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Boilers are often referred to as steam generators, because the
primary function of a boiler is to generate steam.Boiler is a heart
of Fossil Fuel Plant. They seem destructive but they are not.
All boilers have something in common:
all boiler use fossil fuel all generate hot gases all use water
to generate steam
INTRODUCTION:
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The first boilers were not much complicated
They were open, water-filled containers heated by the combustion
of fuel.
When water in the containerabsorbs enough heat, it boils and
produces steam
Steam is produced but there are a number of inefficiencies
caused by its design: an open container only produces steam at
atmospheric pressure. an open container rapidly boils & dry the
flow of steam is impossible to control
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Closing the container allows steam to produce at higher pressure
Constant supply of feed water is introduced thus minimizing the
chances of damaging the container Flow of steam and water can be
controlled by providing valve in steam and feed water line steam
can now be directed to a desired location
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The system still does not make efficient use of the heat
producedMuch of the heat of combustion is lost to surroundings
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Efficiency of a boiler has a direct bearing on the cost of power
generation Power Plant boilers must be efficient Boilers with only
forced draft fan are called pressurized furnace boilers.
Boilers that use both forced draft and induced draft fans are
called balanced-draft boilers.
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Boiler ActsIndian Boiler RegulationThe Indian Boilers Act was
enacted to consolidate and amend the law relating to steam boilers.
Indian Boilers Regulation (IBR) was created in exercise of the
powers conferred by section 28 & 29 of the Indian Boilers
Act.IBR Steam Boilers Means any closed vessel exceeding 22.75
liters in capacity and which is used expressively for generating
steam under pressure and includes any mounting or other fitting
attached to such vessel, which is wholly, or partly under pressure
when the steam is shut off.IBR Steam Pipe Means any pipe through
which steam passes from a boiler to a prime mover or other user or
both, if pressure at which steam passes through such pipes exceeds
3.5 kg/cm above atmospheric pressure or such pipe exceeds 254 mm in
internal diameter and includes in either case any connected fitting
of a steam pipe.
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Definition of Boiler (as per I.B.R.)Any closed vessel exceeding
22.75 litres (5 gallons) in capacity which is used exclusively for
generating steam under pressure and includes mounting or fitting
attached to such vessel, which is wholly or partly under pressure
when steam is shut off.INTRODUCTION:
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There are two kinds of boilers: 1- Fire-tube boilers 2- Water
tube boilers Water tube boilers come in two basic types: i) drum
type : single drum & bi drum ii) once through boilers
1.0-TYPES OF BOILERS
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Types of Furnances Dry Bottom type Wet Bottom type
Firing Systems Stroke Fired Pulverized Fuel Fired CFBC Draft
Systems Natural Draft Induced Draft Forced Draft Balanced Draft
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The hot gases are made to flow through its various components
and come in contacts with water wall tubes, heater tubes ,
convection tube, economiser coils , air heater, ESP and ID fans.
Boilers are also classified on the basis of circulation i.e.
Natural circulation boilers Forced circulation boilers
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Drum-type boilers are re-circulating boilers i.e. water that is
not converted to steam continues to flow through within it
Once-through type boilers convert water to steam in one pass.
Instead of drum these type boilers have moister separator.
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FIRE TUBE BOILER
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WATER-TUBE BOILER
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GENERAL ARRANGEMENT
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A coal-fired boiler produces two types of ash: (1) bottom ash
and (2) fly ash Bottom Ash: Bottom ash is made up of heavy
particles that fall to the bottom of the furnace in the boiler as
combustion take place.Bottom ash also includes molten ash,which
forms on the boilers furnace walls and runs down to the bottom of
the boiler.Bottom ash poses two major problems: 1-It can build up
on the boiler furnace wall, which insulates the tubes and reduces
the amount of heat that reaches the boiler tubes.2-Large chunk of
ash , called clinkers, can drop through the furnace and damage
boilers internal components.
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Fly AshFly ash is made up of light ash particles that are
carried through the boiler in the gas flow.It also causes two major
problems:1-Being highly abrasive in nature it causes wear in the
boiler tubes.2-It can collect on and around the boiler tubes thus
prevents free flow of gases through the boiler. It also reduces
amount of heat that reaches boiler tubes.
