L6= Irrigation Networks Hydraulics
The Islamic University of Gaza-Civil Engineering Department Irrigation and Drainage-ECIV 5327
Lecture 6: Irrigation networks hydraulics
Prepared by HusamAl-Najar Irrigation network Irrigation network Head (Energy) Losses When a fluid is flowing through a pipe, the fluid experiences some resistance due to which some of energy (head) of fluid is lost.
Energy Losses (Head losses)
Major Losses Minor losses
The roughness The properties The mean The pipe The pipe of the pipe of the fluid velocity, V diameter, D length, L A. Major losses
Useful Formulas to find the Major losses
1.Darcy-Weisbachformula 2.The Hazen -Williams Formula 3.The Manning Formula 4.The ChezyFormula The Hazen -Williams formula It has been used extensively for designing of water-supply systems
0.63 0.54 V = 0.85 CHW Rh S 1.85 L æ Q ö V = mean velocity (m/s) h = 10.7 ç ÷ Rh = hydraulic radius f 4.87 ç ÷ hf D CHW S = head loss per unit length of pipe = è ø L 1.85 CHW = Hazen-williamsCoefficient L æ V ö ç ÷ h f = 7.62 ç ÷ D è CHW ø 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: 0.63 0.54 V = 0.85 CHW Rh S
RH = D/4 = 0.2/4 = 0.05 m
CHW = 110 from previous table 30x(10)-3 V = Q/A = 0.63 0.54 = V=0.85(110)(0.05) (hf /100) (3.14 / 4)(0.2)2
hf = 0.68 m The Manning Formula 1 V = R23/ S12/ n h Q 2 h = 10.3n2 L f D16 /3 L h = 6.35 n 2V 2 f D1.33 B. Minor losses It 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
é V 2 ù It has the common form êKL ú ë 2g û 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:Thedischarge through each pipe is the same
Q = A1V1 = A2V2 = A3V3
Q = A1V1 = A2V2 = A3V3 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 Q1, L1, D1, f1
Q2, L2, D2, f2
Q3, L3, D3, f3
• Discharge: 3 Q = Q1 + Q2 + Q3 = åQi i=1
• Head loss: the head loss for each branch is the same
hL = h f 1 = h f 2 = h f 3
2 2 2 L1 V1 L2 V2 L3 V3 f1 = f2 = f3 D1 2g D2 2g D3 2g 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.
1.85 1.85 æ L öæ Q ö æ 250 öæ 0.04 ö Sol: h = 10.7 ç ÷ = 10.7 = 2.5m f1 ç 4.87 ÷ç ÷ ç 4.87 ÷ç ÷ è D øè Chw ø è 0.2 øè 120 ø 1.85 æ 500 öæ 0.04 ö h f 2 = 10.7ç ÷ç ÷ = 1.7m è 0.254.87 øè 120 ø 1.85 æ 250 öæ 0.04 ö hf 3 = 10.7ç ÷ç ÷ = 0.35m è 0.34.87 øè 120 ø
\h f -total = 2.5 +1.7 + .35 = 4.55m 1.85 1.85 æ L öæ Q ö æ 1000 öæ 0.04 ö h = 10.7 ç ÷ Þ 4.55 = 10.7 f ç 4.87 ÷ç ÷ ç 4.87 ÷ç ÷ è D øè Chw ø è D øè 120 ø \ D = 0.235m Q 0.04 \v = = = 0.922m / s A p 2 ´ 0.235 4 0.63 0.63 0.54 æ 0.235 ö 0.54 v = 0.85Chw Rh S Þ 0.922 = 0.85´120´ ç ÷ ´ S è 4 ø \ S = 0.0045 = 0.45% Pumping Systems 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.
Pump Classification
Positive displacement Dynamic
Reciprocating Rotary Centrifugal Special E l ec t r o m e c h a n i al D i a p h r gm Pl un g er Pi s t on S crew Ej ec t or G a s li ft V a ne Ge ar R a d i al A x i al M ix 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 Open Closed Installation of centrifugal pump either submersible (wet) or dry
Dry execution situation (vertical and horizontal) Installation of centrifugal pump either submersible (wet) or dry
Wet execution (vertical and submersible) 2.Positive Displacement pumps A. Screw pumps In the screw pump a revolving shaft fitted with blades rotates in an inclined trough and pushes the water up the trough.
el. motor
Gear box
Sec. A-A
Guide rim
Lining
Touch point
Alternative drive with gear box and belt drive. 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 minorlosses 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. V 2 H = H + h + åh + h + åh + d t stat fd md fs ms 2g
hfs : 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 selectthe 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, andtherefore the proper pump that satisfy all required performance characteristic is selected. Pump Characteristic Curves Pumps in Pipe Systems
system operating point 120
100
80 Head vs. discharge system curve curve for pump hp 60
H ea d ( m) 40 Static head 20
0 0 0.2 0.4 0.6 0.8 Discharge (m3/s)
What happens as the static head changes (a tank fills)? 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.
Qtotal =Q1+Q2+Q3 HTotal =H1+H2+H3
Pumps in Parallel Pumps in Series Irrigation wells Historical Background
Long arm short arm Wooden rod pivot Load
Well Water pocket
Low water level Rotating wheel Gears with water pockets
Channel
Low water level Irrigation well pumps
Line shaft turbine pump Submersible pump
Motor
Pump Shaft
Well
Submersible Turbine Pump Pump and motor Submersible Well pumps components 1 Discharge pipe 2 Discharge bowl 3 Discharge bearing 4 Intermediate bowl 5 Impeller 6 Up thrust collar 7 Intermediate bowl bearing 8 Lock collets 9 Pump shaft 10 Suction inlet 11 Suction adapter 12 Pump/motor coupling 13 Electric motor Line Shaft turbine pump Single-stage turbine pump Two stage turbine pump
Table 1. Characteristics of Turbine Pumps
Head per stage (m) Capacity m3/h Model Speed rpm Pipe diameter to from to from (mm) 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)
N Q 1440 0.0631 Ns = 51.64 , s 3/ 4 N = 51.64 3/ 4 = 993 hp 50
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. 1440 0.0631 2 stages: Ns = 51.64 = 1671 253/ 4 1440 0.0631 3 stages: Ns = 51.64 = 2264 16.673/ 4
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. 1440 0.0631 The specific speed at 5 stages = Ns = 51.64 = 3322 103/ 4
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 pipediameter 300 mm, 5 stages at speed 1440 rpm