Summer Training Report On
“National Thermal Power Corporation”
Submitted by :NITISH BHARDWAJ B .TECH {EE} THAPAR UNIVERSITY,PATIALA
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ACKNOWLEDGEMENT With deep reverence and profound gratitude. I express my sincere thanks to Mr. S. DASfor giving me an opportunity to do training at NTPC Barh. I also would like to thank Mr. PANKAJ KUMAR who has helped me at the working sites, explaining and giving me all the information I needed to complete this report. I am also very much thankful to Mr. PANKAJ KUMAR, helping me throughout the training. At last I would like to convey my thanks to all the members of the staff of NTPC Barh(patna) who have helped me at every stage of training. Training Period: July 1 to 30 NITISH BHARDWAJ ROLL No:-101404077 B.TECH {SECOND YEAR} ELECTRICAL ENGINEERING.
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ABSTRACT I was appointed to do 4 week training at this esteemed organization from 1 July 2016 to 30july, 2016. I was assigned to visit various division of plant, which were; Boiler Maintenance Department (BMD) Turbine Maintenance Department (TMD) Plant Auxiliary Maintenance (PAM) These 4 weeks training was a very educational adventure for me. It was really amazing to see the plant by yourself and learn how electricity, which is one of our daily requirements of life, is produced. This report has been made by my experience at NTPC Barh. The material in this report has been gathered from my textbook, senior student reports and trainers manuals and power journals provided by training department. The specification and principles are as learned by me from the employees of each division of NTPC. 3
TABLE OF CONTENTS Contents
Page no.
Acknowledgement
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Certificate
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Abstract
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List of figures
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1. Introduction
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1.1 Company overview
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1.2 Training overview
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2. Product/Process details
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2.1 Operation of a power plant
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2.2 Basic steps of electricity generation
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2.3 Rankine cycle
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3. Details of training
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3.1 Department/Section Detail
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3.1.1 Boiler Maintenance Department
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3.1.2 Plant Auxiliary Maintenance
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3.1.3 Turbine Maintenance Department
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3.2 Coal Handling Department
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LIST OF FIGURES Figure Figure 1.1 Growth of NTPC Installed Capacity & Generation Chart
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Figure 1.2 Power Contribution chart of NTPC in INDIA
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Figure 1.3 Strategies Chart of NTPC
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Figure 2.1: Block Diagram Of NTPC Power Plant
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Figure 2.2 thevarious parts of the coal thermal power plants
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Figure 2.3 Operation of a Rankine cycle
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Figure 2.4 T-S diagram of a typical Rankine cycle
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Figure 2.5 Boiler Drum
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Figure 3.1 Reheater
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Figure 3.2 Economizer
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Figure 3.3 Air pre-heater
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Figure 3.4 Pulverizer
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Figure 3.5 Ash handling system
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Figure 3.6 Water treatment plant
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Figure 3.7 Demineralization
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Figure 3.8 Operating principle of steam turbine
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Figure 3.9 steam cycle diagram
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Figure 3.10 Turbine & Turbine cycle
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Figure 3.11 A Typical water cooled condenser
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Figure 3.12 A Deaerator
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Figure 3.13 Coal cycle diagram
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Figure 3.14 Coal handling system
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Figure 3.15 Coal handling division at NTPC
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Figure 3.16 A Idler
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Figure 3.17 Coal Storage Area of the NTPC 40
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1. INTRODUCTION 1.1Company Overview NTPC is the largest thermal power generating company of India. India’s largest power company,NTPC was set up in 1975 to accelerate power development in 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 MaharatnaCompany in May, 2010, one of the only four companies to be awarded this status. The total installed capacity of the company is 39,174 MW (including JVs) with 18 coal based and 7 gas based stations, located across the country. In addition under JVs, 7 stations are coal based & another station uses LNG as fuel. The company has set a target to have an installed power generating capacity of 128000 MW by the year 2032. The capacity will have a diversified fuel mix comprising 56% coal, 16% Gas, 11% Nuclear and 17% Renewable Energy Sources(RES) including hydro. By 2032, non-fossil fuel based generation capacity shall make up nearly 28% of NTPC”s portfolio. NTPC has been operating its plants at high efficiency levels. Although the company has 17.75% of the total national capacity, it contributes 27.40% of total power generation due to its focus on high efficiency.
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Figure 1.1 Growth of NTPC Installed Capacity & Generation Chart 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. The rest is held by Institutional Investors and the Public.
