coal to electricity with thermal power plant

7/30/2019 coal to electricity with thermal power plant

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Coal to electricity with Thermal Power Plant

Basic Requirements of power plant

The basic requirements of the conventional power station are decided on the type,size and other essential specifications of the station to be constructed. It isnecessary to know the capacity of the plant that will be required for the immediate

development as well as for the period to follow thereafter. The capacity of the plantfor immediate development gives the instruction for planning the initialdevelopment and the capacity anticipated during the period followinghelps to select the site area sufficiently large for the ultimate development and

services, railway sidings, water supply access and transmission connection to bedeveloped in the most economic manner for the future requirement.

The following factors are to be taken into consideration.

Station building Coal store and siding

Cooling Towers Switch yard compound Surrounding area and approaches

GeologyThe geology of the site should be reasonable as this affects the cost of thefoundations.Modern power plants with their heavy structures impose a heavy load on the subsoil

and hence are to be supported with suitable foundations

Water for Power StationsThe water requirement for thermal stations come under two main groups, the firstrequirement is the water required for steam generation and the second requirementis for cooling purposes.

The amount of water required for condensation is quite significant. In once throughsystem of circulating water the amount required will be approx. 20,000 m3hr/1OOMW.This includes small portion of requirement for cooling of generator and othermachines.


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Coal for Power Station

In India, the principal source of commercial energy is coal amounting to over 95% ofthe

total primary energy resources of the country. The coal resources existing in ourcountryare of the order of 1,30,000 million tonnes or even more and new res erves are beinglocated. The main areas where coal mines are located are eastern region i.e Bihar,Bengal, Central region, Singrauli Coal fields, Tamil Nadu, Neyveli and small sourcesofcoal are located in rest of the country as well. The location of thermal Power stationburning high ashcoals is therefore of great importance since about 50 to 60% of the cost ofgeneration ofelectric power is due to the delivered cost of coal at the generating stations

Climatic ConditionsThe humid conditions with fluctuating temperature lead to dew point and hence thecondensation which results in corrosion of insulation. It is a well-known fact that fortropical countries insulation of electrical machines has different standards and iscostly

Coal to Electricity

Coal is a fuel that is found in the ground. It is made of the remains ofplants that died millions of years ago. Soil piled up on top of the remainsand that weight compacted it into a more dense material, called coal.The energy in the coal came from the sun and was stored in the plants.

When the coal is burned, it gives up that energy as heat.The coal's heatenergy can then be turned into electrical energy


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This happens at a power plant.

1.First the coal is mined and taken to a power plant.2.Then the coal is burned in a boiler which causes the water in the

boiler pipes to become steam.3.The steam travels through the pipes to the turbine.4.The steam spins the turbine blades.5.The spinning blades turn a shaft connected to the generator.6. In the generator, big magnets spin close to coils of wire.7.When this happens, electrical current is produced in the wires.8.Then the electricity goes out through wires to homes, schools,

and businesses.

Basic steps to generate electricity

Generating steam from coal

Conversion of thermal energy to

mechanical power

Generation and load dispatch of electricpower.


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Basic Power Plant Cycle

The thermal (steam) power plant uses a dual (vapour +liquid) phase cycle. It is a closed cycle to enable theworking fluid (water) to be used again and again. The cycleused is "Ranking Cycle" modified to include super heatingof steam, regenerative feed water heating and reheating ofsteam .The Rankine cycle is sometimes referred to as apracticalCarnot cyclebecause, when an efficient

turbine is used, theTS diagrambegins to resemble theCarnot cycle. The main difference is that heat addition(in the boiler) and rejection (in the condenser)areisobaricin the Rankine cycle andisothermalin thetheoretical Carnot cycle. A pump is used to pressurizethe working fluid received from the condenser as a liquidinstead of as a gas. All of the energy in pumping the

working fluid through the complete cycle is lost, as ismost of the energy of vaporization of the working fluid inthe boiler. This energy is lost to the cycle because thecondensation that can take place in the turbine is limitedto about 10% in order to minimize blade erosion; thevaporization energy is rejected from the cycle throughthe condenser. But pumping the working fluid throughthe cycle as a liquid requires a very small fraction of theenergy needed to transport it as compared tocompressing the working fluid as a gas in a compressor
http://en.wikipedia.org/wiki/Carnot_cyclehttp://en.wikipedia.org/wiki/Carnot_cyclehttp://en.wikipedia.org/wiki/Carnot_cyclehttp://en.wikipedia.org/wiki/TS_diagramhttp://en.wikipedia.org/wiki/TS_diagramhttp://en.wikipedia.org/wiki/TS_diagramhttp://en.wikipedia.org/wiki/Isobaric_processhttp://en.wikipedia.org/wiki/Isobaric_processhttp://en.wikipedia.org/wiki/Isobaric_processhttp://en.wikipedia.org/wiki/Isothermal_processhttp://en.wikipedia.org/wiki/Isothermal_processhttp://en.wikipedia.org/wiki/Isothermal_processhttp://en.wikipedia.org/wiki/Isothermal_processhttp://en.wikipedia.org/wiki/Isobaric_processhttp://en.wikipedia.org/wiki/TS_diagramhttp://en.wikipedia.org/wiki/Carnot_cycle


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On large turbines, it becomes economical to increase thecycle efficiency by using reheat, which is a way of partiallyovercoming temperature limitations.

