water circulation system

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9 July 2011 PMI Revision 00 1

Water

Circulation

System




Circulation in Boiler The steam generator has to produce steam at highest purity,

and at high pressure and temperature required for the

turbine. Water must flow through the heat absorption surface

of the boiler in order that it be evaporated into steam Natural

circulation is the ability of water to circulate continuously,

with gravity and changes in temperature being the only

driving force known as "thermal head³.

Cold feedwater is introduced into the steam drum where, because the density of

the cold water is greater, it descends in the 'downcomer' towards the lower

bottom ring header, displacing the warmer water up into the front tubes.

Continued heating creates steam bubbles in the front tubes, which are naturally

separated from the hot water in the steam drum, and are taken off.

The ratio of the weight of water to the weight of steam in

the mixture leaving the heat absorption surfaces is called

Circulation Ratio.




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,




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




3.Combined circulation system:

Circulation ratio=2

No use of drum

Use of controlled circulation pump

Use for pressure above 200kg/cm2.




Difference in the Circulation

System




Economiser

Boiler Economiser are feed-water heaters in which the heatfrom waste gases is recovered to raise the temperature of

feed-water supplied to the boiler.




Tube Bank Arrangement




Finned

Economizers




6oC raise in feed water

temperature, by economizers

corresponds to a

1% saving in fuel consumption

Advantages of E

conomiser

220 C reduction in flue gas

temperature increases boiler

efficiency by 1%




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 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




Type of Construction

� Plain Tube : Several banks of tubes with either-in-line or

staggered type formation. Staggered arrangement induces

more turbulence than the in-line arrangement. This gives a

higher rate of heat transfer and requires less surface but atthe expense of higher draught loss.

� Welded Fin-tube : Fin welded design is used for improving

the heat transfer.




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




SPE

CIF

ICAT

ION-

500MWMATERIAL - CARBON STEEL

LENGTH - 22070 MM

OD - 2130 MMID - 1778 MM

DESIGN.PR - 204 KSC

WEIGHT(INT) -246 TONNES

WITHOUT(INT) ±224 TONNES




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 ANDWATER WALLS LOWER HEADER.

� APPROXIMATELY 50% OF THE HEAT RELEASED BY THECOMBUSTION OF THE FUEL IN THE FURNACE IS ABSORBED BYTHE WATER WALLS.




Construction of Water WallsMembrance Wall A number of tubes

are joined by a process of fusion

welding or by means of steel strips

called 'fins pressurised furnace ispossible with the related Advantages

�Increase in efficiency

�Better load response simpler

combustion control.

�Quicker starting and stopping

�Increased availability of boiler.�Heat transfer is better

�Weight is saved in refractory and

structure

�Erection is made easy and quick




WATER WALL

SPECIFICATION

FRONTWALL

SIDEWALLS

REAR WALLS

ROOF

NO

283

444

283

142

OD(MM)

51

51

51

57




Hydro Testing

� Need:

� Method:

± Ensure pressure instrumentation is ready.

± Water Filling(10ppm Ammonia, 200 ppm Hydrazine, pH10)

± 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 wetpreservation.




IBR Requirement for Hydraulic test

� Every boiler shall be hydraulically tested after erection at site inpresence of the Inspector to 1¼ times the maximum workingpressure as certified by the Inspecting Authority, to be stampedon the boiler, as free from any indication of weakness or defects.

� The test pressure shall be raised gradually under proper controlat 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.




IBR Requirement for Hydraulic test

� 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.




THANK YOU

water circulation system

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