ABOUT NTPC

NTPC Limited is the largest thermal power generating company of India, Public Sector

Company. It was incorporated in the year 1975 to accelerate power development in the country

as a wholly owned company of the Government of India. At present, Government of India holds

89.5% of the total equity shares of the company and the balance 10.5% is held by FIIs, Domestic

Banks, Public and others. Within a span of 31 years, NTPC has emerged as a truly national

power company, with power generating facilities in all the major regions of the country.

NTPC's core business is engineering, construction and operation of power generating plants and

providing consultancy to power utilities in India and abroad.

The total installed capacity of the company is 31134 MW (including JVs) with 15 coal based and

7 gas based stations, located across the country. In addition under JVs, 3 stations are coal based

& another station uses naphtha/LNG as fuel. By 2017, the power generation portfolio is expected

to have a diversified fuel mix with coal based capacity of around 53000 MW, 10000 MW

through gas, 9000 MW through Hydro generation, about 2000 MW from nuclear sources and

around 1000 MW from Renewable Energy Sources (RES). NTPC has adopted a multi-pronged

growth strategy which includes capacity addition through green field projects, expansion of

existing stations, joint ventures, subsidiaries and takeover of stations.

NTPC has been operating its plants at high efficiency levels. Although the company has 18.79%

of the total national capacity it contributes 28.60% of total power generation due to its focus on

high efficiency. NTPC’s share at 31 Mar 2001 of the total installed capacity of the country was

24.51% and it generated 29.68% of the power of the country in 2008-09. Every fourth home in

India is lit by NTPC. 170.88BU of electricity was produced by its stations in the financial year

2005-2006. The Net Profit after Tax on March 31, 2006 was INR 58,202 million. Net Profit after

Tax for the quarter ended June 30, 2006 was INR 15528 million, which is 18.65% more than for

the same quarter in the previous financial year. 2005).

NTPC has set new benchmarks for the power industry both in the area of power plant

construction and operations. It is providing power at the cheapest average tariff in the country.

NTPC is committed to the environment, generating power at minimal environmental cost and

preserving the ecology in the vicinity of the plants. NTPC has undertaken massive a forestation

in the vicinity of its plants. Plantations have increased forest area and reduced barren land. The

massive a forestation by NTPC in and around its Ramagundam Power station (2600 MW) have

contributed reducing the temperature in the areas by about 3°c. NTPC has also taken proactive

steps for ash utilization. In 1991, it set up Ash Utilization Division

A graphical overview

Technological Initiatives

Introduction of steam generators (boilers) of the size of 800 MW.

Integrated Gasification Combined Cycle (IGCC) Technology.

Launch of Energy Technology Centre -A new initiative for development of technologies

with focus on fundamental R&D.

The company sets aside up to 0.5% of the profits for R&D.

Roadmap developed for adopting ‘Clean Development.

Mechanism’ to help get / earn ‘Certified Emission Reduction.

Corporate Social Responsibility

As a responsible corporate citizen NTPC has taken up number of CSR initiatives.

NTPC Foundation formed to address Social issues at national level

NTPC has framed Corporate Social Responsibility Guidelines committing up to 0.5% of

net profit annually for Community Welfare.

The welfare of project affected persons and the local population around NTPC projects

are taken care of through well drawn Rehabilitation and Resettlement policies.

The company has also taken up distributed generation for remote rural areas.

Partnering government in various initiatives

Consultant role to modernize and improvise several plants across the country.

Disseminate technologies to other players in the sector.

Consultant role “Partnership in Excellence” Programme for improvement of PLF of 15

Power Stations of SEBs.

Rural Electrification work under Rajiv Gandhi Garmin Vidyutikaran.

Environment Management

All stations of NTPC are ISO 14001 certified.

Various groups to care of environmental issues.

The Environment Management Group.

Ash Utilization Division.

Afforestation Group.

Centre for Power Efficiency & Environment Protection.

Group on Clean Development Mechanism.

NTPC is the second largest owner of trees in the country after the Forest department.

JOURNEY OF NTPC

NTPC was set up in 1975 with 100% ownership by the Government of

India. In the last 30 years, NTPC has grown into the largest power

utility in India.

In 1997, Government of India granted NTPC status of “Navratna’ being

one of the nine jewels of India, enhancing the powers to the Board of

Directors.

NTPC became a listed company with majority Government ownership

of 89.5%.

NTPC becomes third largest by Market Capitalization of listed

companies

The company rechristened as NTPC Limited in line with its changing

business portfolio and transforms itself from a thermal power utility to

an integrated power utility.

National Thermal Power Corporation is the largest power generation

company in India. Forbes Global 2000 for 2008 ranked it 411th in the

world.

National Thermal Power Corporation is the largest power generation

company in India. Forbes Global 2000 for 2008 ranked it 317th in the

world.

NTPC has also set up a plan to achieve a target of 50,000 MW

generation capacity.

1975

1997

2005

2004

2008

2009

2012

NTPC has embarked on plans to become a 75,000 MW company by

2017.

ABOUT BTPS

Badarpur thermal power station started working in 1973 with a single 95 mw unit. There were 2

more units (95 MW each) installed in next 2 consecutive years. Now it has total five units with

total capacity of 720 MW. Ownership of BTPS was transferred to NTPC with effect from

01.06.2006 through GOI’s Gazette Notification .Given below are the details of unit with the year

they are installed.

Address: Badarpur, New Delhi – 110 044

Telephone: (STD-011) – 26949523

Fax: 26949532

Installed Capacity 720 MW

Derated Capacity 705 MW

Location New Delhi

Coal Source Jharia Coal Fields

Water Source Agra Canal

Beneficiary States Delhi

Unit Sizes 3X95 MW

2X210 MW

Units Commissioned Unit I- 95 MW - July 1973

Unit II- 95 MW August 1974

Unit III- 95 MW March 1975

Unit IV - 210 MW December 1978

Unit V - 210 MW - December 1981

2017

Transfer of BTPS to NTPC Ownership of BTPS was transferred to NTPC with effect

from 01.06.2006 through GOI’s Gazette Notification

BASIC STEPS OF ELECTRICITY GENERATION

The basic steps in the generation of electricity from coal involves following steps:

Coal to steam

Steam to mechanical power

Mechanical power to electrical power

COAL TO ELECTRICITY: BASICS

The basic steps in the generation of coal to electricity are shown below:

Coal to Steam

Coal from the coal wagons is unloaded in the coal handling plant. This Coal is transported up to

the raw coal bunkers with the help of belt conveyors. Coal is transported to Bowl mills by Coal

Feeders. The coal is pulverized in the Bowl Mill, where it is ground to powder form. The mill

consists of a round metallic table on which coal particles fall. This table is rotated with the help

of a motor. There are three large steel rollers, which are spaced 120( apart. When there is no

coal, these rollers do not rotate but when the coal is fed to the table it packs up between roller

and the table and ths forces the rollers to rotate. Coal is crushed by the crushing action between

the rollers and the rotating table. This crushed coal is taken away to the furnace through coal

pipes with the help of hot and cold air mixture from P.A. Fan.

P.A. Fan takes atmospheric air, a part of which is sent to Air-Preheaters for heating while a part

goes directly to the mill for temperature control. Atmospheric air from F.D. Fan is heated in the

air heaters and sent to the furnace as combustion air.

