DRAFT
1 INSPECTION PROCEDURES FOR
EVALUATION OF ELECTROSTATIC
PRECIPITATOR CONTROL SYSTEM
8 PERFORMANCE
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EPA-340/1-78-007
FINAL DRAFT
INSPECTION MANUAL
FOR EVALUATION OF
ELECTROSTATIC PRECIPITATOR
PERFORMANCE
by
Michael F. Szabo and Yatendra M. Shah
PEDCo Environmental, Inc.
11499 Chester Road
Cincinnati, Ohio 45246
Contract No. 68-01-4147
Task No. 36
EPA Project Officer: Kirk E. Foster
Division of Stationary Source Enforcement
Research Triangle Park, North Carolina 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Division of Stationary Source Enforcement
Washington, D.C. 20460
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DISCLAIMER
This report was furnished to the U.S. Environmental Protec-
tion Agency by PEDCo Environmental, Inc., Cincinnati, Ohio, in
fulfillment of Contract No. 68-01-4147, Task No. 36. The con-
tents of this report are reporduced herein as received from the
contractor. The opinions, findings, and conclusions expressed
are those of the authors and not necessarily those of the Envi-
ronmental Protection Agency.
11
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CONTENTS
Disclaimer ii
Figures v
Tables vii
Acknowledgment viii
1. Introduction 1-1
2. Overview of Electrostatic Precipitation 2-1
2.1 Introduction 2-1
2.2 Charging Mechanisms 2-3
2.3 Resistivity 2-5
2.4 Types of Electrostatic Precipitators 2-6
2.5 Basic ESP Components 2-13
2.6 Methods for Sizing of ESP Systems 2-25
2.7 Design and Sizing Parameters 2-29
2.8 Electrical Energization 2-33
References 2-35
3. Instrumentation and Records 3-1
3.1 ESP Performance Parameters 3-1
3.2 ESP Instrumentation 3-2
4. Operation, Maintenance, and Common Problems 4-1
4.1 Normal Operating Procedures 4-1
4.2 Maintenance Requirements 4-11
4.3 Precipitator Malfunctions 4-14
4.4 Reporting ESP Malfunctions 4-14
4.5 Operation, Maintenance, and Common Problems
of Wet ESP's 4-21
References 4-27
5. Inspection Procedures 5-1
5.1 Performing the Periodic Inspection 5-1
References 5-33
6. Performance Evaluation 6-1
6.1 ESP Efficiency Estimates 6-1
References 6-5
111
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CONTENTS (continued)
7. Case Histories 7-1
7.1 Cold-side Electrostatic Precipitator on a
Coal-fired Utility Boiler 7-1
7.2 Electrostatic Precipitator to Control Particu-
late Emissions from Cement Kilns 7-5
References 7-8
Appendices
A. Startup and Shutdown Procedures and Maintenance
Schedule for Electrostatic Precipitators (ESP's) A-l
B. Types of Electrostatic Precipitator Malfunctions B-l
IV
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FIGURES
No. Paqe
2-1 Basic processes involved in electrostatic precipita-
tion 2-2
2-2 Typical temperature-resistivity relationship 2-7
2-3 Typical electrostatic precipitator with top housing 2-8
2-4 Three types of wet electrostatic precipitators 2-11
2-5 Various designs of collection electrodes 2-15
2-6 Typical forms of discharge or corona electrodes 2-17
2-7 Precipitator charging system and wire hanging system 2-18
2-8 Supported electrode structures 2-19
2-9 Various combinations of electrical sectionalization
in an ESP 2-21
2-10 Tumbling hammer assembly for use with rigid discharge
electrode system 2-23
2-11 Action of guide vanes in preventing gas flow separa-
tion at flue turn and at flue expansion 2-24
2-12 Effect of two different methods of gas distribution
on flue characteristics in an ESP 2-26
2-13 Precipitator efficiency versus specific collection
area and precipitation rate w 2-32
2-14 Precipitator efficiency as a function of specific
collection area and modified precipitation rate
parameter w, 2-32
3-1 Typical ESP control panel 3-4
3-2 Example of ESP control panel console 3-5
3-3 ESP instrumentation diagram 3-6
v
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FIGURES (continued)
No. ' Page
3-4 Positions of measuring instruments 3-7
3-5 Connection diagram of the opacity monitoring system 3-13
4-1 ESP current wave form with and without silicon con-
trolled rectifiers 4-3
4-2 Typical fly-ash precipitator voltage-current char-
acteristics, five fields in series, no ash resistivity
problem 4-6
4-3 Typical precipitator operating voltage as a function
of gas temperature 4-8
4-4 Vibrator and rapper assembly, and precipitator high-
voltage frame 4-9
5-1 A sample opacity chart 5-22
6-1 Relationship between collection efficiency and
specific corona power for fly ash precipitators,
based on field test data 6-2
6-2 Efficiency versus specific corona power extended to
high collection efficiencies, based on field test
data on recently installed precipitators 6-2
A-l Precipitator log sheet A-6
B-l Shrouds for wire-weighted discharge electrodes B-4
VI
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TABLES
No. Page
2-1 Range of Basic Design Parameters in Operating Fly-ash
Precipitators 2-30
2-2 Design Factors for Precipitator Specification and
Evaluation 2-31
4-1 Maintenance Schedule for Electrostatic Precipitators 4-12
4-2 Summary of Problems Associated with ESP' s 4-15
4-3 Manufacturer's Suggested Maintenance Schedule for
Wet Precipitators 4-23
5-1 Preinspection Checklist for Electrostatic Pre-
cipitators 5-3
5-2 Plume Characteristics and Operating Parameters for
Coal-fired Boilers 5-14
5-3 Effects of Changes in Normal Operation on ESP Control
Set Readings 5-19
5-4 Recommended Recordkeeping Requirements 5-25
5-5 Inspection Checklist for Electrostatic Precipitators 5-26
5-6 Important Compliance Parameters and Conditions for
Issuance of a Citation 5-32
Vll
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SECTION 1
INTRODUCTION
The Division of Stationary Source Enforcement (DSSE) of
the U. S. Environmental Protection Agency (EPA), the Enforce-
ment Divisions of the EPA Regional Offices, and various
state and local agencies delegated authority to enforce
Federal emission standards have the responsibility for
enforcing emission standards for regulated stationary source
and for ensuring continued compliance with the standards.
An important part of the agency's stationary source compliance
monitoring and surveillance program is the inspection of
sources. The EPA field enforcement staff inspect sources at
regular intervals and determine their compliance status by
observing major operating parameters of the emission control
systems and process parameters affecting emissions.
At the present time, no guidelines exist for determining
whether a control system is adequately designed or is being
properly operated and maintained. Therefore, DSSE is plan-
ning to prepare technical guidelines for inspection and per-
formance evaluation of control systems. These guidelines
will provide the inspector with necessary background and
information relating to inspection and performance evaluation
of a particular control system. At present, the emphasis is
on sources already in operation. Another set of guidelines
applicable to new sources are being planned that will provide
technical information and procedures specifically for the pre-
construction engineering review of control equipment design
and construction and proposed operational parameters.
This report consists of six major sections. The first
four sections orient the reader to the basics of ESP operation,
maintenance, and sizing; the remaining two sections present
guidelines for inspection and performance evaluation.
1-1
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Section 2 presents the basics of ESP's. A brief history of
electrostatic precipitators is followed by descriptions of ESP
types and components, as well as sizing information. Section 3
deals with ESP instrumentation, the major operational parameters,
and operational records.
Operation and maintenance practices are outlined in Section
4. The discussion includes matching of various operating param-
eters for optimum performance, typical ESP maintenance schedules,
maintenance of peripheral accessories, and common ESP malfunc-
tions and their effects on performance.
Section 5 provides a detailed procedure for inspection of
ESP's, with an inspection checklist that covers all ESP com-
ponents and peripheral systems. Frequency and duration of in-
spections and operating conditions during inspection are dis-
cussed.
Section 6 presents the procedure for evaluating ESP perform-
ance on the basis of the inspection results. It describes the
significance of each inspection parameter with respect to per-
formance of the ESP and the interrelationships of various oper-
ating parameters. Performance case histories of two ESP instal-
lations are included to provide insight into ESP performance
evaluation.
The appendices provide supporting information for the
performance evaluation of ESP's. Appendix A outlines a typical
maintenance schedule for ESP's; and Appendix B discusses the
common ESP malfunctions.
1-2
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SECTION 2
OVERVIEW OF ELECTROSTATIC PRECIPITATION
2.1 INTRODUCTION1'2
The three basic processes involved in electrostatic precip-
itation are: (1) the transfer of an electric charge to suspended
particles in the gas stream, (2) establishment of an electric
field for removing the particles to a suitable collecting elec-
trode, and (3) removal of the particle layers from the precipita-
tor with as little loss to the atmosphere as possible. Figure 2-
1 illustrates the basic processes involved in electrostatic
precipitation.
In a single-stage precipitator, negative ions produced by a
high-voltage direct current (d.c.) corona are responsible for
charging the suspended particles and creating the electric field.
Two basic requirements of the precipitation process are:
(1) the precipitation chamber, in which the particles are elec-
trified and removed from the gas and (2) the high-voltage trans-
former and rectifier, which function to create the strong elec-
trical field in the chamber.
In practice, the precipitation chamber consists of a shell
made of metal, tile, or other similar material. Suspended within
the shell are grounded steel plates (collecting electrodes)
connected to the grounded steel framework of the supporting
structure and to an earth-driven ground. Suspended between the
plates are metal rods or wires (discharge electrodes), insulated
from ground and negatively charged at voltages ranging from
70,000 to 105,000 volts (70 to 105 kV). The great difference in
voltage of the wires and the collecting plates sets up a powerful
electrical field between them, which imparts a negative charge to
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the solid particles suspended in the gas stream. The negatively
charged particles are attracted to the collecting plates, which
are at ground potential.
Liquid particles, such as acid mists or tars, coalesce on
the collection plate and drain into a sump at the bottom of the
precipitator. The solid dust particles are removed by periodic
rapping of the collection plate so that the dust falls in sheets
into a receiving hopper. Solid material can also be removed by
irrigation of the collection electrode with water or other fluid
in what is termed a "wet" precipitator.
2.2 CHARGING MECHANISMS '2
Particle charging and subsequent collection take place in
the region between the boundary of the corona glow and the col-
lection electrode, where gas particles are subjected to the
generation of negative ions from the corona process. Charging is
generally done by field and diffusion mechanisms. The predomi-
nant mechanism varies with particle size.
In field charging, ions from the corona are driven onto the
particles by the electric field. As the ions continue to impinge
on a dust particle, the charge on it increases until the local
field developed by the charge on the particle causes distortion
of the electric field lines such that they no longer intercept
the particle, and no further charging will take place. This is
the predominant mechanism for particles larger than about 0.5
micrometers (ym).
The time required for a particle to reach its saturation
charge, varies proportionally to the ion density in the region
where charging takes place. Under normal conditions with sus-
tained high-current levels, charging times are only a few milli-
seconds. Limitation of current because of high resistivity or
other factors can lengthen charging times significantly and cause
the particles to travel several feet through the precipitator
before saturation charge is approached.
2-3
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The waveform of the secondary voltage can further affect the
charging times. The rectified unfiltered voltage has peaks and
lows occurring at regular intervals, which match with the fre-
quency of the primary voltage. Thus, the electric field varies
with time, and the dust particles in the interelectrode region
are subjected to time-varying fields. The particle charging is
interrupted for that portion of the cycle during which the charge
on the particle exceeds that corresponding to the saturation
charge for the electric field existing at the time. This further
lengthens the charging times and, in the case of high-resistivity
2
dust, degrades precipitator performance.
Diffusion charging is associated with ion attachment re-
sulting from random thermal motion, and is the dominant charging
mechanism for particles below about 0.2 ym. As with field charg-
ing, diffusion charging is influenced by the magnitude of the
electric field, since ion movement is governed by electrical as
well as diffusional forces. Neglecting electrical forces, an
explanation of diffusion charging is that the thermal motion of
molecules causes them to diffuse through a gas and contact the
particles. The charging rate decreases as a particle acquires a
charge and repels additional gas ions, but charging continues to
a certain extent because there is no theoretical saturation or
limiting charge other than the limit imposed by the field emis-
sion of electrons. This is because the distribution of thermal
energy ions will always overcome the repulsion of the dust par-
ticle.
The particle size range of about 0.2 to 0.4 ym is a transi-
tional region in which both mechanisms of charging are present
but neither is predominant. Fractional efficiency test data for
precipitators have shown reduced collection efficiency in this
transitional size range, where diffusion and field charging
overlap.
2-4
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2.3 RESISTIVITY
Since dust resistivity can greatly limit precipitator
performance when it exceeds about 10 ohm-cm, it is a major
factor in precipitator technology. Corona current flows through
the collected dust layer to reach the collection electrode.
With dry ESP's, high resistivity affects ESP efficiency by lim-
iting the current and voltage at which the ESP operates; this
limitation is caused by the electrical breakdown of the collected
dust layers across which corona current must flow.
If ESP electrodes are clean, the high-tension voltage can be
increased until a sparking condition is reached. The maximum
voltage is determined principally by the gas composition and ESP
dimensions. If dust is deposited on the collection electrode,
however, the voltage at which sparking occurs is reduced because
of the increased electric field at the dust surface. Further
increasing the dust resistivity decreases the voltage at which
12
sparking occurs. At very high values of resistivity (10 ohm-
cm) , the voltage can be reduced enough that sparks will not
propagate across the inter-electrode space, and dust breakdown
can occur at very low values of current and voltage. This can
result in a back corona in which positive ions form and flow back
toward the discharge electrode, neutralizing the negative charge
previously applied, and thereby limiting ESP performance.
In addition to reducing the performance of an ESP, high
resistivity dust can cling more tenaciously to collection elec-
trodes than particles with an intermediate resistivity. A much
greater rapping acceleration must then be applied to the elec-
trode to remove the dust layer. This can cause severe reentrain-
ment or damage to a precipitator that is not designed to with-
stand such high acceleration.
Fly ash resistivity depends primarily on the chemical com-
position of the ash, the ambient flue gas temperature, and the
4
amounts of water vapor and SO., in the flue gas. High resis-
tivity, which is characteristic of low-sulfur coal, causes
2-5
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uncertainty in sizing cold-side ESP's, which generally operate at
temperatures of 120° to 175°C (250°-350°F).
At low temperatures [<80°C (<175°F)], current conduction
occurs principally along the surface layer of the dust and is
related to the absorption of water vapor and other conditioning
agents in the flue gas. For fly ash, resistivity is primarily
related in an inverse manner to the amount of SO_ and moisture in
the flue gas. Burning of low-sulfur coal releases smaller
amounts of SO , which is oxidized to SO,.. A higher resistivity
fly ash results, except at temperatures below about 80°C (175°F),
where significant amounts of SO., are absorbed onto the fly ash
particles.
At elevated temperatures [<200°C (<400°F)], conduction takes
place primarily through the bulk of the material, and resistivity
depends on the chemical composition of the material. For fly
ash, resistivity above 200°C (400°F) is generally below the
critical value of about 10 ohm-cm, although it has been shown
to decrease with increasing amounts of sodium, lithium, and
5
iron.
The range of operation of cold-side fly ash precipitators is
80° to 200°C (250° to 400°F), a range in which conduction takes
place by a combination of the surface and bulk mechanisms and
resistivity of the ash is highest. Figure 2-2 illustrates this
relationship.
2.4 TYPES OF ELECTROSTATIC PRECIPITATORS
Two types of electrostatic precipitators are used for par-
ticulate control: dry and wet electrode precipitators. Within
each of these categories precipitators can be further classified
by electrode geometry. Most precipitators in use today are the
dry type with plate-type collection electrodes and pyramidal
hoppers. Gas flow is normally horizontal through the ESP,
Figure 2-3 presents an example of this type of precipitator.
Another common electrode arrangement is a wire-pipe (cylindrical)
ESP.
2-6
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SURFACE \ ,
RESISTIVITY^ \ 'VOLUME
. RESISTIVITY
COMPOSITE"'
|OF SURFACE!
AND VOLUME,
(RESISTIVITY]
70 150 250 400
100 200 300
600 800 1000
TEMP., °C
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Figure 2-2. Typical temperature-resistivity relationship.
2-7
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Transformer-Rectifier
Ground Switch Box
on Transformer
Top End
Frames
High Voltage
Conductor
High Tension
Support Insulators
Perforated
Distribution
Plates
Bottom End
Frames
Upper H.T. Hanger Assembly'
(Hanger and Hanger Frame)
Upper H.T. Wire
Support Frame
Discharge Electrode
Vibrator
Collecting Electrode
M.I.G.I. Rappers
Top Housing
Hot Roof
Access Door
Hot Roof
Side
Frames
Discharge
Electrode
Access Door
Between
Collecting Plate
Sections
Precipitator
Base Plate
Slide Plate
Package
Support Structure
Cap Plate
Horizontal
Bracing Strut
Steadying Bars
Lower H.T.
Steadying Frame
Collecting
Electrodes
Figure 2-3. Typical electrostatic precipitator
with top housing.
2-1
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2.4.1 Dry Precipitators
Dry precipitators are installed in industries with widely
varying conditions of gas temperature and pressure, such as
electric utility power boilers, cement kilns, and metallurgical
furnaces. In the electric utility industry, precipitators are
classified as cold or hot, depending on the location of the ESP
relative to the air preheater.
The cold-side ESP, located downstream of the air preheater,
operates in the range of 100° to 200°C (200° to 400°F). The
greatest disadvantage of a cold-side ESP is that its efficiency
varies with fuel composition and boiler firing conditions,
whereas the efficiency of a hot-side ESP is independent of these
factors.
