Unit-2
Storage and distribution of water
The function of carrying the water from the treatment plant to the individual homes is accomplished through a well-planned distribution system.
A Distribution system main consists of pipeline (mains, sub mains, branches or laterals), valves for controlling flow in pipes. Hydrants for or providing connections with the water mains for releasing water during fire, metres for measuring discharges, service connection to the individual homes, pumps for lifting and forcing the water into the distribution pipes, distribution or service providers for storing the treated water to be fed into the distribution pipes etc.
2.1.1 Requirement of a good distribution system
Various requirements for proper functioning of a distribution system as
● It should be capable of supplying water at all the intended places within the city with the reasonably sufficient pressure head.
● Capable of supplying required amount of water for fire fighting.
● Cheap with least capital construction cost (70% cost of total cost).
● Simple and easy to operate and repair.
● Safe against any future pollution of water (away from sewerage lines).
● Safe so as not to cause failure of pipeline by busting.
● Must be water tight so as to keep ‘losses due to leakage’ to minimum.
2.1.2 Arrangements of distribution pipes and other accessories
Distribution pipe system consists of supply mains, sub mains, branches and laterals, usually made of cast iron and jointed by spigot and socket joints.
The distribution pipes are bid on one side of the roads and streets, usually below footpaths and at about 2 m above and 3 m away from C were so as to avoid future contamination.
2.1.3 Layout of distribution networks
Four types, any of which singly or in combination can be used depending on local condition and orientation of roads.
2.1.3.1Dead end system
It is also called tree system. When main supply pipe, from which, at right angles, originates and number of sub main pipes. Sub main divide into branch pipes or literals from which service connections are given to consumers.
Adopted for old town that is unplanned. A number of dead ends are formed.
2.1.3.1.1 Advantages
a) Distribution network can be solved easily and we can easily calculate the discharge and pressure at different points.
b) Less number of sluice valves are cut off valves are required.
c) Shorter pipe lengths are needed and lying of pipes is easy.
d) Cheap and simple, can be extended or expanded easily.
2.1.3.1.2 Disadvantages
a) Only one route so any damage in pipe will completely stop water supply.
b) Numerous dead ends are there which prevents free circulation of water which may degrade its quality. This state of water has to be removed preorder cli by providing scour valves.
c) Supplies during firefighting cannot be increased.
2.1.3.2 Grid iron system
It is also known as interlaced system or reticulation system. The mains, submains and branches are interconnected full stop suitable for well-planned town and cities. It is used in Chandigarh.
2.1.3.2.1 Advantages
a) Since water reaches at different places through more than one route, the discharge to be carried by each pipe, the friction loss and size of the pipe get reduced.
b) For repair, very small area will be devoid of complete supply.
c) Because of different interconnections, the dead ends are eliminated.
d) During fire, more water can be diverted towards affected point.
2.1.3.2.2 Disadvantages
a) This system requires more length of pipe line and large number of sluice valve and cut-off valve and gate valve.
b) Construction is costlier.
c) Design is difficult and costly year. Calculation for sizes of pipe and pressure at key points required design experts.
2.1.3.3 Ring system
Also called circular system full stop a closed ring either circular or rectangular off main pipes is formed around the area to be served. Distribution area is divided into rectangular or circular blocks. It is suitable for town and cities.
Sometimes it is used as looped feeder placed centrally around the high demand area along with grid iron system.
Advantages and disadvantages are same as grid iron system.
2.1.3.4 Radial system
If a city has system of radial roads emerging from different centres, the pipelines are laid in radial method by placing distribution reserve wires at these corners. Here, water is taken from mains, supplied by a radially laid distribution pipe.
Key Takeaways:
Main objective of distribution system is to develop adequate water pressures at various points of consumer’s tabs. Depending upon the level of sources of water and that of the city, topography of the area and other local conditions, the water may be forced into the distribution system in three ways: -
Water from high level source is distributed to consumers at lower levels by action of gravity without pressure. The difference of had available between the source and the localities must be sufficient enough to maintain advocate pressure at consumers door step, after allowing frictional and other losses.
2. Pressure system
The treated water is directly pumped into distribution mains without storing it anywhere. It is also called “pressure without storage system”.
High lift pumps are required which have to operate at various speeds so as to meet variable demand of water. Continuous attendance is needed at pressure station. Not used much.
3. Combined gravity and pressure system
Treated water is pumped at constant rate and stored in elevated distribution reserve wires from where it is distributed to consumer by action of gravity. It is also called “pressure with storage system”.
