THERMAL POWER PLANT USING STEAM

Introduction:A thermal power station is a power plant in which the prime mover is steam driven. Water is heated, turns into steam and spins a steam turbine which drives an electrical generator. After it passes through the turbine, the steam is condensed in a condenser; this is known as a Rankine cycle. The greatest variation in the design of thermal power stations is due to the different fuel sources. Some prefer to use the term energy center because such facilities convert forms of heat energy into electrical energy.Almost 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.Such power stations are most usually constructed on a very large scale and designed for continuous operation.History:Reciprocating steam engines have been used for mechanical power sources since the 18th Century, with notable improvements being made by James Watt. The very first commercial central electrical generating stations in New York and London, in 1882, also used reciprocating steam engines. As generator sizes increased, eventually turbines took over due to higher efficiency and lower cost of construction. By the 1920s any central station larger than a few thousand kilowatts would use a turbine prime mover.Efficiency:The electric efficiency of a conventional thermal power station, considered as saleable energy produced at the plant bus bars compared with the heating value of the fuel consumed, is typically 33 to 48% efficient, limited as all heat engines are by the laws of thermodynamics. The rest of the energy must leave the plant in the form of heat. This waste heat can be disposed of with cooling water or in cooling towers. If the waste heat is instead utilized for e.g. district heating, it is called cogeneration. An important class of thermal power station are associated with desalination facilities; these are typically found in desert countries with large supplies of natural gas and in these plants, freshwater production and electricity are equally important co-products.Since the efficiency of the plant is fundamentally limited by the ratio of the absolute temperatures of the steam at turbine input and output, efficiency improvements require use of higher temperature, and therefore higher pressure, steam. Historically, other working fluids such as mercury have been experimentally used in a mercury vapour turbine power plant, since these can attain higher temperatures than water at lower working pressures. However, the obvious hazards of toxicity, and poor heat transfer properties, have ruled out mercury as a working fluid.Steam generator or boiler:

