CHAPTER 1

Electricity Generating Industry

1.1 Introduction

A power station (also referred to as a generating station, power plant, or powerhouse) is an industrial facility for the generation of electric power.

Almost all coal, nuclear, geothermal, solar thermal electric, and waste incineration plants, as well as many natural gas power plants are thermal. Natural gas is frequently combusted in gas turbines as well as boilers. The waste heat from a gas turbine can be used to raise steam, in a combined cycle plant that improves overall efficiency. Power plants burning coal, oil, or natural gas are often referred to collectively as fossil-fuel power plants. Some biomass-fueled thermal power plants have appeared also. Non-nuclear thermal power plants, particularly fossil-fueled plants, which do not use cogeneration, are sometimes referred to as conventional power plants.

A thermal power station is a power plant in which the prime mover is steam driven. Water is heated, turns into steam and spins a steam turbine which either drives an electrical generator or does some other work, like ship propulsion. After it passes through the turbine, the steam is condensed in a condenser and recycled to where it was heated; this is known as a Rankine cycle. The greatest variation in the design of thermal power stations is due to the different fuel sources. Some prefer to use the term energy center because such facilities convert forms of heat energy into electrical energy.

In thermal power stations, mechanical power is produced by a heat engine that transforms thermal energy, often from combustion of a fuel, into rotational energy. Most thermal power stations produce steam, and these are sometimes called steam power stations. Not all thermal energy can be transformed into mechanical power, according to the second law of thermodynamics. Therefore, there is always heat lost to the environment. If this loss is employed as useful heat, for industrial processes or district heating, the power plant is referred to as a cogeneration power plant or CHP (combined heat-and-power) plant. In countries where district heating is common, there are dedicated heat plants called heat-only boiler stations. An important class of power stations in the Middle East uses by-product heat for the desalination of water.

1.2 Classification of Thermal Power Plants

Thermal power plants are classified by the type of fuel and the type of prime mover installed.

1.2.1 By Fuel

• Nuclear power plants use a nuclear reactor's heat to operate a steam turbine generator.
• Fossil fuelled power plants may also use a steam turbine generator or in the case of natural gas fired plants may use a combustion turbine.
• Geothermal power plants use steam extracted from hot underground rocks.
• Renewable energy plants may be fuelled by waste from sugar cane, municipal solid waste, landfill methane, or other forms of biomass.
• In integrated steel mills, blast furnace exhaust gas is a low-cost, although low-energy-density, fuel.
• Waste heat from industrial processes is occasionally concentrated enough to use for power generation, usually in a steam boiler and turbine.
• Solar thermal electric plants use sunlight to boil water, which turns the generator.

1.2.2 By Prime Mover

• Steam turbine plants use the dynamic pressure generated by expanding steam to turn the blades of a turbine. Almost all large non-hydro plants use this system.
• Gas turbine plants use the dynamic pressure from flowing gases to directly operate the turbine. Natural-gas fuelled turbine plants can start rapidly and so are used to supply "peak" energy during periods of high demand, though at higher cost than base-loaded plants. These may be comparatively small units, and sometimes completely unmanned, being remotely operated. This type was pioneered by the UK, Prince town being the world's first, commissioned in 1959.
• Combined cycle plants have both a gas turbine fired by natural gas, and a steam boiler and steam turbine which use the exhaust gas from the gas turbine to produce electricity. This greatly increases the overall efficiency of the plant, and many new base load power plants are combined cycle plants fired by natural gas.
• Internal combustion Reciprocating engines are used to provide power for isolated communities and are frequently used for small cogeneration plants. Hospitals, office buildings, industrial plants, and other critical facilities also use them to provide backup power in case of a power outage. These are usually fuelled by diesel oil, heavy oil, natural gas and landfill gas.
•  Micro turbines, Stirling engine and internal combustion reciprocating engines are low cost solutions for using opportunity fuels, such as landfill gas, digester gas from water treatment plants and waste gas from oil production.

1.3 Efficiency

Power is energy per time. The power output or capacity of an electric plant can be expressed in units of megawatts electric (MWe). The electric efficiency of a conventional thermal power station, considered as saleable energy (in MWe) produced at the plant busbars as a percent of the heating value of the fuel consumed, is typically 33% to 48%

efficient. This efficiency is limited as all heat engines are governed by the laws of thermodynamics (See: Carnot cycle). The rest of the energy must leave the plant in the form of heat. This waste heat can go through a condenser and be disposed of with cooling water or in cooling towers. If the waste heat is instead utilized for e.g. district heating, it is called cogeneration. Important classes of thermal power station are associated with desalination facilities; these are typically found in desert countries with large supplies of natural gas and in these plants, freshwater production and electricity are equally important co-products.

