Pumping viscous liquids can present some difficult problems. When selecting a pump, fluid viscosity must always be considered. The effects of viscosity can be better understood by looking at the behavior of viscous liquids when subjected to force.
Newtonian Liquids—Viscosity remains constant regardless of changes in shear rate or agitation. As pump speed increases, flow increases proportionately. Liquids displaying Newtonian behavior include water, mineral oils, syrup, hydrocarbons, and resins.
Pseudoplastic Liquids—Viscosity decreases as shear rate increases, but initial viscosity may be great enough to prevent flow from starting in a typical pumping system. Typical pseudoplastic liquids are gels, latex paints, and lotions.
Dilatant Liquids—Viscosity increases as shear rate increases. Pumps can bog down and stall after initially pumping dilatant liquids. Some liquids showing dilatant behavior are slurries, clay, and candy compounds.
Thixotropic Liquids—Like a pseudoplastic liquid, viscosity decreases as shear rate or agitation increases. When agitation is stopped or reduced, hysteresis occurs and viscosity will increase. Often the viscosity will not return to its initial value. Some examples of thixotropic liquids are soaps, tars, vegetable oils, shortening, glue, inks, peanut butter, and some slurries.