vvEPA
United States
Environmental Protection
Agency
Region V
230 South Dearborn
Chicago. IL 60604
EPA-905/2-80-002
May, 1980
Air
Design Considerations For
Minimizing Operation And
Maintenance Problems Of
Participate Control
Equipment
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DESIGN CONSIDERATIONS FOR
MINIMIZING OPERATION AND
MAINTENANCE PROBLEMS OF
PARTICULATE CONTROL EQUIPMENT
by
PEDCo Environmental, Inc.
Donald J. Henz
Project Director
Contract No. 68-02-2535
Task No. 7
Project Officers
Henry Onsgard and Dr. Indur Goklany
Air .Programs Branch
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
REGION V
230 S. DEARBORN STREET
CHICAGO, ILLINOIS 60604
March 1980
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CONTENTS
Page
1. Introduction 1
2. Electrostatic Precipitators 2
Electrodes 2
Prevention of failure 2
Minimization of downtime 4
Dust removal 5
Design 5
Rapper operation 7
Insulators 7
Prevention of high-voltage tracking 7
Detection of high-voltage tracking 8
Minimization of downtime 8
Gas sneakage 9
Additional comments 9
3. Fabric Filters 11
Bag failure 11
Reduction 11
Detection 12
Minimization of downtime 12
Compartment dampers 13
Construction materials 13
Design 13
Shakers 14
Design 14
Minimization of downtime 15
Additional comments 15
Protection Agency
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CONTENTS (continued)
Page
4. Dust Hoppers and Conveyors 17
Plugging or jamming in hoppers and screw
conveyors 17
Hopper vibrator failure 20
Hopper heater failure 21
Dust valve failure 22
Fan failure 23
Reentrainment 24
5. Scrubbers 26
Water Maldistribution 26
Design 27
Detection and correction 27
Erosion and corrosion 28
Prevention 29
Minimization of downtime 30
Slowdown line clogging 30
Prevention 31
Detection 31
Minimization of downtime 32
Additional comments 32
Appendix Questionnaire Sent to Manufacturers 34
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DISTRIBUTION AND DISCLAIMER STATEMENT
This report is issued by the United States Environmental
Protection Agency (U.S. EPA) to report technical data of interest
to a limited number of readers. Copies are available free of
charge to grantees, selected contractors, and Federal employees
in limited quantities from the Library Services Office (MD - 35),
Research Triangle Park, North Carolina, 27711, or for a fee from
NTIS 5285 Port Royal Road, Springfield, Virginia, 22161.
This report was furnished to the U.S. EPA by PEDCo Environ-
mental, Inc., 11499 Chester Road, Cincinnati, Ohio, 45246, in
fulfillment of Contract No. 68-02-2535. The contents of this
report are reproduced herein as received from PEDCo Environmental,
Inc. The opinions, findings, and conclusions expressed are those
of the authors and are not necessarily those of the U.S. EPA.
Mention of company or product name is not to be considered as an
endorsement by the U.S. EPA.
IV
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SECTION 1
INTRODUCTION
This report addresses various operation and maintenance
problems frequently associated with three types of air pollution
control equipment: electrostatic precipitators, scrubbers, and
fabric filters. The discussion presented is limited to a summary
of information gathered by the Industrial Gas Cleaning Institute,
from its members, for PEDCo Environmental. The purpose of this
document is to present the comments and suggestions of major
manufacturers of the control equipment as indicated in responses
to a questionnaire (see Appendix).
The report discusses instrumentation, materials of construc-
tion, and design considerations in particulate control equipment
that improve performance. Such improvements include reduction or
prevention of malfunction, early detection of malfunction, and
easier maintenance and operation of equipment. Because some
problems can never be completely eliminated, methods of reducing
downtime are also addressed.
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SECTION 2
ELECTROSTATIC PRECIPITATORS
2.1 ELECTRODES
Electrostatic precipitators (ESP's) use two types of elec-
trodes: discharge and collection. The discharge electrodes are
held at a high electrical potential during precipitator operation
to ionize the gas and establish electrical fields for particle
charging and precipitation. Collection electrodes are usually
solid sheets with a baffle arrangement to minimize gas velocities
near the dust surface and provide stiffness to the plate.
Electrode failure usually results from electrical erosion,
mechanical fatigue, or hopper buildup. The following are recom-
mendations for prevention of electrode failure and replacement of
electrodes.
2.1.1 Prevention of Failure
2.1.1.1 Electrical Erosion—
Electrical erosion can usually be prevented or minimized by
a few precautionary measures. Although failure sometimes results
from manufacturing defects, this type of failure can be prevented
by attention to detail in the design and manufacturing of elec-
trode systems. Abnormal spark rates, high-current arcs, or high-
energy sparks can cause electrical erosion. Prevention requires
properly sized automatic voltage controls, which keep operating
voltage close to the sparking threshold and never -.How contin-
uous arcing. Controls should be capable of limiting sparking
current surges, so that they are no greater than two or three
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times the normal peak current. Controls should be closely
matched in size to the expected operating power levels, should
react quickly to each spark event (within one cycle or less),
and should operate effectively at very low sparking rates (i.e.,
below 10 sparks/minute). The automatic power control equipment
should also be checked regularly as part of the preventive
maintenance program. Manual operation of controls for extended
periods should be avoided.
Discharge and collection electrodes should be properly
aligned during equipment erection, and electrical contact between
the stabilizing frame and emittin-g electrode should be good.
One manufacturer recommends the use of a rigid frame design to
minimize electrical failures. The physical connection of the
electrodes to the support frame must be tight to prevent spit-
arc erosion. When weighted wire electrodes are used, wires
extending above or below the collection plates should have
shrouds to prevent erosion. Because close clearances may cause
electrode erosion, care must be taken to space plates and wires
uniformly throughout the length of the emitting electrode. The
edges of all high-voltage and grounded surfaces between electrodes
should be smooth and rounded.
2.1.1.2 Mechanical Fatigue--
Mechanical fatigue can be prevented by selecting the proper
materials of construction, eliminating welds, especially in
high-stress areas, minimizing flexing, reducing cross-sectional
area at junction points, and providing strong vibration- and
stress-resistant connections to eliminate stress during normal
rapping. Rapping frequency and intensity should also be con-
trolled to minimize mechanical fatigue. Mechanical stress can
be prevented by ensuring that electrodes are adequately ten-
sioned, having a mounting arrangement that permits the ends of
the electrodes to rotate slightly, and keeping the unbraced
length of the electrode relatively short. Wire electrodes must
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not be scratched or nicked during replacement because such
accidents can cause fatigue failure. Corrosion resistance is
also important to help prevent fatigue failures. In some in-
stances the use of alloys should be considered for electrode
wires.
2.1.1.3 Hopper Buildup—
Because high dust levels can cause excessive sparking and
undervoltage, high-voltage power systems should provide for
automatic shutdown if such levels occur. High dust levels in
the hoppers can also cause wire weights to float and wires to
slacken; this may lead to kinking and permanent damage if wires
are of soft steel. Hard spring wire is recommended to prevent
this from occurring. However, if hopper ash levels are allowed
to buildup, evacuation of ash will not necessarily result in
electrode realignment, possibly necessitating repairs to the
high-voltage structure, collecting surfaces, or both.
