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XVI. ELECTROSTATICPRECIPITATORS

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Contents

1. WORKING PRINCIPLE OF ELECTROSTATIC PRECIPITATOR

2. DESIGN CONSIDERATIONS

3. CONSTRUCTION DETAILS

4. ELECTRICAL SYSTEM

TABLE I ULTIMATE ANALYSIS OF TYPICAL INDIAN COALS

TABLE II CHEMICAL COMPOSITION OF COAL ASH FROM INDIANCOALS

TABLE III TYPICAL FLY ASH ANALYSIS ENTERING THE DUST

COLLECTOR (Pulverised Fuel Bituminous Coal)

TABLE IV RANGE OF BASIC DESIGN PARAMETERS IN PRACTICE

FOR FLY ASH PRECIPITATORS

5.CHECK LIST FOR PRE-COMMISSIONING/COMMISSIONING OFELECTROSTATIC PRECIPITATOR

6.OPERATIONAL PROBLEMS AND TROUBLE SHOOTING

7.MAJOR OVERHAUL OF ELECTRO STATIC ECIPITATORS

FIG. XVI-1 GENERAL ARRANGEMENT OF ELECTROSTATIC PRE-

CIPITATOR

FIG. XVI-2 CASING

FIG. XVI-3 HOPPERS

FIG. XVI-4 GAS DISTRIBUTION SCREEN

FIG. XVI-5 COLLECTING SYSTEM SUSPENSION AND RAPPING

FIG. XVI-6 EMITTING SYSTEM RAPPING MECHANISM WITH SIDE

DRIVE

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FIG. XVI-7 EMITTING SYSTEM RAPPING MECHANISM WITH

VERTICAL DRIVE

FIG. XVI-8 TRANS DUCTOR CONTROL BLOCK SCHEMATIC

DIAGRAM

FIG. XVI-9 SCHEMATIC DIRGRAM MODERN HV RECTIFIER SET

WITH SCR TYPE AUTOMATIC CONTROL FOR EB

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XVI. ELECTROSTATIC PRECIPITATORS

1. WORKING PRINCIPLE OF ELECTROSTATIC

  PRECIPITATOR

Of all the devices used for solid-gas separation, electrostatic precipitator finds wide application

because of its inherent advantage over all other devices. Electrostatic precipitators can handle

large volume of gases from which solid particulates are to be removed. Their technical

superiority lies in low pressure drop, high efficiency for small particles size, and relatively

easy removal of the collected particulates.

There are four different steps in the process of precipitation:

i) lonisation of gases and charging of dust particles.

ii) Migration of the particle to the collector.

iii) Deposition of charged particles on the collecting surface.

iv) Dislodging of particles from the collecting surface.

The electrostatic precipitator essentially consists of two sets of electrodes, one in the form of 

thin wires called discharge or emitting electrodes and other set called collecting electrodes in

the form of pipes or plates. The emitting electrodes are placed in the centre of pipes or midway

between two plates and are connected usually to negative polarity of high voltage d.c. source

of the order of 25-100kV. The collecting electrodes are connected to the positive polarity of 

the source and grounded. The high electric field in the vicinity of the emitting electrodes

creates ‘corona discharge’ ionising the gas molecules. The dust particles entrained in the gas

acquire negative charge and experience a force which drives them toward the collecting

electrodes where they get deposited. The collected material is dislodged by knocking the

electrode by a process called ‘ rapping.’

Figure XVI-1 shows the details of a typical precipitator used for collection of fly ash.

The collection efficiency (h) of a precipitator is given by an empirical formula. - (W 

K  SCA)1/2

=1 -e

Where WK has the dimension of velocity and is known as migration velocity SCA is

Specific collecting electrode area.

Total projected collecting area

  =

Gas flow rate

This equation indicates that higher collection efficiency can be obtained by increasing the

size of the precipitator or increasing the total collecting surface area.

 

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The migration velocity, W K  is influenced by the electrical power input, electrical resistivity of 

the dust particles, dust burden, grain size distribution, temperature. This value varies from

15cms/sec. to 50cms/sec. Depending on the collection efficiency requirement, specific

collecting electrode area may vary from 50 to 120 m2 /m2 /sec.

The performance of the electrostatic precipitator depends on several factors among which the

prominent are:

i) Characteristics of dust :

a) Particle size distribution.

b) Dust loading.

c) Chemical composition.

d) Electrical resistivity.

e) Adhesive/cohesive properties.

ii) Characteristics of gases:

a) Temperature.

b) Chemical composition.

c) Moisture content.

d) Quantity to be handled.

e) Pressure.

Electrostatic precipitator finds its application in a number of processes and metallurgical

industries. This is because it can be designed in a larger number of types to suit the process

conditions.

The precipitator can be basically classified into the following types:

(i) Dry or wet (irrigated)

(ii) Horizontal or vertical flow.

(iii) Plate type or tubular type.

