1 1.1 Introduction
A thermal power plant is an industrial facility for the generation of electric power. It is also termed as energy centre because it more accurately describes what the plants do, which is the conversion of other forms of energy like chemical energy, heat energy into electrical energy.
Energy exists in various forms i.e. mechanical, thermal, electrical etc... One form of energy can be converted into other by the use of suitable arrangements. Out of all these forms of energy, electrical energy is preferred due to the following advantages:-
Can be easily transported from one place to another. Losses in transport are minimum.
Can be easily subdivided. Economical in use.
Easily converted into other forms of energy.
Power is primarily associated with mechanical work and electrical energy. Therefore, power can be defined as the rate of flow of energy and can state that a power plant is a unit for production and delivery of a flow of mechanical and electrical energy. In common usages, a machine or assemblage of equipments that produce and delivers a flow of mechanical or electrical energy is power plant.
A thermal power station is a power plant in which the prime mover is steam driven. Water is heated, converted into steam and spins a steam turbine which drives an electrical generator. After it passes through the turbine, the steam is condensed in a condenser and recycled to where it was heated; this is known as Rankine Cycle. The greatest variation in the design of thermal power station is due to the different fossil fuel resources generally used to heat the water. Certain thermal power plants are also designed to produce heat energy for industrial purposes district heating or desalination of water, in addition to generating electrical power. Globally, fossil fuelled thermal power plant produce a large part of man-made CO2 emission to the atmosphere, and efforts to reduce these are many, varied and
widespread.
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 electric generators to produce Alternating Current (AC) electric power at a frequency of 50Hz (hertz, which is an AC sine wave per second).
1.2 Concept of Thermal Power Station
Thermal power plant converts energy rich fuel into electricity and heat. Possible fuels include coal, natural gas, petroleum products, agricultural waste and domestic waste. Other sources of fuel include landfill gas and bio gases. In some plants renewal fuels such as biogas are co-fired with coal.
2
Coal and lignite accounted for about 57% of India’s installed capacity. However, wind energy depends upon wind speed, and hydropower energy on water level, thermal power plant accounts for over 65% of India’s generated electricity, India’s electricity sector consumes about 80% of the coal product in the county.
India expects that its projected rapid growth in electricity generation over the next couple of decades is expected to be largely met by thermal power plant.
Fig.1.1 Total Installed Power Generation Capacity in India 1.3 History of Thermal Power Plant
The initially developed reciprocating steam engine has been used to produce mechanical power since the 18th century, with notable improvements being made by James Watt. When the first commercially developed central electrical power stations were established in 1882 at Pearl Street in New York and Holborn viaduct power station in London, reciprocating steam engines were used. The development of steam turbine in 1884 provided large and more efficient machine designs for central generating stations. By 1892 the turbine was considered a better alternative to reciprocating engines; turbines offered higher speeds, more compact machinery, and stable speed regulation allowing for parallel synchronous operation of generators on a common bus. After about 1905, turbines entirely replaced reciprocating engines in large central power stations.
The largest reciprocating steam engine-generator sets ever built were completed in 1901 for the Manhattan Elevated Railway. Each of seventeen units weighed about 500tonnes and was rated 6000kilowatts; a contemporary turbine set of similar rating would have weighed about 20% as much.
3 2.1 Introduction
A steam power plant converts the chemical energy of the fossil fuels (Coal, Oil and Gas) into mechanical/electrical energy. This is achieved by raising the steam in the boilers, expanding it through the turbines and coupling the turbines to the generator which convert mechanical energy to electrical energy.
Fig.2.1 Production of Electricity by Steam Power Plant. The following two purposes can be served by a steam power plant: To produce electric power.
To produce steam for industrial purposes besides producing electrical power. The steam may be used for varying industries such as textiles, food manufacturing, paper mills, sugar mills and refineries etc.
2.2 General Layout of Thermal Power Plant
The general layout of a thermal power plant consists of mainly four circuits. The four main circuits are:
1. Coal and ash circuit. 2. Air and gas circuit.
3. Feed water and steam flow circuit. 4. Cooling water circuit.
4
Fig.2.2 Layout of Thermal Power Plant.
A thermal power station using steam as working fluid basically works on the Rankine cycle. Steam is generated in a boiler, expanded in the prime mover and condensed in condenser and fed into the boiler again with the help of pump. However, in actual practice, there are numerous modifications and improvements in the cycle with the aim of affecting heat economy and to increase the thermal efficiency of the plant.
1. Coal and ash circuit. In this circuit, the coal from the storage is fed to the boiler through coal handling equipment for the generation of steam. Ash produced due to combustion of coal is removed to ash storage through ash-handling system.
2. Air and gas circuit. Air is supplied to the combustion chamber of the boiler either through F.D or I.D fan or by using both. The dust from the air is removed before supplying to the combustion chamber. The exhaust gases carrying sufficient quantity of heat and ash are passed through the air-heater where the exhaust heat of the gases is given to the air and then it is passed through the dust collectors where most of the dust is removed before exhausting the gases to the atmosphere through the chimney. 3. Feed water and steam circuit. The steam generated in the boiler is fed to the steam
prime mover to develop the power. The steam coming out of prime mover is condensed in the condenser and then fed to the boiler with the help of pump. The condensate is heated in the feed-heaters using the steam tapped from different points of the turbine. The feed heaters may be of mixed type or indirect heating type. Some
5
of the steam and water is lost passing through different components of the system; therefore, feed water is supplied from external source to compensate this loss. The feed water supplied from external source is passed through the purifying plant to reduce the dissolved salts to an acceptable level. The purification is necessary to avoid the scaling of the boiler tubes.
4. Cooling water circuit. The quantity of cooling water required to condensate the steam is considerably large and it is taken either from lake, river or sea. The cooling water is taken from the upper side of the river; it is passed through the condenser and discharged to the lower side of the river. Such system of cooling water supply is possible if adequate cooling water is available throughout the year. This system is known as open system. When adequate water is not available, then the water coming out from the condenser is either cooled in the cooling pond or cooling tower. The cooling is affected by partly evaporating the water. This evaporative loss is nearly 2 to 5% of the cooling water circulated in the system. To compensate the evaporative loss, the water from the river is continuously supplied. When the cooling water coming out of the condenser is cooled again and supplied to the condenser, then the system is known as closed system. When the water coming out of the condenser is discharged to river downward side directly, the system is known as open system.
2.3 Working Principle of Thermal Power Plant
A thermal power station works on the basic principle that heat liberated by burning fuel is converted into mechanical work by means of a suitable working fluid. The mechanical work is converted into electric energy by the help of generators.
Steam is generated in the boiler of the thermal power plant using the heat of the fuel burned in the combustion chamber. The steam generated is passed through steam turbine where part of its thermal energy is converted into mechanical energy which is further used for generating electric power. The steam coming out of the steam-turbine is condensed in the condenser and the condensate is supplied back to the boiler with the help of the feed pump and the cycle is repeated.
