Published on

Published in: Business, Technology
  • Be the first to comment

No Downloads
Total views
On SlideShare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide


  2. 2. ACKNOWLEDGEMENT I would hereby like to express my profound sense of gratitude to G.M. (T) IPGCL & PPCL, Mr. Y.P. Arora for giving me the opportunity to carry out my industrial training at Rajghat Power House under their experienced and highly qualified staff. Equally thankful I am to Mr. Premraj Singh (Manager,Electrical) for his invaluable guidance during the training period. Thanks are due to Mr. Surendra Vishwakarma (Asst. Mgr., Electrical), Mr. Praveen Kumar (Asst. Mgr., Electrical), Mr. H.C. Pandey (Asst. Mgr., Electrical) and Mr. R.K. Prajapati (Asst. Mgr., Cont. & Inst.) who despite of their busy schedule and workload, were able to find some time for us and imparted us the knowledge that paved a way for better understanding of the fundamentals and their applications alike. I duly acknowledge the help, direct or indirect of the whole department and staff members of the organization for providing all the facilities for the training. The knowledge gained herein and the practical experiences learnt will be invaluable in the long run. Conveyor:Ashutosh Kumar Maurya ROLL NO.:0712831028 B.TECH 7th SEMESTER ELECTRONICS & COMMUNICATION ENGINEERING BHARAT INSTITUTE OF TECHNOLOGY BY PASS ROAD PARTAPUR, MEERUT 2
  3. 3. PREFACE With day to day advantage of new technology the older machinery are being replaced by new machinery . Now it has not been the work of semi skilled persons .It has opened a new horizon for degree holder engineers. But to do the job properly a suitable training is needed. The knowledge of entire system is must for an engineer to do the trouble shooting in the quickest possible way so that the production does not get affected. So for engineering the industrial training is playing a vital role in developing the practical knowledge. The industrial training is not merely an academic requirement but a professional necessity too. With the increasing demand and utilization of electricity an electrical engineer should be well versed or at least must be familiar with the generation ,transmission and distribution of electricity, at the same time must be capable of fault detection and elimination. It is thus the responsibility of an electrical engineer to deal with the sophistication and make the maximum possible utilization. 3
  4. 4. CONTENTS S.NO 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 Title of Heading About the company Introduction of RAJGHAT POWER STATION SECTION-A POWER PLANT BASICS Overview of thermal power plant Steam Rankine Cycle Steam Turbine (Prime Over) Impulse Turbine Reaction Turbine Turbo-Generator Arrangement Bearings Valves The steam & water circuits Deareator Fuel air & Flue Ȃgas circuits Generator Section-B Control & Instrumentation in Power Plant Combustion and Draught Control Fan Control Flame Monitoring Feed Water Control Deaerator Control Level Control Steam Temperature Control Turbine Control & Monitoring LVDT Accelerometer 4 Page No. 06-06 07-08 09-09 10-11 11-11 12-13 14-15 15-16 16-16 16-17 17-18 18-18 19-21 22-22 23-25 25-25 26-26 27-29 30-30 31-32 33-37 37-38 38-40 41-44 45-48 48-49 49-50
  5. 5. S. No. 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 Title of Heading Velocity Transducers Casing & Cylinder Expansion 505 Turbine Control Communication RTD Thermocouple Pressure Transducers Level Measurement Flow Meters Automation of Process PLC Appendix-A Appendix-B Appendix-C Conclusion Page No. 50-51 52-52 52-54 54-55 56-57 57-58 58-60 60-62 62-64 65-65 66-70 71-71 72-72 73-73 74-74 5
  6. 6. About the company Beholding the grimness of power situation in Delhi and failure of all previous attempts in restoring the quality of service to the citizens Delhi Electricity Board Regulatory Commission (DERC) was constituted in May 1999 whose prime responsibility was to look into the entire gamut of existing activity and search for various ways of power sector reforms. This was followed with a Tripartite Agreement which was signed by the government of Delhi, DVB employees to ensure the cooperation of stakeholders in this reform process. The tripartite agreement sent off very positive vibes to the people in general as well as to the investor community about the sincere and hassle-free objectives of power reforms. The Government of India on July 1, 2002, implemented the reforms by unbundling DVB into six companies, one holding company, one generation company (GENCO), one transmission company (TRANSCO) and three distribution companies (DISCOMS). The government handed over the management of the business of electricity distributions to there private companies since July 1, 2002 with 51% equity with the private sector. It was thus that IPGCL (GENCO) came into existence with the aim of meeting the power demands of a city which is the capital of one of the most populated countries of the world and whose resources fall much below its demand and since then its contribution to the power sector has been beyond the expectations as is evident from the current power situation in the city. The following facts may summarize the success story of a never before fundamental reform: y y y 330 MW capacity Pragati Power Station was commissioned in the year 2002-03 and performing excellently. Achieved 100.4% PLF during the month of Jan.,ǯ05 and 88.27% PLF (Deemed PLF 95%) during the year 2004-05, Pragati Power Corporation Ltd. paid dividends of Rs.17.5 Cr., for FY 2003-04 & Rs.14 Cr. for FY 2004-05 as well as 2005-06. The performance of Indraprastha Gas Turbine Power Station which was 47.24% in 2001-02 increased to 70.76% (deemed PLF 75.35%) in 2005-06. This is the best performance of the station in a year since its commissioning in 1985-86. The station also achieved highest ever generation in a day, 5.743 MU (84.86% PLF) on 26.12.05 and highest ever generation in a month, 166.227 MU (79.23% PLF) in October, 2005. The forced outages of the station have also reduced from 17.75 % to 5.2 %. The overall performance of GENCO increased from 45.90% during the year 2001-02 to 64.35% in 2005-06. 6
  7. 7. RAJGHAT POWER STATION Two Units of 67.5 MW were installed in 1989-90 at Rajghat Power House as Replacement of old Units and the present generation capacity of this Station is 135 MW. Further, the station has acquired the following certifications: y y y ISO-9001:2000 for Quality Management ISO-14001:2004 for Environment Management. OHSAS-18001:2007 for Occupational Health & Safety Management Rajghat Power House Specifications 1. Location: Rajghat 2. Type: Coal based thermal power plant 3. Installed Capacity: 2 x 67.5 MW = 135 MW 4. Land Detail a. Plant area: b. Ash dump area: 40 acres 24 acres 5. Cooling Water a. Source: River Yamuna 7
  8. 8. b. Cooling Method: Closed cycle with cooling tower 6. Fuel: Coal a. Type: b. Linked coal mines: c. Gross calorific value: d. Ash content: e. Sulphur content: f. Requirement i. With design coal: ii. With actual coal: g. Stockyard a. Area: b. Capacity: Bituminous Piparwar 3180-4280 kcal/kg 35-42.2 % 0.5 % 2000MT/day 2400MT/day 72000 sq. mt. 20000MT 7. Oil a. Type: b. Nearest service outlet: LSHSLDO Mathura-Shakurbasti oil depot 8
  9. 9. SECTION A Power Plant Basics 9
  10. 10. 1. OVERVIEW OF THERMAL POWER PLANT Schematic of a thermal power plant Power plants generate electrical power by using fuels like coal, oil or natural gas. A simple power plant consists of a boiler, turbine, condenser and a pump. Fuel, burned in the boiler and superheater, heats the water to generate steam. The steam is then heated to a superheated state in the superheater. This steam is used to rotate the turbine which powers the generator. Electrical energy is generated when the generator windings rotate in a strong magnetic field. After the steam leaves the turbine it is cooled to its liquid state in the condenser. The liquid is pressurized by the pump prior to going back to the boiler. Thus the main inputs required by a plant are: y y y Coal: Should have high calorific value and low ash content Water: De-mineralized water for steam generation Air 10
  11. 11. In this chapter first steam and its properties are described which are essential to the understanding of the underlying principle of the thermal plant, the Rankine cycle which is presented next. Steam Steam power is fundamental to what is by far the largest sector of the electricity-generating industry and without it the face of contemporary society would be dramatically different from its present one. We would be forced to rely on hydro-electric power plant, windmills, batteries, solar cells and fuel cells, all of which are capable of producing only a fraction of the electricity we use. Steam is produced by boiling of water and it is achieved at atmospheric pressure at 100 degree Celsius. Let us consider a quantity of water that is contained in an open vessel. Here, the air that blankets the surface exerts a pressure on the surface of the fluid and, as the temperature of the water is raised, enough energy is eventually gained to overcome the blanketing effect of that pressure and the water starts to change its state into that of a vapour (steam). If the pressure of the air blanket on top of the water were to be increased, more energy would have to be introduced to the water to enable it to break free. In other words, the temperature must be raised further to make it boil. To illustrate this point, if the pressure is increased by 10% above its normal atmospheric value, the temperature of the water must be raised to just above 102 °C before boiling occurs. The information relating to steam at any combination of temperature, pressure and other factors may be found in steam tables, which are nowadays available in software as well as in the more traditional paper form. These tables were originally published in 1915 by Hugh Longbourne Callendar (1863-1930), a British physicist. Because of advances in knowledge and measurement technology, and as a result of changing units of measurement, many different variants of steam tables are today in existence, but they all enable one to look up, for any pressure, the saturation temperature, the heat per unit mass of fluid, the specific volume etc. Steam becomes superheated when its temperature is raised above the saturation temperature corresponding to its pressure. This is achieved by collecting it from the vessel in which the boiling is occurring, leading it away from the liquid through a pipe, and then adding more heat to it. This process adds further energy to the fluid, which improves the efficiency of the conversion of heat to electricity. As stated earlier, heat added once the water has started to boil does not cause any further detectable change in temperature. Instead it changes the state of the fluid. Once the steam has formed, heat added to it contributes to the total heat of the vapour. This is the sensible heat plus the latent heat plus the heat used in increasing the temperature of each kilogram of the fluid through the number of degrees of superheat to which it has been raised. In a power plant, a major objective is the conversion of energy locked up in the input fuel into either usable heat or electricity. In the interests of economics and the environment it is important to obtain the highest possible level of efficiency in this conversion process. 11
