Water Distribution Systems
The purpose of distribution system is to deliver water to consumer with appropriate quality, quantity and pressure. Distribution system is used to describe collectively the facilities used to supply water from its source to the point of usage.
Requirements of Good Distribution System
1. Water quality should not get deteriorated in the distribution pipes.
2. It should be capable of supplying water at all the intended places with sufficient pressure head.
3. It should be capable of supplying the requisite amount of water during fire fighting.
4. The layout should be such that no consumer would be without water supply, during the repair of any section of the system.
5. All the distribution pipes should be preferably laid one metre away or above the sewer lines.
6. It should be fairly water-tight as to keep losses due to leakage to the minimum.
Layouts of Distribution Network
The distribution pipes are generally laid below the road pavements, and as such their layouts generally follow the layouts of roads. There are, in general, four different types of pipe networks; any one of which either singly or in combinations, can be used for a particular place. They are:
Dead End System
Grid Iron System
Distribution reservoirs, also called service reservoirs, are the storage reservoirs, which store the treated water for supplying water during emergencies (such as during fires, repairs, etc.) and also to help in absorbing the hourly fluctuations in the normal water demand.
Functions of Distribution Reservoirs:
• to absorb the hourly variations in demand.
• to maintain constant pressure in the distribution mains.
• water stored can be supplied during emergencies.
Location and Height of Distribution Reservoirs:
• should be located as close as possible to the center of demand.
• water level in the reservoir must be at a sufficient elevation to permit gravity flow at an adequate pressure.
Types of Reservoirs
1. Underground reservoirs.
2. Small ground level reservoirs.
3. Large ground level reservoirs.
4. Overhead tanks.
Storage Capacity of Distribution Reservoirs
The total storage capacity of a distribution reservoir is the summation of:
1. Balancing Storage: The quantity of water required to be stored in the reservoir for equalising or balancing fluctuating demand against constant supply is known as the balancing storage (or equalising or operating storage). The balance storage can be worked out by mass curve method.
2. Breakdown Storage: The breakdown storage or often called emergency storage is the storage preserved in order to tide over the emergencies posed by the failure of pumps, electricity, or any othe mechanism driving the pumps. A value of about 25% of the total storage capacity of reservoirs, or 1.5 to 2 times of the average hourly supply, may be considered as enough provision for accounting this storage.
3. Fire Storage: The third component of the total reservoir storage is the fire storage. This provision takes care of the requirements of water for extinguishing fires. A provision of 1 to 4 per person per day is sufficient to meet the requirement.
The total reservoir storage can finally be worked out by adding all the three storages.
Pipe Network Analysis
Analysis of water distribution system includes determining quantities of flow and head losses in the various pipe lines, and resulting residual pressures. In any pipe network, the following two conditions must be satisfied:
1. The algebraic sum of pressure drops around a closed loop must be zero, i.e. there can be no discontinuity in pressure.
2. The flow entering a junction must be equal to the flow leaving that junction; i.e. the law of continuity must be satisfied.
Based on these two basic principles, the pipe networks are generally solved by the methods of successive approximation. The widely used method of pipe network analysis is the Hardy-Cross method.
This method consists of assuming a distribution of flow in the network in such a way that the principle of continuity is satisfied at each junction. A correction to these assumed flows is then computed successively for each pipe loop in the network, until the correction is reduced to an acceptable magnitude.
If Qa is the assumed flow and Q is the actual flow in the pipe, then the correction is given by
d=Q-Qa; or Q=Qa+d
Now, expressing the head loss (HL) as
we have, the head loss in a pipe
=K.[Qax + x.Qax-1d + .........negligible terms]
=K.[Qax + x.Qax-1d]
Now, around a closed loop, the summation of head losses must be zero.
\ SK.[Qax + x.Qax-1d] = 0
or SK.Qax = - SKx Qax-1d
Since, d is the same for all the pipes of the considered loop, it can be taken out of the summation.
\ SK.Qax = - d. SKx Qax-1
or d =-SK.Qax/ Sx.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. Hence,
or d =-SK.Qax/ S l x.KQax-1 l
or d =-SHL / x.S lHL/Qal
where HL is the head loss for assumed flow Qa.
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)