pressure variation in the drip irrigation
Published on: Mar 4, 2016
Transcripts - Pressure variation
Pressure is a form of energy imparted on the
molecules of water.
The pressure is the force applied by a
fluid on a surface.
PRESSURE VARIATION ALONG
DRIP IRRIGATION LINE:
If the drip irrigation line is laid on slopes ,
slopes will affect the pressure variation.
Pressure change due to elevation
When the line is laid upslope , it will loose
When the line is laid down slope , it will gain
➢ The loss or gain in pressure is linearly
proportional to the slope and length of the
➢ This can be expressed as follows ( Wu &
Gitlin , 1974 )
dh/dl = sf = +_
Where Sf = energy slope , the slope of the
energy gradient line
So = line slope
Pressure measurement in a
When water flows in a pipe
, the pressure head at any
point can be easily found
by inserting a vertical pipe
called piezometer as
shown in fig.
PRESSURE LOSS DUE TO FRICTION:
➢ When water passes through a horizontal
pipe , water looses pressure along the
direction of flow.
➢ This is because of friction between the
water if the pipe material and also friction
between water molecules.
HEAD LOSS DUE TO
THERE ARE TWO EQUATIONS AVAILABLE FOR
FRICTIONAL LOSS CALCULATIONS.
✓ Hazen-williams equation
Friction loss in plain pipes
S.NO PIPE MATERIAL FRICTION COEFFICIENT
1. Plastic 150
2. Epoxy coated steel 145
3. Cement asbestos 140
4. Galvanized steel 135
5. Aluminum &steel 130
6. Steel (or)concrete 100
Values of Hazen williams C
DESIGN CRITERIA FOR
SYSTEM BY USING
Design criteria for drip irrigation
➢ There is no definite rule about the design
criteria for drip irrigation
For most of the designers are using below
✓ for design criteria laterals for
emitter flow variation is 10%
✓ for design criteria sub mains for
lateral flow variation is 10%,then emitter
flow variation for whole field is20%.This
will give an application efficiency of 90%
for whole filed
➢ When the sub mains is designed with less
than 10%lateral flow variation ,the application
efficiency of the whole filed will be larger than
➢ IN CASE, the lateral is designed for 20%
emitter flow variation emitter flow variation
and sub main is designed for a10% lateral
flow variation , the over all irrigation
application efficiency still will be about 85%.
Hydraulic variation calculated based on highest
and lowest pressure in the pipe.
This equation for the finding out the
Vh =hydraulic variation
Hmax=max pressure in sub mains or
Hmin=min pressure in sub mains or
h=avgpressure in sub-mains or laterals
✓ For lateral design,the allowable hydraulic variation
✓ For sub main design,the allowable hydraulic
variation is 10%
STATISTICAL VARIATION OF
PRESSUREIt is find out by the coefficient of variation of
pressure in the pipe
Where n is the no of drippers or laterals
➢ For lateral design, the allowable statistical
variation is 7%
➢ For sub mains design the allowable
variation is3.5 to7%
➢ The relation ship between Vh and Vs
➢ In case of laterals are laid upslope, the
maximum pressure head at the start of the
laterals and minimum pressure occurs at the
end of the lateral
➢ In case of laterals and sub-mains are laid in
down slope, the location of minimum
pressure and maximum pressure may be any
where in the pipe depending on the magnitude
and of friction loss and elevation gain along
•To use these figures for the design following
procedure is used .
• First , deside on a lateral length (L)and an
operating pressure head at the start of the
• Find out the ratio L/H in vertical axis .
• Move horizontally till you cut the slopes
percent of the land
• Then move upwards into the first quadrant,
till the desired location of hydraulic variation in
➢ Then Draw horizontal line into quadrant -2.
➢ Then mark L/H horizontal axis.
➢ Draw vertical line from the point
➢ Both lines will intersect at a point
➢ Find out the value of ΔH/L corresponding to
➢ For this value, find out the design diameter
using the nomograph given below
• Note: The nomogram is based on the Prandti - Coalbrook formula using a k factor of k= 0.007 mm.
Factors applicable to other flow formulae are: Hazen Williams c = 150, Manning n = 0.010 and
Darcy roughness factor, = 0.007
Design for drip by using nomograph
SwapnaPriya . S. v.
Pavan Kumar . c