Chapter 1

INTRODUCTION

NSPCL (NTPC-SAIL Power Company Limited) is a joint venture company of NTPC limited and SAIL to

generate power for various steel plants throughout India.NTPC and SAIL joined forces in March 2001 and

took over a captive power plant (consisting of 2*60 MW generators) located at the Durgapur Steel Plant and

another (also 2*60 MW ) at the Rourkela Steel Plant.

NTPC formed another joint venture company with SAIL on in March 2002 in the name of Bhilai Electric

Supply Company Ltd.(BESCL).BESCL took over a captive power plant (comprising 2*30 MW generators)

located at the Bhilai Steel Plant from SAIL.Effective 11 September 2006,BESCL became part of

NSPCL.Since 2006,NSPCL has provided all power required by the Bhilai,Durgapur and Rourkela steel

plants.To meet growing demands,NSPCL commissioned an expansion project at Bhilai comprising two

250MW generators during 2008-2009,and brought the units online in 2009-2010.Additional growth to

generate an additional 1750MW is anticipated at other SAIL facilities.

1.1 Durgapur Captive Power Plant

NTPC Durgapur is located near Waria railway station,5 km from Durgapur city in West Bengal.The power

plant is one of the coal based power plants of Durgapur.

Fig. 1.1 Durgapur Captive Power Plant

Its objective is to supply power to Durgapur Steel Plant of Steel Authority of India Limited (SAIL) from its

coal based captive power plant-II at Durgapur (West Bengal) 2*60 MW on captive basis.

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Chapter 2

LAYOUT OF THE PLANT

Fig.2.1 Layout of the Plant

2.1 Description of layout of the plant

A thermal power plant is based on the Rankine Cycle.A plant layout study is an engineering study used to

analyze different physical configurations for an manufacturing plant.It is also known as Facilities Planning

and layout. A thermal power station is one that takes chemical energy and forms heat (thermal) and

then converts that heat into electrical energy. Here, the prime mover is mainly steam driven. Water is

heated, turns into steam and spins a steam turbine which drives an electrical generator. After the steam

passes through the turbine, the steam is condensed in a condenser. This principle is known as Rankine

Cycle.

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Chapter 3

SITE SELECTION OF THE PLANT

For steam stations,the choice of plant location is governed by the following considerations:

3.1.1 Transmission of energy:

A power plant should be located as near the load centre as possible.This reduces the transmission costs and

losses in transmission.

3.1.2 Cost of real estate and taxes:

Steam stations need lot of space for installation of equipment and storage of fuel.The cost of land near a load

centre may be very high as compared to that at a remote place.In addition to the fixed cost on the capital

invested in real estate,the taxes on land should be taken into account.

3.1.3 Transportation of fuel:

Steam stations need lot of coal every day.The site should be such that coal can be transported easily from

mines to the plant.It has been seen often that the railways are used to deliver coal from coal mines to the coal

yards of the plant.

3.1.4 Availability of water:

An ample supply of water must be available for condenser cooling water.Thus,sites adjacent to large bodies

of water are preferable.Alternatively tube wells and cooling towers have to be installed and their cost must

be taken into account.

3.1.5 Disposal of ash:

A steam station produces huge quantity of ash.A site where ash can be disposed off easily will naturally be

advantageous.

3.1.6 Reliability of supply:

The generating stations should be located in different areas of the state so that reliability of supply is good at

all points.

3.1.7 Pollution and Noise:

A site near a load centre may be objectionable from the point of view of noise and pollution.(Ref. 1)

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Chapter 4

SELECTION OF FUEL

Coal,the most abundant fossil fuel,was formed by the decomposition of vegetation which was buried under

the earth million of years ago.The conversion of vegetation into coal requires ages of time.Coal contains

moisture,carbon,hydrogen,sulphur,nitrogen,oxygen and ash.Semi-bituminuous coal has properties in

between those of bituminous and anthracite coal and is widely used in power plants.Its calorific value is

about 27000kJ/Kg.

4.1 Selection of coal for power plants:

The selection of coal for a power plant depends on a number of factors.Some of them are:

4.1.1 Calorific value:

It represents the amount of energy in a given mass.Coal with a higher calorific value is,obviously,preferable.

4.1.2 Weatherability:

It is a measure of the ability of coal to withstand exposure to environment without excessive

crumbling.Every power plant has considerable storage of coal.If coal crumbles severly during storage,the

small particles will be washed away in rainstorms causing financial and energy loss and pollution of the

surroundings.

4.1.3 Sulphur content:

Sulphur is one of the combustible elements in the coal and produces energy.However its primary combustion

product,sulphur dioxide,is a health hazard.It is difficult and expensive to remove sulphur from the coal or to

remove sulphur dioxide from the combustion products.

4.1.4 Grindability Index:

The grindability index is inversely proportional to the power required to grind the coal to a certain

fineness.Coal with a high grindability index is preferable.

4.1.5 Ash content:

Ash is an impurity,produces no heat and must be removed from the furnace and disposed off.

4.1.6 Particle size:

Coal must be reduced to small size (approx 20 mm) to promote rapid and complete combustion.(Ref. 1)

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Chapter 5

COAL HANDLING PLANT

NTPC Durgapur has a coal yard of capacity one lakh tonnes in its coal handling plant.Coals are available

there from various coal mines through railways via Waria railway station and unloaded by the wagon tippler

and then transferred to the crusher through conveyor belts.

