WATER DISTRIBUTION
SYSTEM
Module 3 – part 3
WATER DISTRIBUTION SYSTEM
• After the treatment process, the wholesome water has to be taken to
the roads and streets in the city, and finally to the individual houses.
• The function is to carry water from treatment plant to individual homes
is accomplished through a well planned distribution system
• Distribution system should deliver water to consumer with appropriate,
quality, quantity and pressure
• Water may be supplied to public either continuously or supplied
intermittently during certain fixed hours
• It may consist of:
• Pipelines – of various sizes for carrying water
• Valves – for controlling the flow in pipes
• Hydrants – hydrants for providing connections with the water mains for
releasing during fires
• Meters – for measuring discharges
• Service connections – to the individual homes
• Pumps – for lifting and forcing the water into distribution pipes
• Distribution/service reservoirs – for storing the treated water to be fed to
distribution pipes
Good quality distribution system:
• Quality should not get deteriorated
• Should be capable of supplying water at all intented places with
sufficient pressure head
• Should be capable of supplying requisite amount of water during fire
fighting
• Should be such that no consumer would be without water supply,
during the repair of any section of the system
• Distribution pipes should be preferably laid one metre away or above
the sewer lines
• It should be fairly water tight as to keep losses due to leakage to a
minimum.
Layout of distribution system
1. Dead end system
2. Grid iron system
3. Ring system
4. Radial system
1. Dead End System
• Also called tree system
• Consist of one main pipe and a number of sub main pipes.
• Each sub mains is divided into several branches called laterals
• From laterals, service connections are provided to the consumers.
• This system result into a number dead ends
• It is used for older towns without properly planned roads
• Suitable for localities which expand irregularly
Advantages
• Distribution network can be solved easily
• Ease calculation of discharge and pressure at different points
• Lesser number of cut off valves
• Require short pipe
• Cheap and simple , and can be expanded easily
Disadvantages
• Water reaches a particular point only through one route
• Damage or repair at a point completely stops supply in that area
• Leads to number of dead ends which prevent free circulation of water
• Water accumulated at the dead ends should be removed frequently as it
is polluted
• Supply during fire cannot be increased
2. Grid iron system (Interlaced system or
reticular system)
• Mains, sub mains and branches are all interconnected with each other
• Used for well-planned towns and cities
• The principle of grid iron system can be applied to dead end system by
closing the loops
Advantages
• Water reaches one point through more than one route
• During repair, only a small section is affected
• Dead ends are eliminated
• Water remains in continues circulation
• For fire, water can be diverted towards the affected pointed from
various points by controlling the cut off valves
Disadvantages
• Require more length of pipes
• Large number of valves is required
• Construction is costlier and design is difficult
• Calculation of discharge and pressure is a tedious job
3. Ring system
• Also known as circular system
• Suitable for cities having well planned roads
• It consist of closed ring of main pipe used around the area to be served
• Sub main pipes are placed according to the points to be served
• Used for towns with well-planned roads
Advantages
• Water reaches one point through more than one route
• During repair, only a small section is affected
• Dead ends are eliminated
• Water remains in continues circulation
• For fire, water can be diverted towards the affected pointed from
various points by controlling the cut off valves
Disadvantages
• Require more length of pipes
• Large number of valves is required
• Construction is costlier and design is difficult
• Calculation of discharge and pressure is a tedious job
4. Radial system
• Suitable for cities having system of radial roads emerging from
different centres.
• Pipe lines are laid radially from distribution reservoir at the center
• Water from main pipe is pumped into the reservoir placed at different
centers of towns
• Water is then supplied through radial pipes
• Highly efficient system
• Calculation and design is simple
METHODS OF DISTRIBUTION
• The water is distributed by different methods based on the level of
source and city to be served
1. Gravitational system
2. Pumping system
3. Combined gravitational and pumping system
1. Gravitational system
• Water from high level source is distributed to a low level city under the
action of gravity without any pumping
• Economical and reliable method
• It needs a lake or river as supply source
• Suitable for cities situated at the foothills.
