prof. dr. muhammad zulfiqar ali khan engr. muhammad...
TRANSCRIPT
Water Distribution Systems & Analysis 1
Environmental Engineering-I
Prof. Dr. Muhammad Zulfiqar Ali Khan
Engr. Muhammad Aboubakar Farooq
References
Water Supply & Sewerage (P-143 to 152)
by TERENCE J. McGHEE
Elements of Public Health Engg.(P-159 to 162)
by K.N. Duggal
Water Distribution Systems & Analysis
2
Content
Water Distribution Systems
Design of Water Distribution Systems
Pipe Network Analysis
Water Distribution Systems & networks
3
Part A
Water Distribution Systems
Water Distribution Systems & networks
4
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)
Supply System-Gravity
HGL or EGL
Water Distribution Systems
5
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
6
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)
Supply System-Pumped
HGL or EGL
Water Distribution Systems
7
Source
(River/Reservoir)
(Consumers)
HGL or EGL
Disadvantages of pumped supply
Complicated 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
8
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
9
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
10
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
11
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
12
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
13
• 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.
Disadvantages:
Water Distribution Systems
14
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
15
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
18
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 Phases
Hydraulic Analysis
Preliminary Studies
Network Layout
Design of Water Distribution Systems
20
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
21
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
22
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. It varies from 150 to 600 lpcd.
• 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
23
How to predict the increase of population?
The total amount of water for domestic use is a function of:
Population increase
Geometric-increase model Use
P P r n 0 1( )P0 = recent population
r = rate of population growth
n = design period in years
P = population at the end of the design
period.
The total domestic demand can be estimated using:
Qdomestic = Qavg * P
Design of Water Distribution Systems
24
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 total
water supply (10-30%).
Design of Water Distribution Systems
25
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
26
Fire demand Formulas
)01.01(1020 PPQF QF = fire demand US Gallon/min
P = population in thousands
ACQF *18.223QF = fire demand flow lit/min
A = areas of all stories of the building
under consideration (m2 )
C = constant depending on the type of
construction;
Minimum and Maximum Storage Required are 4 hours & 10
hours Respectively.
Design of Water Distribution Systems
27
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
Pressure
Average Water Consumption & Future Demand
Design of Water Distribution Systems
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Velocity
• Not be lower than 0.25 m/s to prevent
sedimentation - WASA
• Not be more than 2 m/s to prevent erosion and
high head losses near large fires.
• Commonly velocity is kept in between (1 - 1.5
m/s).
Design of Water Distribution Systems
30
Pressure • Pressure which is required to be maintained depends
upon:
– Height of Highest Building
– Distance of locality from the distribution reservoir
– Supply is metered or not
– Pressure required for Fire Hydrants
– Funds available for project
Design of Water Distribution Systems
31
Pressure • Pressure in municipal distribution systems ranges from 150-
300 kPa in residential districts with structures of three stories or less and 500 kPa in commercial districts.
• Also, for fire hydrants the pressure should not be less than 150 kPa (15 m of water) to ensure proper flow in other buildings and to avoid infiltration in system.
• Moreover, the maximum pressure should be limited to 70 m
of water.
Design of Water Distribution Systems
32
Pipe sizes • Primary feeders form skeleton of distribution system. They convey water
from Pumping station to & from OHR to various parts of city. They should be provided with Air-Relief & Blow-off Valves. Size is generally 300 mm (12 in to 60 in).
• Secondary feeder carry water from primary feeder to various parts of city to provide normal supplies. These are located at a few (2-4) blocks apart to provide water for fire fight without excessive loss. Sizes are 200 mm, 250 mm, 300 mm.
• The size of the small distribution mains is seldom less than 150 mm (6 in) and dictated by fire flow.
• Sizes of Domestic Supply Lines are generally 100mm, 80mm.
Design of Water Distribution Systems
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Design of Water Distribution Systems
34
Head Losses
• Optimum range is 1-4 m/km.
• Maximum head loss should not exceed 10
m/km.
Design of Water Distribution Systems
35
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
36
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, manually) 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
37
References
Water Supply & Sewerage (P-143 to 152)
by TERENCE J. McGHEE
Elements of Public Health Engg.(P-159 to 162)
by K.N. Duggal
Water Distribution Systems & Analysis
38
Part C
Pipe Network Analysis
Water Distribution Systems & networks
39
Pipe Network Analysis
Pipe Network Analysis
40
Pipe Networks
• A hydraulic model is useful for examining the impact of design and operation decisions.