Hot-side ESP's are located upstream of the boiler air pre-
heater. The operating range is generally between 300° and 450°C
(600° to 800°F). At this temperature, the high resistivity of
the low-sulfur coal ash is significantly lessened and its col-
lectibility is greatly increased. Also, because of the ESP
location, the heat transfer surfaces of the air preheater are
less likely to be fouled by fly ash. There are corresponding
reductions in the need for soot blowing of the air preheater and
in hopper plugging. One drawback of locating the ESP upstream of
the air preheaters is that the soot blown from the air preheater
cannot be captured; this may result in occasional increased
emissions.
The typical hot-side precipitator operates at lower voltage
than a cold-side unit. If designed correctly, it operates at
much higher current densities and is characterized by a rela-
tively high-power density and by stable, current-limited opera-
tion.
Thermal expansion has been a problem with hot ESP's. After
construction at ambient temperatures, the internals are main-
tained during operation at approximately 350° to 400°C (650° to
750°F), while the externals remain near ambient temperatures.
2-9
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Adequate provision for differential movement of the precipitator
on its support structure, proper insulation, and adherence to
design stress values have largely eliminated this problem.
Some ESP manufacturers favor cold-side installations, others
stress hot-side units; there is no adequate rule of thumb for
choice of either type. The selection is usually based on oper-
ability and economics.
For new construction at a coal-fired power plant, if the
specific collection area (SCA) required for a cold-side ESP is
23 2 *
greater than 100 m /m /sec (500 ft /1000 acfm), a hot-side unit
would be the proper choice. If the SCA can be smaller, a cold-
side unit could be used.
2.4.2 Wet Precipitators
Wet precipitators are used primarily in the metallurgical
industry, usually operating below 75°C (170°F). Until the late
1960's, their use was restricted mostly to acid mist, coke oven
off-gas, blast furnace, and detarring applications. Their use in
other areas is rapidly increasing as air pollution control codes
for those areas are becoming more stringent. The newer applica-
tions include sources with sticky and corrosive emissions that
must meet these standards. Because of inherent temperature range
limitations, they are not used for boiler installations.
The fundamental difference between a wet and a dry ESP is
that a thin film of liquid flows over the collection plates of a
wet ESP to wash off the collected particulates. In some cases,
the liquid is also sprayed in the gas flow passages to provide
cooling, conditioning, or sometimes a scrubbing action. When the
liquid spray is used, it is precipitated with the particles,
providing a secondary means of wetting the plates. Three dif-
ferent wet ESP configurations are discussed below and illustrated
in Figure 2-4.
Metric SCA (m2/m3/sec) = 0.197 x British SCA (ft2/1000 acfm)
2-10
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HATER SUPPLY
SPRAY NOZZLES
CORONA WIRES9 T0 DMIN
A. Plate type (horizontal flow)
GAS FLOW
GAS FLOW IN
SAS FLOW OUT
HIGH
VOLTAGE
LEADS
WATER PIPES
C. Conventional pipe type
GAS FLOW
HOOD AND STACK
TRANSITION
PRECIPITATOR
SECTION
TYPICAL METAL
ELECTRODE
DISCHARGE CAGE
CONTINUOUS FILM OF
LIQUOR FLOWS DOWN
POSITIVE COLLECTION
ELECTRODE SURFACES
(CYLINDER WALLS)
VENTURI INLET TO
PRECIPITATOR SECTION
BASE
B. Concentric plate type
Figure 2-4. Three types of wet electrostatic precipitators,
2-11
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Plate-type (Horizontal Flow)--
The effluent gas stream is usually preconditioned to reduce
temperature and achieve saturation. As the gas enters the inlet
nozzle, its velocity decreases because of the diverging cross
section. At this point, additional sprays may be used to create
good mixing of water, dust, and gas as well as to ensure complete
saturation before the gas enters the electrostatic field.
Baffles are often used to achieve good velocity distribution
across the inlet of the precipitator.
Within the charging section, water is sprayed near the top
of the plates in the form of finely divided drops, which become
electrically charged and are attracted to the plates, coating
them evenly. Simultaneously, solid particles are charged; they
"migrate" and become attached to the plates. Since the water
film is moving downward by gravity on both the collecting and
discharge electrodes, the particles are captured in the water
film, which is disposed of from the bottom of the precipitator in
the form of slurry.
Concentric-plate ESP—
The concentric-plate ESP consists of an integral tangential
prescrubbing inlet chamber followed by a vertical wetted-wall
concentric-ring ESP chamber. Concentric cylindrical collection
electrodes are wetted by fluids dispensed at the top surface of
the collection electrode system. The discharge electrode system
is made of expanded metal with uniformly distributed corona
points on the mesh background. This system is intended to com-
bine the high, nearly uniform, electric field associated with a
parallel plate system and the nearly uniform distribution of
corona current density associated with closely spaced corona
points. Higher gas flows can be handled by adding concentric
electrode systems and by increasing the length of each electrode.
Conventional Pipe-type ESP—
This system consists of vertical collecting pipes, each
containing a discharge electrode (wire type), which is attached
' 2-12
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to the upper framework and held taut by a cast-iron weight at the
bottom. A lower steadying frame keeps the weights and thus the
wires in position.
The upper frame is suspended from the high-voltage insula-
tors housed in the insulator compartments, which are located on
top of the precipitator shell (casing). Heating and ventilating
systems help to prevent accumulation of moisture and dust in the
insulator compartments.
The washing system usually consists of internal nozzles
located at the top of the plates. At specified intervals, the
tubes are washed thoroughly. During the washing, the louver
damper to the exhaust fan is closed to prevent carryover of
droplets.
2.5 BASIC ESP COMPONENTS
This section briefly describes major ESP components and
presents current nomenclature for a typical ESP configuration.
2.5.1 Precipitator Casing
The precipitator casing is of gas-tight, weatherproof
construction. Major casing parts are the inlet and outlet
connections, the shell and hoppers, inspection doors, and in-
sulator housing. The casing is fabricated of steel of a type
suitable for the application (type of process, heat range, etc.).
The shell is reinforced to handle maximum positive or negative
pressure, support the weight of the internals, and sustain en-
vironmental stresses such as those imposed by wind, snow, and
earthquake. The shell and insulator housing form a grounded
steel chamber, completely enclosing all the voltage equipment to
ensure the safety of personnel.
2.5.2 Dust Removal Systejti
Dust hoppers are required for temporary storage of the
collected dust. They should be large enough to hold the dust
2-13
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collected in a 24-hour period. The most common hopper design is
pyramidal, converging to a square or round discharge area.
To prevent plugging, hoppers should be kept clean, dry, and
if the dust is moisture-laden, warm. Many hoppers do not require
vibrators, but it is economical to install mounting devices for
future installation of vibrators should operation show that they
are necessary. If insulation does not keep the hopper warm
enough, additional heating of the hoppers may be required for
effective performance.
Frequently, hoppers are baffled at the division between two
dust-plate sections to prevent gas from bypassing the precipita-
tor.
Access to hoppers should be by external, key-interlocked
doors to prevent dangerous dust accumulations on the interior
side of the door. Enough "poke hole" ports should be provided to
allow for cleaning a blockage at the discharge.
Systems for removal of dusts accumulated in hoppers include
containers, dry vacuum, wet vacuum, screw conveyors, and scrape
bottom systems.
2.5.3 Collection Electrodes
Collection electrodes are the grounded components upon which
the dust collects. Many shapes of flat collecting electrodes are
used in ESP's, as shown in Figure 2-5, and some ESP's are de-
signed with cylindrical collection surfaces. All plate con-
figurations are designed to maximize the electric field and to
minimize dust reentrainment. All collection plates have a baffle
arrangement to minimize gas velocities near the dust surface as
3
well as to provide stiffness. Collection plates are commer-
cially available in lengths ranging from 1 to 3 meters (3 to 9
feet) and heights from 3 to 15 meters (9 to 50 feet). Generally,
these panels are grouped within the precipitator to form in-
dependently rapped collection modules. The total effective
length of these plates divided by their effective height is
referred to as the aspect ratio. Aspect ratios larger than 1.0
2-14
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GAS_
FLOW
WIRE DISCHARGE
ELECTRODES
OPZEL PLATES
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ROD
CURTAIN
_
FLOW
00000000
ZIG-ZAG
PLATE
DISCHARGE
ELECTRODE!
V-POCKETS
OFFSET SHIELDED
CHANNEL PLATES PLATES
GAS
V-PLATES
GAS < <« «
FLOW
I T
DUST
TULIP PLATE
ELECTRODES
Figure 2-5. Various designs of collection electrodes.
2-15
-------
provide longer residence time for the gas and increase collection
efficiency, all other factors being equal.
2.5.4 Discharge Electrodes
The discharge electrodes are maintained at high electrical
potentials during ESP operation. This ionizes the gas and estab-
lishes the electric field, which imparts a charge to the particle
and causes precipitation.
Discharge electrodes are also referred to as corona elec-
trodes, corona wires, cathodes, and high-voltage electrodes.
Discharge electrodes are metal, the type determined by the
composition of the gas stream. The form of the electrodes may be
cylindrical or square wire, barbed wire, or stamped or formed
strips of metal of various configurations (see Figure 2-6).
Discharge electrodes are mounted in a. variety of ways. They
may be suspended from an insulating superstructure with weights
at the bottom holding them tightly in place, or they may be
rigidly mounted on mats or frames. Regardless of how they are
mounted, they must be stabilized against swinging in the gas
stream. Examples of the wire weight and rigid wire systems are
shown in Figures 2-7 and 2-8.
2.5.5 Electric Power Supplies
The electrical energizing sets consist of high-voltage
silicon diode power packs developed specifically for the high
loads required for supplying high-voltage direct current to
electrostatic precipitators.
The power supply system consists of four components: a step-
up transformer, a high-voltage rectifier, a control element, and
a control system sensor. The system is designed to provide
voltage at the highest level without causing arc-over (sparking)
between the electrode and the collection surface. The automatic
control system maintains optimum voltage value, adjusting to
fluctuations in characteristics and concentrations of the dust.
A more detailed description of the operation of an automatic
control system is given in Section 3.1.
2-16
-------
Figure 2-6. Typical forms of discharge or corona electrodes.'
2-17
-------
SUPPORT INSULATOR
HOUSING
VIBRATION
ISOLATORS
HIGH VOLTAGE
BUS DUCT
BUS CONDUCTOR
HIGH VOLTAGE
SWITCH
TRANSFORMER-
RECTIFIER
PROTECTOR
TUBE
TENSIONING WEIGHT
DISCHARGE ELECTRODE
SUPPORT FRAME
DISCHARGE ELECTRODE
WEIGHT GUIDE FRAME
Figure 2-7. Precipitator charging system and wire
hanging system.
2-18
-------
SUPPORT FRAME
FRAME-TYPE
CORONA ELECTRODES
MAIN MAST SUPPORT
WIRES
Figure 2-8. Supported electrode structures,
2-19
-------
The electrical system of an ESP is arranged in bus sections,
each section representing any portion of the ESP that can be
energized independently. This is accomplished by subdividing the
high-voltage system and arranging the support insulators. Some
power supply arrangements are shown in Figure 2-9.
The sectionalization of precipitators is very important.
First, if the precipitator is sparking, less of the precipitator
is disabled during the spark interval if the system is highly
sectionalized. Higher average voltage and higher electric field
are maintained, and precipitator efficiency is not reduced.
Also, the smaller electrical sets have higher internal imped-
ances, which facilitate spark quenching and minimize the tendency
of a spark to arc. Smaller precipitator sections localize the
effects of electrode misalignment and permit higher voltages in
the remaining sections. Finally, with adequate sectionalization
in very large precipitators, reasonably good collection effi-
ciencies can be maintained if a section must be deenergized
because of wire breakage or other electrical trouble. All new
ESP's are designed so that if one field shorts out, the overall
ESP efficiency will not fall below specifications.
2.5.6 Rappers
Rapping systems are incorporated in an ESP to dislodge dust
from the collecting and discharge surfaces; their effectiveness
and reliability are essential. Generally available types of
rappers are pneumatic or electromagnetic impulse units, electric
vibrators, and mechanical hammers.
Rapper systems are designed to be compatible with the
internal suspension system and the number of surfaces affected
by the rapping shock. Pneumatic rappers supply the most shock
and dislodge tenacious dusts most readily. It is important in
any rapper system that all hardware is designed to withstand high
energy forces.
Pneumatically or electromagnetically operated rappers may be
of the impact or vibratory type. The impact type rapper functions
2-20
-------
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by lifting a weight to a controlled height and then allowing it
to fall against an anvil, which transmits the shock to the dis-
charge and collection surfaces. Vibratory rappers impart vibra-
tions to the discharge and collecting surfaces by means of rods
extending through the precipitator shell.
Rapping hammers, which are used with rigid discharge elec-
trode ESP's, remove dust very efficiently, but when installed in
a moving gas stream may require frequent maintenance. One type
of tumbling hammer design is illustrated in Figure 2-10.
The number and size of rappers and rapping frequencies vary
with the manufacturer and the nature of the dust. Generally one
2 2
rapper unit is required for 110 to 150 m (1200 to 1600 ft ) of
collecting area. Discharge electrode rappers serve from 350 to
2000 m (1000 to 7000 ft) of wire per rapper. Intensity of
rapping ranges from about 35 to 70 joules (25 to 50 ft-lb), and
rapping intervals are adjustable over a range of approximately 30
to 600 seconds.
The paramount consideration in rapping is to provide ac-
celeration to dislodge the dust without causing excessive re-
entrainment. Operation of rappers is discussed further in
Section 3.1.
2.5.7 Gas Flow Distribution
Proper gas flow distribution is critical for optimum pre-
cipitator performance. The plant flue system and its connections
to the ESP are more important than the precipitator itself in
determining the quality of gas flow through the precipitator. A
set of guide vanes, which is the most common device used to
direct gas flow, allows a streamlined flow of gas. Figure 2-11
illustrates how guide vanes prevent flow separation.
Diffusion screens and baffles are also used to reduce
turbulence and maintain uniform gas flow. A diffuser consists of
a woven screen or a thin plate with a regular pattern of small
openings. The effect of a diffuser is to break large-scale
turbulence into a large number of small-scale turbulent zones,
2-22
-------
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2-23
-------
FLOW SEPARATION.
FLOW SEPARATION-^
FLOW CONFIGURATION WITHOUT GUIDE VANES
Figure 2-11. Action of guide vanes in preventing gas flow
separation at flue turn and at flue expansion.
2-24
-------
which in turn decay rapidly and in a short distance coalesce into
a relatively low-intensity turbulent flow field. Two or three
diffusers may be used in series to provide better flow than could
be achieved with only one diffusion plate (see Figure 2-12).
2.6 METHODS FOR SIZING OF ESP SYSTEMS
Methods in current use for sizing electrostatic precipita-
tors include design by analogy to similar installations and by
theoretical application of fundamental precipitation principles.
Design by analogy is most reliable for industrial process ESP
installations where few process variabilities influence the
collection conditions. This method cannot be applied to sizing
of ESP's for the collection of fly ash emitted from coal-fired
sources, primarily because of the wide range of coals, boilers,
and operational methods.
The first equation for predicting particle collection
probability was developed by Anderson in 1919. It was derived
again by Deutsch, who used a different method, in 1922. In
various forms, this equation, n = 1-e w/v> f has become the
basis for estimating precipitator efficiency on the basis of gas
flow, precipitator size, and precipitation rate parameter. In
this equation, n is the precipitator collection efficiency, A is
the total collecting electrode surface area, V is the gas flow
rate, and w is the migration velocity of the particles. When
determined empirically, the precipitation rate parameter, w,
includes effects of rapping losses, gas flow distribution, and
particle size distribution.
The Deutsch-Anderson model assumes that particulate con-
centration is uniform in any cross section perpendicular to the
gas flow of an ESP. This assumption is made because of the
turbulence of the gas, which takes the particles near the collec-
tion surface and allows them to become electrically charged. A
serious limitation in use of the Deutsch-Anderson equation is
that it does not account for changes in the particle size dis-
tribution and subsequently the effective migration velocity as
2-25
-------
(3) PERFORATED
DISTRIBUTOR PLATES
RIGHT
(1) PERFORATED
DISTRIBUTOR PLATE
:HI6H FLOW:
.LOW FLOW
GAS FLOW
WRONG
Figure 2-12. Effect of two different methods of gas
distribution on flue characteristics in an ESP.7
2-26
-------
precipitation proceeds. This limitation affects the accuracy of
sizing estimates for units operating at very high efficiencies
(approximately 98% and above) because of the change in w with
particle size.
A semiempirical modification of the Deutsch-Anderson equa-
tion that essentially removes the size dependency from w was
developed by Matts and Ohnfeldt. This equation is n = l-e~wk( ' '
In most cases, k equals approximately 0.5. The modified migra-
tion velocity, w, , can be treated as being independent of charg-
ing voltage and current levels and of particle size distribution
within an ESP, as precipitation proceeds in the direction of gas
flow. Other changes, however, such as in properties of the gas
entering the ESP, resistivity, and size distribution, produce a
change in w, just as they change the conventional w.
JC
In practice, factors such as particle reentrainment and gas
leakage cannot be accounted for theoretically. In addition, some
of the most important physical and chemical properties of the
particles and gases often are not known. Therefore, most de-
signers use an effective precipitation rate parameter, w , that
g®
is based mainly on field experience rather than theory. Data
from operating installations form a general basis for selection
of w , and these data are modified to fit the particular situa-
tion being evaluated. Thus, w becomes a semiempirical parameter
that can be used in the Deutsch-Anderson equation or its deriva-
tives to estimate the collection area required for a given effi-
ciency and gas flow.
