Water and Steam Manual

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1. Introduction

A. System Function

The water and steam system is the principal system available for the

transfer of heat energy from the boiler. It has the following major functions:

• To control feed water ow to meet requirements during start-up,

normal, and emergency operations.• To preheat feed water to increase eciency !economi"er#.

• To generate high pressure steam by transferring the heat of

combustion in various heat transfer sections in the system and to

the uid such as water-steam.• It also accepts chemical dosing to maintain p$ and reduce scaling.

B. System Overview

The steam generating system operates according to the natural circulation

principle. The feed water delivered by the boiler feed pump enters the boiler

drum after owing through the economi"er. The water ows through

individual downcomers to the evaporator inlet headers and passes from

there into the boiler enclosing walls through which it ows upwards. %ll boiler

enclosing walls are arranged as evaporator heating surfaces. In addition,

screen, the supporting tubes and evaporator tube bundle are arranged as

evaporator heating surfaces.

The uid leaving the evaporator walls, screen, supporting tubes and

evaporator tube bundle is passed through a system of overow tubes to the

boiler drum where the saturated steam is separated from the boiling water.

The superheater is arranged downstream of the evaporator screen. The

superheater is subdivided into three stages. Two spray-type attemperator for

controlling the superheated steam temperature are located between the

stages. The steam is supplied to the turbine via &ain 'team (ipe.

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2. Flow Path Description

+igure ): ater and 'team 'ystem +low (ath

The feed water which is supplied from the deaerator and delivered by oiler+eed ater (ump passes through a ontrol +eed 'tation in which feed water

is regulated. It is then passes through the /conomi"er in which it is

preheated to increase eciency. The feed water enters in the boiler steam

drum.

The oiler is a natural circulation drum type. The density di0erence between

the colder water in the downcomers and the hotter steam-water mi1ture in

the riser tubes ensures the circulation of the steam-water mi1ture bac2 into

the drum. In the steam drum, the mi1ture is separated into saturated steam,

which ows out of the drum, and remaining water, which mi1es with the

incoming feed water and ows through the downcomers to start the

circulation process once again.

The saturated steam that e1its the drum is further heated to the superheater

), 3 and 4 in order increase steam temperature and to produce the rated

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steam for steam turbine. The superheater is a heat e1changer that overheats

!superheats# the saturated steam. y superheating saturated steam, the

temperature of the steam is increased beyond the temperature of the

saturated steam, and thus the eciency of the energy production process

can be raised. The boiler has two '$ %ttemperators. 6ne is located on the

connecting pipe from '$-) to '$-3 and the other is the same from '$-3 and

'$-4. The function of '$ %ttemperators are to control steam temperature so

that the required parameters of steam delivered at the turbine is in the

acceptable condition.

The steam generator is equipped with 7rain 'ystem adequately so that drain

does not remain inside the steam generator. The drain from the 'team

8enerator is collected at 7rain Trough and ows to the lowdown Tan2.

The steam generator is equipped with 9ent line adequately so that air doesnot remain inside the steam generator. %ll of the vents from the steam

generator are collected at 9ent Through and ow to the blowdown tan2.

In itrogen 'ystem, a nitrogen blan2et is formed by 5lling the boiler

completely with water and pressuri"ing with nitrogen gas. The nitrogen gas

forms an inert barrier, and minimi"es corrosive o1ygen intrusion into the

water phase. This system is use for the purpose of a long term standstill of

the steam generator.

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3. omponent Description

Feed !ater ontrol Station " this comprises a set of control valves. +eed

control valves are meant to regulate and maintain required feed water ow

to the boiler. These valves help to maintain drum level at a desired set point.

+eed water control station has three valves !31);;< and )1=;< of ow#. The

=;< valve is for the start-ups as the ow are low whereas );;< are for

normal loadings. Two );;< numbers of valves should be installed because

you need one running and one standby.

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+igure 3: This 5gure shows the partial (>I7 of +eed ater ontrol 'tation.

?ed mar2s indicate the 4 control valves !31);;< and )1=;


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reduces the potential of thermal shoc2 and strong water temperature

uctuations as the feedwater enters the boiler drum or waterwalls.

+igure 4:

This 5gure shows the partial part of (>I7 for /conomi"er

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+igure =: %n e1ample of an /conomi"er. !%ctual /conomi"er may vary#

Boiler Steam Drum " The steam drum is the most important part of a

boiler. It secures natural circulation. (rimary function of steam drum is to

e0ectively separate steam from water. It also mi1es the chemicals that are

put into the drum for the purpose of corrosion control and water treatment. It

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provides the source for a blowdown system where a portion of the water is

rejected as a means of controlling the boiler water chemistry and reducing

the solids content.

