Lecture 6: Irrigation networks hydraulics

Prepared by

Husam Al-Najar

The Islamic University of Gaza- Civil Engineering DepartmentIrrigation and Drainage- ECIV 5327

Irrigation network

Irrigation network

Energy Losses(Head losses)

Major Losses Minor losses

The roughness of the pipe

The properties of the fluid

The mean velocity, V

The pipe diameter, D

The pipe length, L

Head (Energy) LossesWhen a fluid is flowing through a pipe, the fluid experiences some resistance due to which some of energy (head) of fluid is lost.

A. Major losses

1. Darcy-Weisbach formula2. The Hazen -Williams Formula3. The Manning Formula4. The Chezy Formula

Useful Formulas to find the Major losses

The Hazen -Williams formulaIt has been used extensively for designing of water- supply systems

V C R SHW h= 0 85 0 63 0 54. . .

85.1

87.47.10

=

HWf C

QD

Lh

85.1

62.7

=

HWf C

VDLh

V = mean velocity (m/s)Rh = hydraulic radiusS = head loss per unit length of pipe = CHW = Hazen-williams Coefficient

hLf

Hazen-Williams Coefficient, CHW, for different types of pipe

Example 1: A 100 m long pipe with D = 20 cm. It is made of riveted steel and carries a discharge of 30 l/s. Determine the head loss in the pipe using Hazen-Williams formula.

Solution:

V C R SHW h= 0 85 0 63 0 54. . .

54.063.0 )100/()05.0)(110(85.0 hfV=

RH = D/4 = 0.2/4 = 0.05 m

CHW = 110 from previous table

V = Q/A = 2

3

)2.0)(4/14.3()10(30 −x

=

hf = 0.68 m

The Manning Formula

V n R Sh=1 2 3 1 2/ /

3/16

223.10

DQLnh f =

2233.135.6 Vn

DLh f =

B. Minor lossesIt is due to the change of the velocity of the flowing fluid in the magnitude or in direction [turbulence within bulk flow as it moves through and fitting]

The minor losses occurs at :

• Valves • Tees• Bends• Reducers• And other appurtenances

It has the common form

g

VKL 2

2

The loss coefficient for elbows, bends, and tees

Compound Pipe flow The system is called compound pipe flow: When two or more pipes with

different diameters are connected together head to tail (in series) or connected to two common nodes (in parallel)

A. Flow Through Pipes in Series• pipes of different lengths and different diameters connected end

to end (in series) to form a pipeline

• Discharge:The discharge through each pipe is the same

332211 VAVAVAQ ===

332211 VAVAVAQ ===

B. Flow Through Parallel Pipes:

If a main pipe divides into two or more branches and again join together downstream to form a single pipe, then the branched pipes are said to be connected in parallel (compound pipes).

• Points A and B are called nodes.

Q1, L1, D1, f1

Q2, L2, D2, f2

Q3, L3, D3, f3

• Discharge:

• Head loss: the head loss for each branch is the same

∑=

=++=3

1321

iiQQQQQ

Q1, L1, D1, f1

Q2, L2, D2, f2

Q3, L3, D3, f3

321 fffL hhhh ===

gV

DL

fg

VDLf

gV

DLf

222

23

3

33

22

2

22

21

1

11 ==

Example 2.Three pipes connected in series have to be replaced by one pipe of the same total length. The diameters are 200mm, 250mm, and 300mm, and the lengths are 250 m, 500 m, and 250 m, respectively. Determine the slope of the new pipe that can transport flow of 40 l/s. All pipes are galvanized iron.

Sol: mCQ

DLh

hwf 5.2

12004.0

2.02507.107.10

85.1

87.4

85.1

87.41 =

=

=

mh f 7.1120

04.025.05007.10

85.1

87.42 =

=

mh f 35.0120

04.03.0

2507.1085.1

87.43 =

=

mh totalf 55.435.7.15.2 =++=∴ −

120

04.010007.10.554 7.1085.1

87.4

85.1

87.4

=⇒

=

DCQ

DLh

hwf

mD 235.0=∴ sm

AQv /922.0

235.04

04.02

=×

==∴π

54.063.0

54.063.0

4235.012085.0922.0 85.0 SSRCv hhw ×

××=⇒=

%45.00045.0 ==∴ S

Pump Classification

DynamicPositive displacement

Centrifugal SpecialReciprocating Rotary

Radial

Mix

Axial

Ejector

Electrom

echanical

Gas lift

Vane

Screw

Gear

Diaphragm

Plunger

Piston

Definition: Water pumps are devices designed to convert mechanical energy to hydraulic energy. They are used to move water from lower points to higher points with a required discharge and pressure head.

Pumping Systems

All forms of water pumps may be classified into two basic categories:

1. Turbo-hydraulic (Dynamic) pumps : Which includes three main types:

A. Centrifugal pumps ( Radial - flow pumps ).

B. Propeller pumps ( Axial - flow pumps ).

C. Jet pumps ( Mixed - flow pumps ).

Different types of impellers

Semi open ClosedOpen

Installation of centrifugal pump either submersible (wet) or dry

Dry execution situation (vertical and horizontal)

Wet execution (vertical and submersible)

Installation of centrifugal pump either submersible (wet) or dry

A. Screw pumps

Guide rim

Lining

el. motor

Touch point

Alternative drive with gear box and belt drive.

