© Irrigation Association Effect of Irrigation. © Irrigation Association Calculate Hydraulic...

© Irrigation Association

Effect of Irrigation


Calculate Hydraulic

Requirements

Chapter 4:


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


Supply (Main) Line – 1,000 gpm – 8” (approx) pipe


1,000 gpm Total – Identical Outlets at 40’ – 8” (approx.) Pipe


1,000 gpm total – 125 gpm end gun – sprinklers at 20’ 8” (approx) pipe

2


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


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


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).


b

gbh

b

sh Q

QQLLor

Q

QLL


Multiple Sized Laterals

8 inch 6-5/8 inch

Page 66-78


Additional Hydraulic Calculations

Calculating Velocity & Velocity HeadPressure Required at the Pivot

PointSimple Energy ConservationImpact of Pressure Regulators

Elevation Considerations

Page 80


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


Practice ExampleDesign Project

Minimum Sprinkler Pressure

Elevation Rise Nozzle Height Above

Ground Pivot Pressure at

Ground Level

Page 88


• 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


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


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


End Guns & Corner Systems Have Significant Effects on Hydraulics


Varying Pivot Flow Rates(Affects Pump Selction)


Affect on Pumping Requirements


Mainline Layout and Sizing

Layout of clusters Economic vs.

hydraulically “best” solution

More small diameter pipe vs. less larger pipe

Page 89


Mainline Configuration

a. Length of feeder pipe = 6 x lateral length

b. Length of feeder pipe

= 7.7 x lateral length

Page 90


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


Screening Requirements


Practice Example

Pages 94-96

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