A

VOCATIONAL TRAINING PROJECT REPORT

ON

Study of Thermal Power Plant At

Under Guidance of:-

1. Sh. S.S.JHA AGM (MMD)2. Sh. P.K.SHARMA ENGINEER (TMD)

Submitted in partial fulfillment of the requirementFor the award of degree

Of

BACHELOR’S OF ENGINEERING (MECH.)SESSION (2012-2016)

SUBMITTED TO: - SUBMITTED BY:-KBUNL/MTPS, KANTI. TARUN KUMARMuzaffarpur, Bihar Mechanical Engineering

University Roll No -1712917 Haryana College of Technology & Management, Kaithal, Haryana

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Certificate This is to certified that Ms. TARUN KUMAR. Roll number 1712917 student of 2012-2016 batch of mechanical engineering 7th semester of Haryana College of Technology & Management, Kaithal, Haryana has successfully completed his industrial training at Kanti Thermal Power Station (a joint venture of NTPC Ltd. and BSEB) Muzaffarpur for one month from 15/07/2015 to 14/08/2015.He has completed the whole training as per schedule.

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ACKNOWLEDGEMENT

It is my pleasure to be indebted to various people, who directly or

indirectly contributed in the development of this work and who influenced my

thinking, behavior, and acts during the course of study.

I express my sincere gratitude to Mr. S.S JHA (AGM) MM, for providing me

an opportunity to undergo summer training at NTPC-KBUNL, Muzaffarpur.

I am thankful to Mr. P.K. SHARMA Eng.TMD for his support, cooperation,

and motivation provided to me during the training for constant inspiration,

presence and blessings.

I also extend my sincere appreciation to Mr. S.S.SAMANTA

Dy. Manager (HR) who provided his valuable suggestions and precious time

in accomplishing my project report.

Lastly, I would like to thank the almighty and my parents for their

moral support and my friends with whom I shared my day-to-day experience

and received lots of suggestions that improved my quality of work.

TARUN KUMAR

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ABSTRACT

Any thermal power plant is converting the chemical energy of fossil Fuel (coal) into electrical energy. The process involved for this conversion is based upon the Modified Rankin cycle is

Boiler feed pump, The steam generator water walls (evaporator), Steam generator super heater, Steam turbine, Reheater, Condenser Regenerative feed heaters etc.

All component of power generating cycle are vital and critical in operations in modified Rankin cycles, the two most important aspects that is added are reheating & regenerative heating. By reheating we used to send the steam coming from exhaust of the turbine back to the reheat of the boiler so that its enthalpy increases and more work can be done by this steam and other purpose is to make steam dry so that no harm will be done to the blades of the turbine

In MTPS Kanti, we have three turbines in Tandem coupling namely one H.P Turbine, one I.P Turbine & one L.P Turbine coupled with the generator to which is synchronized with the grid to produce electricity at 50Hz

In all my modesty, I wish to record here that a sincere attempt has been made for the presentation of this project report. I also trust that this study will not only prove to be a academic interest but also will be able to provide an insight into the area of technical management.

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Sl.No. Description Page-No.01 An Over View -6-02 Process of Generation of Electricity -9-03 Light Up Process -12-04 Milling System -13-05 Main Boiler Components -14-06 Electrostatic Precipitator -20-07 Air Heaters -20-08 Types of Fan -21-09 Types of Pump -22-10 Types of Cycle -23-11 Types of Heater -25-12 Types of Turbine -26-13 Flame Scanner -27-

14 Important Control Loops in a Thermal Power Plant -28-

15 Unit Control Desk and Panels -33-16 References -34-

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AN OVERVIEW

NTPCNTPC Limited (also known as National Thermal Power Corporation Limited) is an Indian central public sector undertaking (CPSU) under the ministry of power, government of India, engaged n the business of generation of electricity and allied activities. It is a company incorporated under the company’s act 1956 and a “Government company” within the meaning of act. The headquarters of company is situated at New Delhi. NTPC core business is generation and sale electricity to state-owned power distribution companies and state electricity Boards in India. The company also undertakes consultancy and turnkey project contracts that involve Engineering, project

Management, construction management and operation and management of power plants. The company has also ventured into oil and gas exploration and coal mining activities. It is the largest power company in the India with an electric power generation capacity of 43,803 MW. Although the company has approx 18% of total national capacity it contributes to over 27% of total power generation due to its focus on operating its power plants at higher efficiency levels (approx 83%against the national PLF rate of 78%)

It was founded by government of India in1975, which now holds 70% of its equity shares on 13 May 2015 (after divestment of its stake in 2004, 2010, 2013 and 2015).In May 2010 NTPC was conferred Maharatna status by the union government of India.Is is ranked 424th in the Forbes global 2000 for 2014

Environment management

Liquid water treatment plants at Farakka and Kahalgaon. Ash water recycling system at Kahalgaon and Korba to reduce water requirement for

ash disposal at these station

PLANT INTRODUCTION

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Salient Features Of Boiler Plant1. General a) Type of boiler Single drum tangential firing & reheat type.

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b) Type of fuel used Pulverized coal (Main Fuel)Heavy oil & L.D.O. (for light up & flame stabilization)

c) No. of Mills 6

d) Type of Mills Pressurized type Bowl Mill

e) Furnace Balanced draught

f) P.A. Fans 2 nos. (each 60% capacity)

g) F.D. Fans 2 nos. (each 60% capacity)

h) I.D. Fans 3 nos. (one standby)(each 60% capacity)

i) Air Heater 2 nos.

j) Type of Air Heater Trisector regenerative

k) Electrostatic Precipitator 1 nos.

