Project Report on Industrial Summer Training at NTPC Simhadri

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Industrial Training Project Report

On

“Coal - Fired Steam Power Plants”

National Thermal Power Corporation SIMHADRI (Visakhapatnam)

(Submitted towards completion of industrial training at NTPC SIMHADRI)

Under the guidance of: Submitted by:

Shri B.Venkata Rao, Uppu Ashish,

DGM, Ash Handling Plant, B.Tech, Mechanical Engg.

NTPC SIMHADRI, (4th sem),

Visakhapatnam. GITAM University,

Visakhapatnam.

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TRAINING SCHEDULE

DEPARTMENT PERIOD

BOILER MAINTAINANCE11.05.2015

to16.05.2015

TURBINE MAINTAINANCE18.05.2015

to23.05.2015

OFFSITE MAINTAINANCE25.05.2015

to30.05.2015

ASH HANDLING PLANT01.06.2015

to09.06.2015

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CERTIFICATE

This is to certify that UPPU ASHISH, a student of 2012-2016 Batch

of B.Tech,Mechanical Engineering in 4th Year of GITAM University,

Visakhapatnam has successfully completed his industrial training at

NTPC Simhadri, Visakhapatnam for four weeks from 7th May to 9th

June 2015. He has completed the whole training as per the training

report submitted by him.

HR Manager

NTPC Simhadri,

Visakhapatnam

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Acknowledgment“It is not possible to prepare a project report without the assistance &

encouragement of other people. This one is certainly no exception.”

On the very outset of this report, I would like to extend my sincere &

heartfelt obligation towards all the personages who have helped me in

this endeavor. Without their active guidance, help, cooperation &

encouragement, I would not have made headway in the industrial

training

I am ineffably indebted to Mr. K.N. Reddy, AGM (MM-BMD); Mr.

D.Shravan, Dy. Manager (BMD-PP); Mr. Piyush Kanwar, Dy. Manager

(BMD-Mills); Mr. Balaji, Dy. Manager (BMD-RM); Mr. T.Prem Das, AGM

(MM-TMD & OS); Mr. Shridhar, Dy. Manager (MM-TMD) for

conscientious and encouragement to accomplish this assignment.

I am extremely thankful and pay my gratitude to my guide Mr. B.Venkata

Rao for his valuable guidance and support on completion of this project

in its presently.

I extend my gratitude to NTPC Ltd Simhadri and HR-EDC Dept. of NTPC

Ltd Simhadri for giving me this opportunity.

I also acknowledge with a deep sense of reverence, my gratitude

towards my parents, who has always supported me morally as well as

economically.

Any omission in this brief acknowledgement does not mean lack of

gratitude.

Thanking You

Ashish Uppu

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TABLE OF CONTENTS

1. About NTPC……………………………………………… 62. About NTPC SIMHADRI……………………………. 143. NTPC power stations in India…………………… 184.Principal and Operation of a Thermal Power

Plant…………………………………………………………. 195.Principal components of a 500MW Thermal

Power Plant………………………………………………. 296.The Layout of NTPC Simhadri……………………. 457.Boiler and its auxiliaries……………………………. 488.The Steam Turbine Theory……………………… 1189. Turbine and its auxiliaries……………………… 12810. DM treatment

plant……………………………………………………….. 16111. Cooling Towers…………………………………. 16912. Circulating Water System…………………. 17413. Principal components of CWS………….. 17814. Ash Handling System……………………….. 18315. Ways to increase the thermal efficiency of

power plants………………………………………….. 18716. Losses during operation & maintenance of

a power plant…………………………………………. 1905

TABLE OF CONTENTS






a power plant…………………………………………. 1905

TABLE OF CONTENTS






a power plant…………………………………………. 190

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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. NTPC is emerging as a diversified

power major with presence in the entire value chain of the power

generation business. Apart from power generation, which is the mainstay

of the company, NTPC has already ventured into consultancy, power

trading, ash utilization and coal mining. NTPC ranked 341st in the ‘2010,

Forbes Global 2000’ ranking of the World’s biggest companies. NTPC

became a Maharatna company in May, 2010, one of the only four

companies to be awarded this status.

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 15coal based and 7 gas based stations, located across the country.

In addition under JVs, 3 stations are coal based & another station uses

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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 1000MW

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 2001of 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. The 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 is as

second best utility in the world.

In October 2004, NTPC launched its Initial Public Offering (IPO)

consisting of 5.25% as fresh issue and 5.25% as offer for sale by

Government of India. NTPC thus became a listed company in November

2004 with the Government holding 89.5% of the equity share capital. In

February 2010, the Shareholding of Government of India was reduced

from 89.5% to 84.5% through Further Public Offer and the balance 10.5%

is held by FIIs, Domestic Banks, Public and others.

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NTPC LimitedType Public

Founded 1975

Headquarters Delhi, India

Key people R S Sharma, Chairman & Managing Director

Industry Electricity generation

Products Electricity

Revenue INR 416.37 billion (2008)

Net income INR 70.47 billion (2008)

Employees 23867 (2006)

Website http://www.ntpc.co.in

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Founded 1975









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Founded 1975









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Strategies of NTPC

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 fordevelopment 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 μClean Development.

Mechanism to help get / earn μCertified Emission Reduction.

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Corporate Social Responsibility

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

NTPC Foundation formed to address Social issues at nationallevel

NTPC has framed Corporate Social Responsibility Guidelinescommitting up to 0.5% of net profit annually for CommunityWelfare.

The welfare of project affected persons and the local populationaround NTPC projects are taken care of through well drawnRehabilitation and Resettlement policies.

The company has also taken up distributed generation for remoterural areas

Partnering government in various initiatives

Consultant role to modernize and improvise several plants acrossthe country.

Disseminate technologies to other players in the sector.

Consultant role Partnership in Excellence Programme forimprovement 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 tilization Division.

Afforestation Group.

Centre for Power Efficiency & Environment Protection.

Group on Clean Development Mechanism.

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

Vision

“To be the world’s largest and best power producer, powering India’sgrowth.”

Mission

“Develop and provide reliable power, related products and servicesat competitive prices, integrating multiple energy sources with

innovative and eco-friendly technologies and contribute to society.”

Core Values – BE COMMITTED

B Business ethics

E Environmentally and Economically Sustainable

C Customer Focus

O Organizational and Professional Pride

M Mutual Respect and Trust

M Motivating Self and Others

I Innovation and Speed

T Total Quality for Excellence

T Transparent and Respected Organization

E Enterprising

D Devoted

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Journey of NTPC

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Journey of NTPC

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Journey of NTPC

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A Qualitative study of the Company

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About NTPC Simhadri

Simhadri Super Thermal Power Plant is a coal-fired power plant

located in the Visakhapatnam district of the Indian state of Andhra

Pradesh. The power plant is one of the coal fired power plants of NTPC,

a Government of India enterprise. The coal for the power plant is

sourced from Kalinga Block of Talcher Coal fields in Odisha. Power

generated by units 1 and 2, making up for 1,000 MW, is dedicated to

power distribution companies owned by the Government of Andhra

Pradesh. The remainder 1,000 MW, generated by units 3 and 4, is

allocated to the states of Odisha, Tamil Nadu, and Karnataka. Their

shares are decided arbitrarily, with unsold power being sold to Andhra

Pradesh.

NTPC Simhadri is a modern coal-fired power plant, and is a combination

of four independent generation units, with common water and fuel

sources, and common ash ponds. Each of the four units has a

nameplate capacity of 500 MW. Units 1 and 2 were built in the first

phase of development, and were commissioned in February 2002 and

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August 2004, respectively, to meet urgent needs of power in the largely

agrarian Coastal Andhra and North-Coastal Andhra regions. Units 3 and

4 were built in the second phase, and commissioned in March 2011 and

March 2012, respectively. Since the operator of this plant is a

Government of India enterprise, and since the plant was built with

central government funds, power generated by units 3 and 4 are sold to

distribution companies based in neighboring states of Odisha, Tamil

Nadu, and Karnataka, over the National Grid, as power stocks. The

allocations are decided between NTPC and the three states' discoms.

Unsold units are offered to discoms of Andhra Pradesh for purchase at

market prices.

Coal for NTPC Simhadri is sourced from Talcher Coal Fields, Odisha,

and transported by East Coast Railway (ECoR), over the Kolkata-

Chennai trunk line, with a spur heading towards the plant at Duvvada.

NTPC Simhadri uses fresh water sourced from the Yeluru Canal as

working fluid (steam which turns the turbines). For cooling, however, the

plant uses seawater pumped in from the Bay of Bengal. Seawater, with

its salt content, is unfit to be used as working fluid, without desalination.

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PROJECT PROFILE

Approved Capacity 2000 MW (4 X 500 MW)

Location Paravada Mandal, Visakhapatnam, AP

Source of Finance JBIC Loan and Internal Resources

Fuel Source Mahanadi Coal Fields, Talcher

Fuel Requirement 5.04 Million Tons of Coal per annum

Mode of Transportation Rail

DM Water Source Water from Yelluru Canal

Sweet Water Requirement 600 m3 / hr

Cooling Water Source Sea Water from Bay of Bengal

Sea Water Requirement 9100 m3 / hr

Main Contractor M/s BHEL

Power Evacuation AP TRANSCO (Via Kalpaka)

Beneficiary State Andhra Pradesh

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PROJECT PROFILE














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PROJECT PROFILE














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Salient Features of NTPC Simhadri

• First Coastal Based Coal fired thermal Power Project of NTPC

• Biggest Sea Water Intake-Well in India (For Drawing Sea Waterfrom Bay of Bengal)

• Use of Sea Water for Condenser Cooling and Ash Disposal

• Asia’s Tallest Natural Cooling Towers (165 m), 6th in theWorld

• Use of Fly-Ash Bricks in the Construction of all Buildings

• Coal Based Project of NTPC Whose Entire Power is allocated toHome State (AP)

• Use of Monitors and Large Video Screens (LVS) as Man MachineInterface (MMIs) for Operating the Plant

• Use of Process Analysis, Diagnosis and Optimization (PADO) for thefirst time in NTPC

• Flame Analysis of Boiler by Dedicated Scanners for all CoalBurners

• Boiler Mapping By Acoustic Pyrometers

• Use of Distributed Digital Control and Management InformationSystem (DDCMIS)

• Totally Spring Loaded Floating Foundation for all MajorEquipments Including TG

• Use of INERGEN as Fire Protection System for the 1st time inNTPC

• Use of Digital Automatic Voltage Regulator (DAVR)

• Use of VFD in ID Fan

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NTPC POWER STATIONS IN INDIA

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Principle and Operation of a Thermal PowerPlant

Principle:

Any Steam Power Plant operates under the Simple Rankine Cycle.

Hence the Rankine cycle is often termed as Basic Power Plant Cycle.

The Rankine CycleThe Rankine cycle is a thermodynamic cyclewhich 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. The thermal (steam) power plant

uses a dual (vapour+liquid) phase cycle. It is a closed cycle to enable

the working fluid (water) to be used again and again.

The basic principle of the working of a Thermal Power Plant is quite

simple. The fuel used in the plant is burnt in the boiler, and the heat

generated is then used to boil water which is circulated through several

Layout of a SimpleRankine Cycle

T-S diagram of a SimpleRankine Cycle

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Principle:
















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Principle:
















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tubes, the steam that is generated is used to drive a turbine, which in

turn is coupled with a generator, which then produces the electricity.

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 a 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

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of choice due to its favorable properties, such as nontoxic and un

reactive 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.

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

T-S diagram of a TypicalRankine cycle

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matched.


0.06bar and 50bar.



to the right


21












matched.


0.06bar and 50bar.



to the right


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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 vapor.

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 1-2and 3-4 would be represented by vertical lines

onthe Ts diagram. 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.

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

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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 two turbines work in series on a common shaft. The first accepts

vapor from the boiler at a high pressure. After the vapor has passed

through the first turbine (also referred as H.P turbine), it renters the

boiler and is reheated before it is allowed to pass through the second

turbine (often referred to as L.P turbine).It prevents the vapor from

condensing during its expansion which can intensely damage the turbine

blades, and improves the efficiency of the cycle by decreasing the net

work output. To protect the reheat tubes, steam is not allowed to expand

Rankine Cycle with superheating

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deep into the two-phase region before it is taken for reheating, because

in that case the moisture particles in the steam while evaporating would

leave behind solid deposits in the form of scale which is difficult to

remove. A low reheat pressure may bring down the cycle efficiency.

Again, a high reheat pressure increases the moisture content at turbine

exhaust. Thus the reheat pressure is optimized. By increasing the

number of reheats, still higher steam pressures could be used, but

mechanical stresses increase at a higher proportion then the increase in

pressure, also increase. Hence more than two reheats have not been

used so far.

Regenerative Rankine Cycle

The main aim of the Regenerative Rankine cycle is to improve the cycle

efficiency by decreasing the net heat input. In Regenerative Rankine

cycle, after emerging from the condenser (possibly as a sub cooled

liquid) the working fluid is heated by steam tapped from the hot portion

of the cycle (i.e. from the intermediate stages of the turbine). On the

Rankine Cycle with Reheat

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diagram shown, the fluid at 2 is mixed with the fluid at 4 (both at the

same pressure) to end up with a saturated liquid at 7.

Reheat-Regenerative Cycle

The reheating of steam is adopted when the vaporization pressure is

high. The effect of reheat alone on the thermal efficiency of the cycle is

very small. Regeneration or the heating up of feed water by steam

extracted from the turbine has a marked effect on cycle efficiency. The

Reheat-Regenerative Rankine cycle (with minor variants) is commonly

used in modern steam power stations. Another variation is where 'bleed

steam' from between turbine stages is sent to feed water heaters to

preheat thewateron its way from the condenser to the boiler.

Regenerative Rankine Cycle

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Factors affecting thermal cycle efficiency

1. Initial steam pressure

2. Initial steam temperature

3. Reheat pressure and temperature, if reheat is used

4. Condenser pressure

5. Regenerative feed water heating

Operation-Fundamentals of Coal to Electricity:

Reheat – Regenerative RankineCycle

Operation of a Steam Power Plant26









Operation of a Steam Power Plant26









Operation of a Steam Power Plant

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MM

Mechanical Power to Electric Power

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

turbine also rotates .It causes rotation of the exciter which produces an induced emf

(electric power)

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Principle components of a 500MW thermalpower plant

Any 500MW thermal power plant comprises of the followingcomponents:

1. Cooling tower

2. Cooling water pump

3. Transmission line (3-phase)

4. Unit transformer (3-phase)

5. Electric generator (3-phase)

6. Low pressure turbine

7. Feed Water Pump

A typical 500MW Thermal PowerPlant

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1. Cooling tower






7. Feed Water Pump


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1. Cooling tower






7. Feed Water Pump


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8. Condenser

9. Intermediate pressure turbine

10. Steam governor valve

11. High pressure turbine

12. Deaerator

13. Feed heater

14. Coal conveyor

15. Coal hopper

16. Pulverized coal mill

17. Boiler drum

18. Ash hopper

19. Super heater

20. Forced draught fan

21. Re heater

22. Air intake tower

23. Economizer

24. Air pre heater

25. Electrostatic Precipitator (ESP)

26. Induced draught fan

27. Flue Gas

1. Cooling TowerCooling towers are heat removal devices used to transfer process

waste heat to the atmosphere. Cooling towers may either use the

evaporation of water to remove process heat and cool the working

fluid to near the wet-bulb air temperature or in the case of closed

circuit dry cooling towers rely solely on air to cool the working fluid to

near the dry-bulb air temperature. However, evaporative type cooling

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towers are most commonly used. Common applications include

cooling the circulating water used in oil refineries, chemical plants,

power stations and building cooling. The towers vary in size from

small roof-top units to very large hyperboloid structures that can be

up to 200 meters tall and 100 meters in diameter, or rectangular

structures that can be over 40 meters tall and 80 meters long.

Smaller towers are normally factory-built, while larger ones are

constructed on site. The absorbed heat is rejected to the atmosphere

by the evaporation of some of the cooling water in mechanical

forced-draft or induced Draft towers or in natural draft hyperbolic

shaped cooling towers as seen at most nuclear power plants.

2. Cooling Water PumpIt pumps the water from the cooling tower to the condenser.

3. Three Phase Transmission lineThree phase electric power is a common method of electric power

transmission. It is a type of polyphase system mainly used to power

motors and many other devices. A three phase system uses less

conductive material to transmit electric power than equivalent single

phase, two phase, or direct current system at the same voltage. In a

three phase system, three circuits reach their instantaneous peak

values at different times. Taking current in one conductor as the

reference, the currents in the other two are delayed in time by one-

third and two-third of one cycle .This delay between “phases” has the

effect of giving constant power transfer over each cycle of the current

and also makes it possible to produce a rotating magnetic field in an

electric motor. At the power station, an electric generator converts

mechanical power into a set of electric currents, one from each

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electromagnetic coil or winding of the generator. The current are

sinusoidal functions of time, all at the same frequency but offset in

time to give different phases. In a three phase system the phases are

spaced equally, giving a phase separation of one-third of one cycle.

Generators output at a voltage that ranges from hundreds of volts to

30,000 volts.

4. Unit transformer (3-phase)At the power station, transformers step-up this voltage to one more

suitable for transmission. After numerous further conversions in the

transmission and distribution network the power is finally transformed

to the standard mains voltage (i.e. the “household” voltage). The

power may already have been split into single phase at this point or it

may still be three phase. Where the step-down is three phase at the

receiving stage, the output of this transformer is usually star

connected with the standard mains voltage being the phase-neutral

voltage. Another system commonly seen in North America is to have

a delta connected secondary with a center tap on one of the

windings supplying the ground and neutral. This allows for 240 V

three phase as well as three different single phase voltages( 120 V

between two of the phases and neutral , 208 V between the third

phase ( or wild leg) and neutral and 240 V between any two phase)

to be available from the same supply.

A unit Transformer

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5. Electrical generatorAn Electrical generator is a device that converts kinetic energy to

electrical energy, generally using electromagnetic induction. The task

of converting the electrical energy into mechanical energy is

accomplished by using a motor. The source of mechanical energy

maybe water falling through the turbine or steam turning a turbine (as

is the case with thermal power plants). There are several

classifications for modern steam turbines. Steam turbines are used in

our entire major coal fired power stations to drive the generators or

alternators, which produce electricity. The turbines themselves are

driven by steam generated in "boilers “or "steam generators" as they

are sometimes called. Electrical power stations use large steam

turbines driving electric generators to produce most (about 86%) of

the world’s electricity. These centralized stations are of two types:

fossil fuel power plants and nuclear power plants. The turbines used

for electric power generation are most often directly coupled to their-

generators .As the generators must rotate at constant synchronous

speeds according to the frequency of the electric power system, the

most common speeds are 3000 r/min for 50 Hz systems, and 3600

r/min for 60 Hz systems. Most large nuclear sets rotate at half those

speeds, and have a 4-pole generator rather than the more common

2-pole one.

An electric generator with an excitor

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6. Low Pressure TurbineEnergy in the steam after it leaves the boiler is converted into

rotational energy as it passes through the turbine. The turbine

normally consists of several stages with each stages consisting of a

stationary blade (or nozzle) and a rotating blade. Stationary blades

convert the potential energy of the steam into kinetic energy and

direct the flow onto the rotating blades. The rotating blades convert

the kinetic energy into impulse and reaction forces, caused by

pressure drop, which results in the rotation of the turbine shaft. The

turbine shaft is connected to a generator, which produces the

electrical energy. Low Pressure Turbine (LPT) consists of 2x6

stages. After passing through Intermediate Pressure Turbine steam

is passed through LPT which is made up of two parts- LPC REAR &

LPC FRONT. As water gets cooler here it gathers into a HOTWELL

placed in lower parts of turbine.

