chapter 3: water distribution systems & networks university of palestine engineering hydraulics...
TRANSCRIPT
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CHAPTER 3: Water Distribution Systems & networks
CHAPTER 3: Water Distribution Systems & networks
University of PalestineEngineering Hydraulics2nd semester 2010-2011
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ContentWater Distribution Systems
Design of Water Distribution
Systems
Pipe Network Analysis
Water Distribution Systems & networks
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Part A
Water Distribution Systems
Water Distribution Systems & networks
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IntroductionTo deliver water to individual consumers with
appropriatequality, quantity, and pressure in a community
setting requiresan extensive system of:Pipes.Storage reservoirs.Pumps.Other related accessories.
Distribution system: is used to describe collectively the facilities used to supply water from its source to the point of usage .
Water Distribution Systems
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Methods of Supplying Water• Depending on the topography
relationship between the source of supply and the consumer, water can be transported by:
•Canals. •Tunnels.•Pipelines.
• The most common methods are:•Gravity supply•Pumped supply•Combined supply
Water Distribution Systems
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Gravity Supply
• The source of supply is at a sufficient elevation above the distribution area (consumers).
so that the desired pressure can be maintained
Source
(Reservoir)
(Consumers)
Gravity-Supply System
HGL or EGL
Water Distribution Systems
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Advantages of Gravity supply
• No energy costs.• Simple operation (fewer mechanical
parts, independence of power supply, ….)
• Low maintenance costs.• No sudden pressure changes
Source
HGL or EGL
Water Distribution Systems
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Pumped Supply Used whenever:• The source of water is lower than the area to which we need
to distribute water to (consumers) • The source cannot maintain minimum pressure required.
pumps are used to develop the necessary head (pressure) to
distribute water to the consumer and storage reservoirs.
Source
(River/Reservoir)
(Consumers)
Pumped-Supply System
HGL or EGL
Water Distribution Systems
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Source
(River/Reservoir)
(Consumers)
HGL or EGL
Disadvantages of pumped supplyComplicated operation and maintenance.Dependent on reliable power supply. Precautions have to be taken in order to enable
permanent supply:• Stock with spare parts• Alternative source of power supply ….
Water Distribution Systems
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Combined Supply(pumped-storage supply)
• Both pumps and storage reservoirs are used.
• This system is usually used in the following cases: 1) When two sources of water are used to supply water:
Source (1)
Source (2)
City
Gravity
Pumping
HGL
HGL
Pumping station
Water Distribution Systems
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Combined Supply (Continue) 2) In the pumped system sometimes a storage
(elevated) tank is connected to the system.
Elevated tank
Source
Pipeline
High consumption
Pumping station
• When the water consumption is low, the residual water is pumped to the tank.• When the consumption is high the water flows back to the consumer area by gravity.
Low consumption
City
Water Distribution Systems
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Combined Supply (Continue) 3) When the source is lower than the consumer area
Reservoir
Pumping
Pumping Station
• A tank is constructed above the highest point in the area, • Then the water is pumped from the source to the storage tank (reservoir).• And the hence the water is distributed from the reservoir by gravity.
Gravity
City
HGL
HGL
Water Distribution Systems
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Distribution Systems (Network Configurations )
• In laying the pipes through the distribution area, the following configuration can be distinguished:
1. Branching system (Tree)2. Grid system (Looped)3. Combined system
Water Distribution Systems
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Branching System (tree system)
Branching System
Source
Submain
Main pipe
Dead End
Advantages:• Simple to design and build.• Less expensive than other systems.
Water Distribution Systems
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• The large number of dead ends which results in sedimentation and bacterial growths.
• When repairs must be made to an individual line, service connections beyond the point of repair will be without water until the repairs are made.
• The pressure at the end of the line may become undesirably low as additional extensions are made.
Source
Dead End
Disadvantages:
Water Distribution Systems
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Grid System (Looped system)
Grid System Advantages:• The grid system overcomes all of the difficulties of the branching system discussed before. • No dead ends. (All of the pipes are interconnected).• Water can reach a given point of withdrawal from several directions.
Water Distribution Systems
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Disadvantages:• Hydraulically far more complicated than
branching system (Determination of the pipe sizes is somewhat more complicated) .
• Expensive (consists of a large number of loops).
But, it is the most reliable and used system.
Water Distribution Systems
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Combined System
• It is a combination of both Grid and Branching systems
• This type is widely used all over the world.