  12. 12. Rankine Cycle: The Working Principle The Carnot cycle postulates a cylinder with perfectly insulating walls and a head which can be switched at will from being a conductor to being an insulator. Even with modifications to enable it to operate in a world where such things are not obtainable, it would have probably remained a scientific concept with no practical application, had not a Scottish professor of engineering, William Rankine, proposed a modification to it at the beginning of the twentieth century. The concepts that Rankine developed form the basis of all thermal power plants in use today. Even today¶s combined-cycle power plants use his cycle for one of the two phases of their operation. The Rankine cycle in a steam-turbine power plant In the system shown in figure, water is heated in feed heaters (A to B) using steam extracted from the turbine. Within the boiler itself, heat is used to further pre-warm the water (in the economiser) before it enters the evaporative stages (C) where it boils. At D superheat is added until the conditions at E are reached at the turbine inlet. The steam expands in the turbine to the conditions at point F, after which it is condensed and returned to the feed heater. The energy in 12
  13. 13. the steam leaving the boiler is converted to mechanical energy in the turbine, which then spins the generator to produce electricity. The diagram shows that the energy delivered to the turbine is maximised if point E is at the highest possible value and F is at the lowest possible value. THERMAL EFFICIENCY 13
  14. 14. 2. TEAM TURBINE (PRIME MOVER) A steam turbine at Rajghat Power station 2.1 Raising steam Steam is mostly raised from fossil fuel sources, mostly coal but also oil and gas, in a combustion chamber. Recently these fuels have been supplemented by limited amounts of renewable biofuels and agricultural waste. The chemical process of burning the fuel releases heat by the chemical transformation (oxidation) of the fuel. This can never be perfect. There will be losses due to impurities in the fuel, incomplete combustion and heat and percentage of the available energy in the fuel. 2.2 Working Principles High pressure steam is fed to the turbine and passes along the machine axis through multiple rows of alternately fixed and moving blades. From the steam inlet port of the turbine towards the 14
  15. 15. exhaust point, the blades and the turbine cavity are progressively larger to allow for the expansion of the steam. The stationary blades act as nozzles in which the steam expands and emerges at an increased speed but lower pressure. (Bernoulli's conservation of energy principle - Kinetic energy increases as pressure energy falls). As the steam impacts on the moving blades it imparts some of its kinetic energy to the moving blades. There are two basic steam turbine types, impulse turbines and reaction turbines, whose blades are designed control the speed, direction and pressure of the steam as is passes through the turbine. 2.2.1 Impulse Turbines The steam jets are directed at the turbine's bucket shaped rotor blades where the pressure exerted by the jets causes the rotor to rotate and the velocity of the steam to reduce as it imparts its kinetic energy to the blades. The blades in turn change the direction of flow of the steam however its pressure remains constant as it passes through the rotor blades since the cross section 15
  16. 16. of the chamber between the blades is constant. Impulse turbines are therefore also known as constant pressure turbines. The next series of fixed blades reverses the direction of the steam before it passes to the second row of moving blades. 2.2.2 Reaction Turbines The rotor blades of the reaction turbine are shaped more like aerofoils, arranged such that the cross section of the chambers formed between the fixed blades diminishes from the inlet side towards the exhaust side of the blades. The chambers between the rotor blades essentially form nozzles so that as the steam progresses through the chambers its velocity increases while at the same time its pressure decreases, just as in the nozzles formed by the fixed blades. Thus the pressure decreases in both the fixed and moving blades. As the steam emerges in a jet from between the rotor blades, it creates a reactive force on the blades which in turn creates the turning moment on the turbine rotor, just as in Hero's steam engine. (Newton's Third Law - For every action there is an equal and opposite reaction). The turbine employed at the station comprises of 47 stages one of which is impulse and the rest are reaction stages. 2.3 Turbine-Generator arrangement The plant employs a 2-pole synchronous generator with a synchronous speed of 3000 rpm. The output frequency is determined by the relation ܰ ൌ ͳʹͲ ‫כ‬ ݂ ܲ where N is the rotor speed in RPM f is the output frequency of generator P is the number of poles of generator The generator is air cooled and the bearings are cooled by oil which is supplied by various oil pumps EOP, JOP, BOP etc. The detailed description of the generator unit is provided in Appendix-C. The following figure shows the arrangement of turbine and generator set. 16
  17. 17. Overall turbine and generator arrangement 2.3.1 Bearings Steam turbines are provided with journal bearings and thrust bearings. Journal bearings are at each end of each rotor to support the weight of the rotor. One thrust bearing is provided for the entire steam turbine to maintain the axial position of the rotor. Journal Bearings. The journal bearings are constructed of two halves that enclose the shaft. The inside of the bearing adjacent to the shaft is lined with babbitt metal. Babbitt is an alloy of tin, copper, and antimony that has antiseizing qualities and a natural oiliness. The journal bearings are oil-pressure lubricated. Oil flow is controlled to limit oil temperature rise to a set value. Thrust Bearings. The thrust bearing consists of babbitt metal lined, stationary shoes that run against the rotor thrust runner. Shoes on both sides of the runner prevent movement in either 17
  18. 18. axial direction. The thrust bearing compartment is oil-pressure flooded with the oil introduced near the shaft and flowing outward by the centrifugal action of the runner. Oil flow is controlled to limit oil temperature rise to a set value. 2.3.2 Valves Major control valves associated with the steam turbine and their operations are as follows: Main Steam Stop (Throttle) Valves: The steam from the steam generator flows to the main steam stop or throttling valves. The primary function of the stop valves is to provide backup protection for the steam turbine during turbine generator trips in the event the main steam control valves do not close. The energy in the main steam and steam generator can quickly cause the turbine to reach destructive overspeed on loss of the generator load. The main steam stop valves close from full open to full closed in 0.15 to 0.5 s. The stop valves are also closed on unit normal shutdown after the control valves have closed. A secondary function of the stop valves is to provide steam throttling control during startup. The stop valves have internal bypass valves that allow throttling control of the steam from initial turbine roll to loads of 15% to 25%. During this startup time, the main steam control valves are wide open and the bypass valves are used to control the steam flow. Main Steam Control (Governor) Valves: The steam flows from the stop valves to the main steam control or governor valves. The primary function of the control valves is to regulate the steam flow to the turbine and thus control the power output of the steam turbine generator. The control valves also serve as the primary shutoff of the steam to the turbine on unit normal shutdowns and trips. 18
  19. 19. 3. THE STEAM AND WATER CIRCUITS 3.1 Steam Generation and use The steam generation occurs in banks of tubes that are exposed to the radiant heat of combustion. The steam leaves the drum and enters a bank of tubes where more heat is taken from the gases and added to the steam, superheating it before it is fed to the turbine. The superheater, comprises a single bank of tubes but in many cases multiple stages of superheater tubes are suspended in the gas stream, each abstracting additional heat from the exhaust gases. In boilers (rather than HRSGs), some of these tube banks are exposed to the radiant heat of combustion and are therefore referred to as the radiant superheater. Others, the convection stages, are shielded from the radiant energy but extract heat from the hot gases of combustion. 19
  20. 20. Schematic of a boiler After the flue gases have left the superheater they pass over a third set of tubes (called the economizer), where almost all of their remaining heat is extracted to pre-warm the water before it enters the drum. Finally the last of the heat in the gases is used to warm the air that is to be used in the process of burning the fuel. The major moving items of machinery shown in the diagram are the feed pump, which delivers water to the system, and the fan which provides the air needed for combustion of the fuel (in most plants each of these is duplicated). In a combined-cycle plant the place of the combustion-air fan and the fuel firing system is taken by the gas turbine exhaust. 3.2 Feed water-condensate cycle Inside the plant, the steam and water system forms a closed loop, with the water leaving the condenser being fed back to the feed pumps for reuse in the boiler. However, certain other items of plant now become involved, because the water leaving the condenser is cold and contains entrained air which must be removed. Air becomes entrained in the water system at start-up (when the various vessels are initially empty), and it will appear during normal operation when it leaks in at those parts of the cycle which operate below atmospheric pressure, such as the condenser, extraction pumps and low pressure feed heaters. Leakage can occur in these areas at flanges and at the sealing glands of the rotating shafts of pumps. Air entrainment is aided by two facts: one is that cold water can hold greater amounts of oxygen (and other dissolved gases) than can warm water; and the other is that the low-pressure parts of the cycle must necessarily correspond with the low temperature phases. 20