Dust hopper buildup can be minimized or eliminated in
several ways. First, the hopper and removal system must be
properly designed. This should include a hopper with the cor-
rect valley angle for the material being collected, collecting
and discharge electrodes that allow effective rapping, poke
holes in the hopper neck, level indicators, adequate insulation
and/or heaters for each hopper, and in some instances (e.g.,
collection of oil soot) insulation for the conveyer system. A
properly sized and operating conveying system and hopper vibra-
tors can help minimize dust buildup in the hopper. Also, the
power control system must automatically shut down when high dust
levels cause excessive arcing.
2.1.2 Minimization of Downtime
When electrodes fail, quick replacement is desirable. Use
of weighted wire electrodes can prevent excessive downtime. By
contras.t, rigid frame electrodes are difficult to straighten
when bent, and harp frames are difficult to replace in many
designs, although wires can be cut out. If rigid electrodes are
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used they should be of one-piece construction, so that only top
and bottom connections have to be made inside the ESP if re-
placement is required.
Designs suggested by manufacturers for easy replacement of
electrodes include pin-and-loop construction and use of spiral
wires in a rigid frame. Also, ribbon electrodes can be so
"tuned" in a rigid frame that breakage or removal of an electrode
does not distort the frame.
Walkways between the fields should also be provided for
quick and easy identification of failed electrodes. Hopper ac-
cess doors should be wide enough to accommodate ladders, and
access platforms at the hopper manhole level should be high
enough that a ladder can reach the lower high-voltage frame from
the base of the hopper. Installation of individual large-element
rigid electrodes requires use of side-access doors and position-
ing from above. Adequate clearance between collecting surfaces
and interior walkways must be provided to permit this procedure.
2.2 DUST REMOVAL
2.2.1 Design
Many of the measures for prevention of dust hopper buildup
also apply to dust removal. These include adequate insulation
and heaters to maintain flue gas above the dewpoint and prevent
condensation, vibrators tied into the ash removal system so that
rapping cannot occur unless the hopper is being evacuated, poke
holes or strike plates to make pluggage elimination easier, and
an adequate hopper level indicator. Although helpful in some
cases, vibrators may cause dust compaction leading to clogging.
Thus, care must be taken in selecting a vibrator. Installation
of hopper vibrator vacuum interlocks may help prevent compaction
of collected ash. One manufacturer also suggested installation
of activated panels, which act like vibra-screws, on hopper walls
where bridging is likely.
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Dust removal can also be facilitated by designing the
hopper so that the valley angle is greater than 55 degrees, the
sides are reasonably smooth, and the transformation from rectan-
gular hoppers to round outlets is accomplished without ledges or
projections.
One point particularly stressed by the manufacturers is that
hoppers should never be considered a storage device. Operator
control and response to alarms is also very important in pre-
venting hopper blockage and ensuring continuous dust removal.
Proper design and maintenance of the conveying systems is also
important. One recommendation is to use suction, rather than
pressure, conveying systems because of wearing of upper valve
seats in pressure systems. This wearing allows air leakage
through the valves, causing reentrainment and plugging. Another
suggestion was that screw conveyers be designed to carry normal
amounts of dust at 30 percent loading, so that extra capacity is
available for flooded hoppers. Also, all conveyer systems should
be powered sufficiently to provide for a flooded, compacted dust
load condition. A minimum width of 0.3 m (12 in.) is recommended
for conveyors to prevent bridging. If 0.225-m (9-in.) conveyors
are used under trough hoppers, however, flared troughs are recom-
mended. Screw conveyers should be operated at speeds from 15 to
30 rpm, because they are likely to require less maintenance than
conveyers operated at higher speeds. The conveyer system should
be equipped with failure alarms. Zero speed indicators are
suggested for screw conveyers, and pressure sensors are suggested
for pneumatic systems. In addition, the lateral strength of
guide or rapping bars should be sufficient to resist permanent
damage when dust pressure causes side thrust. Usually, good
operator control and a good preventive maintenance program
forestall many problems.
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2.2.2 Rapper Operation
Poor ESP performance can result from rapping/vibration
forces that are either too mild or too severe. The rapping/vi-
bration sequence and intensity must be variable to optimize dust
removal and prevent reentrainment. Each field should be equipped
with separate controls for rapping frequency, duration, and
intensity. One method of determining whether rappers are prop-
erly adjusted is to install recording opacity meters or other
optical instruments in outlet ducts to determine severity and
interval of rapper puffs. The use of individual rapper ground
fault detection and annunciation -and accelerometer checks of
rapping equipment during initial startup can also help ensure
proper adjustment during operation of rappers. Complete elec-
trical instrumentation, such as primary and secondary current and
voltage meters, should be used in each electrical section to help
optimize rapper intensity and frequency. Unduly mild rapping/vi-
bration forces are normally detectable through a gradual dete-
rioration in the electrical readings. Noting electrical charac-
teristics of given high-voltage electrical sections allows de-
termination of dust buildup, which affects resistivity, power,
and collection efficiency. Several weeks may be required to
verify dust buildup. When buildup is suspected, rapping intens-
ity should be increased for 1 or 2 hours without deenergizing the
precipitator. If the power level increases, the rapping inten-
sity should be reduced to slightly above the original level, and
the observations should be repeated.
2.3 INSULATORS
2.3.1 Prevention of High-Voltage Tracking
If an insulator is of adequate length and design, high-
voltage tracking is usually caused by moisture and/or conductive
dust. A properly designed purge and/or pressurization system
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should be provided for the insulator compartments in addition
to heaters for the insulators. Screen tubes under the insulators
can also be used to prevent flow into the insulator. Thermal
insulation of the insulator compartments is suggested to reduce
heating power requirements. In addition, some designs may re-
quire preheating the purge air. Thermocouples and automatic
controls to energize heaters and provide continuous temperature
readings should be provided, as well as temperature interlocks on
transformer/rectifier controls to prevent premature energization.
Condensibles on insulators may be avoided by using heated,
unglazed insulators, which are sealed from the flue gas. One
manufacturer, however, still recommends positive displacement of
air in insulator compartments even when the insulators are not in
direct contact with the flue gas. Periodic cleaning of insula-
tors is also recommended to prevent high-voltage tracking.
2.3.2 Detection of High-Voltage Tracking
Insulator problems can be detected by monitoring high-
voltage control functions, such as spark rate, amperage, and
voltage, with trained operators. With properly functioning
controls, voltage should be reduced before any serious damage is
done. The location of arcing in an insulator can often be
detected audibly, especially with the nelp of a mechanic's
stethoscope. Some designs allow visual detection of insulator
arcing by observing reflected light through the air purge holes
at the top of the insulator. Insulator housing can also be
visually inspected for water leakage.
