AIR PRE HEATER
FANS
ELECTROSTATIC PRECIPITATOR
BHARAT HEAVY ELECTRICALS LIMITED
RANIPET – 632 406
CONTENTS
EDC - AIR PREHEATERS
An ISO 9001 Company
Sl No
Description
Page No
Contents
1.0
Introduction
01
2.0
Modes of Heat Transfer
01
3.0
Use of APH in Boiler
02
3.1
Heat Energy Saved by APH in 210 MW Boiler
03
4.0
Tubular Air Preheater
03
5.0
Steam Coil Air Heater
04
6.0
Working Principle of Regenerative Air Preheater
04
7.0
RAPH in India by BHEL
04
8.0
Range of RAPH
05
9.0
Designation of RAPH
05
10.0
Detailed description of RAPH
05
11.0
Trisector RAPH
09
12.0
Leakage
10
13.0
Improvements made in Air Preheater Design
11
14.0
Storage & Preservation of Heating Element Baskets
13
15.0
Air Preheater Fires
13
16.0
Recommended lubrication Chart
14
17.0
Trouble Shooting
15
18.0
Modification of Primary Air Opening from 50° to 72° in
Trisector APH
17
19.0
Questionnaire for Enquiry
19
20.0
Cascade Evaporator
20
Sketches
01
Exploded View of Trisector Air Preheater
21
02
Support Bearing Assembly - External
22
03
Support Bearing Assembly - Internal
23
04
Guide Bearing Assembly
24
05
RAPH Sealing System
25
06
Sector Plate Static Seal Arrangements
26
07
Plate Type Adjuster for Sector Plate
27
08
Rotor Drive Assembly
28
09
Cleaning Device - Swivel Joint & Piping
29
10
Cascade Evaporator
30
CONTENTS
EDC - AIR PREHEATERS
A n I SO 9001 Comp an y
AIR PREHEATER
1.0 Introduction
The Air Preheater is defined as a heat exchanger used to transfer heat directly
from combustion gases to combustion air without the use of an intermediate heat
transfer fluid. Air Preheaters are used as important auxiliary equipment in the modern
industrial and power boilers.
1.1 Ty pes of Air P reheaters
Air Preheaters can be classified as Recuperative type (heat exchangers without
storage) and Regenerative types (heat exchangers with storage).
In Recuperative type Air Preheaters two fluids flow at different temperatures in a
space separated by a solid partition. Heat is transferred by Convection & Conduction
through the separating wall.
Example : Tubular Air Preheater, Plate Type Air Preheater & Steam Coil
Air Preheater (SCAPH).
In Regenerative type Air Preheaters one heating surface is exposed at certain
intervals of time first to a hot fluid and then to a cold one. The surface of the
Regenerative Air Preheater first removes heat from the hot fluid and is itself heated in
the process, then the surface gives up this heat to the cold fluid. Thus the process of
heat transfer taking place in a Regenerative Air Preheater is always of a transient
nature, while steady state conditions are typical on the whole of Recuperative Air
Preheaters.
Example : Ljunstrom Air Preheater and Rothemuhle Air Preheater.
Both the regenerators and recuperators are often also referred to as surface type
heat exchangers, because the process of heat transfer in them is invariably linked
with the surface of a solid.
2.0 Modes of heat transfer:
The three modes of heat transfer are conduction, convection, and radiation.
2.1
Conduction:
Heat transfer by Conduction is accomplished via two mechanisms. The first is
that of molecular interaction whereby molecules at relatively higher energy levels
(indicated by their temperature) impart energy to adjacent molecules at lower energy
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levels. This type of transfer will occur in systems where molecules of solid, liquid, or
gas are present and in which a temperature gradient exists.
The second Conduction heat transfer mechanism is via 'free' electrons that are
present primarily in pure metallic solids. The concentration of free electrons varies
considerably for metallic alloys and is very low for non-metals. The ability of solids to
conduct heat varies directly with the free electron concentration, thus we would
expect pure metals to be best heat conductors, and our experience has proven this
to be so.
2.2 Convection:
Convection heat transfer involves energy exchange between a bulk fluid and a
surface or interface. Two kinds of Convective processes exist : (a) Forced
Convection in which motion past a surface is caused by an external agency such as
a pump or fan, and (b) Natural or Free convection in which density changes in the
fluid resulting from the energy exchange cause a natural fluid motion to occur.
2.3 Radiation:
Heat transfer by Radiation requires no medium for propagation. Radiant
exchange between surface is, in fact, maximum when no material occupies the
intervening space. Radiant energy exchange can occur between two surfaces,
between a surface and a gas / participating medium, or it may involve a complex
interaction between several surfaces and intervening fluid constituents. Energy
transfer by radiation is an electromagnetic phenomenon and the exact nature of this
transfer is not known. It is possible, however, to treat this complex subject with
reasonable accuracy.
3.0 Use of APH in a Boiler:
Fuel saving and the ability to burn low grades of fuel more efficiently are the two
principal advantages of using an Air Preheater in a Steam Generator.
Improved combustion leaves very less carbon deposit that normally fouls a
furnace and limits the boiler output. For every 20°C drop in flue gas temperature (by
heat recovery), the boiler efficiency will increase by 1%.
The use of an Air Preheater allows a smaller boiler to produce the same amount
of steam, as that of a larger boiler not equipped with an Air Preheater. For coal fired
furnaces, an Air Preheater system provides hot air for fuel drying.
For steam generators, Air Preheaters
1.
2.
Saves as much as 15% of fuel cost
Preheats air for coal drying
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3.
4.
Enables efficient burning of lower grade fuels
Saves energy by allowing lower excess air operation.
3.1 Heat Energy Saved By APH in a 210 MW Boiler
Q
= m Cp (T 2-T1)
= 950 000 x 0.27 (350-150) Kcal/hr
≈ 51.3 x 106 Kcal/hr
≈ 51.3 x 106 / 3500
= 14657 Kg/hr COAL
≈ 14.657 x Rs 1000/-
= Rs 14 657 per Hour
≈ Rs 14 657 x 8000
= Rs 11,72,56,000/-
≈ Rs 11.7 Crores per year.
4.0 Tubular Air Preheater
The Tubular Air Preheater consists of a number of tubes expanded at each end
into tube plates to form banks. The tubes are enclosed in steel casing which forms
the passage for airflow.
A "C" type Expansion Bellow is provided between the casing and the top tube
sheet to take care of the difference in expansion of tubes and casing. Another "S"
type Expansion Bellow is provided between the Air Preheater block and the Gas inlet
duct, which takes care of the block expansion.
If the tube length is more Middle Tube Sheet will be provided to minimise Tube
Vibration. If necessary, Baffle Plates will be provided along the airflow to minimise
the Acoustic Vibration.
The supporting frame welded to the bottom tube sheet is to be rested on the
supporting beams provided by the customer. Depending on the erosive nature of the
ash in flue gas tube extension with Castable Refractory will be provided at Gas inlet.
Each Air Preheater block will be manufactured in one or more modules
depending upon the transportation limitations. These modules shall be assembled &
seal welded at site.
Generally, Gas flows inside the tubes & Air over the tubes with cross flow
arrangement. Based on the performance requirement Air will have single pass or
multi-pass arrangement.
Guide Vanes shall be provided particularly in the air ducts to ensure even
distribution of air over the cross section of the Air Preheater.
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EDC - AIR PREHEATERS
5.0 Steam Coil Air Heater
Steam coil Air Preheater (SCAPH) is used to protect the Cold End Heating
Elements / components of an Air Preheater from Low Temperature Corrosion. It is
located in the cold air duct between the FD fan and the Air Preheater.
SCAPH is a Finned Tube heat exchanger. The straight finned tubes are welded
to the steam inlet and outlet Headers. Steam passes through the tubes and the air
flows over the fins. The heat is transferred from steam to cold air.
Heated air entering Air Preheater maintains the Average Cold End Temperature
of Air Preheater, well above the Acid Dew Point Temperature. Normally SCAPH will
be in operation during boiler start-up and upto 30% boiler load. Required steam will
be taken from auxiliary boiler / adjacent boiler in operation.
6.0 Working Principle of Regenerative Air Preheater
The Regenerative Air Preheater absorbs waste heat from flue gas and transfers
this heat to the incoming cold air by means of continuously rotating Heat Transfer
Elements of specially formed metal plates. Thousands of these high efficiency
elements are spaced and compactly arranged within 12 / 24 sector shaped
compartments of a radially divided cylindrical shell called the rotor.
The housing surrounding the rotor is provided with duct connections at both the
ends, and is adequately sealed by Radial & Axial Sealing members forming an Air
Passage through one half of the Preheater and Gas Passage through the other.
As the rotor rotates, it slowly rotates the mass of heating elements alternatively
through the air and gas passages. The heat is absorbed by the element surfaces
while passing through the hot gas stream, and then as the same surfaces are carried
through the air stream, they release the stored up heat to the air, thus increasing the
temperature of the incoming air.
7.0 RAPH in India by BHEL
BHEL introduced Ljungstrom Air Preheaters in the seventies by a technical
licensing agreement with M/s CE-APCO, USA. Since then we have supplied over
500 heaters and the capacity ranges upto 500 MW Utility Boilers.
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8.0 Range of RAPH
SIZE
TYPE
ROTOR DIA (Meters)
HEAT DUTY(M.K.cals/hr)
7 - 16.5
K
1.2 - 3.0
2.5 - 60
17 - 18.5
S
3.2 - 3.8
50 - 70
19 - 24
R
4.2 - 6.6
70 - 200
LARGE
6.9 - 16.0
> 200
24.5 - 36
9.0 Designation of RAPH
27
VI
M
T
2000
Element depth in mm
Trisector
Modular Rotor
Vertical Shaft; Inverted gas Flow
Size number
10. 0 Detailed Description of RAPH
Each Regenerative heat exchanger shall consist of the following salient
components / assemblies.
1. Cellular / Modular Rotor
2. Rotor Housing and Connecting Plates
3. Heating Elements
4. Sealing system
5. Support Bearing and Guide Bearing
6. Lubricating Oil circulation system
7. Drive Mechanism including Auxiliary Drive
8. Access Doors
9. Observation Port and Light
10. Cleaning and Washing Devices
11. Rotor Stoppage Alarm
12. Deluge System
13. Element Handling Arrangement
14. Fire Sensing Device
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EDC - AIR PREHEATERS
10.1 Cellular / Modular Rotor
Cellular Rotor : The rotor is made up of 12 sectors. The rotor consists of a
central rotor post and cellular rotor shipped in separate pieces to be assembled at
site for completeness of the rotor. The baskets containing heating elements are to be
installed in the completed rotor at site.
Modular Rotor : The rotor is made up of 12 numbers of full sector modules that
are attached to the rotor post by pinned connections. The modules are loaded with
elements and shipped to site for easier, speedy and quality erection.
10.2 Rotor Housing & Connecting Plates
The Housing is octagonal in shape and consists of two Main Pedestals, two Side
Pedestals, four other Panels and Connecting Plates with integral support beams.
Sandwiched between the top and bottom Connecting Plates are the 8 panels, which
form an integral structure to take axial & radial loads, and also form a gas tight
enclosure for the flow of fluids.
10.3 Heating Elements
Each air heater is provided with multi-layers of heating elements. The Cold End
Elements are basketed for easy removal and replacement from the sides. Hot End
Elements are removable from the top of the gas ducts.
Generally the Hot End & Hot Inter elements are of DU type and Cold end
element is of NF type. The material for Hot End & Hot Inter is Carbon Steel and
material for Cold End is Low Alloy Corrosion Resistance (Corten) Steel.
The Heating elements are rolled on special purpose rolling machine, which is a
one-of-its-kind machine in the country, set up exclusively for this application.
10.4 Sealing System
Over many years of continuous operation, the sealing system has proved to be
effective with minimum maintenance requirements. The design takes advantage of
normal thermal growth to keep sealing surfaces in proper alignment.
The rotor is divided into equal sectors each forming a separate air or gas
passage through the rotor. Fixed Leaf Type Metal Seals are Radially & Axially
attached to the rotor structure between each sector. Sector shaped Unrestrained
Radial Sealing Plates provides the sealing surfaces that divide the rotor into air and
gas passages.
Because the seals are applied to the shortest leakage path and the sealing
surfaces are externally adjustable, the most effective & continuous leakage control is
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EDC - AIR PREHEATERS
assured. A Circumferential Bypass Seal is provided to prevent air & gas from
bypassing the rotor through the small space between the rotor and housing.
The sealing surfaces are adjustable from outside by loosening the lock nuts. But
this adjustment is to be done with care. The Circumferential Bypass Seals can be
adjusted only from inside of the rotor. As these seals control only bypassing of flow
through rotor and the leakage in that path is being controlled by axial seals, there is
no need to adjust them from outside.
The sealing arrangement consists of Radial, Axial, Circumferential, Axial seal
plate to Sector plate, and Rotor past seals designed to minimise leakage between
the Gas & Air sides of the Regenerative Air Preheater.
The Radial seals are located along the edges of the radial division plates and
bear against the Sector plates.
The Axial seals are located axially along the outer edges of the radial division
plates and bear against the axial seal plates.
The Circumferential (or Bypass) seals are located in the housing around the
periphery of the rotor and bear against the T bar attached to the periphery of the
rotor.
The Axial seal Plate to Sector plate seals is attached to the Axial seal plates and
bear against the Sector plates.
The Rotor post seals are attached to the ends of the Rotor Post and bear against
the Sector plates.
10.5 Support Bearing & Guide Bearing
The Support Bearing is of Spherical Roller Thrust Bearing Type and is located at
the bottom Connecting Plate.
The Guide Bearing is of Spherical Roller Type and is located at the top
Connecting Plate.
The Bearing Housings are designed to act as oil reservoirs for provision of
integral oil circulation system.
10.6 Lubricating Oil Circulation System
Both the Support & Guide Bearings are provided with independent oil circulation
systems. The Oil Circulation system consists of Oil Pump, Oil Cooler, Pressure &
Temperature Indicators and Flow Switches.
The lubricating oil system proposed is a proven design. An identical unit is also
connected as standby.
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EDC - AIR PREHEATERS
10.7 Drive Mechanism including Emergency Drive
The drive system envisaged is of peripheral Pin Rack - Pinion Type. It consists of
a two input Speed Reducer with built-in Over-Running Clutch, one Electric Motor for
main drive, an Air Motor which is used as emergency drive, Fluid Coupling / Flexible
Coupling and a Pinion for meshing with the Pin Rack of the rotor.
Normally drive is through the Electric Motor and in the event of electrical trip out,
the Air Motor comes into operation automatically, compressed air being admitted
through solenoid valve. The air line is fitted with necessary Filter Lubricator.
10.8 Access Doors
Adequate numbers of access doors are provided, both at the inlet and outlet
ducts and also in the housing panels for inspection and maintenance.
10.9 Observation Port & Light
Observation Port & Vapour Proof Light are provided. These are suitably located
at the air inlet side to have a complete view of the Cold End Elements while in
operation.
10.10 Cleaning & Washing Devices
10.10.1 Cleaning Device
The heat exchanger is provided with a Twin Nozzle Swiveling Arm type Power
Driven Cleaning Device at gas outlet side, for on load cleaning of air heater
elements.
The cleaning device unit is located on the housing wall with the swiveling arm
nozzle transversing horizontally in an area across the radius of the rotor, a short
distance away from the element packs.
10.10.2 Off-Load Water Washing Device
Two fixed multi-nozzle washing pipes are fitted ; one above and one below the
rotor. Terminal points of the pipes to which surface connection can be given are
located adjacent to rotor housing.
10.11 Rotor Stoppage Alarm
Rotor stoppage alarm is provided to indicate the slowing down of the rotor. This
consists mainly of control unit, vane operated Limit Switch and Vanes, which are
mounted on the Trunnion. If the vanes fail to pass under the Limit Switch within the
set time interval, the timer in the control unit transfers its contact to give an alarm in
the control panel, to warn the operator that the rotor is slowing down.
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10.12
EDC - AIR PREHEATERS
Deluge System
Two fixed multi-nozzle fire fighting manifolds are fitted : one above and one
below the rotor. Terminal points of the pipes to which surface connection can be
given are located adjacent to the rotor housing. During an air heater fire, both fire
fighting and water washing manifolds must be used.
10.13 Element Handling Arrangement
Heat exchanger is provided with a hand-operated Pulley Block with Trolley for
handling of hot end elements from inside of the Air Preheater to the Air Preheater
operating floor.
10.14 Thermocouple for APH Fire Sensing
Shell type Thermocouple elements mounted on the Connecting Plate Center
Sections are arranged (in radial direction) in the air outlet & gas outlet ducts close to
rotor face, such that there is a measuring point between each tangential walls of the
rotor.
The increase in temperature, due to fire, causes a momentarily and recurring
increase of the thermo-electric voltage and the signal released by thermocouple is
given to customer DDCMIS at UCB for suitable alarm / annunciation. In the event of
a fire alarm, the deluge system valves and water wash system valves shall be
opened.
11.0 Trisector RAPH
The trisector design permits a single rotary Regenerative Air Preheater to
perform two functions : Coal Drying and Combustion Air heating.
As the name implies, the Trisector design has three sectors - one for flue gas,
one for primary air (the air which dries the coal) and one for secondary air (the air
which goes to the boiler for combustion).
With this design, if there is a large variation in primary air flow, there is relatively
little effect on heat recovery since heat that is not recovered in the primary section
can be picked up in the secondary section. This is a highly desirable feature, since it
minimises heat losses when alternate fuels are burnt.
11.1 Advantages of trisector Air Preheaters
a) The addition of a primary section to form the trisector design is a practical means
of providing both primary and secondary air from a single Air Preheater.
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EDC - AIR PREHEATERS
b)
The trisector Air Preheater can supply the highest available primary air
temperature because the primary air section is located immediately following the
flue gas section and ahead of the secondary air section in the direction of rotation.
c)
The trisector Air Preheater yields a more economical duct arrangement than
separate primary and secondary Air Preheaters. Trisector heater requires just a
single gas duct common for heating both primary and secondary air.
d) It is flexible in meeting operational changes and is easily adaptable to varying coal
moisture content, which is a highly desirable feature for Indian monsoon
conditions
e) A lower KW per ton of coal pulverized can be realised by the elimination of the hot
air fan and / or exhauster.
f) Less total cost because
-
By combining primary & secondary Air Heating systems in one unit, an
appreciable saving is made in the plant space and structural
requirements
-
Less electrical wiring & controls are required for the trisector Air
Preheater arrangement.
-
One set of Cleaning & Washing equipment including air and / or steam
and water piping is required instead of two or more sets.
-
Fewer expansion joints are required.
-
Less insulation required.
12.0 Leakage
Air heater leakage is inherent in all air - to - gas heat exchangers to varying
degrees. Simply stated, the driving force that causes leakage is the difference in
static pressure levels between the air & gas streams. In addition, the quantity of
leakage is dependent on seal clearance and the length of the seals separating the
two sides. Because of the thermal gradients that are inherent in any heat exchanger,
structural deformation takes place resulting in clearances or gaps between seal and
sealing surfaces.
12.1 Definition of Leakage :
Direct leakage is the quantity of air that passes in to the gas stream between
the seals and sealing surface as a result of the static pressure differential between
the air and gas streams. The amount of leakage across the sealing system is directly
proportional to the square root of the pressure differential and is also dependent on
fluid density.
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Entrained leakage is the quantity that is contained in the rotor as the rotor
passes from the air side to the gas side and from the gas side to the air side. The
quantity of entrained leakage is dependent on the rotor depth, rotor diameter and
rotor speed.
12.2 Effects of Leakage :
Leakage whether direct or entrained has no effect on the heat transfer efficiency
of the regenerative air preheater. There is no difference in the number of KCals
transferred to the air stream from the gas stream because of leakage. However, the
gas temperature leaving the preheater is diluted or decreased by 5 to 10°C by the
mixture of the cooler air with the hotter gas stream.
12.3 % Leakage :
Weight of Air leaking to gas side
% of Leakage = ----------------------------------------------Total weight of Gas entering APH
13.0 Improvements Made in Air Preheater Design
IMPROVEMENTS
ADVANTAGES
A. Rotor
a) Split rotor post converted into Solid
rotor post.
a) Better reliability and Cost reduction.
b) Assembly of trunnions in shop
b) Improved quality.
c) Tubular Support trunnion in place of
Solid trunnion for modular Air
Preheaters.
c) Weight reduction.
d) Modular rotor for size 26.5, 27 for 210
MW & 500 MW primary Air Preheaters
d) Better quality & cycle time reduction
e) Double Seal with 24 sector rotor for 27
& 26.5 sizes.
e) Reduction in Direct Leakage by 20%.
in erection.
B. Elements
a) 0.8 mm DU elements replacing 1.2
mm NF elements in cold end.
a) Improved Air Preheater effectiveness
b) Developed FNC (Flat Notched
Crossed )
b) 12% reduction in pressure drop, 10%
C. Rotor Stoppage Alarm for the
indication of slowing down or stopping
of rotor.
Better reliability & cautioning.
for coal fired boilers.
reduction in weight, 14% reduction in
height.
