Post on 07-Apr-2018
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9 July 2011 PMI Revision 00 1
Water
Circulation
System
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Circulation in Boiler The steam generator has to produce steam at highest purity,
and at high pressure and temperature required for the
turbine. Water must flow through the heat absorption surface
of the boiler in order that it be evaporated into steam Natural
circulation is the ability of water to circulate continuously,
with gravity and changes in temperature being the only
driving force known as "thermal head³.
Cold feedwater is introduced into the steam drum where, because the density of
the cold water is greater, it descends in the 'downcomer' towards the lower
bottom ring header, displacing the warmer water up into the front tubes.
Continued heating creates steam bubbles in the front tubes, which are naturally
separated from the hot water in the steam drum, and are taken off.
The ratio of the weight of water to the weight of steam in
the mixture leaving the heat absorption surfaces is called
Circulation Ratio.
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9 July 2011 PMI Revision 00 4
Criticality of Drum Level
control
� Boiler drum level control is critical for both plant protectionand equipment safety and applies equally to high and lowlevels of water within the boiler drum.The purpose of thedrum level controller is to bring the drum up to level at boiler start-up and maintain the level at constant steam load. Adramatic decrease in this level at constant steam load. Adramatic decrease in this level may uncover boiler tubes,allowing them to become overheated and damaged. Anincrease in this level may interfere with the process of
separating moisture from steam within the drum,
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Forced Circulation
However, when the pressure in the
water-tube boiler is increased, the
difference between the densities of the
water and saturated steam falls,
consequently less circulation occurs.
To keep the same level of steam
output at higher design pressures, the
distance between the Bottom ring
header and the steam drum must be
increased, or some means of forced
circulation must be introduced.
Therefore natural circulation is limited
to boiler with drum operating pressure
around 175 Kg/cm2.
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Natural Circulation &
Forced Circulation
Natural Circulation
The downcomer contain relativelycold water, whereas the riser tubecontain steam water mixture,whose density is comparitivelyless .this density difference is thedriving force ,for the mixture.(thermo-siphon principle)
Circulation takes place at such ahigh rate that the driving force andfrictional resisitance in water wallare balanced. Natural Circulation Forced Circulation
Forced Circulation
Beyond 180 Kg/cm2 of pressure, circulation is to be assisted withmechanical pumps, to overcome frictional losses. To regulate the flowthrough various tubes, orifice plates are used. This system is applicable inthe high sub-critical regions (say 200 Kg/cm2).
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Why controlled
circulation is required?� AS THE PRESSURE
INCREASES,THE
DIFFERENCE IN DENSITY
BETWEEN WATER AND
STEAM REDUCES .
� THUS THE HYDROSTATICHEAD AVAILABLE WILL
NOT BE ABLE TO
OVERCOME THE
FRICTIONAL RESISITANCE
FOR A FLOW
CORRESPONDING TO THE
MINIMUM REQUIREMENT
OF COOLING OF WATER
WALL TUBES.
� NATURAL CIRCULATION
IS LIMITED TO 175KSC
Diff in Density
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2.Controlled circulation system:
� use of controlled circulation pump
� used for pressure up to 194kg/cm2 (sub critical pr.)� circulation ratio=6-9
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3.Combined circulation system:
Circulation ratio=2
No use of drum
Use of controlled circulation pump
Use for pressure above 200kg/cm2.
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Difference in the Circulation
System
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Economiser
Boiler Economiser are feed-water heaters in which the heatfrom waste gases is recovered to raise the temperature of
feed-water supplied to the boiler.
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Tube Bank Arrangement
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Finned
Economizers
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6oC raise in feed water
temperature, by economizers
corresponds to a
1% saving in fuel consumption
Advantages of E
conomiser
220 C reduction in flue gas
temperature increases boiler
efficiency by 1%
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Location and Arrangement� Ahead of air-heaters
� Following the primary super-heater or re-heater
� Counter-flow arrangement
� Horizontal placement (facilitate draining)
� Supported to prevent sagging, undue deflection andexpansion .