+igure B: This 5gure shows a schematic diagram of boiler steam drum.

Downcomer " 7owncomers are pipes leading from the top to the bottom of

the boiler. 7owncomers carry the water from steam drum to the bottom part

of the boilers where it enters the distribution headers to be heated in the

combustion "one. 7owncomer tubes are placed outside the boiler to prevent

the water from evaporating, which could decrease the driving force of natural

circulation !decrease average density in downcomer tube#.

Distri%ution headers E 7istribution headers are big pipe headers that

transfer water from the downcomers to the risers through a combustion "one

where the water gets partially heated to steam from the heat of fuel

combustion.

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&iser'Furnace tu%es " ?isers are pipes leading from the bottom to the top

of the boiler. ater and steam generated in the combustion "one, runs

through the risers up to the steam drum.

#vaporator " In boilers with low steam pressure, the share of the heat

needed for evaporation is bigger than when considering a high-pressure

boiler. Thus the furnace-wall evaporator cannot provide enough heat for

evaporation process in low-pressure boilers. onvection evaporators supply

the supplementary heat needed for complete evaporation. They are normally

placed after the superheater stage in boiler process. onvection evaporators

can cause local tube overheat problems with partial loads.

Furnace Screen " The purpose of the screen tubes in a water tube boiler is

to shield the superheater tubes from e1cessive heating so that the heat

radiated from the ames in the furnace do not damage them. $ence they arehaving a large si"e !diameter# which is necessary for 2eeping the

temperatures low.

Superheater " 6nce water gets converted to steam, its temperature needsto be raised further to pump in more energy in the steam and this is done byimparting thermal energy to the steam in the form of superheat. Thesuperheater is a heat e1changer that overheats !superheats# the saturatedsteam. y superheating saturated steam, the temperature of the steam isincreased beyond the temperature of the saturated steam, and thus the

eciency of the energy production process can be raised. The superheaternormally consists of tubes conducting steam, which are heated by ue gasespassing outside the tubes. The tubes are usually connected in parallel usingheaders, with steam entering from one header and e1iting in another header.In the plant, there are 4 'uperheaters found. % schematic diagram in 5gure Cillustrates the arrangement of superheaters.

Attemperator " %ttemperator reduces steam temperature to suit the

requirements for equipments such as steam turbine by bringing superheated

steam into direct contact with water. The steam is cooled through the

evaporation of the water. The water used for attemperation is supplied fromthe feed water line. There are two attemperators in the steam and water

system as shown in the schematic diagram in 5gure C. ater is sprayed after

the 'uperheater ) and 'uperheater 3.

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+ig

ure C: 'chematic 7iagram showing the 'pray %ttemperator %rrangement

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Blowdown (alve " % number of factors determine the need for boiler

blowdown. The amount of total dissolved solids !T7'# is one. 6ther

controlling factors for determining blowdown requirements are al2alinity,

suspended solids, and other chemical factors. 'olids are brought into the

boiler by the feedwater. /ven though the water is treated, either

mechanically or chemically, neither means of treatment is capable of

removing all substances. % small amount of crystalli"ed solids will be present

in the boiler water, which can tend to encrust surfaces. Internal chemical

treatment is therefore required in order to 2eep harmful scale and sludge

from forming. 'cale has a low heat transfer value and acts as an insulating

barrier. Insulation retards heat transfer, which not only results in lower

operating eciency but, more importantly, can cause overheating of boiler

metal. 6verheating of boiler metal may results in tube failure or other

pressure vessel metal damage. 'cale is caused primarily by calcium and

magnesium salts, silica, and oil. %ny calcium and magnesium salts, alongwith organic materials in the boiler water, are generally precipitated by the

use of sodium phosphate in order to maintain these precipitates or @sludgeA

in a uid form.

a. ontinuous Blowdown " this valve opens continuously to maintain

the dirty material level to a minimum value. The opening of the valve

is varied as per requirements with a prede5ned control signal. The

motori"ed actuator can also receive manual commandsH the method of

control is the operators prerogative.

%. Intermittent Blowdown E this type of valve blows down dirty water

as necessary. Its operation may be prede5ned, based on cyclic or time

framed full openJclose signal or manual command from the operator

with a motori"ed actuator. oiler drum conductivity may be one of the

parameters to operate this valve in automatic control.