Water from the boiler feed pump passes through economizer and reaches the boiler drum. Water

from the drum passes through down comers and goes to the bottom ring header. Water from the

bottom ring header is divided to all the four sides of the furnace. Due to heat and density

difference, the water rises up in the water wall tubes. Water is partly converted to steam as it

rises up in the furnace. This steam and water mixture is again taken to thee boiler drum where

the steam is separated from water.

Water follows the same path while the steam is sent to superheaters for superheating. The

superheaters are located inside the furnace and the steam is superheated (540(C) and finally it

goes to the turbine.

Flue gases from the furnace are extracted by induced draft fan, which maintains balance draft in

the furnace (-5 to –10 mm of wcl) with forced draft fan. These flue gases emit their heat energy

to various super heaters in the pent house and finally pass through air-preheaters and goes to

electrostatic precipitators where the ash particles are extracted. Electrostatic Precipitator consists

of metal plates, which are electrically charged. Ash particles are attracted on to these plates, so

that they do not pass through the chimney to pollute the atmosphere. Regular mechanical

hammer blows cause the accumulation of ash to fall to the bottom of the precipitator where they

are collected in a hopper for disposal.

Steam to Mechanical Power

From the boiler, a steam pipe conveys steam to the turbine through a stop valve (which can be

used to shut-off the steam in case of emergency) and through control valves that automatically

regulate the supply of steam to the turbine. Stop valve and control valves are located in a steam

chest and a governor, driven from the main turbine shaft, operates the control valves to regulate

the amount of steam used. (This depends upon the speed of the turbine and the amount of

electricity required from the generator).

Steam from the control valves enters the high pressure cylinder of the turbine, where it passes

through a ring of stationary blades fixed to the cylinder wall. These act as nozzles and direct the

steam into a second ring of moving blades mounted on a disc secured to the turbine shaft. The

second ring turns the shafts as a result of the force of steam. The stationary and moving blades

together constitute a ‘stage’ of turbine and in practice many stages are necessary, so that the

cylinder contains a number of rings of stationary blades with rings of moving blades arranged

between them. The steam passes through each stage in turn until it reaches the end of the high-

pressure cylinder and in its passage some of its heat energy is changed into mechanical energy.

The steam leaving the high pressure cylinder goes back to the boiler for reheating and returns by

a further pipe to the intermediate pressure cylinder. Here it passes through another series of

stationary and moving blades.

Finally, the steam is taken to the low-pressure cylinders, each of which enters at the centre

flowing outwards in opposite directions through the rows of turbine blades through an

arrangement called the ‘double flow’- to the extremities of the cylinder. As the steam gives up its

heat energy to drive the turbine, its temperature and pressure fall and it expands. Because of this

expansion the blades are much larger and longer towards the low pressure ends of the turbine.

Mechanical Power to Electrical Power

As the blades of turbine rotate, the shaft of the generator, which is coupled to tha of the turbine,

also rotates. It results in rotation of the coil of the generator, which causes induced electricity to

be produced.

BASIC POWER PLANT CYCLE

A simplified diagram of a thermal power plant

The thermal (steam) power plant uses a dual (vapour+ liquid) phase cycle. It is a close cycle to

enable the working fluid (water) to be used again and again. The cycle used is Rankine Cycle

modified to include superheating of steam, regenerative feed water heating and reheating of

steam.

On large turbines, it becomes economical to increase the cycle efficiency by using reheat, which

is a way of partially overcoming temperature limitations. By returning partially expanded steam,

to a reheat, the average temperature at which the heat is added, is increased and, by expanding

this reheated steam to the remaining stages of the turbine, the exhaust wetness is considerably

less than it would otherwise be conversely, if the maximum tolerable wetness is allowed, the

initial pressure of the steam can be appreciably increased.

Bleed Steam Extraction: For regenerative system, nos. of non-regulated extractions is taken from

HP, IP turbine.

Regenerative heating of the boiler feed water is widely used in modern power plants; the effect

being to increase the average temperature at which heat is added to the cycle, thus improving the

cycle efficiency.

FACTORS AFFECTING THERMAL CYCLE EFFICIENCY

Thermal cycle efficiency is affected by following:

Initial Steam Pressure.

Initial Steam Temperature.

Whether reheat is used or not, and if used reheat pressure and temperature.

Condenser pressure.

Regenerative feed water heating.

RANKINE CYCLE

The Rankine cycle is a thermodynamic cycle which converts heat into work. The heat is supplied

externally to a closed loop, which usually uses water as the working fluid. This cycle generates

about 80% of all electric power used throughout the world, including virtually all solar thermal,

biomass, coal and nuclear power plants. It is named after William John MacquornRankine, a

Scottish polymath..

Description

Physical layout of the four main devices used in the Rankine cycle

A Rankine cycle describes a model of the operation of steam heat engines most commonly found

in power generation plants. Common heat sources for power plants using the Rankine cycle are

coal, natural gas, oil, and nuclear.

The Rankine cycle is sometimes referred to as a practical Carnot cycle as, when an efficient

turbine is used, the TS diagram will begin to resemble the Carnot cycle. The main difference is

that a pump is used to pressurize liquid instead of gas. This requires about 1/100th (1%) as much

energy as that compressing a gas in a compressor (as in the Carnot cycle).

The efficiency of a Rankine cycle is usually limited by the working fluid. Without the pressure

going super critical the temperature range the cycle can operate over is quite small, turbine entry

temperatures are typically 565°C (the creep limit of stainless steel) and condenser temperatures

are around 30°C. This gives a theoretical Carnot efficiency of around 63% compared with an

actual efficiency of 42% for a modern coal-fired power station. This low turbine entry

temperature (compared with a gas turbine) is why the Rankine cycle is often used as a bottoming

cycle in combined cycle gas turbine power stations.

The working fluid in a Rankine cycle follows a closed loop and is re-used constantly. The water

vapour and entrained droplets often seen billowing from power stations is generated by the

cooling systems (not from the closed loop Rankine power cycle) and represents the waste heat

that could not be converted to useful work.

Note that cooling towers operate using the latent heat of vaporization of the cooling fluid. The

white billowing clouds that form in cooling tower operation are the result of water droplets

which are entrained in the cooling tower airflow; it is not, as commonly thought, steam. While

many substances could be used in the Rankine cycle, water is usually the fluid of choice due to

its favourable properties, such as nontoxic and unreactive chemistry, abundance, and low cost, as

well as its thermodynamic properties.

One of the principal advantages it holds over other cycles is that during the compression stage

relatively little work is required to drive the pump, due to the working fluid being in its liquid

phase at this point. By condensing the fluid to liquid, the work required by the pump will only

consume approximately 1% to 3% of the turbine power and so give a much higher efficiency for

a real cycle.

The benefit of this is lost somewhat due to the lower heat addition temperature. Gas turbines, for

instance, have turbine entry temperatures approaching 1500°C. Nonetheless, the efficiencies of

steam cycles and gas turbines are fairly well matched.

Processes of the Rankine cycle

T-s diagram of a typical Rankine cycle operating between pressures of 0.06bar and 50bar.

There are four processes in the Rankine cycle, each changing the state of the working fluid.

These states are identified by number in the diagram to the right

i. Process 1-2: The working fluid is pumped from low to high pressure, as the fluid is a

liquid at this stage the pump requires little input energy.

ii. Process 2-3: The high pressure liquid enters a boiler where it is heated at constant

pressure by an external heat source to become a dry saturated vapour.

iii. Process 3-4: The dry saturated vapor expands through a turbine, generating power. This

decreases the temperature and pressure of the vapor, and some condensation may occur.

iv. Process 4-1: The wet vapor then enters a condenser where it is condensed at a constant

pressure and temperature to become a saturated liquid. The pressure and temperature of

the condenser is fixed by the temperature of the cooling coils as the fluid is undergoing a

phase-change.