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IMPROVEMENTS
ADVANTAGES
D. Seals
a) Thicker Rotor post seals 6 mm instead a) Better sealing & longer life compared
of 1.6 mm
b) Corten A - 2.5 mm thick Radial seals
to Ceramic coated or Stellite coated
seals of lower thickness.
b) Longer life when compared to other
materials like XAR - 30, Stellite
caoted & Liquid Nitrided.
c) Three layers of Bypass seal at the
c) Less Erosion.
leading edge of the sector plate in gas
side
E. Guide Bearing
a) Pressure testing of trunnion with
a) Better reliability.
adapter sleeve at shop.
b) Seal air connection in Guide bearing
b) Better sealing & less erosion.
assembly and pinion air seal.
F. Support Bearings
a) Introduction of adopter plate in the
a) Easy erection at site.
bearing assembly
b) Fabricated bearing housing in place of
casting.
c) Spherical roller thrust bearing in place
of Kingsbury bearing.
b) Facilitates internal mounting of
bearing assembly in the centre
section. Weight reduction.
c) Easy maintenance and cost
reduction.
G. Connecting Plate Assy
a) Increasing PA sector from 50° to 72°
in second generation Air Preheaters
from 1980 onwards.
b) Roller supported inboard end of hot
end sector plates. 360° static spool &
modified tracking arrangement with
kao-wool packing at the inboard of hot
end.
c) Modified Static seals on both sides of
a) Less pressure drop across PA
sector. Less erosion in Air Heater
internals, seals and hot PA duct.
b) Leakage area eliminated. Erosion of
lug plate eliminated. Hot air leakage
near guide bearing reduced to nil.
c) Better sealing. Less erosion.
hot end sector plates.
d) Plate type adjusters in hot end
d) Better quality. Easy erection.
H. Thermocouples for APH fire sensing
Air heater fire can be detected at an
early stage.
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14.0 Storage & Preservation of Heating Element Baskets
It is preferred that element baskets be stored inside if space is available.
However, outdoor storage can be achieved with proper protection. The crates or
boxes as shipped shall be opened and the elements should be sprayed with the
recommended oil given below.
If rust scales are noticed, the baskets can be cleaned first with a jet of
compressed air (6 kg/cm2 (g) ; a nozzle can be attached to the hose to be effective)
and then oil is applied over the rust which will absorb the oil and deter further rusting.
Baskets after coating with oil shall be supported on timbers high enough to be
free from the defects of surface water. Timbers ( 100 mm x 100 mm ) or other means
should be placed on top of the basket piles. Protective covering should be applied
over the piles and extending down the sides and securely fastened to prevent
deterioration. It will be necessary to periodically check the covering for deterioration
and the baskets for rusting. If rusting is observed, the areas should be re-sprayed
with rust inhibiting oil.
Anti rust oils for use on heating surfaces :
v RP 102 of Indian Oil Corporation
v KOTE 203 or RUSTOP 173 of Hindustan Petroleum
v RUSTROL 152 of Bharat Petroleum
v RUSGARD P-214 of Plastipeel Chemicals & Plastic Ltd, Thane.
Note : A pressure pump with garden type spray can of 5 to 10 litres capacity is
suitable to apply anti-rust oils with good penetration. In the above oils, the 'Carrier' oil
is volatile, and it usually evaporates in several weeks under ambient conditions,
leaving a protective coating. This coating is water soluble and for this reason the
elements thus coated must be stored indoors or under protective covering at
outdoors. The elements need not be cleaned with water at the time of light up, if the
protective coating was applied before 3 months.
15.0 Air Preheater Fires
Air preheater fires are rare. A fire may occur during cold start up on oil or start-up
following hot stand-by because of poor combustion of the fuel. The improper
combustion results in unburned or partially burnt oil condensing and depositing on
the Air preheater element surface. As the gas temperature entering the Air preheater
increases, this deposit is baked to a hard varnish-like material.
These deposits can ignite as temperature increases to 315-370°C range. This
ignition usually starts in a small area of the deposit. During the early stages of
deposit ignition, external effects are not very apparent. The deposit restricts the flow
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of gas or air so that very little of the heat generated is carried away from the area of
its origin. Downstream mixing of the fluids further minimises any external effect. Most
of the heat generated is absorbed by the metal heat transfer element nearby. The
actual temperature build up during this period is relatively slow. If the condition can
be detected at that time, the amount of water required to reduce the temperatures
quickly to below the ignition temperature is much less.
If this ignited deposit remains undetected it will continue to generate heat until
the metal heat transfer element reaches 730-765°C. At this point, metal may ignite
with temperatures reaching 1650°C & higher, in a matter of minutes.
Metal fires are self-sustaining and would require more water than is normally
available to drop the temperature to a reasonable level. It should be noted that CO2 ,
halon and other extinguishing agents are ineffective under these circumstances.
To detect the air heater fire in the early stages, we have envisaged fire detecting
thermocouples strategically located in the hot end and cold end connecting plates,
which will continuously monitor the abnormal increase in temperatures of gas leaving
and air leaving the APH. The milli-volt signals from these thermocouples are taken to
and processed in DDCMIS to generate APH fire alarm. In the event of a fire alarm,
the operator should open the valves in the deluge pipes.
16.0 Recommended Lubrication Chart
Sl
Equipment
Description
01
HPC
BP
Qty/
APH
Fre. Of
Change
Main Drive Elec. Servogem 3
Motor Brgs.
Lithon 3
MP Grease 3
0.4 kg
6 month
02
Fluid Coupling
Servo Prime 46
Turbinal 46
Bharatturbol 46
4.5 lts
yearly
03
Main drive
reducer
a) Gear case
Servomesh SP
220
Parathan EP
220
Amocam 220
90 lts
6 month
b) Bearings
Servogem 3
Lithon 3
Bharat MP 3
0.3 kg
6 month
a) Couplings
Servogem 3
Lithon 3
MP grease 3
1 kg
yearly
b) Gear case
Gear Oil SAE 90
0.5 lt
6 month
c) Bearings
Servogem 3
04
IOC
Air motor :
d) Compressed Servo Sys 32
air line
-
-
Lithon 3
MP grease 3
0.25 kg yearly
Enklo 32
Actuma T Oil 10w 0.5 lt
regular
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CONTENTS
EDC - AIR PREHEATERS
A n I SO 9001 Comp an y
Sl
Equipment
Description
IOC
HPC
05
Support Bearing
Servocyl C-680
06
Guide Bearing
07
Lub Oil Pump &
Motor bearings
08
Cleaning Device.
Qty/
APH
Fre. Of
Change
Cyndol TC-680 Bharat Engol
J - 680
90 lts
6 month
Servocyl C-680
Cyndol TC-680 Bharat Engol
J - 680
25 lts
6 month
Servogem 3
Lithon 3
MP Grease 3
0.5 kg
6 month
a) Motor
bearings &
worm gear
reducer
Servogem 3
Lithon 3
MP Grease 3
2 kg
6 month
b) Two stage
worm gear
box
Servomesh SP
460
Gear Oil ST
140
Spirol 140 EP
1 lt
6 month
c) Sleeve
Bearing
Servogem HT
XX
0.2 kg
6 month
---
BP
---
Note : Check Oil Level after Stopping of Lub Oil Systems.
Caution : Over filling of Oil should be avoided. Over Filling of Oil in Guide Bearing
Assembly may Cause Air Preheater Fire.
17.0 Trouble Shooting
This section contains trouble-shooting procedures that will assist maintenance
personnel. Each fault that is likely to occur is listed along with suggested remedies.
When any fault occurs, which does not have a logical remedy, the manufacturer
should be consulted.
Sl
Fault
Probable Causes
Remedy
01
Excessive leak
between gas & air side
Leaking seals
Inspect all seals and adjust or
replace if necessary.
02
Excessive noise from
rotor
i) Warped rotor due to
excessive expansion.
i) Reduce boiler load or admit
more cold air.
ii) Foreign object in rotor.
ii) Stop rotor & visually inspect it
iii) Bad bearings
iii) Replace bearings.
03
Circulating oil foamy
Air leak into the oil system Check for an air leak on the
suction side of the oil system.
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CONTENTS
EDC - AIR PREHEATERS
A n I SO 9001 Comp an y
Sl
Fault
Probable Causes
Remedy
04
Rotor does not turn
i) Defective drive unit
i a) Ensure power is available to
drive unit
i b) Check pinion gear and pin
rack for binding.
ii) Foreign object in rotor
ii) Inspect rotor and remove any
foreign object.
05
Noisy Bearings
Inadequate lubrication
Check that oil is flowing through
the sight glass and check oil line
up, if necessary.
06
Drive unit overheats
i) Inadequate lubrication in
drive unit.
i) Lubricate drive unit
components
ii) Pinion gear and pin rack ii) Adjust drive unit to proper
meshing too hard due to
clearances.
expansion
07
Oil temperature too
high
iii) Hard rubbing seals
iii) Adjust seals *
iv) Bad bearings in gear
reducer.
iv) Change the bearings.
i) Insufficient cooling
i) Check that adequate water
flow is going into oil cooler
ii) Improper oil viscosity
being used.
ii) Replace oil with proper type
of oil.
iii) Absorption of radiation
heat due to improper
insulation.
iii) Insulate properly.
08.
Low oil level
Leak in oil system
Locate and arrest the leak.
09.
Soot blower not
functioning
i) Power failure
i) Check electrical connection.
ii) Swivel header travel
incorrect.
ii a) Check the speed reducer
moveable parts for binding.
ii b) Adjust travel of nozzle pipe.
iii) Incorrect steam
pressure
iii a) Ensure adequate steam
supply is available.
iii b) Check nozzle pipe for bad
nozzles.
* Slight overheating due to hard rubbing seals may be relieved when the Air preheater
reaches normal running temperature or after the seals have worn in properly.
Pag e 1 6 o f 3 0
CONTENTS
EDC - AIR PREHEATERS
A n I SO 9001 Comp an y
Sl
Fault
Probable Causes
Remedy
10.
Abnormal increase in
the temperatures
Starting up of fire inside
the Air preheater
Immediately isolate air heater
and check for fire and admit
large quantity of water. Refer
section 'Air preheater Fires' in
this manual. CO2 foam, steam
should not be used.
11.
Increase in gas outlet
temperature and steady
fall of air outlet
temperatures.
i) Air preheater stalled.
i) Isolate Air pre heater and
check for troubles in the drive
system.
ii) Heating elements
plugged
ii) Clean heating elements.
12.
Excessive pressure
build up by Lub oil
pump.
Oil filter choked.
Clean or replace the cartridge.
13.
Variation (periodic kick)
in the main drive
ammeter current
i) Excessive rubbing of
some seals.
i) Locate faulty seals and
replace.
ii) Faulty bearings
ii) Rectify or replace the
bearings.
Excessive pressure
drop
i) Badly fouled Air
Preheater / Excessive
plugging
i a) Operate soot blower &
check soot blower steam for
proper pressure & temperature.
14.
i b) If soot blower is not
effective, heater can be isolated
and water washed off load
i c) in worst case, remove the
basket and clean them outside.
ii) Thinned down and
damaged elements
ii) Rectify & replace the element
baskets.
18.0 Modification of Primary Air Opening from 50° to 72° in
Trisector APH
Earlier we have provided 50° primary air opening in Trisector Air Preheaters
based on the quality of coal (4000 to 4500 Kcals/Kg) prevailed at that time. But the
coal available today is of lower quality (3000 to 3500 Kcals/Kg) which requires more
quantity of coal to be fired. This necessitates supply of more quantity of primary air to
mills. Increased primary air flow results in higher-pressure drop across the air
preheater leading to higher loading on PA Fans.
Pag e 1 7 o f 3 0
CONTENTS
EDC - AIR PREHEATERS
A n I SO 9001 Comp an y
In order to overcome the above problem, we have increased the PA opening
from 50° to 72° in our Air Preheaters supplied after 1982. We have also modified the
PA opening from 50° to 72° in the Trisector APHs supplied earlier in the following
projects.
13.1
Project
Capacity
Status
Ø
Singrauli # 1, 2 & 3
3 x 200 MW
Executed
Ø
Badarpur # 4 & 5
2 x 200 MW
Executed
Ø
Obra # 9 to 13
5 x 200 MW
Executed
Ø
Tuticorin # 2
1 x 210 MW
Executed
Ø
Nasik # 3
1 x 210 MW
(Under Execution)
Ø
Parli # 3
1 x 210 MW
(Under Execution)
Ø
Talcher # 5
1 x 110 MW
(Under Execution)
Ø
Kota # 1 & 2
2 x 110 MW
(under proposal)
Ø
Tuticorin # 1
1 x 210 MW
(under proposal)
Ø
Vijayawada # 1 & 2
2 x 210 MW
(under proposal)
Modifications Required for Conversion of PA Opening from 50° to 72°
v Replacement of primary centre section at hot and cold ends including sector
plates.
v Replacement of Air side ducts of connecting plate assembly at hot and cold ends.
v Replacement of Air side housing panels.
v Replacement of hot end sector plates with roller supported type sector plates
v Replacement of stationary spool assembly at hot and cold end.
v Modification in guide bearing assembly and supply of modified tracking
arrangement for hot end sector plates.
v Supply of thicker rotor post seal with back-up ring.
v Modification of Air inlet & outlet ducts.
v Modification of dampers.
v Replacement of air side expansion bellows at the inlet and outlet of air preheater.
v Relocation of support beam on air side side-pedestal.
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CONTENTS
EDC - AIR PREHEATERS
A n I SO 9001 Comp an y
13.2 Advantages
v
Reduced primary air pressure drop across air preheater.
v
Reduced PA fan loading.
v
Reduction in auxiliary power consumption.
v
Reduced erosion in APH parts due to reduced primary air velocity.
v
Improvement in mill air temperature.
19.0 Questionnaire for Enquiry
01. Name of Project & Location
:
02. Name & Address of the Customer
:
03. Application
:
04. Number of APH required
:
05. Fuel
:
06. % of Sulphur in Fuel
:
07. Gas flow direction
: Horizontal / Vertical
08. Flows
Kg/hr
a) Air leaving APH
:
b) Gas Entering APH
:
°C
09. Temperatures
a) Air entering APH
:
b) Gas entering APH
:
c) Desired Gas leaving (uncorrected) :
10. Pressure
mmWC
a) Gas pressure entering APH
:
b) Air pressure leaving APH
:
c) Desired Air side / Gas side / Total
pressure drop
:
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CONTENTS
EDC - AIR PREHEATERS
A n I SO 9001 Comp an y
20.0 Cascade Evaporator
BHEL also manufactures cascade evaporators, which is a direct contact type
heat exchanger, exclusively used in chemical recovery boiler used in paper industry.
Cascade Evaporator is an auxiliary equipment for Chemical recovery boiler. This
is a direct contact type heat exchanger used to evaporate the water content of weak
black liquor. Concentrated black liquor will be used as a fuel in the boiler.
Cascade evaporator consists of one or two cylindrical wheels. Each wheel is
made up of parallel tubes that are plugged and welded in to two round tube plates in
the concentric circular pattern. The wheel assembly is supported on two water-cooled
bearings. These bearings are cast iron pillow blocks having bronze liners in the
bottom half only.
The wheel assembly is rotated by a drive system consist of chain and sprockets.
As the wheel rotates, the tube bank dip in to a bath of liquor maintained in the lower
housing and carries the liquor on its surface to expose it to the hot gases which
passes through the tube bank in the upper housing. Evaporation occurs at rapid rate
because of the turbulent gas flow established by the tube bank. Rotational speed of
the wheel is such that the tubes always remain wet.
---o0o---
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CONTENTS
A n I SO 9001 Comp an y
EDC - AIR PREHEATERS
Exploded View of Trisector Air Preheater
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CONTENTS
A n I SO 9001 Comp an y
EDC - AIR PREHEATERS
Support Bearing Assembly - External
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CONTENTS
A n I SO 9001 Comp an y
EDC - AIR PREHEATERS
Support Bearing Assembly - Internal
Pag e 2 3 o f 3 0
CONTENTS
A n I SO 9001 Comp an y
EDC - AIR PREHEATERS
Guide Bearing Assembly
Pag e 2 4 o f 3 0
CONTENTS
EDC - AIR PREHEATERS
A n I SO 9001 Comp an y
RAPH Sealing System
Pag e 2 5 o f 3 0
CONTENTS
A n I SO 9001 Comp an y
EDC - AIR PREHEATERS
Hot End Static Seal Arrangement
Cold End Static Seal Arrangement
Pag e 2 6 o f 3 0
CONTENTS
A n I SO 9001 Comp an y
EDC - AIR PREHEATERS
Plate Type Adjuster for Sector Plate
Pag e 2 7 o f 3 0
CONTENTS
EDC - AIR PREHEATERS
A n I SO 9001 Comp an y
Rotor Drive Assembly
Pag e 2 8 o f 3 0
CONTENTS
A n I SO 9001 Comp an y
EDC - AIR PREHEATERS
Cleaning Device - Swivel Joint & Piping
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CONTENTS
EDC - AIR PREHEATERS
A n I SO 9001 Comp an y
Cascade Evaporator
Pag e 3 0 o f 3 0
FANS
TRAINING PROGRAMME
COURSE MATERIAL
BHARAT HEAVY ELECTRICALS LIMITED
BOILER AUXILIARIES PLANT
RANIPET – 632 406, TAMIL NADU
CONTENTS
DESCRIPTION
GENERAL DESCRIPTION ABOUT FANS
FAN DESIGNATION
CLASSIFICATION OF FANS AGAINST APPLICATION
FAN PARAMETERS
SELECTION OF FANS
FLOW CONTROLS
ACCESSORIES
STORAGE & PRESERVATION OF FAN COMPONENTS
CONSTRUCTIONAL FEATURES OF AP FANS
PRE-COMMISSIONING CHECKS OF AP FANS
START PERMISSIVE, ALARM & TRIP VALUES (AP FANS)
PARALLEL OPERATION OF AP FANS
CONSTRUCTIONAL FEATURES OF AN FANS
PRE-COMMISSIONING CHECKS OF AN FANS
START PERMISSIVE, ALARM & TRIP VALUES (AN FANS)
CONSTRUCTIONAL FEATURES OF RADIAL FANS
PRE-COMMISSIONING CHECKS OF RADIAL FANS
START PERMISSIVE, ALARM & TRIP VALUES (RADIAL FANS)
VIBRATION – GENERAL DESCRIPTION
VIBRATION IDENTIFICATION TABLE
LIMITS OF VIBRATION
BALANCING
TYPES OF UNBALANCE
SINGLE PLANE BALANCING
FAN TESTING FACILITIES
GENERAL DESCRIPTION ABOUT FANS
'Fan' is one of the many types of turbo machines used for energy transfer. It can be
defined as a rotating machine with a bladed impeller, which maintains a continuous
flow of air or gas. It is continuous because the flow at entry and exit and through
the impeller is steady.
PRINCIPLE OF WORKING:
It is possible that energy transfer can be from the machine to the flowing fluid or
vice-versa. Fans, Blowers, Compressors, Pumps etc., fall under
one category
where energy transfer occurs from the Machine to the Fluid, i.e. Mechanical Energy
is converted to Fluid Energy.
The principal distinction between Fans, Blowers,
Compressors, and Pumps is that the Pump handles liquid, whereas the others handle
air or gas.
Turbines fall under another category, where energy transfer is from the fluid to the
Machine.
In other words, the first category (i.e. Fans) consume power as they
rotate with the help of prime mover and energizes the flowing fluid whereas
turbines rotate due to the fluid energy imparted to it and helps in generating power.
CLASSIFICATION OF FANS
Fans may be classified into two major types - Axial Flow and Radial Flow.
AXIAL FLOW FANS:
In axial flow fans the main flow is parallel to the axis of rotation of the fan both at
entry and exit. Axial fans may be classified further into Impulse Type and Reaction
Type fans.
CONTENTS
-2-
Axial Fan
Impulse Type
Reaction Type
Single Stage
Double Stage
REACTION FAN:
In the reaction type of Axial Fans, most of the energy coming out of the impeller is
in the form of Pressure Energy. It is known that Degree of Reaction.
It is expressed as
R=
Static Pressure Rise Across the impeller.
Total Pressure Rise
The pressure rise for an individual fan can be increased multifold by arranging two
or more impellers in series in the same housing depending upon the requirement.
This is called staging. In reaction fans, so far two stages have been given by BHEL.
Suppose the total pressure rise required is `P’ then individual impeller is designed
to develop P/2, whereas the flow rate remains the same for both the impellers.
CONTENTS
-3-
IMPULSE FAN:
In the impulse type fans, most of the energy coming out of the impeller is Kinetic
Energy. It is converted into Pressure Energy in the Outlet Blades and the diffuser.
Hence, these fans are called Impulse Fans.
CONTENTS
-4RADIAL FANS:
Radial Fan
FORWARD CURVED
RADIAL BLADED
BACKWARD CURVED
Based on the configuration of the blade with respect to the direction of rotation of
the impeller it is called Backward Curved, Forward Curved and Radial Bladed
impeller. For better understanding it can be mentioned that the blade angle at exit
is less than 90', equal to 90' and greater than 90' in B.C. Bladed, Radial Bladed and
F.C. Bladed Impellers respectively.
