EPA-600/2-77-Q06
JANUARY 1977
Environmental Protection Technology Series
ELECTROSTATIC PRECIPITATOR
MALFUNCTIONS IN THE ELECTRIC
UTILITY INDUSTRY
Industrial Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into five series. These five broad
categories were established to facilitate further development and application of
environmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL PROTECTION
TECHNOLOGY series. This series describes research performed to develop and
demonstrate instrumentation, equipment, and methodology to repair or prevent
environmental degradation from point and non-point sources of pollution. This
work provides the new or improved technology required for the control and
treatment of pollution sources to meet environmental quality standards.
EPA REVIEW NOTICE
This report has been reviewed by the U.S. Environmental
Protection Agency, and approved for publication. Approval
does not signify that the contents necessarily reflect the
views and policy of the Agency, nor does mention of trade
names or commercial products constitute endorsement or
recommendation for use.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-77-006
January 1977
ELECTROSTATIC PRECIPITATOR
MALFUNCTIONS IN THE
ELECTRIC UTILITY INDUSTRY
by
Mike Szabo and Richard Gerstle
PEDCo. -Environmental Specialists, Inc.
Atkinson Square, Suite 13
Cincinnati, Ohio 45246
Contract No. 68-02-2105
ROAPNo. 21BAV-081
Program Element No. 1AB012
EPA Project Officer: Dennis C. Drehmel
Industrial Environmental Research Laboratory
Office of Energy, Minerals, and Industry
Research Triangle Park, NC 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, DC 20460
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ACKNOWLEDGEMENT
This report was prepared for the U.S. Environmental
Protection Agency, Industrial Environmental Research Labora-
tory, Research Triangle Park, North Carolina, by PEDCo-
Environmental Specialists, Inc., Cincinnati, Ohio. The
project director was Mr. Richard W. Gerstle and the project
officer was Mr. Norman J. Kulujian. Principal investigators
were Messrs. Norman J. Kulujian and Lario V. Yerino. The
principal authors of the report were Messrs. Michael F.
Szabo and Richard W. Gerstle. Editorial review was provided
by Ms. Anne Cassel. Graphics were prepared under the
direction of Ms. Nancy Wohleber.
Dr. Dennis Drehmel was the project officer for the U.S.
Environmental Protection Agency, Industrial Environmental
Research Laboratory, Research Triangle Park, North Carolina.
The author appreciates the assistance and cooperation pro-
vided by Dr. Drehmel, and the other utilities and precipita-
tor manufacturers, who wish to remain anonymous but contri-
buted information to this project.
11
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ABSTRACT
A comparison of the advantages and disadvantages of hot
and cold precipitators is followed by a discussion of design
considerations that apply to hot and cold precipitators.
Common malfunctions found with precipitators operating on
coal-fired boilers in the electric utility industry and
corrective or preventive measures are summarized. A pre-
cipitator operation and maintenance procedure for minimizing
malfunctions and downtime is presented, procedures followed
by utilities during startups and malfunctions are described,
and costs of precipitator maintenance are discussed. Pro-
cedures for inspection of a precipitator at a utility
operating a coal-fired boiler are outlined. Appendices
compare precipitator operation and maintenance guidelines
recommended by precipitator manufacturers versus the utility
which operates the precipitator; an operating history of
precipitators at a major utility is also presented.
111
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TABLE OF CONTENTS
Page
ACKNOWLEDGMENT ii
ABSTRACT iii
1.0 INTRODUCTION 1-1
2.0 TYPES OF ELECTROSTATIC PRECIPITATOR (ESP) 2-1
SYSTEMS
2.1 Cold-side and Hot-side ESP's 2-1
2.2 Design Considerations for Major ESP 2-4
Components
3.0 MALFUNCTIONS 3-1
3.1 Types of ESP Malfunctions 3-1
3.2 Conditions Specific to Power Plants that 3-16
Cause Problems in Precipitators
3.3 Reduced ESP Collection Efficiency as 3-24
Related to Number of Bus Sections Not
in Operation
3.4 Maintainability of ESP Equipment as 3-26
Related to Frequency of Malfunctions
4.0 MAINTENANCE 4-1
4.1 Maintenance Program for Precipitators 4-1
4.2 Utility Procedures and Recordkeeping 4-19
During Startup and Malfunctions
4.3 Costs of Cold Side ESP Maintenance and 4-23
Operation
v
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TABLE OF CONTENTS (continued).
Page
5.0 INSPECTION TECHNIQUES FOR EVALUATING MAINTENANCE 5-1
PROCEDURES
5.1 Typical ESP Inspection Procedure 5-1
5.2 Inspection Checklist for Electrostatic 5-11
Precipitators in the Electric Utility
Industry
APPENDIX A ESP MANUFACTURERS SUGGESTED MAINTENANCE A-l
PROCEDURES
APPENDIX B UTILITY ESP MAINTENANCE PROCEDURES B-l
APPENDIX C EXAMPLE OPERATING HISTORY OF COLD-TYPE C-l
PRECIPITATORS
VI
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LIST OF FIGURES
No. Page
2-1 ESP Particle Charging System and Wire Hanging 2-8
System
2-2 ESP Plate Hanging System 2-9
2-3 Cutaway View of a Typical ESP and Arrangement 2-16
of Field and Cells
3-1 Shrouds for Wire-weighted Discharge Electrodes 3-7
3-2 Typical Sparking Levels When Precipitating 3-19
Dusts with Different Resistivities
3-3 Typical Operating Curve to Meet Emission 3-25
Regulations with Partial Malfunctions of ESP
5-1 Electrostatic Precipitator Collection 5-8
Efficiency vs. Delivered Power
5-2 Cold-side ESP. SCA vs. % S 5-10
LIST OF TABLES
No. Page
3-1 Summary of Problems Associated with ESP's 3-2
4-1 Maintenance Items for Electrostatic Precipi- 4-2
tators
4-2 Troubleshooting Chart for ESP's 4-15
4-3 Summary of Characteristics and Assumption For 4-26
Model Plants
4-4 Coal Analyses Assumed for ESP Cost Evaluation 4-28
4-5 Capital Costs for Electrostatic Precipitators 4-29
4-6 Annualized Costs for Electrostatic Precipi- 4-31
tators
5-1 Plume Characteristics and Operating Parameters 5-2
for Coal-fired Boilers
5-2 Recommended Recordkeeping Requirements 5-5
vii
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1.0 INTRODUCTION
When an electrostatic precipitator installed on a
utility boiler fails to achieve design efficiency, operators
must determine the causes for poor performance. Although
the reasons for poor performance are numerous, they can be
grouped in two distinct categories: degradation of ESP
efficiency is attributable either to hardware malfunctions
or to improper operation. The purpose of this report is to
examine precipitation malfunctions in the electric utility
industry. Under EPA sponsorship PEDCo-Environmental Special-
ists has prepared concurrently another document discussing
electrostatic precipitator performance related to operational
and maintenance practices.
It is assumed that the reader has a working knowledge
of electrostatic precipitators. The various types of preci-
pitators in the electric utility industry are discussed in
Section 2.0, along with design considerations. Major types
of malfunctions are summarized in Section 3.0. For each
type of malfunction, the possible cause, duration, correc-
tive action, and preventive measures are stated. Section
4.0 presents the maintenance procedures that can minimize
the probability of malfunctions occurring. Section 5.0
1-1
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describes inspection techniques for evaluating ESP main-
tenance procedures and describes in detail the items to be
checked during inspection of power plant precipitators.
Appendix A presents a typical precipitator manufac-
turer's recommended operation and maintenance procedure;
Appendix B is an example of a concientious utility precipi-
tator operation and maintenance schedule. The precipitator
operating history of a major U.S. utility is summarized in
Appendix C.
1-2
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2.0 TYPES OF ELECTROSTATIC PRECIPITATOR (ESP) SYSTEMS
This section discusses the various types of electro-
static precipitators, presenting a detailed discussion of
the major ESP components. It is assumed that the reader
understands the fundamentals of ESP operation and theory,
123
which are described in many references. ' '
2.1 COLD-SIDE AND HOT-SIDE ESP'S3'4'5
The two categories of ESP's for use with coal-fired
boilers are based in location relative to the air preheater
and thus are temperature-dependent. The cold-side ESP,
currently predominant in the utility industry, is located
downstream of the air preheater, operating in the temperature
range of 200 to 400°F. It is used mostly with high-sulfur
coals, since the high resistivity associated with most low-
sulfur coal in the-operating temperature range would require
larger plate areas in cold-side ESP's.
The hot-side ESP is located upstream of the air pre-
heater and operates at temperatures above 230°C (450°F).
The gas flow upstream of the air preheater at 371°C (700°F)
is about 1.5 times the volume of air downstream of the air
preheater; a relatively -larger ESP is usually required to
handle gases at the higher temperature. With many low-
2-1
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sulfur coals, however, because of their high resistivity in
the 93 to 204°C (200 to 400°F) range, it is the smaller gas
volume that requires the larger ESP. The greatest advantage
of the hot-side ESP over the cold-side ESP, is its constant
efficiency under varying fuel conditions. Changes in the
fuel fired in a boiler are necessitated by such things as
contract variances, price differentials, and availability.
Since the hot-side precipitator is located ahead of the air
preheaters, it operates at the temperatures of the boiler
flue gas exhaust, and the fly ash resistivity is reduced to
levels that allow better precipitation. At this higher
temperature resistivity is not sensitive to the fuel's
sulfur content.
Since hot-side ESP is located on the hot gas side of
the air preheater, the fouling of heat transfer surfaces by
ash should be eliminated, the plant should operate more
efficiently, and requirements for soot blowing from the air
4
preheater should be reduced. Use of high-sulfur coal might
introduce a detrimental factor, since the fly ash often acts
to remove any sulfur trioxide present in the gas stream and,
if the particulate is removed ahead of the air preheater,
there is a potential for corrosive attack in the preheater.
The typical hot-side precipitator operates at relatively
lower voltages, but, if properly designed, it operates at
much higher current densities; it is characterized by rela-
2-2
-------
tively high power density, and by stable, current-limited
operation, with sparking usually confined to inlet sections
where dust concentrations are high.
The cold-side ESP does not undergo the thermal expan-
sion associated with a pronounced temperature increase, as
does the hot-side ESP. The expansion can result in extreme
misalignment or even duct discontinuities. Such failures
have been traced to inadequate provision for differential
thermal expansion between the lower shell and support
structure, and between the precipitator shell and roof
housing. These problems can be minimized with provisions
for differential movement of the precipitator on its support
structure, proper insulation, and adherence to design
stresses, particularly in regions where temperature gradients
cannot be avoided.
Some ESP manufacturers favor cold-side installations,
whereas others stress hot-side units; there is no clear-cut,
3 4
all-inclusive criteria for choice of either type. ' The
selection is usually based on operability and economics. In
general, fcr new construction, if the cold-side unit requires
a specific collection electrode area (SCA) greater than 500
to 600 square feet per thousand cubic feet of gas flow per
minute, then a hot-side unit would be the proper choice. If
the SCA can be smaller, a cold-side unit could be used.
2-3
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The situation is somewhat more complicated in retrofit
installations. A hot-side unit requires addition of duct-
work to transport the gas stream from the air preheater
inlet to the ESP and back to the air preheater. The consi-
derable expense that can be involved tends to swing the
economics toward a cold-side installation for retrofit.
systems.
With respect to a specific installation the following
guidelines for selection are suggested:
(1) Determine resistivity as a function of temperature.
(2) Evaluate severity of the potential resistivity
problem considering consistency of coal supplies
and variation in coal characteristics.
(3) Conduct comparative cost estimates with emphasis
on retrofit difficulty.
Neither type of ESP installation can provide perfect
service. Each requires regular attention to ensure good
service and to minimize malfunctions.
2.2 DESIGN CONSIDERATIONS FOR MAJOR ESP COMPONENTS6
2.2.1 Rapping Systems;
Rappers are incorporated in the ESP to remove dust from
the collecting and discharge surfaces; effectiveness and
reliability of the rappers are essential. The following
types are generally available:
2-4
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0 Electromagnetic impulse, either single or multiple
0 Electric vibrators
0 Pneumatic impulse
0 Various mechanical hammers, usually associated
with foreign designs, but sometimes furnished by
others for special applications.
Each ESP manufacturer develops rapper designs for
compatibility with his suspension system and rapper schedule
(number of surfaces per rapper), based on experience and
tests. Generally, pneumatic rappers impart more energy than
either electromagnetic rappers or electric vibrators and
remove tenacious dusts more readily. It is important,
however, to be certain that all hardware in the system is
designed to withstand such high energy forces. Changing
from electrical to pneumatic rappers in an attempt to improve
operation without also strengthening the hardware has led to
structural failure.
Current designs for horizontal rapping hammers impart
more energy to the plates than do conventional designs;
these rappers remove fly ash from the plates in a very
efficient manner.
Mechanical hammers also are often very effective, but
moving parts in a dirty gas stream require frequent mainten-
ance. Repairs require shutdown of an entire chamber or
system.
2-5
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The number and size of rappers required for a par-
ticular installation vary with precipitator manufacturer and
nature of the dust. Requirements for collecting surface
area range from 110 to 550 m2 (1200 to 6000 square feet) per
rapper. Discharge electrode rappers serve from 1000 to 7000
feet of wire per rapper. Rapper intensity ranges from about
35 to 70 J (25 to 50 foot-pounds) per cycle. Rapping inter-
vals are adjustable over a range of approximately 30 to 600
seconds between raps.
The paramount consideration in rapping is to provide
ample acceleration to dislodge the dust without excessive
reentrainment. Accelerations of 30 to 50 g per rap, as
measured on the collection electrode, are required for
removal of fly ash. Both cycle and rapping intensity are
usually adjusted in the field to optimize rapping operations
for maximum precipitator performance.
2.2.2 Wire and Plate Hanging Mechanisms
2.2.2.1 Wire Weight System - The wire weight system con-
sists of individual electrode wires suspended from an upper
support frame. The wires are best shrouded in some fashion
to prevent arcing to the exposed, sharp ground edges, or
where the electrical clearance is reduced by passing the
tops and bottoms of the collecting dust plates. The wires
are held taut by weights suspended from their bottoms. The
weights in turn are spaced by a guide frame. The frame must
2-6
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be stabilized against swinging, an action that may be gener-
ated mechanically by the gas stream, by "electrical wind,"
by an improperly functioning automatic voltage-control, or
by some combination of these.
Commonly, stabilization is accomplished by trusses
extending from the upper support frame to the guide frame.
Rapper energy, transmitted through the trusses, aids in
keeping the lower guide frames clean. Any design of the
guide frames that permits enough dust buildup to raise the
weights may cause slackening of the wires, arcing, and
eventual wire failure.
Stabilization of guide frames by ceramic or other
insulators from the casings or hoppers can cause a mainte-
nance problem. Dust buildup on the insulators during opera-
tion, although resistive in some cases, presents a source of
leakage to ground. Moisture gathered during shutdown (or
low-load operation) might lead to complete failure of an
insulator. Figures 2-1 and 2-2 illustrate the wire and
plate hanging mechanisms of a typical ESP.
Electrode wire failure can be virtually eliminated by:
° Reasonable care, during erection, in alignment of
the casings and surfaces.
0 A well-designed support, guide, and stabilizer
system.
0 Reliable, properly adjusted automatic voltage-
controls.
2-7
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SUPPORT INSULATOR
HOUSING
VIBRATION
ISOLATORS
HIGH VOLTAGE
BUS DUCT
BUS CONDUCTOR
HIGH VOLTAGE
SWITCH
TRANSFORMER-
RECTIFIER
TENSIONING WEIGHT
DISCHARGE ELECTRODE
SUPPORT FRAME
DISCHARGE ELECTRODE
WEIGHT GUIDE FRAME
(Source: Rcf. 3)
Figure 2-1. ESP particle charging system and wire
hanging system.
2-8
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END PANEL OR
INTERIOR GIRDER
•COLLECTING SURFACE
SUPPORT
COLLECTING SURFACE
(Source: Ref. 3)
Figure 2-2. ESP plate hanging system.
2-9
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0 Good operating maintenance of the dust-handling
system.
2.2.2.2 Rigid Wire Frame - The rigid wire frame design was
furnished by U.S. suppliers prior to 1950 and then was
abandoned (in favor of the wire-weight design) because of
reliability and operating problems. Recently U.S. licensees
of foreign manufacturers have reintroduced frame electrodes
to this country; installations are now in operation and on
order (as wire-weight designs are being installed in European
plants).
The rigid frame requires a high degree of quality
control, both in fabrication and erection, and is intrin-
sically more costly. Replacement or repair is expensive and
time-consuming, similar to replacement of a dust-collecting
plate.
At lower temperatures, up to 204°C (400°F), warpage of
the frames is uncommon, but for operating temperatures above
204°C (400°F), or with cyclical operation, potential deforma-
tion of the frames becomes serious.
The rigid frame entails wider gas lanes, or ducts, to
provide electrical clearance between the frame and the dust-
collecting plate. This requirement leads to larger casings
to house the required surface areas.
It is important that the engineer be fully aware of the
differences and the requirements of each design philosophy
2-10
-------
in detail, so that he avoids incorrect evaluations of one
versus another.
The erection sequence usually consists of casings and
hoppers first, followed by collecting surfaces and then
discharge systems. If the casings are not erected to true
dimensions, plumbed vertically, and square cornered in the
plan view, attempts are often made to compensate during
installation of collecting surfaces, i.e., using guides that
should be free of frictional loads as "jacks," and so
forth. Then, the discharge system, which should hang
freely, is stabilized in an offset position to maintain, as
best possible, the wire-to-plate centers. Such a construc-
tion will probably involve difficulties from the first day
of operation. There is great need, therefore, to provide
step-by-step quality control and inspection of the installa-
tion, regardless of the pressure of construction schedules.
Discharge systems are supported from the casing through
standoff electrical insulators. These must be kept clean
and dry during operation to prevent accumulation of dust or
moisture coatings, which provide a path for leakage to
ground. Wet accumulations are common during shutdown, as
the moisture in the gas condenses. They can be prevented by
provision of warmed, filtered pressurizing-air supplies.
