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Best steam power plant

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Page 1: Best steam power plant

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Course Name

Power Plant Engineering

Course number

MEC 403

Submitted To

Md. Abul Basher

Submitted By

  Md. Enamul Hoque Khan

Id#121070182

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Steam power plant

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Objective

• To know the basic knowledge about steam power plant• To know the working principle of steam power plant• To know the equipment used in steam power plant• To know principle of steam power plant design• To know cost about steam power plant and their layout• To know environment control for steam power plant• To know advantages and disadvantages of steam power

plant

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Introduction

The steam engine is a heat engine converting heat energy to work. The conversion is performed by a cycle of processes. Heat created externally by pump is transferred to water in a steam generating unit or boiler. The steam carries energy to the expander (engine), part of the heat energy is converted to mechanical energy, which is work. The steam leaves the expander and is condensed back to water at condenser, rejecting some heat energy. This is known as an external combustion engine. These processes must conform to the First Law of Thermodynamics expressed in the General Energy Equation. This cycle is called the Rankine cycle and is the accepted standard of comparison for steam plants today

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Essentials of steam power plant equipments

A steam power plant must have following equipment :• A furnace to burn the fuel.• Steam generator or boiler containing water. Heat

generated in the furnace is utilized to convert water into steam.

• Main power unit such as an engine or turbine to use the heat energy of steam and perform work.

• Piping system to convey steam and water.

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In addition to the above equipment the plant requires various auxiliaries and accessories depending upon the availability of water, fuel and the service for which the plant is intended.

The flow sheet of a thermal power plant consists of the following four main circuits :

• Feed water and steam flow circuit. • Coal and ash circuit. • Air and gas circuit. • Cooling water circuit.

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A steam power plant using steam as working substance works basically on Rankine cycle. Steam is generated in a boiler, expanded in the prime mover and condensed in the condenser and fed into the boiler again.

The different types of systems and components used in steam power plant are as follows :

• High pressure boiler

• Prime mover

• Condensers and cooling towers

• Coal handling system

• Ash and dust handling system

• Draught system

• Feed water purification plant

• Pumping system

• Air preheater, economizer, super heater, feed heaters.

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• Figure shows a schematic arrangement of equipment of a steam power station. Coal received in coal storage yard of power station is transferred in the furnace by coal handling unit. Heat produced due to burning of coal is utilized in converting water contained in boiler drum into steam at suitable pressure and temperature. The steam generated is passed through the superheater. Superheated steam then flows through the turbine. After doing work in the turbine the pressure of steam is reduced. Steam leaving the turbine passes through the condenser which is maintained the low pressure of steam at the exhaust of turbine. Steam pressure in the condenser depends upon flow rate and temperature of cooling water and on effectiveness of air removal equipment. Water circulating through the condenser may be taken from the various sources such as river, lake or sea. If sufficient quantity of water is not available the hot water coming out of the condenser may be cooled in cooling towers and circulated again through the condenser. Bled steam taken from the turbine at suitable extraction points is sent to low pressure and high pressure water heaters.

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Steam condensing system consists of the following:

• Condenser

• Cooling water

• Cooling tower

• Hot well

• Condenser cooling water pump

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• Air extraction pump

• Boiler feed pump

• Make up water pump

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Application

Steam is used in a wide range of industries. Common applications for steam are, for example, steam heated processes in plants and factories and steam driven turbines in electric power plants, but the uses of steam in industry extend far beyond this.

Here are some typical applications for steam in industry:

• Heating/Sterilization

• Propulsion/Drive

• Motive

• Atomization

• Cleaning

• Miniaturization

• Humidification

For higher production of electricity, Steam power plant is better than diesel power plant

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Principle of steam power plant

Working fluid cycle steam power plant is a closed cycle, which uses the same fluid repeatedly. First, the water is filled into the boiler to fill the entire surface area of heat transfer. In the boiler water is heated by the hot gases of combustion fuel with air so that turned into vapor phase.

