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.
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 |
Viscous liquid behavior
Pumping viscous liquids with:
Peristaltic pumps
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:
- Slow down the speed of your pump. Increasing the speed beyond a certain point will have a diminishing effect on flow rate. The maximum efficient speed of the pump decreases as viscosity increases and tubing size decreases.
- Choose a larger size tubing than required to pump water.
- Choose a firm tubing such as Chem-Durance® Bio, Norprene®, PharmaPure®, PharMed® BPT, Tygon® E-LFL, GORE® STA-PURE® Series PCS or GORE® STA-PURE® Series PFL. Performance will be better because the firmer tubing returns to its original shape quickly after pump head occlusion creating the suction needed to pump the viscous fluid. For L/S®, and I/P® sizes, choose high-performance precision tubing—the thicker wall tube sizes also return more quickly to their original shape than precision tubing. The quicker return allows liquid to be pulled into the tubing with greater force.
- Select a tubing with a smooth bore. A smooth bore will minimize frictional forces. BioPharm, BioPharm Plus, Tygon® E-Lab, Tygon® E-LFL, Puri-Flex® or silicone formulations are good choices.
- Decrease the viscosity of your fluid. Heat your fluid, if possible, at the source or in an immersion bath. Viscosity typically decreases with temperature and if the fluid will not be compromised by heating this will make it easier to pump.
Gear pumps
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
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.