500 mw familiarisation
DESCRIPTION
gfTRANSCRIPT
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IG/
For restricted Circulation Only
500 MW FAMILIARISATION
Power Management Institute Noida
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CONTENTS
S. No. Subject Page No.
1. Salient Features of Boiler 1
2. Boiler Pressure Parts 8
3. Once through Boiler 25
4. Fuel Firing System 35
5. Air/Draught System 60
6. Furnace Safeguard and Supervisory System 80
7. Soot Blowing System 99
8. Data Sheet of 500 MW Boiler and Auxiliaries. 106
9. Salient Features and Constructional details of KWU Steam
Turbine. 113
10. Turbine Oil System 127
11. Turbine Control Fluid System 135
12. Constructional Features of Turbine Governing System and
H.P./ L.P. Bypass System 143
13. Turbine Tripping Devices and Turbine Metal Temp. limit
Curves 158
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14. Automatic Turbine RUN UP System-(ATRS) 164
15. Data Acquisitions System (DAS) 177
16. Feed Regenerative System 193
17. Boiler Feed Pump and Condensate Pump 200
18. Data-Sheet of 500 MW Turbine and its Auxiliaries Turbine
Metal Temp. Limit curves. 217
19. Design and Constructional Feature of 176 500 MW Generator 227
20. Excitation System and Auto Voltage Regulator 247
21. Protections of Generator 268
22. Generator Auxiliaries 273
23. Data Sheet of Generator 284
24. Unit Start up and Shut-Down Procedures 285
25. Major Differences between 210 MW and 500 MW Units 332
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1. Salient Features Of Boiler
BOILER UNITS
The boiler is a radiant reheat, controlled, circulation. Single drum, dry-bottom type unit.
The general arrangement of boiler and its auxiliaries is shown in the Figure no 1. The
boiler units are designed for the following terminal conditions (MCR):
Evaporation a) SH Outlet : 1.725 t/hr
b) RH Outlet : 1.530 t/hr
Working Pressure after stop valve : 178 kg/cm (g)
Steam Temperature at SH
Outlet : 540oC
Steam Temperature at RH inlet : 344.1o C
Steam Temperature at RH Outlet : 540o C
Steam pressure at RH inlet : 42.85 kg/cm (g)
Steam Pressure at RH Outlet : 43.46 kg/cm (g)
Feed water Temperature at ECO : 256o C
Furnace Design Pressure : + 660 mmwc (g)
The boilers are of single furnace design, circulating pumps to provide assisted
circulation.
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Each boiler corner is fitted with tilting tangential burner boxes comprising four high
energy arc igniters, four light-up heavy oil fired burners and eight pulverised coal
burners. The angle of tilt from the horizontal is about-30 to +30.
Feed water to the boiler passes through HP feed heaters into the economiser and then
to the steam drum from where it flows into the suction manifold and furnace wall circuits
via the three boiler circulating pumps, returning to the steam drum as a water/steam
mixture. This mixture is separated in three stages, the first two stages are incorporated
into the turbo separators and the final stage takes place at the top of the drum just
before the steam enters the connecting tubes comprising of first stage superheating,
Within the steam circuit there are a further four stages of (superheating, making five in
total. There are also three stages of reheat.
Superheater temperature control is provided by spray attemperation situated in the
connecting link between the superheater low temp. pendant outlet header and the
superheater division panel inlet headers.
Reheat temperature control is provided by titling burners or spray attemperators
installed prior to the first stage reheater.
PULVERISED COAL SYSTEM
The system for direct firing of pulverised coal utilises bowl mills to pulverise the coal and
a tilling tangential firing system to admit the pulverised coal together with the air
required for combustion (secondary air) to the furnace.
AS crushed coal is fed to each pulveriser by its feeder, primary air is supplied from the
primary air fans which dries the coal as it is being pulverised and transports the
pulverised coal through the coal piping system to the coal nozzles in the wind box
assemblies.
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The pulverised coal and air discharged from the nozzles is directed towards the center
of the furnace to form firing circle.
Fully preheated secondary air for combustion enters the furnace around the pulverised
coal nozzles and through the auxiliary air compartments directly adjacent to the coal
nozzle compartments. The pulverised coal and air streams entering the furnace are
initially ignited by suitable ignition source at the nozzle exit. Above a predictable
minimum loading condition the ignition becomes self sustaining. Combustion is
completed as the gases spiral up in the furnace.
PRIMARY AIR SYSTEM
The primary air (P.A.) draught plant supplies hot air to the coal mills to dry and convey
pulverised coal to the burners. Cold air ducts, however, are included in the system to
regulate mill temperatures and seat mill components against any ingress of coal dust.
The P.A. system comprises two P.A. fans, two Steam Coil Air Preheater (SCAPH) and
two regenerative type primary air preheaters. Each fan, which is of sufficient rating to
support 60% MCR load, discharges through a SCAPH into a common bus duct that has
four outlets, two directing air into the primary air preheater for heating, two direct cold air
straight to the pulverising mills. On the other side of the primary air preheaters, the
outlet ducts combine to form a hot air crossover duct which outlets to the mills at the
L.H.S. and R.H.S. of the boiler furnace, This arrangement of bus duct and cross over
duct ensures continued plant operation even if one fan and/or one primary air preheater
is out of service. The SCAPHs located in the fan discharge ducts, ensure that the
primary air preheaters combined cold end temperature (gas leaving temperature plus air
entering temperature) does not fall below the specified minimum to avoid "Cold End
Corrosion'.
Seal air fans boost up the primary air pressure and are provided for supplying sealing
air to each mill to maintain sufficient differential between primary air and seal air thereby
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safeguarding the lub oil from being contaminated by coal dust.
SECONDARY AIR SYSTEM
The secondary air draught plant supplies the balance of air required for pulverised coal
combustion, air for fuel oil combustion, and overfire air to minimise the production of
nitrous oxides(NOx).
The Secondary air system comprises two forced draft (F.D.) fans, two steam coil air
preheaters (SCAPH) and two regenerative type secondary air preheaters. Each fan.
which is of sufficient rating to support 60% boiler MCR load, discharges through a
SCAPH into a common bus duct that has two outlets each directing air through a
secondary air preheater. Hot air from secondary air preheaters is sent to wind boxes at
each side of the boiler furnace for proper combustion as secondary and overfire air.
Overfire air can be admitted to the furnace through the upper levels of furnace wind
boxe nozzles to assist in reducing the amount of NOx formed in the furnace. Control of
unit air flow is obtained by positioning the FD fans blades while the distribution of
secondary air from wind box compartment to furnace is controlled by secondary air
dampers.
The SCAPHs are located in the FD fan discharge ducts to ensure that the secondary
airpreheaters combined cold end temperature (gas leaving temperatures plus air
entering temperature) does not fall below the specified minimum to protect against cold
end corrosion.
FLUE GAS HANDLING SYSTEM
The flue gas handling plant draws hot flue gases from the furnace and discharges them
to atmosphere through the chimney. During its passage to the chimney, flue gas is
passed through a feed water economiser and four regenerative airpreheaters to
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improve boiler efficiency, and through four electrostatic precipitators to keep dust
emission from chimney within prescribed limits.
Flue gases travel upward in the furnace and downward through the rear gas pass to the
boiler outlet (boiler rear gas pass below the economiser). It then passes through the
primary and secondary air preheaters, the electrostatic precipitators and induced
draught (I.D.) fans to the chimney. Since primary and secondary streams are provided
with separate bisector regenerative air heaters, control dampers at the outlet of the air
preheaters are provided to regulate the gas flow through these streams to get same gas
outlet temperature.
Three I.D. Fans, each of which is of sufficient rating to support 60% boiler MCR load,
are served by a common inlet bus duct to ensure that plant operation continues even
when two fans are out of service. During normal usage, two ID fans will be operational
and one available as standby.
SOOT BLOWING SYSTEM
On load, gas side cleaning of boiler tubes and regenerative air heaters is achieved
using 126 electronically controlled soot blowers which are disposed around the plant as
follows.
88 - Furnace Wall Blower - Steam
34 - Long Retractable Soot Blower - Steam
4 - Air heater Soot Blowers for - Steam
Primary and Secondary Air
heaters
The boiler water wall panels are provided with suitable wall boxes for future
accommodation of an extra sixteen furnace wall blowers and twenty-four long-
retractable soot blowers for upper furnace, arch and rear pass zone, if necessary.
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Steam for soot blowing is taken from division panels superheater outlet header. Steam
is then passed through a pressure control valves where the steam pressure is reduced
to the required limit of soot blowing. However, to soot blow the regenerative air
preheaters during boiler start up, a separate connection is also provided from the
auxiliary Steam System.