2.3.3 Minimization of Downtime
Replacement of insulators should be considered in designing
the ESP. Adequate access should be provided, and minimal dis-
assembly should be required. Electrode supports should be
designed to allow sufficient room inside insulator compartments
for easy access and insulator removal and inside the precipitator
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housing for access and ease in temporarily supporting the elec-
trode support frame during insulator replacement. Some designs
provide special holes in the roof and support hooks for temporar-
ily supporting the frame. Replacement is quicker and easier if
the proper tools and lifting equipment are on hand and if plant
maintenance personnel are properly trained. Also, in some low-
temperature installations operating at a moderate negative
pressure, insulators can be replaced while the process stays on
line.
2.4 GAS SNEAKAGE
Gas sneakage results when a portion of the particulate-laden
gas stream bypasses the collection zones in the ESP. The highest
potential for sneakage exists around the outboard sides of col-
lecting surface chambers and in hoppers under collecting sur-
faces. Proper design and placement of baffles are the primary
means of avoiding gas sneakage. Baffles should be placed at the
top, bottom, and sides of each active area of the precipitator.
Also, hopper design and configuration can help prevent gas
sneakage. Suggestions include the use of shadow baffles on
outlet fields, sneak baffles at the perimeter of each bay, weir
baffling of the inlet field distribution screens, and hopper
baffles. Baffles must be solid, and open gaps around structural
interferences should be as small as possible. Hoppers should be
baffled when a hopper spans more than one mechanical field.
Before startup, smoke wands and confetti can be used to ensure
the lack of gas sneakage and determine whether airflow through
the precipitator is uniform.
2.3 ADDITIONAL COMMENTS
Preventive maintenance and regular inspection are important
in ensuring proper equipment performance and long life. Also,
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copying specifications from one job to another without consider-
ing relative size should be avoided, as well as combining fea-
tures from several manufacturers.
Online washing of air heaters was cited as a potential
problem. This sometimes causes cemented fly ash deposits in the
first field of the precipitator that require washing to restore
normal operations.
Other problems can result from air inleakage and from lack
of provisions or improper provisions for temperature expansion.
Gas inleakage may occur through the ash disposal system or
through holes in the precipitator shell. This can be avoided by
proper design of the hopper system and use of proper construction
materials to fabricate the ESP.
Clinkers are likely to form at one time or another on
discharge electrodes. The clinkers consist partly of oil soot
carried over into the ESP after boilers started by oil flameout.
These can grow until they contact a collecting surface, elec-
trically grounding the associated power supply. The clinkers can
also cause tracking on insulators and create a fire hazard. This
problem can be minimized if the precipitator is partially ener-
gized to the extent necessary to maintain compliance and if oper-
ating voltage is kept below the spark threshold. Precautions
should also be taken to prevent carryover of raw oil in the event
of a flameout.
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SECTION 3
FABRIC FILTERS
3.1 BAG FAILURE
Bag failure can usually be attributed to two major causes:
chemical attack and abrasion. A fabric filter may be subjected
to highly corrosive environments. Temperatures may range from
121° to 260°C (250° to 500°F), and such gases as sulfur dioxide,
sulfur trioxide, and hydrochloric acid may be present, in addi-
tion to various other reactive gases and particulates. Complex
reactions can occur between the fabric fibers and the gases,
liquids, or solids, leading to rapid bag degradation. Below are
presented suggestions to extend bag life, detect bag wear, and
minimize downtime caused by bag failure.
3.1.1 Reduction
Bag life can be increased by properly sizing the dust
collector. This entails using a low filter velocity to keep
operating pressure drops small and a conservative air-to-cloth
ratio to reduce mechanical agitation. Mechanical agitation and
abrasion can also be eliminated by using the proper thimble
height and bag tension.
Chemical attack can usually be minimized by proper selection
of fabric and fabric finish. Heavier media and stronger fibers
can help reduce bag replacement. The bag environment, however,
should also be carefully controlled. Because high temperatures
generally reduce bag life, gas inlet temperatures should be kept
as low as possible without reaching the dewpoint. Startup and
shutdown are critical periods for temperature control.
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Damper valves should be tight when a single module or com-
partment in a fabric filter is removed for inspection or mainte-
nance, especially when the system is operating near atmospheric
pressure or under positive pressure. If the section is removed
and the valve is not tight, hot moist gases from the system leak
in and condense on the walls, floors, and hoppers, causing corro-
sion. When the filter bag is replaced, the condensate can cause
severe damage to filter fabric. If a bag is torn, it should be
tied off immediately before any chain reaction ensues.
Maintenance people should be well instructed in procedures
for keeping the fabric collector operating properly and in good
condition. They should also be instructed to maintain a complete
inspection, repair, and replacement record for bag life history.
Spare bags should be stored where they are not likely to be dam-
aged and extra care should be taken in handling fiberglass bags.
3.1.2 Detection
Many times bag failure can be detected visually. Because
the weakest point in a bag is usually the seam, the bag should be
installed so that the seam is visible from access walkways. This
makes inspection much easier. Pressure gauges can also be used
in individual compartments to detect blinding or tearing of bags.
Blinding increases backpressure, whereas tearing decreases pres-
sure across the filter. Leaks can also be detected by observing
stack discharge or monitoring opacity. If a leak is suspected,
it can be located by injecting a" special dust sensitive to ultra-
violet light, in the exhaust gas stream.
3.1.3 Minimization of Downtime
The initial design of the fabric filter should incorporate
features that allow easy maintenance. One suggestion is to mini-
mize the rows of bags between walkways (e.g., four rows between
two walkways or two rows between wall and walkway). Another sug-
gestion is to provide a movable ladder for fast access to more
inaccessible rows.
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The use of compartmentalized fabric filters that allow
isolation and shutdown of only one section for inspection or
repair was cited as a major way of reducing downtime. Another
way is to use bags requiring no tools for replacement, such as
those with sewn-in snap bands or clips. The use of prebagged
cages and bags requiring no tools for tension adjustment can help
minimize downtime. One manufacturer has developed and recommends
the use of a cartridge instead of filter bags to reduce changing
time by approximately a factor of seven. Provisions for in-place
repair of small holes in bags with inside filtration can also
reduce downtime.
Forced ventilation cooling systems may be used with high-
temperature fabric filters to minimize the cooling time required
before maintenance.
3.2 COMPARTMENT DAMPERS
3.2.1 Construction Materials
The type of construction material used depends on the par-
ticular application. Although stainless steel is used occasion-
ally, more often a good abrasion-resistant alloy is required.
3.2.2 Design
Dampers should be sized for a conservative flow velocity to
minimize valve wear and if possible should be placed on the clean
side of the collector. If they are on the dirty side, anti-
fouling slide gates are recommended. Dampers should incorporate
a self-cleaning mechanism and should be installed such that the
mechanism is not located on the bottom of the duct. Designs
should allow quick replacement of parts likely to wear, such as
bearings and packing glands, which are serviceable from the
outside. Adequate power to move the damper under all conditions
is essential. Also suggested is the proper design of air-purged
packing glands and valve seats.
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Several manufacturers recommend poppet valves for high
reliability and low maintenance even in highly abrasive and cor-
rosive applications. The following items are recommended for
poppet damper designs:
Rigid shaft support
Blade flex capability
Adequate shaft strength
Shaft guide bearings
Mechanical lockout
Adequate air supply
Heat-traced compressed air lines or desiccant dryer
All gaskets and O-rings in dampers should be replaced
periodically (i.e., every 2 years).
