Hydraulics 4 Design of Irrigation Systems

HYDRAULICS_4

• Design of Irrigation Systems

• by

• László Ormos

DESIGN OF IRRIGATION SYSTEM

• Options

• The rule of 20% is applied to all the sprinklers on the same subplot. Any excess pressure over 20% between subplots is controlled by flow pressure regulators.

• The rule of 20% is applied to a single lateral, and pressure regulators control the difference pressure between the laterals.(It is common in drip systems.)

• The difference pressure along the lateral exceeds the 20% head loss by any desired amount. Flow regulators inside the emitters or pressure regulator in the sprinkler riser control the excess pressure. (It enables the use of a longer lateral pipe or smaller diameter than permitted by options 1 or 2.)

DESIGN OF IRRIGATION SYSTEM Option 1

• A PE lateral pipe with 10 micro-sprinklers is spaced 10m

• apart. The first sprinkler is 5m away from the inlet of lateral.

• The flow rate of selected sprinkler is qs=120l/h, the required

• pressure head is hs=20m. The riser height is 0.15m. What

• is the appropriate lateral pipe diameter if options 1, 2, and 3

• are taken into consideration?

• The length of lateral is as follows:

• The flow rate of the lateral pipe is:

DESIGN OF IRRIGATION SYSTEM Option 1

• For a 20mm PE pipe the hydraulic gradient out of a slide

• ruler or monograph is J=19%.

• 3. The head loss of a 20mm plain PE pipe is

• The head loss for 10 sprinklers is as follows where coefficient F10=0.384:

• Since hfhmax=0.2 hs the diameter of lateral pipe is too small.

DESIGN OF IRRIGATION SYSTEM Option 1

• For a 25mm PE pipe the hydraulic gradient out of a slide

• ruler or monograph is J=6.2%.

• 5. The head loss of a 25mm plain PE pipe is

• 6. The head loss for 10 sprinklers is as follows where coefficient F10=0.384:

• Since hf<hmax=0.2 hs the diameter of lateral pipe is good.

• 7. The required inlet lateral pressure is:

DESIGN OF IRRIGATION SYSTEM Option 2

• The permitted pressure variation along the lateral can be 4m,

• therefore, a combination of different diameter pipes can be

• used. The applied pipes are 25mm and 20mm diameter

• ones.

• Pressure regulators will be installed by the inlets of laterals to

• compensate greater head loss than 20%.

• Let the first 25mm diameter pipe be along 35m (n25=4) and 20mm diameter pipe be along 60m (n20=6) as it can be seen below:

DESIGN OF IRRIGATION SYSTEM Option 2

• 2. The flow rate for 25mm pipe along 60m is calculated as follows:

• 3. The hydraulic gradient out of a table or slide ruler for 25mm pipe and flow rate of 0.72m3/h is 2.5%.

DESIGN OF IRRIGATION SYSTEM Option 2

• 4. The head loss for 25mm pipe along 60m is as follows:

• 5. The head loss for 6 sprinklers is as follows where coefficient F6=0.458:

• 6. Previously the head loss for 25mm pipe along 95m, 10pcs of sprinklers and Q=1.2m3/h has been computed which was 2.261m (see Option 1), then the head loss for 25mm pipe along 35 with four sprinklers is

DESIGN OF IRRIGATION SYSTEM Option 2

• The flow rate for 20mm pipe along 60m with six sprinklers is as follows:

• The hydraulic gradient out of a table or slide ruler for 20mm pipe and flow rate of 0.72m3/h is 8%.

• The head loss for 20mm pipe along 60m is as follows:

• The head loss for 20mm pipe with six sprinklers is:

DESIGN OF IRRIGATION SYSTEM Option 2

• The total head loss along the combined lateral pipe with 35m length of 25mm pipe and 60m length of 20mm pipe is as follows:

• Since hf,95<hmax=0.2 hs the diameter of lateral pipe is good.

• 12. The required inlet pressure by the last lateral is:

DESIGN OF IRRIGATION SYSTEM Option 3

• The lateral pipe can be designed either with flow regulators

• or pressure regulators in every emitters, the sprinklers or

• the drippers.

• Therefore, the diameter of pipe can be reduced. The cost

• of reduced pipe must be less than the additional cost for

• energy and regulators.

• Principal theory of Option 3 is that unlike Option 1 and

• Option 2, in case of Option 3 the entire head loss along

• lateral pipe is added to the lateral inlet pressure.

• In case of 20mm diameter, the head loss hf=6.93m (see Option 1).

• Therefore, the required inlet pressure of the lateral pipe is as follows:

References