VISCOSITY. HOW IT AFFECTS YOUR CENTRIFUGAL PUMP V031
Viscosity is a measurement of resistant to flow. The higher the viscosity of a liquid the thicker it is, and the more difficult it is to pump.
- In the United States, we use an instrument called the Saybolt Universal Viscometer to measure low to medium viscosities. The meter measures the time it takes for a certain amount of liquid to flow through a glass tube, and the result is viscosity stated in Seconds Saybolt Universal (SSU).
- For high viscosity liquids we switch to the Saybolt Furol Viscometer that uses the units SSF
- Two of the more popular units of measuring viscosity are centipoises (cP) and centistokes (cSt). There are conversion tables to correlate these units
Centrifugal pumps are used to pump liquids with viscosities up to 500 SSU and occasionally higher (water has a viscosity of 32 SSU). There is no clear rule as to when you should shift from a centrifugal to a positive displacement pump.
A liquids viscosity can change with a change in temperature, or as a result of agitation.
- The viscosity of dilatants increases with agitation (clay slurries, cream becomes butter)
- The viscosity of Thixotropics decreases with agitation (non-drip paint, auto wax)
- Newtonian fluids are not affected by agitation.
As viscosity increases, the operational characteristics of a centrifugal pump will also change. As an example:
- Flow, head and efficiency are reduced.
- The brake horsepower required is increased.
- These changes are largely due to an increase in the fluid friction and the “disk” losses that occur due to viscous drag on the impeller.
- The increased fluid friction reduces head and flow while viscous drag increases the horsepower required.
Take a look at the following table describing the affect of viscosity on a small centrifugal pump.
In January 2006, the hydraulic Institute introduced the ANSI standard HI 9.6.7 to predict the affect of viscosity on your centrifugal pump.