Page 27 - Centrifugal Pumps 2E

Specific Speed and Modeling Laws 15

Table 2-4
Tabulated Performance at 4,000 RPM
GPM H(ft) Eff. % bhp
0 445 0 37
113 443 28 45
226 438 48 52
337 427 52 60
452 412 70 66
565 381 74 73
678 330 73 77
732 298 72 76

the pump specific speed. To obtain suction specific speed continue from
the rotating speed to NPSHR and vertically to the suction specific speed.
Pump specific speed is the same for either single-suction or double-suc-
tion designs.
For estimating the expected pump efficiencies at the best efficiency
points, many textbooks have plotted charts showing efficiency as a func-
tion of specific speed (N s) and capacity (GPM). We have prepared similar
charts, but ours are based on test results of many different types of pumps
and many years of experience.
Figure 2-3 shows efficiencies vs. specific speed as applied to end-suc-
tion process pumps (API-types). Figure 2-4 shows them as applied to sin-
gle-stage double-suction pumps, and Figure 2-5 shows them as applied to
double-volute-type horizontally split multi-stage pumps.
Figure 2-5 is based on competitive data. It is interesting to note that
although the specific speed of multi-stage pumps stays within a rather
narrow range, the pump efficiencies are very high, equal almost to those
of the double-suction pumps. The data shown are based on pumps having
six stages or less and operating at 3,560 RPM. For additional stages or
higher speed, horsepower requirements may dictate an increase in shaft
size. This has a negative effect on pump performance and the efficiency
shown will be reduced.
As can be seen, efficiency increases very rapidly up to N s 2,000, stays
reasonably constant up to N s 3,500, and after that begins to fall off
slowly. The drop at high specific speeds is explained by the fact that hy-
draulic friction and shock losses for high specific speed (low head)
pumps contribute a greater percentage of total head than for low specific
speed (high head) pumps. The drop at low specific speeds is explained by
the fact that pump mechanical losses do not vary much over the range of
specific speeds and are therefore a greater percentage of total power con-
sumption at the lower specific speeds.