For recovery of valuable material, dry type precipitator is normally chosen.

 

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2. DESIGN CONSIDERATIONS

2.1 Application

An electrostatic precipitator is designed differently for different applications. Its height and

width are selected to suit the volume of gas to be treated. Its length and number of electrically

separated fields/ zones are varied according to the collection efficiency requirements.

The basic data required for design of electrostatic precipitator are:

a) Flue gas quantity.

b) Temperature of flue gas.

c) Inlet dust burden.

d) Collection efficiency required.

e) Coal analysis.

- Proximate

- Ultimate

f) Ash analysis.

- Particle size distribution.

- Chemical characteristics.

g) Particle resistivity.

2.2 Properties of Fly Ash

It would appear possible from physical and chemical tests on coal and dust to obtain sufficient

information to predict at least broadly, the behaviour of a precipitator when used for collecting

a particular fly ash. Fly ash from fossil fuel burning varies markedly in composition depending

on the source of coal and degree and type of combustion. In addition to substantial quantities

of oxides of silicon, aluminium, iron and calcium, as many as 30 to 40 additional elements

are present in traces to significant quantities.

Typical chemical properties of the Indian low sulphur coals and resulting fly ash are given

in Table I & II.

a) Particle Size

The size distribution of the fly ash entering the inlet of the electrostatic precipitators play a

major role in the performance of EP Typical particle size distribution is given in Table Ill.

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b) Resistivity

For temperatures below about 1600C, the resistivity is dominated by the surface conduction

over the fly ash particles which in turn is greatly influenced by the chemical composition of 

the flue gas. (i.e. H2O, SO

3etc.) At higher temperatures, or in a perfectly dry atmosphere the

fly ash behaves as semi-insulator.

2.3 Design of Precipitator for Fly AshRemoval

A fundamental task in precipitation technology is the design of optimum precipitator systems

for given applications. The basic design problem for precipitators is the determination of the

principal parameters for precipitators sizing, electrode arrangement and electrical energisation

needed to provide specified level of performance. Other factors such as rappers, gas flow

control methods, dust removal systems and performance monitoring must also be considered.

The collecting efficiency in actual operation depends strongly on such quality factors as

accuracy of precipitator electrodes alignment, uniformity and smoothness of gas flow through

the precipitator, rapping of the electrodes and the size and electrical stability of the rectifiersets. The range of basic design parameters in practice for fly ash precipitators are tabulated in

Table IV. The design incorporates certain features considered essential or desirable when

selecting the electrostatic precipitator for a given application.

3. CONSTRUCTION DETAILS

The major fundamental parts of the electrostatic precipitator consist of the following

i) Casing

ii) Hoppers

iii) Gas distributor screen

iv) Collecting system

v) Emitting system

vi) Rapping mechanism for collecting system

vii) Rapping mechanism for emitting system

viii) Insulator housing.

3.1 casing

The precipitator casing is designed for horizontal gas flow. It is an all-welded steel construction,

assembled from prefabricated wall and roof panels (Figure. XVI-2) using panel construction.

The main part of the fabrication is done in the workshop. This assures better tolerance and

quality control.

The gas pressure and temperature and the wind load will cause the casing structure to flex.

Problem free precipitator operation requires that the electrode contained in and supported by

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the casing remain perfectly aligned. Therefore excessive flexing of the casing Must be avoided.

The casing design philosophy is to minimise distortion rather than using the maximum

allowable stress in the steel.

Each electrical section is available for inspection and maintenance through suitably located

doors.

To provide for heat expansion, the casing is supported by roller bearing supports.

The precipitator internals are suspended in the roof panels, which also carry all the equipment

on top of the roof. These loads are then transferred through the side panel columns and roller

bearings to the support structure. The casing is usually insulated with mineral wool and covered

with aluminium/G.I. lagging. The insulation thickness is determined from case to case based

on gas temperature, acid dew point and prevailing ambient temperatures. This insulation

must cover the entire casing including hoppers and side columns. The top insulation is covered

by a checker plate roof. This roof is walkable and is an ideal surface for maintenance work.

3.2 Hoppers

The hoppers are of pyramidal type (Figure XVI-3). Also rough type and flat-bottom

precipitators with scraper conveyors are available for some applications. The valley angle of 

the hoppers (angle between hopper corner and horizontal) is never less then 550 and offer

more to ensure easy dust flow down to the feed out flange.

All hoppers have gas baffles.

The upper portions of the two adjacent hoppers have a reinforced ridge to support the hoppers

across the precipitator width.

To ensure free flow of ash into the disposal system lower portions of the hoppers are provided

with electrical heaters with thermostatic control.