In an ideal Rankine cycle the pump and turbine would be isentropic, i.e., the pump and turbine

would generate no entropy and hence maximize the net work output. Processes 1-2 and 3-4

would be represented by vertical lines on the T-s diagram and more closely resemble that of the

Carnot cycle.

The Rankine cycle shown here prevents the vapour ending up in the superheat region after the

expansion in the turbine, which reduces the energy removed by the condensers.

Real Rankine cycle (non-ideal) :Rankine cycle with superheat

In a real Rankine cycle, the compression by the pumpand the expansion in the turbine are not

isentropic. In other words, these processes are non-reversible and entropy is increased during the

two processes. This somewhat increases the powerrequired by the pump and decreases the power

generated by the turbine.

In particular the efficiency of the steam turbine will be limited by water droplet formation. As

the water condenses, water droplets hit the turbine blades at high speed causing pitting and

erosion, gradually decreasing the life of turbine blades and efficiency of the turbine. The easiest

way to overcome this problem is by superheating the steam. On the T-s diagram above, state 3 is

above a two phase region of steam and water so after expansion the steam will be very wet. By

superheating, state 3 will move to the right of the diagram and hence produce a dryer steam after

expansion.

Rankine cycle with reheat

In this variation, two turbineswork in series. The first accepts vapourfrom the boilerat high

pressure. After the vapour has passed through the first turbine, it re-enters the boiler and is

reheated before passing through a second, lower pressure turbine. Among other advantages, this

prevents the vapour from condensing during its expansion which can seriously damage the

turbine blades, and improves the efficiency of the cycle.

Regenerative Rankine cycle

The regenerative Rankine cycle is so named because after emerging from the condenser

(possibly as a subcooled liquid) the working fluid is heated by steam tapped from the hot portion

of the cycle. On the diagram shown, the fluid at 2 is mixed with the fluid at 4 (both at the same

pressure) to end up with the saturated liquid at 7. The Regenerative Rankine cycle (with minor

variants) is commonly used in real power stations.

Another variation is where 'bleed steam' from between turbine stages is sent to feedwater heaters

to preheat the water on its way from the condenser to the boiler.

I. BOILERMAINTENANCEDEPARTMENT

Boiler and Its Description

The boiler is a rectangular furnace about 50 ft (15 m) on a side and 130 ft (40 m) tall. Its walls

are made of a web of high pressure steel tubes about 2.3 inches (60 mm) in diameter. Pulverized

coal is air-blown into the furnace from fuel nozzles at the four corners and it rapidly burns,

forming a large fireball at the centre. The thermal radiation of the fireball heats the water that

circulates through the boiler tubes near the boiler perimeter. The water circulation rate in the

boiler is three to four times the throughput and is typically driven by pumps. As the water in the

boiler circulates it absorbs heat and changes into steam at 700 °F (370 °C) and 3,200 psi

(22.1MPa). It is separated from the water inside a drum at the top of the furnace.

Boiler Side of the Badarpur Thermal Power Station, New Delhi

The saturated steam is introduced into superheat pendant tubes that hang in the hottest part of the

combustion gases as they exit the furnace. Here the steam is superheated to 1,000 °F (540 °C) to

prepare it for the turbine. The steam generating boiler has to produce steam at the high purity,

pressure and temperature required for the steam turbine that drives the electrical generator.

The generator includes the economizer, the steam drum, the chemical dosing equipment, and the

furnace with its steam generating tubes and the superheater coils. Necessary safety valves are

located at suitable points to avoid excessive boiler pressure. The air and flue gas path equipment

include: forced draft (FD) fan, air preheater (APH), boiler furnace, induced draft (ID) fan, fly ash

collectors (electrostatic precipitator or baghouse) and the flue gas stack.

For units over about 210 MW capacity, redundancy of key components is provided by installing

duplicates of the FD fan, APH, fly ash collectors and ID fan with isolating dampers. On some

units of about 60 MW, two boilers per unit may instead be provided.

Schematic diagram of a coal-fired power plant steam generator

SPECIFICATIONS OF THE BOILER

1. Main Boiler (AT 100% LOAD):

i. Evaporation 700 tons/hr

ii. Feed water temperature 247(C

iii. Feed water leaving economizer 276(C

2. Steam Temperature:

i. Drum 341(C

ii. Super heater outlet 540(C

iii. Reheat inlet 332(C

iv. Reheat outlet 540(C

3. Steam Pressure:

i. Drum design 158. 20 kg/cm2

ii. Drum operating 149.70 kg/cm2

iii. Super heater outlet 137.00 kg/cm2

iv. Reheat inlet 26.35 kg/cm2

v. Reheat outlet 24.50 kg/cm2

4. Fuel Specifications

A) Coal

i. Fixed Carbon 38%

ii. Volatile Matter 26%

iii. Moisture 8.0%

iv. Ash 28%

v. Grindability 55HGI

vi. High Heat 4860 Kcal/Kg

vii. Coal size to Mill 20 mm

B) Oil

i. Low Heat value 10000 kcal/kg

ii. Sulphur 4.5% w/w

iii. Moisture 1% w/w

iv. Flash point 660 C.

v. Viscosity 1500 redwood at 37.80 C.

vi. Sp. Weight 0.98 at 380 C.

5. Heat Balance

i. Dry gas loss 4.63%

ii. Carbon loss 2%

iii. Radiation loss 0.26%

iv. Unaccounted loss 1.5%

v. H2 in air and H2O in fuel 4.9%

vi. Total loss 13.3%

vii. Efficiency 86.7%

AUXILIARIES OF THE BOILER

1. FURNACE

Furnace is primary part of boiler where the chemical energy of the fuel is converted to

thermal energy by combustion. Furnace is designed for efficient and complete

combustion. Major factors that assist for efficient combustion are amount of fuel inside

the furnace and turbulence, which causes rapid mixing between fuel and air. In modern

boilers, water furnaces are used.

2. BOILER DRUM

Drum is of fusion-welded design with welded hemispherical dished ends. It is provided

with stubs for welding all the connecting tubes, i.e. downcomers, risers, pipes, saturated

steam outlet. The function of steam drum internals is to separate the water from the steam

generated in the furnace walls and to reduce the dissolved solid contents of the steam

below the prescribed limit of 1 ppm and also take care of the sudden change of steam

demand for boiler.

The secondary stage of two opposite banks of closely spaced thin corrugated sheets,

which direct the steam and force the remaining entertained water against the corrugated

plates. Since the velocity is relatively low this water does not get picked up again but

runs down the plates and off the second stage of the two steam outlets.

From the secondary separators the steam flows upwards to the series of screen dryers,

extending in layers across the length of the drum. These screens perform the final stage of

the separation.

Once water inside the boiler or steam generator, the process of adding the latent heat of

vaporization or enthalpy is underway. The boiler transfers energy to the water by the

chemical reaction of burning some type of fuel.

The water enters the boiler through a section in the convection pass called the

economizer. From the economizer it passes to the steam drum. Once the water enters the

steam drum it goes down the down comers to the lower inlet water wall headers. From

the inlet headers the water rises through the water walls and is eventually turned into

steam due to the heat being generated by the burners located on the front and rear water

walls (typically). As the water is turned into steam/vapour in the water walls, the

steam/vapour once again enters the steam drum.

