Esp 2l2e5z

INTRODUCTION TO ESP

Introduction to ESPs: Glossary ACCESS DOORS: A hinged or detached cover provided with a hand operated fastening device where accessibility is required. Typically it is interlocked using a key interlock system to prevent access to the ESP when the ESP is energized. ACFM: Actual Cubic Feet per Minute of gas volume at the actual condition temperature, pressure and composition. See gas flow rate. ACID DEW POINT: The temperature at which combustion gases are saturated with sulfuric acid. AIR LOAD: This term refers to energizing the ESP during an outage period at ambient temperatures. The object is to the electrical integrity of the bus section, to obtain electrical readings of voltage and current and to determine whether sparkover occurs before reaching a specific limit of the TR Set rating. The advantage of this type of measurement is to relate the internal integrity of the ESP BUS SECTIONS to one another as to electrode damage or extent of ash buildup. 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 ages 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 AGES 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 AGES 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. ANODE: Positive electrical terminal of high voltage power supply; this is the collecting plate surface (ash surface) which is maintained at ground potential. Precipitator (ESP) sparking starts at the anode. ANTI-SWAY INSULATOR: These insulators are used to prevent the bottom high voltage frames, which position and retain the DISCHARGE ELECTRODES, from swinging or drifting out of alignment. The insulators are either a ceramic bar or a shaft type and are

usually secured to the hopper wall. Some designs connect directly between the collecting plates and the lower high voltage frames. ANTI-SNEAKAGE BAFFLES: Internal baffle elements within the precipitator to prevent the gas from bying the active field or causing hopper re-entrainment. ARC: A severe electrical breakdown between the negative high voltage component and ground, usually caused by, or aided by, some internal mechanical defect. A poor AUTOMATIC VOLTAGE CONTROL circuit, as well as operation of the control in the MANUAL MODE, can aggravate this problem. Meter swings are usually 2 to 3 times greater those than observed during normal SPARKOVER. Length of breakdown could last several CYCLES. 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 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 CORONA: A term that signifies that the ash layer on the collector surface has reached a level of RESISTIVITY that produces a flow of positive IONS back towards the negative high voltage discharge electrode. Most commonly observed back corona conditions result in SPARKOVER and a reduction of POWER INPUT. SEVERE BACK CORONA (generically referred to as back corona and also referred to as reverse ionization), this condition is extremely detrimental to precipitator performance and will result in dramatic re-entrainment of material for the collecting plates at high current levels. 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 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 s 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 age 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 age. 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 ages). 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: A gaseous discharge found near an ESP discharge electrode resulting in a faint glow caused by ionization of gas molecules due to the electric field. CORONA CURRENT: This term signifies all the measured current flow ing from the CORONA DISCHARGE of the DISCHARGE ELECTRODE through the gas space of the ESP to ground, for a collecting plate area controlled by a single TR Set. It is also commonly known as the secondary DC current and is read on the SECONDARY AMMETER either as milliamperes or fraction of an amp. 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. Corona onset will vary with electrode design and gas age spacing. Electrons coming out of this localized tuft of activity primarily produce the negative gas IONS required to properly charge the ash particles negatively so they can migrate toward the positive ground COLLECTING SURFACE. CORONA POWER (KW): The product of secondary current and secondary voltage. Power density is generally expressed in of: (1) watts per square foot of collecting surface, or (2) watts per 1000 ACFM of gas flow.

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. ELECTROSTATIC PRECIPITATOR (ESP): A single precipitator is defined by all parts that are contained by an independent casing. EMISSION: Release of pollutants into the air from a source. EXCEEDANCE: Defined as a condition that is detected by the CAM plan monitoring that provides data in of an emissions limitation or standard and that indicates that emissions (or opacity) are greater than the applicable emissions standard, consistent with any specified averaging period. EXCESS AIR: Air in excess of the amount necessary to combust all the available fuel. EXCURSION: Defined as a departure from an indicator or indicator range established for monitoring under the CAM plan, consistent with any specified averaging period. 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 of minutes. There are several units of measurement: 

ACFM (Actual Cubic Feet Per Minute): The actual gas flow measured.

 

SCFM (Standard Cubic Feet Per Minute): The gas flow volume reduced to 70OF (standard temperature) by calculation. DSCFM (Dry Standard Cubic Feet Per Minute): The gas flow reduced to 70O (standard temperature) and without volume of steam or water vapor contained in the exhaust gas.

GAS AGE: Is the age 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 age 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 age 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 of ft./sec. And is computed as follows: Velocity =

Gas Volume (ft3/sec.) Effective cross-section area (ft2)

Effective cross-section is construed to be the effective field height X width of gas age X number of ages. 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 ed 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 age 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) ing 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 plan of any state (unless it is proved that such limitations are not achievable) or the most stringent emission limit achieved in practice, whichever is most stringent. 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 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 s 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 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 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 in 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 milliamp meter 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 ages 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. PREVENTATIVE 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 of how the voltage relates to the current than by the product of the two . 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 ing 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: Re-entrainment, usually is associated with rapper re-entrainment, which refers to the reintroduction of particulate back into to the gas stream from the discharge electrodes and collecting surfaces during rapping. Re-entrainment can also result from gas sweepage when gases by 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 re-entrainment. 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 ing 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 re-entrained 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 its 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 ing . Frame shapes can vary from rectangular tubular pipes with horizontal cross to Mast frames with a vertical primary and horizontal cross 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 ed 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 its subsequent effect 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 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 ing through the ground path on its 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 age. Prevention of localized sparkover at these critical points eliminates premature wire failure due to electric erosion. SILICON DIODE BRIDGE: The diode assembly converts the high voltage AC to DC voltage. These bridges are normally made up individual diodes connected in series. The number of diodes used will vary by manufacturer from 20 to several hundred. 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 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. INSULATOR: This term refers to the ceramic component that s 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 AGE 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 AGE of most weighted wire precipitators; 77,300 volts AC (65 KV DC average) for most of the 12″ (305mm) wide GAS AGE 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 its 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 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 age. 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 Weather Enclosure – 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 Weather Enclosure – 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 age. WEIGHTED WIRE DESIGN: This term refers to precipitators utilizing the wire and weight for its discharge electrode rather than a rigid type of high voltage system.

Introduction to Electrostatic Precipitators As you may know, particulate matter (PM) is a common industrial air pollutant that is harmful to human health and must be controlled in order to minimize its harmful effects. In many industrial plants, particulate matter created in the industrial process is carried as dust in the hot exhaust gases. These dust-laden gases through an electrostatic precipitator that collects most of the dust. Cleaned gas then es out of the precipitator and through a stack to the atmosphere. Precipitators typically collect 99.9% or more of the dust from the gas stream. Particle pollution is not isolated to a few geographic locations or even a few industries; but is pervasive across a wide variety of industries all over the world. In the United States the Environmental Protection Agency (EPA) regulates particulate emissions for various industries, and as a result, provoked industry to respond with various control devices that enable these regulations to be met. Of the major particulate collection devices available today, electrostatic precipitators (ESPs) are one of the more frequently used options. It is designed to trap and remove dust particles from the exhaust gas stream of an industrial process. These devices depend on electrostatic attraction to capture particles from industrial processes, and are adaptable to many applications. Depending on the design, ESPs are capable of handling large gas volumes across a wide range of temperatures, pressures, dust volumes, and acid gas conditions. They can collect a wide range of particle sizes, in either dry or wet states, and are cable of achieving collection efficiencies greater than 99.5%. Though they are not always the appropriate PM removal solution, they are popular among these industries:     

Power/Electric Cement Chemicals Metals Paper

Electrostatic precipitators function by electrically charging PM within a ing gas stream, inducing the particles to be attracted to and deposited on grounded plates or some other grounded collecting surface. When a layer of dust has accumulated, the collectors are shaken to dislodge the particles, causing them to fall with the force of gravity to hoppers below. The collected PM is then removed from the ESP by a conveyor system for disposal or recycling. Depending upon flue gas chemistry and the gas volume to be treated, there are many different sizes, types and designs of electrostatic precipitators. However, they all operate using the same principles and key components. Electrostatic precipitation removes particles from the exhaust gas stream of an industrial process. Often the process involves combustion, but it can be any industrial process that would otherwise emit particles to the atmosphere. Six activities typically take place:  

Ionization – Charging of particles Migration – Transporting the charged particles to the collecting surfaces

   

Collection – Precipitation of the charged particles onto the collecting surfaces Charge Dissipation – Neutralizing the charged particles on the collecting surfaces Particle Dislodging – Removing the particles from the collecting surface to the hopper Particle Removal – Conveying the particles from the hopper to a disposal point

The major precipitator components that accomplish these activities are as follows:      

Discharge Electrodes Power Components Precipitator Controls Rapping Systems Purge Air Systems Flue Gas Conditioning / Sorbent Injection Systems

Introduction to ESPs: Theory of Operations All PM either has or can be given a charge – positive or negative. Let’s suppose we impart a negative charge to all the particles within a gas stream. Then suppose we set up a grounded plate having a positive charge in the same vicinity. What would happen? The negatively charged particle would migrate to the grounded collection plate and be captured due to electrostatic attraction, this is the same phenomenon that causes a balloon to stick to the wall after rubbing it on your hair. As particles collect on the plate, a dust layer would form and accumulate until it was removed with some force that is able to overcome the attractive forces, which we could do by rapping the plate or spraying it with a liquid. Charging, collecting, removing – that’s the basic idea of an ESP.