FURNACE SAFEGUARD SUPERVISORY SYSTEM
The Furnace Safeguard Supervisory System (F.S.S.S) is a major component of station
safety monitoring equipment. It permits the remote (Control Equipment Room) and
partly local (adjacent to the boiler) light-up and shut down of all oil burners and igniters
together with continuous monitoring, fault detection and associated shut down of any or
all burners upon fault disclosure.
The system also incorporates the logic sequences required for enforcing proper purging
of the steam generator and for tripping the master fuel relay system.
The pulverised coal burners and their associated mills are controlled by a separate mill
control sequencing system which is provided with essential information regarding milling
plant status from loc. instrumentation as well as start and run permissives for each mill
system from the F.S.S.S.
Both systems integrate with the Analogue Control System (A.C.S.) to provide full on-line
firing safety, optimum operational control and in-depth system awareness.
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2. Boiler Pressure Parts INTRODUCTION
The boiler units are of the balanced draught single drum radiant furnace type that
include an arch between the furnace and the rear gas pass. The water circuit is of
controlled circulation design incorporating boiler circulating pumps in unheated down
comers at the front of the boiler and utilising refill bore tubing in sections of the furnace
wall panels. Boiler units of 500 MW units are identical in design and comprise a single
furnace, three superheater stages, three reheater stages and a feed water economiser,
BARE TUBE ECONOMISER
The Function of the economiser is to preheat the boiler feed water before it is
introduced into the Steam drum by recovering some of the heat of the flue gas leaving
the boiler.
The economiser is located in the boiler back pass. It is composed of two banks of 156
parallel tube elements arranged in horizontal rows in such a manner that each row is in
line with the row above and below. All tube circuits originate from the inlet header and
terminate at outlet headers which are connected with the economiser outlet headers
through three rows of hanger tubes.
Feed water is supplied to the economiser inlet head via feed stop and check valves.
The feed water flow is upward through the economiser, that is, counter flow to the hot
flue gases. Most efficient heat transfer is, thereby, accomplished, while the possibility of
steam generation within the economiser is minimised by the upward water flow. From
the outlet header the feed water is lead to the steam drum through the economiser
outlet links.
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The economiser recirculalting line, which connects the economiser inlet header with the
furnace lower rear drum, provide a means-of ensuring a water flow through the
economiser during startups. This helps prevent steaming. The valves in these lines
must be open during unit startup until continuous feed water flow is established.
WATER COOLED FURNACE Welded Wall Construction
The furnace walls are composed of tubes. The space between the tubes are fusion
welded to form a complete gas tight seal. Some of the tube ends are swaged to a
smaller diameter while other tubes are bifurcated where they are welded to the outlet
headers and lower drum nipples.
The furnace arch is composed of fusion welded tubes.
The back pass walls and roof are composed of, fin welded tubes.
The furnace extended side walls are composed of fin welded tubes.
The back pass front (furnace) roof is composed of tubes peg fin welded.
All peg finned tubes are normally backed with a plastic refractory and skin casing which
is seal welded to form a gas tight envelope.
Where tubes are spread out to permit passage of superheater elements, hanger tubes,
observation ports, soot blowers, etc., the spaces between the tubes and openings are
closed with fin material so a completely metallic surface is exposed to the hot furnace
gases.
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Poured insulation is used at each horizontal buckstay to form a continuous band around
the furnace thereby preventing flue action of gases between the casing and water walls.
Bottom Construction
Bottom designs used in these coal fired units are of the open hopper type, often
referred to as the dry bottom type. In this type of bottom construction two furnace water
walls, the front and rear walls, slope down toward the centre of the furnace to form the
inclined sides of the bottom. Ash and/or slag from the furnace is discharged through the
bottom opening into an ash hopper directly below it. A seal is used between the furnace
and hopper to prevent ambient air being drawn into the furnace and disturbing
combustion fuel/air ratios. The seal is effected by dipping seal plates, which are
attached around the bottom opening of boiler furnace, into a water trough around the
top of the ash hopper. The depth of the trough and seal plates will accommodate
maximum downward expansion of the boiler (predicated 320.3 mms).
WATER AND SATURATED STEAM CIRCUITS
In a controlled circulation Boiler, circulating pumps placed in the downcomer circuits
ensure proper circulation of water through the waterwalls. Orifices installed in the inlet of
each water circuit maintain an appropriate flow of water through the circuit. Feed water
enters the unit through the economizer elements and is mixed with boiler water in the
steam drum. Water flows from the drum through the downcomers to the pumps suction
manifold. The boiler circulating pumps take water from the suction manifold and
discharge it. via the pump discharge lines, into the furnace lower front inlet header.
Furnace lower waterwall right and left side headers assure proper distribution to the rear
header.
In the waterwall inlet headers the boiler water passes through strainers and then
through orifices which feed the furnace wall tubes, the economiser recirculating lines.
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The water rises through furnace wall tubes where it absorbs heat. The front wall tubes,
rear tubes, rear wall hanger tubes, rear arch tubes, rear screen tubes, extended side
wall tubes and side wall tubes from parallel flow paths.
The resulting mixture of water and steam collects in the waterwall outlet headers and is
discharged into the steam drum through the riser tubes. In the steam drum the steam
and water are separated, the steam goes to the superheater, and the water is returned
to the water side of the steam drum to be recirculated.
BOILER CIRCULATION SYSTEM
Boiler water circulates from the steam drum into unheated down comer pipes, then from
the down comers into heated furnace wall tubes back into the drum. The furnace walls
will absorb radiant heat from the furnace and then discharge a saturated steam /water
mixture into the drum. Inside the drum, saturated steam is separated from the water,
then directed into superheater tubing for further temperature increase. Water separates
from the steam will combine with incoming boiler feed water, then re-enter the down
comers to repeat the cycle.
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Fig No 2. FLOW PATH IN DRUM
The boilers are designed with a controlled circulation system which incorporates boiler
water circulation pumps, smooth and rifled bore furnace wall tubing, and orifice plates
at the inlet to furnace wall tubing.
Water flows from the bottom of the steam drum via six large bore downcomers into a
suction manifold common to three parallel mounted boiler water circulation pumps. The
manifold has connections at both ends to the chemical clean pipework, and at three
points along its length to feed individual circulation pump suctions. Water will flow from
the pumps through two discharge pipes into the front leg of the water wall inlet headers
at the bottom of the furnace. Each discharge pipe is fitted with a circulating pump
Discharge Stop/Check Valves which are controlled via sequence equipment to open
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and close as the pump is taken in and out of service. If, however all three pumps are
out of service all of the valves will open to enable thermosyphonic circulation to take
place. Initiating any pump to restart will cause them all to close again then continue with
the in and out of service regime. Controls for the pumps are located in the U.C.B. and
comprise a SEQUENCE pushbutton, ammeter and a DUTY/ STANDBY selector. Pump
status is indicated on RUN/STOP lamps on Panel. The operating regime for the boiler
water circulation pumps is two duty/one standby.
From the Waterwall inlet headers, water travels upward through furnace wall tubing via
furnace upper front rear and side headers into riser tubes which direct a saturated
steam/water mixture into the steam drum. Furnace wall tubing is manufactured from a
combination of both smooth and rifled bore tubing which permits the use of lower tube
flow rate whilst still retaining full tube protection. The required distribution of water to
give the correct flow rates through the various furnace wall circuits is achieved and
maintained by the use of suitably sized orifices installed inside the water wall inlet
headers at the inlet to each furnace wall tube. Orifice size varies for different circuits or
groups of circuits depending on the circuits length, arrangement and heat absorption.
Perforated panel strainers are also located inside the water wall inlet headers to prevent
the orifices blocking and to ensure an even distribution of water around the other inlet
headers. Refer to fig no.2.
The saturated steam/water mixture enter the steam drum on both sides behind a water
tight inner plate baffle which directs the mixture around the inside surface of the drum to
provide uniform heating of the drum shell. This eliminates thermal stresses from
temperature differences through the thick wall of the drum, between the submerged and
unsubmerged portions. Having travelled around this baffle the mixture enters two rows
of steam separators where a spin is imparted. This forces the water to enter the outer
edge of the separator where it is separated from the steam. Nearly dried, the steam
leaves the separators and passes through four rows of corrugated plate baskets where
by low velocity surface contact, the remaining moisture is removed by wetting action on
the plates. From the baskets, steam flows out of the drum into superheater pipework.