Fig: Schematic diagram of typical coal-fired power plant steam generator highlighting the air preheater (APH) location. (For simplicity, any radiant section tubing is not shown.)The steam generating boiler has to produce steam at the high purity, pressure and temperature required for the steam turbine that drives the electrical generator. The generator includes the economizer, the steam drum, the chemical dosing equipment, and the furnace with its steam generating tubes and the superheater coils. Necessary safety valves are located at suitable points to avoid excessive boiler pressure. The air and flue gas path equipment include: forced draft (FD) fan, air preheater (APH), boiler furnace, induced draft (ID) fan, fly ash collectors (electrostatic precipitator or baghouse) and the flue gas stack. For units over about 200 MW capacity, redundancy of key components is provided by installing duplicates of the FD fan, APH, fly ash collectors and ID fan with isolating dampers. On some units of about 60 MW, two boilers per unit may instead be provided.Boiler furnace and steam drum:Once water inside the boiler or steam generator, the process of adding the latent heat of vaporization or enthalpy is underway. The boiler transfers energy to the water by the chemical reaction of burning some type of fuel.The water enters the boiler through a section in the convection pass called the economizer. From the economizer it passes to the steam drum. Once the water enters the steam drum it goes down the downcomers to the lower inlet waterwall headers. From the inlet headers the water rises through the waterwalls and is eventually turned into steam due to the heat being generated by the burners located on the front and rear waterwalls (typically). As the water is turned into steam/vapor in the waterwalls, the steam/vapor once again enters the steam drum. The steam/vapor is passed through a series of steam and water separators and then dryers inside the steam drum. The steam separators and dryers remove the water droplets from the steam and the cycle through the waterwalls is repeated. This process is known as natural circulation.The boiler furnace auxiliary equipment includes coal feed nozzles and igniter guns, soot blowers, water lancing and observation ports (in the furnace walls) for observation of the furnace interior. Furnace explosions due to any accumulation of combustible gases after a trip-out are avoided by flushing out such gases from the combustion zone before igniting the coal.The steam drum (as well as the superheater coils and headers) have air vents and drains needed for initial startup. The steam drum has internal devices that removes moisture from the wet steam entering the drum from the steam generating tubes. The dry steam then flows into the superheater coils.Geothermal plants need no boiler since they use naturally occurring steam sources. Heat exchangers may be used where the geothermal steam is very corrosive or contains excessive suspended solids. Nuclear plants also boil water to raise steam, either directly passing the working steam through the reactor or else using an intermediate heat exchanger. Fuel preparation system:In coal-fired power stations, the raw feed coal from the coal storage area is first crushed into small pieces and then conveyed to the coal feed hoppers at the boilers. The coal is next pulverized into a very fine powder. The pulverizers may be ball mills, rotating drum grinders, or other types of grinders.Some power stations burn fuel oil rather than coal. The oil must kept warm (above its pour point) in the fuel oil storage tanks to prevent the oil from congealing and becoming unpumpable. The oil is usually heated to about 100C before being pumped through the furnace fuel oil spray nozzles.Boilers in some power stations use processed natural gas as their main fuel. Other power stations may use processed natural gas as auxiliary fuel in the event that their main fuel supply (coal or oil) is interrupted. In such cases, separate gas burners are provided on the boiler furnaces.Air path:External fans are provided to give sufficient air for combustion. The forced draft fan takes air from the atmosphere and, first warming it in the air preheater for better combustion, injects it via the air nozzles on the furnace wall.The induced draft fan assists the FD fan by drawing out combustible gases from the furnace, maintaining a slightly negative pressure in the furnace to avoid backfiring through any opening. At the furnace outlet, and before the furnace gases are handled by the ID fan, fine dust carried by the outlet gases is removed to avoid atmospheric pollution. This is an environmental limitation prescribed by law, and additionally minimizes erosion of the ID fan.Auxiliary systems:Fly ash collection:Fly ash is captured and removed from the flue gas by electrostatic precipitators or fabric bag filters (or sometimes both) located at the outlet of the furnace and before the induced draft fan. The fly ash is periodically removed from the collection hoppers below the precipitators or bag filters. Generally, the fly ash is pneumatically transported to storage silos for subsequent transport by trucks or railroad cars.Bottom ash collection and disposal:At the bottom of every boiler, a hopper has been provided for collection of the bottom ash from the bottom of the furnace. This hopper is always filled with water to quench the ash and clinkers falling down from the furnace. Some arrangement is included to crush the clinkers and for conveying the crushed clinkers and bottom ash to a storage site. Boiler make-up water treatment plant and storage:Since there is continuous withdrawal of steam and continuous return of condensate to the boiler, losses due to blow-down and leakages have to be made up for so as to maintain the desired water level in the boiler steam drum. For this, continuous make-up water is added to the boiler water system. The impurities in the raw water input to the plant generally consist of calcium and magnesium salts which impart hardness to the water. Hardness in the make-up water to the boiler will form deposits on the tube water surfaces which will lead to overheating and failure of the tubes. Thus, the salts have to be removed from the water and that is done by a water demineralising treatment plant (DM). A DM plant generally consists of cation, anion and mixed bed exchangers. The final water from this process consists essentially of hydrogen ions and hydroxide ions which is the chemical composition of pure water. The DM water, being very pure, becomes highly corrosive once it absorbs oxygen from the atmosphere because of its very high affinity for oxygen absorption.The capacity of the DM plant is dictated by the type and quantity of salts in the raw water input. However, some storage is essential as the DM plant may be down for maintenance. For this purpose, a storage tank is installed from which DM water is continuously withdrawn for boiler make-up. The storage tank for DM water is made from materials not affected by corrosive water, such as PVC. The piping and valves are generally of stainless steel. Sometimes, a steam blanketing arrangement or stainless steel doughnut float is provided on top of the water in the tank to avoid contact with atmospheric air. DM water make-up is generally added at the steam space of the surface condenser (i.e., the vacuum side). This arrangement not only sprays the water but also DM water gets deaerated, with the dissolved gases being removed by the ejector of the condenser itself.Steam turbine-driven electric generator:The steam turbine-driven generators have auxiliary systems enabling them to work satisfactorily and safely. The steam turbine generator being rotating equipment generally has a heavy, large diameter shaft. The shaft therefore requires not only supports but also has to be kept in position while running. To minimise the frictional resistance to the rotation, the shaft has a number of bearings. The bearing shells, in which the shaft rotates, are lined with a low friction material like Babbitt metal. Oil lubrication is provided to further reduce the friction between shaft and bearing surface and to limit the heat generated. Barring gear:Barring gear (or "turning gear") is the mechanism provided to rotate the turbine generator shaft at a very low speed after unit stoppages. Once the unit is "tripped" (i.e., the steam inlet valve is closed), the turbine coasts down towards standstill. When it stops completely, there is a tendency for the turbine shaft to deflect or bend if allowed to remain in one position too long. This is because the heat inside the turbine casing tends to concentrate in the top half of the casing, making the top half portion of the shaft hotter than the bottom half. The shaft therefore could warp or bend by millionths of inches.This small shaft deflection, only detectable by eccentricity meters, would be enough to cause damaging vibrations to the entire steam turbine generator unit when it is restarted. The shaft is therefore automatically turned at low speed (about one revolution per minute) by the barring gear until it has cooled sufficiently to permit a complete stop.Condenser:

Fig: Diagram of a typical water-cooled surface condenser.The surface condenser is a shell and tube heat exchanger in which cooling water is circulated through the tubes. The exhaust steam from the low pressure turbine enters the shell where it is cooled and converted to condensate (water) by flowing over the tubes as shown in the adjacent diagram. Such condensers use steam ejectors or rotary motor-driven exhausters for continuous removal of air and gases from the steam side to maintain vacuum.For best efficiency, the temperature in the condenser must be kept as low as practical in order to achieve the lowest possible pressure in the condensing steam. Since the condenser temperature can almost always be kept significantly below 100oC where the vapor pressure of water is much less than atmospheric pressure, the condenser generally works under vacuum. Thus leaks of non-condensable air into the closed loop must be prevented. Plants operating in hot climates may have to reduce output if their source of condenser cooling water becomes warmer; unfortunately this usually coincides with periods of high electrical demand for air conditioning.The condenser generally uses either circulating cooling water from a cooling tower to reject waste heat to the atmosphere, or once-through water from a river, lake or ocean.

Feed water heater:

Fig: A Rankine cycle with a two-stage steam turbine and a single feedwater heater.In the case of a conventional steam-electric power plant utilizing a drum boiler, the surface condenser removes the latent heat of vaporization from the steam as it changes states from vapour to liquid. The heat content (btu) in the steam is referred to as Enthalpy. The condensate pump then pumps the condensate water through a feed water heater. The feed water heating equipment then raises the temperature of the water by utilizing extraction steam from various stages of the turbine.Preheating the feed water reduces the irreversibilities involved in steam generation and therefore improves the thermodynamic efficiency of the system. This reduces plant operating costs and also helps to avoid thermal shock to the boiler metal when the feed water is introduced back into the steam cycle. Superheater:As the steam is conditioned by the drying equipment inside the drum, it is piped from the upper drum area into an elaborate set up of tubing in different areas of the boiler. The areas known as superheater and reheater. The steam vapor picks up energy and its temperature is now superheated above the saturation temperature. The superheated steam is then piped through the main steam lines to the valves of the high pressure turbine.Deaerator:

Fig: Diagram of boiler feed water deaerator (with vertical, domed aeration section and horizontal water storage section.

A steam generating boiler requires that the boiler feed water should be devoid of air and other dissolved gases, particularly corrosive ones, in order to avoid corrosion of the metal.Generally, power stations use a deaerator to provide for the removal of air and other dissolved gases from the boiler feed water. A deaerator typically includes a vertical, domed deaeration section mounted on top of a horizontal cylindrical vessel which serves as the deaerated boiler feed water storage tank.There are many different designs for a deaerator and the designs will vary from one manufacturer to another. The adjacent diagram depicts a typical conventional trayed deaerator. If operated properly, most deaerator manufacturers will guarantee that oxygen in the deaerated water will not exceed 7 ppb by weight (0.005cm/L).Auxiliary systems:Oil system:An auxiliary oil system pump is used to supply oil at the start-up of the steam turbine generator. It supplies the hydraulic oil system required for steam turbine's main inlet steam stop valve, the governing control valves, the bearing and seal oil systems, the relevant hydraulic relays and other mechanisms.At a preset speed of the turbine during start-ups, a pump driven by the turbine main shaft takes over the functions of the auxiliary system.Generator heat dissipation:The electricity generator requires cooling to dissipate the heat that it generates. While small units may be cooled by air drawn through filters at the inlet, larger units generally require special cooling arrangements. Hydrogen gas cooling, in an oil-sealed casing, is used because it has the highest known heat transfer coefficient of any gas and for its low viscosity which reduces windage losses. This system requires special handling during start-up, with air in the chamber first displaced by carbon dioxide before filling with hydrogen. This ensures that the highly flammable hydrogen does not mix with oxygen in the air.The hydrogen pressure inside the casing is maintained slightly higher than atmospheric pressure to avoid outside air ingress. The hydrogen must be sealed against outward leakage where the shaft emerges from the casing. Mechanical seals around the shaft are installed with a very small annular gap to avoid rubbing between the shaft and the seals. Seal oil is used to prevent the hydrogen gas leakage to atmosphere.The generator also uses water cooling. Since the generator coils are at a potential of about 22 kV and water is conductive, an insulating barrier such as Teflon is used to interconnect the water line and the generator high voltage windings. Demineralized water of low conductivity is used.Generator high voltage system:The generator voltage ranges from 11 kV in smaller units to 22 kV in larger units. The generator high voltage leads are normally large aluminum channels because of their high current as compared to the cables used in smaller machines. They are enclosed in well-grounded aluminum bus ducts and are supported on suitable insulators. The generator high voltage channels are connected to step-up transformers for connecting to a high voltage electrical substation (of the order of 110 kV or 220 kV) for further transmission by the local power grid.The necessary protection and metering devices are included for the high voltage leads. Thus, the steam turbine generator and the transformer form one unit. In smaller units, generating at 11 kV, a breaker is provided to connect it to a common 11 kV bus system.Other systems: Monitoring and alarm system:Most of the power plant operational controls are automatic. However, at times, manual intervention may be required. Thus, the plant is provided with monitors and alarm systems that alert the plant operators when certain operating parameters are seriously deviating from their normal range.