2. M.C.R. Parameter M.C.R. Value

a) S.H. Outlet Steam Flow 375 T/Hr

b) R.H. Steam Flow 331 T/Hr

c) Pressure at S.H. Outlet 141.5 Ata

d) Temp. at S.H. Outlet 540oC

e) Pressure at R.H. Inlet 37 ata

f) Pressure at R.H. Outlet 32.9 ata

g) Temp. at R.H. Inlet 369oC

h) Temp. at R.H. Outlet 540oC

i) Pressure in Drum 148.69 ata

j) Design Pressure 158.0 kg/cm2

k) Flue Gas temp. leaving

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Economiser 350oCl) Flue Gas temp. leaving Air Heater 142oC

m) Feed Water Temp. before Economizer 235oC

Salient Feature Of Turbine1. General

a) Type Of Turbine Reheat

b) No. Of Cylinders 3 (HP,IP & LP)

c) No. Of LP Heater 5

d) No. Of HP Heater 2

e) Deaerator 1 (Variable pressure type)

f) No. Of Extraction pump 3 (one standby)

g) No. Of BFP 2 (one standby)

2. M.C.R. Parameter M.C.R. Value

a) Rated output 110 MW

b) M.S. Pressure at H.P. turbine inlet 130 ata

c) M.S. temp. at H.P. turbine inlet 535oC

d) H.R.H. temp. at I.P. turbine inlet 535oC

e) Turbine speed 3000 rpm

f) Condenser Vacuum 0.1 kg/cm2 (abs)

g) No. of Extraction 7

h) Quantity of cooling water 15,400 m3/hr

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Salient Feature Of Generator

a) Rating Continuous

b) Active Output 110 MW

c) Rated Voltage 11000 +/- 5% V

d) Rated Current 7220 A

e) Power Factor 0.8 Lagging

f) Frequency 50 Hz

g) Excitation System Static type

h) Field Current at rated output 1335 A

i) Type of Cooling System Hydrogen Cooled

j) Hydrogen Pressure 2 ata

k) No. of H2 Cooled elements 6

l) Cooling Medium for H2 Soft water

PROCESS OF GENERATION OF ELECTRICITY

MTPS Kanti is a Thermal Power Plant. The functioning of every thermal power plant is based on the following processes: -

1. Coal To Steam.2. Steam To Mechanical Power3. Power Generation, Transmission & Distribution.

Coal To Steam

Coal and Water are primary inputs to a thermal power plant.

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This process of conversion of water to steam by using the heat energy Produced by burning coal for producing heat takes place in the boiler

and its auxiliaries. Coal burns in a furnace located at the bottom part of the boiler. Feed water is supplied to the boiler drum by boiler feed pumps, where water is heated and converted into saturated steam This is further superheated in the super heaters.

water Coal

SM

To Condensers GENERATOR TRANSFORMER(400KV)

Steam to Mechanical Power.

This is the most important process of a power plant. The superheated Steam produced in the boiler at high pressure and temperature is feed to the turbine. The steam expands in the turbine giving up heat energy, which is

transformed into mechanical energy on turbine shaft. Thus, Mechanical power is obtained from the turbine shaft.

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BOILER

GENERATOR

TURBINE

Turbine

Power Generation, transmission & Distribution

Mechanical power produced at the shaft of the turbine is used torotate the rotor of an electrical generator that produces electric power. The electric power produced by the generator is boosted to a higher voltage by a generator transformer to reduce the transmissionlosses.This power at EHV i.e. 400 kV is transmitted and distributed by EHV transmission lines.

GENERATOR

A generator consists of rotor which consists of carbon brushes. The rotor rotates at 3000rpm in case of any fault if production of plants stops then we have bearing motor which rotates shaft of turbine continuously and rotor at 65rpm. This is because if shaft doesn’t rotates then due to load it may bend.

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As generator produces 110MW or generates 11kv output. The output of generator is step up to 220kv by using step up transformer or generating transformer. Three phase is fed to station transformer. There are two station transformer1 and 2 which is step down transformer. Here 220kv is step down to 6.6kv for internal purpose. This 6.6kv is step down to 415v for low rating motors. At generating transformer we are using lighting arrestor which protects G.T from lighting. This 220kv is given to grid substation. In grid substation we are using some protective system before distribution we have Bus isolator, SF6 breaker, Line isolator, CT, lightning arrestor. Similarly we have two unit auxiliary transformer UAT-1 and UAT-2, which will step down voltage from 11kv to 6.6kv and it will supply to unit auxiliary board 1BA, 1BB.

Similarly station transformer will supply to station board 9BA, 9BB. One unit is tie with other unit because during the failure of any one of the unit other unit will able to supply.

LIGHT UP PROCESS

MTPS Kanti has direct firing system. In this system, a controlled quantity of crushed coal is fed to each bowl mill (pulveriser) by its respective feeders and primary air is supplied from the primary air fans which dries the coal as it is being pulverized and transports the pulverized coal through the coal piping system to the coal burners.

There are six pulverisers out of which four are used and two remains in standby. The raw coal feeders supply 74 TPH of coal to each mill.

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The pulverized coal and air discharged from the coal burners is directed towards the center of the furnace to form firing ball.There are 24 tilting, tangentially fired coal burners fitted at the four corners of the boiler at six elevations.

