Water Conveyance Loss and Designing Conveyance System - Practices of Irrigation and On-farm Water Management - page 32

The pipe must have the capacity to supply peak demand ( Q ). After calculating
the maximum size required, the second step is to calculate the head loss per unit
length (say 100 m) and also for the whole irrigation farm. Extra power and cost
requirement for the head loss should be calculated for the entire effective life of the
pipe. Similar calculations should be performed for the next available pipe sizes (2
or 3 nos).
Now, compare the prices of the pipes (present or first installment) and extra cost
for head loss for the entire life (transferred into present value). Choose the least-
cost one.

1.3.8 Sample Workout Problems

Example 1.5
Compute the size of best quality cast-iron pipe that will carry 0.03 m 3 /s discharge
with a head loss of 2 m per 1,000 m.

Solution

V 2
2 g

L

We get, h f = f ×

D ×

Given,

0.03 m 3 /s Q =

=

for L 1,000 m, h f = 2m

D 2 /4)

= = π

V Q / A Q /(

thus,

(0.03) 2 (0.03) 2

1000
D

1000 × 16 ×

2 = 0.023 × × = 0.023 ×

D 2 4) 2

2

D 5 ( π / π ×

Or, D 5 = 0.016779
Or, D = 0.4415m = 44.15 cm
That is, pipe dia = 44.15 cm (Ans.)

Relevant Journals

- Journal of Soil and Water Conservation
- Journal of Hydraulic Engineering
- Journal of Hydrology
- Agricultural Water Management