Limit switches should be installed to ensure proper valve
operation in addition to timer operation. Control panel indica-
tor lights can be used to indicate malfunctions of the damper.
Also routine observations of compartment pressures on magnehelic
gauges can be used to detect damper malfunctions.
3 .3 SHAKERS
Because shakers are dynamic devices, they tend to last a
limited time. The reasons for shaker failure include misaligned
components, insufficient lubrication, improperly tensioned bags,
and improper maintenance of filter media. The following are
design recommendations and operating suggestions to prolong
shaker life and reduce downtime during failures.
3.3.1 Design
The shaker should be designed for anticipated load to pre-
vent overstressing. The thickness of the metal composing the
shaker shaft, method of construction, bearing linkage, size of
connection pins, and number and size of bags hung from any single
shaft should be carefully evaluated. Heavy-duty construction is
suggested for bearings, bag suspension hooks, and bag clamping
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devices. Materials of construction are also important. Shaker
fabric filters used where chlorides are present should not be of
stainless steel.
Shaker systems should include as few moving parts as pos-
sible. One design involves many bags mounted on a single pan
rather than individual rows of shaker shafts. Another design
using an intermittent pneumatic cleaner totally eliminates
shaker parts and replaces them with a compressed air system.
Proper installation of a shaker requires careful attention
to manufacturer instructions to ensure appropriate design tol-
erances and accurate initial dimensions.
Overshaking should be avoided to minimize shaker failures.
In addition, damper seals should be tight, so that cleaning com-
partments are not subjected to airflow in the filtering direction
during shaking. If the latter condition exists, the shaker is
overworked. Resilient bushings in the shaker mechanism should be
replaced as soon as they wear out.
3.3.2 Minimization of Downtime
All mechanical parts, such as linkages, bearings, and
drives, should be located on the exterior shell of the collector
to allow for quick maintenance and replacement of shakers. Knife
edge bearings are recommended, because they can be replaced
quickly. Shaker design incorporating zone isolation features
also permit on-line repair of most items.
3.4 ADDITIONAL COMMENTS
The most prevalent problem cited by manufacturers is the
tendency of plant personnel to ignore maintenance of the fabric
filter system because it is not productive process equipment.
Manufacturers stressed that maintenance personnel should be thor-
oughly educated in all aspects of fabric filter operation and
maintenance and given adequate time to perform preventive mainte-
nance. Operation and maintenance manuals should contain complete
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operation and maintenance information on the fabric filter and
auxiliary components, as well as suggestions for preventive
maintenance and stocking of spare parts to reduce downtime.
Fabric filter vendors should retain contact with buyers and
provide components and systems that reduce maintenance and down-
time.
Many problems occur because a fabric filter is not the cor-
rect equipment for a job or is improperly applied (e.g., the
correct ratio, fabric, or type of cleaning is not used). Because
the supplier is responsible for providing information about
proper application, the buyer should make sure that the supplier
is reputable and proven.
The use of a second collector in series with the primary one
was suggested for critical applications. This can provide a
means to keep the process on-line if the first collector fails or
is being serviced.
Other recommendations include minimizing the frequency that
bags pass through the sulfuric acid dewpoint (the maximum is four
times per year) and increasing efforts to develop a low-cost
replacement for teflon-coated fiberglass bags. One candidate
material noted was Aramid.
Finally, only abrasion-resistant materials should be used at
points subject to errosion. An alternative is to use bolted,
replaceable surfaces of less costly metal.
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SECTION 4
DUST HOPPERS AND CONVEYORS
The following subsections discuss common problems affecting
dust hoppers. They suggest ways of detecting these problems and
of eliminating or reducing them with a minimum of downtime.
4.1 PLUGGING OR JAMMING IN HOPPERS AND SCREW CONVEYORS
Various design features can help prevent dust hopper clog-
ging. The hopper should be designed with as steep a valley angle
as possible. Angles from 55 to 60 degrees are recommended. The
design should also include as large a discharge opening as possi-
ble, smooth coatings (e.g., epoxy or teflon) on inside surfaces,
and a minimum of ledges or other obstructions on sidewalls. The
top of the hopper slope should be at least one bag diameter below
the tube plate to allow proper dust discharge, and at least one
foot of clearance should be provided between hopper walls and any
internal partitions to provide easy discharge.
Adequate heaters and insulation should be installed in the
hoppers to prevent condensation and caking. Additionally, jets
of hot air will keep the material in the hopper in a fluid
state. A temperature of 121°C (250°F) is recommended for the
inner walls.
Plugging can be prevented if the hopper is not used as a
storage vessel and if filter bags (in the case of fabric filters)
are frequently cleaned to avoid large slugs of material falling
into the hopper. Continuous evacuation of the hopper can be
facilitated by properly sized airlocks and conveyors.
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In some applications (e.g., where dust agglomerates easily),
vibrators can be used to prevent plugging. Several manufacturers
recommended against the use of vibrators in most applications,
claiming that they tend to pack the dust. Devices such as sledge
hammer plates and poke holes are helpful. The provision of
inspection doors and other means of access to hoppers and con-
veyors also helps in the event of plugging.
Pressure indicators on pneumatic conveying systems, motion
switches for rotary valves, and hopper level indicators can
signal hopper buildup, but no known device can predict a hopper
buildup. Inspection doors can be provided to allow periodic
visual inspection.
A screw conveyor should be adequately sized to prevent plug-
ging or jamming. Although no more than 30 percent loading should
be required to carry normal amounts of dust at aerated density, a
conveyor should be powered for 100 percent loading at compacted
density. One manufacturer suggests that the screw conveyor be
designed with a reserve capacity to allow for catch-up in a
reasonable time when the conveyor system is shut down and the
dust collector continues to operate. The conveyor should also be
designed to allow expansion, as well as easy accessibility during
maintenance or repair. In one design, the trough can be dropped
down for easy access. Care must be taken, however, to reseal the
trough properly, so that dust and air cannot leak into it.
Other design considerations can help eliminate plugging or
jamming. A minimum of parts should be used in the dust area
(e.g., use of piggyback screw conveyors avoids internal hanger
bearings). Special consideration should be given to shaft size
and support bearing spacings for conveyors. Also, internal
hangers and moving components should be minimized. Conveyors and
hoppers should be heated to prevent condensation. Vibrators or
aeration stones are recommended for some applications to prevent
formation of large clumps and ensure a free flow of material into
the conveyor system. A coarse screen should be installed to keep
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large foreign objects from falling into the screw conveyor.
Oversized rotary valves and rotary valve motors are also recom-
mended. One manufacturer recommends that screw conveyors in-
stalled under trough hoppers should have ribbon flight to ensure
uniform drawdown and to prevent tooling of dust surfaces. Box
conveyors should be a minimum of 0.3 m (12 in.) wide, and flared
troughs should be used where 0.225-m (9-in.) conveyors are used.