Backward curved impellers are the best efficient among the
mostly used.
three and hence
Forward Curved impellers have the overloading characteristic and
are more power consuming. Because of the self cleaning characteristics of the
Forward Curved Bladed impeller they are used in some ID application of
recovery bollers.
.
CONTENTS
-5-
FAN DESIGNATION:
BHEL is manufacturing fans of both Axial and Radial type. The designations
used for different fans are as follows:-
RADIAL FAN:
NDV
22
T
Type of Wheel.
Nominal tip dia of impeller in decimeter.
Radial single suction, simply supported impeller.
NDZV
47
S
Type of Wheel.
Nominal tip dia of impeller in decimeter.
Radial Double suction-Simply supported impeller.
CONTENTS
-6-
AXIAL FAN
AN
25
e6
Type of Diffuser.
Nominal tip dia of impeller in decimeter.
Axial Non-profiled bladed Impeller (constant thickness).
A P
2
20
12
Nominal tip dia of HUB in decimeter.
Nominal tip dia of impeller in decimeter.
Number of Stages
Axial Profile bladed Impeller (Aerofoil).
CONTENTS
-7-
CLASSIFICATION OF FANS AGAINST APPLICATION:
FORCED DRAFT FAN
This is used for supplying secondary air to the furnace for combustion.
INDUCED DRAFT FAN
This sucks the product of combustion from the furnace through the system and
delivers to the chimney.
PRIMARY AIR FAN
This supplies air to carry the fuel from the mills to the burners.
GAS RECIRCULATION FANS
This sucks the flue gas from the Boiler Second pass and delivers it to the bottom of
the furnace helping steam temperature control when the boiler is an oil fired one.
CONTENTS
-8-
PARAMETERS FOR FANS:
The various parameters of fans are as follows:
1. Volume (flow rate)
Q cu.m/s
2. Differential pressure
H mmwc
3. Temperature
T deg C
4. Density of medium
kgf/cu.m
5. Medium handled
Fresh air/ Flue gas etc.,
SELECTION OF FANS:
It is not possible to give fresh design for every contract. Already designs have been
standardized for various sizes.
One will have to select the standard size for the
given operating parameters.
Fan parameters are fed to fans designs department by Boiler Performance and
Proposals department for each and every contract. With the use of standard charts
and tables the fans are sized. Mechanical limits are also checked for the selected
fans are sent for approval to customer/consultant if any deviation is taken from
their specifications.
The types of fans for the above mentioned applications are decided after considering
many factors like:
(a) Customer/Consultant requirement
(b) Standardization
(c) Optimum Design
(d) Layout restrictions (if fans are given for old boilers)
(e) Field of application (either industrial or power plants etc.)
For 110 MW units axial fans will be recommended for FD and ID applications. PA
fans will be of radial type. For
FD and
ID it is preferable to give axial fans
because of lower weight, better efficiency and hence lower power consumption,
smaller size, smaller foundations, easy maintenance etc.,
CONTENTS
-9For 200 & 210 MW units, it is recommended as below:
FD
Axial Reaction Type (AP)
ID
Axial impulse Type (AN) or
Radial double suction (NDZV)
PA
Single Suction Radial (NDV)
For 250 mw & 500 MW units the fans given are
FD
Axial Reaction Type (single stage)
ID
Radial Double Suction
PA
Axial Reaction Type (double stage)
For all boilers, if there is to be GR fan it is always radial because of the system
resistance characteristic of GR fans.
FAN PARAMETERS FOR A TYPICAL 250 MW & 500MW BOILER
250 MW
500 MW
CONTENTS
Q
∆p
γ
M3/S
Kg/m2
Kg/m3
FD FAN
105
528
1.0786
PA FAN
83
1322
ID FAN
250
n
P
RPM
KW
45
1480
700
1.0786
45
1480
1400
487
0.7995
154
740
1750
Q
∆p
γ
n
P
M3/S
Kg/m2
Kg/m3
RPM
KW
FD FAN
248
360
1.0503
50
980
1150
PA FAN
185
1200
1.0507
50
1480
2775
ID FAN
558
485
0.8
150
580
3650
T°c
T°c
- 10 -
FLOW CONTROLS
Different types of controls employed for fans can be listed as:
(1) Damper control
(2) Inlet guide vane control
(3) Speed Control
(4) Blade pitch control
DAMPER CONTROL
This is least efficient of all the controls. It is actuated by a power cylinder or
electrical servo motor.
INLET GUIDE VANE CONTROL
This control is used invariably in Axial impulse type (AN) fans and radial fans. This
is more efficient than damper control.
SPEED CONTROL
This is achieved either by a variable frequency drive or hydraulic coupling.
As
Q is proportional to N2D2
H is proportional to ND 3
(Where Q=Volume, H=Pressure, N=Speed and D=Dia of fan)
BLADE PITCH CONTROL
This is the most efficient of all the controls. The impeller blades are tilted during
operation and hence
the angle
of
entry
is
varied
to
vary the performance.
The hydraulic servomotor helps in achieving the control with the help of an external
oil system.
CONTENTS
- 11 -
ACCESSORIES:
For different applications some accessories have to be supplied for the fans. They
may be classified as:
1.Oil System
2.Silencer
3.Slow Turning Mechanism and
4. Air Filter
OIL SYSTEM
External Forced oil system are given for AP fans and radial fans used in
PA,ID,and GR application.
In AP fans the system is used both for Blade Pitch Control and Lubrication. In
Radial fans the (PA,ID & GR) the system is used for lubrication purpose.
The system as a whole is a compact unit with necessary pumps, motors, coolers,
filters and
suitable instruments.
Generally, the recommended oil used for lubricating the bearing is ISO VG 68
SILENCER
Inlet silencers are given for FD and PA fans where the noise level is exceeding the
recommended value as per International standards. For each size and type of fan
silencers are designed.
SLOW TURNING MECHANISM
For fans operating at very high temperatures (more then 300
turning mechanism is provided to rotate
to 350'C) a slow
the rotor slowly in case there is total
failure of AC supply or even during a shut down. This is to avoid sagging of the
hot shaft.
AIR FILTER
Where the environment is dusty, air filters are provided at the fan suction to filter
the dusty particles. Clean air will be delivered to the system
CONTENTS
- 12 -
STORAGE PRESERVATION OF FAN COMPONENTS AT SITE:
STORAGE :
Due attention given towards storage of fan components shall pay rich dividends. Fan parts
comprises of many components are expected to perform certain specific functions. They
require due care and attention from the time they are received at the site. Proper awareness
in this sphere has averted costly repairs and delay in commissioning of projects.
MINIMUM GENERAL STORAGE REQUIREMENTS :
SL.NO.
1.
2.
3.
4.
5.
6.
CONTENTS
FAN COMPONENT
STORAGE LOCATIONS
FREQUENCY
INSPECTION (d)
Suction Chamber
Outdoor (a)
Monthly
Diffuser assembly
Outdoor (a)
Monthly
Silencer
Weather
Protected (b)
Monthly
OGV Assembly
Weather
Protected (b)
Monthly
Rotor @
Indoor (c)
Monthly
Rigiflex coupling
Indoor (c)
Monthly
OF
- 13 SL.NO.
FAN COMPONENT
STORAGE LOCATIONS
FREQUENCY
INSPECTION (d)
7.
Fan shaft
Indoor
Monthly
8.
Fan Impeller
Indoor
Monthly
9.
Fan Bearings
Indoor
Monthly
10.
Primary packers &
shims
Indoor (c)
11.
Instruments
Indoor(c)
Monthly
12.
Lub oil system
Indoor(c)
Monthly
13.
Motor
Indoor(c)
Monthly
OF
Monthly
NOTE:
(a)
(b)
(c)
(d)
@
Above ground, on blocks, exposed to weather.
Out door, above ground, on blocks covered With Tarpaulins and
vented for air circulation
Clean and dry warehouse.
Inspect the components at the given frequency and re preserve it
suitably.
Specific attention should be given to ENSURE that the ROTOR is
stored inside the covered shed and preserved properly. The AP fan rotor
is assembled in our works in a AIR CONDITIONED HALL.
CONTENTS
- 14 -
RECOMMENDED PRESERVATIVE COMPONENTS :
APPLICATION
BRAND NAME
Machined surfaces ( Indoor storage )
RP 102 - Rust preventive Oil
Rust Guard - P226 / P214 / P230
CN19 / CN22, Cantopeel
Machined surfaces ( Outdoor storage )
TRP
Weldments
De-oxy Aluminates
Bearings
RP 102 – Rust Guard P214
PRESERVATION TILL ERECTION :
Disassembled fan parts are to be stored in their ex-works packing. Welded plate parts are to
be protected against the influence of weather under the covers of tarpaulin and on square
timber protected against splash water and rain. These parts should not be piled up. They
must be stored piece by piece so that no deformation can occur.
The areas where paint is suspected to be giving way to the atmospheric action must be
repainted after cleaning with fine emery paper.
Parts packed in boxes are to be stored in covered sheds for protection against mechanical,
chemical damage and contamination. These parts are to be unpacked in the sequence of
their erection just before they are installed. Wherever possible, these parts should be
transported to site as per erection sequence in packed condition.
The active and effective life of protective media has its own life. In such cases, the
protected items may need periodic re-inspection and reapplication of the protective. To
ensure perfect safety of the equipment the instruction given in this regard must be strictly
followed.
CONTENTS
- 15 The shaft with bearing assembly is filled with preservative oil and despatched from our
works. The shaft should be hand rotated for few turns once in a week in order to lubricate
the upper part of the bearings and avoid pitting of bearings. If by any chance, necessity
arises to preserve the bearing assembly for a longer period, the preservative oil should be
drained and refilled with fresh oil once in a year.
PRESERVATION OF FAN UPTO COMMISSIONING :
After erection of fan at site, conservation must be ensured upto commissioning and during
trial operation. All bright surfaces must be sprayed with anticorrosive agents/rust
preventives.
PRESERVATION OF FANS DURING LONG SHUT DOWN :
No special preservation is required for a shut down period up to 2 weeks.
Since the maximum effective life of rust preventive oil/ anti corrosive agents, when applied,
has its own life (3 months), re preservation has to be carried out after 3 months.
The following procedure shall be adopted for long shut downs / stand still
period.
•
Rotate the rotor by hand for few revolutions ONCE IN TWO DAYS and set
rest at a new position 90 Deg away from the original position. This is done to avoid
permanent set of the rotor as well as pitting of shaft and elements of bearings.
•
Blow the rotor with compressed air ONCE IN TEN DAYS to remove hard
deposits on the blades.
•
ONCE IN TEN DAYS, run the lub oil pump and operate the servomotor from
blades full open to blades full close several times.
•
Run the fan ONCE IN FIFTEEN DAYS for at least half an hour. After each
running, repeat the above steps.
PROCEDURE FOR PRESERVATION OF MACHINED SURFACES OF FAN
SHAFTS AT SITE :
CONTENTS
- 16 Inspect the machined surfaces of the Fans shafts immediately on receipt at site.
Clean the machined surfaces if required by using kerosene or mineral turpentine. Exposed
rust to be removed by rust removing solution (Phosphoric acid 10%).
Re-preserve by applying Candopeel strippable coating clear by brushing.
Tarpaulin and wooden pieces dis-assembled are not to be used at site.
Candopeel applied surfaces shall be wrapped with HDPE (High Density Poly
Ethylene) sheets. Keep the shafts in the covered storage.
Inspect the machined
surfaces every month and re preserve as mentioned above.
PRESERVATION :
PROCEDURE FOR PRESERVATION OF ANTI-FRICTION BEARINGS :
a) The store- room must be free from dust.
b) Ideal ambient temperature should be 20 to 30 dog c.
c) Relative air humidity should not exceed 60X. It may be necessary to install air
dehumidifier in places where relative humidity is high.
d) If the bearing is found to be dry and dirty, it should be thoroughly washed and cleaned
before re-packing.
e) The bearing should be first kept in a vessel filled with kerosene for about half an hour
and then washed to take out the dirt.
f) It should then be cleaned in another vessel with filtered Kerosene.
g) The final cleaning is to be done by using petrol of mineral turpentine oil.
h) It should be then allowed to dry completely.
NOTE : Pressurised air for cleaning and drying purpose not recommended.
i) The washed and dry bearing is to be dipped In anticorrosive oil RUST GARD P214.
J) The excess oil should be allowed to escape and bearing should be repacked In a
water proof sealable plastic bag and put in to the carton again.
CONTENTS
- 17 -
CONSTRUCTIONAL FEATURES OF ‘AP’ FANS:
When looking in flow direction, the fan consists of the following Components.
*
Suction Chamber
*
Impeller housing
*
Rotor consisting of shaft, one/ two impellers with adjustable blades with
pitch control mechanism.
*
Main bearings (Anti-friction bearings)
*
Outlet Guide Vane housing with guide vanes
*
Diffuser
STATOR:
Suction chamber, fan housing and diffuser are welded structural steel fabrications,
reinforced by flanges and gussets, resting on the foundation on supporting feet. The
supporting feet are fixed on the foundation In such a way that they slide and
without clearance at the sliding supports of suction chamber and diffuser. On its
impeller side, the suction chamber is designed as an inlet nozzle. Guide vanes of
axial flow type are installed in the fan and guide vane housings, in order to guide the
flow.
Suction Chamber and Fan Housing, Outlet Guide Vane housing and diffuser are
flexibly connected via expansion joints.
Fan and Outlet guide vane housings are horizontally split, in order to facilitate access
of the rotor for dismantling and assembly.
Those parts of the pitch control unit which are arranged in the guide vane and
diffuser cores are accessible through assembly openings.
The fan is driven from the inlet side. The shaft runs on Anti-friction bearings. The
main bearings are accommodated in the core of the bearing housing. The impellers
are fitted to the shaft on either side of the bearing assembly in overhung position.
CONTENTS
- 18 -
The centrifugal and the setting forces of the impeller blades are absorbed by the
blade bearings. For this purpose, the blade shafts are being held in combination of
radial and axial anti-friction bearings. Each blade bearing is sealed off by means of
several seals, in both directions (towards the inside and outside).
ROTOR:
The rotor consists of a shaft, one/ two impellers, and a servomotor.
The rotor is accommodated In Cylindrical Roller bearing. In addition, Angular
Contact Ball bearings are arranged on the drive side in order to absorb the axial
thrust.
Double contact thermometers and resistance thermometers monitor the bearing
temperature. These thermometers must be connected to signaling instruments at
the site.
Lubrication and cooling of these bearings are ensured by a combination of oil bath
and circulating lubrication.
Simmer rings are always installed, filled with grease. The oil ventilated through the
lube oil return pipe, which opens into the oil reservoir. The ventilation hood is
mounted on the reservoir.
THE ROTOR IS ASSEMBLED IN AN AIR-CONDITIONED HALL AT OUR
WORKS. THE ROTOR ASSEMBLY IS TO BE STORED IN A CLOSED SHED AND
PRESERVED PROPERLY AT SITE.
CONTENTS
- 19 -
PRE-COMMISSIONING CHECKS (AP FANS):
Before commencing commissioning activity on the Fan, the following points should
have been verified. Some of the check points may not be possible to be verified after
completing the assembly of the Fan. However, the connected agencies should
confirm that these have been ensured during various stages of erection/
Overhauling.
ENSURE THE FOLLOWING:
ü The bases of Fan parts are tightened with out strain.
ü Tightness of all foundation bolts of Fan/motor.
ü Tightness of connecting bolts of bearing housing and impeller housing.
ü Verticality of impeller housing/horizonitality of shaft within 0.04 mm/rn.
ü Proper clearance and gap at free supports.
ü The impeller blades of each impeller are of the same series, also ensure that the
blades are mounted in the serial order and that the rounded nose of the blades
are on the suction side of each impeller.
ü Tightness of blade fixing screws to the required torque values.
ü For the given blade position, the blades are Identical in the impellers
ü Radial clearance of blade tip with respect to longest blade. Radial clearances of
blade root and impeller hub for both full open and full close positions of blades.
ü Axial gaps between rotor and stator parts.
ü Provision of locating pins and machined washers at parting planes of impeller
housing.
ü Match marks on the coupling parts.
ü Tightness of coupling bolts of regiflex coupling.
ü Correct setting of coupling gap.
ü Alignment of couplings to be within 0.05 mm.
ü Free movement of hinge heads.
ü Correct assembly of links.
CONTENTS
- 20 -
ü Acid cleaning of all incoming and outgoing oil pipes.
ü Gravity inclination of all return oil pipes.
ü The oil tank is adequately filled with correct quality of oil.
ü Early commissioning of the oil system to facilitate checks on the impeller.
CAUTION:
NEVER OPERATE THE SERVOMOTOR WITH OUT CONTROL OIL FLOW.
ü Proper connection of inlet and outlet of lube oil piping to the main bearings.
ü Proper connection of oil pipes to the oil head assembly.
ü There is no pre-load in oil head assembly introduced by flexible hoses.
ü The servomotor stroke length is maintained as per the drawing and mechanically
stopped.
ü The independent operation timing of actuator from zero to 100 % and vice versa
is more than the servomotor timing (From full close to full open and vice versa).
ü Linearity of blades position w.r.t. local/control room Indication.
ü Proper setting of mechanical stoppers.
ü Rotate rotor by hand and ensure free rotation without any mechanical rubbing.
ü Removal all transport stiffeners (Yellow painted).
ü Calibration of all instruments for Fan, Motor and Lub oil system.
ü Proper functioning of interlocks/protections.
ü Proper assembly of all expansion joints.
ü No foreign matter is left inside Fan casing.
ü No loose pieces are found in the suction side ducting of the Fan.
ü Precautions laid down by Motor/Oil system suppliers are complied with.
ü Start lube oil pump and allow it to run for about two hours. Clean filters. Check
level in tank. Ensure proper oil flow. Check the pressures and setting of relief
valve.
ü Check proper functioning of “Emergency Off” switch.
ü Proper direction of rotation of motor.
ü Never run the Fan/Motor with out protections/ interlocks.
CONTENTS
- 21 -
START UP AND OPERATION:
START PERMISSIVE:
(a)
Control oil pressure
= > 8 ata
(b)
Lub oil pressure
= > 0.8 ata
(c)
Impeller blades in minimum position
(d)
Discharge dampers full close
ALARM & TRIP VALUES:
When the fan is kept under service, the following values may be applied as reference
limit and in case the values are found exceeding, suitable corrective actions may be
carried out to maintain the following values below alarm limit.
DESCRIPTION
ALARM
FAN BEARING TEMPERATURE
TRIP
=/> 95 DEG C
=/> 105 DEG C
FAN BEARING VIBRATION (VEL – =/> 6.3 MM/S
=/> 12.6 MM/S
PEAK)
DIFFERENTIAL
PRESSURE
ACROSS
OIL FILTER
=/> 0.6 BAR
****
LUB OIL PRESSURE
=/< 0.4 BAR
****
CONTROL OIL PRESSURE
=/< 6.0 BAR
****
OIL LEVEL IN THE TANK
< 50 %
****
CONTENTS
- 22 -
PARALLEL OPERATION:
During the single fan operation, the volume and pressure demanded by the system
will be high. Under this condition, if the second fan is started as per the starting
sequence mentioned above, it is expected that the second fan will handle some
amount of flow. Correspondingly, the volume handled by the first fan, which is
operating, reduces. Then pressure developed by the first fan increases along its
characteristic line at the same blade position. At this point, there is a chance that the
second fan will go to unstable zone and as a result, the second fan will remain
unloaded irrespective of its blade position. Therefore in order to bring equal load on
both fans, it is required to follow certain procedure given below.
STARTING A SECOND FAN:
One fan (Fan A) is running and another fan (Fan B) is to be started. Even here the
sequence of operation must be such that neither of the two fans runs in the unstable
range of the performance graph. The ranges in which parallel fan (Second Fan) can
be started is cross-hatched as shown in the typical performance characteristic of an
axial reaction fan
(Fig – 2).
Fig – 2
CONTENTS
- 23 Refer Fig – 3. Let us assume Fan A runs at point “ A1” that is the pressure
developed is more than the deepest point “s” of the stall line.
Deepest point of
STALL LINE
Pr
Unstable
Zone
s
FIG – 3
Fan A – Operating
point “A1”
Vol à
Fan B is now shut off from the system by means of damper/ gate. Before starting
Fan B, the pressure developed by the Fan A should be brought down to
“A2” , (Ref fig – 4) that is less than the deepest point (“s”) of the stall line.
Deepest point of
STALL LINE
Pr
Unstable
Zone
“s”
Reduce
load
on fan A to
Point - A2
FIG – 4
Vol à
CONTENTS
- 24 -
Then Fan B is started as per the starting sequence. By opening of blades in Fan B, it
will deliver certain volume. As Fan B is sharing certain volume, the volume handled
by the Fan A will be decreased and there will be slight increase in pressure at the
same blade position. So, close the blade position of Fan A by approximately 5%. As
the aim is to make both fan operating at the same duty point on the performance
graph, alternatively open the blades of Fan B by approximately 5% and close the
blades of Fan A by approximately 5%. Continue this operation step by step, till the
duty points like blade position and Motor current are same in both fans.
From this point onwards, increasing/ decreasing the load on the fans will be
simultaneous and similar in both fans. If not, the phenomena called “LOADING/
UNLOADING OF FANS” will occur.
