Electrostatic Precipitator Dust Removal System

The ash handling system keeps the material (ash etc.) out of the hoppers!Why is it important to maintain the operation of the ash handling system?

Make sure the dust removal system is functioning without any problems. Basically, the material collected by the electrostatic precipitator has to be removed at a faster rate than it is being collected. If not, the material will build up in the precipitator hoppers and eventually short out the high voltage sections. Even worse, if the material builds up into/between the collecting plates, the weight of the material can force the collecting plates apart and induce permanent damage (bowing) in the plates.

The first row of hoppers is going to be the most challenged due to the fact that approximately 70 to 80% of the total material collected by the electrostatic precipitator will be removed in the first field. The plant must pay particular attention to ensure that these hoppers don't fill up.

• In summary, the plant operators should: o Make sure the ash handling system is in good operating condition o Make sure all ash valves work properly and no air leaks are present o Maintain operation of hopper heaters, vibrators, etc. o Constantly observe the level alarms and immediately resolve any high level alarms

Precipitator Hopper sketch showing grounding chains. If the level of dust is too high in the hopper, the TR set grounds out. This prevents the dust to damage the lower frame and collecting electrodes

by pushing up on them.

Electrostatic Precipitator Rappers

• Why is the correct operation and setting of the rapper system essential to good Electrostatic Precipitator performance?

Maintain operation of the rappers. Always check (on a daily basis) that the rappers are operating. One of the best ways is to simply walk up on the roof of the ESP and listen to the rappers hitting. Loss of rapper operation will result in poor precipitator performance.

Both over rapping and under rapping will deter from optimal performance of the electrostatic precipitator.

Over rapping will cause re-entrainment resulting in poor ESP performance and high stack emissions.

Under rapping will cause an excessive amount of build-up on the collecting plates and discharge electrodes, which will reduce electrical power in the ESP thus resulting in poor ESP performance and high stack emissions.

Importance of maintaining rapper ground straps and boot seals

The complete grounding of the rapper coil assembly and housing is essential to complete the entire grounding of the Electrostatic Precipitator. The shaft connections on the rappers are usually not sufficient to assure a complete ground to the rapper coils. Transient voltages can occur in the ESP and if the rappers are not properly grounded, these voltages can induce electrical surges into control and power cables, which may damage the controls. This is especially true with the high voltage discharge electrode rappers. These rappers are isolated by a shaft insulator. Any leakage through this insulator will bleed through to the rapper coil and it is obvious that a good ground connection is required to make sure that any transient voltage is grounded.

During an ESP walk down, always make sure that the rapper ground straps are securely in place. These ground straps frequently break due to the impact of the rappers.

Rapper boot seals are important to insure that no rain or cold air is allowed to enter the ESP. If the rapper boot seals are leaking, this will allow cold outside air to enter the ESP and create corrosion and possibly damage to the insulators.

Always make sure that the rapper boot seals are in good condition and that the clamps are tight. Replace any boot seals that are torn or cracked. A small item such as a torn boot seal could cause a considerable amount of damage to an ESP.

How to Limit Corrosion of your Electrostatic Precipitator

• What causes corrosion in an Electrostatic Precipitator?

Corrosion is always a problem with an ESP. Sometimes corrosion is hard to avoid. The flue gases going through a precipitator usually contain corrosive materials (such as SO2). The best method to avoid/reduce corrosion is to make sure all access doors are properly gasketed and securely tightened. This is to avoid cold air inleakage that can mix with the hot gases in the ESP and create a corrosive atmosphere (namely sulfuric acid).

Electrostatic Precipitator Gas Flow Distribution

The performance of any electrostatic precipitator (ESP) is contingent on the uniform distribution of the gases and suspended particulate entering the ESP.

Poor gas flow will result in poor ESP performance and high stack opacity. Good gas flow distribution is essential to optimum ESP efficiency. The best-constructed and aligned ESP will not provide the performance levels expected if the gas flow distribution is not within standards.

If the gas flow is not uniform through the precipitator, this will result in high velocity zones in the gas treatment region resulting in opacity spikes and excursions as a result of re-entrainment and scouring. Hopper sweepage can also occur if the gases are being forced in the hoppers. Also, the total effective collection zone of the ESP will not be utilized if the gases are not being distributed through the ESP uniformly. This will underutilize the ESP and result in poor performance.

