NTPC badarpur mechanical report

JUNE - JULY

2010

SUBMITTED BY:DEVESH SINGHIIIrd B.TECHMECHANICAL ENGINEERINGROLL NO. 0704540015H.B.T.I. KANPUR

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Summer Training Report

ACKNOWLEDGEMENT

With profound respect and gratitude, I take the opportunity to convey my thanks to complete the training here.

I do extend my heartfelt thanks to Ms. Rachna Singh Bahal for providing me this opportunity to be a part of this esteemed organization.

I am extremely grateful to all the technical staff of BTPS / NTPC for their co-operation and guidance that has helped me a lot during the course of training. I have learnt a lot working under them and I will always be indebted of them for this value addition in me.

I would also like to thank the training incharge of H.B.T.I., Kanpur and all the faculty members of Mechanical Engineering Department for their effort of constant co- operation, which have been a significant factor in the accomplishment of my industrial training.

DEVESH SINGH

H.B.T.I., KANPUR

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CERTIFICATE

This is to certify that DEVESH SINGH, student of 3rd B.Tech Mechanical Engineering, H.B.T.I., Kanpur, has successfully completed his Industrial Training at National Thermal Power Corporation, New Delhi for 6 week from 22nd June to 31st July 2010. He has completed the whole training as per the training report submitted by him.

Training Incharge NTPC, Badarpur, New Delhi

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TRAINING AT BTPS

I was appointed to do 6 week training at this esteemed organization from 22nd June to 31st

July, 2010. I was assigned to visit various division of the plant, which were:

Boiler Maintenance Department (BMD I/II/III)

Plant Auxiliary Maintenance (PAM)

Turbine Maintenance Department (TAM)

Coal Handling Department (CHD/NCHP)

These 6 weeks training was a very educational adventure for me. It was really

amazing to see the plant by yourself and learn how electricity, which is one of our daily

requirements of life, is produced.

This report has been made by my experience at BTPS. The material in this report has

been gathered from my textbook, senior student reports and trainers manuals and power

journals provided by training department. The specification and principles are as learned by

me from the employees of each division of BTPS.

DEVESH SINGH

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INDEX

ABOUT NTPC

ABOUT BTPS

BASIC STEPS OF ELECTRICITY GENERATION

RANKINE CYCLE

BOILER MAINTENANCE DEPARTMENT

PLANT AUXILIARY MAINTENANCE

TURBINE MAINTENANCE DEPARTMENT

MAINTENANCE PLANNING DEPARTMENT

COAL HANDLING DEPARTMENT

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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.

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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. Its 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

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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.

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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.

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

2017.

ABOUT BTPS

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1975

1997

2005

2004

2008

2009

2017

2012

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

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:

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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.

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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 (540C) 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

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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.

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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

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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 Macquorn Rankine, a Scottish polymath..

Description

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

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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 vapor 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 favorable 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.

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Processes of the Rankine cycle

Ts 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 Ts diagram and more closely resemble

that of the Carnot cycle.

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

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

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

In a real Rankine cycle, the compression by the pump and 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 power required 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 Ts 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 turbines work in series. The first accepts vapor from the boiler at high

pressure. After the vapor 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 vapor from condensing during its expansion which can seriously damage the

turbine blades, and improves the efficiency of the cycle.

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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. BOILER MAINTENANCE DEPARTMENT

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.

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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.

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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 247C

iii. Feed water leaving economizer 276C

2. Steam Temperature:

i. Drum 341C

ii. Super heater outlet 540C

iii. Reheat inlet 332C

iv. Reheat outlet 540C

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

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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

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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 30-40C 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

30

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

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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.

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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

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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

34

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.

35

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.

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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

37

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

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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.

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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.

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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

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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

42

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

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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.

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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.

45

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

46

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.

47

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: 535C after reheating at inlet.

Steam flow: 670T / hr.

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

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.

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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.

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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.

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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

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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 /cm2 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

52

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

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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 dlflectors 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)

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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 vapor 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 vapor 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.

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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

Turbine Oil Lubricating System

This consists of main oil pump, starting oil pump, emergency oil pump and each per unit.

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IV. MAINTENANCE PLANNING DEPARTMENT

PREDICTIVE MAINTENANCE

The art of predictive maintenance is to monitor the machine with the appropriate

technologies, frequently enough to detect the anticipated failure modes.