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Water which separates from the saturated steam
drains back to combine with incoming boiler feed
water from the economiser then re-enters the
downcomers to repeat the cycle.
Boiler Water Circulation Pumps
Each boiler-Water Circulation pump consists of a
single stage centrifugal pump on a wet stator
induction motor mounted within a common
pressure vessel. The vessel consists of three
main parts, a pump casing, motor housing and
motor cover as shown in Fig No. 3.
The motor is suspended beneath the pump
casing and is filled with boiler water at full system
pressure. No seal exists between the pump and
motor, but provisions is made to thermally isolate
the pump from the motor in the following respect:
a) Thermal Conduction: To minimize heat conduction a simple restriction in
the form of thermal neck is provided
b) Hot Water Diffusion: To minimize diffusion of boiler water, a narrow
annulus surrounds the rotor shaft, between the hot
and cold regions. A baffle ring restricts solids entering
the annulus
c) Motor Cooling: The motor cavity is maintained at a low temperature
by a heat exchanger and a closed loop water
circulation system, thus extracting the heat conducted
from the pump.
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In addition this water circulates through the stator and bearing extracting the heat
generated in the windings and also provides bearing lubrication. An internal filter
is incorporated in the circulation system.
d) In emergency conditions if low-pressure coolant to the heat exchanger fails, or is
inadequate to cope with heat flow from the pump case, a cold purge can be
applied to the bottom of the motor to limit the temperature rise
The pump comprises a single suction and dual discharge branch casing. The case is
welded into the boiler system pipe work at the suction and discharge branches with the
suction upper most.
Within the pump cavity rotates a key driven, fully shrouded, mixed flow type impeller,
mounted on the end of the extended -motor shaft. Renewable wear rings are fitted to
both the impellers and pump case. The impeller wear ring is the harder component to
prevent galling.
The motor is a squirrel cage, wet stator. Induction motor, the stator, wound with a
special water-tight insulated cable. The phase joints and lead connections are also
moulded in an insulated material. The motor is joined to the pump casing by a pressure-
tight flange joint and a motor cover completes the pressure tight shell.
The motor shell contains all the moving parts, except for the impeller. Below the impeller
is situated an integral heat baffle which reduces the heat flow, a combination of
convection and conduction, down the unit. A baffle wear ring cum-sleeve above the
baffle forms a labyrinth with the underside of the impeller to limit sediment penetration
into the motor. Should foreign matter manage to pass the labyrinth device into the motor
enclosure, it is strained out by a filter located at the base of the cover-end bearing
housing.
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The motor design is such that for ease of maintenance, the stator shell, complete with
the stator pack, the rotor assembly, can be withdrawn from the motor in sequence, after
removal of the motor from the pump case. Removal lifting lugs are supplied for
attachment to permanent lugs on the side of the motor case for securing hoists for the
raising and lowering of the motor.
SUPERHEATER AND REHEATER
The arrangement, tube size and spacing of the Superheater and Reheater elements are
shown in the Figure No. 4.
Superheaters
The superheater is composed of three basic stages of sections; a finishing Pendant
section (34), a Division Panel Section (30) and a Low Temperature Section including
LTSH (23), the Backpass Wall and Roof Sections (12)(13)(14)(19)(21)(17)(7)(8).
The finishing Section (34) is located in the horizontal gas path above the furnace rear
arch tubes.
The Division panel Section (30) is located in the furnace between the front wall and the
Pendant Platen Section. It consists of six front and six rear panel.
The Low Temperature Section (23) and (24) are located in the furnace rear backpass
above the Economiser Section.
The Backpass wall and Roof Section forms the side (7) (8) front (12) and rear (19) walls
and roof (14) of the vertical gas pass.
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Reheater
The reheater is composed of 3 stages or sections, the Finishing Section (46) the Front
Platen Section (47) and the Radiant Wall Section (40)(41).
The Finishing Section (46) is located above the furnace arch between the furnace
screen tubes and the Superheater Finish (34).
The Reheater Front and side Radiant Wall (40) & (41) is composed of tangent tubes on
the furnace width.
Steam Flow
The course taken by steam from the steam drum to the superheater finishing outlet
header can be seen in Fig. No. 4. The elements, which make up the flow path, are
essentially numbered consecutively. Where parallel paths exist, first one and then the
other circuit is numbered. The main steam flow is:
Steam drum - SH connecting tubes (1)- Radiant roof inlet header (2) - First pass roof
front (3) - Rear (4) Radiant tube outlet header (5)-SH SCW inlet header side (G)-
Backpass sidewall tubes (7) & (8)-Backpass bottom headers (9), (10) & (11)- backpass
Front, and rear (12) (21)-Backpass screen (13) Backpass roof (14)-Backpass SH &
Eco.. supports(15) SH & Eco support headers(16)-LTSH support tubes (17)-SH Rear
Roof tubes (18)-SHSC Rear wall tubes (19)-LTSH inlet header (22)-LTSH banks
(23)(24)-LTSH outlet header(25)-SH DESH link (26). SH DESH (27)-Division panel (30)-
Division panel (30)-Division panel outlet header (31)-SH Pendent assembly (34)-SH
outlet header (35).
After passing through the high pressure stages of the turbine, steam is returned to the
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reheater via the cold reheat lines. The reheater desuperheaters are located in the cold
reheat lines. The reheat flow is.
Reheater radiant wall inlet header (38) (39)- radiant wall tubes (40) (41) reheater
assemblies (46) (47)-reheater outlet header (48)-Reheater load (49).
After being reheated to the design temperature, the reheated steam is returned to the
intermediate pressure section of the turbine via the hot reheat line.
Protection and Control
As long as there is a fire in the furnace, adequate protection must be provided for the
Superheater and Reheater elements. This is especially important during periods when
there is no demand for steam, such as when starting up and when shutting down.
During these periods of no steam flow through the turbine, adequate flow through the
superheater is assured by means of drains and vents in the headers, links and main
steam piping. Reheater drains and vents provide means to boil off residual water in the
reheater elements during initial firing of the boiler.
Safety valves on the superheater main steam lines set below the low set drum safety
valve provide another means of protection by assuring adequate flow through the
superheater if the steam demand should suddenly and unexpectedly drop Reheater
safety valves, located on the hot and cold reheat piping serve to protect the reheater if
steam flow through the reheater is suddenly interrupted.
A power control valve on the superheater main steam line set below the low ser super
heater safety valve is provided as a working valve to give an initial indication of
excessive steam pressure. This valve is equipped with a shut off valve to permit
isolation for maintenance. The relieving capacity of the Power Control Valve is not
included in the total relieving capacity of the safety valves required by the Boiler Code.
During all start-ups, care must be taken not to overheat the superheater or reheater
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elements. The firing rate must be controlled to keep the furnace exit gas temperature
from exceeding 540 C. A thermocouple probe normally located the upper furnace side
wall should be used to measure the furnace exit gas temperatures.
NOTE
1. Gas temperature measurements will be accurate only if a shielded, aspirated
probe is used. If the probe consists of a simple bare thermocouple, there will be
an error, due to radiation, resulting in a low temperature indication. At 588 C
actual gas temperature, the thermocouple reading will be approximately 10
degrees low. Unless very careful traverses are made to locate the point of
maximum temperature, it is advisable to allow another 10 degrees tolerance,
regardless what type of thermocoupie probe is used.
2. The 540 C gas temperature limitation is based on normal start-up conditions,
when steam is admitted to the turbine at the minimum allowable pressure
prescribed by the turbine manufacturer. Should turbine rolling be delayed and
the steam pressure to permitted to build up the gas temperature limitation should
be reduced to 510 C when the steam pressure exceeds two-thirds of the design
pressure before steam flow through the turbine is established.
Thermocouples are installed on various Superheaters and Reheater terminal
tubes, above the furnace roof, serve to give a continuous indication of element
metal temperatures during start-ups (Superheater) and when the unit is carrying
load (Superheater and Reheater). In addition to the permanent thermocouples,
on some units temporary thermocouples provide supplementary means of
establishing temperature characteristics during initial operation.
Steam temperature control for Superheater and Reheater outlet is provided by
means of windbox nozzle tilts and desuperheaters.
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DESUPERHEATERS
General
Desuperheaters are provided in the superheater connecting link and the reheater inlet
leads to permit reduction of steam temperature when necessary and to maintain the
temperatures at design values within the limits of the nozzle capacity. Temperature
reduction is accomplished by spraying water into the path of the steam through a nozzle
at the entering end of the desuperheater. The spray water comes from the boiler feed
water system. It is essential that the spray water be chemically pure and free of
suspended and dissolved solids, containing only approved volatile organic treatment
material, in order to prevent chemical deposition in the desuperheaters and reheater
and carry-over of solids to the turbine.
