Steam turbineFrom Wikipedia, the free encyclopediaJump to: navigation, search

A rotor of a modern steam turbine, used in a power plant

A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion. Its modern manifestation was invented by Sir Charles Parsons in 1884.[1]

It has almost completely replaced the reciprocating piston steam engine primarily because of its greater thermal efficiency and higher power-to-weight ratio. Because the turbine generates rotary motion, it is particularly suited to be used to drive an electrical generator – about 80% of all electricity generation in the world is by use of steam turbines. The steam turbine is a form of heat engine that derives much of its improvement in thermodynamic efficiency through the use of multiple stages in the expansion of the steam, which results in a closer approach to the ideal reversible process.

Contents[hide]

1 History 2 Types

o 2.1 Steam Supply and Exhaust Conditions

o 2.2 Casing or Shaft Arrangements 3 Principle of Operation and Design

o 3.1 Turbine efficiency 3.1.1 Impulse turbines 3.1.2 Reaction turbines

o 3.2 Operation and Maintenance o 3.3 Speed regulation

4 Direct drive 5 Marine propulsion 6 Locomotives 7 See also 8 References 9 Further reading

10 External links

[edit] History

2000 KW Curtis steam turbine circa 1905.

The first device that may be classified as a reaction steam turbine was little more than a toy, the classic Aeolipile, described in the 1st century by Greek mathematician Hero of Alexandria in Roman Egypt.[2][3][4] More than a thousand years later, in 1543, Spanish naval officer Blasco de Garay used a primitive steam machine to move a ship in the port of Barcelona.[5] In 1551, Taqi al-Din in Ottoman Egypt described a steam turbine with the practical application of rotating a spit. Steam turbines were also described by the Italian Giovanni Branca (1629)[6] and John Wilkins in England (1648).[7] The devices described by al-Din and Wilkins are today known as steam jacks.

Parsons turbine from the Polish destroyer ORP Wicher .

The modern steam turbine was invented in 1884 by the Englishman Sir Charles Parsons, whose first model was connected to a dynamo that generated 7.5 kW (10 hp) of electricity.[8] The invention of Parson's steam turbine made cheap and plentiful electricity possible and revolutionised marine transport and naval warfare.[9] His patent was licensed and the turbine scaled-up shortly after by an American, George Westinghouse. The Parson's turbine also turned out to be easy to scale up. Parsons had the satisfaction of seeing his invention adopted for all major world power stations, and the size of generators had increased from his first 7.5 kW set up to units of 50,000 kW capacity. Within Parson's lifetime the generating capacity of a unit was scaled up by about 10,000 times,[10] and the total output from turbo-generators constructed by his firm C. A. Parsons and Company and by their licensees, for land purposes alone, had exceeded thirty million horse-power.[8]

A number of other variations of turbines have been developed that work effectively with steam. The de Laval turbine (invented by Gustaf de Laval) accelerated the steam to full speed before running it against a turbine blade. Hence the (impulse) turbine is simpler, less expensive and does not need to be pressure-proof. It can operate with any pressure of steam, but is considerably less efficient.

One of the fundators of the modern theory of steam and gas turbines was also Aurel Stodola, a Slovak physicist and engineer and professor at Swiss Polytechnical Institute (now ETH) in Zurich. His mature work was Die Dampfturbinen und ihre Aussichten als Wärmekraftmaschinen (English The Steam Turbine and its perspective as a Heat Energy Machine) which was published in Berlin in 1903. In 1922, in Berlin, was published another important book Dampf und Gas-Turbinen (English Steam and Gas Turbines).

The Brown-Curtis turbine which had been originally developed and patented by the U.S. company International Curtis Marine Turbine Company was developed in the 1900s in conjunction with John Brown & Company. It was used in John Brown's merchant ships and warships, including liners and Royal Navy warships.

[edit] TypesSteam turbines are made in a variety of sizes ranging from small <1 hp (<0.75 kW) units (rare) used as mechanical drives for pumps, compressors and other shaft driven equipment, to 2,000,000 hp (1,500,000 kW) turbines used to generate electricity. There are several classifications for modern steam turbines.

[edit] Steam Supply and Exhaust Conditions

These types include condensing, noncondensing, reheat, extraction and induction.

Noncondensing or backpressure turbines are most widely used for process steam applications. The exhaust pressure is controlled by a regulating valve to suit the needs of the process steam pressure. These are commonly found at refineries, district heating units, pulp and paper plants, and desalination facilities where large amounts of low pressure process steam are available.

