CHAPTER-1INTRODUCTIONAlmost all coal, nuclear, geothermal, solar thermal electric, and waste incineration plants, as well as many natural gas power plants are thermal. Natural gas is frequently combusted in gas turbines as well as boilers. The waste heat from a gas turbine can be used to raise steam, in a combined cycle plant that improves overall efficiency. Power plants burning coal, fuel oil, or natural gas are often called fossil-fuel power plants. Some biomass-fueled thermal power plants have appeared also. Non-nuclear thermal power plants, particularly fossil-fueled plants, which do not use co-generation, are sometimes referred to as conventional power plants.Commercial electric utility power stations are usually constructed on a large scale and designed for continuous operation. Electric power plants typically use three-phase electrical generators to produce alternating current (AC) electric power at a frequency of 50Hz or 60 Hz. Large companies or institutions may have their own power plants to supply heating ordriven power plants have been used in various large ships, but are now usually used in large naval ships. Shipboard power plants usually directly couple the turbine to the ship's propellers through gearboxes. Power plants in such ships also provide steam to smaller turbines driving electric generators to supply electricity. Shipboard steam power plants can be either fossil fuel or nuclear. Nuclear marine propulsion is, with few exceptions, used only in naval vessels. There have been perhaps about a dozen turbo-electric ships in which a steam-driven turbine drives an electric generator which powers an electric motor for propulsion.Combined heat and power plants (CH&P plants), often called co-generation plants, produce both electric power and heat for process heat or space heating. Steam and hot water lose energy when piped over substantial distance, so carrying heat energy by steam or hot water is often only worthwhile within a local area, such as a ship, industrial plant, or district heating of nearby buildings.

THERMAL PLANT OVERVIEW

CHAPTER-2SECTORS IN THERMAL POWER STATION:Thermal power plant is manly classified into three: Boiler and its auxiliaries Turbine and its auxiliaries Generator and common auxiliariesBOILER AND AUXILARIES:BOILER ACESSORIES: Boiler drum Economizer Super heater Reheaters Water wells De-super heatersBOILER AUXILARIES: Mills Fans Electrostatic precipitators Air pre heatersECONOMISER:Economizers are mechanical devices intended to reduce energy consumption, or to perform another useful function such as preheating a fluid. The term economizer is used for other purposes as well. Boiler, power plant, heating, ventilating, and air conditioning uses are discussed in this article. In simple terms, an economizer is a heat exchanger. It increases the feed water temperature by extracting the temperature from flue gases.ADVANTAGES Coal can be saved from 15% to 20%. Increase in 1% Thermal Efficiency for every 6C change in temperature. Feed water changes to steam quickly. Increases Boiler Life. Decreases thermal stress of Boiler Internal Parts. Decrease in combustion rate SUPER HEATERFossil fuel power plants often have a superheater section in the steam generating furnace. The steam passes through drying equipment inside the steam drum on to the superheater, a set of tubes in the furnace. Here the steam picks up more energy from hot flue gases outside the tubing and its temperature is now superheated above the saturation temperature. The superheated steam is then piped through the main steam lines to the valves before the high pressure turbine.Nuclear-powered steam plants do not have such sections but produce steam at essentially saturated conditions. Experimental nuclear plants were equipped with fossil-fired super heaters in an attempt to improve overall plant operating cost. It increases the temperature of Main steam with the help of temperature of flue gases to get Saturated Steam admitted to the HPTADVANTAGES: Super heats the steam from Boiler before admitting it to turbine. Removes the moisture contents from the steam to avoid the corrosion and breaking of turbine blade tipsREHEATER:Power plant furnaces may have a reheater section containing tubes heated by hot flue gases outside the tubes. Exhaust steam from the high pressure turbine is passed through these heated tubes to collect more before driving the intermediate and then low pressure turbines. It heats the temperature of steam outlet from HPT with the help of Flue gas temperature

ADVANTAGES: Re-heats the steam to increase the thermal efficiency. Increases the energy in the steam to perform additional work before exhausting into condenser from LPTWATER WALLSWater walls carry feed water from ring headers to Boiler Drum through raiser tubes. ADVANTAGES: Increase in efficiency. Better heat transfer. Easy and quick erection. Increased availability of BoilerBOILER AUXILARIES:FANS:Supply air for combustion in the furnace and for evacuation of the flue gases formed from the combustion. Maintain Balanced Draft inside the furnace. Supply air for cooling of equipments working in hot zones. Supply air for sealing of gates, feeders & mills bearings etc. Air used for combustion is divided into 2 parts:PRIMARY AIRPortion of total air sent through mills to the furnace. This air dries the pulverized coal and transport it to the furnace for combustion.