Battery supplied emergency lighting and communication:A central battery system consisting of lead acid cell units is provided to supply emergency electric power, when needed, to essential items such as the power plant's control systems, communication systems, turbine lube oil pumps, and emergency lighting. This is essential for a safe, damage-free shutdown of the units in an emergency situation.Transport of coal fuel to site and to storage:Most thermal stations use coal as the main fuel. Raw coal is transported from coal mines to a power station site by trucks, barges, bulk cargo ships or railway cars. Generally, when shipped by railways, the coal cars are sent as a full train of cars. The coal received at site may be of different sizes. The railway cars are unloaded at site by rotary dumpers or side tilt dumpers to tip over onto conveyor belts below. The coal is generally conveyed to crushers which crush the coal to about inch (6mm) size. The crushed coal is then sent by belt conveyors to a storage pile. Normally, the crushed coal is compacted by bulldozers, as compacting of highly volatile coal avoids spontaneous ignition.The crushed coal is conveyed from the storage pile to silos or hoppers at the boilers by another belt conveyor system.Boiler or Steam generator:A boiler or steam generator is a device used to create steam by applying heat energy to water. Although the definitions are somewhat flexible, it can be said that older steam generators were commonly termed boilers and worked at low to medium pressure(1300 psi/0.06920.684 bar; 6.8952,068.427 kPa), but at pressures above this it is more usual to speak of a steam generator.A boiler or steam generator is used wherever a source of steam is required. The form and size depends on the application: mobile steam engines such as steam locomotives, portable engines and steam-powered road vehicles typically use a smaller boiler that forms an integral part of the vehicle; stationary steam engines, industrial installations and power stations will usually have a larger separate steam generating facility connected to the point-of-use by piping. A notable exception is the steam-powered fireless locomotive, where separately-generated steam is transferred to a receiver (tank) on the locomotive.The steam generator or boiler is an integral component of a steam engine when considered as a prime mover; however it needs be treated separately, as to some extent a variety of generator types can be combined with a variety of engine units. A boiler incorporates a firebox or furnace in order to burn the fuel and generate heat; the heat is initially transferred to water to make steam; this produces saturated steam at ebullition temperature saturated steam which can vary according to the pressure above the boiling water. The higher the furnace temperature, the faster the steam production. The saturated steam thus produced can then either be used immediately to produce power via a turbine and alternator, or else may be further superheated to a higher temperature; this notably reduces suspended water content making a given volume of steam produce more work and creates a greater temperature gradient in order to counter tendency to condensation due to pressure and heat drop resulting from work plus contact with the cooler walls of the steam passages and cylinders and wire-drawing effect from strangulation at the regulator. Any remaining heat in the combustion gases can then either be evacuated or made to pass through an economizer, the role of which is to warm the feed water before it reaches the boiler.Boiler types:1. According to the contents in the tube.1. Fire tube boiler.1. Water tube boiler.4. According to the position of the furnace.1. Internally fired boiler.1. Externally fired boiler.7. According to the axis of the shell.1. Vertical axis.1. Horizontal axis.