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NTPC contributes more than one-fourth of India’s total power generation with less than one-fifth capacity
TOTAL CAPACITY AS ON 2016
GENERATION 2015-2016 27.4 ALL India
17.75
72.6
82.25 Rest of India
NTPC
NTPC
NTPC 220.54* BUs
308308* MW
ALL India India 805.5* Bus
1,73,626 MW
Figure 1.2Power Contributions chart of NTPC in INDIA
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NTPC ALL
Figure 1.3 Strategies Chart of NTPC
JOURNY OF NTPC
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Table 1.1 Chart Journey of NTPC
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1.2 Training Overview ABOUT NTPC Barh Super Thermal Power Station or NTPC Barh is located in Barh in the Indian state of Bihar. NTPC Barh is located barely four kilometres east of the Barh sub-division on National Highway-31 in Patna district. The project has been named a mega power project, and is owned by Indian energy company National Thermal Power Corporation. The 1,980MW (3x660 MW) Barh Stage-1 is being built by Russian firm Technopromexport (TPE), and 1,320MW (2x660 MW) Barh Stage-2 extension is being built by BHEL. Bihar's share is 1183 MW from NTPC Barh(26% from stage 1 and 50% from stage 2). The main power plant and the township is spread over an area of 1,186 acres.The legal possession of 1,186 acres of land has been acquired for setting up the main power plant and its township which includes 12 villages. The then PM, Atal Bihari Vajpayee, had laid the foundation stone of the main plant of stage-1 of NTPC Barh on March 6, 1999. The formal inauguration of its site office and laying of the foundation stone of the training centre at the plant site was done in September 2003. Former Union power minister Sushil Kumar Shinde had inaugurated the main plant house of stage-2 of NTPC Barh on May 29, 2006
Project Cost The plant is to produce 3,300MW of power at a cost of over Rs 26,000 crore. The total approved cost of stage-1 (660MWx3) has Rs 8,692.97 crore. The total approved cost of stage-2 (unit 1) plant is Rs 7,688.12 crore.
Capacity Unit-1 of stage-2 commissioned in November 2013. Sikkim and West Bengal were allocated some share of power from NTPC Barh units. Inland Waterways Authority of India received bids in 2013 September for coal movement on 1620 km National Waterway 1 (India) from Haldia to NTPC's Barh which is about 1000 km away.
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Sta ge 1st
Unit numb er 1
2 3 2nd
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EPC Contractor
Date of Capac Status commissio ity in ning (MW) Tecchnopromex 660 Delayed as port Techniproexport is saying it has entered a lossmaking contract and is asking for $570 million before it can fulfil its obligation to which NTPC objected. Technopromexp 660 Delayed ort Technopromexp 660 Delayed ort Bharat Heavy 2013 Commissioned Electricals November 660 in 2013 November but commercial production yet to start as there are technical issue of multiple tube leakage which is yet to be resolved as of April 2014. This is the first 660 MW unit supplied by BHEL. Commercial operation starts
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Bharat Heavy 2015 Electricals March
Tota 5 l
660
in Nov 2014 after resolving faults. Running
3300
2. PRODUCT/PROCESS DETAILS 2.1 Operation of a power plant Basic Principle:As per FARADAY’S Law-“Whenever the amount of magnetic flux linked with a circuit changes, an EMF is produced in the circuit. Generator works on the principle of producing electricity. To change the flux in the generator turbine is moved in a great speed with steam.” To produce steam, water is heated in the boilers by burning the coal. In Barh Thermal PowerStation, steam is produced and used to spin a turbine that operates a generator. Water is heated, turns into steam and spins a steam turbine which drives an electrical generator. After it passes through the turbine, the steam is condensed in a condenser; this is known as a Rankin cycle. The electricity generated at the plant is sent to consumers through high-voltage power lines The Barh Thermal Power Plant has Steam Turbine-Driven Generators which has a collective capacity of 660MW. The fuel being used is Coal which is supplied from the Jharia Coal Field in Jharkhand. Water supply is given from the Ganga.
2.2 Basic steps of electricity generation The basic steps in the generation of electricity from coal involves following steps: Coal to steam Steam to mechanical power Mechanical power to electrical power
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Figure 2.1: Block Diagram Of NTPC Power Plant
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The various parts of the coal thermal power plants are
Figure 2.2 thevarious parts of the coal thermal power plants 1. Cooling Tower: Cooling 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. 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-
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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 forceddraft or inducedDraft towers or in natural draft hyperbolic shaped cooling towers as seen at most nuclear power plants. 2. Cooling Water Pump:it pumps the water from the cooling tower which goes to the condenser. 3. Three phase transmission line:Three phase electric power is a common method of electric power transmission. It is a type of poly phase 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 eachelectromagnetic 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 3 phase, 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 centre 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. 5. Electrical generator:An 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 may be 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
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"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 theirgenerators.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-polegenerator rather than the more common 2-pole one. 6. Low Pressure Turbine:Energy 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 therotating 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 4x2 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. Condensation Extraction Pump:A Boiler 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 positivedisplacement type. 8. Condenser:The 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. 9. Intermediate Pressure Turbine:Intermediate Pressure Turbine (IPT) consists of 11 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 Valve:Steam 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
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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 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 Turbine:Steam coming from Boiler directly feeds into HPT at a temperature of 540°C and at a pressure of 136 kg/cm². Here it passes through 12 different stages due to which its temperature goes down to 329°C and pressure as 27 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. Deaerator:A Deaerator is a 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 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 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 deaerator vendors will guarantee that oxygen in the deaerated water will not exceed 7 ppb by weight (0.005 cm3/L) 13. Feed water heater:A 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 boilermetal when the feed water is introduced back into the steam cycle. In a steam power (usually modeled as a modified Rankin 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 conveyor:Coal 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
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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. 15. Coal Hopper:Coal Hoppers are the places which are used to feed coal to Fuel 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 Fuel Mill:A pulveriser is a device for grinding coal for combustion in a furnace in a fossil fuel power plant. 17. Boiler drums:Steam 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. 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 (probablystainless) 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 indicator and fuse plug. 18. Ash Hopper:A 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 Heater:A 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 were applied to steam locomotives in quantity from the early 20th century, to most steam vehicles, and also stationary steam engines including power stations. 20. Force Draught Fan:External fans are provided to give sufficient air for combustion. The forced draught fan takes air from the atmosphere and, warms it in the air preheated for better combustion, injects it via the air nozzles on the furnace wall. 21. Reheater:Reheater is a heater which is used to raise the temperature of steam which has fallen from the intermediate pressure turbine 22. Air Intake:Air is taken from the environment by an air intake tower which is fed to the fuel.