2-11
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The system must provide distribution to a multiplicity
of insulators, none of which may be allowed to "starve"
because of disproportionate flow. This design problem is
similar to that of balancing an air-conditioning system.
Some method for checking distribution should be provided to
the operators. Maintenance routines for changing filters
and checking heater elements should be established as soon
as the system is operational.
Most of the commonly used electrical insulators lose
dielectric strength as temperature increases. Although the
maximum temperature varies with the insulating material,
204°C (400°F) is a probable limit. Therefore, the electrical
insulators must be isolated thermally from hot gases. The
purge air system normally suffices, but insulators mounted
on hot steel casing may be affected by conduction, at least
for several inches along the length. Fortunately, most
electric insulators retain structural strength under higher
temperatures, and also act somewhat as thermal insulators so
that if the electrical path is long enough, the effect of
the conducted heat is limited to a short distance up the
insulator.
2.2.3 Aspect Ratio
An important variable in precipitator design is the
aspect ratio (ratio of length to height of gas passage).
Space requirements often determine the overall precipitator
2-12
-------
dimensions. Wherever possible, the engineer should select
an aspect ratio that will result in ample opportunity for
reentrained dust from the first sections to be recollected.
The aspect ratio is integrally related to the overall design
of the precipitator, and also depends on such variables as
3 2
gas velocity m /min (acfm), total plate collection area m
2 232
(ft ), specific collecting area [m /1000 m /min (ft /1000
o 2
acfm)], and power density [watts/m (watts/ft ) of collec-
ting plate]. All other factors being equal, higher ratios
of length to height provide better performances. Historically
this value varies between 0.5 and 1.5 with a present day
average of 0.3. Plate heights usually range from 7 to 14 m
(24 to 45 ft).
Precipitator collection plates are made in standardized
size ranges, typically 7-9-11 m (24-30-36 ft) height by
0.9-1.2 m (3-4 ft) length. Once the collection area is
selected, the design will incorporate enough collecting-
plate sections to yield the required surface area.
Plate area requirements are governed primarily by the
properties of the dust and gas and the desired dust collec-
tion efficiency. Efficiency is related to the collection
plate area and gas volume by the relationship
n = 1 - e~ ( | w)
which is the conventional Deutsch-Anderson equation, where A
is the plate area, V the gas flow, and w the precipitation
2-13
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rate parameter of migration velocity. A serious limitation
in use of the Deutsch-Anderson equation is that the particle
size distribution and, subsequently, the effective migration
velocity change as precipitation proceeds. The Deutsch-
Anderson equation does not account for this change.
A recent empirical modification of the equation by
Matts and Ohnfeldt essentially removes the size dependence
v
on w. The equation is: n = 1 - exp (~w, A/Q) where k is
said to be equal to about 0.5 in most cases. This equation
is an improvement over the Deutsch-Anderson equation because
w, can be treated as a constant in any given application.
2.2.4 Field/Bus Section/TR Set Breakups
The electrical system of an ESP is arranged in bus
sections, each bus section representing any portion of the
ESP that can be energized independently. This is done by
subdividing the high-voltage system and arrangement of the
support insulators.
The number of fields, which is the number of bus
sections arranged in the direction of gas flow, is cal-
culated as follows: as a rule of thumb, manufacturers use
one field for up to 90 percent collection efficiency, two
fields for up to 97 percent, three fields for up to 99
percent, and four or more fields for efficiencies above 99
percent.
2-14
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The number of cells, or the number of bus sections
arranged in parallel, is established so that if any one
field shorts out the overall ESP efficiency will not fall
below specifications. Figure 2-3 illustrates the arrange-
ment of fields and cells in a typical ESP.
Sectionalization is of greatest significance in very
large precipitators for several reasons. First, if the
precipitator is operating in a sparking mode, increased
Sectionalization will cause less of the precipitator to be
disabled during the interval of the spark. This results in
higher average voltage, higher electric field, and better
precipitation. Also, the smaller electrical sets have
higher internal impedances, which give better spark quench-
ing and minimize the tendency of a spark to develop into an
arc. Third, the effects of localized electrode misalign-
ments are limited to smaller precipitator sections and
thereby permit higher voltages in the remaining sections.
Finally, in very large precipitators, reasonably good
collection efficiencies can still be maintained even if one
section must be deenergized because of wire breakage or
other electrical trouble.
Increasing the number of electrical sections leads to
increased costs because the cost of the high-voltage power
supply is not linearly related to power handling capability.
2-15
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K)
I
M
CTi
INSULATOR COMPARTMENT
ROOF
END
TRANSFORMER
RECTIFIER
GAS
DISTRIBUTION
DEVICE
GAS FLOW
COLLECTING SURFACE
BUS DUCT
RAPPER INSULATOR
HIGH VOLTAGE SYSTEM
SUPPORT INSULATOR
COLLECTING SURFACE
RAPPER
ISCHARGE ELECTROD:
RAPPER
-SIDE
-&KS
DISCHARGE ELECTRODE
HOPPER
(Source: Ref. 8)
Figure 2-3. Cutaway view of a typical ESP and arrangement of field and cells,
-------
The greater portion of the cost is in providing the high-
voltage equipment. Increased power can then be provided by
using larger components. Hence it is less expensive to
provide fewer large power supplies than to power the preci-
pitator from a greater number of small sets. Because of the
lower average voltage, however, the precipitation rate
parameter would be lower, and the necessity for providing
larger collecting surface area would partially offset the
lower cost of the larger set.
Often, multiple cells are energized from a common high-
voltage electrical set. However, no more than one field in
any cell should be energized from the same high-voltage
electrical set, because a short would affect more than one
field in the same cell, causing a substantial reduction in
collection efficiency. In general, one high-voltage elec-
trical set is used for up to 2,320 m2 (25,000 ft2) of col-
2 2
lecting surface. About 611 mA/1000 m (55 mA/1000 ft ) of
collecting surface is supplied.
2.2.5 Ash Hoppers
Whether suspended from the casing or supported directly
on the substructure that is interposed between the casing
and the support steel, hoppers are required for collection
and temporary storage of the collected dust. The simplest
and most common hopper is pyramidal, converging to a round
2-17
-------
or square discharge. Frequently, the hoppers are baffled at
the division between two dust-plate sections, to prevent gas
bypassing the treater.
Hoppers must be kept clean and dry. Although many
designs do not require vibrators, which are costly and
require maintenance, it may be prudent to install mounting
provision for vibrators initially, to avoid later costly
removal of insulation and lagging if operation shows that
vibrators are needed.
Moisture-laden dust that hits cold steel hoppers has a
tendency to stick. Therefore, insulation of hoppers is
vital. Insulation is sometimes not sufficient, however, and
additional heating of the hoppers may be required for
effective performance.
When a baffle extends too far down into a hopper, it
may act as a "choke," causing bridging between the baffle
and one or both sides of the hopper. Stopping the baffle a
liberal distance 0.6 m (2 ft) clear of the sloping hopper
wall should prevent gas bypassing. A gas sweep under a
baffle of this type, considering the pressure drop of the
turn, is probably a symptom of poor gas distribution to the
precipitator (that is, a downward jet at the entrance).
Access to hoppers should be by external, key-inter-
locked doors. Bolt-on doors through baffles should be
2-18
-------
avoided because of the dangerous possibility of dust accumu-
lation on the far side of the door. Liberal "poke-hole"
ports should be provided to allow for clearing a blockage at
the discharge.
Level alarms are extremely valuable, provided they are
kept in working order. Too often, because they are located
near the top of the hoppers (even so high as to place them
above the bottom of the structural steel supporting the
precipitator), they are inaccessible for periodic inspection
and maintenance. Also, the temperature in this confined
area may be so high as to cause the alarm mechanisms to
fail; this point should be considered critically and in
detail before installation of the alarm instrument.
Hopper capacity should be checked carefully to provide
reasonable time for minor maintenance of the dust-removal
system. Generally, hoppers are designed to accumulate a 24-
hour load of particulate.
Alignment of the conveyors is important, depending to a
great extent on the alignment of hopper connections. Because
of the difficulty of erecting multiple hoppers to close
alignment tolerances, field-adjustable flange connections
are recommended. Also, the designer should not overlook
provision for expansion between hopper connections and
conveyor troughs.
2-19
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3.0 MALFUNCTIONS
Many ESP equipment components are subject to failure or
malfunction, leading to an increase in emissions. These
malfunctions may be caused by faulty design, installation,
or operation of the ESP; they may entail minor or severe
problems with the ESP system. This section identifies
several types of ESP malfunctions, giving probable causes
and corrective actions. A survey of ESP operating experience
of 63 electric utilities is analyzed.
3.1 TYPES OF ESP MALFUNCTIONS
ESP malfunctions can be classified as electrical, gas
flow, rapping, or mechanical problems. Table 3-1 lists
common problems associated with ESP's, their effect on
emissions, corrective actions, and preventive measures.
3.1.1 Discharge Wire Breakage
Probably the most common problem associated with
suspended wire electrode type ESP's is wire breakage, which
typically causes an electrical short circuit between the
high-tension discharge wire system and the grounded collec-
tion plate. This electrical short trips the circuit break-
er, disabling a section of the ESP, which will remain dis-
3-1
-------
Table 3-1. SUMMARY OF PROBLEMS ASSOCIATED WITH ESP'S
&1function
Cause
Effect on ESP Efficiency*
Corrective
action
Preventive
measure*
X. Poor electrode aligruaent
2. BrcXea electrodes
U)
I
3.
Distorted or skewed
electrode plates
Vibrating or
electrodes
•winging
1) Poor design;
2) Ash buildup on frame hoppers;
3} Poor gas flow
1) Wire not rapped clean, causes an arc
which embrittles ^..id burns through
the wire
2) Clinktzred wire. Causes: a) poor flow
area, distribution through unit is
uneven; b) excess free carbon due to
excess air above co.T-bustion require-
ments or fan capacity insufficient
for demand required; c) wires not
properly centered; d) ash buildup re-
sulting in bent frame, same as c) ;
e) clinker bridges the plates & wire
shorts out; f) asti buildup, pushes
bottle weight up causing sag in the
wire; g) "J" hooks have improper
clearances to the hanging wire; h) bot-
tle weight hangs up during cooling
causing a buckled wire; i) ash build-
up on bottle weight to the frame
forms a clinker and burns off the wire
1) Ash buildup in hoppers
2) Gas flow irregularities
3} High temperatures
1) Uneven gas flow
( 2} Broken electrodes
Can drastically affect performance,
and lower efficiency
Reduction in efficiency due to reduced
power input, bus section unavailability
Realign electrode*
Correct gas flow
Replace electrode
Reduced efficiency
Decrease in efficiency due to reduced
power input
Repair or re-
place plates
Correct gas flow
Repair electrode
Check hoppers frequently
tor proper operation
Boiler problens: check space
between recording stea.3 * air
flow pens, pressure gauges; fouled
screen tubes.
Inspect hoppers
Check electrodes frequently for vea
Jnopect rappers frequently
Check hoppers frequently for
proper operation; check electrode platafl
during outages
Check electrodes frequently
for wear
* The effects of precipitation problem can only be discussed on a qualitative basis. There are no known *mi«»ioa
teats of precipitator* to determine performance degradation as a function of operational problem*.
-------
Table 3-1 (Continued). SUMMARY OF PROBLEMS ASSOCIATED WITH ESP'S
Malfunction
Cause
Effect on ESP Efficiency*
Corrective
action
Preventive
measures
U>
I
u>
5, Inadequate level
ef paver inptit
{voltage too low)
6. Back corona
7. Broken or cracked insulator
or flower pot bushing
leakage
8. Air inleakaga throu9h
hcpp«rs
9. Air inleakag* through ESP
shell
10, Gas bypass around ESP:
- dead passage above
plates
- around high tension
f rane
11. Corrosion
1) High dust resistivity
2) Excessive ash on electrodes
3) Unusually fine particle size
A] Inadequate power supply
5) Inadequate sectionalization
6) Improper rectifier end control operation
7) Misalignment of electrodes
1) Ash accumulated on electrodes •» causes
excessive sparking requiring reduction
in voltage charge
1) Ash buildup during operation causes
leakage to ground
2) Moisture gathered during shutdown
Or low load operation
1) From dust conveyor
1) Flange expansion
1) Poor design - improper isolation
of active portion of ES?
1} Temperature goes below dew point
Reduction in efficiency
Reduction in efficiency
Reduction in efficiency
Lower efficiency - dust reentrained
through ESP
Same as above, also causes intense
sparking
Only few percent drop in efficiency
unless severe
Negligible until precipitation interior
plugs or plates are eaten away; air leaks
may develop causing significant drops in
performance.
- Clean electrodes;
gas conditioning
in temp, to re-
duce resistivity?
Increase section-
alization
Sane as above
Clean or replace
insulators £
bushings
Seal leaks
Baffling to direct
gas into active
ES? section
Maintain flue gas
temperature above
dew point.
Check rangd of voltages
frequently to nuke sure they
are correct
In situ resistivity measurements
Cheek frequently
Clean and dry as needed; check for
adequate pressurixaticn of top housing
Identify early by increase in ash concen-
tration at bottom of exit to ESP
Identify early by measurement of gas
flow in suspected areas
Energite precipitator after boiler systea has been
on line for »apl« period to r*ise flu* gas tam-
erature &bov« acid d»w point.
* The effects of precipitation problems can only be discussed on a qualitative basis. There are no known emission
t*«t» of precipitator* to determine performance degradation as * function of operational problem*.
-------
Table 3-1 (Continued). SUMMARY OF PROBLEMS ASSOCIATED WIH ESP'S
Malfunction
12. Hopper pluggag.*
13. Inadequate rapping t
vibrators fail
14. Too intense rapping
15. Control failures
16. Sparking
Cause
1) Wires, plates , insulators fouled
because of low temperature
2} Inadequate hopper insulation
4} Boiler leaks causing excess ir.oisture
malfunction ) blower ir.alf unction
} solenoid valves
7} Material dropped into hopper - from
bottle weights
8) Solenoid, timer malfunction
9) Suction blower filter not changed
1) Ash buildup
2) Poor design
3) Sappers roisadjusted
1) Poor design
2) Rappers nisadjusted
3) Improper rapping force
1) power failure in primary system
a. insulation breakdown in trans-
former
b. arcing in transformer between
high voltage switch contacts
d. insulating field contamination
1 ) Inspection door ajar
2) Boiler leaks
3) Plugging of hoppers
4) Dirty insulators
Effect on ESP Efficiency*
Reduction in efficiency
Resulting buildup on « lac t rode & Day
reduce efficiency
Been trains ash, raducas efficiency
Reduced efficiency
Corrective
action
Provide proper
flow of ash
Adjust rappers with
ESP exit Btrean
Same as No. 13
Find source of
failure and
repair or replace
in boiler; unplug
hoppers ; clean
insulators
measures
Frequent checks for adequate operation
of hoppers.
Provide h*at«r thenaal insulation
to avoid &oistur« conder.sition
Frequent checks for adequate operation of
Same as No, 13
Reduce vibrating or impact forco
room instrumentation to spot deviations
normal readings
Regular preventive nuintenance will *\11«
control
froa
viate
U)
I
• The effect! of precipitation problems can only be discussed on a qualitative basis. There are no known emission
tests of precipitator. to determine performance .degradation as a function of operational problems.
-------
abled until the broken discharge wire is removed from the
unit.
Following are the principal causes of discharge wire
breakage:
1) Inadequate rapping of the discharge wire causing
an arc, which can embrittle the wire and eventu-
ally break it completely.
2) Clinkered or improperly centered wires causing a
continual spark from the wire to the bracing.
3) Clinker or a wire that bridges the collection
plates and shorts out the wire.
4) Ash buildup under the wire, causing it to sag and
short out.
5) Improper clearance of "J" hooks to the wire, caus-
ing it to short out.
6) Hangup of a bottle weight during cooling, causing
a wire to buckle.
7) Fly ash buildup on a bottle weight, which forms a
clinker or burns off the wire.
8) Corrosion around cooler areas of the wire caused
by condensation.
9) Excessive localized sparking causing erosion of
the wire.
Electrical erosion, the predominant cause of failures,
occurs when repeated electrical sparkovers or arcs occur in
a localized region. A sparkover causes localized heating
and vaporization of a minute quantity of metal with each
spark. If the sparkover occurs at random locations, no
serious degradation of the discharge electrode occurs. If
3-5
-------
the sparkover occurs repeatedly at the same location, how-
ever, significant quantities of material can be removed,
with subsequent reduction of cross-sectional area and
ultimate failure at that point.
Localized sparking can be caused by misalignment of the
discharge electrodes during construction or by electric
field variations caused by "edge" effects where the discharge
and collection electrodes are adjacent to each other at the
top and the bottom of the plates. Corrective measures for
eliminating failure at these points are adding shrouds, such
as those shown in Figure 3-1, and providing a rounded sur-
face at the edge of the collection electrode to reduce the
tendency for sparking.
Electrical erosion can also be caused by "swinging"
electrodes, which can occur when the mechanical resonance
frequency of the discharge wire and weight system is harmon-
ically related to the electrical frequency of the power
supply. The power supply adds energy to the swinging wire
and it continues to approach the collection plate with
sparking occurring at each close approach. This action
leads to erosion of the electrode and mechanical failure.3
Poor workmanship during construction can also cause
electrical failures of the discharge electrode. If pieces
of the welding electrode remain attached to the collection
3-6
-------
(Source: Rof. 3)
Figure 3-1. Shrouds for wire-weighted discharge
electrodes.
3-7
-------
plate, localized electric field deformation can lead to
sparking and ultimate failure of the discharge electrode.
Mechanical fatigue occurs at points where wires are
twisted together and a continued mechanical motion occurs at
one location. This situation is found at the top of a dis-
charge electrode where the wire is twisted around the sup-
port collar. Methods of reducing mechanical fatigue include
selection of discharge electrode material that is less
susceptible to cold work annealing after attachment or
modification of the design of the corona wire attachment.