Condensing turbines are most commonly found in electrical power plants. These turbines exhaust steam in a partially condensed state, typically of a quality near 90%, at a pressure well below atmospheric to a condenser.

Reheat turbines are also used almost exclusively in electrical power plants. In a reheat turbine, steam flow exits from a high pressure section of the turbine and is returned to the boiler where additional superheat is added. The steam then goes back into an intermediate pressure section of the turbine and continues its expansion.

Extracting type turbines are common in all applications. In an extracting type turbine, steam is released from various stages of the turbine, and used for industrial process needs or sent to boiler feedwater heaters to improve overall cycle efficiency. Extraction flows may be controlled with a valve, or left uncontrolled.

Induction turbines introduce low pressure steam at an intermediate stage to produce additional power.

[edit] Casing or Shaft Arrangements

These arrangements include single casing, tandem compound and cross compound turbines. Single casing units are the most basic style where a single casing and shaft are coupled to a generator. Tandem compound are used where two or more casings are directly coupled together to drive a single generator. A cross compound turbine arrangement features two or more shafts not in line driving two or more generators that often operate at different speeds. A cross compound turbine is typically used for many large applications.

[edit] Principle of Operation and DesignAn ideal steam turbine is considered to be an isentropic process, or constant entropy process, in which the entropy of the steam entering the turbine is equal to the entropy of the steam leaving the turbine. No steam turbine is truly “isentropic”, however, with typical isentropic efficiencies ranging from 20%-90% based on the application of the turbine. The interior of a turbine comprises several sets of blades, or “buckets” as they are more commonly referred to. One set of stationary blades is connected to the casing and one set of rotating blades is connected to the shaft. The sets intermesh with certain

minimum clearances, with the size and configuration of sets varying to efficiently exploit the expansion of steam at each stage.

[edit] Turbine efficiency

Schematic diagram outlining the difference between an impulse and a reaction turbine

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.

[edit] Impulse turbines

An impulse turbine has fixed nozzles that orient the steam flow into high speed jets. These jets contain significant kinetic energy, which the rotor blades, shaped like buckets, convert into shaft rotation as the steam jet changes direction. A pressure drop occurs across only the stationary blades, with a net increase in steam velocity across the stage.

As the steam flows through the nozzle its pressure falls from inlet pressure to the exit pressure (atmospheric pressure, or more usually, the condenser vacuum). Due to this higher ratio of expansion of steam in the nozzle the steam leaves the nozzle with a very high velocity. The steam leaving the moving blades is a large portion of the maximum velocity of the steam when leaving the nozzle. The loss of energy due to this higher exit velocity is commonly called the "carry over velocity" or "leaving loss".

[edit] Reaction turbines

In the reaction turbine, the rotor blades themselves are arranged to form convergent nozzles. This type of turbine makes use of the reaction force produced as the steam accelerates through the nozzles formed by the rotor. Steam is directed onto the rotor by the fixed vanes of the stator. It leaves the stator as a jet that fills the entire circumference of the rotor. The steam then changes direction and increases its speed relative to the speed of the blades. A pressure drop occurs across both the stator and the rotor, with steam accelerating through the stator and decelerating through the rotor, with no net change in steam velocity across the stage but with a decrease in both pressure and temperature, reflecting the work performed in the driving of the rotor.

[edit] Operation and Maintenance

When warming up a steam turbine for use, the main steam stop valves (after the boiler) have a bypass line to allow superheated steam to slowly bypass the valve and proceed to heat up the lines in the system along with the steam turbine. Also, a turning gear is engaged when there is no steam to the turbine to slowly rotate the turbine to ensure even heating to prevent uneven expansion. After first rotating the turbine by the turning gear, allowing time for the rotor to assume a straight plane (no bowing), then the turning gear is disengaged and steam is admitted to the turbine, first to the astern blades then to the ahead blades slowly rotating the turbine at 10 to 15 RPM to slowly warm the turbine.

Problems with turbines are now rare and maintenance requirements are relatively small. Any imbalance of the rotor can lead to vibration, which in extreme cases can lead to a blade letting go and punching straight through the casing. It is, however, essential that the turbine be turned with dry steam - that is, superheated steam with a minimal liquid water content. If water gets into the steam and is blasted onto the blades (moisture carryover) rapid impingement and erosion of the blades can occur, possibly leading to imbalance and catastrophic failure. Also, water entering the blades will likely result in the destruction of the thrust bearing for the turbine shaft. To prevent this, along with controls and baffles in the boilers to ensure high quality steam, condensate drains are installed in the steam piping leading to the turbine.