The secondary air heating system supplies secondary air for combustion in the furnace around the pulverized coal burners and through auxiliary air compartments directly adjacent to the coal burner compartments. There are 12 air-atomizing ignitors per boiler, which initially ignite the coal and air mixture. Above a predictable minimum loading condition, the ignition becomes self-sustaining. Combustion is completed as the gases spiral up in the furnace.

MILLING SYSTEM

1. COAL BUNKER: -These are in-process storage silos used for storing crushed coal coming from the coal handling plant through conveyor belts.There are six coalbunkers supplying coal to each mill and are located at top of the mills to aid in gravity feeding of the coal. Each bunker can store coal, which can be used for 12hrs.

2. COAL FEEDER: - The purpose of coal feeder is to transfer coal at a pre- determined rate, from coalbunker to the mill.The coal feeder comprises two continuous chains with L sections flight bars mounted between the chains at every fifth link .The chains runs on sprockets mounted at each end of the feeder to given an upper strand movement towards the driven ends and a lower strand movement in the opposite direction. The drive shaft is supported on two self aligning bearing mounted in the Plummer block on support out side the feeder casing, shaft sealing is achieved by the lip seals in the sealing housing and mounted in board of the bearing to abut the feeder casing. The tail sprocket shaft is mounted in adjustable bearing blocks adjacent to the feeder casing with positioned which allow the feeder chain to be tensioned.Both upper and lower strands run over full width carrying plates with the lower strands located by angle section guides mounted on the feeder wall. The upper and lower carrying plates and the inside wall are protected from wear by replaceable stainless steel panels, chains are kept clean by rubber wiper.Feeder input is achieved by roller chain drive to the conveyor via a fixed speed electric motor driving a variable speed gear box, torque limiter and fixed out put gear box The electric motor is flanged mounted to variable speed gear box, coupled to the fixed output gear box by a flexible coupling and torque limiter.

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The principle of operation of coal feeder is that coal flows from the bunker into the chain feeder via feed hopper and is conveyed to the mill, when the feeder is in the operation, the conveyor chain drag a fixed head of coal towards the driven ends of the feeder. At the end of the carrying plates the coal falls through the conveyor onto the bottom plate, where it is picked up by the returning flight bars and dragged back along the feeder to fall into the mill.

3. PULVERISER MILL :-

There are six mills located adjacent to the furnace at 0 m level .These mills pulverize coal to desired fineness to be fed to the furnace for combustion . The main structure of the pulverisering mill is fabricated from mild steel in three cylindrical sections, the bottom section (the mill housing support )which support the entire unit and encloses the mill drive gear unit, a center section (the mill housing)that contains the rotary grinding element and upper section (the classifier housing )comprising an accommodate the gas loading cylinders of the mill loading gear .A platform around the upper section provide an access to an inspection door and to the top of the mill routine maintenance and is served by detachable ladder .

The grinding element comprises of 3 rotatory rollers.

The raw coal enter the mill through inlet and fall into the grinding zones ,where rotating bottom grinding and transport coal through the grinding element into the primary air stream .The primary air enters through the inlet duct in the mill while goes to the furnace from four outlet ducts at the top of the mill.

The ground fuel particle are picked up by the primary air stream after it is passed through the throat plates and carried upwards towards the classifier .The larger particle are initially carried upwards by the air stream and circulate over the upper grinding ring before falling back into the grinding zone by virtue of their weight .The coal /air mixture then passes into the classifier ,where any remaining oversize particle are separated out and fall down to the return skirt until their commutative weight is sufficient to deflect the flaps and return them into the grinding zone .

The setting of the classifier vane control the fineness of the ground product . Heavy material such as pyrites and tramp iron which has passed through grinding zone without being pulverized is carried around throat plate and discharged through a counter balance relief gate into the space below the yoke .

. Main Boiler ComponentsThe major accessories of a steam-generating unit are listed as below:

Furnace.

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Economizer. Boiler drum. Down comers. Water walls. Riser tubes. Super heaters. Reheaters. Desuperheaters.

1. Furnace

A boiler furnace is the first pass of the boiler in which fuel is burned and from which the combustion products pass to the super heater and second pass of boiler.The combustion process is a continuous process, which takes place in first pass of the boiler and controlled by fuel input through coal feeders. It is a radiant type and water-cooled furnace and enclosure is made up of water wall.

The furnace is open at the bottom to allow ash/clinkers to fall freely into the furnace bottom ash hopper (through a ‘furnace throat’), and at the top of its rear wall, above the arch, to allow hot gases to enter the rear gas pass.The basic requirements that a furnace must satisfy are:

1. Proper installation, operation and maintenance of fuel burning equipment.2. Sufficient volume for combustion requirements.3. Adequate refractories and insulation.

2. EconomiserThe function of an economizer in a steam-generating unit is to absorb heat from the flue gases and add this as sensible heat to the feed water before the feed water enters the evaporative circuit of the boiler. This additional heating surface in the path of the feed water increases the efficiency of the steam generating cycle, saving in fuel consumption, thus this additional surface was named as ‘economizer’.

The economizer is placed in the path of the flue gases leaving the boiler, in the boiler rear gas pass below the rear super heater. The economizer is continuous ‘unfinned loop type’ and water flows in upward direction and gas flows in the downward direction. Since water flow is from bottom to top so if any steam is formed during the heat transfer it also moves along with water and prevent the lock up steam which will cause overheating and failure of economizer tube.

A recalculation line with a stop valve and non return valve is incorporated to keep circulation in economizer into boiler drum when there is fire in furnace but it prevents the feed water flow into the boiler drum.