The most important parameters that determine w in practice
are resistivity, particle size distribution, gas velocity dis-
tribution through the ESP, particle loss due to reentrainment,
rapping and gas sneakage, ESP electrical conditions, and required
efficiency.
Another design technique applied to existing installations
or new processes to aid in a full-scale design is the pilot-scale
precipitator. The use of analog methods for the investigation of
gas flow and precipitation rate may be advisable in view of the
2-27
-------
complexity of the phenomena and the asymmetry of many gas-flow
systems. Theoretical calculations have various limitations. The
general principles of fluid mechanics and similitude may be
applied to yield useful results by using laboratory size models.
Flow patterns can be readily determined by the use of micro-pitot
tubes and various types of electrical anemometers. Pilot models
enable the use of smoke and several other techniques for flow
9
pattern determination. The main problem with use of a pilot-
scale ESP is that the pilot unit almost always performs better
g
3
3 8
than a full-scale unit. ' This can be attributed to better gas-
flow distribution, sectionalization, and electrode alignment.
The result is operation at higher current densities and voltages
than in a full-scale unit. Application of a scale-up factor, as
in spark-limited operation of a pilot-scale ESP, can cause un-
certainties in sizing the full-scale ESP. Therefore, pilot
precipitator data should be supplemented as fully as possible by
basic data on particle and gas properties, especially resis-
tivity.
Combustors are also used in conjunction with pilot scale
ESP's for design purposes. Use of combustors can allow develop-
ment of design data without transporting tons of coal to a power
plant for use in pilot-scale precipitators. Information gained
from operation of combustor-precipitator combinations is usually
qualitative, and much additional information is needed for
Q
application of the data to full-scale precipitator design.
Precipitator designers also make use of laboratory-scale
gas-flow models, which are very important to proper operation of
a full-scale ESP. Modeling techniques are well documented, and
close correlation of gas flow performance in laboratory models
g
and field installations is often achieved.
The Industrial Environmental Research Laboratory (IERL) of
U=S= EPA has done significant work in the area of pilot particu-
late collection systems. Pilot-scale versions of conventional
control devices such as electrostatic precipitators, fabric
filters, and wet scrubbers are mounted in mobile vans and carried
to the test sites.
2i~2 8
-------
The mobile ESP facility is designed for the purpose of de-
termining the effects of dust properties, plate spacing, elec-
trode spacing, rapping, and dust resistivity on electrostatic
precipitation parameters. Precipitation studies can be conducted
at gas flows up to 3000 acfm with a total system pressure dif-
f
ferential up to 26 inches of water and at gas temperatures up to
1000°F. Collection efficiencies of 96 percent and higher are
possible for particles having diameters in the submicron range.
The five sections of the ESP operate independently of one another
and can be switched off for maintenance and service.
The mobile facility can provide adequate data regarding the
dust characteristics. However, the facility cannot study the gas
flow characteristics and power input characteristics. The gas
flow characteristics depend upon the duct configuration of the
actual ESP, the power input characteristics depend upon the
layout of the electrical system. Each individual application
will require a full-scale pilot model, including the actual duct
configuration of the actual ESP, for studying the behavior under
actual operating conditions.
2.7 DESIGN AND SIZING PARAMETERS
Table 2-1 summarizes ranges of values for important basic
design parameters that are discussed in this section. Additional
factors that should be considered in precipitator design specifi-
cations and evaluations are summarized in Table 2-2. The three
most important parameters that affect the size of an ESP are
collection area, gas velocity, and aspect ratio.
2.7.1 Collection Area
Some variation of the Deutsch-Anderson equation is generally
used to estimate the required collection plate area. Figures 2-
13 and 2-14 present the relationships of specific collecting
areas (SCA's) developed with the Deutsch-Anderson w and Matts
"
Ohnfeldt w,, respectively.
2-29
-------
TABLE 2-1. RANGE OF BASIC DESIGN PARAMETERS IN
OPERATING FLY-ASH PRECIPITATORS
8
Parameter
Range of values
Plate spacing
Precipitation rate parameter for
Deutsch-Anderson equation (w )
Collection surface
Gas velocity
Aspect ratio (length °f plate)
Aspect ratio (height of plate)
Corona power
Corona current
Plate area per electrical set
No. of high-tension sections in
the direction of gas flow
Degree of sectionalization
20 - 30.5 cm
0.015 - 0.18 m/sec
18 - 145 m2/m3/sec
1.2 - 2.4 m/sec
0.5 - 1.5
30 - 300 watts/1000 m /sec
2
50 - 750 microamps/m
450 - 7400 m2
2-8
0 25 - 2 5 high~tension ^>us sections
100,000 m3/sec
2-30
-------
TABLE 2-2. DESIGN FACTORS FOR PRECIPITATOR
SPECIFICATION AND EVALUATION8
1. Corona electrodes: type and method of supporting.
2. Collecting electrodes: type, size, mounting, mechani-
cal, and aerodynamic properties.
3. 'Rectifier sets: ratings, automatic control system,
number, instrumentation, and monitoring provisions.
4. Rappers for corona and collecting electrodes: type,
size, range of frequency and intensity settings,
number, and arrangement.
5. Hoppers: geometry, size, storage capacity for col-
lected dust, number, and location.
6. Hopper dust removal system: type, capacity, protection
against air inleakage, and dust blow-back.
7. Heat insulation of shell and hoppers, and precipitator
roof protection against weather.
8. Access doors to precipitator for ease of internal
inspection and repair.
9. '.. Provisions for obtaining uniform, low-turbulence gas
flow through precipitator. This will usually require
a high-quality gas flow model study made by experienced
people in accord with generally accepted techniques,
with full report to precipitator purchaser before field
construction.
10. Quality of field construction of precipitator, includ-
ing adherence to electrode spacing and rigidity re-
quirements.
11. Warranties: performance guarantees, payment schedules,
adequate time allowance for performance tests, penalties
for nonperformance.
12. Support insulators for high-tension frames: type,
number, reliability. Air venting, if required.
13. Inlet and outlet gas duct arrangements.
14. Structure and foundation requirements.
2-31
-------
o
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Q_
o
o
99.9
99.8
99.5
99
98
97
95
90
80
50
0
TOO 200 300 400 500 600
SPECIFIC COLLECTION AREA, FT2/1000 ACFM
METRIC CONVERSION: FT2/1000 ACFM x .055- m2/1000 m3/S«e
Figure 2-13. Precipitator efficiency versus specific
collection area and precipitation rate we.°
QL
UJ
O.
O
LU
O
o
99.9
99.8
99.7
99.5
99
98
97
95
90
80
70
50
I
100 200 300 400 500 600
SPECIFIC COLLECTION AREA, Fr/1000 ACFM
METRIC CONVERSION:
r i /
2/1000 ACFM x .OS5 - m2/1000
Figure 2-14. Precipitator efficiency as a function of specific
collection area and modified precipitation rate paramater w,.8
2-32
-------
2.7.2 Gas Velocity
Designers usually calculate a hypothetical average value for
gas velocity from gas flow and cross section of the precipitator,
ignoring the localized variances within the precipitator. The
primary importance of the hypothetical gas velocity is to mini-
mize potential losses through rapping and reentrainment. Above
some critical velocity, these losses tend to increase rapidly
because of the aerodynamic forces on the particles. This criti-
cal velocity is a function of gas flow, plate configuration,
precipitator size, and other factors such as resistivity. Values
for gas velocity in fly ash precipitators range from 0.9 to 1.2
m/s (3.0 to 4.0 ft/s) in high-resistivity, cold-side ESP applica-
tions, and in all low-resistivity applications (hot or cold
side). For most other applications, the values range from 0.9 to
1.7 m/s (3.0 to 5.5 ft/s).
2.7.3 Aspect Ratio
Aspect ratio is defined as the ratio of the length to the
height of gas passage. Although space limitations often deter-
mine precipitator dimensions, the aspect ratio should be high
enough so that the reentrained dust carried forward from inlet
and middle sections can be collected. In practice, aspect ratios
range from 0.5 to 1.5. For efficiencies of 99 percent or higher,
the aspect ratio should be at least 1.0 to 1.5 to minimize carry-
over of collected dust.
2.8 ELECTRICAL ENERGIZATION
The way in which a precipitator is energized has a strong
effect on its performance. Electrical energization involves the
number and size of the transformer-rectifier (T-R) sets, number
of electrical sections in half wave/full wave (HW-FW) operation,
and changes in the voltage current characteristics as precipita-
tion proceeds in the direction of gas flow.
2-33
-------
Along with gas flow and total collection area, a third basic
design parameter is the power density (watts/m /min) (watts/1000
cfm) required to establish the appropriate voltage-current char-
acteristics of the corona for the type of fly ash entering the
precipitator. Power density is a function of electrical resis-
tivity and particle size, and of the composition, temperature,
and pressure of the gas. Power density is often conveniently
linked with resistivity such that for a moderate resistivety of
9 2
10 ohm-cm, the value will be approximately 8.2 watts/m (2.5
2
watts/ft ). For a high resistivity, the design value will be in
the neighborhood of 1.6 to 3.3 watt/m2 (0.5 to 1.0 watt/ft2).
If the current density and operating voltage are known, the
2 2
current density in mA/m (mA/ft ) can be calculated. This size
of the TR sets is selected to provide lower current density at
the inlet, where corona suppression is likely to decrease collec-
tion efficiency, and higher current density at the outlet, where
there is a greater percentage of fine particles.
2-34
-------
REFERENCES SECTION 2
White, H.J. Electrostatic Precipitation of Fly Ash, Part
I, Journal of the Air Pollution Control Association.
January 1977.
O^Lesby, Sabert, Jr. and Grady B. Nichols. Electrostatic
Precipitation In: Air Pollution, 3rd Edition, Vol. IV.
Engineering Control of Air Pollution. Academic Press. New
York, San Francisco, London. 1977. pp 189-256.
Smith, Wallace B. et al. Procedures Manual for Electro-
static Precipitator Evaluation. Southern Research Insti-
tute. Birmingham, Alabama. EPA 600/7-77-059. June 1977.
White, H.J. Electrostatic Precipitation of Fly Ash, Part
II, Journal of the Air Pollution Control Association.
February 1977.
Bickelhaupt, R.E. Journal of the Air Pollution Control
Association. 1974.
White, H.J. Electrostatic Precipitation of Fly Ash, Part
IV. Journal of the Air Pollution Association. April 1977.
Bump, R.L. Electrostatic Precipitators in Industry. Chem-
|j|al Engineering. January 7, 1977.
White, H.J. Electrostatic Precipitation of Fly Ash, Part
III. Journal of the Air Pollution Control Association.
March 1977.
White, H.J. Industrial Electrostatic Precipitation.
Addison-Wesley Publishing Company, Inc. Reading, Mass.
1963. 376 p.
2-35
-------
SECTION 3
INSTRUMENTATION AND RECORDS
The continued optimum performance of an ESP depends upon
effective control of operating parameters, which can vary because
of the physical characteristics of an ESP or changes in charac-
teristics of the flue gas being treated. Recent ESP installa-
tions are generally equipped with instrumentation for monitoring
and recording the major operating parameters. The ESP instrumen-
tation and records are the major indicators of ESP performance to
the inspector; the inspector should thoroughly understand the
function of each instrument and record to evaluate the ESP
performance.
This section deals with the instrumentation and records for
the major ESP parameters. A general discussion of the parameters
affecting ESP performance is followed by the discussion of in-
strumentation for each parameter. The types of records for the
proper operation of an ESP are presented.
3.1 ESP PERFORMANCE PARAMETERS
The various equations for ESP efficiency, discussed in
Section 2, indicate that efficiency is a function of gas flow
rate, Q, total collection area, A, and precipitation rate para-
meter, w. The precipitation rate parameter depends on the
sulfur concentration of flue gas, flue gas temperature, and
particle size distribution of the particulate matter in the flue
gas. The total collection area, which is fixed by the ESP manu-
facturer, is constant and cannot be varied. The flue gas charac-
teristics — temperature, flow rate, and particle size distribu-
tion -- depend on process or combustion conditions.
3-1
-------
The ESP manufacturer considers these efficiency parameters
during the design stages. The plant designates the ranges for
the various process or combustion conditions and flue gas charac-
teristics. For coal-fired boiler applications the plant provides
detailed analyses of types of coal fired and flue gas streams
under different operating conditions. The ESP manufacturer
designs the ESP on the basis of this information. The plant is
responsible for supplying accurate process and combustion data;
any deviations from these design values during the operating life
of an ESP may lead to unsatisfactory performance.
The ESP manufacturers maintain a large bank of data on
performance and operation of the ESP's they have constructed.
They refer to the data bank often during the design stages and
attempt to prevent any problems that may have developed with
similar ESP's.
In addition to efficiency parameters, satisfactory operation
of an ESP depends equally on various instantaneous operating
parameters. The power input to an individual discharge elec-
trode, for example, directly affects ESP performance. Spark rate
is another power input parameter, one that should be controlled
at a predetermined level for optimum performance. The other
major parameter is effectiveness of dust removal from hoppers.
Maintaining the various operating parameters within the
predetermined ranges requires a proper instrumentation and
record system.
3.2 ESP INSTRUMENTATION1
ESP instrumentation consists mainly of monitors for power
input, gas flow, rapper intensity, and hopper dust levels. The
power input parameters are precipitator current, voltage, and
spark rate. Gas flow parameters are the input gas flow rate and
input gas temperature.
The ESP instruments are generally located in close vicinity
of the ESP unit; when a plant has more than one ESP, a centrally
3-2
-------
located control room houses the instrumentation for all the ESP
units. The location of the ESP control room depends upon the
availability of space at the plant; however, it is generally
located close to ESP units. Two common control room locations
are: on top of the ESP units and directly under the ESP unit for
an elevated unit. The ESP control room is generally located
close to the ESP's and separately from the plant control room to
avoid long cable runs from the ESP to the plant control room.
The instruments for each ESP unit are assembled and housed
in a simple sheet metal cabinet. A typical ESP control room may
consist of a number of control cabinets arranged in a line and
each cabinet labeled to identify the ESP unit it serves. A
typical ESP control panel and control panel console are shown in
Figures 3-1 and 3-2, respectively. ,r~
An ESP instrumentation block diagram is shown in Figure
3-3. Figure 3-4 shows the positions of various measuring instru-
ments in an ESP circuit. The ESP in Figure 3-3 has four T-R
sets, and each T-R set has two bus sections. Primary voltage and
primary current are measured for each T-R set; secondary voltage,
secondary current, and spark rate are measured for each bus
section. Gas flow rate and temperature are measured at the ESP
inlet. An opacity measurement at the ESP outlet roughly indicates
emission levels. The ESP shown in Figure 3-3 has four primary
voltmeters and four primary ammeters; the secondary side instru-
mentation consists of eight secondary voltmeters, eight secondary
ammeters, and eight spark rate meters. The instrumentation
schemes for different ESP installations may vary slightly from
the basic scheme shown in Figure 3-3.
The relative location of each instrument in the circuit is
decided by its function. The primary ammeter, for example, is
always located ahead of the transformer primary in order to
indicate the current available for transformation. The inspector
should understand the relative positions of various ESP instru-
ments. The function of each ESP instrument is discussed in the
following paragraphs.
3-3
-------
Figure 3-1. Typical ESP control panel,
(Courtesy of Babcock and Wilcox Co.)
3-4
-------
Figure 3-2. Example of ESP control panel console,
(Courtsey of Babcock & Wilcox Company)
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3.2.1 Primary Instrumentation
Primary Voltmeter—
Energization power for an ESP is supplied at the primary
side of the T-R set. The a.c. electrical power is normally
supplied at 220 or 460 volts. Each T-R set for an ESP is normal-
ly provided with a voltmeter and an ammeter to indicate input
voltage and input current.
The range of a voltmeter dial for primary voltage is 0 to
480 volts. Normal operating voltage is around 220 or 460 volts,
depending on the selection of supply voltage. A temporary mar-
ginal deviation of about 5 percent from the rated supply voltage
is common and should not be considered a major operating problem.
Generally, a label on the primary voltmeter indicates the maximum
permissible voltage reading.
The voltmeter on the primary side is located ahead of the
T-R set but after the power control circuit, linear reactor, and
feedback network. This positioning of the voltmeter ensures
measurement of regulated voltage available at the T-R set for
transformation.
An indication of no voltage on the primary side may be due
to an open primary circuit. The circuit breaker may be open or
tripped, or the reactor secondary may be open. The open circuit
breaker can be closed with power on, but replacement of the
reactor secondary, requires power switchoff and ESP shutdown.
An indication of high voltage on the primary could result
from an open transformer primary or improper connection of an
ESP. A faulty, open, or disconnected precipitator, an open bus,
or a faulty rectifier will cause indication of high primary
voltage. An ESP shutdown is mandatory for correcting these
faults.
An indication of low voltage on the primary side could
result from several conditions such as a leak in the high-voltage
insulation, high dust level in hoppers, excessive dust on elec-
trodes, or swinging electrodes. Correction of the fault may
require shutdown.
3-8
-------
Primary Ammeter—
The ammeter on the primary side indicates the current drawn
by the ESP. The current and voltage readings on the primary
indicate the power input to a particular section of an ESP.
An ammeter for primary current measurement is located
between the T-R set and the power control circuit, linear reac-
tor, and feedback network. Figure 3-4 indicates the location of
an ammeter for primary current. Positioning the ammeter between
the T-R set and the control circuit ensures measurement of the
current available for transformation.