CAUTION
During start-up of the unit. if desuperheating is used to match the outlet steam
temperature to the turbine metal temperatures, care must be exercised so as not to
spray down below a minimum of 10 above the saturation temperature at the existing
operating pressure. Desuperheating spray is not particularly effective at the low steam
flows of start-up. Spray water may not be completely evaporated but be carried through
the heat adsorbing sections to the turbine where it can be the source of considerable
damage. During start-up, alternate methods of steam temperature control should be
considered.
The location of the desuperheater helps to ensure against water carry-over to the
turbine. It also eliminates the necessity for high temperature resisting materials in the
desuperheaters construction.
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Superneater Desuperheaters
Two spray desuperheaters are installed in the connecting link between the superheater
low temperature pendant outlet header and the superheater division panel inlet
headers.
Reheater Desuperheaters
Two spray type desuperheaters are installed in the reheater inlet leads near the
reheater radiant wall front inlet header.
STEAM DRUM INTERNALS
The function of the steam drum internals is to separate the water from the steam
generated in the furnace walls and to reduce the dissolved solids contents of the steam
to below the prescribed limit. Separation is generally performed in three stages, the first
two stages are incorporated into the turbo-separators, the final stage takes place at the
top of the drum just before the steam enters the connecting tubes.
The steam-water mixture entering the top of the drum from the furnace riser tubes as
shown in Fig No. 5 sweeps down along both sides of the drum through the narrow
annulus formed by a baffle extending over the length of the drum. The baffle is
concentric with the drum shell and effects adequate velocity and uniform heat transfer,
thereby maintaining the entire drum surface at a uniform temperature. At the lower end
of the baffle, the steam-water mixture is forced upward through two rows of turbo
separators.
Each turbo separator consists of a primary stage and a secondary stage. The primary
stage is formed by two concentric cans. Spinner blades impart a centrifugal motion to
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the mixture of steam and water flowing upward through the inner can, thereby throwing
the water to the outside and forcing the steam to the inside. The water is arrested by a
skim-off lip above the spinner blades and returned to the lower part of the drum through
the annulus between the two cans. The steam proceeds up to the secondary separator
stage.
The secondary stage consists of two opposed banks of closely spaced thin, corrugated
matel plates which direct the steam through tortuous path and force entrained water
against the corrugated plates. Since the velocity is relatively low, this water does not get
picked up again, but runs down the plates and off the second stage lips at the two
steam outlets.
From the secondary separators, the steam flow is upward to the third and final stage of
separators. These consists of rows of corrugated plate dryers extending the length of
the drum with a drain through between me rows. The steam flows with relatively low
velocity through the tortuous path formed by the closely spaced layers of corrugated
plates, the remaining entertained water is deposited on the corrugated plates, the water
is not picked up again but runs down the plates into the drain through Suitably located
drain pipes return this water to the water side of the drum.
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STEAM DRUM INTERNALS ARRANGEMENT
Figure No. 5
RECOMMENDED
OPERATING RANGE
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3. Once Through Boiler GENERAL PRINCIPLE
In a drum boiler, the flow rate of water passing through the steam generating tube walls
is different from the flow rate of steam produced. It passes through a loop consisting of
the drum, the downcomers and the steam generating tube walls, and it is far greater
than the flow rate of steam produced because of the recirculating of the circuit: In the
drum type circulation: the recirculating coefficient is high (generally) between 3 and 10):
the tubes and piping have relatively large diameters and the water velocities are low.
The drum constitutes a fixed point the thermodynamically speaking, the steam
generating tube walls are at the saturation temperature and all the steam superheat
must be performed in the heat exchangers independent of the steam generating tube
walls.
In a once through boiler, the steam generating tube walls are in series with the
economiser and the superheaters, and the same flow rate of water passes successively
through the economiser, the evaporator and the super heaters.
In the once through circulation; there is no recirculating, the tubes are of small diameter
and the water velocities are high.
Different Types Of Once Through Boilers
There are three types of once through boilers as shown in Fig. No. 6 and the difference
between them lies in the principle of circulation in the evaporator. Let us examine them
successively.
In the first type. the same flow rate of water passes through the economiser, the
evaporator and the superheaters at normal ratings, but at low loads a minimum flow of
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water is maintained constant in the economiser and evaporators by the use of
recirculalting pumps installed beneath the separator.
In the second type we have the same operating principle, but the minimum water flow
rate of the low ratings is obtained by the use of the feed water pumps themselves; the
non-vaporized excess water, i.e. the drains from the separator being sent to the
deaerator via a heat exchanger called the "Starting exchanger".
In the third type, the circulation in the evaporator is performed at all loads by the boiler
circulating pumps which are installed at the evaporator inlet after the water coming from
the economiser has been mixed with the saturated water from the separator.
Thus, in the evaporators of these three types of boilers, the proportion of steam in the
emulsion is very high (up to 100% for the first and second types, up to 80-90% for the
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third type) and it is impossible to aviod calefaction or D.N.B (departure from nucleate
boiling). We have to live with this, and it is therefore necessary to have a large speed
per unit mass in the evaporator tubes (3.50 m/sec. for the water at the inlet) and
consequently a high-pressure drop in this apparatus (15 to 20 bars).
Furthermore, the diagrams of the first two types eliminate the (thermodynamically
speaking) fixed point created by the drum. Which makes it possible to carry out the start
of superheat in the final part of the steam generating tube walls. The superheaters are
reduced in consequence, and this has a very beneficial effect on the cost of the boiler,
especially in high-pressure cycles where the evaporation part is reduced and the
superheat part amply increased due to the rapid decrease in the enthalpy of the
saturated steam for pressures greater than 140 bars.
Design Of Once Through Boiler
Let us examine, especially with regard to operation with low loads and during startups.
The Figure No. 7 represents the boiler circulation system during these special types of
operation.
At full load, the final part of the evaporator corresponds to a first superheat stage with a
final temperature of 395, i.e. about 30C above the saturation temperature. Between 35
and 100% of load, the separator plays no role at all since only dry steam passes
through it.
The separator operates as an emulsion separator only for ratings where the steam
flowrate is below 35%. as the water flowrate passing through the evaporator is then
maintained constant, and the difference between the feedwater flowrate and the
flowrate of steam produced must be recirculated via the feedwater pumps (which then
function as controlled circulation boiler pumps).
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Fig No. 7
FLOW DIAGRAM THROUGH STEAM GENERATOR The starting heat exchanger is sized so as to obtain a water temperature the inlet of the
feedwater tank which is very close to-the saturation temperature present there
(difference of about 12C.
The figure shows the control valves designated as AA, AN and ANB:
Valve AA is sized solely for cold start-ups and low-pressure start-ups; it is closed and blocked in the off position at a pressure of 60 bars.
Valves AN and ANB are sized for high- and medium-pressure Start-ups and for low ratings (below 35% of full load).
ANB is sized so that it can cope alone with the low ratings between 35 and 11 % (technical minimum with fuel oil alone):
AN is used for all start-ups (cold and hot) so as to send to the condenser the flowrate of water coming from the expansion of the evaporator water at the start
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of evaporation: this flowrate is high for 5 to 6 minutes.
For this same reason AA is used for cold start-ups. AN is also used for cleaning the boiler circuit water in the condensate treatment
station at start-ups and at low loads, when the quality of the feedwater is
inadequate.
This type of one through circulation has the following advantages:
Elimination of the boiler circulating pumps, which tends to reduce the maintenance expenses.
No risk of leakage into the starting heat exchanger which operates with a very slight pressure difference between the two circuits (25 bars maximum).
The terminal part of the evaporator is in fact a superheater, which represents an economical solution since it avoids covering the upper part of the tube walls of a
superheater or a wall reheater (bringing a very expensive double
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Operation On Sliding Pressure
One of the characteristic features of the once-through boiler is its small storage
capacity, which permits sliding pressure operation (i.e. with constant opening of the
turbine inlet safety valves) by maintaining possibilities of sufficiently rapid load
variations. In addition, this characteristic is further improved by limiting the opening of
the turbine inlet safety valves to 90%.
Sliding pressure operation has the following advantages:
The turbine operating conditions are better since the HP body operates in the same way as the IP or LP body. with much lower mechanical stresses:
At partial loads, the mechanical stresses in the boiler are also lower since the pressure is lower:
At partial loads, the steam consumption per kW of the turbine is slightly smaller. It is easy to obtain the reheated steam temperature a the intermediate loads
because of the virtually constant steam temperature at the HP body outlet:
It is easy to obtain the nominal superheated steam temperature at the intermediate loads because of the lower service pressure.
