Civil Engineering Reference
In-Depth Information
the water being treated. In some cases, dilution with unsoftened water increases the
potential for scaling if the pH is elevated.
Liquid feed systems typically are recommended for use:
When low chemical quantities are required
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With less stable chemicals
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With chemicals that are fed more easily as a liquid
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Where handling of dusty chemicals or dangerous chemicals is undesirable
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With chemicals available only as liquids
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Liquid feeders usually are metering pumps or orifices. These metering pumps may
be positive displacement, plunger, or diaphragm-type pumps. Examples of plunger and
diaphragm pumps are given in Figure 24-7, and pictures of two styles of pump are
included in Figures 24-7a and 24-7b. Positive displacement pumps can be set to feed
over a wide range by adjusting the pump stroke length. Diaphragm metering pumps
include mechanical, hydraulic, and solenoid-activated type units. Solution piping sys-
tems must have pulsation dampers when using plunger or diaphragm metering pumps.
Piping systems associated with these types of pumps should include pressure relief
arrangements to protect against over pressurization. In some cases, control valves and
rotameters may be all that is needed, while in other cases the rotating dipper-type
feeder may be satisfactory. For uses such as lime slurry feeding, however, centrifugal
pumps with open impellers are employed. The type of liquid feeder used depends on
the viscosity, corrosivity, solubility, suction and discharge heads, and internal pressure
relief requirements of the chemical. Chemical metering can also be accomplished using
progressive cavity- and peristaltic-type pumps.
Another system that has grown in popularity is the automatic polymer blending
unit, which mixes polymer and water to attain the correct concentration of the feed
solution. These units rely on the pressure of the feed water to provide the mixing and
the hydrodynamic shear energy to disperse the polymer particles and allow them to
hydrate fully. These units have been used successfully in numerous locations around
the country. Typical units are shown in Figure 24-8.
Solid Chemical Feeding
There are a number of dry or solid chemical feeding sys-
tems available that involve direct feed of solutions or slurries. The solutions or slurries
generally can be fed using pumping equipment, eductors, and other hydraulic control
devices as described for liquid systems. The solutions and slurries typically are not so
easy to handle as the liquid forms because they tend to form scales and precipitates.
Proper solution strength and proper mixing of the dry material must be achieved prior
to pumping. The proper solution strength is attained through accurate measurement of
the chemical by chemical feeders. Chemical feeders must accommodate the minimum
and maximum feeding rates required. Manually controlled feeders have a common
range of 10:1, but this range can be increased to about 20:1 or 30:1 with dual-control
systems.
Solids characteristics vary considerably, and the selection of a feeder must be made
carefully, particularly in a smaller-sized facility where a single feeder may be used for
more than one chemical. In general, provisions should be made to keep all dry chem-
icals cool and dry. Dryness is important, as hygroscopic (water-absorbing) chemicals
may become lumpy, viscous, or even rock hard; other chemicals that absorb water less