EFFECT ON FAN PERFORMANCE DURING SUDDEN CHANGE IN SYSTEM RESISTANCE:
Normally when two axial reaction fans are running parallel under stable load
conditions, there is a chance of sudden variation in the system resistance. For
example, tripping of one mill or accidental closure of any damper in the down stream
of the fan etc., the effect of such sudden variation in system demand on the fan
operation is explained below.
During the sudden variation in system demand as explained above, when two PA
fans are running under stable condition at higher blade position, the volume required
to be handled by the Two PA fans reduces. Under this condition, both the PA fans
are likely to get unloaded (Fan delivery pressure goes down drastically) when the
blade positions are kept at same angle. The normal control provided in the C & I
scheme is that, the blade pitch angle will be further open when the fan delivery
pressure goes down. Some times the blade position go to 100 % open. Even in this
condition, the pressure developed by the fan will remain same at lower value. Cases
were reported that the operator switches off the PA fans during such conditions.
This has resulted in tripping of unit due to low-low header pressure.
CONTENTS
- 25 -
In order to over come such situation, it is required to understand the parallel
operation of this type of Fans. Under this condition the following guideline may be
adopted.
When two PA fan run in unloaded condition, bring the blade position to
approximately 60% - 50% by closing the blades in any one fan (Say Fan – A). Close
the blade fully in other fan (Say Fan – B) and keep running it. Now, the Fan A in
which the blade position is 60% - 50% open will operate at stable zone. Set the Fan
out let pressure/ Header pressure at the required level. Bring in the second fan as
and when required by following the parallel operation procedure.
REMEMBER THE FAN SHOULD NOT BE PERMITTED TO RUN ON OR ABOVE
STALL LINE OF THE PERFORMANCE GRAPH CONTINIOUSLY.
If the operating (duty) point of both fans is same in the performance graph, the auto
control system will adjust both fans identically to meet the system demand. To
achieve this, the following MECHANICAL checks should have been performed and
logged prior to commissioning of the fans.
1. The servomotor operating time is less than the operating time of the actuator in
de-coupled condition.
2. After coupling with actuator, the servomotor operating time is identical in both
fans
3. The servomotor stroke length is identical in both fans
4. The actual blade position is calibrated w.r.t control room reading and are
identical in both fans
5. There is no mechanical backlash in the linkages of Servomotor – Actuator.
CONTENTS
- 26 -
CONSTRUCTIONAL FEATURES OF ‘AN’ FANS:
The major sub-assemblies of the fan are as follows:
1. Stator Parts
2. Impeller with shaft assembly
3. Flow Regulating Device
4. Coupling
STATOR PARTS :
SUCTION CHAMBER
The suction chamber is of welded plate construction. The suction chamber is
provided with manhole for checking the inlet of fan and removal of shaft.
IMPELLER HOUSING
Impeller housing is fabricated from sheet metal and is of undivided construction. A
peep-hole is provided in the housing for checking the wear on the impeller. A
conical piece is provided in the housing which is supported by a set of permanent
support and outlet guide blades.
The conical piece supports the inner bearing
assembly. The outlet guide blades are removable type in order to replace the
eroded blades during operation.
DIFFUSER
Diffuser is of welded sheet metal construction with a core inside. The core of the
diffuser is flanged with core of the impeller housing on one end and supported by
struts on the other end.
CONTENTS
- 27 -
A diffuser wedge is provided with two openings in between core and case of
diffuser. One opening as man-hole to inspect the inner bearing and other one for
grease lubricating pipe assembly for inner bearing. The diffuser core and diffuser
wedge are insulated inside.
A cooling pipe is provided from the cooling air blower upto the inner bearing
through the diffuser wedge to have forced cooling of the inner bearing.
The
grease lubricating pipe as well as thermometer leads for the inner bearing are
brought outside through cooling pipe and diffuser wedge for easy access. The core
is provided with a monorail to hold the shaft during the removal of the impeller.
FREE/ SLIDE SUPPORTS
Free or slide supports are provided under diffuser and suction chamber. These
supports are provided to accommodate the thermal expansion of suction
chamber/diffuser during operation.
ROTOR - IMPELLER WITH SHAFT ASSEMBLY:
The impeller hub is of welded sheet metal construction on which the non-profiled
solid twisted blades are welded with proper welding sequence to avoid distortion.
All critical welds are inspected thoroughly by NDT methods. The impeller is stress
relieved upon completion of all welding operations and is dynamically balanced
after final machining.
The shaft is a hollow tube with two solid forged pins shrunk fit at both ends of the
shaft. A flange is welded to the shaft to fix the impeller by means of screw ring.
The completely machined shaft is dynamically balanced. The critical speed of the
shaft is kept well above the operating speed.
CONTENTS
- 28 -
FLOW REGULATING DEVICE (IGV UNIT) :
The inlet guide vane control (IGV) assembly of the fan consists of a number of
aerofoil vanes fixed to individual shafts, each supported by bearing pedestal.
These guide vane shafts are connected to a regulating ring by means of angular
joints. The regulating ring is guided to rotating mechanism by a set of rollers and
spring assemblies called suspension assemblies. A control lever is connected to the
regulating ring which is operated by the electrical actuator.
Note :
For hot medium fans, the rollers of suspension assembly should have radial gap in
cold condition with outer surface of IGV casing in order to accommodate the
expansion of casing during operation.
COUPLING :
The fan and the drive motor is connected by means of PIN TYPE FLEXIBLE
coupling. The distance between the two halves of the coupling is to be maintained
such that the expansion of the shaft will also be considered.
SILENCER :
Fans handling atmospheric air are provided with silencers to attenuate air borne
noise. The silencer consists of baffles fabricated from perforated steel sheets with
sound absorbing wool held in wire mesh inside the baffles.
CONTENTS
- 29 -
PRE-COMMISSIONING CHECKS (AN FANS):
ENSURE THE FOLLOWING:
- The bases of fan parts are tightened with out strain.
- Tightness of all foundation bolts of fan/motor.
- Verticality of impeller housing.
- Tightness of connecting bolts of bearing housing and impeller housing.
- Radial clearance of blade tip with respect to the longest blade.
- Axial gaps between rotor and stator parts.
- Tightness of coupling bolts.
- Correct setting of coupling gap.
- Alignment of couplings as per the Field Quality Check Sheet.
- Bearing radial clearances
- Axial gap at the outer bearing.
- The bearing housing is adequately filled with correct quality of grease.
- Check for proper operation of Inlet Guide Vane unit. All the vanes should be in
Identical position.
NOTE: Vanes of IGV shall CLOSE in the same direction of rotation of
impeller viewing from suction.
- Correct assembly of actuator links.
- Linearity of IGV position w.r.t. local/control room indication.
- Check the condition of balancing weight in impeller and shaft.
- Rotate rotor by hand and ensure free rotation without any mechanical rubbing.
- Calibration of all instruments for Fan/Motor/oil system.
- Proper functioning of interlocks/ protections.
- Proper assembly of all expansion joints.
- Removal all transport stiffeners (Yellow painted).
- No foreign matter is left inside fan casing.
- No loose pieces are found in the suction side ducting of the fan.
- Check for proper earthing of motor body. All statutory conditions stipulated by
motor manufacturer must be adhered.
CONTENTS
- 30 - Check proper functioning of "Emergency Off" switch.
- Proper direction of rotation of motor. The fan shall rotate in anti clock wise when
viewed from drive end.
- Never run the Fan/Motor with out protections/ interlocks.
CONTENTS
- 31 -
START PERMISSIVE:
The following conditions must be satisfied for starting the fan.
* Inlet Guide Vanes full close.
* Discharge damper in full closed position.
ALARM & TRIP VALUES :
-----------------------------------------------------------------------------ALARM
TRIP
-----------------------------------------------------------------------------FAN BEARING TEMPERATURE (Deg C)
95
105
____________________________________________________
FAN BEARING VIBRATION
(Pk VELOCITY IN MM PER SECOND)
CONTENTS
6.3
12.6
- 32 -
CONSTRUCTIONAL FEATURES OF RADIAL (NDV/ NDZV) FANS:
NDV fans are single stage single inlet centrifugal machines and NDZV fans are single
stage, double inlet centrifugal machines. The rotor is simply supported by bearings
located on both side of the impeller.
The major sub-assemblies of the fan are as follows:
1. Impeller with shaft assembly
2. Bearings and thermometers.
3. Suction chamber and spiral casing.
4. Flow regulating devices.
5. Shaft seals.
6. Couplings.
IMPELLER WITH SHAFT ASSEMBLY :
The impeller is a completely welded structure and is made from high tensile steel
with backward curved blades. The selection of the material and thickness for the
impeller/shaft are computed on the basis of the stress analysis/critical speed
programme carried out for each impeller and shaft. The impeller consists of center
plate, blade, cover plate and impeller ring. All weldments are inspected thoroughly
by NDT methods. The back plate of the impeller and the shaft flange has a
machined groove, which ensures correct location of the wheel relative to the shaft
during assembly. Impeller is bolted to the shaft flange and locked by means of
locking plates. Conical cover plates are provided at the inlet (bolted to the center
plate) to guide the inlet flow and to protect the fasteners from exposure to the
medium handled by the fan. The impeller is stress relived upon completion of all
welding operation and is dynamically balanced after final machining. Replaceble
Wear liners are provided in the impeller blades.
The shaft is a hollow tube with two forged solid end pins shrunk fit at both end of
the shaft and welded. The shaft is machined to a high degree of surface finish for
location of impeller, bearings and coupling half etc. The completed shaft is
CONTENTS
- 33 dynamically balanced. The critical speed of the rotor is well above the operating
speed.
BEARING AND THERMOMETERS:
The fan rotor is supported in between a fixed bearing and a free bearing .The fixed
bearing is arranged on the coupling side. The fans are provided with SLEEVE
bearings or anti-friction bearings and oil lubrication.
Provision for mounting temperature gauges (Mercury in steel thermometers and
RTDs) are available on the bearing housings for local and remote (UCB) indication of
bearing temperatures. Platinum resistance thermometers (RTD) are provided with
alarm and trip connections and for remote indication (control room) of bearing
temperatures.
SUCTION CHAMBER AND SPIRAL CASING :
Suction chamber and spiral casing are fully welded structures and are fabricated
from sheet steel with adequate stiffeners. These are split suitably to facilitate easy
handling and maintenance of rotor, etc. The oblique cone, which forms the entrance
to the impeller, helps in accelerating the flow. The lower part of fan casing rests on
the supporting brackets on the foundation.
FLOW REGULATING DEVICES:
The fan output (flow) is regulated by speed control either by HYDRAULIC COUPLING
or VARIABLE SPEED MOTOR or by inlet vanes.
The inlet vane assembly is located at the inlet of the suction chamber for regulating
the flow through the fan for different system demand. It consists of single piece
casing, vanes, bearings for vane shafts and actuating lever. The bearing housings
are supported on the walls of the casing. Levers and links for connection to the
actuating lever connect the vane shafts. The vanes are actuated by means of an
actuator. A graduated plate indicates the vanes position in degrees.
CONTENTS
- 34 -
PRE-COMMISSIONING CHECKS (RADIAL FANS):
ENSURE THE FOLLOWING
- The bases of fan parts are tightened
with out strain.
- The direction of rotation of impeller with respect to casing.
- Tightness of all foundation bolts of fan/motor.
- Tightness of connecting bolts of bearing housing.
- Horizontality of the shaft within 0.04 mm/m.
- Squareness (faceout) and the tighteness of the
thrust collar.
- Bearing clearances.
- Oil level and recommended grade of oil for fan bearing.
- Free rotation of oil rings.
- Cooling water flow to the bearings.
- No water leakages inside the bearing housing.
- Impeller clearances with respect to stator parts.
- Direction operation of inlet damper flaps with respect to the direction rotation of
impeller.
Note: The Direction of OPENING of inlet damper flaps shall be in the same
direction of rotation of impeller viewing from suction end.
- Power cylinder/actuator for proper actuation of suction damper (full open to full
close) .
- Linearity of the damper flaps position in control room with respect to local
indication.
- Proper operation of inlet damper and outlet gate/damper.
- Tightness of coupling bolts.
- Correct setting of coupling gap.
- Alignment of couplings as per the FQA logsheets.
- Rotate rotor by hand and ensure free rotation without any mechanical rubbing.
- Condition of balancing weight in impeller and shaft.
- Removal all transport stiffeners (Yellow painted).
CONTENTS
- 35 - Calibration of all instruments for Fan/Motor.
- Proper functioning of interlocks/protections.
- Proper assembly of all expansion joints.
- No foreign matter is left inside fan casing.
- No loose pieces are found in the suction side ducting of the fan.
- Proper earthing of motor body and all statutory conditions stipulated by motor
manufacturer are complied with.
- Proper functioning of "Emergency Off" switch.
- Proper direction of rotation of motor.
- Never run the Fan/Motor with out protections/ interlocks.
CONTENTS
- 36 -
START PERMISSIVE (RADIAL FANS):
* The inlet damper and discharge gate/damper of the fan should be in closed
position (if provided). Open discharge damper/gate simultaneously after starting the
motor.
* Lub oil level in the bearing should be ensured.
* Cooling water flow to the bearings.
ALARM & TRIP VALUES :
ALARM
TRIP
FAN BEARING TEMPERATURE (Deg C)
SLEEVE BEARING
77
82
ANTI-FRICTION BEARING
95
105
________________________________________________________
FAN BEARING VIBRATION (PEAK VELOCITY IN MM/SEC)
6.3
CONTENTS
12.6
- 37 -
VIBRATION AND SAFE RUNNING OF FANS:
(GENERAL DESCRIPTION)
VIBRATION ANALYSIS AND BALANCING
An attempt is made to bring out the basic concepts of
concentration of this paper is more
towards the
vibration and
the
problem solving in fans.
General problems encountered in rotating machines and the identification of the
causes are, to the possible extent analysed.
WHAT IS VIBRATION?
The product of the force spectrum and its mobility spectrum yields what is
known as VIBRATION SPECTRUM.
All dynamic machines vibrate in the process of channeling energy in the job to be
preformed, forces are generated which will also excite vibration on the individual
components of the machine, directly or via the structure.
CHARACTERISTICS OF VIBRATION
Amplitude
(Displacement/ Velocity/ Acceleration)
à
Indicates of how much vibration is
present
Frequency
à
Determines the cause of vibration
Phase
à
Aids in identifying the defective
parts
CONTENTS
- 38 WHY VIBRATION TO BE MEASURED?
From the world wide experiments conducted on the necessity
of vibration
measurement and monitoring, it has been determined that the economy and
availability of the industries such as Power Plants, Chemical and Paper Plant, Oil
drilling, Aircraft industries
comparatively
low
etc., improved to an appreciable extent
expenditure
towards
the
with
vibration instrumentation,
analysis and maintenance costs.
Any failure can be predicted well in advance eliminating possible catastrophe
of sophisticated and costly machines.
Also it has been proved that "ON
CONDITION MAINTENANCE" improves the life of a machine considerably when
compared to "TIME BASED PREVENTIVE" or "RUN DOWN MAINTENANCES".
Effects of vibration are often serious. Humans subjected to vibration can be
affected by blurred vision, loss of balance and consequent lack of ability to do
their job properly. In some cases, certain vibration frequencies and levels can
permanently damage internal body organs. Machinery can be damaged by
vibration. In some cases as in an aircraft, this can be disastrous.
Noise resulting from vibration is also often a serious problem and can be a health
hazard to people exposed to it for long periods.
Further, vibration will not stay at one place unless
special steps are being taken
to isolate it.
The
above factors
vibration.
CONTENTS
show the importance of measuring and controlling
- 39 WHO HAS TO MEASURE?
The person measuring must understand the vitality of his work. He must be trained
in the following aspects:
•
How to hold the pick up?
•
Where to hold the pick up?
•
A basic understanding of the consequences of not following the specified terms
for measurements.
•
Handling the vibration instrument being used.
•
A clear picture of the total system of machines with which he is dealing.
•
The analysing person must be aware of the total technical details like rotational
speeds, number of rolling elements in bearings, gear specifications etc.
•
Aware of reporting system
•
Basic aptitude to perform his job
Depending on number of locations and measuring points, staff requirement can
be decided.
Relevant training programs must be planned for newly recruited staff, both for
measurement and analysis as well.
WHERE TO MEASURE?
Measuring
at any location of a dynamic machine will yield
a vibration
value.
"But does it correspond to maximum severity of the component? is the query. Ideal
would
be the measurement on the
Otherwise, the measurement should
transmitted
rotating/moving
be at the nearest
part itself(say
shaft).
point where the load is
from the moving component to the static part(say, on bearing
housing nearer to the bearing location).
Further, the direction in which the stiffness of the system is least will output higher
CONTENTS
- 40 vibration amplitudes.
Direction of measurement can be standardised as radial
and axial.
More the number of points more will be the details
of forces acting at that
location but time and cost factors restrict measurement in 3 special directions (2
radial right
angular directions
called horizontal
& vertical and
one axial
direction) represented as X,Y and Z directions.
For deciding the measurement locations, the systems
to be monitored have to
be studied thoroughly.
Pick up can
be fixed permanently or mounted while making measurements.
The deciding factors for continuous monitoring (permanent pick up fixing) are:
- the role of that particular machine in the complete system
- the cost of that machine
- the accessibility
If periodical measurements are considered sufficient, then portable instruments
can be utilised.
WHEN TO MEASURE?
Periodicity of measurement is determined by the factors below:
- disturbing forces acting in the system
- the vitality of the machine in the system
- cost of the machine
- dependence on maintenance schedules
- rate at which deterioration takes place
- number of locations and machines to be monitored
- staff availability
- time and cost factors involved in measurement and monitoring
CONTENTS
- 41 -
Intervals of periods can be increased or decreased based on the immediate
requirement. For example,if a suspicion on deterioration of a part arises, then
the intervals
can be compressed and number of measurements
can be
increased to predict the trend and time of failure.
HOW TO ANALYSE AND SOLVE?
This part consists of 3 stages.
Collection stage
Collect all kinds of information without any bias,
that is unassuming any
particular type of vibration. Put them in a concrete and compact form. The
person who is in charge of analysing and solving the problem must be present in
that place so that any extra information needed can be pointed
out
then and
there itself.
Analysing stage
For this we need the following:
-
Standards for comparison
-
Chart depicting the problems and likely causes
Nothing
can equal the experience obtained in dealing with
a particular type of
machine.
Base line signatures can be maintained for all machines
commissioned.
Further changes can be
when they are newly
watched closely and the point where
action has to be taken can be fixed based on international or local standards.
A chart showing a list of general problems encountered in rotating machines
and the identification of the causes are given below followed by the vibration limits
as per VDI 2056.
CONTENTS
- 42 -
VIBRATION IDENTIFICATION TABLE
Cause
Amplitude
Proportional to
Unbalance
Unbalance.
Largest in radial
direction
Large in axial
Misalignment
of coupling or direction. 50%
bearings and or more radial
vibration
bent shaft
Frequency
1 X RPM
Usually 1 X RPM Single,
often 2 X RPM Double
&
3 X RPM Triple
Damaged
Unsteady
rolling element
bearings (Ball,
Roller, etc.,)
Very
high.
Several times of
RPM
Mechanical
looseness
2 X RPM
Electrical
Aerodynamic
forces
CONTENTS
Variable
Phase
Single
reference
mark
1 X RPM or
Proportional to 1X , 2X
Synchronous
the load.
frequency
Blade & Vane
Variable
passing
frequencies and
harmonics
Remarks
Most common
cause
of
vibration
A
or fault
common
Bearing
Erratic
responsible,
most likely the
one nearest to
the point of
largest
high
frequency
vibration.
Usually
accompanied
Two
by Unbalance
reference
and
or
marks
Misalignment.
Single
or Disappears
when power is
Double
turned off.
rotating
marks
Rare
as
a
cause
of
trouble except
in
case
of
resonance.
- 43 -
LIMITS OF VIBRATION
CONDITION OF MACHINE AS PER
VDI 2056
GOOD
G
R
O
U
P
“G”
G
R
O
U
P
“T”
Velocity(Pk) In
mm/sà
540
rpm
740
Displacement
rpm
(Pk-Pk)
990
in microns
rpm
For
1480
Speed =
rpm
2980
rpm
Velocity(Pk) In
mm/sà
540
rpm
740
Displacement
rpm
(Pk-Pk)
990
in microns
rpm
For
1480
Speed =
rpm
2980
rpm
Satisfactory
Just
Satisfactory
Unsatisfactory
OPERATING
INSTRUCTION
ALARM
TRIP
Upto
Above
Upto
Above
Upto
Above
At &
Above
At
2.54
2.54
6.3
6.3
15.55
15.55
6.3
12.7
80
80
200
200
500
500
200
400
64
64
160
160
400
400
160
320
48
48
120
120
300
300
120
240
32
32
80
80
200
200
80
160
16
16
40
40
100
100
40
80
3.96
3.96
9.9
9.9
25.4
25.4
9.9
19.8
126
126
320
320
800
800
320
640
100
100
250
250
640
640
250
500
75
75
188
480
480
480
188
375
50
50
125
125
320
320
125
250
25
25
63
63
160
160
63
125
Putting together one's own experience with the available charts, the following can be
achieved:
a) Weaning out the surplus data obtained and pick out the important and related
data for analysis.
b) List out all the probable causes.
c) Eliminate from the above ones which are not possible practically in that
particular situation or which have been taken care of already.
d) Try to in interrelate the remaining and arrive at the important causes.