Combined System
Water Distribution Systems
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Part B
Design of Water Distribution
Systems
Water Distribution Systems & networks
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Design of Water Distribution Systems
Main requirements :• Satisfied quality and quantity standards Additional requirements :• To enable reliable operation during irregular
situations (power failure, fires..)• To be economically and financially viable, ensuring
income for operation, maintenance and extension.• To be flexible with respect to the future extensions.
A properly designed water distribution system should fulfill the following requirements:
Design of Water Distribution Systems
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The design of water distribution systems must undergo through different studies and steps:
Design PhasesDesign Phases
Hydraulic Analysis
Preliminary Studies
Network Layout
Design of Water Distribution Systems
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Preliminary Studies:
4.3.A.1 Topographical Studies:
Must be performed before starting the actual design:
1. Contour lines (or controlling elevations).
2. Digital maps showing present (and future)
houses, streets, lots, and so on..
3. Location of water sources so to help locating
distribution reservoirs.
Design of Water Distribution Systems
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Water Demand Studies:
Water consumption is ordinarily divided into the following categories:
Domestic demand.
Industrial and Commercial demand.
Agricultural demand.
Fire demand.
Leakage and Losses.
Design of Water Distribution Systems
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Domestic demand • It is the amount of water used for Drinking,
Cocking, Gardening, Car Washing, Bathing, Laundry, Dish Washing, and Toilet Flushing.
• The average water consumption is different from one population to another. In Gaza strip the average consumption is 70 L/capita/day which is very low compared with other countries. For example, it is 250 L/c/day in United States, and it is 180 L/c/day for population live in Cairo (Egypt).
• The average consumption may increase with the increase in standard of living.
• The water consumption varies hourly, daily, and monthly
Design of Water Distribution Systems
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How to predict the increase of population?
The total amount of water for domestic use is a function of:
Population increase
Geometric-increase Geometric-increase modelmodel
Use
P P r n 0 1( ) P0 = recent populationr = rate of population growthn = design period in yearsP = population at the end of the design period.The total domestic demand can be
estimated using:Qdomestic = Qavg * P
Design of Water Distribution Systems
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Industrial and Commercial demand
• It is the amount of water needed for factories, offices, and stores….
• Varies from one city to another and from one country to another
• Hence should be studied for each case separately.
• However, it is sometimes taken as a percentage of the domestic demand.
Design of Water Distribution Systems
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Agricultural demand
• It depends on the type of crops, soil, climate…
Fire demand
• To resist fire, the network should save a certain amount of water. • Many formulas can be used to estimate the amount of water
needed for fire.
Design of Water Distribution Systems
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Fire demand Formulas
)01.01(65 PPQF QF = fire demand l/s P = population in
thousands
Q PF 53QF = fire demand l/s P = population in
thousands
Q C AF 320 *
QF = fire demand flow m3/d A = areas of all stories of the
building under consideration (m2 ) C = constant depending on the
type of construction;The above formulas can be replaced with local
ones (Amounts of water needed for fire in these
formulas are high).
Design of Water Distribution Systems
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Leakage and Losses
• This is “ unaccounted for water ”(UFW)• It is attributable to:
Errors in meter readings
Unauthorized connections
Leaks in the distribution system
Design of Water Distribution Systems
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Design Criteria
Are the design limitations required to get the most efficient and economical water-distribution network
Velocity
PressurePipe Sizes
Hea
d Los
ses
Design Period
Average Water Consumption
Design of Water Distribution Systems
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Velocity • Not be lower than 0.6 m/s to prevent
sedimentation• Not be more than 3 m/s to prevent
erosion and high head losses. • Commonly used values are 1 - 1.5 m/sec.
Design of Water Distribution Systems
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Pressure
• Pressure in municipal distribution systems ranges from 150-300 kPa in residential districts with structures of four stories or less and 400-500 kPa in commercial districts.
• Also, for fire hydrants the pressure should not be less than 150 kPa (15 m of water).
• In general for any node in the network the pressure should not be less than 25 m of water.
• Moreover, the maximum pressure should be limited to 70 m of water
Design of Water Distribution Systems
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Pipe sizes • Lines which provide only domestic flow may be as small as
100 mm (4 in) but should not exceed 400 m in length (if dead-ended) or 600 m if connected to the system at both ends.
• Lines as small as 50-75 mm (2-3 in) are sometimes used in small communities with length not to exceed 100 m (if dead-ended) or 200 m if connected at both ends.
• The size of the small distribution mains is seldom less than
150 mm (6 in) with cross mains located at intervals not more than 180 m.
• In high-value districts the minimum size is 200 mm (8 in) with cross-mains at the same maximum spacing. Major streets are provided with lines not less than 305 mm (12 in) in diameter.