The ammeter is generally labeled to indicate the normal
range of primary current. Any deviation from this range indi-
cates abnormal operation of the following ESP section. A reading
that shows no primary current associated with no primary voltage
indicates an open primary circuit, which may be due to open
circuit breakers or an open reactor secondary. Minimal primary
current associated with high primary voltage is generally caused
by an open transformer primary or open secondary circuit.
Precipitator shutdown is mandatory to correct these faults.
Indication of low primary current with low primary voltage
results from an open d.c. reactor. Again, precipitator shutdown
is mandatory.
Irregular primary current coupled with low primary voltage
indicates a high resistance short in the circuit. Possible
causes are an electrode short with dust in the hopper, excessive
dust on collecting surfaces, excessive dust on electrodes,
support insulator arcing, and the presence of foreign materials.
A broken swinging electrode causes an intermittent short, which
is indicated by low primary voltage and cycling primary current.
3.2.2 Secondary Instrumentation
Instrumentation on the secondary side of the T-R set indi-
cates the power input parameters to an individual ESP bus section,
This instrumentation is of primary importance to the inspector.
The secondary instrumentation generally consists of a voltmeter,
3-9
-------
an ammeter, and a spark rate meter for each bus section. The
*
combination of these meter readings indicates the overall opera-
ting conditions on the secondary side of the T-R set.
Secondary Voltmeter—
The secondary voltmeter is calibrated in kilovolts to mea-
sure the high-voltage of the power input to the discharge elec-
trodes. The secondary voltmeter is labeled to indicate the upper
limit and normal range of the operative voltage.
The secondary voltmeter is located between the rectifier
output side and discharge electrodes to indicate the d.c.
voltage across the discharge electrodes.
An indication of no voltage on the secondary may be due to
an open primary circuit. The circuit breaker may be open or
tripped, or a reactor secondary may be open. As indicated
earlier, the circuit breaker can be closed with power on, but
replacement of a reactor secondary will require power switchoff
and ESP shutdown. A faulty, open, or disconnected precipitator,
an open bus, or a faulty rectifier will indicate high voltage on
the primary side and no voltage on the secondary side. Shutdown
of the ESP is mandatory for correcting these faults.
Low voltage on the secondary side coupled with the low
voltage on the primary side could result in several operating
problems such as those mentioned earlier, a leak in the high-
voltage insulation, excessive dust in the hoppers or on elec-
trodes, or swinging electrodes. Correction of the fault may
require shutdown.
Secondary Ammeter—
The ammeter on the secondary side indicates the current
being supplied to the discharge electrodes. The combination of
current and voltage readings on the secondary side indicates the
power input to the discharge electrodes. The location of the
secondary ammeter is shown in Figure 3-4.
The secondary current is stepped down in the transformer and
is measured in milliamperes. The secondary ammeter is therefore
3-10
-------
calibrated in milliamperes and is labeled to indicate the maximum
value and normal range of secondary current. Deviation from the
normal operating range indicates improper operating conditions in
the precipitator.
A combination of no secondary current with no secondary
voltage indicates an open primary circuit. Minimal secondary
current associated with high voltage is generally due to an open
transformer primary or open secondary circuit. Precipitator
shutdown is mandatory to correct these faults. An open d.c.
reactor will cause a low current flow and low voltage in the
circuit.
As with primary circuitry, irregular secondary current
coupled with low secondary voltage indicates a high-resistance
short in the circuit; possible causes are the same, usually
related to excessive dust, foreign materials, or arcing. Again,
a broken swinging electrode causes an intermittent short, indi-
cated by low voltage and cycling current.
Spark Rate Meter—
The spark rate meter on an ESP is a major indicator of its
performance. The spark rate meter is generally connected in the
secondary circuit; it indicates the number of sparks in the
precipitator section and is calibrated in number of electrical
sparks per minute.
The number of sparks together with secondary voltage and
secondary current give a fair presentation of the ESP operating
condition. Theoretically, the power input to a section is maxi-
mum when no sparks occur and voltage and currents are at their
rated maximum; in actual practice, however, the optimum power
input occurs at a preset spark rate. For ESP's on coal-fired
utility boilers, the optimum spark rate is around 100 sparks per
minute. Optimum spark rate depends on the physical and design
characteristics of the ESP.
Because excessive sparking indicates power loss, less power
is available for particle charging. A sparking rate below the
optimum level indicates a power supply rate below optimum.
3-11
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3.2.3 External Instrumentation
Certain instruments external to the ESP indicate parameters
that relate strongly to ESP operation. These include instruments
that measure inlet gas-flow rate, inlet gas temperature, opacity
of flue gas at the ESP outlet, and hopper ash discharge.
Inlet Gas Flow and Temperature--
The gas-flow rate and temperature are indicators of ESP
loading. Any variations from the normal design ranges will
affect ESP performance and should be investigated. At most
installations, the gas flow and temperature measurements are
taken at the exit of the process unit or boiler. The instruments
for recording the flow and temperature are generally located in
the main control room. Some installations maintain continuous
records of gas flow and temperature.
Opacity at the ESP Outlet—
Federal and state regulations generally regulate the opacity
of the flue gases in addition to limiting particulate emissions.
The more recent ESP installations are equipped with continuous
opacity recorders.
A general configuration of the opacity monitoring system is
shown in Figure 3-5. The system includes an opacity detector
unit that senses the opacity of the gases passing through the
ductwork. This unit includes a light source and a detector, the
difference between the amount of light transmitted by the light
source and that received by the detector indicates the particu-
late concentration of the gases. The difference is calibrated as
percent by the opacity meter. An opacity reading of zero percent
indicates a minimal particulate concentration, an opacity reading
of 100 percent indicates a maximum particulate concentration.
The variation in opacity gases cannot be solely attributed to the
particulate matter; hov/ever,- it gives a fair indication of
particulate concentration.
3-12
-------
ESP UNIT
TRANSITION
DUCT
|~h LIGHT SOURCE
i \
i-
DUCT TO STACK
• i
MDETECTOR
O
O
O
CONTROL UNIT
O
O
O O
STRIP CHART RECORDER
Figure 3-5. Connection diagram of the opacity monitoring system.
3-13
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SECTION 4
OPERATION, MAINTENANCE, AND COMMON PROBLEMS
The following sections provide the inspector with operating
procedures, maintenance requirements, and common malfunctions of
modern ESP's. This information is presented to orient the
inspector with the operation and maintenance requirements that
are necessary to obtain the highest possible collection effi-
ciency. This will aid the inspector in determining whether the
company has an adequate maintenance program. The ESP can then be
maintained at its highest performance level with minimum loss of
time because of malfunctions. Much of the discussion is based on
the wire-weight type of ESP as applied in removal of fly ash.
However, the inspector can apply many of these procedures to
ESP's on other processes. Further, the discussion chiefly
concerns dry ESP's since these units are more widely used than
wet ESP's and many of the operating problems for different
applications are similar. Section 4.5 deals with some procedures
specific to operation of wet ESP's.
4.1 NORMAL OPERATING PROCEDURES1
Because the basic functions of an ESP are charging and
collection of particles, the components and control associated
with the power supply, rappers, and vibrators, are the most
important operating systems.
4.1.1 Power Supply
During normal operation, the power to the precipitator is
optimized by automatic power controls, which vary the power
parameters in response to a signal generated by the spark rate.
4-1
-------
The automatic controls also make the circuit sensitive to over-
load and provide safety control in the event that spark level
cannot be reached.
The automatic control circuit controls spark rate, current,
and voltage. Although earlier ESP's used saturable reactors for
power control, modern ESP's use silicon controlled rectifiers
(SCR's); this report discuses only SCR's. The components of an
automatic control system were previously shown in Figure 3-2.
The silicon controlled rectifiers provide a wide range of pre-
cipitator current, and the current-limiting reactor limits the
swinging of current during precipitator sparking.
Silicon Controlled Rectifiers (SCR's)—
The SCR's act as a variable impedance in controlling the
flow of power in the circuit. An SCR is a three-junction semi-
conductor that is normally an open circuit until an appropriate
signal is applied to the gate terminal, at which time it rapidly
switches to the conducting state. Its operation is equivalent to
that of a thyroton. The flow of current is controlled by the
forward blocking ability of the SCR's, which in turn is con-
trolled by the firing pulse to the gate of the SCR. The current-
limiting reactor reshapes the wave form of the current and the
peak that occurs during sparking. Current wave form with and
without SCR's is illustrated in Figure 4-1.
Sparking Control—
Conventional spark control is based on storing electrical
pulses in a capacitor for each spark that occurs in the precipi-
tator. If the voltage of the capacitor exceeds a preset reference
value, an error signal will phase the mainline SCR's back to a
point where the sparking will stop. Usually this snap-action
type of control tends to overcorrect and thus leads to longer
downtime than is desirable. At low sparking rates, about 50
sparks per minute, the overcorrection is more pronounced and
voltage is reduced for a longer period, with subsequent loss of
dust and low ESP efficiency.
4-2
-------
Proportional control, another method of spark control, is
also based on storing electrical pulses for each spark that
occurs in the precipitator. In this system, however, phaseback
of the mainline SCR's is proportional to the number of sparks in
the precipitator. The main advantage of proportional control
over conventional spark control is that the precipitator deter-
mines its own optimum spark rate, based on four factors: temper-
ature of the gas, dust resistivity, dust concentration, and
internal condition of the precipitator. With proportional spark
rate control, therefore, the precipitator determines the optimum
operating parameters, whereas with conventional spark control,
the operator selects the parameters, which may not be optimum.
Some precipitators operate at the maximum voltage or current
settings on the power supply with no sparking. In collection of
low-resistivity dusts, where the electric field and the dust
deposit are insufficient to initiate sparking, a no-spark condi-
tion may arise. This condition does not necessarily indicate
that the unit is underpowered, since it may have sufficient power
to provide charging and electric fields without sparking.
Voltage Limit Control--
The voltage-limit unit of the automatic control module
limits the primary voltage of the high-voltage transformer to its
rated value. A potential transformer across the primary circuit
supplies a voltage signal that is compared with a preset voltage
value. The voltage control is set at the primary voltage rating
of the high-voltage transformer. Primary voltage above this
value generates a signal that retards the firing pulse of the
firing module and brings the primary voltage back to the control
setting.
Current-limit Control—
For current-limit control, a current transformer in the
primary circuit of the high-voltage transformer monitors the
primary current. The voltage from this current transformer is
4-4
-------
compared with the setting of the current control, which corre-
sponds to the rating of the T-R unit. Any primary current that
exceeds the unit's rating generates a signal that retards the
firing pulse of the firing circuit (as with spark control) and
reduces the current to the current-limit setting.
With all three control functions properly adjusted, the
control unit energizes the precipitator at its optimum or maximum
level at all times. This level is determined by conditions
within the precipitator and results in one of the three automatic
control functions operating at its maximum, i.e., primary voltage,
primary current, or spark rate. When one of the three maximums
is reached, the automatic control prevents any increase in power
to reach a second maximum. If changes within the precipitator so
require, the automatic control will switch from the maximum limit
of one function to that of another.
The system also includes secondary overload circuits and an
undervoltage trip device that operates when voltage on the pri-
mary of the high-voltage transformer falls below a predetermined
level and remains below that level for a period of time. Another
device provides a delay period in the annunciator circuit while
the network of contacts is changing position to stabilize the
circuit in response to undervoltage. The corona voltage-current
characteristics of ESP's are controlled basically by gas and dust
loading, electrode geometry and alignment, and size of the
individual section energized.
The electrical equipment ratings must be properly matched
2
with load requirements. Over a wide range of gas temperatures
and pressures in different applications, practical operating
voltages range from 15 to 80 kV . at average corona current
av „ „
densities of about 100 to 3200 mA/1000 m (10 to 300 mA/1000 ft )
of collecting area. Figure 4-2 illustrates typical voltage-
current characteristics of a five-field fly-ash precipitator
without ash resistivity problems.
4-5
-------
1075
10
20 30 40
PRECIPITATOR, kV (AVERAGE)
Figure 4-2. Typical fly-ash precipitator voltage-current
characteristics, five fields in series, no ash
resistivity problem.2
4-6
-------
The following are some of the problems that can occur when
2
power supply and load are mismatched:^
0 The ESP is underpowered because of too few electrical
sets, sets of wrong capacity, or too much collecting
area energized from a single set.
0 Reduction of operating voltage with gas temperature, as
shown in Figure 4-3, can result from failure to fully
evaluate effects of gas density and temperature on
required operating levels. While voltage goes down,
the current demands go up. At high pressures the
reverse can occur.
0 A rectifier set larger than is required for the appli-
cation, as when 1500 mA saturable reactor sets are
operating on high resistivity ash at perhaps 100 mA,
can lead to loss of control, excessive sparking, and
poor efficiency.
4.1.2 Rappers
The rapper system is electrically operated to remove dust
continuously from the precipitator's collecting plates. The most
common system consists of magnetic-impulse, gravity-impact
rappers that are energized periodically to rap the collecting
plates to remove dust deposits. The main components of the
system are the rappers and the electrical controls.
The magnetic-impulse, gravity-impact rapper, shown in
Figure 4-4, is a solenoid electromagnet consisting of a steel
plunger surrounded by a coil, both enclosed in a watertight steel
case. The control unit contains all the components (except the
rapper) needed to distribute and control the power to the rappers
for optimum precipitation. The electrical controls provide
separate adjustments so that the rappers can be assembled into
different groups, each of which can be independently adjusted
from zero to maximum rapping intensity. The controls are adjusted
manually to regulate the release of dust from the collecting
plates and prevent undesirable "puffing" from the stack.
During normal operation, a short-duration d.c. pulse through
the coil of the rapper supplies the energy to move the steel
plunger. The plunger is raised by the magnetic field of the coil
4-7
-------
70
60
50
40
30
20
10
0
23 cm (9 inch) DUCT SPACING
0.277 cm (0.109 inch) DIAMETER WIRES
NEGATIVE POLARITY
1 atm, ,
0 38 98
(100) (200)
149 204 260 315
(300) (400) (500) (600)
GAS TEMPERATURE, °C (°F)
371 426
(700) (800)
Figure 4-3. Typical precipitator operating voltage as a
function of gas temperature.
4-8
-------
DISCHARGE
ELECTRODE
VIBRATOR
DISCHARGE
ELECTRODE
VIBRATOR
DISCHARGE ELECTRODE VIBRATOR
AND INSULATOR ASSEJJBLY
COLLECTING
ELECTRODE
RAPPER
RAPPER
COUPLING
COLLECTING ELECTRODE RAPPER
AND INSULATOR ASSEMBLY
Figure 4-4. Vibrator and rapper assembly, and
precipitator high-voltage frame.1
4-9
-------
and then is allowed to fall back and strike a rapper bar, which
is connected to a bank of collecting electrodes within the pre-
cipitator. The shock transmitted to the collecting electrodes
dislodges the accumulated dust.
In some applications, the magnetic-impulse, gravity-impact
rapper is used to clean the precipitator discharge wires. For
this purpose, the rapper strikes the electrode supporting frame
in the same manner, except that an insulator isolates it from the
high voltage of the frame, as shown in Figure 4-4.
Some installations have mechanical rappers, in which a
single hammer assembly mounted on a shaft raps each frame (see
Figure 2-10) . A low-speed gear motor is linked to the hammer
shaft by a drive insulator, fork, and linkage assembly. Inten-
sity of rapping is governed by the hammer weight, and frequency
is governed by the speed of rotation of the shaft.
4.1.3 Vibrators
A vibrating system creates vibrations in either the collect-
ing plates or the discharge wires to dislodge accumulations of
particles. Vibrators are not normally used to clean the collect-
ing electrodes of precipitators that collect fly ash.
The vibrator is an electromagnetic device, the coil of which
is energized by alternating current. Each time the coil is
energized, the resulting vibration is transmitted through a rod
to the high-tension wire supporting frame or collecting plates
(see Figure 4-3). The number of vibrators depends on the number
of high-tension frames or collecting plates in the system.
The control unit contains all devices for operation of the
vibrators, including means of adjusting the intensity and period
of vibration. Alternating current is supplied to the discharge
wire vibrators through a multiple cam-type timer to provide
sequencing and duration for energization of the vibrators.
For each installation, a certain intensity and period of
vibration will produce the best collecting efficiency. Low
intensity will result in heavy buildup of dust on the discharge
4-10
-------
wires. Dust buildup reduces the sparkover distance between the
electrodes, thereby limiting the power input to the precipitator.
It also tends to suppress the formation of negative corona and
the production of unipolar ions required for precipitation.
Further, dust buildup alters the normal distribution of electro-
static forces in the treatment zone and can lead to oscillation
of the discharge wires and the high-tension frame.
Recent studies have investigated reentrainment caused by
rapping in terms of the percentage of material reentrained and
3 4
its particle size distribution. ' One report describes the
testing of six full-scale electrostatic precipitator installa-
tions. Losses from rapping ranged from over 80 percent of the
total mass emissions from one hot-side unit to 30 percent of
emissions from cold-side units. The losses consist mostly of
relatively large particles, primarily those larger than 2.0 ym
in diameter.
Tests of a pilot-scale precipitator showed that rapping
4
emissions decreased as time between raps was increased.
Because reentrainment from rapping can be a significant
portion of the total emissions, it is important that the rapping
system is adjusted to minimize reentrainment.
4.2 MAINTENANCE REQUIREMENTS5'6'7'8
At each precipitator installation, the inspector should
require that a preventive maintenance schedule be kept, listing
the precipitator parts to be checked and maintained daily, weekly,
monthly, quarterly, and in specified situations. Table 4-1
summarizes the maintenance procedures discussed in this section
and can be used by an inspector as an example to be compared with
procedures used by the company.