Since the efficiency of the plant is fundamentally limited by the ratio of the absolute temperatures of the steam at turbine input and output, efficiency improvements require use of higher temperature, and therefore higher pressure, steam. Historically, other working fluids such as mercury have been experimentally used in a mercury vapour turbine power plant, since these can attain higher temperatures than water at lower working pressures. However, the obvious hazards of toxicity, and poor heat transfer properties, have ruled out mercury as a working fluid.

CHAPTER 2

THERMAL POWER PLANT

We are well aware that electricity is a form of energy. There are number of methods by which electricity can be produced, but most common method of production of electrical energy is to rotate a conductor in a magnetic field continuously cutting of magnetic lines will cause E.M.F. to be generated at the ends of conductor. If these terminals are connected through load then electricity will start flowing through that conductor.

        Now let us see what we are doing in Thermal Power Station for the purpose of production of Electricity. Actually speaking we are doing conversion of energies from form to another form, and our ultimate aim is to get Electrical energy.

 For this purpose the rotation movement is required to rotate the magnetic field so that it may cut the stationery conductors of the machine. To be more precise this rotational or mechanical energy is derived from a machine to which we call Turbine which is actually capable enough to convert heat energy to rotational energy.

 For obtaining heat energy we have to make use of the chemical energy, to which we call fossil fuel i.e. coal, oil, gas etc. This is achieved in a plant to which we call furnace or sometimes Boiler.

For transportation of heat energy from furnace to turbine inlet, we require a medium and we have chosen water as media. This water is converted into steam in furnace. Quality of steam is always monitored properly process of Electrical generation.

So we see that the rotational movement required to rotate the magnetic field of the electric generator is produced by the steam turbine. The power to the steam turbine is given by steam generator in the form of high pressure and high temperature steam.

The steam after doing work on the turbine shaft is condensed and condensate is pumped back into Boiler as high pressure and low temperature water, by means of Boiler feed pump. So if we represent whole process in a block diagram this will look like as given below.    

2.1 How Electricity is generated

The complete and complex process of electricity generation in TPS can be divided into four major cycles for the sake of simplicity. The main systems are discussed in these cycles in a step by step manner and some useful drawings are also enclosed. The four cycles are

The simplest of the above four cycles is the coal cycle. In this cycle as explained earlier crushed coal of about 20mm is transported by conveyor belts to the coal mill bunkers. From here the coal goes to coal mills through raw coal feeders. In the coal mills the coal is further pulverized (crushed) to powder form. The temperature of the coal mills are maintained at 180-200 degree centigrade by a suitable mixture of hot & cold air.

The air comes from Primary Air fans (P.A FANS) which are 2 in Nos. - A&B. The outlet duct after combining gets divided into two. One duct goes to the Air Heaters (A.H- A&B) where primary air is heated by the hot flue gases in a Heat Exchanger. This duct provides hot air & the other one provides cold primary air. A suitable mixture of this hot & cold air is fed to the coal mills to maintain their temperature. This is done to remove moisture of coal. More over this primary air is also used for transportation of powdered coal from coal mills to the four corners of the boiler by a set of four pipes. There are six coal mills – A, B, C, D, E&F   and their   outlets in the Boiler are   at different elevations. The high

Temperature of the primary air does not allow the air coal mixture to choke the duct from mill to boilers. A portion of the primary air is further pumped to high pressure and is known as seal air. It is used to protect certain parts of mills like bearings etc. where powered coal may pose certain problems in the functioning of the mill. When the air coal mixture enters the boiler it catches fire in the firing zone and some ash along with clinkers settles down. This is removed periodically by mixing it with water to make slurry.

2.1.2 Oil Cycle

In the oil cycle the oil is pumped and enters the boiler from four corners at three elevations. Oil guns are used which sprays the oil in atomized form along with steam so that it catches fire instantly. At each elevation and each corner there are separate igniters which ignite the fuel oil. There are flame sensors which sense the flame and send the information to the control roam.

2.1.3 Air & Flue Gas Cycle

                 For the proper combustion to take place in the boiler right amount of Oxygen or air is needed in the boiler. The air is provided to the furnace in two ways - Primary Air & Secondary Air. Primary air is provided by P.A. fans and enters the boiler along with powdered coal from the mills. While the secondary air is pumped through Forced Draft fans better known as F.D Fans which are also two in numbers A&B. The outlet of F.D fans combine and are again divided into two which goes to Steam coiled Air pre heaters (S.C.A.P.H) A&B where its temperature is raised by utilizing the heat of waste steam. Then it goes to Air Pre heater-A&B where secondary air is heated further utilizing the heat of flue gases. The temperature of air is raised to improve the efficiency of the unit & for proper combustion in the furnace. Then this air is fed to the furnace.

