200MW-VOLUME-1

KORBA SIMULATOR

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KORBA SIMULATOR

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FOREWORD

Power is the single most important necessity for common people and industrial development of the nation. Electricity can bring a sea change in the quality of life style. Gamut of operating the power plant specially large thermal units having very sophisticated technology and complex control, need to be managed and experience shared by the trained and developed human resources. The Simulators are computer based training tool that are modeled mathematically to provide practical on-job training at real time environment, improve retentivity levels to more than 75%, tuned considerably for high confidence level, over and above the training is completely risk free. The Simulators have been developed to function as the replica of power plant (200MW of Korba unit) and they give the feeling of operating real power plant in a clean and pleasing environment without making use of the auxiliary equipments. NTPC firmly believes that the engineers and the officials operating or having intensions to manage the power plant should be trained regularly through Simulators having features as above. The operation manuals provide the adequate reference information to augment systematic hands-on training. The operational manuals of 200MW Simulator in two volumes (Vol-I & Vol-II) should prove valuable to all the participants of the Simulator Institute, be they the fresh executives under the Executive Trainee schemes of the Company or the experienced power plant engineers particularly operating the power stations or working in the power projects under construction and commissioning phase. It will provide a direct appreciation of basics of thermal power plant operations and encourage them to take on such responsibilities far more sincerely and effectively. The manuals in your hand have been revised suitably based on the feedbacks received from various participants who have undergone training in our Simulator Institute. The revised volume I & II bring together the information from manuals of original equipment manufacturers, theory and course materials & texts from Instruments and Control suppliers/manufactures, efficiency related power plant literatures, water treatment and chemical plants etc. I appreciate the time spent in making the manuals and the exhaustive efforts in bringing these out within the shortest time by the Simulator In-charge, Senior Managers, Faculty Members and the office staff of the Central Simulator Training Institute, KORBA. I hope the readers of the operational manuals will find the contents stimulating and helpful in understanding and managing thermal power plants especially in the operation activities. I believe that in spite of all sincere efforts and care, some areas of improvement might have remained. The suggestions and comments are welcome.

General Manager

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Few words from HOD Simulator & EDC

It has been our endeavour to ensure that the persons responsible for operation of Power Plant should have accessibility to all technical information as presently only running the machine is not enough, but running it efficiently and economically will always give an edge over other power utilities. One of the thrust areas of NTPC management has been skill up gradation and imparting every possible knowledge to concerned employees, which will take the organization into brighter tomorrow. Training gets the utmost priority in our organization. With a view to make the learning easy and spontaneous, need was felt to consolidate the 200MW Power Plant Operation manual in the soft form. At present, Computer and E-Communication have become our essentialities and we have adapted ourselves to it. As a first step towards making our operation engineers equipped with knowledge of operational aspect of power plant and using, it as a knowledge-refreshing tool and making the information available in a CD was thought of. I am pleased to present this CD, the operational manuals of 200MW Simulator in two volumes (Volume I & II), to the personnel who are associated with power plant activities. I sincerely hope that the contents are going to be helpful in upgrading the knowledge and skill of Power Plant employees.

Dy. General Manager

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CONTENTS CHAPTER NO.

TOPIC

1.

FEATURES OF THE SIMULATOR

2.

DATA ACQUISITION SYSTEM (DAS)

3.

CONDENSATE AND FEED WATER SYSTEM

4.

5.

6.

7-13 15-25

CONDENSATE EXTRACTION PUMP

29-31

BOILER FEED PUMP

33-40

BOILER SYSTEM BOILER: GENERAL

43-54

AIR PRE-HEATER

55-57

ID FAN

59-62

FD FAN

63-64

PA FAN

65-66

PULVERISER

67-69

HP AND LP BYPASS SYSTEM HP BYPASS

73-78

LP BYPASS

79-86

TURBINE SYSTEM STEAM TURBINE: GENERAL

7.

PAGE NO

89-100

TURBINE GOVERNING SYSTEM

101-138

TURBINE PROTECTION

139-152

AUTOMATIC TURBINE TEST

153-164

TURBINE STRESS EVALUATOR

165-178

GENERATOR SYSTEM GENERATOR AND AUXILIARIES

181-194

STATIC EXCITATION

195-205

GENERATOR PROTECTION

207-228

8.

MEASUREMENT AND CONTROL

229-338

9.

BOILER WATER CHEMISTRY

339-350

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FEATURES OF THE SIMULATOR

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FEATURES OF THE SIMULATOR Studies on technical feasibility and economical viability of fossil power plants have led to the construction of gigantic sized units equipped with sophisticated control schemes. Increased protection and safety for reliable and efficient operation of these units at the same time ensuring the maximum availability has thrust tremendous responsibility on the operating personnel. The operator is the key to efficiency and safety. As our power plant operation becomes more sophisticated with complex controls, the problem of running a plant profitably and safely becomes critical. Conventional on the job training hitherto imparted has become impracticable and inefficient to develop the skills of a reliable, confident and efficient operator demanded by these modern units. Training simulator plays a vital role by not only training the operator but also by tuning his reflexes in a real time environment. BENEFITS OF TRAINING SIMULATOR Power plant simulator is an effective training tool, with which the actual characteristics of a power plant can be generated through real time execution of mathematical models of various systems on a computer. The trainee operator quickly gains experience in normal, abnormal and emergency operation of power plant through Simulator Training. Operator confidence is increased, resulting in improved efficiency of power plant operating personnel, better equipped to respond to problems and emergencies. The hands-on training in a highly realistic environment provided by the training Simulator cannot be substituted by any other form of training. A welltrained operator runs a plant safely and expensive downtime caused by operator error is significantly reduced. In a highly automated plant, refresher training on Simulator also helps experienced operators to maintain a high level of proficiency. THE PROCESS OF SIMULATION The Simulator creates a realistic representation of any process in an interacting manner. All the engineering systems of power plant are programmed in a computer in the form of mathematical models. The main hardware elements of the simulator system consist of two Main Computers, Shared Memory, Input-Output System and the UCB panel. The computer used for the process simulation is called the Master computer. It is used for computation of various simulation parameters. All the processes and interlocks of the unit are defined by the math models and are iterated by the computer. The inputs from the UCB panel are scanned at a very fast rate and transmitted to the Master computer by the input-output system through a highspeed data link. The computer in accordance with the math models calculates the output parameters and the effects are displayed on the panel in the form of lampoutputs, annunciations, meter/recorder indications etc and updated dynamically. Simulation is based on predicted plant design data. It displays the parameters and provides the necessary alarms or protective system action when plant limits are approached or exceeded. Emergency conditions can be inserted at any time during an exercise or prior to the start of the exercise. The simulator responds dynamically to all changes in the process from within or imposed from outside. KORBA SIMULATOR

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Simulator design includes equipments, instrumentation and controls. This enables an operator to function in all modes of the specified coal fired power plant operation including normal, transient and emergency operating conditions except where specifically noted. Responses resulting from operator actions, automatic plant controls and inherent operating characteristics are copied realistically so that the operator cannot observe any difference (within limits of performance criteria) between the simulator control room indicators and those of the actual power station. The computer used for DAS is called the Slave computer. The data needed for computation of DAS parameters is stored in a common memory called shared memory. The DAS computer has access to this memory and uses this data for calculation of various parameters. All the important parameters are displayed and updated dynamically on various CRTs by means of CRT controllers. This computer also executes several other programs, which provide various facilities like Mimic diagrams, bar charts, group displays of DAS points, video trends etc to the trainee operator. The simulation software is structured and organised in well-defined modules and levels. The various modules are: Computer system software, Simulation executive software and Application software. The system software consists of the operating system (MPX-32, Rev 1.5C) and utilities like text editors, file manager, debugger etc. System level services like management of computer memory, processor time etc is provided by the operating system. The simulation executive software controls the rate of execution of math models and helps in debugging process by tracking the execution sequence. The Application software is further consists of plant simulation software, Instructor station software and DAS software. The Plant simulation software consists of various math-models and subroutines, which are written using FORTRAN-77 and Assembly. The instructor station software enables the operation of instruction station through which simulation is initialised and various facilities of the same become available. The DAS software enables the functioning of its various facilities and features. 200MW SIMULATOR: COMPUTER SYSTEM

Computer

32 bit, GOULD SEL - 32/77

Supplier

GOULD /ENCORE COMPUTER CORP., USA.

Software

MPX - 32 Rev. 1.5 C

Supplier

GOULD / ENCORE COMPUTER CORP., USA

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HARDWARE FEATURES OF THE SIMULATOR Full size Replica Control Room The control panel exactly resembles that of the actual plant. (200MW: unit-I of Korba). All the Switches, Push Buttons, Indicating Lamps and Instruments, Recorders, Annunciations are located precisely at the same position on the simulator control panel as in the real plant. Computer Complex The heart of the Simulator is the Computer and its associated software. Two 32-bit Computers (GOULD SEL-32/77) are the driving force behind the 200MW Simulator. One Computer is used for the simulation of plant system and the other is for Data Acquisition purpose. Instructor Station This is the place from where the instructor is able to control the training process. He can create a number of plant conditions, inject malfunctions, monitor and analyze the trainee’s performance. Several functions are available to the instructor by which he can utilise the training potential to the Simulator to a maximum. Computer Interface This consists of an Input / Output System by means of which data can be transferred from the Computer to the control Panel and vice versa at extremely high speeds. SCOPE OF SIMULATION Following are the systems covered in simulation: • Condensate and Feed Water System • Air and Flue Gas System • Fuel System (Oil and Coal) • Furnace Safeguard and Supervisory System (FSSS) • Steam Generator System • Turbine System • Automatic Turbine Run-up System (ARTS) • Cooling Water System • Electrical Unit Distribution System • Hydrogen and Seal Oil System • Analog Control System (ACS) • Main Generator and Auxiliaries System KORBA SIMULATOR

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SIMULATOR OPERATIONAL FACILITIES From a remote console, the instructor can implement following training features: •

Initial condition

•

Snapshot

•

Remote operator functions

•

Freeze/Run

•

Malfunction activation and removal

•

Programmable Response Time (Real time, Slow time and Fast time)

•

Backtrack

•

Record/Replay

•

Remote control

•

Computer Assisted Exercise (CAE)

•

Trainee Proficiency Review (TPR)

Initial Condition/Snapshot It is a programmed status of the plant from where simulation is to start. There are fifteen initial conditions, which can be chosen for starting simulation. The feature of snapshot allows storing the plant status at a given instant during simulator operation for later use as initial condition. Remote Operator Functions The instructor serves as an auxiliary operator in providing the operation of manual valves etc. located outside the main control room and other controls not provided on UCB panel. Freeze/Run This feature allows all dynamic actions to be suspended during the simulation status remaining intact. This gives the instructor time to discuss the frozen simulated plant condition. Malfunction Activation and Removal Malfunctions simulate fault conditions, which can occur within the plant. Instructor introduces them in the process from the console or hand-held remote transmitter. Programmable Response Time Normally simulator runs in one to one correspondence with real time. Instructor can select either slow time or fast time. Real time expansion is slow time; simulation provides an apparent increase in time for fast changing phases of plant operation KORBA SIMULATOR

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such as feed water process, unit trip sequences, characterised by short time constants. When activated all math models are called at one tenth their normal rate causing all apparent operations to slow to one tenth real time rate. Real time compression is fast time simulation providing an apparent decrease of time intervals for less dynamic phases of plant operation such as turbine warm up which usually takes long intervals. Some of the models run at ten times the normal rate under this condition. Backtrack In automatic snapshot of the simulator, plant status condition will occur at a time interval of one minute for a period of 60 minutes. This feature allows the instructor to select any one of these past 60 selected set of conditions and initialise the simulator at that specific backtrack time. This feature is known as backtrack. Record/Replay This feature permits the status of the control panel displays and indications to be saved during operation for future replaying by instructor. Up to one hour of simulator can be saved. Computer Assisted Exercise (CAE) CAE permits the instructor to develop and store training scenarios, including malfunctions and remote functions, for uniformity of performance testing. Trainee Proficiency Review (TPR) TPR permits automatic monitoring of instructor selected parameters for their deviations during operation, above or below the selected/set limits of safety and efficiency and also the time for which the parameter remained out of contact can also be computed and recorded.

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DATA ACQUISITION SYSTEM (DAS)

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DATA ACQUISITION SYSTEM (DAS) Data Acquisition System (DAS) in the case of 200 MW units is for monitoring of process data. DAS system is not used here for process control. This means that the person on desk can get readily available information about the different process parameters on different display devices as well as on the printers but he cannot use the DAS system for the control of any parameter. PROCESS INPUTS Process inputs to the DAS system are fundamentally of two types: Analog Inputs and Digital Inputs. Analog Inputs The types of analog inputs are as follows: • 0-10 volt analog inputs. • 4-20 mA inputs. • Thermocouple inputs. • RTD inputs. • All other inputs of analog nature as may vary from plant to plant. Digital Inputs Digital inputs have only two states. Any input with only two states namely OPEN/CLOSE, ON/OFF, TRIP/ NORMAL, HIGH/LOW etc falls in this category. ANALOG INPUTS The number of analog inputs in the case of Simulator is of the order of 650. These are the direct inputs coming from the process. In addition to these process inputs, there are some more inputs of analog type, which are not directly coming from the process but are derived from the inputs coming from the process. These inputs are called Calculated Inputs. Miscellaneous Calculation Inputs Calculated inputs are of two types. One type of calculation is mostly averaging or differentials. Say for instance, we are measuring the casing temperature for BFP and these points are directly coming to the DAS. We can calculate the difference between the upper and lowercasing temperature to get the casing differential temperature. Also we can add the coal flow of all the mills per hour and get the total coal flow per hour. We can also add the economiser outlet FW temperature left and right and divide by two to get the average economiser outlet FW temperature.

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DATA ACQUISITION SYSTEM (DAS): BLOCK DIAGRAM

Thus we can derive the following three derived inputs which can be treated as DAS inputs but which are not directly coming from the process. These are: 1.

BFP casing differential temp.

2.

Tons of coal fired/hour.

3.

Economiser outlet FW temp.

This type of calculation is called Miscellaneous Calculation. Performance Calculation Inputs This consists calculation Inputs •

Terminal temperature difference of different heaters.

•

Excess air percentage.

•

Turbine efficiency.

•

Boiler efficiency by different methods.

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•

Cycle efficiency.

•

Heat rate deviation from standard.

The total number of calculated input in the case of analog point is approximately 200. This includes both miscellaneous calculation and performance calculation. ANALOG SCANNING, ALARMING The analog inputs coming from the process are scanned by the computer at various rates depending upon the criticality of the parameter and the computer capability. The different rates of scanning of analog inputs are 1 sec., 2 sec, 12 sec, 30 sec and 60 sec corresponding to each analog input, there are two limits, one high and the other low. It is of course not necessary that each and every analog input will have both high as well as low limits. There are some inputs, which have only high limits and not low limits and vice versa. If a high or low limit is kept at a defined value then if the analog input varies very near to that limit and oscillate, this will cause the alarm to appear, on different display devices at one time and vanish at the next time. To avoid this, each point having alarm associated with it is provided with a dead band. The alarm appears whenever the value goes beyond high or low limit but the alarm stays so long as the value does not come below the dead band value. All analog points which are having alarms have three types of alarms, both for high as well as low. They are 1 HI, 2 HI, 3 HI or 1 LO, 2LO, 3LO. The alarm described above is 1HI or 1LO, 2HI or 2LO. Alarm comes when the value increases beyond 1HI or 1LI. The value between 2 HI & 1 HI or 1 LO & 2 LO is called repeat increment. The 3HI or 3LO alarm comes when the value deviated further from the normal value. Beyond this (3HI or 3LO) there is a digital status, which will cause tripping of the particular device if of course, tripping is provided for that device. Variable Limit Alarms However, there may be analog point whose alarm value is dependent on the load. That means the alarm value will change depending upon the MW generated. For that we have variable limit of alarms. This load dependent variable limit, calculations are also part of miscellaneous calculations, which has already been discussed. Alarm Cutouts Let us consider a case where BFP-C is not running & BFP-C flow is 0. This is a condition where BFP flow low alarm will appear, even though this is not an alarm. So some means are required to avoid these alarms. For that, there are cut out equations, which will see the digital status of the equipment before displaying the alarm. The alarm will be displayed only when that equipment is running. This is more important for mills, feeders, BFPs, etc.

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PERFORMANCE CALCULATION The performance calculation points are treated in a different way than miscellaneous calculation points. The miscellaneous calculation points are scanned all the time at the same rate as the normal process points. For performance calculation, past ten minutes value of the points that are used for the performance calculation is stored in the memory. It is not required to store each and every scanned value of past ten minutes but a number of values. Whenever the operator asks for the log of performance calculation points, these values are averaged and this averaged value is used for the calculation. For example, if we want the boiler efficiency, then all the input scanned points required for this calculation are averaged for last ten minutes and this averaged value is then used for the calculation. To elaborate, take the example of boiler efficiency measurement by input, output method. The inputs for this calculation are: 1. FW Flow. 2. Reheat spray flow. 3. Cold reheat flow. 4. Tons of coal fired per hour. 5. Heavy oil supply flow. 6. Heavy oil return flow Except point 4, all in the above are process points. Point 4 is a miscellaneous calculation point is obtained by adding the coal flow through each feeder. The scanned values of the above points are stored and whenever the calculation is demanded, first the averaging of past ten minutes is done for all the above points. In addition to the above, we need the following information: Blow down flow. Heating value of coal. Heating value of fuel oil. Heat added other than chemical. SH outlet enthalpy (Function of SH outlet temp. and pressure). FW enthalpy at economiser inlet (function of FW temp. and pressure). RH outlet enthalpy (function of RH outlet steam temp. and pressure) HPT exhaust enthalpy (function of RH outlet steam temp. and pressure) Blow down enthalpy (function of drum pressure).

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The first four are the constant point, which are entered by the engineer and remains constant unless changed and the computer does enthalpy calculation from the steam table entered. However, for performance calculation points, it should be noted that all the above calculations, which are done on the basis of performance test code, are valid only from 30% to 100% load. So the calculation is not done below 30% load. DIGITAL INPUTS The number of digital inputs is of the order of 1300. These are the process inputs coming directly from various equipments like pipes, ducts etc. Since the digital points are having two states, the scanning of these points are much faster than the analog points, All digital points under normal circumstances are scanned at every one see. Whereas number of analog points scanned per sec. is of the order of 110. Of the process points there are some two hundred inputs, which are high-resolution digital inputs. These inputs are different from other inputs in that if anyone of the above inputs goes to the alarm states then all high-resolution digital inputs are scanned at a much higher rate of 5-milli sec. This scanning goes on until there is no change in status of the high-resolution digital inputs for a period of two minutes. After that it gives a sequence of events recording on printer. In the digital, it is not necessary that all inputs will have an alarm associated with it. For instance, a BFP off is not an alarm state. In general, the set point for digital is set at a value slightly higher than the corresponding analog value so that the engineer comes to know about alarm in advance and takes necessary action. The different type states in the case of digital points are HI-NORMAL, LOW-NORMAL, HIHI-NOT HIHI, LOLO-NOT LOLO, TRIP-NORMAL, ON-OFF. GENERAL DESCRIPTION OF SIMULATOR DAS Any DAS point in the case of NTPC 200 MW unit consists of five characters, two alphabets followed by three numerics. The example is: SF005, ET005, FT003,

TV501

The first two characters represent the system. In the above, S represents the steam generator point, E represents electrical point, F represents feed water point and T represents turbine point. The second letter represents the parameter measured. In the above case (SF005), F represents vibration. The next three numeric characters represent whether it is an analog point or a digital point. In NTPC philosophy, any points from 001 to 449 are analog points and 500 to 999 are digital points. Thus in the above SF005 and FL003 are analog points where as ET501 are digital points. Any points whether analog or digital; if the first two characters are KC or KV then they are calculated points. If the first two characters are KC, then they are miscellaneous calculation points. If it is KV, it is performance calculation point, which is only analog point. All the points, which start with KN, are constant points, which are used for performance calculation. The AV points are averaged values of the process points, which are used in the performance calculation.

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CAPABILITIES OF THE SIMULATOR DAS SYSTEM The Simulator DAS system is having the following capabilities: a.

Point detail of any point digital or analog.

b.

Review of analog or digital points.

c.

Group display of any point analog/digital

d.

Turbine message display

e.

Display graphics of various systems

f.

Acknowledge of alarm

In the DAS system, for the output purpose, there are four colour CRT' s and two printers. The CRT on the unit controller' s desk is called operator console-1 or in short OPCON 1. The one on the UCB section 2 i.e. on the middle of the UCB panel is called OPCON 2 and the one on UCB section 1 of is called the UTILITY CRT. There is a fourth CRT in UCB section 3, called as ALARM CRT. There are two keyboards. One is on the unit controller' s desk and the other with the OPCON 2 CRT i.e. section 2. The alarm CRT is dedicated to alarms. The analog points, which cross its normal limits or the digital points, which are having an alarm, will blink as long as alarm is not acknowledged. The new alarm will come on the top of the first page. In total there are eight pages of alarms. There are previous page and next page buttons through which we can go to any page of alarm. But if while going to previous page or next page, a new alarm comes, it reverts back to the first page. New alarm appears on the top of the first page pushing the previous alarms down the page. The number of alarms in each page is 20. But if all eight pages are full with alarms and new alarm comes, the last alarm in the last page vanishes creating space for the subsequent alarm to come down so that the latest alarm appears on the top of the first page. When the alarm acknowledge button on any of the two keyboards is pressed, then all alarms, which are flashing, get acknowledged and the flashing stops. If an alarm returns normal, then its colour changes to green. When the acknowledge button is pressed, the alarms which returned to normal value vanishes. The alarm compress button on the keyboard can be used to compress the empty space. It may be noted that other than compressing, acknowledging, next page and previous page, there is no other control of the alarm CRT. That means this CRT is dedicated for alarms only and no other display is possible on this CRT. The other CRT' s can be used for any display. With the latest DAS software modifications, alarm CRT can be used for display. However, if it is a communication between operator & CRT, that communication is possible only through OPCON 1 and OPCON 2 and not through any other CRT. GRAPHICS The system can display the P&ID diagram in the form of graphics with dynamic capabilities. Dynamic capabilities mean, the value and the status of the graphic displayed are updated continuously and it is not necessary to call the graphic every time to get the latest information. In the graphics arrangement, status of various equipments, pipe ducts, valves, dampers etc are shown by different colour. For KORBA SIMULATOR

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example colour of the ducts are yellow when there is flow of air through it and it is half intensity white when there is no flow. The colour of a PA Fan is green when off, red when running and white when tripped. Thus, just by seeing the colour, the status of equipment can be ascertained. A valve open is shown in red colour and a valve closed is shown by green colour. For selecting graphics, first one will have to select a system. There are ten systems here, namely: boiler air, boiler steam, feed water, boiler gas, condenser, circulating water, turbine, coal mill, water steam cycle, boiler water. FUNCTION MENU The functions of this menu are: •

To update the time of the Computer

•

To bring any alarm page from alarm CRT to any other CRT.

•

Display points of different groups on CRT.

•

To display analog parameter in the form of bar chart.

•

To assign any analog parameter to the trend recorders.

•

To enable or display post trip log.

•

To display any analog parameter in the form X-Y plot.

Update Time This is a communication format by which the operator can enter present date and time. The operator in the ‘fill in the blanks’ format can insert the present date and time. Alarm Paging Supervisor Using the function menu, we can go to this supervisor through which by fill in the blanks format the operator can select any page of alarm CRT to appear on any other CRT. Log Supervisor By this we can get a point out of a log group in the printer. A log consists of some 6 to 7 groups each having some 20 points. Example for the logs are: Boiler run-up log, Turbine run-up log, hourly log, summary log, Turbine Generator diagnostic log, performance log etc. Boiler start-up and turbine start-up logs are automatic in the sense that they start automatically. When boiler is lighted up, collections for boiler start-up log start and it gives printout after desired number of collections. The same is the case with turbine run up, which starts when the turbine rolling starts. Most of the logs are automatic which causes these logs to come after a specified event or after a specified time interval. Through log supervisor, we can assign any point to a group and also we can assign any group to any log. We can, instead of taking the log on printer, get the display on CRT.

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Group Supervisor There are a number of points (16), which are assigned to a group. In all we have 40 groups. From function menu through group supervisor, we display any group on the selected CRT. This is an important function because we can display both analog and digital parameters on the CRT, which will dynamically update. We can assign any point to any group or a number of points to the same group. Bar Chart Supervisor The bar chart supervisor is to display bars in the form of horizontal or vertical, of a group of points, which are of similar type if it is a vertical bar and or any type if it is a horizontal bar. We can go to bar chart supervisor through function menu. The vertical bars are included in the graphics in the case of Simulator DAS whereas horizontal bars are under chart supervisor. In one page there can be 8 particular variables. Under normal conditions the bars are green, if the value exceeds beyond high or low limit then the colour change to red and dead band is shown in yellow colour. Trend Pen Supervisor The recorders on the Panel are assigned to some particular analog variables. The DAS gives a facility to assign any analog variable required to be assigned to any of the nine pens. In all nine analog variables can be assigned to all the nine pens at a time. The assignment may be changed as and when required by the operator through man machine communication format. Post Trip Supervision Whenever a unit is started, enabling post trip log through this supervisor, will give a print out of all points of the post trip log when the unit trips. Post trip log is having some 100 analog points, which are important ones and which gives a clear picture of parameters changed from 3 minutes prior to tripping to 5 minutes after tripping. All the points, which are coming in this log, are of such type that any time 5 values of all the variables of last 5 minutes are always available. When a trip occurs considering that to be zeroth time values of all these variables are collected up to a time five minute after tripping. Then the printout of post trip log starts. Thus post trip log print out comes only after trip occurs and it gives ten values for all variable starting from five minutes before tripping five minutes after tripping. Post trip log points are all analog points. X-Y Plot Supervisor This supervisor is to display a process variable with respect to time and with respect to total generation. Each display consists of seven points plotted with respect to time. The plot of the variable is for past ten minutes. In each display gross generated MW is plotted. Each plot is identified from the other by the colour because each variable plot colour is different from other.