Startup and shutdown procedures and a maintenance schedule
for ESP's are presented in Appendix A. These procedures can be
applied to wire-weight and rigid-frame precipitators, with the
noted differences regarding rappers. This information is in-
4-11
-------
g
TABLE 4-1. MAINTENANCE SCHEDULE FOR ELECTROSTATIC PRECIPITATORS
Enter on daily log
1. Boiler operating parameters
2. Flue gas analysis
3. Coal characteristics
4. Particulate collector control readings
5. Transmissometer calibration
Check daily
1. T-R control set readings
2. Rapper and vibrator control settings
3. Ash removal system
4. T-R control room ventilation system
Check weekly
1. Operation of rappers and vibrators
2. Control sets (for internal dirt)
3. Air filters to control sets and precipitator top
housing
Check monthly
1. Pressurization of precipitator top housing
2. Standby fan operation (manually)
Perform quarterly
1. Clean and dress contact surfaces of HW-FW electrical
distribution
2. Lubricate pivots
Perform semiannually
1. Clean and lubricate access door hinges and test
connections
2. Inspect exterior for loose insulation, corrosion, loose
joints, other defects
(continued)
4-12
-------
TABLE 4-1. (continued).
3. Check for points of gas leakage (in or out)
Perform annually
1. Thorough internal inspection:
Check for possible leaks of oil, gas, or air at gas-
keted connections
Check for corrosion of any component
Check for broken or misaligned wires, plates, insulators,
rappers, etc.
Check high-voltage switchgear and interlocks
Clean all insulators and check for hairline cracks or
tracking
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
4-13
-------
tended to give the inspector an idea of what the company should
be doing to properly operate and maintain their ESP.
4.3 PRECIPITATOR MALFUNCTIONS
Many ESP equipment components are subject to failure or
malfunction that can cause an increase in emissions. Malfunctions
may be caused by faulty design, installation, or operation of the
ESP. They may involve electrical, gas flow, rapping, or mechani-
9
cal problems, which can be minor or severe. The inspector
should orient himself with the common ESP malfunctions, their
effects on emissions, corrective actions, and preventive measures.
Table 4-2 lists common problems associated with fly-ash ESP's.
Appendix B identifies and describes the major types of ESP mal-
functions, giving probable causes and corrective actions. Two
surveys of ESP operating experience are summarized.
4.4 REPORTING ESP MALFUNCTIONS
Generally, plant officials are required to submit a report
of excess emissions caused by ESP malfunctions; the inspector
should review these reports to keep informed of the ESP operating
practices at the plant. Part 60 of Title 40, Code of Federal
Regulations, Section 60.7, as amended, December 16, 1975,
requires that a source report excess emissions caused by malfunc-
tions or other reasons in a quarterly report to the EPA Adminis-
trator.1^ The report is to include the magnitude of excess
emissions above the applicable standards; it is to give the date
and time of beginning and end of each period of excess emissions.
Periods of excess emissions resulting from startup, shutdown, and
malfunction are to be specifically identified. The nature and
cause of any malfunction, if known, and the corrective action
taken, or preventive measures adopted, are to be reported. Each
quarterly report is to be submitted within 30 days following the
end of the calendar quarter.
4-14
-------
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4-19
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A reportable malfunction is considered to be any sudden or
unforeseen malfunction that causes or could cause any of the
plant's sources to exceed specified particulate emission limits
for a period of 4 or more hours. When this occurs, a procedure
such as the one listed below should be followed:
0 The malfunction should be reported by phone or telegram
to the EPA regional office and to state or local
officials. The air quality branch of the company should
also be notified.
0 The plant superintendent should submit a report to the
EPA regional office, with copies to various branches of
the utility. The report should include the following:
Time and date excess emissions began and ended
Time and date the breakdown causing the excess
emissions began and ended
Type of emission, estimated rate, and copies of
the opacity monitor records
Cause of the malfunction
Operation and maintenance procedures, prior to
and during the malfunction, designed to prevent
such an occurrence
Additional steps taken to minimize the extent or
duration of the malfunction
Plans to minimize the possibility of a similar
malfunction in the future
Monthly records should be kept by plant and unit of all
malfunctions, total hours that T-R sets are operated, number of
hours T-R sets are not operating (in intervals of 24 hours and
>24 hours), maximum number of sets out at one time, and monthly/
yearly availability of the ESP unit (in percent). Daily logs
should be kept on each ESP unit, with remarks on outages of
various sections of the ESP.
4-20
-------
4.5 OPERATION, MAINTENANCE, AND COMMON PROBLEMS OF WET ESP' S
The several available types of wet precipitators can be
categorized with regard to structure as the plate type or the
pipe type and also with regard to the method in which water is
introduced. Plate units have either a combination presaturation
continuous spray or a spray with intermittent wash and gas flow
in the horizontal or vertical directions. The pipe type usually
has a weir-over-flow arrangement or a continuous spray, with gas
flow vertically downward. Brief descriptions of the operation of
these three types of wet ESP,s were presented in Section 2.4.2.
This section describes the operation and maintenance related
facets of typical wet ESP installations. Wet precipitators are
made in different configurations; they are less widely applied
than the more conventional devices, and there is little published
information on operating and maintenance practices. When the
components of a wet precipitator are similar to those of a dry
precipitator, the operation and maintenance procedures outlined
in Section 4.2 and Appendix A are considered applicable.
4.5.1 Operation and Maintenance During Normal Operation
Heaters and blowers are usually energized first during
normal operation. The spray system is always activated just
before the high-voltage system is energized. Gas flow is moni-
tored by damper. Operators must monitor the pH of the water at
the waste discharge.
Because inspection and maintenance of wet ESP's are highly
specific to the system, operators should follow closely the
instructions provided by the manufacturer. Since precipitators
operate with very high voltage, precautions must be exercised to
ground the precipitator internals properly. The gas flow must be
stopped and the unit cooled to a safe temperature before any
person enters the precipitator. Protective apparatus such as a
respirator may be needed.
4-21
-------
The inspector should familiarize himself with the general
inspection and maintenance practices that the company should be
following; these are briefly outlined below.
Mechanical Maintenance—
All internal components should be checked for alignment,
dust buildup, tightness of bolts, structural soundness of welds,
and structural integrity of cross bracing and other support
members. Since the support insulators perform such a vital
function in electrostatic precipitation, the structural support
end of the high-voltage insulator in the high-voltage housing
must be thoroughly inspected for cracks, chips, or other defects.
Water System--
All pumps, internal spray nozzles, and related valving and
piping should be checked. Nozzles are subject to plugging and
therefore should be routinely disassembled, cleaned, and/or
replaced as necessary. Other required checks include the main
supply pumps for water pressure, all pipe joints for leaks, and
all couplings for tightness. Nozzle orientation should be checked
and adjusted as necessary to maintain the intended spray pattern.
Electrical System—
Inspection should include the high-voltage control panel,
heater and blower control panel, high-voltage insulators, heater
system thermostats, T-R sets, and all related electrical connec-
tions. When components of wet precipitator systems are similar
to those of dry units, many of the inspection and maintenance
practices outlined earlier will apply.
Schedule--
Table 4-3 summarizes a typical maintenance schedule, as
recommended by the manufacturer and given to the plant at the
time of wet ESP installation.
4.5.2 Wet ESP Operating Problems
Data on operating problems of wet ESP's are minimal.
However, the following information obtained on a recent sinter
4-22
-------
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4-25
-------
plant wet ESP installation is considered typical of the problems
an inspector might encounter.
Pilot tests of a wet ESP on a sinter plant in New York State
showed that use of acidic recirculated" water involves great
potential for corrosion of reasonably priced alloys such as 316L
1?
stainless steel. Operation of pH greater than 7.0, however,
caused buildup of calcium and magnesium carbonate scale on spray
nozzles and other critical components of the ESP, rendering it
inoperable. In pilot tests of a second brand of wet ESP, the
water distributor became plugged and recirculation of liquor
containing acidic solids caused deposition of solids.
In full-scale testing at the same sinter plant, however, the
first wet ESP performed very well with regular weekly maintenance
and inspection. Considerable attention to the recirculating
water system was required to maintain the water quality that is
12
needed for successful wet ESP operation.
4-26
-------
REFERENCES - SECTION 4
1. Szabo, M.F., and R.W. Gerstle. Operation and Maintenance of
Particulate Control Devices on Coal-Fired Utility Boilers.
PEDCo Environmental, Inc., Cincinnati, Ohio. EPA-600/
2-77-129. July 1977.
2. Hall, H.J. Design and Application of High Voltage Power
Supplies in Electrostatic Precipitation, H. J. Hall Asso-
ciates, Inc. Presented at Symposium on Electrostatic
Precipitators for the Control of Fine Particles. Pensacola
Beach, Florida. EPA-650/2-75016. September 30-October 2,
1974.
3. Spences, Herbert W. III. Rapping Reentrainment in a Nearly
Full Scale Pilot Precipitator. EPA-600/2-76-140. May 1976.
4. Sparks, Leslie E. et al. Studies of Particle Reentrainment
From Electrode Rapping, presented at the 2nd US/USSR
Symposium on Particulate Control. Research Triangle Park,
North Carolina. September 26-29, 1977.
5. Bibbo, P.P., and M.M. Peacos. Defining Preventive Mainte-
nance Tasks for Electrostatic Precipitators, Research
Cottrell, Inc. Power. August 1975, pp. 56-58.
6. Engelbrecht, H.L. Plant Engineer's Guide to Electrostatic
Precipitator Inspection and Maintenance, Air Pollution Div.
of Wheelabrator Frye, Inc., Plant Engineering. April 1976.
pp. 193-196.
7. Szabo, M.F., and R.W. Gerstle. Electrostatic Precipitator
Malfunctions in the Electric Utility Industry. PEDCo
Environmental, Inc., Cincinnati, Ohio. EPA-600/2-77-006.
January 1977.
8. Hesketh, H.E., and F.L. Cross, Jr. (ed.) Handbook for the
Operation and Maintenance of Air Pollution Control Equipment.
Technomic Publishing. Westport, Connecticut. 1975.
9. Oglesby, Sabert, Jr. A Manual of Electrostatic Precipitator
Technology. Southern Research Institute. August 1970.
10. Federal Register, Part 60 of Title 40, Section 60.7 as
amended December 16, 1975.
4-27
-------
SECTION 5
INSPECTION PROCEDURES
The most important task of an environmental control agency
in terms of ESP performance is to establish and execute a program
to ensure that ESP's at each plant continue to operate in com-
pliance with particulate emission regulations. This program
should include (1) a system by which the plant reports key infor-
mation to the control agency, and (2) random unannounced inspec-
tions as well as periodic announced inspections. The frequency
of inspection will vary with the policy of each control agency
and with the size and quality of the enforcement staff. Quarterly
inspections are recommended for most plants, but many control
2
agencies can manage only an annual inspection.
Comparison of operating parameters observed during inspec-
tion with those recorded in the performance test should indicate
whether emissions are within allowable limits. Major emphasis is
placed on checking plant records, ESP instrumentation, and
emission monitors.
The following sections present step-by-step inspection
procedures and a checklist of key process and control equipment
components. These guidelines are designed to aid enforcement
personnel in conducting on-site assessment and evaluation of ESP
performance and operating conditions. In general, a coal-fired
power plant serves as an example of a facility regulated by a
State Implementation Plan (SIP).
5.1 PERFORMING THE PERIODIC INSPECTION
5.1.1 File Review
Unless it is a first-time inspection, the control agency
5-1
-------
should have a file containing operating data on the power plant
and existing ESP's. Based on a review of these data, the in-
spector should know whether or not the plant can achieve com-
pliance under normal circumstances. Particular attention should
be given to pending compliance schedules and any construction
and/or operating permits pertaining to processes or ESP's within
the plant. The inspector should also review the number of emis-
sions violations, malfunctions of the ESP, and complaints since
the last inspection.
If not already present, the inspector should prepare a con-
cise file summarizing the basic plant and ESP information,
process descriptions, flow sheets, and ranges of acceptable
operating parameters. An example of these type data is presented
in Table 5-1.4'5
5.1.2 Arranging for the Inspection
If the inspection is to be announced in advance, lead
periods of one day to one week are usually adequate to insure
that the necessary personnel will be available. The inspector
should obtain a schedule of plant operations during the inspec-
tion period, and should request that plant records are current
and available for inspection.
Basic equipment the inspector should have to perform his job
both properly and safely, are listed below. Optional equipment
is marked with an asterisk.
Plume evaluation equipment
Polaroid camera
Compass
Wind-speed indicator
Flashlight
Thermometer (50°-800°F)
Stop watch
Tape measure
6-foot rule
Hard hat
Safety glasses
Safety shoes
Asbestos gloves
*Manometer or pressure/vacuum gage (0-30 in. Hg and 0-10 in.
H20)
5-2
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*Gas detection equipment
*Fuel sample containers
thermocouple
*Portable millivolt meter (temperature compensated)
5.1.3 External Plant Inspection
Before entering the plant, the inspector should observe the
plume and determine its opacity using revised method 9, as con-
tained in the Federal Register. Opacity of the plume is the most
indicative guide to the performance of the ESP. Water vapor con-
densation should not be mistaken for particulate emission. If
visible emissions are exceeding applicable standards, the inspec-
tor should use the standard form and follow established pro-
cedures for recording the violation. Table 5-2 lists possible
operating factors that may be causing visible emissions.
The inspector should also note any visible emissions,
odors, or dust fall in the surrounding area. He should document
all significant observations, noting the date, time of day, and
weather conditions.
5.1.4 Plant Entry
Upon arrival to the plant offices, the inspector should
contact a responsible plant official to gain access to the plant.
Visitor release forms should not be signed because this restricts
insurance coverage for the inspector and may disqualify him from
inspecting certain portions of the plant. The inspector has the
legal right to fully inspect the air pollution sources within the
plant without signing a release form. If entry to the plant is
refused, the inspector should obtain a search warrant.
5.1.5 Preinspection Interview
The purpose of the inspection should be discussed with the
appropriate plant official. Any changes in plant management
should be noted, and data sheets on the process and ESP should be
updated to confirm that operational parameters on file still
pertain. This includes items such as the results from perform-
ance tests of changes in operation since the last inspection.
5-13
-------
TABLE 5-2. PLUME CHARACTERISTICS AND OPERATING
PARAMETERS FOR COAL-FIRED BOILERSa
Stack
plume
Associated
pollutant
Occurrence
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 in-
sufficient atomizing
pressure; improper coal
size or type
Excessive furnace tempera-
ture, burner configuration,
too much excess air
Highrsulfur content in
fuel
Insufficient excess air
Based on data from Reference 3.
5-14
-------
Examples of operational changes in the process or ESP that
an inspector might check are summarized below:
Process Operational Changes
1) Has the rate of production increased or decreased? (Tons
per hour, megawatts generated increased or decreased, or any
other measure of production changed.)
2) Has there been a change in the product mix? For a boiler
this would include a new fuel source, a change in the chem-
ical analysis of the coal or oil. For a cement plant it
would include a change in the moisture content of the slurry,
or a different coal or oil as a heat source.
3) Has there been a conversion from gas or oil to coal?
4) Operating temperatures could have been changed by the addi-
tion of energy conservation retrofits. Examples are the
addition of economizers to a boiler, or the addition of air
or raw material preheaters in a molten metal process, or in
a cement process it would include the addition of more
chains in the kiln.
5) Have there been any changes in startup or shut-down proce-
dures? Is there any new day/night schedule changes such as
"Bottling Up" the boiler overnight; or, has the unit been
shifted from "Baseload" to "Standby" status?
6) Have there been any changes such as the addition of new
forced-draft fans or new induced-draft fans? Any addition
or removal of afterburners?
7) Has there been any change made in the use of the collected
dust in the process, such as, reinjection or return of the
dust to the raw material mix?
8) Has the amount of excess air been changed? Has the soot
blowing schedule been revised? Where applicable, has the
angle of burner tilt been changed?
9) Has there been any change in the size of the fuel, or has
there been any change in the fuel preparation and distri-
bution to the burners?
Changes in ESP Operation
1) Has the effective size of the dust collector been altered?
(ESP fields out of service, or new fields added.)
5-15
-------
2) Have power input levels to the dust collector been increased/
decreased? (Additional power supplies, new ESP power
controls, additional reactors, power factor or rectification
revisions.)
3) Have new sources of emissions been added to the dust collec-
tor such as vents from tanks in a paper mill?
4) Have any retrofits been added or removed in the ESP system?
Examples are gas conditioning, sectionalization of power,
additional or more powerful rappers, power off rapping, new
rapping sequence controls, new power controller systems,
etc.
5) Have there been changes in the dust removal system, power
off rapping, new dust conveying equipment or arrangement of
dust conveyors, vacuum/pressure system changes, evacuation
sequence changes, combination or isolation of dust removal
systems from several units?
Maintenance of Production Equipment and Dust
1) Are there established quality control and periodic mainte-
nance schedules set up for the operations that are pertinent
to the dust collector performance. Examples are fuel pulver-
ization, fuel distribution to the burners, air louver main-
tenance, temperature, pressure drop, oxygen content meter
calibration, and air heater cleaning.
5.1.6 Inspection Inside the Plant - Safety Considerations
On the first visit to the plant, the inspector should review
all safety rules with plant personnel. He should never tour the
plant without an escort, and should not open furnace doors,
manipulate valves or controls, or in any way try to change the
operating characteristics of the plant equipment. Obviously, the
high-voltage electricity used in the ESP is extremely dangerous,
and all practical safety measures must be observed even though
the system incorporates interlocks and other safety devices.