Today's ESPs built by Hamon research-Cottrell are provided with the proper gas flow corrective devices to insure good gas flow distribution as defined by ICAC. These devices are carefully selected by conducting a gas flow model study of the ESP and related ductwork and scaling the results to the full size unit.

If your Electrostatic Precipitator electrical readings are good and your stack emissions are high, this condition may suggest a problem with the gas flow distribution to the ESP.

• Tips for maintaining good gas flow distribution o During an outage, make sure that all gas flow devices are clean and not built up with dust. o Make sure all perforated plate holes are open and not plugged off. o Make sure that the bottom floors of all plenums are not built up with dust. Sweep or

vacuum out all dust that may be built up on the plenum floors. o Make sure all rappers or vibrators installed on perforated plates are in good working order. o Make sure all horizontal surfaces (such as splitter vanes and egg crates) are not built up

with dust. o During an ESP inspection, look at the collecting plate surfaces for any signs of scouring.

This may be suggestive of high velocity zones.

Electrostatic Precipitator Electrical Readings

Maintaining and recording electrical readings are important.

Why should plant personnel record ESP electrical readings?

They are a valuable tool in maintaining electrostatic precipitator performance. They also provide a diagnostic tool in assessing the overall performance and operational status of the ESP. A history of electrical readings should be maintained by the plant. Indeed, this is a good diagnostic tool for a service person to reference when onsite for an ESP problem or before an outage.

Take daily electrical readings of the voltage and current levels in the electrostatic precipitator. Note any change in readings such as an increase in sparking, a drop in voltage, etc. The trend of the readings should stay fairly consistent. A change in the readings is an indication that something may be going wrong.

• In summary, the plant operators should: o Record electrical readings on a daily basis o Keep a history of electrical readings o Note the trend of the readings. If readings change dramatically, this may be an indication

that a fault may be developing with the ESP.

Plant and Electrostatic Precipitator Operation

• Process Conditions

Maintain proper process conditions (i.e. boiler operation). Upset conditions in the process will affect the performance of the Electrostatic Precipitator. Also, avoid thermal excursions or other similar conditions that may cause damage to the ESP (such as operating above design temperatures). This may stress and thus bend the collecting plate.

• Seal Air System

Maintain operation of penthouse seal air system. The seal air system must be operating continuously. The purpose of the seal air system is to maintain the insulators in the penthouse free from the dirty flue gases. Keeping the penthouse pressurized will avoid any influx of the flue gas up into the insulators, which will result in insulator failure.

Make sure that the heater is operating properly. Loss of heater operation will result in cold air in the penthouse that will create moisture resulting in sticky build-ups. This will also cause insulator failure and corrosion.

Maintain air intake filters. Clean or change out as necessary. A clogged filter will starve the blower and result in reduced pressurization and possibly heater failure.

• Insulation and Lagging

Maintain the integrity of the insulation and lagging. Loss of insulation on the ESP will result in cold spots and eventually corrode your electrostatic precipitator.

Many old designed precipitators have poor The recommended fix below suppresses any

penthouse insulation, which creates cold spots possible cold spots formation

• Safety Key Interlocks

Make sure all safety key interlocks are in good working order and all interlocks work correctly. Make sure no one has tampered and or defeated any interlocks. Safety key interlocks are installed for personnel safety and must be treated accordingly. Make sure all dust caps are on the lock cylinders to keep out any dirt that would otherwise damage the lock cylinder.

• Periodically, insert dry graphite powder into the lock cylinder to keep the lock mechanism lubricated.

Electrostatic Precipitator (ESP) Ozone Safety

What is Ozone?

Ozone is a naturally occurring gas created by the force of corona discharge during a lightning storm, by UV light from the Sun and by the operation of electrostatic precipitation.

Ozone (O3) is an allotrope of Oxygen (O2). It is 1.5 times as dense as oxygen and 12.5 times more soluble in water and leaves no residuals or byproducts except oxygen and a minimal amount of carbon dioxide and water. It can be manufactured from dry air or from oxygen by passing these gases through an electric field of high potential sufficient to generate a "corona" discharge between electrodes (of an ESP).