START UP
One of the main advantages of once through boiler lies in the possibility of performing
rapid and frequent start-ups and rapid load variations. This is particularly useful for
disturbed or small electric networks.
The start-up of a once through boiler can be much more rapid than that of a natural or
controlled circulation boiler because of the elimination of the large, thick drum which are
the origin of unacceptable heat stresses during the rapid pressure variations which
occur when rapid start-ups are desired.
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In the once through boilers, the pressure parts have been sized so that the thicknesses
are at all points less than 70 mm. The temperature gradients can then rise by up to 5 C
per minute for a total of 5000 cycles.
Moreover, these requirements for rapid start-ups have led the boiler constructor to
review certain more traditional parts of the equipment.
Boiler Supports
To withstand a temperature gradiant of 5C per minute during the planned 5000 cycles,
the support of the furnace consist of a large number of vertical flats of small size fixed
against the tubes of the steam generating tube walls by a large number of welded flats
so as to improve contact between the vertical flats and the tubes and to permit the
vertical flats to follow without delay the temperature of the tubes. Laboratory fatigue
tests have confirmed the appropriateness of the design chosen.
Steam Piping
Large thick nesses must be avioded by paying attention either to the number of pipes in
parallel or to the quality of the steel.
From the economical point of view, the ideal arrangement would be to have a single
connecting pipe between the boiler and the turbine. But in a 500 MW unit, this would
lead to thicknesses greater than 80 mm, which is not acceptable.
We therefore installed two pipes and we determined a trace which gives the assurance
of having the some temperature in both of these pipes in all cases of operation.
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Hp By-Pass
During hot start-ups, the first steam supplied by the boiler is at a temperature much
lower than that of the pipes.
To avoid problems of heat stress in the pipes, the HP by-pass must be installed close to
the boiler and before the piping connecting pieces.
Start-Up Times
The start-up times which are summarized in the table below can thus be achieved with
this type of boiler.
Time after first lgnition First coupling Full load
Cold start-up 50 min 3 to 4 hours
After 36 hours week-end shut-down 30 min 1 to 2 hours
After 8 hours overnight shut-down 25 min 45 to 75 minutes
CONSTRUCTION OF BOILER AND HEAT EXCHANGERS
With regard to boiler construction, the major differences between natural or controlled
circulation boiler and once through boiler lie in the spiral design of the tube walls.
The spiral tube zone between the end of the ash pit hopper and the start of the superheater zone : it consists of 404 tubes of 33.7 mm diameter which each
encircle the furnace twice. So that there is a good homogeneity of the steam
temperature at the outlet of these tubes. From this point of view, the temperature
homogeneity thus achieved is certainly better than that which would be obtained
with vertical tubes which take account of the differences in heat flow at the
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various points of the furnace; homogeneity of the steam temperatures is esential
with once through boiler when the steam is superheated at the furnace outlet.
two parts consisting of vertical tubes - one in the lower part of the furnace and the other in the upper part of the boiler - to form the box around the
superheaters, reheater and economiser.
The number of vertical tubes is three times the number of spiral tubes. The junction is
performed with forged and drilled pieces forming a three-legged pipe. as shown in the figure No, 8.
The arrangement of the furnace supports consists of:
- The system of structures which maintain the furnace tube walls and the fixation
of these structures onto the comer pieces by rods permitting horizontal
expansion.
- The system of hangers for the spiral tube walls consisting of vertical flats which
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remain at the same temperature as the tubes.
The vertical tubes in the upper part of the boiler are designed to support the weight of
the spiral parts of the tube walls via junction pieces.
Generally speaking, the superheaters and reheaters are sized in the same way as in a
drum boiler.
Nevertheless, there is a slight difference for the superheaters which are calculated here
with an additional steam temperature margin of 20 degree to take account of the
regulation of the steam temperature at the separator outlet in the feedwater control
system.
CONCLUSION
The once through system and the type of operation of the control systems and the
precautions taken in tie construction of the boiler enable high temperature gradients to
be achieved in full safety for a large number of cycles.
Finally this type of boiler, which operates with a high-characteristic water-steam cycle,
thus enables peak electricity to be produced with low fuel consumption.
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4. Fuel Firing System INTRODUCTION
This chapter relates to the fuel (oil & Coal) systems and fuel/combustion equipment
under supply of BHEL for 500 MW boiler.
Fuel Oil System
The Fuel Oil System prepares any of the two designated fuel oil for use in oil burners
(16 per boiler, 4 per elevation) to establish initial boiler light up of the main fuel
(pulverized Coal) and far sustaining boiler low load requirements up to 15% MCR load.
To achieve this, the system incorporate fuel oil pumps, oil heaters, filters, steam tracing
lines which together ensure that the fuel oil in progressively filtered, raised in
temperature, raised in pressure and delivered to the oil burners at the requisite
atomising viscosity for optimum combustion efficiency in the furnace.
Coal System
The coal system prepares the main fuel (pulverised coal) for main boiler furnace firing.
To achieve this the raw coal from overhead hopper is fed through pressurised coal
valve, SECOAL nuclear monitor, gravimetric feeder and into mills where it in crushed
and reduced to a pulverised state for optimum combustion efficiency. The pulverised
coal is mixed with a primary air flow which carries the coal air mixture with a primary air
flow which carries the Coal Air mixture to each of 4 corners of the furnace burner
nozzles and into furnace.
Burner Nozzles
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Both the oil and coal burner nozzles fire at a tangent to an imaginary circle at the
furnace centre. The turbulent swirling action this produces, promotes the necessary
mixing of the fuels and air to ensure complete combustion of the fuel. A vertical tilt
facility of the burner nozzles, which in controlled by the automatic control system of
boiler ensures a constant reheat outlet steam temperature at varying boiler loads.
TILTING TANGENTIAL FIRING SYSTEM
General
In the tangential firing system the furnace itself constitutes the burner. Fuel and air
introduced to the furnace through four windbox assemblies located in the furnace
comers. The fuel and air streams from the wind box nozzles are directed to a firing
circle in the centre of the furnace. The rotative or cyclonic action that is characteristic of
this type of firing is most effective in turbulently mixing the burning fuel in a constantly
changing air and gas atmosphere.
Air And Fuel Nozzle Tilts
The air and fuel streams are vertically adjustable by means, of movable air deflectors
and nozzles tips, which can be tilted upward or downward through a total of approx. 60
degrees. These movements effected through connecting rods and tilting mechanism in
each windbox compartment, all of which are connected to a drive unit at each corner
operated by automatic control. Provision is given in UCB to know the position of nozzle
tips during operation. The tilt drive units in all four corners operate in unison so that all
nozzles have identical tilt positions,
Windbox Assembly
The fuel firing equipment consist of four windbox assemblies located in the furnace
corners.
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Each windbox assembly is divided in its height into number of sections or compartment.
The coal compartments (fuel air compartment) contain air (intermediate air
compartments). Combustion air (secondary air) is admitted to the intermediate air
compartments and each fuel compartment (around the fuel nozzle) through sets of
lower dampers. Each set of dampers is operated by a damper drive cylinder located at
the side of the windbox. The drive cylinders at each elevation are operated either
remote manually or automatically by the Secondary Air Damper Control System in
conjunction with the Furnace Safeguard Supervisory System.
Some of the (auxiliary) intermediate air compartments between coal nozzles contains oil
gun.
Retractable High energy Arc (HEA) ignitors are located adjacent to the retractable oil
guns. These ignitors directly light up the oil guns.
Optical flame scanners are installed in flame scanner guide pipe assemblies in the
auxiliary air compartments. The scanners sense the ultraviolet (UV) radiation given off
by the flame and thereby prove the flame. They are used by Furnace safeguard
Supervisory System to initiate a master fuel top upon detection of flame failure in the
furnace,
AIR FLOW CONTROL AND DISTRIBUTION
Total air (tow control is accomplished by regulating fan dampers or fan speed. Air
distribution is accomplished by means of the individual compartment dampers. The
airflow to the air boxes can be equalised by observing-and equalising the reading of the
flow meters located in the hot air duct to windbox.
TOTAL AIR FLOW
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In order to ensure safe light-off conditions, the pre-optional purge air flow (at least 30%
of full load volumetric air flow) is maintained during the entire warm-up period until the
unit is on the line and the unit load has reached the point where the air flow must be
increased to accommodate further load increase. To provide proper air distribution for
purging and suitable air velocities for lighting off, all auxiliary air dampers should be
open during the purge period, the lighting-off and the warm-up period.