5.1.7 Procedures for External Inspection of ESP's Control Sets
The first item that the inspector should check is the con-
trol sets for the ESP.- which are usually located in a room on top
of, or remote from, the ESP. Plant personnel should provide a
diagram showing which fields are served by which TR sets, as a
guide to determining out-of-service fields when reading the TR
5-16
-------
sets. Control panels can include primary and secondary current
and voltage meters, and a spark rate meter. If the ESP has four
sections, the voltage, amperage, and spark rate should be recorded
for each section. The control set readings should be compared
with calibrated or design values for each section. The inspector
should check the daily log of control readings to determine
whether the readings have been drifting from normal. Drift is
indicative of such problems as air inleakage at air heaters or in
ducts leading to the ESP, dust buildup on ESP internals, and/or
deterioration of electronic control components. He should also
make note of inoperative meters, the number of power supplies on
"manual" control, and TR sets on "auto" that are held to operating
levels below design specifications (such as might be done to
reduce wire breakage).
The inspector can utilize the meters to aid in diagnosing
other problems with an ESP. General examples of the effect of
changing conditions in the gas stream and within the ESP on con-
trol set meters are presented below:
1) When the gas temperature increases, the voltage will in-
crease, and the current will decrease. Arcing can develop.
When the gas temperature decreases, the voltage will de-
crease, and the current will increase.
2) When the moisture content of the gases increases for any
given condition, the current and voltage will also tend to
increase in value.
3) If reduced voltage exists because of a sparkover, a rise in
moisture may allow for an increase in the precipitator
voltage level.
4) An increase in the concentration of the particulate will
tend to elevate voltages and reduce current flow.
5) A decrease in the particle size will tend to raise voltage
while suppressing current flow.
6) A higher gas velocity through the precipitator will tend to
raise voltages and depress currents.
7) Air inleakage may cause sparkover in localized areas result-
ing in reduced voltages.
5-17
-------
8) A number of precipitator fields in series will show varying
readings with voltage-current ratio decreasing in the direc-
tion of gas flow.
9) If a hopper fills with dust causing a short, the voltage
will be drastically reduced, and the current will increase.
10) If a discharge electrode breaks, violent arcing can be
observed with the meters swinging between zero and normal.
11) If a transformer-rectifier unit shorts, voltage will be zero
at a high current reading.
12) If a discharge system rapper fails, the discharge wires
build up with dust; the voltage increases to maintain the
same current level.
13) If a plate rapper fails, the voltage decreases to maintain a
current level under sparking conditions.
Table 5-3 presents specific examples of the effect of chang-
ing conditions on ESP control set readings. These examples are
typical of what the inspector could expect to find. He should
become familiar with these meter reading techniques, so that he
is aware of problems during his inspection.
5.1.8 Control Room Ventilation
Next the inspector should check control room temperature to
see that ventilation is adequate, as well as general housekeeping.
He should check the control sets internally for dirt, which can
cause false signals and cause components to deteriorate.
5.1.9 T-R Sets, Rapper/Vibrators, and Insulators
Many times the T-R sets and rappers and/or vibrators are
located in the same room as the control sets, on top of the ESP.
In any event, the T-R sets, insulators, and rapper/vibrators,
should be inspected next.
The inspector should examine insulators for moisture and
tracking from arc-over. Cracks can be spotted with a bright
light during inspection." Corrosion of the insulator compartment
(if applicable) is another indication of moisture buildup. The
inspector should check to see that the pressurization fan for the
5-18
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-------
top housing or insulator compartment is operating properly, and
that air filters for control sets and top housing are not plugged.
He should also note the condition of access hatch covers.
The inspector should check rapper and vibrator action visu-
ally and/or by feel. A uniform rhythmic tapping of metal to
metal should be noted for rappers, and a loud buzzing sound from
vibrators. Any irregular sounds are an indication of improper
operation of rappers or vibrators. The plant should provide a
diagram of the rapping system sequence so that the inspector can
verify that all of the rappers or vibrators are operating proper-
ly. Rapping intensity should be checked against design as stack
opacity can be improved by reducing rapping intensity. Therefore,
indications that rapping intensity has been reduced, should be
questioned.
5.1.10 Exterior Condition of ESP
The inspector should next examine the exterior of the ESP
for corrosion, loose insulation, exterior damage, and loose
joints. The ducts entering the ESP should be checked; if they
show corrosion, the interior of the ESP may also be corroded.
The inspector should check for fugitive emissions at loose
joints, and as a result of other exterior damage.
5.1.11 Ash Handling
The inspector should next check to see that the evacuation
rate for the ash hoppers is often enough to prevent buildup of
ash over the tops of the hoppers. If level alarms are used, he
should determine that they are operating properly. The inspector
can check the temperature at the hopper throat with the back of
his hand, and if one is comparatively low in temperature, this
could indicate that a malfunction of the ash removal system could
cause bridging or plugging of the hoppers and subsequent ash
buildup.
Problems the inspector should look for in the ash evacuation
and removal system include water pump failure, disengagement of
5-20
-------
vacuum connections, malfunction of rotary air lock valves, and
failure of sequencing controls.
If ash is removed from a collection silo by truck, the
inspector should insure that the discharge pipe extends far
enough into the truck to minimize fugitive emissions.
5.1.12 Process Instrumentation
After finishing with the ash handling system, the inspector
should then proceed to check a number of process parameters that
can affect the ESP performance. For example, readings of gas
flow, gas velocity, excess air, gas temperature, pressure drop
across the ESP, mositure content, flue gas analysis (O , CO ,
etc.) soot blowing intervals, and opacity should be taken if
possible. Many of these instruments are located in the boiler
control room and have continuous readouts. An example trace from
a continuous opacity monitor is shown in Figure 5-1. Variations
in readings from process instruments from the normal design
ranges should be investigated as to their possible effect on ESP
performance, in conjunction with ESP control set readings, and
visual observations made during the inspection.
5.1.13 Internal Inspection of an ESP During Scheduled or
Emergency Outages
If an ESP is down for scheduled maintenance or because of a
malfunction, and an inspection is being done, the inspector
should take time to perform some checks in addition to those
already mentioned. These inspection techniques are similar to
those the company should be doing during an annual inspection,
but are not as detailed. These techniques are discussed further
in Appendix A.
As mentioned previously in this section, no inspection
should be undertaken until it is certain that the ESP is de-
energized and grounded, and necessary precautions are taken to
ensure that the equipment cannot be energized during the in-
spection.
5-21
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5-22
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5.1.14 Collection Plates/Discharge Wires
The inspector should observe the dust accumulation on both
plates and wires. The discharge wires should only have a slight
coating of dust with no corona tufts (doughnut-shaped ash accumu-
lations) . Thickness of dust buildup on plates is normally
between 0.3 and 0.6 cm (1/8 to 1/4 in.). If the plates have more
than 0.6 cm (1/4 in.) of dust, the rappers are not cleaning prop-
erly. If the collecting plates are almost metal clean, this may
be an indication of high gas velocity, extremely coarse fly ash,
too high a rapping intensity, or too low an operating voltage for
good precipitation. The inspector may notice this if a section
has been shorted out prior to his inspection.
The inspector should note whether or not the discharge elec-
trodes are centered between the collecting plates from top to
bottom to ensure optimum performance. He should note any broken
or missing discharge electrodes. Records of wire breakage should
be kept by the company to help determine the cause of continued
wire breakage in the same area, possibly caused by alignment
problems. Random wire breakage is probably caused by dust buildup
on wires or plates.
The inspector can check for air inleakage from door openings
by noting the amount of corrosion on collecting plates adjacent
to inspection hatches, and from hoppers by checking on the lower
portion of the collecting plates. Air inleakage also causes non-
uniform gas flow, and can reduce efficiency.
5.1.15 Hoppers
The inspector should have plant personnel open the hopper
access door and check for corrosion, which indicates air inleak-
age as mentioned previously. He should check for dust buildup in
the upper corners of the hopper, and debris such as fallen wires
and weights in the hopper bottom and valves. If discharge elec-
trode weights have dropped 3 inches or more, this indicates a
broken wire. Chronic buildup of ash is an indication of low
operating temperatures, insufficient heat insulation, or inade-
quate hopper emptying.
5-23
-------
5.1.16 Review of Operating Records
The inspector should review operating records from the
process and ESP, both for completeness and for changes in opera-
tion that may have affected ESP performance. Table 5-4 lists a
number of items for which records should be kept. Malfunctions
from both the process and the ESP should be discussed with plant
officials, and the inspector should determine what plant officials
are doing to remedy any recurrent problems.
5.1.17 Inspection Form
Data obtained during an inspection should be summarized on a
form such as that presented in Table 5-5. The form also serves
as a record of the inspection.
5.1.18 Compliance Action
If conditions observed during the inspection indicate that a
citation is warranted, the inspector must clearly state to plant
officials the grounds for such a citation. An onsite citation is
justified only by clear-cut violations such as excessive opacity
or failure of the plant to report malfunctions or to maintain or
provide required records for review. Table 5-6 lists important
compliance parameters and conditions for issuance of citations.
After a review of the inspection report by the inspector's
supervisor, a copy of the inspection checklist should be sent to
the utility with a letter confirming that the inspection was
made, stating any deficiencies, requesting that they be corrected
within a reasonable time. Recommendations can be given for
further improvements in operation and maintenance of the ESP,
although emphasis should be towards indications of trouble and
not the solution.
The utility should know what is expected and the time frame
it will have to accomplish the required action. Some type of
implementation plan may be requested.
5-24
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5-32
-------
REFERENCES SECTION 5
1. Test Manual for Fossil Fuel-Fired Steam Generators. Pre-
pared by PEDCo Environmental, Inc., Cincinnati, Ohio, for
Environmental Protection Agency. January 1977.
2. Industrial Air Pollution Guide. Chapter 7.0. Prepared by
PEDCo Environmental, Inc., Cincinnati, Ohio, for Environ-
mental Protection Agency. 1978.
3. Devitt, T.W., and Norman J. Kulujian. Inspection Manual for
the Enforcement of New Source Performance Standards: Fossil-
Fuel-Fired Steam Generators. PEDCo Environmental, Inc.
January 1975.
4. Szabo, Michael F., and Richard W. Gerstle. Electrostatic
Precipitator Malfunctions in the Electric Utility Industry.
PEDCo Environmental, Inc., Cincinnati, Ohio, EPA- 600/2-77-
006. January 1977.
5. Devitt, T.W., R.W. Gerstle, and N.J. Kulujian. Field
Surveillance and Enforcement Guide: Combustion and Incin-
eration Sources. PEDCo Environmental, Inc., Cincinnati,
Ohio. June 1973.
6. Englebrecht, H.L. Electrostatic Precipitator Inspection and
Maintenance. Plant Engineering. April 29, 1976 p. 193-196.
7. Dismukes, Edward B. Techniques for Conditioning Fly Ash.
In: Symposium on Particulate Control in Energy Processes.
EPA-600/7-76-010.
8. Szabo, Michael F. , and Richard W. Gerstle. Operation and
Maintenance of Particulate Control Devices on Coal Fired
Utility Boilers. PEDCo Environmental, Inc., Cincinnati,
Ohio. EPA-600/2-77-129. July 1977.
5-33
-------
SECTION 6
PERFORMANCE EVALUATION
6.1 ESP EFFICIENCY ESTIMATES
Recent test data should be used to determine compliance
after ascertaining that present operating conditions are con-
sistent with design or test conditions (e.g., boiler load, ash
and sulfur content of coal, ESP operating temperature, ESP cur-
rent, and voltage levels). If test data are not available, ESP
overall mass efficiency can be estimated by use of power data
from the ESP control panel or design data for the ESP, as de-
scribed below. Efficiency as a function of particle size can be
estimated using a programmable calculator.
1) Power Data
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 precipitator 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. Calculate corona power input as watts
per 1000 m3/s (1000 ft3/min) of flue gas. Obtain precipitator
collection efficiency value from Figure 6-1 or 6-2.
6-1
-------
£ 99
LU
g 98
°- 97
S 95
S 90
u_
u_
LU
z 80
2 70
O
o
50
30
0
• TEST DATA
THEORETICAL CURVE
FOR k - 0.55
25 50 75 100 125 150
CORONA POWER, WATTS/1000 acfm
METRIC CONVERSION: (watts/1000 acfm) (2500) watts/1000 m3/sec
Figure 6-1. Relationship between collection efficiency and
specific corona power for flv ash precipitators, basad
on field test data.1
o
I—I
o
99.9
99.8
99.7
99.5
99
98
95
90
80
70
50
0 100 200 300 400 500 600
CORONA POWER, WATTS/1000 acfm
3.
METRIC CONVERSION: (watts/1000 acfm) (2500) - watts/1000 m /sec
Figure 6-2. Efficiency versus specific corona power extended
to high collection efficiencies, based on field test data on
recently installed precipitators.2
6-2
-------
If one or more bus sections are not operating, this will be
reflected in the reduced total corona power available, resulting
in lower collection efficiency.
2) Design Data
From ESP design data, obtain gas flow, temperature at ESP
inlet, total ESP plate area, plate area for each bus section, and
precipitation rate parameter (may need to be recalculated when
coal sulfur content has changed significantly). Make sure that
the design values for gas flow and temperature at ESP inlet are
still accurate for the current operating conditions.
Using the Duetsch equation or one of its derivatives (see
Section 2), calculate the ESP collection efficiency, accounting
for loss in plate area because of bus section outage. Consider
the following example (English units):
0 precipitation rate parameter (w) = 0.35 ft/sec (21
ft/min).
0 gas flow (V) = 300,000 acfm @ 300°F.
0 Total plate area (A) = 84,000 ft2 (21,000 ft2/bus
section, four bus sections total). One bus section is
out of service, reducing effective plate area to 63,000
ft2.
From the Deutsch equation:
A
V
p^
n = 1 - exp - — w
= 1 - exp - ( 63'OOQ) (21)
1 exp 1300,000; U1;
= 1 - 0.0122 = 98.78%
Note that the outlined procedures are based on generalized
or design data and are not precise enough to reflect quanti-
tatively the changes caused by readily measurable variations in
operating parameters. Thus emission estimating procedures are
normally used as a part of the initial permit evaluation process
to judge the adequacy of the system for complying with emission
regulations.
6-3
-------
3) Calculator Program
The inspector can estimate fractional efficiency of an ESP
whenever particle size data are available. The fractional effi-
ciency for an ESP is expressed in terms of overall penetration
using the following equation:
PtQ = ;°° Pt1 (d)f (d)dd
where Pt = Overall penetration, which = (1 - efficiency)
Pt1(d) = Penetration corrected for sneakage and
reentrainment
This equation can be solved for finite limits "d." and "d,.";
Reference 4 gives a calculator program for obtaining the penetra-
tion between particle size limits d. and df. This program,
written for Texas Instruments calculator model SR-52, solves the
penetration equation using the trapesoidal rule of integration
technique.
6-4
-------
REFERENCES SECTION 6
1. Oglesby, Sabert Jr. A Manual of Electrostatic Precipitator
Technology. Southern Research Institute, Birmingham,
Alabama. August 1970.
2. White, H.J. Electrostatic Precipitation of Fly Ash.
JAPCA. March 1977.
3. Devitt, T.W., R.W. Gerstle, and N.J. Kulujian. Field
Surveillance and Enforcement Guide: Combustion and Incinera-
tion Sources. APTD-1449. June 1973.
4. Sparks, L.E. SR-52 Programmable Calculator Programs for
Venturi Scrubbers and Electrostatic Precipitators. EPA-
600/7-78-026. March 1978.
6-5
-------
SECTION 7
CASE HISTORIES
The purpose of presenting these case histories is to famil-
iarize the inspector with some actual operating problems of ESP's
in the field, and the attempts made to correct these problems.
Although it is not the responsibility of the inspector to tell
the company how to resolve problems with a precipitator, he may
be able to provide some suggestions as to what a problem might
be, and methods that could be used to rectify it.
7.1 COLD-SIDE ELECTROSTATIC PRECIPITATOR ON A COAL-FIRED
UTILITY BOILER
This section presents the operating history of a cold-side
ESP installation at a coal-fired power plant located in the
Southern United States. The information presented here was
obtained during a site visit and from discussions with engineer-
ing personnel.
7.1.1 System Description and Operating History
The precipitator was originally installed in 1960 and was
2 3
guaranteed for 90 percent efficiency with an SCA of 18 m /m /s
2
(91 ft /1000 acfm). Actual precipitator efficiency was 55 percent
or less at the normal gas temperature of 132°C (270°F) and about
90 percent at 155°C (310°F). This is a common effect observed in
marginally designed precipitators operating at average gas
velocities of approximately 2.4 m/s (8 ft/s) with medium- to
high-sulfur coals. Studies with ammonia injection showed a sig-
nificant improvement in performance at 132°C (270°F), with
efficiencies ranging from 85 percent at 0.0009 m /s (2 scfm)
7-1
-------
ammonia per ESP to 95 to 97 percent with 0.005 to 0.01 m3/s (10
to 20 scfm) ammonia.
In 1970-72, the ESP was modified to improve performance.
The SCA was increased from 18 m2/m3/s to 27 m2/m3/s (91 ft2/1000
2
acfm to 136 ft /1000 acfm), and the number of T-R sets per col-
lector was increased from 2 to 5. The present arrangement con-
sists of four separate collectors, each with four fields in the
direction of gas flow. Collection efficiencies above 98 percent
were obtained with ammonia injection in 1972, when the utility
began using ammonia on a full-time basis.
Recently, however, performance has decreased to 90 percent
or less, and outlet emissions are several times greater than the
state regulation of 47.26 ng/J (0.11 lb/106 Btu). Some of the
major causes for poor performance at this unit are summarized
below.
