Note the marked efficiency drop from near 80 percent on water to approximately 50 percent on 1,000 SSU for the pump in Image 6. Consideration for these are outlined in ANSI/HI 9.6.7. Not shown in Image 6 but also a concern is the increase of the required net positive suction head (NPSH) where a 3 percent head loss is noted (NPSH3) as well as increased required starting torque with viscous liquids. The procedure is important so that the suitable pump and driver is selected for a required duty on viscous liquids. The HI method enables pump users and designers to estimate performance of a particular rotodynamic pump on liquids of known viscosity, given the performance on water. Centrifugal pump “water” performance and corrected viscous performance This standard prescribes an empirical method based on test data available from sources throughout the world. ANSI/HI 9.6.7 was used to correct the performance as shown in Image 6. The viscous data should be corrected from the water performance test per Hydraulic Institute standard ANSI/HI 9.6.7 Effects of Liquid Viscosity on Rotodynamic Pump Performance. The performance curve in Image 6 shows the water performance and the corrected viscous performance for the application liquid, which has a viscosity of 1,000 SSU and specific gravity of 0.9. A marked increase in input power due to reduced efficiency and a reduction in head and rate of flow occurs with viscous liquids compared to water. The performance of a centrifugal pump is affected when handling viscous liquids because of the increased friction when the impeller rotates and the resistance to flow compared to water test. It is industry standard to test the performance of centrifugal pumps with clear water per ANSI/HI 14.6 Rotodynamic pumps for Hydraulic Performance Tests. Rotary (screw) pump Centrifugal Pump Viscous Pumping Considerations
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