CONTENTS
- 44 e) List out the steps to be taken further to eliminate the step (d) cause.
f) Pass on this properly to the man who is going to practically deal with the
rectification.
Solution Stage
Ensure that all that suggested remedial
measures
are being implemented
properly. If the vibration has not come down, see whether any point has evaded
the attention and try to rectify In general, the expert must be aware of the
following:
- The system's foundation design
- The details of the entire system itself
- What are the interactions of the near by foreign systems.
-
How this system affects other ones.
Now let us take the particular case of fan. Fan problems are generally classified
under three broad categories:
a. Electrical Problems
b. Mechanical Problems
c. Civil problems
In fans, most of the problems will reflect generally on a balancing or bearing
defect vibrations. Before launching on balancing or bearing change, it is better
to ensure the elimination of the remaining possible and probable causes and
notice the reduction in vibration. 'Resonance' is one of mechanical origin but it
behaves like an 'electrical' one. Here the phase measurements will clearly separate
'Resonance' from electrical based faults.
In general, it is very easy to talk about 'individual' problems and their symptoms but
in practice, the vibration spectrum consists of a combination of all vibrations listed
in the chart.
Solving in one attempt, in most cases, is impossible.
development will solve the problem.
CONTENTS
Only
a step by step
- 45 -
BALANCING:
Unbalance exists in a rotor when vibratory force or motion is imparted to its
bearings as a result of unequal centrifugal forces developed about the shaft axis
of the rotor when it is spinning.
Unbalance is caused by non-symmetrical mass distribution about the rotational axis
of the rotor so that the heavier side exerts a larger centrifugal force than the lighter
side.
Balancing therefore consists of the process of redistributing the mass of the rotor
(generally by adding or removing weight) so that its mass becomes symmetrically
distributed about its designed rotational or shaft axis. Centrifugal forces about the
shaft axis, generated when the rotor is spinning will then be in equilibrium and the
rotor will run without vibration.
From an engineering standpoint it may be stated that, unbalancing exists in a
rotor when its principal inertia axis (mass axis) does not coincide with its shaft
axis.
Balancing therefore involves redistributing the mass of the rotor so that its
principal inertia axis coincide with its designed rotational or shaft axis.
Unbalance is generally corrected by adding weight to the lighter side or removing
weight from the heavier side of the rotor.
CONTENTS
- 46 -
TYPE OF UNBALANCE:
a.
STATIC UNBALANCE: The central principal axis is
displaced parallel to the
rotating centre line.
Axis of rotation
Central principal axis
Unbalance mass
b.
COUPLE
UNBALANCE:
Central
principal
axis
intersects
rotating centre line at the centre of gravity.
Axis of rotation
Centre
gravity
CONTENTS
of
Central Principal Axis
the
- 47 b.
QUASI-STATIC UNBALANCE: Central principal axis
intersects the rotating
centre line but not at the centre of gravity.
Central principal axis
Axis of rotation
Centre of
gravity
d.
DYNAMIC
UNBALANCE
:
The
central
principal
axis
and
the
rotating centre line (Axis of rotation) do not coincide or touch.
A.R
C. P. A
METHODS OF BALANCING
Major divisions are double plane balancing and single plane balancing
DOUBLE PLANE BALANCING:
This can be performed easily by doing simple plane balancing in two planes
separately. But this might call for too many trails.
So simultaneous dealing on
both the planes by vector method is a better way of balancing in double plane.
CONTENTS
- 48 -
SINGLE PLANE BALANCING:
Vector method of balancing using phase measurement.
0
0
B
O=4
270
0
A
90
R = 10
O+T=8
180
0
C
CALCULATION:
ORIGINAL VIBRATION MEASURED
(O = AB)
= 4 mm/s
0
ORIGINAL PHASE ANGLE MEASURED
= 35
TRIAL WEIGHT ADDED ON THE ROTOR
= 300 gms.
VIBRATION MEASURED AFTER
ADDING TRIAL WEIGHT
(O+T = AC)
PHASE ANGLE MEASURED AFTER ADDING TRIAL WEIGHT
CONTENTS
= 8 mm/s.
= 130
0
0
- 49 -
RESULTANT MEASURED FROM
THE ABOVE VECTOR DIAGRAM
(R = BC)
CORRECTION WEIGHT
= 10 mm/s
= ((O / R ) * TW)
= ((4 / 10) * 300)
= 120 gms.
CORRECTION ANGLE
(Angle ABC)
= 400
THE TRIAL WEIGHT (300gms) SHOULD BE REMOVED FROM THE ROTOR.
THE CORRECTION WEIGHT (120 gms.) IS TO BE ADDED ON THE ROTOR AT 400
ANTI-CLOCKWISE FROM THE TRIAL WEIGHT LOCATION AT THE SAME RADIUS OF
THE TRIAL WEIGHT.
CAUTION:
EXTREME CARE SHOULD BE EXCERSIED TO ENSURE TO EARTH THE IMPELLER
DIRECTLY WITH THE WELDING MACHINE DURING WELDING OF BALANCING
WEIGHTS. THE VIBRATION PICK UPS SHOULD NOT BE PLACED ON THE BEARINGS
DURING WELDING OF BALANCING MASS.
To the possible extent, the 5W's and 1H are answered here briefly.
Further,
balancing procedure is also outlined above. This might serve as a good
start for
those who are interested deeply in this field.
*************************************
CONTENTS
44
FANS
`````````````````````````````````````````````````````````````````````````
FAN TESTING FACILITIES AT BHEL RANIPET
BHEL Ranipet is equipped with fan test facility in which performance test of any fan can
be done to near its operating speed. The fan under test will be supported on temporary
steel box supports and are fastened to the fitter bars embedded in the concrete.
Following equipments / facility are available.
1. DRIVE MOTOR: Variable frequency drive system having motor capacity 3440 KW
at the base speed of 800 rpm. The motor speed can be varied from 200 rpm to
1700 rpm.
2. TEST TRACKS: Six test tracks having diameter of 400mm, 500mm, 800mm,
1400mm, 2800mm, and 3500mm are available with its own conical typethrottling device. The test track diameters are selected based on the
volume to be handled by the fan. The test track dimensions are as per BS
848 / AMCA 210 standard.
3. TORQUE TRANSDUCERS: Seven sizes of torque transducers of different ranges
up to the torque 5000 kgm are available. By these torque transducers power to the
fan shaft can be measured directly.
4. MANOMETERS: The volume handled by the fan is measured using Pitot tubes
positioned in the test track. The Pitot tube is connected to digital manometer
available in fan test station.
5. BUILDING: The fan test facility is provided with the floor area of 30Mx 60M.
The maximum height of the building is 31 meters. Also provided with sufficient
compressed air, water facility.
6. MATERIAL HANDLING: The fan test facility is provided with an E O T crane of
capacity 10/30 tons with the maximum hook level up to 20
meters.
7. MAN POWER: The fan test facility is provided with 2 engineers and 15 workmen
for fan erection and testing.
8. ACHIVEMENTS: We have completed the performance testing of 116 fans in the
past 17 years. The largest diameter of fan ever tested was 5000mm.
45
FANS
```````````````````````````````````````````````````````````````````
RECENT DEVELOPMENTS ON FANS
Ø Rotor dynamic analysis for assessing mechanical strength of NDZV 47 sidor
has been carried out successfully using ANSYS5.3 for 500 MW ID fan of
Taelcher project
Ø Finite Element analysis was carried out for 16 Hub size AP fan for cast
blades and forged blades and enhancing indigenisation.
Ø Finite Element analysis was carried out on Rigiflex coupling for Unchahar
Project and enhancing indigenisation.
Ø New series of aero-foil bladed radial fans have been developed successfully
and implemented to fill the gap in our manufacturing range as detailed below.
1
Item
Type
Appln
Projects implemented (examples)
Aero – foil bladed fan
BAB1
ID & FD
IFFCO Phulpur, ACC Wadi, Nalco Damanjodi,
IPCL Barauni / Gandhar, Renusager 10,11
2
Aero-Foil
BAB2 FD
ACC Wadi, Aditya cements, Renusagar10,11
SF
Recovery Boiler – APPM, OPM, Jk Paper
bladed fan
3
Forward
ID
curved bladed fan
4
Plate bladed radial fan
Mills, TNPL Kagithapuram,
C2
ID
Chettinadu cements/Karur, Dalmia
magnasite, BASL, ACC Chanda, ACC
Madhukarai
5
High pressure
HPSF SA
All 210/250 MW seal air fan
N
ACC Chanda/ Madhukarai (PA), BSNL,
Seal Air Fan
6
High pressure fan
(narrow width impeller)
SA
Shamanur sugars (OFA), Raichur 5,6 (SA)
CONTENTS
TRAINING MANUAL
TRAINING MANUAL FOR ELECTROSTATIC PRECIPITATOR
INDEX
DESCRIPTION
SL. NO.
PAGE
NO.
1.
PRINCIPLE OF ELECTROSTATIC PRECIPITATOR OPERATION
2.
SELECTION AND SIZING OF ELECTROSTATIC PRECIPITATORS FOR
3
10
COAL FIRED BOILER
3.
MECHANICAL DESIGN OF ELECTROSTATIC PRECIPITATOR
16
4.
FLOW
29
MODEL
STUDIES
IN
THE
FIELD
OF
ELECTROSTATIC
PRECIP1TATOR
5.
ELECTRICAL SYSTEMS FOR ELECTROSTATIC PRECIPITATORS
33
6.
PRE-COMMISSIONING STABILISATION OF EP AND PERFORMANCE
38
TESTING OF EP
7.
EP OPERATION AND MAINTENANCE
41
8.
RECENT DEVELOPMENTS IN ELECTROSTATIC PRECIPITATOR
45
Page 1
CONTENTS
TRAINING MANUAL FOR ELECTROSTATIC PRECIPITATOR
1.PRINCIPLE OF ELECTROSTATIC PRECIPITATOR OPERATION
INTRODUCTION
Electrostatic precipitation utilizes the forces acting on electrically charged particles
in the presence of an electric field to effect the separation of solid or liquid particles
from a gas stream. In the precipitation process dust suspended in the gas is
electrically charged and passed through an electric field where electrical forces
cause the particles to migrate towards the collection surface (fig-1). The dust
separated from the gas by retention on the collection electrode and subsequently
removed from the precipitator. Various physical configurations are used to
accomplish the followings:
(a)
(b)
(c)
(d)
Corona generation
Particle charging
Particle collection
Particle removal
CORONA GENERATION
Corona as applied to electrostatic precipitators is a gas discharge phenomenon
associated with the ionization of gas molecules by electron collision in regions of
high electric field strength. The process of corona generation requires a nonuniform electric field which is obtained by the use of a small diameter wire as one
electrode and a plate or cylinder as the other electrode. The application a high
voltage to this electrode configuration results in a high electric field near the wire.
The electric field decreases inversely with the radius from the wire surface.
The corona process is initiated by the presence of electrons in the high field region
near the wire. Electrons for corona initiation are supplied from natural radiation or
other sources and since they are in a region of high electric field they are
accelerated to high velocities and possess sufficient energy so that on impact with
gas molecules in the region they release orbital electrons from the gas molecules.
The additional free electrons are also accelerated and enter into the ionization
process. This avalanche process continues until the electric field decreases to the
point that the electrons released do not acquire sufficient energy for ionization.
Page 2
CONTENTS
TRAINING MANUAL FOR ELECTROSTATIC PRECIPITATOR
Within the region defined by the corona glow discharge where ionization is taking
place, there are free electrons and positive ions resulting from electron impact
ionization. The behavior of these charged particles depends upon the polarity of the
electrodes, and the corona is termed negative corona if the discharge electrode is
negative or positive corona if the discharge electrode is positive.
Both positive and negative coronas are used in industrial gas cleaning application,
however the negative corona is most prevalent within the temperature range of most
industrial applications.
In the case of the negative corona, positive ions generated in the corona region as a
result of electron impact are attracted towards the negative wire electrode and
electrons towards the positive plate or cylinder electrode. Beyond the corona glow
region the electric field diminishes rapidly and if electronegative gases are present
electrons will be captured by ht egas moleculeson impact. The negative ions thus
generated move towards the collection electrode and serve as the principal means
for charging the dust.
In the corona process there must be a source of electrons to initiate and maintain
the avalanche process. The electrons are supplied from naturally occurring ionizing
radiation photo-ionization due to the presence of the corona glow, and in the case of
high temperature operations, from thermal ionization at the electrode surface. For
negative corona, electrons are also provided by secondary emission from the
impacts between the positive ions and the discharge electrodes.
In most industrial gas cleaning applications, there are sufficient quantities of
electronegative gases such as Oxygen so that practically all of the electrodes are
attached to gas molecules. Gases such as nitrogen, helium, argon etc., do not form
negative ions and hence a stable negative corona is not possible in these gases.
In positive corona the electrons generated by the avalanche process, flow toward the
collection electrode. Since the positive ions are the charge carriers, they serve to
provide an effective space charge and the presence of an electronegative gas is not
required for positive corona. Sources of electrons for initiating and maintaining
avalanche in a positive corona are cosmic radiation and photo ionization due to the
corona glow.
Page 3
CONTENTS
TRAINING MANUAL FOR ELECTROSTATIC PRECIPITATOR
Positive and negative coronas differ in several important aspects. In appearance the
positive corona is rather uniform sheath surrounding the discharge electrode. In
contrast negative corona appears as localized discharges from points on a clean
wire and as localized tufts along the dust coated electrode. The voltage-current
characteristics of the negative corona are superior to those of positive corona at the
temperature at which most precipitators operate. Higher operating voltages and
currents can be reached prior to disruptive sparking.
Most industrial gas cleaning precipitators utilise negative corona because of its
inherently superior electrical characteristics, which leads increase in efficiency at
the temperatures at which they are used.
Geometry of the electrodes, gas composition and gas conditions have important
influences on corona generation. The diameter of the discharge wire and the
electrode spacing determine the voltage gradient and hence the variation in electric
field strength. The electric field varies as the reciprocal of the radius near a small
diameter wire. Hence with a very small wire, the electric field near the surface can
be quite high often in the range of 50-100 kV/cm. The avalanche process requires
the presence of high electric field over a given distance. In general the small
diameter wire requires high electric field strength for initiation of corona. For a
given spacing, however the onset of corona occurs at a lower voltage for the smaller
diameter wire. Also for a given voltage higher currents are obtained with smaller
diameter discharge electrodes.
Temperature and pressure influence the generation of corona by changing the gas
density. In the avalanche process the time available for accelerating an electron
between collisions is a function of gas density. With increased molecular spacing,
higher velocities can be achieved between collisions. Thus ionizing energy can be
achieved with low electric fields for low gas densities.
A second effect, in the case of the negative corona is that the increased molecular
spacing results in the penetration of free electrons further into the inter-electrode
region before capture to form a negative ion. Thus results in an increased average
mobility in the inter-electrode space and hence higher current.
Corona generation studies of basic nature are most often made with clean
electrodes under laboratory conditions. These conditions are highly idealized in
comparison to industrial precipitator. In practical precipitators, the presence of a
dust entering the electrodes space becomes charged by attachment of negative are
Page 4
CONTENTS
TRAINING MANUAL FOR ELECTROSTATIC PRECIPITATOR
positive ions. Because of the much lower mobility of the charged dust it constitutes
a significant space charge,
The magnitude of the space charge depends upon the size and quantity of the dust
and magnitude of its charge. The effect of the space charge is to reduce the electric
field in the vicinity of the corona glow region and thus it tends to quench the corona
and reduce the current. This effect is particularly significant at the inlet section of a
precipitator where dust concentrations are highest.
A second important consideration of the effects of dust on corona generation is the
deposit formed on both collection and discharge electrodes. On the collection
electrodes dust deposits alter the electric field and sparking conditions as a result
of the voltage drop with in the dust layer. This effect limits the voltage and current
at which the predpitatorcan operate and is its chief influence on corona generation.
PARTICLE CHARGING
There are two physical mechanisms by which gas ions impart charge to dust
particles in the precipitator. Particles in an electric field cause localized distortion of
the field so that electric field lines intersect the particles. Ions present in the field
tend to travel in the direction of maximum voltage gradient which is along electric
field lines. Thus ions will be intercepted by the dust particles resulting in a net
charge flow to the particles. The ion will be held to the dust particle by an induced
image charge force between the ion and dust particles. As additional ions collide
with and are held to the particle, it becomes charged to a value sufficient to divert
the electric field lines such that they do not intercept the particle. Under this
condition no ions contact the dust particle and it receives no further charge. The
electrostatic theory of the process shows that the saturation value of the charge on
the particle is related to the magnitude of the electric field-in the region where
charging takes place, the size of the particle and the dielectric constant of the
particle.
The saturation charge is proportional to the square of the particle diameter thus
larger particles are more easily collected than small ones. This mechanism of
charging is called field- dependent charging.
For small particles (diameter less than 0.2 microns) field dependent charging
mechanism is less important and collision between the particles and gas ion is
governed primarily by thermal motion of the ions. The factors influencing charging
rate are particle diameter free ion density and thermal velocity for the ions.
Page 5
CONTENTS
TRAINING MANUAL FOR ELECTROSTATIC PRECIPITATOR
Since the range of thermal velocities has no upper boundary there is no saturation
value associated with diffusion charging. However, as the charge on a particle
increases the probability of impact decreases so that there is a decreasing charging
rate associated with an increasing particle charge, this second charging process is
called diffusion charging.
In practical precipitators, field dependent charging is usually of most interest but in
some applications, particles are present in the range where diffusion charging is
predominant mode (less than 0.2 microns) as well as the area in which both
mechanisms are significant.
Particle charging theory indicates several important factors governing precipitator
performance. Since the magnitude of the particle charge is dependent upon the
magnitude of the electric field in the field dependent mode it is important that field
strength be kept as high as practical in the region where charging takes place.
A second factor of importance is the rate of charging of the particles. Practical
precipitators generally introduce heavy concentrations of uncharged dust in the
inlet section of the precipitator. The electric field in the precipitator determines the
maximum value of the particle charge due to field dependent charging and also the
force acting on a charged particle.
Electric field strength is determined by the electrostatic component, which is related
to the precipitator geometry and the applied voltage and by the space charge
component which is related to the presence of charged particles (ions an charged
particulate) in the inter electrode space. The design of the precipitator can be varied
to the alter the geometry of the discharged electrode and the electrode spacing. This
factor can determine the magnitude of the electrostatic component. Variation in the
electrode geometry can also alter the corona current, which in turn influences the
electric field by changing the space charge contribution.
PARTICLE COLLECTION:
The forces acting on a charged particle in a precipitator are gravitational, inertia,
electrical and aerodynamic. The latter two are the principal ones of the importance
in electrostatic precipitation.
If a particle is suspended in a laminar gas flow stream in a pipe and wire
precipitator a force due to the electric field and particulate charge will act on the
particle in the direction of the collection electrode. This force is opposed by the
viscous drag force of the gas.
Page 6
CONTENTS
TRAINING MANUAL FOR ELECTROSTATIC PRECIPITATOR
In sufficient time, which is short for small particles, the particle would reach a
terminal velocity at which point the electrical and viscous drag forces could be
equal. In precipitator terminology, this is called the migration velocity. The other
force acting on the particle is the aerodynamic force by the gas stream. The motion
of the particle will be along the line defined by the vector sum of these two forces.
Under laminar flow all particles would be collected in a given length of the
precipitator and the collection efficiency for shorter lengths would be linearly
related to precipitator length.
In practical size precipitators, however, laminar force is practically never achieved.
Consequently the turbulent gas flow causes particles to flow random path through
the precipitator. The magnitude of the forces due to the turbulent gas flow is large
compared to the electrical forces. However at the boundary layer, the gas flow is
laminar and particles entering boundary layer will be collected. The collection
efficiency is therefore related to the probability of a particle entering the boundary
layer.
Studies by Anderson, Deutsch and White of particle collection in turbulent gas
stream have shown theoretically that collection efficiencies are exponentially related
to the collection surface, the gas volume handled and the migration velocity of the
particle. The quantum known generally as the Deutsch, Anderson of the form
(efficiency = 1-Exp).
A principal practical use of the Deutsch-Anderson equation has been in relating
measured collection efficiency to the collecting surface area and gas volume. In
such cases the term 'W as calculated from the Deutsch-Anderson equation is a
parameter rather than the migration velocity given by theoretical considerations. In
this case it is called effective migration velocity or precipitation rate parameter.
The term is useful in describing the effectiveness with which a given dust can be
collected and is widely used in design and analysis of precipitators.
From a theoretical as well as a practical standpoint the distribution of particles
within the precipitator is important. There is some evidence to indicate that particle
distribution with in the precipitator may not be uniform and that diffusional forces
may also play a role in collection efficiency.
Page 7
CONTENTS
TRAINING MANUAL FOR ELECTROSTATIC PRECIPITATOR
REMOVAL
Once collected the dust must be removed from the precipitator. This can be
accomplished by flowing liquid down the collection electrode to wash the collected
dust or by rapping the electrodes to impart an acceleration to dislodge the dust,
which falls into a hopper for subsequent removal.
