All pumps are characterized at very specific conditions. Both pump parameters and application parameters (such as the properties of the fluid) affect the performance, so they are always specified in the Flow-Pressure Chart. Changing any of these properties (e.g. mounting different impellers on a certain motor, or cooling down the fluid) might affect the amount of flow rate or pressure that can be delivered.

Effect of Fluid Properties

Levitronix characterizes the performance of its pumps using water at 40 degrees Celsius. These conditions are chosen because they are practical to be obtained in a lab, and are similar to the most common life science industry end-users cases.

Nonetheless, in many applications the fluids used are very different from water (in terms of viscosity and density) or are elaborated at different temperatures.

It’s worth noting that the parameters are correlated: an increase in temperature will cause a decrease of viscosity and density, so the various effects will sum up on top of each other. For practical purposes, when a new fluid (very different from water) is used, the pump performance should be re-evaluated.

Effect of Fluid Density on the Pump Performance

Increasing the density of the fluid will cause a proportional increase in pressure generated. The flow rate will theoretically remain the same (see Figure 1). This means that the pump confers more hydraulic power to deliver the same flow rate.

Since both localized and distributed pressure losses are linearly dependent on the density, the higher pressure generated will be dissipated, bringing no benefit to the system performance.

It is important to mention that the head (pressure expressed as the height of a column of fluid) will not change in case of a change of the fluid density. To explain so, one could imagine a pump developing a certain pressure to balance the weight of a static column of water (see system in Figure 2). In this case, the pressure P at the bottom of the water column is proportional to the height of the column H, the fluid density rho, and the gravity acceleration constant g.

\[P=\rho g H\]

If the density (rho) increase, the pressure (P) will proportionally increase as well, but the height of fluid column (H) will remain constant. This means that the pump will always be able to balance a column of fluid of height H no matter what is the density of the fluid.

Effect of Fluid Viscosity on the Pump Performance

Increasing the viscosity will cause a deterioration of the performance: the added friction within the fluid and between fluid and surfaces will dissipate some of the hydraulic power conferred by the pump. This will cause a reduction of both flow rate and pressure.

Figure 3 shows a comparison of the performance of a certain pump, when it runs at a certain fixed speed, with fluids of different viscosity. As the viscosity increases, it becomes increasingly difficult to deliver high flow rates and high pressures.

Effect of Fluid Temperature on the Pump Performance

An increase of the fluid temperature will cause an increase of the motor temperature, which will cause a limit in the power deliverable. This means that the “power limit of the pump” line will be translated closer to the origin. Additionally, the higher the fluid temperature, the higher the risk of cavitation, further limiting the maximum flow rate.

The shape of the flow pressure curves will also change, since an increase of temperature usually causes a change of the fluid properties (viscosity in particular, density is affected on a smaller scale)

Effect of Pump Parameters and Scalability

The shape of the impeller and of the volute affect how efficiently the pump converts the mechanical power in hydraulic power. Levitronix optimizes the geometry to reach e.g. high flow rates or high pressures. In Figure 4, it is shown as a certain pump, operating at a certain speed, reaches different performance points depending on the different impeller and volute geometries.

If a certain geometry is proved to be successful, it is possible to scale up the size of the pump to increase the performance. The most influential parameter is the impeller outer diameter, since the flow rate increases with the cube of the diameter, while the pressure increases with the square of the diameter.

This means that an increase of 25% of the diameter will cause a multiplication of the power by a factor 3 (the power scales with the impeller diameter to the power of 5).

Effect of Operating Speed

Once a certain geometry is defined, the pump can be operated at various speeds. Increasing the speed will increase both the flow (linearly with the speed), and the pressure generated (with the square of the speed).

As an example, in Figure 6 are shown the performance of a pump at two rotational speeds (4000 rpm and 8000 rpm), and the system load of a closed hydraulic loop.

It is possible to observe that doubling the rotational speed will double the flow rate delivered, and multiply the pressure developed by a factor 4 (the pressure grows with the square of the rotational speed).