Although price increases with decreasing conveyor speeds, a con-
veyor run at lower speeds (i.e., 15 to 30 rpm) requires much less
maintenance than a conveyor run at higher speeds (i.e., 45 to 60
rpm). The use of Redlar conveyors instead of screw conveyors is
suggested for some applications. Some suggestions for prevention
of hopper buildup also apply to prevention of plugging or jamming
in screw conveyors.
Conveyors should also be designed for easy access to the
screw to minimize downtime in case of clogging. Conveyor housing
should be equipped with removable bottom plates for this purpose.
Also important are the inclusion of as few moving parts (such as
intermediate screw shaft bearings) as possible and the adequate
supply of spare parts for quick repairs. Operation of the con-
veyor is another factor to consider. Vibrators should never be
operated with hopper valves in the closed position. If a free
flow can be achieved, metering devices such as dump or rotary
valves are recommended to make certain that gathering screws are
never more than 90 percent loaded. Venting should be provided
for escape of overload materials; or automatic shutdown should
occur in case of overload or lack of material movement.
An inoperative conveyor can be detected by the installation
of zero speed switches on the conveyor or discharge valve shaft.
Motion switches should be interlocked to shut off motors when the
conveyor stops to prevent drive burnout. Pressure drops in the
collector should also be monitored closely, so that a failure in
the cleaning mechanism can be detected. Failure of the cleaning
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mechanism causes material buildup on the bags that could overload
the conveyor. Adequate inspection doors are also recommended for
visual detection of failures.
4.2 HOPPER VIBRATOR FAILURE
Several manufacturers recommended against the use of hopper
vibrators. If not properly installed, vibrators may cause fail-
ures at welds and in metal corners. The installation should al-
low transmission of vibrations throughout the hopper and should
be dampened to maximize equipment life. Recommendations for
installing vibrators include using hoppers and pads of double
thickness metal with rounded corners on all vibrator pads and
welding according to specified techniques. It was also suggested
that vibrators be used only periodically and that aeration stones
or live-bottom bin devices be substituted for constantly required
vibrators. Heat tracing equipment is required if the hopper is
located in a cold environment.
The primary means of preventing hopper vibrator failure is
frequent inspection and proper maintenance in accordance with
manufacturer instructions. Resetting or replacement of gap shims
should be done at regular intervals. Vibrators should be of
heavy-duty design with a mounting mears that allows quick re-
placement. Some types of air or electric vibrators require only
simple bolt-on attachment, and downtime is shortest with total
unit replacement. Some electric vibrators, however, are diffi-
cult to repair because of construction features that prevent
dust, oil, and water ingress. The use of teflon piston liners
may also help reduce maintenance.
Hopper vibrator failure may be detected with instrumentation
that measures electric vibrator coil current or air vibrator
exhaust pressure. One suggestion is to place a light emitting
diode (LED) in parallel with a set of contacts to note operation.
Alarms or hopper level indicators can also be used to detect a
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possible vibrator malfunction. Downtime can be reduced by mount-
ing vibrators on brackets for fast replacement.
4.3 HOPPER HEATER FAILURE
Hopper heater failures are often caused by hotspots in the
heater system, which may be prevented by equal heat distribution
in the system. Two suggestions for prevention of hopper heater
failure are better design and placement of the heater. Design
recommendations included using low-watt-density heaters that
allow uniform heat input over relatively large surface areas
(e.g., electric blanket type heaters). Heaters with panelized
insulation are also recommended for easy removal, which allows
better access to heaters. Improper installation is another cause
of heater failure. Heaters should be installed in such a way
that they are not subjected to high levels of moisture and vibra-
tion. Heaters and thermostat sensors should also be installed so
that the thermostat sensor is not in a cooler location. If a
heater is located between a filled portion of the hopper and good
insulation and if the thermostat sensor is in a cooler location,
the heater may overheat and malfunction. This problem can also
be aggravated if the heater is not attached to the hopper steel
in a manner that enhances dissipation of heat into the steel.
Proper sizing and thermostatic control of heaters is essen-
tial for good performance and long life. Use of circuit breakers
in hopper heater contactors is important in preventing premature
failures. One manufacturer recommends external tracing with
steam or by electrical means to avoid abrasion and condensate
that internal heaters may be subjected to. Another method of
avoiding this problem is to circulate air at 400°F between inner
and outer walls with an electrically powered or steam-driven heat
exchanger. This method is, however, more expensive. It should
also be noted that although in some installations the use of
steam coils rather than heaters can eliminate potential for
overheating, condensate can cause freezing problems.
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Heater failure can be detected most easily by using tempera-
ture monitors to note low temperatures. Hopper heater operation
can also be determined by monitoring element current. Modular
or blanket heaters, instead of cable heaters, are recommended to
reduce downtime because they are more easily serviced and replaced
4.4 DUST VALVE FAILURE
Dust valves must be properly sized for compatibility with
the conveying system to prevent failure. If a conveying system
is unable to remove material at the same rate the dust valve
operates, the valve is forced to operate against a full head of
dust, which may result in severe abrasion. Designing extra large
dust valves to be run at slow speeds is the best preventive step.
The dust valve should also be operated continuously to keep the
hopper empty at all times.
Dust valve failure may result from excessive weight on
counterweight arms, failure to lubricate linkage, bridging of
dust in the hopper directly above the valve, and erosion of valve
discs or gates by abrasive fly ash. Periodic inspection and
proper maintenance reduces or eliminates such valve failures.
Bridging of dust in the hopper can also be prevented by providing
proper insulation and heating to deter condensation and by pre-
venting dust buildup in the hopper. The valves should be of a
good, rugged design with easy access to replace seals and bear-
ings. Hardened valve discs or gates such as Ni-Hard adjustable
seats on rotary valve rotors help eliminate or minimize dust
abrasion. Other suggestions are that rotary valves not be used
in abrasive dust applications and that they be checked at the
highest anticipated operating temperature for free movement after
maximum expansion of the rotor. Use of oversized valves and/or
motors can also help prevent dust valve failures.
Dust valve failures also occur when foreign objects get
caught between the rotor and housing. If discharged dust is
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exposed to moisture, a pumice-type material may be formed, caus-
ing dust valve jamming. Screens can be installed to prevent
foreign objects or large pieces of particulates from falling into
the valve. Care must be taken, however, to prevent plugging of
the hopper throat in this area when screens are used. Also, care
should be taken to prevent moisture from entering or forming in
the system.
Walkby inspections should be performed daily to make sure
the system is operating properly. Dust valve rotation indicators
with pilot lights on the control panel can be installed to detect
valve failures.
Dust valves should be easily accessible for maintenance to
minimize downtime. Rotary valves should be fitted outboard bear-
ings for ease of maintenance, and extra valves should be kept on
hand for fast replacement. Dust valve failures can be detected
with zero speed switches.
4.5 FAN FAILURE
Fan failure usually results from system vibration or from
blade or wheel cracking and failure. Vibration can be caused by
a shift of the impeller on the shaft, but is usually caused by a
buildup of waste material on the impeller. Buildup of materials
on the impeller can be prevented by regularly maintaining fan
filters, starting fans with dampers closed, and having damper
controls interlocked to amperage control on the motor. Vibration
detectors and continuous vibration analysis should reduce un-
scheduled downtime. In addition, fan performance can be deter-
mined by monitoring amperage, and fan failures can be detected by
using motion switches.
