Effects of Viscosity & Specific Gravity on Pump Curve


The standard pump performance curve is plotted with reference to water as a fluid medium. However, in many cases, pumps are used for fluids other than water, and the characteristics of those fluids can have significant effects on the pump’s performance. Viscosity and Specific Gravity are two essential variables in fluids that can impact the performance of any pump.

Viscosity and Specific Gravity Explained

water-flowThe viscosity of a fluid is the resistance it has towards changing its shape at a given temperature. In other words, fluid with high viscosity resists flow, making it slower to flow from one point to another. Honey is an excellent example of a fluid with high viscosity. The viscosity of a fluid is measured with a viscometer.

Specific Gravity is the relative weight of the fluid with reference to the weight of water in equal volume. The Specific Gravity of fluids is 1 when it is similar to that of water. Fluids that are heavier than water have a Specific Gravity of more than one, and fluids lighter than water have a value of less than one.

In simple terms, viscosity determines the stickiness of the fluid, and Specific Gravity shows how heavy the fluid is. In this article, we will explore the effects of Viscosity and Specific Gravity on pump performance.

Effect of Viscosity on the Pump Curve

As discussed, viscous fluids exhibit increased resistance to flow. We often see the correlation in the form of thickness, as highly viscous fluids tend to be thicker.

For pumping viscous liquids, the pump must be adjusted to account for the resistance generated by such fluids. If you compare the pump curve of non-viscous fluids with viscous fluid, you’ll notice that as the fluid becomes more viscous, the pressure head suffers, and the pump consumes more power. In these conditions, centrifugal pumps tend to impart more shear energy into the liquid, leading to more energy loss in the form of friction.

Positive displacement pumps are often the preferred choice for viscous liquids as they can operate at lower speeds. However, picking the correct type of pump is a bit more complex. To land on a safe and suitable choice, you must reference the data sheets that compare the pump curve with viscosity, flow rate, and pump size.

You can also tweak existing pumps to work with viscous fluids, provided you know the variables involved. The viscous correction charts can serve as a valuable guide when adjusting the pump for a fluid of certain viscosity.

Effect of Specific Gravity on the Pump Curve

A change in Specific Gravity points to a change in the fluid's density. Denser fluids offer more resistance in comparison to lighter fluids.

Generally, the effort put out by a pump is directly proportional to the weight of the liquid. Hence, liquids with higher Specific Gravity need more horsepower from the pump to reach a certain height compared to water.

Another key factor between Viscosity and Specific Gravity is that if the fluid has a similar viscosity to that of water, its Specific Gravity will not affect the pump's performance in any way. However, it will cause the pump to draw more energy to transport that particular fluid. Hence, choosing a pump should be based on the fluid's Specific Gravity.

Need Assistance in Selecting the Right Pump?

Choosing a pump while referencing Viscosity and Specific Gravity charts is complex and requires expertise. At Hayes Pump, our experts provide all the necessary assistance to customers in choosing the right pump for the fluid they wish to move.

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