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23. Economizers:Economizer, or in the UK economizer, 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 boilers efficiency. They are devices fitted to a boiler which save energy by using 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 (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 Preheater :Air preheated 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 preheater 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. 25. Precipitator: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 largest coal-fired boiler applications. The original parallel plateWeighted 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 Fan:The 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
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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 stacks: A 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 combustiondevice. Flue gas is usually composed of carbon dioxide (CO2) and water vapour 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 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. The flue gases are exhausted from stoves, ovens, fireplaces or other small sources within residential abodes, restaurants, hotels through other stacks which are referred to as chimneys.
2.3 RANKINECYCLE:The Rankine cycle is a thermodynamics cycle which converts heat into work. The heat is supplied externally to a closed loop, which usually uses water as the working fluid. This cycle generates about 80% of all electricity power used throughout the world, including virtually all solar thermal, biomass, coal and nuclear power plants. It is named after William John MacqueenRankine, a Scottish polymath.
DESCRIPTION:A Rankine cycle describes a model of the operation of a steam heat
Figure 2.3Operation of Rankine cycle
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that a pump is used to pressurize liquid instead of gas. This requires about 1/100 th (1%)as much energy 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 T-S diagram will begin to resemble the Carnot cycle. The main difference is 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 temperature are around 30°C. This gives a theoretical Carnot efficiency of around63% compared with an actual efficiency of 42% for a modern coal-fired power station. This low turbine entry temperature (compared with a gas turbine) is why the Rankine cycle is often used as a bottoming cycle in combined cycle gas turbine power stations. The working fluid in a Rankine cycle follows a closed loop and is re-used constantly. The water vapour and entrained droplets often seen billowing from power stations is generated by the cooling systems (not from the closed loop Rankine power cycle) and represents the waste heat that could not be converted to useful work. Note that cooling towers operate using the latent heat of vaporization of the cooling fluid. The white billowing clouds that form in cooling tower operation are the result of water droplets which are entrained in the cooling tower air flow; it is not, as commonly thought, steam. While many substances could be used in the Rankine cycle, water is usually the fluid of choice due to its favourable properties, such as nontoxic and uncreative 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.
Figure 2.4T-S diagram of a typical Rankine cycle 22
T-S diagram of a typical Rankine cycle operating between pressures of 0.06bar and 50bar .There are four processes in the Rankine cycle, each changing the state of the working fluid. These states are identified by number in the diagram to the right. i. Process 1-2: The working fluid is pumped from low to high pressure, as the fluid is a liquid at this stage the pump requires little input energy. ii. Process 2-3: The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a dry saturated vapour. iii. Process 3-4: The dry saturated vapour expands through a turbine, generating power. This decreases the temperature and pressure of the vapour, and some condensation may occur. iv. Process 4-1: The wet vapour then enters a condenser where it is condensed at a constant pressure and temperature to become 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. Process 1-2 and 3-4 would be represented by vertical lines on the T-S diagram and more closely resemble that of the Carnot cycle. The Rankine cycle shown here prevents the vapour ending up in the super heated region after the expansion in the turbine, which reduces the energy removed by the condensers.