Blade or wheel failures may be caused by inadequate material
properties (i.e., failure of fan to resist erosive, corrosive,
and expansive forces), structural inadequacy resulting from poor
welding procedures and materials, and engineering design problems
such as fatigue or stress.
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Erosion, not corrosion, is the usual problem with precipi-
tator fans, because the gases passing through the fan are often
dirty and abrasive. Where fans are in dirty air, fan speed
should be lowered (a maximum of 680 rpm is suggested). Some-
times, applying wear pads to all or part of the fan blades may be
desirable to minimize erosion. Wear pads are made of materials
with high Brinell numbers, such as Firmex or Wearaloy. On highly
abrasive applications, tungsten carbide may be used to cover the
wear areas of the blades and centerplate.
Structural weaknesses in the fan caused by poor welding pro-
cedures and/or materials can be prevented only by a good quality
control program at the manufacturing plant.
Fan bearings are another source of system failure. Such
failure can be prevented by minimizing thrust loads through
proper design of inlet and outlet ducts. The use of forced
lubrication systems and temperature detectors on fan bearings can
also help prevent bearing failures.
Fans should also be properly sized. Oversized fans operat-
ing in medi-stable conditions may surge and become self-destruc-
tive .
Other methods to eliminate downtime in case of fan failure
include fan redundancy and provision of adequate access for fan
wheel removal.
4.6 REENTRAINMENT
The most common cause of reentrainment is the buildup of
high dust levels in the hopper. The hopper must be evacuated
continuously with a properly sized conveying system to avoid
this. A drum should always be kept under the hopper of units
designed for intermittent dumping.
Baffling is important in the design of dust hoppers. Proper
baffling can prevent reentrainment of dust by reducing large
eddies and preventing gas from "diving" into the hopper. This
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can be done by installation of baffles to direct airflow away
from the dust in the hopper. Baffles should be placed both
parallel and perpendicular to gas flow direction. Another recom-
mendation is the installation of a tight link-chain-type baffle
that deflects to allow easy dust flow, but at the same time mini-
mizes gas sneakage in this region. The velocities of gases con-
taining abrasive materials, such as clinkers and alumina, should
be kept below 15.2 m/s (3000 ft/min) to avoid reentrainment and
prevent excess wear.
Uniform gas distribution throughout the gas cleaning unit is
important. Proper hopper design, flue geometry, and treatment
velocities help ensure uniform gas distribution. Main gas
streamlines should not be forced to turn abruptly near the unit,
because this may create hopper crossflows. Less restrictive duct
geometries and/or perforated plates allowing variable porosity
should be used to eliminate crossflows. Also, gas velocities
should be kept below 1.65 m/s (5.5 ft/s), because greater treat-.
ment velocities may result in hopper eddy flows.
Air inleakage should be kept to a minimum to deter reen-
trainment. All seals and gaskets should be properly maintained.
Equipment of hoppers with positive sealing dust valves, such as
rotary locks, tipping valves, or motor-operated double-flap
gates, is suggested. These should be regularly inspected for
wear.
Inadequate static periods in a fabric filter's cleaning
cycle may result in reentrainment by not allowing adequate time
for dust to settle in the hopper. Timing between the cleaning
cycles for this type of unit should be increased to remedy this
problem. Other recommendations include the following: installing
ash handling system vents. These vents should be goose-necked,
bell-mouthed, and aimed upstream; using ceilings on dust handling
equipment; and timing vibrators to be compatible with the hopper
evacuation system.
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SECTION 5
SCRUBBERS
5.1 WATER MALDISTRIBUTION
A scrubber removes participates basically by impingement of
participates upon water droplets. The efficiency of particulate
removal depends mainly on the physical contact of solids and
water, thus dictating the importance of water distribution in the
scrubber. A variety of devices and mechanisms can be used to
distribute the water. Spray nozzles are the most common type of
distribution device. Various types of nozzles are used alone or
in conjunction with other devices or mechanisms to atomize the
water. Another common water distribution method involves use of
a venturi throat to atomize the water for contact with a high-
velocity influent gas stream; the water is usually injected
tangentially just before the venturi throat. This method,
referred to as gas atomization, may be used alone or with nozzles.
The most obvious cause of uneven water distribution is
pluggage or erosion of nozzles or pipes. Another cause might be
an insufficient number of properly operating water inlets for the
size of the scrubber throat. An improperly leveled scrubber with
a tangential liquid inlet may alter water distribution. A sharp
radius turn in the ductwork before a tangential liguid inlet may
also cause uneven water distribution by diverting a majority of
the gas to one side of the scrubber and thus drying up the liquid
on that side. Several suggestions to prevent uneven water dis-
tribution, as well as methods to detect and correct this problem,
have been noted. These are discussed below.
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5.1.1 Design
Various design alternatives can be used to minimize uneven
water distribution. The throat section, for example, should be
properly sized; an opening of 27.9 to 35.6 cm (11 to 14 in.) has
been suggested. Nozzles and pipes should be designed to avoid
clogging; large-diameter feed openings and use of clean water
help minimize plugging. One manufacturer includes a disc around
which are injection points that feed water to the scrubber
throat. This disc is flooded with water from large-diameter feed
pipes. Provision of an adequate number of water inlets in
relation to the throat size is also important for even water
distribution across the scrubber throat.
For round Venturis in which water enters tangentially at the
top, injection of water onto a shelf is recommended to ensure
even distribution of water. Also, scrubbers with tangential
liquid inlets should be equipped with leveling adjustments to
prevent uneven water distribution. The use of splash or deflec-
tion plates onto which water is sprayed may help spread the water
evenly. In addition, the use of underflow weirs to prevent
solids deposition in the liquid distribution system should help
prevent maldistribution of the water. Weep holes should be used
to minimize buildup in weirs, and water pressure should be kept
low to prevent hydraulic heads.
In large scrubbers a plumb bob can be used to help distri-
bute the water evenly over the scrubber throat. This is done by
placing a spray nozzle over the apex of the plumb bob. As the
water flows down the conical sides of the plumb bob, an annular
throat is created that distributes the water evenly in the
scrubber throat.
5.1.2 Detection and Correction
The most straightforward way to determine maldistribution
across the scrubber throat is by visual observation when the
scrubber is off stream. This problem, however, might be detected
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while the scrubber is on stream. Although a change in stack
opacity can signal a water distribution problem, it is not a
selective signal and can result from other problems. Maldistri-
bution can be more definitively detected by installing a pressure
differential instrument across the scrubber throat or a pressure
gauge or sensor on the pipe header leading to the spray nozzles.
If the pressure gauge on the pipe header shows an abnormally high
pressure, a plugged nozzle is indicated, whereas an abnormally
low pressure indicates nozzle erosion. Also, the installation of
a flowmeter in the water line can help determine maldistribution.
