sprinkler irrigation design exercise (by zechariah hatinanyika)
DESCRIPTION
An exercise on designing a farm irrigation system.TRANSCRIPT
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Irrigation & Water ManagementIrrigation & Water Management
PRINCIPLES OF
SPRINKLER IRRIGATION
DESIGN
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Sprinkler Irrigation Design Sprinkler Irrigation Design ExerciseExercise
A semi-portable sprinkler system is to be designed for a farm in Mvurwi where the maximum Eto is 6mm/d. The soils are loamy with AWC of 100mm/m, and average infiltration rate of 6mm/h. Wheat, with a Kc value of 1.1 and root depth of 0.8m is to be grown. Wind speed max is 11km/h. MAD is 50%.
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432m
312m
800m Prev. wind
Dam elev 100m
Fig. 1
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1. Irrigation water 1. Irrigation water requirementsrequirements
Dnet = AWC * Rz * MAD = mm
Dgross = Dnet / System Eff• = mm
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2. Irrigation cycles2. Irrigation cycles
Etc = Kc * Eto= mm/d
Irrigation interval = Dnet / Etc = days.
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3. Sprinkler selection3. Sprinkler selection
1. Application rate < infiltration rate to avoid runoff
2. Choose pressures 2.5 – 4 bars, as long as CU>85%
3. Consider effect of wind on max sprinkler spacing
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Cont’Cont’
For infiltration rate of 6mm/hr, selecting 4mm nozzle @ 350kPa, 12*18 spacing
Wetted dia = 30.5m, Qspr = 1.16m3/hr.
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Acceptability testAcceptability test
40% of 30.5m = 12.2m > 12m60% of 30.5m = 18.3m > 18m
Application rate = 5.37mm/h < 6mm/h
Therefore ACCEPTABLE!
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Set timeSet time
Set time = Dgross / Ppt rate= hrs.
System can do sets
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System layoutSystem layout
1. Laterals placed across prevailing wind direction and perpendicular to predominant slope for uniform head losses.
2. Mainline to run through the middle for reduced frictional losses.
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Cont’Cont’
No. of sprinklers per lateral = Lateral length / Spr. Spacing
= spr / lateral
No. of lateral positions = 2 { (Field length / Lateral spacing)}
= positions.
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contcont
No. of hydrants = 1/6* lateral posns= hydrants.No. of laterals per posn = No. of
shifts * Irrig Interval = moves.No. of laterals = No. posn / No.
moves = laterals
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System capacitySystem capacity
Qsystem = Spr/lat * No. laterals * Qspr = m3/h
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Pipe size determinationPipe size determination
Friction loss in lateral to be kept at minimum.
Hfriction + Hstatic <= 20% operating pressure
‘F’ friction reduction used for multiple outlets
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Lateral & Mainline friction Lateral & Mainline friction losseslosses
Qlat = No. spr/lat * Qspr = m3/h
Using 3” Ali, HL= mHeader line loss= m
MainlineUsing 125/6, HL= m
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Total Dynamic Head (TDH)Total Dynamic Head (TDH)Static head =Operating presssure = Riser height =Ali friction losses =PVC friction losses =Suction lift =Total =+ 10% loss fittings =TDH =
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Pump selectionPump selection
Power (kW)
= Qsystem * TDH 360*Pump Efficiency
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Bill of materialsBill of materials
Item QtySprinklers c/w riserAli pipe c/w riser T/OAli pipe plainHydrant control valveHydrants
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BOM contBOM cont
End plugsPVC pipePumpAli 90deg bendFittingsetc