Fig. 5.1 Layout of Coal handling plant

5.1

COAL HOPPER:

Coals, from colliery, come to the loco wagon by rail and road. From rail wagon, coal is tripped by using

wagon tippler to the coal hopper. The hopper is equipped with the vibrator which places the coal on the

conveyer in a controlled manner. The hopper does not allow the larger sized coals to pass through and they

are needed to be broken manually using a hammer. The conveyer, carrying the coal from hopper, allows it to

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fall through a chute (generally telescopic) when dozing operation is done. Then it falls through another

hopper & the 2nd conveyor belt carries the coal to the crusher. The capacity of the belt is 250 tons/hr.

Fig.5.2 Coal handling plant control panel

5.2 CRUSHER:

In the crusher the coal is broken into pieces of dimension of about 25mm when the ring hammers suspended

from the suspension bars of the rotors heat the coal on the breaker plate mounted on cage frame in the

crusher body. Ring hammers may be teethed or plain. The teethed ones guide the coal & plain ones break

them.The crusher rotor has 84 no. of ring hammers with equal no of each type. Crushed coal is supplied by

the conveyor belt to 6 bunkers. From bunkers coal is supplied to the coal feeder which controls the amount

of coal supplied to the mill. There are suspension magnets to the separate magnetic material from coal before

feed into mills.

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Fig.5.3 Crushed coal

5.3 COAL MILLS:

The coal is put in the boiler after pulverization.For this pulverizer is used.A pulverizer is a device for

grinding coal for combustion in a furnace in a power plant.

5.3.1 Types of Pulverizers:

5.3.1.1 Ball and Tube Mill

Ball mill is a pulverizer that consists of a horizontal rotating cylinder, up to three diameters in length,

containing a charge of tumbling or cascading steel balls, pebbles, or rods.Tube mill is a revolving cylinder of

up to five diameters in length used for fine pulverization of ore, rock, and other such materials; the material,

mixed with water, is fed into the chamber from one end, and passes out the other end as slime.

5.3.1.2 Ring and Ball

This type consists of two rings separated by a series of large balls. The lower ring rotates, while the upper

ring presses down on the balls via a set of spring and adjuster assemblies. Coal is introduced into the center

or side of the pulverizer (depending on the design) and is ground as the lower ring rotates causing the balls

to orbit between the upper and lower rings. The coal is carried out of the mill by the flow of air moving

through it. The size of the coal particals released from the grinding section of the mill is determined by a

classifer separator. These mills are typically produced by B&W (Babcock and Wilcox).

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In unit-4, 5 out of 8 mills are always in service. The purpose of coal mill is to pulverize the coal employing

in 3- heavy rollers in each mill. The pulverized coal is flown away by the primary air & injected in the

furnace. For better combustion pulverized coal is preheated by primary air. Cold primary air is used to

prevent over heating & burning of coal in the mill. In the furnace, first oil is burnt by using a spark plug for

initiation & then coal firing is started & when coal starts burning, oil firing is withdrawn.NTPC Durgapur

has ball ring type pulverizer.

5.4 COAL FEEDER:

From bunker, the coal comes to the coal feeders to protect the mill (Pulverizer) from large size stones or

metallic impurities.

5.5 CONVEYOR BELT:

Conveyor belts are used to carry the coal, first, from wagon tippler to the raw coal yard, then from raw coal

yard to crusher house, and from crash coal yard to the coal bunker. The belts rest over rollers. The rollers are

placed in a “V” shape in forward direction, and flat rollers are used in reverse direction. A Dead weight is

hanged from two pulleys to allow the extension and contraction of the belt.

Fig.5.4 Conveyor belt

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Chapter 6

WATER TREATMENT PLANT

This is the place, where, the water of the river Damodar, is prepared through a number of steps for using in

the plant and for the colony adjacent to it. The water of the river, known as the Raw water, contains many

impurities, like gases causing erosion and corrosion of metal,hard salts leading to overheating by forming

hard scales,soft salts forming scales and dissolved solids,organic matters, silt, clay, silica and other

impurities in colloidal form.Various steps are taken to eliminate these impurities and prepare usable water.

The water coming from the station service pump is first passed through a multi stage surface spreader. As a

result of this, the water gets more surface contact with the air and atmosphere and so more air is dissolved

into it.The water is then mixed with Chlorine to kill the harmful bacteria. This mixing process is done with

great care as excess amount of chlorine is harmful for both machines and humans.While water from S.S

pump flows to clarifier bed,chlorine gas is added to water with continuous stream of water which flows

through a pipeline. In the pipeline,there is a nozzle and when water flows through the nozzle, pressure falls

at the outlet of the nozzle.This pressure (or lack of it) is the driving force to carry chlorine from its storage.

After this, Alum is fed into this water. The alum mixes with low density particles to form slag which floats

on the surface of water. First, Alum is mixed with water to prepare a solution in a tank with motor driven

stirrers. Then, this solution is mixed with water flowing through S.S Pump to clarifier. Then the filter is sent

to respective pumps for distribution.After this, the water is taken to clarifier bed where coagulation and

precipitation of suspended particles occur. The water is then sent to Sand filled gravity filters, where it gets

filtrated with the help of filter beds made of sand, gravels etc. The filtrated water is then supplied to different

fields by various pumps for demineralisation.