In dry removal system rapping of the collection electrode to remove the dust is
normally done on periodic basis. Successful rapping depends upon accumulation of
sufficient thickness of the material on the plate so that it falls in large agglomerates
into the hopper. There is always some re-entrainment of the dust so that effective
rapping must minimize the amount material reentrained in the gas stream.
The acceleration required to remove the collected dust varies with the properties of
the dust and gas stream. Forces of cohesion and adhesion consist of molecular,
electrical and mechanical forces. Some dusts adhere tenaciously to the collection
surface and require substantial acceleration to dislodge them. Variations in
operating temperature gas composition or both can alter the forces required for
successful rapping.
Electrical forces which are related to current density and dust resistivity are also
significant in holding the collected material to the plate and therefore affect the
forces required for rapping. Since current densities are higher at the discharge
electrode than at the collecting electrode greater forces are often required to
maintain them relatively free of dust deposits than are required at the collection
plates.
Reentrainment of the dust during rapping is evidenced by increased dust loadings
at the precipitator exit following a rap. To minimize this effect only small section of
the precipitator are rapped at one time.
CONCLUSION
The basic principle of electrostatic precipitator operation remain the same
irrespective of the process of application and they fall broadly on the topics covered
above.
Page 8
CONTENTS
TRAINING MANUAL FOR ELECTROSTATIC PRECIPITATOR
2. SELECTION AND SIZING OF ELECTROSTATIC PRECIPITATORS FOR COAL
FIRED BOILER INTRODUCTION
A fundamental task in precipitation technology is the design of optimum
precipitator system for given applications. Precipitator design has changed in
character during the past several years from a routine and casual function to a
more serious enterprise involving high performance and high financial stakes. This
change has been forced by the implementation of stringent air pollution control
standards which require substantially invisible stack emissions.
FACTORS AFFECTING THE SIZE OF ESP
Precipitator
performance
depends
fundamentally
on
physical
and
chemical
properties of the gas and particulate treated. In a power plant these properties are
governed by the coal burnt the furnace design and the overall operation of the
boiler. Precipitator design and performance are strongly dependent on the
properties of the coal burnt in the furnace. All coals of Indian origin contain
significant amount of ash or residues of combustion consisting chiefly of inert
oxides and silicates. Characteristic of coal vary greatly because of the wide
distribution of coal deposits and the many different geological formation in which
these deposits occur. The variability and uncertainty of coal properties are reflected
in the ash generated and these uncertainties and variations can make the problem
of fly ash collection singularly difficult. A typical value of Indian coal and ash
analysis is furnished in Annexure. In order to cope successfully with particulate air
pollution from coal fired power plants it is necessary to apply consistently a high
order of appropriate technology.
DESIGN PARAMETERS
Basic parameter used in the precipitator design are gas flow, electrical resistivity,
specific collection area, gas velocity, aspect ratio, treatment time and number of
fields in gas flow direction. The value of these parameter vary with particle and flue
gas properties with gas flow and with required collection efficiency. The migration
velocity achieved in actual operation depends strongly on many factors such as
accuracy of precipitator electrodes alignment uniformity and smoothness of gas flow
rapping of electrodes and size and electrical stability of the T/R sets.
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GAS FLOW
The total quantity of gas flow is a fundamental factor in determination of the size
and performance of the electrostatic precipitator. In general precipitator size for a
given efficiency is proportional to the gas volume but for a given precipitator size
efficiency drops off with increasing gas volume in accordance with the equation.
? =1- e- (w *A)/Q
Where ? = efficiency of precipitator in percent
A = collection area in Sq.Metre
Q = flue gas volume in Cub.Metre / Sec.
W = migration velocity in M/sec.
The quantity of combustion gas produced in a boiler depends on the composition of
coal burnt the excess air used for combustion and the air in leakage through the
furnace, flue gas ducts, air preheater and electrostatic precipitator. The flue gas
flow through the precipitator also is a function of gas temperature and pressure.
Actual operating gas flows may be more than the design value due to the reasons
enlisted above and consequently increased stack emissions.
ELECTRICAL RESISTIVITY OF ASH
Experience over many years has shown that fly ash from low sulphur coals similar
to that of ours usually has high electrical resistivity and is difficult to precipitate.
Theory and experience indicate that when the dust resistivity exceeds a critical
value of about 10 to the power of 10 Ohm-cm the precipitator operating voltage is
limited which in turn reduces precipitator efficiency. The loss in performance
increases quite rapidly for resistivities greater than 10 to the power of 10 ohm-cm
and resistivity is there fore a major factor in precipitator technology. Detailed
studies made by us indicate resistivity of the order of 10 to the power of 10 ohm-cm
for fly ashes resulting from combustion of coal. Higher electrical resistivities of the
fly ash result in much lower values of migration velocities and consequently a
precipitator having large specific collection surfaces for meeting the prescribed
performance guarantees.
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COLLECTION SURFACE (SPECIFIC COLLECTION AREA)
The collection surface required for a given flow and efficiency is usually expressed
as specific collection area (SCA) i.e. the collecting surface provided for unit gas flow
rate. Practical values of SCA usually range between 100 to 200 for efficiency range
of 99% and 99.9%.
GAS VELOCITY
This is relationship between the total gas flow and cross sectional area provided for
a precipitator. The cross section as taken the open area available for gas flow
between the end collecting plates disregarding the plate baffles. Primary important
of the gas velocity through the precipitator is its relation to rapping and reentrainment losses. Above some critical velocity these loss tend to increase rapidly
because of the aerodynamic force on the particles. The critical velocity depends on
the quality of the gas flow plate configuration, precipitator size and other factors
but for most fly ash precipitators velocity is 1.2 m/sec.
ASPECT RATIO
This parameter is defined as the ratio of the total length of the electrode zone to the
height of the electrode. It is important in precipitator design because of its effect on
rapping losses. Collected dust released from the plates is carried forward by the
flow of the gas. If the length of the collecting zone is too short compared to the
height some of the falling dust will be carried out of the precipitator before it
reaches the hoppers thereby substantially increasing the dust loss.
For efficiencies of 99 % and higher the aspect ratio should be at least 1 to 1.5 to
minimize carry over of collected dust.
TREATMENT TIME
This parameter is defined as the time taken by the flue gas to pass through the
length of the collecting electrode zone. Some of the dust can be carried out of the
precipitation zones due to insufficient treatment when gas velocities exceed about
1.2m/sec. and the duct length is less than 9 metres. The treatment time in that
case is only about 7 sec. for efficiencies of 99% and higher the treatment time
should be at least 15 seconds to ensure satisfactory treatment and collection of the
dust.
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NUMBER OF FIELDS IN SERIES
Theory and practical experience confirms the fact that precipitator performance improves with the number of fields in series (degree of high-tension sectionalisation).
There are several fundamental reason for this improvement. Electrical alignment
and spacing are more accurate for smaller sections. Smaller rectifiers needed are
inherently more stable under sparking conditions and the sparks which occur are
less intense and damaging to performance. Outage of one or two electrical sections
has a much smaller effect on efficiency where a relatively large number of hightension sections are used. The optimum degree of high-tension sectionatisation is a
balance between the increase in efficiency obtained with more sections and the
increased cost of providing the additional sections. This balance is highly dependent
on ash property, gas temperature, efficiency required and the space availability.
EMISSION REGULATIONS
The central! board for the prevention and control of water pollution Act-1984
stipulates the permissible emission limits for thermal power plants are as under:
Boiler Size
< 200 MW
>= 200 MW
Old
600
mg/Nm3
-----
After 1979
General
Protected area
3
350 mg/Nm
150 mg/Nm3
150 mg/Nm3
Existing
(SPM)
150
mg/Nm3 *
150 mg/Nm3
As per the Environment (Protection) Second Amendment Rules, 1993 Schedule VI
(Part-D) enforced from January 1, 1994. The Amendment empowers State Boards to
specify more stringent standards for the relevant parameters with respect to specific
industry or location. Andhra Pradesh State pollution control board made the limit
as 115 mg/Nm3.
A protected area is one that is already polluted from being in a metropolitan
industrial location or the area is sensitive because of proximity to national parks/
forests/ historical monuments/health resorts etc. While specifying the emission or
the collection efficiency the compliance with the stipulations shall be ensured.
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CONCLUSION
In this paper the effect of various critical parameters have been discussed and adequate amount of
design conservatism shall be adopted for obtaining the desired level of efficiency. This is more
demanding in case of retrofit application where the conditions are varying widely.
ANNEXURE
01.
02.
03.
PROXIMATE ANALYSIS
RANGE
Total Moisture
%
9-10
Volatile matter
%
23 – 25
Fixed carbon
%
29 – 33
Ash
%
39 – 32
Carbon
%
40 – 45
Hydrogen
%
2.5 – 3
Nitrogen
%
0.8-1.0
Sulphur
%
0.4 - 0.8
Oxygen
%
8.3 - 8.7
Total moisture
%
9-10
Ash
%
39 – 32
Silica
%
59
Alumina
%
21
Iron Oxide
%
7.5
Calcium Oxide
%
6.5
Magnesium Oxide
%
Sodium Oxide
%
Potassium oxide
%
Phosphorous pentoxide
%
Sulphur trioxide
%
Titanium
%
ULTIMATE ANALYSIS
ASH ANALYSIS
3
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3.
MECHANICAL DESIGN OF ELECTROSTATIC RECIPITATOR
INTRODUCTION
Electrostatic Precipitator is a dust cleaning system provided after coal/oil fired
boiler. It removes fly ash from flue gas coming out of the boiler. Due to the stringent
particulate emission regulation the present efficiency requirements are in the region
of 99.3% to 99.9%. It is rather essential to provide a correctly sized ESP with a
sound internal arrangement. The alignments of internals, effective rapping system
and uniform flue gas distribution are important requirements apart from healthy
electrical system. The performance of internals can be achieved by proper and
careful erection. Faulty erection method will lead to improper alignment of
internals, which cannot be rectified at later stage.
SUPPORTING STRUCTURE AND SUPPORT BEARINGS
The supporting structure of ESP is a rigid frame structure capable of supporting the
load of entire ESP collected dust and additional vertical loads because of horizontal
forces due to wind and earthquake (fig 1). Diagonal members are provided to
transfer the horizontal forces on the ground without generating any moments in the
members. So all the members area designed for axial forces alone. Site welding of
the joints are critical and should be carried out with great care. Support bearings
are provided between casing columns and supporting structure to ensure that the
casing moves freely over supporting structure due to thermal expansion. These
structural bearings are provided with PIPE lining to take horizontal movement and
spherical surface to take angular movement. Side guides are provided to take
horizontal forces coming on the support. The guides of bearing should be kept
parallel to the line joining fixed foot of EP and the particular support point. Mirror
finished surfaces should be protected from any damage.
CASING
Casing is made of 6mm mild steel plates with required stiffeners. Internal bracings
are provided to transfer the horizontal forces due to wind and earthquake to the
support bearing level. Casing columns are subjected to axial compression. The
entire internals collected dust self-weight and additional load due to horizontal
forces are supported by casing. Casing walls are designed to take lateral load due to
wind and under pressure. Casing strength is calculated for the above mentioned
loads at higher temperature (normally 150° C and exceptionally 300° C). All the
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internal bracings and columns will come between the electrical fields and the gaps
are maintained in such a way, that no mechanical fouling or electrical sparking
takes place. Both emitting and collecting system are hung from the top of casing.
The site welding of casing components should be carried out carefully so those no
leakage exists. The alignment of internals depends to a great extent on the
alignment of casing. Bolted connections are provided between the components to
facilitate erection. All the joints are to be welded before the internals are loaded.
OLD CASING
NEW CASING
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This type of casing is known as IB casing . Following are features of the IB casing:
•
The side walls are made of horizontal panels
•
It has two types of roof beam known as longitudinal and transverse roof
beamThe columns are sent separately and site assembled internal horizontal
and diagonal bracings are provided in between the electrical fields
•
Casing columns are positioned in such a manner that the portal beam
immediately below bearings is avoided.
The above change in the design of casing has resulted in considerable reduction in
erection of casing time.
HOPPERS
Pyramidal hoppers are provided under the casing of ESP to collect the dust. The
hoppers should not be treated as storage place for dust. It is preferred to evacuate
the hoppers at the earliest. Long storage of dust leads to clogging of hoppers. The
hoppers are designed with a valley angle of not less than 55° to facilitate free fall of
dust in hopper. Hopper bottoms are
provided
with
electrical heaters
to
avoid any condensation of moisture
resulting
in
clogging
of
hopper.
Hoppers are made of mild steel plate
with adequate stiffness to take up dust
pressure. The hoppers are connected
to each other in the form of ridge. The
ridges are made of wide flange rolled,
reams and rolled channels only. Both
manufacturing and erection are easy.
According to the size of hopper it is sent in number of wall panels which are to be
welded together at site to form hopper. The welding should be proper so that
leakages do not exist. The wall stiffeners should be outside the hopper. Depending
on customer option the bottom part of hopper is provided with smooth stainless
steel inside liner, poke holes or combination of the above. Hoppers are provided
with inspection door. On customers option hoppers are provided with ash level
indicators.
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EMITTING SYSTEM
Emitting system consists of rigid emitting frame (like a cage) suspended from four
points on the top and emitting electrodes in the form of open spiral (fig 4). The four
suspension points are supported on support insulators to give electrical insulation
to the emitting frame. The frames are designed to take up the retention forces of
emitting electrodes which is 20 Kgs
force
per
emitting
electrode.
frame
Members
are
of
generally
rectangular hollow section. The frame
parts
are
manufactured
in
special
fixtures to obtain a closer tolerances on
the dimensions. Special packings are
provided for frame parts. Since this is a
Live part of ESP which will be at 70 kV
(peak) no sharp projection is desirable.
Care
should
be
manufacturing
and
taken
while
erecting
these
components. The weld joints of frame
should be made carefully since these
frames are subjected to rapping and any crack in the weld will lead to failure in due
course of time.
By using four points suspension as mentioned earlier the frame design is totally insensitive to expansion and so rigid that the operation and maintenance crew can
climb on it without disturbing the alignment. The advantages with our rigid frame
design for the emitting system are:
No electrodes are passing the top or lower collecting electrode edges thus spark
erosion hazard is thereby totally eliminated.
No need for ceramic stabilizers at the bottom part of the emitting system for perfect
positioning.
A detailed sketch of the emitting system is enclosed. Support insulators are housed
in weather tight insulator housings which are provided with electrical space heaters
and thermostats. Heaters prevent any condensation on the insulator surface.
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EMITTING ELECTRODES
The discharge electrodes consist of hard drawn spiral wires. The spiral discharge
electrodes are sent to the erection site in the form of spring like coils. At the site
these coils are attached, and stretched out between top and bottom holder in each
level of the discharge framework.
These spiral electrodes are fastened with hooks
to the discharge frame.
Several advantages of this type of electrode are:
Because of their coil spring form the
emitting
electrodes
tensioning
are
(approximately
self
15-20
kgs/spiral which means they resist
these
electrical
field
and
remain
positioned on the centre line of the
gas
passage.
positioning
This
permits
the
stabilized
highest
possible operating voltage.
By utilizing the self-tensioning effect
of
the
spiral
electrode
coils
no
weights are required to keep the electrode hanging plumb. The absence of weights
makes it possible to terminate the discharge electrodes prior to their passing the
edge of the collecting plates. This eliminates flash-over and the need for shield in
the discharge electrodes.
The self-tensioning spiral discharge electrodes allow for better transmission of the
rapping forces. Because of the intermediate frames each separate discharge
electrode is kept short. Short wires well tensioned are not prone to swinging. The
spiral wire electrode provides a uniform current distribution over the full height of
the collecting plates since the corona discharge will occur around the entire surface
of the wire as opposed to a solid emitter with pronounced peaks where the corona
discharge will occur only at the tip of each peak.
RAPPING MECHANISM FOR DISCHARGE ELECTRODE
During electrostatic precipitation a traction of the dust will be collected on the
discharge electrodes and the corona will be suppressed as the dust layer grows. It is
therefore necessary to rap the discharge electrodes Occasionally. This rapping is
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done with a rapping system employing tumbling hammers which are mounted on a
horizontal shaft In a staggered fashion. These hammers hit specially designed shock
beams to which one intermediate part of the discharge frame is attached. In this
manner the vibrations generated by the hammers are transmitted to the discharge
electrodes.
One such rapping mechanism is
provided
for
each
electrical
bus
section. The drive of the rapping
mechanism
is
through
a
shaft
insulator which is installed in one of
the insulator compartments located
on the roof of the precipitator (fig 6).
The operation of the gear motor for
the
rapping
controlled
mechanism
by
is
synchronous
programmer which is adjusted to optimum conditions at the time of commissioning.
Subsequent adjustments can easily be carried out during operation should
conditions vary.
COLLECTING SYSTEM
The design of the collecting system is based on the concept of dimensional stability.
The upper edges of the collecting plates are provided with hooks which are hung
from support members welded to the roof structure. The lower edge of each plate
has a shock receiving lug which is securely guided by the rapping system. By using
an eccentrically mounted suspension hook for the collection on plates good and
positive contact between the shock iron and the shock bar is guaranteed. This
results in a dimensionally stable collecting system compatible with the discharge
system.
In order to maintain the collection efficiency at the design level it is essential that
the discharge electrode and the collecting system are dimensionally stable.
Collecting system mainly consists of collecting suspension frames collecting
electrodes and shock bar (fig 7) Collecting suspension frames are made of slotted
angles which are to be fixed to the roof beams. They should be properly aligned with
the emitting system. Special care is taken during manufacturing to get a closer
tolerance on the dimension. They are properly packed and sent to site.
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Collecting electrodes are made of 1.6mm thick mild steel sheets formed in G profile
of 400mm width. A special roll forming machine is used to get a closer tolerance on
G profile. Hook and guide are welded on one end and shock iron (which goes inside
the shock bar) on the other end on a fixture. Side slot if required, are made on the
roll forming machine itself. The electrodes are bundled together and dipped in rust
preventive oil tank. Collecting electrode bundles should be properly handled to
avoid any damage to the electrodes. Minor local dents can be rectified at site with
the help of correcting tool. Before erecting the electrode should be checked for any
damage rusting and straightness.
Shock bar is provided to transmit the rapping acceleration effectively to all the
collecting electrodes in the row. It is suspended under the Collecting electrodes and
guided transversely by shock bar guides. Collecting electrodes are loosely connected
to the shock bars. The anvil portion of shock bar is stress relieved after welding.
The shock bar should be checked for straightness before erection.
RAPPING MECHANISM FOR COLLECTING ELECTRODE
An essential parameter when designing the internal equipment of a precipitator is
the design of the rapping mechanism for the collecting system. It is essential that
this system is thoroughly cleaned during rapping. The acceleration of the plate
resulting from the rapping action has the greater influence of the cleaning
efficiency. In order to achieve efficient cleaning the rapping system is constructed so
as to provide the required accelerations
over all the plates.
Each collecting plate of the system
offered has a shock lower end. The
plates in one row of each field are
interfaced to one another by means of
these shock receiving plates located in
slots in the shock bar maintaining the
required spacings. The shock bars are
kept in alignment by means of guides.
Each collecting plate is hung on an
eccentric
hook
to
ensure
that
the
shock receiving plate of the collecting electrode is constantly resting against the
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shock bar. In this manner the highest possible energy is transferred to the
collecting plate when the tumbling hammer hits the corresponding shock bar.
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 alt of the
collecting plates in one row because of their direct contact with the shock bar. A
uniform rapping effect is therefore provided over the whole row of collecting plates.
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 design of our
rapping mechanism, the electrodes are given 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 shearing action
greatly reduces the possibility of dust re-entrainment during rapping.
The rapping frequency should be as low as possible in order to minimize the dust
re-entrainment from rapping. The frequency of the rapping mechanism offered by
us is adjustable within wide limits. There is one set of rapping equipment provided
for each series field 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 side access passages, before, between and after the collecting plates.
All physical data essential for designing plate suspension eccentricity and rapping
intensity for this type of dust has been tested in our laboratories.
This type of tumbling hammers rapping mechanism has been used by our
collaborators for fly ash application for over 20 years as well as in all other
precipitator applications.
From full scale tests carried out in our laboratory the acceleration in any point of a
system similar to the one quoted has been determined. Table 1 shows the
effectiveness of the rapping mechanism measured on the collecting plates. The
numbers 2 and 6 in the direction of flow indicates the position of the plate in
relation to the tumbling hammer.
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When judging the effectiveness of the collecting system it is also essential to keep in
mind the total collecting area being rapped at any instance. The higher the
percentage of the total collecting is being rapped at any time the greater the reentrainment of dust into the gas. With out tumbling hammer rapping mechanism
only a very small percentage of the collecting area for each precipitator is rapped at
one time.
This improves the overall efficiency of the precipitator and avoids puffing at the
stack outlet. The functional capabilities of the tumbling hammer system and its
operational reliability have made it a Flakt standard utilised in all installations.
TABLE 1
COLLECTING ELECTRODE PLATE
ACCELERATION IN Gs
PLATE HEIGHT
COLLECTING PLATE No.