After the unit is on the line, the total required amount of air (total air flow) is a function of
the unit load. Proper air flow at a given load depends on the characteristics of the fuel
fired and the mount of excess air required to satisfactory burn the fuel. Excess air can
best be determined through flue gas analysis (Orsat measurements).
The optimum excess air is normally defined as the 02 at the economiser outlet produces
the minimum capacity. Operation below the optimum excess air will result in high
capacity due to unburned carbon where as operation above the optimum excess air will
result in high capacity due to excessive H2 S04 condensation. Operation below
recommended range will result in excessive black smoke and operation above this
range will result in excessive white smoke.
NOTE
The most suitable amount of excess air for a particular unit, at a given load and with a
given fuel must be determined by experience. This is best done from observation of
furnace slagging conditions. Slagging tendency of a particular fuel may dicatate an
increase of operating excess air.
AIR FLOW DISTRIBUTION
The function of the windbox compartment dampers is to proportion the amount of
secondary air admitted to an elevation of fuel compartments in relationship to that
admitted to adjacent elevation of auxiliary air compartments.
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Windbox compartment damper positioning affects the air distribution as follows:
Opening up the fuel-air dampers or closing down the auxiliary air dampers increases the
air flow around the fuel nozzle. Closing down the fuel air dampers and opening the
auxiliary air dampers decreases the air flow directly around the fuel stream.
Proper distribution of secondary air is important for furnace stability when lighting off
individual fuel nozzle, when firing at low rates and for achieving optimum combustion
conditions in the furnace at all loads.
Proper distribution of secondary air also has an effect on the emission of pollutants from
coal tired units. As the unit load increases the quantity of Nitrogen Oxide (NO) Produces
in a furnace (due to the oxidation of nitrogen in the fuel) increases and the upper
elevations of fuel nozzles are placed in service. The quantity of NO produced can be
reduced by limiting the amount of air admitted to the furnace adjacent to the fuel and
increasing the quantity of air admitted above the fire (over fire air). When the unit has
reached a predetermined load (app.50%) the over-fire air dampers should open and
modulate as a function of unit load until, at maximum continuous rating (MCR) when up
to 15% of the total air is admitted to the furnace as over fire air. The optimum ratio of
over fire air to fuel and auxiliary air, as well as the optimum tilt position of the over-fire
air nozzles, to produce a minimum NO emission consistent with satisfactory furnace
performance must be determined through flue gas testing (i.e measurement of NO)
during intial operation of the unit.
The correct proportion of air between fuel compartment and auxiliary air compartments
depends primarily on the burning characteristics of the fuel. It influences the degree of
mixing, the rapidity of combustion and the flame pattern within the furnace. The
optimum distribution of air for each individual installation and for the fuel used must be
determined by experience
The wind-box compartments are normally provided with drive (except end air
compartments) so they may be operated by a secondary air damper and overfire air
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control system in conjunction with the furnace safeguard supervisory system. When on
automatic control the system should provide modulation of the auxiliary air dampers as
required to maintain a pre-set windbox-to-furnace differential pressure. The fuel air
dampers should be closed prior to and during light off. When the fuel elevation is proven
in service, the associated fuel air dampers should open and be positioned in proportion
to the elevation firing rate. Normally the end air compartments are provided with manual
adjustment which can be kept in the required position during commissioning of the unit.
FUEL OIL FIRING SYSTEM
Fuels
A coal tired unit incorporates oil burners also to a minimum oil firing capacity of 15% of
boiler load for the reasons of:
a. To provide necessary ignition energy to light-off coal burner
b. To stabilise the coal flame at low boiler/burner loads
c. As a safe startup fuel and for controlled heat input during light-off.
Auxiliary steam is utilised in boiler for following purposes:
a. For atomising the HFO at the cil gun
b. For tank heating, main heating and heat tracing of HFO
c. To preheat the combustion air at the steam coil air heater and to warm up the
main air heater (this reduces Sulphuroxide condensation and thus cold end
corrosion of main air heater)
With above provisions and with proper oil steam and combustion air parameters at the
burner, HFO is safely fired in a cold furnace.
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Burner Arrangement
In a tangentially fired boiler, four tall windboxes (combustion air boxes) are arranged,
one at each corner of the furnace as shown in Figure No. 9. The coal burners or coal
nozzles are located at different .levels on elevations of the windboxes. The number of
coal nozzle elevations are equivalent to the number of coal mills. The same elevation of
coal nozzle at 4 corners .are led from a single coal mill.
The coal nozzles are sandwitched between air nozzles or air compartments. That is, air
nozzles are arranged between coal nozzles, one below the bottom coal nozzle and one
above the top coal nozzle. If there are 'Q' numbers of coal nozzles per corner there will
be (n+1) numbers of air nozzle per corner.
Air Nozzles - 9 Lower 7 + 2
3 AA EA
The coal fuel and combustion air streams from these nozzles or compartments are
directed tangential to an imaginary circle at the centre of the furnace. This creates a
turbulent vortex motion of the fuel air and hot gases which promotes mixing ignition
energy availability, combustion rate and combustion efficiency.
The coal and air nozzles are tillable 30 about horizontal, in unison, at all elevations
and corners. This shifts the flame zone across the furnace height for the purpose of
steam temperature control.
The air nozzles in between coal nozzles are termed as Auxiliary Air Nozzles, and the
top most and bottom most air nozzles as End Air Nozzles.
The coal nozzle elevations are designated as A.B.C.D etc., from bottom to top, the
bottom end air nozzle as AA and the top end air nozzles as XX. The auxiliary air
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nozzles are designated by the adjacent coal nozzles, like AB. BC.CD.DE. etc. from
bottom to top.
The four furnace corners are designated as 1,2.3 and 4 in clockwise dilection looking
from top and counting front water wall left corner as '1'.
Each par of coal nozzle elevations is served by one elevation of oil burners located in
between. For example in a boiler with 8 mills or 8 elevations of coal nozzles, there are
16 oil guns arranged in 4 elevations, at auxiliary air nozzles AB.CD.EF, & GH.
Heavy fuel oil can be fired at the oil guns of all elevations.
Each oil gun is associated with an high energy are ignitor.
Combustion Air Distribution
Of the total combustion air a portion is supplied by primary air fans that goes to coal mill
for drying and pulverising the coal and carrying it to the coal nozzles. This 'Primary Air'
flow quantity is decided by the coal mill load and the number of coal mills in service. The
primary air flow rate is controlled at the air inlet to the individual mills by dampers
The balance of the combustion air, referred as 'Secondary Air" is provided by FD fans.
A portion of secondary air (normally 30% to 40%) called 'Fuel Air' is admitted
immediately around the coal fuel nozzles (annular space around the casting insert) into
the furnace. The rest of the secondary air called 'Auxiliary Air' is admitted through the
auxiliary air nozzles and end air nozzles. The quantity of secondary air (fuel air+auxiliary
air) is dictated by boiler load and controlled by FD fan blade pitch.
The proportioning of air flow between the various coal fuel nozzles and auxiliary air
nozzles is done based on boiler load, individual burner load, and the coal/oil burners in
service, by a series of air damper. Each of the coal fuel nozzles and auxiliary and end
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air nozzles is provided with a knock knee type regulating dampers, at the air entry to
individual nozzle or compartment. On a unit with 8 mills there will be 8 fuel air dampers,
7 auxiliary air dampers, 2 end air dampers and 2 over fire air dampers per corner.
Each damper is driven by an air cylinder positioner set, which receives signal from
'Secondary Air Control System'. The dampers regulate on elevation basis, in unison, at
all corners.
Furnace Purge
Traces of unburnt fuel air mixture might have been left behind inside the furnace of
some fuel or might have entered the furnace through passing valves during shutdown of
the boiler.
Lighting up a furnace with such fuel air accumulation leads to high rate of combustion,
furnace pressurisation and to explosions at the worst. This is avoided by the "Furnace
Purge' operation during which 30% of total air flow is maintained for above 6 minutes to
clear off such fuel accumulations and fill the furnace with clean air, before lighting up.
During furnace purge, all the elevations of auxiliary and end air dampers are opened to
have a uniform and thorough purging across the furnace volume.
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Fuel Air Dampers
Its operation is independent of boiler load.
All fuel air dampers are normally closed. They open fifty seconds after the associated
feeder is started and a particular speed reached; that it modulates as function of feeder
speed.
Fifty seconds after the feeder is removed from service, the associated fuel air dampers
close.
The fuel air dampers will open fully when both FD fans are off.