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3. Boiler drumThe boiler drum is a cylindrical pressure vessel with hemispherical ends. It contains two rows of cyclone separators, four rows of drier boxes, a perforated feed water distribution pipe, and a chemical dosing pipe. The boiler drum is located on the upper front of the boiler. It is suspended from roof steelwork by two u-shaped slings.It forms a part of the water circulation system of the boiler. The drum serves mainly two functions:

The first and primary one is that it separates steam from the mixture of water and steam discharged into it and to reduce the dissolved solid contents of the steam to below the prescribed limit of 1 ppm.

Secondly, the drum houses all equipments used for purification of steam after being separated from water. These equipments are known as ‘drum internals’.

Drum internalsThese are the equipments, which are used to separate water from steam and to direct the flow of water and steam to obtain an optimum distribution of drum metal temperature in boiler operation. The drum internals consists of baffle arrangements, devices which change the direction of flow of steam and water mixture, separators employing spinning action for removing water from steam or steam purifiers as washers and screen dryers.Steam output. So, the drum size is determined by the space required to accommodate the steam separating and purifying equipments. The level of water in the stream is monitored by control and interlock equipment.

Drum level is monitored by four level transmitters which are connected across individual drum mounted, short-range constant head chambers. Water level gauge is mounted on each end of the steam drum. If water level goes outside of the prescribed operating limit then the boiler is tripped.

Transformer :- A transformer is an electrical device which works on the principle of mutual induction. The autotransformer used in power station. It has three windings primary, secondary and tertiary. The 220kv voltage is fed as input to primary by step down 132kv fed MTPS as input

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4. Downcomers

Down comers provide a passage for water from the boiler drum to bottom ring header. From bottom ring header the water goes to water walls for heat absorption and conversion into steam heating .To achieve the circulation of water into water wall Boiler circulation pumps are provided in down comers.

5. Waterwalls

Water walls are the necessary elements of the boiler. They serve as the means of heating and evaporating the feed water supplied to the boiler from the economizers via boiler drum and down comers.In large boilers, water walls completely cover the interior surfaces of the furnace providing practically complete elimination of exposed refractory surface. They usually consist of vertical tubes membrane and are connected at the top and at the bottom to headers. These tubes receive water from the boiler drum by means of down comers connected between drum and water walls lower header.Water walls absorb 50 percent of the heat released by the combustion of fuel in the furnace, which is utilized for evaporation of feed water. The mixture of water and steam is discharged from the top of the water walls into the upper wall header and then passes through riser tubes to the steam drum.The design and construction of the water walls depends upon the combustion and steam conditions and the size of the boiler.

6. Riser tubes

A riser is a tube through which the mixture of water and steam pass from an upper water wall header to the steam drum.

7. SuperheaterThe steam generated by the boiler is usually wet or at the most dry saturated because it is in direct contact with water. So, in order to get superheated steam, a device known as superheater has to be incorporated in the boiler. The function of the superheater system, is to accept dry saturated steam from the steam drum and to supply superheated steam at the specified final temperature of 540oC, by means of a series of heat transfer surfaces arranged within the boiler gas passes. A superheater is a surface type heat exchanger generally located in the passage of hot flue gases. The dry saturated steam from the boiler drum flows inside the superheater tubes and the hot flue gases flows over the tubes and in this way its temperature is increased at the same pressure.

The super heater consists of three sections classified as primary super heater, secondary super heater and final super heater. In Kanti, there are 14 super heater coils which are divided into above different sections where temperature is increased from approx. 340oC to 540oC.

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Dry saturated steam from the drum passes through the three sections of super heater, increasing the temperature to approx. 540oC as it travels through each section.

8. Reheater

A reheater is a device that is incorporated in the upper arch of the boiler near the gooseneck in the path of the outgoing flue gases. As the name indicates, it reheats the outlet steam from the HP turbine and thus increasing its temperature up to the desired value. The reheater accept cold reheat steam from the HP turbine exhaust and supply hot reheat steam at the specified outlet steam temperature of 540oC by means of heat transfer surfaces arranged within the boiler gas passes.

The reheater consists of 2 heating coils which finally raise the temperature of the steam to the required level.

Steam from the HP turbine exhaust enters the reheater system through two parallel mounted spray water desuperheaters liners located in the cold reheat pipe work, then passes through reheater, increasing the temperature as it travels through it. Reheater outlet temperature is controlled by raising or lowering the angle of burner tilt.When this reheated steam enters the IP turbine, the net efficiency of the cycle is increased.

9. DesuperheatersA.A. Superheater DesuperheaterSuperheater Desuperheater

The superheater desuperheater is fitted after 10The superheater desuperheater is fitted after 10thth coil to control the superheated steam at coil to control the superheated steam at the specified terminal temperature of 540the specified terminal temperature of 540ooC. The maximum design temperatureC. The maximum design temperature reduction at the superheater desuperheater is from 446 reduction at the superheater desuperheater is from 446 ooC to 388 C to 388 ooC.C.

The desuperheater comprises a spray nozzle shell and associated spray assemblyThe desuperheater comprises a spray nozzle shell and associated spray assembly projecting into a section of the superheater steam line. This section of the steam lineprojecting into a section of the superheater steam line. This section of the steam line forms the desuperheater shell. Steam assisted spray nozzle assembly provides a fineforms the desuperheater shell. Steam assisted spray nozzle assembly provides a fine spray of water which attemperates the steam passing through the desuperheater.spray of water which attemperates the steam passing through the desuperheater.Spray water for desuperheater is taken from the boiler feed water pump discharge. In addition, spray water regulating stations are provided further downstream in each line.

