Page 213 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

JWCL344_ch05_154-193.qxd 8/2/10 9:44 PM Page 175

5.3 Capacity and Size of Conduits 175

being investigated for tripling the flow: Either place a pump on the existing line or install an addi-
tional parallel line between the two reservoirs:
1. If a pump is placed, compute the required water head for the added pump.
2. If an additional line is installed, calculate the required size of the added pipe.
Solution:
First let’s determine the existing flow in the 150-mm pipe:
2 2
P A Z A (v A ) >2g P B Z B (v B ) >2g h f
0 (Z A Z B ) 0 0 0 h f
h f 13 m
s h f L 13 3,000 4.3%
3
The nomogram (C 100) of Fig. 5.6 for D 150 mm and s 4.3% will give Q 0.010 m /s
(10 L/s).
1. The required water head for the added pump:
3
Future flow Q 2 3Q 3 0.010 0.030 m /s (30 L/s)
3
The nomogram (C 100) of Fig. 5.6 for D 150 mm and Q 2 0.030 m /s will give
s 30%.
h f sL 30% 3,000 90 m
2 2
P A Z A (v A ) >2g h p P B Z B (v B ) >2g h f
0 (Z A Z B ) 0 h p 0 0 h f
13 h p 90
h p 90 13 77 m
The required water head for the pump 77 m.
2. The required size of the added pipe:
3
Q pipe 2 0.030 0.010 0.020 m /s
3
The nomogram (C 100) of Fig. 5.6 for Q pipe 2 0.020 m /s and s 4.3% will give
D 200 mm.
The required size of the added pipe is 200 mm.

EXAMPLE 5.10 CAPACITY AND SIZE OF PARALLEL PIPES

A town is receiving its water supply from a groundwater tank located at the treatment plant
through a gravity main that terminates at an elevated water tank adjacent to the town. The dif-
ference between the levels of the tanks is 100 m, and the distance between them is 50 km. The
3
tanks were originally connected by a single pipeline designed to carry 13,000 m /day. It was
3
later found necessary to increase the supply to 19,500 m /day and consequently the decision
was made to lay another pipeline of the same diameter alongside part of the original line and
cross-connected to it.
Calculate (given C for all pipes 100):
1. The diameter of the pipes
2. The length of the second pipe, which was necessary to install.