From the combustion chamber the fuel gases travel to the upper portion of the boiler and give a portion of heat to the Platen Super Heater. Further up it comes in contact with the Reheater and heats the steam which is inside the tubes of reheater. Then it travels horizontally and comes in contact with Final Super Heater. After imparting the heat to the steam in super heater flue gases go downward to the Economizer to heat the cold water pumped by the Boiler Feed Pumps (B.F.P.) these all are enclosed in the furnace. After leaving the furnace the fuel gases go to the Air Heaters where more heat of the flue gases is extracted to heat primary and secondary air. Then it goes to the Electrostatic Precipitators (E.S.P.) Stage A&B where the suspended ash from the flue gases is removed by passing the fuel gas between charged plates. Then comes the induced draft fan (I.D Fan) which sucks air from E.S.P. and releases it to the atmosphere through chimney. The pressure inside the boiler is kept suitably below the atmospheric pressure with the help of 1.0. Fans so that the flame does not spread out of the openings of boiler and cause explosion. Further very low pressure in the boiler is also not desirable because it will lead to the quenching of flame.

2.1.4 Steam Water Cycle

The most complex of all the cycles is the steam & water cycle. Steam is the working substance in the turbines in all the thermal and nuclear power plants. As there is very high temperature and pressure inside the boiler, initially water has to be pumped to a very high pressure. Water has also to be heated to a suitably high temperature before putting it inside the boiler so that cold water does not cause any problem. Initially cold water is slightly heated in low pressure heaters. Then it is pumped to a very high pressure of about 200 Kg/Cm2 by boiler feed pumps A & B. After this it is further heated in high pressure heaters by taking the heat from the high pressure steam coming from various auxiliaries and / or turbines. Then this water goes to the economizer where its temperature is further raised by the flue gases.

        This hot water then goes to the boiler drum. In the boiler drum there is very high temperature and pressure. It contains a saturated mixture of boiling water and steam which are in equilibrium. The water level in the boiler         is maintained between certain limit. From here relatively cold water goes down to the water header situated at the bottom, due to difference in density. Then this cold water rises gradually in the tubes of the boiler on being heated. The tubes are in the form of water walls. These tubes combine at the top in the hot water header. From here the hot water and steam mixture comes back to the boiler drum completing the small loop.

        From the boiler drum hot steam goes to platen super heater situated in the upper portion of the boiler. Here the temperature of the steam is increased.  Then it goes to final super heater. Here its temperature is further increased.

The turbine is a three cylinder machine with high pressure (H.P), intermediate pressure (I.P) & low pressure (L.P) casings taking efficiency into account the .The turbine speed is controlled by hydro dynamic governing system. The three turbines are on the same shaft which is coupled with generator. The generator is equipped with D.C excitation system. The steam from the final super heater comes by main steam line to the H.P turbine. After doing work in the H.P turbine its temperature is reduced. It is sent back to the boiler by cold reheat line to the reheater. Here its temperature is increased and is sent to the I.P turbine through hot reheat line. After doing work in the I.P turbine steam directly enters L.P turbine.

The pressure of L.P turbine is maintained very low in order to reduce the condensation point of steam. The outlet of L.P turbine is connected with condenser. In the condenser, arrangement is made to cool the steam to water. This is done by using cold water which is made to flow in tubes. This secondary water which is not very pure gains heat from steam & becomes hot. This secondary water is sent to the cooling towers to cool it down so that it may be reused for cooling. The water thus formed in the condenser is sucked by condensate water pumps (C.W. PUMPS) and is sent to deaerator. A suitable water level is maintained in the hot well of condenser.

Water or steam leakages from the system are compensated by the make up water, line from storage tanks which are connected to the condenser. The pressure in side condenser is automatically maintained less then atmospheric pressure and large volume of steam condense here to form small volume of water. In the deaerator the water is sprayed to small droplets & the air dissolved in it is removed so that it may not cause trouble at high temperatures in the Boiler. Moreover, the water level which is maintained constant in the deaerator also acts as a constant water head for the boiler feed pumps. Water from deaerator goes to the Boiler feed pumps after the heated by L.P. Heaters. Thus the water cycle in the boiler is completed and water is ready for another new cycle. This is a continuous and repetitive process.