B. Reheater DesuperheaterThe reheater desuperheater is only brought into use when the reheater outlet temperature rises above the normal temperature.The reheater desuperheater comprises of a spray nozzle shell and associated spray nozzle assembly projecting into a section of the steam line between the HP turbine outlet and the reheater inlet headers. This section of the steam line forms the desuperheater shell. Water is fed into the shell from the discharge side of the boiler feed pumps via a reheater desuperheater spray water regulating station.

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When the reheater desuperheater is called into service water is fed via the water tube and passes through the spray nozzle thereby forming a spray which attemprates the steam passing through the desuperheater and thus decreasing the quantity of water in the boiler. Drum is relatively small compared to the total steam output. So, the drum size is determined by the space required to accommodate the steam separating and purifying equipments.

Electrostatic precipitatorThe ash content in the Indian coal is of the order of 30% to 40%. When coal is fired in the boiler, ashes are liberated and about 80% of ash is carried along with the flue gases. If this ash is allowed to atmosphere, it is hazardous to health. So, it became necessary to incorporate an electrostatic precipitator in the path of the flue gases going in the atmosphere. The electrostatic precipitators are preferred to mechanical precipitators because they are capable of precipitating particles from sub micron to large sizes of particles. The efficiency of the modern ESP’s is of the order of 99.9%.

The electrostatic precipitator consists of a large chamber, which comprises of parallel rows of sheet type collecting electrodes suspended from the precipitator casing with wire type discharge electrodes arranged mid-way between them. At the inlet of the chamber, gas distributor screens for uniform distribution of the gases in the chamber, are provided.

The collectors are connected to earth at positive polarity while the discharge electrodes are connected to a high voltage dc supply at negative polarity. When dust-laden gas flows between the electrodes, the corona discharge causes the dust particles to become charged, the particles then being attracted towards and, eventually, deposited on the collector electrodes.

This dust falls as the collecting electrodes are continuously rapped through a rapping system and is collected into the pyramid type hoppers, located beneath each collecting electrodes, from where it is removed by the ash handling system

Air heatersAir heater is a heat transferring device in which air temperature is raised by transferring heat from flue gases. Air heaters are capable of reclaiming heat from the flue gases at low temperature levels and thus reducing the amount of heat rejected to chimney. This results in increasing the boiler efficiency. For every 20 0C drop in flue gas exit temperature, the boiler efficiency increases by about 1%. In MTPS Kanti, regenerative type of air heaters is mainly used. In regenerative air

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heaters, the heating medium i.e. flue gases flows through a closely packed matrix structure and then air is passed through the matrix to pickup the heat. There are two regenerative type of main air heaters for heating up the air from fans.

TYPES OF FAN

A fan is a device by which the air is made to flow at required velocity and pressure in a defined path imparting K.E of its impellers to air/flue gases . This pressure boost is used to create a draught in the air and flue gas system. Fans mainly performs two functions:

i. They supply air required for combustion in the furnace with required pressure & flow.

ii. They evacuate the product of combustion i.e. flue gases into the atmosphere via chimney.

1. P. A. FAN

The primary air fan supplies heated air to the coal mills known as primary air, to give dry and pulverized coal to the furnace for efficient combustion. There are two P.A fans per boiler. The fan impeller is a double inlet, centrifugal wheel with backward curved plate blades.

Ambient air is drawn into the P.A duct by two 50% duty, motor driven centrifugal fans. The air from each fan discharges into a hot air crossover duct via a steam air heater. This duct extends around to each side of the boiler to supply the hot air to mills duct, both of which are branched to supply hot air to four coal mills.

2. F.D. FAN

The forced draught fan system is provided to supply secondary air required for pulverized coal combustion in the furnace, air for fuel oil combustion and over fire air to minimize Nox production.The F.D fan system comprises of two single stage axial flow, constant speed, and auto variable pitch fans per boiler. These fans provide pressurized atmospheric air to the boiler for combustion.Ambient air is drawn into the secondary air system by two 50% duty, motor driven, axial flow forced draught fans with variable pitch control. The air from each fan discharges into a hot air crossover duct via a main air heater. This duct extends around to each side of the boiler furnace to form two secondary air to burners ducts. At the sides of the furnace, each duct split to supply air to two corners.

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3. I.D. FAN

The induced draught fan system comprises of three centrifugal double inlet fans per boiler, two operating and one standby. Each fan unit consists of a backward curved plate bladed impeller, which is driven by an electric motor through a variable speed hydraulic coupling. The I.D fan serves the purpose of evacuating the products of combustion or the flue gases in the atmosphere via chimney. The flue gases after being cleaned in the precipitators is directed towards the atmosphere through the chimney.

TYPES OF PUMP

1. Condensate Extraction Pump (CEP)

The function of Condensate extraction pumps is to pump out the condensate to the deaerator through, LP heaters. The steam from the LP cylinders exhausts into the condenser shells where it is constrained to flow across the water tubes, through which cooling water is circulated.

The steam condensed on the tubes drain to the bottom of the shell and is collected in a hotwell .The condensate is retained in the hotwell by means of the condenser level control valve. The water in a condenser provides a head of water for the condensate extraction pump to suppress cavitations in its suction impellers.

There are two 100% duty extraction pumps, one remains in duty and one remains stand by. With all the necessary instruments such as suction and discharge valve isolating and dump valves to insure efficient operation.

The thrust bearings in the driving motors have temperatures sensor, which can trip the motors automatically.