Design of Water Distribution Systems
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Head Losses • Optimum range is 1-4 m/km. • Maximum head loss should not
exceed 10 m/km.
Design of Water Distribution Systems
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Design Period for Water supply Components
• The economic design period of the components of a distribution system depends on
•Their life.•First cost. •And the ease of expandability.
Design of Water Distribution Systems
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Average Water Consumption• From the water demand (preliminary)
studies, estimate the average and peak water consumption for the area.
Design of Water Distribution Systems
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Network Layout
• Next step is to estimate pipe sizes on the basis of water demand and local code requirements.
• The pipes are then drawn on a digital map (using AutoCAD, for example) starting from the water source.
• All the components (pipes, valves, fire hydrants) of the water network should be shown on the lines.
Design of Water Distribution Systems
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Part C
Pipe Network Analysis
Water Distribution Systems & networks
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Pipe Network Analysis
Pipe Network Analysis
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Pipe Networks
• A hydraulic model is useful for examining the impact of design and operation decisions.
• Simple systems, such as those discussed in last chapters can be solved using a hand calculator.
• However, more complex systems require more effort even for steady state conditions, but, as in simple systems, the flow and pressure-head distribution through a water distribution system must satisfy the laws of conservation of mass and energy.
Pipe Network Analysis
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The equations to solve Pipe network must satisfy the following condition:
• The net flow into any junction must be zero
• The net head loss a round any closed loop must be zero. The HGL at each junction must have one and only one elevation
• All head losses must satisfy the Moody and minor-loss friction correlation
Pipe Networks
0Q
Pipe Network Analysis
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Node, Loop, and PipesNode
Pipe
Loop
Pipe Network Analysis
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After completing all preliminary studies and layout drawing of the network, one of the methods of hydraulic analysis is used to
•Size the pipes and •Assign the pressures and velocities required.
Hydraulic Analysis
Pipe Network Analysis
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Hydraulic Analysis of Water Networks
• The solution to the problem is based on the same basic hydraulic principles that govern simple and compound pipes that were discussed previously.
• The following are the most common methods used to analyze the Grid-system networks:
1. Hardy Cross method.2. Sections method. 3. Circle method.4. Computer programs (Epanet,Loop, watercad...)
Pipe Network Analysis
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Hardy Cross MethodThis method based on:
0 0Loop
fJunction
hQ
1- A distribution of flows in each pipe is estimated such that the total inflow must be equal to the outflow at each junction throughout the network system The interflow in the network has +ve signThe outflow from the network has -ve sign
Pipe Network Analysis
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2- Neglect Minor loss
3- In each loop
4- If the direction of flow is clockwise it take +ve sign, otherwise it take –ve sign
5- If the flow is correct other wise, the assumed flow must be corrected as the flowing:
0 Loop
fh
0 Loop
fh
Pipe Network Analysis
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WilliamHazenn
ManningDarcyn
kQh nF
85.1
,2
)1(
)2( oQQ
....
2
1
2& 1
221f
no
no
no
no
n Qnn
nQQkQkkQh
from
1f n
ono
n nQQkkQh
0
0
1
nn
on
loop
n
loopF
nkQkQkQ
kQh
Neglect terms contains 2
For each loop
Pipe Network Analysis
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o
F
F
no
no
Q
hn
h
nkQ
kQ1
6- After calculation correct Qo and check 0 Loop
fh
Pipe Network Analysis
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Assumptions / Steps of this method:
1. Assume that the water is withdrawn from nodes only; not directly from pipes.
2. The discharge, Q , entering the system will have (+) value, and the discharge, Q , leaving the system will have (-) value.
3. Usually neglect minor losses since these will be small with respect to those in long pipes, i.e.; Or could be included as equivalent lengths in each pipe.
4. Assume flows for each individual pipe in the network.5. At any junction (node), as done for pipes in parallel,
outin QQ Q 0or
Pipe Network Analysis
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6. Around any loop in the grid, the sum of head losses must equal to zero:
– Conventionally, clockwise flows in a loop are considered (+) and produce positive head losses; counterclockwise flows are then (-) and produce negative head losses.
– This fact is called the head balance of each loop, and this can be valid only if the assumed Q for each pipe, within the loop, is correct.
• The probability of initially guessing all flow rates correctly is virtually null.
• Therefore, to balance the head around each loop, a flow rate correction ( ) for each loop in the network should be computed, and hence some iteration scheme is needed.
h floop
0
Pipe Network Analysis
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7. After finding the discharge correction, (one for each loop) , the assumed discharges Q0 are adjusted and another iteration is carried out until all corrections (values of ) become zero or negligible. At this point the condition of :
is satisfied.