The function of the boiler is to generate the steam. The function of the condenser is to condensate the steam coming out of steam turbine at low pressure. The function of the steam turbine is to convert part of heat energy of steam into mechanical energy. The function of pump is to raise the pressure of the condensate from the condenser pressure (0.015 bars) to boiler pressure (8 bars). The other components like economiser, superheater are used in the primary circuit to increase the overall efficiency of the thermal power plant.
6
Fig.2.3 Rankine Cycle.
The working fluid in a Rankine cycle follows a closed loop and is reused constantly. The water vapour with condensed droplets often seen billowing from power stations is created by the cooling systems (not directly from the closed-loop Rankine power cycle) and represents the means for (low temperature) waste heat to exit the system, allowing for the addition of (higher temperature) heat that can then be converted to useful work. This 'exhaust' heat is represented by the "Qout" flowing out of the lower side of the cycle.By condensing the
working steam vapour to a liquid the pressure at the turbine outlet is lowered and the energy required by the feed pump consumes only 1% to 3% of the turbine output power and these factors contribute to a higher efficiency for the cycle.
7
The different processes of the Rankine cycle are described below:
1. The point‘d’ represents the water at condenser pressure p2 and corresponding
saturation temperature T2. The process ‘de’ represents the adiabatic compression of
water by the pump from condenser pressure to boiler pressure. There is slight rise in temperature of water during the compression process.
2. During the process ‘ea’ and ‘ab’, heat is supplied by the boiler to the water to convert into steam. The process ‘ea’ represents the supply of heat at constant pressure till the saturation temperature of water is reached corresponding to boiler pressure. The process ‘ab’ represents the addition of heat to the water at constant pressure till the water completely converts into steam. The final condition of steam may be wet, dry saturated or super heated depending upon the quantity of heat supplied by the boiler. 3. The process ‘bc’ represents the isentropic expansion of steam in the prime mover.
During this expansion process, external work is developed and the pressure of steam falls from p1 to p2 and its temperature will be T2.
4. The process ‘cd’ represents the condensation of steam coming out from the prime mover in the condenser. During the condensation of steam, the pressure is constant and there is only change of phase from steam to water as the latent heat of steam is carried by circulating water in the condenser. Again the process ‘de’ represents the adiabatic compression of water by the pump from the pressure p2 to p1 and the cycle is
repeated.
Let hb = enthalpy of steam per kg at point ‘b’
hc = enthalpy of steam per kg at point ‘c’
vw = specific volume of water at point 1 or 2 as there is much change in
specific volume during this process
hfe = enthalpy of water per kg at point ‘e’
hfa = enthalpy of water per kg at point ‘a’
hfd = enthalpy of water per kg at point ‘d’
Total heat supplied by the boiler per kg of steam generated = hb - hfe
= hb – (hfd + wp)
Where wp is the work done by the pump per kg of water supplied.
Work done per kg of steam in the prime mover = hb - hc
Work done by the pump per kg of water supplied to the boiler wp = [vf2 (p1 – p2)] J/kg where p is in N/m2
8 Net work available per kg of water
= (hb – hc) – vfa
– = (hb – hc) – wp
The Rankine efficiency of the cycle is given by ɳr =
= [(hb – hc) – wp] / [hb – (hfd + wp)]
The pump work is always neglected for all practical purpose as it is very small compared with other heat quantities.
9 3.1 Selection of Fuel
1. Solid Fuel
It is not so simple to suggest the general trend of suitability of coals for steam generation. The firing qualities of coal are very important when combustion equipment is being considered.
The slower burning coal of low volatile content generated high-bed temperature and therefore requires forced draught. The high fuel bed temperature may damage the grate unless it is protected by adequate ash.
The fast burning coals of high volatile content require large combustion chambers for the combustion of the volatiles. Such coals are more suitable for meeting sudden demand for steam because the liberation of combustible volatile gas burns rapidly than the solid fuel on the grate.
The most important factors which are considered for the selection of coal are the sizing, caking, swelling properties and ash fusion temperature. The sulphur content in the coal also carries considerable importance in most of the cases.
Electro-static preceptors work (ESP) better with high sulphur coal because of improved resistivity of the flue gases. However, for other systems, a little SO2 can raise the
acid DPT dramatically and this raise can retard the corrosive effects on the equipments. The larger size coal should be used when the draught is low and some moisture percentage must be essentially maintained if the percentage of fineness of coal is high. The use of anthracite coal as fuel requires forced draught furnaces incorporating means for admitting steam to cool the fire bars and hardened clinker.
2. Liquid Fuel
The liquid fuel is used in thermal power plants to generate the steam instead of coal as it offers many advantages over coal as listed below:
1) Excess air required for complete combustion is less as uniform mixing of fuel and air is possible.
2) The storage and handling is much easier compared with coal. 3) The changes in load can be met easily and rapidly.
4) There is no problem of ash disposal. 5) The system is very clean.
6) The operational labour required is less and therefore overheads are considerably less.
All the commercially used liquid fuels are furnished by petroleum and its by-products. The petroleum or the crude oil consists of 83-87% carbon, 10.14% hydrogen and various percentages of sulphur, nitrogen, oxygen and metallic derivatives.
10
The fuel oils used for industrial or domestic purposes are obtained by refining the crude oil. The refining process separates and recombines the hydro-carbons into specialised products like gasoline, fuel oil, etc. The distillation process is generally used to separate into different groups of fuel. The typical fractions from light to heavy are naptha, gasoline, kerosene and gas oil and the remainder is heavy fuel oil which is commonly used for steam generation.
Constituent Carbon Hydrogen Sulphur Oxygen Nitrogen Moisture
Percentage 84 22 2 1 0.5 3.5
Table.3.1 Fuel Oil Analysis.
The important properties of liquid fuel considered are specific heat, viscosity, pour points, flash point, volatility, carbon residue, heating value, ash, moisture, sediments and sulphur.
The pour point indicates the case of handling the oil flow through the lowest temperature at which the oil will flow under specified conditions.
The viscosity is the measure of resistance to the oil flow through the pipes and nozzles. This affects the cost of pumping the fuel. The viscosity of the fuel oil is generally determined by standard viscometer.
The flash point of fuel oil decides the safety of fuel and it is also an indication of ease of ignition.
The percentage of sulphur should be as minimum as possible as it results in the corrosion of different parts in the plants and reduces the life.
For better combustion of fuel, the moisture and sediments must be as small as possible. The ash content in the fuel oil is not very important in steam power plant.
As the fuel oil contains more percentage of hydrogen as compared to coal, therefore, the moisture carried by the gas per kg on fuel burned is considerably more. This results in overall lower combustion efficiency of the plant as compared to coal burning.
