The electrostatic precipitation process bases on the interaction of an electrically charged particle and an electric field. Dust suspended in the flue gas, conducted to the ESP through aduct, is electrically charged and passed through an electric field between discharge electrodes and collecting plates. The collecting plates are (+) earthed, and the discharge electrodes connected to a (–) end of the direct current supply unit (transformer-rectifier).The value of secondary voltage supplied into the electrical fields varies typically between 50-80 kV, depending on the process conditions. High voltage applied to the discharge electrodes results in a corona discharge. The corona phenomenon causes free electron emission, which ionize the gas ions. Negatively charged ions migrate towards the collecting plates influenced by electric field forces. The charged ions migrating towards the collecting plates collide with the dust particles flowing in the gas stream charging them by tacking to. Negatively charged dust particles change their direction under the influence of electric field forces and get collected onto the collecting plates.

After contact with the collecting surface, or with already collected layer of dust, dust particles discharge and remain on the collecting surface forming thicker and thicker layers. These layers of collected dust are removed from surfaces by rapping systems and fall down to theESP bottom hoppers. A number of cations generated near the corona discharge have are latively short way towards the (-) charged discharge electrodes, causing a smaller amount of dust particles to get collected on the surface of the discharge electrodes. The dust collected on the discharge electrodes is also removed by means of a rapping system.Dust removed from collecting surfaces and discharge electrodes fall into the bottom hoppers, or flat bottom scraper located under the ESP electrical fields, and gets conveyed out from the ESP by a collecting convey or and a rotary feeder. For inspection and repair purposes, the ESP has a number of manholes and access doors inthe ESP casing and in the insulator chambers.

Normal maintenance inspections of the ESP can be classified in three (3) different type of inspections, which are:
1. Daily inspections
2. Monthly inspections
3. Annual inspections

2.1 Daily inspections (during normal operation)
During normal operation of the ESP plant it is advised to visually check/monitor the ESP condition and operation in overall manner. All found defects should be correctedor repaired at once when noticed. When doing this, Safety Precautions must always be followed (see Section 3 below).

Areas to be daily checked/monitored are e.g.:
- See, that all ESP safety grounding equipment are in their own places,
- See, that all ESP manholes are closed and that the insulating cushions are in their places,
- See, that all stairs and walkways are not used as storage areas, but free from all foreign objects, It is also recommended to keep a daily operating log for certain ESP operating parameters, which can include:
- Flue gas operating temperature,
- Pressure drop across the ESPs,
- kV- and mA- values for each of the T/R units,
- Rapping system’s ON/OFF- signals,
- Conveyors and rotary feeders operation in the bottom hopper. *)
*) It is extremely essential to make sure at all times, that the bottom hoppers are continuously discharged from collected fly-ash. It must be stressed, that the hopperis designed for continuous discharging – not to act as fly-ash silo.

2.2 Monthly inspections (during normal operation)
  It is further recommended to perform following operations on monthly basis:
- Check, that the rapping gear-drives do not make any disturbing noise,
- Check, that all rapping systems are really operating by listening the sound of the hammers hitting at the same time, when each respective gear-drive is ON,
- Check, that all thermal insulations are in good condition,
- Check, that the T/R unit control cabinets are free from dust,
- Check, that the T/R units oil level is within the limit lines,
- Check, that all switches and relays are in their correct positions.

2.3 Annual inspections (during shutdown)
During annual shutdowns it is further recommended to perform the following inspections:
- Measure the operating current (A) from all motors and heaters,
- Visually check the condition of all porcelain insulators. If cracked insulators are found, they should be replaced with new ones,
- Clean all insulators free from dust e.g. with a wet cloth,
- Check, that all manhole sealings are in good condition and no leakage is visible,
- Check, that there are no dust deposits inside the ESP and in the hopper, *)
-> If dust accumulation is found in the hoppers, then they should be removed before next boiler start-up.
- Check, that the alignment of the electrodes with respect to the collecting plates is equal in each gas passage, *)
- Check, that the rapping systems are operating correctly, and that no abnormal wearing is visible, *)
- Check, that there is no visible corrosion or air leakages inside the ESP, *)
- Check, that there is no broken discharge electrodes in the electrical field, *)*)

Safety Precautions must be followed when entering the ESP!!!