2.2 Elements of Thermal Power Station

• D.M. Plant   

For the generation of steam De-mineralize water prepared removing minerals & impurities to remove the minerals several chemicals are used.

• Deaerator

Deaerator is placed at the height of 26 m to provide the appropriate suction pressure for boiler feed pump. The main function of deaerator is:-

1. To remove the air bubbles from the water entered into boiler feed pump.
2. To provide the suction head to the boiler feed pump.

• Boiler feed pump 

Boiler feed pump pumps the water coming from deaerator to the H.P. heater.  Boiler feed pump consists of a motor coupled with the pump through hydraulic coupling. On passing through the boiler feed pump the pressure of the water becomes about ten times of the suction pressure.

• Economizer

It consists of a large number of closely spaced parallel tubes of thin walls and smaller diameter. The feed water is passed through the economizer before supplying it to boiler. The heat of flue gases which would be lost is used to raise the temperature of the feed water due to which the efficiency of the boiler increases.

• Air Pre-Heater

In the second path of flue gases, just below the economizer Air pre-heater is placed.  It  raise  the  temperature of   the   atmospheric  air, coming   from   the  PA  and   FD fans  ,  for   the  dryness  of   the   coal ,  which  confirms  the  proper   combustion  of  coal  used. To  raise   the  temperature of  the  air heat of  flue gases  is used , hence the efficiency of the  plant is  increased.

• Boiler

 Boiler is used for the generation of steam from the feed water. After passing through economizer feed water enters into the boiler drum. From drum, with the help of down commers it enters into the water walls where the heat coming from the furnace converts water into the steam.

• Super heater

A number of super heaters are used to make a super- heat steam coming from the boiler drum. There are ten super heaters, one de-super heat one   Platon and a final super heater to convert the wet steam into the super heated steam. Heat of flue gases is used to dry the wet steam.

• Turbine

Turbine converts the heat energy of the steam into mechanical energy. The  super heated  steam works  on   the  blades of  the turbine  and  hence the blades starts  rotating  to produce  the  mechanical  energy . The   mechanical energy then converted into the electrical energy with the help of generator. A series of three turbines is used to convert the heat energy into mechanical energy.

        1) High pressure turbine

        2) Intermediate Pressure turbine

        3) Low pressure turbine

• Condensor

The function of condenser is to create suction at very low pressure to the exhaust of turbine thereby it permits the expansion of steam in primary to a very low pressure. The exhaust steam is condensed in the condenser and then again   fed into the boiler.

Elements of a coal fired thermal power plant

Typical diagram of a coal-fired thermal power station

CHAPTER 3

PROCESS: COAL TO ELECTRICITY

We will see how the whole process of generation of electricity from the initial stage i.e. when coal burns. For burning the coal we require three T’s as shown in diagram below.

Unless until these three T’s are well in proportion fire or combustion of source of chemical energy cannot take place. For providing a suitable atmosphere for combustion we take help of well designed furnace for given fuel in which after combustion of fuel heat is released. And this heat energy is transported through a medium i.e. steam.

The essential components of the plant are:

1. Boiler
2. Steam turbine couples with electric generator
3. The condenser
4. The pump to send back condensed water to boiler

Now let us have close look of the working of each equipments of thermal power plant.

1. Feed water enters the boiler at the high pressure and low temperature and it is converted into high pressure and high temperature. Steam in the boiler. The heat required to convert feed water to steam is obtained from the heat released from the combustion of fuels burned in the furnace.  
2. High pressure and high temperature steam from the boiler passes through the turbine blades and expands from boiler pressure, to the condenser pressure. The work performed in this process is transmitted through the shaft to the shaft of the electric generator, where the mechanical energy is converted to electrical energy.
3. The low pressure and low temperature exhaust steam from turbine is condensed into water in a condenser. The heat removal for condensation is done by cooling water through circulating water pumps.
4. The condensate from the condenser is pumped, by the boiler feed pump (B.F.P) as high pressure and low temperature water which is feed to boiler.

           And this cycle goes on.  

The following medium for thermal power plant cycle is steam and before we go into the details of the steam power cycle, we should know about steam.

The use of steam can be traced back as far 56 AD when it provided the mysterious-motive-power of Greek temple after the sacred fires had been lit. It may have been used even earlier for the same purpose by Egyptians but it was not until 1712 that any development of an industrial nature took place.

In those pioneer days of boiler development the life of an operator was not without dangers because explosions were frequent.

This led to the development of steam generators and also the establishment of the excellent codes of safety which we know today.

