45187825 Power Plant Instrumentation Unit 1

7/29/2019 45187825 Power Plant Instrumentation Unit 1

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Unit 1Brief survey of methods of power generation: 1. Hydro power plant:Hydroelectricity is electricity generated by hydropower, i.e., the production ofelectrical power through the use of the gravitational force of falling or flowing water. It is the most widely used form of renewable energy. Once a hydroelectric complex is constructed, the project produces no direct waste, and has a considerably lower output level of the greenhouse gas carbon dioxide (CO 2 ) than fossil fuel powered energy plants. Worldwide, an installed capacity of 777 GWe supplied 2998 TWh of hydroelectricity in 2006. This was approximately 20% of the world's electricity, and accounted for about 88% of electricity from renewable sources.

Electricity generation:

Most hydroelectric power comes from the potential energy of dammed water drivinga water turbine and generator. In this case the energy extracted from the waterdepends on the volume and on the difference in height between the source and the water's outflow. The amount of potential energy in water is proportional to the head. To deliver water to a turbine while maintaining pressure arising from the head, a large pipe called a penstock may be used.

Calculating the amount of available power:A simple formula for approximating electric power production at a hydroelectricplant is: P = h

gk, Where P is Power in watts,

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is the density of water (~1000 kg/m3 ), h is height in meters, r is flow rate incubic meters per second, g is acceleration due to gravity of 9.8 m/s2 , and k is a coefficient of efficiency ranging from 0 to 1. Efficiency is often higher (that is, closer to 1) with larger and more modern turbines.

Advantages:1. The major advantage of hydroelectricity is elimination of the cost of fuel. 2. Hydroelectric plants also tend to have longer economic lives than fuel- firedgeneration, with some plants now in service which were built 50 to 100 years ago. 3. Operating labour cost is also usually low, as plants are automated and have

few personnel on site during normal operation. 4. Since hydroelectric dams do not burn fossil fuels, they do not directly produce carbon dioxide (a greenhousegas). 5. According to a study, hydroelectricity produces the least amount of greenhouse gases and externality of any energy source. Coming in second place was wind, third was nuclear energy, and fourth was solar photovoltaic.

Disadvantages:1. Dam failures have been some of the largest man- made disasters in history. Also, good design and construction are not an adequate guarantee of safety. 2. Almost all rivers convey silt. Dams on those rivers will retain silt in their catchments, because by slowing the water, and reducing turbulence, the silt will fallto the bottom. Siltation reduces a dam's water storage so that water from a wetseason cannot be stored for use in a dry season. 3. Large reservoirs required f

or the operation of hydroelectric power plants result in submersion of extensiveareas upstream of the dams, destroying biologically rich and productive lowlandand reverie valley forests, marshland and grasslands. 4. Lower positive impactsare found in the tropical regions, as it has been noted that the reservoirs ofpower plants in tropical regions may produce substantial amounts of methane andcarbon dioxide. This is due to plant material in flooded areas decaying in an anaerobic environment, and forming methane, a very potent greenhouse gas. 5. Another disadvantage of hydroelectric dams is the need to relocate the people livingwhere the reservoirs are planned.


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2. Nuclear power plant:Nuclear powe r is power (generally electrical) produced from controlled (i.e., non-explosive) nuclear reactions. Electric utility reactors heat water to producesteam, which is then used to generate electricity. In 2009, 15% of the world'selectricity came from nuclear power, despite concerns about safety and radioactive waste management. Nuclear fusion reactions are widely believed to be safer than fission and appear potentially viable, though technically quite difficult.

Use

Historical and projected world energy use by energy source, 1980-2030. As of 2005, nuclear power provided 2.1% of the world's energy and 15% of the world's electricity, with the U.S., France, and Japan together accounting for 56.5% of nuclear generated electricity. Many military and some civilian (such as some icebreaker) ships use nuclear marine propulsion, a form of nuclear propulsion. A few space vehicles have been launched using full- fledged nuclear reactors: the SovietRORSAT series and the American SNAP-10A.

Nuclear reactor technology:Just as many conventional thermal power stations generate electricity by harnessing the thermal energy released from burning fossil fuels, nuclear power plants

convert the energy released from the nucleus of an atom, typically via nuclear fission. When a relatively large fissile atomic nucleus (usually uranium-235 or plutonium-239) absorbs a neutron, fission of the atom often results. Fission splits the atom into two or more smaller nuclei with kinetic energy (known as fission products) and also releases gamma radiation and free neutrons. A portion of these neutrons may later be absorbed by o ther fissile atoms and create more fissions, which release more neutrons, and so on.3

This nuclear chain reaction can be controlled by using neutron poisons and neutron moderators to change the portion of neutrons that will go on to cause more fission. A cooling system removes heat from the reactor core and transports it to

another area of the plant, where the thermal energy can be harnessed to produceelectricity or to do other useful work. Typically the hot coolant will be used as a heat source for a boiler, and the pressurized steam from that boiler will power one or more steam turbine driven electrical generators.