Cycle efficiency can be improved by either:1.Increasing the average temperature during heataddition (Tin )

2.Decreasing the condenser temperature (Tout)

Rankine cycle with reheatIn this variation, twoturbineswork in series. The firstacceptsvapourfrom theboilerat high pressure. After thevapour has passed through the first turbine, it re-enters theboiler and is reheated before passing through a second, lowerpressure turbine. Among other advantages, this prevents thevapour fromcondensingduring its expansion which canseriously damage the turbine blades, and improves the
http://en.wikipedia.org/wiki/Turbinehttp://en.wikipedia.org/wiki/Turbinehttp://en.wikipedia.org/wiki/Turbinehttp://en.wikipedia.org/wiki/Vaporizationhttp://en.wikipedia.org/wiki/Vaporizationhttp://en.wikipedia.org/wiki/Vaporizationhttp://en.wikipedia.org/wiki/Boilerhttp://en.wikipedia.org/wiki/Boilerhttp://en.wikipedia.org/wiki/Boilerhttp://en.wikipedia.org/wiki/Condensationhttp://en.wikipedia.org/wiki/Condensationhttp://en.wikipedia.org/wiki/Condensationhttp://en.wikipedia.org/wiki/Condensationhttp://en.wikipedia.org/wiki/Boilerhttp://en.wikipedia.org/wiki/Vaporizationhttp://en.wikipedia.org/wiki/Turbine


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efficiency of the cycle, as more of the heat flow into the cycleoccurs at higher temperature.

By returning partially expanded steam, to a reheat, the

average temperature at which heat is added, isincreased and, by expanding this reheated steam to theremaining stages of the turbine, the exhaust wetness isconsiderably less than it would otherwise beconversely, if the maximum tolerable wetness isallowed, the initial pressure of the steam can beappreciably increased.Bleed Steam Extraction. Forregenerative system, nos. of non-regulated extractions

are taken from HP, IP turbine.

Regenerative Rankine cycle

The regenerative Rankine cycle is so named becauseafter emerging from the condenser (possibly asasubcooled liquid) the working fluid is heatedbysteamtapped from the hot portion of the cycle. Onthe diagram shown, the fluid at 2 is mixed with the fluidat 4 (both at the same pressure) to end up with thesaturated liquid at 7. This is called "direct contactheating". The Regenerative Rankine cycle (with minorvariants) is commonly used in real power stations.

Another variation is where bleed steamfrom betweenturbine stages is sent tofeedwater heatersto preheatthe water on its way from the condenser to the boiler.These heaters do not mix the input steam andcondensate, function as an ordinary tubular heatexchanger, and are named "closed feedwater heaters".

The regenerative features here effectively raise thenominal cycle heat input temperature, by reducing the

addition of heat from the boiler/fuel source at therelatively low feedwater temperatures that would exist
http://en.wikipedia.org/wiki/Subcooled_liquidhttp://en.wikipedia.org/wiki/Subcooled_liquidhttp://en.wikipedia.org/wiki/Steamhttp://en.wikipedia.org/wiki/Steamhttp://en.wikipedia.org/wiki/Steamhttp://en.wikipedia.org/wiki/Feedwater_heaterhttp://en.wikipedia.org/wiki/Feedwater_heaterhttp://en.wikipedia.org/wiki/Feedwater_heaterhttp://en.wikipedia.org/wiki/Feedwater_heaterhttp://en.wikipedia.org/wiki/Steamhttp://en.wikipedia.org/wiki/Subcooled_liquid


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without regenerative feedwater heating. This improvesthe efficiency of the cycle, as more of the heat flow intothe cycle occurs at higher temperature.This process

ensures cycle economy.Regenerative heating of the boiler feed water is widely usedin modern power plants; the effect being to increase theaverage temperature at which heat is added to the cycle,thus improving the cycle efficiency*''

Factors Affecting Thermal Cycle Efficiency

Thermal cycle efficiency is affected by following:

Initial steam Pressure

Initial Steam Temperature

Whether reheat is used or not, and if used reheatpressure and temperature

Condenser pressureRegenerative feed water heating

Coal from the coal wagons is unloaded in the coal handling plant. ThisCoal is transported upto the raw coal bunkers (1) with the help of belt

conveyors. Coal rs transported to Bowl Mills (3) by Coal feeders (2)The coal is pulverised in the Bowl Mill,where it is ground to a powderform. The mill consists of a round metallic table on which coalparticles fall. This table is rotated with the help of a motor. There arethree large steel rollers which are spaced 120" apart. When there is nocoal, these rollers does not rotate but when the coal is fed to the tableit packs up between roller and the table and this forces the rollers torotate. Coal is crushed by the crushing action between the rollers and

rotating table. This crushed coal is taken away to the furnace throughcoal


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pipes (4) with the help of hot and cold air mixture from P.A. Fan (5).P.A. Fan taken atmospheric air, a part of which is sent to Airpreheaters(7) for heating while a part goes directly to the mill for temperaturecontrol. Atmospheric air from F.D. Fan (18) is heated in the air heaters

(7) and sent to the furnace (6) as combustion air. Water from theboiler feed pump passes through economiser (8) and reaches theboilerdrum (9). Water from the drum passes through down commersand goes to bottom ring header. Water from the bottom ring header isdivided to all the four sides of the furnace.Due to heat and- the density difference the water rises up in the water

wall tubes (12). "Water is partly converted to steam as it rises up inthe furnace. This steam and water

mixture is again taken to the boiler drum (9) where the steam isseparated from water. Water follows the same path while the steam issent to superheaters for superheating.The superheaters are locatedinside the furnace and the steam is superheated ( 540"C)and finally it goes to turbine.Flue gases from the furnace is extractedby induced draft fan (14) which maintains balance draft in the furnace(-5 to -10mm of wcl) with forced draft fan (18). These flue gases emitstheir heat energy to various superheaters in the pant house (15) andfinally passes through air preheaters (7)and goes to electrostaticprecipitator (16) where the ash particles are extracted. Electrostaticprecipitator consists of metal plates which are electrically charged. Ashparticles are attracted on to these plates, so that they do not passthrough the chimney (17) to pollute the atmosphere. Regularmechanical hammers blows cause the accumulation of ash to fall to thebottom of the precipitator where they are collected in a hopper fordisposal. This ash is mixed with water to form a slurry and is pumped


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to ash POND.


