2.21 Pumping high viscosity liquids with positive displacement pumps

Displacement pumps for highly viscous Iiquids

Pumping high viscosity liquids with positive displacement pumps there are some considerations to make. At high viscosities the speed of the displacement pump must be reduced. In principle the speed should be chosen in inverse proportion to the viscosity, but this results in too high a purchase cost for the pump. As a rule the speed of the pump is reduced in inverse proportion to the viscosity raised to about 0,7. Figure 2.211 shows an example of speeds used for gear pumps with internal gears.

Figure 2.211 Example of pump speeds at high viscosity for gear pumps with internal gears

When choosing pumps for highly viscous liquids the design of the suction side must be studied with respect to the admixture of air, etc. Large pumps with large dimensions and Iow speeds are to be preferred. For liquids with extrem viscosity feed devices may be necessary. Displacement pumps which can handle maximum pumpable viscosities, approx. 1 million mm2/s are eccentric screw pumps, lobe rotor pumps and piston pumps, see further under technical features in Chapter 3 Pumps.