The pump discharge the condensate to the LP heater system with a pressure increased to approx. 18 kg/sq. cm from 70-75 mm of Hg.

2. Air Extraction Pump (AEP)The function of the air extraction pump is to raise and maintain the vacuum conditions in the turbine main condensers, and to remove air and other non-condensable gases vented to the condenser from various parts of the turbine and feedwater heating system.

3. Boiler Feed Pump (BFP)

Boiler feed pump is the most critical component of a power plant. It is a rotary machine,

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which is coupled to a motor through variable speed coupling or turbo coupling.

Feed water is supplied to the boiler of each turbo-generator set by three 50% tandem boiler feed water pump sets. Under normal conditions two 50% boiler feed water pump sets are run in parallel to undertake the complete load of feeding the boiler, while the third 50% pump set is on standby duty.

Feed water suppied to the boiler drum should have high pressure which is achieved when passed through boiler feed pump. Whenever the pressure of water is to be raised, BFP is used.

The discharge pressure of a boiler feed pump is approx. 150 kg/sq. cm.

TYPES OF CYCLE

1. Steam Cycle

Drum S. H. H. P. T. R. H. I. P. T.

Condenser L. P. T.

A thermal power plant is based upon the principle of conversion of heat energy (steam energy) into mechanical energy. For this conversion of energy a power plant requires a turbo machine.

A turbo machine is a power producing thermodynamic machine. It requires a suitable working fluid, a source of high-grade energy and a sink for low-grade energy. In a thermal power plant water is used as a working fluid and it is converted into steam.

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Booster Pump

Motor Hydraulic Coupling

Deaerator

BFP

The steam turbine is a device that converts heat energy of the steam coming from the boiler into the mechanical energy (kinetic energy) with the help of which we rotate our shaft.

Steam is formed in the rising tubes of water walls and is collected in the upper portion of the drum which is separated by the water in the drum by the drum internals. This steam contains some water droplets which is to be removed before reaching turbines. This steam is heated in superheater (primary superheater, secondary superheater & final superheater) which makes this steam free from water droplets.Main steam is then 1st applied to high pressure turbine (HPT) at temp. approx. 540oC & pressure 138kg/cm2. The steam coming out from HPT has low temp. & pressure and required to raise this temp. & pressure before applying to next turbine. Hence, it is passed through reheater due to which its temp. & pressure is raised enough and is then applied to intermediate pressure turbine(IPT) and steam coming out of this turbine is directly used to rotate the final low pressure turbine(LPT).Steam from this turbine has very low pressure & temp. and can’t be further used to rotate the turbine. So, it is condensed and converted to water before sending to the drum for reuse.

2. Water Cycle

Hotwell C. E. P. L. P. H. Deaerator

Drum Economizer H. P. H. B. F. P.

Water cycle starts from the condenser and ends to the drum.Steam from the LPT is condensed in the condenser while condensed steam known as condensate is collected in a hotwell having temp. about 40o C & pressure 70-75 mm of Hg. The pressure of this condensate is increased to approx. 18 kg/cm2 by using CEP while the temp. is increased to approx. 80oC by using low pressure heater(LPH). To remove the dissolved oxygen from condensate, deaerator is used and then it is passed through BFP to raise its pressure approx. to the drum water. To further increase its temp. upto 130oC, it is passed through high pressure heater (HPH). Then finally before sending it to drum its temp. is raised to approx. 250oC by passing it through economizer. Hence, the extra steam is condensed and reused.

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3. Flue Gas Cycle

Furnace S. H. R. H. Economizer

Chimney E. S. P. A. P. H

The fuel such as coal when heated in the furnace produces smoke and ash. This smoke produced is known as FLUE GAS whose temperature is very high and so used to heat few systems such as superheater & reheater. The flue gas is produced in the furnace. It then heats superheater & reheater after which it heats economizer and air preheater (APH). Since now also its temperature is quite high and also contains some dust hence, it is precipitated in electrostatic precipitator (ESP) before leaving chimney.

TYPES OF HEATER

1. High Pressure Heater (HPH)

In the water cycle, temperature of feed water from BFP is increased to approx. 130oC by heating it in HP heater. As the heating of the feed water in HP heater is done by the extra steam coming out of the High Pressure Turbine(HPT) hence, it is named as High Pressure Heater(HPH).

2. Low Pressure Heater (LPH)

In the water cycle, temperature of condensate from CEP is raised to approx. 80oC by heating it in LP heater. As the heating of the condensate in LP heater is done by the extra steam coming out of the Low Pressure Turbine(LPT) hence, it is named as Low Pressure Heater(LPH).

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TYPES OF TURBINE

1. HP Turbine HP turbine is a single flow design with eight stages of blading .Each stage comprises stationary and moving blades which are positioned into the rotor mounted on the diaphragms, directs steam into the rotor mounted on the moving blades. H.P. turbine is double shell construction comprising inner and outer casing. H.P steam enters the H.P. turbine inner casing through vertical inlet connection are mounted on the top and bottom outer casing .The steam directed through the diaphragm expands through the rotor blades and diaphragm towards the fronts of the cylinder. The steam exhausts through the two branches in the bottom half casing and returns to the boiler to be reheated to increase the temperature of the steam to 538oC so that the efficiency of Rankine Cycle increases.

2. I.P. Turbine:-

Intermediate pressure turbine is a double flow design with seven stage of blading on either side of central steam inlet. Each stage comprises stationary and moving blades which are positioned so that the stationary blades mounted on diaphragm, directs the steam into the rotor mounted moving blades .

