Monitoring
High Voltage Energization System
Maintain proper operation of the high voltage energization system
Always make sure the high voltage energization system is operating properly. This includes the high voltage controls and transformer-rectifiers. The high voltage system of the electrostatic precipitator is the "heart" of the operation. Any precipitator problems or inefficiencies will be reflected in the operation of the high voltage energization system.
- Make sure all controls are energized and not in alarm
- If in alarm, recognize and correct all alarms
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:
- Record electrical readings on a daily basis
- Keep a history of electrical readings
- Note the trend of the readings. If readings change dramatically, this may be an indication that a fault may be developing with the ESP.
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:
- Make sure the ash handling system is in good operating condition
- Make sure all ash valves work properly and no air leaks are present
- Maintain operation of hopper heaters, vibrators, etc.
- 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.
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 ESP
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 in-leakage that can mix with the hot gases in the ESP and create a corrosive atmosphere (namely sulfuric acid).
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 under-utilize 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
- During an outage, make sure that all gas flow devices are clean and not built up with dust.
- Make sure all perforated plate holes are open and not plugged off.
- 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.
- Make sure all rappers or vibrators installed on perforated plates are in good working order.
- Make sure all horizontal surfaces (such as splitter vanes and egg crates) are not built up with dust.
- During an ESP inspection, look at the collecting plate surfaces for any signs of scouring. This may be suggestive of high velocity zones.