3.3 Gas Distribution Screen

The gas velocity in the precipitator is approximately 1 /10th of the velocity in the ducting

before the precipitator. It is therefore essential that the precipitator has arrangements to give

an even gas distribution over its entire cross sectional area. A good gas distribution cannot be

achieved solely through the design of the ducts. Special gas distribution screens are therefore

located at the inlet of the precipitator (Figure XVI-4). The screens are of modular design and

hang within a frame work in the precipitator casing inlet. During the final checking of the gasflow pattern additional deflector plates are added on to the screens, it necessary. A maximum

of 20% standard deviation can be tolerated for the velocity distribution in the precipitator.

3.4 Collecting System

The ‘ G ’ profiled collecting electrode is based on the concept of dimensional stability. The

upper edge of the collecting plates are provided with hooks, which are hung from support

angles welded to the roof structure (Figure. XVI-5). The lower edge of each plate has a shock 

receiving plate, which is securely guided by the shock bar arrangement. This results in a

stable collecting system similar to the emitting system. In order to maintain the collectingefficiency at the design level it is essential that the emitting and collecting systems are

dimensionally stable.

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The collecting plates are made of 1.6 mm steel plate and shaped in one piece by roll forming.

Rigidity is the main purpose for the special design of the collecting plate edges.

In order to assure the most rigid construction, taller collecting plates (10m) are connected to

one another by transverse guides, thereby preventing any swinging tendencies.

3.5 Emitting System

The emitting system is an important part of the precipitator. The emitting framework is

thoroughly braced and forms a rigid box-like structure (Figure XVI-6). The frame is assembled,

adjusted and welded to its final position inside the casing, which makes it possible to obtain

and maintain highly accurate electrode spacing.

The frame work has a four point suspension effectively taking care of the expansion when hot

gas is entering. All sharp edges and ends of frame parts are rounded to avoid excessive flash

overs.

Prefabricated sub frames, suitably sized for shipment provide the most economical design

at highest quality.

The emitting electrodes are spiralized from semi-hard stainless steel wire. The spiral electrodes

are sent to the erection site as closely wound coils with one hook mounted at each end. At the

erection, the coils are stretched and attached by means of a special stretching device between

top and bottom holders in each stage of the frame work. The following are the advantages of 

this type of electrode :

i) Wire type electrodes give the best current distribution. Therefore they are the ones best

suited for difficult dusts with high electric resistivity.

ii) They are self-tensioning. Therefore no weights are needed to keep them stretched and

taut. Such weights would have to be placed beneath the electrode system and would

require long wires (the entire precipitator height) which latter would have to pass the

lower collecting electrode edge. (a spark erosion hazard).

iii) Since no weights are used the wire can be 0 in height. Short spirals well tensioned

(150-200 N) are much less prone to swinging than long loose wires (50-100 N) from

weight. Spirals are easy to instal with perfect and permanent alignment.

iv) The taut wires are susceptible to rapping accelerations and to stay clean. It is difficult to

accelerate a large number of weights.

An essential part of the internal equipment in a precipitator is the design of rapping mechanisms

for both the emitting and collecting systems.

It is essential that these systems be thoroughly cleaned during rapping and the parameter,

which has greatest influence upon the cleaning efficiency is the acceleration of the electrode

as a result of the rapping action. In order to achieve efficient cleaning, the rapping systems

have to be constructed to provide the required accelerations throughout.

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3.6 Rapping Mechanism for Collecting System

Each collecting plate has a shock receiving plate at its lower end. The plates in one row Of 

each field are interfaced to one another by these shock receiving irons resting in slots in the

shock bar, thus maintaining the required spacings. The shock bars are kept in alignment with

guides located at the front and rear of each shock bar. Each collecting plate is hung on an

eccentric positioned hook to ensure that the shockreceiving iron of   the collecting plate isconstantly resting against the shock bar. In this manner the highest possible energy is transferred

to the collecting plate when the” tumbling hammer” hits the corresponding. shock bar. A

review of the plate rapping system is as follows :

The system employs “ tumbling hammers” which are mounted on a horizontal shaft in a

staggered fashion, with one hammer for each shock bar. As the shaft rotates slowly each of 

the hammers in turn over balances and tumbles, hitting its associated shock bar. The shock 

bar transmits the blow simultaneously to all of the collecting plate in one row because of their

direct contact with the shock bar. A uniform rapping effect is provided for all collecting

plates in one row.

It is of prime importance in any rapping system to avoid excessive re-entrainment of the dust

into the gas stream during the rapping procedure. With the tumbling ,hammer rapping

mechanism the plates are given an acceleration, which causes the collected dust to shear

away from the collecting plates and fall down in large agglomerates. These large agglomerates,

which result from a single shock shearing action greatly reduce the possibility of dust

re-entrainment during rapping.

The rapping frequency should be as low as possible in order to minimize dust losses from

rapping. The frequency of each rapping system is adjustable within a wide range. There is one

set of rapping equipment provided for each bus section so that the frequency can be suited to

the conditions in that individual area.

All internal parts of the rapping mechanism are accessible for inspection, being placed in

wide access passages, before, between and after the collecting fields.