Chemical attack is caused by a corrosive material in
the flue gas, as is the case with high-sulfur coal and low
flue gas exit temperatures near the acid dew point. Another
cause of corrosion is use of ambient air to purge insulator
compartments, causing the temperature to drop below the acid
dew point in a localized region. Corrosion can be minimized
with higher flue gas temperatures or by use of hot, dry air
to purge insulator compartments. Use of good insulation on
the ESP shell to maintain high temperature also provides
adequate protection within the usual range of temperatures
and sulfur contents.
The other causes of discharge wire failure, such as
inadequate rapping, could be minimized by routine checking
of vibrators and rappers. Inspection may help to prevent
3-8
-------
wire failures and tripouts by detecting potential problems
before they become serious. Because of the large number of
wires contained in an ESP, however, some discharge wire
failures can be expected even with good design and preven-
tive maintenance.
3.1.2 Collection Hoppers and Ash Removal
Hoppers and ash removal systems often constitute pro-
blems in precipitator operation. If the hoppers become
full, the collected dust may short-circuit the precipitator.
The power through the dust may fuse the dust, forming a
large clinker-type structure called a "hornet's nest."
This structure further interferes with ash removal and must
be removed. Most problems associated with hoppers are
related to providing for proper flow of the dust. Improper
adjustment of the hopper vibrators or failure of the conveyor
system are the usual causes of failure to empty the hoppers.
It may be necessary to provide heat and/or thermal insula-
tion for the hoppers to prevent moisture condensation and
resultant cementing of the collected dust.
Malfunctions of the evacuation and removal system
include ash water pump failure, water jet nozzle failure,
disengagement of vacuum connections, and failure of sequenc-
ing controls.
3-9
-------
The best preventive measure for an ash removal system,
aside from proper design, is a good program for operation
and maintenance. Since dust buildup can affect so many of
the ESP components, proper ash removal will eliminate or
minimize many of the most common ESP malfunctions.
Gas flow problems affect hoppers as a result of the
inleakage of air into hoppers from the dust conveyor systems.
This results in reentrainment of collected dust, which is
carried back into the ESP. Air inleakage can also occur
through the ESP shell or inlet flanges if operation is at
less than atmospheric pressure. Often enough air is bled in
to cause intense sparking.
'Gas sneakage1 is a term used to describe gas flow that
bypasses the effective ESP section. It can occur through
dead passages of the ESP above the collector plates, around
the high-tension frame, or through the hoppers. Gas sneakage
will reduce ESP efficiency by only a few percent unless it
is unusually severe. Gas sneakage can be identified by
measuring gas flows in suspected areas in a nonoperating or
cold test. Corrective measures usually involve baffling to
direct gas into the active ESP section.
Reentrainment of dust from hoppers caused by air in-
leakage or gas sneakage is often indicated by an increase in
dust concentration at the bottom of the exit to the ESP.
3-10
-------
Corrective measures for air leakage would include proper
design and fit of components and sealing of areas where the
inleakage occurs.
3.1.3 Rappers or Vibrators
Rapping is required for both discharge and collection
electrodes. These systems normally consist of electric or
pneumatic vibrators or electromagnetic or mechanical-impact
rappers.
In dry removal systems, rapping of the collection
electrode to remove dust is normally done periodically.
Successful rapping depends upon accumulation of material on
the plate thick enough that it falls in large agglomerates
into the hopper. Although there is always some reentrainment
of dust, effective rapping must minimize the amount of
material reentrained in the gas stream. Poor performance
can result from rapping forces either too mild or too severe.
Rapping that is too intense and frequent can result in a
clean plate, with the collected dust being reentrained
rather than falling into the hopper. Excessive gas velocity
or poor gas distribution can lead to turbulence, scouring of
the receiving electrode, and reentrainment of particles.
Inadequate rapping of the discharge electrodes can
result in heavy dust buildup with localization of the
corona, low corona current, and excessive sparking, as
3-11
-------
discussed previously. Poor gas flow and the condition of
the dust also can cause formation of deposits on discharge
electrodes. Often, deposits can reach to 5 cm (2 in) thick-
ness; they are generally composed of the finer dust particles
and often cling tenaciously to the discharge wire. Deposits
on the discharge wire do not necessarily lead to poor perfor-
mance, although depending on resistivity, power supply
range, and uniformity of the deposit, efficiency may be
reduced.
Variations in design of the support structure and of
the electrodes can also result in inadequate rapping.
Recent investigations of rapping acceleration in fly ash
ESP's have shown measured accelerations of 5 g when as much
9
as 30 g may be required. The first step in dealing with
problems related to rappers and vibrators would be to
determine the adequacy of the rapping acceleration with an
accelerometer mounted on the plates. A common method of
adjusting rappers is with the use of an optical dust-measur-
ing instrument in the ESP exit gas stream.
Discharge electrodes should be kept as clean as pos-
sible. Rapping intensity in this case is limited only by
the possibility of mechanical damage to the electrodes and
support structure.
3-12
-------
Generally, the vibratory types of cleaning mechanisms
require more maintenance than the impulse types.
3.1.4 Insulator/Bushing Failure
Suspension insulators are used to support and isolate
the high-voltage parts of an ESP. Inadequate pressurization
of the top housing the insulators can cause ash deposits
and/or moisture condensation on the bushing, which may
result in electrical breakdown at the typical operating
potential of 45 kv-DC.
Corrective or preventive measures include inspection of
ventilation fans for the top housing and availability of a
spare fan for emergencies. Frequent cleaning and checking
for damage of the fans by vibration is also necessary to
ensure trouble-free operation.
3.1.5 Inadequate Electrical Energization
Since an ESP operates on the basis of electric field
and electric charge, electrical energization must be ade-
quate to provide for particle charging, maintenance of the
electric field, and holding the collected dust to the
collection plates.
Among several possible causes for inability to achieve
the required level of power input to the ESP, the following
are most common:
1) high dust resistivity
3-13
-------
2) excessive dust accumulation on the electrodes
3) unusually fine particle size
4) inadequate sectionalization
5) improper rectifier and control operation
6) misalignment of electrodes
7) inadequate power supply range
If a precipitator is operating at a spark-rate-limited
condition but with low current and voltage, the problem
commonly can be traced to high-resistivity dust, electrode
misalignment, or uneven corona due to buildup on the dis-
charge electrode.
The effects of high resistivity are discussed in more
detail in Section 3.2 in terms of conditions specific to
utility industry, where resistivity presents the greatest
problem.
Because of the importance of resistivity in the pre-
cipitation process, in situ resistivity measurements should
be made as one of the first steps in troubleshooting. If
the resistivity is found to be high (more than 10 ohm-cm),
most of the difficulty may be due to this cause. If resis-
tivity is not high, other potential causes of abnormally low
currents should be 'investigated.
Typical values for normal power supply operation range
from 33 to 103 mA/1000 m (10 to 31 mA/1000 ft) of collecting
3-14
-------
surface. The spark rate should be adjusted to give about 10
to 100 sparks/min per section. The spark rate should be set
to give maximum average high-tension voltage, usually
resulting in spark rates in the range shown above.
If a precipitator is operating in a spark-limited mode
with abnormally low voltage on dust with resistivities less
than 10 ohm-cm, the problems are likely to be associated
with misalignment of electrodes, uneven deposits on the
discharge wire, or broken corona wires.
Occasionally, precipitators may be found to operate at
the maximum voltage or current settings on the power supply
with no sparking. This condition is likely to be associated
with the collection of low-resistivity dusts, where the
electric field in the deposit is insufficient to initiate
sparking. These installations are referred to as "power
hogs." The fact that the precipitator is not sparking does
not necessarily mean that the unit is underpowered. These
installations may have sufficient power to provide adequate
charging and collection electric fields without sparking.
If ESP efficiencies are low and tests show that sufficient
power is provided, then other disruptive conditions should
be sought.
Failures in ESP controls can prevent the system from
achieving the level of power required for normal operation.
Following are the most common malfunctions in controls:
3-15
-------
1) Power failure in the primary system.
2) Transformer or rectifier failure in secondary
system.
a. insulation breakdown in transformer
b. arcing in transformer between high voltage
switch contacts
c. leaks or shorts in high voltage structure
d. contamination of the insulating field
The most effective measure for correction of control
failures is a good maintenance program in which the controls
are checked periodically for proper operation. A daily log
of instruments that register current, voltage, and spark
rate can also indicate potential problems.
3.2 CONDITIONS SPECIFIC TO POWER PLANTS THAT CAUSE PROBLEMS
IN PRECIPITATORS
A number of operating conditions specific to power
plants can cause ESP problems, often requiring special
equipment designs. The major operating problems are caused
by startup, variable fuel quality, boiler malfunctions, fly
ash resistivity, temperature fluctuations, and large gas
volumes. Each is briefly discussed below.
3.2.1 Startup
Normally, during startup of a coal-fired steam genera-
tor equipped with an ESP, operation of the ESP must be
delayed until a certain exit gas temperature (about 110°C
(230°F)) is attained. This delay is necessary to protect
the ESP from corrosion and plugging, and to prevent secondary
3-16
-------
combustion (fires) due to unburned carbon in the flue gas
and ESP sparking. The latter is particularly important when
secondary liquid fuels are used during startup.
3.2.2 Fuel Quality Variability
Variable fuel quality is a major cause of changes in
operation that lead to fluctuating emissions. The quality
of fuel from a given source is continually changing. As
sulfur and ash content vary, so does the efficiency of the
ESP. Excess moisture in the coal can lead to wet fly ash,
which could interfere with the dampers and solidify in the
hoppers. Ash grindability, fineness and other coal charac-
teristics can cause combustion to deviate from the optimum,
requiring changes in such operating variables as register
settings and excess air.
Although the variability of sulfur and ash contents in
coal cannot be readily controlled, proper drying of the coal
before combustion can minimize the possibility of wet fly
ash and the resultant problems.
3.2.3 Boiler Malfunctions that Indirectly Affect ESP
Performance
Firebox flameout, coked or burned burner impellers,
improper combustion due to faulty fans or dampers, irregular
fuel flow to coal mills, pulverizer problems, excess slag
buildup in the firebox, and soot blower usage all can upset
precipitator performance and cause emissions to increase.
3-17
-------
Usually, these upsets are of short duration. Proper main-
tenance can minimize these types of boiler malfunctions
and increase ESP reliability.
3.2.4 High Resistivity of Fly Ash Resulting 'from Low
Sulfur Content in Coal
High resistivity, which is characteristic of low-sulfur
coal, causes uncertainties in proper sizing of a cold ESP.
In addition, many operating problems can be traced directly
or in part to high resistivity.
High dust resistivity affects ESP efficiency principally
by limiting the voltage and current at which the ESP operates,
If the ESP electrodes are clean, the high-tension voltage
can be increased until a sparking condition is reached. The
maximum voltage is determined principally by gas composition
and ESP dimensions.
If dust is deposited on the collection electrode, the
voltage at which sparking occurs is decreased because of the
increased electric field at the dust surface. If the re-
sistivity of the dust layer is increased, the voltage at
which sparking occurs will be further reduced, as shown in
Figure 3-2. Finally, at very high values of dust resistivity
12
(10 ohm/cm), the voltage will be reduced enough that
sparks will not propagate across the interelectrode space.
Under these conditions, the gas in the interstitial regions
of the dust layer will break down at very low values of
3-18
-------
10'
u
5
I
O
«
>
t-
>
to
IL)
oe
5x10
to
10"
SPARK
SPARK
10 20 30
APPLIED VOLTAGE, K.V.
40
50
(Source: Ref. 10)
Figure 3-2. Typical sparking levels when precipitating dusts
with different resistivities.
3-19
-------
applied voltage and current density, resulting in a back
corona. The positive ions resulting from this corona flow
toward the discharge electrode and neutralize the negative
charge previously applied to the dust particles; performance
of the ESP is thereby limited.
Back corona results in an increase in current at low
voltage and is manifested visibly as a diffuse glow at the
surface of the dust layer. Although visual verification is
usually very difficult, back corona can be observed under
very dark conditions.
If the electrical resistivity of the particulate is in
the intermediate range (10 -10 ohm-cm), the electrical
behavior is somewhat different in that the electrical break-
down occurs at a somewhat higher applied voltage. If the
applied voltage is sufficiently high when the electrical
breakdown occurs in the dust layer, then an electrical
sparkover between the collection and corona electrode occurs.
The electrical breakdown in the layer tends to begin in a
localized region. This localized breakdown behaves as a
small radius of curvature electrode (a point). This leads
to electrical sparkover at reduced voltages in the operating
device, again forcing the electrostatic precipitator to
operate at reduced voltage and current density.
3-20
-------
The reduction in electrical conditions is much more
severe with back corona than with sparkover at reduced
voltage.
In addition to electrically limiting the performance of
an ESP, high-resistivity dust can cling much more tena-
ciously to collection electrodes than an intermediate-
resistivity dust. Therefore, a much greater rapping accel-
eration must be applied to the electrode to remove the dust
layer. This increased acceleration may be so great as to
cause severe reentrainment of the dust, or damage to the
precipitator if it is not designed to accommodate such high
acceleration.
Corrective procedures for ESP's that are limited by
high-resistivity ash include collection at low temperatures
[105-110°C (220-230°F)J, use of very large ESP's, increased
sectionalization, and use of conditioning agents. "Cond-
itioning agents" have been used for many years to improve
the collection of particulate substances in electrostatic
I O
precipitators. ' Normally, the use of a conditioning agent
is expected to overcome the problems associated with high
electrical resistivity. In some instances, however, condi-
tioning agents may alleviate other problems stemming from
adverse particulate properties, one being unacceptably low
resistivity.
3-21
-------
The best known conditioning agent is sulfur trioxide or
the chemically equivalent compound, sulfuric acid. In most
applications, sulfur trioxide is effective in lowering
electrical resistivity by surface deposition along with
water vapor on gas-borne particles. In conditioning fly ash
in power plants, it supplements the small quantity of sulfur
trioxide that is produced naturally when low-sulfur coals
12 13
are burned. ' On the other hand, in some power plants
where sulfur content of the coal is not especially low and
the fly ash resistivity is not low enough to be detrimental
to electrostatic precipitation, the use of sulfur trioxide
as a conditioning agent may be of value in increasing the
cohesiveness of fly ash particles and thus minimizing re-
entrainment losses from the collection electrodes. Evidence
of sulfur trioxide conditioning through this mechanism has
been reported in a publication from the Central Electricity
14
Research Laboratories in Great Britain; further evidence
of this effect has been obtained in recent studies by
Southern Research Institute.
Other conditioning agents, not as well known as sulfur
trioxide, include ammonia, ammonium sulfate, ammonium bi-
sulfate, and sulfamic acid. Of these compounds, ammonia has
been most widely used in the utility industry. Experience
with ammonium sulfate, ammonium bisulfate, and sulfamic acid
3-22
-------
has thus far been relatively limited.16 These compounds
occur at normal temperatures and pressures as solids. They
are injected into flue gas in the form of either a fine
powder or an aqueous solution.
As an alternative to the above-listed corrective pro-
cedures, use of a hot ESP will largely eliminate the prob-
lems.
3.2.5 Temperature Fluctuations
During frequent startup and shutdown operations, or
with a boiler that is used mostly for peak loads, the flue
gas exhibits a large temperature gradient. If the tempera-
ture drops below the dew point of sulfuric acid, corrosion
can occur. In such operations, control of temperature
changes is difficult but corrosion can be minimized by
covering the interior surfaces of side frames, end frames,
and roof of the ESP with gunite.
Another effect of temperature fluctuations is reentrain-
ment of fly ash, resulting in excessive fouling of wires,
plates, and insulators. This fouling leads to ash hopper
plugging, high current, leakage, and excessive power require-
ment for the discharge electrodes. The most effective
measure for dealing with unfavorable temperature fluctua-
tions is to evaluate the range of expected gas temperatures
when designing the ESP and to provide a means of reaching
3-23
-------
the optimum gas temperature for proper ESP operation. After
a unit is installed, temperature changes can be dealt with
by increasing the size of the ESP with add-on equipment.
3.2.6 Large Gas Volumes
Utilities treat larger volumes of gas than most other
industries. Since large gas volumes require large precipi-
tators, space considerations may become critical. In
addition, as more bus sections are required, the precipitator
becomes more complex and the chances for problems increase.
3.3 REDUCED ESP COLLECTION EFFICIENCY AS RELATED TO
NUMBER OF BUS SECTIONS NOT IN OPERATION
Although ESP collection efficiency is reduced by
malfunctions such as discharge wire breakage and deteriora-
tion of power supply equipment, rectifiers, insulators, and
similar equipment, a unit can often be kept in compliance
with particulate emission regulations by reducing boiler
load. Figure 3-3 (top graph) illustrates collection effi-
ciency of a four-field ESP with 24 bus sections as a func-
tion of the gross boiler load, depending on the number of
bus sections out and whether they are in series or parallel.
The bottom graph shows the efficiency needed by the ESP to
meet a state regulation of 0.16 g/MJ (0.38 Ib/MM Btu) as a
function of the ash content of coal [(assuming 25.6 MJ/kg
coal (11,000 Btu/lb coal)].
3-24
-------
u>
KJ
en
S9.0
I 98.0
o
*-l
u.
u.
Ul
97.0
36.0
95.0
»«
98-°
<*3 97.0
s*
I"
gK 96.0
u.
Ul
95.0
140
10
CURVE A
160 180
200 220 240
GROSS LOAD - MW.
260
MOISTURE..
280 300
CURVE B
MA A4B AB4A AB4B BAA BAB
A3A A3B AB3A AB3B B3A B3B
A2A A2B AB2A AB2B B2A B2B
A1A A1B ASIA AB1B B1A BIB
COLLECTOR
SECTIONS OUT
0
1 OUT
2 OUT IN PARALLEL
3 OUT IN PARALLEL
4 OUT IN PARALLEL OR
2 OUT IN A SERIES
5 OUT IN PARALLEL OR
2 OUT IN A SERIES & 1 OUT IN PARALLEL
6 OUT IN PARALLEL OR
2 OUT IN A SERIES & 2 OUT IN PARALLEL
EXAMPLE: LOAD 290 MW.