External View of an Industrial Boiler at BTPS, New Delhi

The steam/vapour is passed through a series of steam and water separators and then

dryers inside the steam drum. The steam separators and dryers remove the water droplets

from the steam and the cycle through the water walls is repeated. This process is known

as natural circulation.

The boiler furnace auxiliary equipment includes coal feed nozzles and igniter guns, soot

blowers, water lancing and observation ports (in the furnace walls) for observation of the

furnace interior. Furnace explosions due to any accumulation of combustible gases after a

tripout are avoided by flushing out such gases from the combustion zone before igniting

the coal.

The steam drum (as well as the superheater coils and headers) have air vents and drains

needed for initial start-up. The steam drum has an internal device that removes moisture

from the wet steam entering the drum from the steam generating tubes. The dry steam

then flows into the superheater coils. Geothermal plants need no boiler since they use

naturally occurring steam sources.

Heat exchangers may be used where the geothermal steam is very corrosive or contains

excessive suspended solids. Nuclear plants also boil water to raise steam, either directly

passing the working steam through the reactor or else using an intermediate heat

exchanger.

3. WATER WALLS

Water flows to the water walls from the boiler drum by natural circulation. The front and

the two side water walls constitute the main evaporation surface, absorbing the bulk of

radiant heat of the fuel burnt in the chamber. The front and rear walls are bent at the

lower ends to form a water-cooled slag hopper. The upper part of the chamber is

narrowed to achieve perfect mixing of combustion gases. The water wall tubes are

connected to headers at the top and bottom. The rear water wall tubes at the top are

grounded in four rows at a wider pitch forming g the grid tubes.

4. REHEATER

Reheater is used to raise the temperature of steam from which a part of energy has been

extracted in high–pressure turbine. This is another method of increasing the cycle

efficiency. Reheating requires additional equipment i.e. heating surface connecting boiler

and turbine pipe safety equipment like safety valve, non return valves, isolating valves,

high pressure feed pump, etc: Reheater is composed of two sections namely the front and

the rear pendant section, which is located above the furnace arc between water-cooled,

screen wall tubes and rear wall tubes.

Tubes of a reheater

5. SUPERHEATER

Whatever type of boiler is used, steam will leave the water at its surface and pass into the

steam space. Steam formed above the water surface in a shell boiler is always saturated

and become superheated in the boiler shell, as it is constantly. If superheated steam is

required, the saturated steam must pass through a superheater. This is simply a heat

exchanger where additional heat is added to the steam.

In water-tube boilers, the superheater may be an additional pendant suspended in the

furnace area where the hot gases will provide the degree of superheat required. In other

cases, for example in CHP schemes where the gas turbine exhaust gases are relatively

cool, a separately fired superheater may be needed to provide the additional heat.

6. ECONOMIZER

The function of an economizer in a steam-generating unit is to absorb heat from the flue

gases and add as a sensible heat to the feed water before the water enters the evaporation

circuit of the boiler.

Earlier economizer were introduced mainly to recover the heat available in the flue gases

that leaves the boiler and provision of this addition heating surface increases the

efficiency of steam generators. In the modern boilers used for power generation feed

water heaters were used to increase the efficiency of turbine unit and feed water

temperature.

An economizer

Use of economizer or air heater or both is decided by the total economy that will result in

flexibility in operation, maintenance and selection of firing system and other related

equipment. Modern medium and high capacity boilers are used both as economizers and

air heaters. In low capacity, air heaters may alone be selected.

Stop valves and non-return valves may be incorporated to keep circulation in economizer

into steam drum when there is fire in the furnace but not feed flow. Tube elements

composing the unit are built up into banks and these are connected to inlet and outlet

headers.

7. AIR PREHEATER

Air preheater absorbs waste heat from the flue gases and transfers this heat to incoming

cold air, by means of continuously rotating heat transfer element of specially formed

metal plates. Thousands of these high efficiency elements are spaced and compactly

arranged within 12 sections. Sloped compartments of a radially divided cylindrical shell

called the rotor. The housing surrounding the rotor is provided with duct connecting both

the ends and is adequately scaled by radial and circumferential scaling.

An air preheater

Special sealing arrangements are provided in the provided in the air preheater to prevent

the leakage between the air and gas sides. Adjustable plates are also used to help the

sealing arrangements and prevent the leakage as expansion occurs. The air preheater

heating surface elements are provided with two types of cleaning devices, soot blowers to

clean normal devices and washing devices to clean the element when soot blowing alone

cannot keep the element clean.

8. PULVERIZER

A pulverizer is a mechanical device for the grinding of many types of materials. For

example, they are used to pulverize coal for combustion in the steam-generating furnaces

of the fossil fuel power plants.

A Pulverizer

Types of Pulverizer

i. Ball and Tube mills

A ball mill is a pulverizer that consists of a horizontal cylinder, up to three diameters in

length, containing a charge of tumbling or cascading steel balls, pebbles or steel rods.

A tube mill is a revolving cylinder of up to five diameters in length used for finer

pulverization of ore, rock and other such materials; the materials mixed with water is fed

into the chamber from one end, and passes out the other end as slime.

ii. Bowl mill

It uses tires to crush coal. It is of two types; a deep bowl mill and the shallow bowl mill.

An external view of a Coal Pulverizer

Advantages of Pulverized Coal

Pulverized coal is used for large capacity plants.

It is easier to adapt to fluctuating load as there are no limitations on the combustion

capacity.

Coal with higher ash percentage cannot be used without pulverizing because of the

problem of large amount ash deposition after combustion.

Increased thermal efficiency is obtained through pulverization.

The use of secondary air in the combustion chamber along with the powered coal helps in

creating turbulence and therefore uniform mixing of the coal and the air during

combustion.

Greater surface area of coal per unit mass of coal allows faster combustion as more coal

is exposed to heat and combustion.

The combustion process is almost free from clinker and slag formation.

The boiler can be easily started from cold condition in case of emergency.

Practically no ash handling problem.

The furnace volume required is less as the turbulence caused aids in complete

combustion of the coal with minimum travel of the particles.

II. PLANT AUXILIARY MAINTENANCE

1. WATER CIRCULATION SYSTEM

Theory of Circulation

Water must flow through the heat absorption surface of the boiler in order that it be evaporated

into steam. In drum type units (natural and controlled circulation), the water is circulated from

the drum through the generating circuits and then back to the drum where the steam is separated

and directed to the super heater. The water leaves the drum through the down corners at a

temperature slightly below the 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. The ratio of the weight of the water to the weight of the steam in the

mixture leaving the heat absorption surface is called circulation ratio.

Types of Boiler Circulating System

i. Natural circulation system

ii. Controlled circulation system

iii. Combined circulation system

i. Natural Circulation System

Water delivered to steam generator from feed water is at a temperature well below the saturation

value corresponding to that pressure. Entering first the economizer, it is heated to about 30-40(C

below saturation temperature. From economizer the water enters the drum and thus joins the

circulation system. Water entering the drum flows through the down corner and enters ring

heater at the bottom. In the water walls, a part of the water is converted to steam and the mixture

flows back to the drum. In the drum, the steam is separated, and sent to superheater for

superheating and then sent to the high-pressure turbine. Remaining water mixes with the

incoming water from the economizer and the cycle is repeated.

