In general, the flow rate provided by a pump is inversely proportional to pressure, which means that the flow rate will increase as pressure decreases (see Figure 2). In order to select a pump with appropriate head pressure, the pump curve provided by the pump manufacturer should be plotted over the system pressure drop curve. The system will operate at the intersection of the two curves. In our example, the pump will operate at 1.6 GPM and 13.5 psi (see Figure 3) because the two plotted lines intersect at this point.
If the pressure drop of the system is known for one point, the curve can be estimated by drawing a straight line from no flow and no pressure drop to the known pressure drop point. The line's intersection with the pump curve provides a good estimate of the expected flow rate. In our example, assume a system pressure drop number of 2 GPM and 18 psi is known (see Figure 4). Using this method, the estimated system flow rate is 1.5 GPM, close to the 1.6 GPM determined using the more precise method.