Reading and understanding centrifugal pump curves is the key to proper pump selection and more importantly reliable, efficient, pump applications. There are four important curves shown on the standard performance curve from the manufacturer. They are listed below and shown on the manufacturers curve see (figure 4).
d. Net positive suction head required (NPSHR)
Figure 4: System Curve
Various manufacturers and industries display the information on their curves with slight variations. One very common variation is shown below in figure 5. Instead of an efficiency curve, the efficiency is broken into several iso- efficiency lines each line representing a constant efficiency. It is read much like a topographic map, with the eye so efficiency lines corresponding to elevation lines on the map.
Figure 5: System Curve w/ Iso-Efficiency Curves
Another common curve variation is to show multiple impeller diameters on the same curve. As can be seen in the curve in figure 6, as the impeller diameter decreases in size, the performance is reduced. This allows the pump performance to be modified to meet your specific application requirements. Additionally we can see that a reduction in diameter reduces the pump power requirement. A reduction of only 10% in the impeller diameter can result in a 27% reduction in power requirements.
Figure 6: Multi-Trim Curve
Steepness of the Performance Curve
Pump head curves with very flat head flow characteristics can make the pump difficult to control. Small changes in system resistance can create large changes in the pumps flow rate. This makes the control of the pump difficult in situations that have a flat system loss curve. The problem is exacerbated when variable-frequency drives are used to control the pump operating point.
Pumps operate where the pump curve meets the system curve. We would like to size the pumps to run as close as possible to its best efficiency flow rate. This not only makes the pump more efficient but also improves the reliability of the pump. This means that accurate sizing requires that both pump curves be fairly accurate. Pump curves are generally fairly accurate. Minor variances of the manufacturerâ€™s tolerances and slight efficiency exaggerations by the marketing departments of the pump companies may slightly overstate the pumps performance, but all curves have a tolerance of approximately Â±3%. System curves have a much wider range of inaccuracy see due to the wide variation in pipe and fitting friction losses from various manufacturers. The note at the bottom of Cameron Hydraulic Data book pipe tables advises that a 15% to 20% increase in loss should be used above the loss levels shown in their tables. This inaccuracy and the rising cost of power make it imperative that larger pumps be field tested to determine actual flow rate. Excessive flow causes excessive friction driving up the power consumption and operating costs. Gone are the days of letting the pump be oversized and run back on its curve at a low efficiency low reliability operating point. Field testing allows the system calculations to be confirmed and the pump to be modified to meet the actual system conditions not just the paper system conditions.