Fluid viscosity and density: A pump user’s guide

 PHOTO: Many people think that all fluids act just like water in a pump (pictured: rapeseed oil), but that misunderstanding can lead to costly errors.
Viscosity & Density

Why are viscosity and density important in relation to pumps?

Because you might need to specify a pump that delivers 10 bars of pressure, but if you do not consider the pumped liquid’s properties, it is highly unlikely you will make the right choice.

“If you don’t account for the differing viscosity and density of liquids, you may end up with too big a pump, which is overly expensive to buy and run and takes up a lot of space,” says Søren Mortensen, Grundfos Application Manager. “If the pump is too small, you simply won’t be able to move the amount of liquid required. And if the motor chosen is too small for the job in hand, it will either burn out or the electrical security system will shut off the pump.”

Okay, will you please remind me what “viscosity” and “density” mean?

Viscosity is a measure of the “thickness” of a liquid, whereas density is a measure of the liquid’s weight.

The thicker the fluid, the more difficult it is to pump?

A fluid’s viscosity or “thickness” certainly affects how it will behave in a pump, but it’s complicated because the viscosity of fluids can change under different conditions. There are four main groups of fluids – each group acts a certain way despite its specific viscosity.

First, you have fluids like water, oil, alcohol and paint thinner. With these so-called “Newtonian fluids,” it does not matter how fast they move or how much you agitate them, they flow the same.

The second type is “dilatant fluids” like cream and butter, whose viscosity increases with agitation until they become almost solid. “They are impossible to pump in standard centrifugal pumps and require special measures to move them,” notes Søren Mortensen.

Third, “plastic fluids” have a yield value (the point of resistance) that must be exceeded before they can start flowing. After that point, viscosity decreases as agitation increases. “The best example of a plastic fluid is ketchup,” says Søren Mortensen. “Think about how you have to shake it up in a bottle before it starts to move, but once it is moving, it flows easily.”

The fourth group, “thixotropic fluids,” is the hardest to explain… and to deal with in pumps. Glues, non-drip paint, greases, cellulose compounds, soaps, starches and tar all fall into this category. These fluids are viscous (thick) when standing still, but will become less viscous (thinner) over time with constant agitation.

Very difficult, indeed. So how do you measure viscosity?

Well, there are two types viscosity – dynamic and kinematic…

“Many people think that all fluids act exactly like water in a pump. This is the misunderstanding we come across 95 per cent of the time, and it can lead to very costly errors.”

Søren Mortensen, Grundfos Application Manager – Industry Segment

Wait … you’re losing me. What is most important for me to know about viscosity when it comes to pumps?

So-called “kinematic viscosity” is a real physical factor that influences pump curves, so this is something you need to know about.

Kinematic viscosity is denoted in centistokes (cSt) and is measured in mm2/s. For instance, water has a low viscosity of 1 cSt at 20°C, while motor oil has a high viscosity of more than 500 cSt at the same temperature.

“Viscosity is essentially resistance to flow and this has implications for pumps,” says Søren Mortensen. “If you have a high-viscosity, sticky fluid on your hands, it clings and it’s much harder to remove than a low-viscosity fluid. It is the same in a pump, where you have impellers to move the fluid. If you are pumping fluids like motor oil that stick to the surface of the impeller, this is a problem that needs to be considered.”

Okay, that is viscosity. Tell me more about density.

Different liquids may have different densities (or weights, measured in g/cm3 or kg/m3) and the density of a single type of fluid does vary with temperature.

When we are sizing pumps, we pay particular attention to the “head,” or how many metres up in the air we can pump a given fluid at one bar of pressure. Heavier fluids will need more pressure to pump them.

As you can see in Figure 1 below, this can vary significantly. Brine (1,300 kg/m3) is much heavier (more dense) than diesel oil (800 kg/m3), while water at 20°C (997 kg/m3) is heavier than water at 95°C (960 kg/m3).

FIGURE 1: Pumping four different liquids at 1 bar at the discharge side of the pump results in four different heads (m), hence four different duty points. Liquids represented: A) Brine at 20°C, B) Water at 20°C, C) Water at 95°C, D) Diesel oil at 20°C.

What is the most common misunderstanding about viscosity and density in a pump context?

“Many people think that all fluids act exactly like water in a pump,” says Søren Mortensen, “but, as we can see, that isn’t the case. This is the misunderstanding we come across 95 per cent of the time, and it can lead to very costly errors.”

How can Grundfos help me avoid all of this trouble?

Go to the Grundfos Product Center, type in the details of the fluids you need to be pumped and our Liquids Guide will guide you to the solutions that can handle the liquid. Try it!

Do you have a pump-related topic you have always wondered about but were afraid to ask? Write to us on our Grundfos Engineering Facebook page.  

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