3.1 As the Electrostatic Precipitator operates at high voltages, it is extremely important that the persons working with the ESP shall be informed of the following safety regulations besides local mill general safety regulations:

1. The operating voltage of any hand lamps taken inside the ESP must not exceed 50 V.
2. If some maintenance work is to be carried out inside the ESP, the tools power supply must have a safety fusing system against any type of electrical shocks.
3. Keys of the manhole and control cabinet locks must be kept in a safe place, where they do not get in hands of any unauthorized personnel.
4. Always before touching or entering any part of the ESP, a safety grounding of the ESP unit must be completed.
5. Check regularly that all ESP grounding equipment and grounding cables are in good and solid condition, and that they have a good contact to the grounding points.
6. Check regularly that the grounding switches of the high-voltage rectifiers arein good condition – if any.
7. The following measures should be followed before entering/inspecting the ESP internally, or the high-voltage rectifier:

- Make sure, that the high-voltage rectifier is switched OFF. Check also from the auxiliary control cabinet, that the rapping motors are not set in operation.
- Switch off the main switch of the TR set and lock the main switch.
- Hang a sign “DO NOT SWITCH ON, or MEN AT WORK” in a clearly visible place inthe control panels.
- Set the safety switch of the high-voltage rectifier and rapping motors in OPEN position and lock the switch.
- Check that the safety grounding equipment are in good condition.
- Ground the high-voltage part of the rectifier by using safety grounding equipment through the hatched opening in the insulator chamber.
- Ground also the discharge electrode system through the manhole in the ESP casing by using safety grounding equipment.
- Make sure that there is no dust deposits which may fall down and cause injuries or burns.
- Make sure that the bottom hopper is empty of dust, and specifically that there is no clogged ash accumulations at sight.
- When entering the ESP use proper safety clothing and a respirator.
- When entering the ESP a multifunctional meter should be used to check the O2, CO and H2S -levels.
- A safety person must be located outside the manhole where work is performed at all times.

3.2 Electrostatic Precipitator Warning Signs
In order to ensure safety at work, the precipitator has been provided with proper warning signs. Attached to the insulating cover plates of the manholes, there must be placed a warning sign: “HIGH VOLTAGE…DISCONNECT AND GROUND…”. These warning signs must also be placed on top of the ESP roof around manholes. A “HIGH VOLTAGE”- sign should be placed to the insulating covers of each of the insulating compartments.

4.1 Starting after a long shutdown
It is recommended that at least before 24 hours of planned start all heating elements of the insulator housings and bottom hopper are connected ON in order to dry all possible mist from these areas.
Before starting of the ESP, check
- that bottom hoppers are empty from dust,
- that all manholes and insulator compartments are closed and locked,
- that all safety groundings have been removed,
- that the safety locks are removed from the T/R unit main switches, and that they are in the closed position,
- that no alarms are valid on T/R sets or other equipment,
- that all fuses are being put back into their locations.

4.2 Starting after a short shutdown
It is recommended not to turn off any of the insulator heaters during a shorts hutdown of the ESP.
Before start, only areas where there has been work/repairs involved require further attention.

4.3 Starting of the ESP
After completing of all preliminary measures, it is advised to start the ESP in following order:
1. Start the dust conveying systems (dust conveyors and rotary feeder),
2. Turn the main switches ON from all T/R units,
3. Start the T/R units from the controller panel. After starting of the T/R units check that the ESP voltage and current rise at least 30% calculated from the maximum operating values.
Note! As the air inside the ESP may be humid, there may occur sparking in all fields for a short period.

5.1 Actions to be taken before stopping of the ESP

1. Switch-off the T/R units
2. Turn the T/R unit main switch in 0- position and place a safety lock to the switch.
3. Hang a “Men at Work – Do Not Start” warning sign on the cabinet door.
4. After switching of the T/R units, operate the rapping systems and fly-ash conveyors at least 30 minutes before turning them OFF.
5. Stop the rappers and the fly-ash conveyors.
6. The heaters of the ESP may be left ON, especially if the shutdown will last only for acouple of days.
This prevents moisture to react with possible dust on the insulator surfaces, thus preventing them from cracking once ESP starts again.

6.1 Visual checks
- Check from the T/R unit control panel the A, mA and kV meters, and that the ESP operation is stable,
- All abnormal readings and variations should be recorded into a logbook for possible later examination.
- Check, that there is fly-ash coming out from the bottom hopper conveyors.

6.2 Adjustments and T/R settings
- All T/R set parameters can be changed from the T/R control panel. For parameter settings, please read the T/R Controller’s product manual.
- The auxiliary control cubicle includes time relays for the rapper controls. The rapping timing and interval will be set at estimated values when the plant is commissioned. It may, however, be occasionally required to change the rapping parameters due to different/changing operating conditions.
- When starting the rappers, note that each of the rapper is started by the program in a specific sequence.
- Under normal boiler operation, the T/R unit of the inlet field is typically operating atlower mA -level, appr. 30…60% of the max. value, due to the reason of treating dusty flue gases. In the ESP’s 2nd and 3rd field the mA -values rise gradually towards maximum set value.
- For recovery boiler ESPs it is typical, that in the 1st field of the ESP there will occur lot more sparking compared to the remaining ESP fields. Normal and accepted sparking level can be as high as 100-120 sparks/min.

7.1 Typical functional troubles finding chart for ESP