10 Frequently Asked Questions 10 Frequently Asked Questions about Center Pivotsabout Center Pivots

Gulf Coast Irrigation ConferenceGulf Coast Irrigation ConferenceSinton, November 18, 2008 Sinton, November 18, 2008

Guy FippsProfessor and Extension Agricultural Engineer

Biological and Agricultural Engineering

Texas AgriLife Extension Service

1) How much water do I need?2) Is my pond large enough to supply my irrigation

water requirements?3) What kind of sprinkler packages are available?4) What type of water applicator is best?5) What are the most common design problems

seen with center pivots?6) What size should my mainline pipe be?7) Do I need pressure regulators?8) What are the key design considerations for a

pivot? 9) How can I check to see if my pivot is operating

at design specifications?10) What are the considerations when renozzling a

pivot?

How Much Water Do I Need?How Much Water Do I Need?

Irrigation Systems are designed to supply peak Irrigation Systems are designed to supply peak water demand of crops water demand of crops

(inches per day, inches per week etc.)(inches per day, inches per week etc.)

Bulletin 6019Bulletin 6019Crop water demand information useful for sizing irrigation Crop water demand information useful for sizing irrigation

systems (shown are values for South Texas)systems (shown are values for South Texas)

(copy of bulletin are posted at (copy of bulletin are posted at http://texaset.tamu.edu))

CropCrop Peak Demand Peak Demand

(inches/day)(inches/day)

cottoncotton 0.23 0.23

corncorn 0.320.32

citruscitrus 0.160.16

sorghumsorghum 0.220.22

perennial pasture perennial pasture 0.250.25

small grainssmall grains 0.260.26

vegetables vegetables 0.160.16

Example: Pasture/forage in South TexasExample: Pasture/forage in South Texas

Peak water demandPeak water demand

0.25 inches/day = 6789 gal/acre/day0.25 inches/day = 6789 gal/acre/day

(note: 1 ac-in = 27,154 gal)(note: 1 ac-in = 27,154 gal)

Total Gallons Needed per dayTotal Gallons Needed per day

10 acres10 acres 50 acres50 acres 100 acres100 acres 120 acres120 acres

67,88567,885

(2.5 ac-in)(2.5 ac-in)

339,425339,425

(1.04 ac-ft)(1.04 ac-ft)

678,850678,850

(12.08 ac-ft)(12.08 ac-ft)

814,600814,600

(2.5 ac-ft)(2.5 ac-ft)

Example: Pasture/forage in South TexasExample: Pasture/forage in South Texas

Peak water demandPeak water demand

0.25 inches/day = 6789 gal/acre/day0.25 inches/day = 6789 gal/acre/day

Pumping rate – 24 hours @ 100% efficiencyPumping rate – 24 hours @ 100% efficiency

10 acres10 acres 50 acres50 acres 100 acres100 acres 120 acres120 acres

47 gpm47 gpm 235 gpm235 gpm 470 gpm470 gpm 566 gpm566 gpm

Example: Pasture/forage in South TexasExample: Pasture/forage in South Texas

Peak water demandPeak water demand

0.25 inches/day = 6789 gal/acre/day0.25 inches/day = 6789 gal/acre/day

Pumping rate – 12 hours @ 80% efficiencyPumping rate – 12 hours @ 80% efficiency

10 acres10 acres 50 acres50 acres 100 acres100 acres

117 gpm117 gpm 587 gpm587 gpm 1175 gpm1175 gpm

Pumping rate and irrigation Pumping rate and irrigation capacitycapacity

From Table 2 of From Table 2 of Center Pivot Irrigation Center Pivot Irrigation in in notebook (assumes irrigating 24 hours per day)notebook (assumes irrigating 24 hours per day)

GPM/acre Inches per day Inches per week1.5

1.5 .08 .55

2.0 .11 .75

3.0 .16 1.10

4.0 .21 1.50

5.0 .27 1.85

6.0 .32 2.25

7.0 .37 2.60

8.0 .42 2.97

Is my pond (tank) large enough to supply my Is my pond (tank) large enough to supply my water requirements?water requirements?

Using an on-farm pond (or water storage facility) Using an on-farm pond (or water storage facility)

to provide 0.25 inches/day? to provide 0.25 inches/day?

Calculate pond size:Calculate pond size:

surface area x average depth = water volume surface area x average depth = water volume

(acres)(acres) (feet) (feet) (ac-ft)(ac-ft)

Example: Pasture/forage in South TexasExample: Pasture/forage in South Texas

Is your pond large enough to provide 0.25 inches/day?Is your pond large enough to provide 0.25 inches/day?