  21. 21. Steam and Water circuit 21
  22. 22. 3.3 The Deareator The deaerator removes dissolved gases by vigorously boiling the water and agitating it, a process referred to as 'stripping'. The water entering at the top is mixed with steam which is rising upwards. The steam, taken directly from the boiler or from an extraction point on the turbine, heats a stack of metal trays and as the water cascades down past these it mixes with the steam and becomes agitated, releasing the entrained gases. The steam pressurizes the deaerator and its contents so that the dissolved gases are vented to the atmosphere. Minimizing corrosion requires the feed-water oxygen concentration to be maintained below 0.005 ppm or less and although the deaerator provides an effective method of removing the bulk of entrained gases it cannot reduce the concentration below about 0.007 ppm. For this reason, scavenging chemicals are added to remove the last traces of oxygen. Principle of a deaerator 22
  23. 23. 4. THE FUEL, AIR AND FLUE-GAS CIRCUITS 4.1 Corner Fired Boiler: The plant employs corner fired (tangential) boilers in which the burning fuel circulates around the furnace, forming a large swirling ball of burning fuel at the centre. The burners are placed at five equidistant levels (named A, B, C, D and E) along the height of boiler on all four corners. A certain degree of tilt can be provided to the burners to direct the fireball to a higher or lower position within the furnace, and this has a significant effect on the temperature of the various banks of superheater tubes, and therefore on steam temperature. 4.2 Air Heater: It works as a heat exchanger and transfers the heat remaining in the exhaust gases to the air being fed to the furnace. The reuse of heat in the exhaust gases results in improvement of efficiency of the plant. However, the exchanger suffers from leakage of heat and perfect transfer of heat cannot be obtained. Draught-plant arrangement 23
  24. 24. 4.3 Pressurized Bowl Mill: The coal that is discharged from the storage hoppers is fed down a central chute onto a table where it is crushed by a grinding rotor assembly and is ground down to a very fine powder (called pulverized fuel (PF)). Air is blown into the crushed coal and carries it, via adjustable classifier blades, to the PF pipes that transport it to the burners. The air that carries the fine particles of coal to the burners is supplied from a fan called a 'primary-air fan'. This delivers air to the mill, which therefore operates under a pressure which is slightly positive with respect to the atmosphere outside. Pressurized bowl mill For supplying the air to the mill, cool air and heated air are mixed to achieve the desired temperature. This temperature has to be high enough to partially dry the coal, but it must not be so high that the coal could overheat (with the risk of the coal/air mixture igniting inside the mill 24
  25. 25. or even exploding while it is being crushed). The warm air is then fed to the mill (or a group of mills) by means of yet another fan, called a 'primary air fan'. It should be noted that the cooler of the two air streams is commonly referred to as 'tempering air' since, because it is obtained from the FD fan exhaust it may already be slightly warm, and its function is to temper the mixture. GENERATOR A steam turbine generator In electricity generation, an electrical generator is a device that converts mechanical energy to electrical energy, generally using electromagnetic induction. The reverse conversion of electrical energy into mechanical energy is done by a motor, and motors and generators have many similarities. A generator forces electric charges to move through an external electrical circuit, but it does not create electricity or charge, which is already present in the wire of its windings. It is somewhat analogous to a water pump, which creates a flow of water but does not create the water inside. The source of mechanical energy may be a reciprocating engine, a steam or gas turbine, water falling through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, the sun or solar energy, compressed air or any other source of mechanical energy. 25
  26. 26. SECTION B Control & instrumentation in Power Plant For the power plant to run and produce electricity, the equipment in each category must be placed into operation to perform its intended function. The operation of the equipment must be coordinated to meet the demand of the electricity production process, and the production process must be regulated so that the cycle and the equipment are operated within design conditions. The plant control systems provide the necessary tools to enable the operators to orchestrate plant operation for the reliable and efficient production of electricity. This section provides an overview of the plant control system, its functions, and the type of control equipment used in the Rajghat Power House. 5. COMBUSTION AND DRAUGHT CONTROL 26
  27. 27. In a fired boiler the control of combustion is extremely critical. In order to maximize operational efficiency combustion must be accurate, so that the fuel is consumed at a rate that exactly matches the demand for steam, and it must be executed safely, so that the energy is released without risk to plant, personnel or environment keeping in mind that the amount of energy involved in a power plant is considerable: in each second of its operation a large boiler releases around a billion joules, and in a process of this scale the results of an error can be catastrophic. 5.1 Load control strategy for pressurized mills The approach is to provide closed-loop control of the primary-air flow, as shown. Here, because the system detects and immediately reacts to changes in PA flow, and adjusts the flow-control damper to compensate, disturbances to steam production are minimized. Again, a feeder-speed signal, representing fuel flow, is fed back to the master system to provide closed-loop correction of speed changes, which would otherwise introduce disturbances to the steam pressure. Closed loop control of PA flow 5.2 Mill temperature control Control objective: It is very important that the temperature of the air in the mill should be maintained within close limits. For many reasons, including inadequate drying of the coal, combustion efficiency will be reduced if the temperature is too low, while too high a temperature can result in fires or explosions occurring in the mill. 27
  28. 28. Control Technique: The control technique involves mixing hot and cold air streams to achieve the correct temperature. Pressurized coal mills require the use of two dampers for this purpose one controlling the flow of hot air, the other the cold air. Mill Temperature Control 5.3 Draught control In a fired boiler, the air required for combustion is provided by one or more fans and the exhaust gases are drawn out of the combustion chamber by an additional fan or set of fans. The control of all these fans must ensure that an adequate supply of air is available for the combustion of the fuel and that the combustion chamber operates at the pressure determined by the boiler designer. All of the fans also have to contribute to the provision of another important function--purging of the furnace in all conditions when a collection of unburned fuel or combustible gases could otherwise be accidentally ignited. Such operations are required prior to light-off of the first burner when the boiler is being started, or after a trip. 5.3.1 Maintaining the furnace draught 28
  29. 29. Apart from supplying air to support combustion, the FD fans have to operate in concert with the ID fans to maintain the furnace pressure at a certain value. The heavy solid line of figure below shows the pressure profile through the various sections of a typical balanced-draught boiler system. It shows the pressure from the point where air is drawn in, to the point where the flue gases are exhausted to the chimney, and demonstrates how the combustion chamber operates at a slightly negative pressure, which is maintained by keeping the FD and ID fans in balance with each other. If that balance is disturbed the results can be extremely serious. Such an imbalance can be brought about by the accidental closure of a damper or by the sudden loss of all flames. It can also be caused by mal-operation of the FD and ID fans. The dashed line on the diagram shows the pressure profile under such a condition, which known as an 'implosion'. The results of an implosion are extremely serious because, even though the pressures involved may be small, the surfaces over which they are applied are very large and the forces exerted become enormous. Such an event would almost certainly result in major structural damage to the plant. Draught profile of a boiler and its auxiliary plant 5.3.2 Fan control The throughput of two fans operating together can be regulated by a common controller or by individual controllers for each fan. Although a single controller cannot ensure that each fan delivers the same flow as its partner, this configuration is much simpler to tune than the alternative, where the two controllers can interact with each other and make optimization extremely difficult. 29
  30. 30. Controlling the windbox pressure 5.4 Binary control of the combustion system So far, we have considered only the modulating systems involved with the combustion plant. In practice, these systems have to operate in concert with binary control systems such as interlocks and sequences. The purpose of an interlock is to co-ordinate the operation of different, but interrelated plant items: tripping one set of fans if another set trips, and so on. The purpose of a sequence system is to provide automatic start-up or shutdown of the plant, or of some part of it. 5.4.1 Flame monitoring Monitoring the status of a flame is not easy. The detector must be able to discriminate between the flame that it is meant to observe and any other in the vicinity, and between that flame and the hot surfaces within the furnace. The detector must also be able to provide reliable detection in the presence of the smoke and steam that may be swirling around the flame. To add to the problems, the detector will be required to operate in the hot and dirty environment of the burner 30