Breeding:As opposed to current light water reactors which use uranium-235 (0.7% of all natural uranium), fast breeder reactors use uranium-238 (99.3% of all natural uranium). Another alternative would be to use uranium-233 bred from thorium as fission fuel in the thorium fuel cycle. Thorium is about 3.5 times as common as uranium in the Earth's crust, and has differe nt geographic characteristics. This would extend the total practical fissionable resource base by 450%.

Nuclear wastes: 1. Solid waste:The safe storage and disposal of nuclear waste is a significant challenge and yet unresolved problem. The most important waste stream from nuclear power plantsis spent fuel. A large nuclear reactor produces 3 cubic metres (2530 tonnes) of spent fuel each year. It is primarily composed of unconverted uranium as well assignificant quantities of transuranic actinides (plutonium and curium, mostly).The actinides (uranium, plutonium, and curium) are responsible for the bulk of the long term radioactivity, whereas the fission products are responsible for thebulk of the short term radioactivity.


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2. High-level radioactive waste:Spent fuel is highly radioactive and needs to be handled with great care and forethought. However, spent nuclear fuel becomes less radioactive over the course of thousands of years of time. Spent fuel rods are stored in shielded basins of water (spent fuel pools), usually located on-site. The water provides both cooling for the still-decaying fission products, and shielding from the continuing radioactivity. The amount of waste can be reduced in several ways, particularly reprocessing.

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3. Low-level radioactive waste:The nuclear industry also produces a huge volume of low-level radioactive wastein the form of contaminated items like clothing, hand tools, water purifier resins, and (upon decommissioning) the materials of which the reactor itself is built.

3. Solar power plant:Solar ene rgy, radiant light and heat from the sun, has been harnessed by humanssince ancient times using a range of ever-evolving technologies. Solar poweredelectrical generation relies on heat engines and photovoltaics. To harvest the solar energy, the most common way is to use solar panels. Solar technologies arebroadly characterized as either passive solar or active solar depending on the w

ay they capture, convert and distribute solar energy. Active solar techniques include the use of photovoltaic panels and solar thermal collectors to harness theenergy. Passive solar techniques include orienting a building to the Sun, selecting materials with favourable thermal mass or light dispersing properties, anddesigning spaces that naturally circulate air.

Energy from the Sun:

The Earth receives 174 petawatts (PW) of incoming solar radiation at the upper atmosphere. Approximately 30% is reflected back to space while the rest is absorbed by clouds, oceans and land masses. The spectrum of solar light at the Earth'ssurface is mostly spread across the visible and near- infrared ranges with a sm

all part in the near- ultraviolet. Earth's land surface, oceans and atmosphere absorb solar radiation, and this raises their temperature.

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Warm air containing evaporated water from the oceans rises, causing atmosphericcirculation or convection. When the air reaches a high altitude, where the temperature is low, water vapour condenses into clouds, which rain onto the Earth's surface, completing the water cycle. Sunlight absorbed by the oceans and land masses keeps the surface at an average temperature of 14 C. By photosynthesis greenplants convert solar energy into chemical energy, which produces food, wood andthe biomass from which fossil fuels are derived.

The total solar energy absorbed by Earth's atmosphere, oceans and land masses isapproximately 3,850,000 exajoules (EJ) per year. Solar energy can be harnessedin different levels around the world. Depending on a geographical location the closer to the equator the more "potential" solar energy is available.

Applications of solar technology:Solar energy refers primarily to the use of solar radiation for practical ends.However, all renewable energies, other than geothermal and tidal, derive their energy from the sun. Solar technologies are broadly characterized as either passi


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ve or active depending on the way they capture, convert and distribute sunlight.Active solar techniques use photovoltaic panels, pumps, and fans to convert sunlight into useful outputs. Passive solar techniques include selecting materialswith favourable thermal properties, designing spaces that naturally circulate air, and referencing the position of a building to the Sun. Active solar technologies increase the supply of energy and are considered supply side technologies, while passive solar technologies reduce the need for alternate resources and aregenerally considered demand side technologies.

4. Wind power plant:Wind powe r is the conversion of wind energy into a useful form of energy, suchas using wind turbines to make electricity, wind mills for mechanical power, wind pumps for pumping water or drainage, or sails to propel ships. Several countries have achieved relatively high levels of wind power penetration (with large governmental subsidies), such as 19% of stationary electricity production in Denmark, 13% in Spain and Portugal, and 7% in Germany and the Republic of Ireland in2008. Large-scale wind farms are connected to the electric power transmission network; smaller facilities are used to provide electricity to isolated locations.Wind energy as a power source is attractive as an alternative to fossil fuels,because it is plentiful, renewable, widely distributed, clean, and produces no greenhouse gas emissions.6

The Earth is unevenly heated by the sun, such that the poles receive less energyfrom the sun than the equator; along with this, dry land heats up (and cools down) more quickly than the seas do. The differential heating drives a global atmospheric convection system reaching from the Earth's surface to the stratospherewhich acts as a virtual ceiling. Most of the energy stored in these wind movements can be found at high altitudes where continuous wind speeds of over 160 km/h(99 mph) occur.