The use of oil for steam generation has no scope in India due to limited resources of oil which are badly needed for industrial and transport purposes.
3. Gaseous Fuel
The gaseous fuel may be either natural or manufactured. The manufactured gas is costly, therefore, only natural gas is used for steam generation.
The natural gas generally comes out of gas wells and petroleum wells. It contains 60.95% of methane with small amounts of other hydrocarbons such as ethane, napthene and aromatic, CO2 and nitrogen. The natural gas is carried through pipes to distances which are
11
The natural gas is colourless, odourless and non-poisonous. Its C.V. lies between 25000kJ to 50,000 kJ/m3 according to percentage of methane in gas.
The various manufactured gases are coal gas, coke oven gas, blast furnace gas, water gas and producer gas. The coal gas and coke oven gas are produced by carbonizing high volatile bituminous coal. The blast furnace gas is produced as by-product from blast furnace used in steel industry. The water gas is produced by passing steam and air through a bed of incandescent carbon.
The gaseous fuels have all the advantages of oil fuels except ease of storage. The major disadvantage of using natural gas as fuel is that the power plant must be located near the natural gas field otherwise transportation and cost of transportation play an important part in selecting the fuel for thermal power plant.
3.2 Coal Handling, Storage & Feeding
The coals from the coal mines to the power station are transported by sea or river or rail or road. The supply of coal by road is limited to a small capacity power plant and this mode of handling the coal does not play much important part in modern capacity power plant. a) Transportation by Sea or River. If the power plant is situated on the bank of river or near the sea shore, it is often economical to transport the coal by ships. The coal brought by the ships is unloaded mechanically by cranes at the site of the power plant. The unloaded coal form the ship is either sent to storage yard or directly to the conveyer system which carries coal directly to the combustion chamber hopper. b) Transportation by Rail. The transportation of coal by rail is the most important
means of transportation in common use. The coal supply to Indian power plants is mainly by rail as unfortunately river transportation is not available. This mode of transport plays very important role for power stations which are located interior. A railway siding line is taken to the power station and coal is either delivered to the storage yard or close to the point of consumption.
c) Transportation by Ropeways. This is very efficient method of transporting the coal from the mine to the power station. This is particularly used when the distance between the mine and power station is less than 10 kilometres. The major advantage of this system is, it supplies the cola continuously and free from workers’ strike which is common with rail transport.
d) Transportation by Road. The transportation of coal by road is used only for small capacity plants. The major advantage of road transport is that the coal can be carried directly into the power house upto the point of consumption. This is better system for small capacity plants as traffic restrictions are comparatively less.
The selection of proper method of coal supply from the coal mines to the power stations depends upon the system capacity in tonnes per hour, location of the plant with respect to rail or water facilities available and location of available outside storage and overhead coal bunkers.
12
e) Transportation of Coal by Pipeline. The power demand throughout the world is increasing quite faster and coal is going to be the only fuel to run the thermal plants as liquid fuel prices are escalating day by day. In India it is expected that the power demand will be doubled by the end of the century and it is physically impossible for the railway to haul the required coal to the coming-up power plants.
Transportation of coal by pipeline is considered most speedy method among all available. Pipe lining of coal slurries from remote mines to strategically located generating plants shows great promise for future development.
The pipeline coal transport system offers many advantages as listed below: i. It is a continuous transport system unaffected by the climate and weather. ii. It is capable of transporting very large quantities of coal.
iii. It has high degree of reliability and safety as the moving machines are limited to the stationary pumping and boosting stations.
iv. It is easy to carry the pipeline through difficult terrain like hills, valleys and swamps compared with other modes of transport.
v. Man-power required is low and maintenance charges are also low. vi. Loss of coal during transport due to theft totally eliminated.
vii. Requirement of large areas as in the case of railway system for site dumping and storage is eliminated.
viii. It produces the least environmental disturbance as noise and dust problem and traffic congestion is drastically reduced.
ix. It provides simplicity in installation and increased safety in operation.
x. The impact of inflation on the operating cost is less than other modes of transport. Some of the disadvantages of the system are listed below:
i. It requires large quantity of water as 1 kg of coal requires one kg of water.
ii. Preparation of coal at the pumping terminal as well as dewatering and recovery of the coal at the delivery terminal requires high capital and operating cost.
iii. Consumer must be able to use coal with added surface moisture (10%). This also results in some loss in the useful heat of coal.
3.3 Storage of Coal at Plant Site
The purpose of coal storage is twofold. First, fuel storage is an insurance against complete shutdown of a power plant occurring from failure of normal supplies. Second, the storage permits choice of the date of purchase allowing the management to take advantage of seasonal market condition. Storage of coal protects the plant failure in case of coal strikes, failure of the transportation system and general coal storages.
The storage of coal is undesirable, because it costs more as there is risk of spontaneous combustion, possibility of loss and deterioration during storage, interest on capital cost of coal laying dormant, cost of insurance, handling cost required by storage and
13
reclamation, cost of area required, cost required to protect the stored coal from deterioration and many others. With all this disadvantages of coal storage, it is more important to public service stations as light and power have become vital and essential in every day domestic and industrial life.
To store the coal containing high sulphur is more troublesome because local heating further aggravates the reaction between sulphur, air and water causing rapid deterioration of coal and increases the chances of spontaneous combustion. Lower rank coals (lignite and bituminous) have a higher tendency to ignite spontaneously compared with high rank coals (anthracite and graphite).
The coal is stored by using one of the following methods to reduce the chances of oxidation and combustion:
1. Stocking the Coal in Heaps. The ground used for stocking should be dry and level. Generally concrete floored are is used to prevent the flow of air from the bottom. The coal is piled at height of 10m to 12m. During storage of coal in heaps, the coal should be compacted in layer of 15 to 30 cm in thickness by means of bulldozers and rubber-tired scrapers. This effectively prevents the sir circulation in the interior of the pile. Another method of removing the heat of oxidation is, the air is allowed to move through the layers evenly so that the heat of reaction is carried away and the temperature of coal is maintained below the combustion temperature (70˚C.).
2. Under-Water Storage. The possibility of slow oxidation and spontaneous combustion can be completely eliminated by storing the coal under water. The dock basins can be used for storage of coal under water.
The following points should be kept in mind during selecting the site for storage and piling.
i. The storage area should be free from standing water.
ii. The artificial drainage in the storage area should be provided if well drained area is not available.
iii. The storage area should be cleared of all foreign matter such as wood, paper, rags, waste oil or materials having a low ignition temperature.
iv. The storage site should be selected in such a way that the handling cost is minimum.
v. The piles should be built-up in successive layers and as far as possible compact.
vi. The piles should be dressed to prevent rain from penetrating into pile. vii. Alternate wetting and drying of coal are undesirable.
viii. Storage on hot bright days should be avoided.
ix. A provision for temperature check at different point should be made. x. A conical piling should be avoided.