• Steam produced by boiler with pressure and temperature are directed to do work on the turbine to produce mechanical power in the form of rotation.

 • The former steam out of the turbine, and then flowed into the

condenser to be cooled with cooling water that turned to water. Condensate water is then used again as boiler feed water. Thus the cycle goes on and repeats.

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Rotation of turbine is used to turn a generator that is coupled directly to the turbine. So when the turbine rotates, the generator output terminals generate electricity. Although working fluid cycle is a closed cycle, but the amount of water in the cycle would decrease. The reduction is due to the leakage of water either intentional or unintentional. steam power plant working on rankine cycle

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There are four processes in the Rankine cycle. These states are identified by numbers (in brown) in the above Ts diagram.

• Process 1-2: The working fluid is pumped from low to high pressure. As the fluid is a liquid at this stage, the pump requires little input energy.

• Process 2-3: The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a dry saturated vapour. The input energy required can be easily calculated using mollier diagram or h-s chart or enthalpy-entropy chart also known as steam tables.

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• Process 3-4: The dry saturated vapour expands through a turbine, generating power. This decreases the temperature and pressure of the vapour, and some condensation may occur. The output in this process can be easily calculated using the Enthalpy-entropy chart or the steam tables.

• Process 4-1: The wet vapour then enters a condenser where it is condensed at a constant pressure to become a saturated liquid

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• In an ideal Rankine cycle the pump and turbine would be isentropic, i.e., the pump and turbine would generate no entropy and hence maximize the net work output. Processes 1-2 and 3-4 would be represented by vertical lines on the T-S diagram and more closely resemble that of the Carnot cycle. The Rankine cycle shown here prevents the vapor ending up in the superheat region after the expansion in the turbine, [1] which reduces the energy removed by the condensers.

• The actual vapor power cycle differs from the ideal Rankine cycle because of irreversibilities in the inherent components caused by fluid friction and heat loss to the surroundings; fluid friction causes pressure drops in the boiler, the condenser, and the piping between the components, and as a result the steam leaves the boiler at a lower pressure; heat loss reduces the net work output, thus heat addition to the steam in the boiler is required to maintain the same level of net work output.

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Essential equipments

1. Cooling tower.

2. Cooling water pump.

3. Transmission line (3-phase).

4. Unit transformer (3-phase).

5. Electric generator (3-phase).

6. Low pressure turbine.

7. Condensate extraction pump.

8. Condenser

9. Intermediate pressure turbine.

10. Steam governor valve.

11. High pressure turbine.

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12. Deaerator

13. Feed heater.

14. Coal conveyor.

15. Coal hopper.

16. Pulverised fuel mill.

17. Boiler drum.

18. Ash hopper.

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19. Superheater.

20. Forced draught fan.

21. Reheater.

22. Air intake.

23. Economiser.

24. Air preheater.

25. Precipitator.

26. Induced draught fan.

27. Chimney stack.

28. Feed pump.

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Deaerator

A deaerator is a device that is widely used for the removal of oxygen and other dissolved gases from the feed water to steam-generating boilers. In particular, dissolved oxygen in boiler feed waters will cause serious corrosion damage in steam systems by attaching to the walls of metal piping and other metallic equipment and forming oxides (rust). Dissolved carbon dioxide combines with water to form carbonic acid that causes further corrosion. Most deaerators are designed to remove oxygen down to levels of 7 ppb by weight (0.005 cm³/L) or less as well as essentially eliminating carbon dioxide.

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• There are two basic types of deaerators, the tray-type and the spray-type:

• The tray-type (also called the cascade-type) includes a vertical domed deaeration section mounted on top of a horizontal cylindrical vessel which serves as the deaerated boiler feed water storage tank.

• The spray-type consists only of a horizontal (or vertical) cylindrical vessel which serves as both the deaeration section and the boiler feedwater storage tank.