[edit] Speed regulation

The control of a turbine with a governor is essential, as turbines need to be run up slowly, to prevent damage while some applications (such as the generation of alternating current electricity) require precise speed control.[11] Uncontrolled acceleration of the turbine rotor can lead to an overspeed trip, which causes the nozzle valves that control the flow of steam to the turbine to close. If this fails then the turbine may continue accelerating until it breaks apart, often spectacularly. Turbines are expensive to make, requiring precision manufacture and special quality materials. During normal operation in synchronization with the electricity network, power plants are governed with a five percent droop speed control. This means the full load speed is 100% and the no-load speed is 105%. This is required for the stable operation of the network without hunting and drop-outs of power plants. Normally the changes in speed are minor. Adjustments in power output are made by slowly raising the droop curve by increasing the spring pressure on a centrifugal

governor. Generally this is a basic system requirement for all power plants because the older and newer plants have to be compatible in response to the instantaneous changes in frequency without depending on outside communication.[12]

[edit] Direct drive

A small industrial steam turbine (right) directly linked to a generator (left). This turbine generator set of 1910 produced 250 kW of electrical power.

Electrical power stations use large steam turbines driving electric generators to produce most (about 80%) of the world's electricity. Most of these centralised stations are of two types, fossil fuel power plants and nuclear power plants, but some countries are using concentrating solar power (CSP) to create the steam. Steam turbines can also be used directly to drive large centrifugal pumps, such as feedwater pumps at a thermal power plant.

The turbines used for electric power generation are most often directly coupled to their generators. As the generators must rotate at constant synchronous speeds according to the frequency of the electric power system, the most common speeds are 3000 RPM for 50 Hz systems, and 3600 RPM for 60 Hz systems. In installations with high steam output, as may be found in nuclear power stations, the generator sets may be arranged to operate at half these speeds, but with four-pole generators.[13]

[edit] Marine propulsion

The Turbinia - the first steam turbine-powered ship

Another use of steam turbines is in ships; their small size, low maintenance, light weight, and low vibration are compelling advantages. A steam turbine is only efficient when operating in the thousands of RPM, while the most effective propeller designs are for

speeds less than 100 RPM. Therefore precise (thus expensive) reduction gears are generally used, although several ships, such as Turbinia and RMS Titanic, had direct drive from the steam turbine to the propeller shafts. The purchase cost is offset by much lower fuel and maintenance requirements and the small size of a turbine when compared to a reciprocating engine having an equivalent power. However, diesel engines are capable of higher efficiencies: steam turbine cycle efficiencies have yet to break 50%, yet diesel engines routinely exceed 50%, especially in marine applications.[14][15][16]

Nuclear-powered ships and submarines use a nuclear reactor to create steam and either use a steam turbine directly for main propulsion, with generators providing auxiliary power, or else employ turbo-electric propulsion, where the steam drives a turbine-generator set with propulsion provided by electric motors. Nuclear power is often chosen where diesel power would be impractical (as in submarine applications) or the logistics of refuelling pose significant problems (for example, icebreakers). It has been estimated that the reactor fuel for the Royal Navy's Vanguard class submarine is sufficient to last 40 circumnavigations of the globe – potentially sufficient for the vessel's entire service life.

[edit] LocomotivesMain article: Steam turbine locomotive

A steam turbine locomotive engine is a steam locomotive driven by a steam turbine.

The main advantages of a steam turbine locomotive are better rotational balance and reduced hammer blow on the track. However, a disadvantage is less flexible power output power so that turbine locomotives were best suited for long-haul operations at a constant output power.[17]

The first steam turbine rail locomotive was built in 1908 for the Officine Meccaniche Miani Silvestri Grodona Comi, Milan, Italy. In 1924 Krupp built the steam turbine locomotive T18 001, operational in 1929, for the German National Railroad Company.

[edit] See also Balancing machine Droop speed control Steam engine Tesla turbine

[edit] References1. ̂ Encyclopædia Britannica (1931-02-11). "Sir Charles Algernon Parsons (British

engineer) - Britannica Online Encyclopedia". Britannica.com. http://www.britannica.com/EBchecked/topic/444719/Sir-Charles-Algernon-Parsons. Retrieved 2010-09-12.