  31. 31. front, and it will be subjected to additional heat radiated from the furnace into which it is looking. With their attendant BMS¶s, flame scanners of a boiler are vital to the safety and protection of the plant. If insufficient attention is paid to their selection, or if they are badly installed or commissioned, or if their maintenance is neglected, the results can be, at best, annoying. The problems will include nuisance trips, protracted start-up of the boiler and the creation of hazardous conditions that could have serious safety implications. A flame scanner is a complex opto-electronic assembly, and modern scanners incorporate sophisticated technologies to improve flame recognition and discrimination. Although the electronics assembly will be designed to operate at a high temperature (typically 65 °C), unless great care is taken this value could easily be exceeded and it is therefore important to take all possible precautions to reduce heat conduction and radiation onto the electronic components. Typical flame scanner 5.4.2 The requirements for purge air The purge air that is supplied to the scanner serves two purposes: 1. it provides a degree of cooling and 2. it prevents dust, oil and soot from being deposited on the optical parts of the unit. The air should be available at each burner, even if the burner itself is not operating. It should therefore be obvious that the air used for purging should be cool, dry and clean, and that it should be available at all times. Purge air can be obtained from the instrument-air supply, or it can be provided by dedicated blowers. In some cases it is taken from the FD fan discharge. Each of 31
  32. 32. these is viable, provided the requirements outlined above have been thoroughly considered. It is also important that the presence of the purge-air supply should be monitored and its loss transmitted to the DCS, because failure of the air supply could result in expensive and possibly irreparable damage to the scanners. Modern scanners include self-monitoring circuits that will warn of overheating. The scanner system should be fail-safe, as a failed system represents the loss of a critical link in the plant's safety chain. If it is overridden, the operator can become used to operating without it in place, and such lapses can eventually create a severe hazard. 6. FEED-WATER CONTROL 32
  33. 33. 6.1 Controlling the flow Control Objective: To supply enough water to the boiler to match the evaporation rate for the widest practical range of operation in a safe and cost-effective manner. Difficulties: 1. Measurements of drum level cannot be made easily. 2. Interactions in the boiler system and uneven nature of these interactions. Control requirement: The purpose of the drum is not only to separate the steam from the water but also to provide a storage reservoir that allows short-term imbalances between feed-water supply and steam production to be handled without risk to the plant. As the level of water in the drum rises, the risk increases of water being carried over into the steam circuits. The results of such 'carry-over' can be catastrophic: cool water impinging on hot pipework will cause extreme and localized stresses in the metal and, conversely, if the level of water falls there is a possibility of the boiler being damaged, partly because of the loss of essential cooling of the furnace waterwalls. Therefore, the target of the feed-water control system is to keep the level of water in the drum at approximately the midpoint of the vessel. Given this objective, it would appear that the simplest solution would appear to be to measure the level of water in the drum and to adjust the delivery of water to keep this at the desired value--feeding more water into the drum if the level is falling, and less if the level is rising. Unfortunately, the level of water is affected by transient changes of the pressure within the drum and the sense in which the level varies is not necessarily related to the sense in which the feed flow must be adjusted. In other words, it is not sufficient to assume that simply because the level is increasing the feed-water flow must be decreased, and vice versa. This strange situation is due to effects known as 'swell' and 'shrinkage'. Boiling water comprises a turbulent mass of fluid containing many steam bubbles, and as the boiling rate increases the quantity of bubbles that is generated also increases. The mixture of water and bubbles resembles foam, and the volume it occupies is dictated both by the quantity of water and by the amount of the steam bubbles within it. If the pressure within the system is decreased, the saturation temperature is also lowered and the boiling rate therefore increases (because the temperature of the mixture is now higher in relation to the saturation temperature than it was before the pressure change occurred). As the boiling rate increases, the density of the, water decreases, but since the mass of steam and water has not changed the decrease in density must be accompanied by an increase in the volume of the mixture. By this mechanism the level of water in the drum appears to rise, a phenomenon referred to as 'swell'. The rise of level is misleading: it is not indicative of a real increase in the mass of water in the system, which would require the supply of water to be cut back to maintain the status quo. In fact, if the drop in pressure is the result of the steam demand suddenly increasing, the water supply will need to be increased to match the increased steam flow. 'Shrinkage' is the opposite of swell: it occurs when the pressure rises. The mechanism is exactly the same as that for swell, but in the reverse direction. Shrinkage causes the level of water in the drum to fall when the steam flow decreases, and once again the delivery of water to the boiler must be related to the actual need rather than to the possibly misleading indication provided by the drum-level transmitter. 33
  34. 34. If a slow change of steam flow occurs, all is well because the pressure within the system can be controlled. It is when rapid steam-flow changes happen that problems occur since, due to swell or shrinkage, the drum level indication provides a contrary indication of the water demand. Following a sudden increase in steam demand, which causes the pressure to drop (and therefore the drum level to rise), a simple level controller would respond by reducing the flow of feed water. Equally, a sudden decrease in steam flow, which would be accompanied by a rise in pressure and an attendant fall in the drum level, would cause a level controller to increase the flow of water. Both actions are, of course, in the incorrect sense. The effects of swell and shrinkage, in addition to being determined by the rate of change of pressure, also depend on the relative size of the drum and the pressure at which it operates. If the volume of the drum is large in relation to the volume of the whole system the effect will be smaller than otherwise. If the system pressure is low the effect will be larger than with a boiler operating at a higher pressure, since the effect of a given pressure change on the density of the water will be greater in the low-pressure boiler than it would if the same pressure change were to occur in a boiler operating at a higher pressure. Control technique: Remembering that the basic requirement of a feed-water control system is to maintain a constant quantity of water in the boiler, it is apparent that one way of addressing the problem would be to maintain the flow of water into the system at a value which matches the flow of steam out of it. The flow is controlled using a valve which maintains the rate of water flowing through the valve at a figure which is directly proportional to the demand signal from the controller (i.e. if the demand signal varies linearly from 0 to 100%, the flow rate also changes linearly between 0 and 100%). Such a valve is said to have a 'linear characteristic' and is employed in conjunction with a transmitter that produces a signal proportional to steam flow. Used together, these two devices keep the parameters in step. If the transmitter produces a signal which is equal to the steam flow at all loads and if the flow through the valve is matched with this signal at every point in the flow range, a controller gain of unity will ensure that, throughout the dynamic range of the system, the flow of water will always be equal to the flow of steam. However, the flow through a valve depends both on its opening and on the pressure drop across it. In a feed-water system, the pressure drop across the valve varies from instant to instant, and the flow through it at any given opening will therefore vary. One method of correcting for the error produced by the feed valve is the addition of a third element to the system-a measurement of feedwater flow. Three-element feed-water control system Here a cascade control technique is applied. The blocks are described below: Item 1: Flow Transmitter for feed water flow rate 34
  35. 35. Item 2: Flow transmitter for steam flow rate Item 3: Level transmitter for drum level Item 4: A gain block to adjust for any range difference between the steam-flow and feed-flow transmitters Item 5: Drum-level controller (proportional only) Item 6: Error generator Item 7: Closed-loop feed-water controller 6.2 Valves A control valve consists of many components which may conveniently be considered as falling into one of two groups: the valve body and the actuator. The former is the part through which the water flows and this flow is controlled by adjusting the resistance offered to the water. This is done by moving the position of a plug in relation to its seat. The position of the plug is controlled by an actuator which acts via the stem. Figure shows a small-bore feed-water control valve body with a contoured trim (the 'trim' being the part of the valve which is in flowing contact with the water). The contour determines the relationship between the position of the plug and the flow of water past it. The type of trim will be dictated by the application, such as the need to minimize acoustic noise or cavitation, the rangeability needed etc. In addition the trim design will determine the valve characteristic, which is the curve relating the stem position to the rate of flow of water through the valve. This is an 35