Introduction to ESPs: Theory of Operations: Charging Charging

A typical ESP has thin wires called discharge electrodes (DE), which are evenly spaced between large plates called collection electrodes (CE). Think of an electrode as something that can conduct or transmit electricity. In the case of an ESP, a negative, high-voltage, pulsating, direct current is applied to the DEs in order to create a negative electric field and induce ionization of the ing PM. For the sake of understanding the charging process, the negative electric field can be mentally divided into three regions. The first region is right next to the discharge electrode, where the field is the strongest. The second region includes space between the DE and CE called the inter-electrode region, and is weaker than the first. The third region is located near the collection electrode and has the weakest field strength of all. Corona Discharge: Free Electron Generation The first region is where the particle charging process begins, and in this small area immediately surrounding the discharge electrode several things happen very rapidly (in a matter of a millisecond). As voltage applied to the DE is increased, it eventually reaches a point when the electric field around the conductor is high enough to form a conductive region, but not high enough to cause electrical breakdown or arcing to nearby objects. This phenomenon is commonly referred to as corona discharge and can be seen by the human eye as a luminous blue glow surrounding the DE. As free electrons created by the corona

discharge are repulsed by the negative electric field, they move faster and faster away from the DE. This acceleration causes the electrons to literally crash into ing gas molecules and occasionally knock off some of their electrons. As these gas molecules lose electrons that are negatively charged, they become positively charged ions. So, this is the first thing that happens – gas molecules are ionized, and electrons are liberated. All this activity occurs very close to the discharge electrode, and as the process continues, it creates more and more free electrons and positive ions. The name for all of this electron generation is avalanche multiplication. Ionization of Gas Molecules As electrons leave the strong electrical field region surrounding the discharge electrode, they enter the inter-electrode region where they begin to lose energy and slow down. Though there are still gas molecules in the inter-electrode region, the electrons kind of bump up to them and get captured instead of violently colliding with them, creating negative gas ions. Now we have ionization of gas molecules happening near the discharge electrode as well as in the inter-electrode area, but with a big difference. The ions created near the discharge electrode are positive and remain in that area. However, because the ions created in the inter-electrode area are negative, they want to move with the electrons in the direction opposite the strong negative field. Charging & Migration Before PM can be captured, it must first acquire a negative charge and the negative gas ions created in the inter-electrode region play a crucial role in this process. When PM and negative gas ions cross paths, the gas ions stick to the particles and impart a negative charge to them. At first the charge is fairly insignificant, as most particles are huge compared to a gas molecule, but many gas ions can fit on a single particle, and they do. Small particles (less than 1 μm diameter) can absorb “tens” of ions, while large particles (greater than 10 μm) can absorb “tens of thousands” of ions. Eventually, there are so many ions sticking to the particles, that the particles begin to emit their own negative electric field. When this happens, the negative fields surrounding the saturated particles start to repulse negative gas ions and no additional ions are acquired. This is called the saturation charge and is responsible for inducing the PMs inescapable pull of electrostatic attraction, or migration. Bigger particles have a higher saturation charge and are consequently pulled more strongly to the collection plate than smaller particles that have a smaller saturation charge. Regardless of size, the particles eventually encounter the CE and stick due to adhesive and cohesive forces.

Introduction to ESPs: Theory of Operations: Collecting Collecting Let’s stop here and reassess what’s taking place inside the precipitator. Gas molecules around the discharge electrode are positively ionized and generate an avalanche of free electrons. These free electrons race as fast as they can away from the strong negative field that surrounds the discharge electrode and get captured as they encounter gas molecules in the inter-electrode area. This creates negative gas ions that then accumulate on nearby PM and the net result is negatively charged particles that are repulsed by the negative electric field surrounding the DE and strongly attracted to the CE. This induces the negatively charge PM to migrate toward the grounded collection plate where it accumulates, and all of this happens in the blink of an eye. Precipitation of Particulate Matter The point when PM sticks to the CE and starts to form a layer is called precipitation. The chemistry definition of precipitation is “the process of separating a substance from a solution as a solid”. In the case of ESPs, PM matter is separated from the flue-gas solution by electrostatic attraction that causes it to precipitate onto the collection plate.

Neutralization & Clamping Forces Once charged PM reaches the grounded CE, the charge on the particle is partially discharged and slowly leaked to the grounded collection plate. It is this retained charge that contributes to the inter-molecular adhesive and cohesive forces that holds the PM onto the CE. Adhesive forces cause the particles to physically hold on to each other because of their dissimilar surfaces. Newly arrived particles are held to the collected particles by cohesive forces; particles are attracted and held to each other molecularly. The accumulating dust layer is allowed to build up on the plate until a desired thickness is achieved and the particle removal cycle is initiated.

Introduction to ESPs: Theory of Operations: Removal Particulate matter that has accumulated to a certain thickness on the CE is removed by one of two processes, depending on the type of CE used. Collection electrodes in ESPs can be either plates or tubes, with plates being the more common of the two. Tubes-type ESPs are usually cleaned by water sprays, while plates can be cleaned by either water sprays or a process called rapping. We will focus on the latter. Rapping: Dislodging Particulate Matter

Rapping is a process whereby deposited, dry particles are dislodged from the CE by sending mechanical impulses, or vibrations, to the plates. Precipitator plates are rapped periodically while maintaining the continuous flue-gas cleaning process. In other words, the plates are rapped while the ESP is on-line; the gas flow continues through the ESP and the applied voltage remains constant. Plates are rapped when the accumulated dust layer is relatively thick (0.08 to 1.27 cm) so that the dust layer is coaxed to fall off the plates as large aggregate sheets instead of small sections, helping to minimize re-entrainment. Most precipitators have adjustable rappers that allow rapper intensity and frequency to be changed according to the dust concentration in the flue gas. Precipitator fields with heavy dust concentrations require more frequent rapping than fields with light dust concentrations.

Recycle or Disposal As PM is dislodged from CEs, is falls into the hopper. A hopper is a dedicated collection bin with sides sloping approximately 50 to 70o so that PM is allowed to flow freely from the top of the hopper to the discharge opening in the bottom. Particulate matter collected in hoppers should be removed as soon as possible in order to avoid packing that is very difficult to remove. Most hoppers are emptied by some type of discharge device and then a conveyor transports the collected PM to its final destination for recycling or disposal. In an ESP using liquid sprays to remove accumulated PM, the sludge collects in a holding basin at the bottom of the vessel. The sludge is then sent to settling ponds of lined landfills for proper ultimate disposal. Spraying can occur while the ESP is on-line and is typically intermittent. While water is generally used as the spraying liquid in wet ESPS, other liquids could be used if absorption of gaseous pollutants is also a goal.