SECONDARY AIRLarge portion of total air sent to furnace to supply necessary oxygen for the combustion.Types of Fans :- 1. Axial Fan with two subgroup as i) Impulse ii) Reaction.2. Radial fans or Centrifugal fan. (Single suction or double suction) FD FANS: Supplies secondary air to the furnace through APH to assist in combustion. Supply total air flow to the furnace except where an independent atmospheric P.A fan is used. Provides air for sealing requirement and excess air requirement in the furnace. Axial fan-reaction type with blade pitch control is use in the Pulverized fired boiler(210/250/500 MW)PA FANS: Supply high pressure primary air through APH needed to dry & transport coal directly from the coal mills to the furnace. Primary air for mills is divided into cold & hot primary air. Axial fan-double stage-reaction type with blade pitch control is use in the Pulverized fired boiler (210/250/500 MW). ID FANS: Suck the gases out of the furnace and throw them into the stack by creating sufficient negative pressure in the furnace (5-10 mmwc) in the balanced draft units. Located in between the ESP and Chimney in the flue gas path. Radial Fans -double suction-backward curved vane with inlet guide vane(IGV)control and VFD control is use in all boilers Handles large volume hot dust/ash laden flue gas (temp up to 150 deg C) from furnace and all leakages occurring in the system till the inlet of the fan. Overcome the pressure drop inside the furnace, Super heater, Re -heater, Economizers, Gas ducting & ESP. Consumes max. Power in all boiler auxiliaries as it handles the large volume and heavy pressure drop of the flue gas.MILLS:The most efficient way of utilizing coal for steam generation is to burn it in pulverized form and this can be considered as most significant engineering achievements, in boiler technology. The purpose of mills is to pulverize crushed coal to fineness such that 70-80 % passes through a 200 mesh (74 micron) sieve and to facilitate supply the same either to burners of boiler by air at required temperature or to storage bins.ADVANTAGES OF PULVERISATION: Efficient utilization of cheaper low grade coals Flexibility in firing with ability to meet fluctuating loads Ability to design and build high unit capacity steam generators Better reaction to automatic controlsDISADVANTAGE: The main disadvantage is that initial cost and maintenance cost of the equipments for preparation of the PF is high.FACTORS THAT AFFECT THE PERFORMANCE OF MILLS: Grind ability of coal Moisture content Fineness of output required Size of coal input

MOISTURE CONTENT:The drying ability of mill varies widely between mills .Since the drying capacity of mill reduces with higher moisture content in coal; this will limit the output capacity of mill when high moisture coal is used. MILL capacity variation is shown in the next figure.SIZE OF RAW COAL:Larger the size of raw coal fed to mill, amount of the work per unit mass is increased to get fine coal of same fineness. Hence the mill capacity varies inversely with size of the coal. Coal passing through 20mm mesh is found suitable for Indian conditions. However 80 to 85% through 200 meshes may be required for low volatile coal. Excessive fineness only wastes mill power. MILL OUTPUT varies inversely with fineness. VARIATION in MILL output is shown in the adjacent fig.

TYPES OF MILLS:The MILLS are classified based upon their speeds SLOW SPEED MILLS: The mills usually rotating between 15 to 25 rpm are called slow speed mills. E.g. Ball tube mills. These are also called Horizontal mills (eg.,Mills of Stage- II &III, VTPS ) MEDIUM SPEED MILLS: These will be operating at 50 to 100 rpm. E.g. Bowl mills and Ball and Race mills These are also called vertical mills as their axis of rotation is vertical.(eg.,Mills of VTPS STAGE I & IV,VTPS ). HIGH SPEED MILLS: These are directly coupled to the driving motor and run at 750 to 1000 rpm. Beater wheel mills are High speed mills and are in service at Neyveli Project.BALL TUBE MILLS They grind coal mainly by impact of the balls falling on the coal due to attrition as particles slide overeach other as well as over the liners and due to crushing as balls roll over the liners with the coal particlesbetween them. They do not lose any of their grinding characteristics with time and provide constant fineness throughout the service life of their wear parts. They are used for very abrasive and high-ash and low volatile coals which require very fine grinding.BALL TUBE MILLS PRINCIPLE OF GRINDINGThe raw coal from the Feeder discharge falls into a mixing-box where it is pre-dried by hot by-pass air. The raw coal then enters the mill through the trunnion tube, whose screw conveyor conveys the coal into the mill. This is then pulverized by the movement of the balls inside the rotating mill body. The hot PA (primary air) introduced into the mill through the central tube of the sc completes the coal drying and carries the PC(pulverized fuel) to the Boiler through the trunnion annulus around sc hot air tube. The PC/PF and the PA mixed with the by-pass air from the mixing box enters the Classifier installed above the mills.

ELECTROSTATIC PRECIPITATORIt is used to separate suspended particles from flue gases

WORKINGIt essentially consists of two sets of electrodes which are completely insulated from each other and a high voltage electrostatic field is maintained across them. One set, called the emitting electrode or discharge electrode is in the form of thin wires and the other set called collecting electrode. The emitting or discharge electrodes are placed in the centre of a pipe in case of tubular type precipitator and are connected to negative polarity of HVDC while the collecting electrodes are connected to the positive polarity of the source and are earthed. High electrostatic field thus set up between the two sets of electrodes creates a corona discharge and ionizes the gas molecules as the flue gas flows through the tube or in between the plates. The dust particles in the gas acquire negative charge and attracted to the electrodes connected to the positive polarity (collecting electrodes) and deposited there. The deposited dust is made to fall off the electrodes when rapped mechanically.