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Flue Gases
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Steam
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Three common fossil fuels are used in boilers:
(1) Natural Gas, (2) Oil and (3) CoalNatural gas is the cleanest
of the three common fuels but it is in short supply for power/steam
plants.It causes no maintenance problems.The major maintenance
problem with oil is soot accumulation.Coal is the cheapest and most
available fossil fuel, but it causes major maintenance
problems.These problems are primarily a result of the ash produced
when coal burns.Soot: Fine black particles, produced by incomplete
combustion of coal / oil
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2.0-Furnace ComponentsWATER WALLS Water-tube boilers have water
walls Water walls are subject to overheating, because they are
close to fire
overheating occurs if there is misalignment of burners resulting
in tube failure Tube rupture also occur from flame impingement
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Tangential/ Membrane tubes
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MATERIAL SPECIFICATIONEco & W/wall tubes: SA 210 Gr A1 / SA
192
Bank tubes: SA 192
Superheater tubes: SA 209 T1 SA 213 T11 SA 213 T22 SA 213 TP
347H SA 213 T91
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Tube ruptures and leaks are also caused by clinker impact, soot
blowers-due to their improper functioning corrosion , erosion,
overheating due to water side scale formation etc.
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3.0-HOT GAS PATH COMPONENTS 3.1-SUPERHEATERS: Superheaters can
be classified in three ways: 1) by the way they absorb heat; 2) by
their tube arrangement; 3) by their location in the boiler Based on
above classification, boilers have one or two or all the following:
Radiant superheater Platen & Pedant superheater Horizontal
Superheater or Primary and secondary superheaters
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CLAMP TACK WELDED WITH SHIELD (SEE DETAIL A) PROTECTION FROM
LRSB9.66 M SuperHeater TubesShielding ArrangementGas Flow
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CLAMP 2 MM THK,25 MM WIDTH, SUITABLE LENGTH FOR TUBE O. D. S.
STEEL HALF SEGMENT SHIELD2 MM THICKDETAIL - ASTITCH WELDShielding
Arrangement
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Radiant superheaters are positioned in direct sight of
furnace.
Tubes in Platen superheater are widely spread to prevent
bridging.
Since Pendant superheaters located in the direct path of hot
gases, subject to deposits, tube failure due to erosion, and ash
problem.
In pendant superheaters spacers are provided to keep the tubes
at proper place.Bridging is accumulation of gas to the point that
it bridges the gap between the tubes.
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3.2-ECONOMIZER: Economiser heats feed water as it first enters
the boiler.
Economizers have some unique maintenance problems,though the gas
flow velocity and temp. are less than that of super-heater
area.
Economisers particularly tube bends , become corroded, and that
can lead to failure.
Ash accumulation occurs to a greater degree on the economiser
tubes than on the superheaters.
Another economiser problem is unconsumed coal or oil, that
sometimes gets mixed in with the accumulated ash .The combustible
material catch fire and explodes.
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4.0-WATERSIDE COMPONENTS STEAM DRUMS & MOISTURE
SEPARATORS
The function of a steam drum is to hold large volume of water
and separate steam from water.
Inside the drum are numerous components for maintaining water
level in the drum, separating steam from water, and keeping steam
and water free from impurities.
There are four basic connections coming off the outside steam
drum are :1-Boiler water supply line 2-Chemical dosing line3-Steam/
water mixture from the water walls4-Steam outlet line
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Steam drum has drum internals: turbo-separators; screen, tubes
and pipes
Outside of drum is insulated. If water collects inside the
insulation, corrosion starts.
Another problem with steam drum metal can crack due to heating
and cooling during repeated startups and shutdowns.
Corrosive sludge accumulations may also crate problem Steam drum
is also subject to erosion . It is caused by flow of water through
the tube to the drum.STEAM DRUMS continued
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5.0-THE OUTSIDE OF THE BOILER5.1-BOILER CASING
Outside covering of boiler is called boiler casing Casings steel
plate is the outside layer Inside the casing is a layer of
refractory Purpose of casing is to contain fire and hot gases
inside the boiler Buckling of casing may cause maintenance problem.
Sometimes buckling process may be violent and casing might
rupture
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Down comers are pipes, usually 6 to 8 inches dia. which run from
drum to the bottom of the furnace ring header. The maintenance
problem found with the downcomers is corrosion.5.2-DOWN COMERS,
VENTS ,DRAINS ANDCIRCULATING PUMPS Vents and drains are other
sources of maintenance They are located on drums, flash tanks,
headers, economisers, superheaters etc.
