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GLOSSARY OF TERMS FOR ELECTROSTATIC
PRECIPITATORS
These readings can be obtained with or without
fan operation, or at different temperatures but, all conditions
should be recorded. Measurements can be recorded as maximum readings
or as V-I CURVES. Generally, SPARKOVER should not be observed during
these readings. ALIGNMENT:
This term refers to the maintenance of optimum clearances
between the high voltage system and the grounded surfaces of the
precipitator. Operation at the highest possible ESP voltages depends
to a large measure on good mechanical integrity and elimination of
reduced distances where sparkover could occur. This means the high
voltage frame of a BUS SECTION must be held plum relative to the
grounded collecting surfaces. Ideally, discharge electrodes should
be centered in the gas passages between the collecting plate
surfaces, and centered between the vertical ribs / baffles of the
collecting plates. On a practical basis for the 9" (228mm) wide
GAS PASSAGES commonly found in WEIGHTED WIRE ESP designs, proper
alignment generally means no corona producing wire closer than
4" (102mm) to the flat surface of the collector plate or 5
1/2" (140mm) to any vertical rib of the collector plate.
The emitters, or corona producing elements of RIGID DISCHARGE
ELECTRODES in GAS PASSAGES spaced at 12" (305mm) should
generally be within 1/2" of center (51/2", 140mm) of the
flat surface of the collecting plates, and be a minimum of 6" -
71/2" (152 - 190mm) to any vertical rib of the collector plate.
Allowable tolerances will vary somewhat by manufacturer,
electrode design, and from a practical standpoint, by age and
condition of the ESP. ASPECT
RATIO: The ratio obtained by dividing effective length of the
precipitator by the effective height. AUTOMATIC
MODE: Modern controls will often feature the ability to transfer
from an automatic mode of AUTOMATIC VOLTAGE CONTROL, which is normal
operation, to a MANUAL MODE for maintenance or trouble periods. The
automatic mode allows the TR Set voltage control to adjust for the
level of sparkover. AUTOMATIC
VOLTAGE CONTROL: The normal method of controlling the amount of
secondary current to the ESP is by controlling the magnitude of
voltage on the primary winding of the TR Set. This is accomplished
by detecting the transient disruption in the electrical circuitry
caused by sparkover, or an arc, in the ESP. A feedback circuit then
adjusts the gate signal of an SCR (thyristor) so as to provide a
level of voltage necessary to maintain the desired sparkover rate. BACK BACT: (Best
Available Control Technology) An emission limitation based on the
maximum degree of emission reduction achievable.
Under Title I of the CAAA, EPA will establish BACT standards
for serious, severe and extreme non-attainment areas. BALANCED
DRAFT: The condition where the absolute pressure in a boiler
furnace is exactly equal to the absolute atmospheric pressure
outside the furnace or it is slightly negative. BUS:
A conductor enclosed within a grounded duct. BUS
SECTION: Is the smallest portion of high voltage structure,
containing a fixed group of DISCHARGE ELECTRODES, that can be
independently energized by a single Transformer (TR). More than one
bus section can be controlled through a TR either in parallel or
series arrangement. CASING: The
precipitator shell or casing is designed to confine the flue gas
within a specific collection zone, and it must provide structural
support for the discharge and collecting electrode systems, rapping
systems, gas distribution system, and other precipitator components.
The precipitator casing is usually constructed of fabricated
steel panels fitted with external columns, beams, and stiffeners and
is designed so that the final assembly provides a gas tight unit
able to withstand both internal and external loading.
The precipitator casing includes ACCESS DOORS located in the
side walls and on the roof that permit access to the precipitator
interior. A key
interlock system prevents opening the doors while the
precipitator is energized. Access
walkways, platforms, stairs, and ladders are attached to the casing
at various internal and external locations. CATHODE:
This is the negative polarity, high voltage DISCHARGE ELECTRODE of a
precipitator. It is the
cathode that suffers metal erosion due to ESP repetitive sparkover
– a common cause of localized wire thinning, draw-out due to
weight tension, and ultimate breakage with sharp points. CELLS: A
cell is an arrangement of bus sections across the width of the ESP.
Typically the number of cells times the number of fields
equals the number of bus sections. CFM:
Cubic Feet (of any gaseous matter) per Minute.
See gas flow rate. CHAMBER:
Refers to a gas tight longitudinal subdivision of the
precipitator (a precipitator without any internal dividing walls is
a single chamber precipitator; a precipitator with a single internal
dividing wall is a two chamber precipitator, etc.).
Very wide precipitators may have non gas tight load bearing
walls that are used for structural purposes.
Technically these non gas tight walls would not be considered
as chamber dividing walls. CHEVRON DESIGN: Refers
to the inlet transition design that places 2 parallel ESPs at a
small angle to each other.
This
layout typically is used to minimize space requirements.