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Economizer

Economizers  are mechanical devices intended to reduce energy consumption, or to perform 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

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Precipitator

A precipitator is a filtration device that removes fine particles, like dust and smoke, from a flowing gas using the force of an induced electrostatic charge minimally impeding the flow of gases through the unit.

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Chimney stack

• A chimney is a structure which provides ventilation for hot flue gases or smoke from a boiler, stove, furnace or fireplace to the outside atmosphere. Chimneys are typically vertical, or as near as possible to vertical, to ensure that the gases flow smoothly, drawing air into the combustion in what is known as the stack, or chimney, effect. The space inside a chimney is called a flue. Chimneys may be found in buildings, steam locomotives and ships. In the United States, the term smokestack is also used when referring to locomotive chimneys or ship chimneys, and the term funnel can also be used.

• The world's tallest chimney, of GRES-2 inEkibastuz, Kazakhstan(419.7 metres)

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Working procedure

 Coal is conveyed (14) from an external stack and ground to a very fine powder by large metal spheres in the pulverised fuel mill (16). There it is mixed with preheated air (24) driven by the forced draught fan (20). The hot air-fuel mixture is forced at high pressure into the boiler where it rapidly ignites. Water of a high purity flows vertically up the tube-lined walls of the boiler, where it turns into steam, and is passed to the boiler drum, where steam is separated from any remaining water. The steam passes through a manifold in the roof of the drum into the pendant superheater (19) where its temperature and pressure increase rapidly to around 200 bar and 570°C, sufficient to make the tube walls glow a dull red.

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.The steam is piped to the high pressure turbine (11), the first of a three-stage turbine process. A steam governor valve (10) allows for both manual control of the turbine and automatic set-point following. The steam is exhausted from the high pressure turbine, and reduced in both pressure and temperature, is returned to the boiler reheater (21). The reheated steam is then passed to the intermediate pressure turbine (9), and from there passed directly to the low pressure turbine set (6). The exiting steam, now a little above its boiling point, is brought into thermal contact with cold water (pumped in from the cooling tower) in the condensor (8), where it condenses rapidly back into water, creating near vacuum-like conditions inside the condensor chest.

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The condensed water is then passed by a condensate pump (7) to a deaerator (12), then pumped by feed water pump (28) and pre-warmed, first in a feed heater (13) powered by steam drawn from the high pressure set, and then in the economiser (23), before being returned to the boiler drum. The cooling water from the condensor is sprayed inside a cooling tower (1), creating a highly visible plume of water vapor, before being pumped back to the condensor (8) in cooling water cycle. The three turbine sets are sometimes coupled on the same shaft as the three-phase electrical generator (5) which generates an intermediate level voltage (typically 20-25 kV). This is stepped up by the unit transformer (4) to a voltage more suitable for transmission (typically 250-500 kV) and is sent out onto the three-phase transmission system (3). Exhaust gas from the boiler is drawn by the induced draft fan (26) through an electrostatic precipitator (25) and is then vented through the chimney stack (27).

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Principle of steam power plant design

The essential principles of steam power plant design are as follows:

• Low capital cost• Reliability• Low operating and maintenance cost• High thermal efficiency• Accessibility• A simple design

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The power plant should be designed such that it can be expanded if required. It should be simple in design. Use of automatic equipment is desired to reduce the labor cost. Heat recovery devices should be used wherever possible. Total capacity of the plant should be subdivided into four or five generating units so that during reduced load periods some of the units may be stopped.

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Factors affecting steam power plant design

The various factors which affect the design of a steam power plant are as follows:

• Steam pressure and temperature• Capacity of power plant• Ratings of generating units• Thermodynamic cycle• Voltage generation

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The trend is towards using higher pressure and temperatures of steam. This improves the thermal efficiency. The capacity of the plant can be determined by studying the load duration curve and anticipated future load demand. The size of turbo-generator depends on the following:

• Rate of growth of load• Availability of condensing water• Space available

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Large type turbo generator sets should be used.• The voltage of generation is usually 11kV, though 22kV

and 33 kV are also used. In central power plants water tube boilers are commonly used. The type of fuel used in a boiler will influence the design and efficiency of the boiler plant considerably.