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6.0- AIR PRE HEATERS REQUIREMENT,TYPES, CONSTRUCTION Air pre
heater is another important Boiler auxiliary which preheats
combustion air for rapid and efficient combustion in the furnace
Its function is to recover the waste heat from the outgoing flue
gas Flue gas temp at economiser outlet is around 3800C.Every 55 0C
drop in flue gas temperature improves the boiler efficiency by
about 2.5% . Having air heater in the down stream of economiser,
the boiler efficiency is considerably improved.
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6.1- AIR PRE HEATER TYPESTypes of air pre-heaters Static
recuperative type : tubular air heater Rotary regenerative type:
Ljungstrom air heater In the recuperative type the flue gas is
transferred to the air through the heat transfer surface normally
in forms of tubes / plates In regenerative type the gas flows
through a closely packed heat transfer element
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CONSTRUCTION DETAILSTubular air heater consists of large no. of
tubes of 40 to 65 mm dia welded / expanded to plates at the
end.Tube bank is placed in a shellBaffles are generally provided
between tube plates to guide the air flow across the entire
surface.Gas flows inside the tubes , heating the tube surface and
air flows over the tubes picking the heat.
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ADVANTAGES OF TUBULAR AIRHEATER No moving parts so less
maintenance No auxiliary power consumptionDISADVANTAGES Occupies
more area Deposits on the tube surface reduce the heat transfer
Pressure drop of flue gas across the air heater is high Cross flow
Heat transfer is less efficient Severely affected due to cold end
corrosion Puncture in tubes results in leaking air into flue gas
stream thereby increasing load of fans Replacement of punctured
tube is major task requiring more downtime on the boiler
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LJUNGSTROM AIR PRE HEATERS ROTARY AIR PREHEATER3900C Flue Gas
Inlet Temp3100C2900C1800C Flue Gas Outlet TempCONSTRUCTION
DETAILS
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The Ljungstrom airpreheater consist the following major
components: Rotor Bearing Housing Connecting plates Sealing
arrangements Drive units Cleaning devices safety devices
Ljungstrom air preheater rotates 2 to 3 rpm and alternately
passes through the gas and air passes.The heating elements pick up
the heat from the flue gas and transfers it to the air when they
move through the air pass.
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ADVANTAGES OF LJUNGSTROM AIRHEATER Compact and hence saves space
& structural cost Economical.As the boiler size increases heat
transfer area also increases Initial cost and operating cost is
lower Holes in the elements due to corrosion etc. will not
materially affect the performance of the heater Deposit on elements
does not reduce the heat transfer Pressure drop across the elements
can be kept nearly constant during its operation with on load
cleaning Hot primary air for coal drying in mills is possible with
tri-sector design
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The Trisector design permits a single Ljungstrom regenerative
air preheater to perform two functions : coal drying and combustion
air heating The Trisector airpreheater is flexible in meeting
operational changes and is easily adaptable to varying coal moister
contents etc.
DISADVANTAGES Moving parts increases possibility of outages.
Leakage of air into gas , and gas and dust into air because of
imperfect sealing. To keep the elements clean both soot blowing and
water washing facilities are provided.
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BOILER MAINTENANCE
COMMON PRACTICES
1. Annual Overhauling
2. Running Maintenance
3. Shutdown Maintenance
4. Breakdown Maintenance
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BOILER MAINTENANCEBoiler overhauling is a planned, preventive
maintenance activity.Objective of this periodic long maintenance is
to avoid forced outages of the unit in between two consecutive
overhauls i.e. to increase MTBF.1.0-PLANNING Overhauling is a
multifaceted activity. It requires proper planning &
co-ordination of various agencies well before unit is taken out for
the purpose. OVERHAULING
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PLANNING INVOLVES: 1.1 - Identification of Areas to be
attended.1.2-Identification of spares.1.3-Identification of tools
and tackles.1.4-Identification of required manpower.1.1- AREAS TO
BE ATTENDED Area wise detailed list of the jobs to be done, is
prepared, well in advance on the basis of:previous years
experience,history log books of the equipment,
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For Example:
Furnace AreaConvection ZoneEconomizer CoilsDrum
InternalsBurnersSoot BlowersAir Pre-heaterCoal MillsFansDucts
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1.2- SPARE PARTSBefore unit is taken for overhauling, the spares
required are to be arranged, based on previous records of
consumption.1.3-TOOLS AND SCAFFOLDINGAll lifting tools like
hook-chucks, pulleys, chain blocks, jacks, spanners and wrenches of
various size, hammers, hacksaw frames with spare blades etc. are
ensured and made available at site store.