The electrostatic precipitator may be (1) Plate or tubular type (2) Horizontal flow or vertical flow type (3) Dry or wet type. In case of wet or irrigated precipitator, the deposited dust is removed by a water film flowing down on the inner side of the collecting electrodes. Usually, a dry type electrostatic precipitator is employed.The electrostatic precipitator has the advantages of high efficiency (up to 99.5%), low pressure drop, easy removal of collected particles and capacity of handling large volume of flue gases. The limitations of this system are high capital and operating costs, and requirement of more space. The maximum collecting efficiency is maintained only for one value of gas velocity for which the system is designed ELECTROSTATIC PRECIPITATION

TURBINE AND ITS AUXILARIESA Turbineis a rotary mechanical device that extractsenergyfrom afluid flow and converts it into usefulwork A turbine is aturbo machine with at least one moving part called a rotor assembly, which is a shaft or drum withblades attached. Moving fluid acts on the blades so that they move and impart rotational energy to the rotor.CLASSIFICATION OF TURBINES:STEAM TURBINES: Steam turbine is a rotating machine which converts heat energy of steam to mechanical energy.PRINCIPLES OF STEAM TURBINE: 1. Conversion of heat energy into kinetic energy. 2. Depends upon the dynamic action of the steam. 3. Drop in pressure of steam through some passage resulting in increase in velocity.4. Change in direction of motion which gives rise to a change of momentum or force.5. This is driving force of the rotor.Depending on steam condition and method used to cause the steam turbines classified into two general groups: IMPULSE TURBINES REACTIVE TURBINES

IMPULSE TURBINE:Impulse: In impulse turbine the steam expands in fixed nozzles. The high velocity steam issuing from nozzles does work on the moving blades which causes the shaft to rotate. Pressure drop occurs in the nozzles only, and there is no pressure drop over the moving blades. Maximum steam velocity (Impulse) is created at the inlet of moving blade. This means high Kinetic Energy. This energy is utilized for rotation of moving blade Steam Velocity can be maximized by having maximum pressure drop in the Nozzles. Hence in 100% Impulse steam Turbine, whole pressure drop will be in stationary blades or nozzles To sustain high velocity impulse stage should be very robust in construction.REACTION TURBINE: Reaction is created due to action of change of direction of steam through bucket (Blades). This thrust or reaction causes the driving force This requires whole pressure drop in moving blades 100% Impulse or Reaction stage is purely a theoretical assumption ,not practically feasible.Steam turbines classified into different categories based on following conditions:1. According to direction of steam flow:Axial turbines:In which the steam flow in a direction parallel to axis of turbine.Radial turbines:In which the steam flow in the direction perpendicular to axis of turbine2.According to the number of cylinders: Single, Double, Three, Four.3. According to method of governing:a) turbines with throttle governing steam flow is controlled by opening snd closing of all control valves simultaneously to the extent required by load and admitting the steam to group of nozzles located at entire periphery.b)turbines with nozzle governing steam flow is controlled by sequential opening or closing of control valves slowing the steam to flow to associated nozzle groupsc) turbine with bypass governing in which steam besides fed to the first stage is also directly led to one, two or even three intermediate stages of turbine.4. According to heat balance arrangements:a) Condensing turbine with regeneration:In these turbines steam at a pressure less than atmospheric is directed to a condenser.The turbine auxiliaries are Main air ejector Condensate extraction pump LP&HP Heaters Deaerators HP &LP bypass system Boiler feed pumpMAIN AIR EJECTORMain ejectors are used for extracting non-condensable gases from the coldest zone of the condenser. The working medium for MAIN EJECTOR is steam of 6.5 kg/cm2 & temp. of 2100c.The energy of the steam is retrieved, as the ejectors are interposed in the feed heating cycle &improves the overall efficiency. Ejectors consist of 3 compression stages with inter coolers &after cooler. 1st stage of suction chamber is connected to the condenser.Main assemblies are: suction chamber shell Water chamber Tube system Air measuring system Suction chamber has been divided into 3 parts and all the compression stages consisting of nozzle & diffuser are mounted inside chamber. Water box has also been divided into different zones in such a way that cold condensate first flow through the first stage inter cooler and there after through the 2nd stage &3rd stages. The convergent divergent nozzle has been designed to accelerate the steam flow & thereby reduces the pr. Of the steam to 0.03kg/cm2 abs. in the first stage of Suction chamber. This low pr. in the Suction chamber sucks the non condensable gases from the condenser. The steam gas mixture enters the diffuser inlet and while passing through the diffuser, the Kinetic head is converted into pr. head. The steam gas mixture flows over the tube nest and steam gets condensed, while the non condensable gases flow to suction chamber of 2nd stage. Allows a condensate of capacity of max. 253 T/hr. & a min. of 103 T/hr. 1 No. ejector is said to be in operation to maintain vacuum in the condenser, when rate of air ingress is < 20 kg/Hr.CONDENSATE EXTRACTION PUMPS:Function of this pump is to pump the condensate to the deaerator through ejectors, gland steam cooler, drain cooler and LP heaters. In 210MW unit 3 pumps are installed in which 1 is standby. LP AND HP HEATERS:Regenerative system is provided to improve the thermal cycle by increasing feed water temperature. In regenerative feed water heating part of steam is bled (extracted) after partial expansion in the turbine and is used to heat up the feed- water going to steam generator. Usually feed-water is heated to 0.65to0.75 times saturation temperature in 5 to 9 heaters by regenerative heating. In 210MW, KWU units 3no.LP heaters and 2no.HP heaters are used.DEAERATOR:The main function of deaerators is to remove the dissolved non condensable gases and to heat boiler feed water. It consists of pressure vessel in which pressure and steam are mixed in a controlled manner. Deaerators protects the feed pumps, piping, boiler and any other equipment that is in the boiler feed and return cycle from the effects of corrosive gases i.e. O2 & CO2.Principle of deaerators and operation:Here is a law, that a solubility of a gas in a liquid decreases with an increase in temperature of a liquid. This is the principle involved in operation of deaerators. The deaerators use the spraying system, it sprays the water in to atmosphere of steam in preheated (first stage), and next it mixes with steam in deaerators section (second stage). In the first stage the water is heated to 2-40C of steam temperature where the O2 & CO2 gets separated. From the 1st stage water flows to 2nd stage contains minute traces of dissolved gases. In this section consists of separate layers of trays. The steam passes in to this stage and is mixed with pre heated water.The steam then rises in to first stage and carries residual gases. In 1st stage most the steam is condensed and any residual gases are there they are sent to atmosphere through vent. The water which leaves the second stage falls to storage tank where it can be stored.HP-LP BYPASS SYSTEMThe HP-LP bypass system is such an important system that any body whoever wants to start the turbine should understand its functions, its necessity and the procedure to keep it in service along with the interlocks for keeping the system and to maintain the system to continue in service till its function is over during the process of warming up the turbine and rolling the turbine. When once we are familiar to keep the HP/LP bypass system into service the balance process of turbine operation is very easy and it is almost that the theory/process of turbine operation is totally understood by us, because all the turbine systems will be in service by the time the HP-LP bypass system is taken into service. As seen from the figure the main turbine system i.e. HP-IP-LP turbines are bypassed and the boiler is operated as though the turbine system is in service for all practical purposes. Purpose of HP/LP bypasses systems: To provide reheater protection by establishing steam flow through reheater while going for coal firing to raise the boiler parameters. To achieve turbine matching steam parameters during hot/warm start up operations. To warm up the main steam, cold reheat and hot reheat steam pipe lines and to facilitate warming up of ESVs, IVs, HPT casing and IPT casing during all types of turbine start up operations. Pre conditions for keeping HP/LP bypass system in to service: a) The condenser vacuum should be above 540 mm of Hg. b) The bypass valves opening position shall be not less than 2%. c) The condensate dumping valves are kept opened. d) The CRH NRVs and their bypass valves should be in closed position. e) The oil supply units for the operation of the bypass valves shall be in service. f) The Generator transformer circuit breaker shall be in open position.