7. Feed Water PumpA Boiler feed water pump or simply a feed water pump is a specific

type of pump used to pump water into a steam boiler. The water may

be freshly supplied or returning condensation of the steam produced

by the boiler. These pumps are normally high pressure units that use

suction from a condensate return system and can be of the

centrifugal pump type or positive displacement type. Feed water

pumps range in size up to many horsepower and the electric motor is

usually separated from the pump body by some form of mechanical

coupling. Large industrial condensate pumps may also serve as the

feed water pump. In either case, to force the water into the boiler, the

pump must generate sufficient pressure to overcome the steam

pressure developed by the boiler. This is usually accomplished

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through the use of a centrifugal pump. Feed water pumps usually run

intermittently and are controlled by a float switch or other similar

level-sensing device energizing the pump when it detects a lowered

liquid level in the boiler. Some pumps contain a two-stage switch. As

liquid lowers to the trigger point of the first stage, the pump is

activated. If the liquid continues to drop, (perhaps because the pump

has failed, its supply has been cut off or exhausted, or its discharge

is blocked) the second stage will be triggered. This stage may switch

off the boiler equipment (preventing the boiler from running dry and

overheating); trigger an alarm, or both.

8. CondenserThe steam coming out from the Low Pressure Turbine (a little above

its boiling pump) is brought into thermal contact with cold water

(pumped in from the cooling tower) in the condenser, where it

condenses rapidly back into water, creating near Vacuum-like

conditions inside the condenser chest allowing it to be pumped. If the

condenser can be made cooler, the pressure of the exhaust steam is

reduced and efficiency of the cycle increases. The surface

condenser is a shell and tube heat exchanger in which cooling water

is circulated through the tubes. The exhaust steam from the low

pressure turbine enters the shell where it is cooled and converted to

condensate (water) by flowing over the tubes as shown in the

adjacent diagram. Such condensers use steam ejectors or rotary

motor-driven exhausters for continuous removal of air and gases

from the steam side to maintain vacuum.

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9. Intermediate Pressure TurbineIntermediate Pressure Turbine (IPT) consists of 12 stages. When the

steam has been passed through HPT it enters into IPT. IPT has two

ends named as FRONT & REAR. Steam enters through front end

and leaves from Rear end.

10. Steam Governor ValveSteam locomotives and the steam engines used on ships and

stationary applications such as power plants also required feed water

pumps. In this situation, though, the pump was often powered using

a small steam engine that ran using the steam produced by the boiler

a means had to be provided, of course, to put the initial charge of

water into the boiler (before steam power was available to operate

the steam-powered feed water pump).The pump was often a positive

displacement pump that had steam valves and cylinders at one end

and feed water cylinders at the other end; no crankshaft was

required. In thermal plants, the primary purpose of surface

condenser is to condense the exhaust steam from a steam turbine to

obtain maximum efficiency and also to convert the turbine exhaust

steam into pure water so that it may be reused in the steam

generator or boiler as boiler feed water. By condensing the exhaust

steam of a turbine at a pressure below atmospheric pressure, the

steam pressure drop between the inlet and exhaust of the turbine is

increased, which increases the amount heat available for conversion

to mechanical power. Most of the heat liberated due to condensation

of the exhaust steam is carried away by the cooling medium (water

or air) used by the surface condenser. Control valves are valves

used within industrial plants and elsewhere to control operating

37

conditions such as temperature, pressure, flow and liquid level by

fully or partially opening or closing in response to signals received

from controllers that compares a “set point” to a “process variable”

whose value is provided by sensors that monitor changes in such

conditions. The opening or closing of control valves is done by

means of electrical, hydraulic or pneumatic systems.

11. High Pressure TurbineSteam coming from Boiler directly feeds into HPT at a temperature of

540°C and at a pressure of 170 kg/cm2. Here it passes through 12

different stages due to which its temperature goes down to 350°C

and pressure as 45 kg/cm2. This line is also called as CRH – COLD

REHEAT LINE. It is now passed to a REHEATER where its

temperature rises to 540°C and called as HRH-HOT REHEATED

LINE.

12. DeaeratorA Deaerator is a boiler feed device for air removal and used to

remove dissolved gases (an alternate would be the use of water

treatment chemicals) from boiler feed water to make it noncorrosive.

A deaerator is an open type feed water heater. A dearator typically

includes a vertical domed deaeration section as the deaeration boiler

feed water tank. A steam generating boiler requires that the

circulating steam, condensate, and feed water should be devoid of

dissolved gases, particularly corrosive ones and dissolved or

suspended solids. The gases will give rise to corrosion of the metal.

The solids will deposit on the heating surfaces giving rise to localized

heating and tube ruptures due to overheating. Under some

conditions it may give rise to stress corrosion cracking. Deaerator

38

level and pressure must be controlled by adjusting control valves the

level by regulating condensate flow and the pressure by regulating

steam flow. If operated properly, most deaerators will guarantee that

oxygen in the deaerated water will not exceed 7 ppb by weight

(0.005 cm3/L).

13. Feed water heaterA Feed water heater is a power plant component used to pre-heat

water delivered to a steam generating boiler. Preheating the feed

water reduces the irreversibility involved in steam generation and

therefore improves the thermodynamic efficiency of the system. This

reduces plant operating costs and also helps to avoid thermal shock

to the boiler metal when the feed water is introduced back into the

steam cycle. In a steam power (usually modeled as a modified

Rankine cycle), feed water heaters allow the feed water to be

brought up to the saturation temperature very gradually. This

minimizes the inevitable irreversibility associated with heat transfer to

the working fluid (water).

14. Coal conveyorCoal conveyors are belts which are used to transfer coal from its

storage place to Coal Hopper. A belt conveyor consists of two

pulleys, with a continuous loop of material- the conveyor Belt – that

rotates about them. The pulleys are powered, moving the belt and

the material on the belt forward. Conveyor belts are extensively used

to transport industrial and agricultural material, such as grain, coal,

ores etc.

39

15. Coal HopperCoal Hoppers are the places which are used to feed coal to Coal Mill.

It also has the arrangement of entering Hot Air at 200°C inside it

which solves our two purposes:

1. If our Coal has moisture content then it dries it so that a proper

combustion takes place.

2. It raises the temperature of coal so that its temperature is more

near to its Ignite Temperature so that combustion is easy.

16. Pulverized Coal MillA pulverizer is a mechanical device for grinding coal for combustion

in a furnace in a Thermal power plant.

17. Boiler drumSteam Drums are a regular feature of water tube boilers. It is

reservoir of water/steam at the top end of the water tubes in the

water-tube boiler. They store the steam generated in the water tubes

and act as a phase separator for the steam/water mixture. Usually,

the boiler drum is at an elevation of 75m. The difference in densities

between hot and cold water helps in the accumulation of the “hotter”-

water/and saturated –steam into steam drum. Made from high-grade

steel (probably stainless) and its working involve temperature of

390°C and pressure well above 350psi (2.4MPa). The separated

steam is drawn out from the top section of the drum. Saturated

Steam is drawn off the top of the drum. The steam will re-enter the

furnace in through a super heater, while the saturated water at the

bottom of steam drum flows down to the mud-drum /feed water drum

by down comer tubes accessories include a safety valve, water level

40

indicator and fuse plug. A steam drum is used in company of a mud-

drum/feed water drum which is located at a lower level. So that it

acts as a sump for the sludge or sediments which have a higher

tendency at the bottom.

18. Ash HopperA steam drum is used in the company of a mud-drum/feed water

drum which is located at a lower level. So that it acts as a sump for

the sludge or sediments which have a tendency to accumulate at the

bottom.

19. Super HeaterA Super heater is a device in a steam engine that heats the steam

generated by the boiler again increasing its thermal energy. Super

heaters increase the efficiency of the steam engine, and were widely

adopted. Steam which has been superheated is logically known as

superheated steam; non- superheated steam is called saturated

steam or wet steam. Super heaters are being applied most stationary

steam engines including power stations. The dry steam coming out

of the boiler drum passes through three stages of superheating.

Initially the main steam is passed through a low temperature super

heater followed by a divisional panel super heater and finally through

a platen super heater. The resulting steam obtained will be at 540o C

this is sent to the inlet of the HP turbine.

20. Force Draught FanExternal fans are provided to give sufficient air for combustion. The

forced draught fan takes air from the atmosphere and, warms it in the

41

air pre heater for better combustion, injects it via the air nozzles on

the furnace wall.

21. Re heaterRe heater is a heater which is used to raise the temperature of steam

which has exhausted from the high pressure turbine. The steam

entering the re heater is known as Cold Reheat (CR). The steam

leaving the re heater is known as Hot Reheat (HR).

22. Air IntakeAir is taken from the environment by an air intake tower which is fed

to the fuel.

23. EconomizerEconomizers are mechanical devices intended to reduce energy

consumption, or to perform another useful function like preheating a

fluid. The term economizer is used for other purposes as well-Boiler,

power plant, heating, ventilating and air-conditioning. In boilers,

economizer are heat exchange devices that heat fluids , usually

water, up to but not normally beyond the boiling point of the fluid.

Economizers are so named because they can make use of the

enthalpy and improving the boiler’s efficiency. They are devices fitted

to a boiler which save energy by using the heat from the exhaust

gases from the boiler to preheat the cold water used to fill it (the feed

water). Modern day boilers, such as those in cold fired power

stations, are still fitted with economizer which is decedents of

Green’s original design. In this context there are turbines before it is

pumped to the boilers. A common application of economizer in steam

power plants is to capture the waste heat from boiler stack gases

42

(flue gas) and transfer thus it to the boiler feed water thus lowering

the needed energy input , in turn reducing the firing rates to

accomplish the rated boiler output . Economizer lower stack

temperatures which may cause condensation of acidic combustion

gases and serious equipment corrosion damage if care is not taken

in their design and material selection.

24. Air Pre heaterAir pre heater is a general term to describe any device designed to

heat air before another process (for example, combustion in a boiler).

The purpose of the air pre heater is to recover the heat from the

boiler flue gas which increases the thermal efficiency of the boiler by

reducing the useful heat lost in the flue gas. As a consequence, the

flue gases are also sent to the flue gas stack (or chimney) at a lower

temperature allowing simplified design of the ducting and the flue gas

stack. It also allows control over the temperature of gases leaving the

stack (chimney).

25. Electrostatic Precipitator (ESP)An Electrostatic precipitator (ESP) or electrostatic air cleaner is a

particulate device that removes particles from a flowing gas (such as

air) using the force of an induced electrostatic charge. Electrostatic

precipitators are highly efficient filtration devices, and can easily

remove fine particulate matter such as dust and smoke from the air

steam. ESPs continue to be excellent devices for control of many

industrial particulate emissions, including smoke from electricity-

generating utilities (coal and oil fired), salt cake collection from black

liquor boilers in pump mills, and catalyst collection from fluidized bed

catalytic crackers from several hundred thousand ACFM in the

43

largest coal-fired boiler applications. The original parallel plate-

Weighted wire design (described above) has evolved as more

efficient (and robust) discharge electrode designs, today focus is on

rigid discharge electrodes to which many sharpened spikes are

attached , maximizing corona production. Transformer –rectifier

systems apply voltages of 50-100 Kilovolts at relatively high current

densities. Modern controls minimize sparking and prevent arcing,

avoiding damage to the components. Automatic rapping systems and

hopper evacuation systems remove the collected particulate matter

while on line allowing ESPs to stay in operation for years at a time.

26. Induced Draught FanThe induced draft fan assists the FD fan by drawing out combustible

gases from the furnace, maintaining a slightly negative pressure in

the furnace to avoid backfiring through any opening. At the furnace

outlet and before the furnace gases are handled by the ID fan, fine

dust carried by the outlet gases is removed to avoid atmospheric

pollution. This is an environmental limitation prescribed by law, which

additionally minimizes erosion of the ID fan.

27. Flue gas stackA Flue gas stack is a type of chimney, a vertical pipe, channel or

similar structure through which combustion product gases called flue

gases are exhausted to the outside air. Flue gases are produced

when coal, oil, natural gas, wood or any other large combustion

device. Flue gas is usually composed of carbon dioxide (CO2) and

water vapor as well as nitrogen and excess oxygen remaining from

the intake combustion air. It also contains a small percentage of

pollutants such as particulates matter, carbon mono oxide, nitrogen

44

oxides and sulphur oxides. The flue gas stacks are often quite tall, up

to 400 meters (1300 feet) or more, so as to disperse the exhaust

pollutants over a greater area and thereby reduce the concentration

of the pollutants to the levels required by government's

environmental policies and regulations.

45

The Layout of NTPC Simhadri

The plant consists of two stages: Stage 1 (consisting of unit 1 and

unit 2) and Stage 2 (consisting of unit 3 and unit 4).Each unit has an

average capacity of 500MW.The boilers used in all the units are sub

critical type and employ tilting tangential firing. Each unit of stage 1

comprises of nine coal mills (bowl mills) while each unit of stage 2

consists of ten coal mills. In addition to, an HP turbine and an LP

turbine the plant uses an IP turbine too. Each pressure part in a unit

employs three pumps out of which one is a standby and two are

under service. Similarly, each unit uses four air pre heaters; two are

under service while the other two are for standby. The plant uses DM

water for steam generation and raw water for cooling purpose. The

plant uses Natural Draught Cooling System. The lube oil that is used

for lubrication and cooling purpose is Servo prime 46. For governing

the speed of the turbine throttle governing is employed. The output of

the plant is distributed and transmitted through a three phase

transmission system (Switch yard). The switch yard is of a one and

half breaker bus configuration. It uses Global Positioning System for

time synchronization. The plant uses a two pole synchronous

brushless generator. (Water cooled stator and hydrogen cooled

rotor).

46A GENERAL LAYOUT OF A UNIT OF NTPC SIMHADRI

47

BOILER MAINTAINANCE

DEPARTMENT

48

Boiler and its auxiliaries

Boiler:

According to IBR, any closed vessel exceeding 22.75 liters in capacity

and which is used expressively for generating steam under pressure and

includes any mounting or other fitting attached to such vessel, which is

wholly, or partly under pressure when the steam is shut off can be

termed as a steam boiler. A boiler is the central or an important

component of the thermal power plant which focuses on producing

superheated steams that is used for running of the turbines which in turn

is used for the generation of electricity. A boiler is a closed vessel in

which the heat produced by the combustion of fuel is transferred to

water for its conversation into steam of the desired temperature &

pressure. The steam generating boiler has to produce steam at the

highest purity, pressure and temperature required for the steam

turbine that drives the electrical generator.

The heat-generating unit includes a furnace in which the fuel is burned.

With the advantage of water-cooled furnace walls, super heaters, air

heaters and economizers, the term steam generator was evolved as a

better description of the apparatus.

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 (60mm) 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 center. 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

49

absorbs heat and changes into steam at 370 °C and 3,200 psi (22.1MPa). It

is separated from the water inside a drum at the top of the furnace. 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 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 super heating 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 pre heater (APH),

boiler furnace, induced draft (ID) fan, fly ash collectors (electrostatic

precipitator or bag house) and the flue gas stack.

Construction of boilers is mainly of steel stainless steel a n d

wrought i ron. In l ive steam models, copper or brass is often use.

An internal section of a boiler49

















An internal section of a boiler49

















An internal section of a boiler

50

For utility purpose, it should generate steam uninterruptedly at operating

pressure and temperature for running steam turbines.

Boilers may be classified on the basis of any of the following

characteristics:

Use

Pressure

Materials

Size

Tube Content

Tube Shape and position

Firing

Fuel

Fluid

Circulations

Furnace position

Furnace type

General shape

Trade name

Special features.

Use: The characteristics of the boiler vary according to the nature of

service performed. Customarily boiler is called either stationary or

mobile. Large units used primarily for electric power generation are

known as control station steam generator or utility plants.

Pressure: To provide safety control over construction features, all boilers

must be constructed in accordance with the Boiler codes, which

differentiates boiler as per their characteristics. Boilers with operating

pressures above 224 kgf/cm2 are known as supercritical boilers, while

51

boilers with operating pressures below 224 kgf/cm2 are known as

subcritical boilers.

Materials: Selection of construction materials is controlled by boiler code

material specifications. Power boilers are usually constructed of special

steels.

Size: Rating code for boiler standardize the size and ratings of boilers

based on heating surfaces. The same is verified by performance tests.

Tube Contents: In addition to ordinary shell type of boiler, there are two

general steel boiler classifications, the fire tube and water tube boilers.

Fire tube boiler is boilers with straight tubes that are surrounded by

water and through which the products of combustion pass. Water tube

boilers are those, in which the tubes themselves contain steam or water,

the heat being applied to the outside surface.

Firing: The boiler may be a fired or unfired pressure vessel. In fired

boilers, the heat applied is a product of fuel combustion. A non-fired

boiler has a heat source other than combustion.

Fuel: Boilers are often designated with respect to the fuel burned.

Fluid: The general concept of a boiler is that of a vessel to generate

steam. A few utility plants have installed mercury boilers.

Circulation: The majority of boilers operate with natural circulation. Some

utilize positive circulation in which the operative fluid may be forced

'once through' or controlled with partial circulation.

Furnace Position: The boiler is an external combustion device in which

the combustion takes place outside the region of boiling water. The

relative location of the furnace to the boiler is indicated by the

description of the furnace as being internally or externally fired.

The furnace is internally fired if the furnace region is completely

surrounded by water.

52

Furnace type: The boiler may be described in terms of the furnace type.

General Shape: During the evaluation of the boiler as a heat producer,

many new shapes and designs have appeared and these are widely

recognized in the trade.

Trade Name: Many manufacturers coin their own name for each boiler

and these names come into common usage as being descriptive of the

boiler.

Special features: Sometimes the type of boiler like differential firing and

Tangential firing are employed. NTPC Simhadri uses tangential firing.

Boilers are generally categorized as follows:

• Steel boilers

• Fire Tube type

• Water tube type

• Horizontal Straight tube

Fire tube boiler type:

Fire-tube boilers rely on hot gases circulating through the boiler inside

tubes that are submerged in water. These gases usually make several

passes through the tubes, thereby transferring their heat through the

tube walls and causing the water to boil on the other side. Fire-tube

boilers are generally available in the range of 20 through 800 boiler

horsepower (BHP) and in pressures up to 150 psi.

Water tube boiler type:

Here the heat source is outside the tubes and the water to be heated is

inside. Most high-pressure and large boilers are of this type. In the

water-tube boiler, gases flow over water-filled tubes. These water-filled

tubes are in turn connected to large containers called drums.

53

The boiler mainly has natural circulation of gases, steam and other

things. They contain vertical membrane water. The pulverized fuel which

is being used in the furnace is fixed tangentially. They consume

approximately 350 ton/hr of coal of about 1370kg/cm2 of pressure

having temperature of 540oC. The first pass of the boiler has a

combustion chamber enclosed with water walls of fusion welded

construction on all four sides. In addition there are four water platens to

increase the radiant heating surface.

Beside this platen super heater re heater sections are also suspended in

the furnace combustion chamber. The first pass is a high heat zone

since the fuel is burn in this pass.

The second pass is surrounded by steam cooled walls on all four sides

as well as roof of the boiler. A horizontal super heater, an economizer &

two air heaters are located in the second pass.