CLR -
CURRENT LIMITING REACTOR: This is primarily a ballast of
inductance placed in the low voltage circuit to provide current
limiting ability under sparkover in the ESP. Another major advantage
of a properly sized reactor is to better shape the waveform of the
input voltage to the TR Set thereby gaining a greater conduction
angle of secondary current flow. This feature has benefits for ash
or dust layers that exhibit high resistivity characteristics. COLLECTION
EFFICIENCY: The weight of dust collected per unit time divided
by the weight of dust entering the precipitator during the same unit
time expressed in percentage. The
computation is as follows:
(Dust in) – (Dust out)
Efficiency =
(Dust in)
X 100 COLLECTING
SURFACE: Is the term for the sheet metal collector plate that
serves as the point of deposition for the particulate that is
negatively charged within the gas passage of the ESP.
Collecting surface plate design differs between manufacturers
but, all are secured to the shell of the ESP at ground potential and
serve as the positive anode of the gas passage. COLD SIDE ESP: An ESP which is installed downstream of the air heaters COLLECTION
SURFACE AREA: The total flat projected area of collecting
surface exposed to the active electrostatic field (length x height x
2 x number of gas passages). CONDUCTION
ANGLE: The CORONA CURRENT flows in pulses rather than as pure
direct current. The percent conduction during each half cycle, out
of an available 8.33 milliseconds, (60 Hz), represents the length of
time this current flows relative to the off time of the CYCLE.
Operating at the current rating of the TR Set usually means a
conduction angle of 86% exists. As the operating level of the TR Set
is reduced, this angle decreases in a somewhat linear fashion. CONTROL
EQUIPMENT: The necessary electrical components required to
regulate the potential of the high voltage system by regulating the
voltage or current applied to the primary of the high voltage
transformer (also metering and protection equipment, and controls
for auxiliary electrical items such as rappers, heaters, etc.). CONTROL
CABINET: This cabinet contains the control and monitor apparatus
of the power supply. Features mainly involve low voltage breaker,
overload controls, metering, and the automatic voltage control
components. CORONA
DISCHARGE: This term represents an electrical breakdown of the
flue gas at localized small zones on the surface of the DISCHARGE
ELECTRODE. Approximately 20 - 25 thousand volts is required to start
this process on a smooth 0.1" (2.54mm) diameter wire. CURRENT
DENSITY: The amount of secondary current per unit of ESP
collecting surface. Common
units are ma/ft2 and nA/cm2. CURRENT
WAVE SHAPE: Usually refers to the pattern of the pulsating
secondary CORONA CURRENT as observed on an oscilloscope. As
conduction angles decrease, the waveshape tends toward peakiness. As
conduction angles increase, the wave shape approaches the look of a
sine-wave. CYCLE: Generally
refers to an alternating current of 60 cycles per second which is
the standard energizing mode of the TR Sets. This means that 3600
alternating cycles per minute are rectified into 7200 half cycles
per minute and are fed into the area of the precipitator controlled
by one TR Set. DEW
POINT: The temperature at which the equilibrium vapor pressure
of a liquid is equal to the existing partial pressure of the
respective vapor. (For
air containing water vapor, it is the temperature at which liquid
water begins to condense for a given state of humidity and pressure
as the temperature is reduced. For
flue gaining water vapor and SO it is the set of conditions at which
liquid sulfuric acid begins to condense as the temperature is
reduced.) DIELECTRIC
FLUID: A substance used to keep the transformer operating at
moderate temperature levels, and as a dielectric where space is
concerned. DISCHARGE ELECTRODE: Refers to the high voltage component which ionizes the process gases and creates the electric field. It is shaped to provide a corona discharge when the impressed voltage breaks the gas down at the electrode surface. This breakdown of the gas creates corona tufts on the discharge surface. Typically, voltage applied to the discharge electrode is of negative polarity. In many WEIGHTED WIRE designs, the discharge electrode is a smooth round wire slightly larger than 0.1" (2.54mm) in diameter. Barbed wire is also frequently used in part, or all of the ESP, to enhance corona characteristics. Discharge Electrodes of the RIGID and RIGID FRAME or Mast variety are also widely available. They can vary widely in style and shape Deutsch-Anderson
Equation: The Deutsch-Anderson Equation is a mathematical
formula that can be used to determine the collection efficiency of
an ESP. It states that
efficiency is related to the ratio of collecting area (A) divided by
the gas volume (V) multiplied by the particle migration velocity
(w). This formula is the
basis for all ESP design and performance models.