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. Therefore natural

circulation is limited to the boiler with drum operating pressure around 175 kg/ cm2.

ii. Controlled Circulation System

Beyond 80 kg/ cm2 of pressure, circulation is to be assisted with mechanical pumps to overcome

the frictional losses. To regulate the flow through various tubes, orifices plates are used. This

system is applicable in the high sub-critical regions (200 kg/ cm2).

2. ASH HANDLING PLANT

The widely used ash handling systems are:

i. Mechanical Handling System

ii. Hydraulic System

iii. Pneumatic System

iv. Steam Jet System

Ash Handling System at Badarpur Thermal Power Station, New Delhi

The Hydraulic Ash handling system is used at the Badarpur Thermal Power Station.

Hydraulic Ash Handling System

The hydraulic system carried the ash with the flow of water with high velocity through a channel

and finally dumps into a sump. The hydraulic system is divided into a low velocity and high

velocity system. In the low velocity system the ash from the boilers falls into a stream of water

flowing into the sump. The ash is carried along with the water and they are separated at the

sump. In the high velocity system a jet of water is sprayed to quench the hot ash. Two other jets

force the ash into a trough in which they are washed away by the water into the sump, where

they are separated. The molten slag formed in the pulverized fuel system can also be quenched

and washed by using the high velocity system. The advantages of this system are that its clean,

large ash handling capacity, considerable distance can be traversed, absence of working parts in

contact with ash.

Fly Ash Collection

Fly ash is captured and removed from the flue gas by electrostatic precipitators or fabric bag

filters (or sometimes both) located at the outlet of the furnace and before the induced draft fan.

The fly ash is periodically removed from the collection hoppers below the precipitators or bag

filters. Generally, the fly ash is pneumatically transported to storage silos for subsequent

transport by trucks or railroad cars.

Bottom Ash Collection and Disposal

At the bottom of every boiler, a hopper has been provided for collection of the bottom ash from

the bottom of the furnace. This hopper is always filled with water to quench the ash and clinkers

falling down from the furnace. Some arrangement is included to crush the clinkers and for

conveying the crushed clinkers and bottom ash to a storage site.

3. WATER TREATMENT PLANT

As the types of boiler are not alike their working pressure and operating conditions vary and so

do the types and methods of water treatment. Water treatment plants used in thermal power

plants used in thermal power plants are designed to process the raw water to water with a very

low content of dissolved solids known as ‘demineralized water’. No doubt, this plant has to be

engineered very carefully keeping in view the type of raw water to the thermal plant, its

treatment costs and overall economics.

A water treatment plant

The type of demineralization process chosen for a power station depends on three main factors:

i. The quality of the raw water.

ii. The degree of de-ionization i.e. treated water quality.

iii. Selectivity of resins.

Water treatment process is generally made up of two sections:

Pretreatment section.

Demineralization section

Pretreatment Section

Pretreatment plant removes the suspended solids such as clay, silt, organic and inorganic matter,

plants and other microscopic organism. The turbidity may be taken as two types of suspended

solid in water; firstly, the separable solids and secondly the non-separable solids (colloids). The

coarse components, such as sand, silt, etc: can be removed from the water by simple

sedimentation. Finer particles, however, will not settle in any reasonable time and must be

flocculated to produce the large particles, which are settle able. Long term ability to remain

suspended in water is basically a function of both size and specific gravity.

Demineralization

This filter water is now used for demineralizing purpose and is fed to cation exchanger bed, but

enroute being first dechlorinated, which is either done by passing through activated carbon filter

or injecting along the flow of water, an equivalent amount of sodium sulphite through some

stroke pumps. The residual chlorine, which is maintained in clarification plant to remove organic

matter from raw water, is now detrimental to action resin and must be eliminated before its entry

to this bed.

A demineralization tank

A DM plant generally consists of cation, anion and mixed bed exchangers. The final water from

this process consists essentially of hydrogen ions and hydroxide ions which is the chemical

composition of pure water. The DM water, being very pure, becomes highly corrosive once it

absorbs oxygen from the atmosphere because of its very high affinity for oxygen absorption. The

capacity of the DM plant is dictated by the type and quantity of salts in the raw water input.

However, some storage is essential as the DM plant may be down for maintenance. For this

purpose, a storage tank is installed from which DM water is continuously withdrawn for boiler

make-up. The storage tank for DM water is made from materials not affected by corrosive water,

such as PVC. The piping and valves are generally of stainless steel. Sometimes, a steam

blanketing arrangement or stainless steel doughnut float is provided on top of the water in the

tank to avoid contact with atmospheric air. DM water make-up is generally added at the steam

space of the surface condenser (i.e., the vacuum side). This arrangement not only sprays the

water but also DM water gets deaerated, with the dissolved gases being removed by the ejector

of the condenser itself.

4. DRAUGHT SYSTEM

There are four types of draught system:

i. Natural Draught

ii. Induced Draught

iii. Forced Draught

iv. Balanced Draught

Natural Draught System

In natural draft units the pressure differentials are obtained have constructing tail chimneys so

that vacuum is created in the furnace. Due to small pressure difference, air is admitted into the

furnace.

A natural draught system

Induced Draft System

In this system, the air is admitted to natural pressure difference and the flue gases are taken out

by means of Induced Draught (I.D.) fans and the furnace is maintained under vacuum.

An induced draught system

Forced Draught System

A set of forced draught (F.D.) fans is made use of for supplying air to the furnace and so the

furnace is pressurized. The flue gases are taken out due to the pressure difference between the

furnace and the atmosphere.

A forced draught system

Balanced Draught System

Here a set of Induced and Forced Draft Fans are utilized in maintaining a vacuum in the furnace.

Normally all the power stations utilize this draft system.

5. INDUSTRIAL FANS

ID Fan

The induced Draft Fans are generally of Axial-Impulse Type. Impeller nominal diameter is of the

order of 2500 mm. The fan consists of the following sub-assemblies:

Suction Chamber

Inlet Vane Control

Impeller

Outlet Guide Vane Assembly

An ID fan

FD Fan

The fan, normally of the same type as ID Fan, consists of the following components:

Silencer

Inlet Bend

Fan Housing

Impeller with blades and setting mechanism

An FD fan

The centrifugal and setting forces of the blades are taken up by the blade bearings. The blade

shafts are placed in combined radial and axial anti-friction bearings, which are sealed off to the

outside. The angle of incidence of the blades may be adjusted during operation. The

characteristic pressure volume curves of the fan may be changed in a large range without

essentially modifying the efficiency. The fan can then be easily adapted to changing operating

conditions.

The rotor is accommodated in cylindrical roller bearings and an inclined ball bearing at the drive

side absorbs the axial thrust.

Lubrication and cooling these bearings is assured by a combined oil level and circulating

lubrication system.

Primary Air Fan

PA Fan if flange-mounted design, single stage suction, NDFV type, backward curved bladed

radial fan operating on the principle of energy transformation due to centrifugal forces. Some

amount of the velocity energy is converted to pressure energy in the spiral casing. The fan is

driven at a constant speed and varying the angle of the inlet vane control controls the flow. The

special feature of the fan is that is provided with inlet guide vane control with a positive and

precise link mechanism.

It is robust in construction for higher peripheral speed so as to have unit sizes. Fan can develop

high pressures at low and medium volumes and can handle hot-air laden with dust particles.