10. According to the number of tubes.1. Single tube boilers.1. Multitubular boilers.

13. According to the method of circulation of water and steam.1. Natural circulation boilers.1. Forced circular boilers.16. According to the use.1. Stationary boilers.1. Mobile boilers. Combustion:The source of heat for a boiler is combustion of any of several fuels, such as wood, coal, oil, or natural gas. Nuclear fission is also used as a heat source for generating steam. Heat recovery steam generators (HRSGs) use the heat rejected from other processes such as gas turbines.Solid fuel firing:In order to improve the burning characteristics of the fire, air needs to be supplied through the grate, or more importantly above the fire. Most boilers now depend on mechanical draft equipment rather than natural draught. This is because natural draught is subject to outside air conditions and temperature of flue gases leaving the furnace, as well as chimney height. All these factors make effective draught hard to attain and therefore make mechanical draught equipment much more economical. There are three types of mechanical draught:1. Induced draught: This is obtained one of three ways, the first being the "stack effect" of a heated chimney, in which the flue gas is less dense than the ambient air surrounding the boiler. The denser column of ambient air forces combustion air into and through the boiler. The second method is through use of a steam jet. The steam jet or ejector oriented in the direction of flue gas flow induces flue gases into the stack and allows for a greater flue gas velocity increasing the overall draught in the furnace. This method was common on steam driven locomotives which could not have tall chimneys. The third method is by simply using an induced draught fan (ID fan) which sucks flue gases out of the furnace and up the stack. Almost all induced draught furnaces have a negative pressure.1. Forced draught: draught is obtained by forcing air into the furnace by means of a fan (FD fan) and ductwork. Air is often passed through an air heater; which, as the name suggests, heats the air going into the furnace in order to increase the overall efficiency of the boiler. Dampers are used to control the quantity of air admitted to the furnace. Forced draught furnaces usually have a positive pressure.1. Balanced draught: Balanced draught is obtained through use of both induced and forced draft. This is more common with larger boilers where the flue gases have to travel a long distance through many boiler passes. The induced draft fan works in conjunction with the forced draft fan allowing the furnace pressure to be maintained slightly below atmospheric. Water treatment:Feed water for boilers needs to be as pure as possible with a minimum of suspended solids and dissolved impurities which cause corrosion, foaming and water carryover. Various chemical treatments have been employed over the years, the most successful being Porta treatment. This contains a foam modifier that acts as a filtering blanket on the surface of the water that considerably purifies steam quality.Boiler safety:Many steam engines possess boilers that are pressure vessels that contain a great deal of potential energy. Steam explosions can and have caused great loss of life in the past. While variations in standards may exist in different countries, stringent legal, testing, training and certification is applied to try to minimise or prevent such occurrences.Failure modes include:1. overpressurisation of the boiler1. insufficient water in the boiler causing overheating and vessel failure1. pressure vessel failure of the boiler due to inadequate construction or maintenance. Essential boiler fittings:1. Safety valve1. Pressure measurement1. Blowdown Valves1. Main steam Stop Valve1. Feed check valves1. Fusible Plug1. Water gauge1. Low-Water Alarm1. Low Water Fuel Cut-out1. Inspector's Test Pressure Gauge Attachment1. Name Plate1. Registration Plate1. Feedwater pump Steam accessories:1. Main steam stop valve1. Steam traps1. Main steam stop/Check valve used on multiple boiler installations.Combustion accessories:1. Fuel oil system.1. Gas system.1. Coal system.1. Automatic combustion systems.Steam turbine:Introduction:A steam turbine is a prime mover in which rotary motion is obtained by the gradual change of momentum of the steam. In steam turbine, the force exerted on the blades is due to the velocity of steam. This is due to the fact that the curved blades by changing the direction of steam receive a force or impulse. The dynamical pressure of steam rotates the vanes, buckets or blades directly. The turbine blades curved in such a way that the steam directed upon them enters without shock, though there is always some loss of energy by the friction upon the surface of blades. The basic principle of operation of a steam turbine is the generation of high velocity steam jet by the expansion of high pressure steam and then conversion of kinetic energy, so obtained into mechanical work on rotor blades.Classification of steam turbine:The steam turbines may be classified into the following types:1. According to the mode steam action1. Impulse turbine and 1. Reaction turbine.1. According to the direction of steam flow1. Axial flow turbine and1. Radial flow turbine.

1. According to the exhaust condition of steam1. Condensing turbine and1. Non-condensing turbine.1. According to the pressure of turbine1. High pressure turbine.1. Medium pressure turbine and1. Low pressure.1. According to the number of stages1. Single stage and1. Multi-stage turbine.

