Emitter Selection

Emitter typesLong path emitters,Short orifice emitters,Vortex emitters,Pressure compensating emitters,Porous pipe or tube emitters.

Further classificationPoint sourceLine source Sprays

Microsprinkler/SpraysOrifice control emitters Flow rate at any given pressure is governed primarily by the

orifice diameter Turbulent flow devices - flow rate is regulated by dissipating

energy. Flow velocities are greater and the potential for clogging is less

than for laminar flow devices. Flow rates are less sensitive to pressure (emitter exponent is

about 0.5) and less sensitive to water temperature than are laminar flow devices.

Vortex Control Emitters• Less sensitive to pressure variations than laminar or turbulent flow

emitters (emitter exponent is about 0.4). • Low pressure area formed in the center where the orifice is located

caused by vortex – reducing energy of water at the discharge point & a controlled flow rate.

• Emitter flow rate is controlled by vortex design and orifice diameter.Pressure Compensating Emitters

• Excess inlet pressure used to deform a diaphragm to control the flow rate.

• As the pressure increases, the diaphragm restricts the passage diameter.

• Pressure compensating emitters are designed to discharge at a fairly constant rate over a wide range of pressures (emitter exponent is normally less than 0.1).

• Drawbacks - the elasticity of the diaphragm may change over time.

• Diaphragms often retain some moisture when the pressure is off and bacteria growth ants seeking food source may result in clogging or destruction of diaphragm.

Stake Assemblies Stake assemblies raise emitter 8

inches above the ground. o larger wetting patterno water dispersed over weeds and grass.

4 mm ID tubing made of vinyl or polyethylene (PE).

Spaghetti tubing length depends on grower preference, but typically is 2 to 4 ft long

Wetting Patterns spinner

spray

• Important consideration in sandy soils or where root zones are shallow.

• Larger wetting patterns are often preferred for tree crops. o Emitters flow needs to correspond

with diameter to manage them effectively.

o When discharge is ≤0.08 in/hr, it requires very long run times to move the water into the mid and lower root zone.

o Potential for more wind drift, evaporation, and wetting of non-productive areas as the diameter increases.

• High density plantingso Most effective to provide each tree

with a smaller pattern emitter than to install larger pattern emitters on every other tree.

o Wetting pattern from larger diameter emitters is often distorted by interference from tree trunks and low branches

o However, small wetting patterns associated with low flow rates can lead to more plugging problems, particularly with the orifice control emitters.

Catch distribution patterns

(a) spinner

(b) spray

• Spinners have much higher application uniformities than the spray-type emitters.

• Both types have higher uniformity with high pressure 20 psi or higher compared 15 psi.

• Spinners - most of the wetted area receiving near-average application depths, with nearly continuous wetting throughout the pattern.

• Spray- wetted spokes radiating from the emitter with 50-75% of the area within the coverage diameter receiving little or no wetting.

• Lateral movement of water in the soil may help compensate for this in the root zone to varying degrees depending on the soil type

Class Activity

Emitter Selection Criteria1. Inexpensive2. Closeness of discharge-pressure

relationship to design specifications. 3. Easy to Install4. Susceptibility to clogging5. Pressure compensating 6. Not affected by temperature and

solar radiation7. Reliablity of discharge-pressure

relationship over a long period of time

Manufacturing variationThe variations in emitter passage

size, shape, and surface finish that do occur are small in absolute magnitude but represent a relatively large percent variation.

Emitter manufacturing variability

Coefficient of manufacturing variation (CV)is a statistical description of how uniformly the flow rate of each manufactured emitter is in relation to one another

meandeviation Standard

CV

Example

DRIP AND SPRAY EMITTERS CV’s CLASSIFICATION

CV < 0.05 Excellent

0.05 < CV < 0.07 Average

0.07 < CV < 0.11 Marginal

0.11 < CV < 0.15 poor

0.15 < CV unacceptable

LINE-SOURCE TUBING CV’s CLASSIFICATION

CV < 0.10 Good

0.10 < CV < 0.20 Average

0.20 < CV Poor to unacceptable

Flow rateFlow is characterized by the following equation

Where:q = flow rate (gph)P = pressure (psi)x = emitter exponetK = flow constant

xq KP

Emitter Exponentis important and critical to the

design, management and uniformity of the Micro system

The exponent (x) measures the flatness of the discharge-pressure curve.

Flow rate/pressure relationship for a laminar flow emitter (X=1.00)

Flow rate/pressure relationship for a turbulent flow emitter (X=0.50)

Flow rate/pressure relationship for a pressure compensated flow emitter (X=0.0)

PC emittersEven the best PC emitters only have a

certain range of pressures over which they provide good pressure compensation.

