Limits of the Flow Pressure Pump Curves
The Flow-Pressure chart shows in which operating ranges a certain pump can work. The combinations of flow and pressure points describe a field, which is enclosed among some limit curves. In this topic the limits of the pump are described, together with the physical limits governing them.
Lower Speed Limit
In many charts there is a lower limit in the range of a few thousands rpm (see Figure 1). This is because the pumps used in microelectronics applications typically operate at higher speeds.
In the life science industry, however, there is a growing demand for pumps that generate low shear stresses on the fluid. These pumps typically operate at lower speeds. As a result, some Levitronix pumps’ flow pressure charts show speeds as low as 1000 rpm.
There is a physical low speed limit (in the order of few tens of rpm), below which the pump doesn’t run smooth. This limit is hard coded in the firmware for safety reasons.
Upper Speed Limit
On the upper side of the chart, the performance is limited at a certain maximum speed (see Figure 2). This limit often comes from stability issues of the impeller. If the impeller is stable, the speed can be increased until the physical motor limit (when the induced voltage reaches its limit, or when the torque limit is reached).
Levitronix already includes a safety margin (ranging between a few hundred to 2000 rpm) in the flow pressure charts, so that the user doesn’t operate the pump at its physical limits and stable operation is guaranteed.
Power Limit
On the right side of the chart, the curves are limited by the power deliverable by the pump (see Figure 3). In fact, displacing large flow rates at high pressures requires high torque values. The torque heavily depends on the intensity of the drive current running in the motor windings. This current is limited by the power electronics and motor components. For this reason at high speeds (and therefore high pressure) the flow rate available is reduced.
A safety margin is included in this limit as well, to guarantee the pump life time.
In case the pump finds itself operating at the power limit, a “Drive Current Limit Warning” would be issued by the controller to notify the user of the status.
System Load Limit
The last limit (not always present on characterization charts) appears in the bottom right corner of the chart (low speeds, high flow rates, as shown in Figure 4). This limit is due to system load curve limitations.
In fact, the test bench setup provides a considerable pressure loss due to friction even when the control valve is fully open, therefore at high flow rates it is very difficult to reach low differential pressures.
With a reduced system load, one could therefore operate the pump at flow higher than the charted maximum. The risk of cavitation, however, must be taken into account (it increases with the flow rate, as described in the Cavitation and NPSH lesson).