All physical data essential for designing plate suspension eccentricity and rapping intensity

for this type of dust has been tested from full scale tests carried out in laboratory. The

acceleration in any point of a system similar to the one quoted has been determined. When

 judging the effectiveness of the collecting system, it is also essential to keep in mind the total

collecting area being rapped at any one time. The higher the percentage of the total collecting

area being rapped at any time, the greater the re-entrainment of dust into the gas. With theBHEL design of tumbling hammer rapping mechanism, a very small percentage of the

collecting area for each precipitator is treated at one time. This enhances the overall efficiency

of the precipitator and avoids puffing at the stack outlet.

3.7 Insulator Housing

Each electric bus section is supported from four insulators located in insulated compartments.

These compartments are provided with top opening covers to make easy access to the insulators

for inspection and service. There is special tooling arrangement for each insulator compartment,

which makes it possible to suspend the emitting system from a temporary jacking hook if theinsulator must be exchanged.

To keep the insulator temperature above the dew point of the gas, thermostatically controlled

 

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electric heaters are provided in each insulator compartment.

A screen tube is installed immediately below Find in connection with the support insulator. It

prevents fouling of the insulator by dust.

3.8 Rapping System for Emitting Electrodes

During electrostatic precipitation, a fraction of the dust will be collected on the emittingelectrodes and the corona will gradually be suppressed as the dust layer grows. It is therefore

necessary to rap the emitting electrodes occasionally. This rapping is done with a rapping

system employing “Tumbling Hammers” Which are mounted on a horizontal shaft in a

staggered fashion (Figure XVI-6 and 7). These hammers hit specially designed shock beams

to which the intermediate part of the emitting frame of each duct is attached. In this manner

the shock energy generated by the hammer is transmitted to the emitting electrodes.

One rapping mechanism is provided per electrical bus section. The driving arrangement for

the rapping mechanism is located either on the roof or on the side wall of the precipitator. The

operation of the gear motor for the rapping mechanism is controlled by a programme relay,which is adjusted to optimum conditions at the time of commissioning. Subsequent adjustments

can easily be carried out during operation, should operating conditions vary.

4. ELECTRICAL SYSTEM

The precipitator presents a non-linear load characteristics which again fluctates with numerous

variables such as size, velocity and nature of dust particles, temperature in the precipitator,

humidity of the gases etc. For optimum functional efficiency of the precipitator, the supply

voltage should be maintained near about the flash over level between the precipitator electrodes.

This can be achieved by an electronic control system which raises the output voltage to flashover level and reduces it automatically by a small amount in the event of a flash over.

An additional increase in voltage beyond the normal operating zone produces a disproportionate

increase in current accompanied by heavy sparking and a rapid reduction in dust collection

efficiency. Experience has shown that the maximum dust collecting efficiency is related to

the amount of minor sparking that occurs on the electrodes.

Thus the function of effective control system is:

a) To operate the precipitator by a current and voltage that will vary according to the

conditions in the precipitator, maintaining a high efficiency by controlling the spark 

rate.

b) To provide an inherent arc suppression by arranging for the power supply output to

reduce practically to zero for the duration of an arc.

c) To provide back up protection against sustained power arc or persistent low voltage

conditions by means of an under voltage alarm circuit.

d) To indicate when the power supply is inadequate or a power arc is sustained due to fault

condition by means of visual and audio alarms.

e) Provision of manual and automatic circuits.

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The rectifier-control cubicle supplied by BHEL provides all the modern controls besides a

spark rate controller unit which controls a spark rate of 5 to 10 sparks per minute to maintain

optimum dust collection efficiency. The rectifier system provides a smoother control of output

current from 10% to 100% of the rated value and also maintains the constant current output.

4.1. Principle of Operation

4.1.1 Transductor Control

AC input is applied to the main transformer through the load windings of the two transductors

(Figure XVI-8). The control windings are connected in series and the control current is provided

by a thyristor bridge circuit. The thyristor bridge is I driven by pulses generated by an electronic

pulse generator which is in turn controllable by electronic control system. Basically transductor

is employed as a variable impedance device which controls the input applied to the main

transformer. If the main A.C. is applied, with zero excitation to the transductor control winding

the transductor impedance will be high compared to the reflected load impedance on the

primary of the main transformer and the H.V. output from the set will be low. As the controlcurrent is gradually increased, the impedance of the transductor falls, thus enabling, application

of increased A.C. voltage to the main transformer. Control current to the transductor is in turn

under the control of a semi-controlled thyristor bridge. Two controllers are employed in the

system, one manual and the other automatic.

4.1.2 S.C.R. Control

A.C. input is applied to the main high voltage transformer through silicon controlled rectifier

regulator. The regulator supplies controlled A.C. voltage to transformer-rectifierset (Fig.