Also known as “Condition Based Maintenance” results in:

Increased uptime

Decreased unexpected breakdowns

Reduced Maintenance Costs

Maintenance is performed and it is planned

Improved Plant Safety

Machines normally give off some signs before failing

The sign may be change in sound level, vibration, pressure, temperature etc.

Change in the performance

Metal particles in the lubricant

Change in motor current etc.

PROACTIVE MAINTENANCE

Also known by different names including “Precision Maintenance” and “Reliability based

Maintenance”. The motto here is “Fix it once & Fix it Right”

How an effective Predictive Maintenance Strategy can Improve Plant Efficiency:

Reduction in Lost Production

Reduced Cost of Maintenance

Less Likelihood of Secondary Damage

Reduced Inventory

Extending the Life of Plant Items

Improved Product Quality

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KNOWLEDGE BASED MAINTENANCE

Shared information among all users

Root cause analysis (proactive) - design problems out of machines, fix the problem

not the symptom

TECHNOLOGIES USED

Vibration monitoring & analysis and balancing.

Motor current signature analysis (mcsa).

Thermography monitoring & analysis.

Accoustic monitoring & analysis.

Checking of ht/lt motors in electrical workshop

VIBRATION ANALYSIS AND UNBALANCE

Amplitude proportional to the amount of unbalance

Vibration high normally in radial direction (may be also in axial direction in case of

overhung and flexible rotors).

1* RPM vibration is greater than 80% (normally) of the overall reading.

Horizontal and vertical 1* RPM amplitude should be nearly same, although it also

depends on system rigidity on the particular direction.

Other frequency peaks may be less than 5% of the 1* RPM amplitude

Phase shift of 90 deg. When sensor moves from horizontal to vertical.

MOTOR ROTOR BAR ANALYSIS

On-line detection of broken rotor bars of Induction Motors.

Detects Rotor bar faults at early stage using supportive software

Avoids Motor Breakdown and hence forced reduction of unit load.

FACTORS LEADING TO ROTOR BAR CRACKS

Most motor failures are preceded by long periods of wear.

More starts & stops or rapidly fluctuating loads leads to Excess stresses

Hardening of Joints of rotor bars and end ring

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THERMOGRAPHY

IRT camera has been very useful tools in predictive maintenance for detecting hot

spots in electrical & switchyard equipment.

The thermal scanning survey in switchyard & other identified areas is done to monitor

the healthiness of electrical equipment.

Machine image captured to know the Thermal distribution

ACCOUSTIC ANALYSIS

What is acoustic emission?

Acoustic emission is a naturally occurring phenomenon wherin external mechanical

loading generates sources of elastic waves.

What is acoustic analysis?

Acoustic analysis or noise analysis is a two step process involving the acquistion and

interperatation of machinery acoustic data.

Its purpose is :

To determine the condition of the machine and pin-point any specific mechanical or

operational defects i.e. Pressure or vacuum leaks (pneumatic, gas& steam) or arcing

and corona (electrical problems).

V. COAL HANDLING DEPARTMENT

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As coal is the prime fuel for thermal power plant, adequate emphasis should be given for its

proper handling and storage. Also it is equally important to have a sustained flow of this fuel

to maintain uninterrupted power generation. Coal is used as the fuel because of the following

advantages.

Advantages of coal as fuel:

Abundantly available in India

Low Cost

Technology for power generation well developed.

Easy to handle, transport, store and use.

COAL CYCLE

Coal Transport by M.G.R.

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Each of the NTPC Project requires transportation of large quantities of coal from the coal

mines to power station’s site of the order of 30,000 tonnes/ day for a typical 2,000 MW

station. This enormous coal requirement is being met from open cast mines.

Techno economic study conducted for coal transportation from mines to power station have

revealed that Merry-Go-Round (MGR) rail transportation system is most economical and is

also reliable. This system calls for high speed load outstation at the mines which have the

following advantages:

High loading enables loading of trains quickly; thus achieving high turn-over of

wagons and reduction rolling stock requirement,

Top open railway wagons are loaded with maximum possible load consistently and

accurately.

Simple loading arrangement at a single point avoids the need for a big marshalling

yard with cumbersome operational system.