Large boiler capacities are often specified in terms of tons of steam

evaporated per hour under specified steam conditions.

Raw materials for boilers:

• Coal from mines

• Ambient air

• Water from natural resources (river, ponds)

• Generating heat energy

• Air for combustion

• Working fluid for steam generation, possessing heat energy

A 500MW steam generator consumes about 8000 tons of coal everyday. It will be considered good, if it requires about 200 cubic meter of

DM water in a day. It will produce about 9500 tons of Carbon dioxide

every day.

54

Specifications of the boiler (at 100% load)

1) Boiler type: radiant reheat, controlled circulation with rifle tubing, dry

bottom, single drum, dry-bottom type unit, top supported, balanced

draft furnace. (BHEL make).

2) Evaporation SH outlet : 1.725 t/hr

RH outlet : 1.530 t/hr

3) Water Pressure after stop valve : 178 kgf/cm2

4) Steam Temperature at SH outlet: : 5400C

5) Steam Temperature at RH inlet: : 344.10C

6) Steam Temperature at RH outlet: : 5400C

7) Steam Pressure at RH inlet : 42.85 kgf/cm2

8) Steam Pressure at RH outlet: : 43.46 kgf/cm2

9) Feed Water Temperature at ECO : 2560C

10) Furnace Design Pressure : +660 mmwc

55

Boiler drum

It is a type of storage tank much higher placed than the level at which

the boiler is placed, and it is also a place where water and steam are

separated. First the drum is filled with water coming from the

economizer, from where it is brought down with the help of down-

comers, entering the bottom ring headers. From there they enter the

riser, which are nothing but tubes that carries the water (which now is a

liquid-vapor mixture), back to the drum. Now, the steam is sent to the

super heaters while the saturated liquid water is again circulated through

the down-comers and then subsequently through the risers till all the

water in the drum turns into steam and passes to the next stage of

heating that is superheating.

NOTE: For a 660 MW plant, the boiler does not employ any drum;

instead the water and steam go directly into the super heater because

the pressure employed being higher than the critical pressure of water

on further stages of heating will eventually turn completely into steam

without absorbing any latent heat of vaporization since the boiling part in

the T-s curve no longer passes through the saturation dome rather its

goes above the dome.

Sub-critical boiler Super-critical boiler

56

The boiler drum is of fusion-welded design with welded hemi-spherical

dished ends. It is provided with stubs for welding all the connecting

tubes i.e. down comers, risers, pipes, saturated steam out let .

The funct ion of s team drum internals is to separate th e

water from the steam generated in the furnace walls and to reduce the

dissolved solid contents of the steam below the prescribed limit of 1ppm

and also take care of the sudden change of steam demand for boiler.

The secondary stage of two opposed 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 f lows upwards

to the series of screen dryers, extending in layers across the length of

the drum. These screens perform the final stage of separation.

In the boiler drum, steam volume increases to 1,600 times from waterand produces tremendous force

Steam Drum Internals

57

In the boiler drum, the steam volume increases to 1,600 times from

water and produces tremendous force. The working fluid within the boiler

drum undergoes evaporation. It is supported on U-structures suspended

on a rigid supporting beam.

Boiler Drum Specifications

Boiler drum lifting in progress

57







57







58

The steam drum contains steam separating equipment and internal

piping for distribution of chemicals to the water, for distribution of feed

water and for blow down of the water to reduce solids concentration.

Steam drum internal view

Steam separator

58





Steam separator

58





Steam separator

59

Once water enters 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/vapor in

the water walls, the steam/vapor once again enters the steam drum.

The steam/vapor 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 trip out are avoided by flushing out such gases from the combustion

zone before igniting the coal. The steam drum (as well as the super

heater 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 super heater coils.

60

Boiler Furnace

Furnace is primary part of boiler where the c h e m i c a l e n e r g y o f

f u e l i s c o n v e r t e d t o t h e r m a l e n e r g y b y

c o m b u s t i o n . F u r n a c e i s d e s i g n e d f o r e f f i c i e n t

a n d c o m p l e t e combustion. Major factors that assist for

efficient combustion are amount of fuel ins ide the furnace

and turbulence, which causes rapid mixing between fuel and air.

In modern boilers, water-cooled furnaces are used. In general, oil fired

furnace is employed in the boiler. Normally about 65% of furnace volume

is enough for an oil-fired boiler as compared to the corresponding P.F.

fired boiler. Oil-fired furnace is generally closed at the bottom, as

there is no need to remove slag as in case of P.F. fired boiler. The

bottom part will have small amount of slope to prevent film boiler

building in the bottom tubes. If boiler has to design for both P.F. as

well as oil, the f u r n a c e h a s t o b e d e s i g n e d f o r c o a l , a s

o t h e r w i s e h i g h e r h e a t loading wi th P.F. wi l l cause

slogging and high furnace exi t gas temperature.

The furnace walls are composed of tubes. The space between the tubes

is fusion welded to form a complete gas tight seal. The furnace arch is

composed of fusion welded tubes. The furnace extended side walls are

composed of fin welded tubes. The back pass front (furnace) roof is

compared of tubes peg fin welded. The spaces between the tubes and

openings are closed with fin material so a completely metallic surface is

exposed to the hot furnace gases. Poured insulation is used at each

horizontal buck stay to form a continuous band around the furnace

thereby preventing flue action of gases between the casing and water

walls. Bottom designs used in these coal fired units are of the open

hopper type, often referred to as the dry bottom type.

61

A water cooled furnace

62

Super Heaters

The steam from the boiler drum is then sent for superheating. This takes

place in three stages. In the first stage, the steam is sent to a simple

super heater, known as the low temperature super heaters (LTSH), after

which the second stage consists of several divisional panel super

heaters (DPSH) or radiant pendent super heaters (RPSH). The final

stage involves further heating in the Platen super heaters (PLSH), after

which the steam is sent through the Main Steam (MS) piping for driving

the turbine.

Superheating is done to increase the dryness fraction of the exiting

steam. This is because if the dryness fraction is low, as is the case with

saturated steam, the presence of moisture can cause corrosion of the

blades of the turbine. Super heated steam also has several merits such

as increased working capacity, ability to increase the plant efficiency,

lesser erosion and so on. It is also of interest to know that while the

super heater increases the temperature of the steam, it does not change

the pressure. There are different stages of super heaters besides the

sidewalls and extended sidewalls. The first stage consists of LTSH (low

temperature super heater), which is conventional mixed type with upper

& lower banks above the economizer assembly in rear pass. The other is

Divisional Panel Super heater which is hanging above in the first pass of

the boiler above the furnace. The third stage is the Platen Super heater

(placed above the furnace in convection path) from where the steam

goes into the HP turbine through the main steam line. The outlet

temperature & pressure of the steam coming out from the super heater

is 5400 Celsius & 157 kg/cm2. After the HP turbine part is crossed the

steam is taken out through an outlet as CRH (Cold Re-heat steam) to be

re-heated again as HRH (Hot Re-heat steam) and then is fed to the IPT

63

(Intermediate pressure turbine) which goes directly to the LPT (Low

pressure turbine) through the IP-LP cross-over.

The enthalpy rise of steam in a given section of the super heater should

not exceed

250 – 420 kJ/kg for High pressure. > 17 MPa

< 280 kJ/kg for medium pressure. 7 Mpa – 17 MPa

< 170 kJ/kg for low pressure. < 7 MPa

Convective Super heaters

64

Platen Super heaters

Pendant Super heaters

65

Super heater specifications

LTSH DPSH PSHNo. of tubes 744 432 400Outer dia inmm

44.5 44.5 54.0

Joining Butt Butt ButtMax. steamtemperature

405 (H)444 (P)

513 550

Max. gastemperature

450 (H)469 (P)

524 629

66

Water walls

The water from the bottom ring header is then transferred to the water

walls, where the first step in the formation of steam occurs by absorbing

heat from the hot interior of the boiler where the coal is burned

continuously. This saturated water steam mixture then enters the boiler

drum.

In a 500 MW unit, the water walls are of vertical type, and have rifled

tubing whereas in a 660 MW unit, the water walls are of spiral type till an

intermediate ring header from where it again goes up as vertical type

water walls. The advantage of the spiral wall tubes ensures an even

distribution of heat, and avoids higher thermal stresses in the water walls

by reducing the fluid temperature differences in the adjacent tubes and

thus minimizes the sagging produced in the tubes.

The above figure depicts the difference between the vertical waterwall and the spiral water wall type of tubing where the vertical waterwalls have the rifle type of tubes to increase the surface area unlikethe spiral ones that have plain, smooth surfaces.

67

Heating and evaporation of feed water supplied to the boiler from the

economizers takes place within the water tubes. These are vertical tubes

connected at the top and bottom to the headers. These tubes receive

water from the boiler drum by means of down comers connected

between drum and water walls lower header. Approximately 50% of the

heat released by the combustion of the fuel in the furnace is absorbed

by the water walls.

Tangent tube The construction consists of water wall placed side by

side nearly touching each other. An envelope of thin sheet of steel called

"SKIN CASING" is placed in contact with the tubes, which provides a

seal against furnace leakage.

Membrane Water tube A number of tubes are joined by a process of

fusion welding or by means of steel strips called 'fins pressurized

furnace is possible with the related Advantages

Tangent water tube

67







by the water walls.








Tangent water tube

67







by the water walls.








Tangent water tube

68

• Increase in efficiency

• Better load response simpler combustion control.

• Quicker starting and stopping

• Increased availability of boiler.

• Heat transfer is better

• Weight is saved in refractory and structure

• Erection is made easy and quick

Down comers

There are six down comers in (500 MW) which carry water from boiler

drum to the ring header. They are installed from outside the furnace

to keep density difference for natural circulation of water & steam.

Membrane water tube

68








Down comers




Membrane water tube

68








Down comers




Membrane water tube

69

Water wall specifications

Front

Wall

Side

Wall

Rear

Wall

Roof

OD (mm) 51 51 51 57

D.thickness 5.6 5.6 5.6 6.3

Joining BUTT BUTT BUTT BUTT

Design pressure

of tube

208.8 208.8 208.8 203.7

Max. Pressure

of tube

197.8 197.8 197.8 192.7

DES.MET.TEMP 394 394 394 412

70

Safety valves

Device attached to the boiler for automatically relieving the pressure of

steam before it becomes great enough to cause bursting. The common

spring-loaded type is held closed by a spring designed to open the valve

when the internal pressure reaches a point in excess of the calculated

safe load of the boiler. Safety valves are installed on boilers according to

strict safety norms and IBR recommendation.

Boiler stop valves

A steam boiler must be fitted with a stop v a l v e ( a l s o k n o w n

a s a c r o w n v a l v e ) w h i c h i s o l a t e s t h e

s t e a m boiler and i ts pressure from the process or plant. I t

is general ly an angle pattern globe valve of the screw-down variety.

A spring loaded safety valve

70

Safety valves







Boiler stop valves






70

Safety valves







Boiler stop valves






71

The stop valve is not designed as a t h r o t t l i n g v a l v e , a n d

s h o u l d b e f u l l y o p e n o r c l o s e d . I t s h o u l d always be

opened slowly to prevent any sudden rise in downstream pressure and

associated water hammer, and to help restrict the fall in boiler

pressure and any possible associated priming.

Three types of safety valves are commonly employed at NTPC Simhadri

Electrically operated valve

Pneumatically operated valve

Manually operated valve

Boiler stop valve

72

Economizer

The economizer is a tube-shaped structure which contains water from

the boiler feed pump. This water is heated up by the hot flue gases

which pass through the economizer layout, which then enters the drum.

The economizer is usually placed below the second pass of the boiler,

below the Low Temperature Super heater. As the flue gases are being

constantly produced due to the combustion of coal, the water in the

economizer is being continuously being heated up, resulting in the

formation of steam to a partial extent. Economizer tubes are supported

in such a way that sagging, deflection & expansion will not occur at any

condition of operation. In other words, Boiler Economizers are feed-

water heaters in which the heat from waste gases is recovered to raise

the temperature of feed-water supplied to the boiler. It reduces the

exhaust gas temperature and saves the fuel. Modern power plants use

steel-tube-type economizers. It is divided into several sections of 0.6 –

0.8 m gap.

An Economizer

73

6oC raise in feed water temperature by the economizer corresponds to a

1% saving in fuel consumption. 220 C reduction in flue gas temperature

increases the boiler efficiency by 1%.

Location and arrangement

Ahead of air-heaters

Following the primary super-heater or re-heater

Counter-flow arrangement

Horizontal placement (to facilitate draining)

Stop valve and non-return valve incorporated to ensure

recirculation in case of no feed-flow

Plain tube: Several banks of tubes with either-in-line or staggered

type formation which induces more turbulence than the in-line

arrangement. This gives a higher rate of heat transfer and requires

less surface but at the expense of higher draught loss.

74

Welded Fin- tube: Fin welded design is used for improving the heat

transfer.

Feed pipe: Any pipe or connected fitting wholly or partly under pressure

through which feed water passes directly to a Boiler and which does not

form an integral part thereof.

Steam pipe: Any pipe through which steam passes from a Boiler to a

prime mover or other user or both, if the pressure at which steam passes

through such pipe exceeds 3. 5 Kilograms per square centimeter above

atmospheric pressure or such pipe exceeds 254 millimeters in internal

diameter.

Economizer Specifications

Material Carbon steel

SA210 GRA1

No. of coils 184

Outer diameter of tubes (in mm) 38.1

Actual thickness 5.3

Des.pr of tubes 217.8

Des.pr of headers 219.7

Fin welded design

74


transfer.








diameter.



SA210 GRA1

No. of coils 184





Fin welded design

74


transfer.








diameter.



SA210 GRA1

No. of coils 184





Fin welded design

75

Deaerator

A deaerator is a device that is widely used for the removal of air and

other dissolved gases from the feed water to steam-generating boilers.

In particular, dissolved oxygen in boiler feed water will cause serious

corrosion damage in steam systems by attaching to the walls of metal

piping and other metallic equipment and forming oxides (rust). Water

also combines with any dissolved carbon dioxide to form carbonic acid

that causes further corrosion. Most deaerators are designed to remove

oxygen down to levels of 7 ppb by weight (0.005 cm³/L) or less.

There are two basic types of deaerators, the tray-type and the spray-

type:

The tray-type (also called the cascade-type) includes a vertical

domed deaeration section mounted on top of a horizontal

cylindrical vessel which serves as the deaerated boiler feed water

storage tank.

The spray-type consists only of a horizontal (or vertical) cylindrical

vessel which serves as both the deaeration section and the boiler

feed water storage tank.

76

Re heater

Purpose: to re-heat the steam from HP turbine to 5400C

It is composed of three sections:

radiant wall re heater arranged in front & side water walls

rear pendant section arranged above goose neck

front section arranged between upper heater platen & rear water

wall hanger tubes

The arrangement and construction of a re-heater is similar to that of a

super-heater. In large modern boiler plant, the reheat sections are mixed

equally with super-heater sections. The pressure drop inside re-heater

tubes has an important adverse effect on the efficiency of turbine.

Pressure drop through the re-heater should be kept as low as possible.

The tube diameter is to be kept between 42 – 60mm. Its design is similar

to convective super-heaters. The Overall Heat Transfer Coefficient lies

between 90 – 110 W/m2 K. Reheating is another method of increasing

the cycle efficiency.

Re heater specifications

Max. operating pressure in kgf/cm2 46.7

Design pressure in kgf/cm2 52.4

Max. steam temperature in 0C 540

Max. gas side mean temp in 0C 593

Outer diameter (in mm) 54.0

Total no. of tubes 888

Joining butt

77

Coal system: Coal burners

Coal burners comprise of a coal nozzle, steel tip, seal plate and tilting

link mechanism, housed in coal compartment in all four corners of the

furnace and connected with coal pipes. One end (outlet) is rectangular

and another end is cylindrical. The burner can be tilted on a pivot pin.

The angle of tilt for the burner is about -300 to +300. The nozzle tip has

separate coal and air passages. Coal and air passages are divided into

several parts. Each boiler of one unit consists of eight pulverized coal

burners. The pulverized coal is mixed with primary air flow which carries

the coal mixture to each of the four corners of the furnace burner

nozzles and into the furnace. Coal is pulverized to achieve optimum

efficiency.

Coal burners

78

Fuel- Oil system

Purpose:

(a) To establish initial boiler light up.

(b) To support the furnace flame during low load operation up to 15%

MCR load.

The Fuel oil system consists of

Fuel oil Pumps

Oil heaters

Filters

Steam tracing lines

The main objective is to get filtered oil at correct pressure and

temperature.

The Fuel Oil system prepares any of the two designated fuel oil for use

in oil burners (16 per boiler, 4 per elevation) to establish the above two

stated purposes. To achieve this, the system incorporates fuel oil

pumps, oil heaters, and filters, steam tracing lines which together ensure

that the fuel oil is progressively filtered, raised in temperature, raised in

pressure and delivered to the oil burners at the requisite atomizing

viscosity for optimum efficiency in the furnace.

Both the oil and coal burner nozzles fire at a tangent to an imaginary

circle at the furnace centre. The turbulent swirling action thus produces,

promotes the necessary mixing of the fuels and air to ensure complete

combustion of the fuel. A vertical tilt facility of the burner nozzles, which

is controlled by the automatic control system of the boiler, ensures

constant reheat outlet steam temperature at varying boiler loads.

79

In the tangential firing system the furnace itself constitutes the burner.

Fuel and air are introduced through the furnace through four wind box

assemblies located in the furnace corners. The fuel and air streams from

the wind box nozzles are directed to a firing circle in the centre of the

furnace. The rotative or cyclonic action that is the characteristic of this

type of firing is most effective in turbulently mixing the burning fuel in a

constantly changing air and gas atmosphere.

Oil burners:

Design Considerations

• Atomization of oil

• Properly shaped jet

• Complete combustion

• Excess air should be minimum

• Ready accessibility for repairs

Tangential Firing in a boiler furnace

80

The three main oils used in the oil burners are:

a) Light Diesel Oil

b) Heavy fuel oil

c) Low sulphur heavy stock (LSHS).

Heavy oil guns are used for stabilizing flame at low load carrying. Warm

up oil guns are used for cold boiler warm up during cold start up and

igniters are used for start up and oil flame stabilizing.

Operating Principle (Atomization):

Atomization breaks the fuel into fine particles that readily mixes with the

air for combustion. Oil should be divided up into small particles for

effective atomization.

The advantages of atomization are:

a) Atomizing burners can be used with heavier grades of oil.

b) Can be adopted to large applications because of its large capacity

range.

c) Complete combustion is assured by the ability of the small particles to

penetrate in turbulent combustion.

Atomization of fuel oil is done by means of oil guns.

Oil burners are classified according to the method used for atomization,

as follows:

a) Air-atomized burners

b) Steam-atomized burners

c) Mechanically atomized burners

81

Air atomizing systems are not recommended for heavy oil system as

they tend to chill the oil and decrease atomization quality.

Steam atomization system uses auxiliary steam to assist in the

atomization of the oil. The steam used in this method should be slightly

superheated and free from moisture. As in the case of air atomizing

system, the steam here is used for both atomizing as well as heating the

fuel as it pass through the tip and into the furnace. The main advantages

of steam atomizing burners over other are:

a) Simplicity of its design

b) Initial cost of installation is low

c) Low pumping pressure

d) Low preheating temperature.