XVI-9). This A.C. voltage is stepped up by the transformer and rectified by silicondiode

bridge. The thristor regulator along with different control ensures the constantcurrent output

irrespective of the changes in precipitator condition. The thyristor whichhas fast response

and switching characteristics tics offers full control on voltage output.

The transformer rectifier set houses the high voltage transformer, silicon diode bridgerectifier

stock and a choke all immersed in insulating oil.

The electronic controller houses all the power and control circuits. Current feedback is taken

from the secondary of the transformer rectifier. The control system includes a number of 

control cards which senses the feed backs and gives out suitablecontrol signals to the SCR

driver which in turn controls the firing pulses of the mainSCR.

4.2. Auxiliary Control Panels

The auxiliary control panels regulate the operation of rapping motors and heating elements of 

the system pertaining to one pass of the precipitator. Opening and closing of DTPA/DTPB

valve and operation of conveyor system. Each auxiliary control panel consists of relays,

contactors, master controllers, timers, switches, indicatinglamps etc., necessary for the control

of :

a) Heating for the precipitator hopper.

b) Heating for shaft insulators of the rapping mechanism of emitting electrodes.

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c) Heating for the support insulators of the precipitator.

d) Controlling the operation of the rapping motors of collecting and emitting elec-

trodes.

e) Provision of potential free contactors for connection to external annunciation of the tripping of rapping motors.

Healing elements provided on the hoppers ensure free flow of ash from the hoppers by

maintaining the temperature of ash above dew point.

The insulators are also provided with heating elements in order that the insulators are kept

free from condensation. The control circuit -for the operation of the rapping motor is provided

with master controller and timers. The master controller and timer, control the sequence and

frequency of operation of the rapping motors of the collecting and emitting systems of the

different fields of the precipitator. During maintenance schedule the operation of the individualrapping motors can be tested. For this purpose the respective toggle switch in the master

controller should be changed over to the continuous operation position.

4.3 Interlocking System

This system is designed for the safety of the personnel and protection of equipment during the

operation and Maintenance. This system will not operate unless the instructions are followed

sequentially.

The system consists of rotary switches, interlocks and key exchange boxes. The exchange

boxes are located in control room and at prominent places on the precipitator casing.

In ‘the interlocking system, the insulator housings, inspection doors, hopper doors, HV isolating

switches are provided with key interlocks.

Each key designation consists of numbers and letters representing the unit involved, type of 

unit and its location.

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TABLE I

ULTIMATE ANALYSIS OF TYPICAL INDIAN COALS

Singareni Kampte Korba

Moisture 10.00 10.00 5.50

Hydrogen 2.50 3.40 2.40

Carbon 38.00 45.70 37.30

Sulphur 0.50 0.40 0.30

Nitrogen 1.50 0.70 0.80

Oxygen (diff) 7.50 11.80 7.20

Ash 40.00 28.00 46.50

HHV (K.Cal /Kg) 3610 4700 3560

Kg Ash/106 K.Cal 110 60 130

TABLE II

  CHEMICAL COMPOSITION OF COAL ASH FROM INDIAN COALS

Singareni Pench West Kampte Seam

Fe2O3 3.20 7.90 12.50

Sio2

61.01 62.70 59.00

AI20

331.06 24.80 23.00

CaO 0.86 0.88 1.10

MgO 0.13 0.62 0.50

Ti02

2.24 1.48 1.40

P20

50.10 0.11 0.17

Na20 0.16 0.16 0.14

K20 0.79 0.90 0.88

SO3 0.45 0.45 1.31

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TABLE III

TYPICAL FLY ASH ANALYSIS ENTERING THE DUST COLLECTOR

(Pulverised Fuel Bituminous Coal)

Micron Size% Distribution by Weight

< 10 3210 - 20 24

20 - 30 14

30 - 40 10

Over 40 20

TABLE IV

RANGE OF BASIC DESIGN

PARAMETERS IN PRACTICE FOR FLY ASH

PRECIPITATORS

Parameter Range of Values

Duct spacing 200—300 mm

Precipitation rate 1.5-20 cm /sec

Collecting surface per zone 800-3800 m2

Gas velocity 0.75-1.5 m/sec

Length of-duct

Aspect ratio (0.5-1 ..)

Height of duct

Specific collecting area 850-1950 M2 /1000 m3/mt

(50-120 m2 /m3 /sec)

Corona current density 85-250 A /m2

Corona power ` 1765-17650 watts /1000m3 / 

min

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5. CHECK LIST FOR PRE-COMMISSIONING/ 

COMMISSIONING OF ELECTROSTATIC PRECIPITATOR

(For precipitator with integrally housed rectifiers)

Checks before commissioning (starting up) of the precipitator shall include inspection of 

the following:

i) Internals of the precipitator and functional testing of the rapper drives etc.

ii) Insulator chamber, Insulators, disconnecting switch, rectifier connections, func-

tional checking of heaters.

iii) Inspection doors.

iv) Earthing connection.

v) Air load test on precipitators (volt, current characteristics).

vi) Oil level in all driving units and grease in bearings.

vii) Walkways, stairs and ladders.

viii) Rapping frequency of the rapping mechanisms.

ix) Test runs of Mechanical equipment.

