Handbook of Civil Engineering Calculations

FIGURE 20. Part of the fluid flow is diverted from the

main pipe. (Peerless Pumps.)

not through pipe 3. Plot the combined system-head curve by adding, at a given flow
rate, the head losses for pipes 1 and 3. With Q = 300 gal/min (18.9 L/s), pipe 1 = 500
ft (152.4 m) of 10-in (254-mm) pipe, and pipe 3 = 50 ft (15.2 m) of 6-in (152.4-mm)
pipe.

ft m

At 1500 gal/min (94.6 L/s) through pipe 1, friction loss 11 3.35

Friction loss for pipe 3 (1500 - 300 = 1200 gal/min) (75.7 L/s) _8 2.44

Total friction loss at 1500-gal/min (94.6-L/s) delivery 19 5.79

9. Plot the effect of pump wear
   When a pump wears, there is a loss in capacity and efficiency. The amount of loss de-
   pends, however, on the shape of the system-head curve. For a centrifugal pump, Fig. 21,
   the capacity loss is greater for a given amount of wear if the system-head curve is flat, as
   compared with a steep system-head curve.
   Determine the capacity loss for a worn pump by plotting its H-Q curve. Find this
   curve by testing the pump at different capacities and plotting the corresponding head. On
   the same chart, plot the H-Q curve for a new pump of the same size, Fig. 21. Plot the sys-
   tem-head curve, and determine the capacity loss as shown in Fig. 21.

Row rote

Combined

system

curve

Heod

Head

FIGURE 21. Effect of pump wear on pump capacity. (Peer-

less Pumps.)

Flow rote

Capacity loss (flat system curve)

Capacity loss (steep system curve)

Worn pump head-capacity curve

New pump head-capacity curve

Steep system curve Flat system curve