In this article we will discuss about:- 1. Purpose of Distribution Reservoirs 2. Types of Distribution Reservoirs 3. Location 4. Storage Capacity.
Purpose of Distribution Reservoirs:
Distribution or service reservoirs are used in a distribution system to provide storage to meet fluctuations in demand of water, to provide storage for firefighting and emergencies such as breakdowns, repairs, etc., and to stabilize pressures in the distribution system.
These reservoirs may be constructed of brick masonry, stone masonry, and cement concrete-plain, reinforced or pre-stressed and steel. These reservoirs are always covered to avoid contamination and prevent algal growths. Further suitable provisions are made for manholes, mosquito-proof ventilation, access ladders, scour and overflow arrangements and water level indicator.
The various purposes served by service or distribution reservoirs are as follows:
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1. If pumps are used, the provision of these reservoirs makes it possible to run the pumps at uniform rate.
2. In the case of gravity system of supply of water, the provision of these reservoirs will result in the reduction of the size of distribution mains.
3. These reservoirs provide the facility of storage of water for meeting the fluctuations in the hourly demand of water.
4. They help in maintaining constant pressure in distribution mains. In their absence the pressure in distribution mains will fall as the demand of water will increase.
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5. The provision of these reservoirs results in an overall reduction in the size of pumps, pipes and treatment units. Thus the distribution system becomes economical.
6. These reservoirs serve as storage for emergencies such as breakdown of pumps, bursting of mains, heavy fire demand, interruption in power supply, temporary floods, etc.
Types of Distribution Reservoirs:
According to the situation with respect to ground, the distribution reservoirs are classified in the following three types:
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Surface reservoirs are circular or rectangular in shape. These reservoirs are constructed at ground level or below ground level and hence these are also called ground reservoirs or non-elevated reservoirs. The treated water stored in these reservoirs is pumped to elevated reservoirs from which it is supplied to the consumers.
However, if surface reservoirs are located at high points in the distribution system then water may be supplied to the consumers directly from these reservoirs by gravity, as far as possible surface reservoirs should be located at high points in the distribution system.
It is usual practice to construct a surface reservoir in two compartments, so that one can be used while the other is being cleaned or repaired. The two compartments are connected with each other by control valves. Overflow pipes are provided at full supply level so as to maintain a constant level of water in the reservoir.
Ventilators are provided in the roof slab so as to affect free circulation of air over the water surface in the reservoir. Although treated water is stored in the reservoir, yet some sludge may be present in the stored water which will be deposited in the reservoir. The deposited sludge can be removed by occasional cleaning through the washout pipes provided at the bottom of the reservoir. The outlet pipes are placed at a slightly higher level, say at least 10 cm, than that of the washout pipes.
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Elevated reservoirs are constructed at an elevation from ground level. These reservoirs are also known as overhead tanks. These reservoirs may be rectangular, circular or elliptical in shape. However, with the advancement in structural analysis it is possible to construct the elevated reservoirs in any shape to suit the architectural requirements.
An R.C.C. tank known as Intz tank is very commonly adopted these days.
Water is pumped to elevated reservoirs from surface reservoirs and then supplied to the consumers.
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The various accessories provided for elevated reservoirs are as indicated below:
(i) Inlet pipe for the entry of water.
(ii) Outlet pipe for the exit of water.
(iii) Overflow pipe for the exit of water above full supply level.
(iv) Ladders to reach the top of reservoir and then to the bottom of reservoir for inspection.
(v) Manholes in top cover or roof of reservoir for providing entry to the inside of reservoir for inspection.
(vi) Ventilators for free circulation of air.
(vii) Washout pipe (or drain pipe) for removing water after cleaning of reservoir.
(viii) Water level indicator for indicating from outside the depth of water in reservoir.
(ix) A lightning conductor for protection against lightning.
A standpipe is a vertical cylindrical tank resting just above the ground. The diameter of standpipe varies from 10 to 15 m and its height varies from 15 to 30 m. Standpipes are made of steel or R.C.C. Steel standpipes are more common as it is very difficult to construct watertight R.C.C. standpipes under heads greater than 15 m. Alike elevated reservoirs, standpipes are also provided with inlet pipe, outlet pipe, overflow pipe, washout pipe and various other accessories for their efficient working, inspection and maintenance.
However, in the case of standpipe the outlet pipe is located in the tank with its entrance being kept above the bottom of the tank at an elevation such that the storage of water created in the tank above this elevation gives the necessary pressure for distribution of water. The volume of water stored in the tank above the entrance of the outlet pipe can only be used and hence it is the useful storage of standpipe.
On the other hand the lower portion of the storage lying below the entrance of the outlet pipe cannot be ordinarily used and it only serves as a support for the useful storage and hence it is termed as supporting storage. However, the supporting storage can also be effectively used by providing boosters or for fire protection with the help of fire engines. Further standpipes are usually located on a high ground so as to successfully utilize its entire storage.