14 4.1 Introduction
Steam is mainly required for power generation, process heating and space heating purposes. The capacity of the boilers used for power generation is considerably large compared with other boilers.
Due to the requirement of high efficiency, the steam for power generation is produced at high pressures and in very large quantities. They are very large in size and are of individual design depending on the type of fuel used.
A steam generator popularly known as boiler is a closed vessel made of high quality steel in which steam is generated from water by the application of heat. The water receives heat from the hot gases through the heating surface of the boiler. The hot gases are formed by burning fuel, may be coal, oil or gas. Heating surface of the boiler is that part of the boiler which is exposed to hot gases on one side and water or steam on the other side. The steam which is collected over the water surface is taken from the boiler through super heater and then suitable pipes to turbine. Usually boilers are coal or oil fired.
According to American Society of Mechanical Engineers (A.S.M.E.) a ‘steam generating unit’ is defined as:
“A combination of apparatus for producing, furnishing or recovering heat together with the apparatus for transferring the heat so made available to the fluid being heated and vaporised”.
The steam generated is employed for the following purposes: i. For generating power in steam engines or steam turbines.
ii. In the textile industries for sizing and bleaching etc. and many other industries like sugar mills; chemical industries.
iii. For heating the building in cold weather and for producing hot water for hot water supply.
The primary requirements of boiler are: i. The water must be contained safely.
ii. The steam must be safely delivered in desired condition. 4.2 Classification of Boiler
The boilers may be classified as follows: 1. Horizontal, Vertical or Inclined
If the axis of the boiler is horizontal, the boiler is called as horizontal, if the axis is vertical, it is called vertical boiler and of the axis is inclined it is known as inclined boiler. The parts of a horizontal boiler can be inspected and repaired easily but it occupies more spaces. The vertical boiler occupies less floor area.
15
Fig.4.1 Horizontal Boiler. 2. Fire Tube And Water Tube
In the fire tube boilers, the hot gases are inside the tubes and the water surrounds the tubes. Examples: Cochran, Lancashire and Locomotive boilers.
In the water tube boilers, the water is inside the tube and hot gases surround them. Examples: Babcock and Wilcox, Stirling boiler etc.
Fig.4.2 Fire Tube Boiler. 3. Externally Fired And Internally Fired
The boiler is known as externally fired if the fire is outside the boiler shell. Examples: Babcock and Wilcox, Stirling boiler.
In case of internally fired boilers, the furnace is located inside the boiler shell. Example: Cochran, Lancashire boiler etc.
16 4. Forced Circulation And Natural Circulation
In forced circulation type boilers, the circulation of water is done by a forced pump. Examples: Velox, Lamont, Benson boiler etc.
In natural circulation type boilers, circulation of water in the boiler takes place due to natural convention currents produced by the application of heat. Examples: Lancashire, Babcock and Wilcox boiler etc.
Fig.4.4 Forced Circulation and Natural Circulation Boiler. 5. High Pressure And Low Pressure Boiler
The boiler which produce steam at a pressure of 80 bar and above are called high pressure boiler. Examples: Babcock and Wilcox, Velox, Lamont, Benson boilers.
The boilers which can produce steam at a pressure below 80 bars are called low pressure boilers. Examples: Cochran, Cornish, Lancashire and locomotive boilers.
6. Single Tube And Multi-Tube Boiler
The fire tube boilers are classified as single tube and multi-tube boilers, depending upon whether the fire tube is one or more than one. The examples of former type are Cornish, simple vertical boiler and rest of the boilers are multi-tube boilers.
17 4.3 Selection of a Boiler
The following factors should be considered while selecting a boiler: i. The working pressure and quality of steam required. ii. Steam generation rate.
iii. Floor area available.
iv. Accessibility for repair and inspection. v. Comparative initial cost.
vi. The probable load factor. vii. The fuel and water available. viii. Operating and maintenance cost. 4.4 Performance of Boiler
Evaporative Capacity
Performance of boiler is expressed in terms of evaporative capacity which is defined as the amount of water evaporated or steam produced in kg per hour.
Boiler Efficiency
Boiler efficiency is the ratio of heat actually utilised in generation of steam to the heat supplied by the fuel in the same period.
i.e., boiler efficiency = [ma (h - hf1)]/C
Where, ma = mass of water actually evaporated into steam per kg of fuel at the
working pressure,
h = enthalpy of steam per kg under the generating condition, hf1 = specific enthalpy of water at a given feed temperature, and
C = calorific value of fuel in kJ/kg.
If the boiler, economiser, and superheater are considered as a single unit, then the boiler efficiency is termed as overall efficiency of the boiler plant.
4.5 Boiler Mountings 4.5.1 Introduction
Different fittings and devices necessary for the operation and safety of a boiler are known as boiler mountings. The safety valve, water level indicator, and the fusible plug are the devices used for safety operation of the boiler. The pressure gauge, feed check valve, blow-off cock and steam stop valve fall under the category of fittings and these are essential for the operation of the boiler.
18 4.5.2 Safety Valve
When there is a sudden drop in steam requirements, the steam pressure in the boiler will increase. The main function of a safety valve is to prevent under such a condition, an increase in the steam pressure in the boiler exceeding a predetermined, maximum pressure for which the boiler is designed. This is automatically done by opening of the valve and discharging the steam to the atmosphere as soon as the pressure inside the boiler increases above the predetermined value. The safety valves are directly placed on the top of the boiler shell.
Spring Loaded Safety Valve
This type of safety valve is commonly used now-a-days for stationary as well as mobile boilers. It is loaded with spring instead of weights. The spring is made from a square steel rod in helical form.
Spring loaded safety valve consists of two valves, each of which is placed over a valve seat fixed over a branch pipe. The two branch pipes are connected to a common block which is fixed on the shell of the boiler. The lever has two pivots each of which is placed over each respective valve. The lever is attached with a spring at its middle which pulls the lever in downward direction. The lower end of the spring is attached to the back. Thus the vales are held tight to their sates by the spring force.
Fig.4.6 Safety Valve.
These valves are fitted against the spring when the steam pressure is greater than the working pressure and allows the steam to escape from the boiler till the pressure in the boiler reaches its working pressure. The lever has an extension which projects into the driver’s cabin. The driver can release the pressure if required just by raising the lever. The lever is connected loosely by a link to the block. This limits the valve opening and prevents the lever blowing off in case of spring failure.
19 4.5.3 Water Level Indicator
It is an important fitting which indicates water level inside the boiler to the observer. Usually two water level indicators are fitted in front of the boiler. The water indicator shows the level or water in the boiler drum and warns the operator if by chance the water level goes below a fixed mark, so that corrective action may be taken in time to avoid any accident.