3. DETAILS OF TRAINING 3.1 Department/Section Detail 3.1.1Boiler Maintenance Department (BMD) Boiler and Its Description: The boiler is a rectangular furnace about 50 ft (15 m) on a side and 130 ft (40 m) tall. Its walls are made of a web of high pressure steel tubes about 2.3inches (60 mm) in diameter. Pulverized coal is air-blown into the furnace from fuel nozzles at the four corners and it rapidly burns, foaming a large fireball at the centre. The thermal radiation of the fireball heats the water that circulates through the boiler tubes near the boiler perimeter. The water circulation rate in the boiler is three to four times the throughput and is typically driven by pumps. As the water in the boiler circulates it absorbs heat and changes into steam at700 °F (370 °C) and 3200psi (22.1MPa). It is separated from the water inside a drum at the top of 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 1,000 °F (540°C)to prepare it for the turbine. The steam generating boiler has to produce steam at the high purity, pressure and temperature required for the steam turbine that drives the electrical generator. The generator includes the economizer, the steam drum, the chemical dosing equipment, and the furnace with its steam generating tubes and the superheated coils. Necessary safety valves are
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located at suitable points to avoid excessive boiler pressure. The air and flue gas path equipment include: forced draft (FD) fan, air preheated (APH), boiler furnace, induced draft (ID) fan, fly ash collectors (electrostatic precipitator or bag house) and the flue gas stack. For units over about 210MW capacity, redundancy of key components is provided by installing duplicates of the FD fan, APH, fly ash collectors and ID fan with isolating dampers. On some units of about 60MW, two boilers per unit may instead be provided.
AUXILARYIES OF BOILER: I. FURNACE Furnace is primary part of boiler where the chemical energy of the fuel is converted to thermal energy by combustion. Furnace is designed for efficient and complete combustion. Major factors that assist for efficient combustion are amount of fuel inside the furnace and turbulence, which causes rapid mixing between fuel and air. In modern boilers, water furnaces are used.
II. BOILER DRUM
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Figure 2.5 Boiler Drum Drum is of fusion-welded design with welded hemispherical dished ends. It is provided with stubs for welding all the connecting tubes, i.e. down comer, risers, pipes, saturated steam outlet. The function of steam drum internals is to separate the water from the steam generated in the furnace walls and to reduce the dissolved solid contents of the steam below the prescribed limit of 1ppm and also take care of the sudden change of steam demand for boiler. The secondary stage of two opposite banks of closely spaced thin corrugated sheets, which direct the steam and force the remaining entertained water against the corrugated plates. Since the velocity is relatively low this water does not get picked up again but runs down the plates and off the second stage of the two steam outlets. From the secondary separators the steam flows upwards to the series of screen dryers, extending in layers across the length of the drum. These screens perform the final stage of the separation. Once water inside the boiler or steam generator, the process of adding the latent heat of vaporization or enthalpy is underway. The boiler transfers energy to the water by the chemical reaction of burning some type of fuel. The water enters the boiler through a section in the convection pass called the economizer. From the economizer it passes to the steam drum. Once the water enters the steam drum it goes down the down comers to the lower inlet water wall headers. From the inlet headers the water rises through the water walls and is eventually turned into steam due to the heat being generated by the burners located on the front and rear water walls (typically). As the water is turned into steam/vapour in the water walls, the steam/vapour once again enters the steam drum. The steam/vapour is passed through a series of steam and water separators and then dryers inside the steam drum. The steam separators and dryers remove the water droplets from the steam and the cycle through the water walls is repeated. This process is known as natural circulation. The boiler furnace auxiliary equipment includes coal feed nozzles and igniter’s guns, so out 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. Geothermal plants need no boilers incest they use naturally occurring steam sources. Heat exchangers may be used where the geothermal steam is very corrosive or contains excessive suspended solids. Nuclear plants also boil water to raise steam, either directly
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passing the working steam through the reactor or else using an intermediate heat exchanger.
III. WATER WALLS: Water flows to the water walls from the boiler drum by natural circulation. The front and the two side water walls constitute the main evaporation surface, absorbing the bulk of radiant heat of the fuel burnt in the chamber. The front and rear walls are bent at the lower ends to form a watercooled slag hopper. The upper part of the chamber is narrowed to achieve perfect mixing of combustion gases. The water wall tubes are connected to headers at the top and bottom. The rear water wall tubes at the top are grounded in four rows at wider pitch forming the grid tubes.
IV.REHEATER: Reheater is used to raise the temperature of steam from which a part of energy has been extracted in high-pressure turbine. This is another method of increasing the cycle efficiency.Reheating requires additional equipment i.e. heating surface connecting boiler and turbine pipe safety equipment like safety valve, non return valves, isolating valves, high pressure feed pump, etc; Reheater is composed of two sections namely the front and the rear pendant section, which is located above the furnace arc between water-cooled, screen wall tubes and rear wall tubes.
Figure 3.1 Reheater V. SUPERHEATER:
Whatever type of boiler is used, steam will leave the water at its surface and passing to the steam space. Steam formed above the water surface in a shell boiler is always saturated and become superheated in the boiler shell, as it is constantly. If superheated steam is required,the saturated steam must pass through a super heater. This is simply a heat exchanger where additional heat is added to the steam. In water-tube boilers, the super heater may be an additional pendant suspended in the furnace area where the hot gases will provide the degree of superheat required. In other
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cases, for example in CHP schemes where the gas turbine exhaust gases are relatively cool, a separately fired super heater may be needed to provide the additional heat.