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POINT REVIEW FUNCTION Unlike the group supervisor and the log supervisor the point review gives the points display in the alphanumeric sequences. This gives one-shot value of the points in either CRT or printer. Here values of both analog and digital, in the same review and in the same sequence can be obtained. The values of all points, scan analog & digital points, calculated analog & digital points and constant points along with the limit set for all scan and calculated analog points can be seen in the display and in the printer. POINT DETAIL The point details function is to get the value information about a particular point. The point detail of a point gives much more information, but they are not relevant to the operator. Step Next Point When we are in point details then pressing step next point button causes the display of the next point in analog/digital in alphanumeric sequence. There are some more keys: OPCON 2, Utility CRT. These keys are for selecting the particular CRT to which subsequent instructions are to go. TURBINE MESSAGE DISPLAY When turbine is rolled, pressing this button gives the criteria not satisfied in a step with description. This is an added feature and included because the ATRS console in KWU turbine does not give the description of the criteria in the step. The status changes dynamically in the display as in the case of group review and this can be displayed in any of the three CRTs. SYSTEM MENU This is also another means to display graphics. Instead of selecting the system by the keys say mill or boiler air etc., we can go to system menu and select the system and then through display list or display diagram we can display graphics. This gives a brief idea of DAS system in general and the DAS system in the simulator. The keyboard operation described pertains to the simulator DAS system that may vary from system to system. ANALOG SCANNING, ALARMING The analog inputs coming from the process are scanned by the computer at various rates depending upon the criticality of the parameter and the computer capability.

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CONDENSATE AND FEED WATER SYSTEM

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CONDENSATE EXTRACTION PUMP (CEP) INTRODUCTION The condensate extraction pump (CEP) is a centrifugal, vertical pump, consisting of the pump body, the can, the distributor housing and the driver lantern. A rising main of length depending upon NPSH available, is also provided. The pump body is arranged vertically in the can and is attached to the distributor body with the rising main. The rotor is guided in bearings lubricated by the fluid pumped, is suspended from the support bearing, which is located in the bearing pedestal in the driver lantern. The shaft exit in the driver lantern is sealed off by one packed stuffing box. Casing It is split on right to the shaft and consists of suction rings and 4 no. of guide vane housing. Casing components are bolted together and sealed off from one another by ' O' rings. For internal sealing of individual stages, the casing components are provided with exchangeable casing wear rings in the arc of impeller necks. In each guide vane casing, a bearing bush is installed to guide the shaft of pump. Rotor The pump impellers are radially fixed on the shaft by keys. The impellers are fixed in position axially by the bearing sleeves and are attached to the shaft by means of impeller nut. Impellers are single entry type, semi-axial and hydraulically balanced by means of balance holes in the shroud and throttle sections at suction and discharge side. A thrust bearing located in the motor stool absorbs residual axial thrusts. Bearings In each guide vane housing the shaft is guided by a plain bearing. These bearings do not absorb any axial forces. Pump bearings consist of bearing sleeve, rotating with the shaft and bearing bush, mounted in guide vane housing. The intermediate shaft is guided in bearing spider and shaft sleeve. The arrangement of bearing corresponds to the bearings of pump shaft. They are lubricated by condensate itself. A combined thrust and radial bearing is installed as support bearing to absorb residual thrust. Axial load is transmitted to the distributor casing via the thrust bearing plate, the thrust bearing and bearing housing. A radial bearing attached to the bearing is installed in an enclosed housing and is splash lubricated by oil filled in the enclosure. Built-in cooling coils in the bath and cooling water control oil temp. Shaft sealing The drive shaft passage in the distributor casing is sealed off by packed stuffing box with lantern ring. During operation, the packed stuffing box reduces the leakage flow in the clearance between shaft protecting sleeve and stuffing box housing. The shaftprotecting sleeve is sealed off from shaft by ' O'rings. To prevent air entry during standstill operation or during reduced pressure operation as well as for cooling stuffing box sealing water is fed into the stuffing box via lantern ring. Connecting line to suction relieves the shaft-sealing chamber.

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CEP CONNECTIONS

Coupling The pump shaft coupling is connected to the driver shaft by a rigid clamp coupling. A flexible claw coupling is used to transmit the torque from driver to drive shaft. Venting/sealing/cooling fluid lines A vent line connects the suction compartment to the top of suction vessel, with an isolating valve. This is to vent the pump off any gas formation, which might interfere with smooth running of pump. From the pressure relief line a partial flow is branched off as sealing fluid. Pump Lubrication System Motor bearings are grease lubricated. The pump bearing is a thrust-cum-journal bearing sub-merged in an oil bath. The oil is cooled by clarified water.

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TECHNICAL DATA Pump Design Type

WKTA 200/4

Speed

1480 rpm. 3

Discharge Capacity

Overload 1480 3

Head

610 m /hr. 190 M

710 m /hr 170 M

Power

402 KW

429 KW

Temperature of medium handled

Water at 40 oC 3.5 M

NPSH Motor Power

470 KW

Supply

6.6 KV, 3-phase, 50 Hz, PF=0.85 lag.

Current

51 Amps.

Speed

1480 rpm.

Class of Insulation

F

CONDENSATE SYSTEM

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BOILER FEED PUMP (BFP) INTRODUCTION The weir type FK8D30 pressure stage pump is an eight stage horizontal centrifugal pump of the barrel casing design. The pump internals are designed as a cartridge, which can be easily removed for maintenance without disturbing the suction and discharge pipe work, or the alignment of the pump and the turbo coupling. The pump shaft is sealed at the drive end and non-drive end by Crane mechanical seals, each seal being flushed by water in a closed circuit and the water is circulated by the action of the seal retaining ring. The flushing water is cooled by passing through seal coolers, (two coolers per seal, one working and one standby), and each seal cooler being circulated with clarified cooling water. The rotating assembly is supported by plain white metal lined journal bearings and axially located by a Glacier double tilting pad thrust bearing.

BFP CONNECTIONS

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TECHNICAL DATA BOILER FEED PUMP Manufacturer

: Weir Pumps Ltd

Pump Serial Number

: 11723-001/9

Type

: FK8D30

No of stages

: 8

Direction of rotation viewed at drive end

: Anti-clockwise

Liquid pumped

: Boiler feed water

S.G. at suction temperature Suction temperature oC Suction pressure, Kg/cm

2

Discharge pressure, Kg/cm

2

Differential pressure, Kg/cm Differential, m

2

NPSHA above impeller eye, m 3

Design

Duty

0.904

0.906

163

161

14.42

15.32

212.66

188.86

198.24

173.54

2196.8

1918.9

96.8

101.7

381

Flow rate, m /h Speed, RPM

4750

Power, KW

2542

BFP MOTOR

Design

Output

3500 KW

Current

361 A

Supply

6.6 KV, 3 phase, 50 Hz.

Power factor

0.8 - 0.85

Efficiency

95.5%

Speed

1485 rpm.

Type of cooling

Closed air circulation

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BFP DESCRIPTION Pump Casing It consists of forged steel barrel with welded suction discharge branches and mounting feet. A suction guide closes the drive end of casing and it is located on the inner casing by a spigot. For prevention of leakage between suction annulus and barrel casing a MS gasket is located between the suction guide spigot and casing inner face. An ' O' ring is also provided on the periphery of suction guide for preventing leakages. Leakage between suction annulus and the drive end is prevented by an ' O'ring and gasket located on the insert ring, secured to the casing by studs/nuts. A discharge cover closes the non-drive end with an ' O'ring secured to base plate pedestals by spacer pieces, washers and holding down bolts, allowing expansion. Transverse key on the drive end feet and longitude keys under the casing transfer moments and thrust to the base plate, allowing for expansion. Discharge cover It also forms the balance chamber; which do the non-drive end water jacket and mechanical seal-housing close. Discharge cover is in close fit in casing bore and is held in place by a ring of studs/nuts. A spring disc is located between the last stage diffuser and discharge cover balance drum bush to provide force required to hold ring section assembly in place against the drive end of the barrel before start up. After starting the discharge pressure assists the spring disc in holding the ring section in place. Last stage diffuser can slide freely over the balance drum bush which is shrunk on to the discharge cover bore to minimise flow of liquid to balance chamber. Two radial holes drilled through the periphery provide outlet connections for balancing chamber lead off to pump suction. Two similar holes are provided for cooling water connection for water jacket. Non-drive end bearing housing is attached to bearing bracket secured to outer face of discharge cover by stud/nuts and dowel pins. Suction Guide It closes the drive end of casing and forms the suction annulus. Ring section assembly, the discharge cover and the spring disc hold suction guide against an internal shoulder in the casing. The drive end water jacket and mechanical seal housing close the suction guide. Two tapped holes are provided in the suction guide for cooling water connections to water jacket. The drive end bearing housing to be attached to bearing jacket secured to the outer face of suction guide by studs, nuts and dowel pins. Ring Section Assembly The ring section assembly consists of seven ring sections which locate one to another by spigots and are secured to each other by socket head screws in counter-bored holes, sealing being effected by metal to metal joint faces and ' O'rings with back-up rings located in grooves in the ring section spigots. Diffusers are dowels and spigot KORBA SIMULATOR

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located to ring sections and to the suction guide, and the last stage diffuser is secured to the last stage ring section with socket head screws in counter bored holes. Packing rings are shrunk into bore of ring section and grub screws locate diffusers. These prevent recirculation of pumped liquid between stages. Ring sections and diffusers form transfer passages from the impeller outlet of one stage of the pump to other impeller inlet of next stage. Rotating Assembly Dynamically balanced rotating assembly consists of shaft, impellers, abutment rings, keys rotating parts of mechanical seals, shaft nuts, balance drum, thrust collar and pump half coupling. Chromium plated shaft at each end is supported by journal bearings and its diameter increases in increments from the non drive end towards drive end to facilitate fitting and removal of impellers. Impellers are of single entry shrouded inlet type and are keyed and shrunk onto the shaft, keys per impeller are being alternately fitted on diametrically opposite sides of the shaft to maintain balance. The hub of each impeller butts against a split abutment ring fitted in a groove in the shaft. The balance drum is keyed and shrunk onto the shaft and held in position against shaft locating shoulder by balance drum nut and lock washer. Inner end of drum is recess and the bore of recess is a close fit over the last stage impeller hub. Provisions are made to inject oil for removal of drum and tapped holes are provided for withdrawal. The rotating parts of mechanical seals are fitted to the shaft where it passes through the seal housing. Seal sleeves are keyed to shaft and are clamped in position by seal sleeve nuts and lock nuts. Thrust collar is keyed to the non-drive end of shaft and is secured against a shoulder on the shaft by the thrust collar nut locked by lock washer. Mechanical Seals They comprise a seal body assembly secured to seal housing, which contains rotating components of seal. Each seal consists of rotating tungsten carbide seat mounted in a carrier, running against a stationary carbon face. Contact between the face and seal is maintained by hydraulic pressure during running and by spring pressure during start-up. Other leakage paths are sealed with ' O'rings. The seal is designed to recirculate the pumped product through seal water coolers to maintain acceptable temperature in the region of seal face. Journal and thrust bearing The rotating assembly is supported at each end of the shaft by a white metal lined journal bearing and a tilting pad double thrust bearing mounted at non-drive end of pump carries the residual thrust. Journal bearing shells are of mild steel, white metal lined, thin wall type and are split on horizontal plane through the shaft axis. Thrust bearing is fitted in non-drive end bearing housing and has eight white metal line tilting pads, held in split ring positioned on each side of thrust collar. The carrier rings are prevented from rotating along with shaft by dowel pins in each ring, which engage in slots in the bearing top half. Thrust pads are retained on the carrier rings by special pad stops screwed into the rings. KORBA SIMULATOR

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A split floating oil-sealing ring is located in the groove in the thrust bearing housing to restrict the escape of lubricating oil from the thrust-bearing chamber. To ensure thrust bearing remains flooded, an orifice is fitted at the oil outlet. Hydraulic balance Due to differential pressures acting on the impeller the rotating assembly is subjected to axial thrusts. The balance drum located at the non-drive end is designed to keep these forces neutralised and only the residual thrust remains, which is taken up by thrust bearing. The main components of hydraulic balancing arrangement are the balance chamber machined in discharge cover, the balance drum secured to the shaft and balance drum bush fitted in the bore of discharge cover. The thrust caused by the suction pressure acting on the area inside the wear ring on inlet side of each impeller is overcome by much greater thrust caused by the discharge pressure acting on the equivalent area on the outlet side of each impeller. The resultant thrust is therefore towards drive end of pump. Thrust force varies with load on the pump but hydraulic balance arrangement will reduce its effect enabling residual thrust to be taken by fitting pads of thrust bearing. The hydraulic balance arrangement operates as follows. The pumped feed water passes from last stage of the pump between the balance drum and the bush and enters the balance chamber at a pressure approximately equal to the suction pressure. Two ports in the discharge cover allow the product to be piped back to the pump suction side. The pressure differential across the balance drum is therefore equal to that across the impellers. The cross sectional area of the balance drum is sized to give a small residual thrust towards the drive end of the pump. Flexible coupling Between the turbo coupling (hydro-coupling) and pump shaft, a flexible coupling, consisting of two hubs flexibly connected through laminated steel elements to a tubular spacer, is provided. This can accommodate a certain amount of misalignment between turbo couplings and pump shafts to which hubs are fitted.

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FEED WATER SYSTEM

DEAERATOR

SYSTEM

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BFP BOOSTER PUMP Introduction Each pump set consists of a weir type booster pump directly driven from one end of the shaft of an electric driven motor and a weir type pressure stage pump (Main Pump) driven from the opposite end of motor shaft through a VOITH type variable speed turbo-coupling. The drive is transmitted, in each case through a Torsiflex spacer type flexible coupling, each coupling being enclosed in a split, fabricated guard. The bearings in booster and main pump and in motor are lubricated from the forced lubricating oil system incorporated in the turbo coupling. Each pump set is supplied with a metallic suction strainer, a NRV on main pump discharge pipe and minimum flow recirculation system comprising a pneumatic valve and a non-return valve. Each pump, motor and turbo-coupling is mounted on its own base plate and on a common grillage. The pump set is provided in each case, with instrument panel and instrumentation for monitoring feed water pressure, temperatures, bearing temperatures, lub oil pressure etc. The booster pump is a single stage horizontal, axial split casing type, having the suction and discharge lines on casing bottom half, thus allowing the pump internals to be removed without disturbing suction and discharge pipe work on the alignment between the pump and driving motor. The pump shaft is sealed at drive and non-drive end by Crane mechanical seals, which are cooled by a supply of clarified water. The rotating assembly is supported by plain white metal lined journal bearings and axially located by a glacier double fitting pad thrust bearing. TECHNICAL DATA PUMP Manufacturer

Weir Pumps Ltd

Pump Serial Number

11723-010/018

Type

FA1F56

Direction of rotation viewed at drive end

Anti-clockwise

Liquid pumped

Boiler feed water

S.G. at suction temperature Suction temperature oC Suction pressure, Kg/cm

2

Discharge pressure, Kg/cm

2

Differential pressure, Kg/cm

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2

Design 0.904

Duty 0.906

163

161

7.263

7.07

15.118

16.02

7.855

7.95

39

Differential, m

87

88

NPSHA above impeller eye, m

16.85

16.0

381.6

346.2

3

Flow rate, m /h 3

Leak-off flow, m /h Efficiency, %

95.4

Speed, RPM

1485

Power, KW

104.5

78

75.5 99.4

DESCRIPTION Pump Casing The cast steel pump casing is of double volute type, split on horizontal centre line. The bottom half pump casing has the suction and discharge branches and support feet cast integrally. A flanged air vent connection is provided on top half casing for initial venting of air during line-up. Connections are also provided on suction and discharge pipes for pressure gauges and drain. Rotating Assembly The dynamically balanced assembly consists of the shaft, impeller, nuts, keys, seal sleeves, thrust collar, rotating part of mechanical seals and pump coupling. The double entry impeller is keyed to the shaft and is located axially. Journal & Thrust Bearings The rotating assembly is supported at each end of the shaft by a white metal lined journal bearing and residual axial thrust is taken up by a tilting pad double thrust bearing mounted at the non-drive end of the pump. The bearings are supplied with lubricating oil from forced lubrication oil system. Mechanical Seals The drive and non-drive end stuffing boxes are fitted with mechanical seals located within seal cooling jackets to prevent feed water escaping along the shaft. Clarified water flow is maintained through cooling water jackets.

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BOILER SYSTEM

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BOILER: GENERAL DESCRIPTION Steam generator is radiant reheat, dry bottom, natural circulation, single drum, semi outdoor type, direct fired, balance draft, top supported type, has provision for firing coal as the principal fuel and is of Combustion Engineering, USA design. Super-Heater / Re-Heater Section The super heater steam system has mainly three sections, the low temperature superheater (LTSH) the radiant platen superheater and final superheater. Two numbers of de-superheater have been provided in between the LTSH and platen superheater (in the connection links) for controlling the superheated steam temperature over a wide load range. The complete second pass of the boiler up to economiser has been covered with steam cooled superheater wall sections. The complete reheater is in one section; which has been located in the horizontal pass of boiler, in between platen and final superheater sections. An emergency reheater de-superheating unit has been provided at the inlet of reheater. Flue Gas heat recovery System The economiser is non-steaming continuous finned type arranged between the LTSH and air heater section. The boiler has two numbers regenerative air heaters of the trisector type for the last stage of heat recovery. The flue gas occupies (1800), i.e., half of the portion. 120o is occupied by the secondary air and the rest (i.e. 600), is occupied by primary air. Two steam coil air pre-heaters are also provided in each of FD Fan discharge ducting to heat up the secondary air prior to entering LUNGSTORM Air Pre-heaters. The SCAPHs are to be charged to maintain cold end temperature of air heater to avoid cold end corrosion. Draught System The draught system includes two induced draught fans, forced draught fans, ignitors, scanner air fans and steam coil air pre-heaters. The forced draught system provides the air required for combustion of fuel, and induced draught system expels the flue gases through stages maintaining balance draught. This system also supplies air to scanner cooling and for lighting up ignitors. Two axial flow reaction type forced draught fans are provided to supply the necessary secondary air for combustion. Two axial flow impulse type induced draught fans are provided to evacuate the flue gases from boiler. Four electrostatic precipitators are provided in each flue gas path, to remove the fly ash from flue gas before it enters ID Fans. ESP passes comprises seven collecting zones. In addition to above, the system includes various ducts and dampers required for maintaining the desired flow, pressure of air or the flue gas depending on demand.

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GENERAL ARRANGEMENT OF BOILER

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1.

ECONOMISER

7.

PLATEN SUPER HEATER

2.

BOILER DRUM

8.

FINAL SUPER HEATER

3.

DOWN COMERS

9.

REHEATER

4.

WATER WALLS

10.

BURNERS

5.

WATER WALL PLATEN

11.

IGNOTORS

6.

PRIMARY SUPER HEATER

12.

FRS (FEED REGULATING STATION)

FUEL FIRING SYSTEM The boiler has direct pulverised coal firing system, which comprises of raw coalbunkers, R.C Feeders, Bowl Mills, discharge piping, coal nozzles with tilting tangential firing system, primary air fans and seal air fans. Each mill supplies the pulverized coal to all the four corners of an elevation. Thus there are six tiers of coal burners and in all twenty four coal burners. The entire burner assembly for all four

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corners can be tilted in the vertical plane (+ 30o) by a burner tilting arrangement; basically for controlling the steam temperature and particularly the hot reheat temperatures. To ensure increased safety, reliability and care in operation, the fuel firing system is equipped with Furnace Safeguard Supervisory System which facilitates single operator to start, stop and control the complete firing system from remote control panels. In the bowl mill, pre-crushed coal is pulverised to desired fineness and is further directed by the primary air. Cold and hot primary air dampers are provided to regulate the flow/ temperature of the primary air. Boiler is equipped with sophisticated flame sensing scanners mounted on all the four corners at three different elevations. In order to cool these scanners, scanner air fans are provided.

LOCATION OF RADIANT AND CONVECTION SUPER-HEATER

ASH DISPOSAL SYSTEM The bottom ash handling system for each unit is capable of removing bottom ash at a rate not less than 15 T/Hr. and conveys it to trenches in slurry form. The ash removal is done continuously. Both side slag baths are provided with continuously moving feeders for transferring the wet slag ash to the respective clinker grinders and is then discharged with the aid of the high-pressure water jets. Fly ash is collected in each of the ESP, air heaters, and economiser and stack hopper. The flushing equipment serves to mix the ash with low-pressure water and discharge the ash in the form of slurry into the ash slurry pit for further disposal by means of slurry pumps.

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BOILER DRUM AND DRUM INTERNALS

FURNACE TEMPERATURE PROBE It is an electro-mechanical equipment for positioning a thermo-couple element in the furnace gas steam for temperature measurement. The thermo-couple is fixed to the tip of a lance tube, which travels into and out of the gas passage. The lance travels approximately two meters per minute while extending and retracting. The thermocouple can be retracted manually in case of an emergency. Flue gas temperature in the area just before platen superheater or reheater elements at the exit of furnace can be critical during boiler start-ups before steam circulation for the cooling of SH and RH tube material is sufficiently established. By using the furnace temperature probe, continuous measurement of gas temperature is possible and thereby the danger over-heating of tubes can be reduced. For this reason, such probes are known as basic start-up probes. The furnace temperature probe can also be used to obtain gas temperature during low load operation of boiler. This standard furnace probe is equipped with ChromelAlumel thermo-couple installed in a lance tube (with air cooling). Model TFP-1E for travel from 1.5 to 7.3 m has a 76 mm outer diameter (OD) lance Model FTP-11E for travel from 7.4 to 12.2 m has a 108 mm OD lance. Both model probes are available with or without air-cooling.

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The probe may be operated in furnace gas temperature up to 537 oC (1000 o F) and for very short period of time in gas temperature as high as 565 oC (1050 oF) without air-cooling. The temperature probe can be used to measure furnace gas temperature up to max. 815 oC (1500 oF) with lance cooled by air. ARRANGEMENT OF BOILER AUXILIARIES

1. COAL BUNKER

6. BURNER

11. ESP

2. COAL FEEDER

7. FD FAN

12. ID FAN

3. COAL MILL

8. WIND BOX

13. STACK

4. PA FAN

9. SCANNER AIR FAN

14. SEAL AIR FAN

5. AIR PRE-HEATER

10. IGNITOR FAN

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BOILER TECHNICAL SPECIFICATIONS General: Manufacture

BHEL, CE (USA), Design Radiant, reheat, natural circulation, single drum, semi-out-door, balanced draft and direct fired.

Type Type of firing

Tilting, tangential

Type of SH

Pendent, platen, horizontal

Total Aux. power

5400 KW at 91% MCR

Min. mill load with oil support Total water content of boiler

40% MCR 321.5 T (including RH)

Furnace Type

Fusion welded walls.

Water walls

Surface area

Front Wall (EPRS)

618 cm2

Side walls (EPRS)

757 cm2

Real Walls (EPRS)

620 cm2

Roof (EPRS)

122 cm2

Total heat surface (EPRS)

2117 cm2

Tube material

SA-120, Gr. A1

Outer Diameter x Thickness Design metal temp.

63.5 mm x 6.3 mm 40 oC.

Resident time of fuel particles in furnace

2.5 Sec.

Drum Material

SA-229

Elevation of Drum

53340 mm

Overall length

15000 mm approx.

Shell thickness

170/135 mm (Bi-hickness)

Design metal temp.

354 oC

Permissible max. Differential Temp between any parts of drum. Normal Operation 55 oC Accelerated start

55 oC

Water capacity with MCR condition between normal & lowest water level permitted

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25 Sec

50

Super Heater Heating surface LTSH

6490 Sq.m.

PLATEN SH

810 Sq.m.

FINAL SH

823 Sq.m

No of stages

3

Material LT SH

SA210 GrA1, SA209 T1 SA213 T11

PLATEN SH

SA213 T11, SA213 TB

FINAL SH

SA 213 T22, SA213 TB, 347 HH

Type of flow LT SH

Counter

Platen SH

Parallel

Final

Parallel

Maximum Gas side Temperature LT SH

4900C

Platen SH

5700C

Final SH

5890C

SHH Specification Material

SA210 Gr. B, SA335 P12, SA335 P22.

Design Pres.

176.8 Kg/cm

Total Weight

2

68 Ton

Super Heater Attemperator Type

Spray type mixing

Stages

1

Position of spray in steam circuit

LTSH→Attemperation→Platen SH

SH temperature between 60% and 535 oC 100% MCR load Maximum Spray

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15,000 Kg/hr. at 60 % MCR

51

Reheater Total heating area

2630 m2

Number of stages

1

Material

SA 269T, SA233T T22, SA213TP 304 H

Max gas side metal temp.