Electricity generation:In a wind farm, individual turbines are interconnected with a medium voltage (often 34.5 kV), power collection system and communications network. At a substation, this medium- voltage electrical current is increased in voltage with a transformer for connection to the high voltage electric power transmission system. The

ratio of actual productivity in a year to this theoretical maximum is called the capacity factor. Typical capacity factors are 2040%, with values at the upper end of the range in particularly favourable sites.

Turbine placement:Good selection of a wind turbine site is critical to economic development of wind power. Aside from the availability of wind itself, other factors include the availability of transmission lines, value of energy to be produced, cost of landacquisition, land use considerations, and environmental impact of construction and operations.

Small-scale wind power:Small-scale wind powe r is the name given to wind generation systems with the ca

pacity to produce up to 50 kW of electrical power. Wind turbines have been usedfor household electricity generation in conjunction with battery storage over many decades in remote areas.

5. Thermal power plant: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. Afterit passes through the turbine, the steam is condensed in a condenser and recycled to where it was heated. The greatest variation in the design of thermal power


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stations is due to the different fuel sources.7

Introduction:Almost all coal, nuclear, geothermal, solar thermal electric, and waste incineration plants, as well as many natural gas power plants are thermal. The waste heat from a gas turbine can be used to raise steam, in a combined cycle plant thatimproves overall efficiency. Power plants burning coal, oil, or natural gas areoften referred to collectively as fossil-fuel power plants.

Efficiency: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 powerstation is typically 33% to 48%. This efficiency is limited as all heat enginesare governed by the laws of thermodynamics. The rest of the energy must leave the plant in the form of heat. This waste heat can go through a condenser and bedisposed of with cooling water or in cooling towers.

Diagram of a typical coal-fired thermal power station:

Typical diagram of a coal-fire d thermal power station 1. Cooling tower 2. Cooli

ng water pump 3. transmission line (3-phase) 4. Step-up transformer (3-phase) 5.Electrical generator (3-phase) 10. Steam Control valve 11. High pressure steamturbine 12. Deaerator 13. Feed water heater 14. Coal conveyor8

19. Super heater 20. Forced draught (draft) fan 21. Reheater 22. Combustion airintake 23. Economiser

6. Low pressure steam turbine 7. Condensate pump 8. Surface condenser 9. Intermediate pressure steam turbine

15. Coal hopper 16. Coal pulverizer 17. Boiler steam drum 18. Bottom ash hopper

24. Air pre heater 25. Precipitator 26. Induced draught (draft) fan 27. Flue gasstack

Structural representation of power plant:

Steam generator:In fossil- fuelled power plants, steam generator refers to a furnace that burnsthe fossil fuel to boil water to generate steam.

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A fossil fuel steam generator includes an economizer, a steam drum, and the furnace with its steam generating tubes and super heater coils. Necessary safety valves are located at suitable points to avoid excessive boiler pressure.

Boiler furnace and steam drum:Once water gets inside the boiler or steam generator, the process of adding thelatent heat of vaporization 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 t he economizer. From the economizer it passes to the steam drum. Once the water enters the steam drum it g


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oes down to the down comers to the lower inlet water wall headers. From the inlet headers the water rises through the water walls and is eventually turned intosteam due to the heat being generated by the burners located on the front and rear water walls. As the water is turned into steam/vapour in the water walls, thesteam/vapour once again enters the steam drum. The steam/vapour 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 water walls 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 for observation ofthe furnace interior. The steam drum (as well as the super heater coils and headers) have air vents and drains needed for initial start-up. The steam drum has internal devices that remove moisture from the wet steam entering the drum from the steam generating tubes. The dry steam then flows into the super heater coils.

Cooling towers:Cooling towers are heat removal devices used to transfer process waste heat to the atmosphere. Cooling towers may either use the evaporation of water to removeprocess heat and cool the working fluid to near the wet-bulb air temperature orrely solely on air to cool the working fluid to near the drybulb air temperature. Common applications include cooling the circulating water used in oil refineries, chemical plants, power stations and building cooling.10

The towers vary in size from small roof- top units to very large hyperboloid structures that can be up to 200 metres tall and 100 metres in diameter, or rectangular structures that can be over 40 metres tall and 80 metres long. Smaller towers are normally factory-built, while larger ones are constructed on site.

Condenser:The function of the condenser is to condense exhaust steam from the steam turbine by rejecting the heat of vaporisation to the cooling water passing through thecondenser. Once the steam has passed through the turbine, it enters the condenser where heat is removed until it condenses back into liquid water. This is doneby passing the wet steam around thousands of small cold water tubes. The cold w

ater is usually supplied from a nearby sea, lake, river, or from a cooling tower. The condensed steam is collected at the bottom of the condenser and returned to the boiler using feed water pumps, to begin the water-to-steam, Steam- to-water cycle again.

