Liquid Pumps—Viscosity Handling Characteristics

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.

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Viscosities for Typical Liquids

 

Typical liquid	Centipoise (cp)	Centistokes (cSt)	Saybolt Second Universal (SSU)
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

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Viscous Liquid Behavior

Pumping Viscous Liquids with Gear Pumps

Gear pumps are well suited for pumping viscous liquids if the following rules are observed:

1. 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.
2. Suction and discharge lines must be increased by at least one, or better, two pipe sizes over the size of the pump ports.
3. Horsepower of the motor must be increased over the power required for pumping water under the same pressure and flow.

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Pumping Viscous Liquids with Centrifugal Pumps

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.

 

 

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