585 oC.

RH Headers Material : Inlet

SA106 Gr. 3

Outlet

SA 335 P22

Design Pressure

46.0 Kg/cm2

Design metal temperature: Inlet

550 oC 550 oC

Outlet Reheater Temperature Control Angle of tilt

+ 30 degree

Type of tilting

Power tilting Cylinder 540 oC

RH steam temp. 60% - 100% MCR RH Emergency Temp. Control Type

Spray type mixing

Stages

1

Position in steam circuit

In CRH line

Material

SA - 106 Gr. B

Maximum water flow

22 T/Hr.

Economiser Material

SA 210 Gr. A1

Outer Diameter x Thickness

44.5 mm x 4.5 mm

Maximum gas side temp.

295 oC

Headers Material

SA-106 Gr. B

Design Pressure

181.1 Kg/cm

Design metal temp.

310 oC

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Boiler Parameters

Flow (T/Hr)

T/Hr

Conti. Load 120 MW T/Hr

HPHs out & Load 200 MW T/Hr

670.0

603.7

402.0

547.0

598.2

537.7

363.7

540.6

670.0

600.7

394.8

519.7

3.0

7.2

27.3

179.9

167.3

121.5

193.4

57.4

70.0

56.5

43.9

611.4

533.7

371.8

557.1

870.5

792.9

571.7

816.2

144.4

131.6

90.4

135.4

MCR

NCR 200 MW

T/Hr

Superheater outlet Reheater outlet

Description

Steam

Water Feed Water Spray Air

Fuel

(Coal)

TEMPERATURE OF STEAM, WATER, AIR AND GAS (in 0C) Description

MCR

NCR 200 MW

Cont. load 120 MW

HPHs out & Load 200 MW

0C

0C

0C

0C

Steam: a. Sat temp in drum

349

348

344

346

b. LTSH outlet

426

421

417

435

c. SH platen outlet

520

520

523

518

d. Final S/H outlet

540

540

540

540

e. R/H inlet

344

339

328

345

f. R/H outlet

540

540

540

540

243

241

223

164

b. Economiser outlet 386

284

270

234

Water: a. Economiser Inlet

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Air: a. Ambient

50

50

50

50

b. APH outlet (prim.)

325

318

297

282

c. APH outlet (sec.)

318

313

294

277

a. S/H platen outlet

1135

1132

1080

1129

b. R/H front inlet (Furnace exit)

1024

1025

945

1008

c. R/H rear inlet

922

917

837

907

d. Final S/H inlet

758

750

682

747

Gas:

e. LTSH inlet

671

661

603

660

f. Economiser inlet

470

462

433

467

g. APH inlet

354

343

312

307

h. APH outlet (Corrected)

136

134

124

121

OXYGEN, CARBON DIOXIDE (Dry Vol.) and EXCESS AIR (in %) Description

a. b. c. d. e. f.

Oxygen in gas at Eco. outlet (by dry. vol.) Oxygen in gas at APH outlet (by dry vol.) Max leakage of air across APH in % Total air to gas leakage in T/Hr. CO2 in gas at Eco outlet (by dry vol) Excess air in gas Eco outlet

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NCR 200 MW

Cont. load 120 MW

HPHs out & Load 200 MW

%

%

%

3.87

3.87

4.69

4.3

5.56

5.56

6.88

6.01

8.13

8.8

11.5

8.6

77.3

76.2

71.4

76.6

14.94

14.94

14.22

14.56

22

22

28

25

MCR

54

AIR PRE-HEATER DESCRIPTION The boiler is provided with two number of tri-sector type re-generative air pre-heaters by which the primary and secondary air heating is done, utilising the waste heat from flue gases. Each air heater is capable of meeting 60% maximum continuous rating of steam generator. Air pre-heater consists mainly of rotor housing, cylindrical cellular rotor, guide and support bearings, oil systems for guide and support bearings, soot blower system, stationary washing devices, auxiliary air motor drive with over-running clutch, air and gas duct access doors etc. The heating elements of specially formed plates from the baskets, which are, arranged compactly in three layers and within twelve sectors shaped compartments of radially divided cylindrical shell called rotor. The housing surrounding the rotor, is provided with duct connections at both ends and is adequately sealed by radial, circumferential and axial sealing members forming passages, for secondary air, primary air and flue gases. The complete rotor is supported by a thrust bearing at the bottom and guided by the radial bearing at the upper end. A pinion attached to the low speed shaft of power driven reduction gear engages a pin rack, mounted on the rotor shell. An air motor is connected at auxiliary high-speed shaft extension of drive unit. The air motor ensures the continued operation of the air pre-heater, even if power to electric motor is interrupted. It may also be used to control speed of the rotor during water washing of heating surface. BEARING LUBRICATION Support bearing sump is kept filled up with lubrication oil for flood lubrication of Mitchell type thrust bearing. Oil circulating system is provided to supply support bearings with a bath of continuously cleaned oil at proper viscosity. To accomplish this, the bearing oil supply is circulated by means of a motor driven screw pump through an external filtering system. Guide bearing is a double row cylindrical roller type. It is lubricated and cooled by oil filled in the bearing housing.

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AIR PREHEATER EXPLODED VIEW

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TECHNICAL DATA Air Heater Number per boiler / size

:

2 Nos. / 27 VI (T) 80" (72 o)

Max operating temp 0C

:

365 oC

Max air leakage %

:

9.3 %

Bearing guide & support

:

Radial / SPH Roller thrust.

Effective heating surface.

:

9000 m2 (per heater)

Gas flow area.

:

23.9 m2

Airflow area.

:

21.6 m2

Speed of air heater

:

1.42 RPM

Length hot end/cold end

:

864/305 mm

Material hot/cold end

:

Corten ' A' /Corten ' A'

Total Wt. of elements

:

130000 Kg / heater.

Material-shaft

:

Carbon steel.

Material Seals

:

Carbon steel ' A'

:

11.0 KW

:

1500 RPM

:

5.0 HP

Rotor

Motor Power Speed Power of Air Motor

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INDUCED DRAUGHT FAN DESCRIPTION OF FAN Each I.D. Fan is provided with inlet regulating vanes (IGVs) for controlling the loading on fans and inlet and outlet shut-off dampers for isolation to facilitate startup and maintenance of fan. Flue gas interconnection is provided with dampers before Electrostatic Precipitator in order to maintain balanced flow through both the air preheater and second pass when only one I.D. Fan is running. ID Fan mainly consists of a suction chamber, inlet vane control assembly, impeller, outlet guide vane assembly, diffuser bearings and flexible coupling. Suction Chamber The suction chamber is of welded sheet steel construction and is split horizontally for easy assembly and dismantling. A manhole is provided in the suction chamber for checking up the inlet of the fan. Inlet Guide Vane Control Flue gas entering suction chamber passes through the number of inlet axial aerofoil vanes before reaching impeller. Inlet guide vanes adjust the angle at inlet with respect to impeller blade depending on the inlet vane angle setting. The axial inlet vanes fixed to individual shafts, which are connected by means of angular joints to a central ring. The ring is guided by a set of roller and spring assemblies. A control lever is connected to the ring, which is operated by pneumatic power cylinder. The inlet vane control assembly is split to facilitate handling and dismantling. Impeller The impeller body is welded sheet steel construction; with welded on, non-profiled, solid blades. The impeller is dynamically balanced at the works. It is bolted to the flange welded on the hollow shaft. The impeller casing is of undivided type by the conical connection piece connected to the casing is split horizontally such that the top half can be removed for removal of the impeller. A peephole is provided in the casing for checking the wear on impeller. The impeller is supported in between the bearings. Outlet Guide Assembly The outlet guides are fixed in between the core of the diffuser and the casing. These guide vanes serve to direct the flow axially and to stabilise the drift flow caused in the impeller. The outlet blades for fans handling dust-laden gases are of removable type from outside. During operation of the fan, these blades can be replaced one by one. Diffuser Diffuser is of welded sheet steel construction with a core inside. The core of the diffuser houses the inner bearing, which is supported by all the outlet blades. The core of the diffuser is provided with a manhole with access from diffuser casing so that the bearing can be checked even during the operation of the fan. For fans handling hot gases, the diffuser cores are insulated inside. The lubrication pipe as KORBA SIMULATOR

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well as the thermometer for the inner bearing is brought outside through the core for easy access. Bearings The bearings are self-aligning roller type. The flanged bearing on the impeller side is the fixed bearing and the outer bearing is the expansion bearing. Both bearings are grease lubricated and the lubrication points are available on diffuser casing for inner bearing. A grease quantity control ring is provided in each bearing discharge the surplus amount of grease. Contact less thermometers are provided for indicating the bearing temperatures and for initiating alarm/tripping signal when bearing temperature rises to 95oC/105oC respectively. Flexible coupling ID Fan rotor shaft is directly coupled to the motor by flexible pin type coupling (with rubber bushing inserts).

FLUE GAS SYSTEM

ID FAN OIL CIRCULATION SYSTEM This Oil Circulation System is designed and manufactured to cool the bearings of fans, which normally operate at high speeds. Oil is drawn from a reservoir tank by means of Trocholdai driven by electric motors. Oil flows to the bearings to be cooled, through suction filters, oil coolers and pressure filters. Oil level indicator, sight glasses, breather, instrument panel are mounted on to the tank. Pressure gauges, pressure switches, thermometers and valves are provided at all important points.

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This is a complete interlocked system with stand-by motor, pump, oil coolers, filters and automatic pressure control devices. Once all the instruments are set to the required value, this system will run with little supervision. DESCRIPTION OF FAN MOTOR ID Fan motor is 3-phase squirrel cage induction motor having closed circuit aircooling system. Air within the motor is circulated by means of internal centrifugal fans and centrifugal action of rotor itself. Rotor support bearings are hydrodynamic ring assisted oil lubricated type. Each motor is provided with a lub oil system for circulating and externally cooling of lub oil. Contact less thermometers are provided for indicating bearing temperature; also for initiating alarm/tripping signal when bearing temperature goes high. FAN LUBRICATION SYSTEM Each motor of ID Fan is provided with an independent lubricating oil system. The bearings of the ID Fan motor are ring lubricated and hence do not require any force lubrication. TECHNICAL DATA Fan Type and size

:

Axial impulse AN 2806

Orientation

:

Horizontal

Medium handled

:

Flue gas

Location

:

Ground level.

No of fans / boiler

:

2

Capacity

:

225 m3/sec

Total head

:

356 mm wcl.

Temperature of medium

:

136 0C.

Specific weight of medium

:

7966 Kg/cm .

Speed

:

740 RPM

Type of fan regulation

:

Inlet Guide Vane (IGV)

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Motor Type

: Squirrel cage inductor motor

Rated power

: 1100 KW

Rated voltage

: 6.6 KV

Rated frequency

: 50 Hz

No of phases

: 3

Lubricating system

: Forced oil lubrication

Bearing type

: Hydrodynamic ring assisted bearing.

Speed

: 740 RPM

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FORCED DRAUGHT FAN FAN DESCRIPTION Forced draught fan may be operated in partial load range without affecting considerable economic efficiency. The rotor is accommodated in cylindrical roller bearings. In addition an inclined ball bearing at the drive side absorbs the axial thrust. For controlling the bearing temperature, there are contact tele-thermometers connected to signalling instruments. An oil hydraulic servomotor flanged to the impeller and rotating with it, adjusts the blades during operation. This results in a closed flux of force between adjusting forces and oil pressure, so that no forces are released to the outside (bearings, housing, foundation). The servomotor consists of piston; cylinder and The FD fan consists of the following components: •

Silencer

•

Inlet bend

•

Fan housing

•

Impeller with blades & blade pitch control mechanism.

•

Guide wheel casing with guide vanes and diffuser.

FD FAN CONNECTIONS

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The inlet bend is executed as inlet nozzle at its impeller end. The unit is driven from the suction side. In the core of the inlet bend, the shaft is accommodated in a specially designed bearing housing. The impeller is mounted in over-hung position on the shaft. The critical speed of the latter is well above the operating speed. The fans control device. The hydraulic servomotor is controlled by a pneumatic power cylinder, which in turn gets command from UCB via E/P converter. FAN MOTOR DESCRIPTION FD Fan is a 3-phase squirrel cage induction motor having closed circuit air-cooling system. Air within the motor enclosure is made to circulate by the help of internal centrifugal action of rotor itself. Contact less thermometers are provided for indicating bearing temperature also for initiating alarm/tripping signal when bearing temperature goes high. TECHNICAL DATA Fan Fan type

:

Axial reaction type

Fan orientation

:

Horizontal

Location

:

Ground level

Medium handled

:

Air

No of fans/boiler

:

Two

Type of fan regulation

:

Blade pitch control.

Lubrication system

:

Forced oil lubrication

Capacity

:

Total head developed

:

105 m /sec. 510 mm wcl

Temperature of medium

:

50 0C.

Specific weight

:

1.619 Kg/m of medium

Flow (reserve)

:

26 %

Pressure (reserve)

:

50 %

Rating

:

750 KW

Voltage

:

6.6 KV Three-phase

Speed

:

1480 rpm.

Lubrication System

:

Grease lubrication

3

3

Motor

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PRIMARY AIR FAN FAN DESCRIPTION Each boiler is provided with 2 Nos. of PA fans, each fan being capable of catering total air requirement of 3 mills. Fans are of radial type with single entry and horizontal orientation. It takes suction from atmosphere through a double entry silencer. Fan is coupled to the driving motors directly through a rubber-bushing coupling. The fan rotor is placed in two cylindrical roller anti-friction bearing and is provided with a double row inclined ball bearing to take up the axial thrust. All the three bearings are housed in a single housing, which is filled with oil. Silencer, provided at PA Fan suction to damp the noise level, is supported as a separate structure and bolted directly to the fan suction. Regulating the inlet guide vane unit arranged in the suction side controls fan loading. The axial inlet guide vane assembly of the fan consists of a number of aerofoil inlet vanes fixed to individual shafts, which are connected by means of angular joints to a central ring. The ring is guided to rotating position by a set of roller and spring assemblies. A control lever is connected to the ring, which can be operated by a pneumatic actuator. For monitoring the fan bearing temperature, indicators are provided for each of the bearings.

PRIMARY AIR AND SEAL AIR SYSTEM

Hot air from air pre-heaters outlet is connected to a common hot air duct from where toppings are taken for individual mills. Hot air shut-off gate and control dampers are provided in the branch line to each mill. Cold air from both the PA Fans discharge is KORBA SIMULATOR

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led directly into common cold air duct from where tapping are given to individual mill for tempering air, to hot air gates for sealing, to feeders for bearing sealing and to mill discharge pipes for sealing and purging. Isolating gate (hand operated) and regulating dampers are provided in the branch lines of cold air to each mill. Hand operated isolating valve are provided in feeder sealing line and solenoid operated isolating valves are provided in sealing air line to pulverisers discharge piping. Primary air fans are provided with isolating dampers at discharge. A portion of the air discharge by the fans is heated up in the air pre-heater and the remaining air is sent directly as cold air. Both the PA Fan discharge ducting is interconnected before the APHs through interconnecting ducting. Each APH is provided with isolating dampers at primary air inlet and outlet. The interconnecting ducts provide the flexibility of operating the PA Fan in combination with any APH and it makes it possible to distribute the primary airflow to both the APH when only one PA Fan is running. TECHNICAL DATA Fan Fan type

: Single suction, radial fan

Fan orientation

: Horizontal

Medium handled

: Air

Location

: Ground level.

No of fans/boiler

: Two

Fan regulation

: Inlet guide vane control

Capacity Total head developed

: 75 m3/sec. : 1187 mm. of water column

Temperature of medium:

: 50 0C

Specific weight of medium Speed

: 1.019 Kg/cm2 : 1480 rpm

Type of fan regulation

: Inlet guide vanes control.

Lubrication system

:

Type

:

Motor

Forced oil circulation system having 5 Lit/min capacity for lubrication

Rated Power

3-phase, air-cooled, Squirrel cage, induction motor : 1250 KW

Voltage

: 6600 V

Speed

: 1480 RPM

Lubrication

: Grease lubricated.

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PULVERISER DESCRIPTION The bowl mill consists essentially of a reduction gear box, mill side and liner assembly forming air and mill reject chamber, revolving bowl and scrapper, separator body with separator body liner assembly, grinding rolls and journal assembly, pressure spring assembly, classifier, multi port outlet assembly, central feed pipe and separating inner cone. Pre-crushed coal is fed by the RC Feeder through central feed pipe into the revolving bowl of the bowl mill. Centrifugal force feeds the coal uniformly between the bullring and independently rotating spring-loaded rolls to travel through the outer periphery of the bowl. The springs, which load the rolls, impart the pressure necessary for grinding. The partially pulverised coal continues up over the edge of the bowl due to centrifugal force. Hot and cold primary air mixed in the dustings enter the mill side housing below the bowl and is directed upwards past the bowl around the separator body liners which carry pulverised coal upwards into the deflector openings at the top of the inner cone, then out through the venturi and multi port outlet assembly. As air passes upward around the bowl, it picks up the partially pulverised coal. The heavier strike the separator body liners and are returned to the bowl immediately for further grinding. The lighter particles are carried up through the deflector opening impart the spinning action to the material with the degree of spin set by the angle of opening of the blades, determining the size of the pulverised coal. Any tramp iron or dense foreign material in the raw coal feed which is difficult to grind, if carried over to the top of the bowl, is dropped out through the air stream to the lower part of the mill side housing. Pivoted scrappers attached to the bowl hub sweep the tramp iron or other material around to the tramp iron spout through normally open pyrite hopper first by closing the inner gate and opening the outer gate of the hopper. The motor is coupled directly to worm shaft of the reduction gear, which rotates the bowl at a reduced speed and transmits the total power required for pulverizing the coal. LUBRICATION SYSTEM Pulveriser and roller bearings are oil lubricated. Pulveriser radial bearings receive oil supplied by the helical pump mounted on bottom of lower half of mill journal in the oil bath. Rollers are filled with oil independently. Worm shaft reduction gear is dipped in oil bath. Motor is grease lubricated

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BOWL MILL

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Pulveriser Specifications Air flow per mill

:

60 T/Hr

Air temperature at mill inlet

:

260 0C

Mill outlet temperature

:

77 0C

Coal flow per mill

:

36 T/Hr

Fineness of coal milled

:

70 % through 200 mesh

Primary air pressure inlet/outlet

:

650/244 mm wcl

COAL MILL ARRANGEMENT

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HP AND LP BYPASS SYSTEM

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HIGH PRESSURE (HP) BYPASS SYSTEM The HP Bypass system in coordination with LP Bypass enables boiler operation and loading independent of the turbine. This allows quick raising of steam parameters to a level acceptable to turbine for rolling during start up. Steam is bypassed from main steam line to cold reheat line through HP-Bypass and from hot reheat line to condenser through LP bypass. The HP bypass valve can handle a maximum of 60% of the full load turbine steam flow. The possible phases of operation of HP bypass station can broadly be classified as follows: •

Boiler start-up with TG set at standstill.

•

Raising of steam parameters to a level acceptable for TG rolling at a relatively faster rate than otherwise is possible.

•

Turbine loading while steam flow gets transferred to the turbine.

•

Parallel operation with turbine on load rejection.

•

Allowing boiler operation following turbine trip, provided boiler load < 60%.

•

Preventing safety valves opening at raised steam pressures.

Description The HP Bypass system consists of two parallel branches that divert steam from the M.S. line to CRH line. The steam pressure on the valve upstream side can be maintained at the desired level. The steam is de-superheated in order to keep the steam temperatures in cold reheat line within limits, below 345 oC. The M.S. pressure ahead of the turbine is maintained by two nos. of pressure reducing valves BP-1 and BP-2 with valve mounted electro-hydraulic actuator. The steam temperatures downstream of the HP-Bypass station is maintained by 2 nos. of spray water temperatures control valves BPE-I and BPE-2 with valve mounted electro-hydraulic actuators. The spray water is available from the BFP discharge line. There is also a spray water pressure control valve with valve mounted electro-hydraulic actuator. HP Bypass System (Hydraulics) Oil Supply unit The oil supply unit provides the hydraulic actuation energy for the complete actuating system, and functions as follows: An axial-piston oil pump draws the oil through a suction strainer and pumps it through a pressure filter and via a non-return valve into the accumulator. A safety relief valve protects the system against over pressure. The accumulator is of the bladder type and consists of a steel pressure vessel containing a nitrogen filled rubber bladder, which separates the oil from the gas. The accumulator supplies pressurised oil to the system and covers the entire peak supply requirement. The oil KORBA SIMULATOR

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pump therefore, is sized only for the mean supply requirement and it is switched off when the accumulator is fully charged. From the accumulator the oil is fed through the supply manifold with the pressure reducing valve and the pressure is set and controlled. The pressure switch monitors the oil pressure in the accumulator and provides the signals to switch on the oil pump.

Servo valve

HP BYPASS SYSTEM

The two-stage servo valve is actuated by the torque motor, which is controlled from an analogue-positioning amplifier or from a manual desk control. The torque motor moves the control fork (of the servo valve) and operates the pilot stage (1st stage), which controls the position of the control piston (2nd stage). A mechanical override acting directly on the control piston permits local manual operation of the valve. Blocking Unit The electro hydraulically pilot-operated blocking unit is mounted between the servo valve and the actuator. It closes off both ports to the actuator if electrically deenergized or with insufficient oil pressure, and holds the piston of the actuator (disregarding some leakage drift) in its last position. A mechanical override on the blocking unit permits local manual de-blocking.

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Actuator The actuator consists of a double acting cylinder with piston and piston rod. An intermediate yoke connects this cylinder with the valve, and a solid coupling connects the valve stem with the piston rod. A feedback transmitter unit is mounted onto the coupling yoke and is connected to the valve stem by a linkage system. Technical Data Oil Pump (Type)

OV 16

OV 32

Oil Supply

12

24

Litre/min

Speed

1500

1500

RPM

Power

4

7.5

KW

Voltage

380

380

V

Frequency

50

50

Hz

Phase

3

3

No load speed

1500

1500

RPM

Oil Tank volume

45

70

Litres

Useable volume

20

50

Litres

Motor

Oil Tank

Hydraulic Accumulator (Standard) Nominal volume

10 lit

Pressure rating approx.

200 bar

Ambient temperature 15 oC 65 oC

min. max. Operating gas

Nitrogen

Bladder material

Perbunan (Synthetic Rubber)

Available Oil Pressures The controlled system pressure (set with the 25 to 120 bar pressure reducing valve) The maximum oil pressure (limited with the 50 to 180 bar pressure relief valve) Pressure Switch 4 micro-switches for the set points: Pump motor - on Pump motor - off Pressure too low Pressure too high

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Electrical Rating: 20

Amp at 488 V AC

10

Amp at 125 V AC

0.25

Amp at 250 V DC

0.5

Amp at 125 V DC

Mode of Operation The HP bypass system is intended to ensure reheater protection, minimum super heater safety valve lifting under emergency conditions, adequate steam in CRH for auxiliary steam consumers, if taken from CRH, to retain the boiler under fire in case of turbine load rejections and to follow boiler control system during certain operation. The control system is designed to maintain the steam pressure ahead of bypass valve to the given set value. The pressure set point can be adjusted from UCB. The steam temperatures at the downstream of valves are automatically controlled to the given set value. The temperature set point can be adjusted from UCB. Operation of the HP bypass station is manipulated by the pressure and temperature set points and is independent of LP bypass operation. Depending upon the initial pressure condition at the time of boiler firing, the pressure set point is to be adjusted to a value equal to the steam pressure ahead of bypass valve minus a bias pressure. This will result in opening of the valves. The pressure controller would then maintain the set pressure by allowing a flow matching with the steam flow. As the firing rate increases, the set point needs to be manipulated in the same manner to allow flow sufficient through RH. This however, shall be possible till the maximum flow capability of the valve is reached at any particular pressure and temperature at upstream. Upon reaching the target steam parameter for turbine rolling, the boilerfiring rate will be maintained at that level. Before admission of steam into turbine, the HP bypass shall be set to maintain the relevant steam pressure ahead of valves plus a bias pressure. Consequent upon steam admission in the turbine, the pressure ahead of bypass valve would tend to fall in view of constant firing rate. This would result in proportional closing of bypass valve during pressure controller action. The process shall continue till the set pressure up stream is reached. After this, further loading of the set can be achieved by increasing the firing rate. Thereafter, the bypass set point shall be raised to live steam pressure plus a bias pressure of 2-5 ata. With this, the HP bypass station would automatically open and balance the discrepancy between steam generation and consumption, acting out of load rejection under normal operation of the unit.