Gear box

Sec. A-A

In the screw pump a revolving shaft fitted with blades rotates in an inclined trough and pushes the water up the trough.

2. Positive Displacement pumps

B. Reciprocating pumps

Pumps System Curve

System Characteristic Curve• It is a graphic representation of the system head and is developed by plotting the

total head, Ht , over a range of flow rates starting from zero to the maximum expected value of Q.

• This curve is usually referred to as a system characteristic curve or simply system curve.

• For a given pipeline system (including a pump or a group of pumps), a unique system head-capacity (H-Q) curve can be plotted.

• The total head, Ht , that the pump delivers includes the elevation head and the head losses incurred in the system. The friction loss and other minor losses in the pipeline depend on the velocity of the water in the pipe, and hence the total head loss can be related to the discharge rate.

H H h h h hV

gt stat f d md f s msd= + + ∑ + + +∑2

2hfs : is the friction losses in the suction pipe. hfd : is the friction losses in the discharge (delivery) pipe.hms : is the minor losses in the suction pipe.hmd: is the minor losses in the discharge (delivery) pipe.

Pump Characteristic Curves

• Pump manufacturers provide information on the performance of their pumps in the form of curves, commonly called pump characteristic curves (or simply pump curves).

• In pump curves the following information may be given:• the discharge on the x-axis,• the head on the left y-axis,• the pump power input on the right y-axis,• the pump efficiency as a percentage,• the speed of the pump (rpm = revolutions/min).• the NPSH of the pump.

• The pump characteristic curves are very important to help select the required pump for the specified conditions.

• If the system curve is plotted on the pump curves we may produce. • The point of intersection is called the operating point. • This matching point indicates the actual working conditions, and therefore the proper

pump that satisfy all required performance characteristic is selected.

Pump Characteristic Curves

system curve

0

20

40

60

80

100

120

0 0.2 0.4 0.6 0.8Discharge (m3/s)

Hea

d (m

)

system operating point

Static head

Head vs. dischargecurve for pump

What happens as the static head changes (a tank fills)?

ph

Pumps in Pipe Systems

Multiple-Pump Operation• To install a pumping station that can be effectively operated over a

large range of fluctuations in both discharge and pressure head, it may be advantageous to install several identical pumps at the station.

Pumps in Parallel Pumps in Series

Qtotal =Q1+Q2+Q3 HTotal =H1+H2+H3

Long arm short arm

Low water level

Water pocket

Well

pivotWooden rod Load

Irrigation wells

Historical Background

GearsRotating wheel with water pockets

Channel

Low water level

Motor

Pump Shaft

Turbine Pump

Well

Submersible Pump and motor

Line shaft turbine pump Submersible pump

Irrigation well pumps

Submersible Well pumps components

Electric motor13Pump/motor coupling12Suction adapter11Suction inlet10Pump shaft9Lock collets8Intermediate bowl bearing7Up thrust collar6Impeller5Intermediate bowl4Discharge bearing3Discharge bowl2

Discharge pipe1

Line Shaft turbine pump

Two stage turbine pumpSingle- stage turbine pump

Table 1. Characteristics of Turbine Pumps

Pipe diameter (mm)

Capacity m3/h

fromfrom toto

Head per stage (m)Speed rpm

Model

Figure 1. Classification of pumps based on specific speed

Example 3

It is required to abstract 227 m3/ h water from well at 50 m depth for irrigation. Select the best and the most efficient irrigation pump for this purpose

1. The required flow = 227/ 0.2271 = 1000 Gpm = 0.0631 m3/s

2. For deep wells it is preferred to use turbine pumps

3. Turbine pumps are easy to maintain)

4. Specific speed based on one stage at 1440 rpm (Table 1)

5. Use figure 1. the efficiency = 78%, while the highest efficiency 83% could be reached at specific speed 2000. So we have to try 2 or 3 stages. On each stage the head should be divided by 2 and 3, respectively.

, 64.51 N 4/3sph

QN= 993

500631.01440 64.51 N 4/3s ==

2 stages:

3 stages:

6. Refer to figure 1. three stages will reach to the best efficiency, therefore the best pump for this purpose is Model 12S (Table 1) with head around 10 m for each stage and provide the required flow. So we need 5 stages pump of this model to cover the required head.

The specific speed at 5 stages =

7. At figure 1. (Q = 1000 Gpm, Ns= 3322, The efficiency equals 81% the best we can reach.

From Table 1. The best pump is Turbine pump, 12S model with pipe diameter 300 mm, 5 stages at speed 1440 rpm

1671 25

0631.01440 64.51 N 4/3s ==

2264 67.16

0631.01440 64.51 N 4/3s ==

3322 10

0631.01440 64.51 N 4/3s ==