SECTIONS OUT A1A, A2A, AB2A, BAB
(CURVE A) EFFICIENCY AT 290 MW. WITH
2 OUT IN SERIES & 2 OUT IN PARALLEL • 95.3
COAL - ASH 14%
MOISTURE 10X
(CURVE E) EFFICIENCY REQ'D. TO MEET
STATE REGULATIONS - 96.5%
TO MEET STATE REGULATIONS REDUCE
LOAD TO 210 KM.
12
14 16 18 20
PERCENT ASH IN COAL
22
24
Figure 3-3. Typical operating curve to meet emission regulations with
partial malfunctions of ESP.
-------
These types of graphs are extremely helpful to a utility
operator. Knowing the ash content of the coal he is firing
and knowing which bus sections of his ESP are inoperative,
he can easily tell from the top graph how much the boiler
load must be reduced to keep emissions in compliance with
regulations. Charts of this type must be developed for each
boiler-ESP combination.
3.4 MAINTAINABILITY OF ESP EQUIPMENT AS RELATED TO
FREQUENCY OF MALFUNCTIONS '
To assess the experience of the major industries iri
recent years in operation and maintenance of ESP's, the TC-1
Committee of the APCA embarked upon a survey in 1974.
Four major industries were canvassed: electric utilities,
cement, paper, and metallurgical. This section is restricted
to data from the electric utility industry.
Sixty-three electric utilities reported on eighty-eight
ESP's. The service life of this equipment was not given,
but the average service for the study ranged from 7 to 10
years.
The first point of inquiry dealt with the overall
experience with ESP's from an operational and maintenance
viewpoint. Responses were as follows:
Excellent
Good
Fair
Poor
Operation
14.8%
45.5%
29.5%
10.2%
Maintenance
13.6%
52.3%
13.6%
20.5%
3-26
-------
The second question dealt with specific areas of poten-
tial difficulty. With respect to failure of discharge
electrodes, results were as follows:
Frequency of Discharge Electrode Failures
Frequent - 29.5%
Infrequent - 38.6%
Very Seldom - 28.4%
Of the three major types of failures normally experi-
enced (fatigue, corrosion, electrical arcing), 61.7 percent
of all industries indicated that electrical erosion (arcing)
was the major cause. Corrosion and fatigue ranked second
and third respectively. This ranking is probably typical of
the electric utility industry.
Failure in the rapping system, which usually includes
electric or pneumatic vibrators or electromagnetic or mechani-
cal impact-type rappers, was evaluated as follows:
Frequency of Rapper or Vibrator Failures
Frequent - 9.1%
Infrequent - 38.6%
Very Seldom - 47.7%
As would be expected, the data indicated that the
vibratory type of mechanism requires more maintenance than
the impulse type.
Frequency of problems created by collecting surfaces
was listed as follows:
3-27
-------
Frequent - 4.5%
Infrequent - 7.9%
Very Seldom - 68.2%
The major cause cited for collecting surface failure was
fatigue at the points of plate suspension. Corrosion was
ranked as the second major cause.
Removal of dust, once precipitated, is historically one
of the major causes of precipitator malfunction, contribu-
ting also to other difficulties, such as discharge electrode
failure. The survey showed the following frequency of dust
removal problems:
Frequent - 36.4%
Infrequent - 42.0%
Very Seldom - 20.5%
By far, the majority of problems cited were with plugging of
the dust hopper. Difficulties with screw conveyors and dust
valves were ranked second and third.
Suspension insulators, manufactured of glazed porcelain,
fused silica or alumina oxide, are used to support and
isolate the high-voltage elements of a precipitator. These
insulators are vulnerable to failure due to electrical arc-
over resulting from accumulations of dust or moisture.
Regarding problems with suspension insulators, the utilities
indicated the following:
Frequent - 8.0%
Infrequent - 34.1%
Very Seldom - 48.9%
3-28
-------
It is apparent that this is not a significant source of
operational difficulty.
The final phase of the survey asked the user's opinion
of which ESP components were the major cause of trouble in
his experience, in terms of both reliability and expense.
The responses, again based on 63 utilities reporting on 88
ESP's, were as follows:
Major Maintenance Problems
Discharge Electrodes - 35.2%
Dust Removal Systems - 5.7%
Rappers or Vibrators - 13.6%
Collecting Plates - 31.8%
Insulators - 1.1%
Several conclusions may be drawn from the results of
this survey:
1. Although precipitator manufacturers obviously can
improve their products, most of the utilities
reporting are satisfied with the precipitator as
a functioning piece of equipment. Only 10.2
percent gave a "Poor" rating.
2. Discharge electrodes are the principal source of
malfunction, requiring application of design
expertise. Recognizing this, ESP manufacturers
are concentrating on design improvements.
3. Design, operation, and maintenance of the dust
removal system are extremely important. The
utilities reported a high incidence of discharge
electrode failure along with a high degree of
hopper pluggage. Dust buildup into the high-
voltage system, in addition to inhibiting perfor-
mance, can accelerate failure of discharge elec-
trodes.
3-29
-------
The TC-1 Committee of the APCA suggested close coopera-
tion between user and supplier, coupled with exchange of
information between the various user industries. Such a
program could lead to mutual development of an electrostatic
precipitator that fills the needs of the users.
The survey was only the initial phase of a comprehensive
study of experience with high-efficiency collectors of
various types. The survey data are considered preliminary,
in that considerably more detail can be derived statistic-
ally. The TC-1 Committee is continuing work with this data
base and intends to report additional findings, conclusions,
and recommendations.
3-30
-------
4.0 MAINTENANCE
This section presents a program for ESP surveillance
and maintenance that could enable a utility to reduce mal-
functions and downtime. It describes typical procedures
followed by utilities during startups and malfunctions, and
discusses costs of ESP maintenance.
4.1 MAINTENANCE PROGRAM FOR PRECIPITATORS
Table 4-1 lists items to consider in establishing
maintenance procedures for ESP's. Table 4-2, given at the
end of this subsection, is a troubleshooting chart for use
in determining the cause of common ESP malfunctions, with
suggestions for remedying these problems.
4.1.1 Operational Procedure
9
Prestartup Inspection - Before implementing startup proce-
dures, all precipitator components must be thoroughly in-
spected to ensure that equipment is ready for operation.
General
1. Visually inspect the mechanical dust collector units,
induced draft fans, and dust handling equipment.
2. Close and secure all access hatches.
3. Determine that all internal areas are completely free
of tools, scrap and foreign material before the fan(s)
is started.
4-1
-------
Table 4-1. MAINTENANCE ITEMS FOR ELECTROSTATIC PRECIPITATORS
A. Daily Log
1. Boiler operating parameters
2. Flue gas analysis
3. Coal characteristics
4. Particulate collector control readings
5. Transmissometer calibration
B. Daily
1. T-R electrical control set readings
2. Rapper and vibrator control settings
3. Ash removal system
4. Check T-R control room ventilation system
C. Weekly
1. Check rappers and vibrators visually for proper
operation
2. Check control sets internally for dirt
3. Make sure air filters to control sets and precipi-
tator top housing are not plugged
D. Monthly Log
1. Check precipitator top housing for pressurization
2. Check standby fan operation manually
E. Quarterly
1. Clean and dress HW-FW electrical distribution
contact surfaces
2. Lubricate pivots
F. Semi-Annually
1. Clean and lubricate access door hinges and test
connections
4-2
-------
Table 4-1 (cont'd). MAINTENANCE ITEMS FOR
ELECTROSTATIC PRECIPITATORS
2. Perform exterior inspection for loose insulation,
corrosion, loose joints, etc.
3. Check for gas leakage points in or out
G. Annually
1. Perform thorough internal inspection
a. check for possible leaks of oil, gas or air
at gasketed connections
b. check for corrosion of any component
c. check for broken or misaligned wires, plates,
insulators, rappers, etc.
d. check high voltage switch gear and interlocks
e. clean all insulators and check for hairline
cracks or tracking
f. check expansion joints on hot precipitators
2. Check for signs of hopper leakage, reentrainment
of particulate, and poor gas distribution
3. Check for dust buildup in inlet and outlet flues
4. Check for dust buildup in hoppers
4-3
-------
4. Verify that primary power is available to thermostati-
cally controlled heaters if provided. Circuit breakers
for this equipment may have to be energized several
hours prior to system operation.
5. Check all interlocks and voltage control modules.
6. Check main OFF/TEST Selector Switch and place in OFF
position.
7. Check grounding connections.
Rapper System
1. Ground the power unit in the control cubicle.
2. Check distributor switch rapper connections.
3. Check ground return leads for proper connections to
sectiorialized control adjustments.
4. Check for proper mechanical adjustment.
5. Adjust each manual sectional control for proper rapping
intensity.
6. Check spark rate feedback circuit and signals for
proper connections.
Rectifiers and Transformers
1. Check all connections, switches, and insulators.
2. Check oil (liquid) levels.
3. See that high-tension duct vent ports are installed and
free.
4. Be sure grounds are completed on transformer-rectifiers,
bus duct, and conduits.
Routine Startup
If hot gases are to be passed through the precipitator,
the system should be warmed to operating temperature before
gas flows are started. (See Section 4.2). The following
procedures are then performed.
4-4
-------
Close all inspection ports and adjust dampers for
proper air flow.
Energize high-voltage current.
Start collector and discharge electrode rappers, if
provided on the system.
Turn on ash discharge system.
Bring fan to full rpm with exit damper closed.
Adjust damper for desired gas flow.
Record system pressure drop and fan pressure drop.
If the system is not equipped with external heating
facilities, the procedure should be reversed so that the
inlet gases enter before the precipitator is energized.
When the precipitator reaches operating temperature, turn on
the high voltage power.
If the precipitator contains air and a potentially
explosive gas mixture is introduced into the unit, the
system must first be purged with an inert gas.
Any conveying systems that follow the hopper conveyor
must be turned on before the hopper conveyor.
Routine Operation
Check pressure drops to prevent unusually high or low
values.
Maintain precipitator current at normal level. Correct
any deviations greater than about 5 percent of normal current.
4-5
-------
Maintain sparking rate at optimum density.
Maintain rapper frequency and intensity to give maximum
collection efficiency.
Check to see that thermostatically controlled heaters
are properly operating.
If combustible gases are present in the precipitator,
check the gas composition to be sure it is not in the
explosive range.
See that all water cooling requirements are properly
met, including any water-cooled bearings or pump stuffing
boxes. Check all drive belt tensions daily. Periodically
test fan inlet dampers to verify that the dampers are free
to move to fully closed and fully opened positions. Inspect
all electrode and collector rapper mechanisms daily for
defective systems. Examine insulators daily for potential
deterioration. Lubricate hopper conveyors and valves daily.
Lubricate dampers and louvers to see that they function
freely.
Instrumentation to register current, voltage, and
spark rates to the precipitator can be red-lined to indicate
norms. Abnormal readings indicate trouble; if these signs
are ignored, serious equipment failures can result in exces-
sive emissions and plant shutdown. Shutdowns can usually be
prevented if operators heed warning signals.
4-6
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Routine Shutdown
Shutdown is performed in reverse fashion from startup.
Deenergize the precipitator, purge if necessary, then shut
off gas flow. When all collectors and electrodes have been
rapped clean, discontinue use of rappers. When hoppers are
empty, turn off conveyors and discontinue any liquid washing.
Q TO
4.1.2 Maintenance '
Maintenance of precipitators falls into categories of
preventive maintenance and maintenance to correct failures.
A preventive maintenance schedule should be established for
each installation, detailing the precipitator parts to be
checked and maintained daily, weekly, monthly, quarterly,
semiannually, annually, and on a situational basis.
Daily - It is obvious that gross departures from normal
readings on the transformer-rectifier meter and transmis-
someter indicate trouble. It is not so widely recognized
that small variations, often too slight to be noticed with-
out checking of daily readings, can indicate impending
trouble.
Problems that usually have a gradual, rather than
sudden, influence on precipitator performance include (1)
air inleakage at air heaters or in ducts leading to the
precipitator, (2) dust buildup on precipitator internals,
and (3) deterioration of electronic-control components.
4-7
-------
Such problems can be indicated by small, but definite, drift
of daily meter readings away from baseline values.
Grossly abnormal readings indicate a serious problem,
and also may aid in diagnosing the probable cause. For
example, sudden tripout of an apparently normal electrical
set probably indicates a short or ground in the secondary
circuitry. A low but steady voltage reading indicates a
high-resistance ground - such as that from discharge wires
to ground through ash accumulating above a plugged hopper or
from clinker formation on a wire.
Fluctuating voltage, dipping to low values, suggests a
broken and swinging discharge electrode. Fluctuation of
spark-rate meter readings does not necessarily indicate a
problem unless there is confirmation by fluctuating voltage
and/or current readings.
Operators should never try to correct deviant meter
readings by adjusting control set points. An automatic-
control response range should accommodate normal variations
in load conditions. If major changes occur, such as would
result from switching to a coal substantially different from
that for which the precipitator was designed, the precipita-
tor manufacturer should be called in to retune the installa-
tion. If no such major changes have occurred, then variant
meter readings indicate problems that must be detected and
corrected.
4-8
-------
Probably 50 percent of all electrical set tripouts are
caused by ash buildup. Short of set tripout, buildup above
the top of hoppers can cause excessive sparking that erodes
discharge electrodes. Further, the forces created by grow-
ing ash piles can push internal components out of position,
causing misalignment that may drastically affect performance.
Field engineers note that utilities sometimes attempt to
preserve alignment by welding braces to hold collecting-
electrode plates in position. This practice is inadvisable,
since restraining the plates interferes with the effective-
ness of the rapping action that keeps them clean.
Although various indicators and alarms can be installed
to warn of hopper-ash buildup and of ash-conveyor stoppage,
the operator car doublecheck by testing skin temperature at
the throat of the hopper with the back of the hand. If the
temperature of one or more hoppers seems comparatively low,
the hopper heaters may not be functioning properly. Gener-
ally, however, low temperature indicates that hot ash is not
flowing through the hopper and that bridging, plugging, or
failure of an automatic dump valve has held ash in the
hopper long enough for it to cool. The ash collected sub-
sequently will pile up at the top.
If the temperature of all hoppers appears low to the
touch, the operator should check the ash-conveyor system to
4-9
-------
see if it has stopped or if dust agglomeration is so great
that the conveyor can no longer handle all of the fly ash.
Hopper plugging is sometimes caused by low flue-gas
temperature, which permits moisture condensation. This
results from carrying the boiler exit-gas temperature too
low or from excessive leakage of ambient air into the flue-
gas duct. Hoppers are particularly prone to plugging during
startup after an outage, when they are cold and normally
damp.
Daily checking of the control-room ventilation system
minimizes the possibility of overheated control components,
which can cause drift of control set points and accelerated
deterioration of sensitive solid-state devices.
Weekly - Solenoid-coil failures, fairly common when
high voltage was used, are rare with modern low-voltage i
equipment. Still, a weekly check of all units is advisable.
Rapper action should be observed visually, and vibrator
operation confirmed by feel. In addition, since rapping
accelerations of 30 g are often required for proper collec-
tion, an accelerometer mounted on the plates should be
checked to verify that rapping acceleration is adequate.
This is best done on a pretest check.
Control sets must be checked internally for deposits of
dirt that may have penetrated the filter. Accumulation of
4-10
-------
dirt can cause false control signals and can damage such
large components as contactors and printed circuits.
Finally, filters in the air supply lines to control
cabinets and the precipitator top housing should be checked
and cleaned if necessary to prevent plugging.
Monthly - Most new precipitators incorporate pressurized
top housings that enclose the bushings through which high-
voltage connections are made to the discharge electrodes
within the precipitator box. Pressurization assures that,
if there is gas leakage where the bushings penetrate the
precipitator hot roof, gas flow will be into the precipita-
tor rather than out from it. Leakage from the precipitator
into the housing could cause ash deposits and/or moisture
condensation on the bushings, with risk of electrical
breakdown at the typical operating potential of 45 kV d.c.
Inspect bushings visually and by touch for component
vibration. Check differential pressure to be sure that the
fan that pressurizes the housing is in good operating condi-
tion. Also, operate manually the automatic standby fan to
make sure it is service-ready.
Quarterly - Quarterly maintenance includes inspection
of electrical-distribution contact surfaces, which should be
cleaned and dressed and the pivots lubricated, if this is
not done even more frequently. These could cause false
4-11
-------
signals. Further, since transmissometer calibration is
subject to drift, calibration should be verified to avoid
the possibility of false indications of precipitator per-
formance.
Semiannually - Routine inspection, cleaning, and lubri-
cation of hinges and test connections should be done semi-
annually. If this task is neglected, extensive effort
eventually will be required to free up test connections and
access doors, involving expensive downtime. Performance
tests may be required at any time, and should not be delayed
while connections are made usable. An effective preventive
measure is to recess fittings below the insulation.
Exterior inspection for corrosion, loose insulation,
exterior damage, and loose joints can identify problems
while repair is still possible. Special attention should be
given to points at which gas can leak out as fugitive emis-
sions.
Annually - Scheduled outages must be of sufficient
duration to allow thorough internal inspection of the preci-
pitator. Checks should be made for (1) possible leakage of
oil, gas, or air at gasketed connections, (2) corrosion
where heat loss is great or gas temperatures are low, and
(3) possible misalignment of internal components. Also,
high-voltage switchgear should be inspected for possible
4-12
-------
binding, misalignment, or defeated interlocks - defects that
create a safety hazard in addition to reducing performance.
All insulator support bushings, rapper insulators, and
antisway insulators should be cleaned and inspected for
hairline cracks and evidence of tracking. Faulty insulators
can cause excessive sparking and voltage loss, and can fail
abruptly, possibly even explode, if allowed to deteriorate.
If the precipitator is located between the air heater
and the boiler, expansion joints must be checked and slide
plates lubricated. Finally, if necessary, all collection
plates and electrode wires should be cleaned manually.
Situational - Certain preventive-maintenance and safety
checks are so important that they should be performed during
any outage of sufficient length, without waiting for sched-
uled downtime. Air flow readings should be compared with
baseline values to detect possible performance deteriora-
tion. Further, meter readings taken immediately upon
restoring the precipitator to service can serve as a check
on any changes that may have resulted from maintenance done
during the outage.