2. ̂ turbine. Encyclopedia Britannica Online3. ̂ A new look at Heron's 'steam engine'" (1992-06-25). Archive for History of

Exact Sciences 44 (2): 107-124.4. ̂ O'Connor, J. J.; E. E. Roberston (1999). Heron of Alexandria. MacTutor5. ̂ Blasco de Garay's 1543 Steamship, Rochester History Resources, University of

Rochester, 1996, http://www.history.rochester.edu/steam/garay/. Retrieved 2008-04-24.

6. ̂ "Power plant engineering". P. K. Nag (2002). Tata McGraw-Hill. p.432. ISBN 0070435995

7. ̂ Taqi al-Din and the First Steam Turbine, 1551 A.D., web page, accessed on line October 23, 2009; this web page refers to Ahmad Y Hassan (1976), Taqi al-Din and Arabic Mechanical Engineering, pp. 34-5, Institute for the History of Arabic Science, University of Aleppo.

8. ^ a b [1][dead link]

9. ̂ [2][dead link]

10. ̂ Parsons, Sir Charles A.. "The Steam Turbine". http://www.history.rochester.edu/steam/parsons/part1.html.

11. ̂ Whitaker, Jerry C. (2006). AC power systems handbook. Boca Raton, FL: Taylor and Francis. p. 35. ISBN 9780849340345.

12. ̂ Speed Droop and Power Generation. Application Note 01302. 2. Woodward. Speed

13. ̂ Leyzerovich, Alexander (2005). Wet-steam Turbines for Nuclear Power Plants. Tulsa OK: PennWell Books. p. 111. ISBN 1593700326.

14. ̂ "MCC CFXUpdate23 LO A/W.qxd" (PDF). http://ansys.com/assets/testimonials/siemens.pdf. Retrieved 2010-09-12.

15. ̂ "New Benchmarks for Steam Turbine Efficiency - Power Engineering". Pepei.pennnet.com. http://pepei.pennnet.com/display_article/152601/6/ARTCL/none/none/1/New-Benchmarks-for-Steam-Turbine-Efficiency/. Retrieved 2010-09-12.

16. ̂ https://www.mhi.co.jp/technology/review/pdf/e451/e451021.pdf17. ̂ Streeter, Tony: 'Testing the Limit' (Steam Railway Magazine: 2007, 336), pp. 85

[edit] Further reading Cotton, K.C. (1998). Evaluating and Improving Steam Turbine Performance. Traupel, W. (1977) (in German). Thermische Turbomaschinen. Thurston, R. H. (1878). A History of the Growth of the Steam Engine. D. Appleton

and Co..

[edit] External links

Wikimedia Commons has media related to: Steam turbines

Steam Turbines: A Book of Instruction for the Adjustment and Operation of the Principal Types of this Class of Prime Movers by Hubert E. Collins

Tutorial: "Superheated Steam" Flow Phenomenon in Steam Turbine Disk-Stator Cavities Channeled by Balance

Holes Extreme Steam- Unusual Variations on The Steam Locomotive Interactive Simulation of 350MW Steam Turbine with Boiler [3] , The Pride of World Engineering

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Thermal power stationFrom Wikipedia, the free encyclopediaJump to: navigation, search

It has been suggested that Fossil fuel power station be merged into this article or section. (Discuss)

Republika Power Plant, a thermal power station in Pernik, Bulgaria

Mohave Generating Station, a 1,580 MW thermal power station near Laughlin, Nevada fuelled by coal

Geothermal power station in Iceland

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 either drives an electrical generator or does some other work, like ship propulsion. After it passes through the turbine, the steam is condensed in a condenser and recycled to where it was heated; 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.

Contents[hide]

1 Introductory overview 2 History 3 Efficiency 4 Cost of electricity 5 Diagram of a typical coal-fired thermal power station 6 Steam generator

o 6.1 Boiler furnace and steam drum o 6.2 Superheater o 6.3 Reheater o 6.4 Fuel preparation system o 6.5 Air path o 6.6 Auxiliary systems

6.6.1 Fly ash collection 6.6.2 Bottom ash collection and disposal 6.6.3 Boiler make-up water treatment plant and storage

7 Steam turbine-driven electric generator o 7.1 Barring gear o 7.2 Condenser o 7.3 Feedwater heater o 7.4 Superheater o 7.5 Deaerator o 7.6 Auxiliary systems

7.6.1 Oil system 7.6.2 Generator heat dissipation 7.6.3 Generator high voltage system

8 Other systems o 8.1 Monitoring and alarm system o 8.2 Battery supplied emergency lighting and communication

9 Transport of coal fuel to site and to storage 10 See also 11 References

12 External links

[edit] Introductory overviewAlmost 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, oil, or natural gas are often referred to collectively as 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 cogeneration are sometimes referred to as conventional power plants.

