The Importance Of Fluid Viscosity In Choosing Equipment

While selecting your equipment for pumping fluids, it’s important to have a full understanding the properties of the fluids that will pass through it. Whether it’s pipes to transfer liquids, pumps to mix and move them or the different types of filters needed, the viscosity of the fluid is a factor that will determine the equipment you need. Understanding viscosity (and other properties of fluids) and the reactions between the fluids and the material of the equipment will help you select the right type and size of equipment to get the best performance.

Broadly, fluids are classified into Newtonian and non-Newtonian fluids. Newtonian fluids have a viscosity that remains constant, regardless of the amount of shear applied. (Shear is defined as the amount force that is applied parallel-ly, creating friction between the particles of the fluid because of its viscosity.) Non-Newtonian fluids, therefore, are the opposite. This means when shear is applied the viscosity of the fluid changes. Gasoline, water, alcohol, and oils are examples of Newtonian fluids. Suspensions, slurries and gels are examples of non-Newtonian fluids.

With non-Newtonian fluids, the impact on viscosity happens in one of a few ways: if the viscosity increases with shear, it’s called dilatant, for example, quicksand; if viscosity reduces with shear, it’s called pseudoplastic, like regular paints. Temperature and time also have a bearing on liquid viscosity, making for a variety of complex, non-Newtonian fluids. The last group falls into this category and is called thixotropic fluids. These are complex, both to define and deal with in pumping. Glues, soaps and starches belong to this group. An overview definition of these fluids is that they are are viscous if still, but will thin with time and agitation.

Once you know the viscosity, (as well as some other operating parameters) you can choose the type of pump needed. Typically, centrifugal pumps are used for low viscosity fluids because of the high shear generated, which as defined makes a thinner liquid that’s easier to pump.

With fluids of greater viscosity, pumps need adjustment to account for their resistance to shear. The best type of pumps in the industry for more viscous fluids are positive displacement pumps (different types of these pumps are available depending on the application at hand). These pumps operate slower and generate more shear.

Friction determines the choice and size of pipes, filters, valves and instrumentation. Plenty of excellent data is available to give you information on loss of friction for different viscosity and flow rates. More than anything else, however, it is experience with the liquids in question that gives you the best possible indication of what to expect and which kinds of equipment to choose. So ensuring your team includes people knowledgeable in the field of fluid viscosity (especially your specific fluids) is a good place to start. Taking the data available against characteristics of the fluids being pumped will help an experienced team member design the ideal fluid process system and make the right selections for the all the equipment involved.