Civil Engineering Reference
In-Depth Information
Basins and Wet Wells
Receiving basins and pump wetwells for finished water
should be designed as finished water storage structures.
Other Design Consideration
Pressure Variation
To limit water service pressure changes to about 14 psi, the
maximum desirable variation between high and low levels in storage structures that
are hydraulically directly connected to or ''float'' on a distribution system is approx-
imately 30 feet (9.1 m). Larger variations can be tolerated if other system features,
such as pressure-reducing valves, are used to minimize pressure fluctuations to cus-
tomers.
Level Controls
Adequate controls should be provided to maintain levels in distri-
bution system storage structures, as follows:
Telemetering equipment should be used for storage structures when pressure-type
controls are employed in control stations and any appreciable headloss occurs in
the distribution system between the source and the storage structure.
•
Altitude valves or equivalent controls may be desirable to prevent overflowing of
a second and subsequent storage structures hydraulically connected on the system.
High-water levels should be set 2-3 feet (0.60-0.91 m) below the overflow level.
•
Water Quality Considerations
Good circulation of water through reservoirs can
assist in the control of tastes and odors, maintenance of residual disinfectant, and
control of slime growths. Reservoirs should be cleaned periodically to prevent accu-
mulation of sediments and the creation of chlorine-demanding residues. Other design
considerations are as follows:
Water circulation through reservoirs should be promoted by the use of baffles, or
by placing inlets and outlets on opposite sides of the reservoir with inlets near
the top and outlets near the bottom.
•
All pipes except the overflow should have valves.
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The minimum reservoir water depth should be about 12 feet (3.66 m).
•
More than one reservoir should be provided, to give storage during outages; or,
if there is a single reservoir, it should be divided into compartments so at least
one section is available for use at all times.
•
Reservoirs should be drained and cleaned approximately every two to three years,
depending upon local conditions.
•
Hydropneumatic Tanks
In some cases, it may not be financially feasible to provide gravity storage in some
areas of the water system. In such cases, hydropneumatic tanks often are installed as
a limited means for equalizing flow and pressure and reducing the frequency of pump-
cycling.
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When used, they should meet ASME Code requirements for unfired pressure vessels
and they should comply with state and local laws and regulations. The following