Classification:As single pressure or multi-pressure, depending on whether the cooling water flow path creates one or more turbine backpressures; By the number of shells (whichis dependent on the number of low-pressure turbine casings); and

As either single pass or two-pass, depending on the number of parallel water flow paths through each shell.

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Types of cooling systems:Some power plants have an open cycle (once through) cooling water system where water is taken from a body of water, such as a river, lake or ocean, pumped through the plant condenser and discharged back to the source. Power plants in remotedry areas without economic water supplies use closed cycle dry cooling systemsthat do not require water for cooling. Hybrid cooling systems are used in particular circumstances.


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Open cycle cooling systems:In the open system, water pumped from intakes on one side of the power plant passes through the condensers and is discharged at a point remote from the intake (to prevent recycling of the warm water discharge).

Open cycle with auxiliary cooling tower:In this system, cooling towers are installed on the discharge from open systemsin order to remove part of the waste heat, so that the load on the receiving waters is contained within pre set limits.

The auxiliary cooling towers are used in the warmer summer periods to limit thetemperature of the discharged cooling water, usually to less than 30C.12

Closed cycle wet cooling systems:In closed cycle wet cooling systems, the waste energy that is rejected b y the turbine is transferred to the cooling water system via the condenser. The waste heat in the cooling water is then discharged to the atmosphere by the cooling tower. In the cooling tower, heat is removed from the falling water and transferredto the rising air by the evaporative cooling process. The falling water is broken up into droplets or films by the extended surfaces of the tower 'fill'.

The major components of a closed cycle wet cooling water system are: Cooling towers - two types are commonly used, concrete natural draught towers and mechanical draught towers; and Pumps and pipes.

Natural draught towers:

Natural draft, which utilizes buoyancy via a tall chimney. Warm, moist air naturally rises due to the density differential to the dry, cooler outside air. Warmmoist air is less dense than drier air at the same pressure. This moist air buoyancy produces a current of air through the tower.

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Natural draught towers are only economic in large sizes, which justify the costof the large concrete shell. The cooling towers have two basic configurations for the directions of the flow of air in relation to the falling water through thetower fill: The counter- flow tower where the air travels vertically up throughthe fill. The cross- flow tower where the air travels horizontally through thefill.

Categorization by air-to-water flow: Cross flow:Cross flow is a design in which the air flow is directed perpendicular to the water flow. Air flow enters one or more vertical faces of the cooling tower to meet the fill material. Water flows (perpendicular to the air) through the fill by

gravity. The air continues through the fill and thus past the water flow into anopen plenum area. Gravity distributes the water through the nozzles uniformly across the fill material.

Counter flow:In a counter flow design the air flow is directly opposite of the water flow. Air flow first enters an open area beneath the fill media and is then drawn up vertically. The water is sprayed through pressurized nozzles and flows downward through the fill, opposite to the air flow.


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Common to both designs:

The interaction of the air and water flow allows a partial equalization and evaporation of water. The air, now saturated with water vapour, is discharged from the cooling tower. A collection or cold water basin is used to contain the waterafter its interaction with the air flow.

Both cross flow and counter flow designs can be used in natural draft and mechanical draft cooling towers.

Mechanical draught cooling towers:In mechanical draught cooling towers, large axial flow fans provide the airflow.

While fans have the disadvantage of requiring auxiliary power, typically 1.0MW to 1.5MW for a 300MW steam turbine-generator unit, but they have the advantage ofbeing able to provide lower water temperatures than natural draught towers, particularly on hot dry days.

Mechanical draft, which uses power driven fan, motors to force or draw air throu

gh the tower.

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Induced draft : A mechanical draft tower with a fan at the discharge which pullsair through tower. The fan induces hot moist air out the discharge.

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Forced draft: A mechanical draft tower with a blower type fan at the intake. Thefan forces air into the tower, creating high entering and low exiting air velocities. Another disadvantage is that a forced draft design typically requires more motor horsepower than an equivalent induced draft design.

Dry cooling systems:In the dry cooling system, heat transfer is by air to finned tubes. The steam condensing pressures and temperatures of a dry cooled unit are significantly higher than a wet cooled unit, due to the low transfer rates of dry cooling and operation at the dry bulb temperature.15

There are two basic types of dry cooling systems: The direct dry cooling system;and The indirect dry cooling system.

Direct dry cooling system:

In the direct dry system, the turbine exhaust steam is piped directly to the air-cooled, finned tube, condenser. The finned tubes are usually arranged in the form of an 'A' frame or delta over a forced draught fan to reduce the land area.

Indirect dry cooling system:With indirect dry cooling, known as HELLER System, cooled water from the coolingtower flows through recovery hydraulic turbines connected in parallel and is used in preferably a direct contact (DC) jet condenser to condense steam from thesteam turbine. The major part of the flow, discharged by the circulating water pumps, is returned to the tower for cooling. The cooling deltas (water-to-air-hea


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t exchangers) dissipate the heat from the cycle.

