Distribution Reservoirs 6k701y

3.1 Distribution reservoirs Water storage in the distribution system is provided for the following purposes. Equalizing supply and demand: The demand for water normally changes throughout the day and night. Water stored in the reservoirs during low consumption will be released to equalize the demand during high consumption. Increasing operating convenience. In some situations it may be more practical and economical to build a larger plant and operate it only for 8 hours a day. In this situation, water treated during this period would be stored for use when the plant is not operating. Leveling out pumping requirements. With out storage, pumps need to match the changing demand by frequently turning on and off. This cycling of the pump causes increased wear on pump controls and motors, as well as increased electrical costs. Storage reservoir is required to overcome this situation. Providing water during source or pump failure. There are times when all pumps may not be available when needed, because of power failure, mechanical breakdown, or preventive maintenance. Sudden increase in demand can be caused by main breaks, broken hydrants, or similar problems. To meet these situations about 25% of the storage capacity must be reserved. Maintaining pressure levels within acceptable ranges: elevated storage facilities maintain pressure at required levels during peak demand periods. The following figure illustrates this. Providing water to meet fire demands: One of the major purposes for distribution system storage is meeting fire demands. Although fire demands may not occur very often or last very long, they can be much greater than consumer peak demands. Increase detention times: The added detention time provided by having water storage has two major advantages: 

Disinfection continues even at low chlorine levels



Sand particles, flocs, or precipitated solids settle out before reaching the mains and customers. Blending water sources: Some water systems use water from different sources that can vary in quality. Blending these different sources together in a reservoir will often improve the quality of marginally acceptable water. It also provides a more uniform quality of water to the customers, rather than water that changes in taste and composition from day to day. Types of storage facilities Distribution storage facilities can be of three types- ground storage, underground reservoir and elevated storage (elevated tanks or standpipes). The choice between these storage types depends on topography, size of community, reliability of water supply, and economy. Ground storage facilities on hills high enough to provide adequate pressure are preferred; the economy and desirability of ground storage with booster pumping, as compared with elevated storage, must be determined in each individual area. In general, elevated tanks are most economical and are

recommended for small water systems. Reservoirs and booster pumping facilities are often less expensive in large systems where adequate supervision can be provided. Important storage facility design considerations and details  Minimum capacity: Equalizing storage + Emergency reserve (about 25%) + Fire storage. The equalizing storage can be estimated by a mass curve method.  Location: normally, it is more advantageous to provide several smaller storage units in different location than an equivalent capacity at a central site. Ground storage must be located near major transmission mains. If pumping is required, the ground storage facility should be located adjacent to the pumping station and its bottom should be higher than the pump centerline. The top of the reservoir should be set at an elevation close to that of the hydraulic gradient of the influent transmission mains. The location of elevated storages should be such that zones of high consumption lie between the pumping station and the tanks.  Aesthetics: the style and color of the storage facilities should reduce visual impacts. The issue aesthetics is sensitive that engineers should leave to architects or community leaders.  Ventilation: storage facilities must be vented to allow air to flow into and out of the reservoir as the water level rises and falls. The vent should have an insect proof screen and rain cover incorporated to protect water quality.  Overflow: storage facilities may inadvertently become filled beyond their capacity. To protect the structural integrity of the facility, an overflow system that incorporate screens or flap valves to protect the water quality from insects and animal intrusion should provided. The overflow should not cause damage from ponding or flooding in adjacent areas.  Security and safety: storage facilities should be located on a secured site and protected by relevant measures.  Corrosion control: storage tanks constructed of steel must be protected from corrosion by epoxy painting systems designed for potable water service or cathodic protection system. Accessories of Service Reservoirs The service reservoirs are to be provided with the following accessories: 1. Inlet Pipe : For the entry of water 2. Ladder : To reach the top of the reservoir and then to the bottom of the reservoir, for inspection and cleaning 3. Lightening Conductor : In case of elevated reservoirs for the age of lightening 4. Manholes : For providing entry to the inside of reservoir for inspection and cleaning 5. Outlet pipe: For the exit of water 6. Outflow Pipe : For the exit of water above full supply level 7. Vent pipes : For free circulation of air 8. Washout pipe : For removing water after cleaning of the reservoir 9. Water level indicator: To know the level of water inside the tank from outside.

Design Capacity of Service Reservoirs The three major components of service storage are: i) Equalizing or operating storage ii) Fire reserve iii) Emergency reserve Equalizing or operating capacity: can be obtained: 1) Analytically by finding out maximum cumulative surplus during the stage when pumping rate is higher than water consumption rate and adding to this maximum cumulative deficit which occurs during the period when the pumping rate is lower than the demand rate of water. 2) By drawing mass curves (graphical method) Mass curve method A mass diagram is the plot of accumulated inflow (i.e. supply) or outflow (i.e. demand) versus time. The mass curve of supply (i.e. supply line) is, therefore, first drawn and is superimposed by the demand curve. The procedure to construct such diagram is as follows: • From the past records, determine the hourly demand for all 24 hours for typical days (maximum, average and minimum). • Calculate and plot the cumulative demand against time, and thus plot the mass curve of demand.