Turbine is double shell construction inner casing , two diaphragm carries the ring , and outer casing .The first 4 stage of each flow are located within the inner casing and remaining stage within the diaphragm carries the ring .The inner casing, diaphragm carrier ring and outer casing are made in halves bolted together in the horizontal centre.

3. L.P. Turbine:-

LP turbine is of double flow design incorporating six stages in each of its front and rear flow paths. Each stage consist of number of stationary blades incorporating in the diaphragm located in the casing and a set of rotating blades mounted on a rotor disc .

A spray water system design to operate automatically ,ensure that excessive temperature are not produced in the exhaust flow during prolonged operation at low turbine load /low condenser vacuum.

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ASSEMBLY OF TURBINE GENERATOR EXCITER AND BARRING GEAR

D B IPT LPT 1 )

HPT Exciter

C A Electrical Barring Gear Generator (15.7 KV, 3000 rpm) Steam Condenser (3 phase, 50 Hz)

CRH C & D :- STOP VALVESA & B :- GOVERNING VALVE

FLAME SCANNERS

In a flame there are three zones.1.Visible zone2. UV zone3.Infrared zone

The flame scanner consists of UV light sensitive tube and UV light sensitive element filled inside the tube on which 700 DC volt is supplied.Initially there is no contact between the two electrode on which 700 DC volt is supplied. As there is UV light sensitive element present inside the tube, it sacns the UV zone of the flame. When it scans the UV zone, the UV element present inside the tube conducts and the two electrode are in contact. Now, the supplied voltage is reduced to zero. Hence, whenever it scans the UV zone, the supplied voltage becomes 0V otherwise it is 700V. Therefore, on an average the scanner shows 400V – 450V which confirms the presence of flame inside the furnace.

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As there are two types of fuel which are the main source of burning. Hence, basically there are two types of flame scanners depending upon the fuel used. So, to sense the flame due to oil used in the furnace there are oil flame scanners and to sense the flame due to coal used are known as coal flame scanners.

There are 12 oil guns present in the four corners of the furnace in the elevations AB, CD & EF known as oil elevations. Hence, there are 12 oil flame scanners present corresponding to each oil guns. There are 16 coal flame scanners present inside the furnace, four on each corners between AB, BC, DE & EF where A,B,C,D,E & F are the coal ducts. Coal flame scanners are also known as fireball scanners.

IMPORTANT CONTROL LOOPS IN A THERMAL POWER PLANT

Basic Block Diagram Of Any Closed Control Loop

Set Point Error

Process: The equipment whose present level, pressure and other values is to be measured is known as process.

Set Point: The required value of parameter is set by the manual which is to be maintained in order to protect the process from damage.

Measurement: The present value of parameter in process is measured here. Generally, capacitance type of measurement is used. Two tapping from the process, one at high pressure & other at low pressure, is taken and transmitted through isolating diaphragm and silicon oil fill fluid to a sensing diaphragm in the centre of the differential pressure cell. The sensing diaphragm deflects in response to differential pressure. The position of the sensing diaphragm is deflected by capacitor plates on both sides of the sensing diaphragm. The differential capacitor between the sensing diaphragm and the capacitor plate is converted electronically to a 4-20 mA signal and transmitted to comparator. This measurement sometimes also known as transmitter.

Comparator: It compares the signal between set point and measured value. If the two values differ from one another, an error signal is generated and sent to the controller.

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+ Controller Final Control Element

Process

Measurement

Controller: It is an electronic card which, according to the error signal sent by comparator, gives a current signal between 4-20 mA to final control element.

Final Control Element: It is that portion of the loop which directly changes the value of the manipulated process variable and finally do some work to maintain the set point of the process.

1. Drum Level Control

Set Point Error

The required drum level is set at the set point. The present drum level is then measured which is done by capacitor type transmitter. Two tapping, one at the bottom in water while other at the top in steam, is made and allowed to flow to the transmitter. Since, the two elements are in different states, steam is condensed and collected in a constant head unit (CHU) before going to the transmitter where the present drum level is measured and converted to current signal between 4-20 mA. This set value and measured value are then compared in a comparator and an error signal, if any, is generated and sent to controller which finally directs the final control element to control the drum level. Here, the final control element is a control valve through which a fluid passes that adjusts the size of the flow passage as directed by a signal from controller to modify the rate of flow of the fluid. Hence, the drum level is controlled.

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+ Controller Feed Water Control

Process

Measurement/FB

2. D.P. Across Feed Control Station

Set Point Error

In order to maintain the linear characteristics of the feed regulating valves under different loads, the differential pressure(D.P.) control loops maintains a fixed differential across the regulating valves and BFP discharge pressure is varied by changing BFP motor speed through hydraulic scoop tube device which is the final control element here. The D.P. across feed station (comparator) is sensed and is fed to the controller. The controller are automatically adjusted as function of steam flow to achieve stable condition. The reserve Boiler Feed pump scoop tube automatically follows the running pump scoop tube and the changeover to the reserve BFP takes place with the scoop tube in the same position of the scoop tube.

3. Combustion Control

The combustion control proposed for this boiler comprises of the following loops:

a. Master pressure controlb. Pulverized coal flow controlc. Combustion air flow control d. Oxygen trim controle. Mill temperature and air flow control

a. Master Pressure Control

The turbine throttle pressure which is a measure of turbine and boiler mismatch, is maintained by proper fuel and air flow control to the burners. Actual steam pressure at turbine inlet is measured and error against a set value is fed to the individual pulverize control loop through controller.