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HP BYPASS: ELECTRO HYDRAULIC SERVO SYSTEM

The control loop for the steam temperature at downstream of HP bypass valve can be operated by modulating set point as required for different mode of start-ups governed by boiler/turbine characteristic as well as warm-up requirements of steam piping. Pressure Control The signal for the HP bypass station is sensed from the main steam and converted to proportional current signal by transmitters. The actual pressure is displayed at the desk by indicators. The set point can be varied from the desk by a push button module and is indicated on the console itself. KORBA SIMULATOR

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Temperature Control The control positioners for the spray valves are designed in the same way as those for the steam valves. In addition PI controllers are connected up to the control positioners. The temperature signal from transmitters is compared at the PI controllers with the common temp. set point. According to particular control deviation the PI controller forms a rated signal for the control positioners of the associated spray control valves. The electro-hydraulic actuators make it possible to attain short positioning time for the spray water control valves and then allow the temperature control to intervene fast enough in the event of quick opening of the HP bypass valves. To offset the time delay of temp measurement and to achieve favourable conditions when reaching on the spray water-cooling system rapid adjustment to temp input of the injection valve controller by the associated bypass valves positioning monitor. Thus, independent of the temperature signal, a certain amount of water is injected during the opening of the bypass valve. Manual operation of the bypass spray water temperature control valve is effected by means of push button modules. The valve position and the control deviation are indicated on the desk. In order to ensure proper spray cooling on BP-1 and BP-2 (under different steam flow rates), the spray water control valves BPE-1 and BPE-2 are reset to a constant pressure feed water supply through the BD Valve. Interlocks for the HP Bypass System HP Bypass valve BP-1 or BP-2 opening less than 2% will automatically close the spray water pressure control valve (BD valve). If opening of either of the bypass valves BP-1 or BP-2 is above 2%, the control of spray water pressure control valves and temperature control valves BPE1 & BPE2 shall be changed to ' AUTO'mode irrespective of their initial conditions. If BP valve position drops < 2% open, it will receive auto close command to ensure positive shut-off. If the steam temperature downstream of the BP valves becomes 380oC, the closing signal for these valves are initiated accompanied with an alarm. In this case, the BP controller will transfer itself from AUTO to MANUAL. The following will activate the ' Fast Opening'Signal: •

Generator Circuit Breaker Open.

•

Turbine Load Shedding Relay operated.

• HP BP Pressure controller deviation more than (+) 10%. • Depressing of the ' FAST OPEN'push button.

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LOW PRESSURE (LP) BYPASS SYSTEM Low Pressure bypass system enables to establish an alternative pass for dumping the steam from reheater outlet directly into condenser at suitable steam parameters. The controls for LP BYPASS system are essentially a combination of electrical and hydraulic system. Electro-hydraulic converter provides the necessary link between hydraulic actuators and the electrical system. The control of LP bypass system is hooked up by the same control, which is used for turbine governing system. The LP bypass valves are two in number. The double shut-off arrangement separates the reheater from the condenser during normal operation. In addition to these, two steam pressure control valves, four injection water valves are provided for de-superheating purposes. This injection water is taken from condensate extraction pump discharge.

LP BYPASS SYSTEM

Set Point Formation Two set points, the fixed set point and the variable set point are formed for the LP Bypass controller, the effective set point under any set of operating conditions being the greater of the two. The fixed set point can be set manually from the control panel to a point between 0 120 % of the maximum LP Bypass pressure with the aid of a motorised set point adjuster. It can also be regulated automatically by means of the ' Automatic Control KORBA SIMULATOR

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Interface'during the start-up phase and is normally used to set the lower limit for pressure set point. The pressure upstream of the H.P. blading, required for reference variable set point formation, is measured by a pressure transducer and transmitted to a matching amplifier which sets the characteristic for the reference variable as a function of the pressure upstream of H.P. blading i.e. throttle pressure.

LPBP EHC POSITION

Vs VARIOUS VALVE OPENING

PRESSURE CONTROL FOR LP BYPASS SYSTEM The reheat steam pressure before interceptor valve is the controlling variable for the LP bypass system. Control of this parameter can be done in the ' MANUAL'mode by changing the electro-hydraulic controller (EHC) output as required by means of the OPEN/CLOSE push buttons located on the control module. In the ' auto'mode, the controller matches the hot reheat pressure with the effective set point (either FIXED or VARIABLE) by modulating the LP Bypass control valves as necessary. A tracking controller is provided so that the control mode (manual or auto) not in service automatically follows the effective controller. This facilitates bump less changeover, between the modes. But when charging over from ' MANUAL'to ' AUTO'care must be taken for matching the set point and actual value, otherwise, a jerk in the system will be felt due to the error present (which the AUTO controller tries to bring to zero).

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LP BYPASS CONTROLLER

AUTOMATIC CONTROL INTERFACE DEVICE (ACI) During the start-up, it is intended to avoid a very high level of set point. For this purpose, the Automatic Control Interface Device has been introduced. For the Automatic Control Interface to come in action, it must be switched on by means of the ON/OFF push button provided on the control panel. Also the Bypass controller must be in auto mode. When the Automatic Control Interface is switched ON, it brings the fixed set point down to 3 Kg/cm2, in case the actual reheat pressure is below 3 Kg/cm2. When the actual reheat pressure exceeds 3 Kg/cm2 the ACI opens the LP Bypass control valves + 25% and they remain locked in 25% position up to a reheat pressure of 12 Kg/cm2. During this time, the fixed set point tracks the actual reheat pressure so that the output of LP Bypass "auto" Controller is zero. Once the ACI has brought the fixed set point 12 Kg/cm2, it gets automatically switched off. The fixed set point remains static at 12 Kg/cm2 and the LP Bypass controller modulates the control valve to maintain this set pressure. Any change in the reheat pressure can now be brought only by manually varying the fixed set point to the desired value. Two Stage Water Injection To prevent undue overloading of condensate pumps under normal shutdown/start-up conditions, the injection water demanded from CEPs is staggered in two stages. This arrangement opens the injection valves (INV-2, 4) via the pressure switch (LPPS), solenoid valve (SVV) & slide valve SV-2/4 when the steam pressure upstream at the expansion orifice exceeds value corresponding to 45% of maximum bypass flow.

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Protective Closing of LP Bypass System (Condenser Back-up Protection) The LP Bypass valves will close automatically under the following conditions to prevent damage to the condenser. •

Condenser vacuum is low (> 0.4 Kg/cm2 abs)

•

Spray water pressure is low (< 10 Kg/cm2 or both condensate pumps off).

•

Condenser wall temperature is high (> 90oC).

•

The steam pressure downstream of LP BP is greater than 19 Kg/cm2.

High exhaust hood temperature will automatically switch on the exhaust hood spray water. In case of condenser wall temperature protection operation, the ' RESET BYPASS TRIP' -Pushbutton for solenoids SV-1 and SV-2 are to be depressed to reset the TRIP command. LP Bypass Control (Hydraulic) Due to difference between set and actual HRH pressure the electro-hydraulic LP bypass governor generates a proportional signal voltage in the moving coil of the converter (EHC). With increasing signal voltage, the jet pipe of the converter moves towards right and the amplifier piston (KA-08) moves down. A feedback mechanism stabilises the amplifier piston for a given voltage change. The sleeves (KA04) of followup piston valves (KA02/KA03) also move down increasing the signal oil Pressure of water injection Valves, there by opening them, in the beginning of control operation.

LP BYPASS CONTROL SYSTEM

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LP bypass stop valves (LPSV-1, 2) open up with a slight time delay after injection valves are opened; due to rising oil pressure in follow-up pistons KA02 (assuming piston KA07 of bypass limiting regulator is in upper position). LPBP Steam control valves (LPCV-1, 2) open up due to hydraulic feedback between actuator pistons and pilot valves (PV-1, 2).

1. 2. 3. 4. 5. 6.

Electric LP bypass governor Plunger coil measuring system Jet pipe Adjusting spring Adjusting screw Jet pipe regulator

a. Control fluid a1. Control fluid under control piston of differential pressure relay a2. Control fluid above control piston of differential pressure relay

Electro Hydraulic Converter for LP Bypass

LP bypass limiting regulator (LPLR) has priority over (EHC). As soon as condensate at required pressure is available with sufficient vacuum in condenser, its jet pipe swings to right and its piston KA07 moves to upper position. This increases the signal oil pressure in KA02 (follow-up pistons), releasing steam Stop Valves and Control Valves to open. In case of condensate water pressure low and condenser pressure high the reverse action takes place and the spring of KA02 is de-tensioned to such an extent that LP bypass valves are unable to open, Refer to Figure. 1. 2. 3. 4. 5. 6. 7. 8. 9.

Jet pipe Jet pipe regulator Adjusting spring Adjusting screw Corrugated measuring system Adjusting spring Corrugated measuring Corrugated measuring Adjusting spring

a. Control fluid a1. Control fluid above control piston of limit pressure amplifier a2. Control fluidunder control piston of limit pressure amplifier k. Condensate from hydraulic pressure switch of injection water pressure monitor l. Vacuum signal from bypass steam piping behind bypass control valve

LP Byapss Limiting Regulator

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LP BYPASS PROTECTIONS Low Vacuum Safety Device

1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Capnut Adjusting csrew Cover Compression spring Diaphragm Valve Valve sleeve Casting Can Lever

a Bypass signal oil from converter a1 Signal oil to bypass valve c Oil drain l Vacuum from condenser

A low vacuum safety device is installed in the signal oil line from follow-up piston KA02 to bypass valves'pilots (PV-1, 2) and (PV-3,4) If vacuum drops below a preset value; the valve of the safety device moves downwards due to increasing pressure above it. The valve thus blocks off the signal oil line and opens the oil between itself and PV-1, 2 & 3, 4 to drain, closing the LP bypass stop and control valves. As vacuum increases, bypass operation is restored in reverse sequence when the preset vacuum has built up. Low Injection Water Pressure Protection A pressure switch (WPS) is installed in the signal oil line from KA02 to PV-1, 2 & PV3,4 of bypass valves, to protect the condenser in the event of water injection failing. If the injection water pressure drops below a preset value, the valve of the pressure switch (WPS) moves down, blocking off the signal oil line and de-pressuring the oil between itself and PV-1, 2 & PV 3, 4. The LP bypass valves are thus closed, due to low condensate water pressure. Bypass operation is restored in the reverse sequence when injection water pressure becomes normal.

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1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Hood Bellos Pushrod Knife edge lever Cam shaft Compression spring Fitted key Shaft Scale Cylindrical pin Nozzle Slide valve Valve bushing Compression spring Bearing bushing Torsion spring Lever

a Control oil a1 Control oil a2 Control oil to pilot valve of bypass valve c Return flow l Injection water pressure

Low Injection Water Pressure Protection

High Condenser Wall Temperature Protection At a preset condenser wall temperature the two thermocouples mounted in steam dome opposite bypass steam inlet transmit a switching pulse to the associated solenoid valves (SOLV-1, 2). 1. Solenoid 2. Compression spring 3. Solenoid valve 4. Compression spring 5. Solenoid 6. Compression spring 7. Main valve 8. Compression spring 9. Limit switch for injection a Control oil b Signal oil to Stop and Control valve operator of bypass SV/CV c Drain

The solenoid valves block off the depressive signal oil and close bypass valves in the event of high condenser wall temperatures. The bypass valves can be opened from the control room manually only after the solenoids are manually reset after the temperature has become normal.

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TURBINE SYSTEM

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STEAM TURBINE: GENERAL DESCRIPTION 210MW capacity turbines at Korba station are of Kraft Werk Union (KWU-Germany) design and supplied by BHEL. The turbine is condensing, tandem compounded, horizontal, reheat type, single shaft machine. In has got separate high pressure, intermediate and low-pressure parts. The HP part is a single cylinder and IP & LP parts are double flow cylinders. The turbine rotors are rigidly coupled with each other and with generator rotor. HP turbine has throttle control. The steam is admitted through two combined stop and control valves. The lines leading from HPT exhaust to reheater have got two cold reheat swing check NRVs. The steam from reheater has got two cold reheat swing check NRVs. The steam from reheater is admitted to IP turbine through two combined stop and control valves. Two crossover pipes connect IP and LP cylinder.

210 MW

KWU TURBINE

Blading The entire turbine is provided with reaction blading. The moving blades of HPT, LPT and front rows of LPT have inverted T roots and are shrouded. The last stages of LPT are twisted; drop forged moving blades with fir-tree roots. Highly stressed guide blades of HPT and IPT have inverted T roots. The other guide blades have inverted Lroots with riveted shrouding. Bearings The TG unit is mounted on six bearings HPT rotor is mounted on two bearings, a double wedged journal bearing at the front and combined thrust/journal bearing adjacent to front IP rotor coupling. IP and LP rotors have self-adjusting circular journal bearings. The bearing pedestals of LPT are fixed on base plates where as HPT front and rear bearing pedestals are free to move axially. Pedestals at machine level support the brackets at the sides of HPT. In axial KORBA SIMULATOR

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direction, HP & IP parts are connected with the pedestals by means of a casing guide. Radial expansion is not restricted. HP & IP casings with their bearing pedestals move forward from LPT front pedestal on thermal expansion. HP TURBINE

1. TURBINE ROTOR 2. OUTER SEAL RING

3. BARREL CASING 4. GUIDE BLADE CARRIER

5. THREADED RING 6. CASING COVER

HP TURBINE SECTIONAL VIEW

HP Turbine is of double cylinder construction. Outer casing is barrel type without any axial/radial flanges. This kind of design prevents any mass accumulation and thermal stresses. Also perfect rotational symmetry permits moderate wall thickness of nearly equal strength at all sections. The inner casing is axially split and kinematically supported by outer casing. It carries the guide blades. The space between casings is filled with the main steam. Because of low differential pressure, flanges and connecting bolts are smaller in size. Barrel design facilitates flexibility of operation in the form of short start-up times and higher rate of load changes even at high steam temperature conditions.

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IP TURBINE

1. TURBINE ROTOR 2. OUTER CASING 3. OUTER CASING

4. INNER CASING 5. INNER CASING 6. EXTRACTION NOZZLE

7. INLET NOZZLE

IP TURBINE SECTIONAL VIEW

IP Turbine is of double flow construction. Attached to axially split out casing is an inner casing axially split, kinematically supported and carrying the guide blades. The hot reheat steam enters the inner casing through top and bottom centre. Arrangement of inner casing confines high inlet steam condition to admission breach of the casing. The joint of outer casing is subjected to lower pressure/temperature at the exhaust. Refer to Figure.

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LP TURBINE Double flow LP turbine is of three-shell design. All shells are axially split and are of rigid welded construction. The inner shell taking the first rows of guide blades is attached kinematically in the middle shell. Independent of outer shell, middle shell is supported at four points on longitudinal beams. Two rings carrying the last guide blade rows are also attached to the middle shell. Refer to Figure.

1. OUTER CASING 2. OUTER SHELL 3. INNER SHELL

4. INNER SHELL 5. OUTER SHELL 6. DIFFUSER

7. OUTER CASING

LP TURBINE SECTIONAL VIEW

Fixed Points (Turbine Expansions) a. Bearing housing between IP and LP b. Rear bearing housing of LP turbine c. Longitudinal beam of LP turbine d. Thrust bearing.

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Front/rear housing of HPT can slide on base plates. Any lateral movements perpendicular to machine axis are prevented by fitted keys. Bearing housings are connected to HP-IP casings by guides, which ensure central position of casings while axially expanding and moving. The LPT casing is located in centre area of longitudinal beam by fitted keys cast in the foundation cross beams. Axial movements are not restricted. The outer casing of LP turbine expands from its fixed points towards generator. Bellows expansion couplings take the differences in expansion between the outer casing and fixed bearing housing. Hence HPT rotor & casing expands towards bearing no (1) while IPT rotor expands towards generator. The LPT rotor expands towards generator. The magnitude of this expansion is reduced by the amount by which the thrust bearing is moved in the opposite direction due to IPT casing expansion.

1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

HP FRONT PEDESTAL HP REAR PEDESTAL LP FRONT PEDESTAL LP REAR PEDESTAL HPT OUTER CASING IPT OUTER CASING LPT OUTER CASING HP FRONT PEDESTAL BASE PLATE HP REAR PEDESTAL BASE PLATE LP FRONT PEDESTAL ANCHOR POINT

11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

LP REAR PEDESTAL ANCHOR POINT LP OUTER CASING ANCHOR POINT HPT INNER CASING IPT INNER CASING LP INNER OUTER CASING LP INNER OUTER CASING HP INNER CASING ANCHOR POINT IP INNER CASING ANCHOR POINT LP INNER –OUTER CASING ANCHOR POINT LP INNER –INNER CASING ANCHOR POINT

TURBINE ANCHOR POINTS AND EXPANSIONS

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Front/rear housing of HPT can slide on base plates. Any lateral movements perpendicular to machine axis are prevented by fitted keys. Bearing housings are connected to HP-IP casings by guides, which ensure central position of casings while axially expanding and moving. The LPT casing is located in centre area of longitudinal beam by fitted keys cast in the foundation cross beams. Axial movements are not restricted. The outer casing of LP turbine expands from its fixed points towards generator. Bellows expansion couplings take the differences in expansion between the outer casing and fixed bearing housing. Hence HPT rotor & casing expands towards bearing no (1) while IPT rotor expands towards generator. The LPT rotor expands towards generator. The magnitude of this expansion is reduced by the amount by which the thrust bearing is moved in the opposite direction due to IPT casing expansion. Turbine Oil Supply In the 200MW KWU turbines, single oil is used for lubrication of bearings, control oil for governing and hydraulic turbine turning gear. During start-ups, auxiliary oil pump (2 Nos.) supplies the control oil. Once the turbine speed crosses 90% of rated speed, the main oil pump (MOP) takes over. It draws oil from main oil tank. The lubricating oil passes through oil cooler (2 nos.) before can be supplied to the bearing. Under emergency, a DC oil pump can supply lub oil. Before the turbine is turned or barred, the Jacking Oil Pump (2 nos.) supplies high-pressure oil to jack-up the TG shaft to prevent boundary lubrication in bearing. Refer to the figure.

TURBINE LUBRICATING OIL SYSTEM

The oil systems and related sub-loop controls (SLCs) can be started or stopped automatically by means of SGC oil sub-group of automatic control system. The various logics and SLCs under SGC oil are given in the ATRS section.

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MAIN OIL PUMP The main oil pump is situated in the front bearing pedestal and supplies the entire turbine with lubricating oil and control oil, which is connected to the governing rack.

1. 2. 3. 4. 5. 6. 7. 8.

Threaded ring Pump casing, upper Journal Bearing Oil pipe Bearing bushing Seal ring Impeller Feather key

9. 10. 11. 12. 13. 14. 15. 16.

Feather key Journal + Thrust Brg Ring Vent pipe Oil inlet vessel Hyd. Speed Xter Oil line Turbine shaft

17. 18. 19. 20. 21. 22. 23.

Coupling Elect. Speed Xter Permanent Magnet Pump shaft Spacer sleeve Pump casing, lower Oil tube

TURBINE TURNING GEAR The turbine is equipped with a hydraulic turning gear assembly comprising two rows of moving blades mounted on the coupling between IP and LP rotors. The oil under pressure supplied by the AOP strikes against the hydraulic turbine blades and rotates the shaft at 110 rpm (220 rpm under full vacuum condition). In addition, provisions for manual barring in the event of failure of hydraulic turning gear, have also been made. A gear, machined of the turning gear wheel, engages with a Ratchet & Pawl arrangement operated by a lever and bar attachment.

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HYDRAULIC BARRING GEAR AND MECHANICAL BARRING GEAR

TURBINE GLAND SEALING Turbine shaft glands are sealed with auxiliary steam supplied by an electro2

hydraulically controlled seal steam pressure control valve. A pressure of 0.01 Kg/cm (g) is maintained in the seals. Above a load of 80 MW the turbine becomes selfsealing. The leak off steam from HPT/IPT glands is used for sealing LPT glands. The steam pressure in the header is then maintained constant by means of a leak-off control valve, which is also controlled by the same electro-hydraulic controller, controlling seal steam pressure control valve. The last stage leak-off of all shaft seals is sent to the gland steam cooler for regenerative feed heating. Refer the Figure.

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TURBINE SEAL STEAM SYSTEM

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TURBINE SPECIFICATIONS Type: Three cylinders reheat condensing turbine having: i. Single flow HP turbine with 25 reaction stages. ii. Double flow IP turbine with 20 reaction stages per flow. iii. Double flow LP turbines with 8 reaction stages per flow. Rated Parameters Nominal rating

: 210 MW

Peak loading (without HP heaters)

: 229 MW

Rated speed.

: 3000 RPM

Main steam flow at full load (With HP heaters in service).

: 630 tons/hr.

Main steam pressure/ temperature at full load.

: 147.1 kg/cm2. 535 oC.

HRH pressure/ temp at full load.

: 34.23 kg/cm2. 535 oC.

Permissible SH / RH temp variations.

o : 543 C. (Long time value but keeping within annual mean 535oC.) : 549 oC. (400 hours per annum) o : 536 C. (80 hours per annum & max. 15 min in individual case) : 76 mm Hg with CW inlet temp 33 oC.

Condenser pressure. STEAM TEMPERATURE

80-hr/ annum maximum. per 15 min., in individual cases oC

Rated value Annual mean value

Long time value keeping 400h within annual annum mean value

oC

oC

oC

Initial steam

535

543

549

563

IPT SV Inlet

535

543

549

563

HPT exhaust

343

359

Extraction 6

343

359

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500 + (special 425 case) 500 + (special 425 case) 98

Extraction 5

433

438

473

Extraction 4

316

326

366

Extraction 3

200

211

255

Extraction 2

107

127

167

Extraction 1

62

82

127

LPT exhaust

49

70

100

70

* Long-time operation: Upper limit value permissible without time limit Valid only for the no-load period with high reheat pressure after trip-out from fullload operation. For the individual case approx. 15 min. Provision for this is that the turbine is immediately reloaded or the boiler immediately reduced to minimum load if no-load operation is maintained. Permissible differential temperature - No time limitation between parallel steam supply lines - Short time period

: :

17 K. 28 K.

In the hottest line the limitations indicated for initial steam and reheat temperature must not be exceeded. Turbine Extractions (Pressure/ Temperature) at 200 MW Extraction Pres. (bar)

Temp.

1.

Extraction No. 6 (from HPT exhaust)

39.23

343

2.

Extraction No. 5 (from 11 the stage IPT)

16.75

433

3.

Extraction No. 4 (from IPT exhaust)

7.06

136

4.

Extraction No. 3 (from 3rd stage LPT)

2.37

200

5.

Extraction No. 2 (from 5th stage LPT)

0.858

107

6.

Extraction No. 1 (from 7th stage LPT)

0.216

62

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C.

99

Alarm and Limiting Values of some Important Parameters Parameters

Alarm value

Limit value

HPT Diff. Expansion.

+4.5 mm

+5.5 mm

- 2.5 mm

- 3.5 mm

+5.0 mm

+ 6.0 mm

-2.0 mm

- 3.0 mm

+25.0 mm

+30.0 mm

-5.0 mm

- 7.0 mm

HPT exhaust casing temperature

480 oC

500 oC

LPT outer casing metal temperature

90 oC

110 oC

IPT Diff. Expansion. LPT Diff. Expansion.

Metal temp diff. between upper & lower casing +/- 30 oC (HPT front middle, IPT front, rear).

+/- 45 oC

Turbine Bearing Metal Temperature 76 oC

Maxm Oil Temperature before coolers Whose normal operating temp is 75 oC

90 oC

120 oC

Whose normal operating temp is 85 oC

100 oC

120 oC

Turbine bearing housing vibration

35 microns

45 microns

Turbine absolute shaft vibration

30 microns

200 microns

Condenser vacuum (absolute)

120 mm Hg

200 mm Hg

Turbine axial shift

±0.3 mm

±0.6 mm

Turbine over speed

51.5 Hz

55.5 Hz

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TURBINE GOVERNING SYSTEM In order to maintain the synchronous speed under changing load/grid or steam conditions, the KWU turbine supplied by BHEL at NTPC Korba is equipped with electro-hydraulic governor; fully backed-up by a hydraulic governor. The measuring and processing of electrical signal offer the advantages such as flexibility, dynamic stability and simple representation of complicated functional systems. The integration of electrical and hydraulic system is an excellent combination with following advantages: •

Exact load-frequency droop with high sensitivity.

•

Avoids over speeding of turbine during load throw offs.

•

Adjustment of droop in fine steps, even during on-load operation.

Elements of Governing System The main elements of the governing system and the brief description of their functions are as follows: •

Remote trip solenoids (RTS).

•

Main trip valves (Turbine trip gear).

•

Starting and Load limit device.

•

Speeder Gear (Hydraulic Governor).

•

Aux. follow-up piston valves.

•

Hydraulic amplifier.

•

Follow-up piston valves.

•

Electro-Hydraulic Converter (EHC).

•

Sequence trimming device.

•

Solenoids for load shedding relay.

•

Test valve.

•

Extraction valve relay.

•

Oil shutoff valve.

•

Hydraulic protective devices.

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REMOTE TRIP SOLENOIDS (RTS) The remote trip solenoid operated valves are two in number and form a part of turbine protection circuit. During the normal operation of the turbine, these solenoids remain de-energised. In this condition, the control oil from the governing rack is free to pass through them to the main trip valves. The solenoids gets energised whenever any electrical trip command is initiated or turbine is tripped manually from local or UCB. Under energised condition the down stream oil supply after the remote trip solenoids gets connected to drain and the upstream will be blocked. By resetting Unit Trip Relays (UTR) from UCB, these solenoids can be reset. Refer to Figure.