• Read the storage required as the sum of the two maximum ordinates between demand and supply line. • Repeat the procedure for all the typical days (maximum, average and minimum), and determine the maximum storage required for the worst day. 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 of the 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 ing this storage. 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 5 to 10% of the total storage is sufficient to meet the requirement. The total reservoir storage can finally be worked out by adding all the three storages. Example 1 Design a service reservoir if Q daymax is 2400 m 3 . Two pumps are working at constant rate of 150 m 3 /hr. Determine for how many hours pumping should be done. Solution Step 1. Determine pumping hours ▫ Pumping hour = Q dmax /(2 x pumping capacity) ▫

= 2400/(2 x 150)

▫

= 8 hrs for each pump

• Step 2. determine reservoir capacity ▫ Balancing requirement Pumping is done for a total of 16 hrs = 2400 m 3 But demand for 16 hrs is 2400 x 16/24 = 1600 m 3 Excess that needs to be stored = 2400-1600 = 800 m 3 Emergency requirement = 0.25 x 800 = 200 m 3 Ignore fire demand = 0 Total reservoir volume = 1000 m 3 Provide 2 reservoirs of each 500 m 3

Example 2 A small town with a design population of 1600 is to be supplied water at 150liters per capita per day. The demand of water during different periods is given in the following table:

Determine the capacity of a balancing reserve if pumping is done 24 hours at constant rate. Solution: Per capita water consumption = 150l/c/d Total water demand = demand * population = 150*1600 = 240,000liters Rate of pumping = 240,000/24 = 10,000lit/hr = 30,000lit/3hr

Example 3: Consider example 1, if the pumping is done for: a) Eight hours from 8 hrs to 16 hrs b) Eight hrs from 4 hrs to 8 hrs and again 16 hours to 20 hrs. Calculate the capacity of the balancing reserve. Solution: Total water demand = 240,000lit/hr Rate of pumping = 240,000/8 = 30,000l/h = 90,000lit/3hrs a) Eight hours from 8 hrs to 16 hrs

3.2 PUPMS AND PUMPING STATIONS Purpose and types of pumps Pump is a mechanical machine. It is used for lifting water or any fluid to a higher elevations or at higher pressures. The operation of lifting water or any fluid is called pumping. Pumping may be adopted for following purposes in a water supply scheme. 1. To increase the water pressure at certain points in the distribution system. 2. To lift treated water to elevated storage tanks, so that it may flow automatically under gravity into distribution system. 3. To lift raw river water or lake water to carry it to treatment plant. 4. To lift water available from wells to an elevated storage tank in stages. 5. To pump water directly into the distribution system. 6. To take out water from basins, sumps, tanks etc. Types of pumps (A) Classification based on mechanical principle of operation (i) Displacement pumps (ii) Centrifugal pumps (iii) Air lift pumps (iv) Miscellaneous pumps

(B) Classification based on type of power required (i) Steam engine pumps (ii) Diesel engine pumps (iii) Electrically driven pumps (C) Classification based on the type of service called for (i) Low lift pumps (ii) High lift pumps (iii) Deep well pumps (iv) Booster pumps Selection of a particular type of pump • Capacity of pump • Number of pump units required • Suction conditions • Lift (total head) • Discharge conditions and variations in load • Floor space requirement • Flexibility of operation • Starting and priming characteristics • Type of drive required • Initial costs and running costs. Centrifugal pumps Centrifugal Pumps are rotodynamic pumps which convert Mechanical energy into Hydraulic energy by centripetal force on the liquid. Figure 4.1 shows a typical centrifugal pump. Centrifugal pumps are classified in to three categories: Radial flow, Mixed flow, and Axial flow. Radial Flow - a centrifugal pump in which the pressure is developed wholly by centrifugal force. Mixed Flow - a centrifugal pump in which the pressure is developed partly by centrifugal force and partly by the lift of the vanes of the impeller on the liquid. Axial Flow - a centrifugal pump in which the pressure is developed by the propelling or lifting action of the vanes of the impeller on the liquid. The discharge of a centrifugal pump is fixed by the design of the impeller and by the speed of rotation. Typical operating curves for a centrifugal pump are shown in Fig

Pumps Application terminology Capacity (Q): volume of water pumped per unit of time [m3/s ]. Head (H): the head against which pump must works when water is being pumped, [m]. Power: the power the pump consumes when it pumps up [KN.m/s = Kilowatt = KW].

Pump Head-Capacity Curve (Pump Characteristics Curve) The head (TDH) that a pump can produce at various flowrates (Q) and constant speed is established in pump tests conducted by the pump manufacturer.

During the tests, the capacity is varied and the corresponding head is measured. Also, the efficiency and the input power are measured.