Primary air fan

6. COMPRESSOR HOUSE

Instrument air is required for operating various dampers, burner tilting, devices, diaphragm

valves, etc: in the 210 MW units. Station air meets the general requirement of the power station

such as light oil atomizing air, for cleaning filters and for various maintenance works. The

control air compressors and station air compressors have been housed separately with separate

receivers and supply headers and their tapping.

A compressor house

Instrument Air System

Control air compressors have been installed for supplying moisture free dry air required for

instrument used. The output from the compressors is fed to air receivers via return valves. From

the receiver air passed through the dryers to the main instrument airline, which runs along with

the boiler house and turbine house of 210 MW units. Adequate numbers of tapping have been

provided all over the area.

Air-Drying Unit

Air contains moisture which tends to condense, and causes trouble in operation of various

devices by compressed air. Therefore drying of air is accepted widely in case of instrument air.

Air drying unit consists of dual absorption towers with embedded heaters for reactivation. The

absorption towers are adequately filled with specially selected silica gel and activated alumina

while one tower is drying the air.

An air drying unit

Service Air Compressor

The station air compressor is generally a slow speed horizontal double acting double stage type

and is arranged for belt drive. The cylinder heads and barrel are enclosed in a jacket, whih

extends around the valve also. The intercooler is provided between the low and high pressure

cylinder which cools the air between tag and collects the moisture that condenses.

A service air compressor

Air from L.P. cylinder enters at one end of the intercooler and goes to the opposite end

wherefrom it is discharged to the high-pressure cylinder; cooling water flows through the nest of

the tubes and cools the air. A safety valve is set at rated pressure.

Two selector switches one with positions auto load/unload and another with positions auto

start/stop, non-stop have been provided on the control panel of the compressor. In auto start-stop

position, the compressor will start.

III. TURBINE MAINTENANCE DEPARTMENT

TURBINE CLASSIFICATION:

1. Impulse turbine:

In impulse turbine steam expands in fixed nozzles. The high velocity steam from nozzles

does work on moving blades, which causes the shaft to rotate. The essential features of

impulse turbine are that all pressure drops occur at nozzles and not on blades.

2. Reaction turbine:

In this type of turbine pressure is reduced at both fixed and moving blades. Both fixed

and moving blades act like nozzles. Work done by the impulse effect of steam due to

reverse the direction of high velocity steam. The expansion of steam takes place on

moving blades.

A 95 MW Generator at BTPS, New Delhi

COMPOUNDING:

Several problems occur if energy of steam is converted in single step and so compounding is

done. Following are the type of compounded turbine:

i. Velocity Compounded Turbine:

Like simple turbine it has only one set of nozzles and entire steam pressure drop takes

place there. The kinetic energy of steam fully on the nozzles is utilized in moving

blades. The role of fixed blades is to change the direction of steam jet and too guide

it.

ii. Pressure Compounded Turbine:

This is basically a number of single impulse turbines in series or on the same shaft.

The exhaust of first turbine enters the nozzles of next turbine. The total pressure drop

of steam does not tae on first nozzle ring but divided equally on all of them.

iii. Pressure Velocity Compounded Turbine:

It is just the combination of the two compounding and has the advantages of allowing

bigger pressure drops in each stage and so fewer stages are necessary. Here for given

pressure drop the turbine will be shorter length but diameter will be increased.

MAIN TURBINE:

The 210MW turbine is a cylinder tandem compounded type machine comprising of H.P. and I.P

and L.P cylinders. The H.P. turbine comprises of 12 stages the I.P turbine has 11 stages and the

L.P has four stages of double flow. The H.P and I.P. turbine rotor are rigidly compounded and

the I.P. and L.P rotor by lens type semi flexible coupling. All the 3 rotors are aligned on five

bearings of which the bearing number is combined with thrust bearing.

The main superheated steam branches off into two streams from the boiler and passes through

the emergency stop valve and control valve before entering the governing wheel chamber of the

H.P. Turbine. After expanding in the 12 stages in the H.P. turbine then steam is returned in the

boiler for reheating.

The reheated steam from boiler enters I.P. turbine via the interceptor valves and control valves

and after expanding enters the L.P stage via 2 numbers of cross over pipes.

In the L.P. stage the steam expands in axially opposed direction to counteract the thrust and

enters the condenser placed directly below the L.P. turbine. The cooling water flowing through

the condenser tubes condenses the steam and the condensate the collected in the hot well of the

condenser.

The condensate collected the pumped by means of 3x50% duty condensate pumps through L.P

heaters to deaerator from where the boiler feed pump delivers the water to the boiler through

H.P. heaters thus forming a closed cycle.

STEAM TURBINE

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

converts it into useful mechanical work.

From a mechanical point of view, the turbine is ideal, because the propelling force is applied

directly to the rotating element of the machine and has not as in the reciprocating engine to be

transmitted through a system of connecting links, which are necessary to transform a

reciprocating motion into rotary motion. Hence since the steam turbine possesses for its moving

parts rotating elements only if the manufacture is good and the machine is correctly designed, it

ought to be free from out of balance forces.

If the load on a turbine is kept constant the torque developed at the coupling is also constant. A

generator at a steady load offers a constant torque. Therefore, a turbine is suitable for driving a

generator, particularly as they are both high-speed machines.

A further advantage of the turbine is the absence of internal lubrication. This means that the

exhaust steam is not contaminated with oil vapour and can be condensed and fed back to the

boilers without passing through the filters. It also means that turbine is considerable saving in

lubricating oil when compared with a reciprocating steam engine of equal power.

A final advantage of the steam turbine and a very important one is the fact that a turbine can

develop many time the power compared to a reciprocating engine whether steam or oil.

OPERATING PRINCIPLES

A steam turbines two main parts are the cylinder and the rotor. The cylinder (stator) is a steel or

cast iron housing usually divided at the horizontal centerline. Its halves are bolted together for

easy access. The cylinder contains fixed blades, vanes and nozzles that direct steam into the

moving blades carried by the rotor. Each fixed blade set is mounted in diaphragms located in

front of each disc on the rotor, or directly in the casing. A disc and diaphragm pair a turbine

stage. Steam turbines can have many stages. A rotor is a rotating shaft that carries the moving

blades on the outer edges of either discs or drums. The blades rotate as the rotor revolves. The

rotor of a large steam turbine consists of large, intermediate and low-pressure sections.

In a multiple-stage turbine, steam at a high pressure and high temperature enters the first row of

fixed blades or nozzles through an inlet valve/valves. As the steam passes through the fixed

blades or nozzles, it expands and its velocity increases. The high velocity jet of stream strikes the

first set of moving blades. The kinetic energy of the steam changes into mechanical energy,

causing the shaft to rotate. The steam that enters the next set of fixed blades strikes the next row

of moving blades.

As the steam flows through the turbine, its pressure and temperature decreases while its volume

increases. The decrease in pressure and temperature occurs as the steam transmits energy to the

shaft and performs work. After passing through the last turbine stage, the steam exhausts into the

condenser or process steam system.

The kinetic energy of the steam changes into mechanical energy through the impact (impulse) or

reaction of the steam against the blades. An impulse turbine uses the impact force of the steam

jet on the blades to turn the shaft. Steam expands as it passes through thee nozzles, where its

pressure drops and its velocity increases. As the steam flows through the moving blades, its

pressure remains the same, but its velocity decreases. The steam does not expand as it flows

through the moving blades.

STEAM CYCLE

The thermal (steam) power plant uses a dual (vapor+liquid) phase cycle. It is a closed cycle to

enable the working fluid (water) to be used again and again. The cycle used is ‘Rankine cycle’

modified to include superheating of steam, regenerative feed water heating and reheating of

steam.