Collection
Efficiency
N = 1 - e – (A / V) w
DUST OR
MIST CONCENTRATION: The weight of dust or mist contained in a
unit of gas, e.g. pounds per thousand pounds of gas, grains per
actual cubic foot of gas, or grains per standard dry cubic foot (the
temperature and pressure of the gas must be specified if given as
volume). EFFLUENT:
A discharge or emission of a fluid (liquid or gaseous). ELECTRONIC TUBE RECTIFIER: A rectifier consisting of high vacuum rectifier tubes. Can be either Air-cooled or Liquid-immersed ELETROSTATIC
PRECIPITATOR: (ESP) A single precipitator is defined by all
parts that are contained by a independent casing. EMISSION:
Release of pollutants into the air from a source. exceedance:
is defined as a condition that is detected by the EXCESS
AIR: Air in excess of the amount necessary to combust all the
available fuel. excursion:
is defined as a departure from an indicator or indicator
range established for monitoring under the FIELD: Refers to an arrangement of one or more bus sections, oriented perpendicular to the direction of flue gas flow, which is energized by one TR SET. The number of TR sets / power supplies positioned in series (parallel to the gas flow), each one controlling the collection of particles in a specific area, will typically identify the number of fields of a precipitator. FLY ASH:
Particulate matter entrained in the flue gas stream leaving a fossil
fuel fired boiler. It
consists of both ash and combustible matter. FOSSIL
FUELS: Coal, oil and natural gas; so-called because they are the
remains of ancient plant and animal life. FUGITIVE
EMISSIONS: Emissions not caught by a capture system. FULL
WAVE: This electrical term means that the 7200 rectified half
cycles per minute are fed into the full precipitator area energized
or controlled by one TR Set. FUME: Solid
particulates generated by condensation from the gaseous state,
generally after volatilization from molten metal, and often
accompanied by a chemical reaction, such as oxidation.
Fumes flocculate and sometime coalesce. GAS
DISTRIBUTION DEVICES: Internal elements in the transition or
ductwork to produce the desired velocity contour at the inlet and
outlet face of the precipitator example: turning vanes or perforated
plates. GAS FLOW RATE, CUBIC FEET PER MINUTE (CFM): The volume of process gas at any point of the plant exhaust system measured in terms of minutes. There are several units of measurement:
ACFM-The actual
gas flow measured (Actual Cubic Feet per Minute: SCFM-The gas flow volume reduced to 70OF (standard
temperature) by calculation
(Standard Cubic Feet per Minute) DSCFM-The gas flow reduced to 70O (standard temperature) and without volume of steam or water vapor contained in the exhaust gas (Dry Standard Cubic Feet per Minute GAS
PASSAGE: Is the passage formed by two adjacent collector plates,
normally on 9" - 10" (228 - 254mm) centers with WEIGHTED
WIRE systems; 11" - 16" (279-406mm) with Rigid Discharge
and Rigid Frame Electrode systems. The passage can be considered to
consist of two capacitors with the negative DISCHARGE ELECTRODE at
centerline and the positive ground collecting plates forming the
other electrode. This passage is where the action takes place within
the precipitator. GAS
VELOCITY: A figure obtained by dividing the volume rate of gas
flow through the precipitator by the effective cross-sectional area
of the precipitator. Gas
velocity is generally expressed in terms of ft./sec. And is computed
as follows: Velocity
=
Gas Volume (ft3/sec.) Effective cross-section is construed to be the
effective field height X width of gas passage X number of passages. GRAIN:
A dust weight unit commonly used in air pollution control.
Equal to one seven thousandth of a pound.
One grain = 1/7000 lb. GRAIN
LOADING: The rate at which particles are emitted form a
pollution source. Measurement
is made by the number of grains per cubic foot of gas emitted. HALF
WAVE: This term means the TR Set is energizing more than one BUS
SECTIONS and that these separate areas of the precipitator are
receiving alternate pulses or 3600 rectified half cycles per minute.
The TR Set will always have two outlet bushings with this mode of
hook-up. Even though each bus section operates electrically
independent, the overall operating level of the TR Set is controlled
by the weakest point of the areas controlled. On balance, with the
larger TR Sets in use today, the half-wave mode is not generally
recommended. HIGH
VOLTAGE BUS SYSTEM: The high voltage (HV) bus system is used to
transfer power from the power supplies (Transformer Rectifiers) to
the HV discharge electrode frames.
The bus is the conductor and is usually made of pipe/bar,
cable, or a combination of the two.
Bus runs between the interlocked insulator compartments or
penthouses are enclosed in watertight bus duct.
The bus is supported with insulators, usually of the standoff
/ post insulator type. Thru-bushing insulators may or may not be
used at the insulator compartment / penthouse and switch housing
penetrations. Ground
and/or disconnect switches may be part of the HV bus arrangement. HIGH
VOLTAGE CONDUCTORS: Conductor to transmit the high voltage from
the transformer-rectifier to the precipitator high voltage system. HIGH
VOLTAGE SELECTOR SWITCH: Is the means to selectively energize a
separate bus section when more than one bus is controlled by a TR
Set. There are several methods of high voltage isolation, but all
must be accomplished with the TR Set shut down and properly locked
out. One type of switch
is internal to the transformer and immersed in the same oil as the
transformer winding. Another type of switch, external to the TR
tank, isolates the high voltage circuit with a blade mechanism by
withdrawing a blade from a clip or pan disc. A third mode of
isolation on the high voltage side involves actual disconnection of
a flexible lead from one TR output bushing and physically placing a
jumper between the two bushings (if one is not already in place). HOPPERS:
Hoppers located at the bottom of the precipitator casing and are
used to collect the material that has been collected and that falls
off of the internal components that are cleaned.