• However, more complex systems require more effort, 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
41
The equations to solve Pipe network must
satisfy the following condition:
• The net flow into any junction must be zero.
Inflow should be equal to Outflow.
• The net head loss a round any closed loop must be
zero.
Pipe Networks
0Q
Pipe Network Analysis
42
Node, Loop, and Pipes Node
Pipe
Loop
Pipe Network Analysis
43
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
44
Hydraulic Analysis of Water Networks
• The following are the most common methods used to
analyze the Grid-system networks:
1. Hardy Cross method.
2. Equivalent Pipe Method.
3. Sections method.
4. Computer programs (EPAnet Software,...)
Pipe Network Analysis
45
Hardy Cross Method
In this method, the rate of flow is ASSUMED and
the ERROR in the assumed flow is computed on the
account of IMBALANCE OF HEAD LOSSES IN
THE CIRCUIT/LOOP. After correcting the flows,
final head loss through various pipes is checked to
assess the adequacy of the design. If needed, the
pipe diameters are changed to result in REQUIRED
PRESSURE AT THE OUTLET.
Pipe Network Analysis
46
Hardy Cross Method
This method based on:
0 0Loop
f
Junction
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 sign
The outflow from the network has -ve sign
Pipe Network Analysis
47
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
48
WilliamHazenn
kQh n
F
85.1
)1(
)2( oQQ
....
2
1
2& 1
221
f
n
o
n
o
n
o
n
o
n Qnn
nQQkQkkQh
from
1
f n
o
n
o
n nQQkkQh
0
0
1
nn
o
n
loop
n
loop
F
nkQkQkQ
kQh
Neglect terms contains 2
For each loop
Pipe Network Analysis
49
o
F
F
n
o
n
o
Q
hn
h
nkQ
kQ
1
6- After calculation correct Qo and check 0 Loop
fh
Pipe Network Analysis
50
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.
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
51
Assumptions / Steps of this method:
6. Compute head loss in each pipe.
7. Find out SUM of total Head loss in pipe having
CLOCKWISE DIRECTION of flow---call it as ∑H1.
8. Find out SUM of total Head loss in pipe having
ANTICLOCKWISE DIRECTION of flow---call it as ∑H2.
9. Find out ∑H1 - ∑H2 .
10. Around any loop in the grid, the sum of head losses must
equal to zero:
Pipe Network Analysis
52
h floop
0
10. 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.
Pipe Network Analysis
53
Assumptions / Steps of this method:
Assumptions / Steps of this method:
11. Calculate hf / Q0 for each pipe and sum for loop S hf / Q0 .
12. Calculate using:
NOTE:
For Common members between two loops:
Pipe Network Analysis
54
0
85.1
Q
h
h
f
f
12
21
2 loopin member common for
1 loopin member common for
13. 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. Q = Q0 + ∆ 14. Repeat the procedure till:
<0.2m3/min or
<10% of flow in that pipe
is satisfied.
h floop
0 0.
Pipe Network Analysis
55
Assumptions / Steps of this method:
• Note that if Hazen Williams (which is generally used in this
method) is used to find the head losses, then
h k Qf 185. (n = 1.85) , then
0
85.1Q
h
h
f
f
• If Darcy-Wiesbach is used to find the head losses, then
h k Qf 2
0
2Q
h
h
f
f(n = 2) , then
o
F
F
n
o
n
o
Q
hn
h
nkQ
kQ1
Pipe Network Analysis
56
• 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 = KQ1.85 .
If hf = 0, then the solution is correct.
If hf 0, then apply a correction factor, Q, to all Q.
For practical purposes, the calculation is usually terminated
when Q < 0.2 m3/min.
A reasonably efficient value of Q for rapid convergence is
given by;
Summary
QH
HQ
L
L
85.1
Pipe Network Analysis
57
Example 1
D L pipe
150mm 305m 1
150mm 305m 2
200mm 610m 3
150mm 457m 4
200mm 153m 5
1
2 3 4
5
37.8 L/s
25.2 L/s
63 L/s
24
11.4
Solve the following pipe network using Hazen William Method CHW =100
Pipe Network Analysis
58
Solve the following pipe
network using Hazen William
Method CHW =120
Example 2 Pipe Network Analysis
59
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
60
61
62
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 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
63
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
64
Assume T= 150C
65
Assume flows magnitude and direction
66