MAIN TURBINE

The 210 MW turbine is a tandem compounded type machine comprising of H.P. and I.P.

cylinders. The H.P. turbines comprise of 12 stages, I.P. turbine has 11 stages and the L.P. turbine

has 4 stages of double flow.

The H.P. and I.P. turbine rotors are rigidly compounded and the L.P. motor by the lens type semi

flexible coupling. All the three rotors are aligned on five bearings of which the bearing no. 2 is

combined with the thrust bearing

The main superheated steam branches off into two streams from the boiler and passes through

the emergency stop valve and control valve before entering the governing wheel chamber of the

H.P. turbine. After expanding in the 12 stages in the H.P. turbine the steam is returned in boiler

for reheating.

The reheated steam for the boiler enters the I.P> turbine via the interceptor valves and control

valves and after expanding enters the L.P. turbine stage via 2 nos of cross-over pipes.

In the L.P. stage the steam expands in axially opposite direction to counteract the trust and enters

the condensers placed below the L.P. turbine. The cooling water flowing throughout the

condenser tubes condenses the steam and the condensate collected in the hot well of the

condenser.

The condensate collected is pumped by means of 3*50% duty condensate pumps through L.P.

heaters to deaerator from where the boiler feed pump delivers the water to boiler through H.P.

heaters thus forming a close cycle.

The Main Turbine

TURBINE CYCLE

Fresh steam from the boiler is supplied to the turbine through the emergency stop valve. From

the stop valves steam is supplied to control valves situated in H.P. cylinders on the front bearing

end. After expansion through 12 stages at the H.P. cylinder, steam flows back to the boiler for

reheating steam and reheated steam from the boiler cover to the intermediate pressure turbine

through two interceptor valves and four control valves mounted on I.P. turbine.

After flowing through I.P. turbine steam enters the middle part of the L.P. turbine through cross-

over pipes. In L.P. turbine the exhaust steam condenses in the surface condensers welded directly

to the exhaust part of L.P. turbine.

The Turbine Cycle

The selection of extraction points and cold reheat pressure has been done with a view to achieve

a high efficiency. These are two extractors from H.P. turbine, four from I.P. turbine and one from

L.P. turbine. Steam at 1.10 and 1.03 g/sq. cm. Abs is supplied for the gland sealing. Steam for

this purpose is obtained from deaerator through a collection where pressure of steam is regulated.

From the condenser, condensate is pumped with the help of 3*50% capacity condensate pumps

to deaerator through the low-pressure regenerative equipments.

Feed water is pumped from deaerator to the boiler through the H.P. heaters by means of 3*50%

capacity feed pumps connected before the H.P. heaters.

SPECIFICATIONS OF THE TURBINE

Type: Tandem compound 3 cylinder reheated type.

Rated power: 210 MW.

Number of stages: 12 in H.P., 11 in I.P. and 4*2 in L.P. cylinder.

Rated steam pressure: 130 kg /sq. cm before entering the stop valve.

Rated steam temperature: 535(C after reheating at inlet.

Steam flow: 670T / hr.

H.P. turbine exhaust pressure: 27 kg /sq. cm., 327(C

Condenser back pressure: 0.09 kg /sq. cm.

Type of governing: nozzle governing.

Number of bearing; 5 excluding generator and exciter.

Lubrication Oil: turbine oil 14 of IOC.

Gland steam pressure: 1.03 to 1.05 kg /sq. cm (Abs)

Critical speed: 1585, 1881, 2017.

Ejector steam parameter: 4.5 kg /sq. cm.

Condenser cooling water pressure: 1.0 to 1.1 kg /sq. cm.

Condenser cooling water temperature: 27000 cu. M /hr.

Number of extraction lines for regenerative heating of feed water; seven.

TURBINE COMPONENTS

Casing.

Rotor.

Blades.

Sealing system.

Stop & control valves.

Couplings and bearings.

Barring gear.

TURBINE CASINGS

HP Turbine Casings:

Outer casing: a barrel-type without axial or radial flange.

Barrel-type casing suitable for quick startup and loading.

The inner casing- cylindrically, axially split.

The inner casing is attached in the horizontal and vertical planes in the barrel casing so

that it can freely expand radially in all the directions and axially from a fixed point (HP-

inlet side).

IP Turbine Casing:

The casing of the IP turbine is split horizontally and is of double-shell construction.

Both are axially split and a double flow inner casing is supported in the outer casing and

carries the guide blades.

Provides opposed double flow in the two blade sections and compensates axial thrust.

Steam after reheating enters the inner casing from Top & Bottom.

LP Turbine Casing:

The LP turbine casing consists of a double flow unit and has a triple shell welded casing.

The shells are axially split and of rigid welded construction.

The inner shell taking the first rows of guide blades is attached kinematically in the

middle shell.

Independent of the outer shell, the middle shell, is supported at four points on

longitudinal beams.

Steam admitted to the LP turbine from the IP turbine flows into the inner casing from

both sides.

ROTORS

HP Rotor:

The HP rotor is machined from a single Cr-Mo-V steel forging with integral discs.

In all the moving wheels, balancing holes are machined to reduce the pressure difference

across them, which results in reduction of axial thrust.

First stage has integral shrouds while other rows have shroudings, riveted to the blades

are periphery.

IP Rotor:

The IP rotor has seven discs integrally forged with rotor while last four discs are shrunk

fit.

The shaft is made of high creep resisting Cr-Mo-V steel forging while the shrunk fit discs

are machined from high strength nickel steel forgings.

Except the last two wheels, all other wheels have shrouding riveted at the tip of the

blades. To adjust the frequency of thee moving blades, lashing wires have been provided

in some stages.

LP Rotor:

The LP rotor consists of shrunk fit discs in a shaft.

The shaft is a forging of Cr-Mo-V steel while the discs are of high strength nickel steel

forgings.

Blades are secured to the respective discs by riveted fork root fastening.

In all the stages lashing wires are provided to adjust the frequency of blades. In the last

two rows, satellite strips are provided at the leading edges of the blades to protect them

against wet-steam erosion.

BLADES

Most costly element of the turbine.

Blades fixed in stationary part are called guide blades/ nozzles and those fitted in moving

part are called rotating/working blades.

Blades have three main parts:

o Aerofoil: working part.

o Root.

o Shrouds.

Shroud are used to prevent steam leakage and guide steam to next set of moving blades.

VACUUM SYSTEM

This comprises of:

Condenser: 2 for 200 MW unit at the exhaust of LP turbine.

Ejectors: One starting and two main ejectors connected to the condenser locared near the

turbine.

C.W. Pumps: Normally two per unit of 50% capacity.

CONDENSER

There are two condensers entered to the two exhausters of the L.P. turbine. These are surface-

type condensers with two pass arrangement. Cooling water pumped into each condenser by a

vertical C.W. pump through the inlet pipe.

Water enters the inlet chamber of the front water box, passes horizontally through brass tubes to

the water tubes to the water box at the other end, takes a turn, passes through the upper cluster of

tubes and reaches the outlet chamber in the front water box. From these, cooling water leaves the

condenser through the outlet pipe and discharge into the discharge duct.

Steam exhausted from the LP turbine washes the outside of the condenser tubes, losing its latent

heat to the cooling water and is connected with water in the steam side of the condenser. This

condensate collects in the hot well, welded to the bottom of the condensers.