Ozone is highly unstable and must be generated on-site. The measure of an oxidizer and its ability to oxidize organic and inorganic material is its oxidation potential (measured in volts of electrical energy). Ozone's oxidation potential (-2.07V) is greater than that of hypochlorite acid (-1.49V) or chlorine (-1.36V), the latter agents being widely used in water treatment practice.

EPA Safety Guidelines

Ozone is a toxic gas and, like chlorine, can cause severe illness and death if inhaled in sufficient quantity. One safety advantage is the physical characteristic of ozone that allows it to be detected (smelled) at concentrations much lower than harmful levels.

Recommended Exposure Limit to Ozone:

A study of the health effects of ozone exposure was conducted by the United States Air Force. Another summary of the health effects of ozone was compiled by the American Society for Testing and Materials (ASTM) in support of their recommended standard for limiting human exposure to ozone. The reported biological effects range from dryness of mouth and throat, coughing, headache, and chest restrictions at concentrations near the recommended limit, to more acute problems at higher concentrations.

The recommended ambient ozone exposure levels have been proposed by the Occupational Safety and Health Administration (OSHA), the American National Standards Institute/American Society for Testing and Materials (ANSI/ASTM), the American Conference of Governmental Industrial Hygienist (ACGIH), and the American Industrial Hygiene Association (AIHA) as follows:

Control occupational exposure such that workers will not be exposed to ozone concentrations in excess of a time weighted average of 0.2 mg/m3 (0.1 ppm by volume) for eight hours or more per workday, and that no worker be exposed to a ceiling concentration in excess of 0.6 mg/m3 (0.3 ppm by volume) for more than 10 minutes. These recommended limits for ozone concentration are higher than the concentrations at which ozone can typically be smelled. Generally, an individual can detect ozone at concentrations ranging from 0.02 to 0.1 mg/m3 (0.01 to 0.05 ppm by volume). The more often a person is exposed to ozone the higher the required concentration for detection.

Electrostatic Precipitator Ozone Safety

An electrostatic precipitator creates appreciable amounts of ozone due to the nature of its operation. High Voltage, negatively charged discharge electrodes create a corona discharge within a confined area between two positively charged collecting electrodes (gas passages). When this occurs in the presence of oxygen ozone is created. After the precipitator has been off-line for repairs or a unit outage, it is a common practice to "air-load" the precipitator to ensure that it is ready for service and free of grounds or other problems before the unit is brought back into operation. Many times work can still be in progress throughout the unit and its auxiliaries. If this is the case make sure that all personnel are out of the flue gas areas and stack downstream of the precipitator and that all access doors are closed before an electrical air load test is performed. This is also true of the immediate ductwork upstream of the precipitator. Levels of ozone produced during the air load test can be very high. Whenever there is a possibility that ozone will affect any personnel conducting operations in adjacent areas, the use of a continuous monitor, with an audible alarm is recommended. This monitor (monitors) should be placed in the work area at a level that will allow it to measure air concentrations at the workers' breathing zone. As with all aspects of industrial operations, safety should always be the first consideration.

Electrostatic precipitation has been a reliable technology since the early 1900's. Originally developed to abate serious smoke nuisances, the manufacturers of zinc, copper, and lead quickly found electric gas cleaning a cost efficient way to recover valuable product carried out of the stacks from furnace operations. Today electrostatic precipitators are found mainly on large power plants, cement plants, incinerators, and various boiler application.

In the wood products industry, the dry electrostatic precipitator preceded by multi clones is now normally considered the best available control technology for wood fired boiler emissions.

Wet electrostatic precipitators have found renewed interest from OSB, particle board, and plywood veneer manufactures for controlling dryer exhaust.

DESIGN AND OPERATION

A precipitator is a relatively simple device. The main components are as follows:

• An insulated and lagged shell • Collection plates or tubes • Discharge electrodes • Collection Plate Rappers/Electrode Vibrators

• Hoppers

Dust laden gases are pushed or pulled through the box with the assistance of a fan. The air flow is channeled into lanes formed by the collection plates or tubes. Discharge electrodes are centered between each collection plate/tube to provide a negative charge to the surrounding dust particles. The collection plates/tubes are positively grounded and act as a magnet for the negatively charged dust particles. The collected dust is transported down the collection plates and electrode with the assistance of a rapper or vibrator system into the collection hopper.

