Viscosity correction, centrifugal pumps

External fires, 450 Unwetted gas only vessels, fire, 454 Viscosity correction, centrifugal pumps, 203-207... 

Mdien dscous liquids are handled in centrifugal pumps, the brake horsepower is increased, the head is reduced, and the capacity is reduced as compared to the performance with water. The corrections may be negligible for viscosities in the same order of magnitude as water, but become significant above 10 centistokes (10 centipoise for SpGr = 1.0) for heavy materials. While the calculation m.ethods are accepta oly good, for exact performance charts test must be run using the pump in the service. 

Figure 3-56. Viscosity performance correction chart for centrifugal pumps. Note do not extrapolate. For centrifugal pumps only, not for axial or mixed flow. NPSH must be adequate. For Newtonian fluids only. For multistage pumps, use head per stage. (By permission. Hydraulic Institute Standards for Centrifugal, Rotary, and Reciprocating Pumps, 13th ed.. Hydraulic Institute, 1975.)...

Figure 32.26 demonstrates a well-known method of correcting for fluid change from water for a centrifugal pump. This allows an engineer to predict change in performance if the kinematic viscosity of the liquid to be pumped is known and the water test data are available. 

Figure 7.15. Effects of viscosity on performance of centrifugal pumps (a) Hydraulic Institute correction chart for pumping liquids, (b) Typical performances of pumps when handling viscous liquids. The dashed lines on the chart on the left refer to a water pump that has a peak efficiency at 750 gpm and 100 ft head on a liquid with viscosity 1000 SSU (220 CS) the factors relative to water are efficiency 64%, capacity 95% and head 89% that of water at 120% normal capacity (1.2QH).

Meyerhofer (4) examines the problem of spinning a two component system which is composed of solids" and a volatile "solvent". He includes the effect of solvent evaporation and a viscosity which depends on solvent concentration. In this model, the film height decreases with time not only because of the centrifugal pumping, but also because of the evaporation from the surface. As the film is depleted in solvent, the viscosity increases and less fluid is lost because of pumping. Eventually a finite film thickness can be reached because all of the solvent evaporates, and the film consists of only an immobile solid phase. The present analysis is similar to Meyerhofer s. His treatment of the evaporation is corrected, as is his conclusion on the effect of spin speed (which apparently is due to numerical error on his part), and some of his assumptions in developing the equations are relaxed. 

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