The high speed load outstation consists of one or two loading sites depending upon the coal

requirement of the linked power station. The handling capacity of the loading site is such that

it fills at least one big rake of wagon and in some cases, two rakes.

Coal transportation

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COAL HANDLING SYSTEM

In the coal handling system of NTPC, three coal paths are normally available for direct

conveying of coal. These are:

Path A: from track hopper to boiler bunker.

Path B: from track hopper to stockyard..

Path C: from stockyard to boiler bunkers.

The Coal Handling System

The storage facilities at the stockyards have been provided only for crushed coal. The coal

handling system is designed to provide 100% standby for all equipments and conveyors.

The 200 mm coal as received at the track hopper is fed to the crusher house for crushing.

Crusher of 50% capacity is provided and these are preferred to two crushers of 100%

capacity because of increased reliability and possible higher availability. A series of parallel

conveyors are designed thereafter to carry crushed coal directly to the boiler bunkers or to

divert it to the stockyard.

To feed coal into bunkers, mobile trippers have been provided over bunkers on conveyors,

coal mil & therefore the bunker conveyors of 200 MW units of the earlier projects are

provided between boiler and turbo-generator building. However for better mill maintenance,

accessibility, and to reduce coal dust nuisance; the turbine plant area, coal mills, bunker

conveyors are now being placed between boiler and electrostatic precipitator.

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Coal Handling Division of BTPS, New Delhi

COAL HANDLING EQUIPMENTS

i. Pulley

They are made of mild steel. Rubber lagging is provided to decrease the friction factor

in between the belt and pulley.

ii. Scrapper

Conveyors are provided with scrappers at the discharge pulley in order to clean the

carrying side of the belt built up material on idler rolls. Care should be taken to ensure

that the scrapper is held against the belt with the pressure sufficient to remove

material without causing damage to the belt due to excessive force exerted by the

wiper.the following categories of scrapper are common in use:

Steel blade scrapper

Rubber/fabric blade scrapper

Nylon brush scrapper

Compressed air blast scrapper.

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iii. Idlers

These essentially consist of rolls made out of seamless steel tube enclosed fully at

each end and fitted with stationary shaft, anti-friction bearings and seals. They support

the belt and enable it to travel freely without much frictional losses and also keep the

belt properly trained.

Idler

iv. Conveyor Belt

The conveyor belt consists of layers or piles of fabric duck, impregnated with rubber

and protected by a rubber cover on both sides and edges. The fabric duck supplies the

strngth to withstand the tension created in carrying the load while the cover protects

the fabric carcass. Heat resistant belting is always recommended for handling

materials at a temperature over 66C.

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Coal Storage Area of the Badarpur Thermal Power Station, New Delhi

v. Vibrating Screen

The function of vibrating screen is to send the coal of having size less than 20 mm to

the crusher. The screen is operated by four v-belts connected to motor.

vi. Crusher

The role of crusher is to crush the coal from 200 mm to 20 mm size of coal received

from the vibrating screen. This is accomplished by means of granulators of ring type.

There are about 37 crushing elevations; each elevation has 4 granulators-2 of plain

type and 2 of tooth type, arranged alternately.

The granulators are made of manganese steel because of their work hardening

property. The coal enters the top of the crusher and is crushed between rotating

granulators and fluid case path. The crushed coal through a chute falls on belt feeder.

Normally these crushers have a capacity round 600 tonnes/ hr.

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A Crusher

vii. Magnetic separators

This is an electromagnet placed above the conveyor to attract magnetic materials.

Over this magnet there is one conveyor to transfer these materials to chute provided

for dumping at ground level. Because of this, continuous removal is possible. It can

remove any ferrous impurity from 10 gms to 50 kg.

Wagon Tripler at Badarpur Thermal Power Station, New Delhi

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viii. Vibrating Feeder

It is used to feed the coal on the underground conveyor belt from where coal goes to

bunker. Coal from the stockyard with the help of bulldozer is taken to the vibrating

feeder via reclaim hopper and underground conveyor belts. A tripper is provided in

the conveyor to stack the material at desired location on either side or along the

conveyor with the help of chute or chute fitted with the tripper itself. The tripper is

provided with wheels, which move on rails parallel to conveyor.

These trippers are of three types mainly:

Motorized tripper

Bell-Propelled Manually operated Tripper.

Winch driven tripper

Screening and Separation Unit of Coal Handling Division

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