Water Supply in Water Supply in Weeks Weeks

( at 100% irrigation ( at 100% irrigation efficiency)efficiency)

50 acre50 acre

fieldfield

100 acre100 acre

fieldfield

120 acre120 acre

fieldfield

11 7 ac-ft7 ac-ft 15 ac-ft15 ac-ft 17 ac-ft17 ac-ft

22 15 ac-ft15 ac-ft 30 ac-ft30 ac-ft 35 ac-ft35 ac-ft

33 22 ac-ft22 ac-ft 45 ac-ft45 ac-ft 55 ac-ft55 ac-ft

Example: Pasture/forage in South TexasExample: Pasture/forage in South Texas

Is your pond large enough to provide 0.25 inches/day?Is your pond large enough to provide 0.25 inches/day?

Water Supply in Water Supply in Weeks Weeks

( at 80% irrigation ( at 80% irrigation efficiency)efficiency)

50 acre50 acre

fieldfield

100 acre100 acre

fieldfield

120 acre120 acre

fieldfield

11 9 ac-ft9 ac-ft 19 ac-ft19 ac-ft 21 ac-ft21 ac-ft

22 19 ac-ft19 ac-ft 38 ac-ft38 ac-ft 44 ac-ft44 ac-ft

33 28 ac-ft28 ac-ft 56 ac-ft56 ac-ft 69 ac-ft69 ac-ft

typetype Natural Natural gasgas

ElectricElectric

turbineturbine

ElectricElectriccentrifugalcentrifugal

DieselDiesel

Cost Cost $1.49$1.49 $2.00$2.00 $2.52$2.52 $3.07$3.07

Fuel Fuel cost*cost*

$11 MCF$11 MCF $0.11 kwh $0.11 kwh $2.65 gal$2.65 gal

How much money can I save by reducing my How much money can I save by reducing my operating pressure?operating pressure?

Typical Pumping Costs in Texas:Typical Pumping Costs in Texas:

Acre-inch per 100 ft head (or 43 psi)Acre-inch per 100 ft head (or 43 psi)

* my fuel costs on Oct 22, 2008* my fuel costs on Oct 22, 2008

pressurepressure 15 15

psipsi

3030

psipsi

4545

psipsi

6060

psipsi

9090

psipsi

cost cost

(per ac-ft)(per ac-ft)

$10.44$10.44 $20.76$20.76 $31.20$31.20 $41.64$41.64 $57.72$57.72

Costs of Pressurizing WaterCosts of Pressurizing Water

Per acre-foot of water, Per acre-foot of water,

electric centrifugal pump

at 0.11 kwh

pressurepressure 15 15

psipsi

3030

psipsi

4545

psipsi

6060

psipsi

9090

psipsi

cost cost

(per ac-ft)(per ac-ft)

$1253$1253 $2492$2492 $3744$3744 $4997$4997 $6926$6926

Costs of Pressurizing WaterCosts of Pressurizing Water

For applying 1 ft of water on a 120 acre field: For applying 1 ft of water on a 120 acre field:

electric centrifugal pump

at 0.11 kwh

What types of sprinkler packages are available?What types of sprinkler packages are available?(1)(1) high pressure impactshigh pressure impacts

(2)(2) medium elevation spray applicators medium elevation spray applicators (MESA)(MESA)

(3)(3) low energy precision applicatorslow energy precision applicators(LEPA)(LEPA)

(4)(4) low elevation spray applicatorslow elevation spray applicators(LESA)(LESA)

Water-move pivot

Older pivot with high pressure impact sprinklers

MESA (medium elevation spray applicators)

MESA (medium elevation spray applicators)

Over-pressured MESA system

LEPA (low energy precision applicators)

bubble modebubble mode

LEPA with alternate row furrow dikes

LESA (low elevation spry applicators)

What types of applicator is best?What types of applicator is best? LEPA and LESA: LEPA and LESA:

– have the lowest operating pressures (6 – 10 psi) have the lowest operating pressures (6 – 10 psi) – the highest efficiencies (90-98%)the highest efficiencies (90-98%)– But require higher skill level for proper irrigation But require higher skill level for proper irrigation

water managementwater management LEPA may require practices to control runoffLEPA may require practices to control runoff

What types of applicator is best?What types of applicator is best? MESAMESA

– About 10% of the water applied to high canopy crops About 10% of the water applied to high canopy crops is lost to evaporationis lost to evaporation

– Typical efficiencies are 80-90% with proper designTypical efficiencies are 80-90% with proper design

What are the most common design problems seen with center pivots?

• mainline too small

• elevation changes in field not considered in the design

• end gun added

• system designed for incorrect flow rate

What size should my mainline pipe be?What size should my mainline pipe be? For the same diameter pipe:

– The higher the flow rate, the higher the friction losses

Try to keep friction loses to 10 psi if practical

Consider telescoping for larger flow rates and for half mile systems

What size should my mainline pipe be?What size should my mainline pipe be?

Friction loss in center pivot mainlines (psi)Friction loss in center pivot mainlines (psi)

Flow rate,GPM

Mainline pipe diameter, inches

6 6 5/8 8

500 8 5

600 11 7

800 18 11 4

1000 28 17 7

1200 39 24 9

What size should my mainline pipe be?What size should my mainline pipe be?