1. Initial design was marginal, calling for a small ESP to
operate at excessively high gas velocities for the sulfur and
temperature conditions. The expected level of performance for
the system at the time of the modifications was only 95 percent.
2. Poor equipment availability and component malfunctions.
3. Unstable T-R electrical sets, limited power output
capability, poor match to load conditions, and uncontrolled
sparking, causing excessive dust reentrainment.
4. The ammonia injection rate is now limited to 8 to 10
ppm because sticky ash buildup on ID fans has caused severe
imbalances and buildup in hoppers has caused difficulty in ash
removal. Higher levels of ammonia injection are needed to pre-
vent excess ash reentrainment, which is caused by low ash resis-
tivity with the high-sulfur coal used by the utility at gas
temperatures of 116° to 138°C (240° to 280°F). The untreated ash
can be readily collected but is easily reentrained before being
rapped off of plates into the hopper.
5. Cyclic boiler load operation aggravates the problems of
condensation and sticky ash, especially in ash hoppers, which are
7-2
-------
marginally sized and have no heaters. In addition, boiler tubes
leak frequently.
6. Possibly serious gas sneakage under the plates through
the top of the hopper has resulted in untreated gas and or reen-
trainment of ash that has built up in the hopper. The plant
reprots that although ash buildup occurs often, there is little
problem with T-R sets being shorted out by the ash. This sug-
gests the possibility that gas sneakage carries dust across the
top of the hoppers and out of the precipitator, thus preventing
ash buildup beyond a certain level.
7. Possible electrode misalignment, loose wires, or other
factors prevent operation at optimum electrical conditions.
During the plant visit about five T-R sets (25% of total) were
operating far below normal capability.
8. A recent preliminary test with ammonia indicated a
performance that was typical of the expectations without an addi-
tive. It is possible that many nozzles are plugged with ash
encrustations and the ammonia distribution is poor enough to
negate its usefulness in retention of collected ash.
Some pertinent recommendations to improve performance,
suggested by a well-know ESP consultant, are as follows:
1. A major reduction in the amount of dust reentrained is
needed. This involves many aspects:
a. Proper hopper baffling and baffles between fields to
eliminate gas flow under collecting plates and dust
sweepage out of hoppers. Proper gas distribution
should be assured. Gas must enter the ESP horizontally
with no significant upward or downward vectors, par-
ticularly the latter, which can reentrain hopper dust.
Regions of excessively high gas velocity in the ESP
should be eliminated.
b. Continued use of a properly functioning ammonia injec-
tion system with good distribution and design to min-
imize ash deposition and pluggage. A high injection
velocity should be maintained through the nozzles into
the gas stream [minimum 60 m/s (200 ft/s)]. The minimum
amount of ammonia to effect desirable agglomeration and
ash retention should be used. At low boiler loads,
less ammonia may be needed, but caution should be taken
7-3
-------
to ensure that plugging does not occur. If the ammonia
system is shut off, suitable air flow through nozzles
should be maintained.
c. Increasing gas temperature to a mimimum of 140°C (280°F),
preferably to 155° to 160°C (310° to 320°F), can be a
corrective measure in lieu of ammonia injection. The
objective is to maintain ash resistivity at a reason-
able level to aid retention. The correct level is one
that will not seriously degrade electrical conditions.
High power input aids collection and retention of dust,
but less power is required for a somewhat higher ash
resistivity than is prevalent with high-sulfur coal and
low gas temperature.
d. Boiler additives to chemically absorb excess SO3/H2S04
in flue gas are also a possible alternative to injec-
tion or increased gas temperature. The advantage of
such additives is some control of ash resistivity at
low gas temperature where boiler efficiency is good.
Possibilities could be investigated.
e. The location of the ID fan suggests the possibility of
downward pull of gases through the precipitator if
outlet vaning is not adequate. A vertical profile of
dust concentration in outlet or precipitator is useful.
Perhaps a 0.9 to 1.2 m (3 to 4 ft) high baffle at the
outlet of the ESP would be helpful.
2. The voltage ratings of the T-R sets and saturable
reactor type control should be improved to provide a better match
to the requirements of the precipitator load. The electrical
control circuits can be modified to improve stability and re-
sponse and to provide additional corona power capability. It is
recommended that linear reactors be added in the primary circuit
in series with the present saturable reactors.
For conditions at this plant, electrical sets should be
operated with very little or no sparking. It is particularly
important in the outlet sections to operate at just under the
sparking threshold.
3. Rapping effectiveness and optimum donditions (duration,
frequency, and force) should be investigated. A program to
accomplish this is now underway. Determining the extent of dust
being reentrained is the first step. With regard to rapping
7-4
-------
frequency and force, fairly frequent, light blows are probably
best. With vibrators installed at this plant, the operating
duration must be reduced to the mimimum possible. Outlet section
rapping can also be very critical; usually only the minimum
amount of rapping needed to maintain electrical conditions
used.
4. The best possible effort should be made to control
condensation and sticky ash.
7.2 ELECTROSTATIC PRECIPITATOR TO CONTROL PARTICULATE EMISSIONS
FROM CEMENT KILNS
This case history covers the use of an ESP on a cement kiln
in a plant located in the Eastern United States. The information
was obtained during a site visit and from discussions with engi-
neering personnel.
7.2.1 System Description
The plant has two cement kilns, which consume over 500
metric tons of coal per day. The ESP, which controls particulate
emissions from both kilns, was manufactured by Western Precipi-
tation and installed in 1962. It is designed to handle 172
m3/s (430,000 cfm) at approximately 260°C (500°F) and, according
to plant personnel, operates within +_ 15 percent of design rate.
Gas flow is split into an upper an a lower precipitator.
Both have four active fields and an empty one. The first three
upper and lower fields are each energized by one T-R set, whereas
the outlet fields on the upper and lower precipitators are each
energized by a separate T-R set. Thus, the entire system contains
a total of five T-R sets.
Approximately 70 percent of the collected dust is recycled
to the kilns, and the remainder is sluiced to a spray pond.
Sluice water is recycled.
7.2.2 Operating History
Many modifications have been made to the precipitator since
1972 to meet state particulate emission regulations. The
7-5
-------
modifications have eliminated most maintenance problems, and the
collection efficiency is reported to be as high as 99.5 percent
with outlet loadings as low as 16 kg/h (35 Ib/h). Following is a
brief discussion of the major maintenance problems that have
plagued this installation and the efforts that have been made to
alleviate these problems:
1. A 1972 inspection revealed that the discharge and
collection electrode alignment were not correct at the
bottom of the frame because of expansion and contrac-
tion of the frame and hoppers, and weak guide supports;
warpage and buckling of some plates were also noted.
Correction of the frame misalignment has markedly
improved collection efficiency.
2. Air inleakage into screw conveyors continued to cause
reentrainment of dust and corrosion of metal on the
screw conveyors. Holes are patched as they occur, and
a new design of screw conveyor with better slide gate
seals is gradually being substituted. Because expan-
sion joints at the inlet and outlet of the precipitator
initally were not insulated, the metal rotted out and
caused air in-leakage. Proper insulation solved this
problem.
3. Because insulator compartments on top of the ESP are
not insulated, pressurized, or heated, moisture con-
densation has caused severe corrosion in a number of
the compartments and occasional failure of an insula-
tor. The company has designed a new insulator com-
partment, which is insulated and may be pressurized
with heated air. Present insulator compartments are
gradually being replaced.
4. Shafts of vibrators on discharge wires have failed
repeatedly. Two men are engaged full time in welding
cracked vibrator shafts. A new Syntron vibrator used
on some sections of the ESP has been successful, re-
quiring almost no maintenance.
5. Difficulties caused by control panel overheating were
compounded by numerous failures of fans used to cool
the control compartments. This was solved by moving
the saturable reactors to a better ventilated room
behind the control compartment.
6. Excessive sparking in the inlet fields of the ESP has
caused failure of a large number of wires because of
arcing. The company is considering replacing the
saturable reactors on these sections with silicon
7-6
-------
controlled rectifier/linear reactor combinations in an
attempt to improve current limit control and current
wave form to reduce sparking.
7. Gas distribution has always been a problem with this
ESP because the gas flow is split into an upper and
lower ESP. The addition of a damper in the common
ductwork on top of the ID fan has improved the balance
of flow; turning vanes would probably improve gas
distribution even further, but the company believes
they are too expensive.
8. The rapping system was improved by installing stronger
supports for the discharge electrode vibrators on the
top level and on bent collecting electrode supports.
The rapping duration and intensity were then readjusted
until satisfactory results were obtained.
7.2.3 Maintenance Procedures
A regular maintenance schedule is followed, and malfunctions
are recorded. The state environmental control agency is notified
when serious problems arise.
7.2.4 Conclusion
Plant personnel have shown great initiative in making the
modifications needed to improve the performance of the precipi-
tator and bring the kiln emissions into compliance with state
particulate emission regulations. By following a regular main-
tenance schedule and continuing to improve the quality of the
precipitator components, plant personnel are confident that the
present high level of performance can be maintained indefinitely.
7-7
-------
REFERENCES SECTION 7
1. Oglesby, Sabert Jr. A Manual of Electrostatic Precipitator
Technology. Southern Research Institute, Birmingham,
Alabama. August 1970.
2. White, H.J. Electrostatic Precipitation of Fly Ash.
JAPCA. March 1977.
3. Devitt, T.W., R.W. Gerstle, and N.J. Kulujian. Field
Surveillance and Enforcement Guide: Combustion and Incinera-
tion Sources. APTD-1449. June 1973.
7-8
-------
APPENDIX A
STARTUP AND SHUTDOWN PROCEDURES AND
MAINTENANCE SCHEDULE FOR
ELECTROSTATIC PRECIPITATORS (ESP's)
A-l
-------
PRECIPITATOR STARTUP AND SHUTDOWN PROCEDURES
Operation of an electrostatic precipitator (ESP) involves
dangerously high voltage. Although all practical safety measures
are incorporated into the equipment, extreme caution should be
exercised at all times. An ESP is, in effect, a large capacitor
that when deenergized can retain dangerous electrical charges.
Grounding mechanisms provided at each access point should there-
fore be used before entering the precipitator.
Preoperational Checklist - Before placing the equipment in oper-
ation, plant personnel should perform a thorough check and visu-
ally inspect the system components in accordance with the manu-
facturer's recommendations. Some of the major items that should
be checked are summarized below:
Control Unit
Proper connections to control
Silicon Rectifier Unit
Rectifier-transformer insulating liquid level
Rectifier ground switch operation
Rectifier high-voltage connections made
High-voltage bus transfer switch operation
High-Tension Connections
High-tension bus duct
Proper installation
Vent ports properly installed
Equipment Grounding
Precipitator grounded
Transformer grounded
Rectifier controls grounded
High-tension guard grounded
Conduits grounded
Rapper and vibrator ground jumpers in place
Air Load Tests - After the precipitator is inspected (i.e.,
preoperational check adjustment of the rectifier control and
check of safety features), the air load test is performed. Air
A-2
-------
load is defined as energization of the precipitator with minimum
flow of air (stack draft) through the precipitator. Before
introduction of an air load or gas load (i.e., entrance of dust-
laden gas into the precipitator), the following components should
be energized:
Collecting plate rappers
Perforated distribution plate rappers
High-tension discharge electrode vibrators
Bushing heaters - housing/compartments
Hopper heaters - vibrators - level indicators
Transformer rectifier
Rectifier control units
Ventilation and forced-draft fans
Ash conveying system
The purpose of the air load test is to establish reference
readings for future operations, to check operation of electrical
equipment, and to detect any improper wire clearances or grounds
not detected during preparation inspection. Air load data are
taken with the internal metal surfaces clean. The data consist
of current-voltage characteristics at intervals of roughly 10
percent of the T-R milliamp rating, gas flow rate, gas tempera-
ture, and relative humidity.
For an air load test, the precipitator is energized on
manual control. The electrical characteristics of a precipitator
are such that no sparking should occur. If sparking does occur,
an internal inspection must be made to determine the cause.
Usually, the cause is (1) close electrical clearances and/or (2)
the presence of foreign matter, such as baling wire, that has
been left inside the precipitator.
After the precipitator has been in operation for some time,
it may be necessary to shut it down to perform internal inspec-
tions. At such times, it would be of interest to take air load
data for comparison with the original readings.
Gas Load Tests - The operation of a precipitator on gas load
differs considerably from operation on air load with respect to
voltage and current relationships. The condition of high current
and low voltage characteristizes the air load, whereas low cur-
A-3
-------
rent and high voltage characterize the gas load. This effect
governs the operation of the precipitator and the final setting
of the electrical equipment.
MAINTENANCE SCHEDULE FOR ESP's
An accurate log should be kept on all aspects of precipi-
tator operation including electrical data, changes in rapper and
vibrator operation, fuel quality, and process operations. Such a
log can provide clues to the probably cause of any change in
performance.
For example, it is obvious that gross departures from
normal readings on the T-R meter and transmissometer indicate
trouble. It is not so widely recognized that small variations,
often too slight to be noticed without checking daily readings,
can indicate impending trouble.
Problems that usually affect precipitator performance
gradually, rather than suddenly, include (1) air inleakage at
heaters or in ducts leading to the precipitator, (2) dust buildup
on precipitator internals, and (3) deterioration of electronic
control components. Such problems are often indicated by slight
but definite drift of daily meter readings away from baseline
values.
Grossly abnormal readings, usually indicating a serious
problem, 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, which may be caused to
ground by accumulation of ash above a plugged hopper or formation
of clinker on a wire.
Fluctuating voltage that dips to low values suggests a
broken and swinging discharge electrode. Fluctuation of spark-
A-4
-------
rate meter readings does not necessarily indicate a problem
unless voltage or current readings fluctuate also.
An operator should never try to correct deviant meter
readings by adjusting control set points. An automatic-control
response range should accommodate normal variations in load.
When major changes occur, such as would result from firing a coal
substantially different from that for which the precipitator was
designed, the precipitator manufacturer should be called in to
retune the installation. If no such major changes have occurred,
then variant meter readings indicate problems that must be
detected and corrected. Figure A-l exemplifies a log of electri-
cal readings that are checked several times each day at a coal-
fired utility. These readings are used in troubleshooting
problems with ESP operation.
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 growing ash piles
can push internal components out of position, causing misalign-
ment that may drastically affect performance. Sometimes utility
operators attempt to preserve alignment by welding braces to hold
collecting-electrode plates in position. This practice is inad-
visable because restraining the plates reduces the effectiveness
of the rapping action that keeps them clean.1
Although various indicators and alarms can be installed to
warn of hopper-ash buildup and of ash-conveyor stoppage, the
operator can doublecheck by testing 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.1 Generally, however, low tempera-
ture 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
subsequently will pile up at the top.1
A-5
-------
PRECIPITATOR LOG SHEET
HO 1 PRECIPITATOR
UJ
UJ
Ul
CM
8i
K-
UJ
PPTR C*BLE 1 KV
AVG SPARK RATE
PPTR CABLE 2 KV
TRANS PRI VOLTS V
PPTR. AVG CURRENT MA
TRANS PR! CURRENT
PPTR CABLE 1 KV
AVG SPARK RATE
PPTR CABLE 2 KV
TRANS. PRI VOLTS V
PPTR AVG CURRENT MA
TRANS PRI. CURRENT
PPTR CABLE 1 KV
AVG SPARK RATE
PPTR CABLE 2 KV
TRANS PRI. VOLTS V
PPTR AVG. CURRENT MA
TRANS PRI CURRENT
PPTR. CABLE 1 KV
AVG. SPARK RATE
PPTR CABLE 2 KV
TRANS PRI VOLTS V
PPTR AVG CURRENT MA
TRANS PRI. CURRENT
NO, 2 PRECIPITATOR
00
a
r-
i/>
to
t-
uj
in
u
tn
PPTR CABLE 1 KV
AVG SPARK RATE
PPTR CABLE 2 KV
TRANS PRI VOLTS V
PPTR AVG CURRENT MA
TRANS PRI CURRENT
PPTR CABLE 1 KV
AVG SPARK RATE
PPTR CABLE 2KV
TRANS PRI VOLTS V
PPTR AVG CURRENT MA.
TRANS PRI CURRENT
PPTR. CABLE t KV
AVG. SPARK RATE
PPTR CABLE 2 KV
TRANS. PRI VOLTS V
PPTR AVG CURRENT MA
TRANS PRI CURRENT
PPTR CABLE 1 KV
AVG. SPARK RATE
PPTR CABLE 2 KV
TRANS PRI VOLTS V
PPTR AVG CURRENT MA
TRANS PRI CURRENT
12 MID
3AM
6 AM
9AM
12 NOON
3PM
6PM
9PM
8-4.
DATE
Figure A-l. Precipitator log sheet,
A-6
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If the temperatures of all hoppers seem low to the touch,
the ash-conveyor system should be checked; the system may have
stopped or dust agglomeration may be so great that the conveyor
can no longer handle all of the fly ash.
Hopper plugging is sometimes caused by low flue gas temper-
ature, which permits moisture condensation. Temperature of gas
at the boiler exit may be too low, or ambient air may be leaking
into the flue gas duct. Hoppers are particularly prone to plug-
ging during startup after an outage, when they are cold and
usually damp.
Daily checking of the control room ventilation system
minimizes the possibility of overheated control components, which
can cause the control set points to drift and can accelerate
deterioration of sensitive solid-state devices.
Weekly
Solenoid-coil failures, fairly common when high voltage was
used, are rare with modern low-voltage equipment. Still, a
weekly check of all units is advisable. Rapper action should be
observed visually, and vibrator operation confirmed by touch. If
inadequate rapping force is suspected, an accelerometer mounted
on the plates should be used to verify that rapping acceleration
is adequate (often, up to 30 g is required). 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 dirt can
cause false control signals and can damage such large components
as contactors and printed circuits.