5.1.1 Ducting

a) Check all inlet and outlet ductings of precipitator for foreign materials.

b) Check all welded joints for leakage.

5.1.2 Gas Distributor Screen

a) Check for removal of all temporary arrangements.

b) Check for the positioning and locking of deflection plates and throttling

plates on the screen sheets.

c) Check for the minimum distance of 100 mm between the bottom of screen

and the front cable to prevent dust (c) deposits in the inlet duct of E.P.

5.1.3 Collecting System

a) Check for the straightness of Collecting (e) electrodes. The collecting

 electrodes shall be free from undue buckling and twisting etc.

b) Also check that suspension hooks are in their notches.

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c) Check the spacing of collecting plates

d) Check pendulum movement of collecting plates.

e) Check for locking of bolts (beneath the shock bar) of the first and last collecting

plates in each row.

f) Check the shock bar guiding for right position, right pitches, and lockings.

g) Shock bar shall run free and easy. No friction is allowed obstructing the free

movement of the shock bar. Such obstruction may be due to lack of clearance on

the bolt or due to heavy twisting of the collecting plates.

h) Check for alignment of rapper hammers with the shock bar.

i) Check for proper direction of shaft rotation with respect to hammers.

 j) Check the rapper shaft with hammers for free rotation.

k) Check that hammer falls and hits the corresponding shock pad at the required

place-height and sideways

l) Check for the proper erection of bearings so that the hammer is not cutting.

m) Check for proper locking of bolts to the bearing and on the inner arms.

5.1.4 Emitting System

a) Check for the correct position of emitting system-height, length and side ways.

b) Check the collecting plate-to-emitting wire spacing (voltage distance) at top,

bottom and in the middle with use of template.

c) Check emitting wires for proper tensioning spring back.

d) Check that the electrodes are erected in correct attachment not sloping.

e) Check for proper locking of bolts and nuts. Check all welds on the emittingsystem. Check that there are no sharp or protruding weld edges.

f) Check for proper position of screen tube.

  g) Check for foriegn material clinging to emitting wires and collecting plates

 5.1.5 Emitting System Rapping Mechanism

a) Check for proper functionning of rapping machanism (hammer falling and

hitting correctly on the shock beam

d) Check for the proper hand and direction of screw coil. (The screw is meant for

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casing). Check for centering of the screw coil.

e) Check that screw advances outwards.

f) Check alignment of rapper shaft.

g) Check for proper Installation of shaft insulators.

h) Check for the proper direction of shaft rotation with respect to hammers.

i) Check packing boxes etc. for leakage.

5.1.6 Gas Screening

a) Check for the clearance between the wall and the collecting electrodes.

b) Check for the screening, guide plate on the supporting beam of the emittingsystem.

c) Check for screening in the hopper (hopper deflection plates).

5.1.7 Insulator Chambers

a) Check the insulators for cracks etc.

b) Check the insulators both sides for cleanliness, foreign material and position

of high tension hanger rods.

c) Check for the function of heaters and that heating is O.K.

d) Check for the rectifier connection.

e) Check the insulator compartments for debris, foreign materials and dryness.

f) Check the function of disconnecting switch.

g) Check for the tightness and closing of the insulator housing sliding door.

h) Check that the screw legs and nuts provided for the placement of alignment jig

(meant for alignment in case of future replacement of insulator) have been properly

welded around the Insulator base.

5.1.8 Inspection Doors

a) Check all inspection doors and latches to ensure tightness.

b) Check the doors to ensure free swinging-easiness to open and close, noseizingnuts.

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c) Check the gaskets (asbestos rope) on the inspection door for proper tightness.

5.1.9 Earthing System

a) Check that all earthing cables are properly connected.

b) Check that the earthing cables are properly grounded.

5.1.10 Safety Interlock System

a) Before start-up of the unit, all keys must be in master key board.

b) Check all key locks to ensure that the, system is operating.

5.1.11 Driving Unit and Bearing

Check all motors, bearings, gear reducers etc. for proper lubrication. Check for proper oillevel in the gear units, grease in bearings etc.

NOTE :-The plain bearings inside the precipitator for the rapping shafts shall never be greased.

  Walkways, Stairs and Ladders Check for rigidity of floor chequered grill plates,

  stairs, ladders, hand rails, toe plates etc.

5.1.13 Outer Roof 

Check for proper welding of the outer roof panels.