Hybrid systems:16

There are two common hybrid systems, which have been developed to overcome someof the disadvantages of the full wet and full dry systems.

1. Wet with part dry:One of the problems with wet towers is that in cold and humid climates the towers plume can create fog. In the part dry or plume abatement tower, a dry sectionabove the wet zone provides some dry cooling to the exhaust plume to remove thecondensing water vapour.

2. Dry with part wet:Problems with full dry towers are centred on loss of performance in hot weather.With the part wet towers, there is provision for water sprays to evaporative cool the finned tubes for short periods of extreme temperature.

Surface condenser:Surface condenser is the commonly used term for a water-cooled shell and tube heat exchanger installed on the exhaust steam from a steam turbine in thermal power stations. These condensers are heat exchangers which convert steam from its ga

seous to its liquid state at a pressure below atmospheric pressure. Where cooling water is in short supply, an air-cooled condenser is often used.

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In thermal power plants, the primary purpose of a surface condenser is to condense the exhaust steam from a steam turbine to obtain maximum efficiency and alsoto convert the turbine exhaust steam into pure water so that it may be reused inthe steam generator or boiler as boiler feed water.

Why is it required?

The steam turbine itself is a device to convert the heat in steam to mechanicalpower. The difference between the heat of steam per unit weight at the inlet tothe turbine and the heat of steam per unit weight at the outlet to the turbine represents the heat which is converted to mechanical power. Therefore, the more the conversion of heat per pound or kilogram of steam to mechanical power in theturbine, the better is its efficiency. By condensing the exhaust steam of a turbine at a pressure below atmospheric pressure, the steam pressure drop between the inlet and exhaust of the turbine is increased, which increases the amount of heat available for conversion to mechanical power. Most of the heat liberated dueto condensation of the exhaust steam is carried away by the cooling medium (water or air) used by the surface condenser. For the convenience of cleaning and maintenance, cooling water flows through the tubes and steam condenses outside thetubes. At each end there are tube sheets into which the water tubes are rolled.

This prevents leakage of circulating water into the steam. An expansion joint allows for the d ifferent rates of expansion between the tubes and shells. Thereare vertical plates at intermediate points between the two tube sheets to provide support to the long tubes and to prevent tube vibration. In a single pass condenser, cooling water flows through the tubes once, from one end to another.

Steam turbine:A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion. Because the turbine generates rotary motion, it is particularly suited to be used to drive an electrical generat


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or about 80% of all electricity generation in the world is by use of steam turbines.

Principle of Operation and Design:An 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 onthe application of the turbine. The interior of a turbine comprises several setsof 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 connectedto the shaft.

Deaerator:18

A deaerator is a device that is widely used for the removal of air and other dissolved gases from the feed water to steam- generating boilers. In particular, dissolved oxygen in boiler feed waters will cause serious corrosion and forming oxides (rust). Water also combines with any dissolved carbon dioxide to form carbonic acid that causes further corrosion. 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 thedeaerated 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 feed water storage tank.

Tray-type deaerator:

Boiler feedwater enters the vertical dearation section above the perforated trays and flows downward through the perforations. Low-pressure dearation steam enters below the perforated trays and flows upward through the perforations. The steam strips the dissolved gas from the boiler feedwater and exits via the vent atthe top of the domed section. The vent line usually includes a valve and just enough steam is allowed to escape with the vented gases to provide a small and visible telltale plume of steam. The deaerated water flows down into the horizontalstorage vessel from where it is pumped to the steam generating boiler system.

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Spray-type deaerator:

The typical spray-type deaerator is a horizontal vessel which has a preheating section (E) and a deaeration section (F). The two sections are separated by a baffle(C). Low-pressure steam enters the vessel through a sparger in the bottom ofthe vessel.The boiler feedwater is sprayed into section (E) where it is preheated by the rising steam from the sparger. The purpose of the feedwater spray nozzle (A) and the preheat section is to heat the boiler feedwater to its saturationtemperature to facilitate stripping out the dissolved gases in the following deaeration section. The preheated feedwater then flows into the dearation section (F), where it is deaerated by the steam rising from the sparger system. The gases


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stripped out of the water exit via the vent at the top of the vessel. The deaerated boiler feedwater is pumped from the bottom of the vessel to the steam generating boiler system.

Feed water heater:

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A feedwater heater is a power plant component used to pre-heat water delivered to a steam generating boiler. Preheating the feedwater reduces the irreversibility involved in steam generation and therefore improves the thermodynamic efficiency of the system. In a steam power plant, feedwater heaters allow the feedwaterto be brought up to the saturation temperature very gradually.

