Water Conveyance Loss and Designing Conveyance System - Practices of Irrigation and On-farm Water Management - page 25

1.3.3 Hydraulic and Energy Grade Line for Pipe Flow

Hydraulic calculations are required to design irrigation pipes. A hydraulic grade line
analysis is required for all designs to ensure that water flows through the pipes in
the manner intended.
We learned that the total energy of flow in a pipe section (with respect to a ref-
erence datum) is the sum of the elevation of the pipe center (elevation head), the
pressure exerted by the water in the pipe expressed or shown by the height of a col-
umn of water (pressure head, or piezometric head, if a piezometer is provided in the
pipe) and the velocity head. The total energy of flowing water when represented in
figure is termed as energy grade line or energy gradient . The pressure of water in
the pipe represented by elevation when drawn in line is termed as hydraulic grade
line or hydraulic gradient (Fig. 1.10 ) .

Energy line

h L , head loss

V 2 /2g

Hydraulic
gradient

y = p/ρg

Centre
line

z

Datum

Fig. 1.10 Schematic showing of hydraulic and energy line in pipe flow

1.3.4 Types of Flow in Pipe - Reynolds Number

Flow of water in pipe is of two types: laminar and turbulent. In laminar flow, the fluid
moves in layers called laminus. In turbulent flow, secondary random motions are
superimposed on the principal flow, and mixing occurs between adjacent sectors. In
1883, Reynolds introduced a dimensionless parameter (which has since been known
as Reynolds number) that gives a quantitative indication of the laminar to turbulent
transition. Reynolds number R N is

R N = ρ

Vd
μ

(1.17)