Learn the difference between viscosity and shear.
by Jim Elsey
May 23, 2018

For Newtonion fluids, the relationship between shear stress and shear rate is linear. For non-Newtonion fluids, the relationship is different since it is mostly nonlinear and can also be time dependent.

For Newtonion fluids, temperature is the only thing that changes the viscosity. For non-Newtonion fluids, temperature still changes the viscosity, but more importantly viscosity changes with applied shear stresses (agitation or pressure).

The viscosity of a dilatant fluid becomes more viscous when agitated (shear stressed).

This is very important when pumping clay slurries. Clay slurries are widely used in the paper and ceramics industries. Dilatants are known as shear thickening fluids.

The viscosity of thixotropic fluids will decrease with increased stress and be easier to pump after some time and effort to get them started, but if allowed to rest will return to the more resistant state. In these cases, make sure you size the driver for the initial load of getting the fluid flow initialized. That is, size the driver torque and brake horsepower (BHP) for the apparent viscosity in lieu of the normal working viscosity. Size the pump hydraulics for the working viscosity.

Lastly—and just because I grew up on a dairy farm—do not confuse density with viscosity. Cream is thicker than milk, but is less dense.

Conclusion

Fluid behavior and rheology are much more complicated than this column would indicate. The purpose here is simply to make you aware of the basics and to be cognizant as to the potential deleterious effects on the pump and the system.

References

Fluid Mechanics (6th Edition), Frank M. White
Cameron Hydraulic Data Book (19th Edition)

To read other articles in the 'Common Pumping Mistakes' column, go here.

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