What is Viscosity and Why is it Important?

When sizing a pump, one of, if not the most important piece of data you should have at your fingertips is the viscosity of the product being pumped. Whether it’s a centrifugal pump, an air operated double diaphragm pump, a positive displacement pump or a scrape surface heat exchanger, the viscosity of your product is likely the first piece of information you’ll be asked to provide. But what is viscosity? Typically, we try to make these blogs about 500 words, but I could wrap this one up pretty quickly. In short, viscosity is the measure of a product’s thickness. But what fun would that be? Let’s do a little deeper dive and try to get to the 500-word mark.

So what is viscosity really? The viscosity of a fluid is a measure of how resistive the fluid is to flow. This is similar to the friction created between solid bodies. This resistance is an obstacle to flow- a product with a higher viscosity is relatively “harder” to flow than a lower viscosity fluid. The ease with which a fluid pours is an indication of its viscosity. For example, heavy molasses pours very slowly. This is an example of a high viscosity fluid. Water, on the other hand, pours very easily. This is an example of a low viscosity fluid. Higher viscosity fluids will require a greater force than lower viscosity fluids to get them to move (i.e. to pump them). This should make some intuitive sense- the greater the resistance, the more energy it takes to overcome.

There are two basic types of viscosity- absolute (or dynamic) viscosity and kinematic viscosity. Dynamic viscosity is the measurement of the fluid’s internal resistance to flow, while kinematic viscosity refers to the ratio of dynamic viscosity to density. The relationship between the two looks something like this:

Kinematic Viscosity (cSt) = Absolute Viscosity (cP)/Specific Gravity

One thing you might notice here are units. Absolute viscosity is typically measured in centipoise (or cps). Water at 1 atmosphere at 20 C has a viscosity of 1 cps. Kinematic viscosity, on the other hand, typically uses units of centistokes of cSt. Here again, water at 1 atmosphere and 20 C has a value of 1 cSt. This would change, however, if we had a slightly “heavier” or more dense fluid (specific gravity greater than 1), like milk. Typically when sizing a pump, an engineer will ask you for a viscosity in centipoise

Ok, that’s great. But why is viscosity important? For Triplex Sales Engineers, viscosity is important because it helps us figure out how much “oomph” we need to pump your fluid. It helps us determine what kind of pumps we can use and what size motors we need to drive those pumps. For thin, or low viscosity fluids (typically up to 1000 cps), we will use a centrifugal pump. For fluids over probably about 500 cps, we’ll start considering a PD pump. And as we go to even higher viscosity fluids, viscosity will play an important role in how fast we want to run a pump and ultimately which model we select.

So now that I’ve hit the 500-word mark, hopefully you have a much better understanding of what viscosity is and why it’s so important that you know it- even if it’s only an approximation. Unfortunately, 500 words just isn’t enough to cover all of the reasons viscosity is important and how it can impact pump sizing and selection. So stay tuned for future posts where we’ll talk more about types of fluids and other fluid characteristics that play an important role when sizing a pump. And as always, if you have any questions, contact a Triplex Sales Engineer today!

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