  36. 36. important feature, since the characteristic determines the gain of the valve system, which forms part of the overall loop gain. A typical feed-water control valve body For a given opening, the flow through the valve will be determined by the delivery pressure of the feed pump and the resistance that the boiler pipework offers to the flow. To simplify the task of selecting the correct valve size and characteristic, it is necessary to relate everything to a definable set of conditions. This is achieved by determining what the flow through the valve would be if a fixed differential pressure were to be maintained across it. This is termed the inherent characteristic of the valve. Once the valve is operating on the actual plant, the position/flow relationship achieved in practice will not match the inherent characteristic, because in the real world the inlet pressure and system resistance will vary, producing a pressure drop which is different from the value that was used to define the inherent characteristic. The pressure/flow relationship achieved in actual operation is called the installed characteristic. 36
  37. 37. Inherent characteristics of valves 6.3 Deareater control Steam admitted to the deaerator rises upwards past metal trays over which the water is simultaneously cascading downwards. As the water and steam mix and become agitated, entrained gases are released. The dissolved gases are vented to the atmosphere because the vessel is pressurised by the steam. The deaerator is situated in the water circuit between the discharge of the condenser extraction pump and the inlet of the feed pumps. It will be evident that two control functions are required by the deaerator: y to maintain the steam pressure at the optimum value y to keep the storage vessel roughly half full of water. 6.3.1 Steam pressure control The pressure of the steam entering the deaerator is maintained by a simple controller whose measured-value signal is obtained from a transmitter measuring the steam pressure in the deaerator. The set value of the controller is fixed. 37
  38. 38. 6.3.2 Level control The storage vessel provides a measure of reserve capacity for the plant. To achieve this function the level of water in it must be maintained at roughly the midpoint. This is achieved by means of a level controller whose measured-value signal is obtained from a differential-pressure transmitter or from capacitive probes which would normally be connected to tappings of an external water column which is in turn connected to the top and bottom of the deaerator storage vessel. If there were no losses in the system, the amount of water would be constant and the level in the deaerator storage vessel would remain at the correct value set during commissioning. However, losses are inevitable (for example, due to leakages at pump glands or during sootblowing or blowdown operations), and a supply of treated water must therefore be made available. The deaerator level controller output adjusts the opening of a valve that admits this make-up water to the condenser, as shown. The make-up supply is conventionally fed into the system at the condenser. Figure shows that interaction between the level controllers of the deaerator and condenser is inevitable. The situation is made more complex because the condenser extraction pump has to be provided with a bypass arrangement to maintain a minimum flow through the pump at all times. In fact, the conditions which cause the deaerator level controller to call for more water to be added to the system will also cause the condenser level to fall, and so the two systems do not act in opposite senses. Nevertheless they do interact, and care must be taken to minimise the instability that is likely to arise. Principle of deaerator level control system 38
  39. 39. a. Single element water level control, Only single sensor i.e. level sensor is used to control the level b. Double Element water level control 39
  40. 40. This uses two sensors level and flow sensors. It is better and precise than single element control. Other Temperature, Flow and Pressure control is done by Feedback loop- This uses a single feedback loop which finds out error from the set point and finds out controller output. Cascaded Feedback loop control This uses two controller: 1. Master 2. Slave The set point of slave controller is decided by the controller output of the master. 7. STEAM TEMPERATURE CONTROL 40
  41. 41. 7.1 Temperature control Control Objective: To maintain the temperature of steam at a precise value over the entire load range. Control Requirement: The steam turbine requires the steam temperature to remain at a precise value over the entire load range, and it is mainly for this reason that some dedicated means of regulating the temperature must be provided. Since different banks of tubes are affected in different ways by the radiation from the burners and the flow of hot gases, an additional requirement is to provide some means of adjusting the temperature of the steam within different parts of the circuit, to prevent any one section from becoming overheated. Difficulties: The long time constants associated with the superheater do not permit a simple control strategy based on measuring the temperature of the steam leaving the final superheater, and modulating the flow of cooling water to the spray attemperator so as to keep the temperature constant at all flow conditions. This form of control would produce excessive deviations in temperature, and a more complex arrangement is required. Two time constants are associated with the superheater. One represents the time taken for changes in the firing rate to affect the steam temperature, the other is the time taken for the steam and water mixture leaving the attemperator to appear at the outlet of the final superheater. In terms of temperature control it is the latter effect which predominates because, although changes in heat input will affect the temperature of the steam, a fast-responding temperature-control loop will be able to compensate for the alterations and keep the temperature constant. It is the reaction time between a change occurring in the spray-water flow and the effects being observed in the final temperature that determines the extent of the temperature variations that will occur. Control technique: Another problem with a simple system is that it does not permit any monitoring and control of the temperature to occur within the steam circuit--only at the exit from the boiler. These difficulties are addressed by the use of a cascade control system as shown. Since it is the temperature of the steam leaving the secondary superheater that is important, this parameter is measured and a corresponding signal fed to a three-term controller (proportionalplus-integral-plus-derivative). In this controller the measured-value signal is compared with a fixed desired-value signal and the controller's output forms the desired-value input for a secondary controller. (Because the output from one controller 'cascades' into the input of another, this type of control system is commonly termed 'cascade control'.) The secondary controller compares this desired-value signal with a measurement representing the temperature of the steam immediately after the spray-water attemperator. Because the steam temperature sensors used are subjected to the high pressures and temperatures of the superheater, they have to be enclosed in substantial steel pockets. Even with the best designs, pockets are usually slow-responding, with the result that any high-speed fluctuations in the measured-value signal will be smoothed out and the resultant signal will be fairly stable. The 41
  42. 42. use of a derivative term is therefore easier than in, say, flow measurement applications where small-scale but sudden changes in flow can occur. When rapid input changes are differentiated, the controller output changes by a large amount, and for this reason tuning three-term flow controllers for optimum response can become difficult. This is not a problem with the temperature controllers used here, and the application of derivative action is viable. Steam-temperature control with a single interstage spray attemperator 7.2 Spray Water Attemperator: The high-pressure cooling water is mechanically atomized into small droplets at a nozzle, thereby maximizing the area of contact between the steam and the water. With this type of attemperator the water droplets leave the nozzle at a high velocity and therefore travel for some distance before they mix with the steam and are absorbed. To avoid stress-inducing impingement of cold droplets on hot pipework, the length of straight pipe in which this type of attemperator needs to be installed is quite long, typically 6 m or more. With spray attemperators, the flow of cooling water is related to the flow rate and the temperature of the steam, and this leads to a further limitation of a fixed-nozzle attemperator. Successful break-up of the water into atomized droplets requires the spray water to be at a pressure which exceeds the steam pressure at the nozzle by a certain amount (typically 4 bar). Because the nozzle presents a fixed-area orifice to the spray water, the pressure/flow 42
  43. 43. characteristic has a square-law shape, resulting in a restricted range of flows over which it can be used (this is referred to as limited turn-down or rangeability). The turn-down of the mechanically atomized type of attemperator is around 1.5:1. The temperature of the steam is adjusted by modulating a separate spray-water control valve to admit more or less coolant into the steam. Mechanically atomised desuperheater 7.3 Temperature control with tilting burners The burning fuel in a corner-fired boiler forms a large swirling fireball which can be moved to a higher or lower level in the furnace by tilting the burners upwards or downwards with respect to a mid-position. The repositioning of the fireball changes the pattern of heat transfer to the various banks of superheater tubes and this provides an efficient method of controlling the steam temperature, since it enables the use of spray water to be reserved for fine-tuning purposes and for emergencies. In addition, the tilting process provides a method of controlling furnace exit temperatures. 43
  44. 44. 8. TURBINE CONTROL AND MONITORING The steam turbine generator is controlled and monitored by several interrelated systems: 1. Turbine governor system: automatically controls turbine speed, acceleration, and load; 44