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Steam to mechanical Power

As can be seen from figure, from the boiler, a steam piped)conveys steam to the turbine through a stop valve (which canbe used to. shut off steam in an emergency) and throughcontrol valves that automatically regulate the supply of steam totheturbine. Stop valve and control valves are located in asteamchest and a governor driven from the main turbine shaft,operates the control valves to regulate the amount of steamused (This depends upon the speed of the turbine and theamount of electricity required from the generator).

Steam from the control valves enters the high pressure cylinderof the turbine, where it passes through a ring of stationaryblades fixed to the cylinder wall .These act as nozzles anddirect the steam into a second ring of moving blades mountedon a disc secured to the turbine shaft. This second ring turnsthe shafts as a result of the force of the steam. The stationaryand moving blades together constitute a 'stage' of the turbineand in practice many stages are necessary, so that the cylindercontains a number of rings of stationary blades with rings ofmoving blades arranged between them. The steam passesthrough each stage in turn until it reaches the end of the highpressure cylinder and in its passage some of its heat energy ischanged into mechanical energy.

The steam leaving the high pressure cylinder goes back to theboiler for reheating and returns by a further pipe to theintermediate pressure cylinder. Here it passes through anotherseries of stationary and moving blades.Finally, the steam istaken to the low pressure cylinders, each of which it enters atthe centre .Flowing outwards in opposite directions through therows of turbine blades - an arrangement known as double flow -to the extremities of the cylinder. As the steam gives up its heatenergy to drive the turbine, its temperature and pressure fall

and it expands. Because of this expansion the blades are much


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larger and longer towards the low pressure ends of the turbineThe turbine shaft usually rotates at 3,000 revolutions perminute. This speed isdetermined by the frequency of the electrical system used inthis country and is thespeed at which a 2- pole generator must be driven to generatealternating current at a frequency of 50 cycles per second.When as much energy as possible has been extracted from thesteam it is exhausted directly to the condenser. This runs thelength of the low-pressure part of the turbine and may bebeneath or on either side of it. The condenser consists of alarge vessel containing some 20,000 tubes, each about 25mm

in diameter. Cold water from the river,estuary, sea or coolingtower is circulated through these tubes and as the steam fromthe turbine passes round them it is rapidly condensed intowater condensate. Because water has a much smallercomparative volume than steam, a vacuum is created in thecondenser. This allows the steam to reduce down to pressurebelow that of the normalatmosphere and more energy can beutilized.

From the condenser, the condensate is pumped through lowpressure heaters by theextraction pump, after which its pressure is raised to boilerpressure by the boiler feed pump. It is passed through furtherfeed heaters to the economiser and the boiler for reconversioninto steam.Where the cooling water for power stations is drawnfrom large rivers, estuaries or the coast, it can be returneddirectly to the source after use. Power stations situated on

smaller rivers and inland do not have such vast waterresources available, so the cooling water is passed throughcooling towers (where its heat is removed by evaporation) andre-used.


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The turbo- Generator

Switching and

Transmission

The electricity is usually produced in the stator windings of

large modem generators at about 25,000 volts and is fed


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through terminal connections to one side of a generatortransformer (1) that steps up the voltage to 132000,220000 or400000 volts. From here conductors carry it to a series of threeswitches comprising an isolator (2), a circuit-breaker (3) andanother isolator (4).

The circuit-breaker, which is a heavy-duty switch capable ofoperating in a fraction of a second, is used to switch off thecurrent flowing to the transmission lines. Once thecurrent has been interrupted the isolators can be opened.These isolate the circuitbreaker being applied to its terminals.Maintenance or repair work can then be carried out safely.

From the circuit-breaker the current is taken to the busbars (5)-conductors which run the length of the switching compound -and then to another circuit-breaker (6) with its associatedisolators (7), before being fed to the Grid (8). Each generator ina power station has its own transformer, circuit-breaker andassociated isolators but the electricity generated is fed into acommon set of busbars.

Circuit-breakers work like combined switches and fuses butthey have certain special features and are very different fromthe domestic switch and fuse. When electrical current isswitched off by separating two contacts, an arc is createdbetween them. At the voltage used in the home, this arc is verysmall and only lasts for a fraction of a second but at the veryhigh voltages used for transmission, the size and power of the

arc is considerable and it must be quickly quenched to preventdamage.

One type of circuit breaker has its contacts immersed ininsulating oil so that whenthe switch is opened, either bypowerful electrical coils or mechanically by springs the arcis quickly extinguished by the oil. Another type works bycompressed air, which operates the switch, and at the same

time 'blows out' the arc.


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Three wires are used in a 'three-phase' system for large powertransmission as it ischeaper than the two wire 'single-phase'system that supplies the home.

The centre of the power station is the control room (9). Hereengineers monitor the output of electricity, supervising andcontrolling the operation of generating plant and high voltageswitch gear and directing power to the Grid system as required.Instrument on the control panels show the output and conditionwhich exits on all the main plant and a miniature diagramindicates the precise state of the electrical system.

Boiler Fundamentals


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The primary function of oil and coal burning systems in the process of steamgeneration is to provide controlled efficient conversation of the chemical energy ofthe fuel into heat energy which is then transferred to the heat absorbing surfaces of

the steam generator.Composition of air: the supply of oxygen for combustion is obtained from air. This isasimportant as the supply of fuel. The average composition of air is79% nitrogen and 21% oxygen by volume77% nitrogen and 23% oxygen by weightIgnition: Fuel must be ignited before it can burn. Combustion is brought about byraising the temperature of the fuel to its ignition temperature. This temperature varieswith different fuels.The following factors in efficient combustion are usually referred to as "The three T's.

Time: It will take a definite time to heat the fuel to its ignition temperature and havingignited, it will also take time to bum. Consequently sufficient time must be allowed forcomplete combustion of the fuel to take place in the chamber.