Fig.6.1 Water treatment plant

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CHAPTER 7

DEMINERALISATION PLANT

This Plant is used to remove all the minerals in the form of silicates chlorides and sulphates from the water.

These salts are responsible for the hardness of the water. If they are not removed, then they form scales on

the surface. Maintaining the PH level is also one of the important reasons of demineralization. It is the last

step, through which water passes before reaching the demineralized water tank. The steps are as follows:

7.1 Activated Carbon Filter:

It is a closed tank, that has activated carbon within, that absorb residual chlorine and other non volatile gases

present in the water. It has three steps, backwash, rinse and Service.

Fig.7.1 Activated carbon filter

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7.2 Cationic Exchanger:

As the name suggests, it exchanges cation (Ca++ and Mg++ ion) present in the water in the form of their

salts with H+ ions.The exchanger is fed with Resin (R-SO3-H). As Water comes in contact with it,

the hydrogen ion of resin gets replaced with calcium or magnesium ions resulting in free hydrogen

ion, that causes the water to be slightly acidic.

2R-SO3-H + X2+ (R-SO3)2X + H3O+

After some period, the resin gets deactivated due to replacement of all the hydrogen atoms. At that

time Sulphuric Acid or Hydrochloric Acid is needed to be added to activate the resin back.

(R-SO3)2X + H2SO4 XSO4 + 2R-SO3H

After this, water is sent for service. So Cationic exchanger has four steps, Backwash, Regeneration,

Rinse, and Service.

Fig.7.2 Cation exchanger

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7.3 Anionic Exchanger:

This exchanger works similarly like a cationic exchanger except that it removes anions like chlorides,

sulphates, silicates etc. The functional group of resin here is OH. Like cationic exchanger, it also has 4

steps, i.e. Backwash, Regeneration, Rinse and Service.

R-SO3-OH + HCl R-SO3-Cl + H2O

In anionic exchanger, resin is re generated by using NaOH.

R-SO3-Cl + NaOH R-SO3-OH + NaCl

Fig.7.3 Anion exchanger

7.4 Mixed Bed Type:

In actual practice, both the cationic and anionic exchangers are not able to completely remove the

cationic and anionic parts, therefore a further processing is needed in the form of a mixed bed which

removes both the cationic and anionic part. This bed contains both type of resins.

12

In reality, completely 100% pure water is not achievable but however a certain industrial level of purity

and quality is maintained that is Ph of 6.8,zero hardness,silica level 0.01 ppm.

13

Now the water is stored in a storage tank called Degassed tank,from here it is feed to the boiler for the

further processes.

Fig.7.4 Degassed tank

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Chapter 8

COMPONENTS USED IN THE PLANT

8.1 Boiler:

Now that pulverized coal is put in boiler furnance.Boiler is an enclosed vessel in which water is heated and

circulated until the water is turned in to steam at the required pressure.

Coal is burned inside the combustion chamber of boiler.The products of combustion are nothing but

gases.These gases which are at high temperature vaporize the water inside the boiler to steam.Some times

this steam is further heated in a superheater as higher the steam pressure and temperature the greater

efficiency the engine will have in converting the heat in steam in to mechanical work. This steam at high

pressure and tempeture is used directly as a heating medium, or as the working fluid in a prime mover to

convert thermal energy to mechanical work, which in turn may be converted to electrical energy. Although

other fluids are sometimes used for these purposes, water is by far the most common because of its economy

and suitable thermodynamic characteristics.

8.1.1 Classification of boilers:

8.1.1.1 Fire tube boilers :

In fire tube boilers hot gases are passed through the tubes and water surrounds these tubes. These are

simple,compact and rugged in construction.Depending on whether the tubes are vertical or horizontal these

are further classified as vertical and horizontal tube boilers.In this since the water volume is more,circulation

will be poor.So they can't meet quickly the changes in steam demand.High pressures of steam are not

possible,maximum pressure that can be attained is about 17.5kg/sq cm.Due to large quantity of water in the

drain it requires more time for steam raising.The steam attained is generally wet,economical for low

pressures.The outut of the boiler is also limited.

Fig. 8.1 Fire tube boiler

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8.1.1.2 Water tube boilers:

In these boilers water is inside the tubes and hot gases are outside the tubes.They consists of drums and

tubes.They may contain any number of drums. Feed water enters the boiler to one drum (here it is drum

below the boiler).This water circulates through the tubes connected external to drums.Hot gases which

surrounds these tubes wil convert the water in tubes in to steam.This steam is passed through tubes and

collected at the top of the drum since it is of light weight.So the drums store steam and water (upper

drum).The entire steam is collected in one drum and it is taken out from there.As the movement of water in

the water tubes is high, so rate of heat transfer also becomes high resulting in greater efficiency.They

produce high pressure , easily accessible and can respond quickly to changes in steam demand.These are

also classified as vertical,horizontal and inclined tube depending on the arrangement of the tubes.These are

of less weight and less liable to explosion.Large heating surfaces can be obtained by use of large number of

tubes.We can attain pressure as high as 125 kg/sq cm and temperatures from 315 to 575 centigrade. (Ref. 2)

Fig. 8.2 Water tube boilers

NTPC Durgapur has two water tube boilers and total boiler drum length is 11 m.Each of them is situated at

height of 52m from the ground.