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consumables like shims, gaskets, asbestos ropes, welding
electrodes, gas cylinders, lubricants etc. are also ensured and
kept at site store.Enough scaffolding items like pipes, connecting
clamps are kept ready.1.4MANPOWERSince most of the maintenance
activities involve skilled jobs, it is recommended to ensure
sufficient skilled manpower. It is also recommended to have
exclusive gang for each equipment :
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2.0-CHECKSBefore a unit is taken out for overhauling a detailed
study of pre-outages and outages during operation, checks are
commended.2.1-Pre-outage ChecksFor this purpose a walk down survey
of the boiler, to identify the areas, which require to be attended
is done.Broadly following areas are checked:Areas of gas, air, oil,
steam & coal leakages. Erosion of Primary and Secondary Super-
heater Tubes
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Erosion of Convection Bank Tubes Leakage through Expanded
JointsBurners tilting arrangements, ignitersSeal air system for any
leakagesExpansion joints, indicatorsSafety valves and other
valvesSoot blower system for passing from valves, or for
operational defectsHangers & supports checks for failure of
components, fouling etcInsulation on pressure parts, non-pressure
parts.Various controls and interlocksConditions of auxiliaries.
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2.2-Operating ChecksThe operating checks on unit are to be
performed for operation parameters given in suppliers manuals.The
operating data are noted down and compared with design data given
in the manual.For example a consistent high super heater outlet
temperature shows fouling of furnace wall. It requires for checking
of entire furnace wall and thorough cleaning.Once again compare new
operating data with the pre-outage data.
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Drums Drums to be inspected for any scaling of the inside
surface.Check that internals are properly fitted.Check for the
orientation of the holes of chemical dosing pipers.Check turbo
separators, steam separating screen.Headers to be checked and
cleaned.Check for rolled tube ends.
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3.0-SHUT DOWN CHECKSDuring shutdown the following areas should
be thoroughly checked. 3.1-Pressure PartsFurnaceAfter thoroughly
cleaning the furnace walls for slag deposits, check for alignment
of water wall tubesAny bow observed should be correctedCheck for
any erosion of water wall tubes particularly near to wall
blowers
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Check for tube thickness.Furnace bottom slope tubes to be
checked for any damage due to falling of slag form the
furnace.Check for the inside condition of water wall tubes for
corrosion, scale formation by cutting a sample tube. If scale are
noticed, this requires for improvement of water qualityFurnace roof
tubes are to be checked for any bowed tubesCheck all soot blower
points for any damage to sleeve.Check for all hangers and
supports
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CONVECTION BANK TUBESShielding ArrangementGas FlowSuperheater
sideEconomiser side50.8 mm O.D. Tubes Spacing50.8 mm O.D. Tubes
SpacingLRSBLRSB
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HALF SEGMENT M.S. SHIELD 3.15 mm THKSTITCH WELDCLAMP 2 MM THK,25
MM WIDTH, SUITABLE LENGTH FOR TUBE O. D. SHIELDING ARRANGEMENTGas
Flow
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Expanding of Convection TubesF.I.D.= Final internal diameter of
tube = I.D. + 2 x t x (% of wall reduction) + hole clearance
100where, I.D. = Internal diameter of tube t = Thickness of tubeFor
example:Tube O.D. = 2,t = 0.200Wall reduction = 8%, Hole clearance
= 0.032then calculate F.I.D.
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Superheaters Check that all tubes are in alignment and gap
between the panels is equalCheck for the erosion of tubes near the
region of soot blower and corrosion of tubes due to flue gasCheck
for swelling of tubes due to overheatingCheck for the soundness of
all attachment weldsCheck for refractories below rear water wall
tubes.
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Economiser Conduct a survey by measuring tube thickness and
diameter of tubes especially at the bends.Check for the corrosion
of tubes due to low flue gas temperature in that region.Check for
all attachment welds and supports.Check for the condition of
baffles and restore and original condition if required.
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Economiser CoilOutletHeaderInletHeader
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PROTECTION COVERover Economiser bends
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ECONOMISER BEND
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SHIELDING ARRANGEMENT over Economiser tubes
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Modifications for Maximum Plant Availability Economizer Failure
Due To Gas Side Erosion Provision of spare coil assemblies.
