Thermal Power Plant

1. 1
Presentation by
Ashrant Dass

2. 1. Boiler Drum is a pressure vessel it is used to
Separate steam and water mixture
2. Mixing feed water with water separated from
steam-water mixture and re-circulate through the
evaporating tubes.
3. Reduce dissolved solid contents of the steam
( Blow down)
4. Storage of water
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3. There is a considerable difference
between the density of water and steam at
low pressure but as the pressure
increases towards critical point the density
difference decreases.
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4. 1. Primary Separation
2. Secondary Separation
3. Tertiary Separation
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5.  Baffle plates are used to reverse flow direction of
the mixture and acts as impact plate.
 Reversing hoods combine the desirable features
of baffles and change of direction principles.
Steam & water from the active generating tubes
are directed behind a baffle into slotted reversing
hoods.
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6.  Mixture passes through the Turbo separator.
 The resulting centrifugal force causes the higher
density water from a layer against the cylinder
walls and the steam moves to the core of the
cylinder and then upward. The water flows
downward through the annular can below the
drum level
 Further steam passes through a small corrugated
scrubber attached with the Turbo separator for
additional separation.
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7.  To arrest the residual moisture if any steam is
passed through the screen dryer before leaving
the drum.
 Closely spaced corrugated or bent plates, screens
or mats of woven wire mesh can be used as dryer
surface materials. The screen type dryers are
commonly used.
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11. 1. Wet steam entering the drum from the riser tubes
is collected in a compartment called separating
chamber
2. From this compartment the steam is first led
through two rows of turbo separators.
3. Each turbo-separator consists of a primary stage
and a secondary stage
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12. 4. Two concentric cans form the primary stage.
Spinner blades impart a centrifugal motion to the
mixture of steam and water flowing upward
through the inner can, thereby throwing the water
to the outside and forcing the steam to inside.
The water is arrested by a skim-off lip and flows
down through the annular between the two cans.
The steam proceeds up to the secondary
separator stage.
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13. 5. The secondary stage consists of two opposed
banks of closely spaced thin corrugated sheets,
which direct the steam through a tortuous path,
and separate the remaining entrained water.
6. Since the velocity is relatively low, this water
does not get picked up again, but runs down the
plates and flows off the second stage lips at the
outlets
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14. 7. From the secondary separators the steam flows
uniformly and with relatively low velocity upward
to the series of screen dryers, extending in layers
across the length of the drum. These screens
perform the final stage of separation.
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15. CBD Lines
To keep concentration of impurities within
specified limit it is necessary to drain a portion of
water from the drum continuously and
compensate the same with fresh make up water
having lower impurities so that no scaling occurs
in side boiler tubes and to prevent silica carry
over. Normally % of CBD will be maximum 1% of
the Steam generation.
CBD % =
TDS in make up water in PPM
TDS allowed in boiler water in PPM
=% of make up
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16. Emergency Blow down Lines
During certain operating condition water level may
raise above the NWL which may lead to carry over
of water by Steam and at times the separators
may be submerged into the water. In such
situation to bring the water level to normal, this
blow down provision is made in the drum. The line
remained open till the level comes back to normal.
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17. Chemical Dosing Line Purpose:
1. To reduce chances of scale forming salt by
converting it to sludge and removal through low
point drain. Tri Sodium Phosphate is used to react
with scale forming salts like Calcium Chloride,
Calcium Sulphate etc. and convert them into
sludge.
3CaCl2 + 2NA3PO4 Ca3 (PO4) 2 +6NaCl
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18.  Chemical Dosing Line Purpose:
2. Tri sodium Phosphate Dosing improves the pH value of
boiler water and produce Sodium Hydroxide
NA3PO4 + H2O NaOH + NA2HPO4
Higher the pH value also limits the Silica carry Over.
However excess dosing may cause free caustic deposits
in boiler tubes called caustic embrittlement in boiler
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21.  Length : 27998 mm
 ID : 1778 mm
 Wall Thk. : 185 mm / 155mm
 Weight : 247.67 MT
 Material : SA 299
 Turbo separators : 110 Nos.
 Screen Dryers : 148 Nos.
 Riser Connections : 80 nos.
 Super Heater Connection : 26 Nos.
 Down comers : 6 nos.
 Safety valve : 6 nos.
 Feed Nozzle : 3 nos.
 Drum Elevation: : 80.561 m
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22.  MAIN CONNECTIONS
1. Feed Lines
2. Down Comers
3. Risers
4. Super heater supply tubes
 AUXILLARY CONNECTIONS
1. Blow Down Lines
2. Chemical Dosing Lines
3. Instrumentation Tappings
4. Air Vents
5. Safety valves
6. Nitrogen Filling Line
7. High & Low level trips
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23.  Feed Lines
Feed water from economiser outlet is supplied to drum
through feed lines and connected to the feed nozzles
mounted with the drum. Feed nozzles in turn are
connected to the feed header provided inside the drum.
 Downcomers
The downcomer pipe are connected to the bottom of the
drum. The water from drum is circulated to the
evaporating tubes through the downcomers which are
kept external to the heating zone of the boiler. There are
6 nos of down comers at 600MW boiler drum.
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24.  Riser Tubes
The water steam mixtures formed inside the
evaporating tubes is collected in the top headers
from where water steam mixture enters the drum
through risers tubes.
 Super Heater Supply Tubes
The dry saturated steam separated inside the
drum is circulated to Superheaters through these
tubes connected at the top of the drum
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25.  The water entry from drum to down comers
connected at the bottom leads to the formation of
vortex at the inlet. This may cause drawl of steam
into down comers and water hammering. To
prevent this vortex formation anti-vortex spider in
the form of cruciform are located at the down
comers inlet.
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26.  Air Vents :Vent pipes fitted with valves and open
to atmosphere are mounted on the top of the
drum. The vents are required to evacuate the air
while filling water in drum and to kill the vacuum
during shutdown due to condensing of steam in
drum.
 Nitrogen filling line: To charge nitrogen into drum
when boiler is under shut down for preservation
purpose.
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27. Safety Valve
To prevent drum explosion from excessive
pressure, drum is provided with spring loaded
safety valve. The valves open automatically when
the drum pressure increases more than the set
pressure of the valve by the action of steam itself
without any operator interference and relieve
excess pressure.
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28. ◦ Boiler drum is a heavy wall pressure vessel having
hemispherical ends
◦ Heavy thickness plate are rolled/pressed and welded to
construct the drum
◦ It consists of cylindrical portion with two hemispherical
end
◦ Generally made of carbon steel plate conforming
specification SA 299 or SA 515-70
◦ Located at the Top of the Boiler hanging from Ceiling
Girder by means of U rods, bolts & nuts.
◦ It is the first pressure parts element to be erected after
completion of necessary structural work.
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29.  The fitting and alignment of drum internals are
very important for its efficient performance. Any
misalignment with gap will lead to heavy carry-
over of impurities into steam which will get
deposited in the super heater . The alignment
should be checked and drum internals should be
set right at every annual overhaul. In between
also, it is desirable to check the drum internals fit
up during any shut down for a few days.
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30. THANK YOU
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