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Table no.1: Boiler Drum Specifications

Inner diameter 1600mm

Thickness 100mm

Outside diameter 1800mm

Overall length 11500mm

Steam Capacity 12.6 cu.m

Number of boiler drums 02

Boiler drum height from the ground 52m

Orientation of water tubes Vertical

8.2 Air preheater :

The remaining heat of flue gases is utilised by air preheater.It is a device used in steam boilers to transfer

heat from the flue gases to the combustion air before the air enters the furnace. Also known as air heater; air-

heating system. It is not shown in the lay out.But it is kept at a place near by where the air enters in to the

boiler. The purpose of the air preheater is to recover the heat from the flue gas from the boiler to improve

boiler efficiency by burning warm air which increases combustion efficiency, and reducing useful heat lost

from the flue. As a consequence, the gases are also sent to the chimney or stack at a lower temperature,

allowing simplified design of the ducting and stack. It also allows control over the temperature of gases

leaving the stack (to meet emissions regulations, for example).After extracting heat flue gases are passed to

elctrostatic precipitator.NTPC Durgapur has regenerative type of air preheater.

Fig.8.3 Air preheater

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8.3 Economiser :

Flue gases coming out of the boiler carry lot of heat.Function of economiser is to recover some of the heat

from the heat carried away in the flue gases up the chimney and utilize for heating the feed water to the

boiler.It is placed in the passage of flue gases in between the exit from the boiler and the entry to the

chimney.The use of economiser results in saving in coal consumption,increase in steaming rate and high

boiler efficiency but needs extra investment and increase in maintenance costs and floor area required for the

plant.This is used in all modern plants.In this a large number of small diameter thin walled tubes are placed

between two headers.Feed water enters the tube through one header and leaves through the other.The flue

gases flow outside the tubes usually in counter flow.

Fig.8.4 Economiser

8.4 Superheater:

Most of the modern boliers are having superheater and reheater arrangement. Superheater is a component of

a steam-generating unit in which steam,after it has left the boiler drum, is heated above its saturation

temperature. The amount of superheat added to the steam is influenced by the location, arrangement, and

amount of superheater surface installed, as well as the rating of the boiler. The superheater may consist of

one or more stages of tube banks arranged to effectively transfer heat from the products of

combustion.Superheaters are classified as convection , radiant or combination of these.

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Fig.8.5 Superheater

8.5 Electrostatic precipitator :

It is a device which removes dust or other finely divided particles from flue gases by charging the particles

inductively with an electric field, then attracting them to highly charged collector plates. Also known as

precipitator. The process depends on two steps. In the first step the suspension passes through an electric

discharge (corona discharge) area where ionization of the gas occurs. The ions produced collide with the

suspended particles and confer on them an electric charge. The charged particles drift toward an electrode of

opposite sign and are deposited on the electrode where their electric charge is neutralized. The phenomenon

would be more correctly designated as electrodeposition from the gas phase.

The use of electrostatic precipitators has become common in numerous industrial applications. Among the

advantages of the electrostatic precipitator are its ability to handle large volumes of gas, at elevated

temperatures if necessary, with a reasonably small pressure drop, and the removal of particles in the

micrometer range. Some of the usual applications are: (1) removal of dirt from flue gases in steam plants; (2)

cleaning of air to remove fungi and bacteria in establishments producing antibiotics and other drugs, and in

operating rooms; (3) cleaning of air in ventilation and air conditioning systems; (4) removal of oil mists in

machine shops and acid mists in chemical process plants; (5) cleaning of blast furnace gases; (6) recovery of

valuable materials such as oxides of copper, lead, and tin; and (7) separation of rutile from zirconium sand.

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Fig.8.6 ESP

8.6 Condenser :

Steam after rotating steam turbine comes to condenser.Condenser refers here to the shell and tube heat

exchanger (or surface condenser) installed at the outlet of every steam turbine in Thermal power stations of

utility companies generally. These condensers are heat exchangers which convert steam from its gaseous to

its liquid state, also known as phase transition. In so doing, the latent heat of steam is given out inside the

condenser. Where water is in short supply an air cooled condenser is often used. An air cooled condenser is

however significantly more expensive and cannot achieve as low a steam turbine backpressure (and

therefore less efficient) as a surface condenser.

The purpose is to condense the outlet (or exhaust) steam from steam turbine to obtain maximum efficiency

and also to get the condensed steam in the form of pure water, otherwise known as condensate, back to

steam generator or (boiler) as boiler feed water.

The steam turbine itself is a device to convert the heat in steam to mechanical power. The difference

between the heat of steam per unit weight at the inlet to turbine and the heat of steam per unit weight at the

outlet to turbine represents the heat given out (or heat drop) in the steam turbine which is converted to

mechanical power. The heat drop per unit weight of steam is also measured by the word enthalpy drop.

Therefore the more the conversion of heat per pound (or kilogram) of steam to mechanical power in the

turbine, the better is its performance or otherwise known as efficiency. By condensing the exhaust steam of

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turbine, the exhaust pressure is brought down below atmospheric pressure from above atmospheric pressure,

increasing the steam pressure drop between inlet and exhaust of steam turbine. This further reduction in

exhaust pressure gives out more heat per unit weight of steam input to the steam turbine, for conversion to

mechanical power. Most of the heat liberated due to condensing, i.e., latent heat of steam, is carried away by

the cooling medium. (water inside tubes in a surface condenser, or droplets in a spray condenser (Heller

system) or air around tubes in an air-cooled condenser).