Economizer coil assemblies are replaced every year instead of
on-site repair. Survey time was brought down from 50 days to 18
days.Erosion prone areas are pre-shielded over the straight length.
The bends are protected by cover shields.Removed coil assemblies
are repaired in workshop and kept ready for reuse in next
survey.Flue gas velocity in boiler no. 1 & 2 was reduced from
17m/sec to 12m/sec by increasing the depth of economizer. More coil
assemblies were added.Lower erosion.Reduced draft loss (gain of 17
mm)Gain in feed waster temperature (3oC)Reduction in tube
leakageFrom 1969 to 1989 fifty six (56) leakage were detected. From
1990 to Jan. 2001 only nine leakage occurred.
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Drums Drums to be inspected for any scaling of the inside
surface.Check that internals are properly fitted.Check for the
orientation of the holes of chemical dosing pipers.Check turbo
separators, steam separating screen.Headers to be checked and
cleaned.Check for rolled tube ends.
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3.2-Duct SystemGas DuctsGas ducts in the Boiler are vulnerable
to corrosion and erosion wear, they need a thorough checking. These
ducts are: i ) Economizer to air heater. ii ) Air-heater to E.S.
Precipitator iii) ESP outlet to ID Fan inlet iv) ID Fan outlet to
chimneySupports and braces provided for the purpose, should also be
checked.Check all the expansion joints physically for any permanent
deformation due to excess loading.
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Air DuctsCheck wind box compartmentCheck struts and hangersCheck
the condition of measuring devices such as venturies, nozzles
etc.Check for damaged insulation
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Coal pipes are subjected to much abrasive wear due to erosive
nature of our coal.
Bends are also subjected to severe erosion.
If the erosion is less, patch welding of bends can be carried
out. For longer radius bends (R>3D) basalt lining could be
provided.
All the victaulic coupling joints are to be checked.
Check all supports of pipes for load sharing while replacing
portion of pipe 3.3-Coal Piping
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3.4-Dampers Gates and VanesNormally gates are meant for shut off
purpose and dampers for control purpose.These are to be checked for
their proper functioning.
Check manually for free movement of dampers. Check that flap to
flap sealing is perfect.
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Check for proper linkage connectionsGates to be checked for any
wearCheck gates for full opening and closingCheck gland packing of
damper shaftsCheck inside surface of driving air cylinder for any
corrosionCheck damaged O ringsCheck the condition of connecting
pipe
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3.5-Fuel Systems A-Pulverizers and Feeders Pulverizers are more
susceptible to wear than other auxiliaries of Boiler.In pulverizers
check for following areas for wear out:separator body liners,
bottom liners, bull ring segments, grinding rolls scrapers and
scraper guards, mill discharge valves/ valve assembly classifiers
deflector vanes, inner cones.If amount of wear is excessive,
replace, if wear is less the same could be built by hard facing
electrodes.
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Analyze lubricating oil used in : (i) Journals and (ii) Vertical
Shaft Check the condition of bearings of (i) Journal Assembly (ii)
Vertical Shaft (iii) Worm Shafts Check the pump hub, clean and fit
it Check the oil cooler tubes for any choking or leakages Check the
condition of worm shaft and worm gear
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Check air sealing housing for any coal / ash built upCheck
whether journal springs are in good conditions.Check thoroughness
of tramp iron spoutVisually inspect feeders for any wear
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Seal Air System Check:All seal air lines for thoroughness and
cleanlinessSeal air hoses for any damageSeal air damper for proper
closing and openingCheck and clean seal air line filtersService the
fan bearingsDifferential pressure switches and other instrument
tapping
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Nozzle and Tilting Mechanism Check all the nozzles for slagging
and clean the slag Check for the damages to the nozzle due to the
following: (a)Warping it occurs due to excess temper(b)Erosion
and(c)Welds Breaking
Refractory throat Shape Move nozzle up and down-check there is
no restriction to its free movement.
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Check the pattern of wear on coal nozzles.Check that all pins
are OKCheck that coal pipe load does not come on to nozzleCheck
that nozzles move in unison in both the directionsCheck free
movements of secondary air dampers. Set for full closing.
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Flame Scanners Clean the detector tube lens and check for any
damage in flexible hoses.Check location of the scanner head with
reference to the nozzle tip.Check whether cooling air path is
through.Check the shutter for proper functioning. Check the card
and control wires.Check service air fans, its bearing and
motor.