Temperature: A fuel will not burn until it has reached its ignition temperature. Thespeed at which this Temperature will be reached is increased by preheating thecombustion air. The temperature of the flame of the burning fuel may vary with thequantity of air used. Too much combustion air will lower the flame temperature andmay cause unstable ignition.Turbulence: Turbulence is introduced to achieve a rapid relative motion between theairand the fuel particles. It is found that this produces a quick propagation of the flameand its rapid spread throughout the fuel/air mixture in the combustion chamber.

Boilers, their classification and typesBoiler is a device for generating steam for power, processing or heating purposes.Boiler is designed to transmit heat from an external combustion source containedwithin the boiler itself.

Use

Pressure

Materials

SizeTube Content

Tube Shape and position

Firing

Heat Source

Fuel

Fluid

Circulations

Furnace position

Furnace type

General shapeTrade name


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Special features.

Categorisation of Boilers: Boilers are generally categorised asfollows :

Steel boilers b) Fire Tube typec) Water tube typed) Horizontal Straight tubee) Bent tubef) Natural Circulationg) Positive Circulationh) Shell typei) Cast Iron Boilersj) Special Design Boilers

k)Nuclear Reactors

Water Circulation SystemTheory of CirculationWater must flow through the heat absorption surface of the boiler in order that it beevaporated into steam. In drum type units (natural and controlled circulation) thewateris circulated from the drum through the generating circuits and back to the drumwheresteam is separated and directed to the superheater. The water leaves the drumthrough

the downcomers at a temperature slightly below saturation temperature. The flowthrough the furnace wall is at saturation temperature. Heat absorbed in water wall islatent heat of vaporization creating a mixture of steam and water. The ratio of theweightof water to the weight of steam in the mixture leaving the heat absorption surfaces iscalled Circulation Ratio

Types of Boiler Circulation SystemThe 3 systems of circulation

Natural circulation system- Water delivered to a steam, generator from feed

heaters is at a temperature well below the saturation value corresponding tothat pressure. Entering first, the economiser, it is heated to about 30 to 40 deg


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C below saturation temperature. From economiser the water enters the drumand thus join the circulation system. Water entering the drum flows downthrough the downcomer and enters ring header at the bottom.

Controlled circulation system

Combined circulation system- Beyond the critical pressure, phasetransformation is absent, and hence once through system is adopted.However, it has been found that even at supercritical pressure, it isadvantageous to re-circulate the water through the furnace tubes at low loads.

EconomiserThe function of an economiser in a steam generating unit is to absorb heat fromthe flue gases and add this as sensible heat to the feed water before the waterenters the evaporative circuit of the boiler. Earlier the economisers were introducedmainly to recover the heat available in flue gas that leaves the boiler and provision of

this additional heating surface increased the efficiency of steam generation, saving infuel consumption, thus the name " Economiser " christened. In the modern boilersused for power generation feed water heaters were used to increase the efficiency ofturbine unit and feed water temperature and hence the relative size of economiser isless than earlier units.

Type of Construction

Plain Tube : Plain tube economizers have several banks of tubes with either-in-lineor staggered type formation. Staggered arrangement induces more turbulence in thegas than the in-line arrangement. This gives a higher rate of heat transfer andrequires less surface for a given duty but at the expense of higher draught loss.Economiser can be supported in a water or steam cooled coils which can also beused to support primary superheater or reheater.

Welded Fin-tube : Large number of variations in this type is available. In earlierdays cast iron shrouds were shrunk on mild steel tube for use as economiser instocker fired boilers. This type has a good resistance against gas side corrosion but

heavy in weight.


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Steam Circulation SystemRiser TubesA riser is a tube through which water and steam pass from an upper waterwallheaderto a steam drum.SuperheaterSuperheaters are usually classified according to the shape of the tube banks and theposition of the header; also according to whether they receive heat by radiation orconvention, although in some instance it may be a combination of both methods.Types of superheaters : Depending on the firing method, fuel fired etc., thesuperheaters are placed in the boiler flue passes, horizontally, vertically or

combined.Pendant type : The superheaters may be of pendant type, hanging from andsupportedby their headers.Horizontal type : The superheaters may be of the horizontal type with tubes arrangedacross the boiler. This type of superheater is self-draining which is an advantageduringlighting up and for this reason they are now favoured by designers for the primarysection superheaters. The platen is a plane surface receiving heat from both sides.The ratio of longitudnal pitching to transfer pitching is very low for platensuperheater.

Desuperheater/Attemperator


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type air pre-heaters, there exists a steam coil air pre-heater. These are located in thesecondary pass of the furnace at a height of around '16' M level. Each 200 MW unitisprovided with two such air pre-heaters.Burners : As evident from the name itself, these are used for burning pulverised coal

oroil. Every unit has a set of such burners located at different elevations of the furnace.F.D.Fan : The forced draft fans (2 per unit - 50% capacity each) are designed forhandling secondary air for the boiler. These fans are located at '0' M level near thePAFan.Wind Box : These act as distributing media for supplying secondary/excess air to thefurnace for combustion. These are generally located on the left and right sides of thefurnace while facing the chimney.Scanner Fan : These fans, two per boiler, supply requisite air for scanner cooling.Ignitor Fan : These fans, again two per boiler, are used to supply air for cooling

ignitorsand combustion of ignitor air fuel mixture.Electrostatic precipitator : These are generally two plate type located between boilerand the chimney. The precipitator is arranged for horizontal gas flow and isconstructedwith welded steel cas ings.ID Fans : There are two induced Draft fans per boiler located between theElectrostaticprecipitator and the chimney. These fans are used for sucking flue gas from furnace.Chimney : These are tall RCC structures with single/multiple flues (one flue per 200MW Unit). The height of these chimneys vary depending on the locationaconsiderations; anywhere between 150 m. to 220 m.