Impulse turbine:An impulse turbine, as the name indicates, is a turbine which runs by the impulse of steam jet. In this turbine, the steam is first made to flow through a nozzle, then the steam jet impinges on the turbine blades. The action of the jet of steam, impinging on the blades, is said to be an impulse and the rotation of the rotor is due to the impulsive forces of the steam jets. The steam impinges on the buckets with kinetic energy. De-Level turbine is the simplest type of impulse turbine.Reaction turbine:In a reaction turbine, the steam enters the wheel under pressure and flows over the blades. The steam while gliding, propels the blades and make them to move, as a matter fact, the turbine runner is rotated by the reactive forces of steam jets. The backward motion of the blades is similar to the recoil of gun. The steam glides over the moving vanes with pressure and kinetic energy. The steam flows first through guide mechanism and then through the moving blades. It may be noted that an absolute reaction turbine is rarely used in actual practice.

Impulse Turbine Stage:

As the steam passes through a nozzle from a high pressure pipeline to a lower pressure region, the velocity of the steam increases as the thermal energy is converted to kinetic energy. The velocity of the flow depends upon the difference in pressures between the high pressure and the low pressure regions. The weight rate of flow depends upon the velocity and the cross sectional area of the nozzle throat. This is illustrated in figure 1-2 below. The decrease in thermal energy as the steam passes through the nozzle equals the increase in kinetic energy, which is proportional to the square of the velocity.

KE = WV / 2gWhere KE = kinetic energy in ft lbW = weight of the steam in lbV = velocity of the steam in ft/secg = 32 ft /sec

If an obstruction, such as a turbine blade, is placed in the path of the flowing steam, the steam will exert a force, or an "impulse" on the blade in an amount equal to the weight rate of flow and the velocity of the steam. As the blade moves due to this force, work is performed on the blade in an amount equal to the force times the distance the blade moves due to this force. If the blade is one of a series of such blades connected to a rotor, the rotor will spin as the continuous flow of steam impinges on each successive blade. Such a device would constitute a simple form of a turbine called an impulse turbine.

In order to obtain the maximum amount of work from the steam, all of its kinetic energy must be converted to work. In other words, the steam must leave the blade with zero absolute velocity. Assuming a frictionless blade, the velocity of the steam relative to the blade must be the same entering as leaving, but reversed in direction. Also, the blade velocity must be one half of the entering steam velocity. (Refer to figure 1-3). There are actually two different forces acting on the blade. The first is the force of the steam jet striking the blade, as described. The second is the reactive force due to the change in direction of the steam flow between the entrance and exit from the blade. In actual turbines, it is impractical to utilize the full advantage of complete reversal of the steam. In a conventional impulse stage, the blades project radially from the wheel and the nozzles are placed so that the steam flow is at an angle to the plane of rotation.

The pressure and velocity changes taking place are shown in figure 1-4. The only pressure drop occurs in the nozzle. The pressure entering the blades is the same as that leaving the blades.

Force Vector Diagram for Impulse Stage:

Now consider the force vector diagram of following figure. A mass of steam entering a row of moving blades with a relative velocity V2 will have a momentum which will exert an impulse force F1 in the direction V2. The same mass of steam leaving the row of moving blades with a relative velocity V3 will exert a reactive force F2 on the blade opposite to the direction of V3. If V2 is equal to V3 in magnitude, assuming no friction between the steam and the blade, and if the blade entrance angle () is equal to the blade exit angle (), the components T1 and T2 will be equal and opposite, and the resultant will produce zero axial thrust on the turbine wheel. In actual design, angle () is made slightly larger than angle () to account for friction between steam and blade.

T1F1 R1R2 T2F2

Turbine Efficiency

To maximize turbine efficiency the steam is expanded, generating work, in a number of stages. These stages are characterized by how the energy is extracted from them and are known as either impulse or reaction turbines. Most steam turbines use a mixture of the reaction and impulse designs : each stage behaves as either one or the other, but the overall turbine uses both. Typically, higher pressure sections are impulse type and lower pressure stages are reaction type.

Figure 2.28 : Schematic Diagram Outlining the difference between an Impulse and a Reaction Turbine

Advantages of the Steam Turbine over Reciprocating Engine

(a) Thermal Efficiency of a Steam Turbine is higher than that of a Reciprocating Engine.

(b) The Steam Turbine develops power at a uniform rate and hence does not require Flywheel.