VI. ECONOMIZER: The function of an economizer in a steam-generating unit is to absorb heat from the flue gases and add as a sensible heat to the feed water before the water enters the evaporation circuit of the boiler. Earlier economizer were introduced mainly to recover the heatavailableinthe flue gases that leaves the boiler and provision of this addition heating surface increases the efficiency of steam
Figure 3.2 Economizer
generators. In the modern boilers used for power generation feed water heaters were used to increase the efficiency of turbine unit and feed water temperature. Use of economizer or air heater or both is decided by the total economy that will result in flexibility in operation, maintenance and selection of firing system and other related equipment. Modern medium and high capacity boilers are used both as economizers and air heaters. In low capacity, air heaters may alone be selected. Stop valves and non-return valves may be incorporated to keep circulation in economizer into steam drum when there is fire in the furnace but not feed flow. Tube elements composing the unit are built up into banks and these are connected to inlet and outlet heaters.
VII.AIR PREHEATER:
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Air preheater absorbs waste heat from the flue gases and transfers this heat to incoming cold air, by means of continuously rotating heat transfer element of specially formed metal plates. Thousands of these high efficiency elements are spaced and compactlyarranged within 12 sections. Sloped compartments of radially divided cylindrical shell called the rotor. The housing surrounding the rotor is provided with duct connecting both the ends and is adequately scaled by radial and circumferential scaling. Special sealing arrangements are provided in the air preheater to prevent the leakage between the air and gas sides. Adjustable plates are also used to help the sealing arrangements and prevent the leakage as expansion occurs. The air preheater heating surface elements are provided with two types of cleaning devices, soot blowers to normal devices and washing devices to clean the element when soot blowing alone cannot keep the element clean.
Figure 3.3 Air preheater VIII.PULVERIZER:A pulverizer is a mechanical device for the grinding of many types of materials. For example, they are used to pulverize coal for combustion in the steam-generating furnaces of the fossil fuel power plants.
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Figure 3.4Pulverizer 3.1.2 PLANT AUXILIARY MAINTENANCE I. WATER CIRCULATION SYSTEM 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 units (natural and controlled circulation), the water is circulated from the drum through the generating circuits and then back to the drum where the steam is separated and directed to the super heater. The water leaves the drum through the down corners at a temperature slightly below the saturation temperature. The flow through the furnace wall is at saturation temperature. Heat absorbed in water wall is latent heat of vaporization creating a mixture of steam and water. The weight of the water to the weight of the steam in the mixture leaving the heat absorption surface is called circulation ratio. Types of Boiler Circulating System: i. ii. iii.
Natural circulation system Controlled circulation system Combined circulation system
I. Natural Circulation System: Water delivered to steam generator from feed water is at a temperature well below the saturation value corresponding to that pressure. Entering first the economizer, it is heated to about 30-40C below saturation temperature. From economizer the water enters the drum and thus joins the
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circulation system. Water entering the drum flows through the down corner and enters ring heater at the bottom. In the water walls, a part of the water is converted to steam and the mixture flows back to the drum. In the drum, the steam is separated, and sent to superheat for superheating and then sent to the high-pressure turbine. Remaining water mixes with the incoming water from the economizer and the cycle is repeated. As the pressure increases, the difference in density between water and steam reduces. Thus the hydrostatic head available will not be able to overcome the frictional resistance for a flow corresponding to the minimum requirement of cooling of water wall tubes. Therefore natural circulation is limited to the boiler with drum operating pressure around 175 kg/cm². II. Controlled Circulation System: Beyond 80 kg/cm² of pressure, circulation is to be assisted with mechanical pumps to overcome the frictional losses. To regulate the flow through various tubes, or if ice plates are used. This system is applicable in the high sub-critical regions (200 kg/cm²).
II. ASH HANDLING PLANT The widely used ash handling systems are: i. Mechanical Handling System ii. Hydraulic System iii. Pneumatic System iv. Steam Jet System
Figure
3.5Ash
handling
Hydraulic Ash handling system is used at the Barh super Thermal Power Station.
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system
The hydraulic system carried the ash with the flow of water with high velocity through a channel and finally dumps into a sump. The hydraulic system is divided into a low velocity and high velocity system. In the low velocity system the ash from the boilers falls into a stream of water flowing into the sump. The ash is carried along with the water and they areseparated at the sump. In the high velocity system a jet of water is sprayed to quench the hotash. Two other jets force the ash into a trough in which they are washed away by the water into the sump, where they are separated. The molten slag formed in the pulverized fuel system can also be quenched and washed by using the high velocity system. The advantage of this system are that its clean, large ash handling capacity, considerable distance can be traversed, absence of working parts in contact with ash. Fly Ash Collection: Fly ash is captured and removed from the flue gas by electrostatic precipitators or fabric bag filters (or sometimes both) located at the outlet of the furnace and before the induced draftfan. The fly ash is periodically removed from the collection hoppers below the precipitators or bag filters. Generally, the fly ash is pneumatically transported to storage silos for subsequent transport by trucks or railroad cars. Bottom Ash Collection and Disposal: At the bottom of every boiler, a hopper has been provided for collection of the bottom ashfrom the bottom of the furnace. This hopper is always filled with water to quench the ash andclinkers falling down from the furnace. Some arrangement is included to crush the clinkers and for conveying the crushed clinkers and bottom ash to a storage site.