Critical internal alignments should be checked whenever
an outage allows and immediate corrective action taken if
misalignment is discovered. Control-cabinet and top-housing
interiors should be checked during any outage of 24 hours or
4-13
-------
more and cleaned if necessary. Any outage of more than 72
hours provides an opportunity to check grounding devices,
alarms, interlocks, and other safety equipment, and to clean
and inspect insulators and bushings.
Safety
It is obvious that high-voltage electricity can be
extremely dangerous. Therefore, all practical safety
measures must be observed even though the system incorporates
interlocks and other safety devices.
The system should never be adjusted with the high-
voltage power on.
Rectifiers and diodes have heat sinks that could seri-
ously shock a person touching them.
The rapper circuitry, which is independent of the high-
voltage circuitry, is nonetheless also dangerous and must be
treated as such.
Spark-rate feedback signals are often taken from the
primary of the high-voltage supply and can be 400 V a.c. or
more. Fuses on these lines should be removed before main-
tenance or adjustment is attempted.
Explosive gas mixtures could be created if air is
introduced into systems. If necessary, the system should be
purged with an inert gas before introducing air. In all
cases, a system should be purged with fresh air before it is
entered.
4-14
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Table 4-2. TROUBLESHOOTING CHART FOR ESP'S
Symptom
Probable cause
Remedy
1. No primary voltage
No primary current
No ESP current
Vent fan on
No primary current
No ESPcurrent
Vent fan off
Alarm energized
Control unit trips
out an overcurrent
when sparking
occurs at high
currents
High primary current
No ESP current
DC overload condition
Misadjustment of current
limit control
Overdrive of rectifiers
Fuse blown or circuit breaker
tripped
Loss of supply power
Circuit breaker defective
or incorrectly sized
Overload circuit incorrectly
set
Short circuit condition in
primary system
Too high ESP voltage for
prevailing operating
conditions
High-voltage circuit
shorted by dust buildup
between emitting &
collecting electrodes
Check overload relay setting
Check wiring and components
Check adjustment of current
Limit control setting
Check signal from firing circuit module
Replace fuse or reset circuit breaker
Check supply to control unit
Check circuit breaker
Reset overload circuit
Check primary power wiring
Lower the ESP voltage
Remove dust buildup
-------
Table 4-2 (continued). TROUBLESHOOTING CHART FOR ESP'S
Symptom
I
M
CTi
5. Low primary voltage
High secondary cur-
rent
Probable cause
Slack or broken emitting
electrode wire shooting
the high "V" circuit
Circuit comnonent failure
Trouble in ESP
1) Dust buildup in hopper;
check meters:
- ammeter very high
- kv meter very low
(1/2 normal)
- milliamperes very high
2) Metallic debris left in
unit during shutdown for
maintenance
3) Unhooked collecting plate
touching emitting frame
4) Broken support insulator
5) Excessive dust buildup
on hopper beams or cross
member
Short circuit in secondary
circuit or pptr.
Remedy
Deenergized ESP & remove or replace broken
or slack wire
Check transformer-rectifier & ESP: ground T-R high "V"
connector to ESP
Clean off dust buildup
Deenergize ESP and remove
Repair
Repair
Clean
Check wiring and components in high voltage
circuit; Check ESP for:
-------
Table 4-2 (continued). TROUBLESHOOTING CHART FOR ESP'S
Symptom
I
M
-J
6.
7.
Abnormally low ESP
current and p
voltage with no
sparking
Spark meter reads
high-off scale
Probable cause
Misadjustment of current and/or
voltage limit controls
Misadjustment of firing circuit
control
Heavy coating on emitting
electrode wires
Stream of cold air entering
ESP from defective door gasket
duct opening, inlet gas
system rupture - condensation
Wet dust clinging to wires
causes extremely low milli—
ammeter readings
Severe arcing in the ESP
without tripping out the unit
Continuous conduction of
spark counting circuit
Remedy
Interior dust buildup
Full hoppers
Broken wires
Ground switch left on
Ground jumper left on
Foreign material on high voltage frame or wires
Broken insulators
Check settings of current and voltage limit controls
Turn to maximum and check setting of current
and voltage limit controls
Check emitting frame vibration and emitting
vibration shaft insulator
Repair
Eliminate source of condensation
Eliminate cause of arcing
Deenergize, allow integrating capacitor to discharge
and re-energize
-------
Table 4-2 (continued). TROUBLESHOOTING CHART FOR ESP'S
Symptom
>£>
I
M
00
10.
11.
Low primary voltage
and current
No spark rate indi-
cation
Spark meter reads
high
Primary voltage and
currant very un-
stable
No spark rate indi-
cation voltmeter and
ammeter unstable
indicating sparking
No response to vol-
tage limit adjust-
ment
Does respond to
current adjustment
No response to spark
rate adjustment
Does respond to
other adjustment
Probable cause
Spark counter counting
60 cycles peak
Misadjustment
Loss of limiting control
Failure of spark meter
Failure of integrating
capacitor
Spark counter sensi-
tivity too low
Controlling on current
limit or spark rate
Controlling on voltage
or current
Remedy
Readjust controls
Readjust
Replace control
Replace spark meter
Replace capacitor
Readjust sensitivity
None needed if unit is operating at maximum current
or spark rate
Reset current and spark rate adjustment if neither
is at max
None needed if unit is operating at maximum voltage
or current
Reset voltage and current adjustment if neither
is at max
-------
Records
Accurate daily logs should be kept of all aspects of
precipitator operation, including electrical data, changes
in rapper and boiler operation, and variations in fuel
quality. Such logs aid the preventive-maintenance effort by
providing clues to probable causes for changes in perform-
ance.
Following the prescribed maintenance procedures and
maintaining accurate logs will provide benefits that justify
the effort.
4.2 UTILITY PROCEDURES AND RECORDKEEPING DURING STARTUP
AND MALFUNCTIONS
4.2.1 Utility Startup Procedures
Upon restarting of a coal-fired boiler that has been
out of service for a period of time, it is fired with oil or
gas for 4 to 5 hours. During this time the pulverizers are
turned on for about 5 minutes at a time until operation is
sustained and stable; more than one mill is always run.
About 8 hours is required to bring a unit on line, i.e. when
the steam pressure reaches 2.8 MPa (400 psi); the turbines
are then turned over. The ESP is not energized until the
temperature reaches the design range, which is about 107-
135°C (225-275°F) for cold-side ESP's. The precipitator is
turned on manually, usually 1 hour after the unit is firing
coal.
4-19
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Times required to bring the boiler to proper operating
temperature vary, but the described procedure is representa-
tive of that for a coal-fired boiler.
4.2.2 Utility Procedure and Recordkeeping During
Malfunctions
Part 60 of Title 40, Code of Federal Regulations,
Section 60.7, as amended, December 16, 1975, requires that
a utility report excess emissions caused by malfunctions or
other reasons by submitting a written report to the Admini-
19
strator for each calendar quarter. The report is to
include the magnitude of excess emissions as measured by the
required monitoring equipment, reduced to the units of the
applicable standards; it is to give the date and time of
commencement and completion of each period of excess emis-
sions. Periods of excess emissions due to startup, shut-
down, and malfunction are be be specifically identified.
The nature and cause of any malfunction if known, the cor-
rective action taken, or preventive measures adopted are to
be reported. Each quarterly report is to be submitted by
the 30th day following the end of the calendar quarter.
This section compares the methods of reporting ESP
malfunctions practiced by two U.S. utilities. The reporting
procedures of most other U.S. utilities probably encompass
similar features.
4-20
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In the case of the first utility, a reportable malfunc-
tion is considered to be any sudden or unforeseen malfunc-
tion of particulate control equipment that causes or could
cause any of the utility's units to exceed specified limits
for a period of 4 or more hours. When this occurs the
following procedure is followed:
1. The malfunction is reported by phone or telegram
to the EPA regional office and to state or local
officials. The air quality branch of the utility
is also contacted.
2. The plant superintendent submits a report to the
EPA regional office, with copies to various
branches of the utility. The report includes the
following:
a. Time and date excess emissions began and
ended.
b. Time and date the breakdown causing the
excess emissions began and ended.
c. Type of emission, estimated rate, and copies
of the opacity monitor records.
d. Cause of the malfunction.
e. Operation and maintenance procedures, prior
to and during the malfunction, designed to
prevent such an occurrence.
f. Additional steps taken to minimize the extent
or duration of the malfunction.
g. Future plans to minimize the possibility of
similar malfunction.
Monthly records are kept, by plant and unit, of all
malfunctions, total hours transformer-rectifier (T-R) sets
are operated, number of hours T-R sets are not operating
4-21
-------
(broken down into 24 hours and >24 hour intervals), maximum
number of sets out at one time, and the monthly/yearly
availability of the ESP unit in percent. In addition, daily
logs are kept on each ESP unit, with remarks on outages of
various sections of the ESP. Costs for operation and
maintenance on all ESP's are also tabulated.
With the second utility, an ESP is considered to be
malfunctioning if opacity is 20 percent or greater. There-
fore, an ESP could be operating at less than its design
efficiency and still remain in operation. If 20 percent
opacity is reached, EPA is subsequently notified, but the
boiler and ESP are not taken out of service until the
following weekend; the necessary maintenance is then per-
formed. Corrective actions and preventive measures are
reported to EPA in a brief letter, which may not be sent
until a month or two after the malfunction has occurred.
4.2.3 Outages - Forced and Scheduled
Forced outages result from unpredicted malfunctions
requiring immediate shutdown. Planned outages are scheduled
for maintenance and inspection.
Forced outage malfunctions, by definition, involve
shutdown and startup. Some malfunctions, however, can be
resolved online and do not require a shutdown. In these
instances boiler operation may be reduced to as low as 10
percent of design load without appreciably increasing emis-
sions.
4-22
-------
Planned outages require complete shutdown of the unit
to enable maintenance personnel to perform such tasks as
slag cleanout, ESP repair, and boiler tube repair.
4.3 COSTS OF COLD SIDE ESP MAINTENANCE AND OPERATION20
The annualized costs of maintenance and operation of 18
model cold-side ESP's have been estimated. These annualized
costs are comprised of the following items:
Utilities, including water for ash slurries and clean-
ing; electricity for fans, valves, lighting, controls,
hoppers, rappers, and charge of plates.
Operating labor, including supervisory and skilled and
unskilled labor required to operate, monitor, and
control the ESP.
Maintenance and repairs, consisting of manpower and
materials to keep the unit operating efficiently. The
function of maintenance is preventive and corrective.
Overhead, a business expense that is not charged
directly to a particular part of a process but is
allocated to it. Overhead costs include administra-
tive, safety, engineering, legal, and medical services;
payroll; employee benefits; and public relations.
Fixed Charges, which continue for the estimated life of
the system and include costs of the following:
0 Depreciation - the charge for losses in physical
assests due to deterioration (wear and tear,
erosion and corrosion) and other factors, such as
technical changes making the physical assets
obsolete.
0 Interim replacement - costs expended during the
year for temporary or provisional replacement of
equipment that has failed or malfunctioned.
4-23
-------
0 Insurance - costs of protection from loss by a
specified contingency, peril, or unforeseen event.
Required coverage could include losses due to
fire, personal injury or death, property damage,
embezzlement, explosion, lightning, or other
natural phenomena.
0 Taxes - including franchise, excise, and property
taxes leveed by a city, county, state, or Federal
government.
0 Capital costs due to interest on borrowed funds.
4.3.1 Basis of ESP Annualized Cost Estimates for 18 Model
Plants
The capital and annualized costs of electrostatic
precipitators can vary significantly with design philosophy
and site-specific factors. Factors having a major cost
impact are plant size (capacity), remaining life, and capa-
city factor; sulfur and ash content and heating values of
the coal; maximum allowable particulate emission rate;
control system status (new plant or retrofit); and replace-
ment power requirements.
As a means of illustrating the impact of site and
process factors on total installed and annualized costs of
ESP's, 18 model plants have been defined and cost estimates
prepared for each. The coverage here is restricted mainly
to annualized operation and maintenance costs. These esti-
mates, presented in the following sections, are in January
1976 dollars and do not include escalation through project
completion or replacement power.
4-24
-------
The 18 model plants analyzed for ESP costs were selected
to incorporate three factors that affect costs: plant size
(capacity), installation status, and degree of participate
control required. Boiler capacities of 150 MW, 300 MW, and
450 MW were considered. Both new and existing ESP applica-
tions are considered for each boiler size. Each plant size
is also analyzed in terms of three particulate control
requirements: 94 percent control on Eastern high-sulfur
coal and corresponding to a specific collecting area (SCA)
of 640 (200); 99 percent control on Eastern low-sulfur coal,
corresponding to an SCA of 1920 (600); and 99.9 percent (10%
opacity) control on Western low-sulfur coal, corresponding
to an SCA of 4570 (900). Specific collecting area (SCA) is
the ratio of the area of the collecting plates in the ESP to
033 23
the flue gas flow rate in thousands [m /10 m /min (ft /10
acfm)].
Other variables such as remaining plant life and plant
capacity factor are selected to be representative of each
model plant. Operating costs for such items as utilities,
which vary with geographical location, are considered repre-
sentative of a midwest location. Table 4-3 identifies the
characteristics and major assumptions for the model plants.
Table 4-4 presents the analyses of the coals used in the
study. Table 4-5 gives capital costs for all 18 model
plants.
4-25
-------
Table 4-3. SUMMARY OF CHARACTERISTICS AND ASSUMPTION FOR MODEL PLANTS
Model plant parameters
I
to
Plant capacities, megawatts
Plant status
Particulate control require-
ment
Boiler data
Capacity factor
Heat rates, flue gas flow
rates, and remaining life
Operating cost factors
Electricity cost
Taxes
Characteristics and assumptions
150, 300, and 450 (single boilers)
Existing (retrofit.) and new
Assumed levels of 99.0 percent control required
on Eastern high sulfur coal 99.0 percent control
required on Eastern low sulfur coal; and 99.9 percent
control (10% opacity) required on Eastern low sulfur coal.
Assumed 0.6 for all plants
Capacity
MW
150 existing
150 new
300 existing
300 new
450 existing
450 new
Flue gas Remaining
Heat rate^, flow rate, boiler
Btu/kWh acfm/MW life, yrs.
10,000 3,400 10
9,300 3,200 35
9,500 3,275 15
9,200 3,175 35
9,300 3,140 20
9,200 3,080 35
Based on averages for midwest region
20 mills/kWh
4%
a MJ/kWh - Btu/kWh x 1055.87 J/Btu * 10° J/MJ
b m3/min/MW = acfm/MW x ( ' £•£—-)
-------
I
to
-J
Table 4-3 (Continued)
SUMMARY OF CHARACTERISTICS AND ASSUMPTIONS
FOR MODEL PLANTS
Capital cost
Retrofit characteristics
Capacity derating
Energy penalty
Replacement capacity cost
9%
Longer duct runs, tight space'constraints, increased
construction labor costs
ESP - 0.5 percent
Eastern coal 0 percent
ESP - 0.5 percent
S400/KW
-------
I
CO
00
Table 4-4. COAL ANALYSES ASSUMED FOR ESP COST EVALUATION
Coal type
Eastern high sulfur
Eastern low sulfur
Sulfur content,
percent
3.0
1.0
Ash content,
percent
15
15
a
Heating value ,
Btu/lb
11,000
11,000
MJ/Kg - Btu/lb x
105'|;*7
Ib
0.4535924 Kg
1000 J
MJ
-------
Table 4-5. CAPITAL COSTS FOR ELECTROSTATIC PRECIPITATORS
Cost
element
Existing Plants
0 Electrostatic
precipitator
@ 200 SCA
0 Electrostatic
precipitator
@ 600 SCA
0 Electrostatic
precipitator
@ 900 SCA
New Plants
e Electrostatic
precipitator
@ 200 SCA
0 Electrostatic
precipitator
@ 600 SCA
0 Electrostatic
precipitator
@ 900 SCA
Plant size/capital cost
150 MW
$, MM
3.74
8.35
12.14
3.34
6.69
9.93
$AW
24. -9
55.7
80.9
22.3
44.6
66.2
$ per ft*
plate area
35.73
27.32
26.46
33.95
23.25
23.00
300 MW
$, MM
7.75
13.94
20.21
6.13
11.63
17.15
$/kW
25.8
46.5
67.4
20.4
38.8
57.2
$ per ft*
plate area
38.17
23.67
22.87
31.15
20.36
20.02
450 MW
$, MM
12.06
20.13
29.15
9.90
17.14
25.18
$/kW
26.8
44.7
64.7
22.0
38.1
56.0
$ per ft*
plate area
41.16
23.76
22.93
34.44
20.63
20.20
I
NJ
VO
Metric conversion: SCA - 1 ft /10 acfm x 3.2 = 1 m2/103 m /min.
-------
4.3.2 Annualized Costs
Annualized costs for the 18 model plants are presented
in Table 4-6. The annual costs in mills per kilowatt hour
decrease as the size of the units increases for most cases.
Costs for retrofit cases are higher because of the effects
of the higher capital cost for retrofitting.
4-30
-------
Table 4-6. ANNUALIZED COSTS FOR ELECTROSTATIC PRECIPITATORS
I
U>
Cost
element
Existing Plants
Electrostatic
pfecipitator
@ 200 SCA
Electrostatic
precipitator
@ 600 SCA
Electrostatic
precipitator
@ 900 SCA
New Plants
Electrostatic
precipitator
@ 200 SCA
Electrostatic
precipitator
§600 SCA
Electrostatic
precipitator
@ 900 SPA
Plant size/annual cost
150 MW
$, MM
1.11
2.43
3.50
0.77
'
1.50
2.19
mills/kWh
1.41
3.08
4.44
0.97
1.89
2.77
300 MW
$, MM
1.98
3.57
5.15
1.35
2.57
3.76
mills/kWh
1.25
2.26
3.26
0.85
1.62
2.38
450 MW
$, MM
2.83
4.79
6.91
2.14
3.75
5.49
mills/kWh
1.19
2.02
2.92
0.90
1.58
2.32
23 233
Metric conversion: SCA - 1 ft /10 acfm x 3.2 = 1 m /10 m /min.