5.2 Part 11 - Inspection -Electrical

Upon confirmation that electric supply to the installation is connected and approved, inspection

of the following shall be carried out:

i) Inspect the control panel and check that all Motor and heater control circuits function

properly.

ii) Check the operation and periods of rest of the rapping mechanisms are in accordance

with instructions issued by the manufacturer.

iii) Arrange that all time relays, bi-metal relays etc. be set properly and that the functioning

of alarm signals be checked

iv) Check that all electric heaters function and set the thermostats.

v) Inspect HV rectifier units with regard to oil level etc. (Refer to instructions from recti-

fier manufacturer).

vi) check all moter and gear motors for proper rotation

vii) Megger ail transformer rectifier units. (Insulation resistance value shall be around 200

magohm).

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proper sequence.

ix) Electrical testing of the precipitator on air load (voltage current characteristics):

a) Before testing the precipitator with high tension observe all safety precautions

(including the precautions furnished in the operation and maintenance manual

and additional precautions as necessitated by the circumstances in the erectionsite). Personnel working around the precipitator shall be informed of the tests

beforehand.

b) Connect one rectifier unit at a time to the corresponding emitting system of the

precipitator.

x) Energise the rectifier (refer to the instruction manual from the rectifier manufacturer).

xi) Record voltage and current value at different settings for each emitting system. If un-

due &parking occurs an internal inspection has to be made to determine the cause of operation.

Usually the cause is:

a) Close electrical clearances.

b) Foreign matter clinging to the wire that has been left in the precipitator or both. It

is important to Measure and record the air temperature when taking air load date.

These readings shall be kept as ‘ reference ‘ for comparison in future.

After the test, observe all safely precautions stated for “ Shutting-down the plant”.

xii) Test run all the rapping motors for one or two days.

NOTE : Wrong direction of rotation on rapping mechanism may give disastrous results.

6. OPERATIONAL PROBLEMS AND TROUBLE SHOOTING

When an electrostatic precipitator fails to achieve its design efficiency, it is necessary to

examine the causes for its poor performances and take corrective action whenever possible.

The number of causes for poor precipitator performance is so large that it is impractical to

exact problem. Diagnosis of the problems and corrective action require a good understanding

of precipitation theory, as well as practical experience in. precipitator operation. The probable

reason may be associated with :

-Boiler overload.

-Excessive air leakage through the ducting.

-Temperature of exhaust gases very high.

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-Inadequate maintenance.

-Changes in gas flow due to dust accumulation.

-Misalignment and warping of electrodes.

-Dust deposits on electrodes not being dislodged effectively.

-Re-entrainment of dust.

Problems include wire snapping, ash buildup in hoppers, rain water leakage, ash disposal

system failure and failure of rapping system.

6.1 Trouble Shooting Techniques

i) For overcoming high resistivity the following can be adopted. Maintain the electrode

systems in such a state of cleanliness that the dust layer is too thin to affect the process.

For this, ensure that rapping system is in good operating condition.

ii) The problem of wire snapping is due to one or more of the following reasons:

a) Misaligned electrode system (i.e.,) emitting electrodes are not central in the

space between the collecting electrodes.

b) Operating of the rectifier unit at a high spark rate so that the wire snaps due

to electrical erosion.

The problem of wire snapping can be overcome by:

Aligning the emitting system frame midway between collecting electrodes within the

acceptable tolerance e.g. for a duct width of 250 mm, the distance between collecting and

emitting electrodes should be 125 + 5 mm and 150 + 5 mm for 300 mm pitch.

iii) The leakage of rain water into the precipitator chamber may be due to :

  a) Bolted and leaky joints on the roof cover.

  b) Roof panels not welded properly.

 iv) Failure of rapping system.

At the time of each inspection, the dust deposits on the collecting plates should be

observed before any cleaning of the precipitator is started. The normal thickness of the

collected fly ash should be about 3 mm thick with occasional build-ups of 6mrn. If the

build-upsexceed this amount the following checks can be done :

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  a) Proper working of the rapping mechanism drive arrangements.

  b) For proper fixing of hammers to the shafts (Bolts and nuts may be checked for

  wear and looseness).

  c) Inadequate rapping.

  The frequency of rapping can be adjusted to maintain minimum thickness of 

  dust deposit.

(v) In case discharge/emitting electrode rapping, failure in addition to the checks

given under class (iv) (a), (b) and (c) the shaft insulator connecting the rapping

shaft with drive mechanism can be checked for breakage.

7. MAJOR OVERHAUL OF ELECTRO STATIC ECIPITATORS

7.1 Objective

The overhaul of the equipment is carried out to have high availability of the equipment. Once

overhaul is carried out it is expected that the equipment runs smoothly without troubles.

Major overhauls are done once in two years of operation of the equipment.

The job of major overhaul has to be carried out in a shortest possible time as during this

period there will be outage of equipment and outage time is to be minimum.

In order to carry out the job efficiently and in a shortest possible time, it is necessary to plan

the work to be done in advance and for this a list of work to be carried out has to be prepared.

The history of the equipment has also to be consulted for to know the problem and attendingthe same during overhaul.