Fuel Oil Atomisation
Atomisation is a process of spraying the fuel oil into fine mist, for better mixing of the
fuel with the combustion air. While passing through the spray nozzles of the oil gun, the
pressure energy of the atomising steam breaks up the oil stream into fine particles.
Poorly atomised fuel oil would mean bigger spray particles, which takes longer burining
time results in carryovers and make the flame unstable due to low rate of heat liberation
and incomplete combustion.
Viscosity of the oil is another major parameter which decides the atomisation level. For
satisfactory atomisation the viscosity shall be less than 28 centistokes.
External mix type steam atomised oil guns suitable for both LFO and HFO have been
provided. Atomisers of this type are widely known as J-tips. The atomiser assembly
consists of nozzle body welded on to the gun body, back plate, spray plate and cap nut.
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SYSTEM REQUIREMENT
The maximum total output of oil burners is 30% of the boiler MCR. This meets the
turbine synchronisation needs before firing coal burners.
Each oil burner capacity is about 2% of boiler MCR.
For coal burner ignition and coal flame stablisation a minimum oil burner output,
equivalent to 10-20% of maximum coal burner capacity is required. This roughly
corresponds to 40 to 50 % rating of an oil burner.
The oil burner output is a function of oil pressure at the oil gun and the normal turndown
range of the oil burner is 3:1.
For steam atomised oil burner, the oil pressure at the oil gun shall not fall below 2.5
kg/sq cm (g) to ensure good atomisation and stable flames.
The oil burners have to be operated at loads, lower than the maximum rating for
reasons mentioned below:
a) during cold startups of the boiler, to have a controlled and gradually increasing
heat loading, to avoid temperature stresses on pressure part materials, as
dictated by boiler startup curves.
b) To conserve fuel. oil by operating the oil burners just at the "Coal flame
stabalisation" requirements.
Igniters
High Energy Arc type electrical igniters are provided, which can directly ignite the heavy
fuel oil. The main features of this system are :
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a. An exciter unit which stores up the electrical energy and releases the energy at a
high voltage and short duration.
b. A spark rod tip which is designed to convert the electrical energy into an
intensive spark.
c. A pneumatically operated retract mechanism which is used to position the spark
rod in the firing position and retract to the non-firing position.
Each discrete spark provides a large burst of ignition energy as the current reaches a
peak value of the order of 2000 amps. These sparks are effective in lighting of a well
atomised oil spray and also capable of blasting off any coke particle or oil muck on the
surface of the spark rod.
For a reliable ignition of oil spray by the HEA ignitorjt is very much necessary to
maintain '.he following conditions:
a. The atomisation is maintained at an optimum level. All the atomising parameters
such as oil temperature, steam pressure, clean oil gun tips etc., are maintained
without fail. The atomising steam shall be with 20C superheat minimum.
b The cold legs are minimum. The burner fittings are well traced and insulated
c The spark rod tip is located correctly at the optimum location.
d The oil gun location with respect to the diffuser and the diffuser location with
respect to the air nozzle, is maintained properly.
e. The control system is properly tuned wit ignitor operation. The time of
commencing of all the operational sequences is properly matched.
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f. It may become necessary to close the air behind the ignitors, during the light of
period for reliable ignition. This must be established during the commissioning of
the equipment and proper sequences must be followed.
The following facts must be borne in mind to understand the igniters and the system
clearly:
a. The spark rod life will be drastically reduced if left for long duration in the
advanced condition when the furnace is hot.
b. Too much retraction of spark rod inside the guide tube will interfere with nozzle
tilts and may spoil the guide tube.
c. A minimum discharge of 300 kg/hr of oil is essential for a reliable ignition.
d. A plugged oil gun tip may result in an unsuccessful start.
e. A cold oil gun and hoses cause quenching of oil temperatures and may lead to
an unsuccessful start. In such cases warming up by Scavenging prior to start is
necessary.
Fuel Oil Gun Advance/Retract Mechanism
The atomiser assembly of an operating oil gun is protected for the hot furnace radiation
by the flowing fuel oil/steam which keeps it relatively cool. Once the burner is stopped
there is no further flow of oil/steam. Under such situation it is required to withdraw the
gun from firing position to save it from possible damage due to over healing.
In the system provided, the oil gun is auto advance, auto retraceable. It is driven by a
pneumatic cylinder and a 4 way dual coil solenoid pilot control valve, with a stroke
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length of 330 mm. There are three position limit switches, one for, "gun engaged"
position, another for "gun advanced" and the third for "gun retracted" position, which
have been suitably interlocked into furnace safeguard supervisory System logics for
safe and sequenced operation.
Steam Scavenging Of Fuel Oil Guns
Before stopping the oil burner, the oil gun is scavenged with steam to keep the small
intricate passages of the atomiser parts clean.
* In the auto programmed burner stop sequence, a planned shut down is followed
by steam scavenging the oil side for quite sometime, to achieve this requirement.
* During emergency tripping of the burners or boiler, the oil gun is neither
scavenged nor retracted automatically. Normally such emergency trips may last
only for a short while and the fuel oil guns shall be re-started or local manually
scavenged immediately on resuming boiler operation.
HFO Lumping System
The screw pump is a constant quantity pump and when only a small quantity of oil is
fired, the excess oil from the constant quantity pump should be by-passed. This is done
automatically by pneumatic operated, pressure maintaining cum regulating valve by by-
passing the excess quantity through the return oil line to storage tank. The delivery
pressure of oil is maintained constant at the pump outlet, whatever be the quantity of oil
fired,
Set the pressure control valve for maintaining adequate and constant pressure at the
upstream of the HFO flow control valve at maximum firing rate.
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PMI, NTPC 50
The flow control valve upstream pressure required is the sum of the following at
maximum firing rate:
a. Oil pressure at the gun inlet.
b. Static head between flow control valve and top level of burners, and frictional
pressure drop in these lines.
c. Flow control valve pressure drop, for best turndown.
HEAVY FUEL OIL HEATING SYSTEM
Three 150% duty steam-oil heat exchangers and three duplex strainers are provided for
operation in combination.
The HFO temperature control valve and the trap station for heaters, steam jacket of
strainers and line tracers are provided in the system.
All these equipment are laid out on the floor The drain points are to be suitably piped up
to the drain pit from the drain trays.
Steam Heaters and Strainers
The steam heaters are of fixed tube sheet, U tube type, with oil on shell side and steam
on the tube side. The oil space is protected against exceeding of allowable pressure by
low lifting spring loaded safety valve. The exchanger is equipped with the valves
needed for air release and draining.
The duplex basket type discharge strainers are at the heater outlet, with fine mesh of
250 micron filtration. The fine filtering prevents chocking of lines, valves and burner
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atomisers. The burner tip wearing rate is also reduced. When the pressure drop across
the strainer exceeds about 0.5 kg/sq. cm. (corresponding to 60% clogged status), the
standby strainer section is put into service and it is taken for cleaning.
SCANNER AIR SYSTEM
The scanner viewing heads are located in the burners and they are exposed to furnace
radiation continuously. The scanner heads cannot withstand high temperatures that will
arise due to this exposure. A constant cooling air is required around the scanner head
to cool it to a safe working temperature to ensure a reliable operation and long life. The
scanner head cannot be exposed to a continuous temperature of 175C without cooling
air.
PULVERIZED COAL SYSTEM GENERAL
The system for direct firing of pulverised coal utilizes Pulverisers to pulverize the coal
and a Tiling Tangential Firing System to admit the pulverized coal together with the air
required for combustion (secondary air) to the furnance.
As crushed coal is fed to each pulverizer by its feeder (at rate to suit the load demand)
primary air is supplied from the primary air fans. The primary air dries the coal as it is
being pulverized and transports the pulverized coal through the coal piping system to
the coal nozzles in the windbox assemblies.
A portion of the primary air is pre-heated in the bisector air heater. The hot and cold
primary air are proportionally J-nixed, prior to admission to the pulveriser, to provide the
required drying as indicated by the pulveriser outlet temperature. The total primary air
flow is measured in the inlet duct and controlled to maintain the velocities required to
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transport the coal through the pulveriser and coal piping. The total primary air flow may
constitute from approximately 15% to 25% of the total unit combustion air requirement.
The pulverised coal and air discharged from the coal nozzles is directed toward the
centre of the furnace to form a firing circle. Fully preheated secondary air for
combustion enters the furnace around the pulverised coal nozzles and through the
auxiliary air compartments directly adjacent to the coal nozzle compartments. The
pulverized coal and air streams entering the furnace are initially ignited by a suitable
ignition source at the nozzle exit. Above a predictable minimum loading condition the
ignition becomes self sustaining. Combustion is completed "as the gases spiral up in the
furnace.