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4.0FUEL OIL SYSTEM - LIQUIDIGNITORS Check physical condition of
ignitor hornsCheck for proper functioning of jamesburry valveCheck
whether transformer is in healthy conditionClean the filters in oil
and air linesClean orifice and check for flow of required quantity
of oilCheck for air leakage from the wind box and stop leakage
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Oil Guns Remove oil guns and check for choking and erosion of
mixing plate and spray plate holes. Thoroughly clean and fit
back.Check the condition of flexible hoses for any damage. Replace
the damaged hoses. While fitting back, avoid kinks.Check for free
operation of retract mechanism. Service all sliding parts.Check the
condition of solenoid valve and replace damaged O ring. Check for
electrical operations.
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Check whether limit of retract mechanism act properly.Check and
maintain required gap between gun tip and diffuser face.Check all
hand operated and control valves of oil system for their proper
functioning and attend any defect.
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5.0-AIR HEATERCheck the condition of hot and cold end heating
surfaces for any deposits pluggage.To remove corrosive deposits,
carryout water washing of heating elements.Check for any heavy plug
gage at outer baskets. This may indicate the incomplete coverage of
soot blower. A check of the sweep should be made and mechanism
adjusted.Check the condition of various seals such as radial,
circumferential, bypass axial seals. Any erosion observed, change
the seal.
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Check the condition of sector plates and adjust rods.Check the
condition of support bearing guide bearings and lubricating oil for
any contamination. Check for proper operation of lubricant oil
pumps and thermostat.Check for any ash entry to drive gear
box.Service electrical motor and air motor for their operating
condition.
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6.0-FansCheck for erosion wear in fan casing, rotating parts,
discharged guide vanes etc.Clean entire system free from ash. All
the eroded portion to be restored.Check the condition of bearings
and its lubricants.In order to safeguard the bearings, ground the
welding machine directly to the impeller.
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Check the tightness of all foundation bolts.Check for proper
operation of inlet control vanes, inlet & outlet dampers. Set
for proper limits of opening and closing by adjusting limit
switches.Check for vane linkages of control vanes and grease.Check
complete bearing lubrication system.
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Check the conditions of supports and its welds with the casing
and ducting.Run the fan with all interlocks and check the record of
vibration. If vibration are high identify the cause and
eliminate.
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7.0-VALVESCheck all critical valves like low point drains, blow
off, S.H. Drains, spray isolation, main steam and feed isolation,
control valves.Valves in which passing was reported by operation or
noted by maintenance, should be opened and checked for the
condition of disk or seat. If needed, they should be lapped.Check
the sealing rings, glands and change if required.
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Check for free travel of valve stem & for any bend is
stem.Check for erosion in valve body and in other location.For
remote operated valves, check and set position of torque
switches.In control valves, check the contour of disc and trim for
any damage.For safety valves, check the condition of springs and
valve setting.
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8.0-SOOT BLOWERS AND FURNACE PROBESCheck valve seat and
discCheck for free movement of lanceCheck for any bend in lance
tubeCheck the gland packing and change, if requiredService the
driving and rotating motor and also the gear boxClean the chain
from ashSet the proper blowing pressure and limit switches for each
blowers
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Service the valves in pressure reducing station of soot
blowerFor all wall blowers check the correct distance of the nozzle
center line from furnace wall (38 mm)Check and simulate the soot
blower control panel for proper functioning with all
protection.Check free movement of the furnace probe and condition
of thermocouple.Check and ensure working of auto retract interlock
of furnace probe when gas temp exceeds preset value.
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9.0INTERLOCKS, CONTROLS AND INSTRUMENTATIONFor safe and steady
operation of the unit, it is essential that various controls and
interlocks should function properly. A-To ensure proper functioning
of boiler utmost care is required in the following: Furnace purge
supervisionIgniter controlPulverizer controlSecondary air damper
controlFlame scanner intelligenceBoiler trip protection
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B.Check and simulate the following controls also:Feed water
controlSteam temperature controlCombustion controlFurnace draft
control
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10.0MISCELLANEOUSPent house, lower dead spaces and arch
enclosures. A- Pent House Check condition of hanger rods.Check
whether all hanger rods are properly loaded. Check the rods for
corrosion and thinning.Check the condition of roof casing for any
crack and missed welds on roof expansion joints and joints between
roofs.