Drain (alve E during start-up of the plant after a prolonged shutdown or

cold start-up, the pipelines and various equipments need to be warmed upbefore loading the boiler. To achieve this, heating steam is admitted phase-

by-phase in a very slow manner to avoid dissimilar heat causing e1pansion of

various casings and pipes. hile heating metal wor2s, the steam gets

condensed and collected at the bottom of the pipeline with a siphon-type of

design at various strategic locations. %t the bottom, condensed water is

drained out through this valve with a motori"ed actuator when the level in

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the drain pipe reaches a prede5ned value to avoid frequent operation. Kevel

switches are provided for automatic operation.

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+igure D: lowdown and 7rainage 'chematic 7iagram. The diagram

illustrates the di0erent drains of water and steam system.

Sa)ety *Pop+up, (alve " these valves are immense importance to the

safety of the plant and its personnel. henever there is a pressure build-up

in the pipeline beyond the limit, the valve should operate or pop up to

release steam to the atmosphere until the pressure comes down to a safe

value. %lthough a loss of energy and mass of wor2ing uid occurs, it is

inevitable during any untoward situation rendered uncontrollable by a normal

control system. There are various types of safety valvesH electromatic !or

relief#, spring-loaded, dead weight, fusible plug, etc.

a. #lectromatic Sa)ety *or &elie), (alve " this is a pilot solenoid valvethat is energi"ed automatically from the pressure switch at a very high

set point, which allows wor2ing uid to operate the actuator of the

safety valve. It can be operated through remote manual command as

well from the control roomJtower.

%. Sprin-+oaded Sa)ety (alve " normally this valve operates as a last

resort to the safety system against high pressure. Lnder normal

operation, the spring tension is high enough to hold the valve plug on

its seat to ensure a closed position until a very high pressure set point

is reached. %t this point and above, the force against the spring lifts

the plug over its seat to allow e1tra steam to escape, unless the steam

pressure comes down to a normal value. The discharge capacity should

be selected so that it is equal to the evaporative capacity to avoid

frequent build-up of pressure !actuation of this valve#. 6ther types of

safety valves are no longer in useH hence, they are not discussed.

Start+up (ent (alve E this type of valve is in the main steam header and,

as the name implies, is required for the start-up period only. %s a cold boiler

is 5red up, steam is produced but cannot initially be sent to the turbine untilthe pressure and temperature has stabilised. 7uring this time it is necessary

to e1haust the steam through the vent valve.

/itro-en appin- " % nitrogen blan2et is formed by 5lling the boiler

completely with water and pressuri"ing with nitrogen gas. The nitrogen gas

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forms an inert barrier, and minimi"es corrosive o1ygen intrusion into the

water phase.

0. Operation

a. Start+up

%. /ormal Operation

c. Shutdown

d. #mer-ency Procedures

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. ontrol and Instrumentation

ontrol and Instrumentation

eneral

The purpose of ontrol and Instrumentation system will provide safe, reliable

and ecient operation of power plant. It includes the coordinated control of

the turbine and boiler, boiler controls, boiler au1iliaries controls, turbine

controls, turbine au1iliaries controls, boiler and turbine protection, electrical

systems control and other commonJstation plant control systems.

The control and operation of the power plant will be accomplished using

state-of-the-art microprocessor based 7istributed ontrol 'ystem !7'#.

Operation Philosophy

The plant operation is based on the principle of central control. The

Instrumentation and ontrol 'ystem will facilitate operation and monitoring

of the equipments from ?. 6ne of this equipment is the oiler.

Boiler ontrols

The 7' will perform the normal start-upJshutdown and emergency

operation of the oiler and its associated au1iliaries.

%ll the open and closed loop control functions including automatic boiler

controls such as drum level control, steam pressure control, combustion air

ow control, steam temperature control, turbine bypass control and

operation of the fanJpumps such as +orced 7raft +ans will be performed. The

interloc2 and protection system for the oiler will operate in coordination

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with the operational and protection requirements of oiler, 'team Turbine

8enerator and their au1iliaries.

Steam emperature ontrol

'pray %ttemperator ater Kine starts from its connection on the feed water

pipe and terminate at the 'pray %ttemperator-) and the 'pray %ttemperator-

3 respectively. In 5gure G, the 'pray %ttemperator ater Kine has a function

to convey spray water to two %ttemperators. The +unction of the control

valve is to supply the demanded spray water. The motor valve is closed

when both control valves are closed in order to shut o0 the spray water

completely. The motor valve is opened in a regulated manner when the

detected outlet temperature e1ceeds the speci5ed temperature of the &ain

'team.

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+igure G: 'hows typical schematic diagram of 'pray %ttemperator ontrol

Boiler evel ontrol

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Documents

Water and Steam Manual