HFO being a highly viscous fluid is atomized using auxiliary steam. Upon

passing hot steam, the temperature of HFO increases, this decreases

the viscosity of HFO and hence the oil can be freely transported from the

oil sump to the boiler furnace. This process is known as Steam Tracing.

82

Wind box assembly

The fuel firing equipment consists of four wind box assemblies located in

the furnace corners. Each wind box assembly is divided in its height into

a number of sections or compartments. The coal components (fuel air

compartment) contain air (intermediate air compartments). Combustion

air (secondary air) is admitted to the intermediate air compartments and

each fuel compartment (around the fuel nozzle) through sets of lower

dampers. Each set of dampers is operated by a damper drive cylinder

located at the side of the wind box. The drive cylinder at each elevation

(25 m to 35 m) are operated either remote manually or automatically by

the secondary air damper control system. Some of the (auxiliary) air

components between coal nozzles contain oil guns. Retractable High

Energy Arc (HEA) igniters are located adjacent to the retractable oil

guns. These igniters directly light up the oil guns.

Wind box Arrangement

83

All auxiliary air dampers regulate the wind box to furnace DP as per the

set point which is generated with respect to Boiler Load Index. All fuel air

dampers regulate in proportion to the fuel firing rate. Oil dampers are

used to maintain a rich mixture of air/oil at the time of Oil Firing. Over fire

dampers are used to reduce SOx & NOx percentage.

The function of the wind box component dampers is to proportion the

amount of secondary air admitted to an elevation pf fuel components in

relation to that admitted to adjacent elevation of auxiliary air components

Wind box Arrangement

84

An overview of Firing System

85

Coal bunkers and Feeders

Coal Bunker: These are in process storage silos used for stor ing

crushed coal f rom the coal handl ing system. General ly,

these are made up of welded steel plates. Normally, there are six such

bunkers supplying coal of the corresponding mills. These are located on

top of the mills so as to aid in gravity feeding of coal.

Coal Feeder: Coal feeders are used to regulate the flow of coal from

bunker to the pulverizer. Each mill is provided with a drag link

chain/ rotary/ gravimetric feeder to t ransport raw coal f rom

the bunker to the in let chute, leading to mill at a desired rate.

There are principally three types of feeders namely:

Chain Feeder

Belt Feeder or gravimetric feeder

Table type belt Feeder

NTPC Simhadri employs gravimetric pulverizer to feed the Coal from

Bunker to Pulverizer as per requirement. It comprises of a leveling bar to

check the level of coal in the bunker. It uses a specialized belt conveyer

whose belt speed can be varied as per the requirement. The amount of

Coal entry is controlled by the speed of the drive pulley. The drive pulley

is connected through the motor with variable speed drive. Either a DC

Motor or a Motor with Magnetic clutch is used.

Gravimetric feeder

86

\

Gravimetric Feeder

Bunker and feederarrangement

Gravimetric Feeder used in NTPC Simhadri

87

Coal mills (Pulverizers)

As the name suggests the coal particles are grinded into finer sized

granules. The coal which is stored in the bunker is sent into the mill,

through the conveyor belt which primarily controls the amount of coal

required to be sent to the furnace. It on reaching a rotating bowl in the

bottom encounters three grinding rolls which grinds it into fine powder

form of approx. 200 meshes per square inch. the fine coal powder along

with the heated air from the FD and PA fan is carried into the burner as

pulverized coal while the trash particles are rejected through a reject

system.

Types of coal pulverizers include:

Impact

Attrition

Crushing

Sometimes these pulverizers employ all the three techniques all

together.

XRP(BHEL)

E MILLS(BABCOCK)

MPS

BOWL/BALL & RACE

VERTICAL SPINDLE

PRESSURIZED

TUBE

CLASSIFICATION OF MILLS

88

Classification as per speed

The plant uses high speed bowl mills for crushing the coal.

Necessity of pulverizing the coal: The economic motives for the

introduction and development of pulverized fuel firing are:

i) Efficient utilization of cheaper low grade coals.

ii) Flexibility in firing with ability to meet fluctuating loads.

iii) Elimination of breaking losses.

iv) Better response to automatic control.

v) Ability to use high combustion air temperature for increasing the

overall efficiency of boiler.

vi) High availability.

v) Ability to burn a wide variety of coals.

Operating principle: The coal is to be ground is fed into the mill at or

near the centre of the revolving bowl. It passes between the grinding ring

in the revolving bowl and rolls as centrifugal force causes the material to

travel towards the outer perimeter of the bowl. The springs, which load

the rolls, impart the pressure necessary for grinding. The partially

pulverized coal continues up over the edge of the bowl.

88



















88



















89

Hot air enters the mill side housing below the bowl, is directed upward

past the bowl, into the deflector liners, then upward again into the

deflector openings at the top of the inner cone, then out through the

venturi and multiple port outlet assembly. As the air passes upward

around the bowl it picks up the partially pulverized coal. The lighter

particles are carried up through the deflector openings. The deflector

blades in the openings impart a spinning action to the material with the

degree of spin, set by the angle of opening of the blades, determining

the size of the finished product. Any oversize material is returned down

the inside of the inner cone to the bowl for additional grinding. When

pulverized to the desired extent, the pulverized fuel air mixture leaves

the mill and enters the piping system. Either constant airflow or variable

airflow methods are adopted. Any tramp iron or dense, difficult to grind

foreign material in the feed, if carried over the top of the bowl it drops out

through the air steam to the lower part of the mill side housing. Pivoted

scrapers attached to the bowl hub sweep the tramp iron or other material

around to the tramp iron discharge spout. The tramp iron spout is fitted

with a valve. Under normal operation, this valve remains open and

material is discharged into a sealed pyrite hopper. The valve is closed

only while the hopper is being emptied. Excessive spillage of coal with

rejects indicates that a mill is not functioning properly and remedial steps

should be taken as soon as possible to correct the situation. Normally

the causes for excessive spillage are a) Over feeding b) Too low a

journal spring pressure c) Too low airflow d) Too low a mill outlet

temperature e) excessively worn out grinding elements or improper mill

setting. The pulverizer operates under positive pressure, except the

suction mills. Seal air system provides clean air to a Chamber

surrounding seal and seal chamber to prevent hot air and coal dust from

escaping to the atmosphere or contaminating the gear bore lube oil.

90

Seal air is also supplied to each roller journal trunnion shaft to prevent

coal dust from entering the roller journal bearings.

Factors affecting the performance of the mill:1. Size of the raw coal

2. Raw coal grind ability

3. Raw coal moisture content

4. Pulverized fuel fineness

5. Mill wear

6. Percentage ash in raw coal

The Bowl Mill is one of the most advanced designs of coal pulveriser

presently manufactured by BHEL. It possesses the following

advantages:

i) Low Power consumption.

ii) Reliability.

iii) Minimum maintenance and time required.

iv) Wide capacity with good turndown ratio

v) Ability to handle wide range of coals

vi) Quite and vibration less operation.

Design considerations:a) Air temperatures up to 400 ° C can be used in these mills enabling the

mills to efficiently dry, grind and classify high moisture coals.

b) Expected wear surfaces are lined with removable type wear resisting

plates/ liners. Suitable access doors are available for easy replacement.

c) Undesirable foreign materials/ difficult to ground materials from coal

fall out and removed through tramp iron spout. This greatly reduces the

possibility of damage to mill parts.

d) Mill output can be raised from minimum to maximum in small

increments depending on boiler needs by varying the output of the

feeder and mill is sensitive to these variations in load. In order to obtain

91

rated capacity of the mill, it is necessary to have sufficient hot air

entering the mill to dry the coal and classifier deflector vanes set so as to

obtain the required fineness reasonably close to the value for which the

mills are designed.

e) Some size of mills is provided with built in lubrication system and

some size of mills with external lube oil system. However for all sizes of

mills the water cooler is fixed in the gear case, except for the HP series

of mills, where the cooler is also external. The journal bearings are

lubricated by oil filled in through the hole in the shaft. The oil level and

quality in the sump is to be maintained within the specified limits.

f) Sufficient journal spring pressure with not more than 0.5-mm clearance

between spring assembly head and journal head must be there to

achieve rated capacity at the required fineness. Because of space

limitation double coil springs are used, inner coil carrying approximately

25% of the total load, while the outer coil carries 75% of load. The

springs are wound in opposite direction to prevent possible interlocking

of the coils. Ring-roll clearances for efficient operation are obtained by

adjusting the stop bolts. If proper compression and ring-roll clearances

are not set, mill capacity reduces and the coal spillage increases.

g) Trunnion shaft supporting the journal assembly is mounted on

Trunnion shaft bushings. Rubber is bonded in between the two

concentric metal bearings and is capable of accommodating oscillating

motions, vibration etc. without wear or lubrication. Worm gear drive is

selected for bowl mills.

Bowl mill designation:Suction type mills are designated as XRS whereas pressurized mills as

XRP and HP.

The nomenclature of each letter is as follows:

X - Frequency of power supply (50 cycles /sec)

92

R - Raymond, the inventor of bowl mills.

S - Suction type with exhauster coming after the mill

P - Pressurized type, with primary air fan coming before the mill.

H - High Performance mills.

The size of the mill is designated by the three numericals that follow the

above. For example, XRP 803 means, it is a Raymond Pressurized Bowl

Mill having the nominal bowl diameter of 80 inches with three numbers of

rollers grinding assemblies.

Constructional features:a. Mill Drive and Bowl Assembly :

Mill Drive and Bowl Assembly consist of the main vertical shaft assembly

with Bearings, Worm gear, Worm shaft, Worm shat bearing etc.

Lubricant is maintained to the level of the centre line of the worm gear in

the Mill base. This lubricates the Bearings and Worm Gear- Worm shaft

in the Mill Base, when the Mill is in operation. The Bowl Assembly

consists of Bull Ring Assembly (Mounted on the Bowl), Skirt Scrapper

Assembly and vane wheel assembly (Attached to the Bowl). In

conventional design mills the fixed air guide vanes are provided in place

of rotating vane wheel assembly.

b. Mill Side and Liner Assembly :

The Hot Primary Air required for drying and carrying pulverized coal

enters the Mill, in the Mill side and air inlet housing. The Mill side and

Liner Assembly are insulated to prevent heat loss from primary air to the

atmosphere, or to the gearbox.

c. Separator Body Assembly :

The Separator body assembly consists of Journal Pressure Spring

Assemblies. Classifier Assembly and Deflector, Intermediate and Journal

Frame Liner Assemblies of Vane Wheel Assembly OR Separator body

93

liner separator bottom liners and air guide vanes of the conventional

design.

d. Roller Journal Assembly :

The Roller Assembly consists of Journal Shaft, Journal Bearings,

Journal Housings, Grinding Roll and Journal Head and Trunnion Shaft

Assembly and Vane Wheel Liners for Journal Head and Upper Journal

Housing. Three roller assemblies are there in a mill. Lub oil in the

Journal Assembly provides Stand Oil Lubrication for the bearings.

e. Mill Discharge Valve Assembly :

The Mill Discharge Valve Assemblies consists of four Multiport Outlet

and Mill Discharge Valves mounted on the multiple port outlet plate. Air

Cylinders operate the flaps in the Mill Discharge Valves. Solenoid Valves

and Limit Switches are provided to effect and indicate the open or close

position of the flap.

f. Coupling :

The Mill and Motor are coupled together by a flexible coupling. (Gear

Type or Bibby Type) for effecting the transmission from the motor to the

Mill. This type of coupling is also known as resilient coupling.

g. Tramp Iron Spout Assembly :

The Tramp Iron Spout Assembly consists of Tramp Iron Spout Body.

Tramp Iron Spout Adapter and Valve Gate. This assembly is mounted on

Mill base to guide the rejects from the Mill side and Liner Assembly to

pyrite Hopper assembly.

h. Pyrite Hopper Assembly :

The Pyrite Hopper Assembly consists of Pyrite Hopper Body and an

outlet valve, which is manually operated. The Pyrite Hopper Body will be

mounted with Tramp Iron spout Assembly. Using the outlet valve, the

rejects can be removed from Pyrite Hopper through a conveyor or wheel

barrow for every half an hour of mill operation. In a pressurized mill

94

before opening the Flap valve of Pyrite Hopper, the Tramp Iron Valve

should be closed to prevent hot primary air leaking into the atmosphere.

Specification of bowl mill:Capacity 66.3T/Hr

Pulverizer Speed 600 RPM

Power 525 KW

Rolls 3

Coal 55 HGI, 14% Moisture

Fineness 70% thru 200 Mesh

Principle features of bowl mill:

Grinding chamber

Classifier mounted above it

Pulverization takes place in rotating bowl

Rolls rotating free on journal do the crushing

Heavy springs provide the pressure between the coal and the rolls

Rolls do not touch the grinding rings

Tramp iron and foreign material discharged.

Internal and external features of a bowl mill

95

Seal air fan

Seal air fan is provided to mills (rollers and gear box) and feeders

(bearings) to prevent ingress of coal dust into area of application and to

protect the bearings from coal particle deposition. Suction of Seal air fan

is taken from PA fan discharge. It is located at 0 meter in boiler area.

Internal view of a bowl mill

96

Air System

The mill produces Pulverized coal 80% of which passes through 200

mesh. Primary air mixed with Pulverized coal (PF) is carried to the coal

nozzle in the wind box assembly. PF from coal nozzle is directed

towards the centre of boiler burning zone. Pre-heated secondary air

enters boiler and surrounds the PF and help in combustion. The primary

air is supplied by Primary Air (PA) fan and the secondary air is supplied

by Forced Draft (FD) fan. Also to dispose the flue gases into the

atmosphere and to maintain a negative pressure, for combustion, within

the boiler furnace an Induced Draft (ID) fan is employed. A fan is

capable of imparting energy to the air/gas in the form of a boost

in pressure. The boost is dependent on density for a given fan at

a given speed. The higher the temperature, the lower is

the boost. Fan performance (Max. capability) is represented as volume

vs. pressure boost.

The basic information needed to select a fan is:

Air or Gas flow (Kg/hr).

Density (function of temperature and pressure).

System, resistance (losses).

Classification of FansIn boiler practice, we meet the following types of fans.

Axial fans: In th is type the movement of a i r or gas is

paral le l to i ts exi t of rotation. These fans are better suited to

low resistance applications. T h e a x i a l f l o w f a n u s e s t h e

s c r e w l i k e a c t i o n o f a m u l t i p l i e d rotating shaft, or

propeller, to move air or gas in a straight through path. Here both

97

the axes inlet air and outlet air flow are parallel to the axis of the

fan.

Centrifugal (Radial) fans: This fan moves gas or air perpendicular

to the axis of rotat ion. There are advantages when the

air must be moved in a system where the frictional resistance is

relatively high. T h e b l a d e w h e e l w h i r l s a i r

c e n t r i f u g a l l y b e t w e e n e a c h p a i r o f blades and forces it

out peripherally at high velocity and high static pressure. More air

is sucked in at the eye of the impeller. As the air leaves the

revolving blade tips, part of its velocity is converted into additional

static pressure by scroll shaped housing. Here the axis of the inlet

air is parallel to the fan axis and that of the outlet air is

perpendicular to the fan axis.

Axial fan

Radial fan

98

Classification of blades

There are three types of blades:

Backward curved blades.

Forward curved blades.

Radial blades.

Fans used in Thermal Power PlantUsually, there are three fans used in any thermal power plant. They are:

1. Induced draught 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 or housing

Inlet Vane Control or Inlet dampers

rotor with two sleeve bearings

Outlet Guide Vane Assembly

Shaft seal

There are two induced draught fans per boiler, both operating. In 500

MW fans are single-stage, double-inlet centrifugal fans (NDVZ type). The

outlet guides are fixed in between the case of the di f fuser and the

casing. These guide vanes serve to direct the flow axially and to

stabilize the draft-flow caused in the impel ler . These out let b lades

are removable type f rom outs ide. During operation of the fan

itself these blades can be replaced one by one. Periodically, the outlet

blades can be removed one at a time to find out the extent of wear on

the blade. If excessive wear is noticed the blade can be replaced by a

new blade. The inlet dampers can be adjusted externally. The rotor

consists of a hollow shaft with an impeller joined by means of a flange.

The fan housing is sealed at the shaft passage to the outside by means

of labyrinth seals. The rotor is placed between oil-lubricated sleeve

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bearings. The fan is adapted to ten changing operating conditions by

varying the speed of the fan and also by adjustable inlet dampers

arranged in the front of the impeller on either side. The main purpose of

an ID fan is to suck the flue gas through all the above mentioned

equipments and to maintain the furnace pressure. ID fans use 1.41% ofplant load for a 500 MW plant. It also maintains the furnace draft.

ID fan specifications

Fan type: NDZV 47 S No. of boilers: Two

Medium: Flue Gas Temperature: 150°c

Capacity: 587m3/s Total head: 490mmwc

Density: 0.793 kg/m3 Speed: 545 rpm

Coupling: REYNOLDS Fan Regulation: VFD & IGV

Motor Rating: 4000 kW

An ID fan

100

ID fan designationNDZV 47 S

Here NDZV implies Radial Double Suction simply supported

47 implies Impeller Tip diameter in decimeter

S implies Type of Impeller

2. Forced draught (FD) fan: There are two FD fans per boiler. The fan,

normal ly of the same type as ID Fan, consists of the

following components:

Suction bend

Inlet housing

Fan housing

Main bearings (anti-friction bearings)

Impeller with adjustable blades and pitch control mechanism

Guide vane casing with guide vanes

Diffuser.

The centrifugal and setting forces of the blades are taken up bythe blade bearings. The blade shafts are placed in combined radial andaxial antifriction bearings which are sealed off to the outside. The angleof-incidence of the blades may be adjusted during operation. T h ec h a r a c t e r i s t i c p r e s s u r e v o l u m e c u r v e s o f t h e f a n m a yb e c h a n g e d i n a l a r g e r a n g e w i t h o u t e s s e n t i a l l ym o d i f y i n g t h e e f f i c i e n c y . T h e f a n c a n t h e n b ee a s i l y a d a p t e d t o c h a n g i n g operating conditions.

An FD fan

101

The rotor is accommodated in cylindrical roller bearings and an

inclined ball bearing at the drive side adsorbs the axial thrust. An oil-

hydraulic servo motor (also known as a power cylinder) flanged to the

impeller and rotating with it adjusts the blades during operation

lubrication and cooling these bearings is assured by a combined

oil level and circulating lubrication system. Turbine oil with a viscosity of

61.2 – 74.8 mm2/sec at 400C is employed.

FD fan SpecificationsFan type: AP1-26/16 No. of boilers: Two

Medium: Atmospheric Air Capacity: 267m3/s

Total head: 410mmwc Density: 1.060 kg/ m3

Speed: 980 rpm Coupling: Spacer Type

Fan regulation: Blade Pitch Control Motor rating: 1430 kW

Volts: 3300 volt

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The forced draft fans, also known as the secondary air fans are used to

provide the secondary air required for combustion, and to maintain the

wind box differential pressure. The features of the FD fans are: axial

flow, single stage, impulse fan. FD fans use 0.36% of plant load for a500 MW plant.FD fan designation:The model no. of the FD fan used at NTPC Simhadri is AP1 26/16,

where A refers to the fact that it is an axial flow fan, P refers to the fan

being progressive, 1 refers to the fan involving a single stage, and the

numbers 26 and 16 refer to the distances in decimeters from the centre

of the shaft to the tip of the impeller and the base of the impeller,

respectively. Similar designation is followed for PA fans.

