The Islamic University of GazaFaculty of

EngineeringCivil Engineering Department

Hydraulics - ECIV 3322

Chapter 4

Water Distribution Systems

Introduction

- To deliver water to individual consumers with

appropriate - quality, quantity, and pressure in a community

setting requires - an 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 .

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

Gravity Supply

- The source of supply is at a sufficient elevation

above the distribution area (consumers). - so that the desired pressure can be maintained

HGL or EGL

Source (Reservoir)

(Consumers)

Gravity-Supply System

Advantages of Gravity supply

HGL or EGL

Source

- No energy costs.
- Simple operation (fewer mechanical parts,

independence of power supply, .) - Low maintenance costs.
- No sudden pressure changes

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.

HGL or EGL

(Consumers)

Source (River/Reservoir)

Pumped-Supply System

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 .

HGL or EGL

(Consumers)

Source (River/Reservoir)

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

Pumping

Source (1)

Gravity

HGL

HGL

Pumping station

City

Source (2)

Combined Supply (Continue)

- 2) In the pumped system sometimes a storage

(elevated) tank is connected to the system.

- 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

High consumption

Elevated tank

Pumping station

Pipeline

City

Source

Combined Supply (Continue)

- 3) When the source is lower than the consumer area

- 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.

Pumping

HGL

Gravity

HGL

Reservoir

Pumping Station

City

Source

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Distribution Systems (Network Configurations )

- In laying the pipes through the distribution

area, the following configuration can be

distinguished - Branching system (Tree)
- Grid system (Looped)
- Combined system

Branching System (tree system)

Branching System

- Advantages
- Simple to design and build.
- Less expensive than other systems.

Disadvantages

- 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.

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.

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.

Combined System

Combined System

- It is a combination of both Grid and Branching

systems - This type is widely used all over the world.

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Design of Water Distribution Systems

A properly designed water distribution system

should fulfill the following requirements

- 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.

- The design of water distribution systems must

undergo through different studies and steps - Design Phases

Preliminary Studies

Network Layout

Hydraulic Analysis

Preliminary Studies

Must be performed before starting the actual

design

- 4.3.A.1 Topographical Studies

- Contour lines (or controlling elevations).
- Digital maps showing present (and future) houses,

streets, lots, and so on.. - Location of water sources so to help locating

distribution reservoirs.

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.

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

The total amount of water for domestic use is a

function of

Population increase

- How to predict the increase of population?

Geometric-increase model

Use

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

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.

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.

Fire demand Formulas

QF fire demand l/s P population in

thousands

QF fire demand l/s P population in

thousands

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).

Leakage and Losses

- This is unaccounted for water (UFW)
- It is attributable to

Errors in meter readings

Unauthorized connections

Leaks in the distribution system

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Design Criteria

- Are the design limitations required to get

the most efficient and economical

water-distribution network

Pressure

Pipe Sizes

Design Period

Velocity

Head Losses

Average Water Consumption

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.

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

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.

Head Losses

- Optimum range is 1-4 m/km.
- Maximum head loss should not exceed 10 m/km.

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.

Average Water Consumption

- From the water demand (preliminary) studies,

estimate the average and peak water consumption

for the area.

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.

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 Networks

- 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

Node, Loop, and Pipes

Pipe

Node

Loop

Hydraulic Analysis

- 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 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 - Hardy Cross method.
- Sections method.
- Circle method.
- Computer programs (Epanet,Loop, Alied...)

Hardy Cross Method

- This method is applicable to closed-loop pipe

networks (a complex set of pipes in parallel).

- It depends on the idea of head balance method
- Was originally devised by professor Hardy Cross.

Assumptions / Steps of this method

- Assume that the water is withdrawn from nodes

only not directly from pipes. - The discharge, Q , entering the system will have

() value, and the discharge, Q , leaving the

system will have (-) value. - 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. - Assume flows for each individual pipe in the

network. - At any junction (node), as done for pipes in

parallel,

or

- 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.

- 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.

How to find the correction value ( )

Neglect terms contains

For each loop

- Note that if Hazen Williams (which is generally

used in this method) is used to find the head

losses, then

(n 1.85) , then

- If Darcy-Wiesbach is used to find the head

losses, then

(n 2) , then

Example

Solve the following pipe network using Hazen

William Method CHW 100

24

12.6

11.4

39

D L pipe

150mm 305m 1

150mm 305m 2

200mm 610m 3

150mm 457m 4

200mm 153m 5

25.2

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Example

Solve the following pipe network using Hazen

William Method CHW 120

Iteration 1

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Iteration 2

Iteration 3

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.