The typical shape is pyramidal with the sides of each hopper
being steep sloped and the outlet opening is sized so that fly ash
may be easily removed by an ash removal system.
Baffles are usually placed in the hoppers; they extend below
the dust level to minimize undesirable gas sneakage below the
collection plates. Typically
hopper are equipped with level detectors to alarm high levels and
hopper heaters which are used to reduce corrosion and to keep the
material fluidized. Hoppers
are also equipped with access doors, strike plates for manually
rapping the hopper walls, and poke holes to unclog the hopper
outlets. HOPPER
CAPACITY: Total volumetric capacity of hoppers measured from a
plane 10” below high voltage system or plates, whichever is lower. HOT SIDE
ESP: An ESP which is installed upstream of the air heaters. HOT WIRE
ANEMOMETER: A device used for the measurement of flow velocities
and turbulence in an ESP and its associated ductwork.
It has the advantage of high sensitivity at very low
velocities and produces an electrical readout. HUMIDITY,
ABSOLUTE: The weight of water vapor per unit volume, pounds per
cubic foot or grams per cubic centimeter. HUMIDITY,
RELATIVE: The ratio of the actual partial pressure of water
vapor in a space to the saturated pressure of pure water vapor in a
space to the saturated pressure of pure water at the same
temperature. INLET
DUST LOADING: A measure of the particulate matter entering an
ESP expressed in grains of particulate matter per actual cubic foot
of flue gas. IONS:
Generally refers to the flue gas molecules within the gas
passage that become primarily charged negatively by the action of
free electrons initiated from the CORONA DISCHARGE. It is this ionic
flow that basically charges and pushes the ash particles toward the
positive ground COLLECTING SURFACE under the influence of the
VOLTAGE FIELD. In the
complex action of the gas space, some positive gas ions are also
formed which tend to promote particle deposition on the negative
discharge electrode. Negative ions by far are the most numerous in
the gas space and help constitute a space charge in the
precipitation process. IN SITU
RESISTIVITY: Particle resistivity as determined by a probe
inserted into the flue gas stream.
See Resistivity INSULATOR
COMPARTMENT: Enclosure for the insulator(s) supporting the high
voltage system (may contain one or more insulators, but not
enclosing the roof as a whole). LAER
(Lowest Achievable Emission Rate): The rate of emissions which reflects either the most
stringent emission limit contained in the implementation LINEAR
REACTOR (CLR): This is primarily a ballast of inductance placed
in the low voltage circuit to provide current limiting ability under
sparkover in the ESP. Another major advantage of a properly sized
reactor is to better shape the waveform of the input voltage to the
TR Set thereby gaining a greater conduction angle of secondary
current flow. This feature has benefits for ash or dust layers that
exhibit high resistivity characteristics. key
interlock system: A system of locks that prevents opening
the ESP access doors while the precipitator is energized. MACT:
(Maximum Achievable Control Technology) – the standard with which
source of HAPs will have to comply; the CAAA defines MACT as “the
maximum degree of reduction in emissions… achievable for new or
existing sources… taking into account the cost of achieving such
reductions.” MACT
standards for existing sources must be at least as stringent as the
average level of control achieved at the best controlled 12 percent
of facilities, and MACT for new sources will have to be even
stricter. MANUAL
MODE: This refers to the ability to remove the automatic voltage
control features from the electrical circuit by a switch in the
control cabinet. In this mode, the amount of power input to the ESP
is fixed at the manual setting chosen. If excessive sparkover occurs
at this manual setting the control will not recognize nor correct
this condition. This could cause deterioration in performance as
well as possible internal damage to the DISCHARGE ELECTRODES. MECHANICAL
COLLECTOR: Devices that are functionally dependent on the laws
of mechanics governing the motion of bodies in space.
They can be operated dry or wet.
When operated wet, devices are generally called scrubbers.
Examples of mechanical collectors are cyclones, settling
chambers and various types of impingement collectors. MECHANICAL
RECTIFIER: A device consisting of a disc or arms with
appropriately placed contacts rotating at a synchronous speed to
produce an unidirectional voltage at its output. MIGRATION
VELOCITY: A parameter in the Deutsch-Anderson equation used to
determine the required size of an electrostatic precipitator to meet
specified design conditions. Other
terminology used are “W” value and precipitation rate.
Values are generally stated in terms of ft/min or cm/sec NOX
(NITROGEN OXIDES): Chemical compounds containing nitrogen and
oxygen; react with volatile organic compounds, in the presence of
heat and sunlight, to form ozone.
They are also a major precursor to acid rain.
Nationwide, approximately 45% of NOX emissions
come from mobile sources, 35% from electric utilities and 15% from
industrial fuel combustion. OHMS LAW: The
formula used to determine the relationship between Voltage (V),
Current (I) and Resistance (R).
V = IR
OPACITY:
Refers to the amount of light that can pass through expressed in
percent reduction of light intensity.
