipment, and motors. For example, lubricating grease tends to be highly shear thinning. It's not necessary to size the system to it's "at rest viscosity" because as soon as the pump turns on and the liquid is in motion the grease's viscosity drops significantly. For any liquid, understanding its viscosity is important. But for non-Newtonian liquids it's also important to understand shear and its effect on that viscosity.