TM 5-815-1/AFR 19-6
stack. In smaller plants, stacks may be a
pressure piping. Considerations must also be
single wall steel construction with insulation
made for weatherproofing against freezing
and lagging on the outer surface. For wet
conditions.
(2) Pumps. Centrifugal pumps are used to
scrubbing practice, chimneys for vapor-satu-
rated gases containing corrosive substances
supply the scrubbing liquid or recycled slurry
may be made of rubber-lined steel,
to the scrubber nozzles at the required
fiberglass-reinforced resin or other
volume flow rate and pressure. Where no
corrosion-resistant material. With materials
solids are present in the liquid, bare metal
that have a limited maximum temperature,
pumps, either iron or stainless steel
provisions must be made to protect the stack
construction, are used. In recycle systems
from high temperatures because of loss of
with solids in the liquid, special rubber-lined
scrubbing liquid. Chimney or stack velocities
or hard-iron alloy pumps are used to control
are generally 30 ft/ sec to prevent re-
erosion of the pump internals. These are
entrainment of moisture from the stack wall
generally belt driven to allow selection of the
which would rain down around the plant.
proper speed necessary for the design
Sometimes cones are fitted at the top to give
capacity and head. Solids content must still
exit velocities as high as 75 ft/sec. The chief
be controlled to limit the maximum slurry
reason for high velocities is to eject the gases
consistency to meet the scrubber and pump
well away from the top of the stack to
requirements.
c. Entrainment separation. After the wetted gas
increase the effective height and to avoid
downwash. Downwash can damage the
stream leaves the scrubbing section, entrained liquid
metal structure supporting the stack, the
droplets must be removed. Otherwise they would rain
stack itself, or the outside steel of a lined
out of the stack and fall on the surrounding area.
metal stack. (For a more detailed analysis of
Removal can be by gravity separation in an expanded
the meteorological considerations involved
vessel with lowered velocity or a cyclonic separator
in stack design, see chapter 4.)
can swirl out the droplets against the vessel wall.
(2) Fans. In a wet scrubber system the preferred
Knitted wire or plastic mesh demisters or chevron or
location for the boiler or incinerator
"zig-zag" vanes can be located at the scrubber outlet to
induced-draft fan is upstream of the
catch any droplets.
d. Process measurement and control. The scrubber
scrubber. This eliminates the need for
special corrosion-resistant construction
control system should be designed to follow variations
required to handle the wet downstream gas.
in the boiler or incinerator gas flow and contaminant
The fan should be selected to resist build-up
load to maintain outlet emissions in compliance with
of dry ash or erosion of the rotor surfaces.
selected criteria.
(1) Measurements. Measurement of data from
For high dust load applications a radial blade
or radial tip blade fan is more durable. In a
the process to provide proper control should
dry scrubber application the fan should be
include inlet gas flow rate, temperature and
downstream of the scrubber in the clean gas
pressure, scrubber gas pressure drop, liquid
stream. Here a more efficient air-foil or
pressure, flow rate, solids consistency, pH,
squirrel-cage rotor can be used.
and outlet gas temperature. Selection of
b. Liquid transport.
instrumentation hardware should be on an
(1) Pipework. For most scrubbing duties, the
individual application basis.
(2) Control. Pressure drop across a scrubber can
liquid to be conveyed will be corrosive.
There exists a wide variety of acid resistant
be referenced as an indication of
pipework to choose from, but generally
performance following initial or periodic,
speaking, rubber-lined steel pipe has high
outlet gas testing. In a variable throat
versatility. It is easy to support, has the
venturi, for instance, this pressure drop can
strength of steel, will withstand increases in
be used to control the throat opening,
temperature for a short time and will not
maintaining constant performance under
disintegrate from vibration or liquid
varying gas volume flow rates. Measurement
hammer. Fiberglass filament wound plastic
of scrubber slurry solids consistency can be
pipe is also suitable for a very wide range of
used to control bleed-off of high solids slurry
conditions of temperature, pressure, and
and make-up with fresh water. If sulfur
chemicals. The chief disadvantage of rubber-
dioxide (SO2) is being controlled then
lined pipe is that it cannot be cut to size and
measurement of scrubber liquid pH can
has to be precisely manufactured with
control make-up of caustic to maintain
correct lengths and flange drilling. Site
efficiency of SO2 removal. Complete
fabrication is not possible. Most piping is
specification or design of a control system
manufactured to ANSI specifications for
must be on a case-by-case basis.
7-8
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