At the stack it normally refers to the degree of visibility
of an exhaust plume. Normally measured by opacity monitors mounted
in the ductwork or stack. EPA method 9 is used to measure it
visually at the stack. OZONE:
A compound consisting of three oxygen atoms, that is the primary
constituent of smog. It
is formed through chemical reactions in the atmosphere involving
volatile organic compounds, nitrogen oxides and sunlight.
Ozone can initiate damage to the lungs as well as damage to
trees, crops and materials. There
is a natural layer of ozone in the upper atmosphere, which shields
the earth from harmful ultraviolet radiation. PARTICLE
SIZE: The diameter in mms
(micrometers) of a particular piece of particulate matter. PARTICLE
MATTER: Solid or liquid particles entrained a gas stream. PENTHOUSE:
A weatherproof gas-tight enclosure over the precipitator to
contain the high voltage insulators. PIGGY-BACK LAYOUT: Refers
to the orientation of two or more parallel ESPs in a piggy back
arrangement (one on top of the other). This layout typically is used
to minimize space requirements.
PITOT
TUBE: A common instrument used for velocity determination in
ducts leading to and from air pollution control devices. PM 10:
A EPA standard for measuring the amount of solid or liquid
matter suspended in the atmosphere (“particulate matter”).
Refers to the amount of particulate matter under 10
micrometers in diameter. The
smaller PM10 particles penetrate to the deeper portions of the lung,
affecting sensitive population groups such as children and people
with respiratory diseases. PM
2.5: New EPA standard that limits the amount of
particulate matter emissions under 2.5 micrometers in diameter. PPM
(PARTS PER MILLION): The number of parts of a given pollutant in
a million parts of air. Units
are expressed by weight or volume. PRECIPITATOR
CURRENT: The rectifier or unidirectional average current to the
precipitator measured by a milliampmeter in the ground return leg of
the rectifier. PRECIPITATOR DIMENSIONS: 1. Effective Length: Total length of collecting surface measured in the direction of gas flow. Length between fields is to be excluded. 2. Effective Height: Total height of collecting surface measured from top to bottom. 3. Effective Width: Total number of gas passages multiplied by spacing dimension of the collecting surface. 4.
Effective Cross-Sectional Area:
Effective width times effective height. PRECIPITATOR
VOLTAGE: The average DC voltage between the high voltage system
and grounded side of the precipitator. Preventive
maintenance (PM): This includes
the actions that detect, preclude, or mitigate degradation of
functional equipment to sustain or extend its useful life by
controlling degradation and failures to an acceptable level. There
are three types of preventive maintenance: periodic, predictive, and
planned. Periodic maintenance
is a form of preventive maintenance consisting of servicing, parts
replacement, surveillance, or testing at predetermined intervals of
calendar time, operating time, or number of cycles. Predictive maintenance
is a form of preventive maintenance performed continuously or at
intervals governed by observed condition to monitor, diagnose, or
trend the equipment’s functional or condition indicators. Results
indicate current and future functional ability or the nature and
schedule for planned maintenance. Planned maintenance
is a form of preventive maintenance consisting of refurbishment or
replacement that is scheduled and performed prior to failure of the
equipment. Corrective maintenance
includes actions that restore, by repair, overhaul, or replacement,
the capability of any failed equipment so it can function within
acceptance criteria. PRIMARY
AMMETER: This meter measures the current flow through the low
voltage primary winding of the TR Set in alternating current
amperes. The meter normally receives its signal from a current
transformer in the primary circuit. Dividing this indicated current
by the turns-ratio of the TR Set will provide the level of AC
current in the secondary winding. PRIMARY
CURRENT: Current in the transformer primary as measured by an AC
ammeter. PRIMARY
VOLTAGE: The voltage as indicated by an AC voltmeter across the
primary of the transformer. PRIMARY
VOLTMETER: This meter measures the voltage drop across the
primary winding of the high voltage transformer in the TR Set. The
voltage can be measured in various manners, but the object is not to
include any other equipment or apparatus within the measurement
point located at the main power cables going directly to the TR Set.
With recent SCR controls, the true value of this voltage varies with
the waveform at different levels of load current. POWER
INPUT: Generally refers to the corona power of the ESP which is
the average DC voltage multiplied by the corona DC current. Without
actual secondary circuit meters, an approximate DC power input in
watts would be between 65 to 70% of the primary AC circuit
volt-amperes. A key point here is not to rely on power as the
criterion of good precipitation. Good precipitation requires
adequate VOLTAGE FIELDS; the current observed will be a reflection
of many factors. Thus, we have to evaluate the values of proper
power input more in terms of how the voltage relates to the current
than by the product of the two terms. RACT
(Reasonably Available Control Technology): An emission limitation on existing sources in
nonattainment areas, defined by EPA in a Control Techniques
Guideline (CTG) and adopted and implemented by states. Under Title I
of the CAAA, EPA will establish RACT standards for marginal,
moderate and serious nonattainment areas. RAPPERS
- COLLECTING SURFACES: These are devices, generally located at
the top of the ESP or bottom of the collecting plates, which
periodically impart a shock to the collecting surfaces to help
dislodge the collected material into the hopper system. The final
collection efficiency of the precipitator is often determined by how
well this process is conducted. The object is to dislodge the
material from the collector surface in small clumps or patches
without building excessive dust layer thicknesses. This is a complex
part of precipitation, but it is more important to know that
reliability of rapper operation holds priority over timing, impact
force and other aspects of this system. RAPPERS
- HIGH VOLTAGE: These rapper devices impart a vibration or shock
to the high voltage frame supporting the discharge electrodes. The
object is to keep the buildups on these electrodes from affecting
the corona discharge pattern. The discharge electrodes will
generally exhibit irregular coatings of various size and shape.