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Check the condition of pent house roof for insulation.Check the
adequacy of braces and supports.Check attachment for integrity.The
following areas should also be closely watched:The four water wall
headers cornersThe under side casing adjacent to the roof tube
inlet headerThe junction points on the side wall casingBuck
StaysBottom Arches
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Boilers Tube Failure / Repairs
Main function of tubes is to transfer the heat produced by
burning fuel to water or steam.
The heat passes in through the wall of the tube and is picked up
by the water or steam.
Boiler tube leaks are a common occurrence in all types of
boilers TubeFurnace Flame
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1.1-Overheating Overheating is the most common cause of boiler
tube failures.
Tubes overheat as result of:
Deposits from boiler water Inadequate flow of water or steam
through the tubesImproper flow of hot gases through the
boilerRefractory failureImproper boiler operation1.0-Causes of
Boilers Tube Failures
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Boiler tubes are constantly exposed to heat intense enough to
cause them to melt
Circulation of water and steam inside the tube keep them
cool
deposits from the boiler water, that coat the inside of tube can
prevent water / steam flow from cooling the tube
Scale is an example of boiler water deposits
Inadequate flow of water or steam caused by blockage can also
cause a tube to overheat, resulting in tube failure
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1.2-Corrosion
corrosion is deterioration of metal through chemical action
-
It can affect both the inside and outside of boiler tubes by
thinning the walls and thus reducing the strength of the tube
Reasons of tube corrosion:
Improper treatment of water by using wrong chemicals, incorrect
amount of chemicals
Oxygen in the boiler also corrodes inside boiler tubes
Rust is another form of corrosion
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Sources of corrosion on the outside of the tube are: Corrosive
slag and Corrosive acid
Corrosive slag is formed by contaminants in the boiler fuel
Moisture that combines with the sulfur found in ash or soot
produces acid. This acid attacks the outside of the tube and
corrodes it
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1.3-Erosion Erosion is the gradual wearing away of tube metal It
can occur on the inside and /or outside of the tube On the inside
of the tube erosion is caused by the flow of water or steam On the
outside it is produced by the flow of gas and ash In coal fired
boiler s, it is severe because of large amounts of abrasive ash
produced
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Mechanical stress can be caused by pressure, thermal expansion,
weight of the tubes and vibration
Boiler tube are built to withstand a certain amount of
mechanical stress, but excessive stress leads to mechanical
failure.
Thermal expansion takes place when boiler is fired. Tubes expand
when they are heated and contract when they are cooled
Stress occurs when expansion and contraction can not take place
freely, 1.4-Mechanical Stress
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1.5-Material Defects
Material defects are weak spots in the tube that occur at the
time of tube manufacturing.
Defective tube fail during normal operation
These can be avoided by careful inspection
Tube wall lamination is the most common example of material
defects in boiler tubes. This occurs during fabrication of the
tube.
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2.0-Types of Failure and Deformities2.1-Boiler Tube Failures
There are three basic kinds of ruptures Thin-lipped rupture
Thick-lipped rupture Double-ended rupture
Most boiler failures are due to tube failures
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Thin-lipped rupturesThin-lipped ruptures are caused by sudden
and severe over heating.There is no evidence of cracking of the
metal in a thin lipped rupture.Thick-lipped ruptures Thick lipped
ruptures are caused by less intense, but more prolonged heating
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Thermal cracks: Thermal cracks are also called creep cracks.
These are developed by prolonged mild overheating or repeated short
time heating
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Pinhole leaks:Pinhole leaks can result from overheating,
corrosion, erosion, mechanical stress or material defectsSteam
leakage from pin whole leak, if it is directed against another tube
, it will erode other tube. It is called steam gouging
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Running MaintenanceGenerally problems are faced in following
equipment:
1. Ignitors
2. Oil Guns
3. Wall Deslaggers- Steam leakage- Mechanical jamming 4. Valves-
Steam leakage from glands- Steam passing5. Bi Colour Gauge
Glass
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MAINTENANCE MANAGEMENT
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MAINTENANCE OF BOILER WALL TUBESOpen the bottom hopper entrance
door and ensure that hydraulic gate of bottom ash hopper is locked,
in open position.Ensure that contractor has erected proper
scaffolding and climbing arrangements in the entire furnace.Ensure
that contractor has provided wire mesh at different stages as a
safety measure.Ensure that Electrical Maintenance Department has
provided sufficient lighting arrangement at different stages of
scaffolding.