-------
5.0 INSPECTION TECHNIQUES FOR EVALUATING
MAINTENANCE PROCEDURES
This section describes procedures for inspection of an
ESP at a utility operating a coal-fired boiler. The circled
numbers correspond to those on the example inspection check-
list in Section 5.2.
5.1 TYPICAL ESP INSPECTION PROCEDURE
21
(A) Observe the plume before entering the plant''
Opacity of the plume is the most indicative guide to the
performance of an ESP. If plume opacity is greater than it
was under similar boiler load conditions at an earlier time,
either the collection efficiency of the ESP has decreased or
the fuel quality has decreased.
Determine the plume's equivalent opacity. (Do not
mistake water vapor condensation for particulate emission.)
Table 5-1 illustrates possible operating factors that may be
causing a visible emission. If visible emissions exceed
applicable standards, use the standard form and follow
established procedures for recording the violation.
(1) Obtain basic boiler data or update boiler data
from the previous inspection. Check for changes in fuel
quality that might affect ESP operation and emissions.
5-1
-------
Table 5-1. PLUME CHARACTERISTICS AND OPERATING
PARAMETERS FOR COAL-FIRED BOILERS
Stack
plume
Associated
pollutant
Occurrence
Coal
Possible operating
factors to investigate
White
Gray
Black
Reddish-
brown
Bluish-
white
Yellow
or
brown
Particulate
Particulate
Particulate
Nitrogen
dioxide
Sulfur
trioxide
Organics
common
common
common
rare
rare
rare
Excessive combustion air
Inadequate air supply or
distribution
Lack of oxygen; clogged
or dirty burners or
insufficient atomizing
pressure; improper coal
size or type
Excessive furnace tempera-
ture, burner configuration,
too much excess air
High sulfur content in
fuel
Insufficient excess air
(Source: Ref. 21)
5-2
-------
(3) Obtain or update general ESP data, noting any
efficiency tests since the last inspection or changes in the
operating parameters. Find out what operating problems the
ESP has had. The plant should provide a diagram showing
what fields are served by what transformers as a guide to
determining what fields are out when reading ESP controls.
(T) Check control set readings and compare with cali-
brated values for these controls. Many times problems that
develop gradually can be recognized by small variations from
normal. Check daily log to determine whether readings have
been drifting from normal. Drift is indicative of such
problems as:
a. Air inleakage at air heaters or in ducts leading
to the ESP.
b. Dust buildup on ESP internals.
c. Deterioration of electronic control components.
If grossly abnormal readings are noted, they indicate a
serious problem and can aid in diagnosing the probable
cause. Following are some examples:
a. One section grounded out - A voltage drop will be
observed in the precipitator (kV, d.c.), and
primary transformer (V, a.c.). There will be an
increase in the primary transformer amps (I, a.c.),
in the average precipitator amps, (I, d.c.), and
in the precipitator spark rate.
b. Ash buildup on wires and plates will reflect high
amps.
5-3
-------
c. A broken wire, not grounded, that is bouncing from
one collecting plate to another would show a
decrease in precipitator voltage (kV, d.c.), the
transformer primary voltage (V, a.c.), would
increase. Also, the needles will bounce as the
wire travels from one collecting plate to another.
d. A broken wire to the insulator (shorted out) will
be most noticeable if the T-R set is on full wave.
The primary transformer amps (I, a.c.) will decrease,
precipitator average amps (I, d.c.) will decrease,
transformer primary voltage (V, a.c.), will
increase, and precipitator voltage (kV, d.c.)
will increase.
Typical ranges of ESP control readings for proper
operation are given below:
minimum maximum typical
1. Primary voltage, volts 460/480 + 5%
2. Primary current, amps 50 200 125
3. ESP voltage kilovolts 30 100+ 40-65
4. ESP current milli amps 250 1500+ 750
5. Spark rate/min 10 100+ 75
It may be difficult to determine whether a section is
out by reading the ESP controls. You may need the help of
the utility's malfunction records to determine which sec-
tions are experiencing problems.
(5) Check pressure drops through the system and com-
pare with the normal pressure drops.
(j) Check ash hoppers for proper operation. Determine
the interval between hopper cleanouts. Check hopper skin
temperature with the back of your hand. Comparatively low
temperature of one or more hoppers could indicate a malfunc-
tion of the ash removal system, causing bridging or plugging
of the hoppers and subsequent ash buildup.
5-4
-------
(l) If possible, check the precipitator control room
and make sure that ventilation is adequate; check control
sets internally for dirt, which can cause false signals and
cause components to deteriorate.
(¥) If possible, check insulators for signs of deteri-
oration such as moisture and tracking from arc-over. Make
sure air filters to control sets and top housing are not
plugged.
Qf) If possible, check rapper action visually and
confirm vibrator operation by feel. You will not be able to
tell whether all of the rappers are operating properly
unless you know the sequence of rapping action.
^B) Check exterior of ESP for corrosion, loose insula-
tion, exterior damage, and loose joints. Give special
attention to points where gas can leak, causing fugitive
emissions.
dQ) Review operating records for all aspects of preci-
pitator operation including electrical data, changes in
rapper and boiler operation, and variations in fuel quality.
Table 5-2 lists recommended recordkeeping requirements.
These records can provide clues to probable causes of changes
in performance. Malfunctions since the last inspection
should be evaluated. The inspector should spot-check these
records to ensure that the plant is adhering to proper
operating procedures between inspections.
5-5
-------
Table 5-2. RECOMMENDED RECORDKEEPING REQUIREMENTS
Item
Frequency
Comments
en
I
(Ti
ESP Controls
Instrument calibration
Primary current, A
Primary voltage, V
Operating current, mA
Operating voltage, kV
Spark rate, sparks/min
Pressure drop through system,
in.
Rapper operation
Insulator condition
Fuel quality
Sulfur, %
Ash, %
HHV, Btu/lb
Changes in boiler operation
Flue gas analysis,
% by vol.
(Circle CO2 or O2)
Soot blowing intervals
Malfunctions
Initial measurement
Daily
Daily
Daily
Daily
Daily
Daily
Daily
Daily
Monthly
As occurring
Spot checks
Daily
As occurring
Compare daily measurements
with red-lined readings.
Check for gross misreadings
or slow drift from redline.
Compare with initial
pressure drop measurement
Check frequency and intensity
Check for deterioration
State range of values and
average
State hours or blows per day
Use standard form for
describing malfunctions
-------
Qj) Estimation of ESP control efficiency
Use design or, preferably, test efficiency after ascer-
taining that present operating conditions are consistent
with design or test conditions (e.g., boiler load, ash and
sulfur contents of coal, precipitator operating temperature).
If such data are not available, perform the following calcu-
lation:
Read secondary currents and voltages for each field of
the precipitator.
Calculate delivered corona power for each section
according to the following formula:
Delivered power = (secondary voltage) x
(secondary current)
If there are no meters for secondary voltage and
current, calculate delivered power for each precipi-
tator field as follows:
Delivered power = (input power) x (power supply
efficiency)
Input power = (primary current) x (primary voltage)
Typical power supply efficiency is 90 percent.
Determine total corona power input by summing the
delivered power for each section.
33 33
Calculate corona power input per 10 m /sec (10 ft /min)
of flue gas (i.e. watts per 103 m3/sec (103 ft3/min).
Obtain precipitator collection efficiency value from
Figure 5-1.
If power data are not available from meters on the
precipitator power supply panel, perform the following
calculation:
5-7
-------
LU
I—i
o
I—I
U_
u_
LU
IB
z
I—I
I—
o
o
o
25 50 75 100 125
CORONA POWER, WATTS/1000 CFM
150
Figure 5-1. Electrostatic precipitator collection
efficiency vs. delivered power.
Metric conversion: w/10 m /min = w/103 ft3/min f 0.028
5-8
-------
. Determine total square feet of precipitator collecting
area (plate area) from manufacturer's specifications.
Obtain sulfur content of coal being burned from opera-
tor.
Use these values to determine expected collection
efficiency from Figure 5-2.
Actual emissions. Actual emissions are computed
according to the following formula.
AE = (UE) (100-E)
where:
AE = actual emissions [(kg/hr) (lb/hr)]
UE = uncontrolled emissions [ (kg/hr) (lb/hr)]
E = control device efficiency, percent
^J) Comments
Use this section for describing items too long to be
entered on the form, such as deficiencies found during the
inspection and malfunctions occurring since the last inspec-
tion.
The results of the inspection could also be summarized
here. A copy of the entire checklist could be sent to the
utility with a letter that confirms that the inspection was
made, states any deficiencies, asks that they be corrected,
and makes recommendations for further improvement in opera-
tion and maintenance of the ESP.
5-9
-------
700 -
o
«c
o
o
o
Bituminous
Pulverized coal
Coal:
Boiler:
Note: each curve represents
a band of values which could
be expected to deviate above
or below the curve.
1.0 2.0
% SULFUR IN COAL
3.0
4.0
Figure 5-2. Cold-side ESP.
SCA vs. % S
Metric conversion: SCA - 1 ft2/103 acfm x 3.2 = 1 m2/103 m3/min
5-10
-------
5.2 INSPECTION CHECKLIST FOR ELECTROSTATIC PRECIPITATORS IN
THE ELECTRIC UTILITY INDUSTRY
FACILITY IDENTIFICATION
Facility Name:
Facility Address:
Inspection Date:
Person to Contact:
Source Code Number:
PREINSPECTION DATA SHEET
Adequate information
Inadequate information (Obtain needed data during first
inspection)
(1) PREENTRY DATA
Stack Plume - Equivalent Opacity
(Circle one):
0 20 40 60 80 100
Opacity regulation
In compliance
Smoke
White Grey
Reddish Brown Bluish White
BOILER DATA
a) Service: Baseload, standby,
floating, peak:
b) Total hours operation (19 ):
c) Average capacity factor (19 ):
d) Year boiler placed in service:
Not in compliance
Black or Brown
Yellowish Brown
5-11
-------
e) Generating capacity (MW):
f) Served by stack No.:
g) Fuel consumption:
Coal Mg/yr (ton/yr)
Oil m3/yr (bbl/hr)
Gas mcm/yr (mcf/hr)
Primary fuel composition: Circle one Coal
Oil
Gas
Range
Ash % to
%
Average
%
Sulfur % to %
J/Kg (Btu/lb)
J/l (Btu/gal)
J/m3 (Btu/ft3)
T) ELECTROSTATIC
to
to
to
PRECIPITATOR -
GENERAL DATA
ESP No.:
Manufacturer:
Type:
Efficiency (Design/Actual):
Mass emission rate:
g/acm (gr/acf)
Kg/hr (#/hr)
Kg/MJ (#/MM Btu)
No. of cells or individual bus sections:
No. of fields:
No. of cells:
Total plate area:
5-12
-------
Flue gas temperature @ inlet to
ESP @ 100% load °C (°F):
Stack diameter:
Stack height:
Stack gas exit temperature:
(?) CONTROL PANEL READINGS
Present Calibrated Present Calibrated
operating operating operating operating
voltage
voltage
current
voltage
Sparks/min.
Spark rate:
(D AIR FLOW READING
Pressure before ESP
Pressure after ESP
(?) HOPPERS
Interval between hopper cleanouts
Field 1
Field 2
Field 3
Field 4
Field 5
Field 6
Pascals (in)
Pascals (in)
hours.
Cleanout and transport
procedure
General housekeeping
Satisfactory Unsatisfactory
D D
5-13
-------
CONTROL ROOM
Satisfactory Unsatisfactory
Ventilation
Control sets condition || j I
(D CONDITION OF INSULATORS
(?) RAPPER OPERATION
id) ESP EXTERIOR CONDITION
l]) MAINTENANCE AND OPERATIONS
Records Kept: Yes No
Instrumentation calibration
Collector control readings | | ["""]
Fuel analysis, changes in quality | I [""]
Pressure drop through system j | j |
Rapper operation, changes | | | ]
Boiler operation, changes | | Q]
Flue gas analysis | [ P"]
Soot blowing intervals | | r~j
Malfunctions | j r~j
) ESTIMATION OF ESP EFFICIENCY
Kg/hr g/scm Kg/MJ
(Ib/hr) (gr/scf) (Ib/MM Btu)
Uncontrolled emissions
Actual emissions
Control device efficiency _ %
) COMMENTS
5-14
-------
REFERENCES
1. Oglesby, Sabert, Jr. A Manual of Electrostatic Pre-
cipitator Technology. Southern Research Institute,
Birmingham, Alabama. August 25, 1970.
2. White, Harry J. Industrial Electrostatic Precipitation.
Addison-Wesley. 1963.
3. The Electrostatic Precipitator Manual. The Mcllvaine
Co. Copyright 1976.
4. Walker, A. B. Characteristics and Electrostatic
Collection of Particulate Emissions from Combustion of
Low-Sulfur Western Coals. Research-Cottrell, Inc.
(Paper 74-11, presented at Air Pollution Control
Association 67th Annual Meeting. Denver, June 9-13,
1974).
5. Walker, A. B. Experience with Hot Electrostatic
Precipitators for Fly Ash Collection in Electric
Utilities. Research-Cottrell, Inc. (Presented at
American Power Conference. Chicago, April 29-May 1,
1974) .
6. Schneider, Gilbert G., et al. "Selecting and Specify-
ing Electrostatic Precipitators." Chemical Engineering.
May 26, 1975.
7. Matts, S. and P-0 ohnfeldt, "Efficient Gas Cleaning
with SF Electrostatic Precipitators."
8. Industrial Gas Cleaning Institute, Inc., "Terminology
for Electrostatic Precipitators."
9. Hesketh, Howard E., and Frank L. Cross, Jr. Handbook
of Air Pollution Control Equipment.
10. Aimone, R. J. et al. "Experience with Precipitators
when Collecting Ash from Low-Sulfur Coals," presented
at the 36th annual meeting of the American Power
Conference, Chicago, Illinois. April, 1974.
5-15
-------
11. Dismukes, E. B. "Conditioning of Fly Ash with Ammonia,"
Southern Research Institute, presented at the Symposium
on Electrostatic Precipitators for the Control of Fine
Particles, in Pensacola Beach, Florida. September 30-
October 2, 1974.
12. Busby, H. G. T., and K. Darby. Efficiency of Electro-
static Precipitators as Affected by the Properties and
Combustion of Coal. J. Inst. Fuel (London) . 36^184-197,
May, 1963.
13. Dismukes, E. B. A Study of Resistivity and Condition-
ing of Fly Ash. Southern Research Institute, Contract
CPA 70-149, Environmental Protection Agency. Publica-
tion Number EPA R2-72-087. February, 1972. NTIS PB
212607. 138 p.
14. Dalmon, J. and D. Tidy. The Cohesive Properties of Fly
Ash in Electrostatic Precipitation, Atmos. Environ.
(Oxford, England), 6:81-92, February, 1972.
15. Dismukes, E. B. Conditioning of Fly Ash with Sulfur
Trioxide and Ammonia. Southern Research Institute for
Environmental Protection Agency and Tennessee Valley
Authority. 1975.
16. Dismukes, E. B. Conditioning of Fly Ash with Sulfuric
Acid, Ammonium Sulfate, and Ammonium Bisulfate.
Southern Research Institute, Contract No. 68-02-1303,
Environmental Protection Agency. October, 1974.
17. Bump, Robert L. "Electrostatic Precipitator Mainten-
ance Survey," TC-1 Committee of Air Pollution Control
Association.
18. Bibbo, P. P., and M. M. Peaces. "Defining Preventative-
Maintenance Tasks for Electrostatic Precipitators,
Research Cottrell, Inc., Power, August, 1975. pp. 56-
58.
19. Federal Register, part 60 of Title 40, Section 60.7 as
amended December 16, 1975.
20. PEDCo-Environmental Specialists, Inc., "Electrostatic
Precipitator Cost Study," prepared for Division of
Stationary Source Enforcement, U.S. Environmental
Protection Agency, Research Triangle Park, North
Carolina. August, 1976.
5-16
-------
21. Devitt, Timothy W., Richard W. Gerstle, and Norman J.
Kulujian. "Field Surveillance and Enforcement Guide:
Combustion and Incineration Sources." PEDCo-Environ-
mental Specialists, Inc. June, 1973.
5-17
-------
APPENDIX A
ESP MANUFACTURERS SUGGESTED MAINTENANCE
PROCEDURES
-------
APPENDIX A
ESP MANUFACTURERS SUGGESTED MAINTENANCE PROCEDURES
Guidelines for ESP maintenance from five manufacturers
have been evaluated in compilation of a list of typical
recommended maintenance procedures for all types of ESP's.
These procedures, typical of those the manufacturer presents
to the purchaser of a new ESP, include the following cate-
gories:
A.I Description of major ESP components and general
maintenance
A.2 Preliminary or preoperational checkout and testing
A.3 Startup
A.4 Routine of preventive maintenance on a daily,
weekly, monthly, quarterly, and annual basis.
A.I DESCRIPTION OF ESP COMPONENTS AND GENERAL MAINTENANCE
A.1.1 Gas Distribution System
A gas distribution system, composed of one or more rows
of distribution plates, is located in the inlet duct immedi-
ately before the ESP. This distribution system ensures that
an even flow of dust-laden gas enters the precipitator, thus
providing optimum operating efficiency.
A.1.2 Precipitator Shell
Combustion of coal usually produces a small amount of
S02 and SO- as well as C02, 02, and moisture. The traces of
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SO- can cause fairly rapid corrosion of the interior of gas
ducts, fans, and dust-collecting equipment if these interior
surfaces become cool for any reason. It is therefore recom-
mended that thorough internal inspection be made during the
first year of operation. If interior corrosion is noted,
some means of correction should be applied as soon as
possible. Applying heat insulation to exteriors of the
corroded components will normally correct this condition.
In installations where the boiler periodically operates at
low loads, covering the interior surfaces of side frames,
end frames, and roof with gunite will prevent any corrosive
damage to the steel.