Functions of HP-LP bypass system: a) Main steam from boiler outlet is admitted into cold reheat lines through HBP-1 & HBP-2, thus bypassing the HP turbine. b) The temperature of steam entering the cold reheat lines was reduced to below 3800 C by the HP bypass spray water injection valves HBPE-1 & HBPE-2 (Feed water pressure was reduced to 90 kg/cm2 by the BD valve). c) Then the steam undergoes reheat cycle in the boiler and comes back to hot reheat lines. d) HRH steam is admitted into condenser through LBP-1 & LBP-2, thus bypassing the IPT & LPT. e) Temperature of steam entering the condenser was reduced to less than 2000C by the LP bypass spray water injection valves LBPE-1 & LBPE-2. (Water drawn from the CEP discharge header was directly used through LBPE-1 & LBPE-2).

BOILER FEED PUMP:Pump: It is a device which converts mechanical energy into pressure energy due to which fluid moves from one point to other point. A boiler feed water pump is a specific type of pump used to pump feed water into a steam boiler. The water may be freshly supplied or returning condensate produced as a result of the condensation of the steam produced by the boiler. These pumps are normally high pressure units that take suction from a condensate return system and can be of the centrifugal pump type or positive displacement type or hydrogen and phosphate dozing pumps.WORKING PRINCIPLE OF BOILER FEEDER PUMP:The water with the given operating temperature should flow through the pump under a certain minimum pressure, water passes through the suction branch into the intake spiral and from here directed to the first impeller. After leaving through the impeller it passes through the distributing passages of the diffuser where it get certain pressure rise and floe over to guide vanes to the inlet of next impeller. This process repeats from the stage to the other till it passes through the last impeller and end diffusers. Thus the feed water arrives through the discharge space develops the necessary operating pressure. PRINCIPLE OF CENTRIFUGAL PUMP:A centrifugal pump is defined as a machine which increases the pressure energy of the fluid with the help of centrifugal action. Whirling motion is imparted to the fluid by means of blades mounted on the disc known as impeller. It consists mainly one or more impellers equipped with vanes mounted on a rotating shaft and enclosed by a casing. Fluid enters into the impeller axially near the shaft and has energy both kinetic and potential imparted to it by vanes. As the fluid leaves the impeller at a relatively high velocity it is collected at volute or series of diffusing passages which transforms the kinetic energy into pressure. After the conversion is completed the fluid is discharged from a machine.In centrifugal pumps there are two types of pumpsVolute casing pumps and Diffuses vane pumps