  45. 45. 2. Trip system: provides protection through trips and ranbacks; 3. Supervisory instrumentation system: provides past and present operating data through parameter sensing, indicating, and recording; and 4. Excitation system: controls generator voltage 8.1 Turbine Governor: The turbine governor system is a hybrid electrical-hydraulic system. This type of control system is significantly more advanced and preferable to the older mechanical-hydraulic designs in that the linkages and cams of the mechanical designs have been replaced with electrical logic and fast-acting hydraulic servomotors. The hydraulic fluid is supplied to the stop and control valve servomotors by a high-pressure power pumping unit. The fluid is flame retardant to minimize fire hazards in the event of a leak. The system also includes controls and instrumentation. The turbine governor facilitates control of the turbine over the full operational range by positioning the turbine control valves and the interceptor valves to control turbine speed, load, and throttle pressure. 8.2 Trip System: The trip system initiates protective tripping of the turbine by sensing potentially damaging operating conditions. Typical parameters sensed for initiation of turbine protective functions include the following: ‡ ‡ ‡ ‡ ‡ ‡ ‡ ‡ ‡ ‡ ‡ ‡ ‡ ‡ Over speed governor trip Manual trip device Generator trip Generator protection trips (loss of coolant, high stator temperature, etc.) High differential expansion Solenoid trip Turbine over speed Thrust bearing failure Low lubricating oil or hydraulic fluid pressure Operator manual trip Governor system protective trips Condenser low vacuum trip High exhaust temperature trip High vibration trip 8.3 Supervisory Instrumentation System: The supervisory instrumentation system includes devices to sense, indicate, and record parameters necessary to monitor the operation of the machine. The following parameters are monitored along with others: ‡ ‡ ‡ ‡ ‡ Shaft speed; Governor or control valve position; Shaft eccentricity; Radial (X-Y) shaft vibration at all turbine, generator, and Shell, rotor, and differential expansion; 45
  46. 46. ‡ ‡ ‡ ‡ ‡ ‡ ‡ ‡ Shell and valve chest temperatures; Water induction thermocouple temperatures; Vibration phase angles; Bearing metal temperature, including the thrust bearing; Generator winding temperatures; Generator gas temperatures; Generator cooling water temperatures; and Exciter temperatures. These parameters are measured, recorded, displayed, and alarmed by hard-wired monitors in the system cabinets. High differential expansion, turbine over speed, and thrust bearing wear alarms are provided to the trip system. Rotor vibration alarms are displayed in the main control room. Turbine Supervisory Equipment Application 8.4 Sensors and Measurement Techniques Turbine supervision is an essential part of the day-to-day running of any power plant. There are many potential faults such as cracked rotors and damaged shafts, which result from vibration and expansion. When this expansion and vibration is apparent in its early stages the problem can usually be resolved without any of the disruption caused when a turbine has to be shut down. By appropriate trending of the various measurement points and the identification of excessive 46
  47. 47. vibration or movement, scheduled equipment stoppages or outages can often be utilised to investigate and resolve the failure mechanism. 8.4.1 The Eddy Current Proximity Probe The principle of operation, as the name implies, depends upon the eddy currents set up in the surface of the target material - shaft, collar, etc. adjacent to the probe tip. The Eddy probe tip is made of a dielectric material and the probe coil is encapsulated within the tip. The coil is supplied with a constant RF current from a separate Eddy Probe Driver connected via a cable, which sets up an electromagnetic field between the tip and the observed surface. Any electrically conductive material within this electromagnetic field, i.e. the target material, will have eddy currents induced in its surface. The energy absorbed from the electromagnetic field to produce these eddy currents will vary the strength of the field and hence the energizing current, in proportion to the probe target distance. Such changes are sensed in the driver where they are converted to a varying voltage signal. The whole probe, extension cable and driver system relies for its operation on being a tuned circuit and as such is dependent on the system¶s natural frequency. Thus each system is set up for a fixed electrical/cable length. Eddy probe systems are usually supplied with 2, 5, 9 or 14 metre total cable lengths. The probe types available are generally according to the API670 standard. Three main variants are straight mount, reverse mount and disc type probes. The main difference between the straight and reverse mount is the location of the thread on the probe body and the fixing nut. Reverse mount tend to be used exclusively with probe holders, while straight mount are the more common and are used on simple bracketry or mounting threads where adjustment to the target is achieved through use of the thread on the probe body in conjunction with a moveable lock nut. The maximum measurement range available on this type of probe is typically 8mm. The disc probe mounts the encapsulated coil on a metal plate with fixed mounting holes, making a very low profile assembly with a side exit cable. Larger coils can be mounted on this plate; for 47
  48. 48. example, the 50mm diameter tip probe can provide a measurement range of beyond 25mm. However, care must be taken to ensure the target area is sufficient to obtain the required linear response. Note the relationship opposite ± between linear range, probe tip and target area. Application: In rotating plant, the variations in shaft/bearing distance created by vibration, eccentricity, ovality etc. are measured by probes mounted radially to the shaft. When the target is stationary the measured voltage can be used to set the probe/target static distance. Shaft speed can also be measured by placing the probe viewing a machined slot or a toothed wheel. 8.4.2 LVDT (Linear Variable Differential Transformer) The LVDT is an electromechanical device that produces an electrical signal whose amplitude is proportional to the displacement of the transducer core. The LVDT consists of a primary coil and two secondary coils symmetrically spaced on a cylindrical former. Schematic of an LVDT Core displacement characteristics A magnetic core inside the coil assembly provides a path for the magnetic flux linking the coils. The electrical circuit is configured as above with the secondary coils in series opposition. When an alternating voltage is introduced into the primary coil and the core is centrally located, then an alternating voltage is mutually induced in both secondary coils. The resultant output is zero, as the voltages are equal in amplitude and in 180º opposition to each other. When the core is moved away from the null position the voltage in the coil, towards which the core is moved, increases due to the greater flux linkage and the voltage in the other primary coil decreases due to the lesser flux linkage. The net result is that a differential voltage is produced across the secondary tappings, which varies linearly with change in core position. An equal effect is produced when the core is moved from a null in the other direction but the voltage is 180º different in phase. 48
  49. 49. Application: The LVDT can be operated where there is no contact between the core and extension rod assembly with the main body of the LVDT housing the transformer coils. This makes it ideal for measurements where friction loading cannot be tolerated but the addition of a low mass core can. Examples of this are fluid level detection with the core mounted on a float and creep tests on elastic materials. This frictionless movement also benefits the mechanical life of the transducer, making the LVDT particularly valuable in applications such as fatigue life testing of materials or structures.This is a distinct advantage over potentiometers which are prone to wear and vibration. The principle of operation of the LVDT, based on mutual inductance between primary and secondary coils, provides the characteristic of infinite resolution. The limitations lie within the signal processing circuitry in combination with the background noise. Commercially available LVDT¶s 8.4.3 The Accelerometer The accelerometer is based on the electrical properties of piezoelectric crystal. In operation, the crystal is stressed by the inertia of a mass. The variable force exerted by the mass on the crystal produces an electrical output proportional to acceleration. Two common methods of constructing the device to generate a residual force are compression mode and shear mode respectively. A residual force is of course required to enable the crystal to generate the appropriate response, moving in either direction on a single axis. An accelerometer operates below its first natural frequency. The rapid rise in sensitivity approaching resonance is characteristic of an accelerometer, which is an un-damped singledegree-of-freedom spring mass system. Generally speaking, the sensitivity of an accelerometer and the ratio between its electrical output and the input acceleration is acceptably constant to approximately 1/5 to 1/3 of its natural frequency. For this reason, natural frequencies above 30KHz tend to be used. 49
  50. 50. Typical frequency response of an accelerometer Although the piezoelectric accelerometer is a self-generating device, its output is at a very high impedance and is therefore unsuited for direct use with most display, analysis, or monitoring equipment. Thus, electronics must be utilised to convert the high impedance crystal output to a low impedance capable of driving such devices. The impedance conversion electronics may be located within the accelerometer, outside of but near the accelerometer, or in the monitoring or analysis device itself. 8.4.4 Velocity Transducer The velocity transducer is inherently different to the accelerometer with a conditioned velocity output. This device operates on the spring-mass-damper principle, is usually of low natural frequency and actually operates above its natural frequency. The transducing element is either a moving coil with a stationary magnet, or a stationary coil with a moving magnet. A voltage is produced in a conductor when the conductor cuts a magnetic field and the voltage is proportional to the rate at which the magnetic lines are cut. Thus, a voltage is developed across the coil, which is proportional to velocity. 8.4.5 Absolute vibration Absolute vibration monitoring is perhaps the primary method of machine health monitoring on steam turbines. The type of transducer used is seismic (ie vibration of turbine relative to earth) and can either be a velocity transducer or an accelerometer. Vibration monitoring is nearly always in terms of velocity or displacement and can therefore be obtained by an accelerometer or a velocity transducer. Particular care needs to be taken when double integrating an accelerometer signal to provide a displacement measurement. Problems usually occur below 10Hz when double integrating and 5Hz when single integrating. In the frequency ranges normally monitored on steam turbines this is not a problem. These measurement issues can be reduced by integrating the signal at source rather than after running the signal through long cables (ie having picked up noise on route). Pedestal vibration is measured in the two axes perpendicular to the shaft direction where the bearing is under load, providing complete measurement coverage. In some instances the thrust direction is also monitored depending on turbine configuration. 50