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The steam power plants needs a lot of space for• Storing the fuel• Elaborated arrangements for fuel handling• Ash handling • Lay out of high-pressure boilers• Steam turbines• Generators• Control switch boards• Cooling water arrangements

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In steam power plant the A.C. generators are driven by turbines with condensing arrangement at a high vacuum of about 73.3 cm Hg . The size of the generating set may vary between 10MW to 500MW or even higher. Modern generators are 2 pole with 3000 R.P.M. as speed and 50 cycle per second as frequency.

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Efficiency:The energy efficiency of a conventional thermal power

station, considered salable energy produced as a percent of the heating value of the fuel consumed, is typically 33% to 48%.[citation needed] As with all heat engines, their efficiency is limited, and governed by the laws of thermodynamics. By comparison, most hydropower stations in the United States are about 90 percent efficient in converting the energy of falling water into electricity.

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The energy of a thermal not utilized in power production must leave the plant in the form of heat to the environment. 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 co-generation. 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.

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The Carnot efficiency dictates that higher efficiencies can be attained by increasing the temperature of the steam. Sub-critical fossil fuel power plants can achieve 36–40% efficiency. Super critical designs have efficiencies in the low to mid 40% range, with new "ultra critical" designs using pressures of 4400 psi (30.3 MPa) and multiple stage reheat reaching about 48% efficiency. Above the critical point for water of 705 °F (374 °C) and 3212 psi (22.06 MPa), there is no phase transition from water to steam, but only a gradual decrease in density

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Currently most of the nuclear power plants must operate below the temperatures and pressures that coal-fired plants do, since the pressurized vessel is very large and contains the entire bundle of nuclear fuel rods. The size of the reactor limits the pressure that can be reached. This, in turn, limits their thermodynamic efficiency to 30–32%. Some advanced reactor designs being studied, such as the very high temperature reactor, advanced gas-cooled reactor and supercritical water reactor, would operate at temperatures and pressures similar to current coal plants, producing comparable thermodynamic efficiency

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The overall efficiency of steam power plants depends upon the efficiency of boiler , turbine and alternator. The heat produced due to the burning of coal is not fully utilized for generating electrical energy because there are heat losses in the boiler turbine and mechanical and electrical losses in the turbine. The overall thermal efficiency of steam power station is given by the following relation:

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Cost of steam power plant:

In recent days vast improvements have been made in generating electric power from steam. About 0.45 kg of coal is need to produce 1kWh of electricity. It is observed that larger capacity power plants can utilize the thermal energy more efficiency than the smaller capacity plant. In the design of a thermal power station future availability of coal and its price has to be taken into account.

A steam power station may cost about TK 1600 per kW of capacity. A typical sub-division of investment cost of a steam power station is as follows:

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The investment cost include the following costs:• Costs of land • Cost of building• Cost of mechanical and electrical equipment and their

installation

Conditions affecting the investment depend on a number of factors:

• Characteristics of the site• Size and number of power generating units• Fuel storing and fuel handling methods

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Plant Layout:

Here show the layout of power plant. The turbine room should be sufficiently wide to accommodate various auxiliaries. In boiler room enough space should be left for repairs and maintenance. Various equipment should be so placed that access to all the parts is easy

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Basic arrangements of various equipments used in power station differ from station to station. The following factors should be considered while installing various components.

• All similar items such as turbines, boiler, transformer, bunker bays and other mechanical and electrical components are arranged in parallel lines and at right angles rows individual boiler turbo-generator, transformers etc.

• Chimneys should be erected independently of the station building with chimney serving two or more boilers.

• Main flue, draft fans and outdoor precipitators should be located behind the boiler house.

