HYDRAULICS_4 Design of Irrigation Systems by László Ormos

DESIGN OF IRRIGATION SYSTEM Options 1.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. 2.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.) 3.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 q s =120l/h, the required pressure head is h s =20m. The riser height is 0.15m. What is the appropriate lateral pipe diameter if options 1, 2, and 3 are taken into consideration? 1.The length of lateral is as follows: 2.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 4.The head loss for 10 sprinklers is as follows where coefficient F 10 =0.384: Since h f  h max =0.2 h s 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 F 10 =0.384: Since h f <  h max =0.2 h s 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%. 1.Let the first 25mm diameter pipe be along 35m (n 25 =4) and 20mm diameter pipe be along 60m (n 20 =6) as it can be seen below: D=20mm D=25mm 35m 95m Pressure regulator

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 0.72m 3 /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 F 6 =0.458: 6.Previously the head loss for 25mm pipe along 95m, 10pcs of sprinklers and Q=1.2m 3 /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 7.The flow rate for 20mm pipe along 60m with six sprinklers is as follows: 8.The hydraulic gradient out of a table or slide ruler for 20mm pipe and flow rate of 0.72m 3 /h is 8%. 9.The head loss for 20mm pipe along 60m is as follows: 10.The head loss for 20mm pipe with six sprinklers is:

DESIGN OF IRRIGATION SYSTEM Option 2 11.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 h f,95 <  h max =0.2 h s 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 h f =6.93m (see Option 1). Therefore, the required inlet pressure of the lateral pipe is as follows:

References Azenkot, A.(1998):”Design Irrigation System”. Ministry of Agricul- ture Extension Service (Irrigation Field service), MASHAV Israel Dr. Avidan, A.(1995):”Soil-Water-Plant Relationship”. Ministry of Agriculture Extension Service (Irrigation Field service), CINADCO, Ministry of Foreign Affairs, MASHAV, Israel Sapir, E.-Dr. E. Yagev (1995):”Drip Irrigation”. Ministry of Agricul- ture and Rural Development, CINADCO, Ministry of Foreign Affairs, MASHAV, Israel Sapir, E.-Dr. E. Yagev (2001):”Sprinkler Irrigation”. Ministry of - culture and Rural Development, CINADCO,Ministry of Foreign Affairs, MASHAV, Israel Eng. Nathan, R. (2002):”Fertilization Combined with Irrigation (Fertigation)”. Ministry of Agriculture and Rural Development, CINADCO,Ministry of Foreign Affairs, MASHAV, Israel