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.
Viscosities for typical liquids
Typical liquid Centipoise
Water 1 1 31 Milk 3 4 40 No. 4 fuel oil 12.6 15.7 80 Cream 20 20.6 100 Vegetable oil 40 43.2 200 SAE 10 oil 88 110 500 Tomato juice 180 220 1,000 SAE 30 oil 352 440 2,000 Glycerine 800 1,100 5,000 Honey 1,500 2,200 10,000 Glue 3,000 4,500 20,000 Mayonnaise 5,000 6,250 28,000 Molasses B 8,640 10,800 50,000 Sour cream 15,000 19,000 86,000
Viscous liquid behavior
The design of peristaltic pumps makes them suitable for abrasive and/or viscous fluids such as suspensions, creams, slurries, lubricants, paints, liquid waste, and more. To maximize the pumping efficiency of viscous fluids, follow these steps:
Gear pumps are well suited for pumping viscous liquids if the following rules are observed:
- Pump speed (rpm) must be reduced if viscosity is above 100 cp. When pumping viscosities above 100 cp, call our Application Specialists to determine optimal rpm and other operating parameters.
- Suction and discharge lines must be increased by at least one, or better, two pipe sizes over the size of the pump ports.
- Horsepower of the motor must be increased over the power required for pumping water under the same pressure and flow.
Centrifugal pumps are generally not suitable for pumping viscous liquids. They are best suited for pumping liquids with viscosities less than 200 cp. Volume and pressure capabilities of the pump are reduced with increasing viscosity.