Feedwater heaters can also be open and closed heat exchangers. An open feedwaterheater is merely a direct-contact heat exchanger in which extracted steam is allowed to mix with the feedwater. This kind of heater will normally require a feed pump at both the feed inlet and outlet since the pressure in the heater is between the boiler pressure and the condenser pressure. A deaerator is a special case of the open feedwater heater which is specifically designed to remove noncondensable gases from the feedwater. Closed feedwater heaters are typically shell and tube heat exchangers where the feedwater passes throughout the tubes and is h

eated by turbine extraction steam. These do not require separate pumps before and after the heater to boost the feedwater to the pressure of the extracted steamas with an open heater.Feedwater heaters are used in both fossil- and nuclear-fuelled power plants. An economiser serves a similar purpose to a feedwater heater, but is technically different. Instead of using actual cycle steam for heating, it uses the lowest-temperature flue gas from the furnace (and therefore does not apply to nuclear plants) to heat the water before it enters the boiler proper.

Pulverizer:A pulverize r is a mechanical device for the grinding of many different types ofmaterials. For example, they are used to pulverize coal for combustion in the steam- generating furnaces of fossil fuel power plants.

Types of pulverizer: Ball mill:A ball mill is a pulverizer that consists of a horizontal rotating cylinder, upto three diameters in length, containing a charge of tumbling or cascading steelballs, pebbles, or rods

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.There are three stages in the pulverization process: A. feeding B. drying C. grinding The feeding system controls the fuel feed rate according to boiler demand

and the required air rate (primary air) for drying, and then transporting the pulverized fuel and primary stream to the burner. Since coals have varying amountof moistures and in order that lower rank coals can be used, dryers are an integral part of the pulverizing equipment. Part of the hot air from the air preheater (primary air) is forced into the pulverizer at about 350 0 C by the primary air fan. There it is mixed with the coal as it is being circulated and ground. Theheart of the equipment is, however, the pulverizer or the tube mil. Grinding isperformed by crushing. Pulverizers, commonly used , are classified by speed: (A): low speed (below 75 rpm), the ball tube mill; (B): medium speed (75 to 225 rpm), the ball and race mill (C): high speed (above 225 rpm), the impact or hammer


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mill. The figure is shown above: The ball tube mill is a hollow horizontal cylinder with conical ends and wear resistance liners revolving slowly at about 20-30rpm with 20-35% of its volume being filled with forged steel balls of mixed size 30-60 mm. Coal is pulverized by attrition and impact as the steel balls and coal rise up and fall down with cylinder position. Primary air is blown over the charge to carry the pulverized coal to classifiers, which feedback the coarser particles for regrinding. The ball mill is reliable and requires low maintenance,but it is bulky and heavy in construction.

Bottom ash:Bottom ash refers to the non-combustible constituents of coal with traces of combustibles embedded in forming clinkers and sticking to hot side walls of a coal-burning furnace during its operation. The portion of the ash that escapes up thechimney or stack is, however, referred to as fly ash. The clinkers fall by themselves into the water or sometimes by poking manually, and get cooled.22

Superheater:A superheater is a device used to convert saturated steam or wet steam into drysteam used for power generation or processes. There are three types of superheaters namely: radiant, convection, and separately fired. A radiant superheater isplaced directly in the combustion chamber. A convection superheater is located in the path of the hot gases. A separately fired superheater, as its name implies

, is totally separated from the boiler. Whatever type of boiler is used, steam will leave the water at its surface and pass into the steam space. Steam formed above the water surface in a shell boiler is always saturated and cannot become superheated in the boiler shell, as it is constantly in contact with the water surface. If superheated steam is required, the saturated steam must pass through asuperheater. This is simply a heat exchanger where additional heat is added tothe saturated steam.

Super heater temperature limitations:Though thermodynamically there is no limit on superheating steam, the maximum temperature to which can be heated in a boiler is dictated by the metallurgy of the super heater tubes, which have withstand the high temperature. Considering availability economy in initial cost and maintenance cost generally ferrite, pearli

te and very limited amount of austenite steels only can be chosen for superheater and reheater tubes. Because of this reason the present trend is to limit the steam temperature value at 5400C both in superheater as well in reheater.

Reheater:Development of large capacity steam turbines with more number of stages posed aproblem of retaining the steam within vapour phase till the last stage. It is because even with a larger steam turbine the inlet steam temperature is kept at 5400C only due to superheater limitations. To overcome this problem it becomes necessary to raise the temperature of steam after part of its energy is extracted from it in the23

steam turbine. This is called the reheating of the steam which increases the cycle efficiency. This reheating of the steam is done in the boiler which suppliessuper heated steam to the turbine itse lf at the heating surfaces called 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 isrerouted to go inside the reheater tubes to pickup more energy to go drive intermediate or lower pressure turbines.

Economizer:


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In simple terms, an economizer is a heat exchanger. In boilers, economizers areheat exchange devices that heat fluids, usually water, up to but not normally beyond the boiling point of that fluid. They are a device fitted to a boiler whichsaves energy by using the exhaust gases from the boiler to preheat the cold water used to fill it (the feed water). Flue gases from large boilers are typically450 - 650F.Stack Economizers recover some of this heat for pre-heating water. The water is most often used for boiler make-up water or some other need that coincides with boiler operation. Stack Economizers should be considered as an efficiency measure when large amounts of make-up water are used or there is a simultaneous need for large quantities of hot water for some other use.