Maintenance

EMR Rapper Maintenance Rapping is an essential component of the collection and removal processes and an integral part of precipitator performance. Ineffective rapping can lead to build-up, re-entrainment, and reduced power levels. Periodically checking your rappers to ensure proper connections and functioning can help consistent precipitator performance. Below are some helpful tips to ensure the continued proper operation of your equipment. How can I check the operation of my EMR2 rappers? 1. Check your lift height. This will be the primary indicator if the rapper needs to be replaced. See our video on how to check lift height. 2. Make sure it’s properly seated on the shaft. The rapper shaft needs to seat up against the strike plate in the mounting of the rapper to assure transfer of rapping energy. 3. Check for corrosion on the housing and around the junction box. Water in-leakage will corrode the internals and cause coil failures. 4. Check for missing or loose bolts at the flange. Replace or tighten as necessary. 5. Check electrical connections in the junction box.

Precipitator Inspections Inspections & Maintenance An effective electrostatic precipitator inspection is part of routine equipment maintenance. For example, at the municipal utilities power station in Sikeston, Missouri, precipitator blast-cleaning is done every year along with a walk-through and routine rapper tune-ups. Complete detail inspections and larger repairs are scheduled every other year during longer outages. “By keeping up with this maintenance, and also doing a more thorough inspection every other year, we get a lot more longevity out of the run,” explains Ed Boyd, Sikeston’s electrical supervisor and project engineer, who has been in the industry for 34 years. Any inspection, whether a quick walk-through or a more lengthy check-up, requires some legwork up front, and a plan for prioritization during and afterward. This article outlines best practices for a smooth inspection. If you’re doing the inspection yourself, treat this as a checklist. If you’re hiring a contractor, make sure these steps are part of their routine procedure. First, clarify desired outcomes of the inspection with whoever will get a report afterward and is authorized to act on recommendations. Having a clear objective up front will save time, money and headaches later. The scope of work for a preventative maintenance (PM) inspection will be different from one focused on troubleshooting specific problems. Next, get background information from people responsible for operating each area of the equipment (electrical, mechanical, ash handling). Ask these people if there are specific issues they’re troubleshooting. Piecing this information together will provide a picture of current operating conditions and possible problem areas. For example, electrical issues might be traceable back to hopper pluggage. “It’s easy to get sidetracked or impatient looking for people who have this information,” notes Jim Parsons, Senior Environmental Consultant at Neundorfer, who has been doing inspections for 30 years. “But it’s worth spending the time gathering background information and clarifying expectations before jumping in the ESP box.” Background information gathered ahead of the inspection should include original design specifications. These specs enable you to create an accurate precipitator layout form (a grid showing the location of plates, wires and other components) to mark with notes during the inspection. A layout form is especially vital if you’re inspecting a large precipitator; without a map showing where things are located, the job can be daunting. “After a while, it’s like walking around in a cave with sunglasses on,” Parsons says. “You can easily get overwhelmed if you don’t have a clear idea of what to look for.” With design specs, information about troubleshooting efforts, and understanding about expectations, you can effectively prioritize where to begin and what to look for during the inspection.

Before actually heading into the precipitator, of course, make sure all safety protocols– including tag-out and lock-out–are being followed. This is high-voltage equipment and not something to mess around with. Usually, the inspection scope of work includes a standard list of maintenance items and at least a few requests to look for specific problems. At Sikeston, inspection work is prioritized first based on job tickets, such as a report of rappers not operating properly. It’s also standard practice to check for broken insulators. “We start with anything that’s obvious from the outside,” Boyd explains. “Then, we go inside and look for anything unusual.” Things can get pretty interesting during an inspection if you are troubleshooting specific issues, but even a routine maintenance walk-through will turn up items in one area that impact performance elsewhere and require further investigation. “If an ash line is clogged and we find foreign material when clearing it, during the inspection we go look for where that material came from,” Boyd notes, citing one troubleshooting example. “Or, if we do a quick inspection of the high voltage area and find arcing, we know there is moisture and we need to look for leaks.” Finally, an exit review meeting should be part of any inspection. During this meeting, give the person you’re reporting to a punch list of items that need to be addressed right away. A punch list is different from the more detailed report you’ll submit later, which will focus on longer-term follow-up. For example, if you find bowed plates in a section of the precipitator, the punch list might recommend pulling wires in those areas to prevent electrical shorts. The longer report would specify that the plates need to be straightened or replaced. It’s important that any inspection be performed by experienced professionals who understand how all components of the system work, know what to look for, and are informed about how to prioritize what needs maintenance immediately and what can wait. Purpose Internal inspections of Electrostatic Precipitator’s (ESP) are a necessary part of maintaining the “mechanical portion of the electrical” to obtain maximum performance. In some instances, when performance is deemed acceptable prior to a scheduled outage, inspections often overlook specific internal areas that can detract from collection efficiency during the next on-line period of the ESP. Detailed internal inspections are required to properly schedule current and future maintenance and as a prelude to failures. The following are recommendations, generic in nature, for the conduction of an inspection that will “time freeze” the condition of the internal ESP components.

Legal Notice CAUTION – Inspections should only be accomplished under the direction of personnel familiar with all Electrostatic Precipitator Safety procedures. It is important that proper grounding procedures and removal of all sources of power be accomplished prior to entering the Precipitator. LETHAL VOLTAGES CAN BE PRESENT AND CAN LEAD TO DEATH BY ELECTROCUTION IF THE PROPER PROCEDURES ARE NOT FOLLOWED. Hot dust may be present on the interior surface of the ESP. These dust deposits may be cool on the surface, but the interior of the deposit may retain sufficient heat to cause SEVERE BURNS ESPECIALLY WHEN OPENING HOPPER DOORS. CONSULT YOUR PLANT’S SAFETY MANUAL FOR PROPER PROCEDURES. Neundorfer, Inc. is not responsible for any damages or injury as the result of this document and the use of the document is at the sole risk of the and NOT Neundorfer, Inc. Pre-Inspection Preparation 1. Take electrical readings from all High Voltage Controls (MC) at normal operating conditions. 2. If possible, take a set of V-I Curves for each MVC identifying the “corona” onset voltage. (First flow of secondary current). 3. Record all Rapper Control Parameters including cycle times, time between raps and intensities. 4. Develop “Cell Diagrams” of each ESP Field showing every Collecting Electrode (CE) and Discharge Electrode (DE) with numbers on CE’s and Gas ages (GP). 5. Develop “Roof Plan Diagrams” of the ESP showing the Transformer-Rectifiers (TR), CE and DE Rappers and locations of High Voltage and Rapper Shaft Insulators. 6. As soon as the Fuel supply is turned off (I.E. coal mills stopped, wood/wood waste feeders stopped, Black Liquor Guns pulled, etc.) Turn off the Rapper Controls. 7. Leave the T-R sets on until the Fans are shutdown. 8. After Fan shutdown, turn off the MVC’s. 9. Remove all ash from the Hoppers. 10. Follow specific owner and ESP Manufacturer instructions for Key Interlock LockOut and Tag-Out of the Transformer-Rectifiers. 11. Follow specific owner and ESP Manufacturer instructions for entry into the Precipitator following all grounding, entry and Personal Protection Equipment (PPE) procedures. Dirty Inspection The purpose of the Dirty Inspection is to examine dust deposits on the ESP internals. It becomes a guide for determining the effectiveness of the Rapping System, Flue Gas Conditioning System (if equipped), Gas Flow Distribution and unusual build-up’s, etc. Dust deposits not compacted by the ESP electrical forces are often quite fragile so minimum disturbance is recommended.

1. Observe dust deposits in inlet and outlet ductwork and diffs (I.E. Nozzles and/or Plenums, etc.), gas distribution devices (such as Turning Vanes, Perforated Distribution Plates) and any other internal struts, braces, etc. 2. Conduct a GP by GP inspection noting the depth of dust deposits on the CE’s and the characteristics of the particulate. (I.E. Color, Density, Muddy, Cement like, etc.) Note if deposits are more in a given area. (Measuring depths is recommended.) Inspect all lower and mid-point CE horizontal spacers and CE Alignment devices attached to the Casing. 3. Inspect all DE’s noting dust deposits on the DE’s and Lower Discharge Electrode Frame (LDE). (DE deposits could be light, donuts, smooth, pointy, etc.) Examine the condition of the lower Anti-Sway insulators is so equipped, especially for carbon and tracking. 4. Measure and record all CE to DE spacing. 5. Look down into the hoppers and note any buildups in the corners, valley angles and baffles. 6. From the inside of the Penthouse or Insulator Compartments inspect all High Voltage and Rapper Shaft Insulators noting dust deposits, cracked and/or tracking. 7. Inspect all other High Voltage Insulators such as Standoff, Feed-Through, etc, for dust deposits and tracking. 8. Inspect all Buss Ducts and High Voltage Conductors and connections. 9. Observe dusting on the interior surfaces of the Penthouse or Insulator Compartments. Observe any air-in and water-in leakages. 10. Open “Hot Roof” access doors noting gasket, hold-downs and corrosion.

