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How Viscosity Can Impact Centrifugal and Positive Displacement Pumps

How Viscosity Can Impact Centrifugal and Positive Displacement Pumps

Which pump type can handle viscous fluids more effectively?

Article written by FCX Performance, Inc

Understanding the Sweet Relationship Between Viscosity and Your Pump

Viscosity is a measurement of a liquid's resistance to flow. From a molecular standpoint, a liquid with a high viscosity will be met with a notable amount of friction when in motion. If someone were to pour honey in one of your hands and water in the other, it would take more time for the honey to run off your hand than it would for the water. Honey is a more viscous fluid and thus it is met with more friction; the honey will stick to your hand and be harder to remove.

Imagine that your hands are the pump’s impellers. If you are working with a viscous fluid, it could stick to the impeller and make it harder to move through the pumping system. The viscosity affects the friction drag on the impeller thus impacting the head and the flow of the pump. How should the viscosity of the fluid moving through the system impact your decision on which pump type to use? Let’s find out.

How Viscosity Can Affect Centrifugal Pumps

There is a manufacturer’s recommended maximum viscosity level for centrifugal pumps where beyond that, the fluid can become too thick for it to perform efficiently. Centrifugal pumps are better suited for low viscosities because they are not designed to impart as much energy into a thick fluid as a thinner fluid. If the fluid is too viscous, your pump will not provide the desired flow or pressure outcome.

How Viscosity Can Affect Positive Displacement Pumps

Positive displacement pumps are not impacted the same way as centrifugal pumps when it comes to a fluid's viscosity. Positive displacement pumps are designed to better handle thicker fluids. Regardless of the viscosity of the fluid in a positive displacement pump, the output flow, or GPM (gallon per minute), will remain the same, but the power input can be increased to achieve the same required flow/GPM.

A thicker fluid demands more motor horsepower for the pump to achieve the required flow rate. Higher viscous fluid tends to have minimal/non-existent internal slip within the pump, making it a much more efficient pump selection versus a centrifugal pump.

Centrifugal pumps are typically better suited for thinner fluids, although it is best practice to refer to the pump manufacturer’s viscosity limit. Positive Displacement pumps can be more efficient when selecting a pump for viscous fluids because the power input can be adjusted to accommodate the required flow regardless of viscosity. Keep in mind that the more viscous a fluid is, the more input horsepower will be required to transfer that fluid within your pumping system.

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