Air and Draft SystemBasics of FansThe air we need for combustion in the furnace and the flue gas that we must evacuatewould not possible without using fans. A fan is capable of imparting energy to the air/gasin the form of a boost in pressure. We overcome the losses through the system bymeans of this pressure boost. The boost is dependent on density for a given fan at agiven speed. The higher the temperature, the lower is the boost. Fan performance (Max.capability) is represented as volume vs pressure boost

Classification of FansIn boiler practice, we meet the following types of fans.a) Axial fansb) Centrifugal (Radial) fans

Draft SystemBefore a detailed study of industrial fans it is in the fitness of things to understand thevarious draft systems maintained by those fans.The terms draft denotes the difference between the atmospheric pressure and thepressure existing in the furnace. Depending upon the draft used, we havea) Natural Draftb) Induced Draftc) Forced Draft and


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d) Balanced Draft System

Natural Draft : In natural draft units the pressure differentials are obtained hvconstructing tall chimneys so that vacuum is created in the furnace Due to tin'pressure

difference, air is admitted into the furnace.Induced Draft : In this system the air is admitted to natural pressure differenc e andtheflue gases are taken out by means of induced Draft fans and the furnace ismaintainedunder vacuum.Forced Draft : A set of forced draft fans are made use of for supplying air to thefurnaceand so the furnace is pressurised. The flue gases are taken out due to the pressuredifference between the furnace and the atmosphere.Balance Draft : Here a set of Induced and Forced Draft Fans are utilized in

maintaininga vacuum in the furnace. Normally all the power stations utilize this draft system

Main Turbine - Steam TurbineTheory

A turbine, being a form of engine, requires in order to function a suitableworking fluid, a source of high grade energy and a sink for low-grade energy.

When the fluid flows through the turbine, part of the energy content is

continuously extracted and converted into useful mechanical work. Steam andgas turbines use heat energy, while water turbines use pressure energy.

Steam turbine as a Prime moverThe steam turbine offers many advantages over other prime movers, boththermodynamically and mechanically. From a .thermo-dynamic point of view, themain advantage of the steam turbine over, say a reciprocating steam engine is thatin the turbine the steam can be expanded down to a lower back pressure, therebymaking available a greater heat drop.

Operating Principles


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A steam turbine's two main parts are the cylinder and the rotor. The cylinder (stator)is a steel or cast iron housing usually divided at the horizontal centerline. Its halvesare bolted together for easy access. The cylinder contains fixed blades, vanes, andnozzles that direct steam into the moving blades carried by the rotor. Each fixedblade set is mounted in diaphragms located in front of each disc on the rotor, or

directly in the casing. A disc and diaphragm pair a turbine stage. Steam turbines canhave many stages. The rotor is a rotating shaft that carries the moving blades on theouter edges of either discs or drums.

Steam CycleThe thermal (steam) power plant uses a dual (vapour + liquid) phase cycle. It is aclosed cycle to enable the working fluid (water) to be used again and again. Thecycle used is "Ranking Cycle" modified to include super heating of steam,regenerative feed water heating and reheating of steam shows this cycle and is selfexplanatory.

Factors Affecting Thermal Cycle Efficiency :Thermal cycle efficiency is affected by following:

Initial Steam Pressure

Initial Steam Temperature

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

Condenser pressure.

Regenerative feed water heating

Classification of Turbines

According to the number of pressure stages:Single-stage turbines with one or more velocity stages usually of small powercapacities, these turbines are mostly used for driving centrifugal compressors,blowers and other similar machinery.

Multistage impulse and reaction turbines; they are made in a wide range ofpower capacities varying from small to large.According to the direction of steam flow:

Axial turbines in which the steam flows in a direction parallel to the axis of theturbine;

Radial turbines in which the steam flows in a direction perpendicular to the axisof the turbine with one or more low pressure stages in such turbines being axial.

According to the number of cylinder:Single-cylinder turbines.

Double-cylinder turbines.

Three-Cylinder turbines and

Four-Cylinder turbines:Multi-Cylinder turbines which have their rotors mounted on one and the same shaftandcoupled to a single generator are known as single shaft turbines. Turbines withseparaterotor shafts for each cylinder placed parallel to each other are known as multi-axial

turbines.According to the method of governing:


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Turbines with throttle governing in which fresh steam enters through one or more(depending on the power developed) simultaneously operated throttle valves.

turbines with nozzle governing in which fresh steam enters through two or moreconsecutively opening regulators.

turbine with bypass governing in which steam, besides being fed to the first stage

is also directly led to one, two or even three intermediate stages of the turbine.According to the principle of action of steam:

Impulse turbines.

Axial reaction turbines.

Radial reaction turbines with stationary guide blades.

Radial reaction turbines without any stationary blades.According to the heat drop process:

Condensing turbines with regeneration; In these turbines steam at a pressureless than atmospheric pressure is directed to a condenser, steam is alsoextracted from intermediate stages for feed water heating. The number of such

extractions usually vary from 2-3 to as much as 8-9. The latent heat of exhauststeam during the process of condensation is completely lost in these turbines.Small capacity turbines of earlier design often do not have regenerative feedheating.

Condensing turbines with one or two intermediate stage extractions at specificpressure for industrial and heating purposes.

Back pressure turbines: The exhaust steam from which is utilised for industrial orheating purposes. To this type of turbines with deteriorated vacuum, can also beadded (in relative sense); the exhaust steam of which may be used for heatingand process purposes.

Topping turbines: These turbines are also of the back pressure type with the

difference that the exhaust steam from these turbines is further utilised inmediumand low-pressure condensing turbines. These turbines, in general,operate at high initial conditions of steam pressure and temperature, and aremostly used during extension of power station capacities, with a view to obtainbetter efficiencies.

Back-pressure turbines with steam extraction from intermediate stages at specificpressure; turbines of this type are meant for supplying the consumer with steamof various pressure and temperature conditions.