REMOTE TRIP SOLENOIDS

MAIN TRIP VALVES The main trip valves (two in numbers) are the main trip gear of the turbine protective circuit. All turbine tripping take place through these valves. The control oil from remote trip solenoids is supplied to them. Under normal conditions, this oil flows into two different circuits, called as the Trip Oil and Auxiliary Trip Oil. The Trip Oil is supplied to the Stop Valves (of HP Turbine and IP Turbine), Auxiliary Secondary Oil circuit and Secondary Oil circuits. The Auxiliary Trip Oil flows in a closed loop formed by main trip valves and turbine hydraulic protective devices (Over Speed trip device, Low Vacuum trip device and Thrust Bearing trip device). The construction of main trip valves is such that when aux. trip oil pressure is adequate, it holds the valves' spools in open condition against the spring force. Whenever control oil pressure drops or any of the hydraulic protective devices are actuated, the main trip valves are tripped. Under tripped condition, trip oil pressure is drained rapidly through the main valves; closing turbine stop and control valves. Refer to the figure below.

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MAIN TRIP VALVES

STARTING AND LOAD LIMIT DEVICE The starting and load limit device is used for resetting the turbine after tripping, for opening the stop valves and releasing the control valves for opening. The starting device consists of a pilot valve that can be operated either manually by means of a hand wheel or by means of a motor from remote. It has got port connections with the control oil, start-up oil and auxiliary start-up oil circuits. The starting device can mechanically act upon the hydraulic governor bellows by means of a lever and link arrangement. Before start-up, the pilot valve is brought to its bottom limit position by reducing the starting device to 0% position. This causes the hydraulic governor bellows to be compressed thus blocking the build-up of secondary oil pressure. This is known as control valve close position. With the valve in the bottom limit position (starting device = 0%) control oil flows into the auxiliary start-up circuit (to reset trip gear and protective devices) and into the start-up oil circuit (to reset turbine stop valves). A build-up of oil pressure in these circuits can be observed, while bringing the starting device to zero position. When the pilot valve i.e. the starting device position is raised, the start-up oil and auxiliary start-up oil circuits are drained. This opens the stop KORBA SIMULATOR

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valves; ESVs open at 42% and IVs open at 56% positions of the starting device. Further raising of the starting device release hydraulic governor bellows which is in equilibrium with hydraulic governor' s spring tension and primary oil pressure (turbine speed), and raises the aux. sec. oil pressure; closing the aux. follow-up drains of hydraulic governor.

STARTING DEVICE ACTING ON SPEEDER GEAR

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SPEEDER GEAR The speeder gear is an assembly of a bellow and a spring, the tension of which can be adjusted manually from UCB by an electric motor or locally by a hand wheel. The bellow compression depends upon the position of the starting device and the speeder gear position, which alters the spring tension on the top of the bellow. The bellow is also subjected to the primary oil pressure, which is the feedback signal for actual turbine speed. The zero position of speeder gear corresponds to 2800 rpm i.e. hydraulic governor comes into action after 2800 RPM. The bellow and spring assembly is rigidly linked to the sleeves of the auxiliary follow-up piston valves. The position of the sleeve changes with the equilibrium position of the bellow.

SPEEDER GEAR

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HYDRAULIC SPEED TRANSMITTER The hydraulic speed transmitter runs in the MOP bearing and operates on the principle of a centrifugal pump. The variation of pressure in the discharge line is proportional to the square of the machine speed. This primary oil pressure acts as the control impulse for the hydraulic speed governor. The transmitter is supplied with control oil via an oil reservoir. An annular groove in the speed transmitter ensures that its inside is always covered with a thin layer of oil to maintain a uniform initial pressure. Excess oil drains into the bearing pedestal.

CURVE SHOWING TURBINE SPEED Vs PRIMARY OIL PRESSURE

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AUXILIARY FOLLOW-UP PISTON VALVES Two Auxiliary Follow-up pistons are connected in parallel and the trip oil is supplied to them through orifice. The sleeves of these valves are attached to the speeder gear bellow link. The position of the sleeve determines the draining rate of trip oil through the ports. Accordingly the trip oil pressure downstream of these valves changes. Oil downstream of auxiliary follow-up pistons circuit is termed as AUXILIARY SECONDARY OIL. Hence, aux. follow-up piston valves can be said to control auxiliary secondary oil pressure.

SEQUENCE TRIMMING DEVICE The function of the sequence trimming device or HP/IP TRIM DEVICE is to prevent any excessive HP turbine exhaust temperature due to churning. It changes response 2 of main and reheat control valves. When the reheat pressure is more than 32 Kg/cm and load less than 20% the IP turbine tends to get loaded more than HP turbine. The steam flow through HP turbine tends to fall to very minimum, causing a lot of churning and excessive exhaust temperature. The trim device operates at this moment trimming the IP turbine control valve. The control valves of HPT open more to maintain flow of steam, reducing the HPT exhaust temperature. It consists of a spring-loaded piston assembly, which is supported by control oil pressure from beneath, under normal conditions. The control oil is supplied via an energised solenoid valve. When the turbine loads is less then 40 MW and hot reheat 2 pressure is more than 32 kg/cm the solenoid valve gets de-energised cutting out the control oil supply to the trim device. The trim device trips under spring pressure. The trim device is connected to the follow-up piston valves of IP control valves by means of a lever. Upon tripping, the trim device alters the spring tension of follow-up pistons of IP pistons control valves, draining the secondary oil. The IP control valves openings are trimmed down.

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HYDRAULIC AMPLIFIER Hydraulic Amplifier consists of a pilot valve and an amplifier piston. The position of the pilot valve spool depends upon the aux. secondary oil pressure. Depending upon the pilot spool position, the control oil is admitted either to the top or the bottom of the amplifier piston. The other side of amplifier is connected to the drain. The movements of the amplifier piston are transformed into rotation of a Camshaft through a piston rod and a lever assembly. A feedback linkage mechanism stabilises the system for one particular aux. secondary oil pressure.

1. 2. 3. 4. 5. 6. 7. 8. 9.

Amplifier piston Follow-up piston Sleeve Shaft Lever Feedback lever Pilot valve Compression spring Adjusting screw

a : Control oil b : Secondary oil b1 : Aux. Sec oil c : Return oil

HYDRAULIC AMPLIFIER

SOLENOIDS FOR LOAD SHEDDING RELAY A pair of solenoid valves has been incorporated in the IP Sec oil line on control valves and Aux Sec. oil line, in order to prevent the turbine from reaching high speed in the event of sudden turbine load throw-off. The control valves are operated (closed) by the load-shedding relay when the rate of load reduction exceeds a certain value. The solenoid drains the IPCV secondary oil directly. Direct draining of IP Sec oil circuit causes the reheat valves to close without any significant delay. The HP control valves are closed due to draining of aux. secondary oil before the hydraulic amplifier, by the second solenoid valve. The extraction stops valves controlled by IP secondary oil acting through extraction valves relays also get closed. After an adjustable time delay (approx. 2 seconds) the solenoid valves are re-closed and secondary oil pressure corresponding to reduce load builds-up in the HP and IP turbine secondary oil lines.

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FOLLOW-UP PISTON VALVES The trip oil is supplied to the follow up piston valves through orifices and flows in the secondary oil piping to control valves. The secondary oil pressure depends upon position of sleeves of follow-up piston valves; which determines the amount of drainage of trip oil.

FOLLOW-UP PISTON VALVES

There are in all twelve follow-up piston valves. Six of them are associated with hydraulic amplifier and six of them with EHC in the governing system. The follow-up piston valves constitute a minimum value gate for both the governors. This means the governor with lower reference set point, is effectively in control. This is also termed as HYDRAULIC MINIMUM SELECTION of governors. The drain port openings of follow-up pistons of hydraulic amplifier depends on auxiliary secondary oil pressure, upstream of aux. follow-up pistons; and that of electro hydraulic converter, on the piston of pilot spool valve of the elector-hydraulic converter (i.e. EHC output).

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TEST VALVE

1. Bolt 2. Hand wheel 3. Spindle 4. Cover 5. Oil Seal 6. Bushing 7. O-ring 8. Valve Cover 9. Valve Body 10. Trip Oil 11. Piston sleeve

12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

EXTRACTION N.R.VS AND

Trip Oil Piston valve Spring plate Spring Spacer Bottom cover Trip oil Drain Trip oil Startup oil

Each of the HP and IP stop valves' servomotors receives trip oil through their associated test valves. The test valves have got port openings for trip oil as well as start-up oil. The test valves facilitate supply of trip oil pressure beneath the servomotor disc. (Stop valve open condition, under normal operation). For the purpose of resetting stop valves after a tripping, startup oil pressure is supplied to the associated test valves, which moves their spool downwards against the spring force. In their bottom most position the trip oil pressure starts building up above the stop valve servomotor piston while the trip oil beneath the disc gets connected to drain. When start-up oil pressure is reduced the test valve moves up draining trip oil above the servomotor piston and building the trip oil pressure below the disc, thus opening the stop valve. A hand wheel is also provided for manual operation of test valves.

EXTRACTION VALVE RELAY

Four pair of swing check valves are provided in the extraction lines to the feed heaters (LP Heaters No: 2,3, Deaerator and HPH No: 5) to prevent back flow of condensed steam into the turbine from heaters on account of high levels in the heaters. There are two NRVs provided in each of these extraction lines and is force closing type. Both these valves are free-swinging check type, however the first valve is equipped with an actuator. In case of flow reversals, both the valves are closed automatically. The actuator assists the fast closing of the first valve. The mechanical design of force-closed valves is such that they are brought into freeswinging position by means of trip oil. They are open as soon as differential pressure is sufficient. If the trip oil pressure falls, the spring force closes the valve when steam pressure either falls or is lowered (reduced load).

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The extraction valve relay, its changeover valve and its solenoid valve control the trip oil to each of the actuators of force closing type valves. Extraction valve relay actuates the FCNRVs in proportion to secondary oil pressure. By suitable adjustment of its spring, the secondary oil pressure at which the FCNRVs will be released for opening can be set. However, swing check FCNRVs will also open without the release action, also if the steam pressure is more than the spring force. But in this case the pressure loss shall be more leading to loss of efficiency. In case of turbine trip or sudden load reduction, by energising the associated solenoid valve, draining of trip oil pressure through extraction valve relay assists closing movements of FCNRVs. In both the cases the actuator is devoid of trip oil and its spring force closes the NRV. Extraction (4) FCNRV solenoid is also energised additionally by lower differential pressure in the extraction line.

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b

:

Control Oil

c

:

Return Oil

:

Trip Oil

:

Trip Oil

b1

:

Secondary Oil

x

b2

:

Secondary Oil

x1

COLD REHEAT SWING CHECK VALVE Two numbers of swing check valves are provided on the CRH lines from which the steam is drawn for HPH-6. Their pilot valves via their rotary servomotor in proportion to secondary oil pressure operate the CRH NRVs. They open out fully when main control valves open up corresponding to 5-10% of maximum turbine out-put. Only when the control valves are closed to this threshold again, the NRVs return into steam flow by the hydraulic actuator, so that when the steam flow ceases in the normal direction, they are closed by the torque of rotary servomotor. Even when the pressure of secondary oil has not built up sufficiently, NRVs can be opened up like safety valves when the upstream pressure rises above the downstream side pressure by one bar.

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VACUUM BREAKER

The function of the vacuum breakers is to cause an increase in condenser pressure by conducting atmospheric air into the condenser together with the steam flowing from the LP Bypass. When the pressure in the condenser increases, the ventilation of the turbine balding is increased, which causes the turboset to slow down so that the running down time of the turboset and the time needed for passing through critical speeds are shortened.

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HYDRAULIC AND ELECTRO-HYDRAULIC GOVERNING OF TURBINES Power produced by any power plant is sent out on utility grid (Transmission line and control equipments) together with power from other plants through process of synchronization with the grid and to distribution systems and then to the consumer. Control of system frequency on the grid or interconnected grid/pool is a major responsibility of load dispatchers. When a Turbo-generator is connected to grid, the speed of each machine in the grid remains same to all other machines connected to the grid. When an increase of load is required, more steam is admitted by opening/controlling the steam control valves. A basic understanding of turbine speed governors is necessary to maintain the central control of system parameters like speed, frequency, load, system voltages etc. In the paragraphs that follow, the turbine governing has been explained using theoretical information, figures and descriptions of governing systems. All turbines are equipped with speed governors. The purpose of the governor is to sense the instantaneous speed of the turbine in revolutions per minute, and to transmit a signal to the turbine control valves to open or close and maintain the desired speed. Most governors do not hold absolutely constant speed as load changes, but are designed to permit the speed to drop as the load is increased. As load is increased on the generator, the turbine speed tends to slow down. The speed governor spins slower (control arm moves toward “LOW” position), which results in the control mechanism in increasing steam flow to the turbine (control valve opens). The governors therefore control the steam supply to the turbine as well as ensure maximum safety of the machine and to the operating people when the turbine is on load. Basically, the governors perform functions such as: •

Parallel operation/working of machines with other turbine-generators connected together in a grid.

•

Output of each individual unit is controllable due to governing actions.

•

The governor enables the electrical grid system to be to some extent selfcompensating to changes in load demand.

•

The governor enables the turbine-generators not connected together, in a grid, run as single unit. (Before synchronisation), and also enables speed of turbine, kept under control.

•

The governor controls the rise in speed of all turbines irrespective of duty, in instances of losing its’ electrical loads.

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Turbine Governor System type-1 Governors of the turbines basically control the steam flow to the turbine. The governor usually takes the form of spring-loaded weights mounted on a shaft assembly that is driven by a worm & worm wheel from end of the H.P. shaft. The weights, which are held by springs, tend to move outwards due to centrifugal force and this movement is dependent upon the speed of the turbine shaft. The movement of the weights is arranged to operate on oil relay valve and this valve through an oil pressure relay system, opens or closes valves that admit steam to the turbine. When an increase of load is required, more steam is admitted to the turbine by opening the steam valves.

Simple turbine governor type-2 The governor (A) is driven from the turbine shaft. An arm pivoted at (B) has attached to it, the governor weights and a moveable sleeve (C). Sleeve (C) is connected to a floating lever (D) to which is attached the spindle (E) of the pilot relay valve and the spindle (F) of the main steam valve. If the turbine shaft speed increases, the governor weight will move outwards causing sleeve C to lift; this also tilts floating lever (D). These movements uncover the port (G) of the pilot valve thereby allowing oil pressure to act on the top of the power piston (H). At the same time port (I) in the pilot valve, allows oil to drain from the bottom (J) of the power piston. Due to this operation, the steam valve will move towards the closed position, thus admitting less steam to the machine. During installation and also afterwards, the governor springs are adjusted periodically, so as to keep the range at which the governor operates between limits.

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Loading on the machine is done/carried out by operating the hand wheel (K) thus opening the steam valve. The hand wheel (K) is normally on remote operation from the control panel by means of a reversible motor known as the “speeder motor”. Such governors do not use the electro-hydraulic governors, which control the operation by electrical interfacing units i.e. the electro-hydraulic converter. For detailed working of Governor, the drawing as shown below should be referred.

The percentage of control valve opening on each turbine depends upon the electrical output from that individual T.G, and in turn the entire system at the same speed (frequency). The system frequency decreases, as more electrical load is required. To regain the previous frequency/speed, the amount of fuel fed to the steam generator is increased adequately. Since with more customer load on the system, the frequency tends to decrease then the governors on all the system turbines need to operate (to open) the control valves to admit more steam to Turbine and allow the system to supply the extra load. Mechanical –Hydraulic System Block Diagram: The speed acts on the radial spring governor, this in turn, affects the hydraulic relay and also, the anticipatory derivative system (acceleration component). Local or remote adjustment on the speeder gear output is algebraically summed to act with the speed component, thus the gain that is also regulated by local adjustment of governor reputation through the pilot oil regulating valve, passes through a minimum selector that has been provided with another signal of locally/remotely controlled load limiting device; minimum signal thus obtained from here is acted upon the Auxiliary and main relays of governor valves of H.P and I.P control valves and the pressure switching & relaying that effects to operate the release and bled steam check valve. The feedback signal of S.V pressure, vacuum unloading gear and anti-motoring device act on check valve and also for differential pressure switching (it compares the minimum selector O/P as explained above); this forms the speeder gear

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runback as the feedback also. H.P and I.P control valves’ position are derived for valve offset adjustments. The figure below shows the block diagram of mechanical-hydraulic system.

The hydraulic oil used in the governor system is at a pressure up to 20 Bar. Better control can be achieved by increasing this pressure (more than 35 Kg/cm2 pressure) but this leads to leaks and fires. For this reason some turbines in use today utilize the Fire Resistant Fluid (F.R.F) system and thus the pressures can be increased without the risk of fires. Turbine bearings are lubricated with oil at between 0.3 and l.4 bar pressure depending upon the make and type of machine. A high-pressure oil pump normally supplies this oil and then pressure of oil is reduced as above. Emergency governors (often referred as the Over speed Governor): The emergency governor is the final line of defense to protect the turbine from dangerous over speeds. This device, when actuated rapidly closes all valves associated with steam supply to the turbine. Emergency governors are normally set to operate instantaneously if turbine speed reaches 110% of rated (3300 rpm on a two pole turbine generator) or higher speeds. The emergency governor shuts off the steam supply in the event of rotor speed increasing by more than 10% above its normal speed. A sliding bolt or an eccentric ring is attached to the shaft. These are held in position by means of a retaining spring. KORBA SIMULATOR

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The bolt or the ring flies out of the normal position .In doing so, it operates a trip and releases the relay oil pressure, which is holding the emergency, valve open. The emergency valve then shuts off the steam supply . The emergency governor is tested at periods by deliberately over-speeding the machine when the load has been taken off. Each of the twin bolts or rings is operated in turn. The one not being tested is made inoperative by a selector lever.

Droop of Turbo-generators: Speed regulations of turbine also called the Droop, (or the proportional band), is defined as the amount of speed change from no load to full load divided by the rated speed. Turbine Droop can be set in turbines either mechanically or electrically (In KWU turbines the provision of droop is made to range from 2.5% to 8.0% and to match the grid frequency, chosen setting is 5%). If the governor speed regulation is required to be set at 5% then for a 3000 rpm turbine, the control valves will be open wide at a speed of 2925 rpm or 2½ % below 3000 rpm. And likewise in other side of 50 Hz frequencies, the control valves will be fully closed, at a speed of 3075 rpm, or 2½ % above 3000 rpm. The droop setting in electronic system of EHG has been incorporated in a module connected in series which receives input as the load controller/comparator forming the error (MV-DV), and the droop corrected/incorporated signal is fed to the final load controller module of the load control loop. The amount of the inherent decrease in speed from no load to full load is called speed regulation, droop, or proportional band. The Droop is necessary in the control system in order to sense a change in speed and thus to reposition the valves. In KWU turbine (of SSTPS droop is set at 5%, i.e. = ±2.5% from 3000 rpm, or 50 Hz KORBA SIMULATOR

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frequency), the droop is set such that a biased zone is maintained from 3000 r.p.m to 3075 rpm. Beyond this speed until 3225 rpm, the droop gets affected automatically for unloading. Most grids operate automatically, to sense a change in system frequency as load goes up or down and to provide continuous signal to the controlled generating units in order to maintain the desired 50 Hz system frequencies. If the cost of generation at given moment on the grid is such that a load of 100 MW should be generated by that unit, that is the load that the automatic control will attempt to maintain The frequency bias of all controlling turbine generators on the grid is added up to determine the system frequency bias. In order to view the economical loading on the sets connected in parallel an example of a single unit can be considered for understanding the cost controlled situation. If the cost of generation at given moment on the grid is such that a load of 100MW should be generated by that unit, that is the load that the automatic control will attempt to maintain. The frequency bias of all controlling turbine generators on the grid is added up, to determine the system frequency bias. Our single unit example was being cost controlled to provide 100MW and it went to 104MW when system frequency dropped 1/10th of a cycle. With a 0 bias setting, as soon as the load increased to 104MW, the cost control would close the control valves to restore 100MW. At this point, the cost control is acting to oppose frequency correction back to 50 Hz. Further, let us review the frequency effects and the frequency bias on a particular unit , if it has been set to 4 MW per 0.1 Hz deviations. As soon as the system frequency drops to 49.9 Hz, the cost signal representing desired generation from this unit changes from 100 MW to 104 MW, under the added influence of frequency bias. If we can again assume that the turbine governor would again have picked up 4 MW, no control action occur to reduce generation back to 100 MW and system frequency should return to 50 Hz. Of course, if no automatic load frequency control is being used, then the dispatcher must manually direct an increase or decrease in generation from the units under his control, in order to restore system frequency to 50 Hz. In this case, the dispatcher “corrects” system frequency in order to provide the correct frequency on a 24-hour basis. This is usually done fairly close to midnight of each day. Instrumentation will advice him how far above or below 50 Hz the system has been operating for the past 24 hours. Knowing his system frequency bias, the dispatcher can then order more or less load to be generated for a given period in order to restore system frequency to an average of 50 Hz for the past 24 hours. This phenomenon is particularly important for controlling system frequency specially in view of controlling power generation with ABT.

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Transient speed rise (TSR): When load rejection takes place, 8-

settling down to steady state value

6-

TSR gives the % speed rise on full

4-

load throw-off

2-

TSR

speed shoots up temporarily before

O-

Steady state Time

Steady State Regulation:

nmax.

It is defined as the Ratio of % speed

nmin.

change (from no load to Full load) to the nominal rated speed. %Regulation=100x(nmax–

min)/nnom

0%

Load

100%

Load Frequency Control is shown in the figure below; it shows the single turbogenerator system supplying an isolated load. Main component are; 1. Fly ball Speed governor system 2. Hydraulic Amplifier 3. Linkage Mechanism 4. Speed changer Increase in frequency f causes the fly balls to move outwards so that B moves downwards by a proportional amount k2’ f. The net movement of C is therefore yC = k1 kC PC + k2 f and movement D, yD= k3 yC + k4 yE. The movement yD depending upon its sign opens one of the ports of the pilot valve admitting highpressure oil into the cylinder thus moving the main piston & opening the steam valve by yE.

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In KWU turbines, the stop valve & control valve (one set) share a common body. The piston of the servomotor is subjected to disc spring force in the close direction and Hydraulic pressure in the opening direction. Hydraulic Governor controls the steam supply by operating the control valves. The fluid pressure under the piston determines the position of the valve; this is controlled by pilot valve of the turbine governor & the secondary fluid oil system. Electro-Hydraulic Governor (EHG) Electro-Hydraulic Governor (EHG) works in parallel with Hydraulic governor at all times of requirements. Basically the Electro-Hydraulic Converter (EHC) is the connecting element between the electrical and hydraulic parts of the turbine governing control system for carrying out the Electro-Hydraulic Governing of the turbine. The Electro-Hydraulic Governor (EHG) is beneficial in:• • • •

Offering the flexibility, dynamic stability, dependability, excellent operational reliability, Low transients and steady-state speed deviations at all instances. Maintaining exact load frequency droop with high sensitivity. Providing reliable operation at times of grid isolation conditions. Operating the turbo-generator Safely in conjunction with TSE.

In KWU turbines, Electro-Hydraulic Governing has been achieved through various electronic / selector modules configured in four modes of controls:

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• • • •

Admission Control mode, Speed Control mode, Load Control mode Pressure Control mode.

The Hydraulic governor and the EHG system have been designed such that the governor with lower set point takes over or assumes the system control, as such normally, the set point of the Hydraulic Governor must be set above that of the Electro-Hydraulic Governor when EHG is effective. In cases, when EHG fails to cause shut-off, the set point that is, affected is that of Hydraulic Governor. In such situations the Tracking Device provides a revised set point of 5-10% above the EHG set point and it causes increase in small load when the control is transferred to Hydraulic-Governor. The tracking device is either switched on or off manually but when EHG failure or turbine trip occurs, the tracking device is switched off automatically thus tracking under faulted operation mode is prevented or prohibited. More details on tracking actions are covered in the follow-up circuits of the speed/load control modes. Electro Hydraulic Converter details: Electro Hydraulic Converter converts the electrical signal in to the hydraulic signals and large positioning forces are generated in control valves. The electrical signal from governor control circuit operates the sleeve and pilot valve spool; this regulates the trip fluid drain. Under steady state condition pilot is at central position; in deflected position, the control oil is admitted above or below the amplifier piston. The motion of the amplifier piston is transmitted via a lever to a camshaft, which actuates the sleeves of follow-up piston valves, causing secondary oil pressure to change. The speed, load, and pressure signals are measured and converted into conditioned signal in electronic modules.

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Admission Valve (spool) Controller Admission Valve (spool) Controller also referred as the position controller is Common for all three modes of EHG, and it supplies the operating current for driving the plunger coil. The Position controller loop uses a PID control mode for processing outputs that provide the driving current signal to the plunger and regulate the oil drains of HP/IP control valves (CV) ; thereby it controls steam supply into the turbine. The current in the plunger coil is increased for closing the HP /IP CV and vice versa for opening of the HP /IP Control Valve. The reference signal therefore works in reverse manner (rise in the coil current for low reference condition). By using two Nos of differential transformer (housed in EHC), feedback signal from the valve lift is derived to ensure proper stationing of plunger spool. Whenever current through the plunger coil gets interrupted or the electrical feedback circuit gets faulted, the reference value of the Hydraulic controller determines the actual valve position. Although the force to the plunger coil and to the control sleeve is, considerably smaller, but the regulating signal to the secondary auxiliary oil flow as transformed is quite large. The figure below gives various connections and modules used in EHG.