A typical water cooled condensor

EJECTORS

There are two 100% capacity ejectors of the steam eject type. The purpose of the ejector is to

evacuate air and other non-condensating gases from the condensers and thus maintain the

vacuum in the condensers.

The ejector has three compartments. Steam is supplied generally at a pressure of 4.5 to 5 kg /cm 2

to the three nozzles in the three compartments. Steam expands in the nozzle thus giving a high-

velocity eject which creates a low-pressure zone in the throat of the eject. Since the nozzle box of

the ejector is connected to the air pipe from the condenser, the air and pressure zone. The

working steam which has expanded in volume comes into contact with the cluster of tube

bundles through which condensate is flowing and gets condensed thus after aiding the formation

of vacuum. The non-condensing gases of air are further sucked with the next stage of the ejector

by the second nozzle. The process repeats itself in the third stage also and finally the steam-air

mixture is exhausted into the atmosphere through the outlet.

CONDENSATE SYSTEM

This contains the following

i. Condensate Pumps: 3 per unit of 50% capacity each located near condenser hot well.

ii. LP Heater: Normally 4 in number with no.1 located at the upper part of the condenser

and nos. 2,3& 4 around 4m level.

iii. Deaerator; one per unit located around 181 M’ level in CD bay.

Condensate Pumps

The function of these pumps is to pump out the condensate to the desecrator through ejectors,

gland steam cooler and LP heaters. These pumps have four stages and since the suction is at a

negative pressure, special arrangements have been made for providing sealing. The pump is

generally rated for 160 m3/ hr at a pressure of 13.2 kg/ cm2 .

L.P. Heaters

Turbine has been provided with non-controlled extractions, which are utilized for heating the

condensate, from turbine bleed steam. There are 410 W pressure heaters in which the last four

extractions are used. L.P. Heater-1 has two parts LPH-1A and LPH-1B located in the upper parts

of the condenser A and condenser B, respectively. These are of horizontal type with shell and

tube construction. L.P.H. 2,3 and 4 are of similar construction and they are mounted in a row of

5m level. They are of vertical construction with brass tubes the ends of which are expanded into

tube plate. The condensate flows in the ‘U’ tubes in four passes and extraction steam washes the

outside of the tubes. Condensate passes through these four L.P. heaters in succession. These

heaters are equipped with necessary safety valves in the steam space level indicator for visual

level indication of heating steam condensate pressure vacuum gauges for measurement of steam

pressure, etc:

Deaerator

The presence of certain gases, principally oxygen, carbon dioxide and ammonia, dissolved in

water is generally considered harmful because of their corrosive attack on metals, particularly at

elevated temperatures. One of the most important factors in the prevention of internal corrosion

in modern boilers and associated plant therefore, is that the boiler feed water should be free as far

as possible from all dissolved gases especially oxygen. This is achieved by embodying into the

boiler feed system a deaerating unit, whose function is to remove the dissolved gases from the

feed water by mechanical means. Particularly the unit must reduce the oxygen content of the feed

water to a lower value as far as possible, depending upon the individual circumstances. Residual

oxygen content in condensate at the outlet of deaerating plant usually specified are 0.005/ litre or

less.

A Deaerator

PRINCIPAL OF DEAERATION

It is based on following two laws.

Henry’s Law

Solubility

The Deaerator comprises of two chambers:

Deaerating column

Feed storage tank

Deaerating column is a spray cum tray type cylindrical vessel of horizontal construction with

dished ends welded to it. The tray stack is designed to ensure maximum contact time as well as

optimum scrubbing of condensate to achieve efficient deaeration. The deaeration column is

mounted on the feed storage tank, which in turn is supported on rollers at the two ends and a

fixed support at the centre. The feed storage tank is fabricated from boiler quality steel plates.

Manholes are provided on deaerating column as well as on feed storage tank for inspection and

maintenance.

The condensate is admitted at the top of the deaerating column flows downwards through the

spray valves and trays. The trays are designed to expose to the maximum water surfaces for

efficient scrubbing to affect the liberation of the associated gases steam enters from the

underneath of the trays and flows in counter direction of condensate. While flowing upwards

through the trays, scrubbing and heating is done. Thus the liberated gases move upwards

alongwith the steam. Steam gets condensed above the trays and in turn heats the condensate.

Liberated gases escapes to atmosphere from the orifice opening meant for it. This opening is

provided with a number of delflectors to minimize the loss of steam.

FEED WATER SYSTEM

The main equipments coming under this system are:

Boiler feed Pump: Three per unit of 50% capacity each located in the ‘0’ meter level in

the T bay.

High Pressure Heaters: Normally three in number and are situated in the TG bay.

Drip Pumps: generally two in number of 100% capacity each situated beneath the LP

heaters.

Turbine Lubricating Oil System: This consists of the Main Oil Pump (MOP), Starting

Oil Pump (SOP), AC standby oil pumps and emergency DC Oil Pump and Jacking Oil

Pump (JOP). (one each per unit)

Boiler Feed Pump

This pump is horizontal and of barrel design driven by an Electric Motor through a hydraulic

coupling. All the bearings of pump and motor are forced lubricated by a suitable oil lubricating

system with adequate protection to trip the pump if the lubrication oil pressure falls below a

preset value.

The high pressure boiler feed pump is a very expensive machine which calls for a very careful

operation and skilled maintenance. Operating staff must be able to find out the causes of defect

at the very beginning, which can be easily removed without endangering the operator of the

power plant and also without the expensive dismantling of the high pressure feed pump.

Function

The water with the given operating temperature should flow continuously to the pump under a

certain minimum pressure. It passes through the suction branch into the intake spiral and from

there; it is directed to the first impeller. After leaving the impeller it passes through the

distributing passages of the diffuser and thereby gets a certain pressure rise and at the same time

it flows over to the guide vanes to the inlet of the next impeller. This will repeat from one stage

to the other till it passes through the last impeller and the end diffuser. Thus the feed water

reaching into the discharge space develops the necessary operating pressure.

Booster Pump

Each boiler feed pump is provided with a booster pump in its suction line which is driven by the

main motor of the boiler feed pump. One of the major damages which may occur to a boiler feed

pump is from cavitation or vapour bounding at the pump suction due to suction failure.

Cavitation will occur when the suction pressure of the pump at the pump section is equal or very

near to the vapour pressure of the liquid to be pumped at a particular feed water temperature. By

the use of booster pump in the main pump suction line, always there will be positive suction

pressure which will remove the possibility of cavitation. Therefore all the feed pumps are

provided with a main shaft driven booster pump in its suction line for obtaining a definite

positive suction pressure.

Lubricating Pressure

All the bearings of boiler feed pump, pump motor and hydraulic coupling are force lubricated.

The feed pump consists of two radial sleeve bearings and one thrust bearing. The thrust bearing

is located at the free end of the pump.

High Pressure Heaters

These are regenerative feed waters heaters operating at high pressure and located by the side of

turbine. These are generally vertical type and turbine based steam pipes are connected to them.

HP heaters are connected in series on feed waterside and by such arrangement, the feed water,

after feed pump enters the HP heaters. The steam is supplied to these heaters to form the bleed

point of the turbine through motor operated valves. These heaters have a group bypass protection

on the feed waterside.

In the event of tube rupture in any of the HPH and the level of condensate rising to dangerous

level, the group protection devices divert automatically the feed water directly to boiler, thus

bypassing all the 3 H.P. heaters.

An HP heater