  51. 51. 8.4.6 Eccentricity & shaft vibration Eccentricity monitoring can be subdivided into shaft vibration and bent shaft monitoring. Bent shafts normally result when the turbine is stationary and thermal arching or bowing of the shaft occurs or the shaft sags under its own weight. The Turbine is rotated slowly (barring) to prevent this happening or to straighten the shaft after it has occurred. One of the difficulties encountered when using shaft displacement transducers be they the eddy current probe or the older inductive probes, is the problem of ³runout´. Runout is the error signal generated by mechanical, electrical or metallurgical irregularities of the shaft surface.These error signals are generally of a low magnitude in comparison to the vibration signal and are often at a much higher frequency. 8.4.7 Rotor differential expansion & shaft position The eddy current probe, as well as providing ac vibratory information, also provides dc information of the probe to target gap. This makes it ideal for measuring rotor to casing differential expansion via a non-contact method. Two probes monitoring expansion by observing a tapered collar 8.4.8 Speed - overspeed - zero speed monitoring The eddy current probe as well as being used for shaft vibration and differential expansion can also be used as a speed monitoring transducer. The eddy current probe gives a large voltage output, which is independent of shaft speed. 8.4.9 Casing & cylinder expansion These techniques require a larger measurement range than can be offered through standard proximity probe equipment, the necessary probe target is also not easy to achieve. This is where LVDTs are used to provide the expansion measurements required. A total range of 50mm 51
  52. 52. usually suffices and the various mounting options available with LVDTs makes installation straightforward. The movement of the turbine pedestals on the cylinder sole plates is a relatively easy measurement to make requiring an LVDT mounted on the turbine and the extension rod fixed or sprung onto the slides. The environment is not hostile although care must be taken to prevent mechanical damage to the transducer. LVDT monitoring cylinder casing expansion 8.4.10 Valve position monitoring The Linear Variable Differential Transformer (LVDT) is ideally suited for valve position monitoring. In this type of application the LVDT is used to provide positional feedback to the governor control system to effect a closed loop system. The electrical properties of the LVDT therefore play a key role in determining the system response. Linearity is obviously key as well as a robust construction with flexible mounting options. AC type devices (as opposed to DC) are exclusively used for this type of application, which permit long cable runs and offset adjustment with gain control. 8.5 Woodward 505 Enhanced Digital Control for Steam Turbines The 505 Enhanced controller is designed to operate industrial steam turbines of all sizes and applications. This steam turbine controller includes specifically designed algorithms and logic to start, stop, control, and protect industrial steam turbines or turbo-expanders, driving generators, compressors, pumps, or industrial fans. The 505 control¶s unique PID structure makes it ideal for applications where it is required to control steam plant parameters like turbine speed, turbine load, turbine inlet or exhaust header pressure, or tie-line power. The control¶s special PID-to-PID logic allows stable control during normal turbine operation and bumpless control mode transfers during plant upsets, minimizing process over- or undershoot conditions. The 505 controller senses turbine speed via passive or active speed probes and controls the steam turbine through one or two (split-range) actuators connected to the turbine inlet steam valves. The plant employs one controller for each turbine. 52
  53. 53. Description The 505 control is packaged in an industrial hardened enclosure designed to be mounted within a system control panel located in a plant control room or next to the turbine. The control¶s front panel serves as both a programming station and operator control panel (OCP). This user-friendly front panel allows engineers to access and program the unit to the specific plant¶s requirements, and plant operators to easily start/stop the turbine and enable/disable any control mode. Password security is used to protect all unit program mode settings. The unit¶s two-line display allows operators to view actual and setpoint values from the same screen, simplifying turbine operation. Turbine interface input and output wiring access is located on the controller¶s lower back panel. Unpluggable terminal blocks allow for easy system installation, troubleshooting, and replacement. System Protection y y y y y Integral Overspeed Protection Logic First-out Indication (10 individual shutdown inputs) Bumpless transfer between control modes if a transducer failure is detected Local/Remote Control priority and selection Fail-safe Shutdown Logic Control The following PIDs are available to perform as process controllers or limiters: 53
  54. 54. y y y Speed/Load PID (with Dual Dynamics) Auxiliary PID (limiter or control) Cascade PID (Header Pressure or Tie-Line Control) Control Specifications Inputs y Power: 18±32 Vdc, 90±150 Vdc, 88±132 Vac (47±63 Hz), 180±264 Vac (47±63 Hz) y Speed: 2 MPUs (1±30 Vrms) or proximity probes (24 Vdc provided), 0.5 to 15 kHz y Discrete Inputs: 16 Contact Inputs (4 dedicated, 12 programmable) y Analog Inputs: 6 Programmable Current Inputs (4±20 mA) Outputs y Valve/Actuator Drivers: 2 Actuator Outputs (4±20 mA or 20±160 mA) y Discrete Outputs: 8 Relay Outputs (2 dedicated, 6 programmable) y Analog Outputs: 6 Programmable Current Outputs (4±20 mA) Communication y Serial: 2 Modbus (ASCII or RTU) Comm Ports (RS-232, RS-422, or RS-485 compatible) 54
  55. 55. Temperature The hotness or coldness of a piece of plastic, wood, metal, or other material depends upon the molecular activity of the material. Kinetic energy is a measure of the activity of the atoms which make up the molecules of any material. Therefore, temperature is a measure of the kinetic energy of the material in question. 1. RTD ( Resistance temperature detector ) - The resistance of an RTD varies directly with temperature: - As temperature increases, resistance increases. - As temperature decreases, resistance decreases. · RTDs are constructed using a fine, pure, metallic, spring-like wire surrounded by an insulator and enclosed in a metal sheath. 55
  56. 56. · A change in temperature will cause an RTD to heat or cool, producing aproportional change in resistance. The change in resistance is measured by a precision device that is calibrated to give the proper temperature reading. The RTD used in RPH is Pt-100. 2. THERMOCOUPLE Thermocouples will cause an electric current to flow in the attached circuit when subjected to changes in temperature.The amount of current that will be produced is dependent on the temperature difference between the measurement and reference junction; the characteristics of the two metals used; and the characteristics of the attached circuit. 56
  57. 57. Heating the measuring junction of the thermocouple produces a voltage which is greater than the voltage across the reference junction. The difference between the two voltages is proportional to the difference in temperature and can be measured on the voltmeter (in mill volts). For ease of operator use, some voltmeters are set up to read out directly in temperature through use of electronic circuitry. Only J and K type of thermocouple is used in RPH. Pressure Transducers Bellows-Type Detectors The need for a pressure sensing element that was extremely sensitive to low pressures and provided power for activating recording and indicating mechanisms resulted in the development of the metallic bellows pressure sensing element. The metallic bellows is most accurate when measuring pressures from 0.5 to 75 psig. However, when used in conjunction with a heavy range spring, some bellows can be used to measure pressures of over 1000 psig. 57
  58. 58. Bourdon tube The bourdon tube consists of a thin-walled tube that is flattened diametrically on opposite sides to produce a cross-sectional area elliptical in shape, having two long flat sides and two short round sides. The tube is bent lengthwise into an arc of a circle of 270 to 300 degrees. Pressure applied to the inside of the tube causes distention of the flat sections and tends to restore its original round cross-section. This change in cross-section causes the tube to straighten slightly. Since the tube is permanently fastened at one end, the tip of the tube traces a curve that is the result of the change in angular position with respect to the center. Within limits, the movement of the tip of the tube can then be used to position a pointer or to develop an equivalent electrical signal to indicate value of the applied internal pressure. 58
  59. 59. Level Measurement Ball Float The operation of the ball float is simple. The ball floats on top of the liquid in the tank. If the liquid level changes, the float will follow and change the position of the pointer attached to the rotating shaft. 59
  60. 60. Pressure head method The differential pressure (_P) detector method of liquid level measurement uses a _P detector connected to the bottom of the tank being monitored. The higher pressure, caused by the fluid in the tank, is compared to a lower reference pressure (usually atmospheric). This comparison takes place in the _P detector. Figure illustrates a typical differential pressure detector attached to an open tank. 60
