# Boiler Water & Steam Cycle

• January 2021
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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 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 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

PASS W.W O/L HDRS

PASS W.W

2ND PASS UPPER C-HDR

2nd PASS LOWER C-HDRS

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

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