Notes:• The flows in pipes common to two loops are
positive in one loop and negative in the other.• When calculated corrections are applied, with
careful attention to sign, pipes common to two loops receive both corrections.
h floop
0 0.
Pipe Network Analysis
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• Note that if Hazen Williams (which is generally used in this method) is used to find the head losses, then
h k Qf 1 85.
(n = 1.85) , then
h
h
Q
f
f185.
• If Darcy-Wiesbach is used to find the head losses, then
h k Qf 2
h
h
Q
f
f2
(n = 2) , then
o
F
F
no
no
Q
hn
h
nkQ
kQ1
Pipe Network Analysis
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Example 1
pipeLD
1305m150mm
2305m150mm
3610m200mm
4457m150mm
5153m200mm
1
234
5
37.8 L/s
25.2 L/s
63 L/s
24
39 11.4
12.6
25.2
Solve the following pipe network using Hazen William Method CHW =100
Pipe Network Analysis
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24.064.085.1
28.01
o
F
F
Qh
n
h
1002.6
1000
0.71
L/s ,100
0.71
852.1
852.187.4
9
852.1
87.4852.1
852.1
87.4852.1
QKh
QD
Lh
Q
DC
Lh
inQCQDC
Lh
f
f
HW
f
HW
HW
f
57.043.085.1
45.02
o
F
F
Qh
n
h
1
23 4
5
12
21
2 loopin 2 pipefor
1 loopin 2 pipefor
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15.064.085.1
18.01
o
F
F
Qh
n
h 09.0
42.085.1
07.01
o
F
F
Qh
n
h
1
23 4
5
12
21
2 loopin 2 pipefor
1 loopin 2 pipefor
Pipe Network Analysis
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Solve the following pipe network using
Hazen William Method CHW =120
Example 2Pipe Network Analysis
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Iteration 1
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0050
75154851
531
022071101851
14
00604179851
840
01408638851
011
4
3
2
1
...
.Δ
...
.Δ
...
.Δ
...
.Δ
o
f
f
no
no
Q
hn
h
nkQ
kQΔ
1
1Iteration for
Pipe Network Analysis
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Iteration 2
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Iteration 3
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Example• The figure below represents a simplified pipe
network.
• Flows for the area have been disaggregated to the nodes, and a major fire flow has been added at node G.
• The water enters the system at node A.
• Pipe diameters and lengths are shown on the figure.
• Find the flow rate of water in each pipe using the Hazen-Williams equation with CHW = 100.
• Carry out calculations until the corrections are less then 0.2 m3/min.
Pipe Network Analysis
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General Notes• Occasionally the assumed direction of flow will be
incorrect. In such cases the method will produce corrections larger than the original flow and in subsequent calculations the direction will be reversed.
• Even when the initial flow assumptions are poor, the convergence will usually be rapid. Only in unusual cases will more than three iterations be necessary.
• The method is applicable to the design of new system or to evaluation of proposed changes in an existing system.
• The pressure calculation in the above example assumes points are at equal elevations. If they are not, the elevation difference must be includes in the calculation.
• The balanced network must then be reviewed to assure that the velocity and pressure criteria are satisfied. If some lines do not meet the suggested criteria, it would be necessary to increase the diameters of these pipes and repeat the calculations.
Pipe Network Analysis
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• Assigning clockwise flows and their associated head losses are positive, the procedure is as follows: Assume values of Q to satisfy Q = 0. Calculate HL from Q using hf = K1Q2 .
If hf = 0, then the solution is correct.
If hf 0, then apply a correction factor, Q, to all Q and repeat from step (2). For practical purposes, the calculation is usually
terminated when hf < 0.01 m or Q < 1 L/s. A reasonably efficient value of Q for rapid
convergence is given by;
QH2
HQ
L
L
Summary
QH2
HQ
L
L
Pipe Network Analysis
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Example• The following example contains nodes with different
elevations and pressure heads. • Neglecting minor loses in the pipes, determine:
• The flows in the pipes.
• The pressure heads at the nodes.