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Clean each tube of all the four walls by scrappers, wire brush,
sander grinder from top to bottom and ensure that no dust / slag
remains on the outside surface of tubes.Number all tubes of north
& south side wall east to west at different stages.Number all
tubes of east and west side wall north to south at different
stages. Inspect visually each tube of all four walls for dent mark,
mis-alignment and bulging and record it in drawing with actual
location.Measure erosion in each tubes and bends of all four walls
and record in measurement book (MB).
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Inspect all soot blower points for any damage to sleeve and
record it in the log book.Replace / rectify all damaged shields.
Mark the position of soot blower box with the help of centre punch
in X and Y axis before cutting the soot blower box. Remove soot
blower after disconnecting electrical connections and cut soot
blower box for cutting the tubes which are connected with soot
blower box.Mark the length at top and bottom from both inside and
outside on the tube to be cut.
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Cut the tackings of the tube with buck stay or scallop
bar.Puncture the tube from inside and outside of the furnace at top
and bottom positions by grinding.Cut the tube length by hacksaw
blade or saber saw and take it out.Plug the cut ends of the tubes
with wooden plugs.Shift the cut tube lengths to Boiler shed after
marking on the tube, tube number and direction of wall from where
it is cut, by white paint. Fabricate new bend as per the cut
bend.
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Fabricate new lengths as per the straight cut lengths and make V
for butt welding by grinder or bevelling machine.Prepare V edge in
both the cut ends tubes insitu.Clean and portion (externally and
internally) of new length and cut ends at location.Place new length
in cut position and check alignment. Get the joints welded.Ensure
that the welding work is carried out by high pressure certified
welders.
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Ensure that the welding of joints is carried out according to
the recommended welding procedures as per combustion engineering
welding manual and with suitable welding electrodes.Issue clearance
to radiography contractor to carryout radiography of all the new
welded joints.Examine all the radiography films for any type of
welding defect.Get the defective joints repaired.Ensure that no
defect persists in all the repaired joints by getting it
re-radiographed.Clean the dog house by vacuum cleaning through Ash
Plant Maintenance Department.
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Inspect the screen tubes, hanger tubes roof tubes and record it
in log book.Inspect all elevation buckstay for any deformation and
record it in log book.Inspect all wall tubes for misalignment or
dislocation and record it in log book.Check all the welds of all
the tubes with buckstay for any damage at all elevations and record
it in log book.Replace the particular beam of the buck stay which
is found deformed as per instruction of Maintenance I/C or HOD.
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Get the alignment done of each particular tube which is found
misalignment as per instruction of Maintenance I/C or HOD by using
screw jack, truss, wooden block and sleeper. Get the tubes tack
welded to the corresponding buckstay beam in contact to the tube
after the tubes are straightened by jacking.Ensure that the tack
welding of the tube to the buckstay beam is carried out by highly
skilled special grade welder or certified high pressure welder.
Issue clearance to Civil Maintenance Department to carryout
refractory work in all inspection door/peep hole area, burner zone,
rear arch, bottom hopper, roof tubes, soot blower bends and other
instrument probe points.
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Replace or rectify all soot blower sleeves which were found
defective as per instruction of Maintenance I/C or HOD. After
removing soot blowe and cutting the soot blower box.Fit all the
soot blower cover boxes and fully weld them after checking that
refractory material is provided inside the box by the Civil
Maintenance Department.Fit soot blowers which were removed to
carryout tube replacement/sleeve replacement work in respective
position.
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Welding Electrodes & Filler Wires
Material
Filler wire
Electrodes
SA 192
TGSM (ER70S-G)
Overcord-S
(E6013)
SA 210 Gr A1
TGSM (ER70S-G)
Overcord-S
(E6013)
SA 209 T1
TGSM (ER70S-G)
Supratherm Spl. (E7018-A1)
SA 213 T11
TGS1CM (ER80S-G)
Chromotherm-2 (E9018-B3)
SA 213 T22
TGS2CM (ER90S-G)
Chromotherm-1 (E8018-B2)
SA213 TP347H
S.S. 120S (ER347)
Superinox-1B
(E347)
SA 213 T91
TGS9cb (ER90S-B9)
CM9cb
(E9018-B3)
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Calculation of Minimum Wall Thickness of TubesAs per ASME
:PDt=+2f + P0.005D + CAs per IBR :t=PD2f + P+ Cwhere,t = Minimum
Wall ThicknessP = Working Pressuref = Maximum Allowable StressC =
ConstantD = Outside Diameter