We used coals as fuel for the generation of heat energy. As the water in the Boiler evaporated due to the intense heat, it becomes high-pressurized steams.  

And the steams are passing through a conduit (there is a turbine at the other end of the tunnel), it forces its way through the Turbine, thus rotating the Turbine. (As the steams are high-pressurized, the Turbine will rotate very fast.) 

The Turbine is connected to a Generator via a coupler. As the Turbine is rotating (from the force of the steams), electrical energy is being produced. 

After the steams have passed through the turbine, it enters a Condenser. The Condenser has got a cooling agent (namely seawater) and the steam will go through the cooling agent via a pipe. The steam thus changes back to its liquid form and returns to the Boiler. 

And the whole process repeats.

Diagram of the Basic Operation of a Thermal Power Station

CHAPTER 4

INSTRUMENTATION IN THERMAL POWER PLANT

4.1 Introduction

The first step in the field of Instrumentation engineering is to understand the process involved in an industry for which the parameters are to be measured and the man on the job should understand

1. Why the measurement is made
2. What should the measurement mean
3. What measurement is made
4. What is the result of making this measurement

        Instrumentation provides a better product in quality and quantity at lower cost in less time with full safety to personnel’s and equipments.

The instrumentation can be classified as under

1. By way of circuitry employed

Such as electrical, optical, hydraulic, pneumatic etc.

2. By way of kind of measuring values

Such as pressure, displacement, temperature, flow, level, vacuum etc.

3. By way of method of measurement

Such as hand adjusted, indicators, recorders, signaling instruments, transmitting instruments etc.

4. By way of class of precision

Amount of deviation of instrument from true value decides the accuracy of the instrument. Normally we call it error and is denoted in percentage.

5. By way of use

Such as operational instruments, sub standard and standard instruments

6. By way of their indication                        

        Such as local instruments and remote instruments etc.

7. By way of their operation

Automatic operated, manual operated wheat stone bridge for measurement of resistance.        

4.2 Classification of Instrumentation

The instrumentation of a large thermal power station has been classified as under

1. Manometry Instrumentation

For the measurement of pressure, flow, level, vacuum etc.

2. Pyrometry Instrumentation

For the measurement of temperature

3. Gas & Salt Instrumentation

For the measurement of O  in flue gas, hydrogen purity, conductivity meters, oxygen in deaerator water etc.

4. Turbo –Generator supervisory/monitoring instruments

For measurement of

1. Axial shift
2. Differential expansion
3. Shaft eccentricity
4. Vibration of turbine generator bearings
5. Casing expansion

4.2.1 Pyrometry Instrumentation

        Pyrometry Instrumentation is the measurement of the temperature in the power plant industry. The most important parameter in thermal power plant is temperature and its measurement plays a vital role in safe operation of the plant. Rise of temperature in a substance is due to the resultant increase in molecular activity of the substance on application of heat; which increases the internal energy of the material. Therefore there exists some property of the substance, which changes with its energy content. The change may be observed with substance itself or in a subsidiary system in thermodynamic equilibrium, which is called testing body and the system itself is called the hot body.

1.Solid Rod Thermometers 

A temperature sensing - Controlling device may be designed incorporating in its construction the principle that some metals expand more than others for the same temperature range. Such a device is the thermostat used with water heaters

2.Bi-Metalic Strip 

Bi-metal strips are composed of two metals, as the name implies, whose coeficients of linear expansion are dissimilar. These two metal plates are welded together as asandwich. When heated, both metals expand, but the metal with greatest coeficient of linearexpansion wil expand more causing the sandwich to curl up or down depending on the position of this metal.  

3.Glass Envelope

 4.Liquid in Glass Thermometers 

The coeficient of cubical expansion of mercury is about eight time greater that of glass. Therefore, a glass container holding mercury, when heated, wil expand far less than the mercury it contains. At a high temperature the mercury will occupy a greater fraction of the volume of the container than it will at a low temperature.

Under normal atmospheric conditions mercury normally boils at a temperature of (347°C). To extend the range of mercury in glass thermometer beyond this point the top end of a thermometer bore opens into a bulb which is many times larger in capacity than the bore. This bulb plus the bore above the mercury, is then filed with nitrogen or carbon dioxide gas at a sufficiently high pressure to prevent boiling at the highest temperature to which the thermometer may be used.

5.Mercury in Steel

The range of liquid in glass thermometers although quite large, does not lend itself to al industrial practices. This fact is obvious by the delicate nature of glass also the position of the measuring element is not always the best position to read the result. Types of Hg in Steel Thermometers are:

• Bourdon Tube : Most common and simplest type
• Spiral type : More sensitive and used where compactness is necessary
• Helical Type: Most sensitive and compact.

