Solution:
1. Field Area:
The field is given to be rectangular.
Note that there are 43,560 ft
2
/acre.
The irrigated
area is the length of the mainline (1,320 ft) multiplied by twice the length of one lateral
(2 x 600 ft):
(1,320)(1,200)
36.4 acres
43,560
=
2. System Capacity:
Use Eq. 5.4 and the given data:
s
Ad
(36.4)(1.9)
Q
453
453
373 gpm
fT
(7)(12)
=
=
=
3. Velocity Checks:
Table 8.4: 5-inch aluminum pipe has an inside diameter of 4.900 inches (0.408 ft).
Note that the maximum recommended velocity, in general, for sprinkler systems is 5 to
7 fps.
3(a). Full system capacity in the mainline:
s
s
Q
3
2
Q
4(373 gpm)
V
6
A
(60 s/min)(7.481gal/ft )(0.408 ft)
=
=
=
π
.36 fps
3(b). Half system capacity in the mainline:
s
s
Q /2
3
2
Q
2(373 gpm)
V
3
2A
(60 s/min)(7.481gal/ft )(0.408 ft)
=
=
=
π
.18 fps
3(c). Half system capacity through a hydrant valve:
s
hydrant
3
2
Q
2(373 gpm)
V
4
2A
(60 s/min)(7.481gal/ft )(0.333 ft)
=
=
=
π
.77 fps
All of the above velocities are below 7 fps, so they are found to be acceptable.

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