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CHEMICALS AND CHEMICAL HANDLING 15.31

The recirculating line must be run as close as possible to the points of application to
minimize any additional length of piping where optimum velocities cannot be maintained.
Takeoffs that are periodically closed, as in refilling day tanks, must be tapped and valved
off the top of the pipe to prevent settling in the static line behind the closed valve. Throt-
tling valves in continuous service may be tapped from the side or bottom of the line. Flow
stability through these diversion valves requires some back pressure in the recirculating
line; this is normally accomplished by installing a throttling valve in the line as it returns
to storage.
Although ball valves and plug-type valves perform reasonably well in open-close ser-
vice, they are not recommended for throttling service. Similarly, diaphragm valves tend
to collect compacted sludge behind their weirs. Pinch valves operate well in throttling ser-
vice, provided they are properly sized and not subject to a vacuum. These valves are avail-
able with manual, electric, hydraulic, or pneumatic operators. Flow modulation is possi-
ble with each type of power operator.
Mild steel piping is satisfactory for most slurry lines where rigid piping is preferred.
Tees and crosses should be used as elbows to facilitate cleaning. A freshwater flushing
system should be installed to flush out piping, pumps, and valves when the system is shut
down for any reason. Reduction of line diameter must not be so abrupt that it causes a
violent hydraulic disturbance that can result in dewatering and compaction of the lime.
Pipeline designers usually use a coefficient of C = 100 for slurry lines carrying up to
3 lb/gal (594 g/L) of hydrated lime. Piping should not be excessively oversized initially
to accommodate estimated future system capacity. The penalty for oversizing is usually
increased maintenance problems during the early years. Dissolved solids tend to be pre-
cipitated out of the process water where lime slurry or any other alkaline substance is ap-
plied. For this reason, an air gap is preferable between the end of the feed line and the
surface of the water being treated. If a submerged application cannot be avoided, some
means must be provided for periodic cleaning of the end of the slurry pipe to break up
the precipitated mass, which may eventually plug the feed line.
Slurry Pumps. Slurry pumps generally fall into two categories: centrifugal pumps
and positive displacement, or controlled-volume, pumps. Centrifugal pumps are generally
employed for low-head transfer or recirculating service. With proper selection of casing
and impeller material and an appropriate shaft seal, satisfactory service can usually be at-
tained at a reasonable cost. Replaceable liners and semiopen impellers are preferred. It is
important that the pump design allow easy dismantling for cleanout and repair. Lime slurry
requires the lowest speed of rotation (1,725 rpm or less) consistent with hydraulic re-
quirements to control impeller plating. Using water-flushed seals on centrifugal lime slurry
pumps is not recommended, because it usually results in localized scaling.
Controlled-volume pumps are typically used where metering or positive control of
slurry flow is required, such as at the point of application to the process. Several types
are available for slurry service. Piston-type pumps where the slurry is in direct contact
with the cylinder walls are not recommended for slurry service because of uncontrolled
wear and abrasion. Similarly, peristaltic or squeeze-type pumps are subject to wear and
excessive tubing replacement and are not normally used for slurry pumping.
Dipper-Wheel Feeders. The rotating dipper feed has been a longtime favorite for
feeding slurries where gravity feed is possible between the feeder and the point of appli-
cation. The feeder consists of a tank in which the slurry level is maintained by a float
valve (or overflow weir if gravity retum to slurry storage is practical), a dipper wheel
with variable-speed drive, and a totalizer to register wheel revolutions. The dipper wheel
is usually divided into eight segments, or dippers, each containing about 500 mL of slurry
liquid. As the wheel rotates, an agitator bar maintains the slurry in suspension. The inlet
float valve to the tank must be routinely cleaned, particularly if it is connected to a pres-
surized slurry recirculating system. An overflow connection must be provided when a