� Stop valve and non-return valve incorporated to ensurerecirculation in case of no feed-flow
� Ash hopper below as flue gas takes a turn
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Type of Construction
� Plain Tube : Several banks of tubes with either-in-line or
staggered type formation. Staggered arrangement induces
more turbulence than the in-line arrangement. This gives a
higher rate of heat transfer and requires less surface but atthe expense of higher draught loss.
� Welded Fin-tube : Fin welded design is used for improving
the heat transfer.
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DRUM Lifting in progress
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DRUM SERVES TWO MAIN
FUNCTIONS
SEPERATING STEAM FROM THEMIXTURE OF WATER AND STEAM
2. IT HOUSES ALL EQUIPMENTS USED
FOR PURIFICATION OF THE STEAM AFTER
BEING SEPERATED FROM THE WATER
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SPE
CIF
ICAT
ION-
500MWMATERIAL - CARBON STEEL
LENGTH - 22070 MM
OD - 2130 MMID - 1778 MM
DESIGN.PR - 204 KSC
WEIGHT(INT) -246 TONNES
WITHOUT(INT) ±224 TONNES
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The steam drum
contains steam
separating
equipment andinternal piping for
distribution of
chemicals to the
water, for distribution
of feedwater and for
blowdown of the
water to reduce
solids concentration.
Drum Internals
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Drum Internals
SteamSeparator
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Downcomers� There are six down comers in (500 MW) which
carry water from boiler drum to the ring header.
� They are installed from outside the furnace to keep
density difference for natural circulation of water &
steam.
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WATER
WALLS� HEATING AND EVAPORATING THE FEED WATER SUPPLIED TO
THE BOILER FROM THE ECONOMISERS.
� THESE ARE VERTICAL TUBES CONNECTED AT THE TOP ANDBOTTOM TO THE HEADERS.
� THESE TUBES RECEIVE WATERFROM THE BOILER DRUM BYMEANS OF DOWNCOMERS CONNECTED BETWEEN DRUM ANDWATER WALLS LOWER HEADER.
� APPROXIMATELY 50% OF THE HEAT RELEASED BY THECOMBUSTION OF THE FUEL IN THE FURNACE IS ABSORBED BYTHE WATER WALLS.
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Construction of Water WallsMembrance Wall A number of tubes
are joined by a process of fusion
welding or by means of steel strips
called 'fins pressurised furnace ispossible with the related Advantages
�Increase in efficiency
�Better load response simpler
combustion control.
�Quicker starting and stopping
�Increased availability of boiler.�Heat transfer is better
�Weight is saved in refractory and
structure
�Erection is made easy and quick
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WATER WALL
SPECIFICATION
FRONTWALL
SIDEWALLS
REAR WALLS
ROOF
NO
283
444
283
142
OD(MM)
51
51
51
57
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Hydro Testing
� Need:
� Method:
± Ensure pressure instrumentation is ready.
± Water Filling(10ppm Ammonia, 200 ppm Hydrazine, pH10)
± Drainable parts (economizer, water wall and drum)
± Non Drainable portions filling
± First pressurization through boiler Fill pump
± Later pressurization through special purpose pumps. ± Test pressure and rate of pressure rise and rate of pressure drop as per IBR.
± After completion system drained and kept under wetpreservation.
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IBR Requirement for Hydraulic test
� Every boiler shall be hydraulically tested after erection at site inpresence of the Inspector to 1¼ times the maximum workingpressure as certified by the Inspecting Authority, to be stampedon the boiler, as free from any indication of weakness or defects.
� The test pressure shall be raised gradually under proper controlat all times so that it never exceeds by more than 6% of therequired pressure and maintained for 30 minutes whereuponthe pressure shall be reduced to maximum allowable workingpressure and maintained for sufficient time to permit closevisual inspection for leakage of pressure parts.
� The boiler shall satisfactorily withstand such pressure withoutappreciable leakage or undue deflection or distortion of its partsfor at least ten consecutive minutes.
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IBR Requirement for Hydraulic test
� At the first hydraulic test of a boiler prior tothe issue of an original certificate deflectionmeasurements shall be made before, during
and after test of each furnace length, fire-boxand flat end or other plates.
� After the application of the hydraulic test theInspector shall carefully examine the boiler
inside and outside and satisfy himself that ithas satisfactorily withstood the test.
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THANK YOU