III. WATER TEATEMENT PLANT: As the types of boiler are not alike their working pressure and operating conditions vary andso do the types and methods of water treatment. Water treatment plants used in thermal power plants used in thermal power plants are designed to process the raw water to water with a very low content of dissolved solids known as µdematerialized water. No doubt, this plant has to be engineered very carefully keeping in view the type of raw water to the thermal plant, its treatment costs and overall economics.
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Figure 3.6Water treatment plant
The type of demineralization process chosen for a power station depends on three main factors: i. The quality of the raw water. ii. The degree of de-ionization i.e. treated water quality. iii. Selectivity of resins. Water treatment process is generally made up of two sections: Pretreatment section Demineralization section
PRETREATEMENT SECTION: Pretreatment plant removes the suspended solids such as clay, silt, organic and inorganic matter, plants and other microscopic organism. The turbidity may be taken as two types of suspended solid in water; firstly, the separable solids and secondly the non-separable solids (colloids). The coarse components, such as sand, silt, etc; can be removed from the water by simple sedimentation. Finer particles, however, will not settle in any reasonable time and must be flocculated to produce the large particles, which are settle able. Long term abilityto remain suspended in water is basically a function of both size and specific gravity .
DEMINERALIZATION: This filter water is now used for dematerializing purpose and is fed to cation exchanger bed, but enroots being first dechlorinated, which is either done by passing through activated carbon filter or injecting along the flow of water, an equivalent amount of sodium sulphite through some stroke pumps. The residual chlorine, which is maintained in clarification plant to remove organic
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matter from raw water, is now detrimental to action resin and must be eliminated before its entry to this bed.
Figure 3.7 Demineralization
3.1.3 TURBINE MAINTENANCE DEPARTMENT:
TURBINE CLASSIFICATION: 1.Impulse Turbine:In impulse turbine steam expands in fixed nozzles. The high velocity steam from nozzles does work on moving blades, which causes the shaft to rotate. The essential features of impulse turbine are that allpressure drops occur at nozzles and not on blades.
2. Reactionturbine: In this type of turbine pressure is reduced at both fixed and moving blades. Both fixed and moving blades act like nozzles.Work done by the impulse effect of steam due to reverse the direction of high velocity steam. The expansion of steam takes place on moving blades.
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Figure 3.8 A95 MW GENERATOR AT NTPC, BARH
MAIN TURBINE: The 210MW turbine is a cylinder tandem compounded type machine comprising of H.P, I.P and L.P cylinders. The H.P. turbine comprises of 12 stages the I.P turbine has 11 stages and the L.P has four stages of double flow. The H.P and I.P. turbine rotor are rigidly compounded and the I.P. and L.P rotor by lens type semi flexible coupling. All the 3 rotorsare aligned on five bearings of which the bearing number is combined with thrust bearing.The main superheated steam branches off into two streams from the boiler and passes through the emergency stop valve and control valve before entering the governing wheel chamber of the H.P. Turbine.After expanding in the 12 stages in the H.P. turbine then steam is returned in the boiler for reheating.The reheated steam from boiler enters I.P. turbine via the interceptor valves and control valves and after expanding enters theL.P stage via 2 numbers of cross over pipes. In the L.P. stage the steam expands in axially opposed direction to counteract the thrust and enters the condenser placed directly below the L.P. turbine. The cooling water flowing through the condenser tubes condenses the steam and the condensate the collected in the hot well of the condenser. The condensate collected the pumped by means of 3x50% duty condensate pumps through L.P heaters to deaerator from where the boiler feed pump delivers the water to the boiler through H.P. heaters thus forming a closed cycle.
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STEAM TURBINE: A steam turbine is a mechanical device that extracts thermal energy from pressurized steamand converts it into useful mechanical work.From a mechanical point of view, the turbine is ideal, because the propelling force is applied directly to the rotating element of the machine and has not as in the reciprocating engine to be transmitted through a system of connecting links, which are necessary to transform a reciprocating motion into rotary motion. Hence since the steam turbine possesses for its moving parts rotating elements only if the manufacture is good and the machine is correctly designed, it ought to be free from out of balance forces.If the load on a turbine is kept constant the torque developed at the coupling is also constant. A generator at a steady load offers a constant torque. Therefore, a turbine is suitable for driving a generator, particularly as they are both high-speed machines.A further advantage of the turbine is the absence of internal lubrication. This means that the exhaust steam is not contaminated with oil vapour and can be condensed and fed back to the boilers without passing through the filters. It also means that turbine is considerable saving in lubricating oil when compared with reciprocating steam engine of equal power. A final advantage of the steam turbine and a very important one is the fact that a turbine can develop many time the power compared to a reciprocating engine whether steam or oil.
