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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. |
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Last updated: May 10, 2009.
Copyright © 1997 TRK Engineering
Services, Inc. All rights reserved.
For more information contact: TRK Engineering Services - 95 Clarks Farm Road - Carlisle,
MA 01741 - Telephone: 978-287-0550 - Fax: 978-287-0569 - email: trkeng@apcnetwork.com