Water Conveyance Loss and Designing Conveyance System - Practices of Irrigation and On-farm Water Management - page 31

1.3.6.6 Sample Workout Problem

Example 1.4

Determine the head loss due to friction in an irrigation pipe having 150 m length
and 25 cm diameter. The velocity of flowing water in the pipe is 2.0 m/s

Solution

Head loss according to Darcy-Weisbach formula:

V 2
2 g

L
D ×

h f = f ×

Given,
V 2.0 m/s

Assuming f

=

= 0.025 (as of cast iron)

(2) 2

150

h f = 0.025 ×

0.25 ×

2 × 9.81

= 3.058 m of water (Ans.)

1.3.7 Designing Pipe Size for Irrigation Water Flow

Selection of pipe size should be based on the following:

•

hydraulic capacity (discharge) requirement

•

head loss, and

• economy

In the short run, a small diameter pipe may require lower initial cost, but due to
excessive head loss, it may require higher cost in the long run.
Pipe size based on hydraulic capacity can be found as

Q
V

A =

where

cross-sectional area of the pipe (m 2 )

A =

required discharge (m 3 /s)

Q =

V = permissible velocity of flow (m/s)

D 2 /4, that is, D Diameter of the pipe can be found from the relation, A = π =

4 A
π

.