3. Primary air (PA) fan: There are two primary air fans per boiler. The fan

consists of the following components:

Suction bend

Fan housing with guide vanes (stage 1)

Main bearings (anti-friction bearings)

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Rotor, consisting of shaft, impellers with adjustable blades and

pitch control mechanism.

Guide vane housing with guide vanes.

Diffuser

On its impeller side, the suction bend is designed as an inlet nozzle.

Guide vanes of axial flow type are installed in the fan and guide vane

housings, in order to guide the flow. Suction bend and diffuser are

connected to the fan housing via expansion joints. The fan is driven from

the inlet side.

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 antifriction bearings which are sealed off to the outside. The angle

of-incidence of the blades may be adjusted during operation.

The rotor is accommodated in cylindrical roller bearings and an

inclined ball bearing at the drive side adsorbs the axial thrust. An oil-

hydraulic servo motor (also known as a power cylinder) flanged to the

impeller and rotating with it adjusts the blades during operation

Lubrication and cooling these bearings is assured by a combined

oil level and circulating lubrication system. Turbine oil with a viscosity of

61.2 – 74.8 mm2/sec at 400C is employed.

PA fan has a flange mounted design, single stage suction, NDFV

type, backward curved bladed radial fan and operates on the principle of

energy transformation due to centrifugal forces. Some amount of

the velocity energy is converted to pressure energy in the spiral

c a s i n g . T h e f a n i s d r i v e n a t a c o n s t a n t s p e e d

a n d t h e f l o w i s controlled by varying the angle of the inlet vane

control. The special feature of the fan is that is provided with inlet

guide vane control with a positive and precise link mechanism. The

primary air fans are used to carry the pulverized coal particles from the

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mills to the boiler. They are also used to maintain the coal-air

temperature. The specifications of the PA fan used at the plant under

investigation are: axial flow, double stage, reaction fan. A PA fan uses

0.72% of plant load for a 500 MW plant.

PA fan Specifications

Fan type: AP2-20/12 No. of boiler: Two

Medium: Atmospheric Air Capacity: 186m3/s

Total head: 1195mmwc Density: 1.060 kg/ m3

Speed: 1480 rpm Coupling: Spacer Type

Fan regulation: Blade Pitch Control Motor rating: 2800 kW

Volts: 11000 volt

A PA fan

105

Air Pre heater

Air pre heater absorbs waste heat from the flue gases and transfers

this heat to incoming cold a ir , by means of cont inuously

rotat ing heat t ransfer element of specially formed metal plates.

Thousands of these high efficiency elements are spaced and

compactly arranged within 12 sections. Sloped compartments of

radially divided cylindrical shell called the rotor. The housing

surrounding the rotor is provided with duct connect ing both the

ends and is adequate ly scaled by rad ia l and circumferential

scaling.

Air pre heaters can further be classified as:

Primary air pre heater (size: 27.5)

Secondary air pre heater (size: 30)

Location and Functioning of an air pre heater

106

Air pre heater is a general term to describe any device designed to heat

air before another process (for example combustion in a boiler). It is a

heat transfer surface in which air temperature is raised by transferring

heat from other medium such as flue gas. The purpose of the air pre

heater is to recover the heat from the flue gas from the boiler to improve

boiler efficiency by burning fuel with warm air which increases

combustion efficiency, and reduces useful heat lost from the flue. As a

consequence, the gases are sent to the chimney or stack at a lower

temperature (to meet emission norms, for example) allowing simplified

design of the ducting and stack.

APH is the last heat exchanger in the boiler flue gas circuit. To achieve

maximum boiler efficiency maximum possible useful heat must be

removed from the gas before it leaves the APH. However certain

minimum temperature has to be maintained in the flue gas to prevent

cold end corrosion.

Functions:An air pre-heater heats the combustion air where it is economically

feasible. These are used for pre-heating the primary and secondary air

before entering the furnace.

The pre-heating helps the following:

Igniting the fuel.

Improving combustion.

Drying the pulverized coal in pulverizer.

Reducing the stack gas temperature and increasing

The boiler efficiency.

Advantages:1. Increase in boiler efficiency.

2. Stability of combustion increases by use of hot air.

107

3. Intensify and improved combustion. Intensified combustion permits

faster load variation and fluctuation.

4. Permitting to burn poor quality of coal.

5. High heat transfer rate in the furnace and hence lesser heat

transfer area requirement.

6. Less un burnt fuel particle in flue gas thus combustion and both

efficiency is improved.

In the case of pulverised coal combustion, hot air can be used for

heating the coal as well as for transporting the pulverised coal to

burners. This being a non-pressure part will not warrant shutdown of unit

due to corrosion of heat transfer surface which is inherent with lowering

of flue gas temperature.

Types:1. Recuperative type

a. Tubular air heater

b. Plate type air heater

2. Regenerative type

a. Ljungstrom type

b. Rothemuhle type

The APH used at NTPC Simhadri is a Ljungstrom regenerative type

APH.

Construction:Air Pre heater consists of:

Connecting plates

Housing

Rotor

Heating surface elements

108

Bearings

Sector plates and Sealing arrangement

In this design, the whole air pre heater casing is supported on the boiler

supporting structure itself with necessary expansion joints in the ducting.

The vertical rotor is supported on thrust bearings at the lower end and

has oil bath lubrication. Oil in bath is cooled by water circulating in coils

inside a cooler. The top end of the rotor has a simple roller bearing to

hold the shaft in a vertical position.

The rotor is built up on the vertical shaft with radial supports and cages

for holding the baskets in position. Radial and circumferential seal plates

are also provided to avoid leakages of gases or air between the sectors

or between the duct and the casing while in rotation. Air pre heater

baskets elements are made up of zigzag corrugated plates pressed into

a steel basket giving sufficient annular space in between for the gas to

pass through. These plates are corrugated to give more surface area per

Guide Bearing Assembly Support Bearing Assembly

108

Bearings
















108

Bearings
















109

unit mass for efficient heat transfer and also to give it the rigidity for

stacking them into the baskets.

The Heating Elements used are Hot End Baskets, Hot Intermediate

Baskets and Cold End Baskets. The material used for Cold end in the

basket is a special type of steel (corten steel (trade name)) which has

high resistance to the low temperature sulphur corrosion, thus

prolonging operational life. In the hot end mild steels are used. The

Radial seal

110

optimal geometric shape is usually corrugated and sizes are

determined based on design modeling and experimental data. The

turbulence of air and gas flow through the package increases the heat

transfer rate.

The air pre heater is rotated by means of an electric drive motor through

a rack and a pinion. The power from the motor is transmitted via a shaft

to the rack and then the pinion. The power from the pinion is transmitted

to the rotor assembly of the APH through another shaft. In case, the

electric motor fails an air motor is used in its place which is driven by

compressed air from the compressor house. The air motor can be put up

to 3 hrs of service as a temporary drive till the electric motor is repaired.

Arrangement of Heating Elements

A Regenerative air pre heater

111

Working:A regenerative type air pre-heater absorbs waste heat from flue gas and

transfers this heat to the incoming cold air by means of continuously

rotating heat transfer elements of specially formed metal sheets. In other

words, the flue gas flows through a closely packed matrix with

consequent increase in matrix temp. And subsequently air is passed

through the matrix to pick up the heat. A bi-sector APH preheats the

combustion air. Thousands of these high efficiency elements are spaced

and compactly arranged within sector shaped compartments of a radially

divided cylindrical shell called the rotor. The housing surrounding the

rotor is provided with duct connections at both ends, and is adequately

sealed by radial and axial sealing members forming an air passage

through one half of the APH and a gas passage through the other. The

rotor itself is the medium of heat transfer in this system, and is usually

composed of some form of carbon steel structure. As the rotor slowly

Ljungstrom Regenerative Air Pre heater

112

revolves the elements alternately pass through the air and gas

passages; heat is absorbed by the element surfaces passing through the

hot gas stream, then as the same surfaces pass through the air stream,

they release the heat to increase the temperature of the combustion of

process air. It rotates quite slowly in order (around 1-5 RPM) to allow

optimum heat transfer first from the hot exhaust gases to the element,

then as it rotates, from the element to the cooler air in the other sectors.

During initial startup of the boiler flue gases are not readily produced but

it is required to pre heat the air hence special air pre heaters called

Steam Cold Air Pre heaters (SCAPH) are used. These air pre heaters

use auxiliary steam to pre heat the incoming air into the boiler during

initial start up. Once, combustion in the boiler takes place flue gases are

released which are diverted to APHs for preheating of air.

113

Advantages of Ljungstrom Regenerative Air Pre heater:

Significant reduction in overall size and weight.

Easy and economic replacement of heating surface with separate

cold end and hot end packs.

Min. metal temp. at cold end is higher. This metal temp. oscillates

some 20-22ºC above and below mean of air entering temp. and gas

exit temp.

Problems:

High Air leakages resulting high fan power.

Dust carry over to furnace is high causing ash erosion of boiler tubes

in burner panels.

Baskets are subjected to abrasive wear, hence frequent replacement

of the baskets are called for

Prone to air heater fire, the problem is aggravated during oil firing

APH Performance:

Higher than expected leakage would decrease the condition ofimproved working.

Higher inlet flue gas temperature is rather rare, but this could be onereason for high exit temperature.

Optimum flue gas temperature is required for effective ESPperformance

Unequal temperature at air heater exit should be investigated.

Working of an air pre heater

114

Performance of APH may be degraded due to the following reasons:

Seal Leakage, Erosion, Corrosion, High Press Drop Across APH, APH

Fire.

APH SpecificationsNumber of air pre heater per unit: 2

Heater size: 27-VI-(T)-74” casing

A p p r o x h e a t i n g s u r f a c e : 1 9 0 0 0 m 2 e a c h

Rotor drive motor: 15 H.P.

Speed reduct ion rat io : 110:1

A p p r o x o i l c a p a c i t y : 1 3 G a l l o n s

S o l e n o i d d r i v e : e l e c t r i c a l & a i r m o t o r

M e c h a n i s m : r a c k & p i n i o n

H E A T I N G E L E M E N T S

H o t e n d : c a r b o n s t e e l d u t y p e

H o t i n t e r m e d i a t e : c a r b o n s t e e l d u t y p e

C o l d e n d : C o r t e n s t e e l n f t y p e

V a l u e 1 1 0 V , A . C

A i r c i r c u l a t i o n S ys t e m Ar r a n g e m e n t :P r i m a r y a i r s y s t e m : Ambient air is drawn into the primary air

ducting by two 50% duty, motor driven axial reaction fans. Air

discharging from each fan is divided into two parts, one passes first

through an air pre-heater then through a gate into the P.A bus duct. The

second goes to the cold air duct. The mix of both is used to carry the

pulverized coal to the boiler.

Secondary air system: Ambient air is drawn into the secondary air

system by two 50% duty, motor driven axial reaction forced draft fans

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with variable pitch control. Air discharging from each fan passes first

through an air preheated then through an isolating damper into the

secondary air bust duct. The cross over duct extends around to each

side of the boiler furnace to form two secondary air to burner ducts. At

the sides of the furnace, the ducts split to supply air to two corners. Then

split again to supply air to each of nineteen burner/air nozzle elevations

in the burner box.

Energy Losses in the boiler

Heat loss from furnace surface. Unburned carbon losses. Incomplete combustion losses. Loss due to hot ash. Loss due to moisture in air and fuel. Loss due to combustion generated moisture. Dry Exhaust Gas Losses.

Primary and secondary air systems

115







in the burner box.




115







in the burner box.




116

Overview of air system

Arrangement of Boiler Auxiliaries

117

TURBINE MAINTAINANCE

DEPARTMENT

118

Steam Turbine theoryA steam turbine is a mechanical device that extracts thermal energy

from pressurized steam, and converts it into useful mechanical work.

Here steam expands from high pressure to low pressure. The steam

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

thermodynamic efficiency from the use of multiple stages in the

expansion of the steam.

Characteristics of a Steam turbine:

It can be operated from <1 MW to >1300MW

High-pressure steam flows through the turbine blades and turns the

turbine shaft.

The shaft of the turbine is coupled to the generator shaft to produce

electricity.

Power output is proportional to the steam pressure drop in the

turbine.

Basic operation of a Steam turbine

119

Operating Principle:

A steam turbine’s two main parts are the cylinder (casing) and the rotor.

As the steam passes through the fixed blades or nozzles it expands

and its velocity increases. The high-velocity jet of steam str ikes

the f i rs t set of moving b lades. The k inet ic energy of the

steam changes into mechanical energy, causing the shaft to rotate. The

steam then enters the next set of fixed blades and 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 s t a g e , t h e s t e a m e x h a u s t s i n t o t h e

c o n d e n s e r o r p r o c e s s s t e a m system. The kinetic energy of

the steam changes into mechanical erringly through the impact

(impulse) or reaction of the steam against the blades.

Turbine classificationBased on the principle of action of steam turbines nay be classified as:

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 t u r b i n e a r e

t h a t a l l p r e s s u r e d r o p s o c c u r a t n o z z l e s a n d n o t

o n blades. This is obtained by making the blade passage of constant

cross-section area. A simple impulse turbine is not very efficient

because it does not fully use the velocity of the steam. Many impulse

turbines are velocity compounded. This means they have two or

more sets of moving blades in each stage. A single-stage impulse

120

turbine is known as the de Laval turbine. Tip leakage is a major problem

in an impulse turbine. For higher efficiency, twisted (or warped) blades

are used in the later stages of the turbine. Steam velocity can be

maximized by having maximum pressure drop in the nozzles. Hence in

100% Impulse steam Turbine, whole pressure drop will be in stationary

blades or nozzles. To sustain high velocity impulse stage should be very

robust in construction.

Reaction Turbine:

In this type of turbine pressure is reduced at both fixed & m o v i n g

b l a d e s . B o t h f i x e d & m o v i n g b l a d e s a c t a s n o z z l e s .

The expansion of s team takes place on moving blades. A

reaction turbine uses the "kickback" force of the steam as i t

leaves the moving b lades and f ixed b lades have the s a m e

s h a p e a n d a c t l i k e n o z z l e s . T h u s , s t e a m e x p a n d s ,

l o o s e s pressure and increases in velocity as it passes through both

sets of blades. The pressure drop suffered by steam while passing

through moving blades causes additional conversion of pressure energy

into kinetic energy within these blades, thus giving rise to reaction and

adding to the propelling force. The blade passage cross-sectional area is

varied (converging type). All reaction turbines are pressure-

compounded turbines. A 100% Impulse or Reaction stage is purely a

theoretical assumption not practically feasible.

Parson’s turbine is a special reaction turbine in which equal enthalpy

drops occur in the fixed and moving blades.

In a reaction turbine, with reduction of inlet pressure, specific volume

increases, thus also increasing the volume flow rate, thereby requiring

increased flow area. This requires increased blade height and mean

121

wheel diameter. For higher efficiency, twisted (or warped) blades are

used in the later stages of the turbine.

S t e a m t u r b i n e s m a y a l s o b e c l a s s i f i e d i n t o t h e

f o l l o w i n g c a t e g o r i e s :

According to the direction of steam flow

Axial turbines

Radial turbines

A c c o r d i n g t o t h e s t e a m c o n d i t i o n s a t

i n l e t t o turbines

Low-pressure turbines

Medium -pressure turbines

High-pressure

Turbines of very high pressures

121






Axial turbines

Radial turbines





High-pressure


121






Axial turbines

Radial turbines





High-pressure


122

Turbines of supercritical pressures

According to their usage in industry

Turbines with constant speed of rotat ion pr imar i ly

used for driving alternators.

S t e a m t u r b i n e s w i t h v a r i a b l e s p e e d m e a n t f o r

d r i v i n g t u r b o blowers, air circulators, pumps etc.

Turbines with variable speed: Turbines of this type are usually

employed in s teamers, ships and rai lway locomot ives

(turbo locomotives)

Compounding:In a steam turbine, if steam is allowed to expand in a single row of

nozzle, the velocity at exit from the nozzles is very large.

Subsequently, the rotational speed of the turbine can be high, in

the range of 30,000 rpm. Such high rotational speeds cannot be

properly utilized due to friction losses, centrifugal stresses, and

energy losses at exit. Therefore, steam turbines are

compounded by expanding the steam in a number of stages.

Following are the types of compounded turbine:

Velocity Compounded Turbine:

Like simple turbine it has only one set or row of nozzles & entire

steam pressure drop takes place there. The kinetic energy of steam on

the nozzles is utilized in moving the blades. The role of fixed blades is to

change the direction of steam jet & to guide it.

Pressure Compounded Turbine:

This is basically a no. of single impulse turbines in series or on the same

shaft. The exhaust of first turbine enters the nozzle of the next

turbine. Total pressure drop of steam does not take on first

123

nozzle ring but divided equally on all of them. The pressure drop

occurs only in the nozzles, not in the moving blades.

Pressure Velocity Compounded Turbine:

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

of allowing bigger pressure drops in each stage & so fewer

stages are necessary. Here for g iven pressure drop the

turbine will be shorter length but diameter will be increased.

Pressure Compounding of a steam turbine

124

The turbine cycleFresh steam from boiler is supplied to the turbine through the

emergency stop valve. From the stop valves steam is supplied to

control valves situated on HP cylinders on the front bearing end.

After expansion through 12 stages at the HP cylinder steam flows back

to boiler for reheating and reheated steam from the boi ler

cover to the intermediate pressure turbine trough two

interceptor valves and four control va lve s mounted on the

IP turbine.

After flowing trough IP turbine steam enters the middle part of the LP

turbine through cross over pipes. In LP turbine the exhaust steam

condenses in the surface condensers welded directly to the exhaust part

of LP turbine.

The selection of extraction points and cold reheat pressure has been

done with a view to achieve the highest efficiency. These are two

extractions from HP turbine, four from IP turbine and o n e f r o m

L P t u r b i n e . S t e a m a t 1 . 1 0 t o 1 . 0 3 g / c m 2 ( a b s ) i s

supplied for the gland sealing. Steam for this purpose is

Velocity compounding of a Steam Turbine

125

obtained f rom deaerator through a co l lect ion where

pressure of s team is regulated.

From the condenser condensate is pumped with the help of 3x50%

capacity condensate pumps to deaerator through the low pressure

regenerative equipments.

Feed water is pumped from deaerator to the boiler through the HP

heaters by means of 3x50% capacity feed pumps connected before the

HP heaters.

Governing of Steam TurbinesFundamentally governing means to control the output of the turbine by

varying the inlet steam flow by means of throttling valves of the turbine.

The valves are controlled by the governor.

The basic functions of Turbine governing are:

The Turbine Cycle

126

1. Safe start up & shut down of machine

2. To change the output of the machine as per requirement

3. To protect the machine from damage

4. To protect the machine from over speeding during load throw off

5. To control speed and load on the turbine (operation of control

valves)

6. To ensure safety of the turbine under unacceptable operating

conditions (operation of emergency stop valves and NRVs)

Types of governing

Throttle governing: In throttle controlled turbines, steam flow is

controlled by opening and closing of all the control valves

simultaneously to the extent required by load and admitting the

steam to the group of nozzles located on the entire periphery.

Nozzle governing: In nozzle controlled turbines, steam flow is

controlled by sequential opening or closing of control valves allowing

steam to flow to associated nozzle groups.

Types of governing systems

The governing system can be one of the following types:

• Mechanical: In mechanical governing system, the speed

transducer is a mechanical centrifugal type speed governor, which

actuates control valves through mechanical linkages. Currently,

purely mechanical governing systems for utility turbines are

obsolete.