Condensers are classified as (i) Jet condensers or contact condensers (ii) Surface condensers.

In jet condensers the steam to be condensed mixes with the cooling water and the temperature of the

condensate and the cooling water is same when leaving the condenser; and the condensate can't be recovered

for use as feed water to the boiler; heat transfer is by direct conduction.

In surface condensers there is no direct contact between the steam to be condensed and the circulating

cooling water. There is a wall interposed between them through heat must be convectively transferred.The

temperature of the condensate may be higher than the temperature of the cooling water at outlet and the

condnsate is recovered as feed water to the boiler.Both the cooling water and the condensate are separetely

with drawn.Because of this advantage surface condensers are used in thermal power plants.Final output of

condenser is water at low temperature is passed to high pressure feed water heater,it is heated and again

passed as feed

water to the boiler.Since we are passing water at high temperature as feed water the temperature inside the

boiler does not decrease and boiler efficiency also maintained.

Fig.8.7 Condenser

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8.7 Cooling tower:

The condensate (water) formed in the condeser after condensation is initially at high temperature.This hot

water is passed to cooling towers.It is a tower- or building-like device in which atmospheric air (the heat

receiver) circulates in direct or indirect contact with warmer water (the heat source) and the water is thereby

cooled. A cooling tower may serve as the heat sink in a conventional thermodynamic process, such as

refrigeration or steam power generation, and when it is convenient or desirable to make final heat rejection

to atmospheric air. Water, acting as the heat-transfer fluid, gives up heat to atmospheric air, and thus cooled,

is recirculated through the system, affording economical operation of the process.

Two basic types of cooling towers are commonly used.One transfers the heat from warmer water to cooler

air mainly by an evaporation heat-transfer process and is known as the evaporative or wet cooling tower.

Fig.8.8 Cooling tower

Evaporative cooling towers are classified according to the means employed for producing air circulation

through them:atmospheric, natural draft, and mechanical draft. The other transfers the heat from warmer

water to cooler air by a sensible heat-transfer process and is known as the non evaporative or dry cooling

tower.

Nonevaporative cooling towers are classified as air-cooled condensers and as air-cooled heat exchangers,

and are further classified by the means used for producing air circulation through them. These two basic

types are sometimes combined, with the two cooling processes generally used in parallel or separately, and

are then known as wet-dry cooling towers.

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Evaluation of cooling tower performance is based on cooling of a specified quantity of water through a given

range and to a specified temperature approach to the wet-bulb or dry-bulb temperature for which the tower is

designed. Because exact design conditions are rarely experienced in operation, estimated performance

curves are frequently prepared for a specific installation, and provide a means for comparing the measured

performance with design conditions.

Fig.8.9 Cooling tower fans

Fig.8.10 Sectional view of cooling tower

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8.8 Turbine:

A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it

into rotary motion. Because the turbine generates rotary motion, it is particularly suited to be used to drive

an electrical generator – about 80% of all electricity generation in the world is by use of steam turbines. The

steam turbine is a form of heat engine that derives much of its improvement in thermodynamic efficiency

through the use of multiple stages in the expansion of the steam, which results in a closer approach to the

idealreversibleprocess.

Non condensing or backpressure turbines are most widely used for process steam applications. The exhaust

pressure is controlled by a regulating valve to suit the needs of the process steam pressure. These are

commonly found at refineries, district heating units, pulp and paper plants, and desalination facilities.

Condensing turbines are most commonly found in electrical power plants. These turbines exhaust steam in a

partially condensed state, typically of a quality near 90%, at a pressure well below atmospheric to a

condenser.

Reheat turbines are also used almost exclusively in electrical power plants. In a reheat turbine, steam flow

exits from a high pressure section of the turbine and is returned to the boiler where additional superheat is

added. The steam then goes back into an intermediate pressure section of the turbine and continues its

expansion.

Extracting type turbines are common in all applications. In an extracting type turbine, steam is released from

various stages of the turbine, and used for industrial process needs or sent to boiler feedwater heaters to

improve overall cycle efficiency. Extraction flows may be controlled with a valve, or left uncontrolled.

Induction turbines introduce low pressure steam at an intermediate stage to produce additional power.

To maximize turbine efficiency the steam is expanded, generating work, in a number of stages. These stages

are characterized by how the energy is extracted from them and are known as either impulse or reaction

turbines. Most steam turbines use a mixture of the reaction and impulse designs: each stage behaves as either

one or the other, but the overall turbine uses both. Typically, higher pressure sections are impulse type and

lower pressure stages are reaction type.