Working:Boiler stack economizers are simply heat exchangers with hot flue gas on one side and water on the other. Economizers must be sized for the volume of flue gas,its temperature, the maximum pressure drop allowed through the stack, what kindof fuel is used in the boiler, and how much energy needs to be recovered.

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Electrostatic precipitator:An electrostatic precipitator (ESP), or electrostatic air cleane r is a collection device that removes particles from a flowing gas (such as air) using the force of an induced electrostatic charge. Electrostatic precipitators are highly eff

icient filtration devices that, and can easily remove fine particulate matter such as dust and smoke from the air stream.

Flue gas stacks:A flue gas stack is a type of chimney, a vertical pipe, channel or similar structure through which combustion product gases called flue gases are exhausted to the outside air. Flue gases are produced when coal, oil, natural gas, wood or anyother fuel is combusted in an industrial furnace, a power plant's steam-generating boiler, or other large combustion device. Flue gas is usually composed of carbon dioxide (CO 2 ) and water vapour as well as nitrogen and excess oxygen remaining from the intake combustion air. It also contains a small percentage of pollutants such as particulate matter, carbon monoxide, nitrogen oxides and sulfuroxides. The flue gas stacks are often quite tall, up to 400 meters (1300 feet) o

r more, so as to disperse the exhaust pollutants over a greater area and therebyreduce the concentration of the pollutants to the levels required by governmental environmental policy and environmental regulation.

Flue gas stack draft (or draught):The combustion flue gases inside the flue gas stacks are much hotter than the ambient outside air and therefore less dense than the ambient air. That causes thebottom of the vertical column o f hot flue gas to have a lower pressure than the pressure at the bottom of a corresponding column of outside air. That higher pressure outside the chimney is the driving force that moves the required combustion air into the combustion zone and also moves the flue gas up and out of the chimney. That movement or flow of combustion air and flue gas is called "naturaldraft (or draught)", "natural ventilation", "chimney effect", or "stack effect".

The taller the stack, the more draft (or draught) is created. The equation below provides an approximation of the pressure difference, P, (between the bottom and the top of the flue gas stack) that is created by the draft:

where: P = available pressure difference, in Pa

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C = 0.0342 a = atmospheric pressure, in Pa h = height of the flue gas stack, in


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m To = absolute outside air temperature, in K Ti = absolute average temperatureof the flue gas inside the stack, in K

Draught (or draft ) system:Large amount of air are needed for combustion of the fuel. The gaseous combustion products in huge quantity have also to be removed continuously from the boilerfurnace. To produce the required flow of air or combustion gas, a pressure differential is needed. The term draft or draught is used to define the static pressurein the furnace, in the various ducts, and the stack. The function of the draughtsystem is basically twofold: 1. To supply to the furnace the required quantityof air for complete combustion of fuel. 2. To remove the gaseous products of combustion from the furnace and throw these through chimney or stack to the atmosphere. There are two ways of producing draught: 1. Natural draught 2. Mechanical draught

1. Natural draught:The natural draught is produced by a chimney or a stack. It is caused by the density difference between the atmospheric air and the hot gas in the stack.

P = g H (a g) Where: a = density of atmospheric air, kg/m326

g = average gas density in the chimney, kg/m3 g = acceleration due to gravity, 9.

81m/s2 In modern boilers, the fuel burning rate is high, and the rate of air supply as well as the flue gar removal is high. There are also various heat exchangers like superheater, reheater, economizer, and air preheater on the way to cause large pressure losses for which stacks alone are insufficient and fans are added for producing mechanical draught. Stacks have thus two functions: 1. To assist the fans in overcoming pressure losses. 2. to help disperse the gas effluent into the atmosphere at a sufficient he ight to cause minimum atmospheric pollution.

2. Mechanical draught:Mechanical draught is produced by fans. There are two types of fans in use today: forced draught (FD) and induced draught (ID) fans. When either one is used alone, it should overcome the total air and gas pressure losses within the steam ge

nerator. Forced draught fans are installed at inlet to the air preheater. They handle cold air. So they have less maintenance problems, consumes less power andtherefore, their operating costs are lo wer. For good reliability, two forced draught fans operating in parallel are normally used, each capable of undertakingat least 60% of full load air flow when the other is out of service. The forceddraught fan if used alone, as in many large steam genera tors and almost all marine applications, maintains the entire system up to the stack entrance under positive gauge pressure. Induced draught fans are normally located at the foot of the stack. They handle hot combustion gases. Their power requirements are, therefore, greater than FD fans. In addition they must cope with corrosive combustionproducts and fly ash. When both FD and ID fans are used in a steam generator, the FD fans push atmospheric air through the air preheater, dampers, various air ducts, and burners into the furnace, and the ID fan sucks out the flue gases thro

ugh the heat transfer surfaces in the superheater, reheater, economizer, gas-side air preheater and dust collectors and discharge into the stack. The stack because of its height, adds a natural driving pressure of its own.