Control Transfer of various controllers: Three selectors have been used for specific functioning Speed controller output (hrnc) and the load controller output (hrpc) are passed through a Maximum selector (MAXKORBA SIMULATOR

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1) and the selected signal passes to a minimum selector (MIN-1) in such a fashion that at times of over-speeding of turbine (during load throw-off situations), the input to the minimum selector: MIN–1: takes care of transient condition of the load throwoff and is sufficient to check the turbine from over speeding. (During sudden load throw-off, over speeding of turbine is effected and since 10.5 V is generated by a potentiometer that gets algebraically summated with hrnc then it outputs voltage which is less than that of the speed/load signal as selected from the MAX-1:) The signal from the Minimum selector: MIN–1: passes through another Minimum selector: MIN–2: that receives the Pressure Controller output (hrPrc) signal as explained in pressure controller loop. Finally through the last minimum selector: MIN–2:, the control signal connects the Admission Valve (spool) Controller loop which outputs the driving current for the EHC plunger coil. Operation of EHG in various modes Start-up Switching the supplies ON and setting the speed/load setter to zero puts the EHG in Operational condition. The hydraulic speed control eqpt and the start up eqpt of the hydraulic controller are set in upper end position. The actual speed is sensed since turbine already is in barring gear and by slow rising of speed reference the speed controller works /is in service; the turbine speed is then brought up situation for synchronising TG with grid using speed controller. Operation under load Load controller can be taken in service after turbine is synchronised to control load in quick response and high linearity either as per LDC/AFDC or using various modes/sub loops explained in Load control. Frequency change is selected via the integral action load controller to corresponding droop values and a sensitivity of 5 Milli-Hz is obtained which meets the operational requirements of the present day large grid. The output signal of the speed controllers is automatically matched to the output signal of load controller from rated power on down to station load. The speed controller then remains in standby mode only and stands ready to provide station load in of load shading. Shutdown During normal shut down operation, the load controller is set to zero value. After the speed controller has assumed control of TG set, the unit can be disconnected from the grid. Load shedding In case of load shedding i.e. sudden separation of the generator, from the grid, the output signal of the load controller is immediately reduced to value below that of speed controller. Consequently due to minimum selection, the speed controller assumes control and returns turbine back to the set reference speed.

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This reference speed practically coincides with the rated speed, since the speed controller is set to provide the station load during the start of operation under load. This provision improves the dynamic response of the closing of the main steam stop valve /control valve and keeps the turbo set speed from rising along the droop characteristic. An additional effect is the reduction of the speed oscillations. In case of automatic reclosing of the generator CB, the reduction of the load controller output signal below the speed controller output signal below the speed controller output signal is cancelled and the initially selected load level restored. Speed Control Mode Speed Control Mode works during • • • • • •

Rolling (start-up or shut-down of the turbine), Speeding up of turbine until synchronisation, For effecting block loading & full loading of TG set at exceptional emergency situations House-loading operation during fast load throw-off For Controlling the TG set during rapid/large frequency fluctuations. Regulating during Over-speeding;(When the speed of the TG set rises slightly above synchronous speed, the control action in speed control mode quickly reduces the turbine speed very close to synchronous speed) During load shedding with subsequent operation of the TG set in an isolated grid situation, (The speed controller assumes continuous TG set control in such situations)

Speed reference signal (nR) is varied (In the range of 0-3600 rpm): • • •

Manually by Raise/Lower push buttons (using motorized potentiometer, By the synchronizer (when selected) or By follow up signal (explained separately). The speed reference (nR) can not be raised when follow-up condition exists and dn/dt is less than monitoring (in this situation lowering of nR gets slowed down.

The reference nR is varied in the range of 0-3600 rpm and for minute operation during synchronizing, above the speed of 2800 rpm; a reducing gear lowers the speed of the motorized potentiometer to ¼ rate for exact speed adjustment. The speed reference (nR) cannot be raised when follow-up condition exists and dn/dt is less than monitoring rather in this condition raising or lowering phenomena of nR gets slowed down when the speed reference (nR) is less than 2800. Two indicators have been provided in UCB panel for monitoring speeds; of narrow range (2700-3300) and wide range (0-3600). The Time-dependent speed reference signal ( nRTD ) The Time-dependent speed reference signal (nRTD) also referred as nR lim. influences the speed reference nR considerably. During start-up of turbine this nRTD allows rising of the turbine speed at the highest permissible rate consistent with the conservative operation as decided by the TSE computed margin signal introduced between a DC amplifiers. The Integrator module performs this function rising with time like a ramp. The slope of the integrator ramp can be adjusted over a wide range KORBA SIMULATOR

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and is optimized during commissioning. Fast mode or the stop action facility, modify the final nR .The output nRTD of the integrator module, is transmitted to the speed controller and displayed on the desk in the range 0-3600 rpm. Quantum of Follow-up signal is the difference between actual speed (nact) and offset of 120 rpm and is effected (switched automatically) if load controller is operative, final load reference (hrpc) is more than final speed reference (hrnc) by 10% and frequency is between 49-51 Hz OR if turbine is tripped (time elapsed) and speed reference (nr) is equal to actual speed (nact) minus 60 rpm. During follow up, the quantum of the follow-up signal is derived from the actual less the off-set (60-120 rpm) speed reference (nR) and difference is further added or subtracted as per the magnitude to cause change in speed reference (nR ). ‘Blocking ‘ or the ‘Stop nRTD ‘ of the speed signal is generated by an AND module in conditions i) speed >2850rpm, ii) nR is more than nRTD by 300 rpm and iii) an OR ed output of many conditions as given below:1. TSE influence gets faulted (goes out of order or switched off) or EHC fault condition appears AND turbine speed is more than 2950rpm. . 2. During the transition of control from electric to Hydraulic, the speed reference signal becomes less than actual speed and if is more than 50 rpm, i.e. (nR nact ) < 50 rpm.; 3. If nR > nRTD; pressure controller is in action OR Generator breaker not ON. This Block signal stops the integration (further) function of time dependent speed reference integrator, it blocks the already generated nRTD , and thus the speed controller input signal remains stay-put during stop action. During rolling of the turbine, if between the speeds of 600-2820 rpm the rate of speed rise is very low i.e. less than 100 rpm per minute, then the dn/dt is less than monitoring signal appears to alarm the operator; it also blocks any further rise in speed and brings back the speed reference to 600 rpm. dn/dt less than monitoring alarms the operator and takes care of low acceleration rate in turbine during rolling by suitable output from the speed reference setting module, and at the critical speeds (between 600-2829 rpm) of the turbine. The dn/dt is less than monitoring is derived from an AND gate module, its conditions are i) nR is more than 600 rpm, ii) nact is less than 2850 rpm, iii) MSV is open (>0%), vi) speed controller is selected & in action, v) Generator breaker is not on and a feedback signal of dn/dt + 225 mm wcl )

•

Main steam temperature trip ( < 480 o C )

•

Trip from functional group control (ATRS shut-down programme)

•

Generator trip

Like low vacuum tripping (electrical) the low steam temperature protection also comprises ' Arming'and ' Disarming'features to facilitate re-start of turbine, under low main steam temperature conditions. Over Speed Trip Device Two hydraulically operated over speed trips are provided to protect the turbine against over speeding in the event of load coincident with failure of speed governor.

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OVER SPEED TRIP DEVICE

1. 2. 3. 4. 5.

Bearing pedestal Spindle Spring Piston Piston body

6. 7. 8. 9. 10.

Spring Pawl Over speed trip bolt Shaft journal Limit switch

c: Return Oil u: Auxiliary Stratup Oil x: Auxiliary Trip Oil

When the preset over speed is reached, the eccentric fly bolt activates the piston and limit switch via a pawl. This connects the auxiliary trip oil to drain thereby depressurising it. The loss of auxiliary trip medium pressure causes the main trip valve to drop, which in turn causes the trip oil pressure to collapse. Low Vacuum Trip Device

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In the hydraulic low vacuum trip device, a compression spring set to a specific tension pushes downwards against diaphragm, the topside of which is subject to the vacuum. If the vacuum is too weak to counteract the spring tension, the spring moves valve 6 downwards. The pressure beneath valve is thereby dispersed and the auxiliary trip medium circuit is connected to drain. The resultant depressurisation of the auxiliary trip oil actuates main trip valves MAX51 AA 005 and MAX51 AA 006 thereby closing all turbine valves. The electrical tripping on low vacuum occurs through a pressure switch on the vacuum line to mechanical hydraulic low vacuum trip device also at the same condenser pressure. When turbine is started up again, this pressure switch is interlocked against a second pressure switch, which monitors this condition and prevents continuation of tripping initiation when condenser pressure is high. Thrust Bearing Trip Device The function of the thrust bearing trip is to monitor the shaft position in the bearing pedestal and, if a fault occurs, to depressurize the auxiliary trip medium and thus the trip oil in the shortest possible time, thereby tripping the turbine. 1. Compression spring 2. Bearing pedestal 3. Piston 4. Valve body 5. Turbine shaft 6. Pawl 7. Torsion spring 8. Piston 9. Compression spring 10. Limit switch 11. Knob a: c: u: x:

Test Oil Return Oil Aux. Startup Oil Aux. Trip Oil

The two rows of tripping cams, which are arranged on opposite sides of turbine shaft, have a specific clearance, equivalent to the permissible shaft displacement, relative to pawl of the thrust-bearing trip. If the axial displacement of the shaft exceeds the permissible limit, the cams engage pawl, which releases a piston to depressurise the auxiliary trip oil and at the same time to actuate limit switch. Electrical tripping of turbine is achieved by fire protection along with closure/stoppage of total control oil supply to turbine governing system by tripping the emergency stop valve on the control oil line. The fire protection trip is achieved by manual Pushbutton in UCB or automatically by very low MOT level (- 150 mm below the normal working level ' O' ). Please refer to the associated logics at the end of this chapter. Also fire protection-1 (automatic actuation) gets bypassed if the barring gear valve is ' not closed' .

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FIRE PROTECTION-1 CHANNEL-1

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FIRE PROTECTION-1 CHANNEL-2

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FIRE PROTECTION-2 CHANNEL-1

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FIRE PROTECTION-2 CHANNEL-2

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FIRE PROTECTION OIL TANK LEVEL MONITOR

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AUTOMATIC TURBINE TESTING (ATT) INTRODUCTION Under the present crunch of power crisis, the economy dictates long intervals between turbine overhauls and less frequent shutdowns. This warrants testing of equipments and protection devices at regular intervals, during normal operation. The steam stop valves and control valves along with all the protective devices on the turbine must be always maintained in serviceable condition for the safety and reliability. The stop and control valves can be tested manually from the location but this test does not cover all components involved in a tripping. Also, manual testing always poses a risk of mal-operation on the part of the operator, which might result in loss of generation or damage to machine components. These disadvantages are fully avoided with the Automatic Turbine Test. A fully automatic sequence for testing all the safety devices has been incorporated which ensures that the testing does not cause any unintentional shutdown and also provides full protection to turbine during testing. SALIENT FEATURES The Automatic Turbine Tester is distinguishable by following features: Individual testing of each protective device and stop/control valve assembly. Automatic functional protective substitute devices that protect turbine during ATT. Only its pretest is carried out without any faults i.e. if the substitute circuit is healthy, the main test begins. Monitoring of all programme steps for execution within a predefined time. Interruption if the running time of any programme step is exceeded or if tripping is initiated. Automatic re-setting of test programme after a fault Full protection of turbine provided by special test safety devices. Automatic Turbine Testing extends into trip oil piping network where total reduction of trip oil pressure due to actuation of any protective device, is the criteria for the satisfactory functioning of devices. During testing, general alarm or the cause of tripping is also initiated so that this part of alarm annunciation system also gets tested. Also, during testing, two electrically formed values of 3300 rpm take over protection of turbine against over speed.

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The testing system or ATT is sub divided in two functional sub-groups.

Each sub-group contains the device and all associated transmission elements for initiation of a trip. AUTOMATIC TESTING OF PROTECTIVE DEVICES ATT sub group for protective devices covers the following devices. 1. Remote trip solenoid-1. 2. Remote trip solenoid-2. 3. Over speed trip device. 4. Hydraulic low vacuum trip device. 5. Thrust bearing trip device. During normal operation, protective devices act on the stop/control valves via the main trip valves. Whenever any tripping condition (hydraulic/electrical) occurs, the protective device concerned is actuated. It drains the control/aux. trip oil, closing the main trip valves. The closure of main Trip Gear drains the trip oil, causing stop/control valves to close. During testing, trip oil circuit is isolated and changed over to control oil by means of test solenoid valves and the changeover valve. This control oil in trip circuit prevents any actual tripping of the machine. However, all alarm/annunciation are activates as in case of an actual tripping. Refer Fig. ATT for protective devices broadly incorporates the following sub programmes: a. Preliminary test programme. b. Hydraulic test circuit establishment. c. Main test programme. d. Reset programme.

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ATT SAFETY DEVICES

Preliminary Test In preliminary test programme, the substitute circuit elements and the circuit are tested for their healthiness; the turbine is fully protected against any inadvertent tripping during ATT. If any fault is present; further testing is inhibited. During preliminary test, following steps are performed.

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Test solenoids (TSX) become energised. Build-up of control oil pressure upstream of changeover valve is monitored. Test solenoids de-energised one by one & drop of control oil pressure is monitored. If all steps are executed within a specified time period pre-test is said to be successfully.

Hydraulic Test Circuit Establishment If no fault is present during preliminary test; command is automatically given to establish hydraulic test circuit (substitute circuit). The hydraulic test circuit is responsible for the supply of control oil in trip oil circuits. The test solenoids valves are again energised building up the control oil pressure upstream of changeover valve. At this moment another solenoid (SVX) gets energised, draining control oil and creating differential pressure across the changeover valve, it assumes upper (test) position and annunciation is flashed to this effect. With changeover valve in its test position, control oil flows in the trip oil piping. After successful establishment of hydraulic test circuit command goes to initiate the main test, in which individual devices can be checked.

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Main Test During main test programme, the associated hydraulic test signal transmitter with the exception of remote trip solenoids provides the necessary signal to actuate protective devices. The protective device under test operates and drains the aux. trip oil. Turbine trip gear (main trip valves) is closed after trip oil pressure drains and associated alarms flash. Reset Programme The resetting programme automatically starts after the main test is over. The reset solenoid valves energise and supply control oil in aux. start-up oil circuit to reset main trip valves and protective devices, which have tripped from their normal positions. Once they return to their normal position, trip oil and aux. trip oil pressure can be built-up and monitored. If oil pressure is satisfactory, reset solenoids along with test solenoid valves and SVX get de-energised, deactivating hydraulic test circuit and resetting circuit. TESTING OF PROTECTIVE DEVICES The main trip valves and remote trip solenoid valves have already been discussed in previous chapters; hence the remaining ones will be taken up here. Over speed Trip Device Trip consists of two eccentric bolts fitted on the shaft with centre of gravity displaced from the shaft axis. They are held in position against centrifugal force by springs whose tensions can be adjusted corresponding to 110% - 111% over speed. When over speed occurs, the fly weights (bolts) fly out due to centrifugal force and strike against the pawl and valves, draining aux. trip oil pressure and tripping the turbine.

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HYDRAULIC TEST SIGNAL TRANSMITTER (HTT) FOR OVER SPEED TRIP DEVICE

During ATT, the associated hydraulic

test signal transmitter

I. II. III. IV. V.

Control Oil Test Oil Aux. Trip Oil Aux. Startup Oil Drain Oil

1. 2. 3. 4.

Limit switch (normal) Limit switch (test) Valve for Test Oil Actuator

(HTT) becomes ' on' ; spool valve slowly moves down to gradually build-up test oil (control) pressure beneath the flyweights. At pre-defined test oil pressure fly weight one and two operate to actuate individual pawl and spool arrangements bringing in the associated alarm. For resetting, spool moves-up and when test oil pressure is fully drained, aux. start up oil (control oil from ' reset'solenoids) pressure resets the devices to their normal position.

Low Vacuum Trip Device With deterioration of vacuum, pressure builds-up over the diaphragm, the spool valve move down, causing valve also to move toward lower position. The aux. trip oil pressure drains, tripping main trip valves and the turbine stop/control valves. During ATT, after hydraulic test circuit is established, the HTT (Hydraulic Test signal Transmitter) gets energised and connects the space above diaphragm to atmospheric pressure through an orifice. The device operates, bringing in the associated alarm. As soon as reset programme starts, HTT is de-energised and vacuum trip device is automatically reset, Field adjustment facilities and checks have been provided when turbine is stationary and there is no vacuum in the condenser. Thrust Bearing Trip Device This device operates in case of excessive axial shift ( >0.6 mm) or excessive thrust pad wear. Two rows of tripping cams on the shaft engage with the pawl) under high axial shift condition. Valves spool moves up draining aux. trip oil and tripping the trip gear and turbine. During ATT, associated ATT solenoid is energised and test oil pressure is supplied to test piston valve. The piston rod actuates the pawl and spool valve assembly, bringing in the associated alarms. During resetting, HTT is deenergised and aux. start-up oil (control oil) resets the device back into normal position.

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AUTOMATIC TESTING OF STOP/CONTROL VALVES The combined stop/control valves are final control elements of the turbine governing system. They must be maintained in absolutely workable condition for safety and reliability of turbine. All the four stop and control valve assemblies are tested individually.

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During ATT of stop/control valves, they are actually closed. In order to prevent large fluctuations of initial pressure or load on the machine, it is essential that Electro Hydraulic Governor is in service and machine load is less than 160 MW and load controller is ' ACTIVE' .

As soon as test programme is initiated, the positioner motor of control valve servomotor' s pilot starts. The control oil supply pressure beneath the servomotor piston drops and control valve starts closing. After the control valve is fully closed, command goes to energise solenoid valve (1). The trip oil pressure drains beneath the disc of stop valve servomotor piston. The stop valve closes. After the stop valve is closed, automatically a command goes to energise another solenoid (2). This supplies trip oil to the test valve such that test valve moves down gradually to admit trip oil pressure above the servomotor piston. As soon as piston sits on the disc, there is a sudden rise in trip oil pressure, which is sensed by pressure switches. After these solenoids (1) & (2) de-energise, the test valve moves up to admit trip oil beneath the disc and connecting the space on top of the piston to drain. This pressure difference causes the stop valve to open. Once the stop valve is opened, next command goes to the positioner motor to move in reverse direction; opening the control valve. All along this test, the other control valves are operated by the governor, so as to keep the load and pressure reasonably constant. Should any turbine trip occur during the test, all solenoids are de-energised and tripping takes place in the usual manner.

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TESTING SCHEDULE All important turbine components must be tested at regular intervals. The operating reliability and availability can only meet the high requirements if testing is undertaken at the scheduled times, as recommended below. Testing Intervals Tests are scheduled according to the following Testing Interval Categories. 1. Testing Interval 0

Fortnightly

2. Testing Interval I

Quarterly

3. Testing Interval II

Six-monthly

4. Testing Interval III

Annually

5. Testing Interval IV

After operation interruptions more than 12 month

6. Testing Interval V

After or during overhauls

Testing Interval Category 0 applies to devices, which can be tested automatically without interrupting operation. The tables show the allocation of the Testing Interval Categories to the test. Controller System / Device

Test

Test Conditions

Operation Turbine controller Load shedding relay Bypass controller Pressure controller Oil temp controller

Function Adjustment

Test Interval 0

I

II

III

x

x

x X*

Standstill

x

x x

Load rejection Operation

Function

V

Load Rejection Standstill

Function

IV

x X*

x

x

Load rejection

x X*

Adjustment Standstill

x

x

Function

Operation

x

x

x

Function

Operation

x

x

x

X*: Recommended; not required by manufacturer

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Sub loop Control of Pumps System / Device

Oil Pumps

Test

Test Conditions

Test Interval O

I

II

Function

Shutdown

x

Start-up Pressure

Operation

x

III

IV

V

x

x

Valves System / Device

Test

Test Interval

Test Conditions

O

I

II

III

Standstill Stop Valves

Freedom of movement Leak Test

Operation

Freedom of movement Leak Test

V

x

x

x

x

x

x

x

x

x

x

x

x

x

Operation ATT x Shutdown Start-up

/

x

Standstill Control Valves

IV

Operation

x

Operation ATT x Shutdown / Start-up

LP Bypass Control Valves

Freedom of movement

Standstill

Extraction Valves

Freedom of movement

Operation

Safety valves

Actuating valves

Operation/ Standstill

Vacuum breaker

Function

Standstill

x

x

x x

x

x

IV

V

x

x

x

x

Protection and Safety Devices System / Device

Test

Main Trip Valves (Gear) Function Remote Trip Solenoids

Function

KORBA SIMULATOR

Test Conditions

Test Interval O

Standstill Operation ATT Standstill Operation ATT

I

II

III

x

x

162

Over Speed Trips

Function Actuating value

Hydraulic Function Low Vacuum Actuating Trip value Electrical Function Low Vacuum Actuating Trip value

x

Function

LP Bypass Condenser Protection

Function Actuating value

Reverse Power Protection Fire Protection

x

x

Rated speed x ATT Standstill Operation ATT

x

x

Standstill

ATT

x

x

Standstill

Thrust Bearing Trip

x x

x

x

Standstill

x

x

Function

Shutdown

x

x

Function

Shutdown

x

x

Safety Function Devices for Actuating Reverse Flow value Low Lub Oil Pressure Protection Device

Over speed after load operation Rated speed

Function Actuating value

Protection Devices

Too high Steam Pr.

Function Actuating value

Too low Steam Pr.

Function Actuating value

KORBA SIMULATOR

Operation

x

x

Standstill

x

x

x

Standstill

x

x

Standstill

x

x

163

Alarms and Measuring Devices System / Device

Alarms for all system Digital Signal Transmitter

Test Conditions

Test

Function

Operation

Actuating Value

Standstill

Function Actuating Value

Standstill

Speed

Test Interval O

I

II

III

IV

V

x x

x

Operation

x

x

x

Measuring Devices

Pressure

x

x

Temperature

x

x

x

x

Expansion Vibration Oil Level

Accuracy of indication

Standstill

Valve Position

x

x

x

x

x

x

x

x

x

Measurement of Important Operating Parameters System / Device

Steam Temperature. Steam Flow Internal Efficiency Condenser Leak Tightness Bearing Metal Temp.

Test Conditions

Test Interval

O

Long Term Monitoring

Steam Pressure

Test

I

II

III

IV

V

x x x x x Operation

x

x

Expansion

x

x

Vibration

x

x

Oil Levels

x

x

Oil Pressure

x

x

Oil Temperature

x

x

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TURBINE STRESS EVALUATOR (TSE) SIGNIFICANCE OF TURBINE STRESS MONITORING It is important for the operator to know how quickly his turbine can be started up and what changes in load he can make without the fear of over-stressing the turbine components; thereby causing excessive fatigue. Whenever steam inlet temperature changes within the turbine, the metal temp. follows the steam temp. with a certain delay. This causes differential thermal expansions within the turbine casing and shaft & corresponding stress in the metal. Thermal over-stressing can reduce useful operation life of turbine and its components. Turbine Stress Evaluator measures and calculates the relevant temperature values and evaluates them in an analog computing circuit and determines the allowable conditions of operation so that useful life of the turbine shall not be unduly reduced. Thus it allows the operation of the turbine at the highest possible rates of load/speed change while limiting the stresses within permissible values. The results of TSE, which are the appropriate operating instructions, are displayed by means of an indicating instrument. TASK OF TSE If the turbine is to be operated so that there is no undesirable material fatigue, these thermal stresses must be kept within acceptable limits. The optimum balance between longevity on one hand and material flexibility of operation on the other is achieved when the permissible range of material stress can be utilised to the fullest extent. The turbine stress evaluator provides the basis of continuously calculating permissible values for desired changes in operating conditions at all times and under all operating states and by displaying temperature margins, within which the speed/load can be changed during loading/unloading of the machine. Signals from the TSE are also fed to the speed and load reference limiter of the turbine controller for use in set point and gradient (speed and load) control. MEASURED VALUE ACQUISITION AND PROCESSING Wall temperature sensors (thermocouples) are used to sense the temperature of various turbine components and these signals are given to the TSE as the input temperature signal. The wall temperature sensors measure temperature of the surface, which is in contact with steam (Ts, at 95% depth) and mean wall temperature in the middle section of the components (Tm, at 55% depth). The thermal stress on the individual components can be ascertained from difference between the two temperatures of the component. Specially designed wall temperature sensors are used on combined stop and control valves of HP turbine for measuring these temperatures. These sensors have two measuring points. These comprise a screwed sleeve containing a measuring insert. The screwed sleeves are inserted in a through hole in the wall of casing and welded on outside. It is made of the material having temperature characteristics similar to those of casing. This ensures good thermal contact and same thermal gradients through temperature sensor as surrounding wall.