An electrostatic precipitator can consistently provide 99%+ removal reducing emissions levels to 0.002 - 0.015 grains per dry standard cubic foot of exhaust gas.

Precipitators are designed to handle gas flow form 10,000 cfm to 300,000 cfm and can operate at temperatures as high as 750 degrees F. Normal gas flow through a precipitator is 2-5 feet per second, consequently, the pressure drip is only 0.5" wc. When replacing existing scrubber systems the fan horsepower to operated the precipitator can usually be decreased to one fourth of the scrubber system, which may have a pressure drop as high as 20" in order to deliver comparable efficiencies.

Insulated Steel Housing: The development of modular, factory built units has significantly lowered the installed cost of precipitators. Dry precipitators are normally fabricated from 3/16" thick steel plate, insulated and lagged with aluminum. The electrodes are made of

steel tubing and the collection plates are made of rolled steel. Since no moving parts are in contact with the gas stream, the housing can last 15-20 years. Wet precipitators are traditionally fabricated of stainless steel for corrosion resistance.

Discharge Electrodes: The advancement of the discharge electrode has solved many of the maintenance complaints associated with precipitators in the past. Originally, the dust particles were charged by a series of small diameter wires which were suspended from a ceiling rack and weighted at the bottom. This maze of electrodes was subject to electric erosion. Replacement was slow, cumbersome and required the unit to be off-line.

Today, discharge electrodes are rigid and constructed of 2" steel tubing, securely fastened to the upper rack and guided at the bottom. Ten years of continuous service is the expected norm.

Rappers and Vibrators: Heavy duty rappers are used by PPC in the wood industry. They consist of 30 pound piston hammers designed to rap small sections of collection plates. A timer periodically releases the rapper to transfer the dust on the collection plates to the hopper.

Electric vibrators are placed on the electrode rack to transfer any collected dust to the hopper and are operated by a timer.

Power: A typical precipitator will take 480 volt AC and with he assistance of transformer/rectifier converts the power to operated the discharge electrode's at 55-70 kv DC. This leads most inquirers to conclude they are huge electricity consumers. In reality, the electrostatic precipitator is the lower power consumer available to accomplish the job. Electrostatic forces are applied directly to the particles and not the entire gas stream. Combining this feature with the low pressure drop (0.5" wc) across the system results in power requirements approximately 50% of comparable wet systems and 25% of equivalent bag filter systems.

Power Consumption ChartACFM KW Hourly Operating Cost20,000 10 $0.5050,000 21 $1.05120,000 62 $3.10

COMPETING TECHNOLOGIES

A dry electrostatic precipitator operates at temperatures above 700oF and maintains the fly ash in its natural dry condition simplifying material handling.

Wet Scrubbers: A scrubber saturates the gas stream in order to remove the dry fly ash. The wet ash has to separate from the water in settling ponds or through a de-sludging unit which increases the annual labor and operating cost.

It is not uncommon to see 150 to 300 hp fans on scrubber installations in the wood industry. The energy necessary to separate the particulate from the gas stream can require 15" - 20" wc of pressure drop through a typical venturi. These are huge and wasteful power consumers, increasing the plant's overall operating cost.

Wet scrubbers have to contend with freezing in northern climates and

equipment corrosion. Finally, regulatory authorities are moving towards zero water discharge from operating plants.

Baghouses: The high temperatures and periodic cinders from the plant boiler can cause fire problems with baghouses. Periodic bag replacement is a definite operating cast consideration which affects the overall cost of a baghouse installation.

NEW APPLICATIONS

Dryers: OSHA and state regulatory authorities are beginning to look at dryer emissions. There is an inside emission problem because the fly ash accumulates on top of the veneer. There is also an external emission problem from the dryer vent stack.

A dry precipitator can be installed after the burner blend chamber to remove the fly ash. The recirculation of the dryer air stream is then cleansed of incoming ash. This arrangement produces cleaner veneer and eliminates dryer vent emissions.

