Viscosity will affect positive displacement and rotodynamic pumps differently. In positive displacement pumps, higher viscosity limits leakage (slip), increasing the displacement volume and volumetric efficiency.
However, there will be friction losses that will offset volumetric efficiency gains. So, at a certain viscosity, the efficiency will decrease. In general, positive displacement pumps will be affected less by viscosity than rotodynamic pumps.
When a liquid of high viscosity is pumped by a rotodynamic pump, the performance is changed in comparison to performance with water, due to increased friction losses. The reduction in performance on viscous liquids may be estimated by applying correction factors for head, rate of flow and efficiency to the water performance as outlined in ANSI/HI 9.6.7 “Rotodynamic Pumps—Guideline for Effects of Liquid Viscosity on Performance.”
As detailed in the standard, the curves for head and efficiency for viscous liquids (vis) are estimated from the head, flow and efficiency measured with water (W) by applying the correction factors CH, CQ and Cƞ, respectively (Equation 2).
An updated version of the standard will be published in 2021. In preparation for publication, the committee found the new data sources to be within the uncertainty range of data used to develop the empirical correction factors. HI continues to seek data on viscous performance compared to water test data to refine and expand the correction factors. Contact hitechnical@pumps.org if you can support this request.
For more information, refer to ANSI/HI standard 9.6.7 “Rotodynamic Pumps—Guideline for Effects of Liquid Viscosity on Performance” at www.pumps.org/standards.