• Hydro-mechanical: In hydro-mechanical governing system, speed

transducer is usually mechanical centrifugal type speed governor.

It is connected to hydraulic system either hydraulically or

mechanically. In hydraulic system, signal is amplified so that

control valve servomotors can be actuated.

127

• Hydraulic: In hydraulic governing system, speed transducer is a

centrifugal pump, whose discharge pressure is proportional to

square of speed. This signal is sent to hydraulic converter /

transformer which generate a signal proportional to valve opening /

closure required. Before applying the signal to control valve

servomotors, the same is suitably amplified

• Electro-hydraulic: This system provides very good combination

of electrical measuring & signal processing and hydraulic controls.

It offers many advantages over other three types of governing

systems and is popular in large steam turbine units due to growing

automation of turbine and generator sets.

Thus the individual TG governing system imply a need to

withstand a full load rejection safely

Provide appropriate contributions to system frequency control.

128

Turbine and its auxiliariesThe Main turbineThe 500MW turbines is predominantly of reaction-condensing- tandem-

compound, three cylinder- horizontal, disc and diaphragm, reheat type

with throttle governing and regenerative system of feed water heating

and is coupled directly with A.C. Generator. The turbine is suitable for

sliding pressure operation to avoid throttling losses at partial loads. It

comprises of separate HP, IP and LP cyl inders , whose

rotors are mounted on a s ingle shaf t . The HP turb ine is a

s ingle cyl inder and comprises of 18 stages whereas the IP and LP

turbines re double flow cylinders having 12 stages & 6 stages

respectively. The individual turbine rotors and the Generator rotor are

connected by rigid couplings. The HP & IP turbine rotor are rigidly

compounded & IP rotor by lens type semi flexible coupling. All the

three rotors are aligned on four bearings of which the bearing no.2 is

combined with thrust bearing.

The main superheated steam branches off into two streams from

the boiler and passes through four combined emergency stop

v a l v e ( m a i n s t o p v a l v e s ) a n d c o n t r o l v a l v e s b y a

s i m p l e t h r o t t l e g o v e r n i n g s y s t e m , b e f o r e e n t e r i n g t h e

g o v e r n i n g w h e e l chamber of the HP turbine. After expanding

in the 12 stages in the HP turbine the steam returned in the boiler for

reheating. On the two exhaust lines of HP turbine, swing check valves

are provided to prevent hot steam from the re heater flowing back into

the HP turbine.

The reheated steam from the boiler enter IP turbine via interceptor

valves and control valves and after expanding enters the LP

turbine stage via 2 numbers of cross over pipes.

129

In the LP stage the steam expands in axially opposite direction to

counteract the trust and enters the condenser placed d i r e c t l y

b e l o w t h e L P t u r b i n e . T h e c o o l i n g w a t e r f l o w i n g

t h r o u g h o u t t h e c o n d e n s e r t u b e s c o n d e n s e s t h e s t e a m

a n d t h e condensate collected in the hot well of the condenser.

The condensate collected is pumped by means of 3x50% duty

condensate pumps through LP heaters to deaerator f r o m

w h e r e t h e b o i l e r f e e d p u m p d e l i v e r s t h e w a t e r t o

b o i l e r t h r o u g h H P h e a t e r s t h u s

f o r m i n g a c l o s e d c y c l e .

HP Turbine

The outer casing of the HP turbine is of the barrel type, which prevents

mass accumulation with high thermal stresses, and has neither axial nor

a radial flange. Barrel-type casing permits quick startup and high rate of

change of load. The guide blade carrier is axially split and kinematically

supported. The space between the outer casing and the inner casing is

A 500 MW Steam Turbine (Cross-sectional view)

130

fed from admission steam to HP turbine. This steam is drained through

HP casing during start up which promotes quicker heating of inner

casing which results in lesser problems of differential expansion. The

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

casing so that it can freely expand radially in all directions and axially

from a fixed point (HP- inlet side). The HP turbine is provided with a

balance piston in the admission side to counter act the axial thrust

caused by steam forces. HP turbine is provided with 18 stages of

reaction blades. The HP casing is made of creep resisting Chromium-

Molybdenum-vanadium (Cr-Mo-V) steel casing. The steam chests which

accommodate the control valves are also made of the same material in

the form of castings. The HP rotor is machined from single Cr-Mo-V steel

forging with integral discs. The blades are attached to their respective

wheels by ‘T’ root fastenings. In all the moving wheels, balancing holes

are also machined to reduce the axial thrust. The HP turbine rotor is also

fitted with a balancing drum to eliminate the axial thrust.

A HP turbine

130

















A HP turbine

130

















A HP turbine

131

Characteristics of a HP turbine

• Single flow

• double shell casing

• Inner casing : Vertically split

• Outer casing: Barrel type

• Single exhaust in L/H

• Mono block rotor

• Reaction blading with integral shroud

• Rigid coupling

• Casing mounted valves

• Transported as single unit

IP Turbine

It is of double flow construction and consists of two casinos. Both are

axially split and the inner casing kinematically supported and carries the

guide blades. The inner casing is attached to the outer casing in such a

manner as to be free to expand axially from a fixed point and radially in

all directions. IP turbine has 12 stages per flow. The IP turbine casing is

made of two parts. The front part is made of creep resisting Chromium-

Molybdenum-Vanadium steel casings and the exhaust part is of steel

fabricated structure. The two parts are connected by a vertical joint. The

control valves of IP turbine are mounted on the casing itself. In an IP

turbine the nozzle boxes are cast integral with the casing itself. The IP

rotor has seven discs integrally forged with rotor while the last four discs

132

are shrunk fit. The shaft is made of high creep resisting Cr-Mo-V steel

forgings. The blades on the integral discs are secured by ‘T’ root

fastenings while on shrunk fit disc by ‘fork root’ fastenings. It provides

opposed double flow in the two blade sections and compensates axial

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

Outer casing is subjected to only low pressure and low temperature

conditions

An IP Turbine

Cross sectional view of an IPturbine

An IP turbine

132







conditions

An IP Turbine


An IP turbine

132







conditions

An IP Turbine


An IP turbine

133

Characteristics of IP turbine

• Single / double flow

• Double shell casing

• Horizontally split

• Two exhaust in L/H


• Reaction blading

• Rigid coupling

• Usually transported as single unit

LP Turbine

The casing of the double-flow LP cylinder is of three-shell design. The

shells are axially split and of rigid welded construction. The inner shell

taking the first row 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. LP turbine is provided with 6 reaction

stages/flow. The LP turbine rotor consists of shrunk fit discs mounted on

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 fastenings. The LP turbine casing consists of three

parts i.e. one middle part and two exhaust parts. The three parts are

fabricated from weld able mild steel. The exhaust casings are bolted to

the middle casings by a vertical flange. The casings are divided in the

horizontal plane through the turbine centre line.

134

Steam enters the middle casing from top and then divides into twp

equal, axially opposed flows, to pass through four stages. The last but

one stage on each side is ‘Baumann’s stages’. They expand a part of

the steam down to the condenser pressure and allow rest of the steam

to expand through the last stages. To protect the IP cylinder against

excessive internal pressure, four atmospheric relief valves are provided

in the exhaust hoods. Each valve assembly has 1 mm thick gasket ring

clamped between valve seats and valve disc. If due to some reasons the

pressure at exhaust hood rises to 1.2 abs, then the valve disc tries to lift

and thereby rupture the gasket ring, thus allowing the steam to exhaust

into the atmosphere in the turbine hall

Characteristics of an LP turbine

• Double flow

• Three shell casing

• Horizontally split


• Reaction blading

• Rigid coupling

135

An LP turbine

Cross-sectional view of an LPturbine

An LP turbine

136

Blading

The entire turbine is provided with reaction blading. The guide blades

and moving blades of the HP and IP parts and the front rows of the LP

part with inverted T-roots and shrouding are milled from one piece. The

last stages of the LP part consists of twisted, drop-forged moving blades

with fir tree roots inverted in corresponding grooves of the rotor and

guide blade rows made of sheet steel.

Bearings

The HP turbine is supported by two bearings, a journal bearing at the

front end of the turbine and a combined journal and thrust bearing

directly adjacent to the coupling with the IP rotor. The IP and LP rotors

have a journal bearing each at the end of the shaft. The thrust bearing

takes up residual thrust from both the directions. The bearing

temperatures are measured by thermocouples. No of general bearing

are 6 and the no of thrust bearing are 1. These Bearings are usually

forced lubricated and have provision for admission of jacking oil. The

function of the journal bearing is to support the turbine rotor. The journal

bearing consists of the upper & lower shells, bearing cap, spherical

block, spherical support and key. The bearing shells are provided with a

Babbitt face. Bearing is pivot mounted on the spherical support to

prevent the bending movement on the rotor. A cap which fits in to the

corresponding groove in the bearing shell prevents vertical movement of

the bearing shell. The bearing shells are fixed laterally by key. Each key

is held in position in the bearing pedestal by two lateral collars. The

Temperature of the bearings at every instant is monitored. Upper and

lower shell can be removed without the removal of Rotor. To do this

shaft is lifted slightly by means of jacking device but within the clearance

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of shaft seal. The lower bearing shell can be turned upward to the top

position and removed. The thrust bearing is normally Mitchell type and is

usually combines with a journal bearing, housed in spherically machined

steel shell. The bearing between the HP and IP rotors is of this type,

while the rest are journal bearings.

Sealing glandsTo eliminate the possibility of steam leakage to atmosphere from the

inlet and exhaust ends of the cylinder, labyrinth glands of the axial

clearance type are provided which provide a trouble free frictionless

sealing. These glands seal the steam in the cylinders against

atmosphere.

Each gland sealing consists of a number of thin sealing strips which in

the HP and IP parts are alternatively caulked into grooves in the shafts

and surrounding sealing rings. The sealing strips in the LP part are only

caulked into the sealing rings. These rings are split into segments which

are forced radially against a projection by helical springs and are able to

yield in the event of rubbing.

Labyrinth seal glands

138

Emergency Stop Valves and Control ValvesThe turbine is equipped with emergency stop valves to cut off steam

supply and with control valves to regulate steam supply. Emergency

Stop Valves (ESV) are provided in the main stream line and the

interceptor valves (IV) are provided in the hot reheat line.

Emergency stop valves are actuated by servomotor controlled by the

protection system. ESV remains fully open or fully close. The stop valves

are spring- operated single-seat type. Control valves are actuated by the

governing system through servomotors to regulate steam supply as

required by the load. Valves are single seat type.

The HP turbine is equipped with four initial steam stop and control

valves. A stop and control valve with stems arranged at right angles to

each other are combined in a common body.

The IP turbine has four combined reheat stop and control valves. The

reheat stop valves are spring loaded single-seat valves. The control

valves operate in parallel and are fully open in the upper load range. In

the lower load range, they control the steam flow to the IP turbine and

ensure stable operation even when the turbo set is supplying only the

station load.

Turbine Governing SystemThe turbine has an electro-hydraulic governing system. An electric

system measures and controls speed and output, and operates the

control valves hydraulically in conjunction with an electro-hydraulic

converter. The electro-hydraulic governing system permits run-up control

of the turbine up to the rated speed and keeps speed swings following

sudden load shedding low. The linear-output characteristic can be very

closely set even during operation.

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Barring GearThe barring gear is mounted on the LP rear bearing cover to mesh with

spur gear on the LP rotor rear coupling. The primary function of the

barring gear is to rotate the turbo-generation rotors slowly and

continuously during start-up and shut down periods when changes in

rotor temperature occur.

When a turbine is shut down, cooling of its inner elements continues for

many hours. If the rotor is allowed to remain standstill during this cooling

period, distortion of rotor begins almost immediately. This distortion is

caused by flow of hot vapors to the upper part of the casings, resulting in

upper half of the turbine beings at a higher temperature, than lower half.

Hence to eliminate the possibility of distortion during shut-down, barring

gear is used to keep the rotor revolving until the temperature change has

stopped and casings have become cool.

An Electro Hydraulic Governor

140

The same phenomenon is also observed during starting of the turbine,

when steam is supplied to the sealing to create the vacuum. If the rotor

is stationary, there would be non- uniform heating of the rotor which will

result in distortion of rotors. The barring gear during starting of turbine,

would slowly rotate the turbine-generator rotor, and thereby resulting in

the uniform heating of rotor. Thus any distortion on the rotor would be

avoided. During starting, period operation of the barring gear eliminates

the necessity of ‘breaking away’ the turbine generator rotors from stand-

still and thereby provides for a more uniform, smooth and controlled

starting.

Turbine Oil systemFunctions:

1. For lubricating and cooling the bearings.

2. Driving the hydraulic turning gear during interruptions to operation,

on start up and shutdown.

3. Jacking up the shaft at low speeds (turning gear operation, start-up

and shut-down)

Oil SystemWhen the machine is running, the main oil pump situated in the bearing

pedestal draws oil from the main oil tank by injectors and conveys it to

the pressure system for lubrication. The return oil is drained into the

tank. During ‘the start up and shut down condition’, one of the two full

load auxiliary oil pumps circulates the oil.

When the main and full load auxiliary oil pumps fail, the lubrication oil is

maintained by a DC- driven emergency oil pump.

The jacking oil required for supporting the shaft system is supplied by

one of the two jacking oil pumps, which takes its suction from the main

oil tank. Two oil vapor extractors are mounted on the MOT to produce

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slight vacuum in the main oil tank and the bearing pedestals to draw off

any oil vapors. There are 2x100% oil coolers and a duplex filter on the

oil line to thrust bearings. Main oil tank is provided with a basket filter.

Oil specification

Name of Oil Servo Prime 46

Specific Gravity at 500C 0.852

Kinematic viscosity at 500C 28 centistokes

Flash point 2010C (min.)

Pour point -6.60C (max.)

Ash percentage by weight 0.01%

Mechanical impurities Nil

Main Oil TankThe main oil tank not only serves as a storage tank but also for

detrainment the oil.

The capacity of the tank is such that the full quantity of oil is circulated

not more than 8 times per hour. This results in a retention time of

approx. 7 to 8 minutes from entry into the tank to suction by the pump.

This time allows sedimentation and detrainment of the oil.

Oil returning to the tank from the oil supply system first flows through a

submerged inlet into the riser section of the tank where the first stage

deaeration takes place as the oil rises to the top of the tank. Oil flows

from the riser section through the oil strainer into the adjacent section of

the tank where it is then drawn off on the opposite side by the suction

pipe of the oil pumps.

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Main Oil PumpThe main oil pump is situated in the front bearing pedestal and supplies

the entire turbine with oil that is used for bearing lubrication, cooling the

shaft journals and as primary and test oil. It is coupled with turbine rotor

through a gear coupling. The main oil pump is driven direct from the

turbine shaft via the coupling. These pumps also convey oil in the

suction branches of the main oil pump for oil injector, which maintains a

steady suction flow to main oil pump. It takes over when the turbine

speed is greater than 2800 rpm.

Auxiliary Oil PumpThe auxiliary oil pump is a vertical one stage rotary pump with a radial

impeller and spiral casing. It is fixed to the cover of the oil tank and

submerges into the oil with the pump body. It is driven by an electric

motor that is bolted to the cover plate of the main oil tank. The pump

shaft bearing in the pump casing and a grooved ball bearing in the

bearing yoke. The bearings are lubricated from the pressure chamber of

the pump; the sleeve bearing via a bore in the casing; the grooved ball

bearing via lube line. Generally, three in number, two AC motor driven

and one DC motor driven. Supplies oil during turbo-set starting and

stopping when the turbine is running at speed lower than 2800 rpm

supplies oil to governing system as well as to the lubrication system. It

also serves as standby to main centrifugal oil pump.

Emergency Oil pumpThis is a centrifugal pump, driven by D.C. electric motor. It is vertical

type. This automatically cuts in whenever there is failure of A.C. supply

at power station and or the lub oil pressure falls below a certain value.

This pump can meet the lubrication system requirement under the

conditions mentioned above.

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Shaft lift oil pump (Jacking Oil Pump)The lift oil pump is a self-priming screw-spindle pump with three spindles

and internal bearings. It is a jack-screw immersion pimp situated on the

tank. The pump supplies high pressure (about 12 kgf/cm2) oil from the

main oil tank in order to lift the turbine rotor at low speeds, thus

preventing damage to the bearings when shaft speeds are too low for

hydrodynamic lubrication to take place. The pump is driven by a three

phase A.C. motor. The pressure oil piping of the lifting oil pump that is

not in operation is closed by the check valves. The pressure in the

system is kept constant by means of the pressure limiting valve. When

the turbine is started up or shut down, the hydraulic lifting device is used

to increase or maintain the oil film between the rotor and bearings. The

necessary torque from the hydraulic turning device or from the manual

turning device is reduced in this way. The bearings are relieved by high

pressure oil that is forced under the individual bearing pins, thus raising

the rotor. In order to avoid damage to the bearings, the lifting oil pump

must be switched on below a certain speed. The drain from the bearings

is connected back to the oil tank only.

Oil coolersThe oil of the lubrication and governing system is cooled in the oil

coolers. The cooling medium for these coolers is circulating water. It

Consists of tube nest, inner & outer shell & water boxes. The pressure of

the cooling water is kept lower than that of oil to avoid its mixing with oil

in the event of tube rupture.

Five oil coolers have been foreseen, out of which four are for continuous

operation and one remains as a standby, provided the cooling water

temperature is not more than 360C. The oil coolers are in parallel for

maintenance purposes, the oil and cooling water system to any one of

the oil coolers may be cut off. Oil temperature controller is employed for

144

maintaining the lub oil temp at rated value by controlling the flow through

the coolers.

Duplex oil filterIt is provided to filter the oil before supply. The duplex filter consists of

two filter bodies and is fitted with a changeover device which enables the

filters to be switched as desired.

Three-way control valve

It is electrically driven and has the function of regulating the lubricating

oil temperature. Possible oil flow paths for regulating the oil temperature

are:

1. All lubricating oil flows through the oil cooler.

2. Lubricating oil flows through oil cooler and by-pass piping

3. All lubricating oil flows through the by-pass piping.

Condensate systemCondensate: The steam after condensing in the condenser is known as

condensate, and is extracted out of the hot well by condensate

extraction pump and taken to the deaerator through drain cooler, gland

steam condenser and series of LP heaters.

This contains the following:

Low Pressure heatersTurbine is provided with non-controlled extractions which a r e

u t i l i z e d f o r h e a t i n g t h e c o n d e n s a t e f r o m t u r b i n e

b l e e d i n g s y s t e m . There are four 10W pressure LP heaters in

which the last four extractions are used. LP Heater-1 has two parts LPH-

1A a n d L P H - 1 B l o c a t e d i n t h e u p p e r p a r t s o f

c o n d e n s e r A a n d condenser B respectively. These are of

horizontal type with shell and tube construction. LP heaters 2, 3 and

4 are of similar construction and they are mounted in a row at

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4m level . They are of vert ica l 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

are employed for measurement of steam pressure etc.

LPT

1ST STAGE

Stages of LP heating

An LP heater145









LPT

1ST STAGE


An LP heater145









LPT

1ST STAGE


An LP heater

146

Condensate Extraction Pumps

The function of these pumps is to pumps out the condensate to the

desecrator through ejectors, gland steam cooler, a n d L P h e a t e r s .

