boiler water & steam cycle

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Boiler Water & Steam

Cycle

30.09.2008



Boiler/ steam generatorSteam generating device for a specific purpose.

Capable to meet variation in load demand

Capable of generating steam in a range of operatingpressure and temperature

For utility purpose, it should generate steamuninterruptedly at operating pressure and temperaturefor running steam turbines.



Water Circulation System

Theory of Circulation: The water leaves thedrum through the down comers at a temperatureslightly below saturation temperature. The flowthrough the furnace wall is at saturation temperature.Heat absorbed in water wall is latent heat ofvaporization creating a mixture of steam and water.



Circulation ratioIt may be defined as The ratio of the weight of water to

the weight of steam in the mixture leaving the heatabsorption surfaces is called Circulation Ratio .

CR = 30-35 Industrial boilersCR = 6-8 Natrual cir. BoilersCR = 2-3 Forced cir. Boilers

CR = 1 Once thru boilers (Sub critical)CR = 1 Supercritical boilers



Type of CirculationNatural circulation(upto 175 ksc)

Forced/ Controlledcirculation (180-200ksc)

Once Through1. Sub critical2. Supercritical

Density difference &height of water column

Assisted by externalcirculating pump (CCpump)

Below 221.5 bar

240-360 bar



NATURAL CIRCULATION SYSTEM

The downcomer contain relatively cold water,whereas the riser tube contain steam water mixture,whose density is comparatively less .this densitydifference is the driving force ,for the mixture.(thermo-siphon principle)circulation takes place at such a high rate that thedriving force and frictional resistance in water wall

are balanced.



NATURAL CIRCULATION SYSTEM

As the pressure increases , the difference in densitybetween water and steam reduces .thus the hydrostatic head available will not be able toovercome the frictional resistance for a flowcorresponding to the minimum requirement ofcooling of water wall tubes.Natural circulation is limited to 175ksc



CONTROLLED CIRCULATION SYSYTEM

Beyond 180 Kg/Cm2 circulation is to be assistedwith pumps to overcome the frictional losses.

ONCE THROUGH CIRCULATION SYSTEM

Beyond the critical pressure ,phase transformation isabsent ,hence once through system is adopted.however even at super critical pressure it isadvantageous to recirculate the water at low loads.Typical operating pressure for such a system is260ksc



Nucleate Boiling

As the heat flux increases, the water temperaturenear the surface increases and reaches, saturationtemperature. At this point a change from liquid tovapor occur locally. But since the bulk of water doesnot reach saturation temperature the steam bubblescollapse giving up their latent heat to raise thetemperature of water. Nucleate boiling regimes arecharacterized by high heat transfer coefficients.



DNB

Beyond nucleate boiling region (i.e at still higher heatfluxes) the bubbles form a film of steam inside theheating surfaces. This condition is known as filmboiling. The point, beyond which film boiling occursis known as Departure from nucleate Boiling (DNB).Till the Occurrence of DNB metal temperature isslightly above the water temperature. When waterstarts boiling, the metal temperature is slightly abovethe saturation temperature. But when DNB occurs,

the metal temperature increases much higher thanthe saturation temperature.



Representation of steam/ waterparameters on T-S diagram

3

2 1

Entropy

374.16 oC

1. Sub critical parameter2. Critical parameter,

(221.65 bar/ 374.16 oC)

3. Supercritical parameter



Hot well

BFP

B.DRUM

LP HEATERS

ECO

WATER WALL

DOWN COMER

HPH

URH

BRH

DEAERATOR

Flow chart of WCS

FRS

CEP



Water Circulation System

EconomizerBoiler drum

Down ComersWater walls



Economiser

The function of an economiser in a steam

generating unit is to absorb heat from the

flue gases and add this as sensible heat to

the feed water before the water enters

the evaporative circuit of the boiler.



Economiser

FORMS PART OF FEED WATER CIRCUIT

PRE HEAT BOILER FEED WATER

RECOVERY OF HEAT FROM FLUE GAS

LOCATED IN BOTTOM OF REAR PASS

NO STEAM FORMATION



Economiser Re-circulation

A recirculation line with a stop valve and non-returnvalve may be incorporated to keep circulation ineconomiser into steam drum when there is fire in

furnace but no feed flow. (e.g. During start-up).



Drum

The boiler drum forms a part of the circulationsystem of the boiler. The drum serves twofunctions, the first and primary one being thatof separating steam from the mixture of waterand steam discharged into it. Secondly, thedrum houses all equipments used forpurification of steam after being separatedfrom water. This purification equipment iscommonly referred to as the Drum Internals.



Drum

TO SEPARATE WATER FROM STEAM

TO REMOVE DISSOLVED SOLIDS

TO PROTECT WATER WALLS FROM

STARVATION

ACTS AS TEMPORARY PRESSURE

RESERVOIR DURING TRANSIENT LOADS



DRUM INTERNALS

1. PRIMARY SEPERATORS

CONSISTS OF BAFFLE ARRANGEMENTDEVICES WHICH CHANGE THE

DIRECTION OF FLOWOF STEAM AND WATER MIXTURE

2. SECONDARY SEPERATORS

SEPERATORS EMPLOYING SPINNING

ACTION

3. SCREENING DRYERS



DOWN COMERS

• There are six down comers which carrywater 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 WATERSUPPLIED TO THE BOILER FROM THE ECONOMISERS.

