Distribution of water

B Y W A Q A S A H M A D
Distribution of water

Methods of distribution system:
water is distributed to the consumers by
1. Gravity Distribution
2. Pumping with storage
3. Direct pumping (without storage)

1) Gravity distribution:
• Used when source of water supply is situated at some
elevation a
2) Pumping with storage:
• used in plain areas & designed on the basis of pressure
available at the end
• Excess of water is pumped during period of low
consumption is stored in on elevated or overhead
reservoir & consumed again during high consumption

3) Direct Pumping without storage:
• Water is directly pumped to the distribution system
without any storage
it is least desirable system as:
i. Power failure would cause complete interruption in
water supply
ii. as consumption varies, pressure is likely to fluctuate,
hence several pumps are needed
iii. High rate of pumping cause high power consumption
& high cost

Layout of distribution system
 There are 4 methods:
1. Dead end system or Tree system
2. Grid iron system
3. Circle or belt system
4. Radial system

1) Dead end system:
• Consist of one main taking water from the plant to town
from which submains are taken out
• The service connections are given to houses from
submain
• Mains & submains are laid parallel along the road, layout
is in the form of a tree

2) Grid iron system:
• Also known as re-circulation or interlaced system
• In this mains, submains & branches are
interconnected with each other which prevents
stagnation of water
• This system is designed by HARDY CROSS method
which is based on the fact that some of head loses of
a pipe forming a loop is zero
• Used in new cities having well planned roads

3) Circle or belt system:
In this method
• A loop/ring of main is formed
• Distribution area is divided into a no of rectangular or
circular blocks
• Submains are laid out on the periphery of these blocks
• Suitable for well planned cities

4) Radial system:
• Water is taken from main or well & pumped into
distribution reservoirs located at centre of various blocks
of the town, the water is then supplied through radially
laid pipe

Design of distribution systems:
 It means to find the diameters of various pipes to
carry certain discharge under necessary pressure
various methods of analysis & design of
distribution systems are:
1. Equivalent pipe method
2. The cut circle or cut contour method
3. Freemann graphical method
4. The Cobb 3 dimensional method
5. Hardy Cross method of analysis

Design Formula:
 Hazen William Formula:
 𝑣 = 𝐾𝐶𝑅0.63 𝑆0.34
Where v= velocity of water
R= hydraulic radius
S= slope of hydraulic gradient line (S=H/L)
C= Hazen William roughness coeff= 100 for cast iron pipe
K= unit correction constt
= 0.849 for MKS system
= 1.382 for KPS system

 Modified formula for circular pipe:
 𝑄 = 𝐴𝑉
𝑄 = (
𝝿
4
𝑑2)(0.849𝐶𝑅0.63 𝑆0.54) where𝑅 =
𝑑
4
& 𝑆 =
ℎ 𝐿
𝐿
 ℎ 𝐿 = 10.68
𝑄
𝐶
1.55 𝐿
𝑑4.87 𝑓𝑡
In english system of units:
 ℎ 𝐿 = 4.72(
𝑄
𝐶
)1.55 𝐿
𝑑4.87 𝑚
 Sometimes nomograms are used
a nomogram is a graph relating Q, hl/L, d and v
if 2 parameters are known, the other 2 can be taken from the graph

Hardy Cross method:
 Used on the fact that the sum of head losses for
closed loop or network is = 0
 Used for grid iron system (network of
distribution pipes)
o Assumptions:
1) Sum of inflows at a node/point is equal to outflow
∑inflow = ∑outflow or ∑total= 0
2) Algebric sum of head loses in close loop = 0
3) Clockwise flows are positive
4) Counter clockwise flows are negative

Derivation:
 Any formula of pipe (manning, chazy, Hazen) can
be expressed generally as:
• 𝐻 = 𝐾𝑄 𝑥………………………(i)
Where H= head loss, Q=discharge through pipe
K= constant depending on the dia, length of pipe
x= exponent whose value is generally 1.85
From Hazen William formula:
𝐻 = 10.68(
𝑄
𝐶
)1.85
𝐿
𝑑4.87
𝐻 = 𝐾𝑄1.85
=>𝐾 =
10.68𝐿
𝐶1.85∗𝑑4.87

 For any pipe in a close loop
 𝑄 = 𝑄1 + 𝜟……………………………(ii)
Where Q= Actual flow
𝑄1=assumed flow
𝜟= required flow correction
From (i) & (ii)=>𝐻 = 𝐾(𝑄1 + 𝜟) 𝑥
=𝐾 𝑄1
𝑥
+
𝑥𝑄1
𝑥−1
𝜟
𝐿1
+
𝑥 𝑥−1 𝑄1
𝑥−2
𝜟2
𝐿2
As 𝜟 is very small as compared to Q we can neglect 𝜟2
𝜟=-𝛴H/x.(𝛴H/Q))………………………(iii)
eq (iii) is used in Hardy Cross method

Procedure for Hardy Cross method:
 Assume dia of each pipe in the loop
 Assume the flow in the pipe such that
 sum of inflows= sum of outflows
At any junction/node (V= V1 + V2) or (Q= Q1 + Q2)
Compute head loses in each pipe by Hazen William eq

 Conventionally clockwise flows are +ive & hence
head loses are also positive & vice versa
 With attention to sign compute the total head
loses around each loop
 i.e. ∑H= ∑KQ^x
 Compute without regard to the sign for the same
loop ∑H/Q1 or ∑Kx Q^x-1
 Apply the correction obtained from the equation

Compound pipe system
Compound pipe
Series comp pipe
Q1=Q2=Q3
H=H1+H2+H3
Parallel comp pipe
Q=Q1+Q2+Q3
H1=H2=H3

Distribution of water

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