11. Inspect the Upper Discharge Electrode Frame (UDE) and tops of CE’s noting dust build-up and dust characteristics. 12. Check the UDE Frame for vertical position from the tops of the CE’s in four locations. 13. Check the High Voltage Insulators from the top of the UDE frame and the corona shield (if so furnished) noting any corrosion or miss-alignment of the UDE Rod or Pipe. Check all welds of the Rod/Pipe to the UDE Frame. 14. Check all DE connections to the UDE Frame noting any corrosion or electrical erosion. Look for bent or twisted DE’s and bent emitter pins (Rigid Discharge Electrodes). 15. At approximately the four outermost corners of the UDE Frame Measure the DE to CE Spacing in the direction and perpendicular to Gas Flow. This will determine the relative position of the UDE Frame with regard to the CE System. Place a level on the UDE frame between Rods/Pipes and record. 16. Check all CE suspension bolts and Rapper Shaft connections to the CE’s and/or CE Suspension. Inspect all welds. 17. Look for any water-in leakage and/or corrosion on the sidewalls and girders of the casing and underside of the hot roof. Pay close attention to any penetrations in the hot roof including High Voltage Lifting Rod Nipples and Rapper Shafts. 18. From the Hopper Access Doors Inspect all internal hopper surfaces and Door Condition. 19. Exhausted and dirty, clean up and take a well-deserved break while transposing notes to the Cell and Roof Plan Diagrams. Interpretation of Dirty Inspection Results 1. From the dust deposits and DE to CE Spacing recorded on the Cell Diagrams, compare these to the electrical readings taken ahead of the shutdown. Using a “rule of thumb” of 10 KV/Inch of clearance between the DE’s, CE’s and other grounded surfaces a general agreement should be noted. 2. DE deposits usually result in higher KV and lower MA. 3. CE deposits usually result in lower KV and MA. 4. Dust deposits that are found to be heaver in one part of a Field as compared to other parts dramatically detract from performance. 5. Schedule maintenance/repairs from recorded non-operating Rappers. 6. Re-align DE’s to CE’s to achieve the ESP Manufacturer’s Tolerances. 7. Unusual dust deposits in Ductwork, Diffs and Gas Distribution Media or “sand blasted clean” indicate attention to Gas Flow Dynamics and/or Process Conditions. 8. Schedule repair of corrosion and/or Access Door Gaskets to minimize air-in leakage (especially with ESP’s operating under suction). Clean Inspection 1. Prior to the inspection clean down the ESP internals in accordance with your specific Plant’s instructions. If minor or major maintenance is to be undertaken during this outage a thorough cleaning is normally required prior to work being accomplished.

2. Inspect all Rappers. If external electro-magnetic style, insure Rappers are Plumb, all rods and nuts are tight. If Pneumatic or Shaft Mounted electro-magnetic style, check Rappers for Plumb. If Vibrators, check all attachment bolts and nuts. If internal rapping system check all hammers for wear, side-to-side motion and the strike anvils. 3. Check all boot seals/stuffing boxes and ground connections on ESP’s with External Rapping Systems. 4. Check stuffing boxes and external drives/motors on ESP’s with Internal Rapping Systems. 5. Run the Rappers at least one full cycle and observe rapping action. Measure lifts where Rapper plungers are visible (electro-magnetic styles). 6. Check the Purge Air System fan and dampers. Operate briefly to determine damper operation, if so equipped. Ohm check Blast Heaters, if so equipped. (Some ESP’s are equipped with Calrod Heaters at the Base of the HV Insulators.) 7. Check all key interlocks for ease of operation, chains and/or mounts, transfer blocks, etc. 8. Check all Transformer-Rectifiers (T-R) noting any dielectric fluid leaks, tank holddowns and grounding. Record nameplate data. Open the 480 V JB and check the surge arrestors. 9. Open up the T-R buss duct inspection covers and check all insulators and HV conductors noting any dust, corrosion and or evidence of sparking. 10. Check the Penthouse or Insulator Compartment Access Doors for gasket resilience, hold-downs and corrosion. 11. Inspect all Grounding Sticks and connections. 12. From the underside of the Hot Roof re-inspect all UDE Frames, DE’s, CE Suspension and Rapper Rod connections. Check for corrosion especially under each HV Insulator. 13. Inspect all Side Access Doors for gaskets, hold-downs and corrosion. Check all key interlocks. (Not all ESP’s have Side Access Doors) 14. Inspect all Grounding Sticks outside of the Side Access Doors is so equipped. 15. Re-Inspect all internal components from the Side Access Doors and or the Hoppers. Spot measure again all DE to CE Clearances. Check all CE alignment devices. Check for corrosion everywhere. 16. Inspect Hopper Access Doors, gaskets, and key interlocks. 17. Check Hopper Vibrators (if so equipped) and operate. 18. Check all hopper poke holes and caps. 19. Ohm check hopper heaters. 20. Re-Inspect all ductwork, diffs and gas distribution devices. Look for corrosion. 21. Inspect all control cabinets. 22. Exhausted again, clean up and transpose all notes to the Diagrams. Post Inspection 1. If internal Repairs were made, megger each Electrical Buss Section, after disconnecting the HV lead, to check for dead shorts. 2. Lock out the ESP following the Plant’s procedures. 3. Operate the Purge Air System and Heaters for a minimum of 1 hour prior to turning on the T-R Sets.

4. Turn on the AHVC’s, one-at-a-time with Secondary KV settings at 20 to 25 KV. Observe readings. Troubleshoot as required. Raise the Secondary KV to the normal setting. A secondary limit of KV or MA should be reached without sparking. Troubleshoot at required. 5. Run V-I curves for each set noting the Corona Onset Voltage. Compare with the one’s taken prior to the outage. 6. Record all AHVC Parameters/Settings. Tune each set as required to match the internal conditions of the ESP. 7. Review Rapper Control Parameters/Settings. Adjust as required. 8. After Process Stabilization, take V-I Curves and compare with those taken just before startup. 9. Take V-I Curves after approximately 1 month of operation and compare with those taken just before the outage. Electrostatic Precipitator Outage Startup Best Practices Prior to an outage, be sure to record a set of benchmark T/R readings, and particularly for a major outage, a set of V/I curves as well. Then, plan to run a set of post-outage readings, once the unit is up and running normally, for comparison. By recording T/R readings even for short outages, you will be able to see potential areas that need attention during this outage or the next one. You may also be able to address an issue that could have caused you to reduce process production if left unattended. The V/I curves provide a benchmark of clearance issues in the box and show which T/R’s are functioning properly versus those that need to be addressed. V/I curves will also help show the effects of other performance factors such as coal and temperature changes.

T/R Set Preventative Maintenance T/R sets play an important role in the performance of precipitators, serving as the workhorses of the process. However, they are often neglected from preventative maintenance. It is important to pay attention to their operations and maintenance in order to avoid future problems that impact the precipitator when it is in operation, increasing the risk of non-compliance. One measure to add to your preventative maintenance routine is to test the T/R set oil annually after 5 years of service and document the results. NWL, a manufacturer of T/R sets, recommends you monitor nine gases: hydrogen, nitrogen, oxygen, carbon monoxide, carbon dioxide, acetylene, methane, ethylene and ethane. As T/R’s are in operation, the insulating paper begins to breakdown in the oil, moisture can infiltrate, semiconductors begin to breakdown, and the normal sparking and arcing inside the ESP cause gasses to form and dissolve in the oil. Good proper preventative maintenance practices will assure long reliable, consistent performance from your T/R sets. For more information, please us or your T/R manufacturer.