FEED PUMP CONSISTS OF FOLLOWING MAIN PARTS:1. ROTOR2. INSIDE STATOR3. MECHANICAL SEAL4. BALANCING DEVICELUBE OIL SYSTEM:-The primary function of the Lube Oil system is to supply lubricating oil at the proper pressure and temperature to the main propulsion turbines and reduction gears. The lube oil not only lubricates the machinery, but it also cools and helps reduce rusting. Secondary functions include purifying oil that has become contaminated and transferring oil to or from the sump tank, gravity tank, storage tank, settling tank, sludge tank, or deck connections.The lube oil service system is provided with two lube oil service pumps. One vertical rotary, motor driven and one reciprocating pump connected so that one unit is available for standby operation while the other unit is supplying the system. The lube oil service pump has a capacity of 435 gpm and lube oil standby 450 gpm, at a discharge pressure of 57.5 psig, and one pump by itself is capable of supplying the entire oil requirements of the main propulsion unit under any power conditions.ADVANTAGES:-It provides safety for other equipment.

GENERATORS:A power station (also referred to as a generating station, power plant, powerhouse or generating plant) is an industrial facility for the generation of electric power. At the center of nearly all power stations is a generator, a rotating machine that converts mechanical power into electrical power by creating relative motion between a magnetic field and a conductor. The energy source harnessed to turn the generator varies widely. It depends chiefly on which fuels are easily available, cheap enough and on the types of technology that the power company has access to. Most power stations in the world burn fossil fuels such as coal, oil, and natural gas to generate electricity, and some use nuclear power, but there is an increasing use of cleaner renewable sources such as solar, wind, wave and hydroelectric.PRINCIPLE OF GENERATORS:-The a.c generator or alternator is based upon the principle of electromagnetic induction and consist generally of a stationary part called stator and a rotator part called rotor. The stator housed the armature windings. The rotor houses the field winding D.C. voltage is applied to the field winding when the rotor is rotated ,the lines of magnetic flux cut through the stator windings .this induces an electromagnetic force in the stator windings . The generator should have same frequency ,number of poles increases with decrease in speed and vice versa. MAJOR COMPONENTS OF GENERATOR:- Stator rotor hydrogen coolersSTATOR AND ROTOR:-The stator frame is used to hold the armature windings in alternators, and in case of larger diameter alternators (which are slow speed) the stator frame is cast out of sections and there are holes for ventilation in the casting its self. The recent trends towards such stator construction are more in favor of using mild steel plates which are welded together rather than using castings. Another integral part of the stator is the stator core which is supposed by the stator frame. Actually you can see the combined diagram of both the stator core as well as the stator frame the core is constructed in the form of laminations and the material used for the same is either magnetic iron or steel alloy. The main purpose of lamination is to prevent loss of energy in the form of eddy currents. There are different types of armature slots provided in the core to insert the conductors and the three various types are as follows Wide open type slots Semi closed type slots Close type slots Whereas the wide open slots are easier from the winding and repair perspective, they are also having a disadvantage that improper distribution of air gap flux. The fully closed one on the other hand are good from this air gap flux perspective but obviously the initial construction and labour involved in such winding is substantially more in this case. The semi closed presents an optimum mix of both these but then the actual design used is decided by the design specialist based on combined effect of all factors involved.HYDROGEN COOLERS:-The hydrogen cooler is a shell and tube type heat exchanger, which cools the hydrogen gas in the generator. The heat removed from the hydrogen, is dissipated through the cooling water .the cooling water flows through the tubes ,while the hydrogen is passed around the finned tubs .the coolers is subdivided into section, which are horizontally mounted in the stator housing. The cooling water flows through the tubes according to the cross flows counter current principle as referred to the hydrogen flow. The water flows through the cooler in a horizontal direction .the cold cooling water flows to return water channel on the cold gas side and the heated water is returned on the hot gas side .this cooling water flow passage is obtained through a partition in the inlet/outlet water channel.

