Loading documents preview...
Boiler Water & Steam Cycle 30.09.2008
Boiler/ steam generator
Steam generating device for a specific purpose.
Capable to meet variation in load demand
Capable of generating steam in a range of operating pressure and temperature
For utility purpose, it should generate steam uninterruptedly at operating pressure and temperature for running steam turbines.
Water Circulation System
Theory of Circulation:
The water leaves the drum through the down comers at a temperature slightly below saturation temperature. The flow through the furnace wall is at saturation temperature. Heat absorbed in water wall is latent heat of vaporization creating a mixture of steam and water.
Circulation ratio It may be defined as The ratio of the weight of water to the weight of steam in the mixture leaving the heat absorption surfaces is called Circulation Ratio.
CR = 30-35 Industrial boilers CR = 6-8 Natrual cir. Boilers CR = 2-3 Forced cir. Boilers CR = 1 Once thru boilers (Sub critical) CR = 1 Supercritical boilers
Type of Circulation
Natural circulation (upto 175 ksc)
Density
difference & height of water column
Assisted by external Forced/ Controlled circulating pump (CC circulation (180-200 pump) ksc) Once Through Below 221.5 bar 1. Sub critical 240-360 bar 2. Supercritical
NATURAL CIRCULATION SYSTEM
The downcomer contain relatively cold water, whereas the riser tube contain steam water mixture, whose density is comparatively less .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 and frictional resistance in water wall are balanced.
NATURAL CIRCULATION SYSTEM
As the pressure increases , the difference in density between water and steam reduces . thus the hydrostatic head available will not be able to overcome the frictional resistance for a flow corresponding to the minimum requirement of cooling of water wall tubes. Natural circulation is limited to 175ksc
CONTROLLED CIRCULATION SYSYTEM
Beyond 180 Kg/Cm2 circulation is to be assisted with pumps to overcome the frictional losses.
ONCE THROUGH CIRCULATION SYSTEM
Beyond the critical pressure ,phase transformation is absent ,hence once through system is adopted. however even at super critical pressure it is advantageous to recirculate the water at low loads. Typical operating pressure for such a system is 260ksc
Nucleate Boiling
As the heat flux increases, the water temperature near the surface increases and reaches, saturation temperature. At this point a change from liquid to vapor occur locally. But since the bulk of water does not reach saturation temperature the steam bubbles collapse giving up their latent heat to raise the temperature of water. Nucleate boiling regimes are characterized by high heat transfer coefficients.
DNB
Beyond nucleate boiling region (i.e at still higher heat fluxes) the bubbles form a film of steam inside the heating surfaces. This condition is known as film boiling. The point, beyond which film boiling occurs is known as Departure from nucleate Boiling (DNB). Till the Occurrence of DNB metal temperature is slightly above the water temperature. When water starts boiling, the metal temperature is slightly above the saturation temperature. But when DNB occurs, the metal temperature increases much higher than the saturation temperature.
Representation of steam/ water parameters on T-S diagram 1. Sub critical parameter 3 2. Critical parameter, oC) (221.65 bar/ 374.16 374.16oC 2 1 3. Supercritical parameter
Entropy
Flow chart of WCS Hot well
BFP
B.DRUM
CEP
LP HEATERS
HPH
FRS
DOWN COMER
URH
DEAERATOR
ECO
BRH
WATER WALL
Water Circulation System
Economizer Boiler drum Down Comers Water 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-return valve may be incorporated to keep circulation in economiser 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 circulation system of the boiler. The drum serves two functions, the first and primary one being that of separating steam from the mixture of water and steam discharged into it. Secondly, the drum houses all equipments used for purification of steam after being separated from water. This purification equipment is commonly 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 ARRANGEMENT DEVICES WHICH CHANGE THE DIRECTION OF FLOW OF STEAM AND WATER MIXTURE 2. SECONDARY SEPERATORS SEPERATORS EMPLOYING SPINNING ACTION
3.
SCREENING DRYERS
DOWN COMERS •
•
There are six down comers 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 AND BOTTOM TO THE HEADERS.
THESE TUBES RECEIVE WATER FROM THE BOILER DRUM BY MEANS OF DOWNCOMERS CONNECTED BETWEEN DRUM AND WATER WALLS LOWER HEADER.
APPROXIMATELY 50% OF THE HEAT RELEASED BY THE COMBUSTION OF THE FUEL IN THE FURNACE IS ABSORBED BY THE WATER WALLS.
Water wall construction
Made of carbon steel (Grade-C) hollow circular tubes and DM water flows inside Waterwalls are stiffened by the vertical stays and buck stays to safeguard from furnace pressure pulsation & explosion/ implosion The boiler as a whole is hanging type, supported at the top in large structural columns. Vertical expansion is allowed downwards and provision is made at bottom trough seal near ring header.
RISER TUBES
A.RISER IS A TUBE THROUGH WHICH WATER AND STEAM PASS FROM AN UPPER WATER WALL HEADER TO A STEAM DRUM
Steam Circulation System
Primary super heater Platen super heater Final super heater Reheater
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 FURNACE IN CONVECTIVE PATH
ABOVE
Superheaters
Convection Superheaters Radiant Superheaters
Convection Super heaters
Convection super heaters absorb heat mainly by the impingement of flow of hot gas around the tubes. . A purely convection super heater has a rising steam temperature characteristic.
Radiant Super heaters
Radiant super heater absorb heat by direct radiation from the furnace and are generally located at the top of the furnace. a radiant super heater has a falling characteristic, the steam temperature drops as the steam flow rises.
Desuperheater/Attemperator
Desuperheating or attemperation is the reduction or removal of superheat from steam to the extent required. a superheater which receives its heat lay covnection from gas flowing over it, is rising temperature with increasing output. A desuperheaters may be used to reduce 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.
DPNL SHTR
Platen SHTR
Drum
Reheater S C R E E n Gooseneck
LTSH
Chimney
Downcomer waterwall
Fireball
Economiser
ID fan
APH
Bottom Ash
ESP
210 MW Boiler: Water and Steam Circuit
LTSH
Final SH. Platen SH. 500-540C
330-37
375C-425C
Economizer Water Wall 240-310C 310C
M.S
H. R. H
C.R.H
FROM F.R.S BOTTOM RNG HDR & Z-PANEL
1ST
1ST
ROOF I/L HEADER
PASS W.W O/L HDRS
PASS W.W
2ND PASS UPPER C-HDR
2nd 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
2ND
ECONOMISER
PASS ROOF O/L HDR(REAR
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