Prof. Dr. Muhammad Zulfiqar Ali Khan Engr. Muhammad...

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



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


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


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


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 ….


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


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


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


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


12

Branching System (tree system)

Branching System

Source

Submain

Main

pipe

Dead End

Advantages:

• Simple to design and build.

• Less expensive than other 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:


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.


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.


16

Combined System

• It is a combination of both Grid and Branching

systems

• This type is widely used all over the world.

Combined System


17

Part B



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:


19

The design of water distribution systems must

undergo through different studies and steps:

Design Phases

Hydraulic Analysis

Preliminary Studies

Network Layout


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.


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.


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.


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


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%).


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.


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.


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


28

Design Criteria

are the design limitations required to get the most efficient and economical water-distribution network

Velocity

Pressure

Average Water Consumption & Future Demand


29

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).


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


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.


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.


33


34

Head Losses

• Optimum range is 1-4 m/km.

• Maximum head loss should not exceed 10

m/km.


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.


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.


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



39



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.


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


42

Node, Loop, and Pipes Node

Pipe

Loop


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


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,...)


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.


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


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


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


49

o

F

F

n

o

n

o

Q

hn

h

nkQ

kQ

1

6- After calculation correct Qo and check 0 Loop

fh


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


51


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:


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.


53



11. Calculate hf / Q0 for each pipe and sum for loop S hf / Q0 .

12. Calculate using:

NOTE:

For Common members between two loops:


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.


55


• 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


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


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


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.


60

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.


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.


64

Assume T= 150C

65

Assume flows magnitude and direction

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