Purpose
The main purpose of a storage reservoir is to provide sufficient amounts of water to equalize water demand. The volume of storage depends on satisfying the demands for the following:
Peak Day Storage
On days of heavy demand, the reservoir supplies water to the system. As a result, other water facilities such as treatment plants, pump houses, supply lines, and distribution mains can be reduced in size. The storage reservoir compensates for flows needed over and above that which can be normally provided by the water treatment plant and related pumping facilities.
Emergency Storage
In the event that the water treatment plant is shut down, the reservoir supplies water to the system. The volume of emergency storage depends on the reliability of the water production facilities but normally equals 15% to 25% of the AVERAGE DAY water demand.
Fire Storage
During fires, large volumes of water are needed. The largest proportion of storage in a reservoir is normally reserved for fire use. The volume of storage is equal to the highest fire flow rate predicted times the probable duration that the fire flow will be needed.
Types of Storage
Common types of storage facilities include underground reservoirs, elevated tanks, and standpipes. In smaller systems, hydropneumatic tanks are used. A description of each follows:
- Underground Reservoirs – Underground reservoirs are usually constructed of concrete. They are built into the ground and buried to provide protection against freezing. If incorporated as part of a water treatment plant, the reservoir also provides process water for the plant operation and is called a CLEAR WELL. Underground reservoirs vary in depth ranging from a minimum of 3 m (10 feet) to over 7.7 m (25 feet). The deeper depths predominate in large storage facilities normally found in cities.
- Elevated Tanks – Elevated tanks provide a smaller storage volume than other comparable storage facilities and are chiefly used to maintain system pressure. The height or relative elevation of the elevated tank is determined by the desired system pressure. Water is pumped into the elevated reservoir during times of low flow and flows out during periods of high demand. Flow in and out of the tank is controlled by level control switches and flow control valves. When the level in the tank reaches the full level, flow into the reservoir is stopped.
- Standpipes – Standpipes are normally constructed using steel or concrete. They are circular with their heights exceeding their diameters. The standpipe is built to a pre-determined height equal to the pressure desired at that location. Available pressures vary with the depth of water in the standpipe. Lower levels in the standpipe, however, provide only marginal pressure to the distribution system.
- Hydropneumatic Tanks – Hydropneumatic tanks are small volume pressurized tanks that supply water to the system under pressure. Typical applications are found in smaller distribution systems. Because the volume contained in the tank is relatively small, hydropneumatic tanks do not significantly reduce the required capacity of other water facilities.
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