A.1.3 Collecting Plates
The gas flows horizontally in the precipitator through
individual gas ducts formed by the collecting plates. The
discharge electrodes are located midway between the plates
for the purpose of ionizing the gases and imparting an
electrical charge to the dust particles. It is important
that the plate and electrode spacing be held to close
tolerances. If not, close clearances can cause high local-
ized sparking, which reduces the maximum precipitator vol-
tage and thus the collection efficiencies.
Whenever the precipitator is out of service and internal
inspections are possible, the collecting plates should be
A-2
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checked for proper alignment and spacing. Hangers should be
checked, and spacers at the bottom of the plates should not
bind plates to prevent proper rapping. The lower portions
of all plates and the portion of plate adjacent to any door
opening should be checked for signs of corrosion. Corrosion
usually is indicative of air inleakage through hoppers or
around doors. Causes of such inleakage should be repaired
at once.
At each inspection, the dust deposits on the collecting
plates should be observed before any cleaning of the preci-
pitator is started. The normal thickness of the collected
fly ash should be about 3.2 mm (0.125 in), with occasional
buildups to 6.4 mm (0.25 in). If the buildup exceeds this
amount, the intensity of the plate rappers should be in-
creased. If the collecting plates are almost metal clean,
however, the lack of dust buildups may indicate high gas
velocity, extremely coarse fly ash, or an operation voltage
too low for good precipitation. This condition may be noted
if a section has been shorted out prior to the inspection.
A.1.4 Discharge Electrodes
The discharge electrodes are small-diameter wires
suspended from a structural steel wire supporting frame,
held taut by individual cast iron weights at the lower end
and stabilized by a steadying frame at the top of the cast
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iron weights. Whenever possible, the condition of the
discharge electrodes should be checked with regard to dust
buildup. The amount of buildup will indicate whether the
high-tension vibrators, when furnished, are operating at the
proper intensity.
The discharge electrodes should be perfectly centered
between the plates from top to bottom for optimum precipi-
tator operation. Any broken discharge electrodes should be
removed and, if time permits, replaced with new wires.
A.1.5 Rapping Equipment
The purpose of the rapping or vibrating equipment is to
dislodge the collected material from plates and/or wires
before the accumulation becomes so heavy that it interferes
with electrical operation. The "Operation and Maintenance
Manual" supplied by the seller for each installatiDn provides
complete descriptions and instructions for operation and
maintenance of the rapping equipment and their controls.
A.1.6 Hopper Emptying
It is extremely important that a regular schedule of
hopper emptying be established at the start of operation and
followed as closely as possible, preferably once each
shift. If the hoppers are allowed to fill over a 24-hour
period or longer, electrical components may short out and
precipitation will cease. Also, if a fly ash hopper is
A-4
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allowed to stand for more than 24 hours, the dust tends to
pack, cool, and absorb some moisture from the gases. The
dust is then extremely hard to remove, and the moisture can
start corrosion of the hopper steel. Dust often tends to
build up in the upper corners of the hoppers, especially if
they have been filled completely at any time. Any abnormal
buildups should be removed. If this condition becomes
chronic, it is an indication of low operating temperatures,
insufficient heat insulation, or inadequate hopper emptying.
Heat tracing of the hopper will usually correct this condi-
tion. In any event, scheduled hopper emptying is critical
to efficient ESP operation.
A.1.7 Insulator Compartments or Housing
The insulator enclosures are vented with air to prevent
flue gases from entering this space, which houses the
supporting insulators. If the precipitator is under nega-
tive pressure, the air is admitted through open vents in the
housing sides. If it is under positive pressure, the venti-
lating air is introduced by means of a ventilating fan,
sized to maintain a pressure within the housing slightly
higher than the precipitator pressure. This air flows
downward around the inside of the bushings, which separate
the treating zone from the insulator enclosures; the flow of
air prevents the gases from entering these cooler enclosures
and condensing on the interior surfaces and also helps keep
A-5
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the insulators clean. The interior condition of the en-
closures should be carefully noted. All insulators should
be cleaned and the exterior and interior of the bushings
cleaned if necessary. The interior of the bushings can be
cleaned easily with an air lance. The interior surfaces of
'-he enclosures should be carefully inspected for signs of
corrosion. Signs of corrosion or an abnormal buildup of
dust in the enclosure can indicate insufficient ventilation.
All high-tension connections to the bus beams should be
checked to see that all connections are secure. If heaters
are provided, they should be serviced as described in the
maintenance manual for insulator compartment heaters.
-.1-8 Transformer-Rectifier Power Supply
a. The transformer-rectifier power supply is con-
tained in an oil-filled tank and consists of the
following equipment:
1. High-voltage supply transformer.
2. Silicon rectifier assembly.
3. Inductor in series with the high-voltage
output bushing.
4. Low-voltage bushing for primary supply.
5. Metering and d.c. ground connection.
The transformer rectifier tanks are maintained by
checking for leaks and for proper oil level; if Askarel is
used as the dielectric, any spills must be cleaned up care-
fully because Askarel is flammable.
A.2 PRELIMINARY CHECKOUT AND TESTING
1. Check the line voltage for proper phase and mag-
nitude.
A-6
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2. Inspect the transformer-rectifier tanks for any
signs of oil leakage or physical damage. Check
the oil tank gauge and refill if necessary.
Follow manufacturer's instructions for pertinent
information. DO NOT OVERFILL THE OIL TANK.
3. Inspect the dust-conveying equipment and the
hopper discharge valves.
4. Inspect main exhauster (if applicable).
5. Follow the procedure outlined under "Key Interlock
System" to gain access to the precipitator.
6. Inspect the rapper motors prior to and during the
initial equipment startup for proper rotation and
alignment.
7. Inspect any gear motor that has been mechanically
serviced for proper rotation and alignment.
8. Check the position of each collecting surface
rapper hammer. These hammers must be in a posi-
tion that conforms to the normal function of the
hammer shaft. A hammer that has been manually
tripped in advance of its normal function may
cause damage upon gear motor startup.
9. Inspect the precipitator control cabinets and the
transformer-rectifier for evidence of loose con-
nections.
10. Inspect the precipitator chamber for foreign
material, such as tools, rags, cleaning material,
etc.
11. Disconnect the high-voltage conductor at the
support insulator and check the discharge wires to
ground prior to initial startup. Resistance to
ground should be 100 megohms or greater.
12. Check the condition of all explosion relief doors
(if applicable).
13. Check all access doors for operation and alignment
and then lock them. Return the door keys to their
proper location in the key interlock transfer
block.
A-7
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14. Operate the insulator heaters a minimum of 2 hours
before energizing the precipitator. The ammeters
on the heater control panel should be balanced and
should read the equivalent of approximately 4kW/
line voltage.
a. For a precipitator operating with positive
pressure, the pressurizing fan(s) must be
started prior to starting the main exhauster.
b. The high-voltage heaters should not be turned
off until after the precipitator has reached
operating temperature.
c. Energize precipitator.
A.3 STARTUP
1. Switch on the dust-handling system.
2. Switch on the discharge electrode and plate
rapping systems.
3. Switch on the precipitator control circuit breaker.
a. Allow precipitator high-voltage insulator
heaters to warm up before switching on high
voltage.
Possible explosions are avoided by not
switching on the high-voltage power while a
combustible mixture is in the precipitator.
4. Place the precipitator power supply on automatic
and press "ON" button.
A.4 ROUTINE PREVENTIVE MAINTENANCE
A program for maintaining the precipitator and its
auxiliary equipment is recommended to ensure proper opera-
tion of the unit and to prevent outages caused by lack of
maintenance.
A-8
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Inspection of the unit on a daily, weekly, monthly,
quarterly, and annual schedule is recommended. Data sheets
and instructions supplied to the customer are of a recom-
mended format, which may be altered by the customer to suit
specific conditions. Following is a typical list of main-
tenance procedures that an ESP manufacturer might provide.
A.4.1 Daily
Control House
1. Check all precipitator control panels for vent fan
operation.
2. Note conditions of filters on control panels.
3. Take precipitator control panel readings.
4. Maintain a daily log for reference.
Auxiliary Control Panels
1. Check insulator heaters for operation mode.
2. Record ammeter readings of each insulator heater.
3. Check all "Push to Test" lights on panel, replace
as necessary.
4. Check all selector switches for proper operation
in manual and automatic mode.
5. Check all rapper timers for operation. Rapper
"on" time and "off" time is set by the service
engineer and should not be changed except by
authorized plant personnel. If times are re-
adjusted, this should be noted on maintenance
records. Record initial settings and final set-
tings.
6. Test annunciator panel for operation. Replace any
bad lights as necessary.
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Precipitator
Dust Removal Level
1. Check hopper/dust-removal equipment for
operation or signs of leakage. Record any
faulty areas.
Side Access Level
1. Collecting Surface Rapper Drives
a. Check all collecting surface rapper
drive motors and reducers; note any
leakage of reducer lubricant.
b. Check all couplings for adequate lubri-
cation.
c. Check for operation temperature of
reducer and motor. If rapper drive is
operating, listen for rapping sound of
hammers.
d. Check any auxiliary equipment on this
level.
Gas Inlet Level
1. Gas Distribution System Rapper Drives if
equipped - same procedure as side access
level.
Roof Level
1. Transformer-Rectifier
a. Check all units for proper oil level.
See instruction book for type, amount,
and method of adding oil, if necessary.
b. Record transformer-rectifier oil tem-
perature.
c. Note and report any leaks on tank of
transformer-rectifier. If dielectric is
Askarel (G.E. - Pyranol - Westinghouse -
Inerteen), the manufacturer should be
contacted immediately and extreme cau-
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tion should be taken in cleanup of
spill. See instruction book concerning
handling of Askarel dielectric material.
Discharge Surface Rapper Drives
1. Check all discharge surface rapper drive motors
and reducers. Note any leakage of reducer lubri-
cant.
2. Check for operational temperature of reducer and
motor.
3. Check all couplings for adequate lubrication.
4. Inspect each discharge surface cam drop mechanism
for wear and operation. Check all rollers on cam
drops for binding or restriction of movement when
they drop off cam. If roller slides down face of
cam, roller must be adjusted or disassembled and
cleaned to eliminate this condition.
A.4.2 Weekly
1. Clean all insulators.
2. Check access doors for tightness.
A.4.3 Monthly
1. Check grounding switches on rapping cubicle doors
and lubricate gate switches.
2. Check that safety interlocks operate freely.
3. Check rapping chains for slackness and grease.
4. Check rapping gear boxes and lubricate cam tips.
5. Check electrical contacts and connectors in the
high-voltage control panel.
6. Check sealing bellows on connector drop rod
rappers.
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A.4.4 Quarterly
Control House
A. Precipitator Controls and Auxiliary Control Panels
1. Clean inside all panels.
2. Check all electrical components for signs of
overheating.
3. Check for loose electrical connections.
4. Lubricate all door latches and adjust as
necessary.
5. Check relays for freedom of movement.
6. Check vent fan for operation and check
clearances between blades and shroud.
7. Install new air filters in control panel.
Side Access Level
A. Collecting Surface Rapper Drives
1. Check reducer for leaks.
2. Check coupling for signs of excess wear.
A.4.5 Annual
1. Remove dust buildup on wires and plants, if any.
2. Adjust vibrator and/or rapper intensity and cycle
to prevent serious material buildup.
3. Inspect perforated diffuser screen and breeching
for dust buildup.
4. Perform maintenance and lubrication of pressurized
fans; check for leaks in the pressurized system.
5. Check for loose bolts in frames, verify that
suspension springs are in good order, and examine
wearing parts, hammers, anvils, etc.
A-12
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6. Inspect discharge wires for tightness and signs of
burning, and discharge system for correct align-
ment, broken parts, and welds.
7. Clean lead through insulators on underside.
8. Check all insulators for cracks.
9. Check complete collector grounding bonding wires
and connections.
10. Drain oil, wash out, and refill gear boxes.
11. Check transformer fluid and dielectric strength.
12. Check relays, contactors, and starter contacts.
A.4.6 Recommended Spare Parts
Following is a typical list of spare parts recommended
by manufacturers.
- support insulator/gaskets
- shaft insulator
- emitting electrodes
- H.V. bushings
- emitting electrode weights
- cap and pin insulators
- H.V. resistor assembly
- Lamp bulbs
- contractor operating cord and contact set
- shunts
- diodes
- filter circuit
- transformers
- relays
- capacitor
- silicon diode
- potentiometer
- resistors
- fuses
- printed circuit card
A-13
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APPENDIX B
UTILITY ESP MAINTENANCE PROCEDURES
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APPENDIX B
UTILITY ESP MAINTENANCE PROCEDURES
This section presents an example of a conscientious ESP
maintenance procedure for utilities. Although this level of
ESP maintenance is not practiced by all utilities, neglect
of proper maintenance can lead to degradation of performance
and ultimately to higher maintenance costs. These procedures
are considered reasonable and representative of sound ESP
maintenance practices for utility applications.
B.I ASH REMOVAL SYSTEM
The document, "Operating and Maintenance Instructions,"
prepared by the Allen-Sherman-Hoff Co., Inc., provides
detailed instructions for operation and maintenance of the
ash-handling system. Operators of this equipment should be
thoroughly familiar with information given in their manual
and with the following supplementary items, required to
ensure successful operation and maintenance of the ash
removal equipment.
1. Obtain from the manual the recommended values of
settings for timers, water pressure and flow, air
pressure, and vacuum high and low settings.
2. Determine that compressed air supplied to this
system is clean and moisture free.
B-l
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3. Values assigned to settings for timers and vacuum
high and low settings are theoretical and are
listed as a starting point. Therefore, some field
adjustment may be required for optimum operation.
4. Observe the air separator tanks overflow at least
once weekly, since this may signal restricted ash
slurry flow.
5. Give particular attention to vacuum highs and lows
observed, since these readings will help in detec-
ting worn hydrovactors and excessive air leakage.
6. At least once each day, determine whether each ash
hopper is emptying and whether all control panel
indicator lights are working. This can be accomp-
lished by observing the panel lights, locating
each hopper on the vacuum recorder sheet and
looking for abnormal deviations in operation, and
by touch to determine whether a hopper is full,
empty, or evacuating ash at the appropriate time.
7. During outages, refer to the maintenance section
of the manual and perform the preventive main-
tenance recommended.
B.2 ELECTROSTATIC PRECIPITATOR INSPECTION AND MAINTENANCE
B.2.1 General
To keep abreast of current operating problems and
internal faults, review precipitator operating logs daily.
This will ensure cognizance of unusual conditions and will
expedite inspection and repair procedures, especially during
emergency outages.
To maintain optimum collection efficiencies, it is
essential that internal faults be corrected at the first
unit outage following their discovery. External faults can
be corrected as they occur.
B-2
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If the plant staff cannot determine the reason for a
fault, request immediate assistance from the central office.
Any unusual condition or equipment problems should also be
brought to the attention of the central office staff.
Make a thorough inspection of each precipitator during
each scheduled outage and summarize the findings and cor-
rective actions in the outage report. To facilitate the
detailed inspection it is usually necessary to wash down the
precipitator internals. If this is not done, it is almost
impossible to inspect all components because of the fly
ash buildup on the internals.
B.2.2 Clearance Procedures
Follow established clearance procedures in tagging out
and placing grounds on the precipitator before any inspec-
tion or maintenance work is performed.
When a unit is shut down, keep the precipitator plate
and wire rappers and ash-removal system in service for 24
hours to ensure that all loose dust is removed. During
short outages it may not be possible to adhere to this
procedure because of maintenance work inside the precipita-
tor.
B.2.3 Insulator Heaters
Do not turn off the insulator heaters until the insula-
tors have been wiped clean of fly ash accumulation. Other-
B-3
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wise, the accumulated ash will become sticky from absorbed
moisture and will form a conductive path to ground. Also,
once the ash has absorbed moisture it is very difficult to
remove. If the heaters have been turned off following
cleaning of the insulators during an extended outage, they
should be energized at least 24 hours prior to firing the
unit to ensure that they are dry and above the acid dewpoint.
A conductive path across an insulator assembly could result
in a catastrophic failure of the insulator from a high-
voltage flashover.
B.2.4 Removal of Foreign Materials
Remove all scaffold boards and other foreign material
from the precipitator before it is released for service.
B.2.5 Prestart Tests
Conduct air load tests following maintenance work on
the precipitator to ensure that all sections are clear of
grounds and alignment is satisfactory. During the air load
test, increase the voltage slowly on manual control to
preclude severe arc-over, which could damage an insulator.
Compare data with initial and previous air load data.
During unit startups place the ash-removal system in
service before a coal fire is established. Energize the
precipitator as soon as possible after the pulverizers are
placed in service. Each plant should determine optimum time
B-4
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for energizing the precipitators and prepare appropriate
operation instructions.
B.2.6 Inspections and Maintenance During ESP Operation
Inspection and maintenance of components such as those
in the penthouse of the precipitators may be done with the
unit in service/ Work in any section of the caged area will
require deenergizing and grounding of the transformer-
rectifier set serving that section. Since each transformer-
rectifier serves at least two bus sections, at least two bus
sections must be out. If the remaining sections of the
precipitator are in service and at their normal operating
level of 40 kV or above, particulate emission limits can be
met provided unit loads do not exceed the recommended limit.
The high-voltage section must be grounded when inspecting
the second or third fields because, even though these are
deenergized, they can pick up a significant static charge
from the upstream fields.
B.2.7 Critical Components - Caged Area
Inspect and maintain the critical components in the
caged area on a routine basis. These include the upper
portion of the insulators, insulator heaters and thermostats,
high-voltage support insulators, high-voltage bus standoff
insulators, voltage dividers, and the plate and wire rapper
machanism. It is very important that the insulator heaters
B-5
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and thermostats be kept in good working order since their
failure could result in tracking and insulator failure
during operation at low gas temperature. Attention shall
always be given the following:
a. Wipe insulators clean and inspect for cracks and
air infiltration through the asbestos sealing
rings.
b. Wipe high-voltage support and standoff insulators
clean and inspect for cracks or evidence of track-
ing.
c. Check plate and wire rapper drive shaft bearings,
drive motors, speed reducers, and chains and
sprockets to ensure that they are properly lubri-
cated.
d. Examine rapper cams and lift plates for wear and
inspect the wire rapper shaft insulator for
cracks; wipe clean.
e. Check resilient bellows seals on the wire and
plate rappers for cracks and evidence of air
infiltration; replace as needed.
f. Clean voltage dividers and examine for oil leakage.