4.5.4 Pressure Gauge
A pressure gauge is used to measure the pressure of steam inside the boiler. The commonly used pressure gauge is known as Bourdon type pressure gauge. It consists of an elastic metallic type of elliptical cross-section and is bent in the form of circular arc. One end of the tube is fixed and connected to the steam space of the boiler and other end is connected to a sector wheel through a link. The sector remains in mesh with a pinion fixed on a spindle to read the pressure on a dial gauge.
Fig.4.7 Pressure Gauge.
When high pressure steam enters the elliptical tube, the tube section tries to become circular which causes the other end of the tube to move outward. The movement of the closed end of the tube is transmitted and magnified by the link and sector. The magnitude of the movement is indicated by the pointer on the dial.
4.5.5 Fusible Plug
The main objective of the fusible plug is to put off the fire in the furnace of the boiler when the water level in the boiler falls below an unsafe level and thus avoids the explosion which may take place due to overheating of the tubes and shell. This plug is generally fitted over the crown of the furnace or over the combustion chamber.
20
Under normal water level condition in the boiler, this plug is covered with water which keeps the temperature of the fusible metal below its melting point. But when the water level in the boiler falls low enough to uncover the plug; the fusible metal between the plug quickly melts and drops out. The opening so made allows the steam to rush the water into the furnace and extinguish the fire. The steam rushing out puts out the fire and gives warning that the crown of the furnace is in danger of being overheated.
4.5.6 Feed Check Valve
The function of the feed check valve is to allow the supply of water to the boiler at high pressure continuously and to prevent the back flow of water from the boiler when pump pressure is less than boiler pressure or when pump fails.
Fig.4.9 Feed Check Valve.
It is fitted to the shell slightly below the normal water level of the boiler. The lift of the non-return valve is regulated by the end position of the spindle which is attached with the hand wheel. The spindle can be moved upward or downward with the help of hand wheel as the upper portion of the spindle is screwed to a nut.
At normal working condition, the non-return valve is lifted due to the pressure of water from the pump and the water is fed to the boiler. But when the pump pressure falls below boiler pressure or if the pump stops, non-return valve is closed automatically due to the pressure of the steam from the boiler and prevents the escape of water form the boiler.
4.5.7 Blow-Off Cock
The blow-off cock used for dual functions:
1. To empty the boiler when necessary for cleaning, repair and inspection.
2. To discharge the mud and sediments carried with the feed water and accumulated at the bottom of the boiler.
21
By periodic blow-off, the salt concentration in the boiler is also reduced. Even with a small amount of dissolved salt, over a period of time, due to the evaporation of water, the salt accumulates in the boiler, raising the salt concentration.
It is fitted to the lowest part of the boiler either directly with the boiler shell or to a pip[e connected with the boiler.
Fig.4.10 Blow-Off Cock.
It consists of a conical plug fitted accurately into a smaller casing. The plug has a rectangular opening which may be brought with the line of the passage of the casing by rotating the plug. This causes the water to be discharged from the boiler. The discharging of water may be stopped by rotating the plug again.
The blow-off cock should be operated only when the boiler is on if the sediments are to be removed. This is because; the sediments are forced out quickly due to the high steam pressure in the boiler.
4.5.8 Steam Stop Valve
It is the largest valve on the steam boiler and usually fitted to the highest part of the boiler shell. The function of the stop valve is to regulate the flow of steam from the boiler to the turbine as per requirement and shut off the steam flow when not required.
22
The main body is made of cast steel. The valve, valve seat and the nut through which the valve spindle works, are made of brass for smooth working. The spindle is passed through a gland to prevent the leakage of steam. The spindle is rotated by means of hand wheel. Due to the rotation of hand wheel, the valve may move up or down and it may close or open the passage fully or partially for the flow of the steam.
4.6 Boiler Accessories 4.6.1 Introduction
Accessories are the auxiliary plants required for steam boilers for their proper operation and for the increase of their efficiency. Water feeding equipments, air-preheater, economisers and super heaters are some of the essential accessories of the boiler.
In the present age of costly fuel, it has become necessary to conserve the fuel by utilizing the wasted energy to the atmosphere. This is done in all modern power plants by incorporating economiser and air preheater. By increasing the temperature of feed water passing through the economiser using waste heat of gas, the quantity of heat given per kg of steam generated in the boiler is reduced. Similarly, the temperature of air is also increased by passing through the air preheater using remaining waste energy of the gases. The preheated air increases the combustion efficiency in the furnace and reduces the fuel loss. In both equipments, the quantity of fuel is reduced by extracting the heat from the exhaust gases.
The common equipments used in thermal power plants to increase the thermal efficiency are economisers, and air pre-heaters. The heat carried with the flue gases is partly recovered in air-preheater and economiser and reduces the fuel supplied to the boiler. The preheating of air with gases increases the combustion efficiency and reduces the fuel consumption.
The adoption of one or both equipments depends upon the economical justification. It is also equally essential to maintain the performance of these equipments by preventing corrosion and fouling from inside and outside; otherwise the gain from these equipments reduces rapidly with respect to time. The corrosion is generally prevented by using proper materials for the equipments and controlling the flue gas temperature to avoid the condensation of corrosive gases carried by the exhaust gases.
4.6.2 Economisers
An economiser is a device used for heating the feed water by means of flue gases from boiler. The economiser usually extracts the waste heat of the chimney gases to preheat the water before it is fed into the boiler.
A boiler producing between 10 to 100tonnes of steam per hour and operating at 30% or more loads should be evaluated for possible retrofitting with an economiser. The cost benefits depend upon the boiler size; type of fuel used and exhausts gas temperature. It has been estimated that about 1% fuel can cost can be saved for every 6˚C rise in temperature of
23
Fig.4.12 Return Bend Economiser.
the boiler feed water. Saving upto maximum 20% can be achieved by incorporating economiser where boiler operates very effectively.
Fig.4.13 Flue Gas Temperature Entering The Economiser On Fuel Saving.
When more heat is available, that can be used in increasing the sensible heat of the feed water or pass it through an air heater. However, in most economisers, the feed water is not heated higher than to within 25˚C of the temperature corresponding to the saturation temperature of steam in the boiler thus preventing steam formation in the economiser.
A water temperature of 85˚C in the hot well is the maximum at which the feed pump works satisfactorily, as there is slight negative pressure on the suction side of the pump. At temperature over 85˚C, steam bubbles begin to form and the boiler feed pump will not be able to pump steam and water flow stops. Therefore, the feed water is pumped through and heated in the economiser. Since it is on the pressure side of the pump, the water can be heated to a much higher temperature than the hot well temperature. The maximum temperature to which water can be heated in the economiser is 25˚C below steam forming temperature in the boiler. The following table gives the maximum temperature for varying pressures and the possible fuel savings for different hot well temperatures.