2
9M
12M
18M
Top part of the plate
460
400
360
Middle part of ptate
560
480
430
Bottom part of plate
880
880
880
Top part of the plate
190
160
150
Middle part of ptate
230
200
180
Bottom part of plate
360
360
360
COLLECTING PLATE NO.
6
The latest design is with 250mm wide collecting electrodes of 1.5mm thick sheet (fig
8). The hook will be provided on the collecting suspension frames and the slots will
be provided on collecting electrode. The shock bar will be firmly connected to the
collecting electrodes with the help of huck bolts (similar to rivet). Three emitting
electrodes per collecting electrode will be provided instead of two as in the case of
400mm electrodes.
GAS DISTRIBUTION SYSTEM
The gas velocity within the precipitator is approximately 1/20 of the velocity in the
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dueling before the precipitator. It is therefore essential that the precipitator is
equipped with arrangements that will give an even gas distribution over its entire
cross sectional area. The desirable gas distribution cannot be achieved solely
through the design of the ducts. Special gas distribution plates are placed before
the precipitator itself (fig 9).
Recognizing the importance of preventing areas of high gas velocities construction
of the gas distribution arrangement consists of two separate rows of baffles located
at the inlet of the casing.
The velocity distribution within the precipitator casing is checked prior to
commissioning. During these gas distribution tests any necessary alterations to the
flow pattern will be made by the installations of horizontal baffle plates sealing off
required areas of the gas distribution screens.
In addition to the gas distribution plates at the inlet of the precipitator the outlet
funnel of the precipitator is also provided with one row of gas screening plates to
improve the flow pattern near the outlet.
The gas distribution screens at the inlet of the precipitator are provided with a
rapping Mechanism if required. This rapping mechanism is similar to rappers used
for the emitting and collecting systems described earlier.
INSULATOR COMPARTMENTS
Each electrical bus section is supported from insulators located in insulator
compartments outside the casing roof. The weather tight insulator compartments
for the high voltage support insulators are of double walled construction with
thermal insulation between the walls. Each insulator compartment is furnished
with an access door for
inspection and service. To
avoid
dust
entering
up
into the insulator a screen
tube
is
installed
immediately below it.
Each of the insulator for
the electrical bus sections
has associated with it a
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one kW heating element which effectively heats the air space of the insulator
compartments and prevents condensation and deposition of the moisture on the
insulator. The elements are of tubular type and are formed to encircle each
insulator in order to provide uniform heating within the chamber. The electrical
heaters are thermostatically controlled. There is a special arrangement in each
insulator compartment which makes it possible to suspend the discharge electrode
system by means of a temporary jacking hook if the insulator must be exchanged.
INLET AND OUTLET NOZZLES
Inlet and Outlet nozzles are provided for each precipitator fabricated of 6mm mild
steel plate. The nozzles are adequately stiffened and braced to stresses due to wind
load earthquake load and suction pressure.
INTERLOCK SYSTEM
The ESP is a high voltage (70 kV) system and hence proper protection devices
should be provided to prevent any operational maintenance personnel to enter into
ESP when it is charged. An elaborate mechanical key type interlock system is
provided for the purpose. All the inspection doors insulator housing and
disconnecting switches are interlocked to rectifier transformer control panels.
Unless the rectifier transformers are de energized and the fields are grounded, a
person cannot open any inspection door disconnecting switch or insulator housing.
PERIPHERALS
Apart from the earlier mentioned features, ESP is provided with galleries and stairs,
rectifiers handling system, pent-house (optional), hopper approach platform
(optional), outer roof, rectifier transformer with controls, auxiliary control panels LT
distribution board, disconnecting switches etc.
Galleries and stairs are provided to make all the inspection doors, electrical
equipments etc. accessible for operation and maintenance purpose. It is designed to
take live load of 500 Kg/M2 of load. Platform and stair widths are generally not less
than 1.0 metre and 0.80 metre respectively.
Rectifier handling system is provided on top of ESP to lift T/R sets from ground to
the top or vice versa. A hand-operated pulley block is provided on a monorail placed
in such a way that it can handle any T/R set on the ESP.
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Pent-house is provided on top of ESP on customer's option. This provides coverage
for insulator housings, disconnecting switches, TR sets etc.
Hopper approach platform (customer option) is provided under ESP to make the
hopper inspection doors and hopper heaters easily accessible. This is required for
maintenance purposes,
Water washing system (customer option) is an elaborate system for washing the
collecting electrode surfaces (mainly) when any maintenance work is required to be
carried out. Washing is possible only when ESP is not charged. Clean water with
low chlorine content should be used for cleaning the ESP internals.
A chequered plate outer roof is provided on top of ESP as maintenance platform.
ESP top insulation is provided in between roof panel of ESP casing and outer roof. If
the pent-house is provided the outer roof would be flat, otherwise, it would have a
two degree slope for rain water drainage. It is designed to take up 500 kg/m 2 of live
load. Rainwater drainage channels are provided to prevent rainwater to fall
hazardously.
THERMAL INSULATION SLAGGING
The precipitator casings, the hoppers and the roof of the precipitator will be
insulated as per the company's standard procedures with both sides Gl wire netting
of mineral wool mattress (slag) of adequate thickness to ensure a maximum surface
temperature of 65°C over an ambient temperature of 45°C and air velocity of 3
metres/second (figs 11 & 12).
The insulator materials and protective covering wilt be new and unused and is
guaranteed to withstand continuously and without deterioration the maximum
temperature to which they wilt be subjected under the specified applications. The
density of the insulation with mineral wool blanket insulating material will be 150
kg/cu.m. The mattresses will be installed by using stud of 6mm. Casing support
binding wires and insulation retainers. The mattresses are held by studs and the
joints of the mattresses are sewn together.
The wool mattress is tied by galvanized binding wires of 186 across the hooks/
studs. After application of insulation the outer casing will be laid. The sheathing
material for all insulation will be aluminum sheet of thickness not less than 1 mm.,
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ELECTRICS
Rectifier transformer (TR) sets are provided on top of ESP. The control panels
(electronic controllers) are housed in EP control room situated on the ground. They
are connected by power and control cables. TR sets are connected to emitting
system through disconnecting switches with the help of bus duct. Disconnecting
switches are provided to enable maintenance personnel to disconnect and ground
the emitting systems before any maintenance work is taken up.
Auxiliary control panels (ACP) housed in EP control room are provided to give power
supply and control the auxiliary equipments of ESP like heaters, rapping motors
etc. These field mounted equipments are connected to ACP by cables.
LT distribution board housed in EP control room is provided to distribute the power
supply to different panels. The above mentioned features are applicable for a typical
electrostatic precipitator for collecting fly ash. The EPs used for the collection of
soda ash coming out of a soda ash recovery boiler (paper industries) are different in
many ways as compared to the EPs used for fly ash collection.
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4. FLOW MODEL STUDIES IN THE FIELD OF ELECTROSTATIC PRECIP1TATOR
ABSTRACT
Flow mode! studies is an effective method to ensure uniform distribution of gases in
the electrode section of the electrostatic precipitator. A good understanding of the
field dynamic behavior is important to arrive at a reliable inference from such
studies. The need for approach to results and conclusion of model studies
conducted are outlined.
INTRODUCTION
The basic design philosophy of electrostatic precipitator emphasizes the need for
good gas distribution inside the electrode chamber.
THEORETICAL ASPECTS
Large installation of these days handling over three million cubic meters of gas
every hour with their associated limitations on space result in layouts giving rise to
serious flow distribution problems. The precipitator by itself is a low pressure
device, hence the flow pattern established by the inlet flue system upstream the
precipitator established the flow pattern within the electrode chamber.
Poor gas flow which includes unbalanced gas velocities, flow separation jet and
pulsating flow result in reduction in precipitator performance by unbalanced
loading and by re-entrainment.
Conducting studies in site and introducing necessary corrective measures are both
laborious time consuming, constantly and impracticable. Flow model studies thus
offers an apt method to tackle flow distribution at the design stage itself enabling
design of flow correcting devices and better design of flue system.
APPROACH TO MODEL STUDY
The following model requirements are considered while constructing the
model :i). Geometric similarity -the same scale being used for ail parts comprising the
model. As regards the surface roughness the effect of scale was compensated by
using materials of smooth surface.
ii).Kinematic similarity - the flow lines in model and full scale plant shall have
similar patterns.
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iii). Dynamic similarity - Here it was assumed that from a practicality point,
is sufficient to ensure that the Reynold's number was maintained in the region of
turbulent flow at all section of the model, A critical area being that of the electrode
section.
The following assumptions are made ;ii) The temperature distribution at the inlet of the precipitator is equalized by
mixing in the preceding duct work.
ii) Dust is evenly distributed in the gas and that a good gas distribution at the inlet
to the electrode chamber will suffice. Hence, a primary aim of the model study is to
ensure an even distribution of gases in the electrode chamber.
TEST SET-UP AND INSTRUMENTATION
The model comprises of the electrostatic precipitator, the inlet flues and the outlet
flues scaled down to 1/10. The main shell is fabricated from 1.6 mm sheet
steel/flexi glass. Flexi glass and windows and test measurement points provided at
various sections to enable measurements and visualization of flow patterns. Vane
type anemometers are used to measure the velocity distribution inside the electrode
chamber.
Prandtl tubes are used in conjunction with electronic micro-manometer to measure
the distribution ducts. Computer is used through keyboard terminals to evaluate
and analyze the measured values.
TEST PERFORMANCE:
The
testing
involves
the
measurement
and
analysis
of
flow
distribution,
introduction of flow correcting devices, flow pattern visualization using smoke
stream and measurement and analysis of flow pattern after introduction of flow
correcting devices.
Flow correction is done in stages measuring, analysing and correcting section by
section starting from the inlet dueling then the outlet dueling and finally the
precipitator chamber. Guidevanes installed for improving the flow pattern in ducts
are basically designed based on the literature available on duct losses and at the
times extrapolation on these.
The vanes in the funnel inlets to the electrostatic precipitator is arrived at an
experimental basis keeping in line with the recommendation given in literature. The
deflector plates provided on the screens are arrived at totally on experimental basis-
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GAS DISTRIBUTION IN DUCTS:
Measurements are made in various duct streams before the precipitator to check for
the equal distribution of flow in the various streams. Where necessary guide vanes
are installed in addition to those already installed to obtain the required flow
distribution between screens.
GAS DISTRIBUTION IN PRECIPITATOR CHAMBER:
After distribution in the various streams of the ducts have been nearly equalised
the distribution in the precipitator chamber is studied. Vanes are provided based on
literature available in the entry funnel. Thereafter repeated tests are conducted with
various locations of deflector plates on the distribution screens till the required
locations for the acceptable distribution is obtained.
Distribution studies are made keeping the Reynold number in the gas duct region
between the collection electrodes (which is critical region in the model) above the
critical value i.e. 4000 at 60% gas flow conditions. Smoke screens are also used to
check visually the flow stream inside the casing.
DUST DROP OUT STUDIES:
Dust drop out studies are carried out by injecting dust into the duct keeping the
flow well below 50% of the normal flow. This ensure that dust drop out takes place
in the ducts. Thereafter the flow is slowly increased to normal flow. Region on dead
zone areas and duct build zones are identified by those regions where dust remains
even after normal flow is obtained.
PRESSURE DROP STUDIES:
Pressure drop measurements are made as a difference of the total pressure at the
test sections of interests. In the case of precipitator the total pressure difference
between the test section in the duct near the inlet funnel of the precipitator and the
test section at the duct near the outlet funnel of the precipitator gives the pressure
drop in the model. This is further scaled to full scale plant by correcting at for the
flow conditions like density, temperature and gas flow.
ACCEPTANCE CRIETERIA FOR GAS DISTRIBUTION :
1). Minimum number of velocity readings taken in any section inside the
precipitator shall not be less than one ninth the area of that section in sq. feet.
2). Readings shall cover a minimum of every third gas page in section.
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3). The spacing between two levels of readings by the sections shall not be greater
than 10% of the collecting electrode heights.
4). The test section shall be at the leading edge of the first field and the leading edge
of last field.
5). The final velocity pattern in any section shall have 80% of the readings not more
than 1.15 times the average value of that section.
6). The average value of the various parallel streams shall be within + or -10 degree,
of the total average.
7). Low velocity may be accepted in the top and bottom levels in view of gas effect.
CONCLUSION :
The performance requirement of the present day precipitators is welt above 99%.
The basic design of the precipitator is to provide required collecting area for every
unit of gas volume to be handled. This brings out the importance of gas
distribution. The model study is the tool available to study this distribution and to
ensure the recovery level of distribution.
The flow correction devices like guide vanes in ducts, vanes in the inlet funnel of
the precipitator and the deflector plates on the screens are finally adopted in the
full scale plant and a final test conducted at a full scale plant to ensure that the
distribution is as desired and determined in the model studies.
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5. ELECTRICAL SYSTEMS FOR ELECTROSTATIC PRECIPITATORS
The following is a general description of the electrical equipment offered as a part of
the electrostatic precipitator for each Boiler. All the equipments specified herein are
designed in accordance with the accepted engineering practices.
HIGH VOLTAGE TRANSFORMER - RECTIFER { HVR )
The HVR Unit is an assembly consisting of a 415 volts single-phase high
transformer and a full wave rectifier bridge designed for electrostatic precipitator
service and contained in a tank filled with insulating oil. The tank also houses a
current limiting linear reactor connected in series with the primary winding of the
transformer and a HF choke connected to the negative terminal of Rectifier Bridge.
A high voltage measuring and feedback resistor column is also mounted in the
transformer tank.
The linear reactor limits the short circuit current during sparking in side the
precipitator to safe value. The HF choke protect the transformer rectifier from
surges occurring during sparking inside the precipitator.
Regulated AC input voltage available from electronic controller is fed to the
transformer primary and full wave rectified negative DC out put is taken through a
HV bushing.
The positive end of the rectifier bridge is connected to earth through current
feedback shunt. The transformer rectifier is designed for heavy duty operation of 24
Hrs. a day with frequent sparking inside the precipitator.
The transformer tank is a fully welded construction suitable for outdoor service.
The insulating and cooling medium of HVR is an oil of dielectric strength and good
heat transfer characteristics and is adequately protected from contamination.
The HVR is fitted with the following instrumentation and appurtenances.
1.
BUCHCHOLZ RELAY.
2. OIL TEMPERATURE INDICATOR.
3. WEATHER PROOF TERMINAL BOX HOUSING "LV" TERMINALS
FOR MEASURING AND CONTROL CIRCUITS, POSITIVE TERMINAL
ALONG WITH PROTECTIVE SURGE ARRESTORS, SHUNT
RESISTOR, SPARK DETECTOR.
4. A HIGH VOLTAGE DC NEGATIVE BUSHING WITH
PROVISION OF BUS DUCT CONNECTIONS.
5.
CONSERVATOR WITH OIL LEVEL INDICATION AND BREATHER.
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6.
ROLLERS.
7.
RATING PLATE.
8.
LIFTING LUGS.
ELECTRONIC CONTROLLER
The electronic controller unit feeding regulated input to HVR is a microprocessor
based controller suitable for indoor application and of dead front free standing floor
mounted type.
The electronic controller houses the following components:
* Switch fuse unit
* Main contractor
* A pair of thyristor in anti parallel connection
* Microprocessor
annunciation system.
based
automatic
control
module
with
fault
* Firing card.
* Overload relay.metere,control transformers, CTs etc.
* Cable glands lugs and control terminal blocks.
The panel front is fitted with BAPCON (BHEL’S ADVANCED PRECIPITATOR
CONTROLLER) comprising of digital display for measurement of secondary voltage
(average, peak, valley), current, spark counter, current set push buttons,
management net work /stand alone selector switch and also analog meters for
current and voltage measurement.
The automatic voltage control system operates so as to maintain constant current
form
transformer
rectifier
unit
under
dynamic
conditions
of
electrostatic
precipitators load. BAPCON controls the precipitator by changing the ignition angle
of the thyristors connected to the primary of the transformer rectifier set.
Precipitator current voltage and phase angle of the primary voltage are used as
Input data. The actual precipitator current which varies continuously due to field
conditions is compared with set value and the error signal is processed to control
firing angle of the thyristof to regulate the input voltage to the transformer to
achieve set constant current inside the precipitator. When a spark occurs the
current is interrupted for a preset time to allow deionizatlon and rebuilds to a value
slightly lower than the current at which the spark occurred. The current decrease
and the rate of rise to the set value are deciding the spark rate.
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BAPCON Provides facility like intermittent charging and peak sensing mode of
operating to suit the process for improving precipitator operation or for energy
saving.
Electronic controller provides the following protections /indications.
1. Protection for T/R from oil temperature high and internal faults.
2. Thermal over load protection.
3. Under voltage indication.
4. DC Voltage high.
5. Transformer temp.high.
6. AC Current high.
7. Peak detector.
A detailed write up on BAPCON is given below.
BAPCON is a microprocessor based unit for regulation and control of the electrical
power input to the HVR /Electrostatic precipitator.
BAPCON maintains the spark rate at a suitable level for
great variations of gas temperature, dust compositions,
flow rate etc.
It regulates the rectifier in such a way that the current
through the precipitator Is corrected constantly as the
conditions for the sparking are changed there by minimising the loss of energy.
The electrostatic precipitator functioning can be monitored on the BAPCON control
module. The figures on the control module display shows the precipitator current,
voltage, spark rate current limit etc. The figures are obtained in succession by
pressing the display select push buttonsAUXIL1ARY CONTROL PANELS (ACP)
The auxiliary control panels provided for Electrostatic Precipitators houses the
power and control circuits required for energizing rapping motors and heating
element of the precipitator.
The complete unit is modular and draw out type with individual modules for each
feeder. Each module houses the power and control circuits with push buttons, and
indicating lamps mounted on the door of the compartments.
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The heating elements for hoppers and support insulators are thermostatically
controlled. Indications for operation of heating element as well as ammeters to read
line currents of heater feeders are provided. The operation of rapping motor is
sequentially controlled through programmers. Annunciation/indication for rapper
motor 0/L trip/rapper motor ON or OFF are provided. The rapper motors are also
provided with local start/stop facility,
GEARED MOTORS
Each collecting and emitting system is provided with one geared motor unit coupled
to the rapper shaft and located out side the casing. The geared motor consists of
helical reduction gear with an integral DOL Start squirrel cage induction motor as
prime mover. The motors are suitable for 415V,3 phase. 50Hz AC input and of
weather proof IP 55 enclosure. The geared box unit is proving with oil filling drain
plugs and level indicators.
DISCONNECTING SWITCH (OFF LOAD ISOLATER)
HV disconnecting switch is provided for isolation of associated transformer rectifier.
In the OFF Position of the dis-connecting switch the emitting system of the
associated bus section is earthed there by proving safety for the personnel during
any maintenance work on the dead HV SYSTEM.
INSULATORS
Following insulators are used in the electrostatic precipitator:
1. Each bus section consists of four support insulators for supporting the emitting
system and are located inside individual insulator housing mounted on roof of
precipitators. These insulators are surrounded by heating elements to prevent
condensation of deposits causing any flash over.
2.Shaft insulator is provided to isolate each geared assembly from the associated
emitting rapping shaft, this also is provided with heating elements to avoid any
condensation on its surface which could result in flash over of the same,
3.0ne bushing insulator is provided corresponding to each field and is mounted in
the +insulator housing, this act as a bushing in the H.V bus duct system providing
necessary support and clearance for the H.V bus section. The insulators are of high
quality porcelain designed to with stand the operating temperatures.
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LT SWITCH BOARD
LT switch board shall be single /double front, dead front. floor mounting and
modular type. All the incomer feeders and bus coupler shall be fully drawout type.
The out going feeders (Switch fuse units) Shall be fixed type. This feeds power to
electronic controllers and auxiliary control panels.
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6.
PRE-COMMISSION1NG STABILISATION OF EP AND PERFORMANCE
TESTING OF EP
INTRODUCTION
Once the erection of EP is over, electrical work can start. Electrical jobs include the
erection of HVR sets electronic controllers, the auxiliary control panels geared
motors the heaters and the laying of inter connecting cables. After this process precommissioning activities involved in the pre commissioning of EP include.
*
*
*
*
*
*
*
*
Checking up of internals for tolerance
Checking up of internals for completion
Checking of heaters
Trial run of rapping motors
OCC of HVR sets
Air load test of fields
Gas distribution test on EP
Gas loading
The internals should be checked for any debris or welding protrusions which will
cause heavy sparking during charging. Any of the internals should be checked for
welding completion as given in the drawings. It should be checked whether all the
items as given in the drawings have been properly erected. This is particularly
applicable for screw for the shaft insulator where the inter-changing is quite
possible between left and right types.
Next, we can take-up charging of heaters and the rapping gears. It should be
ensured that ail these requirements are properly earthed so that safety of the
operating personnel is ensured. The rapping frequency for the various collecting
and emitting field should be set as given in the manual. The motors can be put on
trial run for Eight hours continuously,
To find out the healthiness of the HVR sets OCC test has to be done. After OCC is
done, the transformers may be connected to the fields and VI curves on static air
load as well as with ID and FD fans running may be taken,
With ID and FD fans running at rated current the gas distribution test is conducted
on EP to find out the co-efficient of variations below 20 %. To achieve this limits the
guide vans and blanking plates given have to be made use off. Finally these plates
should be welded to the screens.