GENERATOR AUXILARIES:- Seal oil system Gas system Excitation systemGAS SYSTEM:-In thermal power plants the hydrogen is used in gas system. Hydrogen treatment plant supplies hydrogen to the generator for cooling purpose. The hydrogen gas is transferred in cylinders at a pressure of 150 kg/cm2.The pressure is reduced to 3.5 kg/cm2 and supplied at the top portion of the generator. The generator gets heated up due to various losses and the huge amount of current flowing through the stator. Hydrogen absorbs the heat generated and in turn, it gets heated up. The hydrogen gas is now termed as the HOT GAS. The air blower (fan) collects the hot gas and forces it towards the cooler two vertical coolers are being placed at the turbine end. The cooler consists of many tubes through which chill water flows. The hot gas passes through the coolers and gets cooled. Once the gas is cooled, it is being termed as COLD GAS. The chill water, flowing in the tubes, inside the cooler, is being controlled by two valves (70% valve and 30% valve). The 30% valve is an auto valve which is controlled by using a microprocessor. The 70% valve is being controlled manually.SEAL OIL SYSTEM:-The bearings have to be leak-tight. A hermetic seal, usually a liquid seal, is employed; turbine oil at pressure higher than the hydrogen inside is typically used. A metal, e.g. brass, ring is pressed by springs onto the generator shaft, the oil is forced under pressure between the ring and the shaft; part of the oil flows into the hydrogen side of the generator, another part to the air side. The oil entrains a small amount of air; as the oil is recalculated, some of the air is carried over into the generator. This causes a gradual air contamination buildup and requires maintaining hydrogen purity. Scavenging systems are used for this purpose; gas (mixture of entrained air and hydrogen, released from the oil) is collected in the holding tank for the sealing oil, and released into the atmosphere; the hydrogen losses have to be replenished, either from gas cylinders or from on-site hydrogen generators. Degradation of bearings leads to higher oil leaks, which increases the amount of air transferred into the generator; increased oil consumption can be detected by a flow meter associated to each bearing.EXCITATION SYSTEM:-Modern generators with field coils are self-excited, where some of the power output from the rotor is used to power the field coils. The rotor iron retains a residual magnetism when the generator is turned off. The generator is started with no load connected; the initial weak field creates a weak voltage in the stator coils, which in turn increases the field current, until the machine "builds up" to full voltage. Starting Self-excited generators must be started without any external load attached. An external load will continuously drain off the buildup voltage and prevent the generator from reaching its proper operating voltage.FIELD FLASHING:If the machine does not have enough residual magnetism to build up to full voltage, usually provision is made to inject current into the rotor from another source. This may be a battery, a house unit providing direct current, or rectified current from a source of alternating current power. Since this initial current is required for a very short time, it is called "field flashing". Even small portable generator sets may occasionally need field flashing to restart. The critical field resistance is the maximum field circuit resistance for a given speed with which the shunt generator would excite. The shunt generator will build up voltage only if field circuit resistance is less than critical field resistance. It is a tangent to the open circuit characteristics of the generator at a given speed.

COAL HANDLING PLANT:PURPOSE OF COAL HANDLING PLANTS To have a reliable system for handling and, or, processing To economically handle large quantities, as compared to other modes of handling To serve as a buffer stock so that the unevenness & intermittent arrival of the incoming material is smoothened out and the demand, either continuous or intermittent of the user, is met To homogenize the different types of coal for obtaining a consistent quality product To size or segregate the material to make is convenient & economical to handle furtherAIM AND OBJECTIVES Feeding to bunkers Emptying of wagons Safe operation of equipmentCoal is transported the power station by rail or road from the mines. Loading and unloading of the wagons is automatic while the wagons are moving at a predetermined speed. Coal is unloaded from wagons into track hopper. From the track hopper conveyer takes the coal to crusher for crushing. Before the crusher there is one set of magnetic separator and magnetic supply to remove magnetic materials. After crusher coal is taken by a system of conveyors to the bunker floor, also there is a provision for stacking as well as reclaiming the crushed coal by means of stacker cum reclaimed?

FACTORS TO BE CONSIDERED IN THE DESIGN OF COAL HANDLING SYSTEMS Objective Function Technologies available Characteristics of material Quantity of material to be handled Availability of space Constraints Flexibilities and redundancies Future requirements Buffer stocks Safety and environment Climatic conditions Operational controls Optimization and cost functions Reliability and Maintainability Integration of Eqpt. into a systemMAJOR ISSUES IN COAL HANDLING PLANT OPERATION AND MAINTENANCE SPILLAGES FROM BELT CONVEYOR SYSTEMS

CAUSES Conveyor overloading and material pour outs Poor design of the chute/ belt transfer zone often leading to material turbulence and material splash-outs Poor chute design (slope, flow area, lining etc.) leading to blockages and associated overload spillages Poor skirt design and maintenance Improper Installation and poor conveyor alignments Poor belt cleaning and collection systems Changes in the material flow properties Improper belt geometry in locations where the flat to trough transition length is either too short or too steep