Whether the buildups observed during outage inspections are
detrimental can usually be determined by an analysis of electrical
readings during periods of operation. It is usually better to
operate with some buildup than employ excessive rapping forces that
can result in failure of DISCHARGE ELECTRODES. RAPPER
INSULATOR: A device to electrically isolate discharge electrode
rappers yet transmit mechanically, forces necessary to create
vibration or shock in the high voltage system. RAPPING
INTENSITY: The “g” force measured at various points on
collecting or discharge electrodes.
Measured forces should be specified as longitudinal or
transverse. REENTRAINMENT:
Reentrainment, usually is associated with rapper reentrainment,
which refers to the reintroduction of particulate back into to the
gas stream from the discharge electrodes and collecting surfaces
during rapping. Reentrainment can also result from gas sweepage when
gases bypass the treatment zone of the ESP and disturb collection
zones such as hoppers. High velocity zones and external influences,
such as ambient air infiltration of the casing or hopper, can also
promote particulate reentrainment. Reentrainment can substantially
lower the ESP collection efficiency. RESISTIVITY:
This term is most critical for the fly ash precipitator
because it directly controls the levels of voltage and current
observed at most installations. Resistivity refers to the electrical
resistance of the ash layer after it forms on the positive ground
COLLECTING SURFACE. If the resistance level is high, the corona
current passing through the ash layer must be generally reduced or
BACK CORONA effects will reduce performance of the ESP. The range of
resistivity is primarily affected by the chemistry of the ash,
moisture in the flue gas, levels of sulfur trioxide, and flue gas
temperature. Resistivity effects are generally observed by the
occurrence of SPARKOVER on most ESP fields at some reduced level of
voltage and current. Operation in a good zone of resistivity allows
the ash layer on the collector plate to bond sufficiently for
optimum ESP performance and helps to reduce REENTRAINMENT.
When resistivity drops to low levels, the ash layer on the
collecting surface allows current to flow through it without
restriction and it is easily reentrained back into the gas stream.
This condition is generally characterized by high corona
current levels without the occurrence of sparkover. RIGID
DISCHARGE ELECTRODE (RDE) DESIGN: This term refers to
precipitators utilizing rigid discharge electrodes, such as the pipe
and spike variety, for it's discharge electrode rather than a
weighted wire type of high voltage system. RIGID
FRAME DESIGN: This term refers to precipitators utilizing rigid
frames with tensioned discharge electrodes between supporting
members. Frame shapes can vary from rectangular tubular pipes with
horizontal cross members to Mast frames with a vertical primary
support and horizontal cross members in a T configuration. Electrode
styles and shapes can also vary widely. Rigid frame designs are
almost exclusive to European design precipitators and are typically
rapped by tumbling hammer rappers located within the gas stream. SAFETY
GROUNDING DEVICE: A device for physically grounding the high
voltage system prior to personnel entering the precipitator.
(The most common type consists of a conductor, one end of
which is grounded to the casing, the other and attached to the high
voltage system using an insulated operating lever. SATURABLE
CORE REACTOR: This is a method of voltage control for the
precipitator that has been superseded in recent years by the SCR.
The saturable core reactor passed the primary current through a
substantial winding on an iron core. The voltage output of this
device varied by the level of DC current through a control winding
and it's subsequent affect on the saturation flux of the reactor
core. This control was used extensively for years and still exists
at some locations. A disadvantage was a slower response to sparkover
conditions. SCA – SPECIFIC COLLECTING AREA: The quotient of the total collecting area (A) divide by the total gas volume (V) handled by the ESP multiplied by 1000. SCA is commonly expressed as ft2/1000 acfm (m2 / (m3/sec). SELENIUM
RECTIFIER: A rectifier consisting of selenium cells (fluid
immersed). SPECIFIC
COLLECTING AREA (SCA): A figure obtained by dividing total
effective collecting surface of the precipitator by gas volume,
expressed in thousands of actual cubic feet per minute. SPECIFIC
CORONA POWER: The quotient of the total corona power of all
precipitator bus sections divided by the total gas volume handled by
the precipitator, multiplied by 1000 Units are expressed as
watts/1000 acfm. SCFM
(STANDARD CUBIC FEET PER MINUTE): The volume that a gas would
occupy at standard temperature and pressure conditions (70OF
and 14.7 PSIA). See gas flow rate. SCR: Selective
Catalytic Reduction is the most advanced and efficient process
for NOx reduction. SCR
technology is based upon the conversion (reduction) of NOx (NO2 and
NO) with ammonia (NH3) into water (H2O) and nitrogen (N2).