Excess water during low demand gets stored in reservoir and get supply during high demand periods. Pumps work at constant rate at their rated capacities does increasing the efficiency and reducing wear and tear. Rate of flow is adjusted in such a way that the axis quantity of water stored in the reservoir during low consumption nearly equals to extra demand during high consumption.
Universally adopted due to following advantages: -
a) Balancing reserve of distribution reservoir can be supplied to the places of fire.
b) Pumps work at uniform rate those results in increase in efficiency.
c) Reliable as even during power failure or pump failure, certain amount of water can be supplied from storage or service reservoir.
d) Overall cheap, efficient and reliable and hence adopted practically everywhere.
2.2.1 Systems of supply
Water may be supplied either
2.2.1.1 Disadvantages of intermittent system
a) In many cities the intermittent system may fail to give any savings.
b) They do not cover fire risk during non supply periods.
c) Inconvenience to consumers.
d) Require bigger size distribution means for 6 to 8 hour supply as water is supplied to whole town at same time.
e) When supply is stopped and water is drawn off, a partial vacuum may be created in pipe which induces section through leaky joints so as that dirt may enters to the pipe.
f) More air valves and sluice valves are required which will have to be operated daily requiring more ward staff and maintenance cost.
Key Takeaways: -
It is also called service reserve wires and is the storage reservoir which stores the treated water for supplying water during emergencies like fire, breakdown and repairs etc. and also to help in absorbing the hourly fluctuations in normal water demand.
2.3.1 Functions of distribution reservoirs
2.3.2 Types of distribution reservoirs
It may be made of steel, RCC or masonry. Depending upon their elevation with respect to ground, they may be classified into following types: -
2.3.2.1 Surface reservoirs
They are circular or rectangular tank is constructed at ground level or below ground level. It is also called “ground reservoirs”.
2.3.2.2 Elevated reservoirs
It may be rectangular circular or elliptical overhead tanks erected at contain suitable elevation above ground level and supported on towers.
Accessories are
Key Takeaways:
Total storage is summation of: -
a) Balancing storage or equalizing or operating storage.
b) Breakdown storage.
c) Fire storage.
2.4.1 Balancing storage or equalizing storage
Main function of reserve wire is to meet fluctuating demand with constant date of supply from treatment plant. The quantity of water required to be stored in reserve wire for equalizing for balancing this variable demand against constant supply is known as balancing reservoir or balancing storage or the storage capacity of a balancing reservoir. This is calculated by utilizing hydrographs of inflow and outflow by following two methods:
2.4.1.1 Mass curve method
It is also called mass diagram method and it is the plot of accumulated in flow that is supply outflow that is demand v/s time. The mass curve of supply is first drawn and is superimposed by demand curve. The amount of balancing storage can be determined by adding the maximum ordinates between demand and supply lines.
2.4.1.1.1 Steps: -
2.4.1.1.2 Analytical solution
The cumulative hourly demand and cumulative hourly supply are tabulated for 24 hours. The early access of demand and supply are then work doubt. The submission of maximum of excess of demand and supply will give required storage capacity.
2.4.2 Breakdown storage
These are also called emergency storage and are the storage reserved in order to bind over the emergency is possessed by the failure of pumps, the electricity or any other mechanism driving the purpose. Depends on frequency and extent of failures and also upon the time required for carrying out the repairs.
A value of 25% of total storage capacity of reservoir or 1.5-2 times of average hourly supply may be considered for this storage.
2.4.3 Fire storage
Total volume of water required for firefighting is 125 l/p/d. Three jet streams of each giving a discharge of 1100 ltr/min must be thrown on fire.
Under normal conditions in India, we may store about 1-4 l/p/d as the necessary fire storage, depending upon the importance of the city.
Total reservoir storage can be obtained by adding balancing breakdown and fire storage.
2.4.4 Location and height of distribution reservoir
a) They must be located in heart of city so as to command the maximum area on around and place near points of heaviest demand.
b) Must be located at higher elevation to maintain advocate pressure in distribution system.
c) With respect to position of pumping stations and distribution area, the reservoirs may be located in two different ways i.e.,
Key Takeaways:
Depending upon various factors such as relative levels of the different zones of the city, the layout of the roads etc, the type of distribution network is decided.
2.5.1 Fixing the size of pipes of simple distribution system
After the layout plan of distribution system pipes and location of other appurtenances (Valves, hydrants) is finalized, the sizes of the pipes are computed. The terminal pressure in pipes must remain above the minimum allowable pressure and not exceed maximum allowable pressure.