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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.

Summary

- 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 lt 0.01 m or ?Q lt 1

L/s. - A reasonably efficient value of ?Q for rapid

convergence is given by

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.

Assume T 150C

Assume flows magnitude and direction

First Iteration

- Loop (1)

Pipe L (m) D (m) Q (m3/s) f hf (m) hf/Q (m/m3/s)

AB 600 0.25 0.12 0.0157 11.48 95.64

BE 200 0.10 0.01 0.0205 3.38 338.06

EF 600 0.15 -0.06 0.0171 -40.25 670.77

FA 200 0.20 -0.10 0.0162 -8.34 83.42

S -33.73 1187.89

First Iteration

- Loop (2)

Pipe L (m) D (m) Q (m3/s) f hf (m) hf/Q (m/m3/s)

BC 600 0.15 0.05 0.0173 28.29 565.81

CD 200 0.10 0.01 0.0205 3.38 338.05

DE 600 0.15 -0.02 0.0189 -4.94 246.78

EB 200 0.10 -0.01 0.0205 -3.38 338.05

S 23.35 1488.7

Second Iteration

14.20

14.20

7.84

14.20

- Loop (1)

14.20

Pipe L (m) D (m) Q (m3/s) f hf (m) hf/Q (m/m3/s)

AB 600 0.25 0.1342 0.0156 14.27 106.08

BE 200 0.10 0.03204 0.0186 31.48 982.60

EF 600 0.15 -0.0458 0.0174 -23.89 521.61

FA 200 0.20 -0.0858 0.0163 -6.21 72.33

S 15.65 1682.62

Second Iteration

7.84

14.20

7.84

7.84

- Loop (2)

7.84

Pipe L (m) D (m) Q (m3/s) f hf (m) hf/Q (m/m3/s)

BC 600 0.15 0.04216 0.0176 20.37 483.24

CD 200 0.10 0.00216 0.0261 0.20 93.23

DE 600 0.15 -0.02784 0.0182 -9.22 331.23

EB 200 0.10 -0.03204 0.0186 -31.48 982.60

S -20.13 1890.60

Third Iteration

- Loop (1)

Pipe L (m) D (m) Q (m3/s) f hf (m) hf/Q (m/m3/s)

AB 600 0.25 0.1296 0.0156 13.30 102.67

BE 200 0.10 0.02207 0.0190 15.30 693.08

EF 600 0.15 -0.05045 0.0173 -28.78 570.54

FA 200 0.20 -0.09045 0.0163 -6.87 75.97

S -7.05 1442.26

Third Iteration

- Loop (2)

Pipe L (m) D (m) Q (m3/s) f hf (m) hf/Q (m/m3/s)

BC 600 0.15 0.04748 0.0174 25.61 539.30

CD 200 0.10 0.00748 0.0212 1.96 262.11

DE 600 0.15 -0.02252 0.0186 -6.17 274.07

EB 200 0.10 -0.02207 0.0190 -15.30 693.08

S 6.1 1768.56

After applying Third correction

Velocity and Pressure Heads

pipe Q (l/s) V (m/s) hf (m)

AB 131.99 2.689 13.79

BE 26.23 3.340 21.35

FE 48.01 2.717 26.16

AF 88.01 2.801 6.52

BC 45.76 2.589 23.85

CD 5.76 0.733 1.21

ED 24.24 1.372 7.09

13.79

23.85

1.21

21.35

6.52

26.16

7.09

Velocity and Pressure Heads

Node p/gZ (m) Z (m) P/g (m)

A 70 30 40

B 56.21 25 31.21

C 32.36 20 12.36

D 31.15 20 11.15

E 37.32 22 15.32

F 63.48 25 38.48

13.79

23.85

21.35

1.21

6.52

7.09

26.16

- 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 friction - factor of 0.0163. Assume that minor losses can

be neglected.

- 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

First trial Since ?HL gt 0.01 m,

then correction has to be applied.

Pipe Q (L/s) HL (m) HL/Q

AB 60 2.0 0.033

BC 40 0.886 0.0222

CD 0 0 0

AD -40 -0.886 0.0222

? 2.00 0.0774

Second trial Since ?HL 0.01 m, then it is

OK. Thus, the discharge in each pipe is as

follows (to the nearest integer).

Pipe Q (L/s) HL (m) HL/Q

AB 47.08 1.23 0.0261

BC 27.08 0.407 0.015

CD -12.92 -0.092 0.007

AD -52.92 -1.555 0.0294

? -0.0107 0.07775

Pipe Discharge (L/s)

AB 47

BC 27

CD -13

AD -53