STEAM CYCLE: The thermal(steam) power plant uses a dual(vapor + liquid) phase steam, regenerative feed water heating and re heating of steam cycle. It is a closed cycle to enable the working fluid (water) to be used again and again. The cycle used is ‘Rankine cycle’ modified to include superheating of
Figure 3.9 Steam cycle diagram
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MAIN TURBINE: The 210MW turbine is a tandem compounded type machine comprising of H.P and I.P cylinders. The H.P turbines comprise of 12 stages, I.P turbine has 11 stages and the L.P turbine has 4 stages of double flow. The H.P and I.P turbine rotors are rigidly compounded and the L.P. motor by the lens type semi flexible coupling. All the three rotors are aligned on five bearings of which the bearing no. 2 is combined with the thrust bearing. The main superheated steam branches off into two streams from the boiler and passes through the emergency stop valve and control valve before entering the governing wheel chamber of the H.Pturbine. After expanding in the 12 stages in the H.P turbine the steam is returned in boiler for reheating. The reheated steam for the boiler enters the I.P turbine via the interceptor valves and control valves and after expanding enters the L.P turbine stage via 2 nos of cross-over pipes. In the L.P. stage the steam expands in axially opposite directionto counter act the trust and enters the condensers placed below the L.P turbine. The cooling water flowing throughout the condenser tubes condenses the steam and the condensate collected in the hot well of the condenser.The condensate collected is pumped by means of 3*50% duty condensate pumps through L.P heaters to deaerator from where the boiler feed pump delivers the water to boiler through H.P heaters thus forming a close cycle.
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Figure 3.10 Turbine & Turbine Cycle The selection of extraction points and cold reheat pressure has been done with a view to achieve a high efficiency. These are two extractors from H.P turbine, four from I.P turbine and one from L.P turbine. Steam at1.10 and 1.03 g/sq.cm .As is supplied for the gland scaling. Steam for this purpose is obtained from deaerator through a collection where pressure of steam is regulated. From the condenser, condensate is pumped with the help of 3*50% capacity condensate pumps to deaerator through the low-pressure regenerative equipments.Feed water is pumped from deaerator to the boiler through the H.P. heaters by means of 3*50% capacity feed pumps connected before the H.P. heaters.
TURBINE COMPONENTS:
Casing. Rotor Blades Sealing System Stop & control valves Coupling & Bearing Barring Gear
TURBINE CASINGS: HP Turbine Casing:
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Outer casing: a barrel-type without axial or radial flange. Barrel-type casing suitable for quick startup and loading. The inner casing- cylindrically, axially split. The inner casing is attached in the horizontal and vertical planes in the barrel casingso that it can freely expand radially in all the directions and axially from a fixed point(HPinlet side) . I.P Turbine Casing: The casing of the IP turbine is split horizontally and is of double-shell construction. Both are axially split and a double flow inner casing is supported in the outer casingand carries the guide blades.
ROTORS: HP Rotor: The HP rotor is machined from single Cr-Mo-V steel forging with integral discs. In all the moving wheels, balancing holes are machined to reduce the pressuredifference across them, which results in reduction of axial thrust. First stage has integral shrouds while other rows have surroundings, riveted to the blades are periphery. I.P Rotor: The IP rotor has seven discs integrally forged with rotor while last four discs areshunk fit.
BLADES: Most costly element of the turbine. Blades fixed in stationary part are called guide blades/ nozzles and those fitted inmoving part are called rotating/working blades. Blades have three main part: i. Aerofoil: working part. ii. Root. iii. Shrouds. Shroud is used to prevent steam leakage and guide steam to next set of moving blades.
VACUUM SYSTEM: This comprises of: Condenser: 2 for 200MW unit at the exhaust of L.P turbine. Ejectors: One starting and two main ejectors connected to the condenser located near the turbine. C.W Pumps: Normally two per unit of 50% capacity.
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CONDENSER: There are two condensers entered to the two exhausters of the L.P. turbine.These are surface-type condensers with two pass arrangement. Cooling water pumped into each condenser by a vertical C.W. pump through the inlet pipe.
Figure 3.11 A Typical Water Cooled Condenser Water enters the inlet chamber of the front water box, passes horizontally through brass tubes to the water tubes to the water box at the other end,takes a turn, passes through the upper cluster of tubes and reaches the outlet chamber in the front water box. From these, cooling water leaves the condenser through the outlet pipe and discharge into the discharge duct.Steam exhausted from the LP turbine washes the outside of the condenser tubes, losing its latent heat to the cooling water and is connected with water in the steam side of the condenser. This condensate collects in the hot well, welded to the bottom of the condensers.
