How are Flow-Pressure Curves determined?

During the characterization phase, the pumps are installed in a closed hydraulic loop featuring a control valve at the outlet of the pump (see Figure 1). The control valve is initially fully open. Pressure indicators are installed at the inlet and outlet of the pump to measure the differential pressure, and a flow-meter is installed before the control valve to measure the flow rate.

At the beginning of the test, the pump is turned on and set at a certain fixed rotational speed (speed control mode). The hydraulic loop is designed with as little restrictions as possible (flat system load curve) to reach high flow rates.

Maintaining the speed constant, the valve is gradually closed more and more to steepen the system load curve, and for each valve position the operating point is recorded (the parameters of interest are speed, flow rate and pressure difference between outlet and inlet).

The process continues until the valve is fully closed. Once a rotational speed is fully characterized, the control valve is opened again, and the process is repeated for another speed.

As a result, it is possible to plot the Flow-Pressure curves (one curve for each rotational speed).

Working Point Determination

By overlaying the system load curve with the flow pressure curves and selecting the flow rate needed by the application, it is possible to calculate what is the required speed at which the pump needs to operate.

In the example case shown in Figure 3, a certain pump installed in a high system load plant (A) will have to operate at 7000 rpm to generate 40 lpm. The same pump, if installed in a low system load plant (B) will need to run at only 5000 rpm to deliver the same 40 lpm flow rate.