MINIMIZING SPILLAGE IN COAL CONVEYING SYSTEMS Improved design of chutes Longer skirt-boards with segmental and adjustable sealing systems Provision of DS system/ Dry fog system if the moisture in the coal is a problem Use of spillage conveyors below discharge pulleys Use of deeper idler toughing angles Use of impact pads and garland idlers at feed locations on conveyors Various types of belt cleaning systems successfully used all over the world are: Treatment of belt face, or the top wearing layer of the rubber cover, so as to reduce adhesion. Devices for rapping, brushing, wiping or frictional stripping for the removal of adhering material with the aid of rollers, brushes or rotors. Belt turning, washing, spraying and air jetting equipment. Belt scrap devices with continuous liner contact by means of bar, strips, beams, or wires. Belt scraping devices based on intermittent linear contact by means of individually adjustable and in some instances, spring loaded scraper blades, usually metallic. Scrapper chain conveyorsASH HANDLING SYSTEM:ASH BOTTOM HANDLING SYSTEM: Water impounded W type bottom ash hopper to receive and stores furnace ash for its periodical removal (once in a shift). Bottom ash gets uenched as it enters in to the water and has minimizing the clinker formation. The mixture of ash and water is discharged through feed gate to clinker grinder, which sizes and clinkers. Crushed clinkers in slurry form falls to hydro ejector through ejector feed pump. Hydro ejector provides jeering action by means of high-pressure water flowing through it to convey the slurry through pipelines to slurry sump for its further disposal to dump area. Each section of bottom ash hopper is provided with two outlets. Each is having one number each of hydraulically operated discharge gate, clinker grinder and hydro ejector. Flushing header with nozzles provided in bottom ash agitates and removes settled ash from the hopper. When the hopper is empty it is refilled for maintaining the hopper temperature and same overflows to overflow sump. From there it will be transported to the slurry sump by means of overflow jumps. CHALLENGES OF ASH HANDLING:Indian coal presents high ash content generally which tends to be inconsistent. Design of the system has to adequately cover anticipated variations and be capable of handling the worst scenario. System has to be environmentally friendly. System has to be reliable with least maintenance problems. System has to be energy efficient.COMPRESSED AIR SYSTEMS:INSTRUMENT AIR SYSTEMCOMPONENTS Air compressor Air receiver tank Air drying unit Automatic drain valves Ball valves & gate valves Pipe lines

OPERATION PHILOSOPHY Atmospheric air is compressed to the required pressure of 7 kg/cm2 by a compressor (two stage double acting balanced opposed piston type) This air is stored in air receiver tank. The moisture present in compressed air is drained out by automatic & manual drain valves & drain traps. The leftover moisture is removed by passing the air through air drying units, where the desiccant silica gel adsorbs the moisture to give a dew point of -40 deg. C This dry air , called the instrument air, is used for operation of all pneumatic control valves of boiler & turbine, dampers of pa & fd fans, girth gear lubricating system of mills, silo system etc.

Types of compressed air system:Positive displacement compressor:In this type air pressure is decreased b decreasing the value of velocity. It is used mostly. It is of two typesRotary compressorReciprocating compressorReciprocating compressor:These are very efficient, particularly in large sizes used mostly in various sectors these are most used and these are chosen for high load factors, these may be either singe acting or double acting. When the compression takes place both upwards and downward strokes of the piston. Most large compressors are double acting.Rotary compressor:Rotary machines have moving parts simple and reliable, but have a lower efficiency than reciprocating types; they are used when load factors are not so high, often employed as mobile units.Instrument air is used for operating various dampers and burner tilting device, air heater emergency device. The control and station air compressors have been housed separately with separate receivers and supply headers and their tapings.PROTECTION SYSTEM

COOLING TOWER:Cooling water is used to work as heat sink.Types of cooling towerThere are several types of cooling towers based on the two air and water systems.There are three main types of towers natural draught, forced draught, end dry although the majority of cooling towers used are in the first category Natural draught In this type of cooling towers a natural draught of air induced by a chimney effectFlows up wards against a falling spray of water. as this occurs the heat is removed from from the water partly by heat transfer (due to the temperature difference between the air and water) but mainly by the evaporation of a small percentage of the water this is taken up in the raising air stream as water vapor, taking with it the latent heat of vaporizationTo improve the heat transfer the water is broken up in to a spray or fine film so increasing its surface area and providing better cooling. This effect is produced by allowing the water to fall through a cooling stack, which either breaks the water up into droplets or causes the water to flow as a film. The stack is sometimes known as a packing and the two types are called flash and film packing.In a typical natural draught type the tower can be 115 m (375 ft) high and 90 m (300ft)Base diameter. The characteristic shape is designed to give a good upward draught of air the shell of the tower is supported on legs to allow air to flow in to the tower stack from the base. The tower is sited in or over a pond, which acts as a reservoir for the cooling water pumps. The lower 10m (30ft) of the tower is filled by a stack which we mentioned just now-through which the water falls. A spray eliminator is fitted on the top of the stackTo reduce amount of water which could be carried up by the raising current of air . the two types of packing are ;splash packing, used in the majorities of the cooling towers, are usually made up from treated timber which resists rot caused by moisture ; film pickings ,a resent development where asbestos cement sheets are used instead of timber. To give increased strength the asbestos cement sheets are arranged to split the water up in to films instead of in to droplet as in the timber design the main advantage of asbestos cement is the increased life which can be obtained over the timber type of packing.The water is distributed over the cooling stack by a system of asbestos cement pipe and spray nozzles, or along through with holes in the bottom and splash caps beneath the holes. The troughs may be made from treated timber or asbestos cement, the latter is often used for increased life as in the packing.Forced draughtIn this type of tower the draught is provided by fans instead of by the natural draught from the chimney effect. Until recently the capacity of forced draught towers has limited their use to small installation, and this together with a much higher capital cost, has tended to deter their use on modern plant.The main advantage of this type of tower is that the physical size of tower for a given capacity is very much smaller than would be required by a natural draught type.Dry cooling towersSitting power stations and providing adequate cooling water supplies, particularly on inland sites is becoming increasingly difficult. Even when cooling towers are used, because of in adequate water supplies, the amount of make-up to a tower system for a 2000 MW power station is in the region of million liters (12 million gallon) per day. If dry cooling system can be used then no make-up to the system is required which, as you can appreciate, allows a greater choice in selecting a site for a power station.In these towers, heat exchanger units are fixed at the base of the towers. The heat exchanger units are approximately 15 m (48ft) high and are built up from a number of elements bolted together.The heat exchange units can be operated as four separate sections, so allowing the amount of cooling surface in service to be varied according to the ambient temperature conditions or to the load generated by the unit