V = 0.85 CH R0.63 S0.54
S = 
HL = 
1.85 
2.5.2 Analysis of complex pipe networks
In a pipe network, the following two conditions must be satisfied: -
a) The algebraic sum of the pressure drops around a closed loop must be zero that is there can be no discontinuity in the pressure.
b) The slow entering a junction must be equal to the floor living the same junction that is the law of continuity must be satisfied.
c) Based on the principle, the pipe networks are solved by the following methods of successive approximation as analytical solution is not possible
2.5.2.1 Hardy cross method
The procedure suggested by hardy and cross requires that the flow in each pipe is assumed by designer in magnitude as well as in direction in such a way that the principle of continuity is satisfied at a junction (i.e., the inflow at any junction becomes equal to the outflow at that junction).
A connection to these as wind flow is then computed successively for each pipe loop in the network, until the correction is reduced to an acceptable magnitude.
Qa = Assumed Flow, Q = Actual flow in the pipe
Now, expressing the head loss (HL) as
HL=K.Qx
We have the head loss in a pipe
=K.(Qa+x)x
=K.[Qax + x.Qax-1 + .........negligible terms]
=K.[Qax + x.Qax-1]
Now, around a closed loop, the summation of head losses must be zero.
K.[Qax + x.Qax-1] = 0
K.Qax = -Kx Qax-1
Since, d is the same for all the pipes of the considered loop, it can be taken out of the summation.
Or, K.Qax = -Kx Qax-1
Or, =-K.Qax/ x.KQax-1
Since d is given the same sign (direction) in all pipes of the loop, the denominator of the above equation is taken as the absolute sum of the individual items in the summation.
The numerator in the above equation is the algebraic sum of the head losses in the various pipes of the closed loop computed with assumed flow. Since the direction and magnitude of flow in these pipes is already assumed, their respective head losses with due regard to sign can be easily calculated after assuming their diameters. The absolute sum of respective KQax-1 or HL/Qa is then calculated. Finally the value of d is found out for each loop, and the assumed flows are corrected. Repeated adjustments are made until the desired accuracy is obtained.
The value of x in Hardy- Cross method is assumed to be constant (i.e. 1.85 for Hazen-William's formula, and 2 for Darcy-Weisbach formula)
2.5.2.1.1 Steps
K.Qa2
4. Apply the corrections obtained to the floors in each pipe. Pipes common to two loops will receive both corrections with due regard to sign.
2.5.2.2 Equivalent pipe method
It is used for large network of pipes where it is convenient to replace the different small loops by a single equivalent pipe having same discharging capacities and causing same head loss.
Same discharging capacities and causing same head loss.
It is also called head balanced method of analyzing water distribution five networks.
The pipe net circuit can be reduced into a single equivalent pipe by using two principle of hydraulics: -
a) The loss of head caused by a given flow of water through the pipe is connected in series is additive.
b) The quantity of discharge flowing through the different pipes connected in parallel will be such as to cause equal head loss through each pipe.
2.5.2.3 Other methods
Electrical analyzer method, the method of sections, the circle method, the pitot meter distribution studies method etc. are also used but they are not of much utility and rarely used.
Key Takeaways:
References:
1. Manual on Water Supply and Treatment, C. P. H. E. E. O., Ministry of Urban Development,
Government of India, New Delhi
2. Manual on Sewerage and Sewage Treatment, C. P. H. E. E. O., Ministry of Urban
Development, Government of India, New Delhi
3. Steel and McGhee: Water Supply and Sewerage
4. Fair and Geyer: Water Supply and Wastewater Disposal
5. Hammer and Hammer Jr.: Water and Wastewater Technology
6. Raju: Water Supply and Wastewater Engineering
7. Rao: Textbook of Environmental Engineering
8. Davis and Cornwell: Introduction to Environmental Engineering
9. Kshirsagar: Water Supply and Treatment and Sewage Treatment Vol. I and II
10. Punmia: Water Supply and Wastewater Engineering Vol. I and II
11. Birdie: Water Supply and Sanitary Engineering
12. Ramalho: Introduction to Wastewater Treatment Processes
13. Davis Mackenzie L., Cornwell, David A., “Introduction to Environmental Engineering”
McGraw Hill Education (India) Pvt. Ltd., New Delhi.
14. Birdie: Water Supply and Sanitary Engineering
15. Ramalho: Introduction to Wastewater Treatment Processes
16. Parker: Wastewater Systems Engineering
17. A.K. Jain, Environmental Engineering, Khanna Publishing House