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Shaft temperature simulation If the thermal stress in rotor is to be monitored, the surface temperature on the inside of casing surrounding the rotor is measured by a single thermocouple at a point where the dynamic behaviour of temp of the shaft corresponds to that of casing. It is taken as the surface temperature of shaft itself. The corresponding mean shaft temp is simulated, values derived from the measured surface temperature, depending upon machine load, steam temperature and time lapsed. The mean internal (mid metal) shaft temperature can be calculated with an adequate degree of accuracy by means of the following mathematical equation. Tm = Ts [ 1- (0.692 e -t/T1 + 0.131 e -t/T2 + 0.177 e -t/Tk ) ] Where,

Ts Tm t

: : :

Surface Temperature Mid metal Temperature Time in minutes

T1 T2 Tk

: : :

2408.31 457.08 56.62

Time constants

Various constants used in the above equation are derived from the shaft diameter and thermal diffusivity of the rotor material. The solution of this equation is realised by means of three integrators and one summing amplifier. Normally 5 measuring points feed the TSE. First two measuring points are located in the body of combined Stop/Control valves are called ADMISSION sensors. The next two are located in the HPT cylinder adjacent to the first drum stage and are called HPT wall temp sensors. The last measuring point is in the flange of IPT cylinder inner casing, before the last drum stage to represent the surface temp of the shaft. FUNCTIONING OF TSE The mV output from thermocouples is fed into the signal conditioning cabinet where the transducers give out 4-20 mA signals as temp signals. The measured values are processed in an analog computing circuit with 3 channels namely ADMISSION, HP TURBINE and IP TURBINE channels. Each computing channel determines ∆Ta between surface and mean (mid) temperatures. The thermal stress is proportional to this temp difference. The calculated temp difference is compared against the permissible mean temp difference ∆Tp, which is derived from function generator for each computing channel. The difference between ∆Tp and ∆Ta is called MARGIN. Comparing ∆Ta against ∆Tp on the positive side, we get upper margin and that on the negative side we get lower margin. The smallest of the respective upper and lower temperature margins calculated for Admission and Turbine area, are selected as representative margins and are displayed by TSE indicator and used for further processing. Till the machine load Pact remains < 2% PMCR, the TSE display selection remains in the ADMISSION / SPEED mode, in which the actual speed and temperature margins either form admission or turbine channels, as sleeted, are displayed. At Pact >2% PMCR , the TSE changes over to TURBINE or LOAD mode in which the actual load and load upper and lower margins are indicated. The load margins are calculated from the available temperature margins and changes in the casing temp differential. KORBA SIMULATOR

168

KORBA SIMULATOR

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TSE BLOCK DIAGRAM (200MW KORBA UNITS)

TSE INDICATOR The indicator is divided vertically into two sections one for starting and one for load operation. It comprises of three discs. - One Circular Scale disc, partly calibrated in Speed - and Two Load partly coloured glass discs. These discs are controlled by means of three electrical servomotors equipped with feedback potentiometers. The circular scale is controlled by the actual value for either Speed or Load. The upper red coloured disc is controlled by the upper available margin either for the temperature or the load. The lower red coloured disc is controlled by the lower available margin either for the temperature or the load. The module for control of the potentiometer-equipped servomotor is located in the TSE indicator and receives three impressed currents of ± 1 mA for the computing circuits. Power supply for the TSE indicator is ± 24 V. Before the generator is synchronised, or the load is below 2% MCR the actual speed and temperature margins are displayed on the left side of the display. After the generator is synchronised and load is greater than 2%, the actual load and load margins are displayed on the right side. The effective section is illuminated according to the operating mode.

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The row of alarm windows located across the top of the two section of the indicator shows which computing channel is on line. The red window in the middle is the fault alarm. Illumination of the appropriate chosen window indicates whether the displayed margins are being supplied by admission or turbine channel. Additional LEDs located above and below are the symbols for HP and IP turbine in the turbine related window and indicate from which turbine upper margin (Upper LED) and the lower margin (Lower LED) is originated. The appropriate LEDs then show a red light. During start-up, speed and temperature margins are displayed on the left hand section. Speed is indicated on a circular moving scale. The upper and lower temperature margins are covered on the screen. These screens have two zones in different colour; the red area represents a warning or prohibited range, while the white indicates the permissible zone of operation. During load operation, the right hand section displays the actual load and the load margins. The actual load is shown on a rotating disc marked in MW. Two rotating red discs indicate the permissible load range. The turbine is operating within the permissible stress as long as the actual temperatures, load values are located within the transparent region between the discs.

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The opaque red section between the discs covers the prohibited range. The upper boundary of the transparent sector indicates the upper margins (for start-up and increasing load); the lower boundary indicates lower margins (for decreasing load and shut-down).

Changing Section of the TSE Indicator During speed operation, the TSE indicator can be switched manually by means of two push buttons on the control desk to indicate the respective margin either from the admission or from the turbine channels as desired. During speed operation, if the non-illuminated green alarm window flashes, this indicate that the temp margins originating from one of the channels related to the flashing window has been reduced to less than 100K. The flashing lamp instructs the operator to changeover to the range with the smaller margins. When changeover has been affected, the green alarm window changes the flashing to steady light. When the unit is synchronised, the TSE indicator automatically changes over to the right hand section, which displays the load margins calculated from the temperature margins. Two red discs indicate the permissible load change.

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TSE INDICATOR SELECTION

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IMPORTANCE OF THE MARGINS The temperature margin is a measure of the degree of thermal stresses, which a turbo-set can be subjected to, during rapid increase in speed during synchronisation etc. The load margin is the greatest step change in load based on the instantaneous stress condition, which the turbine can withstand without being over-stressed. If the margin is consumed, this means that the component is being stressed to its permissible limit. The condition is indicated by the relative edge of the red disc horizontal position. Further, increase in speed or load should then only be made at a rate, which will enable the disc to maintain their position. In this way optimum startup or load change is achieved without over-stressing the component. If the indicated actual load becomes covered by the upper or lower red discs, the component material is being subjected to excessive stresses which means an intensified effect on the material fatigue. If the excessive stresses are to be reduced, the steam temperatures are to be brought closer to the turbine temperature that is to say, if the upper margin is exceeded the steam temperature must be reduced and conversely, the lower margin is exceeded the steam temperature is to be increased. Any reduction in steam flow leads to less effective heat transfer and thus tends to reduce temperature differential. In many cases, it is sufficient to wait or to reduce the rate of change of load and temperature until an adequate margin is obtained. All counter measures must be directed towards protecting the component, which is in the greatest danger of overstressing. Before synchronising is carried out, it is necessary to have temperature margins of more than 300K available, so that the minimum load on set can be achieved immediately after synchronisation. Load Margin Calculation The upper and lower load margins are computed by using the respective temperature margins of the turbine computing channels. The upper load margin represents the minimum value determined from the individually computed available upper margins of HPT and IPT. dPu

=

Minimum value out of dPuH and dPuII(M)

Where, dPuH dPuI

=

dPuI + dPuII(H)

= dTuHT min . (A + B. Pact)

dPuII(H) =

dPuII(M) =

dTuHT min. . (V2 + W2. Pact) eH (TH – Tm HT min) dTuMT . (V1 + W1. Pact) eM (TM – Tm MT)

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Values of various constants are: A

=

0.110

TM

=

5.833

V2

= 1.296

B

=

0.090

TH

=

8.250

W1

= 0.090

eH

=

3.000

V1

=

0.315

W2

= -0.150

eM =

6.000

The load lower margin available is calculated as follows: dPl = dTlT min . (C + D. Pact)

Where,

A = 2.725

B = -0.150

TSE TEST Panel Testing A test programme for the Turbine Stress Evaluator is available for testing the correct functioning of individual computing channels, from the input amplifiers to the display unit. For this purpose, fixed voltages are introduced into the computing circuit through relays, instead of the measured temperature and load signals. Testing can be done only if ' Enable'or ‘Release’ signals from EHC (electro hydraulic turbine controller) and FGA (functional group automatic control) are present. If the TSE is functioning correctly, the indicator must show specific known values for each computing channels. If there is a deviation from the tolerance value, it indicates that there is some fault/error in the evaluator.

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The following table can be used while performing the TSE test. The buttons needed to be depressed, for testing of each category, are shown in shaded. Sl. No.

Selector Pushbutton

Test Programme

Computing Pushbuttons MSV

MCV

Channel

(ADMISSION/TURBINE)

Initial condition HPC HPS

IPS

Turbine Rolling

Turbine on Load

1

Admission HP Stop Valve

A

A

2

Admission HP Control Valve

A

A

3

Turbine HPT Casing (Rolling)

T

T

4

Turbine HPT Shaft (Rolling)

T

T

5

Turbine IPT Shaft (Rolling)

T

6

Turbine HPT Casing (Load)

T

7

Turbine HPT Shaft (Load)

T

8

Turbine IPT Shaft (Load)

T

For example, if it is required to start HPT Casing (Sl. No:3) test while the machine is on load, the pushbuttons A and HPC are to be pressed simultaneously. Test Results for 200MW TSE MSV

MCV

HPS

HPC

IPS

Tu

30 K

21 K

96 K

60 K

104 K

Tl

79 K

99 K

6K

13 K

13 K

Pu

230 MW

200 MW

157 MW

Pact

100 MW

100 MW

100 MW

Pl

79 MW

50 MW

49 MW

TSE influence can be switched off from the EHTC control cabinets and under such conditions turbine should be operated in accordance with the recommendations of the manufacturer within permissible temperature differences.

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Dynamic Test (Monitoring) of TSE This facility has been provided for continuous monitoring of the healthiness of all the input signals, computational values, and output signals. This testing is automatically carried out all the time. For this test, a testing device is incorporated in all the signals, which monitors the rate of change of those signal values. If any signal changes at an unrealistic rate, then those devices generate a TSE fault alarm. Consequently PRTD setter will freeze avoiding the erroneous values entering into the load controller loop. PRTD can be reset and made free after TSE influence is switched off.

TSE Output Signals 1) To ATRS a) From Step No. 14 to Step No. 15. If TSE upper margin is less then 300 K, further speed rise from 600 rpm to 3000 rpm is not possible. b) SGC Turbine start up programme gets switched off if TSE upper margin is less than 00K with turbine speed > 600 rpm < 2800 rpm while rolling. c) Switching off TSE influence will make SGC turbine programme off Fault in TSE does not make SGC Turbine programme off. 2) EHC a) Speed Controller Only upper TSE margin is used. Lower TSE Margin is not used, as coasting down is natural. The time dependent speed reference signal (NRTD) allows rising of turbine speed at the highest permissible rate consistent with the conservative operation as decided by TSE computed speed margin signal introduced D.C. amplifier. b) Blocking or ‘stop NRTD’ of the speed signal if Speed > 2850 rpm NR > nRTD by 300 rpm AND TSE Influence sets faulted.

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c) Load Controller Both lower and upper margins used. These margins determine the gradient at which PRTD varies. -ve upper margin can unload the machine whereas reduced lower margin can prevent turbine from deloading. Load signal gets blocked in case TSE going out of order when influence is on. 3) CMC Unit load rate (set in CMC module) is going to Guided Target Indicator gets compressed with TSE margin. Minimum of TSE lower margin and unit load rate considering NO RUNBACK situation goes to GNI which ultimately gives the rate at which the unit should be unloaded. Maximum of TSE upper margin and UNIT load Rate goes to GNI which ultimately gives us the rate at which the unit should be loaded.

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GENERATOR SYSTEM

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GENERATOR AND AUXILIARIES DESCRIPTION The 200MW generator is a three phase, horizontally mounted two-pole cylindrical rotor type, synchronous machine driven by steam turbine. The stator winding is cooled by de-mineralised water flowing through the hollow conductor while the rotor winding is cooled by hydrogen gas maintained inside the machine. Fans mounted on the generator rotor facilitate circulation of hydrogen inside the machine. Four coolers mounted inside the machine cool the hydrogen gas. The generator winding is provided with epoxy thermo-setting type insulation. The machine is provided with completely static thyristor controlled excitation system, fed from terminals of the machine. Hydrogen being a light gas with good heat carry away capacity is used for cooling the rotor winding, rotor and stator core. Two hydrogen driers are provided to facilitate moisture removal. Hydrogen is circulated through them via the fans in dry condition. Normally one drier is kept in service and other is on regeneration. Four hydrogen coolers are provided to cool the hot gas to maintain the cold gas temperature at 40oC. Liquid Level Detectors (LLDs) are provided to indicate liquid in generator casing. This provision is to indicate leakage of oil or water inside the generator. It can be drained through drain valve. H2 gas purity is to be maintained of very high order i.e. more than 97%. STATOR WATER-COOLING SYSTEM The stator winding of the generator is cooled by de-mineralised water circulating through hollow conductors of stator winding bars in a closed loop. The cooling water system consists of 2x100% duty AC motor driven pumps, 2x100% duty water coolers, 2x100% duty mechanical filters, 1x100% duty magnetic filter, expansion tank, polishing unit and ejector system. The stator water pump drive the water through coolers, filters and winding and finally discharges into the expansion tank situated at a height of about 5m above the TG floor. It is maintained at a vacuum of about 250 mm Hg by using water ejectors. A gas trap is provided in the system to detect any traces of hydrogen gas leaking into the stator water system.

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WATER PATH OF STATOR WINDING AND TERMINALS

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SEAL OIL SYSTEM To prevent leakage of hydrogen from generator housing, ring type shell seals are provided at both ends of the generator. During normal operation the AC seal oil pump draws the seal oil from the seal oil tank and feeds it into the shaft seals via 2x100% capacity coolers and 2x100% capacity filters. The differential pressure 2 regulator maintains seal oil pressure differential of 1.3 Kg/cm over the hydrogen pressure irrespective of the value of hydrogen pressure. The seal oil is supplied to the shaft seals into the annular groove of seal ring via the passage in the seal ring carrier. The clearance between shaft and seal ring is such that frictional losses and seal oil temperature rise are minimum. Oil film is of sufficient thickness to provide proper sealing. Higher-pressure ring relief oil is fed in the annular groove in the airside seal ring carrier. Thus gas and oil pressure acting on the seal ring are balanced and friction between seal ring and seal ring carrier is minimized. The seal ring is free to adjust its position according to shaft position. Airside seal oil is directly returned to the seal oil tank via a float valve. The oil drained towards the hydrogen side is first collected in pre-chamber and then passed to the intermediate oil tank in order to separate any trace of hydrogen present in seal oil. The oil from this tank also is returned to the seal oil tank via a float valve. Any possible traces of gases or vapour etc. are removed by vacuum pump from top of the seal oil tank. In case of failure of DPRV-A or AC as well as DC seal oil pumps failure, DPRV-B will come into service and governing oil is used as seal oil.

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GENERATOR SEAL OIL SYSTEM

SPECIFICATIONS OF THE GENERATOR Rated parameters: Maximum continuous KVA rating

247, 000 KVA

Maximum continuous KW rating

210, 000 KW

Rated Terminal Voltage

15, 750 V

Rated Stator Current

9050 A

Rated Power Factor

0.85 Lag

Excitation voltage at MCR condition

310 V

Excitation current at MCR condition

2600 A

Excitation voltage at no load

102 V

Excitation current at no load

917 A

Rated speed

3000 RPM

Rated frequency

50 Hz.

Stator winding resistance per phase at 20 oC. Rotor winding resistance per phase at 20 oC.

0.00155 Ω 0.0895 Ω

Efficiency at MCR condition

98.49 %

Short circuit ratio

0.49

Rise in voltage with 100% load throw off

22.40 KV (without AVR)

Negative phase sequence current capability

1

Direction of rotation when viewed from slip ring

Anti clockwise

Phase connection

Double star

No of terminal brought out

9 (6 neutral, 3 phase)

Generator gas volume

56 m

Nominal pressure of hydrogen

3.5 kg/cm

Permissible variation of gas Pressure Nominal temperature of cold gas

± 0.2 kg/cm 40 oC. (Alarm)

Purity of hydrogen

> 97 %

Relative humidity of H2 at nominal pressure

2 2

t A pre-selected value (15%) is added directly to the output of GNI (Ao) to obtain an immediate change in the load set point to the boiler, and the signal gets slowed down as the target value of the GNI is corrected and the output runs towards the target with a selected gradient. The target value Zo of the GNI is limited by the Maximum of the turbine load signal and the runback limit signal. The Electronic Module XU 02 with catalogued program through X 203 works as a GNI, which is the heart of the load set point guidance sub-loop

CMC-GUIDED TARGET INTEGRATOR INPUTS & OUTPUTS

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324

While the unit load demand thus generated as above is used for directly the boiler master and the firing rate control, it is also further sent to turbine side for controlling the turbine generator load through electro hydraulic governor. The frequency influence is incorporated in CMC loop by suitable setting in the function generator module. The frequency deviation signal modifies the Unit load demand generated by the GNI module, through a proportional module P and is further compared with run back limit signal in a minimum selector. Frequency corrected Mw load demand signal finally forms the input of GNI module for further processing. Final load demand signal is passed on for signalling to the system (healthiness) and also indicated in the dual point indicators. The unit load demand received either from LDC or set by the operator from the unit master is also modified by the deviation in the system frequency. The correction provides a change in unit load demand equivalent to the expected change in MW output due to any deviation in system frequency. System frequency deviations within a small range (±5%) are passed directly and larger deviations are routed through the GNI, influencing the electrical load demand signal. BOILER MASTER CONTROLLER The boiler master controller is provided to maintain the throttle pressure constant during various plant conditions. The output of this controller is used as a demand signal to the boiler firing rate and air controls. The signals steam flow to turbine (1st stage pr.) in case of Boiler Follow Mode and electrical load demand signal in case of coordinated mode and turbine follow mode are used as a feed forward signal. Under transient operating conditions, the throttle pressure is allowed to vary within a small range to increase turbine output by utilizing storage capacity of the boiler and to raise or to lower the energy level of boiler by over or under firing. However, any larger variations in throttle pressure shall restrict the turbine output till boiler has produced additional output to match the increased demand. An additional digital indicator has been provided to monitor the Mw load demand in megawatt scale / display. Two nos. of maximum limit and minimum limit value setters, are available to limit the Mw load demand within limited range as desired (Max – 250 MW & Min 100 MW) and output of the unit master is within these two limits. The frequency deviation K. δF > +/-15% corrects the MW load demand signal that forms input to the GNI. FIRING RATE CONTROL The firing rate demand signal generated from the coordinated control system is divided into two controls loops/ sub-systems of Fuel flow control and Airflow control to establish the fuel and air flow control systems. The firing rate demand signal to the fuel master stream is compared with total air flow in a minimum selector card 15 and the lower of the two signals form the set point and also the feed forward signal to fuel control stream.

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Also the same firing rate demand signal to the Airflow control stream is compared with total fuel flow signal in a minimum selector card 15 and the lower of the two signals form the set point and also the feed forward signal to Airflow control FUEL FLOW CONTROL (COAL FEEDER SPEED CONTROL) The Coal feeder speed signal is 4 to 20 mA and is obtained from coal feeder speed sensors. There are two redundant speed signals for each feeder, one already isolated, and for the other an isolator card is provided. The isolated feeder signal for each feeder is given to signal monitor and distributor card 30 from where 4 to 20 mA decoupled signal is sent for DAS and for various indicators. A provision is made for selecting either of the two speed signals to be used, one for control and the other for switching the feeder speed control to manual upon failure of the signals in use. This is achieved by using the 24-volt release signal from signal monitor card 30 and analog switch 51. The supply and return oil flows are measured by flow meters in the heavy fuel oil supply and return lines. The oil flow to the burners is the difference between the two measurement signals of HFO flow to burners and the return oil back to the HFO tanks. The sum of the feeder speed and the oil flow becomes the Total fuel flow signal (Actual value) in fuel master loop. Calorific value Cv correction is included to modify the fuel flow signal as per the quality of the coal received, this signal is not other than the multiplying constant adjusted at the Cv potentiometer in the derived fuel flow for comparison with the firing rate demand as desired value signal. The fuel master controller has the provision for receiving the Auto release signal from (master controller logic card 02) the feeder speed controller to ensure that unless at least one of the slave feeder controllers is in Auto, the fuel master controller shall not operate in Auto. A uniform dynamic response of the system outputs for the entire load range is maintained and achieved by automatically changing the appropriate control system gain, by Proportional-amplifiers card 16 and analog switch 51 as pulverizes are taken in and out of service.

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CMC : FIRING RATE CONTROL (COAL FEEDER-TYPICAL SCHEME)

A separate controller controls the heavy fuel oil header pressure by comparing the measured value with the set point adjusted by the operator. The output signal reaches to the heavy fuel oil flow control valve. A/M station MA1012 (Oil flow) , MA1040 (Fuel master), MA 1016, MA 1020, MA 1024, MA1028, MA1042, MA1036.(Coal feeder speeds) have been used The signal monitors(for feeder on Auto, set minimum, speed>30%) and system output contacts are provided for each feeder. Release for totalizing circuit, Release feeder speed to Auto, Set feeder speed to minimum; signals are provided for each feeder.

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CMC- FIRING RATE : FUEL FLOW /COAL FEEDER SPEED CONTROL

AIR FLOW CONTROL (F.D.FAN CONTROL) The secondary airflow is measured with two redundant flow transmitter s for left and right side. Each signal is pressure and temperature compensated in computer card 22.The excess air controller 03 via the proportional adjuster 07 influences the set point signal. The set point for the excess air Controller is developed by function generator card 12, according to the firing rate demand Signal as a load characteristic and compared to the measured oxygen value in a PI-step-Controller 03. To ensure a minimum airflow for furnace purge and low load operation a maxselector 15 is provided. Cross limiting by comparison of the master Signal with total fuel and air flow to ensure excess air condition under all circumstances is included in the air flow control. On load pick up, the airflow will be increased first and on load drop, the fuel flow must be decreased first. Provision is also made for selecting either of the two oxygen analysers to be used for control & for switching the oxygen control to manual upon failure of the oxygen content measured signal in use with the help of Signal monitoring card 30 & Analogswitch 51and for selecting either of the redundant secondary airflow signals or for rejecting FD blade pitch control to manual upon failure of the Signals in use. A uniform dynamic response of the output for the entire load range is maintained by automatically changing the appropriate control system gains in P-amplifier card 16 as and when F.D. fans are taken in and out of service. A/M station (MA1001) and (MA1002) for FD fan A /B with provision for manual biasing and (MA10013) for Excess air control has been provided.

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CMC- FIRING RATE: SECONDARY AIR FLOW CONTROL

The interlocks STOP +y/-y in case of furnace pressure too high or to low from DRG6 and MAN-transfer in case of failure of the corresponding ID fan is provided. Signal monitor (SMD 019) for fuel air deviation high & very high and SMD 020 for airflowless than 30% & less than 40% and the system input contacts of ‘CLOSE’ & ‘OPEN’ and system output contacts of Lock-in position Signal to final pneumatic actuator in case of Controller failure for both F.D. fans A & B are provided. PULVERISERS TEMPERATURE

& AIR FLOW CONTROL

The pulveriser temperature control loop has been used to operate the hot and cold air dampers in tandem and in a pre-determined relationship, such that adjustments in the ratio of hot and cold air result in a minimum disturbance to primary airflow. It means the hot air damper opens and the cold air damper closes to maintain the required temperature of the PA to mills. Flow sensing tubes (pilot tube) in each primary air duct sense and measure the primary airflow to each Pulveriser; the temperature is compensated by a computer card 22, which also performs the square root extractor function. The measured primary airflow Signal is applied to the PIController 04 and is compared with the desired set point. The Controller 04 generates a control Signal to position both the hot and cold air damper together for the regulation of primary airflow. The measured temperature Signal is applied to the PID Controller 04, which generates a control Signal to position both the hot and cold air damper to regulate the temperature. Signal monitors and alarm contacts have been provided for temperature low and high. An interlock is provided to automatically open the cold air damper and close hot air damper on Pulverisers trip conditions. Lock-in position is used for cold and hot air KORBA SIMULATOR

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damper Controller failures. Auto Release for cold and hot air dampers, Open. for cold air damper, Close for hot air damper, Set To Min. for cold air damper etc. are available for interlock and other requirements.

PULVERISER ( MILL ) TEMPERATURE CONTROL

PRIMARY AIR PRESSURE CONTROLLER Primary air to all Pulverisers is maintained by controlling the primary air pressure (and so the air quantity) at the common discharge duct of the two PA fans A and B. The primary air pressure is measured and the output signal is given to Signal monitoring and distributing card 30. The output Signal from this Signal monitoring card is compared in a ‘P’ amplifier card 16 with the desired pressure set point from the A/M station MA-1010 and the resulting error is amplified and processed in the Controllers for finally driving the E/P converters of the power cylinders for regulating the positions of the PA fan A/B inlet dampers/vanes. A uniform dynamic

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response is maintained by automatically changing the appropriate control system gain depending on the number of fans in AUTO. An interlock is provided on both Controllers to automatically close the inlet vanes on start command of PA fans. Lock-in position Signal for pneumatic actuator of PA fan A and B and the CLOSE Signal for PA fan A and B are made available in this control loop for required applications.

PRIMARY AIR PRESSURE CONTROL

BOILER FURNACE PRESSURE CONTROL Boiler furnace pressure is controlled by positioning the inlet vanes of the two Induced Draft fans. Two redundant furnace pressure transmitters have been used. The two transmitters are connected through separate taps on the furnace. A comparison network is provided to supervise the outputs of the transmitters with Signal monitors card 17.This comparison network automatically selects the draft control system to manual and also actuates an alarm contact upon detection of high deviation between the outputs of the two transmitters. Furnace pressure is measured and compared to the desired set point in P-amplifiers card 16 and error is amplified and processed to drive the E/P converters of the inlet vane position power cylinders. A uniform dynamic response of the system for the entire load range of the unit is maintained by automatically changing the appropriate control system gain in proportional amplifier card 16, depending upon the number of fans in Auto. The furnace pressure controller also receives the signal of total airflow as a feed forward Signal for an immediate interaction in case of any change in the airflow. KORBA SIMULATOR

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One cabinet mounted switch is provided for selection of one of the transmitters to be used for control. A second cabinet mounted switch is provided to detect the monitor when it is necessary to operate with one transmitter out of service. A/M stations MA-1004 & 1006 have been used for ID Fan A/B vane drives. Pressure switches for high and low furnace pressure and one monitor card 17 is provided for interlock & protection Signal for alarm on High differential between pressure transmitters &Defeat is provided in the control loop. An interlock is provided to block the automatic control from increasing the induced draft fan inlet vanes and decreasing the forced draft fan bled pitches when furnace pressure is excessively low. Conversely, when furnace pressure is excessively high, induced draft fan decreases and forced draft fan increase is blocked. Interlocks are provided to automatically close or open the ID fan inlet vanes.

