![]() ![]() The volute is a region that expands in cross-sectional area as it wraps around the pump casing. The vanes of the rotating impeller impart a radial and rotary motion to the liquid, forcing it to the outer periphery of the pump casing where it is collected in the outer part of the pump casing called the volute. The pump casing guides the liquid from the suction connection to the center, or eye, of the impeller. The pump casing has suction and discharge penetrations for the main flow path of the pump and normally has small drain and vent fittings to remove gases trapped in the pump casing or to drain the pump casing for maintenance.įigure 1 is a simplified diagram of a typical centrifugal pump that shows the relative locations of the pump suction, impeller, volute, and discharge. The pump casing provides a pressure boundary for the pump and contains channels to properly direct the suction and discharge flow. Centrifugal pumps enjoy widespread application partly due to their ability to operate over a wide range of flow rates and pump heads.Ĭentrifugal pumps basically consist of a stationary pump casing and an impeller mounted on a rotating shaft. The PD pump manufacturer should be consulted about the predicted or actual viscous performance characteristics.įor more information on the viscous performance of rotodynamic pumps, refer to ANSI/HI 9.6.7 at Read more HI Pump FAQs by clicking here.Centrifugal pumps are the most common type of pumps found in DOE facilities. This increase in volumetric efficiency is offset by other mechanical and friction losses. Increased viscosity can reduce the amount of slip, which can result in increased output flow per shaft revolution, and an increase the volumetric efficiency of the PD pump. In PD pumps, a certain amount of volume of liquid will leak (called slip) to a lower pressure region. Rotodynamic pumps have an impeller that increases the velocity of the liquid to induce flow, but PD pumps capture a volume of liquid and transfer it directly during every revolution of the shaft. PD pumps will behave differently than rotodynamic pumps handling viscous liquids. The physics of how PD pumps and rotodynamic pumps operate is different. Example performance chart for single-stage rotodynamic pump (Image courtesy of Hydraulic Institute)įor positive displacement (PD) pumps, this same relationship is not applicable. Note that head, flow and efficiency will decrease with the increased viscosity, and the pump input power will have a corresponding increase. Image 1 is an example of water test data for total head, power and efficiency with respect to flow rate corrected for the viscous liquid that will be used in the process. This considers the fluid viscosity, pump total head, flow rate at best efficiency and the rotation speed. The correction factors are based on the “B” parameter calculation. The ANSI/HI 9.6.7 Rotodynamic Pumps–Guideline for Effect of Liquid Viscosity on Performance establishes these correction factors based on empirical data for viscosities up to 3,000 centistokes (cSt), but allows the procedures to be used up to 4,000 cSt with increased uncertainty. These correction factors are used to correct the water performance to viscous performance. Viscous correction factors are defined in Equation 1 (CH for head, CQ for flow, and Cη for efficiency): These correction factors determine the head and efficiency curves for the pump when handling viscous liquids. This performance reduction can be estimated by applying correction factors for head, rate of flow and efficiency to the performance with water. When a highly viscous liquid such as a heavy oil is pumped by a rotodynamic pump, the performance can be significantly changed in comparison to performance with water, due to increased losses. What effect does pumping a high viscosity fluid have on the pump system? ![]()
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