B.2.8 Critical Components - Outside Caged Area
Critical components outside the caged area include the
transformer-rectifier sets and their control cabinets and
the rapper and insulator heater control cabinets. To
ensure optimum performance of the precipitator controls, a
plant electrician who is familiar with the precipitator
controls should inspect them daily and make any required
adjustments. This requirement is necessary to detect pro-
blems such as excessive sparking that are not evident from
the daily operating logs.
B-6
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B.2.9 Inspections and Maintenance During Shutdowns
B.2.9.1 Short Emergency Outages - Internal ground faults in
a section that indicates high amperes and zero or very low
voltage on the control panels are usually due to broken
discharge electrodes, ash hopper buildup, or bridging of ash
between the high-voltage frames and the collection plates.
Such faults can also occur from cracked high-voltage insula-
tors or from debris such as welding rods or pieces of wire
that are left inside the precipitator during maintenance.
These faults should be corrected during short emergency
outages.
Cut broken wires at the unbroken end and remove them.
Replacement of a cracked or failed insulator will required
placing that section on temporary suspension. Check the
alignment of this section following replacement of the
insulator and removal of the section from temporary suspen-
sion. Minimum and maximum distances between the high-
voltage frames and the collection plates are shown in the
manufacturer's handbook.
Install the asbestos sealing rings in the insulators
with care to ensure a good seal and prevent the inleakage of
air, which will result in internal corrosion. The insulator
assembly is shown in the manufacturer's handbook.
B-7
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Clean all insulators if time permits. If there is
sufficient time after completion of the work mentioned,
inspect other sections of the precipitator. Look for indi-
cations of electrical tracking; air infiltration; unusual
accumulations of dust on the plates and wires, which may
indicate ineffective rapping; misalignment; loose bolts,
particularly on the center mast brace assemblies and rapper
anvils; and any indications of corrosion or distress on the
precipitator internals. Document the results of each in-
spection to aid in future inspections and planning for
maintenance work.
B.2.10 Long Scheduled Outages
B.2.10.1 Prewash Inspection - Inspect the precipitator
internals as soon as possible after the unit comes off the
line before it is washed. Note any unusual dust accumula-
tions; polished areas, which indicate gas bypassing; swept
areas, which may be the result of air infiltration; and
other items that may require significant repair work.
Arcing between the wire mast and the high-voltage support
frame resulting from loose bolts is shown by dark spots in
the fly ash adhering to these areas; such spots can readily
be seen before the precipitator is washed. Wipe insulators
clean before washing. After cleaning the insulators, shut
off the heaters to preclude grounding during washing or
B-8
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possibly breakage of heater insulators from cold raw water
impingement. This work can usually be done while prepara-
tions are being made to wash the precipitator.
B.2.10.2 Washing - Thoroughly wash down the precipitator
internals with raw water. Washdown pads are provided for
this purpose. Washing should be done with a fog nozzle, and
personnel must be instructed not to direct the spray into
the insulators. The asbestos sealing rings will become
soaked with water and lose their resiliency upon drying.
Also, they might not dry completely before the unit is
returned to service.
Following the washing, begin repairs on the known
faults. While this work is in progress, perform a detailed
inspection of the other sections. Some items to be inspected
in each area of the precipitator are as follows.
B.2.10.3 Upper Area
Insulators - Check for cracks or any signs of arc-over
on both the lower and outer insulators. Replace broken or
severely cracked insulators. Reseal insulators that show
indications of significant air infiltration. If the sealing
rings have deteriorated, these must be replaced. There
should be some clearance between the inner and outer insula-
tors.
B-9
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Wire Masts - Check for loose bolts where the masts are
connected to the upper high-voltage support frame. Tighten
loose bolts. Exercise caution in tightening these because
this can distort the mast, causing misalignment between the
mast and the collecting plates. Cut out and remove any
broken discharge wires accessible from this area. Keep
records of wires that are removed. Loose or bowed wires can
be tightened by crimping in the direction of gasflow. This
must be done carefully so as not to distort or bend the mast
arms.
High-Voltage Support Frame - Check for loose bolts and
nuts and broken or failed springs; repair as needed.
Collection Plates - Check for loose nuts on plates and
plate hanger eyebolts, broken or failed springs, and loose
nuts on the plate rapper anvils. Check anvil striker plate
for wear. Visually check plates for bowed or wavy areas and
plate-to-wire misalignment.
General - Note any beams or frames that have slipped or
tilted; broken welds; and the general condition of the
internals, shell, access doors, and gaskets.
B.2.10.4 Lower Walkway Area
Alignment - Check alignment between the masts and the
collection plates. Gross misalignment can usually be
detected by visual observation. Questionable areas should
B-10
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be checked with a rule or gauge. Note any out-of-tolerance
spacing. Also check the horizontal distance between the
center mast bracing frame and the edge of the plates. Note
any burned areas resulting from arcing.
Correct the alignment to the tolerances noted in manu-
facturer's handbook.
Plates - Note any warped, buckled, or wavy areas.
Check lower plate foot and guide to ensure that plates are
hanging free. Note any missing bolts or rivets in the plate
assembly.
Wire Masts - Inspect wire masts for broken, loose, or
bowed wires. Cut and remove broken wires and tighten bowed
or loose wires by crimping. Check for loose or missing
bolts on the lower mast spacer frame. These bolts must not
be tight but should be snug. Close checks of these and any
repairs will require a scaffold board in the ash hoppers for
access.
General - Note any broken welds; evidence of air infil-
tration leakage; baffle distortion; ash hopper condition;
and the general condition of the internals, shell, access
doors, and gaskets.
Ducts - Inspect inlet ducts; inlet distribution baffle
half rounds; and outlet ducts for distortion, corrosion,
leakage, and ash buildups.
B-ll
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B.2.11 Records
Maintain detailed records of all inspections and
maintenance performed on fly ash collectors. Copies of
reports or other applicable records should be available at
the plant for review by state air pollution control represen-
tatives or others and should also be maintained in the
central office files.
B.2.12 Conclusions
This example of a utility precipitator maintenance
requirements sets forth concise and detailed instructions,
emphasizing the components that need the greatest attention
in order for an ESP to operate properly. If used properly,
those recommended maintenance procedures can serve as an
excellent supplement to the manufacturer's procedures,
providing further guidelines for an ESP operator in solving
operating difficulties.
B-12
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APPENDIX C
EXAMPLE OPERATING HISTORY
OF
COLD-TYPE PRECIPITATORS
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APPENDIX C
EXAMPLE OPERATING HISTORY OF COLD-TYPE PRECIPITATORS
This section is a review of maintenance and operational
problems a major U.S. utility has encountered with cold-type
precipitators over a number of years. Their plants are well
maintained, and extensive records are kept. The operation
and maintenance problems they report are a typical example
of what could be expected at other power plants utilizing
precipitators and operating under similar conditions.
In assessment of equipment operability, the best infor-
mation concerning operation and maintenance of precipitators
most likely will come from utilities with well-maintained
plants, such as the one discussed in this section. Utilities
with moderately or poorly maintained plants probably do not
keep comprehensive records, and many problems may not be
recognized or reported.
C.I INTRODUCTION
This utility began the change from mechanical collectors
to ESP's on new plant construction in the late 1950's.
Further retrofitting of ESP's on existing units was made in
1967 and subsequent years.
C-l
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Sixteen different types of ESP's serve 51 generating
units, ranging in capacity from 60 to 1300 MW. These ESP's
are supplied by five manufacturers, four domestic and one a
domestic subsidiary of a foreign manufacturer. Coal comes
from eastern and midwestern sources. Sulfur contents of the
eastern coal range from 0.5 to 3.0 percent; sulfur content
of the midwestern coal ranges from 3.0 to 5.0 percent. Each
type of coal imposes special problems for ESP performance
and reliability.
C.2 RELIABILITY AND MAINTENANCE EXPERIENCE
Very few of this utility's precipitator installations
have demonstrated the expected reliability or maintenance
cost. Major problem areas affecting reliability and main-
tenance involve (1) physical features of the collector
design, (2) ash removal problems, (3) operating conditions
such as gas temperature and coal sulfur, and (4) operating
and maintenance practices.
All plants equipped with precipitators maintain operat-
ing and maintenance logs, which provide information relating
to electrical operating conditions, ash removal operations,
maintenance activities, faulty conditions, and sectional
outage times. From these records, periodic summaries of
precipitator operation are prepared for management review.
These summaries contain various indices of precipitator
C-2
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reliability, including a precipitator availability factor
defined as the ratio of the average bus section hours of
operation to the hours of unit operation.
Figure C-l shows the average availability record for
all precipitators for a recent year of operation. The
overall weighted average availability for this period was
92.6 percent. Only 6 of the 37 units in service during this
period had 100 percent precipitator service availability.
Four of these 6 units were 60-MW standby units, which oper-
ated only for short periods of time. The other two units
with 100 percent availability were in the 150-MW class.
The principal cause of unavailability of precipitator
bus sections was grounding of the high-voltage electrode
systems resulting from malfunctions of the ash removal
system and excessive ash accumulations on the electrodes.
These problems accounted for 50 percent of the total bus
section unavailability. The second most serious cause of
bus section unavailability was discharge wire failures,
which accounted for 36.5 percent of the unavailability. All
other fault conditions such as failures of controls, switch-
gear, transformer rectifiers, and support insulators accounted
for the balance, or 13-5 percent of the unavailability.
C.2.1 Ash Removal Problems
Failure to maintain adequate evacuation of the collector
ash hoppers can lead not only to collector malfunction but
C-3
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O
I
S2 4
o
of.
AVERAGE
92.6
65- 70- 75- 80- 85- 90- 95- 97- 98- 99-
69.9 74.9 79.9 84.9 89.9 94.9 96.9 97.9 98.9 99.9
BUS SECTION AVILABILITX X
100
37 UNITS TOTAL
Figure C-l. Precipitator availability.
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also to possible wire burning, formation of ash clinker,
distortion of the high-voltage frames, and misalignment of
collecting plates. Most ash removal problems result from
insufficient capacity and flexibility of the ash removal and
disposal systems. Other contributing factors are operation
of low gas temperatures in combination with high-sulfur
coal, inadequate hopper insulation, substantially higher
quantities of ash in recent coal receipts, and less-than-
desirable operating and maintenance practice.
Most of this utility's precipitators are equipped with
sequentially operated dry ash removal systems. These systems
have proved acceptably reliable when they are designed,
operated, and maintained properly. For satisfactory opera-
tion, the precipitator hopper ash removal and disposal
systems should be completely divorced from other refuse
handling systems and should be of adequate capacity and
arrangement to permit evacuation of each ash hopper often
enough to prevent any appreciable accumulation in the hop-
pers. Unsatisfactory operation and maintenance of ash water
pumps, water jet nozzles, vacuum connections, and sequencing
controls can lead to loss of ash removal efficiency and to
possible precipitator malfunction.
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C.2.2 Discharge Wire Failure
The impact of wire failures on precipitator avail-
ability is a function not only of the frequency of failures,
but also of the degree of sectionalization and the diffi-
culty of removing failed wires from the precipitator during
unit operation. Most precipitators do not have suitable
isolation dampers to permit safe internal access while the
boiler is in operation; therefore, only broken wires that
can be reached from access hatches can be removed during
unit operation. The attachment designs for some types of
discharge wires require unit shutdown to make replacements;
some attachments, however, are simple loops or hooks, which
permit removal of broken wires during operation of some
units.
The incidence of wire breakage varies from essentially
zero on some collectors to several failures a day on other
collectors. The most severe case of wire failures occurred
on collectors serving cyclone furnace boilers burning coal
with about 4 percent sulfur and operating with about 150°C
(300°F) exit-gas temperature. The wire failures, which
occur immediately below the top hanger hook and corona
shield, are characterized by a progressive thinning of the
wire until failure occurs. This condition became pronounced
after about the first year of operation and led to the
ultimate failure or replacement of nearly all wires.
C-6
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Figure C-2 shows the frequency rate of wire failures on this
installation for two operating periods during the second
year of collector operation. These collectors have also
undergone widespread failure of collecting plate support
members, which were fabricated of carbon steel rather than
the low-alloy, corrosion-resistant steel that was specified.
The collector manufacturer attributes all of these problems
to low gas temperature and high-sulfur coal, although operat-
ing conditions are very close to the specified design condi-
tions. The total maintenance cost on these collectors for
the first 3 years of operation was 25 percent of the purchase
cost of the collectors. This is substantially higher than
the acceptable annual cost of 1.5 percent suggested by one
manufacturer.
Wire failures on other classes of precipitators occur
at a frequency of about 0.3 percent a year; nevertheless,
even this rate of failure can seriously impair collector
availability on units that do not permit removal of broken
wires during unit operation and on collectors of low sec-
tionalization.
C.2.4 Tran s f o rimer - Re cti f i e r Fai1ure s
Power set failures occur at an annual average frequency
rate of about 0.6 percent of the total number of sets
installed and contribute a small fraction of total collector
C-7
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O
I
oo
100
80
8 60
40
20
-J ,
ELECTROSTATIC PRECIPITATOR
WIRE FAILURE EXPERIENCE
_l_
_L
15 20 . 25
NUMBER OF SUCCESSIVE BUS SECTION FAILURES
30
35
Figure C-2. Number of successive bus section failures,
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unavailability. Most failures involved insulation break-
down, arcing between internal high-voltage switch contacts,
and contamination of the insulating fluid.
One spare 700-mA power set is maintained as an emer-
gency replacement for about 300 sets ranging in size from
400 to 1400 mA. Some large collector installations present
problems in the removal of power supplies, but recent
installations incorporate original design provisions for
handling power sets.
C.2.5 Support Insulator Failures
High-voltage electode system support insulators of the
cylindrical-tub type show a high rate of failure (about 5
percent a year) on installations operating with low gas
temperature and high-sulfur coal. This type of insulator is
subject to excessive fouling and to arc-over. Most insulator
failures involve fine hairline surface cracking and not
complete physical collapse. Insulator fouling and cracking
reduce effective voltage levels and collector performance
but rarely completely decommission a bus section.
C. 2. 6 Rapper and Vibrator Problems
Inadequacies of design, installation, operation, and
maintenance of rappers and vibrators can lead to excessive
dust accumulations on the electrodes, impaired performance,
and possible grounding of the high-voltage system. A fre-
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quent trouble source is binding of the collecting plate
rapper or vibrator rods at the points of roof penetration.
This trouble is commonly neglected and may lead to ineffect-
ual rapping. Pneumatic impact rappers installed on two of
the collectors have the highest maintenance cost and the
poorest reliability of all types. Severe misalignment has
occurred on one type of collector in which the collecting
plate assemblies are supported by the rapper rods.
C.2.7 Miscellaneous Items
Other common problems adversely affecting performance
and reliability are overheating and failure of automatic
control components and breakers, mechanical instability of
high-voltage electrode systems, oscillation of wires, mis-
alignment of wires and plates, infiltration of air, bypass
of dust through inactive zones, and reentrainment of dust
from hoppers. Precipitators that follow mechanical collec-
tors are sometimes erroneously charged with poor performance
as a result of air leakage and impaired efficiency of the
mechanical collectors.
C.3.8 Conclusions
The unsatisfactory performance of some of this utility's
early collectors resulted from inadequate definition of the
requirements with respect to the range of operating condi-
tions and also from the marginal sizing of collectors for
the specified operating conditions. Recent specifications
C-10
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more realistically define the expected operating conditions
and contain minimum acceptable sizing parameters and other
inducements for contractors to provide conservatively
designed precipitators.
Unfavorable reliability experience of some precipitator
installations has resulted mainly from shortcomings of ash
removal systems and excessive failures of discharge wires.
Adverse operating conditions of coal sulfur content and gas
temperature also contribute to poor reliability.
The emphasis of this utility on performance and reliabi-
lity problems is not meant to suggest that electrostatic
precipitators are not satisfactory for control of fly ash
emissions. The utility officials state that successful
installations are economically attainable by realistic
definition of the requirements, proper design, and adequate
operation and maintenance.
Oil
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/« TECHNICAL REPORT DATA
(fiease read Instructions on the reverse before completing)
. REPORT NO.
EPA-600/2-77-006
2.
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
Electrostatic Precipitator Malfunctions in the
Electric Utility Industry
5. REPORT DATE
January 1977
6. PERFORMING ORGANIZATION CODE
1. AUTHOR(S)
Mike Szabo and Richard Gerstle
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
PEDCo. -Environmental Specialists, Inc.
Atkinson Square, Suite 13
Cincinnati, Ohio 45246
10. PROGRAM ELEMENT NO.
1AB012; ROAP 21BAV-018
11. CONTRACT/GRANT NO.
68-02-2105
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Final; 9/75-8/76
14. SPONSORING AGENCY CODE
EPA-ORD
15. SUPPLEMENTARY NOTES JERL-RTP project officer for this report is D. C. Drehmel,
919/549-8411 Ext 2925, Mail Drop 61.
. ABSTRACT
report discusses precipitation malfunctions in the electric utility indus-
try. When a utility electrostatic precipitator (ESP) fails to achieve its design effi-
ciency, there must be a reason. Although the reasons are numerous, they can be
placed in two distinct categories: ESP degradation is attributable either to hardware
malfunctions or operation under improper conditions. The report discusses the vari-
ous types of ESPs in the electric utility industry , along with design considerations .
It summarizes the different types of malfunctions. For each type of malfunction, it
gives the cause, duration, corrective action, and preventive measures. The report
also gives the maintenance required to minimize the probability of malfunction. It
describes inspection techniques for evaluating maintenance procedures , including
what an inspector should look for during power plant ESP inspections.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Air Pollution Maintenance
Electrostatic Precipitators
Failure Inspection
Electric Utilities
Flue Gases
Dust
Air Pollution Control
Stationary Sources
Particulate
13B
14D
21B
11G
15E
13. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (ThisReport)
Unclassified
1. NO. OF PAGE
20. SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
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