A PC emitter may retain its compensating abilities at very high pressures. But when pressures exceed 35 psi or so, emitters tend to pop off the hose, or hoses tend to pop out of their fittings

System EU for a PC emittersThe EU for a PC emitter is still

dependent on the manufacturing variation CV

They do not have a discharge exponent of exactly 0.0, even though that is what is claimed

How to get x and KK and x may be obtained from manufacture or

calculated

For sprinkler x is nearly always 0.5For pressure compensating x ~ 0.0

xPqK

PPqq

x

log

log

2

1

2

1

ExampleGiven: q1 = 1.5 gph, q2 = 2.0 gph,

P1 = 12 psi, P2 = 20 psi

Find: x and K

Solution

37.020

2 56.0

2012log

25.1log

56.0

xPqKx

Practice problem

Emitter spacing

Optimum SpacingOptimum spacing is

approximately 80% of the wetted area (7.2*.8= 5.76ft)

Wetted area overlapWhat is the optimum emitter spacing?Closest together? Farther away?

The optimum area is a rectangle

Soil wetted areaKind of soil layers

SoilDepthAnd

Texture

Homogeneous Varying layers,Generally

Low density

Varying layers,Generally

Medium density

S’e x Sw = Aw (ft2) S’e x Sw = Aw (ft2) S’e x Sw = Aw (ft2)

Depth 2.5 ft 2.5 ft 2.5 ft

Coarse 1.2 x 1.5 = 1.8 2.0 x 2.5 = 5.0 2.8 x 3.5 = 9.8

Medium 2.4 x 3.0 = 7.2 3.2 x 4.0 = 12.8 4.0 x 5.0 = 20.0

Fine 2.8 x 3.5 = 9.8 4.0 x 5.0 = 20 4.8 x 6.0 = 28.8

Depth 5 ft 5 ft 5 ft

Coarse 2.0 x 2.5 = 5.0 3.6 x 4.5 = 16.2 4.8 x 6.0 = 28.8

Medium 3.2 x 4.0=12.8 5.6 x 7.2 = 39.2 7.2 x 9.0 = 64.8

Fine 4.0 x 5.0=20.0 5.2 x 6.5 = 33.8 6.4 x 8.0 = 51.2

1 Based on an emitter flow rate of 1 gph (3.785 L), the estimated Aw is given as a rectangle with the wetted width (Sw) equal to the maximum expected diameter of the wetted circle and the optimum emitter spacing (S’e) equal to 80 percent of that diameter.

Some times two or more rows are needed

Number and spacing of emitters.

◦…..shall be adequate to provide water distribution to the plant root zone and percent plant wetted area (Pw).

Example problemGiven: Tree spacing of 24’x24’, Root depth

4’Single drip hoseLoam soil ( medium texture,

Homogeneous)Desired wetted area 50%

Find: # of emitters and emitter spacing

SolutionNumber of emittersFrom table 7-14 Emitter wetted area = 7.2Tree area = 24x24 =576Desired wetted area = 576 *.5 =288Required emitters = 288/7.2= 40

Practice problem

System capacity.

◦….shall be adequate to meet the intended water demands during the peak use period

◦….shall include an allowance for reasonable water

losses (evaporation, runoff, and deep percolation) during application periods.

◦…shall have the capacity to apply a specified amount

of water to the design area within the net operation period.

System capacity Continued

◦should have a minimum design capacity sufficient to deliver the peak daily irrigation water requirements in 90% of the time available, but not to exceed 22 hours of operation per day.

Emitter Flow rate qa

( / / )

T = set time (hrs)

q

( / )

a

Where:

emitter flow rate (ave) number of emitters

F gallons per day per plant

a

g d plant

gp da

a

e

Fq

T e

Watering strategiesSelect emitter based on water

requiredCalculate set time

adgp

a qeF

T )/(

Adjust flow rate or set time

If Ta is greater than 22 hr/day (even for a single-station system), increase the emitter discharge

If the increased discharge exceeds the recommended range or requires too much pressure, either larger emitters or more emitters per plant are required.

Select the number of stations If Ta ≈ 22 h/d, use a one-station system (N =

l), select Ta < 22 hr/day, and adjust qa accordingly.

If Ta <11 h/d, use N = 2, select a Ta <11, and adjust qa accordingly.

If 12 < Ta < 18, it may be desirable to use another emitter or a different number of emitters per plant to enable operating closer to 90 percent of the time and thereby reduce investment costs.

Determine average emitter pressure head (Pa)

1x

aa

qPK

Where:

qa= average emitter flow rate (gph)Pa = average pressure (psi)x = emitter exponentK = flow constant

Average depth applied

Where:Fn = Average applied (in)e = number of emittersqa=average emitter flow rate (gph)Ta = set time (hrs)Sp = Plant spacing (ft)Sr = Row spacing (ft)

1.604 a an

p r

eq TF

S S

Determine total system flow rate

Where:A = field area, ac.e = number of emitters per plant.N = number of operating stations.qa = average or design emission rate, gph.Sp = plant spacing in the row, ft.Sr = distance between plant rows, ft

rp

as SS

qeNAQ 726

Practice problem