Telescoping ExampleTelescoping Example::

Pivot is 1316 ft long with a flow rate of 1100 GPMPivot is 1316 ft long with a flow rate of 1100 GPM

Case 1: all 1316 ft is 6 5/8-inch pipe Friction loss is 19 psi

Case II: 640 ft of 8-inch and 676 ft of 6 5/8-inch Friction loss is 10 psi

Do I need pressure regulators?Do I need pressure regulators?

Depends upon theDepends upon the:: Elevation change in the field Elevation change in the field

(maximum change in elevation from pivot point to highest/lowest (maximum change in elevation from pivot point to highest/lowest point in fieldpoint in field

System design pressure System design pressure (pressure at the nozzle) (pressure at the nozzle)

Regulators require about 3 psi more than their Regulators require about 3 psi more than their pressure rating to operate properlypressure rating to operate properly

A 6 pound regulator requires an operating pressure of 9 A 6 pound regulator requires an operating pressure of 9 psi psi

Do I need pressure regulators?Do I need pressure regulators? Pivots should be designed to maintain less than 20%

variation in system design pressure

Table 5. % variation in design pressure Table 5. % variation in design pressure Elevation change, ft

System design pressure (psi)

10 20 30 40

2.3 10.0 5.0 3.3 2.5

4.6 20.0 10 6.6 5.0

9.2 40.0 20.0 13.3 10.0

13.9 30.0 20.0 15.0

18.5 26.6 20.0

What are the key design considerations for a pivot?

• elevation changes in field considered

• mainline pipe sized correctly

• efficient water applicators

• matched to available water supply

• matched to water requirements of crop

How can I check to see if my pivot is operating at design specifications?

• obtain a copy of the pivot design printout• note the following:

1) total flow rate 2) pressure at the pivot point3) pressure at the last nozzle or pressure

regulator

How can I check to see if my pivot is operating at design specifications?

• Do you have enough water (gpm) as measured by your flow meter?

• Is your flow meter accurate?• is the meter installed in a straight

section which is at least 15 pipe diameters long?

How can I check to see if my pivot is operating at design specifications?

• Does the pressure match at the pivot point?

• Install a pressure gage in the last drop just above the water applicator or pressure regulator

• Is the pressure equal to or greater than what’s required?

Considerations when renozzlingConsiderations when renozzling(see Section 15 of Pivot Publication)(see Section 15 of Pivot Publication)

Actual lowest and highest elevation in field in relationship to the pivot point is used in the computer design printout.

Actual measured flow rate and pressure available by pump (or water source) is used in the computer design printout.

Friction loss in pivot mainline for quarter-mile long systems is no greater than 10 psi.

Considerations when renozzlingConsiderations when renozzling(see Section 15 of Pivot Publication)(see Section 15 of Pivot Publication)

Mainline outlets spaced:Mainline outlets spaced:– a maximum of 60 to 80 inches apart a maximum of 60 to 80 inches apart – or alternately, 2 times the crop row spacing.or alternately, 2 times the crop row spacing.

For non-leveled fields:For non-leveled fields:– less than 20% pressure variation in system design less than 20% pressure variation in system design

operating pressure is maintained when pivot is operating pressure is maintained when pivot is positioned at highest and lowest point in the field positioned at highest and lowest point in the field

– (computer design printout provided for each case). (computer design printout provided for each case).

Considerations when renozzlingConsiderations when renozzling(see Section 15 of Pivot Publication)(see Section 15 of Pivot Publication)

Pressure regulators are evaluated for fields Pressure regulators are evaluated for fields with more than 5 feet elevation change from with more than 5 feet elevation change from pad to the highest or the lowest point in the pad to the highest or the lowest point in the field.field.

Copy of pivot design printout provided by Copy of pivot design printout provided by dealer.dealer.

Considerations when renozzlingConsiderations when renozzling(see Section 15 of Pivot Publication)(see Section 15 of Pivot Publication)

No end gun. No end gun. Consideration was given to equipping the Consideration was given to equipping the

pivot with either LEPA or LESA applicatorspivot with either LEPA or LESA applicators

Considerations when renozzlingConsiderations when renozzling(see Section 15 of Pivot Publication)(see Section 15 of Pivot Publication)

Propeller flow meter or other type of flow Propeller flow meter or other type of flow measurement devicemeasurement device– with an accuracy of with an accuracy of + + 3% – installed in water supply pipeline near pivot

point in a straight section 10 pipe diameters upstream and 5 pipe diameters downstream from the flow meter.

Considerations when renozzlingConsiderations when renozzling(see Section 15 of Pivot Publication)(see Section 15 of Pivot Publication)

Two pressure gaugesTwo pressure gauges– one on the mainline near the pivot point, one on the mainline near the pivot point, – one in the last drop, located just above the one in the last drop, located just above the

applicator or pressure regulator.applicator or pressure regulator.

Copy of this presentation will be Copy of this presentation will be posted atposted at

http://gfipps.tamu.eduhttp://gfipps.tamu.edu