Hydraulics 4 Design of Irrigation Systems



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HYDRAULICS_4


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




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