EJECTORS: There are two 100% capacity ejectors of the steam eject type. The purpose of the ejector is to evacuate air and other non-condensation gases from the condensers and thus maintain the vacuum in the condensers. The ejector has three compartments. Steam is supplied generally at a pressure of 4.5 to 5 kg/cm² to the three nozzles in the three compartments. Steam expands in the nozzle thus givinga high-velocity eject which creates a low-pressure zone in the throat of the eject. Since the nozzle box of the ejector is connected to the air pipe from the condenser, the air and pressure zone. The working steam which has expanded in volume comes into contact with
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the cluster of tube bundles through which condensate is flowing and gets condensed thus after aiding the formation of vacuum. The non-condensing gases of air are further sucked with the next stageof the ejector by the second nozzle. The process repeats itself in the third stage also andfinally the steam-air mixture is exhausted into the atmosphere through the outlet .
Deaerator: The presence of certain gases, principally oxygen, carbon dioxide and ammonia, dissolved in water is generally considered harmful because of their corrosive attack on metals, particularly at elevated temperatures. One of the most important factors in the prevention of internal corrosionin 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
Figure 3.12a Deaerator PRINCIPAL OF DEAERATION: It is based on following two laws. Henry’s Law Solubility The Deaerator comprises of two chambers:
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Deaerating column Feed storage tank
Boiler Feed Pump: This pump is horizontal and of barrel design driven by an Electric motor through a hydraulic coupling. All the bearings of pump and motor are forced lubricated by a suitable oil lubricating system with adequate protection to trip the pump if the lubrication oil pressure falls below a preset value.
3.2.2 COAL HANDLING DEPARTMENT: As coal is the prime fuel for thermal power plant, adequate emphasis should be given for its proper handling and storage. Also it is equally important to have a sustained flow of this fuel to maintain uninterrupted power generation. Coal is used as the fuel because of the followingadvantages .
Advantages of coal as fuel:
Abundantly available in India Low Cost Technology for power generation well developed. Easy to handle,transport, store and use.
COAL CYCLE:
Figure 3.13 Coal cycle Diagram COAL HANDLING SYSTEM:
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In the coal handling system of NTPC,three coal paths are normally available for the diret conveying of coal. These are: Path A: From track hopper to boiler bunker. Path B: From track hopper to stock yard. Path C: From stock yard to boiler bunkers.
Figure 3.14 Coal handling system The storage facilities at the stockyards have been provided only for crushed coal. The coal handling system is designed to provide 100% standby for all equipments and conveyors.The 200 mm coal as received at the track hopper is fed to the crusher house for crushing.Crusher of 50% capacity is provided and these are preferred to two crushers of 100% capacity because of increased reliability and possible higher availability. A series of parallel conveyors are designed thereafter to carry crushed coal directly to the boiler bunkers or to divert it to the stockyard.
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Figure3.15Coal handling division at NTPC,Barh
COAL HANDLING EQUIPMENTS i.
PULLEY : They are made of mild steel. Rubber lagging is provided to decrease the friction factor in between the belt and pulley. SCRAPPER: Conveyors are provided with scrappers at the discharge pulley in order to clean the carrying side of the belt built up material on idler rolls. Care should be taken to ensure that scrapper is held against the belt with the pressure sufficient to remove material without causing damage to the belt due to excessive force exerted by the wiper. The following categories of scrapper are common in use : Steel blade scrapper Rubber/fabric blade scrapper Nylon brush scrapper Compressed air blast scrapper.
ii.
iii.
IDLERS: These essentially consist of rolls made out of seamless steel tube enclosed fully at each end and fitted with stationary shaft,anti-friction bearing and seals. They support the belt and enable it to travel freely without much frictional losses and also keep the belt properly trained.
Figure3.16 an Idler
iv.
CONVEYOR BELT:
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The conveyor belt consists of layers or piles of fabric duck, impregnated with rubber and protected by a rubber cover on both sides and edges. The fabric duck supplies the strength to with stand the tension created in carrying the load while the cover protects the fabric arecas. Heat resistant belting is always recommended for handling materials at a temperature over 66˚ C.
Figure 3.17 Coal Storage Area of the barh superThermal Power Station, v.
VIBRATING SCREEN:The function of vibrating screen is to send the coal of having size less than 20 mm to the crusher. The screen is operated by four v-belts connected to motor. vi. CRUSHER:The role of crusher is to crush the coal from 200 mm to 20 mm size of coal received from the vibrating screen. This is accomplished by means of granulators of ring type. There are about 37 crushing elevations; each elevation has 4 granulators-2 of plain type and 2 of toothtype, arranged alternately.The granulators are made of manganese steel because of their work hardening property. The coal enters the top of the crusher and is crushed between rotating granulators and fluid case path. The crushed coal through a chute falls on belt feeder. Normally these crushers have a capacity round 600tonnes/hr. vii. MAGNETIC SEPAROTERS: This is an electromagnet placed above the conveyor to attract magnetic materials. Over this magnet There is one conveyor to transfer these materials to chute provided for dumping at ground level.Because of this, continuous removal is possible. It can remove any ferrous impurity from 10gms to 50kg.
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