an Engineering & Service Bulletin

Technical Topics

J.H. WRIGHT & ASSOCIATES

VISCOSITY AS APPLIED TO PUMPING AND MIXERS

Most of us have a general idea of the term "viscosity" from high school physics, but perhaps a little review of the subject as it applies to pumping and mixing equipment might be of interest.

At best, viscous is only a relative term, applied as a measure of a fluid's tendency to resist a shearing force.

The viscosity of most liquids (Newtonian) does not change when they are exposed to shearing forces. A few liquids react to shear by becoming thinner or less viscous (Thixotropic) and a very few become thicker or more viscous when sheared (Dilatent). Fortunately most Thixotropic and Dilatent liquids return to their original viscosity when the shearing force is removed.

There are two rare liquid types that don't revert to their original viscosity when the shearing force is removed (Rheopectic and Pseudoplastic), but they are so rare you probably will never run into either one of them.

There are so many scales and ways of measuring viscosity that confusion is easy. Most of these scales grew out of a need in a specific industry or process for their own use. Ever heard of Barbey, Casper's Tin Plate, Gardner-Lithographics, or Pratt & Lambert F? Some of these measure viscosity by timing the flow from a hole in the bottom of a can, others by rotating a spindle immersed in the liquid and measuring the torque. If you send back the "bingo card" at the bottom of this Technical Topics, I'll send you a compilation of all the viscosity scales I've been collecting for thirty years.

Examples of Thixotropic liquids are ketchup and paint. Both seem very thick but flow nicely when sheared; ketchup thins when stirred or shaken and paint flows easily off the brush. Seventy percent (70%) Kaolin Clay slurry used for coating paper is very Dilatent. It's not hard to visualize the problems that pumping and mixing devices can have with Thixotropic and Dilatent materials. After all, they can hardly operate without causing some shear. In very, very simple terms - what do you need to know from the standpoint of pumping these viscous liquids with centrifugal pumps?:

Newtonian: Don't attempt much above 2,500 SSU.

Thixotropic: Usually no problem - if you get the liquid to the pump.

Dilatent: Careful, but can be done by opening up the impeller clearances.

How about mixing?:

Newtonian: No problem at any viscosity - just pick the proper type turbine.

Thixotropic: No problem, but your mixerexpert is going to want viscosities at several spindle speeds to properly size the mixer.

Dilatent: Careful - you must use slow speeds and low shear type impellers.

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