Handbook of Civil Engineering Calculations

2. Choose the most energy-efficient pump
   Use Fig. 14, entering at the bottom at 1000 gal/min (3800 L/min) and projecting vertical-
   ly upward to a total head of 150 ft (45.5 m). The resulting intersection is within area 1,
   showing from Table 4 that a single-stage 3500-r/min electric-motor-driven pump would
   be the most energy-efficiency.
   Related Calculations. The procedure given here can be used for pumps in a variety
   of applications—chemical, petroleum, commercial, industrial, marine, aeronautical, air-
   conditioning, cooling-water, etc., where the capacity varies from 10 to 1,000,000 gal/min
   (38 to 3,800,000 L/min) and the head varies from 10 to 10,000 ft (3 to 3300 m). Figure 14
   is based primarily on the characteristic of pump specific speed Ns = NQ
   2
   IH
   314
   , where N =
   pump rotating speed, r/min; Q = capacity, gal/min (L/min); H= total head, ft (m).
   When N 5 is less than 1000, the operating efficiency of single-stage centrifugal pumps
   falls off dramatically; then either multistage or higher-speed pumps offer the best effi-
   ciency.
   Area 1 of Fig. 14 is the densest, crowded both with pumps operating at 1750 and 3500
   r/min, because years ago, 3500-r/min pumps were not thought to be as durable as 1750-
   r/min ones. Since the adoption of the AVS standard in 1960 (superseded by ANSI B73.1),
   pumps with stiffer shafts have been proved reliable.
   Also responsible for many 1750-r/min pumps in area 1 has been the impression that
   the higher (3500-r/min) speed causes pumps to wear out faster. However, because im-
   peller tip speed is the same at both 3500 and 1750 r/min [as, for example, a 6-in (15-cm)
   impeller at 3500 r/min and a 12-in (30-cm) one at 1740 r/min], so is the fluid velocity, and
   so should be the erosion of metal surface. Another reason for not limiting operating speed

Flow, gal/min

FIGURE 14. Selection guide is based mainly on specific speed, which indicates impeller
geometry. (Chemical Engineering.)

Total head,

m

Total head,

ft

Single stage

Single stage

( 2 pole)

High speed single stage or Multistage

Single stage limit

Multistage

(4)

Reciprocating

Multistage

25,0OO

suction specific speed

Flow, L/s