Understanding Viscosity and Why It's Important

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In this edition of The Pump Report - Chad walks through the importance of understanding liquid viscosity. To learn more visit our website at vikingpump.com

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Viscosity is a measurement of a liquid's resistance to flow. Water is a great example to start with. When you pour it or spill it, you can observe that it flows very easily. It's a great example of a low viscosity, or thin liquid.

Honey, on the other hand, does not flow easily. When it's poured out it flows much more slowly. Honey would be regarded as a high viscosity, or viscous liquid.

Just like you can measure a liquid's temperature or its density, you can also measure its viscosity. To do so is going to require a tool called a viscometer. The simplest versions feature a cup with an orifice at the bottom. The cup is filled and then a time measurement is taken. The longer the cup takes to empty, the more viscous the liquid. These types of viscometers are called kinematic viscometers and for liquids like water, oil, and even paint, this method works acceptably well.

For other liquids this method isn't practical. Here I have a jar of viscous silicone and inside is a marble. Now it may not look like the marble is moving but in actuality it's slowly sinking to the bottom as the liquid flows around it. To show you just how long it takes for this marble to sink to the bottom, here's a time lapse video of that same jar taken over a period of 12 hours. This silicone is over 100,000X more viscous than water so measuring it with a kinematic viscometer isn't practical.

For some liquids a dynamic viscometer is used. This type of viscometer uses a rotating spindle submerged in the sample. This measures the shear stresses as the liquid creates viscous drag. This has the added benefit of measuring a liquid's shear sensitivity (but that's a topic for another day).

In either case, what is returned is a quantified value of a liquid's viscosity. Here are some common liquids and their viscosities at room temperature

Remember though, temperature has a big impact on viscosity. With very rare exception, the colder the temperature, the higher the viscosity. It's important that the viscosity measurements you take are done at the pumping temperature.

Alright, so how does this all relate to pumps.

Pump type, the physical size, and the speed are all affected by viscosity. Typically, the more viscous the liquid, the larger and slower the pump.
Drive sizing will also be impacted. The more viscous the liquid, the more horsepower required.
Piping sizes may be impacted. Viscous liquid means viscous drag in pipes - which means pressure may be higher and NPSH may be lower.
Pump construction may also be affected, but you'll need to check with the pump manufacturer to confirm that it's suitable for the viscosity.

Knowing your liquid viscosity is critical to ensuring you get the right pump and drive selected for a pump application. Here at Viking Pump we're experts at handling any viscosity, from water to silicone and everything in between.

To learn more about liquids or to see other Pump Reports, please visit our website at VikingPump.com.