T h e s e p u m p s a r e o f v e r t i c a l b a r r e l o r c a n i s t e r ,

d o u b l e s u c t i o n , m u l t i s t a g e , d i f f u s e r t y p e . T h e

s u c t i o n b a r r e l i s i n s t a l l e d o n t h e p u m p f l o o r . I n t e r n a l

b e a r i n g s ( L e a d e d b r o n z e b e a r i n g s ) i n s t a l l e d i n a

c o l u m n p i p e a n d t h e t o p c a s i n g i s p r o v i d e d f o r

s u p p o r t i n g t h e p u m p s h a f t a g a i n s t t h e r a d i a l l o a d .

U p p e r a n d l o w e r b e a r i n g s ( l e a d e d b r o n z e ) a r e

i n s t a l l e d i n t h e s t u f f i n g b o x a n d s u c t i o n b e l l . T h e

w e i g h t o f t h e p u m p r o t o r a n d t h e h y d r a u l i c t h r u s t

a c t i n g o n t h e r o t o r i n t h e a x i a l d i r e c t i o n s a r e

s u p p o r t e d b y t h e t h r u s t b e a r i n g s i n t h e m o t o r . T h e s e

p u m p s a r e d r i v e n b y 1 1 2 0 K W i n d u c t i o n m o t o r ,

d e l i v e r i n g 8 1 0 0 0 0 k g / h r o f c o n d e n s a t e w a t e r a g a i n s t

3 0 7 m o f t o t a l d y n a m i c h e a d a t t h e r a t e d c o n d i t i o n .

T h e s e p u m p s h a v e f o u r s t a g e s a n d s i n c e t h e suction is

147

at a negative pressure, special arrangements have been m a d e

f o r p r o v i d i n g s e a l i n g . T h i s p u m p i s r a t e d g e n e r a l l y

f o r 160m3/ hr. at a pressure 13.2 Kg/cm2. They are 3 per unit of 50%

capacity each located near the condenser hot well. Here the suction is

under vacuum.

Hot well

Condenser Extraction pumps

148

Specifications of CEP

A Condenser Extraction Pump148


A Condenser Extraction Pump148


A Condenser Extraction Pump

149

Deaerator

One per unit located around 18m level in CD bay. 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 dissolved gases

from the feed water by mechanical means. Particularly the unit must

reduce the oxygen content of the feed water to as low as possible or

desired, depending upon the individual circumstances, residual oxygen

content in condensate at the outlet of deaerating plant usually specified

0.005/liter or less. Water is sprayed in atmosphere of steam. Oxygen

and free CO2 removed. This preheated water having minute traces of

dissolved gases flows into second stage where water is in contact with

fresh steam. The steam then rises to first stage and carries residual

gases. Water is stored in storage tank for further use. The main sources

of this steam are Extraction steam, CRH (cold reheat steam), auxiliary

steam.

Parts of a deaerator are

• Tubular type gauge glass.

• High level alarm switch.

• Low level alarm switch.

• Pressure gauge.

Deaerator level controller

150

• Safety valves

• Isolating valves for steam pipes.

Specifications of a deaerator

• Design pressure - 9.0 kg/cm2

• Operating pressure - 6.8 kg/cm2

• Capacity - 170 m3

• No. of trays - 576

• No. of spray valves - 108

• No. of safety valves - 6

An Overview of deaerator and hot well

151

A Deaerator

152

Feed water system

The main equipments coming under this system are:

Booster PumpEach 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. By the use of

a booster pump in the main pump suction line, always there will be

positive suction pressure which will remove the possibility of Cavitation.

Each pump set consists of a Weir type FAIE 64 booster stage pump and

a Weir type FK4E36 pressure stage pump.

The Weir type FAIE 64 booster stage pump is a single stage, horizontal,

axial split casing type, having the suction and discharge branches

integrally cast in the casing lower half, thus allowing the pump internals

to be removed without disturbing the suction and discharge pipe work or

the alignment between the pump and discharge.

The pump shaft is sealed at the drive end and non-drive end by Crane

mechanical seals. The rotating assembly is supported by plain white

metal lined journal bearings and axially located by Glacier double tilting

pad thrust bearing.

153

Specifications Single stage, horizontal, axial split casing

Aim: to obtain positive suction pressure in order to avoid cavitation

suction temp : 164 0c

suction pressure : 9.06 bar

discharge pressure : 20.3 bar

speed : 1494 rpm

power consumption : 608 kW

Boiler Feed PumpsThey are three per unit of 50% capacity each located in the 0m level in

the TG bay. The pump is Weir type FK4E36 pressure stage pump. It is a

multi- stage pump. This pump is horizontal at zero level 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 pump internals

are designed as a cartridge which can be easily removed for

maintenance without disturbing the suction and discharge pipe work, or

the alignment of the pump and the turbo coupling. The pump is sealed

at the drive end and non-drive end by labyrinth glands.

The pump casing consists of a forged steel barrel with welded suction,

discharge branches; inter stage tapping and mounting feet.

The high-pressure boiler feed pump is very expensive m a c h i n e

w h i c h c a l l s f o r a v e r y c a r e f u l o p e r a t i o n a n d s k i l l e d

maintenance. The safety in operat ion and ef f ic iency of the

feed pump depends largely on the reliable operation and

maintenance. Operating staff must be able to find out the causes of

154

defect at the very beginning which can be easily removed without

endangering the operator of the power p lant and a lso wi thout

the expensive dismantling of the high pressure feed pump.

The feed pump consists of pump barrel, into which is mounted the inside

stator together with rotor. The hydraulic part is e n c l o s e d b y t h e

h i g h p r e s s u r e c o v e r a l o n g w i t h t h e b a l a n c i n g device.

The suct ion s ide of the barrel and the space in the

high pressure cover behind the balancing device are enclosed by the

low pressure covers along with the stuffing box casings. The

brackets o f t h e r a d i a l b e a r i n g o f t h e s u c t i o n s i d e a n d

r a d i a l a n d t h r u s t bearing of the discharge side are fixed to

the low pressure covers. T h e e n t i r e p u m p s a r e

m o u n t e d o n a f o u n d a t i o n f r a m e . T h e hydraulic

coupling and two claws coupling with coupling guards are also

del ivered a long wi th the pump. Water cool ing and o i l

lubricating are provided with their accessories. The use of Mechanical

seal reduces the losses of feed water in the stuffing box to maintain and

working ability of the feed pump increases. Cooling is carried out by the

circulation of water between the stuffing box space and the cooler. Even

after stopping the pump stuffing box cooling should be continued as its

cooling circuit is different from the seal coolers. Coolers are designed to

keep the stuffing box space temperature below 800C. The rotating

assembly is supported by plain white metal lined journal bearings and

axially located by Glacier double tilting pad thrust bearing. BFP have two

main uses namely, to give the required pressure to the feed water before

entering into boiler and to supply water for de superheating in the boiler.

155

Specifications

single cylinder turbine

axial flow type

No of stages 14

Normal speed 5275 rpm

Steam pr. 6.33 kg/cm2

Output 5732 kW

Steam cons. 36 tons/hr

Turbine Driven Boiler Feed Pump

The single cylinder turbine is of the axial flow type. The live steam

flows through the emergency stop valve and then through the main

Control Valves 5 nos. (Nozzle governing). These valves regulate the

steam supply through the turbine in accordance with load

Sectional view of a Boiler Feed pump

156

requirements. The control valves are actuated by a lift b a r

w h i c h i s r a i s e d o r l o w e r e d v i a a l e v e r s y s t e m b y t h e

r e l a y cylinder mounted on the turbine casing.

The journal bearings supporting the turbine shaft are arranged in the

two bearing blocks. The front end -bearing block also houses the

thrust bearing, which locates the turbine shaft and takes up "the

axial forces”. There are 14 stages of reaction balding. The

balancing piston is provided at the. Steam admission side to

compensate the axial thrust to the maximum extent. Since the

axial thrust varies with the load, the residual thrust is taken up

by the thrust bearing. The leak of f f rom the balancing p is ton

is connected back to the turbine after 9th stage. The turbine is

provided with hydraulic and electro-hydraulic governing system. A

primary oil pump is used as a speed sensor for hydraulic governing and

shall Probes are used as a speed sensor for electro hydraulic governing.

Whenever steam is drawn from the cold reheat line or auxi l iary

supply, s team f low is control led by auxi l iary cont rol va lve.

Dur ing th is per iod the main control va lves (4 nos. ) wi l l

remain fu l ly opened and the bypass va lve across i t wi l l

remain closed. (Bypass remains c losed for a short per iod

when change, over from IP steam to CRH takes place).The steam

exhaust for the BFP- Turbine is connected to the main condenser and

the turbine glands are sealed by gland steam.

The turbine is provided with a hand barring facility. The turbine rotor is

connected to the pressure pump through detachable coupling and to the

booster pump through a set of reduction gears. A plate type filter is

provided and either one can be isolated during the running of the

turbine. The control oil pressure is around 5 to 8 ata and the lubricating

157

oil pressure is 0.8 to 1.7 atm. The oil temperature after the coolers is to

be maintained at 450C to 480C.

Turbine driven Boiler Feed Pump

158

High Pressure Heaters

They are three in number and are situated in the TG bay. These are

regenerative feed water heaters operating at h i g h p r e s s u r e a n d

l o c a t e d b y t h e s i d e o f t u r b i n e . T h e s e a r e generally

vertical type and turbine bleed 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 suppl ied to these heaters 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 f tube rupture in any of the HPH and the level of the

condensate rising to dangerous level, the group protection

device d i v e r t s a u t o m a t i c a l l y t h e f e e d w a t e r

d i r e c t l y t o b o i l e r , t h u s bypassing all the three HP heaters.

Following fittings are generally provided on the HP heaters

Gauge glass for indicating the drain level.

Pressure gauge with three way cock.

Air Vent cock.

Safety valve shell side.

Seal pot.

Isolating valves.

High level alarm switch.

An HP heater

159

An HP heater

160

OFFSITE MAINTAINANCE

161

DM water treatment plantAs 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 t reatment plants used in t h e r m a l p o w e r

p l a n t s a r e d e s i g n e d t o p r o c e s s t h e r a w w a t e r t o

water low in dissolved solids known as "dematerialized w a t e r " . N o

d o u b t , t h i s p l a n t h a s t o b e e n g i n e e r e d v e r y

c a r e f u l l y k e e p i n g i n v i e w t h e t y p e o f r a w w a t e r t o

t h e t h e r m a l p l a n t , i t s treatment costs and overall economics.

T h e t y p e o f d e m i n e r a l i z a t i o n p r o c e s s chosen for a

power station depends on three main factors:

The quality of the raw water.

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

Selectivity of resins.

W a t e r t r e a t m e n t p r o c e s s w h i c h i s g e n e r a l l y m a d e u p

o f t w o sections:

Pretreatment section

Demineralization section

Pretreatment sectionPretreatment plant removes the suspended solids such as clay, silt,

organic and inorganic matter, plants and other microscopic

organism. The turbidity may be taken as of two types of suspended

solids in water. Firstly, the separable solids and s e c o n d l y t h e

n o n s e p a r a b l e s o l i d s ( c o l l o i d s ) . T h e

c o a r s e 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 p a r t i c l e s w h i c h a r e a b l e t o s e t t l e . L o n g t e r m

162

a b i l i t y t o r e m a i n suspended in water is basically a function of

both size and specific g r a v i t y . T h e s e t t l i n g r a t e o f t h e

c o l l o i d a l a n d f i n e l y d i v i d e d (approximately 0.01 to 1

micron) suspended matter is so slow that removing them from

water by plain sedimentation is tank shaving ord inary dimensions is

impossib le. Set t l ing veloc i ty of f ine ly divided and collide

particles under gravity also are so small that ordinary

sedimentation is not possible. It is necessary, therefore, to use

procedures which agglomerate the small particles into

larger aggregates, which have pract ica l set t l ing ve loc i t ies.

The term "Coagulation" and "flocculation" have been used

indiscriminately to describe process of turbidity removal.

"Coagulation" means to bring together the suspended particles.

The process describes the e f f e c t p r o d u c e d b y t h e a d d i t i o n

o f a c h e m i c a l A l ( S P ) g t o a c o l l o i d a l d i s p e r s i o n

r e s u l t i n g i n p a r t i c l e d e s t a b i l i z a t i o n b y a reduction of force

tending to keep particles apart. Rapid mixing is i m p o r t a n t a t t h i s

s t a g e t o o b t a i n . U n i f o r m d i s p e r s i o n o f t h e

c h e m i c a l a n d t o i n c r e a s e o p p o r t u n i t y f o r p a r t i c l e s t o

p a r t i c l e c o n t a c t . T h i s o p e r a t i o n i s d o n e

b y f l a s h m i x e r i n t h e clarifier. Second stage of

formation of settle able particles f r o m d e s t a b i l i z e d

c o l l o i d a l s i z e d p a r t i c l e s i s t e r m e d a

"flocculation". Here coagulated particles grow in size by attaching to

each other. In contrast to coagulation where the primary force is

e l e c t r o s t a t i c o r i n t r i n s i c , " f l o c c u l a t i o n " o c c u r s

b y c h e m i c a l bridging. Flocculation is obtained by gentle and

prolonged mixing w h i c h c o n v e r t s t h e s u b m i c r o s c o p i c

c o a g u l a t e d p a r t i c l e i n t o discrete, visible & suspended

163

particles. At this stage particles are l a r g e e n o u g h t o s e t t l e

r a p i d l y u n d e r t h e i n f l u e n c e o f g r a v i t y anomaly be removed.

This is best at pH ~6.5 - 7.0 & higher retention time.

For removing the organic matter chlorine as a biocide is dosed in

clarifier. It is essential to remove organic matter because it may lead to

fouling of ion exchange resin in DM Plant. Also the organic matter at

high temperature may get converted to CO2 & cause metal corrosion in

boiler system. To completely eliminate the organic matter a slight excess

of chlorine is dosed (~ 0.5ppm at Clarifier O/l).The clarified water so

produced is passed through filter beds (Graded Sand / Anthracite can be

used) to remove any floating turbid matter. This is called filtered water.

This water is being used for drinking purpose & for demineralization.

I f p r e t r e a t m e n t o f t h e w a t e r i s n o t d o n e

e f f i c i e n t l y t h e n t h e consequences are as follows:

Si02 may escape wi th water which wi l l increase the

anion loading.

Organic matter may escape which may cause organic fouling i n

t h e a n i o n e x c h a n g e r b e d s . I n t h e ' p r e - t r e a t m e n t

Raw water being pre treated

164

p l a n t c h l o r i n e a d d i t i o n p r o v i s i o n i s n o r m a l l y

m a d e t o c o m b a t organic contamination.

Cation loading may unnecessary increase due to addition

of Ca (OH)2 in excess of calculated amount for raising the pH

of the water for maximum f loe format ion and a lso

AKOrDg m a y p r e c i p i t a t e o u t . I f l e s s t h a n

c a l c u l a t e d a m o u n t o f C a ( O H ) 2 i s a d d e d ,

p r o p e r p H f l o c c u l a t i o n w i l l n o t b e obtained and

silica escape to demineralization section will occur, thereby

increasing load on anion bed.

Demineralization sectionThis filter water is now used for de mineral iz ing purpose and

is fed to cat ion exchanger bed, but enroute being first de

chlorinated, which is either done by passing through activated

carbon filter or injecting along the flow of water, an equiva lent

amount of sodium sulphi te through some stroke pumps.

Excess chlorine is removed in ACF.At ACF O/l Turbidity <0.1 NTU &

Free Cl2 <0.1ppm. The absorbed chlorine is released by backwash

whenever Free Cl2 >0.1ppm or the end of rated cycle whichever is

earlier. 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. Normally, the

typical scheme of demineralization up to the .mark against average

surface water is three bed systems with a provision of removing

gaseous carbon dioxide from water before feeding to Anion

Exchanger.

Resins, which are built on synthetic matrix of a styrene divinely benzene

copolymer, are manufactured in such a way that these have the ability

165

to, exchange one ion for another, hold it temporarily in chemical

combination and give it to a strong electrolytic solution. Suitable

treatment is also given to them in such a way that a particular resin

absorbs only a particular group of ions. Res ins, when absorbing

and releas ing cat ionic port ion of d i s s o l v e d s a l t s , i s

c a l l e d c a t i o n , e x c h a n g e r r e s i n a n d w h e n removing

anionic portion is called anion exchanger resin. Preset trend is of

employing 'strongly acidic cation exchanger resin and strongly

basic anion exchanger res in in a DM Plant of modern

thermal power stat ion. We may see tha t the chemical ly

act ive group in a cationic resin is SOx-H (normally represented

by RH) and in an anionic resin the active group is either tertiary

amine or quaternary ammonium group (normally the resin is

represented by ROH). The react ion of exchange may be

further represented as below

Cation Resin

R-H + Na R-Na + H2SO4

K K HCl

Mg Mg

Ca Ca HNO3

In the form of Resins in Removed inSalts H2CO3 degasser tower

Anion ResinR-OH + H2SO4 R-SO4 + H2O

HCl

HNO3

Mineral acid obtained Resins in

from cation exhausted form

166

The water from the ex-cation contains carbonic acid also sufficiently,

which is very weak acid difficult to be removed by strongly basic anion

resin and causing hindrance to remove silicate ions from the bed. It is

therefore a usual practice to remove carbonic acid before it is led to

anion exchanger bed; this is done in a degasser.

In the degasser, the ex-cation water is trickled in fine streams

from top of a tall tower packed with, rasching rings, and compressed

air is passed from the bot tom. Carbonic ac id breaks into C0 2

and water mechanica l ly ( H e n r y ' s L a w ) w i t h t h e

c a r b o n d i o x i d e e s c a p i n g i n t o t h e atmosphere.

The water is accumulated in sui tab le storage tank below the

tower, called degassed water dump from where the same is led to anion

exchanger bed, using acid resistant pump.

H2CO3 H2O + CO2

The ex-anion water is fed to the mixed bed exchanger (regenerative

type ion exchanger resin beds both strong and weak) containing

both cationic resin and anionic resin. This bed not only takes

care of sodium slip from cation but also silica slip from anion

exchanger very effectively. The final output from t h e m i x e d

b e d i s E x i r a - o r d i n a r i l y p u r e w a t e r h a v i n g l e s s

t h a n 0.2/mho conductivity 7.0 and silica content less than 0.02 pm. Any

DM plant storage tanks and degasser towers

167

deviation from the above quality means that the resins in

mixed b e d a r e e x h a u s t e d a n d n e e d r e g e n e r a t i o n ,

r e g e n e r a t i o n o f t h e mixed bed first calls for suitable, back

washing and settling, so that the two types of resins are separated from

each other. Lighter anion resin r ises to the top and the heavier

cat ion res in set t les to the bottom. Both the resins are then

regenerated separately with alkali and acid, rinsed to the desired value

and air mixed, to mix the resin a g a i n t h o r o u g h l y . I t i s t h e n p u t

t o f i n a l r i n s i n g t i l l t h e d e s i r e d quality is obtained. It may be

mentioned here that there are two types of st rongly basic anion

exchanger. Type I I res ins a re s l ight ly less basic than type I,

but have higher regeneration efficiency than type I. Again as type II

resins are unable to remove silica effectively, type I res ins a lso

have to be used for the purpose. As such, the general

condition so far prevailing in India, is to employ type II resin in

anion exchangers bed and type I resin in mixed bed (for the

anionic portion). It is also a general convention to regenerate the above

two resins under through fare system i.e. the caustic soda

entering into mixed bed for regeneration, of type I anion resin, is

ut i l ized to regenerate type I I res in in an ion exchanger bed.