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An impulse turbine has fixed nozzles that orient the steam flow into high speed jets. These jets contain

significant kinetic energy, which the rotor blades, shaped like buckets, convert into shaft rotation as the

steam jet changes direction.As the steam flows through the nozzle its pressure falls from inlet pressure to the

exit pressure (atmospheric pressure, or more usually, the condenser vacuum). Due to this higher ratio of

expansion of steam in the nozzle the steam leaves the nozzle with a very high velocity. The steam leaving

the moving blades is a large portion of the maximum velocity of the steam when leaving the nozzle. The loss

of energy due to this higher exit velocity is commonly called the "carry over velocity" or "leaving loss"

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Fig.8.11 Turbine

In the reaction turbine, the rotor blades themselves are arranged to form convergent nozzles. This type of

turbine makes use of the reaction force produced as the steam accelerates through the nozzles formed by the

rotor. Steam is directed onto the rotor by the fixed vanes of the stator. It leaves the stator as a jet that fills the

entire circumference of the rotor. The steam then changes direction and increases its speed relative to the

speed of the blades. A pressure drop occurs across both the stator and the rotor, with steam accelerating

through the stator and decelerating through the rotor, with no net change in steam velocity across the stage

but with a decrease in both pressure and temperature, reflecting the work performed in the driving of the

rotor.

When warming up a steam turbine for use, the main steam stop valves (after the boiler) have a bypass line to

allow superheated steam to slowly bypass the valve and proceed to heat up the lines in the system along with

the steam turbine. Also a turning gear is engaged when there is no steam to the turbine to slowly rotate the

turbine to ensure even heating to prevent uneven expansion. After first rotating the turbine by the turning

gear, allowing time for the rotor to assume a straight plane (no bowing), then the turning gear is disengaged .

Problems with turbines are now rare and maintenance requirements are relatively small. Any imbalance of

the rotor can lead to vibration, which in extreme cases can lead to a blade letting go and punching straight

through the casing. It is, however, essential that the turbine be turned with dry steam. If water gets into the

steam and is blasted onto the blades (moisture carryover) rapid impingement and erosion of the blades can

occur, possibly leading to imbalance and catastrophic failure. Also, water entering the blades will likely

result in the destruction of the thrust bearing for the turbine shaft. To prevent this, along with controls and

baffles in the boilers to ensure high quality steam, condensate drains are installed in the steam piping leading

to the turbine.(Ref. 4)

Fig.8.12 Types of turbines

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8.9 Alternator:

An alternator is an electromechanical device that converts mechanical energy to alternating current electrical

energy. The A.C Generator or alternator is based upon the principle of electromagnetic induction & consists

a stationary part called Stator & a rotating part called Rotor. The armature windings are placed in the stator.

Field winding is placed in the Rotor & DC supply is given to the field winding. When rotor is rotated, the

lines of magnetic flux (viz magnetic field) cut through the stator windings. This induces an electromagnetic

force (e.m.f) in the stator windings.

The magnitude of e.m.f (E) = 4.44 f f N volts

f = Strength of magnetic field in webers

N = Number of turns in a coil of stator winding

f = Frequency in Hz = P n/120

where n = revolutions of rotor

P = Number of poles. For same frequency increase in number of poles decrease the

machine speed & vice versa. Hence low speed hydro turbines have 14 to 20 poles & high speed steam

turbines have generally 2 poles.

There are three basic requirements for the generation of voltage /e.m.f / electricity. These are magnetism,

motion and conductors. When a coil moves relative to a magnetic field, a voltage is produced; generation

systems are based on this concept.

When a conductor cuts through a magnetic field, a current is produced in that conductor. These two concepts

are very closely connected. Keep in mind that it makes no difference if the magnetic field is stationary and

the conductor moves or whether the conductor is stationary and the magnetic field moves. The important

aspect is that there is relative motion/ there must be a relative motion between two windings of the

Generator.

The simplest generator consists of a loop of wire rotating between two permanent magnet poles. An AC

synchronous generator is significantly more complex than the simple generator of a wire loop rotating

between two permanent magnets. An AC synchronous generator consists of four main components and/or

systems field (rotor),armature (stator),exciter and automatic Voltage Regulator.

Essentially the process of generating voltage goes in the following order. The exciter provides DC current to

the rotor windings. DC current through these wires creates magnetic flux. Magnetic flux generates an AC

voltage in the nearby stator windings when there is relative motion between the two. The regulator then

senses this output and controls the exciter current. In generators, the rotor (the source of the magnetic field)

rotates inside a stationary armature called a stator. One reason for using a stationary armature and a rotating

magnetic field is the difficulty of taking 3-phase current from a rotating armature. The rotor is rotated by a

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prime mover. The Turbine has been used as a prime mover for Generators; that is why it is called as TG

(Turbo Generator).

The rotor contains magnetic poles with windings wrapped around them to form coils. These coils are called

field coils or field windings because they create a magnetic field when excited with a DC current. Typically,

the generator field windings contain many turns. A magnetic field radiates out from the rotor as lines of

magnetic flux. As the rotor rotates, so does the magnetic field. When this moving magnetic field comes

across a stator winding, an AC voltage is produced. The magnetic field is strongest at the center of the north

and south poles where the lines of magnetic flux are concentrated. Therefore, the closer a pole is to a stator

winding, the higher the voltage produced in that stator winding. It is important to note voltage is a function

of flux change per time, not only the proximity to the field.