Major Components

Major Components: Collecting Plate Electrode Collecting plate electrodes (CE) are designed to receive and retain the precipitated particles until they are intentionally removed into the hopper. Collecting plates are also part of the electrical power circuit of the precipitator. These collecting plate functions are incorporated into the precipitator design. Plate baffles shield the precipitated particles from the gas flow while smooth surfaces provide for high operating voltage. Collecting plates are suspended from the precipitator casing and form the gas ages within the precipitator. While the design of the collecting plates varies by manufacturer, there are two common designs:  

Plates ed from anvil beams at either end. The anvil beam is also the point of impact for the collecting rapper Plates ed with hooks directly from the precipitator casing. Two or more collecting plates are connected at or near the center by rapper beams, which then serve as impact points for the rapping system

Top, center, or bottom spacer bars may be used with either design to maintain collecting plate alignment and sustain electrical clearances to the discharge system.

Major Components: Discharge Electrodes Discharge electrodes (DE) emit charging current and provide voltage that generates an electrical field between the discharge electrodes and the collecting plates. The electrical field forces dust particles in the gas stream to migrate toward the collecting plates. The particles then precipitate onto the collecting plates. Common types of discharge electrodes include:       

Straight round wires Twisted wire pairs Barbed discharge wires Rigid masts Rigid frames Rigid spiked pipes Spiral wires

Discharge electrodes are typically ed from the upper discharge frame and are held in alignment between the upper and lower discharge frames. The upper discharge frame is in turn ed from the roof of the precipitator casing. High-voltage insulators are incorporated into the system. In weighted wire systems, the discharge electrodes are held taut by weights at the lower end of the wires.

Major Components: Ductwork Ductwork connects the precipitator with upstream and downstream equipment. The design of the ductwork takes into consideration the following: 1. Low Resistance to Gas Flow – Achieved by selecting a suitable cross-section for the ductwork and by installing gas flow control devices, such as turning valves and flow straighteners 2. Gas Velocity Distribution – Gas flow control devices are used to maintain good gas velocity distribution 3. Minimal Fallout of Fly Ash – Fallout can be minimized by using a suitable transport velocity 4. Minimal Stratification of the Fly Ash – A suitable transport velocity also reduces fly ash stratification in the gas stream 5. Low Heat Loss – The goal is to reduce the heat loss of the flue gas to a level that will prevent acid or moisture condensation in the downstream equipment, requiring the use of thermal insulation protected by external siding. 6. Structural Integrity – Ductwork structure s its total load, including wind and snow loads. The design also allows for accumulated fly ash, negative/positive operating pressure, and gas temperature. Expansion ts are used to accommodate thermal growth.

Major Components: Gas Distribution Systems One electrical field or bus section of an electrostatic precipitator is by itself an independent precipitator. Its operation is governed by the inlet gas and dust conditions, as well as the collecting plate and discharge electrode geometries. Within this electrical field or bus section, one gas age is also an independent precipitator – governed by the same factors. (Note that the gas age shares the voltage level with the adjacent gas ages of the same electrical field or bus section, but not the corona current level, which can be different in each gas age.) This points to the importance of creating similar gas and dust conditions 1) at the inlet of each electrical field or bus section, and 2) further at the inlet of each gas age of the electrical field or bus section. Ideally, uniformity is desired in:   

Gas velocity Gas temperature Dust loading

Gas velocity distribution can be most effectively influenced by the use of gas distribution devices. The quality of gas velocity distribution can be measured in a scaled-down model of the precipitator and its ductwork, and also in the precipitator itself. Typical criteria are based

on ICAC (Institute of Clean Air Companies) recommendations using average gas velocities or on a calculated RMS statistical representation of the gas velocity pattern. In general, gas distribution devices consist of turning vanes in the inlet ductwork, and perforated gas distribution plates in the inlet and/or outlet fields of the precipitator.

Major Components: Hoppers Precipitator hoppers are designed to completely discharge dust load on demand. Typically, precipitator hoppers are rectangular in cross-section with sides of at least 60-degree slope. These hoppers are insulated from the neck above the discharge flange with the insulation covering the entire hopper area. In addition, the lower 1/4- 1/3 of the hopper wall may be heated. Discharge diameters are generally 8″ – 12″. 1. Insulation – Insulation provides protection for facility personnel and also works to retain as much hopper wall temperature as possible. Hopper wall temperature retention discourages condensation on the inside of the hopper. Heaters are added to ensure that metal surfaces immediately above the fly ash discharge are hot. 2. Facilitating Hopper Discharge – Hopper discharge problems are caused by compaction of the fly ash in the hopper. Moisture content, particle size and shape, head of material, and vibration all affect PM’s compaction characteristics. The flow of PM out of the hopper can be facilitated by the use of external vibrators. These can operate on the outside wall of the hopper or on an internal hopper baffle. 3. Hopper Fluidizers – Hopper fluidizers have a membrane that permits airflow to the PM directly above. This airflow fills the voids between the fly ash particles at a slight pressure, changes the repose angle of the particles, and promotes gravity flow. 4. Ash Handling System – The fly ash handling system evacuates the fly ash from the hoppers, and transports the fly ash to reprocessing or to disposal. The ash handling system should be designed and operated to remove the collected fly ash from the hoppers without causing re-entrainment into the gas flow through the precipitator. The design of the ash handling system should allow for flexibility of scheduling the hopper discharges according to the fly ash being collected in these hoppers. Either the precipitator hopper or the feeder hopper is used for temporarily storing material prior to discharge. Three types of handling systems are in use:  

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Negative Pressure or Vacuum System – Connects to the hopper by a simple discharge valve Positive Pressure Dilute Phase System – Uses an airlock-type feeder; the feeder is separated from the hopper by an inlet gate and from the conveying line by a discharge gate Positive Pressure Dense Phase System – Connects to the hopper with an airlock type feeder.

Understanding Major Electrostatic Precipitator Components The major components of the precipitator are part of the power circuit, rapping system, collecting system, hoppers and evacuation system, insulator compartment, and gas flow distribution systems.

Major Components: Power Supplies & Controls The power supply system is designed to provide voltage to the electrical field (or bus section) at the highest possible level. The voltage must be controlled to avoid causing sustained arcing or sparking between the electrodes and the collecting plates. Electrically, a precipitator is divided into a grid, with electrical fields in series (in the direction of the gas flow) and one or more bus sections in parallel (cross-wise to the gas flow). When electrical fields are in series, the power supply for each field can be adjusted to optimize operation of that field. Likewise, having more than one electrical bus section in parallel allows adjustments to compensate for their differences, so that power input can be optimized. The power supply system has four basic components:   

Automatic voltage control Step-up transformer High-voltage rectifier

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Sensing device

Voltage Control Automatic voltage control varies the power to the transformer-rectifier in response to signals received from sensors in the precipitator and the transformer-rectifier itself. It monitors the electrical conditions inside the precipitator, protects the internal components from arc-over damages, and protects the transformer-rectifier and other components in the primary circuit. The ideal automatic voltage control would produce the maximum collecting efficiency by holding the operating voltage of the precipitator at a level just below the spark-over voltage. However, this level cannot be achieved given that conditions change from moment to moment. Instead, the automatic voltage control increases output from the transformerrectifier until a spark occurs. Then the control resets to a lower power level, and the power increases again until the next spark occurs. Theory 

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Optimize Power Application The primary purpose of a voltage controller is to deliver as much useful electrical power to the corresponding electrostatic precipitator field(s) as possible. This is not an easy job; electrical characteristics in the field(s) are constantly changing, which is why a voltage controller is required. Spark Reaction When the voltage applied to the electrostatic precipitator field is too high for the conditions at the time, a spark over will occur. Detrimentally high amounts of current can occur during a spark over if not properly controlled, which could damage the fields. A voltage controller will monitor the primary and secondary voltage and current of the circuit, and detect a spark over condition. Once detected, the power applied to the field will be immediately cut off or reduced, which will

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stop the spark. After a short amount of time the power will be ramped back up, and the process will start over. Protect System Components by Adhering to Component Limitations The Transformer Rectifier set (TR set) can be damaged by excessive amounts of current or voltage flowing through it. Each TR set has voltage and current limits established by the manufacturer, which are labeled on an attached nameplate. These nameplate limit values (typically primary and secondary current, and voltage) are programmed into the voltage controller. Through metering circuits, the voltage controller will monitor these values, and ensure these limits are not exceeded. Tripping When a condition occurs that the voltage controller cannot control, often times the voltage controller will trip. A trip means the voltage controller (by way of the or) will shut off the individual precipitator power circuit. A short inside the electrostatic precipitator field caused by a fallen discharge electrode (wire), or a shorted out Silicone Controlled Rectifier are examples of conditions that a voltage controller cannot control.