Finally, filters in the lines supplying air to control
cabinets and to 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
A-7
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are made to the discharge electrodes within the precipitator box.
Pressurization ensures that if gas leakage occurs where the
bushings penetrate the precipitator hot roof, gas will flow into
the precipitator rather than out from it. Leakage from the
precipitator into the housing could cause ash deposits or mois-
ture condensation on the bushings, with risk of electrical break-
down at the typical operating potential of 45 kV d.c.
Monthly maintenance also should include inspection of
bushings visually and by touch for component vibration, checks of
differential pressure to ensure good operation of the fan that
pressurizes the housing, and manual operation of the automatic
standby fan to make sure it is service-ready.
Quarterly
Quarterly maintenance includes inspection of electrical-
distribution contact surfaces. These should be cleaned and
dressed and the pivots should be lubricated quarterly if not more
frequently,-'- since faulty contacts could cause false signals.
Further, because transmissometer calibration is subject to drift,
calibration should be verified to prevent false indications of
precipitator performance.
Semiannually
Inspection, cleaning, and lubrication of hinges and test
connections should be done semiannually. If this task is ne-
glected, extensive effort eventually will be required to free
test connections and access doors, often involving expensive
downtime. Performance tests may be required at any time; they
should not be delayed while connections are made usable. An
effective preventive measure is to recess fittings, below the
insulation.
Inspection of the exterior for corrosion, loose insulation,
surface 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 emissions. ^
A-8
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Annually
Scheduled outages must be long enough to allow thorough
internal inspection of the precipitator. Following is a summary
of items to be checked during an annual inspection, abstracted
primarily from Reference 2.
1. Dust Accumulation - The upper outside corners of a
hopper usually show the greatest accumulation. A spotlight can
be used to check for dust buildup, eliminating the need to enter
the hopper.
2. Corrosion - Inaccessible parts of the ESP are often
attacked by corrosion. Access doors and frames, which are diffi-
cult to insulate, are usually attacked first. Condensation can
occur in penthouses that contain support insulators; the pent-
houses are at lower temperature than the gas, and moisture is
added also by purge air from the outside.
Corrosion can occur at several places in the ESP housing:
the underside of roof plates, the outside wall, the space between
outside collecting surface plates and sidewall, the back of
external stiffening members that act as heat sinks, and any area
not continuously subject to gas flow, such as corners and the
upper portion of the hopper connection to inlet and outlet ducts.
All gas connections should be checked for inleakage of oil, gas,
or air.
Corrosion in these areas can be minimized by keeping interi-
or surfaces hot and by effective thermal insulation of outside
surfaces, use of heaters during routine shut downs or operation
at low loads also may help prevent corrosion.
3. Rappers - Maintenance of the magnetic-impulse, gravity-
impact rapper has been discussed. Many rigid-wire ESP's, how-
ever, have mechanical rappers. The drives for collecting and
discharge electrode rappers should be checked for high motor
temperature, unusual noise, and level and condition of the
lubricant.
A-9
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Mechanical rappers should be checked for excessive wear,
shifting of point of impact, free movement of wire-frame rapper
release, free movement of hammers, and wear on hammer shaft
bushings.
4. Hoppers - On both wire-weight and rigid-wire precipi-
tators, the hopper discharge should be checked for such objects
as broken pieces of rappers, wires, shotcrete, and scale. Pres-
ence of foreign objects indicates a problem that should be
investigated further.
5. Gas Distribution Plates - Although perforated plates
usually do not become plugged, uneven distribution of uneven low
gas load can sometimes cause plugging of a portion of the plates.
If a rapping system is not used, manual cleaning is required.
6. Discharge Electrodes - Frames in rigid-frame, dis-
charge-electrode-type precipitators should be centered between
two rows of collecting surface plates with a maximum deviation of
+ 0.6 cm (+ 0.15 in.). Discharge wires must be straight and
securely connected to the discharge frame.
Wire-weight precipitators should be checked for missing or
dropped weights. Common causes of wire failure and remedial
action are discussed in the following section on malfunctions.
Removal of a broken wire that is not replaced should be recorded
on a permanent log sheet.3 Discharge wires should be cleaned
manually as required.
7. Collecting Electrodes - Collection plates should be
inspected for warping due to excessive heat. Corrosion of lower
portions of the plates and portions of plates adjacent to door
openings indicates air inleakage through hoppers or around doors.
Plates should be cleaned manually as required.
8. Suspension Insulators - When insulators become heavily
coated with moisture and dust, they may become conductive and
crack under high-voltage stress. Cracks can be spotted with a
bright light during inspection. Faulty insulators can cause
excessive sparking and voltage loss and can fail abruptly or even
explode if allowed to deteriorate.
A-10
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9. Housing - Thick dust deposits on interiors of housings
indicate high gas velocities resulting from excessive gas vol-
umes, a condition that should be corrected.
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 Maintenance
Certain preventive maintenance and safety checks are so
important that they should be performed during any outage of
sufficient length, without waiting for scheduled downtime. Air
load readings should be compared with baseline values to detect
possible deterioration in performance. Readings taken immedi-
ately upon restoring the precipitator to service can serve as a
check on any changes resulting from maintenance done during the
outage.
Critical internal alignments should be checked whenever an
outage allows; any misalignment warrants immediate corrective
action. Interiors of control cabinets and top housing should be
checked during any outage of 24 hours or more and cleaned if
necessary. Any outage of more than 72 hours provides an oppor-
tunity to check grounding devices, alarms, interlocks, and other
safety equipment and to clean and inspect insulators and bushings.
Safety
Because high-voltage electricity can be extremely dangerous,
all practical safety measures must be observed even though the
system incorporates interlocks and other safety devices.4
The system should never be adjusted with the high-voltage
power on.
Rectifiers and diodes have heat sinks that could seriously
shock a person touching them.
The rapper circuitry, which is independent of the high-
voltage circuitry, is nonetheless also dangerous and must be so
treated.
A-ll
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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 maintenance or adjustment
-, 4
is attempted.
Explosive gas mixtures could be created if air is introduced
into the systems. If necessary, a system should be purged with
an inert gas before air is introduced. A system should always be
purged with fresh air before it is entered.
A-12
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REFERENCES - APPENDIX A
1. Bibbo, P.P., and M.M. Peaces. Defining Preventive Mainten-
ance Tasks for Electrostatic Precipitators, Research
Cottrell, Inc. Power. August 1975, pp. 56-58.
2. Engelbrecht, H.L. Plant Engineer's Guide to Elec-
trostatic Precipitator Inspection and Maintenance, Air
Pollution Division of Wheelabrator Frye, Inc., Plant
Engineering. April 1976, pp. 193-196.
3. Szabo, M.F., and R.W. Gerstle. Operation and Maintenance of
Particulate Control Devices on Coal-Fired Utility Boilers.
PEDCo Environmental, Inc., Cincinnati, Ohio. EPA-600/2-77-129
July 1977.
4. Hesketh, H.E., and F.L. Cross, Jr. (ed.) Handbook for the
Operation and Maintenance of Air Pollution Control Equip-
ment. Technomic Publishing. Westport, Connecticut. 1975.
A-13
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APPENDIX B
TYPES OF ELECTROSTATIC
PRECIPITATOR MALFUNCTIONS
B-l
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TYPES OF ELECTROSTATIC PRECIPITATOR MALFUNCTIONS
Discharge Wire Breakage
Probably the most common problem associated with suspended
wire electrode ESPs is wire breakage, which typically causes an
electrical short circuit between the high-tension discharge wire
system and the grounded collection plate. The electrical short
trips the circuit breaker and disables a section of the ESP,
which remains disabled 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 causes an arc,
which can embrittle the wire and eventually break it
completely.
2) Clinkered or improperly centered wires cause a contin-
ual spark from the wire to the bracing.
3) A clinker or wire bridges the collection plates and
shorts out the wire.
4) Ash buildup under the wire causes it to sag and short
out.
5) Improper clearance of "J" hooks to the wire causes it
to short out.
6) Hangup of a bottle weight during cooling causes a wire
to buckle.
7) Fly ash buildup on a bottle weight forms a clinker or
burns off the wire.
8) Corrosion caused by condensation around cooler areas of
the wire.
9) Excessive localized sparking causes erosion of the
wire.
Electrical erosion, the predominant cause of failures,
occurs when repeated electrical sparkovers or arcs occur in a
localized region. Keating and vaporization of a minute quantity
of metal occur with each spark. Sparkover at random locations
will cause no serious degradation of the discharge electrode.
B-2
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Repeated sparkover at the same location, however, can remove
significant quantities of material, 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 variations in the
electric field resulting from "edge" effects of adjacent dis-
charge and collection electrodes at the top and the bottom of the
plates. Measures that will eliminate failure at these points are
adding shrouds, such as those shown in Figure B-l, and providing
a rounded surface at the edge of the collection electrode to
reduce the tendency for sparking.1
Electrical erosion can also be caused by "swinging" of
electrodes, which can occur when the mechanical resonance fre-
quency of the discharge wire and weight system is harmonically
related to the electrical frequency of the power supply. The
power supply adds energy to the swinging wire, and sparking
occurs with each close approach to the collection plate. This
action leads to erosion of the electrode and mechanical failure.
Poor workmanship during construction can also cause electri-
cal failure of the discharge electrode. If pieces of the welding
electrode remain attached to the collection plate, localized
deformation of the electric field can lead to sparking and fail-
ure of the discharge electrode.
Mechanical fatigue occurs at points where wires are twisted
together and mechanical motion occurs continually at one loca-
tion. This occurs at the top of a discharge electrode where the
wire is twisted around the support collar. Methods of reducing
mechanical fatigue include selection of discharge electrode
material that is resistant to cold work annealing after attach-
ment.
Chemical attack is caused by a corrosive material in the
flue gas, which can occur when high-sulfur coal is burned and
flue gas exit temperatures are low and near the acid dew point.
Use of ambient air to purge insulator compartments also can cause
B-3
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Figure B-l. Shrouds for wire-weighted discharge electrodes.'
B-4
-------
the temperature to drop below the acid dew point in a localized
region. Corrosion can be minimized by operation at higher flue
gas temperatures or- by use of hot, dry air to purge insulator
compartments. Use of good insulation on the ESP shell to main-
tain high temperature also provides adequate protection within
the usual range of operating temperatures and fuel sulfur con-
tents. -1
The other causes of discharge wire failure, such as in-
adequate rapping, could be minimized by routine checking of
vibrators and rappers. Inspection helps to prevent wire failures
and tripouts when potential problems are detected before they
become serious. Because an ESP contains many wires, however,
some discharge wire failures are to be expected, even with good
design and preventive maintenance.
Collection Hoppers and Ash Removal
Hoppers and ash removal systems often constitute problems in
precipitator operation. If the hoppers become full, the col-
lected dust may short-circuit the precipitator. Electrical power
may fuse the dust, causing formation of a large, clinkerlike
structure called a "hornet's nest," which must be removed. Most
problems associated with hoppers are related to flow of the dust.
Flow may be inhibited by improper adjustment of the hopper vi-
brators or failure to empty the hoppers. Heat and/or thermal
insulation of the hoppers may be required to prevent condensation
of moisture and resultant cementing of the collected dust.
Malfunctions of the ash evacuation and removal system
include water pump failure, water-jet nozzle failure, disengage-
ment of vacuum connections, and failure of sequencing controls.
The best measure for preventing malfunction of an ash
removal system, aside from proper design, is a good program of
operation and maintenance. Since dust buildup can affect so many
of the ESP components, proper ash removal will minimize or
eliminate many of the most common ESP malfunctions.
B-5
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Gas flow problems occur as a result of the inleakage of air
into hoppers from the dust conveyor systems. This results in
reentrainment of collected dust into the ESP, as discussed ear-
lier. Air inleakage can also occur through the ESP shell or
inlet flanges if operation is at pressures lower than atmospheric.
Often enough air is bled in to cause intense sparking.
The term "gas sneakage" describes gas flow that bypasses the
effective ESP section. Sneakage can occur through dead passages
of the ESP above the collector plates, around the high-tension
dust concentration at the bottom of the outlet section of the
frame, or through the hoppers. It 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 action.
Reentrainment of dust from hoppers caused by air inleakage
or gas sneakage is often indicated by an increase in dust concen-
tration at the bottom of the outlet section of the ESP. Correc-
tive measures for air leakage would include proper design and fit
of components and sealing of areas where inleakage occurs.
Rappers or Vibrators
In dry removal systems, rapping of the collection electrode
to remove dust is normally done periodically. Effective rapping
can occur only when the accumulation of material on the plate is
thick enough that it falls in large agglomerates into the hopper.
Although there is always some reentrainment of dust, effective
rapping must minimize it. As discussed earlier, rapping forces
that are either too mild or too severe can cause poor perform-
ance.
Poor gas flow and the condition of the dust can also cause
formation of deposits on discharge electrodes, often as much as 5
cm (2 in.) thick. Their deposits 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
B-6
-------
poor performance, but efficiency may be reduced, depending on
resistivity, power supply range, and uniformity of the deposit.
Design of the support structure and of the electrodes can
also cause inadequate rapping. Recent investigations of rapping
acceleration in fly-ash ESP's have shown measured accelerations
2
of 5 g when as much as 30 g may be required. The first step in
dealing with problems related to rappers and vibrators is 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-measuring instrument
in the exit gas stream of the ESP.
Discharge electrodes should be kept as clean as possible.
Rapping intensity is limited only by the possibility of mechani-
cal damage to the electrodes and support structure.
The vibratory types of cleaning mechanisms usually require
more maintenance than the impulse types.
Insulator/Bushing Failure
Suspension insulators support and isolate the high-voltage
parts of an ESP. As mentioned earlier, inadequate pressurization
of the top housing of the insulators can cause ash deposits or
moisture condensation on the bushings, which may cause electrical
breakdown at the typical operating potential of 45 kV d.c.
Corrective or preventive measures include inspection of fans
that ventilate 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.
Inadequate Electrical Energization
Since an ESP operates on the basis of electric field and
electric charge, electrical energization must be adequate to
charge the particles, maintain the electric field, and hold the
collected dust to the collection plates.
B-7
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Among several possible causes of failure to achieve the
required level of power input to the ESP, the following are most
common:
0 High dust resistivity
0 Excessive dust accumulation on the electrodes
0 Unusually fine particle size
0 Inadequate sectionalization ,
0 Improper rectifier and control operation
0 Misalignment of electrodes
0 Inadequate power supply range
If a precipitator is operating at a spark-rate-limited
condition but current and voltage are low, the problem can
commonly be traced to high-resistivity dust, electrode misalign
ment, or uneven corona resulting from buildup on the discharge
electrode.
The effects of high resistivity were discussed in more
detail in Section 2, in terms of conditions specific to utility
industry, where resistivity presents the greatest problem.
Because of the importance of resistivity in the precipita-
tion process, a first step in troubleshooting should be in situ
resistivity measurements. High resistivity (more than IQlO ohm-
cm) may be causing the abnormally low currents. If resistivity
is not high, other potential causes should be investigated.
Failures in ESP controls can prevent the system from achiev-
ing the level of power required for normal operation. Following
are the most common malfunctions in controls:
1) Power failure in the primary system
2) Transformer or rectifier failure in secondary system
caused by:
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 fail-
ures is a good maintenance program in which the controls are
checked periodically for proper operation. A daily log of
B-8
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instruments that register current, voltage, and spark rate can
also indicate potential problems.
SURVEYS OF PRECIPITATOR MALFUNCTIONS
Two surveys are cited to support the preceding information
on precipitators, one by the Industrial Gas Cleaning Institute
(IGCI)3 and the other by the TC-1 Committee of the Air Pollution
Control Association (APCA).4 Both surveys give similar results.
The IGCI survey lists the following problems, in order of
frequency and severity:
0 Discharge electrode failure
0 Rapper malfunction
0 Insulator failure
0 Dust buildup (causing electrical shorts)
0 Hopper plugging
0 Transformer rectifier malfunctions
The APCA survey covers four major industries: electrical
utilities, cement, paper, and metallurgical. The equipment
reported on had been in service ranging from 3 months to 50
years, with an average service life of 7 to 10 years.
Responses from 174 users indicate the following problem
areas in order of importance and cost:
Discharge electrodes 30.5%
Dust removal systems 25.1
Rappers or vibrators 21.4
Collecting plates 14.8
Insulators 5.8
The TC-1 Committee notes that manufacturers are making
design improvements in discharge electrodes, the largest source
of problems. They also cite the importance of the design, opera-
tion, and maintenance of dust removal systems, noting the indus-
try that reported the highest incidence degree of hopper plugging.
The committee concludes that although problems do Occur with
precipitator equipment, most users are satisfied with the ESP as
a functioning unit. They suggest close cooperation between user
and supplier, coupled with exchange of information among the user
industries to facilitate development of an ESP that meets the
needs of all users.
B-9
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REFERENCES - APPENDIX B
1. Electrostatic Precipitator Manual. The Mcllvaine Co. 1976.
2. Hesketh, H.E., and F.L. Cross, Jr. (ed.) Handbook for the
Operation and Maintenance of Air Pollution Control Equip-
ment. Technomic Publishing Co. Westport, Connecticut.
1975.
3. Engelbrecht, H.L. Plant Engineer's Guide to Electrostatic
Precipitator Inspection and Maintenance. Plant Engineering.
April 1976, pp. 193-196.
4. Bump, Robert L. Electrostatic Precipitator Maintenance
Survey. TC-1 Committee of the Air Pollution Control
Association. 1974.
B-10
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