The load for this system could be divided among three, four, or more pumps, if de-
sired. To achieve the best results, the number of pumps chosen should be based on
achieving the proper head and capacity requirements in the system.
- Construct a system-head curve
Based on the known flow rate, 80 gal/min (7.4 L/s) at 13-ft (3.96-m) head, a system-head
curve can be constructed using the fact that pumping head varies as the square of the
change in flow, or Q 2 IQi = H 2 IH 1 , where Q 1 = known design flow, gal/min (L/s); Q 2 = se-
lected flow, gal/min (L/s); HI = known design head, ft (m); H 2 = resultant head related to
selected flow rate, gal/min (L/s).
Figure 19 shows the plotted system-head curve. Once the system-head curve is plot-
ted, draw the single-pump curve from Fig. 17 on Fig. 19, and the paralleled-pump curve
from Fig. 18. Connect the different pertinent points of concern with dashed lines, Fig. 19.
The point of crossing of the two-pump curve and the system-head curve is at the re-
quired value of 80 gal/min (7.4 L/s) and 13-ft (3.96-m) head because it was so planned.
But the point of crossing of the system-head curve and the single-pump curve is of partic-
ular interest.
The single pump, instead of delivering 40 gal/min (7.4 L/s) at 13-ft (3.96-m) head will
deliver, as shown by the intersection of the curves in Fig. 19, 72 gal/min (6.67 L/s) at 10-ft
(3.05-m) head. Thus, the single pump can effectively be a standby for 90 percent of the re-
quired capacity at a power input of 0.5 hp (0.37 kW). Much of the time in heating and air
conditioning, and frequently in industrial processes, the system load is 90 percent, or less. - Determine the single-pump horsepower input
In the installation here, the pumps are the inline type with non-overload motors. For larg-
er flow rates, the pumps chosen would be floor-mounted units providing a variety of
GALLONS PER MINUTEFIGURE 19. System-head curve for parallel pumping.
Head,m