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Water
Circulation
System
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Contents of Presentation� Theory of circulation
� Types of circulation
� Boiling fundamentals� Economizer
� Water walls
� Drum and its internals
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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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Types of Drum level control� The Three main options available for drum level
control are Single element drum level control
� Two element drum level control
� Three-element drum level control
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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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Nucleate boiling, DNB
and Film Boiling
� Boiling is the formation of vapor bubbles at theheating surface. These
bubbles form atnucleation sites whosenumber and locationdepend upon the surfaceroughness or cavities,
fluid properties, andoperating conditions. Theboiling heat transfer coefficient is verysensitive to the
temperature difference.
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Economiser � The economizer preheats the feed water by utilizing the residual heat of the
flue gas.
� It reduces the exhaust gas temperature and saves the fuel.
� Modern power plants use steel-tube-type economizers.
� Design Configuration: divided into several sections : 0.6 ± 0.8 m gap
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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
Economiser
220 C reduction in flue gas
temperature increases boiler
efficiency by 1%
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6oC raise in feed water temperature,
by economizers corresponds to a
1% saving in fuel consumption
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Location andA
rrangement� 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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Boiler Drum� It is an enclosed Pressure
Vessel� Heat generated by
Combustion of Fuel is
transferred to water to
become steam� Process: Evaporation
Steam volume increases to
1,600 times from water and
produces tremendous force
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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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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 AND
WATER 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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THANK YOU