WATER TREATMENT:This is also called LP dosing system. Natural water is seldom pure. Water gets contaminated due to contact with air, soil or industrial effluent. For high pressure boilers, pre treatment is key to successful operation of industrial power plants.SOURCES: Surface water--- contains high suspended matter and dissolved solids. Ground water--- free from suspended matter but high TDS Sea water---contain high dissolved mineral saltsFor thermal power stations surface water is preferred, due to low TDS and high abundance.PURITY REQUIREMENTS: Modern high pressure steam generating systems requires Conductivity at 25oC (max): 0.1micro mhos/CM Total silica (max):0.01 PPM (10 PPB) Total dissolved solids: 0.02 PPM (20 PPB) Total sodium: 0.015 PPM (15 PPB) METHOD OF TREATMENTRAW WATER PRETREATMENT CLARIFIED WATER FILTRATIONTO REMOVE SUSPENDED SOLIDS

TO REMOVE DISSOLVED SOLIDS DEMINERALISATION DM WATER

PRE-TREATMENT FOR NON - IONIC IMPURITIES Gravity settling Filtration Different Media: Sand, Anthracite Active Carbon Polymer Bead Multimedia Filters Iron Removal Filters Pre-Treatment for Non - Ionic Impurities1. Chemical Coagulation2. Inorganic Coagulant : 3. Alum, Iron Salts, etc.4. Organic flocculent :5. Cationic, Anionic, Nonionic, Polyelectrolyte COAGULATION Alum,Aluminum sulphates are quickly added and mixed into the water to begin the process of removing dirt and particles.

FLOCCULATIONGentle mixing of the Aluminum Sulphate or Alum in the water causes dirt particles to stick together and become heavy, forming rust-colored "floc"

SEDIMENTATIONThe heavy, sticky floc clumps settle to the bottom of the basins, where they are vacuumed up by the moving bridge. The remaining water flows over a raised baffle, then moves to the filters

FACTORS AFFECTING COAGULATION: pH Temperature Nature of particles present in raw water Chemical composition of raw water The effective coagulation takes place at a pH 6.0 to 7.0 for aluminum sulphate as coagulant. Waters having outside this range to be treated with acid or alkali with coagulant for effective coagulation. CHLORINATION:Liquid chlorine is added to clarified water to remove organic matter and microorganisms. Chlorine reacts with water produces hypochlorous acid. Cl2 + H2O --- HCl + HOCl Hypochlorous acid liberates oxygen which prevents growth of micro organisms. Chlorination enhances heat exchange efficiency. 0.2 ppm of residual chlorine in clarifier out let is to be maintained.

Treatment for Dissolved Ionic Impurities Chemical Precipitation Hardness and alkalinity removal by lime soda process (hot or cold) Iron removal Ion Exchange Technique Membrane Technique Electro dialysis Reverse Osmosis Evaporation

ION EXCHANGE:A reversible stoichiometric process. Every ion which is removed from the solution is replaced by equivalent amount of another ion of same sign. After the ion exchange, the exchanger material can be brought back to original form by suitable reaction, called regeneration.Advantages i. Ambient temperature operationii. Instantaneous treated wateriii. Take care of fluctuation of loadiv. Easy waste disposalv. Cheaper to operate

DE MINERALIZATION:

Selectivity: At low conc. in aq. medium and at ordinary temperature - The exchange potential increased with increasing valence. Na+ < Ca++ < Al+++ < Th++++ If valence is constant, exchange potential increases with increase in atomic number.Li < Na < K < Rb < CsMg < Ca < Sr < Ba F < Cl < Br < IRegeneration Process The process to bring back the exhausted resin to original or usable form is regeneration. The reaction is opposite to service reaction. There are two methods for regeneration co-current and counter current method.