24 Pressure in the boiler in (Kgf/cm2) Saturation temp. in the boiler in (˚C) Max economiser outlet temp. in ( ˚C)
Percentage fuel saving using economiser with different hot well
temperature.
45˚C 65˚C 85˚C
8 174.5 149.5 17 14 10
13 194.1 169 20 17 14
18 208.8 183 23 19 16
Table.4.1 Maximum Pressure for Varying Pressures and the Possible Fuel Savings for Different Hot Well Temperatures.
Design Requirement for an Economiser. The design requirements must satisfy the following conditions:
The heat transfer surface should be minimum.
It must be able to extract maximum possible heat from exhaust gases.
The height of the tube banks should be minimum so the cleaning on load can be done effectively.
The gas side pressure loss should be minimum to reduce the running expenses of I.D. fans.
There must be uniform water flow to avoid the steam formation in the economiser. The pressure loss of water side must be also minimum to reduce the running expenses of the pump.
There must be connection from steam and water drum to the economiser inlet header, to permit the free circulation of water around the economiser to prevent the overheating and boiling during the period when there is no feed-flow during early pressure rising stages.
Types of Economisers. Basically there are two types of economisers:
1) Plain Tube Economiser. Plain tube types are generally used under natural draught condition. The tubes are made of cast iron to resist corrosive action of the flue gases and their ends are pressed into top and bottom headers.
An economiser consists of a group of these cast iron tubes located in the main flue between the boiler and the chimney. The waste flue gases flow outside the economiser tubes and heat is transferred to the feed water flowing inside the tubes. The external surfaces of the tubes are continuously cleaned by soot scrapers moving up and down. 2) Gilled Tube Type Economiser. A reduction in economiser size together with
increase in heat transmission can be obtained by casting rectangular gills on the bare tube walls. Cast iron gilled tube economiser can be used upto 50bar working pressure and such economisers are indigenously available. At higher pressure steel tubes are used instead of cast-iron gilled sleeves are shrunk to them.
25
Corrosion of Economiser and Its Prevention. The corrosion and its prevention are very important for safe and efficient working of the economiser. Internal and external corrosion are the primary enemies of an economiser and dissolved O2 and CO2 are the major culprits.
A properly designed deaerator, combined with water treatment plant, virtually eliminates internal corrosion in the economiser tubes. Deaeration removes 95% dissolved O2 and CO2
from the feed water. Vigorous steam scrubbing with chemical assist should follow deaeration to ensure complete O2 removal and corrosion control.
CO2 forms carbonic acid (H2CO3) when it dissolves in water. This compound is unstable
and ionizes into H2 ion (H+) and bicarbonate radical (H CO3-). The H CO3- further ionizes to
form the H+ ion and carbonate ion (CO3 -). The H2CO3 is the only one that exerts gas
pressure; therefore, CO2 must be removed by deaeration at low pH levels.
NH3 gas forms NH4 OH (ammonium hydroxide) upon dissolving in water. NH4 OH
ionizes to form NH4+ and OH- ions. Therefore, NH4 OH is responsible for exerting gas
pressure and it must be removed by deaeration at higher pH.
The pH value of water passing through the economiser should be maintained between 8 and 9 to reduce its effect of acid. CO2 removal is achieved at low pH and NH3 removal is
achieved at high pH, therefore complete degasification of flow containing combination of two is very difficult to achieve through deaeration alone.
Advantages of Economiser. There are several indirect advantages obtained by installing an economiser with a boiler plant as listed below:
1) The feeding of the boiler with water at a temperature near the boiling point reduces the temperature differences in the boiler, prevents the formation of stagnation pockets of the cold water and thus reduces greatly the thermal stress created in the pressure part of the boiler and the boiler and promotes better internal circulation.
2) When the feed water is not as pure as it should be, the temporary hardness is deposited on the inside of the economiser tubes and while this necessitates internal cleaning of the economiser, the evil is not as great as internal cleaning of the boiler.
3) Due to the reduction in the combustion rate of the furnace, the boiler will be more efficient and the actual fuel saving will be greater than the theoretically calculated.
4) The flow of flue gases over the economiser tubes acts indirectly as a grit arrester and large portion of the soot and fly-ash is deposited on the tubes and scraped off into the soot chamber. This reduces the omission of soot and fly-ash.
26 4.6.3 Air Preheater
Air preheater, recovers some portion of the waste heat of the flue gases. Air supplied to the combustion chamber is preheated by using the heat in the waste flue gases. Airs preheater is placed after the economiser and before the gases enter the chimney.
The heat carried with the flue gases coming out of the economiser is further utilised for preheating the air before supplying to the combustion chamber. It has been found that an increase of 20˚C in the air temperature increases the boiler efficiency by 1%.
The air heater is not only considered in terms of boiler efficiency in modern power plants, but also as a necessary equipment for supply of hot air for drying the coal in pulverised fuel systems to facilitates grinding and satisfactory combustion of fuel in the furnace.
The use of preheater is much economical when used with pulverised fuel boilers because the temperature of flue gases going out is sufficiently large and high air temperature is always desirable for better combustion.
Air heaters are usually installed on steam generators that burn solid fuels but rarely on gas or oil fired units. By contrast, economisers are specified for most boilers burning liquid or gas or coal whether or not an air heater is provided.
The principal benefits of preheating the air are: I. Improved combustion.
II. Successful use of low grade fuel. III. Increased thermal efficiency. IV. Saving in fuel consumption.
V. Increased steam generation capacity of the boiler.
The air preheater are generally divided into two groups as recuperative and regenerative type.
The recuperative heaters continuously transfer the heat from hot gases to cold air. The regenerative heater alternately gets heated and cooled by hot gases and cold air. Unlike the recuperative type, the regenerative is discontinuous in action and operates on cycle. In rotary regenerative type, the cyclic action applies to the heating and cooling of an individual element of the surface but the flowing steam of air receives heat continuously.
The two recuperative types of heat-exchangers which are commonly used for air-heating are described below:
Tubular Air-Heater. The flue gases flow through the tubes and air is passed over the outer surface of the tubes. The horizontal baffles are provided to increase time of contact which will help for higher heat transfer. The steel tubes 3 to 10 m in height and 6 to 8 cm in diameter are commonly used.
27
Fig 4.14 Tubular Air-Heater.
Plate Type Air-Heater. It consists of rectangular flat plates spaced from 1.5 to 2.5 cm apart leaving alternate air and gas passage. This type of air-heater is not used in modern installation as it is more expensive both as to flat cost and maintenance cost compared with tubular air-heaters.
Regenerative Heat Exchangers. The transfer of heat from hot gases to cold air is divided into two stages. In the first stage, the heat of the hot gases flowing through the heat exchanger is transferred to the packing of the heater and it is accumulated in the packing and the hot gases are cooled to sufficiently low temperature before exhaust to atmosphere. This stage is referred to as ‘Heating period’. In the second stage, the cold air is passed through the hot packing where the heat is accumulated and the heat from the packing is transferred to the cold air. This stage is known as ‘Cooling period’.