Operation To maximize electrostatic precipitator efficiency, a voltage controller usually attempts to increase the electrical power delivered to the field. However in some conditions a voltage controller must just maintain power at a constant level. Increased electrical power into the electrostatic precipitator directly correlates with better precipitator performance, but there is a limit. If too much voltage is applied for a given condition (as mentioned in the spark reaction section), a spark over will occur. During a spark over precipitator performance in that field will drop to zero, rendering that field temporarily ineffective. To overcome the crippling effect that spark over has to increasing the electrical power in the precipitator field, spark response algorithms have been developed that will interrupt power upon detection of a spark, then ramp power back up to a high level. These response algorithms can greatly influence overall precipitator performance. 

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Transformer/Rectifiers – The T/R set rating should be matched to the load imposed by the electrical field or bus section. The power supply will perform best when the T/R set operates at 70 – 90% of the rated capacity, without excessive sparking. This reduces the maximum continuous-load voltage and corona power inputs. Practical operating voltages for T/R sets depend on: 1. Collecting plate spacing 2. Gas and dust conditions 3. Collecting plate and discharge electrode geometry At secondary current levels over 1500 mA, internal impedance of a T/R set is low, which makes stable automatic voltage control more difficult to achieve. The design of the T/R set should call for the highest possible impedance that is commensurate with the application and performance requirements. Often, this limits the size of the electrical field or bus section. It is general practice to add additional impedance in the form of a current-limiting reactor in the primary circuit. This reactor will limit the primary current during arcing and also improve the wave shape of the voltage/current fed into the T/R set.

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Corona current density should be in the range of 10 – 100 mA/1000 ft2 of plate area. (Calculate this using secondary current divided by collecting area of the electrical field or bus section.) The actual level depends upon: 1. Location of electrical field or bus section to be energized 2. Collecting plate area 3. Gas and dust conditions 4. Collecting electrode and discharge wire geometry

Major Components: Rapping Systems Rappers are time-controlled systems provided for removing dust from the collecting plates and the discharge electrodes as well as for gas distribution devices (optional) and for hopper walls (optional). Rapping systems may be actuated by electrical or pneumatic power, or by mechanical means. Tumbling hammers may also be used to dislodge ash. Rapping methods include:     

Electric vibrators Electric solenoid piston drop rappers Pneumatic vibrating rappers Tumbling hammers Sonic horns (do not require transmission assemblies)

Discharge Electrode Rapping In general, DE should be kept as free as possible of accumulated particulate. As a result, the rapping system for the discharge electrodes should be operated on a continuous schedule with repeat times in the 2 – 4 minute range, depending on the size and inlet particulate loading of the precipitator. Collecting Plate Rapping Collecting plate rapping must remove the bulk of the precipitated PM. The collecting plates are ed from anvil beams or directly with hooks from the precipitator casing. With anvil beam , the impact of the rapping system is directed into the beams located at the leading and/or trailing edge of the collecting plates. For direct casing , the impact is directed into the rapper beams located at or near the center of the top of the collecting plates. The first electrical field generally collects about 60-80% of the inlet dust load. The first field plates should be rapped often enough so that their precipitated layer of particulate is about 3/8 – 1/2″ thick. There is no advantage in rapping more often since the precipitated dust has not yet agglomerated to form a sheet, which requires a minimum layer thickness. Sheet formation is essential to make the dust drop into the precipitator hopper without reentrainment into the gas stream. Rapping less frequently typically results in a deterioration of the electrical power input by adding an additional resistance into the power circuit. Once an optimum rapping cycle has been found for the first electrical field (which may vary across the face of a large precipitator), the optimum rapping cycles for the downstream electrical fields can be established. The collecting plate rapping system of the first field has a repeat time T equal to the time it takes to build a 3/8 – 1/2″layer on the collecting plates. The plates in the second field should have a repeat time of about 5T, and the plates in the third field should have a repeat time of 25T. Ideally, these repeat times yield a deposited layer of 3/8-1/2″ for the plates in all three fields. Adjustment may be required for factors such as dust resistivity, dust layer cohesiveness, gas temperature effects, electrical field height and length, and the collecting area served by one rapper.

Gas Distribution & Hopper Wall Rapping The gas distribution plates used in an ESP should also be kept free of excessive particulate buildup and may require rapping on a continuous basis with a cycle time in the 10-20 minute range, depending on the inlet particulate loading of the precipitator and the nature of the PM. Gas distribution plates in the outlet of the precipitator may be rapped less often (every 30 – 60 minutes) due to the smaller concentration of PM that reach this area.

Process Influences & Keys to Performance

Combustion Process Improvements for Power Plants The primary contributors to combustion process conditions and their effects include: Coal    

Flue gas flow rate Flue gas moisture content Fly ash resistivity Fly ash inlet loading

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Fly ash particle size

Coal Mills 

Fly ash particle size

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Unburned carbon (LOI)

Furnace 

Base load/swing load operation

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Flue gas flow rate

Burners  

Flue gas temperature Fly ash resistivity

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Unburned carbon (LOI)

Air Pre-Heaters   

Rotation Gas flow pattern Gas temperature pattern

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SO3 distribution pattern

Coal Bituminous coals from Eastern mines, sub-bituminous and lignite coals from Western mines, and lignites from Texas mines are substantially different from each other in the combustion process. Coal blending is now used for operational and financial benefits. This results in a wide range of boiler and precipitator operating conditions.

Precipitating fly ash from difficult coals can be improved with conditioning systems. However, the furnace and its associated equipment can still cause problems in the precipitator, particularly coal mills, burners, and air pre-heaters. Coal Mills The setting of the coal mills and classifiers defines the coal particle size which in turn impacts the fly ash particle size. Larger coal particles are more difficult to combust, but larger fly ash particles are easier to collect in the precipitator. Furnace Base-load operation of the boiler is usually better for precipitator operation than swingload operation due to more stable operating conditions. Boiler operation at low loads may be as problematic for the precipitator as operating the boiler at its maximum load level, due to fallout of fly ash in the ductwork, low gas temperatures, and deterioration of the quality of the gas velocity distribution. If low load operation cannot be avoided, the installation of additional gas flow control devices in the inlet and outlet of the precipitator may prove beneficial. Coal Burner The operation of coal burners, together with the setting of the coal mills and their classifiers, affects the percentage of unburned carbon (LOI or UBC) in the fly ash. The use of Lo-NOx burners increases this percentage, and causes re-entrainment and increased sparking in the precipitator. Further, the UBC tends to absorb SO3, which in turn increases the fly ash resistivity. Over-fire air optimization or coal-reburn systems may reduce UBC in the fly ash. Air Pre-heater Regenerative air pre-heaters cause temperature and SO3 stratification in the downstream gas flow. This problem is more severe in closely coupled systems, where the precipitator is located close to the air pre-heater. Depending upon site-specific conditions, flow mixing devices may be installed in the ductwork to the precipitator, or flue gas conditioning systems may be used to equalize the gas flow characteristics. Fly Ash & Flue Gas Conditioning Flue gas and fly ash characteristics at the inlet define precipitator operation. The combination of flue gas analysis, flue gas temperature and fly ash chemistry provides the base for fly ash resistivity. Typically, fly ash resistivity involves both surface and volume resistivity. As gas temperature increases, surface conductivity decreases and volume resistivity increases.

In lower gas temperature ranges, surface conductivity predominates. The current ing through the precipitated fly ash layer is conducted in a film of weak sulfuric acid on the surface of the particles. Formation of the acid film (from SO3 and H2O) is influenced by the surface chemistry of the fly ash particles. In higher gas temperature ranges, volume conductivity predominates. Current conduction through the bodies (volume) of the precipitated fly ash particles is governed by the total chemistry of the particles. Fly ash resistivity can be modified (generally with the intent to reduce it) by injecting one or more of the following upstream of the precipitator:  

Sulfur trioxide (SO3) Ammonia (NH3)

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Water

Sulfur Trioxide & Ammonia Conditioning Systems In most cases, a sulfur trioxide conditioning system is sufficient to reduce fly ash resistivity to an acceptable level. The source of sulfur trioxide can be liquid sulfur dioxide, molten elemental sulfur, or granulated sulfur. It is also possible to convert native flue gas SO2 to SO3. In some instances, ammonia alone has been proven a suitable conditioning agent. It forms an ammonia-based particulate to increase the space charge. The source of ammonia may be liquid anhydrous or aqueous ammonia, or solid urea. Finally, sulfur trioxide and ammonia may be used in combination. This solution has been successful because it can lower fly ash resistivity and also form ammonia bisulfate. The latter increases the adhesion of particles, and thus reduces re-entrainment losses. Water Injection The injection of water upstream of the precipitator lowers the gas temperature and adds moisture to the flue gas. Both are beneficial in cold-side precipitator applications. However, care must be taken that all of the water is evaporated and that the walls in the ductwork or gas distribution devices do not get wet.