Wet Units: Wet electrostatic precipitators have seen renewed interest in the wood products industry as OSB, particle board, and veneer plants are required to control VOC emissions form their dryer exhaust. The wet est serves to remove particulate emissions and "condensible" VOC's (pinenes, terpenes, cymene, toluene, etc.) from their dryer exhaust. Depending upon the exhaust temperature and partial pressure considerations of

each component, the wet esp can reduce the VOC emission by 20 - 40% while solving all opacity problems. If additional VOC removal efficiencies are required, the wet precipitator may be a necessary pre-treatment item for incinerators or biofiltration systems which do not handle particulate concentrations very well.

The purpose for the "wet" electrostatic system is to mainly prevent fires. The particulate carry over form an OSB dryer can represent large fiber stands which can be ignited by the sparking inside a dry precipitator setting off a hopper fire. The wet precipitators apply a water quench to the gas stream before entering the collecting tubes. The collection tubes are also continuously sprayed with water in order to wash the particulate off the tubes thereby eliminating any chance of combustion.

Pre-cleaner: Wet electrostatic precipitators are excellent particulate removal devices for use ahead of RCO's, TRO's, and Biofilters. These VOC removal devices are sensitive to particulate in the flue gas stream. Since wet electrostatic precipitators can provide emission levels as low as 0.003 gr/dscf, they prevent fouling of the VOC removal devices.

Thermal Oil Heater: Electrostatic precipitators are fast becoming the device of choice for controlling emissions from thermal oil heaters. Several of the recent Canadian OSB plants have selected PPC for the electrostatic precipitator to control the particulate from combustion of wood. The electrostatic precipitator's low power consumption combined with low maintenance make it an excellent choice for plants with a limited number of operating personnel. The boiler PLC can be configured with eh 10-12 channels required to completely automat the operation of the electrostatic precipitator. The PLC provides

the plant with a written chart for submittal to the regulatory agencies that require continuous emissions monitoring.

CONCLUSION

Electric Gas Cleaning is an old proven technology for removing particulate emissions from exhaust stacks. Electrostatic precipitators have proven to be reliable workhorses in the wood products industry as more companies focus on profits.

ELECTRICAL ENGINEERING FOR POLLUTION CONTROL

Electrostatic Precipitators for Power Plants

Many countries around the world, including our own, depend on coal and other fossil fuels to produce electricity. A natural result from the burning of fossil fuels, particularly coal, is the emission of flyash. Ash is mineral matter present in the fuel. For a pulverized coal unit, 60-80% of ash leaves with the flue gas. Historically, flyash emissions have received the greatest attention since they are easily seen leaving smokestacks.

Two emission control devices for flyash are the traditional fabric filters and the more recent electrostatic precipitators. The fabric filters are large baghouse filters having a high maintenance cost (the cloth bags have a life of 18 to 36 months, butcan be temporarily cleaned by shaking or backflushing with air). These fabric filters are inherently large structures resulting in a large pressure drop, which reduces the plant efficiency. Electrostatic precipitators have collection efficiency of 99%, but do not work well for flyash with a high electrical resistivity (as commonly results from combustion of low-sulfur coal). In addition, the designer must avoid allowing unburned gas to enter the electrostatic precipitator since the gas could be ignited.

The salt & pepper collector/selector, and repelling balloon experiments serve to illustrate the basis of an electrostatic precipitator. In these experiments a type of electrostatic collector and electrostatic selector are created. This same principle is used to keep the environment clean today. A description of a more elaborate demonstration of how an electrostatic precipitator works using a Van de Graaff generator may be found at http://www.physics.umd.edu/lecdem/services/demos/demosj2/j2-15.htm.

Top View of ESP Schematic Diagram [Source: Powerspan Corp.].

The fluegas laden with flyash is sent through pipes having negatively charged plates which give the particles a negative charge. The particles are then routed past positively charged plates, or grounded plates, which attract the now negatively-charged ash particles. The particles stick to the positive plates until they are collected. The air that leaves the plates is then clean from harmful pollutants. Just as the spoon picked the salt

and pepper up from the surface they were on, the electrostatic precipitator extracts the pollutants out of the air. For a more detailed overview of an electrostatic precipitator, see Powerspan Corp. (formerly Zero Emmisions Technology) homepage at www.powerspancorp.com/news/precipitator.shtml which also includes a couple of nice schematics. Photographs of electrostatic precipitators at coal-fired power plants can be found at http://www4.ncsu.edu/~frey/apcespph.html.