4.6.4 Superheater
The function of the superheater in the thermal power plant is to remove the last traces of moisture (1 to 2%) from the saturated steam coming out of boiler and to increase its temperature sufficiently above saturation temperature. The superheating raises overall cycle efficiency as well as avoids too much condensation in the last stages of the turbine which avoids the blade erosion.
The heat of combustion gases from furnace is utilised for the removal of moisture from steam and to superheat the steam. Super heaters usually have several tube circuits in parallel with one or more return bends, connected between headers.
28
Fig.4.15 Superheater. Superheated steam has the following advantages:
I. Steam consumption of the turbine is reduced.
II. Losses due to condensation in the cylinder and the steam pipe are reduced. III. Erosion of turbine blade is reduced.
IV. Efficiency of the steam plant is increased. Types of Superheater
There are two types of super heaters: 1. Convective superheater 2. Radiant superheater
Convective superheater makes use of heat in flue gases whereas a radiant superheater is placed in the furnace and a wall tube receives heat from the burning fuels through radiant process. The radiant type of superheater is generally used where a high amount of superheat temperature is required.
Heat from the hot gases to the vapour in the superheater is transferred at high temperatures. Therefore primary section of superheater is arranged in counter flow and secondary section in parallel flow to reduce the temperature stressing of the tube wall. The metal used for superheat tubes must have high temperature strength, high creep strength and high resistance to oxidation as superheater tubes get rougher service than water wall of the modern boilers. Carbon steels (510˚C) and chromium-molybdenum alloys (650˚C) are commonly used for superheater tubes.
The superheater tubes are subjected to corrosion when they are exposed to oxidising and reducing conditions alternately. This destroys the protective oxide film and exposes the metal surface open to further corrosion. The alkali deposits formed also have corrosion effect
29
on steel depending upon its temperature and composition. Low chromium ferritic steels confer some corrosion resistance but marked resistance is obtained by the use of austenitic alloys.
4.6.5 Steam Separator
The steam available from a boiler may be wet, dry; or superheated; but in many cases there will be loss of heat from it during its passage through the steam pipe from the boiler to the engine tending to produce wetness. The use of wet steam in an engine or turbine is uneconomical besides involving some risk; hence it is usual to need to separate any water that may be present from the steam before the latter enters the engine. This is accomplished by the use of a steam separator. Thus the function of a steam separator is to remove the entrained water particles from the steam conveyed to the steam turbine.
30 5.1 Introduction
The steam turbine is a prime mover in which the potential energy of the steam is transformed into kinetic energy and latter in its turn is transformed into the mechanical energy of rotation of the turbine shaft. The turbine shaft, directly or with the help of a reduction gearing, is connected with the driven mechanism. Depending on the type of the driven mechanism a steam turbine may be utilised in most diverse fields of industry, for power generation.
The steam turbines are mainly divided into two groups as: a) Impulse turbine
b) Reaction turbine
In both types of turbine, first the heat energy of the steam at high pressure is converted into kinetic energy passing through the nozzles. The turbines are classified as impulse or reaction according to the action of high velocity steam used to develop the power.
In impulse turbine, the steam coming out at a very high velocity through the fixed nozzles impinges on the blades fixed on the periphery of a rotor. The blades change the direction of the steam flow without changing its pressure. The resulting motive force (due to the change in momentum) gives the rotation to the turbine shaft.
Fig 5.1 Impulse and Reaction Turbine.
In the reaction turbine, the high pressure steam from the boiler is passed through the nozzles. When the steam comes out through this nozzles the velocity of the steam increases relative to the rotating disc. The resulting reaction force of the steam on nozzle gives the
31
rotating motion to the disc and the shaft. The shaft rotates in the opposite direction to the direction of the steam jet.
In an impulse reaction turbine, the steam expands both in fixed and moving blades continuously as the steam passes over them. Therefore, the pressure drop occurs gradually and continuously over both moving and fixed blades.
5.2 Compounding of Steam Turbine
If the entire pressure drop from boiler pressure to condenser pressure is carried out in single stage nozzle, then the velocity of the steam entering into the turbine could be very high of the order of 1500 m/sec. The turbine rotor velocity will be very high, of the order of 30,000 r.p.m as it is directly proportional to the steam entering velocity. Such high R.P.M. of the turbine rotor is not useful for practical purpose and a reduction gear is necessary between the turbine and external equipment driven by the turbine. There is also danger of structural failure of the blade due to excessive centrifugal stresses. Therefore the velocity of the blades is limited to 400 m/sec.
The velocity of the steam at the exit of the turbine is sufficiently high when single stage blades are used. This gives a considerable loss of kinetic energy (about 10 to 12%). The above-mentioned difficulties associated with the single stage turbine can be solved by compound. The combinations of stages are known as compounding. The different methods of compounding are:
1. Velocity Compounding 2. Pressure Compounding
3. Pressure And Velocity Compounding
1. Velocity Compounding. There is only one set of nozzles and two or more rows of moving blades. There is also a row of fixed blades in between the moving blades. The function of fixed blade is only to direct the steam coming out from first moving row to next moving row. The heat energy drop takes place only in the nozzle at the first stage and it converts into kinetic energy. The kinetic energy of the steam gained in the nozzles is successively used by the rows of moving blades and finally exhausted from the last row of the blades on the turbine rotor. The function of the fixed blades is merely to turn the steam into the direction required for entry into the next row of rotor blades without altering pressure and velocity of the steam. A turbine working on this principle is known as velocity compounded impulse turbine.
2. Pressure Compounding. A number of simple impulse turbine sets arranged in series is known as pressure compounding. In this arrangement, the turbine is provided with one row of fixed blades at the entry of each row of moving blades. The total pressure drop of the steam does not take place in a single stage nozzle but is divided equally in all the rows of fixed blades which work as nozzles.
32
Fig.5.2 Pressure Compounded Steam Turbine.
3. Pressure and Velocity Compounding. This compounding is a combination of pressure and velocity compounding. The total pressure drop of the steam from boiler to condenser pressure is divided into a number of stages as done in pressure compounding and velocity obtained in each stage is also compounded. This arrangement requires less stages and compact turbine can be designed for a given pressure drop. This compounding has an advantage of pressure compounding to provide higher pressure drop in each stage and hence less number of stages and an advantage of velocity compounding to reduce the velocity of each blade row.
33
Advantages and Disadvantages of Velocity Compounding Advantages:
1. It requires less number (2 to 3 only) of stages, therefore initial cost is less. 2. The space required is less.
3. The system is easy to operate and more reliable.
4. The turbine housing need not be made strong as pressure in the housing is considerably less because the total pressure falls in the nozzle only.