Establishing Design & Performance Requirements for Precipitators Deg a precipitator for optimum performance requires proper sizing of the precipitator in addition to optimizing precipitator efficiency. While some s rely on the precipitator manufacturer to determine proper sizing and design parameters, others choose to either take a more active role in this process or hire outside engineering firms. Precipitator performance depends on its size and collecting efficiency. Important parameters include the collecting area and the gas volume to be treated. Other key factors in precipitator performance include the electrical power input and dust chemistry. 

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Precipitator Sizing The sizing process is complex as each precipitator manufacturer has a unique method of sizing, often involving the use of computer models and always involving a good dose of judgment. No computer model on its own can assess all the variables that affect precipitator performance. Collecting Efficiency Based on specific gas volume and dust load, calculations are used to predict the required size of a precipitator to achieve a desired collecting efficiency. Power Input Power input is comprised of the voltage and current in an electrical field. Increasing the power input improves precipitator collecting efficiency under normal conditions.

Keys to Equipment Improvement The objectives of equipment improvements are to optimize corona power, reduce reentrainment, and optimize gas velocity distribution inside the precipitator. Some important topics to consider when planning equipment improvements include: 1. Precipitator Size – When sizing the precipitator, it is important to provide a crosssection that will maintain an acceptable gas velocity. It is also important to provide for enough total discharge wire length and collecting plate area, so that the desired specific corona current and electrical field can be applied. 2. Gas Velocity Distribution – Improving gas velocity distribution in the precipitator reduces particle re-entrainment and boosts precipitator efficiency. Typically, a uniform gas velocity is desired, but there are site-specific exceptions. Gas velocity distribution can be modified by using flow control devices and baffles. Refer to the special section on gas velocity distribution. 3. Corona Power – The separation of dust particles from the gas flow in an electrostatic precipitator depends on the applied corona power. Corona power is the product of corona current and voltage. Current is needed to charge the particles. Voltage is needed to an electrical field, which in turn transports the particles to the collecting plates. For additional information, refer to Corona Power.

4. Sectionalization – The precipitator is divided into electrical sections that are crosswise and parallel to the gas flow to accommodate spatial differences in gas and dust conditions. Optimization of corona power involves adjusting the corona power (secondary voltage and current) in each electrical section for optimum conditions. 5. Particle ReEntrainment – Minimizing reentrainment of dust particles is important to improvement of precipitator efficiency. Most precipitator equipment affects the reentrainment level. For a detailed discussion, visit the special section on reentrainment. 6. Additional Equipment – Performance improvement options include the installation of a second precipitator in series with the existing precipitator; using fabric filters downstream of the precipitator; and adding a second particle collector in parallel with the existing collector. Other possibilities include sonic or electrostatic particle agglomerators upstream of the precipitator; a mechanical upstream collector; or an electrostatically-enhanced or mechanical collector, or a filter downstream of the precipitator. 7. Review the General Equipment Requirements – Reviewing the Neundorfer Knowledge Base sections on equipment will provide additional insight into performance improvements.

Improve Flow Distribution Maintain Design Temperatures The air heater is usually the last piece of equipment before a cold-side ESP. It transfers some heat from the flue gas back into the boiler system to pre-heat combustion air. This significantly reduces the temperature of flue gas heading downstream to the ESP, from about 700℉ to about 300℉. Flue gas temperature is critical to ESP performance; both ash resistivity and gas volume increase with higher temperatures. The optimal operating temperature for a cold-side ESP is in the 280℉-320℉range. Temperatures above 350℉ usually have a significantly negative impact on ESP performance. At most plants, air heater exit temperature readings are readily available in the DCS. If air heater temperatures are higher than normal, testing the combustion system is an effective way to determine why. High exit gas temperatures can be associated with excess air—indicating poor combustion control, which wastes energy and increases flue gas volume. Poor balance across the furnace is another likely cause. Mill performance, as well as ash chemistry, can alter particle size distribution, which can significantly impact ESP performance. Combustion system testing to find the cause of high exit temperatures may be combined with checking the air heater itself to find out if flow quantities are optimal and/or if air inleakage is a problem.

Minimize Air In-Leakage Anything that increases the volume of gas being treated by the ESP is bad for collection efficiency. Finding air in-leakage can be complicated, since it can get into the system at many locations. If tramp air is suspected, the hunt for a source usually starts by observing oxygen (O2) meter readings. At most plants, O2 meters are located at the economizer exit; readings from this location are used primarily for combustion control. Under normal conditions, the boiler exit meter will read around 3% oxygen. Some plants also have an O2 meter in the stack as part of a continuous emission monitoring system (CEMS). Normally, the stack meter reads a bit higher: between 5% and 6.5%. If the stack O2 meter reading is above 8%, air in-leakage may be a significant problem. Regular testing for air in-leakage at multiple points should be part of every plant’s operating policy. Test results help direct repairs to duct-work and equipment, ensuring that whatever maintenance is done will have maximum impact. Optimize Flow Distribution Devices

The goal of the flow distribution devices in an ESP are to distribute the flue gases across the ESP as evenly as possible. When process changes or air in-leakage occur the flow profile of the flue gases change and the flow distribution devices may no longer be able to carry out this job efficiently. Another possibility is that the flow distribution devices get PM built up on them or deteriorate from corrosion. In any one of these situations the flow distribution is skewed and leads to poor ESP performance. In order to optimize flow it is important to check your devices and make sure they are clean and in good condition. If buildup is present, one option may be to add some type of rapping that will periodically

clean these systems. If permanent changes to the process have changed, it may be a good idea to have a flow study done in order to find out how the new conditions affect the ESP and what can be done to correct this issue.

Improving Rapping System Performance All precipitator rapping systems allow adjustment of rapping frequency, normally starting with the highest frequency (the least time between raps), progressing to the lowest frequency. The times that are actually available may be limited. Rapping systems with pneumatic or electric actuators allow variations of the rapping intensity. Pneumatic or electric vibrators allow adjustments of the rapping time. State-of-the-art rapper controls allow selection of rapping sequences, selection of individual rappers, and provide anticoincidence schemes which allow only one rapper to operate at a given time. Rapping systems can be optimized for top precipitator performance using precipitator power input and stack opacity as criteria. Optimization of the rapping system starts with the discharge electrode rapping system operating on its own time schedule, for example with repeat times of 2 – 4 minutes. The rapping system for the gas distribution screens in the inlet and outlet of the precipitator should then be operated with repeat times of 2-3 minutes for the inlet and 2 – 3 hours for the outlet screens. The only rapping system requiring optimization is the collecting plate rapping system. The optimization should start with the Collecting Plate Rapping Schedule determined above. Next, the rapping frequency of the inlet field should be increased or decreased until the electrical power input of the inlet field remains constant. Next, the rapping frequency of the other fields should be adjusted in sequence until their electrical power inputs remain constant. If the stack opacity trace shows rapping spikes, the rapping intensity should be reduced while observing the electrical power input of the precipitator. The adjustment of the rapping system for optimum precipitator performance is a slow process. It requires a substantial amount of time for stabilization after each adjustment.

Process Influences & Keys to Performance

The different areas of a power utility or industrial plant are sometimes operated as if they’re isolated from one another, when in fact every piece of equipment in the system eventually impacts every other piece of equipment. This section looks at some of the ways front-end operations (mills, boiler) impact the performance of electrostatic precipitators on the back end.