Pointer may be mounted direct on end of helix which rotates, thus eliminating backlash and lost motion.

6.Thermometer Bulbs 

        The thermometer bulb may take many forms dependent on the application .For example; if the temperature of a large enclosure is to be measured the bulb may be in the form of a U or of a considerable length of small tube into a spiral. This type of bulb presents the surface area necessary for measuring the temperature of a gas and is therefore used in this application.

A. Plain Bulb B. Union Bulb C. Pocket Bulb D. Wall Mounting

E. Short Coil F. Long Coil G. Finned Straight H. Watch Capsule

7.Thermometer- Wels. Pockets or Sheaths

        Plant conditions, quite often, necessitate the use of wells, pockets or sheaths, in order to protect the bulb, also to facilitate removal of the bulb without detriment to plant operation. The materials, from which these protective elements are manufactured, are dependent upon the nature of the protection required, i.e. anti-corrosive, anti-abrasive etc. Gas Thermometers: As already stated, in Effect of Heat, the volume of a gas. At constant pressure, will change with relation to temperature change and that at constant volume the pressure changes in relation to temperature. Therefore, if a bulb, capillary and bourdon tube enclose a certain volume of gas and the both of that assembly is subjected to heat, or change of the same, the changes of pressure, affected by the heat, within the system, can be directly related to temperature. The later will, of course be shown though the movement of the free end of the bourdon tube.

4.2.2 Manometry Instrumentation

1.Pressure Measurement

        The U-Tube or Manometer: Liquid contained in a tube bend in the form of a U will responds to a difference in pressure across the two limbs. A glass tube of uniform cross-sectional-area is bent to form a U and partly filed with a liquid of known density’d’ P.S.I

2.Measurement of Atmospheric Pressure

         Atmospheric pressure will support calcium of Mercury approximately 30 inches in a U-tube provided a good vacuum is maintained in one limb. If a U-tube is replaced with a straight limb about 35 inches long, one end closed, then being filed with clean, dry mercury and then inverted in a container of mercury open to the atmosphere, the mercury would fall in the tube forming a good vacuum.

3.Single Tube Manometer

 This is used for measuring low pressure and for testing and recalibration low-pressure instruments of al types. If the ratio of the area of one tube is considerably greater than other, then practically al the movement takes place in the small manometer tube and for al practical purposes only the one limb need be read.

 4.Kenotometer 

The low pressures produced in steam condensers are usually measured in inches of mercury, marking downwards from atmospheric pressure. A high working vacuum of 29.5 inches of mercury is the same as an absolute pressure of 0.5 inch of mercury, or approximately 0.25 p.s.i (absolute). One device for measuring the absolute pressure in a condenser is the Kenotometer.

5.Bourdon Pressure Gauge

        This is the most commonly used of al pressure measuring devices. (Range 10- 80,000 p.s.i). Here, a tube of oval section is bent into a circular arc. One end is sealed and the other end fixed to a solid block into which the applied pressure is fed. The tube will "uncurl" as the pressure (operating Force) increases, or will 'Curl up' as the vacuum increases; so giving a movement of the free end which is proportional to the change in pressure. The Controlling Force will depend upon the thickness of the tube and the material from which it is made.

6.Special Types of Pressure Gauges

6.1 Spiral Tube

        This type is used for low-pressure indication and recording when a C-shaped Bourden tube is not suitable and where power is required. By making the oval tube in the form of a spiral an enlarged movement of the free end is achieved and thus the tube becomes more sensitive over pressure ranges below 10 p.s.i.

6.2 Helical Tube

        For higher pressures the tube is wound in the form of a helix and is often used in pressure recorders. Range 0-80,000 p.s.i.

 6.3 Critical Type

         This is used in boiler houses to enable distant reading of the steam pressure to be made to the nearest 1 p.s.i over a range of say ± 15 p.s.i. The movement of a pressure sensitive element is transmitted to a pointer and scale via linkages, which only allow the pointer to operate over a selected range of pressure to either side of the normal steam pressure.

CHAPTER 5

AUTOMATIC CONTROL

5.1 Introduction to Control Engineering and Terminology

An automatic control scheme compares a control condition value with a desired value and automatically corrects any deviation. There are three basic types of controls and they are as follows

• Proportional
• Integral
• Derivative

Various combinations of these basic types may be employed to suit the plant characteristics.