Disadvantages:
1. The friction losses are too larger due to the high velocity of steam.
2. The maximum blade efficiency and efficiency range decreases with an increase in number of stages.
3. The power developed in each successive blade row decreases with an increase in number of rows, even though all the rows require same space, material and initial cost. Therefore all the stages are not economically used. Velocity compounded steam turbines are generally used as drives for centrifugal compressors, centrifugal pumps, and small generators and feed pumps of high capacity power plants.
5.3 Losses in Steam Turbine
The causes for the energy losses in steam turbines are listed below:
1. Residual Velocity Loss. The steam leaves the turbine with some absolute velocity. The energy loss due to absolute exit velocity of steam is equivalent to Vaex2/2gJ kJ/kg,
where Vaex is absolute velocity of steam leaving the turbine.
The residual velocity loss is 10to 12% in a single stage impulse turbine. This loss is reduced by using the multistage.
2. Loss Due To Friction and Turbulence. Friction loss occurs in nozzles, turbine blades and between the steam and rotating discs. The friction loss in the nozzle is taken into account with introducing factor ‘nozzle efficiency’. The loss due to friction and turbulence is about 10%.
3. Leakage Loss. The leakage of steam occurs at the points mentioned below: a) Between the turbine shaft and bearing.
b) Between the shaft and stationary diaphragms carrying nozzle in case of reaction turbine.
c) Leakage at the blade tips in the glands. d) Leakage of steam through the glands. The total leakage loss is about 1 to 2%.
4. Loss Due To Mechanical Friction. The loss due to friction between the shaft and bearing comes under this category. Some loss also occurs in regulating the valves. This friction loss can be reduced with the help of an efficient lubricating system.
34
5. Radiation Loss. The heat is lost from the turbine to the surroundings as its temperature is higher than atmospheric temperature. Usually the turbines are highly insulated to reduce this loss. The loss due to radiation is always negligible.
6. Loss Due To Moisture. The steam contains water particles passing through the lower stages of the turbine as it becomes wet. The velocity of the water particles is less than the steam and therefore the water particles have to be dragged along with the steam and consequently part of the K.E. of the steam is lost.
5.4 Governing of Steam Turbine
The main function of the governing is to maintain the speed constant irrespective of load on the turbine. The different methods which are commonly used for governing the steam turbines are listed below:
1. Throttle Governing.
2. Nozzle Control Governing. 3. By-Pass Governing.
4. Combination of Throttle and Nozzle Governing. 5. Combination of Throttle and By-Pass Governing.
1. Throttle Governing. The quantity of steam entering into the turbine is reduced by the throttling of the steam. The throttling is achieved with the help of double heat balanced valve which is operated by a centrifugal governor through the servo-mechanism. The effort of the governor may not be sufficient to move the valve against the piston in big units. Therefore an oil operated relay is incorporated in the circuit to magnify the small force produced by the governor to operate the valve.
Fig.5.4 Throttle Governing.
2. Nozzle Control Governing. In this method of control, the steam supplied to the different nozzle groups is controlled by uncovering as many steam passages as necessary to meet the load by poppet valves.
An arrangement often used for large steam power plants is shown in fig. The numbers of nozzles supplying the steam to the turbine are divided into three groups and the supply to these nozzles is controlled by the three valves.
35
Fig.5.5 Nozzle Control Governing.
3. By-Pass Governing. More than one stage is used for high pressure impulse turbine to reduce the diameter of the wheel. The nozzle control governing cannot be used for multi stage impulse turbine due to small heat drop in first stage. It is also desirable in multistage impulse turbine to have full admission into high pressure stages to reduce the partial admission losses. In such cases by-pass governing is generally employed.
Fig 5.6 By-Pass Governing. 5.5 Turbine Troubles
The following troubles may occur during the running of turbines which may cause the damage to the turbines:
1) Loss of blade shrouding. 2) Damage of the seal.
3) Failure of a bearing or whipping of shaft because of improper lubricating-oil pressure; temperature or viscosity.
36
Sudden increase in the vibration of the turbine is the most usual indication of any trouble caused during running of the turbine.
5.6 Blade Materials for Turbines
The creep phenomenon is the main criteria in selection of blade material especially for high temperature region. 1% Cr-Mo-V alloy and stainless steels having 12% Cr are widely used. Austenite alloys are preferred for still higher temperature.
Blades of L.P. stage, though, at the low temperature end have to withstand the effect of corrosion and erosion due to water droplets (0.25mm), about 10-12% stainless iron is commonly used. New materials such as titanium, plastics reinforced with carbon having a lower specific weight and higher strengths are also considered as they have high tensile strength (70 kgf/mm2).
5.7 ALTERNATOR 5.7.1 Introduction
The alternator is universally used in automotive applications. It converts mechanical energy into electrical energy, by electro-magnetic induction.
In a simple version, a bar magnet rotates in an iron yoke which concentrates the magnetic field. A coil of wire is wound around the stem of the yoke. As the magnet turns, voltage is induced in the coil, producing a current flow. When the North Pole is up, and South is down, voltage is induced in the coil, producing current flow in one direction.
Fig.5.7 Alternator.
As the magnet rotates, and the position of the poles reverses, the polarity of the voltage reverses too, and as a result, so does the direction of current flow. Current that
37
changes direction in this way is called alternating current, or AC. The change in direction occurs once for every complete revolution of the magnet.
5.7.2 Theory of Operation
Alternators generate electricity by the same principle as DC generators. When magnetic field lines cut across a conductor, a current is induced in the conductor. In general, an alternator has a stationary part (stator) and a rotating part (rotor). The stator contains windings of conductors and the rotor contains a moving magnetic field. The field cuts across the conductors, generating an electrical current, as the mechanical input causes the rotor to turn.
Fig.5.8 Alternator Working Principle.
The rotor magnetic field may be produced by induction (in a "brushless" generator), by permanent magnets, or by a rotor winding energized with direct current through slip rings and brushes. Automotive alternators invariably use brushes and slip rings, which allows control of the alternator generated voltage by varying the current in the rotor field winding. Permanent magnet machines avoid the loss due to magnetizing current in the rotor but are restricted in size owing to the cost of the magnet material. Since the permanent magnet field is constant, the terminal voltage varies directly with the speed of the generator. Brushless AC generators are usually larger machines than those used in automotive applications.
5.7.3 Alternator Protection
An alternator is an important aspect of a power plant's electrical system. Any kind of obstacle in its performance can mar the working of the power plant's overall electrical system. It is for this reason that it requires adequate protection systems to prevent any kind of hindrance to the power plant's functionality.
The main types of protection system are: 1. Over Current Protection
2. Reverse Power Protection
Over Current Protection. Every alternator has an over current protection. With the help of this trip, the alternator and distribution system can be protected from various faults
