Excerpt from the June 2022 Pumps & Systems Article by Kyle Clark, the Hydraulic Institute
Think again if you are pumping thick fluids using water pump performance curves.
Viscosity is a fundamental property of a liquid. It is a fluid’s resistance to flow and is higher for thicker fluids. For example, a fluid with high viscosity, such as maple syrup, is thicker and resists flow more when compared to a fluid with a lower viscosity, like water.
Typically, pump manufacturers use water to obtain the values for their pump performance curves, even if the intended service of the pump is for a fluid with properties that are different from water. But what happens when the fluid’s viscosity significantly deviates from water? This is where engineers need to adjust the pump performance curves to account for the difference in viscosity between water and the actual fluid in the pump.
Pump performance curves describe the head added to a fluid, pump power and net positive suction head required (NPSHr) at a variety of different volumetric flow rates. Due to the importance of centrifugal pump performance in every fluid industry, it is imperative that accurate corrections are used when a centrifugal pump uses a thicker fluid than what the pump manufacturer used to evaluate the performance. A more viscous fluid will generally experience a decrease in volumetric flow rate, head and efficiency compared to water at the same pump speed. Likewise, pump power and NPSHr increase as viscosity increases.
Pump manufacturers that only provide water performance curves for pumps should consider providing performance curves for thicker fluids. Engineers who have been correcting the water performance curves when pumping thicker fluids should consider using the American National Standards Institute/Hydraulic Institute (ANSI/HI) 9.6.7-2015 guidelines.
While it is preferred to use actual performance curve data from pump manufacturers for thicker fluids, ANSI/HI 9.6.7-2015 provides a commonly used guideline to correct pump performance based on viscosity. This guideline has an acceptable amount of uncertainty, but it is imperative to understand the uncertainties of this method to ensure its correct application in pumping systems. This article summarizes technical findings and discussion as to why the guideline is acceptable despite the uncertainties.
Viscosity corrections rely on empirical methods using test data to properly account for a pump performance when the service fluid has a different viscosity than the reference fluid, typically water. As with many empirical methods, uncertainty inevitably exists and falls into one of the following categories:
- The use of a dimensionless number to characterize complex phenomenon
- The limited data set used to create the empirical model
- The reliability of data measurement equipment
Before going into more detail, it is worth discussing why performance decreases when pumping a viscous fluid.
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