Air preheater:

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An air preheater (APH) is a general term to describe any device designed to heat


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air before another process (for example, combustion in a boiler) with the primary objective of increasing the thermal efficiency of the process. The purpose ofthe air preheater is to recover the heat from the boiler flue gas which increases the thermal efficiency of the boiler by reducing the useful heat lost in theflue gas. As a consequence, the flue gases are also sent to the flue gas stack (or chimney) at a lower temperature, allowing simplified design of the ducting and the flue gas stack. It also allows control over the temperature of gases leaving the stack (to meet emissions regulations, for example).

Types of Boilers:

Boiler systems are classified in a variety of ways. They can be classified according to the end use, such as for heating, power generation or process requirements. Or they can be classified according to pressure, materials of construction,size tube contents (for example, waterside or fireside), firing, heat source orcirculation. Boilers are also distinguished by their method of fabrication. Sometimes boilers are classified by their heat source. For example, they are often referred to as oil- fired, gas- fired, coal- fired, or solid fuel fired boilers. Let us take a look at some typical types of boilers.

Fire tube boilers:Firetube boilers consist of a series of straight tubes that are housed inside awater-filled outer shell. The tubes are arranged so that hot combustion gases flow through the tubes. As the hot gases flow through the tubes, they heat the wat

er surrounding the tubes. The water is confined by the outer shell of boiler. Toavoid the need for a thick outer shell firetube boilers are used for lower pressure applications. Generally, the heat input capacities for firetube boilers arelimited to 50 mbtu per hour or less, but in recent years the size of firetube boilers has increased.

Firetube boilers are subdivided into three groups.28

Horizontal return tubular (HRT) boilers typically have horizontal, self-contained firetubeswith a separate combustion chamber.

Scotch, Scotch marine, or shell boilers have the firetubes and combustion chamber housedwithin the same shell.

Firebox boilers have a water-jacketed firebox and employ at most three passes ofcombustion gases.These boilers contain long steel tubes through which the hot gases from the furnace pass and around which the hot gases from the furnace pass and around which the water circulates. Firetube boilers typically have a lower initial cost, are more fuel efficient and are easier to operate, but2

they are limited generally to capacities of 25 tonnes per hour and pressures of17.5 kg per cm .

Watertube boiler:Watertube boilers are designed to circulate hot combustion gases around the outside of a large number of water filled tubes. In the older designs, the tubes were either straight or bent into simple shapes. Newer boilers have tubes with complex and diverse bends. Because the pressure is confined inside the tubes, watertube boilers can be fabricated in larger sizes and used for higher-pressure applications. Small water tube boilers, which have one and sometimes two burners, ar


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e generally fabricated and supplied as packaged units. Because of their size andweight, large water tube boilers are often fabricated in pieces and assembled in the field. In water tube or water in tube boilers, the conditions are reversed with the water passing through the tubes and the hot gases passing outside the tubes. These boilers can be of a single- or multiple-drum type. They can be builtto any steam capacity and pressures, and have higher efficiencies than firetubeboilers. Almost any solid, liquid or gaseous fuel can be burnt in a water tube boiler. The common fuels are coal, oil, natural gas, biomass and solid fuels suchas municipal solid waste (MSW), tire-derived fuel (TDF) and RDF. Coal- fired water tube boilers are classified into three major categories: stoker fired units,PC fired units and FBC boilers.

Package water tube boilers come in three basic designs: A, D and O type. The namesare derived from the general shapes of the tube and drum arrangements.

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All have steam drums for the separation of the stea m from the water, and one ormore mud drums for the removal of sludge. Fuel oil- fired and natural gas-firedwater tube package boilers are subdivided into three classes based on the geometry of the tubes. The A design has two small lower drums and a larger upper drum for steam- water separation. In the D design, which is the most common, the unit ha

s two drums and a large- volume combustion chamber. The orientation of the tubesin a D boiler creates either a left or right-handed configuration. For the O desig, the boiler tube configuration exposes the least amount of tube surface to radiant heat. Rental units are often O boilers because their symmetry is a benefit intransportation.

Electric boilers:Electric boilers can use electric resistance heating coils immersed in water andare normally very lowcapacity units. Other types of electric boilers are electrode-type units that generate saturated steam by conducting current through the water itself. Boiler water conductivity must be monitored and controlled. If theconductivity is too low, the boiler will not reach full operating capacity. Whenthe conductivity is too high, over-current protection will normally shut off th

e power. Proper conductivity and high-quality water as well as effective water treatment is required. Solids from the saturated steam tend to accumulate slowlyon the insulators supporting the electrodes from the grounded shell. The unit must be shut down periodically so that the insulators can be washed off to preventarcing. Finally, voltages of up to 16 kV may be used. Protection is needed forground faults, over-current and, for three-phase systems, loss of phase. The main electrical disconnect switch must be locked out before performing maintenanceon the boiler.

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45187825 Power Plant Instrumentation Unit 1