Low water flow indicates a plugged nozzle. Using the saturation
temperature downstream of the scrubber throat as an indicator of
water distribution has also been suggested. Poor throat coverage
might be indicated by a higher temperature than adiabatic satura-
tion. Usually maldistribution can be remedied by replacing or
reaming out the defective nozzles. One manufacturer prevents
plugging by using nozzle reamers that are operated by an air
cylinder that sequentially clears each water jet. If insuffi-
cient nozzles are available for the throat size, additional water
injection points are required. As previously discussed, improper
duct design may also cause maldistribution. One way to remedy
the specific problem mentioned (i.e., a sharp radius turn prior
to the scrubber throat) is to install turning vanes in the duct
to channel the gas properly.
5.2 EROSION AND CORROSION
A major concern of scrubber manufacturers is to avoid
scrubber erosion and corrosion. In most applications, one or
more of the following conditions are encountered:
Low pH
High pH
High abrasive materials
High temperatures
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The most erosive and corrosive conditions in a scrubber
system seem to exist downstream of the mist eliminator. Harsh
conditions, however, can also be found in the scrubber inlet,
scrubber section before the mist eliminator, and scrubber water
system. The following are suggestions to prevent erosion and
corrosion and to minimize downtime during repair of eroded and
corroded parts.
5.2.1 Prevention
The consensus of manufacturers is that erosion and corrosion
can best be prevented by good design and proper selection of
materials of construction for the operating environment. Econom-
ic considerations prompt many buyers to purchase equipment that
requires lower capital investment, but may need increased main-
tenance because of improper construction materials. Materials of
construction should be chosen to withstand the abrasiveness and
chemical nature of the substances handled.
If the solution is very acidic or contains chlorides, equip-
ment either should be lined with rubber, glass, or fiber-rein-
forced plastic (FRP) or should be made of FRP. If this type of
equipment is not suitable, high-nickel alloys such as Hastelloy C
or Inconel should be used. Rubber lining is also desirable on
scrubber pumps and valves for erosion and corrosion protection,
and Teflon material can be used in nonclogging spray nozzles.
Venturi scrubbers for abrasive applications such as blast
furnaces and basic oxygen furnaces require special designs to
protect against erosion. High-alumina tile or silicon carbide
lining systems provide exceptional erosion protection in these
applications.
The use of wet or flooded elbows before an entrainment
separator and the use of pinch valves rather than ball, plug, or
diaphragm valves can alleviate erosion and corrosion of internal
surfaces. Also, the use of Teflon-type materials in nozzles can
reduce erosion and corrosion. Other steps to prevent erosion and
corrosion include:
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Use of low-speed pumps
Prevention of pump cavitation
Installation of proper filters in the pump line
Reduction of solids concentrations by using tanks or
thickeners or by increasing bleed stream from the
scrubber
Maintenance of proper pH control systems
Installation of spray headers with timers that inter-
mittently spray mist eliminators and fan impellers
Low pipeline velocities (but not low enough to cause
settling)
Periodic maintenance of equipment
5.2.2 Minimization of Downtime
The most obvious way to reduce downtime during repairs is to
provide backup systems and spares. The use of easily replaceable
wear plates has also been suggested as a means to reduce down-
time. Parts and equipment most likely to require periodic re-
placement or maintenance should be made easily accessible (e.g.,
mist eliminators and spray headers should be removeable through
manholes for servicing). Downtime may also be minimized by the
detection of a problem before it has caused much damage. For
instance, devices to monitor reductions in thickness can be
mounted on the outside of equipment. With these devices, zones
requiring maintenance could be detected before they corroded or
eroded away completely. These devices are, however, costly.
5.3 SLOWDOWN LINE CLOGGING
Because of negative pressure in a scrubber system, a water
head equal to the negative pressure must be maintained to ensure
proper drainage. If the vertical drainage section below the
scrubber is too short, liquid backs up in the scrubber shell,
causing problems with air distribution and efficiency. Solids
buildup in blowdown lines, resulting in clogging, is another
problem frequently encountered. Solids buildup may result from
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physical action such as settling or from chemical action such as
the precipitation of a low-solubility compound. Measures to
prevent and detect blowdown line clogging and to minimize down-
time caused by clogging are discussed below.
5.3.1 Prevention
Several plumbing guidelines should be followed in designing
a blowdown line system. As many elbows, tees, and other poten-
tial "dead spots" as possible should be eliminated. One way to
accomplish this is by using 45 degree fittings rather than 90
degree fittings. Solids buildup in the lines can also be mini-
mized by designing pipe sizes to maintain relatively high veloci-
ties [2.12 to 2.42 m/s (7 to 8 ft/s)]. If settling is a problem,
installation of pipe with smaller diameter increases pipe veloci-
ty and prevents settling. The blowdown lines should also be
designed with an adequate slope to the discharge point. The use
of pinch valves instead of other restrictive valves (e.g., ball,
gate, and butterfly valves) is another suggestion.
The chemistry of the blowdown water is another important
consideration for preventing clogging. Limiting solids con-
centration in the water and controlling pH help prevent solids
buildup and precipitation. Solids buildup can be prevented by
increasing the bleed rate of the blowdown water and using decant
and thickener systems to minimize the solids recirculated.
Dissolved solids should not be concentrated above 50 percent of
their solubility at design temperature. This prevents localized
precipitation in wet-dry zones of the scrubber, which can poten-
tially plug the drain.
5.3.2 Detection
Clogging of blowdown lines can be detected by measurement of
flow of the blowdown. Sight flowgauges or magnetic flowmeters
may be used in the blowdown lines, and a low-flow alarm may be
installed. High-liquid-level sensors may be used in a scrubber
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with an integral recycle tank to detect buildup in blowdown
lines. Pressure gages with/without high-pressure alarms could
also be used to detect buildup in pipeline. An increase in
pressure indicates solids buildup.
5.3.3 Minimization of Downtime
As noted with prevention of line plugging, the plumbing of
the blowdown system is important in reducing downtime if a line
becomes plugged. Suggestions for blowdown lines include placing
cleanout ports at strategic locations (e.g., crosses with blind
flanges at all turns) using many unions and flanges for easy
disassembly and cleaning of lines, using flexible connectors for
fast disconnection at lines, using quick-connection fitting pip-
ing (Dresser or Victaulic), and using rubber hoses at long-radius
turns (blockages can be broken up by external hammering).
If a line becomes clogged, the best method of cleaning de-
pends on the nature of the deposit. In many cases, running clean
water through the system or injection of compressed air suffices
to clean the line. If using water and air fails or is not ap-
plicable, rodding out the pipe or acid cleaning may be required.
In many instances, replacing a plugged line with a clean one and
then cleaning the plugged lines may be preferable. This can
reduce downtime significantly. In installations where downtime
costs are high, a standby blowdown system may be justified.
5.4 ADDITIONAL COMMENTS
Adequate and properly maintained instrumentation is con-
sidered necessary for the efficient operation and maintenance of
a scrubber system. Proper instrumentation allows close observa-
tion of system operation and quick detection of changes in per-
formance. That operating personnel know what is happening at the
time it is happening is essential. All instrumentation should be
checked for operation and calibration on a routine schedule. One
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manufacturer recommends that this be done through a maintenance
and service contract with a major instrument vendor. Another
recommendation is that all instrumentation come from one source
to ensure compatibility of equipment and enable quick assessment
of equipment malfunction. A specific suggestion was use of
temperature detection devices in conjunction with emergency spray
activation in high-temperature scrubbers. Also suggested was the
use of detectors for fan vibration and fan bearing temperature.