A catalyst is used to speed up the conversion rate. The term
SCR is used to describe both for the technology and the apparatus
that that is used. SCR
CONTROLS (Silicon-Controlled
Rectifier): Silicon controlled rectifiers are the most
extensively used method of voltage control in recent years, and
consists of two silicon rectifiers mounted in an inverse parallel
fashion in the primary AC circuit of the TR Set. Thyristors are also
used instead of silicon diodes, but the principle is basically
identical. These devices are normally open in both directions until
a small gate signal is applied which allows the SCR to conduct in
one direction. The output is controlled by the strength of the
current flow in the gate circuit which receives it's signal from
either the AUTOMATIC or MANUAL mode operation of the voltage
controller. SCRUBBER:
A device that uses a liquid spray to remove aerosol and gaseous
pollutants from an air stream. The
gases are removed either by absorption or chemical reaction.
Solid and liquid particulates are removed through contact
with the spray. Scrubbers
are used for both the measurement and control of pollution. SECONDARY
AMMETER: This meter measures the average DC secondary current,
which is actually the precipitator corona current passing through
the ground path on it's return to the rectifier connection of the TR
Set so as to complete the electrical circuit. This meter has a low
resistance movement and the scale reads in milliamperes or amps
depending on the size of the TR Set. The secondary current waveform
can usually be observed by connecting an oscilloscope across the
meter. There is usually a shorting device or surge arrestor across
the meter for protection. Under no circumstance should the leads be
removed from this type of meter with the TR Set energized. Another
method generally used is a meter measuring a voltage across a
resistor and calibrated as a current meter. SECONDARY
VOLTMETER: This measurement is made between the rectifier output
and the outlet bushing of the TR Set by use of a voltage divider
installed inside the tank. With older TR Sets, it is possible to
obtain the average precipitator voltage, usually read as average DC
kilovolts, by installation of a retrofit voltage divider at the
outlet bushing of the TR Set. The indicated voltage represents the
voltage from the discharge electrode to ground, comprising both the
voltage drop across the gas space as well as the ash layer on the
collecting surface. It actually is a measurement of the dielectric
resistance and represents all the characteristics of the
precipitator load. SHROUD:
Refers to a steel tube or rod installed at the top and
bottom of a wire discharge electrode. The purpose of this shroud,
approximately 3/8" (9.5mm) outside diameter, is to eliminate
the corona discharge opposite the top and bottom termination of the
collector plate. The enlarged radius of the shroud would require a
higher voltage than required by the wire to initiate a corona tuft.
The shrouds usually extend about 4 - 6 inches (102 - 152mm) into the
gas passage. Prevention of localized sparkover at these critical
points eliminates premature wire failure due to electric erosion. SNCR:
Selective Non-Catalytic Reduction, As the name implies this form of
NOx reduction technology does not use a catalyst as with SCR NOx
reduction technology. With
the SNCR technology, ammonia or ammonia based compounds, such as
urea, are injected into the furnace at specific temperatures. These
temperatures are much higher than those for an SCR, the optimum
being around 1400 to 2000 F.
The NOx reduction reactions are very sensitive to temperature
and ammonia slip is a common problem. SO2:
Sulfur dioxide is an invisible, nonflammable acidic gas, formed
during combustion of fuel containing sulfur. SO3:
Sulfur trioxide oxidized from SO2; combines with
atmospheric moisture to form sulfuric acid mist (H2SO4). SPARKOVER:
Is a localized electrical breakdown in the gas space
between the high voltage system and ground. This generally occurs
between the DISCHARGE ELECTRODE and COLLECTING SURFACE. This
breakdown, or flashover, can occur when the physical clearance has
been reduced so that the operating voltage is greater than the space
will allow. More often, sparkover will occur when the resistivity of
the ash layer on the collecting surface reaches critical levels.
Premature sparkover at extreme low levels of voltage can often be
observed with a combination of higher resistivity and internal
difficulties such as reduced electrical clearances. During a significant sparkover, the basic
collapse of the VOLTAGE FIELD occurs which should always cause a
downward flick of the VOLTMETER needles. A small number of sparks
per minute is generally desirable in fly ash precipitators as long
as it occurs at reasonable levels of voltage and current. SPARKMETER:
This meter attempts to represent the number of sparks per
minute by integrating these transient surges by some type of
capacitance circuit. In most locations where spark-meters still
exist, replacement of the automatic voltage controls will
discontinue its use. In all cases, representation of the ESP
sparkover by a meter can be misleading. For all practical purposes,
it is recommended that a visual count or evaluation of the flicks of
the voltmeter needle be made on a per minute basis to better gauge
the spark rate. Spark rates as high as 60 to 70 per minute can be
easily observed. This is generally at the higher range that should
exist on most modern installations. At this higher sparkover level,
the meter needle must still come to rest many times during any given
minute. Structural
Steel: The precipitator casing is support by a structure
steel system that is typically only tied to the ESP in one fixed
location to allow the ESP casing to grow and expand independently
during normal operation. The
casing columns rest on a slide
plate or slide bearing in all
other areas. SUPPORT
INSULATOR: This term refers to the ceramic component that
supports and isolates the high voltage frame from ground potential.
