Water+Circulation+System

8/6/2019 Water+Circulation+System

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2 June 2011 PMI Revision 00 1

Water

Circulation

System




Contents of Presentation� Theory of circulation

� Types of circulation

� Boiling fundamentals� Economizer

� Water walls

� Drum and its internals






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,




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




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.




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).




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




2.Controlled circulation system:

� use of controlled circulation pump

� used for pressure up to 194kg/cm2 (sub critical pr.)� circulation ratio=6-9




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.






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




Finned

Economizers




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%




6oC raise in feed water temperature,

by economizers corresponds to a

1% saving in fuel consumption




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




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




DRUM Lifting in progress




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




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




Drum Internals

SteamSeparator




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.




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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Water+Circulation+System

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