Low-pressure (exhaust-pressure) turbines in which the exhaust steam fromreciprocating steam engines, power hammers, presses, etc. is utilized for powergeneration purposes.

Mixed pressure turbines with two or three pressure stages, with supply ofexhauststeam to its intermediate stages. The turbines enumerated under 'a' and'h' usually have extractions for regeneration of steam at specific pressures forother purposes.According to the steam conditions at inlet to turbines:

Low-pressure turbines, using steam at pressure of 1.2 to 2 ata.

Medium-pressure turbines, using steam at pressure of upto. 40 ata.

High-pressure turbines, utilising steam at pressure above 40 ata.

Turbines of very high pressures, utilising steam at pressures of 170 ata andhigher and temperature of 535 deg. C and higher.

Turbines of supercritical pressures, using steam at pressure of 255 ata andabove.


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According to their usage in industry:

Turbines with constant speed of rotation primarily used for driving alter nators:

Steam turbines with variable speed meant for driving turbo-blowers, aircirculators, pumps etc.

Turbines with variable speed: Turbines of this type are usually employed in

steamers, ships and railway locomotives (turbo-locomotives)

Turbine ComponentsTurbine CasingsHigh Pressure Casing: The high pressure casing is made of creep resistingChromium -Molybdenum-vanadium (Cr-Mo-V) steel casting. The top and bottom halves of thecasing are secured together at the flange joint by heat tightened studs to ensure aneffective seal against steam leakage. Four steam chests, two on top and two onsides

are welded to the nozzle boxes, which in turn are welded to the casing at the middlebearing end. The steam chests accommodate four control valves to regulate the flowofsteam to the turbine according to the load requirement. Nozzle boxes and steam

chests are also made of creep resisting Cr-Mo-V steel castings.

RotorsHigh Pressure Rotor: The HP rotor is machined from a single Cr-Mo-V steel forgingwithintegral discs. The rotor forging is thermally stabilised to prevent abnormal defection/The blades are attached to their respective wheels by "T" root fastening. In all themoving wheels, balancing holes are machined to reduce the pressure differenceacross


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them, which results in reduction of axial thrust. First stage has integral shrouds whileother rows have shroudings, rivetted to the blades are periphery. The number ofbladesconnected by a single strip of shrouding is called a blade packet and the number of

blades per packet is decided from vil

Intermediate Pressure Rotor: The IP rotor has seven discs integrally forged with rotorwhile last four discs are shrunk fit. The shaft is made of high creep resisting Cr-Mo-Vsteel forging while the shrunk fit disc are machined from high strength nickel steelforgings. The blades on the integral disc are secured by "T" root fastenings while onshrunk fit disc by 'fork root' fastening. Except the last two wheels, all other wheelshaveshroundings rivetted at the tip of the blades. To adjust the frequency of the movingblades, lashing wires have been provided in some stages.Low Pressure Rotor: The LP rotor consists of shrunk fit discs a shaft. The shaft is aforging of Cr-Mo-V steel while the discs are of high strength nickel steel forgings.

Blades are secured to the respective discs by rivetted fork root fastening. In all

thestages lashing wires are providing to adjust the frequency of blades . In thelast tworows satellite strips are provided at the leading edges of the blades to protectthemagainst wet steam erosion.

BladesBlades are single most costly element of turbine. Blades fitted in the stationary partarecalled guide blades or nozzles and those fitted in the rotor are called moving orworkingblades. The following are three main types of blades:

Cylindrical (or constant profile) blade

Tapered cylindrical (tapered but similar profile).

Twisted and varying profile blades.Blades have three main parts(a) Aerofoil: It is working part of blade and is one of the types described above,(b) Root: It is portion of the blade which is held with the disc, drum or casing and(c) Shrouds.

Emergency Stop Valves and Control ValvesTurbine is equipped with emergency stop valves to cut off steam supply and with

control valves regulate steam supply. Emergency stop valves (ESV) are providedin the mainstream line ad Interceptor valves (IV) are provided in the hot reheatline.Emergency stop valves are actuated by servomotor controlled by the protectionsystem. ESV remains either fully open or fully close.Control valves are actuated by the governing system through servomotors toregulate steam supply as required by the load.Valves are either single seat type or double seat type Single seat type valves arepreferred though these required higher force for opening or closing.

Couplings

Since the shaft (rotor) is made in small parts due to forging limitations and othertechnological and economic reasons, the couplings are required between any


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two rotors. The coupling permits angular misalignment, transmits axial thrust andensures axial location. The couplings are either rigid or semi flexible The formerneither permits angular nor lateral deflection while the later permits only angulardefection Number of critical speeds depend upon the modes of vibration andhence the type of coupling provided between rotors. Generally in 200/210 MW

turbines, coupling between HPT and IPT is of rigid type and between IPT andLPT is of semi-flexible lens type.

Turbine AuxiliariesArrangement Of Turbine AuxiliariesTo facilitate proper functioning of turbine the auxiliaries are arranged at differentlocations peeping in view the easy installation, proper operation andmaintenance and technical requirements,The turbine cycle can be viewed in the form of different systems as given inthe following paragraphs

Vacuum System This comprises ofCondenser - 2 per 200MW unit at the exhaust of LP turbine.

Ejectors - One starting and two main ejectors connected to the condenserlocated near the turbine.

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

Condensate System This contains the following :

Condensate Pumps - 3 per unit of 50% capacity each located near thecondenser hot well. LP Heaters - Normally 4 in number with no. 1 located at the upper part ofthe condenser and nos. 2,3 & 4 around 4m level. Deaerator- One per unit located around 18 'M' level in CD bay.Feed Water System The main equipments coming under this system are:


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Boiler Feed Pump : Three per unit of 50% capacity each located in the '0'meter level in the TG bay. High Pressure Heaters: Normally three in number and are situated in theTG bayDrip Pumps : Generally two in number of 100% capacity

Turbine Lub Oil System This consists of Main Oil Pump (MOP) Starting Oil Pump(SOP), AC standby oil pumps and emergency DC oil pump and Jacking Oil Pump(JOP) (one each per unit).Auxiliary Steam System The main 16 ata header runs parallel to BC bay at thelevelof around 18 'M'.