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+ Controller BFP Scoop Process

Measurement

b. Flue Flow Control

In order to maintain an air rich furnace, air flow demand signal is superimposed over total fuel flow signal through a high limiter unit. This way when master demand signal increases and if air flow is low, fuel flow is not straightaway increased. Instead, main demand signal first increases the air flow and only when demand signal is low as compared to air flow, tie selector unit in the fuel control loop increases the fuel flow. When the master demand calls for a reduction of combustion, fuel flow and air flow are reduced simultaneously with fuel flow leading air flow, thus ensuring always an air rich furnace.

c. Combustion Air Flow Control

Total air flow signal is fed to control and this controller output adjusts the FD fan vanes. Provision also exists to ensure a minimum air flow (30% of maximum) through a high signal selector.

d. Oxygen Trim Control

To ensure some percentage of excess air for optimum combustion, Oxygen trim control is employed. Oxygen contain in flue gas before air preheater is measured and error is fed to controller. A maximum/minimum limiter is introduced so that should the oxygen supply fall, a minimum disturbance is introduced in the flue/air control loop.

e. Mill Temperature And Flow Control

This control loop is envisaged to maintain constant air flow to the mills and also to maintain constant mill outlet temperature. Primary air flow and mill outlet temperature signals are measured and fed to the controller respectively. Output of the controllers are connected to each of the two error modules, the output of which are going to coal and hot air dampers through respective auto manual stations. The provision of variable air flow supply exists in the hardware supplied and shall be adopted on site, if required.

4. Furnace Draft Control

The furnace draft is maintained by modulating the I D fan Hydraulic coupling (3 nos.). Furnace draft at combustion chamber outlet is measured and the error is fed to the controller. Output of this controller accordingly positions the vanes to maintain constant furnace pressure to improve the system dynamic response. An anticipator total air flow signal is alsoadded in the loop.

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5. Primary Air Header Pressure Control

A control loop always ensure sufficient PA to the pulveriser hot air duct at the set pressure and achieves the same by modulating the PA fan vanes (2 nos.).

6. Superheat Steam Temperature Control

The superheater steam temperature control system makes use of three parameters, secondary superheater outlet temperature, total steam flow and superheater inlet temperature. The final superheaters temperature error is fed to controllers which positin spray control valves left & right sides to maintain constant superheater outlet temperature.

7. Reheat Steam Temperature Control

Reheat outlet steam temperature is maintained by tilting the burner to increase/decrease heat absorption in the reheat section of the boiler. Additional emergency reheat spray is used to maintain temp. when burner tilts are unable to reduce the temp. sufficiently.Left and right reheater temp. signals are averaged and fed to controller. Controller output is indexed with total steam flow signal and through an auto-manual station drives nozzle tilt drive. In case of differential reheater temp. difference above allowable limits, respective spray control valve left or right are used to bring balance in left and right side R.H. steam temperatures.

8. BFP Minimum Flow Recirculation Control

In order to ensure safety of the pump against overheating, the minimum flow is to be maintained when the pump flow reduces below a preset limit. This is achieved by a reliable pneumatically operated minimum flow recirculation control valve with built-in pressure breakdown device. The control envisaged is an on-off control, the operation of which is initiated by a low range DP switch sensing the boiler feed pump flow. Whenever the flow falls below 100 T/hr., the minimum circulation valve is opened and when the flow increases above 200 T/hr., this valve is kept closed. Indication is provided on the UCB to indicate the operator the status of this valve by open-close position indication lamp.

9. Hotwell Level Control

Hotwell level is maintained by recirculation of the condensate after steam jet air ejector through a level controller and split-range control valves. Any excess condensate is, therefore, fed to the deaerator.

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10. Deaerator High & Low Level Control

The deaerator low level control acts on the condenser make-up control valve to add DM water in the hotwell and the high level control acts on the excess condensate to Unit condensate floating tank. Two separate control loops have been provided for the above.

11. Secondary Air Damper Controls

The function of a secondary air damper control system is to distribute the secondary air from the windbox requirements. Electronic analog control system is used for this application. Duplicate transmitters of 4-20 mA dc output for Heavy oil and furnace/wind box differential pressure are used to improve system availability. The 4-20 mA dc output from the control system is converted into pneumatic signal using E/P signal converter to position power cylinder operated dampers at all the elevations on four corners. Electric to pneumatic (E/P) signal converters are field mounted type.

UNIT CONTROL DESK & PANELSThe operation of each unit is envisaged from the central unit control room. It is located in the control bay at 9.0m TG floor. It is adequately illuminated and is centrally air conditioned. For operational convenience, the control room front wall has complete glass paneling for TG hall view and the two double doors for entry from TG hall.

The control board has a special profile with three sloping surfaces for mounting a large facias, instruments and controls. The automatic control station and drive contrl switches & Indications are located on the first sloping surface. The process indicators/recorders and ammeters are mounted on the second sloping surface and the alarm annunciation window facias are mounted on the top i.e. third sloping surface. The unit control board are arranged in logical operating sequence from the left to right starting with (i.)Air & Flue Gas, (ii.)Fuel oil, (iii) Bowl Mills, (iv)Steam & Feed water, (v)Regenerative System, (vi)Turbine and (vii) Generator.

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References:

‘Modern power station practice-Volume-B. -Volume-C.

‘Power plant Engg.’ By P.K.NAG

Control & Instrumentation-Volumel

Operation & Maintenance Manual (MTPS)-Volume H

THANK YOU

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