Post on 31-Dec-2015
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Effect of Irrigation
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Calculate Hydraulic
Requirements
Chapter 4:
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Predicting Friction Losses
Calculating friction lossesHead (pressure) required at the
pivot pointBased on what the pivot “thinks” it is
irrigatingCalculating pressures, sprinkler by
sprinklerMainline layout and sizing
Page 59
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Supply (Main) Line – 1,000 gpm – 8” (approx) pipe
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1,000 gpm Total – Identical Outlets at 40’ – 8” (approx.) Pipe
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1,000 gpm total – 125 gpm end gun – sprinklers at 20’ 8” (approx) pipe
Page 62
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Practice Session on Friction Loss
Hydraulic Length Lateral Size (single
size) C = 145 (why?) Lateral friction loss Span Lengths
(Obstacles?)
Page 66
Friction Loss
• Hazen-Williams equation
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1.852 1.852
4.87
1002.207f
QH
C D
01
00
20
03
00
40
05
00
60
07
00
80
09
00
10
00
11
00
12
00
13
00
13
23
13
20
14
000100200300400500600700800900 GPM w/o Endgun
GPM w/Endgun
System Length (feet)
Gal
lon
s P
er M
inu
te
Center Pivot Flow Rate Distribution
• In a system having an end gun, the flow rate at the end of the pipe is equal to the discharge of the end gun, rather than zero
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Hyrdualic length
where:
Lh = the equivalent hydraulic length of the center-pivot lateral, ft (or m)
L = the physical length of the center-pivot lateral, ft (or m)
Qb = the “base” discharge for the basic circle, gpm (or L/s)
Qs = the discharge for all of the area irrigated by the center pivot lateral, gpm (or L/s) (generally, Qs = Qb + Qg)
Qg = the discharge of the end gun, if one is present, gpm (or L/s).
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b
gbh
b
sh Q
QQLLor
Q
QLL
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Multiple Sized Laterals
8 inch 6-5/8 inch
Page 66-78
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Additional Hydraulic Calculations
Calculating Velocity & Velocity HeadPressure Required at the Pivot
PointSimple Energy ConservationImpact of Pressure Regulators
Elevation Considerations
Page 80
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Friction Losses at Tower Joint?H-W “C”?
Page 84
Energy Balance EquationBernoulli
Accounts For Total Energy of water between two points
• Pressure head• Elevation head• Velocity head• Friction head
Energy Balance EquationBernoulli - Head
P1 + z1 + V12 / 2g = P2 + z2 + V2
2 / 2g – Hf 1-2 EQ. 4-16
Where:P1 = Pressure head at point 1, feet Z1 = Elevation head at point 1, feetV1
2 / 2g= Velocity head at point 1, feetP2 = Pressure head at point 1, feet z2 = Elevation head at point 1, feetV2
2 / 2g= Velocity head at point 1, feetHf 1-2 = Friction loss between points 1 and 2, feet
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Practice ExampleDesign Project
Minimum Sprinkler Pressure
Elevation Rise Nozzle Height Above
Ground Pivot Pressure at
Ground Level
Page 88
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• Look at each section of one-diameter pipe separately
• Compare to a pivot with all that size pipe
• Add the friction losses together
Calculating PressureSection by Section
Page 89
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Section-by-section
50
52
54
56
58
60
62
64 P
ressure
, P
SI
0 200 400 600 800 1000 1200 Distance along lateral, feet
Line (Lateral) Pressure Nozzle Pressure (drops)
Section-by-Section Calculation
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0 2 4 6 8
10 12 14 16
Nozzle
Dis
charg
e, gpm
0 200 400 600 800 1000 1200 Distance along lateral, feet
Required Est. (Manuf.)
Est. (Calc.) Est. (Complete.)
Section-by-Section Calculation Method
Nozzle discharge
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End Guns & Corner Systems Have Significant Effects on Hydraulics
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Varying Pivot Flow Rates(Affects Pump Selction)
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Affect on Pumping Requirements
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Mainline Layout and Sizing
Layout of clusters Economic vs.
hydraulically “best” solution
More small diameter pipe vs. less larger pipe
Page 89
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Mainline Configuration
a. Length of feeder pipe = 6 x lateral length
b. Length of feeder pipe
= 7.7 x lateral length
Page 90
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Supply (Main) Line Selection
Friction Losses in Supply (Main) LinePressure required at start
Pipe diameter (velocity method)Friction lossesSizing using economic criteriaSurge Pressures (Water hammer)
Appendix DValve placement, Air-vac, Pressure relief,
Sediment traps
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Screening Requirements
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Practice Example
Pages 94-96