Pipe Network Analysis
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Assume T= 150C
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Assume flows magnitude and direction
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First Iteration
• Loop (1)
PipeL
(m)
D
(m)
Q
(m3/s)f
hf(m)
hf/Q
(m/m3/s)
AB6000.250.120.015711.4895.64
BE2000.100.010.02053.38338.06
EF6000.15-0.060.0171-40.25670.77
FA2000.20-0.100.0162-8.3483.42
-33.731187.89
L/s20.14/sm01419.0)89.1187(2
73.33 3
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First Iteration
• Loop (2)
PipeL
(m)
D
(m)
Q
(m3/s)f
hf(m)
hf/Q
(m/m3/s)
BC6000.150.050.017328.29565.81
CD2000.100.010.02053.38338.05
DE6000.15-0.020.0189-4.94246.78
EB2000.10-0.010.0205-3.38338.05
23.351488.7
L/s842.7/sm00784.0)7.1488(2
35.23 3 77
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Second Iteration
• Loop (1)
PipeL
(m)
D
(m)
Q
(m3/s)f
hf(m)
hf/Q
(m/m3/s)
AB6000.250.13420.015614.27106.08
BE2000.100.032040.018631.48982.60
EF6000.15-0.04580.0174-23.89521.61
FA2000.20-0.08580.0163-6.2172.33
15.651682.62
L/s65.4/sm00465.0)62.1682(2
65.15 3
14.20
14.20
14.20 7.8414.20
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Second Iteration
• Loop (2)
PipeL
(m)
D
(m)
Q
(m3/s)f
hf(m)
hf/Q
(m/m3/s)
BC6000.150.042160.017620.37483.24
CD2000.100.002160.02610.2093.23
DE6000.15-0.027840.0182-9.22331.23
EB2000.10-0.032040.0186-31.48982.60
-20.131890.60
L/s32.5/sm00532.0)3.1890(2
13.20 3
14.20 7.84
7.84
7.84
7.84
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Third Iteration
• Loop (1)
PipeL
(m)
D
(m)
Q
(m3/s)f
hf(m)
hf/Q
(m/m3/s)
AB6000.250.12960.015613.30102.67
BE2000.100.022070.019015.30693.08
EF6000.15-0.050450.0173-28.78570.54
FA2000.20-0.090450.0163-6.8775.97
-7.051442.26
L/s44.2/sm00244.0)26.1442(2
05.7 3
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Third Iteration
• Loop (2)
PipeL
(m)
D
(m)
Q
(m3/s)f
hf(m)
hf/Q
(m/m3/s)
BC6000.150.047480.017425.61539.30
CD2000.100.007480.02121.96262.11
DE6000.15-0.022520.0186-6.17274.07
EB2000.10-0.022070.0190-15.30693.08
6.11768.56
L/s72.1/sm00172.0)56.1768(2
1.6 3 81
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After applying Third correction
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Velocity and Pressure Heads:
pipeQ
(l/s)
V
(m/s)
hf(m)
AB131.992.68913.79
BE26.233.34021.35
FE48.012.71726.16
AF88.012.8016.52
BC45.762.58923.85
CD5.760.7331.21
ED24.241.3727.09
1.2121.35
13.79 23.85
6.52
26.16
7.09
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Velocity and Pressure Heads:
Nodep/+Z(m)
Z
(m)
P/(m)
A703040
B56.212531.21
C32.362012.36
D31.152011.15
E37.322215.32
F63.482538.48
1.2121.35
13.79 23.85
6.52
26.16
7.09
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Example For the square loop shown, find the discharge in all the pipes.All pipes are 1 km long and 300 mm in diameter, with a frictionfactor of 0.0163. Assume that minor losses can be neglected.
Pipe Network Analysis
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•Solution:Assume values of Q to satisfy continuity
equations all at nodes.
The head loss is calculated using; HL = K1Q2
HL = hf + hLm
But minor losses can be neglected: hLm = 0
Thus HL = hf
Head loss can be calculated using the Darcy-Weisbach equation
g2
V
D
Lh
2
f
Pipe Network Analysis
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First trial
Since HL > 0.01 m, then correction has to be applied.
554'K
Q'KH
Q554H
3.0x4
Qx77.2
A
Q77.2H
81.9x2
Vx
3.0
1000x0163.0H
g2
V
D
LhH
2L
2L
22
2
2
2
L
2
L
2
fL
PipeQ (L/s) HL (m)HL/Q
AB602.00.033
BC400.8860.0222
CD000
AD-40-0.8860.0222
2.000.0774
Pipe Network Analysis
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Second trial
Since HL ≈ 0.01 m, then it is OK.Thus, the discharge in each pipe is as follows (to the nearest integer).
s/L92.120774.0x2
2
QH2
HQ
L
L
PipeQ (L/s) HL (m)HL/Q
AB47.081.230.0261
BC27.080.4070.015
CD-12.92-0.0920.007
AD-52.92-1.5550.0294
-0.01070.07775
PipeDischarge (L/s)
AB47
BC27
CD-13
AD-53
Pipe Network Analysis
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http://www.haestad.com/library/books/awdm/online/wwhelp/wwhimpl/java/html/wwhelp.htm
89