Editor's Note: This is the fourth article the year-long series, Understanding NPSH. To read the previous article, click here. To read the next article, click here.
NPSHR Shown as a Single Line as a Function of Capacity Only.
Figure 1 is an example of single‑line performance characteristics of a centrifugal pump. The independent variable capacity is plotted on the horizontal scale. Power, head, efficiency and NPSHR are plotted on vertical scales-each as a function of capacity.
Figure 1. The centrifugal pump family of characteristic curves
Note that the NPSH curve is roughly a lazy "U," reaching a minimum value at a capacity Q about 40 percent of the best‑efficiency capacity. Although most published curves do not show the increased NPSHR at low flows, all centrifugal pumps exhibit such a characteristic. The NPSHR always rises as the pump capacity approaches shut‑off (zero flow rate).
Figure 2 is a typical published performance curve. In this example, the NPSH curve is shown as a single line. (Note that it incorrectly shows the NPSHR to be a minimum at shut‑off.)
Figure 2. Typical published performance curve. Single-line NPSH curve.
The Effect of Impeller Diameter on NPSHR
Figure 3 shows all parameters in the same fashion except for NPSH. Instead of a single curve that applies to all impeller diameters, lines of "iso‑NPSH" are shown. Note that for the same capacity, smaller diameter impellers require more NPSH.
Figure 3. Typical published performance curve. NPSH values increase as impeller diameter decreases.
It is normal to experience a rise in NPSHR as the impeller diameter is reduced since a 3 percent drop of a smaller head (resulting from a smaller diameter impeller) is a smaller head drop. By our definition of NPSHR, we are therefore permitting less cavitation with smaller diameter impellers. Currently there is no known way to predict this increase in NPSHR. Testing must therefore be relied on to establish NPSHR at each impeller diameter.
Is it fair to penalize the smaller diameter impellers? Some pump vendors reason that it is not-the single‑line NPSHR curve, obtained with the maximum‑diameter impeller, should apply to all diameters supplied for that pump.
What if one of those smaller‑diameter impellers is supplied in a smaller‑diameter casing (with a duplicate inlet passage)? We would be required to base our NPSHR on a 3 percent drop of a lower pump head, resulting in a higher NPSHR for the same impeller. Such contradictions account for some of the confusion surrounding this confounding subject of NPSH.