BOILER FURNACE PRESSURE ( SUCTION ) CONTROL

BOILER DRUM LEVEL (FEED WATER FLOW & STEAM FLOW) CONTROL The feed water flow to the boiler is controlled by the low range and high ranges FWControl valves that maintain the drum level as desired. The boiler drum level is measured on the left and right side. The boiler drum level signal is pressure compensated in computer card 22. The left or right drum level signal is compared with the set point in the PI-Controller 02 for the FW low range control valve and in KORBA SIMULATOR

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the PI-PI Controller 23 for the FW high range control valve. Provisions are there for selection of the level transmitters (mounted in right and left sides) and also automatic transfer to Auto for the low range to high range Controller in case of Feed Water -flow less than 20% or larger 30% conditions. Following output contacts are provided for alarm/signaling:Drum pressure left. L/LL Drum pressure right.L/LL High-Diff.of right/left Alarm Drum pressure left L/H

SM-D021; Drum level left. H/HH SM-D023 SM-D024 Drum level left. SM-D024 SM-D025 (switch defeat) SM-D036 Drum pressure right L/H SM-D037

The feed water flow is measured in both pipes doubly by orifice/flow elements, which create differential pressure (Delta-Pi) proportional to the feed water flow. The ‘DP’ signals are temperature compensated and square root extracted in computer card 22. Total FW-flow thus obtained, becomes the feed back signal to the FW high range valve controller for further processing.

BOILER DRUM LEVEL CONTROL

The Main steam flow is measured on the left and right sides of main steam pipes. In each case the delta-P signals are square root extracted, pressure-and temperature compensated in the computer card 22. The two signals are added in the operational amplifier card 16 and processed for various uses. The auxiliary steam flow is measured and added to the signal ‘steam flow to turbine. The steam pressure signals PT-D003 and D004 are supervised for Low and High values with signal monitors SM D060 and D061 the difference between both signals is monitored with KORBA SIMULATOR

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SM D062 and alarm contact is provided. The feed flow and the steam flow should match in normal conditions FEED WATER CONTROL VALVE DIFFERENTIAL PRESSURE CONTROL The Feed Water control valve differential pressure control is implemented to ensure an optimal reaction of F/W flow control by holding the differential pressure (DeltaP) above F.S valve, as constant. The Delta- P is measured and compared in an OpAmp with that of the set point value. The resulting signal reaches to a following amplifier with provisions for gain change if two boiler feed pumps are in operation. The control deviation signal passes to the Delta-P master Controller. Main steam flow signal is used as a feed forward signal. The power amplifier drives the electromechanical actuators that are mounted on the hydraulic coupling of BFPs.

BOILER FEED WATER DIFFERENTIAL PRESSURE CONTROL

The Controller output is connected to the Scoop Controllers of Boiler feed pump A, B, C .The UCB operator can adjust the scoop position set point via proportional adjuster for balancing the scoop position controllers of individual pumps that are in operation. The output signal of these position Controllers are going via function generator card 12 to the power amplifier KE 01 (40) for each scoop tube of BFP hydraulic coupling. Scoop tube can be set to minimum position during starting.

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Auto/manual stations of DP master (MA 1009), BFP scoop Position Controller A,B,C (MA 1046, 1047, 1048) have been provided in this control loop. SUPERHEATED STEAM TEMPERATURE CONTROL The Superheated (S/H) steam temperature is maintained constant by controlling the outlet steam temperature during various plant conditions with the help of spray control valves. The S/H outlet steam temperature is measured on the left and right side. This signal is compared with the set point in a Proportional plus Integral controller. Additional to the outlet steam temperature signal the S/H or De -S/H outlet steam temp. is measured on both sides and used in the control loop. Main steam flow signal is connected as feed forward signal via a function generator card. The individual temperature signals are monitored. Modules are there for obtaining high-diff. Alarm between left and right side. The hand/auto stations of S/H outlet steam temp. Controller left/right MA 1042/1043 with provision for set point adjustment are provided in the control system. The alarm contacts are provided i.e. S/H outlet steam temp.left High/Low (SM-D026), Low Low(SM-D066) , S/H outlet steam temp Right H/L (SM-D067), Right LL(SM-D027) De S/H outlet steam temp.left H/L (SM-D028) right H/L (SM-D029) High diff. Alarm of S/H outlet temp.(. SM-D031)

SUPERHEATED TEMPERATURE CONTROL

REHEAT STEAM TEMPERATURE. CONTROL The R/H steam temperature control maintains the R/H outlet steam temperature constant during various plant conditions. The R/H temperature is measured on the left and right side and supervised for high/low values. Average temp. of both left and right side forms the measured variable and it is compared with the set point in the burner master Controller. The controller output signal drives the I/P converter of the four burner tilt power cylinders. The control deviation of the burner master

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controller is also connected to the R/H spray master controller. Main steam flow signal is used as a feed forward signal. In case of high negative control deviation, that means the R/H outlet temp. is too high, the Reheat spray control valves (left and right) are opening to avoid further increase. Under normal operation the R/H spray control valves are closed The interaction is suppressed by a fixed input signal ( i.e. 5%). Provisions are made for balancing the left/right R/H spray valve. The position of the R/H spray control valves are monitored and contacts below minimum are provided for opening or closing of additional. Block valves. Interlocks for the position controller are included. The position of the burner tilts is measured and a network is included to produce the signals i.e. Position out of synchronization (SM-D071), Nozzle tilt horizontal (SMd071) , R/H outlet temperature left& right H/L (SM-D032&33) , High diff. Temp Alarm between left/right SM-D034, R/H spray valve position left/right below MIN. SM-D042/044 alarms. DEAERATOR LEVEL AND CYCLE MAKE-UP CONTROL The deaerator level is measured by level transmitter and compared with the set point value in Controller card. The Controller output signal is used to position the D.M. make up control value. Deaerator level is maintained by modulating the D.M. make up control valve (condensate surge-tank outlet line) and thus regulates the make up water to condenser.

CYCLE MAKE-UP AND DEAERATOR LEVEL CONTROL

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DEAERATOR PRESSURE CONTROL

CONDENSER HOT WELL LEVEL AND MINIMUM RE-CIRCULATION The hot well level is measured by level transmitter and compared with the set point value in Controller card. The Controller output signal is applied to the pneumatic converters of the control valves MC-27 and MC-33. Condenser hot well level is maintained by regulating the condensate re-circulation flow control valve MC-33 to full open condition by diverting the condensate water to the condenser up to the condition that a minimum flow of 210 T/H flows through condensate extraction pumps. As soon as the CEP flow increases beyond this, the condensate flow control valve MC-27 starts opening and simultaneously the MC-33 starts closing until MC – 27 opens to 40%, by this time MC-33 is fully closed and the hotwell level is further maintained through modulation of control valve MC-27.

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CONDENSER HOTWELL LEVEL CONTROL

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BOILER WATER CHEMISTRY

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BOILER WATER CHEMISTRY It is important to maintain proper quality of feed water and boiler water for trouble free operation of boilers. The quality requirements become more stringent for highpressure boilers, as they are designed to closer tolerances. Since the boiler, water is primary output; it is not possible to achieve desirable steam purity for trouble free operation of boiler and turbine, without proper control of boiler water chemistry. In spite of good water management the internal surfaces in a boiler become dirty over a period of operation. It is essential to periodically clean the boiler during overhauls etc. The principal objective of boiler water treatment is to prevent. a. Scaling b. Corrosion c.

Steam contamination

SCALING Water contains many impurities like dissolved salts and/or suspended matter. The suspended impurities such as biological growth, mud and bacterial growth can be removed easily as compared to dissolved solids. These dissolved impurities are insoluble at elevated temperatures. When temperature rises, solubility of these dissolved salts decreases and some precipitation occurs locally. These precipitations are sticky in nature and form coating on the metallic surface. This is called scaling. It can also be described as a continuous, adherent layer of foreign material formed on the waterside of a surface through which heat is exchanged. Scales are objectionable because of their heat insulating effect. The co-efficient of conductivity of several metals and some compounds of which scales are made of, are tabulated below: CONDUCTIVITY OF METALS Metal

3

x 10 µ mho/cm

Copper Carbon steel Bessemer steel Scales Aluminium oxide fused Calcium Carbonate Ferric oxide Calcium sulphate Magnetite Magnesium Phosphate

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920 110 98 (A12 O3) (Ca CO3) (Fe2 O3) (CaSO4) (Fe3 O4) Mg3 (PO4)2

8.0 2.2 1.4 3.1 6.9 5.1

341

From the above table it is very clear that how insulating these scales are. It is sometimes said that a thin layer of Ca CO3 (calcium carbonate) should be maintained to protect the surface from corrosion. But it is impossible to lay down uniform thickness of scale, because the scale thickness depends upon the amount of heat being transferred, which is not same in all sections of boiler. Any scale in the boiler, however, is absolutely undesirable. Scales and deposits are formed because the compounds, of which they are composed, are insoluble under high temperatures prevailing in the boiler. Certain anhydrous calcium salts especially sulphate, decrease in solubility as temperature and pressure increase. Similarly, solubility of Ca CO4 decreases rapidly with increasing temperature producing extremely hard, adherent coating on boiler tubes, especially in locations where heat flux is high. Accumulation in boiler drums is most often in the form of mud or sludge. When oil is present as a contamination in boiler water, loose scales may form particularly in water tubes. Oil serves as a nucleus and binder of scaling at hot spots. The ' oil balls'found in steam drum and water wall headers are typical formation in turbine flow sections. Prevention of Scaling The most effective method for prevention of scaling is to eliminate scale-forming elements from the feed water, or to transform them by some means into an innocuous form. That is the reason why dematerialised water is used in the system. Demineralises can produce water quality with nearly zero hardness. All ionised salts are removed in these processes, which greatly minimise the potential for boiler deposits, corrosion and turbine fouling. On-line removal of scales forming salts is done by phosphate treatment. These salts are inevitably present is the boiler water in the form of residual hardness even after demineralisation. The tri-sodium phosphate, which is used for phosphate treatment, tends to increase the pH value while di-sodium phosphate formed as a by-product; is a neutral salt. Tri-sodium phosphate reacts with salts of Magnesium and Calcium to form sludge (Calcium & Magnesium Phosphate). The reaction is as follows: 2NA3 PO4 + CaSO4 Na3 PO4 + H2O Na2 HPO4 + H2O

= = =

Ca3 (PO4)2 NaOH NaOH

+ 3Na2 SO4 + Na2 HPO4 + Na3H2PO4

The advantages of phosphate treatment are: •

Adequate alkalinity can be maintained in the boiler water.

•

Na2 HOP4 is additionally available to form phosphate sludge.

•

Self-containing hydrolysis, hence proper control over pH (above pH =10.2 reactions reverts to left) can be observed.

•

No Problem of caustic corrosion.

The only disadvantage of phosphate treatment is Phosphate hideout.

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Phosphate Hideout Sometimes, objections are raised against co-ordinate phosphate treatment because of the phenomenon of the phosphate hideout. At higher loads phosphate comes out of the solution because of its low solubility at raised temperature. But when the load is reduced it goes back into the solution adding to the total phosphate content of water. This may lead to excessive total dissolved solids content of boiler water in drum. However, there is no danger in phosphate hideout. It is nothing more than a nuisance to the operators for control of boiler water chemistry regarding (PO4) - level control. Any phosphate salts, hidden out, are available to take care of any hardness in water. It is never advisable to resort to any boiler water treatment with free caustic, even in small amounts. It does much more damage to the boiler tubes than any ' phosphate hide-out' . The recommended boiler water limits for phosphate & drum water pH are as follows: Phosphate should be

<

3 - 10 ppm

pH

=

9.4 - 9.7

CORROSION Scattered pitting in the presence of oxygen is sometimes observed in the water line in the steam drum and in the down-corner tubes of boilers. Economiser, on account of their high temperature, is also susceptible to corrosion by oxygen. The mechanism of pitting in a metallic surface produced by a bubble of air is shown in the figure 10.3. Two stages in the formation of a pit are represented by the following chemical half reactions Cathodic 2e- = 1/2 O2 = H2O = 2 OH Anodic

20H = Fw = Fe (OH)2 = 2e-

It ultimately forms a cell and is continuous in nature unless surface of oxygen is removed. Corrosion of iron and copper in condensate systems leads to formation of porous deposits under which salts in boiler water concentrate and damage the underlying surface of boiler steel. Then, even in absence of deposits, caustic gauging can occur owing to the concentration of sodium hydroxide particularly in places where the rate of heat transfer is unusually high. Other possibilities are the corrosion of stressed metal. The severity of these effects can be controlled to some extent by reducing the concentration of oxygen and free alkali, and by eliminating products introduced from pre-boiler system. Corrosion is the oxidation of metal by some oxidising agent in the environment. The area over which the metal is oxidized is called the anode and at which the oxidising agent is reduced is called cathode. These areas are necessarily separated but usually are not far apart. As corrosion products, electrons flow between these areas through the metal while ions migrate through the solution. This system constitutes an electro-mechanical cell. In boiler the oxidation of iron is accompanied by the reduction of hydrogen ions supplied by the hot water. 3Fe + 4H2O = Fe3O4 + 4H2) KORBA SIMULATOR

(A) 343

In case of acidic water 2 H+ + Fe

= Fe ++ + H2

(B)

The reaction (A) is self-limiting on account of the barrier of Fe3 O4 that forms on the surface of the metal. The reaction (B) on the contrary, continues until the supply of hydrogen ions is depleted in boilers. Both reactions are posed by an irreversible potential called the hydrogen over-voltage, which is affected, by the condition of the surface of the metal. PREVENTION OF CORROSION Removal of Oxygen Oxygen is introduced into boilers dissolved in feed water. When this water enters the steam drum, most of the oxygen flashes into steam space, producing characteristics pitting at the water wall lines and in the vicinity of the discharge of the feed line. In addition in high-pressure boiler, several local corrosion, pinhole failures and pitting in the rear furnace wall tubes are attributable to attack by the dissolved oxygen. The concentration of dissolved oxygen in feed water should be less than 0.03 ppm and preferably less than 0.005 ppm in water for high-pressure boilers. Cold water saturated with air contains about 10 ppm of oxygen. This can be reduced to 0.3 to 0.7 ppm in an open heater and about 0.01 in a spray type deaerator normally used inn power plants. The greater part of corrosive gases, and carbon dioxide and oxygen that are dissolved in water can be removed by de-aeration. Open heaters are suitable for low pressure but spray type deaerating heaters are commonly used. In these units, steam heats the feed water in primary heater and also scrubs the heated water. Hot spray flows down through a baffle arrangement against a rising flow of steam that sweeps the liberated gases out through a vent at the top of the vessels, while deaerated water collects in a storage section at the bottom. The vent is equipped with a condenser through which cold feed water flows to prevent excessive wastage of steam. The oxygen concentration of less than 0.007 ppm can be obtained through this method. Because of volatilisation of CO2 and thermal decomposition of bicarbonate, the pH of deaerated water is normally maintained 8.5 - 9.5. 2 HCO3

=

CO3 + CO2 + H2O

H+ + HCO3

=

CO2 + H2O

So far we have discussed the mechanical method of deaeration. Let us see how effectively the corrosive oxygen is removed with the help of chemicals. There are two chemicals, which are primarily used to remove oxygen. a.

Sodium Sulphate

b.

Hydrazine

Sodium Sulphate is commonly used in boilers operating at less than 60 Kg/cm2 KORBA SIMULATOR

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while hydrazine is the reducing agent at higher pressures. The reaction of hydrazine is as follows: N2H4 + O2

=

N2 + 2H2O

3N2H4

=

4NH3 + N2 (at 200

0

C)

H2O 2NH3 + CO2

(NH4)2 (At 270

0

C)

Nitrogen being inert gas gets liberated and is removed as non-condensable gas. Advantages of hydrazine treatment are o

Low equivalent weight.

o

Does not increase dissolved solid content of drum water

Disadvantages of hydrazine treatment are o

Vapour toxic nature.

o

Excess of hydrazine at high temp disintegrates into ammonia.

o

Concentrated solution of hydrazine is flammable.

At the economiser inlet the concentration of hydrazine is to be limited to 0.050 ppm. The presence of porous deposits on the waterside of boiler tubes lead to serious corrosion, especially when there is free alkali in the water. Various conditioners are added to disperse insoluble materials and prevent the accumulation of sludge on surface, where the heat is transferred. Recently poly-crystallites and other synthetic polymers have come into use. Small amount of particulate matter comprising finely divided oxides of copper often contaminate condensate. These oxides, besides causing foaming, deposit on boiler tubes at a rate proportional to the heat flux. The rate of deposition increases rapidly above 55 Kg/cm2. The presence of these deposits causes over heating of the tubes and sometimes ductile gauging. Direct reaction of steel with particles of ferric oxide is also possible. 4Fe2O3 + Fe+ = 3Fe3O4 Monitoring Feed/Condensate Water pH The metallic surface on the waterside of a boiler tube is naturally protected by a thin film of magnetite formed by the action of hot water on steel. 3Fe + 4H2O =

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Fe3O4 + 4H2

345

Ideally there is no further oxidation of metal after the protective layer is formed. The minimum rate of corrosion is realised at pH value 11 to 12. At lower pH values hydrogen ions are discharged whereas at values greater than 12, the magnetite layer thickens, peptises to some extent and is made porous by diffusion of ions from underlying metal. Above pH value of 13, the magnetite layer is completely destroyed. 2 With the pressure above 40 Kg/cm , hydrogen may diffuse into metal, blistering and weakening it severely. Hydrogen atoms react with the carbon in steel to form methane. The pressure generated in this may cause fissures along the grain boundaries, so ideally the pH of 8.5 to 9.5 should be maintained. The re-circulation of a small amount of alkaline boiler water through BFP has been recommended but this can lead to plugging of feed lines in economiser, feed water heaters by insoluble phosphate. At lower pH values than 8.5 in the drum, the removable sludge formed by phosphate treatment of scale forming salts becomes very sticky itself. Also at lower pH values the silica carry over (Distribution ratio x1/pH) increases very rapidly, not to say a rapidly increasing rate of corrosion due to pH values lower than those recommended. STEAM CONTAMINATION Carry over of salts in steam occur either due to mechanical or vapour carry over. Efficient drum internals can only reduce mechanical carry over. Silica is always carried over in vaporous form. The vaporous carry over of remaining salts mainly 2 sodium salts is significant only at pressure above 180 Kg/cm . The carry over may occur in four types. They are: a.

Leakage carryover.

b.

Spray or mists carry over.

c.

Priming.

d.

Foaming.

Leakage Carryover Leakage of water droplets through seams or gasket joints of steam purifying equipment cause this type of carry over. It is usually highly localised and may not be detected in steam purity test unless the sampling points are very near to the leakage point. This will be revealed in steam purity measurement by the fact; that with increase in load condition the purity of steam will deteriorate. The change in water level may not alter the degree of contamination. As this type of carry over is most frequently localised, it is responsible for localised super heater failure. If suspected, then new gasket or seal welding may be required to eliminate this problem.

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Spray or Mist Carry-over In this case, atomised droplets of water will be carried with the steam. This is common in all boilers to some extent. Spray or mist carry over can be avoided by installing steam purification equipment. If the steam purification equipment is under designed, this will be present even after the installation of these equipments. Priming It is relatively unimportant in present power plant boilers and is rarely encountered. Carry over of this type is characterised by a sudden carry over of gross quantities of boiler water, which would show up a drastic deterioration of steam purity. It can be caused by the variation in pressure such as large pressure drop. In this case the water in the boiler would swell due to expansion of steam and formation of additional steam. This action is similar to the” bumping" experienced when water is boiled in an open beaker. It is more violent spasmodic action, resulting in the throwing of slugs of boiler water with the steam flow. Foaming The important factors that affect the carry over in steam are: 1.

Drum and its internals.

2.

Water level in the drum.

3.

Boiler water concentration.

4.

Foaming and vaporous and carry over.

To achieve the quantity or purity of steam required for power and industrial units, mechanical contrivances are provided inside the boiler drum. These are known as drum internals. They distribute and mix feed and chemicals added to boiler water while removing entrained moisture from steam as it leaves the drum. The three basic effects by the internal arrangements are: 1. Centrifugal action to produce separation force, which is many times greater than gravity. 2. To direct the steam water mixture so that the upward velocity vector is zero. 3. Provision of drainable wetted surface in which fine spray can coalesce. Foaming is the condition resulting from the formation of bubbles on the surface of boiler water. The foam produced may entirely fill the steam space of the boiler or may be relatively minor depth. In either case this foaming condition causes appreciable entrainments of boiler water with steam. Generally presence of organic matter and/or oil will promote foaming.

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Silica Carry Over Certain dissolved solids in the boiler water are carried away with the steam as vapour and the internals have no control over such vaporous carry over. One of the detrimental constituents is silica. In order to limit silica carry over, the concentration of silica in the drum water must be limited to a specified value for a given operation pressure range. In order to control the silica in boiler water, the most effective method used is blow down. Blowdown As steam leaves the boiler, solids introduced in feed water are concentrated in the water left behind in the drum. If this concentration were allowed to continue, the less soluble components in the water eventually crystallize on the internal surface and in addition, the steam would contaminate. In ideal operation, the concentration of solids is allowed to reach the limit after which the concentrated boiler water is bled off at such a rate that the amount of solids entering in feed water is exactly balanced by that method in bleed stream. This process is called continuous blow down. Pressure

Dissolved Solids

Suspended Matter

Alkalinity

Silica

1000 psi

500

10

50

10

1500 psi

150

3

0

3

2000 psi

50

1

0

1

Suspended solids in the presence of iron tend to collect as sludge in the lower parts of boiler i.e. down comers or ring header. Opening the intermittent blow down can blow out this concentrated sludge. The turbulence caused by opening the valve disperses the sludge. So there is no point in leaving the valve open longer than 15 sec. It is a usual boiler practice that water wall headers should never be blow down because the circulation or water through them is usually critical when the boiler is on load. The outlet for CBD should be below the level where the riser tubes enter the steam drum because this is where the dissolved solids in re-circulating water are most concentrated. Chemicals for internal treatment (phosphate, sulphate etc.) should be introduced above the down comer tubes to prevent sludging on the hot risers, and also to promote mixing and reaction with saline in the entering feed water. If the chemicals are injected near the blow down outlet, short-circuiting will occur. Continuous blow down is the most effective way for controlling the amount of solids in boiler water after the rate of withdrawal has been adjusted properly. If the rate of blow down is too high, heat and water are wasted, if too low the permissible limits will be exceeded.

KORBA SIMULATOR

348

% Blow down (In terms feed water)

=

% Blow down (In terms of steam)

=

Total Solid in feed Total solid in water Total solids in feed

x 100

Total solid-Total solid in feed

RECOMMENDED BOILER / FEED WATER LIMITS The following are the generally recommended feed water and boiler water limits for high pressure drum type boilers: Total solids

50 ppb

(max.)

Total iron

10 ppb

(max.)

Total copper

10 pbb

(max.)

Total silica

20 ppb

(max.)

Total oxygen

5 ppb

(max.)

Feed water pH

9.2 to 9.4

Phosphate

5-10 ppm

Boiler water pH

9.4 to 9.7

The recommended boiler and feed water limits for 210MW Korba boiler is as follows: Recommended Feed Water Limits 2

Operating drum pressure in Kg/cm (g)

60-100

100% & above

Hardness

Nil

Nil

pH at 25 oC (copper alloy pre-boiler system)

8.8-9.2

8.8-9.2

pH at 25 oC (copper free pre-boiler system)

9.0-9.4

9.0-9.4

Oxygen (max.) ppm

0.007

0.007

Total iron (max.) ppm

0.01

0.01

Total copper (max.) ppm

0.01

0.005

Total CO

Nil

Nil

Total silica (max.) ppm

0.02

0.02

Sp. conductivity after cation exchanger (mho/cm)

0.5

0.3

Residual Hydrazine ppm (before economiser)

0.01-0.02

0.01-0.02

Oil

Not allowed.

KORBA SIMULATOR

349

Recommended Boiler Water Limits 2

60-125

125-165

165-180

Total dissolved solids (max.) in ppm

100

50

25

Sp. Conductivity at 25oC (mho.cm) max

200

100

50

Phosphate residual ppm

5-20

5-10

3-7

pH at 25 oC

9.1-10

9.1-9.8

9.1-9.8

Silica (max.) ppm

To be controlled on the basis of Silica distribution / Drumpressure curves to maintain less than 0.02 ppm in steam leaving the drum

Drum operating pressure in Kg/cm

KORBA SIMULATOR

350