A fluid’s viscosity, or thickness, will affects how it will behave in a pump. Fluids are classified into four main groups Newtonian, Pseudoplastic, Dilatant and Thixotropic.

fluid viscosity

One of the most important aspect to understand before any pump selection is the viscosity of the fluid which is going to be pumped.

A fluid’s viscosity, or thickness, will affects how it will behave in a pump. However, this is where things start becoming complicated because the viscosity of fluids/liquids can change under different conditions.

Generally, fluids are classified into four main groups:

  • Newtonian
  • Pseudoplastic
  • Dilatant
  • Thixotropic

Newtonian

water
Viscosity remains constant regardless of changes in shear rate or agitation. As pump speed increases, flow increases proportionately. It does not matter how fast they move or how much you agitate them, they flow the same.

Typical Newtonian fluids:

  • water
  • mineral oils
  • alcohol
  • hydrocarbons

Pseudoplastic

tomato paste
Viscosity decreases as shear rate increases, but initial viscosity may be great enough to prevent flow from starting in a typical pumping system. Taking tomato paste as an example, you will have to shake the bottle before the tomato paste starts to move, but once it is moving, it flows fairly easy.

Typical pseudoplastic fluids:

  • tomato paste
  • latex paints
  • lotions

Dilatant

Butter
Viscosity of fluid increases with agitation until they become almost solid. As a result, pumps can bog down and stall after initially pumping dilatant liquids.

Typical dilatant fluids:

  • slurries
  • clay
  • cream
  • butter

Thixotropic

peanut butter
Like pseudoplastic fluids, 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.

Typical thixotropic liquids:

  • soaps
  • tars
  • shortening
  • glue
  • inks
  • peanut butter

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

What is most important about viscosity during pump selection?

Kinematic viscosity is a real physical factor that influences pump curves, and thus the choice of pump.

Kinematic viscosity is denoted in centistokes (cSt) and is measured in mm2/s.

Based on the table above, water has a low viscosity of 1 cSt at 20°C, while motor oil has a high viscosity of more than 500 cSt at the same temperature.

Viscosity is essentially resistance to flow and this has implications for pumps. If you have a high-viscosity, sticky fluid on your hands, it clings and it’s much harder to remove than a low-viscosity fluid.

It is the same in a pump, where you use impellers instead of your hands, to move the fluid. If you are pumping fluids like motor oil that stick to the surface of the impeller, this is a problem that needs to be considered.

So what’s next?

If you need help with your pump selection, please contact us through our online enquiry form or give us a quick call.

Our sales team will be available for your assistance.