Side view of ESP Schematic Diagram [Source: Powerspan Corp.].

Electrostatic precipitators are not only used in utility applications but also other industries (for other exhaust gas particles) such as cement (dust), pulp & paper (salt cake & lime dust), petrochemicals (sulfuric acid mist), and steel (dust & fumes).

As we can see Electrical Engineers can play an important part in the fight against pollution. Through devices such as the electrostatic precipitator, electrical engineers can protect the environment from harm. Such a design also appeals to the general public as the electricity can be produced cheaply. The electrostatic precipitator is just one example of a device designed by electrical engineers to help the environment. Engineers are responsible for considering environmental impact as part of their original design work.

BAGHOUSES vs PRECIPITATORS

Precipitators have many operational advantages over baghouses. The net result may be that the environment is far better off with a precipitator than it is with a baghouse.

As operators of two large incineration plants with three large rotary kiln incinerators in each, Montgomery County, Ohio, staff and management operated large electrostatic precipitators for many years. Their engineers and managers visited similar waste combusting facilities across the nation and developed a distinct and well founded preference for the electrostatic precipitator.

Unfortunately, Federal Legislators and regulators spurred on by unscientific "environmentalists" and unsupportable evidence have, since 1989, passed legislation which has effectively eliminated the electrostatic precipitator as an air pollution control tool for waste combustion.

Consider the following observations:

Precipitators meeting the same emission standards require substantially less energy consumption during operation than do baghouses. Energy is part of our total environment and it should not be wasted. A baghouse typically requires six times the energy to pull the flue gases through the fabric filters. This is very energy wasteful on a system that handles hundreds of thousands of cubic feet per minute.

Baghouses are relatively more sensitive to temperature and humidity variations in the process gas stream. Incinerator gases are inherently variable in temperature and humidity due to the varying nature of trash. While electrostatic precipitators have some sensitivity to these variables as well, continuous static voltage regulators can accommodate the usual range of temperature and humidity without failure. Baghouses have been known to blind, crust over, melt, or burn under conditions which would not have harmed an electrostatic precipitator.

Baghouses can catch fire. During the operation of our existing electrostatic precipitators, we have observed particulate continuing combustion in the electrostatic precipitator enclosure, even after being carried through the wet conditioning system. You can imagine what this condition will do to a baghouse. The author has witnessed a brand new industrial baghouse on fire only two days after its startup.

Very fine particulate material is prone to "blind" the fabric of a baghouse. The nature of incineration particulate ("fly ash") is such that more than 50% of it is sub-micron in size. Blinded bags increase head loss across the filter (requiring the use of even more energy) and must be replaced.

Baghouses tend to "crust over." During the operation of the County incinerator precipitators, staff occasionally needed to wire brush crusty fly ash off the collector plates. Operators of baghouses acknowledge that they have similar crusty buildups on

occasion on the bags. Mechanical removal from the bags (wire brushing?) is sometimes possible, but that causes damage to the filter bags themselves.

Baghouse performance deteriorates over a period of 15 months after new bag installation. Electrostatic precipitator performance, on the other hand, stays uniform for years after the precipitator has been started up without any deterioration. This has been observed in testing at a number of installations. Bags rubbing against their wire frame holders in the course of being cleaned can develop small holes.

Flue gases then zip through those holes without any cleaning action. Since the rest of the unit is slowly blinding and crusting over, the differential pressure may offset one another and sensing instruments on each side of the filter may not detect a net change in the back pressure. Thus, the baghouse may be continued in service long after its cleaning ability has substantially deteriorated.

Baghouse maintenance can be a threat to plant staff health. When baghouse bags need changing, plant staff is intimately exposed to the very materials we are trying to keep away from the ambient environment. Frequent changes requires frequent contact. While we can provide safety training and special clothing for careful bag changing, the fact remains that the contacts between staff and fly ash will be more frequent with a baghouse than with a precipitator.

All in all, County staff was well pleased with saving over $70,000 of electricity each year by using precipitators. Mother Nature cannot long afford to squander energy on fruitless endeavors.

The nation needs to reassess its standards, base the regulations on human health requirements, and encourage engineers to select the most efficient control tools available.