5.1.1 Proportional Control

This type of control is used where the deviation is not very large or the deviation is not sudden. The control gives a change in regulator position which is directly proportional to a change in conditions.

          The regulator position is directly related to the deviation and for every controlled condition value there is a regulator position which is dependent upon the control sensitivity. The regulator takes up a position tending to reduce the deviation, the amount of excursion from its initial setting being dependent upon the sensitivity setting. If the deviation is increasing rapidly the regulator will apply the correction rapidly. The regulator position resulting from a deviation of the variable from a desired value depends upon the position it occupies when there is no deviation. This latter setting is about 50% regulator travel.

The range of values of the variable which the regulator to cover its full range of travel is proportional band. The band is inversely proportional sensitivity of the control so that at 100% proportional measuring index must travel through the full scale instrument to move the regulator through its full travel.

As a high proportional sensitivity (narrow proportional band) enables the regulator to move a large amount for a very small deviation, it is possible to reduce the offset to negligible amount if a sufficiently small proportional band is permissible. Normally, the proportional band must be made wide to avoid hunting or instability, so as alternative method of deviating offset must sometimes be used (proportional plus integral control).

5.1.2 Integral Control

With Integral Control the controller is only at rest when the controlled condition is at the desired value. The regulator moves, when there is a deviation, in a direction which applies correction and continues to move until either the extreme regulator position is reached or the variable returns to the desired value. The speed of movement of the regulator is directly proportional to the amount of deviation, and can be adjusted to give any required speed per unit deviation. This adjustment is known as Integral Action Time adjustment. The speed of regulator movement is related to the amount of deviation and not, as in proportional control, to the rate of deviation. For certain integral action time sensitivity the speed of travel of the regulator for a one unit deviation is half the speed of travel for a two unit deviator.

The term "integral" is derived from the mathematical consideration of this type of control. Integral calculus considers the sum of an infinite number of small increments; the actual regulator position at any instant is dependent on the amount of deviation and the time for which the deviation has been maintained.

Integral control can be used in a system but dead-time results in a sustained hunting unless the sensitivity is drastically reduced. The system's main attribute is that the regulator position is not rigidly tied to the set point. Therefore, if used with proportional control, integral control provides automatic elimination of offset.

 5.1.3 Derivative Control

Using this control the regulator is not influenced by the desired value but moves in accordance with the direction and with rate of change of the deviation. If the change in the variable is a sudden step movement, its rate of change is infinitely fast and the regulator travels (moves) gradually at a constant rate, the regulator will move by an amount proportional to that rate and then stop until the rate of change of deviation alters.

Derivative control is not used alone but normally in conjunction with proportional or proportional plus integral control.

5.1.4 Combination of Proportional, Integral and Derivative Control

The combination of proportional and integral control provides automatic elimination of the offset. When a deviation occurs, the regulator moves under proportional control by an amount proportional to the deviation. The regulator then continues to move under integral control at a constant rate towards its extreme position. The combined integral and proportional wave lags behind the proportional wave by a value of less than 90 degrees and is dependent upon the relative sensitivities. Therefore, a more stable form of control is provided.

The integral function is derived from the proportional function. The time required for the integral action to increase the control output to the regulator, by an amount equal to the output change caused by the proportional action, is termed the Integral Action Time.        

5.2 Requirement of Control System

        A control system, to be effective, must satisfy requirements. It must be possible to measures the condition to be preferably by the standard application of a proven instrument.

The regulator must be capable of handling the plant under all load conditions and at all probable desired value settings, preferably with a little range to spare; if the system is continually out ranging the regulator, satisfactory control will be impossible.

The measuring point must be as close as possible to the regulator in order to minimize lags.

5.3 Sensitivity Adjustments

Two major requirements of an automatically controlled plant are:

1. The variable must be returned to the desired value as quickly as possible after a disturbance.
2. The control system must be stable tendency to hunt.

        Following gradual deviation of the measured variable the each method of control previously described has its particular advantages regarding sensitivity requirements, and these may be summarized as follows:

1. Proportional control is a stable system but does not necessarily ensure that the measured variable is always at the desired value under various load conditions.

2. Integral control always returns the measured variable to the desired value, but tends to make the control loop less stable and the inherent frequency of plant oscillation lower.

3. Derivative control tends to make some control loops more stable (this depends upon the plant characteristics and increases the inherent frequency of plant oscillation. It is not concerned, however, with the absolute value of the controlled variable.

Thus, the overall sensitivity of particular method or combination or methods becomes a compromise between stability and the requirement of returning to the desired value quickly. The relative sensitivities of the methods employed in a combined system are derived by compromises to achieve the best results.