(c) No internal lubrication is required for Steam Turbine as there are no rubbing parts inside.

(d) No heavy foundation is required for Turbine because of the perfect balancing of the different parts.

(e) If the Steam Turbine is properly designed and constructed then it is the most durable Prime Mover.

(f) Much higher speed may be developed and a far greater range of speed is possible than in the case of Reciprocating Engine.

(g) There are some frictional losses in Reciprocating Engine as some arrangements are required for conversion of Reciprocating Motion into circular motion. But in Steam Turbine no friction losses are there.

(h) Steam Turbines are quite suitable for large Thermal Power Plant as they can be built in size from few Horse Powers to over 200000 HP in single unit.

Advantages of Steam Turbines

(a) High efficiency at full load.

(b) Mechanical simplicity and hence potential reliability.

(c) Conventional reciprocating steam locomotives give a varying torque through the cycle, resembling a sine characteristic. This makes wheel slip at starting much more likely.

(d) Conventional steam locomotives have substantial reciprocating masses such as connecting rods and valve gear. This creates fore-and-aft forces that cannot be completely balanced without unacceptably increasing the up-and-down forces on the track.

Disadvantages of Steam Turbines

(a) High efficiency is only obtained at full-load. Naval vessels very often had cruising turbines which could be run at full output while the main turbines were shut down.

(b) High efficiency is only obtained when the turbine exhausts into a near-vacuum, generated by a condenser. These are very large pieces of equipment to carry around.

(c) Turbines cannot run in reverse. Ships carried separate turbines solely for reversing, and locomotives had to do the same.

Model Steam TurbineIn this activity we will demonstrate how different energy sources can be used to spin a turbine. Remember that the sole purpose of spinning a turbine at a power plant is to rotate an electrical generator. The turbine in this activity is not strong enough to operate an electrical generator; however, we can still experience how the force of steam is used to make a turbine spin. We will also be constructing a device that produces steam in a manner similar to that used at a steam-driven power plant. We will recall from the theory that the actual steam production technology at a power plant is extremely sophisticated and produces steam at very high pressures. However, this activity works well enough to get the point across.

Steam turbines have come very much to the fore during recent years, especially for marine propulsion. In principle they are far simpler than cylinder engines, steam being merely directed at a suitable angle on to specially shaped vanes attached to a revolving drum and shaft. In the Parsons type of turbine the steam expands as it passes through successive rings of blades, the diameter of which rings, as well as the length and number of the blades, increases towards the exhaust end of the casing, so that the increasing velocity of the expanding steam may be taken full advantage of. The De Laval turbine includes but a single ring of vanes, against which the steam issues through nozzles so shaped as to allow the steam to expand somewhat and its molecules to be moving at enormous velocity before reaching the vanes. A De Laval wheel revolves at terrific speeds, the limit being tens of thousands of turns per minute for the smallest engines. The greatest efficiency is obtained, theoretically, when the vane velocity is half that of the steam, the latter, after passing round the curved inside surfaces of the vanes, being robbed of all its energy and speed. The turbines to be described work on the De-Laval principle, which has been selected as the easier for the beginner to follow.

A Very Simple Turbine:

We will begin with a very simple contrivance, shown in Figure. As a "power plant" it is confessedly useless, but the making of it affords amusement and instruction. For the boiler select a circular pipe, of diameter 4 inches, so as to give plenty of heating surface, and at least 6 inches deep, to ensure a good steam space and moderately dry steam, welded both side to make it water tight. A shallow boiler may "prime" badly, if reasonably full, and fling out a lot of water with the steam. Clean the metal round the joints, and drill three holes on the upper portion, one for water inlet, one for steam outlet and last one is for mounting pressure gauze, two half an inch in diameter and one inches in diameter. For the turbine blades take a piece of thin sheet GI; flatten it out. Then scratch a series of marks on the sheet and turn it to seven numbers of pieces of equal dimensions and make a slight bend from the two nearest edges to make them in the form of curved shape. And directly welded them in equal distance on the circumference of the turbine hub. Turbine hub and shaft are made up of stainless steel with two numbers of ball bearings and the whole assembly is supported on stand. A check valve is fitted on steam outlet to control the flows of steam through the pipelines and turbine. Pipeline is made up of mild steel, half an inch in diameter. Nozzle is fitted in the pipe line. A rigid frame made up of mild steel is made to support the whole establishment.

VANENOZZLEOUTLET PIPE LINE

P.G.