The content of ut i l iz ing the above res in and mode of

regenerat ion is now days being switched over from the

economy to a higher cost s o a s t o h a v e m o r e s t r i n g e n t

q u a l i t y c o n t r o l o f t h e f i n a l D M water.

R-OH + HCl RCl + H20

2 R-OH + H2SO4 R2SO4 +2H20

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At anion O/l, pH 8-9, Conductivity < 20 umhos/cm , Silica< 200 ppb will

be achieved.

Internal Treatment

This final D.M. effluent is then either led to hot well of the condenser

directly as make up to boilers, or being stored in D.M. Water

storage tanks first and then pumped for makeup purpose to boiler feed.

As the D.M. Water has a good affinity to absorb carbon dioxide and

oxygen, and both are extremely harmful to metal surfaces for their

destruction like corrosion, these have to b e r e m o v e d b e f o r e i t

i s f e d t o b o i l e r . T h i s i s b e i n g d o n e i n desecrator .

St i l l the res idual oxygen which is remaining in the water is

neutralized by a suitable doze of hydrazine, at the point af ter

desecrator . To have fur ther min imum corros ion, the pH

of feed water is to be maintained at around 9.0 for which

purpose ammonia in suitable doze is added to this make up water at a

point along with hydrazine as stated above.

Cation and anion exchangeresin unit in a DM plant

169

Cooling towersNecessityCooling water system plays a vital role in dissipation of waste heat in

power station. More than 60 % of total heat input to the plant is finally

dissipated as waste heat. The waste heat from the power plant is carried

away by circulating water and ultimately gets dissipated in cooling tower.

Types

Natural draught cooling tower (NDCT): These are structures

supported on RCC columns, Most of the structure is empty shell

but the lower portion contains a cooling stack over which hot water

is distributed by RCC channel or pipe system. The lower portion of

the shell is open to allow the air to go to the cooling stack

supported on the RCC columns, which are designed for horizontal

load due to wind. A pond is constructed below the toer to catch the

cooled water and make-up water for circulation. As the warm water

falls in the stack, it gives its heat to the air there, which becomes

Where water supply is not consistent, closed loop cooling system with cooling toweris used.

170

lighter than the ambient air and a draft is created due to chimney

action. In this case, cooling is dependent on dry bulb temperature

i.e. better in humid conditions. Natural draft cooling towers are

normally adopted near coastal areas where humidity is generally

very high. But the capital cost of NDCT is about 60% than that of

IDCT and FDCT put together.

Induced draught cooling tower (IDCT): In this system the fan is

located at the top and air enters from the openings located at the

ground level. Air, mixed with vapors, is discharged through a fan

stack located at the top of the tower. In this case, moist air is

discharged higher in the atmosphere thereby dispersing to a

greater distance from the tower. There is a cylindrical RCC

structure supported on RCC columns. Hot water is taken to the top

of the tower by steel pipes and discharged on the packing with

distribution system of precast RCC trough and tubes. Eliminators

of asbestos are provided at the top to arrest the droplets. The fan

is located at the top to draw air from the cylinder for dispersion.

Hot water is cooled by the induced air travelling up. Cold water is

collected in the pond located below the cooling tower where make

– up water is also discharged.

Forced draught cooling tower (FDCT): Here, motor driven fans

located at the base, i.e. ground level, below air into the tower from

the sides. The top of the tower is open to the air vapor discharge.

The main draw back in this type of tower is that exit velocity is low

and this results in recirculation of hot air into the fan intake. Thus,

the efficiency of the tower is reduced. The other disadvantages of

FDCTs are: High velocity from the fan located at the base makes it

difficult to distribute air evenly over the whole of packing. Low

height, low velocity of air and low wind velocity generally results in

171

recirculation of hot air. This results in rise in cold water

temperature and reduction in efficiency. Frequent clogging due to

organic matter and thus reduction in efficiency.

At NTPC Simhadri, each unit has one Natural Draft Cooling Tower.Principle:

Natural Draft CT depends on the airflow caused by natural driving

pressure due to the density difference between the cool outside air and

hot humid air inside. The driving pressure “P” is given by

P = (density (o) – density (i) fill exit)* H

Normally the density difference is low. Hence “H” has to be more in order

to achieve “P”. The Hyperbolic profile of NDCT offers great resistance to

outside wing loading and superior strength when compared to other

forms. It has little to do with inside air flow.

DETAILS OF NDCTs OF STAGE-I (2X500 MW)

• No. of NDCTs: 2

• Height of NDCT: 165 m

• Bottom diameter: 100 m

• Top diameter: 70 m

• Total no of Racker columns: 88 per NDCT

• Shell thickness: 300-350 mmA Natural Draught Cooling Tower

172

An Induced Draft Cooling Towers

Types of Cooling Towers

A Forced Draft Cooling Towers

173

Inner view of an NDCT

Drift Eliminators

174

Circulating water systemModern high capacity thermal power stations require enormous quantity

of water for steam production. This steam has to be recycled again to

generate power. For recycling steam, it has to be condensed into water.

Circulating water is a system that is used for condensing the steam.

USES OF CIRCULATING WATER

Condensing of steam

Cooling of dm cooling water

Ash evacuation

Bottom de ashing

Fly ash removal

Circulating Water System at NTPCSimhadri

174






Condensing of steam


Ash evacuation

Bottom de ashing

Fly ash removal


174






Condensing of steam


Ash evacuation

Bottom de ashing

Fly ash removal


175

Theory of circulation

Water must flow through the heat absorption surface of the boiler in

order that it is evaporated into steam. In drum type uni ts

(natura l and cont rol led c i rcu lat ion) 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 comers at a

temperature s l ight ly below the saturat ion temperature. The

f low through the furnace wal l is at saturat ion temperatur e.

Heat absorbed in water wal l is latent heat of vapor izat ion

creat ing 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.

Water circulation system in a ThermalPower Plant

176

The types of boiler circulating system are:

Natural circulation system

Controlled circulation system

Combines circulation system

Natural circulation systemWater delivered to steam generator from feed heater is at a

temperature wel l below the saturat ion va lue corresponding

to that pressure. Entering first the economizer it is h e a t e d t o

a b o u t 3 0 - 4 0 ˚ C b e l o w s a t u r a t i o n t e m p e r a t u r e . F r o m

economizer the water enters the drum and thus joins the circulation

system. Water entering the drum flows through the down

comer and enters ring heater at the bottom. In the water walls a part of

the w a t e r i s c o n v e r t e d t o s t e a m a n d t h e m i x t u r e f l o w s

b a c k t o t h e drum. In the drum, the steam is separated, and sent to

super heater f o r s u p e r h e a t i n g a n d t h e n s e n t t o t h e

h i g h p r e s s u r e t u r b i n e . R e m a i n i n g w a t e r m i x e s

w i t h t h e i n c o m i n g w a t e r f r o m t h e economizer and

the cycle is repeated. The circulation in this case takes place on the

thermo-siphon principle. The down comers contain relatively cold water

whereas the riser tubes contain a steam water mixture. Circulation

takes place at such a rate that the driving force and the frictional

resistance in water walls are balanced.

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 t o t h e

m i n i m u m r e q u i r e m e n t o f c o o l i n g o f w a t e r w a l l t u b e s .

Therefore natura l c i rculat ion is l imi ted to the boi ler wi th

drum operating pressure around 175 kg/cm².

177

Controlled circulation systemBeyond 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, orifice plates are used. This system is

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

Combined circulation systemBeyond the critical pressure, phase t r a n s f o r m a t i o n i s a b s e n t ,

a n d h e n c e o n c e t h r o u g h s y s t e m i s adopted.

However, i t has been found that even at super

cr i t ica l pressure, i t is advantageous to re c i rcu late the

water through the f u r n a c e t u b e s a n d s i m p l i f i e s t h e

s t a r t u p p r o c e d u r e . A t y p i c a l operating pressure for such a

system is 260 kg/cm².

Natural circulating system

178

Principal Components of CWSCondenserThere are two condensers entered to the two exhausters of the LP

turbine. These are surface type condensers with two pass arrangement.

Cooling water pumped into each condenser by a vertical CW pump

through the inlet pipe. Water enters the inlet chamber of the front water

box, passes horizontally through the brass 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 there, cooling

water leaves the condenser through the outlet pipe and discharged into

the discharge duct.

Steam exhausted from the LP turbine washing the outside of the

condenser tubes losses its latent heat to the cooling water in the steam

side of the condenser. This condensate collects in the hot well, welded

to the bottom of the condensers.

Sectional view of a condenser

179

EjectorsThere are two 100% capacity ejectors of the steam eject type. The

purpose of the ejector is to evacuate air and other non-condensing

gases from the condensers and thus maintain the vacuum in the

condensers.

A 3 stage ejector using steam from the deaerator with 11 ata header as

the working medium is employed. In addition to the main ejectors there

is a single starting ejector which is used for initial pulling of vacuum up to

500mm of Hg. It consists of nozzle through which the working steam

expands; the throat of the nozzle is connected to the air pipe from the

condenser.

C.W. pumpsThe pumps which supply the cooling water to the condensers are called

circulating water pumps. There are two such pumps for each unit with

requisite capacity.

These pumps are normally vertical, wet pit, mixed flow type, designed for

continuous heavy duty; suitable for water drawn through an open gravity

intake channel terminating in twin-closed ducts running parallel to the

main building.

The fluid through the suction bow/eye provided with stream lined guide

vanes, whose function is to prevent pre-whirl and impart hydraulically

correct flow to the liquid. The propeller, in turn, imparts motion to the

fluid. The purpose of the discharge bowl provided with streamlined

diffuser vanes, is to direct the flow of water into the discharge column.

Bulk requirement of water is used in thermal plants for the purpose of

cooling the steam in condensers. The requirement of water for this

purpose is of the order of 1.5-to2.0 cusecs/MW of installation where

sufficient water is available once through system is used.

180

Specifications Discharge : 31000m3/hr

Head:28m

rpm:330rpm

2 pumps per unit (60%)

An Overview CW system

A CW pump

An Overview of CW system

A Plate Heat Exchanger forcooling auxiliary cooling

water

181

Auxiliary cooling water systemUsually a part of the water to condenser is tapped off and supplied for

the following sub-systems:

Turbine lub oil and gas cooler directly from CW pump discharge

Bearing cooling system

DM plant

General services and miscellaneous cooling.

182

ASH HANDLING PLANT

183

Ash Handling SystemThe ash produced in the boiler is transported to ash dump area by

means of sluicing type hydraulic ash handling system, which consists

of Bottom ash system, Ash water system and Ash slurry system.

Bottom ash systemIn the bottom ash system the ash d i s c h a r g e d f r o m t h e

f u r n a c e b o t t o m i s c o l l e c t e d i n t w o w a t e r compounded

scraper through installed below bottom ash hoppers. The ash is

continuously transported by means of the scraper chain conveyor

onto the respect ive c l inker gr inders which reduce the l u m p

s i z e s t o t h e r e q u i r e d f i n e n e s s . T h e c r u s h e d a s h f r o m

t h e bottom ash hopper from where the ash slurry is further transported

to operat ion, the bot tom ash can is discharged d irect ly

into the sluice channel through the bifurcating chute bypass the

grinder. The position of the flap gate in the bifurcating chute

bypasses the grinder. The position of the flap gate in the bifurcating

chute is to be manually changed. The main types of hoppers used in

power stations are:

1. Water Filter Hoppers: This consists of a tank made of steel plate.

The bottom ash from the boiler falls into water filled tank and is

immediately quenched large pieces of ash break up due to

thermal shock, thus the ash collected will be fairly small size and

during the disposal not much difficulty in terms of crushing aspects

will be encountered. These hoppers may or may not be lined with

refractory which goes off too frequently due to temperature

variations. The unlined hoppers have problems on corrosion for

which special coating are recommended.

184

2. Quencher cooled Ash hopper: This uses a series of quenchers

located near the top of the hoppers which provide fine spray of

water. This ensures that the ash is cooled sufficiently to prevent

after combustion and simitering within the hopper. The spray water

also keeps the refractory lining of the hopper cool. The quencher

type hoppers are not very effective as far as the breaking up of ash

due to thermal shocks is concerned. If there is a tendency of slag

accumulation of large pieces clinker grinders are normally used.

Fly ash systemThe flushing apparatus are provided under E . P . h o p p e r s ( 4 0

n o s . ) , e c o n o m i z e r h o p p e r s ( 4 n o s . ) , a i r p r e heaters (2

nos.) , and stack hoppers (4 nos.) . The f ly ash gets mixed

with flushing water and the resulting slurry drops into the ash

sluice channel . Low pressure water is appl ied through the

n o z z l e d i r e c t i n g t a n g e n t i a l l y t o t h e s e c t i o n o f p i p e t o

c r e a t e t u r b u l e n c e a n d p r o p e r m i x i n g o f a s h

w i t h w a t e r . F o r t h e maintenance of flushing apparatus

plate valve is provided between apparatus and connecting tube.

Bottom ash handling system

185

Ash water systemHigh pressure water required for bottom ash hopper quenching

nozzles, bot tom ash hopper spraying, c l inker grinder

sealing scraper bars, cleaning nozzles, bottom ash hopper seal

through f lush ing, economizer hopper f lushing nozzles and

sluic ing t rench jet t ing nozzles is tapped f rom the h igh

pressure water ring mainly provided in the plant area. Low pressure

water required for bottom ash hopper seal through make up, scraper

conveyor make up, flushing a p p a r a t u s j e t t i n g n o z z l e s f o r

a l l f l y a s h h o p p e r s e x c e p t i n g economizer hoppers, is

t rapped f rom low pressure water r ings mainly provided in the

plant area.

Ash slurry systemBottom ash and fly ash slurry of the system is sluiced up to ash pump

along the channel with the acid of high pressure water jets located

at suitable intervals along the channel. Slurry pump suction line

consisting of reducing elbow with drain v a l v e , r e d u c e r a n d

Fly ash handling system

186

b u t t e r f l y v a l v e a n d p o r t i o n o f s l u r r y p u m p delivery line

consisting of butterfly valve, pipe & fitting has also been provided.

Ash slurry pump

Electrostatic Precipitator with fly ash hoppers

187

Ways to increase the thermal efficiency ofpower plants:The basic idea behind all the modifications to increase the thermal

efficiency of a power cycle is the same: Increase the averagetemperature at which heat is transferred to the working fluid in theboiler, or decrease the average temperature at which heat isrejected from the working fluid in the condenser. That is, the

average fluid temperature should be as high as possible during heat

addition and as low as possible during heat rejection.

Lowering the Condenser Pressure (Lowers Tlow,avg): Steam exists as

a saturated mixture in the condenser at the saturation temperature

corresponding to the pressure inside the condenser. Therefore, lowering

the operating pressure of the condenser automatically lowers the

temperature of the steam, and thus the temperature at which heat is

rejected. The effect of lowering the condenser pressure on the Rankine

cycle efficiency is illustrated on a T-s diagram in Fig.1. For comparison

purposes, the turbine inlet state is maintained the same. The colored

area on this diagram represents the increase in net work output as a

result of lowering the condenser pressure from P4 to P4’. The heat input

requirements also increase (represented by the area under curve 2-2),

but this increase is very small. Thus the overall effect of lowering the

condenser pressure is an increase in the thermal efficiency of the cycle.

188

Effect of lowering of the condenser pressure on efficiency

Superheating the Steam to High Temperatures (Increases Thigh, avg):The average temperature at which heat is transferred to steam can be

increased without increasing the boiler pressure by superheating the

steam to high temperatures. The effect of superheating on the

performance of vapor power cycles is illustrated on a T-s diagram in

Fig.2. The colored area on this diagram represents the increase in the

net work. The total area under the process curve 3-3 represents the

increase in the heat input. Thus both the net work and heat input

increase as a result of superheating the steam to a higher temperature.

The overall effect is an increase in thermal efficiency, however, since the

average temperature at which heat is added increases.

Effect of superheating the steam to high temperatures

189

Increasing the Boiler Pressure (Increases Thigh, avg): Another way of

increasing the average temperature during the heat-addition process is

to increase the operating pressure of the boiler, which automatically

raises the temperature at which boiling takes place. This, in turn, raises

the average temperature at which heat is transferred to the steam and

thus raises the thermal efficiency of the cycle. The effect of increasing

the boiler pressure on the performance of vapor power cycles is

illustrated on a T-s diagram in Fig.3. Notice that for a fixed turbine inlet

temperature, the cycle shifts to the left and the moisture content of

steam at the turbine exit increases. This undesirable side effect can be

corrected, however, by reheating the steam, as discussed in the next

section.

Effect of increasing boiler pressure to increase efficiency

190

Losses during operation & maintenance ofa power plant1) SURFACE ROUGHNESS:It increases friction & resistance. It can be due to Chemical deposits,

Solid particle damage, and Corrosion Pitting & Water erosion. As a

thumb rule, surface roughness of about 0.05 mm can lead to a decrease

in efficiency of 4%.

2) LEAKAGE LOSS: Inter stage Leakage

Turbine end Gland Leakages

About 2 - 7.5 kW is lost per stage if clearances are increased by

0.025 mm depending upon LP or HP stage.

3) WETNESS LOSS: Drag Loss: Due to difference in the velocities of the steam &

water particles, water particles lag behind & can even take

different trajectory leading to losses.

Sudden condensation can create shock disturbances & hence

losses.

About 1% wetness leads to 1% loss in stage efficiency.

4) OFF DESIGN LOSSES:

Losses resulting due to turbine not operating with design terminal

conditions.

Change in Main Steam pressure & temperature.

191

Change in HRH pressure & temperature.

Condenser Back Pressure

Convergent-Divergent nozzles are more prone to Off Design

losses then Convergent nozzles as shock formation is not there in

convergent nozzles.

5) PARTIAL ADMISSION LOSSES: In Impulse turbines, the controlling stage is fed with means of

nozzle boxes, the control valves of which open or close

sequentially.

At some partial load some nozzle boxes can be partially open /

completely closed.

Shock formation takes place as rotor blades at some time are full

of steam & at some other moment, devoid of steam leading to

considerable losses.

6) LOSS DUE TO EROSION OF LP LAST STAGE BLADES: Erosion of the last stage blades leads to considerable loss of

energy. Also, it is the least efficient stage.

Erosion in the 10% length of the blade leads to decrease in 0.1%

of efficiency

192

ConclusionAs an undergraduate of GITAM University I would like to say that this

training program is an excellent opportunity for us to get to the ground

level and experience the things that we would have never gained

through going straight into a job. I am grateful to GITAM University and

NTPC Ltd Simhadri for giving us this wonderful opportunity. The main

objective of the industrial training is to provide an opportunity to

undergraduates to identify, observe and practice how engineering is

applicable in the real industry. It is not only to get experience on

technical practices but also to observe management practices and to

interact with fellow workers. It is easy to work with sophisticated

machines, but not with people. The only chance that an undergraduate

has to have this experience is the industrial training period. I feel I got

the maximum out of that experience. Also I learnt the way of work in an

organization, the importance of being punctual, the importance of

maximum commitment, and the importance of team spirit. The training

program having several destinations was a lot more useful than staying

at one place throughout the whole one month. It was an advantage for

me to be in the O & M-MM Division where I have boosted up my skills

and abilities. The conclusion that I can make is that NTPC Ltd Simhadri

is the right place for students to do their industrial training. In my opinion,

I have gained lots of knowledge and experience needed to be successful

in a great engineering challenge, as in my opinion, Engineering is after

all a Challenge, and not a Job.

Date post:	11-Aug-2015
Category:	Engineering
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