Condensate SystemCondensate PumpsThe function of these pumps is to pumps out the condensate to the deaerator thru'ejectors, gland steam cooler, and L.P. heaters. These pumps have four stages andsince the suction is at a negative pressure, special arrangements have been madeforproviding sealing. This pump is rated generally for 160 cu.m hr. at a pressure 13.2Kg/sq.cm.

L.P. HeatersTurbine has been provided with non-controlled extractions which are utilised forheating the condensate, from turbine bleed steam. There are 4 low pressure heatersin

which the last four extractions are used.


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Feed Water SystemThis system plays an important role in the supply of feed water to the boiler atrequisite pressure and steam I water ratio. This chapter describes the variousauxiliaries under this system starting from Boiler Feed Pump to FeedRegulating Station Via HP heaters.

Boiler Feed PumpsThis pump is horizontal and of barrel design driven by an Electric motor through ahydraulic coupling. All the bearings of pump and motor are forced lubricated by asuitable oil lubricating system with adequate protection to trip the pump if the lubricationoil pressure falls below a preset value.The high-pressure boiler feed pump is very expensive machine which calls for a verycareful operation and skilled maintenance. The safety in operation and efficiency of thefeed pump depends largely on the reliable operation and maintenance. Operating staffmust be able to find out the causes of defect at the very beginning which can be easilyremoved without endangering the operator of the power plant and also without theexpensive dismantling of the high pressure feed pump.The feed pump consists of pump barrel, into which is mounted the inside stator togetherwith rotor. The hydraulic part is enclosed by the high pressure cover alongwith thebalancing device. The suction side of the barrel and the space in the high pressure

cover behind the balancing device are enclosed by the low pressure covers alongwiththe stuffing box casings. The brackets of the radial bearing of the suction side and radial


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and thrust bearing of the discharge side are fixed to the low pressure covers. The entirepumps is mounted on a foundation frame. The hydraulic coupling and two claw couplingwith coupling guards are also delivered alongwith the pump. Water cooling and oillubricating are provided with their accessories.Mechanical Seal The use of mechanical sealreduces the loses of feed water in thestuffing box to a minimum and working ability of the feed pump increases. Cooling ofstuffing box space should be perfect by the use of mechanical seal.Cooling is carriedout by the circulation of water between the stuffing box space and the cooler. Even afterstopping the pump stuffing box cooling should be continued as its cooling circuit isdifferent from the seal cooler. Coolers are designed to keep the stuffing box spacetemperature below .80 deg.C.Function The water with the given operating temperature should flow continuously tothe pump under a certain minimum pressure. It passes through the suction branch intothe intake spiral and from there is directed to the first impeller. After leaving the impellerit passes through the distributing passages of the diffuser and thereby gets a certainpressure rise and at the same time it flows over to the guide vanes to the inlet of thenext impeller. This will repeat from one stage to the other till it passes through the last

impeller and the end diffuser. Thus the feed water reaching into the discharge spacedevelops the necessary operating pressure.Balancing Device A small portion of the feed water in the order of about 10% whichisnot calculated to the guaranteed delivery capacity is taken of 'f from the spacebehindthe last impeller for the operation of the automatic balancing device to balance thehydraulic axial thrust of the pum p rotor. The purpose of the balancing device is totakeup thrust pressure in a similar way as the thrust hearing. It is evident from thefunction

of the balancing device that behind the balancing disc the pressure must not rise,otherwise the hydraulic equibilirum will be broken and therefore equilising pipingmusthave a sufficient flow capacity A pressure gauge connection is given for the controlofpressure in the equalising piping. For safe operating of the balancing device, thepressure value on this pressure gauge should be 0.5 to 2 atm, higher than the intakesuction branch pressure. With the pressure rise in the balancing space by 5 atm.abovethe suction pressure it is necessary to trip the pump in order to find out the cause ofdefect and to rectify it.

Lubricating SystemAll the bearings of boiler feed pump, pump motor and hydrauliccoupling, are force lubricated. The feed pump consists of two radial sleeve bearingsandone thrust bearing. The thrust bearing is located at the free-end of the pump. Thefeedpump driving motor consists of two radial sleeve bearings. Feed pump is coupledwithits driving motor through hydraulic coupling which serves the purpose of controllingthespeed of feed pump for maintaining a definite delivery head and qualtity of feedwater


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as per requirement of the boiler.This hydraulic coupling consists of four radialbearingsand two tilting pad bearings.Booster Pump: Each boiler feed pump is provided with a booster pump in its suctionline which is driven by the main motor of the boiler feed pump. One of the major

damages which may occur to a B.F. pump is from cavitation or vapour bounding atthepump suction due to suction failure. Cavitation will occur when the suction pressureofthe pump at the pump at the pump suction is equal or very near to the vapourpresureof the liquid to be pumped at a particular feed water temperature. By the use of abooster pump in the main pump suction line, always there will be positive suctionpressure which will remove the possibility of Cavitation

Turbine Driven Boiler Feed Pump

The single cylinder turbine is of the axial flow type. The live steam flows through theemergency stop valve and then through the main Control Valves (5 nos. (Nozzlegoverning). These valves regulate the steam supply through theturbine in accordance with load requirements. The control valves are actuated by aliftbar which is raised or lowered via a lever system by the relay cylinder mounted ontheturbine casing.The journal bearings supporting the turbine shaft are arranged in the two bearingblocks.The front end-bearing block also houses the thrust bearing, which locates the turbineshaft and takes up "the axial forces.There are 14 stages of reaction blading. The balancing piston is provided at the.Steamadmission side to compensate the axial thrust to the maximum extent. Since theaxialthrust varies with the load, the residual thrust is taken up hy the thrust bearing. Theleak off from the balancing piston is connected back to the turbine after 9th stage.The turbine is provided with hydraulic and electro-hydraulic governing system. Aprimaryoil pump is used as a speed sensor for hydraulic governing and shall Probes are

used asa speed sensor for electro hydraulic governing.Whenever steam is drawn from the cold reheat line or auxiliary supply, steam flow iscontrolled by auxiliary control valve. During this period the main control valves (4nos.)will remain fully opened and the bypass valve across it will remain closed. (Bypassremains closed for a short period when changeover from IP steam to CRH takesplace).The steam exhaust from the BFP- Turbine is connected to the main condenser andtheturbine glands are sealed by gland steam.