Commercial electric utility power stations are most usually constructed on a very large scale and designed for continuous operation. Electric power plants typically use three-phase or individual-phase electrical generators to produce alternating current (AC) electric power at a frequency of 50 Hz or 60 Hz (hertz, which is an AC sine wave per second) depending on its location in the world. Other large companies or institutions may have their own usually smaller power plants to supply heating or electricity to their

facilities, especially if heat or steam is created anyway for other purposes. Shipboard steam-driven power plants have been used in various large ships in the past, but these days are used most often in large naval ships. Such shipboard power plants are general lower power capacity than full-size electric company plants, but otherwise have many similarities except that typically the main steam turbines mechanically turn the propulsion propellers, either through reduction gears or directly by the same shaft. The steam power plants in such ships also provide steam to separate smaller turbines driving electric generators to supply electricity in the ship. Shipboard steam power plants can be either conventional or nuclear; the shipboard nuclear plants are mostly in the navy. 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.

In some industrial, large institutional facilities, or other populated areas, there are combined heat and power (CHP) plants , often called cogeneration plants, which produce both power and heat for facility or district heating or industrial applications. AC electrical power can be stepped up to very high voltages for long distance transmission with minimal loss of power. 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 or facility, such as steam distribution for a ship or industrial facility or hot water distribution in a local municipality.

[edit] HistoryReciprocating 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.

[edit] EfficiencyPower is energy per unit time. The power output or capacity of an electric plant can be expressed in units of megawatts electric (MWe). The electric efficiency of a conventional thermal power station, considered as saleable energy (in MWe) produced at the plant busbars as a percent of the heating value of the fuel consumed, is typically 33% to 48% efficient. This efficiency is limited as all heat engines are governed by the laws of thermodynamics (See: Carnot cycle). The rest of the energy must leave the plant in the form of heat. This waste heat can go through a condenser and be disposed of with cooling water or in cooling towers. If the waste heat is instead utilized for 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.

[edit] Cost of electricitySee: Relative cost of electricity generated by different sources

[edit] Diagram of a typical coal-fired thermal power station

Typical diagram of a coal-fired thermal power station 1. Cooling tower 10. Steam Control valve 19. Superheater

2. Cooling water pump 11. High pressure steam turbine 20. Forced draught (draft) fan

3. transmission line (3-phase) 12. Deaerator 21. Reheater4. Step-up transformer (3- 13. Feedwater heater 22. Combustion air intake

phase)5. Electrical generator (3-phase) 14. Coal conveyor 23. Economiser

6. Low pressure steam turbine 15. Coal hopper 24. Air preheater7. Condensate pump 16. Coal pulverizer 25. Precipitator

8. Surface condenser 17. Boiler steam drum 26. Induced draught (draft) fan

9. Intermediate pressure steam turbine 18. Bottom ash hopper 27. Flue gas stack

[edit] Steam generatorIn fossil-fueled power plants, steam generator refers to a furnace that burns the fossil fuel to boil water to generate steam. In the nuclear plant field, steam generator refers to a specific type of large heat exchanger used in a pressurized water reactor (PWR) to thermally connect the primary (reactor plant) and secondary (steam plant) systems, which of course is used to generate steam. In a nuclear reactor called a boiling water reactor (BWR), water is boiled to generate steam directly in the reactor itself and there are no units called steam generators. In some industrial settings, there can also be steam-producing heat exchangers called heat recovery steam generators (HRSG) which utilize heat from some industrial process. 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. A fossil fuel steam generator includes an economizer, a steam drum, and the furnace with its steam generating tubes and 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.[1][2][3]

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 generating steam from the reactor (BWR) or else using an intermediate heat exchanger (PWR).

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.

[edit] 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 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.

[edit] Superheater

Fossil fuel power plants can have a superheater and/or reheater section in the steam generating furnace. Nuclear-powered steam plants do not have such sections but produce steam at essentially saturated conditions. In a fossil fuel plant, after the steam is conditioned by the drying equipment inside the steam drum, it is piped from the upper drum area into tubes inside an area of the furnace known as the superheater, which has an elaborate set up of tubing where the steam vapor 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.

[edit] Reheater

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 rerouted to go inside the reheater tubes to pickup more energy to go drive intermediate or lower pressure turbines.

[edit] 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 100 °C 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.