5.4 Automatic Control System

An Automatic Control System is one in which the actual value of a controlled condition such a temperature, conductivity, oxygen content, flow, pressure level or PH is continuously compared with a desired value and corrective action automatically taken which is dependent on the deviation between the two values. The system can be either 'open loop' or 'closed loop’, depending on the location of the control elements with respect to the plant.

5.4.1 The open loop system

The open loop system is used when it is not possible to measure the actual value of a condition which is to be controlled; a reasonable degree of control may still be achieved by measuring the actual value of a related conditions and applying correction according to this value. This system is only regarded as satisfactory in the simplest of control schemes because changes in load or in any other variables in the scheme will necessitate a changing proportional relationship in the correction applied by the open loop.

5.4.2 The closed loop system

The closed loop system performs the following functions:

a)    Measures the condition or quantity.

b)    Compares the measurement with the desired value.

c)    Re-positions the regulator according difference obtained, in such a way as to the condition at     the desired value.

Power amplification and a mean of introducing one or more mathematical functions, such as multiplication or reversal are normally required between stages (b) and (c).

5.5 Automatic Control Techniques

A control process can be considered as the operation of balancing inflow of energy against outflow of energy. The response of a plant to variations of input energy occasioned by changes in the regulator position can normally be represented by an S-shaped curve. The curve has to characteristics, one representing the lag during which the response of the variable is either delayed in the plant and/or is so small as to give no indication of the much more rapid response which is to flow. The other represents the maximum subsequent reaction of the variable.

The shape of this curve has a bearing controllability of the plant. The smaller the lag and the slower reaction rate, the more simple control system needed.

5.6 Selection of a Controller

1. Proportional control is used where

• Load changes are small
• Offset can be tolerated
• The process reaction rate is such as to permit a narrow PO since this reduces the amount of offset.

2. Integral action is used where

• Offset must be eliminated
• Integral saturation due to sustained deviation is not objectionable.

3. Derivative action is used where

• Large transfers of distance velocity logs are present.
• It is necessary to minimize the amount of deviation caused by plant load changes

P+I are chosen when criteria of A & B are met.

P+D are chosen when criteria of A & C are met.

P + I + D are chosen when criteria of AB & C are met.

Guidelines for the settings of P I and D actions cannot be given to any accuracy, because prop, band depends upon the range of controller as well as plant characteristics.

5.7 Summary of the Different Control Actions

CHAPTER 6

NTPC---Environment Management

6.1 Environment management Policy

• NTPC  has a well defined Environment management Policy

• NTPC has actively gone for adoption of best international practice on environment, occupational health and safety areas and has pursed EMS system ISO 14001 and OHSAS 18001. All NTPC Stations have ISO 14001 & OHSAS 18001 certifications by International certifying agencies.

• Pollution controls systems like ESP, Flue gas Stacks, Low Nox burners, Neutralization pits, Coal settling pits and DE & DS systems for better air quality are in place in all power plants.

• Cooling Towers, Ash dyke and Ash disposal systems and Ash recycling is installed for better water management and saving fresh water.

6.2 Monitoring of Environmental parameters

• All Pollutants discharged from the power plant such as Stack emission, ash pond effluents, main plant effluents, domestic effluents are monitored at stipulated frequencies. Ambient air, Surface water and ground water quality are also regularly monitored.

• Upgradation and Retrofitting of Pollution control systems In order to keep pace with changing environmental norms retrofitting and up gradation of pollution monitoring and control facilities in existing plants is also taken up.

• On Line data Management is in place for environmental performance of the power stations.

6.3 Reducing and Recycling Waste

• Hazardous waste management as per GOI rules of 1989 is done and recyclable wastes and non recyclable waste are suitably treated. The non recyclable wastes are sent to the State Pollution Control Board for common treatment, storage and disposal facility.

• Land Use/ Bio-Diversity… As a Policy NTPC lays special emphasis on land use. Development of green belts, energy plantations, reclamation of Ash ponds and EIA studies for ecological monitoring in the project areas and its surroundings.

• Reclamation of Abandoned Ash Ponds… Extensive plantations have been undertaken on dry ash mound at NTPC DADRI.

6.4 Afforestation

• Forest Banks concept

• NTPC  has created a Green wealth of 18.2 Million Trees by Summer 2007

• Plantation of 1.25 Million trees around Ramagundam Project has resulted in Summer Peak temperature reduction by 3 deg C

• Ash Mound Reclamation and Dry ash disposal system