Introduction

A properly maintained electrostatic precipitator is important to ensure clean gas enters the contact section of the acid plant. Failure of the WESP to perform its function will shorthen the life of downstream equipment because the acid mist that carries forward causes accelerated corrosion. The following describes some typical problems with WESP's.

Maintenance and Inspection Schedule

Daily

• Note and record switch board readings at least once per shift • Check that all flushing system components are working properly

• Ensure insulator compartment ventilating system is operating

Weekly

• Remove dust and foreign matter from electrical equipment • Check control system and note any alarm conditions

• Clean or replace insulator compartment ventilation system filters

Shutdown

• Clean the insulators in the insulator compartments • Thoroughly inspect the interior of the unit. Particular attention should

be given to the high voltage electrodes.

• Check FRP components for damage

Insulator Compartment Ventilating System

The following maintenance is required:

• A periodic check of the heater and temperature controller should be made to ensure their proper operation.

• The fan and motor bearing should be checked for vibration. • Check air flow to each compartment and rebalance the system to ensure equal flow to

each compartment. • Clean or replace air filters

Insulators

All insulators in the system must be cleaned at regular intervals to remove accumulation of dust, acid or moisture.

Wipe the insulators with a dry clean cloth to remove all foreign matter. If acid is to be removed, clean with ammonia and polish with a clean dry cloth. Never use wire brushes as they may damage the surface finish of the insulators. Remove any accumulation of dust and dirt inside the insulator compartments.

Any cracked or damaged insulators must be replaced immediately.

Air Load Test

An air load test of the precipitator provides a baseline for the comparison of precipitator performance. An air load test should be performed on every unit at initial start-up and after any maintenance, repair or inspection is done on the unit.

The test is performed with air and not process gas. The unit is energized and operated in manual. The current is increased in increments of 10% of the transformer/rectifier nameplate rating. All voltage and current readings are recorded at each increment until the rated current is reached or until sparking/arcing occurs.

Problems

Tubesheet Leakage

In the old style WESP's the lead collection tubes were welded to a carbon steel tubesheet overlayed with lead. As the WESP ages the weld between the tubesheet and tubes may become damaged and crack. The result is a leak path for weak acid to pass through the protective lead covering and begin attacking the carbon steel underneath. The easiest way to see when this is occurring is to look for signs of liquid running down from the tubesheet corroding and staining the outside of the tubes. Ususally when this occurs the WESP unit is approaching the end of its useful life. There will be other problems in addition to the tubesheet leaks which the plant maintenance staff will need to contend with. The tubesheet leaks can be fixed by fixing the tube-to-tubesheet weld.

Collection Tube Burn Through

The discharge electrode should be centered in each of the collecting tubes to ensure optimum operation of the unit and maximum collection efficiency. If one of the discharge electrodes is off-centre then it will affect the over all operation of the unit. Spark-overs will be more frequent and in the worse case, the electrical system will short out. Off-centreing may be caused by gas turbulence causing the lower support frame to swing, loss of the weight that holds the discharge electrode taught, movement in the collecting tube, etc. When a discharge electrode is off-centre, excessive sparking will occur and the sparking may cause the collecting electrode to burn through as illustrated in the photo. When WESP's are inspected, each tube should be checked to ensure that the discharge electrode is exactly centred in the collecting tube. If it isn't, either the discharge or collecting electrode will need to be re-aligned.

Broken Discharge Electrode

Broken discharge electrodes may cause the high voltage system to electrically short and become ineffective if the broken electrode remains lodge in the collecting tube. Broken discharge electrodes are a problem with lead star wires but is less of a problem with the rigid discharge electrode systems. The broken discharge electrode must be removed and replaced or the tube blanked off until repairs can be made. The broken wire is indicated by the fallen weight in the back of the photo.

Bulging Tubesheet

The top support tubesheet of lead type precipitators are lead lined to protect the carbon steel from corrosion. To reduce costs, the lead was often spot bonded to the carbon steel tubesheet. In most cases weak acid eventually penetrates through the lead lining through cracks or pin hole and corrodes the steel. The resulting iron sulphate occupies a greater volume and causes the lead lining to bulge upwards as shown in the photo. The lead lining may crack further leading to even more weak acid penetrating the tubesheet. A homogeneously lead lined tubesheet would have prevented this type of damage.