water circulation system 1

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26 May 2012 PMI Revision 00 1


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Theory of circulation

Types of circulation

Boiling fundamentals

Economizer

Water walls

Drum and its internals


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The steam generator has to produce steam at highest purity,

and at high pressure and temperature required for theturbine. Water must flow through the heat absorption surfaceof the boiler in order that it be evaporated into steam Naturalcirculation is the ability of water to circulate continuously,with gravity and changes in temperature being the onlydriving force known as "thermal head.

Cold feedwater is introduced into the steam drum where, because the density ofthe cold water is greater, it descends in the 'downcomer' towards the lowerbottom ring header, displacing the warmer water up into the front tubes.Continued heating creates steam bubbles in the front tubes, which are naturallyseparated from the hot water in the steam drum, and are taken off.

The ratio of the weight of water to the weight of steam inthe mixture leaving the heat absorption surfaces is calledCirculation Ratio.


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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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The Three main options available for drum levelcontrol are Single element drum level control

Two element drum level control

Three-element drum level control


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However, when the pressure in thewater-tube boiler is increased, thedifference between the densities of thewater and saturated steam falls,

consequently less circulation occurs.To keep the same level of steamoutput at higher design pressures, thedistance between the Bottom ringheader and the steam drum must beincreased, or some means of forced

circulation must be introduced.

Therefore natural circulation is limitedto boiler with drum operating pressurearound 175 Kg/cm2.


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Natural Circulation The downcomer contain relatively

cold water, whereas the riser tubecontain steam water mixture,whose density is comparitivelyless .this density difference is the

driving force ,for the mixture.(thermo-siphon principle) Circulation takes place at such a

high 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 with

mechanical 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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AS THE PRESSUREINCREASES,THEDIFFERENCE IN DENSITYBETWEEN WATER ANDSTEAM REDUCES .

THUS THE HYDROSTATICHEAD AVAILABLE WILLNOT BE ABLE TOOVERCOME THEFRICTIONAL RESISITANCEFOR A FLOW

CORRESPONDING TO THEMINIMUM REQUIREMENTOF COOLING OF WATERWALL TUBES.

NATURAL CIRCULATION

IS LIMITED TO 175KSC

Diff in Density


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


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Circulation ratio=2

No use of drumUse of controlled circulation pumpUse for pressure above 200kg/cm2.


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Boiling is the formation ofvapor bubbles at theheating surface. These

bubbles form atnucleation sites whosenumber and locationdepend upon the surfaceroughness or cavities,

fluid properties, andoperating conditions. Theboiling heat transfercoefficient is verysensitive to the

temperature difference.


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Boiler Economiser are feed-water heaters in which the heatfrom waste gases is recovered to raise the temperature offeed-water supplied to the boiler.


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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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Out side diameter : 25 38 mm.

Tube thinckness: 3 5 mm

Transverse spacing : 2.5 3.0

Longitudinal spacing : 1.5 2.0

The water flow velocity : 600 800 kg/m2 s

The waterside resistance should not exceed 5 8 %.Of drum pressure.

Flue gas velocity : 7 13 m/s.


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6oC raise in feed watertemperature, by economizerscorresponds to a

1% saving in fuel consumption

220 C reduction in flue gas

temperature increases boilerefficiency 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 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 and

expansion . Stop valve and non-return valve incorporated to ensure

recirculation 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 orstaggered type formation. Staggered arrangement inducesmore turbulence than the in-line arrangement. This gives ahigher rate of heat transfer and requires less surface but atthe expense of higher draught loss.

Welded Fin-tube : Fin welded design is used for improvingthe heat transfer.


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It is an enclosed PressureVessel

Heat generated byCombustion of Fuel istransferred to water tobecome steam

Process: Evaporation

Steam volume increases to1,600 times from water and

produces tremendous force

if i i


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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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SPECIFICATION-

500MWMATERIAL - CARBON STEEL

LENGTH - 22070 MM

OD - 2130 MM

ID - 1778 MM

DESIGN.PR - 204 KSC

WEIGHT(INT) -246 TONNES

WITHOUT(INT)224 TONNES


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The steam drumcontains steamseparatingequipment andinternal piping fordistribution ofchemicals to thewater, for distributionof feedwater and for

blowdown of thewater to reducesolids concentration.

Drum Internals


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DRUM INTERNALS

1. PRIMARY SEPERATORS

CONSISTS OF BAFFLE ARRANGEMENT

DEVICES WHICH CHANGE THE

DIRECTION OF FLOWOF STEAM AND WATER MIXTURE

2. SECONDARY SEPERATORS

SEPERATORS EMPLOYING SPINNING

ACTION

3. SCREENING DRYERS


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SteamSeparator


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There are six down comers in (500 MW) whichcarry 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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HEATING AND EVAPORATING THE FEED WATER SUPPLIED TOTHE BOILER FROM THE ECONOMISERS.

THESE ARE VERTICAL TUBES CONNECTED AT THE TOP ANDBOTTOM TO THE HEADERS.

THESE TUBES RECEIVE WATER FROM 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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Tangent tubeTheconstruction consists ofwater wall placed side byside nearly touching each

other. An envelope of thinsheet of steel called"SKIN CASING" is placedin contact with the tubes,which provides a sealagainst furnace leakage.


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Membrance Wall A number of tubesare joined by a process of fusionwelding or by means of steel stripscalled 'fins pressurised furnace is

possible with the related AdvantagesIncrease in efficiencyBetter load response simplercombustion control.Quicker starting and stopping

Increased availability of boiler.Heat transfer is betterWeight is saved in refractory andstructureErection is made easy and quick


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FRONTWALL

SIDEWALLS

REAR WALLS

ROOF

NO

283

444

283

142

OD(MM)

51

51

51

57


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Need: Method:

Ensure pressure instrumentation is ready. Water Filling(10ppm Ammonia, 200 ppm Hydrazine, pH

10) 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 wet

preservation.


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Every boiler shall be hydraulically tested after erection at site inpresence of the Inspector to 1 times the maximum workingpressure as certified by the Inspecting Authority, to be stampedon the boiler, as free from any indication of weakness ordefects.

The test pressure shall be raised gradually under proper control

at 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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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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water circulation system 1

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