A good operation and maintenance manual and a good preventive
maintenance program were cited as important considerations for
efficient scrubber operation and long life. On the other hand,
poor plant management attitudes and policies were cited as
frequent causes of shorter equipment life and increased unsched-
uled downtime.
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APPENDIX
QUESTIONNAIRE SENT TO MANUFACTURERS
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INSTRUCTIONS FOR FILLING OUT THE QUESTIONNAIRES
The questionnaires are designed to address different
operation and maintenance problems associated with three
types of air pollution control equipment. They are general
questions and are not meant to apply to any one industry or
source within an industry. The questionnaires are not
exhaustive but simply illustrative of some common problems.
Seperate questionnaires have been drafted for each of the
following types of control equipment: electrostatic pre-
cipitator, scrubber, and fabric filter.
Responses to the questions should point out instrumen-
tation, materials of construction and methods of improving
design of particulate control equipment which will either
reduce or prevent malfunction, detect malfunction before
they occur, enable easier and faster troubleshooting and
allow for quicker and easier maintenance and operation of
such equipment. Since it is realized that many problems and
failures of the equipment can never be completely eliminated,
emphasis should be placed on methods that allow for the
quickest repair of the equipment. When possible incremental
costs that could result from such design changes and in-
strumentation requirements should be noted.
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ELECTROSTATIC PRECIPITATORS
What measures can be taken to prevent electrical ero-
sion, mechanical fatique, and dust hopper buildup which
are three (3) common causes of electrode failure? Also,
what are the best methods or construction techniques
that allow for quick electrode replacement.
2. Inadequate dust removal is a major cause of precipita-
tor malfunctions and is usually a result of either
improper adjustment of the hopper vibrators, failure of
the conveyor system or low flue gas temperature which
permits moisture condensation and plugging of the
hopper. This dust buildup can cause excessive sparking
and sometimes pushes internal components out of posi-
tion. What design measures or operation and mainten-
ance procedures can help reduce this problem or rectify
the problem quickly?
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ELECTROSTATIC PRECIPITATORS (contd)
3. Poor performance can result from rapping/vibrator
forces that are either too mild or too severe. What
instrumentation can be used to detect and help adjust
rappers properly?
4. Problems with insulators such as high-voltage tracking
on the insulators as a result of the formation of con-
densables is often a problem. What design features can
help prevent this or what instrumentation can detect
this before it becomes to severe? When insulators must
be replaced, what methods allows for quickest re-
placement?
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ELECTROSTATIC PRECIPITATORS (contd)
5. What construction methods are available to reduce gas
sneakage to a minimum?
6. Additional comments - types of construction, material,
instrumentation, operating practices, or preventative
maintenance not covered above or relating to other
problems?
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FABRIC FILTERS
1. Bag replacement is the most expensive maintenance
operation. What procedures or materials are available
to reduce bag replacement, possibly predict bag re-
place-ment, or reduce or eliminate downtime during bag
replacement?
Filter compartment dampers are a high-maintenance
problem. What construction and/or materials can be
utilitized to reduce problems with dampers or detect
and/or troubleshoot problems with dampers and repair
or correct the problem quickly?
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FABRIC FILTERS (contd)
Dust hoppers are a common problem in any fabric filter.
What type of special construction or materials can be
used to prevent clogging a buildup? What type of
instrumentation can be used to predict such a problem
before it becomes an operational problem or what methods
allow for corrections of the problem?
4. What construction methods are available to help reduce
shaker mechanism failures or to allow for quick re-
placement or maintenance?
40
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5. Additional comments - types of construction, materials,
instrumentation, operating practices, or preventative
maintenance not covered above or relating to other
problems?
41
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SCRUBBERS
Uneven water distribution across scrubber discs can
cause low efficiency. What construction methods exist
to help prevent this? What methods or instrumentation
permits detection of such an occurrence and/or permit
quick correction of the problem?
Erosion and corrosion of internal surfaces such as,
nozzles, demisters, valves, pumps, and impellers can
cause malfunctions. Except for repairing as necessary
are there any other means of preventing this situation
or reducing downtime during repair?
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SCRUBBERS (contd)
3. Solids can buildup in blowdown lines. What means exist
for preventing this from occuring or detecting it in
its early stages of development? Which methods of
cleaning allow for the shortest downtime?
4. Additional comments - types of construction, materials,
instrumentation, operating practices, or preventative
maintenance not covered above or relating to other
problems?
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QUESTIONS PERTAINING TO BOTH
ELECTROSTATIC PRECIPITATORS AND FABRIC FILTERS
The following items are common problems with both ESP's
and fabric filters. What methods of construction or in-
strumentation can help eliminate or reduce these problems
and what practices allow to repair with the shortest down-
time?
A. Plugging or jamming of screw conveyors in hoppers?
B. Hopper vibrator failures?
44
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C. Hopper heater failures?
D. Dust valve failures?
£. Fan failures?
F. Keentrained dust from dust hopper*?
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TECHNICAL REPORT DATA
(Please read Inurucnovs on the reverse before completing)
1 REPORT NO.
EPA-905/2-80-002
2.
3. RECIPIENT'S ACCESSION NO.
4 TITLE AND SUBTITLE
5. REPORT DATE
Design Considerations for Mininrizinn Operation and
Maintenance Problems of Particulate Control Eauioment
6. PERFORMING ORGANIZATION CODE
7. AUTHORIS)
8. PERFORMING ORGANIZATION REPORT NO
9. PERFORMING ORGANIZATION NAME AND ADDRESS
PEDCo Environmental, Inc.
11499 Chester Road
Cincinnati, Ohio 45246
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
68-02-2535. Task No. 7
12. SPONSORING AGENCY NAME AND ADDRESS
U.S. Environmental Protection Agency - Region V
Air Programs Branch
230 S. Dearborn Street
Chicago, Illinois 60604
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
Project Officers: Henry Onsgard and Dr. Indur Gokl
anv
16. ABSTRACT
This report addresses various operation and maintenance problems frequently associated
with three types of air pollution control equipment: electrostatic precipitators,
scrubbers, and fabric filters.
The report discusses instrumentation, materials of construction, and design considera-
tions in particulate control equipment that improve performance. Such improvements
include reduction or prevention of malfunction, early detection of malfunction, and
easier maintenance and operation of equipment. Because some problems can never be
completely eliminated, methods of reducing downtime are also addressed.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS C. COSATI Field/Group
Air Pollution
Air Pollution Control Equipment
Electrostatic Precipitators
Dust Collector
Scrubber
Design
Fabric Filters
Particulates
13B
18. DISTRIBUTION STATEMENT
19. SECURITY CLASS (Thu Report)
None
21. NO. OF PAGES
50
Unlimited
20. SECURITY CLASS (This page)
None
22. PRICE
EPA Form 2220-1 (t-73)
46
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