Editor's Note: This is the fourth article the year-long series, Understanding NPSH. To read the previous article,
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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.
A Method for Eliminating the NPSH Variation Caused by Different Impeller Diameters
Because pump head is a function of the impeller diameter, D2, the traditional 3 percent‑head‑drop NPSH varies as the impeller diameter varies. With the same degree of cavitation in the impeller eye, a pump with a maximum‑diameter impeller will exhibit a smaller percentage of head drop than the same pump with a reduced‑diameter impeller.
Gongwer (3) recognized this shortcoming in our definition of NPSHR. To normalize the NPSH requirements in his tests, he created a "constant" pseudo‑head on which to base his head loss. Rather than use the actual head of the pump, he calculated the head that would be developed with an impeller diameter that was twice the diameter of the impeller eye (D2=2D1). A pump develops a head that is approximately U22/2g. This pseudo‑head is therefore (2U1)2/2g, or 2 U12/g.
The 3 percent head loss therefore converts to 0.03x2UI2/g = 0.06 UI2/g. In U.S. units, this would convert to a head loss, for the 3 percent loss, of ΔH = (DlxN/5300)2. A 5‑in diameter eye, rotating 3,600-rpm, would have an allowable head loss of 11.5-ft (completely independent of impeller diameter and pump head).
Adoption by the pump industry of this revised definition of head loss would be a significant step in normalizing NPSH characteristics. It would be of value to pump users and manufacturers.
Such an adjustment would result in NPSH3 curves being lifted for all pumps with (discharge) specific speeds less than about 1,500, and a lowering of NPSH3 curves for pumps with (discharge) specific speeds above that value.
Prior to the concept of suction specific speed, S, the pump industry attempted to use the Thoma‑Moody concept, sigma, which stated that NPSHR was proportional to pump head. It is not. Numerous authors have so confirmed. S enabled us to begin decoupling NPSHR from H. Adoption of the above suggested criterion would be a further step in decoupling these two independent characteristics.
Prior to the development of the NPSH concept, a performance curve would sometimes show the inlet characteristics of a centrifugal pump as lines of constant "maximum suction lift," as can be seen in Figure 4.