BOILER C.V. W.I. HUB

SHAFT

HEAT*

SUPPORT

C.V. = Check ValveW.I. = Water InletP.G. = Pressure GaugeFig: Steam Turbine Model

Model of steam turbine:Main components:Boiler: The boiler is made from a mild steel circular pipe, of diameter 4 inches, so as to give plenty of heating surface, 6mm thickness and at least 6 inches deep, to ensure a good steam space and moderately dry steam, welded both sides (upper and lower) to make it water tight. A shallow boiler may "prime" badly, if reasonably full, and fling out a lot of water with the steam. Clean the metal round the joints, and drill three holes on the upper portion, one for water inlet, one for steam outlet and last one is for mounting pressure gauze, two half an inch in diameter and one inches in diameter. One socket of inches in diameter is fixed by welding joint, two numbers of half inches socket are also welded with upper portion of boiler. Boiler is filled with water in such a manner that space for steam must be there above the water surface. Water is heated by electric heater of 1000W. Heater is fitted below the boiler and supply heat at a constant rate. Formation of Dry steam is occurred just above the water surface. Boiler must be sustain heat and steam pressure due to safety reason. For that precaution must be followed.Steam outlet pipe: Simple mild steel pipe of half inches in diameter is used as a steam outlet pipe. It is connected with check valve. Nozzle is fitted with this pipe to increase the velocity of the steam by decreasing the pressure. Steam outlet pipe must be sustain the steam pressure.Check valve: Check valve is fitted between the outlet pipe and the socket. It is used to check the flow of steam or in other words to control the flow of steam. It is made up of brass material due to safety reason. It is fitted just above the boiler shell and manually operated. We can control the flow of steam by revolving the wheel of valve.Pressure gauge: A Bourdon type pressure gauge is fitted on the boiler to know the steam pressure inside the boiler. Pressure gauge is one of the important mounting. Pressure gauge must be fitted on every boiler due to safety reason or in other words to minimum the risk or hazards. Pressure gauge shows reading in both C.G.S. system and F.P.S. system (in both kg/cm2 and psi).Nozzle: Nozzle is a device which is used to increase the velocity of fluid by decreasing pressure. Convergent-divergent nozzle is commonly used for this purpose. We construct this nozzle from a mild steel bar of half inches diameter. Drilled a 3mm hole through the center. On the outside end, enlarge this hole to 5mm to a depth of 200mm. Nozzle is fitted with outlet pipe so that the steam may expand and gain velocity as it approaches the blade.

Turbine wheel: Turbine wheel consists of shaft, hub and blades. Hub is made up of stainless steel and blades are welded on the circumference of the hub in equal distance. Shaft is fitted in the hub and rotates along its axis with the aid of steam force. Shaft is made up of mild steel material. To decrease the friction between the shaft and inner surface of the hub two ball bearings are fitted between them. Components of turbine wheel: 60. Shaft.60. Hub.60. Bearings.60. Blades or vanes.Blades: Seven numbers of vanes are fitted on the hub of the turbine. Construction procedures of making blades are to mark out a piece of GI sheet to form seven rectangles, 2.5 inches by 6 inches. Cut very carefully according to the marking line. In the edge of a piece of hard wood 1 inch thick file a notch 3/8 inch wide and 1/8 inch deep with a 1/2-inch circular file and procure a metal bar which fits the groove loosely. Each blade is laid in turn over the groove, and the bar is applied lengthwise on it and driven down with a mallet, to give the blade the curvature of the groove. When all the blades have been made and shaped, weld them on the circumference of the hub. True up the long edges of the blades with a file, and bring them off to a sharp edge, removing the metal from the convex side. Frame: A rigid frame is made from 1x1 inch MS angle to support the whole arrangement. Frame must have enough strength to give proper support.

Conclusion:In this activity we will demonstrate how different energy sources can be used to spin a turbine. Since this activity is a simple demonstration, the full scientific method outline is not called for here. The turbine in this activity is not strong enough to operate an electrical generator; however, we can still experience how the force of steam is used to make a turbine spin. We will also be constructing a device that produces steam in a manner similar to that used at a steam-driven power plant. We will recall from the theory that the actual steam production technology at a power plant is extremely sophisticated and produces steam at very high pressures. However, this activity works well enough to get the point across. Our main motto is to make a working model of steam turbine which may be in use for demonstration purpose.

Bibliography:1. Steam Turbine and Steam Power Plant --by R. Yadav1. A text book of Thermal Engineering --by R.S. Khurmi & J. K. Gupta

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