Recent designs involve an alumina cylinder that also acts as a gas
seal at the top frame locations. The surface of the insulator is
sensitive to electrical leakage to ground if condensation or
contamination is allowed to occur. Purge air and heater applications
are two methods used to minimize insulator failures. TR
RATING: The transformer - rectifier (TR) should be sized to
supply sufficient current for the area of the precipitator to which
it is connected. The nameplate shows a KVA size, primary and
secondary voltage ratings, and primary and secondary current
ratings. These values are of most interest. The nameplate should
show whether the primary winding is tapped for more than one voltage
connection. A key point
here is that the actual electrical performance of the ESP may in no
way resemble any of the values shown on the nameplate. TR SET: Is
the term for the high voltage transformer and rectifier that
provides the electrical energy for a given precipitator area. These
components involve a specially wound transformer that supplies a RMS
secondary voltage sized on the basis of GAS PASSAGE spacing and
discharge electrode design. An RMS secondary voltage of about 53,500
volts AC (45 KV DC average) is utilized for the 9" (228mm) wide
GAS PASSAGE of most weighted wire precipitators; 77,300 volts AC (65
KV DC average) for most of the 12" (305mm) wide GAS PASSAGE of
rigid electrode precipitators. This AC voltage is usually rectified through a
silicon diode bridge circuit in most existing TR Sets. Rated DC
voltages are usually specified at the 45,000 - 50,000 volt level for
9"-10" (228 - 254mm) plate spacing; the 55,000 - 65,000
volt level for 11"-12" (279 - 305mm) plate spacing; the
70,000 - 90,000 volt level for 15"-16" plate spacing.
Other pertinent data can be observed on the metal nameplate of each
tank. While the voltages are generally similar between TR Sets, the
current ratings vary greatly based on the anticipated load
requirements of the particular ESP FIELD. While the KVA rating is
used, it is also common practice to specify the size of the TR Set
by it's corona current rating in milliamperes. TR TURNS
RATIO: Expresses the number of turns in the secondary winding of
the transformer for every turn in the primary winding. For example,
a TR Set with a 400 volt primary rating relative to a 53,500 volt
secondary would have an approximate ratio of 133 turns in the
secondary winding. TRANSITION:
An aerodynamically designed inlet or outlet duct connection to
the precipitator. Transitions
are normally included as part of the precipitator. TREATMENT
TIME: A figure, in seconds, obtained by dividing the effective
length, in feet, of a precipitator by the precipitator gas velocity
figure calculated above. TURNING
VANES: Vanes in ductwork or transition to guide the gas and dust
flow through the ductwork in order to minimize pressure drop and to
control the velocity and dust concentration contours. V-I
CURVE: Usually refers to a plot of secondary voltage versus
secondary current for a single TR set in which the shape of the
plotted curve might indicate a number of internal operating
characteristics of the precipitator. An important part of these
measurements is the indicated voltage at the threshold of corona
current. While normally obtained during air load, these curves
sometimes can be developed during operating periods VOLTAGE
DIVIDER: A means for supplying a low voltage feedback signal
that is proportional to the KV output of the TR. VOLTAGE
FIELD: Refers to the high voltage field generated between the
negative discharge electrode and positive collecting surface at
ground potential. This field supplies the charging mechanism and
driving force for the removal of the particles from the flue gas
stream. Desirable values of this field strength would lie between 4
to 5 kilovolts per inch of space. VOLTAGE
WAVE SHAPE: The pulsating DC voltage in the precipitator will
show peak and minimum values that vary in magnitude about the
average as observed on a secondary KV meter. The peak voltage is
based somewhat on the peak AC magnitude while the minimum voltage is
based on the capacitance resistance effect on the decay
characteristics of the voltage field in the gas passage. The
conduction time of the secondary current will be a factor. With low
capacitance conditions, the minimum point might coincide with the
threshold corona producing voltage on the discharge electrode. WEATHER
ENCLOSURE: Upper – A non gas-tight enclosure on the roof
of the precipitator to shelter equipment (TR sets, rappers, purge
air fans, etc.) and maintenance personnel. Lower – A non gas-tight enclosure at base of
precipitators to protect hoppers from wind and/or detrimental
weather condition. WEIGHTS:
This term refers to the cast iron weight attached to the
bottom of the wire discharge electrode to keep it taut, much like
the effect of a plumb-bob. These weights are about 25 lbs. (11.3kg)
for most installations. The weights are positioned and retained in a
bottom guide frame for maintenance of wire alignment at the center
line of the gas passage. WEIGHTED
WIRE DESIGN: This
term refers to precipitators utilizing the wire and weight for it's
discharge electrode rather than a rigid type of high voltage system. |