Fig.8.13 An Alternator

Table No.2: Alternator ratings (Ref. 5)

Parameter Rating

Rated Power 81250 KVA

RPM 3000

Insulation Class F

Stator voltage 11500 V

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Rotor voltage 186.6 V

Stator current 4879.1 A

Rotor current 1550 A

Power factor 0.8

Phase connection Y

Frequency 50 Hz

Cooling hydrogen Temp. 42. oC

8.10 Fan or draught system:

In a boiler it is essential to supply a controlled amount of air to the furnace for effective combustion of fuel

and to evacuate hot gases formed in the furnace through the various heat transfer area of the boiler. This can

be done by using a chimney or mechanical device such as fans which acts as pump.(Ref. 3)

8.10.1 Natural draught 

When the required flow of air and flue gas through a boiler can be obtained by the stack (chimney) alone, the

system is called natural draught. When the gas within the stack is hot, its specific weight will be less than the

cool air outside; therefore the unit pressure at the base of stack resulting from weight of the column of hot

gas within the stack will be less than the column of extreme cool air. The difference in the pressure will

cause a flow of gas through opening in base of stack. Also the chimney is form of nozzle, so the pressure at

top is very small and gases flow from high pressure to low pressure at the top.

8.10.2 Mechanized draught

There are 3 types of mechanized draught systems:

Forced draught: – In this system a fan called Forced draught fan is installed at the inlet of the boiler.

This fan forces the atmospheric air through the boiler furnace and pushes out the hot gases from the

furnace through superheater, reheater, economiser and air heater to stacks.

Induced draught: – Here a fan called ID fan is provided at the outlet of boiler, that is, just before the

chimney. This fan sucks hot gases from the furnace through the superheaters, economiser, reheater

and discharges gas into the chimney. This results in the furnace pressure lower than atmosphere and

affects the flow of air from outside to the furnace.

Balanced draught:-In this system both FD fan and ID fan are provided. The FD fan is utilized to

draw control quantity of air from atmosphere and force the same into furnace. The ID fan sucks the

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product of combustion from furnace and discharges into chimney. The point where draught is zero is

called balancing point.

8.11 ASH HANDLING PLANT:

The furnace is a dry bottom furnace characterized by a huge amount of fly ash (about 80%) removed

by the suction of Induced Duct fan (I.D Fan) and about 20% bottom ash that falls through furnace

bottom. The exit temperature of gases must be less than ash fusion temperature. Different components of

ash handling plant are ash hopper, scraper conveyor, clinker grinder, Electrostatic Precipitator (ESP),

flushing apparatus, sump, ash slurry pump, ash pond, etc.The various processes that take place in Ash

Handling Plant are:

8.11.1 Removal of Bottom-Ash: The ash collected in bottom hopper is quenched by spraying water

andcontinuously discharging water through impounded scraper conveyor. The ash is then transported to

respective clinker grinder to reduce the lump size.

Fig.8.14 Ash Handling Plant

The crushed ash falls below ash sluice trench and then transported to ash slurry pump by gravity associated

with high pressure water jet.

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8.11.2 Removal of fly-Ash: Fly ash collected in each of the E.S.P hopper, economizer hopper, air pre

heater hopper and stack hopper drops continuously toflushing apparatus where water mixes with

ash and the resulting slurry drops into ash sluice trench. The ash slurry is later transported to ash slurry

sump in the same way as bottom ash.

8.11.3 Disposal of Ash Slurry: Three ash slurry pumps have provided for disposal of slurry sump to the

disposal area .Each set comprise two stage of pumping. Two streams of disposal pipe lines of 250dia and

length about 3 km are provided for three slurry pump sets, out of which one pipe line will be stand by.

Chapter 9

SWITCHYARD

Fig.9.1 Switchyard

9.1 SWITCHYARD EQUIPMENTS:

Isolators

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Current transformer

CVT

Lightening arrestor

Earthing switch

Wave traps

Power transformers

Bus reactor

Circuit Breaker

Relays

9.1.1 Isolator

Operates under No Load Condition

Interlocked with Breakers and Earth Switches

Local as well as Remote Operation possible

Isolates Sections for Maintenance

Used to select Bus Bars

Fig. 9.2 Isolator

9.1.2 CT

To step-down the high magnitude of current to a safe value to incorporate protection .

Current Transformers are used for the instrumentation of power systems.

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Fig. 9.3 Current Transformer

It provides a simple, inexpensive and yet accurate means of sensing current flow in power conductors.

9.1.3 CVT

The Capacitor Voltage Transformer (CVT) is used for supplying voltage signal to the measuring

instruments and protective relays.

This is also used as coupling capacitor for Power Line Carrier Communication (PLCC).

o Primary voltage is applied to a series of capacitors group. The voltage across one of the

capacitor is taken to Aux PT. The secondary of the Aux PT is taken for measurement and

protection.

Fig. 9.4 CVT

9.1.4 Lighting arrestor

• To discharge the high voltage surges in the power system due to Lightning to the ground.

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• Not in circuit in normal operation.

Offers low resistance at abnormal voltages

The earthing screen and ground wires can well protect the electrical system against direct lightning strockes

but they fail to provide protection against travelling waves which may reach the terminal apparatus . The

lightning arresters or surge diverters provide protection against such surges.

A lightning arresters or surge diverter is a protective device which conducts the high voltage surges on the

power system to the ground.

It consists of a spark gap in series with a non linear resistor. One end of diverter is connected to the terminal

of the equipment to be protected and the other end is effectively grounded. The length of gap is so set that

normal line voltage is not enough to cause an arc across the gap but a dangerously high voltage will break

down the air insulation and form an arc. The property of non linear resistance is that its resistance decreases

as the voltage (or current) increase and vice-versa.