2. Pumping system (Pumping without
storage)
• Treated water is pumped directly into the distribution system without
any storage – pumping without storage system
• High lift pump is required, which have to operate at variable speeds to
meet the variable demand of water
• Used for source at low level and city at high level
• The system fails if power breaks
3. Combined gravitational and pumping
system (pumping with storage)
• Treated water is pumped into an elevated storage reservoir
• From this reservoir, water is distributed to the consumers by the action
of gravity
• Pumping with storage system
• Excess water during low demand periods gets store in reservoir and
gets supplied in high demand periods
ANALYSIS OF PRESSURE IN
DISTRIBUTION SYSTEM:
• The following methods are used for the analysis of pressure in the
distribution system:
1. Equivalent pipe method
2. Hardy cross method
3. Method of sections
4. Graphical method
Conditions:
1. Law of continuity must be satisfied: at each junction, the total
inflow must be equal to total outflow
2. Head loss balance criteria: the algebraic sum of pressure drops
around a closed loop must be zero
HARDY CROSS METHOD
• In this method, the rate of flow is assumed by the designer, and the
error in the assumed flow is computed on the account of
IMBALANCE OF HEAD LOSSES IN THE CIRCUIT/LOOP.
• A correction to these assumed flows is computed successively for each
pipe loop in the network, until the correction is reduced to an
acceptable limit
• 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
HARDY CROSS METHOD:
• Select separate circuit and assume any distribution of flow such that
flow entering a junction is equal to flow leaving the junction
• Corrections are applied to the assumed flow
if Qa is assumed flow and Q is the actual flow, the correction given
should be Δ=Q-Qa
Q=Qa + Δ
• Compute head loss in each pipe considering clockwise flow as positive
and anticlockwise flow as negative.
•
Hazen Williams equation:
• Around a closed loop, summation of head losses = 0
• Assumed flow in each pipe are corrected
• Pipe common to two loops will receive both corrections.
• Repeat the process until we get accurate and precise value
• The value of x is assumed to be constant, x=1.85 for Hazen William’s
formula and x=2 for Darcy Weisbach formula.
Procedure:
•
Q. Determine the distribution of flow on the pipe network shown in
figure using hardy cross method with Darcy-Weisbach formula.
Head loss by Darcy – weisbach formula is given by
Assume the flow as given below
EQUIVALENT PIPE METHOD:
• In this method a complex system of pipes is replaced by a single
hydraulically equivalent pipe.
• The equivalent pipe is one which will replace a given system of pipes
with equal head loss for a given flow.
• If the pipes are in series, the head losses are addictive
• If the pipes are parallel, then head losses are identical
EQUIVALENT PIPE METHOD:
• Equivalent pipe is a method of reducing a combination of pipes into a
simple pipe system for easier analysis of a pipe network, such as a
water distribution system.
• An equivalent pipe is an imaginary pipe in which the head loss and
discharge are equivalent to the head loss and discharge for the real
pipe system.
• There are three main properties of a pipe: diameter, length, and
roughness
• Hydraulically equivalent pipe means it will have same capacity.(Q). It
will have same amount of head loss.(hf )
• When the pipes are connected in series, the total head loss is equal to
the summation of the individual head losses.
When using darcys formulae
William Hazen Nomogram
• Nomogram gives the relation between discharge, diameter, head loss
and velocity of flow.
• Different nomograms are used for different values of C.
• There are 4 scales:
• 1st scale: discharge in Gallons/min and L/s
• 2nd scale: Pipe diameter in inches and centimeter
• 3rd scale: Head loss in feet/1000 feet or meter/1000 meter and
kilopascal/1000m
• 4th scale: mean velocity in feet/sec and meter/sec.
William Hazen Nomogram for c=80
William Hazen
Nomogram for
c=100
1. Solve the following pipe network using Hazen William’s Equation
A flow of 60 l/s be assumed to be flowing through the routes ABC and
ADC. Using Hazen-William’s nomogram, the head losses into the two
circuits will be,
Now, if the routes ABC and ADC are replaced by 300 mm. dia. pipes
each, then the length of each pipe required can be obtained from the fact
that a 300 mm dia. pipe offers a head loss of 4.5 m per 1000 m length
William Hazen
Nomogram for
c=100
•
• Hence, the total loop ABCD can thus be replaced by a single
equivalent pipe of 300 mm dia and 530 m length, irrespective of the
flow discharges.
