The Impact of Fluid Viscosity on Pump Performance and Selection

Fluid viscosity is a critical parameter in pump selection, affecting the calculation of pressure loss, pump speed, efficiency, and power, among other factors.

Viscosity is a measure of this frictional resistance, a physical datum quantifying the strength of viscous force.

For fluids with lower viscosity, such as water, flow through pipelines is relatively unimpeded. Highly viscous fluids, on the other hand, move sluggishly. In such instances, positive displacement pumps, functioning much like bulldozers, can be used to push the fluid.

The viscosity of a liquid is not constant; it typically decreases with rising temperature and increases with rising pressure. With centrifugal pumps, an increase in liquid viscosity leads to a decrease in flow and head, as well as increased energy consumption. With positive displacement pumps, higher viscosity means reduced backflow and increased volumetric efficiency, leading to higher flow rates, but the overall efficiency of the pump decreases, and power consumption increases.

To ascertain the viscosity of a material, You can refer to the table below:

In the petrochemical industry, centrifugal pumps selected for use are generally tested using water at 20°C (with a kinematic viscosity of 1 cSt) as the pumping medium. When the fluid's kinematic viscosity exceeds 20 cSt, the pump's head and flow rate will slightly decrease, power requirements will sharply increase, and efficiency will greatly decrease. Accordingly, performance adjustments to the pump are necessary.

Two common methods for performance adjustment are graphical and formula correction methods.

Here, we focus on the steps for the formula correction method:

1. Calculate parameters using the corresponding equations.

2. Adjust the flow rate of viscous liquids using the corresponding equations.

3. Adjust the head for pumping viscous liquids using the corresponding equations.

4. Adjust the efficiency for pumping viscous liquids using the corresponding equations.

Following these four steps will calibrate the performance for transporting high-viscosity oils. When the performance of the pump when pumping water and the kinematic viscosity of the viscous liquid are known, the actual performance for pumping viscous liquids can be determined using steps 1 and 4.

More conveniently, many pump manufacturers provide a selection software system with a built-in viscosity correction feature. Below is an example from a selection system. In the graph, the upper crosshairs indicate the performance curve for clear water, and the lower crosshairs show the corrected performance curve. It can be observed that after viscosity correction, the efficiency decreases and power consumption increases.