A large portion of the ash is carried out of the furnace with the flue gas; the remainder is
discharged through the furnace bottom into the ash pit.
COMBUSTION OF PULVAREED COAL IN TANGENTSALLY RRED FURNACES
The velocity of the primary air and coal mixture within the fuel nozzle tip exceeds the
speed of flame propagation. Upon the nozzle tip the stream of coal and air rapidly
spreads out with a corresponding decrease in velocity, especially at the outer fringes
where eddies form as mixture occurs with the secondary air. Here flame propagation
and fuel speeds equalize, resulting in ignition. As the stream advances in the furnace,
ignition spreads until the entire mass is burning completely.
The speed which the air and coal mixture ignites after leaving the windbox nozzles
depends largely on the amount of volatile matter in the fuel. Heat released by oxidizing
the volatile components in the coal accelerates of the fixed carbon to its ignition
temperature.
The key to complete combustion consists of bringing a successive stream of oxygen
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molecules into contact with carbon particles, the smallest of which are relatively large by
comparison with he oxygen molecules. As combustion of the carbon progresses it
becomes increasingly difficult to bring about contact with the diminishing oxygen supply
in the limited time available, which for this type of firing is in effect greater due to the
longer travel taken by the gases.
The cyclonic mixing action that is characteristic of this type of firing is most effective in
turbulently mixing the burning coal particles in a constantly changing air and gas
atmosphere. As the main part of the gases spiral upward in the furnace, the relatively
dense solid particles are subjected to a sustained turbulence which is effective in
removing the products of combustion from the particles and in assisting the natural
diffusion of oxygen through the gas film that surrounds the particles.
PULVERIZERS
The pulverizer, exclusive of its feeder, consists essentially of a grinding chamber with a
classifier mounted above it. The pulverizing takes place in a rotating bowl in which
centrifugal force is utilized to move the coal. delivered by the feeder, outward against
the grinding ring (buil ring) as shown in fig no. 10. Rolls revolving on journals that are
attached to the mill housing pulverize the coal sufficiently to enable the air stream
through the pulverizer to pick it up. Heavy springs, acting through the journal saddles,
provide the necessary pressure between the grinding surfaces and the coal. The rolls
do not touch the grinding rings, even when the pulveriser is empty. Tramp iron and
other foreign material is discharged through a suitable spout.
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R P PULVERIZER GENERAL ARRANGEMENT Figure No. 10
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LEGEND 1. HOT AIR CONTROL DAMPER
2. COLD AIR CONTROL DAMPER
3. HOT AIR SHUTOFF GATE
4. COLD AIR SHUTOFF GATE
5. PULVERIZER DISCHARGE VALVES
8. PULVERIZER DISCHARGE SEAL AIR VALVE
7. FEEDER SEAL AIR SHUTOFF VALVE
8. PULVERIZER SEAL AIR SHUTOFF VALVE
9. FAN - ISOLATING VALVES
SEAL AIR A. TO FEEDER
B. TO PULVERIZER DISCHARGE VALVES (COAL PIPES)
C. TO HOT AIR SHUT OFF GATE ft CONTROL DAMPER
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The air and coal mixture passes upward the classifier with its deflector blades where the
direction of the flow is changed abruptly, causing the coarse particles to be returned to
the bowl for further grinding. The fine particles, remaining in suspension, leave the
classifier and pass on through the coal piping to the windbox nozzles.
FEEDERS
The raw crushed coal is delivered from the bunkers to the individual feeders, which, in
turn feed the coal at a controlled rate to the pulverisers.
In order to avoid overloading the pulveriser motor due to overfeeding, an interrupting
circuit should be used to reduce the coal feed if the motor should become overloaded
and to start the coal feed again when the motor load becomes normal.
PULVERISED COAL DRYING
For satisfactory performance, the temperature of the primary air and coal mixture
leaving the classifier should be kept at approximately 77C for our coals. To low a
temperature may not dry the coal sufficiently; too high temperature may lead to fires in
the pulveriser. The outlet temperature must not exceed 90C in any case. The moisture
content of coals varies considerably. Therefore the best operating conditions for an
particular installation must be determined by experience.
Figure No. 11 shows the location of dampers, shutoff gates and valves generally
utilised. The hot air control damper (No.1) and the cold air control damper (No.2)
regulate the temperature entering the pulveriser, by proportioning the air flow from the
hot air and cold air supply ducts. These dampers also regulate the total primary air flow
to the pulveriser.
The hot air shutoff gate (No.3) is used to shutoff the hot air to the pulveriser. The hot air
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gate drive must be interlocked with the pulveriser motor circuit so that the gate will be
closed any time the pulveriser is not in service. It must also be interlocked with the
temperature controller to effect closing of the hot air gate when the pulveriser outlet
temperature exceeds 90C.
The pulverise discharge valves (No.5), the cold air shutoff gate (No.4) and the seal air
shutoff valves (No.8) are always kept wide open. They are closed only when isolation of
a pulveriser or feeder is required for maintenance. Pulveriser discharge valves are also
closed on loaded, idle pulverisers when other pulverisers are being restarted after an
emergency fuel trip.
An adequate supply of clean seal air for the pulveriser trunion shaft bearing, etc.,
normally is assured by installing two booster fans and a filter in the seal air system. One
fan normally runs continuously, however it may be isolated for maintenance by closing
its inlet shutoff damper (No.9). The filter in this system is an inertial separator type
which discharges approximately 90% of its input as clean air. A bleed off system, with a
control valve, will control the amount of air being bled from the filters, so that the
differential pressure between the filter air outlet and the filter bleed air outlet is zero.
The control valve should be installed so the valve fails open with a loss of instrument
air.
The coal pipe seal air valve (No.6) is utilized to admit seal air to the coal pipes for
cooling when the pulveriser is isolated. The seal air valve is open whenever the
pulveriser discharge valve air closed an vice versa.
Primary air velocity requirements in the pulveriser and coal piping preclude wide
variations in system air flows. Therefore a constant air flow is maintained over the entire
pulveriser load range. The air flow should be low enough to avoid ignition instability and
high enough at avoid settling and drifting in the pulverised coal piping or excessive
spillage of coal from the pulveriser through the tramp iron spout.
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Coal spillage may also be caused by overfeeding, insufficient heat Inputs for
drying, too low a hydraulic pressure on the rolls or excessive wear of the grinding
elements.
GRAVIMETRIC FEEDERS
The STOCK Model 7736 gravimetric feeder is designed-to supply 4366 to 76,408 Kgs.
of coal to the pulveriser per hour while operating on 415 volt, 3-phase, 50 Hertz power
supply.
BELT AND DRIVE SYSTEM
The feeder belt is supported by a machined drive pulley near the outlet, a slotted take-
up pulley at the inlet end, six support rollers beneath* the feeder inlet, and a weighted
idler in the middle of the feeder. A counterweighted scraper with replaceable rubber
blade continuously cleans the carrying surface of the belt after the coal is delivered to
the outlet. Proper belt tracking is accomplished by crowning the take-up pulley: in
addition, all three pulley faces are grooved to accept the molded V-guide in the belt. The
pulleys are easily removable for belt changing and bearing maintenance.
Belt tension is applied through downward pressure exerted by the tensioning idler on
the return strand of the belt. Proper tension is obtained when the round protrusion at the
center of the tension roll is in line with the center indicator mark on the tension indicator
plate. The tension roll indicator is found on the drive motor side of the feeder and is visible through the viewing port in the tension roll access door. Tension adjustments can
be made with the feeder operating or at rest by turning the two belt take-up screws
which protrude through the inlet and access door.
CHANGES IN HUMIDITY OR TEMPERATURE MAY CAUSE VARIATIONS IN BELT
LENGTH. BELT TENSION SHOULD ALWAYS BE MAINTAINED WITHIN THE TWO
EXTREME MARKS ON THE TENSION INDICATOR PLATE.
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The belt drive system consists of Louis-Allis 5 HP variable speed DC shunt wound
motor with a speed range of 100-1750 rpm. The motor is housed in a totally-enclosed,
non-ventilated enclosure with Class II epoxy coated insulation with tropical protection,
severe duty house down provisions, and a 150 watt space healer wired for 240 VAC
operation.
The motor operates through a multiple-reduction gearbox to a total reduction of 149.6:1.
A reluctance-type magnetic sensor is provided on the motor drive to detect motor
speed. This data is used for motor speed control feed back information, for zero speed