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Finally the EP can be charged with flue gas. The EP can be charged once the flue
gas temperature is above due point and the combustion of coal/oil is complete. So
that chances for fire is avoided. The various fields may be set with different current
values as given in the manual. Also the spark limit of 5-10 sparks/minute have to
be ensured by proper S&T pot setting. Some coals may lead to back corona problem
because of high resistivity. In such coals, intermittent charging with 'ON' - 'OFF'
controller of ICE controller will help.
PERFORMANCE TESTING
In the performance testing of EP measurement of the operating flue gas volume,
temperature and the concentration of the dust is done both at the inlet and outlet.
This dust sampling should be done in such a way that velocity of sampling is same
as the flue gas velocity in the duct at that point. Pitot traverse is done after splitting
the ducts to number of equal areas using the Pitot tubes and micro-manometer.
Here velocity is measured as the dynamic pressure
Next dust sampling is done using the fiber glasses thimbles with proper size
nozzles. vacuum pump and the gas meter in series. The thimble entraps all the
dust coming alongwith the sampled flue gas and the gas meter records the gas
volume sampled.
Then concentration of dust (c) is
Where :
m
C = ——
x
m = mass of dust collected
x = volume of gas sampled
If C1 is the inlet dust concentration and C2 is the outlet dust concentration the
efficiency of EP is,
C1 -C2
n = ——
C1
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CONCLUSION
As the performance level of equipment goes up, the methods of measurement are
also keeping apace. The measurement accuracy will depend on the equipments
used for resting. Hence a proper selection of equipments and procedure will go a
long way in establishing a foolproof method.
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7. EP OPERATION AND MAINTENANCE
INTRODUCTION
Now-a-days, Electrostatic Precipitators are the main pollution control equipments
for the utility steam generators and they are very much suitable for treating large
volume of flue gas emanating from steam generators at very high efficiencies of the
order of 99.8% plus with low pressure drop. Though it is rugged and very simple in
construction still it needs some maintenance works for its proper upkeep. Points
that need attention during short and major shutdowns are discussed below.
CAUSES FOR POOR PERFORMANCE:
The main parameters to judge the EP performance is the emission from the
chimney. However, it will be very difficult to differentiate when the emissions are
below 150 mg/Nm3. High ash emission may result from one or more of the
following:
1. Fields not in service due to electrode snapping.
2. HVRs/Control panels are not working.
3. Ash removal system not working.
4. Voltage-current level is low.
One of the major reasons for the field tripping is ash bridging- this is caused by the
poor/ inadequate ash handling system, and non- functioning of the surface heaters.
Snapped electrodes also cause the field to trip. Reverse rotation of the rapping
system also may lead to slippage of the collecting electrodes from the hooks and
thereby snorting the fields.
The HVRs are placed on EP roof top and control panels in the air conditioned
control room. Any malfunction of either of the HVR or the panel affects the EP field
and hence EP performance. One of the ways to overcome this situation would be
put this particular field to the adjacent transformer alongwith its own field i.e. to
put two fields in parallel to a HVR. Though it is not a long term solution this will
help to reduce the emission until the defective HVR/EC is rectified.
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The voltage current level in the EP may be low because of:
* High ash deposit on the emitting/collecting electrodes
* Poor alignment of emitting and collecting system.
* Heavy sparking caused by high resistivity dust.
Continuous rapping of say for 10-15 minutes will reduce the ash deposit on the
electrodes. Initial deposits on the emitting electrodes some times, takes six months
to peel off. The problem of poor alignment can be attended only during-a shutdown.
Problem associated with high resistivity dusts can be solved by resorting to either
conditioning by SO3 & NH 3 or to pulse charging.
One need not over emphasize the importance of adequately designed ash evacuation
system. ash accumulation in hoppers not only leads to bridging of emitting and
collecting system causing the field to trip but also damage to the internals.
Cases of failures of rapping shafts, displacement of shockbar guides, bending of
collecting electrodes because of ash reaching the level of manhole door have come
to our notice. Proper upkeep of heating elements in the hoppers and operating the
ash removal system regularly will obviate this problem.
OPERATION OF EP AND LOG MAINTENANCE
Electrostatic precipitator is one of the equipments which has very few moving
/rotating parts and hence needs minimum maintenance. Nevertheless, reliable and
sustained good performance will result if a little attention is paid to the operation
and maintenance of this rugged and magnificent equipment. Hourly logging of the
voltage current levels of the various fields as well as the operation of the heaters,
rapping motors will aid in analyzing problem if any at a later date. Typical log sheet
is enclosed (Annexure-1).ON LINE MAINTENANCE
*
*
*
*
*
Drive for rapping system
High voltage power supply system
Heating elements for hopper insulator housing and
Ash level indicator
Ash handling system
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SHORT AND LONG
Whenever the unit is under shut down the opportunity may be availed to inspect
the internals, During a short shutdown extending say upto a fortnight the following
Jobs can be attended to.
*
*
*
*
*
Snapped emitting electrodes, if any can be removed
Ash deposit on the shaft support insulators can be removed.
visual inspection of alignment between emitting and collecting
system as well as rapping systems can be made.
Places of air/water leakage if any, can be identified
Minor deformation to the collecting electrodes can be rectified.
However if there is any major defect /damage in the internals that can be attended
to only during major overhauls.
SPARES
One has to plan well for spares before starting the shut down jobs on electrostatic
precipitators. It is found that items like shock bar guides and raping hammers etc;
are required for replacement. Hence adequate stock of these items as well as other
spares will be a wise proposition. The annexure II gives a recommended list of
spares for three years operation.
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ANNEXURE-1
RECOMMENDED SPARES FOR ELECTROSTATIC PRECIPITATOR 200 MW
Sl.no
Items
Qty/Boiler
1.
Support insulator
10
2.
Shaft Insulator Assy.
16
3.
Bushing Insulator Assy
04
4.
Heating Element/Hopper
06
5.
Heating Element for Insulator housing
06
6.
Heating Element for Shaft Insulator
06
7.
Foot mounted Gear Motor
06
8.
Timers
06
9.
Synchronous Programmer
02
10.
Thermostat for Insulator Housing
02
11.
Thermostat for Hoppers
06
12.
Emitting electrode
13.
Inner arm
40
14.
Outer arm for Emitting Electrode
16
15.
Outer arm for Collecting Electrode
16
16.
Plain bearing
06
17.
Set ring
08
18.
Sleeve Pin for Shock bar guide
02
19.
Shock bar
08
20.
Carbon Bush
06
21.
Shock bar guide front
24
22.
Shock bar guide rear
14
23.
Sleeve for Shock bar guide
06
24.
Pin Insulator for disconnecting switch
10
600
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8. RECENT DEVELOPMENTS IN ELECTROSTATIC PRECIPITATOR
INTRODUCTION
In recent years there has been an increasing demand for fly ash removal efficiencies
of 99.8% and above. Air (prevention and control of pollution)Act 1981 stipulates
emission standard which requires the collection efficiency of the electrostatic
precipitator to be very high. In India, we use mainly tow sulphur coals. Because of
the high resistive nature it becomes difficult to collect Indian fly ash and hence the
size of EP is large. Studies conducted by us resulted in development of ICE-C
controller, BAPCON controller and pulse rectifier, which is well suited for low
sulphur coals.
Recent experimental work has shown that wider spacing EP results in increased migration velocity. The potential of plate spacing as a parameter for optimizing design
can be evaluated only by conducting experiments. If migration velocity can be
increased as plate space is increased it might be possible to hold collection
efficiency constant and reduce the specific collection area and therefore the initial
cost of a precipitator. BHEL has taken up R&D experiments on wide spacing with
pilot and full scale tests.
Establishing of basic research facilities to analyze the various parameters and to
evaluate the various components is required for any company to attain strong
technological
base.
BHEL
has
already
established
its
ELECTROSTATIC
PRECIPITATOR LABORATORY to analyze various parameters and help in selecting,
designing, evaluating improving and maintaining a trouble free long lasting and
economic electrostatic precipitator.
RECENT DEVELOPMENT
The development of new ESP technology notably in the high voltage supply area Is
opening up new possibilities for substantial energy savings reduced dust emissions
and increased availability. Today there are plants in commercial operation in which
energy savings of between 70 and 90 percent are achieved and where emissions are
reduced up to 85 percent.
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The efficiency of dust collection in an ESP follows an exponential curve. Any desired
collecting efficiency can be achieved If the precipitator is sized large enough. The
following formula Is used universally for calculating the required collection area (A)
m2 for a given dust collecting efficiency and a given volume flow(Q) m3/s
n =1-e- (WK x A)/Q
In the formula K is a constant and WK a dust constant characteristic of a given dust
composition. WK is called the migration velocity. If the dust is difficult to collect i.e.
if it has a low WK a high A is required to achieve the desired collecting efficiency. WK
is determined largely on the basis of the ESP supplier's experience from operating
plants.
Developments have taken place recently in the field of electrostatic precipitator
controllers to obtain substantial energy savings reduced dust emissions increased
availability and cost reduction. Major developments are as follows:** Development of ICE-C controller
** Devolopment of BAPCON controller
** Development of multipulse transformer set
** Introduction of wide spacing ESP
DEVELOPMENT OF BAPCON CONTROLLERS
BHEL’S
ADVANCED
PRECIPITATOR
CONTROLLER
(BAPCON)
is
based
on
intelligent microprocessor to regulate and control power input to the electrostatic
precipitator. Thyristor controlled rectifiers of any make can be connected to
BAPCON. For different gas temperatures, dust compositions, gas flows etc.,
BAPCON maintains the spark rate at optimum level. For different conditions of
sparking current through the electrostatic precipitator will be corrected by controlling the primary current of the transformer rectifier. The EP functioning can be
monitored on the BAPCON control panel. If the value of the parameter is out of set
limits BAPCON gives an audio and visual alarm. Co-ordinated control of all the EP
functions can be achieved by connecting number of BAPCON units to a centralized
management system called INTEGRATED OPERATING SYSTEM.
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BAPCON controls the EP power by controlling the firing angle of the rectifier
thyristors. EP current and voltage as well as phase angle of the primary voltage are
used as control inputs. Initially EP current is slowly increased toward the set
current limit.
When a spark occurs the EP current is blocked for one cycle and then the CP
current is restarted. At this point the EP current suddenly raises to (IS-S) level. The
spark rate will be controlled by the set values of S&K controls.
INTERMITTENT CHARGING:
The number of half cycles the thyristors should conduct can be selected. The
selected number indicates the number of half cycles the thyristor should conduct.
Intermittent Charging operation be selected for improving precipitation or energy
saving. Often both are possible.
BASE CHARGE:
When we adapt very high charge ratio, the valley voltage drop and thereby average
voltage will also go down. To improve the valley voltage and also to eliminate the
effect of uni-polar charging of HVRs base charging facility is provided. The small
charging is introduced (base charge) in between the two main intermittent charging
pulses to increase the valley voltage.
AUTOMATIC OPTIMISATION:
The BAPCON controllers are provided with the feature of automatic optimisation.
BAPCON samples the VI characteristics of the fields at regular programmable
intervals and selects the best possible chrge ratio.
Figure-3 shows the general arrangement of BAPCON controllers developed by BHEL
which are in operation at Tuticorin TPS and DESU power station. The improved
performance has been obtained by using BAPCON controllers. The major
advantages are:1. Reduced power consumption.
2. Increased efficiency and performance of EP
3. Can be retrofitted in existing power controllers of EP with thyristor control.
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4. Automatic selection of best intermittent charge ratio for varying operating
conditions
5. Base Charging facility increase the valley voltage and hence the
performance
DEVELOPMENT OF MULTIPULSE TRANSFORMER SET
A recently developed technology for Electrostatic precipitators is pulsed energisation
extensive research and development during the last decade has introduced several
pulse concepts of which a few are commercially available today reduced outlet
emissions have been emphasized but tests by BHEL shows that remarkable power
savings also can be obtained. When upgrading or retrofitting existing ESPs to
comply with new emission regulations the ESP. Pulsed energization now offers a
number of specific benefits in addition to improved performance and power savings.
These includes installation with mum ESP downtime, minimum supervision and
maintenance and low investment since only the transformer/rectifier (T/R) set is
changed and the internals of the ESP do not have to be modified. Different designs
of internals have been used successfully and found that the deterioration of EP
efficiency due to poor current distribution and back corona conditions can be
reduced by pulsed operation.
As it takes some time in the order of some seconds for a high resistivity dust for the
charges in the dust layer to disappear the current density where back corona starts
is equal to the time average current density as a first approximation. Thus high
sharp peak currents at low frequencies from the discharge electrode have little
effect on back corona. By increasing the voltage for short periods intensive corona is
formed and good current distribution is obtained at regular intervals.
MULTIPULSE CONCEPT
Figure-4 shows the circuit diagram of the Flakt pulse supply. A storage capacitor
placed after high voltage rectifier is charges. The energy is transferred via an
inductance to the ESP by thyristors. The energy oscillates between the ESP and the
storage capacitor until an essential portion has been used by the ESP.
Figure-5 shows the high voltage wave form for the MFC system. When the energy.
When the energy generated in the storage capacitor is released, the pulse is
generated. The pulse amplitude declines in line with progressive use of the energy
to form corona in the ESP. By means of pulses peak voltages higher than used for
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conventional energization are used in the ESP without leading to sparking. Better
corona current distribution results in enhanced efficiency and due to the
application of pulsed currents back corona formation is suppressed.
A separate conventional base rectifier is not required for the DC since the
multipulse unit is designed to supply an inherent base voltage up to the corona
onset voltage. Any change in the corona onset voltage is automatically tracked.
Table At shows technical data for the MPC units at Gnsted unit-2 power station.
TABLE 1
Pulse amplitude
40 kV
Pulse width
90 Micro sec.
Pulse frequency up to
300 Hz
Corona current
200mA
Pulse/burst up to
8
Size
1.2x1.2x1m
Weight
1,470kg,
RAPCON FOR PRECISION TIMINGS OF RAPPER MOTORS
RAPCON(rapper controller) is a microprocessor based unit that controls and
Surveys the
operation of rapping motors in Electrostatic Precipitators. One
RAPCON unit can control up to 16 rapping motors. RAPCON starts and stops the
rappi1lg motors as programmed and will give an alarm if a rapping motor fails.
RAPCON is a component of BHEL's Integrated Operating System (IOS), but can also
used as a stand alone unit. In BHEL 's Integrated Operating System a maximum of
eight RAPCONs will communicate on a data with other control units.
From RAPCON each rapping motor can be manually started or stopped during
operation, without interfering with other rapping motors. The Operation State of
each rapping motor is indicated with light emitting diodes on the RAPCON Panel.
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The RAPCON is pre-programmed with number of Rapping Sequences, any of which
can be selected depending on the field failure condition of the Electrostatic
Precipitator.
INTEGRATED OPERATING SYSTEM
Integrated Operating System (IOS) is a PC based management system developed for
Electrostatic Precipitator applications and is one of the most sophisticated systems
available today. Controlling and supervising an Electrostatic Precipitator with IOS
ensures an effective control of the entire process in the precipitator. Remote control
of entire ESP operation can be achieved from a single point e.g. at UCB. The set
point of the various BAPCONs and RAPCONs can be changed from IOS. The status
of the ESP and printout of the same can be obtained in the IOS-PC. The ESP can
also be optimised for best performance with the help of IOS, using optional control
algorithms to achieve,
•
Maximum dust collection, by optimising charge ratio.
•
Minimum power comsumption with allowed emission, by increasing charge ratio
and reducing current.
•
.Automatic ESP startup and shutdown procedure at minimum time and cost.
The IOS is adaptable to the varying requirements of the different
objectives like above.
The IOS is using Distributed Digital Control (DDC) co,ncept. It incorporates
specialised sub-controllers which are independent and can control and supervise
their part following their local preset, paramenters. The comprehensive control of
the precipitator can be set in the IOS. On-line help is also available in the IOS with
easy access.
INTRODUCTION OF WIDE SPACING EP
The Electrostatic precipitators can remain competitive with other particulate control
devices like fabric filter if their capital cost can be reduced while keeping their
performance. Recent experimental work has shown that wider spacing EP results in
increased migration velocities. Presently we use spacing of 300mm between any two
collecting electrode rows. World wide many suppliers have started using wide
spacing EP.BHEL has taken up evaluation and demonstration of wide spacing EPs
in India. Full scale trails wee conducted with 400mm spacing at Tuticorin thermal
power plant. The performance results were obtained with full scale experiments at
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Tuticorin and Fig.-7 shows the comparison of performance for wide spacing EP.
Results shows that by using 400mm spacing performance of EP can be maintained,
when compared to 300mm spacing, the following improvements/changes may be
obtained with wide spacing EPs.
**
Increasing the pitch to an optimum value to
get higher migration velocity.
**
Higher operating voltage.
**
More stable electrical operation.
**
Permissible range of alignment error can be increased.
**
Maintenance will be easier. Personnel can
enter the inter electrode space easily.
High collecting performance even for
sub-micron particles.
**
**
Total weight is reduced.
BHEL is introducing 400 mm spacing for EP applications. In addition R & D
experiments with 600mm wide spacing in Indian power plant for evaluating the
performance of this modification is also being taken-up. Based on the trials to be
conducted EP spacing for Indian application, can be optimized.
LABORATORY FACILITIES AVAILABLE AT BAP-RANIPET
A. R&D LABORATORY ;BHEL has established its own R&D laboratory at its manufacturing plant. The total
cost of the facility is Rs.50.0 lakhs including building, electrical accessories ad
laboratory equipments. The following facilities are available.
a) FLOW MODEL TEST FACILITY
b) EP COMPONENT TEST FACILITY
FLOW MODEL TEST FACILITY.
Flow model test facility is used for obtaining better air distribution patterns by
conducting tests on scale models of electrostatic precipitator and associated
dueling. The results are useful for BHEL and customer. Such model studies are
conducted based on the contractual requirement of customers. The facility is used
for: ** Obtaining better gas distribution inside EP chamber for improved performance.
** Optimized pressure drop in the system which will result in a reduction in the
operation cost of the boiler system.
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EP COMPONENT TEST FACILITY
Facilities were established to analyze the characteristics of dust and to obtain better
performance and reliability by developing and testing new profiles of collecting and
emitting electrodes and other components of EP. Table-1 gives a list of various
equipments and facilities available and their uses.
TABLE-1
LIST OF VARIOUS EQUIPMENTS/FACILITIES AVAILABLE AND THEIR
USES
Sl.no.
Test facility
Major use
1.
Resistivity meter
Resistivity of fly ash with varying air
temperature & dew point.
2.
Bahco classifier
Fly ash size distribution from 3 microns
and above.
3.
Current distribution test rig
Current distribution on collecting
electrode
4.
Corona study rig
Corona aspect of various electrode system.
5.
Spark erosion test rig
Spark erosion endurance of emitting
electrode
6.
Thermal relaxation testrig
Thermal relaxation of emitting electrode
7.
Image intensifier
Recording of corona discharge and spark
discharge.
8.
Ozone level monitor
Monitoring of ozone level due to corona.
9.
Portable smoke density meter
smoke density measurement
10.
Data logger
11.
Acceleration measuring system
12.
HV-HF active divider
On line computation and logging of data
for flow model tests & current distribution
test.
To measure rapping accelerations on
electrodes of EP.
To measure repetitive and fast HV pulses.
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B. PILOT ELECTROSTATIC PRECIPITATOR
The performance of EP can be described in terms of dust characteristics gas flow
temperature and electrical characteristics. But is difficult to conduct experiments to
evaluate the effectiveness of each variable in full scale EPs. It is desirable to
construct a dedicated pilot EP with enough flexibility for an experimental
investigation of the effects of individual functional units on overall performance.
BHEL has acquired a pilot scale EP which is being tested at Ennore thermal power
station. Major design features are :** Gas temperature can be varied from ambient to 350 Deg.C.
** Gas velocity variation from 0.3m/sec. upwards.
** Sampling ports at outlet of EP fly ash collected in each field can be measured
from its hoppers.
** Total gas volume flow is measured at EP outlet.
** The collection plates are 2Mtr. high plate to plate spacing can be varied. Width of
gas path is 1 metre.
** Specific collection area of EP is 64M2/M3/Sec. at a plate spacing of 250mm and
gas velocity of 0.6m/sec,
There are three electrical sections in the direction of gas flow with only one gas
path. Pilot EP can be moved to any power station and the performance
characteristics can be obtained.
C. COLLECTING ELECTRODE TEST TOWER
We have established this test facility for acceleration and life testing of collecting
electrodes, guides, shock bars and hammers.
SUMMARY
Presented till now are important developments that have taken place around the
world and also- in India. BHEL has taken up efforts for better customer satisfaction
and also keeping up with the various new trends in the world. Based on R&D
results on wide spacing EP, BHEL's ESP collaborator M/s. Flakt Industri, Sweden
has started supplying electrostatic precipitators with 400mm wide spacing EP and
is expected as a standard for few contracts. BHEL's laboratory facilities will be used
for
analyzing
and
understanding
the
various
programmes
associated
with
electrostatic precipitator operation.
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