[edit] 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

[edit] Auxiliary systems

[edit] 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.

[edit] Bottom ash collection and disposal

At the bottom of the furnace, there is a hopper for collection of bottom ash. 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.

[edit] 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 blowdown and leakages have to be made up to maintain a desired water level in the boiler steam drum. For this, continuous make-up water is added to the boiler water system. 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. Any ions in the final water from this process consist essentially of hydrogen ions and hydroxide ions, which recombine to form pure water. Very pure DM water becomes highly corrosive once it absorbs oxygen from the atmosphere because of its very high affinity for oxygen.

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 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 an air ejector attached to the condenser.

[edit] Steam turbine-driven electric generator

Rotor of a modern steam turbine, used in a power stationMain article: Turbo 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.

[edit] 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 percent rated speed) by the barring gear until it has cooled sufficiently to permit a complete stop.

[edit] Condenser

Main article: Surface condenser

Diagram of a typical water-cooled surface condenser.[2][3][4][5]

The surface condenser is a shell and tube heat exchanger in which cooling water is circulated through the tubes.[2][4][5][6] 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 100 °C where the vapor pressure of water is much less than atmospheric pressure, the condenser generally works under vacuum. Thus leaks of non-condensible 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.

[edit] Feedwater heater

Main article: Feedwater heater

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 (joules or Btu) in the steam is referred to as enthalpy. The condensate pump then pumps the condensate water through a Air ejector

condenser and Gland steam exhauster condenser. From there the condensate goes to the deareator where the condenstae system ends and the Feedwater system begins.[2][3]

Preheating the feedwater reduces the irreversibilities involved in steam generation and therefore improves the thermodynamic efficiency of the system.[7] This reduces plant operating costs and also helps to avoid thermal shock to the boiler metal when the feedwater is introduced back into the steam cycle.

[edit] 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.

[edit] Deaerator

Main article: Deaerator

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 feedwater. A deaerator typically includes a vertical,

domed deaeration section mounted on top of a horizontal cylindrical vessel which serves as the deaerated boiler feedwater storage tank.[2][3][8]

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.[8][9] If operated properly, most deaerator manufacturers will guarantee that oxygen in the deaerated water will not exceed 7 ppb by weight (0.005 cm³/L).[8][10]

[edit] Auxiliary systems

[edit] 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.

[edit] 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.

[edit] 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 115 kV to 520 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.

[edit] Other systems

[edit] 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.

[edit] 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.

[edit] Transport of coal fuel to site and to storageMain article: Fossil fuel power plant

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 ¾ inch (6 mm) 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.

[edit] See also

Boiler Combined heat and power Cooling tower system Flue gas stacks Fossil fuel power plant

Geothermal power

Energy portal

Nuclear power Power station Relative cost of electricity generated

by different sources Levelised energy cost Water-tube boiler

Integrated Gasification Combined Cycle

[edit] References1. ̂ British Electricity International (1991). Modern Power Station Practice:

incorporating modern power system practice (3rd Edition (12 volume set) ed.). Pergamon. ISBN 0-08-040510-X.

2. ^ a b c d e Babcock & Wilcox Co. (2005). Steam: Its Generation and Use (41st edition ed.). ISBN 0-9634570-0-4.

3. ^ a b c d Thomas C. Elliott, Kao Chen, Robert Swanekamp (coauthors) (1997). Standard Handbook of Powerplant Engineering (2nd edition ed.). McGraw-Hill Professional. ISBN 0-07-019435-1.

4. ^ a b Air Pollution Control Orientation Course from website of the Air Pollution Training Institute

5. ^ a b Energy savings in steam systems Figure 3a, Layout of surface condenser (scroll to page 11 of 34 pdf pages)

6. ̂ Robert Thurston Kent (Editor in Chief) (1936). Kents’ Mechanical Engineers’ Handbook (Eleventh edition (Two volumes) ed.). John Wiley & Sons (Wiley Engineering Handbook Series).

7. ̂ Fundamentals of Steam Power by Kenneth Weston, University of Tulsa8. ^ a b c Pressurized deaerators9. ̂ Tray deaerating heaters10. ̂ Deaerator Presentation

[edit] External links Conventional coal-fired power plant Power plant diagram Power Plant Reference Books Steam jet ejectors Steam jet ejector performance guidelines First and second video lectures by S. Banerjee on "Thermal Power Plants"

Retrieved from "http://en.wikipedia.org/wiki/Thermal_power_station"Categories: Power station technology | Energy conversion | Chemical engineering | Steam power

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