Handbook of Civil Engineering Calculations

Capacity, L/s

FIGURE 17. Negative lift (gravity head). (Peerless Pumps.)

is 50 ft (15.2 m). Flows up to 7200 gal/min (454.3 L/s) are obtained by gravity head
alone. To obtain larger flow rates, a pump is needed to overcome the friction in the piping
between the tanks. Compute the friction loss for several flow rates as follows:

ft m

At 5000 gal/min (315.5 L/s) friction loss in 1000 ft (305 m) 25 7.6

of 16-in (406.4-mm) pipe

At 7200 gal/min (454.3 L/s), friction loss = available 50 15.2

gravity head

At 13,000 gal/min (820.2 L/s), friction loss 150 45.7

Using these three flow rates, plot the system-head curve, Fig. 17.

6. Plot the system-head curves for different pipe sizes
   When different diameter pipes are used, the friction loss vs. flow rate is plotted independ-
   ently for the two pipe sizes. At a given flow rate, the total friction loss for the system is
   the sum of the loss for the two pipes. Thus, the combined system-head curve represents
   the sum of the static head and the friction losses for all portions of the pipe.
   Figure 18 shows a system with two different pipe sizes. Compute the friction losses as
   follows:

Capacity, 1000 gal/min

Head,

m

Head,

ft

Capacity, L/s

FIGURE 18. System with two different pipe sizes. (Peerless Pumps.)

Capacity, 1000 gal/min

Head,

m

Head,

ft