THESE ARE VERTICAL TUBES CONNECTED AT THE TOP AND BOTTOM TO THE HEADERS.

THESE TUBES RECEIVE WATER FROM THE BOILERDRUM BY MEANS OF DOWNCOMERS CONNECTEDBETWEEN DRUM AND WATER WALLS LOWER HEADER.

APPROXIMATELY 50% OF THE HEAT RELEASED BY THECOMBUSTION OF THE FUEL IN THE FURNACE IS

ABSORBED BY THE WATER WALLS.



Water wall constructionMade of carbon steel (Grade-C) hollow circulartubes and DM water flows insideWaterwalls are stiffened by the vertical stays

and buck stays to safeguard from furnacepressure pulsation & explosion/ implosionThe boiler as a whole is hanging type,supported at the top in large structuralcolumns.Vertical expansion is allowed downwards andprovision is made at bottom trough seal nearring header .



RISER TUBES

A.RISER IS A TUBE THROUGHWHICH WATER AND STEAM PASS FROMAN UPPER WATER WALL HEADER TO ASTEAM DRUM



Steam Circulation System

Primary super heaterPlaten super heater

Final super heaterReheater



SUPER HEATER

RAISE STEAM TO HIGHER TEMPERATURE

ARRANGED IN 3 STAGES• LTSH LOCATED ABOVE ECONOMISER• RADIANT PENDENT TYPE (DIV PANEL) ABOVE

FURNACE

• CONVECTIVE FINAL SUPER HEATER ABOVE

FURNACE IN CONVECTIVE PATH



SuperheatersConvection SuperheatersRadiant Superheaters



Convection Super heaters

Convection super heaters absorb heat mainly by theimpingement of flow of hot gas around the tubes. . Apurely convection super heater has a rising steam

temperature characteristic.



Radiant Super heaters

Radiant super heater absorb heat by direct radiationfrom the furnace and are generally located at the topof the furnace. a radiant super heater has a falling

characteristic, the steam temperature drops as thesteam flow rises.



Desuperheater/Attemperator

Desuperheating or attemperation is the reduction orremoval of superheat from steam to the extentrequired.

a superheater which receives its heat lay covnectionfrom gas flowing over it, is rising temperature withincreasing output. A desuperheaters may be used toreduce the steam temperature



RE HEATER

This is the part of the boiler which receives steam

back from the turbine after it has given up some of

its heat energy in the high pressure section of the

turbine. The reheater raises the temperature of this

steam, usually to its original value, for further

expansion in the turbine.



DPNLSHTR

P l a t e n

S HT R

SCREEn

LTSH

ESP APH

ID fan

Chimney

Economiser

Bottom Ash

Downcomer

Drum

waterwallFireball

Gooseneck

Reheater

210 MW Boiler: Water and Steam Circuit



Platen SH.

375C-425C

Final SH.

500-540C

Economizer

240-310C

LTS

330-3

Water Wall

310C

210 MW Boiler: Water and Steam Circuit



H



BOTTOM RNG HDR & Z-PANEL 1 ST PASS W.W

1ST PASS W.W O/L HDRS ROOF I/L HEADER

2 ND PASS UPPER C-HDR 2 nd PASS LOWER C-HDRS

LTSH I/L HEADER LTSH O/L HEADER

D.P.I/L HEADER D.P.O/L HEADER

S.H. HEADER R.H.HEADER

2 ND PASS ROOF O/L HDR(REAR ECONOMISER

M.S

H.R.HC.R.H

FROM F.R.S



DESCRIPTION UNIT HP HEATER IN

NCR 210 MW

SAT STEAM TEMP. IN DRUM ° C 348

STEAM TEMPERATURE AT LTSH OUTLET ° C 400

STEAM TEMPERATURE AT SH PLATEN OUTLET°

C 510

STEAM TEMPERATURE AT FINAL SH OUTLET ° C 540

STEAM TEMP. AT RH INLET ° C 337

STEAM TEMP. AT RH OUTLET ° C 540

WATER TEMP. AT ECO INLET ° C 244

WATER TEMP. AT ECO OUTLET ° C 274

AMBIENT AIR TEMPERATURE°

C 35

AIR TEMPERATURE AT AH OUTLET (PRI. SIDE) ° C 354

AIR TEMPERATURE AT AH OUTLET (SEC. SIDE) ° C 348

FLUE GAS TEMPERATURE AT SH PLATEN INLET ° C 1165

FLUE GAS TEMPERATURE AT RH FRONT INLET ° C 1016

FLUE GAS TEMPERATURE AT RH REAR INLET ° C 868

FLUE GAS TEMPERATURE AT SH FINISH INLET ° C 753

FLUE GAS TEMPERATURE AT LTSH INLET ° C 638

FLUE GAS TEMPERATURE AT ECO INLET ° C 473

FLUE GAS TEMPERATURE AT AH INLET ° C 379

FLUE GAS TEMPERATURE AT AH OUTLET ° C 146

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