  61. 61. Flow meters Differential Flow Transmitter Orifice plates · Flat plates 1/16 to 1/4 in. thick · Mounted between a pair of flanges · Installed in a straight run of smooth pipe to avoid disturbance of flow patterns due to fittings and valves. Venturi tube · Converging conical inlet, a cylindrical throat, and a diverging recovery cone · No projections into the fluid, no sharp corners, and no sudden changes in Contour. Dall flow tube 61
  62. 62. · Consists of a short, straight inlet section followed by an abrupt decrease in the inside diameter of the tube · Inlet shoulder followed by the converging inlet cone and a diverging exit cone · Two cones separated by a slot or gap between the two cones. Electromagnetic flow meter The electromagnetic flow meter is similar in principle to the generator. The rotor of the generator is replaced by a pipe placed between the poles of a magnet so that the flow of the fluid in the pipe is normal to the magnetic field. As the fluid flows through this magnetic field, an electromotive force is induced in it that will be mutually normal (perpendicular) to both the magnetic field and the motion of the fluid. This electromotive force may be measured with the aid of electrodes attached to the pipe and connected to a galvanometer or an equivalent. For a given magnetic field, the induced voltage will be proportional to the average velocity of the fluid. However, the fluid should have some degree of electrical conductivity. 62
  63. 63. Ratio control is used to ensure that two flows are kept at the same ratio even if the flows are changing. E.g. Air-Fuel Ratio where the proper ratio between air and fuel i.e. pulverized coal has to be maintained. Controller feed is adjusted according to the ratio of wild feed. The implementation is shown above. utomation of Processes The main purpose of automation is to minimize human intervention to reduce the errors. Large processes like power plant has large scope for errors. 63
  64. 64. The automation of industrial processes is carried by PLCs and SCADA. The above figure shows the hierarchy setup of industrial automation. The field devices are connected to the PLC and all the PLCs are connected to the SCADA server based in control room. The SCADA server is connected via LAN to ERP (Enterprise Resource Planning) and the data can be accessed from any where on internet. PLC (Programmable Logic Controller) Digital electronic device that uses a programmable memory to store instructions and to implement specific functions such as logic , sequencing , timing etc to control machine and processes. Salient features ‡ Cost effective for controlling complex systems. 64
  65. 65. ‡ Flexible and can be reapplied to control other systems quickly and easily. ‡ Computational abilities allow more sophisticated control. ‡ Trouble shooting aids make programming easier and reduce downtime. ‡ Reliable components make these likely to operate for years before failure. PLC HARDWARE Many PLC configurations are available, even from a single vendor. But, in each of these there are common components and concepts. The most essential components are: Power Supply ± This can be built into the PLC or be an external unit. Common voltage levels required by the PLC (with and without the power supply) are 24Vdc, 120Vac, 220Vac. CPU (Central Processing Unit) ± This is a computer where ladder logic is stored and processed. I/O (Input/Output) ± A number of input/output terminals must be provided so that the PLC can monitor the process and initiate actions. Indicator lights ± These indicate the status of the PLC including power on, program running, and a fault. These are essential when diagnosing problems. The configuration of the PLC refers to the packaging of the components. Rack - A rack is often large (up to 18´ by 30´ by 10´) and can hold multiple cards. When necessary, multiple racks can be connected together. These tend to be the highest cost, but also the most flexible and easy to maintain. INPUTS AND OUTPUTS Inputs to, and outputs from, a PLC are necessary to monitor and control a process. Both inputs and outputs can be categorized into two basic types: 65
  66. 66. logical or continuous. Consider the example of a light bulb. If it can only be turned on or off, it is logical control. If the light can be dimmed to different levels, it is continuous. Continuous values seem more intuitive, but logical values are preferred because they allow more certainty, and simplify control. As a result most controls applications (and PLCs) use logical inputs and outputs for most applications. Hence, we will discuss logical I/O and leave continuous I/O for later. Outputs to actuators allow a PLC to cause something to happen in a process. A short list of popular actuators is given below in order of relative popularity. · Solenoid Valves - logical outputs that can switch a hydraulic or pneumatic flow. · Lights - logical outputs that can often be powered directly from PLC output boards. · Motor Starters - motors often draw a large amount of current when started, so they require motor starters, which are basically large relays. · Servo Motors - a continuous output from the PLC can command a variable speed or position. Outputs from PLCs are often relays, but they can also be solid state electronics such as transistors for DC outputs or Triacs for AC outputs. Continuous outputs require special output cards with digital to analog converters. Inputs come from sensors that translate physical phenomena into electrical signals. Typical examples of sensors are listed below in relative order of popularity. · Proximity Switches - use inductance, capacitance or light to detect an object logically. · Switches - mechanical mechanisms will open or close electrical contacts for a logical signal. · Potentiometer - measures angular positions continuously, using resistance. · LVDT (linear variable differential transformer) - measures linear Displacement 66
  67. 67. Step 1-CHECK INPUT STATUS-First the PLC takes a look at each input to determine if it is on or off. In other words, is the sensor connected to the first input on? How about the second input? How about the third... It records this data into its memory to be used during the next step. Step 2-EXECUTE PROGRAM-Next the PLC executes your program one instruction at a time. Maybe your program said that if the first input was on then it should turn on the first output. Since it already knows which inputs are on/off from the previous step it will be able to decide whether the first output should be turned on based on the state of the first input. It will store the execution results for use later during the next step. Step 3-UPDATE OUTPUT STATUS-Finally the PLC updates the status of the outputs. It updates the outputs based on which inputs were on during the first step and the results of executing your program during the second step. Based on the example in step 2 it would now turn on the first output because the first input was on and your program said to turn on the first output when this condition is true. 67
  68. 68. Deadman Switch A motor will be controlled by two switches. The Go switch will start the motor and the Stop switch will stop it. If the Stop switch was used to stop the motor, the Go switch must be thrown twice to start the motor. When the motor is active a light should be turned on. The Stop switch will be wired as normally closed. APPENDIX A DesignRated Parameters 1. Generation: 2. Load: 3. Boiler steam pressure: 1.62 (MU) max. 67.5 (MU) max. 95 kg/cm2 68
  69. 69. 4. Boiler steam temperature: 5. Opacity: 6. Unburnt carbon in fly ash: 7. Unburnt carbon in bottom ash: 8. Feed water temperature after HPH: 9. Steam temp. at throttle valve before ESV: 10. Steam pressure ESV (kg/cm2): 11. Vaccum: 12. Exhaust hood temp.: 13. Circulating water rise of temp.: 14. Fuel oil pressure (LSHS): 15. Generator stator temp.: 16. Steam flow before ESV: 17. Steam consumption per MW: 18. Turbine heat rate: 19. DM water conductivity: 20. DM water silica: 21. Clarifier water outlet turbidity: 22. Feed water conductivity: 23. Feed water silica: 24. Boiler water conductivity: 25. Boiler water silica: 26. Moisture in turbine oil (at filter point): 27. Auxiliary consumption: 28. Make-up water consumption: 29. Clarifier water inlet conductivity: 540 deg C 150 mg/Nm < 1.0 % < 3.5 % 236.85 deg C 535 deg C 86.49 kg/cm2 695 mm 44.25 deg C 8 deg C 7 kg/cm2 100 deg C 257.5 T/hr 3.81 T/hr 2232 kcal/kWh < 1.0 mho/cm < 20 ppb 10 N.T.U 6 m mho/cm 0.02 ppm 30-70 mho/cm < 1 ppm 100 ppm 11.2 % per day (both units) 624 kl/day (both units) 600-1700 m mho/cm APPENDIX B Details of Fans and Pumps used in the plant Fans Equipment ID Fan FD Fan Type Radial Double Suc. Radial Double Suc. Capacity 81 m3/sec 57 m3/sec 69 Make BHEL BHEL
  70. 70. B.F.P C.E.P PA Fan Radial Double Suc. Centrifugal Radial Single Suc. 318.4 T/hr 283.2 T/hr 18 m3/sec BHEL KSB BHEL Equipment ID Fan FD Fan PA Fan Bowl Mills B.F.P C.E.P Type Squirrel cage I.M. Squirrel cage I.M. Squirrel cage I.M. Squirrel cage I.M. Squirrel cage I.M. Squirrel cage I.M. Capacity (KW) 400 400 190 260 2000 250 Make BHEL BHEL BHEL BHEL BHEL BHEL Drives APPENDIX C Turbine Specifications Make: BHEL 70
  71. 71. Capacity: Exhaust Temperature: Critical speed: Governing: Efficiency: Actual heat consumption: 67.5 MW 44.25 deg C 1800 RPM Hydraulic 95 % 3.85 kg/KWh Generator Specifications Make: Type: Rated Capacity: Power factor: Frequency: Stator: Rotor: Rotor S.C. ratio: Type of Cooling: BHEL- TARI Turbo 673.5 MW/ 84.375 MVA 0.8 lag 50 Hz 10.5 KV, 4639 A 303 V (max.), 601 A 0.56 Air CONCLUSION:During our visit to power plant station from June 07,2010 to July 24,2010. The modernization of world as we see today would not become possible if electrical power do not come into picture. Today each area of Science & technology is highly affected by the use of electrical energy. Electrical energy is the only reliable form of Energy which is easily converted into any form of energy whether it is Mechanical, Chemical, Light, and Sound etc. 71
  72. 72. The electrical energy has been used not only in industrial areas but also in residential and commercial application as well. The reason behind the popularity of this form of energy is because it can be easily transferred to one place to another with efficiency. So at the outset I would like to conclude that there is no doubt without the development of this form of energy, we would never achieved the faster pace of growth rate as today the world is growing and this has become possible only due to the development of power system. THANK YOU 72