Flue gas volume, temperature and composition, as well as particulate size and composition, is largely determined by how well combustion equipment is operated and maintained. These two sets of factors have a direct, significant impact on a plant’s ability to comply with air pollution limits set by Federal and state regulators. Gas characteristics and particle properties define how well a precipitator will work in a given application. The main process variables to consider are: 

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Gas Flow Rate – The gas flow rate in a power plant is defined by coal quality, boiler load, excess air rate and boiler design. Where there is no combustion, the gas flow rate will have process-specific determinants. A precipitator operates best with a gas velocity of 3.5 – 5.5 ft/sec. At higher velocity, particle re-entrainment increases rapidly. If velocity is too low, performance may suffer from poor gas flow distribution or from particle dropout in the ductwork. Particle Size – The size distribution in a power plant is defined by coal quality, the coal mill settings and burner design. Particle size for non-combustion processes will have similar determinants. A precipitator collects particles most easily when the particle size is coarse. The generation of the charging corona in the inlet field may be suppressed if the gas stream has too many small particles (less than 1 µm). Very small particles (0.2 – 0.4µm) are the most difficult to collect because the fundamental field-charging mechanism is overwhelmed by diffusion charging due to random collisions with free ions. Particle Resistivity – Resistivity is resistance to electrical conduction. The higher the resistivity, the harder it is for a particle to transfer its electrical charge. Resistivity is influenced by the chemical composition of the gas stream, particle temperature and gas temperature. Resistivity should be kept in the range of 108 – 1010 ohm-cm. High resistivity can reduce precipitator performance. For example, in combustion processes, burning reduced-sulfur coal increases resistivity and reduces the collecting efficiency of the precipitator. Sodium and iron oxides in the fly ash can reduce resistivity and improve performance, especially at higher operating temperatures. On the other hand, low resistivity can also be a problem. For example

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(in combustion processes), unburned carbon reduces precipitator performance because it is so conductive and loses its electrical charge so quickly that it is easily re-entrained from the collecting plate. Gas Temperature – The gas temperature is affected by heat transfer efficiency in the boiler as well as the air-preheater efficiency and air in-leakage. The effect of gas temperature on precipitator collecting efficiency, given its influence on particle resistivity, can be significant.

Optimizing Corona Power Optimize Controls Precipitator corona power is the useful electrical power applied to the flue gas stream to precipitate particles. Either precipitator collecting efficiency or outlet residual can be expressed as a function of corona power in Watts/1000 acfm of flue gas, or in Watts/1000 ft of collection area. Corona power is the product of corona current and voltage. Current is needed to charge the particles, while voltage is needed to an electrical field that in turn transports the particles to the collecting plates. In the lower range of the collecting efficiencies, relatively small increases in corona power result in substantial increases in collecting efficiency. On the other hand, in the upper ranges, even large increases in corona power will result in only small efficiency increases.

In order to maximize corona power in an ESP, the power controls must be optimized for the process conditions at that time. This includes setting the secondary voltage and current limit, spark rate, spark rate setback and any other controllable variables that are available on the installed controls. Sectionalize Electrostatic precipitators are divided into electrical sections that are cross-wise and parallel to the gas flow in order to accommodate spatial differences in gas and dust conditions. Optimization of corona power involves adjusting the corona power (secondary voltage and current) in each electrical section for optimum conditions. When voltage controllers sense a spark they momentarily cut power to the affected field in order to quench the spark. During the quench, collection efficiency of that field is also momentarily lost; however increasing sectionalization of the ESP can minimize this effect. This means splitting the fields and adding additional T/R sets so that a smaller section is affected when a spark is quenched by the voltage controllers.

Methods to Reduce ReEntrainment Optimize Rapping All ESP rapping systems allow for some adjustment of rapping frequency, normally starting with the highest frequency (the least time between raps), progressing to the lowest frequency. Although, variability between individual rappers or the actual frequencies that are available may be limited. For rapping systems that have pneumatic or electric actuators, variations of the rapping intensity can also be modified along with the rapping frequency. Today, state-of-the-art rapper controllers allow complete customization of the rapping sequences, including selection of individual rappers, their intensity and frequency, and also provide anti-coincidence schemes that allow only one rapper within a given lane to operate at the same time.

An optimized rapping system starts with the DE rapping system operating on its own time schedule, and having typical repeat times in the range of 2 – 4 minutes. The same is true for the gas distribution rapping system in the inlet and outlet of the precipitator, however they should be operated with repeat times of 2-3 minutes for the inlet and 2 – 3 hours for the outlet screens. Because the goal for DE and Gas distribution rapping systems are to make sure that these components stay as clean as possible, these systems require the least amount of effort for optimization.

The only rapping system that truly requires optimization is the CE rapping system, with precipitator power input and stack opacity being excellent criteria for adjustment. Typically, CE rapping optimization should start with the Collecting Plate Rapping Schedule determined above, followed by the rapping frequency of the inlet field increased or decreased until the electrical power input of the inlet field remains constant. Once the inlet field is satisfactory, the rapping frequency of the other fields should be adjusted in sequence until their electrical power inputs also remain fairly constant. If during this process the stack opacity trace shows spikes that correspond with rapping events, the rapping intensity should be reduced while simultaneously observing the electrical power input of the precipitator. Adjustment of the rapping system for optimum precipitator performance is a methodical process, and is often considered more of an art than a science. Doing it correctly is a slow process that requires a substantial amount of time for stabilization after each adjustment. Improve Hopper Reliability Often times the hoppers and hopper evacuation system are overlooked as crucial elements to overall system performance. However, when hopper systems fail it can be the source of several issues that can lead to excessive sparking or performance killing re-entrainment. As hopper levels get too high, the PM within gets closer to the discharge electrodes and the flue gas moving through the precipitator. This can cause sparking between the DE and PM in the hopper, or allow the ash to be picked up by the faster moving air and carried below the CEs out of the hopper. This type of re-entrainment is cause sneakage because the PM sneaks below the collecting area and out of the ESP. Improve Resistivity

Resistivity is dependent on temperature, particle size, and flue gas chemistry. As a result, the process can be modified to change the resistivity of PM. Typically PM resistivity involves both surface and volume resistivity. Larger particles capture charged gases easier and therefore have a lower resistivity. Smaller particles are less probable to capture charged gases and have a higher resistivity. The effect of temperature can be see in the graph below, and may be modified by improving boiler and air-preheater efficiency. As gas temperature increases, surface conductivity decreases and volume resistivity increases. In lower gas temperature ranges, volume surface conductivity predominates. The current ing through the precipitated PM layer is conducted in a film of weak sulfuric acid on the surface of the particles. Formation of the acid film (from SO3 and H2O) is influenced by the surface chemistry of the particles. In higher gas temperature ranges, volume conductivity predominates. Current conduction through the bodies (volume) of the precipitated PM is governed by the total chemistry of the particles. The third influence of resistivity, flue gas chemistry, can be modified through Implementation of flue gas conditioning systems that inject either sulfur trioxide (SO3), ammonia (NH3), or water (H2O). Reduce Unburned Carbon (Loss on Ignition) Electrostatic precipitators (ESPs) don’t do a very good job of collecting carbon; these particles re-entrain easily because their resistivity levels are low. ESP performance starts to noticeably degrade, and stack opacity levels to rise, when the ash contains more than 10 percent unburned carbon. High carbon concentration can also cause increased sparking and reduce secondary voltage levels in the ESP. At most plants, carbon content in ash is measured regularly. If not, it is a relatively easy matter to send ash samples from the ESP hoppers to a lab (such as Neundorfer’s) for analysis. When coal is not pulverized properly, and/or there is poor combustion air distribution, the result is incomplete combustion and unburned carbon. Reducing unburned carbon means you are wringing more power out of the same amount of fuel. Optimizing your combustion system isn’t just about cutting emissions; it might also reduce fuel costs.

Improving Your Precipitator Operation Improvement or optimization of precipitator operation can result in significant savings. Many specific situations encourage a review of precipitator operation:     

Deterioration of existing equipment Tightening of air pollution emission regulations Changes in products and/or production rates Frequent forced outages De-rating of production

The key areas for performance improvement are reducing re-entrainment, improving gas flow distribution, and optimizing corona power.

Troubleshooting Precipitator Solutions The general process for troubleshooting starts with identifying if the issue is likely a process issue or a precipitator issue. Changes in performance that are isolated to one or a few fields are typically indicative of precipitator issues where general performance issues and powering issues across the box are typically indicative of a process issue.