When doing a pump selection for a specific application, it is important to know whether the product to be pumped has a higher viscosity than water. Viscosity of fluid is defined as that property which offers resistance to flow due to internal fricti on within the liquid. Fluids with high viscosity move slowly and resist deformation, such as honey, syrup, and peanut butter. Fluids with low viscosity such as water, milk, beer, and olive oil move easily and have low resistance in pipework.
Temperature often has a dramatic effect on the viscosity of high viscosity fluids, with the viscosity getting less as the product is heated. Also be aware that there are liquids that are thixotropic (also known as stir thinning). A thixotropic liquid becomes less viscous (thinner or flows more easily) when shaken, stirred, agitated or otherwise stressed.
When it comes to pumping low viscosity fluids, the pump selection is not as complex as for high viscosity fluids. Centrifugal pumps are best suited for low viscosity fluids as the pump’s performance is not greatly affected by small changes in viscosity. For liquids slightly thicker than water, pump performance may be reduced, and there is typically an increase in power draw. However, high viscosity has a huge effect on centrifugal pump performance as an increase in the viscosity of the liquid will quickly decrease the pumps efficiency, reducing head and flow, so it is crucial to ensure you are using the right pump for your application.
Whether you are pumping high viscosity fluids like honey or oil, positive displacement pumps such as gear pumps, diaphragm pumps, vane pumps, and hose pumps must be used. These pumps are positive displacement and can operate at lower speeds, with variable flow rates to compensate for the products thickness. Positive displacement pumps deliver a constant flow of fluid at your desired speed if the viscosity (temperature) remained constant, but as the viscosity increases, the resistance to flow increases which means these pumps require more power. Peristaltic pumps can effectively handle both high and low viscosity products, can run dry without risk of pump failure and have a flow rate that is proportional to the rotation speed, making them suitable for repeatable dosing applications.
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There are two other important considerations to consider when pumping high viscosity products. It is important there is no restriction on the suction side and a flooded suction is imperative. Secondly the system pipework size will very likely need to be increased because of the extra drag or friction loss.
Viscosity is normally expressed in units of SSU or cSt. Provided the viscosity of the liquid to be pumped is known, there are correction charts available for each type of pump to determine the performance you can expect from the pump. We can offer the best solution for your desired application. Get in touch with us today to discuss your product viscosity, pipe size and flow rates so we can help you make the best pump selection.