A lightning arrestor is used for to protect the insulation on system from damaging effect of lighting. When a

lighting surge travels down the power system to the arrestor, the current from surge is diverted to ground and

the system is protected.

Fig. 9.5 Surge Arrester

9.1.5 Earthing Switch

These are also called as Grounding Switches.

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Used To ground the Sections required for

maintenance .

To ground induction voltages.

Interlocked with Breakers and Isolators.

Motor driven as well as Hand driven.

9.1.6 Wavetrap

Wave trap is used for protection of the transmission lines and communication & data

transmission between the Substations.

Major part of PLCC i.e. Power Line Carrier Communication.

Sends inter-trip signal to the other end CBs so that fault can be isolated at the earliest time.

9.1.7 Circuit Breaker

Device capable of making and breaking an electrical circuit under normal and abnormal conditions.Main

Parts of a Circuit Breaker :-

Fixed Contact

Movable Contact

Operating Mechanism and control circuit

Arc quenching medium

Types of circuit breakers

Oil CB

Air Blast CB

Vacuum CB

SF6 CB

Circuit breakers are mechanical devices designed to close or open contact members, thus closing or opening

of an electrical circuit under normal or abnormal conditions.Automatic circuit breakers, which are usually

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employed for the protection of electrical circuits, are equipped with a trip coil connected to a relay or other

means, designed to open the breaker automatically under abnormal condition, such as over current. The

automatic circuit breakers perform the following duties:

(a) It carries the full-load current continuously without overheating or damages,

(b) It opens and closes the circuit on no load,

(c) It makes and brakes the normal operating current and

(d) It makes and brakes the shorts-circuits currents of magnitude upto which it designed.

Fig.9.6 Circuit Breaker

9.1.8 Bus bar

When a number of lines operating at the same voltage have to be directly connected electrically, bus bars are

used as the common electrical component. Bus bars are copper or aluminium bars ( generally of rectangular

X section ) and operate at constant voltage. The incoming and outgoing lines in a substation are connected to

the bus bars. The most commonly used bus bar arrangements in substations are :

Single bus bar arrangement.

Single bus bar system with sectionalisation.

Double bus bar arrangement.

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Fig. 9.7 Bus Bars

9.1.9 Relays

It is an electrical device designed to initiate isolation of a part of electrical installation, or to operate an alarm

signal, in the event of an abnormal condition or a fault.

9.1.10 Transformers

A transformer is a device that transfers electrical energy from one circuit to another through inductively

coupled electrical conductors. A changing current in the first circuit (the primary) creates a changing

magnetic field; in turn, this magnetic field induces a changing voltage in the second circuit (the secondary).

By adding a load to the secondary circuit, one can make current flow in the transformer, thus transferring

energy from one circuit to the other.

The secondary induced voltage VS, of an ideal transformer, is scaled from the primary VP by a factor equal to

the ratio of the number of turns of wire in their respective windings:

By appropriate selection of the numbers of turns, a transformer thus allows an alternating voltage to be

stepped up by making NS more than NP or stepped down, by making it less.

Transformers are some of the most efficient electrical 'machines', with some large units able to transfer

99.75% of their input power to their output.Transformers come in a range of sizes from a thumbnail-sized

coupling transformer hidden inside a stage microphone to huge units weighing hundreds of tons used to

interconnect portions of national power grids. All operate with the same basic principles, though a variety of

designs exist to perform specialized roles throughout home and industry

Transformers are used to step up and step down the voltage or current in the substations.

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Fig.9.8 Generator transformer

Fig. 9.9 Station transformer

Chapter 10

CONTROL ROOM

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Fig.10 Control panel and monitors

Control room or control unit is for the controlling of equipments like alternator,switchyard its components

and their protection.This unit also helps in identifying type of faults in the system and eradicate it from the

system.It also tells us about the power factor and the ratings as well as the characteristics of an

alternator.The panels of the control unit are always has three push buttons namely accept (A),test (T) and

reset (R).This unit plays a vital role in any plant as after generation,controlling is done by this unit.

CONCLUSION

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The first phase of practical training has proved to be quiet fruitful. It provided an opportunity for encounter

with such huge machines like turbines and generators.

The architecture of the power plant the way various units are linked and the way working of whole plant is

controlled make the student realize that engineering is not just learning the structured description and

working of various machines, but the greater part is of planning proper management.

It also provides an opportunities to learn low technology used at proper place and time can cave a lot of

labour e.g. wagon tippler (CHP).

But there are few factors that require special mention. Training is not carried out into its tree sprit. It is

recommended that there should be some project specially meant for students where presence of authorities

should be ensured. There should be strict monitoring of the performance of students and system of grading

be improved on the basis of work done.

However training has proved to be quite fruitful. It has allowed an opportunity to get an exposure of the

practical implementation to theoretical fundamentals.

REFERENCES

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1. Generation of electrical energy “B R Gupta” page no. 5 and 83

2. http://powerelectrical.blogspot.com/2007/03/thermal-power-plant-layout-and.html

3. indianpowersector.com/home/power-station/thermal-power-plant/

4 . http://powerplantstechnology.blogspot.in/2010/04/steam-turbine-use-in-power-plant.html

5.Materials provided at NSPCL Durgapur

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