To carry out the above jobs, the manpower requirement has to be worked out and if necessary

contractor has to be fixed Up. The list of tools for carrying out the job and the anticipated list

of spares for the precipitator are kept in hand prior to start of work. As seen from the above all

the above jobs start a few months in advance of the start of the major overhaul.

Let us examine the general list of jobs to be carried out for the precipitator. The jobs can be

divided into two viz. Mechanical and Electrical.

7.2 Mechanical Jobs

a) The collecting electrodes are to be cleaned by water washing.

b) The discharge electrodes are also to be cleaned.

c) Cleaning of the supporting insulators of discharge electrodes over its inner faces has to

be carried.

d) Cleaning of ash collected in the hopper and arranging for disposing of the same that

had, collected in the EP area as otherwise this May cause dust nuisance to the people aswell as to the equipment adjacent to the precipitator.

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The above lour jobs are the first and fore most thing to be carried out as it will facilitate the

further working on the precipitator. Subsequent jobs are :

7.3 Collecting Electrode System

Inspect all the collecting electrode rows and ensure that the electrodes are in line. Wherever

any buckling or twisting of electrodes affecting the clearance between discharge and collectingelectrode are to be attended and if necessary. replacement of collecting plates to be made.

7.4 Discharge Electrode System

a) Check the suspension arrangement of the discharge electrode system and, ensure the

tightness of all the bolts and nuts in this.

b) Clean the supporting insulators and check for any crack and replace if defects are no-

ticed.

c) Inspect the discharge electrode frames for its proper fixing and any bends 7.7 noticed

are to be rectified.

d) Inspect for missing of discharge electrodes and the new electrodes to be put in this

place.

The cause for missing electrodes may be due to wire snapping and this has to be inves-

tigated with reference to its location and any defects noticed have tobe attended to.

e) Loose discharge electrodes noticed have to be replaced with new ones.

f) Check alignment of the discharge electrode and wherever out of alignment. align the

same.

7.5 Rapping System of Discharge Electrode

a) Make a thorough inspection of the rapping system of discharge electrode to ensure that

all the components are intact.

b) Clean all the insulators of discharge electrodes rapping and check for any crack and

replace the insulators,  If found defective.

c) Check all the bearings of the discharge electrode rapping for wear out and replace, if 

necessary.

d) Carry out the overhaul on the motor and gear box of the rapping as prescribed by the

supplier of these equipments. The general procedures for this are :

7.6 Motor

Take out the rotor from the stator and check the bearing for any defect.

-Clean the bearing and renew the grease.

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- Check the HT, LT and the earth connections of the transformer for any change

  in colour due to heat or corrosion etc. and clean the same and reconnect the

  terminals properly.

-Check the functional aspect of thermometer and Bucholz’s relay.

-Check the contacts of the thermometer which has been provided for protecting  purpose.

-Measure the insulation resistance of the windings and record.

  -Conduct magnetising current test on the transformer.

7.11.2 Control Panels

-Blow the control panel for removing the dust.

-Check the contacts of the contractor for any pitting and if necessary replace the same.

-Remove all the ammeters and voltmeters of the panel, calibrate the same and fit back.

-Clean all the contact points of the control cards with CTC.

-Check for proper tightness of all the connecting leads to the components.

-Carry out functional check of the control cards viz. stabilise manual ccntrol card arc

  suppresser, spark rate and kindervoltage.

  -Check the alarm and indicator circuit

7.11.3 Disconnecting Switch

Operate the disconnecting switch sad check for proper contact between the contacts at

both on and off position.

Check the earth connection of the earthing insulators.

Check the insulators and examine for any crack.

7.12 Recommissioning of the Precipitator after Overhaul

a) Energise transformer alone from the panel.

b) Conduct the air load test on the EP zones.

c) Putting EP into service when the boiler goes on coal firing and observe Its per-

formance.

Having known the above jobs the material requirement, tool requirement, manpowerrequirement, etc., can be prepared and major overhaul can be completed successfully.

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FIG. XVI-1 GENERAL ARRANGEMENT OF ELECTROSTATIC

PRECIPITATOR

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FIG. XVI-2 CASING

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FIG. XVI-3 HOPPERS

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FIG. XVI-4 GAS DISTRIBUTION SCREEN

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FIG. XVI-5 COLLECTING SYSTEM SUSPENSION AND RAPPING

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  FIG. XVI-6 EMITTING SYSTEM RAPPING MECHANISM WITH

SIDE DRIVE

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  FIG. XVI-7 EMITTING SYSTEM RAPPING MECHANISM WITH

VERTICAL DRIVE

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FIG. XVI-8 TRANS DUCTOR CONTROL BLOCK SCHEMATIC

DIAGRAM

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FIG. XVI-9 SCHEMATIC DIRGRAM MODERN HV RECTIFIER SET

WITH SCR TYPE AUTOMATIC CONTROL FOR EB