Since large variations in pressure are undesirable in a distribution system, fluctuation of the water level in a standpipe is usually limited to 10 m or less. Generally standpipes of height more than 15 m are not economical since the lower portion of a standpipe serves only to support the upper useful portion.
The economic limit of height for standpipes is reached when the supporting structure for an elevated reservoir becomes less costly than the lower ineffective portion of the standpipe.
Location of Distribution Reservoirs:
Distribution reservoirs should be located centrally or as close as possible to the areas to be served by them. A central location of a distribution reservoir will reduce friction losses in the distribution pipes due to reduction in the length of pipes.
Further in this case the pressure over the entire distribution area will be uniform during periods of both high and low demands. On the other hand if a distribution reservoir is not located centrally there will be large head loss, and by the time water reaches the tail end of the area served by it the pressure will be too low for satisfactory supply of water to the consumers at the tail end.
Storage Capacity of Distribution Reservoir:
The storage capacity of a distribution reservoir to be provided is based on the following requirements:
(i) Balancing Storage (or Equalising Storage or Operating Storage):
The main function of a distribution reservoir is to provide storage to meet the fluctuating demand of water with a constant rate of pumping of water into the reservoir. The quantity of water required being stored in the reservoir for balancing or equalising this variable demand of water against the constant rate of pumping is known as balancing storage or balancing reserve.
The balancing storage of a distribution reservoir can be determined by the following two methods:
(a) Hydrograph Method:
In this method hourly demands of water for a typical maximum day are plotted against the respective hours of the day and a hydrograph is obtained as shown in Fig. 10.9. For a uniform 24 hours pumping, the pumping rate will be equal to the mean hourly demand of water and the same is plotted as shown by line PQ in Fig. 10.9. The storage required for distribution reservoir is then obtained by determining the area (shown shaded) between the curve BCDE and the line PQ. This area when converted into volume units gives the storage required for distribution reservoir.
From the hydrograph it is clear that in the absence of a distribution reservoir a very high rate of pumping would be required, but by providing a distribution reservoir of suitable storage capacity the required rate of pumping would be reduced by almost 30 to 40 per cent.
The storage required for a distribution reservoir may also be determined analytically. Moreover, when pumping is done only for limited hours of the day then analytical method is more convenient for determining the storage required for a distribution reservoir.
(b) Mass Curve Method:
In this method a mass curve of demand is drawn which is a plot of accumulated demand of water against time. By continuously adding the hourly demand of water for a typical maximum day, values of accumulated demand of water at successive hours are obtained and the same are plotted against the corresponding hours to obtain mass curve of demand as shown in Fig. 10.10. A mass curve of demand continuously rises. The steepness of mass curve of demand indicates a higher rate of demand, while flat portion shows a lower rate of demand.
If the two ends A and B of the mass curve of demand are joined by a line AB, then line AB represents the mass curve of pumping into the distribution reservoir if the pumping is done at a uniform rate for all the 24 hours, and the slope of line AB represents this uniform rate of pumping. In order to determine the storage required for distribution reservoir, draw tangents through lowest point C and highest point D, parallel to line AB.
The maximum vertical distance EF between the two tangents gives the storage required for distribution reservoir. This is so because at C there is maximum excess supply equal to CC’ which should be stored, and at D there is maximum deficit DD’ which will be drawn from the storage and that is replenished again. The storage S required for distribution reservoir will thus be the sum of the maximum excess supply through pumping Ep (i.e., CC’ and maximum excess demand (or maximum deficiency) Ed (i.e., DD’).
Thus,
S = Ep + Ed … (10.1)
If pumping is done at a uniform rate but only for a limited period then on the mass curve of demand, the mass curve of pumping represented by line CD is drawn from the beginning of the pumping period to its end as shown in Fig. 10.11. In this case CC represents the maximum excess demand (or maximum deficiency) Ed and DD’ represents the maximum excess supply Ep through pumping.
The storage S required for distribution reservoir is again given by Eq. 10.1 as –
S = Ep + Ed
(ii) Break Down Storage or Emergency Storage:
The break down storage or emergency storage is the storage required to be provided in a distribution reservoir to take care of emergencies which may arise due to failure of pumps, failure of electric supply, etc. It is, however, difficult to assess the magnitude of the storage to be provided to meet this requirement, because it depends on frequency and extent of failures and also on time required for carrying out repairs. As such for this storage a lump sum provision of about 25% of the total storage capacity of the distribution reservoir, or about 1½ to 2 times the average hourly supply is usually made.
A provision of fire storage in a distribution reservoir is required to be made to provide water for firefighting purposes. The fire fighting requirements are based on the recommendations indicated there the fire storage for a distribution reservoir may be provided.