High Pressure Heaters :


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These are regenerative feed water heaters operating at high pressure and located bytheside of turbine. These are generally vertical type and turbine bleed steam pipes areconnected to them.HP heaters are connected in series on feed waterside and by such arrangement, the

feed water, after feed pump enters the HP heaters. The steam is supplied to theseheaters form the bleed point of the turbine through motor operated valves. Theseheaters have a group bypass protection on the feed waterside. In the event f tuberupture in any of the HPH and the level of the condensate rising to dangerous level,thegroup protection device diverts automatically the feed water directly to boiler, thusbypassing all the 3 H,P. heaters.

Following fittings are generally provided on the HP heaters :

Gauge glass for indicating the drain level.

Pressure gauge with three way cock.

Air Vent cock.

Safety valve shell side.

Seal pot.

Isolating valves.

High level alarm switch.

Drip/Drain SystemThe steam, bleed from the turbine, after condensation is termed as drip/ drain.The drain is cascaded from H.P.-5 will go to deaerator ofL.P-H.-4 depending on theshell pressure and load on the machine. The drain ofL.P.H.-2 from where the drain is

pumped back into the condensate line going to deaerator. The drain from L.P.H.-2canbe regulated to condenser in case the level in L.P.H.-2 rises to a predeterminedlevel.All the L.P.Heaters drains are having manual by-pass which can be operated in caseof any individual regulator fail. The drain from L.P.H.-l is only connected to thecondenserby U-tube water seal.

H.P. - L.P. By Pass SystemHP/LP by pass system can be broadly classified in two groups (Ref. fig. 45).HP bypass station This is utilised for the following tasks. To establish flow at the outlet ofsuperheater (SH) for raising boiler pa rametersduring start-up. To maintain or control steam pressure at per-set value in main steam line duringstart-up. To warm up the steam lines. To control steam temperature down stream of HPBypass at the preset value. To dump steam from boiler into condenser in case the generator circuit breakerLP bypass station The same is utilised of the following tasks.

Control of steam pressure after reheater.

Establish flow of steam from reheat lines to condenser by its opening,proportional to the opening of HP bypass valves.


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Release of steam entrapped in HPT and reheater circuit in case generator circuitbreaker opens.

The interconnections of the above stations with the turbine power cycle are as under

Description Up stream steam Down streamConnection Steam connection

HP bypass Main steam Cold reheat linesstation ahead of MSV

LP bypass Hot reheat lines Steam throw offahead of IV device build in

Condenser

Generator And AuxiliariesThe transformation of mechanical energy into electrical energy is carried outby the Generator. This Chapter seeks to provide basic understanding aboutthe working principles and development of Generator.

Working PrincipleThe A.C. Generator or alternator is based upon the principle of electromagneticinduction and consists generally of a stationary part called stator and a rotating partcalled rotor. The stator housed the armature windings. The rotor houses the fieldwindings. D.C. voltage is applied to the field windings through slip rings. When the rotor

is rotated, the lines of magnetic flux (viz magnetic field) cut through the stator windings.This induces an electromagnetic force (e.m.f.) in the stator windings. The magnitude ofthis e.m.f. is given by the following expression.

E = 4.44 /O FN volts0 = Strength of magnetic field in webers.F = Frequency in cycles per second or Hertz.N = Number of turns in a coil of stator windingF = Frequency = Pn/120Where P = Number of polesn = revolutions per second of rotor.From the expression it is clear that for the same frequency, number of poles increases

with decrease in speed and vice versa. Therefore, low speed hydro turbine drivesgenerators have 14 to 20 poles where as high speed steam turbine driven generators


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have generally 2 poles. Pole rotors are used in low speed generators, because the costadvantage as well as easier construction.

Generator Components

RotorThe electrical rotor is the most difficult part of the generator to design. It revolves inmost modern generators at a speed of 3,000 revolutions per minute. The problem ofguaranteeing the dynamic strength and operating stability of such a rotor iscomplicatedby the fact that a massive non-uniform shaft subjected to a multiplicity of differentialstresses must operate in oil lubricated sleeve bearings supported by a structuremounted on foundations all of which possess complex dynamic be behaviourpeculiar tothemselves. It is also an electromagnet and to give it the necessary magneticstrengththe windings must carry a fairly high current.

Rotor windingSilver bearing copper is used for the winding with mica as the insulation betweenconductors. A mechanically strong insulators such as micanite is used for lining theslots. Later designs of windings for large rotor incorporate combination of hollow

Rotor balancingWhen completed the rotor must be tested for mechanical balance, which means thatacheck is made to see if it will run upto normal speed without vibration. To do this itwould have to be uniform about its central axis and it is most unlikely that thiswill be so to the degree necessary for perfect balance. Arrangements are thereforemade in all designs to fix adjustable balance weights around the circumference ateach end.

Generator Cooling And Sealing SystemThe Generator is provided with an efficient cooling system to

avoid excessive heating and consequent wear and tear of its main componentsduring operation. This Chapter deals with the rotor-hydrogen cooling systemand stator water cooling system along with the shaft sealing and bearingcooling systems.


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coal to electricity with thermal power plant

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