• Circulating water supply , coal supply to bunkers and lifting equipment should be properly placed.

• Outdoor generator and unit transformers should be in front of the turbine house.

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Modern steam power station

Central steam power station of larger capacities use higher boiler pressure with super-heated steam as the super heated steam contains more heat energy than saturated steam at the same pressure and provides more energy to the turbine for conversion into electric energy. They use large capacity boiler , and improved methods of coal firing such as stoker firing or pulverized coal firing. The coal used is of good quality as low grade coal such as those having high ash contents ( about 18-20% and above) and higher moisture contents (about 30% or more) are not

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preferred. High ash contents in coal reduce the heating value of coal and more labour is required for the removal of ash from the furnace . Ash with low fusing , or melting temperatures forms clinkers when subjected to high temperatures of fuel bed. The clinkers so formed interfere with the movement of fuel on the stokers. The coal to be burnt on hand fired grates and on the stokers should have a minimum of about 4 to 6% of ash. Moisture also reduces the heat value of coal. Coals with 7 to 12% moisture are generally burnt on chain grate and travelling grate stokers. Coal analysis is done to know its composition.

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Thermal power station in Bangladesh:

A thermal power station is a power plant in which the prime mover is steam driven. Water is heated, turns into steam and spins asteam turbine which drives an electrical generator. 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 fossil fuel resources generally used to heat the water. Some prefer to use the term energy center because such facilities convert forms of heat energy into electrical energy. Certain thermal power plants also are designed to produce heat energy for industrial

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purposes of district heating, or desalination of water, in addition to generating electrical power. Globally, fossil fueled thermal power plants produce a large part of man-made CO2 emissions to the atmosphere, and efforts to reduce these are varied and widespread.

In Bangladesh, Barapukuria Power Station which is coal fired power plant and capacity is about 250MW

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Environmental control

Fuel burnt in at thermal power plants contain harmful impurities which are ejected into the environment as gaseous and solid components of combustible products and can adversely affect the atmosphere and water. Toxic substances contained in the flue gases discharge from chimneys of thermal power plants can produce harmful effects on the whole of complex of living nature.

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The flue gases may contain the following:• Oxides of carbon and hydrogen• Fly ash• Solid particles of unburnt fuel• Oxides of sulphur and nitrogen

Thermal power plants consume more than 1/3 of all the fuels produced and thus can significantly affect the local environment and the whole of bio-shepre comprising the atmospheric layer near the earth’s surface and upper layers of soil and water basins.

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Advantages and disadvantages

Advantages

• The fuel used is quite cheap.• Less initial cost as compared to other generating plants.• It can beinstalled at any place iirespective of the

existence of coal. The coal can be transported to the site of the plant by rail or road.

• It require less space as compared to Hydro power plants.

• Cost of generation is less than that of diesel power plants.

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Disadvantages

• It pollutes the atmosphere due to production of large amount of smoke and fumes.

• It is costlier in running cost as compared to Hydro electric plants.

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 Present situation in Bangladesh:

At present , Barapukuria Power Station which is coal fired and produced electricity about 250 MW. There are several oil and gas fired power plant among them largest one is Ghorasal and produced electricity about 950 MW and Ashuganj power station is also largest power station which produce electricity about 638 MW . Bangladesh have lots of gas resources. That’s why there have many gas turbine powered power plants.There are one hydroelectric power plants which produced electricity about 230MW and having five turbine. Ruppur Nuclear Power Plant is a proposed 2,000 MW nuclear power plant. Planned to go into operation by 2020, it will be the country's first nuclear power plant.

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Conclusion

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Steam power plants are located at the water and coal available places. Steam is utilized to run the turbines, in turn gives the power to generator and generator produces the electricity, the electricity is utilized for lighting, running the industries, for lighting of offices, schools, etc. Boiler is an important component of the power plants, it produces the steam.

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Any Questions???

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4

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